Occupied Gap States Research Articles

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.

(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.

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.

Defect induced gap states in monolayer MoS2 control the Schottky barriers of Pt-mMoS2 interfaces

Vacancies can significantly affect the performance of monolayer MoS2 (mMoS2) nanodevices because defect induced gap states can introduce large Schottky barriers at Pt-mMoS2 interfaces. Effects of adsorbed gases at S-vacancies on the defect induced gap states and Schottky barriers of Pt-mMoS2 interfaces have been studied by first-principles calculations. The defect induced gap states are occupied (unoccupied) ones when electron-rich (electron-poor) gases adsorb at S-vacancies. The occupied gap states in mMoS2 result in n-type Schottky barriers, whereas unoccupied gap states cause p-type Schottky barriers. Moreover, both the n-type and p-type Schottky barriers of Pt-mMoS2 interfaces decrease when the gap states are closer to the valence bands of mMoS2 because the gap states determine the direction and the amount of charge transfer at interfaces. The n-type and p-type Schottky barriers of Pt-mMoS2 interfaces are reduced to 0.36 and 0 eV by adsorbing high concentrations Cl2 and CO, respectively. Furthermore, adsorbing electron-poor gases (CO and NO) at S-vacancies change the n-type Pt-mMoS2 interfaces to p-type ones. These findings provide guidance to develop approaches to design high performance metal-mMoS2 interfaces with low Schottky barriers.

Structural Defects Control the Energy Level Alignment at Organic/Organic Interfaces

The dependence of the energy level alignment (ELA) on structural defects at an organic/organic heterojunction (OOH) of perfluoropentacene (PFP)‐on‐diindenoperylene (DIP) was investigated using X‐ray scattering and ultraviolet photoelectron spectroscopy. The density of structural defects near the interface between the PFP and DIP layers was varied by changing the growth temperature of the DIP film. A direct relationship was found between the defect density and the ELA at the OOH; the ELA together with the change in the electrostatic potential (quasi‐interface dipole layer) at the OOH varies systematically with the defect density near the interface. This indicates that a key factor affecting the ELA is the electrostatic potential change across the OOH interface, which is produced by electron transfer from DIP occupied gap states to PFP unoccupied gap states. These gap states originate from the defects and are effectively controlled by adjusting the growth conditions of the organic films. As a result, the ELA at OOH interfaces can be controlled by the density of structural defect, which is important for organic devices employing OOHs, such as organic photovoltaic cells.

Solution‐Processed Hydrogen Molybdenum Bronzes as Highly Conductive Anode Interlayers in Efficient Organic Photovoltaics

Highly efficient and stable organic photovoltaic (OPV) cells are demonstrated by incorporating solution‐processed hydrogen molybdenum bronzes as anode interlayers. The bronzes are synthesized using a sol‐gel method with the critical step being the partial oxide reduction/hydrogenation using an alcohol‐based solvent. Their composition, stoichiometry, and electronic properties strongly correlate with the annealing process to which the films are subjected after spin coating. Hydrogen molybdenum bronzes with moderate degree of reduction are found to be highly advantageous when used as anode interlayers in OPVs, as they maintain a high work function similar to the fully stoichiometric metal oxide, whereas they exhibit a high density of occupied gap states, which are beneficial for charge transport. Enhanced short‐circuit current, open‐circuit voltage and, fill factor, relative to reference devices incorporating either PEDOT‐PSS or a solution processed stoichiometric molybdenum oxide, are obtained for a variety of bulk heterojunction mixtures based on different polymeric donors and fullerene acceptors. In particular, high power conversion efficiencies are obtained in devices that employed the s‐HxMoO2.75 as the hole extraction layer.

The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics

Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxide's electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/extraction layers was achieved as a result of favorable energy level alignment at the metal oxide/organic interface and enhanced charge transport through the formation of a large density of gap states near the Fermi level.

Barrierless hole injection through sub-bandgap occupied states in organic light emitting diodes using substoichiometric MoOx anode interfacial layer

In this letter, highly efficient hole injection was demonstrated in hole only devices based on organic semiconductors with different highest occupied molecular orbital level and transport properties. The barrierless hole injection was achieved by using a substoichiometric MoOx thin film (consisting of 65% Mo+6 and 35% Mo+5) as a higly effective anode interfacial layer. The current in these devices was found to be space charge limited, achieved due to the formation of highly efficient anode ohmic contact via the excellent band alignment through occupied gap states at the ITO/MoOx and MoOx/organic semiconductor modified interface. Quite remarkably, the efficiency of hole injection was found to be almost independent of the MoOx thickness, which is indicative of perfect band alignment at the anode interface.

Evaluating the C, N, and F Pairwise Codoping Effect on the Enhanced Photoactivity of ZnWO4: The Charge Compensation Mechanism in Donor–Acceptor Pairs

We perform first-principles density function theory calculations to study the geometric and electronic structures and photoactivity of C, N, and F monodoped and pairwise codoped ZnWO4. By incorporating single C, N, or F into ZnWO4, considering substitutional as well as interstitial locations of dopants in the ZnWO4 lattice, we find that the photon transition energy decreases to varying degrees, while the partially occupied states induced by the impurity are located in the gap, which may act as recombination centers and reduce the photoinduced current density. By analyzing the defect wave function character, we propose several pairwise codoped ZnWO4 systems, such as Cs+2Fs-, Ns+Fs-, Ci+2Ns-, and Ni+Fs-codoped ZnWO4, to passivate the partially occupied states in the monodoping systems by the charge compensation effect in the donor–acceptor pairs, resulting in occupied states in the gap and reducing the formation energy compared with the monodoping systems. The Cs+2Fs and Ns+Fs codoping do not narrow the host band gap of ZnWO4, while they decrease the excitation energy from the occupied gap states to the conduction band minimum to some extent. The Ci+2Ns and Ni+Fs codoping in the ZnWO4 can red shift the transition energy of photoexcited electrons from the impurity levels to the conduction band minimum to the ideal visible-light region; therefore, the Ci+2Ns codoping can decrease the host band gap of ZnWO4 by about 0.3 eV. Our results show that the partially occupied states in the monodoping ZnWO4 can be passivated by the charge compensation effect in the donor–acceptor pairs, thus improving the photoactivity and reaching the visible-light photoactivity for ZnWO4.

Adsorption-induced gap states ofh−BNon metal surfaces

The formation of hexagonal boron nitride (h-BN) monolayers on Ni(111), Rh(111), and Pt(111) has been studied by a combination of x-ray emission, angle-resolved valence band photoemission, and x-ray absorption in search for interface-induced gap states of h-BN. A significant density of both occupied and unoccupied gap states with N 2p and B 2p characters is observed for h-BN/Ni(111), somewhat less for h-BN/Rh(111) and still less for h-BN/Pt(111). X-ray emission shows that the h-BN monolayer is chemisorbed strongly on Ni(111) and very weakly on Pt(111). We associate the gap states of h-BN adsorbed on the transition metal surfaces with the orbital mixing and electron sharing at the interface because their density increases with the growing strength of chemisorption.

Characterization of Gap Defect States in Hydrogenated Amorphous Silicon Materials

ABSTRACTAn enhanced simulation model based on the carrier recombination through these states was developed to characterize the gap defect states in hydrogenated amorphous silicon materials (a-Si:H). The energy dependent density of electron occupied gap states, kN(E), was derived directly from Dual Beam Photoconductivity (DBP) measurements at different bias currents. Through Gaussian de-convolution of kN(E), the energy peaks of the multiple defect states, including both neutral and charged states, were obtained. These energy levels, together with the information on the capture cross sections, were used as known input parameters to self-consistently fit the subgap absorption spectra, the electron mobility-lifetime products over a wide range of generation rates, as well as the energy dependent density of electron occupied gap state spectra. Accurate gap state information was obtained and the nature of the defect states was studied. Simulation results on light degraded hydrogen diluted, protocrystalline a-Si:H show that the density of charged states is 2.3 times that of neutral states. The two states close to the midgap act as effective recombination centers at low generation rates and play key roles in photoconductivity studies.

Structural model of amorphous silicon annealed with tight binding

We present a model of amorphous silicon generated by extensive annealing of a continuous random network structure using a molecular dynamics simulation with forces computed by a tight-binding total energy method. We also produce a refined model by relaxing the annealed model using density functional theory. Our annealed structure is primarily a fourfold coordinated continuous random network, with a few coordination defects. The first peak of the pair correlation function of the annealed structure is sharper and more symmetric than the unannealed structure, a result confirmed in the density-function-theory relaxed structure and in good agreement with static disorder results from recent x-ray diffraction analysis by Laaziri et al. The density, bond angle distribution function, elastic constants, and vibrational density of states of the initial and annealed structures are similar. The energy of the annealed structure is lower for both tight-binding and density-functional theory, indicating that the structure with coordination defects is energetically favored. The electronic structure of the annealed structure, computed with both charge-self-consistent tight-binding and density-functional theory, has a wide gap with several occupied gap states.

Passivation of oxygen vacancy states in HfO2 by nitrogen

Nitrogen is known to reduce leakage currents and charge trapping in high-dielectric-constant gate oxides such as HfO2. We show that this occurs because nitrogen, substituting for oxygen atoms next to oxygen vacancy sites, repels the occupied gap states due to the neutral and positively charged oxygen vacancies out of the band gap into its conduction band. The state of the negatively charge vacancy is also repelled upwards but remains as a shallow gap state. This occurs because the vacancy becomes effectively positively charged; the adjacent Hf ions relax outwards from the vacancy and shift its states upwards. We show this using ab initio calculation methods which do not require an empirical correction to the band gap.

Electronic structure calculations of an oxygen vacancy inKH2PO4

We present first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of an oxygen vacancy in KH{sub 2}PO{sub 4} (KDP). Even though the overall DOS profiles for the defective KDP are quite similar to those of the perfect KDP, the oxygen vacancy in the neutral and +1 charge states induces defect states in the band gap. For the neutral oxygen vacancy, the gap states are occupied by two electrons. The difference between the integral of the total density of states (DOS) and the sum of the DOS projected on the atoms of 0.98 |e|, indicates that one of the two electrons resulting from the removal of the oxygen atom is trapped in the vacancy, while the other tends to delocalize in the neighboring atoms. For the +1 charge oxygen vacancy, the addition of the hole reduces the occupation of the filled gap-states in the neutral case from two to one electron and produces new empty states in the gap. The new empty gap states are very close to the highest occupied states, leading to a dramatic decrease of the band gap. The difference between the integral of the total DOS and the sum of the DOS projected on the atoms is 0.56 |e|, which implies that more than 56% of the redundant electron is trapped in the oxygen vacancy, and 44% spreads over the neighboring atoms. In sharp contrast, no defect states appear in the energy gap for the +2 charge O vacancy. Thus, the addition of the two holes completely compensates the two redundant electrons, and removes in turn the occupied gap states in the neutral case.

Dynamics of a bond-disordered Peierls chain and mixed gap states

We numerically apply a femtosecond electric pump pulse to a bond-disordered Su-Schrieffer-Heeger model and observe the formation of excitations like bound and separate soliton pairs as well as neutral localized breathers. Bound soliton pairs though overall charge neutral show an electric dipole moment oscillating with the transition frequency between the partially occupied gap states. Furthermore, the dipole moment amplitude pulsates while the solitons are interacting with the localized breathers. We investigate the ground state and dynamics of the bound soliton pairs as mixed gap states. In their ground state a discontinuous phase transition from bound to separate soliton pairs is found. The dynamics of these states is very similar to the dynamics under the pump-pulse excitation. We discuss their ground state and the dynamics in terms of the soliton-pair solution of the continuum model.

Implication of hydrogen-induced boron passivation in wet-chemically cleaned Si(111):H

Atomically-flat, hydrogen-terminated Si(111) surfaces with a boron concentration range of 4.5 ∼ 11 × 10 18 cm −3 have been prepared by a standard RCA cleaning step and subsequent 40% NH 4F immersion followed by pure water dipping. The surface Fermi level for as-prepared Si(111) lies near midgap as confirmed by X-ray or ultraviolet excited photoelectron spectroscopy (XPS/UPS) and Kelvin probe measurements although occupied gap states density as low as 10 11 cm −2 at most is verified from an extremely low photoelectron yield for photons with energies below ∼ 5 eV. Annealing at 360°C for 5 min causes the Fermi level to shift towards the valence-band maximum top by ∼0.2 eV without an appreciable increase in the gap state density and a change in hydrogen surface termination. These observations imply a hydrogen-induced passivation of boron atoms in a surface layer during wet-chemical cleaning and the reactivation of such passivated boron atoms by 360°C annealing. For annealing temperatures above 460°C, hydrogen desorption from the surface results in a true pinning of the Fermi level close to midgap because of the generation of surface gap states.

Oxide growth on a-Si:H and its influence on surface gap states

The density of occupied band gap states at the interface between a-Si:H and a-SiO x:H was measured as a function of oxide thickness for differently prepared oxides by means of photoelectric yield spectroscopy and ellipsometry. For naturally grown oxides we found a decrease in the density of occupied defects from 6·10 12 cm −2 to values below 5·10 11 cm −2. This could be observed mainly during the growth of the first 0.8 nm of the oxide. In this thickness range we also found strong decrease of the photoelectric yield suggesting that occupation and charging of the gap states is altered. Oxides prepared by plasma exposure show a different and unstable interface however.

Photoemission study of near-surface band gap states in a-Si:H

We use total yield photoelectron spectroscopy in combination with the Kelvin probe to study changes in the work function and the distribution of occupied band gap states upon boron doping and oxidation of amorphous hydrogenated silicon (a-Si:H) surfaces. We observe 3×1011 occupied surface states/cm2 in clean undoped a-Si:H in an 0.4-eV wide band centered 5.0 eV below the vacuum level, which we ascribe to intrinsic surface defects. Incorporation of boron removes these states and exposes the intrinsic valence band tail, which is exponential over four orders of magnitude in the density of states. In contrast to c-Si, where adsorbed oxygen decreases the large intrinsic surface state density (4×1014 states/cm2 ) on the clean, cleaved surface, the relatively low density of intrinsic surface states on a-Si:H allows the observation of oxygen-induced surface band gap states. The origin of these states is discussed.

Subbandgap absorption in a-Si:H from photoconductivity spectra

The spectral distribution of the optical absorption coefficient, α(hν), in glow-discharge-deposited a-Si:H films has been determined from measurements of the photoconductivity σ ph(h ν) performed in the constant-photocurrent mode. In spite of the fixed quasi-Fermi level the decay of σ ph, after cutting off the steady-state photon flux F, depends considerably on the photon energy hν. We determine the time τH in which σ ph decays to 1 2 of its initial value and assume that its spectral distribution is the same as that of the response time τ R. The absorption coefficient α is then proportional to 1/F(hν)τH(hν). We propose a simple model which allows to determine the density of occupied gap states, N(E), simply by differentiating the α(hν) spectrum by hν.

Direct observation of occupied gap states in amorphous Si 1- xGe x: H alloys by SXS

Si Kβ emission band have been analysed from a-Si 1- x Ge x : H alloys for Ge concentrations varying from 0.05 to 0.60. Localized states with p character are observed in the pseudo-gap near the valence band edge.