Some Electrophysical Properties of Polycrystalline Silicon Obtained in a Solar Oven

The article describes the results of the study of the microstructure and some electrophysical properties of silicon obtained by re-melting in a solar oven. It was found that the granularity of polycrystalline silicon consists of Si atoms with a size of 10–15 μm, the roughness of its surface. It is shown that at T ≤ 600 K the concentration of charge carriers increases due to an increase in the concentration of ionized impurity atoms, which, in turn, leads to a decrease in the resistivity of polycrystalline silicon. The position at T ~ 600–700 K is based on the decrease in the free path of the charge carriers as a result of thermal vibrations of the crystal lattice. The situation at T ≥ 700 K was explained by the emergence of new recombination centers specific to localized traps. Polycrystalline silicon heated by sunlight does not create a barrier effect of traps localized in the grain boundary regions from polycrystalline silicon obtained by other methods. This can expand the possibilities of creating highly efficient semiconductor devices, solar cells, thermoelectric materials for micro- and nanoelectronics, photovoltaics.


Introduction
At present, the microstructure and their influence on the electrophysical, photoelectric and thermoelectric properties of polycrystalline silicon (PS) semiconductors can be considered sufficiently well studied from both experimental and theoretical points of view (see, for example, [1][2][3][4][5][6][7][8][9][10][11], as well as references given in them). It has been reliably established that the physical properties of PS, arising under certain conditions, depend on the microstructure of the between grain boundaries (BGB). That is, localized traps or segregated impurity atoms in BGB create a barrier effect that leads to a non-monotonic change in the electrophysical properties and shunting of the p-n junction [1][2][3][4][5][6][7][8][9][10], which simultaneously leads to a decrease in the efficiency of semiconductor devices or solar cells. For example, the process of charge transfer can occur through the BGB or along the BGB (Fig. 1) [4][5][6][7][8][9][10]. In the first case, localized traps create a barrier effect (Fig. 1a). At the same time, charge carriers are captured in localized traps, which leads to an increase in the height of the potential barrier (φ) in the BGB and this simultaneously leads to an increase in the resistivity (ρ) of PS [1,2,[4][5][6][7]. And if the BGB is located as shown in Fig. 1b. That is, the BGB has a columnar structure and the charge transfer process occurs along the BGB region, the charge carriers move along localized traps (E ip and (E in ). This process leads to the manifestation of the local conductivity of the trap, as well as to the shunting of the p-n-junction [4,[8][9][10]. Despite such a disadvantage of PS, the results of the study demonstrated the possibility of creating relatively inexpensive and resistant to external influences semiconductor devices, solar cells and thermoelectric materials. In this regard, one of the important tasks is the study of technologies for the creation of polycrystalline semiconductor materials and the study of physical processes related to the regions of BGB.
There are a number of methods for obtaining PS, of which the method of obtaining by heating in a solar oven [11] is the most attractive for polycrystalline semiconductors. In [11], we experimentally proved and patented that the method makes it possible to obtain thermoelectric material based on granular silicon. The present work is devoted to solving this problem for PS obtained by heating in a solar furnace using powder technology.

Research Method
It is known that powder technology is one of the promising methods for the production of polycrystalline semiconductor devices and thermoelectric materials [4,5,11]. The fundamental novelty of the research lies in the study of the electrophysical properties and charge transfer processes of PS obtained by heating silicon particles under the action of concentrated solar radiation, from which silicon particles are made on the basis of powder technology. The production of PS consists in the production of silicon particles by powder technology, which are heated by concentrated solar radiation at a temperature of 1523 К [11]. This method is a relatively inexpensive technology compared to technologies such as molecular light beam or cast silicon. To study the microstructure of PS, X-ray microprobe analysis was used, and to study the electrophysical properties, modern four-probe and modernized Wonder-Pauw methods were used in the range from 300 to 800 K [9,10].

Features of the Method of Obtaining Polycrystalline Silicon
Unlike traditional powder technology, in this study, for the manufacture of silicon particles, a heat-resistant dielectric material was used, which does not contain metal materials in it. This prevents contamination of silicon particles with various impurities that may come from the outside environment. As a raw material, samples of single monocrystalline silicon p type of conductivity, o ∼ 1 − 10(Ohm • sm) were selected. This method makes it possible to obtain silicon particles ranging in size from 3 microns to 0.5 microns. Prepared silicon particles are washed with ethyl alcohol and dried under vacuum.
The interconnections of silicon particles are carried out in a "solar concentrator," which with a focal temperature is 1573 К (Fig. 2). First, a mixture of silicon particles is prepared using 70% ethyl alcohol. The mixture (2) is then inserted into a tray of heat-resistant dielectric housing (1). This process is performed by pressing the hand with little force. This process is carried out by pressing with little force. As a result, the silicon particles take the form of a substrate. It should, be noted that the use of a dielectric body base of any shape as a base makes it possible to produce samples of any shape. It should also, be noted, that the thickness of the mixture of silicon particles in the substrate should not exceed 2.5-3 mm. When the thickness of the mixture is less than 2.5-3 mm, instead of combining silicon particles, the degree of liquefaction increases. Conversely, when it is higher, the temperature is unevenly distributed over the volume. Due to the difference in temperatures of combining silicon particles in the surface areas, it is high and weak below. As a result, a brittle surface is formed on which silicon particles are not firmly attached to the base surface where the sample is not exposed to sunlight.
The use of sunlight (hγ)-also has a specific process. Concentrated solar rays (hγ) are supplied perpendicular to silicon particles and the heat-resistant dielectric body moves along the focus of the solar concentrator. At the same time, the temperature of sunlight incident on silicon particles slowly shifts from 873 К to 1523 К. It should be noted that this method, unlike conventional methods, does not perform high-pressure pressing or heat treatment (specification) using special devices. It should be noted that this method, unlike conventional methods, does not perform high-pressure pressing or heat treatment using special devices. Figure 3a shows the photomicrograph for the silicon sample heated by a sunlight. And Fig. 3b shows the photomicrograph for the monosilicon, that is, the original monosilicon.

Mechanism of Polycrystalline Structure Formation
As can be seen, the silicon sample has a polycrystalline structure with a rough grain surface of ~ 10-15 μm (Fig. 3a). Figure 4 also shows the X-ray spectral characteristics of impurity atoms. It was found that the grain of the crystal consists of Si atoms, and its surface is rough. The results can be explained as follows.
It is known that the grain size and the formation of roughness on its surface depend on the technology for producing PS. In the process of obtaining PS, waste and uncontrolled impurities belonging to metallurgical silicon or impregnated with the external environment can be retained (for example, Si → 98-99%; Fe, Au, B, P, Ca, Cr, Cu, Mg, Mn, Ni, Ti, V → 1-2%) [1][2][3][4][5][6][7][8][9][10]. These impurities can lead to the formation of defects of granular size and in the regions of inter BGB, from point defects to complex shapes.
It has been reported previously that PS consists of Si atoms, while the roughness of its surface consists of the residual impurity atoms [4][5][6][7][8][9][10]. It was found that their amount increased from nucleus to surface, while conversely, the amount of silicon atoms decreased. In addition, it has been reported previously in [4][5][6] that the samples obtained by powder technology are fundamentally different from PS obtained by casting. For example, for the samples prepared by casting, the grain size (α) found to have values between 200 and 300 µm, while those obtained by powder technology, α has ~ 100 µm. This is explained on the basis that it depends on the chemical processes and thermodynamic conditions in the crystallization process. In the casting method, the raw material is carried out at or above the melting point of silicon. Crystallization due to thermodynamic changes is not the same in all parts, which leads to an uneven distribution of impurities, leading to formation of various defects between the grain boundary regions in the crystal lattice. In powder technology, pressed samples are subjected to heat treatment close to the melting point of the material, for  [4,5]. During heat treatment, the powders may melt but they will not liquefy. Therefore, they do not change the shape they take during the pressing process. Accumulation of impurities or the segregation process is explained by the formation of roughness.
Thus, in the process of heating with sunlight, the silicon particles combine to form grains. This ensures that the silicon has a polycrystalline structure. It should be noted that silicon particles 0.5-3 µm in size were used to prepare the sample. In our opinion, small-sized silicon particles combine into relatively large-sized particles. In the work [4,12], we proposed a structural model of silicon particles, obtained by us on the basis of powder technology. According to the model, the heating process is observed in silicon particles obtained by powder technology. The heating process does not occur uniformly across the particle crystal size. That is, there is a temperature difference from the particle core to the surface (T1 ≥ T2 ≥ T3, Fig. 4). In this case, the structure of the silicon particle can be divided into 3 parts according to the temperature difference (Fig. 5). The rough surface (1) has a relatively high temperature, the core of the particle (3) and consists of a separating region (2). The surface area temperature is high, it consists of empty or broken bonds, and the energy of defect formation in the field also increases. In contrast to the nucleus, in the region close to the surface, the initial region of atomic decay of the crystal lattice appears (region 2). The atomic structure of this field is radically different from the atomic structure of fields 1 and 3. In this case, the reactivity of the surface of a silicon particle increases from the center to the surface in accordance with its atomic structure [4,12]. According to the model, the reactivity of a small-sized particle nucleus is close to the reactivity of areas 1 and 2 of a large-sized particle. In this case, particles of small size accumulate around large particles. The result is areas of a certain size of granularity and BGB.

Charge Transfer Mechanism
It is known that the electrophysical properties of polycrystalline semiconductors are explained by charge transfer processes between the grain boundary areas [1][2][3][4][5][6][7][8][9][10]. In Fig. 6 illustrates the temperature dependence of the (ρ) resistivity of the sample (p-type conductivity, 1 o ∼ 191Ohm • sm) . To compare the results obtained, we will use the results presented (p-type conductivity, 2 and 3 o ∼ 1 − 10Ohm • sm , obtained by two casting methods, 4o ∼ 100Ohm • sm by the method powder metallurgy) in [4,5]. The dependence of ρ on the temperature change can be conditionally divided into a-b, b-c, and c-d.
In polycrystalline semiconductors, localized traps in the BGB area create a barrier effect. The capture and emission of charge carriers in localized traps leads to a change in the height of the potential barrier (φ), which, in turn, changesρ. According to the Setto model, the following expression can be written for ρ [1,2]: here, q -electronic charge, k -Boltzmann constant, 〈a -grain size, A* -Richardson's efficiency constant, 5 Simplified scheme of silicon particle; 1-Rough surface area with uneven atomic structure, 3-granule core, 2-the area separating the core (1) from the field, T3 ≤ T2 ≤ T1 is the temperature difference in the corresponding areas Fig. 6 Temperature dependence of resistance: T -temperature, φ -is the height of the potential barrier between the grain boundary areas. It is known that φ depends on the amount of charges trapped in localized traps (Q i ): where is the N G -concentration of electrically active doped impurities, ε and ε o are the relative dielectric constant of the medium and the absolute dielectric constant of the vacuum, respectively.
Equation (2) shows that the increase in charges (Q i ) trapped in localized traps leads to an increase in φ, which in turn leads to an increase in ρ (case a -b', lines 2, 3, 4). However, the sample prepared by heating with sunlight ρ (line 1) [4,5] is radically different from the results given in the study (lines 2, 3, 4). That is, at T ≤ 600 K ρ decreases and then increases, at T ≥ 700 K it stably changes (line 1).
It is known from semiconductor physics that the dependence of electrical conductivity on temperature change is expressed by the mobility and concentration of charge carriers. Under the influence of temperature, the concentration of charge carriers increases due to an increase in the concentration of ionized impurity atoms. This leads to a decrease in resistivity. At later stages of increasing the temperature, the impurity atoms are ionized, the concentration of charge carriers does not change, since the phonon energy is insufficient to ionize the main atoms of the semiconductor. It is also observed that the free running path of the charge carriers is reduced as a result of the thermal vibration of the crystal lattice. In this case, a decrease in the mobility of the charge carriers leads to an increase in the resistivity. As the temperature increases further, the major atoms of the semiconductor ionize and a special conductivity is formed.
The sample prepared by heating with sunlight (line 1, Fig. 6) confirms the observations given by the change in temperature ρ. In our opinion, at T ≤ 600 K, the concentration of charge carriers increases due to an increase in the concentration of ionized impurity atoms, which, in turn, leads to a decrease in ρ (case a-b, line 1, Fig. 6). At T ~ 600-700, the decrease in the free path of the charge carriers as a result of thermal oscillations of the crystal lattice leads to an increase in ρ (case b-c, line 1). At T ≥ 700 K, a constant change in ρ (c-d condition, line 1) may be due to the appearance of new recombination centers specific to localized traps. For example, at T ~ 600-700 K, oxygen atoms can diffuse to the surface, leading to the formation of various forms of silicon oxide (SiO x , Si y O x ) compounds on the surface of the silicon crystal [1][2][3][4][5][6][7][8][9][10][12][13][14].
Although oxygen is electrically neutral in silicon, it has a significant effect on charge carriers during temperature changes. That is, as the ionization process increases, the ρ at T ≥ 700 K changes steadily at the expense of oxygen. In other words, the results obtained for PS heated by sunlight due to this mechanism are fundamentally different from the results obtained for PS obtained by other methods.

Conclusion
Polycrystalline semiconductor materials form a barrier effect of localized traps or segregated impurity atoms in BGB, which leads to shunting of the p-n junction. It also has a negative effect on the efficiency of semiconductor devices or solar cells made on the basis of PS. In our opinion, PS heated by sunlight does not create a barrier effect of traps localized in the BGB regions from PS obtained by other methods. This can expand the possibilities of creating highly efficient semiconductor devices, solar cells, thermoelectric materials for micro-and nanoelectronics, photovoltaics. These results can be used to create highly efficient p -n structures based on polycrystalline semiconductors, as well as to explain the electronic properties of the BGB.