Postdeposition annealing effect on atomic-layer-deposited Al2O3 gate insulator on (001) β-Ga2O3

• Published in: Journal of vacuum science and technology. B, Nanotechnology & microelectronics Nanotechnology & microelectronics, 2021-12, Vol.39 (6)
• Main Authors: Hiraiwa, Atsushi, Horikawa, Kiyotaka, Kawarada, Hiroshi, Kado, Motohisa, Danno, Katsunori
• Format: Article
• Language: English
• ISSN: 2166-2746; 2166-2754
• DOI: 10.1116/6.0001360

β-Ga2O3 is gaining increasing attention from power device engineers owing to its wide bandgap and fabrication potential of low-cost, large-diameter substrates. Atomic-layer-deposited (ALD) Al2O3 has application potential for the gate insulation and surface passivation of β-Ga2O3 devices, which cannot incorporate a well-established SiO2/Si system. To improve the device performance and reliability, the effect of postdeposition annealing (PDA) on the gate insulation characteristics of Al/ALD-Al2O3/(001) β-Ga2O3 capacitors was comprehensively investigated. As in previous studies, PDA at 700 °C and higher sharply reduced the capacitor leakage current by three orders of magnitude. This threshold temperature was 100 °C lower than that for GaN devices. Space-charge-controlled field emission analysis revealed that the current reduction was achieved via conduction-band-offset enhancement from 1.45 to 2.2 eV. These changes were caused by Al2O3 crystallization, which started at 650 °C according to an x-ray diffraction analysis. Selective-area electron diffraction (SAED) patterns showed that the crystallized films comprised twinned γ-Al2O3, wherein the (111) planes are parallel to the sawtooth β-Ga2O3 (101) planes with epitaxial relations of γ-Al2O3 [ 0 1 ¯ 1 ] || β-Ga2O3 [010] and γ-Al2O3 [ 01 1 ¯ ] || β-Ga2O3 [010]. This epitaxy was realized by three-dimensional oxygen sublattice matching with relatively small misfits of less than 1%, 1%, and 8% along the γ-Al2O3 [ 2 1 ¯ 1 ¯ ], [111], and [ 01 1 ¯ ] directions, respectively. Furthermore, the SAED patterns displayed diffraction spots specific to triaxially tripled γ-Al2O3. This is yet to be identified as δ-Al2O3. Contrary to expectations, PDA magnified the bias instability of β-Ga2O3 capacitors, supposedly owing to the Al2O3 and Ga2O3 solid-solution reaction, which contrasts with the previous significant improvement in GaN capacitors. However, PDA negligibly affected the β-Ga2O3 capacitor interface characteristics. This result also contrasts sharply with the previous results obtained for GaN capacitors that experienced a PDA-induced increase in both interface states and flat-band voltage. This apparent thermal stability of Al2O3/(001) β-Ga2O3 interface can be ascribed to the aforementioned small lattice misfit at the γ-Al2O3/(101) β-Ga2O3 interface, which contrasts with the 12% misfit at the γ-Al2O3/(0001) GaN interface. These findings form the foundation for developing technologies to enhance the performance and reli