Metamorphic Buffer Layers Research Articles

E‐Band InAs/GaAs Trilayer Quantum Dot Lasers

The introduction of a closely stacked quantum dot (QD) structure is considered as one of the most effective approaches to extend the emission wavelength of InAs/GaAs QDs beyond 1.3 μm (1300 nm). Herein, a trilayer QD structure (three closely stacked QD layers) is proposed to further extend the emission wavelength of QDs. Room‐temperature (RT) emission at 1418 nm from InAs/GaAs trilayer QDs is demonstrated. Moreover, based on these results, an E‐band InAs/GaAs trilayer QD laser is fabricated on a GaAs substrate, achieving RT oscillation at 1370 nm with a low threshold current density of 96 A cm−2 under continuous‐wave mode operation. The results indicate that the trilayer QD structure is promising for realizing high‐performance QD lasers on GaAs substrate over 1300 nm without incorporating metamorphic buffer layers.

Metamorphic Buffer Layer Platform for 1550 nm Single-Photon Sources Grown by MBE on (100) GaAs Substrate.

We demonstrate single-photon emission with a low probability of multiphoton events of 5% in the C-band of telecommunication spectral range of standard silica fibers from molecular beam epitaxy grown (100)-GaAs-based structure with InAs quantum dots (QDs) on a metamorphic buffer layer. For this purpose, we propose and implement graded In content digitally alloyed InGaAs metamorphic buffer layer with maximal In content of 42% and GaAs/AlAs distributed Bragg reflector underneath to enhance the extraction efficiency of QD emission. The fundamental limit of the emission rate for the investigated structures is 0.5 GHz based on an emission lifetime of 1.95 ns determined from time-resolved photoluminescence. We prove the relevance of a proposed technology platform for the realization of non-classical light sources in the context of fiber-based quantum communication applications.

InAs/InGaAs Quantum Dot Lasers on Multi-Functional Metamorphic Buffer Layers.

With the development of dry fiber over the past two decades, the E-band has become a new telecommunication wavelength. However, owing to material constraints, an effective high-performance semiconductor light source has not yet been realized. InAs quantum dot (QD) lasers on GaAs substrates are in the spotlight as O-band light sources because of their excellent thermal properties and high efficiency. The introduction of a very thick InGaAs metamorphic buffer layer is essential for realizing an E-band InAs QD laser, but it can cause degradation in laser performance. In this study, we fabricate an E-band InAs/GaAs QD laser on a GaAs substrate with an AlInGaAs multifunctional metamorphic buffer layer that realizes the function of the bottom cladding layer of normal thickness in addition to the functions of a metamorphic buffer layer and a dislocation filter layer. The lasing oscillation at a wavelength of 1428 nm is demonstrated at room temperature under continuous-wave operation. This result paves the way toward the realization of highly efficient light sources suitable for E-band telecommunications.

On-Demand Generation of Entangled Photon Pairs in the Telecom C-Band with InAs Quantum Dots

Entangled photons are an integral part in quantum optics experiments and a key resource in quantum imaging, quantum communication, and photonic quantum information processing. Making this resource available on-demand has been an ongoing scientific challenge with enormous progress in recent years. Of particular interest is the potential to transmit quantum information over long distances, making photons the only reliable flying qubit. Entangled photons at the telecom C-band could be directly launched into single-mode optical fibers, enabling worldwide quantum communication via existing telecommunication infrastructure. However, the on-demand generation of entangled photons at this desired wavelength window has been elusive. Here, we show a photon pair generation efficiency of 69.9 ± 3.6% in the telecom C-band by an InAs/GaAs semiconductor quantum dot on a metamorphic buffer layer. Using a robust phonon-assisted two-photon excitation scheme we measure a maximum concurrence of 91.4 ± 3.8% and a peak fidelity to the Φ+ state of 95.2 ± 1.1%, verifying on-demand generation of strongly entangled photon pairs and marking an important milestone for interfacing quantum light sources with our classical fiber networks.

Thermoelectrically-Cooled InAs/GaSb Type-II Superlattice Detectors as an Alternative to HgCdTe in a Real-Time Mid-Infrared Backscattering Spectroscopy System.

We report on the development of thermoelectrically cooled (TE-cooled) InAs/GaSb type-II superlattice (T2SL) single element infrared (IR) photodetectors and exemplify their applicability for real-time IR spectroscopy in the mid-infrared in a possible application. As the European Union’s Restriction of Hazardous Substances (RoHS) threatens the usage of the state-of-the-art detector material mercury cadmium telluride (MCT), RoHS-compatible alternatives to MCT have to be established for IR detection. We use bandgap engineered InAs/GaSb T2SLs to tailor the temperature-dependent bandgap energy for detection throughout the required spectral range. Molecular beam epitaxy of superlattice samples is performed on GaAs substrates with a metamorphic GaAsSb buffer layer. Photolithographic processing yields laterally-operated T2SL photodetectors. Integrated in a TE-cooled IR detector module, such T2SL photodetectors can be an alternative to MCT photodetectors for spectroscopy applications. Here, we exemplify this by exchanging a commercially available MCT-based IR detector module with our T2SL-based IR detector module in a real-time mid-infrared backscattering spectroscopy system for substance identification. The key detector requirements imposed by the spectroscopy system are a MHz-bandwidth, a broad spectral response, and a high signal-to-noise ratio, all of which are covered by the reported T2SL-based IR detector module. Hence, in this paper, we demonstrate the versatility of TE-cooled InAs/GaSb T2SL photodetectors and their applicability in an IR spectroscopy system.

Распределение дислокаций несоответствия и упругих механических напряжений в метаморфных буферных слоях InAlAs разной конструкции

Here we report on computing of the distribution of the equilibrium misfit dislocation density ρ(z) as well as the elastic strain ε(z) along the grow direction for metamorphic buffer layer InAlAs/GaAs(001) with high In content (x ≤ 0.87) and different design of composition profile: step-, linear- and convex-graded. For the computation, an approach based on the iterative finding the system total energy minimum have been used. It was shown, that the significant difference between different types of the buffer layer is observed for the ρ(z) distribution rather than for ε(z). In contrast to traditionally used step- and linear-graded metamorphic buffer layers, which are characterized by homogenous spreading of misfit dislocations, the main part of such dislocations in the convex-graded composition profile is concentrated at the bottom part of the buffer layer near to heterointerface InAlAs/GaAs, and the dislocation density drop by more than one order of magnitude along the layer thickness reaching near the surface the minimal value among the buffer types. Despite the fact, that the significant effect of interaction between misfit dislocations is not taken into account in the computation, the results obtained allowed one to determine the main features of the ρ(z) and ε(z) distributions in the different InAlAs metamorphic buffer layers, which were previously obtained experimentally. Thus, such an approach can be effectively utilized for the development of the metamorphic heterostructure based devices.

Anomalous minority carrier behavior induced by chemical surface passivation solution in p-type GaSb metal–oxide–semiconductor capacitors on Si substrates

p-type GaSb metal–oxide–semiconductor capacitors with thin InAs surface capping layers were prepared on Si(001) substrates. Epitaxial structures with superlattice metamorphic buffer layers were grown by molecular beam epitaxy. Chemical surface treatment and atomic layer deposition methods were employed for a semiconductor surface passivation and Al2O3 high-k oxide fabrication, respectively. Capacitance-voltage measurements and scanning and transmission electron microscopies were used to correlate electrical properties with the oxide-semiconductor interface structure of the capacitors. Unexpectedly, fast minority carrier response present down to liquid nitrogen temperature was observed in the capacitors passivated by an ammonium sulfide solution. This fast response was found to be related to etch pitlike surface morphology developed upon chemical passivation at the surface steps formed by microtwins and antiphase domain boundaries. Preferential InAs etching by ammonium sulfide at the surface defects was confirmed by analytical TEM studies. Very low activation energy of minority carrier response suggests the presence of electron sources under the gate; they result from growth-related surface defects that give rise to potential fluctuations of as high as half the GaSb bandgap.

Photoluminescence of extended defects in heterostructures with InAlAs metamorphic buffer at different excitation wavelengths

We report the observation of photoluminescence (PL) of energy levels presumably associated with extended defects in metamorphic buffer InxAl1–xAs with x = 0.05 → 0.65 at photon energy range of 0.8–1.2 eV, while the quantum well PL was observed at 0.6–0.8 eV. The metamorphic HEMT heterostructures with In0.65Al0.35As/In0.70Ga0.30As/In0.65Al0.35As quantum well (QW) and with different defectiveness degree were studied. Interband excitation by lasers with a wavelength of 409, 450, 532 nm, and subband excitation by infrared (IR) laser with 1064 nm wavelength (with photons energy less than the bandgap of metamorphic buffer layer) both result in the appearance of “defect” PL. Compared to the visible excitation, the IR illumination with the same power density increases “defect” PL intensity in samples with high twins density by an order of magnitude, whereas in more perfect samples with stacking faults and threading dislocations it causes an increase in “defect-to-QW” PL intensities ratio. The study of PL intensity as a function of the pump power in the range of power density 10–3600 W/cm2 reveals that the non-radiative recombination plays an important role during the recombination of photoexcited charge carriers from “defect” levels because the “defect” PL intensity dependence on the pump power density is mainly sublinear while for the QW PL the power law with exponent 1.2–1.4 is observed.

Antimonidation of ultrathin epitaxial aluminum nanofilms for metamorphic growth of Sb-based structures on GaAs substrates

The narrow-bandgap Sb-based semiconductors find their potential in various kinds of device applications such as high-speed, low-power transistors and mid-infrared diode lasers. However, epitaxial growth on the substrates that are commercially available in large sizes (for example, GaAs and Si) faces the fundamental challenge of lattice mismatch. Different from the conventional metamorphic growth employing a thick buffer, we adopt the approach of transforming an ultrathin epitaxial aluminum (Al) nanofilm into epitaxial AlSb to serve as the template for growth of antimonide structures. It is found that the process named “antimonidation” plays a critical role in Sb-based metamorphic growth. Our experimental results provide practical usefulness for growing semiconductor devices on lattice-mismatched substrates without using thick metamorphic buffer layers.

MBE growth of high quality AlInSb/GaSb compound buffer layers on GaAs substrates

The epitaxy properties of GaAs-based GaSb thin films were investigated under different growth conditions to identify suitable smooth areas on GaSb epilayer surface for growing high-quality AlInSb metamorphic buffers. The results showed notable reduction in threading dislocation densities (TDDs) and hillocks densities (HDs) of GaSb buffer layer when the temperature of substrates rising to Tc (surface reconstruction conversion temperature) + 110 °C and growth rate decreasing to 0.25 mono layer per second (ML/s). These conditions led to gradual changes in GaSb epilayer surface morphologies from tiny hillocks to near-parallel steps. The AlInSb/GaSb compound buffers were studied by employing five different structures of AlInSb metamorphic buffer layers grown on optimized GaSb layers, and then analyzed by various analytical methods. The influence of various structural buffer layers on defects were evaluated in order to suppress the formation of defects. It was shown that the buffer layer with low-temperature interface layer and superlattice structure presents obvious inhibitory effects on defects. Among these, specimens with superlattice interface exhibits the lowest TDDs of 3.1 × 107 cm−2 and moderate HDs of 2.4 × 107 cm−2.

Interpretation of photoluminescence spectra of metamorphic InAlAs/GaAs heterostructures

The results of analyzing the photoluminescence spectra of metamorphic In0.75Al0.25As/In0.05-0.8Al0.95-0.2As/GaAs heterostructures grown by molecular-beam epitaxy method are presented. In the spectra of low-temperature photoluminescence, three emission peaks have been detected, which correspond to carrier recombination in different areas of the structure. These include a part of the In0.75Al0.25As “virtual substrate” grown at high temperature, and the area of a triangular potential well formed by the meta-morphic buffer layer and the “virtual substrate”. Photoluminescence related to defects in the high-temperature part of the “virtual substrate” was also measured.

Crystal lattice tilt analysis in gradient composition layers by electron and X-ray diffraction

The combination of X-ray tilt-azimuth figure, reciprocal space map and transmission electron microscopy in selected area electron diffraction mode was applied to analyse the tilt of the MBE-grown 1.2-μm thick convex-graded InxAl1-xAs (x=0.05-0.80) metamorphic buffer layer on singular GaAs (001) substrate. The combination of the techniques is demonstrated to enable the determination of the tilt angle as well as the tilt direction together with monitoring of the tilt development along the growth direction. For the studied metamorphic buffer layer as an example, the tilt direction was deduced to be 20° off the (110) GaAs substrate crystallographic plane. The tilt angle of (002) buffer layer planes was observed to rapidly rise to 0.6° near the layer-substrate interface and then to slowly increase to 1.4° with the approach toward the surface.

In situ study of elastic strain relaxation in metamorphic InAs(Sb)/In(Ga,Al)As/GaAs heterostructures by using reflection high energy electron diffraction

We report on a comparative study of the elastic strain relaxation in InAs(Sb)/In(Ga,Al)As heterostructures grown by molecular beam epitaxy on GaAs substrates via InAlAs metamorphic buffer layer (MBL) with and without a highly strained 5 nm-thick GaAs insertion. Reflection high energy electron diffraction was used for in situ monitoring an in-plane lattice parameter of the epitaxial layers during growth. As a result, critical thickness of the initial stage of the InAlAs metamorphic buffer layer as well as its corresponding composition were determined for the structures with different position of the GaAs insertion within the MBL. The structure with 5 nm-thick GaAs layer inserted in the InAlAs MBL directly after achieving the In content of 37 mol.% demonstrates the highest room temperature photoluminescence at a wavelength of about 3.5 μm.

Effect of growth temperature of GaAsx Sb1–x metamorphic buffer layer on electron mobility of InAs/AlSb heterostructures grown on Si substrate**Project supported by the National Defense Advanced Research Project, China (Grant No. 315 xxxxx301), the National Defense Innovation Program, China (Grant No. 48xx4), the National Key Technologies Research and Development Program, China (Grant No. 2018YFA0306101), the Scientific Instrument Developing Project of

The InAs/AlSb heterostructures with step-graded GaAsxSb1 − x metamorphic buffer layers grown on Si substrates by molecular beam epitaxy are studied. The step-graded GaAsxSb1 − x metamorphic buffer layers are used to relax the strain and block defects at each interface of the layers. Meanwhile, adding Sb to GaAs is also beneficial to suppressing the formation of dislocations in the subsequent materials. The influences of the growth temperature of the step-graded GaAsxSb1 − x metamorphic buffer layer on the electron mobility and surface topography are investigated for a series of samples. Based on the atomic force microscopy (AFM), high resolution x-ray diffraction (HRXRD), reciprocal space map (RSM), and Hall measurements, the crystal quality and composition of GaAsxSb1 − x layer are seen to strongly depend on growth temperature while keeping the Ga growth rate and V/III ratio constant. The results show that the highest electron mobility is 10270 cm2/V·s and the roughness is 4.3 nm for the step-graded GaAsxSb1 − x metamorphic buffer layer grown at a temperature of 410 °C.

Electrically injected 1.64µm emitting In0.65Ga0.35As 3-QW laser diodes grown on mismatched substrates by MOVPE.

We report the characteristics of the strained In0.65Ga0.35As triple quantum well (QW) diode lasers grown by metalorganic vapor phase epitaxy (MOVPE) on lattice-mismatched substrates such as GaAs or Si, by utilizing InP metamorphic buffer layers (MBLs) in conjunction with InAs nanostructure-based dislocation filters. As the lattice-mismatch between the substrate and InP MBL increases, higher threshold current densities and lower slope efficiencies were observed, together with higher temperature sensitivities for the threshold current and slope efficiency. Structural analysis performed by both high-resolution X-ray diffraction (HR-XRD) and transmission electron microscopy indicates graded and/or rougher QW interfaces within the active region grown on the mismatched substrate, which accounts for the observed devices characteristics.

Emission at 1.6 μm from InAs Quantum Dots in Metamorphic InGaAs Matrix

Growth of InAs quantum dots (QDs) in metamorphic InGaAs matrix for long‐wavelength laser applications on GaAs substrates by molecular beam epitaxy (MBE) is demonstrated. Metamorphic InGaAs matrix is based on a five‐step graded InGaAs metamorphic buffer layer (MBL) with a final indium composition of about 40%. Reciprocal space mapping for the asymmetrical (−2 2 4) and (2 2 4) reflections along the [−1 1 0] and [1 1 0] directions shows anisotropic relaxation along these two directions. The metamorphic InGaAs matrix is more relaxed along [−1 1 0] direction but nearly fully strained along [1 1 0] direction. InAs QDs are embedded in two InGaAs confining layers with the same indium composition. Emission at 1.6 μm from metamorphic InAs QDs and 1.42 μm emission from metamorphic InGaAs matrix at room temperature (RT) are observed, respectively. The activation energy of ≈94.6 meV is obtained.

Optical properties of metamorphic type-I InAs1−xSbx/Aly In1−y As quantum wells grown on GaAs for the mid-infrared spectral range

We analyse the optical properties of InAs1−xSbx/Aly In1−y As quantum wells (QWs) grown by molecular beam epitaxy on relaxed Aly In1−y As metamorphic buffer layers (MBLs) using GaAs substrates. The use of Aly In1−y As MBLs allows for the growth of QWs having large type-I band offsets, and emission wavelengths >3 m. Photoluminescence (PL) measurements for QWs having Sb compositions up to x = 10% demonstrate strong room temperature PL up to 3.4 m, as well as enhancement of the PL intensity with increasing wavelength. To quantify the trends in the measured PL we calculate the QW spontaneous emission (SE), using a theoretical model based on an eight-band Hamiltonian. The theoretical calculations, which are in good agreement with experiment, identify that the observed enhancement in PL intensity with increasing wavelength is associated with the impact of compressive strain on the QW valence band structure, which reduces the band edge density of states making more carriers available to undergo radiative recombination at fixed carrier density. Our results highlight the potential of type-I InAs1−xSbx/Aly In1−y As metamorphic QWs to address several limitations associated with existing heterostructures operating in the mid-infrared, establishing these novel heterostructures as a suitable platform for the development of light-emitting diodes and diode lasers.

1080 nm InGaAs laser power converters grown by MOCVD using InAlGaAs metamorphic buffer layers

Metamorphic InGaAs laser power converters have a great potential to be used for a delivery of electrical energy over distances of centimeters up to kilometers. The direct band gap InGaAs material covers monochromatic wavelengths around 1100 nm, which can be supplied by widespread commercial lasers. In this study, the InGaAs-based laser power converter have been designed to absorb a laser at 1080 nm and grown by metalorganic chemical vapor deposition. Step graded InAlGaAs buffer layers were employed to overcome the lattice mismatch problem between InGaAs and GaAs. Various epitaxial growth conditions such as overshoot layer which contains higher indium composition than the final buffer layer, back surface field barrier, growth temperature and V/III ratio were studied to obtain high quality InAlGaAs metamorphic buffer. The fabricated InGaAs-based laser power converter, which had an enhanced metamorphic buffer and 5 μm thick grid metal, showed a high external quantum efficiency of 82.16% at 1080 nm wavelength. As a result, a conversion efficiency of 37.87% was achieved under 1080 nm monochromatic light at a laser power density of 538 mW/cm2. Further improvements are expected when the device structure is modified to reduce the series resistance and the growth condition is optimized to increase the minority carrier life time.

Metamorphic Integration of GaInAsSb Material on GaAs Substrates for Light Emitting Device Applications.

The GaInAsSb material has been conventionally grown on lattice-matched GaSb substrates. In this work, we transplanted this material onto the GaAs substrates in molecular beam epitaxy (MBE). The threading dislocations (TDs) originating from the large lattice mismatch were efficiently suppressed by a novel metamorphic buffer layer design, which included the interfacial misfit (IMF) arrays at the GaSb/GaAs interface and strained GaInSb/GaSb multi-quantum wells (MQWs) acting as dislocation filtering layers (DFLs). Cross-sectional transmission electron microscopy (TEM) images revealed that a large part of the dislocations was bonded on the GaAs/GaSb interface due to the IMF arrays, and the four repetitions of the DFL regions can block most of the remaining threading dislocations. Etch pit density (EPD) measurements indicated that the dislocation density in the GaInAsSb material on top of the buffer layer was reduced to the order of 106 /cm2, which was among the lowest for this compound material grown on GaAs. The light emitting diodes (LEDs) based on the GaInAsSb P-N structures on GaAs exhibited strong electro-luminescence (EL) in the 2.0–2.5 µm range. The successful metamorphic growth of GaInAsSb on GaAs with low dislocation densities paved the way for the integration of various GaInAsSb based light emitting devices on the more cost-effective GaAs platform.

Peculiarities of the energy spectrum of InSb/InAs/InGaAs/InAlAs/GaAs nanoheterostructures revealed by room temperature photomodulation FTIR spectroscopy

Fourier-transform infrared photoreflectance was used to optimize design and growth temperature of complex III–V nanoheterostructures proposed for efficient mid-IR emitters. They comprised an InAs/InGaAs quantum well (QW) with InSb sub-monolayer insertions and were grown by molecular beam epitaxy on GaAs substrates via a convex-graded InAlAs metamorphic buffer layer. The room temperature photoreflectance spectra supported by the full 8 × 8 Kane model calculations showed that the triple InSb insertions enhance the probability of the main optical transitions in the QW, while lowering the growth temperature results in a red shift of the transitions and further increase in their probability.