此論文研究之目的為探討化學吸附在二氧化錫薄膜表面之氧離子造成二氧 化錫薄膜之光電流增強機制。由實驗結果發現二氧化錫薄膜經過氧氣環境下熱退 火後，紫外光雷射激發之光電流會明顯地增強。藉由光電子能譜儀(XPS)以及光 激發螢光光譜儀(PL)分析可得知，經過氧氣環境下熱退火之二氧化錫薄膜表面之 化學吸附氧離子數量會明顯增加。由於化學吸附之氧離子會與二氧化錫薄膜靠近 表面之氧空缺形成空間電荷區及內建電場，我們認為受光激發之電子電洞對會受 此內建電場之影響而快速分離。此電子電洞快速分離的現象可使光激發之電子電 洞對覆合機率降低、並且延長電子電洞對的生命期，因此，我們認為此電子電洞 加速分離的現象為表面化學吸附之氧離子能有效提升二氧化錫薄膜光電流之主 因。另外，我們也從理論計算方面著手研究此電子電洞分離現象。將實際情況下 氧空缺濃度於薄膜中為縱深分布代入並修正poisson’s 公式，可計算出表面空間 電荷區之寬度為4.32 nm。利用此空間電荷區寬度，可計算電子電洞分離並跨越 表面空間電荷區所需之時間，藉此驗證此電子電洞分離現象是否存在。計算所得 之電子電洞分離並跨越表面空間電荷區所需之時間為3.31 10-10 s。計算結果顯 示電子電洞分離並跨越表面空間電荷區所需之時間遠小於一般二氧化錫之激子 的生命期，因此，我們認為以上的計算驗證結果顯示表面空間電荷區可於電子電 洞對覆合前將其分離。由以上實驗及計算的驗證結果可證明表面氧吸附所產生之 表面空間電荷區可有效分離電子電洞並增強光激發載子之生命期。;In this study, we report a photocurrent generation mechanism in the SnO2 thin film by the charged chemisorption O ions on the SnO2 thin film surface induced by O2- annealing. Both XPS and PL results indicate that the amount of the surface chemisorption O ions of the SnO2 thin film increases with being annealed in O2 ambient. The surface chemisorption O ions would form the surface space charge region and the build-in electric field in the SnO2 thin film, which would separate the photo-excited electron-hole pairs by UV-laser irradiation (266 nm) in the SnO2 surface layer. This phenomenon can prolong the lifetime and reduce the recombination probability of the photo-excited electron-hole pairs. That is the key for the photocurrent generation in the SnO2 thin film by the charged chemisorption O ions. We also investigate the phenomenon of the separation of photo-excited electron-hole pairs by the theoretical calculation. We find that the width of space charge region from the results of poisson’s equation do not conform with the common width of the space charge region. Therefore, we assume that the oxygen vacancy concentration is not constant, but a profile distributing from surface into thin films. The calculated width of the space charge region is 4.32 nm. Since the time of the separation of electron-hole pairs is shorter than the time of the recombination, we can study the time of separation of electron-hole pairs by built-in electric field to verify the enhancement of lifetime of photo-carriers. The time of electron drift across the space charge region is calculated to be about 3.31 10-10 s. This results can prove that the space charge region of the surface oxygen adsorptions can effectively separate the electron-hole pairs and enhance the lifetime of photo-carriers.
