Making a systematic effort, we have developed a single-crystalline ZnSnN2 on ZnO (0001) by reactive magnetron co-sputtering. Epitaxial growth was achieved at 350°C by co-sputtering from metal targets in nitrogen...

Nickel oxide, in particular in its doped, semiconducting form, is an important component of several optoelectronic devices. Doping NiO is commonly achieved either by incorporation of lithium, which readily occupies Ni sites substitutionally, producing the Li Ni acceptor, or by supplying reactive oxygen species during NiO film deposition, which leads to the formation of Ni vacancies (V Ni ). However, the energetic position of these acceptors in the NiO band gap has not been experimentally determined until today. In this work, we close this knowledge gap by studying rectifying n ++ p heterojunctions of NiO on top of fluorine-doped tin oxide. These structures show sufficient rectification to perform electric characterization by defect spectroscopic techniques, specifically capacitance-voltage and thermal admittance spectroscopy. Using these methods, the (0/−) charge transition levels are determined to be 190 meV and 409 meV above the valence band edge for the Li Ni and the V Ni acceptor, respectively.

Highly crystalline thin films of (ZnO) 1−x (GaN) x were synthesised using RF magnetron sputtering, with x ranging from 0 to 0.20. The band gap of the alloys showed, as estimated, a significant reduction down to ∼2.5 eV for x > 0.07, by employing UV-VIS transmission measurements and electron energy loss spectroscopy, compared to the band gap energies of the two host materials, i.e. E ZnO g = 3.37 eV and E GaN g = 3.51 eV. The reduced band gap results in an extension of the absorption for the alloys well into the visible part of the spectrum. Structural analysis, utilizing x-ray diffraction, Rutherford backscattering spectrometry and transmission electron microscopy, yielded highly crystalline films, with columnar grains and a good heteroepitaxial relation to the Al 2 O 3 substrate. The unit cell of the alloys was found to be rotated 30 • with respect to the that of the substrate, in order to minimize the lattice mismatch to the substrate. An increase in c-lattice constant as a function of GaN content (x) was found, opposite to that predicted by Vegard's law, and explained in terms of strain, as well a high density of threading dislocations. The effects of thermal annealing in N 2 atmosphere after growth were analysed, both experimentally and using computational calculations employing density functional theory. Optically no large effects were found, especially in the estimated band gap energies. In terms of crystal structure, an increase in grain size was detected, reduced strain and c-lattice parameters approaching the expected values from Vegard's law, reduced dislocation density and an overall increase in crystalline quality. On the other hand, a systematic peak-broadening of the (0002) x-ray diffraction reflection was detected, attributed to an increase in Ga-N bonds. Moreover, for the films with higher x, an interfacial layer with a higher Ga-content compared to the remaining film was detected, attributed to the formation of zinc blende phases resulting from the accumulation of stacking faults. Nano-sized voids consisting of molecular N 2 were also found after post-deposition annealing, where the formation of voids was attributed to the agglomeration of Zn-and Ga-vacancies. The filling of voids with molecular N 2 was found to be a stabilization mechanism for the vacancy clusters, indicating that N is not stable in the O substitutional site. Finally, a deeper investigation of the mechanisms governing the band bowing effect in the (ZnO) 1−x (GaN) x alloys was undertaken, combining experimental and computational results. The results revealed the formation of a GaN-like defect band above the valence band maximum of ZnO to be the cause of the reduced band gap, oppositely to the explanation used in the literature, with orbital repulsions within the valence band, pushing the valence band maximum upward in energy.

It is known that (ZnO) 1−x (GaN) x alloys demonstrate remarkable energy band bowing, making the material absorb in the visible range, in spite of the binary components being classical wide band gap semiconductors. However, the origin of this bowing is not settled; two major mechanisms are under debate: Influence of the orbital repulsion and/or formation of a defect band. In the present work, we applied a combination of the absorption and emission measurements on the samples exhibiting an outstanding nanoscale level of (ZnO) 1−x (GaN) x homogeneity as monitored by the high resolution electron microscopy equipped with the energy dispersive x-ray analysis and the electron energy loss spectroscopy; moreover the experimental data were set in the context of the computational analysis of the alloys employing density functional theory and quasiparticle GW approximation. A prominent discrepancy in the band gap values as deduced from the absorption and emission experiments was observed systematically for the alloys with different compositions and interpreted as evidence for the absorption gap shrinking due to the defect band formation. Computational data support the argument, revealing only minor variations in the bulk of the conduction and valence band structures of the alloys, except for a characteristic "tail" in the vicinity of the valence band maximum. As such, we conclude that the energy gap bowing in (ZnO) 1−x (GaN) x alloys is due to the defect band formation, presumably at the top of the valence band maximum.