目前氮化鋁鎵(AlxGa1-xN)及其相關材料已成為紫外光偵測器及場效電晶體的熱門研究材料，這主要是因為氮化鎵是寬能隙材料，可以在高功率及高溫的環境下穩定操作，並具有高崩潰電壓的特性。然而，較大的閘極漏電流往往會影響元件在高偏壓及高溫環境操作下的特性。其中，蕭特基接觸中的漏電流主要是受到蕭特基位障高度(Schottky barrier height)和材料的缺陷密度所影響。所以，為了要降低蕭特基接觸中的漏電流，必須要提高金屬和半導體之間的蕭特基位障高度，而金屬與半導體之間的蕭特基位障高度，除了跟選用的金屬功函數有關之外，也跟最上層半導體表面狀態有很大的關聯。

　　在本論文的第一部分是探討低溫成長的氮化鎵覆蓋層對蕭特基二極體漏電流的影響。結果發現多了一層低溫成長的氮化鎵覆蓋層之後，可以有效降低氮化鎵蕭特基二極體的漏電流。這是由於低溫成長的氮化鎵覆蓋層可以提高氮化鎵半導體與金屬接觸的蕭特基位障高度，而且對半導體的表面狀態有鈍化(passivation)的作用。之所以認為低溫成長的氮化鎵覆蓋層可以提高金屬與半導體之間的蕭特基位障高度，是由一系列的量測結果所獲得的結論，例如將鎳/金分別鍍在有低溫成長氮化鎵覆蓋層的試片及沒有低溫成長氮化鎵覆蓋層的試片作為蕭特基接觸。實驗結果得到有低溫成長氮化鎵覆蓋層及沒有低溫成長氮化鎵覆蓋層的蕭特基二極體之蕭特基位障高度分別是1.13eV及0.85eV。而之所以認為低溫成長的氮化鎵覆蓋層對半導體的表面狀態有鈍化(passivation)的作用，是由一系列的氮化鋁鎵/氮化鎵異質結構場效電晶體(AlGaN/GaN HFETs)的閘極延遲量測(gate lag measurements)所得到的結果。藉由以上的結果，我們可以將低溫成長的氮化鎵應用在蕭特基二極體紫外光偵測器(Schottky barrier UV photodetectors)及金屬-半導體-金屬紫外光偵測器(metal-semiconductor-metal UV photodetectors)的製作上，來降低元件的漏電流。

　　在本論文中，我們也研究氮化鋁鎵系列p-i-n紫外光偵測器，為了解決p型氮化鋁鎵(AlxGa1-xN)材料高電阻率的問題，我們以低電阻率的鎂摻雜氮化鋁鎵/氮化鎵超晶格結構(Mg-doped AlGaN/GaN SLS)來取代p型氮化鋁鎵，以降低接觸電阻。

　　一般來講，為了更進一步提高不受太陽光幅射干擾(solar-blind)的氮化鋁鎵系列p-i-n 紫外光偵測器之偵測率(responsivity)，必須製作更寬能隙(Eg >4.43 eV)且具有低電阻率的p型氮化鋁鎵材料作為窗戶層(window layer)；然而當鋁含量愈高時，p型氮化鋁鎵材料的電阻率會愈高。傳統的解決方法是利用背面照光來克服p型氮化鋁鎵材料無法高濃度摻雜的問題。在本論文中，我們是利用一層高電子濃度且厚度很薄的氮化銦鎵(n++- In0.3Ga0.7N)與p型氮化鎵形成穿隧層接面(tunneling junction)，再接著成長n-i-p的結構於此一穿隧層接面之上，來解決這些問題。在這個元件中，陽極電極是配置在最底層的n型氮化鎵上，使得外加偏壓時，載子會藉由穿隧機制，而呈現類似歐姆接觸的特性(like an “Ohmic contact”) ，使得此一n-i-p結構的紫外光偵測器可以像傳統的p-i-n紫外光偵測器般操作。

　Aluminum Gallium nitride (AlxGa1-xN) alloys are highly promising for the application of ultraviolet photodetectors and field-effect transistors operating under high power and high temperature due to a nature of wide band gap. However, large gate leakage current, which is especially operated at high temperature or high bias, is one factor limiting device performance. Leakage current in Schottky contacts (SCs) is strongly dependent on barrier height. To reduce the leakage current in SCs, it is necessary to achieve a high Schottky barrier height (SBH) at the metal/semiconductor interface. In addition to the choice of various contact metals, SBH also strongly depends on the properties of the topmost semiconductor layer. The first phase of the dissertation examined the effect of high-resistivity low-temperature-grown(LTG) GaN cap layer on the leakage current of GaN Schottky barrier diodes(SBDs). The presence of the LTG GaN cap layer was found to effectively reduce the leakage current of GaN SBDs. These results will be discussed in Chapter 4 and they were ascribed to a fact that GaN SBDs with LTG GaN cap layer can induce an enhancement of SBH and/or an effect of passivation on surface defects including threading-dislocation-related (TD related) surface pits. The former case can be clarified by a series of measurements of SBH performed on the GaN Schottky contacts. For example, Ni/Au bi-layer metals deposited on LTG caped samples exhibited a barrier height of around 1.13 eV. In contrast, the barrier height of samples without the LTG cap layer exhibited a barrier height of around 0.85 eV. The effect of passivation on surface defects can be explained by the gate lag measurements performed on AlGaN/GaN HFETs. The AlGaN/GaN HFETs exhibited a significant frequency dispersion on the drain current of FETs if the with LTG cap layer was absent. Based on the aforementioned results, UV photodetectors including Schottky barrier photodetectors and metal-semiconductor-metal (MSM) photodetectors were designed with the LTG GaN cap layer to achieve the reduction of dark current.

　For GaN-based p-i-n UV photodiodes, samples used in this dissertation were designed to overcome the inherent high resistivity of p-type AlxGa1-xN. The author applied an low-resistivity Mg-doped AlGaN/GaN SLS structure to GaN -based p-i-n photodiodes to reduce the p-type contact resistance. A detailed study on the electrical and optical properties of these nitride-based p-i-n photodiodes will be discussed herein. In general, to further improve the responsivity of a conventional solar-blind p-i-n photodiode, a wide bandgap (Eg >4.43 eV) and low-resisitivity p-type AlxGa1-xN top layer is necessary. However, the resistivity of p-type AlxGa1-xN layer increases rapidly with increasing the Al content. Fortunately, this problem can be solved by backside illumination. In this dissertation, an inverted structure was designed with a p+/n+ tunneling junction between the p-layer and n-type bottom contact layer to solve this problem. The p+/n+ tunneling junction, with a low-resistivity n++-In0.3Ga0.7N layer associated with a p+-GaN layer deposited on an n-type GaN bottom contact layer, allows the anode electrode contact to be formed directly on the n-type GaN bottom contact layer. When a bias is applied to the device with the aforesaid inverted structure, the tunneling junction will behave like an “Ohmic contact”. Accordingly, an inverted p-i-n UV PD can operate like a conventional device.

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