利用同調兆赫光譜技術研究鎂摻雜氮化銦半導體的光電特性

Abstract

本論文中，我們利用自由空間同調性兆赫波段光譜技術，研究不同載子濃度且成長方向為c軸摻雜鎂的氮化銦與a軸方向氮化銦薄膜在兆赫波段的響應，藉由此方式我們可以求得樣品的折射係數與導電率。 經由實驗的結果，我們發現所有摻雜鎂的氮化銦薄膜，它的穿透率都增加了三倍，這個結果似乎與載子濃度沒有直接關係。而在c軸方向成長出的摻雜鎂的氮化銦薄膜，量測出來的結果可以成功的使用Drude模型來擬合，而根據擬合所得到的參數；我們可以推算出樣品的載子飄移率和載子濃度。由於THz-TDS無法直接判別半導體是n型或是p型，所以我們假設不同條件分別計算出半導體的載子飄移率和載子濃度。經由計算結果，我們假設電子有效質量為0.075時，計算出來的結果與霍爾量測所得到的結果是符合的，而當我們若是利用電洞的有效質量所計算出來的結果與先前其他作者所發表的結果是相符合的。 而a軸方向成長的氮化銦薄膜的實驗結果，我們發現如果電場對於樣品的極化方向不同(垂直和平行c軸)，所量測到的結果也會不同。在a軸方向成長的氮化銦薄膜會有非等向性的結構產生，主要是因為成長過程中引起的定向性缺陷和形變，因此對於這樣的結構；無論是光學或是電學的性質均會有很大的影響。

In this thesis, terahertz time-domain spectroscopy (THz-TDS) has been used to investigate optical and electrical properties of c-plane magnesium doped indium nitride (c-InN:Mg) and undoped a-plane InN epitaxial films. By using this method, we can obtain the frequency dependent complex conductivity and refractive index information. In the experiment on c-InN:Mg, we observed the transmittance for undoped InN film is less than 20%, whereas that for InN films doped by magnesium acceptors was enhanced around three times (>60%). We found the terahertz transmittance does not have particular dependence on carrier concentration. From the analysis, the complex conductivity of c-InN:Mg was obtained and its frequency dependence could be well described by the Drude model. Since THz-TDS cannot provide the type of conductivity of semiconductors, we assumed both p- and n-type conductivity for InN:Mg and calculated the carrier concentration and mobility. With an assumption of electron effective mass of 0.075mo, the electron mobility and electron concentration of the samples were calculated and they are consistent with those measured by Hall effect measurement method. With the assumption of p-type conductivity, the hole concentration and mobility were calculated and they agree with the previously published results. For a-plane InN, we found the anisotropy of refractive index and electrical conductivity along parallel and perpendicular to in-plane c-axis direction. The structural anisotropy formed during the growth of a-plane InN causes the directionality of defects and strain and results in the anisotropy of optical and electrical properties of nonpolar InN.