Misoriented GaAs Substrates Research Articles

Influence of GaAs substrate misorientation on gallium and arsenic evaporation rates

The dependence of the Ga and As evaporation rates on GaAs(111)A substrate misorientation was analyzed using Monte Carlo simulation. The gallium and arsenic desorption rates are proportional to the step density of vicinal substrates at congruent (layer-by-layer) evaporation. With incongruent evaporation (gallium droplet formation on the surface), the arsenic desorption rate weakly depends on the substrate misorientation and the gallium desorption rate is independent of it.

On the Phase Composition, Morphology, and Optical and Electronic Characteristics of AlN Nanofilms Grown on Misoriented GaAs(100) Substrates

Thin AlN nanofilms are produced by reactive ion-plasma deposition onto GaAs(100) substrates misoriented with respect to the 〈100〉 direction to different degrees. It is shown that growth on substrates misoriented with respect to the 〈100〉 direction to different degrees results in the formation of AlN films with different phase compositions and crystal states. An increase in the degree of misorientation of the GaAs(100) substrate used for growth influences both the structural quality of AlN nanofilms and their electronic structure, surface morphology, and optical properties. Thus, the morphology, surface composition, and optical functional characteristics of AlN/GaAs(100) heterophase systems can be controlled using differently misoriented GaAs(100) substrates.

The Growth of InAsxSb1 –x Solid Solutions on Misoriented GaAs(001) Substrates by Molecular-Beam Epitaxy

The effect of a substrate misorientation degree from a singular face on the composition and morphology of layers of InAsxSb1 –x solid solutions obtained by molecular-beam epitaxy on a GaAs surface has been investigated. The substrates of GaAs wafers with the orientation (100) misoriented in the direction [110] by 0°, 1°, 2°, and 5° are used. The heterostructures are grown at temperatures of 310°C and 380°C (respectively, the lower and upper boundaries of the temperature range in which structurally perfect InAsxSb1 –x films form). The effect of the molecular form of arsenic (As2 or As4) on the composition of the layers is studied. The composition and structural properties are investigated using high-resolution X-ray diffractometry (HRXRD) and atomic-force microscopy (AFM). It is established that, in the series of misorientation angles 0° → 5°, the arsenic fraction x increases consecutively when using fluxes of both As2 and As4 molecules. With the As2 molecular flux, the fraction x increases only a little (1.05 times) with increasing degree of misorientation, while, when using the As4 flux, the increase in x is 1.75 times. An increase in the growth temperature leads to growth in the arsenic fraction in the solid solution. The surface morphology improves with an increasing degree of misorientation at a low growth temperature and degrades at a high temperature.

Influence of GaAs substrate misorientation on the characteristics in undulating compositional step-graded AlGaInAs buffers

Undulating compositional step-graded AlGaInAs buffers are applied to transit the lattice mismatch of 3.8% between InP cap layer and GaAs substrates with misorientation of 2°, 7° and 15° toward (111)A by metal organic chemical vapor deposition (MOCVD). The surface morphology and strain relaxation are strongly dependent on the substrate misorientation. The sample grown on 15° GaAs substrate showed a substantial improvement of surface roughness and reduction of dislocation pile-up densities compared with that on the 2° and 7° substrate. The experiment results showed that the substrate with 15° misorientation can maintain a smooth epitaxial film surface by increasing the possibility of atoms incorporating into the lattice and ease the imbalance of the misfit strain stress distributed between different slip systems to make dislocations distributed more uniformly, which can reduce the strain field to exaggerate the channel width for dislocation gliding along the interface and decrease the threading dislocation densities. Overall, this work provides a promising way to obtain virtual substrates for the achievement of desired metamorphic devices.

Gradual Evolution From Quantum‐Well‐Like to Quantum‐Dot‐Like Characteristics in InGaAs/GaAs Nanostructures

Dense arrays of carrier localizing indium‐rich regions (referred to as quantum well‐dots, QWDs) formed inside an indium‐depleted residual quantum well by metalorganic vapor phase epitaxial deposition of 4–16 monolayers (ML) of InxGa1−xAs (0.3 < x < 0.5) on 6° misoriented GaAs (100) substrates are studied. It is shown that in addition to QWDs the deposited layers may contain other objects with size and shape similar to conventional self‐organized quantum dots (QDs). Transmission electron microscopy and photoluminescence studies reveal that the density of QDs grows with the increase in indium composition and average amount of deposited InGaAs. The QWDs show efficient absorption in the optical region of 900–1100 nm, whereas QDs do not contribute to photocurrent spectra and result in a waste of carriers. Optimal indium composition x to form QWDs with high structural and optical quality is about 0.4. For x = 0.3 the QWDs are not yet well developed, while for x = 0.5 the density of QDs becomes too high.

Structural and optical studies of thin films of aluminum nitride grown via ion-plasma sputtering on gallium arsenide substrates with different orientations

IR and UV spectroscopy is used to study the properties of nanostructured aluminum nitride films obtained via reactive ion-plasma sputtering on GaAs substrates with different orientations. Nanostructured thin (100–200 nm) films of cubic aluminum nitride with optical bandgaps of ~5 eV and refractive indices varying from 1.6 to 4.0 in the wavelength range of ~250 nm are fabricated. Growth on a misoriented GaAs(100) substrate (4° with respect to the [110] plane) makes it possible to synthesize AlN films with smaller grains and higher refractive indices (n ~ 4). It is shown that misoriented GaAs substrates allow us to control the morphology, surface composition, and optical functional characteristics of AlN/GaAs heterophase systems.

Growth features and spectroscopic structure investigations of nanoprofiled AlN films formed on misoriented GaAs substrates

Nanostructured aluminum-nitride films are formed by reactive ion-plasma sputtering onto GaAs substrates with different orientations. The properties of the films are studied via structural analysis, atomic force microscopy, and infrared and visible–ultraviolet spectroscopy. The aluminum-nitride films can have a refractive index in the range of 1.6–4.0 at a wavelength of ~250 nm and an optical band gap of ~5 eV. It is shown that the morphology, surface composition, and optical characteristics of AlN/GaAs heterophase systems can be controlled using misoriented GaAs substrates.

Distinctions of the growth and structural-spectroscopic investigations of thin AlN films grown on the GaAs substrates

Using X-ray diffraction analysis, atomic force microscopy, IR and UV spectroscopy, the properties of thin aluminium nitride films (<200nm) that were obtained by ion-plasma reactive sputtering on GaAs substrates with different orientations were studied.The films of aluminium nitride can have a refractive index within the range of 1.6–4.0 for the wavelength band around ~250nm and an optical band-gap of ~5eV. It was shown that the morphology, surface composition and optical functional characteristics of AlN/GaAs heterophase systems can be controlled owing to the use of misoriented GaAs substrates as well choice of the technological parameters used for the film growth.

Structural studies of Al thin layer on misoriented GaAs(100) substrate by transmission electron microscopy

AbstractA thin Al layer has been grown on a misoriented GaAs(100) substrate by molecular beam epitaxy and its structure has been studied by means of transmission electron microscopy. The metal layer is formed as grains of three orientations Al(100), Al(110) and Al(110)R. Digital analysis of dark‐field micrographs made it possible to obtain their grain sizes and relative coverage areas. By comparison with an Al layer grown on an exactly oriented GaAs(100) substrate it has been found that on the vicinal surface the relative coverage area and grain size of the Al(110)R orientation increase and the Al(100) relative coverage area decreases. This is attributed to surface atomic steps, which have been visualized in high‐resolution micrographs. The edge misfit dislocation system at the Al/GaAs interface has been revealed, which is insensitive to the substrate misorientation. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Comparison of single junction AlGaInP and GaInP solar cells grown by molecular beam epitaxy

We have investigated ∼2.0 eV (AlxGa1−x)0.51In0.49P and ∼1.9 eV Ga0.51In0.49P single junction solar cells grown on both on-axis and misoriented GaAs substrates by molecular beam epitaxy (MBE). Although lattice-matched (AlxGa1−x)0.51In0.49P solar cells are highly attractive for space and concentrator photovoltaics, there have been few reports on the MBE growth of such cells. In this work, we demonstrate open circuit voltages (Voc) ranging from 1.29 to 1.30 V for Ga0.51In0.49P cells, and 1.35–1.37 V for (AlxGa1−x)0.51In0.49P cells. Growth on misoriented substrates enabled the bandgap-voltage offset (Woc = Eg/q − Voc) of Ga0.51In0.49P cells to decrease from ∼575 mV to ∼565 mV, while that of (AlxGa1−x)0.51In0.49P cells remained nearly constant at 620 mV. The constant Woc as a function of substrate offcut for (AlxGa1−x)0.51In0.49P implies greater losses from non-radiative recombination compared with the Ga0.51In0.49P devices. In addition to larger Woc values, the (AlxGa1−x)0.51In0.49P cells exhibited significantly lower internal quantum efficiency (IQE) values than Ga0.51In0.49P cells due to recombination at the emitter/window layer interface. A thin emitter design is experimentally shown to be highly effective in improving IQE, particularly at short wavelengths. Our work shows that with further optimization of both cell structure and growth conditions, MBE-grown (AlxGa1−x)0.51In0.49P will be a promising wide-bandgap candidate material for high-efficiency, lattice-matched multi-junction solar cells.

Electron microscopy of an aluminum layer grown on the vicinal surface of a gallium arsenide substrate

A thin Al layer grown by molecular-beam epitaxy on a misoriented GaAs (100) substrate is studied by transmission electron microscopy. Electron diffraction data and bright-field, dark-field, and high-resolution images show that, in the layer, there are Al grains of three types of crystallographic orientation: Al (100), Al (110), and Al (110)R. The specific structural features of the interfaces between the differently oriented grains and substrate are studied by digital processing of the high-resolution images. From quantitative analysis of the dark-field images, the relative content and sizes of the differently oriented grains are determined. It is found that atomic steps at the substrate surface cause an increase in the fraction and sizes of Al (110)R grains and a decrease in the fraction of Al (100) grains, compared to the corresponding fractions and sizes in the layer grown on a singular substrate surface.

Single-Mode Emission From 4–9-μm Microdisk Lasers With Dense Array of InGaAs Quantum Dots

A dense array of InGaAs quantum dots formed by MOCVD on a misoriented GaAs substrate has been used as an active medium of microdisk resonators of various types: a cylindrical disk, an undercut disk, and a suspended disk. Single-mode room temperature lasing in a 9-μm microdisk laser is demonstrated with a dominant line around 1.13 μm. Impact of the resonator design on the lasing threshold and its temperature behavior is discussed.

InGaAs/GaAsP strain balanced multi-quantum wires grown on misoriented GaAs substrates for high efficiency solar cells

Quantum wires (QWRs) form naturally when growing strain balanced InGaAs/GaAsP multi-quantum wells (MQW) on GaAs [100] 6° misoriented substrates under the usual growth conditions. The presence of wires instead of wells could have several unexpected consequences for the performance of the MQW solar cells, both positive and negative, that need to be assessed to achieve high conversion efficiencies. In this letter, we study QWR properties from the point of view of their performance as solar cells by means of transmission electron microscopy, time resolved photoluminescence and external quantum efficiency (EQE) using polarised light. We find that these QWRs have longer lifetimes than nominally identical QWs grown on exact [100] GaAs substrates, of up to 1 μs, at any level of illumination. We attribute this effect to an asymmetric carrier escape from the nanostructures leading to a strong 1D-photo-charging, keeping electrons confined along the wire and holes in the barriers. In principle, these extended lifetimes could be exploited to enhance carrier collection and reduce dark current losses. Light absorption by these QWRs is 1.6 times weaker than QWs, as revealed by EQE measurements, which emphasises the need for more layers of nanostructures or the use light trapping techniques. Contrary to what we expected, QWR show very low absorption anisotropy, only 3.5%, which was the main drawback a priori of this nanostructure. We attribute this to a reduced lateral confinement inside the wires. These results encourage further study and optimization of QWRs for high efficiency solar cells.

Effect of (1 0 0) GaAs substrate misorientation on electrophysical parameters, structural properties and surface morphology of metamorphic HEMT nanoheterostructures InGaAs/InAlAs

The effect of GaAs (100) substrate misorientation on metamorphic high electron mobility transistor nanoheterostructure In0.64Al0.3As/In0.7Ga0.3As properties was studied by the means of Hall measurements, photoluminescence spectroscopy, X-ray diffraction, TEM and STEM measurements. The identical heterostructures with step-graded metamorphic buffer InAlAs were grown by MBE on (100) GaAs substrates exactly oriented and 2° misoriented towards [0−1−1] direction and the detailed comparison of electronic and structural properties was performed. The increase of electron density by 40% was found in the heterostructure grown on misoriented GaAs substrate though Si doping concentration in δ-layers was the same for both samples. In addition the substrate misorientation affected some of the heterostructure structural properties: the QW heterointerfaces were found to be more broadened, the residual strain in the In0.64Al0.36As barrier region was higher and the surface morphology was rougher in the heterostructure on (100)+2° substrate as compared to that on the (100) GaAs substrate.

Study of the Growth and Dislocation Blocking Mechanisms in InxGa1−xAs Buffer Layer for Growing High-Quality In0.5Ga0.5P, In0.3Ga0.7As, and In0.52Ga0.48As on Misoriented GaAs Substrate for Inverted Metamorphic Multijunction Solar Cell Application

AbstractEffects of growth conditions and buffer structures on crystal quality of 1.9-eV In

Effect of growth temperature and GaAs substrate misorientation on the morphology of InAsBi nanoislands grown by metalorganic vapor phase epitaxy

The structural properties of InAsBi nanoislands grown on semi insulating GaAs by atmospheric pressure metalorganic vapor phase epitaxy, using trimethyl indium, trimethyl bismuth, and arsine as precursor sources have been studied. The influence of growth temperature and substrate misorientation on the surface morphologies of these nanostructures have been controlled by means of atomic force microscopy. The results show InAsBi islands formation on the studied samples. The density, shape, size and the size dispersion of these islands vary greatly with growth temperature. So, below 400°C island density increases with increasing growth temperature and accompanied by appearance of ridges. Increasing temperature over 400°C induces a decrease in the island density and enlargement of sizes. In addition, samples grown on 10° misoriented substrates exhibit a clearly ridge on the surface.

InGaP/GaAs Dual-Junction Solar Cell with AlGaAs/GaAs Tunnel Diode Grown on 10° off Misoriented GaAs Substrate

InGaP/GaAs dual-junction solar cells with different tunnel diodes (TDs) grown on misoriented GaAs substrates are investigated. It is demonstrated that the solar cells with P++-AlGaAs/N++-GaAs TDs grown on 10° off GaAs substrates not only show a higher external quantum efficiency (EQE) but also generate a higher peak current density (Jpeak) at higher concentration ratios (185×) than the solar cells with P++-GaAs/N++-InGaP TDs grown on 6° off GaAs substrates. Furthermore, the cell design with P++-AlGaAs/N++-GaAs TDs grown on 10° off GaAs substrates does not generate a disordered InGaP epitaxial layer during material growth, and thus shows superior current–voltage characteristics.

InGaP/GaAs Dual-Junction Solar Cell with AlGaAs/GaAs Tunnel Diode Grown on 10° off Misoriented GaAs Substrate

InGaP/GaAs dual-junction solar cells with different tunnel diodes (TDs) grown on misoriented GaAs substrates are investigated. It is demonstrated that the solar cells with P++-AlGaAs/N++-GaAs TDs grown on 10° off GaAs substrates not only show a higher external quantum efficiency (EQE) but also generate a higher peak current density (Jpeak) at higher concentration ratios (185×) than the solar cells with P++-GaAs/N++-InGaP TDs grown on 6° off GaAs substrates. Furthermore, the cell design with P++-AlGaAs/N++-GaAs TDs grown on 10° off GaAs substrates does not generate a disordered InGaP epitaxial layer during material growth, and thus shows superior current–voltage characteristics.

Optical and morphological study of misoriented GaAs substrates exposed to bismuth flow using in situ spectral reflectance and atomic force microscopy

(100) GaAs substrates with different misorientations were exposed to trimethyl-bismuth (TMBi) flow. The wafers were examined after exposure times of 9 and 43 min. The wafers growth was carried out at atmospheric pressure, in a metalorganic vapor phase epitaxy (MOVPE) reactor, at a temperature of 375 °C. The in situ spectral reflectance (SR) in the spectral range 400–1000nm was used to monitor the deposition. The reflectivity is marked by two temporal phases, an increase followed by a slow decrease. The atomic force microscopy (AFM) measurements show two different growth modes of bismuth, big isolated islands for early exposure time and a high density of small islands for long exposure times. We also note that the islands' density and size change greatly with the misorientation of the substrate. The correlation between the results given by ex situ AFM and in situ SR is performed via theoretical simulations. We show that roughness, as well as optical and thermal effects are mainly responsible for the reflectivity behavior. We have successfully quantified the spectral evolution of the sensitivity σSR of the incident beam wavelength to the surface undulations.

High resolution X-ray diffraction study of InAs layers grown with and without bismuth flow on GaAs substrates by metalorganic vapor phase epitaxy

InAs layers were grown with and without bismuth flow by atmospheric pressure metalorganic vapor phase epitaxy on exactly oriented, 2° and 10° misoriented (100) GaAs substrates. Structural analysis was carried out using high resolution X-ray diffraction. Without bismuth flow, only InAs layers grown on 10° misoriented substrates exhibit a mosaic structure. Layers grown on exactly oriented and 2° misoriented substrates show large full widths at half maxima of their diffraction rocking curves. Growing InAs under bismuth flow leads to the reduction of this full width indicating a clear improvement of their structural quality. Particularly for samples grown on 10° misoriented substrates, a complete disappearance of the mosaic structure was obtained. The crystalline quality improvement is attributed to the contribution of Bi nanodots in relieving strain.