GaAs Buffer Layer Research Articles

Precursor flow rate manipulation for the controlled fabrication of twin-free GaAs nanowires on silicon substrates

Vertically oriented GaAs nanowires (NWs) are grown on Si(111) substrates using metal–organic chemical vapor deposition. Controlled epitaxial growth along the 〈111〉 direction is demonstrated following the deposition of thin GaAs buffer layers and the elimination of structural defects, such as twin defects and stacking faults, is found for high growth rates. By systematically manipulating the AsH3 (group-V) and TMGa (group-III) precursor flow rates, it is found that the TMGa flow rate has the most significant effect on the nanowire quality. After capping the minimal tapering and twin-free GaAs NWs with an AlGaAs shell, long exciton lifetimes (over 700 ps) are obtained for high TMGa flow rate samples. It is observed that the Ga adatom concentration significantly affects the growth of GaAs NWs, with a high concentration and rapid growth leading to desirable characteristics for optoelectronic nanowire device applications including improved morphology, crystal structure and optical performance.

Epitaxial growth of CdTe films on GaAs-buffered (001) Si substrates by metal organic chemical vapor deposition

Epitaxial (001) CdTe thin film was successfully grown on a (001) Si substrate using an atomically smooth, thin (2nm) GaAs buffer layer deposited using the metal organic chemical vapor deposition method. High-resolution transmission electron microscopy studies revealed a periodic distribution of dislocations due to lattice mismatch along the interfaces between CdTe film, GaAs buffer layer, and Si substrate. Most of the dislocation cores were located inside the GaAs buffer layer or at the interface between the CdTe and GaAs layers, indicating that GaAs buffer layer originally coherent to Si, was fully relaxed during the CdTe layer growth. The thin GaAs buffer layer effectively absorbed any strain coming from the large lattice mismatch between a CdTe layer and a Si substrate by forming an array of structural defects in the GaAs layer, which permitted epitaxial growth of CdTe on the Si substrates.

Photoluminescence investigation of GaAs1 − xBix/GaAs heterostructures

In this work the impact of doped GaAs buffer and cap layers on the bismide photoluminescence (PL) spectra has been studied. For this study three types of heterostructures were grown: a thin GaAsBi layer deposited directly onto the nominally undoped GaAs buffer layer, a GaAsBi layer grown onto the GaAs:Be layer, and a GaAsBi layer deposited onto the GaAs:Be layer and capped with the GaAs:Be layer. It has been demonstrated that p-type doping of the GaAs buffer and cap layers is resulting in a significant increase of the PL intensity. This enhancement was explained by a better photoexcited electron and hole confinement in the GaAsBi layer.

Features of the formation of Mn doped InAs/GaAs quantum dots by vapor phase epitaxy

A self-organized InAs/GaAs quantum dot (QD) array is doped with Mn. The effect of the Mn concentration on the morphology and QD luminescence properties is investigated. It is found that Mn deltadoping of the GaAs buffer layer before QD growth with a layer concentration of 1014 cm−2 leads to the formation of an array of large QDs with variable composition In x Ga1 − x As. The effect is explained within a model of In and Ga atom interdiffusion.

A Structural Investigation of CdTe(001) Thin Films on GaAs/Si(001) Substrates by High-Resolution Electron Microscopy

Epitaxial CdTe thin films were grown on GaAs/Si(001) substrates by metalorganic chemical vapor deposition using thin GaAs as a buffer layer. The interfaces were investigated using high-resolution transmission electron microscopy and geometric phase analysis strain mapping. It was observed that dislocation cores exist at the CdTe/GaAs interface with periodic distribution. The spacing of the misfit dislocation was measured to be about 2 nm, corresponding to the calculated spacing of a misfit dislocation (2.6 nm) in CdTe/Si with Burgers vector of a[110]/2. From these results, it is suggested that the GaAs buffer layer effectively absorbs the strain originating from the large lattice mismatch between the CdTe thin film and Si substrate with the formation of periodic structural defects.

Coexistence of type-I and type-II band alignments in antimony-incorporated InAsSb quantum dot nanostructures

Antimony-incorporated InAsSb quantum dots (QDs) are grown by molecular beam epitaxy on GaAs(001) substrates. The QD density increases ∼7 times while the QD height decreases ∼50% due to the increase of QD nucleation sites after Sb incorporation into the GaAs buffer layer and into the InAs QDs. These Sb-incorporated InAsSb QDs show red-shift in the photoluminescence (PL) spectrum and large energy separation between confined energy levels. More interestingly, besides the typical type-I QD transition, an additional peak from the recombination at wetting layer interface develops as the excitation laser intensity increases. This peak clearly exhibits type-II characteristics from the measurement of a large blue-shift of the PL peak and a long PL decay time. Finally, the mechanism of the coexistence of type-I and type-II band alignments is discussed.

Polarization and Excitation Dependence of Photoluminescence of InAs Quantum Wires and Dots Grown on GaAs(631)

We studied the optical properties of self-assembled InAs nanostructures grown by molecular beam epitaxy on GaAs substrates with (631) crystallographic orientation by photoluminescence (PL) spectroscopy. The growth of InAs proceeded on GaAs(631) buffer layers conformed by corrugated surfaces with a high degree of lateral periodicity (uniformly spaced nano grooves). The grooved surface is sustained even after the growth of InAs with thicknesses below 2 monolayers (ML), indicating the formation of InAs quantum wires. The one-dimensional confinement is corroborated by PL polarized along the [113] and [8,19,9] orthogonal directions. The calculated polarization degree, Πd, was 0.13 and 0.125 for InAs thicknesses of 1 and 1.5 ML, respectively. As the InAs thickness is increased, the InAs film reaches the critical thickness and quantum dots aligned along the grooved surface are formed, as observed by atomic force microscopy. Excitation-power-dependent luminescence is observed to a larger extent for low-energy PL lines in addition to a blue shift related to the band-filling effect.

Defect-Free GaAs/AlGaAs Core–Shell Nanowires on Si Substrates

We report straight and vertically aligned defect-free GaAs nanowires grown on Si(111) substrates by metal–organic chemical vapor deposition. By deposition of thin GaAs buffer layers on Si substrates, these nanowires could be grown on the buffer layers with much less stringent conditions as otherwise imposed by epitaxy of III–V compounds on Si. Also, crystal-defect-free GaAs nanowires were grown by using either a two-temperature growth mode consisting of a short initial nucleation step under higher temperature followed by subsequent growth under lower temperature or a rapid growth rate mode with high source flow rate. These two growth modes not only eliminated planar crystallographic defects but also significantly reduced tapering. Core–shell GaAs–AlGaAs nanowires grown by the two-temperature growth mode showed improved optical properties with strong photoluminescence and long carrier life times.

Emission of the terahertz electromagnetic wave from coherent longitudinal optical phonons in a GaAs buffer layer optically masked by a GaSb top epitaxial layer

We demonstrate that, in a GaSb/GaAs epitaxial structure, the coherent longitudinal optical (LO) phonon in the GaAs layer optically masked by the GaSb top layer is observed utilizing terahertz- electromagnetic-wave spectroscopy. It is confirmed from a Raman scattering measurement that only the optical phonon in the GaSb layer is optically observable, where the photon energy of the excitation laser beam was almost the same as that of the femtosecond pulse pump beam for the terahertz wave measurement. In the terahertz wave measurement, the Fourier power spectrum of the terahertz waveform exhibits both the GaAs and the GaSb LO phonons; namely, the coherent LO phonon in the optically masked GaAs buffer layer is observed in the terahertz wave measurement. This fact demonstrates that the instantaneous surface potential modulation originating from the impulsive carrier excitation by the pump pulses reaches the GaAs buffer layer. Consequently, the above-mentioned surface potential modulation generates the coherent GaAs LO phonon.

Surface morphology and magnetic anisotropy in (Ga,Mn)As

Atomic force microscopy and grazing incidence x-ray diffraction measurements have revealed the presence of ripples, aligned along the [11¯0] direction on the surface of (Ga,Mn)As layers grown on GaAs(001) substrates and buffer layers, with periodicity of about 50 nm in all samples that have been studied. These samples show the strong symmetry breaking uniaxial magnetic anisotropy normally observed in such materials. We observe a clear correlation between the amplitude of the surface ripples and the strength of the uniaxial magnetic anisotropy component suggesting that these ripples might be the source of such anisotropy.

THE EFFECT OF In0.1Ga0.9As UNDERLYING LAYER ON THE STRUCTURAL PROPERTIES OF SELF-ASSEMBLED In0.5Ga0.5As QUANTUM DOTS

The effect of a thin In0.1Ga0.9As underlying layer on the structural properties of single layer In0.5Ga0.5As quantum dots (QDs) was investigated using atomic force microscopy (AFM), transmission electron microscopy (TEM) and high-resolution X-ray diffraction (HR-XRD) characterization. The size of dots formed on the surface is uniform but the density increases with the addition of In0.1Ga0.9As underlying between In0.5Ga0.5As QDs and GaAs buffer layer. This is consistent with the TEM characterization. The existence of thin underlying layer has caused the dots to have different crystal orientation as shown in TEM characterization. From the HR-XRD characterization, broad peak of In0.1Ga0.9As underlying layer and QDs has been observed. The wider width of the layer peak than the expected one has been attributed to the strain-relaxation-induced defects. The growth of a thin In0.1Ga0.9As underlying layer in the In0.5Ga0.5As/GaAs structures strongly affects the structural properties, which was also believed to influence the optical properties of QDs.

Influence of growth temperatures on the quality of InGaAs/GaAs quantum well structure grown on Ge substrate by molecular beam epitaxy

Molecular beam epitaxy growth of an InxGa1−xAs/GaAs quantum well (QW) structure (x equals to 0.17 or 0.3) on offcut (100) Ge substrate has been investigated. The samples were characterized by atomic force microscopy, photoluminescence (PL), and high resolution transmission electron microscopy. High temperature annealing of the Ge substrate is necessary to grow GaAs buffer layer without anti-phase domains. During the subsequent growth of the GaAs buffer layer and an InxGa1−xAs/GaAs QW structure, temperature plays a key role. The mechanism by which temperature influences the material quality is discussed. High quality InxGa1−xAs/GaAs QW structure samples on Ge substrate with high PL intensity, narrow PL linewidth and flat surface morphology have been achieved by optimizing growth temperatures. Our results show promising device applications for III—V compound semiconductor materials grown on Ge substrates.

Investigation of pre-structured GaAs surfaces for subsequent site-selective InAs quantum dot growth

In this study, we investigated pre-structured (100) GaAs sample surfaces with respect to subsequent site-selective quantum dot growth. Defects occurring in the GaAs buffer layer grown after pre-structuring are attributed to insufficient cleaning of the samples prior to regrowth. Successive cleaning steps were analyzed and optimized. A UV-ozone cleaning is performed at the end of sample preparation in order to get rid of remaining organic contamination.

GaSb quantum rings grown by metal organic molecular beam epitaxy

GaSb quantum rings (QRs) of different sizes and shapes were grown by metal organic molecular beam epitaxy (MOMBE). Partially capped InAs quantum dots (QDs) grown on a GaAs buffer layer were flattened by In-flushing, followed by regrowth of GaSb which forms ring structure around the InAs seed dots. With the assistance of such an InAs/GaAs groundwork layer, the GaSb QRs exhibit a better defined shape compared to the spontaneously formed rings. First, rims of QRs develop around the boundary of the InAs/GaAs, then additional GaSb QDs grow on top of them preferentially along the [0−11] direction compared to [011].

Self-assembled InAs quantum dots on anti-phase domains of GaAs on Ge substrates

The authors report the formation of self-assembled InAs quantum dots (QDs) grown on GaAs/Ge substrates having anti-phase domains (APDs) by molecular beam epitaxy. The AFM images of InAs QDs grown on different GaAs thicknesses are shown and compared. The samples with InAs coverage of 1.80MLs with GaAs thickness of 300 and 700nm show non-uniformed size distribution of the dots. Due to anisotropic property of quantum dots, ellipsoidal quantum dots appear. Unexpectedly, most of InAs quantum dots align perpendicularly to anti-phase boundary (APB) and to quantum dot alignment formed in adjacent domains. Photoluminescence spectrum excited by 20mW 476-nm Ar+ laser at 20K does not show emission peak of InAs QDs. This is due to defects in the GaAs buffer layer.

Structural optimization of quantum wells used in a 1-μm vertical-external-cavity surface-emitting laser

On the basis of the analysis of material gain, a comprehensive optimization of quantum wells used in a 1-μm vertical-external-cavity surface-emitting laser was carried out. For a single-well structure, the optimized width lies between 8 and 10 nm, the optimized depth is a quantum well with ∼0.1 Al composition in AlGaAs barrier, and the optimized configurations are graded-index quantum well and quantum well with AlGaAs barrier and a GaAs buffer layer. The optimal width of a double- or triple-well structure lies between 6 and 8 nm. Compared to its single- and triple-well counterparts, double-well structure provides higher gain and has more tolerance to the deviation of laser wavelength.

Controlling the self-organization of InAs quantum dots upon growth by means of vapor-phase epitaxy on an antimony δ-doped GaAs buffer layer

This work examines the possibility of controlling the parameters of InAs/GaAs quantum dot arrays obtained by metal-organic chemical vapor deposition (MOCVD) at atmospheric pressure with using antimony as a surfactant. The possibility of controlling the parameters and optical properties of InAs quantum dot arrays by varying the surface concentration of Sb atoms in a GaAs buffer layer surface is demonstrated. A model of quantum dot array formation in the presence of Sb atoms is proposed.

Coherent heteroepitaxy of Bi2Se3 on GaAs (111)B

We report the heteroepitaxy of single crystal thin films of Bi2Se3 on the (111)B surface of GaAs by molecular beam epitaxy. We find that Bi2Se3 grows highly c-axis oriented, with an atomically sharp interface with the GaAs substrate. By optimizing the growth of a very thin GaAs buffer layer before growing the Bi2Se3, we demonstrate the growth of thin films with atomically flat terraces over hundreds of nanometers. Initial time-resolved Kerr rotation measurements herald opportunities for probing coherent spin dynamics at the interface between a candidate topological insulator and a large class of GaAs-based heterostructures.

Effect of temperature on the mutual diffusion of Ge/GaAs and GaAs/Ge

We studied the diffusion of Ge, As and Ga in GaAs/Ge and Ge/GaAs epilayers grown at different temperatures by metal-organic vapor phase epitaxy using iso-butylgermane, arsine and trimethylgallium in hydrogen atmosphere at low pressure. The use of low temperature buffer layers was investigated in order to overcome the diffusion problem.High-resolution X-ray diffraction and transmission electron microscopy were used to assess the crystal quality, while secondary neutral mass spectrometry has been employed to investigate diffusion profiles in the samples.As it is well known, the diffusivity of the atoms (e.g. Ga, As, Ge) and intermixing of layers during sample preparation strongly depend on the substrate temperature. We found that the use of a low temperature GaAs buffer layer reduced the diffusion in GaAs/Ge epitaxy at 600°C; while a Ge low temperature buffer layer was not effective in reducing the interdiffusion in Ge/GaAs epitaxy at 700°C.

Growth of InAs/GaAs quantum dots on Si, Ge/Si and germanium-on-insulator-on-silicon (GeOI) substrates emitting in the 1.3 μm band for silicon photonics

We report the growth of self-assembled InAs/GaAs quantum dots (QDs) on Si, Ge/Si and germanium-on-insulator-on-silicon (GeOI) substrates by metal organic chemical vapor deposition. GaAs layers with lower surface roughness (root mean square roughness of 1 nm) and higher structural quality were obtained on Ge/Si and GeOI compared to those obtained on Si substrate. We showed that the introduction of a QD layer within the GaAs buffer layer was efficient in suppressing the propagation of anti-phase domains to the GaAs surface for both cases of Ge/Si and GeOI substrates. Coalescence-free QDs with densities above 10 10 cm −2 and ground state emission in the 1.3 μm band at room temperature were obtained on all substrates. QDs grown on GeOI yield the highest photoluminescence (PL) intensity, and quite remarkably, have similar PL intensity as those grown on GaAs substrate. These results suggest the better suitability of GeOI substrate compared to Si or Ge/Si substrates for the monolithic integration of QD-based lasers on silicon (or any other III–V photonic device) for silicon photonics.