Fig. 1 Crystal structure of (a) β-, (b) α-, (c) κ, (d) γ-, and (ɛ) δ-Ga2O3.




Fig. 2 Schematic of a typical oxide MOCVD system.




Fig. 3 Schematic of typical processes occurring in the MOCVD Ga2O3 reactor using TEGa and O2 as precursors: (a) precursor breakdown, (b) precursor prereactions, (c) by-product formation, (d) surface adsorption of adatoms, (e) surface desorption of adatoms, (f) surface diffusion of adatoms, (g) incorporation of adatoms at incorporation sites, and (h) gas transport by the carrier gas.




Table 1 Summary of the physical properties and the epitaxial growth methods utilized to grow the five different crystal phases of Ga2O3.

A. F. M. Anhar Uddin Bhuiyan, et al, J. Appl. Phys. 133, 211103 (2023)
https://doi.org/10.1063/5.0147787

(1) Unlike most other oxide materials with complex electron band structures, the β-Ga2O3 conduction band structure is primarily contributed by the Ga s orbitals, which is similar to the ones observed in GaN and GaAs.
(2) All major acceptor candidates for β-Ga2O3 act as deep acceptors indicating a very low activation efficiency and a trap-like behavior of the acceptors.
(3) Holes in many oxides (Ga2O3, In2O3, SnO2, and MgO) prefer the localized, self-trapped form of polarons around the O-site due to the characteristic lattice distortion.
(4) The advantage of scalable native substrates grown from melt is specific to Ga2O3, distinguishing it from other UWBG semiconductors.
(5) Mist-CVD growth technique is considered a low cost and simple approach, which is efficient in reducing the oxygen vacancies as the growth is performed under sufficient overpressure of oxygen using water/alcohol solutions.
(6) In the LPCVD growth of Ga2O3, high purity metallic Ga and O2 are used as sources, and SiCl4 is used as the n-type dopant. Growths are performed in the temperature range between 750 and 1050 °C.
(7) The use of off-axis c-sapphire substrates to overcome the issue caused by lattice-mismatch for heteroepitaxy.
(8) MOCVD growth of β-Ga2O3 has been demonstrated with high crystalline quality and effective n-type doping.
(9) Tetraethylorthosilicate (TEOS) and silane (SiH4) have been used as the Si source, whereas tetraethyltin (TESn) and germane (GeH4/N2) were used as the Sn and Ge sources, respectively.
(10) There is a strong memory effect of Ge and Sn doping dependent on the reactor geometry.
(11) Another key contribution of the MOCVD growth technique is the demonstration of high-quality β-(AlxGa1-x)2O3 epitaxy with high Al incorporation up to 40% on native (010) substrates.
(12) The determined band offsets revealed the formation of type-II (staggered gap) heterojunctions at (100). (001) and (010) oriented β-(AlxGa1-x)2O3/β-Ga2O3 interfaces and type-I (straddling gap) heterojunction between (-201) β-(AlxGa1-x)2O3/β-Ga2O3.
(13) High resolution STEM imaging, DLTS, DLOS, quantitative SIMS mapping, and electron paramagnetic resonance (EPR), together with first principle density functional theory can provide a big picture of defect identification.