THE ROLE OF THE INITIAL NUCLEATION STAGE IN MICROSTRUCTURAL DEVELOPMENT FOR CDTE GROWN ON HEAT-CLEANED 2-DEGREES-OFF (001)GAAS BY METALORGANIC CHEMICAL-VAPOR-DEPOSITION

A transmission electron microscopy (TEM) study of the initial nucleation layers for the two-step growth of CdTe on 2 degrees-off (001) GaAs substrates by metalorganic chemical vapour deposition (MOCVD) is presented. For substrates which are degreased and etched in the usual manner, the initial deposits are oriented with their axes parallel to those of the substrates and the defects are distributed isotropically. Initial deposits grown on substrates which are prepared by thermal etching at 545-585 degrees C, however, contain both (001)- and (111)-oriented grains. When these polycrystalline initial deposits are heated to the ''bulk'' growth temperature, epitaxial regrowth occurs giving a single crystal film with a very distinctive anisotropic defect microstructure with stacking faults and a mixture of 60 degrees-type and 90 degrees-type misfit dislocations parallel to the off-cut axis and subgrain boundaries and 90 degrees-type misfit dislocations in the orthogonal direction. All of the 60 degrees-type misfit dislocations and most of the stacking faults lie on the plane (111) producing a rotation of 5-8 degrees between the substrate and deposit lattices about the off-cut axis. These effects are explained in terms of differences in resolved shear stresses as secondary defects are introduced during the regrowth process.