Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Due to its significant applications in many relevant fields, light detection in the solar-blind deep-ultraviolet (DUV) wavelength region is a subject of great interest for both scientific and industrial communities. The rapid advances in preparing high-quality ultrawide-bandgap (UWBG) semiconductors have enabled the realization of various high-performance DUV photodetectors (DUVPDs) with different geometries, which provide an avenue for circumventing numerous disadvantages in traditional DUV detectors. This article presents a comprehensive review of the applications of inorganic UWBG semiconductors for solar-blind DUV light detection in the past several decades. Different kinds of DUVPDs, which are based on varied UWBG semiconductors including Ga2O3, MgxZn1−xO, III-nitride compounds (AlxGa1−xN/AlN and BN), diamond, etc., and operate on different working principles, are introduced and discussed systematically. Some emerging techniques to optimize device performance are addressed as well. Finally, the existing techniques are summarized and future challenges are proposed in order to shed light on development in this critical research field.

Recent Progress in Solar-Blind Deep-Ultraviolet Photodetectors Based on Inorganic Ultrawide Bandgap Semiconductors

Solar-blind photodetectors are of great interest to a wide range of industrial, civil, environmental, and biological applications. As one of the emerging ultrawide-bandgap semiconductors, gallium oxide (Ga2O3) exhibits unique advantages over other wide-bandgap semiconductors, especially in developing high-performance solar-blind photodetectors. This paper comprehensively reviews the latest progresses of solar-blind photodetectors based on Ga2O3 materials in various forms of bulk single crystal, epitaxial films, nanostructures, and their ternary alloys. The basic working principles of photodetectors and the fundamental properties and synthesis of Ga2O3, as well as device processing developments, have been briefly summarized. A special focus is to address the physical mechanism for commonly observed huge photoconductive gains. Benefitting from the rapid development in material epitaxy and device processes, Ga2O3-based solar-blind detectors represent to date one of the most prospective solutions for UV detection technology towards versatile applications.

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Review of gallium-oxide-based solar-blind ultraviolet photodetectors

In recent years, solar-blind ultraviolet (UV) photodetectors have attracted significant attention from researchers in the field of semiconductor devices due to their indispensable properties in the fields of high-temperature event monitoring, anti-terrorism, security and ad hoc network communication. As an important member of the third-generation semiconductors, β-Ga2O3 is considered to be one of the most promising candidates for solar-blind UV detectors due to its ultra-wide band gap (∼4.9 eV), economic efficiency, high radiation resistance and excellent chemical and thermal stability. Herein, we provide a comprehensive review on Ga2O3-based solar-blind UV photodetectors, with a detailed introduction of the developmental process of material growth methods and device manufacturing in the past decade. We classify the currently reported Ga2O3-based solar-blind UV photodetectors (mainly including photoconductive detectors, heterogeneous PN junction detectors and Schottky junction detectors) and summarize their respective superiorities and potentials for improvement. Finally, considering the actual application requirements, we put forward some meaningful suggestions, including energy band engineering and homogeneous epitaxy, for the future development of Ga2O3 material growth and device manufacturing.

Gallium oxide solar-blind ultraviolet photodetectors: a review

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.

Recent progress on the electronic structure, defect, and doping properties of Ga2O3

Single crystallinity (AlGa)2O3 solar-blind photodetectors are epitaxially grown on sapphire. Oxygen pressure during the growth has a great impact on the Al composition in (AlGa)2O3, which is investigated utilizing X-ray photoelectron spectroscopy and X-ray diffraction measurements. Measured transmittance spectra and responsivity demonstrate that (AlGa)2O3 photodetectors achieve a wider bandgap compared to a Ga2O3 device. An (Al0.12Ga0.88)2O3 device obtains 10 times higher photocurrent Iphoto than a Ga2O3 photodetector. However, as Al composition increases, significant Iphoto degradation is observed in an (Al0.35Ga0.65)2O3 photodetector. Meanwhile, an (Al0.35Ga0.65)2O3 device exhibits the stronger persistent photoconductivity compared to the Ga2O3 control device. Analysis shows that defect states in a bandgap of (AlGa)2O3 might be associated with the performance change in (AlGa)2O3 photodetectors.

(AlGa)_2O_3 solar-blind photodetectors on sapphire with wider bandgap and improved responsivity

In this paper, β-Ga
 2
O
 3
 ultraviolet photodetectors were grown on sapphire utilizing the laser molecular beam epitaxy tool. The impact of oxygen pressure P
 O2
 in growth chamber on the crystal quality, the surface morphology, the chemical component of Ga
 2
O
 3
 films, and the electrical performance of photodetectors are characterized. As the P
 O2
 is increased during growth, the concentration of oxygen vacancy (V
 O
) is effectively reduced. The photodetector grown at the P
 O2
 of 0.05 mbar exhibits the significantly improved photocurrent I
 photo
 and responsivity R characteristics in comparison with the device grown with the P
 O2
 of 0.01 mbar, which is attributed to a reduction in the number of V
 O
. However, as the P
 O2
 continuously increased to 0.09 mbar, I
 photo
 and R of the detector are degraded, which might be due to the fact that the gallium vacancies (V
 Ga
), as the dominant trapping centers, lead to the recombination of photo-generated carriers.

Comparison Study of β-Ga2O3 Photodetectors Grown on Sapphire at Different Oxygen Pressures

We fabricated $\beta $ -Ga2O3 photodetectors on bulk substrate and sapphire. Bulk Ga2O3 photodetector demonstrates the improved responsivity compared with the device on sapphire, due to the higher crystal quality in bulk material. Optical gain is achieved in both the devices. For the first time, we report that the Ga2O3 photodetector epitaxially grown on sapphire achieves a blueshift of bandgap in comparison with bulk device. Based on the measured responsivity characteristics, the direct and indirect $E_{G}$ of bulk Ga2O3 are 4.78 and 4.59 eV, respectively. The Ga2O3 photodetector on sapphire exhibits a maximum cutoff wavelength at 253 nm, corresponding to the direct $E_{G}$ of 4.90 eV. The increment of $E_{G}$ in Ga2O3 on sapphire over bulk material is attributed to the residual strain in the film. The time-dependent photoresponse of the devices suggests that Ga2O3 on sapphire might have more oxygen vacancies than the bulk material.

Comparison Study of $\beta $ -Ga 2 O 3 Photodetectors on Bulk Substrate and Sapphire

β-Ga2O3 epitaxial thin films were deposited by laser molecular beam epitaxy (LMBE) and annealed at 800°C for 30 minutes in air and oxygen atmospheres, respectively. Photodetectors were fabricated using as-grown and annealed Ga2O3 epilayers. The influence of the annealing atmosphere on the crystal structure and optical properties of Ga2O3 films was investigated. X-ray diffraction (XRD) measurements show that the in-plane compressive strain of Ga2O3 thin films could be relaxed after high temperature thermal annealing. Compared with the as-grown sample, the annealed samples exhibit a red shift of absorption edge in the transmittance spectra, indicating a reduced bandgap. According to the XPS measurement results, the atomic ratios of O to Ga also increased for the annealed samples. Moreover, the oxygen-annealed photodetector achieves a larger photocurrent, higher responsivity and better time-dependent photoresponse than the other two samples, which may be attributed to the decrease in the number of oxygen vacancies.

Lu Huang

Papers

(AlGa)_2O_3 solar-blind photodetectors on sapphire with wider bandgap and improved responsivity

Comparison Study of β-Ga2O3 Photodetectors Grown on Sapphire at Different Oxygen Pressures

Comparison Study of $\beta $ -Ga 2 O 3 Photodetectors on Bulk Substrate and Sapphire

Influence of annealing atmosphere on the performance of a β-Ga2O3 thin film and photodetector

Band alignment of SiO 2 /(Al x Ga 1-x ) 2 O 3 (0 ≤ x ≤ 0.49) determined by X-ray photoelectron spectroscopy