Hydrated gallium trioxide precipitated uponneutralization of acidic or basic solution of gallium salt. Also, it is formed on heating gallium in air or by thermally decomposinggallium nitrate at 200–250 °C.
Crystalline Ga2O3 occur in five polymorphs, α, β, γ, δ, and ε. Of these polymorphs β-Ga2O3 is the most thermodynamically stable phase at standard temperature and pressure[8] while α-Ga2O3 is the most stable polymorph under high pressures.[9]
β-Ga2O3epitaxial thin films can be depositedheteroepitaxially on substrates such as sapphire, GaN, SiC, and Si, as well ashomoepitaxially. For example,ALD onsapphire substrates at temperatures between 190 °C and 550 °C have been demonstrated.[10] High-quality β-Ga2O3 films have also been grown using techniques such asMBE,HVPE, andMOVPE (also known as MOCVD or OMVPE).[11] HVPE is preferred for verticalpower semiconductor devices due to its fast growth rate.[12] β-Ga2O3 epitaxial films grown by MOVPE exhibit higherelectron mobilities and lower backgroundcarrier concentrations than those grown by other thin-film growth techniques.[13][14]
Bulk substrates of β-Ga2O3 can be produced, which is one of the major advantages of this material system. Bulk substrates can be produced in multiple orientations and by multiple techniques.[15][16]
Diagram of how gallium oxide is grown by the Czochralski method
α-Ga2O3 can be obtained by heating β-Ga2O3 at 65 kbar and 1100 °C. It has acorundum structure. The hydrated form can be prepared by decomposing precipitated and "aged" gallium hydroxide at 500 °C.Epitaxial thin films of α-Ga2O3 deposited on c-plane (0001), m-plane (1010), or a-plane (1120)sapphire substrates have been demonstrated.
γ-Ga2O3 is prepared by rapidly heating the hydroxide gel at 400–500 °C. A more crystalline form of this polymorph can be prepared directly from gallium metal by a solvothermal synthesis.[17]
δ-Ga2O3 is obtained by heating Ga(NO3)3 at 250 °C.[18]
ε-Ga2O3 is prepared by heating δ-Ga2O3 at 550 °C.[8] Thin films of ε-Ga2O3 are deposited by means ofmetalorganic vapour-phase epitaxy usingtrimethylgallium and water on sapphire substrates at temperatures between 550 and 650 °C[19]
Gallium(III) trioxide isamphoteric.[20] It reacts withalkali metal oxides at high temperature to form, e.g., NaGaO2, and with Mg, Zn, Co, Ni, Cu oxides to formspinels, e.g., MgGa2O4.[21]It dissolves in strong alkali to form a solution of the gallate ion,Ga(OH)− 4.
β-Ga2O3, with a melting point of 1900 °C, is the most stable crystalline modification. The oxide ions are in a distorted cubic closest packing arrangement, and the gallium (III) ions occupy distorted tetrahedral and octahedral sites, with Ga–O bond distances of 1.83 and 2.00 Å respectively.[25]
α-Ga2O3 has the same structure (corundum) asα-Al2O3, wherein Ga ions are 6-coordinate.[26][27]
γ-Ga2O3 has a defect spinel structure similar to that ofγ-Al2O3.[28]
κ-Ga2O3 has anorthorhombic structure and forms with 120° twin domains, resulting in hexagonal symmetry which is often identified as ε-Ga2O3.[30]
β-Ga2O3 can also formalloys withalumina to yield β-(AlxGa1-x)O3.[31] This alloy can be used to form heterostructures and create a two-dimensional electron gas (2DEG).[32]
The β-phase'sbandgap of 4.7–4.9 eV and large-area, native substrates make it a potential competitor toGaN andSiC-based power electronics applications andsolar-blind UVphotodetectors.[7][39] The orthorhombic ĸ-Ga2O3 is the second most stable polymorph. The ĸ-phase has shown instability of subsurface doping density under thermal exposure.[40] Ga2O3 exhibits reduced thermal conductivity and electron mobility by an order of magnitude compared toGaN andSiC, but is predicted to be significantly more cost-effective due to being the only wide-bandgap material capable of being grown from melt.[7][41][42] β-Ga2O3 is thought to beradiation-hard, which makes it promising for military and space applications.[43][44]Gallium(III) oxide has been studied for usage as passive components in lasers,[45] phosphors,[5] and luminescent materials[46] as well as active components for gas sensors,[6] power diodes,[47] and power transistors.[48][49] Since the first publication in January 2012 by theNational Institute of Information and Communications Technology, in collaboration with Tamura Co., Ltd. and Koha Co., Ltd. of the world's first single-crystal gallium oxide (Ga2O3)field-effect transistors, the predominant interest in gallium oxide is in the β-polymorph forpower electronics.[50][7]
Monoclinic β-Ga2O3 has been compared with GaN- and SiC-based power devices.[7] β-Ga2O3Schottky diodes have exceededbreakdown voltages of 2400 V.[47] β-Ga2O3/NiOxp–n diodes have exhibited breakdown voltages over 1200 V.[51] β-Ga2O3MOSFETs have individually achieved figures of merits of fT of 27 GHz,[48] fMAX of 48 GHz,[49] and 5.4 MV/cm average breakdown field.[49] This field exceeds that which is possible in SiC or GaN.
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_oxide_crystal.jpg)
