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.     

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.     

Photoluminescence kinetics slowdown in an ensemble of GaN/AlN quantum dots upon tunneling interaction with defects

The carrier recombination dynamics in an ensemble of GaN/AlN quantum dots is studied. The model proposed for describing this dynamics takes into account the transition of carriers between quantum dots and defects in a matrix. Comparison of the experimental and calculated photoluminescence decay curves shows that the interaction between quantum dots and defects slows down photoluminescence decay in the ensemble of GaN/AlN quantum dots.     

EBIC characterization of light-emitting structures based on GaN

EBIC investigations of light-emitting structures based on InGaN/GaN MQW with different numbers of wells have been carried out. A pronounced dependence of collection efficiency on quantum-well number is observed. A comparison with apparent carrier profiles calculated from C-V curves reveals a correlation between the collection efficiency and location of quantum well inside the depletion region. Defects producing bright EBIC contrast are revealed in the structure with five quantum wells. This contrast is associated with defects locally decreasing the excess carrier recombination inside quantum wells. The text was submitted by the authors in English.     

Efficiency droop behavior of GaN-based light emitting diodes under reverse-current and high-temperature stress

The efficiency droop behavior of GaN-based light emitting diodes (LEDs) is studied when the LEDs are under reverse-current and high-temperature stress tests respectively. It is found that reverse-current stress mainly induces additional non-radiative recombination centers within the active region of InGaN/GaN multiple quantum wells, which degrade the overall efficiency of the GaN LED under test but push the peak-efficiency-current towards higher magnitude. The up-shift of peak-efficiency-current can be explained by a rate-equation model in which the newly-created defects by reverse-current stress enlarge the dominant low-current region of non-radiative recombinations. Comparatively, high-temperature stress mainly increases the series resistance of the LED under test. Although the overall efficiency of the GaN LED also drops, there is no shift of peak-efficiency-current induced by the high-temperature stress.     

On the effect of ballistic overflow on the temperature dependence of the quantum efficiency of InGaN/GaN multiple quantum well light-emitting diodes

The dependences of the quantum efficiency of InGaN/GaN multiple quantum well light-emitting diodes on the temperature and excitation level are studied. The experiment is performed for two luminescence excitation modes. A comparison of the results obtained during photo- and electroluminescence shows an additional (to the loss associated with Auger recombination) low-temperature loss in the high-density current region. This causes inversion of the temperature dependence of the quantum efficiency at temperatures lower than 220–300 K. Analysis shows that the loss is associated with electron leakage from the light-emitting-diode active region. The experimental data are explained using the ballistic-overflow model. The simulation results are in qualitative agreement with the experimental dependences of the quantum efficiency on temperature and current density.     

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.     

Analysis of Nonradiative Carrier Recombination Processes in InN Films by Mid-infrared Spectroscopy

We investigate the reduction in the efficiency of band-edge radiative recombination in InN by two carrier recombination processes via mid-gap states: radiative recombination via deep states and nonradiative recombination (NR). Because of the small band-gap energy value and the existence of the surface electron accumulation layer, the carrier transition processes via deep states cannot be observed easily. We address this problem by using mid-infrared photoluminescence (PL) measurements, and observe an emission peak around 0.32 eV at room temperature, which we interpret as being caused by transition processes via deep-defect states. Since this emission is weaker than the band-edge emission, the dominant carrier recombination process is concluded to be NR by phonon emission. The NR rate is known to be determined by the NR defect density, carrier transport processes to NR defects, and thermal activation processes of carriers. Carrier transport and capture processes by NR defects are investigated using p-type samples for various carrier mobility values. It is concluded that the NR rate is highly affected by the carrier transport, and that the candidates for the NR defect species are point defects and complexes of acceptor nature. We have also observed the correlation between the thermal conductivity and the band-edge PL intensity. As a result, we have found that the NR rate is highly affected by the carrier transport and thermal activation processes in InN.     

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.     

Native defects and dopants in gan studied through photoluminescence and optically detected magnetic resonance

Native defects and dopants in GaN grown by organometallic chemical vapor deposition have been studied with photoluminescence and optically detected magnetic resonance. For undoped samples, the combined results indicate the presence of residual shallow donors and acceptors and deep donors. A model for the capture and recombination among these defects is developed. For Mg-doped samples, the experiments reveal shallow and perturbed acceptors and shallow and deep donors. Hence, shallow and deep states for the native donor or donors appear in all samples. The Mg-acceptor is perturbed from its effective-mass state by nearby point defects.     

GaN发光二极管的老化数学模型及寿命测试方法

GaN发光二极管因其寿命、效率和环保等优点得到了广泛的应用。寿命问题一直是限制GaN发光二极管应用的核心问题。为了研究GaN发光二极管的老化过程,计算了GaN发光二极管物理参数,分析了GaN发光二极管的深能级缺陷和非辐射复合中心增加的老化原理,并且针对该原理的老化过程进行物理原理的分析推导,进而建立了老化数学模型。同时,利用一组实际的GaN发光二极管大应力老化实验的数据进行计算,提出了利用该数学模型的GaN发光二极管寿命的测试方法和数学计算方法,并计算出实验GaN发光二极管的寿命数值。提出的GaN发光二极管老化数学模型对比传统的阿伦纽斯模型具有针对性强、物理意义明显和寿命预测准等优点,具有很好的实际应用价值。     

Properties of compensated GaAs light-emitting diodes following 60Co irradiation

The effect of gamma radiation on the light-emitting properties of compensated GaAs electroluminescent diodes has been investigated. For gamma doses up to 10⁸ rads(Si), a large exponential reduction in the external quantum efficiency is observed, accompanied by a small reduction in the total carrier lifetime. These results are consistent with a model wherein two different defects are introduced through irradiation. These defects in addition to increasing the number of nonradiative recombination centers can also degrade the radiation process directly. Isochronal annealing data on these irradiated diodes suggest that the defect responsible for degrading the radiative process anneals below 300°C, but that the defect responsible for decreasing the nonradiative lifetime is relatively unaffected. The exponential reduction of efficiency with irradiation, including an observed shift of spectral emission peak, as well as the annealing characteristic seen in these diodes suggests that the defect responsible for degrading the radiative process might be similar to the luminescent killer center associated with an arsenic vacancy.     

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.     

激光二极管的位移损伤效应研究

以解析公式的推导、位移损伤实验结果以及位移效应的数值模拟结果为基础,分析了位移效应产生的缺陷作为非辐射复合中心和多数载流子陷阱两种情形下的激光二极管阈值电流、外微分量子效率及I-V特性随辐照注量的变化规律。通常的实验注量范围内,缺陷主要作为非辐射复合中心,导致激光二极管阈值电流随注量呈线性增大,但外微分量子效率基本不变,I-V特性低压区电流增大;当辐照注量较高,引起明显的多数载流子去除效应时,阈值电流随注量的增大不再呈线性关系,同时外微分量子效率下降,I-V特性高压区的电流减小。     

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.     

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.     

Temperature dependence of performance of InGaN/GaN MQW LEDs with different indium compositions

The temperature dependence of performance of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with different indium compositions in the MQWs was investigated. With increasing In composition in the MQWs, the optical performance of the LEDs at room temperature was increased due to an increase in the localized energy states caused by In composition fluctuations in MQWs. As the temperature was increased, however, the decrease in output power for LED with a higher In composition in the MQWs was higher than that of LED with a lower In composition in the MQWs. This could be due to the increased nonradiation recombination through the high defect densities in the MQWs resulted from the increased accumulation of strain between InGaN well and GaN barrier.     

CdTe/CdS、CdTe/ZnS核壳结构半导体量子点制备及荧光特性

核壳半导体量子点材料因其在修复单量子点表面缺陷方面的特殊性能,极大地提高了量子点的光学性能而受到人们的研究。改进了CdTe核心的制作方法,使用小型三口瓶替代传统的小烧瓶作为反应容器,制备碲氢化钠,合成了不同核心尺寸、不同壳层厚度与不同壳层材料的10种CdTe/CdS、CdTe/ZnS核壳结构半导体量子点。对10种核壳结构半导体量子点材料进行紫外可见吸收光谱及荧光光谱测试,并分析其荧光特性。量子点在紫外可见波段的吸收光谱表明随着量子点尺寸的增大,吸收峰发生红移。通过实验结果与分析可推断出CdTe/CdS量子点荧光寿命和强度的不同是由于核心和壳层尺寸的不同量子点在I型和II型中相互转换;CdTe/ZnS的壳层厚度增加时,由于ZnS的壳层降低了核心外表的悬空键和表面缺陷态的数量,使电子空穴对复合机率加大,使得荧光峰位产生了红移。     

宽带隙半导体材料光电性能的测试

主要通过光致发光的实验手段,研究分析了在自支撑GaN衬底上生长的InGaN/GaN多量子阱(InGaN/GaN MQW)有源层中的载流子复合机制,实验中发现多量子阱的光致发光光谱中有一个与有源区中的深能级相关的额外的发光峰。在任何温度大功率激发条件下,自由激子的带边复合占主导地位,并且带边复合的强度随温度或激发功率的下降而减弱;在室温以下小功率激发条件下,局域化能级引入的束缚激子复合占主导地位,其复合强度随温度的下降而单调上升,随激发功率的下降而上升。带边复合在样品温度上升或者激发功率变大时发生蓝移,而局域的束缚激子复合辐射的峰值波长,随样品温度和激发功率的变化没有明显变化。     

Effects of p-type GaN thickness on optical properties of GaN-based light-emitting diodes

The influence of p-type Ga N(p Ga N) thickness on the light output power(LOP) and internal quantum efficiency(IQE) of light emitting diode(LED) was studied by experiments and simulations. The LOP of Ga N-based LED increases as the thickness of p Ga N layer decreases from 300 nm to 100 nm, and then decreases as the thickness decreases to 50 nm. The LOP of LED with 100-nm-thick pG a N increases by 30.9% compared with that of the conventional LED with 300-nm-thick p Ga N. The variation trend of IQE is similar to that of LOP as the decrease of Ga N thickness. The simulation results demonstrate that the higher light efficiency of LED with 100-nm-thick p Ga N is ascribed to the improvements of the carrier concentrations and recombination rates.