Deep Gap States Research Articles

Deep and Shallow Gap States in Reduced and n-Type Doped m-ZrO2

Deep and Shallow Gap States in Reduced and n-Type Doped <i>m</i>-ZrO₂

Heterogeneity of grain boundary properties in Cu2ZnSnS4: A first-principles study

Using first-principles density functional calculations, we investigate the structure and properties of previously unstudied grain boundaries (GBs) in the solar absorber material copper-zinc-tin-sulfide (CZTS). We identify four stable low-Σ value symmetric tilt GBs with low formation energies: Σ3 (111) and Σ5 (201), each with two different GB terminations. Compared to CdTe and CuInSe, GBs in the quaternary semiconductor CZTS exhibit a wider variety of electronic states due to the more complex chemical environment near the GB, including under-coordinated atoms and dangling bonds. Further analysis confirms that strong dangling bonds introduce deep gap states in all GBs studied. We also investigate segregation and electronic properties of intrinsic point defects to GBs and find that one of the Σ3 (111) GBs exhibits an abnormal defect segregation behavior that favors Cu-poor (Zn-rich) GB composition, which is beneficial for its overall performance.

Atomic and electronic structures of interfaces between amorphous (Al2O3)1−x(SiO2)x and GaN polar surfaces revealed by first-principles simulated annealing technique

We report first-principles molecular-dynamics calculations with the simulated annealing technique that clarify the atomic and electronic structures of the semiconductor–insulator interfaces consisting of GaN (0001) and (0001¯) faces and the amorphous (Al2O3)1−x(SiO2)x. We confirm that the obtained interfaces are free from dangling bonds, as predicted by our previous calculations, irrespective of the thickness of the amorphous (Al2O3)1−x(SiO2)x layer. This is due to the high atomic density and large mean coordination number near the interfaces caused by atomic diffusion from inside of the insulator to the interfaces. The calculated local density of states of the (Al2O3)1−x(SiO2)x/GaN system quantitatively shows clear band offsets and, more importantly, the absence of deep states in the GaN energy gap. Interestingly, we find that the band alignment causing the offset is not abrupt at the interface but varies gradually near the interface, predicting the existence of transition layers. We determine the thicknesses of the transition layers in the (Al2O3)1−x(SiO2)x/GaN system to be about 10 Å. We argue that those structural characteristics prevent the formation of the dangling-bond origin carrier traps at the interface, and this is a superior feature of the (Al2O3)1−x(SiO2)x as a gate oxide for the GaN-based metal–oxide–semiconductor devices.

Semiconducting Behaviour and Corrosion Resistance of Passive Film on Corrosion-Resistant Steel Rebars.

Chloride-induced corrosion of steel rebars is one of the major causes of the premature failures of reinforced concrete structures served in different environments. This paper investigates the semiconducting behaviour and corrosion resistance of the passive film formed on the corrosion-resistant rebars exposed to simulated concrete pore solutions with different pH values and chloride concentrations. The electronic properties of the passive film were studied using potentiodynamic measurements and capacitance measurements (Mott-Schottky analysis). The results indicate that, firstly, the passive film of corrosion-resistant steel rebar shows n-type semiconducting behaviour with shallow and deep donor states in the band gap during passivation, and the deep donor energy level of corrosion-resistant steel rebar passive film is not sensitive to the decreasing pH value; secondly, under the same conditions, the passive film of corrosion-resistant rebars has a larger negative flat-band potential and thicker space charge layer than hot-ribbed rebars; thirdly, n-type semi-conductivity at a higher potential disappears once the chloride concentration at the rebar surface attains the chloride threshold value; and finally, a reverse charge layer forms on the surface of CR rebar at 0.50 V potential.

Efficient passivation of surface defects by lewis base in lead-free tin-based perovskite solar cells

Lead-free tin-based perovskites are highly appealing for the next generation of solar cells due to their intriguing optoelectronic properties. However, the tendency of Sn2+ oxidation to Sn4+ in the tin-based perovskites induces serious film degradation and performance deterioration. Herein, we demonstrate, through the density functional theory based on first-principle calculations in a surface slab model, that the surface defects of the Sn-based perovskite FASnI3 (FA = NH2CHNH2+) could be effectively passivated by the Lewis base molecules. The passivation performance of Lewis base molecules in tin-based perovskite is tightly correlated with their molecular hardness. We reveal that the degree of hardness of Lewis adsorbate governs the stabilization via dual effects: first, changing the stubborn spatial distribution of tin vacancy (VSn) by triggering charge redistribution; second, saturating the dangling states while simultaneously reducing the amounts of deep band gap states. Specifically, the hard Lewis base molecules like edamine (N-donor group) and Isatin-Cl (Cl-donor group) would show a better healing effect than other candidates on the defects-contained tin-based perovskite surface with a somehow hard Lewis acid nature. Our research provides a general strategy for additive engineering and fabricating stable and high-efficiency lead-free Sn-based perovskite solar cells.

Interfacial engineering of lattice coherency at ZnO-ZnS photocatalytic heterojunctions

Heterogeneous semiconductor interfaces with built-in electric fields are central to promoting directional carrier migration and separation for charge-induced redox reactions. Unfortunately, most heterointerfaces are structurally incoherent due to large lattice mismatch accompanied with multiple dangling bonds, imposing high energy barriers for charge transport. Here, we report the atomic engineering of ZnO-ZnS heterogeneous interfaces, transforming from incoherent characters to semi-coherent counterparts through annealing-induced spontaneous assembly of ZnS nanoparticles on single-crystalline ZnO nanowires, which is substantiated by aberration-corrected scanning transmission electron microscopy. Theoretical calculations reveal that semi-coherent interfaces are beneficial in annihilating deep local gap states and thus, remarkably reducing carrier transport barrier height by 1.25 eV. This contributes to a 5.8-fold enhanced photocatalytic hydrogen production rate with an improved stability. The study provides physical insights into interfacial lattice engineering of photocatalytic heterojunctions and offers a convenient strategy to convert incoherent interfaces to photocatalytically favorable semi-coherent phase boundaries for solar energy utilization. • The coherency of ZnO-ZnS interfaces was atomically engineered by thermal annealing • Creation of semi-coherent interfaces by grain rotation facilitates carrier transfer • Semi-coherent interfaces endow the composites with enhanced photocatalytic property Interfacial interaction in heterogeneous systems has a profound effect on photocatalytic performance. However, the intrinsic phase boundaries between two dissimilar photocatalytic materials are more prone to establish randomly incoherent interfaces containing multiple dangling bonds, due to the large discrepancy in their lattice parameters. This work opens up a convenient scenario to convert incoherent interfaces into plane-matched semi-coherent counterparts through annealing-induced grain rotation. These lattice-matching semi-coherent interfaces are conducive to annihilating deep gap states, resulting in a substantially reduced barrier height for carrier transport, which contributes to strengthened photocatalytic property and improved photostability. Overall, our report provides a practical paradigm for the atomic engineering of interface coherency of advanced photocatalysts, opening a promising avenue for the construction of heterojunctions. The incoherent ZnO-ZnS photocatalytic heterojunctions could be atomically engineered into semi-coherent phase boundaries by the spontaneous assembly of ZnS nanoparticles on ZnO nanowires via a convenient annealing strategy. The lattice-matching interface coherency reduces carrier transport barrier height by removing interfacial dangling bonds, giving rise to optimized interfacial electronic structure for promoted charge separation and strengthened photocatalytic activity.

Construction of Single-Atom Platinum Catalysts Enabled by CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals (CsPbX3 NCs) have been regarded as promising materials in photocatalysis. Combining metal single atoms with CsPbX3 NCs may be a practical way in exploring perovskite-based catalysts. However, such hybrids have not been achieved experimentally yet, mainly due to the weak interaction between the metal atom and the CsPbX3 surface. Here, we demonstrate that Pt single atoms can be deposited on CsPbBr3 NCs through a photoassisted approach, in which the surface was partially oxidized first, followed by the anchoring of Pt single atoms through the formation of Pt-O and Pt-Br bonds. The deposition of Pt single atoms can significantly change the photophysical properties of CsPbBr3 NCs by inducing the generation of deep trap states in the band gap. The as-prepared Pt-SA/CsPbBr3 can be used as efficient and durable catalysts for photocatalytic semi-hydrogenation of propyne. A CsPbBr3 nanocrystal might be a suitable substrate for anchoring other metal single atoms, such as Cu, Au, Ag, Pd, and so on.

Impact of S-Vacancies on the Charge Injection Barrier at the Electrical Contact with the MoS2 Monolayer.

Making electrical contacts to semiconducting transition metal dichalcogenides (TMDCs) represents a major bottleneck for high device performance, often manifesting as strong Fermi level pinning and high contact resistance. Despite intense ongoing research, the mechanism by which lattice defects in TMDCs impact the transport properties across the contact-TMDC interface remains unsettled. Here we study the impact of S-vacancies on the electronic properties at a MoS2 monolayer interfaced with graphite by photoemission spectroscopy, where the defect density is selectively controlled by Ar sputtering. A clear reduction of the MoS2 core level and valence band binding energies is observed as the defect density increases. The experimental results are explained in terms of (i) gap states' energy distribution and (ii) S-vacancies' electrostatic disorder effect. Our model indicates that the Fermi level pinning at deep S-vacancy gap states is the origin of the commonly reported large electron injection barrier (∼0.5 eV) at the MoS2 ML interface with low work function metals. At the contact with high work function electrodes, S-vacancies do not significantly affect the hole injection barrier, which is intrinsically favored by Fermi level pinning at shallow occupied gap states. Our results clarify the importance of S-vacancies and electrostatic disorder in TMDC-based electronic devices, which could lead to strategies for optimizing device performance and production.

Analytical Drain Current and Capacitance Model for Amorphous InGaZnO TFTs Considering Temperature Characteristics

Analytical drain current and capacitance model is developed for the amorphous InGaZnO (a-IGZO) thin-film transistor (TFT). The numerical Pao-Sah model is presented to describe the temperature characteristics considering the deep and tail trap states in the energy gap of the a-IGZO thin film. The numerical model is successful for the TFT in both the subthreshold regime and the above-threshold regime. In the subthreshold regime, considering that the trapped electron concentration in the deep trap states dominates the Poisson's equation, the surface-potential-based analytical model is presented. In the above-threshold regime, the threshold-voltage-based analytical model is presented. Applying the smooth function to connect the subthreshold model and the above-threshold model, the analytical compact model is obtained. The compact model is verified by the numerical Pao-Sah model and the available experimental data from 253 to 393 K. Furthermore, the temperature characteristics of the field-effect mobility are discussed.

AgBiS2 as a low-cost and eco-friendly all-inorganic photovoltaic material: nanoscale morphology-property relationship.

Solar cells made of low-cost solution-processed all-inorganic materials are a promising alternative to conventional solar cells made of high-temperature processed inorganic materials, especially because many high-temperature processed inorganic materials contain toxic element(s) such as lead or cadmium (e.g., CsPbI3 perovskite, PbS, CdTe and CdS(Se)). AgBiS2 nanocrystals, consisting of earth-abundant elements but without lead and cadmium, have already emerged as a promising candidate in high-performance solar cells. However, the nanoscale morphology–optoelectronic property relationship for AgBiS2 nanocrystals is still largely unknown. Herein, we investigate the electronic properties of various AgBiS2 nanocrystals by using first-principles computation. We show that the optoelectronic properties of bulk AgBiS2 are highly dependent on the M–S–M–S– (M: Ag or Bi) orderings. Moreover, because Ag–S–Ag–S– and Bi–S–Bi–S– in AgBiS2 bulk crystals contribute respectively to the valence band maximum and conduction band minimum, these unique chemical orderings actually benefit easy separation of mobile electrons and holes for photovoltaic application. More importantly, we find that AgBiS2 nanocrystals (NCs) can exhibit markedly different optoelectronic properties, depending on their stoichiometry. NCs with minor off-stoichiometry give rise to mid-gap states, whereas NCs with substantial off-stoichiometry give rise to many deep defect states in the band gap, and some NCs even show metallic-like electronic behavior. We also find that the deep-defect states can be removed through ligand passivation with optimal coverage. The new insights into the nanoscale morphology–optoelectronic property relationship offer a rational design strategy to engineer the band alignment of AgBiS2 NC layers while addressing some known challenging issues inherent in all-inorganic photovoltaic materials.

Suppression of Electron-Hole Recombination by Intrinsic Defects in 2D Monoelemental Material.

The Shockley-Read-Hall (SRH) model, in which the deep trap defect states in the band gap are proposed as nonradiative electron-hole (e-h) recombination centers, has been widely used to describe the nonradiative e-h recombination through the defects in semiconductor. By using the ab initio nonadiabatic molecular dynamics method, we find that the SRH model fails to describe the e-h recombination behavior for defects in 2D monoelemental material such as monolayer black phosphorus (BP). Through the investigation of three intrinsic defects with shallow and deep defect states in monolayer BP, it is found that, surprisingly, none of these defects significantly accelerates the e-h recombination. Further analysis shows that because monolayer BP is a monoelemental material, the distinct impurity phonon, which often induces fast e-h recombination, is not formed. Moreover, because of the flexibility of 2D material, the defects scatter the phonons present in pristine BP, generating multiple modes with lower frequencies compared with the pristine BP, which further suppresses the e-h recombination. We propose that the conclusion can be extended to other monoelemental 2D materials, which is important guidance for the future design of functional semiconductors.

Tuning the density distribution of deep localized states of TiO2 nanotube arrays through decoration with Pt and Pt@DLC

We demonstrate the possibility to tune the distribution of deep localized states and electronic transport properties of TiO2 nanotube arrays (TNA) through decoration its surface with Pt or Pt@DLC (Diamond-like carbon). A quantitative calculation of the position of the deep trap states in the band gap and the amount of accumulated charge in deep traps of Pt/TNA and Pt-DLC/TNA electrodes are presented and compared based on photocurrent-voltage curves. Our findings demonstrate that both Pt NPs and Pt@DLC depositions could decrease the quantity of deep localized states in the band gap by saturating the dangling bonds on the surface of TNA. A decline in photoluminescence intensity in platinized samples confirmed the density of deep trap states acting as recombination centers reduced by these surface modifications. Also, the deep localized states in Pt/TNA and Pt-DLC/TNA electrodes shift down towards valence band, in comparison with pure TNA. The neighborhood of the trapped electrons in deep states to the hole sites (VB) in platinized samples leads to the reduction of the electron- hole life time in these samples. Moreover, higher anodic photocurrent density was observed for pure TNA, revealed that in pure TNA the free photoexcited electrons reached to the conduction band will transport to the external circuit whereas, in Pt/TNAs and Pt-DLC/TNAs some free electrons would transfer to Pt NPs via Schottky junction at the semiconductor/electrolyte interface or via hopping or tunneling of pi-electrons existing in the sp2 nanoregions of DLC. Finally, Pt- and Pt@DLC-modified TNA electrodes absorbing light in the visible range and suppressing photoelectron recombination are worthy photocatalysts. In contrast, the deposition of Pt or Pt@DLC on TiO2 nanotubes resulted in lower PEC response.

Formation of defects and impurities in MoSx and their effect on electronic properties

The authors investigate the thermodynamic and electronic properties of few- and monolayer MoS2 using density functional theory. They calculate the formation energies of S vacancies and the doping effect of C and O under various chemical environments which are commonly observed during MoS2 deposition. The results show that in the oxidizing condition, O readily gets incorporated into the MoS2 layer and assists in C doping. They analyze the bandgap and density of states of MoS2 at different concentrations of defect. They find that the bandgap decreases as the defect concentration increases, wherein the ordering of S vacancies has a more significant influence than that of the other two defects. The S vacancy and C dopants generate deep acceptor gap states within the bandgap of MoS2, while O dopant states are delocalized. They also observe that the carbon dopants form penetrative dimers, which lead to shallow acceptor states. This work contributes to the understanding of defect/impurity incorporation during deposition and prediction of electronic behavior of deposited low-quality MoS2 materials.

The Role of Cation-Vacancies for the Electronic and Optical Properties of Aluminosilicate Imogolite Nanotubes: A Non-local, Linear-Response TDDFT Study.

We report a combined non-local (PBE-TC-LRC) Density Functional Theory (DFT) and linear-response time-dependent DFT (LR-TDDFT) study of the structural, electronic, and optical properties of the cation-vacancy based defects in aluminosilicate (AlSi) imogolite nanotubes (Imo-NTs) that have been recently proposed on the basis of Nuclear Magnetic Resonance (NMR) experiments. Following numerical determination of the smallest AlSi Imo-NT model capable of accommodating the defect-induced relaxation with negligible finite-size errors, we analyse the defect-induced structural deformations in the NTs and ensuing changes in the NTs' electronic structure. The NMR-derived defects are found to introduce both shallow and deep occupied states in the pristine NTs' band gap (BG). These BG states are found to be highly localized at the defect site. No empty defect-state is modeled for any of the considered systems. LR-TDDFT simulation of the defects reveal increased low-energy optical absorbance for all but one defects, with the appearance of optically active excitations at energies lower than for the defect-free NT. These results enable interpretation of the low-energy tail in the experimental UV-vis spectra for AlSi NTs as being due to the defects. Finally, the PBE-TC-LRC-approximated exciton binding energy for the defects' optical transitions is found to be substantially lower (up to 0.8 eV) than for the pristine defect-free NT's excitations (1.1 eV).

Impact of Sb and Na Doping on the Surface Electronic Landscape of Cu2ZnSnS4 Thin Films

Open-circuit voltage deficiency is the key limiting factor in Cu2ZnSnS4 (CZTS) thin-film solar cells, which is commonly associated with band tails and deep gap states arising from elemental disorder. The introduction of dopants such as Na and Sb has led to improvement in device performance, yet their effects on the optoelectronic properties of CZTS are yet to be fully elucidated. In this Letter, we unraveled the effect of Sb and Na:Sb co-doping on the surface energy landscape of solution-processed CZTS films employing energy-filtered photoelectron emission microscopy. In the absence of the additives, 150 nm resolution photoemission maps reveal oscillations in the local effective work function as well as areas of low photoemission energy threshold. The introduction of dopants substantially reshapes the photoemission maps, which we rationalize in terms of Cu:Zn and Sn disorder. Finally, we establish unprecedented correlations between the photoemission landscape of thin films and the performance of over 200 ...

High sub-band gap response of TiO2 nanorod arrays for visible photoelectrochemical water oxidation

Developing new methods to use visible light is important for TiO2 photocatalysis. This research reported that the TiO2 rutile nanorod (NR) array shows high sub-band gap visible light response. By means of electrochemical cyclic voltammetries, the intrinsic occupied gap states of TiO2 NR array were identified. It was seen that the gap states can be switched from filled states to empty states by visible light illumination, which play the role of relay in promoting electrons from the valence band to the conduction band of TiO2 NRs after absorbing two visible photons. In a consequence, the TiO2 NR array shows the obvious anodic photocurrents for water photoelectrochemical oxidation and the photocatalytic degradation of methyl orange under visible light illumination. This study revealed the significance of the deep occupied gap states of TiO2 NR arrays, which are usually considered as recombination centers. Different from conventional photocatalysis arising from inter-band electron transition, the present research shows a different pathway to use visible light in TiO2 photocatalysis effectively, which should be highly meaningful for designing visibly-active TiO2 photocatalysts.

Identification of Ge vacancies as electronic defects in methyl- and hydrogen-terminated germanane

We use a combination of optical and electrostatic surface science techniques to measure electronically active native defects in multilayer GeCH3 and GeH, two-dimensional (2D) functionalized materials. Chemical processing techniques coupled with density functional theory enable us to identify the specific physical nature of both native point defects and synthesis-related impurities which can limit the optical and charge transport properties of these materials. Direct comparison of optical measurements with calculated electronic levels provides identification of these localized, deep level gap states and confirms partial H-passivation of dangling bonds, revealing synthesis and processing methods needed to control specific defects and optimize these 2D materials for emergent solid state-electronics.

Doping-Enhanced Short-Range Order of Perovskite Nanocrystals for Near-Unity Violet Luminescence Quantum Yield.

All-inorganic perovskite nanocrystals (NCs) have emerged as a new generation of low-cost semiconducting luminescent system for optoelectronic applications. The room-temperature photoluminescence quantum yields (PLQYs) of these NCs in the green and red spectral range approach unity. However, their PLQYs in the violet are much lower, and an insightful understanding of such poor performance remains missing. We report a general strategy for the synthesis of all-inorganic violet-emitting perovskite NCs with near-unity PLQYs through engineering local order of the lattice by nickel ion doping. A broad range of experimental characterizations, including steady-state and time-resolved luminescence spectroscopy, X-ray absorption spectra, and magic angle spinning nuclear magnetic resonance spectra, reveal that the low PLQY in undoped NCs is associated with short-range disorder of the lattice induced by intrinsic defects such as halide vacancies and that Ni doping can substantially eliminate these defects and result in increased short-range order of the lattice. Density functional theory calculations reveal that Ni doping of perovskites causes an increase of defect formation energy and does not introduce deep trap states in the band gap, which is suggested to be the main reason for the improved local structural order and near-unity PLQY. Our ability to obtain violet-emitting perovskite NCs with near-perfect properties opens the door for a range of applications in violet-emitting perovskite-based devices such as light-emitting diodes, single-photon sources, lasers, and beyond.

Germanium substitution endowing Cr3+-doped zinc aluminate phosphors with bright and super-long near-infrared persistent luminescence

We present novel near-infrared (NIR) Cr3+-doped non-gallate super-long-persistence phosphors (Zn1+xAl2-2xGexO4:Cr3+) by Germanium substitution in the original ZnAl2O4:Cr3+. Unlike the negligible NIR persistent luminescence of ZnAl2O4:Cr3+ upon UV or visible light excitation, the Ge substituted phosphors feature strong and super-long-persistent luminescence at approximately 650–750 nm for more than 120 h. The relation between the Ge substitution and the defect trapping states is investigated systematically. The experimental results combined with the first-principles calculations reveal that GeZn○○ and GeAl○ in the spinel structure would introduce shallow and deep defect states in the band gap to serve as new efficient traps, which are mainly responsible for the strong and super-long-persistent luminescence upon UV or visible light excitation. The present advanced phosphors is an alternative candidate for applications in biomedical imaging and night-vision surveillance.

Mechanism of high-fluence proton induced electrical degradation in AlGaN/GaN high-electron-mobility transistors

The effects of displacement damage induced by 3 and 6 MeV protons in AlGaN/GaN high-electron-mobility transistors (HEMTs) are investigated. For the 6 MeV protons at a dose of 5 × 1014 cm−2, a 12% decrease in saturation current, a 3.8% decrease in the peak transconductance, a 0.3 V positive shift of the threshold voltage, and a three-to fourfold decrease in reverse gate leakage current are observed compared with the pre-irradiation values. The main degradation mechanism is considered to be the generation of deep trap states in the band gap, which remove electrons and reduce the carrier mobility in a two-dimensional electron gas (2DEG). Both the carrier removal rate and negatively charged trap density can be extracted, which shows that about 3500 proton injections lead to one carrier removal. Proton fluence and energy are found to be two key parameters that affect the degradation characteristics of irradiated GaN HEMTs.