A Kumar

S K Dwivedi

V S Kushwaha

CiteSeerX

Journal of Ovonic Research Vol. 4, No. 5, October 2008, p. 96 - 99 
 
 
 
 
A COMPARATIVE STUDY OF LOCALIZED STATES IN Pb AND Ag DOPED 
Ge-Se GLASSY ALLOYS 
  
 
V. S. Kushwaha, S. K. Dwivedi, A. Kumara* 
Dept. of Physics, Kanpur Institute of Technology, Kanpur, India 
aDept. of Physics, Harcourt Butler Technological Institute, Kanpur, India 
         
                                                      
 
The present paper reports the comparative analysis of localized defect states in glassy 
Ge20Se80-xAgx (x = 0, 10, 15, 20) and (Ge20Se80)100-xPbx (x = 0, 2, 4, 6) systems. The 
density of defect states is determined by space charge limited conduction measurements. It 
is observed that in glassy Ge20Se80-xAgx the density of defect states increases with the 
concentration of Ag, becomes maximum and then decreases with Ag concentration. On the 
other hand, in glassy (Ge20Se80)100-xPbx, the density of defect states decreases with the 
concentration of Pb, becomes minimum and then increases with Pb concentration. 
 
(Received  August 25, 2008; accepted August 30, 2008) 
 
Keywords:  Amorphous Materials, Thin Films, Electronic Properties. 
 
 
1. Introduction 
 
Chalcogenide glasses are an important class of glassy materials as they behave as 
semiconductor and hence can be used for semiconducting devices similar to crystalline ones. 
However, it is found experimentally that it is difficult to have efficient doping (n type or p type) in 
these materials due to large density of defect states in the band gap.  
Though, all chalcogenide glasses could not be doped, the properties of chalcogenide 
glassy semiconductors are usually affected by the addition of impurities when third element is 
added to the binary alloys. Experimental results reported by various researchers have shown that 
the addition of impurity atoms in binary Se- Ge and Se – In systems does change the electrical 
properties of chalcogenide glasses significantly [1-3]. It has also been found that the effect of 
impurities depends strongly on the composition of the glass, the chemical nature of the impurity 
and the method of doping. Several physical properties are found to be improved by the addition of 
certain impurities.  
Only those glass compositions can be used for photoconductive and photovoltaic devices, 
which have relatively smaller density of defect states. Therefore, a search of new materials is still 
continued which have lesser and lesser density of structural defect states. The measurement of 
such defect states is also difficult in these materials because of large density of defects and 
insufficient knowledge of electronic structure of these materials. To measure such defect states, 
different methods have been used with all their advantages and their limitations. One of the most 
direct methods for the determination of defect states involves the measurements of space charge 
limited conduction. In these materials, the density of defect states reported by different groups 
[4-7].               
In our previous work [8, 9], the density of defect states is determined  by using the theory 
of space charge limited conduction in glassy alloys of Ge20Se80-xAgx (x = 0, 10, 15, 20) and 
(Ge20Se80)100-xPbx (x = 0, 2, 4, 6). The aim of present paper is to report the comparative study of 
                                                 
*Corresponding author: dr_ashok_kumar@yahoo.com 
 
 
 
97
localized defect states in these glassy systems as they show quite different composition 
dependence in these systems. 
 
2. Experimental 
 
Glassy alloys of Ge20Se80-xAgx (x = 0, 10, 15, 20) and (Ge20Se80)100-xPbx 
 (x = 0, 2, 4, 6) are prepared by quenching technique. High purity (99.999 %) materials are 
weighed according to their atomic percentages. The actual amount of the constituent elements 
taken, for the preparation of glassy alloys. The materials are then sealed in quartz ampoules (length 
~ 5 cm and internal diameter ~ 8 mm) with a vacuum ~ 10-5 Torr. The detailed procedure for 
obtaining glassy materials is discussed in our previous work [4]. Thin films of these glasses are 
prepared by vacuum evaporation technique keeping glass substrates at room temperature. The 
thickness of the films is ~ 500 nm. The co-planar structure of thin films (length ~ 1.2 cm and 
electrode separation ~ 0.12 mm) is used for the present measurements. The amorphous nature of 
thin films is confirmed by X-ray diffraction technique. 
For the measurements of high field conduction, thin film samples are mounted in a 
specially designed sample holder. A vacuum ~10-2 Torr is maintained throughout the 
measurements. A d. c. voltage (0 to 300 V) is applied across the sample and the resultant current is 
measured by a digital Pico-Ammeter. I – V characteristics are measured at various fixed 
temperatures in these films. The temperature of the films is controlled by mounting a heater inside 
the sample holder and measured by a calibrated copper- constantan thermocouple mounted very 
near to the films. 
  
3. Results and discussion 
 
It is observed that, at low electric fields (<103 V/cm), an ohmic behavior is observed. 
However, at high electric fields (~104 V/cm), a superohmic behavior is observed.  
According to the theory of space charge limited conduction, in case of a uniform 
distribution of localized states g(E) = g0, the current (I) at a particular voltage (V) is given by the 
following relation [10] 
 
                                I = (e Aµn0V/d) exp (SV)                                                     (1)  
 
where d is the electrode spacing, n0 is the density of the thermally generated charge carriers, µ is 
the mobility, e is the electronic charge, A is the area of cross section of thin films and S is given by 
 
                               S=2 εr εo /e g0 k T d2                                                                     (2) 
 
As evident from eqs. (1) and (2), in case of space charge limited conduction, the ln I/V vs 
V curves should be a straight line and slope (S) of these curves should decrease linearly with the 
increase of temperature.  
In the glassy Ge20Se80-xAgx (x = 0, 10, 15, 20) and (Ge20Se80)100-xPbx  (x = 0, 2, 4, 6) 
systems, space charge limited conduction is reported in our previous work [8, 9]. By fitting the 
experimental data to the theory of space charge limited conduction, density of defect states near 
Fermi level is calculated for the above glassy systems. The density of localized states (g0) is 
plotted against Ag concentration in Fig.1 and against Pb concentration in Fig. 2. 
 
 
 
 
98
Ge20Se80-xAgx
0.00E+00
2.00E+14
4.00E+14
6.00E+14
8.00E+14
1.00E+15
1.20E+15
1.40E+15
0 5 10 15 20 25
X %
g o
 (e
V
-1
cm
-3
)
 
Fig. 1 The density of localized states (g0) versus Ag concentration 
 
(Ge20Se80)1-xPbx
0.00E+00
2.00E+14
4.00E+14
6.00E+14
8.00E+14
0 1 2 3 4 5 6 7
 % concentration of Pb
go
 (e
V
-1
cm
-3
)
 
Fig. 2 The density of localized states (g0) versus Pb concentration 
 
In Ge20Se80-xAgx system, the density of defect states increases with the increase of Ag 
concentration upto 10 at % of Ag due to smaller electronegativity of Ag [11] as compared to 
binary Ge20Se80 alloy. However, at higher concentration of Ag, the density of defect states 
decreases with Ag concentration. This can be explained in terms of  change in structure of 
chalcogenide glasses from two-dimensional layered structure to a three-dimensional network 
arrangement due to cross-linking at a particular average co-ordination number <z> = 2.7 as 
reported by Tanaka [12]. In this glassy system, such a transition is observed at <z> = 2.6 which is 
close to the above value of <z>.  
        However, it is interesting to note that in case of (Ge20Se80)1-xPbx system, the density of 
defect states decreases on incorporation of Pb upto 4 at%, though Pb has lesser electronegativity 
than binary Ge20Se80 alloy.  The increase of the density of defect states at higher concentration of 
Pb is also observed in the above glassy system. This type of change in the composition dependence 
of density of defect states is observed at an average co-ordination number <z> = 2.4 which is 
reported in the literature [13] due to the formation of stable glass at this value of <z>.  
 
 
99
The above results indicate that the behaviour of Pb is different in binary Ge20Se80 alloy as 
compared to other additives such as Ag. The different behavior of Pb additive in binary Ge-Se 
system is also well known in the literature [2] as p to n transition is observed at a particular 
concentration of Pb in this glassy system while no such transition has been reported with Ag.    
It is now well established that the metallic additives such as Pb enters the network of 
Ge-Se system as charged species, altering the concentration of valence alteration pairs [14, 15]. 
When the concentration of charged additives exceeds that of valence alteration pairs, the 
chalcogenide glasses exhibit the p to n transition.   
 
     
4. Conclusions 
 
I–V characteristics have been studied in amorphous thin films of Ge20Se80-xAgx (x = 0, 10, 
15, 20) and (Ge20Se80)100-xPbx (x = 0, 2, 4, 6) at different temperatures. At low fields (<103 V/cm), 
an ohmic behavior is observed. However, at high fields (~ 104 V/cm), a super ohmic behavior is 
observed. In Ge20Se80-xAgx system, the density of defect states increases with the increase of Ag 
concentration upto 10 at % of Ag due to smaller electronegativity of Ag as compared to binary 
Ge20Se80 alloy. However, at higher concentration of Ag, the density of defect states decreases with 
Ag concentration. However, in case of (Ge20Se80)100-xPbx (x = 0, 2, 4, 6) system, the density of 
defect states decreases on incorporation of Pb up to 4 at%, though Pb has lesser electronegativity 
than binary Ge20Se80 alloy.  This shows that the behavior of Pb is different in binary Ge20Se80 alloy 
as compared to other additives such as Ag. 
 
 
References 
 
  [1] P. Nagel, H. Ticha, L.Tichy, A. Triska, J. Non-Cryst. Solids 59, 1015 (1983). 
  [2] N. Tohge, H. Matsuuo, T. Minami, J. Non-Cryst. Solids 95, 809(1987). 
  [3] S. Kohli, V. K.Sachdeva, R. M. Mehra, P. C. Mathur, Phy. Stat. Sol. (B) 209,389 (1998).    
  [4] M. A. Majeed Khan, M. Zulfequar, M. Husain, Physica B 366, 1 (2005).  
  [5] S. M. El-Sayed, Vacuum 65, 177 (2002). 
  [6] V. S. Kushwaha, A. Kumar, J. Mater. Sci. 42, 2712 (2007). 
  [7] S. Kumar, R. Arora, A. Kumar, Solid State Commun. 82, 725 (1992). 
  [8] V. S. Kushwaha, S. Kumar, A. Kumar, Euro. Physical J. App. Physics 35, 149 (2006) 
  [9]V. S. Kushwaha, A Kumar, J. Non-Cryst. Solids 353, 4718 (2007). 
[10] M. A. Lampert, P. Mark, Current Injection in Solids. Academic Press, New York: 1970. 
[11] http://www.capital.net/com/vcl/periodic/periodic.htm 
[12] K. Tanaka, Phys. Rev. B 39, 1270(1989). 
[13] J. C. Phillips, M. Thorpe, Constraint theory and stiffness percolation in network glasses,  
        P. M. Duxbury (eds.). Kluwer Academic Press, 155 (1999). 
[14] N. Tohge, T. Minami, Y. Yamamoto, M. Tanaka, J. Appl. Phys. 51, 1048 (1980). 
[15] K. L. Bhatia, G. Parthasarathy, A. Sharma, E. S. R. Gopal, Phys. Rev. B  38, 6342 (1988). 
 


A COMPARATIVE STUDY OF LOCALIZED STATES IN Pb AND Ag DOPED Ge-Se GLASSY ALLOYS

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=09D7CCC73FD94CE485243FDB4D1B141A?doi=10.1.1.1047.9181&rep=rep1&type=pdf

A COMPARATIVE STUDY OF LOCALIZED STATES IN Pb AND Ag DOPED Ge-Se GLASSY ALLOYS

Abstract

Similar works

Full text

Available Versions

CiteSeerX