使用奈米結構改善氮化銦鎵發光元件之光電及結構特性研究

Abstract

本論文初期，我們使用陽極氧化鋁(AAO)遮罩製程技術完成奈米圖案藍寶石基板(NPSS)製備，並成長InGaN發光元件。由SEM側視圖發現，GaN薄膜與high-AR(i.e. ~2)基板界面存在空孔，顯現側向成長機制將主導GaN薄膜成長初期，GaN薄膜將迅速覆蓋奈米圖案，阻絕反應氣體分子進入圖案孔洞內成長GaN。X光繞射分析顯示，GaN薄膜成長於high-AR基板的刃差排密度，約可由1.0×109 cm-2降低至5.9×108 cm−2，全係因為GaN側向成長使得差排缺陷彎曲、最終抵消。然而，由於圖案表面之側向成長以及圖案底面之垂直成長兩者的相互競爭，導致GaN成長在low-AR(i.e. ~0.7)基板的螺旋差排密度增加，造成薄膜品質低劣。此外，雖然high-AR基板擁有較高之光漫反射率，InGaN元件之發光強度卻並未顯著增加；相反地，low-AR基板卻可將發光強度提升~2倍，這係因為low-AR基板可增加與磊晶結構層之接觸面積，奈米圖案能對發射光子進行有效散射。因此，幾何圖案是實現高亮度InGaN發光元件的重要關鍵。 本論文另外研究「具微奈米結構圖案之藍寶石基板」(Hybrid-PSS)，我們利用AAO遮罩將奈米圖案轉印至傳統微米圖案基板(Bare-PSS)，並成長InGaN發光元件(HP-LED)，其差排缺陷將能有效抑制，並獲得較低的漏電流。側視微拉曼光譜顯示，兩者LED皆為應力鬆弛狀態。此外，在高電流注入下產生的波長紅移行為可由量子侷限史塔克效應以及能隙縮減效應進行解釋。由模擬波長-電流曲線得知，內建壓電場在BP-LED及HP-LED分別為1.59 MV cm1及1.57 MV cm1；而預估之晶片溫度在HP-LED中，於600-mA時可由150 °C降低至75 °C，這係因為因為奈米孔洞的崁入，增加基板與磊晶結構層之散熱接觸面積導致的結果。此外，Hybrid-PSS擁有較高的光漫反射率，能增加光子逃逸機率，降低晶片因光子吸收轉變熱能所造成的溫度增加，且額外增加可視角~15°。最終，光輸出功率在360-mA時提升~75%，而顯著抑止光效下降，因此能成為高功率InGaN發光元件之基板。 最後，我們引入「氮氧化銦量子點」(InON nanodot)夾層製備出具歐姆接觸之AZO電極於p-GaN層，其特徵接觸電阻在嵌入InON量子點密度為6.5×108 cm−2時，可達1.12×10−4 Ω•cm2；然而，當樣品在氧氣環境中退火後，其電流傳導轉為蕭特基發射機制。電子能譜儀結果顯示，InON量子點內部的氧空缺在載子傳輸中扮演著關鍵因素。同時，由模擬電流-電壓特徵曲線得知，跳躍傳輸將主導著載子傳輸機制，其缺陷間距及活化能分別為1.2 nm及36.2 meV。我們認為由於高密度的類施體氧空缺存在InON量子點及p-GaN層界面，正電荷將會吸附於p-GaN層表面形成電荷聚積現象，並創造出一個窄空乏區；載子可藉由跳躍傳輸通過InON量子點，最後則穿隧界面進入p-GaN層。因此，藉由缺陷輔助穿隧效應，利用InON量子點將能製備出具歐姆接觸之AZO/p-GaN層。

In this thesis, InGaN-based light-emitting diodes (LED) are grown on the nanoscale-patterned sapphire substrates (NPSSs) prepared with the anodic aluminum oxide (AAO) technique as the dry etching mask. The cross-sectional view of SEM image shows that voids exist between the interface of the GaN thin film and the high-AR (i.e. ~2) NPSS. The formation of voids on the high-AR NPSS is believed to be due to the enhancement of the lateral growth in the initial growth stage, and the quick-merging GaN thin film blocks the precursors from continuing to supply the bottom of the pattern. The edge-type threading dislocation (TD) density can be reduced from 1.0×109 cm-2 for GaN on bare sapphire to 5.9 ×108 cm−2 for GaN on a high-AR NPSS. A large number of bending dislocations terminated together and stopped propagating to the surface of the merged GaN, leading to the dislocation reduction in the GaN thin film. However, the increased screw-type threading dislocation density for GaN on a low-AR NPSS (i.e. ~0.7) is due to the competition of lateral growth on the flat-top patterns and vertical growth on the bottom of the patterns that causes the material quality of GaN thin film to degenerate. Although high-AR NPSS exhibits a higher diffuse reflection rate, the light emitting intensity of LED shows no significant enhancement. In contrast, the light emitting intensity increases by 100% for LED on a low-AR NPSS. Increasing GaN coverage area with the inclined facets of low-AR nanopatterns will effectively scatter off the emitted photons, leading to the increased light emitting intensity of LED. Thus, the specific geometry of a NPSS plays a crucial role in achieving high-brightness InGaN-based LEDs. Besides, a hybrid patterned sapphire substrate (Hybrid-PSS) is prepared by using an AAO etching mask to transfer nano-patterns onto a conventional patterned sapphire substrate (Bare-PSS). The threading dislocations (TDs) suppression of light-emitting diodes (LEDs) grown on a Hybrid-PSS (HP-LED) exhibits a smaller reverse leakage current compared with that of LEDs grown on a Bare-PSS (BP-LED). The cross-sectional micro-Raman spectra of both samples show the GaN thin films in the LED structural layer are strain-free. The combined effects of quantum confinement Stark effect and bandgap shrinkage of the InGaN well layer were considered to explain the large red-shifted EL peak wavelength under high injection currents. The calculated piezoelectric fields are 1.59 and 1.57 MVcm-1 respectively for BP-LED and HP-LED. The estimated LED chip temperatures rise from room temperature to 150C and 75C for BP-LED and HP-LED respectively at a 600-mA injection current. This smaller temperature rise of LED chip is attributed to the increased contact area between sapphire and the LED structural layer because of the embedded nano-pattern. Although the chip generates more heat at high injection currents, the accumulated heat can yet be removed outside the chip effectively. The high diffuse reflection (DR) rate of hybrid-PSS increases the escape probability of photons, resulting in an increase in the viewing angle of LEDs from 130 to 145. The efficiency droop was reduced from 46% to 35%, effects, which can be attributed to the elimination in TDs and enhancement in light extraction by embedded nano-patterns. In addition, the light output power of HP-LED at 360-mA injection currents exhibits a ~75% enhancement, demonstrating that hybrid-PSSs are beneficial to applicate in high-power LEDs. Finally, we develop an Ohmic contact formation method for a ZnO:Al (AZO) contact on p-GaN films involving the introduction of an indium oxynitride (InON) nanodot interlayer. A low specific contact resistance of 1.12×10−4 Ω•cm2 is achieved for a sample embedded with an InON nanodot interlayer with a nanodot density of 6.5×108 cm−2. By contrast, a sample annealed in oxygen ambient exhibits a Schottky behavior. X-ray photoemission spectroscopy results shows that the oxygen vacancy (Vo) in the InON nanodots plays a crucial role in carrier transport. The fitting I–V characteristic curves indicate that the hopping mechanism with an activation energy of 36.2 meV and trap site spacing of 1.2 nm dominates the carrier transport in the AZO/InON nanodot/p-GaN sample. Because of the high density of donor-like oxygen vacancy defects at the InON nanodot/p-GaN interface, positive charges from the underlying p-GaN films are absorbed at the interface. This leads to positive charge accumulation, creating a narrow depletion layer; therefore, carriers from the AZO layer pass through InON nanodots by hopping transport, and subsequently tunneling through the interface to enter the p-GaN films. Thus, AZO Ohmic contact can be formed on p-GaN films by embedding an InON nanodot interlayer to facilitate trap-assisted tunneling.