Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor that has attracted a large amount of interest due to its ultra-large bandgap of 4.8 eV, a high breakdown field of 8 MV/cm, and high thermal stability. These properties enable Ga2O3 a promising material for a large range of applications, such as high power electronic devices and solar-blind ultraviolet (UV) photodetectors. In the past few years, a significant process has been made for the growth of high-quality bulk crystals and thin films and device optimizations for power electronics and solar blind UV detection. However, many challenges remain, including the difficulty in p-type doping, a large density of unintentional electron carriers and defects/impurities, and issues with the device process (contact, dielectrics, and surface passivation), and so on. The purpose of this article is to provide a timely review on the fundamental understanding of the semiconductor physics and chemistry of Ga2O3 in terms of electronic band structures, optical properties, and chemistry of defects and impurity doping. Recent progress and perspectives on epitaxial thin film growth, chemical and physical properties of defects and impurities, p-type doping, and ternary alloys with In2O3 and Al2O3 will be discussed.

over 80% in the visible light range. [2] ITO is widely used as essential transparent conducting electrodes in flat panel displays, touch screens, and solar cells. The global ITO market has an annual growth rate of 15% and is valued at 7 billion USD in 2019. In 2004, Nomura and Hosono et al. made great breakthrough in oxide thin film transistor (TFT) based on amorphous indium gallium zinc oxide (IGZO) grown at room temperature. [3] The amorphous IGZO showed an impressive mobility of 9 cm 2 V −1 s −1 , about 10 times of amorphous hydrogenated Si TFT which was used exclusively for displays at that time. Soon after Hosono's seminal work, IGZO TFT was commercialized by Sharp Corporation in 2012, and then rapidly expanded to mobile phones, tablets and laptops. [4] In 2019, the fifth generation IGZO TFT went to mass production, capable of driving large-area displays (85 in.) with ultrahigh 8 K resolution. [5] Moreover, oxide TFTs are also considered as the most promising transistors for next-generation curved, flexible, or even rollable electronics. [6] The great success of oxide semiconductors is underpinned by their unique electronic structure, amenability for n-type doping, as well as intrinsic stability. Oxide semiconductors have bandgap larger than 3 eV, enabling transparency in the visible spectrum. The conduction band (CB) of oxide semiconductors is typically composed of empty ns-orbitals (n ≥ 4) of heavy posttransition metals. The large, spherical ns-orbitals give rise to a high electron mobility even in amorphous phases, as well as high dopability for hosting a high density of electrons. Therefore, oxide semiconductors are amendable via doping to be a transparent semiconductor or a transparent conductor, depending on the purposes of device applications, e.g., TFT or ITO. However, there are two sides to every coin. The nature of electronic structure of oxide semiconductors also leads to the fundamental limitation of achieving p-type oxide semiconductors, which is exacerbated by the presence of a high background electron density arising from the formation of unintentional defects and impurities. [1a,7] The lack of p-type semiconductor significantly limits the great potential of oxide electronics. [7b] A high electron density and defect states cause detrimental effects on oxide TFT device performance, such as a high off-current, lower mobility, and instability issues. [8] In the past two decades, considerable research efforts have been made to understand the microscopic origin of defect states and background electrons Wide bandgap oxide semiconductors constitute a unique class of materials that combine properties of electrical conductivity and optical transparency. They are being widely used as key materials in optoelectronic device applications, including flat-panel displays, solar cells, OLED, and emerging flexible and transparent electronics. In this article, an up-to-date review on both the fundamental understanding of materials physics of oxide semiconductors, and recent research progress on design of new...

Complex transition-metal oxides (TMOs) are critical materials for cutting-edge electronics and energy-related technologies, on the basis of their intriguing properties including ferroelectricity, magnetism, superconductivity, (photo- and electro-) catalytic activity, ionic...

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