Realization of Conductive n‐Type Doped α‐Ga₂O₃ on m‐Plane Sapphire Grown by a Two‐Step Pulsed Laser Deposition Process

Abstract

Structural and electrical properties of undoped and doped α‐Ga2O3 thin films grown by pulsed laser deposition on m‐plane sapphire in a two‐step process are presented. A buffer layer of undoped α‐Ga2O3 is introduced below the electrically active thin film to improve the crystal quality and enable the stabilization of the α‐phase at lower substrate temperatures for sufficient dopant incorporation. Donor doping of the active layers with tin, germanium, and silicon, respectively, is realized below a critical substrate temperature of 600 °C. Depth‐resolved X‐ray photoelectron spectroscopy measurements on tin‐doped samples reveal a lower amount of tin in the bulk thin film compared to the surface and a lower tin incorporation for higher substrate temperatures, indicating desorption or float‐up processes that determine the dopant incorporation. Electron mobilities as high as 17 cm2 V−1 s−1 (at 1.4 × 10 19 cm − 3 ) and 37 cm2 V−1 s−1 (at 3.7 × 10 18 cm − 3 ) are achieved for tin‐ and germanium doping, respectively. Further, a narrow window of suitable annealing temperature from 680 to 700 K for obtaining ohmic Ti/Al/Au layer stacks is identified. For higher annealing temperatures, a deterioration of the electrical properties of the thin films is observed suggesting the need for developing low temperature contacting procedures for α‐Ga2O3‐based devices.

Bandgap engineering of spinel-structured oxide semiconductor alloys

“…According to the empirical doping rules for oxides, VBM must be higher than about −6.5 eV for hole doping and CBM must be lower than about −3.7 eV for electron doping [64][65][66]. For example, corundumstructured α-Ga 2 O 3 is controllable only for the electron carrier concentration [72][73][74][75][76][77][78][79][80], despite its large bandgap (E g ∼ 5.6 eV) [81]. The asymmetric doping trend of α-Ga 2 O 3 is consistent with the empirical VBM and CBM position criteria [82].…”

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

“…The success in achieving conductive films with band gaps approaching 7 eV suggests the promise of further optimizing conductive high-Al content α-(Al,Ga) 2 O 3 layers through MOCVD. 68) mist-CVD, [29][30][31]63) PLD, 26) and implantation. 69) In this work, the Si-doped (AlGa) 2 O 3 layers with [Si] = 3 × 10 20 cm −3 were grown at 2.6 kPa and 900 °C with the VI/III ratio between 788 and 3193.…”

“…20,24) Common (AlGa) 2 O 3 /Ga 2 O 3 MODFETs consist of a donordoped (AlGa) 2 O 3 barrier layer and an undoped Ga 2 O 3 channel layer. (AlGa) 2 O 3 layers have been grown by using MBE, 21,22,25) pulse-laser deposition (PLD), [26][27][28] MOCVD, 23) and mist CVD. [29][30][31][32][33] High-quality α-(AlGa) 2 O 3 growth favors m-plane (10_10) sapphire substrates.…”

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

“…1,2) High-power applications require high-Al composition in (AlGa) 2 O 3 layers because the E c of (Al x Ga 1−x ) 2 O 3 increases with increasing Al composition (x). 3,4) (AlGa) 2 O 3 layers have been grown on α-Al 2 O 3 and β-Ga 2 O 3 substrates by using metal-organic CVD, [5][6][7][8] mist CVD, [9][10][11][12] pulse-laser deposition (PLD), 13,14) and plasma-assisted MBE (PAMBE). [15][16][17][18][19] Especially, (AlGa) 2 O 3 layers grown on α-Al 2 O 3 substrates are much attractive for high-power applications because of large-scale α-Al 2 O 3 wafers at a low cost.…”

“…Using m-plane (101 ¯0) α-Al 2 O 3 substrates, the (AlGa) 2 O 3 layers can have the α single phase for the whole Al composition range. 5,13,16,23) For α-(AlGa) 2 O 3 , Si, Ge, and Sn atoms are available as donors. 24,25) The Sn-doped α-Ga 2 O 3 layers on (101 ¯0) α-Al 2 O 3 substrates had a higher electron mobility than those on (0001) α-Al 2 O 3 substrates.…”