NiO is a p-type wide bandgap semiconductor of use in various electronic devices ranging from solar cells to transparent transistors. This work reports the controlling of conductivity and increase of work functions by Li doping.

As a contribution to (transparent) bipolar oxide electronics, vertical pn heterojunction diodes were prepared by plasma-assisted molecular beam epitaxy of unintentionally doped p-type SnO layers with hole concentrations ranging from p=1018 to 1019 cm−3 on unintentionally doped n-type β-Ga2O3(−201) substrates with an electron concentration of n=2.0×1017 cm−3. The SnO layers consist of (001)-oriented grains without in-plane epitaxial relation to the substrate. After subsequent contact processing and mesa-etching (which drastically reduced the reverse current spreading in the SnO layer and associated high leakage), electrical characterization by current–voltage and capacitance–voltage measurement was performed. The results reveal a type-I band alignment and junction transport by thermionic emission in forward bias. A rectification of 2×108 at ±1 V, an ideality factor of 1.16, a differential specific on-resistance of 3.9 m Ω cm2, and a built-in voltage of 0.96 V were determined. The pn-junction isolation prevented parallel conduction in the highly conductive Ga2O3 substrate during van-der-Pauw Hall measurements of the SnO layer on top, highlighting the potential for decoupling the p-type functionality in lateral transport devices from that of the underlying n-type substrate. The measured maximum reverse breakdown voltage of the diodes of 66 V corresponds to a peak breakdown field of 2.2 MV/cm in the Ga2O3-depletion region and suggests the low bandgap of the SnO (≈0.7 eV) not to be the limiting factor for breakdown. Higher breakdown voltages that are required in high-voltage devices could be achieved by reducing the donor concentration in the β-Ga2O3 toward the interface to increase the depletion width, as well as improving the contact geometry to reduce field crowding.

NiO layers were grown on MgO(100), MgO(110) and MgO(111) substrates by plasma-assisted molecular beam epitaxy under Ni-flux limited growth conditions. Single crystalline growth with a cube-on-cube epitaxial relationship was confirmed by X-ray diffraction measurements for all used growth conditions and substrates except MgO(111). A detailed growth series on MgO(100) was prepared using substrate temperatures ranging from 20°C to 900°C to investigate the influence on the layer characteristics. Energy-dispersive X-ray spectroscopy indicated close-to-stoichiometric layers with an oxygen content of ≈ 47 at.% and ≈ 50 at.% grown under low and high O-flux, respectively. All NiO layers had a root-mean-square surface roughness below 1 nm, measured by atomic force microscopy, except for rougher layers grown at 900°C or using molecular oxygen. Growth at 900°C led to a significant diffusion of Mg from the substrate into the film. The relative intensity of the quasi-forbidden one-phonon Raman peak is introduced as a gauge of the crystal quality, indicating the highest layer quality for growth at low oxygen fluxes and high growth temperature, likely due to the resulting high adatom diffusion length during growth. Optical and electrical properties were investigated by spectroscopic ellipsometry and resistance measurements, respectively. All NiO layers were transparent with an optical band gap around 3.6 eV and semi-insulating at room temperature. However, changes upon exposure to reducing or oxidizing gases of the resistance of a representative layer at elevated temperature was able to confirm p-type conductivity, highlighting their suitability as a model system for research on oxide-based gas sensing.