Increase In Non-radiative Recombination Research Articles

Impact of crystallographic facet of InGaN micro-LED sidewalls on electro-optical characteristics

InGaN micro-LEDs (μLEDs) with their potential high-volume applications have attracted substantial research interest in the past years. In comparison to other III–V semiconductors, InGaN exhibits a reduced susceptibility toward non-radiative surface recombination. However, efficiency degradation becomes more prominent as dimensions shrink to a few μm or less. Due to the high surface-to-volume ratio of the miniaturized devices, the non-radiative recombination increases and reduces the internal quantum efficiency. While many groups focus on surface passivation to mitigate surface defects, the influence of crystallographic orientation of the μLED sidewall on the efficiency remains unexplored. This study addresses this gap by investigating the impact of crystallographic orientation of the sidewalls on the emission properties of the μLEDs. Hexagonal and elongated μLEDs with dimensions as small as 3.5 μm and sidewalls with crystallographically well-defined m- and a-planes were fabricated. Electrical and optical properties were investigated using photo- and electroluminescence. External quantum efficiency (EQE) is assessed based on well-known carrier recombination models. It can be shown that μLED performance intrinsically depends on the crystallographic orientation of the sidewalls. Comparing hexagonal μLED structures with a-plane and m-plane sidewalls, an increase in the EQE by 33% was observed for structures with a-plane sidewalls, accompanied by reduction in the current density of the peak EQE by a nearly two orders of magnitude compared to structures with m-plane sidewalls. By analyzing the EQE characteristics at the μLED center and near the sidewalls, the improvements can be directly attributed to the increased radiative recombination from sidewalls with a-plane orientation.

Investigation of degradation dynamics of 265 nm LEDs assisted by EL measurements and numerical simulations

We studied four AlGaN-based 265 nm LEDs with increasing QW thickness (1.4, 3, 6 and 9 nm) during a constant current stress at 100 A cm−2. We focused our attention on the parasitic components of the emission spectra at low current levels and on the optical power recovery observed at high current levels. We associated every parasitic peak or band to a region in the device where they can be generated, also demonstrating if they are related to band-to-band emission or radiative emission through defects. At high current levels, we showed the simultaneous effect of the decrease in injection efficiency in the active region and the increase in non-radiative recombination, by fitting the EQE curves with a mathematical model. Moreover, we associated the optical power recovery with a generation of negative charge near the active region, which led to an increase in injection efficiency in the QW.

Improving Perovskite Green Quantum Dot Light-Emitting Diode Performance by Hole Interface Buffer Layers.

Perovskite quantum dot light-emitting diodes (QLEDs) are potential candidates for next-generation displays due to their high color purity and wide color gamut. Due to the strong electron-accepting ability of poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA), quantum dot (QD) films are prone to be charged, which leads to the imbalance of charge injection and the increase of nonradiative recombination, ultimately affecting the performance of the QLEDs. Here, we compared and studied two polymers, poly(methyl methacrylate) (PMMA) and poly(vinyl pyrrolidone) (PVP), as the hole interface buffer layers of QD films, which effectively reduced the defect density, suppressed nonradiative recombination, and greatly improved the efficiency and stability of QLEDs. The devices with PMMA achieved a maximum external quantum efficiency of 20.71%.

Degradation of AlGaN-based UV-C SQW LEDs analyzed by means of capacitance deep-level transient spectroscopy and numerical simulations

The lifetime of deep-ultraviolet light-emitting diodes (LEDs) is still limited by a number of factors, which are mainly related to semiconductor defects, and still need to be clarified. This paper improves the understanding of UV LED degradation, by presenting an analysis based on combined deep-level transient spectroscopy (C-DLTS), electro-optical characterization, and simulations, carried out before and during a constant current stress test. The original results of this paper are (i) C-DLTS measurements allowed us to identify three traps, two associated with Mg-related defects, also detected in the unaged device, and one related to point defects that were generated by the ageing procedure. (ii) Based on these results and on TCAD simulations, we explain the variation in the forward I–V by the degradation of the p-contact, due to Mg passivation. (iii) On the other hand, optical degradation is ascribed to an increase in defectiveness of the active region and surrounding areas, which led to a decrease in injection efficiency, to an increase in non-radiative recombination, and to an increase in trap-assisted tunneling processes.

Size effects of AlGaInP red vertical micro-LEDs on silicon substrate

To study the size effect of AlGaInP red micro-LEDs on the silicon substrate, we fabricated five AlGaInP red micro-LEDs with different pixel sizes (160 × 160, 80 × 80, 40 × 40, 20 × 20, and 10 × 10 µm2) and studied their electrical and optical properties. Smaller micro-LEDs have smaller leakage current and larger series resistance and can withstand higher current density without the current crowding effect. Due to the larger perimeter-to-area ratio of small-sized micro-LEDs, non-radiative recombination increases, which leads to a lower EQE. But smaller micro-LEDs can alleviate the problem of the high-current efficiency droop. In addition, because of a better heat dissipation under a high injection current, smaller micro-LEDs (>80 µm) have a smaller center wavelength shift. These experimental results provide important data for designing and fabricating red micro-LEDs with different pixel sizes for diverse future applications.

MAAc Ionic Liquid-Assisted Defect Passivation for Efficient and Stable CsPbIBr2 Perovskite Solar Cells

Cesium-based all-inorganic perovskite solar cells (PSCs), such as CsPbIBr2 PSCs, have attracted wide attention for good thermal and wet stability, but the low open-circuit voltage (Voc) mainly caused by the inadequate coverage of CsPbIBr2 films is the main reason for limiting their development. The CsPbIBr2 films grown on TiO2 substrate directly have a large number of pinholes, which bring a lot of defects and lead to an increase of nonradiative recombination. In this work, a strategy extended for CsPbIBr2 films by precoating methylammonium acetate (MAAc) ionic liquid onto a TiO2 layer before depositing the CsPbIBr2 film is demonstrated. The uniformly distributed MA+ will be the nucleation center at the bottom of the CsPbIBr2 film, which exhibited a notable impact on the crystallization kinetics of CsPbIBr2 films. This effect simultaneously enhanced the crystal quality of the CsPbIBr2 film and interfacial contact between the electron transporting layer (ETL) and CsPbIBr2 layer. It is instrumental in decreasing the trap state density, suppressing nonradiative recombination, and extracting the charge carriers. Therefore, the stability of the optimized device has been improved considerably, and the champion power conversion efficiency (PCE) is up to 8.85% with a high Voc of 1.26 V. Benefiting from passivation, the PSC with 2 M MAAc IL interfacial modification remains 82% of its initial PCE after 30 days of exposing the device to ambient air at room temperature.

Accurate determination of junction temperature in a GaN-based blue light-emitting diode using nonlinear voltage-temperature relation

We investigate the junction temperature measurements for GaN-based blue light emitting diodes (LEDs) using nonlinear dependence of the forward voltage (Vf) on temperature. Unlike the conventional linear model of the dependence of Vf on temperature, the modeling of the temperature dependent Vf with a quadratic function showed good agreements with measured data in the temperature range between 20 and 100 °C. Using the proposed quadratic model, the junction temperature and thermal resistance of the measured LED could be accurately determined as the ambient temperature varied. It was observed that the junction temperature increment remained almost unchanged as the ambient temperature increased from 20 to 80 °C, which could be attributed to the interplay between the decrease in series resistance and the increase in non-radiative recombination with increasing temperature. The presented method for accurate determination of the junction temperature is expected to be advantageously employed for the thermal management of high-power LEDs.

Improvement of photoluminescence intensity and film morphology of perovskite by Ionic liquids additive

Metal halide perovskites have received much attention for their application in light-emitting diodes (LEDs) and solar cells in the past several years. Among them, 2D and quasi-2D perovskite with organic long-chain cations introduced have drawn significant attention. However, while improving wet and thermal stability, as the grain size becomes smaller, more defects introduced at the grain boundary and surface, resulting in the increase of non-radiative recombination is becoming the main problem which should be faced by 2D/quasi-2D perovskite materials. Here, we report a new strategy employing ionic liquid named 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide(EMB).By adding a small amount of ionic liquid to the precursor, the defect was effectively passivated and the photoluminescence intensity was increased by 11 times and the fluorescent lifetime was increased by about 1.5 times. The flatness of the prepared perovskite thin films has also been effectively improved.

Low-temperature photoluminescence emission of monolayer MoS2 on diverse substrates grown by CVD

Monolayer MoS2 films on three different substrates, SiO2/Si, quartz and sapphire are synthesized by the method of chemical vapor deposition (CVD). Through a series of the optical characterization methods, the films are confirmed to be uniform and continuous on micrometer scale with a reliable quality. Low-temperature photoluminescence (PL) properties of monolayer MoS2 on diverse substrates are primarily studied. As the temperature increases, a decrease in PL intensity and a red shift in PL wavelength are observed. This is because the thermal motion of the phonons becomes stronger, which means that the nonradiative recombination increases and the PL intensity decreases. And the reduction of semiconductor band gap contributes to the redshift of PL. Furthermore, the influence of the lattice structure and thermal conductivity of the substrate on the low-temperature PL of monolayer MoS2 is also discussed.

Impact of strain on periodic gain structures in vertical external cavity surface-emitting lasers

In this article, the impact of strain relaxation on the emission properties of InGaAs/GaAs multiple quantum wells without strain compensation was examined. Structures consisting of different numbers of quantum wells, namely 4, 8, 12 and 16, on top of distributed Bragg reflectors were grown by molecular beam epitaxy as a typical vertical external cavity surface-emitting laser (VECSEL). The relation between emission parameters in the lasing regime and strain relaxation were investigated. A two-step control of the growth rate allowed for obtaining fixed spectral detuning in all structures regardless of the number of quantum wells. The heterostructures varied in its strain and the microcavity length. The other parameters remained unchanged. In consequence, for the first time a unique set of VECSEL-like heterostructures was investigated. The strain was analyzed by reciprocal space mapping using high-resolution X-ray diffractometry. It was found that the degree of structure relaxation caused by misfit dislocation generation depends linearly on the number of quantum wells. By fitting numerical simulations to the experimental results, we have quantitatively determined the extent to which output power was suppressed by increase in non-radiative recombination arising from misfit dislocations. The non-radiative coefficients were determined. Taking output power as a criterion, we determined the optimal number of QWs to be 12 and the maximum tolerable relaxation value of 0.27 for InGaAs/GaAs VECSEL structures with uniformly distributed quantum wells in microcavity. The dependence of the monomolecular recombination coefficient on structure relaxation has been determined.

Structural and photoluminescence properties of Al-doped zinc oxide nanoparticles synthesized in polyol

Al-doped ZnO (AZO) nanoparticles have been prepared using a modified polyol process that makes use of di-ethylene glycol as a solvent. This procedure allows for obtaining nanoparticles with a narrowed size distribution, a controlled morphology and a high crystal quality. The prepared AZO nanoparticles were annealed at 400°C. We studied the effect of doping and annealing on structural and optical properties. The structural investigations of the products confirmed the hexagonal wurtzite structure for all products and having a most preferred orientation along (101) plane. The results obtained by TEM revealed that the average particle size of the products decreases by doping and increases by annealing temperature. Energy dispersive spectroscopy (EDS) confirms the substitution of Al into ZnO lattice. Raman scattering analysis shows that the crystallinity of the material was improved by increasing the concentration of the dopant Al3+ and the photoluminescence spectra shows that the UV emission peak position of AZO nanoparticles exhibited a slight blue shift from 384 to 383nm, and the intensity decreased with increasing the Al concentration, which is attributed to an increase in nonradiative recombination. However the UV emission peak position of AZO (0.2% and 0.6%) nanoparticles annealed at 400°C exhibited a slight red shift due to the influence of the size effect on the energy level of confined excitons, because of the average size of the nanoparticles obviously became bigger with the increase of annealing temperature.

Ageing of InGaN-based LEDs: Effects on internal quantum efficiency and role of defects

This paper describes the degradation of InGaN-based LEDs submitted to constant current stress; based on combined electroluminescence, photoluminescence and deep-level transient spectroscopy we show that: (i) when submitted to constant current stress, LEDs can show a measurable decrease in the optical power, which is more prominent in the low current regime; (ii) the decrease in optical power is strongly correlated to the increase in the Shockley–Read–Hall recombination coefficient A, as estimated by differential lifetime measurements; (iii) stress induces the increase in the concentration of a trap level, with activation energy between 0.6 and 0.7eV, which is supposed to be located next to/within the active region. The results suggest that the optical degradation can be ascribed to the increase in non-radiative recombination, rather than to a decrease in carrier injection efficiency.

Electroluminescence from GeSn heterostructure pin diodes at the indirect to direct transition

The emission properties of GeSn heterostructure pin diodes have been investigated. The devices contain thick (400–600 nm) Ge1−ySny i-layers spanning a broad compositional range below and above the crossover Sn concentration yc where the Ge1−ySny alloy becomes a direct-gap material. These results are made possible by an optimized device architecture containing a single defected interface thereby mitigating the deleterious effects of mismatch-induced defects. The observed emission intensities as a function of composition show the contributions from two separate trends: an increase in direct gap emission as the Sn concentration is increased, as expected from the reduction and eventual reversal of the separation between the direct and indirect edges, and a parallel increase in non-radiative recombination when the mismatch strains between the structure components is partially relaxed by the generation of misfit dislocations. An estimation of recombination times based on the observed electroluminescence intensities is found to be strongly correlated with the reverse-bias dark current measured in the same devices.

Effect of Mg Doping in GaN Interlayer on the Performance of Green Light-Emitting Diodes

A light-emitting diode (LED) structure containing a low-temperature (LT) GaN interlayer between active region and AlGaN electron blocking layer is proposed to improve the performance of InGaN-based green LEDs. The experimental and simulated results show that, as the Mg doping depth in the LT-GaN interlayer increases, the hole injection efficiency gets improved and electron current leakage decreases, while defect-related nonradiative recombination increases. With an optimized Mg doping depth in the LT-GaN interlayer, a substantial suppression of efficiency droop can be achieved compared with the conventional LED.

Degradation of InGaN/GaN laser diodes investigated by micro-cathodoluminescence and micro-photoluminescence

We present an investigation of the degradation of InGaN/GaN laser diodes grown on a GaN substrate. The results indicate that: (i) Ageing induces a significant increase in the threshold current (Ith) of the lasers, which is attributed to an increase in non-radiative recombination; (ii) Ith increase is correlated to a decrease in the micro-cathodoluminescence signal measured (after the removal of the top metallization) in the region under the ridge; (iii) micro-photoluminescence measurements indicate that constant current stress increases non-radiative recombination within the quantum wells (and not only within the barriers), and induces an increase in the emission wavelength of the degraded region.

Effects of two-step Mg doping in p-GaN on efficiency characteristics of InGaN blue light-emitting diodes without AlGaN electron-blocking layers

A light-emitting diode (LED) structure containing p-type GaN layers with two-step Mg doping profiles is proposed to achieve high-efficiency performance in InGaN-based blue LEDs without any AlGaN electron-blocking-layer structures. Photoluminescence and electroluminescence (EL) measurement results show that, as the hole concentration in the p-GaN interlayer between active region and the p-GaN layer increases, defect-related nonradiative recombination increases, while the electron current leakage decreases. Under a certain hole-concentration condition in the p-GaN interlayer, the electron leakage and active region degradation are optimized so that high EL efficiency can be achieved. The measured efficiency characteristics are analyzed and interpreted using numerical simulations.

Optical and electrical variations in high-power white light-emitting diodes stressed with typical operation conditions

We present an analysis of the degradation of the optical and electrical properties of high-power light-emitting diodes (LEDs) used for general lighting applications. The study was conducted by submitting the LEDs to different current and temperature stress conditions. Those conditions are based on typical operating conditions that can be encountered on LED-based luminaires for general lighting. LEDs were stressed under four different operating currents, and two of those were stressed at two junction temperatures, controlled with a thermoelectric cooler. Results described in this paper indicate that the lumen droop due to an increase in nonradiative recombination is correlated with the stress conditions in accordance with the literature. However, the LED samples showed a forward-voltage droop which seems to be independent of the stresses. For all the stress conditions, the LED forward voltages decreased by about 1% after 1000 h of stress time. A link between forward voltage and lumen output was made through LED efficiency. Also, the yellow peak/blue peak ratio was measured and showed an increase after 1000 to 1200 h. This is attributed to LED chip degradation. These observations suggest the use of both current and voltage control to optimize the use of LEDs in general lighting.

Optimal Bandgap Combinations—Does Material Quality Matter?

The balance of photogeneration and recombination gives rise to an optimum bandgap for any solar cell. The radiative limit represents the lowest permissible level of recombination in a solar cell and, therefore, places an upper limit on the voltage that can be attained. Introducing additional nonradiative recombination results in a loss in voltage that can only be compensated for by moving to higher bandgaps. Consequently, the optimal bandgap for solar energy conversion will rise with increasing nonradiative recombination rate. This balance was recognized by Shockley and Queisser for single-junction solar cells and is here extended to multijunction solar cells. A rise in optimal bandgaps has been observed in simulated single-, double-, and triple-junction devices as nonradiative recombination increases. Optimal bandgaps between excellent and poor diode quality devices are shown to differ by 100s of meV under 1-sun illumination with both terrestrial and extraterrestrial spectra but exhibit no significant change at high concentration due to the dominance of the radiative component in the recombination dynamics.

The physical characteristic study on luminance uniformity and temperature for power GaN LEDs chip

In this paper, we study the relationship among current density distribution, heat and temperature based on current continuity equation, ohm law and three-dimensional heat transfer model. The relationship between luminance distribution and current spreading of GaN blue light emitting diode (LED) is studied. Luminance distribution is proved to be an effective method of distinguishing the performance of current spreading. Because of the close relationship among temperature, luminance distribution and current density, a qualitative method of optimizing electrode structure and current spreading is proposed. With different currents and heat sink temperatures, the current non-uniformity and the luminance distribution of LED are analyzed. Temperature or current density crowding results in heat accumulation, increase of non-radiative recombination and the restriction of the emitting photons, hence thermal flux is an important factor influencing the luminance distribution. Through carrier transport mechanism, the reason for the temperature influence on luminance distribution is explained. Optimized contact electrode structure can improve current spreading and luminance uniformity, also considerably increase the reliability of high power LED.

A semi-analytical model for semiconductor solar cells

A semi-analytical model is constructed for single- and multi-junction solar cells. This model incorporates the key performance aspects of practical devices, including nonradiative recombination, photon recycling within a given junction, spontaneous emission coupling between junctions, and non-step-like absorptance and emittance with below-bandgap tail absorption. Four typical planar structures with the combinations of a smooth/textured top surface and an absorbing/reflecting substrate (or backside surface) are investigated, through which the extracted power and four types of fundamental loss mechanisms, transmission, thermalization, spatial-relaxation, and recombination loss are analyzed for both single- and multi-junction solar cells. The below-bandgap tail absorption increases the short-circuit current but decreases the output and open-circuit voltage. Using a straightforward formulism this model provides the initial design parameters and the achievable efficiencies for both single- and multiple-junction solar cells over a wide range of material quality. The achievable efficiency limits calculated using the best reported materials and AM1.5 G one sun for GaAs and Si single-junction solar cells are, respectively, 27.4 and 21.1% for semiconductor slabs with a flat surface and a non-reflecting index-matched absorbing substrate, and 30.8 and 26.4% for semiconductor slabs with a textured surface and an ideal 100% reflecting backside surface. Two important design rules for both single- and multi-junction solar cells are established: i) the optimal junction thickness decreases and the optimal bandgap energy increases when nonradiative recombination increases; and ii) the optimal junction thickness increases and the optimal bandgap energy decreases for higher solar concentrations.