Influence of growth temperature and growth rate of p-GaN layers on the characteristics of green light emitting diodes

We investigated the electrical and structural qualities of Mg-doped p-type GaN layers grown under different growth conditions by metalorganic chemical vapor deposition (MOCVD). Lower 300 K free-hole concentrations and rough surfaces were observed by reducing the growth temperature from 1,040°C to 930°C. The hole concentration, mobility, and electrical resistivity were improved slightly for Mg-doped GaN layers grown at 930°C with a lower growth rate, and also an improved surface morphology was observed. In_0.25Ga_0.75N/GaN multiple-quantum-well light emitting diodes (LEDs) with p-GaN layers grown under different conditions were also studied. It was found from photoluminescence studies that the optical and structural properties of the multiple quantum wells in the LED structure were improved by reducing the growth temperature of the p-layer due to a reduced detrimental thermal annealing effect of the active region during the GaN:Mg p-layer growth. No significant difference in the photoluminescence intensity depending on the growth time of the p-GaN layer was observed. However, it was also found that the electroluminescence (EL) intensity was higher for LEDs having p-GaN layers with a lower growth rate. Further improvement of the p-GaN layer crystalline and structural quality may be required for the optimization of the EL properties of long-wavelength (∼540 nm) green LEDs.     

表面粗化高空穴浓度InGaN:Mg性能及其发光二极管应用

利用金属有机物化学气相淀积(MOCVD)技术在蓝宝石衬底上生长了InGaN:Mg薄膜,对不同源流量InGaN:Mg材料特件进行了研究.光学和电学特性观测表明,当外延生长温度在760℃,三甲基铟(TMIn)摩尔流量不变时,随CP2Mg和Ⅲ族源摩尔比([CP2Mg]/[Ⅲ])增加,当In摩尔成分增加,空穴浓度也线性增加;当...     

Mg fluctuation in p-GaN layers and its effects on InGaN/GaN blue light-emitting diodes dependent on p-GaN growth temperature

Correlation between material properties of bulk p-GaN layers grown on undoped GaN and device performance of InGaN/GaN blue light-emitting diodes (LEDs) as a function of p-GaN growth temperature were investigated. The p-GaN layers of both structures grown by metal-organic chemical-vapor deposition were heavily doped with Mg. As the growth temperature of the bulk p-GaN layer increased up to 1,080°C, N_A-N_D increased. However, above 1,110°C, N_A-N_D sharply decreased, while the fluctuation of Mg concentration ([Mg]) increased. At this time, a peculiar surface, which originated from inversion domain boundaries (IDBs), was clearly observed in the bulk p-GaN layer. The IDBs were not found in all LEDs because the p-GaN contact layer was relatively thin. The change in photoluminescence emission from the ultraviolet band to blue band is found to be associated with the fluctuation of [Mg] and IDBs in bulk p-GaN layers. The LED operating voltage and reverse voltage improved gradually up to the p-GaN contact-layer growth temperature of 1,080°C. However, the high growth temperature of 1,110°C, which could favor the formation of IDBs in the bulk p-GaN layer, yielded poorer reverse voltage and saturated output power of the LEDs.     

低温生长n型氮化镓插入层提高LED抗静电能力

在这篇论文里,我们通过在InGaN/GaN 多量子阱和n型氮化镓层中间插入一层低温生长的n型氮化镓显著提高了LED的抗静电能力。通过引入低温生长的氮化镓插入层使得LED抗击穿电压超过4000V的良品率从9.9%提升到74.7%。低温生长的氮化镓插入层作为后续生长的多量子阱的缓冲层,释放了量子阱中的应力并且改善了量子阱的界面质量。另外,我们证明了在氮气气氛下生长低温氮化镓插入层对于LED抗静电能力的改善要强于氢气气氛,同时也进一步证明低温插入层对量子阱中应力的释放有利于提高LED的抗静电能力。光电测试结果表明,在引入低温nGaN缓冲层后,LED的电学特性并没有衰退,并且LED的光输出功率提高了13.9%。     

Low-operation voltage of InGaN/GaN light-emitting diodes by using aMg-doped Al0.15Ga0.85N/GaN superlattice

Low-resistivity Mg-doped Al_0.15Ga_0.85N/GaN strained-layer superlattices were grown. In these superlattices, the maximum hole concentration is 3×10¹⁸/cm³ at room temperature. Hall-effect measurements indicate high conductivity of this structure in which the high activation efficiency is attributed to the strain-induced piezoelectric fields. This work also fabricated InGaN/GaN blue LEDs that consist of a Mg-doped Al_0.15Ga_0.85N/GaN SLs. Experimental results indicate that the LEDs can achieve a lower operation voltage of around 3 V, i.e., smaller than conventional devices which have an operation voltage of about 3.8 V     

Optimized doping and contact scheme for low-voltage 275-nm deep ultraviolet LEDs

Deep ultraviolet light-emitting diode structures with a peak wavelength of 275 nm, as well as individual AlGaN:Mg layers, were grown by metalorganic chemical vapor deposition on (0001) silicon carbide. Control of the Mg profile in the devices reduced unwanted Mg-related emission at 320 nm to 1/224th of that emitted at the peak wavelength. An additional peak at 410 nm was observed to be related to oxygen incorporation in the film and confirmed with secondary ion mass spectroscopy (SIMS). Also investigated in an effort to improve hole injection were aluminum content, layer thickness, V/III ratio, activation temperature, and properties of the GaN:Mg contact layer and transparent contact metal stack. By optimizing this and other layers of the device, output powers of 0.84 mW at 1.3 A were obtained from packaged devices, with forward voltages as low as 4.9 V at 20 mA.     

InGaN/GaN multiple quantum well green light-emitting diodes prepared by temperature ramping

High-quality InGaN/GaN multiple-quantum well (MQW) light-emitting diode (LED) structures were prepared by a temperature-ramping method during metal-organic chemical-vapor deposition (MOCVD) growth. Two photoluminescence (PL) peaks, one originating from well-sensitive emission and one originating from an InGaN quasi-wetting layer on the GaN-barrier surface, were observed at room temperature (RT). The observation of high-order double-crystal x-ray diffraction (DCXRD) satellite peaks indicates that the interfaces between InGaN-well layers and GaN-barrier layers were not degraded as we increased the growth temperature of the GaN-barrier layers. With a 20-mA and 160-mA current injection, it was found that the output power could reach 2.2 mW and 8.9 mW, respectively. Furthermore, it was found that the reliability of the fabricated green LEDs prepared by temperature ramping was also reasonably good.     

Electrical and luminescent properties and the spectra of deep centers in GaMnN/InGaN light-emitting diodes

Electrical and electroluminescent properties were studied for GaN/InGaN light-emitting diodes (LEDs) with the n-GaN layer up and with the top portion of the n layer made of undoped GaMnN to allow polarization modulation by applying an external magnetic field (so-called “spin-LEDs”). The contact annealing temperature was kept to 750°C, which is the thermal stability limit for retaining room-temperature magnetic ordering in the GaMnN layer. Measurable electroluminescence (EL) was obtained in these structures at threshold voltages of ∼15 V, with a lower EL signal compared to control LEDs without Mn. This is related to the existence of two parasitic junctions between the metal and the lower contact p-type layer and between the GaMnN and the n-GaN in the top contact layer.     

Comparison of GaN and In<Subscript>0.04</Subscript>Ga<Subscript>0.96</Subscript>N <Emphasis Type="Italic">p</Emphasis>-Layers on the Electrical and Electroluminescence Properties of Green Light Emitting Diodes

We have compared the effects of Mg-doped GaN and In_0.04Ga_0.96N layers on the electrical and electroluminescence (EL) properties of the green light emitting diodes (LEDs). To investigate the effects of different p-layers on the LED performance, the diode active region structures were kept identical. For LEDs with p-InGaN layers, the p-In_0.04Ga_0.96N/GaN polarization-related EL peak was dominant at low current levels, while the multiple-quantum-well (MQW) peak became dominant at higher current levels, different from LEDs with p-GaN layers. Also, LEDs with p-InGaN exhibited slightly higher turn on voltages (V _on ) and forward voltages (V _f ) compared to LEDs with p-GaN layers. However, the MQW related EL intensity was much higher and diode series resistance lower for LEDs with p-InGaN layers compared with LEDs with p-GaN, showing possible improvements in output power for LEDs with p-InGaN layers. The diodes with p-GaN layers typically showed V _f of ∼3.1 V at a drive current of 20 mA, with a series resistance of ∼24.7 Ω, while diodes with p-InGaN showed V _f of ∼3.2 V, with a series resistance of ∼18.5 Ω, for device dimensions of 230 μm by 230 μm.     

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.     

Improving the Luminescence of InGaN–GaN Blue LEDs Through Selective Ring-Region Activation of the Mg-Doped GaN Layer

In this study, we used the selective ring-region activation technique to restrain the surface leakage current and to monitor the luminescence characteristics of InGaN-GaN multiple quantum-well blue light-emitting diodes (LEDs). To access the current blocking region after forming a periphery high-resistance ring-region of the Mg-doped GaN layer and to reduce the degree of carrier trapping by the surface recombination centers, we deposited a titanium film onto the Mg-doped GaN epitaxial layer to form a high-resistance current blocking region. To characterize their luminescence performance, we prepared LEDs incorporating titanium films of various widths of the highly resistive current blocking layer. The hole concentration in the Mg-doped GaN epitaxial layer decreased from 3.45times10¹⁷ cm^-3 to 3.31times10¹⁶cm^-3 after capping with a 250-nm-thick layer of titanium and annealing at 700 degC under a nitrogen atmosphere for 30 min. Furthermore, the luminescence characteristics could be improved by varying the width of the highly resistive region of the current blocking area; in our best result, the relative electroluminescence intensity was 30% (20 mA) and 50% (100 mA) higher than that of the as-grown blue LEDs     

Luminescence properties of blue and green dual wavelength InGaN/GaN multi-quantum well light-emitting diode

Blue and green dual wavelength InGaN/GaN multi-quantum well (MQW) light-emitting diode (LED) has wide applications in full color display, monolithic white LED and solid state lighting, etc. Blue and green dual wavelength LEDs, which consist of InGaN strain-reduction layer, green InGaN/GaN MQW and blue InGaN/ GaN MQW, were grown by metal-organic chemical vapor deposition (MOCVD), and the luminescence properties of dual wavelength LEDs with different well arrangements were studied by photoluminescence and electrolumines-cence. The experimental results indicated that well position played an important role on the luminescence evolvement from photoluminescence to electroluminescence.     

Optical properties and recombination mechanisms in GaN and GaN:Mg grown by metalorganic vapor phase epitaxy

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm²/Vs, n=3×10¹⁷ cm⁻³). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.     

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。     

Low resistance and reflective Mg-doped indium oxide-Ag ohmic contacts for flip-chip light-emitting diodes

We have investigated an Mg-doped In/sub x/O/sub y/(MIO)-Ag scheme for the formation of high-quality ohmic contacts to p-type GaN for flip-chip light-emitting diodes (LEDs). The as-deposited sample shows nonlinear current-voltage (I--V) characteristics. However, annealing the contacts at temperatures of 330/spl deg/C-530/spl deg/C for 1 min in air ambient results in linear I--V behaviors, producing specific contact resistances of 10/sup -4/--10/sup -5/ /spl Omega//spl middot/cm/sup 2/. In addition, blue LEDs fabricated with the MIO-Ag contact layers give forward-bias voltages of 3.13-3.15 V at an injection current of 20 mA. It is further shown that LEDs made with the MIO-Ag contact layers give higher output power compared with that with the Ag contact layer. This result strongly indicates that the MIO-Ag can be a promising scheme for the realization of high brightness LEDs for solid-state lighting application.     

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.     

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。     

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。     

Improvement of the efficiency droop of GaN-LEDs using an AlGaN/GaN superlattice insertion layer

With an n-AlGaN(4 nm)/GaN(4 nm) superlattice(SL) inserted between an n-GaN and an InGaN/GaN multiquantum well active layer,the efficiency droop of GaN-based LEDs has been improved.When the injection current is lower than 100 mA,the lumen efficiency of the LED with an n-AlGaN/GaN SL is relatively small compared to that without an n-AlGaN/GaN SL.However,as the injection current increases more than 100 mA,the lumen efficiency of the LED with an n-AlGaN/GaN SL surpasses that of an LED without an n-AlGaN/GaN SL. The wall plug efficiency of an LED has the same trend as lumen efficiency.The improvement of the efficiency droop of LEDs with n-AlGaN/GaN SLs can be attributed to a decrease in electron leakage due to the enhanced current spreading ability and electron blocking effect at high current densities.The reverse current of LEDs at -5 V reverse voltage decreases from 0.2568029 to 0.0070543μA,and the electro-static discharge(ESD) pass yield of an LED at human body mode(HBM)-ESD impulses of 2000 V increases from 60%to 90%.     

InGaN/GaN light emitting diodes activated in O2 ambient

Mg-doped GaN epitaxial layers were annealed in pure O₂ and pure N₂. It was found that we could achieve a low-resistive p-type GaN by pure O₂ annealing at a temperature as low as 400°C. With a 500°C annealing temperature, it was found that the forward voltage and dynamic resistance of the InGaN/GaN light emitting diode (LED) annealed in pure O₂ were both smaller than those values observed from InGaN/GaN LED annealed in pure N₂. It was also found that an incomplete activation of Mg will result in a shorter LED lifetime