Reduction Of Emission Efficiency Research Articles

Recombination efficiency in c-plane (In,Ga)N/GaN quantum wells: saturation of localisation sites versus Auger–Meitner recombination

Abstract Light emitting diodes based on c-plane (In,Ga)N/GaN quantum wells (QWs) can have >90% emission efficiency at modest current densities but this drops significantly at higher excitation, an effect known as efficiency droop that limits device efficacy at high brightness. Several explanations for this have been proposed including the saturation of carrier localisation sites at high excitation densities, resulting in a greater exposure of carriers to defects and hence a significant increase in the associated non-radiative recombination processes. Here, power- and temperature-dependent photoluminescence spectroscopy of c-plane (In,Ga)N/GaN QWs is used to investigate the relationship between the saturation of localised states and emission efficiency. For the samples studied, we find that the saturation of localised sites broadly coincides with the onset of efficiency droop. However, it is also found that as the localised states saturate with increasing excitation, the relative contribution of defect-associated non-radiative processes to overall recombination decreases rather than increases. Based on these observations and on modelling of recombination processes in the QW, it is concluded that the saturation of localised states does not significantly contribute to the reduction in emission efficiency at high excitation. Our studies rather suggest that defect-related non-radiative recombination is out-competed by radiative and Auger–Meitner recombination at the carrier densities required for saturation.

Cathodoluminescence studies of the optical properties of a zincblende InGaN/GaN single quantum well

Zincblende GaN has the potential to improve the efficiency of green- and amber-emitting nitride light emitting diodes due to the absence of internal polarisation fields. However, high densities of stacking faults are found in current zincblende GaN structures. This study presents a cathodoluminescence spectroscopy investigation into the low-temperature optical behaviour of a zincblende GaN/InGaN single quantum well structure. In panchromatic cathodoluminescence maps, stacking faults are observed as dark stripes, and are associated with non-radiative recombination centres. Furthermore, power dependent studies were performed to address whether the zincblende single quantum well exhibited a reduction in emission efficiency at higher carrier densities—the phenomenon known as efficiency droop. The single quantum well structure was observed to exhibit droop, and regions with high densities of stacking faults were seen to exacerbate this phenomenon. Overall, this study suggests that achieving efficient emission from zinc-blende GaN/InGaN quantum wells will require reduction in the stacking fault density.

Influences of InGaN/GaN superlattice thickness on the electronic and optical properties of GaN based blue light-emitting diodes grown on Si substrates

GaN based light-emitting diodes (LEDs) are subjected to a large polarization-related built-in electric field in c-plane InGaN multiple quantum well (MQW) during growth, which causes the reduction of emission efficiency. To mitigate the electric field, a superlattice layer with a numerous good characteristics, such as a small thickness, a high crystalline quality, is embedded in the epitaxial structure of LED. However, the effect of the superlattice thickness on the properties of LED is not fully understood. In this paper, two blue-LED MQW thin film structures with different thickness values of InGaN/GaN superlattice inserted between n-GaN and MQW, are grown on Si (111) substrates by metal-organic chemical vapor deposition. Electronic and optical properties of the two kinds of samples are investigated. The obtained results are as follows. 1) Comparing two samples, it is observed that more serious reverse-bias leakage current exists in the one with thicker superlattice; 2) Room temperature electroluminescence (EL) measurement shows that the emission spectrum peak between two samples is blue-shifted to different extents as the injection current increases. With superlattice thickness increasing, the extent to which the peak is blue-shifted decreases. Nevertheless, there is no obvious discrepancy in the EL intensity between two samples with different thickness values at 300 K. In addition, the V-shaped pit characteristics including density and size, and the dislocation densities of two samples are studied by high-resolution X-ray diffraction, scanning electron microscope, and transmission electron microscope. The experimental data reveal that the reason for a tremendously different in reverse-bias leakage current between two samples is that there are larger and more V-pits in the superlattice sample with a large thickness. Whereas, V-pits also act as preferential paths for carriers, resulting in the fact that the thicker superlattice suffers more serious reverse-bias leakage current. According to reciprocal space X-ray diffraction intensity around the asymmetrical (105) for GaN measurement, the relaxed degree of InGaN quantum well on GaN is proportional to the superlattice thickness. On the other hand, it is useful for increasing superlattice thickness to reduce a huge stress in c-plane InGaN. Owing to joint effects of above factors, the EL intensities of the superlattice sample with different thickness values are almost identical. Our results show the functions of superlattice thickness in electronic and optical characteristics. What is more, the conclusions obtained in the present research indicate the practical significance for improving the performances of LED.

The impact of trench defects in InGaN/GaN light emitting diodes and implications for the “green gap” problem

The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a model that the “green gap” problem might relate to localized strain relaxation occurring through defects.

Improvement of quantum efficiency by employing active-layer-friendly lattice-matched InAlN electron blocking layer in green light-emitting diodes

Improvement of the internal quantum efficiency in green-light emitting diodes has been achieved using lattice-matched InAlN electron-blocking layers. Higher electroluminescence intensities have been obtained due to better electron confinement in the device active region. The device efficiency has also been found to significantly depend on the InAlN growth temperature. Optimized InAlN growth at ∼840 °C results in a lower growth rate and longer growth times than at ∼780 °C. The observed reduction in emission efficiency for InAlN layers grown at higher temperatures is possibly attributed to thermal damage in the green active region.

Aggregation-Induced Emission Enhancement of Polysilole Nanoaggregates

Polysiloles have recently received much attention because of their unusual electronic properties. These unusual optical and electrical properties can be useful in electronic devices, such as electron transporting materials, light-emitting diodes (LEDs), and chemical sensors. Poly(2,3,4,5-tetraphenyl)siloles (1) possess both 2,3,4,5-tetraphenyl-1-silacyclopenta2,4-diene and Si-Si backbone that their absorptions exhibit at 314 nm. However, the unsaturated five-membered ring of the silole shifts their optical absorption and emission spectra into the visible spectral region at 364 and 520 nm, respectively. The aggregation of highly emissive organics and polymers into a solid state causes an emission-quenching effect, since the aggregation of molecules forms less emissive species such as excimers. Reduction of emission efficiency in the solid state has been a major problem in device applications of light-emitting organic molecules. Many attempts to prevent aggregate formation have been made through chemical, physical, and engineering approaches. In contrast, few results on aggregation-induced emission (AIE) properties have been recently reported. Herein, we report an aggregation-induced emission enhancement of polysiloles. The syntheses of 1, dichloro(tetraphenyl)silole (2a), and dimethyl(tetraphenyl)silole (2b) were reported previously. The reduction of 2a with lithium gave molecular weights (Mw) of 5500 (Mw/Mn = 1.1 determined by SEC). Fluorescence emission and excitation spectra were recorded on a Perkin-Elmer Luminescence Spectrometer LS 50B. The solvents were determined to be free of emitting impurities prior to use. The concentration of polysilole aggregates for the fluorescence measurement was 10 μg/L. 2a and 2b emit 380 and 425 nm, respectively. However polysilole 1 shows emission near 520 nm with excitation at 340 nm. Nanoaggregates of 1 are prepared by rapid precipitation from water solution by the injection of THF solution containing polysilole. This procedure is different from the previous report and gives better monodisperse for any given water volume fraction. Polysilole aggregate exhibits a nearly identical emission band, however polysilole nanoaggregates are more highly fluorescent than polysiloles at identical concentration. A photograph of polysiloles in THF solution (left) and polysilole nanoaggregates (right) under a UV lamp is shown in Figure 1, indicating that polysilole nanoaggregates are more highly fluorescent than polysiloles at identical concentration. For polysilole solution in pure THF, the photoluminescence (PL) intensity is very weak and an emission peak near 520 nm was observed. In solutions between 0% and 40% water by volume, the emission intensity of polysilole nanoaggregates does not increased. However, the intensity of the emission band increases by about 17.8 times when the solution is 99% water by volume. As the water fraction is increased, the emission intensity of polysilole nanoaggregates increases dramatically with no shift in the emission wavelength. Polysilole nanoaggregates in water-THF mixtures with 50%-99% water by volume exhibit one emission band at 513 nm when excited at 340 nm as shown in Figure 2. The emission band of polysilole nanoaggregates in water-THF mixtures is blue shifted by 7 nm compared to that of polysilole in THF. A minimum volume-fraction of 50% water is required to increase the luminescence, which indicates the Figure 1. Photograph of polysilole in THF (10 mg/L, left) and polysilole nanoaggregates in water-THF mixture (90 : 10 by volume, 10 mg/L, right) under blacklight.

Band-edge emission of undoped and doped ZnO single crystals at room temperature

Band-edge emission of ZnO at around room temperature was investigated by measuring the temperature dependence of cathodoluminescence spectra at 20–300 K. Undoped crystals grown by a vapor transport method and Al-doped crystals by flux method were employed to elucidate the effect of doping on luminescence properties. For the Al-doped crystals, the free-exciton emission was weak through out the temperature range T<300 K. The most intense emission peak of the Al-doped crystal was energetically close to bound exciton annihilation emission. On the other hand, for undoped crystals, it was found that the most intense emission peak at room temperature was at E≈Eg−60 meV and this peak was not assignable to free-exciton annihilation emission. It was also found that this peak is not a reason for the reduction in emission efficiency.

Temperature dependence of far-infrared electroluminescence in parabolic quantum wells

We have measured the far-infrared emission from parabolically graded quantum wells driven by an in-plane electric field in the temperature range from 20 to 240 K. The peak emission corresponds to the intersubband plasmon in the parabolic potential. Its photon energy (6.6/9.8 meV) remains rather unaffected by temperature variations, the full-width at half-maximum ranges from 1 (T=20 K) to 2 meV (T=240 K). The reduction of emission efficiency with increasing temperature is attributed to the change in the nonradiative lifetime.

Spatially and temporally resolved emission from aggregates in conjugated polymers.

Abstract : We present results of ew, time-resolved and spatially resolved spectroscopic studies of emission and absorption in a model conjugated polymer, poly (p-pyridyl vinylene) (PPyV). The redshifted film spectra suggest the formation of aggregated regions. The approx. 4x reduction in emission efficiency in films vs. solution is attributed to a longer radiative lifetime for aggregate excitons, as is evidenced by time-resolved fluorescence measurements. We present the first direct optical imaging of aggregates in a conjugated polymer via near-field scanning optical microscopy (NSOM). The aggregate emission and absorption are found to be localized to partially aligned regions of the film approx. 200 nm in size.