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Dose-rate effect in GexSe1−x glasses
Volume
152, number
5,6
PHYSICS
LETTERS
A
2 1 January
199
1
Dose-rate effect in Ge,Se, _-xglasses B.V. Andreev, Yu.N. Kostrubov and V.V. Gromov Institute of Physical Chemistry of the Academy of Sciences of the USSR, Leninsky Prospekt 31, Moscow I 17915, USSR Received 27 August 1990; accepted Communicated by J. Rouquet
for publication
19 November
1990
A dose-rate effect in chalcogenide glassy semiconductors (ChGS) predicted earlier by Andreev et al. has been experimentally investigated in the Ge-Se system. The effect consists in the fact that the concentration of radiation-induced paramagnetic centres (RPC) depends strongly on the dose rate of weakly absorbed y-irradiation, other conditions being equal. The effect’s composition dependence obtained from the experiment has been also discussed.
1. Introduction Many properties of chalcogenide glasses, in particular the absence of paramagnetism, are known to be due to the existence of an effective attraction between carriers. This attraction leads to the formation of electron and hole spinless pairs of a small size with a negative effective correlation energy We 0 (negative U-centres), whose absolute value is anomalously large [ 2,3]. It enables chalcogenide glassy semiconductors ( ChGS) to be considered as highly correlated solids. To the latter we relate also superconductors, ferroelectrics, charge-ordered compounds, Mott systems and other solids, the characteristic properties of which are tangibly determined by the existence of an interaction between excitations of the electron subsystem. In terms of our concepts (see, for example, refs. [ 1,4] ) some peculiarities of radiationor photo-induced processes, occurring via excitations of the electron subsystem (or stimulated by them), must observed in highly correlated solids under ionizing (light) radiation. One such peculiarity is the dose-rate effect, the realization of which is connected with two conditions. They are: (i) the decay (or capture) of some electron excitations is one of the mechanisms by which radiation defects are formed in the system; (ii) there exists an effective interaction between the excitations leading to their lifetime being highly 0375-9601/91/s
03.50 0 1991 - Elsevier Science Publishers
dependent on their concentration. It is clear that the occurrence of condition (ii) unambiguously determines the direction of the effect, namely that increasing the dose rate at a constant absorbed dose may only result in a decrease in the concentration of defects formed according to condition (i). According to Klinger’s theory [ 31 anomalous properties of ChGS are explained by the presence of soft atomic configurations on nearly 10% of the regular lattice sites. Electron-phonon interactions on such configurations may, on the one hand, lead to the capture of an excited delocalized carrier with the formation of a paramagnetic centre and, on the other hand, to the recombination into a pair of carriers of one sign with the formation of a negative U-centre. Thus Klinger’s mechanism can ensure the fulfillment of the above conditions in GhGS. The aim of the present work is an experimental observation of the dose-rate effect, first predicted by the authors [ 1 ] in ChGS. For the investigation the Ge-Se system was chosen. Radiation paramagnetic centres (RPC) in this system have been, in part, studied by Chepeleva [ 5 1. Under light or ionizing radiation in the system there appear metastable paramagnetic centres, which at low intensities and doses are due to the rearrangement of charges in existing defects. The observed RPC are thermally stable at 77 K.
B.V. (North-Holland)
293
Volume 152, number 5,6
2. Experimental
PHYSICS LETTERS A
procedure
Glasses of the composition Ge,Se, _.~with x ranging from 0.03 to 0.37 including the stoichiometric composition x=0.33 (corresponding to the compound GeSe*), as well as glassy selenium (g-Se ) were investigated. The GeSe, samples were both glassy and crystalline, as confirmed by X-ray analysis. The samples were synthesized from highly pure raw materials using standard technologies. The ESR spectra were recorded before and after irradiation at 77 K in the X-band with a frequency modulation of 100 kHz using an ERS-230 radiospectrometer. The amount of RPC was determined using a carbon standard. Irradiation was performed at 77 K with a 60Cby-source up to an absorbed dose of 3.2 x lo3 Gy at two dose rates Pm,, = 7.7 Gy s-i and P,,,=O.16 Gy s-l. Dosimetry was performed using a technique described elsewhere [ 6 1.
3. Results Before irradiation the ESR signal is absent from all the samples including g-Se, crystalline and glassy GeSe,. After irradiation there appears a signal, whose form and intensity depends on the composition of the glass (fig. 1). In glasses with ~~0.33 the ESR spectrum changes gradually with increasing Ge content. Its shape resembles that of g-Se. According to earlier work [ 51 such an anisotropic ESR spectrum may be due to two types of RPC, which are related to Se atoms in chains and rings respectively. The change in the ESR spectrum with increasing Ge content consists in a gradual increase in the broad component of the signal and a disappearance of the narrow one. As a result, in glasses with x2 0.33 the signal is a broad (AHx 150 G) isotropic singlet with gz2.07. The detailed origin of such changes is not clear to us and requires additional investigations. In all the glasses RPC are stable at 77 K. When the samples are heated up to T,,,, in g-Se and glasses with ~~0.33, the ESR signals are completely annealed, in samples with xaO.33 there occurs a partial decrease ( !Z60%) in the signal, its shape being preserved. This is also in agreement with the data reported by Chepeleva [ 5 1. In glasses with x < 0.15 as well as in g-Se and glassy 294
21 January 1991
3.2
3.3
3.4
I
3.5
H&G)
Fig. 1. ESR spectra of paramagnetic centres arising under y-irradiation of glasses of the system Ge,Se, _Xup to a dose of 3.2 x 10’ Gy. Irradiation and recording at 77 K. (In all investigated samples the form of the ESR spectrum does not depend on the dose rate. For example see (e).) (a) g-Se (strongly magnified). (b) Ge,. lsSe0.85.(c) Geo.30Seo.70.Cd) Geo.deo.67 (GeSe2 crystal 1. (e) Geo.37Seo.63(broken line: irradiation at P,,,).
GeSe2 the concentration of RPC at a dose of 3.2~ lo3 Gy is low (about lOi spin/cm3). Therefore, we can speak about the dose-rate effect in these compositions only on a qualitative basis: visually the amplitudes of ESR signals induced at Pminare greater than those at Pmax.However, in glasses with x2 0.15 the effect may be estimated quantitatively (fig. 2). The behaviour of GeSez under the given irradiation conditions is quite peculiar. In glassy samples
L
I
I
0.2
0.3
1
0.4
XGe
Fig. 2. Dependence of the concentration of radiation-induced paramagnetic centres (RPC) in Gesei_, glasses on the Ge content under y-irradiation up to a dose of 3.2~ lo3 Gy with the dose rates P,.,= 7.7 Gy s-land Pm,,= 0.16 Gy s-‘. The gap in the curves corresponds to the composition GeSe,.
Volume 152, number 5,6
PHYSICS LETTERS A
the quantity of RPC is negligibly small ( x lOI spin/ cm3 ). In crystalline GeSea the spin density at Pmin is by five times, and at P,,_ by an order of magnitude lower than that expected from the dependences in fig. 2. Though this result adds to the set of anomalous properties of GeSe*, its reasons are not wellunderstood.
4. Discussion In the investigation of radiation defects in solids a trivial explanation of the dose-rate dependences consists in assuming a local warming as a mechanism of radiation-induced annealing of metastable defects. But, firstly, as was shown in previous work [ 5 1, the thermal stability of RPC in glasses of the Ge-Se system increases with a rise of the Ge content, and this fact diminishes consequently the probability of radiation annealing. We have observed an increase of the dose-rate effect with increasing Ge content (fig. 2 ), which contradicts the assumption that the dose-rate effect is caused by the radiation-induced annealing. Secondly, it is already well-known (see, for example, ref. [ 7 ] ) that at the dose rates of y-irradiation used in our experiment the processes connected with local heating are always negligible. However, it should be noted that in order to clarify the role of annealing phenomena in the dose-rate effect in question it is necessary to study the temperature dependences of the latter. We believe that there exist two main ways of evolution for an isolated excited carrier (low-lying delocalized one-article excitation): (i) capture after departure beyond the mobility edge due to an energy loss resulting in the formation of a localized paramagnetic state; (ii) recombination into a pair upon coupling with a similar carrier on a soft atomic configuration which results in the formation of a negative U-centre. Other conditions being equal, the probability of process (ii) depends greatly on the concentration of excited carriers - on the dose rate. Thus, in our opinion, the “driving force” of the dose-rate effect in ChGS under consideration is process (ii). The competition of processes (i) and (ii) at P,,,,, and Pmin in glasses of different composition demonstrates this effect. However, based on the results of our experi-
21 January 1991
ments, it is impossible to say, the interaction of which (localized or delocalized) excited carriers is responsible for the effect observed. We hope that the study of the temperature dependence of the effect will clarify this principal problem. It is well-known that upon doping of GhGS the correlation between the number of electron and hole negative U-centres changes, which ensures pinning of the Fermi-level. It is evident that the electron-lattice interaction leads to repulsion of carriers of opposite sign in the same way as to attraction of carriers of one sign. Thus, the dose-rate effect must manifest itself stronger in cases where the concentration of electron negative U-centres is much higher than the concentration of hole negative U-centres. By this means we can explain the increase of the doserate effect when passing from the stoichiometric composition with x=0.33 to the composition enriched by Ge. The small magnitude of the effect at the compositions with XX 0.15 is explained here apparently by the lower hole mobility in comparison with the electron one. In our opinion, the study of the donor and acceptor impurities’ influence on the magnitude of the dose-rate effect is of exceptional interest. It is evident that the dose-rate effect under discussion can be also manifested in GhGS upon light irradiation. But by virtue of the fact that the efficiency of the excited carriers’ formation is by several orders higher under the influence of light with ho> Eg than upon y-irradiation, the effect’s mechanism proposed by us will manifest itself at intensities of exciting light far less than those which are commonly used in investigation of photo-induced phenomena in ChGS. The dose-rate effect under consideration can also be observed upon light irradiation with jE,< ho
Volume
152, number
56
PHYSICS
dose-rate effect in ChGS with increasing pressure, as well as due to other effects, which can change the mobility edge.
LETTERS
A
cussion and valuable of the results.
21 January
remarks
1991
on the interpretation
References Acknowledgement The authors acknowledge Dr. I.V. Chepeleva for samples and Professor M.I. Klinger for fruitful dis-
296
[ 1] B.V. Andreev et al., J. Phys. Cond. Matter 1 ( 1989) 3359. [2] [3] [4] [5] [6] [ 71
P.W. Anderson, Phys. Rev. Lett. 34 (1975) 953. MI. Klinger, Phys. Rep. 165 (1988) 275. Yu.N. Kostrubov et al., Radiat. Phys. Chem. 30 ( 1987) 63. I.V. Chepeleva, J. Non-Cryst. Solids 97/98 (1987) 1179. B.V. Andreev et al., Khim. Vys. Energ. 20 (1986) 181. L.T. Bugaenko, M.G. Kuzmin and L.S. Polak, Khimiya vysokikh energiy (Khimiya, Moscow, 1988).
152, number
5,6
PHYSICS
LETTERS
A
2 1 January
199
1
Dose-rate effect in Ge,Se, _-xglasses B.V. Andreev, Yu.N. Kostrubov and V.V. Gromov Institute of Physical Chemistry of the Academy of Sciences of the USSR, Leninsky Prospekt 31, Moscow I 17915, USSR Received 27 August 1990; accepted Communicated by J. Rouquet
for publication
19 November
1990
A dose-rate effect in chalcogenide glassy semiconductors (ChGS) predicted earlier by Andreev et al. has been experimentally investigated in the Ge-Se system. The effect consists in the fact that the concentration of radiation-induced paramagnetic centres (RPC) depends strongly on the dose rate of weakly absorbed y-irradiation, other conditions being equal. The effect’s composition dependence obtained from the experiment has been also discussed.
1. Introduction Many properties of chalcogenide glasses, in particular the absence of paramagnetism, are known to be due to the existence of an effective attraction between carriers. This attraction leads to the formation of electron and hole spinless pairs of a small size with a negative effective correlation energy We 0 (negative U-centres), whose absolute value is anomalously large [ 2,3]. It enables chalcogenide glassy semiconductors ( ChGS) to be considered as highly correlated solids. To the latter we relate also superconductors, ferroelectrics, charge-ordered compounds, Mott systems and other solids, the characteristic properties of which are tangibly determined by the existence of an interaction between excitations of the electron subsystem. In terms of our concepts (see, for example, refs. [ 1,4] ) some peculiarities of radiationor photo-induced processes, occurring via excitations of the electron subsystem (or stimulated by them), must observed in highly correlated solids under ionizing (light) radiation. One such peculiarity is the dose-rate effect, the realization of which is connected with two conditions. They are: (i) the decay (or capture) of some electron excitations is one of the mechanisms by which radiation defects are formed in the system; (ii) there exists an effective interaction between the excitations leading to their lifetime being highly 0375-9601/91/s
03.50 0 1991 - Elsevier Science Publishers
dependent on their concentration. It is clear that the occurrence of condition (ii) unambiguously determines the direction of the effect, namely that increasing the dose rate at a constant absorbed dose may only result in a decrease in the concentration of defects formed according to condition (i). According to Klinger’s theory [ 31 anomalous properties of ChGS are explained by the presence of soft atomic configurations on nearly 10% of the regular lattice sites. Electron-phonon interactions on such configurations may, on the one hand, lead to the capture of an excited delocalized carrier with the formation of a paramagnetic centre and, on the other hand, to the recombination into a pair of carriers of one sign with the formation of a negative U-centre. Thus Klinger’s mechanism can ensure the fulfillment of the above conditions in GhGS. The aim of the present work is an experimental observation of the dose-rate effect, first predicted by the authors [ 1 ] in ChGS. For the investigation the Ge-Se system was chosen. Radiation paramagnetic centres (RPC) in this system have been, in part, studied by Chepeleva [ 5 1. Under light or ionizing radiation in the system there appear metastable paramagnetic centres, which at low intensities and doses are due to the rearrangement of charges in existing defects. The observed RPC are thermally stable at 77 K.
B.V. (North-Holland)
293
Volume 152, number 5,6
2. Experimental
PHYSICS LETTERS A
procedure
Glasses of the composition Ge,Se, _.~with x ranging from 0.03 to 0.37 including the stoichiometric composition x=0.33 (corresponding to the compound GeSe*), as well as glassy selenium (g-Se ) were investigated. The GeSe, samples were both glassy and crystalline, as confirmed by X-ray analysis. The samples were synthesized from highly pure raw materials using standard technologies. The ESR spectra were recorded before and after irradiation at 77 K in the X-band with a frequency modulation of 100 kHz using an ERS-230 radiospectrometer. The amount of RPC was determined using a carbon standard. Irradiation was performed at 77 K with a 60Cby-source up to an absorbed dose of 3.2 x lo3 Gy at two dose rates Pm,, = 7.7 Gy s-i and P,,,=O.16 Gy s-l. Dosimetry was performed using a technique described elsewhere [ 6 1.
3. Results Before irradiation the ESR signal is absent from all the samples including g-Se, crystalline and glassy GeSe,. After irradiation there appears a signal, whose form and intensity depends on the composition of the glass (fig. 1). In glasses with ~~0.33 the ESR spectrum changes gradually with increasing Ge content. Its shape resembles that of g-Se. According to earlier work [ 51 such an anisotropic ESR spectrum may be due to two types of RPC, which are related to Se atoms in chains and rings respectively. The change in the ESR spectrum with increasing Ge content consists in a gradual increase in the broad component of the signal and a disappearance of the narrow one. As a result, in glasses with x2 0.33 the signal is a broad (AHx 150 G) isotropic singlet with gz2.07. The detailed origin of such changes is not clear to us and requires additional investigations. In all the glasses RPC are stable at 77 K. When the samples are heated up to T,,,, in g-Se and glasses with ~~0.33, the ESR signals are completely annealed, in samples with xaO.33 there occurs a partial decrease ( !Z60%) in the signal, its shape being preserved. This is also in agreement with the data reported by Chepeleva [ 5 1. In glasses with x < 0.15 as well as in g-Se and glassy 294
21 January 1991
3.2
3.3
3.4
I
3.5
H&G)
Fig. 1. ESR spectra of paramagnetic centres arising under y-irradiation of glasses of the system Ge,Se, _Xup to a dose of 3.2 x 10’ Gy. Irradiation and recording at 77 K. (In all investigated samples the form of the ESR spectrum does not depend on the dose rate. For example see (e).) (a) g-Se (strongly magnified). (b) Ge,. lsSe0.85.(c) Geo.30Seo.70.Cd) Geo.deo.67 (GeSe2 crystal 1. (e) Geo.37Seo.63(broken line: irradiation at P,,,).
GeSe2 the concentration of RPC at a dose of 3.2~ lo3 Gy is low (about lOi spin/cm3). Therefore, we can speak about the dose-rate effect in these compositions only on a qualitative basis: visually the amplitudes of ESR signals induced at Pminare greater than those at Pmax.However, in glasses with x2 0.15 the effect may be estimated quantitatively (fig. 2). The behaviour of GeSez under the given irradiation conditions is quite peculiar. In glassy samples
L
I
I
0.2
0.3
1
0.4
XGe
Fig. 2. Dependence of the concentration of radiation-induced paramagnetic centres (RPC) in Gesei_, glasses on the Ge content under y-irradiation up to a dose of 3.2~ lo3 Gy with the dose rates P,.,= 7.7 Gy s-land Pm,,= 0.16 Gy s-‘. The gap in the curves corresponds to the composition GeSe,.
Volume 152, number 5,6
PHYSICS LETTERS A
the quantity of RPC is negligibly small ( x lOI spin/ cm3 ). In crystalline GeSea the spin density at Pmin is by five times, and at P,,_ by an order of magnitude lower than that expected from the dependences in fig. 2. Though this result adds to the set of anomalous properties of GeSe*, its reasons are not wellunderstood.
4. Discussion In the investigation of radiation defects in solids a trivial explanation of the dose-rate dependences consists in assuming a local warming as a mechanism of radiation-induced annealing of metastable defects. But, firstly, as was shown in previous work [ 5 1, the thermal stability of RPC in glasses of the Ge-Se system increases with a rise of the Ge content, and this fact diminishes consequently the probability of radiation annealing. We have observed an increase of the dose-rate effect with increasing Ge content (fig. 2 ), which contradicts the assumption that the dose-rate effect is caused by the radiation-induced annealing. Secondly, it is already well-known (see, for example, ref. [ 7 ] ) that at the dose rates of y-irradiation used in our experiment the processes connected with local heating are always negligible. However, it should be noted that in order to clarify the role of annealing phenomena in the dose-rate effect in question it is necessary to study the temperature dependences of the latter. We believe that there exist two main ways of evolution for an isolated excited carrier (low-lying delocalized one-article excitation): (i) capture after departure beyond the mobility edge due to an energy loss resulting in the formation of a localized paramagnetic state; (ii) recombination into a pair upon coupling with a similar carrier on a soft atomic configuration which results in the formation of a negative U-centre. Other conditions being equal, the probability of process (ii) depends greatly on the concentration of excited carriers - on the dose rate. Thus, in our opinion, the “driving force” of the dose-rate effect in ChGS under consideration is process (ii). The competition of processes (i) and (ii) at P,,,,, and Pmin in glasses of different composition demonstrates this effect. However, based on the results of our experi-
21 January 1991
ments, it is impossible to say, the interaction of which (localized or delocalized) excited carriers is responsible for the effect observed. We hope that the study of the temperature dependence of the effect will clarify this principal problem. It is well-known that upon doping of GhGS the correlation between the number of electron and hole negative U-centres changes, which ensures pinning of the Fermi-level. It is evident that the electron-lattice interaction leads to repulsion of carriers of opposite sign in the same way as to attraction of carriers of one sign. Thus, the dose-rate effect must manifest itself stronger in cases where the concentration of electron negative U-centres is much higher than the concentration of hole negative U-centres. By this means we can explain the increase of the doserate effect when passing from the stoichiometric composition with x=0.33 to the composition enriched by Ge. The small magnitude of the effect at the compositions with XX 0.15 is explained here apparently by the lower hole mobility in comparison with the electron one. In our opinion, the study of the donor and acceptor impurities’ influence on the magnitude of the dose-rate effect is of exceptional interest. It is evident that the dose-rate effect under discussion can be also manifested in GhGS upon light irradiation. But by virtue of the fact that the efficiency of the excited carriers’ formation is by several orders higher under the influence of light with ho> Eg than upon y-irradiation, the effect’s mechanism proposed by us will manifest itself at intensities of exciting light far less than those which are commonly used in investigation of photo-induced phenomena in ChGS. The dose-rate effect under consideration can also be observed upon light irradiation with jE,< ho
Volume
152, number
56
PHYSICS
dose-rate effect in ChGS with increasing pressure, as well as due to other effects, which can change the mobility edge.
LETTERS
A
cussion and valuable of the results.
21 January
remarks
1991
on the interpretation
References Acknowledgement The authors acknowledge Dr. I.V. Chepeleva for samples and Professor M.I. Klinger for fruitful dis-
296
[ 1] B.V. Andreev et al., J. Phys. Cond. Matter 1 ( 1989) 3359. [2] [3] [4] [5] [6] [ 71
P.W. Anderson, Phys. Rev. Lett. 34 (1975) 953. MI. Klinger, Phys. Rep. 165 (1988) 275. Yu.N. Kostrubov et al., Radiat. Phys. Chem. 30 ( 1987) 63. I.V. Chepeleva, J. Non-Cryst. Solids 97/98 (1987) 1179. B.V. Andreev et al., Khim. Vys. Energ. 20 (1986) 181. L.T. Bugaenko, M.G. Kuzmin and L.S. Polak, Khimiya vysokikh energiy (Khimiya, Moscow, 1988).