Step-graded Buffer Research Articles

C/L-band emission of InAs QDs monolithically grown on Ge substrate

In recent years, the growing demand for silicon based light sources has boosted the research field of III-V/IV hybrid lasers. Here, the C/L-band light emission (1.53 μm-1.63 μm) of InAs/In0.25Ga0.75As quantum dots (QDs) epitaxially grown on Ge substrate by solid-source molecular beam epitaxy (MBE) is reported. By hybrid III-V/IV epitaxial growth, ultra-thin and anti-phase domains (APD) free III-V materials are achieved on Ge substrate. Step-graded InGaAs metamorphic buffer layers are applied to reduce the strain in InAs QDs in order to extend the emission wavelength. At last, a high quality InAs/In0.25Ga0.75As QD structure on Ge(001) substrate is obtained, which has a strong C/L-band emission centered at the wavelength of 1.6 μm with a full-width-half-maximum (FWHM) of 57 meV at room temperature.

Dynamical X-Ray Diffraction Analysis of Triple-Junction Solar Cells on Germanium (001) Substrates

We demonstrate the dynamical x-ray diffraction analysis of metamorphic triple-junction solar cells grown on Ge (001) substrates. The solar cells investigated involve an In0.67Ga0.33P top cell, an In0.17Ga0.83As middle cell, and a Ge bottom cell. A graded buffer layer is inserted between the bottom and middle cells for the purpose of accommodating the lattice mismatch. Linearly-graded, step-graded, and S-graded compositional profiles were considered for this buffer layer. The x-ray rocking curve analysis for a number of hkl reflections including 004, 113, 116, 044, 026, and 117 was conducted for the case of Cu Kα1 radiation. We show that the use of non-destructive x-ray analysis allows determination of the threading dislocation densities in the top two cells. In the cases of S-graded or step-graded buffer layers, the buffer threading dislocation density could also be estimated.

Simultaneous dual-functioning InGaN/GaN multiple-quantum-well diode for transferrable optoelectronics

We propose a wafer-level procedure for the fabrication of 1.5-mm-diameter dual functioning InGaN/GaN multiple-quantum-well (MQW) diodes on a GaN-on-silicon platform for transferrable optoelectronics. Nitride semiconductor materials are grown on (111) silicon substrates with intermediate Al-composition step-graded buffer layers, and membrane-type MQW-diode architectures are obtained by a combination of silicon removal and III-nitride film backside thinning. Suspended MQW-diodes are directly transferred from silicon to foreign substrates such as metal, glass and polyethylene terephthalate by mechanically breaking the support beams. The transferred MQW-diodes display strong electroluminescence under current injection and photodetection under light irradiation. Interestingly, they demonstrate a simultaneous light-emitting light-detecting function, endowing the 1.5-mm-diameter MQW-diode with the capability of producing transferrable optoelectronics for adjustable displays, wearable optical sensors, multifunctional energy harvesting, flexible light communication and monolithic photonic circuit.

Fabrication and Characterization of Si Substrate-Free InGaN Light-Emitting Diodes and Their Application in Visible Light Communications

Visible light communications with InGaN-based light-emitting diodes (LEDs) grown on large-diameter (6-inch) and cost-effective Si (111) substrates are investigated experimentally. During epitaxial growth, the transition layers consisted of the step-graded AlGaN buffers incorporated with three low-temperature-grown (∼900 °C) AlN interlayers on AlN/Si substrates that are used to compensate for thermally induced tensile stress and to maintain a reasonable crystalline quality of GaN-on-Si LEDs. Strong light absorption from Si can be prevented by fabricating a Si substrate-free InGaN LED with a composite metal coating of Al/Ag/Al multilayer, providing improved adhesive strength and reflectivity comparable to the unitary Ag film. In comparison with GaN-on-Si LEDs, stripping Si substrates combined with the use of a highly reflective bottom mirror (Al/Ag/Al multilayer) reflected a more intense emission pattern corresponding to a 2.2 times (@ 190 mA) increase in light output power in thin-film LEDs. In addition, a 1.8 times (@ 160 mA) increase in optical channel bandwidth is achieved by using thin-film LEDs as optical transmitters. A direct line-of-sight optical link using the proposed thin-film LEDs achieved data transmission rates of up to 100 Mb/s over a distance of 100 cm, indicating that the proposed LEDs have potential for use as optical transmitters in indoor visible light communications.

梯度铝镓氮缓冲层对氮化镓外延层性质的影响

梯度铝镓氮缓冲层对氮化镓外延层性质的影响

Comparative Analysis of Strain Fields in Step-graded Buffers of Different Design, based on InxAl1-xAs Ternary Solutions

Two hetero structures with the step-graded buffers of different design grown on (001) GaAs substrates by molecular beam epitaxy were employed to reveal applicability of an extension of phenomenological approach developed for the description of strain relief in single layer hetero structures to multilayer thin film systems. Difference in the design of buffers provided to the formation of dislocation free layers of different thickness. The determination of the residual strains in the epitaxial layers was done using reciprocal space mapping performed with a tripleaxes X-ray diffractometer Smart Lab 9 kW and the following processing of data obtained within the linear theory of elasticity. It was established that, despite the different design of buffers the character of strain spatial distributions in them was similar. It gives possibility to attract a phenomenological rule to describe the strain relief in the final constructive elements of both hetero structures. A correction for a work hardening in the phenomenological rule governing the strain relief in single layer hetero structures was performed.

Intensive Terahertz Radiation from InXGa1-XAs due to Photo-Dember Effect

We have proposed and investigated InyGa1-yAs photoconductor grown by molecular-beam epitaxy on low-temperature step-graded metamorphic buffer. It exhibits superior bandwidth up to 6 THz and provides optical-to-terahertz conversion efficiency up to ~ 10−5 for rather low optical fluence ~ 40 μJ/cm2. The intensity of THz generation for the given structure is two orders higher than for lowtemperature grown GaAs due substantial contribution of photo-Dember effect.

Inverted Metamorphic Multijunction Semiconductors for Exceptionally High Photoelectrolysis Efficiencies: Materials Development and Measurement Challenges

In order to economically generate renewable hydrogen fuel from solar energy using semiconductor-based devices, the U.S. Department of Energy Fuel Cells Technology Office has established technical targets of over 20% solar-to-hydrogen (STH) efficiency with several thousand hours of stability under operating conditions. Our goal is to improve STH efficiency from just over 10% to over 20% via novel tandem semiconductor materials and configurations. Our primary focus is to develop inverted metamorphic multijunction (IMM) III-V semiconductors that have bandgaps optimized for water splitting. The IMM devices are grown by metal organic chemical vapor deposition using a GaAs substrate and use a transparent step-graded buffer layer to allow off-lattice growth. We used IMM growth to achieve a high-quality, lattice-mismatched lower bandgap (1.2 eV) InGaAs bottom cell that captures a larger fraction of the solar spectrum. This 1.8/1.2 eV bandgap combination of the GaInP2/InGaAs configuration is able to generate higher photocurrents, and thus efficiencies, than the 1.8/1.4 eV bandgap combination of the lattice-matched GaInP2/GaAs configuration. We will also discuss measurement challenges in appraising solar-to-hydrogen efficiency, some of which are specific to tandem absorbers. We have recently discovered several systematic errors in measurements made by our group and others that consistently overrate efficiency. Using more stringent measurement standards, we have confirmed over 15% STH on our most advanced IMM devices. Our focus in the next year will be on photon management strategies at the semiconductor/electrolyte interface. Capturing a significant portion of the ~25% of photons lost to reflection at this interface should allow the realization of devices that exceed 20% STH efficiency.

Structure characterization of MHEMT heterostructure elements with In0.4Ga0.6As quantum well grown by molecular beam epitaxy on GaAs substrate using reciprocal space mapping

The crystallographic parameters of elements of a metamorphic high-electron-mobility transistor (MHEMT) heterostructure with In0.4Ga0.6As quantum well are determined using reciprocal space mapping. The heterostructure has been grown by molecular-beam epitaxy (MBE) on the vicinal surface of a GaAs substrate with a deviation angle of 2° from the (001) plane. The structure consists of a metamorphic step-graded buffer (composed of six layers, including an inverse step), a high-temperature buffer of constant composition, and active high-electron-mobility transistor (HEMT) layers. The InAs content in the metamorphic buffer layers varies from 0.1 to 0.48. Reciprocal space mapping has been performed for the 004 and 224 reflections (the latter in glancing exit geometry). Based on map processing, the lateral and vertical lattice parameters of InxGa1–xAs ternary solid solutions of variable composition have been determined. The degree of layer lattice relaxation and the compressive stress are found within the linear elasticity theory. The high-temperature buffer layer of constant composition (on which active MHEMT layers are directly formed) is shown to have the highest (close to 100%) degree of relaxation in comparison with all other heterostructure layers and a minimum compressive stress.

Structural Mapping of Functional Ge Layers Grown on Graded SiGe Buffers for sub-10 nm CMOS Applications Using Advanced X-ray Nanodiffraction.

We report a detailed advanced materials characterization study on 40 nm thick strained germanium (Ge) layers integrated on 300 mm Si(001) wafers via strain-relaxed silicon-germanium (SiGe) buffer layers. Fast-scanning X-ray microscopy is used to directly image structural inhomogeneities, lattice tilt, thickness, and strain of a functional Ge layer down to the sub-micrometer scale with a real space step size of 750 μm. The structural study shows that the metastable Ge layer, pseudomorphically grown on Si(0.3)Ge(0.7), exhibits an average compressive biaxial strain of -1.27%. By applying a scan area of 100 × 100 μm(2), we observe microfluctuations of strain, lattice tilt, and thickness of ca. ±0.03%, ±0.05°, and ±0.8 nm, respectively. This study confirms the high materials homogeneity of the compressively strained Ge layer realized by the step-graded SiGe buffer approach on 300 mm Si wafers. This presents thus a promising materials science approach for advanced sub-10 nm complementary metal oxide-semiconductor applications based on strain-engineered Ge transistors to outperform current Si channel technologies.

Equilibrium Lattice Relaxation and Misfit Dislocations in Continuously- and Step-Graded InxGa1-xAs/GaAs (001) and GaAs1-yPy/GaAs (001) Metamorphic Buffer Layers

The inclusion of metamorphic buffer layers (MBL) in the design of lattice-mismatched semiconductor heterostructures is important in enhancing reliability and performance of optical and electronic devices. These metamorphic buffer layers usually employ linear grading of composition, and materials including InxGa1-xAs and GaAs1-yPy have been used. Non-uniform and continuously graded profiles are beneficial for the design of partially-relaxed buffer layers because they reduce the threading dislocation density by allowing the distribution of the misfit dislocations throughout the metamorphic buffer layer, rather than concentrating them at the interface where substrate defects and tangling can pin dislocations or otherwise reduce their mobility as in the case of uniform compositional growth. In this work we considered heterostructures involving a linearly-graded (type A) or step-graded (type B) buffer layer grown on a GaAs (001) substrate. For each structure type we present minimum energy calculations and compare the cases of cation (Group III) and anion (Group V) grading. In addition, we studied the (i) average and surface in-plane strain and (ii) average misfit dislocation density for heterostructures with various thickness and compositional profile. Moreover, we show that differences in the elastic stiffness constants give rise to significantly different behavior in these two commonly-used buffer layer systems.

Control of threading dislocations by strain engineering in GaInP buffers grown on GaAs substrates

High quality strain-relaxed In0.3Ga0.7As layers with threading dislocation density about 2×106cm−2 and root-mean-square surface roughness below 8.0nm were obtained on GaAs substrates using compositionally undulating step-graded Ga1−xInxP (x=0.48–0.78) buffers. The transmission electron microscopy results reveal that the conventional step-graded GaInP buffers produce high density dislocation pile-ups, which are induced by the blocking effect of the nonuniform misfit dislocation strain field and crosshatched surface on the gliding of threading dislocations. In contrast, due to strain compensation, insertion of the tensile GaInP layers decreases the surface roughness and promotes dislocation annihilation in the interfaces, and eventually reduces the threading dislocation density. This provides a promising way to achieve a virtual substrate with the desired lattice parameter for metamorphic device applications.

Imaging Structure and Composition Homogeneity of 300 mm SiGe Virtual Substrates for Advanced CMOS Applications by Scanning X-ray Diffraction Microscopy.

Advanced semiconductor heterostructures are at the very heart of many modern technologies, including aggressively scaled complementary metal oxide semiconductor transistors for high performance computing and laser diodes for low power solid state lighting applications. The control of structural and compositional homogeneity of these semiconductor heterostructures is the key to success to further develop these state-of-the-art technologies. In this article, we report on the lateral distribution of tilt, composition, and strain across step-graded SiGe strain relaxed buffer layers on 300 mm Si(001) wafers treated with and without chemical-mechanical polishing. By using the advanced synchrotron based scanning X-ray diffraction microscopy technique K-Map together with micro-Raman spectroscopy and Atomic Force Microscopy, we are able to establish a partial correlation between real space morphology and structural properties of the sample resolved at the micrometer scale. In particular, we demonstrate that the lattice plane bending of the commonly observed cross-hatch pattern is caused by dislocations. Our results show a strong local correlation between the strain field and composition distribution, indicating that the adatom surface diffusion during growth is driven by strain field fluctuations induced by the underlying dislocation network. Finally, it is revealed that a superficial chemical-mechanical polishing of cross-hatched surfaces does not lead to any significant change of tilt, composition, and strain variation compared to that of as-grown samples.

High hole mobility InGaSb/AlSb QW field effect transistors grown on Si by molecular beam epitaxy

Growth of InGaSb/AlSb high hole mobility quantum well field effect transistors (QW FETs) on Si substrates with a step-graded GaAsSb metamorphic buffer layer by molecular beam epitaxy is explored. With an optimized growth temperature for the InGaSb/AlSb QW, hole mobility of 770cm2/Vs and 3060cm2/Vs have been achieved at room temperature and 77K, respectively. It is also found that the twins in the samples do not cause significant anisotropic behavior of the InGaSb QW FETs in term of gate direction.

Growth variations and scattering mechanisms in metamorphic In0.75Ga0.25As/In0.75 Al0.25As quantum wells grown by molecular beam epitaxy

Modulation doped metamorphic In0.75Ga0.25As/In0.75Al0.25As quantum wells (QW) were grown on GaAs substrates by molecular beam epitaxy (MBE) with step-graded buffer layers. The electron mobility of the QWs has been improved by varying the MBE growth conditions, including substrate temperature, arsenic over pressure and modulation doping level. By applying a bias voltage to SiO2 insulated gates, the electron density in the QW can be tuned from 1×1011 to 5.3×1011cm−2. A peak mobility of 4.3×105cm2V−1s−1 is obtained at 3.7×1011cm−2 at 1.5K before the onset of second subband population. To understand the evolution of mobility, transport data is fitted to a model that takes into account scattering from background impurities, modulation doping, alloy disorder and interface roughness. According to the fits, scattering from background impurities is dominant while that from alloy disorder becomes more significant at high carrier density.

Control of metamorphic buffer structure and device performance of InxGa1−xAs epitaxial layers fabricated by metal organic chemical vapor deposition

Using a step-graded (SG) buffer structure via metal-organic chemical vapor deposition, we demonstrate a high suitability of In0.5Ga0.5As epitaxial layers on a GaAs substrate for electronic device application. Taking advantage of the technique’s precise control, we were able to increase the number of SG layers to achieve a fairly low dislocation density (∼106 cm−2), while keeping each individual SG layer slightly exceeding the critical thickness (∼80 nm) for strain relaxation. This met the demanded but contradictory requirements, and even offered excellent scalability by lowering the whole buffer structure down to 2.3 μm. This scalability overwhelmingly excels the forefront studies. The effects of the SG misfit strain on the crystal quality and surface morphology of In0.5Ga0.5As epitaxial layers were carefully investigated, and were correlated to threading dislocation (TD) blocking mechanisms. From microstructural analyses, TDs can be blocked effectively through self-annihilation reactions, or hindered randomly by misfit dislocation mechanisms. Growth conditions for avoiding phase separation were also explored and identified. The buffer-improved, high-quality In0.5Ga0.5As epitaxial layers enabled a high-performance, metal-oxide-semiconductor capacitor on a GaAs substrate. The devices displayed remarkable capacitance–voltage responses with small frequency dispersion. A promising interface trap density of 3 × 1012 eV−1 cm−2 in a conductance test was also obtained. These electrical performances are competitive to those using lattice-coherent but pricey InGaAs/InP systems.

Investigation of In0.7Ga0.3As/In0.7Al0.3As metamorphic HEMT- heterostructures by photoluminescence spectroscopy

Low-temperature photoluminescence and photoreflectance have been studied in several metamorphic HEMT- (MHEMT-) heterostructures with the same active regions and different buffer layer designs grown by solid-source molecular beam epitaxy. The indium mole fraction in InAlAs/InGaAs/InAlAs single quantum well (QW) is 0.7. It was found that structures with step-graded metamorphic buffer have better quality. Also it was shown that mismatched superlattices in metamorphic buffer can influence on the half-width of photoluminescence spectra. The possible attribution of photoluminescence and photoreflectance spectral lines and their thermal behaviour are critically discussed.

Comparison of Continuously- and Step-Graded ZnS y Se1−y /GaAs (001) Metamorphic Buffer Layers

The design of metamorphic buffer layers for semiconductor devices with reduced defect densities requires control of lattice relaxation and dislocation dynamics. Graded layers are beneficial for the design of these buffers because they reduce the threading dislocation density by (1) allowing the distribution of the misfit dislocations throughout the buffer layer therefore reducing pinning interactions, and (2) enhancing mobility from the high built-in surface strain which helps to sweep out threading arms. In this work, we considered heterostructures involving a linearly-graded (type A) or step-graded (type B) buffer grown on a GaAs (001) substrate. For each structure type, we studied the equilibrium configuration and the kinetically-limited lattice relaxation and non-equilibrium threading dislocations by utilizing a dislocation dynamics model. In this work, we have also considered heterostructures involving a constant composition ZnSySe1−y device layer grown on top of a GaAs (001) substrate with an intermediate buffer layer of linearly-graded (type C) or step-graded (type D) ZnSySe1−y. For each structure type, we studied the requirements on the thickness and compositional profile in the buffer layer for the elimination of all mobile threading dislocations from the device layer by the dislocation compensation mechanism.

MOCVD-Grown GaP/Si Subcells for Integrated III–V/Si Multijunction Photovoltaics

Enabled by a heteroepitaxial nucleation process that yields GaP-on-Si integration free of heterovalent-related defects, GaP/active-Si junctions were grown by metalorganic chemical vapor deposition. n-type Si emitter layers were grown on p-type (1 0 0)-oriented Si substrates, followed by the growth of n-type GaP window layers, to form fully active subcell structures compatible with integration into monolithic III-V/Si multijunction solar cells. Fabricated test devices yield good preliminary performance characteristics and demonstrate great promise for the epitaxial subcell approach. Comparison of different emitter layer thicknesses, combined with descriptive device modeling, reveals insight into recombination dynamics at the GaP/Si interface and provides design guidance for future device optimization. Additional test structures consisting of GaP/active-Si subcell substrates with subsequently grown GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> step-graded buffers and GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> terminal layers were produced to simulate the optical response of the GaP/Si junction within a theoretically ideal dual-junction solar cell.

Reduction in leakage current in AlGaN/GaN HEMT with three Al-containing step-graded AlGaN buffer layers on silicon

AlGaN/GaN high-electron-mobility transistor (HEMT) structures with two and three Al-containing step-graded AlGaN buffer layers (BLs) were grown on silicon (111) substrates by metal organic chemical vapor deposition. Considerable tensile stress was observed in the GaN grown with only two 0.8 µm AlGaN BLs, while a large in-plane compression in GaN grown with three 2.3 µm AlGaN BLs. The reverse gate leakage current in the HEMT with three AlGaN BLs was approximately 0.1 µA/mm, which was more than one order of magnitude smaller than that for the HEMT with two AlGaN BLs. A three-terminal off-state breakdown voltage of 265 V and a vertical gate-to-substrate breakdown voltage of 510 V were obtained in the HEMT with three AlGaN BLs. Detailed analysis was performed on the basis of the structural properties of AlGaN/GaN heterostructures.