Copper‐oxide compound semiconductors provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from bandgap energies of 2.1 eV to the infrared at 1.40 eV, i.e., right into the middle of the efficiency maximum for solar‐cell applications. Three distinctly different phases, Cu2O, Cu4O3, and CuO, of this binary semiconductor can be prepared by thin‐film deposition techniques, which differ in the oxidation state of copper. Their material properties as far as they are known by experiment or predicted by theory are reviewed. They are supplemented by new experimental results from thin‐film growth and characterization, both will be critically discussed and summarized. With respect to devices the focus is on solar‐cell performances based on Cu2O. It is demonstrated by photoelectron spectroscopy (XPS) that the heterojunction system p‐Cu2O/n‐AlGaN is much more promising for the application as efficient solar cells than that of p‐Cu2O/n‐ZnO heterojunction devices that have been favored up to now.

We present a simple method for the elimination of cracks in GaN layers grown on Si (111). Cracking of GaN on Si usually occurs due to large lattice and thermal mismatch of GaN and Si when layer thicknesses exceeds approximately 1 µm. By introducing thin, low-temperature AlN interlayers, we could significantly reduce the crack density of the GaN layer. The crack density is practically reduced to zero from an original crack density of 240 mm-2 corresponding to crack-free regions of 3×10-3 mm2. Additionally for the GaN layer with low temperature interlayers, the full width at half maximum X-ray (2024) rocking curve is improved from approximately 270 to 65 arcsec.

Spatially separated ZnO pillars, typically 300 nm in diameter and 2 microm in height, are fabricated via a template-directed approach that leads to long-range hexagonal order. The templates of Au nanodisk arrays are obtained by using metal membranes as a lithography mask. The growth of ZnO pillars is performed in a double-tube system through vapor diffusion-deposition. The growth mechanism of the pillars is studied in detail and is proposed to be a combination of vapor-liquid-solid and vapor-solid models. The piezoelectric and optical properties of single pillars are characterized using piezoresponse force microscopy and micro-photoluminescence spectroscopy, respectively. The pillars show strong excitonic emissions up to room temperature, which indicate a relatively low defect density and good crystalline quality. The obtained piezoelectric coefficient d(33) is (7.5+/-0.6) pm V(-1), which is to our knowledge the first reported value for a single nanopillar.

We report on GaN n-type doping using silane, germane, and isobutylgermane as Si and Ge dopants, respectively. A significant increase in tensile stress during growth is observed for Si doped samples while this is not the case for Ge doping. In addition, Ge can be doped up to 2.9 Â 10 20 cm À3 , while Si doping leads to 3-D growth already at concentrations around 1.9 Â 10 19 cm À3. The free carrier concentration was determined by Hall-effect measurements, crystal quality, and structural properties by x-ray diffraction measurements. Additionally, secondary ion mass spectroscopy and Raman measurements were performed demonstrating the high material quality of Ge doped samples. V

No abstract