Effects of electrical stress on the characteristics and defect behaviors in GaN-based near-ultraviolet light emitting diodes

The degradation mechanism of GaN-based near-ultraviolet (NUV, 320-400 nm) light emitting diodes (LEDs) with low-indium content under electrical stress is studied from the aspect of defects. A decrease in the optical power and an increase in the leakage current are observed after electrical stress. The defect behaviors are characterized using deep level transient spectroscopy (DLTS) measurement under different filling pulse widths. After stress, the concentration of defects with the energy level of 0.47-0.56 eV increases, accompanied by decrease in the concentration of 0.72-0.84 eV defects. Combing the defect energy level with the increased yellow luminescence in photoluminescence spectra, the device degradation can be attributed to the activation of the gallium vacancy and oxygen related complex defect along dislocation, which was previously passivated with hydrogen. This study reveals the evolution process of defects under electrical stress and their spatial location, laying a foundation for manufacture of GaN-based NUV LEDs with high reliability.     

金属有机化学气相沉积外延技术生长GaN基半导体发光二极管和激光二极管(Ⅱ)

如今，InGaN／GaN基量子阱发光二极管已经商业化，而且InGaN／GaN基量子阱激光二极管已实现连续波室温运转，使用寿命超过10000小时，虽然如此，但还没有完全搞清楚这些器件的发光机理．试验中通常使用连续输出He-Cd激光器（325nm）作光源，或者使用20—50mA注入电流来研究In—GaN量子阱样品或发光二极管光学性质，本文上篇研究了量子阱厚度与发光二极管发光功率的关系，lnGaN和GaN之间的晶格失配产生压电场，从而导致量子束缚斯塔克效应，而量子束缚斯塔克效应强     

Study on the thermal characteristics of GaN-based laser diodes

In this work, we have analyzed the thermal properties of a GaN-based laser diode (LD) as functions of input powers, cooling conditions, and ambient temperatures. It was found that the thermal resistance has a slight change with input current under the forced cooling condition. In contrast to the forced cooling condition, significant change of thermal resistance was observed under the natural cooling condition. When the ambient temperature was increased from 0 to 50 °C, the measured thermal resistance was increased from 20 to 27.5 K/W.     

GaN-based violet laser diodes grown on free-standing GaN substrate

A violet laser diode (LD) structure is grown on a free-standing c-plane GaN substrate and 4~μ m× 800~μ m ridge waveguide LDs are fabricated. The electrical and the optical characteristics of LDs under different facet-coating and chip-mounting conditions are investigated under pulse mode operation. The active region temperatures of p-side up and p-side down mounted LDs are calculated with different injection currents. The calculated thermal resistances of p-side up and p-side down mounted LDs are 4.6~K/W and 3~K/W, respectively. The threshold current of the p-side down mounted LD is much lower than that of the p-side up mounted LD. The blue shift of the emission wavelength with increasing injection current is observed only for the LD with p-side down mounting configuration, due to the more efficient heat dissipation.     

Current spreading in GaN-based light-emitting diodes

We have investigated the factors affecting the current spreading length(CSL) in GaN-based light-emitting diodes(LEDs) by deriving theoretical expressions and performing simulations with APSYS.For mesa-structure LEDs,the effects of both indium tin oxide(ITO) and n-GaN are taken into account for the first time,and a new Q factor is introduced to explain the effects of different current flow paths on the CSL.The calculations and simulations show that the CSL can be enhanced by increasing the thickness of the ITO layer and resistivity of the n-GaN layer,or by reducing the resistivity of the ITO layer and thickness of the n-GaN layer.The results provide theoretical support for calculating the CSL clearly and directly.For vertical-structure LEDs,the effects of resistivity and thickness of the CSL on the internal quantum efficiency(IQE) have been analyzed.The theoretical expression relating current density and the parameters(resistivity and thickness)of the CSL is obtained,and the results are then verified by simulation.The IQE under different current injection conditions is discussed.The effects of CSL resistivity play a key role at high current injection,and there is an optimal thickness for the largest IQE only at a low current injection.     

Suppression of Q-switching instabilities in broadened-waveguide monolithic mode-locked laser diodes

It is predicted theoretically that broadening the optical confinement layer in monolithic mode-locked semiconductor lasers may suppress Q-switching instability, by increasing the carrier transport time, and lead to emission of shorter, more stable optical pulses.     

Status of GaN-based green light-emitting diodes

GaN-based blue light emitting diodes(LEDs) have undergone great development in recent years,but the improvement of green LEDs is still in progress.Currently,the external quantum efficiency(EQE) of GaN-based green LEDs is typically30%,which is much lower than that of top-level blue LEDs.The current challenge with regard to GaN-based green LEDs is to grow a high quality In GaN quantum well(QW) with low strain.Many techniques of improving efficiency are discussed,such as inserting Al GaN between the QW and the barrier,employing prestrained layers beneath the QW and growing semipolar QW.The recent progress of GaN-based green LEDs on Si substrate is also reported:high efficiency,high power green LEDs on Si substrate with 45.2% IQE at 35 A/cm2,and the relevant techniques are detailed.     

Different influences of u-InGaN upper waveguide on the performance of GaN-based blue and green laser diodes

Performances of blue and green laser diodes(LDs) with different u-InGaN upper waveguides(UWGs) are investigated theoretically by using LASTIP. It is found that the slope efficiency(SE) of blue LD decreases due to great optical loss when the indium content of u-InGaN UWG is more than 0.02, although its leakage current decreases obviously. Meanwhile the SE of the green LD increases when the indium content of u-InGaN UWG is varied from 0 to 0.05, which is attributed to the reduction of leakage current and the small increase of optical loss. Therefore, a new blue LD structure with In_(0.05) Ga_(0.95)N lower waveguide(LWG) is designed to reduce the optical loss, and its slope efficiency is improved significantly.     

利用W形空穴阻挡层降低AlGaN基深紫外激光二极管的空穴泄露

本文设计了V形和W形的空穴阻挡层(HBL)结构,改善空穴在AlGaN基深紫外激光二极管(DUV-LD)n型区的泄露问题.使用Crosslight软件,将参考型矩形、V形和W形三种空穴阻挡层结构进行仿真研究,分别比较了三种不同结构的DUV-LD能带、n区空穴浓度、辐射复合率、电光转换效率、有源区载流子浓度等特性,结果表明,具有W形空穴阻挡层的DUV-LD拥有更高的空穴有效势垒高度、更高的辐射复合率、更低的空穴泄露以及更好的斜率效率,可以有效降低深紫外激光二极管在n型区的空穴泄露,提升其光学和电学性能.     

Technology aspects of GaN-based diodes for high-field operation

This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.     

Highly efficient blue organic light-emitting diodes using various hole and electron confinement layers

In this Letter, blue phosphorescence organic light-emitting diodes(PHOLEDs) employ structures for electron and/or hole confinement; 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene is used as a hole confinement layer and tris-(phenylpyrazole)iridium [IreppzT3] is utilized for an electron confinement layer(ECL). The electrical and optical properties of the fabricated blue PHOLEDs with various carrier-confinement structures are analyzed.Structures with a large energy offset between the carrier confinement and emitting layers enhance the charge-carrier balance in the emitting region, resulting from the effective carrier confinement. The maximum external quantum efficiency of the blue PHOLEDs with the double-ECLs is 24.02% at 1500 cd∕m2and its luminous efficiency is 43.76 cd∕A, which is 70.47% improved compared to the device without a carrier-confinement layer.     

Simulation of current spreading for GaN-based light-emitting diodes

Uniform current spreading is crucial for the performance of light-emitting diodes (LEDs). It has been reported that the reliability and light distribution are affected by non-uniform current spreading. In this paper, the impact of different electrode patterns on the performance of LED chips is investigated. A hybrid modeling method is employed to analyze the electrical and thermal characteristics of LEDs with two different electrode mesa structures. Corresponding experiments are also carried out to validate the calculation results. It is found that increasing amount of p-electrodes in interdigitated electrode patterns is effective in improving the performance of LEDs.     

利用山形空穴存储层结构对深紫外激光二极管的性能优化

为了有效提高深紫外激光二极管的空穴注入效率和减少电子泄露,优化其性能,设计出了在基础矩形空穴存储层结构上改进后的山形空穴存储层和倒山形空穴存储层。使用Crosslight软件模拟仿真倒山形和山形空穴存储层结构的电子浓度、电子电流密度、能带图以及P-I特性曲线。结果显示山形空穴存储层激光器的光学和电学性能优于矩形和倒山形激光器,因此山形空穴存储层激光器能有效地增加有源区空穴注入和减少电子泄露,提高有源区载流子浓度和辐射复合速率,实现了激光器优越的光电性能。     

Temperature-dependent efficiency droop behaviors of GaN-based green light-emitting diodes

The efficiency droop behaviors of GaN-based green light-emitting diodes (LEDs) are studied as a function of temperature from 300 K to 480 K. The overall quantum efficiency of the green LEDs is found to degrade as temperature increases, which is mainly caused by activation of new non-radiative recombination centers within the LED active layer. Meanwhile, the external quantum efficiency of the green LEDs starts to decrease at low injection current level (<1 A/cm2 ) with a temperature-insensitive peak-efficiency-current. In contrast, the peak-efficiency-current of a control GaN-based blue LED shows continuous up-shift at higher temperatures. Around the onset point of efficiency droop, the electroluminescence spectra of the green LEDs also exhibit a monotonic blue-shift of peak energy and a reduction of full width at half maximum as injection current increases. Carrier delocalization is believed to play an important role in causing the efficiency droop in GaN-based green LEDs.     

Design of patterned sapphire substrates for GaN-based light-emitting diodes

A new method for patterned sapphire substrate(PSS) design is developed and proven to be reliable and cost-effective.As progress is made with LEDs' luminous efficiency,the pattern units of PSS become more complicated,and the effect of complicated geometrical features is almost impossible to study systematically by experiments only.By employing our new method,the influence of pattern parameters can be systematically studied,and various novel patterns are designed and optimized within a reasonable time span,with great improvement in LEDs' light extraction efficiency(LEE).Clearly,PSS pattern design with such a method deserves particular attention.We foresee that GaN-based LEDs on these newly designed PSSs will achieve more progress in the coming years.     

Analysis on electrical characteristics of high-voltage GaN-based light-emitting diodes

<正>In order to investigate their electrical characteristics,high-voltage light-emitting-diodes(HV-LEDs) each containing four cells in series are fabricated.The electrical parameters including varying voltage and parasitic effect are studied. It is shown that the ideality factors(IFs) of the HV-LEDs with different numbers of cells are 1.6,3.4,4.7,and 6.4.IF increases linearly with the number of cells increasing.Moreover,the performance of the HV-LED with failure cells is examined.The analysis indicates that the failure cell has a parallel resistance which induces the leakage of the failure cell.The series resistance of the failure cell is 76.8Ω,while that of the normal cell is 21.3Ω.The scanning electron microscope(SEM) image indicates that different metal layers do not contact well.It is hard to deposit the metal layers in the deep isolation trenches.The fabrication process of HV-LEDs needs to be optimized.     

GaN基双波长发光二极管电致发光谱特性研究

通过同时调节同一有源区内不同阱层和垒层的In组分，制备了GaN基单有源区蓝、绿光双波长发光二极管(LED).实现了20mA下蓝、绿光同时发射.实验发现随注入电流由10mA增大到60mA，电致发光(EL)谱中绿光峰强度相对于蓝光峰强度不断增强，峰值波长蓝移也更加明显.同时考虑极化效应和载流子不均匀分布的影响，通过对一维薛定谔方程、稳态速率方程和泊松方程的联立自洽求解.分析了测试电流下蓝、绿光EL谱峰值波长和功率的变化情况.发现理论结果与实验结果有很好地符合. 关键词： 极化 载流子不均匀分布 双波长     

Improved light extraction of GaN-based light-emitting diodes with surface-patterned ITO

The surface patterning of the indium tin oxide (ITO) transparent current layer has been investigated to improve the light extraction efficiency of GaN-based light-emitting diodes (LEDs). LEDs with periodic micro-hexagon patterned ITO have been fabricated utilizing standard lithography techniques and inductively coupled plasma (ICP) technology. The luminance intensity of the LED chips with patterned ITO following 160 s ICP etching was enhanced by about 50% compared to the LED chips with unpatterned ITO. Detailed processing parameters are provided. scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to examine the micro-structures. The results indicate that the surface-patterned ITO technique could have potential applications in high-power GaN-based LEDs.