GaN基蓝绿光LED电应力老化分析

对InGaN/GaN多量子阱蓝光和绿光LED进行了室温20,40,60 mA加速电流下的电应力老化研究,发现蓝光与绿光样品经过60 mA电流老化424 h后,其电学性能表现出一定的共性与差异性:在小测量电流下,绿光样品的光衰减幅度较蓝光样品大~9%;而在较大测量电流 (20 mA)下,两者的光衰减幅度基本相同 (18%)。同时,蓝绿光样品的正向电学性能随老化时间的变化幅度基本一致,反映出它们具有相似的退化机制,绿光样品老化后增多的缺陷大部分体现为简单的漏电行为,而并非贡献于非辐射复合中心。在此基础上对GaN基外延结构进行了优化,优化后的LED长期老化的光衰减幅度较参考样品降低了3%。     

High power and high reliability GaN/InGaN flip-chip light-emitting diodes

High-power and high-reliability GaN/InGaN flip-chip light-emitting diodes (FCLEDs) have been demonstrated by employing a flip-chip design, and its fabrication process is developed. FCLED is composed of a LED die and a submount which is integrated with circuits to protect the LED from electrostatic discharge (ESD) damage. The LED die is flip-chip soldered to the submount, and light is extracted through the transparent sapphire substrate instead of an absorbing Ni/Au contact layer as in conventional GaN/InGaN LED epitaxial designs. The optical and electrical characteristics of the FCLED are presented. According to ESD IEC61000-4-2 standard (human body model), the FCLEDs tolerated at least 10\,kV ESD shock have ten times more capacity than conventional GaN/InGaN LEDs. It is shown that the light output from the FCLEDs at forward current 350mA with a forward voltage of 3.3\,V is 144.68\,mW, and 236.59\,mW at 1.0\,A of forward current. With employing an optimized contact scheme the FCLEDs can easily operate up to 1.0\,A without significant power degradation or failure. The life test of FCLEDs is performed at forward current of 200\,mA at room temperature. The degradation of the light output power is no more than 9\% after 1010.75\,h of life test, indicating the excellent reliability. FCLEDs can be used in practice where high power and high reliability are necessary, and allow designs with a reduced number of LEDs.     

Shallow versus deep nature of Mg acceptors in nitride semiconductors

We investigate the properties of Mg acceptors in nitride semiconductors with hybrid functional calculations. We find that although the thermodynamic transition level is relatively close to the valence band in GaN (260 meV), Mg(Ga) exhibits key features of a deep acceptor: the hole is localized on a N atom neighboring the Mg impurity, inducing a large local lattice distortion and giving rise to broad blue luminescence. We show that the ultraviolet photoluminescence peak attributed to Mg acceptors in GaN is likely related to Mg-H complexes, explaining the results of photoluminescence and electron paramagnetic resonance experiments. Predictions for Mg acceptors in AlN and InN are also presented.     

压缩应变载荷下氮化镓隧道结微观压电特性及其巨压电电阻效应

电子器件可控性研究在日益追求器件智能化和可控化的当今社会至关重要. 基于第一性原理和量子输运计算, 本文研究了压缩应变载荷对氮化镓(GaN)隧道结基态电学性质和电流输运的影响, 在原子尺度上窥视了氮化镓隧道结的微观压电性, 验证了其内在的巨压电电阻(GPR)效应. 计算结果表明, 压缩应变载荷可以调节隧道结内氮化镓势垒层的电势能降、内建电场、电荷密度和极化强度, 进而实现对隧道结电流输运和隧穿电阻的调控. 在-1.0 V的偏置电压下, -5%的压缩应变载荷将使氮化镓隧道结的隧穿电阻增至4倍. 本研究展现了氮化镓隧道结在可控电子器件中的应用潜力, 也展现了应变工程在调控电子器件性能方面的光明前景.     

基于石墨烯-ZnO纳米线的复合电极在GaN LED中的应用

使用一维ZnO纳米线和二维石墨烯复合结构集成到p-GaN表面来同时实现电流扩展和提高LED光提取效率。通过两组有无ZnO纳米线器件的对比,发现ZnO纳米线使器件的光提取效率提高了30%.通过分析两组器件的开启电压、工作电压和反向漏电流等关键参数,验证了本结构应用于GaN LED不会恶化其电性能。本文所采用的复合结构用于GaN LED,同时达到了良好欧姆接触、避免使用ITO和增强出光的效果。     

High-temperature failure of GaN LEDs related with passivation

This paper analyses the thermally-activated failure mechanisms of GaN LED test-structures related with the presence of a hydrogen rich SiN passivation layer, by comparing the electrical and optical behaviour of samples with and without passivation during thermal stress. The analysis was carried out by means of electroluminescence, cathodoluminescence, emission microscopy and current–voltage measurements. Thermal treatment induced degradation only on the samples with passivation: identified degradation modes were an efficiency decrease exponential in time, emission crowding, and a forward voltage increase. On the other side, thermal treatment did not change the behaviour of the LEDs without passivation. An interpretation for the degradation of the passivated samples is the following: as a consequence of passivation deposition, a considerable amount of hydrogen is incorporated in the passivation layer. Heating at 250 C allows this hydrogen to interact with the LED surface, thus worsening the transport properties of p-GaN and of the p-ohmic contact, and then the current and emission distribution, inducing the observed degradation and emission crowding. The activation energy of the degradation process was found to be equal to 1.3 eV. Comparison between spectral electroluminescence and cathodoluminescence measurements shows how the mechanism mentioned above is not the only ageing cause and the thermal worsening of QW confinement and/or the creation of nonradiative centers possibly contribute to the LED damage.     

Low power fluorine plasma effects on electrical reliability of AlGaN/GaN high electron mobility transistor

The new electrical degradation phenomenon of the AlGaN/GaN high electron mobility transistor(HEMT) treated by low power fluorine plasma is discovered. The saturated current, on-resistance, threshold voltage, gate leakage and breakdown voltage show that each experiences a significant change in a short time stress, and then keeps unchangeable. The migration phenomenon of fluorine ions is further validated by the electron redistribution and breakdown voltage enhancement after off-state stress. These results suggest that the low power fluorine implant ion stays in an unstable state. It causes the electrical properties of AlGaN/GaN HEMT to present early degradation. A new migration and degradation mechanism of the low power fluorine implant ion under the off-stress electrical stress is proposed. The low power fluorine ions would drift at the beginning of the off-state stress, and then accumulate between gate and drain nearby the gate side. Due to the strong electronegativity of fluorine, the accumulation of the front fluorine ions would prevent the subsequent fluorine ions from drifting, thereby alleviating further the degradation of AlGaN/GaN HEMT electrical properties.     

Low-temperature activation of Mg-doped GaN with thin Co and Pt films

The activation of metallorganic chemical vapor deposition-grown Mg-doped GaN by N₂ annealing with thin Co and Pt films has been investigated. The Hall effect measurements revealed that both the Co and Pt films enhance activation of Mg acceptors as catalysts at low temperatures. A maximum hole concentration of p-type GaN was achieved at annealing temperature of 600 °C for the samples activated with both the Co and Pt films. It was also revealed that the activation of the acceptors is strongly affected by the thickness of the Co film.     

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.     

硅衬底氮化镓基LED薄膜转移至柔性黏结层基板后其应力及发光性能变化的研究

本文将硅(Si)衬底上外延生长的氮化镓(GaN)基发光二极管(LED)薄膜转移至含有柔性黏结层的基板上, 获得了不受衬底和支撑基板束缚的LED薄膜. 利用高分辨率X射线衍射仪(HRXRD)研究了薄膜转移前后的应力变化, 同时对其光致发光(PL)光谱的特性进行了研究. 结果表明: 硅衬底GaN基LED薄膜转移至柔性基板后, GaN受到的应力会由转移前巨大的张应力变为转移后微小的压应力, InGaN/GaN量子阱受到的压应力则增大; 尽管LED薄膜室温无损转移至柔性基板其InGaN阱层的In组分不会改变, 然而按照HRXRD倒易空间图谱通用计算方法会得出平均铟组发生了变化; GaN基LED薄膜从外延片转移至柔性基板时其PL谱会发生明显红移.     

基于反应动力学的GaN LED参数退化模型的研究

加速实验中, 参数退化模型描述了参数的退化规律, 参数退化规律对应于器件退化机理, 而退化机理又对应于内部的物理化学反应. 因此, 本文基于反应动力学中物理化学反应的温度效应速率模型及反应量浓度随时间的变化规律, 研究并建立了GaN LED参数退化模型. 本模型尝试从物理机理上解释参数退化过程中的退化规律, 包括单调上升或单调下降退化规律、先上升后下降或先下降后上升等非单调退化规律, 解决了实验后拟合方法不能建立非单调退化模型的问题. 然后对GaN LED进行加速实验, 确定模型参数. 同时对GaN LED的退化规律进行分解, 并且量化了GaN LED两种退化规律的退化比例及时间常数. 关键词： 参数退化模型 反应动力学 加速实验 GaN LED     

InGaN/GaN多量子阱结构发光二极管发光机理转变的低频电流噪声表征

本文对InGaN/GaN多量子阱结构发光二极管开启后的电流噪声进行了测试, 结合低频电流噪声的特点和载流子之间的复合机理, 研究了低频电流噪声功率谱密度与发光二极管发光转变机理之间的关系. 结论表明, 当电流从0.1 mA到10 mA逐渐增大的过程中, InGaN/GaN发光二极管的电流噪声行为从产生-复合噪声逐渐接近于低频1/f噪声, 载流子的复合机理从非辐射复合过渡为电子与空穴之间载流子数的辐射复合, 并具有标准1/f噪声的趋势, 此时多量子阱中的电子和空穴之间的复合趋向于稳定. 本文的结论提供了一种表征InGaN/GaN多量子阱发光二极管发光机理转变的有效方法, 为进一步研究发光二极管中载流子的复合机理、优化和设计发光二极管、提高其发光量子效率提供理论依据.     

Current status for light-emitting diode with Eu-doped GaN active layer grown by MBE

Recent progress on light-emitting diode having a Eu-doped GaN active layer is reported. Although the first success on LED using GaN:Eu has been achieved by OMVPE, the factors to be controlled during the crystal growth are not well understood. We found that GaN:Eu co-doped with Mg in NH₃-MBE shows a Eu site which is excited only by the above band-gap excitation. The luminescence intensity is enhanced at least 10 times than that without Mg co-doping. The LED operation fabricated using Mg co-doping technique is successfully demonstrated.     

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。     

AlGaN/GaN 高电子迁移率晶体管漏电流退化机理研究

本文首先制备了与AlGaN/GaN高电子迁移率晶体管 (HEMT) 结构与特性等效的AlGaN/GaN异质结肖特基二极管, 采用步进应力测试比较了不同栅压下器件漏电流的变化情况, 然后基于电流-电压和电容-电压测试验证了退化前后漏电流的传输机理, 并使用失效分析技术光发射显微镜 (EMMI) 观测器件表面的光发射, 研究了漏电流的时间依赖退化机理. 实验结果表明: 在栅压高于某临界值后, 器件漏电流随时间开始增加, 同时伴有较大的噪声. 将极化电场引入电流与电场的依赖关系后, 器件退化前后的 log(I_FT/E)与√E 都遵循良好的线性关系, 表明漏电流均由电子Frenkel-Poole (FP) 发射主导. 退化后 log(I_FT/E)与√E 曲线斜率的减小, 以及利用EMMI在栅边缘直接观察到了与缺陷存在对应关系的“热点”, 证明了漏电流退化的机理是: 高电场在AlGaN层中诱发了新的缺陷, 而缺陷密度的增加导致了FP发射电流I_FT的增加. 关键词： AlGaN/GaN 高电子迁移率晶体管 漏电流 退化机理     

Time-dependent degradation of threshold voltage in AlGaN/GaN high electron mobility transistors

This paper gives a detailed analysis of the time-dependent degradation of the threshold voltage in AlGaN/GaN high electron mobility transistors(HEMTs) submitted to off-state stress. The threshold voltage shows a positive shift in the early stress, then turns to a negative shift. The negative shift of the threshold voltage seems to have a long recovery time. A model related with the balance of electron trapping and detrapping induced by shallow donors and deep acceptors is proposed to explain this degradation mode.     

影响GaN基蓝光LED能带结构与光谱特性关系的仿真研究

本文采用Silvaco TCAD软件对GaN基InGaN/GaN量子阱蓝光发光二极管(LED)的光谱特性进行了仿真研究。研究结果表明:光谱会随着注入电压的增加而产生蓝移现象,并出现0.365μm处的紫外光发光峰;发光效率在正向电流较小时增长很快,随着正向电流进一步增加而逐渐趋于饱和;随着量子阱中In组分和量子阱阱层厚度的增加,发光光谱出现红移现象,并且发光效率下降。仿真结果对GaN基InGaN/GaN量子阱结构蓝光LED的设计和优化提供一定的依据。     

不同掺杂类型的GaN间隔层和量子阱垒层对双波长LED作用的研究

采用软件理论分析的方法对不同掺杂类型的GaN间隔层和量子阱垒层在InGaN/GaN多量子阱双波长发光二极管中对发光光强、内量子效率、电子空穴浓度分布、溢出电流等作用进行模拟分析. 分析结果表明,p型掺杂的GaN间隔层与量子阱垒层的引入同不掺杂和n型掺杂两种类型比较,可以大大减少溢出电子流,极大地提高各量子阱内空穴浓度,提高双波长发光二极管的发光强度,极大的改善内量子效率随电流增大而下降问题. 关键词： GaN 掺杂类型 数值模拟 双波长发光二极管     

高温预生长对图形化蓝宝石衬底GaN薄膜质量的提高

在图形化蓝宝石衬底生长低温缓冲层之前,通入少量三甲基镓（TMGa）和大量氨气进行短时间的高温预生长,通过改变TMGa流量制备了4个蓝光LED样品。MOCVD外延生长时使用激光干涉仪实时监测薄膜反射率,外延片使用高分辨率X射线衍射（002）面和（102）面摇摆曲线估算位错密度,并使用光致发光谱表征发光性能,制备成芯片后测试了正向电压和输出光功率。结果表明,高温预生长可促进薄膜的横向外延,使得三维岛状GaN晶粒在较小的薄膜厚度内实现岛间合并,有利于降低位错密度,提高外延薄膜质量,LED芯片的输出光功率的增强幅度达29.1%,而电学性能无恶化迹象;但高温预生长工艺中TMGa的流量应适当控制,过量的TMGa导致GaN晶粒过大,将延长岛间合并时间,降低晶体质量。     

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.