Conduction- and valence-band offsets at the hydrogenated amorphous silicon-carbon/crystalline silicon interface via capacitance techniques

Abstract

Using a combination of junction capacitance techniques, we measure both the a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H/c-Si conduction- and valence-band offsets in a Schottky diode heterostructure sample composed of a sub-\ensuremath{\mu}m-thick layer of intrinsic hydrogenated amorphous silicon-carbon (a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H) deposited on an n-type crystalline silicon (c-Si) substrate. A series of these heterostructure samples with amorphous optical gaps ranging from 1.75 to 2.1 eV were fabricated using plasma-enhanced chemical-vapor deposition. First, a thermally activated capacitance step due to the response of defects at the amorphous/crystalline interface is evident in capacitance vs temperature (C-T) scans taken on all these samples. The bias dependence of this step's activation energy provides a direct measure of the a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H/c-Si interface potential as a function of the c-Si depletion width in each heterostructure. By application of Poisson's equation, we find that the a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H/c-Si conduction-band offset \ensuremath{\Delta}${\mathit{E}}_{\mathit{C}}$ increases as the optical gap widens and varies in the range of 0.00--0.20 for the samples investigated. Second, while under reverse bias at low temperature, we optically pulsed each sample with c-Si band-gap light to create trapped holes at the a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H/c-Si valence-band offset \ensuremath{\Delta}${\mathit{E}}_{\mathit{V}}$. By noting the threshold for the subsequent optical release of these trapped holes by subband-gap light, we found that \ensuremath{\Delta}${\mathit{E}}_{\mathit{V}}$ increases from 0.65 to 0.83 eV in the alloy range investigated. Finally, using the known crystalline silicon band gap, we directly determine the mobility gap for each of the amorphous samples. \textcopyright{} 1996 The American Physical Society.