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].