首先在紫外光檢測器方面，我們利用高功函數的銥鉑合金作為氮化鎵金半金光檢測器的蕭基接觸。與傳統鎳金接觸電極比較，以銥鉑合金製作的金半金光檢測器可降低兩個數量級的暗電流，同時具有較好的熱穩定性及較高的崩潰電壓。我們也使用銥鉑合金與低溫氮化鎵覆蓋層來製作氮化鎵金半金光檢測器，我們發現雖然光電流減少，但相對的檢測器的暗電流降低，紫外光與可見光的拒斥比升高，而且最低雜訊等效功率及檢測度分別為2.75×10-13 W及2.77×1012 cmHz0.5W-1。之後我們利用半透明的銥鉑合金製作蕭基光檢測器，並研究回火對元件特性的影響。發現回火後，銥鉑合金在波長365nm處的穿透率由43.5%提升到68.7%，且銥鉑合金表面也變的比較粗糙。由能量散射光譜分析，我們認為此結果是由於氧化銥的形成。回火後，蕭基光檢測器的漏電流亦可降低四個數量級。此外，其360nm光響應值及量子效率分別為0.19 A/W及65%，且紫外光對可見光的拒斥比可達1050。
藉由上述的結果，我們更進一步應用低溫氮化鋁覆蓋層及未活化的鎂摻雜氮化鎵覆蓋層來製作氮化鎵光檢測器。與傳統無覆蓋層的蕭基能障光檢測器相比，藉著加入此覆蓋層可以降低暗電流值、增加紫外光與可見光拒斥比及檢測率，此可歸因於覆蓋層可以導致較大與較厚蕭基能障，且對半導體表面能態有鈍化的作用。在-1V的偏壓下，具有低溫氮化鋁及鎂摻雜氮化鎵覆蓋層的拒斥比分別為3.17×103 and 3.52×103。在-5V的偏壓下，無覆蓋層、具有低溫氮化鋁覆蓋層及具有鎂摻雜氮化鎵覆蓋層的光檢測器之檢測度分別為5×107 cmHz0.5W-1, 5.63×1010 cmHz0.5W-1, and 9.34×1011 cmHz0.5W-1。因此，藉由此研究結果，我們知道將低溫氮化鋁及鎂摻雜氮化鎵覆蓋層應用在氮化鎵光檢測器的製作上，可降低元件漏電流且改善元件特性。我們進一步使用氮化銦鎵/氮化鎵多重量子井取代傳統氮化鎵活性層。我們亦發現在多重量子井上覆蓋鎂摻雜的氮化鎵層，其金半金光檢測器的暗電流值依然維持在10-11A左右，但其光響應值可提高到0.366 A/W。此外，多重量子井光檢測器的最低雜訊等效功率及檢測度分別為4.09×10-14 W及1.18×1013 cmHz0.5W-1。
在異質結構場效電晶體方面，我們使用偏角度的藍寶石基板成長氮化鋁鎵/氮化鎵異質結構。可以發現使用偏角度的藍寶石基板成長可以有效的降低其缺陷與差排密度，其蕭基能障二極體的漏電流可降低兩個數量級，同時也具有較高的能障高度及較小的理想因子。此外，藉著電容-電壓的量測，可以發現多數載子被侷限於氮化鋁鎵與氮化鎵界面處。我們亦使用偏角度的試片去製作金半異質場效電晶體，發現可以有效降低閘極漏電流，且其室溫下的最大轉導值為110mS/mm。在1.25μm的閘極線寬下，其電流增益截止頻率及最高震盪頻率分別為7.6及9.8 GHz。因此，使用偏角度基板製作的氮化鋁鎵/氮化鎵金半異質場效電晶體將可適用低雜訊、高功率及高溫的應用。

The main goal of this dissertation is the fabrication and analyses of GaN-based ultraviolet (UV) photodetectors (PDs) and metal-semiconductor heterostructure field effect transistors (MES-HFETs).
First, we use Ir/Pt alloy with high work function as the Schottky contacts of metal-semiconductor-metal (MSM) PDs. Compared to conventional Ni/Au contacts, the dark current of the MSM PDs with Ir/Pt alloys can be reduced by two orders. Besides, the MSM PDs with Ir/Pt contacts had the better thermal stability and higher breakdown voltage. We also fabricated GaN MSM PDs with Ir/Pt contact electrode and LT GaN cap layer. Although the photocurrent was smaller, we achieved smaller dark current and larger UV to visible rejection ratio for the PD with Ir/Pt contact electrode and LT GaN cap layer. Furthermore, noise equivalent power (NEP) and normalized detectivity (D*) were correspondingly determined as 2.75×10-13 W and 2.77×1012 cmHz0.5W-1. Then, we use semi-transparent Ir/Pt alloy to fabricate the Schottky barrier PDs, and research the effect of annealing on devices characteristics. After annealing, the transmittance of Ir/Pt alloy improves from 43.5% to 68.7% at 365 nm, and the surface morphology becomes rougher. We think the result can be attributed to the formation of IrOx. Besides, the leakage current of Schottky barrier PDs also reduce four orders. The responsivity and quantum efficiency at a wavelength of 360 nm are 0.19 A/W and 65%, and UV to visible rejection ratio is 1050.
Based on the aforementioned results, we apply in-situ low-temperature AlN and un-activated Mg-doped GaN layer to the fabrication of GaN-based PDs. Compared with conventional Schottky barrier PDs without cap layer, it was found that we can achieve significantly much smaller dark current, larger UV to visible rejection ratio and larger normalized detectivity by inserting the LT AlN or Mg-doped GaN cap layer. This result could be attributed to the thicker and higher potential barrier and effective surface passivation after inserting in-situ grown cap layer. Under a -1 V bias, it was found that the UV to visible rejection ratios were 3.17×103 and 3.52×103 for PDs with LT AlN and un-activated Mg-doped GaN layer, respectively. We also found that noise behavior for this type of PDs was flicker noise. Under a -5 V applied bias, it is also found that we can achieve a lower noise level and a higherr normalized detectivity by inserting an in-situ grown cap layer. D* for PDs without cap layer, with LT AlN cap layer and with Mg-doped GaN cap layer was 5×107, 5.63×1010, and 9.34×1011 cmHz0.5W-1, respectively. Therefore, PDs with LT AlN cap layer or Mg-doped GaN cap layer can be used to reduce dark leakage current and improve the device characteristics. We also used the InGaN/GaN multi-quantum well (MQW) in place of the conventional GaN active layer. The dark current of MQW MSM PD with un-activated Mg-doped GaN layer is still around 10-11 A. For PDs with un-activated Mg-doped GaN cap layer, the responsivity at 380 nm and UV to visible rejection ratio were 0.366 A/W and 1.93×103 when biased at 5 V, respectively. Besides, NEP and D* are calculated to be 4.09×10-14 W and 1.18×1013 cmHz0.5W-1.
On the part of HFETs, we demonstrate a new technique of growing AlGaN/GaN heterostructures on vicinal-cut sapphire substrates. The vicinal-cut substrates are proved to reduce the threading dislocations (TDs) of our epilayers. AlGaN/GaN Schottky barrier diodes on vicinal-cut sapphire substrates have the smaller leakage current, higher barrier height and smaller ideality factor. We also obtain the carrier distribution profile from C-V measurement and find the majority of carriers is well confined. Then, we fabricate MES-HFETs by AlGaN/GaN heterostructures on vicinal-cut sapphire substrates, and find that the gate leakage current can be reduced effectively. The Gm,max is 110 mS/mm at room temperature. The extrinsic current-gain cut-off frequency (fT) and maximum frequency of oscillation (fmax) were 7.6 and 9.8 GHz, respectively, for 1° sample with 1.25-μm-gate length. Therefore, AlGaN/GaN HFETs on vicinal-cut sapphire substrates are potentially useful for low-noise, high power and high temperature applications.

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Chapter 3
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