Strained Layer Superlattice Structures Research Articles

Investigation of AlGaN/GaN high electron mobility transistors on Silicon (111) substrates employing multi-stacked strained layer superlattice structures

Abstract AlGaN/GaN high electron mobility transistors (HEMTs) on Silicon (111) substrates are grown by using multi-stacked strained superlattices layer structures. Atomic force microscopy, X-ray diffraction and cross-sectional transmission electron microscopy are employed to investigate the structural properties of these AlGaN/GaN HEMT structures. The insertion of AlGaN/AlN strained superlattices layer modified with different numbers of GaN layers is revealed to reduce the pit density and threading dislocation density, resulting in the improvement of epilayer quality. The temperature dependent Hall effect measurements show that the two-dimensional electron gas characteristics are greatly improved by the employed modified superlattices layers in AlGaN/GaN HEMT structures throughout the whole measured temperature range. The superlattices layer engineering is shown to be important for achieving higher carrier mobility (>2340 cm2/V) and sheet carrier density (>8.22 × 1012 cm−2) at room temperature in AlGaN/GaN HEMTs.

Interfacial Mixing Analysis for Strained Layer Superlattices by Atom Probe Tomography

Quantum wells and barriers with precise thicknesses and abrupt composition changes at their interfaces are critical for obtaining the desired emission wavelength from quantum cascade laser devices. High-resolution X-ray diffraction and transmission electron microscopy are commonly used to calibrate and characterize the layers’ thicknesses and compositions. A complementary technique, atom probe tomography, was employed here to obtain a direct measurement of the 3-dimensional spatially-resolved compositional profile in two InxGa1−xAs/InyAl1−yAs III-V strained-layer superlattice structures, both grown at 605 °C. Fitting the measured composition profiles to solutions to Fick’s Second Law yielded an average interdiffusion coefficient of 3.5 × 10−23 m2 s−1 at 605 °C. The extent of interdiffusion into each layer determined for these specific superlattices was 0.55 nm on average. The results suggest that quaternary active layers will form, rather than the intended ternary compounds, in structures with thicknesses and growth protocols that are typically designed for quantum cascade laser devices.

InAs/InAsSb type-II strained-layer superlattices for mid-infrared LEDs

InAs/InAsSb type-II strained-layer superlattice (SLS) and multiple quantum well (MQW) structures have been studied for their suitability in the active region of mid-infrared LEDs operating at room temperature. A series of InAs/InAs1−xSbx superlattices with low antimony content (x = 3.8–13.5%) were grown by MBE on InAs substrates and characterised using x-ray diffraction and photoluminescence (PL). The 4 K PL spectra of these samples exhibit the expected peak shift to longer wavelength and a reduction in intensity as the Sb content is increased. Band structure simulations highlight the effects of changing the antimony content and the layer thicknesses, to tailor the overlap of the electron and hole wavefunctions and maximise the radiative recombination rate. Analysis of the temperature dependence of the PL emission spectra enabled the extraction of quenching energies that demonstrate some suppression of Auger recombination in both the MQW and SLS structures. The MQW samples exhibit a changeover in the dominant radiative recombination process above ~100 K associated with thermal emission of holes into the InAs barriers; this behaviour was not observed in the SLS samples. These SLS structures have the potential for use as the active region in room temperature mid-infrared LEDs.

Temperature-Dependent X-ray Diffraction Measurements of Infrared Superlattices Grown by MBE

Strained-layer superlattices (SLSs) are an active research topic in the molecular beam epitaxy (MBE) and infrared focal plane array communities. These structures undergo a >500 K temperature change between deposition and operation. As a result, the lattice constants of the substrate and superlattice are expected to change by approximately 0.3%, and at approximately the same rate. However, we present the first temperature-dependent X-ray diffraction (XRD) measurements of SLS material on GaSb and show that the superlattice does not contract in the same manner as the substrate. In both InAs/InAs0.65Sb0.35 and In0.8Ga0.2As/InAs0.65Sb0.35 SLS structures, the apparent out-of-plane strain states of the superlattices switch from tensile at deposition to compressive at operation. These changes have ramifications for material characterization, defect generation, carrier lifetime, and overall device performance of superlattices grown by MBE.

Growth and Characterization of In x Ga1−x As/GaAs1−y P y Strained-Layer Superlattices with High Values of y (~80%)

Strained-layer superlattice (SLS) structures, such as InGaAs/GaAsP lattice matched to GaAs, have shown great potential in absorption devices such as photodetectors and triple-junction photovoltaic cells. However, until recently they have been somewhat hindered by their usage of low-phosphorus GaAsP barriers. High-P-composition GaAsP was developed as the barrier for InGaAs/GaAsP strained-layer superlattice (SLS) structures, and the merits of using such a high composition of phosphorus are discussed. It is believed that these barriers represent the highest phosphorus content to date in such a structure. By using high-composition GaAsP the carriers are collected via tunneling (for barriers ≤30 A) as opposed to thermionic emission. Thus, by utilizing thin, high-content GaAsP barriers one can increase the percentage of the intrinsic in a p-i-n structure that is composed of InGaAs wells in addition to increasing the number of periods that can be grown for given depletion width. However, standard SLSs of this type inherently possess undesirable compressive strain and quantum size effects (QSEs) that cause the optical absorption of the thin InGaAs SLS wells to shift to higher energies relative to that of bulk InGaAs of the same composition. To circumvent these deleterious QSEs, stress-balanced, pseudomorphic InGaAs/GaAsP staggered SLSs were grown. Staggering was achieved by removing a portion of one well and adding it to an adjacent well. The spectral response obtained from device characterization indicated that staggering resulted in thicker InGaAs films with reduced cutoff energy. Additionally, these data confirm that tunneling is a very effective means for carrier transport in the SLS.

Growth of type II strained layer superlattice, bulk InAs and GaSb materials for minority lifetime characterization

We have examined the growth of strained layer superlattice (SLS) structures for the purpose of characterizing and improving the minority carrier lifetime. Structures with different SL periods but with same absorption wavelength were first studied. Despite a doubling of the number of interfaces per thickness unit, no significant change was seen in the carrier lifetime. This observation points away from the interfaces as the location of lifetime limiting defect centers. To gain further insights into the spatial location of the defect centers, a series of binary InAs and GaSb layers grown with different substrate temperatures, were studied. We found that higher growth temperatures were beneficial for both binaries, although the improvement for GaSb was less than that of InAs. The substrate temperature was also varied in SLS structures and characterized with high-resolution x-ray diffraction. By using the peak width from the SLS zero-order diffraction as a figure of merit, we found a shallow growth window of ∼±20° around an optimum temperature of 440°C. Outside this temperature window the material quality deteriorated very rapidly. Unfortunately, the substrate temperatures that would provide an improvement in the binary lifetimes fall mainly above the SLS growth window, thus limiting this parameter as a means of improving lifetimes in the SLS. A model that qualitatively relates bulk and SLS lifetimes through native defects is proposed and strategies for improving the lifetimes are discussed.

계면 흡착에 의한 InAs/GaSb 초격자의 응력변조 효과

본 연구에서는 InAs/GaSb 응력초격자(SLS)의 계면 흡착(soaking)에 의한 응력변조 효과를 X선회절(XRD)을 통하여 분석하였다. As과 Sb 흡착에 의하여 유도된 응력의 변화는 XRD 곡선의 기판피크과 0차 위성피크 사이의 분리각으로부터 조사하였으며, As/InAs 흡착은 약간의 GaAs-like 계면층을 유발하는 반면, Sb/GaSb 흡착은 InSb-like 계면상을 유도하는 것으로 분석되었다. Pendellosung 간섭진동의 Fourier 변환 곡선을 이용하여, [InAs/GaSb]-SLS 성장에서 결정성이 가장 우수한 최적 As/InAs와 Sb/GaSb의 흡착시간은 각각 2 sec와 10 sec임을 밝혔다. InAs<TEX>${\rightarrow}$</TEX>GaSb 계면에 As과 Sb를 동시에 흡착시킨 SLS에서 XRD 위성피크가 2개로 분할되는 특이한 쌍정현상이 관측되었는데, 이것은 계면에서 In<TEX>${\leftrightarrow}$</TEX>Ga 및 Sb<TEX>${\leftrightarrow}$</TEX>As 상호혼합에 의한 InSbAs와 GaAsSb의 2종의 결정상이 공존함으로써 발생한 현상으로 추정된다. In this study, the interface soaking effect in InAs/GaAs strained-layer superlattice (SLS) on crystalline phase modulation has been analyzed by the x-ray diffraction (XRD) curve. The strain variation induced by As and/or Sb soaking was determined by the separation angle between the substrate peak and the 0th-order superlattice satellite peak in the XRD spectra. Contrated that the As/InAs soaking arises minor GaAs-like interfacial layer, the Sb/GaSb soaking induces InSb-like one. The Fourier-transformed curves of the Pendellosung interference oscillation shows that the optimum soaking times of As/InAs and Sb/GaSb are 2 sec and 12 sec, at which the highest crystallineity has, respectively. An anomalous twin-peak phenomenon that a satellite peak splits into two peaks was observed in the SLS structure co-soaked by As and Sb at InAs<TEX>${\rightarrow}$</TEX>GaSb interfaces. We suggest that it may be resulted from coexistence of two kinds crystalline phases of InAsSb and GaAsSb due to intermixing of In<TEX>${\leftrightarrow}$</TEX>Ga and Sb<TEX>${\leftrightarrow}$</TEX>As.

Implementation of ZnO/ZnMgO strained-layer superlattice for ZnO heteroepitaxial growth on sapphire

The main challenge in fabrication of ZnO-based devices is the absence of reliable p-type material. This is mostly caused by insufficient crystalline quality of the material and not well-enough-developed native point defect control of ZnO. At present high-quality ZnO wafers are still expensive and ZnO heteroepitaxial layers on sapphire are the most reasonable alternative to homoepitaxial layers. But it is still necessary to improve the crystalline quality of the heteroepitaxial layers. One of the approaches to reduce defect density in heteroepitaxial layers is to introduce a strained-layer superlattice (SL) that could stop dislocation propagation from the substrate-layer interface. In the present paper we have employed fifteen periods of a highly strained SL structure. The structure was grown on a conventional double buffer layer comprising of high-temperature MgO/low-temperature ZnO on sapphire. The influence of the SLs on the properties of the heteroepitaxial ZnO layers is investigated. Electrical measurements of the structure with SL revealed very high values of the carrier mobility up to 210cm2/Vs at room temperature. Structural characterization of the obtained samples showed that the dislocation density in the following ZnO layer was not reduced. The high mobility signal appears to come from the SL structure or the SL/ZnO interface.

Carrier lifetime measurements in short-period InAs/GaSb strained-layer superlattice structures

Minority carrier lifetime and interband absorption in midinfrared range of spectra were measured in InAs/GaSb strained-layer superlattices (SLSs) grown by molecular beam epitaxy on GaSb substrates. The carrier lifetime in 200-period undoped 7 ML InAs/8 ML GaSb SLS with AlSb carrier confinement layers was determined by time-resolved photoluminescence (PL) and from analysis of PL response to sinwave-modulated excitation. Study of PL kinetics in frequency domain allowed for direct lifetime measurements with the excess carrier concentration level of 3.5×1015 cm−3. The minority carrier lifetime of 80 ns at T=77 K was obtained from dependence of the carrier lifetime on excitation power.

고분해능 XRD 분석에 의한 InAs/GaSb 응력초격자 구조의 성장 최적화 연구

InAs/GaSb (8/8-ML) 응력초격자 (SLS)의 성장 변수를 최적화하기 위하여, 다양한 조건 및 모드에서 SLS 구조를 제작하여 고분해능 X선회절 (XRD) 특성을 분석하였다. 본 연구에서는 성장온도, V/III 분자선 비율, 성장일시정지 (growth interruption, GI) 등의 변화를 통하여 SLS 계면층의 응력 변조를 유도하였고, XRD 0차 위성피크의 변위로서 응력의 변화를 고찰하였다. XRD 분석 결과로부터, SLS의 결정성과 응력의 변화를 유발하는 주요 변수는 각각 성장온도와 V/III(Sb/Ga) 비율임을 보여 주었다. 압축변형을 가지고 있는 본 연구에서 제작한 SLS 시료는 V/III(Sb/Ga) 비율의 감소에 따라 인장변형으로 전환됨을 보여 주었으며, GI 모드 및 시간에 따라 응력이 민감하게 변함을 관측할 수 있었다. 본 연구 결과로부터, [InAs/GaSb]-SLS ([8/8]-ML)의 최적 성장온도와 V/III(Sb/Ga) 비율는 각각 <TEX>$350^{\circ}C$</TEX>와 20이고, 결정성을 극대화하고 응력완화를 감소시키기 위해서는 InAs 성장 직전 약 3초 동안의 GI방법이 유효함을 보였다. For the growth optimization of InAs/GaSb (8/8-ML) strained-layer superlattice (SLS), the structure has been grown under various conditions and modes and characterized by the high-resolution x-ray diffraction (XRD) analysis. In this study, the strain modulation is induced by changing parameters and modes, such as the growth temperature, the ratio of V/III beam-equivalent-pressure (BEP), and the growth interruption (GI), and the strain variation is analyzed by measuring the angle separation of 0th-order satellite peak in XRD patterns. The XRD results reveal that the growth temperature and the V/III(Sb/Ga) ratio are major parameters to change the crystallineity and the strain modulation in SLS structures, respectively. We have observed that the SLS samples with compressive strain prepared in this study are show a transition to tensile strain with decreasing V/III(Sb/Ga) ratio, and the GI process is a sensitive factor giving rise to strain modulation. These results obtained in this study suggest that optimized growth temperature and V/III(Sb/Ga) ratio are <TEX>$350^{\circ}C$</TEX> and 20, respectively, and the appropriate GI time is approximately 3 seconds just before InAs growth that the crystallineity is maximized and the strain relaxation is minimized.

Characterization of Strained Layer Superlattice Solar Cells by X-ray Diffraction and Current-Voltage Measurements

AbstractInGaAs can be used to enhance the response of solar cells past the 1.43 eV cutoff of GaAs. Strained-layer superlattice (SLS) structures with high indium and phosphorus compositions (up to 35% and 68% respectively) have been grown successfully. SLS solar cells with indium and high phosphorus compositions (up to 15% and 85% respectively) have been grown successfully. The spectral response of the solar cells has been extended to as low as 1.27 eV. This enhancement is also shown by an increase in the short circuit current, with a small reduction in the short circuit voltage as compared to standard GaAs p-n junction for AM1.5 and one sun.Dark current curves show the extent of recombination in the superlattice. The reverse saturation current in the recombination region (0.2-0.8 V) was determined using a non-linear least squares fitting routine. An Arrhenius plot was generated by finding the reverse saturation current over a temperature range of 300-370 K. The low recombination devices show non-ideality constants of 1.7 with activation energies of 1.3-1.4 eV. The high recombination devices have non-ideality constants (˜2.3) and lower activation energies of 1.1 eV.

Optimization of InAs/GaSb type-II superlattice interfaces for long-wave (∼8 μm) infrared detection

Optimization of various growth parameters for type-II 13 MLs InAs/7 MLs GaSb strained layer superlattices (SLSs) ( λ cut-off ∼8 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.

Visible–blind GaN p–i–n photodiodes with an Al 0.12Ga 0.88N/GaN superlattice structure

GaN ultraviolet photodiodes with (i.e. sample 1) and without (i.e. sample 2) Mg-doped AlGaN/GaN strained-layer superlattice structure were both fabricated. It was found that we could achieve a near constant dark current of around 3 and 7 nA/cm 2 for sample 1 and sample 2, respectively. It was also found that we could achieve a larger photocurrent and a larger photocurrent to dark current contrast ratio from sample 1. The zero-bias peak responsivity was found to be around 0.05 A/W at 356 nm and 0.006 A/W at 363 nm for sample 1 and sample 2, respectively. However, the transient response full-width-half-maximum (FWHM) of sample 1 was found to be longer. For devices with a 500 μm diameter, it was found that the transient response FWHM was 0.6 μs for sample 1 and 0.2 μs for sample 2. The difference in response time between sample 1 and sample 2 should be attributed to the distribution of depletion layer in between the p-layer and the n-type i-layer.

Deformation of Small Volumes of Material Studied Using Strained-Layer Superlattice Structures

AbstractMechanical studies of semiconductor superlattices have shown that the onset of plastic deformation under an inhomogeneous stress is a process that takes place simultaneously across a finite volume of the order of a micron across. The ability to incorporate known internal stresses, and to vary the stress and thickness of individual layers in a semiconductor superlattice, is a very powerful tool, opening up new possibilities for investigations that cannot be achieved by varying external stresses on a specimen that is sensibly homogeneous. In this way, from the initial yield stress of single-crystal strained-layer superlattices under indentation, we demonstrated a new criterion, of which the key feature is that it is to be averaged over a finite volume. Here we show that designing samples with individual layers in bands to form low yield-stress material within the structure can give information about the size and position of the initial yield volume.

Relationship between the lateral length and thickness of the platelets in naturally occurring strained layer superlattice structures

The molecular beam epitaxial growth of InAs0.5Sb0.5 onto (001) surfaces below 430 °C results in the formation of a “natural” strained layer superlattice (n-SLS). Transmission electron micrographs of 〈110〉 cross sections showed the existence of two different alloy compositions that formed a tetragonally distorted interleaved platelet structure in which the interfaces were highly regular and ran approximately parallel to the growing surface. It is found that the structure of the n-SLS can be changed systematically by carefully controlling the InAs0.5Sb0.5 growth conditions. A simple relationship is derived relating the lateral size of the platelet to its thickness that is found to hold experimentally for n-SLS structures. It is suggested that the n-SLS structure occurs because it corresponds to the minimum free energy configuration of the growing crystal. A similar formalization has been applied to periodic modulations in other material systems.

The use of computer controlled group V valved sources for interface type control in InGaSb/InAs strained layer superlattices

The purpose of this research is to demonstrate the necessity of computer controlled valved group V effusion cell sources in the growth of indium gallium antimonide/indium arsenide (InGaSb/InAs). These sources allow enhanced control of the group V flux. This flux control allows the reduction of unwanted cross contamination and complete control of the interface type. For simple structures, this control can be done manually, however, for complicated structures the control must be automated to allow for reproducibility and uniformity. The InGaSb/InAs strained layer superlattice (SLS) is an example of a complicated structure with hundreds of layers that requires interface type control. Arsenic incorporation with typical flux shuttering was found to be a problem in the growth of antimonide layers and limit interface type control. The antimony incorporation was not found to occur for the growth of arsenide layers. In addition, antimony exposure to critical interfaces did not appear to reduce the interface quality. This research demonstrates that the use of computer controlled valve sources is only required for the arsenic source when attempting to create InGaSb/InAs SLS structures.

Cross-sectional scanning tunneling microscopy of InAsSb/InAsP superlattices

Cross-sectional scanning tunneling microscopy (STM) has been used to characterize compositional structures in InAs0.87Sb0.13/InAs0.73P0.27 and InAs0.83Sb0.17/InAs0.60P0.40 strained-layer superlattice structures grown by metal-organic chemical vapor deposition. High-resolution STM images of the (110) cross section reveal compositional features within both the InAsxSb1−x and InAsyP1−y alloy layers oriented along the [1̄12] and [11̄2] directions—the same as those in which features would be observed for CuPt–B type ordered alloys. Typically one variant dominates in a given area, although occasionally the coexistence of both variants is observed. Furthermore, such features in the alloy layers appear to be correlated across heterojunction interfaces in a manner that provides support for III–V alloy ordering models which suggest that compositional order can arise from strain-induced order near the surface of an epitaxially growing crystal. Finally, atomically resolved (11̄0) images obtained from the InAs0.87Sb0.13/InAs0.73P0.27 sample reveal compositional features in the [112] and [1̄1̄2] directions, i.e., those in which features would be observed for CuPt–A type ordering.

Surface charge limit in NEA superlattice photocathodes of polarized electron source

The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.

Strain relaxation of InGaAs/GaAs superlattices by wet oxidation of underlying AlAs layer

The effects of AlAs wet oxidation on overlayers were investigated using InGaAs/GaAs strained-layer superlattice structures grown on an AlAs layer. The superlattice partially relaxes towards its equilibrium spacing as the result of the oxidation of the underlying AlAs layer. Double-crystal x-ray diffraction measurements were used to determine the degree of strain relaxation. Larger relaxation is observed for the sample with a higher indium composition and a thicker AlAs layer.

Picosecond free-electron laser studies of Auger recombination in arsenic-rich InAs1–xSbx strained layer superlattices at 300 K

Room temperature pump-probe transmission experiments have been performed on arsenic-rich InAs/InAs1-xSbx strained layer superlattices (SLS) using a picosecond far-infrared free electron laser. With excitation frequencies well above the fundamental bandgap, near 10 µm, large excited carrier concentrations were obtained, allowing the density dependence of the recombination rate to be determined directly. The results have been interpreted in terms of an 8×8 k.p SLS energy band calculation, including the full dispersion for both k in-plane and k parallel to the growth direction. A comparison with identical measurements on epilayers of InSb, of comparable room temperature bandgap, shows that the Auger processes have been substantially suppressed in the superlattices. In the non-degenerate regime, where the Auger lifetime scales as τaug-1 = C1Ne2, a value of C1 between 10 and 100 times smaller is obtained for the SLS structures.