Transmission electron microscopy characterization of II-VI semiconductor compounds grown by pulsed laser deposition

ZnTe and ZnSe films grown on (001) GaAs substrates by pulsed laser deposition (PLD) were characterized by transmission electron microscopy (TEM). The microstructure and electrical properties of the films were determined and correlated with growth parameters such as laser energy density, partial pressure of the background gas, gas flow rate, and substrate cleaning method. Stacking faults, dislocations, and columnar structures were examined by cross-section TEM. The degree of epitaxiality between film and substrate was determined by high resolution TEM analysis of the interfaces. The film microstructure is a strong function of the gas pressure in the growth chamber during deposition. A background pressure of 50 mtorr or less of N₂ resulted in single crystalline ZnTe films, with stacking faults as the main observable crystalline defect. Growth pressures of 88 or 100 mtorr of N₂ give rise to columnar grains that propogate from an initial layer of heavily faulted single crystal. Evidence of cluster deposition was found in the ZnTe films grown in high pressures of N₂. A similar columnar microstructure was observed in a ZnSe film grown in 1000 mtorr of a He/Cl₂ atmosphere, but no clusters were observed. Growth conditions are responsible for film microstructure rather than the degree of lattice mismatch between the film and substrate. The film-substrate interfaces were semicoherent, with interfacial misfit dislocations spaced so as to accommodate the film-substrate lattice parameter mismatch. The density of stacking faults initiating at the interface was found to be a function of the method used to clean the substrate surface prior to film growth rather than the lattice mismatch. A more effective removal of oxides led to decreased interfacial stacking fault densities. The electrical properties of ZnTe films were significantly improved by rapid thermal annealing (RTA). Point defect annihilation is the most likely cause of this improvement.