Dominant Surface Electronic Properties of SiO2-Passivated Ge Surfaces as a Function of Various Annealing Treatments

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Abstract
The metal-oxide-semiconductor (MOS), capacitance vs applied voltage (C-V) measurement technique was used to investigate SiO2-passivated Ge surfaces as a function of high-temperature annealing treatments in H2, O2, N2, and H2O. Capacitance measurements were made at high frequencies (100 MHz) and/or reduced temperatures (145°, 77°K) to eliminate the capacitive contribution of fast surface states to the ac signal. Since the surface states did equilibrate with the superimposed applied dc voltage, is was possible to determine surface-state type, energy, and densityfrom voltage shifts in the C-V traces. Hydrogen annealing at 600°C or above introduces a high density (>1013/cm2) of fast acceptor states. These states are located at the center of the Ge energy gap. In addition, hydrogen annealing at 700°–800°C introduces acceptors close to the valence band or fixed negative charge. Most of these hydrogen-induced states are removed by annealing in oxygen or water vapor at 230°–440°C. At the same time the oxygen or water vapor exposure introduces an effective positive charge density of 1–5×1012/cm2. This positive charge is due either to donors lying close to the conduction band edge or to fixed positive charge. The above effects are reversible.