Sources of n‑Type Conductivity in Single Crystal β‑Ga₂O₃ and Monoclinic WO₃ Nanoparticles

The source of the commonly observed unintentional n-type conductivity in wide-band-gap oxides is controversial. For high-quality β-Ga₂O₃ single crystals, the physical characteristics of H-containing defects are studied by solid-state nuclear magnetic resonance (ssNMR) and Fourier transform infrared (FT-IR) spectroscopy. NMR observation demonstrated that the hydrogen atoms occur exclusively in the positive charge state, and three trapped protons are housed in two “half vacancies” left by the Ga vacancy to form a V_Ga–H₂₊₁ complex. Protons trapped in the vacancies are hardly ever able to diffuse out due to the strong hydrogen bonds they establish, even at relatively high temperatures. The hydrogen atoms in one of the two “half vacancies” are the bridged protons in Ga–O­(H₂)–Ga bonds, the hydrogen atoms can readily donate electrons to the conduction band, and these hydrogen atoms become the shallow donors. The V_Ga–H₂₊₁ complex provides high-quality β-Ga₂O₃ single crystals with the extra-stability and the n-type conductivity. For monoclinic WO₃ nanoparticles, NMR, FT-IR, and EPR results show that the bridged proton in W–O­(H₂)–W groups was assumed to be responsible for the so-called “hidden” hydrogen, which can be converted into a shallow donor in the course of sample processing. Our results provide a guide to more refined experimental studies of point defects and impurities in wide-band-gap oxides and their influence on the control of n-type conductivity.