Relaxed InAs Research Articles

Lattice‐Mismatched Epitaxy of InAs on (111)A‐Oriented Substrate: Metamorphic Layer Growth and Self‐Assembly of Quantum Dots

In this article, recent developments in the lattice‐mismatched epitaxy of InAs on (111)A‐oriented substrates and related research topics, in which the presence or absence of the misfit dislocations is controlled via prescribed growth sequences, are reviewed. When InAs is grown on GaAs (111)A substrates, a unique lattice‐relaxation mechanism occurs. A misfit dislocation network is formed at the initial stage of InAs growth, that is followed by the layer‐by‐layer growth of relaxed InAs films. The InAs/GaAs (111)A heterostructure is being applied in infrared photodetectors which have a new operating principle that employs the high‐density dislocations at the interface. The InAs/GaAs (111)A heterostructure is also useful for the growth of InGaAs layers: a nearly lattice‐relaxed InGaAs containing different concentrations of indium can be formed by inserting a thin‐InAs layer between the InGaAs and GaAs. These InGaAs layers can be used as virtual substrates with a desired lattice constant for a range of devices. In addition to the formation of lattice‐relaxed structures, dislocation‐free InAs quantum dots (QDs) can be formed on InP (111)A substrates by applying droplet epitaxy. The C3v symmetry of the (111)A surface makes it possible to form symmetric InAs QDs that emit entangled photon pairs at telecommunication wavelengths.

Ab initio calculations of polarization, piezoelectric constants, and elastic constants of InAs and InP in the wurtzite phase

We report first-principle density functional calculations of the spontaneous polarization, piezoelectric stress constants, and elastic constants for the III–V wurtzite structure semiconductors InAs and InP. Using the density functional theory implemented in the VASP code, we obtain polarization values–0.011 and–0.013 C/m2, and piezoelectric constants e33 (e31) equal to 0.091 (–0.026) and 0.012 (–0.081) C/m2 for structurally relaxed InP and InAs respectively. These values are consistently smaller than those of nitrides. Therefore, we predict a smaller built-in electric field in such structures.

Sweeping-rate-dependent photocurrent of GaAs Schottky diode with strain relaxed InAs quantum dots

We demonstrate that the photocurrent of a GaAs Schottky diode can be influenced by the carrier stored in strain relaxed InAs quantum dots (QDs) embedded in the Schottky diode. A potential drop is induced by the charged QDs, and the photocurrent generated from the Schottky diode can be suppressed by the induced potential drop. In this paper, the charging time of the relaxed InAs QDs is obtained from 0.1 to 10 s. So the variation in photocurrent can be detected, if the sweeping rate of applied bias is from 10 to 0.1 V/s. Two kinds of QD samples, relaxed and non-relaxed InAs QDs, are compared, and we found that the photocurrent is obviously influenced in the relaxed InAs QD sample. A equivalent RC circuit modal also is provided to analyze the formula of the photocurrent, and the analysis agrees with the results of measurements. This study is advantageous for the investigation of QD memory devices.

Nanostructured hybrid material based on highly mismatched III–V nanocrystals fully embedded in silicon

InAs quantum dots were directly grown on (100) planar silicon surfaces and embedded in a defect‐free silicon matrix after a multi‐step silicon overgrowth and annealing process performed by molecular beam epitaxy. Detailed high‐resolution transmission electron microscope investigations allow to follow within several steps the formation process of nearly fully relaxed InAs nanocrystals embedded in a defect‐free and planar silicon layer. The lattice mismatch between InAs and Si is almost fully accommodated by closed misfit dislocation loops at the III–V silicon interface, which suppresses the generation of threading dislocations in the embedding silicon matrix. InAs QDs embedded in defect‐free silicon.

Chemical beam epitaxy growth and optimization of InAs/GaAs quantum dot multilayers

This paper reports on an in-situ growth process used to optimize InAs/GaAs quantum dot (QD) multilayer structures grown on (001) GaAs substrate by chemical beam epitaxy (CBE). Defects related to the incoherently relaxed InAs clusters are found to alter the QD nucleation mechanism on the subsequent layers, leading to reduced QD density and photoluminescence intensity. The formation of poor crystalline quality clusters is avoided by growing the GaAs spacer layers in a two-step process. The technique consists in covering the InAs QD layer with a 10nm-thick GaAs layer grown at 465°C, and then removing the excess indium contained in the uncapped portion of the clusters by increasing the temperature to 565°C for 10min before the deposition of the remaining GaAs spacer layer. Morphological investigation shows that the QD density and size distribution obtained in the first layer are preserved up to the tenth layer. The QD integrated photoluminescence intensity is found to increase linearly with the number of stacked layers. These results are very promising for chemical beam growth of high performance intermediate-band solar cells.

Substitutional-Gate MOSFETs With Composite $( \hbox{In}_{0.53}\hbox{Ga}_{0.47}\hbox{As}/\hbox{InAs}/\hbox{In}_{0.53}\hbox{Ga}_{0.47}\hbox{As})$ Channels and Self-Aligned MBE Source–Drain Regrowth

We report enhancement-mode composite-channel (In0.53Ga0.47As/InAs/In0.53Ga0.47As) MOSFETs fabricated using a substitutional-gate process, with n+ relaxed InAs source-drain regions formed by regrowth by molecular beam epitaxy. A device with 70-nm gate length and 2-nm In0.53Ga0.47As/3.5-nm InAs/3-nm In0.53Ga0.47As channel showed a peak transconductance of greater than 0.76 mS/μm at Vds = 0.4 V and showed Id = 0.5 mA/μm at Vds = 0.4 V and Vgs - Vth = 0.7 V. The subthreshold swing at Vds = 0.1 V was 130 mV/dec.

Photoconductivity of InAs/GaAs structures with InAs nanoclusters in the near-infrared region

InAs/GaAs multilayered heterostructures containing dense arrays of the low-defect partially relaxed InAs nanoclusters larger than defect-free quantum dots are fabricated by metal-organic chemical vapor deposition in an atmospheric-pressure reactor. The structures have intense photoconductivity in the wavelength range of 1–2 μm at room temperature. The detectivity of fabricated prototypes of photodetectors is D* = 109 cm Hz1/2 W−1. The relaxation time of photoconductivity at a wavelength of 1.5 μm is less than 10 ns.

Compensation effect and differential capacitance analysis of electronic energy band structure in relaxed InAs quantum dots

The use of a differential capacitance technique for analyzing the effect of strain relaxation on the electronic energy band structure in relaxed InAs self-assembled quantum dots (QDs) is presented. Strain relaxation is shown to induce a deep defect state and compensate the ionized impurity in the bottom GaAs layer, leading to a double depletion width and a long emission time. An expression of capacitance at different frequency and voltage is derived for analyzing the experimental data. It has been shown that the relationship between the low-frequency and high-frequency capacitances can be well explained by a Schottky depletion model with a compensated concentration in the bottom GaAs layer. A simple expression is presented to account for the modulation of the free electrons in the top GaAs layer. This capacitance analysis shows a long low-energy tail for the electron ground state, suggesting not very uniform strain relaxation. The results of this study illustrate a carrier compensation effect of the defect state on the electronic energy band structure near the QDs.

10.1007/s11453-008-3010-9

Multilayered InAs/GaAs quantum dot (QD) heterostructures are produced by metal-organic gas phase epitaxy. The structures exhibit photoluminescence around 1.55 μm at 300 K. The specific feature of the technology is the growth of an InAs layer with an increased effective thickness d eff to form QDs, in combination with low-temperature overgrowth of the QDs with a thin (6-nm) GaAs layer and with the annelaing of defects. By X-ray diffraction analysis and PL studies, it is shown that, in a structure with the increased thickness d eff, a secondary wetting InGaAs layer is produced on top of the QD layer from the growing relaxed large-sized InAs clusters on annealing. A new mechanism of formation of large-sized QDs characterized by a large “aspect ratio” is suggested. The mechanism involves the 2D–3D transformation of the secondary InGaAs layer in the field of elastic strains in previously formed QDs. The specific feature of the array of QDs is the coexistence of three populations of different-sized QDs responsible for the multimode photoluminescence in the range from 1 to 1.6 μm. The potentialities of such structures for infrared photoelectric detectors operating in the range from 1–2.5 μm at room temperature are analyzed.

Photoluminescence up to 1.6 μm of quantum dots with an increased effective thickness of the InAs layer

Multilayered InAs/GaAs quantum dot (QD) heterostructures are produced by metal-organic gas phase epitaxy. The structures exhibit photoluminescence around 1.55 {mu}m at 300 K. The specific feature of the technology is the growth of an InAs layer with an increased effective thickness d{sub eff} to form QDs, in combination with low-temperature overgrowth of the QDs with a thin (6-nm) GaAs layer and with the annelaing of defects. By X-ray diffraction analysis and PL studies, it is shown that, in a structure with the increased thickness d{sub eff}, a secondary wetting InGaAs layer is produced on top of the QD layer from the growing relaxed large-sized InAs clusters on annealing. A new mechanism of formation of large-sized QDs characterized by a large 'aspect ratio' is suggested. The mechanism involves the 2D-3D transformation of the secondary InGaAs layer in the field of elastic strains in previously formed QDs. The specific feature of the array of QDs is the coexistence of three populations of different-sized QDs responsible for the multimode photoluminescence in the range from 1 to 1.6 {mu}m. The potentialities of such structures for infrared photoelectric detectors operating in the range from 1-2.5 {mu}m at room temperature are analyzed.

High-mobility two-dimensional electron gas in InAlAs∕InAs heterostructures grown on virtual InAs substrates by molecular-beam epitaxy

In 0.8 Al 0.2 As ∕ In As heterostructures were grown on virtual InAs substrates consisting of a relaxed InAsyP1−y step-graded buffer grown on InP by molecular-beam epitaxy. Hall measurements revealed the presence of a high-mobility two-dimensional electron gas within the relaxed InAs layer, with a peak electron mobility of 133000cm2∕Vs at 25K. In contrast, identical InAlAs∕InAs heterostructures grown directly on InAs buffers on InP showed only bulk transport characteristics. A combination of transport modeling and electron microscopy demonstrates that reduced dislocation scattering in the channel region is responsible for observing the two-dimensional transport within the relaxed InAs on graded InAsP. These results demonstrate the potential of achieving ultrahigh-speed InAs based high electron mobility transistors using relaxed, virtual InAs substrates on InP.

An empirical potential approach to the structural stability of InAs stacking-fault tetrahedron in InAs/GaAs(1 1 1)

The structural stability of InAs stacking-fault tetrahedron (SFT) in InAs/GaAs (1 1 1) is theoretically investigated. Using an empirical interatomic potential, cohesive energies are calculated for the three types of InAs/GaAs(1 1 1) system where coherent InAs and relaxed InAs with the SFT and misfit dislocations (MDs). The calculated results reveal that InAs with the SFT is more favorable beyond the film thickness of 21 monolayers (MLs) than coherent InAs. The critical film thickness of 21 ML is comparable with that of 8 ML for the MDs generation. This suggests that the SFT appears in InAs thin film layers instead of MDs resulting from lowering the strain energy accumulated in InAs thin film layers.

Characterization of electron emission from relaxed InAs quantum dots capped with InGaAs

The effect of strain relaxation in a relaxed InAs quantum dot (QD) capped with InGaAs is investigated by admittance and deep-level transient spectroscopy (DLTS). Strain relaxation markedly increases the emission time in the QD region and extends carrier depletion to the bottom GaAs layer. The experimental data show the presence of relaxation traps in the QD region and the neighboring bottom GaAs layer. The electron emission from the QD region is governed by a trap located at 0.17–0.21 eV below the QD ground state. The electron-escape process is identified as thermal activation at high temperatures and direct tunneling at low temperatures from the trap. In the bottom GaAs layer near the QD, DLTS reveals a relaxation trap at 0.37–0.41 eV relative to the GaAs conduction band. The energy difference between these two traps is comparable to the QD ground-state energy relative to the GaAs conduction-band edge, suggesting that the two traps may be the same trap which is pinned to the GaAs conduction band. The considerable difference between their properties may result from different atoms surrounding the trap.

Tellurium surfactant effects in the growth of lattice mismatched InAs xP 1−x by metal organic vapor-phase epitaxy

A step-graded relaxed InAs x P 1− x (0< x<0.32) buffer is an integral component of high-efficiency InGaAs/InAsP-based thermophotovoltaic energy converters. The effect of Te on the surface structure and crystallinity of the InAs x P 1− x buffer layers grown on InP by MOVPE has been studied. During the growth of non-intentionally doped InAs x P 1− x , strain energy in the epilayers due to the mismatch with InP is relieved by formation of a cross-hatched misfit dislocation network. Variation of Te incorporation level reveals an evolution of the surface structure such that at Te levels near 10 19 cm −3, the surface morphology includes steps orthogonal to the wafer miscut direction in addition to the expected cross-hatch features. In situ and ex situ reflectance measurements indicate a considerably smoother surface as the Te incorporation is increased, suggesting that the presence of Te at the surface during growth alters the characteristics of the epitaxial growth front. Additionally, an incorporation delay of the dopant measured by SIMS suggests a surfactant mechanism at work.

Strain status of self-assembled InAs quantum dots

Grazing incidence x-ray diffraction experiments employing the asymmetric (202) Bragg diffraction have been performed to characterize self-assembled InAs quantum dots grown by molecular-beam epitaxy. We find that the strain is elastically relaxed with different components. The volume distribution of partially strained InAs inside islands is peaked at intermediate strain values. The fraction of both almost fully strained and totally relaxed InAs is found to be small. In addition, a small volume fraction of relaxed InxGa1−xAs is found.

Two-step method to grow InAs epilayer on GaAs substrate using a new prelayer

The strain relaxed InAs grown on GaAs(0 0 1) under In-rich condition is investigated by X-ray double-crystal diffraction. It is found that the strain is fully relaxed within 10 nm thick InAs layer grown under this condition, which is much faster than the growth of a similar layer of InAs under As-condition. A new growth method to obtain a high-quality InAs layer is suggested.

TEM observation of threading dislocations in InAs self-assembled quantum dot structure

A multilayer structure and a single-layer structure of InAs self-assembled quantum dots were grown by molecular beam epitaxy, and cross-sectional observations were made by transmission electron microscopy. Fringe patterns due to wetting layers in the Stranski–Krastanow mode were observed. Threading dislocations were generated from large incoherent InAs islands, and they often appeared in pairs. The dislocations in a pair had different Burgers vectors and did not form a half-loop. Based on the observations under zone-axis illumination conditions, the following two generation mechanisms of the threading dislocations were deduced: (i) the dislocations originated from the overlap between the two large islands; and (ii) the dislocations were caused by a misfit between the relaxed InAs islands and the GaAs layer, and threaded epilayer in {1 1 1} planes.

Growth mode and strain relaxation of InAs on InP (111)A grown by molecular beam epitaxy

Growth mode and strain relaxation of molecular-beam-epitaxy grown InAs/InAlAs/InP (111)A system have been investigated using reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and photoluminescence measurements. In direct contrast to the well-studied InAs/GaAs system, our experimental results show that the InAs grown on InAlAs/InP (111)A follows the Stranski–Krastanov mode. Both self-organized InAs quantum dots and relaxed InAs islands are formed depending on the InAs coverage. Intense luminescence signals from both the InAs quantum dots and wetting layer are observed. The luminescence efficiency of (111)A samples is comparable to that of (001) samples, suggesting the feasibility of fabricating quantum dot optoelectronic devices on InP (111)A surfaces.

Sequential Operation of Three Distinct Misfit Dislocation Introduction Mechanisms in an Epitaxial Bilayer Film

AbstractMismatch stress relaxation mechanisms in bilayer films of (In, Al)As on InAs on GaP have been examined. Initial edge (90°) misfit dislocations at the InAs/GaP interface appear to be introduced directly at island edges during initial stages of growth. The incomplete mismatch compensation is taken up by later introduction of glissile (60°) dislocations, usually in pairs which combine to form edge dislocations. The edge dislocations in the interface then move laterally to equalize their spacings. The upper (In, Al)As capping layer, which is tensile strained to match the relaxed InAs, exhibits a different mechanism of misfit dislocation introduction. Threading dislocations move by climb, directly introducing sessile edge dislocations at the buried interface. It is believed this is the first time that this mechanism has been observed.

InAs/(GaIn)Sb superlattices for IR optoelectronics: Strain optimization by controlled interface formation

The structural properties of InAs/(GaIn)Sb and (InGa)As/GaSb superlattices (SLs), grown by solid-source molecular-beam epitaxy on (0 0 1) GaAs substrates using a strain relaxed GaSb or InAs buffer layer or directly on (0 0 1) InAs substrates, were analyzed by high-resolution X-ray diffraction and Raman spectroscopy. The residual strain within the SL was found to depend critically on the type of interface bonds, which can be either InSb- or GaAs-like. Thus, to achieve lattice matching to the buffer layer or substrate by strain compensation within the SL stack, the controlled formation of the interface bonds is vital. On the other hand, minimization of the residual strain is shown to be a prerequisite for achieving a high photoluminescence yield and high responsivities for InAs/(GaIn)Sb SL based IR detectors.