Generation Of Non-radiative Recombination Centers Research Articles

Effects of low fluence 212 MeV Ge swift heavy ion irradiation on the structural and optical properties of β-Ga2O3 epitaxial layers

Considering the future applications of the ultra-wide bandgap semiconductor β-Ga2O3 in high-performance electronic devices and aerospace, the effects of low fluence 212 MeV Ge swift heavy ion (SHI) irradiation on the structural and optical properties of β-Ga2O3 epitaxial layers have been studied in this work. The alternations in Raman scattering properties of the epitaxial layer after Ge SHI irradiation indicate the generation of lattice defects. The increased concentration of oxygen vacancies after irradiation leading to an enhanced area ratio of the yellow-red emission band in the PL spectrum of the β-Ga2O3 epitaxial layer. Simultaneously, the PL intensity quenches as the irradiation fluence increases, attributed to the generation of non-radiative recombination centers. Furthermore, HRTEM analysis provides direct evidence of lattice damage caused by 212 MeV Ge SHI irradiation to the samples. The results demonstrate that the spectral behaviors of β-Ga2O3 epitaxial layers can be modulated by low fluence Ge SHI irradiation with less damage caused to the overall crystalline quality and structure.

A Review of the Reliability of Integrated IR Laser Diodes for Silicon Photonics

With this review paper we provide an overview of the main degradation mechanisms that limit the long-term reliability of IR semiconductor lasers for silicon photonics applications. The discussion is focused on two types of laser diodes: heterogeneous III–V lasers bonded onto silicon-on-insulator wafers, and InAs quantum-dot lasers epitaxially grown on silicon. A comprehensive analysis of the reliability-oriented literature published to date reveals that state-of-the-art heterogeneous laser sources share with conventional laser diodes their major epitaxy-related degradation processes, such as the generation of non-radiative recombination centers or dopant diffusion, while eliminating cleaved facets and exposed mirrors. The lifetime of InAs quantum dot lasers grown on silicon, whose development represents a fundamental step toward a fully epitaxial integration of future photonic integrated circuits, is strongly limited by the density of extended defects, mainly misfit dislocations, protruding into the active layer of the devices. The concentration of such defects, along with inefficient carrier injection and excessive carrier overflow rates, promote recombination-enhanced degradation mechanisms that reduce the long-term reliability of these sources. The impact of these misfits can be largely eliminated with the inclusion of blocking layers.

Structure and luminescence of a-plane GaN on r-plane sapphire substrate modified by Si implantation* *Project supported by the Key-Area Research and Development Program of Guangdong Province, China (Grant Nos. 2019B010132001, 2020B010174003, and 2019B121204004), the Basic and Application Basic Research Foundation of Guangdong Province, China (Grant Nos. 2020A1515110891 and 2019A1515111053), and the Fund from the Ion Beam Center (IBC) at HZDR.

We show the structural and optical properties of non-polar a-plane GaN epitaxial films modified by Si ion implantation. Upon gradually raising Si fluences from 5 × 1013 cm−2 to 5 × 1015 cm−2, the n-type dopant concentration gradually increases from 4.6 × 1018 cm−2 to 4.5 × 1020 cm−2, while the generated vacancy density accordingly raises from 3.7 × 1013 cm−2 to 3.8 × 1015 cm−2. Moreover, despite that the implantation enhances structural disorder, the epitaxial structure of the implanted region is still well preserved which is confirmed by Rutherford backscattering channeling spectrometry measurements. The monotonical uniaxial lattice expansion along the a direction (out-of-plane direction) is observed as a function of fluences till 1 × 1015 cm−2, which ceases at the overdose of 5 × 1015 cm−2 due to the partial amorphization in the surface region. Upon raising irradiation dose, a yellow emission in the as-grown sample is gradually quenched, probably due to the irradiation-induced generation of non-radiative recombination centers.

Modeling the degradation mechanisms of AlGaN-based UV-C LEDs: from injection efficiency to mid-gap state generation

In this work, we analyze and model the effect of a constant current stress on an ultraviolet light-emitting diode with a nominal wavelength of 285 nm. By carrying out electrical, optical, spectral, and steady-state photocapacitance (SSPC) analysis during stress, we demonstrate the presence of two different degradation mechanisms. The first one occurs in the first 1000 min of stress, is ascribed to the decrease in the injection efficiency, and is modeled by considering the defect generation dynamics related to the de-hydrogenation of gallium vacancies, according to a system of three differential equations; the second one occurs after 1000 min of stress and is correlated with the generation of mid-gap defects, for which we have found evidence in the SSPC measurements. Specifically, we detected the presence of deep-level states (at 1.6 eV) and mid-gap states (at 2.15 eV), indicating that stress induces the generation of non-radiative recombination centers.

Multiple Angle Analysis of 30-MeV Silicon Ion Beam Radiation Effects on InGaN/GaN Multiple Quantum Wells Blue Light-Emitting Diodes

High-resolution X-ray diffraction, temperature-dependent photoluminescence (PL), time-resolved PL, and positron annihilation spectroscopy are employed to investigate the degradation mechanism of InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) under silicon ion irradiation. Reduction of the quantum-confined Stark effect due to crystalline strain relaxation, enhancement of indium localization due to thermal spike, generation of nonradiative recombination centers (NRCs), and carrier removal effect due to atom displacement are revealed to be critical factors in LEDs postirradiation performance. New NRCs are proven to be the main reason for the degradation of the internal quantum efficiency of MQWs. The increase of the threshold voltage and leakage current in LEDs are caused by the carrier removal effect and new defects in bandgap induced by radiation. In addition, new NRCs are found to appear earlier than indium localization and carrier removal effect with increasing silicon ion fluence. Atom displacement defects are revealed to be located mainly in p-type GaN and MQWs layers. Radiation-induced nitrogen vacancies are considered compensation donors in p-type GaN, whereas all other nitrogen and gallium-related defects are NRCs in MQWs.

A Comparative Study of InGaN/GaN Multiple‐Quantum‐Well Solar Cells Grown on Sapphire and AlN Template by Metalorganic Chemical Vapor Deposition

Two kinds of substrates, sapphire and AlN/sapphire template (AlN template), are used for the growth of InGaN/GaN multi‐quantum‐well solar cell structures by metalorganic chemical vapor deposition, and their material and device properties are investigated. The results show that the samples grown on AlN template have a better crystal quality with a larger in‐plane compressive strain than those on sapphire, and solar cells fabricated on sapphire mostly exhibit better performance than those on AlN template. An analysis of the photoluminescence measurements indicates that a critical InGaN well thickness related to the generation of nonradiative recombination centers, which affects the internal and external quantum efficiencies, is thinner in samples grown on AlN template than in samples on sapphire. The critical thickness is speculated to be related to the large in‐plane compressive strain in the samples on AlN template. By contrast, in comparison between samples with a sufficiently thin InGaN well thickness of 1.0 nm, the sample on AlN template exhibits better solar cell performance than on sapphire. This implies that the improved crystal quality contributes to the improvement of internal quantum efficiency as long as the well layer is thinner than the critical thickness.

Investigation of InGaN/GaN laser degradation based on luminescence properties

Degradation of InGaN/GaN laser diode (LD) is investigated based on the luminescence properties. Gradual degradation of the LD is presented with the threshold current increase and the slope efficiency decrease. The cathodoluminescence and photoluminescence characterizations of the LD show a dislocation independent degradation of the active region under the ridge. Detailed studies on the temperature-dependent micro-photoluminescence and the electroluminescence indicate that the degradation of the LD is attributed to the generation of non-radiative recombination centers in the local multiple quantum well regions with lower indium content. The activation energy of the non-radiative recombination centers is about 10.2 meV.

Optical characterization of voltage-accelerated degradation in CH3NH3PbI3 perovskite solar cells.

We investigate the performance degradation mechanism of CH3NH3PbI3 perovskite solar cells under bias voltage in air and nitrogen atmospheres using photoluminescence and electroluminescence techniques. When applying forward bias, the power conversion efficiency of the solar cells decreased significantly in air, but showed no degradation in nitrogen atmosphere. Time-resolved photoluminescence measurements on these devices revealed that the application of forward bias in air accelerates the generation of non-radiative recombination centers in the perovskite layer buried in the device. We found a negative correlation between the electroluminescence intensity and the injected current intensity in air. The irreversible change of the perovskite grain surface in air initiates the degradation of the perovskite solar cells.

Surface modification in mixture of ZnO + 3%C nanocrystals stimulated by mechanical processing

The photoluminescence (PL), Raman scattering and SEM images for the mixture of ZnO + 3% C nanocrystals (NCs) have been studied before and after of intensive mechanical processing (MP) with the aim to identify the nature of defects. The study reflects the diversity of physical processes occurring at MP: amorphizating the surface of ZnO NCs, crushing the individual ZnO NCs and carbon nanoparticles, covering the ZnO NC surface by the graphene layers, the oxidation partially of the graphene layers, carbon and ZnO NCs etc. Three stages of MP have been revealed which are accompanied by PL spectrum transformations: i) amorphizating the ZnO NC surface together with the generation of nonradiative recombination centers, ii) passivating the ZnO NC surface by the graphene layer with its oxidation partially and iii) further crushing of ZnO NCs, the oxidation of ZnO NCs and the formation of graphene (graphite) oxides. The new PL band peaked at 2.88 eV has been detected after 9 min of MP. Note that the passivation of the ZnO NC surface by graphene layer can be interesting for future technological applications.

Influence of growth temperature, working gas ratio, and buffer layer in ZnO films grown on (001) Si substrates by using rf-sputtering

We report on an extensive research result about the relationship between the growth parameters of growth temperature, working gas ratio, and Low-temperature (LT) ZnO buffer and the physical properties of surface properties, structural properties, and optical properties in the ZnO films grown on (001) Si substrates by rf-sputtering. In the substrate temperature range of room temperature (RT) - 500 °C, a higher temperature is found to lengthen the surface migration lengths of Zn and O atoms and to promote their surface reaction. In the O2/Ar+O2 ratio range of 0–80%, a higher O2/Ar+O2 ratio is found to suppress the generation of nonradiative recombination centers due to the nonstoichiometric point defects such as O vacancies and Zn interstitials. The buffer layers deposited at RT are found to be partially crystallized by annealing at a high temperature and to be able to serve as seeds for the following ZnO film growth. As a result, the ZnO films fabricated at a substrate temperature of 500 °C and an O2/Ar+O2 ratio of 80% with a LT-ZnO buffer annealed under an O2 ambient of 10 mTorr have the highest crystalline quality.

Enhanced room temperature ferromagnetism and photoluminescence behavior of Cu-doped ZnO co-doped with Mn

Cu, Mn co-doped ZnO nanoparticles were successfully synthesized by the sol–gel technique. XRD pattern described that Mn-doping did not affect the hexagonal wurtzite structure of the samples and no secondary phases were found. The reduced crystallite size at Mn=2% is due to the suppression of grain surface growth by foreign impurity. The enhancement of crystal size after Mn=2% is due to the expansion of lattice volume produced by the distortion around the dopant ion. The better dielectric constant and conductivity noticed at Mn=2% are explained by charge carrier density and crystallite size. The suppression of broad UV band by Mn-doping is discussed based on the generation of non-radiative recombination centers. Hysteresis loop showed the clear room temperature ferromagnetism in all the samples and the magnetization increased with Mn-doping. Better electrical and magnetic behavior of Zn0.94Cu0.04Mn0.02O sample is suggested for effective opto-magnetic devices.

Recoverable degradation of blue InGaN-based light emitting diodes submitted to 3 MeV proton irradiation

This paper reports on the degradation and recovery of two different series of commercially available InGaN-based blue light emitting diodes submitted to proton irradiation at 3 MeV and various fluences (1011, 1013, and 1014 p+/cm2). After irradiation, we detected (i) an increase in the series resistance, in the sub-turn-on current and in the ideality factor, (ii) a spatially uniform drop of the output optical power, proportional to fluence, and (iii) a reduction of the capacitance of the devices. These results suggest that irradiation induced the generation of non-radiative recombination centers near the active region. This hypothesis is further confirmed by the results of the recovery tests carried out at low temperature (150 °C).

Further Investigations into the Effects of Temperature on a 975 nm Tapered Laser Bar Using Convolution to Ascertain the Dominant Effect of Temperature on a Laser Bar

Temperature continues to be an issue in the reliability of high power laser diodes. Any effort, therefore, made to understand the dynamics of temperature on the performance of laser diodes is important. This is because it serves as a catalyst for the generation of nonradiative recombination centers, which ultimately kills laser diodes. In this paper, the convolution of simulation results was done to compare it with experimental results, which revealed more details that hitherto, could not have been captured by experiments alone. The inability of experiments to capture these details is due to the limited spatial resolution of the charged-coupled device (CCD) camera, which is approximately 30 µm much larger than the thickness of the quantum well active region used in the experiment. The convolution results showed a further smile-shaped profile within the four groups of array emitters, which in the experimental results, were considered as one emitter due to the limited spatial resolution of the CCD camera. The use of convolution to determine more details was investigated, due to the dominant effect of temperature found across the output power distribution of high power semiconductor laser diode bars.

Optical investigation of degradation mechanisms in AlGaN/GaN high electron mobility transistors: Generation of non-radiative recombination centers

Degradation mechanisms in AlGaN/GaN high electron mobility transistors have been studied under pinch-off conditions. Sites of localized emission of electroluminescence (EL) in the form of hotspots, known to be related to gate leakage currents, are shown to be the result of the generation of non-radiative recombination centers in the AlGaN device layer during device stress. EL from the hotspot site contains both hot-carrier emission from the acceleration of charge carriers in the device channel and defect-related transitions. Gate leakage through the generated centers is the most likely mechanism for the observation of EL hotspots.

Thermal stability of Si-doped InGaN multiple-quantum wells for high efficiency light emitting diodes

Excitation power dependent photoluminescence (PL) spectroscopy was employed to determine the thermal degradation of InGaN quantum wells (QWs) structure with different Si doping in well region. At a low excitation power density, PL intensity of undoped InGaN well was significantly decreased, while those of interfacial and full Si-doped InGaN well were slightly reduced by rapid thermal annealing (RTA) process. However, PL measurement with high excitation power density showed that PL intensities of InGaN QWs regardless of Si doping were almost similar with/without RTA process. In addition, x-ray diffraction results indicated that Si-doping in well could improve the interfacial quality of InGaN QWs. Therefore, we suggest that Si doping suppress the generation of nonradiative recombination centers by thermal degradation at weaker localization states which could be easily filled by low excitation carriers.

Influence of doping on the reliability of AlGaInP LEDs

The aging behavior of red-orange AlGaInP light-emitting diodes (LEDs) is investigated. It is found that the amount of magnesium and tellurium doping as well as of the oxygen incorporation influence the device degradation. The generation of non-radiative recombination centers in the active layer is responsible for the decrease of the LED light output during operation. The kinetics of the light aging as well as the configuration of the non-radiative recombination centers emerging during degradation depend on the doping species. High stress temperatures can accelerate the device degradation. For the aging of Te-rich device, the activation energy of 660 meV for the degradation process is determined. Deep level transient spectroscopy is used to investigate the properties of the non-radiative recombination centers. Four deep traps are detected in aged devices. One trap with activation energy of about 1 eV is found in all aged devices. In the aged O-rich device, one additional deep trap is observed with slightly lower activation energy. In the aged Te-rich device two additional shallower traps emerge during degradation.

Spatial fluctuation enhancement and nonradiative-recombination-center generation due to high Si-doping into GaAs/AlAs short-period-superlattices

Through the time-resolved photoluminescence (TR-PL) measurement for excitons, we have studied the enhancement of spatial fluctuation (SF) and the generation of nonradiative-recombination-centers (NRC) due to high Si-doping into GaAs/AlAs short-period-superlattices (SPS's). We have carried out the exciton transport analysis according to Krivorotov et al. [I.N. Krivorotov, T. Chang, G.D. Gilliland, L.P. Fu, K.K. Bajaj, Phys. Rev. B 58 (1998) 10687]. From this analysis, we have obtained the temperature dependence of the exciton diffusivity, the concentration of the NRC and the average distant between adjacent localized states of excitons. The temperature dependence of the exciton diffusivity is found to be given by the sum of the temperature-independent contribution and the activation-type contribution. For the exciton diffusivity in undoped GaAs/AlAs SPS's, only the activation-type contribution has been observed. Therefore, we point out the possibility that the temperature-independent contribution comes from the tunneling through the impurities. In this experiment, the activation energy and the concentration of the NRC are found to be larger than those of undoped GaAs/AlAs SPS's. We infer that high Si-doping into GaAs/AlAs SPS causes the enhancement of the SF and the generation of NRC.

33.1: Invited Paper: OLED Aging Mechanisms: from Fluorescence Quenchers to Nonradiative Recombination Centers

AbstractA recently introduced technique to measure fixed charge densities inside OLEDs was applied to study the electrical aging phenomenon. Strong linear correlations between luminance efficiency and deeply trapped positive charge were observed. Correlations between these non‐trivially evolving quantities in OLEDs of various chemical composition were interpreted in terms of a common degradation mechanism— generation of nonradiative recombination centers.

Failure Modes and Mechanisms of DC-Aged GaN LEDs

This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.