Donor-acceptor Pair Recombination Research Articles

General algorithm for characterization of donor-acceptor pair recombination processes in solid-state materials

Radiative recombination processes can occur in solid-state systems through the pairing of donor and acceptor defects of the lattice. Recently, donor-acceptor pairs (DAP) have been proposed as promising candidates for quantum applications, and their signature has been observed in emerging low-dimensional materials. Therefore, the identification of such processes is gaining interest and requires methods to efficiently and reliably characterize them. Here, we introduce a general algorithm to identify DAP processes starting from the experimental photoluminescence (PL) emission spectrum and basic material parameters, including the lattice structure and dielectric constant. The algorithm recognizes possible DAP transitions from the emission pattern in the spectrum and returns the characteristic energy of the DAP transition and the separation between the donor and acceptor sites. By testing the algorithm on the photoluminescence spectrum of hexagonal boron nitride (hBN), we show that our method is robust against experimental errors and adds new capabilities to the investigation toolbox of semiconductors and their optical properties.

Effect of Post-Implantation Heat Treatment Conditions on Photoluminescent Properties of Ion-Synthesized Gallium Oxide Nanocrystals.

A novel and promising way for creating nanomaterials based on gallium oxide is the ion synthesis of Ga2O3 nanocrystals in a SiO2/Si dielectric matrix. The properties of nanocrystals are determined by the conditions of ion synthesis-the parameters of ion irradiation and post-implantation heat treatment. In this work, the light-emitting properties of Ga2O3 nanocrystals were studied depending on the temperature and annealing atmosphere. It was found that annealing at a temperature of 900 °C leads to the appearance of intense luminescence with a maximum at ~480 nm caused by the recombination of donor-acceptor pairs. An increase in luminescence intensity upon annealing in an oxidizing atmosphere is shown. Based on data from photoluminescence excitation spectroscopy and high-resolution transmission electron microscopy, a hypothesis about the possibility of the participation of a quantum-size effect during radiative recombination is proposed. A mechanism for the formation of Ga2O3 nanocrystals during ion synthesis is suggested, which makes it possible to describe the change in the luminescent properties of the synthesized samples with varying conditions of post-implantation heat treatment.

Origin of discrete donor–acceptor pair transitions in 2D Ruddlesden–Popper perovskites

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light–matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by μ-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths (≃120μeV) and long decay times (5–8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden–Popper perovskites, (PEA)2(MA)n−1PbnI3n+1, particularly in phases with n≥2, has been identified as a mechanism of donor–acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of ≃10 nm. The analysis of the spectral and electronic characteristics of these single donor–acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions.

Phosphorescence and donor-acceptor pair recombination in laboratory-grown diamonds

Phosphorescence and donor-acceptor pair recombination in laboratory-grown diamonds

High-resolution photoluminescence study on donor-acceptor pair (DAP) recombination in silicon crystals co-doped with phosphorous and gallium

Before lower purity, lower cost silicon (Si) materials, such as compensated Si, can play a role in the terawatt-level (TW) capacity of photovoltaics, a better understanding of the fundamental properties of impurities in compensated Si is essential. In this work, high-resolution photoluminescence (PL) has been used to study the charge carrier radiative recombination through Donor-Acceptor pairs (DAPs) in phosphorus (P) and gallium (Ga) co-doped Si material grown for solar cell applications. The high spectral resolution of our PL system, 0.06 meV, enables us to overcome hitherto prior issues of overlapping spectral lines, giving access to extremely fine structures associated with DA pair (DAP) recombination. Our results confirm the presence of three broad bands and a discrete line structure related to DAP luminescence. The comparison of the discrete line structure due to DAPs recombination in the PL spectra with the theoretically predicted one allows the accurate determination of the Ga ionization energy. Temperature-dependent PL is then used to understand the thermally-induced changes in the DAP luminescence. In particular, we observe that the radiative recombination channel remains active for distant DAPs up to ∼40 K, unlike that for close-range DAPs for which the radiative channel is quenched after only slight increases in the temperature range 10–25 K. Furthermore, the analysis of the temperature dependent changes in the PL intensity of the broad DAP bands up to ∼200 K is used to derive the ionization energy of P donors in compensated Si material. In light of this important information, the significance of using high resolution PL to analyse spectral features in compensated Si is demonstrated.

Temperature-dependent electroluminescence of a gate pulsed silicon carbide metal–oxide–semiconductor field-effect transistor: Insight into interface traps

Switching a silicon carbide (SiC) metal–oxide–semiconductor field-effect transistor between inversion and accumulation with removed drain and grounded source terminals leads to defect-assisted carrier recombination and light emission. The energy spectrum of the emitted photons provides valuable information on the involved defects, located both at the 4H-SiC/SiO2 interface and in the 4H-SiC bulk. Here, we measured and analyzed the emitted light over a broad temperature range between 12 and 297 K. Our results reveal two local maxima in light intensity around 30 and 140 K. Most importantly, the local intensity maxima and the related temperatures correlate with both the overall recombination current and gate capacitance measurements. The spectral analysis allowed us to distinguish between recombinations occurring on 4H-SiC bulk defects and 4H-SiC/SiO2 interface-related defects. We explain an initial increase of light emission with decreasing temperature to competing non-radiative pathways with activation energies of 34 and 60 meV for SiC/SiO2 interface- and 4H-SiC bulk-related emissions, respectively. Based on an extensive literature review, we link the measured photon emission to donor–acceptor pair recombination, the EH6/7 or the Z1/2 defect centers. In addition to that, we could link a prominent peak at 2.915 eV to the L1 line of the D1-center. Most importantly, we conducted our own ab initio simulations revealing that recombination via PbC-centers, previously identified with carbon dangling bonds at the 4H-SiC/SiO2 interface [Cottom et al., J. Appl. Phys. 124, 045302 (2018)], could also provide an explanation for the photon emission around 1.75 eV. Finally, our simulation of an interface-related silicon vacancy VSi,I reveals a radiative transition around 2.8 eV.

Computational study of defect complexes in β-LiGaO2 and their relation to the donor–acceptor-pair recombination

Hybrid functional calculations are presented for defects in LiGaO2 with the fraction of non-local exchange adjusted to reproduce the recently reported exciton gap of 6.0 eV. We study how the defect transition levels of the main native defects change with respect to the band edges compared to earlier calculations which assumed a smaller bandgap near 5.1 eV. In addition, we consider defect complexes formed by combining the main native donor GaLi with the main acceptors, VLi and LiGa antisites as a function of their relative position. These results are used to tentatively identify the photoluminescence bands previously assigned to donor–acceptor-pair recombination.

Electroluminescence Spectra of a Gate Switched MOSFET at Cryogenic and Room Temperatures Agree with Ab Initio Calculations of 4H-SiC/SiO<sub>2</sub>-Interface Defects

To reach the theoretical performance limit of 4HSiCMOSFETs the SiC/SiO2interfacedefects along the inversion channel need to be fully identified in order to be avoided. We employa measurement technique that allows to observe energetically resolved trap states at the SiC/SiO2 interface by measuring the electrolumiscence of a gate pulsed MOSFET. The spectra are recorded at room and cryogenic temperatures with a spectrometer and two different amplitudes of the gate pulse. Comparison of the results to literature allows for identification of the L1 line of the D1 center with an energy of 2.9 eV and suggests donoracceptorpair recombination or Z1/2 to be responsible for the emission around 2.5 eV. Ionization energies of PbC and related vacancy centers determined via ab initio calculations show similar results as the experimental data and provide a possible classification of the trap level around 1.8 eV.

Donor-Acceptor Pairs Recombination as the Origin of the Emission Shift In InGaN/GaN Scintillator Heterostructures Doped with Zn

We report luminescence decay characteristics of the InGaN/GaN scintillator heterostructures doped with Zn. Unusually large shifting of luminescence band caused by Zn acceptors incorporated in InGaN is observed both in time-resolved and excitation-dependent spectra. Origins of the shifts are discussed, and model based on donor-acceptor pair recombination is introduced. The results imply a shrinkage of donor Bohr radius compared to the bulk material caused by quantum confinement effect. The slow decay of Zn band points out to the necessity of Zn impurity elimination in applications requiring fast timing characteristics of a scintillator.

Hydrogen-related 3.8 eV UV luminescence in α -Ga2O3

Temperature-dependent photoluminescence was used to investigate the impact of H on the optical properties of α-Ga2O3 films grown by halide vapor phase epitaxy. An additional UV luminescence line centered at 3.8 eV is observed at low temperatures, which strongly correlates with the concentration of H in the films. This luminescence line is assigned to donor–acceptor pair recombination involving an H-related shallow donor and H-decorated Ga vacancy (VGa-nH) as the acceptor, where n = 1, 2, 3. Previous reports have already suggested the impact of H on the electrical properties of Ga2O3, and the present study shows its clear impact on the optical properties of α-Ga2O3.

Atomic structures and carrier dynamics of defects in a ZnGeP2 crystal

ZnGeP2 (ZGP) crystals have attracted tremendous attention for their applications as frequency conversion devices. Nevertheless, the existence of native point defects, including at the surface and in the bulk, lowers their laser-induced damage threshold by increasing their absorption and forming starting points of the damage, limiting their applications. Here, native point defects in a ZGP crystal are fully studied by the combination of high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and optical measurements. The atomic structures of the native point defects of the Zn vacancy, P vacancy, and Ge-Zn antisite were directly obtained through an HAADF-STEM, and proved by photoluminescence (PL) spectra at 77 K. The carrier dynamics of these defects are further studied by ultrafast pump-probe spectroscopy, and the decay lifetimes of 180.49, 346.73, and 322.82 ps are attributed to the donor Vp+ → valence band maximum (VBM) recombination, donor GeZn+ → VBM recombination, and donor–acceptor pair recombination of Vp+ → VZn-, respectively, which further confirms the assignment of the electron transitions. The diagrams for the energy bands and excited electron dynamics are established based on these ultrahigh spatial and temporal results. Our work is helpful for understanding the interaction mechanism between a ZGP crystal and ultrafast laser, doing good to the ZGP crystal growth and device fabrication.

Narrow-Bandwidth Blue-Emitting Ag-Ga-Zn-S Semiconductor Nanocrystals for Quantum-Dot Light-Emitting Diodes.

I-III-VI type semiconductor nanocrystals (NCs) have attracted considerable attention in the display field. Herein, we realized the synthesis of narrow-bandwidth blue-emitting Ag-Ga-Zn-S (AGZS) NCs via a facile one-pot method. Intriguingly, the Ag/Zn feeding ratio and Ag/Ga feeding ratio are crucial for the realization of narrow-bandwidth AGZS NCs. By choosing a Ag/Zn feeding ratio of 4:1 and Ag/Ga feeding ratio of 1:8, AGZS NCs demonstrate a typical blue emission at 470 nm with a narrow full width at half-maximum (fwhm) of 48 nm, which is mainly generated from the band-to-hole recombination rather than the donor-acceptor pair (DAP) recombination. Furthermore, a solution-processed quantum-dot light-emitting device based on AGZS NCs exhibits a narrow electroluminescent bandwidth of 53 nm and high luminance over 123.1 cd m-2, as well as a high external quantum efficiency (EQE) of 0.40%. Our work highlights AGZS NCs with high color purity as an important candidate for blue-light-emitting devices.

Unraveling the Defect-Dominated Broadband Emission Mechanisms in (001)-Preferred Two-Dimensional Layered Antimony-Halide Perovskite Film

By adding molar-controlled SbCl3 in a Cs3Sb2Cl9 precursor, we employed a low-temperature solution-processed approach to prepare high-quality (001)-preferred Cs3Sb2Cl9 thin film, which demonstrates a stable defect-dominated broadband emission at room temperature. Density functional theory calculations reveal that the defect emission originates from the donor-acceptor pair (DAP) recombination between chlorine vacancy (VCl) and cesium vacancy (VCs). Furthermore, VCl + VCs DAP is more stable on the (001) surface. The improved film quality and the more stable VCl + VCs DAP increase the activation energy related to defect states, resulting in an enhancement of the defect emission for the high-quality (001)-preferred film. This work provides deep insight into the key role of the (001) surface in defect emission and a feasible strategy to enhance the defect emission in 2D halide perovskites A3B2X9 (A = CH3NH3, Cs, Rb; B = Bi, Sb; X = Cl, Br, I) by control of the thin film preferred orientation.

Continuous Flow Aqueous Synthesis of Highly Luminescent AgInS2 and AgInS2/ZnS Quantum Dots

Continuous flow synthesis of semiconductor quantum dots (QDs) holds the promise of being highly reproducible, being scalable, and providing precise control of all reaction parameters. Here, we applied this technique to the aqueous synthesis of the Ag–In–S (AIS) core and AIS/ZnS core/shell QDs and optimized several parameters comprising reaction temperature, pressure, time, nature, and the ratio of precursors. Photoluminescence quantum yield (PLQY) values of 32%/44% (average/best) for the core and 77%/83% for the core/shell system have been obtained in short reaction times (8–15 min). We demonstrate by means of combined structural and optical studies that the high PLQY originates from donor–acceptor pair recombination processes, involving essentially [InAg2+ + 2VAg–] defect complexes whose formation is favored by the large excess of indium used (In:Ag ratio of 4:1), and the low reaction temperature (100–120 °C). The structural disorder is further enhanced during ZnS shell growth, which in addition to surface passivation and removal of nonradiative decay channels leads to the partial diffusion of the added zinc ions into the AIS core and the formation of ZnIn− antisite defects. The presented method provides excellent reproducibility and high scalability, facilitating the large-scale production of highly luminescent AIS/ZnS QDs.

OPTICAL PROPERTIES OF FeGaInS4 SINGLE CRYSTALS UNDER LASER EXCITATION

We experimentally investigated photoluminescence, photocurrent relaxation, and the lux-ampere characteristic of photoconductivity in FeGaInS4 single crystals upon second-harmonic excitation by a pulsed YAG:Nd3+ laser. It is shown that the photoluminescence band can be associated with the recombination of bands on impurity or the recombination of donor-acceptor pairs. The decrease in the lifetime upon excitation is due to the high concentration of nonequilibrium carriers generated by high-power laser radiation. An explanation of the lux-ampere characteristic of photoconductivity is given.

Anisotropic photoluminescence of β-LiGaO2 crystal

Manifestation of the anisotropic properties of β-LiGaO2 crystal revealed in photoluminescence (PL) excitation and emission has been studied. Three exciton bands at 6.043, 6.085 and 5.980 eV, corresponding to three crystallographic axes, have been found in the PL excitation spectrum under the synchrotron irradiation, whose relative contribution depends on crystal orientation. Study of the properties of polarized luminescence 4.43 eV under the excitation with ArF laser (193 nm) with varied intensity in the 10–300 K temperature range has demonstrated that the excitation-induced and temperature-induced band's energy and intensity behaviour depends on the orientation of the crystal in relation to the incident light. The simultaneous coexistence of two radiative processes is proposed for the explanation of the PL properties of β-LiGaO2 crystal: the tunnel recombination of donor-acceptor pairs (DAP) and the recombination of a free electron with an acceptor center (e-A). An assumption is done about the complex nature of the 4.43 eV emission band consisting of subbands corresponding to the DAP radiative recombination in three crystallographic directions, and a subband due to the e-A radiative recombination. Dependence of the luminescence thermal quenching properties on excitation density are discussed.

Highly luminescent air-stable AgInS2/ZnS core/shell nanocrystals for grow lights

Plants need lights in the blue (400–500 nm) and red (600–700 nm) spectral regions for the photosynthesis to grow. Lights to match the mentioned spectral ranges can be provided from a blue LED and red luminescence materials. This article presents the high yield solvothermal syntheses of AgInS2 (AIS) core nanocrystals (NCs) and AIS/ZnS core/shell NCs that emit stably strong luminescence peaking around 665 nm under the excitation of a blue LED. The solvothermal syntheses of AIS core NCs were performed in oleylamine solvent added with 1,3-dimethylthiourea at 200 °C for a relatively short time of 20 min, in a closed Teflon vessel. Then, the ZnS shells with different thicknesses of 1, 3, 5, and 7 monolayers were coated on the same AIS core NCs at 150 °C for 60 min under open air. The synthesised AIS core and AIS/ZnS core/shell NCs were characterised by using X-ray diffraction (XRD), transmission electron microscopy (TEM), and optical spectroscopies (UV–Vis absorption and photoluminescence (PL), including steady state and time-resolved PL). With increasing the thickness of the ZnS shell, AIS/ZnS core/shell NCs become more stable under open air to emit the strong luminescence in the 600–700 nm range, peaking at ∼650 nm with the high luminescence quantum yield (QY) up to ∼60%. The blue-shift of the emission and increase of the QY after shelling AIS core NCs with ZnS are rationalized by the formation of the partially alloyed structure and passivation of surface states. In addition, time-resolved PL spectra exhibit to be red-shifted with the delay time after the excitation that is evident for the nature of donor–acceptor pairs recombination in AIS/ZnS core/shell NCs. Coating of layers of AIS/ZnS core/shell NCs dispersed in transparent epoxy on the 460-nm LED as assays for the grow light sources are also illustrated.

Defect-Induced Luminescence Quenching of 4H-SiC Single Crystal Grown by PVT Method through a Control of Incorporated Impurity Concentration

The structural defect effect of impurities on silicon carbide (SiC) was studied to determine the luminescence properties with temperature-dependent photoluminescence (PL) measurements. Single 4H-SiC crystals were fabricated using three different 3C-SiC starting materials and the physical vapor transport method at a high temperature and 100 Pa in an argon atmosphere. The correlation between the impurity levels and the optical and fluorescent properties was confirmed using Raman spectroscopy, X-ray diffraction, inductively coupled plasma atomic emission spectroscopy (ICP-OES), UV-Vis-NIR spectrophotometry, and PL measurements. The PL intensity was observed in all three single 4H-SiC crystals, with the highest intensities at low temperatures. Two prominent PL emission peaks at 420 and 580 nm were observed at temperatures below 50 K. These emission peaks originated from the impurity concentration due to the incorporation of N, Al, and B in the single 4H-SiC crystals and were supported by ICP-OES. The emission peaks at 420 and 580 nm occurred due to donor–acceptor-pair recombination through the incorporated concentrations of nitrogen, boron, and aluminum in the single 4H-SiC crystals. The results of the present work provide evidence based on the low-temperature PL that the mechanism of PL emission in single 4H-SiC crystals is mainly related to the transitions due to defect concentration.

Charge collection and trapping mechanisms in hexagonal boron nitride epilayers

Understanding charge collection and trapping mechanisms is crucial for using hexagonal boron nitride (h-BN) as active layers for many photonic and electronic devices such as deep UV detectors and emitters, neutron detectors, and single photon emitters. Charge collection and trapping mechanisms in h-BN epilayers have been investigated by probing impurity related optical emissions under an applied electrical field. Our results suggested that the existence of oxygen impurities affects the charge collection efficiency and results in an additional emission peak at 3.75 eV, corresponding to a donor-acceptor pair (DAP) recombination involving ON (oxygen residing on the nitrogen site) donors and the VB-H (boron vacancy bonded with hydrogen complex) deep level acceptors. Experimental results further revealed that the applied electric field induces an anti-correlation between the emission intensity of the DAP transition and the charge collection efficiency from which it was shown that it is possible to find an expression to quantitatively measure the maximum charge collection efficiency in h-BN. The results introduce not only a coherent picture for the relationship between common impurities in h-BN and charge collection and trapping mechanisms but also useful insights into possible approaches to improve the quality, purity, and charge collection of the h-BN epilayers.

Low-temperature luminescence in organic-inorganic lead iodide perovskite single crystals

We present a temperature and laser-power dependent photoluminescence (PL) study of methylammonium lead iodide (CH3NH3PbI3) single crystals in the orthorhombic phase. At temperatures below 140 K, we revealed the multi-component PL emission. In addition to a free exciton with an energy of 1.65 eV, we found emission bands with peaks approximately equal to 1.6 eV, 1.52 eV, and 1.48 eV. Analysis of the thermal evolution of the intensities, peak positions, and linewidths of all the PL bands allowed one to determine their origin. We attributed the PL peak with energy of 1.6 eV to a bound exciton, while the free exciton-bound exciton splitting energy is 50–60 meV. The PL emission with an energy of 1.52 eV can be explained by the donor-acceptor pair (DAP) recombination, where donor and acceptor defects have a depth of about 12 meV and 120 meV, respectively. MA (CH3NH3) interstitials (MA) and lead vacancies (V) are the most suitable for the DAP transition to occur in CH3NH3PbI3 crystals. The 1.48 eV PL emission is consistent with the recombination of self-trapped excitons, and interstitial iodine is likely to be an active trap source. We found the variation of the self-trapped depth from 15 meV (at 80 K) to 53 meV (at 80 K) with increasing the temperature. Although the multi-component PL emission in CH3NH3PbI3 single crystals appears at low temperatures, defects and excitonic traps that cause this emission can affect the photophysics of hybrid perovskites at higher temperatures.