成長與製作高效率氮化鎵系列發光元件

Abstract

近年來，寬能隙氮化鎵系列半導體是製作寬波長發光元件非常重要的材料系統。其中氮化鎵雷射與發光二極體元件已經很廣泛利用於很多領域，如光資訊儲存、液晶顯示器背光光源、交通號誌與固態照明。 在本研究論文中，為了達到製作高效率發光元件與克服晶體成長於不適合機板所產生的問題，首先我們利用原子層沉積技術建立一新穎的方法達成高品質氮化鋁鎵/氮化鎵多重量子井結構之成長。經由原子層沉積後之應變磊晶層可有效阻擋大多數從基板產生之線缺陷。並且插入原子層沉積磊晶層之氮化鎵系列發光二極體與一般發光二極體在操作電流為20mA 相比較下可提升 27% 之外部輸出功率。 我們同時也研究利用缺陷填補法來降低磊晶層缺陷密度。此缺陷填補法是利用化學蝕刻方式定義缺陷位置，將利用沉積二氧化矽薄膜填補蝕刻後的空洞，接者使用側向磊晶方式將其填補。此方法可以有效的降低磊晶層缺陷密度從單位面積1x109/cm2 至 4x107/cm2 。在與一般發光二極體相比較之下，缺陷填補發光二極體可有效提升45%外部發光功率。 在另一方面，我們也提出倒金字塔結構位於氮化鎵與藍寶石積板介面製作高效率紫外光發光二極體，此倒金字塔結構可以同時有效提升光萃取效率與磊晶層品質。在與一般發光二極體相比較之下，此結構之外部發光功率可有效提升85%。除此之外，我們建立了首先利用機械式剝離法製作垂直式發光二極體。並且在拉曼光譜可以顯示氮化鎵磊晶層之應力在此倒金字塔結構中可以有效的被釋放。 最後，我們製作氮化鎵奈米柱結構應用於雷射操作上並討論其雷射特性。首先，利用氮化鎵磊晶層與奈米級鎳金屬遮罩進行蝕刻後形成氮化鎵奈米柱狀結構，接者利用再成長的方式重新成長新的晶格表面於奈米柱側壁。並且在光激發操作下，此結構之臨界條件為122 MW/cm2，雷射發光波長與發光半寬度分別為363 nm與 0.38 nm。我們推測此激發的現象歸因於在氮化鎵奈米柱結構中所產生隨機雷射與同調反饋的現象所致。

Recently, GaN-based wide bandgap semiconductors are very important material system for fabrication of light emitting devices in a wide range of wavelength. The GaN-based laser diodes and light emitting diodes have been widely used in many areas, such as optical storage, backlight in liquid crystal displays, traffic signal and solid state lighting. In this study, in order to achieve high efficiency light emitting devices and overcome the epitaxial issues on foreign substrate, we demonstrated a novel approach for high quality AlxGa1-xN/GaN multiple quantum well epitaxy using atomic layer deposition (ALD) technique. The strain ALD layers effectively block a majority of threading dislocation from the substrate. The light output power for the GaN-based LED with an ALD insert layer at 20 mA was 27% higher than that for a conventional GaN-based LED structure. We also study the defect selective passivation method to block the propagation of threading dislocations. The defect selective passivation is done by using defect selective chemical etching to locate defect sites, followed by silicon oxide passivation of the etched pits, and epitaxial over growth. The threading dislocation density in the regrown epi-layer is significantly improved from 1x109 to 4x107 cm-2. The output power of DP-LED is enhanced by 45% at 20 mA compared to a conventional LED. On other hand, we also proposed the fabrication of high efficiency UV light emitting diodes with inverted pyramid (IP) structures at GaN-sapphire interface. The pyramid structures have significantly enhanced the light extraction efficiency and at the same time also improved the crystal quality. The output power was enhanced by 85% compared to a conventional LED. In addition, we demonstrated the first mechanical lift-off technique for fabrication of vertical type LED with IP structure. Raman spectroscopy analysis revealed that the compressive strain of GaN epilayer effectively relaxed in the IP structures. Finally, the GaN based nanopillars structure for laser operation has been also fabricated and the laser behaviors were also been studied. The nanopillars were fabricated from a GaN epitaxial wafer by self-assembled Ni nanomasked etching, followed by epitaxial regrowth to form crystalline facets on the etched pillars. The lasing action occurs at threshold pumping power density of 122 MW/cm2 with a linewidth of 0.38 nm at 363 nm. The lasing phenomenon could to due to random laser action with coherent feedback in GaN nanopillars.