利用同調兆赫光譜技術研究Ⅲ族氮化物的光電特性

Abstract

我們利用同調兆赫光譜技術研究氮化鎵與氮化銦薄膜和奈米柱樣品在兆赫波段的響應，進而求得樣品的折射係數和導電率，並成功的使用簡單Drude模型來擬合氮化鎵與氮化銦薄膜的導電率得到良好的結果，但氮化銦奈米柱量測結果與簡單Drude模型的預測有偏差，而使用經修正的Drude-Smith模型可得到良好的擬合結果，根據擬合所得的參數可以計算出樣品的載子的飄移率和濃度且跟霍爾量測所得的結果相符。 利用時間解析的光激發兆赫波探測技術，我們也研究氮化銦薄膜和其奈米柱的瞬時載子特性，並發現在奈米柱中的載子有較短的生命期，推測是因奈米柱的結構會產生較多的缺陷而使得載子有較快的捕捉時間，而奈米柱結構在光激發後會有較低的光導電率則是因為光電子會因為量子侷限效應而有較低的漂移率所致。 我們並利用氮化銦薄膜和奈米柱作為兆赫波的光激發射器，研究其兆赫輻射特性。與薄膜相比,實驗結果顯示奈米柱的結構輻射強度提高達三倍以上，並跟奈米柱的直徑大小有關。因為表面空乏區的電場會遮蔽住產生主要輻射的Photo-Dember 電場，所以當奈米柱直徑遠大於表面空乏區深度時，可以更有效率的發射出兆赫波。

Terahertz time-domain spectroscopy (THz-TDS) has been used to investigate THz conductivity of group III-nitride compound semiconductors and their nanostructures including gallium nitride (GaN) and indium nitride (InN). THz conductivities of GaN and InN films are well fitted by the simple-Drude model, while negative imaginary conductivity of InN nanorods should be fitted using the Drude-Smith model. The carrier mobility and concentration of each sample are obtained from the fit parameters. Transient carrier dynamics of InN film and nanorods have been investigated by optical pump-terahertz probe technique. The faster carrier trapping time of nanorods is attributed to the morphology of nanorods with a considerable amount of the defect and trap states. The reduced photoconductivity of nanorods is due to the confinement of excited carriers inside the nanorods. We have also studied terahertz emission from InN nanorods and InN film. THz emission from InN nanorods is at least three times more intense than that from InN film and depends strongly on the size distribution of the nanorods. Surface electron accumulation at the InN nanorods effectively screens out the photo-Dember field in the accumulation layer formed under the surface. The nanorods with considerably large diameter than the thickness of accumulation layer contribute dominantly in the emission of THz radiation from InN nanorods arrays.