| Femtosecond pump-probe spectroscopy is used to investigate the carrier dynamics in GaN. Nonlinear frequency conversion techniques enable us to cover the spectral range from the mid-IR to the near UV. This tremendous flexibility has led us to invent a new approach to monitor the photoinjected carrier dynamics, from generation, thermalization, and cooling, to recombination.; Traditional above bandgap degenerate pump-probe experiments can provide useful information about the carrier dynamics in semiconductors. Because both pulses have the same wavelength, information is very limited. In this thesis, we have performed two non-degenerate pump/probe experiments. In the first experiment, hot carriers are excited across the bandgap by a fixed ultraviolet pump and the hot carrier relaxation dynamics are monitored by a tunable near ultraviolet probe. This experiment provides an extensive picture of hot carrier relaxation dynamics, which cannot be obtained by degenerate pump-probe method. In the second experiment, an infrared pump beam is used to excite the doped sample via free carrier absorption. This novel pump technique overcomes several shortcomings of across bandgap pump-probe experiment. It can isolate the electron (hole) dynamics from hole (electron) dynamics, and also eliminate the contribution from many other processes. It provides not only a direct measurement of the electron dynamics, but also the first time direct measurement of hole dynamics.; The III-V semiconductor GaN has attracted great attention for its many applications in LEDs, LDs. Their ultrafast carrier relaxation dynamics is very important for engineering as well as understanding fundamental semiconductor physics. In this study, across bandgap pump-probe and infrared pump/near ultraviolet probe technique have been used to measure both the electron and hole dynamics in GaN. The hot electron and hot hole relaxation time in GaN is determined to be around 0.6 ps. |
