Electrical resistivity and conductivity

Summary

Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-metre (Ω⋅m). For example, if a 1m3 solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1Ω, then the resistivity of the material is 1Ω.m. Electrical conductivity (or specific conductance) is the reciprocal of electrical resistivity. It represents a material's ability to conduct electric current. It is commonly signified by the Greek letter σ (sigma), but κ (kappa) (especially in electrical engineering) and γ (gamma) are sometimes used. The SI unit o

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Free carriers in nanocrystalline titanium dioxide thin films

Simon Springer

This thesis analyzes the properties of heavily doped nanocrystalline titanium dioxide. Thin films were deposited by magnetron sputtering with either water vapor as reactive gas or a periodically interrupted oxygen supply. The samples were at the same time electrically conducting and transparent. They consisted of mixtures of rutile, anatase and amorphous phases. Titanium dioxide is chemically stable, hard, non-toxic, transparent, and inexpensive. Due to a high refractive index it is often found in optical applications as a thin film coating. For many purposes it is interesting to take advantage of a combination of good transparency and electrical conductivity. An even larger field of applications would open if the electric conductivity of TiO2 could be increased in a controlled manner. Conventional bulk doping is limited, however, by low solubility limits. Highly conducting nanocrystalline TiO2 thin films have been obtained recently by sputtering in presence of water vapor. The present project intends to elucidate the doping mechanisms at work in such films. In addition, an alternative process to achieve grain-boundary doping of TiO2 was explored. Spectrophotometry over a wide spectral range (0.05 to 5 eV) was the main tool in probing the mobile electrical charge carriers. The interpretation of these optical measurements required appropriate models. The models developed took many structural properties into account, such as thickness, surface roughness, density, anisotropy and crystalline phase composition. The phase and morphology of the films were analyzed by X-ray diffraction and reflection, scanning probe microscopy, and electron microscopy. The chemical analyses included electron-probe microanalyses, Rutherford backscattering and elastic recoil detection analysis. The DC electrical properties were measured between room temperature and 250 °C. Most water-deposited samples showed semiconducting behavior with an activation energy of the order of 100 meV. The optical measurements revealed a broad absorption band around 1 eV. The absorption coefficient scaled with conductivity. It was successfully modelled as a Lorentzian. Similar absorption bands have been reported for reduced rutile and anatase single crystals. Water-deposited samples containing anatase exhibited a metallic behavior. In these samples the thermal activation energy was reduced to about 30 meV and the charge carriers displayed a high mobility (3 cm2V-1s-1). The infrared absorption band was about 10 times less important than in the samples devoid of anatase. Thin films of TiO2 intercalated with the equivalent of nanometer-thick layers of Ti were prepared. By changing the period in the oxygen supply the grain size, and thereby the film properties, could be modified. The films obtained in this way had many properties in common with water-deposited films. They showed the same absorption band in the infrared, had comparable charge carrier densities and were electrically conductive (102 to 104 Sm-1 at room temperature). To our knowledge, this was the first time that such high electrical conductivities were achieved by sputtering in an oxygen plasma.

EPFL2004

Titanium dioxide is a cheap, chemically stable and non-toxic material. However its electrical properties are unstable and it is a modest semiconductor and a mediocre insulator. For several applications it would be interesting to make it either more insulating or more conducting. The goal of this work was to modify the electrical properties of nano-crystalline TiO2 thin films deposited by reactive sputtering and to understand the mechanism leading to these modifications. The principal factors that influence the electrical conductivity are on the one hand the concentration and nature of the chemical impurities incorporated in TiO2, and on the other hand the morphology of the thin films. The study was split into two parts. The fist part describes the modifications of the sputtered material obtained by chemical doping of the TiO2 thin films. Niobium, cerium, iron, and fluorine were incorporated successfully in TiO2. The second part describes the modifications obtained by modifying the reactive gas used during the sputtering process: thus oxygen was substituted with water vapor. Several analysis techniques have been used to characterize the TiO2 thin films. They are essentially divided in four categories. The chemical analyses included electron probe microanalyses, x-ray photoemission spectroscopy, and secondary ion mass spectrometry. The structure and morphology analyses of thin films were carried out with x-ray diffraction and atomic force microscopy. The electrical properties in dc or ac mode were measured between room temperature and 350°C. Finally optical transmission provided information on the electronic states and morphology of the thin films. With an appropriate choice of impurities, titanium dioxide can be made more insulating (TiO2:Ce), more conducting with an n-type electrical conductivity (TiO2:Nb) or p-type electrical conductivity (TiO2:Fe). Chemical doping is also the cause of important structure and morphology changes, for example it can force the transformation from the anatase to the rutile structure. It is shown in particular that dopant atoms generate defects such as oxygen vacancies. These defects impede the variation of the electrical conductivity produced by the impurities, thus, in spite of dopant concentration in the percent range, the variation of the electrical conductivity is an increase or a decrease of three order of magnitude at most when compared with undoped TiO2. Thin films deposited with water vapor as the reactive gas present an increase in the electrical conductivity by height order of magnitude compared to samples prepared with oxygen in similar conditions. No atom other than titanium or oxygen could be detected in the thin films in significant concentration. The dramatic conductivity increase is due to the injection inside TiO2 grains of electrons donated by unsaturated titanium atoms lying on the grain surface. A nanometric grain size is a requirement to make such high doping levels possible. Depending on the impurities or reactive gas selected, titanium dioxide can be made either into a good insulator with a high breakdown field and a high permittivity or into a reasonably conducting, transparent film. The electrical conductivity of the samples prepared for this study cover the range from 10-10 to 103 S m-1.

EPFL1999

Semiconductor nanowires are an emerging class of materials with great potential for applications in future electronic devices. The small footprint and the large charge-carrier mobilities of nanowires make them potentially useful for applications with high-integration density, but also to replace thin-film transistors in large-area electronics. This thesis investigates the use of wet-chemically grown ZnO nanowires for the fabrication of high-performance, low-voltage field-effect transistors (FETs). The first part of this thesis addresses the electrical characteristics of the wet-chemically grown ZnO nanowires. The as-grown ZnO nanowires are highly conductive due to a large charge-carrier concentration caused by dopants unintentionally incorporated into the ZnO nanowires during the synthesis. This large charge-carrier concentration makes it difficult to modulate the conductivity of the nanowires by an external electric field, so that the as-grown wires are not suitable for FETs. A post-growth annealing step is employed to dramatically reduce the charge-carrier concentration of the nanowires which makes it possible to fabricate FETs with useful characteristics. ZnO-nanowire FETs in a global back-gate geometry based on annealed ZnO nanowires have a transconductance of 300 nS, an on/off current ratio of 107 and a subthrehold slope of 500 mV/decade. The maximum field-effect mobility of the annealed ZnO nanowires is around 50 cm2/Vs. An important requirement to obtain good FET characterisitcs is to minimize the influence of the source and drain contact resistance on the total device resistance. The material used for the source and drain contacts in this thesis is aluminum. It is demonstrated that the use of a plasma treatment in the contact regions prior to the evaporation of the aluminum contacts can greatly influence the contact properties between ZnO and aluminum. An argon-plasma treatment locally increases the charge-carrier concentration in the ZnO. It is shown that the doping effect of the argon-plasma treatment can be exploited to reduce the contact resistance between alumiunm and ZnO and improve the electrical performance of the FETs. In the second part of this thesis, the influence of the ambient air on the electrical characteristics of the ZnO-nanowire FETs is investigated. The electrical conductivity of the FETs is strongly affected by the distinct atmospheric conditions, which makes it difficult to obtain reliable FET characteristics. The potential of a self-assembled monolayer (SAM) based on fluoroalkylphosphonic acid molecules to passivate the ZnO nanowires against the undesirable effects of the ambient and to stabilize the electrical performance of ZnO-nanowire FETs is demonstrated. The stabilizing effect is attributed to the formation of a densely packed, hydrophobic SAM on the surface of ZnO and aluminum oxide. The quality of the hydrophobic SAMs is confirmed by means of water-contact-angle measurements on SAM-covered ZnO single crystals that show a contact angle of more than 110°. The third part of this thesis is dedicated to the fabrication of high-performance ZnO-nanowire FETs with patterned metal top-gate electrodes. A top-gate fabrication process is developed that uses a very thin gate dielectric consisting only of an alkylphosphonic acid SAM that can be deposited from solution. The insulating quality of the SAM is investigated for top-gate FETs that utilize either gold or aluminum for the top-gate electrode. Gold top-gate FETs show a transconductance of 1 µS, an on/off current ratio of 107, and a steep subthreshold slope of 90 mV/decade. The thin gate dielectric makes it possible to operate the gold top-gate FETs at low voltages of 1 V and simultaneously reduces the undesired gate current by more than three orders of magnitude compared to gold top-gate FETs that have been fabricated without a gate dielectric (MESFETs). The gate current of the gold top-gate FETs with SAM gate dielectric is as low as 1 pA for gate-source voltages between -1 V and 0.5 V. When aluminum is used for the top-gate electrode, a hybrid dielectric is formed at the interface between the SAM and the aluminum, consisting of the SAM and a spontaneously formed aluminum oxide layer. Owing to the hybrid gate dielectric, the aluminum top-gate FETs are operated with gate currents below 1 pA for voltages up to 3 V. The top-gate fabrication process does not require temperatures above 160 °C, which makes it possible to fabricate FETs on unconventional substrates that cannot tolerate high temperatures, such as glass or flexible plastics. Integrated circuits on single ZnO nanowires that are fabricated on glass and plastic substrates are demonstrated. The maximum switching frequency of the inverters is 1 MHz.

EPFL2011