Gallium(III) oxide

Gallium(III) oxide is an inorganic compound and ultra-wide-bandgap semiconductor with the formula Ga₂O₃. It is actively studied for applications in power electronics, phosphors, and gas sensing.^[5][6][7] The compound has several polymorphs, of which the monoclinic β-phase is the most stable. The β-phase’s bandgap of 4.7–4.9 eV and large-area, native substrates make it a promising competitor to GaN and SiC-based power electronics applications and solar-blind UV photodetectors.^[7][8] The orthorhombic ĸ-Ga₂O₃ is the second most stable polymorph. The ĸ-phase has shown instability of subsurface doping density under thermal exposure.^[9] Ga₂O₃ exhibits reduced thermal conductivity and electron mobility by an order of magnitude compared to GaN and SiC, but is predicted to be significantly more cost-effective due to being the only wide-bandgap material capable of being grown from melt.^[7][10][11] β-Ga₂O₃ is thought to be radiation-hard, which makes it promising for military and space applications.^[12][13]

Gallium(III) trioxide

β-Ga2O3 crystal
Crystal structure of β-Ga2O3
Names
Other names gallium trioxide, gallium sesquioxide
Identifiers
CAS Number	12024-21-4 Y
3D model (JSmol)	Interactive image
ChemSpider	139522 Y
ECHA InfoCard	100.031.525
EC Number	234-691-7
PubChem CID	5139834
RTECS number	LW9650000
UNII	46F059V66A Y
CompTox Dashboard (EPA)	DTXSID9031359
InChI InChI=1S/2Ga.3O Y Key: QZQVBEXLDFYHSR-UHFFFAOYSA-N Y InChI=1/2Ga.3O/rGa2O3/c3-1-5-2-4 Key: QZQVBEXLDFYHSR-OGCFUIRMAC
SMILES O=[Ga]O[Ga]=O
Properties
Chemical formula	Ga2O3
Molar mass	187.444 g/mol
Appearance	white crystalline powder
Melting point	1,725 °C (3,137 °F; 1,998 K)[1]
Solubility in water	insoluble
Solubility	soluble in most acids
Structure[2][3]
Crystal structure	Monoclinic, mS20, space group = C2/m, No. 12
Lattice constant	a = 1.2232 nm, b = 0.3041 nm, c = 0.5801 nm α = 90°, β = 103.73°, γ = 90° β-phase
Formula units (Z)	4
Thermochemistry[4]
Heat capacity (C)	92.1 J/(mol·K)
Std molar entropy (S⦵298)	85.0 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	−1089.1 kJ/mol
Gibbs free energy (ΔfG⦵)	−998.3 kJ/mol
Enthalpy of fusion (ΔfH⦵fus)	100 kJ/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Preparation

Gallium trioxide is precipitated in hydrated form upon neutralization of acidic or basic solution of gallium salt. Also, it is formed on heating gallium in air or by thermally decomposing gallium nitrate at 200–250 °C.

Crystalline Ga₂O₃ can 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^[14] while α-Ga₂O₃ is the most stable polymorph under high pressures.^[15]

• β-Ga₂O₃ epitaxial thin films can be deposited heteroepitaxially on substrates such as sapphire, GaN, SiC, and Si, as well as homoepitaxially. For example, ALD on sapphire substrates at temperatures between 190 °C and 550 °C have been demonstrated.^[16] High-quality β-Ga₂O₃ films have also been grown using techniques such as MBE, HVPE, and MOVPE.^[17] HVPE is preferred for vertical power semiconductor devices due to its fast growth rate.^[18] β-Ga₂O₃ epitaxial films grown by MOVPE exhibit higher electron mobilities and lower background carrier concentrations than those grown by other thin-film growth techniques.^[19][20]

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.^[21][22]

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 a corundum 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.^[23]
• δ-Ga₂O₃ is obtained by heating Ga(NO₃)₃ at 250 °C.^[24]
• ε-Ga₂O₃ is prepared by heating δ-Ga₂O₃ at 550 °C.^[14] Thin films of ε-Ga₂O₃ are deposited by means of metalorganic vapour-phase epitaxy using trimethylgallium and water on sapphire substrates at temperatures between 550 and 650 °C^[25]

Reactions

Gallium(III) trioxide is amphoteric.^[26] It reacts with alkali metal oxides at high temperature to form, e.g., NaGaO₂, and with Mg, Zn, Co, Ni, Cu oxides to form spinels, e.g., MgGa₂O₄.^[27] It dissolves in strong alkali to form a solution of the gallate ion, Ga(OH)⁻
₄.

With HCl, it forms gallium trichloride GaCl₃.^[28]

Ga₂O₃ + 6 HCl → 2 GaCl₃ + 3 H₂O

It can be reduced to gallium suboxide (gallium(I) oxide) Ga₂O by H₂.^[29] or by reaction with gallium metal:^[30]

Ga₂O₃ + 2 H₂ → Ga₂O + 2 H₂O

Ga₂O₃ + 4 Ga → 3 Ga₂O

Structure

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.^[31]

α-Ga₂O₃ has the same structure (corundum) as α-Al₂O₃, wherein Ga ions are 6-coordinate.^[32][33]

γ-Ga₂O₃ has a defect spinel structure similar to that of γ-Al₂O₃.^[34]

ε-Ga₂O₃ films deposited by metalorganic vapour-phase epitaxy show a columnar structure with orthorhombic crystal symmetry. Macroscopically, this structure is seen by X-ray crystallography as hexagonal close packed.^[35]

κ-Ga₂O₃ has an orthorhombic structure and forms with 120° twin domains, resulting in hexagonal symmetry which is often identified as ε-Ga₂O₃.^[36]

Phase of Ga2O3	Figure	Crystal structure name
α	Crystal structure of α-Ga2O3[33]	Rhombohedral (Corundum)
β	Crystal structure of β-Ga2O3	Monoclinic
γ	Crystal structure of γ-Ga2O3[37]	Cubic defect spinel
δ	Crystal structure of δ-Ga2O3[38][39]	Body-centered cubic bixbyite
ε	Crystal structure of ε-Ga2O3[40]	Hexagonal
κ (subgroup of ε phase)[41]	Crystal structure of κ-Ga2O3[42]	Orthorhombic

Electronic and thermal properties

Applications

Gallium(III) oxide has been studied for usage as passive components in lasers,^[43] phosphors,^[5] and luminescent materials^[44] as well as active components for gas sensors,^[6] power diodes,^[45] and power transistors.^[46][47] Since the first publication in January 2012 by the National 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 for power electronics.^[48][7]

Monoclinic β-Ga₂O₃ has shown increasing performance since 2012 approaching state of the art GaN and SiC power devices.^[7] β-Ga₂O₃ Schottky diodes have exceeded breakdown voltages of 2400 V.^[45] β-Ga₂O₃/NiO_x p–n diodes have exhibited breakdown voltages over 1200 V.^[49] β-Ga₂O₃ MOSFETs have individually achieved figures of merits of  f_T of 27 GHz,^[46] f_MAX of 48 GHz,^[47] and 5.4 MV/cm average breakdown field.^[47] This field exceeds that which is possible in SiC or GaN.

ε-Ga₂O₃ thin films deposited on sapphire show potential applications as solar-blind UV photodetector.^[8]