Obtaining higher manganese silicide films with high thermoelectric properties

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Introduction
Many research centers worldwide are currently researching the creation of environmentally friendly and low-cost energy sources.Recently, scientists worldwide have focused on developing effective methods for producing environmentally friendly and more costeffective energy sources.In this regard, great results have been achieved in converting wind, light, and heat energy into electrical energy, which has increased the efficiency of the generated photovoltaic and thermoelectric elements [1][2][3][4][5][6][7][8][9][10].
This, in turn, led to very important scientific discoveries in the field of physical electronics, but some issues remain a pressing problem to this day.This is due to the film's low efficiency and high cost of the film-forming process.Among such materials, a film of higher manganese silicide is the most promising.(HSM), the thermoelectric figure of merit can reach 0.4 in the temperature range of 20 -700°C [11][12][13][14][15][16].
A thin Mn 4 Si 7 film can be used as a high-quality thermoelement and shows the possibility of creating nanostructures with high thermal properties based on fundamental research on various physical properties, quantum effects, and size factors.Photocells from such structures are promising when using highly sensitive receivers of electromagnetic waves in optical and IR fields.Photocells of such structures are promising when using highly sensitive receivers of electromagnetic waves in the optical and IR areas.
This article provides information about the Mn 4 Si 7 film formed by magnetron sputtering and its high thermoelectric properties.

Methods
Before the formation of the HCM film, the SiO 2 /Si substrate was cleaned in two ways: 1. Cleaning the surface of silicon wafers SiO 2 /Si (d=60 mm) in an ammonia-peroxide mixture at a temperature of 60-70°C, washing in deionized water, drying in a centrifuge; 2. Vacuum treatment (cleaning) of the surface of a group plate (2-4 pieces) with an argon plasma flow on EPOS-PVD-DESK-PRO.The plasma flow creates a source of ions with a cold cathode at a voltage of 2-3 kV and a current of up to 100 mA in the flow for 3-5 minutes on a rotating tool with plates.HSM films were formed using an EPOS-PVD-DESK-PRO magnetron sputtering apparatus at a pressure of 10 -4 Pa and at room temperature.The purity of Mn 4 Si 7 used as a target was 99.5%, the diameter was 76 mm, and the thickness was 6 mm.[17,18].Before placing the target on the magnetron, their composition and structure were studied with a Quanta 200 3D scanning electron microscope from the Dutch company FEI (Fig. 1).The specific resistance of the formed Mn 4 Si 7 film with a thickness of ~0.7 μm was determined by the four-probe method, and the thermoelectric properties were determined in a vacuum of 10 Pa by the two-probe method [19].The bandwidth of the Mn 4 Si 7 /SiO 2 film was measured on an HR-4000 high-precision spectrometer according to the law of light reflection.It is known that the thermoelectric figure of merit of a material is a dimensionless quantity and is determined by the formula

Results and Discussion
The Mn 4 Si 7 film formed by magnetron sputtering is in an amorphous state before thermal heating, which we can identify by (Fig. 2-a).Silicon and manganese atoms deposited on silicon oxide almost completely covered the substrate.As a result of experiments with an amorphous film, we (see--saw) observed that the film has metallic properties.In the amorphous state, the film's resistance is greater than the resistance in the polycrystalline state.This is because the bond between the manganese and silicon atoms is very weak, and there are point defects on the surface areas that are not completely covered.After the film is annealed, point defects on the surface disappear, the manganese and silicon atoms form a bond, and the resulting structure has semiconductor properties.The specific resistance of the formed Mn 4 Si 7 film is 20•10 -6 Ω•cm at room temperature; when heated to a temperature of 750 K, its resistivity increases to 50• 10 -6 Ω•cm (Fig. 3-a).The specific electrical conductivity of this film is 5•10 4 Ω -1 •cm -1 at room temperature.When heated to 750 K, it can be seen that its electrical conductivity decreases to 1.2•10 4 Ω -1 •cm -1 (Fig. 3-b).After annealing the Mn 4 Si 7 silicide film at 650 K for 1 hour at a pressure of 10 -3 Pa using a special device, the film was cooled in a vacuum until room temperature was reached.The graph of the temperature dependence of the resistivity and electrical conductivity of the Mn 4 Si 7 silicon film is shown in fig. 4. At room temperature, it is 7.86•10 -6 Ohm•cm, and when heated to a temperature of 700 K, it resistivity decreases to 3.9•10 -6 Ohm•cm (Fig. 4-a).The specific electrical conductivity of this film is 0.12•10 6 Ω - 1 •cm -1 at room temperature.When heated to 700 K, it can be seen that its electrical conductivity increased to 0.27•10 6 Ω -1 cm -1 (Fig. 4-b).
It can be seen from the graph the decrease in resistivity with increasing temperature (Fig. 4-a) and the increase in electrical resistivity (Fig. 4-b 5-a) and in the polycrystalline state (Fig. 5-b).The Seebeck coefficient of the film in the polycrystalline state turned out to be approximately 6 times higher than in the amorphous state.These experiments lead us to conclude that an increase in the temperature of the Mn 4 Si 7 silicon film in the polycrystalline state leads to an increase in the concentration of electrons and holes in the film.Based on the laws of reflection from the graphs shown in fig.6, it can be seen that the Mn 4 Si 7 silicide film has a high sensitivity in the visible and IR regions for the corresponding wavelengths.In addition, from these data, using the "Kubelka -Munk" function, it is possible to determine the band gap of the Mn 4 Si 7 silicide film.Where α is the absorption coefficient, hν is the photon energy, R is the reflection coefficient, λ is the light wavelength [23,24].

Conclusion
It has been found that the thermoelectric power of the Mn 4 Si 7 film increases during the transition from the amorphous state to the nanocrystalline state, which is associated with the selective scattering of charge carriers at the boundaries of nanoclusters.It is shown that Mn 4 Si 7 films grown (grew) on a SiO 2 /Si substrate have the highest conversion coefficient, which is explained by the low specific thermal conductivity of SiO 2 /Si k = 149 W/m•K.Films of Mn 4 Si 7 on SiO 2 /Si have a high response speed and high sensitivity and can be used in thermal wave radiation detectors in the visible and IR ranges.

Fig. 4 .
Fig. 4. Graph of the dependence of the resistivity of the Mn 4 Si 7 silicon film in the polycrystalline state a) and the electrical conductivity b) on temperature.

Figure. 5
Figure.5 shows graphs of the temperature dependence of the Seebeck coefficient of the Mn 4 Si 7 film in the amorphous state (Fig.5-a) and in the polycrystalline state (Fig.5-b).The Seebeck coefficient of the film in the polycrystalline state turned out to be approximately 6 times higher than in the amorphous state.These experiments lead us to conclude that an increase in the temperature of the Mn 4 Si 7 silicon film in the polycrystalline state leads to an increase in the concentration of electrons and holes in the film.