III族氮化物半導體之超快光學及兆赫頻波光譜研究

Abstract

本計畫的主要目標是針對III 族氮化物半導體薄膜、奈米結構及其合金材料在光學和兆赫頻 波段之物理性質進行完整?究,其中之主要項目包括載子動力學及介電性質的探討。自從飛秒 雷射問世以來，很多關於半導體的超快物理現象皆透過超快雷射光譜技術而獲得理解，其中 光學pump-and-probe 的技術最早被廣泛應用到光學頻率範圍的半導體載子動力學。最近，由 於兆赫頻波技術的突飛猛進使得光學波pump-兆赫頻波probe 的實驗領域得以實現，更因為 兆赫頻波可與低能量的載子激發?生強交互作用，超快兆赫頻波脈衝特別適合作為物質中與 瞬間電子傳導性質有關的非接觸式量測工作。 III 族氮化物是目前最被看好的白光二極體照元件之材料。由(InGa)N 三元合金半導體所組 成的直接能隙材料，可涵蓋從紅外光、可見光到紫外光的完整光頻範圍，因此極為適合作為 發展光電元件及系統的基礎材料。最近，氮化鎵材料也被發現具有很大的breakdown 場強， 非常適合用來作為室溫高功率兆赫頻波發射器的基板材料。此外，低維次發光材料亦對積 體光學的發展具有很大的影響，目前無應力(strain-free)、低錯位密度(low dislocation density) 的GaN 奈米柱陣列材料已被發現可以被長在矽基板上，由於可與矽技術的相容性而提高了 氮化鎵材料的應用價?。由光奈米材料具有很大的表面積/體積比例及特殊之電荷載子及光 子侷限效應，使得它們與塊材形式的同類材料有決然不同的電性及光性。然而，雖然氮化鎵 奈米材料對於未來兆赫波技術可產生重大的影響，並沒有很多的相關研究成果被報導過。部 分的原因肇始於沒有適當的高品質、低缺陷氮化鎵奈米柱及量子點材料可供研究。因此，對 於未來開發新穎性兆赫波技術而言，奈米材料的缺陷及尺寸的依附性質特別值得深入探討。 利用光學和兆赫頻段波的時間解析光譜技術，我們將可以研究氮化鎵薄膜和奈米柱材料的本 徵特性；而且我們可期待透過這項研究工作，這類材料在光電元件上的應用可以被更完整的 認識。 除了氮化鎵材料之外，氮化銦(InN)是最近頗受注目的III 族氮化物半導體，其主因來自氮 化銦具有窄能隙及優越的電子傳輸性質，使得它可被用到紅外光光電元件、高頻/高速電子 元件及高效率太陽能電池。但是目前對於氮化銦的實驗數據不多，而且常有互相矛盾的狀 況。而在兆赫頻段波的應用領域，氮化銦的高電子遷移速率及高導電率使得單晶氮化銦材料 非常適合用作為高效率兆赫波的產生源。雖然有這些很高的期待，關於氮化銦用作兆赫波源 的應用，目前世界上僅有一份初步的實驗成果報導，對於其基本機制及細節特性都還沒有很 系統的研究成果，非常值得深入探討。在本計畫內，我們將針對氮化銦材料的兆赫波的產生 機制及與其相關的之載子和聲子動力學，利用時間解析光頻和兆赫波時區光譜(THz-TDS)作 一個完整的研究。

A major focus of the proposed project is a comprehensive study of carrier dynamics and dielectric properties of group-III nitride thin films, nanostructures, and their alloys in the optical and terahertz (THz) frequency ranges. Since their first appearance, femtosecond lasers have been served as essential facilities to provide a wealth of information of ultrafast phenomena in semiconductors. Much of work has been done using optical pump-and-probe techniques to investigate carrier dynamics in semiconductors over the optical frequency ranges. Recent remarkable developments of THz techniques make it possible to conduct optical-pump-THz-probe experiments on an extended variety of materials. Since they interact exclusively with low energy carrier excitations, ultrafast THz pulses are particularly suitable for noncontact measurements of transient conductivity in materials, which is dominated by the electron contribution. Group III-nitrides have proved to be one of the most promising material systems for the development of white light emitting diodes (LEDs). The bandgap energies of the GaInN ternary alloys have been shown to cover a wide spectral range from the infrared (IR) to the near ultraviolet (UV) and this wide tunable range is very promising for the applications in many optoelectronic applications and systems. Recently, GaN also proved to be a good substrate material candidate for high-power THz emitter operating at room temperature due to its high breakdown field. Furthermore, low-dimensional-structure light emitting devices are crucial elements for the integrated devices development. At present, strain-free dislocation-free GaN nanorods grown on Si substrates shows the excellent compatibility to Si-based devices. The increase in surface area and the confinement effects of charge carriers and photons have made nanostructured materials quite distinct from their bulk forms in both electrical and optical properties. However, despite their importance in future THz technology development, not much research in the THz regime has been done on GaN nanostructures, partially due to the difficulty related with the growth of high-quality, defect-free GaN nanorods and quantum dots. For the future development of THz devices, the influence of defects of rods and rod-size dependence of electrical and optical properties of nanostructures should be clearly characterized. By using our time-resolved techniques applicable at both the optical and THz frequency regions, we will investigate intrinsic properties of GaN films and nanorods and we expect to clarify the prospects of these materials for the developments of optoelectronic devices InN is another attractive group III-nitride as a very promising material for IR optoelectronics, high-frequency/high-speed electronics, and high-efficiency solar cells. In most of cases, however, the data are very scarce and sometimes contradictory. High electron mobility and high conductivity also enable the single-crystal InN to be an ideal choice for efficient terahertz generation. Despite of its high expectations, the fundamental properties of InN are not yet fully measured or understood. Although there was a report on THz emissions from InN, the systematic information of the emission mechanism dependences on the intrinsic material properties are still unknown. Within this project, THz generation mechanism from InN films and nanorods and its dependence on the surface morphology, and the carrier and phonon dynamics associated with InN will be investigated by using time-resolved spectroscopy and THz time-domain spectroscopy (THz-TDS).