Heteroepitaxial growth of single-phase ε-Ga₂O₃ thin films on c-plane sapphire by mist chemical vapor deposition using a NiO buffer layer

Abstract

In this study, single-phase ε-gallium oxide (Ga2O3) thin films were heteroepitaxially grown on c-plane sapphire substrates.

Ga2O3polymorphs: tailoring the epitaxial growth conditions

“…1 reports the Ga 2 O 3 phase resulting from different Ga : Cl ratio and growth temperature (T g ) for mist-CVD and MOVPE. The values for mist-CVD are obtained from published reports [45][46][47][48][49][50][51] as well as from private communications with the corresponding authors of the cited studies. In mist-CVD the Ga : Cl ratio is calculated considering the molar ratio between the gallium(III) acetylacetonate (Ga(acac) 3 ) and the HCl (37%) used to prepare the feeding solution.…”

“…The Ga 3+ and Cl À ions and H 2 O molecules are supposed to be vaporized, transported in the gas phase and able to reach the substrate with the same ratio defined in the solution. 52 Arata et al 50 used GaCl dissolved in deionized H 2 O, resulting in Ga : Cl = 1. For MOVPE, the work of Sun et al 42 is the only one where HCl is added to the gas phase: in this case the Ga : Cl value results from the ratio between the TEG (triethyl-gallium) and HCl flows, privately communicated by the corresponding author of the paper.…”

“…42,53 Higher temperatures favour the b phase, as it happens for mist-CVD. 50 Chlorine seems to play an important role in the stabilization of different Ga 2 O 3 polymorphs. A secondary ion mass spectroscopy (SIMS) investigation on different Ga 2 O 3 layers 53 evidenced a higher amount of Cl incorporated in the e-phase with respect to the a one (5 Â 10 17 vs. 3 Â 10 15 atoms per cc, respectively); this happens despite the lower O 2 : HCl ratio employed to grow the a-Ga 2 O 3 layer.…”

“…It is noteworthy that the addition of a small buffer layer of NiO(111) on the sapphire substrate permits one to switch from a-Ga 2 O 3 to e-Ga 2 O 3 growth in the whole 400-800 1C range. 50 A lower mismatch between the substrate and the e-Ga 2 O 3 epilayer also appears to facilitate the growth of this phase; with the same HVPE growth conditions that resulted in a-Ga 2 O 3 layers on c-plane sapphire substrates, (001)-oriented e-Ga 2 O 3 was obtained on GaN(0001), AlN(0001), and b-Ga 2 O 3 (% 201), the latter exhibiting superior crystal quality. 29,[88][89][90] A similar trend has been reported for Mist-CVD growth, where the use of MgO(111) instead of YSZ(111) substrates (with 2.5 and 20% nominal mismatch with e-Ga 2 O 3 , respectively) allowed one to obtain single-phase (001)-oriented e layers at much lower temperatures.…”

Formation of High‐Quality Heteroepitaxial β‐Ga2O3 Films by Crystal Phase Transition

“…[ 14–18 ] In contrast, ɛ‐Ga 2 O 3 , the second stable phase of the Ga 2 O 3 polymorphs, has good lattice matching and crystalline symmetry for several hexagonal and quasi‐hexagonal substrates, including sapphire. [ 19–22 ] Thus, ɛ‐Ga 2 O 3 grown on sapphire (or other substrates with hexagonal symmetry) appeared good surface morphology and crystallinity. However, compared to β‐Ga 2 O 3 , achievement of high electrical conductivity by doping and activation is relatively hard in ɛ‐Ga 2 O 3 because of its electrical nature.…”