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

beta-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 beta-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) beta-Ga2O3 capacitors was comprehensively investigated. As in previous studies, PDA at 700 degrees C and higher sharply reduced the capacitor leakage current by three orders of magnitude. This threshold temperature was 100 degrees 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 degrees C according to an x-ray diffraction analysis. Selective-area electron diffraction (SAED) patterns showed that the crystallized films comprised twinned gamma-Al2O3, wherein the (111) planes are parallel to the sawtooth beta-Ga2O3 (101) planes with epitaxial relations of gamma-Al2O3 [ 0 1 over bar 1 ] || beta-Ga2O3 [010] and gamma-Al2O3 [ 01 1 over bar ] || beta-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 gamma-Al2O3 [ 2 1 over bar 1 over bar ], [111], and [ 01 1 over bar ] directions, respectively. Furthermore, the SAED patterns displayed diffraction spots specific to triaxially tripled gamma-Al2O3. This is yet to be identified as delta-Al2O3. Contrary to expectations, PDA magnified the bias instability of beta-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 beta-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) beta-Ga2O3 interface can be ascribed to the aforementioned small lattice misfit at the gamma-Al2O3/(101) beta-Ga2O3 interface, which contrasts with the 12% misfit at the gamma-Al2O3/(0001) GaN interface. These findings form the foundation for developing technologies to enhance the performance and reliability of ALD-Al2O3/beta-Ga2O3 devices. Specifically, based on them, a guideline for reducing the bias instability is proposed.