Model Of Radiative Recombination Research Articles

Radiative recombination model for BiSeI microcrystals: unveiling deep defects through photoluminescence

Abstract Pnictogen chalcohalides are semiconductors that have emerged as promising materials for energy conversion due to their exceptional optoelectronic properties. Their electronic configuration (ns2), particularly for Bi- and Sb-based compounds, can be a key factor in efficient carrier transport and defect tolerance, similarly, to Pb-perovskites. In the present study, the Bi-containing chalcohalide, bismuth selenoiodide (BiSeI) was synthesized via isothermal heat treatment of binary precursors in evacuated quartz ampoules. The synthesized BiSeI microcrystals exhibited a characteristic needle-like morphology and a near-stoichiometric composition. Both indirect and direct band gap energies of BiSeI were determined by ultraviolet–visible–near-infrared diffuse reflectance spectroscopy, with room temperature values of 1.17 eV and 1.29 eV, respectively. This study presents the first experimental investigation of the photoluminescence properties of BiSeI microcrystals resulting in a recombination model involving multiple defect states. This work provides valuable insights into the defect structure and recombination mechanisms within BiSeI, paving the way for further exploration of its potential in optoelectronic devices.

Study of (AgxCu1−x)2ZnSn(S,Se)4 monograins synthesized by molten salt method for solar cell applications

The open circuit voltage (VOC) deficit of Cu2ZnSn(S,Se)4 (CZTSSe) kesterite solar cells is higher than that of the closely related Cu(InGa)Se2 solar cells. One of the most promising strategies to overcome the large VOC deficit of kesterite solar cells is by reducing the recombination losses through appropriate cation substitution. In fact, replacing totally or partially Zn or Cu by an element with larger covalent radius one can significantly reduce the concentration of I–II antisite defects in the bulk. In this study, an investigation of the impact of partial substitution of Cu by Ag in CZTSSe solid solution monograins is presented. A detailed photoluminescence study is conducted on Ag-incorporated CZTSSe monograins and a radiative recombination model is proposed. The composition and structural quality of the monograins in dependence of the added Ag amount are characterized using Energy Dispersive X-ray Spectroscopy and X-Ray Diffraction method, respectively. The Ag-incorporated CZTSSe monograin solar cells are characterized by temperature dependent current-voltage and electron beam induced current methods. It was found, that low Ag contents (x ≤ 0.02) in CZTSSe lead to higher solar cell device efficiencies.

Ultra-small aqueous glutathione-capped Ag-In-Se quantum dots: luminescence and vibrational properties.

We introduce a direct aqueous synthesis of luminescent 2–3 nm Ag–In–Se (AISe) quantum dots (QDs) capped by glutathione (GSH) complexes, where sodium selenosulfate Na2SeSO3 is used as a stable Se2− precursor. A series of size-selected AISe QDs with distinctly different positions of absorption and PL bands can be separated from the original QD ensembles by using anti-solvent-induced size-selective precipitation. The AISe–GSH QDs emit broadband PL with the band maximum varying from 1.65 eV (750 nm) to 1.90 eV (650 nm) depending on the average QD size and composition. The PL quantum yield varies strongly with basic synthesis parameters (ratios of constituents, Zn addition, duration of thermal treatment, etc.) reaching 4% for “core” AISe and 12% for “core/shell” AISe/ZnS QDs. The shape and position of PL bands is interpreted in terms of the model of radiative recombination of a self-trapped exciton. The AISe–GSH QDs reveal phonon Raman spectra characteristic for small and Ag-deficient tetragonal Ag–In–Se QDs. The ability of ultra-small AISe QDs to support such “bulk-like” vibrations can be used for future deeper insights into structural and optical properties of this relatively new sort of QDs.

Photoluminescence Spectra From The Direct Energy Gap of a-SiQDs

A theoretical model for radiative recombination in amorphous silicon quantum dots (a-SiQDs) was developed. In this model, for the first time, the coexistence of both spatial and quantum confinements were considered. Also, it is found that the photoluminescence exhibits significant size dependence in the range (1-4) nm of the quantum dots. a-SiQDs show visible light emission peak energies and high radiative quantum efficiency at room temperature,in contrast to bulk a-Si structures. The quantum efficiency is sensitive to any change in defect density (the volume nonradiative centers density and/or the surface nonradiative centers density) but, with small dots sizes, the quantum efficiency is insensitive to such defects. Our analysis shows that the photoluminescence intensity increases or decreases by the effect of radiative quantum efficiency. By controlling the size of a-SiQDs, we note that the energy of emission can be tuned. The blue shift is attributed to quantum confinement effect. Meanwhile, the spatial confinement effect is clearly observed in red shift in emission spectra. we found a good agreement with the experimental published data. Therefore, we assert that a-SiQDs material is a promising candidate for visible, tunable, and high performance devices of light emitting.

Localized-State-Dependent Electroluminescence from ZnO/ZnS Core–Shell Nanowires–GaN Heterojunction

ZnO is a very important material for excitonic ultraviolet optoelectronic devices operating above room temperature due to its wide band gap and high exciton binding energy. In this paper, the influences of different degrees of the localized state on the photoluminescence and electroluminescence properties of the ZnO/ZnS core–shell nanowires–GaN heterojunction are systematically discussed. The physical model for radiative recombination of localized carriers was proposed to explain these phenomena. Our results indicate that surface-coating of ZnS nanoparticles on ZnO nanowires (NWs) is one of the effective ways to manipulate the localized states, and only the appropriate localized state will result in the optimal optoelectronic properties.

External Luminescence and Photon Recycling in Near-Field Thermophotovoltaics

The importance of considering near-field effects on photon recycling and spontaneous emission in a thermophotovoltaic device is investigated. Fluctuational electrodynamics is used to calculate external luminescence from a photovoltaic cell as a function of emitter type, vacuum gap thickness between emitter and cell, and cell thickness. The observed changes in external luminescence suggest strong modifications of photon recycling caused by the presence of the emitter. Photon recycling for propagating modes is affected by reflection at the vacuum-emitter interface and is substantially decreased by the leakage towards the emitter through tunneling of frustrated modes. In addition, spontaneous emission by the cell can be strongly enhanced by the presence of an emitter supporting surface polariton modes. It follows that using a radiative recombination model with a spatially uniform radiative lifetime, even corrected by a photon recycling factor, is inappropriate. Applying the principles of detailed balance, and accounting for non-radiative recombination mechanisms, the impact of external luminescence enhancement in the near field on thermophotovoltaic performance is investigated. It is shown that unlike isolated cells, the external luminescence efficiency is not solely dependent on cell quality, but significantly increases as the vacuum gap thickness decreases below 400 nm for the case of an intrinsic silicon emitter. In turn, the open-circuit voltage and power density benefit from this enhanced external luminescence toward the emitter. This benefit is larger as cell quality, characterized by the contribution of non-radiative recombination, decreases.

A Model of Radiative Recombination in (In,Al,Ga)N/GaN Structures with Significant Potential Fluctuations

The potential fluctuations in III-nitride quantum wells lead to many effects like emission broadening and S-shape energy vs. temperature dependence. The best description of the energy dependence comes from calculations based on Gaussian density of states. However, in most of the published reports, changes of carrier lifetime with energy and temperature are not taken into account. Since experimental evidence shows that lifetime significantly depends on energy and temperature, here we propose a model that describes two basic parameters of luminescence: lifetime of carries and emission energy as a function of temperature in the case of quantum wells and layers that are characterized by potential fluctuations. Comparison of the measured energy and lifetime dependences on temperature in specially grown InGaN/GaN quantum wells and InAlGaN layer shows very good agreement with the proposed theoretical approach.

Photoluminescence study of defect clusters in Cu2ZnSnS4 polycrystals

Temperature dependent photoluminescence (PL) study of Cu2ZnSnS4 (CZTS) polycrystals was performed. The low temperature PL spectrum consists of two PL bands: PL1 at 0.66 eV and PL2 at 1.35 eV. We propose a new radiative recombination model involving theoretically predicted (CuZn−+SnZn2+) and (2CuZn−+SnZn2+) defect clusters in nearly stoichiometric CZTS. PL1 band at 0.66 eV is proposed to result from a band-to-impurity type recombination related to a deep donor level at 0.66 eV below CBM that originates from the (CuZn−+SnZn2+) defect cluster. The PL2 band at 1.35 eV is found to be the dominating radiative recombination path that results from the recombination between electrons and holes in the potential wells caused by the (2CuZn−+SnZn2+) clusters that induce a significant band gap decrease of 0.35 eV in CZTS.

Radiative recombination model of degenerate semiconductor and photoluminescence properties of 3C-SiC by P and N doping

Based on radiative recombination theory, we have established a recombination model that can be used to calculate photoluminescence (PL) intensity for degenerate semiconductors. Using this model and density functional theory, we calculated photoluminescence excitation (PLE) and PL spectra of intrinsic 3C-SiC, P-doped SiC and N-doped SiC. The violet or near ultraviolet PLE peaks were found to be observed in PLE spectra for Sin−1PCn and SinNCn−1 (n = 4, 8, 12, and 16). Compared to intrinsic 3C-SiC, doped 3C-SiC exhibits higher PL peaks which for P-doped SiC are in the indigo spectral region, near the 3C-SiC’s peak, and for N-doped SiC appear in the green. The phenomena are studied through analyses of band structure, carrier concentration, and absorption. For doped 3C-SiC, the PL properties are mainly improved by the band-gap transformation from indirect to direct and the increase in carrier concentration near the Fermi level.

Photoluminescence and electrical study of fluctuating potentials in Cu2ZnSnS4-based thin films

In this work, we investigated structural, morphological, electrical, and optical properties from a set of ${\text{Cu}}_{2}\text{Zn}\text{Sn}{\text{S}}_{4}$ thin films grown by sulfurization of metallic precursors deposited on soda lime glass substrates coated with or without molybdenum. X-ray diffraction and Raman spectroscopy measurements revealed the formation of single-phase ${\text{Cu}}_{2}\text{Zn}\text{Sn}{\text{S}}_{4}$ thin films. A good crystallinity and grain compactness of the film was found by scanning electron microscopy. The grown films are poor in copper and rich in zinc, which is a composition close to that of the ${\text{Cu}}_{2}\text{Zn}\text{Sn}{\text{S}}_{4}$ solar cells with best reported efficiency. Electrical conductivity and Hall effect measurements showed a high doping level and a strong compensation. The temperature dependence of the free hole concentration showed that the films are nondegenerate. Photoluminescence spectroscopy showed an asymmetric broadband emission. The experimental behavior with increasing excitation power or temperature cannot be explained by donor-acceptor pair transitions. A model of radiative recombination of an electron with a hole bound to an acceptor level, broadened by potential fluctuations of the valence-band edge, was proposed. An ionization energy for the acceptor level in the range 29--40 meV was estimated, and a value of $172\ifmmode\pm\else\textpm\fi{}2$ meV was obtained for the potential fluctuation in the valence-band edge.

Synthesis of nanoparticles in laser ablation of aluminum in liquid

We report the synthesis of aluminum nanoparticles using pulsed laser ablation in water confined plasma. Nanoparticles have spherical shape and size distribution depends on laser fluence. Strong blue photoluminescence peaks at 405 nm (3.06 eV) and 430 nm (2.89 eV) due to oxygen deficient defects (F, F+, and F++ centers) is reported with different UV excitations. A comparative study of plasma in deionized water and air ambient reveals enhanced line broadening and higher electron density in water confined plasma compared to that in air, in agreement with radiative recombination model. The temporal dependence of spectral radiant energy density of plasma is also discussed.

Blue Light Emission from PECVD Deposited Nanostructured SiC

AbstractExperimental data on the room temperature blue light emission from nanocrystalline silicon carbide (nc-SiC) films are presented. The silicon carbide films were deposited on silicon substrates with a plasma enhanced chemical vapor deposition (PECVD) reactor from methyltrichlorosilane at substrate temperature in the range of 200-350°C. The film deposited at 350°C is nanostructured and X-ray diffraction proves the presence of the 3C-SiC crystallites. Infrared absorption spectroscopy in the region of Si-C, C-H, C-O, Si-H and Si-O bonds show the corresponding absorption bands. X-ray photoelectron spectroscopy studies confirm this bond picture. Photoluminescence was measured at 77 and 300 K. The bright blue emission has a double-peak structure at 415 and 437 nm. To clarify the origin of such an emission, tight binding molecular dynamics (TB-MD) simulations of several SiC and Si nanoclusters were carried out. Based on the temperature dependence of the photoluminescence and on the simulation data, a possible model of radiative recombination in nc-SiC films was proposed. According to this model, the emission bands at 415 and 437 nm are assigned to band-to-band and band-to-tail recombination in the nanocrystallite core. The recombination at band tails of the interface and Si-C-O-H amorphous tissue gives rise to a shoulder around 470 nm.

Cathodoluminescence of Cu(In,Ga)Se2 thin films used in high-efficiency solar cells

Cathodoluminescence spectroscopy and spectrum imaging are employed to investigate Cu(In,Ga)Se2 (CIGS) thin films used in high-efficiency solar cells. We have found a nonuniform spatial distribution for the photon energy. The shift by decade of the emission spectrum is also found to depend systematically on the location of excitation. In addition, the photon energy at grain boundaries is not affected by the external excitation. A model for radiative recombination to be applied to these chalcopyrite compounds should explain these results, and some suggestions are considered.

Sharp-line luminescence and absorption in ZnGeP2

Photoluminescence and optical absorption spectroscopies have been used to study bulk ZnGeP2. Two sharp zero-phonon lines with opposite polarizations are resolved in emission at 1.7849 eV (at 70 K) and 1.7784 eV (at 5 K), respectively. Thermalization occurs between these two lines, suggesting a split excited state of the center. Considerable structural detail is resolved in the vibronic sidebands of these lines, revealing phonon energies of 6.3±0.1 meV and 43.7±0.1 meV. Similar phonon energies are resolved in low-temperature absorption spectra. These results for ZnGeP2 are explained by a model of radiative recombination of excitons bound to an isoelectronic defect center. The observed polarization behavior of the sharp-line spectra can be produced by the built-in tetragonal distortion along the c axis associated with the chalcopyrite structure.

Hydrogenated amorphous silicon deposited by glow discharge of SiH4 diluted with He: photoluminescence and electroluminescence in the visible region

Samples of a-Si:H having a wide gap (≥2.1 eV) were prepared using routine rf glow discharge decomposition of silane (SiH4) strongly diluted with He. Also, microwave electron–cyclotron–resonance plasma-enhanced chemical-vapour-deposition was used to prepare wide-gap a-Si:H via decomposition of He diluted SiH4. The wide-gap a-Si:H samples, containing above 30 at.% of hydrogen, exhibit visible photoluminescence at room temperature. From the temperature dependence of photoluminescence intensity and from the observed shift of the maximum with temperature we deduce that the customary model of radiative recombination of carriers localised in tail states is not directly applicable to the case of wide-gap a-Si:H. Instead, we propose that the photoluminescence arises in parallel from tail states and from isolated (extrinsic) centres. Electroluminescence results obtained on a p–i–n structure with wide-gap a-Si:H are reported.

A model of size-dependent photoluminescence in amorphous silicon nanostructures: Comparison with observations of porous silicon

We present calculations using a simple model of radiative recombination in 2D slabs, 1D wires, and 0D spheres of hydrogenated amorphous silicon (a-Si:H) showing a significant size dependence of the photoluminescence. Room-temperature peak emission energies ≳1.8 eV and efficiencies near unity are possible in a-Si:H spheres with diameters <20 Å. Broad homogeneous linewidths ≳0.25 eV are also predicted for these highly confined structures. While the effects are similar to those predicted from quantum confinement, these results are caused by the statistics of spatial confinement. We suggest that these results provide insights into nanostructured a-Si:H structures and porous silicon.

Inner electric field mechanism of tunneling of electron-hole pair in porous silicon

Recently a porous silicon model of radiative recombination of electron-hole pair through luminescence center outside the nanoscale Si unit was proposed by Qin et al. In order to test it we calculated the transmission coefficient (TC) of electron-hole pair in terms of a coupled double quantum box model and an inner electric field mechanism. According to this mechanism an inner electric field will be established as soon as some electrons tunnel through the SiO 2 barrier. This field will reduce the TC of electron, T e and increase the TC of hole, T h . As long as T e > T h , this field will increase monotonically until it reaches an equilibrium inner electric filed, E eq at which the TCs of electron and hole approach the same value, T pair which is just the TC of electron-hole pair in steady state. In this paper, we calculated T pair and E eq as functions of the thickness of SiO 2 barrier for coupled double quantum boxes, lines and layers. The results show that the TC of electron-hole pairs is increased by taking into account the equilibrium inner field. This fact supports the proposal of pair recombining outside the silicon unit, since a sufficiently early TC of pairs is the precondition for efficient recombination. Our results are of importance for further calculations of the quantum efficiency of photoluminescence in porous silicon.

Transient photoconductivity in GaInAs/InP MQW

Transient photoconductivity in nominally undoped (n-type background doping) Ga0.47In0.53As/InP multiquantum well heterostructures is reported. The observed responses exhibited an initial fast decay followed by a much slower subsequent decay. The slow decay was consistent with a kinetic radiative recombination model which included free carriers, excitons and photon recycling. A model describing the fast initial decay is presented in terms of fast barrier radiative recombination. A comparison between the experimental results and the model shows good agreement.

Transient photoluminescence in GaInAs/InP multiple quantum wells

Photoluminescence decay in low-doped lattice-matched GaInAs/InP multiple quantum wells has been studied over a range of excess carrier densities. High carrier densities, which moved the Fermi level up to the top of the wells, could be created without focusing the laser beam. Radiative band-to-band recombination was shown to dominate up to excess carrier densities of 2.5*1012 cm-2 per well. A kinetic radiative recombination model which included free carriers, excitons and photon recycling was used to fit the transient decays in the regime in which the electrons were non-degenerate. At carrier densities greater than 2.5*1012 cm-2 per well, when both electrons and holes were degenerate, the recombination was predominantly non-radiative. An Auger coefficient of 4.0*10-29 cm6 s-1 was estimated for 5 nm quantum wells. No stimulated recombination was detected.

The temperature dependence of in-plane transient photoconductivity in GaInAs/InP MQWs

Transient photoconductivity has been used to study the recombination of free carriers in GaInAs/InP multiple quantum well material. The sublinear increase in the initial amplitude of the photoconductivity with excitation intensity is attributed to the carrier density dependence of the mobility. This nonlinearity is taken into account when the time constant associated with the decay of the free carriers is obtained from the photoconductive decays. For low excess carrier densities, in the non-degenerate regime, the recombination was consistent with a kinetic radiative recombination model which included free carriers, excitons and photon recycling. The time constants, deduced from the experimental data, were in good agreement with the temperature dependence predicted by the model. When the excess carrier density was high enough to make the material degenerate, the initial decay, while the material remained degenerate, was exponential with a relatively short time constant. Even when the wells were full there was no evidence of Auger recombination. An upper limit of 3*10-29 cm6 s-1 could therefore be placed on the Auger coefficient.