Determination Of Barrier Height Research Articles

A theoretical exploration of monomer activation mechanisms in bis-lithium N[sbnd]heterocyclic carbenes complexes for enhanced understanding of charge distribution in ring-opening polymerization of ε-caprolactone

A comprehensive examination was conducted on the ring-opening polymerization (ROP) mechanism of ε-caprolactone (ε-CL) employing bis-lithium N-heterocyclic carbene complexes (bisLiNHCs) in a theoretical framework. The modeled reactions were carried out utilizing density functional theory at B3LYP level. An activated-monomer mechanism was observed, characterized by occurrence of two distinct transition states. Initiating with activation of ε-CL via two lithium atoms from bisLiNHCs catalysts, the ensuing step involved nucleophilic addition of carbonyl group stemming from the lithium benzoxide (LiBnO) initiator. The ring opening ε-CL was accomplished via classical cleavage of acyl-oxygen bond. The determination of reaction barrier heights for ε-caprolactone with bisLiNHCs catalysts involved the examination of potential energy profiles. The computed energy profiles revealed exothermic characteristics for each ROP reaction, with the identification of a rate-determining step occurring at the second transition state, which entailed cleavage of acyl-oxygen bond from monomer. Furthermore, through comparisons catalytic capabilities of bisLiNHCs derivatives and energy barrier heights, it was observed that the length and flexibility of linker chain as well as steric effects from substituent groups, significantly influenced the geometric configuration. These specific variations induce alterations in both distance and angular orientation, which are pivotal aspects influencing the chelation dynamics between lithium and oxygen atoms.

The millimeter-wave spectrum and astronomical search for ethyl methyl sulfide.

Sulfur-containing molecules constitute only 8% of the molecules observed in the interstellar medium (ISM), in spite of the fact that sulfur has been shown to be an abundant element in the ISM. In order to understand the chemical behavior of the ISM and specific cases like the missing sulfur reservoir, a detailed chemical molecular composition in the ISM must be mapped out. Our goal is to investigate the rotational spectrum of ethyl methyl sulfide, CH3CH2SCH3, which ms to be a potential candidate for observation in the ISM since the simpler analogs, CH3SH and CH3CH2SH, have already been detected. Rotational spectrum of ethyl methyl sulfide has been observed before, but its experimental rotational parameters are not precise enough to allow its detection in the ISM. The rotational spectrum of ethyl methyl sulfide in the frequency range 72-116.5 GHz was measured using a broadband millimeter-wave spectrometer based on radio astronomy receivers with fast Fourier transform backends. The spectral searches and identification of the vibrational excited states of ethyl methyl sulfide was supported by high-level ab initio calculations on the harmonic and anharmonic force fields. The rotational spectra for the trans and gauche conformers of ethyl methyl sulfide was analyzed, and a total of 172 and 259 rotational transitions were observed for each one, respectively. The observation of A - E internal rotation splittings allowed the experimental determination of the V 3 hindered internal rotation barrier height for both trans and gauche species. In addition, the vibrational excited states, resulting from the lowest frequency vibrational mode ν 30 were identified for both conformers. The new experimental rotational parameters were employed to search for ethyl methyl sulfide in the warm and cold molecular clouds Orion KL, Sgr B2(N), B1-b and TMC-1, using the spectral surveys captured by IRAM 30 m at 3 mm and 2 mm.

Determination of the barrier height of iridium with hydrogen-terminated single crystal diamond

Abstract

Internal photoemission spectroscopy determination of barrier heights between Ta-based amorphous metals and atomic layer deposited insulators

The energy barrier heights between two recently reported Ta-based amorphous metals (TaWSi and TaNiSi), TaN, and atomic layer deposited Al2O3 and HfO2 insulators are measured in metal/insulator/metal (MIM) structures with Au top electrodes using internal photoemission (IPE) spectroscopy. For Al2O3, the Ta-based metal barrier heights, φBn, increase with increasing metal work function, ΦM, for TaN, TaWSi, and TaNiSi, respectively. For HfO2, however, the barrier heights are relatively constant for all three metals φBn,TaNiSi ≈ φBn.TaWSi ≈ φBn.TaN. The difference between HfO2 and Al2O3 is attributed to enhanced Fermi-level pinning due to a larger dielectric constant. The slope parameter, S, was found to be roughly 0.89 and 0.44–0.69 for Al2O3 and HfO2, respectively. For devices with a TaWSi bottom electrode, a comparison was also made between Al and Au top electrodes. Significantly, smaller barrier heights were obtained with an Au top electrode than with an Al top electrode, 0.6 eV and 0.8 eV lower for HfO2 and Al2O3, respectively. IPE energy barriers are found consistent with current-voltage asymmetry of MIM diodes, whereas Schottky model predictions of barrier heights were inconsistent.

Negative contribution to the reverse bias capacitance of organic diodes due to field dependent mobility: Determination of barrier height and transport parameters

We analyse the capacitance voltage characteristics of a small molecule-based single carrier diode consisting of a p-doped/intrinsic layer interface using m-MTDATA. In such homojunction diodes, in which the intrinsic layer thickness varies between 10 and 50 nm, the capacitance in deep reverse bias falls nearly exponentially and goes below the geometrical capacitance (Cg) beyond a critical electric field. We mainly focus on this reduction of the capacitance, which is interpreted based on charge injection mechanism in reverse bias. The small signal capacitance in such cases is shown to have negative contribution which is directly related to the delay time introduced by the charge transport. Thus, the capacitance characteristics have been modelled in deep reverse bias using the charge transport mechanisms, and the barrier height so estimated is in excellent agreement with the values calculated from current density-voltage (J-V) characteristics. The technique also allows the determination of mobility, which in turn yields the disorder parameters through its temperature and field dependence.

Graphene-insulator-semiconductor capacitors as superior test structures for photoelectric determination of semiconductor devices band diagrams

We report on the advantages of using Graphene-Insulator-Semiconductor (GIS) instead of Metal-Insulator-Semiconductor (MIS) structures in reliable and precise photoelectric determination of the band alignment at the semiconductor-insulator interface and of the insulator band gap determination. Due to the high transparency to light of the graphene gate in GIS structures large photocurrents due to emission of both electrons and holes from the substrate and negligible photocurrents due to emission of carriers from the gate can be obtained, which allows reliable determination of barrier heights for both electrons, Ee and holes, Eh from the semiconductor substrate. Knowing the values of both Ee and Eh allows direct determination of the insulator band gap EG(I). Photoelectric measurements were made of a series of Graphene-SiO2-Si structures and an example is shown of the results obtained in sequential measurements of the same structure giving the following barrier height values: Ee = 4.34 ± 0.01 eV and Eh = 4.70 ± 0.03 eV. Based on this result and results obtained for other structures in the series we conservatively estimate the maximum uncertainty of both barrier heights estimations at ± 0.05 eV. This sets the SiO2 band gap estimation at EG(I) = 7.92 ± 0.1 eV. It is shown that widely different SiO2 band gap values were found by research groups using various determination methods. We hypothesize that these differences are due to different sensitivities of measurement methods used to the existence of the SiO2 valence band tail.

Cross-contamination of the fitting parameters in multidimensional tunneling treatments

In this paper we examine the two-dimensional tunneling formalism used previously to fit the hydrogen-transfer and internal-rotation splittings in the microwave spectrum of 2-methylmalonaldehyde in an effort to determine the origin of various counterintuitive results concerning the isotopic dependence of the internal-rotation splittings in that molecule. We find that the cause of the problem lies in a “parameter contamination” phenomenon, where some of the numerical magnitude of splitting parameters from modes with large tunneling splittings “leaks into” the parameters of modes with smaller tunneling splittings. We show that such parameter contamination, which greatly complicates the determination of barrier heights from the least-squares-fitted splitting parameters, will be a general problem in spectral fits using the multi-dimensional tunneling formalism, since it arises from subtle mathematical features of the non-orthogonal framework functions used to set up the tunneling Hamiltonian. Transforming to a physically less intuitive orthonormal set of basis functions allows us to give an approximate numerical estimate of the contamination of tunneling parameters for 2-methylmalonaldehyde by combining a dominant tunneling path hypothesis with results recently given for the hydrogen-transfer-internal-rotation potential function for this molecule.

Metal-organic semiconductor interfacial barrier height determination from internal photoemission signal in spectral response measurements

Accurate and convenient evaluation methods of the interfacial barrier ϕb for charge carriers in metal semiconductor (MS) junctions are important for designing and building better opto-electronic devices. This becomes more critical for organic semiconductor devices where a plethora of molecules are in use and standardised models applicable to myriads of material combinations for the different devices may have limited applicability. In this paper, internal photoemission (IPE) from spectral response (SR) in the ultra-violet to near infra-red range of different MS junctions of metal-organic semiconductor-metal (MSM) test structures is used to determine more realistic MS ϕb values. The representative organic semiconductor considered is [6, 6]-phenyl C61 butyric acid methyl ester, and the metals considered are Al and Au. The IPE signals in the SR measurement of the MSM device are identified and separated before it is analysed to estimate ϕb for the MS junction. The analysis of IPE signals under different bias conditions allows the evaluation of ϕb for both the front and back junctions, as well as for symmetric MSM devices.

Determination of Schottky barrier height and enhanced photoelectron generation in novel plasmonic immobilized multisegmented (Au/TiO2) nanorod arrays (NRAs) suitable for solar energy conversion applications

We have determined the Schottky barrier height (0.23 eV) and efficient photoelectron generation in novel multisegmented Au/TiO2 nanorod arrays.

Direct Determination of the Barrier Height of Au Contact on p-Type Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films

Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films have a specific film structure, wherein a large number of diamond nanograins are embedded in an a-C:H matrix. The UNCD/a-C:H films possess large optical absorption coefficients of more than 106 cm–1 in the photon energy range between 3 and 6 eV. These optical features are profitable for the application of thin-film based solar cell and UV photodetectors. Although the Metal-Semiconductor-Metal (MSM) geometry is suitable for photodetection by using single-layered UNCD/a-C:H film, the property of film/metal interface hasn’t been investigated in detail. In this study, a direct determination of barrier height of metal contact on p-type UNCD/a-C:H film was performed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). As a result, it was revealed that the down bending occurs on the film/metal interface, which indicates that depletion layer might appear on the surface of p-type UNCD/a-C:H by this structure. Whereas the surface of UNCD/a-C:H films is oxidized, that band-alignment of that does not change. It is possible that that different physical phenomena from that of single crystalline diamond occurs. There are numerous dangling bonds on the surface of UNCD/a-C:H film, therefore it is supposed that the dangling bonds have greatly affected the surface state.

Dopant induced polarity inversion in polar ZnO nanorods

Highly oriented and polarity controlled ZnO nanorod thin films are deposited by chemical bath deposition where the preferred polarity depends on the anionic nature of the Zn salt. Significant differences are noticed in magnesium (Mg) doping concentration on Zn- and O-polar ZnO nanorods. Higher doping concentration is achieved in Zn polar nanorods, which is characterized with different spectroscopic techniques. A noteworthy contribution in morphological changes is noted upon by Mg doping on both Zn- and O-polar ZnO nanorods. Polarity inversion of the individual nanorods are found at certain doping condition, characterized by chemical etching rate measurements and Schottky Barrier Height determination of the individual nanorod.

Review and test of methods for determination of the Schottky diode parameters

This paper deals with the extraction of the Schottky diode parameters from a current-voltage characteristic. 10 analytical methods, 2 numerical methods, and 4 evolutionary algorithms of the series resistance, barrier height, and ideality factor determination are reviewed. The accuracy of the methods is quantified using a wide range of both ideal and noisy synthetic data. In addition, the influencing factors of the parameters extraction accuracy are estimated. The adaptive procedure, which improves the precision of analytical Gromov's method, is suggested. The use of Lambert W function has been shown to reduce the error of parameter extraction by numerical method. Finally, all methods are applied to experimental data. The most reliable and preferred methods are chosen.

Determination of Schottky barrier heights and Fermi-level unpinning at the graphene/n-type Si interfaces by X-ray photoelectron spectroscopy and Kelvin probe

The interface characteristics of graphene/n-type Si samples using X-ray photoelectron spectroscopy (XPS) measurements are investigated. XPS makes it possible to extract a reliable Schottky barrier value. For graphene/n-type Si samples with (without) sulfide treatment, the Schottky barrier height is 0.86 (0.78)eV. The Schottky barrier height was increased from 0.78 to 0.86eV, indicating that sulfide treatment is effective in passivating the surface of Si (owing to the formation of Si–S bonds). To determine the Fermi-level pinning/unpinning at the graphene/n-type Si interfaces with sulfide treatment, an analysis is conducted according to the Schottky–Mott limit and the actual work function of graphene is examined with the Kelvin probe. It is shown that the Fermi energy level is unpinned and the Schottky barrier value is dependent on the work function of graphene. Investigation of graphene/n-type Si interfaces is important, and providing the other technique for surface potential control is possible.

Electron spectro-microscopic determination of barrier height and spatial distribution of Au and Ag Schottky junctions on boron-doped diamond (001)

Electron spectro-microscopic methods were applied as direct methods of determining the Schottky barrier heights (SBHs) and their spatial distribution for Au- and Ag-Schottky junctions fabricated on an acid-treated oxygen-terminated diamond (001) substrate. Metal layers were formed with two ranges of thickness (3–5 nm for thin layers and 13–100 nm for thick layers) for both Au- and Ag-Schottky junctions. Leading X-ray photoelectron spectroscopy (XPS) core-level peaks of either Au 4f7/2 or Ag 3d5/2 for the metal layers and C 1s for diamond were used as measures of the SBH. For the thick-metal samples, spatially resolved XPS measurements were performed over most of the sample surface. It was found that there is a variation in SBH on the order of 0.1 eV for the large (“high barrier”) SBH values and that there are several places where the SBHs were rather small (“low barrier” junction). For the thin-metal samples, less variations in SBH were observed. The average SBH of “high barrier” junctions for the thick-metal samples appeared to be slightly (0.1 eV order) larger than that for the thin-metal samples. XPS images of leading metal core levels tuned for the “high barrier” and “low barrier” SBHs were observed. For the thick-Ag Schottky sample, the resulting Ag 3d5/2 XPS images clearly showed the locations of defective Schottly junctions. It is suggested that the SBHs determined for the thin-metal samples are the average SBHs on the measured surface and that the SBHs determined for the thick-metal samples are the highest SBHs within the measured µm-size metal islands. The presently determined SBHs were compared with previously reported SBHs and reasonable agreement was found. Photoemission electron microscopy (PEEM) images were observed for the thick-Ag Schottky sample and the “low barrier” islands were identified. The methodologies of XPS, XPS imaging, and PEEM used for the thick-metal samples can be applied to any Schottky junction on diamond.

Barrier height estimation of asymmetric metal-insulator-metal tunneling diodes

A method is developed to estimate barrier heights of an asymmetric metal-insulator-metal (MIM) diode exhibiting Fowler-Nordheim tunneling. The method requires determination of slopes and intercepts of a log(|I(V+Δϕ)2|) versus 1V+Δϕ plot rather than a log(|I(V)2|) versus 1V plot (i.e., a conventional Fowler-Nordheim plot), where I, V, and Δϕ refer to tunneling current, applied voltage, and the difference in barrier heights, respectively. As the value of Δϕ directly impacts the applied electric field magnitude, it is a critical component in barrier height determination from a current-voltage measurement. Conventional Fowler-Nordheim plot analysis does not employ Δϕ, which compromises the accuracy of barrier height estimation when Δϕ≠0. Using the described method, the barrier heights of a ZrCuNiAl/Al2O3/Al MIM diode are estimated to be 1.75 V and 1.07 V, respectively, and the Al2O3 tunneling effective mass is estimated to be 0.47. Additional MIM diodes are analyzed to show that the accuracy of MIM diode barrier height and effective mass estimates is highly sensitive to contamination and/or energy imparted during the deposition of the upper electrode.

Accurate Determination of Barrier Height and Kinetics for the F + H2O → HF + OH Reaction

The reaction energy and barrier height of the title reaction are investigated using two high-level ab initio protocols, namely Focal Point Analysis (FPA) and modified High Accuracy Extrapolated Ab Initio Thermochemistry (HEAT) methods. It is concluded from these calculations that despite some multireference character, dynamic electron correlation plays a dominant role near the reaction barrier. Thus, the coupled-cluster method with higher excitations than singles and doubles gives a better description than the multireference configuration interaction method for the barrier height. The FPA and HEAT classical barrier heights, including the spin-orbit and other corrections, are 1.919 and 2.007 kcal/mol, respectively. The rate constants and H/D kinetic isotope effect for the title reaction are determined by semiclassical transition-state theory based on the anharmonic potential energy surface near the saddle point, and the agreement with experiment is excellent. The rate constants are also computed using a quasi-classical trajectory method on a global potential energy surface scaled to the FPA barrier height and a similar level of agreement with experimental data is obtained.

Determination of Barrier Heights for Proton Exchange in Small Water, Ammonia, and Hydrogen Fluoride Clusters with G4(MP2)-Type, MPn, and SCS-MPn Procedures-A Caveat.

Calculation of accurate water-water interaction energies is of fundamental importance in computational modeling of many biological and chemical phenomena. We have obtained benchmark barrier heights for proton-exchange reactions and complexation energies in water clusters (H2O)n (n = 1-6) by means of the high-level W1-F12 procedure. We find that lower-cost composite procedures (e.g., G4(MP2) and G4(MP2)-6X), as well as MP2 and SCS-MP2, exhibit surprisingly poor performance for the barrier heights of reactions involving multiple proton exchanges. Moreover, the performance significantly deteriorates with increasing size of the clusters. Similar observations apply to complexation energies in water clusters, and to barrier heights for proton exchange in ammonia and hydrogen fluoride clusters. We propose a modified version of G4(MP2)-6X (denoted G4(MP2)-6X+) that includes sp- and d-diffuse functions in the CCSD(T) term, which gives excellent proton-exchange barrier heights at a computational cost only slightly greater than that of standard G4(MP2). G4(MP2)-6X+ also leads to a substantial improvement over G4(MP2) and G4(MP2)-6X for the calculation of electron affinities.

Inhomogeneous ohmic contacts: Barrier height and contact area determination

The current-voltage characteristics at a uniform ohmic contact are, by definition, dominated by its series resistance and ordinarily offer little information, beyond an upper-bound, on the Schottky barrier height (SBH) of the metal-semiconductor junction. We demonstrate through temperature-dependent measurements of Au “ohmic” contacts on n-type Si(100)1 × 1-S, however, that quantitative information on not only the magnitude of the SBH, to within ± 0.03 eV, but also the effective conduction area can still be deduced. Details of these analyses, which require the use of a thin Si epi-layer, and the “partisan interlayer” mechanism, which lead to the formation of ohmic contacts, are discussed.

Barrier height determination of silicide-silicon contact by hybrid density functional simulation

Accurate prediction of the Schottky barrier heights (SBHs) from first principles is important for contact and device technologies. Ab initio simulations based on widely used local density approximation and generalized gradient approximation significantly underestimate the SBHs. By modeling CoSi2-Si and NiSi2-Si contacts using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations, we demonstrate that HSE hybrid functional simulations can predict the SBHs in agreement with experiments. Furthermore, the effects of interfacial dopants and metal induced gap states are examined by ab initio hybrid functional simulations.

Band offsets of epitaxial LaAlO3/TiO2 interface determined by X-ray photoelectron spectroscopy

Room-temperature ferromagnetism in doped anatase TiO2 has previously been observed, ferromagnetic semiconductor heterostructures based on anatase TiO2 can thus provide a new opportunity to study spin-dependent transport phenomena at room temperature. An accurate determination of barrier heights or band offsets at the TiO2-based heterojunctions is of great importance for the spintronics application with semiconductors. X-ray photoelectron spectroscopy with high-energy resolution was used to determine the band offsets of epitaxial LaAlO3/TiO2 heterojunction on SrTiO3(001) substrate fabricated by pulsed laser deposition. Results showed an upward band bending of 0.48(0.03) eV when the film thickness of the overlayer LaAlO3 above 4 unit cells. The valence band offset obtained is about 0.35(0.16) eV. Assuming bulk band gaps for the LaAlO3 and TiO2 epitaxial films, the associated conduction band offset is about 2.95(0.16) eV. These results show that LaAlO3 can be an ideal tunneling barrier for TiO2-based heterojunctions.