The influence of oxidation rate and heat treatment on the Si surface potential in the Si-SiO2system

Thermal oxidation of silicon results in a nonuniform distribution of defects in the oxide and in a Si-SiO2interface characterized by a certain disorder. The Si surface potential is determined by the defect structure of the oxide and phase boundary. High temperature postoxidation treatment leads to a redistribution of the oxide defects and rearrangement of the interface. To study these effects, p- and n-type Si was oxidized in dry oxygen at 1145°C. The oxidation was followed by a heat treatment in a dry helium atmosphere at the oxidation temperature for various periods of time (2 minutes to 65 hours). The density of the charged oxide and interface states was determined by MOS capacitance measurements. It was found that the density of the donor states decreases logarithmically with the annealing time. Since such a behavior is typical of adsorption phenomena that are associated with modifications of the surface structure, we attribute this change to a rearrangement of the phase boundary. Further evidence for this is the fact that the density of donor states for quenched samples increases with the oxidation rate within the regime of parabolic growth, n-type specimens also exhibited donor states but their distribution was different from that for p-type units. In the presence of slight traces of water vapor, redistribution of hydroxyls can become the predominant mechanism, especially for longer annealing times.