Gallium oxide is a wide bandgap n-type semiconductor highly interesting for optoelectronic applications (e.g., power electronics and solar blind UV photodetectors). Besides its most thermodynamically stable monoclinic β phase, Ga2O3 can crystallize in different polymorphs; among them the corundum α and the orthorhombic e phases are the most promising ones. In this review we focus on the main aspects that promote the nucleation and stable growth of these Ga2O3 polymorphs. Particular emphasis is given to the e phase since it is recently gaining increasing attention in the scientific community because of: (i) its higher lattice symmetry with respect to β-Ga2O3, which could favour the realization of heterostructures, (ii) the possibility to be grown on cheap sapphire substrates and (iii) its peculiar piezoelectric properties. While the growth of β-Ga2O3 is widely studied and understood, a thorough and comprehensive analysis of the chemical and physical aspects that allow for the stabilization of the metastable Ga2O3 phases with different synthesis methods is still missing. Therefore, the present review aims at filling this gap, by analysing the relevant growth parameters for several growth techniques (MOVPE, HVPE, mist-CVD, MBE, and PLD), highlighting similarities and differences, looking for a unified framework to understand the growth and nucleation of different Ga2O3 polymorphs. As a conclusion, we highlight practical guidelines for the deposition of the different Ga2O3 polymorphs with all the discussed thin film growth techniques.

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Ga2O3 polymorphs: tailoring the epitaxial growth conditions

Gallium oxide (Ga2O3) is rapidly emerging as a material of choice for the development of solar blind photodetectors and power electronic devices which are particularly suitable in harsh environment applications, owing to its wide bandgap and extremely high Baliga figure of merit (BFOM). The Ga2O3 based devices show robustness against chemical, thermal and radiation environments. Unfortunately, the current Ga2O3 technology is still not mature for commercial usage., Thus, extensive research on the growth of various polymorph of Ga2O3 materials has been carried out. This article aims to provide an overview of the current understanding of epitaxial growth of different phases of Ga2O3 by various growth techniques including pulsed laser deposition (PLD), molecular beam epitaxy (MBE), metal-organic chemical vapour deposition (MOCVD), sputtering, mist chemical vapour deposition (Mist CVD) and atomic layer deposition (ALD).The review also investigates the factors such as the growth temperature, pressure, carrier gas, III/V ratio, substrate as well as doping which would influence the synthesis and the stability of meta stable phases of Ga2O3. In addition, a through discussion of growth window is also provided using phase diagrams for aforementioned epitaxial deposition methods.

Recent advances in the growth of gallium oxide thin films employing various growth techniques- A review

Control of phase formation of (AlxGa1 − x)2O3 thin films on c-plane Al2O3

Herein, single-domain κ-Ga2O3 thin films were grown on FZ-grown e-GaFeO3 substrates via a step-flow growth mode. The e-GaFeO3 possessing the same crystal structure and similar lattice parameters as those of the orthorhombic κ-Ga2O3 facilitated the growth of κ-Ga2O3 thin films, as observed by the X-ray diffraction (XRD) analysis. Furthermore, the surface morphologies of the κ-Ga2O3 thin films exhibited a step-terrace and atomically flat structure. XRD φ-scan and transmission electron microscopy with selected area electron diffraction revealed that there is no occurrence of in-plane rotational domains in the κ-Ga2O3 thin films on e-GaFeO3 substrates and that the κ-Ga2O3 thin film comprised a single domain. TEM analysis revealed that there were no clear dislocations in the observation area. Moreover, high-resolution TEM observation showed that the atomic arrangements of the film and the substrate were continuous without the presence of an intermediate layer along the growth direction and were well-aligned in the in-plane direction.

Single-Domain and Atomically Flat Surface of κ-Ga2O3 Thin Films on FZ-Grown ε-GaFeO3 Substrates via Step-Flow Growth Mode.

Films of Ga2O3 were grown by Halide Vapor Phase Epitaxy (HVPE) on bulk heavily B-doped (001)-oriented diamond substrates using thin interlayers of Al2O3 deposited by HVPE or AlN/AlGaN deposited by metalorganic chemical vapor deposition. The growth with AlN/AlGaN was dominated by the formation of a highly conducting ɛ-phase with poor crystalline quality. For these samples, excessive leakage of Schottky diodes and of the Ga2O3/diamond heterojunction prevented meaningful electrical characterization. The film grown with the Al2O3 interlayer was mainly composed of (−201) β-Ga2O3 with an admixture of the ɛ-phase. The film had a low density of residual shallow donors, 5 × 1015 cm−3, with deep electron traps spectra consisting of the well documented centers for β-Ga2O3 near Ec 0.27, Ec 0.7, and Ec 1 eV, all of which are often ascribed to native defects or their complexes. The electrical properties of heterojunctions were mostly determined by the properties of the Ga2O3 films. Both Schottky diodes and heterojunctions showed measurable photosensitivity for 259 nm wavelength excitation, but very low photocurrent for near-UV (365 nm wavelength excitation).

Electrical properties and deep trap spectra in Ga2O3 films grown by halide vapor phase epitaxy on p-type diamond substrates

In this study we compare the growth of gallium oxide films by halide vapor phase epitaxy (HVPE) on various substrates under the same growth conditions. Gallium oxide films were deposited at 500 °C–600 °C on basal plane (0001) planar and patterned sapphire substrates, (0001) 2H-GaN, 4H-SiC, and bulk β-Ga2O3 substrates. The layers were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and cathodoluminescence (CL) techniques. Most of the films exhibit growth features of hexagonal symmetry. Sn-doped Ga2O3 films exhibit n-type electrical conductivity. Heterojunctions composed of n-type hexagonal Ga2O3:Sn and p-type GaN:Mg demonstrate diode-like I-V characteristics and emit light under forward bias.

https://iopscience.iop.org/article/10.1149/2162-8777/ab8b4c/pdf

HVPE Growth and Characterization of ε-Ga2O3 Films on Various Substrates

Gallium oxide with a corundum structure ({\alpha}-Ga2O3) has recently attracted great attention in view of electronic and photonic applications due to its unique properties including a wide band gap exceeding that of the most stable beta phase (\b{eta}-Ga2O3). However, the lower thermal stability of the {\alpha}-phase at ambient conditions in comparison with the \b{eta}-phase requires careful investigation of its resistance to other external influences such as ion irradiation, ion doping, etc. In this work, the structural changes under the action of Al+ ion irradiation have been investigated for a polymorphic gallium oxide layers grown by hydride vapor phase epitaxy on c-plane sapphire and consisting predominantly of {\alpha}-phase with inclusions of {\alpha}(\k{appa})-phase. It is established by the X-ray diffraction technique that inclusions of {\alpha}(\k{appa})-phase in the irradiated layer undergo the expansion along the normal to the substrate surface, while there is no a noticeable deformation for the {\alpha}-phase. This speaks in favor of the different radiation tolerance of various Ga2O3 polymorphs, especially the higher radiation tolerance of the {\alpha}-phase. This fact should be taken into account when utilizing ion implantation to modify gallium oxide properties in terms of development of efficient doping strategies.

A. I. Pechnikov

Papers

HVPE Growth and Characterization of ε-Ga2O3 Films on Various Substrates

Ion-Beam Modification of Metastable Gallium Oxide Polymorphs