| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 89 === In this thesis, ultrafast exciton dynamics in GaN-based semiconductors were investigated using femtosecond transmission pump-probe measurements. An electron and a hole can fall into a lower energy state called an exciton, which is essentially like an atom in which the electron orbits around the hole. Excitons represent the lowest intrinsic excited electronic states of semiconductors.
Recently, the realization of high-brightness blue and green light-emitting diodes and cw blue laser diodes has attracted the efforts of many research groups with the advantages of high luminescence efficiency, quick response time, long lifetime, high reliability, and low energy consumption. Theoretical description of detailed physical mechanisms affecting their active medium behavior is developed taking into account many-body effects, which include plasma screening of the Coulomb potential, bandgap renormalization due to carrier exchange-correlation effects, bandgap shrinkage due to temperature, excitonic or Coulomb enhancement of the interband transition probability, and higher excitonic correlation effects such as excitonic screening. Furthermore, it has also been pointed out that because of the large exciton binding energies in wide bandgap group-III nitride compounds, many-body Coulomb interactions are expected to significantly influence the nonlinear optical gain response in the electron-hole system. Therefore, in this thesis, we report the results of exciton dynamics studies on unintentionally doped GaN thin film at room temperature (RT). Based on observation of free-A-excitonic absorption peak as a strong resonant spectral feature on unintentionally doped GaN thin film at room temperature, we report on, to our knowledge, the first direct measurement of exciton ionization process in wurtize GaN. We present evidence that at RT excitons ionizating into free electron-hole pairs actually exists in this pure GaN sample with a ionization time constant of 100~250 fs, in well agreement with previously measured exciton ionization time in CdZnTe/ZnTe, InGaAs, and GaAs material systems.
Another type of the correlation of electron and hole exists in quantum well system due to the well-defined electron-hole wavefunction overlapping inside the well. Comparing to the femtosecond range exciton dynamics of the above bulk unintentionally doped GaN, at even longer timescale (~hundreds picosecond or even longer), the photoexcited carriers would recombine to reach a true equilibrium, including scattering, diffusion, and recombination processes. Due to the photoresponse of high-frequency device performances being limited by the diffusion process of the photoexcited carriers, understanding the transport properties in GaN material systems is more fundamental. Therefore, in another part of this thesis, we present the diffusion studies of the femtosecond transmission pump-probe experiments on more complicated GaN-based structure of 14 periods InGaN\GaN MQWs at room temperature. We focus on lateral ambipolar diffusion behavior of the photoexcited 2D-carrier gas in InGaN/GaN MQWs. The 2D ambipolar diffusion coefficient, which describes the excess electron-hole pair diffusion in a 2D plane, is measured and ascribe to the previous observation of the giant ambipolar diffusion coefficient in GaAs based n-i-p-i superlattices. In our experiment, a large ambipolar diffusion coefficient is found to increase with increased well width with a value on the order of 3000 cm2/s for a 62 A well-width sample, which is attributed to the enhancement by a strong piezoelectric field.
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