Observation of negative capacitance in a-SiC:H/a-Si:H UV photodetectors

UV photosensors based on a-Si:H/a-SiC:H were deposited by ultra high vacuum plasma enhanced chemical vapour deposition system (UHV-PECVD) over a large area (10 cm · 10 cm) in p–i–n configuration. The devices were characterized in the spectrum range 200–800 nm. They showed a good sensitivity in the UV range and a rejection of visible light. A relation was found between spectral response and the thickness of p- and i-layers. A linear dependence of the photocurrent as a function of photon flux at constant wavelength (365 nm) was found. The electrical properties were correlated with the deposition conditions. Negative values of capacity at high frequency were also found in all devices. These values appear at different critic frequencies and correspond to the maximum of the device conductance. The ability to obtain regular capacitive shifts simply by reversing the signal may open research for novel devices and applications.