半极性面GaN基偏振光LED外延技术及性能研究

With the breakthrough of III-nitrides' growth and chip fabrication technologies, white light emitting diodes (LEDs), based on the GaN-based blue LEDs exciting the yellow phosphors, become gradually an important illumination source. In order to further enhance the luminous efficacy of white LEDs, it is necessary to largely cut down the defect density in materials and to lower down or avoid influence of the polarization electric field on the LED performance. Therefore, it is one choice to fabricate the nonpolar or semipolar GaN-based LEDs. This project will focus on the achievement about the (11-22) semi-polar polarized GaN-based LEDs on m-plane patterned sapphire substrate (PSS) with the cost-effectiveness. We will take an advantage of the microstructures on PSS surfaces to reduce the defect formation during the GaN growth and to enhance the extraction efficiency of GaN-based LEDs. The (11-22) growth orientation will be controlled by the combination of the treatment to sapphire substrate surfaces and the utilities of AlN buffer layers. We will aslo study the growth process of the semipolar GaN via the stepwise growth, and show the influence of the pattern sizes and shapes on the formation and suppression of micro-structural defects. At the same time, we will also investigate the effects of LED layer structures and epitaxial growth conditions on the output performances of LEDs. The polarization properties of the LEDs will be uncovered. Such the polarized GaN-based LEDs will be useful and promising in application for the liquid crystal displays (LCD) as a backlight sources due to the energy savings.

随着宽带隙GaN基材料生长技术和芯片制造技术的突破，以GaN基蓝光LED激发黄色荧光粉制成的白光LED已经成为主要半导体照明光源。为了进一步提高白光LED发光效率，需要大幅减少材料内的缺陷密度和消除极化电场对LED发光效率的影响,为此，制造非极性或半极性面GaN基LED是其选项之一。本项目旨在利用m面图形蓝宝石衬底的凸凹结构抑制缺陷生成和提高LED取光效率，实现低成本(11-22) 半极性面GaN基偏振光LED。通过衬底表面处理和AlN缓冲层的协同作用，控制半极性面GaN的外延生长取向，通过分步外延生长，揭示图形衬底外延生长过程以及图形尺寸、形状对缺陷生成及抑制机理。研究LED外延层结构设计、外延各工艺因素对半极性面LED性能的影响机理，揭示半极性面LED的偏振特性。这种偏振光LED因其更节能，将极具潜力作为液晶显示背光源。

项目利用m面蓝宝石衬底，探索了(11-22)面半极性GaN基LED材料的外延生技术，表征了材料的表面形貌、晶体质量的各向异性，应力分布、微结构缺陷和发光特性等。.利用AlN或GaN缓冲层，制备出了结晶质量优良、表面呈镜面的GaN材料,GaN和衬底取向关系为[1-100]GaN||[1-210]Al2O3 和[-1-123]GaN||[0001]Al2O3。原子力显微镜揭示其表面形貌呈现条纹状，10x10μm^2区域粗糙度仅为4.34nm，这一结果处于世界领先水平。透射电子显微术剖析了衬底－缓冲层－GaN抑制界面的原子排列情况，发现半极性GaN材料内存在大量的位错和层错，其密度分别处于6x10^9~10^10cm^-2和5x10^4~4x10^5cm^-1之间。.高分辨x射线衍射测量显示,半极性GaN的晶体质量呈现各向异性，(11-22)衍射峰半高宽随不同方位角的变化规律呈现M形，沿[-1-123] 方向时最小，沿[10-10]方向时最大。用AlN缓冲层生长的GaN(11-22)衍射峰半高宽在0.2~0.4度，各向异性度为~0.33。而用GaN缓冲层生长的GaN(11-22)衍射峰半高宽为0.11~0.27度，各向异性度却为0.44，显示各向异性与生长工艺有关。半极性InGaN样品的各向异性随In掺入的增加而减少,In组分x为0.19和0.34半极性InGaN样品的各向异性度ξ分别0.2和0.096。在In0.25Ga0.75N多量子阱结构中观察到多级卫星峰，显示量子阱具有陡峭的异质界面。.用室温光致发光谱测量显示,AlN缓冲层上生长(11-22)GaN样品的光偏振度为0.63，而半极性InGaN样品的光偏振度随In掺入的增加而减少，其机理尚不清楚。In组分x为0.19和0.34半极性InGaN样品的偏振度ρ分别为0.238和0.087。.设计和成功生长出了半极性蓝光和绿光LED，在不做出芯片的情况下加电可以直接发光，这是国内首次实现的半极性蓝光和绿光LED，经室温显微光致发光测试，波长~448nm蓝光和~515nm绿光LED的偏振度分别为0.41和0.43。.半极性蓝、绿光LED的实现表明，半极性氮化物材料有潜力解决绿黄光波段c面LED材料难以解决的“绿隙”挑战、LED效率衰减和颜色漂移等问题，它的偏振特性用作LCD显示背光源时节能且无需外加偏振膜。