Optical studies of fast plasma transport in Si

We present two new optical techniques for the investigation of the transport properties of ambipolar plasmas in semiconductors. The first method is a time-of-flight technique: Using surface doping with shallow impurities which provide a characteristic bound exciton emission we introduce optically active spatial markers into the thin Si wafers investigated. Carrier pairs are excited at the undoped surface of the wafer by short laser pulses. From time-resolved studies of the bound exciton emission we obtain average velocity values for the ambipolar transport through the sample. The most attractive feature of the time-of-flight method compared to other spatially resolved measurements is the combination of very high spatial resolution (submicron range) with a high sensitivity. The second method used to study the plasma transport is based on a time-resolved investigation of the Mott transition between the electron-hole plasma and free excitons in Si. Using well-established values for the Mott transition and the temporal evolution of the plasma and the free exciton emissions we obtain values for the velocity with which the plasma expands to the Mott density as functions of the excitation power and the temperature. The advantage of this method is the lack of any need for spatial resolution. Possible extensions of both methods are discussed.