Deep-depletion high-frequency capacitance-voltage responses under photonic excitation and distribution of interface states in MOS capacitors

Probing the deep-depletion mode high-frequency capacitance-voltage (C-V) responses of MOS capacitors under photonic excitation, the distribution (D/sub it/) of the interface states (E/sub t/) at Si/SiO/sub 2/ heterojunctions was characterized. A photon energy E/sub ph/=0.943 eV (less than the silicon energy bandgap) was employed for the characterization of interface states in the photoresponsive energy band. In order to limit the photonic contribution to the change in the gate capacitance by excess carriers excited only from the interface states, a fast dc sweep rate (50 mV/s) with a high-frequency (500 kHz) small-signal was applied. Photonic C-V responses with a slow dc sweep for E/sub ph/=0.943 eV and a fast sweep for E/sub ph/=1.481 eV have been also provided for a comparison. Modulating the surface potential with the gate bias, a U-shaped distribution of D/sub it/ was obtained over the photoresponsive energy band in (E/sub C/-E/sub ph/)<E/sub t/<(E/sub i/-q/spl phi//sub f/+q/spl phi//sub s/) from NMOS and (E/sub i/+q/spl phi//sub s/-q/spl phi//sub f/)<E/sub t/<(E/sub V/+E/sub ph/) from PMOS capacitors. D/sub it,max/ was observed to be /spl sim/1.76/spl times/10/sup 11/ cm/sup -2/eV/sup -1/ on the edges of conduction and valence bands.