xAs Layers Research Articles

Strain study of GaAs/In x Ga 1−x As/GaAs structures grown by MOVPE

Abstract GaAs/InxGa1 − xAs/GaAs strained and partially relaxed structures were grown by metalorganic vapor phase epitaxy and in situ monitored by laser reflectometry (LR). Two structures were formed by a single InxGa1 − xAs layer, and the third comprises three superposed InxGa1 − xAs layers having increasing indium contents. LR plots as function of time were recorded to extract growth rates and thicknesses of active and cap layers. In order to study the strain effect on structural and optical properties of these heterostructures, high resolution X-ray diffraction (HRXRD) and photoreflectance (PR) measurements were performed. HRXRD curves are developed to calculate strain tensor components, indium composition, and thicknesses of strained and partially relaxed layers. Besides, valence-band splitting and band-gap energy shift were measured by best fitting PR spectra at 300 K. Experimental energy values determined as a function of indium composition and relaxation rate were compared to those obtained by the elastic strain theory. For single and superposed InxGa1 − xAs active layers, a good correlation between experimental results and theoretical predictions was obtained.

Electrical control of the spin-orbit coupling in GaAs from single to double and triple wells

We consider a realistic GaAs/Al0.3Ga0.7As well, inside which there are either one or two additional AlxGa1 − xAs barriers embedded, with two occupied electron subbands ν = 1, 2. By varying the Al content x in the AlxGa1 − xAs layer, we investigate the electrical control of the spin-orbit (SO) interaction, i.e., intrasubband Rashba (Dresselhaus) αν (βν) and intersubband Rashba (Dresselhaus) η (Γ), in the course of the transition of our system from single to double and triple wells. At x = 0 (single well), the scenario of SO terms is usual, e.g., α1 and α2 have the same sign and both change almost linearly as functions of an external gate voltage Vg. In contrast, when x away from zero and only one AlxGa1 − xAs barrier embedded (double well), α1 and α2 tend to have opposite signs, and α2 first increases with Vg, while peaks at some point depending on x. For a larger value of x, α2 increases with Vg more abruptly till it peaks. As opposed to α1 and α2, the intrasubband Dresselhaus terms βν have a relatively weak dependence on Vg, and β1 and β2 become close as x increases. As for the intersubband SO terms, at x = 0, the Rashba coupling η remains essentially constant, while the Dresselhaus Γ changes almost linearly with Vg. When x is nonzero, on the one hand, both η and Γ have a sensitive dependence on Vg near the symmetric configuration; on the other hand, right at the symmetric configuration η exhibits the highest while Γ vanishes. In the case of our system having two additional AlxGa1 − xAs barriers (triple well), we find that the gate dependence of SO terms becomes more smooth and βν becomes more stronger. The persistent-spin-helix symmetry of the two subbands is also discussed. These results are expected to be important for a broad control of the SO interaction in semiconductor nanostructures.

Investigation of spin lifetime in strained InxGa1−xAs channels through all-electrical spin injection and detection

We investigated the spin-dependent transport properties of strained InxGa1−xAs (x = 0.04, 0.07, and 0.16) channels grown on GaAs substrates by observing the spin-valve and Hanle signals in a lateral spin-transport device with an Fe spin source. The spin lifetime in the strained InxGa1−xAs channels estimated according to Hanle signals was one order of magnitude smaller than that in GaAs channels, and the spin lifetime depended on the degree of strain induced in the InxGa1−xAs layer. These results are explained by the strain-induced spin–orbit interaction.

Catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on graphene layers using molecular beam epitaxy

We report the catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on large-area graphene layers using molecular beam epitaxy and our investigation of the chemical composition and crystal structure of these heterostructures using electron microscopy. The graphene layers used as the substrate were prepared by chemical vapor deposition and transferred onto SiO2/Si substrates. InAs nanorods and their heterostructures were grown vertically on the graphene layers; electron microscopy images revealed uniform distributions for their diameter, length and density. Cross-sectional electron microscopy images showed that InxGa1−xAs layers, having uniform composition, coated heteroepitaxially the entire surface of the InAs nanorods, without interfacial layers or structural defects. The catalyst-free growth mechanism of InAs nanorods on graphene was investigated using in situ reflection high-energy electron diffraction. Nanorods with InAs cores and InxGa1−xAs shells have been grown catalyst-free on graphene using molecular beam epitaxy by scientists in Korea. Inorganic semiconductor nanorods on graphene layers are promising for producing electronic and optoelectronic devices that are simultaneously flexible and have long lifetimes and high efficiencies. Now, researchers at Seoul National University led by Gyu-Chul Yi have realized catalyst-free molecular beam epitaxy growth of InAs/InxGa1−xAs coaxial nanorods on graphene. This is an important step as catalyst-free molecular beam epitaxy can produce ultrapure single crystals and multilayered heterostructures with precisely-controlled thickness and chemical compositions. Analysis showed that the cores and shells were clearly defined with clean interfaces and had uniform compositions and thicknesses. This demonstration opens up the path for monolithic integration of semiconductor coaxial nanorod heterostructures on two-dimensional materials. We report the catalyst-free growth of InAs/InxGa1−xAs coaxial nanorod heterostructures on large-area graphene layers using molecular beam epitaxy and our investigation of the chemical composition and crystal structure of these heterostructures using electron microscopy. Cross-sectional electron microscopy images showed that InxGa1−xAs layers, having uniform composition, coated heteroepitaxially the entire surface of the InAs nanorods, without interfacial layers or structural defects. The catalyst-free growth mechanism of InAs nanorods on graphene was investigated using in situ reflection high-energy electron diffraction.

Strain-relaxed buffer technology based on metamorphic InxAl1-xAs

A strain-relaxed buffer (SRB) technology that produces close to fully relaxed InxAl1–xAs is reported. The InxAl1–xAs layer was linearly graded from x=0.05 to 0.21. The effect of growth temperature on strain relaxation and surface morphology of InxAl1–xAs layer was studied here. For the strain relaxation study, the residual strain of InxAl1–xAs was measured with (004) and (224) reciprocal space maps along two orthogonal <110> directions. Atomic force microscopy was used to study the changes in surface roughness with growth temperatures. An InxAl1–xAs top-layer with 99.2% mean strain relaxation, surface roughness of 1.46nm in root-mean-square (RMS) value, and a small epilayer tilt of 0.2° was attained when an inverse-graded layer strategy was employed. The remaining strain was anisotropic with +0.27% and −0.27% residual strains in the [1¯10] and [110] directions, respectively.

Formation of III–V semiconductor nanotubes on an InP substrate by using the strain-induced self-rolling Method

Semiconductor nanotube structures have attracted much interest for building blocks of future nanoscale electronic and optical devices. Here, we investigate the structural properties of straininduced self-rolled III–V semiconductor nanotubes. The III–V semiconductor structures for nanotube formation were grown on InP substrates. The bilayer and the quantum-well structures are grown using a metalorganic chemical-vapor deposition system and were fabricated into selfrolled nanotubes. For the self-rolling process, ternary InxGa1−xAs layers were used to produce a lattice-mismatch strain in the nanotube membrane. The experimental observations of the nanotube structures are discussed.

Quantitative strain and compositional studies of InxGa1-xAs Epilayer in a GaAs-based pHEMT device structure by TEM techniques.

In GaAs-based pseudomorphic high-electron mobility transistor device structures, strain and composition of the In x Ga1-x As channel layer are very important as they influence the electronic properties of these devices. In this context, transmission electron microscopy techniques such as (002) dark-field imaging, high-resolution transmission electron microscopy (HRTEM) imaging, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging and selected area diffraction, are useful. A quantitative comparative study using these techniques is relevant for assessing the merits and limitations of the respective techniques. In this article, we have investigated strain and composition of the In x Ga1-x As layer with the mentioned techniques and compared the results. The HRTEM images were investigated with strain state analysis. The indium content in this layer was quantified by HAADF imaging and correlated with STEM simulations. The studies showed that the In x Ga1-x As channel layer was pseudomorphically grown leading to tetragonal strain along the [001] growth direction and that the average indium content (x) in the epilayer is ~0.12. We found consistency in the results obtained using various methods of analysis.

Photoluminescence variation in InAs quantum dots embedded in InGaAs/AlGaAs quantum wells at thermal annealing

Photoluminescence (PL) and its temperature dependence have been studied in MBE grown InAs quantum dots (QDs) embedded in GaAs/Al0.3Ga0.7As/In0.15Ga0.85As/AlxGa1−xAs/GaAs quantum wells (QWs) in dependence on the composition of capping AlxGa1−xAs layers and after the thermal annealing. Two types of capping layers (GaAs and Al0.3Ga0.7As) were investigated and the PL parameters of such structures have been compared. The annealing has been done for some part of the QD structures at 640°C for 2h. It is shown that thermal annealing initiates the shift of PL peak positions into the high energy spectral range and the value of this shift depends on the composition of capping layers.Temperature dependences of PL peak positions in QDs have been analyzed in the range of 10–300K and compared with the temperature shrinkage of the band gap in the bulk InAs crystal. This permits to investigate the efficiency of the Ga(Al)/In inter diffusion processes in dependence on the capping layer compositions and thermal annealing. Experimental and fitting parameters obtained for InAs QDs have been compared with known ones for the bulk InAs crystal. It is revealed that the efficiency of the Ga(Al)/In inter diffusion depends essentially on the capping layer compositions.The thermal quenching of integrated PL intensities has been studied in both types of QD structures as well. The fast thermal decay of the integrated PL intensity in the structure with the GaAs capping layer in comparison with the other one with AlGaAs capping is revealed. Finally the reasons for higher thermal stability of the structure with AlGaAs capping layer have been analyzed and discussed.

Photoluminescence spectroscopy study of excited states in In&lt;SUB align="right"&gt;xGa&lt;SUB align="right"&gt;1&amp;minus;xAs-capped InAs quantum dots

This study reports on photoluminescence (PL) and PL excitation (PLE) spectroscopy studies of excited state transitions in InxGa1–xAs-capped InAs quantum dots (QDs). InAs self-assembled QDs were grown on GaAs (001) wafers using 2.5 mono layer (ML) InAs deposition via molecular beam epitaxy. GaAs-capped QDs showed PL peak position at 1.182 μm at 78 K with a narrow full width at half maximum of 23 meV, indicating a highly uniform QD ensemble. The PLE spectrum of GaAs-capped QDs showed four peaks related to the QD excited state transitions at 1.117, 1.140, 1.192 and 1.224 eV at 8 K. Both ground state and excited state interband transitions were red-shifted when the first 30 ML of GaAs capping layer were replaced by InxGa1–xAs layer. The PL peak of In0.2Ga0.8As-capped QDs reached 1.246 μm (1.335 μm) at 78 K (296 K). Energy differences between the excited and ground states became smaller with increasing In composition of the InxGa1–xAs capping layers because of increasingly lower InAs to InxGa1–xAs band edge discontinuity as a result of the chemical and strain relieving effects. In addition, PLE spectra clearly showed wetting layer and quantum well states caused by the InxGa1–xAs capping layer for GaAs- and InxGa1–xAs-capped QDs, respectively. Based on the current PL, PLE, and the calculated transition energies available in literature, the lowest electron state levels of ~60 meV and ~112 meV above the electron ground state and the lowest hole state levels of ~23 meV and ~55 meV above the hole ground state were extracted.

Dependence of the hardness of AlxGa1−xAs on composition and effect of oxidation

The mechanical properties of semiconductor materials determine the reliability of microelectronics. The hardness of Al1−xGaxAs is investigated together with oxidation and its effect on this parameter. A structure consisting of three AlxGa1−xAs layers of different composition separated by In0.01Ga0.99As etch stops was deposited via metal-organic vapor phase epitaxy. High resolution x-ray diffraction determined the composition of the baseline layers to be Al0.9Ga0.1As, Al0.8Ga0.2As and Al0.7Ga0.3As. Nanoindentation measurements demonstrated that hardness decreases with aluminum content following a linear correlation. To study the effect of oxidation on hardness, two samples of Al0.8Ga0.2As were oxidized in air at 475°C for 2 and 4h. The surface morphology of the oxide imaged using atomic force microscopy was granular. X-ray diffraction simulated curves estimated the Al0.8Ga0.2As thickness decrease due to oxidation. The growth of the oxide layer was linear with time indicating that the process is reaction-rate limited and the layer is porous. The hardness of the (Al0.8Ga0.2)2O3 oxide was extrapolated to be more than twice the value of the baseline sample.

Analysis of the composition of (Al,Ga)As alloys by secondary ion mass spectroscopy and X-ray diffractometry

Calibration lines for the layer-by-layer analysis of the concentration of matrix elements in AlxGa1 − xAs layers are obtained using a TOF.SIMS-5 secondary-ion mass spectrometer. The alloy concentration for the set of test samples was independently measured by high-resolution X-ray diffractometry allowing for deviation of the lattice constants and elastic moduli from Vegard’s law. It is shown that when using Cs+ sputtering ions and a Bi+ beam in secondary-ion mass spectrometry, the dependence of the intensity ratio Y(CsAl+)/Y(CsAs+) on x(AlAs) is close to linear for positive ions; and when detecting negative ions, the dependence Y(Al2As−)/Y(As−) on x is close to linear. These data allow us to normalize the profiles of layer-by-layer analysis in the AlxGa1 − xAs/GaAs system. In addition, a simple variant for the introduction of corrections to the deviation from Vegard’s law in the X-ray data is suggested.

Emission and elastic strain in InAs dot-in-a well InGaAs/GaAs structures

The paper presents the photoluminescence (PL) study of InAs quantum dots (QDs) embedded in the asymmetric GaAs/InxGa1−xAs/In0.15Ga0.85As/GaAs quantum wells (QWs) with the different compositions of capping InxGa1−xAs layers. The composition of the buffer In0.15Ga0.85As layer was the same in all studied QD structures, but the In content (parameter x) in the capping InxGa1−xAs layers varied within the range 0.10–0.25. The In concentration (x) increase in the InxGa1−xAs capping layers is accompanied by the variation non-monotonously of InAs QD emission: PL intensity and peak positions. To understand the reasons of PL variation, the PL temperature dependences and X ray diffraction (XRD) have been investigated. It was revealed that the level of elastic deformation (elastic strain) and the Ga/In interdiffusion at the InxGa1−xAs/InAs QD interface are characterized by the non-monotonous dependences versus parameter x. The physical reasons for the non-monotonous variation of the elastic strains and PL parameters in studied QD structures have been discussed.

High-efficiency graded band-gap AlxGa1−xAs/GaAs photocathodes grown by metalorganic chemical vapor deposition

To enhance the quantum efficiency over the wavelength region of interest, a special graded band-gap structure applied to the AlGaAs/GaAs photocathodes is developed, in which the compositional grade in the AlxGa1−xAs layer is associated with the doping grade in the GaAs layer. The experimental results show that this unique structure can be achieved by the metalorganic chemical vapor deposition technique. As a result of the built-in electric fields arising from the graded band-gap structure, the quantum efficiency in the short-wavelength and long-wavelength regions is significantly increased for the transmission-mode and reflection-mode photocathodes, respectively.

Photoluminescence of i-Ga x In1 − x As/n-GaAs heterostructures containing a random InAs quantum dot array

i-GaxIn1 − xAs/n-GaAs heterostructures containing InAs quantum dots have been grown by ion beam deposition and their photoluminescence has been studied. The photoluminescence spectrum of the heterostructures contains three characteristic peaks. The peak at 1.1 eV has a large width and is due to the array of InAs quantum dots of different sizes, randomly arranged on the surface of the i-GaxIn1 − xAs layer. From the position of another peak (hν = 0.94 eV), we have evaluated the composition of the GaxIn1 − xAs solid solution: x = 0.64. The third photoluminescence peak corresponds to the intrinsic absorption edge of the n-GaAs substrate. The key features of luminescence photoexcitation in the heterostructures are discussed. We show that the Ga0.64In0.36As quantum well may accumulate not only photogenerated electrons but also electrons that come from a thin interfacial n-GaAs layer through ballistic transport.

Strain accumulation in InAs/In x Ga1−x As quantum dots

The effect of strain accumulation in the InAs/InxGa1−xAs quantum dots (QDs) system was studied in this work. It was found that strain in the InxGa1−xAs layer accumulation in the QD layer. This effect resulted in a dramatic reduction of growth mode transition thickness of the QD layer. For InAs/In0.25Ga0.75As QDs, critical thickness is measured to be as low as 1.08 ML. The experimental results in this work highlight the importance of strain accumulation in the design and fabrication of QD-based devices with metamorphic buffer layer involved.

Potential barrier study at the InGaAs/InAs quantum dot interface in asymmetric multi quantum well structures

Abstract The photoluminescence and its temperature dependences have been investigated for the InAs quantum dots embedded in the asymmetric GaAs/InxGa1−xAs/In0.15Ga0.85As/GaAs quantum wells (dot-in-a-well, DWELL structures) as a function of the In content x (x = 0.10–0.25) in the capping InxGa1−xAs layer. The study of PL temperature dependences in the range of 80–120 K has revealed the potential barrier for electrons at the capping InxGa1−xAs/InAs QD interface. The value of mentioned barrier has varied in QD structures with the different InxGa1−xAs capping layer compositions and it was estimated in the studied asymmetric GaAs/InxGa1−xAs/In0.15Ga0.85As/GaAs DWELL structures. It is shown that the barrier for electrons equal to 48 and 24 meV appears in the InGaAs conduction band at the formation of InGaAs QW/InAs QD interface for the In compositions of x = 0.10 and 0.15, respectively. This barrier has not been detected in DWELLs with the In compositions x = 0.20 and 0.25. The energy gap offsets at the InAs/InxGa1−xAs interface in studied structures has been estimated and discussed as well.

Crystalline quality and photovoltaic performance of InGaAs solar cells grown on GaAs substrate with large-misoriented angle

This article reports the quality of InxGa1−xAs (0<x<0.2) layers grown on 15°-off GaAs substrate by metalorganic chemical vapor deposition. The crystalline quality of the InxGa1−xAs epilayers is determined by x-ray reciprocal space mapping (RSM). From the RSM results, the crystalline quality of InxGa1−xAs epilayers grown with small indium composition (x<0.11) is better than that of large indium composition (x>0.11) due to the small strain relaxation. The crystalline quality of InxGa1−xAs epilayer is found to strongly depend on indium content. The photovoltaic performance of p–n structure In0.16Ga0.84As solar cell shows the lower device performance, because the InxGa1−xAs films grown on 15°-off GaAs substrate show a large strain relaxation in the active layer of solar cell. It results in dislocation defects created at the initial active layer/InxGa1−xAs graded layer interface. The performance of In0.16Ga0.84As solar cell with p–n structure can be significantly improved by the p–i–n structure.

Reverse engineering of AlxGa1−xAs/GaAs structures composition by reflectance spectroscopy

AbstractThe paper presents the application of non-modulation reflectance method for composition profiling of epitaxial AlxGa−xAs/GaAs structures. This non-destructive method is based on spectral measurements and theoretical reflectance spectrum matching. This is a very accurate and sensitive method of determining the Al composition in AlxGa1−xAs layers and structures with resolution down to 1 nm. In this work, the authors describe theoretic principles of this method and present experimental results of characterization of different AlGaAs structures to prove the potential of the worked out method.

SIMS depth profiling of semiconductor interfaces: Experimental study of depth resolution function

AbstractReconstruction of original element distribution at semiconductor interfaces using experimental SIMS profiles encounters considerable difficulties because of the matrix effect, sputtering rate change at the interface, and also a sputtering‐induced broadening of original distributions. We performed a detailed depth profiling analysis of the Al step‐function distribution in GaAs/AlxGa1−xAs heterostructures by using Cs+ primary ion beam sputtering and CsM+ cluster ion monitoring (where M is the element of interest) to suppress the matrix effect. The experimental Depth Resolution Function (DRF) was obtained by differentiation of the Al step‐function profile and compared with the ‘reference’ DRF found from depth profiling of an Al delta layer. The difference between two experimental DRFs was explained by the sputtering rate change during the interface profiling. We experimentally studied the sputtering rate dependence on the AlxGa1−xAs layer composition and applied it for a reconstruction of the DRF found by differentiating the Al step‐function distribution: the ‘reconstructed’ and ‘reference’ DRFs were found to be in good agreement. This confirmed the correctness of the treatment elaborated. Copyright © 2010 John Wiley &amp; Sons, Ltd.

TEM characterization of oxidized AlGaAs/AlAs nonlinear optical waveguides

The internal interfaces of multilayer AlxGa1−xAs/AlAs nonlinear optical waveguides are investigated by high-angle annular-dark-field and energy-filtered scanning transmission electron microscopy, before and after partial wet oxidation of AlAs layers. Via a simple phenomenological model, the corresponding roughness parameters allow prediction of the scattering-induced waveguide optical losses, which are in reasonable agreement with the experimental value of 0.5 cm−1. We also find that AlxGa1−xAs layers adjacent to oxidized AlAs tend to be oxidized through the interfaces, even for low Al fraction, with typical oxidation depths of 9 nm for x = 0.7 and 2 nm for x = 0.