Schottky Limit Research Articles

Lowering of the Schottky barrier height of metal/n-type 4H-SiC contacts using low-work-function metals with thin insulator insertion

We examined the lowering of the Schottky barrier height (SBH) of a metal/n-type 4H-silicon carbide (SiC) contact using low-work-function metals to realize an ohmic contact through a low-temperature process. We found that the SBHs of Y, Mg, and Hf/n-type 4H-SiC contacts deviated from the Schottky limit and that the SBH was greater than ∼0.5 eV at minimum. Inserting 0.3 and 0.7 nm thick SiN x layers into a Mg/n-type 4H-SiC interface could help effectively reduce the SBH; Mg/SiN x /n-type 4H-SiC exhibited an SBH as low as ∼0.36 eV. This reduction in the SBH could be attributed to the suppression of the metal-induced gap state manifested at the conduction band edge. Finally, through a quantitative analysis, it was demonstrated that the contact resistivity of a Mg/SiN x (0.3 nm)/4H-SiC interface could be reduced by approximately one order of magnitude compared with that of a Mg/4H-SiC interface at donor concentrations below ∼3 × 1019 cm−3.

In situ grown single crystal aluminum as a nonalloyed ohmic contact to n-ZnSe by molecular beam epitaxy

Novel ohmic contacts to n-ZnSe are demonstrated using single crystal Al films deposited on epitaxially grown ZnSe (100) by molecular beam epitaxy. Electron backscatter diffraction confirmed the single crystalline structure of the Al films. The (110)-oriented Al layer was rotated 45° relative to the substrate to match the ZnSe (100) lattice constant. The as-grown Al-ZnSe contact exhibited nearly ideal ohmic electrical characteristics over a large doping range of n-ZnSe without any additional treatment. The contact resistances are in a range of 10−3Ωcm2 for even lightly doped ZnSe (∼1017cm−3). Leaky Schottky behavior in lightly doped ZnSe samples suggested that Al-ZnSe formed a low-barrier height, Schottky limit contact. In situ grown Al could act as a simple metal contact to n-ZnSe regardless of carrier concentration with lower resistance compared to other reported contacts in literary studies. The reported novel metallization method could greatly simplify the ZnSe-based device fabrication complexity and lower the cost.

Electrical Contact Barriers between a Three-Dimensional Metal and Layered SnS2.

Field-effect transistors derived from traditional 3D semiconductors are rapidly approaching their fundamental limits. Layered semiconducting materials have emerged as promising candidates to replace restrictive 3D semiconductor materials. However, contacts between metals and layered materials deviate from Schottky-Mott behavior when determined by transport methods, while X-ray photoelectron spectroscopy measurements suggest that the contacts should be at the Schottky limit. Here, we present a systematic investigation on the influence of metal selection when electrically contacting SnS2, a layered metal dichalcogenide semiconductor with the potential to replace silicon. It is found that the electrically measured barrier height depends also weakly on the work function of the metal contacts with slopes of 0.09 and -0.34 for n-type and p-type Schottky contacts, respectively. Based on the Kirchhoff voltage law and considering a current path induced by metallic defects, we found that the Schottky barrier still follows the Schottky-Mott limits and the electrically measured barrier height mainly originates from the van der Waals gap between the metal and SnS2, and the slope depends on the magnitude of the van der Waals capacitance.

(Invited) Energy Alignment at the Molecule – Electrode Interface: An Electrochemical-Potential / Hardness View

A major aspect of hetero-interfaces is the relative position the energy levels / bands of two adjacent phases, known as the ‘injection barrier’. From inorganic semiconductors to thin organic films, interfacial energy alignment is accompanied by charge redistribution and therefore, creation of a built-in potential. Seemingly, adsorbed molecules (isolated or within a monolayer) follow similar principles: the injection barrier is equivalent to the tunneling barrier in molecular junctions (ε, all quantities are illustrated below) while charging is expressed as an adsorption-induced molecular dipole (Δ). However, molecules have no free carriers and their nearest energy level is way-off the electrode’s Fermi-level. This explains why the electrode-molecule interface is widely assumed to be ‘inert’: adsorption induces no long-range charge rearrangement beyond the mere chemical bond (Δ=0; left-side illustration). This scenario is equivalent to ‘vacuum level alignment’ (Schottky limit) in bulk semiconductors where the injection (tunneling) barrier is predicted from the work-function of the clean electrode (WF) and the frontier-level of the isolated molecule (IP in the illustration). However, evidence is growing for the opposite: electrodes of largely different work-function hardly affect the tunneling barrier (constant ε) while the molecular dipole varies drastically (Δ ≠ 0; right-side illustration). It is as if the molecules are “Fermi-level-pinned”, but molecules have no Fermi level! Instead the electrochemical potential (μ) of the molecule is a well-defined thermodynamic quantity. I will describe how electrochemical balance combined with the notion of chemical “hardness / softness” (electronic polarizability) provide qualitative guidelines for predicting the energy alignment of adsorbed molecules. This concept will be demonstrated on our own results with molecular monolayers on Si(111) and by reference to literature works on thiol-bonded monolayers on Au. Figure 1

Equivalent ambipolar carrier injection of electrons and holes with Au electrodes in air-stable field effect transistors

Carrier injection from Au electrodes to organic thin-film active layers can be greatly improved for both electrons and holes by nano-structural surface control of organic semiconducting thin films using long-chain aliphatic molecules on a SiO2 gate insulator. In this paper, we demonstrate a stark contrast for a 2,5-bis(4-biphenylyl)bithiophene (BP2T) active semiconducting layer grown on a modified SiO2 dielectric gate insulator between two different modifications of tetratetracontane and poly(methyl methacrylate) thin films. Important evidence that the field effect transistor (FET) characteristics are independent of electrode metals with different work functions is given by the observation of a conversion of the metal-semiconductor contact from the Schottky limit to the Bardeen limit. An air-stable light emitting FET with an Au electrode is demonstrated.

Reduction of Schottky barrier height for n-type Ge contact by using Sn electrode

We have investigated the electrical properties of the Sn/n-Ge contact and estimated its Schottky barrier height (SBH). We prepared metal/n-Ge Schottky diodes by Sn, Al, and Au deposition on Ge substrates at room temperature. The current density–voltage characteristics of Sn, Al, Au/n-Ge contacts were measured, and the SBHs of these interfaces were estimated to be 0.35, 0.55, and 0.59 eV, respectively. The SBH of Sn/n-Ge contacts increases for samples annealed at 150–220 °C. We have also investigated the crystalline structure of the Sn layer on Ge(001) by X-ray diffraction analysis and examined the relationship between the crystalline structure and the SBH of Sn/Ge contacts. We found that a Sn layer deposited on Ge(001) at room temperature exhibits a preferentially oriented structure. We also performed hard X-ray photoelectron spectroscopy measurement of these metal/n-Ge samples and estimated the energy band bending of Ge near these metal/n-Ge interfaces. We found a small band bending of Ge in the Sn/n-Ge contact, which has a small SBH, in contrast to Au and Al/n-Ge contacts showing large band bending. The small SBH of the Sn/n-Ge contact can be attributed to the small work function of Sn and corresponds well to the SBH expected from the Schottky limit. The Sn/n-Ge contact has the potential to alleviate the Fermi level pinning.

Effects of surface properties on barrier height and barrier inhomogeneities of platinum contacts to n-type 4H-SiC

We investigated the Schottky barrier of Pt/4H-SiC contact as a function of 4H-SiC surface properties which effectively controlled by electronic cyclotron resonance hydrogen plasma pretreatment for different periods and annealing. It is found that the effective barrier height monotonically increases with decreasing the degree of Fermi level pinning. Electrically homogeneous contacts are observed when the Fermi level (FL) is “pinned (Bardeen limit)” and “free-pinned (Schottky limit).” However, a partial pinning of FL leads to Gaussian distribution of inhomogeneous barrier height. These results could be correlated with changes in the magnitude and spatial distribution of surface state density after different pretreatments.

First-principles analysis of MoS2/Ti2C and MoS2/Ti2CY2(Y=Fand OH) all-2D semiconductor/metal contacts

First-principles calculations are used to explore the geometry, bonding, and electronic properties of MoS${}_{2}$/Ti${}_{2}$C and MoS${}_{2}$/Ti${}_{2}$C${Y}_{2}$ ($Y$ $=$ F and OH) semiconductor/metal contacts. The structure of the interfaces is determined. Strong chemical bonds formed at the MoS${}_{2}$/Ti${}_{2}$C interface result in additional states next to the Fermi level, which extend over the three atomic layers of MoS${}_{2}$ and induce a metallic character. The interaction in MoS${}_{2}$/Ti${}_{2}$C${Y}_{2}$, on the other hand, is weak and not sensitive to the specific geometry, and the semiconducting nature thus is preserved. The energy level alignment implies weak and strong $n$-type doping of MoS${}_{2}$ in MoS${}_{2}$/Ti${}_{2}$CF${}_{2}$ and MoS${}_{2}$/Ti${}_{2}$C(OH)${}_{2}$, respectively. The corresponding $n$-type Schottky barrier heights are 0.85 and 0.26 eV. We show that the MoS${}_{2}$/Ti${}_{2}$CF${}_{2}$ interface is close to the Schottky limit. At the MoS${}_{2}$/Ti${}_{2}$C(OH)${}_{2}$ interface, we find that a strong dipole due to charge rearrangement induces the Schottky barrier. The present interfaces are well suited for application in all-two-dimensional devices.

Electronic transport and Schottky barrier heights of p-type CuAlO2 Schottky diodes

A CuAlO2 Schottky diode was fabricated and investigated using current density-voltage (J-V) and capacitance-voltage (C-V) methods. It is shown that the barrier height (qϕB) determined from J-V measurements is lower than that determined from C-V measurements and qϕB determined from C-V measurements is close to the Schottky limit. This is due to a combined effect of the image-force lowering and tunneling. Time domain measurements provide evidence of the domination of electron trapping with long-second lifetime in CuAlO2. Carrier capture and emission from charge traps may lead to the increased probability of tunneling, increasing the ideality factor.

Barrier height of Pt–InxGa1−xN (≤x≤0.5) nanowire Schottky diodes

The barrier height of Schottky diodes made on InxGa1−xN nanowires have been determined from capacitance-voltage measurements. The nanowires were grown undoped on n-type (001) silicon substrates by plasma-assisted molecular beam epitaxy. The length, diameter and density of the nanowires are ∼1 μm, 20 nm, and 1×1011 cm−2. The Schottky contact was made on the top surface of the nanowires with Pt after planarizing with parylene. The measured barrier height ΦB varies from 1.4 eV (GaN) to 0.44 eV (In0.5Ga0.5N) and agrees well with the ideal barrier heights in the Schottky limit.

Observation of full shot noise in CoFeB/MgO/CoFeB-based magnetic tunneling junctions

The electron transport through the CoFeB/MgO/CoFeB-based magnetic tunneling junction (MTJ) was studied by the shot noise measurement. The obtained Fano factor to characterize the shot noise is very close to unity, indicating the full shot noise, namely, the shot noise in the Schottky limit, both in the parallel and antiparallel magnetization configurations. This means the Poissonian process of the electron tunneling and the absence of the electron–electron correlation in the low bias regime. The shot noise measurements will be a good guideline to make up tunneling criteria for designing MTJ-based spin devices.

Density functional theoretical study of pentacene/noble metal interfaces with van der Waals corrections: Vacuum level shifts and electronic structures

In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu(111), Ag(111), and Au(111) by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals (vdW) approach [S. Grimme, J. Comput. Chem. 27, 1787 (2006)] is employed to include long-range vdW interactions and is shown to reproduce pentacene-metal distances quite accurately. The pentacene-metal distances for Cu, Ag, and Au are evaluated to be 0.24, 0.29, and 0.32 nm, respectively, and work function changes calculated by using the theoretically optimized adsorption geometries are in good agreement with the experimental values, indicating the validity of the present approach in the prediction of the interface dipole at metal/organic interfaces. We examined systematically how the geometric factors, especially the pentacene-substrate distance (Z(C)), and the electronic properties of the metal substrates contribute to the interface dipole. We found that at Z(C) > or = 0.35 nm, the work function changes (Delta phi's) do not depend on the substrate work function (phi(m)), indicating that the interface level alignment is nearly in the Schottky limit, whereas at Z(C) < or = 0.25 nm, Delta phi's vary nearly linearly with phi(m), and the interface level alignment is in the Bardeen limit. Our results indicate the importance of both the geometric and the electronic factors in predicting the interface dipoles. The calculated electronic structure shows that on Au, the long-range vdW interaction dominates the pentacene-substrate interaction, whereas on Cu and Ag, the chemical hybridization contributes to the interaction.

Interface States and Barrier Heights on Metal/4H-SiC Interfaces

Available data on Schottky barrier heights on silicon and carbon rich faces of 4H-SiC have been carefully analyzed to investigate the mechanism of barrier formation on these surfaces. As in case of 3C and 6H-SiC, the barrier heights depend strongly upon method of surface preparation with a considerable scatter in the barrier height for a given metal-semiconductor system. However, for each metal the barrier height depends on the metal work function and strong pinning of the Fermi level has not been observed. The slopes of the linear relation between the barrier heights and metal work functions varies over a wide range from 0.2 to about 0.75 indicating that the density of interface states depends strongly on the method of surface preparation. By a careful examination of the data on barrier heights we could identify a set of nearly ideal interfaces in which the barrier heights vary linearly with metal work function approaching almost to the Schottky limit. The density of interface states for these interfaces is estimated to lie between 4.671012 to 2.631012 states/ cm2 eV on the silicon rich surface and about three times higher on the carbon rich faces. We also observed that on these ideal interfaces the density of interface states was almost independent of metal indicating that the metal induced gap states (MIGS) play no role in determining the barrier heights in metal-4H-SiC Schottky barriers.

Theory of Metal/Dielectric Interfaces -Breakdown of Schottky Barrier Limits-

Recent development of VLSI technique requires wide variety of controlled interfaces especially in high-k metal gate technologies. As for metal/dielectric interfaces, they have been intensively studied, and a lot of knowledge and concepts have been proposed so far. In this paper, we point out the breakdown of the two Schottky barrier limits of Bardeen and Schottky limits which has been believed for a long time as the intrinsic limit of Schottky barrier heights, by considering the characteristics of metal density of states and the atomistic structures of metal/insulator interfaces. Moreover, we have experimentally confirmed that Fermi level of Au (typical high work function metal) further lowers, when Au is in contact with HfO2, by photoemission measurements, indicating the breakdown of the Schottky limit.

Finite-size effects in a nanotube connected to leads: Super- and sub-Poissonian noise

The injection of electrons in the bulk of a carbon nanotube which is connected to ideal Fermi-liquid leads is considered. While the presence of the leads gives a cancellation of the noise cross correlations, the autocorrelation noise has a Fano factor which deviates strongly from the Schottky behavior at voltages where finite-size effects are expected. Indeed, as the voltage is increased from zero, the noise is first super-Poissonian, then sub-Poissonian, and eventually it reaches the Schottky limit. These finite-size effects are also tested using a diagnosis of photoassisted transport, where a small ac modulation is superposed onto the dc bias voltage between the injection tip and the nanotube. When finite-size effects are at play, we obtain a stepwise behavior for the noise derivative, as expected for normal-metal systems, whereas in the absence of finite-size effects, due to the presence of Coulomb interactions, a smoothed staircase is observed. The present work shows that it is possible to explore finite-size effects in nanotube transport via a zero-frequency noise measurement.

Theoretical study of the insulator/insulator interface: Band alignment at theSiO2∕HfO2junction

While the physics of the Schottky barrier is relatively well understood, much less is known about the band alignment at the insulator/insulator interface. As a model problem we study theoretically the band alignment at the technologically important $\mathrm{Si}{\mathrm{O}}_{2}∕\mathrm{Hf}{\mathrm{O}}_{2}$ interface using density functional theory. We report several different atomic level models of this interface along with their energies and electronic properties. We find that the valence band offset increases near linearly with the interfacial oxygen coordination, changing from $\ensuremath{-}2.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. For the fully oxidized interface the Schottky limit is reached. We propose a simple model, which relates the screening properties of the interfacial layer to the band offset. Our results may explain a somewhat confusing picture provided by recent experiments.

Enhancement of Schottky barrier height on p-type GaN by (NH4)2Sx treatment

Barrier height values of Ni contacts to (NH4)2Sx-treated p-type GaN (p-GaN) were obtained from current-voltage and x-ray photoelectron spectroscopy (XPS) measurements in this study. The induced deep level defect band through high Mg doping led to a reduction of the depletion layer width in the p-GaN near the interface and an increase in the probability of thermionic field emission (TFE). Furthermore, the calculated barrier height value of Ni contacts to (NH4)2Sx-treated p-GaN using the TFE model is close to the Schottky limit, which is in good agreement with the observed result by XPS measurements and suggests that (NH4)2Sx surface treatment leads to the removal of native oxides and the reduction of the surface state related to oxygen-induced and nitrogen-vacancy defects.

Control of interface states at metal/6H-SiC(0001) interfaces

Metal/$6\mathrm{H}\ensuremath{-}\mathrm{SiC}(0001)$ interfaces free of Fermi level pinning were formed by realizing well-ordered atomic arrangements and perfect termination of the surface atoms of SiC substrates. The surfaces and interfaces were investigated by electrical measurements, Auger electron spectroscopy, low energy electron diffraction, x-ray photoemission spectroscopy, scanning tunneling microscopy, and transmission electron microscopy. We used three different regimes for the surface treatments: (i) the conventional procedure of degreasing and HF dipping, (ii) thermal oxidation followed by HF dipping after (i), and (iii) immersion into boiling water after (ii). We found that the dependence of the Schottky barrier height on the metal work function changes drastically following these surface treatments. The Fermi level at the interface prepared using only treatment (i) was almost pinned at \ensuremath{\sim}0.8 eV below the conduction band minimum. On the other hand, for the interfaces formed by treatments (ii) and (iii), the position of the interface Fermi level varied strongly with the metal work function. In particular, treatment (iii) approached the Schottky limit, with a density of interface states of $4.6\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{states}\mathrm{\ensuremath{\cdot}}\mathrm{cm}}^{\ensuremath{-}2}\ensuremath{\cdot}{\mathrm{eV}}^{\ensuremath{-}1}.$ The surface characterization of the SiC surfaces formed by the Schottky-limit treatment (iii) indicated that the surface was atomically flat, the terraces of the surface was terminated by hydrogen atoms, and their step-edges were stable due to passivation by oxygen. An abrupt commensurate epitaxial connection at the Ti/SiC interface was found for treatment (iii), whereas the Ti/SiC interface obtained by employing treatment (i) had a disordered layer with a thickness of \ensuremath{\sim}2 nm, which is the origin of the large density of interface states enough to pin the interface Fermi level.

Ultrathin Ni layers grown epitaxially on SiC(0001) at room temperature

Ultrathin (1-20 ML) Ni layers deposited on 6H-SiC(0001)-3×3 at room temperature (RT) were analyzed in situ by high-resolution medium energy ion scattering (MEIS), reflection high-energy electron diffraction (RHEED), and photoelectron spectroscopy using synchrotron-radiation light. For a Ni coverage of more than 3 ML [I ML for SiC(0001): 1.21×10 1 5 atoms/cm 2 ] uniform Ni(111) layers grow epitaxially at RT in spite of a large lattice mismatch of 20%. There are two domains, (A) Ni-[110]//SiC-[1120] and (B) Ni-[112]//SiC-[1120], and the occupation ratio of (A) to (B) is 5:1. The ion shadowing effect reveals significant expansion of the interplanar distance of Ni(111), which relaxes with increasing the Ni thickness. The MEIS analysis shows that a small amount of Si segregate to the surface and the crystalline Ni(111) layer contains Si atoms (3-15 at. %). The segregation rate is derived to be 0.015 s - 1 by solving a simple rate equation. The uniform stack and epitaxial growth of Ni layers at RT may be responsible for the surface-segregating Si atoms as a surfactant. The two components from Si on top and in the Ni layer are clearly observed in the Si 2p spectra and a higher binding energy shift of the latter relative to the former indicates an electronic charge transfer from Si to Ni. The binding energy shift of the Si 2p level of the bulk SiC gives the Schottky barrier height, which reaches the Schottky limit for a thickness above 3 ML.

Electronic States and Schottky Barrier Height at Metal/MgO(100) Interfaces

Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.