具有相分离InGaN有源层的白光发光二极管

韩国Kwangju理工学院光电子材料中心和材料科学工程系利用相分离InGaN有源层，无需添加荧光材料，制造出了白光发光二极管。这种二极管的白光发射归因于分离相InGaN三元合金中铟组分和类量子点富铟区域尺寸的宽分布。     

Temperature-dependent light-emitting characteristics of InGaN/GaN diodes

Temperature-dependent light-emitting and current-voltage characteristics of multiple-quantum well (MQW) InGaN/GaN blue LEDs were measured for temperature ranging from 100 to 500 K. The measurement results revealed two kinds of defects that have pronounced impact on the electroluminescent (EL) intensity and device reliability of the LEDs. At low-temperature (<150 K), in addition to the carrier freezing effect, shallow defects such as nitrogen vacancies or oxygen in nitrogen sites can trap the injected carriers and reduces the EL intensity. At high temperature (>300 K), deep traps due to the structure dislocations at the interfaces significantly reduce the efficiency for radiative recombination though they can enhance both forward and reverse currents significantly. In addition, the significant enhancement of trap-assisted tunneling current causes a large heat dissipation and results in a large redshift of the emission peak at high temperature.     

Temperature-dependent electroluminescence in InGaN/GaN multiple-quantum-well light-emitting diodes

We present a comparative study on temperature dependence of electroluminescence (EL) of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with identical structure but different indium contents in the active region. For the ultraviolet (UV) and blue LEDs, the EL intensity decreases dramatically with decreasing temperature after reaching a maximum at 150 K. The peak energy exhibits a large redshift in the range of 20–50 meV with a decrease of temperature from 200 K to 70 K, accompanying the appearance of longitudinal-optical (LO) phonon replicas broadening the low energy side of the EL spectra. This redshift is explained by carrier relaxation into lower energy states, leading to dominant radiative recombination at localized states. In contrast, the peak energy of the green LED exhibits a minimal temperature-induced shift, and the emission intensity increases monotonically with decreasing temperature down to 5 K. We attribute the different temperature dependences of the EL to different degrees of the localization effects in the MQW regions of the LEDs.     

Vertical InGaN light-emitting diodes with Ag paste as bonding layer

Vertical InGaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using Ag paste as bonding layer. Vertical LEDs with Ag paste bonding layer were bonded with Si substrate at a low temperature of 140 °C. In addition to the low-temperature bonding process, the soft property of Ag paste could better alleviate thermal stress compared with conventional eutectic metal bonding layer such as Au–Sn. Under the same test conditions, these two LEDs showed similar optical and electrical properties and reliability. However, LEDs with Ag-paste bonding layer were fabricated through a low-temperature bonding process. The characteristic of soft solder enables a relatively wider process window, such as bonding pressure and temperature, and a higher yield as compared with the vertical LEDs with Au–Sn eutectic bonding layer.     

InGaN multiple quantum well based light-emitting diodes with indium composition gradient InGaN quantum barriers

To improve the internal quantum efficiency(IQE) and light output power of In Ga N light-emitting diodes(LEDs), we proposed an In-composition gradient increase and decrease In Ga N quantum barrier structure. Through analysis of its P-I graph, carrier concentration, and energy band diagram, the results showed that when the current was 100 m A, the In-composition gradient decrease quantum barrier(QB) structure could effectively suppress electron leakage while improving hole injection efficiency, re...     

Tunnel injection and power efficiency of InGaN/GaN light-emitting diodes

The results of studying the influence of the finite tunneling transparency of injection barriers in light-emitting diodes with InGaN/GaN quantum wells on the dependences of the current, capacitance, and quantum efficiency on the p-n junction voltage and temperature are presented. It is shown that defectassisted hopping tunneling is the main transport mechanism through the space charge region (SCR) and makes it possible to lower the injection barrier. It is shown that, in the case of high hopping conductivity through the injection barrier, the tunnel-injection current into InGaN band-tail states is limited only by carrier diffusion from neutral regions and is characterized by a close-to-unity ideality factor, which provides the highest quantum and power efficiencies. An increase in the hopping conductivity through the space charge region with increasing frequency, forward bias, or temperature has a decisive effect on the capacitance-voltage characteristics and temperature dependences of the high-frequency capacitance and quantum efficiency. An increase in the density of InGaN/GaN band-tail states and in the hopping conductivity of injection barriers is necessary to provide the high-level tunnel injection and close-to-unity power efficiency of high-power light-emitting diodes.     

Estimation of the degradation of InGaN/AlGaN blue light-emitting diodes

The long-term accelerated degradation of GaN blue light-emitting diodes under current stress was investigated. From the degradation pattern of optical output with respect to time, the dependence of the degradation on current stress and an equation for estimation of the half-life of the diode were obtained. The major factor causing the degradation is the decrease in the radiative recombination probability due to defect level generation.     

Changes in electrical characteristics associated with degradation of InGaN blue light-emitting diodes

Because of the high concentration of threading dislocations, the reverse current-voltage (I–V) characteristics for either homo- or heterojunctions made on GaN-based materials grown on sapphire often show a strong electric field dependence (called a soft breakdown characteristic), which can be described by a power law I=Vⁿ, with n between 4 to 5. We find a significant increase of reverse currents associated with the early degradation of emission in InGaN blue single-quantum-well light-emitting diodes (LEDs) subjected to aging tests (injected current of 70 mA over a total time of about 300 h). The formation of dislocations might be due to the relaxation of strain in the thin InGaN active layer during the aging tests.     

The use of short-period InGaN/GaN superlattices in blue-region light-emitting diodes

Optical and light-emitting diode structures with an active InGaN region containing short-period InGaN/GaN superlattices are studied. It is shown that short-period superlattices are thin two-dimensional layers with a relatively low In content that contain inclusions with a high In content 1–3 nm thick. Inclusions manifest themselves from the point of view of optical properties as a nonuniform array of quantum dots involved in a residual quantum well. The use of short-period superlattices in light-emitting diode structures allows one to decrease the concentration of nonradiative centers, as well as to increase the injection of carriers in the active region due to an increase in the effective height of the AlGaN barrier, which in general leads to an increase in the quantum efficiency of light-emitting diodes.     

Luminescence distribution and hole transport in asymmetric InGaN multiple-quantum well light-emitting diodes

Asymmetric InGaN/GaN multiple-quantum well(MQW) light-emitting diodes were fabricated to expose the luminescence distribution and explore the hole transport.Under electrical injection,the sample with a wNQW active region in which the first QW nearest the p-side(QW1) is wider than the subsequent QWs shows a single long-wavelength light-emission peak arising from QW1.The inverse nWQW sample with a narrow QW1 shows one short-wavelength peak and one long-wavelength peak emitted separately from QW1 and the su...     

Edge and defect luminescence of powerful ultraviolet InGaN/GaN light-emitting diodes

The spectrum of ultraviolet (UV) InGaN/GaN light-emitting diodes and its dependence on the current flowing through the structure are studied. The intensity of the UV contribution to the integrated diode luminescence increases steadily with increasing density of current flowing through the structure, despite a drop in the emission quantum efficiency. The electroluminescence excitation conditions that allow the fraction of UV emission to be increased to 97% are established. It is shown that the nonuniform generation of extended defects, which penetrate the active region of the light-emitting diodes as the structures degrade upon local current overheating, reduces the integrated emission intensity but does not affect the relative intensity of diode emission in the UV (370 nm) and visible (550 nm) spectral ranges.     

Degradation of InGaN blue light-emitting diodes under continuous and low-speed pulse operations

Long-term accelerated degradation tests on InGaN blue light-emitting diodes were performed under continuous and low-speed pulse operations, and the half-life of the optical output was estimated. It was estimated that the lifetime under pulse operation is 2–4 times longer than that in continuous operation. A higher pulse repetition rate confers a longer life.     

对具有 AlGaN 渐变垒的蓝光LED的研究

在此项研究中,我们对具有渐变AlGaN垒的蓝光LED进行了一系列的分析。其中,用APSYS模拟出了载流子分布、能带图、静电场分布及其光输出功率。模拟结果显示具有渐变AlGaN垒的蓝光LED相比普通结构的LED性能更好。     

非对称InGaN多量子阱发光二极管的发光分布和空穴输运

为研究多量子阱中的发光分布和空穴输运,制备了非对称InGaN/GaN 多量子阱（MQW）发光二极管。在电注入下,具有wNQW有源区结构（靠近p的第一个阱QW1比其他QWs较宽）的样品只有一个来自QW1的发光峰,而具有nWQW有源区结构的样品具有一个短波长发光峰和一个长波长发光峰,分别来自QW1和后面的QWs。增加QW1和后面QWs之间的势垒厚度,来自后面QWs的长波长发光峰减弱,总的发光强度也随之减弱。结论是具有nWQW和薄势垒的非对称耦合MQW结构可以改善空穴输运,从而增强后面QW的发光,提高LED内量子效率。     

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.     

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.     

Efficiency enhancement of InGaN/GaN light-emitting diodes with a back-surface distributed bragg reflector

The design, fabrication, and performance characteristics of a back-surface distributed Bragg reflector (DBR) enhanced InGaN/GaN light-emitting diode (LED) are described. A wide reflectance bandwidth in the blue and green wavelength regions is obtained using a double quarter-wave stack design composed of TiO₂ and SiO₂ layers. More than 65% enhancement in extracted light intensity is demonstrated for a blue LED measured at the chip level. Similar improvement in green LED performance is discussed and achieved through simulation. Possible applications of back-surface DBR-enhanced LEDs include surface-mount packages with significantly reduced vertical profiles, resonant cavity LEDs, and superluminescent diodes.     

Effect of n-type barrier doping on steady and dynamic performance of InGaN light-emitting diodes

The steady and dynamic properties are comparatively simulated results show that the n-doped LED exhibits is mainly attributed to the higher carrier radiative experimental results perfectly. investigated for the n-doped and non-doped InGaN LEDs. The the superior luminescence and modulation performance, which rate of n-doped LED. The results can explain the reported     

Defect generation in InGaN/GaN light-emitting diodes under forward and reverse electrical stresses

Electrical and optical degradations of GaN/InGaN single-quantum-well light-emitting diodes (LEDs) under high-injection current (150 A/cm²) and reverse-bias (−20 V) stresses were investigated. A substantial increase in the tunneling components of both forward and reverse currents was observed in the devices subjected to reverse biases. However, the stressed LEDs exhibited minimal degradation of optical characteristics. For devices subjected to high forward currents, a monotonic decrease in light intensities with stress time, accompanied by an increase of forward leakage current, was observed in the low-injection region, but a positive stress effect was found on the light output measured at high currents. These degradation behaviors can be explained by slow generation of point defects in the LEDs via different mechanisms, i.e., thermally induced defect formation in the InGaN active region in the devices subjected to high-injection currents, and destructive microstructual changes as a result of impact ionization in the cladding layer in the devices under high reverse-bias stress.     

Effects of internal fields on deep-level emission in InGaN/GaN quantum-well light-emitting diodes

We report on the important role played by internal quantum well (QW) fields in the anomalous inversion of capacitance transients in InGaN/GaN multi-QW light-emitting diodes (LEDs). This effect was observed by deep-level transient spectroscopy (DLTS) characterization. Deep-level C (E_C-E_T=0.25 eV), a majority carrier trap related to isolated point defects, gives rise to a negative transient when the bias stimulates it only in the bulk region and to a positive transient when the filling pulse is such that the QW region is probed. We explain this behavior by a model based on the confining effects of QW internal fields on the charge emitted by deep levels.