Variation Of Barrier Height Research Articles

Electrical and microstructural properties of (Zn, Nb, Fe)-doped SnO 2 varistor systems

The electrical and microstructural properties of SnO 2-based varistors with the addition of 0.025 and 0.050 mol% of Fe 2O 3 have been characterised. Electric field ( E) versus current density ( J) curves showed that the effect of Fe 2O 3 addition is to increase both the non-linear coefficient and the breakdown voltage. Variations in the potential barrier height were inferred from impedance spectroscopy (IS) analysis. Through transmission electron microscopy (TEM), the presence of precipitates of secondary phases was confirmed. Samples with precipitates displayed poor electrical properties.

Influence of hydrogen treatment and annealing processes upon the Schottky barrier height of Au/n-GaAs and Ti/n-GaAs diodes

After annealing in a H2 atmosphere at different temperatures, ⟨100⟩-oriented n-GaAs substrates were metallized with Au and Ti layers of different thicknesses to form Au/n-GaAs and Ti/n-GaAs Schottky diodes. The Schottky barrier height (SBH) variation and its dependence on subsequent N2 annealing for these Schottky diodes have been studied by different measurement techniques (I–V, C–V and BEEM) to obtain reliable values. These methods show that pre-metallization annealing leads to a less homogeneous metal semiconductor (MS) interface. In case of thick Au layers the effective barrier height is reduced as soon as the H2 annealing temperature reaches 300 °C. However, GaAs samples covered with thin Au layers or Ti layers do not exhibit such a barrier height reduction. The lower value in the case of thick Au layers is attributed to H+ groups, present at the interface due to the annealing in H2 atmosphere, forming interfacial dipoles with Au, leading to an inhomogeneous barrier and a decrease of the effective barrier height. It seems that these dipoles disappear again in the case of thin Au layers or are not formed with Ti. Post-metallization N2 annealing at higher temperatures lowers the barrier height for all samples. The resulting barrier inhomogeneities are explained and analysed using the bond polarization theory of Tung (2001 Phys. Rev. B 64 205310, 2001 Mater. Sci. Eng. R 35), a recent Schottky barrier formation model and the pinch-off model (Tung 1992 Phys. Rev. B 45 13509).

Surface cleaning and annealing effects on Ni∕AlGaN interface atomic composition and Schottky barrier height

Internal photoemission spectroscopy reveals changes in the Schottky barrier height of Ni on AlGaN∕GaN high electron mobility transistor structures with premetallization processing conditions and postmetallization ultrahigh-vacuum annealing. These variations in the internal photoemission Schottky barrier height are correlated with AlGaN near-band-edge emissions from low-energy electron-excited nanoluminescence spectroscopy and Ni∕AlGaN interface impurities by secondary ion mass spectrometry. We show that changes in the Schottky barrier height and the appearance of dual barriers are dominated by changes in the local Al mole fraction. Interfacial oxygen and carbon have secondary but systematic effects as well.

Local transport properties of Co-Fe/Al-O/Co-Fe tunnel junctions with high thermal stability

In order to investigate the annealing temperature dependence of the tunnel magnetoresistance ratio (TMR) for ferromagnetic tunnel junction, the local transport properties were measured for Ta 5nm/Cu 20nm/Ta 5nm/Ni76Fe24 2nm/Cu 5nm/Mn75Ir25 10nm/Co71Fe29 4nm/Al-O junction with metal Al thickness of 0.8nm and oxidization time of 7s. The current histogram statistically calculated from the electrical current image was in good accordance with the fitting result considering the Gaussian distribution of barrier height and Fowler-Nordheim conductance. At high annealing temperature of 340°C, where the TMR ratio of the corresponding MTJ had the maximum value of 44%, the average barrier height φave increased to 1.12eV and the standard deviation σφ decreased to 0.1eV. The increase of TMR ratio after annealing was attributed to the increase of average barrier height and decrease of the barrier height variation. After further annealing at 360°C, the standard deviation σφ increased up to 0.2eV and the TMR decreased to 10%.

Exploring the electronic and mechanical properties of protein using conducting atomic force microscopy.

In interfacing man-made electronic components with specifically folded biomacromolecules, the perturbative effects of junction structure on any signal generated should be considered. We report herein on the electron-transfer characteristics of the blue copper metalloprotein, azurin, as characterized at a refined level by conducting atomic force microscopy (C-AFM). Specifically, the modulation of current-voltage (I-V) behavior with compressional force has been examined. In the absence of assignable resonant electron tunneling within the confined bias region, from -1 to 1 V, the I-V behavior was analyzed with a modified Simmons formula. To interpret the variation of tunneling barrier height and barrier length obtained by fitting with the modified Simmons formula, an atom packing density model associated with protein mechanical deformation was proposed and simulated by molecular dynamics. The barrier heights determined at the minimum forces necessary for stable electrical contact correlate reasonably well with those estimated from bulk biophysical (electroanalytical and photochemical) experiments previously reported. At higher forces, the tunnel barrier decreases to fall within the range observed with saturated organic systems. Molecular dynamics simulations revealed changes in secondary structure and atomic density of the protein with respect to compression. At low compression, where transport measurements are made, secondary structure is retained, and atomic packing density is observed to increase linearly with force. These predictions, and those made at higher compression, are consistent with both experimentally observed modulations of tunneling barrier height with applied force and the applicability of the atom packing density model of electron tunneling in proteins to molecular-level analyses.

Characteristics of Pd/InGaP Schottky Diodes Hydrogen Sensors

Pd/InGaP hydrogen sensors based on the metal-oxide-semiconductor (MOS) and metal-semiconductor Schottky diodes have been fabricated and systematically studied. The effects of hydrogen adsorption on device performances such as the current-voltage characteristics, barrier height variation, hydrogen coverage, and heat of adsorption are investigated. The studied devices exhibit very wide hydrogen concentration detection regimes and remarkable hydrogen-sensing properties. Particularly, an extremely low hydrogen concentration of 15 ppm H/sub 2//air at room temperature can be detected. In addition, under the presence of oxide layers in the studied MOS device structure, the enhancements of barrier height and high-temperature operating capability are observed. The initial heat of adsorption for Pd/oxide and Pd/semiconductor interface are calculated as 355 and 65.9 meV/atom, respectively. Furthermore, the considerably short response times are found in studied devices.

Investigation of hydrogen-sensing properties of Pd/AlGaAs-based Schottky diodes

The hydrogen response characteristics and sensing properties or catalytic Pd/Al/sub 0.3/Ga/sub 0.7/As metal-oxide-semiconductor (MOS) and metal-semiconductor (MS) Schottky diodes are systematically studied. The effects of hydrogen adsorption on device performances such as the current-voltage characteristics, sensitivity, barrier height variation, heat of adsorption, and transient response are investigated. The studied devices can be operated under very wide hydrogen concentration regimes with remarkable hydrogen-sensing properties. Particularly, at an extremely low hydrogen concentration of 15 ppm H/sub 2//air, both steady-state and transient responses at room temperature can be detected. In addition, under the presence of oxide layer in the studied MOS device, a larger change of barrier height and higher hydrogen response are observed. In addition, according to the van't Hoff equation, the initial values of heat adsorption for Pd/semiconductor and Pd/oxide interface are calculated as 7.29 and 49.6 KJ/mole, respectively.

Schottky barrier heights at polar metal/semiconductor interfaces

Using a first-principle pseudopotential approach, we have investigated the Schottky barrier heights of abrupt Al/Ge, Al/GaAs, Al/AlAs, and Al/ZnSe (100) junctions, and their dependence on the semiconductor chemical composition and surface termination. A model based on linear-response theory is developed, which provides a simple, yet accurate description of the barrier-height variations with the chemical composition of the semiconductor. The larger barrier values found for the anion- than for the cation-terminated surfaces are explained in terms of the screened charge of the polar semiconductor surface and its image charge at the metal surface. Atomic scale computations show how the classical image charge concept, valid for charges placed at large distances from the metal, extends to distances shorter than the decay length of the metal-induced-gap states.

On the Origin of Higher Rotational Barriers in Thioamides than in Amides. Remote Substituent Effects on the Conformational Stability of the Thioamide Group in Thioacetanilides

The variations of barrier heights for rotation around the amide C−N bond in para-substituted thioacetanilides was studied in a comparative context with data for a series of acetanilides by applying density functional theory at the B3LYP/6-31G(d,p) level. The shifts of the energy barriers induced by remote substituents in the para position of the aromatic ring are interpreted in terms of changes of the bond lengths and electric-charge parameters for the equilibrium configurations, as well as in terms of charge fluctuations that accompany the internal rotation. A satisfactory explanation for the distinctly greater shifts of rotational barriers in the thioacetanilide series upon substitution in the aromatic ring, as compared with data for the respective acetanilides, is obtained by analyzing the dependences of barriers heights on the charge shifts upon substitution in the aromatic ring and upon rotation. Several linear relationships are established linking the barrier heights with structural and electronic parameters that characterize the amide and thioamide groupings. A common linear equation quantifying the dependence of barrier heights on the shifts of natural bond orbital partial charges at the amide S and O atoms upon substitution in the aromatic ring in the two series is found. The relationship is confirmed by data from a Mulliken population analysis. The results obtained are consistent with the views for a classical amide resonance as being the origin of higher rotational barriers in thioamides than in amides.

A hydrogen sensing Pd/InGaP metal-semiconductor (MS) Schottky diode hydrogen sensor

An interesting hydrogen sensing Pd/InGaP metal-semiconductor (MS) Schottky diode has been fabricated and studied. Both the steady state and the transient condition of the hydrogen adsorption process are investigated. Even at room temperature, an extremely low hydrogen concentration of 15 ppm H2/air can be detected. In addition, the wide operating temperature range of 250 K of the studied Pd/InGaP hydrogen sensor is found. From experimental results, it is shown that the variation of Schottky barrier height increases with the increase of the operating temperature and hydrogen concentration. As the operation temperature is elevated, the water formation effect is also studied in the quasi-equilibrium region under the transient condition.

Characteristics of a new Pt/oxide/In 0.49Ga 0.51P hydrogen-sensing Schottky diode

A new Pt/oxide/InGaP metal-oxide semiconductor (MOS) Schottky diode has been fabricated and studied. Upon exposure to hydrogen, the steady-state and transient responses under different hydrogen concentrations and temperatures are measured. Due to the inherent property of InGaP material, e.g. the wide energy gap, a wide hydrogen-sensing range as large as 300 K (from room temperature to 600 K) is obtained. Even at room temperature, a very high sensitivity over 500% for 9090 ppm hydrogen in air is acquired. Furthermore, the measured absorption response time is less than 1 s at the applied voltage of 0.7 V and 9090 ppm hydrogen concentration atmosphere condition. Simultaneously, based on the analysis of the variation of barrier height and hydrogen coverage, the characteristics of the studied Pt/oxide/InGaP MOS Schottky diode is in good agreement with the Lundström isotherm.

Polymer light emitting diodes based on LiF/Al composite cathode

We report on the performances of polymer light emitting diodes utilizing LiF/Al composite cathodes. Among the devices with the composite cathodes of different concentration of LiF, the layer by layer device is the most efficient, which is different from the results based on small molecule organic light diode devices. The different performances of the devices can be ascribed to the variation of the electron injection barrier heights at the polymer/cathode interface, as suggested by photovoltaic experiments.

Direct evidence for grain-boundary depletion in polycrystalline CdTe from nanoscale-resolved measurements

We use scanning probe microscopy-based methods for direct characterization of a single grain boundary and a single grain surface in solar cell-quality CdTe, deposited by closed-space vapor transport. We find that scanning capacitance microscopy can serve to study polycrystalline electronic materials, notwithstanding the strong topographical variations. In this way, we find a barrier for hole transport across grain boundaries, a conclusion supported by the much more topography-sensitive scanning kelvin probe microscopy, with some variation in barrier height between different boundaries.

A new Pt/oxide/In/sub 0.49/Ga/sub 0.51/P MOS Schottky diode hydrogen sensor

A new and interesting Pt/oxide/In/sub 0.49/Ga/sub 0.51/P metal-oxide-semiconductor (MOS) Schottky diode hydrogen sensor has been fabricated and studied. The steady-state and transient responses with different hydrogen concentrations and at different temperatures are measured. The presence of dipoles at the oxide layer leads to an extra electrical field and the variation of Schottky barrier height. Even at room temperature, a very high hydrogen detection sensitivity of 561% is obtained when a 9090 ppm H/sub 2//air gas is introduced. In addition, an absorption response time less than 1 s under the applied voltage of 0.7 V and 9090 ppm H/sub 2//air hydrogen ambient is found. The roles of hydrogen adsorption and desorption for the transient response at different temperatures are also investigated.

Barrier height determination of SiC Schottky diodes by capacitance and current–voltage measurements

Extractions of barrier heights of 6H and 4H-SiC Schottky diodes have been performed on structures with various gate metallization, using both capacitance–voltage (C–V) and current–voltage (I–V) measurements. The sum of the two barriers extracted by C–V measurements on both n-type and p-type materials is found to be higher than the band gap energy EG, whereas the one extracted by I–V is less than EG. However, above room temperature, temperature variations of barrier heights are in agreement with the variations of EG. We have also computed theoretical I–V characteristics using a two-barrier height model. By taking account of temperature variations of a large number of parameters, e.g., the carrier mobility, free carrier concentration, and barrier height, we have achieved a good fit with experimental data. The model is shown to be valid for n-type Schottky diodes over a wide range of temperatures (from 100 to 500 K).

Electron transport measurements of Schottky barrier inhomogeneities

We report nonmonotonicities in the low-temperature current versus gate voltage characteristics of PtSi/Si Schottky Barrier metal–oxide–semiconductor field-effect transistors. Direct tunneling through the Schottky barrier is shown to limit the current and be superimposed with resonant peaks and oscillations. These structures are attributed to resonant tunneling through impurities located close to the interface and nonuniformities of the heterojunction. We thus demonstrate barrier height variations in electron transport through a relatively large metal/semiconductor contact area. The inhomogeneities result in different average Schottky barrier heights between devices, and cause height variations as a function of carrier concentration within a metal/semiconductor interface.

Ballistic electron emission microscopy of “on-surface” self-assembled InAs dots and wetting layers

In this work ballistic electron emission microscopy and spectroscopy (BEEM/BEES) are employed to investigate electronic properties of self-assembled InAs quantum dots on a GaAs substrate. BEEM spectra are presented for positions on the dots on the one hand and for “off-dot” regions, i.e., on the wetting layer, on the other hand. The local barrier height variations on the dots and on the InAs wetting layer are studied systematically and compared with data from the literature obtained on homogeneous InAs layers of various thickness. In addition, the temperature dependence of the InAs–GaAs barrier height is investigated. At low temperatures, evidence of higher conduction bands both in the GaAs and in the InAs dots is found.

Ballistic electron emission microscopy for local measurements of barrier heights on InAs self-assembled quantum dots on GaAs

In this work, we employ the ballistic electron emission microscopy and spectroscopy (BEEM/S) to investigate the electronic properties of self-assembled InAs quantum dots on a GaAs substrate. The BEEM spectra are taken both on the dots and in the area between the dots, i.e. on the InAs wetting layer, and the corresponding barrier heights are determined. The local barrier-height variations on the dots and on the InAs wetting layer are studied systematically and compared with data from the literature obtained on strained InAs layers of various thicknesses. In addition, the temperature dependence of the InAs/GaAs barrier height is investigated.

Doping dependence of the barrier height and ideality factor of Au/n-GaAs Schottky diodes at low temperatures

The barrier height and ideality factor of Au/n-GaAs Schottky diodes grown by metal-organic vapor-phase epitaxy (MOVPE) on undoped and Si-doped n-GaAs substrates were determined in the doping range of 2.5×10 15–1×10 18 cm −3 at low temperatures. The thermionic-emission zero-bias barrier height for current transport decreases rapidly at concentrations greater than 1×10 18 cm −3. The ideality factor also increases very rapidly at higher concentration and at lower temperature. The results agree quite well with thermionic field emission (TFE) theory. The doping dependence of the barrier height and the ideality factor were obtained in the concentration range of 2.5×10 15–1.0×10 18 cm −3 and the results are well described using TFE theory. An excellent match between the homogeneous barrier height and the effective barrier height was observed which supports the good quality of the GaAs film. The observed variation in the zero-bias barrier height and the ideality factor can also be explained in terms of barrier height inhomogeneities in the Schottky diode.

Local transport properties of ferromagnetic tunnel junctions

The local electrical properties were measured simultaneously with the topography for a SiO 2/Ta(3 nm)/ Fe 20Ni 80(3 nm) /Pt(20 nm)/Fe 20Ni 80(3 nm) /IrMn(10 nm)/Co 75Fe 25(4 nm)/Al(0.8 nm)-oxide junction. The current image became very homogeneous and smooth after annealing at around 300 ∘C for 1 h. Increase of tunneling magnetoresistance ratio of the junction after annealing can be well explained by taking into account both increase of the barrier height and decrease of the barrier height variation. After further annealing over 350 ∘C, the barrier height decreased and leak currents were detected. Reduction of spin polarization of the ferromagnetic electrodes due to interface mixing or damage of the insulator due to the growth of grains of bottom electrode is a possible reason for the drastic decrease of TMR ratio for annealing at temperatures higher than 350 ∘C.