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

  • Mallesham Bandi*
    Mallesham Bandi
    Center for Advanced Materials Research (CMR), University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, United States
    *Email: [email protected] (M.B.).
  • Vishal Zade
    Vishal Zade
    Center for Advanced Materials Research (CMR), University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, United States
    More by Vishal Zade
  • Swadipta Roy
    Swadipta Roy
    Center for Advanced Materials Research (CMR), University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, United States
    Department of Metallurgical, Materials and Biomedical Engineering, University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, United States
    Environmental Molecular Sciences Laboratory (EMSL), Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States
    More by Swadipta Roy
  • Aruna N. Nair
    Aruna N. Nair
    Department of Chemistry, University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, United States
  • Sierra Seacat
    Sierra Seacat
    Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, United States
  • Sreeprasad Sreenivasan
    Sreeprasad Sreenivasan
    Department of Chemistry, University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, United States
  • V. Shutthanandan
    V. Shutthanandan
    Environmental Molecular Sciences Laboratory (EMSL), Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States
  • Chris G. Van de Walle
    Chris G. Van de Walle
    Materials Department, University of California, Santa Barbara, California 93106-5050, United States
  • Hartwin Peelaers
    Hartwin Peelaers
    Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, United States
  • , and 
  • C. V. Ramana*
    C. V. Ramana
    Center for Advanced Materials Research (CMR), University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, United States
    *Email: [email protected] (C.V.R.).
    More by C. V. Ramana
Cite this: Cryst. Growth Des. 2020, 20, 3, 1422–1433
Publication Date (Web):January 23, 2020
https://doi.org/10.1021/acs.cgd.9b00747
Copyright © 2020 American Chemical Society
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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.

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