Formation of structures of sensitive elements of film transducers with given electrophysical parameters
EFFICIENCY OF THERMOELECTRIC ENERGY TRANSDUCER
Considerable interest in compounds based on chalcogenides of elements of the first group is caused by a wide range of them both already realized in practice and potential use in various fields of science and industry. Chalcogenides of the elements of the first group are used in heterojunctions of solar cells, in magnetic information recording devices, and their nanoparticles are promising for creating quantum dots in tunable semiconductor lasers. It is possible to use them in interference optics, as effective membrane materials, in potentiometric sensors, ion-selective sensors for various analytical methods.
When creating transducers, high requirements are imposed on materials, which are dictated by the operational parameters of the instruments [3, 4]. The output characteristics of the transducers are determined by the characteristics of the sensitive elements, which depend significantly on the properties of the alloys or compounds.
The following requirements are imposed on materials for transducers:
• thermo EMF of alloys (compounds) should be sufficiently large, continuous and unambiguous function of temperature that is as close as possible to linear;
• alloys must be corrosion-resistant and resistant to airborne operation;
• during operation, alloys should maintain their thermoelectric characteristic unchanged and be durable enough.
The efficiency of the transducer depends on the electrophysical properties of the materials used: thermo EMF (α), electrical conductivity (σ), thermal conductivity (χ). The most important is the high thermoelectric figure of merit, from which in most cases depends the possibility of using the material in the creation of transducers. It is the value of the thermoelectric figure of merit that determines the efficiency of the thermoelectric energy transducer.
The use of chalcogenides of the elements of the first group allows achieving the required thermoelectric parameters of materials and maximum thermoelectric figure of merit.
The expression for the thermoelectric figure of merit has the following form: Z = f (µ*, βZ, r), where µ* is the chemical potential; r is the scattering factor; βZ is a dimensionless parameter that depends on the effective mass, the phonon component of the thermal conductivity, the mobility of the charge carriers, and the temperature of the selected material.
For each βZ, there exists µ* at which the thermoelectric figure of merit reaches its maximum value, that is, at a certain concentration of charge carriers, the maximum Q-values for a given material can be achieved. To achieve the optimum concentration of charge carriers, one can use impurity materials with a single- or multi-valley spectrum of charge carriers, parabolic bands, equivalent extrema and a power-law dependence of the charge carrier relaxation time on energy with exponent r [5–7].
At a fixed temperature, the value of Z depends on the change in the chemical potential level, the mobility of the charge carriers, the effective mass, and the scattering factor r determined by the scattering mechanism of the charge carriers.
The parameters of the substance, on which the transformation efficiency Z(α, σ, χ) depends, are determined by the concentration of free charge carriers. The carrier concentration is determined from the value of the thermo EMF, taking into account the scattering by acoustic vibrations of the atoms of the crystal lattice, and also from the values of µ*.
Perspective materials with good thermoelectric properties for thermoelectric devices and micro-thermotransducers are chalcogenides of copper and silver. The presence of several regions of homogeneity, polymorphism, and temperature changes of polymorphic transformations of the compounds of the chalcogenides of the elements of the first group, depending on deviations from stoichiometry, indicate a strong relationship between the electrophysical properties and composition. The nature of the dependence of properties is corrected by the change in the crystal-chemical structure and the nature of the chemical bond.
CREATING FILM TRANSDUCERS WITH SPECIFIED CHARACTERISTICS
There are certain difficulties in obtaining such compounds with predetermined electrophysical properties. On the one hand, this is due to sufficiently high growth temperatures of materials, which leads to the creation of a large concentration of intrinsic defects. On the other hand, they tend to self-compensate of defects, which complicates the production of materials with the required electrophysical characteristics. It should also be noted that it is difficult to obtain a compound of the required composition due to volatility and intrinsic defectiveness of chalcogens.
Given the above circumstances, the synthesis of the starting compounds is carried out by direct fusion of the components under vacuum in sealed quartz ampoules. Before use, the ampoules are etched, washed with distilled water and alcohol. The ampoule loaded with a blend of the appropriate composition is pumped to a pressure of 1.33 ∙ 10–2 Pa and then sealed. Then it is placed into an oven and heated to a temperature exceeding the melting point of the synthesized compound by several tens of degrees. Heating is carried out slowly, with hourly exposures at the temperatures of reaction initiation of the initial components. After synthesis, homogenization is carried out by annealing in sealed ampoules.
Analysis of the presence of residual impurities shows that the synthesized crystals have an impurity concentration of <1017 atom/cm3, while the carrier concentration is 1018–1021 cm–3. This allows us to assume that the residual impurities do not affect the physical properties of the compounds obtained.
The most important tasks in creating instrumental structures based on chalcogenides of the first group are to obtain materials with high electrophysical properties and to develop a deposition mode that allows reproducing these properties on a film.
When creating thin-film transducers, the specific problems associated with the reproduction of the parameters of the compounds of the initial material in the film structure are added to the difficulties of synthesizing materials with the necessary properties. Depending on the mode of deposition, the thermoelectric properties of the resulting thin films vary greatly and can significantly differ from the properties of the evaporated material. At elevated temperatures, the volatility of their components differs so much that the chemical composition of the vapor and the film does not coincide with the chemical composition of the evaporated substance. Difficulties in obtaining films are due to the fact that during evaporation, the compound separates into separate components, evaporating at different rates.
The choice of the method for obtaining thin films depends on their purpose and its compatibility with other processes of microelectronic technology. To ensure the reproducibility of the electrophysical properties of thin films, the deposition method should allow the preparation of films of the initial composition.
To obtain thin films of copper and silver chalcogenides of the initial composition, discrete evaporation in vacuum is used. In this method, the substance evaporates instantly and completely, so that in the space above the evaporator the components of the compound are present in the same ratio as they are contained in the initial material, so that the composition of the film that condenses on the substrate will be close to the composition of the starting material. The films obtained undergo thermal treatment in a vacuum immediately after the deposition.
The mechanical and strength properties of film transducers are determined by the substrate. Correctly selected substrate material allows to increase the stability of the structure to the effect of heterogeneous additional loads and external factors.
In the process of manufacturing transducers, the substrate must be technologically compatible with both the production process and the film material, while ensuring a minimum difference in the temperature coefficients of linear expansion and the similarity of their temperature dependences. As a substrate for film converters, it is necessary to use materials that have small losses, good vacuum properties, heat resistance. For example, it is expedient to use polyamide films, which are obtained by centrifugation followed by imidization in vacuum on a polished bronze surface serving as a further base for deposition the heat-sensitive elements.
The purity of the substrate and its processing make a significant impact on the nature of the thin film. Thorough pre-cleaning of the coated surface is extremely important for obtaining high-quality and durable films. Pollution changes the condensation conditions of the deposited material, in particular, the degree of mobility of atoms on the surface of the substrate changes, and as a result, the structure of the film is disrupted.
During the formation of thin films by evaporation in vacuum, the material of the evaporated substance interacts with the atmosphere of the residual gases, and the maximum interaction affecting the properties of the films occurring on the surface of the substrate. Therefore, in order to reduce the impurity concentration during the deposition of films, it is necessary to increase the condensation rate.
As protective layers, films of silicon monoxide are used, which has good dielectric characteristics, mechanical strength and is deposited by evaporation in a vacuum. At the same time, mechanically strong films that are resistant to external influencing factors are formed on the substrate.
The most important task in the development of thin-film transducers is the obtaining of materials with good thermoelectric properties, selection and testing of the technological mode of film deposition, allowing to reproduce these properties in its structure. The correctly selected substrate material increases the resistance to the effects of dissimilar additional loads. The use of alloys makes it possible to obtain thermoelectric parameters of materials, the ratio of which leads to the maximum thermoelectric efficiency. The use of films in the creation of transducers makes it possible to improve the parameters and characteristics of measuring instruments, thermoelectric measuring devices. The film technology allows to perform the elements of the transducers in a constructive and technologically compatible manner. ■