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Crystal growth and the influence of structural defects on the physical properties of CdCr2S4
Journal of Crystal Growth 49 (1980) 67—70 © North-Holland Publishing Company
CRYSTAL GROWTH AND THE INFLUENCE OF STRUCTURAL DEFECTS ON THE PHYSICAL PROPERTIES OF CdCr2S4 S.!. RADAUTSAN, V.E. TEZLEVAN and K.G. NIKIFOROV Institute ofApplied Physics, Academy of Sciences of the Moldavian SSR, Kishiney, USSR Received 26 February 1979;manuscript received in final form 5 October 1979
The crystal growth and the effect of doping and nonstoichiometry defects on the physical properties of the magnetic semiconductor CdCr2S4 are described. It has been shown that these properties may be controlled both by doping during growth or by thermal treatment of the grown crystals.
1. Introduction
cal properties of CdCr2S4 single crystals. These crystals, pure and with the Cd substitutions for monovalent (Cu, Ag) and trivalent (Ga, Gd) metallic atoms (Me), were grown by chemical vapour transport with temperature gradient reversal [10]. The starting material was polycrystalline powder Me~Cdi_~Cr2S4 (where x = 0 ÷0.05) synthesised previously from Cr, S, CdS and Me. The transport agent was chlorine from CrCl3. The dimensions of the reaction ampoule, the CrC13-concentration, and the temperatures of source-zone T1 and growth-zone T2 were changed in order to obtain the most advantageous crystal growth conditions. The optimum technology was: T 1= 1000°C,3,T~ process = 900°C, duration transport 240 h. agent The concentration transport rate 3 mg/cm was about 5 X i0~’g mol/day. CdCr 2S4 single crystals with smooth facets up to 5 mm in size were grown (fig. 1). All crystals were single phase. To change the structural defect concentration some samples were thermally treated in vacuum i0~ Torr or in sulphur saturated vapour at 550°Cfor about 150 h. The ohmic contacts for electrical measurements were obtained by indium soldering in vacuum l0~ Torr at 300—350°C. The temperature dependences of the electrical conductivity for pure (as-grown and vacuumannealed) and Gd-doped CdCr2S4 single crystals are shown in fig. 2. All crystals had n-type conductivity. Their activation energies near room temperatures were close to
A new group 2Cr of ternary semiconductors magnetic spinels A 2X~ has intensively been studied in recent years. The presence of both interconnected ferromagnetic ordering and semiconductor properties in these materials results in a number of specific effects [1—4]. A rather strong dependence of the characteristics on2B6 thecompounds stoichiometric previously noticed in [5] deviations has been observed in chromium A chalcogenide spinels. This dependence may also be —
—
caused by the valency change of chromium atoms [6].The dependence of the physical properties of the mentioned compounds on their growth conditions, thermal treatment, impurity and structural defects have not yet been studied enough. There is only fragmentary information about the electrical properties of donor doped crystals [7,8] and the nonstoichiometry influence on the paramagnetic Curie temperature and the Curie constant for CdCr2S4 [9].
2. Experimental results In this paper we present the results of an investigation of the effect of structural defects on the electri67
68
SI. Radautsan eta!. /Influence of structural defects on CdCr
2S4
~
1
Oh ~
c
.0 -6
6 -0.5 2.
—B Fig. 1. CdCr2S4 single crystals grown by chemical vapour transport.
0.6 eV. In the range T— l50~200 K they decreased, the samples with lower resistivity possessing lower activation energy. The Ga-doped CdCr2S4 crystals exhibit quite different electrical properties (fig. 3). The temperature minimums of electrical conductivity characteristic of magnetic semiconductors were observed here. The position and the value of these minimums depend greatly on Ga-concentration. They are absent in crys-
\~
e~ -6 ~
1 cm
üiim
—8
-10
3/T,K 5 $0 10 Fig. 2. Temperature dependences of conductivity for CdCr 2S4 crystals: curve 1, as-grown; curve 2, vacuumannealed; curve 3, 5 mol% Gd-doped. -i~
~ Fig. 3. Temperature dependences of conductivity for Gadoped CdCr2S4 crystals: curves 1, 2 and 3, with 1, 2 and 3 mol% respectively; curves 4 and 5, with 3 mol% sulphurannealed and vacuum-annealed respectively. Curve 6, the temperature dependence Ga : CdCr2S4(at 5 kOe). of magnetorcsistivity for 2 mol%
tals with low Ga-concentration (up to 1 mol%), they are the highest at about 2 mol% of Ga and become relaxed with increasing Ga content [11]. M increase in conductivity and a reduction of the temperature minimum are observed by vacuumannealing of 3 mol% Ga-doped crystals. The sulphurannealing of such samples gives the opposite effect. The resistivity and activation energy abruptly decrease with the Ga-concentration increase in CdCr 2S4 crystals [11] which is characteristic of impurity conduction. The application of an external magnetic field decreases the resistivity of such crystals. This reduction is particularly strong near the Curie temperature (approximately ten times at S kOe, fig. 3). The temperature dependences of the conductivity for CdCr2S4 crystals doped with acceptor (Cu, Ag) impurities are shown in fig. 4. Doping with 1 mol% Ag leads to an (1.14 increase the resistivity and the vation energy eV),ofthese values being closeactito the parameters of CdCr2S4 crystals [12]. The further
undoped sulphur-annealed increase of Ag-concentra-
SI. Radautsan et a!. / Influence of structural defects on CdCr
2S4
a
grown crystals. The activation energy 0.6 eV (fig. 2) may be accounted for by 2tions electrontotransitions from the conduction the narrow 3d-band of Cr band. With decreasing temperature the transition
Oh,n~’cm~~
-5~
—10
5
69
10
Fig. 4. Temperature dependences of conductivity for acceptor doped CdCr 2S4 crystals: curves 1, 3 and 4, with 1, 2.5 and 5 mol% Ag respectively; curve 2, with I mol% Ag sulphur-annealed; curve 5, with 3 mol% Cu. Curve 6, undoped sulphur-annealed crystals.
form of Cr2+ ~÷Cr3~ transitions may occur. The from decrease bandofconduction the activation to impurity energy with conduction Cr2~ioninconthe centration increase supports the above statement. In 1 mol% Ag-doped crystals the compensation of free charge carriers of donor Cr2~-levelsand acceptor Ag-levels probably occur. The analogous electrical properties of such samples and of the undoped sulpher-annealed crystals demonstrated that the anion
tion results in hole conductivity and a reduction of resistivity and activation energy. The sulphur-annealing of the samples with 1 mol% Ag has the same effect.
vacancy concentration is close to I mol% in CdCr2S4. Further Ag-concentration increase or sulphur-annealing of the samples with I mol% Ag leads to the predominance of hole conductivity. The nature of the temperature minima of conductivity in Ga-doped CdCr2S4 crystals (fig. 3) is in progress. They may be associated with the peculiarities of s—d exchange interaction in high-doped magnetic semiconductors [4,17].
3. Discussion
4. Conclusion
Taking into account both the results obtained by the Japanese authors and by ourselves [10,13] we consider that sulphur vacancies are the dominant structural defects in CdCr2S4 [14]. As was shown in the papers [9,15] chalcogen deficiency in chromium chalcogenide spinels leads to the valency change of 2t some Itchromium and tobythethe appearance ions. may be ions explained adherenceoftoCrthe electrical neutrality conditions. Vacuum-annealing increases the S-vacancy concentration and probably results in an increase in the Cr2~concentration. Doping by trivalent Cd substituting for Gd might cause the appearance of new Cr2t
CdCr2S4 single crystals, pure and doped with Cu, Ag, Ga, Gd, were grown by chemical vapour transport. The electro- and magnetoresistivity of these crystals have been investigated. Our results suggest that: (a) chromium in CdCr2S4 reveals a variable (b) the doping or the variation of the valency; structural defect concentration changes the quantity of the divalent chromium ions: (c) the physical properties of these crystals are defined as impurities and Cr2tions. It has been shown that the physical properties of CdCr 2S4 may be controlled both by doping during growth or by thermal treatment of the grown crystals.
ions or might result in “healing” of some S-vacancies as in the case of CdS doped with elements of the third group [6]. Taking into consideration both the analogous influence of vacuum-annealing and Gd-doping on conductivity of CdCr2S4 crystals and the data of magnetic anisotropy and ferromagnetic resonance in these crystals [16] we interpret results terms 2~ionsour which areinresponof thefor appearance of new Cr sible the conductivity of both these and pure as-
References [1] S. PJ.Methfessel Wojtowicz,and IEEE Trans. Magn. Mag-5der (1969) 840. [21 D. Mattis, Handbuck Physik 18/1 (1968).
70
SI. Radautsan eta!. /Influence of structural defects on CdCr
[3] V.G. Veselago, Colloq. Intern. CNRS No. 242 (1975) 295. [4] E.L. Nagaev, Fizika Magnitnyh Poluprovodnikov (Moscow, 1979). [5] N.A. Goryunova, Slojnye Almazopodobnye Poluprovodniki (Moscow, 1968). [6] N.B. Hannay, Solid State Chemistry (New Jersey, 1971). [7] M. Toda, in: Proc. 3rd Conf. on Solid State Devices, Tokyo, 1971 (Tokyo, 1972) p. 183. [8] P. Larsen, in: Proc. Intern Conf. on Magnetism, Moscow, 1973, Vol. 5 (Moscow, 1973) p. 484. [9] K. Masumoto, T. Kiyosawa and I. Nakatani, J. Phys. Chem. Solids 34(1973) 569. [10] A.S. Averyanov, R. Yu. Lyalikova, K.G. Nikiforov and A.V. Suranov, in: Tezisy Dokladov Vses. Konf. Troynye
2S4
Poluprovodniki i ih Primenenie, Kishiriev, 1976 (Kishinev, 1976) p. 115. [111 K.G. Nikiforov, SI. Radautsan and V.E. Tezlevan, DokI. Akad. Nauk SSSR 239 (1978) 77. [121 S.I. Radautsan, KG. Nikiforov and yE. Tezlevan, lzv. Akad. Nauk SSSR, Neorg. Mater. 14 (1978) 165. [13] K. Ametani, Bull. Chem. Soc. Japan 49 (1976) 450. [14] V.E. Tezlevan, S.I. Radautsan, K.G. Nikiforov, Fiz. i Tekhn. Poluprov. 12(1978) 824. [15] H. Pinch, S. Berger, J. Phys. Chem. Solids 29 (1968) 2091. [16] K.G. Nikiforov, A.G. Gurevich, S.l. Radautsan, yE. Tezlevan and L.M. Emiryan, Fiz. Tverd. Tela 20(1978) 1896. [17) E.L. Nagaev and A.P. Grigin, Phys. Status Solidi B65 (1974) 457.
CRYSTAL GROWTH AND THE INFLUENCE OF STRUCTURAL DEFECTS ON THE PHYSICAL PROPERTIES OF CdCr2S4 S.!. RADAUTSAN, V.E. TEZLEVAN and K.G. NIKIFOROV Institute ofApplied Physics, Academy of Sciences of the Moldavian SSR, Kishiney, USSR Received 26 February 1979;manuscript received in final form 5 October 1979
The crystal growth and the effect of doping and nonstoichiometry defects on the physical properties of the magnetic semiconductor CdCr2S4 are described. It has been shown that these properties may be controlled both by doping during growth or by thermal treatment of the grown crystals.
1. Introduction
cal properties of CdCr2S4 single crystals. These crystals, pure and with the Cd substitutions for monovalent (Cu, Ag) and trivalent (Ga, Gd) metallic atoms (Me), were grown by chemical vapour transport with temperature gradient reversal [10]. The starting material was polycrystalline powder Me~Cdi_~Cr2S4 (where x = 0 ÷0.05) synthesised previously from Cr, S, CdS and Me. The transport agent was chlorine from CrCl3. The dimensions of the reaction ampoule, the CrC13-concentration, and the temperatures of source-zone T1 and growth-zone T2 were changed in order to obtain the most advantageous crystal growth conditions. The optimum technology was: T 1= 1000°C,3,T~ process = 900°C, duration transport 240 h. agent The concentration transport rate 3 mg/cm was about 5 X i0~’g mol/day. CdCr 2S4 single crystals with smooth facets up to 5 mm in size were grown (fig. 1). All crystals were single phase. To change the structural defect concentration some samples were thermally treated in vacuum i0~ Torr or in sulphur saturated vapour at 550°Cfor about 150 h. The ohmic contacts for electrical measurements were obtained by indium soldering in vacuum l0~ Torr at 300—350°C. The temperature dependences of the electrical conductivity for pure (as-grown and vacuumannealed) and Gd-doped CdCr2S4 single crystals are shown in fig. 2. All crystals had n-type conductivity. Their activation energies near room temperatures were close to
A new group 2Cr of ternary semiconductors magnetic spinels A 2X~ has intensively been studied in recent years. The presence of both interconnected ferromagnetic ordering and semiconductor properties in these materials results in a number of specific effects [1—4]. A rather strong dependence of the characteristics on2B6 thecompounds stoichiometric previously noticed in [5] deviations has been observed in chromium A chalcogenide spinels. This dependence may also be —
—
caused by the valency change of chromium atoms [6].The dependence of the physical properties of the mentioned compounds on their growth conditions, thermal treatment, impurity and structural defects have not yet been studied enough. There is only fragmentary information about the electrical properties of donor doped crystals [7,8] and the nonstoichiometry influence on the paramagnetic Curie temperature and the Curie constant for CdCr2S4 [9].
2. Experimental results In this paper we present the results of an investigation of the effect of structural defects on the electri67
68
SI. Radautsan eta!. /Influence of structural defects on CdCr
2S4
~
1
Oh ~
c
.0 -6
6 -0.5 2.
—B Fig. 1. CdCr2S4 single crystals grown by chemical vapour transport.
0.6 eV. In the range T— l50~200 K they decreased, the samples with lower resistivity possessing lower activation energy. The Ga-doped CdCr2S4 crystals exhibit quite different electrical properties (fig. 3). The temperature minimums of electrical conductivity characteristic of magnetic semiconductors were observed here. The position and the value of these minimums depend greatly on Ga-concentration. They are absent in crys-
\~
e~ -6 ~
1 cm
üiim
—8
-10
3/T,K 5 $0 10 Fig. 2. Temperature dependences of conductivity for CdCr 2S4 crystals: curve 1, as-grown; curve 2, vacuumannealed; curve 3, 5 mol% Gd-doped. -i~
~ Fig. 3. Temperature dependences of conductivity for Gadoped CdCr2S4 crystals: curves 1, 2 and 3, with 1, 2 and 3 mol% respectively; curves 4 and 5, with 3 mol% sulphurannealed and vacuum-annealed respectively. Curve 6, the temperature dependence Ga : CdCr2S4(at 5 kOe). of magnetorcsistivity for 2 mol%
tals with low Ga-concentration (up to 1 mol%), they are the highest at about 2 mol% of Ga and become relaxed with increasing Ga content [11]. M increase in conductivity and a reduction of the temperature minimum are observed by vacuumannealing of 3 mol% Ga-doped crystals. The sulphurannealing of such samples gives the opposite effect. The resistivity and activation energy abruptly decrease with the Ga-concentration increase in CdCr 2S4 crystals [11] which is characteristic of impurity conduction. The application of an external magnetic field decreases the resistivity of such crystals. This reduction is particularly strong near the Curie temperature (approximately ten times at S kOe, fig. 3). The temperature dependences of the conductivity for CdCr2S4 crystals doped with acceptor (Cu, Ag) impurities are shown in fig. 4. Doping with 1 mol% Ag leads to an (1.14 increase the resistivity and the vation energy eV),ofthese values being closeactito the parameters of CdCr2S4 crystals [12]. The further
undoped sulphur-annealed increase of Ag-concentra-
SI. Radautsan et a!. / Influence of structural defects on CdCr
2S4
a
grown crystals. The activation energy 0.6 eV (fig. 2) may be accounted for by 2tions electrontotransitions from the conduction the narrow 3d-band of Cr band. With decreasing temperature the transition
Oh,n~’cm~~
-5~
—10
5
69
10
Fig. 4. Temperature dependences of conductivity for acceptor doped CdCr 2S4 crystals: curves 1, 3 and 4, with 1, 2.5 and 5 mol% Ag respectively; curve 2, with I mol% Ag sulphur-annealed; curve 5, with 3 mol% Cu. Curve 6, undoped sulphur-annealed crystals.
form of Cr2+ ~÷Cr3~ transitions may occur. The from decrease bandofconduction the activation to impurity energy with conduction Cr2~ioninconthe centration increase supports the above statement. In 1 mol% Ag-doped crystals the compensation of free charge carriers of donor Cr2~-levelsand acceptor Ag-levels probably occur. The analogous electrical properties of such samples and of the undoped sulpher-annealed crystals demonstrated that the anion
tion results in hole conductivity and a reduction of resistivity and activation energy. The sulphur-annealing of the samples with 1 mol% Ag has the same effect.
vacancy concentration is close to I mol% in CdCr2S4. Further Ag-concentration increase or sulphur-annealing of the samples with I mol% Ag leads to the predominance of hole conductivity. The nature of the temperature minima of conductivity in Ga-doped CdCr2S4 crystals (fig. 3) is in progress. They may be associated with the peculiarities of s—d exchange interaction in high-doped magnetic semiconductors [4,17].
3. Discussion
4. Conclusion
Taking into account both the results obtained by the Japanese authors and by ourselves [10,13] we consider that sulphur vacancies are the dominant structural defects in CdCr2S4 [14]. As was shown in the papers [9,15] chalcogen deficiency in chromium chalcogenide spinels leads to the valency change of 2t some Itchromium and tobythethe appearance ions. may be ions explained adherenceoftoCrthe electrical neutrality conditions. Vacuum-annealing increases the S-vacancy concentration and probably results in an increase in the Cr2~concentration. Doping by trivalent Cd substituting for Gd might cause the appearance of new Cr2t
CdCr2S4 single crystals, pure and doped with Cu, Ag, Ga, Gd, were grown by chemical vapour transport. The electro- and magnetoresistivity of these crystals have been investigated. Our results suggest that: (a) chromium in CdCr2S4 reveals a variable (b) the doping or the variation of the valency; structural defect concentration changes the quantity of the divalent chromium ions: (c) the physical properties of these crystals are defined as impurities and Cr2tions. It has been shown that the physical properties of CdCr 2S4 may be controlled both by doping during growth or by thermal treatment of the grown crystals.
ions or might result in “healing” of some S-vacancies as in the case of CdS doped with elements of the third group [6]. Taking into consideration both the analogous influence of vacuum-annealing and Gd-doping on conductivity of CdCr2S4 crystals and the data of magnetic anisotropy and ferromagnetic resonance in these crystals [16] we interpret results terms 2~ionsour which areinresponof thefor appearance of new Cr sible the conductivity of both these and pure as-
References [1] S. PJ.Methfessel Wojtowicz,and IEEE Trans. Magn. Mag-5der (1969) 840. [21 D. Mattis, Handbuck Physik 18/1 (1968).
70
SI. Radautsan eta!. /Influence of structural defects on CdCr
[3] V.G. Veselago, Colloq. Intern. CNRS No. 242 (1975) 295. [4] E.L. Nagaev, Fizika Magnitnyh Poluprovodnikov (Moscow, 1979). [5] N.A. Goryunova, Slojnye Almazopodobnye Poluprovodniki (Moscow, 1968). [6] N.B. Hannay, Solid State Chemistry (New Jersey, 1971). [7] M. Toda, in: Proc. 3rd Conf. on Solid State Devices, Tokyo, 1971 (Tokyo, 1972) p. 183. [8] P. Larsen, in: Proc. Intern Conf. on Magnetism, Moscow, 1973, Vol. 5 (Moscow, 1973) p. 484. [9] K. Masumoto, T. Kiyosawa and I. Nakatani, J. Phys. Chem. Solids 34(1973) 569. [10] A.S. Averyanov, R. Yu. Lyalikova, K.G. Nikiforov and A.V. Suranov, in: Tezisy Dokladov Vses. Konf. Troynye
2S4
Poluprovodniki i ih Primenenie, Kishiriev, 1976 (Kishinev, 1976) p. 115. [111 K.G. Nikiforov, SI. Radautsan and V.E. Tezlevan, DokI. Akad. Nauk SSSR 239 (1978) 77. [121 S.I. Radautsan, KG. Nikiforov and yE. Tezlevan, lzv. Akad. Nauk SSSR, Neorg. Mater. 14 (1978) 165. [13] K. Ametani, Bull. Chem. Soc. Japan 49 (1976) 450. [14] V.E. Tezlevan, S.I. Radautsan, K.G. Nikiforov, Fiz. i Tekhn. Poluprov. 12(1978) 824. [15] H. Pinch, S. Berger, J. Phys. Chem. Solids 29 (1968) 2091. [16] K.G. Nikiforov, A.G. Gurevich, S.l. Radautsan, yE. Tezlevan and L.M. Emiryan, Fiz. Tverd. Tela 20(1978) 1896. [17) E.L. Nagaev and A.P. Grigin, Phys. Status Solidi B65 (1974) 457.