Epitaxial Growth and Optical Properties of PbTe/CdTe Semiconductor Heterostructures

Abstract

The accessibility of very efficient light sources in the mid-infrared region is crucial for many applications in medicine, environmental protection or molecular spectroscopy. In this spectral region low dimensional structures based on IV-VI semiconductors like PbTe, are attractive candidates for room temperature operating optoelectronic devices. Recently, high crystal and optical quality PbTe/CdTe multilayers were successfully grown by molecular beam epitaxy on GaAs substrates. These semiconductor materials exhibit excellent matching of their lattice parameters but crystallize in different cubic lattice: zinc blende (a0=6.48 A) for CdTe and rock salt (a0=6.46 A) for PbTe. In PbTe/CdTe heterostructures, narrow gap PbTe constitutes quantum well or quantum dot material with wide gap CdTe electronic barriers. The aim of this work is to determine the optimal conditions for epitaxial growth of PbTe/CdTe structures and to experimentally study their basic optical properties. Various PbTe/CdTe multilayers (superlattices, trilayers) were grown by molecular beam epitaxy using compound PbTe and CdTe sources. Two substrates, i.e. semi insulating (100) oriented GaAs with 4 μm thick high quality CdTe buffer layer and (111) oriented BaF2 with PbTe buffer were used. The structures were deposited at substrate temperature varying in the range 250-340 °C. The layer thickness covered the range t=6-1000 nm for PbTe layers and d=7-500 nm for CdTe barriers. The growth was controlled in-situ by reflection highenergy electron diffraction (RHEED) indicating well defined streaky pattern characteristic for 2D growth mode. After growth, in-situ and ex-situ annealing of samples was performed. The crystal quality of the layers was examined by x-ray diffraction method and cross-sectional observation using atomic force microscopy (see Fig.1). The samples were optically characterized at LHe, LN2 and room temperatures by a photoluminescence measurements using 532 and 1064 nm lines of YAG: Nd pulsed laser for excitation. For both GaAs and BaF2 substrates the best crystal quality of PbTe/CdTe multilayers was obtained for samples grown at relatively low temperatures about 250 °C. The PbTe/CdTe superlattices exhibit in xray spectra diffraction satellites up 4 order. In PL spectra, for excitation by 532 nm, we observe strong luminescence at about 0.8 μm deriving from CdTe and, for excitation by 1064 nm line, broad luminescence with maximum at 3.6 μm from PbTe. In the case of CdTe the photoluminescence is related to neutral acceptor bond excitons, whereas in the case of PbTe we attribute the luminescence to band to band transitions. In IR region we have observed the dependence of luminescence energy (blue shift) on the PbTe layer thickness due to quantum size effect. For thin PbTe layer structures the strong influence of post-growth annealing on luminescence was also find out. Fig.1 Cross–sectional AFM image of the PbTe/CdTe