The drift mobility and decay of photocurrent in doped amorphous diamond-like carbon films

This paper presents results of the measurements of dark current and photocurrent, mobility, and decay time of photocurrent of doped and undoped a:DLC films as a function of temperature. The a:DLC films were grown by a r.f. glow discharge technique using methane gas, CH4, as a source of carbon. Several films were doped employing iodine (I-2) as doping gas. a:DLC films have shown the photoconductivity effect for a wide range of temperatures. A maximum photosensitivity ratio sigma(ph)/sigma(d)(sigma(ph) is the conductivity under irradiation and sigma(d) the dark conductivity) of 1100 was achieved for iodine doped a:DLC films (I-DLC) at T= 140 K. The drift mobility was determined by the steady-state photocurrent and photocurrent decay time measurement method as a function of temperature. The highest drift mobility, 4.2 x 10(-7) cm(2) V-1 s(-1), was obtained for I-2 doped films at room temperature. The drift mobility at high temperature is thermally activated with an energy of 0.16 eV for doped and 0.13 eV for undoped films. In this case, it was assumed that the temperature activated behavior results from the interaction of free carriers with traps below the extended state transport levels. However, for low temperatures it was observed that the drift mobility is weakly dependent on temperature. This is due to the hopping transport mechanism. (C) 1997 Elsevier Science S.A.