7268. Ion irradiation damage in noncrystalline FeCo thin films: J P Riviere et al,Radiat Eff, 114, 1990, 145–155

Abstract

The crystalline structure, surface morphology, optical properties and purity of ZnTe layers grown by MOVPE were investigated. Various substrates, different combinations of metalorganics and various growth conditions were studied. The results of three different MOVPE growth systems and reactor cells are compared. A variety of methods were used to study the structure and morphology (e.g. TEM, HRTEM, X-ray diffraction, Nomarski microscopy, photo reflection, Raman scattering). The preparation of the GaAs and ZnTe surfaces is well advanced but problematic for GaSb. For heterostructures like ZnTe on GaAs (001) with about 7% mismatch, the crystalline structure is mainly dominated by interfacial misfit dislocations and threading dislocations penetrating about 300 nm into the ZnTe layer. With better matched substrates (GaSb or ZnTe) or different orientations (GaAs (111)), the threading dislocations can nearly be eliminated.The comparison of growth studies with MOVPE, MBE and ALE reveals that the initial growth of ZnTe on GaAs (001) is mainly determined by the misfit and its relaxation. The first ≈4 monolayers are characterized by a pseudomorphous, 2D growth mode. The following surface roughening is caused by a relaxation through a 3D growth mode with islands. Once the relaxation is completed, the surface smoothens and recovers the 2D growth mode. The initial growth stages, representing the Stranski-Krastanov mechanism, are dependent on the surface termination of the substrate and on the growth method and parameters. The surface morphology of thicker ZnTe layers (>0.1 μm on GaAs (001)) grown by MOVPE is solely determined by the conditions at the growing interface (adsorption and decomposition of the precursors, desorption of undesired species, quantities of released Zn and Te, surface stoichiometry). These interface conditions are dependent on the precursor combination, the kinetic or mass transport limitation and the reactor hydrodynamics.The studies of the layer properties were sometimes bothered by a thin native oxide with the structure ZnTe/Te/ZnO.Purity and optical quality were tested by low temperature PL. The strain induced by the cooling of the ZnTe/GaAs structures grown at 300–400°C renders the identification of the radiative transitions difficult. However, many transitions and radiative centers are now identified. Under many growth conditions, the (A0Asl, X1) transition which is due to As from substrate autodoping, dominates the spectra. Hence, stoichiometry and inhomogeneity of the GaAs substrates are reflected in the spectra. But also precursor combination, partial pressures and growth temperatures have a significant influence on the PL spectra. The substrate type (GaAs, GaSb, ZnTe) is reflected in the spectra by transitions due to outdiffusion and by the Y-lines which are related to the misfit dislocations. Transitions induced by layer contaminations like Cu, Li, O and N were found. With the alkyl combination DEZn/DIPTe, reproducible growth of samples is possible showing PL spectra dominated by free excitons. The frequently appearing I1c and I'1c transitions might be due to a VZn vacancy or a vacancy donor complex. Complex relations to the growth stoichiometry were found.SIMS measurements helped to further identify the layer impurities. The layer purity is affected by extrinsic impurities due to substrate outdiffusion (As, Ga), contaminations from growth system (O, Cu) and precursors (Li). Impurities like H and C (and N) which are intrinsic to the MOVPE process, depend on the dissociation kinetics of the precursors which are themselves dependent on the growth parameters (e.g. temperature, carrier gas). The origins of the extrinsic impurities are identified as far as possible.