Handbook of thin film technology

It has been experimentally shown that water vapor thermal treatment of silicate glasses with silver ions introduced by ion exchange leads to the formation of a silver nanoparticle layer with a high packing density on the glass surface. The results of studying the morphology of samples by atomic force and electron microscopy and X-ray spectral analysis of the composition of nanoparticles, as well as the optical density and luminescence spectra in different stages of the treatment, are presented. Mechanisms explaining the processes responsible for silver nanoparticle formation upon water vapor thermal treatment on the glass surface after ion exchange are proposed.

Formation of silver nanoparticles on the silicate glass surface after ion exchange

Research has recently begun on the use of ultrathin films and nanoclusters as mechanisms for sensing of gases, liquids, etc., because the basic material parameters may change because of film morphology. As films of various materials are applied to the surface of SAW devices for sensors, the conductivity of the films may have a strong acoustoelectric effect, whether desired or not. The purpose of this paper is to reexamine the theory and predictions of the acoustoelectric effect for SAW interactions with thin conducting or semi-conducting films. The paper will summarize the theory and predict the effects of thin film conductivity on SAW velocity and propagation loss versus frequency and substrate material. The theory predicts regions of conductivity which result in extremely high propagation loss, and which also correspond to the mid-point between the open and short-circuit velocities. As an example of the verification and possible usefulness of the acoustoelectric effect, recent experimental results of palladium (Pd) thin films on a YZ LiNbO3 SAW delay line have shown large changes in propagation loss, depending on the Pd film thickness, exposure to hydrogen gas, or both. By proper design, a sensitive hydrogen leak detector SAW sensor can be designed.

Study of the acoustoelectric effect for SAW sensors

The conduction mechanisms in different growth stages of Pd thin films are investigated in detail. Special attention is paid to the transition regions between the different stages of film growth and the dominant conduction mechanisms. Mechanisms of thermally activated tunneling, percolation conduction, and continuous film conduction are considered and a consistent set of model parameters is deduced. This is done by also varying the substrates. Activation energies of tunneling on the order of 50 meV, percolation thresholds ${p}_{c}=0.70\ifmmode\pm\else\textpm\fi{}0.01$, and conduction exponents $\ensuremath{\mu}=2.43\ifmmode\pm\else\textpm\fi{}0.12$ were achieved. The physical meaning of the model parameters is discussed and connected mechanisms are proposed. The minimal film thicknesses of continuous RT-sputtered Pd films were found to be on the order of 1.5 nm.

Conduction mechanisms during the growth of Pd thin films: Experiment and model

The decomposition of thin silver films into clusters on a SiO2 surface during thermal heating in vacuum has been studied. The experimental histograms of distribution of clusters formed from films of different thicknesses over their diameters have been constructed. It has been found that the decomposition of the films with thickness to 10 nm leads to the formation of monoclusters having one predominant diameter in the range from 15 to 20 nm. After a longer term annealing, the array of clusters almost does not change with time, and the predominant diameter is retained. From films with thicknesses ranging from 10 to 130 nm, an array of clusters with two predominant diameters containing monoclusters (40–80 nm) and globular clusters (400 nm) is formed. It is noted that, with increasing annealing time, the array is enlarged due to the coalescence process and is gradually evaporated; in this case, the predominant cluster diameters remain the same.

Specific features of the formation of arrays of silver clusters from a thin film on a SiO2 surface

Photoelectric phenomena in island metal films are investigated. The electrical conductivity and photoconductivity are measured in island films of Ti, W, FeNi, and others grown by the method of radio-frequency sputtering in argon. The photoconductivity effect is observed in metal films whose resistivity varies with temperature by the activation law of ρ ∝ exp(T
0/T)0.5. This law is interpreted within the model of variable range hopping conductivity in the region of a Coulomb quasi-gap. The photoconductivity in metal films is detected in the visible and near-infrared spectral regions. The mechanisms of photoconductivity in an island metal film are discussed.

Detection of photoconductivity in hyperfine metal films in the visible and infrared spectral regions

The low-frequency effective permittivity in a system of Co, W, and FeNi metal nanoislands in an electric field is investigated. The permittivity of films is determined from analyzing the dependences of the susceptance of the films on the temperature and the electric field. It is found that the effective permittivity of the structures under investigation has an anomalously high positive value (107–108) and that an increase in the electric field leads to a decrease in the effective permittivity and an increase in the electrical conductivity of the films. The nature of the high effective permittivity of the structures is governed by the island structure of the metal films and associated with the polarization of dipoles formed by pairs of negatively and positively charged metal nanoislands.

Anomalously high low-frequency effective permittivity in a system of metal nanoislands

The differential conductivity of metallic island films of Ti, Co, W, and FeNi is investigated in the vicinity of liquid nitrogen temperatures. It is found that the temperature dependence of the conductivity of metallic island films in the insulator phase varies in accordance with the activation law σ ∝T

 n
 exp(−E/kT). It is shown that the power of temperature in the preexponential factor varies from n = 2 to 1 upon an increase in the film thickness. In thicker films, in which a transition from the insulator to the metal conductivity phase takes place, the temperature dependence of the conductivity increases in proportion to temperature. The mechanism of conduction in metallic island films is discussed.

Activation conduction in metallic island films

The variation of conduction in island metallic Ti, Co, W, and FeNi films in weak electric field is studied. The variation of the differential conductivity of island metallic films with an electric field at temperatures from T = 77 to 300 K is measured, as well as the temperature dependence of the differential conductivity in the same temperature range. It is shown that a thermally activated conduction mode is realized in such structures. The mechanism of variation of conductivity of island metallic films in a weak electric field is discussed.

A. P. Boltaev

Papers

Detection of photoconductivity in hyperfine metal films in the visible and infrared spectral regions

Anomalously high low-frequency effective permittivity in a system of metal nanoislands

Activation conduction in metallic island films

Effect of weak electric fields on the conduction in thin metal films