Nonradiative Recombination Centers Research Articles

Improved Optical and Electrical Characteristics of GaN-Based Micro-LEDs by Optimized Sidewall Passivation.

GaN-based Micro-LED has been widely regarded as the most promising candidate for next generation of revolutionary display technology due to its advantages of high efficiency, high brightness and high stability. However, the typical micro-fabrication process would leave a great number of damages on the sidewalls of LED pixels, especially for Micro-LEDs, thus reducing the light emitting efficiency. In this paper, sidewall passivation methods were optimized by using acid-base wet etching and SiO2 layer passivation. The optical and electrical characteristics of optimized Micro-LEDs were measured and analyzed. The internal quantum efficiency (IQE) of Micro-LED was increased to 85.4%, and the reverse leakage current was reduced down to 10-13 A at -5 V. Optimized sidewall passivation can significantly reduce the non-radiative recombination centers, improving the device performance and supporting the development of high-resolution Micro-LED display.

Cross‐Sectional Carrier Lifetime Profiling and Deep Level Monitoring in Silicon Carbide Films Exhibiting Variable Carbon Vacancy Concentrations

The carrier lifetime control over 150 μm thick 4H‐SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2 lifetime killer sites is reported. The defect developments upon typical SiC processing steps, such as high‐ and moderate‐temperature anneals in the presence of a carbon cap, are monitored by combining electrical characterization techniques capable of VC depth‐profiling, capacitance–voltage (CV) and deep‐level transient spectroscopy (DLTS), with a novel all‐optical approach of cross‐sectional carrier lifetime profiling across 4H‐SiC epilayer/substrate based on imaging time‐resolved photoluminescence (TRPL) spectroscopy in orthogonal pump‐probe geometry, which readily exposes in‐depth efficacy of defect reduction and surface recombination effects. The lifetime control is realized by initial high‐temperature treatment (1800 °C) to increase VC concentration to ≈1013 cm−3 level followed by a moderate‐temperature (1500 °C) post‐annealing of variable duration under C‐rich thermodynamic equilibrium conditions. The post‐annealing carried out for 5 h in effect eliminates VC throughout the entire ultra‐thick epilayer. The reduction of VC‐related Z1/2 sites is proven by a significant lifetime increase from 0.8 to 2.5 μs. The upper limit of lifetimes in terms of carrier surface leakage and the presence of other nonradiative recombination centers besides Z1/2, possibly related to residual impurities such as boron are discussed.

Direct Observation of Carrier Transportation between Localized States in InGaN Quantum Wells

Despite the large misfit dislocation densities, indium gallium nitride (InGaN) demonstrates high luminous efficiency both for electroluminescence and photoluminescence. The mechanism behind it has been interpreted as the existence of potential minima (i.e., localized states), which will screen the non-radiative recombination centers to avoid carriers being trapped by the defects. The existence of localized states has been testified by many experiments. However, almost all of the observations are indirect observations, and some experiments, such as those focused on whether the indium clusters observed by transmission electron microscopy are localized states, still remain controversial. Here, we report the direct observation of carrier transportation between localized states driven by temperature-dependent photoluminescence (TDPL) and excitation power-dependent PL in InGaN quantum wells. This enriches the experimental evidence on the existence of localized states.

Dislocation Filter Based on LT-GaAs Layers for Monolithic GaAs/Si Integration.

The use of low-temperature (LT) GaAs layers as dislocation filters in GaAs/Si heterostructures (HSs) was investigated in this study. The effects of intermediate LT-GaAs layers and of the post-growth and cyclic in situ annealing on the structural properties of GaAs/LT-GaAs/GaAs/Si(001) HSs were studied. It was found that the introduction of LT-GaAs layers, in combination with post-growth cyclic annealing, reduced the threading dislocation density down to 5 × 106 cm-2, the root-mean-square roughness of the GaAs surface down to 1.1 nm, and the concentration of non-radiative recombination centers in the near-surface GaAs/Si regions down to the homoepitaxial GaAs level. Possible reasons for the improvement in the quality of near-surface GaAs layers are discussed. On the one hand, the presence of elastic deformations in the GaAs/LT-GaAs system led to dislocation line bending. On the other hand, gallium vacancies, formed in the LT-GaAs layers, diffused into the overlying GaAs layers and led to an increase in the dislocation glide rate. It was demonstrated that the GaAs/Si HSs obtained with these techniques are suitable for growing high-quality light-emitting HSs with self-assembled quantum dots.

Radiative Recombination and Carrier Injection Efficiencies in 265 nm Deep Ultraviolet Light‐Emitting Diodes Grown on AlN/Sapphire Templates with Different Defect Densities

The electro‐optical characteristics of deep ultraviolet light‐emitting diodes (DUV LEDs) emitting at 265 nm and grown on AlN/sapphire templates with different threading dislocation densities, i.e., high‐temperature annealed (HTA) AlN, epitaxially laterally overgrown (ELO) AlN, and HTA‐ELO AlN are analyzed. The external quantum efficiency of each individual device is separated into maximum radiative recombination efficiency, carrier injection efficiency, and light extraction efficiency. This is achieved by combining an ABC‐model‐based fit of the current‐dependent external quantum efficiency together with calibrated Monte Carlo ray‐tracing simulations. A maximum radiative recombination efficiency between 50% and 60% is estimated for DUV LEDs grown on ELO and HTA‐ELO AlN/sapphire, whereas the values for devices grown on HTA AlN/sapphire are around 45%. The extracted radiative recombination efficiency does not scale with the measured threading dislocation density (TDD), even when accounting for the inhomogeneous TDD in the AlN base layers. This discrepancy is attributed to the formation of dislocation half‐loops introduced by additional compressive strain caused by the HTA process and may result in the formation of additional nonradiative recombination centers in the AlGaN multi‐quantum well region. In addition, the carrier injection efficiency values ranging from 45% to 55% are determined for devices grown on all three templates.

Photoluminescence Properties of InAs Quantum Dots Overgrown by a Low-Temperature GaAs Layer under Different Arsenic Pressures

We studied the influence of the arsenic pressure during low-temperature GaAs overgrowth of InAs quantum dots on their optical properties. In the photoluminescence spectrum of quantum dots overgrown at a high arsenic pressure, we observed a single broad line corresponding to unimodal size distribution of quantum dots. Meanwhile, two distinct peaks (~1080 and ~1150 nm) at larger wavelengths are found in the spectra of samples with quantum dots overgrown at a low arsenic pressure. We attributed this phenomenon to the high-pressure suppression of atom diffusion between InAs islands at the overgrowth stage, which makes it possible to preserve the initial unimodal size distribution of quantum dots. The same overgrowth of quantum dots at the low arsenic pressure induces intensive mass transfer, which leads to the formation of arrays of quantum dots with larger sizes. Integrated photoluminescence intensity at 300 K is found to be lower for quantum dots overgrown at the higher arsenic pressure. However, a difference in the photoluminescence intensity for the high- and low-pressure overgrowths is not so significant for a temperature of 77 K. This indicates that excess arsenic incorporates into the capping layer at high arsenic pressures and creates numerous nonradiative recombination centers, diminishing the photoluminescence intensity.

Self-Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage.

The photoluminescence (PL) of metal halide perovskites can recover after light or current-induced degradation. This self-healing ability is tested by acting mechanically on MAPbI3 polycrystalline microcrystals by an atomic force microscope tip (applying force, scratching, and cutting) while monitoring the PL. Although strain and crystal damage induce strong PL quenching, the initial balance between radiative and nonradiative processes in the microcrystals is restored within a few minutes. The stepwise quenching-recovery cycles induced by the mechanical action is interpreted as a modulation of the PL blinking behavior. This study proposes that the dynamic equilibrium between active and inactive states of the metastable nonradiative recombination centers causing blinking is perturbed by strain. Reversible stochastic transformation of several nonradiative centers per microcrystal under application/release of the local stress can lead to the observed PL quenching and recovery. Fitting the experimental PL trajectories by a phenomenological model based on viscoelasticity provides a characteristic time of strain relaxation in MAPbI3 on the order of 10-100 s. The key role of metastable defect states in nonradiative losses and in the self-healing properties of perovskites is suggested.

Luminescence enhancement of ZnS:Mn and ZnS:Cu-PVP nanoparticles by laser radiation annealing

Most of the ZnS:M (M: transition metal) nanoparticles synthesized from the S2− source in organic and inorganic precursors cause defects, dangling bonds, and vacancies in the surface of nanoparticles, creating nonradiative recombination centres and reducing the photoluminescence (PL) intensity. ZnS:Mn and ZnS:Cu-PVP nanoparticles were synthesized by different methods with sources of S2− in organic and inorganic precursors of Na2S, Na2S2O3.5H2O or HSCH2COOH (TGA), and PVP. Then, the surface activation methods used ultraviolet radiation at 325 and 337 nm and visible radiation at 532 nm, 632.8 nm, and 650 nm were used to increase the photoluminescence (PL) intensity of the nanoparticles. The results of the XRD pattern analysis showed that the crystal structure and grain size of the samples did not appear to change due to the radiation annealing process. However, the FT-IR spectra showed that samples using the S2− source from the inorganic precursor Na2S, Na2S2O3, exhibited ZnSO4 formation that was attributed to the photochemical phenomena on the surface of the nanoparticles. The samples using S2− source from organic precursor TGA showed the formation of ZnSO4 and polyglycolide [CH2–COO]n (PGA) polymer on the surface of nanoparticles. This was attributed to the polymerization phenomenon. These photochemical and polymerization processes have the effect of reducing the nonradiative recombination processes of the nanoparticle surface defect states and simultaneously increasing the photoluminescence (PL) intensity of the luminescent centres (Mn2+, Cu2+) from 2.0 to 3.6 times for the ZnS:Mn system and 2.7 times for the ZnS:Cu-PVP system in comparison with the samples that were not annealed. The effects of UV and visible radiation, annealing time, and annealing power on the PL intensity of luminescence centres in the nanoparticles were also investigated and explained. The radiation annealing methods for the samples after synthesis are highly important for improving the optical properties of the nanoparticles and for studies on the fabrication of photovoltaic elements, quantum dots, and LEDs.

Near-Unity Superradiant Emission from Delocalized Frenkel Excitons in a Two-Dimensional Supramolecular Assembly.

We demonstrate three general effective strategies to mitigate non-radiative losses in the superradiant emission from supramolecular assemblies. We focus on J-aggregates of 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) and elucidate the nature of their nonradiative processes. We show that self-annealing at room temperature, photo-brightening, and the purification of the dye monomers all lead to substantial increases in emission quantum yields (QYs) and a concomitant lengthening of the emission lifetime, with purification of the monomers having the largest effect. We use structural and optical measurements to support a microscopic model that emphasizes the deleterious effects of a small number of impurity and defect sites that serve as non-radiative recombination centers. This understanding has yielded a room temperature molecular fluorophore in solution with an unprecedented combination of fast emissive lifetime and high QY. We obtain superradiant emission from J-aggregates of TDBC in solution at room temperature with a QY of 82% coupled with an emissive lifetime of 174 ps. This combination of high QY and fast lifetime at room temperature makes supramolecular assemblies of purified TDBC a model system for the study of fundamental superradiance phenomena. High QY J-aggregates are uniquely suited for the development of applications that require high speed and high brightness fluorophores such as devices for high speed optical communication.

Enhanced Red Emission from Amorphous Silicon Carbide Films via Nitrogen Doping.

The enhanced red photoluminescence (PL) from Si-rich amorphous silicon carbide (a-SiCx) films was analyzed in this study using nitrogen doping. The increase in nitrogen doping concentration in films results in the significant enhancement of PL intensity by more than three times. The structure and bonding configuration of films were investigated using Raman and Fourier transform infrared absorption spectroscopies, respectively. The PL and analysis results of bonding configurations of films suggested that the enhancement of red PL is mainly caused by the reduction in nonradiative recombination centers as a result of the weak Si-Si bonds substituted by Si-N bonds.

Vacancy-Regulated Charge Carrier Dynamics and Suppressed Nonradiative Recombination in Two-Dimensional ReX2 (X = S, Se).

Point defects in semiconductors usually act as nonradiative charge carrier recombination centers, which severely limit the performance of optoelectronic devices. In this work, by combining time-domain density functional theory with nonadiabatic molecular dynamics simulations, we demonstrate suppressed nonradiative charge carrier recombination and prolonged carrier lifetime in two-dimensional (2D) ReX2 (X = S, Se) with S/Se vacancies. In particular, a S vacancy introduces a shallow hole trap state in ReS2, while a Se vacancy introduces both hole and electron trap states in ReSe2. Photoexcited electrons and holes can be rapidly captured by these defect states, while the release process is slow, which contributes to an elongated photocarrier lifetime. The suppressed charge carrier recombination lies in the vacancy-induced low-frequency phonon modes that weaken electron-phonon coupling, as well as the reduced overlap between electron and hole wave functions that decreases nonadiabatic coupling. This work provides physical insights into the charge carrier dynamics of 2D ReX2, which may stimulate considerable interest in using defect engineering for future optoelectronic nanodevices.

The role of surface states and point defects on optical properties of InGaN/GaN multi-quantum wells in nanowires grown by molecular beam epitaxy

The optical properties of nanowire-based InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by plasma-assisted molecular beam epitaxy are investigated. The beneficial effect of an InGaN underlayer grown below the active region is demonstrated and assigned to the trapping of point defects transferred from the pseudo-template to the active region. The influence of surface recombination is also investigated. For low InN molar fraction value, we demonstrate that AlO x deposition efficiently passivate the surface. By contrast, for large InN molar fraction, the increase of volume non-radiative recombination, which we assign to the formation of additional point defects during the growth of the heterostructure dominates surface recombination. The inhomogeneous luminescence of single nanowires at the nanoscale, namely a luminescent ring surrounding a less luminescent centre part points towards an inhomogeneous spatial distribution of the non-radiative recombination center tentatively identified as intrinsic point defects created during the MQWs growth. These results can contribute to improve the performances of microLEDs in the visible range.

A Novel Way to Fill Green Gap of GaN-Based LEDs by Pinning Defects in Nanorod Array.

Nanorod array and planar green-emission InGaN/GaN multi-quantum well (MQW) LEDs were fabricated by lithography, nano-imprinting, and top-down etching technology. The defect-pinning effect of the nanostructure was found for the first time. The ratio of the bright regions to the global area in the panchromatic CL images of green MQW samples increased from 30% to about 90% after nano-fabrication. The overall luminous performance significantly improved. Throughout temperature-dependent photoluminescence (TDPL) and time-resolved PL (TRPL) measurements, the migration and recombination of carriers in the MQWs of green LEDs were analyzed. It was proved that nanostructures can effectively prevent carriers from being captured by surrounding nonradiative recombination centers. The overall PL integral intensity can be enhanced to above 18 times. A much lower carrier lifetime (decreasing from 91.4 to 40.2 ns) and a higher internal quantum efficiency (IQE) (increasing from 16.9% to 40.7%) were achieved. Some disputes on the defect influence were also discussed and clarified.

First-principles study of non-radiative carrier capture by defects at amorphous-SiO2/Si(100) interface

Defects have a significant impact on the performance of semiconductor devices. Using the first-principles combined with one-dimensional static coupling theory approach, we have calculated the variation of carrier capture coefficients with temperature for the interfacial defects P b0 and P b1 in amorphous-SiO2/Si(100) interface. It is found that the geometrical shapes of P b0 and P b1 defects undergo large deformations after capturing carriers to form charged defects, especially for the Si atoms containing a dangling bond. The hole capture coefficients of neutral P b0 and P b1 defects are largest than the other capture coefficients, indicating that these defects have a higher probability of forming positively charged centres. Meanwhile, the calculated results of non-radiative recombination coefficient of these defects show that both P b0 and P b1 defects are the dominant non-radiative recombination centers in the interface of a-SiO2/Si(100).

Electric field-induced large strain and photoluminescence properties in SrRECoO4 (RE = Pr, Eu, Sm)-modified BNT-based ferroelectric ceramics

(1-x) (0.935BNT–0.065BaTiO3)-xSrRECoO4 (BNBT6.5-xSREC, RE = Pr, Eu, Sm)–based lead-free ferroelectric ceramics with large electrostrain and photoluminescence effect were prepared in our work. Through introducing the SREC dopant into BNBT6.5, the long-range ferroelectric order of ceramics was disrupted, inducing a weak polar state dominated by polar nanoregions before applying an electric field. Due to the reversible transition of PNRs under the electric field, the electrostrain was greatly promoted. When the content of SREC was 0.012, large field-induced large strains of 0.38–0.44% were obtained under an electric field of 70 kV/cm, equivalent to a large signal d33* (Smax/Emax) of 543–586 pm/V. Moreover, relying on the luminescence effect of RE3+ ions in SREC, the modified ceramics exhibited strong photoluminescence with bright red or orange emissions, depending on the type of RE elements. However, the strong emission was strongly quenched in samples with a higher SREC content due to the photoluminescence quenching effect of the heavy metal (Co3+) ions in the SREC dopant. The quenching model can be based on the formation of non-radiative recombination centers induced by Co3+ ions, which affect the energy transition of RE3+ ions from the excited state to the ground state.

Effect of Cesium for Cu(In,Ga)(S,Se)$_{2}$ Solar Cells Using Photothermal Atomic Force Microscopy Under Various Photoexcitation Conditions

We have performed photothermal (PT) measurements by atomic force microscopy, by which nonradiative recombination of photoexcited carriers can be examined very locally, on Cu(In,Ga)(S,Se) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> (CIGSSe) materials in order to investigate CsF-treatment effects. From the PT signal images taken under various photoexcitation conditions and by comparing those images taken on CIGSSe samples with or without the CsF-treatment, we have found the possibility that the nonradiative recombination centers, which should exist along grain boundaries and may be distributed at a CdS/CIGSSe interface, are well passivated by the CsF-treatment.

Influence of silver nitrate in obtaining highly luminescent sodium-doped carbon dots for white light emitting diodes

Luminescent metal-doping carbon dots (CDs) possess excellent optical properties for light-converting phosphors of white light emitting diode (LED). However, the photoluminescence quantum yields (PL QYs) of CDs still suffer from the trapping defect at the surface of CDs which may generate the detrimental nonradiative recombination centers. Herein, we developed an N doping method to enhance blue light emission of Na+-doped CDs via a modified solid-state reaction, using silver nitrate as nitrogen source. The blue PL intensity form Na+-doped CDs is improved by the increasing molar ratio of silver nitrate precursor. The obtained Na+ and N co-doping CDs exhibited 5-fold enhancement of PL intensity with a PL QYs up to 24% as compared to their undoped counterpart. Based on the temporal evolution of PL and surface structure characteristics, it suggested that the improvement of PL QYs derived from the elimination of the trapping defects at the surface of Na+-doped CDs. This is realized by the introduction of nitrate groups and Ag+ ions, which can reduce the formation of hydroxyl radicals related nonradiative recombination centers. Furthermore, the Na+ and N co-doped CDs is applied with (Gd,Y)3(Ga,Al)5O12: Ce3+ yellow phosphor on a commercial UV-LED chip, achieving strong cold white emission. Our finding provides a new idea to enhance blue PL QYs of Na+-doped CDs for their further potential applications in optoelectronic devices.

Impact of doped barriers on the recombination coefficients of c-plane InGaN/GaN single quantum well light-emitting diodes

Differential carrier lifetime measurements were performed on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different QW indium compositions as well as with and without doped barriers. Mg-doped p-type and Si-doped n-type barriers close to the QW were used to reduce the net internal electric field in the QW, thereby improving the electron–hole wavefunction overlap on the LEDs. LEDs with doped barriers show short lifetimes and low carrier densities in the active region compared to the reference LEDs. The recombination coefficients in the ABC model were estimated based on the carrier lifetime and quantum efficiency measurements. The improvement in the radiative coefficients in the LEDs with doped barriers coupled with the blueshift of the emission wavelengths indeed indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect as a result of the reduced internal electric field. However, doped barriers also introduce non-radiative recombination centers and thereby increase the Shockley–Read–Hall (SRH) coefficient, although the increment is less for LEDs with high indium composition QWs. As a result, at high indium composition (22%), LEDs with doped barriers outperform the reference LEDs even though the trend is reversed for LEDs with lower indium composition (13.5%). Despite the trade-off of higher SRH coefficients, doped barriers are shown to be effective in reducing the internal electric field and increasing the recombination coefficients.

Room-temperature nonradiative recombination lifetimes in c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11 ≤ x ≤ 0.21)

Lattice-matched Al1−xInxN / GaN heterostructures with InN mole fraction (x) of 0.18 have attracted considerable interest for use in GaN-based optoelectronic devices. Because the light emission efficiency (ηemission) of Al1−xInxN alloys is far less than that of InxGa1−xN, understanding its causes is essential. For this purpose, room-temperature photoluminescence lifetime (τPLRT), which almost represents the nonradiative recombination lifetime that limits the internal quantum efficiency in low ηemission semiconductors, of c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11≤x≤0.21) was examined. For the epilayers grown on low threading dislocation density (TDD) GaN substrates (≪107cm−2), τPLRT principally decreased with increasing x, indicating a progressive increase in the concentration of nonradiative recombination centers (NRCs), NNRC. One of the probable causes is the growth temperature (Tg) reduction that is indispensable to incorporate more In, because in insufficient Tg regime higher Tg is preferred for enhancing the surface migration of adatoms to decrease the concentrations of vacancies that compose NRCs. The Al1−xInxN epilayers of the same x but grown on high TDD (&amp;gt;108cm−2) GaN-on-sapphire templates exhibited shorter τPLRT. Because the diffusion length of minority carriers was nearly zero in the Al1−xInxN epilayers, the shorter τPLRT indicates higher bulk NNRC in high TDD epilayers. The Al1−xInxN epilayers of considerably rough surface morphologies exhibited spatially inhomogeneous τPLRT, implying that excited carriers recombined everywhere at InN-rich to InN-poor portions, where NNRC were likely lower to higher, respectively, than the average due to the deviations in the surface stoichiometry at various non-c-plane surfaces at a given Tg.

Effects of the diameter of thermally generated nanopits on carrier dynamics in AlGaN/GaN heterostructures

The size and density of nanopits, generated at the surface of their top layer, strongly affect the electrical and optical properties of AlGaN-based structures. Therefore, the control of the layer quality evolution as a function of the nanopits size/density is a crucial issue to enhance the device performance. In this paper, the effects of the nanopits diameter observed at the surface of AlGaN on carrier dynamics are systematically investigated. The variation of nanopits diam-eter is achieved through thermal annealing of a set of AlGaN/GaN heterostructures at different temperatures. The samples are characterized using the scanning electron microscope (SEM), energy-dispersive x-ray, high-resolution x-ray diffraction, photoluminescence (PL), and time-resolved PL spectroscopies. SEM images have revealed an increase in the nanopits diameter with increasing annealing temperature. In addition, we observed a linear development in the yellow luminescence intensity, accompanied by a deterioration in the PL decay times due to an increase in the density of point-defect complexes that act as nonradiative recombination centers. We also performed temperature-dependent PL measurements to study the impact of the nanopits diameter on electron–phonon scattering processes. Both electron-acoustic- and electron-longitudinal optical phonon interactions enhance with increasing nanopits diameter.