Effect of Titanium Induced Chemical Inhomogeneity on Crystal Structure, Electronic Structure, and Optical Properties of Wide Band Gap Ga₂O₃

Abstract

Tailoring the optical and electronic properties of wide band gap β-Ga2O3 has been of tremendous importance to utilize the full potential of the material in current and emerging technological applications in electronics, optics, and optoelectronics. In the present work, we report the effect of Ti-dopant insolubility driven chemical inhomogeneity on the structural, morphological, chemical bonding, electronic structure, and band gap red shift characteristics in Ga2O3 polycrystalline compounds. Ga2–2xTixO3 (GTO; 0 ≤ x ≤ 0.20) compounds were synthesized using a conventional high-temperature solid state reaction route under variable calcination temperatures (1050–1250 °C) while sintering was performed at 1350 °C. X-ray diffraction analysis of GTO samples reveals that the formation of single-phase compounds occurs only at a very low concentration of Ti doping (<5 at. %), whereas higher Ti doping results in composite formation with a significant undissolved TiO2 rutile phase. However, in sintered samples, fraction of undissolved rutile phase transformed into monoclinic TiO2. Rietveld refinement of intrinsic Ga2O3 and single-phase Ti-doped compound (x = 0.05) confirms that samples are stabilized in monoclinic symmetry with C2/m space group. Surface morphologies of samples reveal that intrinsic Ga2O3 exhibits rod shaped morphology, while Ti-doped compounds exhibit spherical morphology. Moreover, in doped compounds with abnormal grain growth, lattice twinning induced striations were noted in contrast to intrinsic Ga2O3. High-resolution X-ray photoelectron spectroscopic analysis of Ga 2p shows a positive shift compared to metallic Ga due to interaction between the electron cloud of adjacent ions. Ti 2p1/2 spectra show anomalous broadening due to the Coster–Kronig effect. First-principles calculations using hybrid density functional theory show that Ti preferentially substitutes on octahedral Ga sites and that it behaves as a deep donor in Ga2O3. From the optical absorption spectra, a red shift in the optical band gap is observed. Absorption within the band gap of Ga2O3 is attributed to the inclusion of undissolved TiO2, as TiO2 has a type I alignment within the gap of Ga2O3. In addition, the electrocatalytic behavior of GTO compounds was examined. From electrocatalytic studies it is evident that doped compounds exhibit appreciable electrocatalytic activity in contrast to intrinsic Ga2O3.