- Email: [email protected]
Heat capacity measurements on Ge20Se80−xBix glasses
Solid State Communications, Vol. 88, No. 3, pp. 251-254, 1993. Printed in Great Britain.
0038-1098/93 $6.00 + .00 Pergamon Press Ltd
HEAT CAPACITY MEASUREMENTS ON Ge20Ses0_xBix GLASSES M.K. Rabinal, K.S. Sangunni, E.S.R. Gopal* and S.V. Subramanyam Dept. of Physics, Indian Institute of Science, Bangalore 560 012, India * National Physical Laboratory, New Delhi 110 012, India
(Received 5 May 1993 by C.N.R. Rao) The specific heat Cp of glassy Ge20Ses0_xBix (0 < x < 12) samples is investigated. The Cp at 323K and the ACp at glass transition temperature T-l show anomalous features around x = 8 at.%, where p-n conductio~n type inversion also take place. These features are discussed in the light of Phillips model of phase separation in these glasses at the microscopic level. 1. INTRODUCTION SEMICONDUCTORS containing chalcogen atoms as major constituents are named as the lone-pair semiconductors [1]. Anderson's idea of spin pairing on the same site with negative correlation energy is realized in these chalcogenide glasses [2]. Mott et al. [3] and Kastner et al. [4], using alternate approaches, discussed the formation of defects in these glasses which are termed as Valence Alteration Pair (VAP). One of the important feature of these defects is that the Fermi-level is pinned effectively in the middle of the gap. For a long time it was believed that these materials cannot be doped. However, Ovshinsky et al. achieved the break-through and succeeded in doping using non-equilibrium techniques [5]. The main limitation of these methods is that they are useful only in amorphous thin films. Investigations that Bi and Pb atoms modify certain bulk chalcogenide glasses invoked a special interest in the scientific community [6, 7]. The Ge20Sea0_xBix glasses show the conduction inversion around 7-8 at.% of Bi, which is quite closer to the mechanical threshold in these glasses [8]. To unravel this phenomena these glasses are studied extensively using various sophisticated experimental techniques, and they show subtle changes around this critical value [6, 7, 9-11]. The purpose of the present communication is to check such an unusual feature in the specific heat at constant pressure (Cp).
Ge, Se, and Bi (with 5N purity) are sealed in quartz ampules of 8mm i.d. evacuated upto 5 × 10-5 Torr. These ampules are loaded into an electric furnace and the temperature of the furnace is raised slowly to 1273 K. The molten alloy was held at this temperature for about 36h and good homogenization is achieved by rotating the ampule externally. The final homogenized melt is quenched in ice-water+ NaOH solution. The amorphous nature of these samples is confirmed by the X-ray diffraction. It has been well established that the annealing of semiconducting glasses affect the thermodynamical properties to some extent [12]. Therefore, to have identical thermal history, all the samples are heated just below their respective glass transition temperatures and quickly cooled back to the room temperature. These glasses show the phenomena of double glass transition and double crystallization. The energy involved at the Tg2 is very small compared to the TgI [9], hence the Ce measurements are carried out only at Tgl. The Cp is measured on these samples using the constant heating rate method [13] with a fully automated Du-Pont Differential Scanning Calorimeter (DSC-910) under N 2 atmosphere with the heating rate of 20 K min -1 . AI203 powder is used as Cp calibrant and its standard data are taken from
[14].
3. RESULTS AND DISCUSSION 2. EXPERIMENTAL METHODS Bulk glasses of Ge20Ses0_xBix (0 < x < 12) are prepared in steps of x = 2 at.% by quenching the melt. The required amount of high purity elements of
Figure 1 shows the variation of Cp at 323 K as a function of Bi composition. It is clear from Fig. 1 that there is a subtle change in Ce between 6 and 8 at% of Bi and further addition of Bi has little effect. The change in Cp around Tgl for different compositions
251
252
H E A T CAPACITY M E A S U R E M E N T S ON Ge20Se80_~Bi~ GLASSES
_
38
\ \\
37
\\
36
Vol. 88, No. 3
\
.4 I" •
"o. &Cp
44
.44
35 "o
.4C
34
40!
~ 33 36
~ 32 l
o-31
\
32 32
3C
(a)
,
,
360
,
4;0
4X0
410
'
510
'
5;0
' 600
T c r n p e r t a t u r e (K)
29
\X 44
28
44I
I
I
I
I
I
2
4
6
8
10
12
B i ot.
*/.
~
.
"
-40 ~"
~
v -36
Fig. 1. Specific heat, Ce, at 323 K as a function of the composition x. The dotted line is a merely smooth connection of experimental points.
~
36-
~
AC
'~
-
~4
-32 ½ -
32-
are depicted in Fig. 2(a) and 2(b). Initially at x = 2 at.% it shows the broad variation in Cp at Tgl, with increase in x it becomes sharp and is more prominent at x > 8 at.%. The jump in the specific heat ACp = - ( C g - Ctp), where Cg and Cpl are the specific heats of glass and super cooled liquid respectively, are calculated as indicated in Fig. 2(a) and 2(b). Figure 3 shows ACp variation as a function of composition. The ACp shows maxima at 8 at.% Bi. The mechanism responsible for the conduction change in these glasses has become the subject of persisting controversy, and hence a matter of scrutiny. So far various structural models have been put forward. These could conveniently be viewed as two groups, normally the electronic [15] and the structural inhomogeneity [9, 16] groups. The electronic model considers the existence of Bi in various defect configurations, which perturbs the VAP and hence the unpinning of EF. On an other hand, the structural inhomogeneity model assumes these glasses are phase separated into n-type tetradymite Bi2Se3 clusters (with diameter less than 40 A [8]) in the rest of matrix at microscopic level. Hence at the conductivity percolation [16], or the mechanical [8] threshold, these glasses show the conduction change. Measurements on the double stage glass transition and the double stage crystallization in these glasses support the phase separations at the microscopic level [9].
-20
2824
( b ) 360
~ ,
,
400
,
,
440
,
,
480
-24 ,
5)0
5;0
000
Temperature (K)
Fig. 2. The change temperature, Tf~, for (a) O, x = 2; z~, x = (b) ©, x = 8; A, x =
in C. at the glass transition e .. the different compositions: 4; and [~, x = 6 at.% of Bi. 10; and I-3, x = 12 at.% of Bi.
The basic features of the mechanical constraint theory has been discussed in detail by Phillips and Thorpe [17]. Subsequently, it has been modified to the hindrance induced by the medium range order in certain glasses [18]. The Phillips-Thorpe model predicts the critical composition in IVx-VIl00-x binary glasses around xc = 20 at.%. This has been observed in SixTel00_x and in number of other glasses [19]. However, the GexSel00-x glasses shows it at slightly higher composition xc = 23 at.% [20]. This excess amount required is attributed to the fact that not all the bond bending constraints are effective in hindering intercluster motion. At this stage, it is important to mention the phenomena of intercluster induced plastic deformation in nano-crystalline materials. This has been observed and well studied in nano-crystallite and non-crystalline Si [21]. It has been noticed that if the cluster diameter is reduced below a certain value, the non-hydrostatic intercluster stress gives the surface
HEAT CAPACITY M E A S U R E M E N T S ON Ge20Ses0_xBix GLASSES
Vol. 88, No. 3
minimum fragility [23]. Our measurements of Cp at 323 K shows the drastic changes between x = 6 and 8 at.% and the ACp also goes to a maximum at x ----8 at.%, which is closer to the mechanical threshold xc. Thus, these observed features could be attributed to the effects of rigidity percolation in these glasses, although a quantitative calculation is not possible at the present stage.
18
/
I
i/
i/
-~" lz IC3~
i
i /
253
J
E
'~
1
Acknowledgements - - The authors wish to acknowledge Mr R.S. Vaidyanathan for his support to record the measurements. We are also grateful to Mr N. Ramesh Rao and G.N. Raghunath for their help during the experimental work, the IUC-DAE, Govt. of India, and the Department of Science and Technology for financial assistance.
\ xx
I 2
I /.
I 6
I 8
I 10
I 12
Bi a t %
Fig. 3. Variation of ACp as a function of composition x. The dotted line is a merely smooth connection of the experimental points. steps and the stacking faults in Si [8]. Phillips combined this phemomena with the rigidity percolation to explain the conduction inversion in these glasses. According to this model, these glasses can be represented as BixSe80_xGe20 =Bi2zSe3z+Ge20Se80_Sz. At lower Bi percentage, the localized unconstructed Se- defects are present at the Bi2Se3 tetradymite surfaces. These clusters are uniformly dispersed in the rest of GeSe2 and Se flexible chains, which makes these materials p-type. With the increase in x the mechanical stiffness of the combined medium increases. At x = 6 at.%, Bi6Se74Ge20 = Bi6Se 9 + Ge20Se65 and Ge20Se65~ GeySel00_y with y = 23.5 at.% which is very close to the mechanical threshold x c = 23 at.% in these glasses [20]. Thus, for x > 6 at.% the mechanical misfit between these clusters is high, which leads to the plastic deformation of tetradymite clusters; as a result the Se- density increases. The high density of these Se- defects evolve in percolative manner at mechanical threshold and produce the n-type conduction in these glasses. The Cp measurements around the glass transition have been carried out on a number of glasses, such as AsxSel0o_x [22], G e - A s - S e [23] glasses. The ACp is taken as an experimental parameter to check the rigidity percolation in these glasses [23]. The As-Se glasses show a maxima in ACp at mechanical or chemical threshold, whereas G e - A s - S e glasses show a minima at mechanical threshold. The minima in ACp at the critical composition is attributed to the
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
M. Kastner, Phys. Rev. Lett. 28, 355 (1972). P.W. Anderson, Phys. Rev. Lett. 34, 953 (1975). N.F. Mott, E.A. Davis & R.A. Street, Phil. Mag. 32, 961 (1975). M.Kastner, D. Adler & H. Fritzsche, Phys. Rev. Lett. 37, 1504 (1976). S.R. Ovshinsky, "Chemical Modification of Amorphous Chalcogenides", Proc. 7th Int. Conf. Amorph. and Liq. Semiconduct. Edinburgh, Scotland, p. 519 (1977). N. Tohge, T. Minami, Y. Yamamoto & M. Tanaka, J. Appl. Phys. 51, 1048 (1980). N. Tohge, H. Matsuo & T. Minami, J. NonCryst. Solids 95&96, 809 (1987). J.C. Phillips, Phys. Rev. B36, 4265 (1987). M.K. Rabinal, K.S. Sangunni & E.S.R. Gopal, Communication to Phys. Rev. B. S.R. Elliott & A.T. Steel, Phys. Rev. Lett. 57, 1316 (1986). K.L. Bhatia, G. Parthasarathy, E.S.R. Gopal & A.K. Sharma, Solid State Commun. 51, 739 (1984). J.P. Larmagnac, J. Grenet & P. Michon, J. Non-Cryst. Solids 45, 157 (1981). M.J. O'Neill, Anal. Chem. 38, 1331 (1966). G.T. Furukawa, T.D. Doaglas, R.E. McCoskey & D.C. Ginnings, J. Res. Nat. Bur. Std. 57, 67 (1956). S.R. Elliot & A.T. Steel, Phys C." Solid State Phys. 2t), 4335 (1987). L. Tichy, H. Ticha, A. Triska & P. Nagels, Solid State Commun. 53, 399 (1985). J.C. Phillips & M.F. Thorpe, Solid State Commun. 53, 699 (1985). K. Tanaka, Phys. Rev. B39, 1270 (1989). S. Asokan & E.S.R. Gopal, Rev. Solid State Sci. 3, 273 (1989). W. Bresser, P. Boolchand & Suranyi, Phys. Rev. Lett. 56, 2493 (1986).
254 21. 22.
HEAT CAPACITY MEASUREMENTS ON Ge20Seso_xBix GLASSES S. Verprek, Z. Iqbal & F.A. Sarott, Phil. Mag. B45, 137 (1982). R. Blachnik & A. Hoppe, J. Non-Cryst. Solids 34, 191 (1979).
23.
Vol. 88, No. 3
M. Tasumisago, B.L. Halfpap, J.L. Green, S.M. Lindsay & C.A. Angell, Phys. Rev. Lett. 64, 1549 (1990).
0038-1098/93 $6.00 + .00 Pergamon Press Ltd
HEAT CAPACITY MEASUREMENTS ON Ge20Ses0_xBix GLASSES M.K. Rabinal, K.S. Sangunni, E.S.R. Gopal* and S.V. Subramanyam Dept. of Physics, Indian Institute of Science, Bangalore 560 012, India * National Physical Laboratory, New Delhi 110 012, India
(Received 5 May 1993 by C.N.R. Rao) The specific heat Cp of glassy Ge20Ses0_xBix (0 < x < 12) samples is investigated. The Cp at 323K and the ACp at glass transition temperature T-l show anomalous features around x = 8 at.%, where p-n conductio~n type inversion also take place. These features are discussed in the light of Phillips model of phase separation in these glasses at the microscopic level. 1. INTRODUCTION SEMICONDUCTORS containing chalcogen atoms as major constituents are named as the lone-pair semiconductors [1]. Anderson's idea of spin pairing on the same site with negative correlation energy is realized in these chalcogenide glasses [2]. Mott et al. [3] and Kastner et al. [4], using alternate approaches, discussed the formation of defects in these glasses which are termed as Valence Alteration Pair (VAP). One of the important feature of these defects is that the Fermi-level is pinned effectively in the middle of the gap. For a long time it was believed that these materials cannot be doped. However, Ovshinsky et al. achieved the break-through and succeeded in doping using non-equilibrium techniques [5]. The main limitation of these methods is that they are useful only in amorphous thin films. Investigations that Bi and Pb atoms modify certain bulk chalcogenide glasses invoked a special interest in the scientific community [6, 7]. The Ge20Sea0_xBix glasses show the conduction inversion around 7-8 at.% of Bi, which is quite closer to the mechanical threshold in these glasses [8]. To unravel this phenomena these glasses are studied extensively using various sophisticated experimental techniques, and they show subtle changes around this critical value [6, 7, 9-11]. The purpose of the present communication is to check such an unusual feature in the specific heat at constant pressure (Cp).
Ge, Se, and Bi (with 5N purity) are sealed in quartz ampules of 8mm i.d. evacuated upto 5 × 10-5 Torr. These ampules are loaded into an electric furnace and the temperature of the furnace is raised slowly to 1273 K. The molten alloy was held at this temperature for about 36h and good homogenization is achieved by rotating the ampule externally. The final homogenized melt is quenched in ice-water+ NaOH solution. The amorphous nature of these samples is confirmed by the X-ray diffraction. It has been well established that the annealing of semiconducting glasses affect the thermodynamical properties to some extent [12]. Therefore, to have identical thermal history, all the samples are heated just below their respective glass transition temperatures and quickly cooled back to the room temperature. These glasses show the phenomena of double glass transition and double crystallization. The energy involved at the Tg2 is very small compared to the TgI [9], hence the Ce measurements are carried out only at Tgl. The Cp is measured on these samples using the constant heating rate method [13] with a fully automated Du-Pont Differential Scanning Calorimeter (DSC-910) under N 2 atmosphere with the heating rate of 20 K min -1 . AI203 powder is used as Cp calibrant and its standard data are taken from
[14].
3. RESULTS AND DISCUSSION 2. EXPERIMENTAL METHODS Bulk glasses of Ge20Ses0_xBix (0 < x < 12) are prepared in steps of x = 2 at.% by quenching the melt. The required amount of high purity elements of
Figure 1 shows the variation of Cp at 323 K as a function of Bi composition. It is clear from Fig. 1 that there is a subtle change in Ce between 6 and 8 at% of Bi and further addition of Bi has little effect. The change in Cp around Tgl for different compositions
251
252
H E A T CAPACITY M E A S U R E M E N T S ON Ge20Se80_~Bi~ GLASSES
_
38
\ \\
37
\\
36
Vol. 88, No. 3
\
.4 I" •
"o. &Cp
44
.44
35 "o
.4C
34
40!
~ 33 36
~ 32 l
o-31
\
32 32
3C
(a)
,
,
360
,
4;0
4X0
410
'
510
'
5;0
' 600
T c r n p e r t a t u r e (K)
29
\X 44
28
44I
I
I
I
I
I
2
4
6
8
10
12
B i ot.
*/.
~
.
"
-40 ~"
~
v -36
Fig. 1. Specific heat, Ce, at 323 K as a function of the composition x. The dotted line is a merely smooth connection of experimental points.
~
36-
~
AC
'~
-
~4
-32 ½ -
32-
are depicted in Fig. 2(a) and 2(b). Initially at x = 2 at.% it shows the broad variation in Cp at Tgl, with increase in x it becomes sharp and is more prominent at x > 8 at.%. The jump in the specific heat ACp = - ( C g - Ctp), where Cg and Cpl are the specific heats of glass and super cooled liquid respectively, are calculated as indicated in Fig. 2(a) and 2(b). Figure 3 shows ACp variation as a function of composition. The ACp shows maxima at 8 at.% Bi. The mechanism responsible for the conduction change in these glasses has become the subject of persisting controversy, and hence a matter of scrutiny. So far various structural models have been put forward. These could conveniently be viewed as two groups, normally the electronic [15] and the structural inhomogeneity [9, 16] groups. The electronic model considers the existence of Bi in various defect configurations, which perturbs the VAP and hence the unpinning of EF. On an other hand, the structural inhomogeneity model assumes these glasses are phase separated into n-type tetradymite Bi2Se3 clusters (with diameter less than 40 A [8]) in the rest of matrix at microscopic level. Hence at the conductivity percolation [16], or the mechanical [8] threshold, these glasses show the conduction change. Measurements on the double stage glass transition and the double stage crystallization in these glasses support the phase separations at the microscopic level [9].
-20
2824
( b ) 360
~ ,
,
400
,
,
440
,
,
480
-24 ,
5)0
5;0
000
Temperature (K)
Fig. 2. The change temperature, Tf~, for (a) O, x = 2; z~, x = (b) ©, x = 8; A, x =
in C. at the glass transition e .. the different compositions: 4; and [~, x = 6 at.% of Bi. 10; and I-3, x = 12 at.% of Bi.
The basic features of the mechanical constraint theory has been discussed in detail by Phillips and Thorpe [17]. Subsequently, it has been modified to the hindrance induced by the medium range order in certain glasses [18]. The Phillips-Thorpe model predicts the critical composition in IVx-VIl00-x binary glasses around xc = 20 at.%. This has been observed in SixTel00_x and in number of other glasses [19]. However, the GexSel00-x glasses shows it at slightly higher composition xc = 23 at.% [20]. This excess amount required is attributed to the fact that not all the bond bending constraints are effective in hindering intercluster motion. At this stage, it is important to mention the phenomena of intercluster induced plastic deformation in nano-crystalline materials. This has been observed and well studied in nano-crystallite and non-crystalline Si [21]. It has been noticed that if the cluster diameter is reduced below a certain value, the non-hydrostatic intercluster stress gives the surface
HEAT CAPACITY M E A S U R E M E N T S ON Ge20Ses0_xBix GLASSES
Vol. 88, No. 3
minimum fragility [23]. Our measurements of Cp at 323 K shows the drastic changes between x = 6 and 8 at.% and the ACp also goes to a maximum at x ----8 at.%, which is closer to the mechanical threshold xc. Thus, these observed features could be attributed to the effects of rigidity percolation in these glasses, although a quantitative calculation is not possible at the present stage.
18
/
I
i/
i/
-~" lz IC3~
i
i /
253
J
E
'~
1
Acknowledgements - - The authors wish to acknowledge Mr R.S. Vaidyanathan for his support to record the measurements. We are also grateful to Mr N. Ramesh Rao and G.N. Raghunath for their help during the experimental work, the IUC-DAE, Govt. of India, and the Department of Science and Technology for financial assistance.
\ xx
I 2
I /.
I 6
I 8
I 10
I 12
Bi a t %
Fig. 3. Variation of ACp as a function of composition x. The dotted line is a merely smooth connection of the experimental points. steps and the stacking faults in Si [8]. Phillips combined this phemomena with the rigidity percolation to explain the conduction inversion in these glasses. According to this model, these glasses can be represented as BixSe80_xGe20 =Bi2zSe3z+Ge20Se80_Sz. At lower Bi percentage, the localized unconstructed Se- defects are present at the Bi2Se3 tetradymite surfaces. These clusters are uniformly dispersed in the rest of GeSe2 and Se flexible chains, which makes these materials p-type. With the increase in x the mechanical stiffness of the combined medium increases. At x = 6 at.%, Bi6Se74Ge20 = Bi6Se 9 + Ge20Se65 and Ge20Se65~ GeySel00_y with y = 23.5 at.% which is very close to the mechanical threshold x c = 23 at.% in these glasses [20]. Thus, for x > 6 at.% the mechanical misfit between these clusters is high, which leads to the plastic deformation of tetradymite clusters; as a result the Se- density increases. The high density of these Se- defects evolve in percolative manner at mechanical threshold and produce the n-type conduction in these glasses. The Cp measurements around the glass transition have been carried out on a number of glasses, such as AsxSel0o_x [22], G e - A s - S e [23] glasses. The ACp is taken as an experimental parameter to check the rigidity percolation in these glasses [23]. The As-Se glasses show a maxima in ACp at mechanical or chemical threshold, whereas G e - A s - S e glasses show a minima at mechanical threshold. The minima in ACp at the critical composition is attributed to the
REFERENCES 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
M. Kastner, Phys. Rev. Lett. 28, 355 (1972). P.W. Anderson, Phys. Rev. Lett. 34, 953 (1975). N.F. Mott, E.A. Davis & R.A. Street, Phil. Mag. 32, 961 (1975). M.Kastner, D. Adler & H. Fritzsche, Phys. Rev. Lett. 37, 1504 (1976). S.R. Ovshinsky, "Chemical Modification of Amorphous Chalcogenides", Proc. 7th Int. Conf. Amorph. and Liq. Semiconduct. Edinburgh, Scotland, p. 519 (1977). N. Tohge, T. Minami, Y. Yamamoto & M. Tanaka, J. Appl. Phys. 51, 1048 (1980). N. Tohge, H. Matsuo & T. Minami, J. NonCryst. Solids 95&96, 809 (1987). J.C. Phillips, Phys. Rev. B36, 4265 (1987). M.K. Rabinal, K.S. Sangunni & E.S.R. Gopal, Communication to Phys. Rev. B. S.R. Elliott & A.T. Steel, Phys. Rev. Lett. 57, 1316 (1986). K.L. Bhatia, G. Parthasarathy, E.S.R. Gopal & A.K. Sharma, Solid State Commun. 51, 739 (1984). J.P. Larmagnac, J. Grenet & P. Michon, J. Non-Cryst. Solids 45, 157 (1981). M.J. O'Neill, Anal. Chem. 38, 1331 (1966). G.T. Furukawa, T.D. Doaglas, R.E. McCoskey & D.C. Ginnings, J. Res. Nat. Bur. Std. 57, 67 (1956). S.R. Elliot & A.T. Steel, Phys C." Solid State Phys. 2t), 4335 (1987). L. Tichy, H. Ticha, A. Triska & P. Nagels, Solid State Commun. 53, 399 (1985). J.C. Phillips & M.F. Thorpe, Solid State Commun. 53, 699 (1985). K. Tanaka, Phys. Rev. B39, 1270 (1989). S. Asokan & E.S.R. Gopal, Rev. Solid State Sci. 3, 273 (1989). W. Bresser, P. Boolchand & Suranyi, Phys. Rev. Lett. 56, 2493 (1986).
254 21. 22.
HEAT CAPACITY MEASUREMENTS ON Ge20Seso_xBix GLASSES S. Verprek, Z. Iqbal & F.A. Sarott, Phil. Mag. B45, 137 (1982). R. Blachnik & A. Hoppe, J. Non-Cryst. Solids 34, 191 (1979).
23.
Vol. 88, No. 3
M. Tasumisago, B.L. Halfpap, J.L. Green, S.M. Lindsay & C.A. Angell, Phys. Rev. Lett. 64, 1549 (1990).