BaCd 2 P 2 : A Promising Impurity‐Tolerant Counterpart of Gaas for Photovoltaics

We use multimodal microscopy to study carrier recombination in semiconducting tin halide perovskite films based on PEA0.2FA0.8SnI3 (PEA = phenethylammonium; FA = formamidinium). We use the observation of pseudo-first-order photoluminescence (PL) decay kinetics to establish a method for quantifying the hole dopant level and nonradiative recombination rate constant. We find that untreated PEA0.2FA0.8SnI3 films exhibit large hole doping concentrations of p0 ≈ 1019 cm–3, which is reduced to p0 ≈ 1016 cm–3 after SnF2 treatment. While it is well-known that the radiative recombination rates are increased with p0, we reveal that the nonradiative rate is also increased. We find that p-type regions in untreated PEA0.2FA0.8SnI3 films are centers for nonradiative recombination, which are diminished in films with p0 ≈ 1016 cm–3. We discover significant PL heterogeneity even in PEA0.2FA0.8SnI3 films with moderate dopant levels, suggesting that new strategies to eliminate deleterious defects in PEA0.2FA0.8SnI3 must be developed.

Presence and influence of nonradiative recombination (NRR) centers in an intermediate band (IB)‐type material, GaP1–xNx (), are studied by two‐wavelength excited photoluminescence (TWEPL) method and time‐resolved photoluminescence (TRPL) measurement at 77 K. With the use of below‐gap excitation (BGE) light in addition to an above‐gap excitation (AGE), the PL peak intensity is found to increase which indicates the presence of NRR centers and a secondary excitation from the IB to conduction band (CB). Depending on the effect of different BGE energies, an energy diagram on the distribution of NRR centers and NRR process is interpreted. The saturation of PL increase is attributed to the trap‐filling effect in NRR centers, which allows us to modify the rate equation. The NRR parameters are evaluated by a qualitative simulation of the modified rate equations of one‐level model together with the lifetime determined by TRPL. In continuation of evaluating NRR parameters by rate equation analysis, the addition of TRPL measurement improves accuracy and approaches the determination of NRR parameters. A successful characterization of NRR centers leads to a proper optimization of IB‐type solar cells (IBSCs).

The interactions between Bi dopants including Bi-substituting Li (BiLi) and Bi-substituting Nb (BiNb) and the intrinsic antisite defects (NbLi) and Li vacancies (VLi) in LiNbO3 are investigated using local and hybrid density functional theories. Three charge-compensated defect clusters, BiLi4+ + NbLi4+ + 8VLi-, BiLi4+ + 4VLi-, and BiLi4+ + BiNb0 + 4VLi-, are modeled in this work to investigate the effects of the Bi concentration. The most stable cluster configurations, the Bi-doping stability in the clusters, and the electronic state interaction between Bi and intrinsic defects have been studied in detail. It is found that BiLi4+ has a stronger electron-capturing ability than NbLi4+ in Bi-doped congruent LiNbO3. The BiLi-doping-induced local lattice distortion and the electron-trapping behavior remain unchanged with increasing Bi-doping concentration. However, the position of the Bi defect states in the band gap is found to be shifted in congruent LiNbO3. This is mainly attributed to the large lattice relaxation induced by the large number of Li vacancies instead of the ionic level redistribution caused by the direct interaction between Bi and intrinsic defects.

The synthesis of silicon quantum dots (SiQDs) via thermal pyrolysis is considered promising due to its cost-effectiveness. The etching process in this method has the potential to control the size of SiQDs precisely and has thus garnered attention. However, there are varying observations regarding the effect of etching duration on SiQD size. Additionally, the impact of dioxonium hexafluorosilicate (DH), a byproduct of the etching process, on the photoluminescence (PL) quantum yield (QY) of SiQDs remains unclear. This study investigates the effect of etching duration on the physical and optical sizes as well as the PLQY of SiQDs. The results indicate that extending the etching duration decreases the physical size of SiQDs, while the optical size initially increases slightly before decreasing. The SiQDs transition through three phases with increasing etching duration: oxidation removal, shallow over-etching, and deep over-etching. Both amorphous silicon (a-Si) in the oxidation removal phase and DH in the deep over-etching phase act as non-radiative recombination centers, thereby reducing the PLQY of SiQDs. Therefore, optimizing the etching duration to achieve the shallow over-etching phase is essential. This study provides new insights into the effects of etching duration on SiQD size and PLQY, aiding in the preparation of higher-quality SiQDs.

Comparative study on nonradiative recombination (NRR) centers in InAs/GaAs quantum dot (QD) structure generated by 3 MeV proton irradiation is performed by two wavelength excited photoluminescence (TWEPL). The above-gap excitation (AGE) source of 2.33 eV or 1.26 eV is used to excite GaAs host material or InAs QDs, respectively. The QD PL intensity decreased after irradiation of below-gap excitation (BGE) of 0.75 eV over AGE, indicating a pair of NRR centers activated. The proton irradiation at fluence of $7\times 10^{11}\mathrm{cm}^{-2}$ reduces the NRR density, while that of ${4\times 10^{12}\mathrm{cm}^{-2}}$ increases it. Defect formation, carrier injection and their fluence dependence explain experimental results.

Hybrid-perovskite-based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF4 - , introduced via methylammonium tetrafluoroborate (MABF4 ) in a surface treatment for MAPbI3 perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non-radiative charge-carrier recombination with a reduction in first-order recombination rate from 6.5 × 106 to 2.5 × 105 s-1 in BF4 - -treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). 19 F, 11 B, and 14 N solid-state NMR is used to elucidate the atomic-level mechanism of the BF4 - additive-induced improvements, revealing that the BF4 - acts as a scavenger of excess MAI by forming MAI-MABF4 cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF4 - .

In this work we present degradation studies of GaN based blue‐violet laser diodes grown on different substrates. By replacing the SiC substrate by GaN substrate, we change from hetero to homo epitaxy, while the threading dislocation density (TDD) is reduced by 3 orders of magnitude. A detailed analysis of small signal I –V curves shows an increase of non radiative (NR) recombination centers during aging for laser diodes on SiC substrate. This was not observed for lasers on GaN substrate due to the reduced TDD and therefore reduced number of diffusion channels for Mg‐atoms, acting as NR recombination centers in the active region. With an improved epitaxial structure on GaN substrate, we increased the lifetime of our lasers by a factor of 10. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-wavelength excited photoluminescence, a non-destructive and non-contacting method for defect study has been used to investigate the AlGaN multiple quantum well (MQW) emitting at ∼260 nm. We have succeeded to detect the non-radiative recombination (NRR) centers in both wells and barrier layers using a below-gap ex- citation (BGE) light of 1.17 eV. The PL intensity of both well and barrier layers decreases after irradiation of BGE which can be explained by the well-established two-level model. The normalized PL intensity decreases with increasing BGE power density for both cases but more pronounced for the barrier layer. The normalized PL intensity increases for both well and barrier layer's emission with increasing AGE density and becomes saturated at higher excitation due to the fill-up of NRR centers.

Nonradiative recombination (NRR) centers in n-type GaN samples grown by MOCVD technique on a LT-GaN buffer layer and aAlN buffer layer have been studied by two wavelength excited photoluminescence (TWEPL). The near band-edge photoluminescence (PL) intensity decreases due to the superposition of below-gap excitation (BGE) light of energies 0.93, 1.17 and 1.27 eV over above-gap excitation (AGE) light of energy 4.66 eV. The decrease in PL intensity due to the addition of the BGE has been explained by a two levels recombination model based on SRH statistics. It indicates the presence of a pair of NRR centers in both samples, which are activated by the BGE. The degree of quenching in PL intensity for the sample grown on LT-GaN buffer layer is stronger than the sample grown on AlN buffer layer for all BGE sources. This result implies that the use of the AlN buffer layer is more effective for reducing the NRR centers in n-GaN layers than the LT-GaN buffer layer. The dependence of PL quenching on the AGE density, the BGE density and temperature has been also investigated. The NRR parameters have been quantitatively determined by solving rate equations and fitting the simulated results with the experimental data.

A number of groups have studied reliability and degradation processes in GaAs-based lasers, but none of these studies have yielded a reliability model based on the physics of failure. Unsuccessful development of this model originates from the facts that: (i) defects related phenomena responsible for degradation in GaAs-based lasers are difficult to study due to the lack of suitable non-destructive techniques and (ii) degradation process occurs extremely fast after a long period of latency. Therefore, most of laser diode manufacturers perform accelerated multi-cell lifetests to estimate lifetimes of lasers using an empirical model, but this approach is a concern especially for satellite communication systems where high reliability is required of lasers for long-term duration in the space environment. Since it is a challenge to control defects introduced during the growth of laser structures, we studied degradation processes in broad-area InGaAs-AlGaAs strained quantum well (QW) lasers with intrinsic defects as well as those with defects introduced via proton irradiation. For the present study, we investigated the root causes of catastrophic degradation processes in MOCVD-grown broad-area InGaAs-AlGaAs strained QW lasers using various failure mode analysis techniques. A number of lasers were proton irradiated with different energies and fluences. We also studied GaAs double heterostructure (DH) test samples with different amounts of intrinsic defects introduced during MOCVD growth. These samples were proton irradiated as well to introduce additional defects. Deep level transient spectroscopy (DLTS) and time resolved photoluminescence (TR-PL) techniques were employed to study traps (due to point defects) and non-radiative recombination centers (NRCs) in pre- and poststressed lasers, respectively. These characteristics were compared with those in pre- and post-proton irradiated lasers and DHs to study the role that defects and NRCs play in catastrophic degradation processes. Lastly, we employed focused ion beam (FIB), electron beam induced current (EBIC), and high resolution TEM (HR-TEM) techniques to study dark line defects and crystal defects in both post-aged and post-proton irradiated lasers.

High-pressure research on nanostructured materials has been of considerable interest owing to the quantum confinement effect and intrinsic defects in the nanocrystals. Here, we report a pressure-induced reversible structural phase transition in nanostructured Bi2Te3 hierarchical architectures (HAs) that were prepared via a facile solution-phase method. Therein, distinct phases I–IV by respectively adopting crystal structures of rhombohedral (I), monoclinic (II, III), and cubic (IV) were experimentally identified with increasing pressure up to 20.2 GPa in a diamond anvil cell (DAC). It is worthwhile to notice that nanostructured Bi2Te3 HAs ultimately evolved into a fascinating Bi–Te substitutional nonmetallic alloy at pressure even as low as 15.0 GPa, approximately 10 GPa lower than that of the corresponding bulk counterpart. The synergistic effect involving large volume collapse and the unique one-dimensional nanostructures with intrinsic antisite defects was proposed to be responsible for the reduction of transition pressure that is contrary to the general model for most nanomaterials. Our findings may pave a potential pathway for developing future multifunctional nanoalloys that are composed of nonmetallic elements.

The laser heterodyne photothermal displacement (LH-PD) method was used to characterize the nonradiative recombination centers of semiconductors, such as defects and deep-lying electronic levels. When a semiconductor surface is irradiated with a modulated continuous wave laser, the irradiated area is periodically heated and expanded owing to the nonradiative recombination of the photoexcited carriers. The LH-PD can measure an absolute value of surface displacement and its time variation at various excitation beam frequencies (fex). Si and GaAs substrate samples were used to confirm the usefulness of the proposed method. The obtained time variation of the surface displacement was well explained by theoretical calculations considering the carrier generation, diffusion, recombination, heat diffusion, and generated thermal strain. Because nonradiative carrier recombination generates local heat at defects in semiconductors, the LH-PD technique is useful for analyzing defect distributions. Additionally, measurements of intentional Fe-contaminated Si samples confirmed that this technique is suitable for defect mapping. Displacement mapping with changing fex suggests the potential to measure the distribution of nonradiative recombination centers in the sample depth direction.

Optimizing the photoluminescence quantum yield (PLQY) of non-fullerene acceptor (NFA) molecules is critical for reducing the non-radiative recombination energy loss in NFA-based organic solar cells. In this letter, by developing a non-adiabatic dynamical method combined with different electron population rate equations, we separately simulate the radiative and non-radiative recombination process of the NFA molecular excited state, and thus clarify the quantitative correlations of typical characteristics of NFA molecules with their PLQY, including the analyses for the corresponding mechanisms directed against the conventional "energy gap law". The main findings include: weakening the intramolecular electron-phonon coupling and electronic push-pull potential can optimize the competition between radiative and non-radiative recombinations, thus improving PLQY; furthermore, increasing the intermolecular J-aggregation ratio should be an effective strategy to alleviate the aggregation-induced reduction in PLQY. These findings provide clear directions for the rational design of NFA molecules and morphology optimization toward a higher PLQY.

Nonradiative recombination (NRR) centers in AlGaN multiple quantum well samples, grown on sapphire substrate at two different growth temperatures by the MOCVD technique, have been studied by using below-gap excitation (BGE) light in photoluminescence (PL) measurements at about 25 K. The PL intensity decreased by the superposition of BGE light of energies between 0.93 and 1.46 eV over above-gap excitation light of energy 4.66 eV. This is explained by a two-level model based on Shockley–Read–Hall statistics. The model indicates the presence of a pair of NRR centers in both samples, which are activated by the BGE. The degree of PL quenching for the sample grown at 1140 °C is higher than that of the sample grown at 1180 °C for BGE energies 0.93, 1.17, and 1.27 eV. The density ratio of 1.5, for the BGE energy of 1.27 eV, was obtained from a qualitative simulation. This result implies that a slight difference in growth conditions changes defect densities.

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.