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

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

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

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

We studied the growth of Si-and Sn-doped homoepitaxial β -Ga 2 O 3 layers on (010)-oriented substrates by metal organic vapor phase epitaxy (MOVPE). At optimal growth conditions (850 ◦ C, 5 mbar) the layers were smooth with RMS roughness values of ∼ 600 pm. A microstructural study by transmission electron microscopy (TEM) revealed a very high crystalline perfection of the layers. No dislocations or planar defects were observed within the ﬁeld of view of TEM. Using Si as dopant, the free electron concentration could be varied in a range between 1 × 10 17 and 8 × 10 19 cm − 3 , while with Sn the doping range was restricted to 4 × 10 17 − 1 × 10 19 cm − 3 . This was explained by a pronounced Sn memory effect in the MOVPE reactor that hampers achieving low carrier densities and by incorporation issues that limit the doping efﬁciency at high Sn doping levels. The electron mobility for a given doping density increased from ∼ 50 cm 2 /Vs for n = 8 × 10 19 cm − 3 to ∼ 130 cm 2 /Vs for n = 1 × 10 17 cm − 3 independently of the dopant. These values match the best literature data relative to β -Ga 2 O 3 bulk crystals and layers grown by molecular beam epitaxy, setting a new standard for MOVPE-grown layers. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which

https://iopscience.iop.org/article/10.1149/2.0081702jss/pdf

Editors' Choice—Si- and Sn-Doped Homoepitaxial β-Ga2O3 Layers Grown by MOVPE on (010)-Oriented Substrates

Sn-doped gallium oxide (Ga2O3:Sn) films were deposited on β-Ga2O3 (100) substrates by metal organic chemical vapor deposition method. The Sn concentration was varied from 1 to 12 % (atomic ratio). The influences of Sn-doping concentration on structure, raman, and photoelectric properties of the films were investigated in detail. The obtained films were monoclinic β-Ga2O3 homoepitaxial films. A decrease of about eight orders of magnitude in the film resistivity could be achieved through Sn doping. The 10 % Sn-doped film exhibited the best electrical properties and the lowest resistivity of about 1.20 × 10−1 Ω cm, with the hall mobility value of 12.03 cm2 V−1 s−1obtained. The average transmittances of the β-Ga2O3: Sn films in the visible and UV wavelength ranges were all over 85 %. The optical band gap of the films with different Sn-doping concentrations could be modulated from 4.16 to 4.69 eV.

Preparation and characterization of Sn-doped β-Ga2O3 homoepitaxial films by MOCVD

Characterization of β-Ga2O3 thin films on sapphire (0001) using metal-organic chemical vapor deposition technique

Structural and optical properties of heteroepitaxial beta Ga2O3 films grown on MgO (100) substrates

Ultraviolet–green photoluminescence of β-Ga2O3 films deposited on MgAl6O10 (100) substrate

Tin-doped β-Ga2O3 (β-Ga2O3:Sn) films doped with different tin concentrations were deposited on MgO (110) substrates by metal organic chemical vapor deposition (MOCVD) at 700 °C. The effect of doping on the structural, electrical and optical properties of the films was investigated. The 10% Sn-doped film exhibited the best electrical conductivity properties with the lowest resistivity about 5.21 × 10−2 Ω cm, which is over ten orders of magnitude lower than the un-doped film. Micro-structural analysis revealed that the film with 10% Sn content had a clear in-plane relationship of β-Ga2O3 (100) ‖ MgO (110) with β-Ga2O3 (01) ‖ MgO (111). The average transmittance of the samples in the visible range exceeded 87% and the optical band gap of the films varied from 4.12 to 4.80 eV.

Electrical and optical characterizations of β-Ga2O3:Sn films deposited on MgO (110) substrate by MOCVD

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