L. Parthier

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.

Growth and characterization of lattice-matched HgSe

HgSe layer on ZnTe and ZnTe 1-x Se x layers were grown by molecular beam epitaxy. The introduction of a GaSb substrate reduces the lattice mismatch drastically ( f = - 0.11 %) compared to GaAs (f = - 7A%). By spatial variation of the Se content from 0.016 ≤ x ≤ 0.041 the lattice constant of the buffer layer can be varied between that of the GaSb substrate and that of the HgSe layer. The different heterostructure systems were investigated by high-resolution X-ray diffraction to calculate the crystalline perfection and the strain behavior. A 150 nm HgSe layer pseudomorphically grown on a 145 nm ZnTe 0.964 Se 0.036 buffer layer shows pendellosung thickness fringes indicating the high crystalline perfection of the structures, and has a FWHM of 84 arcsec. Only tensile strain is present in this structure. The carrier concentration and the mobility are calculated from Shubnikov-de-Haas oscillations in high magnetic fields up to 12 T. The best samples have a high mobility of up to 2.1 x 10 5 cm 2 /V.s with a carrier concentration of 1.0 x 10 17 cm -3 at 4.2 K.

Incorporation of rare earths into II - VI compounds during MBE growth: optical and structural studies of Sm-doped tellurides

Sm-doped ZnTe and CdTe films with thicknesses of several hundred nanometres were grown by molecular beam epitaxy on GaAs substrates at temperatures between 300 and . Sm concentrations between about and (determined by x-ray microprobe analysis and SIMS) were incorporated by varying the temperature of the Sm source between 400 and . The RHEED patterns show that the lattice structure changes from zinc blende to rocksalt type at about some . This phase transition is also found to manifest itself in the interband reflection spectra studied for ZnTe:Sm. Optical transmission measurements reveal two prominent absorption bands peaking at about 1.45 eV (ZnTe and CdTe) and 2.1 eV (ZnTe) respectively and appearing at Sm concentrations above some . On the basis of SmTe data, we assign these features to 5d transitions. From their high-energy shift, we conclude that, in the zinc blende range, Sm is incorporated on sites surrounded by an octahedron of Te atoms, which are formed from zinc blende interstitial sites during MBE growth. These centres are thought to initiate the transition to the rocksalt phase when the critical concentration is reached.

Strain relaxation in selectively grown ZnSe-epilayers on patterned GaAs-substrates

For the investigation of the strain relaxation in mesoscopic ZnSe structures selectively grown by MBE on patterned and unpatterned (001) GaAs substrates high-resolution X-ray diffraction was used. It is shown that the strain relaxation in the selectively grown structures with a thickness below about 500 nm is up to 9% higher than in the corresponding homogeneous layers. With increasing layer thickness the ratio of the relaxation is inverted, e.g. the relaxation is higher in the unpatterned layer. Reciprocal space mapping shows that the orientation distribution of the lattice net planes for the mesoscopic structures has a decreased width with respect to the unpatterned layers.

The influence of interdiffusion on the lattice misfit of epitaxial structures

Abstract The concept of lattice misfit and strain distribution at a heterointerface is reconsidered, taking into account the interdiffusion of substrate and layer components. Based on calculated compositional profiles for multicomponent solid solutions, the respective lattice-parameter profiles and stress profiles in the interface region of an epilayer system have been derived. For different diffusion coefficients and different influences of substrate and layer components on the lattice parameters, stresses develop due to interdiffusion during growth although the natural lattice matching is perfect. The stresses are formed both in the substrate and the layer and change their sign near the interface. Depending on the diffusion of substrate and layer components and on the sign of the natural misfit more than one stress-free plane can occur. They are shifted away from the interface either into the substrate or into the layer, the distance depending on the diffusion time. The model was applied to LPE-grown (Hg,Cd)Te on different substrates.

MBE-grown heavily Sm-doped ZnTe studied by optical spectroscopy

Abstract Sm-doped ZnTe films with thicknesses of several hundred nm were grown by molecular beam epitaxy (MBE) on GaAs substrates at temperatures between 300 and 400 °C. Total Sm concentrations between 1018 and about 1021 cm−3 as determined by X-ray microprobe analysis and secondary ion mass spectrometry (SIMS) were incorporated by varying the temperature of the Sm source between 400 and 650 °C. Studies of optical transmission spectra between 0.6 and 2.3 eV, and photoluminescence spectra in the excitonic range, are reported. The transmission spectra were measured after removing the substrates by etching, and fixing the films on glass platelets. These spectra were used to evaluate absorption spectra by correcting for Fabry-Perot interferences. Two prominent absorption bands peaking at about 1.45 and 2.1 eV are seen, and these are attributed, by comparison with SmTe data, to Sm2+ 4f6 → 4f5 5d transitions. We argue that the Sm2+ is incorporated on sites surrounded by an octahedron of Te atoms, which are formed from zincblende interstitial sites during MBE growth. The coalescence of these centers occurring above approximately 1020 cm−3 is thought to be the reason for the observation of rocksalt-type reflections in the reflection high-energy electron diffraction (RHEED) patterns in this concentration range.

LO-phonon assisted tunneling from spatially direct to indirect exciton in semimagnetic double quantum well structures

Abstract Relaxation from spatially direct to the spatially indirect exciton through ZnSe barriers of different thicknesses is investigated in (ZnCdMn)Se/ZnSe/(ZnCd)Se asymmetric double quantum wells by use of magneto-optical steady-state photoluminescence (PL) and PL excitation (PLE) experiments. The 1-LO-phonon scattering has been found to be the relevant mechanism for effective electron and hole tunneling.

Growth and characterization of CdTe/ZnTe buffer layers on GaAs substrates

Abstract The MBE growth of ZnTe, CdTe, (Cd,Zn)Te and related superlattice structures on (001)GaAs has been investigated. Growth rate, structural perfection and mixed crystal composition have been determined as functions of different growth parameters. Optical properties and the ability of blocking substrate impurities with a superlattice structure have been studied.

Effect of interdiffusion on dislocation generation in epitaxial layers on CdTe, (Cd,Zn)Te and Cd(Te,Se) substrates

Diffusion modifies the interfacial strain field induced by the composition difference and temperature changes. The strain field is estimated theoretically for liquid-phase epitaxial-grown (Hg,Cd)Te layers on different substrates. The effect on the generation of misfit dislocations is compared with etch pit density profiles.

Incorporation of rare earths into II–VI compounds during MBE growth: optical and EXAFS studies of Sm-doped ZnTe

Abstract Sm-doped ZnTe films with thicknesses several hundred nm were grown by molecular-beam epitaxy (MBE) on GaAs substrates at temperatures between 300 and 400°C. Sm concentrations between about 10 18 and about 10 22 cm −3 , as determined by X-ray microprobe analysis and SIMS, were incorporated by varying the temperature of the Sm source between 400 and 650°C. We report optical absorption spectra obtained from transmission measurements at photon energies below the interband edge. Two prominent absorption bands peak at about 1.45 and 2.1 eV are seen at concentrations above some 10 20 cm −3 . Based on a comparison with SmTe data, we assign these bands to 4f 6 → 4f 5 5d transitions in Sm 2+ ions on sites with octahedral Te coordination. These centers are thought to be formed from zincblende interstitial sites during MBE growth. We have also performed extended X-ray absorption fine structure (EXAFS) studies at the Sm L 3 absorption edge using fluorescence yield detection. The results of these studies are consistent with our incorporation model, leading to a SmTe bond length of 3.25 A for the dominant Sm site. These Sm centers are obviously responsible for the transformation from zincblende to rocksalt structure which is seen in the RHEED patterns of such films as the total Sm concentration is increased above some 10 21 cm −3 .