- Email: [email protected]
Photoconductivity and light stimulated contact potentials in β-RbAg4I5
Solid State Ionics 14 (1984) 159-161 North-Holland, Amsterdam
PHOTOCONDUCTIVITY AND LIGHT STIMULATED CONTACT POTENTIALS IN p-RbAg415 V.G. MURAKHOVSKY * and W. VAN GOOL Inorganic Chemistry Department, State University of Utrecht, Croesestraat 77.4, 3522 AD Utrecht, The Netherlands Received 9 January 1984 Revised manuscript received 29 June 1984
Photoconductivity and photoelectric effects have been established in thin p o l y c r y s t a l l i n e films of ~ - RbAg4I 5. The spectral and the temperature dependence of the p h e n o m e n a have been st +died. The results are interpreted using a model in which ionic processes dominate in the photoconductivity when the exclting photon energy less than 3.2 eV.
I. INTRODUCTION
3. RESULTS AND DISCUSSION
The previous decade was m a r k e d by a sharp increase in the number of publications on ionic superconductors. RbAg4I 5 is a favourite among these. As a rule single crystals or polycrystalline tablets have been investigated, while the number of publications on physical properties of thin polycrystalline RbAg4I 5 films is scarce. This paper presents the results of investigations of photoconductivity and photoelectric voltage in RbAg4I 5 when exposed to light as a function of wavelenghts, light excitation and temperatures. The kinetic parameters of certain relaxation processes in p o l y c r y s t a l l i n e RbAg4I 5 films have been measured.
Figs.1 and 2 represent the spectral dependence of the p h o t o c o n d u c t i v i t y in ~ - RbAg4I 5 films at 120 K. The two curves correspond to the planar and the sandwich configuration. The peak photoccnductivity is at 386 nm which is in g o o d agreement with values found for the optical absorption edge in thin layers of ~ - RbAg4I 5 (2). Fig.2 shows that a large difference in photoconductivity is found as a function of the contact type in the region of 500 - 700 rgn. We assume that a great number of surface d e f e c t states is present with the planar arrangement of the contacts. This leads to the f o r m a t i o n of active clusters in the surface film layer when exposed to light. We assume that the excitation of the carriers frcm these centers leads to the peak of the photoconductivity curve at 520 nm. Ionic
2. EXPERIMENTAL Photoconductivity and photoelectric contact voltage of p o l y c r y s t a l l i n e RbAg4I 5 films have been measured. The films were obtained by thermal v a c u u m e v a p o r a t i o n techniques (I). X - ray diffraction patterns of the films at ambient temperature showed that their cc~nposition c o r r e sponds to ~ - R b ~ 4 I 5. The film thickness was I ~ m. Silver contacts were applied by thermal vacuum deposition. Both planar and "sandwich" geometries were used for the electrodes. The thermal resistance at 120 K was 106 Ohm. All experiments were performed in a low-temperature optical c r y o s t a t under 10 -7 Torr pressure. The samples were excited by a high-power LOMO-lamp in combination w i t h monochromator, or a set of light filters.
i0
-4
t
E i0
-5
10 -6 i
350 Fig.1.
* Permanent address: Technological Institute of Refrigerating Industry, 1/3 Petra Velikogo St, 270000 Odessa, USSR. 0 167-2738/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
i
400
i
450
i
500
i
~,
nm
Spectral d e p e n d e n c e of p h o t o c o n d u c t i v i t y in 8 - RbAg4I 5 absorption edge region: l-planar contact geometry; 2-sandwich contact geometry.
1,1.G. Murakhovsky, W. van Gool / Photoconductivity in #-RbAg4l 5
160
i0
5"10 -6
-4
light
off
1
E ~,
10-5
E 3
LO
© 10 -6
i0-6
Fig.2.
i
i
,
500
600"
Photoconductivity l-planar contact contact geometry.
700
i
i
i
800
~,
0
nm
in the impurity region: geometry; 2-sandwich
MAg4I 5 type superconductors are known for the formation of q u a s i m e t a l l i c centers (clusters) (3). Fig.3 present the p h o t o c o n d u c t l v i t y vs temperature curve at d i f f e r e n t excitation energies. Interesting is the d i s c r e p a n c y in temperature dependence of the peak values of the photocurrent in the regions of 390 and 520 nm. While the super ionic conductors in the region of 390 nm behave as usual (compared to ~ - AgI (4)) the behaviour in the region of 520 nm is far from being standard. This is demonstrated by the fact that in the temperature r e g i o n of 125 - 200 K the photoconductivity peak values at 520 nm increase w i t h increasing temperature. The above behaviour of the p h o t o c o n d u c t i v i t y strengthens our assumption that the r e l e v a n t d e fects exists of quasimetallic silver centers in p o l y c r y s t a l l i n e RbAg4I 5 layers. The p h o t o c o n d u c tivity relaxation kinetics at various temperatures and w a v e l e n g t h s of the excitation light
5'10
-6 2
"
Fig.4.
i
i
.
i00
200
300
t,
s
Kinetics of p h o t o c o n d u c t i v i t y relaxation: 1,2 of excitating light 390 nm, temperature 120 and 160 K, respectively; 3,41 = 520 nm, temperature 120 and 160 K, respectively.
(fig.4) shows two different types of relaxation processes which suggest the presence of several excitation and recombination paths in ~ RbAg4I 5 . For one type of relaxation process the c h a r a c t e r i s t i c time is t1= 50 s; it is m o s t eminent when excitation takes place with I < 410 nm. A second type of decay with t 2 > 50 s takes place over the whole spectral range. The investigation of thin films of RbAg4I 5 leads to another interesting result, viz. a contact voltage of the order 0.15 - 0.25 V when the layer is exposed to light. Such voltage must in fact be the r e s u l t of d i f f e r e n t c o n t a c t potential barriers in the electrode regions of the films. The spectral d e p e n d e n c e of this contact photoelectric voltage is presented in fig.5; m e a s u r e m e n t s were c o n d u c t e d on samples with planar contact geometry only. An inversion of the sign of the c o n t a c t voltage was o b s e r v e d for all samples when the excitation takes place in the
10 -4
i
I
E
£
10-500 O9
>
3
)2 i0
-6
5
I
6
i
i
7
8
i
103 T
Fig.3.
1
10 - 6 i
Photoconductivity versus temperature at different wavelengths of excitation light: I- dark conductivity; 2- 1 = 390 nm; 3- i = 520 nm.
450 -i
Fig.5.
Spectral dependence of contact photoelectric voltage in ~ - RbAg4I 5 .
V.G. Murakhovsky, W. van Gool / Photoconductivity in pRbAg.,Zs
4
2 3
2
> a
F
2
3
4
5
?l_
-1
versus voltage contact Fig.6. Photoelectric and wavelength of light probe: location 2- A = 520 "m; 3- X = l- h = 390 nm; 375 nm.
region The contact of fundamental absorption. photoelectric voltage value depends on the location of the light probe on the film. In order to excite the sample a" optical probe (0.5 mn in diameter) with wavelengths 350, 390 and 520 "m (fig.61 was used. The internal field distribution is in good agreement with that described in (5). Such a distribution is characteristic for silver - containing superionic photoelectrets The value of contact using a weak excitation. voltage decreases with increasing temperature. The results described above lead us to the following conclusions. The mechanism of the physical processes which take place in thin
161
films of the 8 polycrystalline - RbA94I5 superionic conductors when exposed to light does not vary in the wavelength range x >400 nm. In this spectral region the photoelectric contact voltage changes in the same way as the dark voltage. To our opinion this supports the assumption that exposure of 8 - RbAg415 to light produces ?q "photo" ions. Photoionisation of silver atoms localized in the lattice or on for quasimetallic centers may serve as a basis the emergence In the abof such carriers. sorption edge region a change takes place from electronic conduction to predominantly ion CCnwhich leads duction with increasing wavelength, to the inversion of photoelectronic voltage. Cur conclusion is that the photoconductivity ionic supsrin thin films of 6 - RbAg415 conductor is due to the production of silver ions by light. 1. REFERENCES ) V.Borzim, E.Vasilkovskaj, I.Pzola, in Sbornit Nauchnyh Trudov IPM (1975) 27 (in Russian) Materials 9 Inorganic (2) V.Murakhovsky, (1983) 123 ( in Russian). Solid State V.Murakhovsky, (3) V.Drozdov, Phys. 21 (1979) 275 (in Russian). I.Kostadinov, E.Popov, So(4) L.Konstantinov, lid State Ionics 8 (1983) 127. V.Murakhovsky, P.Kovalsky, (5) V.Drozdov, and Microelectronics Semiconductors Phys. (1980) 32 (in Russian). (1
PHOTOCONDUCTIVITY AND LIGHT STIMULATED CONTACT POTENTIALS IN p-RbAg415 V.G. MURAKHOVSKY * and W. VAN GOOL Inorganic Chemistry Department, State University of Utrecht, Croesestraat 77.4, 3522 AD Utrecht, The Netherlands Received 9 January 1984 Revised manuscript received 29 June 1984
Photoconductivity and photoelectric effects have been established in thin p o l y c r y s t a l l i n e films of ~ - RbAg4I 5. The spectral and the temperature dependence of the p h e n o m e n a have been st +died. The results are interpreted using a model in which ionic processes dominate in the photoconductivity when the exclting photon energy less than 3.2 eV.
I. INTRODUCTION
3. RESULTS AND DISCUSSION
The previous decade was m a r k e d by a sharp increase in the number of publications on ionic superconductors. RbAg4I 5 is a favourite among these. As a rule single crystals or polycrystalline tablets have been investigated, while the number of publications on physical properties of thin polycrystalline RbAg4I 5 films is scarce. This paper presents the results of investigations of photoconductivity and photoelectric voltage in RbAg4I 5 when exposed to light as a function of wavelenghts, light excitation and temperatures. The kinetic parameters of certain relaxation processes in p o l y c r y s t a l l i n e RbAg4I 5 films have been measured.
Figs.1 and 2 represent the spectral dependence of the p h o t o c o n d u c t i v i t y in ~ - RbAg4I 5 films at 120 K. The two curves correspond to the planar and the sandwich configuration. The peak photoccnductivity is at 386 nm which is in g o o d agreement with values found for the optical absorption edge in thin layers of ~ - RbAg4I 5 (2). Fig.2 shows that a large difference in photoconductivity is found as a function of the contact type in the region of 500 - 700 rgn. We assume that a great number of surface d e f e c t states is present with the planar arrangement of the contacts. This leads to the f o r m a t i o n of active clusters in the surface film layer when exposed to light. We assume that the excitation of the carriers frcm these centers leads to the peak of the photoconductivity curve at 520 nm. Ionic
2. EXPERIMENTAL Photoconductivity and photoelectric contact voltage of p o l y c r y s t a l l i n e RbAg4I 5 films have been measured. The films were obtained by thermal v a c u u m e v a p o r a t i o n techniques (I). X - ray diffraction patterns of the films at ambient temperature showed that their cc~nposition c o r r e sponds to ~ - R b ~ 4 I 5. The film thickness was I ~ m. Silver contacts were applied by thermal vacuum deposition. Both planar and "sandwich" geometries were used for the electrodes. The thermal resistance at 120 K was 106 Ohm. All experiments were performed in a low-temperature optical c r y o s t a t under 10 -7 Torr pressure. The samples were excited by a high-power LOMO-lamp in combination w i t h monochromator, or a set of light filters.
i0
-4
t
E i0
-5
10 -6 i
350 Fig.1.
* Permanent address: Technological Institute of Refrigerating Industry, 1/3 Petra Velikogo St, 270000 Odessa, USSR. 0 167-2738/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
i
400
i
450
i
500
i
~,
nm
Spectral d e p e n d e n c e of p h o t o c o n d u c t i v i t y in 8 - RbAg4I 5 absorption edge region: l-planar contact geometry; 2-sandwich contact geometry.
1,1.G. Murakhovsky, W. van Gool / Photoconductivity in #-RbAg4l 5
160
i0
5"10 -6
-4
light
off
1
E ~,
10-5
E 3
LO
© 10 -6
i0-6
Fig.2.
i
i
,
500
600"
Photoconductivity l-planar contact contact geometry.
700
i
i
i
800
~,
0
nm
in the impurity region: geometry; 2-sandwich
MAg4I 5 type superconductors are known for the formation of q u a s i m e t a l l i c centers (clusters) (3). Fig.3 present the p h o t o c o n d u c t l v i t y vs temperature curve at d i f f e r e n t excitation energies. Interesting is the d i s c r e p a n c y in temperature dependence of the peak values of the photocurrent in the regions of 390 and 520 nm. While the super ionic conductors in the region of 390 nm behave as usual (compared to ~ - AgI (4)) the behaviour in the region of 520 nm is far from being standard. This is demonstrated by the fact that in the temperature r e g i o n of 125 - 200 K the photoconductivity peak values at 520 nm increase w i t h increasing temperature. The above behaviour of the p h o t o c o n d u c t i v i t y strengthens our assumption that the r e l e v a n t d e fects exists of quasimetallic silver centers in p o l y c r y s t a l l i n e RbAg4I 5 layers. The p h o t o c o n d u c tivity relaxation kinetics at various temperatures and w a v e l e n g t h s of the excitation light
5'10
-6 2
"
Fig.4.
i
i
.
i00
200
300
t,
s
Kinetics of p h o t o c o n d u c t i v i t y relaxation: 1,2 of excitating light 390 nm, temperature 120 and 160 K, respectively; 3,41 = 520 nm, temperature 120 and 160 K, respectively.
(fig.4) shows two different types of relaxation processes which suggest the presence of several excitation and recombination paths in ~ RbAg4I 5 . For one type of relaxation process the c h a r a c t e r i s t i c time is t1= 50 s; it is m o s t eminent when excitation takes place with I < 410 nm. A second type of decay with t 2 > 50 s takes place over the whole spectral range. The investigation of thin films of RbAg4I 5 leads to another interesting result, viz. a contact voltage of the order 0.15 - 0.25 V when the layer is exposed to light. Such voltage must in fact be the r e s u l t of d i f f e r e n t c o n t a c t potential barriers in the electrode regions of the films. The spectral d e p e n d e n c e of this contact photoelectric voltage is presented in fig.5; m e a s u r e m e n t s were c o n d u c t e d on samples with planar contact geometry only. An inversion of the sign of the c o n t a c t voltage was o b s e r v e d for all samples when the excitation takes place in the
10 -4
i
I
E
£
10-500 O9
>
3
)2 i0
-6
5
I
6
i
i
7
8
i
103 T
Fig.3.
1
10 - 6 i
Photoconductivity versus temperature at different wavelengths of excitation light: I- dark conductivity; 2- 1 = 390 nm; 3- i = 520 nm.
450 -i
Fig.5.
Spectral dependence of contact photoelectric voltage in ~ - RbAg4I 5 .
V.G. Murakhovsky, W. van Gool / Photoconductivity in pRbAg.,Zs
4
2 3
2
> a
F
2
3
4
5
?l_
-1
versus voltage contact Fig.6. Photoelectric and wavelength of light probe: location 2- A = 520 "m; 3- X = l- h = 390 nm; 375 nm.
region The contact of fundamental absorption. photoelectric voltage value depends on the location of the light probe on the film. In order to excite the sample a" optical probe (0.5 mn in diameter) with wavelengths 350, 390 and 520 "m (fig.61 was used. The internal field distribution is in good agreement with that described in (5). Such a distribution is characteristic for silver - containing superionic photoelectrets The value of contact using a weak excitation. voltage decreases with increasing temperature. The results described above lead us to the following conclusions. The mechanism of the physical processes which take place in thin
161
films of the 8 polycrystalline - RbA94I5 superionic conductors when exposed to light does not vary in the wavelength range x >400 nm. In this spectral region the photoelectric contact voltage changes in the same way as the dark voltage. To our opinion this supports the assumption that exposure of 8 - RbAg415 to light produces ?q "photo" ions. Photoionisation of silver atoms localized in the lattice or on for quasimetallic centers may serve as a basis the emergence In the abof such carriers. sorption edge region a change takes place from electronic conduction to predominantly ion CCnwhich leads duction with increasing wavelength, to the inversion of photoelectronic voltage. Cur conclusion is that the photoconductivity ionic supsrin thin films of 6 - RbAg415 conductor is due to the production of silver ions by light. 1. REFERENCES ) V.Borzim, E.Vasilkovskaj, I.Pzola, in Sbornit Nauchnyh Trudov IPM (1975) 27 (in Russian) Materials 9 Inorganic (2) V.Murakhovsky, (1983) 123 ( in Russian). Solid State V.Murakhovsky, (3) V.Drozdov, Phys. 21 (1979) 275 (in Russian). I.Kostadinov, E.Popov, So(4) L.Konstantinov, lid State Ionics 8 (1983) 127. V.Murakhovsky, P.Kovalsky, (5) V.Drozdov, and Microelectronics Semiconductors Phys. (1980) 32 (in Russian). (1