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NONMETAL TO METAL TRANSITION IN LIQUID CHALCOGENIDES
H. Radscheit
To cite this version:
H. Radscheit. NONMETAL TO METAL TRANSITION IN LIQUID CHALCOGENIDES. Journal de
Physique Colloques, 1981, 42 (C4), pp.C5-1063-C5-1066. �10.1051/jphyscol:19814233�. �jpa-00220864�
page C4-1063
NONMETAL TO METAL TRANSITION IN LIQUID CHALCOGENIDES
H . Radscheit
Physics Department, Oregon S t a t e University, CorvaZZis, Oregon, U.S.A.
ABSTXACT.
-
From static magnetic susceptibilities it is found that a nonmetal to metal transition occurs in some d o ~ e d liquid chalcogenides. Tiith increasing doping level the transi- tion is shifted to lower temperatures. The density ds of para- nagnetic centers at the transition is constant in both of the systems investigated. The shape of the valence band just below the mobility edge E, is obtained fron X above the transition.90 structure of the valence band has been found for energies .3eV below Ec.
INTP.ODUCTI3iJ.
-
Most liquid chalcogenides exhibit semiconducting pro- perties at tenperatures below 5 5 0 ~ ~ . From transport data (1) it is known that with increasing temnerature the Fermi energy E, shifts to- wards the valence band. At temperatures high enough Ef is-in the va- lence band and a liquid metal model is appropriate. Depending on com-~osition the transition temperature T to the metallic state can be higher than the critical tem~erature \e.g. pure sulfur) or lower than the melting point (e.g. Te rich Se-Te alloys). Liquid Se and S are known to contain chain molecules derived from 2-fold covalently bonded atoms together with dangling bond atoms at chain ends. Although liquid Te is 3-fold coordinated there are reasons to believe that it is 2-fold bonded when alloyed with Se or S (2,3). Thus it is expected that in Se-Te and S-Te alloys copolymerisation occurs and the molecular struc- ture is similar to that of the pure elements. If doping creates band states above the valence band this will decrease the transition tem- nerature.
In order to get further insight into the nonmetal to metal transition static nagnetic susceptibility of (S
.
l -xrIx and (Se - 5 Te.5)1-xMx have been measured with II=Cu,Tl,As,Na or Au and 0<x<0.11. The reasons for choosing the actual conpositions in this investigation are a) the transition tem~eratures are in an experimentally convinient temnera- ture range and b) transport data in (Se.
Te. 5 ) -x'PIx are available for different doping elements :.I.EXPEBIblENTAL.
-
The static magnetic susceptibility were measured by Faraday's method. The apparatus has been described in grevious payers(2,4). The materials used were 99.99% pure. Alloys were made by sealing ap~ropriate quantities of the element into quartz capcules. The renro- ducebility of all data was better than .5-10-~cm~/mole. In some doped samples the temperature range was restricted by a liquid-liquid phase segaration which could be observed by a sudden drop in the susce~tibi- lity X. This additional in.Eormation about phase diagrams will be pub- lished elsewhere.
On leave from Universitat Eeidelberg, Gernany.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814233
C4- 1064 JOURNAL DE PHYSIQUE
FBSULTS AND DISCUSSION.- At sufficiently low temperatures the magnetic susceptibility of liquid S-Te and Se-Te alloys can be well described by a sum of a diamgnetic term
xD
and a paramagnetic contribution (2,3).The paramagnetism of the alloys is mainly due to dangling bonds,so that at temperature T:
x=xD + g 2 N, e So/k -
Eo/kT T < Tt (1
where k is Boltzmann's constant, u B
the Bohr magneton and No Avogadrors
number. E. and So are the energy and entropy required to create a para-
magnetic center.
The same equation a?plies to the systems investiga-
-
ted. In fig. 1, the results
Gio'
of fitting the low tempera--. (Se,5Te.5)1
-,Cux
transition temberature to
the metallic state. In the pure materials Tt=996 X and Tt=853 K for Se.5 T e e 5 and S.3 Te.7 respectively.
A xz.09
X X = OL + X = 01 At high temperatures X is 102-
.
x.00lower than predicted by Eq. 1 and independent of the metal concentration.
In ref. 1 it is found that the distance of the Fermi energy from the nobility edge is a linear function of 1 for Se.5 Teas. Extrapolation to E -E =o yields T=980 Ii which agrees well with Tt=996 f c K . Just above the Similar behaviour is found
for other dopants and for l$- (S.3 Te-7)1-x Mx alloys too.
This cannot be explained by another activated pro- cess but with the inci-
pient transition from semi- 10'-
transition the electrical conductivity o is within the diffusive trans- port region. Then U is proportional to the square of the density of states N (Ep) (5) :
+ X x
. +
' + X• + X X
+ + X
The magnetic susceptibility in this region is given by
conducting the metallic 08 10 12 1 L 16 181031~~~-~
behaviour as T rises. The
arrows in fig. 1 mark the Fig. 1 Paramagnetic susceptibility highest temperature Tt at of liquid (Se- Te. l CuX which Eq. 1 applies. It is
belived that T+ is the vs. inverse temperature.
When eliminating the density of states one obtains
This relation has been observed in a number of liquids. It is also reasonably satisfied in doped S e - 5 T e e s alloys (fig.2). Tt derived from Eqs. 1 and 4 agree within the error limits. Assuming C=l, from the
A = 1.9
.
I o3 ev2 atom2 (R cm) -l is obtained.Although no conductivity data are available in doped S a 3 Te.7 it is assumed that Tt derived from Eq. 1 is the transition temperature in this system too.
(N
)'l3 ay =.ZZfog S. Te which has to be compared to the Mott criterion for the transition to the me- tallic state (5) :
From fig. 1 it can be seen that doping shifts the transition to lower temperatures and increases the density of paramagnetic centers below Tt. In fig.3 the den-
(N
-
a,- = .24 (6)~ h g slight decrease of ds with increasing do- ping level is due to the decreasing dielec- tric constant.
slty of spins ds at the transition norma- lised to the number of -15-
atoms is plotted vs.
the concentration of s dopants. Eqs. 2-4 im- 2 E
plie that this is the
5
concentration of con- '$
ducting species too.
2
Together with density -2C-
data and dielectric constants taken from the literature (6,7,8) one finds
N n = 6.8
-
1020a - -30
for Se
.
~e~ a n d -25-XAlthough Eq. 6 is sa- tisfied the observed is more likely from the Anderson type. In ref. 9 it is shown that doping creates an acceptor band above the valence band. With increasing metal con- centration the accep- tor band broadens and the Ferni energy is shifted towards the valence band. Thus the effects of temperature
I 1 l
iSe.5Te.5 j.96
C u . ~ ~
X/x-XA/%' X./x
Trons~tim to metallic state,/
Tiq. 2 Yagnetic susceptibility of ( s e e s m le. 5 ) . g6Cu.04 VS.
square root of conductivity7.
e521 cm-3
I
5 10 20
N, = 1.5 ' 10 O 1 f ~ 1 Q-lI2 Cm-112 1
Fig. 3 Density of spins vs. doping level Open and filled symbols refer to Se.5 Te.5 and S a 3 T e a alloys .,
C4-1066 JOURNAL DE PHYSIQUE
and doping on Ef are very similar.
From Eq. 3 the density of states within the valence band can be deduced with the help of X. In order to yet an energy scale it is assumed that the linear relation-
ship between E -E f c
and T found below Tt T - T t I K l
is valid above T LOO 350 300 250 200 150 100 50 0 too. In fiq. 4 IJ'?E) ,
l
' I I I I I I lis plotted-versus E-Ec for the S a 5 T e - 5 system doped with Ag.
Different symbols are for different doping levels. Simi- lar results have been obtained for the other dopants.
No particular struc- ture of the valence band can be seen.
There is no effect +A
0 X
of doping on N (E)
.
I I I I I I IAs the zero point of -.300 -250 - 200 -150 -300 -050 .OOO
E-Ec[eVI
energy in fig. 4 be- longs to different
temperatures this Fig. 4. Density of states in means that the tempe- the valence band for rature dependence of (see 5 l'e. 5) l %X-
E- is neqligible. . -
~ k i s is in contrast to some solid amorphous semiconductors (10).
ACKNOWLEDGMENTS.- I wish to thank John A. Gardner for stimulating discussions. This work was supported in part by the National Science Foundation and the Deutsche Forschungsgemeinschaft. I am gratefull for the hospitality of Oregon State University.
REFERENCES
1 . Cutler M. and Fischer R., J. Non Cryst. Sol. 35&36 (1980) 1289 2. Gardner J.A. and Cutler PI., Phys. Rev. B 20 (1979) 529
3. Radscheit H. and Gardner J.A., J. Non Cryst. Sol. 35&36 (1980) 1263
4 . Ritter A.L. and Gardner J.A., Phys. Rev. B 20 (1979) 5252
5. Mott N.F. and Davis E.A., Electronic Processes in Non-Crystalline Materials, Clarendon Press (1979) Oxford
6. Ruska J., in Amorphous and Liquid Semiconductors, Stuke J. and Brenig W. eds., Taylor and Francis (1974) London
7. Fischer R., Schmutzler R.W. and Hensel F., J. Non Cryst. Sol. 35&36 (1980) 1295
8. Rosental S., 7,. Physik 66 (1 930) 653
9. Radscheit H., Fischer R. and Cutler M., J. Physique this issue 10. Beyer W., Plell H. and Overhof H., in Amorphous and Liquid Semicon-
ductors, Spear W.E. ed., Centre for Industrial Consultancy and Liaison ( 1 977) Edinburgh
