成長高品質的氮化鎵薄膜是相當重要的工作，因為磊晶的好壞通常就決定整體元件的特性。目前氮化鎵系列材料大多成長在藍寶石基板上，然而，氮化鎵系列材料與藍寶石基板有極大的晶格不匹配度以及熱膨脹係數不匹配度，因此造成成長後的氮化鎵系列材料具有非常高的缺陷密度，這些缺陷會形成電流漏電路徑而使得元件的崩潰電壓難以提昇，或造成電子電洞對的非輻射結合，導致元件發光效率降低。 本論文之重點在於使用導電式原子力顯微鏡掃描氮化鎵材料表面，藉由高解析度的電流影像圖統計材料表面電流分布情形及其與材料缺漏的關聯性。實驗結果發現電流分布的半高寬能相當敏銳地反應出氮化鎵材料的缺陷密度，並且隨電壓增高這現象變得更加明顯，所以我們可藉由半高寬的變化進行薄膜品質的確認。同時我們也將這實驗結果與X光繞射以及表面蝕刻凹洞 (etching-pit density)的實驗結果比對，得到了一致的趨勢。 藉由本論文的實驗結果，我們建立了一個利用導電式原子力顯微術做為檢測氮化鎵基材品質的技術，可以在奈米尺度下觀察氮化鎵材料的結構與電氣特性，進而判斷材料的優劣。 High quality GaN buffer layer is essential to high performance III-nitride devices grown heterogeneous substrates. It is thus desirable to develop a sensitive, reliable and quick method to assess the properties of III-nitride materials. At present, most of the III-nitride epitaxial films are grown on sapphire substrate, which has large mismatch with GaN in lattice constant as well as expansion coefficient. This results in high density of dislocations, which lower the radiative recombination efficiency and cause leakage current, leading to reduced breakdown voltage. In this work, we employ conductive atomic force microscopy (CAFM) to obtain high-resolution current images of III-nitride surface. From the statistical results of current distribution, it is revealed that the full-width at half-maximum (FWHM) of the distribution is very sensitive to the dislocation density of III-nitride films and significantly increases with the sample bias even the dislocation density is as low as ~108 cm-2. The results acquired by CAFM are shown to be consistent with those obtained from etching-pit density and X-ray diffraction measurements. Our experimental results indicate that CAFM is a promising method for evaluating the electrical and structural properties of III-nitride materials in a nanometric scale.
