H. Stanzl

Growth and doping of ZnTe and ZnSe epilayers with metalorganic vapour phase epitaxy

Abstract In this work we report on the growth and doping of ZnTe and ZnSe layers with metalorganic vapour phase epitaxy (MOVPE). Low restistive p-type ZnTe was grown by doping with arsenic and phosphorus. Acceptor concentrations of up to 3.5 × 1017 cm-3 were achieved in the case of phosphorus-doped samples. For the first time nitrogen acceptors in ZnTe were investigated with photoluminescence at various temperatures and under resonant conditions. ZnSe layers were grown with DTBSe and DMZnTEN. The crystalline quality was investigated by high resolution X-ray diffraction. The halfwidths (150–500 arc sec) depend on the degree of relaxation and therefore on the layer thickness. Undoped ZnSe layers show at 2 K a strong recombination of free and bound excitons and a weak donor-acceptor pair luminescence.

Low temperature MOVPE growth of ZnSe with ditertiarybutylselenide

Abstract The results on the synthesis of the selenium alkyl ditertiarybutylselenide and its application in atmospheric pressure MOVPE are presented. In combination with dimethylzinc-triethylamine, single crystalline ZnSe layers were grown on GaAs at temperatures lower than 350°C. Good morphology, crystalline and interface quality are demonstrated by optical and electron microscopy, X-ray diffraction and Raman spectroscopy. Photoluminescence at 2 K reveals chlorine as an impurity. The electron mobility of 500 cm 2 /V·s at room temperature supports a fairly low compensation.

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.

Phase matched second harmonic generation using thin film ZnTe optical waveguides

Wide gap II‐VI semiconductors have strong second order susceptibilities χ(2) and are therefore promising materials for efficient second harmonic generation. We have grown high quality single crystalline ZnSe/ZnTe/ZnSe/GaAs (001) waveguides by metalorganic‐vapor‐phase‐epitaxy. Using end‐fire coupling we observe a phase matched signal of the 1170.5 nm fundamental wave. The fundamental beam is generated by a tuneable KTP optical‐parametric‐oscillator pumped by a ps‐Ti:sapphire laser system. Phase matching is achieved by coupling the TE0 fundamental mode with the TM2 second harmonic mode within the symmetric ZnTe waveguide.

Luminescence from structural defects in heteroepitaxial MOVPE-grown ZnTe

Abstract Two new strong deep emission bands with a weak phonon coupling were observed in heteroepitaxial ZnTe layers. The study of PL, PLE spectra in the layers with different thicknesses has shown that these emissions are produced in the ZnTe/GaAs interface region. It is suggested that the observed luminescence is related to the recombination of excitons bound to some extended structural defects, probably misfit dislocations. An unusual behavior of these bands - PL ‘fatigue’ effect - was observed under the strong laser excitation.

Exciton recombination in ZnSexTe1−x/ZnTe QWs and ZnSexTe1−x epilayers grown by metalorganic vapour phase epitaxy

We present the optical characterization of MOVPE grown ZnSe x Te 1-x epilayers with 0<x<0.35 and ZnSe x Te 1-x single quantum wells (SQWs) with L z =2.0-8.5 nm. The structures were investigated using photoreflectance and photoluminescence spectroscopy. Transmission electron microscope analysis was used to determine quantum well thickness. The studied ZnSe 0.3 Te 0.7 SWQs show a bright emission band with a halfwidth ∼10 meV in the spectral region of 100-250 meV below the band gap of the corresponding mixed crystal. We suggest a type II bands alignment in ZnSe x Te 1-x /ZnTe single quantum wells

Resonant photoluminescence and excitation spectroscopy on donor and acceptor states in doped ZnTe epilayers

Abstract The paper reports investigations on Ga, Al, I and Cl donors and As, P and N acceptors in ZnTe epilayers. The shallow acceptor nitrogen was studied for the first time. Excited acceptor and donor states were observed with resonant photoluminescence (PL). The parameters μ and δ are derived from excited acceptor states which allow to calculate the Luttinger parameters. We also calculated the donor binding energies and found that the incorporation of group VII elements gives rise to a strong donor acceptor pair luminescence with zero-phonon lines at about 2.24 eV.

Resonant luminescence of excitons, Cl-donors and N-acceptors in epitaxially grown ZnSeGaAs-layers

Abstract In this work, we report on resonant photoluminescence (RPL) investigations of epitaxially ZnSe GaAs layers grown by MBE and MOVPE. Lattice mismatch between ZnSe and GaAs causes a strain in the layer material which is relaxed by nucleation of misfit dislocations during growth. The relaxation process depends on layer thickness and growth conditions. Different thermal expansion coefficients of ZnSe and GaAs lead to a further strain component when the samples are cooled to RT or 2 K. The resulting compressive or tensile strain causes a splitting of exciton states and a shift of exciton recombination energies to higher and lower energies respectively. The strength of this shift depends on the ¦m j ¦ value of holes. We determine the recombination energy of free and bound excitons for different strain conditions (from fully relaxed ZnSe GaAs layers to samples without any strain relaxation) with interband excitation. The resulting deformation potentials are compared to results from measurements with high pressure diamond cells. Additionally, these investigations enable us to identify donor and acceptor bound excitons in biaxially strained ZnSe GaAs layers as heavy or light hole exciton states for compressive or tensile strain respectively. The characteristic binding energies of Cl-donors are investigated with RPL. A comparison of transition energies from the ground state to excited states indoped and undoped materials allows an identification of impurities in as-grown ZnSe. The ground state energy of N-acceptors is determined by temperature dependent measurements. Binding energies of Cl-donors and N-acceptors in biaxially strained ZnSe GaAs layers are compared to their corresponding energy levels in strain-free ZnSe. The strain in the layer lowers the symmetry of acceptor states and changes their binding energy. Excited acceptor states are again investigated by RPL measurements.