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ESR and Mössbauer spectroscopy of iron-doped AgAsS and GeSbSe glasses
Journal of Non-Crystalline Solids 97&98 (1987) 65%662 North-Holland, Amsterdam
659
ESR AND MOSSBAUER SPECTROSCOPY OF IRON-DOPED Ag-As-S AND Ge-Sb-Se GLASSES
I.V. CHEPELEVA, Z.U. BORISOVA, Yu.G. VLASOV and E.A. BYCHKOV Institute of Nuclear Physics, Moscow University, Moscow, USSR Department of Chemistry, Leningrad University, Leningrad, USSR
Thermal doping of Ag-As-S and Ge-Sb-Se chalcogenide glasses (ChG) with iron results in a considerable increase of conductivity (by 4-7 orders of magnitude at dopant concentration of 0.4-2.0 at.%) and in the transition from p- to n-type conductivity. Iron-doped glasses were studied using ESR and Mossbauer spectroscopy. The features of iron local environment are discussed, ESR and Mossbauer spectroscopy data are compared, and small polaron model of charge transport in transition metal doped vitreous alloys is suggested.
I.
INTRODUCTION The impurities of transition metals such as Ni, Fe, Mo, W influence the
properties of semiconducting chalcogenide glasses (ChG) strongly. Both thin films, modified by these metals using r.f. co-sputtering of glass and metal /1,2/ and bulk samples of ChG doped with Fe or Ni from the melt /3-5/ are characterized by a considerable (5-11 orders of magnitude) increase of conductivity. The sign inversion of thermoelectric power and the appearance of impurity conduction region on the temperature dependence of conductivity were also observed, but the optical gap altered negligibly. Some different explanations of such behaviour were put forward 5-8. The present paper deals with conductivity measurements and local environment of impurity iron atoms in thermally doped Ge28Sb12Se60 and AgAsS? glasses. Glass synthesis and experimental details were described earlier 3-5,9.
2.
CONDUCTIVITY Ge28Sb12Se60 glass is a wide-gap electronic insulator. Thermal doping with
iron results in conductivity enhancement by 7 orders of magnitude at dopant concentration 2.0 at.%, but the optical absorption edge is red-shifted by 0.20.3 eV only 3'4. Vitreous smithite AgAsS 2 is a ionic conductor. Wagner's d.c. polarization 10 reveal p-type electronic conductivity of updoped glass, which
measurements
is 5-6 orders of magnitude lower than ionic one 9. Iron additives caused electronic conductivity increase by 103 times, ionic conductivity value remaining constant. Fe-doped AgAsS 2 glassy alloys display n-type electronic conductivity. Compositional dependence of room temperature ionic and electronic conductivities is shown in Fig. I.
0022-3093/87/$03.50© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
660
I.V.. Chepeleva et al. / ESR and M6ssbauer spectroscopy
I
--
n-type
,
~
o
=e
0.0
0.6
1.2
[ Fe ] (ot.%) FIGURE 1 Room temperature ionic (o) and electronic (o) conductivities in AsAgS2:Fe glasses. 3.
c
-1
0 1 2 Velocity (mm/s)
3
FIGURE 2 Typical Mossbauer spectrum in Fe-doped
AgAsS2 g l a s s e s (Ta=295 K).
57Fe MOSSBAUER SPECTROSCOPY Mossbauer spectra of iron-doped ChG reveal a superposition of several
quadrupole doublets (Fig. 2). Broad lines (0.5-0.7 mm/s) and non-Lorentzian line shape are observed. We used a modified computational procedure by Hesse and Rubartsch 11 to find a distribution function of quadrupole splitting P(QS). Selected example of P(QS) is shown in Fig. 3. Slightly asymmetric near-Gaussian distribution function
displays that impurity iron atoms are distributed in glass
network in a random manner. The main component of the spectra (80-95 %) corresponds to high-spin Fe 2+ ions in tetrahedral environment by chalcogens. Magnetic susceptibility results 4 are in a good agreement with such a conclusion. Considerable values of quadrupole splitting (Fig. 3) show that FeX 4 tetrahedra are strongly distorted. Additional absorption is presumably due to high-spin Fe 3+ ions, low-spin iron complexes and octahedrally coordinated Fe + ions. The measurements of magnetic susceptibility and Mossbauer spectra performed in 1.8-300 K temperature range reveal no magnetic order. This means the absence of Fe-rich clusters in doped glasses.
4.
ESR SPECTROSCOPY The X-band ESR spectra of iron ions in the glasses of both systems are quite
similar and reveal no essential change of line positions in temperature interval 20-40 K. The typical ESR spectra are shown in Fig. 4. Some common features are observed: a broad line in low magnetic field, a narrow line with isotropic gfactor g=4.28 (6H-60-65 Oe), and in some samples an asymmetric signal with gll ~2.0-2.2, g± ~J 1.9. The first signal can be attributed to Fe 2+ ions.
I.V. Chepeleua et al. / ESR and M~ssbauer spectroscopy
0,10
I
.~l geft~.8g/4.3 i . ~ ~ ~~
I
661
I
. [Fe] =1.2at-°/o i
I
0.05
K
0.01
I
I
2
3
4
0
OuodrupoteSplitting(mm/s) FIGURE 3 Distribution function of quadrupole splitting P(QS) in Fe-doped glasses.
1
2 H (kG)
3
4
FIGURE 4 Typical ESR spectra of AgAsS2:Fe glasses.
Because of intensive zero-field absorption only an effective g-value cprresponding to the high-field extremum is represented: geff~8 for all glasses. This value 72 for the lowest
is close to the theoretical one gzz=8.97 (gxx=gyy=O calculated
non-Kramers doublet (S'=I/2) of the Fe 2+ ions in a strong cubic crystalline field with orthorhombic distortion. The dependence of g-factor on the low orbital triplet splitting A* due to axial distortion was calculated 13. The value geff~8 corresponds to &*~120-150 cm -I. The ESR data are in a good agreement with Mossbauer spectroscopy results. The line with g=4.28 is due to Fe 3+ ions 14. The asyrmnetric signal at g=2.21.9 with rather irregular parameters was observed in AgAsS 2 glass with 0.5 at,% Fe and in heavily doped Ge-Sb-Se glasses ([Fe]> 0.5 at.%) and can be attributed to both Fe 2+ and Fe 3+ ions in weak crystalline field. The relative amounts of Fe 2+ and Fe 3+ ions depend on the total ion content: as Fe concentration in2+ creases the Fe concentration also increases while relative as well as absolute content of Fe 3+ ions decreases.
5.
CONDUCTIVITY MECHANISM IN DOPED GLASSES Comparison of electric properties and local environment of impurity iron
atoms allow to conclude that charge transport in transition metal (TM) doped ChG is due to small polaron hopping between TM n+ and TM (n+1)+ ions. Firstly, ESR and Mossbauer spectroscopy showed the existence of both Fe 3+ and Fe 2÷ ions in ChG.
l.~Chepe&oa etaL / ESR and M6ssbauerspectroscopy
662
Secondly,
temperature-independent
dicts with simple donor-like
density of Fe 3+ centers in doped glasses contra-
impurity center model
density of ionized donor centers(Fe 3+) temperature.
because in this case the
should exponentially
increase with
It is surprising that according to ESR results Fe 3+ center density
decreased with total iron concentration
increase. However,
one can suppose that
only a part of Fe 3+ centers is involved in small polaron hopping.
REFERENCES I) R. Flasck, M. Izu, K. Sapru, T. Anderson, S.R. Ovshinsky and H. Fritzsche, in Proc. 7th Int. Conf. on Amorphous and Liquid Semiconductors, eds. W.E. Spear and P.C. LeComber (Edinburgh, 1977) p. 524. 2) V.L. Averyanov,
in Glassy Semiconductors
(Russ.),
(Leningrad,
1095, p.224.
3) A.A. Andreev, Z.U. Borisova, E.A. Bychkov and Yu.G, Vlasov, J. Non-Cryst. Solids 35-36 (1980) 901. 4) Yu.G. Vlasov and E.A. Bychkov, Hung. Sci. Instrum. 53 (1982) 35. 5) Z.U. Borisova, V.A. Kochemirovskii, E.A. Zhilinskaya, Yu.G. Vlasov, Fiz. Khim. Stekla 11 (1985) 319.
E.A. Bychkov and
6) H. Fritzsche and M. Kastner, Phil. Mag. B 37 (1978) 285. 7) B.L. Gelmont, B.T. Kolomiets and K.D. Taendin, Phys. Stat. Solidi A92 (1985) 319. 8) Yu.S. Tveryanovich
and Z.U. Borisova,
J. Non-Cryst.
Solids 90 (1987) 405.
9) Z.U. Borisova, E.A. Bychkov, Yu.G. Vlasov and I.V. Chepeleva, in Glassy Semiconductors (Russ.), ed. B.T. Kolomiets (Leningrad, 1985) p.244. 10) C. Wagner, Z. Elektrochem.
60 (1956) 4.
11) J. Hesse and A. Rubartsch,
J. Phys. E7 (1974) 526.
12) M. Tinkham, Proc. Roy. Soc. Roy. Soc. A236 (1956) 535. 13) V.F. Meshcheryakov, B.N. Grechushnikov Fiz. 66 (1974) 1870.
and I.N. Kalinnikova,
14) I.V. Chepeleva, Dokl. Akad. Nauk SSSR 202 (1972) 1042.
Zh. Eksp. Teor.
659
ESR AND MOSSBAUER SPECTROSCOPY OF IRON-DOPED Ag-As-S AND Ge-Sb-Se GLASSES
I.V. CHEPELEVA, Z.U. BORISOVA, Yu.G. VLASOV and E.A. BYCHKOV Institute of Nuclear Physics, Moscow University, Moscow, USSR Department of Chemistry, Leningrad University, Leningrad, USSR
Thermal doping of Ag-As-S and Ge-Sb-Se chalcogenide glasses (ChG) with iron results in a considerable increase of conductivity (by 4-7 orders of magnitude at dopant concentration of 0.4-2.0 at.%) and in the transition from p- to n-type conductivity. Iron-doped glasses were studied using ESR and Mossbauer spectroscopy. The features of iron local environment are discussed, ESR and Mossbauer spectroscopy data are compared, and small polaron model of charge transport in transition metal doped vitreous alloys is suggested.
I.
INTRODUCTION The impurities of transition metals such as Ni, Fe, Mo, W influence the
properties of semiconducting chalcogenide glasses (ChG) strongly. Both thin films, modified by these metals using r.f. co-sputtering of glass and metal /1,2/ and bulk samples of ChG doped with Fe or Ni from the melt /3-5/ are characterized by a considerable (5-11 orders of magnitude) increase of conductivity. The sign inversion of thermoelectric power and the appearance of impurity conduction region on the temperature dependence of conductivity were also observed, but the optical gap altered negligibly. Some different explanations of such behaviour were put forward 5-8. The present paper deals with conductivity measurements and local environment of impurity iron atoms in thermally doped Ge28Sb12Se60 and AgAsS? glasses. Glass synthesis and experimental details were described earlier 3-5,9.
2.
CONDUCTIVITY Ge28Sb12Se60 glass is a wide-gap electronic insulator. Thermal doping with
iron results in conductivity enhancement by 7 orders of magnitude at dopant concentration 2.0 at.%, but the optical absorption edge is red-shifted by 0.20.3 eV only 3'4. Vitreous smithite AgAsS 2 is a ionic conductor. Wagner's d.c. polarization 10 reveal p-type electronic conductivity of updoped glass, which
measurements
is 5-6 orders of magnitude lower than ionic one 9. Iron additives caused electronic conductivity increase by 103 times, ionic conductivity value remaining constant. Fe-doped AgAsS 2 glassy alloys display n-type electronic conductivity. Compositional dependence of room temperature ionic and electronic conductivities is shown in Fig. I.
0022-3093/87/$03.50© Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
660
I.V.. Chepeleva et al. / ESR and M6ssbauer spectroscopy
I
--
n-type
,
~
o
=e
0.0
0.6
1.2
[ Fe ] (ot.%) FIGURE 1 Room temperature ionic (o) and electronic (o) conductivities in AsAgS2:Fe glasses. 3.
c
-1
0 1 2 Velocity (mm/s)
3
FIGURE 2 Typical Mossbauer spectrum in Fe-doped
AgAsS2 g l a s s e s (Ta=295 K).
57Fe MOSSBAUER SPECTROSCOPY Mossbauer spectra of iron-doped ChG reveal a superposition of several
quadrupole doublets (Fig. 2). Broad lines (0.5-0.7 mm/s) and non-Lorentzian line shape are observed. We used a modified computational procedure by Hesse and Rubartsch 11 to find a distribution function of quadrupole splitting P(QS). Selected example of P(QS) is shown in Fig. 3. Slightly asymmetric near-Gaussian distribution function
displays that impurity iron atoms are distributed in glass
network in a random manner. The main component of the spectra (80-95 %) corresponds to high-spin Fe 2+ ions in tetrahedral environment by chalcogens. Magnetic susceptibility results 4 are in a good agreement with such a conclusion. Considerable values of quadrupole splitting (Fig. 3) show that FeX 4 tetrahedra are strongly distorted. Additional absorption is presumably due to high-spin Fe 3+ ions, low-spin iron complexes and octahedrally coordinated Fe + ions. The measurements of magnetic susceptibility and Mossbauer spectra performed in 1.8-300 K temperature range reveal no magnetic order. This means the absence of Fe-rich clusters in doped glasses.
4.
ESR SPECTROSCOPY The X-band ESR spectra of iron ions in the glasses of both systems are quite
similar and reveal no essential change of line positions in temperature interval 20-40 K. The typical ESR spectra are shown in Fig. 4. Some common features are observed: a broad line in low magnetic field, a narrow line with isotropic gfactor g=4.28 (6H-60-65 Oe), and in some samples an asymmetric signal with gll ~2.0-2.2, g± ~J 1.9. The first signal can be attributed to Fe 2+ ions.
I.V. Chepeleua et al. / ESR and M~ssbauer spectroscopy
0,10
I
.~l geft~.8g/4.3 i . ~ ~ ~~
I
661
I
. [Fe] =1.2at-°/o i
I
0.05
K
0.01
I
I
2
3
4
0
OuodrupoteSplitting(mm/s) FIGURE 3 Distribution function of quadrupole splitting P(QS) in Fe-doped glasses.
1
2 H (kG)
3
4
FIGURE 4 Typical ESR spectra of AgAsS2:Fe glasses.
Because of intensive zero-field absorption only an effective g-value cprresponding to the high-field extremum is represented: geff~8 for all glasses. This value 72 for the lowest
is close to the theoretical one gzz=8.97 (gxx=gyy=O calculated
non-Kramers doublet (S'=I/2) of the Fe 2+ ions in a strong cubic crystalline field with orthorhombic distortion. The dependence of g-factor on the low orbital triplet splitting A* due to axial distortion was calculated 13. The value geff~8 corresponds to &*~120-150 cm -I. The ESR data are in a good agreement with Mossbauer spectroscopy results. The line with g=4.28 is due to Fe 3+ ions 14. The asyrmnetric signal at g=2.21.9 with rather irregular parameters was observed in AgAsS 2 glass with 0.5 at,% Fe and in heavily doped Ge-Sb-Se glasses ([Fe]> 0.5 at.%) and can be attributed to both Fe 2+ and Fe 3+ ions in weak crystalline field. The relative amounts of Fe 2+ and Fe 3+ ions depend on the total ion content: as Fe concentration in2+ creases the Fe concentration also increases while relative as well as absolute content of Fe 3+ ions decreases.
5.
CONDUCTIVITY MECHANISM IN DOPED GLASSES Comparison of electric properties and local environment of impurity iron
atoms allow to conclude that charge transport in transition metal (TM) doped ChG is due to small polaron hopping between TM n+ and TM (n+1)+ ions. Firstly, ESR and Mossbauer spectroscopy showed the existence of both Fe 3+ and Fe 2÷ ions in ChG.
l.~Chepe&oa etaL / ESR and M6ssbauerspectroscopy
662
Secondly,
temperature-independent
dicts with simple donor-like
density of Fe 3+ centers in doped glasses contra-
impurity center model
density of ionized donor centers(Fe 3+) temperature.
because in this case the
should exponentially
increase with
It is surprising that according to ESR results Fe 3+ center density
decreased with total iron concentration
increase. However,
one can suppose that
only a part of Fe 3+ centers is involved in small polaron hopping.
REFERENCES I) R. Flasck, M. Izu, K. Sapru, T. Anderson, S.R. Ovshinsky and H. Fritzsche, in Proc. 7th Int. Conf. on Amorphous and Liquid Semiconductors, eds. W.E. Spear and P.C. LeComber (Edinburgh, 1977) p. 524. 2) V.L. Averyanov,
in Glassy Semiconductors
(Russ.),
(Leningrad,
1095, p.224.
3) A.A. Andreev, Z.U. Borisova, E.A. Bychkov and Yu.G, Vlasov, J. Non-Cryst. Solids 35-36 (1980) 901. 4) Yu.G. Vlasov and E.A. Bychkov, Hung. Sci. Instrum. 53 (1982) 35. 5) Z.U. Borisova, V.A. Kochemirovskii, E.A. Zhilinskaya, Yu.G. Vlasov, Fiz. Khim. Stekla 11 (1985) 319.
E.A. Bychkov and
6) H. Fritzsche and M. Kastner, Phil. Mag. B 37 (1978) 285. 7) B.L. Gelmont, B.T. Kolomiets and K.D. Taendin, Phys. Stat. Solidi A92 (1985) 319. 8) Yu.S. Tveryanovich
and Z.U. Borisova,
J. Non-Cryst.
Solids 90 (1987) 405.
9) Z.U. Borisova, E.A. Bychkov, Yu.G. Vlasov and I.V. Chepeleva, in Glassy Semiconductors (Russ.), ed. B.T. Kolomiets (Leningrad, 1985) p.244. 10) C. Wagner, Z. Elektrochem.
60 (1956) 4.
11) J. Hesse and A. Rubartsch,
J. Phys. E7 (1974) 526.
12) M. Tinkham, Proc. Roy. Soc. Roy. Soc. A236 (1956) 535. 13) V.F. Meshcheryakov, B.N. Grechushnikov Fiz. 66 (1974) 1870.
and I.N. Kalinnikova,
14) I.V. Chepeleva, Dokl. Akad. Nauk SSSR 202 (1972) 1042.
Zh. Eksp. Teor.