Transient thermal gratings and carrier-induced gratings in diffusion experiments

Abstract

Complications can arise during measurements of the ambipolar diffusion coefficient, D amb, of charged carriers by the transient grating method (TG) due to the concomitant decay of a transient thermal grating. The thermal diffusion coefficient, D th, of hydrogenated amorphous silicon (a-Si:H) is of the same order of magnitude as D amb, about 10 −2–1 cm 2/s. For calibration purposes we measured the thermal change of refractive index, d n/d T, 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 D amb a good thermal connection to the substrate and a high quality material is needed.