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We present a novel method of modeling the contact between a metal and a two-dimensional semiconductor. Using Au on MoS 2 as an example, we self-consistently solve the Schrödinger and Poisson equations and obtain the charge density in the contact. We consider open boundary conditions using the quantum transmitting boundary method and model the electron current through the contact region. We then investigate the effect of effective Schottky barrier height, electrostatic doping, and length of the overlap region on the contact resistance in a top contact geometry. By using data from experiments or from ab initio calculations for the fitting of parameters, such as the effective Schottky barrier height, the model can be used to efficiently obtain the contact resistance and, therefore, the quality of the contact. Furthermore, we investigate the effect of sampling of the Brillouin zone in the transverse direction on the numerical calculation of key quantities, such as contact resistance and free charge density. Additionally, we show that the boundary conditions applied to the Poisson equation during the calculation of the free charge density have a significant impact on the calculated contact resistance and that the impact is more pronounced in heterostructures with a larger Schottky barrier. We found that the contact resistance may be significantly underestimated, by up to one order of magnitude, when the height of the simulation domain is not large enough. [ABSTRACT FROM AUTHOR]

Memristors have attracted considerable attention as next-generation devices for logic and neuromorphic computing applications, owing to their high on/off current ratio, low power consumption, and high switching speed. Despite the various excellent characteristics of memristors, they suffer from unstable conductive filament-based switching when applied in real-world applications. To address this issue, the effects of Schottky barrier modulation on device performance, in terms of conduction and failure mechanisms of an Ag/WOx/p-Si memristor, were investigated in this study by varying the silicon (Si) doping concentration. Through the temperature analysis of I–V characteristics, different conduction mechanisms are observed according to the doping concentration and resistance state. Moreover, endurance failure with several doping concentrations is analyzed by using filament overgrowth phenomena. The results of this study are expected to help in the development of devices with characteristics suitable for application. [ABSTRACT FROM AUTHOR]

Using a quantum mechanical calculation, we investigate the fundamental limitations of the Schottky emission (SE) model for its applications to electron injection into dielectrics from a metal or semiconductor electrode. This work covers a wide range of electric fields from 0.01 to 10 MV/cm and a large temperature span with many barrier height values (ΦB). We conclude that the SE model is only applicable for a very small class of dielectrics under 0.1 MV/cm and at high temperatures over ∼330 K. For many defective dielectrics with large barrier heights (ΦB ≥ ∼1.5 eV) in back-of-line/middle-of-line/metal–insulator–metal capacitor applications, the corresponding electric fields for the measurable currents far exceed 0.1 MV/cm, and up to 10 MV/cm, the application of the SE model is likely invalid so that the extracted ΦB values may not be correct. We provide a quantitative guide to avoid future misapplications of the SE model. [ABSTRACT FROM AUTHOR]

Chlorine-based gases are used for the reactive ion etching (RIE) of β − Ga 2 O 3. However, the effects of Cl-plasma on the electronic properties of β − Ga 2 O 3 are not known. In order to shed light on this topic, we carried out an experimental and theoretical study on β − Ga 2 O 3 epilayers treated with Cl 2 / Ar or BCl 3 / Ar plasma. We found four traps in the 0.2–0.8 eV energy range below the conduction band edge ( E C ). Two of these, located at E C -0.24 eV and E C -0.28 eV, arise only when the epilayers are treated with BCl 3 / Ar. While the involvement of Cl in their microscopic structure is not discarded, the possibility that these two levels might have an intrinsic nature seems more plausible. Our findings might explain the reported effects on the Schottky barrier diodes of β − Ga 2 O 3 when RIE is employed during processing. [ABSTRACT FROM AUTHOR]

Atomically thin layered materials such as MoS2 have future versatile applications in low power electronics. Here, we demonstrate the growth of a salt-assisted large scale, high-quality monolayer MoS2 toward the realization of a high-performance hysteresis-free field-effect transistor (FET). Density functional theory calculations are implemented to monitor the effects of the Schottky barrier and metal-induced gap states between our metal electrodes and MoS2 for achieving high carrier transport. The role of absorbed molecules and oxide traps on the hysteresis are studied in detail. For the first time, a hysteresis-free intrinsic transistor behavior is obtained by an amplitude sweep pulse I–V measurement with varying pulse widths. Under this condition, a significant enhancement of the field-effect mobility up to 30 cm² V−1 s−1 is achieved. Moreover, to correlate these results, a single-pulse time-domain drain current analysis is carried out to unleash the fast and slow transient charge trapping phenomena. Our findings on the hysteresis-free transfer characteristic and high intrinsic field-effect mobility in salt-assisted monolayer MoS2 FETs will be beneficial for future device applications in complex memory, logic, and sensor systems. [ABSTRACT FROM AUTHOR]

The bulk magnetization, magnetocaloric effect, and heat capacity of polycrystalline NdFe0.5Mn0.5O3 have been studied in the temperature range of 1.5–300 K. The magnetic entropy change (Δ S M) shows a peak near 7 K at 100 kOe with a value of ∼7.22 J kg−1 K−1. Another magnetic entropy change is also observed between 0 and 40 kOe near the spin reorientation region of Fe3+/Mn3+ ions (∼45 K), wherein the entropy change gets suppressed with increasing field. The magnetic heat capacity shows a broad hump near the Néel temperature (∼250 K). Around 5 K, a step-like feature in heat capacity due to the Schottky effect is observed which is associated with the crystal field effects in Nd3+ ions. [ABSTRACT FROM AUTHOR]

In this work, we investigate the effect of different chemical treatments, such as solvents, bases, and acids, on the surface properties and electrical behavior of Schottky diodes fabricated on metalorganic chemical vapor deposition-grown, n-type, N-polar GaN. The I–V and C–V barrier heights of the as-grown Schottky diodes are found to be 0.40 eV and 0.60 eV, respectively, with an ideality factor n = 1.07. It is found that the solvent treatments neither change the surface nor the electrical performance of the Schottky diodes, as expected. However, the treatments by the alkaline photolithography developer and acid—the latter of which is often used to clean the surface of Ga-polar GaN films prior to metal contact deposition—degrade the performance of N-polar GaN. These base and acid treatments severely roughen the surface by creating triangular/hexagonal crystallographic facets. The I–V and C–V barrier heights of these base- and acid-treated diodes are increased to 0.63 eV and 1.00 eV, respectively, with ideality factor values n < 1.2. X-ray photoelectron spectroscopy studies indicate that base- and acid-treated surfaces have lower oxygen content as compared to the as-grown sample surface. It is understood that the increment in the barrier height for base- and acid-treated diodes could be due to the change in polarity, from N-polar to semi-polar/non-polar, on these crystallographic features. All these results demonstrate that, unlike for Ga-polar GaN, the N-polar GaN surface is highly reactive to bases and acids. [ABSTRACT FROM AUTHOR]

We report the lateral and vertical electrical conduction properties of PdCrO2 thin films grown on insulating Al2O3 (001) and conducting β-Ga2O3 (2 ¯ 01) substrates. The c-axis oriented PdCrO2 films on the both substrates showed metallic temperature dependence of in-plane resistivity down to 2 K. In PdCrO2/β-Ga2O3 vertical devices, rectifying current density–voltage (J–V) characteristics revealed the formation of a Schottky barrier at the PdCrO2/β-Ga2O3 interface. The Schottky barrier height (SBH) of 1.2–1.8 eV, evaluated by J–V characteristics, is significantly larger than 0.8 eV expected from the usual Mott–Schottky relation based on the electron affinity of β-Ga2O3 (4.0 eV) and the work function of PdCrO2 (4.8 eV) determined by ultraviolet photoelectron spectroscopy. The enhanced SBH at the PdCrO2/β-Ga2O3 interface indicates the existence of interface dipoles, as in the case of PdCoO2/β-Ga2O3. Besides, we observed a large difference of the SBH between the J–V measurements (1.2–1.8 eV) and capacitance measurements (2.0–2.1 eV). While the SBH is definitely enhanced by the interface dipole effect, the level of enhancement at the PdCrO2/β-Ga2O3 interface is rather inhomogeneous, different from that at the PdCoO2/β-Ga2O3. In fact, two typical types of interfaces were found by a high-angle annular dark-field scanning transmission electron microscope, which would be the origin of the inhomogeneous SBH. Further understanding of the interface formation between delafossite oxides and β-Ga2O3 (2 ¯ 01) will improve the performance of Ga2O3 Schottky junctions as a power diode available at high temperatures. [ABSTRACT FROM AUTHOR]

Bright, red electroluminescence is generated in reverse biased Schottky barrier diodes (SBDs) based on β-Ga2O3 single crystals codoped with chromium and silicon. It is due to intracenter transitions of octahedrally coordinated Cr3+ ions excited by electron impact in the depletion layer of the diodes. The electroluminescence spectrum around 700 nm in wavelength is nearly identical to the spectrum of the well-known photoluminescence of Cr3+ in β-Ga2O3, which is featured by the two lines R1 and R2. In contrast to the photoluminescence, however, in the electroluminescence, each of the R1 and R2 lines is additionally split by 1.5–3 meV. Since the R1/R2 lines correspond to transitions from two Kramers-degenerate states (split 2E excited state) to the ground state 4A2, this splitting cannot be ascribed to the normal Stark effect in the strong electric field of the Schottky barrier diode's depletion layer. Instead, we explain the splitting by the pseudo-Stark effect that occurs because the Cr3+ ions replace Ga3+ at two kinds of energetically equivalent octahedral sites that differ crystallographically by an inversion at the Cr ion. Superposition of the dominating R1 doublet radiation would result in a tunable beat frequency of about 0.4–0.7 THz and might be utilized for a terahertz light source. Moreover, the electroluminescence of chromium is representative of the ability to excite the luminescent states of other transition metals. Hence, high temperature light-emitting SBDs in different colors are a potential application for β-Ga2O3. [ABSTRACT FROM AUTHOR]

Nonmagnetic semiconductor based magnetoresistance (MR) devices combining high performance and low cost have attracted a lot of attention. However, it has been a great challenge to separate the interface MR from the bulk MR in the devices composed of Schottky heterojunctions. In this paper, the MR effect of a silicon-based Schottky heterojunction device had been studied, and its mechanisms were investigated by separating the interface MR effects from the bulk MR effects through combining two-probe and four-probe methods. We find that the bulk MR value is significantly smaller than the total MR value in the avalanche breakdown region in the temperature range of 150 K to 300 K, indicating that the total MR effect mainly originates from the interface MR effect. Theoretical analysis shows that the bulk MR effect is a normal one due to the existence of the Lorentz force on the carriers, and the interface MR effect relates to the suppression of the local plasmas by applying magnetic fields, where the local plasmas form due to the avalanche breakdown in the Ag/SiO2/p-Si Schottky heterojunctions. The total MR effect at room temperature can be further enhanced by reducing the distance between electrodes, and the total MR reaches about 1847% under a magnetic field of 1 T and the MR sensitivity is as large as 118.5 T−1 under 0.1 T. [ABSTRACT FROM AUTHOR]

Schottky's Conjecture posits that the geometric field enhancement produced by a hybrid shape formed from a small perturbation on a larger base is the product of the individual field enhancement factors of the base and perturbation in isolation. This is a powerful concept with practical applications to understanding field emitter design and operation, as actual field emitters have complicated surface shapes with structure and, therefore, contributions to field enhancement, occurring simultaneously on many length scales. Recent studies of the Conjecture imply that the degree to which it accurately predicts a hybrid structure's total field enhancement depends on the degree of self-similarity between the base and perturbation shapes. To explore these aspects of the Conjecture, we have used the zero-potential surface produced by simple charge distributions to produce compound shapes with small perturbations on larger base structures. In the limit of small perturbation strength, these simple models adequately approximate idealized compound shapes, such a hemisphere-on-hemisphere geometry. Changing the base shape and the location of the perturbation allows us to selectively degrade the self-similarity present in the problem geometry. Proofs of the Schottky Conjecture in the limit of small perturbation strength will be provided, using these techniques, for hemisphere-on-hemisphere and half-cylinder-on-half-cylinder systems—the latter being the geometry considered by Schottky and for which the Conjecture was first proposed, but not proven. [ABSTRACT FROM AUTHOR]

Schottky's Conjecture (SC) posits that when a compound conductive structure is formed by placing a protrusion on top of an underlying base, the total field enhancement factor is the product of the field enhancement factors that would be produced by the base and protrusion in isolation. This is a powerful concept, which, in principle, allows separate treatment of the electrostatic effects of geometric features occurring at differing length scales. Recent work suggests that the degree to which the SC holds depends on the shape of the protrusion and base, and, in particular, on their relative sizes and their degree of self-similarity. Here, we use a Line Charge Model (LCM) to study the applicability of the SC to compound, quasiellipsoidal structures. The general features of compound structures produced by the LCM are discussed. The SC consistently overpredicted the computed field enhancement factor but was seen to provide reasonable estimates, correct to within a factor of 2 or better, when the protrusion was sufficiently small compared to the base; a dependence of the threshold protrusion height on the base radius was identified. This range of applicability of the SC is more restrictive than that previously reported in the literature, and potential causes of this are discussed. [ABSTRACT FROM AUTHOR]

Computing inspired by the human brain requires a massive parallel architecture of low-power consuming elements of which the internal state can be changed. SrTiO3 is a complex oxide that offers rich electronic properties; here, Schottky contacts on Nb-doped SrTiO3 are demonstrated as memristive elements for neuromorphic computing. The electric field at the Schottky interface alters the conductivity of these devices in an analog fashion, which is important for mimicking synaptic plasticity. Promising power consumption and endurance characteristics are observed. The resistance states are shown to emulate the forgetting process of the brain. A charge trapping model is proposed to explain the switching behavior. [ABSTRACT FROM AUTHOR]

Computing inspired by the human brain requires a massive parallel architecture of low-power consuming elements of which the internal state can be changed. SrTiO3 is a complex oxide that offers rich electronic properties; here, Schottky contacts on Nb-doped SrTiO3 are demonstrated as memristive elements for neuromorphic computing. The electric field at the Schottky interface alters the conductivity of these devices in an analog fashion, which is important for mimicking synaptic plasticity. Promising power consumption and endurance characteristics are observed. The resistance states are shown to emulate the forgetting process of the brain. A charge trapping model is proposed to explain the switching behavior. [ABSTRACT FROM AUTHOR]

We investigated the influence of space charge region (SCR) on the electrical properties of Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) thin film based capacitor structures. The metal/PLZT/Pt capacitors were prepared by using metal electrodes (Pt, Au, Al) of varying work function as top electrodes, and Pt as a bottom electrode. The SCR formed at the top interface between the PLZT film and the metal differs from each other in three capacitor structures. The results obtained from the hysteresis, dielectric, and leakage current measurements were analyzed to understand the variation in the electrical properties of the capacitors arising from the different SCRs formed at the interfaces. Among the three, the Pt/PLZT/Pt capacitor showed highest polarization and permittivity, and lowest FCC and coercive field. The Al/PLZT/Pt capacitor showed lowest polarization and permittivity, but higher coercive field and FCC. The leakage current increased with a decrease in work function of the top electrode. The Schottky conduction mechanism was found to be dominant in all the three capacitors. Both the photovoltaic parameters (short circuit photocurrent denisty and open circuit photovoltage) increased with the increase in work function of the top electrodes. These studies suggest the existence of wider SCR and higher space charge field at the top interface in the case of capacitor designed with an Al (or Au) top electrode as compared to the capacitor designed with a Pt top electrode. [ABSTRACT FROM AUTHOR]

The presence of copper impurities is known to deteriorate the bulk minority carrier lifetime of silicon. In p-type silicon, the degradation occurs only under carrier injection (e.g., illumination), but the reason for this phenomenon called copper-related light-induced degradation (Cu-LID) has long remained uncertain. To clarify the physics of this problem, a mathematical model of Cu-LID was introduced in Paper I of this article. Within the model, kinetic precipitation simulations are interlinked with a Schottky junction model for electric behavior of metallic precipitates. As this approach enables simulating precipitation directly at the minority carrier lifetime level, the model is verified in this second part with a direct comparison to the corresponding degradation experiments and literature data. Convincing agreement is found with different doping and Cu concentrations as well as at increased temperature, and in the dark, both simulated degradation and measured degradation are very slow. In addition, modeled final lifetimes after illumination are very close to experimental final lifetimes, and a correlation with the final precipitate size is found. However, the model underestimates experimentally observed differences in the degradation rate at different illumination intensities. Nevertheless, the results of this work support the theory of Cu-LID as a precipitate formation process. Part of the results also imply that heterogeneous nucleation sites play a role during precipitate nucleation. The model reveals fundamental aspects of the physics of Cu-LID including how doping and heterogeneous nucleation site concentrations can considerably influence the final recombination activity. [ABSTRACT FROM AUTHOR]

We consider the carrier transport and plasmonic phenomena in the lateral carbon nanotube (CNT) networks forming the device channel with asymmetric electrodes. One electrode is the Ohmic contact to the CNT network and the other contact is the Schottky contact. These structures can serve as detectors of the terahertz (THz) radiation. We develop the device model for collective response of the lateral CNT networks which comprise a mixture of randomly oriented semiconductor CNTs (s-CNTs) and quasi-metal CNTs (m-CNTs). The proposed model includes the concept of the collective twodimensional (2D) plasmons in relatively dense networks of randomly oriented CNTs (CNT "felt") and predicts the detector responsivity spectral characteristics exhibiting sharp resonant peaks at the signal frequencies corresponding to the 2D plasmonic resonances. The detection mechanism is the rectification of the ac current due the nonlinearity of the Schottky contact current-voltage characteristics under the conditions of a strong enhancement of the potential drop at this contact associated with the plasmon excitation. The detector responsivity depends on the fractions of the s- and m-CNTs. The burning of the near-contact regions of the m-CNTs or destruction of these CNTs leads to a marked increase in the responsivity in agreement with our experimental data. The resonant THz detectors with sufficiently dense lateral CNT networks can compete and surpass other THz detectors using plasmonic effects at room temperatures. [ABSTRACT FROM AUTHOR]

Formation of metal-semiconductor (M-S) contacts at sub-20 nanometer range is a key requirement for down-scaling of semiconductor devices. However, electrical measurements of M-S contacts at this scale have exhibited dramatic change in the current-voltage (I-V) characteristics compared to that of conventional (or planar) Schottky contacts. This change is actually attributed to the limited metal contact region where the transferred charge from the semiconductor into the metal is confined to a small surface area, which in turn results in an enhanced electric field at the nano-M-S interface. We here present detailed theoretical models to analyze the nano-M-S junctions at 10 nm contact range and then implement this analysis on the experimental data we conducted under these conditions. Both theoretical and experimental results demonstrate a significant effect of the contact size on the electronic structure of the M-S junctions and thus on the I-V characteristics. This effect is rather prominent when the size of the metal contact is substantially smaller than the width of conventional depletion region of the relevant planar M-S contacts. [ABSTRACT FROM AUTHOR]

Internal photoemission (IPE) across an n-type Schottky junction due to standard AM1.5G solar illumination is quantified with practical considerations for Cu, Ag, and Al under direct and fully nondirect transitions, all in the context of the constant matrix element approximation. Under direct transitions, photoemitted electrons from d bands dominate the photocurrent and exhibit a strong dependence on the barrier energy ΦB but are less sensitive to the change in the metal thickness. Photocurrent is shown to be nearly completely contributed by s-state electrons in the fully nondirect approximation that offers nearly identical results as in the direct transition for metals having a free-electron-like band structure. Compared with noble metals, Al-based IPE has the highest quantum yield up to about 5.4% at ΦB=0.5 eV and a maximum power conversion efficiency of approximately 0.31% due mainly to its relatively uniform and wide Pexc energy spectral width. Metals (e.g., Ag) with a larger interband absorption edge are shown to outperform those with shallower d-bands (e.g., Cu and Au). [ABSTRACT FROM AUTHOR]

This article reports on scaling effects in Schottky contacts on various types of semiconductors, including low resistivity, semi-intrinsic, and deep-level compensated. The investigation was performed using a finite element computation and drift-diffusion transport model. In low resistivity semiconductors, the currents scale with contact area as long as thermionic emission process dominates the current transport, with limited impact of velocity saturation effect. In high resistivity semiconductors, the scaling is much more complex due to the considerable impact of minority carrier contribution. In several cases, the currents scale with contact radius, rather than with area, due to corresponding electric field variations. In some compensated materials, the impact of velocity saturation was shown to boost the current, due to carrier accumulation and corresponding space charge variations. [ABSTRACT FROM AUTHOR]

We investigated the influence of the cation off-stoichiometry on the transport properties of junctions consisting of a metal (Ti or Pt) and a Nb-doped SrTiO3 (Nb-STO) epitaxial film. Ti/Nb-STO junctions with a Ti-deficient Nb-STO film show slightly asymmetric I-V characteristics and a large zero-bias differential junction resistance. Junctions with a Sr-deficient film, in contrast, exhibit nearly ohmic I-V characteristics and a differential resistance comparable to that of junctions with a stoichiometric film. We also found that the cation off-stoichiometry of the Nb-STO film has a strong influence on the rectifying and hysteric I-V characteristics of Pt/Nb-STO Schottky junctions. For Schottky junctions with the Ti-deficient film, the junction currents are largely reduced while the hysteric behavior remains. Junctions with the Sr-deficient film, on the other hand, have hysteric I-V characteristics that are comparable to those of junctions with the stoichiometric film. The results show the strong influence of the cation off-stoichiometry of the Nb-STO film on the junction properties. Based on the experimental results including transient behavior measurements, we discuss how the cation off-stoichiometry affects the transport properties of the metal/Nb-STO junctions. [ABSTRACT FROM AUTHOR]

This article reports on scaling effects calculated with ohmic contacts on various types of high and low resistivity semiconductors, from near-intrinsic to heavily compensated. The investigation was performed using a finite element computation. It is shown that small ohmic contacts exhibit Schottky-like energy band diagrams on compensated and uncompensated semiconductors. It is shown that the currents of ideal ohmic contacts scale with the contact's perimeter below critical contact size and with area for larger contacts. Bulk velocity saturation effect is shown to introduce non-linearity and asymmetry in the current-voltage characteristics even for ideal ohmic contacts. The effect of finite contact recombination velocity is analyzed. In compensated semiconductors, the deep level carrier capture cross-sections have no effect on resistivity, but may play an important role in the device dc characteristics. [ABSTRACT FROM AUTHOR]

Here, we report on the current transport mechanisms in AlN thin films deposited at a low temperature (i.e., 200 °C) on p-type Si substrates by plasma-enhanced atomic layer deposition. Structural characterization of the deposited AlN was carried out using grazing-incidence X-ray diffraction, revealing polycrystalline films with a wurtzite (hexagonal) structure. Al/AlN/ p-Si metal-insulatorsemiconductor (MIS) capacitor structures were fabricated and investigated under negative bias by performing current-voltage measurements. As a function of the applied electric field, different types of current transport mechanisms were observed; i.e., ohmic conduction (15.2-21.5 MV/m), Schottky emission (23.6-39.5 MV/m), Frenkel-Poole emission (63.8-211.8 MV/m), trap-assisted tunneling (226-280 MV/m), and Fowler-Nordheim tunneling (290-447 MV/m). Electrical properties of the insulating AlN layer and the fabricated Al/AlN/p-Si MIS capacitor structure such as dielectric constant, flat-band voltage, effective charge density, and threshold voltage were also determined from the capacitance-voltage measurements. [ABSTRACT FROM AUTHOR]

Pt Schottky contacts on (001)-orientated Nb-doped SrTiO3 (NSTO) in both ambient air and vacuum were investigated by the conductive atomic force microscope. The co-existed TiO2 and SrO termination layers were identified on the terrace-structured NSTO surface, where the former possessed a higher forward current than the latter. In ambient air, the barrier height of Pt/NSTO Schottky junction exhibited periodical variation with cyclic terrace plane and step sites, whereas it became homogeneous in ambient vacuum. We suggested that the oxygen absorption and releasing of surface dangling bonds were the origin for reversible changes in transport properties, which indicates a feasible approach for the surface modulation and band structure tailoring of NSTO based heterojunctions. [ABSTRACT FROM AUTHOR]

We develop a semianalytical model for monolayer graphene field-effect transistors in the ballistic limit. Two types of devices are considered: in the first device, the source and drain regions are doped by charge transfer with Schottky contacts, while, in the second device, the source and drain regions are doped electrostatically by a back gate. The model captures two important effects that influence the operation of both devices: (i) the finite density of states in the source and drain regions, which limits the number of states available for transport and can be responsible for negative output differential resistance effects, and (ii) quantum tunneling across the potential steps at the source-channel and drain-channel interfaces. By comparison with a self-consistent non-equilibrium Green's function solver, we show that our model provides very accurate results for both types of devices, in the bias region of quasi-saturation as well as in that of negative differential resistance. [ABSTRACT FROM AUTHOR]

Voltage and frequency dependent capacitance measurements were performed on epitaxial BaTiO3 and Pb(Zr0.2Ti0.8)O3 thin films deposited on single crystal SrTiO3 substrates with (001) and (111) orientations. The measured capacitors have common bottom SrRuO3 contact and different metals as top electrodes: SrRuO3, Pt, Cu, Al, and Au. The capacitance-voltage characteristics were used to extract information regarding the density of the free carriers and the linear contribution to the static dielectric constant. The frequency dependent impedance was used to develop a suitable equivalent circuit for the epitaxial ferroelectric capacitors. It was found that the frequency dependence of the imaginary part of the impedance can be well simulated, in all cases, using a circuit composed of Schottky-type capacitance related to electrode interfaces, contact resistance, and the R-C parallel connection related to the ferroelectric volume of the film. Values for the components of the equivalent circuit were obtained by fitting the experimental data with the simulated curves. These were then used to extract quantities such as dielectric constant in the ferroelectric volume, the width of the depletion layers, and the apparent built-in potential. It was found that, although the investigated capacitors are of different ferroelectric materials, grown on substrates with different orientations, and having different metals as top electrodes, the values for the capacitance associated with the Schottky contacts and the apparent built-in potential are not very different. The results suggest a strong influence of ferroelectric polarization on the electrode interface properties in the case of epitaxial ferroelectric films. [ABSTRACT FROM AUTHOR]

We report the electrical characteristics of Schottky contacts and high-hole-mobility, enhancement-mode, p-channel metal semiconductor field effect transistors (MESFETs) fabricated on Ge epitaxially grown on Si substrates. The Ge film covers the entire underlying Si substrate at the wafer scale without mesas or limited-area growth. The device performance is characterized primarily as a function of threading dislocation density in the epitaxial Ge film (2×107, 5×107, 7×107, and 2×108 cm-2) and dielectric layers (SiO2, Al2O3, and HfO2) inserted between gate metal and Ge. The thin dielectric layers (∼1.3 nm) are used to unpin the Fermi level. The device performance improves with decreasing threading dislocation density and the use of HfO2. The hole mobility in the Ge film with 2×107 cm-2 dislocation density, obtained from Hall measurements, is 1020 cm2/V-s. Capacitance-voltage measurements on Schottky contacts provide the energy-dependent interfacial trap density of 6×1011 cm-2 eV-1, while current-voltage measurements provide an ON/OFF current ratio of 250. Based on the current-voltage characteristics of p-MESFETs, we have obtained an external transconductance of 7 mS/mm and low-field, effective hole-mobility of 307 cm2/V-s under 0.1 MV/cm at room temperature. The cut-off frequency of MESFETs is 10 GHz at 200K and 2GHz at 300K. These results compare well with other reported transistor performance. [ABSTRACT FROM AUTHOR]

Spectrally and spatially resolved photovoltages were measured by Kelvin probe force microscopy (KPFM) on a Schottky photo-diode made of a 4 nm thin tungsten-carbide (WC) layer on a 500 nm oxygen-terminated boron-doped diamond epitaxial layer (O-BDD) that was grown on a Ib (100) diamond substrate. The diode was grounded by the sideways ohmic contact (Ti/WC), and the semitransparent Schottky contact was let unconnected. The electrical potentials across the device were measured in dark (only 650 nm LED of KPFM being on), under broad-band white light (halogen lamp), UV (365 nm diode), and deep ultraviolet (deuterium lamp) illumination. Illumination induced shift of the electrical potential remains within 210mV. We propose that the photovoltage actually corresponds to a shift of Fermi level inside the BDD channel and thereby explains orders of magnitude changes in photocurrent. [ABSTRACT FROM AUTHOR]

In this study, we present the results of investigations on Schottky Au-GaN diodes by means of conventional DLTS and Laplace DLTS methods within the temperature range of 77 K-350 K. Undoped GaN layers were grown using the plasma-assisted molecular beam epitaxy technique on commercial GaN/sapphire templates. The quality of the epilayers was studied by micro-Raman spectroscopy (µ-RS) which proved the hexagonal phase and good crystallinity of GaN epilayers as well as a slight strain. The photoluminescence spectrum confirmed a high crystal quality by intense excitonic emission but it also exhibited a blue emission band of low intensity. DLTS signal spectra revealed the presence of four majority traps: two high-temperature and two low-temperature peaks. Using the Laplace DLTS method and Arrhenius plots, the apparent activation energy and capture cross sections were obtained. For two high-temperature majority traps, they were equal to E1 = 0.65 eV, σ1 = 8.2 x 10-16 cm² and E2 = 0.58 eV, σ2 = 2.6 x 10-15 cm² whereas for the two low-temperature majority traps they were equal to E3 = 0.18 eV, 3 = 9.7 x 10-18 cm² and E4 = 0.13 eV, =σ4 = 9.2 x 10-18 cm² . The possible origin of the traps is discussed and the results are compared with data reported elsewhere. [ABSTRACT FROM AUTHOR]

As-grown and pre-oxidized silicon carbide (SiC) samples of polytype 4H have been annealed at temperatures up to 1950 °C for 10 min duration using inductive heating, or at 2000 °C for 30 s using microwave heating. The samples consisted of a n-type high-purity epitaxial layer grown on 4° off-axis <0001> n+-substrate and the evolution of the carbon vacancy (VC) concentration in the epitaxial layer was monitored by deep level transient spectroscopy via the characteristic Z1/2 peak. Z1/2 appears at ~0.7 eV below the conduction band edge and arises from the doubly negative charge state of VC. The concentration of VC increases strongly after treatment at temperatures ≥ 1600 °C and it reaches almost 1015 cm-3 after the inductive heating at 1950 °C. A formation enthalpy of ~5.0 eV is deduced for VC, in close agreement with recent theoretical predictions in the literature, and the entropy factor is found to be ~5 k (k denotes Boltzmann's constant). The latter value indicates substantial lattice relaxation around VC, consistent with VC being a negative-U system exhibiting considerable Jahn-Teller distortion. The microwave heated samples show evidence of non-equilibrium conditions due to the short duration used and display a lower content of VC than the inductively heated ones. Finally, concentration-versus-depth profiles of VC favour formation in the "bulk" of the epitaxial layer as the prevailing process and not a Schottky type process at the surface. [ABSTRACT FROM AUTHOR]

We investigated the graphene oxide (GO) based n-type crystalline silicon (c-Si)/conductive poly(ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) Schottky junction devices with optical characterization and carrier transport measurement techniques. The optical transmittance in the UV region decreased markedly for the films with increasing the concentration of GO whereas it increased markedly in the visible-infrared regions. Spectroscopic ellipsometry revealed that the ordinary and extraordinary index of refraction increased with increasing the concentration of GO. The hole mobility also increased from 1.14 for pristine film to 1.85 cm2/V s for the 12-15 wt. % GO modified film with no significant increases of carrier concentration. The highest conductivity was found for a 15 wt. % GO modified PEDOT:PSS film: the c-Si/PEDOT:PSS:GO device using this sample exhibited a relatively high power conversion efficiency of 11.04%. In addition, the insertion of a 2-3 nm-thick GO thin layer at the c-Si/PEDOT:PSS interface suppressed the carrier recombination efficiency of dark electron and photo-generated hole at the anode, resulting in the increased photovoltaic performance. This study indicates that the GO can be good candidates for hole transporting layer of c-Si/PEDOT:PSS Schottky junction solar cell. [ABSTRACT FROM AUTHOR]

Carrier recombination in high-resistivity CdZnTe is investigated using the non-contact Defect Specific Photoconductance (DSPC) method that allows for the separation of the photoconductance components associated with recombination at point defects and defect clusters. Unlike photoconductance component controlled by the recombination at point defects that decays exponentially upon cessation of illumination, the component associated with carrier recombination at defect clusters decays logarithmically with time. It is shown that this decay is associated with formation of a Schottky-type depletion space-charge region around the cluster. In high-resistivity CdZnTe the time constant of the logarithmic decay exceeds 1 ms. Reduction of the height of cluster potential barrier with increasing photo-excited excess carrier density increases carrier recombination through the cluster states and leads to its dominance over the recombination through the point defects. At higher carrier injection densities the contribution of the carrier recombination through the clusters exceeds 80% of the total recombination rate. [ABSTRACT FROM AUTHOR]

We have analyzed the characteristics of hydrogenated amorphous silicon source gated transistors (SGTs) by using numerical simulations and we found that the original SGT characteristics can be reproduced without introducing barrier lowering mechanisms at the Schottky contact. Output characteristics show reduced current increase when pinch-off of the source end of the channel is triggered by increasing Vds, while perfect saturation of the drain current is achieved when pinch-off at the drain occurs. According to our simulations, even in the saturation regime the current at metal-semiconductor interface does not reach the thermionic emission limit and remains diffusion limited. Gate bias dependence of the saturation current can be simply explained as a combination of increased saturation voltage and reduced output conductance, without invoking barrier lowering mechanisms. SGT contact effects were modeled by introducing a distributed diode equivalent circuit for the source contact, which reproduces very well the device characteristics and can be easily implemented in a circuit simulator. [ABSTRACT FROM AUTHOR]

We investigate transport properties of polyacrylic acid (PAA) capped n and p-type silicon nanowire (SiNW) arrays. PAA diluted with deionized water at different concentrations was spun directly on vertically grown SiNW arrays prepared by metal assisted electroless chemical etching. PAA provides mechanical support to electrical contacts and acts as a source of interface doping by creating acceptor like states (holes) on SiNWs surface. PAA capping results in increase in current in p-type SiNWs and decrease in current in n-type SiNWs. Schottky emission model fits current voltage (IV) characteristics of p-type SiNWs/PAA device. Ohmic like conduction at lower voltages followed by space charge limited current (SCLC) with and without traps is observed in p-type SiNWs, n-type SiNWs, and n-type SiNWs/PAA devices. Using SCLC model with exponential distribution of traps, the extracted trap density was 7.20 × 1011/cm3 and 6.0 × 1011/cm3 for p-type SiNWs and n-type SiNWs devices, respectively. Our findings also demonstrate that the carrier concentration in SiNWs depends not only on doping concentration but also depends significantly on density of surface states. [ABSTRACT FROM AUTHOR]

The forward Schottky characteristic method, utilizing the temperature dependence of forward gate-source Schottky junction voltage, has been used to measure the transient temperature rise under DC and cycle pulse for multi-finger AlGaN/GaN high electron mobility transistor. The effect of electrical pulse on channel temperature has been studied. The transient temperature rise of channel under pulses with different duty cycles and frequencies are determined, respectively. The measurement results show that operation under high frequency and/or low duty cycle can improve the lifetime and performance reliability of AlGaN/GaN HEMT devices effectively. Furthermore, the measurement results are found to be consistent with the electro-thermal simulation results performed for the device. [ABSTRACT FROM AUTHOR]

A semi-analytical model is derived for the reverse Schottky gate current in AlGaN-GaN high-electron-mobility-transistors (HEMTs) in off-state at high drain voltage. At elevated drain voltages, the depletion of the two-dimensional electron gas (2-DEG) not only exists directly underneath the gate contact but also extends from the gate towards the drain. This increases the electric fields at the gate edge on the drain side, which causes an increase in the gate leakage current. In this paper, a new method is proposed to calculate the electric fields at the gate edge using conformal mapping of the charge configuration in the device. This method also allows calculation of the influence of charges trapped at the interface between the semiconductor and the passivation layer next to the gate. It is shown that direct tunneling is the dominant current mechanism at high drain voltages, such that the current becomes insensitive to temperature variation. Interface charges next to the gate exert a large influence on the high voltage leakage current. It is argued that temperature variations of the high voltage leakage current are caused by temperature dependent changes in trapped charge. [ABSTRACT FROM AUTHOR]

The mechanism for resistive switching in aluminum oxide (AlOx) based electrochemical metallization memory cells is presented. Copper/AlOx/tungsten (Cu/AlOx/W) cells show reproducible resistive switching with an ON/OFF ratio of about 5 × 102 at a reading voltage of 0.1 V and reliable retention characteristics. Resistive switching occurs due to the formation and rupture of a Cu filament between the active electrode (Cu) and the counter electrode (W). The conduction of the devices was explained through back-to-back Schottky contacts in the OFF state, while it exhibits ohmic behavior in the ON state. Thermionic emission model was used to calculate the barrier heights of the Schottky contacts. The rupture of the Cu filament proved to occur at the weakest point of the filament inside the AlOx. Using Ohms Law, the slope of the linear I-V characteristics in the ON state was used to extract the Cu filament resistance and its diameter was estimated to be between 6 and 23 nm. [ABSTRACT FROM AUTHOR]