Charge storage in undoped hydrogenated amorphous silicon by ambient atomic force microscopy

Hydrogenated amorphous silicon (a-Si:H) layers are prepared by plasma-enhanced chemical vapor deposition on metallized glass substrates. Ambient atomic force microscopy (AFM) is employed for both modification and characterization of a-Si:H films. Voltage pulses of up to 35 V are applied as a cantilever scans the amorphous silicon surface in contact mode AFM. Subsequent detection by Kelvin probe microscopy reveals a persistent negative charge stored in the a-Si:H layers. The stored charge is always negative independent of voltage polarity and results in an upward shift of the Fermi level by as much as 0.1 eV. Only at higher negative voltages (<−15 V) a positively charged oxide layer grows on the surface due to anodic oxidation. A model of the observed phenomena is proposed considering charge transport across a biased contact junction, metastable trapping in band-gap states of a-Si:H, as well as the influence of silicon oxide.