Growth and optical properties of CdTe ultra-thin quantum wells

Summary form only given. CdTe has been considered as an ideal material for solar cells and has been employed in HgCdTe alloys to elaborate infrared photodetectors. However, when few monolayers of CdTe are deposited within ZnTe, the fundamental optical transitions of CdTe/ZnTe quantum wells (QWs) can be designed to be in the 1.5 - 2.3 eV (infrared-green) range at room temperature. In order to consider possible device applications it is necessary a systematic study of the optical and structural properties of ultra-thin quantum wells (UTQWs) of CdTe and the determination of the most favorable growth parameters. In this work we present an investigation by low temperature photoluminescence (PL) and room temperature photoreflectance (PR) of UTQWs of CdTe grown by atomic layer epitaxy (ALE). The epitaxial growth was carried out in a Riber 32P system with Cd, Zn and Te elemental solid sources. Before the deposition of the CdTe QWs a 0.5 /spl mu/m thick ZnTe buffer layer was grown onto the GaAs[100] substrates. The ALE growth of CdTe UTQWs was performed by alternate exposure of the substrate surface to individual fluxes of Cd and Te. After each flux exposure, a 4 sec dead time (flux interruption) was applied to allow re-evaporation and migration of the atomic species. The ZnTe barriers were deposited by molecular beam epitaxy (MBE). The samples were prepared at a substrate temperatures (T/sub s/) of 270/spl deg/C and 290/spl deg/C. They consisted of five CdTe UTQWs made with four and eight ALE-cycles. Under our growth conditions a /spl sim/0.5 ML coverage per cycle is obtained, variations are observed as a function of temperature. This means that the QWs are nominally 2 and 4 ML thick. The growth was monitored in-situ by reflection high energy electron diffraction (RHEED). Information about the kinetics of growth was obtained through the analysis of the temporal evolution of some features of the RHEED pattern of the (2/spl times/1)-Te reconstruction along the [110] azimuth.- The UTQWs, grown at 270/spl deg/C presented very intense and sharp PL emission, while those grown at 290/spl deg/ presented peak structures attributed to thickness fluctuations of the QWs due to a chemical interaction that results in the substitution of Cd atoms by Zn atoms in the near-surface region during the ALE growth; this interaction has been clearly demonstrated by Auger spectroscopy. No evidence of a 3-D growth was observed from in-situ RHEED experiments. Room temperature PR spectra presented optical transitions originating in the CdTe UTQWs, the ZnTe barriers and the GaAs substrate.