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In order to study the impact of negative oxygen ion bombardment on the electronic transport properties of ZnO:Al films, a systematic magnetron sputtering study from ceramic targets with excitation frequencies from DC to 27 MHz, accompanied by strongly varying discharge voltages, has been performed. Higher plasma excitation frequencies significantly improve the transport properties of ZnO:Al films. The effect of the bombardment of the films by energetic particles (negative oxygen ions) can be explained by the dynamic equilibrium between the formation of acceptor-like oxygen interstitials compensating the extrinsic donors and the self-annealing of the interstitial defects at higher deposition temperatures.

The origin of the pronounced radial distributions of structural and electrical properties of magnetron sputtered ZnO:Al films has been investigated. The film properties were correlated with the radially resolved ion-distribution functions. While the positive ions exhibit low energies and a radial distribution with a maximum intensity opposite the center of the target, the negative ions can have energies up to several hundred eV, depending on the target potential, with a radial distribution with two maxima opposite the erosion tracks. The most prominent positive ion is that of the working gas (Ar+), while the highest flux of the negative ions is measured for negative oxygen O−. The radial distribution of the flux of the high-energetic negative ions can clearly be related to the radial variations of the structural (c-axis lattice parameter, crystallite size) and electronic (resistivity) properties for sputtering from the planar target, which points to the decisive role of the high-energetic negative oxygen ions for the film quality. The relation between the negative ion bombardment and the structural as well as electronic properties can be explained by a qualitative model recently developed by us. The same target has also been investigated in the eroded state. In this case, the limited acceptance angle of the mass spectrometer leads to a misinterpretation of the radial distribution of the flux of the high-energetic negative ions. This effect can be explained by a simulation, based on the assumption that the high-energetic negative ions are mainly accelerated in the cathode (target) sheath perpendicular to the uneven substrate surface.

The prospects of scaling current photovoltaic technologies to terawatt levels remain uncertain. All-oxide photovoltaics could open rapidly scalable manufacturing routes, if only oxide materials with suitable electronic and optical properties were developed. A potential candidate material is tin monoxide (SnO), which has exceptional doping and transport properties among oxides, but suffers from a low absorption coefficient due to its strongly indirect band gap. Here, we address this shortcoming of SnO by band-structure engineering through isovalent but heterostructural alloying with divalent cations (Mg, Ca, Sr, Zn). Using first-principles calculations, we show that suitable band gaps and optical properties close to that of direct semiconductors are achievable in such SnO based alloys. Due to the defect tolerant electronic structure of SnO, the dispersive band-structure features and comparatively small effective masses are preserved in the alloys. Initial Sn 1−x Zn x O thin films deposited by sputtering exhibit crystal structure and optical properties in accord with the theoretical predictions, which confirms the feasibility of the alloying approach. Thus, the implications of this work are important not only for terawatt scale photovoltaics, but also for other large-scale energy technologies where defect-tolerant semiconductors with high quality electronic properties are required.

An analytical description of the charge carrier transport, valid for non-degenerated and degenerated semiconductors, was developed, critically reviewed, and fitted to the temperature-dependent Hall mobility data of magnetron sputtered, degenerately doped ZnO:Al films. Our extended model for grain boundary scattering in semiconductors of arbitrary degeneracy is based on previous models from literature and suitable to describe the Hall mobility of the carriers as a function of the free carrier concentration and the temperature at the same time. It is mathematically simple enough for a fast fit procedure, which is not possible with most of the previous models. Applying a combined transport model consisting of ionized impurity scattering, phonon scattering, and grain boundary scattering in degenerate semiconductors, we were able to determine the trap density at the grain boundaries Nt ≈ 3 × 1013 to 5 × 1013 cm−2 and the deformation potential Eac in the range of 5 eV to 9 eV depending on the details of the transport model.

Homoepitaxial and heteroepitaxial ZnO, ZnO:Al, and Zn1-xMgxO:Al films have been grown by magnetron sputtering from ceramic targets at substrate temperatures between 200 °C and 500 °C. We studied the relation between the electronic transport and structural properties for the epitaxially grown films and compared it to the properties of polycrystalline films by means of X-ray diffraction, transmission electron microscopy and optical reflectance and transmittance measurements. The results show that the epitaxial growth of ZnO:Al and Zn1-xMgxO:Al thin films, which has been observed for nearly all films prepared on single crystalline substrates, will not significantly improve the electronic transport properties in comparison to polycrystalline films unless the grain boundaries are eliminated completely. The grain boundary defect densities of about 3 × 1013 cm−2 are nearly independent on the structural quality of the different polycrystalline, hetero- and homoepitaxial films. This clearly proves that the grain boundary defects are not caused by crystallographic defects, but, most probably, by the dopant aluminium.

We report on design of optoelectronic properties in previously unreported metastable tin titanium nitride alloys with spinel crystal structure. Theoretical calculations predict that Ti alloying in metastable Sn 3 N 4 compound should improve hole effective mass by up to 1 order of magnitude, while other optical bandgaps remains in the 1−2 eV range up to x ∼ 0.35 Ti composition. Experimental synthesis of these metastable alloys is predicted to be challenging due to high required nitrogen chemical potential (Δμ N ≥ +1.0 eV) but proven to be possible using combinatorial cosputtering from metal targets in the presence of nitrogen plasma. Characterization experiments confirm that thin films of such (Sn 1−x Ti x ) 3 N 4 alloys can be synthesized up to x = 0.45 composition, with suitable optical band gaps (1.5−2.0 eV), moderate electron densities (10 17 to 10 18 cm −3 ), and improved photogenerated hole transport (by 5×). Overall, this study shows that it is possible to design the metastable nitride materials with properties suitable for potential use in solar energy conversion applications.

The spatial distribution of Al in magnetron sputtered ZnO:Al films has been investigated in depth. Two different kinds of inhomogeneities were observed: an enrichment in the bulk of the film and an enrichment at the interface to the substrate. This has been correlated to the electrical properties of the films: the former inhomogeneities can lead to trap states at the grain boundaries limiting the free carrier mobility. The latter can promote the formation of secondary phases, which leads to an electrical inactivation of the dopant. Furthermore, this effect can contribute to the thickness dependence of the electrical properties of ZnO:Al films.