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_it) of the interface states (E_t) at Si/SiO₂ heterojunctions was characterized. A photon energy E_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_ph=0.943 eV and a fast sweep for E_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_it was obtained over the photoresponsive energy band in (E_C-E_ph)t<(E_i-qφ_f+qφ_s) from NMOS and (E_i+qφ_s-qφ_f)t<(E_V+E_ph) from PMOS capacitors. D_it,max was observed to be ∼1.76×10¹¹ cm^-2eV^-1 on the edges of conduction and valence bands.