Nonradiative Recombination Centers Research Articles

Etching-free pixel definition in InGaN green micro-LEDs

The traditional plasma etching process for defining micro-LED pixels could lead to significant sidewall damage. Defects near sidewall regions act as non-radiative recombination centers and paths for current leakage, significantly deteriorating device performance. In this study, we demonstrated a novel selective thermal oxidation (STO) method that allowed pixel definition without undergoing plasma damage and subsequent dielectric passivation. Thermal annealing in ambient air oxidized and reshaped the LED structure, such as p-layers and InGaN/GaN multiple quantum wells. Simultaneously, the pixel areas beneath the pre-deposited SiO2 layer were selectively and effectively protected. It was demonstrated that prolonged thermal annealing time enhanced the insulating properties of the oxide, significantly reducing LED leakage current. Furthermore, applying a thicker SiO2 protective layer minimized device resistance and boosted device efficiency effectively. Utilizing the STO method, InGaN green micro-LED arrays with 50-, 30-, and 10-µm pixel sizes were manufactured and characterized. The results indicated that after 4 h of air annealing and with a 3.5-μm SiO2 protective layer, the 10-µm pixel array exhibited leakage currents density 1.2 × 10−6 A/cm2 at −10 V voltage and a peak on-wafer external quantum efficiency of ~6.48%. This work suggests that the STO method could become an effective approach for future micro-LED manufacturing to mitigate adverse LED efficiency size effects due to the plasma etching and improve device efficiency. Micro-LEDs fabricated through the STO method can be applied to micro-displays, visible light communication, and optical interconnect-based memories. Almost planar pixel geometry will provide more possibilities for the monolithic integration of driving circuits with micro-LEDs. Moreover, the STO method is not limited to micro-LED fabrication and can be extended to design other III-nitride devices, such as photodetectors, laser diodes, high-electron-mobility transistors, and Schottky barrier diodes.

Slow-Release Effect Assisted Crystallization for Sequential Deposition Realizes Efficient Inverted Perovskite Solar Cells.

The two-step sequential deposition strategy has been widely recognized in promoting the research and application of perovskite solar cells, but the rapid reaction of organic salts with lead iodide inevitably affects the growth of perovskite crystals, accompanied by the generation of more defects. In this study, the regulation of crystal growth was achieved in a two-step deposition method by mixing 1-naphthylmethylammonium bromide (NMABr) with organic salts. The results show that the addition of NMABr effectively delays the aggregation and crystallization behavior of organic salts; thereby, the growth of the optimal crystal (001) orientation of perovskite is promoted. Based on this phenomenon of delaying the crystallization process of perovskite, the "slow-release effect assisted crystallization" is defined. Moreover, the incorporation of the Br element expands the band gap of perovskite and mitigates material defects as nonradiative recombination centers. Consequently, the power conversion efficiency (PCE) of the enhanced perovskite solar cells (PSCs) reaches 20.20%. It is noteworthy that the hydrophobic nature of the naphthalene moiety in NMABr can enhance the humidity resistance of PSCs, and the perovskite phase does not decompose for more than 3000 h (30-40% RH), enabling it to retain 90% of its initial efficiency even after exposure to a nitrogen environment for 1200 h.

Characterizations of Subbandgap Optical Absorption in Undoped‐GaN and 90 nm‐Thick Al1−xInxN Thin Film on Sapphire Substrates Grown by Metal–Organic Chemical Vapor Deposition

Subbandgap optical absorption (SOA) in undoped GaN and 90 nm‐thick Al1−xInxN thin films grown on sapphire substrates is investigated using photothermal deflection spectroscopy (PDS) and photoluminescence (PL). An Al1−xInxN alloy (x = 0.17) is grown on a GaN/sapphire template by metal–organic chemical vapor deposition (MOCVD), and the SOA is observed using PDS. To develop an estimation method for the absorption coefficient (α) of SOA in GaN and Al1−xInxN thin films, the use of a thick GaN substrate is proposed, which is grown by hydride vapor‐phase epitaxy, as a converter of the PDS signal intensity to α, and the accuracies of the estimated α are discussed. Comparing the PDS and PL results, it is revealed that nonradiative recombination centers leading to the reduction of the near‐band‐edge PL intensity are not the dominant sources of SOAs in GaN. Other in‐gap states formed by impurities and/or their complexes with vacancy‐type defects are possible sources of a large SOA in the MOCVD‐grown GaN template. Considering the PDS results and reported peak reflectivity of Al1−xInxN/GaN distributed Bragg reflectors, the α value of sub‐100 nm‐thick Al1−xInxN alloy grown on GaN/sapphire template is expected to be ≈100 cm−1 or less below 3.0 eV.

Modulating secondary growth of perovskite grains through residual solvent evaporation

Over the past decade, perovskite solar cells (PSCs) have attracted enormous attention due to their high performance. One key to fabricating high-quality perovskite films lies in controlling the volatilization rate of residual solvents during the annealing process. This study systematically investigates how different protective substrates affect the volatilization rate of residual solvent in perovskite films. By adjusting the direction and rate of evaporation, the supersaturation time of the solution was precisely controlled, leading to effective recrystallization of the grains. Concurrently, the annealing time was optimized to enhance film quality further. This optimization aimed to increase crystallinity, reduce defects, and thereby minimize non-radiative recombination centers. Implementing these methodologies, particularly the use of filter paper as a protective substrate during a 2-minute annealing process, significantly improved the fill factor (FF) and open-circuit voltage (VOC) of the PSCs. This led to a remarkable 5.26% improvement in power conversion efficiency (PCE) compared to control devices. The strategies employed in this work demonstrate significant potential in improving PSC film quality. This approach not only advances our understanding of film formation dynamics but also provides a practical guideline for future PSC fabrication.

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

Correlative Micro‐Photoluminescence Study on Hybrid Quantum‐Well InGaN Red Light‐Emitting Diodes

To investigate nonradiative recombination processes in indium gallium nitride (InGaN)‐based red light‐emitting diodes (LEDs), an InGaN‐based red LED with a hybrid quantum well (QW) structure consisting of red and blue single quantum wells (SQWs) is characterized by micro‐photoluminescence (μ‐PL) spectroscopy. The μ‐PL mapping of the red emission reveals numerous dark spots with various sizes and contrasts. Not only the red and blue (from a blue SQW) but green emission bands are observed at some red dark spots, suggesting that indium (In) segregation is one of the causes of nonradiative recombination in the red emission. Comparing the blue and green emission images to the red emission image reveals that the dark spots in the intensity map of the red emission can be classified into four types. Through this correlative analysis, the red dark spots associated with the dark areas in the intensity map of the blue emission are attributed to the major nonradiative recombination centers in the red emission.

Tailoring interface and morphology of TiO2 electron transport layer with potassium bitartrate for high-performance perovskite solar cells

The performance of perovskite solar cells (PSCs) has rapidly improved, largely due to advancements in mitigating inherent defects and enhancing interface carrier transport properties. Herein, we provide a facile method to improve the morphology of TiO2 ETLs, suppress defects at interface, and tailor energy level alignment to enhance carrier separation and extraction efficiency by introducing potassium bitartrate (KBT) into the TiO2 electron transport layers (ETLs), achieving high-efficiency and stable PSCs. KBT molecules can enhance the surface smoothness of TiO2 ETLs, resulting in denser and more uniform perovskite films. In addition, the carboxyl groups, hydroxyl groups, and potassium cations containing in KBT improve charge transport efficiency at TiO2/perovskite interface by reducing non-radiative recombination centers in TiO2 and perovskite bottom as well as improving energy level matching. As a result, the target PSC exhibits an obvious improvement in performance with a champion efficiency of 23.35 %, which is greatly improved relative to the control (21.66 %). The long-term operational stability is also remarkably enhanced, after 800 h MPPT, PSCs with KBT remain over 80 % of its initial efficiency. PSCs with KBT-doped ETLs exhibit increased efficiency and stability, highlighting potential of KBT as an interface engineering tool for enhancing PSCs performance.

The Influence Mechanism of Quantum Well Growth and Annealing Temperature on In Migration and Stress Modulation Behavior.

This study explores the effects of growth temperature of InGaN/GaN quantum well (QW) layers on indium migration, structural quality, and luminescence properties. It is found that within a specific range, the growth temperature can control the efficiency of In incorporation into QWs and strain energy accumulated in the QW structure, modulating the luminescence efficiency. Temperature-dependent photoluminescence (TDPL) measurements revealed a more pronounced localized state effect in QW samples grown at higher temperatures. Moreover, a too high annealing temperature will enhance indium migration, leading to an increased density of non-radiative recombination centers and a more pronounced quantum-confined Stark effect (QCSE), thereby reducing luminescence intensity. These findings highlight the critical role of thermal management in optimizing the performance of InGaN/GaN MQWs in LEDs and other photoelectronic devices.

Multifunctional Interface Modification Enables Efficient and Stable HTL-Free Carbon-Electroded CsPbI2Br Perovskite Solar Cells.

In recent years, hole transport layer-free all-inorganic CsPbI2Br carbon-electroded perovskite solar cells (C-PSCs) have garnered significant attention due to a trade-off between stability and photovoltaic performance. However, there are inevitably many defects generated at the surfaces or grain boundaries of CsPbI2Br perovskite films, which will serve as carrier non-radiative recombination centers, and CsPbI2Br perovskite films are sensitive to water molecules to degrade, together with energy level mismatch between CsPbI2Br perovskite and carbon electrodes. Herein, 1-benzyl-3-methylimidazolium hexafluorophosphate (1-B-3-MIMPF6), an imidazolium-based ionic liquid simultaneously containing benzene ring and fluorine atoms, was introduced for the modification of the perovskite/carbon interface. The results showed that it could effectively reduce defects, enhance carrier transfer, mitigate carrier non-radiative recombination, facilitate energy alignment, and block moisture intrusion. Therefore, the photovoltaic performance of the modified PSCs with ITO/SnO2/CsPbI2Br/1-B-3-MIMPF6/carbon architecture has been boosted with a champion power conversion efficiency (PCE) of 13.47 %, open circuit voltage of 1.20 V, short circuit current density of 14.69 mA/cm2, and fill factor of 76 %. Moreover, the unencapsulated modified devices exhibited an improved stability and the PCE maintained 78 % of their initial PCE after 24 h storage at room temperature in a 30 %-35 % humidity environment, whereas that of the pristine devices dropped to almost zero.

Temperature‐ and Excitation Power Density‐Resolved Photoluminescence of AlGaN‐Based Multiple Quantum Wells Emitting in the Spectral Range of 220–260 nm

Internal quantum efficiency (IQE) of AlGaN‐based multiple quantum wells (MQWs) on face‐to‐face‐annealed sputter‐deposited AlN templates is examined by photoluminescence spectroscopy. The excitation power density dependence of IQE is evaluated as a function of temperature under the selective excitation of the quantum wells. The temperature dependences of the maximum IQE and the corresponding excitation power density (EPD) are analyzed based on the rate equation models for carriers and excitons. The decrease of the maximum IQE and increase of the corresponding EPD are mainly due to the increase in the nonradiative recombination rate via nonradiative recombination centers. Furthermore, the nonradiative recombination rate for the MQW with an emission wavelength around 220 nm is activated at a lower temperature than the other samples, which is expected to lead to the lower IQE of the MQW with an emission wavelength around 220 nm.

Effect of Wet Etching on AlGaN‐Based Ultraviolet‐B Laser Diodes Grown on Wet‐Etched Periodic AlN Nanopillars

The effect of wet etching on the characteristics of ultraviolet‐B (UV‐B) lasers fabricated on AlGaN templates grown as crystals or wet‐etched AlN nanopillars is compared in detail. Specifically, a tetramethylammonium hydroxide (TMAH) solution is used to form templates with AlN nanopillars of different diameters, followed by determining the dependence of the dislocation density in AlGaN crystals grown on the templates, the threshold power density (Pth) of an optically pumped UV‐B laser, and the threshold current density (Jth) of a UV‐B laser diode (LD) on the extent of etching. It is observed that the pillar diameter decreases almost linearly as the etching time with the TMAH solution increases. The dislocation density of AlGaN crystals grown on AlGaN templates is almost independent of the etching time. Meanwhile, Pth of the optically pumped UV‐B laser is almost halved upon etching with TMAH. Finally, the Jth of the UV‐B LD tends to decrease by 10%–20% upon etching. The optical gain is estimated by optical excitation; wet etching of AlN nanopillars with TMAH solution results in a larger optical gain at the same excitation intensity, indicating that nonradiative recombination centers, such as point defects, are reduced.

Native point defects in HgCdTe infrared detector material: Identifying deep centers from first principles

We investigate the native point defects in the long-wavelength infrared (LWIR) detector material Hg0.75Cd0.25Te using a dielectric-dependent hybrid density functional combined with spin–orbit coupling. Characterizing these point defects is essential as they are responsible for intrinsic doping and nonradiative recombination centers in the detector material. The dielectric-dependent hybrid functional allows for an accurate description of the bandgap (Eg) for Hg1−xCdxTe (MCT) over the entire compositional range, a level of accuracy challenging with standard hybrid functionals. Our comprehensive examination of the native point defects confirms that cation vacancies VHg(Cd) are the primary sources of p-type conductivity in the LWIR material given their low defect formation energies and the presence of a shallow acceptor level (−/0) near the valence-band maximum. In addition to the shallow acceptor level, the cation vacancies exhibit a deep charge transition level (2−/−) situated near the midgap, characteristic of nonradiative recombination centers. Our results indicate that Hg interstitial could also be a deep center in the LWIR MCT through a metastable configuration under the Hg-rich growth conditions. While an isolated Te antisite does not show deep levels, the formation of VHg–TeHg defect complex introduces a deep acceptor level within the bandgap.

Vacancy‐Type Defects and Their Trapping/Detrapping of Charge Carriers in Ion‐Implanted GaN Studied by Positron Annihilation

Annealing behaviors of vacancy‐type defects in ion‐implanted GaN are studied by positron annihilation. Mg+ and N+ ions are implanted to obtain 700 nm deep box profiles with Mg and N concentrations of 1 × 1018 cm−3, and the samples are annealed using an ultrahigh‐pressure annealing system. For as‐implanted samples, the major defect species is identified as Ga‐vacancy (VGa)‐type defects. For N‐implanted GaN, the size of the vacancies increases as the annealing temperature increases up to 1100 °C and then shrank above 1200 °C. This behavior is attributed to recombinations between N‐vacancy (VN)‐type defects and excess N. For Mg‐implanted GaN, the major defect species after annealing above 1000 °C is vacancy clusters such as (VGaVN)3. Some of them act as nonradiative recombination centers for blue and ultraviolet luminescence. Their energy levels corresponding to the transition from positive to neutral are located between 2.6 eV above the valence band maximum and the conduction band minimum. The thermal activation process of electron detrapping from the vacancy clusters is also studied. For Mg‐implanted GaN, one of the major secondary defects is collapsed vacancy disks forming dislocation loops. They are eliminated by additional N implantation, which is associated with vacancy agglomerations under the annealing in VGa‐rich condition.

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 Comprehensive Review on Defects-Induced Voltage Losses and Strategies toward Highly Efficient and Stable Perovskite Solar Cells

The power conversion efficiency (PCE) of single-junction perovskite solar cells (PSCs) has reached 26.1% in small-scale devices. However, defects at the bulk, surface, grain boundaries, and interfaces act as non-radiative recombination centers for photogenerated electron-hole pairs, limiting the open-circuit voltage and PCE below the Shockley–Queisser limit. These defect states also induce ion migration towards interfaces and contribute to intrinsic instability in PSCs, reducing the quasi-Fermi level splitting and causing anomalous hysteresis in the device. The influence of defects becomes more prominent in large-area devices, demonstrating much lower PCE than the lab-scale devices. Therefore, commercializing PSCs faces a big challenge in terms of rapid decline in working performance due to these intrinsic structural defects. This paper provides a comprehensive review of recent advances in understanding the nature and the classification of defects, their impact on voltage losses, device parameters, intrinsic stability, and defect quantification and characterization techniques. Novel defect passivation techniques such as compositional engineering, additive engineering, post-treatments, dimensionality engineering, and interlayer engineering are also reviewed, along with the improvements in PCE and stability based on these techniques for both small-area devices and large-area roll-to-roll coated devices.

How do the oxygen vacancies affect the photoexcited carriers dynamics in β-Ga2O3？

β-Ga2O3 is a rising star material for solar-blind photoelectronic detectors (PDs) since its excellent properties and lower cost than GaN and SiC. However, the high density of O vacancy (VO) strongly affect the performance of devices. In this work, we investigate the effect of VOs on carrier dynamics in β-Ga2O3 and its meaningful application in solar-blind PDs by the time-dependent density functional theory (TDDFT) and nonadiabatic molecular dynamics (NAMD). We find the three inequivalent VOs (VO1, VO2, and VO3) play different roles in carrier dynamics although they all introduce deep defect states in the bandgap, which significantly shorten the lifetime (τ) of carriers. VO1 and VO3 form nonradiative recombination centers, considerably decreasing the τ from 23.10 ns to 0.12 ns and 0.11 ns, respectively. VO2 forms shallow-level defects, and the τ is 0.47 ns, which weakens the decline of the carrier compared to VO1 and VO3. Our findings provide a different angle to understand the disputation on relationship between point defects and device performance based on β-Ga2O3, and to further improve the device performance by defect engineering.

Regulate defects and energy levels for perovskite solar cells by co-modification strategy

The under-coordinated bonds and deep-level defects on the perovskite surface always act as non-radiative recombination centers and lead to energy loss. The usual surface modification efficiency is limited because of the diversity of surface terminals and defects. Here, a co-modification strategy is proposed for more-thorough modification by reconstructing and post-modifying perovskite surface defects. In detail, the tetrahydrofuran (THF) is firstly implemented to remove the organic component and expose the perovskite surface with uniform Pb terminals and defects, and then the 4-Methylbenzyl Mercaptan (MBM) is used to form strong R-S-Pb bonding with them, which achieves the increase and decrease of under-coordinated Pb donor defects. Correspondingly, the surface energy level of perovskite is observed a consistent change with defects. THF/MBM co-modification changes the perovskite surface energy level from n-type toward p-type, which enhances the holes’ transport efficiency. As a result, the best power conversion efficiency (PCE) increases from 22.23% of the control device to 24.17% of the device with THF/MBM co-modification. Notably, a full-coverage and strong-bonding protective layer is formed by THF/MBM co-modification, which facilitates excellent device stability (Retains 93.7%, after 3360 h in N2). This strategy shows great potential in high-performance PSCs by reconstructing defects and energy level of perovskite surface.

Photoluminescence of bulk α -In2Se3 crystals irradiated by high-energy electrons

The photoluminescence (PL) spectra of bulk α-In2Se3 crystals before and after 10 MeV electrons irradiation with the 1015 and 1017 cm−2 fluences were studied in the temperature range from 7 to 340 K. Three main types of radiative recombinations corresponding to band-to-tail (BT), deep defects, and band-to-band (BB) recombination were manifested in the non-irradiated α-In2Se3 crystals. Also recombinations that can be associated with exciton recombinations at temperatures below 45 K are observed. After electron irradiation, noticeable changes in the PL spectra are observed. We detected a slight increase in activation energy of the BT recombination. An increase in the concentration of deep defects is also noted. The significant decrease in PL intensity of the BB recombinations indicates the formation of non-radiative recombination centers after electron irradiation with the 1017 cm−2 fluence. Our study may be useful for understanding the effects of high-energy electrons irradiation on the performance of electronic and photovoltaic devices based on α-In2Se3.

Fabrication of 1D/3D heterostructure perovskite layers by tetrabutylammonium tetrafluoroborate for high-performance devices

Organic-inorganic halide perovskites achieved tremendous success in solar cells, however, their commercialization has not yet been realized. The primary reason is the intrinsic property of perovskite film. The significant density of defects in the polycrystalline perovskite layers influenced not only the performance of the solar cell as well its long-term stability. Here, we employed a post-surface treatment of the perovskite layer by tetrabutylammonium tetrafluoroborate (TBATFB). TBATFB interacted through hydrogen bonding with the 3D perovskite and formed an in situ a 1D TBAPbI1.66Br0.34BF4 on the surface of the 3D perovskite. This low-dimensional 1D perovskite (perovskitoid) capping layer represses the defects in perovskite by blocking the nonradiative recombination centers and minimizing the surface and grain boundary defects. Our experimental results verified both the electron and hole defects were significantly suppressed by the TBATFB modification. As a result, the 1D/3D heterostructure device exhibited a champion performance of 21.68% PCE compared to 20.03% PCE for the control device. Intriguingly, the 1D TBAPbI1.66Br0.34BF4 capping layer safeguards the 3D perovskite from the infiltration of moisture thus the TBATFB modified devices maintained 99% of their initial performance after 720h in the air (RH = 40–60%).

Growth‐Mode‐Modulated Defects in Non‐Hydrogenated a‐Si Thin Films for Photothermal Conversion

Nanostructured Si exhibits superior photothermal performance and is a cost‐effective solar absorbing material for solar evaporation, photothermal catalysis, and solar power systems. Defects are effective nonradiative recombination centers and are suppressed in amorphous silicon (a‐Si) to benefit radiative recombination. In this research, a bottom‐up approach to optimize the disordered and defective structure of non‐hydrogenated a‐Si by magnetron sputtering for photothermal conversion is proposed. The 2D mesoporous structure is developed in a‐Si thin films under the growth scenario with reduced nucleation density. Local disorder and defects are analyzed based on the vibrational response of Si‐Si bonds. Accompanied by narrowed bandgap (Eg) of 1.06 eV (Eu) and elongated Urbach tail of 0.49 eV, prominent short‐range disorder and substantial amounts of dangling bonds contribute to enhanced solar absorption. Thereby, the full‐spectrum absorptance above 92% can be realized with antireflection coatings of SiO2/Si3N4. The electron–phonon behavior revealed by resonance Raman scattering provides a comparative study on phonon production. Stronger electron–phonon coupling originated from the defective structure can be achieved at larger Eu. Provided with narrowed Eg and elongated Eu, local disorder and coordinative defects benefit both broadband absorption and thermal production, which can be introduced with controlled nucleation density and arrival rates.