Metal oxide catalyzed epitaxy (MOCATAXY) of β-Ga2O3 films in various orientations grown by plasma-assisted molecular beam epitaxy

Abstract

In this work, the growth of (010), (001), and 2¯01 β-Ga2O3 by plasma assisted molecular beam epitaxy was investigated. The presence of an indium flux during growth markedly expands the growth regime for β-Ga2O3 across all orientations to higher growth temperatures and growth rates. This metal oxide catalyzed growth allows for similar growth rates of around 5 nm/min across all three orientations, more than twice that of conventional (010) growth and seven times that of (001) growth without indium. Smooth surface morphologies for (010) and (001) β-Ga2O3 were demonstrated, while 2¯01 was significantly rougher. Additionally, doping with Sn was achieved across all orientations and Hall measurements demonstrated higher electron mobility (55 cm2/Vs for a carrier concentration of 3.3 × 1018 cm−3) for (001) β-Ga2O3 grown via metal oxide catalyzed epitaxy than conventional growth.

Ga2O3polymorphs: tailoring the epitaxial growth conditions

“…Moreover, it is worth mentioning that In-mediated MBE growth on (% 201) b-Ga 2 O 3 single crystalline substrates under growth conditions that would otherwise allow for the synthesis of e layers on c-plane sapphire substrates results in the preservation of the underlying substrate monoclinic structure. 12,13 This is surprising considering that (i) the growth of e-Ga 2 O 3 on c-plane sapphire substrates is usually happening on top of (% 201) b-Ga 2 O 3 interlayers, and (ii) the deposition of e-Ga 2 O 3 layers has been already previously reported on top of b-Ga 2 O 3 bulk substrates via HVPE. 26 One possible explanation relies on the formation of wide facets [i.e., different surfaces not parallel to the (% 201) one] during MBE (% 201)-homoepitaxy, 12,13 which could favour the stabilization of the monoclinic polymorph, similarly to what has been previously discussed in the case of a-Ga 2 O 3 on r-plane sapphire substrates.…”

“…12,13 This is surprising considering that (i) the growth of e-Ga 2 O 3 on c-plane sapphire substrates is usually happening on top of (% 201) b-Ga 2 O 3 interlayers, and (ii) the deposition of e-Ga 2 O 3 layers has been already previously reported on top of b-Ga 2 O 3 bulk substrates via HVPE. 26 One possible explanation relies on the formation of wide facets [i.e., different surfaces not parallel to the (% 201) one] during MBE (% 201)-homoepitaxy, 12,13 which could favour the stabilization of the monoclinic polymorph, similarly to what has been previously discussed in the case of a-Ga 2 O 3 on r-plane sapphire substrates. 82 An interesting contribution about the role of the substrate in the nucleation of the a or e phase is given in ref.…”

“…e-Ga 2 O 3 layers in the presence of the catalysing element Sn were also reported for PLD grown thin films on MgO(111), SrTiO 3 (111), and yttria-stabilized ZrO 2 (111) substrates. 70,79 Notably, MBE In-mediated metal-exchange catalysis has been also applied to widen the growth window of Ga 2 O 3 layers deposited on all the available b-Ga 2 O 3 bulk substrate orientations [i.e., (010), ( 100), (001), and (% 201)] 12,13,80 but differently from the discussed heteroepitaxial growth; in this case the deposited layers always preserved the monoclinic structure of the substrate.…”

“…This problem could be overcome either by growing on 4-61 off-oriented (100) substrates, 10 a methodology that proved to be effective but that ultimately makes the homoepitaxy less attractive, or by selecting a different substrate orientation for the homoepitaxy like (010) 11 or (001). 12,13 For the above reasons, in the past few years Ga 2 O 3 research has been extended towards the two phases that present higher symmetry and easier epitaxial conditions than b-Ga 2 O 3 , namely a and e. Their main properties are summarized in Table 1. Both polymorphs were obtained by a variety of epitaxial techniques on different hetero-substrates, mostly sapphire, as well as on b-Ga 2 O 3 .…”

Bandgap Tailoring of Monoclinic Single‐Phase β‐(AlxGa1−x)2O3 (0 ≤ x ≤ 0.65) Thin Film by Annealing β‐Ga2O3/Al2O3 Heterojunction at High Temperatures

“…The intensity difference may be caused by the nonuniform rate of Al occupation on different β-Ga 2 O 3 surfaces due to the discrepancy in formation energy. [42,43] No matter which method is used to obtain the β-(Al and β ¼ (103.87 þ 0.31x) , which follow Vegard's law [44] and Li et al's [26] report. Considering the monoclinic structure, the (À603) interplanar spacing can be expressed as…”