P. Möck

Self-assembled InSb quantum dots grown on GaSb: A photoluminescence, magnetoluminescence, and atomic force microscopy study

We report a study of self-assembled quantum dots (QDs) of InSb embedded in a GaSb matrix grown by metalorganic vapor phase deposition. Growth temperatures and deposition times have been optimized for maximal photoluminescence peak intensities. Photoluminescence (PL), magneto-PL, and atomic force microscopy (AFM) have been performed to estimate the size of the QDs. The quantum dots luminesce in the midinfrared at around 0.73 eV. The application of magnetic fields up to 15 T both parallel and perpendicular to the growth direction enhanced the wetting layer and bulk PL intensity and enabled an estimate to be made of the QD height and widths as 2–4 and 20–30 nm, respectively. These sizes were confirmed by AFM.

Origin and evolution of background impurity content of materials used in the preparation of (Hg, Cd) Te LPE layers on CdTe substrates

Abstract The presence of unintentional background impurities found in LPE-grown Hg 1- x Cd x Te layers has been traced back to the starting materials and different technological steps in the course of preparation of the layers. The purified elements Cd and Te, the binary compounds HgTe and CdTe synthesized from them, VB-grown CdTe monocrystals, LPE source solutions and the final LPE (Hg 0.78 Cd 0.22 Te/CdTe) layer/substrate structures have been analysed with regard to their impurity content. Spark source mass spectrometry, atomic absorption spectrophotometry and secondary ion mass spectrometry were the analytical techniques employed. Generally, any high-temperature and handling procedures cause an increase in the concentration of most of the impurities. For CdTe Bridgman ingots, a non-uniform distribution with enrichment in the last-to-freeze part of the as-grown crystal is observed. Furthermore, it was found that the carrier concentration and conductivity type of annealed LPE layers are influenced by the varying impurity levels of substrates from different axial positions within the CdTe ingot. The impurity depth profiles of LPE layers show a gettering effect of the layer surface and the layer/substrate interface resulting in a reduced impurity level in the central part of the layers.

Void-mediated formation of Sn quantum dots in a Si matrix

Atomic scale analysis of Sn quantum dots (QDs) formed during the molecular beam-epitaxy (MBE) growth of SnxSi1-x (0.05 less than or equal to x less than or equal to 0.1) multilayers in a Si matrix revealed a void-mediated formation mechanism. Voids below the Si surface are induced by the lattice mismatch strain between SnxSi1-x layers and Si, taking on their equilibrium tetrakaidecahedron shape. The diffusion of Sn atoms into these voids leads to an initial rapid coarsening of quantum dots during annealing. Since this formation process is not restricted to Sn, a method to grow QDs may be developed by controlling the formation of voids and the diffusion of materials into these voids during MBE growth.

MOVPE grown self-assembled and self-ordered InSb quantum dots in a GaSb matrix assessed by AFM, CTEM, HRTEM and PL

Self-assembled InSb quantum dots (QDs) were grown by metal-organic vapour phase epitaxy (MOVPE) in a GaSb matrix. Atomic force microscopy (AFM), conventional diffraction contrast transmission electron microscopy (CTEM), high resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) were used for the assessment of the QDs. Reductions in the III :V ratios and growth rates resulted in a change of the morphology of the InSb islands from hillocks without facets, and a low level of order to dumbbell shaped islands with distinct facets and a higher level of order.

Internal self-ordering in In(Sb,As), (In,Ga)Sb, and (Cd,Zn,Mn)Se nano-agglomerates/quantum dots

Nano-agglomerates of In(Sb,As) in InAs, (In,Ga)Sb in GaSb, and (Cd,Zn,Mn)Se in (Zn,Mn)Se are classified by transmission electron microscopy. In scanning transmission electron microscopy, atomic resolution Z-contrast images reveal different modes of internal compositional modulation on the atomic length scale, resulting for all three material systems in nano-agglomerates of an appropriate size that may constitute a new type of quantum dot. For other nano-agglomerates of In(Sb,As) in InAs and (In,Ga)Sb in GaSb, we observed a second type of nanoscale ordering that results in nano-agglomerates with an internal compositional modulation on a length scale of a few nm. Both types of compositional modulation are discussed as having arisen from a rather long-term structural response to a combination of internal and external strains.

Topography measurements of the critical thickness of ZnSe grown on GaAs

Synchrotron-based x-ray topography (XRT) measurements have been used to study the initial stages of relaxation in ZnSe layers grown by molecular beam epitaxy on vertical gradient freeze Bridgman GaAs substrates. The formation of the very first strain-relieving misfit dislocations in the grown ZnSe layers has been detected in a layer of thickness 100 nm. No such dislocations have been observed in a corresponding layer of 95 nm thickness. The critical thickness for this material system is therefore estimated to be 97.5±2.5 nm, which is markedly lower than the widely accepted value of 150 nm. In contrast to the InGaAs/GaAs system, combined XRT and transmission electron microscopy studies indicate that the initial misfit dislocations observed for ZnSe/GaAs are not, in general, formed by the bending over of pre-existing threading dislocations into the interface, but by other mechanisms such as stacking fault decomposition. The critical thickness data obtained have been used to infer the maximum critical thickn...

Self-ordered CdSe quantum dots in ZnSe and (Zn, Mn)Se Matrices Assessed by transmission electron microscopy and photoluminescence spectroscopy

Single and multilayer sheets of self-assembled CdSe [quantum dots (QDs)] were grown by means of molecular beam epitaxy in both ZnSe and (Zn0.9Mn0.1)Se matrices. Both types of structure were assessed by means of transmission electron microscopy in the scanning, high-resolution, and diffraction-contrast modes. Complementary results from wider sample areas were obtained by means of photoluminescence spectroscopy. In one of the samples analyzed, a fractional monolayer of MnSe was deposited immediately before the CdSe deposition. A second structure grown under identical conditions, but without the MnSe fractional monolayer, was also analyzed. This comparison provides direct evidence for an enhanced size and shape homogeneity of 3D QDs caused by the presence of a tiny amount of MnSe at the interface. In the multilayer structure, we observed the co-existence of highly strained quasi-2D QDs and CdSe rich aggregates with compositional modulations on certain crystallographic planes in close proximity.

In-situ direct measurement of activation energies for the generation of misfit dislocations in the InGaAs/GaAs(001) system

Abstract In-situ X-ray topography (XRT) studies of misfit dislocation generation and movement in epitaxial InGaAs strained-layer structures on (001) GaAs are described. Examination of the changes in dislocation structure during a series of successive post-growth in-vacuo sample anneals has, for the first time, yielded activation energies of 0.7 and 0.8 eV for the formation of α-and β-misfit dislocations (MDs) by the initial glide of substrate threading dislocations (TDs) in the InGaAs epilayer. The introduction of MDs by this method is supplemented by the presence of an additional MD generation process. The activation energy for this is found to be comparable to that required to initiate the glide of a TD. The XRT studies have also confirmed the existence of MD cross-slip events, where α to β cross-slip was found to have an activation energy of 1.2 eV and to be much more common than the reverse β-α cross-slip process.

Thermal processing induced plastic deformation in GaAs wafers

Abstract Different types of dislocation bundles were identified in the (001) GaAs substrates of III–V heterostructures. Comparisons of scanning infrared polariscopy images and X-ray transmission topograms showed a one to one correlation of stripes of reduced residual shear strain and dislocation bundles of the majority type. Visible-light interferometry and Makyoh topography, on the other hand, showed a slip-line distribution that is in correspondence to the distribution of dislocation bundles of the minority type(s). A new model for the plastic deformation of circular GaAs wafers during thermal processing is briefly outlined and its good agreement with the main experimental results demonstrated.

Analysis of thermal-treatment-induced dislocation bundles in GaAs wafers by means of X-ray transmission topography and complementary methods

By means of a heat treatment that was part of a molecular beam epitaxy (MBE) growth procedure, dislocation bundles have been induced in two-inch-diameter undoped (001) GaAs substrates. On the basis of contrast variations in synchrotron-based single-crystal X-ray transmission topograms that were recorded under conditions of high anomalous transmission, these dislocation bundles have been classified into three different types. Dislocation bundles of the majority type start at the sample edges in regions around the four 〈100〉 peripheral areas, glide typically up to about 1.5 cm into the bulk of the wafer following perpendicular 〈110〉 line directions, and form a pseudo-symmetric fourfold set. There are dislocations with two different Burgers vectors in each majority-type dislocation bundle and the extended segments of all of these dislocations are of the 60° type. In order to explain complementary experimental results, it is suggested that dislocation pairs are formed in the majority-type dislocation bundles. Theoretical support for this hypothesis is derived from a model of plastic deformation of GaAs wafers during typical MBE growth. Dislocation bundles of two minority types, on the other hand, are not part of the fourfold set and originate in peripheral areas at and around 〈110〉.