T. Reisinger

High resolution transmission electron microscopy determination of Cd diffusion in CdSeZnSe single quantum well structures

Abstract The diffusion coefficient D(T) of Cd in CdSe ZnSe single quantum well (SQW) structures grown pseudomorphically on GaAs(001) is determined by high resolution transmission electron microscopy of annealed SQWs and subsequent digital analysis of lattice images. SQWs of 2 monolayer (ML) thickness were grown by molecular beam epitaxy. During growth the CdSe quantum wells (QWs) broaden to about 7 ML CdZnSe as measured by reflection high energy electron diffraction (RHEED). We find for the diffusion coefficient of Cd in ZnSe at temperatures between 340 and 400° C D(T) = 1.9 × 10−4 [cm2/s] · exp(− 1.8 [eV]/kT).

Luminescence due to lattice‐mismatch defects in ZnTe layers grown by metalorganic vapor phase epitaxy

Two sharp and intense emission bands with the maxima of 2.185 and 2.150 eV and a weak phonon coupling were investigated in the heteroepitaxial ZnTe layers. The study of the ZnTe layers grown on different substrates with thicknesses of 0.5–3.2 μm has shown that this luminescence is produced in the interface region that contains a high density of structural defects. The temperature dependence of these bands reveals the excitonic character of the recombination. We propose that the observed bands originate from the recombination of excitons localized at structural defects such as dislocations and associated defects.

Transmission electron microscopy and reflected high‐energy electron‐diffraction investigation of plastic relaxation in doped and undoped ZnSe/GaAs(001)

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 d...

Luminescence caused by extended lattice defects in epitaxially grown ZnTe layers

Two strong emission bands (Y1 and Y2), 210 and 250 meV lower than Egap of ZnTe, have been studied in heteroepitaxially grown ZnTe layers. The phonon coupling and the thermalization energy of these emissions are usually small, similar to those of the Y band in ZnSe and CdTe layers. The Y luminescence in ZnTe is emitted above all near the ZnTe/GaAs interface which contains a high density of lattice defects (mainly misfit dislocations). An increasing concentration of point defects leads to a strong decrease of the Y emission. The observation of this luminescence in ZnTe bulk material or in homoepitaxially grown ZnTe is not possible because of a strong dependence of the intensity on the defect concentration and the dislocation density. Both bands are excited only by laser light resonant to or higher than the free exciton energy. The strain dependent energy shift behaves similar to that of bound excitons. The intensity of the Y bands observed at liquid helium temperature decreases under strong laser excitation. A recovery effect of both lines is observed when the sample is heated to liquid nitrogen or room temperature.

Relaxation process and luminescence of lattice defects in epitaxially grown ZnSeGaAs layers

Abstract In this work, we report on the dependence of strain relaxation in epitaxially grown ZnSe GaAs layers on the growth process. New investigations about luminescence signals are presented, which are caused by lattice defects in these layers. The strain is due to different lattice constants of layer and substrate material and is relaxed by the nucleation of misfit dislocations during growth. This relaxation process depends on layer thickness and growth conditions. We determine the residual strain for different layer thicknesses and growth processes with X-ray diffractometry. Furthermore, the created misfit dislocations give rise to a luminescence at 2.6 eV, the so-called Y line. We found that its behaviour in ZnSe is analogous to that of Y lines in epitaxially grown ZnTe GaAs layers. An excitation of the Y luminescence is only possible with photon energies higher than the recombination energy of free excitons. Its intensity decreases with increasing impurity concentration. It also decreases strongly for a steady state excitation at 2 K. A recovery effect of luminescence intensity is observed, if the sample is heated to room temperature. Furthermore, the strain dependence of the Y luminescence in ZnSe is compared with the maxima of bound exciton luminescence from shallow donors and acceptors.

Hole mass determination in ZnSe by observation of standing polariton waves

Interference pattern due to standing polariton waves have been observed in thin planparallel ZnSe layers. The samples consist of pseudomorphic ZnSe/ZnCdSe quantum well structures grown by molecular beam epitaxy with cap layers of various thicknesses. The interference pattern appearing in reflectance and photoluminescence excitation spectra were fitted to calculated spectra applying a simple theoretical model. This fit results in the determination of the deadlayer thickness and the evaluation of the Luttinger parameter γ1 and γ2.

Role of nitrogen precursors in MOVPE growth of ZnSe

Abstract Two amines, Et 3 N and t BuNH 2 have been investigated in their role to improve the quality of MOVPE-grown ZnSe layers. A reduction of growth rate is observed in the kinetic limited range, which is shown to be due to an increase of activation energy, independent of the amine used. An improvement of the layer quality is related to the suppression of nonradiative recombination centers. However, 2D growth, controlled by ex situ RHEED and MBE-regrowth experiments, was only observed without Et 3 N. Under photoirradiation the growth rate increases in the kinetic limited range, where the two amines show remarkable differences. Whereas in this case t BuNH 2 leads to a poor optical layer quality, Et 3 N gives excellent layers at sufficiently high growth rates. The experiments, including RDS growth control, lead to the conclusion that the nitrogen compounds are always adsorbed at Se-terminated surfaces and thus inhibit a quick reaction of Zn and Se on the surface. A model is proposed which describes the presented experiments consistently.

TEM-investigation on the critical thickness anisotropy of MBE-grown ZnSe/GaAs and Zn1−xMgxSe/GaAs

The critical thickness anisotropy of the Zn 1-x Mg x Se/GaAs(001) and ZnSe/GaAs(001) systems grown by molecular beam epitaxy (MBE) is investigated by in situ reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). The increasing FWHM of the specular spot in the RHEED-pattern during growth indicates the initial relaxation stages in strained epitaxial layers as described in an earlier publication [Reisinger et al.. Mater. Sci. Forum 182-184 (1995) 147]. Plan-view TEM-micrographs of ZnSe/GaAs samples show that the perfect 60 -misfit dislocations in the [1 - 10]-direction nucleate prior to the [110]-direction due to their higher mobility. According to the TEM investigation the FWHM increase of the specular spot is connected with the formation of 60 -dislocations perpendicular to the electron beam, whereas dislocations aligned parallel to the beam leave the FWHM unaffected. Finally, a thickness range of the ZnSe-layer from about 225-337 nm is found with misfit dislocations aligned only in the [1 - 1 0]-direction. Furthermore, we present a new in situ RHEED-method to determine the critical thickness of the Zn 1-x Mg x Se/GaAs system separately in both (110)-directions without changing the experimental set-up. The application of this method to various Zn 1-x Mg x Se-samples with different Mg-content x confirms the observation that different critical thicknesses in [110] and [1 - 1 0] are measured related to two different glide systems as observed in ZnSe. Plan-view TEM-micrographs reveal the dislocation structure during the different states of relaxation.

High-resolution X-ray diffraction investigations of epitaxially grown ZnSe/GaAs layers

Single-crystalline ZnSe layers were grown by metal-organic vapour phase and molecular beam epitaxy on (001) oriented GaAs substrates. The lattice mismatch between layer and substrate at growth temperature causes a strain in the layer material, which is relaxed by the nucleation of misfit dislocations. The relaxation process starts at the critical thickness, which depends on the growth conditions. The crystalline quality and the residual strain of the epilayers were investigated with a high-resolution X-ray diffractometer. Additionally, the intensity distribution of the scattered X-rays in the directions perpendicular and parallel to the reciprocal lattice vector (004) was observed by a two-reflection analyser crystal. For the system ZnSe/GaAs, this intensity distribution depends on the degree of strain relaxation, which is dependent on the layer thickness. The results are compared with transmission electron microscopy results.

Investigation of the pressure dependence of subband transitions in ZnSe/Zn1−xMgxSe quantum wells by PLE

Abstract Photoluminescence-excitation (PLE) measurements of wide ZnSe/Zn 1− x Mg x Se single quantum wells (SQW) are presented, performed under high hydrostatic pressure up to 4 GPa. The fully strained samples have been grown by MBE on GaAs(0 0 1) substrates or (1 1 0) substrates. The well transitions 1nH 1s and 1mL 1s with n = 1, 2, 3, 4, 5 and m = 1, 2 were recorded as well as excitonic signals from the barrier material at 2 K and their pressure dependence were determined. A cross-over between the 11L and 13H, as well as between the 12L and 14H transitions, was observed above 1.2 GPa. The transitions were assigned using a modified Kronig Penney model. The exciton binding energy was derived from the energy difference between the 11H 1s and the 11H 2s transitions.