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Solution deposited bulk heterojunction organic solar cells are viewed as one of the most promising alternative energy sources because of their ease of processing and their potential to be produced using large scale techniques such as roll-to-roll, newspaper style, coating. Since organic materials have a relatively low dielectric constant the dissociation of an excited electron–hole pair into free collectable charge carriers is inefficient in many cases. Often the excited electron–hole pairs recombine back to the ground state in a process known as geminate recombination before they ever fully dissociate into free charge carriers. Even after dissociation, free holes and electrons can encounter each other once more and subsequently recombine back to the ground state in a process known as nongeminate recombination. In both cases the incident photon energy is lost and fewer carriers are collected at the electrodes. Hence, charge carrier recombination is one of the key loss mechanisms in organic solar cells. In this review the latest on geminate and nongeminate recombination is discussed.

Co-precipitation in highly alkaline environment resulted in well crystalline ZnO nanoparticles over which ZnS shell was grown in situ in solution at room temperature. Formation of epitaxial hexagonal ZnS over wurtzite ZnO core particles have been confirmed by x-ray diffraction and transmission electron microscopy studies. The core ZnO nanoparticles show prominent UV emission which enhances appreciably due to ZnS shell formation in ZnO/ZnS core/shell particles. Experimental evidence indicates the quenching of non radiative recombination pathways due to shell formation. The results suggest that synthesized epitaxial core/shell particles have charge confinement mainly in the core region with type I characteristics of band alignment and can be useful for device applications.

We report the changes in the structural and optical property of TiO 2 nanoparticles on codoping Ce–Nd ions. X-ray diffraction clearly demonstrates the structural changes occurring in the codoped TiO 2 nanoparticle. Oxygen defects disturb the Ti O bonds in the TiO 6 octahedra and result in the shifting and broadening of the Raman E g peak. Pure TiO 2 nanoparticles show absorption peak in the UV region. However, codoped TiO 2 nanoparticles exhibit absorption peaks in the visible region corresponding to the f–d and f–f electronic transition of Ce 3+ and Nd 3+ in the crystalline environment of TiO 2 . The visible emission peaks of pure and codoped TiO 2 nanoparticles are mainly associated with oxygen vacancies. Incorporation of cerium intensifies the visible emission peaks of TiO 2 nanoparticles. On the other hand, codoping of Nd forms some non radiative recombination centres and increases the possibility of emission energy transfer among dopants, defects, thereby quenching the intensity of the visible emission peaks.

Most of the applications of semiconductors involve either photon absorption to form free carriers (photodetectors or solar cells) or the formation of photons by free carrier recombination (light emittiing diodes or lasers). Both kinds of applications require high quality single crystals having desirable dimensions, devoid of defects, grain boundaries, or impurities that act as electron scatterers, traps and non radiative recombination centers. As a consequence of the requirement of high quality, fabrication and growth conditions for the semiconductors must be carefully controlled for most applications. Hence single crystals of GeSe were grown by Direct Vapor Transport (DVT) technique, in a two zone horizontal furnace with temperature difference of 50 K between growth and source zones. The material crystallizes in the form of shining gray and platelets like crystals at the end of growth cycles. Thermogravimetric analysis (TGA) has been used for many years to evaluate thermal stability of material as it will determine the range of stable operation for a device made up out of these materials under investigation. Thermal characteristics of GeSe crystals were studied employing thermoanalytical techniques, viz. TGA and DTA. Thermal analysis experiments were carried out with constant heating rate of 10 °C/ min in N2. The objective of this study is to determine activation energy and other kinetic parameters of GeSe crystals. Broido and Coats-Redfern (C-R) methods are used to evaluate different kinetic parameters of GeSe crystals viz. activation energy, entropy, enthalpy, Gibbs mean free energy etc.

Nowadays, the nature of the non radiative recombination centres in ZnO is a matter of controversy; they have been related to extended defects, zinc vacancy complexes, and surface defects, among other possible candidates. We present herein the optical characterization of catalyst free ZnO nanorods grown by atmospheric MOCVD by microRaman and cathodoluminescence spectroscopies. The correlation between the defect related Raman modes and the cathodoluminescence emission along the nanorods permits to establish a relation between the non radiative recombination centers and the defects responsible for the local Raman modes, which have been related to Zn interstitial complexes.

There is a growing interest in the mode-by-mode understanding of electron and phonon transport for improving energy conversion technologies, such as thermoelectrics and photovoltaics. Whereas remarkable progress has been made in probing phonon–phonon interactions, it has been a challenge to directly measure electron–phonon interactions at the single-mode level, especially their effect on phonon transport above cryogenic temperatures. Here we use three-pulse photoacoustic spectroscopy to investigate the damping of a single sub-terahertz coherent phonon mode by free charge carriers in silicon at room temperature. Building on conventional pump–probe photoacoustic spectroscopy, we introduce an additional laser pulse to optically generate charge carriers, and carefully design temporal sequence of the three pulses to unambiguously quantify the scattering rate of a single-phonon mode due to the electron–phonon interaction. Our results confirm predictions from first-principles simulations and indicate the importance of the often-neglected effect of electron–phonon interaction on phonon transport in doped semiconductors., Direct measurement of electron-phonon interactions at the single-mode level has been a challenge. Here, Liao et al. use a three-pulse photoacoustic spectroscopy technique to investigate the damping of a single sub-terahertz coherent phonon mode by photo-excited free charge carriers in silicon at room temperature.

International audience; n this paper, we present experimental evidence on the change of the phonon spectrum and vibrational properties of a bulk material through phonon hybridization mechanisms. The phonon spectrum in a finite material is strongly affected by the presence of free surfaces, which is the addition of a contribution from an essentially two-dimensional crystal. The phonon spectrum of a bulk material can hence be altered by a hybridization mechanism between confined phonon modes in nanostructures introduced on the surface of a bulk material and the underlying bulk phonon modes. We measured the heat capacities of bare and surface-structured silicon substrates originating from the same silicon wafer. Then, we deduced important features of the phonon spectra of the samples investigated through a rigorous analysis of the measured heat capacity curves. The results show that the shape and size of the nanostructures made on the surface of the bulk substrate have a strong effect on the phonon spectrum of the bulk material.

We have studied recombination and relaxation dynamics in InAs quantum dots by means of photoluminescence cross-correlation techniques with sub-picosecond time-resolution. We have determined the relative radiative efficiencies for carrier recombination from different energy levels of the quantum dots and show that the radiative efficiency decreases with increasing transition energy, indicating that carrier access to non-radiative recombination centres increases for higher-energy states inside the dots. Cross-correlation measurements with both excitation beams of the same circular polarization and with the beams having opposite circular polarization are shown to give direct insight into the spin relaxation dynamics in the quantum dots and the wetting layer.

In this paper we discuss our experimental data obtained for several Si PV cell samples. From the I-V curves measured under various optical irradiance levels, we extracted the fill factors and the series and shunt resistances of the solar cells. For some of our samples, the solar cell efficiencies decrease even at illuminating power densities less than 1 Sun due to reductions in fill factors. Theoretical analysis suggests the reason for the observed dependences of PV cell circuit parameters on optical injection can be defect-mediated carrier recombination.

Two-wavelength excited photoluminescence, a non-destructive and non-contacting method for defect study has been used to investigate the AlGaN multiple quantum well (MQW) emitting at ∼260 nm. We have succeeded to detect the non-radiative recombination (NRR) centers in both wells and barrier layers using a below-gap ex- citation (BGE) light of 1.17 eV. The PL intensity of both well and barrier layers decreases after irradiation of BGE which can be explained by the well-established two-level model. The normalized PL intensity decreases with increasing BGE power density for both cases but more pronounced for the barrier layer. The normalized PL intensity increases for both well and barrier layer's emission with increasing AGE density and becomes saturated at higher excitation due to the fill-up of NRR centers.

Non-radiative recombination (NRR) of excited carriers poses a serious challenge to optoelectronic device efficiency. Understanding the mechanism is thus crucial to defect physics and technological applications. Here, by using first-principles calculations, we propose a new NRR mechanism, where excited carriers recombine via a Frenkel-pair (FP) defect formation. While in the ground state the FP is high in energy and is unlikely to form, in the electronic excited states its formation is enabled by a strong electron-phonon coupling of the excited carriers. This NRR mechanism is expected to be general for wide-gap semiconductors, rather than being limited to InGaN-based light emitting devices.

Elucidating the decay mechanisms of photoexcited charge carriers is key to improving the efficiency of solar cells based on organo-lead halide perovskites. Here we investigate the spectral dependence (via above-, inter- and sub-bandgap optical excitations) of direct and trap-mediated decay processes in CH3NH3PbI3 using time resolved microwave conductivity (TRMC). We find that the total end-of-pulse mobility is excitation wavelength dependent - the mobility is maximized (172 cm(2) V(-1) s(-1)) when charge carriers are excited by near bandgap light (780 nm) in the low charge carrier density regime (10(9) photons per cm(2)), and is lower for above- and sub-bandgap excitations. Direct recombination is found to occur on the 100-400 ns timescale across excitation wavelengths near and above the bandgap, whereas indirect recombination processes displayed distinct behaviour following above- and sub-bandgap excitations, suggesting the influence of different trap distributions on recombination dynamics.

Recombination is an important loss mechanism in organic solar cells. Here, both free charge and trapped charge carriers are taken into account in order to calculate the recombination rate. Trapped charge carriers are approximated by an exponential tail between the conduction band and valence band, and recombination is calculated as the sum of bimolecular recombination and trap-assisted recombination, the latter being described by recombination via exponential tail states. Then, an optoelectronic model including the effect of tail states is employed by finite element method to simulate a common structure of P3HT:PCBM organic solar cell. We investigate the influence of characteristic temperature on device performance, charge carrier densities and recombination rate. Simulation results reveal that the dominant recombination process could be the Langevin recombination or the recombination via exponential tail, depending on the characteristic temperature and applied voltage. Calculated open circuit voltage versus generation rate demonstrates a good agreement between our modeling prediction for the dominant recombination and experimental reports.

Three InAs quantum dot (QD) samples with dislocation filter layers (DFLs) are grown on Si substrates with and without in-situ annealing. Comparison is made to a similar structure grown on a GaAs substrate. The three Si grown samples have different dislocation densities in their active region as revealed by structural studies. By determining the integrated emission as a function of laser power it is possible to determine the power dependence of the radiative efficiency and compare this across the four samples. The radiative efficiency increases with decreasing dislocation density; this also results in a decrease in the temperature quenching of the PL. A laser structures grown on Si and implementing the same optimum DFL and annealing procedure exhibits a greater than 3 fold reduction in threshold current as well as a two fold increase in slope efficiency in comparison to a device in which no annealing is applied. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Power conversion efficiency (PCE) of bulk heterojunction solar cells is influenced by many factors, such as energy level alignment, light trapping and absorption, exciton diffusion, charge carrier mobility and non radiative recombination rate. Despite significant efforts towards improving all these aspects, the PCE remains relatively low and progress has been slow. Here we report a remarkable 52% relative increase in efficiency of solar cells embedded with small amounts of Mo6S9−xIx nanowires dispersed in P3HT:PCBM matrix. We present a detailed and systematic investigation of the numerous factors influencing this breakthrough increase in PCE. Raman spectroscopy and photocurrent imaging are used to investigate the spatial inhomogeneity of solar cell parameters and correlate them with the device performance. The largest effect appears to be improved hole mobility, which increases by a factor of 2.5. Surprisingly, only cells with highly regioregular P3HT show a dramatic effect with Mo6S9−xIx nanowires, while less regioregular P3HT:PCBM matrices show much smaller effect, pointing to level alignment as the crucial parameter in cell efficiency. A smaller PCE increase is attributed to absorbance of the active layer by surface-deposited Mo6S9−xIx nanowires.

Temperature variation of the recombination rates has been investigated for the electron-hole pairs responsible for defect PL in a defective a-Si:H film as grown. The results are compared with those obtained for a high-quality a-Si:H film after illumination. The results of the nonradiative recombination rate are fitted by a theoretical prediction for the case of strong electron-phonon coupling in the case of the defective a-Si:H film similarly to the case of the illuminated high-quality a-Si:H film. Difference between the frequency of the phonon associated with the non-radiative recombination process in the defective a-Si:H film and that in the illuminated highquality a-Si:H film is discussed by considering the influence of the amorphous network in the a-Si:H films affected by the preparation conditions and the nature of the native and photo-created defects.

The temperature dependences of the piezoelectric photo-thermal (PPT) signals from unintentionally doped p-type GaAsN films grown on semi-insulating GaAs substrate were measured from 80 to 300 K. From the theoretical analysis based on the rate equation for the recombination of photo exited carriers to the localized levels, we identified five majority hole traps, P1-P5 in GaAsN films. Among them, estimated concentrations of the P3 and P5 traps increased with the nitrogen contents. Therefore, we concluded that these two traps were due to nitrogen-related recombination centers in GaAsN.

We present a quantitative analysis of the loss of electroluminescence in light-emitting diodes (LEDs) based on poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) due to the combination of non-radiative trap-assisted recombination and exciton quenching at the metallic cathode. It is demonstrated that for an MEH-PPV LED the biggest efficiency loss, up to 45%, arises from extrinsic non-radiative recombination via electron traps. The loss caused by exciton quenching at the cathode proves only to be significant for devices thinner than 100 nm. Removal of electron traps by purification is expected to enhance the efficiency of polymer LEDs by more than a factor of two. (C) 2012 Elsevier B.V. All rights reserved.

The mechanism of the radiative recombination of hot carriers in narrow-gap semiconductors is analyzed using the example of indium antimonide. It is shown that the CHCC Auger recombination process may lead to pronounced carrier heating at high excitation levels. The distribution functions and concentrations of hot carriers are determined. The radiative recombination rate of hot carriers and the radiation gain coefficient are calculated in terms of the Kane model. It is demonstrated that the radiative recombination of hot carriers will make a substantial contribution to the total radiative recombination rate at high carrier concentrations.

With this paper we propose a detailed study of the gradual degradation of InGaN-based laser diodes and Light-Emitting Diodes submitted to electro-thermal stress. The two device structures have been processed from the same epitaxial wafer. Our purpose is to compare the behavior of the two devices by means of electro-optical measurements, electroluminescence characterization, and near field emission measurements. We demonstrate that: (i) stress induces a decrease in the optical power and an increase in threshold current; (ii) the two failure modes are strictly linked each other, and correlated to the increase in defect-related current components, indicating a substantial increase in defect density during degradation; (iii) analysis of characteristic temperature and near-field emission measurements do not indicate any strong variation of injection efficiency nor current confinement of the devices.

We report on the optical and magnetic characteristics of the Co and Mn doped ZnO submicrometer crystals (SMCs) synthesised by organic thermal decomposition method. The doped ZnO SMCs are hexagon cone in shape with size ranging from 100 to 400 nm. X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy measurements indicate that the Mn2+ and Co2+ are doped into the ZnO lattice with concentration of 5.4 and 4.85%, respectively. Optical absorption spectra of the doped SMCs display additional peaks associated with d–d crystal-field transitions due to the presence of tetrahedral Co2+ or Mn2+ ions. Photoluminescence of the samples shows suppressed band-gap emission, indicating that the Mn or Co ions in the ZnO lattice increase the non-irradiative recombination processes. Magnetic property measurements reveal that the Mn:ZnO SMCs are paramagnetic while the Co:ZnO exhibit weak ferromagnetism at 300 K.

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.

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.

Regular dislocation networks (DN) formed by direct bonding of Si wafers actively participate in recombination of minority carriers. Besides nonradiative recombination (NRR), dislocation-related luminescence (DRL), i.e. radiative recombination could be observed. We show that the type of recombination is defined by the type of the dislocations and the structure of the network. Screw dislocations are mostly DRL active, while NRR is mostly related to edge dislocations. Moreover, the DRL intensity strongly depends on the misalignment angle between the crystal orientations of both bonded wafers. There exists an optimal alignment between bonded Si wafers, for which the DRL intensity has a maximum. Such alignment could be used for dislocation based all-Si light emitting devices in the future on-chip optical communication components. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Internal quantum efficiency (IQE) of InGaN/GaN UV LEDs with patterned sapphire substrates (PSS) was investigated using electroluminescence (EL) and photoluminescence (PL) methods. The physical mechanisms that affect temperature-dependent EL efficiency as a function of injected carrier density were deduced. In order to reduce the density of non-radiative recombination centers to improve quantum efficiency, improvement of crystal quality and reduction in the number of defects are necessary. PSS LEDs showed better EL characteristics than non-PSS LEDs because of the improved epitaxial quality. Contrary to PL IQE, EL IQE was significantly affected by carrier injection efficiency, especially at low temperature when high bias voltage was applied to the p–n junction.

We have investigated the influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs) with an indium composition around 20%. The samples with five-pairs InGaN/GaN MQWs were grown on sapphire substrates by metalorganic chemical vapor deposition. Photoluminescence spectra at 8 K showed that the MQWs grown with a low amount of TEGa flow gave a strong single peak and a higher emission energy. High-resolution X-ray diffraction measurements showed a deterioration of the InGaN/GaN interfaces in the sample grown with the large TEGa flow. The luminescence thermal quenching characteristics suggested that more structural defects acting as non-radiative recombination centers formed in the MQWs when the TEGa flow increased. The results indicate that decreasing the TEGa flow help to build up a new growth balance during the growth of InGaN wells, leading to less structural defects, more homogeneous indium distribution and the abrupt MQWs interfaces.

ZnO and Al-doped ZnO films have been deposited on quartz substrates by ultrasonically assisted chemical vapor deposition technique. Photoluminescence (PL) spectra of the films reveal that Al doping leads to suppression of defect related visible band. Time resolved photoluminescence studies have been carried out for the measurement of lifetime of deep level luminescence. The decay of PL intensity with time has been found to follow biexponential behavior. The relative contributions of fast decay component (τ1) and slow decay component (τ2) in total decay process are found to be ∼99% and ∼1% respectively. The values of τ1 and τ2 are found to decrease with Al doping in ZnO film. The decrease of both τ1 and τ2 is attributed to increase in non-radiative recombination due to reduction in grain sizes and the decrease in radiative recombination due to suppression of defects.

In this paper, the main photocurrent density and power conversion efficiency limiting mechanisms in bulk heterojunction solar cells are discussed with the emphasis on recombination processes. Charge extraction by linearly increasing voltage, time of flight, and other methods that allow the carrier lifetime and recombination to be studied experimentally in operating solar cells are discussed. It is shown that non-Langevin recombination is required for high-performance organic photovoltaic devices, which typically have low charge carrier mobility. Long charge carrier lifetime, exceeding carrier transit time through the film, can be achieved when non-Langevin recombination is observed. Langevin-type recombination dominates in most low-efficiency solar cells, whereas non-Langevin recombination is present in high efficient, e.g., annealed poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester blend devices. The film nanomorphology plays a crucial role governing the charge transport and the carrier lifetime. Double injection current with non-Langevin carrier recombination is demonstrated in high-efficiency devices, which strongly exceeds the injection current with Langevin recombination, due to the high carrier concentration attainable under non-Langevin recombination. Several different models explaining the non-Langevin recombination in organic solar cells are reviewed. Requirements for charge carrier mobility and recombination to maximize power conversion efficiency in organic photovoltaic devices are outlined.

For an InGaN/GaN multiquantum well sample with low threading dislocation density (TDD), temperature-dependent microphotoluminescence measurements were performed. Evaluation of the behavior of the photoluminescence peak energy leads to the conclusion that even at a length scale of about 500?nm we still see the same S-shape of the temperature-dependent energy shift that was observed on a far larger scale. The behavior of the temperature-dependent full width at half maximum of the spectra and the integrated intensity leads to similar conclusions. We therefore demonstrate that the phenomenon responsible for the S-shape acts on a length scale that is far smaller than 500?nm. In addition, the behavior of the integrated intensity shows a fluctuation of local point defect density in the analyzed sample with its low TDD.

The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions.

This paper describes the design and fabrication process of a two-dimensional GaAs-based photonic crystal nanocavity and analyzes the optical characterization of cavity modes at room temperature. Single InAs/InGaAs quantum dots (QDs) layer was embedded in a GaAs waveguide layer grown on an Al"0"."7Ga"0"."3As layer and GaAs substrate. The patterning of the structure and the membrane release were achieved by using electron-beam lithography, reaction ion etching, inductively coupled plasma etching and selective wet etching. The micro-luminescence spectrum is recorded from the fabricated nanocavities, and it is found that some high-order cavity modes are clearly observed besides the lowest-order resonant mode is exhibited in spite of much high rate of nonradiative recombination. The variance of resonant modes is also discussed as a function of r/a ratio and will be used in techniques aimed to improve the probability of achieving spectral coupling of a single QD to a cavity mode.

A comparative study of AlGaN/GaN high-electron-mobility transistor (HEMT) surface passivation using ex situ and in situ deposited SiN x is presented. Performing ex situ SiN x passivation increased the reverse gate leakage and off-state channel leakage by about three orders of magnitude. The in situ SiN x layer was characterized using transmission electron microscopy (TEM) and capacitance–voltage (CV) measurements. Photoluminescence (PL) spectra indicated a reduction of nonradiative recombination centers in in situ SiN x -passivated samples, indicating improved crystal quality. CV measurements indicated a reduction of surface state density as well, and thus better overall passivation using in situ SiN x . Electroluminescence (EL) images of the channel regions in AlGaN/GaN HEMT devices operating in forward blocking mode with up to 400 V drain bias demonstrated reduced channel emission profiles of in situ-passivated devices. Compared with a nonpassivated reference sample, the reduced EL emission profiles correlated with a reduced channel temperature on ex situ SiN x -passivated devices.

The effect of the magnetic field on radiative and non-radiative mechanisms of charge carriers in GaAs/AlGaAs quantum wells (QWs) is investigated via quasi-simultaneous magneto-photoluminescence (PL) and magneto-surface photo-voltage (SPV) spectroscopy. At low-temperature, the luminescence intensity of ultra-low disordered GaAs/AlGaAs QWs generally increases under strong magnetic perturbation. Even at relatively high-temperature (100 K), the magnetic field driven enhancement of PL intensity is observed for thick QWs. On the other hand, it is found that the PL intensity of narrow QWs gradually decreases under a strong magnetic field at 100 K. The magnetic field driven enhancement (suppression) of radiative recombination efficiency for wide (narrow) QWs is investigated by considering the oscillator strength, thermal effects, and carrier re-distribution in energy states. Also, the charge carriers which escape from narrow QWs or are captured by interface defects are probed via magneto-SPV measurements. Radiative recombination and thermionic emission of charge carriers, investigated by magneto-PL and magneto-SPV spectroscopy, provide a clear guideline of the critical QW width which would be essential for magnetic field driven high-temperature operation of advanced emission based-devices.

Alq3 is synthesized by mixing 8-quinolinol and Al(OH)3 in distilled water at about 95 °C. The reaction time of 24 and 100 h leads to α- and γ-rich Alq3, respectively. The α-rich Alq3 is converted to γ-Alq3 after annealing at 400 °C, while sublimation at 330 °C destroys γ-Alq3. The mer- and fac-isomers give the same vibronic structure in the photoluminescence (PL) spectra at 12 K, which is composed of a progression of vibrational energy of 660 cm− 1, but the PL band peak of fac-isomer is blue-shifted by about 34 nm with respect to that of mer-isomer. When the temperature is increased from 12 K to 296 K, the PL intensity is observed to decrease above 100 K by non-radiative bimolecular recombination.

Transient femtosecond absorption spectroscopy and time-correlating single photon counting (TCSPC) photoluminescence (PL) were employed to study InS nanowires (NWs) grown by chemical vapor deposition (CVD) and determine the relaxation mechanisms in these nanostructures. Intensity dependent measurements revealed that Auger recombination plays an important role in the relaxation of photogenerated carriers at fluences larger than 0.4 x 10 15 photons/cm 2 . Calculations provided an estimated of the Auger recombination coefficient to be 1.1 ±0.5 x 10 -31 cm 6 /s. At the low fluence regime TCSPC PL revealed three relaxation mechanisms with time constants ranging from ps to nanosecond providing evidence of the importance of non-radiative decay channels associated with defect/trap states within the NWs.

The development of nanocrystal quantum dots (NQDs) with suppressed nonradiative Auger recombination has been an important goal in colloidal nanostructure research motivated by the needs of prospective applications in lasing devices, light emitting diodes, and photovoltaic cells. Here, we conduct single-nanocrystal spectroscopic studies of recently developed core-shell NQDs (so-called “giant” NQDs) that comprise a small CdSe core surrounded by a 16-monolayer-thick CdS shell. Using both continuous-wave and pulsed excitation, we observe strong emission features due both to neutral and charged biexcitons, as well as multiexcitons of higher order. The development of pronounced multiexcitonic peaks in steady-state photoluminescence of individual nanocrystals, as well as continuous growth of the emission intensity in the range of high pump levels, point toward a significant suppression of nonradiative Auger decay that normally renders multiexcitons nonemissive. The unusually high multiexciton emission efficiencies in these systems open interesting opportunities for studies of multiexciton phenomena using well-established methods of single-dot spectroscopy, as well as new exciting prospects for applications, that have previously been hampered by nonradiative Auger decay.

GaN epitaxial layers and nitride-based light-emitting diodes (LEDs) structures with a conventional single low-temperature (LT) GaN buffer layer and multiple Mg x N y /GaN buffer layers were prepared by metalorganic chemical vapor deposition. It was found that crystal quality of the GaN epilayer prepared on multiple Mg x N y /GaN buffer layers was significantly better than that prepared with conventional low-temperature GaN nucleation layer. With the multiple Mg x N y /GaN buffer layers, it was also found that 20 mA LED output power can be enhanced by 12%, as compared to the conventional LED. Such an enhancement in output power could be attributed to the reduction of threading dislocation induced nonradiative recombination centers using 12-pairs Mg x N y /GaN buffer layers.

We have developed a nano-optical technique using diffraction-limited far-field fluorescence monitoring of near-field scanning optical microscopy (NSOM) experiments. The technique was combined with time-resolved (TR) spectroscopy and was used to study the emission properties of single colloidal CdSe nanowires (NWs) with spatial resolution ∼100 nm. The use of this novel technique reveals a critical role of organic suspensions in optical experiments with single colloidal nanostructures. We observed that organic suspensions form a viscous shell which acts as a nano-lens providing efficient coupling of excited and emitted photons to the NW. We demonstrate that physical distortion of the shell produced by the near-field probe affects the emission dynamics of single NWs. Temperature dependent measurements show that the emission decay of a colloidal NW is dominated by size-dependent non-radiative recombination.

The optical and structural properties of In0.08Ga0.92N/In0.02Ga0.98N multiple quantum wells (MQWs) grown at different temperatures and with different supplies of indium were analyzed by atomic force microscopy and spectrally resolved cathodoluminescence (CL). By comparing the contrasts of monochromatic CL images with high-resolution secondary-electron images of the sample surface, it is shown that almost all contrasts of the CL images can be explained by lateral inhomogeneities of both the thickness and the InN mole fraction of the InGaN layers. Dark contrasts in the CL images solely related to dislocations were not observed, indicating very weak nonradiative recombination correlated with threading dislocations in the InGaN quantum wells. The lateral inhomogeneities of layer thickness and indium incorporation depend strongly on the growth conditions.

The recombination rate of coulombically bound electron–hole pairs depends on their spin configuration. Because of low spin–orbit coupling the spin dynamics of these well separated coulombic pairs is determined by weak hyperfine and exchange interactions. In this case a weak magnetic field produces strong effect on the spin dynamics and hence on the recombination rate of e–h pairs. We have shown that the recombination current in organic semiconductors may have a maximum as a function of recombination constant. For high recombination constants the current is space charge limited and decreases with increasing the e–h recombination constant. This decay of current is due to decrease of the region where the recombination takes place. At a low recombination constant the recombination takes place in the whole volume and the current increases with increasing the recombination constant. The characteristic recombination constant separating those two regimes depends on the thickness of sample, applied voltage, and charge carrier mobilities. The model predictions are consistent with experimental data for magnetoresistance of organic semiconductors.

We have demonstrated that Mg is a controllable acceptor element for p-type GaAsN alloys grown by solid-source molecular-beam epitaxy (MBE), where a hole concentration has been realized up to 1.5×10 20 cm −3 and has been controlled along the vapor pressure curve of Mg using a standard effusion cell. In addition, electrical and luminescence characterizations have been investigated for GaAsN:Mg samples grown by solid-source MBE. The activation energy of Mg for the GaAsN:Mg samples is close to the impurity energy of Mg in GaAs (28 meV). Thus Mg is a very shallow acceptor element for GaAsN alloys. From photoluminescence (PL) measurements, the integrated PL intensity drastically increases with increasing a hole concentration up to 8.6×10 19 cm −3 at 300 K. This result suggests that the hole concentration can increase up to 8.6×10 19 cm −3 without significantly increasing the non-radiative recombination centers related to Mg doping when Mg is the acceptor element.

We review the failure modes and mechanisms of gallium nitride (GaN)-based light-emitting diodes (LEDs). A number of reliability tests are presented, and specific degradation mechanisms of state-of-the-art LED structures are analyzed. In particular, we report recent results concerning the following issues: 1) the degradation of the active layer induced by direct current stress due to the increase in nonradiative recombination; 2) the degradation of LEDs submitted to reverse-bias stress tests; 3) the catastrophic failure of advanced LED structures related to electrostatic discharge events; 4) the degradation of the ohmic contacts of GaN-based LEDs; and 5) the degradation of the optical properties of the package/phosphors system of white LEDs. The presented results provide important information on the weaknesses of LED technology and on the design of procedures for reliability evaluation. Results are compared with literature data throughout the text.

With this paper we analyze the correlation between the degradation of InGaN-based laser diodes (LDs) and the increase in the non-radiative recombination rate in the active region. Several 405 nm MOCVD LDs have been submitted to CW stress, for 2000 h (stress current in the range 40-100 mA, case temperature = 75 °C). During stress, we extensively evaluated the optical characteristics of the LDs: a technique for the evaluation of the non-radiative recombination lifetime (τ nr ) in the active material was developed and used for the analysis of the stress effects. We demonstrate the following: (1) degradation determines the increase in LDs threshold current (I th ) and the decrease in the τ nr ; (2) degradation of I th and τ nr have similar kinetics; and (3) the degradation rate of the LDs is almost linearly related to the stress current level. The degradation process is therefore ascribed to the decrease of internal quantum efficiency caused by the increase of the non-radiative recombination rate in the active region.

The isovalent dopant diffusion from a localized source method was applied for the surface and bulk passivation of In 0.53 Ga 0.47 As (InGaAs) and GaAs layers. The passivation effects are characterized by low-temperature (77 K) photoluminescence (PL) spectroscopy. It has been found that phosphorus and/or indium doping from the gas phase increases effectively the PL intensity of InGaAs and GaAs layers. The phosphorus and indium diffusion into thin (∼100 nm) GaAs layers grown on the Ge substrate was carried out. The PL spectrum of these layers consists normally of a broadband emission line at ∼1.2 eV associated with the donor–acceptor pair recombination. Both donors and acceptors in GaAs are usually assigned to diffuse from the substrate Ge atoms occupying both Ga and As sites. Drastic changes of the PL emission after the phosphorous diffusion were observed: the intensity of the photoluminescence increases gradually, and the blueshift observed with the pumping energy increase is more pronounced. These changes are explained by a decrease in the nonradiative charge carrier recombination rate (e.g. at the GaAs–Ge heterointerface) by incorporation of phosphorus and indium atoms into GaAs during diffusion. This passivation method opens a promising way to improve the characteristics of multi-junction solar cells on Ge substrates.

InGaN/GaN multiple quantum wells (MQWs) blue and violet light-emitting diodes (LEDs) were stressed under the injection current density of 26.5 A/cm 2 for about 400 h at room temperature. Degradation behaviors of blue and violet LEDs were compared. Two stages of degradation were identified in the blue and violet LEDs. At the initial stage, the violet LEDs degraded very fast, but the blue LEDs degraded relatively slow. At later stage, both blue and violet LEDs showed similar degradation rates. Analyses of the L – I and I – V characteristics revealed that the increase of effective nonradiative recombination centers in the active regions was responsible for both blue and violet LED degradations. Rapid degradation of violet LEDs at early stage resulted from dramatical increase of the effective nonradiative centers. More indium in the MQWs might be helpful for better reliability performance of blue LEDs.

Transitions via impurities, in addition to transitions from the valence band to the conduction band in semiconductors increase the generation of electron–hole pairs. These transitions lead to alterations in the electronic density around the impurities, and as a consequence, to modifications in the forces on the impurities. In many cases it leads to an increase in the non-radiative recombination. The understanding of these processes at an atomic scale is very important for the physics of doped semiconductors, and technologically, for optoelectronic and spintronic devices. Using simple approximate models based on first-principles, the charge density and the related processes are analyzed. The results show that an increase in the impurity–host interaction or an increase in the concentration of the impurity leads to small variations in the density around the impurities for different charge states and a decrease in the effective Coulomb repulsion. These facts reduce the mechanism of non-radiative recombination via multi-phonon emission.