Induced Gap States Research Articles

Identifying the Length Dependence of Orbital Alignment and Contact Coupling in Molecular Heterojunctions

Transport in metal-molecule-metal junctions is defined by the alignment and coupling of molecular orbitals with continuum electronic states in the metal contacts. Length-dependent changes in molecular orbital alignment and coupling with contact states were probed via measurements and comparisons of thermopower (S) of a series of phenylenes and alkanes with varying binding groups. S increases linearly with length for phenylenediames and phenylenedithiols while it decreases linearly in alkanedithiols. Comparison of these data suggests that the molecular backbone determines the length dependence of S, while the binding group determines the zero length or contact S. Transport in phenylenes was dominated by the highest occupied molecular orbital (HOMO), which aligns closer to the Fermi energy of the contacts as approximately L(-1), but becomes more decoupled from them as approximately e(-L). In contrast, the decreasing trend in S for alkanedithiols suggests that transmission is largely affected by gold-sulfur metal induced gap states residing between the HOMO and lowest unoccupied molecular orbital.

Tuning of Fermi level position at HfNx/SiO2 interface

We demonstrate that the vacuum work function of HfNx, as measured in situ by Kelvin probe, can be tailored on the entire silicon band gap by changing the nitrogen content of the HfNx alloy. We observed an increase in the effective work function of HfNx on SiO2, extracted from capacitance-voltage measurements, only for x<1, and saturation is reached at a value of 4.6 eV for x>1. The measured value of the effective work function for pure Hf (4.1 eV) can be explained by the metal induced gap states model, while the HfNx behavior is explained by oxidation at the HfNx/SiO2 interface.

Band offsets of a ruthenium gate on ultrathin high-κoxide films on silicon

Valence-band and conduction-band edges of ultrathin oxides (${\text{SiO}}_{2}$, ${\text{HfO}}_{2}$, ${\text{Hf}}_{0.7}{\text{Si}}_{0.3}{\text{O}}_{2}$, and ${\text{Al}}_{2}{\text{O}}_{3}$ grown on silicon) and their shifts upon sequential metallization with ruthenium have been measured using synchrotron-radiation-excited x-ray, ultraviolet, and inverse photoemissions. From these techniques, the offsets between the valence-band and conduction-band edges of the oxides, and the ruthenium metal gate Fermi edge have been directly measured. In addition the core levels of the oxides and the ruthenium have been characterized. Upon deposition, Ru remains metallic and no chemical alteration of the underlying oxide gates, or interfacial ${\text{SiO}}_{2}$ in the case of the high-$\ensuremath{\kappa}$ thin films, can be detected. However a clear shift of the band edges is measured for all samples due to the creation of an interface dipole at the ruthenium-oxide interface. Using the energy gap, the electron affinity of the oxides, and the ruthenium work function that have been directly measured on these samples, the experimental band offsets are compared to those predicted by the induced gap states model.

Effect of contact induced states on minimum conductivity in graphene

The objective of this paper is to point out that contact induced states can help explain the structure dependence of the minimum conductivity observed experimentally even if the samples were purely ballistic. Contact induced states are similar to the well-known metal induced gap states (MIGS) in metal-semiconductor Schottky junctions, which typically penetrate only a few atomic lengths into the semiconductor, while the depth of penetration decreases with increasing band gap. However, in graphene we find that these states penetrate a much longer distance of the order of the width of the contacts. As a result, ballistic graphene samples with a length less than their width can exhibit a resistance proportional to length that is not Ohmic in origin, but arises from a reduced role of contact-induced states. While actual samples are probably not ballistic and involve scattering processes, our results show that these contact induced effects need to be taken into account in interpreting experiments and minimum conductivity depends strongly on the structure and configuration (two- vs. four-terminal) used.

Interfacial properties of Ce0.75Zr0.25O2 supported noble metals (Pd, Pt) from first principles

First-principles electronic structure calculations of noble metals (NM=Pd, Pt)/Ce0.75Zr0.25O2 systems are presented. It is found that: the NM adatoms do not prefer to stay at the atop or the bridge sites of the cations (Ce and Zr), but prefer to be adsorbed at or around the anion sites. The most preferable adsorption sites for both the Pd and Pt adatoms are the O-bridge sites neighboring the Zr dopant. The Pt adatom show much stronger interaction with the Ce0.75Zr0.25O2(111) surface than does the Pd adatom. The interactions of the NM/Ce0.75Zr0.25O2(111) interfaces are stronger than those of the corresponding NM/ceria(111) interfaces. There are some metal induced gap states (MIGS) appeared in the gaps of the NM/Ce0.75Zr0.25O2(111) interfaces, which are important to catalytic properties of the NM/Ce0.75Zr0.25O2(111) catalysts.

Influence of gap states on the electrical stability of pentacene thin film transistors

The influence of oxygen induced gap states on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) was investigated by electrical in situ measurements. Unexposed devices are stable, whereas devices exposed to dry oxygen exhibit a shift of the threshold voltage upon prolonged device operation. Stressing the transistor in the on-state (negative bias) leads to a shift of the threshold voltage towards negative gate voltages, whereas prolonged bias stress in the off-state causes a shift of the threshold voltage in the opposite direction. The gap states are formed 0.18eV (acceptor-like states) and 0.62eV (donor-like states) above the valence band maxima. The charge carrier mobility and the on/off ratio of the transistor are not affected by the gap states. A simple density-of-states model will be presented, which allows for the explanation of the experimental results.

Metal induced gap states on Pt-modified Ge(001) surfaces

Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) we have studied the electronic properties of a novel, planar, metal semiconductor contact. For this purpose we take advantage of the unique properties of the Pt-modified Ge(001) surface, which consist of coexisting metallic and semiconducting terraces. Spatially resolved STS measurements reveal that the higher lying metallic terraces induce electronic states in the band gap region of the lower lying semiconducting terraces.

First-principles study of thePt∕CeO2(111)interface

The adsorption of platinum on the ceria (111) surface may change the reduction and redox properties of $\mathrm{Ce}{\mathrm{O}}_{2}$. The density functional theory with the inclusion of on-site Coulomb interaction has been used to study the properties of Pt adsorbed on the (111) surfaces of stoichiometric and reduced ceria. In all cases, strong interactions are found between the Pt and the surfaces. The Pt-O-Ce bonds may be formed between the Pt adatom and the surface in the preferred adsorption mode. The adsorption on the reduced surface is stronger than on the unreduced surface. The formation energy of an O vacancy at the $\mathrm{Pt}∕\mathrm{Ce}{\mathrm{O}}_{2}(111)$ interface is lower than on the clean $\mathrm{Ce}{\mathrm{O}}_{2}(111)$ surface due to the existence of the lower lying partially occupied metal induced gap states including mainly the $\mathrm{Pt}\phantom{\rule{0.2em}{0ex}}4d$ states below the empty $\mathrm{Ce}\phantom{\rule{0.2em}{0ex}}4f$ band.

First-Principles Investigation of the Structure, Energetics, and Electronic Properties of Ru/HfO2 Interfaces

The electronic properties of metal oxide semiconductor gate electrodes are strong functions of the structure of the metal−dielectric interface. We use density functional theory to investigate the structures, energetics, and electronic properties of Ru/HfO2 interfaces. We find that atom-by-atom deposited Ru films have significantly different interfacial structures from epitaxially connected Ru films on the m-HfO2(001) surface. Atom-by-atom deposited films are rougher than epitaxially connected films, even when forced to form two-dimensional films. These interfacial structural differences lead to differences in the electronic properties of the films and therefore affect electrical properties, such as the work function of the gate electrode. We found that for both types of films the partial densities of states of interfacial layers are significantly different from that of the bulk region. Each shows charge transfer from the interfacial Ru layer to the interfacial dielectric layer, although the magnitude of charge transfer varies. Thus both films exhibit metal induced gap states in the band gap region of the interface localized on atoms of the HfO2 interfacial layer.

Electronic structure of octane on Cu(1 1 1) and Ni(1 1 1) studied by near edge X-ray absorption fine structure

The electronic structure of an octane film grown on Cu(1 1 1) and Ni(1 1 1) was studied using C K-edge near edge X-ray absorption fine structure (NEXAFS). A pre-peak was observed on the bulk edge onset for the 1 ML thick octane films on the metal substrates. The pre-peak originated from metal induced gap states (MIGS) in the band gap of octane. The intensity of the pre-peak for octane/Ni(1 1 1) was the same as that of octane/Cu(1 1 1), suggesting that there was little difference in the density of unoccupied MIGS between the octane film on Ni(1 1 1) and Cu(1 1 1). We discuss the metal dependence of the density of unoccupied MIGS on the band structure of the metals.

Band gap and Schottky barrier heights of multiferroic BiFeO3

Bi Fe O 3 is an interesting multiferroic oxide and a potentially important Pb-free ferroelectric. However, its applications can be limited by large leakage currents. Its band gap is calculated by the density-functional based screened exchange method to be 2.8eV, similar to experiment. The Schottky barrier height on Pt or SrRuO3 is calculated in the metal induced gap state model to be over 0.9eV. Thus, its leakage is not intrinsic.

Copper phthalocyanine buffer layer to enhance the charge injection in organic thin-film transistors

Copper phthalocyanine (CuPC) has been used as the buffer layer for source and drain electrodes to improve the performance of organic thin-film transistors. The mobility was improved by almost onefold after the modification. By incorporating with CuPC, the contact resistance was reduced, deduced from the line-transfer method. The higher hole current observed in the hole-only diode after adding CuPC further confirms the improvement of hole-injection efficiency. It is concluded that the device improvement is attributed to the reduction of contact resistance, which resulted from the Fermi level pinning at the induced gap states at the Au∕CuPC interface.

Quasi-One-Dimensional K-O Chain in PTCDA Thin Films: Evidence from First-Principles Calculations

Using density functional theory calculations we have found that K atoms in a PTCDA (3,4:9,10-perylenetetracarboxylic dianhydride) crystal form a quasi-one-dimensional (1D) K-O chain interacting with carboxylic oxygen of the terminal anhydride groups of PTCDA. The K-K distance in the chain (3.72 Angstrom) is commensurate to the periodicity of the organic semiconductor. We obtain that the K-O structure is stabilized by charge transfer from K to PTCDA molecules, forming prevalently ionic bonds: the electronic density of the chemistry induced gap states is essentially delocalized on the perylene core of PTCDA, while potassium appears spoiled of its charge. Band dispersion along the direction of molecular stack is evaluated to be 0.2 eV in pure PTCDA crystal and 0.5 eV in the K-doped system, confirming that the interaction occurs between different molecular planes.

First-principles molecular dynamics study of Al/Alq3 interfaces

We have carried out first-principles molecular dynamics simulations of Al deposition on tris (8-hydroxyquinoline) aluminum (Alq3) layers to investigate atomic geometries and electronic properties of Al=Alq3 interfaces. Al atoms were ejected to Alq3 one by one with the kinetic energy of 37.4 kJ/mol, which approximately corresponds to the average kinetic energy of Al at the boiling temperature of metal Al. The first Al atom interacts with two of the three O atoms of meridional Alq3. Following Al atoms interact with Alq3 rather weakly and they tend to aggregate each other to form Al clusters. During the deposition process, Alq3 was not broken and its molecular structure remained essentially intact. At the interface, weak bonds between deposited Al atoms and N and C atoms were formed. The projected density of states (PDOS) onto the Alq3 molecular orbitals shows gap states in between the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs), which were experimentally observed by ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). Our results show that even though the Alq3 molecular structure is retained, weak N–Al and C–Al bonds induce gap states.

VUV Induced Doping of Cu-Phthalocyanine Thin Films: A Possibility of n-Type Doping

The new type doping method, VUV-induced doping, has been applied to copper phthalocyanine (CuPc) thin film. The highest occupied molecular orbital (HOMO) peak in the ultraviolet photoelectron spectrum of CuPc film was gradually shifted toward high binding energy side upon the irradiation of He Iα radiation. Furthermore, the radiation induced gap states were observed up to the Fermi level. The shift of the HOMO peak in the low-dose region depends in a semi-logarithmic fashion on the irradiation time, indicating that the intentional n-type doping of CuPc film can be realized by VUV irradiation.

Theoretical Studies on the Physical Properties of Poly-Si and Metal Gates/HfO2 Related High-k Dielectrics Interfaces

We have theoretically investigated poly-Si and metal gates on Hf-related high-k gate dielectrics. First, we have investigated the cause of the substantial threshold voltage (Vth) shifts observed in Hf-related high-k gate stacks with p+poly-Si gates. The oxygen vacancy (Vo) level in ionic HfO2 is located in a relatively higher part of the band gap. If the p+poly-Si-gate is in contact with HfO2, Vo formation in the HfO2 induces a subsequent electron transfer across the interface because of the higher energy level position of Vo, causing a substantial Vth shifts in p+poly-Si gate MISFETs. Next, we also investigate the microscopic electronic structures at metal gates/HfO2 interfaces. We have found that the wave functions of metal induced gap states (MIGS) have large amplitudes both around Hf and O atoms, which may be the cause of unusual work function behaviors of p-like metals.

Electronic properties of the Zr–ZrO2–SiO2–Si(100) gate stack structure

The interface electronic structure of a layered Zr–ZrO2–SiO2–Si(100) system was studied with x-ray (hν=1254eV) and ultraviolet (hν=21.2eV) photoemission spectroscopies. In situ growth and characterization allow the structures to be deposited and studied in a stepwise manner without the risk of contamination. This study discusses the electronic properties including electron affinities and work functions, valence band maxima, band bending in the Si, and internal fields in a layered high-κ gate stack. With this information the band alignments can be reconstructed and compared to predictions of the vacuum alignment models (i.e., the Schottky-Mott model for metal-semiconductor interfaces or the electron affinity model for heterojunctions) and the interface induced gap states model. The vacuum alignment models are first order approaches to determine the electronic barrier height for a heterojunction, and interface bonding can contribute to charge transfer across the interface, affecting the dipole contribution and altering the barrier heights. In this study, the band offsets and vacuum levels are independently measured, thereby determining the deviation from the vacuum level alignment models. The valence band offsets at the Si–SiO2, SiO2–ZrO2, and ZrO2–Zr are found to be 4.4±0.1, 0.67±0.24, and 4.9±0.44eV, respectively. For these same interfaces the deviations from the electron affinity or Schottky-Mott model are determined to be 0.2±0.14, −1.43±0.29, and 1.3±0.39eV, respectively.

Metal induced gap states at tetratetracontane/Cu interface

In order to search for states specific to organic-insulator/metal interfaces, we have studied atomically well-defined tetratetracontane(TTC)/Cu interface with the element-selective near edge x-ray absorption fine structure (NEXAFS). An extra peak is observed below the bulk edge onset for thin TTC films on Cu. We regard the pre-peak as an evidence for the metal-induced gap states (MIGS) formed by the proximity to a metal. An ab initio electronic structure calculation supports the existence of the MIGS.

Electronic structure of intrinsic defects in crystalline germanium telluride

Germanium telluride undergoes rapid transition between polycrystalline and amorphous states under either optical or electrical excitation. While the crystalline phases are predicted to be semiconductors, polycrystalline germanium telluride always exhibits -type metallic conductivity. We present a study of the electronic structure and formation energies of the vacancy and antisite defects in both known crystalline phases. We show that these intrinsic defects determine the nature of free-carrier transport in crystalline germanium telluride. Germanium vacancies require roughly one-third the energy of the other three defects to form, making this by far the most favorable intrinsic defect. While the tellurium antisite and vacancy induce gap states, the germanium counterparts do not. A simple counting argument, reinforced by integration over the density of states, predicts that the germanium vacancy leads to empty states at the top of the valence band, thus giving a complete explanation of the observed -type metallic conduction.

Theoretical investigation of Pt monosilicide and several germanides: electronic structure, surface energetics, and work functions

AbstractWe present a theoretical study of the electronic structure, surface energies and work functions of orthorhombic Pt monosilicide and germanides of Pt, Ni, Y and Hf within the framework of density functional theory (DFT). Our calculated bulk structures are within 1-2% of reported experimental values. Calculated work functsions for the (001) surfaces of PtSi, NiGe and PtGe are 5.12, 4.57 and 4.83 eV, respectively, suggesting that these metals and their alloys can be used as self-aligned contacts to p-type silicon and germanium. Work functions for Y and Hf germanides range from 2.4 to 4.3 eV making them a possible n-type contact material. In addition, we also report an ab-initio calculation of the Schottky-barrier height at the Si(001)/PtSi(001) interface. The p-type Schottky barrier height of 0.28 eV is found in good agreement with predictions of a simple metal induced gap states (MIGS) theory and available experiment. This low barrier suggests PtSi as a low contact resistance junction metal for silicon CMOS technology. We identify the growth conditions necessary to stabilize this orientation.