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In this paper, we present a review of experimental results examining charged defect components in the Al2O3/In0.53Ga0.47As metal-oxide-semiconductor (MOS) system. For the analysis of fixed oxide charge and interface state density, an approach is described where the flatband voltage for n- and p-type Al2O3/In0.53Ga0.47As MOS structures is used to separate and quantify the contributions of fixed oxide charge and interface state density. Based on an Al2O3 thickness series (10-20 nm) for the n- and p-type In0.53Ga0.47As layers, the analysis reveals a positive fixed charge density ( ~ 9 ×1018 cm-3) distributed throughout the Al2O3 and a negative sheet charge density (- 8 × 1012 cm-2) located near the Al2O3/In0.53Ga0.47As interface. The interface state density integrated across the energy gap is ~1 ×1013 cm-2 and is a donor-type (+/0) defect. The density of the fixed oxide charge components is significantly reduced by forming gas (5 % H2/ 95% N2 ambient at 350 °C for 30 minutes) annealing. The interface state distribution obtained from multi-frequency capacitance-voltage and conductance-voltage measurements on either MOS structures or MOSFETs indicates a peak density located around the In0.53Ga0.47As midgap energy, with a sharp increase in the interface state density toward the valance band and evidence of interface states aligned with the In0.53Ga0.47As conduction band. The integrated interface state density obtained from multi-frequency capacitance-voltage and conductance-voltage analysis is in good agreement with the approach of comparing the flatband voltages in n- and p -type Al2O3/In- .53Ga0.47As MOS structures. Finally, this paper reviews recent work based on an optimization of the In0.53Ga0.47As surface preparation using (NH4)2S, combined with minimizing the transfer time to the atomic layer deposition reactor for Al2O3, which indicates interface state reduction and genuine surface inversion for both n- and p -type Al2O3/In0.53Ga0.47As MOS structures.