Rapid thermal annealed InGaN/GaN flip-chip LEDs

Nitride-based flip-chip (FC) light-emitting diodes (LEDs) emitting at 465 nm with Ni transparent ohmic contact layers and Ag reflective mirrors were fabricated. With an incident light wavelength of 465 nm, it was found that transmittance of normalized 300/spl deg/C rapid thermal annealed (RTA) Ni(2.5 nm) was 93% while normalized reflectance of 300/spl deg/C RTA Ni(2.5 nm)/Ag(200 nm) was 92%. It was also found that 300/spl deg/C RTA Ni(2.5 nm) formed good ohmic contact on n/sup +/ short-period-superlattice structure with specific contact resistance of 7.8/spl times/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. With 20-mA current injection, it was found that forward voltage and output power were 3.15 V and 16.2 mW for FC LED with 300/spl deg/C RTA Ni(2.5 nm)/Ag(200 nm). Furthermore, it was found that reliabilities of FC LEDs were good.     

InGaN/GaN MQW双波长LED的MOCVD生长

利用金属有机物化学气相淀积(MOCVD)系统生长了InGaN/GaN多量子阱双波长发光二极管(LED).发现在20 mA正向注入电流下空穴很难输运过蓝光和绿光量子阱间的垒层,这是混合量子阱有源区获得双波长发光的主要障碍.通过掺入一定量的In来降低蓝光和绿光量子阱之间的垒层的势垒高度,增加注入到离p-GaN层较远的绿光有源区的空穴浓度,从而改变蓝光和绿光发光峰的强度比.研究了蓝光和绿光量子阱间垒层In组分对双波长LED的发光性质的影响.此外,研究了双波长LED发光特性随注入电流的变化.     

InGaN/GaN多量子阱LED的电特性研究

针对InGaN/GaN多量子阱LED,分析了占据能态高于势垒的载流子和低于势垒的载流子参与的电流输运机制,从而推导出对应能态电流输运机制下的电流-电压关系,以及理想因子与温度的变化规律.实验结果证实,在低注入强度下,由材料缺陷引入的深能级辅助隧穿输运机制占主导,电流电压特性符合相应的推导结果,随着注入强度的增大,参与扩散-复合输运机制的载流子逐渐增加,温度对输运机制的影响逐渐增大.     

InGaN/GaN LEDs with a Si-doped InGaN/GaN short-period superlattice tunneling contact layer

Nitride-based light-emitting diodes (LEDs) with Si-doped n⁺-In_0.23Ga_0.77N/GaN short-period superlattice (SPS) tunneling contact top layer were fabricated. It was found that although the measured specific-contact resistance is around 1 × 10⁻² Ω-cm² for samples with an SPS tunneling contact layer, the measured specific-contact resistance is around 1.5×10⁰ Ω-cm² for samples without an SPS tunneling contact layer. Furthermore, it was found that one could lower the LED-operation voltage from 3.75 V to 3.4 V by introducing the SPS structure. It was also found that the LED-operation voltage is almost independent of the CP₂Mg flow rate when we grow the underneath p-type GaN layer. The LED-output intensity was also found to be larger for samples with the SPS structure.     

Efficiency improvement of near-ultraviolet InGaN LEDs using patterned sapphire substrates

The use of conventional and patterned sapphire substrates (PSSs) to fabricate InGaN-based near-ultraviolet (410 nm) light-emitting diodes (LEDs) was demonstrated. The PSS was prepared using a periodic hole pattern (diameter: 3 /spl mu/m; spacing: 3 /spl mu/m) on the (0001) sapphire with different etching depths. From transmission-electron-microscopy and etch-pit-density studies, the PSS with an optimum pattern depth (D/sub h/=1.5 /spl mu/m) was confirmed to be an efficient way to reduce the thread dislocations in the GaN microstructure. It was found that the output power increased from 8.6 to 10.4 mW, corresponding to about 29% increases in the external quantum efficiency. However, the internal quantum efficiency (@ 20 mA) was about 36% and 38% for the conventional and PSS LEDs, respectively. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the PSS. Finally, better long-time reliability of the PSS LED performance was observed.     

Tunnel-recombination currents and electroluminescence efficiency in InGaN/GaN LEDs

The mechanism of injection loss in p-GaN/InGaN/n-GaN quantum-well LEDs is analyzed by studying the temperature and current dependences of external quantum efficiency in the temperature range 77–300 K and by measuring transient currents. The data obtained are interpreted in terms of a tunnel-recombination model of excess current, which involves electron tunneling through the potential barrier in n-GaN and the over-barrier thermal activation of holes in p-GaN. At a low forward bias, the dominant process is electron capture on the InGaN/p-GaN interface states. At a higher bias, the excess current sharply increases due to an increase in the density of holes on the InGaN/p-GaN interface and their recombination with the trapped electrons. The injection of carriers into the quantum well is limited by the tunnel-recombination current, which results in a decrease in efficiency at high current densities and low temperatures. The pinning of the Fermi level is attributed to the decoration of heterointerfaces, grain boundaries, and dislocations by impurity complexes.     

快速热退火对真空蒸镀ITO InGaN/GaN蓝光LED的影响

制作了8milX 10mil的InGaN/GaN 蓝光LED(λ=460nm),采用了真空蒸镀在P-GaN上淀积了240nm的ITO。对不同温度下(100℃至550℃)热退火ITO的电学特性和光学特性进行了比较分析。实验发现,450℃下热退火ITO电阻率低至1.19X10_-4Ω?cm,而此温度下得到高透射率94.17%。在20mA注入电流下,正向电压和输出功率分别为3.14V and 12.57mW。另外,550℃ITO退火下制备的LED光通量最大,为0.49lm,这是因为此温度下透射率较大。     

Failure analysis of electrical-thermal-optical characteristics of LEDs based on AlGaInP and InGaN/GaN

The electrical-thermal-optical characteristics of AlGaInP yellow and InGaN/GaN blue LEDs under electrical stresses were studied. Since the increase of effective acceptor concentration on p-type side, the forward voltages of AlGaInP decrease after 3155 h aging. And the operating voltage of high forward bias expansion for InGaN/GaN is due to the increase of the series resistance. Compared with InGaN/GaN, AlGaInP LEDs display different trend for the relationship between optical output and ideality factors. The relationship between ideality factor and radiative recombination is also studied and established. The characteristic of different intermediate adhesive is compared during aging period based on transient thermal test.     

Temperature dependence of performance of InGaN/GaN MQW LEDs with different indium compositions

The temperature dependence of performance of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with different indium compositions in the MQWs was investigated. With increasing In composition in the MQWs, the optical performance of the LEDs at room temperature was increased due to an increase in the localized energy states caused by In composition fluctuations in MQWs. As the temperature was increased, however, the decrease in output power for LED with a higher In composition in the MQWs was higher than that of LED with a lower In composition in the MQWs. This could be due to the increased nonradiation recombination through the high defect densities in the MQWs resulted from the increased accumulation of strain between InGaN well and GaN barrier.     

High performance of InGaN LEDs on (111) silicon substrates grown by MOCVD

We report on the high-performance of InGaN multiple-quantum well light-emitting diodes (LEDs) on Si (111) substrates using metal-organic chemical vapor deposition. A high-temperature thin AlN layer and AlN-GaN multilayers have been used for the growth of high-quality GaN-based LED structure on Si substrate. It is found that the operating voltage of the LED at 20 mA is reduced to as low as 3.8-4.1 V due to the formation of tunnel junction between the n-AlGaN layer and the n-Si substrate when the high-temperature AlN layer is reduced to 3 nm. Because Si has a better thermal conductivity than sapphire, the optical output power of the LED on Si saturates at a higher injected current density. When the injected current density is higher than 120 A/cm/sup 2/, the output power of the LED on Si is higher than that of LED on sapphire. The LED also exhibited the good reliability and the uniform emission from a large size wafer. Cross-sectional transmission electron microscopy observation indicated that the active layer of these LEDs consists of the dislocation-free pyramid-shaped (quantum-dot-like) structure.     

Effects of V-pits covering layer position on the optoelectronic performance of InGaN green LEDs

The impact of the V-pits covering layer (VCL) position on the optoelectronic performance of InGaN-based green light-emitting diodes (LEDs) was investigated. It is found that earlier covering of V-pits will hinder the hole injection via the sidewall of V-pits, and then result in less quantum wells (QWs) participating in radioluminescence. The current-voltage characteristics show that the LEDs with earlier covering of V-pits have higher operating voltage at room temperature, and a more dramatic voltage rise with the reduction of temperature. Meanwhile, more manifested emission peaks for sidewall QWs and deeper QWs near to n-type layer was observed in the sample with earlier coveing of V-pits at cryogenic temperatures, for the reason that the holes being injected via V-pits sidewall have higher kinetic energy and could transport to deeper QWs.     

Double stage low-frequency noise equivalent circuit of green InGaN LEDs for description of noise characteristics

To explain the current dependences of the mean-square value of low-frequency (LF) noise current in green InGaN light emitting diodes (LEDs), a double stage low-frequency noise equivalent circuit of the LED is proposed. It is shown that the nonmonotonic dependence of the LF noise on the injection current in the LED can be explained by the effect of two LF noise generators: a noise current generator, which is localized near the heterojunction and is determined by tunnel-recombination processes at the interface, and a generator determined by recombination processes in the active region of the structure.     

InGaN/GaN多量子阱蓝光LED的p-GaN盖层的MOCVD生长研究

利用金属有机物化学气相淀积（MOCVD）生长了InGaN／GaN多量子阱（MQW）蓝光发光二极管（LED）,研究了不同Cp2Mg流量下生长的p-GaN盖层对器件电学特性的影响。结果表明,随着Cp2Mg流量的提高,漏电流升高,并且到达一临界点会迅速恶化;正向压降则先降低,后升高。进而研究相同生长条件下生长的p-GaN薄膜的电学特性、表面形貌及晶体质量,结果表明,生长p-GaN盖层时,Cp2Mg流量过低,盖层的空穴浓度低,电学特性不好;Cp2Mg流量过高,则会产生大量的缺陷,盖层晶体质量与表面形貌变差,使得空穴浓度降低,电学特性变差。因此,生长p-GaN盖层时,为使器件的正向压降与反向漏电流均达到要求,Cp2Mg流量应精确控制。     

Improvement of photon extraction efficiency of InGaN LEDs utilizing textured ZnO layer deposited by electrospray deposition

Improvement of photon extraction efficiency of InGaN LEDs has been achieved by deposition of textured ZnO layers on InGaN LED bare chips utilizing electrospray deposition. The electrical and spectral properties of the InGaN LED remain unchanged after the deposition of ZnO, while the InGaN LED with textured ZnO layer exhibits a wider far-field angular distribution. The optical power of the InGaN LED capped with ZnO layer is ∼30% higher than that without ZnO, which is due to increased photon escape probability as a result of the increased surface roughness.     

高反射率电极及n-GaN表面粗化的InGaN/GaN多量子阱LED的研制

采用高反射率的Cr/Al/Pd/Au作为LED的p-GaN和n-GaN金属电极,替代低反射率的Cr/Pd/Au电极,减小了金属电极对光的吸收,使入射到高反射率金属电极的光经过反射后增加了出射概率,提高了光萃取效率.通过进一步粗化n-GaN表面,抑制了n-GaN/空气界面的光全反射,提高了光萃取效率.实验结果表明,在350 mA电流下,采用高反射率的Cr/Al/Pd/Au电极的LED相比传统电极LED光输出增加了14.3％;结合n-GaN表面粗化,LED的光输出则增加了35.3％.     

组份渐变电子阻挡层对InGaN/GaN LED光电特性的影响

利用数值模拟方法,研究组份渐变电子阻挡层(EBL)对InGaN/GaN发光二极管电学和光学特性的影响。结果表明,三角形组份渐变EBL结构能有效减小器件的开启电压,提高光输出功率,改善高注入电流水平下发光效率的下降情况。能带模拟结果进一步表明,三角形组份渐变EBL结构显著提高了导带底的电子势垒,可有效限制电子向P型GaN层的泄露,同时减小了价带顶的空穴势垒,可增强P型GaN层的空穴向有源区的注入效率,改善其在量子阱内的浓度分布。     

High brightness InGaN green LEDs with an ITO on n/sup ++/-SPS upper contact

Indium tin oxide (ITO) (260 nm) and Ni (5 nm)/Au (10 nm) films were deposited onto glass substrates, p-GaN layers, n/sup +/-InGaN/GaN short-period-superlattice (SPS), n/sup ++/-SPS and nitride-based green light-emitting diodes (LEDs). It was found that ITO could provide us an extremely high transparency (i.e., 95% at 520 nm). It was also found that the 1.03/spl times/10/sup -3/ /spl Omega/cm/sup 2/ specific contact resistance of ITO on n/sup ++/-SPS was reasonably small. Although the forward voltage of the LED with ITO on n/sup ++/-SPS upper contacts was slightly higher than that of the LED with Ni/Au on n/sup ++/-SPS upper contacts, the 20 mA output power and external quantum efficiency of the former could reach 4.98 mW and 8.2%, respectively, which were much larger than the values observed from the latter. The reliability of ITO on n/sup ++/-SPS upper contacts was also found to be reasonably good.     

Effect of strain relaxation on active-region formation in InGaN/(Al)GaN heterostructures for green LEDs

Semiconductors - Processes of active-region formation for green LEDs on the basis of multilayer strained InGaN/GaN nanoheterostructures have been studied. It is shown that the formation of...     

Iridium-based semi-transparent current spreading layer on short-period-superlattice (SPS) tunneling contact of InGaN/GaN LEDs

Iridium-containing and Ni(4 nm)/Au(6 nm) films were evaporated separately on the n⁺-InGaN–GaN short-period-superlattice (SPS) structure of light-emitting diodes (LEDs). The collective deposition of iridium and other metals as an ohmic contact induces the formation of highly transparent IrO₂, which helps to enhance the light output and decrease the series resistance of LEDs. By comparing different metal films used as current spreading contact layer, Ir/Ni film annealed at 500 °C for 20 min in O₂ ambient renders devices with lowest turn-on voltage at 20 mA and highest luminous intensity. Moreover, we also analyzed films using atomic force microscopy (AFM) with an emphasis on studying how the surface quality of Ir/Ni and Ni/Au films influences the current spreading and luminosity of LEDs.     

A study of thermal processes in high-power InGaN/GaN flip-chip LEDs by IR thermal imaging microscopy

Results of an experimental study of temperature fields generated in high-power AlGaInN heterostructure flip-chip light-emitting diodes (LEDs) via their self-heating at high working currents are presented. The method of IR thermal imaging microscopy employed in the study enables a direct measurement of the temperature distribution over the p-n junction area with a high resolution of ∼3 μm at an absolute measurement error of ∼2 K. It is shown that large temperature gradients may arise in high-power LEDs at high excitation levels as a result of current crowding. This effect should be taken into account when designing lightemitting chips and estimating the admissible operation modes. The method of IR thermal imaging microscopy can also reveal microscopic defects giving rise to current leakage channels and impairing device reliability.