近數十年間在光電科學、光子學、超快電子學與生物醫學等研究領域裡，於自由空間中產生與偵測兆赫波段電磁脈衝(兆赫波、T射線)的技術與應用一直是倍受重視且為人感興趣的，隨著皮秒與飛秒脈衝雷射光源的快速發展，利用這些光源來產生與偵測兆赫輻射波的技術日趨成熟，更加速了此波段電磁輻射於應用上的快速成長。過去十年間，兆赫波已被實際的應用探討許多物質(包含了固態、液態與氣態物質)的光電性質、分子間的振動與轉動能級與物質的基本組成等，最近更成功地被應用在二維的即時影像與三維電腦輔助斷層掃瞄(T-ray computed tomography)上。

本篇論文中首先裝置了一套在自由空間傳播的兆赫波產生與偵測系統，利用不同組成與微結構的半導體材料如SIN+ 結構之GaAs、InAlAs與氧化物-n型摻雜GaAs等，做為輻射器來產生兆赫輻射，藉由分析不同物質與結構所產生之兆赫輻射脈衝的強度與頻寬，尋求產生兆赫輻射之最佳、最有效率之材料與結構。

研究中應用光調制光譜分析存在於半導體微結構中的內建電場，在脈衝雷射的激發下，這些存在於半導體中的內建電場將驅動光激發載子形成一隨時間變化的光電流，產生兆赫輻射波，一般輻射的強度與光激發載子的數目與內建電場的強度有關，本篇論文中，將仔細探討這些材料中此兩個因子對兆赫輻射強度的影響。在低電場極限下，兆赫輻射的強度與內建電場的強度成正比關係，但若電場強度超越了臨界電場，因載子的最高飄移速度隨電場強度的變化趨於緩慢，且速度隨電場強度的增加逐漸緩降，故在這個電場範圍下，兆赫輻射強度並未隨著電場強度的改變而有顯著的變化，有效載子數目的多寡反而主宰了此電場範圍下兆赫輻射強度的高低。

論文中亦透過激發-探測的時間解析技術，藉由監測兆赫輻射強度隨激發光延遲時間的變化，探討光激發載子對半導體表面電場造成的屏蔽效應。載子激發光激發半導體時所形成的偶極電場，會對原半導體表面電場產生屏蔽現象，使得同時入射於半導體的兆赫激發光所產生的兆赫輻射強度，隨延遲時間產生先迅速衰減後逐漸回升的變化，這是因為光激載子造成的屏蔽電場強度在1ps內達到最高值後因載子重新復合逐漸衰減所造成的結果。屏蔽效應的強度隨載子激發光強度的增強而增加，於高電場狀態時，相較於電場強度此效應則可被忽略。於此研究中同時應用了一維Drude-Lorentz模型，以Monte Carlo的擬合方法分析不同延遲時間下屏蔽效應強度變化的情形，以瞭解屏蔽過程中光激載子的傳輸行為，由擬合結果得到載子動量鬆弛時間(momentum relaxation time) τs、載子被捕捉時間(trapping time)τt與載子復和時間(recombination time)τr等特徵時間參數。

最後配合光調制光譜與拉曼光譜探討氧化物－n型摻雜砷化鎵的界面性質，應用不同激發光強度調變下的調制光譜技術得到這些樣品的界面勢壘高度與界面態密度，發現這些利用分子束壘晶技術成長的氧化物－GaAs材料其界面態密度都低至1011cm-2，且Ga2O3(Gd2O3)混合之氧化物與GaAs間的界面態密度更低，只有(1.240.14)1010cm-2，故Ga2O3(Gd2O3)氧化物可有效的去除GaAs的界面態。我們同時也應用了拉曼光譜技術對這些氧化物的結構性質進行分析，亦獲得一致性的結果。在這些樣品的兆赫輻射性質分析中，因Al2O3-、Ga2O3-與Ga2O3(Gd2O3)-GaAs樣品其界面電場強度皆低於臨界電場，故所產生的兆赫輻射強度隨界面電場強度增加而增加，雖然air-GaAs樣品的界面電場強度較Al2O3-GaAs樣品為強，但因其強度已超越了臨界電場值，故所產生之輻射強度較Al2O3-GaAs樣品為弱，估計GaAs的臨界電場值為42.45±0.25kV/cm。

Free space generation and detection of terahertz-bandwidth pulsed electromagnetic radiation (THz wave, T-ray) is a fast-growing field in ultrafast electronics, photonics, optoelectronics and biomedicine. This area has received considerable interest over the past decade. With the rapid development of picosecond and femtosecond pulse laser sources, various methods have been adopted for generating and detecting terahertz waves. During the past decade, THz waves have been used to characterize the electronic, vibrational and compositional properties of solid, liquid and gas phase materials, flames and flows. Recently, real-time 2D and 3D T-ray computed tomography have become feasible.

This work has established a system for generating and detecting terahertz waves. Various semiconductor materials and microstructures have been used as emitters, which radiate the THz waves. The intensities and bandwidths of the THz waves radiated by various emitters are closely examined to find the most effective and efficient material and / or structure for use as emitter for the THz wave radiation. Specifically, GaAs and InAlAs surface-intrinsic-n+ structures (SIN+) and Oxide/GaAs structures are studied.

Built-in electric fields in semiconductor microstructures are determined based on the spectra of modulation spectroscopy of photoreflectance (PR). The time domain and frequency domain THz spectra emitted from microstructures with various built-in electric fields are measured. The static built-in fields provide a natural bias field for the photocurrent to produce the THz radiation. The intensity or amplitude of the radiated field is well known to primarily depend on the bias field and the number of photo-induced free charged carriers. This study closely investigates the dependence of the amplitude of the THz radiation on the built-in electric field and the number of free charged carriers. In low field limit, the amplitude of the radiated field is found to be proportional to the built-in field. However, as the field exceeds the so called critical electric field, the amplitude of the radiation decreases slightly with increasing electric field. The drift velocity of the charged carrier peaks at the critical electric field. Meanwhile, the radiated field is proportional to the number of the photo-induced free charged carriers.

THz wave pump and probe measurements are also performed in this work. Free charged carriers are induced by the femtosecond laser pulse, THz radiation is then generated and measured (pump and probe) at various delay time. The dipole field created by photo-excited electron-hole pairs screens the static electric field in the semiconductor microstructure, causing the amplitude of the THz radiation to first decrease rapidly and then increase slowly. This behavior results from the fact that the screen effect reaches its maximum strength within one picosecond and then decreases due to the carrier recombination. The screen effect strengthens with pump power. At high surface electric field, the screen effect is negligible compared to the surface field. To understand carrier transport behavior, this study employs the one-dimensional Drude-Lorentz model to analyze the screen effect strength at various delay times. Fitting the experimental results to theoretical equations using the Monte Carlo method, the carrier momentum relaxation time s carrier trapping time t and carrier recombination timer are obtained.

Photoreflectance (PR) and Raman spectra are employed to investigate the interfacial characteristics of a series of oxide films on GaAs. The barrier heights across the interfaces and the densities of interfacial states are determined from the PR intensity as a function of the pump power density. The oxide-GaAs structures fabricated by in situ molecular beam epitaxy exhibit low interfacial state densities in the low 1011 cm-2 range. The density of the interfacial states of the Ga2O3(Gd2O3)-GaAs structure is as low as (1.24 ± 0.14)×1010 cm-2. The Ga2O3(Gd2O3) dielectric film has effectively passivated the GaAs surface. Additionally, Raman spectra were used to characterize the structural properties of the oxide films. Oxide-GaAs structures are also used as THz radiation emitters. Since the interfacial electric fields in Al2O3-, Ga2O3-, and Ga2O3(Gd2O3)-GaAs structures are below the critical electric field, the amplitude of THz radiation increases with electric field. Although the interfacial field in air-GaAs structure exceeds that in Al2O3-GaAs structure, the amplitude of its THz radiation is smaller than that of Al2O3-GaAs since its electric field is higher than the critical field. The critical field in GaAs can be estimated and is 42.45±0.25 kV/cm.

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