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

β-Ga2O3 epitaxial thin films were deposited using laser molecular beam epitaxy technique and oxygen atmosphere in situ annealed in order to reduce the oxygen vacancy. Metal/semiconductor/metal structured photodetectors were fabricated using as-grown film and annealed film separately. Au/Ti electrodes were Ohmic contact with the as-grown films and Schottky contact with the annealed films. In compare with the Ohmic-type photodetector, the Schottky-type photodetector takes on lower dark current, higher photoresponse, and shorter switching time, which benefit from Schottky barrier controlling electron transport and the quantity of photogenerated carriers trapped by oxygen vacancy significant decreasing.

Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in β-Ga2O3 solar-blind ultraviolet photodetectors

Ultraviolet (UV) radiation has a variety of impacts including the health of humans, the production of crops, and the lifetime of buildings. Based on the photovoltaic effect, self-powered UV photode...

Self-Powered Ultraviolet Photodetector with Superhigh Photoresponsivity (3.05 A/W) Based on the GaN/Sn:Ga2O3 pn Junction.

Because of the direct band gap of 4.9 eV, β-Ga2O3 has been considered as an ideal material for solar-blind photodetection without any bandgap tuning. Practical applications of the photodetectors require fast response speed, high signal-to-noise ratio, low energy consumption and low fabrication cost. Unfortunately, most reported β-Ga2O3-based photodetectors usually possess a relatively long response time. In addition, the β-Ga2O3 photodetectors based on bulk, the individual 1D nanostructure, and the film often suffer from the high cost, the low repeatability, and the relatively large dark current, respectively. In this paper, a Au/β-Ga2O3 nanowires array film vertical Schottky photodiode is successfully fabricated by a simple thermal partial oxidation process. The device exhibits a very low dark current of 10 pA at −30 V with a sharp cutoff at 270 nm. More interestingly, the 90–10% decay time of our device is only around 64 μs, which is much quicker than any other previously reported β-Ga2O3-based photodet...

Self-Powered Solar-Blind Photodetector with Fast Response Based on Au/β-Ga2O3 Nanowires Array Film Schottky Junction

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

Laser molecular beam epitaxy technology has been employed to deposit β-gallium oxide (β-Ga2O3) on (0001) sapphire substrates. After optimizing the growth parameters, (2¯01)-oriented β-Ga2O3 thin film was obtained. Ultraviolet-visible absorption spectrum demonstrates that the prepared β-Ga2O3 thin film shows excellent solar-blind ultraviolet (UV) characteristic with a band gap of 5.02 eV. A prototype photodetector device with a metal-semiconductor-metal structure has been fabricated using high quality β-Ga2O3 film. The device exhibits obvious photoresponse under 254 nm UV light irradiation, suggesting a potential application in solar-blind photodetectors.

Fabrication of β-Ga_2O_3 thin films and solar-blind photodetectors by laser MBE technology

Au/Nb:SrTiO3/Ti structures were fabricated by depositing Au and Ti electrodes on a single crystal 0.5 wt% Nb:SrTiO3 (NSTO) using rf-magnetron sputtering technique. Resistive switching properties at different temperature were investigated. The Ti/NSTO interface was ohmic contact, which indicated that the resistive switching behavior was attributed to Au/NSTO interface. The resistive switching behavior happened only at the temperature above 180 K, which was possibly caused by the increase of Schottky barrier height with the increase of temperature. The structure showed a semiconductor behavior at high-resistance state (HRS) and a metallic behavior at low-resistance state (LRS). The switching conduction mechanism of Au/NSTO/Ti device is primarily described as space-charge-limited conduction (SCLC) according to the electrical transport properties measurement.

Temperature-dependent resistive switching behavior in the structure of Au/Nb:SrTiO 3 /Ti

The transition metal fluorides KMF3 (M = Mn, Co, and Ni) were synthesized through a simple solution route. The crystal structure, morphology and electrical transport property of the resulting products were investigated. The compound KMF3 crystallizes in a cubic perovskite structure with space group Pm-3m (No. 221). A crystal structure of KMF3 was refined by the Rietveld method based on the X-ray powder diffraction data. The unit-cell parameters are 4.189 46(4), 4.075 58(4), and 4.025 70(2) for KMnF3, KCoF3 and KNiF3, respectively. A metal–insulator transition was observed in temperature-dependent electrical transport characterization in the temperature range from 250 to 280 K for these three compounds, which is considered to be related to spin-exchange in this kind of material.

Crystal structure and electrical transport property of KMF3 (M= Mn, Co, and Ni)

With the aim to get optimised resistive switching properties, the influence of oxygen vacancy concentration of Nb:SrTiO3 (NSTO) on the resistive switching properties of Ag/NSTO/Ti structure was studied. To obtain the NSTO with different oxygen vacancy concentrations, the substrates had been annealed in different atmospheres and temperatures before they were used to fabricate Ag/NSTO/Ti structure. The resistive switching properties were investigated by measuring I–V characteristics, endurance ability and resistance distribution at room temperature. The results show that both annealing in oxygen atmosphere and vacuum atmosphere degraded the switching properties. In our case, the device annealed in air at 700°C displayed the best resistive switching behaviour. After analysis, we suggest that both Schottky barrier height and barrier width, which can be tuned by changing the oxygen vacancy concentration, play important roles in getting high quality resistive switching property.

Influence of oxygen vacancy on resistive switching property of Ag/Nb:SrTiO3/Ti structure

The invention discloses a method for preparing a resistance changing storage unit device, and in particular relates to a method for preparing a resistance random access memory device realizing the multiple-valued storage performance based on interface oxygen vacancy, wherein the method uses metals Au and Ag respectively as an upper electrode and a lower electrode, and uses single crystal SrTiO3 in which 0.5wt% of Nb is doped as a resistance random layer material. According to the method, the Nb-containing single crystal SrTiO3 is used as a substrate, and is ultrasonically cleaned up and naturally dried in the air; the substrate is shielded by a mask plate; a metal gold thin film is sputtered on the substrate by using a radio frequency magnetic control sputtering method and is used as an electrode, and metal silver is used as the other electrode; and then two electrodes are connected by a copper wire. The method has the advantages that the used equipment is a commercial product and does not need to be prepared in the complicated process; the process is high in controllability and easy to operate; and the film is compact in surface, stable and uniform in thickness and continuous in structure. The prepared device structure has a larger memory window, and high in retaining property and multiple-valued storage performance.

G. F. Wang

Papers

Fabrication of β-Ga_2O_3 thin films and solar-blind photodetectors by laser MBE technology

Temperature-dependent resistive switching behavior in the structure of Au/Nb:SrTiO 3 /Ti

Crystal structure and electrical transport property of KMF3 (M= Mn, Co, and Ni)

Influence of oxygen vacancy on resistive switching property of Ag/Nb:SrTiO3/Ti structure

Method for preparing resistance random access memory device realizing multiple-valued storage performance based on interface oxygen vacancy