Average Barrier Height Research Articles

Diffusion-Limited a-IGZO/Pt Schottky Junction Fabricated at 200 $^{\circ}\hbox{C}$ on a Flexible Substrate

Diffusion-limited Schottky junctions were fabricated at 200°C by employing a top amorphous In-Ga-Zn-O (a-IGZO)/bottom Pt electrode structure. Those fabricated on glass exhibited rectification ratios of Ion/off >; 108 and ideality factors close to unity n = 1.04. The temperature dependence values of current-voltage characteristics were small and not explained by a simple thermionic emission theory; instead, a percolation model explains them with an average Schottky barrier height of 1.2 eV and its distribution width of 0.13 eV. Flexible a-IGZO/Pt Schottky junctions were also demonstrated on polyimide substrates with n = 1.2 and Ion/off >; 105.

Thermal annealing behaviour of Pd Schottky contacts on melt-grown single crystal ZnO studied by IV and CV measurements

Abstract Current–voltage (IV) and capacitance–voltage (CV) measurement techniques have successfully been employed to study the effects of annealing highly rectifying Pd/ZnO Schottky contacts. IV results reveal a decrease in the contact quality with increasing annealing temperature as confirmed by a decrease in the zero bias barrier height and an increase in the reverse current measured at −1.5 V. An average barrier height of (0.77 ± 0.02) eV has been calculated by assuming pure thermionic emission for the as-deposited material and as (0.56 ± 0.03) eV after annealing at 550 °C. The reverse current has been measured as (2.10 ± 0.01) × 10−10 A for the as-deposited and increases by 5 orders of magnitude after annealing at 550 °C to (1.56 ± 0.01) × 10−5 A. The depletion layer width measured at −2.0 V has shown a strong dependence on thermal annealing as it decreases from 1.09 μm after annealing at 200 °C to 0.24 μm after annealing at 500 °C, resulting in the modification of the dopant concentration within the depletion region and hence the current flowing through the interface from pure thermionic emission to thermionic field emission with the donor concentrations increasing from 6.90 × 1015 cm−3 at 200 °C to 6.06 × 1016 cm−3 after annealing at 550 °C. This increase in the volume concentration has been explained as an effect of a conductive channel that shifts closer to the surface after sample annealing. The series resistance has been observed to decrease with increase in annealing temperature. The Pd contacts have shown high stability up to an annealing temperature of 250 °C as revealed by the IV and CV characteristics after which the quality of the contacts deteriorates with increase in annealing temperature.

Electrical properties of Poly(ethylene glycol dimethacrylate-n-vinyl imidazole)/Single Walled Carbon Nanotubes/n-Si Schottky diodes formed by surface polymerization of Single Walled Carbon Nanotubes

In this paper we report the electrical characteristics of the Schottky diodes formed by surface polymerization of the Poly(ethylene glycol dimethacrylate-n-vinyl imidazole)/Single Walled Carbon Nanotubes on n-Si. The Single Walled Carbon Nanotubes were synthesized by CVD method. The main electrical properties of the Poly(ethylene glycol dimethacrylate-n-vinyl imidazole)/Single Walled Carbon Nanotubes/n-Si have been investigated through the barrier heights, the ideality factors and the impurity density distribution, by using current–voltage and reverse bias capacitance voltage characteristics. Electrical measurements were carried out at room temperature. Poly(ethylene glycol dimethacrylate-n-vinyl imidazole)/Single Walled Carbon Nanotubes/n-Si Schottky diode current–voltage characteristics display low reverse-bias leakage currents and average barrier heights of 0.61 ± 0.02 eV and 0.72 ± 0.02 eV obtained from both current–voltage and capacitance–voltage measurements at room temperature, respectively.

Investigation of current transport parameters of Ti/4H-SiC MPS diode with inhomogeneous barrier

The current transport parameters of 4H-SiC merged PiN Schottky (MPS) diode are investigated in a temperature range of 300–520 K. Evaluation of the experimental current–voltage (I—V) data reveals the decrease in Schottky barrier height Φb but an increase in ideality factor n, with temperature decreasing, which suggests the presence of an inhomogeneous Schottky barrier. The current transport behaviours are analysed in detail using the Tung's model and the effective area of the low barrier patches is extracted. It is found that small low barrier patches, making only 4.3% of the total contact, may significantly influence the device electrical characteristics due to the fact that a barrier height of 0.968 eV is much lower than the average barrier height 1.39 eV. This shows that ion implantation in the Schottky contact region of MPS structure may result in a poor Ti/4H-SiC interface quality. In addition, the temperature dependence of the specific on-resistance (Ron—sp), T2.14, is determined between 300 K and 520 K, which is similar to that predicted by a reduction in electron mobility.

Temperature dependent transport behavior of n-InN nanodot/p-Si heterojunction structures

The present work explores the temperature dependent transport behavior of n-InN nanodot/p-Si(100) heterojunction diodes. InN nanodot (ND) structures were grown on a 20 nm InN buffer layer on p-Si(100) substrates. These dots were found to be single crystalline and grown along [001] direction. The junction between these two materials exhibits a strong rectifying behavior at low temperatures. The average barrier height (BH) was determined to be 0.7 eV from current-voltage-temperature, capacitance-voltage, and flat band considerations. The band offsets derived from built-in potential were found to be ΔEC=1.8 eV and ΔEV=1.3 eV and are in close agreement with Anderson’s model.

Giant tunneling magnetoresistance with electron beam evaporated MgO barrier and CoFeB electrodes

Electron-beam (EB) evaporated MgO grows with (001) texture on amorphous CoFeB when the deposition rate is kept below 5 pm/s. Magnetic tunnel junctions (MTJs) fabricated using this method exhibit ∼240% magnetoresistance at room temperature for a 2.5 nm thick EB-MgO barrier, which is similar to the value for a radio frequency (rf) sputtered barrier with the same junction geometry. The average barrier height of the EB-MgO is 0.48 eV, which is higher than previously reported values for rf-MgO barriers and it increases with increasing annealing temperature. Our results show that EB-MgO could be a simpler alternative to rf-MgO in MTJs without any compromise in the tunnelling magnetoresistance.

Evaluation of physical parameters of localized states in insulating Y–Ba–Cu–O layers by means of electronic transport measurements

Electronic transport measurements were carried out on YBa 2Cu 3O 7− δ /insulator/Au planar junctions in order to determine physical parameters of the localized states in thin insulating Y–Ba–Cu–O layers. In doing so, 12×5 μm 2 YBa 2Cu 3O 7− δ /insulator/Au junction areas were defined by standard lithographic techniques and Ar ion milling. The analysis of the conductance of the junction at high-temperature or high-bias voltages showed that Mott's variable-range hopping conduction model is appropriated to describe the electrical behavior of the junction in this regime. From the fitting procedure, important physical parameters of the barrier such as the localization length α −1 (∼3 Å), the average barrier height φ (∼0.5 eV) or the variable range hopping length ℓ in (∼20 Å at ∼300 K) were estimated. The experimentally estimated values were physically reasonable and comparable to those reported for other oxide materials.

Temperature dependence of the specific resistance in Ti/Al/Ni/Au Ohmic contacts on (NH4)2Sx treated n-type GaN

The temperature dependence of the specific contact resistance in annealed Ti/Al/Ni/Au multilayers on (NH4)2Sx treated n-type GaN has been studied in the temperature range from 25 to 600 °C by the transmission line technique. It is found that the specific contact resistivity ρc of the sample treated with (NH4)2Sx solution for 5 min at 90 °C decreases with increasing measuring temperature, while the ρc of the sample treated with (NH4)2Sx solution for 25 min at 90 °C increases with increasing measuring temperature. Excellent agreement with the “5 min treated” sample can be obtained by the field emission model with an average Schottky barrier height (SBH) ϕB=1.05 eV. Meanwhile, a field emission model with a temperature-dependent effective SBH is suggested to be responsible for the “25 min treated” sample in which metal/semiconductor interface potential pinch-off may occur. The high-resolution transmission electron microscope results support the above model.

Improved Ni Schottky Contacts on n-Type 4H-SiC Using Thermal Processing

High-temperature processing was used to improve the barrier properties of three sets of n-type 4H-SiC Schottky diodes fabricated with Ni Schottky contacts. We obtained an optimum average barrier height of 1.78 eV and an ideality factor of 1.09 using current–voltage measurements on diodes annealed in vacuum at 500°C for 24 h. Nonannealed contacts had an average barrier height of 1.48 eV and an ideality factor of 1.85. The Rutherford backscattering spectra of the Ni/SiC contacts revealed the formation of a nickel silicide at the interface, accompanied by a substantial reduction in oxygen following annealing.

MESFETs Made From Individual GaN Nanowires

In this paper, we demonstrate novel MESFETs based on individual GaN nanowires. The Pt/Au Schottky gates exhibited excellent two-terminal Schottky diode rectification behavior. The average effective Schottky barrier height was 0.87 eV, with an average ideality factor of 1.6. In addition, the Schottky gates efficiently modulated the conduction of the nanowires. The threshold gate voltages required for complete pinch off were as small as -2.6 V, and transconductances exceeded 1.4 muS. Subthreshold swings approaching 60 mV/decade and on/off current ratios of up to 5times108 were achieved. These results show that the Schottky gate has the potential to significantly improve the performance of GaN nanowire field-effect devices.

Study of SrTiO 3 thin films grown by sputtering technique for tunnel barriers in quasiparticle injection contacts

High-quality, c-axis oriented YBa 2Cu 3O 7 − x /SrTiO 3/Au (YBCO/STO/Au) planar structures were fabricated in situ by direct current/radiofrequency inverted-cylinder magnetron sputtering on (001) STO oriented substrates. The sandwich-type structures were patterned to transistor dimensions by standard ultraviolet-photolithography and Ar etching. The current transport mechanism in the very thin STO barriers (2–30 nm) was examined by measuring the tunneling G as function of temperature ( T), and bias voltage ( V). It was found that resonant tunneling and hopping via a small number of localized states (LS) are responsible for electronic conduction in the insulating material. Elastic tunneling was observed for the case of a nominal 2 nm thick STO-barrier with an energy gap Δ ≈ 20 meV in the (001) direction of YBCO. On the other hand, inelastic hopping transport via n-LS dominated for STO barrier thickness d > 2 nm. G of the lowest-order hopping channel (hopping via two LS) exhibits the characteristic T and V dependences: G 2 hop( T) ∝ T 4/3, G 2 hop( V) ∝ V 4/3, respectively. Increasing the thickness of the STO barriers, hopping channels of higher order contribute more and more to the current transport as proven by measuring the T and V dependences. A crossover to variable range hopping behavior has been observed for junctions with thicker barriers ( d ≥ 20 nm) in the high- V or high- T regime. By fitting the experimental data to theoretical models, physical parameters of the LS could be determined. For instance, the value of the localization length or radius of the localized state was determined to be ~ 4.6 × 10 − 8 cm which corresponds to the lattice constant of the STO unit cell. A value of ~ 6 × 10 19 (eV) − 1 cm − 3 was calculated for the density of LS and the average barrier height was estimated as ~ 0.4 eV.

Methyl Rotation Barriers in Proteins from 2H Relaxation Data. Implications for Protein Structure

Side-chain 2H and backbone 15N relaxation data have been collected at multiple temperatures in the samples of the SH3 domain from alpha-spectrin. Combined analyses of the data allowed for determination of the temperature-dependent correlation times tauf characterizing fast methyl motion. Molecular dynamics simulations confirmed that tauf are dominated by methyl rotation; the corresponding activation energies approximate methyl rotation barriers. For 33 methyl groups in the alpha-spectrin SH3 domain the average barrier height was thus determined to be 2.8 +/- 0.9 kcal/mol. This value is deemed representative of the "fluid" hydrophobic protein core where some barriers are increased and others are lowered because of the contacts with surrounding atoms, but there is no local order that could produce systematically higher (lower) barriers. For comparison, the MD simulation predicts the average barrier of 3.1 kcal/mol (calculated via the potential of mean force) or 3.4-3.5 kcal/mol (rigid barriers after appropriate averaging over multiple MD snapshots). The latter result prompted us to investigate rigid methyl rotation barriers in a series of NMR structures from the Protein Databank. In most cases the barriers proved to be higher than expected, 4-6 kcal/mol. To a certain degree, this is caused by tight packing of the side chains in the NMR structures and stems from the structure calculation procedure where the coordinates are first annealed toward the temperature of 0 K and then subjected to energy minimization. In several cases the barriers >10 kcal/mol are indicative of van der Waals violations. The notable exceptions are (i) the structures solved using the GROMOS force field where tight methyl packing is avoided (3.0-3.6 kcal/mol) and (ii) the structure solved by means of the dynamic ensemble refinement method (Lindorff-Larsen, K.; Best, R. B.; DePristo, M. A.; Dobson, C. M.; Vendruscolo, M. Nature 2005, 433, 128) (3.5 kcal/mol). These results demonstrate that methyl rotation barriers, derived from the experiments that are traditionally associated with studies of protein dynamics, can be also used in the context of structural work. This is particularly interesting in view of the recent efforts to incorporate dynamics data in the process of protein structure elucidation.

Analysis of current–voltage–temperature characteristics in SiC Schottky diodes using threshold-accepting simulated-annealing techniques

Because of their high switching speeds and low power losses, metal-SiC Schottky-barrier diodes (SBD) are important to high performance, high temperature, and high frequency applications in power electronics. The use of 4H-SiC in SBDs is particularly advantageous because it has higher electron mobility than other SiC polytypes. However, due to surface non-homogeneity, the current–voltage characteristics of SiC SBDs are mostly non-ideal, and conventional analysis based on simple thermionic theory often leads to erroneous conclusions. In this work, we examine current–voltage–temperature properties of Ti on 4H-SiC SBDs and develop fitting algorithms to extract diode parameters based on non-uniform barrier height analysis approaches. These algorithms are based on “threshold-accepting simulated-annealing” techniques. The fitting yields a parameter set that is argued to better describe diode behavior: this parameter set is suggested to replace the average barrier height and the ideality factor often obtained from conventional Schottky diode analysis.

The calculation of electronic properties of an Ag/chitosan/n-Si Schottky barrier diode

In this study, the film of chitosan by adding the solution of chitosan being a polymeric compound on the top of an n-Si substrate and then by evaporating solvent was formed. It was seen that the chitosan/n-Si contact demonstrated clearly rectifying behavior and the reverse curves exhibit a weak bias voltage dependence by the current–voltage ( I– V) curves studied at room temperature. Average barrier height and ideality factor values for this structure were determined as 0.94 eV and 1.81, respectively. Furthermore, the energy distribution of the interface state density located in the semiconductor band gap at the chitosan/n-Si substrate in the energy range ( E c−0.785) to ( E c−0.522) eV have been determined from the I– V characteristics. The interface state density N ss ranges from 5.39 × 10 12 cm −2 eV −1 in ( E c−0.785) eV to 1.52 × 10 13 cm −2 eV −1 in ( E c−0.522) eV. The interface state density has an exponential rise with bias from the midgap towards the bottom of the conduction band.

Inductive to capacitive ac behavior of nanocrystalline silicon wires

The ac response of nanometer sized silicon percolating pathways, embedded into a silicon oxide matrix, was studied. CW laser treatments, performed on substoichiometric hydrogenated amorphous silicon oxide films, were used to induce silicon nanocrystals formation by solid state crystallization. A contact structure, probing the material along the laser treated traces, was employed. The real part of admittance showed a resonance-like response. The initial observation of a resistive behavior is followed by a decrease, reminiscent of an inductive effect, eventually turning into a further increase, typical of a capacitive behavior. The imaginary part of impedance and electric modulus showed temperature dependent Lorentzian peaks, 1.14 decades wide at half-maximum, with the same most probable hopping frequency and average barrier height equal to 0.22 ± 0.02 eV. On the other hand, loss tangent spectra addressed resistive losses, only. The conductivity relaxation time was found to be Arrhenius activated, with τ 0 = 8.2 ns, as the most probable hopping frequency. Analysis of such results supported the idea of an “apparent” inductivity related to a frequency dependent polarization state of defects located at the grain boundaries, which are mainly constituted by a residual thin layer of silicon oxide.

Barrier height homogeneity for 4.5 kV 4H-SiC Schottky diodes

4.5 kV SiC Schottky diodes have been fabricated using Ni as the Schottky contact . A manufacturing yield of 40% is reached for the bigger area diodes (1.6×1.6 mm 2 ) and of 70% for the smaller ones (0.4×0.4 mm 2 ). The measured variations of barrier height and ideality factor with temperature do not agree with the thermionic model. This has been interpreted in terms of barrier height inhomogeneities using the Werner model. We extracted an average barrier height ϕ b ¯ = 1 eV and its standard deviation ( σ = 90 mV ) . These two parameters are almost independent of the diode size. The variation of the barrier height distribution with field has also been investigated and shows a dependence similar to that of Schottky diodes realized from other semiconductor materials .

2-Diphenylphosphino-2′-hydroxybiphenyl-based P-ligands and their platinum(II) complexes

2-Diphenylphosphino-2′-hydroxybiphenyl ( 2), become readily available via a ring opening reaction of a dibenzo[ c. e][1,2]oxaphosphorine, was utilized as P-ligand in complexation with dichlorodibenzonitrile platinum to give a Cl 2Pt 2 2 complex ( 4) with trans geometry. The O-benzyl derivative ( 5) could not be involved in complexation. A bidentate P-ligand ( 7) obtained by the interaction of hydroxy-diphenylphosphinobiphenyl 2 with chloro-dibenzo[ c. e][1,2]oxaphosphorine ( 1) formed an 8-ring transition metal complex (Cl 2Pt 7 = 9) in reaction with PtCl 2(PhCN) 2. Under the conditions of chromatography, separation of the diastereomers of 7 was not possible due to a partial isomerisation by rotation around the biphenyl axis of the molecule that is justified by the average barrier height of 6.0 kcal/mol. The stereostructure of P-ligands 2 and 7, as well as platinum complexes 4 and 9 was evaluated by B3LYP quantum chemical calculations.

Prediction of Folding Rates of Small Proteins: Empirical Relations Based on Length, Secondary Structure Content, Residue Type, and Stability

Delineating the determinants of folding of small proteins is essential for decoding the folding code. By considering the literature data for the folding of 45 two-state proteins that differ widely in sequence, structure, and function, this work suggests two empirical relations for the prediction of the folding rate. One relation is based on the content of secondary structure elements, and the other uses the number of residues of each of the following types: hydrophobic, positively charged, and negatively charged. Both relations incorporate the chain length as an indirect descriptor. The correlation between experimental values for folding rates and free energy is poor, providing little support to the "stability gap" hypothesis based on the folding of simple lattice and off-lattice polymers. The average rate-determining barrier height for these two-state proteins is much larger than the small barriers for the transition-state ensemble envisaged by theoretical models.

Epitaxial Bi∕GaAs(111) diodes via electrodeposition

Bismuth films formed by electrodeposition on n-GaAs (111) at 70°C are found to be single crystalline, (0001) oriented, with trigonal surface morphologies typical of high quality single crystals. Diode current-voltage characteristics display low reverse-bias leakage currents and average barrier heights of 0.77±0.02eV (n=1.07). A necessary requirement for single crystalline growth is the presence of ammonium sulfate in the electrolyte.

TiB 2/TiSi 2 bilayer fabrication by various techniques: Structure and contact properties

TiB 2/TiSi 2 films were produced by several techniques in an attempt to evaluate the most appropriate method to fabricate this system. Analysis by X-ray diffraction, Auger electron spectroscopy, transmission and high-resolution transmission electron microscopy indicate that the best method to obtain the above system is by sequential cosputtering of the layers without exposure to air between the two cosputtering sequences. Post-deposition annealing was performed to obtain a low resistive bilayer. Schottky diodes fabricated by this method provided an average barrier height of ∼0.68 V with an ideality factor in the range of 1.0–1.04 (excluding the as-deposited specimen). In specimens fabricated by silicidation of TiB 2/Ti films formation of TiSi 2 was Ti thickness dependent [G. Sade, Ph.D. Thesis, Ben Gurion University of the Negev, Beer Sheva, Israel, 1999]. Small amounts of Ti 5Si 3 were observed at 1123 K. The attempts to obtain a TiB 2/TiSi 2 bilayer from (Ti+B) enriched with Ti at 1073 K resulted in the formation of small amounts of Ti 5Si 3, and some crystallization of the amorphous TiB 2 also occurred. Diodes fabricated by this technique showed Ohmic rather than rectifying character. The shift from rectifying to Ohmic behavior is the result of B out-diffusion to the Si and the consequent change of the substrate from an n- to a p-type Si. The results place the Fermi level of TiB 2 about 0.9 eV below the silicon conduction band. A remedy to this problem could result in a challenging method of fabricating a TiB 2/TiSi 2 bilayer structure in a one-step process.