ISSN 1063�7826, Semiconductors, 2010, Vol. 44, No. 7, pp. 946–948. © Pleiades Publishing, Ltd., 2010.Original Russian Text © A.P. Belyaev, V.P. Rubets, V.V. Antipov, E.O. Eremina, 2010, published in Fizika i Tekhnika Poluprovodnikov, 2010, Vol. 44, No. 7, pp. 978–980.

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A high crystalline quality of films synthesizedunder highly nonequilibrium conditions by themethod of a thermal screen has been previouslyreported [1]; this method makes it possible to use amixture with one composition to obtain (with highmanufacturability) practically the entire range of solidsolutions. The composition of the solution in thismethod of synthesis was varied by a simple change inthe temperature of the thermal�screen zone, i.e., theregion of elevated temperature located between theevaporator and substrate. In what follows, we reportthe results of studies of the influence of the thermalscreen on the properties of binary compounds, i.e.,CdTe films the properties of which, in the case of syn�thesis by the method of a quasi�closed volume, werereported in [2].

We studied the films obtained by evaporation andcondensation in vacuum in highly nonequilibriumconditions. The films were synthesized by the ther�mal�screen method using the technique described in[1], while the method of a quasi�closed volume forsynthesis was used as described in [3]. In both cases,the temperature of the evaporator was Ti = 923 K. Thetemperature of the thermal screen was 100 K higherthan the temperature of the evaporator. Pieces of arti�ficial fluorophlogopite mica were used as substrates.The latter were cooled with liquid nitrogen (highlynonequilibrium conditions). A specific temperature ofthe substrate was chosen in such a way as to ensure thehighest crystalline quality of the films. According toelectron diffraction studies, the films synthesized bythe method of a thermal screen were not different inany way from the films obtained by the method of aquasi�closed volume. The structure of the films canbe assessed from Figs. 1 and 2, where typical elec�tron�diffraction patterns of the films and micropho�

tographs of the surface are shown. The films weregrown in cubic modification with the orientation

(111)[1 0] CdTe|| (0001)[11 0] of mica. The filmswere nominally undoped and were not annealed. Thethickness of the studied films was several micrometers.

Electrical and galvanomagnetic measurementswere carried out in planar configuration. Gold con�tacts deposited by the method of vacuum evaporationwere used in measurements. The distance betweencurrent contacts was 0.6 cm and it was 0.3 cm for theHall contacts. The conductivity was measured using aV7�30 electrometer in the current mode. The lowestmeasured current was 10–14 A. The Hall effect was

1 2

FABRICATION, TREATMENT, AND TESTING OF MATERIALS AND STRUCTURES

Influence of the Method of Synthesis on Properties of Cadmium Telluride Films Synthesized in Highly Nonequilibrium Conditions

A. P. Belyaev^, V. P. Rubets, V. V. Antipov, and E. O. EreminaSt. Petersburg State Technological Institute (Technical University), St. Petersburg, 190013 Russia

^e�mail: Belyaev@lti�gti.ru; Belyaev@tu.spb.ru; Belyaev@spcpa.ruSubmitted October 26, 2009; accepted for publication November 2, 2009

Abstract—The results of comparative studies of electrical and galvanomagnetic properties of cadmium tellu�ride films are reported. The films were synthesized under highly nonequilibrium conditions by the methodsof a thermal screen and a quasi�closed volume. The temperature dependences of the conductivity, Hall coef�ficient, and effective Hall mobility are reported. As a result of experimental data, it is concluded that themethod of a thermal screen favorably affects the stoichiometry of the composition and, thus, is conducive toobtaining films with a lower concentration of defects.

DOI: 10.1134/S1063782610070195

Fig. 1. A typical electron�diffraction pattern for a CdTefilm synthesized in highly nonequilibrium conditions.

SEMICONDUCTORS Vol. 44 No. 7 2010

INFLUENCE OF THE METHOD OF SYNTHESIS 947

studied using an alternating current with the frequency8 Hz, which made it possible to detect the lowest Hallmobility of 0.5 cm2 V–1 s–1. The conductivity type wasdetermined from the emf sign. All measurements werecarried out in vacuum at a residual pressure of ~10–3 Pa.The temperature (T) was kept constant to within 0.1 Kusing a thermocontroller.

The main results of the studies are shown inFigs. 3–5. In Fig. 3, we show the temperature depen�dences of the conductivity σ. Curve 1 represents thetemperature dependence for the sample obtained bythe method of a thermal screen. As can be seen, thiscurve corresponds to a simple exponential functionwith the activation energy E

σ = 0.73 eV. Curve 2 cor�

responds to the temperature dependence of σ for afilm synthesized by the method of a quasi�closed vol�ume. A more complicated shape is characteristic ofthis curve: there are two exponential portions, a high�temperature portion with the activation energy E

σ1 =0.54 eV and a low�temperature portion with the acti�vation energy E

σ2 = 0.34 eV. The resistivity of the sam�ples obtained by the method of a thermal screen washigher by an order of magnitude in the entire temper�ature range than the resistivity of the samples synthe�sized by the method of a quasi�closed volume.

In Fig. 4, we show temperature dependences of theHall coefficient RH. Curve 1 corresponds to the sampleobtained by the method of a thermal screen; this curve

1 μm

Fig. 2. A microphotograph of the surface of a CdTe filmsynthesized in highly nonequilibrium conditions. 2.0 2.5

σ, Ω−1 cm−1

103/T, K−1

10−5

4.0

10−6

10−7

10−8

10−9

10−10

10−11

10−12

3.0 3.5 4.5 5.0 5.5 6.0 6.5

2

1

Fig. 3. Temperature dependences of conductivity of CdTefilms synthesized by the methods of (1) a thermal screenand (2) a quasi�closed volume.

2.4

RH, cm3/C

103/T, K−12.8

109

2.5 2.6 2.9 3.0 3.1 3.2 3.3

2

1

2.7

107

108

106

105

Fig. 4. Temperature dependences of the Hall coefficientfor CdTe films synthesized by the methods of (1) a thermalscreen and (2) a quasi�closed volume.

2.4

μH, cm2/(V s)

103/T, K−12.82.6 3.0 3.2 3.4

2

1102

100

101

Fig. 5. Temperature dependences of the effective Hallmobility in the CdTe films synthesized by the methods of(1) a thermal screen and (2) a quasi�closed volume.

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SEMICONDUCTORS Vol. 44 No. 7 2010

BELYAEV et al.

is nearly exponential. Curve 2 represents the tempera�ture dependence of RH for a sample synthesized by themethod of a quasi�closed volume; the shape of thiscurve corresponds to the so�called “two�band model”[4]; i.e., it includes a maximum. The values of RH werealways positive and were always larger for the samplessynthesized by the thermal�screen method.

In Fig. 5, we show temperature dependences of theeffective Hall mobility μH. At high temperatures, thevalue of μH in both samples was almost independent oftemperature; in contrast, at low temperatures, μHdecreased drastically as temperature was decreased.The values of μH for the samples synthesized by themethod of a thermal screen did not exceed severalhundreds of cm2 V–1 s–1, whereas these values for thesamples obtained by the method of quasi�closed vol�ume were no larger than several tens of cm2 V–1 s–1.

The electrical properties of CdTe films synthesizedin highly nonequilibrium conditions by the method ofa quasi�closed volume were discussed in [2]. It wasshown that the object of the study represents a highlycompensated semiconductor the bands of whichinclude a random potential. As a result, charge�trans�port processes are realized over the percolation level:at high temperatures, over the percolation level of thevalence band and, at low temperatures, over the perco�lation level of the impurity band. The electrical con�ductivity of the material under consideration is activa�tional, and the role of activation energy is played by theenergy separating the Fermi level from the percolationlevel. Pronounced compensation accounts for a largevalue of the activation energy for conductivity (theFermi level is located deep in the band gap) and a smallvalue of effective Hall mobility (a high concentrationof defects). The presence of charge transport overimpurities accounts for the shape of temperaturedependence of the Hall coefficient (the shape charac�teristic of the two�band model) and presence of twoexponential portions in the temperature dependenceof conductivity (one portion corresponds to chargetransport over the valence band, while the second por�tion is related to charge transport over the impurityband).

Properties of the samples synthesized by themethod of a thermal screen are qualitatively similar tothose described previously by Belyaev et al. [2]. Inorder to verify that this is true, it is sufficient to com�pare curves 1 and 2 in Figs. 3–5. The difference con�sists only in the values of conductivity and mobility.Consequently, we may assume that the mechanisms ofcharge transport in both systems also have a similarcharacter; however, then, larger values of mobility ofcharge carriers in the films synthesized by the thermal�screen method are indicative of a lower concentrationof defects in these films. This conclusion is also sup�ported by a higher activation energy for conductivity,since the Fermi level approaches the midgap and theenergy separating it from the percolation level

increases. The lower concentration of defects is alsoresponsible for the absence of the second exponentialportion in the curve representing the temperaturedependence of conductivity (see curve 1 in Fig. 3). Ata low concentration of defects, the impurity bandeither does not appear at all or features a very low con�ductivity; as a result, the contribution of this band tothe total conductance of the film will be significantonly at very low temperatures where the main bandsare found to be almost completely empty.

The defects affecting the mobility of charge carriersin the films under consideration are apparently pointdefects. Otherwise, they should have manifestedthemselves in electron diffraction studies, so that theelectron�diffraction patterns for films with charge car�riers with different mobilities would be different.Dominant point defects in p�CdTe are cadmiumvacancies [5], while, in the case of synthesis from avapor phase in highly nonequilibrium conditions,these defects are related to an excess of tellurium [6].It then follows that the thermal screen is conducive toformation of a stoichiometric vapor phase. The hotwalls of the thermal screen increase the dynamic pres�sure in the system and, thus, make the evaporation ofcadmium telluride closer to the congruent one.

Thus, the results reported in this paper make it pos�sible to state that the method of a thermal screen usedin synthesis of CdTe films in highly nonequilibriumconditions favorably affects the stoichiometry of com�position and, thus, is conducive to obtaining films witha lower concentration of defects.

ACKNOWLEDGMENTS

This study was supported by the Russian Founda�tion for Basic Research, project no. 07�03�00366.

REFERENCES

1. A. P. Belyaev, V. P. Rubets, V. V. Antipov, andKh. A. Toshkhodzhaev, Fiz. Tekh. Poluprovodn. 43,735 (2009) [Semiconductors 43, 706 (2009)].

2. A. P. Belyaev, V. P. Rubets, V. V. Antipov, andV. V. Grishin, Fiz. Tekh. Poluprovodn. 42, 1309 (2008)[Semiconductors 42, 1282 (2008)].

3. A. P. Belyaev, V. P. Rubets, and I. P. Kalinkin, Fiz.Tverd. Tela 39, 382 (1997) [Phys. Solid State 39, 333(1997)].

4. B. I. Shklovskii and F. L. Efros, Electronic Properties ofDoped Semiconductors (Nauka, Moscow, 1979; Springer,New York, 1984).

5. Physics of II–VI Compounds, Ed. by A. N. Georgobianiand M. K. Sheinkman (Nauka, Moscow, 1986) [inRussian].

6. A. P. Belyaev, V. P. Rubets, M. Yu. Nuzhdin, andI. P. Kalinkin, Fiz. Tekh. Poluprovodn. 37, 641 (2003)[Semiconductors 37, 617 (2003)].

Translated by A. Spitsyn