Schottky Barrier Theory Research Articles

Modelling of the impedance behaviour of an amorphous semiconductor schottky barrier in high depletion conditions. Application to the study of the titanium anodic oxide/electrolyte junction

A mathematical model to describe the impedance behaviour of the amorphous semiconductor Schottky barrier in the high band bending region is proposed. The model is based on the admittance theory of the amorphous silicon Schottky barrier and it is valid in the polarisation region where the semiconductor displays a Mott—Schottky like behaviour. We have applied the model to the characterisation of very thin (10–44 nm) amorphous oxides formed electrochemically on titanium. The calculated parameters are shown to be in good agreement with those obtained from the classical (low band bending) amorphous silicon Schottky barrier theory for the same oxides. A comparative discussion on the advantages and handicaps of each model is carried out.

Calculations of the barrier height and charge distribution of a metal–dielectric interface

We present calculations of the barrier height (work function) and charge distribution of a metal–dielectric interface using a self‐consistent density functional method. The metal was modeled by a semiinfinite jellium and the dielectric was characterized by a single dielectric constant. Results show a monotonic decrease of over 2 eV in the work function of high density simple metals such as aluminum with increasing dielectric constant. As the dielectric constant becomes large, the work function approaches a constant value. This change is much larger than the roughly 0.5 eV change found by others using a variational density functional method, which also showed the work function increasing as the dielectric constant becomes large (e≳10). Low electron density metals display a smaller decrease in work function as the dielectric constant increases, due to the smaller electrostatic dipole layer at the interface. As a consequence, the work function difference between high and low electron density simple metals narrows from 1.1 to 0.15 eV in the limit of infinite screening. Similar work function screening effects are expected for noble metals, although the difference in work functions between noble metals may not be reduced as much as simple metals. These results are discussed in relation to current Schottky barrier theories and are compared to other models of metal–semiconductor interfaces.

Development and characterisation of polypyrrole/metal junctions for electronic applications

AbstractPreliminary results are presented on the fabrication and analysis of junctions formed between metals and semi‐conducting and conducting polypyrrole. Rectifying junctions were observed between aluminium and indium with semi‐conducting polypyrrole in a sandwich geometry using current‐voltage (I‐V) and capacitance‐voltage (C‐V) analysis techniques, respectively. All planar junctions analysed were non‐rectifying, while both gold and platinum were observed to form ohmic junctions in both the sandwich and planar geometries. Preliminary analysis using ideal Schottky barrier theory was attempted with various junction parameters determined, with significant deviation from the ideal theory observed. It is proposed that this deviation is due to the synthesis history of the polymer and the presence of an insulating oxide layer between the polymer and the metal.

The de-pinning of Schottky barrier at the In/n-GaAs(1 1 0) interface

The phenomenon of metal/semiconductor interface Fermi level “de-pinning” is observed at the interface of In on n-GaAs(1 1 0). The temperature dependence of Schottky barrier formation at the nonreactive In/ n-GaAs(1 1 0) interface has been studied by photoemission spectroscopy. For the n-GaAs(1 1 0) surface, which is kept at liquid nitrogen temperature during cleave and indium deposition, the surface Fermi level pinning at midgap does not occur until a complete In overlayer is formed. When the In/ n-GaAs(1 1 0) interface formed at low temperature is warmed up to room temperature, part of the GaAs surface band bending disappears, while the In overlayers froms clusters and the GaAs substrate is partly re-exposed. The phenomenon of “de-pinning” may shine new light on the Schottky barrier formation mechanism for nonreactive interface systems. Its implication is discussed in terms of various plausible Schottky barrier theories and models.

Amorphous semiconductor—electrolyte junction. Impedance study on the a-Nb 2 O 5—electrolyte junction

A systematic study of the impedance behaviour of the anodic niobium oxide film/aqueous electrolyte interface was carried out using the lock-in technique at different signal frequencies. The dependence of both components of the impedance on the electrode potential and on frequency is analysed by taking into account the amorphous nature of the films. The lack of long-range order in these oxide layers modifies the physical picture in respect to the case of single crystal semiconductors. A new equivalent circuit has been assumed, based on recent theory of an amorphous semiconductor Schottky barrier. Such a new approach allows the characterization of the interface and the determination of the electronic states distribution within the oxide gap. Possible mistakes arising from the use of models valid for crystalline semiconductors to study these kinds of interfaces are pointed out. The effect of annealing of the films under argon atmosphere as well as the possible nature of the defects in Nb 2O 5 anodic films are discussed.

Characterisation of CoSi2/- and TiSi2/n-GaAs Schottky barriers

The heights of CoSi2/- and TiSi2/n-GaAs Schottky barriers are determined, by various methods, to be 0.77 and 0.64 eV respectively. These values are discussed in view of the recently developed Schottky barrier theories such as the advanced unified defect model and the effective workfunction model. The thermal stability of these contacts is investigated and it is found that the CoSi2/n-GaAs barrier height remains reasonably stable up to 600 degrees C for 10 min anneals and up to 750 degrees C for 1 min anneals. Under oxygen-free conditions and/or for shorter anneal times, this limit value may probably be increased. Consequently, CoSi2 may be an interesting contact material for high-temperature applications such as self-aligned GaAs MESFET fabrication.

The adsorption of Xenon on both low and high work-function metals

The structure of unoccupied electronic states in Xe condensed on differently prepared metal surfaces has been observed by inverse photoemission spectroscopy. The Xe/metal interface is interpreted in the framework of Schottky barrier theory. On low workfunction surfaces a wetting to non-wetting phase transition is inferred from dramatic changes in the unoccupied level structure. Existing data in the literature indicate non-wetting behaviour of Xe on low work-function materials to be a quite general phenomenon.

Amorphous Semiconductor‐Electrolyte Junction. A New Interpretation of the Impedance Data of Amorphous Semiconducting Films on Metals

AbstractOn the basis of the theory of amorphous semiconductor Schottky barrier an equivalent electrical circuit of the amorphous oxide film/electrolyte interface is proposed.—The analytical expressions for the equivalent conductance and capacitance of the barrier are reported in the hypothesis of a constant density of states within the mobility gap.—According to this model, the semiconducting properties and the impedance behaviour at different frequencies of anodic oxide films on Niobium are interpreted by taking into account the amorphous nature of the films.—An explanation for the anomalous behaviour of the Mott‐Schottky plots usually observed with amorphous anodic oxide films is presented.—The possible extension of such a model to different metal/anodic oxide/electrolyte junctions is suggested.

Characteristics of the RuO2-n-GaAs Schottky barrier

It is shown that the RuO2-n-GaAs junction behaves as a Schottky barrier. From impedance measurements and current-voltage measurements as a function of temperature, a barrier height of about 0.7 eV was established. This value is discussed in the light of existing Schottky barrier theories.

On interfacial charges in the oxide layer of mis-type Ag- and Au- n GaAs Schottky barriers

Fixed interfacial charges in MIS type n-GaAs Schottky barriers were investigated by measuring the flatband voltage as a function of the oxide layer thickness in the range of 2–20 nm. The linear relationship obtained allowed the determination of the sign, the location and the density of these charges. In contrast to Si, negative charges were found here. They were located at less than 2 nm from the interface. From the dependence of the charge density on the annealing temperature the conclusion was reached that excess As at the oxide semiconductor interface is at the origin of these charges. The observed changes in work function difference at the interface caused by the annealing support a recent Schottky barrier theory developed by Woodall and Freeouf.

Generalised thermionic-emission theory of carrier transport through thin base of bipolar transistor

Carrier transport through the very thin, inhomogeniusly doped base of a bipolar transistor is analysed. Use is made of the analogy between the Schottky diode and thin base transistor model to find the current/voltage characteristic. So the advanced, generalised thermionic-emission theory of Schottky barrier, taking into account quantum-mechanical effects, is easily adopted for transistor model. The results show there is a considerable level (about 10%) of tunnelling component in the total collector current of contemporary microwave transistors.

Mechanisms of Charge Transfer at the n ‐ GaAs / Room Temperature Molten Salt Electrolyte Interface

Photoelectrochemical (PEC) characterization was carried out on electrodes in contact with a room temperature molten salt electrolyte comprising mixtures of and n‐butyl pyridinium chloride (BPC) in varying molar ratios. The ferrocene/ferricenium ion couple was used as the redox system in these studies. The barrier height for charge transfer, which determines the ultimate efficiency of any photovoltaic system, was determined from dark current‐voltage measurements. These data were in good agreement with previously determined values from capacitance measurements. The residual barrier heights in the illuminated PEC system were determined from open‐circuit voltage and short‐circuit current densities using Schottky barrier theory. A simple method for estimating the reorganization energy (λ) of a redox system from dark current‐voltage measurements is presented. A value of was thus deduced for the ferrocene/ferricenium ion couple in the electrolyte. The acidic electrolyte showed larger dark currents and inferior photoresponse relative to the case of electrodes in contact with basic electrolytes. Using the measured values of λ and the semiconductor bandedges, energy band diagrams were developed for the present PEC system as a function of electrolyte composition. The direct correlation which was seen between PEC performance and electrolyte composition is rationalized on the basis of these diagrams. The larger dark currents in the acid electrolytes were shown to result from greater overlap of the empty redox levels with the conduction bandedge in . On the other hand, in basic electrolyte systems, the filled redox levels overlap better with the valence bandedge leading to more efficient charge transfer on illumination. The role of surface states in mediating charge transfer and degrading PEC performance is discussed in this regard. A key finding of the present work is that the electrical behavior of semiconductor/electrolyte junctions is well described by Schottky barrier theory once the differences in the energy distribution in metals vis‐à‐vis electrolytes, are rationalized.

Schottky-barrier diodes incorporating langmuir-film interfacial monolayers

n+-GaP/stearic-acid/Au MIS structures have been fabricated by depositing monolayers on the chemically etched semiconductor surface using the Langmuir-Blodgett technique. Their electronic and photoelectronic properties are compared with those of n+-GaP/Au diodes. A single monolayer of stearic acid increases the open-circuit photovoltage developed by such devices from 400mV to 900mV without significantly altering the short-circuit photocurrent. The dark I/V characteristics under forward bias conditions and the photoelectric action spectra both show near-ideal Schottky-barrier diode properties and the barrier height as measured by both methods increases from 1.14eV without the monolayer to 1.58eV in its presence. A number of other organic monolayers give very similar results. In the presence of multilayers of stearic acid the behaviour of the MIS device becomes complex and deviates considerably from ideality. In general, the photocurrent due to over-the-barrier excitation is much more affected by the nature and thickness of the interfecial layer than the band-to-band photocurrent. A plausible explanation is offered for the increased barrier height in the presence of a monolayer, based on establised Schottky-barrier theory, but the full range of behaviour observed, and particularly that with multilayers, cannot yet be adequately treated theoretically.

The mercury–indium phosphide diode

en

Comment on "Ionicity and the theory of Schottky barrier"

The theoretical work of Louie, Chelikowsky, and Cohen on Schottky barriers is compared with other theoretical work based on a different approach. A reason is suggested for the apparently different results, and the covalent-ionic transition is discussed in this context.

Effect of externally applied pressure on zinc oxide varistors

A study of the effect of uniaxial pressure on the I-V response of zinc oxide varistors has shown a pressure sensitivity in both prebreakdown and breakdown regions. The general observation is that the conductivity increases with pressure, the effect being more pronounced in the prebreakdown region. Data are in general agreement with the phenomenological model given by Schwertz and Mazenko. Data are also treated on the basis of Schottky barrier and tunneling theories. According to this analysis, the barrier height is found to decrease with pressure.

Schottky barriers on ZnTe

Metal surface barriers consisting of gold, nickel, tantalum, and aluminum on zinc telluride have been investigated. The behavior of gold and nickel barriers agrees with simple thermionic Schottky barrier theory. The barrier height found for gold is 0.64 eV and for nickel 0.65 eV. Tantalum and aluminum barriers were found to have characteristics dominated by recombination currents. The barrier height found for tantalum is 1.50 eV and for aluminum 2.00 eV.