摘要
　　以飛秒雷射脈衝照射各種半導體塊材表面產生兆赫輻射的三種不同機制分別是光電導過程(the photo-conductivity)、photo-Dember效應以及光整流(the optical rectification)。本論文利用自由空間電光取樣(free space electro-optic sampling, FS-EOS)的方法來量測一系列的砷化鎵表面-本質-N型摻雜(surface intrinsic-N+, SIN+)結構所發出的兆赫輻射。利用這種時域(time-domain)的兆赫輻射光譜，可以直接測量到兆赫波的時域電場。此外，我們也確認在我們的實驗條件下樣品的輻射機制為光電導過程；光電導過程裡，半導體表面發出的兆赫輻射是光激載子受到半導體的表面空乏電場或外加電場加速造成的結果。我們首先使用光調制反射率(photoreflectance, PR)的調制光譜來量測此樣品結構的表面電場。實驗結果顯示，當表面電場低於所謂的“臨界電場”(critical electric field)時，兆赫波電場振幅與表面電場及光激載子數的乘積成正比；當表面電場超過臨界電場時，兆赫波的電場振幅與表面電場無關，僅與光激載子數成正比。此臨界電場為載子漂移速率(drift velocity)達到最大時的電場，它與半導體的Γ和L能谷(valley)之間的能量差有關。

Abstract
Terahertz (THz) radiation can be generated from a variety of semiconductors illuminated by femtosecond optical pulses. It is widely believed that three different mechanisms result in THz radiation from surface of bulk semiconductors, i.e. the photoconductivity, the photo-Dember effect, and the optical rectification. In this thesis, we use free space electro-optic sampling (FS-EOS) to characterize the THz radiation from a series of GaAs surface intrinsic-N+ (SIN+) structures. By THz time-domain spectroscopic technique we directly measure the electric field of the THz radiation. In addition, the radiation mechanism of the sample under the experiment condition is identified as the photoconductivity. In the photoconductive process, the THz radiation from semiconductor surface is a consequence of the motion of photo-excited carriers accelerated by the local or bias field which may be an external applied field or an internal field from charge depletion layers, Schottkey barriers, strained layers, and p-n junctions of semiconductors. The modulation spectroscopy of photoreflectance (PR) is used to measure the surface electric field of a series of GaAs SIN+ structures with various intrinsic layers, followed immediately by the measurement of the electric field amplitude of the THz radiation. Experimental results indicate that as the surface field is lower than the so called “critical electric field”, the amplitude of THz radiation field is proportional to the product of the surface field and the number of photo-excited carriers. As the surface field exceeds the critical field, the amplitude is independent of the surface field but is proportional to the product of the critical field and the number of the photo-excited carriers. The critical field corresponds to the field at which the drift velocity is maxima in semiconductor and depends on the energy difference between the Γ to L valley of the semiconductor.

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