This Topical Section in physica status solidi (a) “Emerging Semiconducting Oxides” features 12 peer-reviewed articles and is related to the DPG Spring Meeting 2013 in Regensburg hosting the Focus Session on “Crystalline n-type semiconducting oxides – SnO2, Ga2O3, and In2O3 for novel devices”. The recent development of considering binary semiconducting oxides as wide-band gap semiconductors in their own right sets a focus on the semiconducting properties that open up the path to novel applications as active materials in semiconductor devices.
The Invited Article by M. Higashiwaki et al. reports the state-of-the-art and gives a clear perspective for Ga2O3-based power electronics , but also the less well known Bi2O3 has been investigated as potential absorber material for photovoltaic applications by J. Morasch et al. .
Semiconducting applications require a high degree of crystal quality and purity similar to established semiconductor materials (e.g. Si, GaAs, GaN) which is higher than that usually required for TCO or sensor applications. This quality is achieved by suitable material synthesis using bulk growth methods to realize pure and single crystalline substrates and thin-film epitaxy by MOVPE/MOCVD or MBE. The availability of bulk oxide substrates for homoepitaxial growth ensures the highest quality thin films for devices. To this end, the bulk growth of high-quality single-crystalline SnO2 is described by Z. Galazka et al. , whereas homoepitaxy towards high-quality Ga2O3 by MOVPE is described by G. Wagner et al. .
Controlled conductivity and doping is another key ingredient to successfully implement semiconductor applications such as power electronics. S. Müller et al. demonstrate the systematic conductivity control in Si-doped Ga2O3 by PLD growth conditions . O. Bierwagen and J. S. Speck investigated systematic Sn-doping of In2O3 by MBE, its compensation by point defects, and its doping limits . Structural defects arising from excess Ga-doping of SnO2 and their implication on conductivity have been studied by A. Mogalitenko et al. .
Electrical contacts are a crucial part for electronic devices, including power electronics. D. Splith et al. studied Cu-Schottky contacts on Ga2O3  that could serve as control gate for high-power MESFETs.
Besides directly application-relevant aspects, fundamental physics plays an important role in understanding semiconducting oxides. To this end the exploration of material properties by advanced theory and the experimental determination of these properties in material of highest quality play a major role. As an example, the theoretical contribution by C. Rödl and A. Schleife gives effective masses and photoemission spectra of different n- and p-type semiconducting oxides calculated from first principles . In the case of SnO2, their results on the non-parabolicity of the anisotropic effective electron mass are compared to the experimental results obtained by M. Feneberg et al. who used infrared ellipsometry on single-crystalline SnO2 films controllably doped to different electron concentrations . Single-crystalline bulk In2O3 was investigated by optical spectroscopy by K. Irmscher et al. showing that the optical absorption at 2.7 eV is intrinsic and likely due to an indirect band gap . Point defects play a critical role in semiconducting oxides. One approach taken towards this issue is the scanning tunneling spectroscopy study of single-crystalline In2O3 by D. Braun et al. relating electronic states in the fundamental band gap to point defects .
We hope that the reader enjoys this selection of articles and may pick up a flavor of the current developments and open issues in this rapidly developing field of oxide semiconductors.
Paul-Drude-Insitut für Festkörperelektronik, Berlin, Germany
Saskia F. Fischer
Humboldt-Universität zu Berlin, Germany