The influence of the power density, PD, on the density and structure of defects of undoped a-Si:H thin films, deposited by radio-frequency PECVD, was studied by the constant photo-current method, and by slow positron beam spectroscopy, respectively. The deep-defect density, NDD, remained approximately constant at 1016/cm3 for PD in the range of 7 to 20mW/cm3, as calculated using the constant photo-current method. Out of this range, NDD increased by roughly an order of magnitude for both lower and higher power densities. Positron annihilation spectroscopy revealed the predominance of 2 kinds of vacancy-type defect in the films: large vacancy clusters or voids for PD ~ 7mW/cm3 and small vacancy type defects as PD increased to ~30mW/cm3.

Influence of the a-Si:H Structural Defects Studied by Positron Annihilation on Solar Cells Characteristics. A.Amaral, G.Lavareda, C.Nunes de Carvalho, P.Brogueira, P.M.Gordo, V.S.Subrahmanyam, C.L.Gil, M.D.Naia, A.P.de Lima: Thin Solid Films, 2002, 403-404, 539-42