Induced Gap States Research Articles

Adsorption properties versus oxidation states of rutile TiO2(110)

Using density functional theory we have studied the adsorption properties of different atoms and molecules deposited on a stoichiometric, reduced, and oxidized rutile TiO(2)(110) surface. Depending on the oxidation state of the surface, electrons can flow from or to the substrate and, therefore, negatively or positively charged species are expected. In particular, we have found that a charge transfer process from or to the surface always occurs for highly electronegative or highly electropositive species, respectively. For atoms or molecules with intermediate electron affinity, the direction of the charge flow depends on the oxidation state of the rutile surface and on the adsorption site. Generally, the charging effect leads to more stable complexes. However, the increase in the binding energy of the adsorbates is highly dependent on the electronic states of the surface prior to the adsorption event. In this work we have analyzed in details these mechanisms and we have also established a direct correlation between the enhanced binding energy of the adsorbates and the induced gap states.

Towards barrier height modulation in HfO 2/TiN by oxygen scavenging – Dielectric defects or metal induced gap states?

The band alignment between a dielectric and a metal gate is crucial as it controls the MOSFET threshold voltage as well as the leakage in metal–insulator–metal (MIM) structure. In the ideal Schottky–Mott model the barrier height should be controlled only by the workfunction and the electron affinity of the materials considered. However, this seems the case only for few insulating materials other than SiO 2 (i.e., Fermi level pinning). The most popular explanation invokes metal-induced gap states (MIGS), where electron states from the bulk of a metal tails into the insulator. The MIGS hypothesis explains a rather large series of experimental results and, importantly, predicts that the MI barrier height will mostly be controlled by the energy distribution of electron states in the bulk of the contacting metal and dielectric. In this paper, we analyze the band alignment of contacting metal (TiN) and dielectric (HfO 2) by using internal photoemission. It will be shown that defects in the dielectric rather than MIGS control the barrier height.

Remote phonon and surface roughness limited universal electron mobility of In 0.53Ga 0.47As surface channel MOSFETs

The inversion layer electron mobility in n-channel In 0.53Ga 0.47As MOSFET’s with HfO 2 gate dielectric with several substrate impurity concentrations (∼1 × 10 16 cm −3 to ∼1 × 10 18 cm −3) and various surface preparations (HF surface clean, (NH 4) 2S surface clean and PECVD a-Si interlayer with a HfO 2 gate dielectric) have been studied. The peak electron mobility is observed to be strongly dependent on the surface preparation, but the high field mobility is observed to be almost independent of the surface preparation. A detailed analysis of the effective mobility as a function of electric field, substrate doping, and temperature was used to determine the various mobility components (surface roughness, phonon, and coulombic scattering limited mobility components). For the substrates with high doping concentration, the electron mobility at low vertical electric field is dominated by Coulomb scattering from the substrate dopants, whereas, for lower substrate doping the Coulombic scattering is dominated by the disorder induced gap states. Low temperature measurements were used to determine the surface roughness scattering and phonon components. The results show that room temperature mobility of In 0.53Ga 0.47As surface channel MOSFETs with HfO 2 gate dielectric at high electric field is limited primarily by remote phonons whereas the Al 2O 3 gate dielectric is limited by surface roughness scattering.

Au nanoparticle modified GaN photoelectrode for photoelectrochemical hydrogen generation

Systematic investigations of photoelectrochemical behavior between Au nano-particle modified n- and p-GaN were reported in this study. With Au nanoparticles sputtered on the surface, strong Fermi level pinning caused by the creation of metal induced gap states alters the behavior of electrolyte/GaN interface. Under illumination, the photocurrent of p-GaN at zero bias exhibited 25 times enhancement, whereas that of n-GaN showed slight decrease. The overall hydrogen generation efficiency of p-GaN in HCl solution was increased from 0.02% to around 0.59%. The enhancement can be attributed to the different energy shift of the surface band edge at the interface according to the doping of GaN.

The Adsorption Properties of Cu and Ni on the Ceria(111) Surface

First-principles electronic structures calculations of the adsorption properties of Cu and Ni on the ceria(111) surface are presented. The adatoms (Cu, Ni) are adsorbed strongly at the hollow site on the CeO2(111) support. Metal induced gap states (MIGS) appear in the O2p-Ce4f gaps and the Cu and Ni adatoms are oxidized to Cu+ and Ni+ mainly by their next nearest neighbor Ce ion, which experiences a conversion of Ce4+→Ce3+. The bonding mechanisms for the Cu-ceria(111) and Ni-ceria(111) systems are proposed.

Mechanism of the Fermi level pinning at organic donor–acceptor heterojunction interfaces

We investigate the energy level alignment and the Fermi level pinning mechanism at organic donor–acceptor heterojunctions interfaces by using the model organic–organic heterojunctions (OOHs) with well-defined molecular orientation of the standing copper (II) phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) films on the standing copper-hexadecafluoro-phthalocyanine (F16CuPc) thin films on SiO2. We identify two distinct regions for the energy level alignment by in situ ultraviolet photoelectron spectroscopy investigation. In region (I) where the work function (WF) of the underlying substrate is larger than the ionization potential (IP) of the top organic layers, the substrate Fermi level is pinned at the leading edge of the HOMO peak accompanied by a decreasing of the WF; in region (II) where the WF is smaller than the IP of the top organic layers, a downward shift of both the HOMO and vacuum level is observed. In connection with the defect induced gap states, we provide a detailed explanation for this thickness dependent energy level alignment and Fermi level pinning mechanism at the organic donor–acceptor OOH interface.

(Invited) Electron Spectroscopic Measurements of Band Alignment in Metal/Oxide/Semiconductor Stacks

Valence and conduction band edges of ultra-thin oxides (SiO2, HfO2, Hf0.7Si0.3O2 and Al2O3 grown on silicon) and their shifts upon sequential metallization with three metals (Ru, Ti and Al) have been measured using synchrotron radiation-excited x-ray photoemission, ultra-violet photoemission and inverse photoemission. From these techniques, the offsets between the valence and conduction band edges of the oxides and the metal gate Fermi edge have been directly measured. Upon metallization, consistent shifts of the oxides band edges and core levels are measured, due to the creation of interface dipoles at the metal/oxide interfaces. Using the energy gap, the electron affinity of the oxides and the metal workfunctions that have been directly measured on theses samples, the experimental band offsets are compared to those predicted by the induced gap states model.

The main factors influencing the O vacancy formation on the Ir doped ceria surface: A DFT+U study

The effects of Ir doping on the oxygen vacancy formation energy have been investigated using the DFT+U method, i.e., first-principles density functional theory calculations with the inclusion of the on-site Coulomb interaction. The main factors influencing the reducibility of Ir-doped ceria are studied carefully. It is found that, although the Ir doping induces gap states (MIGS) as do other noble metals (Pd, Pt, Rh), the structural relaxation (instead of the electronic structure relaxation) is the main factor responsible for the decrease of the oxygen vacancy formation energy, i.e., the Ir doping makes structural distortions much more exothermic for the reduced ceria.

Physical origin of dipole formation at high-k/SiO2 interface in metal-oxide-semiconductor device with high-k/metal gate structure

A physical model on dipole formation at high-k/SiO2 interface is proposed to study possible mechanism of flatband voltage (VFB) shift in metal-oxide-semiconductor device with high-k/metal gate structure. Dielectric contact induced gap states (DCIGS) on high-k or SiO2 side induced by high-k and SiO2 contact are assigned to dominant origin of dipole formation. DCIGS induced interface dipole is considered to cause VFB shift through charge transfer effect. Based on the proposed model, directions of dipoles at several high-k/SiO2 interfaces are predicted, and magnitudes of dipoles are approximately calculated. Both directions and magnitudes are in agreement with the reported results.

High-k Al 2O 3 MOS structures with Si interface control layer formed on air-exposed GaAs and InGaAs wafers

This paper attempts to realize unpinned high-k insulator–semiconductor interfaces on air-exposed GaAs and In 0.53Ga 0.47As by using the Si interface control layer (Si ICL). Al 2O 3 was deposited by ex situ atomic layer deposition (ALD) as the high-k insulator. By applying an optimal chemical treatment using HF acid combined with subsequent thermal cleaning below 500 °C in UHV, interface bonding configurations similar to those by in situ UHV process were achieved both for GaAs and InGaAs after MBE growth of the Si ICL with no trace of residual native oxide components. As compared with the MIS structures without Si ICL, insertion of Si ICL improved the electrical interface quality, a great deal both for GaAs and InGaAs, reducing frequency dispersion of capacitance, hysteresis effects and interface state density ( D it). A minimum value of D it of 2 × 10 11 eV −1 cm −2 was achieved both for GaAs and InGaAs. However, the range of bias-induced surface potential excursion within the band gap was different, making formation of electron layer by surface inversion possible in InGaAs, but not possible in GaAs. The difference was explained by the disorder induced gap state (DIGS) model.

Metal/III-V Schottky barrier height tuning for the design of nonalloyed III-V field-effect transistor source/drain contacts

In this work, we introduce a novel nonalloyed contact structure for n-GaAs and n-In0.53Ga0.47As by using single metals in combination with a thin dielectric to tune the effective metal/III-V work function toward the conduction band edge. We reduced the effective Schottky barrier height (ΦB,eff) of Al/GaAs from 0.75 to 0.17 eV through the use of a thin atomic layer deposition Al2O3. Barrier height reduction was verified for a variety of metals (Y, Er, Al, Ti, and W) through direct measurements and deduced from increased diode current and reduced contact resistance. Similar results were observed on n-In0.53Ga0.47As. Two possible underlying mechanisms are discussed: one based on the formation of a dielectric dipole and the other based on the blocking of metal induced gap states. This structure has applications as a nonalloyed low resistance ohmic contact for III-V metal-oxide-semiconductor field-effect transistors (MOSFETs) or high electron mobility transistors (HEMTs), and as a near zero barrier height contact for III-V Schottky barrier field-effect transistors or diodes.

Investigating the origin of Fermi level pinning in Ge Schottky junctions using epitaxially grown ultrathin MgO films

Fermi level (FL) pinning at the Ge valence band results in a high Schottky barrier height for all metal/n-Ge contacts. The origin of this pinning effect has been ascribed to either metal induced gap states or surface states arise from the native defects at the Ge surface, such as dangling bonds. The discrepancy in the reported results/explanations is mainly due to the lack of an explicit characterization of a high quality metal/Ge or metal/ultrathin oxide/Ge junction, which should be ideally single crystalline, atomically smooth and free of process-induced defects or intermixing. We report the Schottky characteristics of high quality metal/MgO/n-Ge junctions with the ultrathin MgO epitaxially grown on Ge. We find the depinning effect displays a weak dependence on the MgO thickness, indicating the interface states due to the native defects on Ge surface are likely to play the dominant role in FL pinning.

Schottky barrier height and conduction mechanisms in ferroelectric bismuth titanate

Band structure of ferroelectric bismuth titanate is calculated by first-principles computations under the framework of density functional theory. Using the metal induced gap state model, the Schottky barrier height on Pt electrode is estimated to be as high as 1.26 eV, which indicates that the Schottky effect may not be the dominant conduction mechanism in bismuth titanate. By further comparisons with the experimental data, we conclude that the leakage current behavior of bismuth titanate films is dominated by bulk limited conduction mechanisms and can be reduced by better processing conditions or doping.

Band alignment at metal–semiconductor and metal–oxide interfaces

AbstractThe mechanisms of Schottky barrier formation are reviewed from the metal‐induced gap state model to the universal defect model, and the chemical reaction model. The recent progress in understanding barrier heights and band offsets in Si – high dielectric constant oxide and metal high dielectric constant oxide systems is then discussed, and interesting they contain components of each model. The greater emphasis on understanding defect reactions has allowed us to separate the effects of intrinsic mechanisms, metal induced gap states (MIGS) and extrinsic mechanisms (defects).

Origin of Schottky Barrier Modification by Hydrogen on Diamond

The origin of the Schottky barrier modification on diamond (100) surfaces by hydrogen is studied based on simplified interface models with the first-principles calculation. It is revealed that the estimated S-factor is rather negative for the ideal contact with no interfacial dangling bonds, while it approaches 1 when the interface is terminated by just hydrogen. The negative S-factor is thought to come from the formation of interfacial chemical bonds between the metal and the diamond. It is suggested that hydrogen-termination prevents such interfacial bond formation and makes the general metal induced gap state theory work, as well as passivates the interfacial dangling bond states which are the strong sources of the pinning of the Schottky barrier height.

Influence of metallic coatings on the photoluminescence properties of ZnO nanowires

AbstractWe report on low‐temperature photoluminescence studies of ZnO nanowires coated with thin metallic films. For all analyzed metals (Al, In, Au, Ni, Cu), we find an increased relative intensity of the green deep‐level emission. This is accompanied by a significant reduction of the relative intensity of the surface exciton band. The observed effects are most likely related to the formation of metal induced gap states in the surface region of the ZnO nanowires. A model for the band structure in the surface region of the metal‐coated nanowires is proposed that successfully explains the changes in the photoluminescence spectra after the coating process. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrathin K∕p-Si(001) Schottky diodes as detectors of chemically generated hot charge carriers

Ultrathin potassium layers were deposited on hydrogen passivated Si(001)−1×1 surfaces at a temperature of 120K in the thickness range from submonolayers up to 3nm. They were investigated with Auger spectroscopy, work function, and current-voltage measurements. After the formation of a wetting layer, potassium deposition leads to island growth. The surface hydrogen atoms are removed by the adsorption process. By attaching an electrical contact to the thin film, the current-voltage characteristics of the Schottky diodes could be determined. The analysis yields a homogeneous Schottky barrier height of 0.55eV for K∕p-Si(001) diodes in agreement with the metal induced gap state model. Exposing the diodes to molecular oxygen generates a strong chemicurrent signal which first increases with exposure, passes a maximum, and drops exponentially. The chemicurrent transient is attributed to a nucleation and growth formation of oxide islands and gives strong evidence for the existence of precurser states prior to oxidation.

Inhomogeneous Ni/Ge Schottky barriers due to variation in Fermi-level pinning

To achieve high performance Ge nMOSFETs it is necessary to reduce the metal/semiconductor Schottky barrier heights at the source and drain. Ni/Ge and NiGe/Ge Schottky barriers are fabricated by electrodeposition using n-type Ge substrates. Current ( I)–voltage ( V) and capacitance ( C)–voltage ( V) and low temperature I– V measurements are presented. A high-quality Schottky barrier with extremely low reverse leakage current is revealed. The results are shown to fit an inhomogeneous barrier model for thermionic emission over a Schottky barrier. A mean value of 0.57 eV and a standard deviation of 52 meV is obtained for the Schottky barrier height at room temperature. A likely explanation for the distribution of the Schottky barrier height is the spatial variation of the metal induced gap states at the Ge surface due to a variation in interfacial oxide thickness, which de-pins the Fermi level.

Quantum Nature of Two-Dimensional Electron Gas Confinement atLaAlO3/SrTiO3Interfaces

We perform density functional calculations to understand the mechanism controlling the confinement width of the two-dimensional electron gas (2DEG) at LaAlO_{3}/SrTiO_{3} interfaces. We find that the 2DEG confinement can be explained by the formation of metal induced gap states (MIGS) in the band gap of SrTiO3. These states are formed as the result of quantum-mechanical tunneling of the charge created at the interface due to electronic reconstruction. The attenuation length of the MIGS into the insulator is controlled by the lowest-decay-rate evanescent states of SrTiO3, as determined by its complex band structure. Our calculations predict that the 2DEG is confined in SrTiO3 within about 1 nm at the interface.

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.