Band Gap Control and Properties of Indium–Zinc Oxynitride Thin Films Grown by Molecular Beam Epitaxy

Abstract

The material system of II–III oxynitride semiconductors has opened new prospects in solar energy harvesting and photocatalysis in recent years due to a tunable band gap and favorable band edge positions with respect to important redox levels. A promising member of this family is In–Zn–O–N (IZNO), as its band gap can be tailored to lower values compared to its better studied cousin Ga–Zn–O–N. We study IZNO thin films grown on sapphire substrates by molecular beam epitaxy (MBE) and investigate their structural, surface morphology, optical absorption, and photoemission characteristics. We investigate the influence of the constituting elements in the alloy on the position of valence band maxima, conduction band minima, and the structural properties. Through precise variation of the composition, samples with a band gap range between 1.0 and 2.6 eV have been deposited and analyzed. Based on our results, Zn and N have been identified to lower the energy of valence and conduction band edges with respect to the vacuum level, while In and O have the opposite effect. Structural characterization reveals that the samples are polycrystalline with grain sizes of about 30 nm, comprising a mixture of cubic and hexagonal crystal phases with distinct short-range disorder. While in the ternary compounds In–O–N and Zn–O–N metal–oxide bonds are dominant, we elucidate the formation of metal–oxynitride bonds in IZNO. Electrical and optical measurements reveal charge carrier concentrations of 1018–1020 cm–3 and absorption coefficients of 105 cm–1 above about 2 eV excitation energy, accompanied by pronounced free carrier absorption found in samples in the upper carrier concentration range in the infrared energy region. Typical Urbach energies are 80–220 meV, with no clear correlation with the elemental composition. By introducing MBE growth for IZNO, we overcome the limitations typically inflicted by the fabrication methods on stoichiometric InN:ZnO solid solutions and provide unprecedented access to new compounds in this material class.