THE METAL-ORGANIC VAPOR-PHASE EPITAXY OF ZNTE .3. CORRELATION OF GROWTH AND LAYER PROPERTIES

The crystalline structure, surface morphology, optical properties and purity of ZnTe layers grown by MOVPE were investigated. Various substr ates, different combinations of metalorganics and various growth condi tions were studied. The results of three different MOVPE growth system s and reactor cells are compared. A variety of methods were used to st udy the structure and morphology (e.g. TEM, HRTEM, X-ray diffraction, Nomarski microscopy, photo reflection, Raman scattering). The preparat ion of the GaAs and ZnTe surfaces is well advanced but problematic for GaSb. For heterostructures like ZnTe on GaAs (001) with about 7% mism atch, the crystalline structure is mainly dominated by interfacial mis fit dislocations and threading dislocations penetrating about 300 nm i nto the ZnTe layer. With better matched substrates (GaSb or ZnTe) or d ifferent orientations (GaAs (111)), the threading dislocations can nea rly be eliminated. The comparison of growth studies with MOVPE, MBE an d ALE reveals that the initial growth of ZnTe on GaAs (001) is mainly determined by the misfit and its relaxation. The first similar or equa l to 4 monolayers are characterized by a pseudomorphous, 2D growth mod e. The following surface roughening is caused by a relaxation through a 3D growth mode with islands. Once the relaxation is completed, the s urface smoothens and recovers the 2D growth mode. The initial growth s tages, 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 mu m on GaAs (001)) grown by MOVPE is solely determined by the conditions a t the growing interface (adsorption and decomposition of the precursor s, desorption of undesired species, quantities of released Zn and Te, surface stoichiometry). These interface conditions are dependent on th e precursor combination, the kinetic or mass transport limitation and the reactor hydrodynamics. The studies of the layer properties were so metimes bothered by a thin native oxide with the structure ZnTe/Te/ZnO . Purity and optical quality were tested by low temperature FL. The st rain induced by the cooling of the ZnTe/GaAs structures grown at 300-4 00 degrees C renders the identification of the radiative transitions d ifficult. However, many transitions and radiative centers are now iden tified. Under many growth conditions, the (A(0)Asl, X(l)) transition w hich is due to As from substrate autodoping, dominates the spectra. He nce, stoichiometry and inhomogeneity of the GaAs substrates are reflec ted 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 contamination s Cu, Li, O and N were found. With the alkyl combination DEZn/DIPTe, r eproducible growth of samples is possible showing PL spectra dominated by free excitons. The frequently appearing I-lc and I'(lc) transition s might be due to a V-Zn vacancy or a vacancy donor complex. Complex r elations to the growth stoichiometry were found. SIMS measurements hel ped to further identify the layer impurities. The layer purity is affe cted by extrinsic impurities due to substrate outdiffusion (As, Ga), c ontaminations from growth system (O, Cu) and precursors (Li). Impuriti es like H and C (and N) which are intrinsic to the MOVPE process, depe nd on the dissociation kinetics of the precursors which are themselves dependent on the growth parameters (e.g, temperature, carrier gas). T he origins of the extrinsic impurities are identified as far as possib le.