Existence Of Interface States Research Articles

Analysis of admittance measurements of Al/Gr-PVA/p-Si (MPS) structure

A metal-polymer-semiconductor (MPS) structure based on 5 wt% graphene (Gr) doped polyvinyl alcohol (PVA) thin film was fabricated by using the electrospinning method. Admittance (Y = G + iωC) based measurements of Al/Gr-PVA/p-Si (MPS) structure were performed in the frequency range from 5 kHz to 5 MHz at room temperature. Significant shifts in capacitance (C) and conductance (G) characteristics were observed for the MPS structure. The C and G value decrease with increase in frequency. The variation of C and G with frequency comes from the existence of interface states. the interface state density (Nss) was estimated by various methods such as low-high frequency capacitance (CLF-CHF), Hill-Coleman and conductance. The Nss value obtained from all methods decreased as the frequency increased. Because as the frequency increases, the interface states cannot follow alternating current (ac) signal. Meanwhile, the measured capacitance and conductance were corrected taking into account the effect of series resistance (Rs).

Interface states and bound states in the continuum in photonic crystals with different lattice constants.

The existence of interface states at the boundary of two semi-infinite photonic crystals (PhCs) with different lattice constants are investigated systematically. Compared to the interface states in the two PhCs with the same period, a band folding effect is observed for the interface states inside the common band gap of the two PhCs with different lattice constants. We demonstrate that these interface states can be predicted by the surface impedance of the two PhCs. The dispersion of interface states can be determined by the condition of impedance matching combined with the band folding effect. Moreover, some part of the folded interface states penetrates the region of projected bulk bands, and they usually leak to the bulk and form resonant states. However, the interface state at the Γ point can be perfectly localized and becomes a bound state in the continuum (BIC) due to the symmetry mismatch. These findings may provide a general scheme for designing BICs in the PhC structures based on the interface states.

Analysis of Electrical Characteristics of Pd/n-Nanocarbon/p-Si Heterojunction Diodes: By C-V-f and G/w-V-f

Diamond films are candidate for a wide range of applications, due to their wide band gap, high thermal conductivity, and chemical stability. In this report, diamond-based heterojunction diodes (HJDs) were fabricated by growing n-type nanocarbon composite in the form of nitrogen-doped ultrananocrystalline diamond/amorphous carbon (UNCD/a-C:H:N) films onto p-type Si substrates. X-ray photoemission and the Fourier transform infrared spectroscopies were employed to examine the contribution of nitrogen atoms from the gas phase into the deposited films. The results indicate the incorporation of nitrogen atoms into the grain boundaries of UNCD/a-C:H film by replacing hydrogen atoms. The capacitance- (C-V-f), conductance- (G/ω-V-f), and series resistance-voltage characteristics of the fabricated Pd/n-(UNCD/a-C:H:N)/p-Si HJDs were studied in the frequency range of 40 kHz-2 MHz. The existence of interface states (Nss) and series resistance (Rs) were attributed to the interruption of the periodic lattice structure at the surface of the fabricated junction as well as the defects on the (UNCD/a-C:H:N)/Si interface. By increasing the frequency (≥500 kHz), the Nss reveals an almost frequency-independent behavior, which indicates that the charges at the interface states cannot follow ac signal at higher frequency. The obtained results demonstrated that the UNCD/a-C:H:N is a promising n-type semiconductor for diamond-based heterostructure diodes.

Designing topological interface states in phononic crystals based on the full phase diagrams

The topological invariants of a periodic system can be used to define the topological phase of each band and determine the existence of topological interface states within a certain bandgap. Here, we propose a scheme based on the full phase diagrams, and design the topological interface states within any specified bandgaps. As an example, here we propose a kind of one-dimensional phononic crystals. By connecting two semi-infinite structures with different topological phases, the interface states within any specific bandgap or their combinations can be achieved in a rational manner. The existence of interface states in a single bandgap, in all odd bandgaps, in all even bandgaps, or in all bandgaps, are verified in simulations and experiments. The scheme of full phase diagrams we introduce here can be extended to other kinds of periodic systems, such as photonic crystals and designer plasmonic crystals.

Electrical characterization of two analogous Schottky contacts produced from N-substituted 1,8-naphthalimide.

The aim of this study was to analyze the interface states (Nss) in pure Al//p-Si/Al, Al/N-F Nft/p-Si/Al and Al/N-T Nft/p-Si/Al Schottky barrier diodes (SBDs). N-Substituted 1,8-naphthalimide thin films were deposited on a p-Si substrate by spin coating and annealed at ∼200 °C for 60 s under an air atmosphere. Al contacts were obtained via reactive magnetron sputtering. The current voltage (I-V) characteristics of the SBDs were measured at room temperature. From the I-V characteristics, the SBDs ideality factor (n) and zero-bias barrier height values (Φb) of 1.27, 1.00, and 1.05 and 0.66 eV, 0.70 eV, and 0.64 eV were observed for the Al//p-Si/Al, Al/N-F Nft/p-Si/Al and Al/N-T Nft/p-Si/Al Schottky barrier diodes, respectively. The interface state density distribution profile (Nss) as a function of (Ess-Ev) was extracted from the forward-bias I-V measurements by considering the effective barrier height and (Φe) and series resistance (Rs) of the Schottky diode. The obtained Nss plot tendency showed that the existence of interface states has no significant effect on the rectifying and capacitance characteristics. The Nss values with the 1,8-naphthalimide layer were lower than that without it. This shows that naphthalimide exhibits a strong contribution by blocking the unwanted states and some traps in the conduction mechanism, which may cause possible cracks or deep paths for carriers to travel along the junction.

Topological Subspace-Induced Bound State in the Continuum

We propose and experimentally realize a new kind of bound states in the continuum (BICs) in a class of systems constructed by coupling multiple identical one-dimensional chains, each with inversion symmetry. In such systems, a specific separation of the Hilbert space into a topological and a nontopological subspace exists. Bulk-boundary correspondence in the topological subspace guarantees the existence of a localized interface state which can lie in the continuum of extended states in the nontopological subspace, forming a BIC. Such a topological BIC is observed experimentally in a system consisting of coupled acoustic resonators.

Nanometer Thick Diffused Hafnium and Titanium Oxide Light Sensing Film Structures

We examine 10 nm thick film structures containing either Hf or Ti sandwiched between two respective oxide layers. The layers are deposited onto heated substrates to create a diffusion region. We observe a high degree of light sensitivity of the electric current through the film thickness for one polarity of an applied voltage. For the other polarity, the current is not affected by the light. We explain the observed phenomenology using the single-particle model based on the existence of interface states on the metal-oxide interfaces.

Electrical and photoresponse properties of Al/graphene oxide doped NiO nanocomposite/p-Si/Al photodiodes

The electrical and photoconducting properties of Al/graphene oxide doped NiO nanocomposite/p-Si/Al photodiodes with various graphene oxide contents were investigated by using current–voltage, transient photocurrent, photocapacitance and photoconductance measurements at various illumination intensities in the range of 10–100 mW/cm2. Graphene oxide doped NiO nanocomposite thin films were prepared by sol–gel spin method. Experimental results indicate that the reverse current of the photodiodes increases with the increasing illumination intensity. The value of transient photocurrent, photocapacitance and photoconductance measured as a function of time increases after illuminating and returns to original value after turning off the illumination. In addition, the frequency-dependent capacitance and conductance measurements was performed to indicate the existence of interface states. The obtained results suggest that the fabricated diode can be used as a photodiode in optoelectronic applications.

Photodiode and photocapacitor properties of Au/CdTe/p-Si/Al device

The electrical and photoconductivity properties of CdTe on p-type silicon photodiode were investigated by current–voltage (I–V) and capacitance time (C–t) measurements. The current–voltage (I–V) and capacitance–time(C–t) characteristics of the fabricated photodiode were recorded at various illumination intensities in the range of 10–100 mW/cm2. It is observed that the reverse current of the photodiode increases with increase in illumination intensity. The value of transient photocurrent, photocapacitance and photoconductance increases after illuminating and returns to original value after turning off the illumination. The frequency dependence of the C and G indicates the existence of interface states at metal-semiconductor interface. The obtained results suggest that the fabricated photodiode can be used for photovoltaic and optoelectronic applications.

Dirac-like cones at k=0

Dirac cones and Dirac points are found at the K (K') points in the Brillouin zones of electronic and classical waves systems with hexagonal or triangular lattices. Accompanying the conical dispersions, there are many intriguing phenomena including quantum Hall effect, Zitterbewegung and Klein tunneling. Such Dirac cones at the Brillouin zone boundary are the consequences of the lattice symmetry and time reversal symmetry. Conical dispersions are difficult to form in the zone center because of time reversal symmetry, which generally requires the band dispersions to be quadratic at k=0. However, the conical dispersions with a triply degenerate state at k=0 can be realized in two dimensional (2D) photonic crystal (PC) using accidental degeneracy. The triply degenerate state consists of two linear bands that generate Dirac cones and an additional flat band intersecting at the Dirac point. If the triply degenerate state is derived from the monopolar and dipolar excitations, effective medium theory can relate this 2D PC to a double zero-refractive-index material with effective permittivity and permeability equal to zero simultaneously. There is hence a subtle relationship between two seemingly unrelated concepts: Dirac-like cone and zero-refractive index. The all-dielectric double zero-refractive-index material has advantage over metallic zero-index metamaterials which are usually poorly impedance matched to the background and are lossy in high frequencies. The Dirac-like cone zero-index materials have impedances that can tune to match the background material and the loss is small as the system has an all-dielectric construction, enabling the possibility of realizing zero refractive index in optical frequencies. The realization of Dirac-like cones at k=0 can be extended from the electromagnetic wave system to acoustic and elastic wave systems and effective medium theory can also be applied to relate these systems to zero-index materials. The concept of Dirac/Dirac-like cone is intrinsically 2D. However, using accidental degeneracy and special symmetries, the concept of Dirac-like point can be extended from two to three dimensions in electromagnetic and acoustic waves. Effective medium theory is also applicable to these systems, and these systems can be related to isotropic media with effectively zero refractive indices. One interesting implication of Dirac-like cones in 2D PC is the existence of robust interface states. The existence of interface states is not a trivial problem and there is usually no assurance that localized state can be found at the boundary of photonic or phononic crystal. In order to create an interface state, one usually needs to decorate the interface with strong perturbations. Recently, it is found that interface state can always be found at the boundary separating two semi-infinite PCs which have their system parameters slightly perturbed from the Dirac-like cone formation condition. The assured existence of interface states in such a system can be explained by the sign of the surface impedance of the PCs on either side of the boundary which can be derived using a layer-by-layer multiple scattering theory. In a deeper level, the existence of the interface state can be accounted for by the geometric properties of the bulk band. It turns out that the geometric phases of the bulk band determine the surface impedance within the frequency range of the band gap. The geometric property of the momentum space can hence be used to explain the existence of interface states in real space through a bulk-interface correspondence.

Electrical characterization of wafer-bonded Ge(111)-on-insulator substrates using four-point-probe pseudo-metal-oxide-semiconductor field-effect transistor method

The electrical properties of wafer-bonded n-type Ge(111)-on-insulator (Ge(111)-OI) substrates were characterized using a four-point-probe pseudo-metal-oxide-semiconductor field-effect transistor (pseudo-MOSFET) method. Average electron and hole mobilities in the Ge(111)-OI channel were measured to be ~1000cm2/Vs in accumulation mode and ~310cm2/Vs in inversion mode, respectively. The measured mobility strongly depended on the sample position, due to the spatially inhomogeneous distribution of the interface states. Despite the existence of interface states, the carrier mobility exhibited a high value demonstrating the prospect of wafer-bonded Ge(111)-OI as a channel material in MOSFETs.

Temperature and voltage dependent current-transport mechanisms in GaAs/AlGaAs single-quantum-well lasers

The quantum-well (QW) lasers are the most important optoelectronic devices in many application fields and the current-transport mechanisms (CTMs) are strongly depend on temperature and voltage. Therefore, we have examined the samples in the temperature range of 80–360 K. The electrical parameters such as saturation current ( I o), zero-bias barrier height ( Φ Bo), ideality factor ( n) and series resistance ( R s), were obtained from the forward bias I– V data. The n and Φ Bo values change from 3.49 and 1.313 eV (at 80 K) to 1.73 and 0.492 eV (at 360 K), respectively. These values of n indicate that the thermionic field emission (TFE) mechanism is dominated the others, particularly at low temperature (80–170 K). High values of n were attributed to image force lowering of the barrier height, tunneling current at low temperature, generation-recombination current within the space charge region and the existence of interface states at metal/semiconductor interface. The Φ Bo values do not have to be absolutely correct, especially when there are more CTMs present in a given temperature interval. Also, n and R s were found to be extremely dependent on the contributions of particular CTMs to the total current; the ratio of which to the TE theory increases with the increasing temperature. The Φ Bo and activation energy ( E a) values decrease with the linearly increasing temperature and voltage. In addition, double-logarithmic plots of the forward bias I– V characteristics have shown two distinct linear regions in the whole temperature range. SCLC mechanism may be dominated CTMs especially at high temperature regime namely regime II.

Electrode-dependent electrical properties of metal/Nb-doped SrTiO3 junctions

In this study, we discuss the electrical properties of junctions consisting of metal electrodes and Nb-doped SrTiO3(001) single crystals. The junctions formed with large work function metals (Ni, Au, Pd, and Pt) resulted in rectifying transport. A hysteretic feature was observed in the current (I)-voltage (V) and capacitance (C)-V characteristics of these junctions upon polarity reversal. The ideal Schottky–Mott rule could not explain the barrier height obtained from the I-V data, indicating the existence of interface states. Analyses of the C-V data revealed that a low dielectric constant layer existed at the interface. The interface states and layers affected the transport and the related resistance switching characteristics of the junctions.

The impact of static and dynamic degradation on SOI “smart-cut” floating body MOSFETs

The impact of static (DC) and dynamic (AC) degradation on SOI “smart-cut” floating body MOSFETs, was investigated by means of deep level transient spectroscopy (DLTS). The study was based on drain current signal recording, immediately after the transistor transition from OFF- to ON-state. In order to isolate the activity of capture/emission carrier mechanisms, undesirable parasitic effects such as drain current overshoot were suppressed by appropriately biasing the transistor substrates. Under DC degradation regime, DLTS spectra disclosed that carrier capture/emission process occurred through discrete traps governed by thermally activated mechanisms. Furthermore, polarization phenomena emerged. Under AC degradation regime, although the existence of interface states in Si-SiO2 interface was dominant, the revelation of shallow traps in low temperature domain was also monitored.

Role of interface bonding in spin-dependent tunneling (invited)

Measured positive values of the spin polarization of the tunneling current from 3d ferromagnetic metals are commonly explained by the dominant s-electron contribution based on symmetry considerations for bulk materials, ignoring the influence of the interfaces. In this work, three different models are considered which suggest that the spin polarization is primarily determined by the electronic and atomic structures of the ferromagnet/insulator interfaces rather than by the bulk properties. A simple tight-binding model demonstrates that the existence of interface states and their contribution to the tunneling current depend on the degree of hybridization between the orbitals on metal and insulator atoms. The decisive role of the interface bonding is further supported by considering spin-dependent tunneling from oxidized Co surfaces through vacuum and in Co∕Al2O3∕Co tunnel junctions within the first-principles Green’s-function approach. For the oxidized Co surface it is found that the Co–O bonding at the surface removes the conducting orbitals forming the bulk Bloch states from the Fermi level, creating an additional tunneling barrier for minority-spin electrons. For the Co∕Al2O3∕Co junctions, two types of the interface O atoms are distinguished: those which saturate Al bonds and those which are adsorbed by Co. The latter bind strongly to Co creating interface states which enhance the tunneling current in the majority-spin channel. In both cases, the spin polarization changes sign and becomes positive, evidencing the crucial role of the interface structure and bonding.

Some properties of Au/n-CdTe Schottky barriers as established by I–V characteristics and photocurrent spectroscopy

A study of the electric and photoelectric behaviour of electrodeposited Au/n-CdTe Schottky diodes in a wide temperature range is presented. The dependence of quantum efficiency on photon energy, temperature, and bias is used to determine the Schottky barrier height, the built-in voltage, the space charge layer width, the field strength and the bulk carrier concentration. The built-in voltage is strongly temperature dependent. At temperatures above 100 K there is no surface inversion. Several arguments indicate the existence of interface states at the Au/CdTe interface. All information is combined to establish the band diagram of the structure and to determine parameters relevant for solar-cell applications.

An Approach to Study Band-gap and Interface States in a-Si:H p-i-n Solar Cells

The defect states in bulk of i-layer and at p/i interface and their energy locations in a-Si:H p-i-n solar cells have been studied by using dark reverse current–voltage ( J–V ), capacitance–voltage–temperature–frequency (C–V–T–ω) and deep level transient spectroscopy (DLTS) measurements. The dark reverse currents as a function of voltage have been analyzed on the basis of thermal generation of the carriers. In current–voltage characteristics, it has been observed that at low reverse voltages (V<5 V) the defect states in the bulk of the i-layer and at high reverse voltages (V∼24 V) the defect states at p/i interface are contributing to the reverse current. Reverse bias annealing (RBA) treatment has been performed on these cells. It has been observed that RBA reduces the defect states more in the i-region near to the p-layer and at the p/i interface as compared to the deep regions in the bulk of i-layer. The calculated defect state density (DOS) is varying from its intrinsic value of 4.8×1016 cm-3 in the bulk of i-layer upto 1.8×1018 cm-3 near and at p/i interface. These values decrease to 1.8×1016 cm-3 and 6.6×1016 cm-3, respectively, in the samples annealed under reverse bias at 2 V. The capacitance as a function of temperature under fixed voltage of 0.5 V and frequency of 100 Hz is divided into three regions depend on the capacitance behavior. The RBA treatment has showed prominent changes only in the third region (T>350 K) which is due to the defects at p/i interface. The DOS in the bulk of i-layer and at p/i interface have been determined. The hole and electron trap states densities have been estimated to be 1.8×1016 eV-1 cm-3 and 1.6×1016 eV-1 cm-3, respectively, at 0.8 eV and 1.12 eV above the valence band from the DLTS measurements. The existence of interface states at 0.55 eV and 1.2 eV above the valence band, respectively, for p/i and i/n interfaces have also been observed. A good correlation has been observed in DOS estimation and their affect by RBA treatment in all the studies.

Electronic Structures of Porphyrins and their Interfaces with Metals Studied by UV Photoemission

The electronic structures of porphyrins and their interfaces with metals were investigated by UV photoemission spectroscopy (UPS). The UPS spectral features of porphine, 5,10,15,20-tetraphenylporphynatozinc(II) (ZnTPP), 5,10,15,20-tetra(4-pyridyl)porphyrin (H2T(4-Py)P) and 5,10,15,20-tetraphenylporphyrin (H2TPP) could be assigned by comparison with MOPAC PM3 calculations. The electronic structures of the porphyrins can be regarded as the superposition of those of porphine and the substituents. The UPS results for ZnTPP/metal (Mg, Al, Ag, Au) interfaces indicated that the energies of the levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface. The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. The level positions of ZnTPP at the interface exhibited drastic change by exposure to oxygen. This change could be explained by the change of the work function of metal surface upon oxidization. For H2T(4-Py)P and H2TPP/metal (Mg, Ag, Au) interfaces, similar linearity was observed between the energy levels of porphyrin and the work function of metal with the slope of much smaller than unity. This deviation of the slope from unity might be explained by the existence of interface state. These findings require the modification of the traditional concept of a common vacuum level at organi/metal interface. The relation between the observed electronic structures and the conduction type of the porphyrins are also discussed.

The electronic structure of porphyrin/metal interfaces studied by UV photoemission spectroscopy

The electronic structures of the interface between porphyrin and metal (Mg, Al, Ag and Au) were studied by UV photoemission spectroscopy (UPS). The samples were 5,10,15,20-tetraphenylporphynatozinc(ZnTPP), 5,10,15,20-tetra(4-pyridyl) porphyrin(H 2T(4-Py)P) and 5,10,15,20-tetra-phenylporphyrin(H 2TPP) which are known as organic semiconductors. We found that the energy levels of these porphyrins relative to the Fermi level of the substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface (Δ). This indicates that the energy levels of these porphyrins are fixed to the vacuum level of substrate metal with an interfacial dipole layer. We also found that the magnitude of Δ is constant at ZnTPP/metal interfaces, but depends on the work function of the metal at H 2T(4-Py)P and H 2TPP/metal interfaces. This work function dependence of Δ might be explained by the existence of interface state.

The electronic structure of porphyrin/metal interfaces studied by ultraviolet photoelectron spectroscopy

Electronic structure of interfaces between 5,10,15,20-tetraphenylporphynatozinc (ZnTPP) and four metals (Mg, Al, Ag, and Au) were studied by ultraviolet photoelectron spectroscopy (UPS). The energy levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with the shift of the vacuum level at interface (Δ). The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. 5,10,15,20-Tetra(4-pyridyl)porphyrin (H 2T(4-Py)P)/metal and 5,10,15,20-tetraphenylporphyrin (H 2TPP)/metal interfaces were also investigated, and similar linearity was observed between the energy levels of porphyrin and the work function of metal with the slope of much smaller than unity. This deviation of the slope from unity might be explained by the existence of interface state.