THERMOELECTRIC-POWER OF AMORPHOUS-SILICON UNDER ILLUMINATION

Both the photoconductivity a and the photothermoelectric power S have been measured on undoped, phosphorus-doped and boron-doped amorphous s ilicon films prepared by r.f. decomposition of the appropriate silane gas mixture. A significant feature of the work was the preparation of suitable contacts to the amorphous film. Considerable care was require d to reduce photovoltaic effects to a sufficiently low level before me asurements could be made. Measurements of a and S are presented in the temperature range 150-500 K at a number of illumination intensity lev els. Room-temperature measurements are also given asa function of inci dent photon energy. For each sample the Q function, defined as Q = 1n sigma + \eS\k, is independent of illumination intensity and varies lin early with reciprocal temperature. It is concluded that photoconductio n between 150 and 500 K occurs through extended states, in the conduct ion band for the undoped and n-type samples and in the valence band fo r the p-type sample. The temperature variation in the photoconductivit y and the energy transported by the carriers as measured by the thermo electric power are determined solely by the position of the quasi-Ferm i level.