Interface States In Metal Research Articles

Reliable and precise determination of interface states in metal–insulator–polymeric semiconductors devices

The major objective of this manuscript is to develop an integrable capacitance-voltage (C-V) model for the extraction of the interfacial states at p-type polymeric semiconductor-insulator-metal (MIPS) capacitors. A polymeric P3HT (3-hexylthiophene) semiconductor-insulator-metal device was fabricated, firstly. Then, an integrable C-V model was developed. The model was based on Fermi-Dirac distribution for holes and experimental C-V of the p-type polymeric MIS. By the least square nonlinear curve fitting, good matches are presented between theoretical and experimental C-V plots for our p-type P3HT MIS capacitor. With nonlinear regression technique, the capacitance of polymeric MIS not only can be explained via voltage, but also can be predicted by frequency. Hence, the static C-V of polymeric MIS was successfully extracted and therefore the surface state density of polymeric MIS was evaluated via quasi-static technique. Because the suggested model is integrable, surface potential and the interfacial state density can be obtained with reliable, precise and simple method and over whole range of the energy gap on polymeric MIS in explicit functional form for the first time. The linear and non-linear electric potential were determined by applying integral of C-V model over whole voltage. The amount of the charge in channel region was determined by integrating model of C-V over all surface potential. This C-V model has been used to detect the charge on per unit area in channel too. Evidence is presented suggesting that more electrical information of polymeric MIS can be obtained via analysis on C-V characteristics using the suggested model.

RF compressibility of topological surface and interface states in metal–hBN–Bi2Se3 capacitors

The topological state that emerges at the surface of a topological insulator (TI) and at the TI-substrate interface are studied in metal–hBN–Bi2Se3 capacitors. By measuring the RF admittance of the capacitors versus gate voltage, we extract the compressibility of the Dirac state located at a gated TI surface. We show that even in the presence of an ungated surface that hosts a trivial electron accumulation layer, the other gated surface always exhibits an ambipolar effect in the quantum capacitance. We succeed in determining the velocity of surface Dirac fermions in two devices, one with a passivated surface and the other with a free surface that hosts trivial states. Our results demonstrate the potential of RF quantum capacitance techniques to probe surface states of systems in the presence of a parasitic density-of-states.

Correlation-Driven Dimerization and Topological Gap Opening in Isotropically Strained Graphene.

The phase diagram of isotropically expanded graphene cannot be correctly predicted by ignoring either electron correlations, or mobile carbons, or the effect of applied stress, as was done so far. We calculate the ground state enthalpy (not just energy) of strained graphene by an accurate off-lattice quantum MonteCarlo correlated ansatz of great variational flexibility. Following undistorted semimetallic graphene at low strain, multideterminant Heitler-London correlations stabilize between ≃8.5% and ≃15% strain an insulating Kekulé-like dimerized (DIM) state. Closer to a crystallized resonating-valence bond than to a Peierls state, the DIM state prevails over the competing antiferromagnetic insulating state favored by density-functional calculations which we conduct in parallel. The DIM stressed graphene insulator, whose gap is predicted to grow in excess of 1eV before failure near 15% strain, is topological in nature, implying under certain conditions 1D metallic interface states lying in the bulk energy gap.

Modification of the Terman method for determination of interface states in metal–insulator–semiconductor structures

In this work a modification of Terman method for determination of interfacial energy states located in the forbidden energy gap of the semiconductor (interface states) convenient for application in metal—high-κ—Si structures is proposed and explained in details. It has been found that usually observed presence of tails in the distribution of interface states towards the band edges is an artifact due to the method of determination of interface state densities. While excluding the contribution of quantum charge in semiconductor close to the interface with dielectric, these tails in the distribution disappear. Simple efficient method for determination of total density of interface states over the silicon bandgap is proposed.

Tailoring the Doping Mechanisms at Oxide Interfaces in Nanoscale.

Here, we demonstrate the nanoscale manipulations of two types of charge transfer to the LaAlO3/SrTiO3 interfaces: one from surface adsorbates and another from oxygen vacancies inside LaAlO3 films. This method can be used to produce multiple insulating and metallic interface states with distinct carrier properties that are highly stable in air. By reconfiguring the patterning and comparing interface structures formed from different doping sources, effects of extrinsic and intrinsic material characters on the transport properties can be distinguished. In particular, a multisubband to single-subband transition controlled by the structural phases in SrTiO3 was revealed. In addition, the transient behaviors of nanostructures also provided a unique opportunity to study the nanoscale diffusions of adsorbates and oxygen vacancies in oxide heterostructures. Knowledge of such dynamic processes is important for nanodevice implementations.

Two symmetric n-type interfaces SrTiO3/LaAlO3 in perovskite: Electronic properties from density functional theory

The first principles study of the (001) two symmetric n-type interfaces between two insulating perovskites, the nonpolar SrTiO3 (STO), and the polar LaAlO3 (LAO) was performed. We have analyzed the formation of metallic interface states between the STO and LAO heterointerfaces by using the all-electron full-potential linearized augmented plane-wave approach based on the density functional theory, within the local density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA), and the Engel-Vosko GGA (EVGGA) formalism. It has been found that some bands cross the Fermi energy level (EF), forming a metallic nature of two symmetric n-type 6.5STO/1.5LAO interfaces with density of states at EF, N(EF) of about 3.56 (state/eV/unit cell), and bare electronic specific heat coefficient (γ) of about 0.62 mJ/(mol cell K2). The electronic band stature and the partial density of states in the vicinity of EF are mainly originated from Ti1,2,3,4-3dxy orbitals. These bands are responsible for the metallic behavior and the forming of the Fermi surface of the two symmetric n-type 6.5STO/1.5LAO interfaces. To obtain a clear map of the valence band electronic charge density distribution of the two symmetric n-type 6.5STO/1.5LAO interfaces, we have investigated the bond's nature and the interactions between the atoms. It reveals that the charge is attracted towards O atoms as it is clear that the O atoms are surrounded by uniform blue spheres which indicate the maximum charge accumulation.

Determination of interface states in metal(Ag,TiN,W)−Hf:Ta2O5/SiOxNy−Si structures by different compact methods

In this work a compact set of analytical methods for determination of the interface state densities of metal-insulator-silicon structures containing ultrathin dielectrics is constructed (high-low method, conductance method using Gps/ω–ω curves, conductance method using Gps/ω–V curves and Terman׳s method). Specific structures metal(Ag,TiN,W)−Hf:Ta2O5/SiOxNy−Si are studied in details.It has been found that the serial measurement mode is more suitable for use than the parallel one. Obtained capacitance values in serial mode can be used in majority of the cases without further corrections, if very high precision is not required. The proposed correction method for serial resistance gives substantial improvements for Gps values.At midgap all four considered methods give similar values. Terman׳s method appears to give substantially overestimated values of Dit for energies other than that of the midgap, owing to the charging of the internal interface between the higk-k layer and the interfacial SiOxNy layer.

The metallic interface between the two band insulators LaGaO3 and SrTiO3

The formation of metallic interface states between the two band insulators LaGaO3 and SrTiO3 is studied by the full-potential linearized augmented plane-wave method based on density functional theory. Structural optimization of the atomic positions points to only small changes of the chemical bonding at the interface. The n-type (LaO/TiO2) and p-type (GaO2/SrO) interfaces turn out to be metallic. Reduction of the O content increases the conductivity of the n-type interface, while the p-type interface can be turned gradually from a hole doped into an electron doped state.

Charge transfer mechanism for the formation of metallic states at the KTaO3/SrTiO3interface

The electronic and optical properties of the KTaO${}_{3}$/SrTiO${}_{3}$ heterointerface are analyzed by the full-potential linearized augmented plane-wave approach of density functional theory. Optimization of the atomic positions points at subordinate changes in the crystal structure and chemical bonding near the interface, which is due to a minimal lattice mismatch. The creation of metallic interface states thus is not affected by structural relaxation but can be explained by charge transfer between transition metal and oxygen atoms. It is to be expected that a charge transfer is likewise important for related interfaces such as LaAlO${}_{3}$/SrTiO${}_{3}$. The KTaO${}_{3}$/SrTiO${}_{3}$ system is ideal for disentangling the complex behavior of metallic interface states, since almost no structural relaxation takes place.

Impact of Interfacial Oxygen Content on Bonding, Stability, Band Offsets, and Interface States of GaAs:HfO2 Interfaces

A theoretical study of the structural and electronic properties of GaAs:HfO2 interface is performed using the density functional theory method. For the interfaces with Ga−O bonds (formed between Ga-terminated GaAs surface and O-terminated HfO2 surface), we found that the interfaces with 10% and 20% interfacial oxygen removed are thermodynamically stable over a wide range of O chemical potential. By gradually decreasing the interfacial O content from 100% to 30% (by changing O chemical potential corresponding to varying the growth condition), the valence band offset increases from 1.06 to 3.34 eV. The analysis of the electronic structures indicates that for interfaces with high interfacial oxygen content, the interface gap states are induced by interfacial Ga dangling bonds and As−As dimer pairs, which are formed by interface atomic structure reconstruction. The decreasing interfacial oxygen content causes the decrease of the charge states of interfacial Hf and Ga leading to metallic interface states. For interfaces with low interfacial oxygen content, we found that As−As, Ga−Ga dimer pairs, and Ga and As dangling bonds are also formed and contribute to the interface gap states.

Improving the Accuracy in Determining the Insulator Capacitance in Metal–Insulator–Semiconductor Structures

The results of the voltage–capacitance spectroscopy of interface states in metal–insulator–semiconductor (MIS) structures are critical functions of the accuracy in determining the insulator capacitance C i, which is typically no higher than a few fractions of a percent. This substantially limits the energy range of the observed spectrum of the interface states (ΔE ≅ 0.5 eV for Si-based MIS structures) and the sensitivity to the density of the interface states at the spectrum edges (ΔN ss ≅ 1 × 1010 cm–2 eV–1). We propose a method for minimizing these errors that is based on a sequential variation of the initial estimate C i ≅ C i 0 → C ij, j = 0, 1, 2, ... and the identification of singular points in the dependences $$\overline {\delta V_g^{ac,in} } $$ and $$\overline \delta _{ac,in} $$ on C ij, where $$\overline {\delta V_g^{ac,in} } $$ are the mean arithmetic values of the voltage difference between the experimental and ideal voltage–capacitance characteristic and $$\overline \delta _{ac,in} $$ are the rms deviations of the voltage $$\overline {\delta V_g^{ac,in} } $$ values taken in the high-accumulation (ac) and inversion (in) regions from $$\delta V_g^{ac,in} $$ values. The highest (≅10–4%) accuracy in determining C i is achieved in the regions of the equidistant experimental and ideal voltage–capacitance characteristic. This method, combined with the technique of Ψ s ′/Ψ s diagrams, ensures an extension of ΔE to ≅0.9 eV at ΔN ss ≅ 1 × 1010 cm–2 eV–1 and the possibility of determining the sign and density of the fixed charge in the gate insulator.

Interface state measurements by the DLS-82E lock-in spectrometer

A calculation procedure to determine the energy distribution of interface states from the deep level transient spectroscopy spectrum measured in metal–insulator–semiconductor structures by means of the DLS-82E lock-in spectrometer is described. Interface states in metal–boron nitride–InP and metal–oxide–Si capacitors have been examined by the DLS-82E spectrometer to demonstrate the practical application of the proposed procedure.

U-shaped distribution of interface states in metal-(Langmuir-Blodgett) Si structures with phthalocyanine Langmuir-Blodgett films

A considerable part of the effort aimed at practical applications of Langmuir-Blodgett (LB) films is based on metal-insulator-semiconductor structures with an incorporated LB layer as the insulator. In accordance with this trend the present communication deals with a study of the metal-LB-Si structures with thin copper tetra-4- t-butylphthalocyanine films deposited by the conventional LB technique onto silicon substrates (ϱ ≈ 5 Ω cm). The experimental methods used were deep-level transient spectroscopy in a charge-modification mode and a feedback charge C-U method. The analysis of the results indicates the existence of energy states at the LB-Si interface. The states form a continuous U-shaped spectrum in the gap of the semiconductor. The high density of the states (about 10 12eV −1 cm −2) is typical of the “wet” technologies to which LB deposition belongs.