探討與改進氮化物材料光電及微電子元件之特性

Abstract

近年來，寬能帶與直接能隙IIIV族氮化物材料愈來愈受到重視，它具有可調變發光波長從紫外到藍綠光的特性，及獨特的高崩潰電壓、高電子飽和速度等材料性質，使得IIIV氮化物已被廣泛的應用在發光二極體、垂直共振腔面射型雷射及高電子遷移場效電晶體等元件上。在發光二極體方面，最期待的固態照明應用仍在發展中，儘管利用圖形化藍寶石基板增加光萃取效率及磊晶品質，已達商業化之標準，但發光二極體在高電流操作下的效率下降問題仍有待解決。在雷射應用方面，與傳統的邊射型雷射比較，垂直共振腔面射型雷射具有低發射角的圓型雷射光點及低製作成本等優點，如何進一步優化垂直共振腔面射型雷射的結構，降低雷射的臨電流及提高斜率效率，乃是許多學者及研究團隊之目標。隨著電力電子的蓬勃發展，未來的高頻段、高功率應用上氮化物材料之高電子遷移場效電晶體會是極佳的選擇，如何降低元件之漏電流及提高崩潰電壓是努力之方向。 在本研究中，我們利用鋁含量漸變增加的電子阻檔層來改善藍光發光二極體的效率下降現象。從APSYS模擬之能帶圖、電場圖、載子分佈及電流密度等圖形，顯示了鋁含量漸變增加之結構可以有效地增加電洞注入效率及侷限電子之能力。並且實驗結果也顯示了效率下降現象大幅改善。 在第二部分中，我們在磊晶結構中應用了成份漸變的超晶格結構取代了傳統的電子阻檔層來降低效率下降。首先利用漸變成份的超晶格電子阻檔層消除在最後氮化鎵量子能障與電子阻檔層間的極化效應，進而同時增加電洞注入及電子侷限能力。在適當的漸變成份下，不但可以減少效率下降現象，更能夠大幅提升高電流操作時的光輸出功率。 接著，為了提高紫外光發光二極體之發光效率及磊晶層之晶格品質，我們利用反應式電漿沉積氮化鋁緩衝層在紫外光發光二極體上。從材料分析結果，顯示氮化鋁緩衝層可以有效抑制缺陷，進而增加內部量子效率。從模擬結果顯示若將此紫外光發光二極體製作成覆晶結構，此氮化鋁緩衝層更可增加光萃取。因此可以增加紫外光發光二極體的光輸出功率及減緩效率下降現象。 在第四部分中，我們在垂直共振腔面射型雷射中應用了能帶工程化概念來降低臨界電流及提高斜率效率。首先，利用漸變成份電子阻檔層減少因極化產生的能帶彎曲效應，進而同時增加電洞注入及減少電子溢流。經過適當的成份漸變電子阻檔層設計，可以降低雷射之臨界電流，更可增加雷射之斜率效率。 在第五部分中，我們使用碳掺雜的氮化鋁/氮化鎵超晶格結構成長在高電子遷移場效電晶體結構。從材料分析結果，顯示氮化鋁/氮化鎵超晶格結構可以提高磊晶品質，並且降低垂直方向之漏電流，因此能有效地增加電晶體的崩潰電壓與改善電晶體特性。 最後期許此篇論文可以幫助解決IIIV氮化物光電及微電子元件所遭遇之問題。

III-Nitride materials has been intensively studied over the past few decades. It has remarkable material properties not only for optoelectronic devices but also application in microelectronic devices. The III-Nitride materials have excellent properties such as wide direct bandgap, high breakage voltage, high electron mobility, and high operation frequency. These properties make III-Nitride materials very attractive for application in blue/ultraviolet light-emitting diodes (LEDs), blue vertical-cavity surface-emitting lasers (VCSELs), and high electron mobility transistors (HEMTs). Although these devices have been development by different techniques, enhanced performance is still essential. The motivation of this work is to figure out the method that can be used to improve the device performance sufficiently. First part, a tapered AlGaN electron blocking layer (EBL) with step-graded aluminum composition is analyzed in blue LED numerically and experimentally. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition (MOCVD) exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design. In the second part, blue LEDs with graded-composition AlGaN/GaN superlattice (SL) EBL were designed and grown by metal-organic chemical vapor deposition. The simulation results demonstrated that the LED with a graded-composition AlGaN/GaN SL EBL have superior hole injection efficiency and lower electron leakage over the LED with a conventional AlGaN EBL or with a normal AlGaN/GaN SL EBL. Consequently, the efficiency droop can be alleviated to be about 20% from maximum at injection current of 15 to 120 mA, which is smaller than that for conventional AlGaN EBL (30%). The corresponding experimental results also confirm that the use of a graded-composition AlGaN/GaN SL EBL can markedly enhance the light output power by 60%. In the third part, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure MOCVD. A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction In the fourth part, the design and fabrication of GaN-based VCSELs with a composition-graded electron blocking layer (GEBL) are revealed experimentally and theoretically. It has been demonstrated that the laser output performance is improved by using a GEBL when compared the typical VCSEL structure with rectangular EBL. The output power obtained at 20 kA/cm2 is enhanced by a factor of 3.8 by the successful reduction of threshold current density from 12.6 kA/cm2 to 9.2 kA/cm2 and the enlarged slope efficiency. Numerical simulation results also suggest that the improved laser output performances are due mainly to the reduction of electron leakage current and the enhanced hole injection efficiency in the multiple-quantum-well (MQW) active region. In the fifth, the carbon-doped AlN/GaN superlattice (AlN/GaN SL) structure was introduced into the epitaxial growth of AlGaN/GaN HEMTs on Si (111) substrates, which could suppress the leakage from channel to substrate. Compared with conventional AlN/AlGaN double under layer (DUL) structure for AlGaN/GaN HEMTs, the results of electric properties imply that the vertical leakage can be dramatically decreased as two and half times as the carbon-doped AlN/GaN SL structure was introduced. Therefore, a threshold voltage of 2.0 V and maximum drain current of 175 mA/mm at the VGS of 2 V could be achieved. The output of this dissertation provide a great help on enhancing performance in GaN-based optoelectronic and microelectronic devices.