Hot off the presses! Mar 15 J Cryst Growth

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Latest Articles Include:

• Contents of The International Conference on Molecular Beam Epitaxy
  - J Cryst Growth 311(7):iii-xii (2009)
  
• Editor's preface
  - J Cryst Growth 311(7):1621-1622 (2009)
  
• Fifteenth International Conference on molecular beam epitaxy: Vancouver, BC, Canada, August 7–14, 2008
  - J Cryst Growth 311(7):1623-1624 (2009)
  
• Quantum dot lasers: From promise to high-performance devices
  - J Cryst Growth 311(7):1625-1631 (2009)
  Ever since self-organized In(Ga)As/Ga(AI)As quantum dots were realized by molecular beam epitaxy, it became evident that these coherently strained nanostructures could be used as the active media in devices. While the expected advantages stemming from three-dimensional quantum confinement were clearly outlined, these were not borne out by the early experiments. It took a very detailed understanding of the unique carrier dynamics in the quantum dots to exploit their full potential. As a result, we now have lasers with emission wavelengths ranging from 0.7 to 1.54 μm, on GaAs, which demonstrate ultra-low threshold currents, near-zero chip and α-factor and large modulation bandwidth. State-of-the-art performance characteristics of these lasers are briefly reviewed. The growth, fabrication and characteristics of quantum dot lasers on silicon substrates are also described. With the incorporation of multiple quantum dot layers as a dislocation filter, we demonstrate lasers! with Jth=900 A/cm2. The monolithic integration of the lasers with guided wave modulators on silicon is also described. Finally, the properties of spin-polarized lasers with quantum dot active regions are described. Spin injection of electrons is done with a MnAs/GaAs tunnel barrier. Laser operation at 200 K is demonstrated, with the possibility of room temperature operation in the near future.
• Nitride-based laser diodes by plasma-assisted MBE—From violet to green emission
  - J Cryst Growth 311(7):1632-1639 (2009)
  We present recent progress in growth of nitride-based laser diodes (LDs) and efficient light-emitting diodes (LEDs) made by plasma-assisted MBE (PAMBE). This technology is ammonia free, and nitrogen for growth is activated by RF plasma source from nitrogen molecules. The recent demonstration of CW blue InGaN LDs has opened a new perspective for PAMBE in optoelectronics. The LDs were fabricated on low threading dislocation density (TDD) bulk GaN substrates at low growth temperatures 600–700 °C. In this work, we describe the nitride growth fundamentals, the influence of the TDD on the layer morphology, the peculiarities of InGaN growth as well as properties of LEDs and LDs made by PAMBE.
• Theoretical and experimental molecular beam angular distribution studies for gas injection in ultra-high vacuum
  - J Cryst Growth 311(7):1640-1645 (2009)
  A dedicated set-up was designed and constructed for the purpose of accurately measuring flux distributions out of gas injectors of various shapes into ultra-high vacuum (UHV). Angular flux profiles of nitrogen exiting through various axially symmetric injectors over a range of flow rates are measured. The intent of these experiments is to validate a self-consistent simulation model based on the cosine law of molecular scattering on the walls. The experimental results give a clear confirmation of the proposed model within the limits of the molecular flow regime. Beyond this limit, the experimental profiles become flow rate dependent and gradually deviate from the model predictions.
• Unintentional aluminum incorporation related to the introduction of nitrogen gas during the plasma-assisted molecular beam epitaxy
  - J Cryst Growth 311(7):1646-1649 (2009)
  We examine unintentional incorporation of Al during the growth of molecular beam epitaxy (MBE) related to N gas introduction. In spite of the closed shutter of Al cell, we observe Al incorporation in the epitaxial layer with a concentration up to 1×1018 cm−3. The concentration depends on the N2 gas flow rate and Al source temperature. The concentration is suppressed by the reduction of the Al cell temperature. Shutter control of the N cell, As beam equivalent pressure and the operation of the RF plasma power have no impact on that. The introduction of the large amount of N can modify the beam dispersion of Al in the MBE chamber, which will cause the extrinsic Al incorporation. The unintentional impurity incorporation can induce material deteriorations. We thus suggest that the growth should be carried out with the decreased Al cell temperature and the N gas flow rate as low as possible.
• Study of the oxygen incorporation in Al0.2Ga0.3In0.5P:Be layers grown by MBE employing a P-cracker cell
  - J Cryst Growth 311(7):1650-1654 (2009)
  In this work the incorporation of oxygen in Al0.2Ga0.3In0.5P:Be layers lattice-matched to GaAs grown by solid source molecular beam epitaxy (SSMBE) was studied by secondary ion mass spectrometry (SIMS) and photoluminescence (PL) spectroscopy. By increasing the temperature of the phosphorous cracking zone from 800 to 1000 °C the amount of oxygen was considerably raised by more than one order of magnitude as measured by SIMS. The increasing oxygen content resulted in the creation of radiative deep-defects in the samples, some of them identified as transitions between O-related moderately deep donor-like states and Be-acceptor levels (DdAP) as observed by low-temperature PL spectroscopy. Other observed lines were associated with Be-acceptor levels (A0,X), shallow impurities (A,X) and the band-to-band transition (B,B). On the other hand, as the beryllium concentration increased the PL intensity of the A0,X and DdAP lines increased, indicating that Be-incorporation in the ! Al0.2Ga0.3In0.5P epilayers enhances the probability that A0,X and DdAP transitions occur. After annealing the samples, the DdAP transition significantly increased due to further activation of O-related moderately deep donor-like states. Also the 15 K-PL spectra from annealed samples showed another lower-energy broad band located at 0.271 eV below the AlGaInP band edge, presumably related with oxygen deep-defect levels (O,DL).
• Gallium beam analysis and implications for the growth of ultra-high-mobility GaAs/AlGaAs heterostructures
  - J Cryst Growth 311(7):1655-1657 (2009)
  The composition of gallium beams originating from an effusion source in an ultra-high-vacuum system is monitored with a mass spectrometer mounted in cross-beam geometry. Here, thermal cycles comparable with operating conditions as in an epitaxial growth chamber can be simulated and beam components with partial pressures of 10−13 Torr can be reliably detected. Using different gallium charges, particular focus was placed on gallium-oxygen- and gallium-hydrogen-compounds, whose signatures fade after a few thermal cycles, but do not disappear entirely. No other impurities have been detected in the same sensitivity range. In parallel, employing Ga loads from the same batches, high-purity simple GaAs/AlGaAs heterostructures exceeding low-temperature electron mobilities of 107 cm2/V s were grown in a separate system. Mobility versus density dependence of these high-mobility samples points to the simple architecture of the heterostructures as being the current limit of the e! lectron mobility. The Ga analysis suggests the Ga2O and GaH3O are the main reasons for background impurities in the very early stages of a growth campaign.
• MBE growth of ultra-low disorder 2DEG with mobility exceeding 35×106 cm2/V s
  - J Cryst Growth 311(7):1658-1661 (2009)
  Two-dimensional electron gas (2DEG) in AlGaAs/GaAs heterostructures, grown by molecular beam epitaxy (MBE), has been a favorite template for numerous research in a field of quantum physics during last several decades. While in the early stages the main efforts were devoted to fabricate extremely high-mobility 2DEG by concentrating on the purity of the grown material, nowadays it became clear that the further progress in the field requires new approaches of heterostructures design and the growth procedure. Here we report on the MBE growth of AlGaAs/GaAs heterostructures using a short-period superlattice (SPSL) doping instead of the more standard n-AlGaAs doping. Such doping process allows the use of a low AlAs mole fraction spacer which, in turn, leads to a lower background of impurities as well as a better interface quality. Mobility exceeding 35×106 cm2/V s was measured in samples with doping introduced on both sides of a quantum well (QW) where the 2DEG was imbedded! in. Most importantly the SPSL doping allows introducing "correlations" between ionized donors and allows controlling the potential disorder landscape that governs the appearance of various fractional quantum Hall states.
• Bandedge absorption of GaAsN films measured by the photothermal deflection spectroscopy
  - J Cryst Growth 311(7):1662-1665 (2009)
  The optical absorption of GaAsN films grown by molecular beam epitaxy on GaAs substrates is measured using the mirage effect photothermal deflection spectroscopy (PDS). The PDS spectra were fitted with a modified Fernelius model, which takes into account multiple reflections within the GaAsN layer and GaAs substrate. This allowed the extraction of bandedge parameters for a series of GaAsN films with N content varying from 0.24% to 1.4% N. All films show a clear Urbach absorption edge with a composition-dependent bandgap consistent with literature and Urbach slope parameters roughly 3 times larger than GaAs values.
• Study of AlGaAs/GaAs quantum wells grown by molecular beam epitaxy on GaAs substrates subjected to different treatments
  - J Cryst Growth 311(7):1666-1670 (2009)
  AlGaAs/GaAs quantum wells (QWs) structures were grown by molecular beam epitaxy (MBE) on semi-insulating- and Si-doped GaAs (1 0 0) substrates subjected to different surface treatments. The substrate treatments employed were: (a) H2SO4-based chemical etching, (b) hydrogen flux exposition at 650 °C under an arsenic overpressure, and (c) ultraviolet (UV) light irradiation in ambient conditions. In order to evaluate the impact of the substrate treatment on the samples properties the QWs were grown directly on the substrates with no buffer layer. Secondary ion mass spectroscopy (SIMS) was used to detect oxygen and carbon impurities at the QWs/substrate interface. From an analysis of the full-width at half-maximum (FWHM) of 10 K-photoluminescence (PL) peaks, we estimated the roughness at the interfaces in each QW. Photoreflectance spectroscopy (PR) at 300 K was employed to evaluate the optical quality of the structures. The lowest concentration of C and O contaminants, and! the QWs with the lowest interfacial roughness were achieved employing the H+As exposition. On the other hand, we found that UV irradiation is effective with brief exposition times (less-than-or-equals, slant1 min), but for longer UV exposition times the QWs are severely damaged.
• Molecular beam epitaxy in a high-volume GaAs fab
  - J Cryst Growth 311(7):1671-1675 (2009)
  Since building its first GaAs wafer fabrication facility in 1997, RF Micro Devices has grown into the world's largest consumer of GaAs substrates. The vast majority of these wafers pass through its internal MBE facility for the deposition of HBT and PHEMT epilayers before being kitted into lots for the 4- and 6-in GaAs wafer fabs. Rapid growth, a continuing push to reduce costs, and commitment to shipping the highest quality epiwafers have been primary forces driving activity within our MBE operation. In this paper, we present an introduction to RFMD and its GaAs HBT and PHEMT manufacturing capabilities and scale. This will be followed by a discussion of strategies applied to increasing operational efficiencies, reducing costs and reducing process variation. This includes maximizing system uptime and developing a process that is well-documented, automated as much as possible, and robust such that output is consistent regardless of having half a dozen different types of! MBE systems, a dozen engineers and several dozen MBE technicians operating the equipment.
• Design elements affecting wafer temperature uniformity in multi-wafer production MBE systems
  - J Cryst Growth 311(7):1676-1679 (2009)
  In this paper we present the results of key experiments that have illuminated our understanding of the interactions between wafers, backing rings, platen and manipulator heater in a 7×6″ multi-wafer production MBE system. We present examples of detailed wafer temperature maps across platens of 6-in diameter GaAs wafers. Results are discussed in terms of a qualitative model that can guide the design of MBE system manipulators, platens and backing rings that will be capable of producing a high degree of temperature uniformity. One of the most important factors is that the backing ring must be of proper width to compensate for heat conducted into the wafer perimeter through contact with the hotter platen. The platen must also be far enough from the radiative heating source, and of low reflectivity, to prevent reflected power from causing localized temperature non-uniformities in the heater. Because of the importance of platen emissivity to the thermal environment of th! e wafers, users should be aware of unintended changes in emissivity as platens become coated through weeks or months of use.
• Strained quantum wells in scrolled structures studied by μ-photoluminescence
  - J Cryst Growth 311(7):1680-1683 (2009)
  We investigate the impact of the specific strain gradient on the optical properties of quantum well (QW) structures. This strain gradient emerges if the layer stack is released from the underlying substrate forming a scroll. As the strain state in this configuration changes from compressive to tensile strain the character of the fundamental optical transition in a strained QW is modified. Polarization-dependent low-temperature μ-photoluminescence studies reveal that in all the examined QWs in scrolled structures the valence band has a mixed heavy/light hole like character in agreement with calculations. Therefore, for the interpretation of optical and electronic transport properties of double heterojunctions with a strongly curved geometry the actual built-in strain has to be considered.
• A study of the doping influence on strain relaxation of graded composition InGaAs layers grown by molecular beam epitaxy
  - J Cryst Growth 311(7):1684-1687 (2009)
  We investigate the role of p- and n-type doping in strain relaxation of graded composition InGaAs layers grown by molecular beam epitaxy. It is found that p-type Be-doping can improve material properties, resulting in smaller surface roughness and lower threading dislocation density, while n-type Si-doping has an opposite effect. The effect is strongly dependent on the grading profile, with linear grading showing small differences, while there is a significant difference when an exponential grading is used. Since doping is essential for many types of devices, these results are useful for improving the material properties and performance of metamorphic devices.
• Critical thickness of MBE-grown Ga1−xInxSb (x<0.2) on GaSb
  - J Cryst Growth 311(7):1688-1691 (2009)
  Several Ga1−xInxSb layers, capped with 1 μm of GaSb, were grown on GaSb(0 0 1) substrates by molecular beam epitaxy in a Varian Gen II Modular system using either the conventional sample growth position with substrate rotation, or a tilted sample position with no substrate rotation. The GaInSb layers were examined by X-ray diffraction (XRD) using both symmetrical and asymmetrical reflections. The "tilted sample method" gave a variation of ±25% in thickness of the Ga1−xInxSb layers, while the indium (In) content varied by ±10% around the nominal value. The disappearance of thickness fringes in 004 XRD scans was used to determine the onset of relaxation, as determining the in-plane lattice constant for tilted samples was found to be difficult. Determining residual strain in samples grown by the tilted method was likewise found to be very difficult. The critical thickness for several In mole fractions between 5% and 19% was determined and was found to be from 2! .2 to 2.7 times higher than predicted by Matthews and Blakeslee (1974) [J. Crystal Growth 27 (1974) 118] but lower than that predicted by People and Bean (1985) [Appl. Phys. Lett. 47 (1985) 322].
• High-temperature growth of heteroepitaxial InSb films on Si(1 1 1) substrate via the InSb bi-layer
  - J Cryst Growth 311(7):1692-1695 (2009)
  To achieve the high-temperature growth of heteroepitaxial InSb films on the InSb bi-layer, we studied the influence of substrate temperature of first layer deposition (Ts1) on the two-step growth procedure. Although the growth at higher Ts1 of 240 and 280 °C is difficult to achieve using the usual procedure due to the desorption of In atoms from the InSb bi-layer, it can be realized by means of the adsorption of excess Sb atoms onto an initial InSb bi-layer prepared via √7×√3-In surface reconstruction. The high-temperature growth of 30°-rotated InSb films at 420 °C was demonstrated on a Si(1 1 1) substrate with a InSb bi-layer. The electron mobility of the InSb film grown at 420 °C was about 20,000 cm2/V s at RT.
• Transport properties of InSb and InAs0.1Sb0.9 thin films sandwiched between Al0.1In0.9Sb layers grown by molecular beam epitaxy
  - J Cryst Growth 311(7):1696-1699 (2009)
  InAs0.1Sb0.9 active layers sandwiched between Al0.1In0.9Sb insulating layers were grown on GaAs(1 0 0) substrates by molecular beam epitaxy (MBE) where the InAs0.1Sb0.9 active layers had no lattice mismatch with the Al0.1In0.9Sb layers. Basic transport properties and electronic properties of the InAs0.1Sb0.9 were studied as functions of InAs0.1Sb0.9 thickness. Very large electron mobility of InAs0.1Sb0.9 active layers and very small thickness dependence at <500 nm were observed. The large lattice mismatch effect observed for InSb active layers grown directly on GaAs substrates and for an InSb active layer sandwiched between Al0.1In0.9Sb was almost completely eliminated.
• Molecular beam epitaxy of AlAsSb/AlAs/InGaAs coupled double quantum wells with extremely thin AlAs center barrier
  - J Cryst Growth 311(7):1700-1702 (2009)
  We realized In0.8Ga0.2As/AlAs/AlAs0.56Sb0.44 coupled double quantum wells with an extremely thin AlAs center barrier in order to decrease the energy of interband transitions to nearly 0.8 eV. By widening the In0.8Ga0.2As well for the extremely thin AlAs center barrier, the interband transition energy was successfully decreased to nearly 0.8 eV while the energy of intersubband transitions was constant at 0.8 eV. Optical and structural characterizations revealed that the In0.8Ga0.2As/AlAs/AlAs0.56Sb0.44 coupled double quantum wells with the extremely thin AlAs center barrier had good optical and structural properties.
• Growth of InAs/GaSb type-II superlattices by gas-source molecular-beam epitaxy
  - J Cryst Growth 311(7):1703-1706 (2009)
  We have investigated the growth of InAs/GaSb type-II superlattices by gas-source molecular-beam epitaxy, where Sb cracker cell and AH3 high-temperature injector are used to provide Sb and As flux, respectively. Sharp InAs/GaSb interfaces can be obtained at optimized growth conditions, revealed by high-resolution transmission electron microscopy. Moreover, it was found that the intrinsic net tensile strain of the InAs/GaSb superlattices can be partially compensated by intentional insertion of a sub-monolayer InSb layer at the interfaces of InAs and GaSb. Finally, based on InAs/GaSb superlattices grown by gas-source molecular-beam epitaxy (GSMBE), we have fabricated a photodiode with cut-off wavelength of 4.9 μm.
• High-resolution X-ray diffraction analysis of InGaAs/AlAsSb coupled double quantum wells grown by molecular beam epitaxy
  - J Cryst Growth 311(7):1707-1710 (2009)
  High-resolution X-ray diffraction was used to study the structural properties of InGaAs/AlAs/AlAsSb coupled double quantum wells (CDQWs) grown on an InP (0 0 1) substrate by molecular beam epitaxy. The InGaAs composition and the AlAs layer thickness were the considered parameters. High-resolution rocking curve measurements and two-dimensional reciprocal space mapping (RSM) were employed in both symmetrical and asymmetrical reflections. All samples showed extended satellite peaks in the rocking curves, although the sample with largest residual strain (03125) had broader satellite peaks. Symmetric RSMs indicated that none of the samples had a crystallographic tilt between the quantum well (QW) layers and the substrate. A clear strain gradient in the CDQW layers was observed in the RSM of the sample 03125, which was caused by a large residual strain.
• Hall effect and magnetoresistance analysis by electron–hole coexisting model in AlInSb/InAsSb quantum wells
  - J Cryst Growth 311(7):1711-1714 (2009)
  Transport properties were investigated for undoped Al0.1In0.9Sb/ InAs0.1Sb0.9 quantum wells (QWs), and two-carrier analyses were applied to the magnetic-field variations of the Hall coefficient and magnetoresistance at 77 K. The Hall coefficient increases with magnetic field, which shows coexistence of electrons and holes. This behaviour is similar to AlGaAsSb/ InAs QWs, which have a type-II band structure. Additionally, the Hall coefficient turns to decrease in more than 1 T, and the behaviour was not observed in the deep InAs QWs. This indicated that there are other electrons with different mobility. These are thought to be electrons extended over the QW, accumulated near the hetero-interface, and thermally excited in the barrier layer.
• Influence of arsenic flux on the annealing properties of GaInNAs quantum wells for long wavelength laser applications around 1.6 μm
  - J Cryst Growth 311(7):1715-1718 (2009)
  The influence of the arsenic flux during molecular beam epitaxy growth of 1.5–1.6-μm-emitting GaInNAs quantum wells on the annealing properties is studied. Reducing the arsenic flux results in an increase of the time needed for optimum annealing and offers an easy possibility for tailoring the annealing behavior of GaInNAs layers. Investigation of the annealing behavior of complete laser structures shows that over-annealing can already occur after the growth of complete laser structures due to the high substrate temperature during growth of cladding layers. Applying these observations to the growth of laser structures results in laser diodes covering the wavelength range from 1460 to 1610 nm with threshold current densities in the range 1.3–3.3 kA/cm2.
• GaInAsN growth studies for InP-based long-wavelength laser applications (TUA3-3)
  - J Cryst Growth 311(7):1719-1722 (2009)
  Growth studies of Ga1−xInxAs1−yNy quantum wells with indium contents covering the full compositional range (xIn=0…1) have been performed using a gas-source MBE and a valved nitrogen plasma source manufactured by the company ADDON. By using GaAs- and InP- as well as InAs substrates, differently strained GaInAsN-layers were produced, offering a complete overview of the relative bandgap shrinkage with respect to nitrogen-free GaInAs structures, after 1% nitrogen is incorporated. We have shown that this relative bandgap shrinkage depends on the absolute value of indium content of the GaInAs host matrix. As a result a new parameter ε was defined describing the relative bandgap shrinkage per percent incorporated nitrogen. When plotting this parameter over the complete range of indium content, a nearly linear decrease occurs.
• Growth and characterization of GaInNAs by molecular beam epitaxy using a nitrogen irradiation method
  - J Cryst Growth 311(7):1723-1727 (2009)
  We propose an innovative technique, making use of the In segregation effect, referred as the N irradiation method, to enhance In–N bonding and extend the emission wavelength of GaInNAs quantum wells (QWs). After the formation of a complete In floating layer, the growth is interrupted and N irradiation is initiated. The majority of N atoms are forced to bond with In atoms and their incorporation is regulated independently by the N exposure time and the As pressure. The effect of the N exposure time and As pressure on the N incorporation and the optical quality of GaInNAs QWs were investigated. Anomalous photoluminescence (PL) wavelength red shifts after rapid thermal annealing (RTA) were observed in the N-irradiated samples, whereas a normal GaInNAs sample revealed a blue shift. This method provides an alternative way to extend the emission wavelength of GaInNAs QWs with decent optical quality. We demonstrate light emission at 1546 nm from an 11-nm-thick QW, using thi! s method and the PL intensity is similar to that of a 7-nm-thick GaInNAs QW grown at a reduced rate.
• The role of Sb and N ions on the morphology and localization of (Ga,In) (N,As) quantum wells
  - J Cryst Growth 311(7):1728-1732 (2009)
  A comparative study of the influence of Sb and N ion concentration on the properties of GaInNAs (GINA) quantum wells (QWs) is presented. We find that for perfect two-dimensional (2D) QW there is no direct perceptible improvement in structural quality due to the introduction of Sb. Moreover, contrast variations in the QW suggest an inhomogeneous Sb incorporation. A different behaviour of the integrated photoluminescence (PL) intensity with temperature is observed when comparing the 2D samples grown with/without Sb. Regarding the GINA QWs exhibiting a three-dimensional (3D) morphology, we observe a significant improvement in structural quality for the samples grown with added Sb, as well as for those Sb-free samples but grown under a low N- ions density. Nevertheless, 2D and 3D samples show clear common properties: the GINA:Sb QWs exhibit an improved PL efficiency, but only at high temperatures. On the contrary, the Sb-free samples grown under a low N- ions concentration! exhibit a brighter PL at all temperatures considered.
• MBE growth and characterization of TlInGaAsN double quantum well structures
  - J Cryst Growth 311(7):1733-1738 (2009)
  In the pursuit of reducing the temperature dependence of the emission wavelengths of devices, TlInGaAsN double quantum well (DQW) structures with different barriers grown on GaAs substrates by molecular beam epitaxy (MBE) were investigated. Higher Tl incorporation, a key parameter to reduce temperature dependence, could be obtained in the TlGaAsN barrier samples. However, the presence of many dislocations and very rough interfaces together with phase separation reduced the photoluminescence (PL) characteristics. DQW structures with combined barriers of TlGaAsN+TlGaAs+TlGaAsN and those consisting of TlGaAsN with reduced N composition showed improved crystalline characteristics. The (2 2 4) reciprocal space maps of these two samples did not show any diffraction corresponding to phase segregation. However, cross-sectional transmission electron microscopy (X-TEM) images revealed the presence of inhomogeneity (i.e., the presence of nearly perfect regions with good interface! s as well as regions with rough interfaces) in these samples.
• Interface properties of (Ga,In)(N,As) and (Ga,In)(As,Sb) materials systems grown by molecular beam epitaxy
  - J Cryst Growth 311(7):1739-1744 (2009)
  Transmission electron microscopy (TEM) techniques are applied to characterize the interfaces of epitaxial III–V semiconductor heterostructures with high spatial resolution and to analyze compositional variations at interfaces. As expected, realistic interfaces of compound semiconductors are not chemically sharp but there is a transition region at the interface. The functional dependence of the smooth change in composition is sigmoidal and can be described using an analytical expression, thus enabling a quantitative characterization of the interface width. The model gives a very good description of the distribution profiles and applies to several material systems, including the interfaces of As-based alloys and those of the non-common-atom InAs/GaSb short-period superlattices. The analysis of (Ga,In)(N,As)/GaAs quantum wells reveals that there is a connection between the interface properties in this material system and the miscibility gap of the alloy.
• Optical characterization of InGaAsN layers grown on InP substrates
  - J Cryst Growth 311(7):1745-1747 (2009)
  Optical characterization of InGaAsN layers on InP substrates grown by molecular beam epitaxy (MBE) was carried out using photoluminescence (PL) and photoreflectance (PR) measurements. The PL wavelength coincides well with the energy gap (Eg) obtained by the PR measurement for the samples grown with various nitrogen compositions and growth temperatures. In particular, the sample which has the longest PL wavelength (2.03 μm at 300 K) shows a clear PR spectrum and the Eg obtained by the PR measurement corresponds well with the PL peak energy, indicating that the emission at 2.03 μm is not a defect related emission but a band-edge emission of the InGaAsN layer.
• MBE growth of GaAsN/GaP(N) quantum wells with abrupt heterointerfaces for photonics applications on Si substrates
  - J Cryst Growth 311(7):1748-1753 (2009)
  We demonstrated an appropriate growth procedure for GaAsN/GaP(N) single quantum wells (SQWs) with abrupt heterointerfaces in solid-source molecular beam epitaxy able to prevent the formation of unwanted As/P intermixing layers caused by residual As pressure. It was clarified that a GaP capping layer, grown on the top of the GaAsN SQW, was effective in avoiding the P/As exchange reaction on the GaAsN SQW surface. In fact, gettering by Ga was effective in removing the residual As2 pressure, thus preventing the formation of the unwanted As/P intermixing layer. Then, we suggested that a GaAs0.965N0.035/GaP0.98N0.02 SQW has a type-I band alignment with the conduction band offset larger than 300 meV. Finally, we fabricated a GaAs0.965N0.035/GaP SQW light-emitting diodes (LEDs) on the Si substrate. The GaAs0.965N0.035/GaP SQW LED emitted in the infrared region at a wavelength at 860 nm. Therefore, the GaAsN/GaP(N) SQW can be used as a direct-transition active layer in light-e! mitting devices on Si, to realize monolithic optoelectronic integrated circuits and systems operating in the wavelength range around 850 nm.
• Molecular beam epitaxy growth of bulk GaNAsSb on Ge/graded-SiGe/Si substrate
  - J Cryst Growth 311(7):1754-1757 (2009)
  We present here the preliminary growth and characterization results on the integration of GaAs/GaNAsSb/GaAs III-V layer structure on Ge/graded-SiGe/Si substrate or simply silicon virtual substrate (SVS). It was found in cross-section transmission electron microscopy (TEM) study that a sub-optimal Ge surface annealing (below 600 °C) was detrimental to the subsequent growth of III-V layer, and surface pitting was observed on the GaAs surface as a result. This surface pit was found to originate from the heterovalent interface between the Ge substrate surface and the GaAs epitaxy. By properly treating the Ge surface with in-situ annealing at 640 °C and low temperature GaAs seeding before epitaxy growth, the above III-V layer structure can be successfully grown with smooth surface morphology. The atomic force microcopy (AFM) results correlated well with the RHEED observation and provided further insight into the quality of three GaAs/GaNAsSb/GaAs samples. The root-mean-sq! uare (rms) surface roughness of the GaAs/GaNAsSb/GaAs layer structures with GaAs grown at 650, 610, and 580 °C were 25.9, 6.2, and 1.7 nm, respectively. In addition, upon annealing at not, vert, similar750 °C for 1 min in N2 ambient, one of the GaAs/GaNAsSb/GaAs sample showed a 5 K photoluminescence (PL) spectrum that was dominated by a peak with emission energy at 980 nm or equivalent to 1.27 eV.
• Real time extraction of quantum dot size from RHEED intensity profiles
  - J Cryst Growth 311(7):1758-1760 (2009)
  We provide an in situ approach to the determination of quantum dots (QDs) size based on the calculated reflection high-energy electron diffraction (RHEED) patterns. We observe the periodic intensity fringes along the RHEED chevron tails predicted earlier by the theoretical model. The calculated and the observed RHEED intensity profiles are analyzed systematically for different QD heights. The periodicity of the intensity fringes is correlated to the average height of QDs. The possibility of monitoring real time the evolution of dot height during the molecular beam epitaxy/chemical beam epitaxy (MBE/CBE) growth of self-assembled QDs is demonstrated.
• In situ X-ray diffraction during stacking of InAs/GaAs(0 0 1) quantum dot layers and photoluminescence spectroscopy
  - J Cryst Growth 311(7):1761-1763 (2009)
  The growth of five layers of InAs quantum dots embedded in GaAs matrices was investigated by in situ synchrotron X-ray diffraction. Time-resolved X-ray diffraction revealed the evolution of the strains and height of quantum dots during the entire growth process including the nucleation of islands and encapsulation with GaAs at various substrate temperatures. Comparisons of in situ X-ray results with postgrowth photoluminescence spectra showed a clear correlation between the structural and optical properties.
• Surface compositional mapping of self-assembled InAs/GaAs quantum rings
  - J Cryst Growth 311(7):1764-1766 (2009)
  The composition profile of self-assembled InAs/GaAs quantum rings is investigated both experimentally and theoretically. Our study is aimed at obtaining information on unburied rings, which cannot be directly accessed in cross-sectional analysis. Two-dimensional surface chemical maps obtained by X-ray photoemission electron microscopy reveal a non-uniform composition profile with a double structure composed of an In-rich core, corresponding to the central hole of the ring, surrounded by a rim with a stronger In–Ga intermixing. These results are substantiated by an atomistic model which, for a given shape, identifies the composition distribution that minimizes the elastic energy of the system. The good agreement between experiment and theory allows us to identify the minimization of strain energy as the main driving force for the formation of quantum rings.
• Differential absorption spectroscopy on coupled InGaAs quantum dots
  - J Cryst Growth 311(7):1767-1769 (2009)
  We report on the electroluminescence (EL) spectra and the differential absorption (Δα) spectra for triple-layer InGaAs quantum dots (QDs) of different GaAs spacer thicknesses. Cross-sectional transmission electron microscopy directly reveals that the InGaAs QDs of 5-nm spacer are well-aligned along the growth direction. The Δα spectra exhibit an increase of absorption change as the spacer layer thickness decreases from 40 to 5 nm. Meanwhile, the amount of absorption change at the photon energy of the excited transition becomes higher than that of the ground-state transition for the QDs of spacer thickness less than 20 nm. The Δα results are consistent with the EL spectra showing the higher emission intensity for the excited transition. From the EL and Δα experiments, the higher optical gain and absorption change for the excited transition suggest that the e2–h2 transition has higher oscillator strength for the vertically coupled QDs.
• Growth of InGaAs/GaNAs strain-compensated quantum dot superlattice on GaAs (3 1 1)B by molecular beam epitaxy
  - J Cryst Growth 311(7):1770-1773 (2009)
  We have studied the structural and optical properties of 10 stacked layers of self-organized In0.4Ga0.6As quantum dots (QDs) grown on GaAs (3 1 1)B substrates by atomic hydrogen-assisted radio frequency (RF)-molecular beam epitaxy. A 40 nm-thick GaN0.007As0.993 dilute nitride, which is used to cover each QD layer acts as a strain-compensation layer (SCL). The density of strain-compensated In0.4Ga0.6As QDs on GaAs (3 1 1)B can be controlled between 2×1010 and 1×1011 cm−2 by simply changing the growth temperature. Closely spaced In0.4Ga0.6As QDs on GaAs (3 1 1)B shows an ordered structure, in which we observe clear peaks in the two-dimensional fast Fourier transformation image. The temperature dependence of photoluminescence (PL) spectra shows a narrower linewidth over the whole temperature range 30−300 K for strain-compensated QDs owing to better uniformity in the QD size.
• Optical studies on InAs/InGaAs/GaNAs strain-compensated quantum dots grown on GaAs (0 0 1) by molecular beam epitaxy
  - J Cryst Growth 311(7):1774-1777 (2009)
  We have investigated the growth of 10 stacked layers of self-assembled InAs quantum dots (QDs) on GaAs (0 0 1) substrates, which were embedded in a combined set of a GaNAs strain-compensating layer (SCL) and an InGaAs strain-reducing layer (SRL). The internal compressive strain induced by each InAs QD layer was compensated by a tensile strain due to a GaNAs SCL. Consequently, the photoluminescence (PL) measured at 30 K showed a narrower linewidth of 60.3 meV for the sample with GaN0.008As0.992 SCL compared to 94.2 meV for InAs/GaAs uncontrolled sample due to an improved QD size uniformity. In order to reduce the nominal lattice mismatch at the heterointerface, we propose to insert an InGaAs SRL between the InAs QD layer and GaNAs SCL. We observed a PL peak redshift up to 1230 nm at room temperature, and increased PL intensity for InAs QD/InGaAs SRL/GaNAs SCL samples.
• Effect of crystallographic orientation of microchannel on low-angle incidence microchannel epitaxy on (0 0 1) GaAs substrate
  - J Cryst Growth 311(7):1778-1782 (2009)
  Low-angle incidence microchannel epitaxy (LAIMCE) is a technique to obtain a lateral overgrowth using molecular beam epitaxy. In order to produce a wide and flat lateral overgrowth of GaAs, the effect of the crystallographic orientation of microchannel on GaAs LIMCE was investigated. The cross-sectional shape of GaAs LAIMCE was studied by scanning electron microscope and found to largely change by the choice of microchannel orientation. For example, the layer grown from a microchannel with the orientation of −10° off from [0 1 0] showed a trapezoidal shape, while the layer grown from a microchannel with that of +10° off from [0 1 0] had an inverted-triangular shape with a sharp peak on the growth front. The layer grown from that of +20° off from [0 1¯ 1] had a vertical side. Detailed observation of the layers revealed that several kinds of facets, such as (1 0 1), (1 1¯ 2)B, (1 1 n¯), (3 1 0), formed on the side and shaped the layers. As adatoms on the surface ! diffuse from these facets to the top of the layers, the lateral growth would be suppressed. Therefore, it is important to control the growth condition not to form facets on the side, at least to suppress the formation of the facets with strong nature of inter-surface diffusion of adatoms form side to top.
• Control of dot geometry and photoluminescence linewidth of InGaAs/GaAs quantum dots by growth conditions
  - J Cryst Growth 311(7):1783-1786 (2009)
  A study on the influence of the growth conditions on the optical and structural properties of In0.6Ga0.4As quantum dots (QDs) grown on GaAs(1 0 0) substrates is presented. We investigated the impact of the substrate temperature, the growth rate, and V/III flux ratio on the full-width of half-maximum (FWHM) of the photoluminescence (PL) spectra and on the density and height of quantum dots. Our results show that the dot density depends strongly on the growth parameters. From them the substrate temperature has the strongest influence on the density and the height of QDs as well as on the PL properties. By varying only this parameter a direct control of the dot density and the height is possible over more than one order of magnitude (from 6×1010 cm−2 at 480 °C to 1×108 cm−2 at 530 °C). The smallest PL linewidth of 36 meV was observed by QDs deposited at low substrate temperatures (480 °C).
• Blue-shift emission in InP-based quantum dots by SiO2 sputtering and rapid thermal annealing
  - J Cryst Growth 311(7):1787-1790 (2009)
  We have investigated the atomic-intermixing effect in In(Ga, Al)As/InP quantum dots (QDs) by tuning the group III compositions. The QDs including InAs, In0.95Al0.05As, and In0.95Ga0.05As were embedded in In0.52Al0.48As matrix. The intermixing process includes the deposition of a sputtered SiO2 layer on the sample surface and a subsequent rapid thermal annealing (RTA) at temperature between 700 and 800 °C. From the room-temperature photoluminescence data, InAs and In0.95Al0.05As QDs show similar wavelength tuning behavior against the RTA temperature. In0.95Ga0.05As QDs exhibit an extra amount of wavelength blue-shift, and have a maximum blue-shift of 281 nm at a RTA temperature of 800 °C. The results suggest an enhanced atomic-intermixing effect by the presence of Ga atoms.
• The effects of rapid thermal annealing on doubled quantum dots grown by molecular beam epitaxy
  - J Cryst Growth 311(7):1791-1794 (2009)
  The effects of different rapid thermal annealing temperatures on the optical properties of InAs double quantum dots (DQDs) grown by molecular beam epitaxy using a partial-capping-and-regrowth process have been investigated. Improvement of the material quality is indicated by enhanced photoluminescence (PL) intensity and narrower PL linewidth. The blueshift of the PL emission peak with increasing annealing temperature is due to the interdiffusion of group III atoms during the annealing process, which is confirmed by the temperature dependence of the PL peak position. Thermal quenching of the PL intensity is observed at temperature over 110 K, and the main activation energy decreases with annealing temperature, consistent with a reduced confining potential from the interdiffusion of group III atoms. All of these results are similar to those of single quantum dots reported in the literature.
• The Kondo effect observed up to TKnot, vert, similar80 K in self-assembled InAs quantum dots laterally coupled to nanogap electrodes
  - J Cryst Growth 311(7):1795-1798 (2009)
  We have fabricated single electron tunneling structures by forming nanogap metallic electrodes directly upon single self-assembled InAs quantum dots (QDs). The fabricated samples exhibited clear Coulomb blockade effects. Furthermore, Kondo effect was observed when strong coupling between the electrodes and the QD was realized using a large QD with a diameter of not, vert, similar100 nm. From the temperature dependence of the linear conductance at the Kondo valley, the Kondo temperature TK was determined to be not, vert, similar81 K. This is the highest TK ever reported for artificial quantum nanostructures. The observed very high TK is due to strong QD–electrode coupling and large charging/orbital-quantization energies in the present self-assembled InAs QD structures.
• Improving size distribution of InAs quantum dots for intersubband devices
  - J Cryst Growth 311(7):1799-1802 (2009)
  We present the growth and characterization of InAs quantum dots on AlxGa1−xAs surfaces for intersubband devices. This requires the quantum dot energy levels in the AlxGa1−xAs matrix to be above the GaAs bandedge. Using standard As4 fluxes (beam equivalent pressure 8e−6 Torr), inhomogeneous broadening of the quantum dot size distribution increases with increasing Al content in the AlxGa1−xAs matrix. Reducing the As4 overpressure during In deposition is found to greatly improve the size distribution of the quantum dots, while producing slightly larger dots and a reduction in the density of small dots (h<1.3 nm). Annealing at the higher standard As4 flux for 30 s, after the reduced As4 In deposition, produced a negligible change in the quantum dot size distribution. Utilizing surface dots on top of 30 layers of self-assembled quantum dots, the maximum quantum dot height for ground state energies above the GaAs bandedge is determined to be 2 nm for Al0.30Ga0.7As an! d 3 nm for Al0.45Ga0.55As.
• Growth and fabrication of quantum dots superluminescent diodes using the indium-flush technique: A new approach in controlling the bandwidth
  - J Cryst Growth 311(7):1803-1806 (2009)
  Broadband superluminescent diodes incorporating multiple layers of InAs quantum dots (QDs), where the dots height was deliberately varied from one layer to another have been grown and characterized. We used the indium-flush process to accurately control the emission energy of each layer of dots, enabling us to reliability and predictably engineer the bandwidth of the overlapped layers. Photoluminescence spectrum of four combined layers of QDs with full-width at half-maximum of 125 nm at peak wavelength energy of 1.06 μm was obtained. A 3 dB bandwidth emission spectrum of 80 nm and output power of 1 mW was obtained under CW operation mode at room temperature.
• Fast carrier relaxation of self-assembled InAs quantum dots embedded in strain-relaxed In0.35Ga0.65As barriers for ultrafast nonlinear optical switching applications
  - J Cryst Growth 311(7):1807-1810 (2009)
  Self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers were grown on GaAs (1 0 0) substrates by molecular beam epitaxy. Relaxation of lattice strain in the In0.35Al0.65As nucleation layer was monitored by in situ reflection high-energy electron diffraction. Self-assembled InAs QDs were successfully formed on the strain-relaxed In0.35Ga0.35As barrier. Twenty-layer stacked InAs QDs showed optical absorption in the wavelength range of 1350–1650 nm. A fast decay of 18 ps was observed in the temporal profile of absorption saturation measurement at a wavelength of 1540 nm, which is expected to be useful for ultrafast nonlinear optical switching applications operating in the View the MathML source waveband.
• MBE growth of In(Ga)As quantum dots for entangled light emission
  - J Cryst Growth 311(7):1811-1814 (2009)
  Radiative biexciton decay in a single semiconductor quantum dot (QD) is a process by which entangled pairs of photons can be generated for quantum information applications. The observation of entangled light from a QD requires minimal splitting of exciton states and the ability to isolate the neutral biexciton and exciton photoluminescence (PL) emission of the individual dot. As a consequence, the growth of QDs for this purpose is subject to simultaneous constraints on areal dot density, dot emission energy, and wetting-layer (WL) emission energy. In this work we will describe modifications to the molecular beam epitaxial (MBE) growth of In(Ga)As QDs performed to address these requirements, for the realization of samples which generate entangled light of increasing quality.
• Gallium-assisted deoxidation of patterned substrates for site-controlled growth of InAs quantum dots
  - J Cryst Growth 311(7):1815-1818 (2009)
  In-situ gallium-assisted deoxidation of ex-situ patterned GaAs (1 0 0) substrates has been investigated, and compared with the more conventionally used hydrogen-assisted deoxidation. A total of 6–8 ML of gallium supplied at a substrate temperature of 420–460 °C has been shown to remove the surface oxide without significantly damaging shallow electron-beam patterned holes. These holes, not, vert, similar20 nm deep and not, vert, similar100 nm wide, have then been successfully used to control the nucleation site of single InAs quantum dots.
• Site-controlled InAs quantum dot formation grown on the templates fabricated by the Nano-Jet Probe method
  - J Cryst Growth 311(7):1819-1821 (2009)
  We have developed a nanoprobe-assisted bottom-up technique that allows to fabricate site-controlled quantum dots (SC-QDs), using a specially designed atomic force microscope probe, referred to as the Nano-Jet Probe (NJP). Beginning with the fabrication of regular QD arrays by using the NJP, we vertically aligned self-assembled InAs QDs using the strain-induced stacking method and produced assembled QD stacks. In order to improve the optical property of the SC-QDs, an AlGaAs barrier layer was introduced in the spacer layer of the stacked structures. The photoluminescence measurements of the fabricated structures revealed that they had good crystallographic qualities.
• Formation of linear InAs/InGaAsP/InP (1 0 0) quantum dot arrays by self-organized anisotropic strain engineering in chemical beam epitaxy
  - J Cryst Growth 311(7):1822-1824 (2009)
  The formation of laterally ordered linear InAs quantum dot (QD) arrays based on self-organized anisotropic strain engineering is demonstrated. An InAs/InGaAsP superlattice (SL) on InP (1 0 0) serves as a template for the QD arrays grown by chemical beam epitaxy. The InAs QD arrays exhibit excellent photoluminescence emission up to room temperature which is tuned into the 1.55-μm telecom wavelength region through the insertion of ultra-thin GaAs interlayers. Stacking of the QD arrays with identical emission wavelength upon adjusting the GaAs interlayer thickness produces a three-dimensionally self-ordered QD crystal.
• Droplet epitaxy of GaAs quantum dots on (0 0 1), vicinal (0 0 1), (1 1 0), and (3 1 1)A GaAs
  - J Cryst Growth 311(7):1825-1827 (2009)
  We study the formation of GaAs quantum dots (QDs) by droplet epitaxy on various GaAs surfaces. The fabrication process consists of two steps. First, liquid Ga droplets are grown in a Volmer-Weber-like mode. This is followed by crystallization under As pressure. We demonstrate fabrication of droplet epitaxial GaAs QDs on (0 0 1), vicinal (0 0 1), (1 1 0), and (3 1 1)A GaAs surfaces. On (3 1 1)A GaAs, QDs are formed with higher density and smaller height compared to (0 0 1) and (1 1 0), which is attributed to a higher energy barrier for surface diffusion. Values of the surface diffusion barrier are determined and its influence on temperature-dependent QD density is studied by means of a QD growth model. On vicinal (0 0 1) surfaces, step bunches are found to act as preferred nucleation sites for GaAs QDs which opens the possibility for a lateral positioning of the QDs by pre-patterning.
• High-density GaAs/AlGaAs quantum dots formed on GaAs (3 1 1)A substrates by droplet epitaxy
  - J Cryst Growth 311(7):1828-1831 (2009)
  We investigated the self-assembly of GaAs/AlGaAs quantum dots (QDs) on GaAs (3 1 1)A substrates by droplet epitaxy. High-density Ga droplets were formed on the (3 1 1)A surfaces due to the short surface migration distance of Ga atoms. The maximum area density exceeded 1011 /cm2. These Ga droplets were crystallized into dot-shaped nanostructures (QDs) even by the irradiation of low As4 flux intensity. The capped GaAs QDs exhibited efficient, narrow PL emission at 5 K, indicating their high quality and uniformity.
• Formation of In0.5Ga0.5As ring-and-hole structure by droplet molecular beam epitaxy
  - J Cryst Growth 311(7):1832-1835 (2009)
  Fabrication of InGaAs ring-and-hole nanostructures was successfully demonstrated by the droplet epitaxy technique using molecular beam epitaxy (MBE). The evolution of surface morphology during growth was monitored in situ by reflection high-energy electron diffraction (RHEED). Droplet-forming conditions were changed by varying substrate temperatures during In0.5Ga0.5 deposition (so-called deposition temperature). Dependence of the ring structural properties on the deposition temperature was investigated. Distributions of InGaAs ring outer diameter, outer height, and inner depth at different deposition temperatures were also examined. It was found that the higher the deposition temperatures, the larger the outer diameter and the higher the outer height of most of the InGaAs rings. However, they had slightly lower densities. Photoluminescence results confirmed the high quality of the nanocrystal.
• Spontaneous formation of a cluster of InAs dots along a ring-like zone on GaAs (1 0 0) by droplet epitaxy
  - J Cryst Growth 311(7):1836-1838 (2009)
  We examine in detail the spontaneous formation process of a cluster of InAs dots on GaAs (1 0 0) by droplet epitaxy. A cluster of dots was fabricated by irradiating an As flux for 30 s to In droplets. The dots were found to be formed mainly along a ring-like zone stretching in the [0 1 1] direction, showing that In atoms migrated from the droplet and reacted with As atoms. This indicates the presence of not only the surface diffusion of In atoms but also the inward diffusion of As atoms. This is supported by the fact that such a ring was not visible when the As irradiation time was shortened (5 s). For photoluminescence measurements the dots were capped with a 50-nm-thick GaAs layer. Two peaks at 1.46 and 1.3 eV were observed in the sample annealed at 750 °C. These peaks were attributed to the wetting layer and the dots, respectively.
• Nanohole formation on AlGaAs surfaces by local droplet etching with gallium
  - J Cryst Growth 311(7):1839-1842 (2009)
  We demonstrate the self-assembled generation of nanoholes on AlGaAs surfaces by local droplet etching (LDE). For the etching process, Ga is deposited on the surface, where liquid droplets are formed in a Volmer–Weber-like growth mode. The etching takes place locally at the interface between droplets and substrate and removes a significant amount of substrate material. The structural properties of the LDE nanoholes are studied with atomic force microscopy as function of etching temperature and Ga coverage. A bimodal depth distribution with flat and deep holes is observed. The formation of flat holes can be almost suppressed by optimized etching parameters. The depth of deep holes was adjusted by the process parameters up to a maximum depth of 15 nm. The density of deep holes is in the range 5×10−7–1×10−8 cm−2 and depends only slightly on the etching parameters. However, the density can be significantly increased by repeated etching.
• Fabrication of In0.15Ga0.85As nanohloes on GaAs by droplet molecular beam epitaxy
  - J Cryst Growth 311(7):1843-1846 (2009)
  This article reports on the fabrication of self-assembled In0.15Ga0.85As nanoholes on GaAs(1 0 0)substrates grown by droplet epitaxy using molecular beam epitaxy. The effects of growth interruption time and substrate temperature were investigated. The surface morphology of In0.15Ga0.85As nanoholes were examined by atomic force microscopy. The results show the dependence of density, depth, and width of nanoholes on the growth interruption time and substrate temperature. This growth technique is simple and flexible. It does not require additional complicated substrate processing and has a potential in developing quantum dots and quantum dot molecules for quantum computation applications.
• Growth and structural characterization of GaAs/GaAsSb axial heterostructured nanowires
  - J Cryst Growth 311(7):1847-1850 (2009)
  GaAs nanowire (NW) heterostructures with four GaAsSb inserts were grown on GaAs(1 1 1)B substrates by Au-assisted molecular beam epitaxy (MBE). Structural characterization of the NWs showed that the transition between the GaAs and GaAsSb heterojunction is atomically abrupt and that the GaAs crystallographic phase is mainly wurtzite with some stacking faults, whereas the GaAsSb phase is zinc blende and free from twinning defects. Interestingly, the growth rates of the GaAs segments and GaAsSb inserts were observed to vary with growth time. We attribute this behavior to the effect of the inclined molecular beams with respect to the substrate surface in the MBE growth chamber. Further, the mole fraction of Sb in the GaAsSb insert was determined by energy-dispersive X-ray spectrometry and was found to increase with the inserts position along the NW.
• InGaAs quantum wires grown on (1 0 0)InP substrates
  - J Cryst Growth 311(7):1851-1854 (2009)
  We have investigated the growth of InGaAs wires on InP substrates by tuning the Ga composition. Nanostructures including InAs, In0.95Ga0.05As, In0.90Ga0.10As, and In0.85Ga0.15As were grown on In0.53Ga0.26Al0.21As buffer layers. For the growth of InAs nanostructures, elliptical quantum dots are observed. As we add Ga composition to the nanostructures, quantum wires arrays along the [0 1¯ 1] direction are formed for In0.95Ga0.05As and In0.90Ga0.10As nanostructures, and the In0.85Ga0.15As nanostructure shows a dash-wire morphology. The In0.90Ga0.10As quantum wires have a photoluminescence spectrum of emission peak at λ not, vert, similar 1680 nm and a narrow full-width at half-maximum of 65 meV at 10 K. The In0.90Ga0.10As quantum wires show a high emission polarization anisotropy of 47%.
• Chemical beam epitaxy of highly ordered network of tilted InP nanowires on silicon
  - J Cryst Growth 311(7):1855-1858 (2009)
  In this work, the growth of undoped InP nanowires on silicon(1 1 1) using gold as the metal seed particle was undertaken by chemical beam epitaxy. Prior to the growth process an ordered array of gold nano dots is integrated on the surface of a silicon substrate using self-assembled (hexagonal compact array) polystyrene nanospheres as the Au evaporation template. The size of the gold nanodots ranged from 20 to 150 nm. The InP nanowires were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). The InP nanowires were found to grow tilted in the left angle bracket1 0 0right-pointing angle bracket direction and exhibited slightly broadened low-temperature photoluminescence emissions.
• Growth of one-dimensional III–V structures on Si nanowires and pre-treated planar Si surfaces
  - J Cryst Growth 311(7):1859-1862 (2009)
  This paper presents a technique that allows the growth of epitaxial GaAs nanowires that are aligned to the crystal directions of the Si substrate. Low-pressure chemical vapor deposition (LP-CVD) grown Si nanowires were used as templates for molecular beam epitaxy (MBE) growth. The growth direction of the nanowires aligns with the [1 1 1] direction perpendicular to the Si substrate. After deposition of 200 nm GaAs in a solid source MBE system, we observed the formation of GaAs whiskers perpendicular to the {1 1 2} sidewalls of Si nanowires. By TEM analysis, the crystal structure of the GaAs nanostructures could be identified as wurtzite, where the growth axis of the wires was in the [0 0 0 1] direction. The whiskers are of good crystalline quality as no dislocations or stacking faults were observed in large areas by TEM, which makes these structures potential candidates for III–V integration on Si. In parallel to these experiments, we found a growth technique that all! ows GaAs nanowires to grow along the [1 1 1] direction of the planar Si [1 1 2] substrates. III–V nanowires with comparable geometry but higher density could be realized.
• Growth of GaInNAs and 1.3 μm edge emitting lasers by molecular beam epitaxy
  - J Cryst Growth 311(7):1863-1867 (2009)
  We show that the use of a low growth rate combined with low N flux and RF power during molecular beam epitaxy (MBE) growth of dilute nitrides can efficiently enhance N incorporation while retaining good optical quality. A maximum light emission wavelength of 1.44 and 1.71 μm has been obtained at 300 K from GaNAs and GaInNAs quantum wells, respectively. We demonstrate high-performance 1.3 μm GaInNAs multiple quantum well edge emitting lasers with record low threshold current densities, a 3 dB modulation bandwidth of 17 GHz at 300 K and capability of being modulated at 10 Gbit/s up to 110 °C without extra coolers. Our results show that MBE is an epitaxial technology suitable for the growth of dilute nitride materials and devices.
• MBE grown GaInNAs-based multi-Watt disk lasers
  - J Cryst Growth 311(7):1868-1871 (2009)
  We report the fabrication and characterization of high-quality GaInNAs/GaAs gain mirror as an active media for semiconductor disk lasers emitting at the 12XX nm spectral range. The structure was fabricated by molecular beam epitaxy using a radio frequency plasma source for incorporating the nitrogen. The growth parameters have been optimized to reduce detrimental effects of nitrogen on the emission efficiency. Using the gain mirror, which comprised 10 GaInNAs quantum wells with a relatively low content of nitrogen, we demonstrated semiconductor disk laser with an output power of not, vert, similar5 W and a differential efficiency of not, vert, similar20%.
• GaAs1-xBix light emitting diodes
  - J Cryst Growth 311(7):1872-1875 (2009)
  GaAs1-xBix light emitting diodes have been grown and characterized. The p–i–n structure uses a View the MathML source intrinsic layer with a central View the MathML source GaAs1-xBix light emitting layer with 1.8% bismuth. The diodes showed peaks in the electroluminescence (EL) emission at View the MathML source from the GaAs1-xBix and View the MathML source from the GaAs. The wavelength of the peak in the EL from the GaAs1-xBix was independent of temperature in the range View the MathML source while the GaAs peak shifted with temperature as expected. Photoluminescence measurements on the same p–i–n structure show temperature dependence of the peak wavelength similar to the temperature dependence of GaAs.
• Improved performance of GaInNAs solar cells grown by molecular-beam epitaxy using increased growth rate instead of surfactants
  - J Cryst Growth 311(7):1876-1880 (2009)
  GaInNAs is potentially useful for increasing the conversion efficiency of multijunction solar cells if low photocurrents and photovoltages can be increased. Wide-depletion width devices generate significant photocurrents using an n–i–p structure grown by molecular-beam epitaxy, but these wide depletion widths are only realized in a region of parameter space that leads to rough surface morphologies. Surfactants are effective at reducing the surface roughness, but lead to increased defect densities and changes in the net acceptor or donor concentration. Here, we show that increasing the growth rate of GaInNAs solar cells leads to smooth surfaces without the use of a surfactant, even at high In compositions and substrate temperatures. No degradation in material quality is observed when increasing the growth rate from 1.5 to 3.0 μm/h, but a shunt resistance does appear for the high-growth-rate samples. This shunt is attributed to increased spitting of the Ga cell, lea! ding to an increase in the oval defect density, at the higher effusion cell temperatures used to achieve high growth rates. As with the case of Bi in GaInNAs, increased growth rates also appear to increase the net donor concentration, but it is not clear if these effects have the same cause.
• Performance of gas source MBE-grown wavelength-extended InGaAs photodetectors with different buffer structures
  - J Cryst Growth 311(7):1881-1884 (2009)
  Wavelength-extended InyGa1−yAs photodiodes with cut-off wavelengths of 2.0, 2.4 and 2.7 μm at room temperature were grown using gas source molecular beam epitaxy with linearly graded InxAl1−xAs (x=0.52 to y) buffer layers and InyAl1−yAs cap layers. A convenient and reliable correlating ramping procedure was developed for the growth. Detector performances were compared with our standard homojunction detectors containing linearly graded InxGa1−xAs buffer layers. The heterojunction detectors showed better performance than the homojunction detectors. Also, the use of wider bandgap buffer and cap made the heterojunction detectors more suitable for both front and back illumination. For the photodiodes with 500 μm mesa diameter at room temperature, the typical dark current (VR=10 mV) and R0A were 74 nA and 104 Ω cm2 at 290 K for the cut-off wavelength of 2.4 μm. Optimization of the buffer structure was necessary for further extension of the response wavelength.
• Photovoltaic characteristics of InAs/InGaAs/GaAs QD heterostructures
  - J Cryst Growth 311(7):1885-1888 (2009)
  In this work, preliminary photovoltaic results of the 10-layer InAs/InGaAs/GaAs quantum dot (QD) heterostructures were presented. As demonstrated, enhancement in the sub-GaAs bandgap spectral response was observed, especially at the 1.0–1.2 eV energy range. This implies that the incorporation of InAs QDs in existing InGaP/GaAs/Ge multijunction solar cells is beneficial for increasing the spectral utilization between Ge (0.67 eV) and GaAs bandgaps (1.42 eV). The open-circuit voltage (VOC) and fill factor (FF) of the device are 0.4 V and 0.51, respectively. The obtained values are smaller than that reported by GaAs solar cells (VOC=1.04 eV and FF=0.85), and the degradation is believed to be due to the accumulated strain from the 10-layer QDs. We believe that, upon optimization, incorporation of the InAs/InGaAs/GaAs QDs into existing InGaP/GaAs/Ge solar cells will result in solar cells with higher efficiency and render solar energy more cost competitive.
• Development of uncooled miniaturized InSb photovoltaic infrared sensors for temperature measurements
  - J Cryst Growth 311(7):1889-1892 (2009)
  This paper reports the development of an InSb photovoltaic infrared sensor (InSb PVS) operating at room temperature for temperature measurements. The InSb PVS consists of 910 InSb p+–p−–n+ photodiodes connected in series on a semi-insulating GaAs (1 0 0) substrate. An Al0.17In0.83Sb barrier layer was grown between the p+ and p− layers to reduce the diffusion of photo-excited electrons. As the InSb PVS operates in a photovoltaic mode, no thermal insulation is required, enabling its miniaturized plastic molding package. The sensitivity of the InSb PVS was 127 μV/K, and a noise equivalent temperature difference (NETD) of 1.0 mK/Hz1/2 was obtained at room temperature. The results demonstrate the potential for the sensor to be used both in non-contact thermometry, as well as human body detection.
• High detectivity AlGaAsSb/InGaAsSb photodetectors grown by molecular beam epitaxy with cutoff wavelength up to 2.6 μm
  - J Cryst Growth 311(7):1893-1896 (2009)
  In this paper, we report AlGaAsSb/InGaAsSb heterojunction p–i–n photodetectors lattice-matched to GaSb substrates grown by solid source molecular beam epitaxy using As and Sb valved crackers. The use of valved crackers greatly facilitated the lattice-matching of the quaternary InGaAsSb absorbing layer to the GaSb substrates, as characterized by X-ray diffraction. The growth temperature of the absorbing layer in the device was kept at 450 °C. The V to III flux ratio was optimized based on comparison studies of the strength of photoluminescence (PL), which indicated excellent material quality of the InGaAsSb active layer lattice-matched to the GaSb substrates. The device structure was designed to be optimized for the maximum quantum efficiency. The p–i–n photodetectors were processed using wet chemical etching and standard photolithographic process. The resulting devices exhibited low dark current and a breakdown voltage of 32 V at room temperature. A record Joh! nson-noise-limited detectivity of 9.0×1010 cm Hz1/2/W was achieved at 290 K. The 50% cutoff wavelength of the device was 2.57 μm. Thus, our result has clearly demonstrated the potential of very high-performance lattice-matched InGaAsSb p–i–n photodetectors for the mid-infrared wavelengths, comparable or superior to the current InGaAs photodiodes on InP substrate with extended wavelengths.
• Optimizing residual carriers in undoped InAs/GaSb superlattices for high operating temperature mid-infrared detectors
  - J Cryst Growth 311(7):1897-1900 (2009)
  The mid-infrared 21 Å InAs/24 Å GaSb superlattices (SLs) designed for the 4 μm cutoff wavelength were grown by molecular beam epitaxy at growth temperatures between 370 and 430 °C in order to reduce residual background carriers. The lowest density of 1.8×1011 cm−2 was obtained from the SLs grown at 400 °C. With increasing growth temperature, in-plane hole mobility decreased from 8740 to 1400 cm2/V s due to increased interfacial roughness, while the photoluminescence (PL) intensity increased due to a decrease in the number of nonstoichiometric nonradiative defects. Further reduction of carrier density to 1×1011 cm−2 was achieved by increasing barrier width. As GaSb layer width increases from 24 to 48 Å, the cutoff wavelength decreased from 4.1 to 3.4 μm, which is still in the mid-infrared detection window. More importantly, a dramatic improvement on the PL intensity and the full width at half maximum was achieved from the SL samples with the wider GaSb widt! hs. All mid-infrared SL samples investigated in our studies were residually p-type.
• Optimization of InAs/GaSb type-II superlattice interfaces for long-wave (not, vert, similar8 μm) infrared detection
  - J Cryst Growth 311(7):1901-1904 (2009)
  Optimization of various growth parameters for type-II 13 MLs InAs/7 MLs GaSb strained layer superlattices (SLSs) (λcut-offnot, vert, similar8 μm at 300 K), grown by solid source molecular beam epitaxy, has been undertaken. This includes a systematic study to investigate the influence of the effect of the growth temperature and the thickness of an InSb layer formed at the GaSb-on-InAs interface on the properties of the superlattice. We present optical and structural characterization of these SLS structures, using high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and cross-sectional scanning transmission electron microscope (STEM). Optimized growth parameters were then used to grow a 2-μm-thick active region for a SLS detector designed to operate in the long-wave infrared (LWIR) region, which demonstrated full-width half-maximum (FWHM) of 16 arcsec for the first SLS satellite peak and nearly zero lattice mismatch between zero-order SLS peak and ! GaSb substrate.
• MBE growth of mid-IR diode lasers based on InAs/GaSb/InSb short-period superlattice active zones
  - J Cryst Growth 311(7):1905-1907 (2009)
  We report on the MBE growth and properties of heterostructures and laser diodes based on 4 ML InAs/3 ML GaSb/1 ML InSb/3 ML GaSb short-period superlattices (SPSLs) for emission in the 3–4 μm wavelength range. We show that the interface configuration has a strong influence on both the structural and electronic properties of the SPSL. Excellent agreement between experimental and simulated X-ray diffraction patterns reveals the excellent crystal quality achieved with such complex SPSLs. Lasing is demonstrated up to 300 K in pulsed conditions and up to 200 K under continuous wave operation. Laser emission is centered at 3.32 μm, a technologically very interesting wavelength. Our results demonstrate the potential of these new active zones for mid-IR laser diodes.
• MBE growth of active regions for electrically pumped, cw-operating GaSb-based VCSELs
  - J Cryst Growth 311(7):1908-1911 (2009)
  Electrically pumped, cw-operating, single-mode GaSb-based VCSELs are attractive light sources for trace-gas sensing systems using tunable diode laser absorption spectroscopy (TDLAS) [A. Vicet, D.A. Yarekha, A. Pérona, Y. Rouillard, S. Gaillard, Spectrochimica Acta Part A 58 (2002) 2405–2412]. Only recently, the first electrically pumped (EP) devices emitting at 2.325 μm in cw-mode at room temperature have been reported [A. Bachmann, T. Lim, K. Kashani-Shirazi, O. Dier, C. Lauer, M.-C. Amann, Electronics Letters 44(3) (2008) 202–203]. The fabrication of these devices employs the molecular beam epitaxy (MBE) growth of GaSb/AlAsSb-distributed Bragg mirrors, a multi-quantum-well active region made of AlGaAsSb/InGaAsSb and an InAsSb/GaSb-buried-tunnel junction. As VCSELs are usually driven under high injection rates, an optimum electrical design of active regions is essential for high-performance devices. In this paper we present an enhanced simulation of current flow! in the active region under operation conditions. The calculation includes carrier transport by drift, diffusion and tunneling. We discuss different design criteria and material compositions for active regions. Active regions with various barrier materials were incorporated into edge emitter samples to evaluate their performance. Aluminum-containing barriers show better internal efficiency compared to active regions with GaSb as the barrier material.
• GaSb-based VCSELs emitting in the mid-infrared wavelength range (2–3 μm) grown by MBE
  - J Cryst Growth 311(7):1912-1916 (2009)
  We report the growth conditions and operations of electrically pumped monolithic Sb-based type-I quantum-well vertical cavity surface emitting lasers (VCSELs) emitting above 2.2 μm. The structures were grown on (0 0 1)-GaSb substrates by molecular beam epitaxy (MBE) and are made of two N-type GaSb/AlAsSb Bragg reflectors, a GaInAsSb/AlGaAsSb multiquantum-well active region and an InAsSb/GaSb tunnel junction. Growth conditions have been optimized for each target wavelength. Laser emission in CW up to 293 K at 2.3 μm and in pulsed regime at 2.52 μm at room temperature (RT) is demonstrated. These are the longest wavelength achieved with electrically pumped VCSELs to date.
• High-power and broadly tunable GaSb-based optically pumped VECSELs emitting near 2 μm
  - J Cryst Growth 311(7):1917-1919 (2009)
  We demonstrate GaSb-based vertical-external-cavity surface-emitting lasers (VECSELs) emitting multi-Watt output power and exhibiting a broad tuning range. A VECSEL gain structure comprising 15 In0.2Ga0.8Sb quantum wells grown on (1 0 0) n-doped GaSb substrate emitted a continuous-wave output power of over 4 W at 1970 nm wavelength operating near room temperature. Lasing was detected even at 50 °C. Optimized for high-power operation, the structure exhibited a tuning range of 75 nm. Using a modified VECSEL gain structure, comprising three different kinds of quantum wells, we achieved a spectral tuning of about 156 nm.
• Quaternary GaInAsSb/AlGaAsSb vertical-external-cavity surface-emitting lasers—A challenge for MBE growth
  - J Cryst Growth 311(7):1920-1922 (2009)
  Molecular beam epitaxial growth and operation of quaternary GaInAsSb/AlGaAsSb-based optically pumped vertical-external-cavity surface-emitting lasers (VECSEL) emitting at wavelengths 2.X μm are reported here. The epitaxial layer sequence of a VECSEL consists of three different sections: a distributed Bragg reflector (DBR), an active region, and a window layer terminated by a capping layer. The Al content of the AlGaAsSb layers varies between 100% in the DBR and 30% in the barriers separating the compressively strained GaInAsSb quantum wells (QW) in the active region. While the active region was grown without growth interruption, growth had to be interrupted at the interfaces between the above-mentioned sections to adjust the group-III cell temperatures for different flux settings. At the growth interruption interfaces secondary-ion mass-spectrometry revealed the unintentional incorporation of In at a concentration equivalent to 1–2 monolayers. Inspite of the formati! on of InAsSb-like interfacial layers between the DBR and the active region on the one hand, and the active region and the window layer on the other, high-performance VECSELs emitting in the 2.0–2.3 μm wavelength range have been grown. A maximum output power of 3.4 W at 2.25 μm and −10 °C heat-sink temperature has been achieved with a maximum optical-to-optical power conversion efficiency of 24%.
• The reproducibility and transferability of a THz quantum cascade laser design between two MBE growth manufacturers' platforms
  - J Cryst Growth 311(7):1923-1928 (2009)
  We report a comprehensive study into the growth of a 2.9 THz quantum cascade laser structure on two different molecular beam epitaxy manufacturers' systems; within a single growth campaign and over multiple growth campaigns. Device performance figures are compared from all the lasers grown, demonstrating excellent structure reproducibility, not only between each growth campaign, but also between the two MBE systems; including using different combinations and types of group III effusion cells. A small systematic shift in the emission frequency of not, vert, similar0.1 THz between the two systems was observed. The mechanisms behind this small shift in emission frequency were investigated and partly attributed to differences in the flux profiles associated with the type of aluminium effusion cells used and inhomogeneous broadening in the structure, possibly due to interface roughness.
• Precise growth control and characterization of strained AlInAs and GaInAs for quantum cascade lasers by GSMBE
  - J Cryst Growth 311(7):1929-1931 (2009)
  The precise growth controls of strained AlInAs and GaInAs for strain-balanced quantum cascade lasers (QCLs) are optimized and developed. The precise measurements of constituent compositions and thicknesses of strained AlInAs and GaInAs are performed on the strain-balanced AlInAs/GaInAs superlattice (SL). The doping incorporation behaviour of strained AlInAs and GaInAs is studied. High-crystalline quality QCL structures with desired two-dimensional electron gas (2DEG) densities in the injector region are achieved through precise composition, thickness and doping controls.
• Al(In)As–(Ga)InAs strain-compensated active regions for injectorless quantum cascade lasers
  - J Cryst Growth 311(7):1932-1934 (2009)
  We present a new design for quantum cascade lasers (QCLs) without the typically used injector between two consecutive active stages. The lasers are realized with the InP-based material system AlInAs/GaInAs. With additional AlAs and InAs layers a significant optimization of the structure can be realized. In this improved structure the possibility of electrons escaping into the quasi-continuum is drastically reduced by the AlAs-blocking layer. On the other hand, InAs, a material with a very low effective mass, significantly prolongs the carrier lifetime, enhancing the population inversion and increasing the dipole matrix element of the transition. Both inserted layers result in an overall improvement of the device properties, basically the threshold current density (jth), maximum operating temperature (Tmax), output power, slope efficiency and characteristic temperature T0. With high reflection coated facets a record threshold current density as low as 450 A/cm2 at 300 K! was achieved in the pulsed mode.
• Optimization of AlInGaAs/InGaAs/InAs strain compensated triangular quantum wells grown by gas source molecular beam epitaxy for laser applications in 2.1–2.4 μm range
  - J Cryst Growth 311(7):1935-1938 (2009)
  A group of AlInGaAs/InGaAs/InAs strain compensated triangular quantum well samples have been grown by using gas source molecular beam epitaxy (GSMBE), and their properties are investigated by X-ray diffraction and photoluminescence (PL) measurements. Through the adjustment of the growth temperature and barrier width, the quality of the quantum wells has been improved distinctly. The maximal PL peak wavelength of 2.38 μm at 300 K has been reached. The X-ray diffraction measurements show good structural properties and the full-width at half-maximum (FWHM) of PL spectrum is 17 meV at 12 K and 33 meV at 300 K. For the sample with larger well width, the transition involving the second sub-energy levels occurs.
• Terahertz quantum cascade lasers based on In0.53Ga0.47As/In0.52Al0.48As/InP
  - J Cryst Growth 311(7):1939-1943 (2009)
  Continuous wave and above liquid-nitrogen temperature operation of MBE grown terahertz quantum cascade lasers based on the In0.53Ga0.47As/In0.52Al0.48As/InP material system are reported. Samples investigated used a bound-to-continuum active region design with a single or double quantum well injector at the wavelength View the MathML source. A precise flux calibration and a modified injector design resulted in a maximum operating temperature in continuous wave and pulsed mode of 63 and 87 K and an optical output power of 4 mW at 10 K.
• Research advances on III–V MOSFET electronics beyond Si CMOS
  - J Cryst Growth 311(7):1944-1949 (2009)
  An overview is given on recent advances of science and devices of III–V based and Si MOS and MOSFET. Firstly, we have integrated molecular beam epitaxy (MBE) with atomic layer deposition (ALD) for the growth of excellent high-κ dielectrics with abrupt interfaces, critical for further complementary metal-oxide-semiconductor (CMOS) scaling beyond the 45 nm node. Secondly, we showed that epitaxial yttrium-doped HfO2 films on GaAs(1 0 0) have stabilized the cubic phase, and led to enhancement of κ over 30. Thirdly, inelastic electron tunneling spectroscopy (IETS) was applied to probe the phonon modes and charge trappings within the high-κ dielectrics. Fourthly, scaling of the high-κ oxides approaching 1.0 nm capacitance equivalent thickness (CET) is achieved in a Ga2O3(Gd2O3)[GGO]/In0.2Ga0.8As (InGaAs) gate stack that has undergone 850 °C rapid thermal annealing, and which has unpinned the surface Fermi level of the III–V semiconductor. Finally, we have demonstrat! ed a self-aligned inversion-channel In0.53Ga0.47As MOSFETs made of Al2O3(2 nm)/GGO(7 nm) gate oxide and TiN metal gate at 1-μm gate length, reaching a world record of drain current and transconductance.
• Interface properties of MBE-grown MOS structures with InGaAs/InAlAs buried channel and in-situ high-k oxide
  - J Cryst Growth 311(7):1950-1953 (2009)
  With the goal to demonstrate feasibility of high-mobility buried channel, we used in-situ high-k deposition approach and show for the first time operational MOSFET with buried HfO2/In0.52Al0.48As/In0.53Ga0.47As/InP channel with mobility 1800 cm2/V-s at 3×1012 cm−2 and e-mode operation. Interface properties are compared with a similar gate stack with 2 monolayer thick InGaAs "passivation" layer between InAlAs and the oxide. The latter gate stack has shown significantly improved gate control and ON/OFF ratio due to reduction of the interface state density.
• Depletion-mode In0.2Ga0.8As/GaAs MOSFET with molecular beam epitaxy grown Al2O3/Ga2O3(Gd2O3) as gate dielectrics
  - J Cryst Growth 311(7):1954-1957 (2009)
  Depletion-mode In0.2Ga0.8As/GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) were fabricated with molecular beam epitaxy (MBE) grown Al2O3/Ga2O3(Gd2O3) as the gate dielectric in two comparable processes. In the "metal-gate-last" process, a 12 μm gate-length depletion-mode n-channel InGaAs/GaAs MOSFET with a Ga2O3(Gd2O3) gate oxide 6 nm thick shows an accumulated drain current density of 135 mA/mm at Vg=2 V. In the other process of "metal-gate-first" process, the device with same gate dielectric, channel, and gate length exhibits a larger drain current density of 175 mA/mm at the same gate bias. In addition, there is a broader transfer characteristics and higher extrinsic peak transconductance of 48 mS/mm in the metal-gate-first process. MOS capacitors from both processes have exhibited excellent capacitance–voltage (C–V) characteristics with minor dispersion, negligible hysteresis, and κ values of 13.7–13.9 in Ga2O3(Gd2O3).
• Inversion-channel enhancement-mode GaAs MOSFETs with regrown source and drain contacts
  - J Cryst Growth 311(7):1958-1961 (2009)
  The use of compound semiconductors as the channel material has recently drawn great attention because of its potential to solve the upcoming Si metal–oxide–semiconductor field effect transistor (MOSFET) scaling problem for device beyond 22 nm node. In this work, a method of fabricating inversion-channel enhancement-mode GaAs n-MOSFET by incorporating molecular beam epitaxy regrown source and drain regions is demonstrated. By using regrown contact layers to avoid high-temperature processes and, thus, preserve the integrity of the oxide–semiconductor interface, the structure allows the fabrication of self-aligned III–V-based MOSFET. The fabricated n-channel enhancement-mode GaAs MOSFET with a 4 μm gate length shows a record high transconductance of 75 mS/mm.
• High-quality III–V semiconductor MBE growth on Ge/Si virtual substrates for metal-oxide-semiconductor device fabrication
  - J Cryst Growth 311(7):1962-1971 (2009)
  We describe the molecular-beam epitaxial (MBE) growth and fabrication of III–V metal-oxide-semiconductor (MOS) devices on Ge/Si virtual substrates. We show that high-temperature in-situ H2 annealing in the chemical-vapor deposition system changes the Ge surface configuration and produces a surface with predominantly double-step-layer conditions, which is crucial for the growth of single-domain GaAs. In addition, the surface morphology of III–V on Ge/Si improved significantly with an annealing treatment of the Ge surface carried out under high arsenic background pressure in the MBE chamber. This facilitates uniform As-monolayer formation on the entire Ge surface. Low-temperature migration-enhanced epitaxy (MEE) and low-temperature conventional GaAs growth not only enhance the growth of single-domain GaAs without Ge outdiffusion but also produce a sufficiently smooth surface for high-k dielectric deposition, achieving low leakage current. A 300-nm-thick GaAs buffer layer was grown, followed by a 10 nm growth of In0.2Ga0.8As high-mobility channel layer. A 7–8-nm-thick Al2O3 layer was deposited ex-situ by atomic-layer deposition (ALD). We verify the quality of III–V growth using transmission electron microscopy (TEM), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and photoluminescence (PL) measurement. The C–V characteristics show unpinning of the Fermi level, which is a necessary condition for gate voltage control of the drain current. This work suggests this materials combination is a promising candidate for the realization of advanced, nonclassical complementary-MOS and optoelectronic devices on Si substrates.
• InSb quantum-well structures for electronic device applications
  - J Cryst Growth 311(7):1972-1975 (2009)
  The small effective mass of electrons in InSb results in a high electron mobility if the densities of crystalline defects and other scattering sources are sufficiently reduced. The performance of geometrical magnetoresistors was used to confirm the high electron mobility in InSb quantum-well structures grown on GaAs substrates with defect-reducing buffer layers. High-resolution X-ray diffraction measurements indicated that strain relaxation in such structures can be anisotropic. Two-dimensional hole systems were realized by doping the barrier layers with Be instead of Si.
• The growth of high electron mobility InAsSb for application to high electron-mobility transistors
  - J Cryst Growth 311(7):1976-1978 (2009)
  High electron mobility and low defect density InAsSb lattice matched to AlSb has been successfully grown on InP substrates by the gas-source molecular beam epitaxy using an AlAsSb/AlSb composite buffer layer structure. The common antimony anion of AlAsSb, AlSb, and InAsSb is believed to effectively improve the film quality of InAsSb and AlSb by providing a surfactant effect. With this composite buffer layer structure, the room temperature electron mobility of InAsSb (lattice matched to AlSb) can reach as high as 18,000 cm2/V s. A high electron-mobility transistor based on this heterostructure shows a high gm of 350 mS/mm (gate length=6 μm) indicating the potential for high-speed applications.
• Monolithic integration of InP-based transistors on Si substrates using MBE
  - J Cryst Growth 311(7):1979-1983 (2009)
  We report on a direct epitaxial growth approach for the heterogeneous integration of high-speed III–V devices with Si CMOS logic on a common Si substrate. InP-based heterojunction bipolar transistor (HBT) structures were successfully grown on Si-on-lattice-engineered- substrate (SOLES) and Ge-on-insulator-on-Si (GeOI/Si) substrates using molecular beam epitaxy. Structurally, the epiwafers exhibit sharp interfaces and a threading dislocation density of 3.5×107 cm−2 as measured by plan-view transmission electron microscopy. HBT devices fabricated on GeOI/Si substrates have current gain of 55–60 at a base sheet resistance of 650–700 Ω/sq, and ft and fmax of around 220 GHz. HBT structures with DC and RF performance similar to those grown on lattice-matched InP were also achieved on patterned SOLES substrates with growth windows as small as 15×15 μm2. These results demonstrate a promising path of heterogeneous integration and selective placement of III–V devic! es at arbitrary locations on Si CMOS wafers.
• Height-selective etching for regrowth of self-aligned contacts using MBE
  - J Cryst Growth 311(7):1984-1987 (2009)
  Advanced III–V transistors require unprecedented low-resistance contacts in order to simultaneously scale bandwidth, fmax and ft with the physical active region [M.J.W. Rodwell, M. Le, B. Brar, in: Proceedings of the IEEE, 96, 2008, p. 748]. Low-resistance contacts have been previously demonstrated using molecular beam epitaxy (MBE), which provides active doping above 4×1019 cm−3 and permits in-situ metal deposition for the lowest resistances [U. Singisetti, M.A. Wistey, J.D. Zimmerman, B.J. Thibeault, M.J.W. Rodwell, A.C. Gossard, S.R. Bank, Appl. Phys. Lett., submitted]. But MBE is a blanket deposition technique, and applying MBE regrowth to deep-submicron lateral device dimensions is difficult even with advanced lithography techniques. We present a simple method for selectively etching undesired regrowth from the gate or mesa of a III–V MOSFET or laser, resulting in self-aligned source/drain contacts regardless of the device dimensions. This turns MBE into an! effectively selective area growth technique.
• MBE growth and patterned backgating of electron–hole bilayer structures
  - J Cryst Growth 311(7):1988-1993 (2009)
  We report on the development of backgated independently contacted two-dimensional electron–hole bilayer devices suitable for carrying out transport measurements at low temperatures. A method for creating patterned backgates using conventional photolithography and a single-sided aligner is outlined along with the use of a conventional digital camera and commercially available infrared LEDs for checking the backside lithography at stages during this process.
• Nitridation of (1 1 1)Al substrates for GaN growth by molecular beam epitaxy
  - J Cryst Growth 311(7):1994-1996 (2009)
  The GaN layers were grown by compound-source molecular beam epitaxy (CS-MBE) on (1 1 1) aluminum (Al) substrates with and without nitridation. The melting point of Al substrates limits the growth temperature. The layers were grown at 650 °C. Reflection high-energy electron diffraction (RHEED) patterns of the layers indicate that the nitridation is effective for GaN growth by CS-MBE on aluminum substrates. Photoluminescence was observed from the layer grown on the Al substrate with nitridation at RT.
• Growth of free-standing GaN layer on Si(1 1 1) substrate
  - J Cryst Growth 311(7):1997-2001 (2009)
  This investigation demonstrates the epitaxial growth of a free-standing GaN layer on Si(1 1 1) using a funnel-like GaN nano-rod buffer structure. The funnel-like GaN nano-rods were directly grown on Si substrates by RF-plasma molecular beam epitaxy. Free-standing GaN layers were achieved through the coalescence of funnel-like GaN nano-rods by the metalorganic chemical vapor deposition. This study examines the structure, optical characteristics and stress of GaN nano-rods and free-standing GaN layers. The c-axis lattice constant of the strain-free Ga-face GaN layer on Si is 5.1844 Å, as determined by high-resolution X-ray diffraction. The fully relaxed band edge at 3.468 eV without deep-level emission around 2.3 eV, was revealed in a free-standing GaN layer on Si, using photoluminescence.
• Strain engineering in GaN layers grown on silicon by molecular beam epitaxy: The critical role of growth temperature
  - J Cryst Growth 311(7):2002-2005 (2009)
  In this work we study both the structural and electrical qualities of AlGaN/GaN high electron mobility transistor heterostructures grown on silicon(1 1 1) by molecular beam epitaxy. Correlations are established between the quality of the structures and the relaxation rate of the mismatch stress in layers grown using ammonia as a nitrogen source. Comparison with layers grown using a nitrogen plasma source confirms the primordial role of the growth temperature for stress relaxation and dislocation filtering.
• GaN on Si with nm-thick single-crystal Sc2O3 as a template using molecular beam epitaxy
  - J Cryst Growth 311(7):2006-2009 (2009)
  The epitaxial growth of GaN on Si (1 1 1) substrates has been performed using plasma-assisted molecular beam epitaxy with a thin (not, vert, similar19.3 nm) single-crystal layer of Sc2O3 as a template/buffer layer. The structural properties and in situ epitaxial growth were studied using reflection high energy electron diffraction (RHEED), high-resolution transmission electron microscopy, and high-resolution X-ray diffraction. An orientation relationship of GaN(0 0 0 2)short parallelSc2O3(1 1 1)short parallelSi(1 1 1) and GaN[1 0 1¯ 0]short parallelSc2O3[4 2¯ 2¯] off 60° with Si[4 2¯ 2¯] was determined. The excellent growth of Sc2O3 on Si and GaN on Sc2O3 was achieved, as observed from streaky and bright RHEED patterns. The Sc2O3 template serves as an effective barrier layer preventing diffusion of Si and Ga during the GaN growth at high temperatures under nitrogen plasma. No cracking was observed in the GaN layer with thickness around 0.2 μm when inspected with! an optical microscope.
• Effect of AlN interlayers in the structure of GaN-on-Si grown by plasma-assisted MBE
  - J Cryst Growth 311(7):2010-2015 (2009)
  The insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si(1 1 1) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between a bottom 0.5 μm GaN layer and the top 1.0 μm GaN layer. The interlayer was effective in reducing the tensile stress to the level required for complete avoidance of microcracks, which were present in high densities in the case of GaN-on-Si heterostructures grown without an AlN interlayer. The strain in all the layers of the heterostructure was analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space mapping in XRD indicated that the 15 nm AlN interlayer was on average coherently strained with the GaN. However, TEM observations showed that the interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. ! In particular, above morphological features such as v-shaped surface depressions, GaN was overgrown with a high density of threading dislocations and inversion domain boundaries. A synergistic relaxation mechanism is proposed for the AlN interlayer leading to an elastically strained interlayer interconnected by plastically relaxed patches.
• The growth and characterization of an InN layer on AlN/Si (1 1 1)
  - J Cryst Growth 311(7):2016-2020 (2009)
  The variation of the strain and structural properties of InN layers grown by molecular beam epitaxy on AlN/Si(1 1 1) substrates were investigated using reflection high-energy electron diffraction (RHEED), atomic force microscopy, scanning electron microscopy, photoluminescence, and X-ray diffraction. The RHEED intensity, the thickness of the InN wetting layer, and the lattice constant of the InN during its initial growth stage were found to be most dependent on the indium flux. Although when using a high indium flux and high growth temperature, growth would be expected to follow the Volmer–Weber growth mode during the early growth stages, in the case of the nitrogen-rich conditions, the initial InN layer grows according to the Stranski–Krastanov mode. The emission peaks are present at 0.83 eV (1500 nm) and 0.79 eV (1569 nm) for the samples made using low and high indium fluxes, respectively. These results provide important information about single-crystal hexagonal! InN.
• Effect of the MgO substrate on the growth of GaN
  - J Cryst Growth 311(7):2021-2024 (2009)
  We investigate the effect of the MgO substrate on the growth of GaN by molecular beam epitaxy with radio frequency plasma source. MgO substrate is advantageous for its closer lattice constant to GaN. However, epitaxial GaN layer grown on MgO (1 1 1) substrate is often inclined about 2ring operator toward a particular a-axis of the GaN. This inclination is found to be caused by the domain structure of the MgO substrates. On the tilted domains of the substrate, the strain is effectively relaxed through the inclination. Another difficulty of MgO substrate is the diffusion of Mg atoms into the GaN layer. This Mg diffusion from the MgO substrate has successfully been suppressed by introducing the thin low-temperature grown AlN buffer layer. The AlN buffer also improves the crystalline quality of the GaN layer.
• Growth of GaN with warm ammonia by molecular beam epitaxy
  - J Cryst Growth 311(7):2025-2028 (2009)
  We demonstrate the growth of GaN by molecular beam epitaxy with warm ammonia as a nitrogen source. Ammonia gas is heated by the tungsten filament located at the open end of the gas-tube installed in the growth chamber. By using this simple structure, the multiple collisions of molecules within the heater, thus the generation of nitrogen molecule, can be suppressed. The crystalline quality of the grown GaN layer is significantly improved by introducing the warm ammonia. This effect can be explained by the enhancement of the two-dimensional growth due to the active nitrogen species such as radical View the MathML source generated by cracking ammonia molecule.
• Advances in quality and uniformity of (Al,Ga)N/GaN quantum wells grown by molecular beam epitaxy with plasma source
  - J Cryst Growth 311(7):2029-2032 (2009)
  We report on the advances in quality and uniformity of GaN layers and (Al,Ga)N/GaN quantum wells grown by molecular beam epitaxy using a nitrogen plasma source. The first purpose of this work is to highlight that radio frequency plasma cell is a well-proven nitrogen source to achieve, on 2 inch wafers, III-nitrides heterostructures with both crystalline quality and optical properties consistent with the state of the art. Preliminary studies demonstrate consistently uniform properties in terms of both barrier composition and photoluminescence energy peaks across the wafer.
• Gallium adlayer adsorption and desorption studies with real-time analysis by spectroscopic ellipsometry and RHEED on A-, M-, and C-plane GaN grown by PAMBE
  - J Cryst Growth 311(7):2033-2038 (2009)
  The absence of spontaneous and piezoelectric polarization in nonpolar-oriented III nitrides can improve emission efficiency in light-emitting devices due to the absence of electrostatic fields. It is therefore of interest to study the growth processes necessary to achieve smooth morphology for nonpolar-oriented films. In this paper, we compare and contrast gallium adsorption and desorption on C-, M-, and A-plane GaN epilayers by plasma-assisted molecular beam epitaxy (PAMBE) coupled with in-situ spectroscopic ellipsometry (SE). Adsorption/desorption of Ga under vacuum and under active nitrogen flux were investigated. Previous studies on C- and M-plane materials have suggested that optimum growth conditions with respect to surface morphology are those giving rise to a Ga bilayer and trilayer, respectively. Under vacuum, the stable Ga coverage was 0.48–0.50 nm for the C-, M-, and A-planes. While under N plasma, C-plane GaN maintained similar Ga coverage, while a thicke! r Ga-stable coverage of 0.7 nm was determined for M-plane. This is in contrast to A-plane GaN, which did not have a consistent Ga adlayer under plasma. We observed that Ga-rich growth of A-plane GaN leads to rough surface morphologies and N-rich conditions are necessary for smooth two-dimensional growth.