Urbach tail in amorphous gallium arsenide films.

Measurements using photothermal deflection spectroscopy (PDS) have been performed on samples of amorphous gallium arsenide deposited by rf sputtering of monocrystalline GaAs targets with and without hydrogen. The trend of the Urbach energy, E0, and Tauc gap, E(g), as a function of some of the deposition parameters is discussed. In particular, the behavior of E0 shows a decrease as a function of the quantity (W/p)1/2, where W is the discharge power and p the pressure of the deposition chamber. E(g) increases with the hydrogen pressure and tends to saturate when p(H-2) is greater than 0.1 Pa. E0 shows exactly the opposite trend.