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.     

Internal quantum efficiency of GaN-based light-emitting diodes grown on silicon substrates determined from rate equation analyses

We present a convenient and reliable method for determining the internal quantum efficiency (IQE) in GaN-based blue light-emitting diodes (LEDs) grown on Si(111) substrates based on the carrier rate equation model. By using the peak point of the efficiency curve in photoluminescence (PL) measurements as the parameter of the rate equation analysis, the IQE can be unambiguously determined without any pre-assumed parameters. The theoretical IQE model is used to fit the measured PL efficiency curves and the IQE of LED samples are determined. The maximum IQE of the LED sample grown on the Si substrate was obtained to be 0.74, which is found to agree well with the results obtained by conventional temperature-dependent PL measurements.     

Optically pumped InGaN/GaN MQW lift-off lasers grown on silicon substrates

Chemical etching and removal of the silicon substrate was used for the creation of optically pumped lift-off InGaN/GaN multiple quantum well (MQW) lasers from heterostructures grown on silicon substrate by MOVPE. Luminescence and laser properties of these heterostructures on silicon substrates as well as those of MQWs lifted-off from their substrate by chemical etching were investigated. The lowest value of the lasing threshold of the lift-off lasers at room temperature was about 205 kW/cm² for a laser wavelength of 463 nm and about 360 kW/cm² for a wavelength of 475 nm. It was shown theoretically that the reduction of internal losses, caused by the absence of absorption in the substrate (resulting from its removal) is most significant for the high order modes having lower values of mirror losses and can lead to a 50% reduction of the threshold (or material gain in InGaN necessary to achieve the threshold).     

AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

Comparison of resonant tunneling diodes grown on freestanding GaN substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy

AlN/GaN resonant tunneling diodes (RTDs) were grown separately on freestanding GaN (FS-GaN) substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy (PA-MBE). Room temperature negative differential resistance (NDR) was obtained under forward bias for the RTDs grown on FS-GaN substrates, with the peak current densities (J_p) of 175-700 kA/cm² and peak-to-valley current ratios (PVCRs) of 1.01-1.21. Two resonant peaks were also observed for some RTDs at room temperature. The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically, showing that lower dislocation densities, flatter surface morphology, and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.     

3D simulation of InGaN/GaN micro-ring light-emitting diodes

We employed the APSYS software to perform 3D electrical and ray-tracing simulations on micro-ring light-emitting diodes (LEDs) to verify previous experimental findings that they have higher extraction efficiency than micro-disk and broad area LEDs. 3D ray-tracing indicates the importance of inter-ring optical interactions. Furthermore we found that the higher light extraction efficiency is at the expense of reduced internal quantum efficiency (IQE) as injection current is increased.     

Hydrogen sensing characteristics of non-polar a-plane GaN Schottky diodes

Pd and Pt Schottky diodes on non-polar a-plane (11–20) GaN layers show large increases in both forward and reverse bias current upon exposure to 4% H₂ in N₂. The barrier height reduction due to hydrogen exposure is 0.11 eV for Pd/GaN and 0.14 eV for Pt/GaN, with long recovery times (>25 min) at room temperature. The sensitivity to hydrogen is significantly greater than for diodes on conventional c-plane (Ga-polar) GaN, but less than for c-plane (N-polar) material. The diode characteristics remain rectifying after exposure to hydrogen, unlike the case of N-polar GaN where Ohmic behavior is observed.     

Growth and fabrication of semi-polar InGaN/GaN multi-quantum well light-emitting diodes on microstructured Si(001) substrates

Semi-polar(1-101) In Ga N/Ga N light-emitting diodes were prepared on standard electronic-grade Si(100) substrates.Micro-stripes of Ga N and In Ga N/Ga N quantum wells on semi-polar facets were grown on intersecting {111} planes of microscale V-grooved Si in metal–organic vapor phase epitaxy, covering over 50% of the wafer surface area. In-situ optical reflectivity and curvature measurements demonstrate that the effect of the thermal expansion coefficient mismatch was greatly reduced. A cross-sectional analysis reveals low threading dislocation density on the top of most surfaces. On such prepared(1-101) Ga N, an In Ga N/Ga N LED was fabricated. Electroluminescence over 5 m A to 60 m A is found with a much lower blue-shift than that on the c-plane device. Such structures therefore could allow higher efficiency light emitters with a weak quantum confined Stark effect throughout the visible spectrum.     

In组分渐变提高InGaN/GaN多量子阱发光二极管发光性能

利用金属有机物化学气相沉积系统在蓝宝石衬底上通过有源层的变温生长,得到In组分渐变的量子阱结构,从而获得具有三角形能带结构的InGaN/GaN多量子阱发光二极管(LED)(简称三角形量子阱结构LED).变温光致发光谱结果表明,相对于传统具有方形能带结构的量子阱LED(简称方形量子阱结构LED),三角形量子阱结构有效提高了量子阱中电子和空穴波函数的空间交叠,从而增加了LED的内量子效率;电致发光谱结果表明,三角形量子阱结构LED器件与传统结构LED器件相比,明显改善了发光峰值波长随着电流的蓝移现象.通过以上     

Simulation of InGaN/GaN light-emitting diodes with patterned sapphire substrate

Blue InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with patterned sapphire substrate (PSS) are simulated by the APSYS software. Approach of combining finite-difference time-domain (FDTD) method and raytracing technique is applied to perform light extraction. The simulation results show that PSS dramatically increases extraction efficiency of light power, in agreement with experiment. It is found that extraction efficiency can be maximized by changing the shape of PSS. This work presents a new approach to combine electrical simulation with FDTD and raytracing in 3D TCAD simulation of GaN-LED.     

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.     

Si(110)和Si(111)衬底上制备InGaN/GaN蓝光发光二极管

分别在Si(110)和Si(111)衬底上制备了In Ga N/Ga N多量子阱结构蓝光发光二极管(LED)器件.利用高分辨X射线衍射、原子力显微镜、室温拉曼光谱和变温光致发光谱对生长的LED结构进行了结构表征.结果表明,相对于Si(111)上生长LED样品,Si(110)上生长的LED结构晶体质量较好,样品中存在较小的张应力,具有较高的内量子效率.对制备的LED芯片进行光电特性分析测试表明,两种衬底上制备的LED芯片等效串联电阻相差不大,在大电流注入下内量子效率下降较小;但是,相比于Si(111)上制备LED芯片,Si(110)上LED芯片具有较小的开启电压和更优异的发光特性.对LED器件电致发光(EL)发光峰随驱动电流的变化研究发现,由于Si(110)衬底上LED结构中阱层和垒层存在较小的应力/应变而在器件中产生较弱的量子限制斯塔克效应,致使Si(110)上LED芯片EL发光峰随驱动电流的蓝移量更小.     

Comparison of InGaN/GaN light emitting diodes grown on m ‐plane and a ‐plane bulk GaN substrates

InGaN/GaN‐based light emitting diodes (LEDs) grown on m ‐plane, a ‐plane and off‐axis between m ‐ and a ‐plane GaN bulk substrates were investigated. A smooth surface was obtained when a ‐plane substrate was applied; however, large amounts of defects were observed. Photoluminescence measurements of the LEDs with a well thickness of 2.5 nm revealed that all the LEDs showed the peak emission wavelength at 389 nm. The PL intensity of the a ‐plane LED is one order of magnitude lower than that of the m ‐plane LED. The a ‐plane LEDs showed significant lower electroluminescence output powers than m ‐plane LEDs, suggesting that excitons are trapped by the defects, which act as non‐radiative recombination centers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced performance of GaN-based light-emitting diodes with InGaN/GaN superlattice barriers

GaN-based multiple quantum well light-emitting diodes （LEDs） with conventional and superlattice barriers have been investigated numerically. Simulation results demonstrate using InGaN/GaN superlattices as barriers can effectively enhance performances of the GaN-Based LEDs, mainly owing to the improvement of hole injection and transport among the MQW active region. Meanwhile, the improved electron capture decreases the electron leakage and alleviates the efficiency droop. The weak polarization field induced by the superlattice structure strengthens the intensity of the emission spectrum and leads to a blue-shift relative to the conventional one.     

Effect of graphene/ZnO hybrid transparent electrode on characteristics of GaN light-emitting diodes

In order to reduce the Schottky barrier height and sheet resistance between graphene(Gr) and the p-GaN layers in GaN-based light-emitting diodes(LEDs), conductive transparent thin films with large work function are required to be inserted between Gr and p-GaN layers. In the present work, three kinds of transparent conductive oxide(TCO) zinc oxide(ZnO) films, Al-, Ga-, and In-doped ZnO(AZO, GZO, and IZO), are introduced as a bridge layer between Gr and p-GaN,respectively. The influence of different combinations of Gr/ZnO hybrid transparent conducting layers(TCLs) on the optical and thermal characteristics of the GaN-LED was investigated by the finite element method through COMSOL software. It is found that both the TCL transmittance and the surface temperature of the LED chip reduce with the increase in Gr and ZnO thickness. In order to get the transmittance of the Gr/ZnO hybrid TCL higher than 80%, the appropriate combination of Gr/ZnO compound electrode should be a single layer of Gr with ZnO no thicker than 400 nm(1 L Gr/400-nm ZnO),2 L Gr/300-nm ZnO, 3 L Gr/200-nm ZnO, or 4 L Gr/100-nm ZnO. The LEDs with hybrid TCLs consisting of 1 L Gr/300-nm AZO, 2 L Gr/300-nm GZO, and 2 L Gr/300-nm IZO have good performance, among which the one with 1 L Gr/300-nm GZO has the best thermal property. Typically, the temperature of LEDs with 1 L Gr/300-nm GZO hybrid TCLs will drop by about 7 K compared with that of the LEDs with a TCL without ZnO film.     

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm² of substrate A and on an area of 1×1 cm² of substrate B. Two traps labeled B1 and B2 are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.