| 摘要: | 目前因氮化鎵與藍寶石基板的晶格常數與熱膨脹係數不匹配，因而產生許多的缺陷在磊晶結構中，缺陷密度達108至1010cm-2，因此造成氮化銦鎵發光二極體元件的晶體結構品質較差，進而造成降低氮化銦鎵發光二極體的外部量子轉換效率。本論文旨在研發使用有機金屬氣相沉積法成長高效率發光輸出功率的氮化銦鎵發光二極體元件，重點分為兩個重要的技術，(一)使用新的緩衝層來成長氮化銦鎵發光二極體元件;(二)使用氮化鎵基板來成長氮化銦鎵的紫外光發光二極體元件，分別概述如下。
首先蒸鍍鈦金屬薄膜於藍寶石基板上再置入低壓式的有機金屬沉積儀，透過氮化的過程形成的氮化鈦緩衝層來磊晶成長發光二極體元件，由於氮化鈦緩衝層的薄膜特性再磊晶成長氮化鎵材料時，可形成奈米級的氮化鎵側向磊晶成長層於氮化鈦緩衝層上，藉由使用X射線與螢光反應實驗可觀察到增進磊晶成長後氮化鎵材料晶體品質的效果，利用氮化鎵側向磊晶成長行為減少缺陷密度的產生進而提升氮化鎵的晶體品質，並且將發光二極體的晶體結構應力釋放出68％相較於傳統結構，因此經由電激發光實驗可獲得較高的發光輸出功率45%與42%分別在輸入電流20安培與100安培下，與減少量子侷限史托克效應，這些量測結果反應出成長元件本身時的應力釋放與高晶體品質可得到較高效率的發光輸出功率。
接著再探討使用氮化鎵基板對發光波長在380 奈米的氮化銦鎵紫外光發光二極體元件在晶體結構品質與光電特性上產生的影響，因此首先成長同質結構的氮化銦鎵紫外光發光二極體在氮化鎵基板上並與藍寶石基板在特性上做比較，成長380奈米氮化銦鎵紫外光發光二極體在氮化鎵基板上經由X射線量測可得到較佳的晶體結構品質，其界面粗糙度為1.55%，除此之外當X射線照射在元件的(0002)與 (101 ̅2)面上的半高寬數值可縮小至90arcsec，元件的結構應力釋放可達0.31Gpa，透過穿透式電子顯微技術可更一步分析此元件在多重量子井的晶體品質，因為沒有觀察到缺陷與雜質，因此推測此元件的缺陷密度為3.6106 cm-2或更少，經由電激發光實驗可獲得較高的發光輸出功率80%與90%分別在輸入電流20安培與100安培下，除此之外在高注入電流下觀察到超低的功率掉落3%的元件特性，因此結論採用氮化鎵基板來成長380 奈米的氮化銦鎵紫外光發光二極體元件可獲得高效率的發光輸出功率與在高注入電流下超低的功率掉落。
最後，本文研發使用氫化物氣相磊晶技術來成長氮化鎵基板後，再磊晶成長380 奈米的氮化銦鎵紫外光發光二極體元件於其上並與市售氮化鎵基板在光電特性上做比較。成長氮化銦鎵紫外光發光二極體於市售的氮化鎵基板上的光輸出功率高於成長元件在使用氫化物磊晶法成長的氮化鎵基板70%在20安培電流輸入下，且氮化銦鎵紫外光發光二極體元件的功率掉落特性也是成長元件在市售的氮化鎵基板比較好，從這個結果，我們認為是因市售的氮化鎵基板製造是從砷化鎵基板磊晶而來，且砷化鎵熱膨脹係數相較於藍寶石基板與氮化鎵的差值較小，也就是說使用砷化鎵基板來成長氮化鎵基板可獲得較低的殘留應力使得紫外光發光二極體元件的發光輸出功率獲得提升。;The epitaxial layer of InGaN-based light-emitting diodes (LEDs) still contain a high defect density (around 108－1010 cm-2) and large strain-induced piezoelectric field due to the large lattice mismatch and the difference in thermal expansion coefficients of GaN films and sapphire substrates, resulting in the reduction in the external quantum efficiency (EQE) of InGaN-based LEDs devices. In this dissertation including two topics, we demonstrated light-output-power (LOP) enhancement of the InGaN-based light-emitting diodes (LEDs) by using metal-organic vapor phase epitaxy. One is using a new buffer layer for the growth of InGaN-based LEDs, the other is using free-standing GaN (FS-GaN) substrate for the growth of InGaN-based ultraviolet light-emitting diodes (UV-LEDs).
First, a low-cost and time-saving buffer layer of nitrided titanium (Ti) achieved through the nitridation of a Ti metal layer on a sapphire substrate was used for the epitaxial growth of LEDs achieved by low pressure metal-organic chemical vapour deposition. It showed that the use of the nitrided Ti buffer layer (NTBL) induced the formation of a nanoscale epitaxial lateral overgrowth layer (NELOG) during the epitaxial growth. The effect of in-situ Ti metal nitridation was then improved on the crystal quality of these InGaN-based LEDs from using X-ray diffraction (XRD) and CL results. When evaluated by Raman spectroscopy, the InGaN-based LEDs with an NTBL exhibited larger in-plane compressive stress releasing 68% than the LEDs with a LT-GaN buffer layer. The electroluminescence (EL) results indicate that the LOP of InGaN-based LEDs with an NTBL can be enhanced by 45% and 42% at 20 mA and 100 mA, respectively. These results suggest that the strain relaxation and quality improvement in the GaN epilayer could be responsible for the enhancement of emission power.
Then we investigate the influence of FS-GaN substrates on the performance of 380 nm InGaN-based UV-LEDs with InGaN/InAlGaN MQWs grown atop by atmospheric pressure metal-organic chemical vapor deposition. High-resolution double crystal X-ray diffraction (HRDCXD) analyses demonstrated high-order satellite peaks and clear fringes between them for UV-LEDs epilayers grown on FS-GaN substrate, from which the interface roughness (IRN) was 1.55%. Besides, the full width at half maximum of the HRDCXD rocking curve in the (0002) and (101 ̅2) reflection were reduced to below 90 arcsecond. The Raman results which the calculated in-plane compressive stress was 0.31 GPa indicate that the UV-LEDs epilayers of strain free are grown. Additionally, the effect of FS-GaN substrate on the crystal quality of UV-LEDs epilayers was examined in detail by transmission electron microscopy (TEM). TEM characterizations revealed no defects and V-pits were found in scanned area of InGaN/InAlGaN MQWs. The total defect density including edge, screw and mixed type was considered to be less than 3.6106 cm-2 or less, which agrees well with our HRDCXD rocking curve data, further proving that homo-epitaxial is an effective measure to improve the crystal quality of UV-LEDs epilayers. The LOP of 380 nm UV-LEDs on FS-GaN substrate can be enhanced drastically by 80% and 90% at 20 mA and 100 mA, respectively. Furthermore, an ultra-low efficiency degradation of about 3% can be obtained for 380 nm UV-LEDs on FS-GaN substrate at high injection current. Conclusively, the use of an FS-GaN substrate is suggested to be effective for improving the emission efficiency and droop of UV-LEDs grown thereon.
Finally, we investigated the free-standing GaN substrate fabricating from sapphire substrate by hydride vapor phase epitaxy (HVPE) and using these wafers to grow 380 nm InGaN-based UV-LEDs with InGaN/InAlGaN MQWs on them. The LOP of 380 nm UV-LEDs grown on FS-GaN substrate manufacturing from GaAs substrate by HVPE is higher by 70% and 105% than the FS-GaN substrate fabricating from sapphire at 20 mA and 100 mA, respectively. Besides, the efficiency droop was reduced from 21% in the UV-LEDs grown on FS-GaN substrate fabricating from sapphire substrate to 3% in UV-LEDs grown on FS-GaN substrate manufacturing from GaAs substrate. This result was attributed to less difference of thermal expansion coefficient of GaAs substrate than sapphire substrate, resulting in the less residual stress in the 380 nm UV-LEDs on FS-GaN substrate manufacturing from GaAs substrate which the strain relaxation degree was about 47% than UV-LEDs grown on FS-GaN substrate fabricating from sapphire substrate. |
