Si-doped (AlGa)₂O₃ growth on a-, m- and r-plane α-Al₂O₃ substrates by molecular beam epitaxy

Abstract

We reported the growth of (AlGa)2O3 layers on (11-20), (10-10), and (10-12) α-Al2O3 substrates using molecular beam epitaxy, and the electrical characterization of Si-doped (AlGa)2O3 layers. The Ga2O3 layers grown on (10-10) and (10-12) α-Al2O3 were α-phase single crystals, while the Ga2O3 layer grown on (11-20) α-Al2O3 consisted of (11-20) α-Ga2O3 and (-201) β-Ga2O3. The Al composition of the (11-20) and (10-10) (AlGa)2O3 layers was controlled by varying the Al flux. The Si-doped (10-10) α-(Al x Ga1-x )2O3 layers with x = 0-0.41 were electrically conducting.

MOCVD growth of Si-doped α-(AlGa)2O3 on m-plane α-Al2O3 substrates

“…The bandgap energy of α-(Al 0.56 Ga 0.44 ) 2 O 3 is estimated to be 6.88 eV. 34) Still, σ of the MOCVD-grown (AlGa) 2 O 3 layers were small, which we expect is likely due to the larger amount of compensating centers incorporated at the higher growth temperatures and additional alloy scattering that reduces the mobility relative to α-Ga 2 O 3 . As previously mentioned, the larger band gap alloys are more amenable to the formation of compensating acceptors like cation vacancies that can complex and incorporate with hydrogen, as well as complex with incorporated Si shallow donors (which are already favorable in the limit of no Al).…”

“…The σ of the Si-doped (AlGa) 2 O 3 layer decreased with increasing Al composition, corresponding to the Si-doped (AlGa) 2 O 3 layers grown by mist CVD and MBE. 31,34) Previous DFT calculations have identified that the ionization energies of several shallow donors tend to increase with Al composition, as well an increased solubility of compensating acceptors like cation vacancies that become more favorable with larger band gaps and donor-doping, which both contribute to a decreased σ due to the small carrier concentration depending on the dopant choice and alloy composition. 30) For example, Sn dopants that have been previously used in the pure Ga 2 O 3 films become inactive at ∼30% Al, indicating the need for Si doping that we have pursued in this work for higher Al contents.…”

“…In a densityfunctional theory (DFT) calculation, Si is expected to act as a more effective donor dopant than Sn and Ge for α-(AlGa) 2 O 3 , acting as a shallow donor across a wider composition range (e. g. up to ∼70% Al in solid solution alloys). 35) The Si doping of α-(AlGa) 2 O 3 layers using MBE and mist CVD showed electrical conductance up to the Al composition of 41% and 30%, respectively, 31,34) which can have the ΔE c of ∼1 eV. However, there are few reports of donor-doped α-(AlGa) 2 O 3 growth by MOCVD.…”

“…[29][30][31][32][33] High-quality α-(AlGa) 2 O 3 growth favors m-plane (10_10) sapphire substrates. 34) In comparison with MBE and PLD, MOCVD and mist-CVD methods have the advantages of a high growth rate and large wafer capability, achieving α-(AlGa) 2 O 3 growth at the growth rate over 1 μm/h. 23,31) MODFETs require the donor-doped α-(AlGa) 2 O 3 layers, which supply electrons in the 2DEG channel and enhance carrier confinement as the Al composition increases.…”