ELECTRICAL PROPERTIES OF SEMICONDUCTOR SILICON CARBON (SiC) AND ZINC OXIDE (ZnO) COMPOSITE POLYCRYSTALS WITH HETEROGENEOUS STRUCTURE Текст научной статьи по специальности «Техника и технологии»

UDC: 621.592

ELECTRICAL PROPERTIES OF SEMICONDUCTOR SILICON CARBON (SiC) AND ZINC OXIDE (ZnO) COMPOSITE POLYCRYSTALS WITH HETEROGENEOUS STRUCTURE

SULEYMANOVA LILIYA GENGHIS

1Mingachevir State University, teacer of the Department of Energy, doctor of philosophy in technology, Azerbaijan, Mingachevir city orcid:0000-0002-8884-3947

Materials with nonlinear conductivity are of great importance for energy. The application of such materials ensures the weakening of harmful voltage waves that can increase by leaps in high voltage lines and substations. It should be noted that SiC and ZnO materials with symmetrical voltampere characteristics are mainly used in modern electrical engineering. By means of the non-linearity effect, it is possible to adjust the attenuation of harmful voltage surges that can increase by leaps in high voltage lines and substations. If we connect such an element in parallel with the device we protect, for example, a transformer, then the surge waves are turned off (weakened) and the device does not feel high transient voltages. It was determined that during the selection of additives, the resistance of crystallites, mole percentages, the effect on the non-linearity of the volt-ampere characteristic of the varistor should be determined in advance and the formation of a potential barrier in varistors should be taken into account. The main reason why ZnO and SiC varistors have nonlinear resistance is the formation of a potential barrier at the crystallite-amorphous phase boundary. Works dedicated to electron-ion and polarization processes in polycrystals, the possible mechanism of high conductivity in varistors made of semiconducting ceramics and doped ZnO were collected, analyzed and systematized. Also, an analysis of the characteristics of the electric conductivity of the ceramic varistors up to the opening (linear field) voltage and of the non-linear field was given, and similar and different characteristics were determined for the doped ZnO - varistors [1-20].

Keywords: ZnO (zinc oxide) and SiC (silicon carbide) composite crystals, polarization, barrier conductivity, inter-crystallite boundary, selection of additives, automatic device for measuring VAX, Pull-Frenkel effect, microvaristors.

Introduction

The height, width, and symmetry of the crystallite economic potential barrier are derived from the electrophysical, material, and physical structures of the amorphous phase (the boundary within the crystallite) that isolates the crystallites from each other. The filling of traps in the amorphous phase determines the symmetry and width of the crystallite potential barrier. Depending on the fields of application of polycrystals, special requirements are placed on the symmetry (polarity) and nonlinearity of the non-ohmic volt-ampere characteristic. An example of this is the role of varistors in the development of protective devices in high voltage technology. Recently, intensive scientific-research work on increasing the amplitude interval of the applied voltage allows the application of voltage and varistor integrated microcircuits for continuous high-voltage line calculations. It should be noted that when selecting semiconductor ceramic materials, for example, SiC, ZnO, for the development of varistors with various functions, the conductivity of their crystallites, the width of the forbidden zone, the conductivity of electric charges, and the parameters of the potential barrier that may arise at the border of amorphous and crystalline phases should be taken into account. An important factor in the development of composite varistors based on polymer-SiC and polymer-ZnO is the width of the band gap of 2.2 eVin SiC and ZnO materials, which are semiconducting ceramic phases of these composites. Therefore, undoped SiC is not a conductive material. Zinc oxide is a semiconducting compound of type A2B6. Its crystal structure has a wurtzite lattice. The width of the forbidden band at room temperature is 3.2 eV. Despite the large width of the band gap, ZnO always

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has n-type conductivity due to the violation of stoichiometry, more precisely, the lack of O2. The melting temperature is equal to ~19750C, the concentration of defects is equal to ~1018 cm-3. The type of electrical conductivity of SiC depends on the concentration of additives in its composition and the group in which it is located in the Mendeleev system. Elements of the fifth group (N, P, As, Bi) of SiC are involved in n-type electrical conductivity, and elements of the second group (Ca, Mg) are involved in p-type. In conditions of abundance of carbon atoms, SiC has p-type conductivity, and Si has n-type conductivity. It should be noted that the application of SiC is not so important in terms of limiting electrical voltages, especially high voltages. Thus, the non-linearity coefficient of the varistor obtained on its basis is < 7. Therefore, when the SiC varistor is connected in parallel with the device we want to protect, it cannot protect it, that is, it cannot limit voltage spikes more effectively. In addition, such varistors have both resistance and losses in the working voltage range. In order to eliminate the mentioned negative factors, electric gas discharge devices are connected in series to the SiC varistor. Experiments show that since the differential resistance of SiC varistors is large,

Rd = their nonlinearity coefficient J3 is small. Despite all this, SiC-based nonlinear elements

combined with electric gas discharge devices can reliably protect voltage (current) sources under the conditions of high voltage impact from lightning voltage surges with an amplitude of 100kA. The electrical conductivity of pure ZnO is determined by the concentration ratios of ionized donors and acceptors, as well as the mobility of charge carriers. ZnO-based varistors, often referred to as metal-oxide varistors, are made of high-quality (natural) powdered ceramics, unlike SiC varistors [7-9,1618]. For its preparation, they mix ZnO powder with submicron grains and a certain amount of other metal oxides. As additives, for example, bismuth, manganese, antimony and chromium oxides are used. On the one hand, they are used to control cooking processes, and on the other hand, they have a certain effect on nonlinear electrical conductivity. After preparation of granules and its molding under a pressure of ~ 5*104MPa, it is baked for several hours at a temperature of 1200 ^ 13000C. The amount of crystallites formed during cooking depends on the concentration of crystallization centers at the beginning of cooking, as well as its duration. These quantities are inversely proportional to the opening voltage. In turn, the opening voltage is not determined by the number of serially connected ZnO contacts per unit length, but by the size of the particles. After obtaining the required structure, they cool down the furnace temperature to room temperature according to a certain temperature-time regime. At this time, a thin insulating layer is formed at the grain boundary, which causes the varistor effect [1-4, 6-10,13-17]. ZnO is currently widely used in high-voltage technology to limit voltage (current) surges caused by various causes. The non-linearity of the Volt-ampere characteristic of the elements based on ZnO varies in the range of 50-70. Such high nonlinearity prevents ZnO varistors from being used in combination with electric gas discharge devices. ZnO varistors are connected in series-parallel and placed in dielectric coating. One of the important factors when applying varistors is their parameter stability. For this purpose, electrothermal aging of varistors is carried out. Varistors are heated to 403-423K under the influence of an electric field, and then cooled to room temperature. This technological operation is repeated until the varistor's opening voltage, nonlinearity coefficient and resistance to opening voltage are stabilized. In order for the parameters of varistors to remain stable, it is important to protect them from moisture. The final operation of making varistors is drawing electrodes on their surface. This operation is carried out as follows: the surface of its elements is chemically cleaned, silver paste is applied to its surface as an electrode and baked. To protect the electrodes from the environment, their surface is varnished. To increase the non-linearity of the voltampere characteristic of varistors, they use cyclic and pulse cooking methods. Additives added to ZnO can be relatively divided into three groups: donors, surfactants and mineralizers. Current difficulties for interpretation indicate very large values of nonlinearity (20<P<70) [11-16].

Experimental

The potential barrier was created as a result of electron exchange with two neighboring crystallites and the amorphous phase between them (intercrystallite boundary) (Fig. 1.a,b). In equilibrium, charges on the surface of crystallites are compensated by electrons stabilized in the amorphous phase, that is, eNs=eNdr, and the height of the potential barrier is determined by the known ratio: [6-15]. Figure 1. shows the intercrystallite potential barrier.

Ф0 =eNdr2/2808r=eNs2/2808Nd (1).

Here:

r - the width of the area where surface loads are generated;

d - average size of crystallites;

Ns - density of levels on the surface of crystallites;

Nd -concentration of donors;

s0 -electrical constant (s = 8,85 • 10 ~2F / m);

S Г -relative electrical permeability;

e - is the charge of the electron (1,6 • 10-19Kl ).

r « d and eNs « eNd • d /2 because the crystallites are relatively reduced.

Electrical conductivity created by crossing the potential barrier formed between crystallites of high conductivity

= TT"av exP (-<o /kT) (2).

N a

is defined as. Here, Na the height <p0 is the number of electric charge carriers crossing the potential barrier.

av = e n ^ - specific conductivity of crystallites

- mobility of electric cargo carriers;

n - is the concentration of transferred electric charges.

The exponential factor refers to the temperature dependence of the mobility of electric charge carriers in a polycrystal. Barrier conduction is the main type of conduction for polycrystalline materials. The non-linearity of their volt-ampere characteristic (VAX) is determined by the change of the height of the potential barrier and the width (r) of the area where space charges are generated in the crystallites depending on the applied electric field voltage. The effect of the conductivity of crystallites is manifested at high voltage, for example: after the opening voltage, and at this time the potential barrier is short-circuited by the tunnel current. Therefore, in the equivalent circuit of polycrystals, crystallites are shown as current limiters. This resistance explains the saturation part of VAX of varistors, and this determines its current carrying capacity after opening voltage. The height of the intercrystallite potential barrier in polycrystals is determined by their doping level and the concentration of crystallite surface levels. Formula 1 shows that the value of the potential barrier varies depending on the concentration of donors (Nd) until the crystallite surface energy levels (Ns) are completely filled. The potential barrier takes its highest value when eNs=eNdd/2. After the traps (stabilization centers) on the surface of the crystallites are completely filled, the height of the potential barrier is affected by the decrease in the width (r) of the area where the space charges are generated and 1/ Nd decreases as a parameter (r). The parameters of the potential barrier of polycrystalline

semiconductors (< and r) depend on their doping degree and the concentration of oxygen traps

(centers with high electronegativity). For example, addition of crystallites of ZnO semiconductor ceramics with group III elements affects the parameters of the varistors based on it both before and

after the turn-on voltage, as well as VAX. The intercrystallite potential barrier in oxygenated semiconductors used for varistor fabrication is mainly related to different forms of chemisorption of oxygen. Intercrystallite potential barriers are formed in enlizone semiconductors, for example, ZnO, and they are stable only in an oxidizing environment and at temperatures T < 673K [14-19].

Figure. 1. Microstructure of polycrystals (a) and energetic diagram (b) of the intercrystallite

boundary U=0 (a), U>0 (b) [1-4].

In the indicated temperature range, there may be transitions between different forms of oxygen chemisorption:

O2 0"2 va 0"2 +e^2 O" (3)

ZnO is a strongly doped (Nd=10-10 sm), poorly compensated semiconductor. Intrinsic defects, such as cations or oxygen vacancies between the nodes of the crystal lattice, act as donors.

In bulk ZnO crystallites, the Fermi level approximately coincides with the conduction band. Therefore, the specific resistance of the crystallites after the opening voltage is very small. The specific resistance of the ZnO polycrystal in the fields up to the opening voltage is determined by the parameters of the intercrystallite potential barrier. A sharp reduction of the intercrystallite potential barrier due to the effect of electric field and temperature increases the electrical conductivity of ZnO >106 times and equals the corresponding parameter of single crystals. Figure 2. shows the energetic characteristics of the tunnel-transparent crystallite interface. According to the Oje-spectroscopy method, the thickness of the irregular region (amorphous phase) separating the crystallites is approximately 2x10-7cm. This region is separated from the volume of crystallites by a surface potential barrier.

When the doping level of crystallites is, Nd=1018 sm-3 the width of the surface area of positive space charges is r = (2 s0s % / e2Nd )1/2 < 2,5 • 10sm

The conductivity of crystallites <JV and the linearity of (VAX) arise at larger values of current

density. In the electrical scheme, the current losses along the inter-crystallite boundary are characterized by the Rs resistance. This resistance arises during the transfer of electric charge carriers by the jump mechanism. ZnO can be doped with trivalent elements (acceptors) to reduce the resistance Rv, which limits the amplitude of the current after the opening voltage of the varistor. The obtained results show that the activation energy of local levels created by additives in the quasi-barrier zone of ZnO is ~0.05 eV , that is, shallow donor levels. Since this effect is of particular importance in the formation of the nonlinearity of VAX of varistors, let's give a broader analysis of the addition of ZnO with ions of different ionic radii and its effect on the Rv parameter. The varistor effect of ZnO-polymer composite is of great importance in determining the formation mechanism.

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(a)

(b)

Figure. 2. Energy diagram of intercrystallite boundary (a) and equivalent electrical circuit (b)

[1-4].

Here, Rf and Rr are the resistance of the intercrystalline wall in the direct and opposite directions of the electric field, Rv is the resistance of the crystallite, and Rs is the total resistance. If we want to apply the known models and C-U characteristics that we have analyzed for polycrystallites to composites, then first of all, the physical structure of the composite and the structures of polycrystals should be compared, and relevant and different factors should be determined. In the initial approach, the role of crystallites in the polycrystal is played by the varistor ceramic particles in the composite, and the phase plays the role of the inter-crystallite boundary. However, it should be noted that the crystallite boundaries of semiconducting ceramic varistors (for example, ZnO, SiC) and ZnO-polymer varistors are quite different due to their chemical and physical structures. In addition, if in the ZnO varistor the crystallites and the phase that separates them differ little in terms of their structure and electrophysical properties, then in the ZnO polymer composite, on the contrary, the structure and electrophysical parameters of the polymer phase that separates the ZnO particles differ sharply from the corresponding parameters of ZnO. A more extensive analysis and study of

these issues will be given in the following chapters. Now let's analyze the characteristics of electrical conductivity of ceramic varistors up to and after the opening voltage and determine their similar and different characteristics for ZnO-polymer varistors. In terms of developing ZnO polymer composite varistors, it is necessary to determine the mechanism of tunneling currents in ZnO semiconductor varistors and the existence of this effect in composites. Experimental study of tunneling conductivity is difficult, as there are many uncertainties in the values of the physical parameters characterizing the inter-crystallite boundary. For example, it is not always possible to distinguish intercrystallite boundary overpotential or tunneling. One of the shortcomings of the tunnel model is that the nonlinearity of VAX of p = dlnI/dInU polycrystals and the probability of tunneling decrease with increasing temperature. Another fundamental issue is the relationship between thermoactivation processes and the tunnel effect in microvaristors. The junction density of microvaristors can be determined through the volt-forad characteristic (C-U). It is known that the voltage dependence of the specific capacitance for a potential barrier

C

2

1

Ai

e2snsvN.

0°r^d J

-eUlFy + (00 + eUlRy

(4).

calculated by the formula.

Here, U/f is the voltage 9k across the potential fence connected in the forward direction, and Ur is the voltage 9r across the potential fence connected in the reverse direction. If we take the total capacitance C=Cr of the microvaristor shown in figure 1.2.b, then the C-U characteristic

C 2C

-)2 =

e WrNd

( 0+ eUR)

(5).

expressed as. Here is C1 = e2s0srNd /8^02 the capacitance of the microvaristor at the zero

value of the voltage. If we apply the properties of the microvaristor to the polycrystal, then

C = C / / N

U* = N /U' = U R ° R 2 '

R 0

(6).

1

2

1

a

and the normalized C-U characteristic is obtained:

C 1 1

C -1)2 =7^(NCT0o + eUR ) (7).

С 2 4N0o

Here, U*R is the external voltage across the entire reverse connected potential wall in the varistor.

For polycrysts

1

C = e2s0srNd !8Naq>2 , Uo = Nap0 /ß (8).

0

and

С 1

(Cr -1)2 = f (Ur ) (9).

The electrical properties of many polycrystalline semiconductors and semiconductor ceramics are determined by the potential barrier created at the intercrystallite boundary. Since the electrical properties are determined by the inter-crystallite Schottky-type double potential barrier (Fig. 3 b), the microscopic study of the processes taking place at the inter-crystallite boundary under the influence of an electric field is of particular importance for revealing the essence of the processes in the interphase layer of the ZnO-polymer composite. For example, the determination of the mechanism of the emergence of the state of high conductivity in ZnO-polymer varistors can be cited as an example. To develop the technology of ZnO -polymer varistors, let's look at the three stages of formation of the conduction state of ZnO.

A) Weak field E<E,<Ec

In order to understand the mechanism of the processes taking place in this area, let us first give a brief explanation of the essence of the Pull-Frenkel effect. The Pull-Frenkel effect is the thermal ionization of local levels with a decrease in the ionization potential under the influence of an electric field. So, thermal ionization is possible in varistors and is one of the factors that ensure the increase of VAX instability of varistors in the E<Ei field. Energy diagram of microvaristors for E<Ei and E>Ei field is shown in figure 3. Electrons freed at crystallite levels cross the contact zone because their free flight path in ZnO is quite large. The ionization energy of the surface levels on the right side of the contact whose dipole moment is antiparallel to the intensity vector of the external field increases, that is, the surface levels on the side of the opposite slip voltage contact are maximally filled and they play little role in the conductivity of the ZnO-based composite varistor.

B) Intermediate area - Ei<E<Ec (fig. 3 b).

As the amplitude of the applied electric field increases (E<Ea), the crystal surface levels on the contact side, under shear stress, change the direction of polarization and do not create an antiparallel dipole moment with the external field. The subsequent increase of the applied voltage undoubtedly affects the distribution of electrons on the right and left sides of the intercrystallite boundary, and the initial stage of a sharp increase in the conductivity of the varistor is formed. At the moment U=Ua, a thermostimulated tunnel junction (Pull-Frenkel effect) is formed and is observed by a sudden increase in the current in VAX [2; 19].

Figure 3. Energy diagram of microvaristors

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a) E<E; b) E>E [19].

The effect of the synthesis temperature on the opening voltage and nonlinearity coefficient of the varistor was studied. It has been shown that as the synthesis temperature of the varistor increases, the value of the opening voltage decreases, and the nonlinearity of the volt-ampere characteristic increases. The temperature dependence of the current passing through the varistor, electrical conductivity, nonlinearity coefficient and opening voltage was studied. As a result of the analysis, it was determined that as the temperature of the varistor increases, its electrical conductivity and current strength increase by 3-4 times. The values of the opening voltage and the nonlinear coefficient decrease. The observed results are explained by the decrease in the height of the potential barrier between the ZnO grains as the temperature and voltage increase, and the increase in the concentration of charge carriers as a result of the ionization of the traps. From the analysis of the experiments, it was determined that the multicomponent additives included in order to increase the nonlinearity of VAX slow down the diffusion processes of cooking at 600-8000C, and intensify it above 9500C due to the formation of fast-melting eutectic compounds located at the border of ZnO crystallites.

The advantages of ZnO-polymer varistors are as follows:

-ZnO composite varistor conducts a current of several kA for 10-8 seconds when the cross-sectional area is 1 cm2;

- The stability of the characteristics of the ZnO varistor and its resistance to wear are high;

- the production technology is relatively simple in small weight and size;

- the nonlinearity coefficient is large;

- the resistance in the area of the volt-ampere characteristic up to the opening voltage is very

small.

Composite varistors based on -ZnO -polymer or SiC -polymer are close to SiC and ZnO semiconductor varistors in terms of the specified characteristics. However, they are superior to them due to a number of mechanical, physical-technological and geometrical parameters:

- the simplicity and low temperature of the acquisition technology;

- light weight, flexibility and the possibility of making it in any size and configuration.

Conclusion

The potential barrier (barrier) formed at the border of ZnO particles causes a time dependence of the voltage at a constant value, which affects the pattern of electrical parameters of ceramics with a small percentage of ZnO addition. The increase of ZnO addition changes the phase collection, structure, nonlinearity of VAX and the height of the potential wall (barrier) formed at the boundary of the particles of the ceramic. Let's look at the effects of the zone width (E) of the SiC and ZnO phase we use as a dispersant, the concentration (N) and conductivity of the electric charge carriers on the properties of the composite varistors. It should be noted that the determination of the effect of the concentration of additives (band width and density) in the forbidden zone of SiC and ZnO on the parameters of the potential barrier formed at the interphase boundary in SiC-polymer and polymer composites in general is an important issue for solving the above problem. The effect of fermi levels at the contact boundary (ZnO -polymer, SiC -polymer) will lead to determining the mechanism of the varistor effect in composites and developing technologies that can increase the parameters of composite varistors.

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