C.A. Hogarth

An electron spin resonance study of reactively evaporated silicon oxide

Abstract An e.s.r. signal in reactively evaporated silicon oxide films at g =2.00 was measured as a function of R p where R is the evaporation rate and p is the pressure. Values for spin density between 10 18 and 10 20 cm -3 were recorded. The signal is thought to be that of an unpaired silicon electron which acts as a centre for Poole-Frenkel emission during d.c. conduction. Oxidation of the unsaturated bond is only slightly sensitive to substrate temperature, indicating a low activation energy for oxidation. Atmospheric storage oxidized the bonds and the effects on spin density have been measured for various R p values. The results are compared with previously reported results on the electrical properties of silicon oxide and certain conclusions about conduction properties have been made.

Effects of annealing on the structure, electron spin resonance and optical energy gap of thin BaO-SiO films

Abstract A study of the effects of changes in composition and of annealing on the electron spin resonance properties of BaO-SiO thin films is presented. The results are found to be compatible with the reduction in the value of the optical energy gap of these materials as the molar fraction of BaO in the SiO thin film increases. The increase in optical energy gap after annealing is interpreted in terms of the removal of voids and a rearrangement of the structure.

Electron emission and related properties of amorphous thin films of mixed barium and silicon oxides

Abstract Co-evaporated thin films of SiO and BaO were prepared and used as the insulator material in MIM and MIMIM structures, where the metal M was Al, Ag or Cu. Electroforming processes, resulting in an increase in the circulating current after the application of a direct forming voltage, were slow when Al electrodes were used but quite rapid with Ag or Cu electrodes. For devices formed with Cu or Ag electrodes the peak circulating current observed was at a voltage of the order of 2.6-3.2 V. At this applied voltage the electron emission current rose sharply and reached values comparable with those measured from MIM structures using SiO as the insulator. The probable existence of high field regions in the insulator was further tested by investigating triode structures and the forming processes were found to be quite complex. Although the polyfilamentary model can explain most of the diode properties, a refinement to this theory is needed to explain the forming effects observed with triode structures and this will involve a more careful consideration of the role of oxygen, as well as the usually considered metal and vacancy diffusion.

A study of the d.c. electrical properties of thin films of the co-evaporated dielectric system SiO/TiO

The d.c. electrical properties of Au-SiO/TiO-Au thin film devices which exhibit a voltage-controlled negative resistance (VCNR) in their current-voltage characteristics have been studied and their properties including electroforming and electron emission into a vacuum have been determined. Typical layer thicknesses are 5000 A for the insulator and 400 A for the gold electrodes. The maximum electron emission current is of the order of 25 μA cm-2 before dielectric breakdown and the peak circulating current is of order 5–30 mA. The filamentary conduction model seems appropriate to explain electron transport in these devices.

A note on the normal distribution of filamentary resistances in formed metal/insulator/metal devices

Abstract It is argued that the resistances of conducting filamentary paths in a formed metal/insulator/metal device are normally distributed. Symmetrically triangular and parabolic distributions which have been previously reported to give the best fit to the experimental data are believed to be derived from this fundamental distribution.

Voltage-temperature memory and pressure-voltage memory effects in coevaporated films of SiOTiO

Abstract Sandwich structures of Cu/(SiOTiO)/Cu where the thickness of the coevaporated insulator is in the range 2000–13 000 A were investigated. Electroformed devices show voltage-controlled negative resistance, electron emission, pressure-voltage memory and thermal-voltage memory effects. The effects of an increase in pressure or a decrease in temperature on the electrical behaviour of a formed sample are similar in form. The results can be interpreted in terms of the filamentary model of electrical conduction as proposed by Dearnaley, Morgan and Stoneham.

Comments on “Potential distributions in electroformed MIM and MIMIM structures”

Abstract A number of workers have suggested that electroformed MIM structures contain a high field region in the insulating layer, which is set up during the forming process. This is supported by experimental potential distribution measurements on MIMIM structures which reveal a highly non-linear potential distribution. In a recent criticism of such measurements, it has been argued that a MIMIM structure acts as two MIM structures in series, and that the potential distribution can be predicted by a model involving a simple load line analysis. In the present paper it is shown that certain consequences of such a model are not confirmed experimentally, and certain experimental results are inconsistent with the predictions of such a model. In particular the load line analysis does not fit the experimental evidence of potential distributions in triode structures, except in cases where the two sections are separately formed.

The trap depth in evaporated polypropylene from measurements of thermally stimulated currents

Abstract The occurrence of thermally stimulated currents in thin evaporated films of polypropylene was demonstrated and an analysis of the curves gave an electron trap depth of 0.33 eV in this material.

The electrical characteristics of porous Al2O3 produced by anodization

Abstract The electrical characteristics of films of Al 2 O 3 prepared by anodizing aluminium substrates were investigated. The Al 2 O 3 consisted of a thin barrier layer of approximate thickness 150–180 A surmounted by a thick porous layer of thickness 5–10 μm. Cobalt ions were deposited in the lower sections of the porous layer by a method of metal pigmentation. A number of different conduction processes were observed, including Schottky-type behaviour, field ion diffusion, space-charge-limited currents, ohmic conductivity and also electroforming and electron emission. A typical barrier height ∅ for Schottky emission at the AlAl 2 O 3 interface of 1.11 eV was calculated, and an activation energy E = 0.50−0.68 eV was estimated for the space-charge-limited process. The observation of electroforming in samples of such thickness was ascribed to the field-induced growth of conducting filaments in regions of the porous material which had previously been destroyed by excess heat dissipation.

Hall effect and magnetoresistance in Au/SiOx thin films

Abstract Measurements of the temperature dependence in the range 300–520 K of the Hall coefficient and magnetoresistance of thin films of SiO containing 2 and 5 at.% Au are reported. The derived carrier density values are combined with earlier measurements of electrical conductivity to determine the temperature variation in mobility; the mobility increases with increasing temperature in the lower part of the range, as expected for a carrier hopping conduction mechanism, but decreases with increasing temperature in the higher range in accordance with the expectations of free-band conduction. Variations in the parameters with magnetic induction up to 130 mT are discussed. The free-carrier density is substantially constant with temperature, as expected for an amorphous or highly disordered material.