Deformation of a charge density wave in the vicinity of a point contact with a normal metal

The current-voltage characteristics of Cu-K_0.3MoO₃ point contacts between a metal and a semiconductor with a charge density wave (CDW) are studied for various diameters of the contacts in a wide range of temperatures T and voltages V. In the interval 80 K ? T ? 150 K, the current-voltage characteristics are correctly described in the framework of a semiconductor model: screening of an external electric field causes CDW deformation, shifts the chemical potential of quasiparticles, and changes the point contact resistance. It is shown that the chemical potential is above the middle of the Peierls gap in equilibrium and approaches the middle upon an increase in temperature. The current-voltage characteristics of point contacts with a diameter d ? 100 Å exhibit a sharp decrease in resistance for |V| > V _t , which is associated with the beginning of local CDW sliding within the contact region. The V _t (d, T) dependence can be explained by the size effect in the CDW phase slip.     

Comparison on plasticity of current-driven vortex lattices in as-grown and heavy ion irradiated microbridge Bi2Sr2Ca1Cu2O8+δ single crystals

From the I-V characteristics for as-grown and irradiated Bi₂Sr₂CaCu₂O_8+δ single crystals at T=5 K in a magnetic field applied parallel to the c axis, we have seen two types of vortex dynamics near the depinning threshold. For the as-grown sample, at low field, the I-V curves show steps that clearly indicate a “fingerprint phenomenon” since they reflect the current dependence of differential resistance R_d=dV/dI. This can be ascribed to vortices flow through uncorrelated channels for the highly defective lattice. As fields sufficiently increase, these peaks merge, giving broader ones, indicating a crossover from filamentary strings to braid river. In contrast, in the irradiated sample, the pinning is found to be individual at low magnetic fields and collective when the vortex-vortex interactions are involved. Our result suggests a dynamic nature of the peak effect, in agreement with recent numerical simulations and experimental works.     

Characterization of conductance under finite bias for a self-assembled monolayer coated Au quantized point contact

We have demonstrated that an experimental cross-wire junction set-up can be used to measure the I-V characteristics of a self-assembled monolayer (SAM) stabilized metal quantized point contact. The increased stability due to the presence of the SAM allows the measurement of the I-V characteristics. However, the SAM also provides additional conductance paths in addition to the pure metal point contact. The presence of the SAM may contribute to the non-integral quantum conductance transition and the non-linear I-V characteristics of the quantum contact. Nonetheless, a straight I-V curve is obtained for the Au quantized point contact from 0 to 1 V with a conductance of approximately 1G₀, in contrast to previous work reported in the literature.     

Molecular control over Ag/p-Si diode by organic layer

Electrical transport properties of Ag metal-fluorescein sodium salt (FSS) organic layer-silicon junction have been investigated. The current-voltage (I-V) characteristics of the diode show rectifying behavior consistent with a potential barrier formed at the interface. The diode indicates a non-ideal I-V behavior with an ideality factor higher than unity. The ideality factor of the Ag/FSS/p-Si diode decreases with increasing temperature and the barrier height increases with increasing temperature. The barrier height (φ_b=0.98 eV) obtained from the capacitance-voltage (C-V) curve is higher than barrier height (φ_b=0.72 eV) derived from the I-V measurements. The barrier height of the Ag/FSS/p-Si Schottky diode at the room temperature is significantly larger than that of the Ag/p-Si Schottky diode. It is evaluated that the FSS organic layer controls electrical charge transport properties of Ag/p-Si diode by excluding effects of the SiO₂ residual oxides on the hybrid diode.     

The effect of interface states, excess capacitance and series resistance in the Al/SiO2/p-Si Schottky diodes

The current-voltage (I-V) characteristics of Al/SiO₂/p-Si metal-insulator-semiconductor (MIS) Schottky diodes were measured at room temperature. In addition the capacitance-voltage (C-V) and conductance-voltage (G-V) measurements are studied at frequency range of 10 kHz-1 MHz. The higher value of ideality factor of 3.25 was attributed to the presence of an interfacial insulator layer between metal and semiconductor and the high density of interface states localized at Si/SiO₂ interface. The density of interface states (N_ss) distribution profile as a function of (E_ss − E_v) was extracted from the forward bias I-V measurements by taking into account the bias dependence of the effective barrier height (Φ_e) at room temperature for the Schottky diode on the order of ≅4 × 10¹³ eV⁻¹ cm⁻²_. These high values of N_ss were responsible for the non-ideal behaviour of I-V and C-V characteristics. Frequency dispersion in C-V and G-V can be interpreted only in terms of interface states. The N_ss can follow the ac signal especially at low frequencies and yield an excess capacitance. Experimental results show that the I-V, C-V and G-V characteristics of SD are affected not only in N_ss but also in series resistance (R_s), and the location of N_ss and R_s has a significant on electrical characteristics of Schottky diodes.     

Nonideality of Au/Si and Au/GaAs Schottky barriers due to process-induced defects

A mechanism of local lowering of the Schottky barrier height (SBH) is proposed, which causes nonideality in nearly ideal Au/n-Si and Au/n-GaAs Schottky barriers. Positively ionized defects generated by the process very close to the interface induce electrons in the metal-induced gap states (MIGS) and lower the SBH locally. The spatial density distribution of the ionized defects obtained from the SBH distribution is determined by the unique interaction with the MIGS. The defects are considered to have the negative-U property and are neutralized at very close positions to the MIGS. The potential distributions close to the interface have a considerable potential drop due to the large defect density. These inhomogeneous potentials are coincident with the energy level scheme of the defect identified as the defect causing the nonideality. This defect is Si self-interstitial in Au/Si SB, and As antisite in Au/n-GaAs SB. This MIGS with process-induced defect model supersedes the previously proposed two major Fermi level pinning models. The mystery of the T₀ effect is solved. The thermionic-field emission current taking place in the strong electric field has influence on the I-V characteristics at low temperatures. Regarding the C-V characteristics of Au/Si SB, the observed extra capacitance under the forward bias is an experimental evidence in accordance with the proposed model.     

Comparison of electrical parameters of Zn/p-Si and Sn/p-Si Schottky barrier diodes

In this study, current-voltage (I-V) and capacitance-voltage (C-V) characteristics of metal-semiconductor (MS) Zn/p-Si and Sn/p-Si Schottky diodes, with high resistivity silicon structures, are investigated. The parameters of series resistance (R_S), the ideality factor (n) and the barrier height (Φ_b) are determined by performing different plots from the forward bias current-voltage (I-V) and reverse bias capacitance-voltage (C-V) characteristics. Thus, the barrier heights (Φ_b) for the Si Schottky diodes obtained between 0.725 and 1.051 eV, the ideality factor (n) between 1.043 and 1.309, and the series resistance (R_S) between 12.594 and 12.950 kΩ. The energy distribution of interface states density was determined from the forward bias I-V characteristics by taking into account the bias dependence of the effective barrier height. It was concluded that the density of interface states in the considered energy range are in close agreement with each other values obtained for Zn/p-Si and Sn/p-Si Schottky diodes.     

Applications of vertically oriented TiO2 micro-pillars array on the electrode of dye-sensitized solar cell

In this research, dye-sensitized solar cells based on TiO₂ micro-pillars fabricated by inductive couple plasma etcher were investigated by analyses of X-ray diffraction (XRD), scanning electron microscopy (SEM), contact angle, ultraviolet-visible absorption spectra (UV-vis), and current-voltage characteristics. X-ray diffraction patterns show that the TiO₂ anatase phase forms while sintering at 450 °C for 30 min. The SEM images reveal that the diameter and height of TiO₂ micro-pillars are about 3 and 0.8 μm, respectively. The measurements of contact angle between TiO₂ micro-pillars and deionized water (DI water) reveal that the TiO₂ micro-pillars is super-hydrophilic while annealed at 450 °C for 30 min.The absorption spectrum of TiO₂ micro-pillars is better than TiO₂ thin film and can be widely improved in visible region with N3 dye adsorbed. The results of current-voltage (I-V) characteristics analysis reveal that dye-sensitized solar cell with TiO₂ micro-pillars electrode has better I-V characteristics and efficiency than TiO₂ film electrodes. This result may be due to the annealed TiO₂ micro-pillars applied on the electrode of dye-sensitized solar cell can increase the contact area between TiO₂ and dye, resulting in the enhancement of I-V characteristics and efficiency for dye-sensitized solar cell.     

Characterization of the low temperature dopant activation behavior at NiSi/silicon interface formed by implant into silicide method

Process temperature and thermal budget control are very important for high-k dielectric device manufacturing. This work focuses on the characteristics of low temperature activated nickel silicide/silicon (M/S) interface formed by implant into silicide (IIS) method. By combining SIMS, C-V, I-V, and AFM measurements in this work, it provides a clear picture that the high dopant activation ratio can be achieved at low temperature (below 600 °C) by IIS method. From SIMS and C-V measurements, high dopant activation behavior is exhibited, and from I-V measurement, the ohmic contact behavior at the M/S junction is showed. AFM inspection displays that under 2nd RTA 700 °C 30 s no agglomeration occurs. These results suggest that IIS method has the potential to integrate with high-k dielectric due to its low process temperature. It gives an alternate for future device integration.     

Inhomogeneities in 130 MeV Au ion irradiated Au/n-Si (1 0 0) Schottky structure

The electrical characteristics of Au/n-Si (1 0 0) Schottky rectifier have been studied in a wide irradiation fluence range using conventional current-voltage (I-V) and capacitance-voltage (C-V) measurements. The I-V characteristics showed an abnormal increase in forward current at low voltage. The device shows a bend in forward I-V and reverses bias C-V characteristics due to extra current, suggesting that there are two independent contributions to thermionic current, corresponding to two levels of the Schottky barrier. It is shown that the excess current at low voltage can be explained by taking into account the role of heavy ion irradiation induced defects at the metal semiconductor interface.     

Transient non-linear phenomena and metastable states of charge-density waves in niobium triselenide

Details are given of the transient response of NbSe₃ at 42 K to currents I(t) of various repetitive pulsed waveforms. Nonlinear conduction, due supposedly to motion of one of the two charge-density waves (CDWs) present, was measured as U(t) = IR₀ ? V, where V(t) is the voltage developed across the specimen and R₀ is its resistance in the Ohmic regime.With unidirectional pulses two threshold currents for nonlinearity were observed. On passing the lower threshold, a gradual rise (time-constant 50 μs) of U towards the d.c. value was seen; this behaviour was shown not to originate in the inertia of the CDW, and probably arose thermally. Only after the second threshold was passed did U appear to rise immediately current was applied. The existence of two thresholds accounts for a discrepancy between pulsed and d.c. measurements of conductivity noted by Brill et al. (1981), and also for a phenomenon previously attributed to “locking” between the two CDWs [6].When the pulses were alternately negative- and positive-going, |U| rose beyond its eventual (d.c.) value, towards which it subsequently decayed. A study of this “overshoot” phenomenon has shown the speciment to be left, after a current pulse, in a long-lived metastable state in which pinning stabilises some distortion of the CDW, presumably similar to that of which recent electron micrographs [11] appear to be evidence.A simple model, associating the overshoot with transitions between metastable states, adequately describes the conditions for its occurrence. However, the origin of the conduction associated with the moving CDW remains uncertain, both the Frohlich mechanism and a single-electron alternative finding some experimental support.     

On the origin of quasi-periodic current noise in charge-density wave conductors: Experiments on orthorhombic tantalum trisulphide

The quasi-periodic components, which appear in the voltage V developed across a charge-density wave (CDW) conductor when the steady current I exceeds the threshold current I_T at which motion of the CDW begins, have been observed in orthorhombic TaS₃ at 77 K at frequencies v down to less than 1 Hz. Although its relation to the nonlinear part of the current when I?I_T indicates that v is, or is a low multiple of, the ‘washboard’ frequency v/λ, where v is the velocity and λ the wavelength of the CDW, the amplitude of the periodic variation of V when v<1 kHz is much too great to represent a modulation of the Frohlich current. The alternative is that the variation of V arises from a periodic modulation of the single-electron conductivity, resulting from distortion of the CDW as it moves through the crystal. Such a modulation of conductivity has been demonstrated experimentally, by interrupting a current I>I_T at different stages of the cyclic variation of V, and then using a current I<I_T to observe the resistance of the specimen when the CDW is at rest. Mechanisms whereby a periodic dependence of resistance on the position of the CDW may arise are briefly discussed.     

Studying of barrier height and ideality factor relation in the nano sized Au-n type Si Schottky diodes

The results of formation of the operating potential barrier height (Φ_в) of inhomogeneous Schottky diodes (SD) in view of an additional electric field in the near contact region of the semiconductor and features of its dependence on the external applied voltage are presented. A correlation, between SD heterogeneity and dependence between potential barrier height (Φ_в) and ideality factor (n), is presented. Using conducting probe atomic force microscope (CP-AFM) techniques, it is shown that Au/n-Si diodes consist of sets of parallel-connected and cooperating nano diodes with the contact surfaces sizes in the order of 100-200 nm. The effective Φ_в and ideality factors of the SD have been obtained from the current-voltage (I-V) characteristics, which were measured using a CP-AFM along a contact surface. It was experimentally shown that the forward and reverse part of I-V characteristics and their effective Φ_в and ideality factors of the identically fabricated nano-SD differ from diode to diode. The Φ_в for the nano-SD has ranged from 0.565 to 0.723 eV and ideality factor from 1.11 to 1.98. No correlation can be found between the Φ_в and ideality factor. The Φ_в distribution obtained from the I-V characteristics has been fitted by a Gaussian function but the ideality factor distribution could not be fitted by a Gaussian function.     

The I-V characteristics of InAs/GaAs quantum dot laser

The I-V characteristics of an InAs/GaAs quantum dot (QD) laser diode have been investigated under both the high and low input current conditions. Under the low current condition, the I-V curve obeys the Shockley equation, from which the forbidden energy gap of the junction can be derived. On the other hand, in the high current range, the I-V characteristics violate the Shockley equation and the device tends to operate as a resistance. In addition, the I-V curve can be used to fit the temperature coefficient of the forward voltage, which is a critical parameter for determining the junction temperature of the laser diode.     

Current conduction mechanism in Al/p-Si Schottky barrier diodes with native insulator layer at low temperatures

The forward bias current-voltage (I-V) characteristics of Al/p-Si (MS) Schottky diodes with native insulator layer were measured in the temperature range of 80-300 K. The obtained zero bias barrier height Φ_B0(I-V), ideality factor (n) and series resistance (R_s) determined by using thermionic emission (TE) mechanism show strong temperature dependence. There is a linear correlation between the Φ_B0(I-V) and n because of the inhomogeneties in the barrier heights (BHs). Calculated values from temperature dependent I-V data reveal an unusual behaviour such that the Φ_B0 decreases, as the n and R_s values are increasing with decreasing absolute temperature, and these changes are more pronounced especially at low temperatures. Such temperature dependence of BH is contradictory with the reported negative temperature coefficient of the barrier height. In order to explain this behaviour we have reported a modification in the expression reverse saturation current I_o including the n and the tunnelling factor (αΧ^1/2δ) estimated to be 15.5. Therefore, corrected effective barrier height Φ_bef.(I-V) versus temperature has a negative temperature coefficients (α = −2.66 × 10⁻⁴ eV/K) and it is in good agreement with negative temperature coefficients (α = −4.73 × 10⁻⁴ eV/K) of Si band gap. In addition, the temperature dependent energy distribution of interface states density N_ss profiles was obtained from the forward bias I-V measurements by taking into account the bias dependence of the Φ_e and n. The forward bias I-V characteristics confirm that the distribution of N_ss, R_s and interfacial insulator layer are important parameters that the current conduction mechanism of MS Schottky diodes.     

Electrical characteristics of polycrystalline Si layers embedded into high-k Al2O3 gate layers

The electrical characteristics of polycrystalline Si (poly Si) layers embedded into high-k Al₂O₃ (alumina) gate layers are investigated in this work. The capacitance versus voltage (C-V) curves obtained from the metal-alumina-polysilicon-alumina-silicon (MASAS) capacitors exhibit significant threshold voltage shifts, and the width of their hysteresis window is dependent on the range of the voltage sweep. The counterclockwise hysteresis observed in the C-V curves indicates that electrons originating from the p-type Si substrate in the inversion condition are trapped in the floating gate layer consisting of the poly Si layer present between the top and bottom Al₂O₃ layers in the MASAS capacitor. Also, current versus voltage (I-V) measurements are performed to examine the electrical characteristics of the fabricated capacitors. The I-V measurements reveal that our MASAS capacitors show a very low leakage current density, compared to the previously reported results.     

A model for the electrical characteristics of metal-ferroelectric-insulator-semiconductor field-effect transistor

An improved theoretical model on the electrical characteristics of metal-ferroelectric-insulator-semiconductor field-effect transistor (MFIS-FET) has been proposed by considering the history-dependent electric field effect and the mobility model. The capacitance-voltage (C-V) characteristics of MFIS structure is evaluated by combining the switching physics of ferroelectric with the silicon physics, and the drain current-gate voltage (I_D-V_GS) and drain current-drain voltage (I_D-V_DS) characteristics of MFIS-FET are modeled by combining the switching physics of ferroelectric with Pao and Sah’s double integral. For two MFIS-FETs with SrBi₂Ta₂O₉ and (Bi,La)₄Ti₃O₁₂ ferroelectric layers, the C-V, I_D-V_GS and I_D-V_DS characteristics are simulated by using the improved model, and the results are more consistent with the previous experiment than those based on Lue model, indicating that the improved model is suitable for simulating the electrical characteristics of MFIS-FET. This work is expected to provide some guidance to the design and performance improvement of MFIS structure devices.     

Temperature dependent transport properties of CuInSe2-ZnO heterostructure solar Cell

Experimental study of dc and ac transport properties of CuInSe₂/ZnO heterostructure is presented. The current-voltage (I-V) and frequency dependent capacitance (C-f) characteristics of CuInSe₂/ZnO heterostructure were investigated in the temperature range 160-393 K. The heterostructure showed non-ideal behavior of I-V characteristics with an ideality factor of 3.0 at room temperature. Temperature dependent dc conductivity studies exhibited Arrhenius type behavior and revealed the presence of trap level. The C⁻²-V plot measured at frequency 50 kHz had shown non-linear behavior. An increase in capacitance with temperature was observed. The capacitance-frequency characteristics exhibited a transition between low frequency and the high frequency capacitance. As the temperature was lowered the transition occurred at lower frequencies. The frequency and temperature dependent device capacitance had shown a defect state having activation energy of 108 meV.     

Electrical properties of metal-GaAs Schottky barriers

Schottky barrier contacts have been fabricated by vacuum evaporation of thin films of six different metals (Mg, In, Al, Au, Bi and Sb) onto chemically cleaned n-type GaAs substrates. Bi and Sb contacts on GaAs have been reported for the first time. The contacts were examined for their C-V and I-V characteristics. The diodes exhibit near ideal characteristics for all the metals with values of the ideality factor n ranging from 1.06 to 1.1. The values of the barrier height obtained from C-V measurements could be brought into agreement with those obtained from I-V measurements only when the ideality factor was also included in the expression of the saturation current. By employing the most recent and reliable values of metal work function φ_M, the dependence of barrier height on φ_M has been investigated and an estimate of the surface state density and the Fermi level at the GaAs surface has been made. The surface state density obtained from this analysis is significantly larger than that reported by previous workers.     

Current-voltage analysis of a-Si:H Schottky diodes

Direct current (dc)-voltage (I-V) characteristics of the hydrogenated amorphous silicon (a-Si:H) Schottky diode have been measured at different temperatures under dark and light. From the fourth quadrant of illuminated characteristics, fill factor (FF) values were obtained for each temperature measured (173-297 K). We have found that FF increases very little as the temperature is decreased. The measured data from I-V characteristics has been analyzed in detail. In particular, from dark I-V characteristics obtained, the density of state (DOS) near the Fermi level was determined using a simple model based on the space-charge limited current (SCLC). On the other hand, from the illuminated I-V characteristics, the density of carriers was calculated for each temperature using the analysis of diode equation as known. A comparison of the carrier density and the measured photocurrent as a function of the reverse temperature was also made and a good correspondence was obtained.