Direct measurement of non-equilibrium carrier diffusion at a Si surface

We have developed an ultrafast carrier dynamics measurement method which pr ovides both a transient reflecting grating signal and a transient reflectiv ity signal. The method was used to measure the carrier dynamics at a silico n surface in the carrier density range of 1.6-3.0 x 10(20) cm(-3), and the diffusion coefficient of non-equilibrium electrons was determined by a comp arison between the transient reflecting grating and transient reflectivity signals. The diffusion coefficients increases as the carrier density become s larger. This tendency is qualitatively in agreement with the theoretical prediction that increase of diffusion coefficients is caused by a temperatu re rise of the carriers and increase of their temperature gradient.