Growth characteristics of vacuum coated thick a-Se films for device applications

There has been a resurgence of interest in the growth characteristics of am orphous Se (a-Se) films due to their recently commercialized applications i n x-ray imaging as x-ray photoconductors. Electronic quality a-Se films req uire the substrate temperature during vacuum deposition to be as high as po ssible but increasing the substrate temperature leads to amorphous films co ntaining crystalline inclusions and eventually to polycrystalline films whi ch have high conductivity and are therefore not useful as photoconductors. We prepared a variety of thick a-Se films on Al substrates by conventional vacuum deposition in which only the substrate temperature T-sb and boat tem perature Tbt Were varied. Experiments indicated the appearance of crystalli ne zones (crystallites) in the amorphous Se film as the substrate temperatu re was increased. We identified an experimental critical T-bt-T-sb boundary between amorphous films with and without crystalline inclusions. A model h as been developed for the critical relationship between T-sb and T-bt, i.e. , T-bt=f(T-sb) based on two competing rate mechanisms: the rate of depositi on (or material condensation on the substrate) and rate of crystal growth i n the film assuming heterogeneous nucleation at favorable substrate sites. Over the narrow substrate temperature range, we approximated the Vogel-Tamm an-Fulcher behavior of the crystal growth rate by an Arrhenius rate with an apparent activation energy E-cr that accounts for most crystallization stu dies on a-Se. The condensation rate was assumed to follow an Arrhenius rate with the activation energy determined by the enthalpy of evaporation, E-ev . The T-bt=f(T-sb) model was particularly sensitive to the E-cr/E-ev ratio. By using the presently accepted values for E-ev and E-cr, it was shown tha t the experimental results could be well fitted to the T-bt=f(T-sb) model. The results have a wider significance than a-Se and can be easily extended to apply to a number of other chalcogenide semiconductor films. (C) 2000 Am erican Vacuum Society. [S0734-2101(00)07202-X].