Heteroepitaxial films of Ga 2 O 3 were grown on c-plane sapphire (0001). The stable phase β-Ga 2 O 3 was grown using the metalorganic chemical vapor deposition technique, regardless of precursor flow rates, at temperatures between 500 • C and 850 • C. Metastable α-and ε-phases were grown when using the halide vapor phase epitaxy (HVPE) technique, at growth temperatures between 650 • C and 850 • C, both separately and in combination. XTEM revealed the better lattice-matched α-phase growing semi-coherently on the substrate, followed by ε-Ga 2 O 3 . The epitaxial relationship was determined to be [1100] IMPACT STATEMENTThis study demonstrates one of the first epitaxial growths of multiple polymorphs of Ga 2 O 3 on sapphire (0001) substrates, including its β-, α-, and ε-phases. Epitaxial relationship is confirmed through HRTEM. ARTICLE HISTORY

The Schottky barriers of Ti, Mo, Co, Ni, Pd, and Au on (100) β-Ga2O3 substrates were analyzed using a combination of current-voltage (J-V), capacitance-voltage (C-V), and current-voltage-temperature (J-V-T) measurements. Near-ideal, average ideality factors for Ti, Mo, Co, and Ni were 1.05–1.15, whereas higher ideality factors (∼1.3) were observed for Pd and Au contacts. Barrier heights ranging from 0.60 to 1.20 eV were calculated from J-V measurements for the metals with low ideality factors. C-V measurements of all Schottky metals were conducted and yielded average barrier heights ranging from 0.78 to 1.98 eV. J-V-T measurements of Ti and Co diodes yielded barrier heights of 0.81 and 1.35 eV, respectively. The results reveal a strong positive correlation between the calculated Schottky barrier heights and the metal work functions: the index of interface behavior, S = 0.70, 0.97, and 0.81 for J-V, C-V, and J-V-T data, respectively.

Over the last decade, beta-phase gallium oxide (β-Ga2O3) has developed an extensive interest for applications such as high-power electronics. Due to its ultrawide bandgap of ∼4.8 eV and predicted breakdown field of ∼8 MV/cm along with its ability to be grown from the melt, this material demonstrates immense promise for high-voltage switching. The pace of development for β-Ga2O3 over these past ten years has been rapid, and ample new information has been generated on metal/β-Ga2O3 interfaces. With the advent of high-quality melt-grown substrates and epitaxial layers, low ideality factors (<1.1) and high Schottky barrier heights (>2.2 eV) have been demonstrated for Schottky contacts. For Ohmic contacts, specific contact resistivities of the common Ti/Au metallization are routinely reported in the 10−5–10−6 Ω cm2 range. This critical and comprehensive review focuses on the fundamental physics of Ohmic and Schottky contacts to bulk and epitaxial β-Ga2O3 in the published literature. It centers on the influence of surface treatments and defects on electrical contacts, Ohmic contacts, and Schottky contacts to β-Ga2O3. Native upward band bending is observed on β-Ga2O3, and the influence of wet and dry etching on band bending along with Schottky barrier height and ideality factor is discussed. Work on Ohmic contacts concentrates on the conventional Ti/Au anneal but additional Ohmic metallizations such as conductive oxides and others are treated as well. Schottky contacts are examined with specific focus on Fermi level pinning, thermal stability of Schottky metallizations, and Schottky barrier inhomogeneity.

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In this study, electrical properties of four metals (W, Mo, Au, Ni) as Schottky contacts on n-type (100)-oriented β-Ga2O3 substrates grown by the Czochralski method are reported. The Schottky barrier heights for each metal contact were calculated from I-V and/or C-V measurements. Two methods were used to cross check the Schottky barrier heights (φB) and ideality factors (n) calculated from I-V measurements. The Schottky barrier height values calculated from C-V and I-V measurements showed excellent agreement with each other and increased with an increase in the metal work functions. Some anomalous behavior of Au contacts, which is similar to behavior reported on (010)-oriented β-Ga2O3, is also described.