Effect of AlGaN interlayer on luminous efficiency and reliability of GaN-based green LEDs on silicon substrate

The effect of AlGaN interlayer in quantum barrier on the electroluminescence characteristics of GaN-based green light emitting diodes(LEDs)grown on silicon substrate was investigated.The results show that AlGaN interlayer is beneficial to improve the luminous efficiency of LED devices and restrain the phase separation of In GaN.The former is ascribed to the inserted AlGaN layers can play a key role in determining the carrier distribution and screening dislocations in the active region,and the latter is attributed to the increased compressive stress in the quantum well.However,when the electrical stress aging tests were performed at a current density of 100 A/cm^2,LED devices with AlGaN interlayers are more likely to induce the generation/proliferation of defects in the active region under the effect of electrical stress,resulting in the reduced light output power at low current density.     

InGaN/GaN超晶格厚度对Si衬底GaN基蓝光发光二极管光电性能的影响

采用有机金属化学气相沉积技术在Si(111)衬底上生长蓝光多量子阱发光二极管(LED) 结构, 通过在量子阱下方分别插入两组不同厚度的InGaN/GaN超晶格, 比较了超晶格厚度对LED光电性能的影响. 结果显示: 随超晶格厚度增加, 样品的反向漏电流加剧; 300 K下电致发光仪测得随着电流增加, LED发光光谱峰值的蓝移量随超晶格厚度增加而减少, 但不同超晶格厚度的两个样品在300 K下的电致发光强度几乎无差异. 结合高分辨X射线衍射仪、扫描电子显微镜、透射电子显微镜对样品的位错密度和V形坑特征分析, 明确了两样品反向漏电流产生巨大差异的原因是由于超晶格厚度大的样品具有更大的V形坑和V形坑密度, 而V形坑可作为载流子的优先通道, 使超晶格更厚的样品反向漏电流加剧. 通过对样品非对称(105)面附近的X射线衍射倒易空间图分析, 算得超晶格厚度大的样品其InGaN量子阱在GaN上的弛豫度也大, 即超晶格厚度增加有利于减小InGaN量子阱所受的应力. 综合以上影响LED发光效率的消长因素, 导致两样品最终的发光强度相近.     

低色温无荧光粉LED光源的可靠性研究

为了对比研究新型低色温无荧光粉LED光源和低色温荧光粉LED光源的可靠性,本文进行了85℃加速老化和温度步进应力实验,测试了各光电参数在老化过程中的变化规律,并分析了有无荧光粉低色温光源的老化衰减机理。实验结果表明,在20 A/cm2工作电流密度、85℃的加速老化条件下,两种类型的光源都表现出很好的稳定性,但随着功率的增加,无荧光粉光源展现出更好的光通量稳定性;随着温度的升高,无荧光粉光源在高达200℃左右仍表现出良好的可靠性,而荧光粉光源则在175℃后出现了明显的光通量衰减和色温升高。电流应力或温度应力提升以后,无荧光粉光源展现出更优异的可靠性。     

V形坑尺寸对硅衬底InGaN/AlGaN近紫外LED光电性能的影响

使用MOCVD在图形化Si衬底上生长了InGaN/AlGaN近紫外LED,通过改变低温GaN插入层的厚度调控V形坑尺寸,系统地研究了V形坑尺寸对InGaN/AlGaN近紫外LED(395 nm)光电性能的影响。结果表明,低温GaN插入层促进了V形坑的形成,并且V形坑尺寸随着插入层厚度的增加而增大。在电学性能方面,随着V形坑尺寸的增大,-5 V下的漏电流从5.2×10~(-4)μA增加至6.5×10~2μA;350 mA下正向电压先从3.55 V降至3.44 V,然后升高至3.60 V。在光学性能方面,随着V形坑尺寸的增大,35 A/cm~2下的归一化外量子效率先从0.07提高至最大值1,然后衰退至0.53。对V形坑尺寸影响InGaN/AlGaN近紫外LED光电性能的物理机理进行了分析,结果表明:InGaN/AlGaN近紫外LED的光电性能与V形坑尺寸密切相关,最佳的V形坑尺寸为120~190 nm,尺寸太大或者太小都会降低器件性能。     

氮化镓基绿光LED中V坑对空穴电流分布的影响

采用实验与理论模拟相结合的方法,研究了氮化镓基绿光发光二极管（LED）中V坑对空穴电流分布的影响。首先,实验获得了V坑面积占比不同的3种样品;然后,建立数值模型,使得理论计算的外量子效率（EQE）及电压与实验测试的变化趋势相匹配,从而确立了所用数值模型的可信性。计算结果显示：V坑改变了空穴电流的分布,空穴电流密度在V坑处显著增加,在平台处明显减小。进一步的分析表明：V坑面积占比在0~10%范围内,V坑空穴电流占比与V坑面积占比之间呈近线性增长（斜率为2.06）,但V坑空穴注入在整个空穴注入的过程中仍未占主导。     

准备层温度对黄光LED光电特性和老化性能的影响

研究了InGaN/GaN超晶格准备层的生长温度对Si衬底GaN基黄光LED光电特性和老化性能的影响。研究发现准备层生长温度较高的样品外量子效率高于准备层生长温度较低的样品。500 mA电流下老化1000 h后,准备层生长温度较高的样品的光衰相对更大。老化前后100 K的电致发光光谱显示高温生长的样品老化后的空穴注入途径发生变化;老化后光衰大的样品非辐射复合中心增加的程度更大。荧光显微镜观察到两个样品老化前均出现大量暗斑,高温样品的颜色更深更黑,低温样品颜色则相对较浅且呈红色。老化后高温样品的暗斑数量有所增加,而低温样品数量变化不大,这可能也是导致超晶格温度高的样品光衰更大的原因之一。     

垒温对硅衬底GaN基蓝光LED发光效率的影响

用MOCVD技术在硅衬底上生长了GaN基蓝光LED外延材料,研究了有源层多量子阱中垒的生长温度对发光效率的影响,获得了不同电流密度下外量子效率(EQE)随垒温的变化关系。结果表明,在860~915℃范围内,发光效率随着垒温的上升而上升。当垒温超过915℃后,发光效率大幅下降。这一EL特性与X光双晶衍射和二次离子质谱所获得的阱垒界面陡峭程度有明显的对应关系,界面越陡峭则发光效率越高。垒温过高使界面变差的原因归结为阱垒界面的原子扩散。垒温偏低使界面变差的原因归结为垒对前一个量子阱界面的修复作用和为后一个量子阱提供台阶流界面的能力偏弱。外延生长时的最佳垒温范围为895~915℃。     

Effects of Hydrogen Treatment in Barrier on the Electroluminescence of Green InGaN/GaN Single-Quantum-Well Light-Emitting Diodes with V-Shaped Pits Grown on Si Substrates

Effect of hydrogen(H_2) treatment during the GaN barrier growth on the electroluminescence performance of green In GaN/GaN single-quantum-well light-emitting diodes(LEDs) grown on Si substrates is experimentally investigated. We prepare two LED samples with different carrier gas compositions during the growth of GaN barrier. In the H_2 free LED, the GaN barrier is grown in full nitrogen(N_2) atmosphere. For the other H_2 treated LED, a mixture of N_2 and H_2 was used as the carrier gas. It is observed that V-shaped pits decrease in size after H_2 treatment by means of the scanning electron microscope. Due to the fact that the p–n junction interface would be closer to the p-GaN as a result of smaller V-shaped pits, the tunneling barrier for holes to inject into the In GaN quantum well would become thicker after H_2 treatment. Hence, the external quantum efficiency of the H_2 treated LED is lower compared to the H_2 free LED. However, LEDs would exhibit a better leakage behavior after H_2 treatment during the GaN barrier growth because of more effective blocking of the threading dislocations as a result of the H_2 etching at V-shaped pits.