- Email: [email protected]
Conductivity and photopolarization properties of vitreous As2S3
JOURNAL OF NON-CRYSTALLINESOLIDS4 (1970) 73-77 © North-Holland Publishing Co., Amsterdam
CONDUCTIVITY AND PHOTOPOLARIZATION PROPERTIES OF VITREOUS As2S3 R. A N D R E I C H I N
Institute of Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria Investigations are described of the electrical, photoelectric and photopolarization properties of vitreous arsenic sulphide, both pure and containing silver and copper impurities. It is shown that this substance is a good photoelectric, and a model of its band structure is suggested. Vitreous A s 2 S 3 , the most highly resistant of the arsenic chalcogenides, has the properties of a semiconductor and of a dielectric at the same time. The latter properties are shown by the existence of the so-called relaxation or high-voltage polarization. The separation of these two effects could provide valuable information about the band structure and the nature of the conductivity of this vitreous substance. So far high voltage polarization and photopolarization have been studied chiefly in crystals showing ionic or electronic conductivity. In amorphous semiconductors they occur in selenium and in the arsenic chalcogenides as was discovered by Kolomiets and Lubin 1); there have been some preliminary investigations in our laboratory2). The change in intensity of the current with time is given by the relation I = Is + Ip
=
Is + Ipo e x p ( -
t/l:p),
where I s is the stabilized current remaining after the accumulation of the bulk polarization charges; Ip is the so-called polarization current whose initial value is lpo- When the potential is removed the depolarization current I d follows the relation Id = /dO e x p ( - -
t/Zd).
The bulk charge accumulated in the dielectric is Q = lpo~p. The same bulk charge is discharged during depolarization. We define the resistance of the substance either as that which determines the initial current I o = I s+ Ipo, or that which determines the current Is after polarization. We consider that I s and ps determine the properties of the substance as a semiconductor; as a measure of the dielectric properties we take the value of the accumulated 73
74
R. A N D R E I C H I N
charge Q; in some cases we take the initial current Ipo or the electromotive force of the polarization Vv = IpoRpo . All our measurements have been made on plates about 0.1 cm thick with the electrodes placed on opposite sides. The experimental results show that both the semiconducting and the initial dielectric conductivity (a s and lpo) depend on the applied electric field. The values of the specific resistance for zero external field calculated from these results are 10 and 2 x 10as f~ cm respectively. Impurities increase the conductivity. Both pure and with Ag and Cu impurities, As2S 3 shows photoconductivity, which within the limits of the concentrations investigated remains of the same order for Cu but decreases if we increase the Ag concentration. The energy gap decreases with the presence of both impurities, but much more rapidly with Cu than with Ag (fig. 1A). The high-voltage polarization shows one peculiarity: it decreases with the addition of Cu but increases with Ag (fig. 1B).
V
Ep
300"
2,q
_200-
2,Z 2,0
1oo.
x\Cu. \ o,oof
o,b~
o.'~
~ at %
o,ob¢
o.bf
o,:
B Fig. 1. (A) Variation of energy gap of vitreous As2S3 with addition of Ag and Cu impurities. (B) Variation of high-voltage polarization with addition of Ag and Cu impurities.
Q
t2
/ /o
KV/c,n
~
K Lx
~
~b
t's cp
~5 Fig. 2. (A) Dependence of accumulated charge on applied electric field for constant illumination. (B) Dependence of accumulated charge on intensity of illumination for constant electric field.
CONDUCTIVITY AND PHOTOPOLARIZATION PROPERTIES
75
Photopolarization phenomena were investigated, and the chief experimental results are as follows 5): 1) At constant intensity of illumination the quantity of accumulated charge depends linearly on the applied field up to 3000 V/cm, after which it shows a tendency towards saturation (fig. 2A). 2) At constant electric field the accumulated charge is linearly proportional to the illumination (up to the investigated intensity of the illumination 15000 Ix, fig. 2B). 3) The charge accumulated under the influence of light does not disappear immediately after cutting off the light but remains with time; (the "photoelectret state"). This can be seen in fig. 3, showing the ratio between the
0,8 0,q 0,2"
h J.b
a0
3b
6
t
Fig. 3. Ratio of depolarization and polarization charges as a function of time after cutting of light. (l) electrodes short-circuited; (2) open circuit, (3) ratio of initial depolarization and polarization currents as a function of time with electrodes short-circuited.
depolarization and the polarization charges as a function of time (in hours), keeping the sample in darkness without potential. Curve 1 is with short circuited electrodes and curve 2 with an open circuit. Curve 3 shows the dependence of the relation of the initial depolarization on the initial polarization current with short-circuited electrodes. The relaxation time of photopolarization and photodepolarization3), "c = e d / 4 n q O m o ~
decreases with the intensity of the light as shown in fig. 4. If the mobility/~ and the lifetime z are constant, we can assume that this curve expresses the dependence of the concentration of the charge carriers on the light intensity (the remaining parameters are constant for a given sample). Structurally vitreous A s 2 S 3 represents a complex of ramified chains, containing connected As and S atoms 4). In an earlier work z), we suggested that Ag penetrates into the chains, disrupting them (which results in a greater number of shorter chains), without any considerable changes in their com-
76
R. ANDREICHIN
position or structure. Cu, however, penetrates them, forming a new semiconducting vitreous compound A s - S - C u , somewhat analogous to a solid solution of AszS 3 and Cu2S or CuS. And so, proceeding from the experimental data summarized here, we assume that both phenomena observed in As2S3 - the semiconducting conductivity and the accumulation of bulk charges - are due to two different mechanisms and to two kinds of traps or states in the forbidden zone. T 10
8 6 q.
2
Fig. 4.
Decrease of the relaxation time associated with the photopolarization or photodepolarization as a function of light intensity.
The semiconducting conductivity is due to electrons in traps or in states comparatively close to the edges of the band where the conductivity mechanism can also be hopping. There is other evidence in favour of this assumption - the quicker initial decay of the photoelectret state, as well as the dependence of the conductivity of the applied electric field. The accumulation of bulk charges - photopolarization and polarization is probably due to reorientation of the chains, or to the electric charges on their being liberated by light or by the electric field and sticking at the ends of the chains according to the most probable mechanism of formation of high-voltage polarization. On the band model this would be described as due to the presence of another group of traps or states, with energies deep in the forbidden zone, where the current carriers have small mobility and can be liberated only by the influence of light or a similar agent. Andriesh et al. 6) show two groups of levels: one close to the band edge, and another about 1.0-1.3 eV deep; Kolomiets et al. 7) report on a group at about 1.15 eV. Both these results are in good agreement with the description given here.
CONDUCTIVITY AND PHOTOPOLARIZATION PROPERTIES
77
References 1) B. T. Kolomiets and V. M. Lubin, Fiz. Tverd. Tela 4 (1962) 401. 2) R. Andreichin, P. Simidchieva and M. Nikiforova, Compt. Rend. Acad. Bulgare Sci. 18 (1965) 995; 21 (1968) 753. 3) V. M. Fridkin and Yu. N. Barulin, Fiz. Tverd. Tela 5 (1963) 1523. 4) A. A. Vaipolin and E. A. Porai-Koshits, Fiz. Tverd. Tela 5 (1963) 683. 5) R. Andreichin, M. Baeva and M. Nikiforova, Fizicheskie Osnovi Electrofotografii (Vilnius, 1969). 6) B. T. Koiomiets, T. F. Mazets, Sh. M. Efendiev and A. M. Andriesh, J. Non-Crystalline Solids 4 (1970) 45. 7) B. T. Kolomiets, T. N. Mamontova and A. A. Babaev, J. Non-Crystalline Solids 4 (1970) 289.
CONDUCTIVITY AND PHOTOPOLARIZATION PROPERTIES OF VITREOUS As2S3 R. A N D R E I C H I N
Institute of Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria Investigations are described of the electrical, photoelectric and photopolarization properties of vitreous arsenic sulphide, both pure and containing silver and copper impurities. It is shown that this substance is a good photoelectric, and a model of its band structure is suggested. Vitreous A s 2 S 3 , the most highly resistant of the arsenic chalcogenides, has the properties of a semiconductor and of a dielectric at the same time. The latter properties are shown by the existence of the so-called relaxation or high-voltage polarization. The separation of these two effects could provide valuable information about the band structure and the nature of the conductivity of this vitreous substance. So far high voltage polarization and photopolarization have been studied chiefly in crystals showing ionic or electronic conductivity. In amorphous semiconductors they occur in selenium and in the arsenic chalcogenides as was discovered by Kolomiets and Lubin 1); there have been some preliminary investigations in our laboratory2). The change in intensity of the current with time is given by the relation I = Is + Ip
=
Is + Ipo e x p ( -
t/l:p),
where I s is the stabilized current remaining after the accumulation of the bulk polarization charges; Ip is the so-called polarization current whose initial value is lpo- When the potential is removed the depolarization current I d follows the relation Id = /dO e x p ( - -
t/Zd).
The bulk charge accumulated in the dielectric is Q = lpo~p. The same bulk charge is discharged during depolarization. We define the resistance of the substance either as that which determines the initial current I o = I s+ Ipo, or that which determines the current Is after polarization. We consider that I s and ps determine the properties of the substance as a semiconductor; as a measure of the dielectric properties we take the value of the accumulated 73
74
R. A N D R E I C H I N
charge Q; in some cases we take the initial current Ipo or the electromotive force of the polarization Vv = IpoRpo . All our measurements have been made on plates about 0.1 cm thick with the electrodes placed on opposite sides. The experimental results show that both the semiconducting and the initial dielectric conductivity (a s and lpo) depend on the applied electric field. The values of the specific resistance for zero external field calculated from these results are 10 and 2 x 10as f~ cm respectively. Impurities increase the conductivity. Both pure and with Ag and Cu impurities, As2S 3 shows photoconductivity, which within the limits of the concentrations investigated remains of the same order for Cu but decreases if we increase the Ag concentration. The energy gap decreases with the presence of both impurities, but much more rapidly with Cu than with Ag (fig. 1A). The high-voltage polarization shows one peculiarity: it decreases with the addition of Cu but increases with Ag (fig. 1B).
V
Ep
300"
2,q
_200-
2,Z 2,0
1oo.
x\Cu. \ o,oof
o,b~
o.'~
~ at %
o,ob¢
o.bf
o,:
B Fig. 1. (A) Variation of energy gap of vitreous As2S3 with addition of Ag and Cu impurities. (B) Variation of high-voltage polarization with addition of Ag and Cu impurities.
Q
t2
/ /o
KV/c,n
~
K Lx
~
~b
t's cp
~5 Fig. 2. (A) Dependence of accumulated charge on applied electric field for constant illumination. (B) Dependence of accumulated charge on intensity of illumination for constant electric field.
CONDUCTIVITY AND PHOTOPOLARIZATION PROPERTIES
75
Photopolarization phenomena were investigated, and the chief experimental results are as follows 5): 1) At constant intensity of illumination the quantity of accumulated charge depends linearly on the applied field up to 3000 V/cm, after which it shows a tendency towards saturation (fig. 2A). 2) At constant electric field the accumulated charge is linearly proportional to the illumination (up to the investigated intensity of the illumination 15000 Ix, fig. 2B). 3) The charge accumulated under the influence of light does not disappear immediately after cutting off the light but remains with time; (the "photoelectret state"). This can be seen in fig. 3, showing the ratio between the
0,8 0,q 0,2"
h J.b
a0
3b
6
t
Fig. 3. Ratio of depolarization and polarization charges as a function of time after cutting of light. (l) electrodes short-circuited; (2) open circuit, (3) ratio of initial depolarization and polarization currents as a function of time with electrodes short-circuited.
depolarization and the polarization charges as a function of time (in hours), keeping the sample in darkness without potential. Curve 1 is with short circuited electrodes and curve 2 with an open circuit. Curve 3 shows the dependence of the relation of the initial depolarization on the initial polarization current with short-circuited electrodes. The relaxation time of photopolarization and photodepolarization3), "c = e d / 4 n q O m o ~
decreases with the intensity of the light as shown in fig. 4. If the mobility/~ and the lifetime z are constant, we can assume that this curve expresses the dependence of the concentration of the charge carriers on the light intensity (the remaining parameters are constant for a given sample). Structurally vitreous A s 2 S 3 represents a complex of ramified chains, containing connected As and S atoms 4). In an earlier work z), we suggested that Ag penetrates into the chains, disrupting them (which results in a greater number of shorter chains), without any considerable changes in their com-
76
R. ANDREICHIN
position or structure. Cu, however, penetrates them, forming a new semiconducting vitreous compound A s - S - C u , somewhat analogous to a solid solution of AszS 3 and Cu2S or CuS. And so, proceeding from the experimental data summarized here, we assume that both phenomena observed in As2S3 - the semiconducting conductivity and the accumulation of bulk charges - are due to two different mechanisms and to two kinds of traps or states in the forbidden zone. T 10
8 6 q.
2
Fig. 4.
Decrease of the relaxation time associated with the photopolarization or photodepolarization as a function of light intensity.
The semiconducting conductivity is due to electrons in traps or in states comparatively close to the edges of the band where the conductivity mechanism can also be hopping. There is other evidence in favour of this assumption - the quicker initial decay of the photoelectret state, as well as the dependence of the conductivity of the applied electric field. The accumulation of bulk charges - photopolarization and polarization is probably due to reorientation of the chains, or to the electric charges on their being liberated by light or by the electric field and sticking at the ends of the chains according to the most probable mechanism of formation of high-voltage polarization. On the band model this would be described as due to the presence of another group of traps or states, with energies deep in the forbidden zone, where the current carriers have small mobility and can be liberated only by the influence of light or a similar agent. Andriesh et al. 6) show two groups of levels: one close to the band edge, and another about 1.0-1.3 eV deep; Kolomiets et al. 7) report on a group at about 1.15 eV. Both these results are in good agreement with the description given here.
CONDUCTIVITY AND PHOTOPOLARIZATION PROPERTIES
77
References 1) B. T. Kolomiets and V. M. Lubin, Fiz. Tverd. Tela 4 (1962) 401. 2) R. Andreichin, P. Simidchieva and M. Nikiforova, Compt. Rend. Acad. Bulgare Sci. 18 (1965) 995; 21 (1968) 753. 3) V. M. Fridkin and Yu. N. Barulin, Fiz. Tverd. Tela 5 (1963) 1523. 4) A. A. Vaipolin and E. A. Porai-Koshits, Fiz. Tverd. Tela 5 (1963) 683. 5) R. Andreichin, M. Baeva and M. Nikiforova, Fizicheskie Osnovi Electrofotografii (Vilnius, 1969). 6) B. T. Koiomiets, T. F. Mazets, Sh. M. Efendiev and A. M. Andriesh, J. Non-Crystalline Solids 4 (1970) 45. 7) B. T. Kolomiets, T. N. Mamontova and A. A. Babaev, J. Non-Crystalline Solids 4 (1970) 289.