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

β-Ga2O3 is an emerging, ultra-wide bandgap (energy gap of 4.85 eV) transparent semiconducting oxide (TSO), which attracted recently much scientific and technological attention. Unique properties of that compound combined with its advanced development in growth and characterization place β-Ga2O3 in the frontline of future applications in electronics (Schottky barrier diodes, field-effect transistors), optoelectronics (solar- and visible-blind photodetectors, flame detectors, light emitting diodes), and sensing systems (gas sensors, nuclear radiation detectors). A capability of growing large bulk single crystals directly from the melt and epi-layers by a diversity of epitaxial techniques, as well as explored material properties and underlying physics, define a solid background for a device fabrication, which, indeed, has been boosted in recent years. This required, however, enormous efforts in different areas of science and technology that constitutes a chain linking together engineering, metrology and theory. The present review includes material preparation (bulk crystals, epi-layers, surfaces), an exploration of optical, electrical, thermal and mechanical properties, as well as device design / fabrication with resulted functionality suitable for different fields of applications. The review summarizes all of these aspects of β-Ga2O3 at the research level that spans from the material preparation through characterization to final devices.

https://iopscience.iop.org/article/10.1088/1361-6641/aadf78/pdf

β-Ga2O3 for wide-bandgap electronics and optoelectronics

In this report, we demonstrate high spectral responsivity (SR) in MBE grown epitaxial beta-Ga2O3-based solar blind metal-semiconductor-metal (MSM) photodetectors (PD). The (-201)-oriented beta-Ga2O3 thin film was grown using plasma-assisted MBE on c-plane sapphire substrates. MSM devices fabricated with Ni/Au contacts in an interdigitated geometry were found to exhibit peak SR > 1.5 A/W at 236-240 nm at a bias of 4V with a UV to visible rejection ratio > 10(5). The devices exhibited very low dark current 10(3). These results represent the state-of-art performance for the MBE-grown beta-Ga2O3 MSM solar blind detector. Published by AIP Publishing.

High responsivity in molecular beam epitaxy grown beta-Ga2O3 metal semiconductor metal solar blind deep-UV photodetector

In this report, we demonstrate high spectral responsivity (SR) in MBE grown epitaxial \b{eta}-Ga2O3-based solar blind MSM photodetectors (PD). (-2 0 1)-oriented \b{eta}-Ga2O3 thin film was grown by plasma-assisted MBE on c-plane sapphire substrates. MSM devices fabricated with Ni/Au contacts in an interdigitated geometry were found to exhibit peak SR > 1.5 A/W at 236-240 nm at a bias of 4 V with a UV to visible rejection ratio > 105. The devices exhibited very low dark current 103. These results represent the state-of-art performance for MBE-grown \b{eta}-Ga2O3 MSM solar blind detector.

High Responsivity in Molecular Beam Epitaxy (MBE) grown \b{eta}-Ga2O3 Metal Semiconductor Metal (MSM) Solar Blind Deep-UV Photodetector

Effect of annealing on β-Ga2O3 film grown by pulsed laser deposition technique

We report on the highest responsivity for III-nitride Metal Semiconductor Metal solar-blind photodetectors on sapphire. Devices on unintentionally doped AlGaN epilayers grown by Metal Organic Chemical Vapor Deposition exhibited sharp absorption cut-off in the range of 245–290 nm. Very high responsivity >5 A/W at 10 V bias was achieved with visible rejection exceeding three orders of magnitude for front illumination. Compared to the responsivity values reported in the literature for state-of-the-art solar-blind photodetectors, this work presents the highest values of responsivity at a given bias and up to sub-250 nm detection threshold. The high responsivity is attributed to an internal gain mechanism operating on these devices. The reverse-bias leakage current across these samples was found to be dominated by thermionic field emission at low biases and Poole-Frenkel emission from a deep trap level (0.7 eV from the conduction band-edge for Al0.50Ga0.50 N) at high biases.

Gain mechanism and carrier transport in high responsivity AlGaN-based solar blind metal semiconductor metal photodetectors

Two-dimensional resistive sensor arrays that utilize shared row and column connections to simplify the interconnect complexity suffer from the crosstalk problem among its elements introduced due to the interconnection overloading. In this letter, we present a method of overcoming the problem of crosstalk by putting all of the row nodes at virtually equal potential using virtual ground of high-gain operational amplifiers (opamps) in negative feedback. The circuit, though it requires a large number of opamps, solves the crosstalk problem to a large extent and provides faster scanning. We verified the circuit functionality with PSPICE simulations. We have also derived the expressions of crosstalk rejection and sensitivity to show that, by using high-gain, low-noise opamps, we may get excellent performance.

Rangarajan Muralidharan

Papers

High responsivity in molecular beam epitaxy grown beta-Ga2O3 metal semiconductor metal solar blind deep-UV photodetector

High Responsivity in Molecular Beam Epitaxy (MBE) grown \b{eta}-Ga2O3 Metal Semiconductor Metal (MSM) Solar Blind Deep-UV Photodetector

Effect of annealing on β-Ga2O3 film grown by pulsed laser deposition technique

Gain mechanism and carrier transport in high responsivity AlGaN-based solar blind metal semiconductor metal photodetectors

Virtual Ground Technique for Crosstalk Suppression in Networked Resistive Sensors