Ultra-wide bandgap semiconductor of β-Ga<sub>2</sub>O<sub>3</sub> and its research progress of deep ultraviolet transparent electrode and solar-blind photodetector

Abstract

Gallium oxide (Ga<sub>2</sub>O<sub>3</sub>), with a bandgap of about 4.9 eV, is a new type of ultra-wide bandgap semiconductor material. The Ga<sub>2</sub>O<sub>3</sub> can crystallize into five different phases, i.e. <i>α</i>, <i>β</i>, <i>γ</i>, <i>δ</i>, and <i>ε</i>-phase. Among them, the monoclinic <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> (space group: C2/m) with the lattice parameters of <i>a</i> = 12.23 Å, <i>b</i> = 3.04 Å, <i>c</i> = 5.80 Å, and <i>β</i> = 103.7° has been recognized as the most stable phase. The <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> can be grown in bulk form from edge-defined film-fed growth with a low-cost method. With a high theoretical breakdown electrical field (8 MV/cm) and large Baliga’s figure of merit, the <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> is a potential candidate material for next-generation high-power electronics (including diode and field effect transistor) and extreme environment electronics [high temperature, high radiation, and high voltage (low power) switching]. Due to a high transmittance to the deep ultraviolet-visible light with a wavelength longer than 253 nm, the <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> is a natural material for solar-blind ultraviolet detection and deep-ultraviolet transparent conductive electrode. In this paper, the crystal structure, physical properties and device applications of Ga<sub>2</sub>O<sub>3</sub> material are introduced. And the latest research progress of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> in deep ultraviolet transparent conductive electrode and solar-blind ultraviolet photodetector are reviewed. Although Sn doped Ga<sub>2</sub>O<sub>3</sub> thin film has a conductivity of up to 32.3 S/cm and a transmittance greater than 88%, there is still a long way to go for commercial transparent conductive electrode. At the same time, the development history of <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> solar-blind ultraviolet photodetectors based on material type (nanometer, single crystal and thin film) is described in chronological order. The photodetector based on quasi-two-dimensional <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> flakes shows the highest responsivity (1.8 × 10<sup>5</sup> A/W). The photodetector based on ZnO/Ga<sub>2</sub>O<sub>3</sub> core/shell micron-wire has a best comprehensive performance, which exhibits a responsivity of 1.3 × 10<sup>3</sup> A/W and a response time ranging from 20 <inline-formula><tex-math id="M2">\begin{document}${\text{μ}}{\rm{s}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181845_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20181845_M2.png"/></alternatives></inline-formula> to 254 nm light at –6 V. We look forward to applying the <i>β</i>-Ga<sub>2</sub>O<sub>3</sub> based solar-blind ultraviolet photodetectors to military (such as: missile early warning and tracking, ultraviolet communication, harbor fog navigation, and so on) and civilian fields (such as ozone hole monitoring, disinfection and sterilization ultraviolet intensity monitoring, high voltage corona detection, forest fire ultraviolet monitoring, and so on).