Substrate Misorientation Research Articles

Dramatically enhanced self-assembly of GeSi quantum dots with superior photoluminescence induced by the substrate misorientation

A dramatically enhanced self-assembly of GeSi quantum dots (QDs) is disclosed on slightly miscut Si (001) substrates, leading to extremely dense QDs and even a growth mode transition. The inherent mechanism is addressed in combination of the thermodynamics and the growth kinetics both affected by steps on the vicinal surface. Moreover, temperature-dependent photoluminescence spectra from dense GeSi QDs on the miscut substrate demonstrate a rather strong peak persistent up to 300 K, which is attributed to the well confinement of excitons in the dense GeSi QDs due to the absence of the wetting layer on the miscut substrate.

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.

Effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of metamorphic In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As HEMT nanoheterostructures

The results of studies of the effect of GaAs (100) substrate misorientation on the electrical parameters and surface morphology of high electron mobility In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As/GaAs nanoheterostructures are reported. Using molecular-beam epitaxy, two identical structures with a stepped compositional profile of the metamorphic In x Al1 − x As (Δ x = 0.05) buffer are grown on substrates of two types: a singular GaAs substrate with the orientation (100) ± 0.5° and a GaAs (100) substrate misoriented by (2 ± 0.5)° in the \(\left[ {0\bar 1\bar 1} \right]\) direction. It is found that, in the case of the misoriented substrate, the concentration of the two-dimensional electron gas is ∼40% higher. Broadening of the photoluminescence spectra and a shift of the peaks to lower photon energies, as experimentally observed in the case of the misoriented substrate, are attributed to the increased roughness of the heterointerfaces and strengthened fluctuations of the quantum-well width.

Surexpression de l’IL-31 dans une forme sporadique de lichen amyloïde : un lien avec le prurit ?

Radiative recombination of carriers in two kinds of GaxIn1−xP/GaAs double-junction tandem solar cell structures was investigated by using room-temperature electroluminescence (EL) and photoluminescence (PL) techniques. Efficient radiative recombination was observed simultaneously in the top and the bottom subcells in both the samples. By studying the behavior of EL and PL spectra, the radiative recombination intensity ΦEL was demonstrated to be reliant on the material-dependent radiative recombination coefficient, base layer doping concentration and thickness. Furthermore, dependence of ΦEL on substrate misorientation in both the subcells was also evidenced, which was explained in terms of the growth-induced variations in microstructure for the GaInP top cell and in potential barrier profile across the p–n junction for the GaAs bottom cell. Based on these observations, the radiative recombination in the two base layers of the subcells was demonstrated to be the major carrier loss mechanism in the GaxIn1−xP/GaAs double-junction tandem photovoltaic devices and should be suppressed.

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.

The impact of substrate miscut on the microstructure and photoluminescence efficiency of (0001) InGaN quantum wells grown by a two-temperature method

The impact of the miscut of a (0001) c-plane substrate on the structural and optical properties of InGaN/GaN quantum wells grown by metal-organic vapour phase epitaxy using a two-temperature method has been investigated. The two-temperature growth method involves exposure of the uncapped InGaN quantum well to a temperature ramp in an ammonia atmosphere before growth of the GaN barrier at a higher temperature. The resulting quantum well, consists of interlinking InGaN strips containing gaps which may impede carrier diffusion to dislocations. By increasing the substrate misorientation from 0° to 0.5° we show that the density of InGaN strips increases while the strip width reduces. Our data show that the PL efficiency increases with miscut and that the peak efficiency occurs at a lower excitation power density.

Growth mode and oxidation state analysis of individual cerium oxide islands on Ru(0001)

The growth of cerium oxide on Ru(0001) by reactive molecular beam epitaxy has been investigated using low-energy electron microscopy (LEEM) and diffraction as well as local valence band photoemission. The oxide islands are found to adopt a carpet-like growth mode, which depending on the local substrate morphology and misorientation leads to deviations from the otherwise almost perfect equilateral shape at a growth temperature of 850 °C. Furthermore, although even at this high growth temperature the micron-sized CeO₂(111) islands are found to exhibit different lattice registries with respect to the hexagonal substrate, the combination of dark-field LEEM and local intensity-voltage analysis reveals that the oxidation state of the islands is homogeneous down to the 10 nm scale.

Effect of AlN buffer layers on the surface morphology and structural properties of N-polar GaN films grown on vicinal C-face SiC substrates

We investigated the effect of AlN buffer layers on hexagonal hillock formation and the crystallinity of N-polar GaN films grown by metalorganic chemical vapor deposition on vicinal C-face SiC substrates misoriented toward <112¯0> by 3.6° and <101¯0> by 4°. As the source input group V/III ratio increased from 650 to 16310 and growth temperature was raised from 1100°C to 1150°C during AlN buffer layer growth on the substrates with <112¯0> miscut direction, the threading dislocation (TD) density of the AlN buffer layers and subsequent N-polar GaN films decreased. TD density of AlN and GaN films grown on <101¯0> misoriented substrates was higher in comparison to that grown on <112¯0> miscut substrates. The hexagonal hillock density of the N-polar GaN films was reduced by increasing the AlN V/III ratio up to 16310 and decreasing the AlN thickness from 90 to 30nm, indicating that nucleation of inversion domain occurs during AlN buffer growth. Hexagonal hillocks were completely suppressed in N-polar GaN films grown on both substrates when the growth temperature of the 30nm thick AlN was increased from 1100°C to 1150°C at a V/III ratio of 16310.

Nucleation-related defect-free GaP/Si(100) heteroepitaxy via metal-organic chemical vapor deposition

GaP/Si heterostructures were grown by metal-organic chemical vapor deposition in which the formation of all heterovalent nucleation-related defects (antiphase domains, stacking faults, and microtwins) were fully and simultaneously suppressed, as observed via transmission electron microscopy (TEM). This was achieved through a combination of intentional Si(100) substrate misorientation, Si homoepitaxy prior to GaP growth, and GaP nucleation by Ga-initiated atomic layer epitaxy. Unintentional (311) Si surface faceting due to biatomic step-bunching during Si homoepitaxy was observed by atomic force microscopy and TEM and was found to also yield defect-free GaP/Si interfaces.

Mechanism of macrostep formation in solution growth of compound semiconductor and the evidence given by space experiment

AbstractThe behavior of macrostep and its formation mechanism are discussed taking solution growth of compound semiconductor as an example. The macrosteps are created by the bunching of atomic steps on a misoriented substrate and they coalesce to form larger macrosteps. At a steady state, the vertical growth of the macrostep terrace is carried out by the atomic steps supplied from a screw dislocation. Space experiments conducted by the group of Professor K. W. Benz showed that the macrostep disappears under a temperature gradient when the growth rate is decreased below a certain value. It is concluded that the non‐uniform bulk diffusion of the solute is the driving force to create the macrostep.

Monte Carlo Study of the Hetero-Polytypical Growth of Cubic on Hexagonal Silicon Carbide Polytypes

We use three dimensional kinetic Monte Carlo simulations on super-lattices to study the hetero-polytypical growth of cubic silicon carbide polytype (3C-SiC) on misoriented hexagonal (4H and 6H) substrates finding that the growth on misoriented (4°-10° degree off) 6H substrates, with step bunched surfaces, can strongly improve the quality of the cubic epitaxial film promoting 3C single domain growths

Radiative recombination of carriers in the GaxIn1−xP/GaAs double-junction tandem solar cells

Radiative recombination of carriers in two kinds of GaxIn1−xP/GaAs double-junction tandem solar cell structures was investigated by using room-temperature electroluminescence (EL) and photoluminescence (PL) techniques. Efficient radiative recombination was observed simultaneously in the top and the bottom subcells in both the samples. By studying the behavior of EL and PL spectra, the radiative recombination intensity ΦEL was demonstrated to be reliant on the material-dependent radiative recombination coefficient, base layer doping concentration and thickness. Furthermore, dependence of ΦEL on substrate misorientation in both the subcells was also evidenced, which was explained in terms of the growth-induced variations in microstructure for the GaInP top cell and in potential barrier profile across the p–n junction for the GaAs bottom cell. Based on these observations, the radiative recombination in the two base layers of the subcells was demonstrated to be the major carrier loss mechanism in the GaxIn1−xP/GaAs double-junction tandem photovoltaic devices and should be suppressed.

Effect of vicinal substrates on the growth and device performance of quantum dot solar cells

During quantum dot (QD) growth, substrate misorientation has been shown to play a role in the QD growth mechanism, changing their size, shape and density. Since various misorientation angles are used in production of solar cells, this work investigates QD enhanced GaAs p-i-n solar cells grown using the Stranski–Krastanov (SK) growth method on substrates misoriented either 2° or 6° off the (100) in the [11¯0] direction. Results of this work show that 2° misoriented samples have a lower critical thickness for InAs QD formation as compared to the 6° misorientation: ∼1.7 monolayers (ML) versus∼1.8ML, respectively. In addition, the 6° substrates showed a more uniform QD density and size distribution of QDs without significant QD coalescence. Photoluminescence of both substrate types shows that the QD ground state transitions are similar in wavelength. Results of the solar cells under one sun illumination show that QD cells grown on both 2° and 6° substrates have higher short-circuit current density than comparable cells without QD and maintain a high open-circuit voltage. The QD contributed short-circuit current density was normalized for QD size and density for both the 2° and 6° samples. Values of 360A/cm2 per cm3 of InAs and 400A/cm2 per cm3 of InAs were found for the 2° and 6° samples, respectively. For both substrate types, the reduced number of coalesced QDs promoted effective strain balancing, while the increased QD density lead to strong QD absorption.

Tensile-strained growth on low-index GaAs

We present a comparative study of the growth of tensile-strained GaP on the four low-index surfaces of GaAs: (001), (110), (111)A, and (111)B. For each surface orientation we outline the growth conditions required for smooth GaAs homoepitaxy. We are able to predict the resulting surface morphology when GaP is deposited onto these four GaAs surfaces by considering the influence of surface orientation on tensile strain relief. GaP deposited on GaAs(001) forms extremely smooth, planar layers. In contrast, the elastic relief of tensile strain on both GaAs(110) and GaAs(111)A leads to the three-dimensional self-assembly of GaP into dislocation-free nanostructures. Similarities between tensile and compressive self-assembly suggest that the kinetics governing many aspects of self-assembled growth is independent of the sign of strain. We show that differences in self-assembly on GaAs(110) and (111)A are the result of unequal adatom diffusion lengths. Tensile-strained self-assembly also occurs on GaAs(111)B, although our use of misoriented substrates resulted in the formation of one-dimensional nanoscale wires. Tensile-strained self-assembly is a versatile, reliable technique that can be extended to a wide range of materials in order to create dislocation-free nanostructures on (110) and (111) surfaces.

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.

Optimization of AlAs/AlGaAs quantum well heterostructures on on-axis and misoriented GaAs (111)B

We report systematic growth optimization of high Al-content AlGaAs, AlAs, and associated modulation-doped quantum well (QW) heterostructures on on-axis and misoriented GaAs (111)B by molecular beam epitaxy. Growth temperatures TG &amp;gt; 690 °C and low As4 fluxes close to group III-rich growth significantly suppress twin defects in high-Al content AlGaAs on on-axis GaAs (111)B, as quantified by atomic force and transmission electron microscopy as well as x-ray diffraction. Mirror-smooth and defect-free AlAs with pronounced step-flow morphology was further achieved by growth on 2° misoriented GaAs (111)B toward [01¯1] and [21¯1¯] orientations. Successful fabrication of modulation-doped AlAs QW structures on these misoriented substrates yielded record electron mobilities (at 1.15 K) in excess of 13 000 cm2/Vs at sheet carrier densities of 5 × 1011 cm−2.

Monte Carlo study of the hetero-polytypical growth of cubic on hexagonal silicon carbide polytypes

In this article we use three dimensional kinetic Monte Carlo simulations on super-lattices to study the hetero-polytypical growth of cubic silicon carbide polytype (3C-SiC) on misoriented hexagonal (4H and 6H) substrates. We analyze the quality of the 3C-SiC film varying the polytype, the miscut angle and the initial surface morphology of the substrate. We find that the use of 6H misoriented (4°–10° off) substrates, with step bunched surfaces, can strongly improve the quality of the cubic epitaxial film whereas the 3C/4H growth is affected by the generation of dislocations, due to the incommensurable periodicity of the 3C (3) and the 4H (4) polytypes. For these reasons, a proper pre-growth treatment of 6H misoriented substrates can be the key for the growth of high quality, twin free, 3C-SiC films.

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.