Crystal orientation dictated epitaxy of ultrawide-bandgap 5.4- to 8.6-eV α-(AlGa) ₂ O ₃ on m-plane sapphire

Abstract

Ultrawide-bandgap semiconductors are ushering in the next generation of high-power electronics. The correct crystal orientation can make or break successful epitaxy of such semiconductors. Here, it is found that single-crystalline layers of α-(AlGa)2O3 alloys spanning bandgaps of 5.4 to 8.6 eV can be grown by molecular beam epitaxy. The key step is found to be the use of m-plane sapphire crystal. The phase transition of the epitaxial layers from the α- to the narrower bandgap β-phase is catalyzed by the c-plane of the crystal. Because the c-plane is orthogonal to the growth front of the m-plane surface of the crystal, the narrower bandgap pathways are eliminated, revealing a route to much wider bandgap materials with structural purity. The resulting energy bandgaps of the epitaxial layers span a broad range, heralding the successful epitaxial stabilization of the largest bandgap materials family to date.

Strain states and relaxation for $$\alpha$$-(Al$$_x$$Ga$$_{1-x}$$)$$_2$$O$$_3$$ thin films on prismatic planes of $$\alpha$$-Al$$_2$$O$$_3$$ in the full composition range: Fundamental difference of a- and m-epitaxial planes in the manifestation of shear strain and lattice tilt

“…In reports on α-(Al x Ga 1−x ) 2 O 3 layers on m-plane sapphire, only one asymmetric reflection is typically evaluated to determine strain state and lattice constants of the epilayer [33]. However, one should be aware that for an accurate determination of those properties, the measurement of several reflections is necessary to obtain correct results due to the aforementioned effects on this epitaxial plane.…”

“…Additionally, it can be grown heteroepitaxially with high quality on costeffective isostructural α-Al 2 O 3 substrates [17][18][19][20][21][22] (sapphire). It can be doped n-type utilizing, e.g., Si, Sn or F as dopants [22][23][24][25][26] and due to it being isostructural to α-Al 2 O 3 the full compositional range of α-(Al x Ga 1−x ) 2 O 3 from Ga 2 O 3 to Al 2 O 3 can be covered without miscibility gaps allowing bandgap engineering for HEMT structures or quantum wells from 5.3 eV to 8.8 eV [22,[27][28][29][30][31][32][33]. In most reports, α-Ga 2 O 3 and α-(Al x Ga 1−x ) 2 O 3 is grown on the basal c-plane of α-Al 2 O 3 [9, 17-22, 24, 28, 29, 34, 35].…”

“…In most reports, α-Ga 2 O 3 and α-(Al x Ga 1−x ) 2 O 3 is grown on the basal c-plane of α-Al 2 O 3 [9, 17-22, 24, 28, 29, 34, 35]. Lately, also the pyramidal r-plane [14,31,[36][37][38][39][40] as well as the prismatic a- [14,27,32,41,42] and m-plane [26, www.mrs.org/jmr © The Author(s) 2021 2 Invited Paper 33,[42][43][44] of sapphire were utilized as epitaxial plane for the material. Especially the last one became interesting for potential device applications due to doped m-plane α-Ga 2 O 3 exhibiting up to three times higher mobility than similar layers on c-plane sapphire [26].…”

“…Especially the last one became interesting for potential device applications due to doped m-plane α-Ga 2 O 3 exhibiting up to three times higher mobility than similar layers on c-plane sapphire [26]. Also, m-plane growth is beneficial for the stabilization of the α-phase due to the suppression of basal facets, where β-Ga 2 O 3 favourably forms [33].…”

“…This should manifest itself in a shear strain present for heterostructures on the m-plane in contrast to the a-plane, if the theoretically calculated C 14 component is correct. However, the epitaxial r-plane is relatively insensitive to the C 14 component [31] and the m-plane α-(Al x Ga 1−x ) 2 O 3 epilayers in literature were typically relaxed [33]. To the best knowledge of the authors, pseudomorphic growth on m-plane sapphire substrates was not yet reported.…”