Density distribution of gap states in extremely thin a-Si:H layers on crystalline silicon wafers

Stimulated by the application of a-Si:H as ultra thin emitter layer in a-Si:H/c-Si heterostructure solar cells the electronic properties of 2–30 nm thin undoped and doped a-Si:H films have been explored using photoelectron yield spectroscopy excited with UV-light (4–7 eV). The films were deposited by plasma enhanced chemical vapor deposition (PECVD) or electron cyclotron resonance chemical vapor deposition (ECR-CVD) onto c-Si wafers. It was found that the Fermi level shift toward the conduction band saturates at 1.47 eV above the valence band edge at a phosphine gas phase concentration of 104 ppm. The Urbach energy increases with doping from 65 to 136 meV. For doped samples the main features of the density of states distribution change only little when the thickness decreases to values as low as 2.8 nm. In nominally undoped material the high Urbach energy and high defect density indicate enhanced disorder.