Nonradiative Recombination Centers Research Articles

Low‐Dimensional Contact Layers for Enhanced Perovskite Photodiodes

AbstractControlling defects and energy‐band alignments are of paramount importance to the development of high‐performance perovskite‐based photodiodes. Yet, concurrent improvements in interfacial contacts and defect reduction simply by tailoring bottom contacts have not been investigated. An effective strategy is reported that can simultaneously improve energy‐band alignments and structural defects by introducing low‐dimensional contact (LDC) layers at the bottom interface. It is found that LDC‐based perovskites considerably suppress undesirable structural defects induced by microstrains, resulting in reduced nonradiative recombination centers and improved carrier lifetimes. Additionally, the resulting LDC‐based interface structures help block minority carrier injection from the electrodes by forming built‐in electric fields. As a consequence, LDC‐based perovskite photodiodes showed improved light detection capabilities. The result opens an avenue to yield highly efficient photodiodes.

Suppressing the luminescence of Vcation-related point-defect in AlGaN grown by MOCVD on HVPE-AlN

AlGaN materials have a great prospect in ultraviolet and deep-ultraviolet optoelectronic devices, while the point-defects impede their applications. In this report, we successfully suppressed the luminescence of Vcation-related point-defect in AlGaN materials. AlGaN epilayers were grown on AlN/sapphire template by metal-organic chemical vapor deposition and the mixed metal-organic flows were used to pretreat the surface of AlN/sapphire template. The luminescence intensity of (Vcation-complex)2− point-defects was reduced by the pretreatment, demonstrating the favorable suppression effect on the luminescence of (Vcation-complex)2− point-defects. The ephemeral metal-rich condition and metal-droplets on the AlN/sapphire template were believed to be partially responsible for the suppression. It also supported the conception that the 3.90 eV luminescence in AlN originated from the Vcation-related point-defects rather than the CN point-defects. The surface morphology was investigated and an optimized pretreating time of 60 s was obtained. The carrier recombination mechanism was also studied and the results revealed that dislocations not only could act as non-radiative recombination centers, but also could bind the excitons.

Temperature dependence of nonradiative recombination processes in UV-B AlGaN quantum well revealed by below-gap excitation light

Nonradiative recombination (NRR) processes through defect states and their temperature dependence in UV-B AlGaN MQW sample on sapphire substrate grown by MOCVD technique have been studied by photoluminescence (PL) spectroscopy. We detected NRR centers by adding a below-gap excitation light with photon energies from 0.93 eV to 1.46 eV on an above-gap excitation light of 4.66 eV. All the BGE energies decreased PL intensity at 25 K, and the most-distinct quenching is observed by 1.27 eV BGE at the same BGE photon number density. The temperature-dependent PL intensity for the BGE energy of 1.27 eV is interpreted by three NRR centers. The one-level model dominates over that of two-level model in the temperature range 58 K < T < 88 K. The two-level model prevails in other region of temperature. The combination of one-level and two-level models is consistent with the spectral peak-energy shift as a function of temperature.

Iodine interstitials as a cause of nonradiative recombination in hybrid perovskites

The identification of deep-level defects that act as detrimental nonradiative recombination centers is critical for optimizing the optoelectronic performance of hybrid perovskites. Although extensive studies have been devoted to revealing the nature of deep-level defects in hybrid perovskites, it is still unclear what defects are responsible for the experimentally observed nonradiative recombination rates. Employing first-principles approaches, we quantitatively show that the iodine interstitial is a dominant nonradiative recombination center in methylammonium-lead iodide. This important insight points to a target for defect engineering in order to further improve the performance of hybrid perovskites.

Using Cross-Sectional Cathodoluminescence to Visualize Process-Induced Defects in GaN-Based High Electron Mobility Transistors

We performed cross-sectional cathodoluminescence (CL) for gallium nitride (GaN)-based high electron mobility transistors (HEMTs) to investigate process-induced defects. The cross-sectional CL measurements at room temperature clearly show the intensity distribution for the 25 nm-thick AlGaN layer, even though it is very thin. We also observed that the intensities of the band-edge emissions from the AlGaN and GaN layers fall near the source and drain regions. Those intensity decays demonstrate that ion implantation at the source and drain regions generates many nonradiative recombination centers and that these defects are not eliminated even by activation annealing after ion implantation. We also observed that the intensity of yellow luminescence (YL) in the GaN layer drops not only at the source and drain regions, but also in deeper regions of the 1.4 μm-thick GaN layer. We consider diffusion of the implantation-generated defects and their interactions with point defects in this deep region, which are responsible for the YL, to be the mechanism responsible for the YL intensity decrease. Ion implantation and subsequent annealing not only activates the dopant atoms, but also changes the point defects, which may affect the device characteristics. These findings show that cross-sectional CL spectral mapping can visualize the process-induced defects in GaN HEMTs and can therefore be used for process optimization and failure analysis of these devices.

Characterization of carrier transport behavior of specific type dislocations in GaN by light assisted KPFM

The direct characterization of carrier transport behavior at different types of dislocations in GaN is still questionable due to the current lack of feasible strategy. Herein, we developed a method by combining ultraviolet light-assisted Kelvin probe force microscope with defect selective etching technology. The dislocation types could be confirmed by the shape of the etching pit, and the photogenerated carrier recombination behavior at specific dislocation. Thus, it can be characterized by ultraviolet light-assisted Kelvin probe force microscope, which paves the way for analysis of carrier transport behavior at different types of dislocations in GaN. The screw dislocations are found to be the main non-radiative recombination centers, and mainly responsible for leakage current in GaN based device. This shows higher surface potential and higher electron concentration due to the donor defects introduced during dislocation growth. Conversely, a barrier is generated around the edge and mixed dislocations, which could suppress the non-radiative recombination at the dislocation. The present work provides a feasible way to direct characterization of specific type dislocations in GaN. Moreover, this method can be used to characterize other III-nitride materials.

Temperature‐Dependent Charge Carrier Diffusion in [0001¯] Direction of GaN Determined by Luminescence Evaluation of Buried InGaN Quantum Wells

Temperature‐dependent transport of photoexcited charge carriers through a nominally undoped, c‐plane GaN layer toward buried InGaN quantum wells is investigated by continuous‐wave and time‐resolved photoluminescence spectroscopy. The excitation of the buried InGaN quantum wells is dominated by charge carrier diffusion through the GaN layer; photon recycling contributes only slightly. With temperature decreasing from 310 to 10 K, the diffusion length in direction increases from 250 to 600 nm in the GaN layer. The diffusion length at 300 K also increases from 100 to 300 nm when increasing the excitation power density from 20 to 500 W cm−2. The diffusion constant decreases from the low‐temperature value of ∼7 to 1.5 cm2 s−1 at 310 K. The temperature dependence of the diffusion constant indicates that the diffusivity at room temperature is limited by optical phonon scattering. Consequently, higher diffusion constants in GaN‐based devices require a reduced operation temperature. To increase diffusion lengths at a fixed temperature, the effective recombination time has to be prolonged by reducing the number of nonradiative recombination centers.

Enhanced performance of InAs/GaAs quantum dot superluminescent diodes by direct Si-doping

It is necessary to improve the output power and spectral width of superluminescent diodes (SLDs) simultaneously. In this paper, we show that both the output power and the spectral width of the SLDs based on InAs/GaAs quantum dots (QDs) can be significantly enhanced by direct Si-doping in the QDs. The maximum output power of the Si-doped QD-SLD reaches 20.5 mW at an injection current of 570 mA, while that of the undoped one with an identical structure is only 17.8 mW at the injection current of 550 mA. Moreover, the broadest spectral width of the doped QD-SLD is 105 nm, while that of the undoped QD-SLD is 93 nm. The enhanced performance of the doped QD-SLDs can be attributed to the direct Si doping that leads to inactivating the nonradiative recombination centers within or near the QDs and provides excess carriers to occupy the higher excited states.

Effect of dislocation density on optical gain and internal loss of AlGaN-based ultraviolet-B band lasers

We investigated the dependence of the lasing threshold power density, optical gain, and internal loss on the dislocation density of an optically pumped AlGaN-based ultraviolet-B band laser. Reducing the dislocation density was found to not only increase the optical gain by reducing the non-radiative recombination centers but also reduce the internal loss. Furthermore, this reduction in internal loss was appropriately explained using an increased scattering model based on an increase in the refractive index fluctuations formed by the dislocations.

Effect of low-temperature GaN cap layer thickness on the optoelectronic performance of InGaN green LEDs with V-shape pits

The low-temperature GaN cap layer was designed after quantum wells for the V-pits contained InGaN-based green light-emitting diodes (LEDs). It is found that the LEDs’ performances are greatly improved with a thicker cap layer. The indium distribution in InGaN/GaN multiple quantum wells (MQWs) becomes more homogeneous. The quality of interface between the quantum well and quantum barrier is promoted, and the carrier confinement effect is enhanced. Thereby the external quantum efficiency (EQE) increases at high current density. However, the sample with a relative thicker cap layer exhibits the lower EQE at low current density, which is attributed to there being more defects at the upper interface of QWs, and the increased probability of carriers meeting at these non-radiative recombination centers. Meanwhile, only the major emission peak emitted from c-plane MQWs can be observed in the electroluminescence spectrum of 100 K, and other emission peaks are hardly visible, which indicates that the sample with the thicker cap has better carrier transport performance.

Superlattice period dependence on nonradiative recombination centers in the n-AlGaN layer of UV-B region revealed by below-gap excitation light

Nonradiative recombination (NRR) centers in n-AlGaN layers of UV-B AlGaN samples with different numbers of superlattice (SL) periods (SLPs), grown on the c-plane sapphire substrate at 1150 °C by the metalorganic chemical vapor deposition technique, have been studied by using below-gap-excitation (BGE) light in photoluminescence (PL) spectroscopy at 30 K. The SLP affects the lattice relaxation of the SL and n-AlGaN layer. The PL intensity decreased by the superposition of BGE light of energies from 0.93 eV to 1.46 eV over the above-gap-excitation light of energy 4.66 eV, which has been explained by a two-level model based on the Shockley–Read–Hall statistics. The degree of PL quenching from n-AlGaN layers of the sample with SLP 100 is lower than those of other samples with SLP 50, 150, and 200. By a qualitative simulation with the dominant BGE energy of 1.27 eV, the density ratio of NRR centers in n-AlGaN layers of 50:100:150:200 SLP samples is obtained as 1.7:1.0:6.5:3.4. This result implies that the number of SLP changes lattice relaxation and determines the density of NRR centers in the n-AlGaN layer, which affects the performance of LEDs.

Optical Investigation of Proton‐Irradiated Metal Organic Chemical Vapor Deposition AlGaN/GaN High‐Electron‐Mobility Transistor Structures

About 2 MeV proton numerical calculation damage indicates that a quasiflat profile is introduced into the GaN buffer layer of high‐electron‐mobility transistor (HEMT) templates. A previous transport study of such HEMT structures showed increased breakdown voltage and reduced GaN buffer layer leakage after proton irradiation. Hyperspectral electroluminescence measurements detected the emission band in the spectral region between 700 and 800 nm. This emission is assumed to be associated with the generation of trap levels responsible for the device failure. To obtain insights into the nature of radiation‐generated traps, detailed low‐temperature photoluminescence (PL) and cathodoluminescence (CL) experiments are conducted in a set of virgin and proton‐irradiated device structure templates. Both the PL and CL spectra show large intensity variations of all emission bands, in the spectral range between 330 and 890 nm, with increasing proton dosage. This is consistent with the introduction of compensating and/or nonradiative recombination centers in the GaN buffer layer. Luminescence measurements carried out under various excitation conditions show different recombination rates for emission bands observed near 420 and 550 nm, indicating different free carrier capture rates. Spectral analysis suggests that these two emission bands have different dependences on proton irradiation dosage, consistent with different chemical natures.

Structural Features and Photoelectric Properties of Si-Doped GaAs under Gamma Irradiation.

GaAs has been demonstrated to be a promising material for manufacturing semiconductor light-emitting devices and integrated circuits. It has been widely used in the field of aerospace, due to its high electron mobility and wide band gap. In this study, the structural and photoelectric characteristics of Si-doped GaAs under different gamma irradiation doses (0, 0.1, 1 and 10 KGy) are investigated. Surface morphology studies show roughen of the surface with irradiation. Appearance of transverse-optical (TO) phonon mode and blueshift of TO peak reflect the presence of internal strain with irradiation. The average strain has been measured to be 0.009 by Raman spectroscopy, indicating that the irradiated zone still has a good crystallinity even at a dose of 10 KGy. Photoluminescence intensity is increased by about 60% under 10 KGy gamma irradiation due to the strain suppression of nonradiative recombination centers. Furthermore, the current of Si-doped GaAs is reduced at 3V bias with the increasing gamma dose. This study demonstrates that the Si-doped GaAs has good radiation resistance under gamma irradiation, and appropriate level of gamma irradiation can be used to enhance the luminescence property of Si-doped GaAs.

Core/Shell Quantum Dots Solar Cells

AbstractSemiconductor nanocrystals, the so‐called quantum dots (QDs), exhibit versatile optical and electrical properties. However, QDs possess high density of surface defects/traps due to the high surface‐to‐volume ratio, which act as nonradiative carrier recombination centers within the QDs, thereby deteriorating the overall solar cell performance. The surface passivation of QDs through the growth of an outer shell of different materials/compositions called “core/shell QDs” has proven to be an effective approach to reduce the surface defects and confinement potential, which can enable the broadening of the absorption spectrum, accelerate the carrier transfer, and reduce exciton recombination loss. Here, the recent research developments in the tailoring of the structure of core/shell QDs to tune exciton dynamics so as to improve solar cell performance are summarized. The role of band alignment of core and shell materials, core size, shell thickness/compositions, and interface engineering of core/thick shell called “giant” QDs on electron–hole spatial separation, carrier transport, and confinement potential, before and after grafting on the carrier scavengers (semiconductor/electrolyte), is described. Then, the solar cell performance based on core/shell QDs is introduced. Finally, an outlook for the rational design of core/shell QDs is provided, which can further promote the development of high‐efficiency and stable QD sensitized solar cells.

Performance deterioration of GaN-based laser diode by V-pits in the upper waveguide layer

Abstract The effect of V-pits in the upper waveguide (UWG) on device performance of GaN-based laser diodes (LDs) has been studied. Experimental results demonstrate that in comparison with the LDs with u-In0.017Ga0.983N/u-GaN multiple UWG or u-In0.017Ga0.983N one, the LDs with a single u-GaN UWG has the best device performance. They have a smaller threshold current density, and a larger and more stable output optical power. The lowest threshold current density is as low as 1.3 kA/cm2, and the optical power reaches to 2.77 W. Furthermore, atomic force microscopy suggests that the deterioration of device performance of former kinds of devices may be attributed to the increase of V-pits’ size and quantity in the undoped-In0.017Ga0.983N UWG layer, and these V-pits could introduce more nonradiative recombination centers and exacerbate the inhomogeneity of injection current. Moreover, theoretical calculation results indicate that the increase of leakage current and optical loss are additional reasons for the device performance deterioration, which may be caused by a reduction of the potential barrier height for electrons in the quantum wells and by an increased background electron concentration in UWG.

Synergy between Ion Migration and Charge Carrier Recombination in Metal-Halide Perovskites

Charge carrier recombination plays a vital role in the CH3NH3PbI3 perovskite solar cell. By investigating a possible synergy between ion migration and charge carrier recombination, we demonstrate that the nonradiative recombination accelerates by an order of magnitude during iodide migration. The migration induces lattice distortion that brings electrons and holes close to each other and increases their electrostatic interactions. The wave function localization in the same spatial region, and the enhanced lattice and iodide movements increase the nonadiabatic coupling. At the same time, quantum coherence lasts longer, because electron and hole energy levels become correlated. All these factors greatly increase the recombination rate. Moreover, the energy level of the iodide vacancy created during the migration moves from inside the conduction band in the equilibrated structure into the band gap, acting as a typical efficient nonradiative charge recombination center. Our work shows that the different dynamic processes are strongly correlated in halide perovskites and demonstrates that defects, considered to be benign, can become very detrimental under non-equilibrium conditions. The reported results strongly suggest that ion migration should be avoided in halide perovskites, both for its own reasons, such as the large current-voltage hysteresis, and because it greatly accelerates charge carrier losses.

Mixed-Dimensional Naphthylmethylammonium-Methylammonium Lead Iodide Perovskites with Improved Thermal Stability

Metal halide perovskite solar cells, despite achieving high power conversion efficiency (PCE), need to demonstrate high stability prior to be considered for industrialization. Prolonged exposure to heat, light, and moisture is known to deteriorate the perovskite material owing to the breakdown of the crystal structure into its non-photoactive components. In this study, we show that by combining the organic ligand 1-naphthylmethylammonium iodide (NMAI) with methylammonium (MA) to form a mixed dimensional (NMA)2(MA)n−1PbnI3n+1 perovskite the optical, crystallographic and morphological properties of the newly formed mixed dimensional perovskite films under thermal ageing can be retained. Indeed, under thermal ageing at 85 °C, the best performing (NMA)2(MA)n−1PbnI3n+1 perovskites films show a stable morphology, a low PbI2 formation rate and a significantly reduced variation of both MA-specific vibrational modes and fluorescence lifetimes as compared to the pristine MAPbI3 films. These results highlight the role of the bulky NMA+ organic cation in mixed dimensional perovskites to both inhibit the MA+ diffusion and reduce the material defects, which act as non-radiative recombination centres. As a result, the thermal stability of metal halide perovskites has been substantially improved.

Near-unity photoluminescence quantum yield in inorganic perovskite nanocrystals by metal-ion doping.

The luminescence and charge transport properties of inorganic CsPbX3 perovskite nanocrystals (NCs) make them attractive candidates for various optoelectronic applications, such as lasing, X-ray imaging, light communication, and light-emitting diodes (LEDs). However, to realize cutting-edge device performance, high-quality NCs with high photoluminescence quantum yields (PLQYs) are essential. Therefore, substantial efforts and progress have been made to attain superior design/engineering and optimization of the inorganic NCs with a focus on surface quality, reduced nonradiative charge carrier recombination centers, and improved colloidal stabilities. Metal-ion doping has been proven to have a robust influence on the electronic band structure, PL behavior, and charge carrier recombination dynamics. Thus, in this perspective, we summarize the recent progress of the significant impact of metal cation doping on the optical properties, including the PL enhancement of CsPbCl3, CsPbBr3, and CsPbI3 perovskite NCs. Moreover, we shed light on the mechanism behind such improved properties. We conclude by recommending possible aspects and strategies to be further explored and considered for better utilization of these doped NCs in thin-film optoelectronic and energy conversion devices.

Statistical Analysis of the Photoelectric Characteristics for InGaN/GaN MQWs Solar Cells Following Proton Irradiation

The statistical photoelectric properties of InGaN/GaN MQWs solar cells under proton irradiation were studied. Three groups of devices were irradiated with 3 MeV proton beams with fluences of 1.0 × 1011 cm−2, 5.0 × 1011 cm−2, and 1.0 × 1012 cm−2. Averaged experiment results showed that the open circuit voltage (VOC) and fill factor (FF) always decreased. In contrast, the short circuit current density (JSC) and photoelectric conversion efficiency (η) increased first and then decreased with the increase of fluence. Firstly, the donor-like defects of InGaN layer in active region played a major role in increasing JSC and η when the fluence was relatively low. Due to the introduction of donor-like defects, extra free electrons were produced, which lead to an increase in the number of effective carriers. With the further increasing of proton fluence, the number of free electrons caused by donor-like defects in the used InGaN material (low indium component) increased slowly due to pinning effect. However, additional point defects (Ga vacancies) increased significantly. Ga vacancies can act as effective non-radiative recombination centers, thereby reducing the number of effective carriers. The combined effect of pinning effect and non-radiative recombination process is responsible for the first increase and subsequent decline of JSC and η.

Perspective: Toward highly stable electroluminescent quantum dot light-emitting devices in the visible range

With significant improvements in external quantum efficiency (EQE) and stability for red, green, and blue devices over the past decade, the future of electroluminescent quantum dot light-emitting devices (QDLEDs) is bright. State-of-the-art QDLEDs have achieved &amp;gt;30% EQE and a &amp;gt;2 000 000 h electroluminescence half-life for an initial luminance of 100 cd m−2, rivaling those of organic light-emitting devices. To date, most of the improvements in QDLED performance have been primarily achieved via advancements in QD synthesis and design that aim at reducing Auger recombination and improving the balance between electron and hole concentrations in the emissive QD layer. However, recent work is starting to reveal the critical role that other device layers, as well as interlayer interfaces, play in limiting QDLED stability. Degradation within the organic hole transport layer (HTL) and near the QD/HTL interface has recently been found to lead to the formation of nonradiative recombination centers that quench excitons in the emissive QD layer and contribute to QDLED failure over time. Looking forward, minimizing degradation in the charge transport layers will likely be crucial for the realization of highly stable QDLEDs and this perspective provides potential avenues to achieve these enhancements. In particular, tailoring the QD energy levels via material selection or interfacial dipoles may reduce charge carrier accumulation in the transport layers and replacing the organic HTL with an inorganic alternative may be an effective approach to circumvent the inherent susceptibility of organic semiconductors to exciton-induced degradation.