Thin GaAs Layer Research Articles

Investigation of shape, strain, and interdiffusion in InGaAs quantum rings using grazing incidence x-ray diffraction

The formation of nanoscopic InGaAs ring structures on a GaAs(001) substrate takes place when InAs quantum dots, grown by Stranski-Krastanov self-organization, are covered by a thin layer of GaAs. The shape transformation into rings is governed by strain, diffusion, and surface tension, physical parameters which are of importance to monitor the magneto-optical and electronic properties of the rings. In this work we report on the characterization of morphology and structure of the rings in three dimensions (such as strain and chemical composition). To this end we apply grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence diffraction (GID). From GISAXS the shape is found to be of circular symmetry with an average outer radius of 26nm, a height of about 1.5nm, and a hole in the middle, in good agreement with atomic force microscopy measurements. Information about strain and interdiffusion is derived from intensity mappings in reciprocal space close to the (220) and (22¯0) reflections done in the surface sensitive GID geometry. From a comparison of the intensity maps with finite-element model calculations the InGaAs interdiffusion profile in the ring is determined. It strongly depends on the crystallographic orientation. In the ring we find a maximum InAs concentration of more than 80% along [11¯0], while along [110] it is below 20%. This results from the preferred diffusion of InAs along [11¯0].

Regrowth of self-assembled InAs quantum dots on nanohole and stripe templates

Self-assembled InAs quantum dots (QDs) are grown on two types of templates by molecular beam epitaxy (MBE). A nanohole template is prepared by first growing InAs QDs on a GaAs substrate by a standard MBE process, then capping the QDs with a thin lattice-mismatched layer of GaAs. Subsequent annealing of the nanohole template results in a stripe template. We are able to transfer the patterns of self-assembled QDs onto these two types of templates: regrowth on the nanohole template results in uniform QDs situated in the nanoholes, while regrowth on the stripe template results in chains of QDs aligned along the stripes. Our results imply that self-assembled QDs can be grown onto in-situ prepared patterned substrates, with limited degree of randomness.

Characteristics of InGaAsN-GaAsSb type-II “W” quantum wells

InGaAsN-GaAsSb type-II “W” quantum well structures have been grown by metalorganic chemical vapor deposition (MOCVD). Photoluminescence and X-ray diffraction measurements indicate that thin layers (2–2.5 nm) of GaAs 1− y Sb y and InGaAs 1− x N x can be grown with compositions of y = 0.3 and x = 0.0 2 . “W” structures with different N contents indicate that emission wavelengths in the 1.4–1.6 μm range can be achieved. The use of GaAs 0.85P 0.15 tensile strained barrier layers is found to significantly improve the photoluminescence intensity of the “W” structure and result in a wavelength blueshift.

Observation of indium segregation effects in structural and optical properties of pseudomorphic HEMT structures

We present a detailed study on the influence of several growth related factors such as indium segregation, spacer layer thickness and Si δ doping on the structural, optical and electrical properties of molecular beam epitaxy grown pseudomorphic high electron mobility transistor (P-HEMT) structures. Simulation of a high resolution x-ray diffraction (HRXRD) rocking curve was performed and compared with the experimental data to determine the quantum well (QW) thickness and composition as well as the indium segregation related changes in the QW composition. It is shown that growth of a thin layer (6 Å) of GaAs on top of the pseudomorphic InGaAs layer followed by ‘flash-off’ at a higher temperature could minimize the indium segregation related degradation of the hetero interface. Photoluminescence (PL) and surface photo voltage (SPV) peaks corresponding to the sub-band transitions are analysed using a self-consistent solution of Schrödinger and Poisson equations. PL results showed the dependence of the transition energies on the spacer layer thickness and the effectiveness of the carrier transfer into the InGaAs well. SPV spectra showed characteristic peaks from all the layers in the HEMT structures and were correlated with the data obtained from PL and HRXRD.

Critical barrier thickness for the formation of InGaAs/GaAs quantum dots

The discovery of self-assembled quantum dots (QDs) has attracte because of its possible applications in optoelectronic devices. The low density of QDs formed makes it useful to grow several layers of dots in the form of a stacked structure. In these structures, the buried islands tend to influence the further nucleation of islands in subsequent layers. It has been experimentally found that, when the number of layers increases, island sizes and shapes become more regular with each successive layer. Theoretical models have been proposed to elucidate the correlated vertical self-organization. The Stranski–Krastanow (SK) growth produces a tensile region, which induces the preferential nucleation of the next SK island just above the buried 3D-island. Experimental evidence of strain below the quantum dots (QDs) has induced us to study the influence of very thin barrier GaAs layers on InGaAs/GaAs QDs structures. This In diffusion due to strain defines the Critical Barrier Thickness for the formation of quantum wells from three dimensional islands. A Critical Barrier Thickness of 6 nm was observed in the case of 1.8 nm In0.5Ga0.5As/GaAs QDs structures. Above this thickness stacked QDs show near perfect alignment, whilst below this thickness modulated QWs are observed. The structural behaviour is supported by photoluminescence (PL) characteristics.

Atomic-scale structure of self-assembled In(Ga)As quantum rings in GaAs

We present an atomic-scale analysis of the indium distribution of self-assembled In(Ga)As quantum rings (QRs) which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20nm which is consistent with the observed electronic radius of QR structures.

Determination of doping concentrations in very thin GaAs layers using micro-Raman spectroscopy on bevelled samples

A new method for determining the doping concentrations in very thin GaAs layers is presented. The method is based on the evaluation and calibration of the changes in ratio of transversal to longitudinal optical phonon intensities measured by micro-Raman spectroscopy at different positions along the bevel prepared through the examined structure. The doping concentrations measurements by micro-Raman spectroscopy on bevelled structures are in good agreement with SIMS depth profiling analysis.

State of the art on epitaxial GaAs detectors

We first briefly review the performances for X-ray detection which are obtained using thin epitaxial GaAs layers. We then show that good detectors can be realized on thick and large area epitaxial GaAs layers which are now available, making them suitable for X-ray imaging. We finally discuss the main limitation imposed by the epitaxial nature of this new material and ways to overcome it.

Dislocation networks adapted to order the growth of III‐V semiconductor nanostructures

We study by transmission electron microscopy shallowly buried dislocation networks (DNs). These DNs are grain boundaries (GBs) that form, between a thin GaAs layer and a GaAs substrate joined together by wafer bonding, in order to accommodate a tilt and a twist between these two crystals. These GBs are composed of a one-dimensional network of mixed dislocations and of a one dimensional network of screw dislocations (whereas a two-dimensional (2D) one is usually observed) forming hexagonal cells. These GBs generate a 2D surface pattern of dilatational and compressive strain adapted to the ordered growth of nanostructures which we demonstrate experimentally for GaAs and InGaAs nanostructures. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stress‐engineered orderings of self‐assembled III–V semiconductor nanostructures

We report lateral orderings of self-assembled GaAs and InGaAs nanostructures by the stress field of a periodic dislocation network (DN) shallowly buried and parallel to the surface. This DN is a grain boundary (GB) that forms, between a thin GaAs layer and a GaAs substrate joined together by wafer bonding, in order to accommodate a tilt and a twist between these two crystals; both these misorientations are imposed in a controlled manner. This GB is composed of a one-dimensional network of mixed dislocations and of a one dimensional network of screw dislocations. The mixed DN accommodates the tilt and a part of the twist, while the screw DN accommodates the remaining twist. The ordered nanostructures we report result from a growth by metalorganic vapor phase epitaxy of a GaAs/InAs/GaAs sequence on the thin bonded layer. Using transmission electron microscope and atomic force microscope images, we prove that these nanostructures have same dimensions and orientations as the cells of the underlying DN and are thus correlated to the latter. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stress-driven self-ordering of III–V nanostructures

We report lateral orderings of III–V nanostructures by the stress field of a periodic dislocation network (DN) shallowly buried and parallel to the surface. These nanostructures consist of thickness modulations of GaAs and InGaAs layers grown by metalorganic vapor-phase epitaxy. We study here two samples for which the growth was performed at two different temperatures. The DNs of these two samples are composed of a one-dimensional network of mixed dislocations and of a one-dimensional network of screw dislocations. These DNs are grain boundaries which form, between a thin GaAs layer and a GaAs substrate joined together by wafer bonding, in order to accommodate a tilt and a twist between these two crystals. Both these misorientations are imposed in a controlled manner. The dimensions and orientation of the nanostructures formed at the surfaces of the samples are identical to those of the cells defined by the underlying DNs. Moreover, we demonstrate that by reducing the growth temperature, we favor the formation of less flat nanostructures which approach the aspect of conventional quantum dots.

Structural and optical properties of InAs/GaAs quantum dots emitting at 1.5 μm

We have demonstrated 1.5 μm light emission from InAs quantum dots (QDs) capped with a thin GaAs layer. The extension of the emission wavelength can be assigned to the large QD height. We also investigate the effect of growth interruption on the PL properties and the shape of InAs QDs fabricated by migration-enhanced growth (MEG). Contrary to expectation, we observed a remarkable blueshift of the emission energy with the growth interruption in MEG mode. Detailed investigations reveal that the blueshift is related to the reduced island height with the growth interruption, which is confirmed by reflection high-energy electron diffraction (RHEED) patterns and atomic force microscopy (AFM) measurement results. Accordingly, the structure changes of the islands are interpreted in terms of thermodynamic and kinetic theories.

Tensile-strained GaAs quantum wells and quantum dots in aGaAsxSb1−xmatrix

We report on the fabrication and analysis of tensily strained thin GaAs layers (``insertion layers'') in a $\mathrm{Ga}{\mathrm{As}}_{x}{\mathrm{Sb}}_{1\ensuremath{-}x}$ matrix $(xl0.05)$. The GaAs insertion layers vary in nominal thickness in the range of 0.6--2 monolayers. They were grown by molecular beam epitaxy on GaSb substrates. Transmission electron microscopy studies indicated either growth of ultra-thin quantum wells (QWs) or self-organized formation of quantum dots (QDs), depending on the nominal thickness of the insertion layers. Two photoluminescence (PL) peaks are observed at photon energies below the $\mathrm{Ga}{\mathrm{As}}_{x}{\mathrm{Sb}}_{1\ensuremath{-}x}$ band edge. One of them, detected near $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for any insertion layer thickness, is attributed to the emission of a type II QW. The second peak shifted toward smaller energies in the thicker insertion layers is assigned to the PL of type II QDs. This assignment is supported by microscopic tight-binding calculations of the QW electron states.

Long-range ordering of III–V semiconductor nanostructures by shallowly buried dislocationnetworks

We account for lateral orderings of III–V nanostructures resulting from aGaAs/InAs/InGaAs/GaAs sequence grown on GaAs by metallorganic vapour phaseepitaxy at two different temperatures. For both samples, the ordering is induced by thestress field of a periodic dislocation network (DN) shallowly buried and parallel to thesurface. This DN is a grain boundary (GB) that forms, between a thin GaAs layer (onwhich growth was performed) and a GaAs substrate joined together by wafer bonding, inorder to accommodate a tilt and a twist between these two crystals; both thesemisorientations are imposed in a controlled manner. This GB is composed of aone-dimensional network of mixed dislocations and of a one-dimensional network of screwdislocations. For both samples, the nanostructures observed by transmission electronmicroscopy (TEM) and atomic force microscopy are ordered by the underlying DNobserved by TEM since they have same dimensions and orientations as the cells of the DN.

Micro-Raman study of photoexcited plasma in GaAs bevelled structures

The properties of the photoexcited plasma in n-GaAs and its influence on micro-Raman spectra were studied on bevelled structures. The modes of the photoexcited free-carriers plasmon–LO–phonon (PLP) coupling have been detected in the surface depletion layer at room temperature in continuum wave (CW) mode. The strong modifications of the intensities of TO–phonon and LO–phonon modes were observed along the bevel. They were discussed in terms of scattering by PLP coupling. The dependence of the ratio of the intensities of TO–phonons and LO–phonons was linear along the bevel in the region of the surface depletion layer. The first derivative of this dependence is a characteristic value for the corresponding doping concentration. Five Si-doped GaAs layers with different doping levels were analyzed in this way to obtain a calibration function for determining the doping concentration profile in the very thin GaAs layers.

Improvement of the uniformity of self-assembled InAs quantum dots grown on InGaAs∕GaAs by low-pressure metalorganic chemical vapor deposition

We report an approach to improve the uniformity of self-assembled InAs quantum dots (QDs) grown on a strained In0.12Ga0.88As buffer layer on GaAs substrates by low-pressure metalorganic chemical vapor deposition. By inserting a thin GaAs layer between the InAs QD layer and the In0.12Ga0.88As buffer layer and examining its thickness effect, we demonstrate that the photoluminescence (PL) inhomogeneous linewidth from the QDs can be improved by increasing the thickness of the thin GaAs layer. The PL inhomogeneous linewidth is significantly decreased from about 70to20meV at 7K as the thickness is increased from 0to2nm. This significant improvement of the PL inhomogeneous linewidth is due to the fact that the QDs change from a bimodal distribution to a monomodal distribution consisting only of large QDs as a result of the inserted thin GaAs layer.

Effect of nonradiative recombination centers on photoluminescence efficiency in quantum dot structures

The influence of dislocations on photoluminescence (PL) intensity in structures with InAs-GaAs quantum dots (QD) has been studied. The structural characteristics of samples were studied by transmission electron microscopy in bright-field and weak-beam dark-field diffraction conditions. At temperatures below room temperature and for moderate excitation density, the PL intensity in a structure containing large clusters with dislocations was about the same as in a structure with a significantly lower density of clusters. In contrast, the measurement of PL intensity at elevated temperatures and high excitation densities allows an accurate estimation of the structural perfection of QD structures. The overgrowth of QDs with a thin (1–2 nm) GaAs layer with subsequent annealing reduces the density of clusters with dislocations and significantly improves the temperature stability of the PL intensity.

The model of a thin semiconductor layer on a thermally thick semiconductor backing for the photoacoustic use

In this paper the thermal model of a thin semitransparent layer on a semitransparent thermally thick backing material is presented. The formula for the temperature of the front surface of the structure that was derived in this paper allowed the computations of the photoacoustic amplitude and phase spectra of a thin GaAs layer on a silicon substrate and a porous silicon layer on a silicon substrate. This formula is a useful tool in the field of the photoacoustic spectroscopy of thin layers.

TEM study of GaAs/GaSb QD heterostructures

We have presented experimental results and theoretical estimations of the electronic structure of the GaAs quantum dots (QD) in a GaSb matrix which have large enough lattice-mismatch (∼7%) to introduce self-organization of QD nanostructures optically active in mid-IR range. This nanostructure is the only type-II QD system with the electron confinement, which meets the 3D-confinement conditions due to anomalously small lateral sizes and height of the QDs. The reason of the occurrence of such small QDs is still questionable, although the tensile stress in a thin GaAs layer, driving the self-organization process in this case, may be responsible for that.

Continuous wavelength tuning of InAs quantum dots on InP [formula omitted] and [formula omitted] substrates by chemical-beam epitaxy

The growth of InAs quantum dots (QDs) on InP (1 0 0) and (3 1 1) A substrates by chemical-beam epitaxy is studied. The InAs QDs are embedded in a GaInAsP layer lattice-matched to InP. We demonstrate an effective way to continuously tune the emission wavelength of InAs QDs grown on InP (1 0 0) . With an ultra-thin GaAs layer inserted between the QD layer and the GaInAsP buffer, the peak wavelength from the InAs QDs can be continuously tuned from above 1.6 μm down to 1.5 μm at room temperature. The major role of the thin GaAs layer is to greatly suppress the As/P exchange during the deposition of InAs and subsequent growth interruption under arsenic flux, as well as to consume the segregated In layer floating on the GaInAsP buffer. Moreover, it is found that InP (3 1 1) A substrates are particularly promising for formation of uniform InAs QDs. The growth of InAs on InP (3 1 1) A consists of two stages: nanowire formation due to strain-driven growth instability and subsequent QD formation on top of the wires. The excellent size uniformity of the InAs QDs obtained on InP (3 1 1) A manifests itself in the narrow photoluminescence line width of 26 meV at 4.8 K .