On a GaN-Based Light-Emitting Diode With a p-GaN/i-InGaN Superlattice Structure

An interesting GaN-based light-emitting diode (LED) with a ten-period i-InGaN/p-GaN (5-nm/5-nm) superlattice (SL) structure, inserted between a multiple-quantum-well structure and a p-GaN layer, is fabricated and studied. This inserted SL can be regarded as a confinement layer of holes to enhance the hole injection efficiency. As compared with a conventional LED device without the SL structure, the studied LED exhibits better current-spreading performance and an improved quality. The turn-on voltage, at 20 mA, is decreased from 3.32 to 3.14 V due to the reduced contact resistance as well as the more uniformity of carrier injection. A substantially reduced leakage current (10^-7-10^-9 A) and higher endurance of the reverse current pulse are found. As compared with the conventional LED without the SL structure, the significant enhancement of 25.4% in output power and the increment of 5% in external quantum efficiency are observed.     

Effect of the joule heating on the quantum efficiency and choice of thermal conditions for high-power blue InGaN/GaN LEDs

The heat model of a light-emitting diode (LED) with an InGaN/GaN quantum well (QW) in the active region is considered. Effects of the temperature and drive current, as well as of the size and material of the heat sink on the light output and efficiency of blue LEDs are studied. It is shown that, for optimal heat removal, decreasing of the LED efficiency as current increases to 100 mA is related to the effect of electric field on the efficiency of carrier injection into the QW. As current further increases up to 400 mA, the decrease in efficiency is caused by Joule heating. It is shown that the working current of LEDs can be increased by a factor of 5–7 under optimal heat removal conditions. Recommendations are given on the cooling of LEDs in a manner dependent on their power.     

Enhanced Luminance Efficiency of Wafer-Bonded InGaN–GaN LEDs With Double-Side Textured Surfaces and Omnidirectional Reflectors

A p-side-up GaN-based light-emitting diode (LED) on a silicon substrate was designed and fabricated using a combination of omnidirectional reflector (ODR) and double-side textured surface (both p-GaN and undoped-GaN) structures via surface-roughening, laser lift-off (LLO) and wafer-bonding technologies. The reflectivity of the designed ODR can reach 99.1% at a wavelength of 460 nm. The textured surface of top p-GaN was achieved under low temperature (LT) conditions using metalorganic chemical vapor deposition. It was found that the GaN LED with an extra 200-nm-thick LT p-GaN layer exhibits a 50% enhancement in luminance intensity. The luminance efficiency of double-side roughened silicon–ODR–GaN LED with a small chip size of 250 $mu {hbox {m}} times {hbox {500}}~mu$m can be improved from 23.2% to 28.2% at an injection current of 20 mA. For the case of 1 mm $times$ 1 mm in chip size, the saturation behavior of the light output power is not observed when an injection current increased from 20 to 350 mA, where the luminance efficiency at 20 mA can reach 28.9%, demonstrating an enhancement by 46%, as compared with that of the conventional GaN–sapphire LEDs. These enhanced results can be attributed to higher reflectivity from the ODR and multiple chances of light emitted from the active region to escape, as well as a centralizing effect of light along the vertical direction.      

A simple and reliable wafer-level electrical probing technique forIII-nitride light-emitting epitaxial structures

We report an easy-to-implement wafer-level electroluminescence characterization technique for InGaN/GaN light-emitting diodes (LED's) epi-wafers by means of multiple electrical probes. By first damaging the p-n junctions of the LED epilayer at localized spots, diode-like current versus voltage characteristics and emission spectra can be obtained at injection currents as high as 100 mA. This allows a relative but reliable comparison of device-related parameters such as differential quantum efficiency, leakage current, and series resistance among LED epi-wafers     

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.     

激光钻孔技术对氮化镓LED特性的影响

研究利用激光钻孔技术应用于氮化镓发光二极管,是利用高能激光束将蓝宝石基板打出孔洞,并在孔洞内壁蒸镀金属层薄膜,藉以利用金属导热良好的特性,将表面热能传导至基板,并利用封装技术配合,使得热能顺利从底座散去,降低热效应带来的影响.通过实验获得:在注入电流700 mA下,打孔较未打孔的氮化镓发光二极管,光输出功率增加约77％...     

On an AlGaInP-Based Light-Emitting Diode With an ITO Direct Ohmic Contact Structure

An interesting AlGaInP multiple-quantum-well light-emitting diode (LED) with a direct ohmic contact structure, formed by an indium–tin–oxide (ITO) transparent film and AuBe diffused thin layer, is fabricated and studied. The direct ohmic contact structure is performed by the deposition of an AuBe diffused thin layer and the following activation process on the surface of a Mg-doped GaP window layer. Experimental results demonstrate that a dynamic resistance of 5.7 $Omega$ and a forward voltage of 1.91 V, under an injection current of 20 mA, are obtained. In addition, the studied LED exhibits a higher external quantum efficiency of 9.7% and a larger maximum light-output power of 26.6 mW. The external quantum efficiency is increased by 26% under the injection current of 100 mA, as compared with the conventional LED without this structure. This is mainly attributed to the reduced series resistance resulted from the relatively uniform distribution of AuBe atoms near the GaP layer surface and the effective current spreading ability by the use of ITO film. Moreover, the life behavior of the studied LED, under a 20-mA operation condition, is comparable to the conventional LED without this structure.      

Characteristics of an AlGaInP-Based Light Emitting Diode With an Indium-Tin-Oxide (ITO) Direct Ohmic Contact Structure

An AlGaInP multi-quantum-well (MQW) light-emitting diode (LED) with a direct Ohmic contact structure, formed by an indium-tin-oxide (ITO) transparent film and AuBe diffused thin layer, is fabricated and studied. By the deposition of an AuBe metallic thin layer on the surface of Mg-doped GaP window layer, followed by a thermal activation process, a direct Ohmic contact between ITO and p-GaP layers can be obtained. Experimentally, under an injection current of 20 mA, a dynamic resistance of 5.7 $Omega$ and a forward voltage of 1.91 V, are obtained. In addition, a higher external quantum efficiency of 9.7% and a larger maximum light output power of 26.6 mW are found for the studied LED. As compared with the conventional LED without this structure, the external quantum efficiency of the studied device is increased by 26% under the injection current of 100 mA. This is mainly attributed to the reduced series resistance resulted from the relatively uniform distribution of AuBe atoms near the GaP layer surface and the effective current spreading ability by the use of ITO film. Moreover, the life behavior is not degraded by using this AuBe diffused layer for the studied LED under a 20 mA operation condition.      

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.     

High-power GaN-mirror-Cu light-emitting diodes for vertical current injection using laser liftoff and electroplating techniques

A large-area (1 /spl times/ 1 mm) vertical conductive GaN-mirror-Cu light-emitting diode (LED) fabricated using the laser liftoff and electroplating techniques is demonstrated. Selective p-GaN top area was first electroplated by the thick copper film, and then an excimer laser was employed to separate the GaN thin film from the sapphire substrate. The luminance intensity of the vertical conductive p-side-down GaN-mirror-Cu LED presented about 2.7 times in magnitude as compared with that of the original GaN-sapphire LED (at 20 mA). The light output power for the GaN-mirror-Cu LED was about twofold stronger (at 500 mA). A more stable peak wavelength shift under high current injection was also observed.     

GaN-based light emitting diodes on nano-hole patterned sapphire substrate prepared by three-beam laser interference lithography

Nano-hole patterned sapphire substrates (NHPSSs) were successfully prepared using a low-cost and high-efficiency approach, which is the laser interference lithography (LIL) combined with reactive ion etching (RIE) and inductively coupled plasma (ICP) techniques. Gallium nitride (GaN)-based light emitting diode (LED) structure was grown on NHPSS by metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) measurement was conducted to compare the luminescence efficiency of the GaN-based LED structure grown on NHPSS (NHPSS-LED) and that on unpatterned sapphire substrates (UPSS-LED). Electroluminescence (EL) measurement shows that the output power of NHPSS-LED is 2.3 times as high as that of UPSS-LED with an injection current of 150 mA. Both PL and EL results imply that NHPSS has an advantage in improving the crystalline quality of GaN epilayer and light extraction efficiency of LEDs at the same time.     

High quality GaN-based LED epitaxial layers grown in a homemade MOCVD system

A homemade 7×2 inch MOCVD system is presented.With this system,high quality GaN epitaxial layers,InGaN/GaN multi-quantum wells and blue LED structural epitaxial layers have been successfully grown. The non-uniformity of undoped GaN epitaxial layers is as low as 2.86%.Using the LED structural epitaxial layers, blue LED chips with area of 350×350μm~2 were fabricated.Under 20 mA injection current,the optical output power of the blue LED is 8.62 mW.     

Inserting a low-temperature n-GaN underlying layer to separate nonradiative recombination centers improves the luminescence efficiency of blue InGaN/GaN LEDs

We have investigated the effects of nonradiative recombination centers (NRCs) on the device performance of InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) inserting low-temperature n-GaN (LT-GaN) underlying layers. Inserting an LT-GaN underlying layer prior to growing the MQWs is a successful means of separating the induced nonradiative recombination centers because a growth interrupt interface exists between the n-GaN template and the InGaN QW. We found that by introducing this technique would improve the external quantum efficiency of the as-grown conventional LEDs. The electroluminescence relative intensity of a blue LED incorporating a 70-nm-thick LT-GaN was 20.6% higher (at 20 mA current injection) than that of the corresponding as-grown blue LED in the best case.     

p-AlGaN/GaN超晶格做p型层350 nm紫外AlGaN基LED

使用p-AlGaN/p-GaN SPSLs作为LED的p型层,在蓝宝石衬底上生长出发光波长为350 nm的AlGaN基紫外LED。[JP+1]由于AlGaN/GaN超晶格的极化效应,使得Mg受主的电离能降低,大幅提高了器件的光学和电学性能。在工作电流为350 mA下发光亮度达到了22.66 mW,相应的工作电压为3.75 V,LEDs的光功率满足了实际应用需求。     

Effect of AlInGaN barrier layers with various TMGa flows on optoelectronic characteristics of near UV light-emitting diodes grown by atmospheric pressure metalorganic vapor phase epitaxy

Near ultraviolet light-emitting diodes (LEDs) with quaternary AlInGaN quantum barriers (QBs) are grown by atmospheric pressure metalorganic vapor phase epitaxy. The indium mole fraction of AlInGaN QB could be enhanced as we increased the TMG flow rate. Both the wavelength shift in EL spectra and forward voltage at 20 mA current injection were reduced by using AlInGaN QB. Under 100 mA current injection, the LED output power with Al_0.089In_0.035Ga_0.876N QB can be enhanced by 15.9%, compared to LED with GaN QB. It should be attributed to a reduction of lattice mismatch induced polarization mismatch in the active layer.     

高亮度AlGaInP红光发光二极管

对用于提高AlGaInP红光发光二极管(LED)出光效率的分布布拉格反射镜(DBR)和增透膜进行了分析,用金属有机物化学气相沉积(MOCVD)生长了包含DBR和增透膜的LED,在20 mA注入电流下,LED的峰值波长为623 nm,光强达到200 mcd,输出光功率为2.14 mW.与常规的LED相比,光强和输出光功率有很大的提高.     

高亮度AIGalnP红光发光二极管

对用于提高AlGaInP红光发光二极管(LED)出光效率的分布布拉格反射镜(DBR)和增透膜进行了分析,用金属有机物化学气相沉积(MOCVD)生长了包含DBR和增透膜的LED,在20mA注入电流下,LED的峰值波长为623nm,光强达到200mcd,输出光功率为2.14mW。与常规的LED相比,光强和输出光功率有很大的提高。     

高反射性电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率

由SiO₂/TiO₂分布布拉格反射镜(DBR)和Al镜组成的混合式反射电流阻挡层用于提高InGaN/GaN发光二极管的光输出功率。混合式反射电流阻挡层不仅增强了电流扩展效应而且有效的将射向p金属电极的光子反射防止其对p电极焊点附近光子的吸收。实验结果表明,淀积在p-GaN上1.5个周期的SiO₂/TiO₂DBR和Al镜在455nm垂直入射时的反射率高达97.8%。在20mA的工作电流下,与没有电流阻挡层的发光二极管相比,生长1.5对SiO₂/TiO₂ DBR和Al镜作为电流阻挡层的发光二极管的光输出功率提高了12.5%,且光输出功率的分布更加均匀。     

注入电流对GaN基LED发光特性的影响

通过调节量子阱中的In组分,制备了GaN基蓝光和绿光发光二极管（LED）。对两种LED进行变电流测试发现,注入电流由3 mA增加到900 mA过程中,波长有蓝移现象,且绿光LED的波长蓝移较明显。这是量子阱限制斯塔克效应（QCSE）造成的。由于绿光LED中In组分含量较大,QCSE较明显。并且发现,光效迅速下降,绿光L...     

透明导电ITO欧姆接触的AlGaInP薄膜发光二极管

提出了一种透明导电氧化铟锡(ITO)欧姆接触的AlGaInP薄膜发光二极管(LED)的结构和制作工艺.在这个结构里,ITO还作为窗口层材料,增强电流扩展,并应用了高反射率的金属作为反光镜.用Au-Sn合金(Au∶Sn＝8∶2,重量比)作为焊料,把带有金属反光镜的AlGaInP LED(RS-LED)外延片倒装键合到GaAs基板上,并去掉外延GaAs衬底,把被GaAs衬底吸收的光反射出去.与常规AlGaInP吸收衬底LEDs(AS-LED)和带有分布布拉格反光镜(DBR)的AlGaInP吸收衬底LEDs(DBR-AS-LED)电、光特性的比较,用透明导电ITO做欧姆接触的AlGaInP薄膜RS-LED结构能极大提高光输出功率和发光强度.正向电流20 mA时,RS-LED的光输出功率分别是AS-LED和DBR-AS-LED的2.4倍和1.7倍;RS-LED 20 mA下峰值波长624 nm的轴向光强达到了179.6 mcd,分别是AS-LED 20 mA下峰值波长627 nm和DBR-AS-LED 20 mA下峰值波长623 nm轴向光强的2.2倍和1.3倍.