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We report growth of Ge nano/micro structures on ultra clean, high vicinal silicon surfaces of Si(5 5 7) and Si(5 5 12) under two substrate heating conditions: direct current (DC) and radiative heating (RH). These were grown under ultra high vacuum conditions while keeping the substrate at a temperature of 600°C. The results for 10 monolayer (ML) and 12 ML thick Ge deposited on the above surfaces show spherical island structures for RH conditions while aligned trapezoidal structures were observed under DC conditions of heating. We find that the longer side of trapezoid structures are along irrespective of DC current direction. In the case of 10 ML Ge deposited on Si (5 5 7), elongated SixGe1−x nanostructures with an average length of ≈300 nm and a length/width ratio of ≈3.3 have been formed along the step edges. Under similar conditions for 10 ML Ge growth on Si(5 5 12), we found aligned SixGe1−x trapezoidal microstructures of length ≈6.25 μm and an aspect ratio of ≈3.0. Scanning transmission electron microscopy (STEM) measurements showed the mixing of Ge and Si at the interface and throughout the over-layer. Detailed electron microscopy studies (scanning electron microscopy (SEM) and STEM) reveal the structural aspects of these microstructures.

In this contribution we have studied the diffusion of Al in AlAs/GaAs distributed Bragg-reflectors using the high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) intensity. The measured intensity is normalized to the intensity of the incoming electron beam using a detector scan. The normalized intensity can be directly compared with a set of frozen lattice simulations yielding specimen thickness in regions with known composition or concentration in regions with known thickness. The thickness was evaluated both from GaAs and AlAs regions yielding that the specimen was about 15 nm thinner in AlAs regions due to oxidation. For the concentration evaluation the thickness was derived from GaAs regions and concentrations up to 1.2 were found due to the overestimated thickness. Concentration profiles were scaled down to 1.0 and fitted to the solution of Fick's laws.

In this contribution we compare optical and structural properties of In0.2Ga0.8N0.024As0.976 quantum wells before and after annealing at 550 °C and 600 °C in an N2 atmosphere. We measure strain and chemically sensitive contrast to determine local indium and nitrogen concentrations using a TEM 3-beam image formed by the 000, 220 and 200 beams. For quantification of the concentrations Bloch wave simulations are used, which include bonding and static atomic displacements. The samples were grown by metal-organic vapour phase epitaxy, prepared with focused ion beams (FIB), thinned with low energy ion milling and investigated in a Cs-corrected Titan 80/300 microscope using an L-shaped objective aperture. Imaging conditions with the Laue circle centre at (0 4.2 0) are used as they show a weak dependence on the lamella thickness. Absorption measurements show a blueshift of the band gap of 19 ± 7meV (550 °C) and 36 ± 7 meV (600 °C) after annealing. The average nitrogen concentration was found to be 2±1% and is unaffected by the annealing temperature. In contrast, the mean indium concentration appears to decrease from 18.5 ± 2% before to 15 ± 1% after annealing. Together with the blueshift, this observation is discussed in terms of a modification of electron structure factors, caused by preferred coordination of N to In atoms.

We report on a Transmission Electron Microscopy 3-beam technique based on the interference of 000, 200 and 220. Nonlinear imaging artefacts are eliminated by Fourier filtering, yielding 200 and 220 lattice fringe images, from which chemically sensitive contrast and strain are measured, respectively. In this way, In and N composition can be mapped at atomic scale in quaternary InGaNAs by comparison with simulated reference data. Our Bloch wave simulations are based on structure factors derived from supercells with 106 atoms, which have been strain-relaxed by valence force field methods. Additionally, the influence of electron redistributions due to chemical bonding is accounted for by modified atomic scattering amplitudes derived from density functional theory. By comparing local compositions in an annealed In0.28Ga0.72N0.025As0.975 sample with its as-grown counterpart, we find homogenisation of InGaNAs layer thickness and –stoichiometry upon annealing.

Investigation of composition in InGaN quantum wells and quantum dots by TEM is hampered by formation of electron beam induced agglomeration of indium, which occurs if the specimen is exposed to the electron beam for a few minutes. In this contribution we demonstrate that compositional analysis of InGaN nanostructures is possible without this artifact if STEM Z-contrast imaging is applied instead of parallel beam illumination. The suggested method for composition analysis in InGaN is based on a comparison of intensity normalized with respect to the incident electron beam with simulated image intensity. Simulations are performed with the STEMsim program using the frozen lattice multislice approximation for which static atomic displacements were taken into account.

In this work we show how the indium concentration in high indium content InxGa1-xN quantum wells, as they are commonly used in blue and green light emitting diodes, can be deduced from high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. This method bases on introducing normalized intensities which can be compared with multislice simulations to determine the specimen thickness or the indium concentration. The evaluated concentrations are compared with atom probe tomography measurements. It is also demonstrated how the quality of focused ion beam prepared TEM- lamellas can be improved by an additional etching with low energy ions.

The effect of strain, composition and disorder in ADF images is systematically studied as a function of detection angle in order to understand the main contrast mechanisms. We demonstrate that the complex phenomenology in ADF images can be accounted for by accurate simulations and modelling. The advantage of an accurate modelling on the image interpretation will be demonstrated in the case of dislocations, chemical analysis of InGaP/GaAs and most noticeably the measurement of both In and N content in quaternary InGaAsN.

We determine the nitrogen concentration of GaN0.01≤x≤0.05As1−x quantum wells by evaluation of high resolution scanning transmission electron microscopy (STEM) images using a high-angle annular dark field detector. Although nitrogen has a smaller atomic number than Ga the image intensity increases with the nitrogen content. This is explained by the influence of static atomic displacements by comparison with frozen lattice simulations. The resulting nitrogen concentrations agree with high resolution X-ray diffraction measurements and strain state analysis applied to STEM images.

We report on the fabrication of a light-emitting diode based on GaN nanorods containing InGaN quantum wells. The unique system consists of tilted N-polar nanorods of high crystalline quality. Photoluminescence, electroluminescence, and spatially resolved cathodoluminescence investigations consistently show quantum well emission around 2.6 eV. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements reveal a truncated shape of the quantum wells with In contents of (15 ± 5)%.

We report the growth of Ge nanostructures and microstructures on ultraclean, high vicinal angle silicon surfaces and show that self-assembled growth at optimum thickness of the overlayer leads to interesting shape transformations, namely from nanoparticle to trapezoidal structures, at higher thickness values. Thin films of Ge of varying thickness from 3 to 12 ML were grown under ultrahigh vacuum conditions on a Si(5 5 12) substrate while keeping the substrate at a temperature of 600 °C. The substrate heating was achieved by two methods: (i) by heating a filament under the substrate (radiative heating, RH) and (ii) by passing direct current through the samples in three directions (perpendicular, parallel and at 45° to the (110) direction of the substrate). We find irregular, more spherical-like island structures under RH conditions. The shape transformations have been found under DC heating conditions and for Ge deposition more than 8 ML thick. The longer sides of the trapezoid structures are found to be along (110) irrespective of the DC current direction. We also show the absence of such a shape transformation in the case of Ge deposition on Si(111) substrates. Scanning transmission electron microscopy measurements suggested the mixing of Ge and Si. This has been confirmed with a quantitative estimation of the intermixing using Rutherford backscattering spectrometry (RBS) measurements. The role of DC heating in the formation of aligned structures is discussed. Although the RBS simulations show the presence of a possible SiO(x) layer, under the experimental conditions of the present study, the oxide layer would not play a role in determining the formation of the various structures that were reported here.

Real time electron microscopy observation on morphological changes in gold nano structures deposited on Si (100) surfaces as a function of annealing temperatures has been reported. Two types of interfaces with the substrate silicon were used prior to gold thin film deposition: (i) without native oxide and on ultra-clean reconstructed Si surfaces and (ii) with native oxide covered Si surfaces. For a \approx 2.0 nm thick Au films deposited on reconstructed Si(100) surfaces using molecular beam epitaxy method under ultra high vacuum conditions, aligned four-fold symmetric nanogold silicide structures formed at relatively lower temperatures (compared with the one with native oxide at the interface). For this system, 82% of the nanostructures were found to be nano rectangles like structures with an average length \approx 27 nm and aspect ratio of 1.13 at \approx 700{\deg}C. For \approx 5.0 nm thick Au films deposited on Si (100) surface with native oxide at the interface, formation of rectangular structures were observed at higher temperatures (\approx 850{\deg} C). At these high temperatures, desorption of the gold silicide followed the symmetry of the substrate. Native oxide at the interface was found to act like a barrier for the inter-diffusion phenomena. Structural characterization was carried out using advanced electron microscopy methods., Comment: 27 pages, 7 figures, 1 Table

GaN nanorods were grown on r-plane sapphire substrates by a two-step approach. Nucleation sites for the nanorods were provided by the formation of AlN islands during nitridation in a metal organic vapor phase system. These islands are a-plane oriented as expected for nitride growth on r-plane sapphire. The nanorods themselves were grown by plasma assisted molecular beam epitaxy. The nanorods show an inclination towards the surface normal of 28.3° and are highly ordered. Studies with high resolution x-ray diffraction polar plots reveal the epitaxial relationship between the substrate and nanorods as a c-direction growth on inclined m-plane facets of the nitridated islands. The determined lattice constants show nanorods which are strain free. The growth direction of the nanorods has been confirmed in a transmission electron microscope by convergent beam electron diffraction patterns to be in the N-polar [Formula: see text] direction.

Al1−xInxN layers with an indium content between x=10.5% and x=24% were grown by metal-organic vapor-phase epitaxy and characterized concerning their optical, structural and morphological properties with regard to the realization of optoelectronic devices. The indium content and the strain of these layers were measured by high resolution x-ray diffraction. Ellipsometric measurements were used to determine the optical constants [refractive index n(λ) and extinction coefficient κ(λ)] in dependence of wavelength and indium content. The values determined for the electronic bandgaps are in good agreement with theoretical predictions and previous publications on this topic but are more focused on AlInN layers which are pseudomorphically grown on GaN. A bowing parameter of b=10.3±0.1 was determined for fully strained layers with an indium content between 13% and 24%. In order to investigate the suitability of these layers for use in distributed Bragg reflectors, the surface morphology is characterized with respec...

Transmission electron microscopy (TEM) was applied to study In segregation during molecular beam epitaxy (MBE) growth of InAs-based quantum-dot (QD) structures with a focus on achieving QD luminescence at the technologically relevant wavelength of 1.3 μm. Quantitative composition analyses were carried out with the composition evaluation by lattice fringe analysis (CELFA) technique and In-segregation efficiencies were determined on the basis of atomic-scale composition profiles. The effect of the InAs-deposition rate on the In distribution was studied in detail which needs to be low, in the 0.005 ML/s regime to achieve long-wavelength emission. Further minimization of In-segregation induced intermixing can be achieved by lowering the substrate temperature during GaAs cap layer growth. Finally, a procedure is presented to extract the real composition of QDs taking into account their three-dimensional morphology and embedding in a GaAs cap layer.

The atomic scattering amplitudes of the various atoms of the systems Ga1−xInxAs, GaAs1−xNx and InAs1−xNx are calculated using the density functional theory (DFT) approach. The scattering amplitudes of N, Ga, As and In in the model systems are compared with the frequently used Doyle and Turner values. Deviation from the latter values is found for small scattering vectors (s

Catalyst- and mask-free grown GaN nanorods have been investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy filtered transmission electron microscopy (EFTEM). The nanorods were grown on nitridated r-plane sapphire substrates in a molecular beam epitaxy reactor. We investigated samples directly after the nitridation and after the overgrowth of the structure with GaN. High resolution transmission electron microscopy (HRTEM) and EFTEM revealed that AlN islands have formed due to nitridation. After overgrowth, the AlN islands could not be observed any more, neither by EFTEM nor by Z-contrast imaging. Instead, a smooth layer consisting of AlGaN was found. The investigation of the overgrown sample revealed that an a-plane GaN layer and GaN nanorods on top of the a-plane GaN have formed. The nanorods reduced from top of the a-plane GaN towards the a-plane GaN/sapphire interface suggesting that the nanorods originate at the AlN islands found after nitridation. However, this could not be shown unambiguously. The number of threading dislocations in the nanorods was very low. The analysis of the epitaxial relationship to the a-plane GaN showed that the nanorods grew along the [000-1] direction, and the [1-100] direction of the rods was parallel to the [0001] direction of the a-plane GaN.

High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is a powerful tool to quantify size, shape, position, and composition of nano-objects with the assessment of image simulation. Due to the high computational requirements needed, nowadays it can only be applied to a few unit cells in standard computers. To overpass this limitation, a parallel software (SICSTEM) has been developed. This software can afford HAADF-STEM image simulations of nanostructures composed of several hundred thousand atoms in manageable time. The usefulness of this tool is exemplified by simulating a HAADF-STEM image of an InAs nanowire.

III/V semiconductors containing dilute amounts of nitrogen are metastable and need to be thermally treated after growth to optimize optoelectronic properties. The influence of thermal annealing on the nitrogen depth profile in metal organic vapor phase epitaxygrown GaNAs/GaAs as well as GaInNAs/GaAs heterostructures is examined on a nanometer scale by combining several high resolution transmission electron microscopy techniques, also with Rutherford backscattering spectrometry. Annealing conditions, which are optimized for quaternary alloys with respect to photoluminescence intensity, do not result in element redistribution for the In containing material. Contrary to the quaternary material, the result of annealing the ternary GaNAs is a pronounced pileup of the nitrogen profile without any out diffusion of nitrogen. These findings have important influence on device structures, which often contain GaNAs barriers for strain-compensation purposes together with GaInNAs active regions. In the light of metastability considerations for the ternary and quaternary alloy, one can conclude that the In contained in the quaternary material stabilizes the material and suppresses phase separation. Consequently GaInNAs is more stable than its ternary counterpart GaNAs .© 2007 American Institute of Physics. DOI: 10.1063/1.2794739

The interface between a -plane GaN, grown by metal organic vapor phase epitaxy and hydride vapor phase epitaxy, and r -plane sapphire was investigated by transmission electron microscopy in [1-100] ...

We studied the influence of surface segregation on optical properties of semiconductor quantum wells. This effect leads to significant deviations of composition profiles from expected rectangular profiles. The model of Muraki is used to simulate composition profiles of InGaAs∕GaAs quantum wells for different segregation efficiencies from which we derive potentials for electrons and holes. To compute eigenenergies the Schrodinger equation is numerically solved. The transition energies are calculated from the energy differences of electrons and holes as a function of segregation efficiency. We find that the optical properties are influenced for segregation efficiencies larger than 0.7.

We compared the optical properties and material nanostructures between several InGaN∕GaN multiple quantum-well (QW) samples of different interfacial layers. In some of the samples, InN interfacial layers were inserted between the wells and barriers to improve the QW quality and hence the light-emission efficiency. Compared with a widely used barrier-doped QW structure, the insertions of the InN interfacial layers (silicon doped or undoped) do enhance the photon emission efficiencies. Of the two samples with InN interfacial layers, the one with intrinsic InN interfacial layers had the higher photoluminescence (PL) and electroluminescence (EL) efficiencies. Cluster structures are clearly observed in this sample, resulting in strong carrier localization. In this sample, we also observed a temperature-dependent S-shape variation in the PL spectral peak, a strong photoluminescence excitation (PLE) intensity, and a steep PL decay time variation beyond its peak as a function of temperature. On the other hand, bo...

The change of the morphology and indium distribution in an In0.12Ga0.88N quantum well embedded in GaN was investigated depending on the duration of electron-beam irradiation in a transmission electron microscope. Strain-state analysis based on high-resolution lattice-fringe images was used to determine quantitatively the local and average indium concentration of the InGaN quantum well. In-rich clusters were found already in the first image taken after 20 s of irradiation. The indium concentration in the clusters tends to increase with prolonged irradiation time. In contrast, the locally averaged indium concentration and the quantum-well width do not change within the first minute.

The mean inner potential of ZnO was measured by means of electron holography in a transmission electron microscope. To overcome the problem of imprecise knowledge of the sample thickness, ZnO nanorods with well-defined geometry and diameter were used in this study. The phase of the transmitted beam of the image wave yields a mean inner potential of 15.9 ±1.4 V for ZnO. This value is in good agreement with the calculated value of 15.8 V using ab initio density functional theory computations.

Dichroism in the transmission of light (the dependence of the transmittance on the direction of polarization of light) is revealed in corrugated GaAs/AlAs superlattices grown on a nanofaceted A(311) surface. It is assumed that the observed effect is associated with the structural anisotropy, i.e., with the formation of an array of GaAs quantum wires. This inference is confirmed by high-resolution electron microscopy. The GaAs/AlAs superlattices containing quantum wires also exhibit polarization anisotropy of the photoluminescence observed in the yellow-red spectral range.

High‐resolution electron microscopy of wurtzite type CdSe epilayers grown on (111) GaAs revealed that interfacial misfit dislocations are perfect 60° dislocations with Burgers vectors parallel to the interface. Glide is therefore limited to the (0001) interface plane and the extension of dislocations into the epilayer is suppressed. Single‐beam bright field imaging shows that the 7% mismatched epilayer is free of a dislocation network in its volume.

We have compared the In distribution in InGaN quantum wells grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE). The samples were studied by conventional and high-resolution transmission electron microscopy (HRTEM). The local and average In concentrations and the In distribution in the quantum wells were determined using the digital analysis of lattice images (DALI) method based on the evaluation of HRTEM lattice-fringe images. Similar lateral fluctuations of the In concentration were observed in MBE- and MOVPE-grown samples. The In concentration varies on a small scale (In-rich clusters with lateral extensions below 4nm) and on a larger scale of a few 10 nm, which is attributed to phase separation. In contrast, the In distribution in growth direction differs significantly in the MBE and MOVPE samples which is explained by different In-segregation efficiencies and In desorption before the GaN cap layer deposition during MBE. (C) 2003 Elsevier B.V. All rights reserved.

The local In distribution in InGaN quantum wells was measured in samples which were grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) varying systematically the metal-flux ratio and the growth rate. The composition analyses were performed by local lattice parameter measurements based on lattice-fringe transmission electron microscopy (TEM) images. Composition fluctuations are observed in all samples on two typical scales: In-rich clusters with lateral sizes below 5 nm with high In concentration and weaker composition fluctuations on a 100 nm scale. The size of the clusters does not depend on the growth conditions, but the amplitude of the composition fluctuations is influenced by the growth rate. In-concentration profiles along the growth direction show the effect of In desorption and In segregation during the growth.

Transmission electron microscopy (TEM) has been applied to analyze the thickness and the In-concentration of InGaN layers in GaN/InGaN/GaN- and AlGaN/InGaN/AlGaN-quantum well (QW) structures. Two series of samples were grown by metal organic chemical vapor deposition varying either only the growth duration for the InGaN QW or by changing the Al- concentration in the buffer layers at unaltered InGaN growth conditions. A rising average In- concentration from 6.5 to 15.4 % and a decreasing growth rate are observed with increasing growth duration. The increase of the Al-concentration in the buffer layers from 0 to 36 % strongly affects the In-incorporation during the InGaN growth, which decreases from 17.5 to 2.5 %. All samples are characterized by an inhomogeneous In-distribution containing In-rich agglomerates with a size of only a few nanometers and less pronounced composition fluctuations on a scale of 100 nm.

Two well distinguishable classes of nanoscale islands were identified in CdSe/ZnSe quantum dot structures by optical spectroscopy and transmission electron microscopy. For 2.1 to 3.1 monolayer CdSe deposition, coherent three-dimensional (3D) islands, formed in the Stranski–Krastanow (SK) mode, are found with typical diameters of ∼16 nm and a coverage-dependent density of up to 3×1010 cm−2. Simultaneously, small islands with lateral extensions below 10 nm and a density of ∼5×1011 cm−2 are formed by strain-modified island growth. Whereas the 3D SK islands dominate the emission properties at room temperature, the latter smaller islands determine the optical properties at temperatures below 120 K.

A quantitative transmission electron microscopy (TEM) investigation of the relaxation of GaN grown on Al2O3(0001) has been carried out. Terminating {112̄0}-substrate fringes observed at the interface of the highly mismatched system have been characterized and the efficiency of the relaxation was measured. Wurtzite type GaN was grown by plasma induced molecular beam epitaxy (MBE) on the (0001) basal plane of Al2O3. The in-plane orientation between GaN and substrate reveals a high lattice misfit of f=-13.9% and therefore the critical thickness of dislocation formation is reached when the first monolayer of GaN is grown. An expected interfacial relaxation process is characterized by the results of high resolution transmission electron microscopy (HRTEM) which reveals misfit dislocations confined in the GaN/Al2O3 interface region. The quantitative evaluation of the HRTEM images on the one hand and Moiré pattern on the other hand shows the effectiveness of the observed relaxation process: here a degree of relaxation δ=(-11.8±0.9)% and a residual strain of εr=(-2.1±0.9)% was measured, and it seems that only εr causes a dislocation density of estimately 1010 cm-2 in the GaN epilayer.

InGaN/GaN structures with dense arrays of InGaN nanodomains were grown by metallorganic chemical vapor deposition. Lasing in vertical direction occurs at low temperatures, indicating ultrahigh gains (∼ 105 cm−1) in the active region. Fabrication of an effective AlGaN/GaN distributed Bragg reflector with reflectivity exceeding 90% enables vertical lasing at room temperature in structures with a bottom distributed Bragg reflector, despite the absence of a well-reflecting upper mirror. The lasing wavelength is 401 nm, and the threshold excitation density is 400 kW/cm2.

We demonstrate exciton waveguiding and short wavelength lasing (460 nm at 300 K) in quantum dot structures based on submonolayer superlattices. High resolution electron microscopy studies reveal that CdSe submonolayer insertion in a ZnSe matrix forms an array of 2D nanoscale islands. Strong modulation in the optical reflectance spectra is observed in a 20 period submonolayer superlattice structure at energy corresponding to the localized exciton state, and the lasing occurs directly in the spectral region of the exciton-induced enhancement of the refractive index.

Stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures are found to induce enhanced radiative recombination visible as pairs of bright spots in microphotoluminescence intensity maps. Structural investigation by atomic-force microscopy and transmission electron microscopy (plan view as well as cross section) reveal that a widening and bending of quantum wells occurs when they are intersected by Frank-type stacking faults. The enlargement of the well width by up to 12 bilayers evokes an efficient localization of excitons. The localizing potential related to Shockley-type stacking-fault pairs is found to be much shallower.

We report on reflected high‐energy electron‐diffraction and transmission electron microscopy plane‐view investigation of the dislocation structure in doped and undoped ZnSe/GaAs(001) grown by molecular‐beam epitaxy and metal‐organic vapor‐phase epitaxy. The thicknesses of the investigated layers vary between 60 and 900 nm. Several stages of dislocation formation are found which occur at distinct layer thicknesses. Frank partial dislocations (up to 500 nm), Shockley partial dislocations (between 130 and 400 nm) with a maximum density at 300 nm, and perfect 60° dislocations (above 300 nm) are observed in samples with perfectly smooth surface. The formation of Shockley partial dislocations is strongly anisotropic which might be due to the higher mobility of α‐type dislocations. An increased roughness of the growing surface yields a suppression of Shockley partial dislocations and an irregular dislocation network with dislocations inclined to the 〈110〉 directions. A regular dislocation network with straight dislocations is found in Cl‐doped samples.

Structural and optical properties of structures with nanoscale InAs islands obtained by submonolayer deposition and embedded in an AlxGa1−xAs matrix is investigated. Deposition of several planes of InAs insertions results in formation of arrays of vertically correlated islands. The lateral size of the islands in a column is about 10 nm. Lasing via the ground states of the islands without external optical confinement is demonstrated.

Results from a structural and optical analysis of buried InxGa1-xAs islands carried out after the process of GaAs overgrowth are presented. It is found that during the growth process, the indium concentration profile changes and the thickness of the wetting layer emanating from a Stranski-Krastanow growth mode grows significantly. Quantum dots are formed due to strong gradients in the indium concentration, which is demonstrated by photoluminescence and excitation spectroscopy of the buried InxGa1-xAs islands. (C) 1997 American Institute of Physics.

GaAs-AlAs corrugated superlattices (CSL) are formed on spontaneously nanofaceted (311)A surfaces. Using high-resolution transmission electron microscopy (HRTEM) along the $$[\bar 233]$$ zone axis with an appropriate image evaluation technique to enhance the contract between GaAs and AlAs we found two distinct lateral periodicities along the $$[0\bar 11]$$ directions for two different CSL layer thickness regimes. For multilayer deposition with GaAs layer thickness exceeding 1 nm the lateral periodicity of 3.2 nm is clearly revealed. The contrast originates from the thickness modulation of both AlAs and GaAs layers with a period of 3.2 nm in the $$[0\bar 11]$$ direction. The corrugation height is about 1 nm and it is symmetric for both upper and lower GaAs-AlAs interfaces. Thicker sections of the thickness-modulated AlAs and GaAs layers of the CSL are shifted by a half period with respect to each other. In the regime when the GaAs deposited average thickness is below 1 nm, which is necessary for complete coverage of the AlAs surface, a lateral periodicity of ≈1.5–2 nm is additionally revealed. We attribute this effect to the formation of local GaAs clusters dispersed on a corrugated (311)A AlAs surface resulting in a local phase reversal of the AlAs surface in their vicinity upon subsequent overgrowth. This reversal can be explained by the same effect as the phase shift of the surface corrugation upon heteroepitaxy on (311)A. In our model AlAs does not wet the GaAs cluster surface, unless different more energetically favorable scenario is possible. This causes accumulation of AlAs in the vicinity of the GaAs cluster and, as a result, the local phase reversal of the AlAs surface. The AlAs corrugated surface domains with different phases coexist on the surface resulting in an additional periodicity revealed in the HREM contrast modulation. Additionally HRTEM studies indicate that the AlAs-GaAs interface inclination angles in both regimes are 40° and 140° with respect to the flat (311) surface in an argreement with the {331} facet geometry model proposed by R. Notzel, N.N. Ledentsov, L. Daweritz, M. Hohenstein, and K. Ploog.

We investigated the strain relaxation process of ternary ZnSxSe1−x layers using high-resolution X-ray diffractometry (XRD), Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). The samples were grown by low pressure MOVPE on (001) GaAs substrates. XRD analysis shows that after exceeding a critical layer thickness strain relaxation occurs first in the [1–10] direction and the layer unit cell is monoclinically distorted. A detailed structural characterization and strain measurements for various compositions and layer thicknesses were carried out. TEM and RBS channeling measurements reveal an anisotropic distribution of defects not confined at the interface. Their contribution to the observed strain relaxation will be discussed. It is demonstrated that the introduction of a thin ZnSe layer between substrate and ZnSxSe1−x epilayer will drastically slow down the relaxation process and hamper the formation of stacking faults at the heterointerface.

Low-dimensional semiconductor heterostructures grown by metal organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) are one of the main research topics in solid-state physics at present. Most applications of semiconductor nanostructures are found in the field of optoelectronic devices such as ligh-emitting diodes and laser diodes. Especially InGaAs/GaAs is being studied intensively owing to the formation of self organized quantum dots (QDs) by Stranski-Krastanow growth mode under certain conditions. The development of QD lasers is expected to lead to an increased quantum efficiency and to lower threshold current densities [1]. Although MOCVD and MBE can, in principle, be used to control expitaxial growth on a monolayer (ML) scale, kinetic growth effects such as segregation [2], diffusion and adatom migration [3] must be taken into account. Especially segregation is crucial in InGaAs/GaAs structures, because it leads to a significant broadening of interfaces at conditions of growth used commonly.