Effects of a prestrained InGaN interlayer on the emission properties of InGaN/GaN multiple quantum wells in a laser diode structure

The electroluminescence (EL) and photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells (MQWs) with a prestrained InGaN interlayer in a laser diode structure are investigated. When the injection current increases from 5 mA to 50 mA, the blueshift of the EL emission peak is 1 meV for the prestrained sample and 23 meV for a control sample with the conventional structure. Also, the internal quantum efficiency and the EL intensity at the injection current of 20 mA are increased by 71% and 65% respectively by inserting the prestrained InGaN interlayer. The reduced blueshift and the enhanced emission are attributed mainly to the reduced quantum-confined Stark effect (QCSE) in the prestrained sample. Such attributions are supported by the theoretical simulation results, which reveal the smaller piezoelectric field and the enhanced overlap of electron and hole wave functions in the prestrained sample. Therefore, the prestrained InGaN interlayer contributes to strain relaxation in the MQW layer and enhancement of light emission due to the reduction of QCSE.     

Efficiency enhancement of InGaN based blue light emitting diodes with InGaN/GaN multilayer barriers

The advantages of InGaN based light-emitting diodes with InGaN/GaN multilayer barriers are studied.It is found that the structure with InGaN/GaN multilayer barriers shows improved light output power,lower current leakage,and less efficiency droop over its conventional InGaN/GaN counterparts.Based on the numerical simulation and analysis,these improvements on the electrical and the optical characteristics are mainly attributed to the alleviation of the electrostatic field in the quantum wells(QWs) when the InGaN/GaN multilayer barriers are used.     

Tuning effects in optimisation of GaAs-based InGaAs/GaAs quantum-dot VCSELs

The comprehensive optical-electrical-thermal-recombination self-consistent simulation of an operation of quantum-dot (QD) VCSELs is used to optimise their structure for GaAs-based oxide-confined QD VCSELs predestinated for the second-generation 1.3-μm optical-fibre communication. It has been found that, contrary to a general belief of lasing thresholds of QD lasers inversely proportional to their density, for any design of QD VCSELs, there exists an optimal QD density ensuring its lowest lasing threshold. Besides, in intentionally strongly detuned QD VCSELs, to reach the desired 1.30-μm radiation, it is superfluous to improve uniformity of their QDs because their lasing thresholds are surprisingly distinctly lower for less uniform QDs. Then for these devices more optimal are somewhat non-uniform QDs and a necessary optical gain may be achieved with the aid of an increasing QD density.     

Interface characterization and indium content of indium-rich quantum dots in InGaN/GaN multiple quantum wells

We report on the interface characterization of InGaN/GaN multiple quantum wells with indium aggregation grown by metalorganic chemical vapor deposition. The interface related microstructure was analyzed by high-resolution transmission electron microscopy, high-resolution X-ray diffraction and high angle annular dark field. Luminescence measurements were carried out by micro-photoluminescence measurement. In addition, quantitative determination of the indium concentration inside the ultra-small dots was attempted. We demonstrate that the quantum dots are coherent and the interfaces remain sharp. The In content inside ∼2 nm InGaN dots is about 65% determined by spectrum imaging in energy-filtered transmission electron microscopy combined with multiple linear least squares fitting, which is slighter higher than the value obtained either from HRTEM or theoretical calculations. This discrepancy is briefly discussed but demands further studies for complete understanding.     

A dual-blue light-emitting diode based on strain-compensated InGaN-AlGaN/GaN quantum wells

A strain-compensated InGaN quantum well(QW) active region employing a tensile AlGaN barrier is analyzed.Its spectral stability and efficiency droop for a dual-blue light-emitting diode(LED) are improved compared with those of the conventional InGaN/GaN QW dual-blue LEDs based on a stacking structure of two In0.18Ga0.82N/GaN QWs and two In0.12Ga0.88N/GaN QWs on the same sapphire substrate.It is found that the optimal performance is achieved when the Al composition of the strain-compensated AlGaN layer is 0.12 in blue QW and 0.21 in blue-violet QW.The improvement performance can be attributed to the strain-compensated InGaN-AlGaN/GaN QW,which can provide a better carrier confinement and effectively reduce leakage current.     

Performance enhancement of an InGaN light-emitting diode with an AIGaN/InGaN superlattice electron-blocking layer

The efficiency enhancement of an InGaN light-emitting diode （LED） with an A1GaN/InGaN superlattice （SL） electron-blocking layer （EBL） is studied numerically, which involves the light-current performance curve, internal quan- tum efficiency electrostatic field band wavefunction, energy band diagram carrier concentration, electron current density, and radiative recombination rate. The simulation results indicate that the LED with an A1GaN/InGaN SL EBL has better optical performance than the LED with a conventional rectangular A1GaN EBL or a normal A1GaN/GaN SL EBL because of the appropriately modified energy band diagram, which is favorable ibr the injection of holes and confinement of elec- trons. Additionally, the efficiency droop of the LED with an AIGaN/InGaN SL EBL is markedly improved by reducing the polarization field in the active region.     

Droop improvement in blue InGaN light-emitting diodes with GaN/InGaN superlattice barriers

GaN/InGaN superlattice barriers are used in InGaN-based light-emitting diodes （LEDs）. The electrostatic field in the quantum wells, electron hole wavefunction overlap, carrier concentration, spontaneous emission spectrum, light-current performance curve, and internal quantum efficiency are numerically investigated using the APSYS simulation software. It is found that the structure with GaN/InGaN superlattice barriers shows improved light output power, and lower current leakage and efficiency droop. According to our numerical simulation and analysis, these improvements in the electrical and optical characteristics are mainly attributed to the alleviation of the electrostatic field in the active region.     

Enhanced performances of InGaN/GaN-based blue light-emitting diode with InGaN/AlInGaN superlattice electron blocking layer

InGaN/AlInGaN superlattice(SL) is designed as the electron blocking layer(EBL) of an InGaN/GaN-based lightemitting diode(LED). The energy band structure, polarization field at the last-GaN-barrier/EBL interface, carrier concentration, radiative recombination rate, electron leakage, internal quantum efficiency(IQE), current–voltage(I–V) performance curve, light output–current(L–I) characteristic, and spontaneous emission spectrum are systematically numerically investigated using APSYS simulation software. It is found that the fabricated LED with InGaN/AlInGaN SL EBL exhibits higher light output power, low forward voltage, and low current leakage compared with those of its counterparts.Meanwhile, the efficiency droop can be effectively mitigated. These improvements are mainly attributed to the higher hole injection efficiency and better electron confinement when InGaN/AlInGaN SL EBL is used.     

Enhanced performances of InGaN/GaN-based blue light-emitting diode with InGaN/AIInGaN superlattice electron blocking layer

InGaN/AIlnGaN superlattice （SL） is designed as the electron blocking layer （EBL） of an InGaN/GaN-based light- emitting diode （LED）. The energy band structure, polarization field at the last-GaN-barrier/EBL interface, carrier concen- tration, radiative recombination rate, electron leakage, internal quantum efficiency （IQE）, current-voltage （l-V） perfor- mance curve, light output-current （L-l） characteristic, and spontaneous emission spectrum are systematically numerically investigated using APSYS simulation software. It is found that the fabricated LED with InGaN/AIInGaN SL EBL exhibits higher light output power, low forward voltage, and low current leakage compared with those of its counterparts. Meanwhile, the efficiency droop can be effectively mitigated. These improvements are mainly attributed to the higher hole injection efficiency and better electron confinement when InGaN/AIlnGaN SL EBL is used.     

Influence of injection current and temperature on electroluminescence in InGaN/GaN multiple quantum wells

Electroluminescence (EL) spectra of blue InGaN/GaN multiple-quantum-well light-emitting diode (LED) have been investigated over a wide range of injection current (0.001–200 mA) and at various temperatures (6–300 K). Surprisingly, with increasing the injection current the EL peak energy shows an initial blueshift accompanied by a broadening of the EL linewidth at low temperatures (below 30 K). This trend differs from the usual photoluminescence (PL) measurement results, which have shown that with increasing the optical excitation power the PL peak energy gave an initial blueshift accompanied by a narrowing of the PL linewidth at low temperatures. The anomalous current behavior of the EL spectra may be attributed to electron leakage results in the failure of Coulomb screening effect and the relative enhancement of the low-energetic localized state filling at low temperatures and low currents. The electron leakage for the LED is further confirmed by both the current dependence of the EL intensity and the temperature dependence of the EL efficiency.     

Phonon–replica transitions in InGaN/GaN quantum well structures

A side-bump feature in a photoluminescence (PL) spectrum of an InGaN compound was widely observed. With reasonable fitting to PL spectra with three Gaussian distributions, the temperature variations of the peak positions, integrated PL intensities, and peak widths of the main and first side peaks of three InGaN/GaN multiple quantum well samples with different nominal indium contents are shown and interpreted. The existence of the side peaks is attributed to phonon–replica transitions. The variations of the peak position separations and the decreasing trends of the first side peak widths beyond certain temperatures in those samples were explained with the requirement of phonon momentum condition for phonon–replica transitions. In the sample with 25% nominal indium content, the phonon–replica transition could become stronger than the direct transition of localized states.     

How exact are simplified scalar approaches to optical fields in oxide-confined stripe-geometry diode lasers?

Optical model, scalar or vectorial one, describing behaviour of an optical field within a diode-laser cavity, is one of the most important parts of modelling of a diode-laser operation. As compared to more accurate vectorial optical approaches, scalar ones are known to be less exact but simultaneously they need much less computation time. Besides, they have been sometimes found to be surprisingly exact even beyond their confirmed range of validity. Therefore, in this paper, real validity limits of their application have been determined by comparing their simplified results with more exact results obtained with the aid of vectorial models. The analysis comprises a comparison of an application of the most popular scalar approach to optical properties of diode lasers, i.e., the effective index method, and the vectorial method of lines for the standard 1.3-μm GaAs-based stripe-geometry diode laser. The scalar model has been found to be quite exact in the case of a determination of the effective refractive index, i.e., the wavelength of emitted radiation, whereas its exactness in the lasing threshold analysis is much worse, especially in the case of higher-order modes. Our analysis is concluded with a determination of the regions where both models give satisfactorily close results.     

InGaN/GaN多量子阱LED载流子泄漏与温度关系研究

通过测量光电流,直接观察了InGaN/GaN量子阱中载流子的泄漏程度随温度升高的变化关系。当LED温度从300K升高到360K时,在相同的光照强度下,LED的光电流增大,说明在温度上升之后,载流子从量子阱中逃逸的数目更多,即载流子泄漏比例增大。同时,光电流的增大在激发密度较低的时候更为明显,而且光电流随温度的增加幅度与激发光子的能量有关。用量子阱-量子点复合模型能很好地解释所观察到的实验现象。实验结果直接证明,随着温度的升高,InGaN/GaN量子阱中的载流子泄漏将显著增加,而且在低激发密度下这一效应更为明显。温度升高导致的载流子泄漏增多是InGaN多量子阱LED发光效率随温度升高而降低的重要原因。     

Temperature dependent growth of InGaN/GaN single quantum well

InGaN/GaN single quantum well (SQW) structures under various InGaN growth temperatures have been grown by metal organic chemical vapor deposition (MOCVD), the surface morphologies and optical properties are investigated. The radius of the typical V-pits on the SQW surface is affected by the InGaN well-temperature, and the surface roughness decreased as the well-temperature reduced. Room-temperature photoluminescence (PL) and cathode luminescence (CL) shows the quantum well and quantum dot (QD)-like localized state light emission of the SQWs grown at 700 and 690 °C, respectively, whereas the samples grown at 670 and 650 °C present hybrid emission peaks. Excitation power dependent PL spectra indicates the QD-like localized state emission dominates at low excitation power and the quantum well emission starts to take over at high excitation power.     

Optically pumped InGaN/GaN MQW lift-off lasers grown on silicon substrates

Chemical etching and removal of the silicon substrate was used for the creation of optically pumped lift-off InGaN/GaN multiple quantum well (MQW) lasers from heterostructures grown on silicon substrate by MOVPE. Luminescence and laser properties of these heterostructures on silicon substrates as well as those of MQWs lifted-off from their substrate by chemical etching were investigated. The lowest value of the lasing threshold of the lift-off lasers at room temperature was about 205 kW/cm² for a laser wavelength of 463 nm and about 360 kW/cm² for a wavelength of 475 nm. It was shown theoretically that the reduction of internal losses, caused by the absence of absorption in the substrate (resulting from its removal) is most significant for the high order modes having lower values of mirror losses and can lead to a 50% reduction of the threshold (or material gain in InGaN necessary to achieve the threshold).     

The exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses

This paper studies the exciton-longitudinal-optical-phonon coupling in InGaN/GaN single quantum wells with various cap layer thicknesses by low temperature photoluminescence (PL) measurements.With increasing cap layer thickness,the PL peak energy shifts to lower energy and the coupling strength between the exciton and longitudinal-optical (LO) phonon,described by Huang-Rhys factor,increases remarkably due to an enhancement of the internal electric field.With increasing excitation intensity,the zero-phonon peak shows a blueshift and the Huang-Rhys factor decreases.These results reveal that there is a large built-in electric field in the well layer and the exciton-LO-phonon coupling is strongly affected by the thickness of the cap layer.     

Optical characterization of InGaN/AlGaN/GaN diode grown on silicon carbide

Electroluminescence (EL) properties of In_xGa_1−xN/Al_yGa_1−yN/GaN/SiC diode were studied. The spectral range for which EL spectra were recorded is 1–3.5 eV. Room temperature EL was obtained for forward bias (3.18 V, 220 μA) at 446.067 nm (blue luminescence band), 606.98 nm (yellow luminescence band) and 893.84 nm (Infrared luminescence band). The EL temperature dependence shows that, BL band is mostly given by e–h recombination corresponding to indium composition equal to 0.17 ± 0.01 and 0.14 ± 0.02 obtained theoretically and experimentally, respectively. The yellow band is generally weak and absent at low temperature. The IRL band is more consistent with the DAP recombination and could be explained by the thermal activation of Mg states. The luminescence bands shift to lower energies is due probably to the larger potential fluctuations effect.     

Two dimensional electron gas in a hybrid GaN/InGaN/ZnO heterostructure with ultrathin InGaN channel layer

We investigated the influence of an ultrathin InGaN channel layer on two-dimensional electron gas (2DEG) properties in a newly proposed hybrid GaN/In_xGa_1−xN/ZnO heterostructure using numerical methods. We found that 2DEG carriers were confined at InGaN/ZnO and GaN/InGaN interfaces. Our calculations show that the probability densities of 2DEG carriers at these interfaces are highly influenced by the In mole fraction of the InGaN channel layer. Therefore, 2DEG carrier confinement can be adjustable by using the In mole fraction of the InGaN channel layer. The influence of an ultrathin InGaN channel layer on 2DEG carrier mobility is also discussed. Usage of an ultrathin InGaN channel layer with a low indium mole fraction in these heterostructures can help to reduce the short-channel effects by improvements such as providing 2DEG with higher sheet carrier density which is close to the surface and has better carrier confinement.     

InGaN/GaN多量子阱热退火的拉曼光谱和荧光光谱

通过拉曼光谱及荧光光谱测量研究了采用低压金属有机化学沉积(MOCVD)方法生长的InGaN／GaN多量子阱高温快速热退火处理对量子阱光学性质的影响。观测到退火后InGaN／GaN量子阱的拉曼光谱E2,A1(LO)模式的峰位置出现了红移,而且该振动峰的半高宽也有微小变化。温度升高退火效果更明显。退火使量子阱内应力部分消除,同时In,Ga原子扩散出现相分离使拉曼谱表现出变化。在常温和低温下的光荧光谱表明,退火处理的量子阱发光主峰都出现了红移;而且低温退火出现红移,退火温度升高相对低温退火出现蓝移;同时在低温荧光光谱里看到经过退火处理后原发光峰中主峰旁边弱的峰消失了。讨论了退火对多量子阱光学性质的影响。     

Effects of V/III ratio on a-plane GaN epilayers with an InGaN interlayer

The effects of V/Ill growth flux ratio on a-plane GaN films grown on r-plane sapphire substrates with an InGaN interlayer are investigated. The surface morphology, crystalline quality, strain states, and density of basal stacking faults were found to depend heavily upon the V/III ratio. With decreasing V/III ratio, the surface morphology and crystal quality first improved and then deteriorated, and the density of the basal-plane stacking faults also first decreased and then increased. The optimal V/III ratio growth condition for the best surface morphology and crystalline quality and the smallest basal-plane stacking fault density of a-GaN films are found. We also found that the formation of basal-plane stacking faults is an effective way to release strain.