Influence of the magnetic field strength and excitation intensity on the shape of microphotoluminescence spectra of quantum-well structures based on GaN/InGaN doped with Sm and Eu + Sm

This paper presents the results of complex measurements of the microphotoluminescence spectra of quantum-well structures based on InGaN/GaN〈Sm〉 and the determination of the concentration and charge state of the Sm dopant. It has been shown that an increase in the magnetic field strength and the excitation intensity of the microphotoluminescence spectra leads to an increase in the luminescence intensity and a shift in the position of the maximum of the emission wavelength toward the short-wavelength region of the spectrum. Measurements of the microphotoluminescence spectra with variations in the external magnetic field strength, as well as with the introduction of paramagnetic and magnetic impurities, provide additional information on the mechanisms of formation of luminescence spectra in the quantum-well structures InGaN/GaN〈Sm〉,〈Eu + Sm〉. In the long-wavelength region, the influence of the magnetic field on the shape of the microphotoluminescence spectra of the InGaN/GaN structures doped with Sm and Sm + Eu is less pronounced than that in the short-wavelength region.     

Influence of europium doping on the sensitization of luminescence in GaAs/AlGaAs and InGaN/GaN quantum-well structures

A correlation between the photoluminescence spectra and structural parameters of Eu-doped quantum- well nanostructures InGaN/GaN and GaAs/AlGaAs is established. It is shown that the incorporation of rare-earth ions initiates lattice (as a rule, compressive) strains. The excitation migration in structures of high perfection stimulates transfer of nonequilibrium carriers to the ⁵ D ₂-⁵ D ₀ atomic levels of the Eu ion. In less perfect structures, the insertion of a rare-earth ion leads to the formation of isovalent traps in GaN layers capable of effectively capturing nonequilibrium carriers, which increases the intensity of photoluminescence of the structure by one order of magnitude.     

Effect of energy migration on the emission line shape in InGaN/GaN-based quantum-well structures

Structures with InGaN/GaN-based quantum wells and inhomogenetities in the lateral plane were used to study the effect of the mechanism of electronic excitation migration and carrier trapping on the formation of the emission line of the quantum wells. The stationary and time-resolved photoluminescence spectra were investigated over a wide spectral range under variation of the measurement temperature (4.2, 77, 300 K). The anisotropy (polarization) of radiation in InGaN/GaN structures with different inhomogeneities was studied. Excitation exchange among the inhomogeneities was shown to play an essential part in the formation of the emission line in a quantum-well system.     

Evaluation of polarization field in InGaN/GaN multiple quantum well structures by using electroluminescence spectra shift

In order to investigate the inherent polarization intensity in InGaN/GaN multiple quantum well(MQW) structures,the electroluminescence(EL) spectra of three samples with different GaN barrier thicknesses of 21.3 nm, 11.4 nm, and 6.5 nm are experimentally studied. All of the EL spectra present a similar blue-shift under the low-level current injection,and then turns to a red-shift tendency when the current increases to a specific value, which is defined as the turning point.The value of this turning point differs from one another for the three InGaN/GaN MQW samples. Sample A, which has the GaN barrier thickness of 21.3 nm, shows the highest current injection level at the turning point as well as the largest value of blue-shift. It indicates that sample A has the maximum intensity of the polarization field. The red-shift of the EL spectra results from the vertical electron leakage in InGaN/GaN MQWs and the corresponding self-heating effect under the high-level current injection. As a result, it is an effective approach to evaluate the polarization field in the InGaN/GaN MQW structures by using the injection current level at the turning point and the blue-shift of the EL spectra profiles.     

The role of built-in electric fields in the InGaN/GaN quantum-well emission

By analyzing the evolution of time-resolved photoluminescence spectra, it is detected experimentally for the first time that there is a correlated effect of built-in electric fields and of long-lived localized states on the formation of emission in quantum wells based on nitrides of Group III elements. It is shown that light-emitting diode structures can be classified for commercial applications by studying time-resolved photoluminescence spectra.     

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.     

Photoluminescence of ZnMnTe quantum-well structures in a magnetic field

Photoluminescence spectra of strained structures Zn_{1 ? x} Mn _x Te/Zn_{1 ? y} Mg _y Te with magnetic quantum wells and nonmagnetic barriers are studied. The Zeeman splitting of the heavy exciton is found to follow an unusual behavior: both spin components shift down in energy. The heavy-exciton photoluminescence Zeeman components are observed to be inversely distributed in intensity, with the higher energy component being stronger than the lower energy component. The Zeeman splitting of the exciton in a magnetic field is calculated. The data obtained permit refinement of some parameters of the energy spectrum and magnetic properties of these structures.     

Temperature dependence of the cathodoluminescence spectra of irradiated light-emitting-diode structures with multiple InGaN/GaN quantum wells

The cathodoluminescence spectra of regions of light-emitting structures with multiple InGaN/GaN quantum wells unirradiated and irradiated with an electron beam are investigated in the temperature range from liquid-nitrogen temperature to room temperature. It is shown that a new emission line with an energy of 2.69 eV emerges as a result of irradiation in addition to the initial 2.6 eV line. The intensity of the emission line associated with Mg in p-GaN also increases after irradiation.     

Properties of Schottky barrier photodiodes based on InGaN/GaN MQW structures

The characterization and fabrication of Schottky barrier photodiodes based on InGaN/GaN multiple-quantum well structures in the active region are presented. These devices allow photodetection based on nitrides from the visible (VIS) to the ultraviolet (UV) ranges to be covered, and offer an alternative to InGaN bulk devices. Indium concentrations in the 8 to 14% range have been used. It has also been shown that in these devices the envelope average electric field in the depletion region can be tailored as a result of the huge polarization fields present in wells and barriers. As a consequence, two different device operation modes, as a function of voltage bias, are possible. By proper well and barrier design, charge accumulation can be produced at the boundaries of the active region and a large responsivity for photons with energies close to the barrier bandgap is found. Photodetectors show a rather sharp detection edge with a contrast of more than four orders of magnitude, reaching peak responsivities in the 0.1–1.0 A/W range. Self-consistent simulations and a discussion on the electric fields in the active regions are also presented.     

Electroreflectance spectroscopy of compressively strained InGaN/GaN multi-quantum well structures

The built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) was investigated using the electric field dependent electroreflectance (ER) spectroscopic method. InGaN/GaN MQW structures were prepared on sapphire substrates of different thicknesses. Thinning the sapphire substrate enables control of the compressive stress by changing the curvature of the wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN/GaN MQW. The flat band voltage, estimated by measuring the applied reverse bias voltage that induces a 180° phase shift in the ER spectra, was decreased from −11.21 V to −10.51 V by thinning the sapphire substrate thickness from 200 to 60 μm. To calculate the piezoelectric field (F_pz) from the compensation voltage, the depletion width was obtained from the capacitance–voltage measurement. The F_pz estimated from the energy shift in ER peak in a bias range from 0 to −12 V was changed by 110 kV/cm.     

Optical excitation study on the efficiency droop behaviors of InGaN/GaN multiple-quantum-well structures

Efficiency droop is generally observed in electroluminescence under high current injection. Optical characterization on efficiency droop in InGaN/GaN multiple-quantum-well structures has been conducted at 12 K. Clear droop behaviors were observed for the sample excited by above-bandgap excitation of GaN with pulse laser. The results show that dislocation is not the crucial factor to droop under high carrier density injection, and Auger recombination just slightly affects the efficiency. The radiative recombination may be mainly affected by a multi-carrier-related process (diffusion and drift with a factor of n ^3.5 and n ^5.5) at the interface between GaN barrier and InGaN well.     

Influence of nanoparticles and thin layers of gold, europium phthalocyanine, and erbium nanoparticles on the formation of luminescence spectra of structures with InGaN/GaN quantum wells

The influence of nanoparticles and thin layers of Au, Eu phthalocyanine, and Er nanoparticles on the formation of luminescence spectra of InGaN/GaN quantum-well structures has been investigated. It has been shown that the influence of localized plasmons on the generation of charge carriers is determined by the size of Au nanoparticles under the assumption that the interaction of plasmons with surface states of the structures plays a dominant role. The influence of Au nanoparticles on the formation of luminescence spectra of multiple quantum-well structures based on InGaN/GaN, unlike the case of Au layers, is determined by the indium concentration. The influence of Eu phthalocyanine films, which are deposited onto the surface of the studied structures, on their photoluminescence spectra is similar to the influence of doping of these structures with europium.     

Electroluminescence spectra of structures based on the InGaN solid solution

The electroluminescence spectra of structures based on the InGaN solid solution (λ_max = 426 nm) have been investigated in the voltage range 3.2–3.6 V. The decrease in the growth rate of luminescence intensity with increasing current through a sample, which is observed in the current-intensity characteristics, is explained. The band gap of the material under study is determined. The change in this parameter with increasing voltage across the structure may be related to a change in the solid solution composition. Specific features of the dependences ?J/??ω = f(?ω) and ?β/?U = f(U) were used to determine the energy of the levels that can be involved in recombination.     

扫场对核磁共振测量的影响

在连续波核磁共振实验中,通常采用低频扫场产生重复再现的共振吸收信号。当处于磁场中的射频线圈引线回路等效面积不为零时,简谐扫场必然引起该闭合回路的磁通量变化,由此而产生同频简谐感生电动势并与核磁共振信号叠加共同构成振荡器输出信号。由于扫场上升和下降过程的感生电动势方向相反,从而扫场前半周和后半周所对应的共振吸收测量信号存在差异。实验结果表明：扫场强度及方向对核磁共振测量影响来源于实验测量技术缺陷,而非核磁共振的物理本质。     

Optical and X-ray diffraction studies of multilayer structures based on InGaN/GaN solid solutions

Multilayer structures based on the In _x Ga_{1 ? x} N/GaN compounds grown by gas-phase epitaxy from organometallic compounds are studied using photoluminescence spectroscopy and high-resolution X-ray diffraction. A method for analyzing the experimental rocking curves of multilayer structures in terms of the Parratt-Speriosu model is developed. This method permits one to determine the thickness, period, and average composition of In _x Ga_{1 ? x} N/GaN layers, as well as the deformation of the active region in the samples under study. The local indium content is determined using the theoretical model which describes the radiation energy as a function of the thicknesses of the InGaN layers taking into account the energy of quantum confinement, the energies of the spontaneous polarization and piezoelectric polarization, and the parameters determined from high-resolution X-ray diffraction data.     

Electric field effects on optical properties in zinc-blende InGaN/GaN quantum dot

The effect of electric field on exciton states and optical properties in zinc-blende (ZB) InGaN/GaN quantum dot (QD) are investigated theoretically in the framework of effective-mass envelop function theory. Numerical results show that the electric field leads to a remarkable reduction of the ground-state exciton binding energy, interband transition energy, oscillator strength and linear optical susceptibility in InGaN/GaN QD. It is also found that the electric field effects on exciton states and optical properties are much more obvious in QD with large size. Moreover, the ground-state exciton binding energy and oscillator strength are more sensitive to the variation of indium composition in InGaN/GaN QD with small indium composition. Some numerical results are in agreement with the experimental measurements.     

光电耦合对InGaN/GaN量子阱光学性能的影响

围绕高性能GaN基垂直腔面发射激光器(VCSELs),设计了两种具有不同光电耦合强度的InGaN/GaN量子阱(QWs)样品,研究了它们的光学性质。样品A在腔模的两个波腹处各放置两个InGaN耦合量子阱,而样品B在腔模的一个波腹处放置5个InGaN耦合量子阱。计算表明样品A具有较大的相对光限制因子1.79,而样品B为1.47。光学测试发现样品A有着更高的内量子效率(IQE)和更高的辐射复合效率。使用两种样品制作了光泵VCSEL结构,在光激发下实现激射,其中基于样品A的VCSEL有着更低的激射阈值。结果表明有源区结构会显著影响量子阱与光场的耦合作用、外延片的内量子效率、辐射复合寿命和VCSEL激射阈值,同时也说明样品A的有源区结构更有利于制作低阈值的VCSEL器件。     

Influence of polar surface properties on InGaN/GaN core-shell nanorod LED properties

InGaN/GaN nanorod core-shell LEDs have shown to be very promising candidates for high efficiency lighting devices. Such nanorods can be grown in different ways, leading to different device geometry and in particular to different structures near the polar Ga- and N-face nanorod surfaces. In this work the influence of the properties of the polar surfaces on the electrical device behaviour is studied qualitatively based on a semiclassical simulation model.