Photofield interface impurity spectroscopy in blocked impurity band Si:B structures

Abstract

The far infrared spectral response of photoconductive boron doped silicon blocked impurity band structures was measured at electric field values up to 7 kV/cm and sample temperatures in the range of 4–15 K. The structures are constructed of adjacent layers of pure (NA=5×1013 cm−3) and boron doped (NA=1018 cm−3) silicon inserted between highly doped ohmic contact layers. A broad-band photoresponse is observed which is attributed to the impurity band of the doped layer. In addition, several peaks of response are observed, even at the lowest sample temperatures, which are attributed to optical transitions from ground states to excited states of isolated impurities. It is shown that these transitions originate from a thin layer at the interface between the doped and the pure silicon regions. When the electric field is oriented towards the doped layer, the photocurrent peaks are attributed to the tunneling of photoexcited holes across excited states into near neighbor atoms within a hole depleted delta sheet, followed by relaxation to the ground state. Then hopping transport occurs across the doped layer to the ohmic contact. In this scheme, the hole depleted layer is generated by the effect of the applied electric field and involves only 109–1010 cm−2 impurities. This study provides evidence of a new mechanism for extrinsic photoconductivity at interfaces.