Barrier Heights Of Metals Research Articles

Optimization of ohmic contact on n-type GaAs by screen-printing silver paste

We used printed electronics technology to print silver paste (SP) on n-GaAs as an electrode replacing conventional alloy electrodes to simplify the fabrication process of solar cell and to reduce cost. The linear transmission line model was used to characterize the performances of SP/semiconductor ohmic contact at different annealing temperatures. The lowest specific contact resistance between SP and n-GaAs of 1.8 × 10−4 Ω cm2 was achieved after annealing at 560 °C, which indicates the appropriate annealing temperature can not only ensure the close contact of silver particles, but also reduce the barrier height of metals and semiconductors to a certain extent. On the basis of these results, n-GaAs with an SP electrode can be promisingly applied to realize highly efficient and simple-manufacturing III–V solar cells.

Correction to "Evaluating Transition Metal Barrier Heights with the Latest Density Functional Theory Exchange-Correlation Functionals: The MOBH35 Benchmark Database".

A new database of transition metal reaction barrier heights (MOBH35) is presented. Benchmark energies (forward and reverse barriers and reaction energy) are calculated using DLPNO-CCSD(T) extrapolated to the complete basis set limit using a Weizmann-1-like scheme. Using these benchmark energies, the performance of a wide selection of density functional theory (DFT) exchange–correlation functionals, including the latest from the Martin, Truhlar, and Head-Gordon groups, is evaluated. It was found, using the def2-TZVPP basis set, that the ωB97M-V (MAD 1.7 kcal/mol), ωB97M-D3BJ (MAD 1.9 kcal/mol), ωB97X-V (MAD 2.0 kcal/mol), and revTPSS0-D4 (MAD 2.2 kcal/mol) hybrid functionals are recommended. The double-hybrid functionals B2K-PLYP (MAD 1.7 kcal/mol) and revDOD-PBEP86-D4 (MAD 1.8 kcal/mol) also performed well, but this has to be balanced by their increased computational cost.

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.

Effect of adding an insulator between metal and semiconductor layers on contact resistance

The authors investigated the effective Schottky barrier heights of metal and silicon contacts after insertion of insulator layers with different conduction band offsets. A decrease in Schottky barrier height after insertion of an insulator layer was observed. In particular, the Schottky barrier height of metal/semiconductor contacts was lowest when a ZnO layer was inserted compared to the other insulator layer types, because the conduction band offset between ZnO and silicon was the lowest among those measured. The authors also investigated current density as a function of the thickness of the insulator and doping concentration of silicon.

Modeling of Mean Barrier Height Levying Various Image Forces of Metal–Insulator–Metal Structure to Enhance the Performance of Conductive Filament Based Memristor Model

Memristor manifests resistive switching based memory as its inherent property. Simple nanoscale structure, nonvolatility, low-power operation, and fast computing make it viable to compete with the emerging memory technologies. In this paper, we design the mean barrier height of metal–insulator–metal structure by imposing various image forces on it. Parabolic, triangular, and rectangular nature of image force potentials are superimposed with reported rectangular potential barrier to see their impact in tunneling current. Results revealed that the triangular image force significantly lower the barrier and results in highest current density among other barriers. Parabolic image force shows a 40% improvement in current density at higher voltage range when compared with reported hyperbolic image forces, whereas rectangular image force yield least current. The resultant relations are deployed in the existing conductive filament based memristor model to investigate the pinched hysteresis I – V characteristics. To validate our design approach, we compared the results with reported experimental data of memristor as well as spice model of memristor, which includes hyperbolic image potential. It is inferred that parabolic image potential builds better agreement with reported data as compared to others and shows a maximum off-switching current $1.74\,\times \,10^{- 3}$ A and on-switching current $0.95\times 10^{- 3}\;{\rm{A}}$ . The improved model of memristor can be employed to various applications like neuromorphic computing, logic design, and memory design.

On the band-structure lineup at Ga2O3, Gd2O3, and Ga2O3(Gd2O3) heterostructures and Ga2O3 Schottky contacts

The interface-induced gap states (IFIGS) are the fundamental mechanism that determines the band-structure lineup at semiconductor interfaces, i.e., the band-edge offsets at semiconductor heterostructures and the barrier heights of metal–semiconductor or Schottky contacts. Both quantities are composed of a zero-charge transfer and an electrostatic-dipole term which are given by the IFIGS’s branch-point energies and the electronegativities of the two solids in contact, respectively. A respective analysis of experimental valence-band offsets of Ga2O3 and Gd2O3 heterostructures results in the empirical p-type branch-point energies of 3.57 and 2.85 eV, respectively. From experimental barrier heights of n-Ga2O3 Schottky contacts an empirical n-type branch-point energy of 1.34 eV is obtained. The p- and n-type branch point energies of Ga2O3 add up to 4.91 eV, the width of the Ga2O3 band gap, as to be expected from the theoretical IFIGS-and-electronegativity concept. The experimental valence-band offsets of Ga2O3(Gd2O3) heterostructures indicate that at their interfaces the chemical composition of the oxide differs from its nominal value in the bulk.

On the band-structure lineup at Schottky contacts and semiconductor heterostructures

The band-structure lineup at semiconductor interfaces is explained by the intrinsic interface-induced gap states (IFIGS) that derive from the complex band structures of the semiconductors. The barrier heights of metal–semiconductor or Schottky contacts as well as the band-edge offsets of semiconductor heterostructures are composed of a zero-charge-transfer term plus an electrostatic-dipole contribution which are determined by the IFIGS branch-point energies of the semiconductors and the electronegativity difference of the two materials in contact, respectively. This concept will be illustrated by experimental core-level shifts induced by metal adatoms on group-IV semiconductor surfaces. Choosing Si and SiO2 Schottky contacts and heterostructures as typical examples, it will be demonstrated that the IFIGS-and-electronegativity concept self-consistently explains the barrier heights of Schottky contacts and the valence-band offsets of heterostructures. The IFIGS-and-electronegativity concept also resolves the alleviation of the Fermi-level pinning by ultra-thin insulator interlayers in Schottky contacts. Finally, the modification of Schottky barriers by atomic interlayers will be discussed.

Work Function Control on High K Metal Gate Stacks

The paper summarizes the development of metal gate materials and the control of the effective work function for use in future Si field effect transistors. The Schottky barrier heights of metals on HfO2 are modeled, for ideal interfaces of various stoichiometries and for interfaces with defects, and also for dipole layers.

Determination of the laterally homogeneous barrier height of metal/ p-InP Schottky barrier diodes

We have reported a study of a number of metal/ p-type InP (Cu, Au, Al, Sn, Pb, Ti, Zn) Schottky barrier diodes (SBDs). Each one diode has been identically prepared on p-InP under vacuum conditions with metal deposition. In Schottky diodes, the current transport occurs by thermionic emission over the Schottky barrier. The current–voltage characteristics of Schottky contacts are described by two fitting parameters such as effective barrier height and the ideality factor. Due to lateral inhomogeneities of the barrier height, both characteristic diode parameters differ from one diode to another. We have determined the lateral homogeneous barrier height of the SBDs from the linear relationship between experimental barrier heights and ideality factors that can be explained by lateral inhomogeneity of the barrier height. Furthermore, the barrier heights of metal–semiconductor contacts have been explained by the continuum of metal-induced gap states (MIGS). It has been seen that the laterally homogeneous barrier heights obtained from the experimental data of the metal/ p-type InP Schottky contacts quantitatively confirm the predictions of the combination of the physical MIGS and the chemical electronegativity.

Band offsets and work function control in field effect transistors

The article summarizes the development of metal gate materials and the control of the effective work function on high dielectric constant (high K) oxides for use in advanced Si field effect transistors. The Schottky barrier heights of metals on HfO2 are calculated accurately for ideal interfaces of various stoichiometries and for interfaces with defects.

Fermi level pinning-free interface at metals/homoepitaxial diamond (111) films after oxidation treatments

Schottky barrier heights of metal (Al, Au, Ni, and Pt) contacts on boron (B)-doped (111) homoepitaxial diamond films are investigated as a function of surface oxidation treatments before metal deposition [after wet-chemical oxidation (WO), WO followed by annealing in Ar atmosphere (WO-AN) and air oxidation]. It is found that the Schottky barrier height (ϕB) depends on the metal work function (ϕM) (Sϕ≡dϕB∕dϕM>0.3) on surfaces only after WO-AN, indicating that the interface at metals/B-doped (111) films after WO-AN has fewer defect states which do not pin the Fermi level. Experimental results and the appearance of a p-type surface conductive layer (SCL) are discussed and it is concluded that the pinning-free nature of the Fermi level is related to the appearance of a SCL.

Some comments on the determination and interpretation of barrier heights of metal–semiconductor contacts

The intrinsic interface-induced gap states are the fundamental theoretical concept which explains the band-structure lineup at all semiconductor interfaces. However, a comparison of experimental results and theoretical predictions requires data of well-characterized interfaces. With respect to the experimental input, two aspects are of importance. First, the evaluation of the measured data has to be performed critically and very carefully and, second, extrinsic and intrinsic effects have to be recognized and then separated. Some examples are given. The interpretation of the experimental data may become misleading if a physical quantity such as the Fermi-level stabilization energy of heavily radiation-damaged semiconductors is considered which derives from the complex semiconductor band-structure as the branch-point of the interface-induced gap states.

Comparison of ZnO metal–oxide–semiconductor field effect transistor and metal–semiconductor field effect transistor structures grown on sapphire by pulsed laser deposition

ZnO thin film field effect transistors with 1.5–20μm gate width were fabricated using either a metal gate [metal–semiconductor field effect transistor (MESFET)] or a metal–oxide–semiconductor (MOS) gate. In both cases we found that use of a thick (∼0.8–0.9μm) ZnO buffer was necessary on the sapphire or glass substrate prior to growing the active layers in order to reduce gate leakage current. Source/drain contacts of e-beam deposited Ti∕Al∕Pt∕Au showed specific contact resistances of 2.18×10−6Ωcm2 without annealing and the interdevice isolation currents were ∼10μA at 40V bias. The MOS structure with 50nm (Ce,Tb)MgAl11O19 gate dielectric showed a 1 order of magnitude lower gate leakage current than the MESFET, due to the relatively low barrier height of metals on n-type ZnO (0.6–0.8eV). Good drain–source current characteristics were obtained from MOS gate structures using P-doped ZnO channels, whereas the metal structures showed very poor modulation.

A method for measuring barrier heights, metal work functions and fixed charge densities in metal/SiO2/Si capacitors

A method for measuring metal barrier heights, work function and fixed charge densities in metal/SiO2/Si capacitors is developed and verified. This technique is based on theoretical studies of tunneling phenomenon through a potential barrier and requires measurement of current versus voltage sweeps at two different temperatures. Unlike the commonly used capacitance method, this method does not require a set of capacitors with different gate oxide thickness for determining work functions and fixed charge densities in metal/SiO2/Si capacitors. Hence, this method provides a fast means for investigating metal work function and fixed charge densities in metal-gated SiO2 capacitors.

Spectroscopic investigations of diamond/hydrogen/metal and diamond/metal interfaces

The formation of metal contacts on diamond surfaces is of decisive importance for the material's application in semiconductor devices. We present an investigation of the barrier heights of metals on diamond (111) single crystal surfaces by a combination of spectroscopic and structural methods. Two metals with different work functions and chemical reactivities, aluminium and gold were chosen as model systems for Schottky and ohmic contacts, respectively. Particular attention was paid to the role hydrogen termination of diamond has for the metal film morphology and the barrier heights. To this end diamond/metal contacts on hydrogenated and dehydrogenated diamond were investigated. Core-level spectroscopy was used to monitor chemical reactions between diamond and metal, metal-induced band bending, and the barrier heights. The morphology of the evaporated metal films was investigated by low energy electron diffraction (LEED), atomic force microscopy (AFM), and secondary electron microscopy (SEM).

Calculation of valence-band offsets of lattice-matched GaInTlP/InP heterostructures and of Schottky barrier heights of metal–GaInTlP contacts

Quaternary Ga0.18(1−y)InyTl0.82(1−y)P alloys are lattice matched with InP and their band gaps were predicted to be direct and to vary from zero width to 1.35 eV, the value of InP. This study calculates the branch points of the interface-induced gap states of the binary compound TlP and of the ternary GaTlP and InTlP as well as some quaternary GaInTlP alloys using an empirical tight-binding approach. Several predictions are made. First, the band lineup of lattice-matched InP/Ga0.18(1−y)InyTl0.82(1−y)P heterostructures is of type I. Second, the conduction-band discontinuities are larger than the valence-band offsets. And third, metal/Ga0.18(1−y)InyTl0.82(1−y)P contacts are naturally ohmic for indium contents y smaller than approximately 0.5.

Hydrogen-Modification of Electronic Surface, Bulk, and Interface Properties of Si

Silicon surfaces may be covered with hydrogen by adsorption of atomic hydrogen and by wet chemical etching. The adsorption of hydrogen induces additional surface dipoles and new surface states. The occupied H-induced surface states are below the valence-band maximum so that the bands should be flat up to the surface. However, after H-termination by wet chemical etching the Fermi level is pinned close to midgap position for n- and p-type bulk doping. This behavior is not caused by any kind of defect states but was explained by a passivation of shallow impurities due to an incorporation of hydrogen from the etching solution. Capacitance–voltage characteristics of lead contacts on H-terminated silicon samples demonstrate a penetration of hydrogen up to 1 μm below the interface. In comparison to what is observed with clean interfaces hydrogen-doping reduces the barrier heights of metal–p-diamond contacts but increases it at Pb–p-silicon interfaces. Hydrogen-induced interface dipoles explain this behavior. They are oppositely oriented at diamond and silicon interfaces since hydrogen is more electronegative than silicon but electropositive compared to carbon.

Schottky contacts on ternary compound semiconductors: Compositional variations of barrier heights

The alignment of the electronic band structures at contacts between solids is defined by the continuum of interface-induced gap states and the charge transfer across the interface. The barrier heights of metal–semiconductor contacts as well as the band edge discontinuities of semiconductor heterostructures are primarily described by the zero-charge-transfer barrier heights. This characteristic semiconductor property equals the energy distance between the majority-carrier band edge and the charge-neutrality level of the interface-induced gap states. The compositional trends of barrier heights of Al1−xInxAs, In1−xGaxAs, and Al1−xGaxAs Schottky contacts and of valence-band offsets of Al1−xGaxAs/GaAs heterostructures are analyzed. The zero-charge-transfer barrier heights of ternary compounds are found to vary linearly as a function of the compositional parameter x.

Experimental determination of the pressure dependence of the barrier height of metal/

The experimentally measured pressure dependence of the Au/[n-type GaAs] barrier height, 9.5 meV/kbar, is consistent with the theoretically calculated value for an interface with a high concentration of As antisite defects while it is inconsistent with the calculated value for an ideal interface. This is conclusive evidence that Fermi-level pinning is dominated by defects at a metal/semiconductor interface, whereas the pressure dependence of the Al/[n-type GaAs] barrier height, 10.5 meV/kbar, is consistent with the prediction of an ideal interface.

Ballistic electron emission spectroscopy of metals on GaP(110)

Ballistic electron emission spectroscopy (BEES), a technique based on the scanning tunneling microscope (STM), was used to measure Schottky barrier heights of metals on cleaved n-type GaP(110). The threshold voltages V0 for current detection in the semiconductor were found to be uniform to within ±0.02 V over the sample surface for any given metal on GaP. A transport model for the current Ic crossing the barrier, that includes both nonclassical transmission across the metal–semiconductor interface and electron scattering in the metal, yields Ic∝(V−V0)5/2 near threshold. The value of V0 extracted from the data, which represents the Schottky barrier height, depends somewhat on the details of the transport model. Our best estimates of the Schottky barrier heights, within ±0.03 eV, are 1.07 (Mg), 1.11 (Ni), 1.14 (Bi), 1.25 (Cu), 1.31 (Ag), and 1.46 eV (Au).