Enhancement of photo- and electro-luminescence of GaN-based LED structure grown on a nanometer-scaled patterned sapphire substrate

In this study, a 2 in. sized a highly periodic nanometer-scaled patterned sapphire substrate (NPSS) was fabricated using nanoimprint lithography (NIL) and inductively coupled plasma etching to improve the light-extraction efficiency of GaN-based light-emitting diodes (LEDs). A blue LED structure was grown on the nanometer-scale patterned sapphire substrates, and the photoluminescence (PL) and electroluminescence (EL) were measured to confirm the effectiveness of the nanometer-scaled patterns on sapphire. An improvement in luminescence efficiency was observed when NPSS was applied; 2 times stronger PL intensity and 2.8 times stronger EL intensity than the LED structure grown on the unpatterned sapphire wafers were measured. These results show highly periodic nanometer-scaled patterns create multi-photon scattering and effectively enhance the light-extraction efficiency of LEDs.     

Study of GaN-Based Light-Emitting Diodes Grown on Chemical Wet-Etching-Patterned Sapphire Substrate With V-Shaped Pits Roughening Surfaces

We investigate the mechanism responding for performance enhancement of gallium nitride (GaN)-based light-emitting diode (LED) grown on chemical wet-etching-patterned sapphire substrate (CWE-PSS) with V-Shaped pit features on the top surface. According to temperature-dependent photoluminescence (PL) measurement and the measured external quantum efficiency, the structure can simultaneously enhance both internal quantum efficiency and light extraction efficiency. Comparing to devices grown on planar sapphire substrate, the threading dislocation defects of LED grown on CWE-PSS are reduced from 1.28 times 10⁹/cm² to 3.62 times 10⁸/cm², leading to a 12.5% enhancement in internal quantum efficiency. In terms of the theoretical computing of radiation patterns, the V-Shaped pits roughening surface can be thought of as a strong diffuser with paraboloidal autocorrelation function, increasing the escape probability of trapped photons and achieving a 20% enhancement in light extraction efficiency. Moreover, according to the measurement of optical diffraction power, CWE-PSS demonstrated superior guided light extraction efficiency than that of planar sapphire substrate, thus an extra 7.8% enhancement in light extraction efficiency was obtained. Therefore, comparing to the conventional LED, an overall 45% enhancement in integrated output power was achieved.     

Influence of GaN material characteristics on device performance for blue and ultraviolet light-emitting diodes

An analysis of blue and near-ultraviolet (UV) light-emitting diodes (LEDs) and material structures explores the dependence of device performance on material properties as measured by various analytical techniques. The method used for reducing dislocations in the epitaxial III-N films that is explored here is homoepitaxial growth on commercial hybride vapor-phase epitaxy (HVPE) GaN substrates. Blue and UV LED devices are demonstrated to offer superior performance when grown on GaN substrates as compared to the more conventional sapphire substrate. In particular, the optical analysis of the near-UV LEDs on GaN versus ones on sapphire show substantially higher light output over the entire current-injection regime and twice the internal quantum efficiency at low forward current. As the wavelength is further decreased to the deep-UV, the performance improvement of the homoepitaxially grown structure as compared to that grown on sapphire is enhanced.     

Laser lift-off technique and the re-utilization of GaN-based LED films grown on sapphire substrate

A thin GaN LED film, grown on 2-inch-diameter sapphire substrates, is separated by laser lift-off. Atom force microscopy （AFM） and the double-crystal X-ray diffraction （XRD） have been employed to characterize the performance of Gan before and after the lift-off process. It is demonstrated that the separation and transfer processes do not alter the crystal quality of the GaN films obviously. InGaN/GaN multi-quantum-wells （MQW＇s） structure is grown on the separated sapphire substrate later and is compared with that grown on the conventional substmte under the same condition by using PL and XRD spectrum.     

Enhancement of light extraction efficiency of GaN-based light-emitting diode using ZnO sol-gel direct imprinting

ZnO nano-structures were formed on transparent conducting oxide layer of GaN LED device on non-patterned (non-PSS) and patterned sapphire substrates (PSS). Since ZnO nano-structures were formed by sol-gel direct imprinting process, plasma etching process, which may create the plasma induced damage, was not used. Due to the ZnO nano-structures, light extracted from active layer was coupled with ZnO nano-structures and thus total internal reflection at the ITO layer was suppressed. According to electroluminescence measurement, the emission intensities of GaN LED devices with ZnO nano-structures, on both non-PSS and PSS sapphire substrates were increased by 20.5% and 19.0%, respectively, compared to GaN LED devices without ZnO nano-structures, due to the suppression of total internal reflection. Moreover, it is confirmed that there is no decrease of light extraction on side direction due to light focusing to vertical axis by nanostructure. Electrical performance of GaN LED devices were not degraded by ZnO sol-gel direct imprinting process.     

Stress Reduction and Enhanced Extraction Efficiency of GaN-Based LED Grown on Cone-Shape-Patterned Sapphire

High-quality InGaN–GaN film was grown on a cone-shape-patterned sapphire substrate (CSPSS) by using metal–oganic chemical vapor deposition. The growth mode of GaN on CSPSS was similar to that of the epitaxial lateral overgrowth (ELOG), because the growth, in the initial stage, proceeds only on flat basal sapphire substrate and there is no preferential growth plane on the cone region. An analysis of X-ray diffraction showed a shorter lattice constant of 5.1877 Å along the $c$-axis for the GaN thin films grown on CSPSS, compared to 5.1913 Å for the samples grown on a conventional sapphire substrate (CSS). This is because the ELOG-like mode of the GaN layer over the cone-shaped region results in less lattice mismatch and incoherency between the GaN layer and the sapphire substrate. The output power of a sideview light-emitting diode (LED) grown on CSPSS was estimated to be 7.3 mW at a forward current of 20 mA, which is improved by 34% compared to that of an LED grown on CSS. The significant enhancement in output power is attributed to both the increase of the extraction efficiency, resulted from the increase in photon escaping probability due to enhanced light scattering at the CSPSS, and the improvement of the crystal quality due to the reduction of dislocation.      

Comparison of InGaN-Based LEDs Grown on Conventional Sapphire and Cone-Shape-Patterned Sapphire Substrate

To improve the external quantum efficiency, a high-quality InGaN/GaN film was grown on a cone-shape-patterned sapphire substrate (CSPSS) by using metal-organic chemical vapor deposition. The surface pattern of the CSPSS seems to be more helpful for the accommodative relaxation of compressive strain related to the lattice mismatch between GaN and a sapphire substrate because the growth mode of GaN on the CSPSS was similar to that of the epitaxial lateral overgrowth. The output power of a light-emitting diode (LED) grown on the CSPSS was estimated to be 16.5 mW at a forward current of 20 mA, which is improved by 35% compared to that of a LED grown on a conventional sapphire substrate. The significant enhancement in output power is attributed to both the increase of the extraction efficiency, resulted from the increase in photon escaping probability due to enhanced light scattering at the CSPSS, and the improvement of the crystal quality due to the reduction of dislocation.     

2×25.4 mm GaN LED外延膜的激光剥离及其衬底的重复利用

利用激光剥离技术实现直径为50.8 mm CaN LED外延膜的大面积完整剥离.激光剥离后的原子力显微镜(AFM)扫描和X射线双晶衍射谱(XRD)表明剥离前后外延膜的质量并未明显改变.并报道了在剥离掉后的蓝宝石(α-Al2O3)衬底上MOCVD外延生长InGaN/CaN多量子阱(MQW's)LED器件结构,通过光致发光谱(PL)和XRD谱对比分析了在相同条件下剥离掉后衬底与常规衬底上生长的CaN LED外延膜晶体质量.     

利用高频Plasma CVD在蓝宝石衬底上生长 立方GaN缓冲层及其光学性质

研究了采用高频PlasmaCVD技术在较低温度下(300—400℃)生长以GaN为基的Ⅲ-Ⅴ族氮化物的可行性,在蓝宝石衬底上生长了GaN缓冲层.热处理后的光致发光谱和X光衍射表明,生长的GaN缓冲层为立方相,带边峰位于3.15eV.在作者实验的范围内,最优化的TMGa流量为0.08sccm(TMAm=10sccm时),XPS分析结果表明此时的Ga/N比为1.03.这是第一次在高Ⅴ/Ⅲ比下得到立方GaN.相同条件下石英玻璃衬底上得到的立方GaN薄膜,黄光峰很弱,晶体质量较好.     

Investigation of light output performance for gallium nitride-based light-emitting diodes grown on different shapes of patterned sapphire substrate

GaN-based blue light-emitting diodes (LEDs) on various patterned sapphire substrates (PSSs) are investigated in detail. Hemispherical and triangular pyramidal PSSs have been applied to improve the performance of LEDs compared with conventional LEDs grown on planar sapphire substrate. The structural, electrical, and optical properties of these LEDs are investigated. The leakage current is related to the crystalline quality of epitaxial GaN films, and it is improved by using the PSS technique. The light output power and emission efficiency of the LED grown on triangular pyramidal PSS with optimized fill factor show the best performance in all the samples, which indicates that the pattern structure and fill factor of the PSS are related to the capability of light extraction.     

Enhanced carrier confinement in AlInGaN-InGaN quantum wells in near ultraviolet light-emitting diodes

To increase carrier confinement, the GaN barrier layer was substituted with an AlInGaN quaternary barrier layer which was lattice-matched to GaN in the GaN-InGaN multiple quantum wells (MQWs). Photoluminescence (PL) and high-resolution X-ray diffraction measurements showed that the AlInGaN barrier layer has a higher bandgap energy than the originally used GaN barrier layer. The PL intensity of the five periods of AlInGaN-InGaN MQWs was increased by three times compared to that of InGaN-GaN MQWs. The electroluminescence (EL) emission peak of AlInGaN-InGaN MQWs ultraviolet light-emitting diode (UV LED) was blue-shifted, compared to a GaN-InGaN MQWs UV LED and the integrated EL intensity of the AlInGaN-InGaN MQWs UV LED increased linearly up to 100 mA. These results indicated that the AlInGaN-InGaN MQWs UV LED has a stronger carrier confinement than a GaN-InGaN MQWs UV LED due to the larger barrier height of the AlInGaN barrier layer compared to a GaN barrier layer.     

预处理蓝宝石衬底上生长高质量GaN显示薄膜

采用化学方法腐蚀部分 c-面蓝宝石衬底,在腐蚀区域形成一定的图案,利用 LP-MOCVD 在此经过表面处理的蓝宝石衬底上外延生长 GaN 薄膜.采用高分辨率双晶X射线衍射(DCXRD)、光致发光光谱(PL)、透射光谱分析GaN薄膜的晶体质量和光学质量.分析结果表明,CaN 薄膜透射谱反映出的 CaN 质量与 X射线双晶衍射测量的结果一致,即透射率越大,半高宽越小,结晶质量越好;对蓝宝石衬底进行前处理可以大大改善GaN薄膜的晶体质量和光学质量,其(0002)面及(1012)面XRD半高宽(FWHM)分别降低到 208.80arcsec 及 320.76arcsec,而且其光致发光谱中的黄光带几乎可以忽略.     

Improved Light Output Power of GaN-Based Light-Emitting Diodes Using Double Photonic Quasi-Crystal Patterns

The enhancement of light extraction from GaN-based light-emitting diodes (LEDs) with a double 12-fold photonic quasi-crystal (PQC) structure using nanoimprint lithography is presented. At a driving current of 20 mA on a transistor-outline-can package, the light output power of an LED with a nanohole patterned sapphire substrate (NHPSS) and an LED with a double PQC structure are enhanced by 34% and 61%, compared with the conventional LED. In addition, the higher output power of the LED with the double PQC structure is due to better reflectance on NHPSS and higher scattering effect on p-GaN surface using a 12-fold PQC structure pattern. These results provide promising potential to increase the output powers of commercial light-emitting devices.      

Edge-emitting electroluminescence polarization investigation of InGaN/GaN light-emitting diodes grown by metal-organic chemical vapor deposition on sapphire (0001)

The edge-emitting electroluminescence (FL) state of polarization of blue and green InGaN/GaN light-emitting diodes (LEDs) grown in EMCORE’s commercial reactors was studied and compared to theoretical evaluations. Blue (∼475 nm) LEDs exhibit strong EL polarization, up to a 3:1 distinction ratio. Green (∼530 nm) LEDs exhibit smaller ratios of about 1.5:1. Theoretical evaluations for similar InGaN/GaN superlattices predicted a 3:1 ratio between light polarized perpendicular (E⊥c) and light polarized parallel (E‖c) to the c axis. For the blue LEDs, a quantum well-like behavior is suggested because the E⊥c mode dominates the E‖c mode 3:1. In contrast, for the green LEDs, a mixed quantum well (QW)-quantum dot (QD) behavior is proposed, as the ratio of E⊥c to E‖c modes drops to 1.5:1. The EL polarization fringes were also observed, and their occurrence may be attributed to a symmetric waveguide-like behavior of the InGaN/GaN LED structure. A large 40%/50% drop in the surface root mean square (RMS) from atomic force microscopy (AFM) scans on blue/green LEDs with and without EL fringes points out that better surfaces were achieved for the samples exhibiting fringing. At the same time, a 25%/10% increase in the blue/green LED photoluminescence (PL) intensity signal was found for samples displaying EL interference fringes, indicating superior material quality and improved LED structures.     

Enhanced luminescence from GaN-based blue LEDs grown on grooved sapphire substrates

Luminescence from GaN-based blue light-emitting diodes grown on grooved sapphire substrates was investigated using cathodoluminescence (CL) and electroluminescence (EL). The 60-nm-deep 2 (ridge) /spl times/4 /spl mu/m (trench) grooves along the <101~0> direction were created by BCl/sub 3/-Cl/sub 2/-based inductively coupled plasma reactive ion etching. Stronger CL and EL from the trench regions of the grooves in GaN and InGaN-GaN multiquantum-wells were observed, confirming its better crystalline quality over the trench regions, further supported by the EL mapping results. Epitaxial lateral growth was believed to initiate from the ridge regions to cover the trench regions at the foremost stage of GaN growth that is similar to the coalescence of islands.     

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes through higher pressure grown underlying GaN layers

The 410-nm near-ultraviolet (near-UV) InGaN-GaN multiple quantum-wells light-emitting diodes (LEDs) with low-pressure-grown (200 mbar) and high-pressure-grown (400 mbar) Si-doped GaN underlying layers were grown on c-face sapphire substrates by metal-organic vapor phase epitaxy. Increasing the growth pressure during the initial growth of the underlying n-type GaN epilayers of the near-UV InGaN-GaN LEDs was found to reduce the amount of threading dislocations that originated from the GaN-sapphire interfaces. The electroluminescence intensity of LEDs with underlying GaN layers grown at a higher pressure was nearly five times larger than that of LED with layers grown at lower pressure. Additionally, two-order reduction of leakage current was also induced for the LEDs grown at a higher pressure.     

GaN基LED与Si键合技术的研究

采用金属键合技术结合激光剥离技术将GaN基LED从蓝宝石衬底成功转移到Si衬底上。利用X射线光电子谱(XPS)研究不同阻挡层对Au向GaN扩散所起的阻挡作用,确定键合所需的金属过渡层。利用多层金属过渡层,在真空、温度400℃和加压300 N下实现GaN基LED和Si的键合,通过激光剥离技术将蓝宝石衬底从键合结构上剥离下来,形成GaN基LED/金属层/Si结构。用金相显微镜及原子力显微镜(AFM)观察结构的表面形貌,测得表面粗糙度(RMS)为12.1 nm。X射线衍射(XRD)和Raman测试结果表明,衬底转移后,GaN基LED的结构及其晶体质量没有发生明显变化,而且GaN与蓝宝石衬底间的压应力得到了释放,使得Si衬底上GaN基LED的电致发光(EL)波长发生红移现象。     

基于蓝宝石纳米图形衬底上氮化镓生长的研究

利用两步生长法在蓝宝石纳米图形衬底（NPSS）上生长得到高质量的氮化镓薄膜。通过XRD和SEM对薄膜质量的表征和研究发现,为得到高质量的氮化镓（GaN）薄膜,在NPSS上生长时得到的最优缓冲层厚度为15nm,而在微米级尺寸的图形衬底（MPSS）上得到的最优缓冲层厚度远大于15nm。同时,在NPSS上生长氮化镓薄膜的过程中观察到一个有趣的现象,即GaN在NPSS上生长的初始阶段,氮化镓晶粒主要在图形之间的平面区域生长,极少量的GaN在衬底图形的侧面上聚集生长。这一有趣的现象明显不同于GaN在MPSS上的生长过程。接着,又在NPSS上生长了GaN基LED结构,并对其光电性能进行了研究。     

GaN外延用蓝宝石衬底的图形化研究进展

因蓝宝石具有良好的稳定性能,且其生产技术成熟,是目前异质外延GaN应用最广泛的衬底材料之一.采用图形化蓝宝石衬底技术可以降低GaN外延层材料的位错密度,提高LED的内量子效率,同时提高LED出光效率提高,近年来引起了国内外的广泛关注.概述了图形化蓝宝石衬底的研究进展,包括图形化蓝宝石衬底的制备工艺、图形尺寸、图形形状及图形化蓝宝石衬底的作用机理;详细介绍了凹槽状、圆孔状、圆锥形、梯形和半球状5种图形形状,并分析了GaN材料在不同图形形状的图形化蓝宝石衬底上的生长机理及不同图形形状对GaN基LED器件性能的影响.对图形化蓝宝石衬底技术的研究方向进行了展望,提出了亟待研究和解决的问题.     

GaN-based optoelectronic devices on sapphire and Si substrates

A recessed gate AlGaN/GaN high-electron mobility transistor (HEMT) on sapphire (0 0 0 1), a GaN metal-semiconductor field-effect transistor (MESFET) and an InGaN multiple-quantum well green light-emitting diode (LED) on Si (1 1 1) substrates have been grown by metalorganic chemical vapor deposition. The AlGaN/GaN intermediate layers have been used for the growth of GaN MESFET and LED on Si substrates. A two-dimensional electron gas mobility as high as 9260 cm²/V s with a sheet carrier density of 4.8×10¹² cm⁻² was measured at 4.6 K for the AlGaN/GaN heterostructure on the sapphire substrate. The recessed gate device on sapphire showed a maximum extrinsic transconductance of 146 mS/mm and a drain–source current of 900 mA/mm for the AlGaN/GaN HEMT with a gate length of 2.1 μm at 25°C. The GaN MESFET on Si showed a maximum extrinsic transconductance of 25 mS/mm and a drain–source current of 169 mA/mm with a complete pinch-off for the 2.5-μm-gate length. The LED on Si exhibited an operating voltage of 18 V, a series resistance of 300 Ω, an optical output power of 10 μW and a peak emission wavelength of 505 nm with a full-width at half-maximum of 33 nm at 20 mA drive current.