Transient thermal gratings and carrier-induced gratings in diffusion experiments

Abstract Complications can arise during measurements of the ambipolar diffusion coefficient, Damb, of charged carriers by the transient grating method (TG) due to the concomitant decay of a transient thermal grating. The thermal diffusion coefficient, Dth, of hydrogenated amorphous silicon (a-Si:H) is of the same order of magnitude as Damb, about 10−2–1 cm2/s. For calibration purposes we measured the thermal change of refractive index, dn/dT, by optical reflection. We monitored the temperature decay after pulsed laser excitation using transient photoreflectance (TPR). A thermal diffusion process is confirmed by a near square-root time dependence. In grating experiments, the initial temperature and its time dependence can be monitored by TPR. In order to determine Damb a good thermal connection to the substrate and a high quality material is needed.