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The effect of charged additives on the conductivity and the thermopower in liquid selenium
Journal of Non-Crystalline Solids 59 & 60 (1983) 1083-1086 North-Holland PublishingCompany
1083
THE EFFECT OF CHARGEDADDITIVES ON THE CONDUCTIVITY AND THE THERMOPOWER IN
LIQUID SELENIUM M. YAO, S. HOSOKAWAand H. ENDO
Department of Physics, Kyoto University, Kyoto 606, Japan The simultaneous measurements of conductivoty ~ and thermopower S for l i q u i d Se and i t s d i l u t e mixtures containing Na, T1 and C1 have been performed in the temperature and pressure range up to 1500°C and 2200 bar. The addition of Na, T1 and C1 give r i s e to a substantial increase in ~ at low temperatures. The value of S of l i q u i d Se is large p o s i t i v e at low temperature. The sign of S is changed to be negative by addition of Na and TI. By the addition of Cl the sign of S remains p o s i t i v e and the value increases. The e f f e c t of charged additives on the transport properties of l i q u i d Se is discussed in connection with the change of the bonding configuration. I . INTRODUCTION Above the melting point Tm the twofold coordination in helical chains of c r y s t a l l i n e Se is l a r g e l y preserved and hence l i q u i d Se near Tm e x h i b i t s s i m i l a r semiconducting behaviour to that of c r y s t a l l i n e Se.
Studies of
magnetic properties I ' 2 indicate that as the temperature is raised the chains are shortened successively and danglingnbond states C~ with an isolated spin are produced.
When the number of the Cy states is s u f f i c i e n t l y large, some of
them are l i k e l y to c o r r e l a t e with twofold states C~ in the neighbouring chain, r e s u l t i n g in the formation of threefold states C~ of rather l o c a l i z e d character.
The study of the effects of the charged impurities such as Na, T1
and C1 on l i q u i d Se in a wide temperature and pressure range may provide a special opportunity to investigate both the defect states 3 and transport properties 4, 5 in l i q u i d semiconductor.
We have made the simultaneous
measurements of the e l e c t r i c a l c o n d u c t i v i t y ~ and the thermopower S for l i q u i d Se containing Na, T1 and C1 impurities in the temperature and pressue range up to 1500°C and 2200 bar. 2. EXPERIMENTAL PROCEDURE Simultaneous measurements of o and S were carried out by using the i n t e r n a l l y heated autoclave which was pressurized with Ar gas by a gas compressor.
The cell was made of the alumina tube with graphite rods as the
electrodes.
The sample containing T1 impurity was prepared by melting the
weighed amounts of Se and T1 in the evacuated qualtz tube.
For the sample
containing Na, Na2Se and Se were weighed in a glove box and t h e i r mixture was 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
M. Yao et al. / The effect o f charged additives
1084
I (°c~
melted in the evacuated quartz tube. I ,
The sample c a n t a i n i n g C1 was prepared by mixing Se and SeCI 4.
i
The p u r i t i e s
I
pureSo
,02i~
i
of o r i g i n a l m a t e r i a l s were 99.99% f o r
2000b+r
iOI
Se and T I , and 99% f o r Na2Se and SeCI4. ~
IoC~bar
3. RESULTS AND DISCUSSION b =
Figure 1 shows ~ at I00, I000 and 2000 bar f o r l i q u i d Se as a f u n c t i o n of r e c i p r o c a l temperature.
\ \\
As seen in
Fig. 1 a increases with temperature and pressure.
\
The temperature v a r i a t i o n s
of a at d i f f e r e n t
+°3I ~'6
n'8
pressure are q u i t e
s i m i l a r to each other.
The value of a
The log a vs I / T curve is
not a s t r a i g h t l i n e .
The slope of the
curve at 2000 bar e x h i b i t s a maximum
600
L
The
present r e s u l t s o f ~ are f a i r l y
,,
T (°C) 1400 1200 1000 800
between I000 and IIO0°C, and at I000 bar between llO0 and 1200°C.
' llo IO),T [~ t)
FIGURE 1 Electrical conductivity at I00, I000 and 2000 bar f o r l i q u i d Se as a f u n c t i o n of r e c i p r o c a l temperature.
becomes 380 Ohm-I cm- l at 1400°C and 2000 bar.
'
60<
good
p~r~s+
agreement with t h a t of Fischer and Schmutzler 6 .
.~ob~ 7
/!
The r e s u l t s of S f o r l i q u i d Se are shown as a f u n c t i o n o f r e c i p r o c a l temperature in Fig.2.
The value of S is
large p o s i t i v e at low temperatures and decreases c o n s i d e r a b l y with increasing temperature and pressure.
O~ 06
t i o n s of S a t d i f f e r e n t
pressures are
s i m i l a r to each other.
The present
¢18 103/T
The v a r i a -
I()
~ -PZ
(K 1)
FIGURE 2 Thermopower S at I00, I000 and 2000 bar f o r l i q u i d Se as a f u n c t i o n of r e c i p r o c a l temperature.
r e s u l t is in e x c e l l e n t agreement with t h a t by Fischer and Schmutzler 6.
Figure 3 shows ~ of l i q u i d Se c o n t a i n i n g 0.2 at.% Na and 0.5 at.% T1 at I00 and 1000 bar as a f u n c t i o n of r e c i p r o c a l l i q u i d Se are also shown f o r comparison. to a s u b s t a n t i a l temperatures.
temperature .
The r e s u l t s f o r pure
The a d d i t i o n of Na and T1 gives r i s e
increase in a and reduction of the slope at low
The e f f e c t of the a d d i t i o n of i m p u r i t y on a becomes less pro-
nounced a t elevated temperatures.
The rate of increase in a at 600°C f o r Na is
about twice as large as t h a t f o r T l .
I t is noticed t h a t the slopes of a and S
M. Yao et al. / The ef]ect o f charged additives
1085
vs I / T curves are nearly the same f o r the l i q u i d Se containing Na and TI. Figure 4 shows o of l i q u i d Se containing 0.5 at.% C1 a t I00 and I000 bar as a function of reciprocal temperature. shown in Fig. 4.
The results f o r pure l i q u i d Se are also
The a d d i t i o n of C1 gives r i s e to increase in o l i k e Na and TI.
Figure 5 shows S o f l i q u i d Se containing 0.2 at.% Na, 0.5 at.% T1 and 0.5 at.% C1 as a function of reciprocal temperature at I000 bar.
The thermopowers
of the l i q u i d Se containing Na and T1 i m p u r i t i e s are large negative at low temperatures and become p o s i t i v e with increasing temperature, while S o f the l i q u i d Se containing C1 i m p u r i t y is large p o s i t i v e at low temperatures and decreases with increasing temperature.
At elevated temperatures S o f the
l i q u i d Se containing charged i m p u r i t i e s are close to t h a t of pure l i q u i d Se. I f S is taken as a guide, l i q u i d Se shows p-type behavior.
The t r a n s p o r t is
considered to be mainly due to the holes a c t i v a t e d to the m o b i l i t y edge in the valence band.
The observed change in the sign of S by the a d d i t i o n o f Na and
T1 implies t h a t the conduction due to the electrons overcomes the conduction due to the t h e r m a | l y a c t i v a t e d holes.
The e f f e c t of doped Na on S is more
prominent than t h a t o f doped T1 as shown in Fig. 3.
We t h i n k t h a t the e l e c t r o -
n e g a t i v i t y o f the dopant should be an important f a c t o r which determines the transport properties. T (°C)
i 0 2 a t %Na o Iooo bar
I
", 100 bar
I
~ot
o Sate. Th
e loco b~r
I
100o J~ar • 1o0 bar
\
i
\
I
• lO0 bar
"
o
i 1 !
i ,d~!
,o-! pureSe
OOba r--
-
Ii
!
"\
S
~0o0 bar
I
. - "%1
p.r~ e ~00 bar
!
:
~3C 103IT
I
(K "1 )
FIGURE 3 E l e c t r i c a l c o n d u c t i v i t y a of l i q u i d Se containing 0.2 at.% Na and 0.5 at.% T1 at !00 and I000 bar as a f u n c t i o n of reciprocal temperature. The r e s u l t s f o r pure l i q u i d Se are also shown f o r comparison, which are denoted by dotted lines.
FIGURE 4 E l e c t r i c a l c o n d u c t i v i t y ~ of l i q u i d Se containing 0.5 at.% C1 at I00 and I000 bar as a function of r e c i p r o c a l temperature. The dotted l i n e s show the r e s u l t s f o r pure l i q u i d Se.
1086
M.
Yao et al. / The effect o f charged additives T (~C)
When Na or T1 is added to l i q u i d Se, the Na or T1 atom gives up an electron
F
bouring Se chain, which produces C2 center with bond d i s t o r t i o n . center acts as a donor.
/
6~x>.
to the antibonding state of the neigh-
400~
/
05at %ct// ~ pureSe i
/
The C7
F
We suppose
that the donor level l i e s near the o[
conduction band since the sign of S of
'~ t -
i
l i q u i d Se changes to be negative and
-2oo.i
the slope of o at low temperatures
\~\
0
-~ooI
TI.
!
2at%Na \ \
T
becomes small by the a d d i t i o n of Na or
4
~\ \
p : Iooo bar
L . . . . .
'\A_
•
\\
.
'~'g
I f C1 is added, C1 absorb an electron from the lone-pair state of Se s i t e which produces C* 2 center changing the helicity.
The C~ center acts as an
acceptor, of which level l i e s near the valence band. I t is expected that the deformed Se + chain around C2 or C2 center is easily broken by raising temperature.
FIGURE 5 Thermopower S of l i q u i d Se containing 0.2 at.% Na, 0.5 at.% T1 and 0.5 at.% Cl at lO00 bar as a function of reciprocal temperature. The r e s u l t f o r pure l i q u i d Se are also shown by dotted line.
This is consitent with the experimental evidence
that the v i s c o s i t y of l i q u i d Se shows a substantial decrease by the addition of + charged impurities 7. At high temperature the number of the C2 or C2 center decreases substantialy with bond breaking and the effects of the charged additives on ~ and S become less pronounced. ACKNOWLEDGEMENT The authors is grateful to Dr. K. Tamura and Professor H. Fukutome for valuable discussions through the course of the present work. REFERENCES I) W. Freyland and M. Cutler, J. C. S. Faraday I I , 76 (1980) 756. 2) M. Misonou and H. Endo, J. Phys. Soc. Jpn. 51 (1982) 2285. 3) N. F. Mott, Philos, Mag. 34 (1976) II01. 4) R. B. P e t t i t and W. J. Camp, Phys. Rev. L e t t . 32 (1974) 369. 5) C. van der Marel and W. van der Lugt, Z. Naturforsch. 34a (1979) 832. 6) R. Fischer and R. W. Schmutzler, The Physics of Selenium and Tellurium, eds. E. Herlach and P. Grosse (Springer Verlag, 1979) p.225. 7) J. C. Perron, J. Rabit and J. F. Riallands, Philos. Mag. B46 (1982) 321.
1083
THE EFFECT OF CHARGEDADDITIVES ON THE CONDUCTIVITY AND THE THERMOPOWER IN
LIQUID SELENIUM M. YAO, S. HOSOKAWAand H. ENDO
Department of Physics, Kyoto University, Kyoto 606, Japan The simultaneous measurements of conductivoty ~ and thermopower S for l i q u i d Se and i t s d i l u t e mixtures containing Na, T1 and C1 have been performed in the temperature and pressure range up to 1500°C and 2200 bar. The addition of Na, T1 and C1 give r i s e to a substantial increase in ~ at low temperatures. The value of S of l i q u i d Se is large p o s i t i v e at low temperature. The sign of S is changed to be negative by addition of Na and TI. By the addition of Cl the sign of S remains p o s i t i v e and the value increases. The e f f e c t of charged additives on the transport properties of l i q u i d Se is discussed in connection with the change of the bonding configuration. I . INTRODUCTION Above the melting point Tm the twofold coordination in helical chains of c r y s t a l l i n e Se is l a r g e l y preserved and hence l i q u i d Se near Tm e x h i b i t s s i m i l a r semiconducting behaviour to that of c r y s t a l l i n e Se.
Studies of
magnetic properties I ' 2 indicate that as the temperature is raised the chains are shortened successively and danglingnbond states C~ with an isolated spin are produced.
When the number of the Cy states is s u f f i c i e n t l y large, some of
them are l i k e l y to c o r r e l a t e with twofold states C~ in the neighbouring chain, r e s u l t i n g in the formation of threefold states C~ of rather l o c a l i z e d character.
The study of the effects of the charged impurities such as Na, T1
and C1 on l i q u i d Se in a wide temperature and pressure range may provide a special opportunity to investigate both the defect states 3 and transport properties 4, 5 in l i q u i d semiconductor.
We have made the simultaneous
measurements of the e l e c t r i c a l c o n d u c t i v i t y ~ and the thermopower S for l i q u i d Se containing Na, T1 and C1 impurities in the temperature and pressue range up to 1500°C and 2200 bar. 2. EXPERIMENTAL PROCEDURE Simultaneous measurements of o and S were carried out by using the i n t e r n a l l y heated autoclave which was pressurized with Ar gas by a gas compressor.
The cell was made of the alumina tube with graphite rods as the
electrodes.
The sample containing T1 impurity was prepared by melting the
weighed amounts of Se and T1 in the evacuated qualtz tube.
For the sample
containing Na, Na2Se and Se were weighed in a glove box and t h e i r mixture was 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
M. Yao et al. / The effect o f charged additives
1084
I (°c~
melted in the evacuated quartz tube. I ,
The sample c a n t a i n i n g C1 was prepared by mixing Se and SeCI 4.
i
The p u r i t i e s
I
pureSo
,02i~
i
of o r i g i n a l m a t e r i a l s were 99.99% f o r
2000b+r
iOI
Se and T I , and 99% f o r Na2Se and SeCI4. ~
IoC~bar
3. RESULTS AND DISCUSSION b =
Figure 1 shows ~ at I00, I000 and 2000 bar f o r l i q u i d Se as a f u n c t i o n of r e c i p r o c a l temperature.
\ \\
As seen in
Fig. 1 a increases with temperature and pressure.
\
The temperature v a r i a t i o n s
of a at d i f f e r e n t
+°3I ~'6
n'8
pressure are q u i t e
s i m i l a r to each other.
The value of a
The log a vs I / T curve is
not a s t r a i g h t l i n e .
The slope of the
curve at 2000 bar e x h i b i t s a maximum
600
L
The
present r e s u l t s o f ~ are f a i r l y
,,
T (°C) 1400 1200 1000 800
between I000 and IIO0°C, and at I000 bar between llO0 and 1200°C.
' llo IO),T [~ t)
FIGURE 1 Electrical conductivity at I00, I000 and 2000 bar f o r l i q u i d Se as a f u n c t i o n of r e c i p r o c a l temperature.
becomes 380 Ohm-I cm- l at 1400°C and 2000 bar.
'
60<
good
p~r~s+
agreement with t h a t of Fischer and Schmutzler 6 .
.~ob~ 7
/!
The r e s u l t s of S f o r l i q u i d Se are shown as a f u n c t i o n o f r e c i p r o c a l temperature in Fig.2.
The value of S is
large p o s i t i v e at low temperatures and decreases c o n s i d e r a b l y with increasing temperature and pressure.
O~ 06
t i o n s of S a t d i f f e r e n t
pressures are
s i m i l a r to each other.
The present
¢18 103/T
The v a r i a -
I()
~ -PZ
(K 1)
FIGURE 2 Thermopower S at I00, I000 and 2000 bar f o r l i q u i d Se as a f u n c t i o n of r e c i p r o c a l temperature.
r e s u l t is in e x c e l l e n t agreement with t h a t by Fischer and Schmutzler 6.
Figure 3 shows ~ of l i q u i d Se c o n t a i n i n g 0.2 at.% Na and 0.5 at.% T1 at I00 and 1000 bar as a f u n c t i o n of r e c i p r o c a l l i q u i d Se are also shown f o r comparison. to a s u b s t a n t i a l temperatures.
temperature .
The r e s u l t s f o r pure
The a d d i t i o n of Na and T1 gives r i s e
increase in a and reduction of the slope at low
The e f f e c t of the a d d i t i o n of i m p u r i t y on a becomes less pro-
nounced a t elevated temperatures.
The rate of increase in a at 600°C f o r Na is
about twice as large as t h a t f o r T l .
I t is noticed t h a t the slopes of a and S
M. Yao et al. / The ef]ect o f charged additives
1085
vs I / T curves are nearly the same f o r the l i q u i d Se containing Na and TI. Figure 4 shows o of l i q u i d Se containing 0.5 at.% C1 a t I00 and I000 bar as a function of reciprocal temperature. shown in Fig. 4.
The results f o r pure l i q u i d Se are also
The a d d i t i o n of C1 gives r i s e to increase in o l i k e Na and TI.
Figure 5 shows S o f l i q u i d Se containing 0.2 at.% Na, 0.5 at.% T1 and 0.5 at.% C1 as a function of reciprocal temperature at I000 bar.
The thermopowers
of the l i q u i d Se containing Na and T1 i m p u r i t i e s are large negative at low temperatures and become p o s i t i v e with increasing temperature, while S o f the l i q u i d Se containing C1 i m p u r i t y is large p o s i t i v e at low temperatures and decreases with increasing temperature.
At elevated temperatures S o f the
l i q u i d Se containing charged i m p u r i t i e s are close to t h a t of pure l i q u i d Se. I f S is taken as a guide, l i q u i d Se shows p-type behavior.
The t r a n s p o r t is
considered to be mainly due to the holes a c t i v a t e d to the m o b i l i t y edge in the valence band.
The observed change in the sign of S by the a d d i t i o n o f Na and
T1 implies t h a t the conduction due to the electrons overcomes the conduction due to the t h e r m a | l y a c t i v a t e d holes.
The e f f e c t of doped Na on S is more
prominent than t h a t o f doped T1 as shown in Fig. 3.
We t h i n k t h a t the e l e c t r o -
n e g a t i v i t y o f the dopant should be an important f a c t o r which determines the transport properties. T (°C)
i 0 2 a t %Na o Iooo bar
I
", 100 bar
I
~ot
o Sate. Th
e loco b~r
I
100o J~ar • 1o0 bar
\
i
\
I
• lO0 bar
"
o
i 1 !
i ,d~!
,o-! pureSe
OOba r--
-
Ii
!
"\
S
~0o0 bar
I
. - "%1
p.r~ e ~00 bar
!
:
~3C 103IT
I
(K "1 )
FIGURE 3 E l e c t r i c a l c o n d u c t i v i t y a of l i q u i d Se containing 0.2 at.% Na and 0.5 at.% T1 at !00 and I000 bar as a f u n c t i o n of reciprocal temperature. The r e s u l t s f o r pure l i q u i d Se are also shown f o r comparison, which are denoted by dotted lines.
FIGURE 4 E l e c t r i c a l c o n d u c t i v i t y ~ of l i q u i d Se containing 0.5 at.% C1 at I00 and I000 bar as a function of r e c i p r o c a l temperature. The dotted l i n e s show the r e s u l t s f o r pure l i q u i d Se.
1086
M.
Yao et al. / The effect o f charged additives T (~C)
When Na or T1 is added to l i q u i d Se, the Na or T1 atom gives up an electron
F
bouring Se chain, which produces C2 center with bond d i s t o r t i o n . center acts as a donor.
/
6~x>.
to the antibonding state of the neigh-
400~
/
05at %ct// ~ pureSe i
/
The C7
F
We suppose
that the donor level l i e s near the o[
conduction band since the sign of S of
'~ t -
i
l i q u i d Se changes to be negative and
-2oo.i
the slope of o at low temperatures
\~\
0
-~ooI
TI.
!
2at%Na \ \
T
becomes small by the a d d i t i o n of Na or
4
~\ \
p : Iooo bar
L . . . . .
'\A_
•
\\
.
'~'g
I f C1 is added, C1 absorb an electron from the lone-pair state of Se s i t e which produces C* 2 center changing the helicity.
The C~ center acts as an
acceptor, of which level l i e s near the valence band. I t is expected that the deformed Se + chain around C2 or C2 center is easily broken by raising temperature.
FIGURE 5 Thermopower S of l i q u i d Se containing 0.2 at.% Na, 0.5 at.% T1 and 0.5 at.% Cl at lO00 bar as a function of reciprocal temperature. The r e s u l t f o r pure l i q u i d Se are also shown by dotted line.
This is consitent with the experimental evidence
that the v i s c o s i t y of l i q u i d Se shows a substantial decrease by the addition of + charged impurities 7. At high temperature the number of the C2 or C2 center decreases substantialy with bond breaking and the effects of the charged additives on ~ and S become less pronounced. ACKNOWLEDGEMENT The authors is grateful to Dr. K. Tamura and Professor H. Fukutome for valuable discussions through the course of the present work. REFERENCES I) W. Freyland and M. Cutler, J. C. S. Faraday I I , 76 (1980) 756. 2) M. Misonou and H. Endo, J. Phys. Soc. Jpn. 51 (1982) 2285. 3) N. F. Mott, Philos, Mag. 34 (1976) II01. 4) R. B. P e t t i t and W. J. Camp, Phys. Rev. L e t t . 32 (1974) 369. 5) C. van der Marel and W. van der Lugt, Z. Naturforsch. 34a (1979) 832. 6) R. Fischer and R. W. Schmutzler, The Physics of Selenium and Tellurium, eds. E. Herlach and P. Grosse (Springer Verlag, 1979) p.225. 7) J. C. Perron, J. Rabit and J. F. Riallands, Philos. Mag. B46 (1982) 321.