Thin films of amorphous bulk Se

0.70

Ge

0.15

Sb

0.15

system are deposited on a quartz and glass substrates at 300K by the thermal evaporation technique.The amorphous films were annealed at 370 and 470K in vacuum ～10

-4

Pa for 1h.The as-deposited and annealed films were checked by X-ray diffraction.On annealing at 470K(in the same time and vacuum)

the films revealed crystalline nature.The optical constants such as refractive index(

n

)

absorption coefficient(

α

)and extinction coefficient(

k

) were determined from the measured transmittance and reflectance at normal incidence of light in the wavelength range 300～2500nm.It was found that both(

n

) and (

k

) depend markedly on the temperature of heat treatment.The analysis of the optical data gave non-direct band gaps(

E

g

non

)of 1.715±0.021

1.643±0.021 and 1.572±0.021eV for as-deposited

370K and 470K annealed samples respectively.The annealing temperatures are decreased the band gap(

E

g

non

)and increasing the band-tail(

E

e

).This effect is interpreted in terms of the density of state model in amorphous solids proposed by Mott and Davis.The Wemple-DiD omenico single oscillator model parameterizes for as-deposited and annealed films are determined.