Conduction Band Of Insulator Research Articles

Impact of CVD chemistry on band alignment at the MoS2/SiO2 interface

Reliable knowledge of energy alignment of electronic bands at interfaces of few layered Molybdenum Disulfide (MoS2) is crucial for designing MoS2 based electronic devices. In this work, we have applied internal photoemission spectroscopy (IPE) to characterize this band alignment. MoS2 films grown on SiO2(50 nm)/Siusing two different methods have been analyzed by IPE to determine the energy position of the semiconductor valence band (VB) relative to the reference level of the insulator conduction band (CB). We found that changing the MoS2 growth method from Vapor Phase Reaction (VPR) of MoO3 and sulfur, to Solid Precursor Film (SPF) of sulfurized AHM-NaOH spin coated MoS2 film, results in significant (≈600 meV) enhancement in the electron barrier. Such effects are ascribed to the interaction of hydroxy groups from NaOH and AHM promoters with the SiO2 surface leading to interface dipole modification.

Unraveling the Mechanisms of Visible Light Photocatalytic NO Purification on Earth-Abundant Insulator-Based Core-Shell Heterojunctions.

Earth-abundant insulators are seldom exploited as photocatalysts. In this work, we constructed a novel family of insulator-based heterojunctions and demonstrated their promising applications in photocatalytic NO purification, even under visible light irradiation. The heterojunction formed between the insulator SrCO3 and the photosensitizer BiOI, via a special SrCO3-BiOI core-shell structure, exhibits an enhanced visible light absorbance between 400-600 nm, and an unprecedentedly high photocatalytic NO removal performance. Further density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) analysis revealed that the covalent interaction between the O 2p orbital of the insulator (SrCO3, n-type) and the Bi 6p orbital of photosensitizer (BiOI, p-type) can provide an electron transfer channel between SrCO3 and BiOI, allowing the transfer of the photoexcited electrons from the photosensitizer to the conduction band of insulator (confirmed by charge difference distribution analysis and time-resolved fluorescence spectroscopy). The •O2- and •OH radicals are the main reactive species in photocatalytic NO oxidation. A reaction pathway study based on both in situ FT-IR and molecular-level simulation of NO adsorption and transformation indicates that this heterojunction can efficiently transform NO to harmless nitrate products via the NO → NO+ and NO2+ → nitrate or nitrite routes. This work provides numerous opportunities to explore earth-abundant insulators as visible-light-driven photocatalysts, and also offers a new mechanistic understanding of the role of gas-phase photocatalysis in controlling air pollution.

Leakage current induced by surfactant residues in self-assembly based ultralow-k dielectric materials

In this work, we studied low-field leakage currents in the self-assembly based spin-on low-k dielectrics (k = 2.2) as it may be affected by the degree of the organic template decomposition. The distinct role of the template residues could be examined due to the remarkably different rate of organosilica matrix cross-linking and template decomposition during the hard-bake process. We found that the incomplete decomposition of the sacrificial organic phase is responsible for increased low-field leakage current through the dielectric. The analysis of photocurrent spectra and the results of electron resonance spectroscopy suggest that the degradation of electrical performance can be attributed to the presence of defect states ∼5 eV below the insulator conduction band induced by the residues in the form of oxidized sp3-carbon chains. The lowest leakage current is attained in the template-free material obtained by introduction of additional broadband UV-assisted curing (λ > 200 nm).

Band alignment and effective work function of atomic‐layer deposited VO2 and V2O5 films on SiO2 and Al2O3

AbstractThe effective work function (EWF) and the energy position of the valence band in 20‐40‐nm thick VO2and V2O5layers grown by atomic layer deposition (ALD) on top of insulating SiO2 and γ‐Al2O3 films were evaluated using the comparison between capacitance‐voltage and internal photoemission measurements. From the capacitance measured at different temperatures on the metal‐VO2(V2O5)‐insulator‐silicon and metal‐insulator‐silicon diodes we found that the both studied vanadium oxides have the same EWF as gold electrodes evaporated on the same oxides. This result is further collaborated by the internal photoemission experiments at the VO2/SiO2 and V2O5/SiO2 interfaces which indicate the energy barrier between the top of the vanadium oxide valence band (in the insulating phase) and the insulator conduction band to be 4.1 ± 0.1 eV. Since the transition from the narrow‐gap VO2 to the wide‐gap V2O5 oxide causes no change in the WF or in the photoemission threshold, we conclude that the ALD‐grown VO2 in its insulating phase represents a a heavily‐doped semiconductor which becomes metallic upon metal‐insulator transition without significant EWF change. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectroscopy of Deep Gap States in High-k Insulators

Spectroscopy of Deep Gap States in High-k Insulators

Modulation of electron barriers between TiNx and oxide insulators (SiO2, Al2O3) using Ti interlayer

Precise evaluation of the effective work function and the built‐in voltage in metal–oxide–metal (MOS) or metal–insulator–metal (MIM) stacks is crucial for attaining the functionality of various oxide‐based electron devices. We studied the impact of a low work function (WF) metal interlayer (IL), i.e. Ti, on the electron barrier height between the metal Fermi level and the conduction band of the oxide insulator. The experiments were performed on structures comprised of a layer of thermal SiO2 or an atomic layer deposited Al2O3 on heavily doped p‐ or n‐Si substrates. In order to evaluate the impact of the Ti IL, 3, 6 or 10 nm thick Ti films have been deposited prior to TiNx sputtering. Spectral curves of electron photoemission from the metal layer into the oxide were analyzed in order to determine the energy barrier for electrons between the Fermi level of the metal and the conduction band of the insulator (Φe). As expected, the insertion of the low WF metal shifts the barrier to lower energies, depending on the IL thickness. However, while a 3‐nm thick Ti IL is sufficient to lower the barrier by 0.5 eV in the SiO2‐based stack, it has no measurable impact on the barrier in the Al2O3/Ti/TiNx stack. For the latter structure, both Ti and the Al2O3 thicknesses influence the Φe lowering trend, suggesting that oxygen scavenging from Al2O3 by Ti and nitridation of the Ti IL by subsequent TiN deposition may affect the band alignment between the metal and the oxide.

Electron heating through a set of random levels in the conduction band of insulators induced by femtosecond laser pulses

We develop a theory describing the heating of electrons in crystalline insulators irradiated by high-intensity laser pulses. In agreement with photoelectron yield versus intensity measurements, we assume that electrons are excited into the conduction band from defect layers and traps. The electron dynamics due to direct inter-branch transitions within the conduction band is simulated by solving of time-dependant Schrodinger equation. The set of levels for this equation is supposed to be random with a distribution function equal to the density of states in the conduction band. The influence of different parameters on the electron heating efficiency is studied. The theory is applied for diamond; the theoretical spectrum is in qualitative agreement with the experimental observations.

Simulation of hole and electron tunnel currents in MIS devices adopting the symmetric Franz-type dispersion relation for the charged carriers in thin insulators

The tunnel current models based on the Franz dispersion relation for carriers in the involved insulators are tested for several types of metal-insulator-silicon structures. The features related to the application of this alternative dispersion relation are analysed. Simplified approaches for tunneling current models are also examined. The positive and negative aspects of the Franz model are described. Special attention is paid to the tunneling of carriers with energies far from the insulator conduction band edge. The results of the new tunneling models are compared to those of the conventional models adopting the parabolic dispersion laws. It turns out that the physically well justified Franz relation yields typically a better agreement with experiments compared to the parabolic relation despite offering less freedom in term of adjustable parameters.

Electron heating in the conduction band of insulators irradiated by ultrashort laser pulses

We investigate the theoretical problem of electron heating in the conduction band of wide band gap insulators and semiconductors induced by intense femto-second Ti:Sapphire laser pulses. We analyze in detail the heating mechanism due to the sequence of direct interbranch transitions in the conduction band, which has been shown to be of crucial importance in previous work. This analysis is fulfilled by resolving the time dependant Schr\odinger equation (TDSE) in a basis of Bloch functions for the CsI crystal. The field is represented semiclassically and the laser-electron interaction is treated in the dipole approximation. The presented approaches are based on a one-active electron approximation. First the TDSE is solved in a basis of Bloch functions, in one dimension, the influence of laser and crystal parameters on the electron spectra is studied. The electron transfer from the lower conduction band to the higher one is already effective at intensity of $3\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. Then the problem is solved in three dimension. The electron spectra is consistent with the experimental results, we note in particular the presence of a large plateau at intensities of the order of the terawatt per square centimeter.

Electron photoemission from conducting nitrides (TiNx,TaNx) into SiO2 and HfO2

The energy barrier between the metal Fermi level and the insulator conduction band is directly determined using internal electron photoemission at the interfaces of metallic nitrides (TiNx,TaNx) with SiO2 and HfO2. TiNx yields high barriers both on SiO2(4.15±0.05eV) and HfO2(3.0±0.05eV) stable upon annealing at temperatures up to 1000 °C. Incorporation of a monolayer HfO2 at the TiNx∕SiO2 interface causes ≈0.2eV barrier enhancement upon annealing. The TaNx∕SiO2 interface barrier (3.55±0.05eV) is found to be stable under annealing up to 1000 °C, but Hf contamination leads to a barrier increase similar to that observed for TiNx∕SiO2. The as-deposited TaNx on HfO2 yields a low barrier (2.1±0.05eV), but it significantly increases upon annealing, suggesting a chemical interaction to occur between TaNx and HfO2.

Creation of high energy electronic excitations in inorganicinsulators by intense femtosecond laser pulses

We measured photoemission spectra for a number of insulators (CsI, Diamond, SiO2, CeF3) excited by femtosecond Ti-Sapphire laser pulses at peak intensities, from 0.5 to 6 TW/cm2, which are at least one order of magnitude below the optical breakdown threshold. An intense and pronounced plateau of high energy electrons appears in the photoelectron spectra in this intensity range, which extends up to 30-40 eV at the highest intensities, which we used. The excitation of electrons at high energies is treated in terms of direct interbranch transitions in the conduction band of insulator. These processes are described using calculations based on numerical solution of time dependant Schrodinger equation (TDSE) for Bloch electrons in electric fields. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Low‐Energy Electron Transport in Insulators

AbstractTransport of excited electrons with an energy less than the band gap value in the conduction band of insulators is described analytically. Interaction of electrons with optical phonons is assumed to be the predominant process in scattering. By solving the diffusion equation obtained on the basis of the kinetic equation it is possible to find the energy distribution of the electrons in the solid and vacuum. Photoelectron spectra for CsI are calculated. Comparison of theoretical and experimental results yields the structure of the density of states in the conduction band, the energy loss intensity, and the free path time of excited electrons with respect to the electron—phonon scattering.

High-field electron capture and emission in nitrided oxides

High-field, constant current injection has been used to study electron trapping and reemission processes in ammonia-annealed (nitrided) silicon dioxide. The gate voltage during injection versus injected charge exhibits two rates. The first provides the capture cross section and trap density information. The second provides information on reemission of trapped charge by tunneling. From the tunneling data, the electron trap depth can be estimated to be 3.5–4 eV below the insulator conduction band. Nitrogen annealing reduces the density of these traps.

Photoemission measurements of graded barrier in thin silicon oxynitride films

Photoemission measurements were performed on metal-insulator-silicon structures having thin thermal oxynitride films as the insulator. The results for the sample with the least oxidized nitride film, yield for the energy intervals from the insulator conduction band to the Fermi level of the aluminum and to the valence band of the silicon, the values of 3.3 and 3.6 eV, respectively. The affinity of the insulator as deduced from these values is larger at the silicon interface than at the aluminum interface by 0.8 eV, indicating that the film exhibits a graded energy barrier for electron injection from the electrodes.

Theory of resonant tunneling through the insulator conduction band of a thin film metal-insulator-metal (MIM) heterojunction

An “atomic” model of an insulating barrier between two free-electron model metals is used to investigate resonant tunneling across the insulator in the presence of a medium to large, externally applied electric field (bias). The exact numerically calculated tunneling current exhibits a pronounced oscillatory bias dependence superposed on the dominant roughly exponential tunneling characteristic. The interpretation of these results in terms of an internal field emission or Fowler-Nordheim type tunneling subject to “periodic deviations” (or interferences) seems plausible and was suggested by Maserjian. To test this conjecture, a trapezoidal barrier model of our “atomic” model analyzed numerically. As expected, the trapezoidal barrier model could only qualitatively reproduce the oscillatory bias dependence of the barrier transmissivity and of the current. Furthermore this limited agreement depends on allowing the effective mass in the barrier to become a strictly adjustable parameter. This failure of the conventional model of the junction can be interpreted as follows: (i) For moderate external (bias) fields the trapezoidal barrier fails to account for the correct position dependence of the Blochwave vector in the insulator's conduction band, hence the correct interference conditions cannot be reproduced. (ii) For large external fields the band model itself begins to fail. An explanation of oscillatory bias dependence at the tunneling current in terms of splitting of the insulator's conduction band into a set of discrete Stark levels is suggested. It is demonstrated that a fit of the oscillatory tunneling characteristics in the “Fowler-Nordheim regime” is not a reliable technique to determine the effective mass in the thin insulating film of tunneling junctions over the energy interval containing the forbidden gap and the adjoining conduction-band.

Barrier Energies in MIM Structures from Photoresponse: Effect of Scattering in the Insulating Film

Scattering of electrons photoexcited into the insulator conduction band prevents photoresponse from following the Fowler relation in MIM structures. However, barrier energies can be obtained without specific knowledge of the scattering process either by measuring the threshold for photoresponse directly, or by applying sufficiently large voltages across the insulator.

Tunneling through thin insulating films in parallel electric and magnetic fields

The most important parameters in the theory of electron tunneling between metal electrodes separated by a thin insulating film are: the barrier heights of the metal-insulator boundaries; s, the barrier thickness; and m i, an effective electron mass in the vicinity of the insulator conduction band edge. Generally, it is not possible to obtain both the barrier thickness s and the effective electron mass m i from the measured current-voltage characteristic of an M-J-M tunneling junction. Besides the barrier heights, only the product, s·√ m i, can be deduced from the observed current-voltage characteristic. In this paper the theory of electron tunneling through thin insulating films is extended to the case where in addition to the electric field a parallel magnetostatic field is applied. It is shown theoretically that the application of a high magnetostatic field should provide a method to determine both the barrier thickness s and the electron effective mass m i. Moreover, the application of a high magnetostatic field should provide a new method to determine the barrier height of the metal-insulator boundaries. The latter result is somewhat similar to the so-called temperature cusp of the tunneling current. However, instead of measuring the J-V characteristic at two different temperatures, the method proposed here requires the measurement of the J-V characteristic at two different magnetic fields.