Nonradiative Centers Research Articles

Hydrogen passivation: a proficient strategy to enhance the optical and photoelectrochemical performance of InGaN/GaN single-quantum-well nanorods

Recently, III-nitride semiconductor nanostructures, especially InGaN/GaN quantum well nanorods (NRs), have been established as a promising material of choice for nanoscale optoelectronics and photoelectrochemical (PEC) water-splitting applications. Due to the large number of surface states, III-nitride NRs suffer from low quantum efficiency. Therefore, control of the surface states is necessary to improve device performance in real-time applications. In this work, we investigated the effect of hydrogen plasma treatment on the optical properties of InGaN/GaN single-quantum-well (SQW) NRs. The low-temperature photoluminescence (PL) studies revealed that yellow and green emissions overlapped and the yellow band is more dominant in the pristine InGaN/GaN SQW NRs. However, the emission corresponding to yellow luminescence was strongly suppressed and the green emission is more intensified in hydrogenated InGaN/GaN SQW NRs. Furthermore, the time-resolved PL spectroscopy studies revealed that the carrier lifetimes of hydrogenated InGaN/GaN SQW NRs are relatively short compared to the pristine InGaN/GaN SQW, indicating the effective reduction of non-radiative centers. From the PEC measurement, the photocurrent density of hydrogenated InGaN/GaN SQW NRs in the H2SO4 solution is found to be 5 mA cm−2 at −0.48 V versus reversible hydrogen electrode, which is 3.5-fold larger than that of pristine ones. These findings shed new light on the significance of surface treatment on the optical properties and thus nanostructured photoelectrodes for PEC applications.

Mechanistic insights into defect generation and tuning of optical properties in Zn1-xFexAl2O4(0.01 ≤ x ≤ 0.40) nanocrystals.

The correlation of several defects and optical and magnetic properties with Fe content in Zn1-xFexAl2O4 (0.01 ≤ x ≤ 0.40) nanocrystals has been scrutinized through X-ray diffraction, O K-edge X-ray absorption near-edge structure, FT-IR, diffuse reflectance, photoluminescence and electron spin-resonance spectroscopies, and vibrating sample magnetometry. Increasing Fe content causes elongation in the octahedral units of the lattice, accompanied by distortion in the octahedral coordination. Fe introduces non-radiative centres in the forbidden gap, thereby tuning the band gap from 4.37 to 3.88 eV and eliminating emission in the visible region. Zn vacancies are found to tail off, while {\rm Fe}_i^{\bullet \bullet \bullet}, {\rm Al}_{\rm Zn}^\bullet and FeAl× antisite defects increase in concentration with increasing Fe content. Inhomogeneous broadening of spin-resonance signals infers strong spin-lattice interactions of Fe3+ ions at distorted octahedral and non-symmetric tetrahedral sites. A transition is observed from paramagnetism to superparamagnetism at higher Fe concentrations. A visual colour change from pearly white to orange-brown is observed in Zn1-xFexAl2O4 nanocrystals with increasing Fe content, revealing its potential candidature for pigments in the paint and dye industries.

Downshifting of highly energetic photons and energy transfer by Mn-doped perovskite CsPbCl3 nanocrystals in hybrid organic/silicon nanostructured solar cells

Abstract The downshifting of highly energetic photons in the ultraviolet region has the benefit of extending the spectral response and decreasing the thermalization effect in Si-related solar cells. Herein, we report low-cost hybrid organic/inorganic solar cells with minimized spectral losses with directly-deposited Mn-doped perovskite CsPbCl3 nanocrystals (NCs) on the surface of Si nanotips with PEDOT:PSS acting as the hole transporting and passivating layer. The CsPbCl3 NCs act as highly efficient radiative and non-radiative energy transfer centers. Manganese doping in CsPbCl3 NCs also drastically enhances photoluminescence quantum yields due to the charge transfer of photoinduced excitons from the CsPbCl3 host to the dopant Mn2+ centers. Finally, the Mn doped CsPbCl3 NCs enhance the cell properties and give a high power conversion efficiency of 13.48%.

1 meV electron irradiation and post-annealing effects of GaInAsN diluted nitride alloy with 1 eV bandgap energy

1 meV electron irradiation and post thermal annealing effects of GaInAsN bulk material with bandgap energy of 1 eV was studied by low-temperature photoluminescence (PL). The nitrogen proportion of the samples was confirmed by high-resolution X-ray diffraction measurement as 0.26%. The result shows that the PL intensity and high activation energy non-radiative centers of GaInAsN materials degraded seriously due to the irradiation induced large number of defects. The PL enhancement phenomena were observed in all samples after rapid thermal annealing (RTA) at 650 ∘C for five minutes. The irradiation and RTA caused PL peak red-shift and blue-shift have been discussed.

Relating Defect Luminescence and Nonradiative Charge Recombination in MAPbI3 Perovskite Films

Nonradiative losses in semiconductors are related to defects. At cryogenic temperatures, defect-related photoluminescence (PL) at energies lower than the band-edge PL is observed in methylammonium lead triiodide perovskite. We applied multispectral PL imaging to samples prepared by two different procedures and exhibiting 1 order of magnitude different PL quantum yield (PLQY). The high-PLQY sample showed concentration of the emitting defect sites around 1012-1013 cm-3. No correlation between PLQY and the relative intensity of the defect emission was found when micrometer-sized local regions of the same sample were compared. However, a clear positive correlation between the lower PLQY and higher defect emission was observed when two preparation methods were contrasted. Therefore, although the emissive defects are not connected directly with the nonradiative centers and may be spatially separated at the nano scale, chemical processes during the perovskite synthesis promote/prevent formation of both types of defects at the same time.

Effect of InGaN growth interruption on photoluminescence properties of an InGaN-based multiple quantum well structure

Abstract Two different green-emitting InGaN-based multiple quantum well (MQW) structures, with and without two-step growth interruption (TSGI) during InGaN well layer growth, were prepared, and their optical properties were investigated by examining the dependences of photoluminescence (PL) spectra on temperature and excitation power. The results indicate that, compared to the MQW structure without TSGI, the MQW structure with TSGI exhibits a higher PL peak energy, narrower linewidth, and a weaker carrier localization effect and quantum confined Stark effect within the chosen measurement range. These results are due to the fact that a TSGI can reduce the average In content and result in a more homogeneous distribution of In atoms in the MQWs due to In re-evaporation and diffusion of In atoms from indium-rich to indium-poor regions, and thus causing a less significant In composition fluctuation and a weaker polarized electric field in the MQWs. It is also consistent with the experimental result, in which the MQW structure with TSGI has a higher internal quantum efficiency compared with the MQW structure without TSGI, due to it having fewer non-radiative centers as confirmed by measuring excitation power-dependent integrated PL intensity.

Wave-shaped temperature dependence characteristics of the electroluminescence peak energy in a green InGaN-based LED grown on silicon substrate

This study aimed to investigate temperature dependencies at different injection currents (ICs) of the electroluminescence (EL) spectra from a green InGaN/GaN light-emitting diode (LED) based on multiple quantum wells (MQWs) grown on a Si substrate in a wide range of ICs (0.001–350 mA) and temperatures (6–350 K). The results show that the temperature-changing characteristic of the EL peak energy gradually evolves from an approximately V-shaped temperature dependence into a wave-shaped (three-step blueshift) dependence with increasing IC. Finally, it emerges as an approximately inverted V-shaped temperature dependence. The behavior reflects the fact that the emission related to InGaN is significantly influenced by the changing recombination dynamics of carriers with rising temperature or IC. This is attributed to the presence in the MQW active region of a stronger carrier localization effect across three zones with different average In contents. Moreover, with the decline of the temperature at lower ICs, the temperature behavior of the external quantum efficiency (EQE) value is dominated by the deactivated non-radiative centers. This phenomenon occurs not only in the higher temperature range but also at lower temperatures due to more In-content-induced structural defects, which are confirmed by measurements of the integrated EL intensity as well as the EQE dependence on IC.

Instabilities in nanodiamond nitrogen-vacancy centre single photon sources under prolonged pulsed excitation

Colour centres in nanodiamonds provide robust sources of fluorescence and can be used as triggered sources of single photons at room temperature. However, practical devices require stability over thousands of hours of operation, and the use of strong pulsed optical excitation, placing significant burden on the robustness of the emitters that requires bespoke testing. In this work we report the response of single NV centres in nanodiamonds of 50 nm and 100 nm diameter to accelerated lifetime testing, exciting the defects close to saturation around 1013 times to simulate the minimum operational lifetime of a practical device. For nanodiamonds 50 nm in diameter, observed changes in the fluorescence intensity and lifetime suggest a progressive size reduction as a result of the pulsed laser excitation, combined with the introduction of non-radiative centres on or near the nanodiamond surface which affect the quantum efficiency of the NV centre and ultimately lead to photobleaching of the emission. We find examples of NV centres in 100 nm nanodiamonds for which triggered single photon emission remains stable for over these accelerated lifetime tests, demonstrating their suitability for use in practical devices.

Ions beam induced luminescence study of variation of defects in zinc oxide during ion implant and after annealing

<sec>The optical and electrical properties of ZnO related on the type and the concentration of defects in ZnO crystal. Ion implantation and annealing can change the type and the concentration of defects in ZnO. To understand the variation of defects in ZnO during ion implantation and after different temperature annealing, in situ luminescence measurements of ZnO crystal samples were carried out by ion beam induced luminescence (IBIL) during ion implantation of 2 MeV H<sup>+</sup> and then after annealing at 473 K and 800 K in vacuum on the GIC4117 tandem accelerator in Beijing Normal University.</sec><sec>IBIL spectra of ZnO showtwo emission peaks: UV emission, which is called near band emission (NBE), and visible emission, which is called deep band emission (DBE).The high-intensity of DBE and weak NBE of IBIL spectra of ZnOmay be due to the NBE is intrinsic to ZnO samples and therefore is just visibly observed from samples that are virtually defect-free. With the ion implantation, the destruction of the crystal structure and the arising of a mass of defects, inducing the weak intensity NBE and intense DBE.In addition, the overall IBIL spectra of ZnOreveal decrease intensity with the ion fluence,which indicates that the concentration of luminescence centersdecreases duringion implantation.With the H<sup>+</sup> fluence, the concentration of the point defects increases. The point defects migrate and subsequently agglomerate into larger defect clusters. These defect clusters serve as traps for catching electrons and holes, which result in the quenching of luminescence centres. Annealing can help todecompose the defect clusters and repair the defects of crystal. However, amounts of defects and clusters still remain in the irradiated sample annealed at 473 K in vacuum, which acted as nonradiative center and suppress the luminescence induced weak intensity of IBIL. Annealing the sample at 800 K in vacuum may facilitate the decomposition of defect clusters during ion irradiation to point defects and the point defect return to the lattice position that can reduce the nonequilibrium defects inside the crystal and improve the crystallinity of the crystal, which increase the intensity of its IBIL.</sec>

Optimization of atomic layer deposition temperature of ZrO2 protective coat for GaInAsP photonic crystal nanolaser sensor

This study examined the effect of atomic layer deposition temperature of a ZrO2 protective coat on GaInAsP semiconductor photonic crystal nanolaser sensors. At low temperatures (around 100 °C), the nonradiative centers at the ZrO2/GaInAsP interfaces were reduced, and the laser operation and refractive index sensing characteristics were stabilized. At high temperatures (around 250 °C), the nonradiative centers increase, which, in turn, improves the ion sensitivity based on surface recombination.

A new sulfur-doped source and synergistic effect with nitrogen for carbon dots produced from glucose**Project by the National Natural Science Foundation of China (Grant Nos. 51571085, 11805052, and 61705062), the Research Project for Basic and Forefront Technology of Henan Province, China (Grant No. 162300410219), and the Doctor Foundation of Henan Polytechnic University, China (Grant No. B2014049).

The nitrogen and sulfur co-doped carbon dots (N, S-CDs) with increased luminescence were synthesized by a hydrothermal process in one green pot by using glucose, and a new sulfur-doping source of sodium sulfite was developed. The synergistic effect of the N and S groups was well discussed through the structure analysis of Fourier transform infrared spectra and x-ray photoelectron spectra. The surface states of N, S-CDs embody more complicated functional groups, and S element exists as –SSO3, –C–SO3, and groups due to the introduction of sodium sulfite. The sulfur-containing groups passivate the surface of the CDs, and the relatively high sulfur groups may reduce the non-radiation centers. The fluorescence is affected by the hydroxyl group of the solvent. The quenching of Fe3+ ion to fluorescence and the sensitivity of fluorescence to pH were also investigated.

Luminescence properties of ZnB2O4:Pb2+ phosphors: Suppression of concentration quenching

The UV-A emitting ZnB2O4:Pb2+ crystalline powder phosphors were produced using the conventional solid-state reaction synthesis technique at 830 °C for 6 h in air. The morphology and structure of the phosphors were explored by SEM, XRD and FT-IR analyses. The optical properties of the phosphors were examined by UV–vis diffuse reflectance and steady-state photoluminescence (PL) analyses. The excitation and emission bands of Pb2+ doped ZnB2O4 was observed at 304 nm and 388 nm, respectively. The effects of dopant concentration on the PL emission intensities of Zn1–xPbxB2O4 (0 ≤ x ≤ 0.15) phosphors were studied. The well-known concentration quenching phenomenon was not observed in the studied range. To explain the observation of suppressed concentration quenching the energy transfer between radiative and non-radiative centers has been discussed. Based on the absorption, excitation and emission results of the samples, a tentative configurational coordinate-energy diagram was proposed. Obtained results show that the ZnB2O4:Pb2+ phosphor is a good candidate for UV-A applications as a rare-earth free wavelength converter.

Photoluminescence properties of blue and green multiple InGaN/GaN quantum wells**Project supported by the National Natural Science Foundation of China (Grant Nos. 51672163, 51872167, and 61504044) and the Major Research Plan of the National Natural Science Foundation of China (Grant No. 91433112).

The photoluminescence (PL) properties of blue multiple InGaN/GaN quantum well (BMQW) and green multiple InGaN/GaN quantum well (GMQW) formed on a single sapphire substrate are investigated. The results indicate that the peak energy of GMQW-related emission (PG) exhibits more significant “S-shaped” dependence on temperature than that of BMQW-related emission (PB), and the excitation power-dependent carrier-scattering effect is observed only in the PG emission; the excitation power-dependent total blue-shift (narrowing) of peak position (line-width) for the PG emission is more significant than that for the PB emission; the GMQW shows a lower internal quantum efficiency than the BMQW. All of these results can be attributed to the fact that the GMQW has higher indium content than the BMQW due to its lower growth temperature and late growth, and the higher indium content in the GMQW induces a more significant compositional fluctuation, a stronger quantum confined Stark effect, and more non-radiative centers.

Raman investigation of N‐implanted ZnO: Defects, disorder and recovery

AbstractUnderstanding defects, disorder and doping due to N implantation in ZnO is one of the most debated issues for the last few years. In the present work, a comprehensive investigation has been carried out using Raman, photoluminescence (PL) spectroscopy and grazing‐incidence X‐ray diffraction on 50 keV N ions implanted granular ZnO with different fluence (approximately up to 6.5% atomic concentration) along with postimplantation annealing. Raman investigation suggests that 275, 510, 643, and 857 cm−1 modes are directly related to nitrogen. Additionally, VZn may have some role in stabilizing 275 cm−1 mode. The broadening (or tailing) of E2low mode is related to vibration of distorted Zn sublattice, which may be a product of ion implantation generated defect cluster like VZn–VO. The distortion starts to reduce with annealing at elevated temperatures. Direct correlation between 555 cm−1 Raman mode and the tailing of E2low mode has been found. More defect clustering is vivid from the reduced PL of the ZnO samples with increasing implantation fluence. So, tailing of E2low Raman mode, increasing intensity of 555 cm−1 mode and nonradiative defect centers are of common origin. Both the ratios E1(2LO)/E1(LO) and E2high/E1(LO) can be used as parameters to measure the defective nature of ZnO after ion implantation/irradiation. Low temperature PL (selected samples) suggests absence of shallow acceptor states, although negative thermal quenching above 175 K has been observed (implantation fluence 1 × 1016 ions/cm2 and annealed at 500°C) which can be a signature of deep acceptors.

Studies of photoluminescence properties and thermal stability of Cu doped ZnCdS/ZnS quantum dots by various host composition

Cu-doped ZnCdS/ZnS (Cu:ZnxCd1−xS/ZnS) core/shell quantum dots (QDs) are synthesized by a facile one-pot noninjection approach to investigate the composition-dependent photoluminescence (PL) properties. The resulting monodisperse Cu:ZnxCd1−xS/ZnS QDs exhibit a tuned band gap energy Eg from 2.84 to 3.13 eV and a wide dopant emission band from 630 to 569 nm when Zn content x increases from 0.2 to 0.5. Furthermore, the decreasing FWHM with increasing Zn content reveals the emission mainly arises from the recombination of electrons in conductor band with localized holes in Cu-related T2 and E level. With Zn content increasing in ZnxCd1−xS host, the relatively perfect lattice match between ZnxCd1−xS core and ZnS shell reduces the PL lifetime and increased PL quantum yield. The temperature dependent PL behavior suggests that the thermal stability of Cu:ZnxCd1−xS/ZnS QDs is enhanced because the interface nonradiative centers are efficiently suppressed by increasing Zn content.

Interplay of carriers and deep-level recombination centers of 275-nm light-emitting diodes - Analysis on the parasitic peaks over wide ranges of temperature and injection density.

The low luminance efficiency, poor reliability and parasitic peaks have greatly limited the commercialization of deep ultraviolet (DUV) light-emitting diodes. Tasks of identifying the culprits of these deficits are of paramount importance but remains unaccomplished. We employ the full-range temperature (20 K -300 K) measurement on 275-nm DUV devices that subjected to a 15-hour current-stress aging. The results suggest that the primary culprit of fast luminous decay is the proliferation of non-radiative centers. The origins of two main parasitic peaks are identified. The 310-nm peak is considered to solely come from deep-level radiative centers (DLRCs) that only dwell in the active region. Whereas, the 400-nm peak is proven to be dual-sources. One is related to the DLRCs in the active region, which only can be observed at very low currents; the other emerging at higher currents are associated with similar kinds of DLRCs located in the p-region, which only are excited when electrons overflow. This new discovery also demonstrates that a thorough investigation on the interplay among carriers and various types of defects should be conducted on the basis of the measurement that is taken under a wide temperature range, as well as under a proper forward voltage. This is to let the quasi-Fermi level shift across deep defect levels, the band-edge, and to over-band, whereby these recombination sites are exposed to deficit, moderate and saturated electron environment so that their natures can be well tested.

Effect of the synthesis conditions on the properties of Co embedded porous Si nanostructures

The electrodeposition of cobalt in the porous silicon (PSi) substrate was investigated in terms of the deposition times and current densities. The PSi/Co samples were characterized by SEM, XRD, Raman, and photoluminescence (PL) spectroscopies. The results indicated that for all current densities, the PL intensities of PSi/Co samples with shorter deposition times (ts ≤ 20 min) increased due to spherical Co nanoparticles (NPs) could be created the new recombination centers, compared to that of the undeposited PSi. On the other hand, the PL intensity of PSi/Co samples significantly decreased at longer deposition times (tl > 20 min) because of larger Co NP cluster promoted the formation of non-radiative centers. The increased PL intensities in samples with ts were attributed to both the quantum confinement effect and surface effects. PL analyses also suggested that after exposure to air for 60 days, PL characteristics of PSi/Co were stabilized depending on deposition time and current density.

Effects of NH3 flow on structural and optical properties of ZnO films grown by atmospheric-pressure chemical vapor deposition

Structural and optical properties of zinc oxide (ZnO) films doped with nitrogen (N) grown on r-plane sapphire substrates by the atmospheric-pressure chemical vapor deposition using zinc, water, and ammonia (NH3) as source materials have been studied. The small amount of NH3 flow was found to be effective for improving the film orientation and surface morphology. Photoluminescence (PL) spectra of the as-grown undoped and N-doped ZnO films were composed of a near-band-edge emission with two peaks at ~376 and ~386 nm and a green band (GB) emission with a peak at ~540 nm. PL and PL excitation (PLE) measurements for the as-grown N-doped films grown at the different NH3 flow rates indicated the possibility of the contribution of the N and/or hydrogen (H) related defect to both the appearance of the peak at ~386 nm and the excitation process of the GB emission. The comparison between the PL and photoacoustic (PA) spectra showed that the N atom occupying oxygen site (NO) or its complex defect is the most conceivable candidate for the non-radiative center responsible for the intra-band-gap PA absorption. After post annealing under the different conditions, a red band (RB) emission appeared at ~720 nm besides the GB emission. Both the RB and GB emissions observed for the annealed films were found to be effectively excited by the band-to-band (inter-band-gap) and free exciton transitions. Regardless of the difference in annealing condition, all the PLE spectra for the RB emissions showed a weak excitation band extending from 460 nm to 680 nm, suggesting the formation of the deep impurity level probably due to the NO acceptor. It was also confirmed by X-ray photoelectron spectroscopy that the NO deep acceptors were formed in the films, but passivated by the H atoms and/or compensated by the H related donors and/or by the nitrogen molecule substituting oxygen site [(N2)O] double donors.

X-ray radiation hardness and influence on blinking in Si and CdSe quantum dots

We study the effect of X-ray irradiation on the photoluminescence (PL) efficiency and intermittency (blinking) of single Si/SiO2 and CdSe/CdZnS quantum dots (QDs). Our results show that the PL efficiency of Si nanocrystals is not significantly altered up to a cumulative fluence of 1020 photons/m2 (corresponding to ∼300 kGy of absorbed dose in SiO2), while CdSe particles become completely dark already after a 17 times lower fluence. In both types of QDs, the statistical nature of blinking ON- and OFF-times remains unaltered: mono-exponential for Si and power-law for CdSe QDs. However, the evolution of the blinking parameters with absorbed dose depends on the choice of material. On average, both ON- and OFF-time constants do not vary in Si nanocrystals, highlighting their radiation hardness. Instead, the ON-time exponent increases while the OFF-time exponent decreases with the increasing dose for CdSe dots, confirming their efficiency quenching. Ensemble measurements did not show PL spectral changes neither indicated removal of surface ligands in irradiated CdSe dots. Thus, ionization-generated non-radiative centers in the core-shell system modify blinking of CdSe dots and eventually rapidly quench their emission, in contrast to robust Si/SiO2 nanocrystals. Our study is important for the future use of luminescent QDs in harsh environments, such as space, and the engineering of their blinking properties via ionizing radiation.

Structural evolution and effective improvement of emission quantum yields for silicon nanocrystals synthesized by femtosecond laser ablation in HF-contained solution

Stable luminescent colloidal silicon (Si) nanocrystals (NCs) with sufficient surface protection are prepared through femtosecond laser ablation in organic solvent containing diverse concentrations of HF solution. The average size of Si NCs shows the decreasing tendency from 6.5 to 2.7 nm when the concentration of HF varies from 0 to 11.1 vol% (volume ratio). In line with the structural evolution, UV–visible absorption, photoluminescence (PL) excitation spectra, and time-resolved PL, we propose that room temperature blue emission peaks at 412 and 440 nm originate from alkyl-related radiative recombination centers. The enhanced PL quantum yield of colloidal Si NCs from 16.3% to 76.5% has been attributed to the effective passivation and suppression of non-radiative defect centers with increasing HF concentration from 0 to 11.1 vol%.