The study of optimizing GaN-based optoelectronic devices
|關鍵字:||發光二極體;垂直型發光二極體;覆晶型發光二極體;高速電子遷移率場效電晶體;light emitting diode;vertical LED;flip-chip LED;HEMT|
本研究中，將針對上述的情況分別提出解決的辦法。首先在提升晶體品質與增加光萃取效率方面，利用皇冠型圖樣化基板與在基板上製作二氧化矽圖樣化結構來成長發光元件，使堆疊缺陷轉彎，不再向上串升，提升晶體品質，改善發光元件效率，其圖形化結構亦能增強光萃取效率。在電流20毫安培下，皇冠型圖樣化基板與在基板上製作二氧化矽圖樣化結構之發光二極體分別使光輸出增加了32%與62%相較於傳統發光二極體；由於常用來做為氮化鎵元件成長的藍寶石基板本身的散熱不佳，在高電流注入的使用上會產生熱累積而使得元件效率下降或毀損，為改善此現象，很多研究改採將藍寶石基板移除之垂直型發光元件，而移除的方式不外乎以高功率紫外光雷射或化學蝕刻法，但這二種方式前者設備昂貴，後者易發生過蝕或蝕刻不足的控制難度，其共通的缺陷是易造成元件損傷。於是我們提出一種方便又傷害低的自然力剝除方法，此方法提升了30%光功率輸出(在350毫安培下)也將製程良率由78.3%提升至91.3% (Ir < 0.2μA at -5V) 相較於使用雷射撥離之發光垂直型二極體；在紫外光波段的開發上，我們使用反應電漿鍍膜技術沉積高品質之氮化鋁層，取代傳統磊晶技術所使用的低溫氮化鎵成核層，減少紫外光易被缺陷吸收的缺點與減少磊晶時產生的螺紋線狀差排缺陷，提升元件發光效率。在電流350毫安培下，與傳統使用低溫氮化鎵成核層之發光二極體多出32%的光功率輸出；在功率元件上，使用反應電漿鍍膜技術沉積之氮化鋁層成核層成長高品質氮化鋁鎵/氮化鎵高電子遷移率場效電晶體結構，並成功製作出適用於可見光通訊發射端模組之高功率氮化鋁鎵/氮化鎵高電子遷移率場效電晶體元件；在高電子遷移率場效電晶體元件熱管理上，我們使用內嵌式單層石墨烯來增加熱擴散，在120度下使得飽和汲極電流衰減約28%。
With developments and requirements of solid-state lightings and high-frequency electronic devices, wide-band gap III-nitrides become a popular materials due to broad wavelength range. However, unavoidable issue existed during the growth process of III-nitride material, such like increment of non-radiative recombination effect owing to crystal defects which is caused by the lattice mismatch between epilayers and substrate. Furthermore, the strain-induced polarization field and its effects conspicuously diminished the LED performance, leading to the poor light emitting efficiency. The efficiency of optoelectronic devices have been affected by structure strain and build-in polarization field. Therefore, the thesis focus on improving crystal quality of optoelectronic devices, reducing structure defects and increasing ligh extraction efficiency. First, growing light emitting devices on crown-shaped pattern sapphire substrate and cone-shaped SiO2 patterned template to bend stacking faults that made less defects attached device structure to improve crystal quality and efficiency of light emitting devices, and the pattern structure also increased the light extracted efficiency. At 20 mA driving current, the light output power of LEDs grown on crown-shaped pattern sapphire substrate and cone-shaped SiO2 patterned template, compared with LEDs grown on planar sapphire substrate, has 32% and 62% enhancement, respectively. Owing to poor thermal conductivity of sapphire substrate which is widely used to be a substrate for growing GaN-based devices, a thermal accumulation makes the efficiency reduction and device damage in high current injection. Generally, the techniques of laser lift-off (LLO) and chemical lift-off (CLO) have been adopted for removing sapphire substrate to build up a VLED structure. Unfortunately, LLO cost higher and CLO is difficult to maintain the etching uniformity and crystal quality simultaneously. In this work, we demonstrate a natural substrate lift-off (NSLO) technique for fabricating high-quality GaN-based VLEDs. The light output power of VLED with NSLO was increased 30% (@350 mA) and yield of the leakage current (Ir < 0.2μA at -5V) was also improved from 78.3% to 91.3% compared to that of VLED with LLO. For UVLED, we utilized ex-situ reactive plasma deposited (RPD) AlN nucleation layer to instead of low temperature GaN(LT-GaN) on patterned sapphire substrate (PSS), which exhibited threading dislocation density (TDD) can be reduced. Furthermore, we demonstrated the light output power of the LEDs with the RPD AlN nucleation layer was increased 32% (@350 mA) than that of LEDs grown on PSS with conventional LT-GaN nucleation layer. In power devices, we successfully grew high quality the AlGaN/GaN high-electron-mobility transistor (HEMT) structure with RPD-AlN on planar sapphire substrat by MOCVD and produed high power AlGaN/GaN HEMT device which was suitable for visible light communication (VLC) transmitter module. For the thermal issue of HEMT devices, we use a single embedded graphene layer to improve heat spreading. The decreasing rate of Idss was as small as 28%. In this thesis, several effective approaches in order to enhance the optoelectronic characteristics have been proposed, and expected that these researches could contribute to the development and progress for GaN-based optoelectronic components.
|Appears in Collections:||Thesis|