Carrier Recombination Centers Research Articles

Theory of the carbon vacancy in 4H -SiC: Crystal field and pseudo-Jahn-Teller effects

The carbon vacancy in $4H$-SiC is a powerful minority carrier recombination center and a major cause of degradation of SiC-based devices. Despite the extensiveness of the literature regarding the characterization and modeling of the defect, many fundamental questions persist. Among them we have the shaky connection of the EPR data to the electrical measurements, the physical origin of the pseudo-Jahn-Teller (pJT) effect, the reasoning for the observed sub-lattice dependence of the paramagnetic states, and the severe temperature-dependence of some hyperfine signals which cannot be accounted for by a thermally-activated dynamic averaging between equivalent JT-distorted structures. In this work we address these problems by means of semi-local and hybrid density functional calculations. We start by inventorying a total of four vacancy structures. Diamagnetic states have well defined low-energy structures, whereas paramagnetic states display metastability. This rich structural variety is traced back to the filling of electronic states which are shaped by a crystal-field-dependent pJT effect. From calculated minimum energy paths for defect rotation and transformation mechanisms, combined with the calculated formation energies and electrical levels, we arrived at a configuration-coordinate diagram of the defect. The diagram provides us with a detailed first-principles picture of the defect when subject to optical and thermal excitations. The calculated acceptor and donor transitions agree well with Z$_{1/2}$ and EH$_{6/7}$ trap energies, respectively. From comparison of calculated and measured $U$-values, and correlating the site-dependent formation energies with the relative intensity of the DLTS peaks in as-grown material, we assign Z$_{1}$ (EH$_{6}$) and Z$_{2}$ (EH$_{7}$) signals to acceptor (donor) transitions of carbon vacancies located on the $h$ and $k$ sub-lattice sites, respectively.

Characterization of Rh:SrTiO3 photoelectrodes surface-modified with a cobalt clathrochelate and their application to the hydrogen evolution reaction

Water is a promising source of clean hydrogen. Besides water electrolysis, the direct photoelectrochemical dissociation of water that combines light absorption and electrochemical water splitting into a single device is actively investigated by the scientific community. We report results on the p-type rhodium-doped strontium titanate (Rh:SrTiO3) surface-modified by adsorption of a 3d-transition metal complex which is known to be a good catalyst of the hydrogen evolution reaction (HER) from water. The tris-dioximate cobalt hexachloroclathrochelate with an encapsulated cobalt(II) ion was selected as a surface HER co-catalyst for this study. Samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The kinetics of the photoelectrochemical water dissociation was studied using the open circuit photovoltage decay (OCPD) and photoelectrochemical impedance spectroscopy (PEIS) methods. Under open circuit conditions, the cobalt cage complex did not substantially affect the HER kinetics compared to the bare doped Rh:SrTiO3 semiconductor. But when a reverse bias was applied, a significant difference caused by presence of the clathrochelate was observed. The rate constants of the charge transfer and the recombination processes were both affected, leading to the conclusion that the macrobicyclic cobalt-encapsulated species not only increased the rate of charge transfer, but also played a role as recombination centers for photoexcited minority carriers. The space charge capacitance was determined under the inversion conditions. Only under strong reverse bias it was found that, whereas bulk Rh:SrTiO3 has a p-type conductivity, the surface of both bare Rh:SrTiO3 and the cobalt clathrochelate-covered Rh:SrTiO3 show the properties of a n-type semiconductor.

A deep-level transient spectroscopy study of gamma-ray irradiation on the passivation properties of silicon nitride layer on silicon

Plasma-enhanced chemical vapor deposited silicon nitride (SiNx) films are extensively used as passivation material in the solar cell industry. Such SiNx passivation layers are the most sensitive part to gamma-ray irradiation in solar cells. In this work, deep-level transient spectroscopy has been applied to analyse the influence of gamma-ray irradiation on the passivation properties of SiNx layer on silicon. It is shown that the effective carrier lifetime decreases with the irradiation dose. At the same time, the interface state density is significantly increased after irradiation, and its energy distribution is broadened and shifts deeper with respect to the conduction band edge, which makes the interface states becoming more efficient recombination centers for carriers. Besides, C–V characteristics show a progressive negative shift with increasing dose, indicating the generation of effective positive charges in SiNx films. Such positive charges are beneficial for shielding holes from the n-type silicon substrates, i. e. the field-effect passivation. However, based on the reduced carrier lifetime after irradiation, it can be inferred that the irradiation induced interface defects play a dominant role over the trapped positive charges, and therefore lead to the degradation of passivation properties of SiNx on silicon.

Activation of amorphous bismuth oxide via plasmonic Bi metal for efficient visible-light photocatalysis

Amorphous semiconductors are seldom exploited as effective photocatalysts, as they are restricted by abundant bulk defects as carrier recombination centers. To activate amorphous bismuth oxide for efficient visible-light photocatalytic performance, a novel and facile strategy was developed. Plasmonic Bimetal-decorated amorphous bismuth oxide (Bi–BiO) was prepared by partial reduction with NaBH4. The content of Bi metal and the photocatalytic activity of the catalysts can be modulated by controlling the concentration of NaBH4 solution. Various techniques were employed to explore the structural features, optical properties, and active species during photocatalysis. The as-synthesized Bi–BiO catalysts were applied in photocatalytic removal of NO in air under and exhibited highly enhanced visible light photocatalytic activity. The significantly increased photocatalytic capability can be attributed to the combined effects of the enhanced visible light absorption and the improved separation efficiency of the charge carriers attributed to the surface plasmon resonance conferred by Bi metal. The advanced Bi–BiO catalysts also exhibited high photochemical and structural stability under repeated irradiation. Moreover, in situ DRIFT was carried out to reveal the time-dependent evolution of reaction intermediates during photocatalytic NO oxidation. A molecular-level photocatalysis mechanism was first proposed for Bi–BiO based on ESR and in situ DRIFT. This work could provide a new perspective in utilizing non-noble-metal Bi as a key activation factor to trigger the photocatalytic ability of amorphous semiconductors.

Photoluminescence Characterization of Carrier Recombination Centers in 4H-SiC Substrates by Utilizing below Gap Excitation

Though the crystal growth technology of SiC is improving steadily, it is still crucial to reduce crystalline defects which act as carrier recombination (CR) centers and deteriorate device performance. We detected CR centers in a p-type 4H-SiC substrate by observing the intensity change of photoluminescence due to the addition of a below-gap excitation (BGE) light of 0.93[eV]. We noticed the temperature and the BGE density dependence of band edge (BE) emission in addition to donor acceptor pair (DAP) emission and discriminated the temperature effect from that of BGE. The BGE density dependence of the PL intensity quenching is different among the BE emission, B0- and C0-lines of the DAP, respectively. It gives us an important clue for understanding CR mechanisms inside the bandgap of SiC.

Electronic structure and optical properties of Ta-doped and (Ta, N)-codoped SrTiO3 from hybrid functional calculations

A systematic study has been carried out to research the effect of Ta monodoping and (Ta, N)-codoping on the electronic structure and optical properties of SrTiO3. The results indicate that the incorporation of N into the SrTiO3 lattice is in favor of the substitution of Ta at a Ti site, which is the most favorable structure with respect to both the energetic stability and high photocatalytic activity. Furthermore, the carrier recombination centers induced by Ta monodoping are passivated in the (Ta, N)-codoped SrTiO3 system with Ta at a Ti site. Simultaneous incorporation of N and Ta results in a band gap decreasing about 0.7 eV due to the appearance of the new states hybridized by N-p states with the O-p states above the valence band. The band alignment verifies that the (Ta, N)-codoped SrTiO3 simultaneously meets the criteria of band-edge energetic positions and band gap for the overall water splitting under visible light.

Performance investigation of black silicon solar cells with surface passivated by BiFeO3/ITO composite film

In order to prepare black silicon material with excellent optical absorption performance for solar cell application, a micro/nano bilayer-structure is formed on the surface of textured silicon wafer by a silver assisted chemical etching method. It is found that the deeper nanoholes could form as the etching time is longer, and the surface reflectivity is reduced obviously due to the increased time of photon reflection from the nanowires. The incident light reflectivity of the prepared black silicon is significantly reduced to 2.3%, showing obviously better optical reflectance characteristics than general monocrystalline silicon wafer, especially in a wavelength range of 300-830 nm. Considering the fact that a large number of carrier recombination centers is introduced into the nanostructured crystal silicon surface, BiFeO3/ITO composite film is coated on the surface of the black silicon solar cell by magnetron sputtering process to optimize the surface defect states and improve the cell performance. The experimental results show that the lengths of the nanowires are predominantly in a range of 180-320 nm for the prepared black silicon with micro/nano double-layer structure. The reflectivity of the incident light is below 5% in a wavelength range from 300 nm to 1000 nm, and reaches a maximal value at about 700 nm. The reflectance increases slightly as BiFeO3/ITO composite film is coated on the surface of black silicon solar cell, but it is still much lower than that of general monocrystalline silicon solar cell. The open circuit voltage and short circuit current density of the black silicon solar cell increase respectively from 0.61 V to 0.68 V and from 28.42 mA/cm2 to 34.57 mA/cm2 after it has been coated with BiFeO3/ITO composite film, and the photoelectric conversion efficiency of the cell increases from 13.3% to 16.8% accordingly. The improvement in performance of black silicon solar cell is mainly due to the promotion of effective separation of photogenerated carriers, thereby enhancing the spectral response of black silicon solar cell in the whole wavelength range. This indicates that the spontaneously polarized BiFeO3 film can play a better role in improving the surface properties of black silicon solar cell. On the other hand, for the BiFeO3 film deposited on the surface of black silicon, a spontaneous polarization positive electric field could be produced, pointing from the film surface to the inside of the solar cell. This polarization electric field could also act as part of built-in electric field to contribute the photoelectric transformation of the black silicon solar cell, leading to the open circuit voltage of cell increasing from 0.61 V to 0.68 V.

Electronic and optical properties study on Fe[sbnd]B co-doped anatase TiO2

We investigate the density of states and optical properties for Fe, 2B and (Fe, 2B) doped TiO2 with DFT calculations. The calculated results reveal mono-doping introduces midgap states which are half-occupied and easy to become the recombination centers of charge carriers, thus inhibiting the enhancement of photocatalystic activity. The coupling of 2p-3d states in the (Fe, 2B) compensated co-doped TiO2 makes gap states couple with the valence bands edge, thus greatly causing the band gap narrowing and higher visible light absorption. Moreover, the gap states cannot become recombination centers of the photoexcited carriers, thus promoting the separation of electron-hole pairs, prolonging the lifetime of carriers. The analysis of electron density indicates more electrons from Fe transfer to adjacent B, realizing the charge compensation and forming a stronger FeB bond. Therefore, the (Fe, 2B) compensated co-doped TiO2 exhibits the higher visible-light photocatalystic activity than those of pure and solely doped TiO2.

First-principles study of twin grain boundaries in epitaxial BaSi2 on Si(111)

Epitaxial films of BaSi2 on Si(111) for solar cell applications possess three epitaxial variants and exhibit a minority carrier diffusion length (ca. 9.4 μm) much larger than the domain size (ca. 0.2 μm); thus, the domain boundaries (DBs) between the variants do not act as carrier recombination centers. In this work, transmission electron microscopy (TEM) was used to observe the atomic arrangements around the DBs in BaSi2 epitaxial films on Si(111), and the most stable atomic configuration was determined by first-principles calculations based on density functional theory to provide possible interface models. Bright-field TEM along the a-axis of BaSi2 revealed that each DB was a twin boundary between two different epitaxial variants, and that Ba(II) atoms form hexagons containing central Ba(I) atoms in both the bulk and DB regions. Four possible interface models containing Ba(I)-atom interface layers were constructed, each consistent with TEM observations and distinguished by the relationship between the Si tetrahedron arrays in the two domains adjacent across the interface. This study assessed the structural relaxation of initial interface models constructed from surface slabs terminated by Ba(I) atoms or from zigzag surface slabs terminated by Si tetrahedra and Ba(II) atoms. In these models, the interactions or relative positions between Si tetrahedra appear to dominate the relaxation behavior and DB energies. One of the four interface models whose relationship between first-neighboring Si tetrahedra across the interface was the same as that in the bulk was particularly stable, with a DB energy of 95 mJ/m2. There were no significant differences in the partial densities of states and band gaps between the bulk and DB regions, and it was therefore concluded that such DBs do not affect the minority carrier properties of BaSi2.

Ambient Engineering for High-Performance Organic-Inorganic Perovskite Hybrid Solar Cells.

Considering the evaporation of solvents during fabrication of perovskite films, the organic ambience will present a significant influence on the morphologies and properties of perovskite films. To clarify this issue, various ambiences of N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and chlorobenzene (CBZ) are introduced during fabrication of perovskite films by two-step sequential deposition method. The results reveal that an ambient CBZ atmosphere is favorable to control the nucleation and growth of CH3NH3PbI3 grains while the others present a negative effect. The statistical results show that the average efficiencies of perovskite solar cells processed in an ambient CBZ atmosphere can be significantly improved by a relatively average value of 35%, compared with those processed under air. The efficiency of the best perovskite solar cells can be improved from 10.65% to 14.55% by introducing this ambience engineering technology. The CH3NH3PbI3 film with large-size grains produced in an ambient CBZ atmosphere can effectively reduce the density of grain boundaries, and then the recombination centers for photoinduced carriers. Therefore, a higher short-circuit current density is achieved, which makes main contribution to the improvement in efficiency. These results provide vital progress toward understanding the role of ambience in the realization of highly efficient perovskite solar cells.

Minority Carrier Recombination Properties of Crystalline Defect on Silicon Surface Induced by Plasma Enhanced Chemical Vapor Deposition

This research investigates the carrier recombination properties of a crystalline defect layer introduced by the plasma enhanced chemical vapor deposition (PECVD) process of amorphous hydrogenated silicon nitride (SiNx) passivation films. A direct PECVD technique was used for SiNx films deposition. A crystalline defect layer existed on the surface of the silicon substrate and is under the SiNx passivation film. The recombination lifetime in this defect layer was obtained by focusing on the thickness of the defect layer and the effective lifetime before and after the defect layer etching. After etching a few nanometer thickness, effective lifetime drastically increased. On the other hands, the carrier recombination center could be electrically inactivated by 600°C annealing after SiNx deposition. According to the depth profile of effective lifetime, it was clarified the high carrier recombination region were concentrated near the surface of silicon substrate.

Photoluminescence study of surface treatment effects on detector-grade CdTe:In

We studied the influence of standard surface treatment techniques on the generation of defects with deep levels that can act as trapping and recombination centers for photo-generated carriers in detector-grade CdTe:In material grown via the Vertical-Gradient-Freeze (VGF) method. We measured room-temperature contactless resistivity, photoconductivity, detector performance and low-temperature photoluminescence dependence on the surface preparation of the material and observed changes in the resistivity and photoluminescence signal after etching a 5 μm thick surface layer. We found four deep levels in the range of 0.8–1.3 eV. The relative ratio of their photoluminescence maxima changes after mechanical polishing and chemical etching treatment. A deep level at ∼0.9 eV seems to be connected to mechanical stress induced by polishing of the sample with a standard 1 μm alumina abrasive and influences the charge collection efficiency of the detector.

Impact of carrier recombination on fill factor for large area heterojunction crystalline silicon solar cell with 25.1% efficiency

We have achieved a certified 25.1% conversion efficiency in a large area (151.9 cm2) heterojunction (HJ) crystalline Si (c-Si) solar cell with amorphous Si (a-Si) passivation layer. This efficiency is a world record in a both-side-contacted c-Si solar cell. Our high efficiency HJ c-Si solar cells are investigated from the standpoint of the effective minority carrier lifetime (τe), and the impact of τe on fill factor (FF) is discussed. The τe measurements of our high efficiency HJ c-Si solar cells reveal that τe at an injection level corresponding to an operation point of maximum power is dominated by the carrier recombination at the a-Si/c-Si interface. By optimization of the process conditions, the carrier recombination at the a-Si/c-Si interface is reduced, which leads to an improvement of the FF by an absolute value of 2.7%, and a conversion efficiency of 25.1% has been achieved. These results indicate that the reduction of carrier recombination centers at the a-Si/c-Si interface should be one of the most crucial issues for further improvement of FF even in the HJ c-Si solar cells with efficiency over 25%.

Degradation Mechanisms of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Molecules

Degradation of organic light-emitting diodes (OLEDs) operated continuously at a constant current density is investigated using photoluminescence techniques. The OLEDs contained the thermally activated delayed fluorescence emitting dopant (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). OLED degradation proceeds mainly on the basis of excited-state instability of host molecules rather than processes related to 4CzIPN. Additionally, the electrochemical instability of radical cations and anions influences long-term OLED degradation. The formation of exciton quenchers and nonradiative carrier recombination centers acts to reduce OLED luminance. These findings highlight the need for new host material development to fabricate more stable TADF-OLEDs.

Period-doubling reconstructions of semiconductor partial dislocations

Atomic-scale understanding and control of dislocation cores is of great technological importance, because they act as recombination centers for charge carriers in optoelectronic devices. Using hybrid density-functional calculations, we present period-doubling reconstructions of a 90° partial dislocation in GaAs, for which the periodicity of like-atom dimers along the dislocation line varies from one to two, to four dimers. The electronic properties of a dislocation change drastically with each period doubling. The dimers in the single-period dislocation are able to interact, to form a dispersive one-dimensional band with deep-gap states. However, the inter-dimer interaction for the double-period dislocation becomes significantly reduced; hence, it is free of mid-gap states. The Ga core undergoes a further period-doubling transition to a quadruple-period reconstruction induced by the formation of small hole polarons. The competition between these dislocation phases suggests a new passivation strategy via population manipulation of the detrimental single-period phase. Theoretical simulations suggest a pathway toward reducing the population of efficiency-draining defects in gallium arsenide (GaAs) solar cells. Modern, multilayered light-harvesting devices can have lattice mismatches at interfaces where the crystals meet, and this can create dislocations - two-dimensional ribbons of atoms that disrupt the bulk crystal structure. Joongoo Kang from DGIST in Daegu, Korea, and collaborators at the USA's National Renewable Energy Laboratory have used sophisticated density-functional calculations to analyse GaAs dislocations that had different patterns of partnered atomic dimers. Repetitive ‘single-period’ dimers had deep gap states that degrade the optoelectronic performance. But less dense ‘double-period’ and newly discovered ‘quadruple-period’ structures can compete with the single-period phase and limit its influence — a passivation strategy that may be realized through shallow p-type doping of GaAs, suggest the researchers. Using hybrid density-functional calculations, we present period-doubling reconstructions of a 90° partial dislocation in GaAs, for which the periodicity of like-atom dimers along the dislocation line varies from one to two, to four dimers. The electronic properties of a dislocation change drastically with each period doubling, suggesting a new passivation strategy by biasing the competition between these phases through the tuning of the carrier density in the dislocation.

Swift heavy ion induced capacitance and dielectric properties of Ni/n-GaAs Schottky diode

The in-situ capacitance and dielectric properties of 25 MeV C4+ ion irradiated Ni/n-GaAs Schottky barrier diode (SBD) were studied at 100 kHz in the fluence range 5 × 1010 – 5 × 1013 ions/cm2. The investigation shows reduction in capacitance and charge density with increase in ion fluence. Consequent changes were observed in other related parameters like conductance, dielectric constant, dielectric loss, loss tangent and electrical modulus. The results were interpreted in terms of generation of swift heavy ion induced acceptor trap states by electronic energy loss mechanism. Besides, the switch over characteristics of depletion to inversion regions in the CV plot reveals minority carrier recombination centers also. The dispersion and relaxation peaks observed in bias dependent dielectric plots were ascribed to the polarization and relaxation mechanism due to the interfacial trap states. The traps and recombination centers were found to alter the barrier characteristics of the fabricated SBD depending upon the ion fluence.

Effect of defects in TiO2 nanotube thin film on the photovoltaic properties of quantum dot-sensitized solar cells

In the liquid-phase-deposition (LPD) method, the deposition temperature is considered to be one of the most important factors in TiO2 nanotube crystal growth. We investigated the effects of the deposition temperature on the surface morphology and defects in TiO2 nanotube (NT–TiO2) thin film electrodes utilizing scanning-electron-microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL), together with the effects of these on the photovoltaic characteristics of CdSe quantum dot (QD)-sensitized NT–TiO2 solar cells. In addition, we studied the effect of these defects on the physical properties, such as the carrier recombination and electron transport at the TiO2 and TiO2/QD interface. NT–TiO2 electrodes prepared at low temperatures have a more uniform surface and lower defects than those prepared at high temperatures. From the PL measurements and the photovoltaic characterization such as shunt resistance (Rsh) and open circuit voltage decay (OCVD), these defects can act as carrier recombination centers. The defect density increases with increasing deposition temperature, leading to an increase in carrier recombination. Series resistances (Rs) of the solar cells with NT–TiO2 electrodes prepared at high temperatures were larger than those of the solar cells with NT–TiO2 electrodes prepared at low temperatures, suggesting that the defects can also affect the carrier transport characteristics. Eventually, CdSe QD-sensitized NT–TiO2 solar cells employing NT–TiO2 prepared at low temperatures showed higher conversion efficiencies than those prepared at high temperatures.

Comparison of phosphorus gettering effect in faceted dendrite and small grain of multicrystalline silicon wafers grown by floating cast method

We attempt to clarify the effect of phosphorus diffusion gettering (PDG) on the differences in microstructures of multicrystalline silicon (mc-Si) wafers. From the results of the floating cast method, the obtained microstructure of mc-Si was determined to contain unique microstructures, consisting of large faceted dendrites and small grains with low and high dislocation densities. The PDG efficiency was evaluated through the change in minority carrier lifetime. As a result, a stronger positive PDG effect was found for large faceted dendrites with low dislocation density, attributed to a small number of defects that can act as recombination centers of carriers. Lifetime improvement was also found in dislocation regions possibly owing to the redistribution of interstitial metals. These results suggest the importance of controlling the microstructure of mc-Si, which could strongly affect the PDG efficiency and existence of incorporated impurities.

Exploring the effect of boron and tantalum codoping on the enhanced photocatalytic activity of TiO2

Aiming to improve the photocatalytic activity, boron and tantalum co-doped TiO2 nanocrystals were prepared by a sol–gel method. Pure and doped titania samples were characterized by X-ray diffraction (XRD) measurements, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy. The results revealed that all the samples have highly crystalline anatase structure, and no any other second phase was observed. It is found that the incorporation of dopants inhibits the growth of TiO2 nanocrystal. Moreover, XPS observations showed that Ta are in the form of Ta5+ while the majority of B species are present at interstitial sites. Finally, photocatalytic activities of all the samples were evaluated by degradation of methylene blue (MB) solutions under simulated solar light irradiation. Ta-doped and (B, Ta)-codoped catalysts exhibit enhanced photocatalytic activities in comparison with pure TiO2, with the codoped sample exhibiting the best photodegradation performance. This indicates that B and Ta co-doping is an effective way to improve photocatalytic performance of TiO2. The catalytic activity of B-doped TiO2 becomes slightly worse than pure TiO2 nanocrystals, which is plausible due to increased charge carrier recombination centers induced by incorporation of boron species.

Response of as grown dislocation structure to temperature and stress treatment in multi-crystalline silicon

Dislocations and dislocation networks in multi-crystalline (mc) silicon wafers for photovoltaic applications act as minority carrier recombination centers and thus limit the efficiency of solar cells. The literature shows results for a massive dislocation reduction by applying an annealing procedure in combination with and without an impurity gettering step. In this work different kinds of annealing experiments with mc silicon samples were carried out at 1200°C and 1365°C for 1h to 96h under an applied stress of up to 4.2MPa under pure inert or boron containing atmospheres. The results show clearly that, under the used process conditions, a dislocation reduction could not be observed via defect selective etching using different kinds of etchants. It was found, that it is essential to carefully select the etching solution as the electrical resistivity of the samples might change after a respective thermal treatment, such that the eventually still present dislocations will not be overseen.