Distribution Of Gap States Research Articles

Topological Phase and Tunable Quantum State Transfer of Su–Schrieffer–Heeger Chain with an Embedded Quantum Ring

AbstractAn extended Su–Schrieffer–Heeger (SSH) model consisting of an SSH chain with an embedded quantum ring (QR) is investigated. In the case of two SSH chains with a symmetric distribution with respect to QR, by calculating the real‐space winding number and energy spectrum, the hopping amplitude‐induced topological phase is discovered. The probability distributions of gap states and the relationship between energy and magnetic flux prove the existence of the Aharonov–Bohm effect in the QR. Moreover, in the case of asymmetric distribution, the model possesses a zero‐energy mode within the energy gap, and it is found that it can realize quantum state transfer. By adjusting the connection site between the right SSH chain and QR, the direction of the output port can be flexibly engineered. Furthermore, it is shown that high‐fidelity quantum state transfer can still be achieved with the increasing of the system size. The tunable quantum state transfer based on the zero‐energy mode can be equivalent to a topological tunable directional switch with properties of non‐directional transfer. This work provides an approach for studying topological phase transition and tunable topological devices in an SSH chain with an embedded QR.

Stability and gap states of amorphous In-Ga-Zn-Ox thin film transistors: Impact of sputtering configuration and post-annealing on device performance

Stability of amorphous In-Ga-Zn-Ox (a-IGZO) thin-film-transistors (TFTs) is studied from a viewpoint of gap states, associated with oxygen-related defects and structural disordering. The a-IGZO gap states are controlled through the film preparation by magnetron sputtering under two different configurations and post-annealing in oxygen atmosphere. The distribution of gap states is then evaluated from the subgap absorption spectrum, which is measured by a constant photocurrent measurement technique. With this known distribution, a-IGZO TFTs are fabricated, and the stability of the TFT performances is examined through the negative bias induced stability (NBIS) experiments. The results suggest that a negative threshold voltage shift, induced by NBIS, is improved by reducing the density of gap states, i.e., narrowing the gap state distribution. The reduction of gap states also contributes to the improvement of subthreshold swing.

Light soaking effect on defect states distribution of Hydrogenated amorphous silicon investigated By means of constant photocurrent technique

In the present paper we have investigated, by using the constant photocurrent method in dc-mode (dc-CPM), the effect of the light soaking (LS) on the deep defect density (Nd) and the slope of the Urbach tail (E0) of a slightly phosphorus-doped hydrogenated amorphous silicon (a-Si:H) film prepared by Plasma-Enhanced Chemical Vapour Deposition (PECVD). By applying the derivative method, we have converted the measured data into a density of states (DOS) distribution in the lower part of the energy gap. The evolution of the sub-band-gap absorption coefficient and the CPMdetermined density of gap-states distribution within the gap versus the illumination time leads to: (i) an increase in the deep defect absorption without any significant changes in the Urbach tail (exponential part), (ii) a presence of more charged than neutral defects as predicted by the defect pool model, and (iii) a saturation point of the degradation of both optical absorption coefficient and density of deep states of slightly P-doped sample measured by dc-CPM. The constant photocurrent technique in dc-mode as a spectroscopy method for the defect distribution determination is, therefore, most reliable to study the light soaking effect on the stability of hydrogenated amorphous silicon layers used in solar cells manufacturing.

Topological phase transition in cavity optomechanical system with periodical modulation

We investigate the topological phase transition and the enhanced topological effect in a cavity optomechanical system with periodical modulation. By calculating the steady-state equations of the system, the steady-state conditions of cavity fields and the restricted conditions of effective optomechanical couplings are demonstrated. It is found that the cavity optomechanical system can be modulated to different topological Su–Schrieffer–Heeger (SSH) phases via designing the optomechanical couplings legitimately. Meanwhile, combining the effective optomechanical couplings and the probability distributions of gap states, we reveal the topological phase transition between trivial SSH phase and nontrivial SSH phase via adjusting the decay rates of cavity fields. Moreover, we find that the enhanced topological effect of gap states can be achieved by enlarging the size of system and adjusting the decay rates of cavity fields.

Gap States in Methylammonium Lead Halides: The Link to Dimethylsulfoxide?

Understanding the origin and distribution of electronic gap states in metal halide perovskite (MHP) thin films is crucial to the further improvement of the efficiency and long-term stability of MHP-based optoelectronic devices. In this work, the impact of Lewis-basic additives introduced in the precursor solution on the density of states in the perovskite bandgap is investigated. Ultraviolet photoemission spectroscopy and contact potential difference measurements are conducted on MHP thin films processed from dimethylformamide (DMF)-based solutions to which either no additive, dimethylsulfoxide (DMSO), or N-methylpyrrolidine-2-thione (NMPT) isadded. The results show the presence of a density of states in the gap of methylammonium lead halide films processed from DMSO-containing solution. The density of gap states is either suppressed when the methylammonium concentration in mixed cation films is reduced or when NMPT is used as an additive, and eliminated when methylammonium (MA) is replaced with cesium or formamidinium (FA). These results are consistent with the notion that reaction products that result from DMSO reacting with MA+ in the precursor solution are responsible for the formation of gap states.

Gap state distribution and Fermi level pinning in monolayer to multilayer MoS2 field effect transistors.

Gap states and Fermi level pinning play an important role in all semiconductor devices, but even more in transition metal dichalcogenide-based devices due to their high surface to volume ratio and the absence of intralayer dangling bonds. Here, we measure Fermi level pinning using Kelvin probe force microscopy, extract the corresponding electronic state distribution within the band gap, and present a systematic comparison between the gap state distribution obtained for exfoliated single layer, bilayer and thick MoS2 FET samples. It is found that the gap state distribution in all cases decreases from the conduction band edge and is in the order of 1019 eV-1 cm-3 and slightly decreases with increasing channel thickness. Strong Fermi level pinning is observed near the conduction band edge, and it decreases as it approaches the middle and lower part of the bandgap.

Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots Using Broadband Electronic Sum Frequency Generation Microspectroscopy

Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the band gap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states, providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of the QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.

Photoresponses of Supported Au Single Atoms on TiO2(110) through the Metal-Induced Gap States.

When a metal single-atom (SA) catalyst is supported on a semiconducting photocatalyst, the charge transfer of the photoexcited carriers to metal SAs can provide a synergetic activity for the co-catalysts. Here, we report the interfacial electronic coupling of the Au SAs on the TiO2(110) surface using scanning tunneling microscopy/spectroscopy, in combination with first-principles calculations. Distinct energy and spatial distributions of the metal-induced gap states (MIGSs) are experimentally revealed for the Au SAs adsorbed at the terminal Ti sites and the oxygen vacancies. The localized MIGS below the Fermi level provides a dedicated channel for the transfer of a photoexcited hole from the TiO2 substrate to the adsorbed Au SAs. The hole can weaken the Ti-Au bonding and activate the diffusion of Au SAs. Our results shed light on combining the advantages of photocatalysis and metal SA catalysis using a co-catalyst, which is promising to promote chemical reactions at low temperatures.

(Invited) Photoemission-Based Characterization of Gate Dielectrics and Stack Interfaces

We have demonstrated quantitative evaluation of dielectric function and optical constants for gate dielectrics such as remote plasma-assisted chemical-vapor-deposition (CVD) SiO2 and plasma-assisted atomic-layer-deposition (ALD) alumina-silicate in the energy range of 5 ~ 30 eV from the combination of the analysis of energy loss signals (ELS) of core-line photoelectrons and the results of Kramers-Kronig transformation of measured ELS. We have also show quantification of the energy distributions of occupied gap states in gate dielectrics and dielectric stack interfaces such as CVD SiO2, ALD HfO2 and ALD hafnia-silicates on GaN by means of total photoelectron yield spectroscopy (PYS) in the photo-excitation energy range from 3 ~ 10eV.

Resonant and Selective Excitation of Photocatalytically Active Defect Sites in TiO2.

It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO2 and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly, resulting in Pauli blocking, and are thereby undetectable under direct current or continuous wave excitation. In the method demonstrated here, the incident light is modulated with an optical chopper, whereas the PC is measured with a lock-in amplifier. Thin (5 nm) films of TiO2 deposited by atomic layer deposition on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent PC spectra, with a broad peak centered around 2.0 eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV-vis absorption spectra of these films show no features at 2.0 eV, photoluminescence (PL) spectra of these photoelectrodes show a similar wavelength dependence with a peak of around 2.0 eV, corresponding to the subband gap emission associated with these defect sites. As a control, alumina (Al2O3) films exhibit no PL or PC over the visible wavelength range. The ac PC plotted as a function of electrode potential shows a peak of around -0.4 to -0.1 V versus normal hydrogen electrode, as the monoenergetic defect states are tuned through a resonance with the HER potential. This approach enables the direct photoexcitation of catalytically active defect sites to be studied selectively without the interference of the continuum interband transitions or the effects of Pauli blocking, which is limited by the slow turnover time of the catalytically active sites, typically on the order of 1 μs. We believe that this general approach provides an important new way to study the role of defects in catalysis in an area where selective spectroscopic studies of these are few.

Impact of a Low Concentration of Dopants on the Distribution of Gap States in a Molecular Semiconductor

We investigate the distribution of valence and tail states in copper phthalocyanine (CuPc) upon the introduction of minute amounts of the p-dopant molybdenum tris[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), using a combination of electron spectroscopy and carrier transport measurements. Density of gap states, conductivity, and hole-hopping activation energy are measured. We observe the progressive filling (and deactivation) of the deepest tail states by charges introduced by the dopants, as well as significant broadening of the CuPc density of states. Simulations relate this broadening to the electrostatic and structural disorder induced by the dopant in the CuPc matrix.

High-sensitivity ultraviolet photoemission spectroscopy technique for direct detection of gap states in organic thin films

We developed ultrahigh sensitivity, low-background ultraviolet photoemission spectroscopy (UPS) technique which does not introduce detectable radiation damages into organic materials. The UPS allows to detect density of states of the order of ∼1016 states eV−1cm−3 even for radiation-sensitive organic films, this results being comparable to electrical measurements of charge trapping centers. In this review we introduce the method of ultrahigh sensitivity photoemission measurement and we present some results on the energy distribution of gap states in pentacene (Pn) films deposited on SiO2 and Au(111) substrate. For Pn/SiO2 thin film the results show that exposure to inert gas (N2 and Ar) atmosphere produces a sharp rise in gap states from 1016 to 1018 states eV−1cm−3 and pushes the Fermi level closer to the valence band (0.15–0.17eV), as does exposure to O2 (0.20eV), while no such gas-induced effects are observed for Pn/Au(111) system. The results demonstrate that these gap states originate from small imperfections in the Pn packing structure, which are induced by gas penetration into the film through the Pn crystal grain boundaries. Similar results were obtained for CuPc/F16CuPc thin films, a prototypical example of donor/acceptor interface for photovoltaic application.

Electrochemical determination of the density of states of nanostructured NiO films.

Mesoporous p-type NiO films were prepared by aerosol-assisted chemical vapor deposition (AACVD) and characterized by X-ray diffraction (XRD). The nanostructure of the films was investigated by field emission gun scanning electron microscopy (FEG-SEM). The density of states (DOS) in these nanostructured films has been determined by means of electrochemical impedance spectroscopy and cyclic voltammetry. The analysis reveals an exponential distribution of band gap states above the valence band that extends around 1.5 eV. In addition, monoenergetic states were also identified which overlap with the exponential distribution. This distribution of states has an enormous influence in the electronic processes of the devices in which NiO electrodes are employed (electrochromism, water splitting or energy storage). Especially, in p-type dye-sensitized solar cells (p-DSCs), it is thought that intra-band-gap states are responsible for the fast observed recombination processes, whose existence and distribution has not been clearly determined yet and are now confirmed and quantified by our analysis. This provides a better comprehension of the recombination events which represent one of the main losses in p-DSCs.

(Invited) Spectroscopy of Deep Gap States in High-k Insulators

The work describes the physical principles of the exhaustive photodepopulation spectroscopy. This method allows one to determine the energy distribution of gap states in insulating materials using the observation of optically-assisted electron removal from the gap states in incremental spectral windows. Examples of the inferred energy distributions of acceptor-type electron states in several high-k oxide insulators (Al2O3, HfO2, GdxAl2-xO3) indicate that these are »1-2 eV broad. At the same time, no detectable density of donor states, expected to be present due to oxygen vacancies, is found in the studied oxides.

Defect states in amorphous Ge2Sb2Te5 phase change material

Steady-state photoconductivity measurements in the temperature range 100–300 K on amorphous Ge2Sb2Te5 thin film prepared by dc sputtering are analyzed. The dark conductivity is thermally activated with a single activation energy that allocates the position of the Fermi level approximately in the middle of the energy gap relative to the valance band edge. The temperature dependence of the photoconductivity ensures the presence of a maximum normally observed in chalcogenides with low- and high-temperature slopes, which predict the location of discrete sets of localized states (recombination levels) in the gap. The presence of these defect states close to the valence and conduction band edges leaves the quasi Fermi level shifts in a continuous distribution of gap states at high temperatures, as evidenced from the γ values of the lux–ampere characteristics.

Low density of gap states and unpinned Fermi level inn-channel organic thin-film transistors

Electronic states within the band gap of a semiconductor have a significant influence on the electronic properties and the operational stability of electronic devices, especially in the case of organic semiconductors. Previous work has shown that the gap-state density in organic thin-film transistors (TFTs) is often so large that the Fermi level is pinned deep within the gap. For $p$-channel organic TFTs, we have recently reported that the density of gap states and hence the pinning of the Fermi level can be greatly reduced by optimizing the fabrication process. For $n$-channel organic TFTs, very little is known about the gap density of states and its impact on the performance and stability of the devices. Here, we report measurements of the density and distribution of gap states in $n$-channel TFTs based on two different organic semiconductors. We have found that the gap-state density in both semiconductors is relatively small (\ensuremath{\sim}10${}^{17}$ cm${}^{\ensuremath{-}3}$) and no greater than in $p$-channel organic TFTs fabricated with the same technology and the same gate dielectric. This is a significant finding, since both the performance and the stability of $n$-channel organic TFTs are often greatly inferior to those of $p$-channel organic TFTs. We have also found that the gap-state density increases over time during air exposure and that this increase in gap-state density correlates with the time-dependent, air-induced decrease in electron mobility.

Variable gap and gap state distribution of film silicon

A method of preparing film silicon with variable optical gap is introduced, and the relevant results are given. An easy way to determine the film optical gap by transmittivity curve tangent is shown, and the preparation craftwork and conditions are given. The gap state distribution maps of various materials are presented. In experiment, it is found that the optical gap width of material is related not only to the quantum size effect, but also to the height and width of the barrier formed by lacuna, and to the length of short range order (atom cluster length) as well. A relationship between optical gap of silicon film and atom cluster length is given. Computations show that electron liquid level lifts in potential well with atom cluster length increasing, and the scattering of carrier is weakened by defect barrier. When the atom cluster length is shorter, the electron liquid level does not always lift with atom cluster length increasing, but fluctuates heavily and forms an indention wave. The computations also show that in the case of a constant ratio of the barrier width to the length of the atomic string, the electronic liquid level is also related to the barrier height.

Electrons in the Band Gap: Spectroscopic Characterization of Anatase TiO2 Nanocrystal Electrodes under Fermi Level Control

Macroscopic properties of semiconductor nanoparticle networks in functional devices strongly depend on the electronic structure of the material. Analytical methods allowing for the characterization of the electronic structure in situ, i.e., in the presence of an application-relevant medium, are therefore highly desirable. Here, we present the first spectral data obtained under Fermi level control of electrons accumulated in anatase TiO2 electrodes in the energy range from the MIR to the UV (0.1–3.3 eV). Band gap states were electrochemically populated in mesoporous TiO2 films in contact with an aqueous electrolyte. The combination of electrochemical and spectroscopic measurements allows us for the first time to determine both the energetic location of the electronic ground states as well as the energies of the associated optical transitions in the energetic range between the fundamental absorption threshold and the onset of lattice absorption. On the basis of our observations, we attribute spectral contributions in the vis/NIR to d–d transitions of Ti3+ species and a broad MIR absorption, monotonically increasing toward lower wavenumbers, to a quasi-delocalization of electrons. Importantly, signal intensities in the vis/NIR and MIR are linearly correlated. Absorbance and extractable charge show the same exponential dependence on electrode potential. Our results demonstrate that signals in the vis/NIR and MIR are associated with an exponential distribution of band gap states.

IR–Spectrophotoelectrochemical Characterization of Mesoporous Semiconductor Films

A combined IR-spectroscopic and electrochemical approach for the study of photo- and bias-induced reactions at the semiconductor/electrolyte interface is presented. Information on the electronic structure of a mesoporous semiconductor nanoparticle network, concretely the density and distribution of band gap states, as well as the nature of solution species are analyzed in situ. It has been shown that under appropriate conditions the electrode potential determines the quasi-Fermi level throughout the mesoporous film and thus the occupation of IR-active band gap states, independently of the type of external perturbation, i.e., application of a bias voltage or electrode exposure to photons exceeding the semiconductor band gap at open circuit. Importantly, electronic properties of the semiconductor and vibrational properties of solution species can be addressed simultaneously by IR-spectroscopy. In addition, electrochemical methods provide a means for the active manipulation (in potentiostatic measurements) or the passive tracking (during open circuit potential decay) of the quasi-Fermi level in the mesoporous film together with the possibility of electron quantification (by charge extraction experiments).

Formation, Morphology, and Effect of Complex Defects in Boron Nitride Nanotubes: An ab initio Calculation

The formation mechanism of the complex defect, consisting of a carbon (C) impurity and an atomic vacancy, and the corresponding effects on structure and properties of boron nitride nanotubes (BNNTs) are studied using density functional theory. It is found that the C impurity and the vacancy tend to bind together to form a pentagon-nonagon pair more stable than the isolated vacancy, because of geometrical and electronic effects. The combination of the cation vacancy and CB (C substitution for B) is preferable in N-rich conditions, while the combination of the anion vacancy and CN (C substitution for N) is favorable in B-rich conditions. The C-doping facilitates the formation of the vacancy in the growth of the BNNT. The interaction between the C impurity and the vacancy on the one hand quenches their respective local moments, making the system nonmagnetic; on the other hand substantially changes the density and distribution of localized gap states, reducing the activity. We suggest that the complex defect system is more appropriate for chemical applications, such as ion sieve and filter, than for physical applications.