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Preparation and properties of FeSi, α-FeSi2 and β-FeSi2 single crystals
Journal of
~OYS ANDC,OMPOL~DS ELSEVIER
Journal of Alloys and Compounds 219 (1995) 93-96
Preparation and properties of FeSi, a-FeSi2 and /3-FeSi2 single crystals Ch. Kloc, E. Arushanov *, M. Wendl, H. Hohl, U. Malang, E. Bucher Faculty of Physics, University of Konstanz, P.O. Box 5560, D-78434 Konstanz, Germany
Abstract
FeSi and a-FeSi2 single crystals were prepared by the Czochralski technique, fl-FeSi2 and a-FeSi2 single crystals were prepared by chemical vapour transport. FeSi single crystals 18 cm3 in size and a-FeSi2 single crystals of a few cubic centimetres were obtained, fl-FeSi2 decomposes peritectically above 970 °C, therefore only needle-like crystals, each about 10 ram long, were grown from the vapour. The temperature dependence of the resistivity was measured from 4.2 K up to room temperature. a-FeSi2 shows a weak temperature dependence of resistivity. The activation energy of FeSi, calculated from In p=f(1/T) is equal to 58.2 meV. the Hall coefficient and resistivity of/3-FeSi2 single crystals was measured between 30 and 300 K. The as-grown, not intentionally doped /3-FeSi2 crystals exhibit n-type conductivity, while Cr and AI doped crystals show p-type conductivity. The Hall coefficient of n-type samples depends on the magnetic field; therefore, the transport properties of/3FeSi2 were explained taking into account two types of carrier, heavy and light. At low temperature (32 K) heavy electrons show a mobility of /zn = 48 cm2 V-1 s-1. An impurity band and an additional deep acceptor level are observed in p-type crystals. The activation energies of the shallow acceptor level and deep acceptor level are equal to 55 meV and 100 meV respectively. The mobility of holes /Zp in p-type crystals reaches a maximum of 1200 cm2 V -1 s -1 at 67 K. Magnetization and magnetic susceptibility measurements in the temperature range 4-320 K show a small positive value of susceptibility and nonlinear magnetization for /3-FeSi2 but no ferromagnetic phase was detected, a-FeSi2 crystals show linear magnetization and susceptibility similar to /3-FeSi2. Magnetization of FeSi is linear but the susceptibility passes through a minimum at about 150-200 K. Keywords: Single crystals; Chemical vapour transport
1. Introduction
The iron-silicon phase diagram has been the subject of controversial considerations for a long time [1,2]. According to a generally accepted, summarized study [2], iron and silicon form four compounds, FezSi, FesSi3, FeSi, ot-FeSi2 and/3-FeSi2. A m o n g them, only FeSi and the low t e m p e r a t u r e modification of FeSi2, the/3-phase, are thermodynamically stable at room temperature. Orthorhombic/3-FeSi2 shows a direct band gas of 0.87 eV [3] and the possibility of p- and n-type doping [4,5]. Cubic FeSi seems to be a narrow band semiconductor [6], or magnetic semiconductor, which exhibits band ferromagnetism [7]. Tetragonal a-FeSi2, though thermodynamically metastable at ambient temperature, does * Permanent address: Institute of Applied Physics, Academy of Sciences of Moldova, Kishinev, Moldova.
0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)05055-4
not easily undergo a phase transition to the low temperature phase /3-FeSi2. a-FeSi2 is metallic [8]. All iron silicides show high resistance to oxidation, low vapour pressure, lack of toxicity and wide occurrence of constituent elements in the Earth. Such an alliance of desired properties makes the silicides attractive for semiconductor technology [9]. For all that, the properties of iron silicides are still not understood well. Many measurements have been taken on sintered samples or on polycrystalline thin films. These data are difficult to interpret because the effects of the surface, grain structure, or defects on the properties are difficult to evaluate. In recent years, single crystals of FeSi and a-FeSi2 have been used for magnetic and transport measurements [8,10] but the preparation procedures have not been reported./3-FeSi2 crystals were p r e p a r e d by chemical vapour transport and used for structure deter-
94
Ch. Kloc et aL / Journal of Alloys and Compounds 219 (1995) 93-96
the apparatus was evacuated to approximately 10 -5 mbar, back-filled with 99.999% pure argon, and again evacuated. Growth runs were carried out in a dynamic vacuum of 10 -5 mbar. Iron (99.998% pure Alfa or at least 99.9% purity remelted powder from Merck) and silicon (99.9999% Atomergic Chemicals) reacted inside an A1203 crucible. An extra 5% silicon above the stoichiometric amount was added to compensate for volatilization of silicon during the growth process. After about half an hour, which was needed for reaction and homogenization, the seed crystal was partly immersed in the melt and the Czochralski growth process was started.
mination [11], however, neither transport nor magnetic measurements have been taken on these crystals. Therefore, our interest in the iron silicides focuses on the preparation of good quality single crystals of FeSi, a-FeSi2 and fl-FeSi:. The crystals were characterized with respect to their structure, and their basic electrical and magnetical properties.
2. Experimental procedure 2.1. Czochralski growth of FeSi and a-FeSi2 The apparatus used to grow FeSi and a-FeSi2 consists of a 40 cm high and 25 cm diameter stainless steel water-cooled chamber. Two sealed shafts for rotation and pulling from the top and bottom allow independent positioning of the crucible and the seed. Melt ranging from 100 to 200 g was contained in a 5 cm high and 5 cm diameter alumina crucible. A six-turns r.f. copper coil was used for inductive heating of the melt. The melting of elements and the growth of silicides were observed though the quartz windows. The temperature of the melt was measured with a pyrometer through one of the two windows. The synthesis and pulling of FeSi or a-FeSi2 was done in one run. Before melting the charge material,
(o)
2.2. Chemical vapour transport of [3- and a-FeSi2 The FeSi2, synthesised by melting elements inside the Czochralski puller as described above, was used as raw charge in the chemical vapour transport of a- or /3-FeSi2. About 5 g of this material and a small sealed quartz capillary containing iodine were placed inside a clean quartz ampoule. The ampoule was 250 mm long with 20 mm inner diameter. Before sealing the ampoule was evacuated to 10 -5 mbar. Prior to positioning the ampoule inside a two-zone electric furnace, the ampoule was shaken to crack the iodine-containing capillary. The iodine concentration inside was roughly
(b)
b (c) Fig. 1. Single crystals of (a) FeSi, grown by the Czochralski technique, (b) /3-FeSi2 and (c) a-FeSi2 grown by chemical vapour transport. The smallest subdivisions are millimetres.
95
Ch. Kloc et al. I Journal of Alloys and Compounds 219 (1995) 93-96 0.14
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Fig. 2. Temperature dependence of resistivity in (a) FeSi and (b) a-FeSi2 single crystals.
5 mg cm -3. The temperature gradient was stabilizated at 1050-750 °C and 1050-950 °C for /3- and a-FeSi2 respectively.
2.3. Annealing of ot-FeSi2 It seemed reasonable to obtain/3-FeSi2 by annealing the a-FeSi2 samples prepared by the Czochralski technique. A few bars, each about 1 0 x 1 × 1 mm 3, were annealed inside the evacuated quartz ampoule at 800 °C for 240 h. The phases formed were analysed by powder X-ray diffraction. 2.4. Characterization After growth, crystals were characterized by etching and reflection Laue measurements. The composition was evaluated by energy dispersive spectroscopy in a scanning electron microscope. Measurements of conductivity and mobility were taken with a Hall apparatus. The magnetic measurements were taken using a Faraday balance between 4 and 300 K in field up to 4000 Oe.
10
20 (b)
;0
~'0 8'0 1~0 Temperature (K)
200
,00
Fig. 3. (a) Resistivity and (b) Hall mobility vs. temperature for ptype /3-FeSi2 crystals.
3. Results and discussion 3.1. FeSi and a-FeSi2 Czochralski crystal growth Single crystals of FeSi and a-FeSi2 were grown by the Czochralski technique. The synthesis and crystal growth were carried out in one run. During melting of iron and silicon in an argon atmosphere, a dull thin layer of oxide covered the surface. Therefore, when the process was continued the layer disturbed the crystal growth. However, after heating in a vacuum, the dull layer slowly disappeared and the surface of the melt became shiny. For this reason, all the crystal growth processes were carried out in a vacuum, even at the risk of evaporation of silicon and a change in composition of the melt. The pulling rate was usually 4 mm h -a, and speed of rotation of the seed was 10 rev min-l. In the early runs, the crucible was rotated in the opposite direction from the seeds, but it was found that rotating the crucible did not improve the growth process and thereafter it was held stationary. The original seed was a bar of polycrystalline material. After making a constriction, a single-crystal ingot was pulled. Fig. l(a)
96
Ch. Kloc et al. / Journal of Alloys and ('ornpounds 219 (1995) 93-96
presents a few Czochralski growth boules of FeSi. During an FeSi crystal growth run, the loss of silicon was about 01-1 g, which was not more than 1% of the starting material. A larger silicon loss was noticed during FeSi~ growth, therefore the growth process was terminated after the boules reached lengths of about 3 cm. 3.2. Annealing of a-FeSi2 The phase transition a- to /3-FeSi2 is very sluggish [4]. FeSi2 formed by the Czochralski process remains metastable tetragonal during cooling. X-ray powder diffraction analysis performed on bars annealed at 800 °C for 240 h indicated that the samples changed to orthorhombic/3-phase. Unfortunately, the samples were useless for measurements because FeSi precipitate always accompanied the/3-FeSi2 phase. Hence, to obtain the desired pure low temperature/3-phase, growth was carried out from vapour below the phase transition temperature of 960 °C. 3.3. Chemical vapour transport growth of a- and [3feSi 2 The chemical vapour deposition of FeSi2 with iodine as transport agent yielded a- or/~-FeSi2 single crystals at deposition temperatures of 950 and 750 °C respectively. The source temperature was 1050 °C in both cases. The formation of/3- or c~-FeSi2 crystals was not affected by the excess silicon in the growth ampoule, which was added during the melting of silicon with iron. Silicon condensed at the cold part of the ampoule and/3-FeSi2 crystals grew partly on the deposited layer of silicon. The orthorhombic/3-FeSi2 grew in the form of thin, frequently twinned needles, each about 10 mm long (Fig. l(b)), while tetragonal a-FeSi2 crystals grew as plates or orthogons (Fig. 1(c)). 3.4. Transport properties Electrical measurements taken at room temperature show a resistivity of 3 × 1 0 - 4 fl cm and 0.1×10-~-0.2×10 -4 ~ cm for FeSi and a-FeSi2 respectively. The temperature dependence of the resistivity was measured from 4.2 K up to room temperature (Fig. 2). The resistivity of a-FeSi2 is virtually temperature independent, similar to that observed previously [8]. The activation energy of FeSi, calculated from In p =f(1/ T) is equal to 58.2 meV, which is roughly consistent with the theoretical value of about 100 meV [6]. The Hall coefficient and resistivity of/3-FeSi 2 single crystals was measured from 30 to 300 K. The as-grown, not intentionally doped /3-FeSi2 crystals exhibit n-type conductivity. Chromium and aluminium doped crystals show p-type. The Hall coefficient of n-type samples
depends on the magnetic field; therefore, the transport properties of/3-FeSi2 were explained taking into account two types of carrier, heavy and light [12]. At low temperature (32 K) heavy electrons show a mobility of tx,,= 48 cm 2 V-~ s-~. In p-type /3-FeSi2 crystals, an impurity band and an additional deep acceptor level are found. The activation energies of the shallow acceptor level and deep acceptor level are equal to 55 meV and 100 meV respectively. The mobility of holes Ixp in p-type crystals reached a maximum of 1200 cm 2 V ~ s ~ at 67 K (Fig. 3). Observed values of txn and txp are up to 10-50 times higher than those previously reported [4,13,14], which may be considered as conformation of the high quality of/3-FeSi2 crystals. Magnetization and magnetic susceptibility measurements in the temperature range 4-320 K show a small positive value of susceptibility and non-linear magnetization for 13-FeSi2, but no ferromagnetic phase transition, mentioned in [151, was detected, a-FeSi2 crystals show linear magnetization and susceptibility similar to /3-FeSi2. Magnetization of FeSi is linear but the susceptibility passes through a minimum at about 150-200 K; this is consistent with previous observations [16]. In conclusion, the possibility of growth of FeSi, aFeSiz and /3-FeSi2 single crystals, which will be used for further studies, has been demonstrated. New electronic parameters, found for 13-FeSi2, differ considerably from previously published values.
References [I] Gmelins Handbuch der Anorganischen Chemic, Eisen, Vol. IA, II, Verlag Chemic, Berlin, 1934-1939, pp. 1740-1758. [2] O. Kubasehewski, Iron-Binary Phase Diagrams; Springer, Belin, 1982, p. 136. [3] M.C. Bost and J.E. Mahan, J. AppL Phys., 58 (1985) 2696. [4] U. Birkholz and J. Schelm, Phys. Status Solidi, 27 (1968) 413. I5] E. Arushanov, Ch. Kloc and E. Bucher, Phys. Rev. B, 50 (1994) 2653. I61 C. Fu, M.P.C.M. Krijn and S. Doniach, Phys. Rev. B, 49 (1994) 2219. [7] P.V. Geld, A.G. Volkov, S.V. Kortov, A.A. Pvzner and V. Yu. lvanov, Dokl. Akad. Nauk SSSR, 320 (1991) 1097. 18] T. Hirano and M. Kaise, J. Appl. Phys., 68 (1990) 627. [9] S.P. Murarka, J. Vac. Sci. Technol., 17 (1980) 775. [10] G. Shirane, J.E. Fisher, Y. Endoh and K. Tajima, Phys. Rev. Lett., 59 (1987) 351. [11] R. Wandji, Y. Dusausoy, J. Protas and B. Roques, C.R. Acad. Sci. Paris, 267 (1968) 1587. [ 12] E. Arushanov, Ch. Kloc, H. Hohl and E. Bucher, J. Appl. Phys., 7.5 (1994) 5106. I13[ D.J. Oostra, C.W.T. Bulle-Lieuwma, D.E.W. Vandenhoudt, F. Felten and J.C. Jans, J. Appl. Phys., 74 (1993) 4347. [14] J.L. Regolini, F. Trincat, I. Sagnes, Y. Shapira, G. Bremond and D. Bensahel, IEEE Trans. Electron Devices, 39 (1992) 200. [15] O. Valasiades, C.A. Dimitriadis and J.H. Werner, J. Appl. Phys., 70 (1991) 890. [16] K. Taiima, Y. Endoh, J.E. Fischer and G. Shirane, Phys, Rev. t:1, 38 (1988) 6954, and references cited therein.
~OYS ANDC,OMPOL~DS ELSEVIER
Journal of Alloys and Compounds 219 (1995) 93-96
Preparation and properties of FeSi, a-FeSi2 and /3-FeSi2 single crystals Ch. Kloc, E. Arushanov *, M. Wendl, H. Hohl, U. Malang, E. Bucher Faculty of Physics, University of Konstanz, P.O. Box 5560, D-78434 Konstanz, Germany
Abstract
FeSi and a-FeSi2 single crystals were prepared by the Czochralski technique, fl-FeSi2 and a-FeSi2 single crystals were prepared by chemical vapour transport. FeSi single crystals 18 cm3 in size and a-FeSi2 single crystals of a few cubic centimetres were obtained, fl-FeSi2 decomposes peritectically above 970 °C, therefore only needle-like crystals, each about 10 ram long, were grown from the vapour. The temperature dependence of the resistivity was measured from 4.2 K up to room temperature. a-FeSi2 shows a weak temperature dependence of resistivity. The activation energy of FeSi, calculated from In p=f(1/T) is equal to 58.2 meV. the Hall coefficient and resistivity of/3-FeSi2 single crystals was measured between 30 and 300 K. The as-grown, not intentionally doped /3-FeSi2 crystals exhibit n-type conductivity, while Cr and AI doped crystals show p-type conductivity. The Hall coefficient of n-type samples depends on the magnetic field; therefore, the transport properties of/3FeSi2 were explained taking into account two types of carrier, heavy and light. At low temperature (32 K) heavy electrons show a mobility of /zn = 48 cm2 V-1 s-1. An impurity band and an additional deep acceptor level are observed in p-type crystals. The activation energies of the shallow acceptor level and deep acceptor level are equal to 55 meV and 100 meV respectively. The mobility of holes /Zp in p-type crystals reaches a maximum of 1200 cm2 V -1 s -1 at 67 K. Magnetization and magnetic susceptibility measurements in the temperature range 4-320 K show a small positive value of susceptibility and nonlinear magnetization for /3-FeSi2 but no ferromagnetic phase was detected, a-FeSi2 crystals show linear magnetization and susceptibility similar to /3-FeSi2. Magnetization of FeSi is linear but the susceptibility passes through a minimum at about 150-200 K. Keywords: Single crystals; Chemical vapour transport
1. Introduction
The iron-silicon phase diagram has been the subject of controversial considerations for a long time [1,2]. According to a generally accepted, summarized study [2], iron and silicon form four compounds, FezSi, FesSi3, FeSi, ot-FeSi2 and/3-FeSi2. A m o n g them, only FeSi and the low t e m p e r a t u r e modification of FeSi2, the/3-phase, are thermodynamically stable at room temperature. Orthorhombic/3-FeSi2 shows a direct band gas of 0.87 eV [3] and the possibility of p- and n-type doping [4,5]. Cubic FeSi seems to be a narrow band semiconductor [6], or magnetic semiconductor, which exhibits band ferromagnetism [7]. Tetragonal a-FeSi2, though thermodynamically metastable at ambient temperature, does * Permanent address: Institute of Applied Physics, Academy of Sciences of Moldova, Kishinev, Moldova.
0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)05055-4
not easily undergo a phase transition to the low temperature phase /3-FeSi2. a-FeSi2 is metallic [8]. All iron silicides show high resistance to oxidation, low vapour pressure, lack of toxicity and wide occurrence of constituent elements in the Earth. Such an alliance of desired properties makes the silicides attractive for semiconductor technology [9]. For all that, the properties of iron silicides are still not understood well. Many measurements have been taken on sintered samples or on polycrystalline thin films. These data are difficult to interpret because the effects of the surface, grain structure, or defects on the properties are difficult to evaluate. In recent years, single crystals of FeSi and a-FeSi2 have been used for magnetic and transport measurements [8,10] but the preparation procedures have not been reported./3-FeSi2 crystals were p r e p a r e d by chemical vapour transport and used for structure deter-
94
Ch. Kloc et aL / Journal of Alloys and Compounds 219 (1995) 93-96
the apparatus was evacuated to approximately 10 -5 mbar, back-filled with 99.999% pure argon, and again evacuated. Growth runs were carried out in a dynamic vacuum of 10 -5 mbar. Iron (99.998% pure Alfa or at least 99.9% purity remelted powder from Merck) and silicon (99.9999% Atomergic Chemicals) reacted inside an A1203 crucible. An extra 5% silicon above the stoichiometric amount was added to compensate for volatilization of silicon during the growth process. After about half an hour, which was needed for reaction and homogenization, the seed crystal was partly immersed in the melt and the Czochralski growth process was started.
mination [11], however, neither transport nor magnetic measurements have been taken on these crystals. Therefore, our interest in the iron silicides focuses on the preparation of good quality single crystals of FeSi, a-FeSi2 and fl-FeSi:. The crystals were characterized with respect to their structure, and their basic electrical and magnetical properties.
2. Experimental procedure 2.1. Czochralski growth of FeSi and a-FeSi2 The apparatus used to grow FeSi and a-FeSi2 consists of a 40 cm high and 25 cm diameter stainless steel water-cooled chamber. Two sealed shafts for rotation and pulling from the top and bottom allow independent positioning of the crucible and the seed. Melt ranging from 100 to 200 g was contained in a 5 cm high and 5 cm diameter alumina crucible. A six-turns r.f. copper coil was used for inductive heating of the melt. The melting of elements and the growth of silicides were observed though the quartz windows. The temperature of the melt was measured with a pyrometer through one of the two windows. The synthesis and pulling of FeSi or a-FeSi2 was done in one run. Before melting the charge material,
(o)
2.2. Chemical vapour transport of [3- and a-FeSi2 The FeSi2, synthesised by melting elements inside the Czochralski puller as described above, was used as raw charge in the chemical vapour transport of a- or /3-FeSi2. About 5 g of this material and a small sealed quartz capillary containing iodine were placed inside a clean quartz ampoule. The ampoule was 250 mm long with 20 mm inner diameter. Before sealing the ampoule was evacuated to 10 -5 mbar. Prior to positioning the ampoule inside a two-zone electric furnace, the ampoule was shaken to crack the iodine-containing capillary. The iodine concentration inside was roughly
(b)
b (c) Fig. 1. Single crystals of (a) FeSi, grown by the Czochralski technique, (b) /3-FeSi2 and (c) a-FeSi2 grown by chemical vapour transport. The smallest subdivisions are millimetres.
95
Ch. Kloc et al. I Journal of Alloys and Compounds 219 (1995) 93-96 0.14
)
-5"
O
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-
FeSi
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i
100
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10
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.
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o
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-
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1-:
0.012
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' 100
(o)
0.006
I 200
I 250
1 300
0,00
J
i
I
I
I
0.01
0.02
0.03
0.04
1/'r (Kl )
(o)
Temperature (K)
0.00078
0,00076,
[]
et-FeSi 2 []
0.00074
0
0
OOOo o
1000 800
[]
o
600
[]
[]
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% D
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150
200
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Temperature (K)
Fig. 2. Temperature dependence of resistivity in (a) FeSi and (b) a-FeSi2 single crystals.
5 mg cm -3. The temperature gradient was stabilizated at 1050-750 °C and 1050-950 °C for /3- and a-FeSi2 respectively.
2.3. Annealing of ot-FeSi2 It seemed reasonable to obtain/3-FeSi2 by annealing the a-FeSi2 samples prepared by the Czochralski technique. A few bars, each about 1 0 x 1 × 1 mm 3, were annealed inside the evacuated quartz ampoule at 800 °C for 240 h. The phases formed were analysed by powder X-ray diffraction. 2.4. Characterization After growth, crystals were characterized by etching and reflection Laue measurements. The composition was evaluated by energy dispersive spectroscopy in a scanning electron microscope. Measurements of conductivity and mobility were taken with a Hall apparatus. The magnetic measurements were taken using a Faraday balance between 4 and 300 K in field up to 4000 Oe.
10
20 (b)
;0
~'0 8'0 1~0 Temperature (K)
200
,00
Fig. 3. (a) Resistivity and (b) Hall mobility vs. temperature for ptype /3-FeSi2 crystals.
3. Results and discussion 3.1. FeSi and a-FeSi2 Czochralski crystal growth Single crystals of FeSi and a-FeSi2 were grown by the Czochralski technique. The synthesis and crystal growth were carried out in one run. During melting of iron and silicon in an argon atmosphere, a dull thin layer of oxide covered the surface. Therefore, when the process was continued the layer disturbed the crystal growth. However, after heating in a vacuum, the dull layer slowly disappeared and the surface of the melt became shiny. For this reason, all the crystal growth processes were carried out in a vacuum, even at the risk of evaporation of silicon and a change in composition of the melt. The pulling rate was usually 4 mm h -a, and speed of rotation of the seed was 10 rev min-l. In the early runs, the crucible was rotated in the opposite direction from the seeds, but it was found that rotating the crucible did not improve the growth process and thereafter it was held stationary. The original seed was a bar of polycrystalline material. After making a constriction, a single-crystal ingot was pulled. Fig. l(a)
96
Ch. Kloc et al. / Journal of Alloys and ('ornpounds 219 (1995) 93-96
presents a few Czochralski growth boules of FeSi. During an FeSi crystal growth run, the loss of silicon was about 01-1 g, which was not more than 1% of the starting material. A larger silicon loss was noticed during FeSi~ growth, therefore the growth process was terminated after the boules reached lengths of about 3 cm. 3.2. Annealing of a-FeSi2 The phase transition a- to /3-FeSi2 is very sluggish [4]. FeSi2 formed by the Czochralski process remains metastable tetragonal during cooling. X-ray powder diffraction analysis performed on bars annealed at 800 °C for 240 h indicated that the samples changed to orthorhombic/3-phase. Unfortunately, the samples were useless for measurements because FeSi precipitate always accompanied the/3-FeSi2 phase. Hence, to obtain the desired pure low temperature/3-phase, growth was carried out from vapour below the phase transition temperature of 960 °C. 3.3. Chemical vapour transport growth of a- and [3feSi 2 The chemical vapour deposition of FeSi2 with iodine as transport agent yielded a- or/~-FeSi2 single crystals at deposition temperatures of 950 and 750 °C respectively. The source temperature was 1050 °C in both cases. The formation of/3- or c~-FeSi2 crystals was not affected by the excess silicon in the growth ampoule, which was added during the melting of silicon with iron. Silicon condensed at the cold part of the ampoule and/3-FeSi2 crystals grew partly on the deposited layer of silicon. The orthorhombic/3-FeSi2 grew in the form of thin, frequently twinned needles, each about 10 mm long (Fig. l(b)), while tetragonal a-FeSi2 crystals grew as plates or orthogons (Fig. 1(c)). 3.4. Transport properties Electrical measurements taken at room temperature show a resistivity of 3 × 1 0 - 4 fl cm and 0.1×10-~-0.2×10 -4 ~ cm for FeSi and a-FeSi2 respectively. The temperature dependence of the resistivity was measured from 4.2 K up to room temperature (Fig. 2). The resistivity of a-FeSi2 is virtually temperature independent, similar to that observed previously [8]. The activation energy of FeSi, calculated from In p =f(1/ T) is equal to 58.2 meV, which is roughly consistent with the theoretical value of about 100 meV [6]. The Hall coefficient and resistivity of/3-FeSi 2 single crystals was measured from 30 to 300 K. The as-grown, not intentionally doped /3-FeSi2 crystals exhibit n-type conductivity. Chromium and aluminium doped crystals show p-type. The Hall coefficient of n-type samples
depends on the magnetic field; therefore, the transport properties of/3-FeSi2 were explained taking into account two types of carrier, heavy and light [12]. At low temperature (32 K) heavy electrons show a mobility of tx,,= 48 cm 2 V-~ s-~. In p-type /3-FeSi2 crystals, an impurity band and an additional deep acceptor level are found. The activation energies of the shallow acceptor level and deep acceptor level are equal to 55 meV and 100 meV respectively. The mobility of holes Ixp in p-type crystals reached a maximum of 1200 cm 2 V ~ s ~ at 67 K (Fig. 3). Observed values of txn and txp are up to 10-50 times higher than those previously reported [4,13,14], which may be considered as conformation of the high quality of/3-FeSi2 crystals. Magnetization and magnetic susceptibility measurements in the temperature range 4-320 K show a small positive value of susceptibility and non-linear magnetization for 13-FeSi2, but no ferromagnetic phase transition, mentioned in [151, was detected, a-FeSi2 crystals show linear magnetization and susceptibility similar to /3-FeSi2. Magnetization of FeSi is linear but the susceptibility passes through a minimum at about 150-200 K; this is consistent with previous observations [16]. In conclusion, the possibility of growth of FeSi, aFeSiz and /3-FeSi2 single crystals, which will be used for further studies, has been demonstrated. New electronic parameters, found for 13-FeSi2, differ considerably from previously published values.
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