Hydrogen and related defects in oxide semiconductors

Marianne Tarun

Oxide semiconductors are an interesting class of materials which occur in a variety of crystal structures and exhibit diverse electronic and optical properties. This thesis is primarily focused on investigating defects in zinc oxide (ZnO) and strontium titanate (SrTiO3), two important wide-bandgap oxide semiconductors. While zinc oxide is a promising material for blue and UV solid-state lighting devices, the lack of p-type doping has precluded ZnO from becoming a dominant material for optoelectronic applications. Over the past decade, numerous reports have claimed that nitrogen is a feasible p-type dopant in ZnO. In this work, nitrogen-doped bulk, single crystal ZnO grown by chemical vapor transport is investigated. Photoluminescence (PL) measurements show broad red luminescence band at ~1.7 eV, with an excitation threshold of ~2.2 eV. This result is in agreement with the calculated configuration-coordinate diagram for nitrogen deep acceptor [Appl. Phys. Lett. 95, 252105 (2009)] that shows an acceptor level of 1.3 eV above the valence band maximum. Comparing the PL spectra for the undoped and N-doped samples confirms the red PL band is correlated with nitrogen acceptors. The intensity of this band increases with the concentration of activated nitrogen acceptors. The deep-acceptor behavior of nitrogen can be explained by the low position of the ZnO valence band maximum relative to vacuum. Hydrogen is a ubiquitous impurity in oxide semiconductors and a potential source of n-type conductivity. Despite its prevalence, little is known about its behavior in oxides such as SrTiO3. Infrared (IR) absorption spectroscopy shows a hydrogen-related defect in SrTiO3 with local vibrational modes (LVMs) at 3355 and 3384 cm-1 at liquid-helium temperatures. Isotope substitution experiments reveal that the defect consists of two hydrogen atoms bound to host oxygen atoms. From the temperature dependence of the LVMs, the complex is ascribed to a strontium vacancy passivated by two hydrogen atoms. The thermal stability of the defect is determined through a series of isochronal annealing experiments on the hydrogenated SrTiO3 sample. These measurements provide evidence of "hidden hydrogen," possibly H2 molecules, in the crystal. Remarkably, in ZnO, a much less ionic crystal, hydrogen also passivates zinc vacancies [Phys. Rev. B 66, 165205 (2002)] and form H2 molecules [Appl. Phys. Lett. 85, 5601 (2004)]. It is therefore suggested that a "universal behavior" of hydrogen exists across a diverse range of oxides.

Hydrogen and related defects in oxide semiconductors

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