Thermal modulation of the optical properties of amorphous semiconducting films

Modulation spectroscopy was performed on amorphous semiconducting films of a-Si:H and a-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ (of thickness ∼3 μm), using a chopped ${\mathrm{Ar}}^{+}$-ion laser beam as the modulating source. The results show that oscillation fringes in the modulated reflection (ΔR) and transmission (ΔT ) are due to the thermal modulation of the index of refraction through the thermal modulation of the band gap $E_g$. Therefore the (peak-to-peak) signal amplitude Δ$R_{\mathrm{p}.\mathrm{p}.\mathrm{}}$ in the ΔR spectrum gives a measure of the quantity ∂$E_g$/∂T. The temperature dependence of Δ$R_{\mathrm{p}.\mathrm{p}.\mathrm{}}$ in a-Si:H between 1.8 and 300 K is found to be consistent with the known behavior of ∂$E_g$/∂T.