Wilfried Favre

Observation by conductive-probe atomic force microscopy of strongly inverted surface layers at the hydrogenated amorphous silicon/crystalline silicon heterojunctions

Heterojunctions made of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) are examined by conducting probe atomic force microscopy. Conductive channels at both (n)a-Si:H/(p)c-Si and (p)a-Si:H/(n)c-Si interfaces are clearly revealed. These are attributed to two-dimension electron and hole gases due to strong inversion layers at the c-Si surface in agreement with previous planar conductance measurements. The presence of a hole gas in (p)a-Si:H/(n)c-Si structures implies a quite large valence band offset (EVc-Si−EVa-Si:H>0.25 eV).

Band bending and determination of band offsets in amorphous/crystalline silicon heterostructures from planar conductance measurements

An analytical model for the calculation of the band bending in amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions is presented and validated by comparison with full numerical simulations. The influence of the various structure properties and parameters, such as the density of states in bulk a-Si:H or at interface defects, the position of the Fermi level in a-Si:H, the temperature dependence of band gaps, is investigated. Significant band offsets imply the presence of a strong inverted layer at the c-Si surface of both (p)a-Si:H/(n)c-Si and (n)a-Si:H/(p)c-Si structures, forming two-dimensional hole and electron gases, respectively. This leads to high sheet carrier densities that have been evidenced from planar conductance measurements. Experimental data obtained on samples coming from various research institutes are analyzed with our model in order to extract the band offsets. We find that the valence band offset ranges between 0.32 and 0.42 eV with an average value at 0.36 eV; the conduction band...