Average Barrier Height Research Articles

Tunneling electroresistance of MgZnO-based tunnel junctions

We investigated the tunneling electroresistance (TER) in metal/wurtzite-MgZnO/metal junctions for applications in nonvolatile random-access memories. A resistive switching was detected utilizing an electric-field cooling at ±1 V and exhibited a TER ratio of 360%–490% at 2 K. The extracted change in the average barrier height between the two resistance states gave an estimation of the MgZnO electric polarization at 2.5 μC/cm2 for the low-temperature limit. In addition, the temperature-dependent TER ratio and the shift of the localized states energies at the barrier interface supported the ferroelectric behavior of the MgZnO tunnel-barrier. From the first-principles calculations, we found a similar effect of the barrier height change coming from the reversal of ZnO electric polarization. The possibility of using metal electrodes and lower growth temperatures, in addition to the ferroelectric property, make the ZnO-based memory devices suitable for CMOS integration.

A study on non-stoichiometric p-NiOx/n-Si heterojunction diode fabricated by RF sputtering: Determination of diode parameters

NiOx thin films were grown on n-Si substrates by radio frequency sputtering technique for the fabrication of a heterojunction p-n diode. X-ray diffraction, scanning electron microscope and atomic force microscope results revealed that NiOx films had nano sized polycrystalline nature. The X-ray energy dispersive analysis was used to determine elemental composition of the NiOx films. High quality vacuum evaporated silver (Ag) (ohmic) layer and nickel (Ni) (measurement electrode) dots were used to make contacts to the p-NiOx/n-Si heterojunction, in such a way that 8 Ni/p-NiOx/n-Si/Ag devices were fabricated. Current-voltage (I–V) and capacitance-voltage (C–V) measurements of the p-NiOx/n-Si heterojunctions showed good diode characteristics and the average barrier height has been calculated as 0.652 eV.

Epitaxial Fe on free-standing GaAs nanowires

Epitaxial Fe contacts have been fabricated onto the top half of free-standing, Te-doped GaAs nanowires (NWs) via electrodeposition. Electrical isolation from the substrate via a polymeric layer enabled the measurement of electrical transport through individual wires. Using a fixed probe within a scanning electron microscope, an average metal-semiconductor diode barrier height of 0.69 ± 0.03 eV (ideality factor 1.48 ± 0.02) was found.

Barrier height inhomogeneity and its impact on (Al,In,Ga)N Schottky diodes

III-N materials, especially ternary and quaternary alloys, are profoundly affected by barrier height inhomogeneity as evidenced by great variability in reported barrier height and Richardson constant values for Schottky diode samples involving epilayers with identical material composition. Research into AlInGaN-based devices is gaining traction due to its usefulness for strain engineering, polarization engineering, and vertical device design. Thus it is important to characterize the Schottky barrier height between AlInGaN and technologically relevant metals like nickel. It is proposed that alloy composition fluctuations inherent to low-temperature III-N alloys result in a Schottky barrier height inhomogeneity, and that the Schottky barrier height follows a Gaussian distribution. Current vs voltage data as a function of temperature was measured for three AlInGaN samples of varying composition. Utilizing a model tailored to thermionic emission over a Gaussian distribution of barriers, both the average barrier height and the standard deviation in the distribution were extracted from experimental data via multiple linear regression. Average barrier height was found to correlate with the AlInGaN band gap, while the standard deviation in barrier height increased with aluminum and indium concentration on the group-III sublattice.

Graphene doped TiO2/p-silicon heterojunction photodiode

The photoresponse properties of Al/p-Si/GO:TiO2/Au diodes were investigated using transient photocurrent and conductance spectroscopy techniques. The average barrier height and ideality factor values of the diodes were found to be 0.601 ± 0.001 eV and 17.9.3 ± 9. It is seen that the electronic parameters of the diodes depend on GO content. The obtained ideality factor is higher than unity due to the low carrier mobility of the organic interlayer. The change in the capacitance of the diodes indicates a continuous distribution of interface states. The photoresponse properties of the diodes are changed with GO content. Al/p-Si/GO:TiO2/Au having 3% GO weight exhibited the highest photosensitivity. The obtained results indicates that the prepared photodiodes can be used as a photosensor in optoelectronic device applications.

Ge-added TiO2–Ta2O5–CaCO3 varistor ceramics

The influence of doping with Ge on the nonlinear coefficient α and the breakdown electric field EB of TiO2–Ta2O5–CaCO3 varistor ceramics was investigated. In this study, TiO2–Ta2O5–CaCO3 varistor ceramics added with Ge was successfully prepared using the traditional method of ball milling–molding–sintering. The electrical performance, including the nonlinear coefficient α, the breakdown electric field EB, and the leakage current JL, are tested using a varistor direct current parameter instrument. The average barrier height ΦB of each sample is calculated using the relevant formula. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning transmission electronic microscopy analyses demonstrated that Ge doping notably changed the microstructure of TiO2–Ta2O5–CaCO3 ceramics, thereby increasing α and decreasing EB. When the doping contents of Ta2O5 and CaCO3 were 0.2 and 0.4mol%, respectively, the optimum doping content of 0.9mol% Ge exhibited high α (10.2), low EB (14.1Vmm−1), and high ΦB (0.95eV). These results are superior to previous findings. In addition, Ge as sintering aid reduced the sintering temperature caused by the low melting point. The optimal sintering temperature was 1300°C for the TiO2–Ta2O5–CaCO3 ceramics doped with Ge.

Current injection from metal to MoS2 probed at nanoscale by conductive atomic force microscopy

Contacts with MoS2 are currently the object of many investigations, since current injection through metal/MoS2 interfaces represents one of the limiting factors to the performance of MoS2 thin film transistors. In this paper, we employed conductive atomic force microscopy (CAFM) to investigate the current injection mechanisms from a nanometric contact (a Pt coated tip) to the surface of MoS2 thin films exfoliated on SiO2. The analysis of local current–voltage (I–V) characteristics on a large array of tip positions provided high spatial resolution information on the lateral homogeneity of the tip/MoS2 Schottky barrier ΦB and of the ideality factor n. From the histograms of the measured ΦB and n values, an average Schottky barrier height of 297meV with standard deviation of 22meV and an average ideality factor of 1.65 with a standard deviation is 0.15 have been estimated. The implications of these lateral variations of ΦB and n in MoS2 nano-Schottky diodes on the electrical properties of macroscopic contacts to MoS2 have been discussed also in relation with recent literature results.

Electrical characterization of metal/diamond-like carbon/inorganic semiconductor MIS Schottky barrier diodes

In this study, electrodeposited diamond-like carbon (DLC) film was used to fabricate 12 identical Au/DLC/p-Si metal–interlayer–semiconductor (MIS) Schottky diodes. Current–voltage (I–V) measurements were carried out to obtain diodes' parameters. The rectification ratios of the MIS diodes were found between 103 and 104. By using the forward bias I–V characteristics, the average ideality factor (n) and barrier height (Φb) values of Au/DLC/p-Si MIS structures were found as 2.06 and 0.84eV, respectively. The Φb value of 0.84eV obtained for Au/DLC/p-Si MIS diode was much higher than the value of 0.34 of conventional Au/p-Si Schottky diode. Cheung–Cheung functions were also used to extract barrier height and series resistance values and the obtained results were compared with I–V method. I–V measurements were also carried out under the hydrostatic pressure in the range of 0.0–2.0kbar. It was found that barrier height and series resistance values were highly pressure sensitive and increased with the ascending pressure. It was proposed that Au/DLC/p-Si Schottky MIS diode can be used as pressure sensor due to high sensitivity to changing pressure.

Effect of bias voltage on tunneling mechanism in Co40Fe40B20/MgO/Co40Fe40B20 pseudo-spin valve

Abstract Bias voltage dependence of tunneling mechanism has been systematically investigated in Co40Fe40B20 (2.1 nm)/MgO (2 nm)/Co40Fe40B20 (1.7 nm) pseudo-spin valve magnetic tunnel junction deposited using the combination of the pulsed DC unbalanced magnetron and RF magnetron sputtering techniques. Structural investigations revealed polycrystalline and partially (001) oriented growth of CoFeB/MgO(001) MTJ with similar low interface roughness on both side of the MgO barrier. The junction with a 25×25 µm2 area indicates a giant tunnel magnetoresistance in the order of 505% at room temperature. The magnetoresistance ratio decreases with increasing applied bias voltage ranging from 0.5 to 1.8 V. Reasonable values for barrier thickness and heights were obtained using the combination of Brinkman and Gundlach models, including average barrier height and symmetry. Both barrier parameters and the tunneling mechanism vary in dependence of applied bias voltage. The tunneling mechanism indicates a change from direct to the FN tunneling, especially when reaching high bias voltages. Effect of the tunneling mechanism on the bias dependence of the magnetoresistance was also discussed.

Improvement in gate bias stress instability of amorphous indium-gallium-zinc oxide thin-film transistors using microwave irradiation

In this study, we evaluated the effects of microwave irradiation (MWI) post-deposition-annealing (PDA) treatment on the gate bias stress instability of amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs) and compared the results with a conventional thermal annealing PDA treatment. The MWI-PDA-treated a-IGZO TFTs exhibited enhanced electrical performance as well as improved long-term stability with increasing microwave power. The positive turn-on voltage shift (ΔVON) as a function of stress time with positive bias and varying temperature was precisely modeled on a stretched-exponential equation, suggesting that charge trapping is a dominant mechanism in the instability of MWI-PDA-treated a-IGZO TFTs. The characteristic trapping time and average effective barrier height for electron transport indicate that the MWI-PDA treatment effectively reduces the defects in a-IGZO TFTs, resulting in a superior resistance against gate bias stress.

Potential variations around grain boundaries in impurity-doped BaSi2 epitaxial films evaluated by Kelvin probe force microscopy

Potential variations around the grain boundaries (GBs) in antimony (Sb)-doped n-type and boron (B)-doped p-type BaSi2 epitaxial films on Si(111) were evaluated by Kelvin probe force microscopy. Sb-doped n-BaSi2 films exhibited positively charged GBs with a downward band bending at the GBs. The average barrier height for holes was approximately 10 meV for an electron concentration n ≈ 1017 cm−3. This downward band bending changed to upward band bending when n was increased to n = 1.8 × 1018 cm−3. In the B-doped p-BaSi2 films, the upward band bending was observed for a hole concentration p ≈ 1018 cm−3. The average barrier height for electrons decreased from approximately 25 to 15 meV when p was increased from p = 2.7 × 1018 to p = 4.0 × 1018 cm−3. These results are explained under the assumption that the position of the Fermi level Ef at GBs depends on the degree of occupancy of defect states at the GBs, while Ef approached the bottom of the conduction band or the top of the valence band in the BaSi2 grain interiors with increasing impurity concentrations. In both cases, such small barrier heights may not deteriorate the carrier transport properties. The electronic structures of impurity-doped BaSi2 are also discussed using first-principles pseudopotential method to discuss the insertion sites of impurity atoms and clarify the reason for the observed n-type conduction in the Sb-doped BaSi2 and p-type conduction in the B-doped BaSi2.

Measurements of tunneling barrier thicknesses for Nb/Al–AlOx/Nb tunnel junctions

The tunnel barrier thicknesses of Nb/Al–AlOx/Nb tunnel junctions were measured using transmission electron microscopy (TEM) and X-ray Reflection (XRR). By investigating the barrier thickness dependence of current density Jc, the barrier height for Nb/Al–AlOx/Nb junctions was calculated. Nb/Al–AlOx/Nb junctions with different Jc were fabricated by controlling the O2 exposure in Al oxidation. The junctions show good tunneling properties with subgap leakage factor Vm larger than 30mV in the range of Jc from tens of A/cm2 to several kA/cm2. TEM images showed clear interface and indicated the AlOx thicknesses ranging from 0.8nm to 1.9nm, and the average barrier height was estimated to be 0.17eV for Nb/Al–AlOx/Nb tunnel junctions.

Electrical properties of TiN on gallium nitride grown using different deposition conditions and annealing

This study evaluates the thermal stability of different refractory metal nitrides used as Schottky electrodes on GaN. The results demonstrate that TiN, MoSiN, and MoN possess good rectification and adhesion strength, with barrier heights of 0.56, 0.54, and 0.36 eV, respectively. After thermal treatment at 850 °C for 1 min, the TiN and MoN electrodes still exhibit rectifying characteristics, while the MoSiN degrades to an ohmic-like contact. For further study, several TiN films are deposited using different N2/Ar reactive/inert sputtering gas ratios, thereby varying the nitrogen content present in the sputtering gas. Ohmic-like contact is observed with the pure Ti contact film, and Schottky characteristics are observed with the samples possessing nitrogen in the film. The average Schottky barrier height is about 0.5 eV and remains virtually constant with varying nitrogen deposition content. After examining Raman spectra and x-ray photoelectron spectroscopy results, the increase in the film resistivity after thermal treatment is attributed to oxidation and/or nitridation. Films deposited with a medium (40% and 60%) nitrogen content show the best film quality and thermal stability.

High-performance ferroelectric memory based on fully patterned tunnel junctions

In tunnel junctions with ferroelectric barriers, switching the polarization direction modifies the electrostatic potential profile and the associated average tunnel barrier height. This results in strong changes of the tunnel transmission and associated resistance. The information readout in ferroelectric tunnel junctions (FTJs) is thus resistive and non-destructive, which is an advantage compared to the case of conventional ferroelectric memories (FeRAMs). Initially, endurance limitation (i.e., fatigue) was the main factor hampering the industrialization of FeRAMs. Systematic investigations of switching dynamics for various ferroelectric and electrode materials have resolved this issue, with endurance now reaching 1014 cycles. Here we investigate data retention and endurance in fully patterned submicron Co/BiFeO3/Ca0.96Ce0.04MnO3 FTJs. We report good reproducibility with high resistance contrasts and extend the maximum reported endurance of FTJs by three orders of magnitude (4 × 106 cycles). Our results indicate that here fatigue is not limited by a decrease of the polarization or an increase of the leakage but rather by domain wall pinning. We propose directions to access extreme and intermediate resistance states more reliably and further strengthen the potential of FTJs for non-volatile memory applications.

Evaluation of potential variations around grain boundaries in BaSi2 epitaxial films by Kelvin probe force microscopy

Potential variations around the grain boundaries (GBs) on the surface in undoped n-BaSi2 epitaxial films on Si(111) and Si(001) were analyzed using Kelvin prove force microcopy. The potentials were higher at GBs than those in the BaSi2 grains on Si(111). The average barrier height was approximately 30 meV at the GBs, indicating that the enhanced potentials repulse photogenerated holes so that the charge carrier recombination can be effectively reduced. In contrast, the potentials were smaller at GBs in the BaSi2 on Si(001), and the average barrier heights were approximately 30 and 50 meV along Si[1–10] and [110], respectively.

Dynamics of hydrogen sensing with Pt/TiO2 Schottky diodes

The dynamics of hydrogen sensing with nanoporous Pt/TiO2 Schottky barriers is studied by time-resolved electronic transport measurements. The development with time of the doping density, the average Schottky barrier height, and the built-in voltage are determined from current-voltage and capacitance-voltage characteristics. The current-voltage characteristics change from exponential towards Ohmic behavior as time proceeds. This is interpreted in terms of local areas that switch from diodic to Ohmic response due to a combination of increasing doping density and decreasing Schottky barrier height.

Tunneling investigation of the electron scattering effect on the momentum-dependent energy gap distribution in MgB2

We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB2 by measuring planar tunnel junctions made from epitaxial MgB2 thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the π (∼1.8 meV) and σ (∼7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The σ gap was enhanced (∼7.9 meV) in MgB2 thin films on SiC substrates which had Tc values over 40 K due to epitaxial tensile strain. As the MgB2 film thickness decreased from 100 nm to 33 nm, the π gap distribution range was narrowed from ∼1.7 meV to ∼1.4 meV, and the fine peak structures disappeared due to the reduced electron mean free path limited by the film thickness. The fine peak structures were also gradually smeared out when the junction resistance increased. The results show that the electron scattering, either from the MgB2 film or the junction interface or barrier, can smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB2 film surface was also found to cause an increase in the π gap value, likely due to an enhancement of interband scattering.

Reactions of Nb2 and Nb3 with CO, D2, N2, and O2: Reconciling experimental kinetics with density functional theory-calculated reaction profiles

Calculated energy profiles for the reactions of neutral Nb(2) and Nb(3) metal clusters with CO, D(2), N(2), and O(2) are presented. In each reaction path, both a physisorption energy minimum, where the reactant remains intact, and a chemisorption energy minimum, where the reactant has dissociated, are calculated and linked by saddle points. We calculate branching ratios for the forward (dissociative) and reverse reactions which we compare with the experimental kinetic data. It is found that a combination of average thermal energies and barrier heights leads to wide variation in branching ratios which compares favourably to previously determined experimental reaction rates.

Strain sensing mechanism of the fabricated ZnO nanowire-polymer composite strain sensors

The nature of contacts between ZnO semiconductor nanowires and metal pads has attracted great research interests since its impact on enhancing sensitivity of the sensors. Herein, ZnO nanowire arrays were synthesized by a scalable wet chemical method and ZnO nanowire-polymer composite strain sensors were fabricated by utilizing Schottky contact. The electrical transport response to strain was attributed to piezoelectric-effect-induced Schottky barrier height changes at the metal–semiconductor junction under various strained states. When the strain increased from 0% to 0.12%, the average barrier height change increased to 26meV, inducing sensitive current changes in these strain sensors.

Carbinolamine Formation and Dehydration in a DNA Repair Enzyme Active Site

In order to suggest detailed mechanistic hypotheses for the formation and dehydration of a key carbinolamine intermediate in the T4 pyrimidine dimer glycosylase (T4PDG) reaction, we have investigated these reactions using steered molecular dynamics with a coupled quantum mechanics–molecular mechanics potential (QM/MM). We carried out simulations of DNA abasic site carbinolamine formation with and without a water molecule restrained to remain within the active site quantum region. We recovered potentials of mean force (PMF) from thirty replicate reaction trajectories using Jarzynski averaging. We demonstrated feasible pathways involving water, as well as those independent of water participation. The water–independent enzyme–catalyzed reaction had a bias–corrected Jarzynski–average barrier height of approximately for the carbinolamine formation reaction and ) for the reverse reaction at this level of representation. When the proton transfer was facilitated with an intrinsic quantum water, the barrier height was approximately in the forward (formation) reaction and for the reverse. In addition, two modes of unsteered (free dynamics) carbinolamine dehydration were observed: in one, the quantum water participated as an intermediate proton transfer species, and in the other, the active site protonated glutamate hydrogen was directly transferred to the carbinolamine oxygen. Water–independent unforced proton transfer from the protonated active site glutamate carboxyl to the unprotonated N–terminal amine was also observed. In summary, complex proton transfer events, some involving water intermediates, were studied in QM/MM simulations of T4PDG bound to a DNA abasic site. Imine carbinolamine formation was characterized using steered QM/MM molecular dynamics. Dehydration of the carbinolamine intermediate to form the final imine product was observed in free, unsteered, QM/MM dynamics simulations, as was unforced acid-base transfer between the active site carboxylate and the N–terminal amine.