Investigation of structure and growth on polar surfaces of wide-bandgap semiconductors

GaN and ZnO show promise for deep hole injection into pentacene films since their valance bands lie below the highest occupied molecular orbital of pentacene. To understand heterojunctions between pentacene and either inorganic material, both the clean surface properties of the inorganic as well as the crystalline structure of pentacene must be understood. We have investigated the clean surfaces of GaN and ZnO as well as the effect of deposition conditions on the structure and size of pentacene crystallites.;For Ga-polar GaN surfaces we examined the Ga-N site exchange critical to the autosurfactant effect during molecular beam epitaxy of GaN. On the GaN(0001) pseudo-(1 x 1), the first site exchange results in N incorporation at subsurface T1 sites, forming ghost islands. The second exchange that converts these islands to bilayer height can be triggered by continued scanning tunneling microscopy imaging. This process frees the second-layer Ga atoms, allowing N to form covalent Ga-N-Ga bonds of a new GaN bilayer.;Low energy electron diffraction shows a (√3 x √3)R30° reconstruction on O-polar ZnO surfaces after a tube furnace annealing process used to create atomic-height steps, and in situ under conditions having an extremely low hydrogen background. X-ray photoelectron spectroscopy suggests H stabilizes the (1 x 1) termination that has been extensively reported. The (√3 x √3)R30° reconstruction is stable against H 2, N2, and air, although its formation is suppressed when preparation occurs under a H2 background. Therefore, caution must be taken when using annealed O-polar ZnO substrates, as reconstruction may affect the growth interface.;Finally, the dependence that deposition conditions have on pentacene crystallite structure and size was studied. The results show that, for pentacene films up to three monolayers thick, all deposition rates result in the thin-film phase even when the substrate temperature is raised to 65°C. When deposition occurs at room temperature the average crystallite size is increased with a decrease in deposition rate. These results suggest that, for the films studied, deposition conditions have no effect on promoting the bulk-phase, but that it is possible to increase the crystallite size by depositing pentacene at slower rates.