CC BY
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https://doi.org/10.29013/ESR-21-1.2-35-38
Otazhonov S. M., Fergana State University, Fergana city, Uzbekistan
Rakhmonulov M. Kh., Fergana State University, Fergana city, Uzbekistan
Khalilov M. M.,
Fergana branch of Tashkent University information technologies named after Muhammad al-Khwarizmi, Fergana city, Uzbekistan
Botirov K. A.,
Fergana State University, Fergana city, Uzbekistan
Yunusov N.,
Fergana State University, Fergana city, Uzbekistan E-mail: [email protected]
EFFECT OF GROUP VII ELEMENTS ON STRAIN SENSITIVITY OF POLYCRYSTALLINE FILMS PBTE, PBS
Abstract. The article describes tensoelectric properties of polycrystalline films based on PbTe and Pb S. It was found that the addition of a dopant increased the current carrier concentrations and mobility. It was shown that by changing the amount of lead chloride introduced into lead sulphide and giving an excess of lead, the concentration of current carriers could be changed within a wide range.
Keywords: polycrystalline film, semiconductor, strain sensitivity, strain, resistivity.
Introduction duction of silicon and germanium strain gauges has
In recent years in our country and abroad inter- been successfully mastered [5]. est in semiconductor strain gauges has increased It is known that the strain factor in semiconductor sharply. The reason of rapid development of semi- strain gauges is related to the constants of strain semiconductor strain gauges is new wide possibilities of tivity, the material from which they are made, which semiconductor strain gauges application in the field are determined from the effect of strain sensitivity. of research of material and constructions strength, in Strain-sensitivity effects in n - PbS monocrystals super miniature transducers of mechanical quanti- were investigated in the low temperature region of ties (force, pressure, deformation, moment, etc.) in 77-300 °K [6]. In polycrystalline samples, the high electric signals [1; 2; 3]. strain effect found in [7] is explained by the appear-
At present time intensive researches of semicon- ance of micro p-n junctions at the grain boundary.
ductor strain gauges and devices using them as trans- However, in all these works there was no compre-
ducers are carried out. Recent investigations have re- hensive study of strain response properties of n- and
vealed a number of strain-sensitive semiconductor p - PbS with different carrier concentrations in the
materials such as silicon, germanium, silicon carbide, temperature range above room temperature. gallium phosphide, indium and gallium antimonide The aim of this work is to investigate the strain-
and others, promising for their use in strain measure- sensitivity effect in PbTe PbS and to develop a
ment for a range of temperatures [4]. The pilot pro- strain-sensitive material for strain gauges based
from it, which has high strain sensitivity and low enough cost.
Experimental methodology
To measure the effect of strain sensitivity in poly-crystalline samples we applied the following methodology. Polycrystalline films were obtained under high vacuum, the methodology given in [8].
The samples ready for measurement were installed in the deforming device, the diagram ofwhich is shown in [8]. Although the installation is not complicated, in order to carry out reliable measurements, great attention should be paid to the following circumstances. The friction losses in the set-up must be as low as possible. The force to deform the sample must produce a uniform mechanical stress field throughout the thickness of the sample and, finally, the setup must be suitable for temperature control.
In terms ofelectrical measurements, more stringent requirements are placed on the quality ofthe contacts. They must be ohmic and their properties must not be strain dependent. Since very small voltages have to be measured when examining the strain effect, the measuring equipment must be accurate to at least 0.1 +0.3 (V when measuring the voltage. To create optimum concentration of current carriers in n-PbS and PbTe we investigated impurities ofgroup VII elements.
Experimental results and discussion
Measurement of strain effect in lead sulfide has some interesting features and can give valuable information about its band structure. In [9] it was shown that the change of effective mass (or strain
potential constant) makes an essential contribution to the strain effect. In n-PbS with carrier concentration 1018 cm3 a noticable temperature dependence of the shear constant p44 , is observed which is close
to —: (T - absolute temperature in K degrees), however, in the whole investigated temperature range (73-293 0K) the corresponding effect is very small.
Since we are going to study strain gauge parameters of PbTe PbS strain gauges as well, it is useful to consider the strain gauge effect and its defining characteristics.
Large values of strain-sensitivity constants of some materials were the basis for the development of strain gauges, widely used to measure strain.
A technique for introducing lead halide impurities into lead telluride was proposed in [10]. They showed that to change the concentration of electrons in telluride lead in a wide range is possible only by the introduction of "double impurity" such as Pbcl2+Pb.
We used this technique for the introduction of impurities in lead sulphide obtained by the synthesis of the starting components. Lead chloride was used as an impurity. Experimental data obtained by us on growing crystals showed that lead sulphide with complex addition of lead chloride and lead metal can be obtained in the form of single crystals only in the case, if for each chlorine atom there is one lead atom. If the excess lead is not complexed with added chlorine, the lead is not incorporated into the lattice and interferes with the growth of monocrystals, and fine crystalline ingots are obtained.
Ser.No. Number of atoms of introduced lead, cm-3 Number of chlorine atoms cm-3 n cm 3 MV a—- degree s cm 1
1. 1•1020 1•1020 8.5 • 1019 -44 3800
2. 1•1020 2•1020 8.2 • 1019 -44 3800
3. 2.5 • 1020 2.5 • 1020 1.8 • 1020 -30 4950
4. 3•1020 3•1020 1.8 • 1020 -30 4950
If, however, more chlorine atoms than lead atoms grow well, but in this case the excess chlorine atoms are introduced with the admixture, the single crystals do not increase the concentration of current carriers.
As can be seen from (Table 1), samples of crystals No. 1 and No. 2 have practically identical carrier concentrations, electric conductivity and coefficient of thermal emf, although with the same amount oflead in the impurity (1 • 1020 cm-3) twice as many chlorine atoms are introduced into crystal No. 2.
Increasing the addition of chlorine and lead above 2.5 • 1020cm-3 does not lead to a further increase in current carrier concentration.
The electron concentration, electrical conductivity and thermal EMF in sample No. 3 (Table 1) and No. 4 are the same, although 2.5 • 1020cm-3 of chlorine and lead were added to the former and 3 • 1020cm-3 to the latter.
Lead increases the concentration of electrons to values corresponding to the concentration of chlorine introduced. By varying the amount of lead chloride introduced and giving an excess of lead, the concentration of current carriers can be varied within a wide range (Fig. 1) (curve 3) shows the dependence of the obtained electron concentration on the number of impurity chlorine and lead atoms introduced. The figure shows that increasing the addition of chlorine and lead beyond 2,5 • 1020 cm3 does not lead to a further increase in the electron concentration. The maximum electron concentration we were able to achieve by the introduction of the complex addition of Pbcl + Pb is n = 1,9 • 10 20cm-3.
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Figure 1. The dependence of Hall effect of conductors in polycrystalline PbS and PbTe on the amount of injected impurity
The ratio between electron concentration for single crystal samples, calculated from Hall effect measurements and the concentration of introduced impurity chlorine atoms, in average varies in the range 0 : 8 for samples with high concentration of current carriers (above 5 • 1019cm-3).
The ultimate solubility of lead halides in lead sulphide depends on the mass of the introduced halogen: chlorine has the maximum solubility, iodine has the minimum. It can be assumed that chlorine enters the lattice of lead sulphide more easily than iodine and bromine.
It appears to be that the ionic radii of chlorine and sulfur (1.05 and 1.03) are very close, and bro-
mine and iodine have larger ionic radii than chlorine (1.6 and 1.4 for bromine and iodine, respectively).
Since chlorine allows deeper doping of lead sulfide than other halogens, we studied strain-gauge properties of PbS samples doped with Pbcl2+Pb in a wide concentration range most completely.
Doped (Pbcl2+Pb) amples have higher absolute values of electron mobility compared to samples doped with bromine and iodine. Maximum carrier mobilities from their concentration in polycrystalline PbS PbCl2 + Pb, oped samples are obtained for samples with electron concentration in the range of 3.1018 1.2.1019cm-3. With a further increase in carrier concentration the mobility of the samples decreases steeply.
Conclusion
On the basis of the results obtained we can say that with the addition of an alloy of group VII elements we can see a change in the value of the tensivity fac-
tor and electron mobility and this is explained by the penetration into the depth of the sample of chlorine and other halogens. Chlorine penetrates the lattice of lead sulphide more easily than iodine and bromine.
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