Gap States Research Articles

Bi-induced photochromism and photo-stimulated luminescence with fast photochromic response for multi-mode dynamic anti-counterfeiting and optical information storage

Materials with photochromic (PC) and photo-stimulated luminescence (PSL) properties have been increasingly recognized as promising candidates in information storage and anti-counterfeiting applications. However, the general reported doping method cannot realize both PC and PSL properties in one inorganic material due to the unmanageable intrinsic electronic structure, which limits its applications in multi-mode information storage. Herein, we report a single phosphor with PC, PSL, thermal luminescence (TL), and afterglow properties to substitute the common composites utilized in multi-mode systems through the systematic defect management strategies. By doping Bi into LiNbO3 (LNO):Pr3+ phosphor, it shows a fast response time (∼1 s) and a large reflectivity difference of 34 % between the bleached and colored states under 365 nm excitation. In addition, it also presents a wavelength-dependent bleaching character. The first-principle calculations reveal that such properties result from the presence of trap states in the band gap of LNO:Pr3+ phosphor through Bi-doping, which enable us to design the device with multiple dimensions of anti-counterfeiting and optical information storage performances. This finding stimulates the exploration for designing multi-mode responsive luminophores for future generation of information storage.

Nitrogen tuned crystal structure, optical band gap, localized states on amorphous and crystalline GeTe films

The nitrogen (N) doping effect on crystal structure, optical band gap, and localized states for amorphous/crystalline GeTe films was studied. The crystallization temperature and sheet resistance were improved by N doping. The local bonding structure of amorphous and crystalline N x ( GeTe ) 1 - x was elucidated by Raman spectra. The changes in optical band gap and localized states density for the films were evaluated by transmittance spectra. The band gap broadening after N doping is crucial to reducing the threshold current for phase change memory.

Enhancement of tunneling currents by isoelectronic nitrogen-atom doping at semiconductor pn junctions; comparison of indirect and direct band-gap systems

Enhancement of tunneling currents by the isoelectronic Al–N/N-atom doping is studied at the pn junctions made of Si, Ge, GaP, InP, and GaAs semiconductors, using the sp 3 d 5 s * tight-binding model and the non-equilibrium Green’s function method. With respect to indirect band-gap systems, doping produces the impurity state in the band gap, and such a state produces resonance with conduction-band states of n-type layers under the electric field. We show that this resonance state works to decrease the tunneling length between valence-band states of p-type layers and conduction-band states of n-type layers and promotes the marked enhancement of tunneling current. As for direct band-gap systems, on the other hand, the N-atom doping not only produces the localized N-atom state in the conduction bands but also reduces the band-gap energy by lowering the conduction-band. We show that the localized N-atom state does not contribute to the tunneling current, while the band-gap reduction shortens the tunneling length a little and slightly increases the tunneling current.

Optical study of Ga2-xSnxO3 (0 ≤ x ≤ 0.7) thin films using spectroscopic ellipsometry and cathodoluminescence

Diagnosis of the semiconductor defect levels is essential for their device applications. The optical and electrical properties of Ga2-xSnxO3 (GTO, 0 ≤ x ≤ 0.7) films were investigated to look into the defect levels. Amorphous GTO thin films were grown on Si substrates with various Sn contents via radio frequency magnetron sputtering deposition. Thermal annealing at 900 °C for two hours was performed after the deposition of films at 500 °C to achieve the polycrystalline phase. We examined the physical properties of amorphous β-Ga2O3 thin films grown using various O2 gas flows during growth, and of the post-annealed GTO films. Using spectroscopic ellipsometry, the optical constants in the spectral range between 1.0 and 6.0 eV were determined. We estimated the optical gap energy of the GTO layers using the Tauc method. The optical gap energy was approximately 4.9 eV for amorphous β-Ga2O3 films. Optical structures owing to defects were found below optical gap energy in the spectra of the complex refractive index. Using cathodoluminescence (CL) spectroscopy, there were several peaks discovered in the range between 1.5 and 3.0 eV. The CL peaks correspond to transitions between valence (conduction band) and defect-induced gap states. We identified the 1.5 − 1.6 eV peak as the transition between defect states and the conduction band. The extinction coefficient (k) peak became sharper at 5.3 eV suggesting a strong excitonic behavior at the fundamental gap for amorphous Ga2O3 films.

Analysis of electrical properties in lateral Schottky barrier diode based on n-GaN and AlGaN/GaN heterostructure

In this paper, the lateral Schottky barrier diodes (SBDs) with small capacitance and low turn-on voltage (Von) were fabricated on n-GaN and AlGaN/GaN heterostructure. The capacitances of lateral n-GaN SBD and lateral AlGaN/GaN SBD are 1.35 pF/mm and 0.70 pF/mm, respectively. Compared with the planar SBDs, the capacitances of lateral SBDs are reduced by about two orders of magnitude without sacrificing the performance of on-resistance (Ron) and reverse leakage current. For the planar and lateral n-GaN SBDs, the value of the Von is similar. However, compared with the planar AlGaN/GaN SBD, the Von of lateral AlGaN/GaN SBD is reduced from 1.64 V to 0.87 V owing to the anode metal directly contacting the two-dimensional electron gas. According to temperature-dependent I-V results, the barrier inhomogeneity of the lateral SBD is more intensive than the planar SBD, which is attributed to etching damage. The withstand voltage of SBD is a very important parameter for power electronic applications. Compared with the breakdown voltage of 73 V in the lateral n-GaN SBD, the lateral AlGaN/GaN SBDs exhibit a breakdown voltage of 2322 V. In addition, we found that Schottky contact introduces anode resistance (RA) by analysing the Ron distribution of lateral SBDs. The experimental results also show that the RA of lateral n-GaN SBD and lateral AlGaN/GaN SBD are 10.5 Ω mm and 9.2 Ω mm respectively, which are much larger than the ohmic contact resistance due to worsening anode contact by metal-induced gap states.

Blue TiO2 with tunable oxygen-vacancy defects for enhanced photocatalytic diesel oil degradation

The proliferation of marine petroleum exploitation has culminated in the undesired release of oils into the aquatic ecosystem, causing detrimental environmental effects. Photocatalysis proffers a solution to degrade recalcitrant diesel oil utilizing abundant solar energy, in which TiO2 is the perennial photocatalyst of choice. However, it suffers from the low utilization of the light spectrum and the rapid recombination of photogenerated charge carriers. In this work, oxygen-vacancy-rich blue TiO2 was employed to overcome the shortcomings of TiO2 in diesel oil degradation which has historically not been attempted. These oxygen vacancies induce favourable mid gap states to extend the optical absorption capabilities and enrich the photocatalyst charge density. Additionally, oxygen vacancies enhance the separation of electron hole pairs by acting as trap states, evident from increased trap-mediated capture fluorescence. Consequently, a 1.60-fold enhancement of diesel oil degradation performance compared to pristine TiO2 was observed. Mechanistic insights revealed the restructuring of diesel alkane distribution upon degradation corollary to the cleavage of hydrocarbon chains, illustrating that short alkanes experience a greater susceptibility to degradation compared to long alkanes. GC–MS analysis elucidates that shorter chained alkanes, alkenes, alcohols and cycloalkanes were produced as a product of the degradation.

Detection of defect levels in vicinity of Al2O3/p-type GaN interface using sub-bandgap-light-assisted capacitance–voltage method

Defect levels in the vicinity of the Al2O3/p-type GaN interface were characterized using a sub-bandgap-light-assisted capacitance–voltage (C–V) method. For metal–oxide–semiconductor (MOS) diodes prepared using p-type GaN (p-GaN) and Al2O3 formed by atomic layer deposition, the C–V curves measured in the dark showed capacitance saturation at a negative bias and a large negative voltage shift compared with ideal curves, which implied the effects of donor-like gap states in the vicinity of the Al2O3/p-GaN interface. Upon illumination with monochromated sub-bandgap light with photon energies higher than 2.0 eV under a large positive bias, the subsequently measured C–V curves showed three plateaus. The plateau under the positive bias voltage due to the surface inversion appeared despite the sub-bandgap illumination, which did not appear at 1.8 eV light illumination, indicating the existence of midgap defect levels. Moreover, the other plateaus were attributed to defect levels at 0.60 and 0.7–0.8 eV above the valence band maximum. For a sample whose surface was prepared by photo-electrochemical (PEC) etching to a depth of 16.5 nm, the C–V curve measured in the dark showed a reduced voltage shift compared with the unetched sample. Furthermore, sub-bandgap-light-assisted C–V curves of the sample with PEC etching showed no plateau at a positive bias, which indicated the reduction in the density of the midgap defect states. Possible origins of the detected defect levels are discussed. The obtained results showed that the interface control can improve the properties of p-GaN MOS structures.

Topological properties of subsystem-symmetry-protected edge states in an extended quasi-one-dimensional dimerized lattice

We investigate theoretically the topological properties of dimerized quasi-one-dimensional (1D) lattice comprising of multi legs $(L)$ as well as multi sublattices $(R)$. The system has main and subsidiary exchange symmetries. In the basis of latter one, the system can be divided into $L$ 1D subsystems each of which corresponds to a generalized $SSH_R$ model having $R$ sublattices and on-site potentials. Chiral symmetry is absent in all subsystems except when the axis of main exchange symmetry coincides on the central chain. We find that the system may host zero- and finite-energy topological edge states. The existence of zero-energy edge state requires a certain relation between the number of legs and sublattices. As such, different topological phases, protected by subsystem symmetry, including zero-energy edge states in the main gap, no zero-energy edge states, and zero-energy edge states in the bulk states are characterized. Despite the classification symmetry of the system belongs to $BDI$ but each subsystem falls in either $AI$ or $BDI$ symmetry class.

Effect of halogen doping on the electronic, electrical, and optical properties of anatase TiO2

Titanium dioxide (TiO2) is one of the most used oxides in renewable energy applications, such as hydrogen production, photovoltaics, and light-emitting diodes. To further improve the efficiency of the devices, doping strategies are used to modify their fundamental properties. Here, we used density functional theory (DFT) simulations to explore the effect of all the halogen dopants on the structural, electronic, and optical properties of TiO2. We investigated both the interstitial and the oxygen substitutional positions, and for the optimized structures, we used hybrid DFT calculations to predict the electronic and optical properties. In all cases, we found that halogen dopants reduce the bandgap of the pristine TiO2 while gap states also arise. The halogen dopants constitute a single acceptor when they occupy interstitial sites, while when they are inserted in oxygen sites, they act as donors. This can be established by the states that form above the valence band. It is proposed that these states contribute to the significant changes in the optical and electronic properties of TiO2 and can be beneficial to the photovoltaic and photocatalytic applications of TiO2. Importantly, the iodine doping of TiO2 significantly reduces the bandgap of TiO2 while increasing its dielectric constant, making it suitable for light-harvesting applications.

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 Inverted Perovskite Solar Cells with a Low‐Dimensional Halide/Perovskite Heterostructure

AbstractThe photovoltaic performance of inverted (positive‐intrinsic‐negative) perovskite solar cells (PSCs) is predominantly limited by interfacial recombination loss. Here, by constructing a low‐dimensional halide/perovskite heterostructure, non‐radiative recombination pathways at the perovskite/C60 contact are effectively eliminated and a voltage loss of only 370 mV is achieved in inverted PSCs. Through molecular engineering of the organic spacer, a strong electronic coupling is enabled at the heterointerface, which effectively shifts the gap states out of the bandgap and leads to a prolonged carrier lifetime of 4.28 µs. Our strategy enables a power conversion efficiency of 24.09% (certified 23.54%) for inverted PSCs with an open‐circuit voltage of 1204 mV, and an efficiency of 21.89% (certified 21.48%) for centimeter‐scale cells. The devices retain 92% of the initial efficiency after 85 °C thermal aging for over 1400 h, and 95% of the initial efficiency after 1008 h of maximum power point operation under AM1.5G illumination in air.

First-principles study of metallic impurities induced 355 nm UV laser absorption in fused silica

To date, it is controversial which metallic impurities play crucial role in the laser induced degradation and damage of fused silica at 355 nm. In this work, a First -principles study for the influences of impurities in fused silica are presented, which are usually contaminated during manufacture and post-processing processes. Our results show that Ca, K, Fe and Ce can lead to obvious optical absorption near 355 nm. Moreover, we find that, for K and Ca, the absorption intensity near 355 nm is highly sensitive to the concentration. Our results indicate that K and Ca can easily induce degradation and damage of fused silica at 355 nm. These two impurities should be concerned during manufacture and post-processing processes. The detailed theoretical study reveal that Ca and K can induce obvious impurity states in band gap and the optical absorption near 355 nm is originated from the transition between impurity states to conduction band. Our results also reveal that, for fused silica with Ca impurity, the exciton effect plays an important role for the optical absorptions near 355 nm, while it is not significant for K. Our work helps to explore laser damage mechanism and improve manufacture and post-processing techniques of fused silica.

Unconventional Floquet Topological Phases from Quantum Engineering of Band-Inversion Surfaces

Floquet engineering provides a toolbox for the realization of novel quantum phases without static counterparts, while conventionally the realization may rely on the manipulation of complex temporal evolution. Here we propose a systematic and high-precision scheme to realize unconventional Floquet topological phases by engineering local band structures in particular momentum subspace called band inversion surfaces (BISs). This scheme is based on a new bulk-boundary correspondence that for a class of generic $d$-dimensional periodically driven systems, the local topological structure formed in each BIS uniquely determines the features of gapless boundary modes. By engineering the BIS configuration we demonstrate a highly efficient approach to realize, manipulate, and detect novel Floquet topological phases. In particular, we predict a two-dimensional (2D) anomalous Floquet valley-Hall phase which carries trivial global bulk topological invariants but features protected counter-propagating edge states in each quasienergy gap. The unconventional nature of this novel 2D phase is further illustrated by the examination of edge geometry dependence and its robustness to disorder scattering. Anomalous chiral topological phases with valley protection in higher dimension are also predicted and studied. Our systematic and highly feasible scheme opens a new route to realize and engineer unconventional Floquet topological phases for ultracold atoms and other quantum simulators.

An Insight of the Theoretical Physics of Ru-Alloyed Iron Pyrite Studied for Energy Generation

Pyrite FeS2 has become the focus of many researchers in thin-film photovoltaics because it has some possibilities in photovoltaics. In this manuscript, we present an experimental and a theoretical study of the electronic structure of pyrite FeS2 alloyed with a small concentration of 1.19% of ruthenium (Fe0.9881Ru0.0119S2) by using the Linear Muffin-Tin Orbital Method in the Atomic-Sphere approximation (LMTO-ASA) calculations and the density of states. We observed that the bandgap of FeS2 increases from 0.90508 to 1.21586 eV when we replace ~1.19% of the Fe atoms with ruthenium atoms x=0.0119 concentration of Ru. We prove that this low concentration of Ru saved the gap states and the electronic and optical properties of FeS2 pyrite. Our calculated electronic bandgap is 1.21586 eV and direct. Our results confirm that the symmetric operation of the space Th6 Pa3 saves electronic structure of iron pyrite when alloyed with ruthenium.

Atomic and electronic structures of nitrogen vacancies in silicon nitride: Emergence of floating gap states

We report the density functional calculations with the hybrid approximation to the exchange-correlation energy that clarify the atomic and electronic structures of the N vacancy (${\mathrm{V}}_{\mathrm{N}}$) in ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$. We find that the gap states induced by the Si dangling bonds around ${\mathrm{V}}_{\mathrm{N}}$ show interesting interplay between the spin splitting and the Jahn-Teller splitting depending on their charges. More surprisingly, we find that a peculiar electron state distributed in the internal space of the host SiN is hidden in the conduction bands and converted to a localized floating state in the fundamental energy gap due to the presence of ${\mathrm{V}}_{\mathrm{N}}$. This theoretical finding indicates ${\mathrm{V}}_{\mathrm{N}}$ being multiply charged from $+1$ to $\ensuremath{-}5$: The charge states from $+1$ to $\ensuremath{-}3$ are stable at respective Fermi-level positions in the gap, whereas the states $\ensuremath{-}4$ and $\ensuremath{-}5$ are metastable for the Fermi level near the conduction band bottom. A possibility of multilevel memory function of ${\mathrm{V}}_{\mathrm{N}}$ in SiN is suggested.

Density functional theory calculations for investigation of atomic structures of 4H-SiC/SiO2 interface after NO annealing

We propose the atomic structures of the 4H-SiC/SiO2 interface for a face (11¯00), m face (112¯0), the C face (0001¯), and Si face (0001) after NO annealing using the OH-terminated SiC surface models. Our proposed structures preferentially form at the topmost layers of the SiC side of the interface, which agrees with the experimental finding of secondary-ion mass spectrometry; that is, the N atoms accumulate at the interface. In addition, the areal N-atom density is of the order of 1015atom/cm2 for each plane, which is also consistent with the experimental result. Moreover, the electronic structure on the interface after NO annealing in which the CO bonds are removed and the nitride layer only at the interface is inserted, is free from gap states, although some interface models before NO annealing include the gap states arising from the CO bonds near the valence band edge of the bandgap. Our results imply that NO annealing can contribute to the reduction in the density of interface defects by forming the nitride layer.

Facile Chemical Vapor Modification Strategy to Construct Surface Cyano-Rich Polymer Carbon Nitrides for Highly Efficient Photocatalytic H2 Evolution.

The surface grafting of electro-negative cyano groups on polymer carbon nitrides (PCNs) is an effective way to tail their electronic structure. Despite the significant progress in the synthesis of cyano group-enriched PCN, developing a simple and efficient method remains challenging. Here, a facile strategy was developed for fabricating surface cyano-rich PCN (PCN-DM) with a porous structure via chemical vapor modification using diaminomaleonitrile. The cyano groups of diaminomaleonitrile substituted the amino groups on PCN surface via a deamination. The hydrogen production rate of the PCN-DM was approximately 17 times higher than that of pristine PCN. This significant increase in photocatalytic performance could be assigned to the fusion of cyano groups in the surface of PCN, forming new gap states that broadened the visible-light harvesting and accelerated charge separation for photoredox reactions. This study unveils a promising approach for incorporating functional units in the design of novel photocatalysts for efficient hydrogen production.

Carrier trapping centers in a two-dimensional silica bilayer: Strongly localized shallow gap states and resonances induced by oxygen vacancies

Structural and electronic properties of neutral native defects in a two-dimensional ${\mathrm{SiO}}_{2}$ bilayer (2D-${\mathrm{SiO}}_{2}$) are examined using the SIESTA ab initio approach. We identify scissor and rotation modes of oxygen atoms, in the middle of Si-O-Si chains, as low-energy structural excitations responsible for the response of the 2D-${\mathrm{SiO}}_{2}$ lattice to the formation of all native defects in the present study. Furthermore, we find that oxygen native defects (single vacancies and interstitials) should be the most abundant defect species in thermal-equilibrated samples. Regarding native-defect electronic states, single oxygen vacancies and interstitials are found to be amphoteric trapping centers in 2D-${\mathrm{SiO}}_{2}$. Silicon vacancies and interstitials introduce several strongly localized states spanning a large fraction of the gap. Generally, we identify a marked tendency for the appearance of strongly spatially localized defect states, be their energies shallow or deep within the band gap, as well as the emergence of strongly localized resonances in the valence and/or conduction bands. Strong spatial localization of defect states, a hallmark of systems displaying trapping and polaronic effects, is a result of quantum confinement and enhanced Coulombic effects in this 2D system. Carrier trapping and polaron formation are thus expected to be a common feature of defect states and added carriers in 2D-${\mathrm{SiO}}_{2}$.

Influence of multi-walled carbon nanotubes in polytetrafluoroethylene on the parameters of electronic structure and absorption of ultra-high-frequency radiation

Using the methods of angular correlation of annihilation radiation (ACAR), attenuation of electromagnetic radiation in 1.5–2.2 GHz frequency range, and optical ellipsometry, it was shown that in composites of polytetrafluoroethylene (PTFE) + multi-walled carbon nanotubes (MWCNTs), a 2% decrease in the probability of annihilation of positrons in free volumes in PTFE leads to changes in other parameters of electronic structure of composites by 8–29%. Polytetrafluoroethylene is transparent to electromagnetic radiation, but after the addition of 10 wt.% or more of MWCNTs, the composites demonstrate 200–410-fold decrease in the electromagnetic radiation intensity when the radiation passes through a specimen with a thickness of ≈2 mm. It was found that the average radius of the free volumes and the probability of annihilation of positrons are determined by the defect and electronic structures of the polymer matrix only. The Fermi angle and the probability of positrons annihilation with free electrons are determined by the analogous structures of MWCNTs only. Since the electronic characteristics of the atoms and defects in the polymer matrix (at least outside the interphase) do not change, the changes in the other ACAR parameters are mainly due to changes in the imperfect MWCNTs’ atomic and electronic structures. The average radius of free volumes reaches its maximum value in the composite with 10 wt.% MWCNTs. It was found that in a specimen with 10 wt.% MWCNTs, the highest density of free electrons is observed due to charge transfer from free volumes to MWCNTs, and the highest electron density is observed on defects. A disorder of MWCNTs and their branched conductive network can form the ‘tails’ of electronic density of states in a band gap. Thus, composite with 10 wt.% MWCNTs has the highest absorption coefficient for electromagnetic radiation.

First-principles investigation of a-line Shockley partial dislocations in wurtzite GaN: core reconstruction and electronic structure

Αtomistic simulations based on density functional theory were performed to investigate Shockley partial dislocations, parallel to the <11–20> direction, in wurtzite gallium nitride. The cores of both 30° and 90° Shockley partial dislocations, their possible reconstructions and their electronic structures were analyzed and compared systematically. Shockley partial dislocations were found to have a variety of core structures while exhibiting different bonding states like: deformed Ga–N bonds, Ga–Ga and N–N homo-nuclear bonds, as well as Ga– and N–dangling bonds. We have demonstrated that the core reconstruction of these dislocations is not always energetically favourable in wurtzite GaN, as it is the case in elemental semiconductors. It results from interplay between the energy gain from eliminating dangling and forming homo-nuclear bonds and the energy excess from the induced strain of hetero-nuclear Ga–N bonds within the core. Depending on their core configurations, Shockley partial dislocations are expected to have different electronic behaviours in wurtzite GaN, as they can act either as sources of parasitic luminescence or non-radiative recombination centres, or give pathways to leakage currents. We have demonstrated that deep states associated with dislocations are not due to the dangling bonds, as generally believed, but rather to Ga–Ga bonds in their cores, and that N–N bonds do not lead to any gap states. These results are of high relevance as guidelines in defect engineering strategies aiming at producing dislocations with a particular electronic behaviour in wurtzite GaN.