Effect of localized tail states in InGaN on the efficiency droop in GaN light-emitting diodes with increasing current density

The mechanism of the internal quantum efficiency droop in InGaN/GaN structures with multiple quantum wells at current densities of up to 40 A cm^?2 in high-power light-emitting diodes is analyzed. It is shown that there exists a correlation between the efficiency droop and the broadening of the high-energy edge of the emission spectrum with increasing current density. It is also demonstrated that the efficiency is a spectrum-dependent quantity and the emission of higher energy photons starts to decrease at higher current densities. The effect of tunneling and thermally activated mechanisms of thermalization of carriers captured into shallow band-tail states in the energy gap of InGaN on the efficiency and the emission spectrum??s shape is considered. Analysis of the results obtained suggests that the efficiency droop occurs at high current densities because of the relative rise in the contribution from nonradiative recombination via defect states as a result of the increasing occupancy of deep band-tail states in InGaN. It is shown that power efficiency close to the theoretical limit can be obtained in the case of low-voltage tunnel injection into localized band-tail states in the InGaN active region.     

Quantum efficiency and formation of the emission line in light-emitting diodes based on InGaN/GaN quantum well structures

The spectra of electroluminescence, photoluminescence, and photocurrent for the In_0.2Ga_0.8N/GaN quantum-well structures are studied to clarify the causes for the reduction in quantum efficiency with increasing forward current. It is established that the quantum efficiency decreases as the emitting photon energy approaches the mobility edge in the In_0.2Ga_0.8N layer. The mobility edge determined from the photocurrent spectra is E _me = 2.89 eV. At the photon energies hv > 2.69 eV, the charge carriers can tunnel to nonradiative recombination centers with a certain probability, and therefore, the quantum efficiency decreases. The tunnel injection into deep localized states provides the maximum electroluminescence efficiency. This effect is responsible for the origin of the characteristic maximum in the quantum efficiency of the emitting diodes at current densities much lower than the operating densities. Occupation of the deep localized states in the density-of-states “tails” in InGaN plays a crucial role in the formation of the emission line as well. It is shown that the increase in the quantum efficiency and the “red” shift of the photoluminescence spectra with the voltage correlate with the changes in the photocurrent and occur due to suppression of the separation of photogenerated carriers in the field of the space charge region and to their thermalization to deep local states.     

Efficiency droop in GaN LEDs at high current densities: Tunneling leakage currents and incomplete lateral carrier localization in InGaN/GaN quantum wells

The phenomenon of the emission efficiency droop of InGaN/GaN quantum wells (QWs) in light-emitting diode p-n structures is studied. The influence exerted by two basic processes on the emission efficiency is considered: tunnel injection into a QW and incomplete lateral carrier localization in compositional fluctuations of the band-gap width in InGaN. The sharp efficiency peak at low currents and the rapid efficiency droop with increasing current are due to tunneling leakage currents along extended defects, which appear as a result of a local increase in the electron hopping conductivity via the depletion n region and a corresponding local decrease in the height of the injection p barrier. A less sharp efficiency peak and a weak, nearly linear, decrease in efficiency with increasing current are caused by incomplete lateral carrier localization in the QW due to slowing-down of the carrier energy-relaxation rate and to the nonradiative recombination of mobile carriers.     

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.     

The effects of interface states on the capacitance and electroluminescence efficiency of InGaN/GaN light-emitting diodes

The capacitance-voltage characteristics and external quantum efficiency of electroluminescence in blue GaN light-emitting diodes (LEDs) with an InGaN quantum well have been investigated in the temperature range 77–300 K. The results obtained are interpreted taking into account the effect of the InGaN/GaN interface states of structural defects and impurities on the capacitance of the GaN LEDs. The nonlinearity of the C^?2(U) characteristics observed at low forward bias is attributed to an increase in the interface charge resulting from tunneling of free electrons and their trapping at the interface states. According to estimates, states with a density of about 3 × 10¹² cm^?2 are present at the interface. A recombination current in the interface region suppresses the injection of charge carriers into the quantum well and decreases the electroluminescence efficiency at high forward bias. Degradation of the optical power of the LEDs, accompanied by an increase in the measured capacitance, is attributed to an increase in the density of charged interface states and changes in their distribution in the band gap.     

A mechanism of electroluminescence in silicon diodes with a high dislocation density

A new concept is suggested to explain the effect of dislocations on electroluminescence in silicon diodes. This concept is based on consideration of the spatial correlation between injected electrons and holes that recombine inside a dislocation core. This correlation leads to an increase in the probability of radiative recombination for electron-hole pairs. Two cases are analyzed. In the first case, the resulting current is controlled mainly by tunneling of electrons and holes along dislocations, which is followed by electron-hole recombination under the conditions of barrier lowering. In this situation, electroluminescence is not related to the fundamental absorption edge and the energy position of the electroluminescence band shifts to shorter wavelengths as either the temperature decreases or the applied voltage increases. In the second case, the diffusion-related component is prevalent in the total current. The radiative recombination of electron-hole pairs occurs in quasi-neutral regions and the electroluminescence exhibits an edge-related character. It is shown that the suggested mechanism may be responsible for a substantial enhancement of both the electroluminescence intensity and the quantum efficiency in silicon diodes with dislocations if the Shockley-Read-Hall lifetime is shorter than 10^?3 s.     

Effect of the electric field on the intensity and spectrum of emission from InGaN/GaN quantum wells

Comparative study of the photoluminescence (PL) from quantum wells (QWs) in forward-biased p-GaN/InGaN/n-GaN structures and electroluminescence from these structures has been carried out. It is shown that, upon application of a forward bias, a characteristic red shift of the spectral peak is observed, together with a broadening of the PL line and simultaneous burning-up of the PL. This results from a decrease in the field strength in the space charge region of the p-n junction and suppression of the tunneling leakage of the carrier from band-tail states in the active InGaN layer. An analysis of the results obtained demonstrated that the tunneling strongly affects the quantum efficiency and enabled evaluation of the internal quantum efficiency of the structures. It is shown that nonequilibrium population of band-tail states in InGaN/GaN QWs depends on the injection type and is controlled by the capture of carriers injected into a QW, in the case of optical injection, and by carrier tunneling “below” the QW under electrical injection.     

Tunnel-recombination currents and electroluminescence efficiency in InGaN/GaN LEDs

The mechanism of injection loss in p-GaN/InGaN/n-GaN quantum-well LEDs is analyzed by studying the temperature and current dependences of external quantum efficiency in the temperature range 77–300 K and by measuring transient currents. The data obtained are interpreted in terms of a tunnel-recombination model of excess current, which involves electron tunneling through the potential barrier in n-GaN and the over-barrier thermal activation of holes in p-GaN. At a low forward bias, the dominant process is electron capture on the InGaN/p-GaN interface states. At a higher bias, the excess current sharply increases due to an increase in the density of holes on the InGaN/p-GaN interface and their recombination with the trapped electrons. The injection of carriers into the quantum well is limited by the tunnel-recombination current, which results in a decrease in efficiency at high current densities and low temperatures. The pinning of the Fermi level is attributed to the decoration of heterointerfaces, grain boundaries, and dislocations by impurity complexes.     

Thermal effect on the electroluminescence of InGaN/GaN multiquantum-well light-emitting devices

A steady-state thermal model is presented to investigate the temperature and injection-current dependence of the electroluminescence (EL) in InGaN/GaN multiquantum-well light-emitting devices. The important mechanisms for the carrier dynamics, including thermal emission, recapturing, radiative and nonradiative recombination, are taken into account in this model. From the measured EL spectra, it is found that the S-shaped temperature dependence of the peak energy disappears at a high injection-current level. The temperature-dependent emission energies of the EL spectra are calculated with this model. The band-filling effect and the heating effect are considered in our investigation of this phenomenon, and the simulation results are in fair agreement with the experimental data. It is observed that both the band-filling and heating effects influence the temperature dependence of the EL emission spectra of InGaN/GaN multiquantum-wells. Quantitative discussion reveals that the heating effect becomes more apparent when the device is working at high injection-currents.     

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     

Characterization of undoped and silicon-doped InGaN/GaN single quantum wells

We investigated the influence of doping and InGaN layer thickness on the emission wavelength and full width at half maximum (FWHM) of InGaN/GaN single quantum wells (SQW) of thicknesses between 1 nm and 5 nm by temperature and intensity resolved photoluminescence (PL). The crystalline quality of the GaN claddings was assessed by low temperature PL. The emission energy of 5 nm Si doped SQW could be tuned from 3.24 eV to 2.98 eV by reducing the deposition temperature. An increase of piezoelectric (PE) field screening with increasing deposition temperature is attributed to an increase of the SiH₄ decomposition efficiency. Piezoelectric (PE) fields between 0.5 MV/cm and 1.2 MV/cm in undoped structures of varying SQW thicknesses were calculated. Two activation energies of 15 meV and 46 meV of the SQW emission could be observed in temperature resolved measurements. The higher value was assigned to the confined exciton binding energy, whereas the activation energy of 15 meV is probably due to a decrease in carrier supply from the absorption zone in the GaN cladding into the SQW.     

InGaN/GaN多量子阱纳米线发光二极管制备及研究

用SiO2纳米图形层作为模板在以蓝宝石为衬底的n-GaN单晶层上制备了InGaN/GaN多量子阱纳米线,并成功实现了其发光二极管器件(LED).场发射扫描电子显微镜(FESEM)的测量结果表明,InGaN/GaN多量子阱纳米线具有光滑的表面形貌和三角形的剖面结构.室温下阴极射线荧光谱(CL)的测试发现了位于461 nm...     

Mechanism of the GaN LED efficiency falloff with increasing current

The quantum efficiency of GaN LED structures has been studied at various temperatures and biases. It was found that an efficiency falloff is observed with increasing current density and, simultaneously, the tunnel component of the current through the LED grows and the quasi-Fermi levels reach the mobility edge in the InGaN active layer. It is shown that the internal quantum efficiency falloff with increasing current density is due to the carrier leakage from the quantum well as a result of tunnel transitions from its band-tail states to local defect-related energy levels within the energy gaps of the barrier layers.     

InGaN/GaN多量子阱蓝光LED电学特性研究

对不同温度(120～363 K)下InGaN/GaN多量子阱(MQW)结构蓝光发光二极管(LED)的电学特性进行了测试与深入的研究.发现对数坐标下I-V特性曲线斜率随温度变化不大.分别用载流子扩散-复合模型和隧道复合模型对其进行计算,发现室温下其理想因子远大于2,并且随着温度的下降而升高;而隧穿能量参数随温度变化不大.这说明传统的扩散-复合载流子输运模型不再适用于InGaN/GaN MQW蓝光LED.分析指出由于晶格失配以及生长工艺的制约,外延层中具有较高的缺陷密度和界面能级密度,导致其主要输运机制为载流子的隧穿.     

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.     

预应变InGaN/GaN多量子阱发光特性调控研究

为了减弱InGaN/GaN量子阱内的压电极化场,在蓝紫光InGaN/GaN多量子阱激光器结构中采用了预应变InGaN插入层,通过变温电致发光和高分辨X射线衍射测量研究了预应变插入层对量子阱晶体质量和发光特性的影响。实验结果显示,常温下有预应变层的量子阱电致发光谱积分强度显著提高。模拟计算进一步表明,预应变层对量子阱内压电极化场有调制效果,有利于量子阱中的应力弛豫,可以有效减弱量子限制斯塔克效应,有助于提高量子阱的发光效率。     

Electroluminescence of InAs-GaSb heterodiodes

The electroluminescence of a Type II InAs-GaSb superlattice heterodiode has been studied as a function of injection current and temperature in the spectral range between 3 and 13 /spl mu/m. The heterodiode comprises a Be-doped midwavelength infrared (MWIR) superlattice with an effective bandgap around 270 meV and an undoped long wavelength infrared (LWIR) superlattice with an effective bandgap of 115 meV. At high injection currents and elevated temperatures the band to band transitions of both superlattices can be observed. By increasing the temperature the intensity of the MWIR emission component shows a well defined thermally activated increase. The activation energy of the Beryllium doping was evaluated to be 28 meV.     

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.     

Kinetics and inhomogeneous carrier injection in InGaN nanolayers

The electrical and optical properties of light-emitting devices with an active region containing several layers of InGaN/GaN quantum dots (QDs) separated by GaN spacers are studied. It is shown that the overgrowth of the QD layer with an InGaN layer that has a reduced In content at higher temperatures raises the confinement energy of carriers in QDs. Furthermore, inhomogeneous carrier injection, predominantly into regions with higher confinement energy, is observed. The electrical and optical properties of p-n junctions and the effect of the inhomogeneities on these properties are studied in detail. It is shown that the shifts of photoluminescence and electroluminescence lines, which are observed when changing the experimental conditions, are related to these properties of the inhomogeneities in the p-n junction.     

线性分布的In组分对紫色InGaN/GaN单量子阱发光特性的影响

采用数值模拟的方法研究了具有相同的平均In组分,但In组分的分布不同的3个紫色InGaN/GaN单量子阱样品的光谱特性.通过分析样品的电致发光谱、能带结构、波函数交叠以及载流子浓度分布等,发现沿生长方向阱内In组分线性增加的单量子阱样品的发光效率最高,而In组分线性减小的样品发光效率最低.这是因为In组分的线性增加能够减弱极化场对价带的影响,使阱内价带变得更加平缓.这不仅降低了空穴的注入势垒高度、增大了阱中的空穴浓度,还增强了阱内电子-空穴波函数的交叠积分,提高了辐射复合几率,从而使In组分线性增加的量子阱的发光效率显著提高.