Yu. N. Andreev and N. P. Yaroslavtsev. Voronezh State Technical University, 394026 Voronezh, Russia. M. V. Bestaev, D. Ts. Dimitrov, V. A. Moshnikov, and Yu.
Study of submicron deposits in polycrystalline materials using the internal-friction method Yu. N. Andreev and N. P. Yaroslavtsev Voronezh State Technical University, 394026 Voronezh, Russia
M. V. Bestaev, D. Ts. Dimitrov, V. A. Moshnikov, and Yu. M. Tairov St. Petersburg State Electrical Engineering University, 197376 St. Petersburg, Russia
~Submitted November 18, 1996; accepted for publication November 25, 1996! Fiz. Tekh. Poluprovodn. 31, 841–843 ~July 1997! An internal friction method is proposed for investigating the kinetics of impurity deposits on grain surfaces in polycrystalline samples. The possibilities of the method have been tested on polycrystalline, gas-sensitive, tellurium-doped layers of tin dioxide. © 1997 American Institute of Physics. @S1063-7826~97!02007-3#
Polycrystalline semiconductor materials are now being used extensively in microelectronics, optoelectronics, and sensor electronics. Reproducible properties are difficult to obtain in polycrystalline samples because of the effect of structural features associated with intergrain boundaries ~IGBs!. Depending on the type of IGBs and the character of their interaction with the dopants, the properties of the semiconductors can differ sharply.1 This is especially strongly manifested in the properties of devices such as gas-sensitive sensors, luminophors, photodetecters, and light emitters. Microsegregation and precipitation of impurities on IGBs make it possible to control the gas sensitivity and selectivity of adsorption semiconductor sensors. The kinetics of segregation and precipitation predetermines the degradation properties. Different models are used to describe segregation: hard spheres,2 structural unit,3 electric,4 molecular dynamics,5 and local electronic sheets.6 The role of the impurity precipitating on a grain boundary in changing the selectivity with respect to an adsorbed gas is studied in Refs. 7 and 8. The experimental investigations of segregation and precipitations of impurities are performed, as a rule, by an electron-probe or other method of surface analysis. Although these methods yield a great deal of information when the probe falls directly on the surface of an IGB, their transverse resolution strongly limits their applicability.9 Our objective in the present paper is to evaluate the possibility of using other integral physical methods which are
FIG. 1. Diagram of the oxidation setup. 1 — Precision regulation valve 2; 3 — bubblers; 4 — quartz tube; 5 — furnace; 6 — substrate; 7 — bubbler with deionized water; 8 — furnace; 9 — output neutralizing bubbler. 714
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sensitive to precipitation of microinclusions. The objects of investigation were gas-sensitive, doped, tin-dioxide-based layers.10 The experimental data on the change in the electrical properties, sensitivity, and selectivity of tellurium-doped samples are explained by redistribution of the impurity. Tin films were deposited on 22KhS ceramic substrates by the method of thermal vacuum sputtering. The substrate temperature was held at 150 °C, which made it possible to avoid the formation of drops of condensate. Two types of charges were used to regulate the tellurium content: pure tin and tin telluride obtained by the iodine method.11 Traces of iodine make it possible to obtain a more developed granularity of the films. The impurity content was fixed by mixing the compositions of the two initial charges. The uniformity of the phase composition was monitored by an x-ray diffraction method using the technique described in detail in Ref. 12. Tin dioxide was obtained by oxidation in two stages. The low-temperature annealing stage corresponded to 210 °C and lasted for 6 h. The high-temperature stage varied in duration from 6 to 30 h and was conducted at a temperature of
FIG. 2. Diagram of the setup for measuring the temperature dependence of the IF by the inverted-pendulum method. Explanations are given in the text.
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FIG. 3. Temperature dependence of the IF in tellurium-doped tin polycrystalline films on ceramic substrates. 1 — Before annealing; 2 — after annealing for 30 h at 450 °C.
450 °C. Oxidation was conducted in a controlled atmosphere. The arrangement of the apparatus is shown in Fig. 1. The oxygen partial pressure was varied from 0.1 to 0.2 atm. As a physical method for investigating the redistribution of tellurium between the volume and the grain boundaries, we employed the internal-friction ~IF! method, which was previously successfully used to study inclusions of a second phase in semiconductors.13–15 The method involves measuring the temperature dependence of the IF by the invertedpendulum method. The arrangement of the apparatus is illustrated in Fig. 2. The experimental sample is secured at one end to base 3 by a collet 2. A collet 4 with the pendulum 5 is secured to the other end of the sample 1. A ring 6 of ferromagnetic material is placed on the top of the pendulum. Coils 7 and 8, which are connected via the switches 9 either to a low-frequency generator 2 or to an amplitude discriminator 11, whose output is connected to the electronic counter 12, are arranged symmetrically near the ring 6. In the first case, coils 7 and 8 are used to excite mechanical oscillations of the pendulum 5 by the interaction of the magnetic field of the coils with the ferromagnetic ring 6; in the second case they are used as a sensor for displacements of the ring 6. A heater 13 is placed near sample 1. The elements 1,2,4–8,13 are placed in an airtight container 14, from which air is pumped out in order to decrease the damping of the oscillations of the pendulum 5. Internal friction measurements were performed on
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samples obtained in a single technological cycle at the stage of deposition of the tin layer and low-temperature oxidation. It was found that in the tellurium-doped samples, in contrast to the samples obtained from pure tin, in the process of hightemperature oxidation an IF peak appears at temperatures close to the melting point of tellurium. The intensity of the peak increases by a factor of 12 as the annealing time increases from 6 to 30 h. These results can be interpreted as a change in the tellurium distribution in the polycrystalline samples. The absence of an IF peak ~Fig. 3! in the case of a short treatment time attests to the presence of tellurium bonds characteristic of SnTe ~melting point 805 °C!. As the annealing time increases, the tellurium diffuses to the grain surfaces, where it precipitates in the form of a second phase. Figure 3 also shows the temperature dependence of the IF for a sample annealed for 30 h. It should be noted that in a number of samples the maximum of the peak was displaced to lower temperatures. In summary, the IF-based method can be used to analyze the kinetics of microsegregations and precipitates in polycrystalline samples. Compared with the electron-probe method, this method can be used to determine submicron precipitates in the entire polycrystalline layer formed. G. Harbeke @Ed.#, Polycrystalline Semiconduction. Physical Properties and Application. Proc. Int. School of Materials Sci. and Technol. at the Etore Majorana Centre, Erice, Italy, July 1–15, 1984, Springer Verlag, N. Y., 1985 @Russian transl., Mir, Moscow,1989#. 2 H. J. Grost and F. Spaepen, J. Physique 43, 73 ~1982!. 3 V. Vitek, A. P. Sutton, D. A. Smith, and R. C. Pond, Grain Boundary Structure and Kinetics, A. S. M., Metal Park, 1980. 4 R. W. Balutti, P. D. Bristowe, and C. P. Sun, J. Amer. Soc. 64, 29 ~1981!. 5 V. Pontikis, J. Physique 43, 65 ~1982!. 6 C. L. Briant and R. D. Mesmer, J. Physique 43, 255 ~1982!. 7 R. S. Morrison, Sensor and Actuators 12, 425 ~1987!. 8 L. Horer, Polprgzewodnikowe materialy ceramiczhe z aktywnymi granicami zranicami ziaru, Warszawa, PWN, 1990. 9 L. C. Feldman and J. W. Mayer, Fundamentals of Surface and Thin Film Analysis, North Holland, N. Y., 1986 @Russian transl., Mir, Moscow, 1989#. 10 M. V. Bestaev, D. Tr. Dimitrov, V. A. Moshnikov, and Yu. M. Tairov, in SnO2 Based Gas Sensitive Sensor. Abstracts E-MRS 1996 Spring Meeting, Strasburg, France, June 4–7, 1996, B-1. 11 R. Assenov, V. A. Moshnikov, and D. A. Yaskov, Cryst. Res. Technol. 21, 1553 ~1986!. 12 N. I. Dolotov, A. B. Zil’berman, Yu. A. Il’in, A. V. Mokhin, V. A. Moshnikov, and D. A. Yas’kov, Neorg. Mater. 30, No. 1, 83 ~1994!. 13 V. I. Mitrokhin, N. P. Yaroslavtsev, S. I. Rebeza, G. S. Pesotski, and N. V. Izmalov, Inventor’s Certificate, 105/42 SSSR G01N11/16 ~1985!. 14 N. V. Izmalov, Yu. L, Il’in, V. A. Moshnikov, V. V. Tomaev, N. P. Yaroslavtsev, and D. A. Yas’kov, Zh. Fiz. Khim. 12, 1370 ~1988!. 15 B. M. Darinski and N. P. Yaroslavtsev, Vysokochistye Veshchestva, No. 3, 80 ~1990!. 1
Translated by M. E. Alferieff
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