氫化物氣相磊晶成長矽摻雜氮化鎵厚膜 應用於發光二極體之研究

Abstract

本論文利用實驗室自組水平式氫化物氣相磊晶系統 (Hydride Vapor Phase Epitaxy, HVPE) 成長不同濃度之n型氮化鎵厚膜於氮化鋁/高深寬比圖案化藍寶石基板，藉此改善平面電極氮化鎵發光二極體之電流擁擠現象，期望應用於大電流下提升整體氮化鎵發光二極體電流擴散之均勻性。 首先分別成長未摻雜氮化鎵厚膜於氮化鋁/圖案化藍寶石基板及氮化鋁/高深寬比圖案化藍寶石基板，由XRD量測結果得知，(002)面的半高寬可由347降至320 arcsecs;量測氮化鎵(102)面的半高寬可由351降至330 arcsecs;單位面積缺陷數可由3.80x108 /cm2降至2.96x108/ cm2，由此證明使用氮化鋁/高深寬比圖案化藍寶石基板成長氮化鎵厚膜可改善薄膜品質。 接著由氮化鎵發光二極體電流擴散原理得知，使n型氮化鎵厚膜與表面ITO導電薄膜之電阻值越匹配，則其電流分佈於氮化鎵發光二極體內部就越均勻，但成長不同濃度之n型氮化鎵厚膜會因為摻雜過多的矽而使薄膜品質降低，由於成長氮化鎵厚膜於氮化鋁/高深寬比圖案化藍寶石基板，可有效降低缺陷密度，因此本研究將直接成長摻雜不同濃度之n型氮化鎵厚膜於氮化鋁/高深寬比圖案化藍寶石基板，可減緩n型氮化鎵厚膜因摻雜矽而增加缺陷密度之程度。 接著我們成長氮化鎵發光二極體結構於實驗樣品上，積分球量測結果可得知，比較LED R與LED II，光輸出功率由4.34mW上升至6.39mW，提升了47.2%;飽和電流由355mA提升至480mA，由於利用HPVE成長氮化鎵厚膜，使缺陷密度下降，厚膜品質提升，且改變發光二極體結構內之n型氮化鎵電阻值，進而改善電流於發光二極體內部分佈情形，使流經MQWs的面積增加，因此提升光輸出效率，同時降低熱效應。變電流對波長量測分析可得知，比較LED I與LED R，藍移量由6.9nm下降至3.9nm，由於氮化鎵厚膜能有效釋放應力，因此在結構中所承受的應力小。比較LED I、LED II與LED III可得知，藍移量由3.9nm上升至5.0nm再上升至6.9nm，隨著摻雜矽的濃度提升，氮化鎵材料晶格內部受擠壓的程度也越大，進而增加結構中之應力。 經本研究結果得知，理論計算擴散長度與實際量測趨勢相符合，改變氮化鎵發光二極體內部各層的電阻值，將會影響實際電流分佈的情形，對n型氮化鎵薄膜與表面ITO薄膜之電阻值最匹配的LED II而言，其電流分佈因此較均勻，在大電流密度注入條件下，降低電流聚集在電極附近，使電流流經MQWs的面積增加，減少元件上的熱效應，進而提高發光二極體之光輸出功率與可靠度。

In this thesis, we studied the growth of different concentrations of nGaN film on a sputtered AlN/high-aspect ratio patterned sapphire substrate (HARPSS) via hydride vapor phase epitaxy (HVPE) to improve the p-side-up mesa-structure of GaN LEDs obtained from the crowding effect. A high current density is necessary to enhance the current spreading uniformity of GaN LEDs. First, thick un-doped GaNs were grown on sputtered AlN/PSS and sputtered AlN/HARPSS. Results revealed that the thick GaN layer grown on sputtered AlN/HARPSS has a lower etching pit density (from 3.80×108 cm−2 to 2.96×108 cm−2) and smaller FWHM (from 347 arcsecs to 320 arcsecs) than the thick GaN layer grown on sputtered AlN/PSS. Thus, film quality can be improved by using sputtered AlN/HARPSS as a substrate. We then used theoretical models to calculate the current distribution of GaN-based LEDs prepared on different concentrations of thick nGaN/AlN/HARPSS templates. The resistances of the nGaN layers and the transparent current layers were matched so that the current is evenly distributed, and high-concentration nGaN layers were grown to reduce their resistance. As heavily Si-doped nGaN layer can degrade film quality, we directly grew thick nGaN templates with different concentrations on AlN/HARPSS to suppress the threading dislocation density effectively. We used a thick GaN template to fabricate GaN-based light-emitting diodes (LEDs). The light output power of LED R improved from 4.34mW to 6.39mW (enhancement of 47.2%) compared with that of LED II at an injection current of 20 mA. The saturation current of LED R also improved from 355 mA to 480 mA compared with that of LED II. The larger LED output power observed is attributed to the enhanced current spreading length, which improves heat dissipation ability and crystal quality. The blue shift decreased from 6.9 nm to 3.9 nm because the GaN thick layer can effectively release stress. Thus, the structure was able to withstand stress. Comparing the performances of LED I, LED II, and LED III, increases in blue shift from 3.9 nm to 5.0 nm and then to 6.9 nm were observed with increasing silicon doping concentration, which increases the compactness of the internal lattice of the GaN material and increases the stress in the structure. In this study, the output power of LED-2 with a 10 μm-thick nGaN template was 47.2% higher than that of LED R. The 10 μm-thick nGaN template resulted in a longer current spreading length, better heat dissipation ability, and better crystal quality. Thus, the light output power of the LED was significantly enhanced.