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Mechanism of extrinsic conductivity in modified vitreous semiconductors
Journal of Non-Crystalline Solids 64 (1984) 279-282 North-Holland, Amsterdam
279
L e t t e r to the E d i t o r
M E C H A N I S M O F EXTRINSIC CONDUCTIVITY IN M O D I F I E D VITREOUS SEMICONDUCTORS V.L. AVERYANOV, B.L. G E L M O N T , B.T. KOLOMIETS, V.M. LYUBIN, O.Yu. P R I K H O D K O and K.D. T S E N D I N A.F. loffe Physico-Technical Institute, 194021 Leningrad, USSR Received 9 August 1983
To reveal the mechanism of the influence of impurities on the properties of chalcogenide vitreous semiconductors (ChVS) an investigation of the impurity concentration dependence of the optical forbidden gap (Eg) and the activation energy (Eo) of conductivity in a typical vitreous semiconductor As2Se 3 has been carried out. The method of modification applied is analogous to that given in ref. 1. Both transition (Ni, Cu, Fe) and nontransition (Bi, Sn) metals were chosen as modifying elements. The concentration of foreign elements ( N ) has been measured using a Camebax X-ray analyzer. X-ray, electron diffraction and electron microscope analysis do not show any crystallization of the samples. As-prepared films were annealed at 450 K for 15 min to stabilize their electrical properties. As can be seen from figs. 1 and 2 the degree of the influence on the electrical and optical properties depends on the type of impurity and its
~ -
Fe Fe
-
O0
;
I
~
'-J.
4
Iv/ 6I
,
8J
~
I iO
(2
L2 1.00
J
Zi
~
4i
Fig. 1. Energy activation versus N in As2Se 3 modified with various metals [2,3]. Fig. 2. Optical gap versus N in modified As2Se 3 [2,3]. 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
~
10
V.L. Averyanov et al. / Mechanism of extrinsic conductivity
280
~ t}O
Cu
I~ 07. °0
O/ 2
q
6
8
iv
Fig. 3. The shift of the Fermi energy versus N in AszSe 3 [2].
concentration [2,3]. In the non-modified As~Se 3 the Fermi level ( E v ) is fixed in the middle of the gap. A shift ( A W = ½Eg - Eo) of the Fermi level from the middle of the gap is shown in fig. 3 as a function of N for different modifying elements. Modifiers can be divided into three groups according to their influence on the position of E v. The transition metals of the first group (Ni, Fe) with the non-occupied d-shell have the most pronounced effect. The thermopower changes its sign with the increase of the Ni and Fe concentration [3]. This is evidence that electrically active centers are donors. In the case of the second group (CU) the shift of E v is less pronounced, There is no trace of saturation of E v at the highest modifier concentration. The third group which consists of non-transition metals (Bi, Sn) do not shift the E v noticeably. We assume that all valence electrons of the non-transition metals are taken up in bonds while in the case of transition metals a few atoms exist that do not satisfy all their valency requirements. These atoms are electrically active. This suggestion is confirmed by the M6ssbauer spectra of the films of AszSe 3 modified with Sn and Fe [4]. Sn has its maximum valence at any concentration while at small concentration of Fe only one M6ssbauer peak corresponding to Fe +3 is observed and the second peak corresponding to Fe +2 appears with the increase of concentration. On the basis of the above described experimental data it is possible to make the following conclusion. Almost all of the transition metal atoms are built in the network of the glass leading to the change of the optical gap. ChVS has a smaller density than its crystalline analogue. This fact may be ascribed to the presence of "microvoids". Some impurity atoms may be situated on the boundary of such a "microvoid" and therefore cannot use all their valence electrons for bonding with neighboring Se atoms. These atoms are responsible for the impurity states. As soon as the M6ssbauer spectrum has only one peak for a small concentration an ionized donor has the same charge (Fe ÷ 3) as the
V.L. Averyanov et al. / Mechanism of extrinsic conductivity
281
1.8 j.
?> q~
c~ o--------o
t~
taj
o
N D
060
5~
. . . . . . . . . .
~
:10
.
.
.
.
.
.
.
15
Fig. 4. E g - E o versus N for As2Se 3 (1), Ge3:Se32Te32As 4 (2), amorphous As (3), modified with Ni
[6].
majority of Fe atoms. Excess electrons occupy the tail of the density of states and shift the E F smoothly upwards to the position of the donor level with the increase of concentration [5]. When the E F approaches the donor level the probability of the ionization decreases and the second peak corresponding to Fe ÷ 2 appears. As follows from fig. 4 the value of Eg - E. does not depend on Ni concentration [6]. Such behaviour can be explained by the appearance of the classical impurity level rigidly connected to one of the bands. The impurity level should shift in parallel with the edge of the band responsible for the gap decrease because both are due to the same bonds of the impurity atoms with the host atoms. Therefore the value of E g - E~ should be independent of concentration at sufficiently large concentration. In fig. 5 the change of Eg and the position of the impurity level are schematically represented as functions of concentration. This conception is also confirmed by analysis [6] of experimental data on the modification of a-As [7] and Ge32Se32Te32As 4 [5] with Ni (see fig. 4). In order to change the conductivity of ChVS noticeably the concentration of electrically active atoms ( N d) should be greater than the concentration
EC _...:7.,. f . . .
o
'
E~ Ev w
-
-
N
-t)..,
Fig. 5. Schematic variation of Eg and the energy of the impurity level versus modifier concentration for the n-type semiconductor.
282
V..L. Averyanov et al. / Mechanism of extrinsic conductivity
of intrinsic defects which is approximately equal to 1018 cm -3 in As2Se 3. On the other h a n d the part impurity atoms in the " m i c r o v o i d s " is N d = N ( o c P,,)/Pc where Pc and Pv are densities of crystalline and vitreous semiconductors correspondingly; hence (Pc - Pv)/Pc = 0.01 [8]. Therefore concentration should not be smaller than 10 20 cm -3 or N >/1 at.%, a value which coincides rather well with experimental data. In conclusion, we shall show for As2Se 3 ( N i ) that the extrinsic conductivity can be described as the conductivity of a highly compensated semiconductor which has the concentration of free electrons n = N c (nd Na-Na) exp( - ~-~) Ed where E d is the d o n o r level, Na is the acceptor concentration and N~ is the effective density of b a n d states. The electron conductivity is as follows -Na ° = e l ' t N c N a Na
exp(-
E d ' ~'O0N a~[a -N'~ -'~)
e x p ( - ff-~);
where/~ is the electron mobility in the conduction band. Extrapolation of the temperature dependence of the conductivity gives the following values of the pre-exponential factor: i 0 -3, 10 -2 and 10 -~ I2 -1 cm -1 for N = 0 . 6 ; 4.3 and 6.7 at. % respectively. Assuming o0 = 10 2 ~2-~ cm -1 we obtain that the values of ( N d - N " ) / N a are equal to 10 -5, 10 -4, 10 -3 for the above-mentioned concentrations. So the degree of compensation is close to unity. This means that donors and acceptors emerge in pairs. As a result, the experimental data and their interpretation show that modified vitreous As2Se 3 can be considered as a heavily doped and highly compensated semiconductor. N o w it is possible to produce both n- and p-type vitreous semiconductors and therefore the fabrication of p - n junctions on their basis is possible.
References [1] R. Flasck, M. Izu, K. Sapru, T. Anderson, S.R. Ovshinsky and H. Fritzsche, Proc. 7th Int. Conf. Amorphous and Liquid Semiconductors, Edinburgh, 1977 (Institute of Physics, Bristol, 1978) p. 524. [2] B.T. Kolomiets, V.L. Averyanov, V.M. Lyubin and O.Yu. Prikhodko, Solar En. Mat. 8 (1982) 1; Pis'ma ZhTF 6 (1980) 577. [3] V.L. Averyanov, V.M. Lyubin, F.S. Nasredinov, O.Yu. Prikhodko and P.P. Seregin, Amorphrile poluprovodniki-82 (Bucharest, 1982) p. 160. [4] V.L. Averyanov, F.S. Nasredinov, P.V. Nistiryuk, O.Yu. Prikhodko and P.P. Seregin, Phys. Chi. Stek. 8 (1982) 541. [5] T. Gomi, Y. Hirose, T. Kurosu, T. Shiraishi, M. Tida, Y. Gekka and A. Kunioke, J. Non-Crystalline Solids 41 (1980) 37. [6] B.L. Gelmont and K.P. Tsendin, Ph.T.P. 17 (1983) 1040. [7] E.A. Davis and E. Mytilineou, Solar En. Mat. 8 (1982) 341. 18] Z.U. Borisova, Chemia stekloobrasnich poluprovodnikov (LGU, 1972).
279
L e t t e r to the E d i t o r
M E C H A N I S M O F EXTRINSIC CONDUCTIVITY IN M O D I F I E D VITREOUS SEMICONDUCTORS V.L. AVERYANOV, B.L. G E L M O N T , B.T. KOLOMIETS, V.M. LYUBIN, O.Yu. P R I K H O D K O and K.D. T S E N D I N A.F. loffe Physico-Technical Institute, 194021 Leningrad, USSR Received 9 August 1983
To reveal the mechanism of the influence of impurities on the properties of chalcogenide vitreous semiconductors (ChVS) an investigation of the impurity concentration dependence of the optical forbidden gap (Eg) and the activation energy (Eo) of conductivity in a typical vitreous semiconductor As2Se 3 has been carried out. The method of modification applied is analogous to that given in ref. 1. Both transition (Ni, Cu, Fe) and nontransition (Bi, Sn) metals were chosen as modifying elements. The concentration of foreign elements ( N ) has been measured using a Camebax X-ray analyzer. X-ray, electron diffraction and electron microscope analysis do not show any crystallization of the samples. As-prepared films were annealed at 450 K for 15 min to stabilize their electrical properties. As can be seen from figs. 1 and 2 the degree of the influence on the electrical and optical properties depends on the type of impurity and its
~ -
Fe Fe
-
O0
;
I
~
'-J.
4
Iv/ 6I
,
8J
~
I iO
(2
L2 1.00
J
Zi
~
4i
Fig. 1. Energy activation versus N in As2Se 3 modified with various metals [2,3]. Fig. 2. Optical gap versus N in modified As2Se 3 [2,3]. 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
~
10
V.L. Averyanov et al. / Mechanism of extrinsic conductivity
280
~ t}O
Cu
I~ 07. °0
O/ 2
q
6
8
iv
Fig. 3. The shift of the Fermi energy versus N in AszSe 3 [2].
concentration [2,3]. In the non-modified As~Se 3 the Fermi level ( E v ) is fixed in the middle of the gap. A shift ( A W = ½Eg - Eo) of the Fermi level from the middle of the gap is shown in fig. 3 as a function of N for different modifying elements. Modifiers can be divided into three groups according to their influence on the position of E v. The transition metals of the first group (Ni, Fe) with the non-occupied d-shell have the most pronounced effect. The thermopower changes its sign with the increase of the Ni and Fe concentration [3]. This is evidence that electrically active centers are donors. In the case of the second group (CU) the shift of E v is less pronounced, There is no trace of saturation of E v at the highest modifier concentration. The third group which consists of non-transition metals (Bi, Sn) do not shift the E v noticeably. We assume that all valence electrons of the non-transition metals are taken up in bonds while in the case of transition metals a few atoms exist that do not satisfy all their valency requirements. These atoms are electrically active. This suggestion is confirmed by the M6ssbauer spectra of the films of AszSe 3 modified with Sn and Fe [4]. Sn has its maximum valence at any concentration while at small concentration of Fe only one M6ssbauer peak corresponding to Fe +3 is observed and the second peak corresponding to Fe +2 appears with the increase of concentration. On the basis of the above described experimental data it is possible to make the following conclusion. Almost all of the transition metal atoms are built in the network of the glass leading to the change of the optical gap. ChVS has a smaller density than its crystalline analogue. This fact may be ascribed to the presence of "microvoids". Some impurity atoms may be situated on the boundary of such a "microvoid" and therefore cannot use all their valence electrons for bonding with neighboring Se atoms. These atoms are responsible for the impurity states. As soon as the M6ssbauer spectrum has only one peak for a small concentration an ionized donor has the same charge (Fe ÷ 3) as the
V.L. Averyanov et al. / Mechanism of extrinsic conductivity
281
1.8 j.
?> q~
c~ o--------o
t~
taj
o
N D
060
5~
. . . . . . . . . .
~
:10
.
.
.
.
.
.
.
15
Fig. 4. E g - E o versus N for As2Se 3 (1), Ge3:Se32Te32As 4 (2), amorphous As (3), modified with Ni
[6].
majority of Fe atoms. Excess electrons occupy the tail of the density of states and shift the E F smoothly upwards to the position of the donor level with the increase of concentration [5]. When the E F approaches the donor level the probability of the ionization decreases and the second peak corresponding to Fe ÷ 2 appears. As follows from fig. 4 the value of Eg - E. does not depend on Ni concentration [6]. Such behaviour can be explained by the appearance of the classical impurity level rigidly connected to one of the bands. The impurity level should shift in parallel with the edge of the band responsible for the gap decrease because both are due to the same bonds of the impurity atoms with the host atoms. Therefore the value of E g - E~ should be independent of concentration at sufficiently large concentration. In fig. 5 the change of Eg and the position of the impurity level are schematically represented as functions of concentration. This conception is also confirmed by analysis [6] of experimental data on the modification of a-As [7] and Ge32Se32Te32As 4 [5] with Ni (see fig. 4). In order to change the conductivity of ChVS noticeably the concentration of electrically active atoms ( N d) should be greater than the concentration
EC _...:7.,. f . . .
o
'
E~ Ev w
-
-
N
-t)..,
Fig. 5. Schematic variation of Eg and the energy of the impurity level versus modifier concentration for the n-type semiconductor.
282
V..L. Averyanov et al. / Mechanism of extrinsic conductivity
of intrinsic defects which is approximately equal to 1018 cm -3 in As2Se 3. On the other h a n d the part impurity atoms in the " m i c r o v o i d s " is N d = N ( o c P,,)/Pc where Pc and Pv are densities of crystalline and vitreous semiconductors correspondingly; hence (Pc - Pv)/Pc = 0.01 [8]. Therefore concentration should not be smaller than 10 20 cm -3 or N >/1 at.%, a value which coincides rather well with experimental data. In conclusion, we shall show for As2Se 3 ( N i ) that the extrinsic conductivity can be described as the conductivity of a highly compensated semiconductor which has the concentration of free electrons n = N c (nd Na-Na) exp( - ~-~) Ed where E d is the d o n o r level, Na is the acceptor concentration and N~ is the effective density of b a n d states. The electron conductivity is as follows -Na ° = e l ' t N c N a Na
exp(-
E d ' ~'O0N a~[a -N'~ -'~)
e x p ( - ff-~);
where/~ is the electron mobility in the conduction band. Extrapolation of the temperature dependence of the conductivity gives the following values of the pre-exponential factor: i 0 -3, 10 -2 and 10 -~ I2 -1 cm -1 for N = 0 . 6 ; 4.3 and 6.7 at. % respectively. Assuming o0 = 10 2 ~2-~ cm -1 we obtain that the values of ( N d - N " ) / N a are equal to 10 -5, 10 -4, 10 -3 for the above-mentioned concentrations. So the degree of compensation is close to unity. This means that donors and acceptors emerge in pairs. As a result, the experimental data and their interpretation show that modified vitreous As2Se 3 can be considered as a heavily doped and highly compensated semiconductor. N o w it is possible to produce both n- and p-type vitreous semiconductors and therefore the fabrication of p - n junctions on their basis is possible.
References [1] R. Flasck, M. Izu, K. Sapru, T. Anderson, S.R. Ovshinsky and H. Fritzsche, Proc. 7th Int. Conf. Amorphous and Liquid Semiconductors, Edinburgh, 1977 (Institute of Physics, Bristol, 1978) p. 524. [2] B.T. Kolomiets, V.L. Averyanov, V.M. Lyubin and O.Yu. Prikhodko, Solar En. Mat. 8 (1982) 1; Pis'ma ZhTF 6 (1980) 577. [3] V.L. Averyanov, V.M. Lyubin, F.S. Nasredinov, O.Yu. Prikhodko and P.P. Seregin, Amorphrile poluprovodniki-82 (Bucharest, 1982) p. 160. [4] V.L. Averyanov, F.S. Nasredinov, P.V. Nistiryuk, O.Yu. Prikhodko and P.P. Seregin, Phys. Chi. Stek. 8 (1982) 541. [5] T. Gomi, Y. Hirose, T. Kurosu, T. Shiraishi, M. Tida, Y. Gekka and A. Kunioke, J. Non-Crystalline Solids 41 (1980) 37. [6] B.L. Gelmont and K.P. Tsendin, Ph.T.P. 17 (1983) 1040. [7] E.A. Davis and E. Mytilineou, Solar En. Mat. 8 (1982) 341. 18] Z.U. Borisova, Chemia stekloobrasnich poluprovodnikov (LGU, 1972).