Nonradiative Recombination Channels Research Articles

Coulomb Contribution to Shockley-Read-Hall Recombination.

A nonradiative recombination channel is proposed, which does not vanish at low temperatures. Defect-mediated nonradiative recombination, known as Shockley-Read-Hall (SRH) recombination, is reformulated to accommodate Coulomb attraction between the charged deep defect and the approaching free carrier. It is demonstrated that this effect may cause a considerable increase in the carrier velocity approaching the recombination center. The effect considerably increases the carrier capture rates. It is demonstrated that, in a typical semiconductor device or semiconductor medium, the SRH recombination rate at low temperatures is much higher and cannot be neglected. This effect renders invalid the standard procedure of estimating the radiative recombination rate by measuring the light output in cryogenic temperatures, as a significant nonradiative recombination channel is still present. We also show that SRH is more effective in the case of low-doped semiconductors, as effective screening by mobile carrier density could reduce the effect.

Short excitonic lifetimes of MoSe2 monolayers grown by molecular beam epitaxy on the hexagonal boron nitride

We present a time-resolved optical study of recently developed narrow-line MoSe2 monolayers grown on hexagonal boron nitride with means of molecular beam epitaxy. We find that the photoluminescence decay times are significantly shorter than in the case of the exfoliated samples, even below one picosecond. Such a short timescale requires measurements with better resolution than achievable with a streak camera. Therefore, we employ an excitation correlation spectroscopy pump-probe technique. This approach allows us to identify two distinct non-radiative recombination channels attributed to lattice imperfections. The first channel is active at helium temperatures. It reduces the lifetime of the neutral exciton to below one picosecond. The second channel becomes active at elevated temperatures, further shortening the lifetimes of both neutral and charged exciton. The high effectiveness of both radiative and non-radiative recombination makes epitaxial MoSe2 a promising material for ultrafast optoelectronics.

Sub-nanosecond free carrier recombination in an indirectly excited quantum-well heterostructure

Nanometer-thick quantum-well structures are quantum model systems offering a few discrete unoccupied energy states that can be impulsively filled and that relax back to equilibrium predominantly via spontaneous emission of light. Here we report on the response of an indirectly excited quantum-well heterostructure, probed by means of time and frequency resolved photoluminescence spectroscopy. This experiment provides access to the sub-nanosecond evolution of the free electron density, indirectly injected into the quantum wells. In particular, the modeling of the time-dependent photoluminescence spectra unveils the time evolution of the temperature and of the chemical potentials for electrons and holes, from which the sub-nanosecond time-dependent electron density is determined. This information allows to prove that the recombination of excited carriers is mainly radiative and bimolecular at early delays after excitation, while, as the carrier density decreases, a monomolecular and non-radiative recombination channel becomes relevant. Access to the sub-nanosecond chronology of the mechanisms responsible for the relaxation of charge carriers provides a wealth of information for designing novel luminescent devices with engineered spectral and temporal behavior.

How Halide Alloying Influences the Optoelectronic Quality in Tin-Halide Perovskite Solar Absorbers.

Halide alloying in tin-based perovskites allows for photostable bandgap tuning between 1.3 and 2.2 eV. Here, we elucidate how the band edge energetics and associated defect activity impact the optoelectronic properties of this class of materials. We find that by increasing the bromide:iodide ratio, a simultaneous destabilization of acceptor defects (tin vacancies and iodine interstitials) and stabilization of donor defects (iodine vacancies and tin interstitials) occurs, with strong changes arising for Br contents exceeding 50%. This translates into a decreased doping which is, however, accompanied by a higher density of nonradiative recombination channels. Films with high Br content show a high degree of disorder and trap state densities, with the best optoelectronic quality being found for Br contents of around 33%. These observations match the open circuit voltage trend of tin-based mixed halide perovskite solar cells, supporting the relevance of optoelectronic properties and chemistry of defects to optimize wide-bandgap tin perovskite devices.

Efficient biphoton emission in semiconductors by single-photon recycling

An efficient biphoton emission process in semiconductors can enable the realization of highly tunable lasers, squeezed light sources, and entangled photon pair sources on an integrated platform. We propose a general single-photon recycling scheme to improve the overall efficiency of the typically weak biphoton emission process in a broad class of semiconductor materials. Using a rate-equation-based analysis, we first frame the general design principles and subsequently design a one-dimensional photonic crystal cavity to reach the ideal photon recycling limit for the spontaneously emitted single photons. The cavity is designed with realistic constituent materials to achieve high biphoton output efficiency in the absence of nonradiative recombination channels.

2D nanosheets of layered double perovskites: synthesis, photostable bright orange emission and photoluminescence blinking.

Lead (Pb)-free layered double perovskites (LDPs) with exciting optical properties and environmental stability have sparked attention in optoelectronics, but their high photoluminescence (PL) quantum yield and understanding of the PL blinking phenomenon at the single particle level are still elusive. Herein, we not only demonstrate a hot-injection route for the synthesis of two-dimensional (2D) ∼2-3 layer thick nanosheets (NSs) of LDP, Cs4CdBi2Cl12 (pristine), and its partially Mn-substituted analogue [i.e., Cs4Cd0.6Mn0.4Bi2Cl12 (Mn-substituted)], but also present a solvent-free mechanochemical synthesis of these samples as bulk powders. Bright and intense orange emission has been perceived for partially Mn-substituted 2D NSs with a relatively high PL quantum yield (PLQY) of ∼21%. The PL and lifetime measurements both at cryogenic (77 K) and room temperatures were employed to understand the de-excitation pathways of charge carriers. With the implementation of super-resolved fluorescence microscopy and time-resolved single particle tracking, we identified the occurrence of metastable non-radiative recombination channels in a single NS. In contrast to the rapid photo-bleaching that resulted in a PL blinking-like nature of the controlled pristine NS, the 2D NS of the Mn-substituted sample displayed negligible photo-bleaching with suppression of PL fluctuation under continuous illumination. The blinking-like nature in pristine NSs appeared due to a dynamic equilibrium flanked by the active and in-active states of metastable non-radiative channels. However, the partial substitution of Mn2+ stabilized the in-active state of the non-radiative channels, which increased the PLQY and suppressed PL fluctuation and photo-bleaching events in Mn-substituted NSs.

Enhancement of excitonic and defect-related luminescence in neutron transmutation doped β−Ga2O3

Neutron irradiation analysis, inductively coupled plasma mass spectrometry (ICPMS), and cathodoluminescence (CL) spectroscopy are used to investigate the influence of transmuted Ge incorporation on the luminescence properties of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ single crystals. Calculations based on ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$-neutron interaction reveal temporal variations of both Ge and Zn concentrations as a function of time during and after neutron irradiation. To produce a concentration of $5\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{Ge}\phantom{\rule{0.16em}{0ex}}\mathrm{donors}/\mathrm{c}{\mathrm{m}}^{3}$ from the neutron transmutation of Ga, the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystal was irradiated for 27 h, which was accompanied by the incorporation of ${10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{Zn}\phantom{\rule{0.16em}{0ex}}\mathrm{acceptors}/\mathrm{c}{\mathrm{m}}^{3}$. These calculated dopant concentrations are confirmed by ICPMS. The $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystals exhibit a UV band at 3.40 eV due to self-trapped holes (STHs) and two blue donor-acceptor pair (DAP) peaks at 3.14 eV (BL1) and 2.92 eV (BL2). In addition to the neutron-induced incorporation of substitutional Ge donors and Zn acceptors on Ga sites, Ga vacancies (${V}_{\mathrm{Ga}}$) were created by high-energy neutrons in the flux, which strongly enhanced the BL1 peak. The ${V}_{\mathrm{Ga}}$ acceptors compensate the neutron-induced Ge donors, making the ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ crystal highly resistive. Concurrent temperature-resolved CL measurements of the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ before and after neutron irradiation reveal a twofold increase in both the STH and BL1 peaks. This result suggests that STHs are preferentially localized at an O site adjacent to ${V}_{\mathrm{Ga}}$, as theoretically predicted by Kananen et al. [Appl. Phys. Lett. 110, 202104 (2017).]. Analysis of the ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ CL temperature dependence reveals that the UV and BL1 bands after the neutron irradiation exhibit an equivalent activation energy of $100\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ due to the presence of a neutron-induced defect that acts as an efficient competitive nonradiative recombination channel. The results also provide evidence that the BL1 and BL2 bands arise from different DAP pairs.

Through-Space Conjugated Supramolecular Polymer Radicals from Spatial Organization of Cucurbit[8]uril: An Efficient Approach for Electron Transfer and Smart Photochromism Materials

Electron transfer-based long-lived radicals are highly challenging because of the limited control over relative orientation, distance, electronic coupling, and nonradiative recombination channels of the donor and acceptor on a molecular level. Herein, the cavity of macrocyclic cucurbit[8]uril (Q[8]) was found to exhibit excellent advantages in controlling the relative orientation and distance of the donor and acceptor moieties via the spatial organization, i.e., the 4-carboxylphenyl appended viologen-derived guest (BcpV2+) was elegantly rearranged as a rigid linear J-type supramolecular polymer by the Q[8] host via noncovalent interactions. Thus, an unprecedented photoinduced electron transfer (PET) triggered through-space conjugated organic radical with distinct photochromism and a NIR-II photothermal effect was observed. Further studies have indicated that the Q[8] encapsulation-triggered PET cycle exhibited good repeatability without significant loss of its efficiency and had potential application in the fabrication of smart windows and erasable printing under photoirradiation or sunlight. These results suggest that the Q[8] host can be used as a new tool in light-energy conversion and photochromism materials science.

Laser‐Induced Secondary Crystallization of CsPbBr3 Perovskite Film for Robust and Low Threshold Amplified Spontaneous Emission

AbstractAll‐inorganic perovskite nanocrystals (NCs) have received extensive attention for next‐generation thin film devices due to their excellent optical properties, such as strong light absorption, high carrier mobility, and defect tolerance. However, significant challenges remain to obtain high‐quality perovskite thin films. Herein, a simple but effective post‐treatment by laser irradiation for CsPbBr3 NCs thin films is reported. Laser‐induced secondary crystallization is observed in CsPbBr3 NCs thin films after treatment. In addition, amplified spontaneous emission (ASE) with a low threshold (5.6 µJ cm−2) and a high gain value (743 cm–1) is achieved. Based on optical measurements, it is attributed to the low defect density, reduced Auger recombination, and weak exciton–phonon interactions, which greatly suppress the nonradiative recombination channels. The ASE from the film after treatment has a high characteristic temperature (134 K), showing a stable optical gain performance that maintains its intensity for 35 h at room temperature (and 12 h at 40 °C). Finally, the proof‐of‐concept demonstration of graphic coding is shown. This study deepens the understanding of the optical gain mechanism of CsPbBr3 perovskite films and provides a simple and convenient laser treatment that enables the fabrication of high‐quality CsPbBr3 perovskite thin films.

Time-resolved terahertz spectroscopy for probing the effects of low-temperature annealing on CsPbBr3 evaporated thin-films

The fine-tuning of the growth conditions and post-deposition treatments is of fundamental importance to improve the efficiency of photovoltaic devices based on all-inorganic metal halide perovskites like CsPbBr3. In this work, we used time-resolved terahertz spectroscopy (TRTS) in combination with optical characterization techniques, x-ray diffraction (XRD), and scanning electron microscopy, to probe the different properties induced by a low-temperature (180 °C) annealing treatment on evaporated CsPbBr3 thin-films. We observed a faster build-up and relaxation dynamics in the annealed sample, accompanied by a remarkable decrease of the photoluminescence intensity and minor changes in the photoconductivity and XRD measurements as compared to the as-deposited sample. We estimated for both the samples a mobility of cm2 V−1 s−1. Our results suggest that the lattice reorganization induced by low-temperature annealing of evaporated CsPbBr3 could lead to a different charge carrier-phonon coupling and to an increased contribution of non-radiative recombination channels. We found that TRTS can be effectively used to follow the changes induced by post-deposition thermal annealing of CsPbBr3.

Effect of manganese incorporation on the excitonic recombination dynamics in monolayer MoS2

Using x-ray photoelectron spectroscopy, atomic force microscopy, and Raman spectroscopy techniques, we investigate the incorporation of manganese (Mn) in monolayer (1L)-MoS2 grown on sapphire substrates by microcavity based chemical vapor deposition method. These layers are coated with different amounts of Mn by pulsed laser deposition technique. The study reveals two contrasting Mn-incorporation regimes. Below a threshold deposition amount, thin Mn-coating with large area coverage is found on MoS2 layers where substitution of Mo ions by Mn is detected through XPS. Dewetting takes place when Mn deposition crosses the critical mark, resulting in the formation of Mn-droplets on MoS2 layers. In this regime, substitutional incorporation of Mn is suppressed, while the Raman study suggests an enhancement of disorder in the lattice with the Mn deposition time. This knowledge can help us in tackling the challenge of doping of 2D transition metal dichalcogenides in general. From the temperature dependent photoluminescence study, it has been found that, even though Mn deposition enhances the density of non-radiative recombination channels for the excitons, the thermal barrier height for such recombinations to take place also rises. The study attributes these non-radiative transitions to Mo-related defects (Mo-vacancies and/or distorted Mo–S bonds), which are believed to be generated in large numbers during Mn-droplet formation stage.

Room‐Temperature Treatment with Thioacetamide Yielding Blue‐ and Green‐Emitting CsPbX3 Perovskite Nanocrystals with Enhanced Photoluminescence Efficiency and Stability

AbstractWhile multiple protocols are currently available for obtaining green‐ and red‐emitting lead halide perovskite nanocrystals (NCs) with near‐unity photoluminescence quantum yield (PLQY), achieving this target for the blue‐emitting NCs is still quite challenging. This is an impediment to the development of perovskite‐based highly efficient white LEDs of which blue is an essential component. Herein, we show that room‐temperature treatment of the blue‐emitting CsPbCl1.5Br1.5, cyan‐emitting CsPbClBr2 and green‐emitting CsPbBr3 NCs with thioacetamide not only enhances the PLQY to near‐unity (∼97–99%) without affecting the PL maximum and bandwidth, but also improves the stability of the systems significantly. The investigation reveals that effective passivation of under‐coordinated surface Pb2+ through formation of Pb−S and Pb−NH2 bonds and removal of excess lead atoms suppress the nonradiative recombination channels and enhances the PLQY and stability of the NCs. Outstanding PL efficiency and superior long‐term stability make these systems potential ingredients for various lighting and display applications.

Beyond the Phase Segregation: Probing the Irreversible Phase Reconstruction of Mixed-Halide Perovskites.

Mixed‐halide perovskites can undergo a photoinduced phase segregation. Even though many reports have claimed that such a phase segregation process is reversible, what happens after phase segregation and its impact on the performance of perovskite‐based devices are still open questions. Here, the phase transformation of MAPb(I1− x Br x )3 after phase segregation and probe an irreversible phase reconstruction of MAPbBr3 is investigated. The photoluminescence imaging microscopy technique is introduced to in situ record the whole process. It is proposed that the type‐I band alignment of segregated I‐rich and Br‐rich domains can enhance the emission of the I‐rich domains by suppressing the nonradiative recombination channels. At the same time, the charge injection from Br‐rich to I‐rich domains drives the expulsion of iodide from the lattice, and thus triggers the reconstruction of MAPbBr3. The work highlights the significance of ion movements in mixed‐halide perovskites and provides new perspectives to understand the property evolution.

Monolithically integrated InGaAs/AlGaAs multiple quantum well photodetectors on 300 mm Si wafers

Near infrared light detection is fundamental for sensing in various application fields. In this paper, we detail the properties of InGaAs/AlGaAs multiple quantum well (MQW) photodetectors (PDs) monolithically integrated by direct epitaxy on 300 mm Si(001) substrates. A MQW high crystalline quality is achieved using 300 mm Ge/Si pseudo-substrates with a low threading dislocation density of 4 × 107 cm−2 from electron channeling contrast imaging measurements. The localized states in the MQW stack are investigated using temperature-dependent photoluminescence. Two non-radiative recombination channels are identified. The first one is due to delocalized excitons generated by potential’s fluctuations because of the InGaAs/AlGaAs interfacial roughness (with an activation energy below 4 meV). The second one is due to exciton quenching because of the presence of numerous threading dislocations. A low dark current density of 2.5 × 10−5 A/cm2 is measured for PDs on Ge/Si substrates, i.e., a value very close to that of the same PDs grown directly on GaAs(001) substrates. A responsivity of 36 mA/W is otherwise measured for the photodiode on Ge/Si at room temperature and at −2 V.

Optical properties of N-polar GaN: The possible role of nitrogen vacancy-related defects

Detailed comparison of optical quality of GaN layers grown homoepitaxially on bulk Ga-polar and N-polar substrates by plasma-assisted molecular beam epitaxy was performed. One order of magnitude lower photoluminescence (PL) intensity and decay time at room temperature was observed for layers grown on N-polar substrates, realized using gallium-rich conditions. This nonradiative recombination channel was very efficient also at liquid helium temperatures, what was evidenced by no improvement in PL time decay. Optical quality of GaN layer (PL intensity, lifetimes) grown on N-polar substrates was greatly improved by using nitrogen-rich growth conditions. We attribute this improvement to suppressed formation of nitrogen vacancy-related point defects. Finally, linewidth below 1 meV and lifetime above 0.1 ns (DX line, He temperature) were obtained for layers grown on N-polar substrate.

Triplet exciton formation for non-radiative voltage loss in high-efficiency nonfullerene organic solar cells

<h2>Summary</h2> Nonfullerene acceptor (NFA) organic solar cells (OSCs) with power conversion efficiency (PCE) reaching up to 18% has shown tremendous potential toward practical applications. However, the non-radiative recombination loss in high-efficiency NFA OSCs is evidently large, and the origin remains unclear. Herein, by combining transient absorption spectroscopy, photovoltaic characterization, and detailed energy loss analysis, we unambiguously show the formation of NFA triplet exciton from free carrier recombination as a main non-radiative recombination and energy loss channel. More importantly, in contrast to the previous energetics point of view, we show side-chain fluorination in high-efficiency NFA OSCs has little effect on charge transfer energy landscapes but efficiently inhibits charge recombination to NFA triplet excitons, leading to higher <mml:math><mml:mrow><mml:mi>V</mml:mi>OC</mml:mrow></mml:math> and PCE. Our findings provide a new view about energy loss in NFA OSCs and pave the way toward OSCs with lower <mml:math><mml:mrow><mml:mi>V</mml:mi>OC</mml:mrow></mml:math> loss and efficiency exceeding 20%.

Ultrafast electron–hole relaxation dynamics in CdS nanocrystals

Surface traps significantly influence the charge carrier dynamics within semiconductor nanocrystals, introducing non-radiative exciton recombination channels which are detrimental for their applications. Understanding the nature of these trap states and modulating them synthetically bears immense potential in designing defect-free colloidal semiconductor nanocrystals for efficient optoelectronic devices. Thus, systems devoid of surface traps can be used to study the relaxation pathways of excitons generated within these nanocrystals. In this work, we study the ultrafast charge carrier relaxation dynamics upon near-edge resonance excitation and above-resonance excitation in CdS nanocrystals using ultrafast transient absorption spectroscopy, in order to understand intraband cooling and mid-gap trap states. The time-resolved studies reveal that the above bandgap excitation results in a three-step process, including instantaneous growth followed by a fast sub-picosecond decay and a long-lived (>1 ns) excited state or a trap state recombination. The large percentage of long-lived excitons in CdS nanocrystals elucidates the defect-free nature of the system arising from the absence of surface states.

Advanced hydrogenation process applied on Ge on Si quantum dots for enhanced light emission

For the development of photonic integrated circuits, it is mandatory to implement light sources on a Si-on-insulator (SOI) platform. However, point defects in the Si matrix and, e.g., at the Si/SiO2 interface act as nonradiative recombination channels, drastically limiting the performance of Si-based light emitters. In this Letter, we study how these defects can be healed by applying an advanced hydrogenation process, recently developed in photovoltaic research for the passivation of performance-limiting defects in Si solar cells. Upon hydrogenation, we observe an increase in the room temperature photoluminescence (PL) yield by a factor of more than three for defect-enhanced quantum dots (DEQDs) grown on float-zone Si substrates, revealing the potential of this technique to passivate detrimental defects. For DEQDs grown using SOI substrates, the PL yield enhancement even exceeds a factor of four, which we attribute to the additional passivation of defects originating from the substrate. The results for SOI substrates are of particular interest due to their relevance for future photonic integrated circuits.

Oxygen annealing induced changes in defects within β-Ga2O3 epitaxial films measured using photoluminescence

In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy induced by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72–0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of V Ga and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.

Midinfrared Emission and Absorption in Strained and Relaxed Direct-Band-Gap Ge1−xSnx Semiconductors

By independently engineering strain and composition, this work demonstrates and investigates direct-band-gap emission in the midinfrared range from ${\mathrm{Ge}}_{1\text{\ensuremath{-}}x}{\mathrm{Sn}}_{x}$ layers grown on silicon. We extend the room-temperature emission wavelength above approximately 4.0 \textmu{}m upon postgrowth strain relaxation in layers with uniform Sn content of 17 at.%. The fundamental mechanisms governing the optical emission are discussed based on temperature-dependent photoluminescence, absorption measurements, and theoretical simulations. Regardless of strain and composition, these analyses confirm that single-peak emission is always observed in the probed temperature range of 4--300 K, ruling out defect- and impurity-related emission. Moreover, carrier losses into thermally activated nonradiative recombination channels are found to be greatly minimized as a result of strain relaxation. Absorption measurements validate the direct band-gap in strained and relaxed samples at energies closely matching photoluminescence data. These results highlight the strong potential of ${\mathrm{Ge}}_{1\text{\ensuremath{-}}x}{\mathrm{Sn}}_{x}$ semiconductors as versatile building blocks for scalable, compact, and silicon-compatible midinfrared photonics and quantum optoelectronics.