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Light-induced changes in electrical properties of As3Se2 thin films
Journal of Non-Crystalline Solids 35 & 36 (1980) 1079-1083 ©North-Holland Publishing Company
LIGHT-INDUCED CHANGES IN ELECTRICAL PROPERTIES OF As3Se 2 THIN FILMS T.Botila,M.L.A!dea Institute
of Physics and Materials Technology Bucharest-Magurele,P.O.Box 5207 ROMANIA
We performed measurements of the photoconductivity (PC) and thermally stimulated depolarization currents (TSDC) before and after photodarkening (PD).Spectral onset of PC is shifted to lower energies after PD.TSDC display a low-temperature peak (0.23eV) independent on PD and a high-temperature peak (0.39eV) before PD and(O.54eV) after PD.The activation energy of the dark conductivity is unchanged by PD.The results could be explained by the presence of states with negative correlation energy of about l.leV. It is well known that some materials of the As-Se and As-S systems have photodarkening properties when irradiated with corresponding wave lengths.These materials do not imply a crystallisation phenomenon during exposure ~,2,3,4,5,8]. As it was experimentally observed,this photodarkening effect has both a reversible and an irreversible components.Photodarkening induces in these materials not only changes in optical properties (a shift of the absorption edge Towards lower energies and increase in the refractive index),but also in structural properties. De Neufville [I] considers that in molecular glasses (As~Seg,AsuSg) the irreversible photostructural Transformations are dfie ~o pho~opolimemization. The mechanism of The reversible shift of the absorption edge is not yet very well understood,although a few models based on photodecomposition[4] ,or on structural changes [~ ,or on Trapping of non-equilibrium carriers on deep levels [i] have been proposed. Gurevitch et al ~]performed photodarkening measurements on thin films of the system As-Se with As excess like AsSe and As~Se? and they shown that higher contrast and better reversibility Was-obtained for As3Se 2 thin films. The aim of this paper is the investigation of the light-induced changes in the Transport properties due to non-equilibrium carriers in As Se .The measurements were carried out on thin films (1.5-2~m) d~pos~te~ by flash evaporation on glass substrates.We used both c o planar and sadwich structures.0n coplanar structure the electrodes were both of aquadag and on sandwich structure the electrodes were either both electrodes of Al,or the bottom electrode of A1 and the top electrode of SnO?.0n these structures we performed The photoconductivity and Thermally stimulated depolarization current measurements before and after photodarkenin~.
1079
1080
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
PHOTOCONDUCTIVITY Because the photosignal on the coplanar structure was very small,we measured the photoconductivity with modulated light (8Hz).0n the sandwich structure we measured the photoconductivity with modulated light (SHz) as well as with continuos light. The results were practically the same. The measurements with modulated light were performed with a lock-in system (Brookdeal) and the measurements with continuous light with a vibrating reed electrometer (Cary 401).As spectral light source we used a low voltage tungsten lamp (100W) and a Carl Zeiss quartz monochromator. The spectral dependence O f the photoconductivity photodarkening is shown in fig.l and 2.
r
d
before
and after
obefore PD + Q f t e r PD
7 30
Figure 1 Spectral dependences of photoconduetivity (unnormalized units) before and after photodarkening
20
10
0
o.4
0.6 o.8
x, (,,u)
In fig.l the spectral dependences of photoconductivity before and after photodarkening are presented without normalization in order to put in evidence the increase of the photosignal after photodarkening In fig.2 simJlare curves for both the sandwich and the eoplanar structure are normalized to their maximum. The degree of photodarkening on the coplanar sample was higher than on sandwich sample and therefore the shift of the spectral dependence of the photoconductivity is more accentuated for the coplanar one.The highest value of this shift reached about 0.3eV compared with O.19eV found by Gurevitch at al[5]. In order to see if trappin~ phenomena could play an important role in photodarkening we investigated the influence of photodarkening on
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
the presence method.
and depth of traps
in As3Se 2 thin films,using
]081
the TSDC
1.0
~0.8
EJ c-
Q..
i
0.6
0.4
02
0 04
0 5
06
07 --
k~)
08
Figure 2 Spectral dependences of photoconductivity (normalized to their m a x i m u m ) % e - c o p l a n a r structure before PD;+-coplanar structure after P D ; A -sandwich structure before P D ; O -sandwich structure after PD TSDC MEASUREMENTS To perform the TSDC measurements,the sample had to be polarized.We polarized the sample in two ways:with light only [6,7] and with light and an external electric field simoultaneously. For the polarizaZion with light,the sample was cooled down to liquid nitrogen temperature and then irradiated with monochromatic light (l=0.5~m or ~=0.63~m) for 20 min. After switching off the light,the sample was short-circuited through an electrometer and heated up at a constant rate (0.1K/sec). Because the polarization with light only in the coplanar structure Was negligebly small,we polarized the sample with both light and an external electric field. The external electric field was applied at room temperature and the sample was cooled down to liquid nitrogen temperature under illumination. Upon heating,we got before photodarkening a TSDC curve which displayed two peaks,one situated at 26OK with an activation energy of (O.23±O.02)eV and another one situated at 298K with an activation energy of (0.39±O.02)eV.Similar curves
1082
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
obtained after photodarkening also displayed two peaks ,the peak situated at low Temperature (0.23eV) being unchanged and the second peak being now situated at 308K with an activation energy of (0.54 + 0. 02 )eV ,fig. 3.
, before PD sampte nr. 2. Uc~=340V
~< T
~=0.5~u
< O
t,.-
O
"-&
+ a f t e r PD sarnpte nr. 5 Ua=340V ~.=0.5 ~
10
2
220
2 70
-
T (K)
3 20
Figure 3 TSDC curves As for the sandwich structure,Al is known to give in almost all cases blocking contacts with chalkogenides.Therefore,in order to avoid complications in the case of polarization by an external electric field,we polarized the sample with light only. Indeed~TSDC measurements on sandwich structures display a very well shaped high-temperature peak whose position and activation energy are identical with those formed in coplanar structure before and after photodarkening. The low-temperature peak is present,but masked by strong residual currents and therefore difficult to evaluate it. DISCUSSION On the one hand fig.l shows that photoconductivity increases after photodarkening. On the other hand TSDC curves show that photodarkening deepens the high-temperature trapping level.We consider these
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
|083
two facts to be connected in the following way:if this deep level acts as a trapping level for minority carriers then after photodarkening this level being deeper,the ~hermal freeing probability for trapped carriers is decreased and this fact may lead to the increasing of the life time of the carriers which are responsible for the photoconductive process. Indeed,bleaching the sample tended to restore the initial situation.As we did not observe any change in the slope of the temperature dependence of the dark conductivity before and after photodarkening,we conclude that all photodarkening processes take place in such a way that the energy between the band which is responsible for the conductivity and the Fermi level remains unchanged. This and the fact That the activation energy of 0.23eV is unchanged after photodarkening could be explained by the presence of states with negative correlation energy[gJof about l.leV which pinn-up the Fermi level at (0.78±O.02)eV.These states can also act as trapes in TSDC measurements.The level of 0.4eV before photodarkening and 0.54eV after photodarkening is probably connected with the shifting band. AKNOWLEDGEMENTS We are very thanksful to prof. R.Grigorovici and T.Stoica for helpful discussions and to Dr. Liubin for samples. REFERENCES [i] De Neufville,J.P.,Moss,S.C.,0vshinsky,S.R.,Jrnl.of Non-Crystalline Solids,13,(Ig74) 191. [~ Tanaka,K.,Iizima,S.,Aoki,K.,Minomura,S.,Proc.6th Innt. Conf. Amorph. Liquid Semie.,Leningrad (1976) ,42. [4 Tanaka,K.,Kikuki,M.,Proc.5th Int. Conf. Amorph. Liquid Semic.,Garmisch Partenkirchen~Ig74) 439(Taylor and Francis LTD-London) [~ Berkes,J.S.,et al,Jrnl. Appl.Phys.,42 (1971) ~gOB. [~ Gurevitch,S.B.,et al,Golografia i obrabotka informatzii(in russian) (1978) 428. [~ BQ%ila~T. ,Thesis (in rom.) (1974). Botila,T.,Croitoru,N.,Phys. Stat. Sol.,(a),19 (1973) 357. E~Tanaka,K,Apll. Phys. Letters,26(Ig75)243. Street,R.A.,Mott,N.F.,Phys.Rev. Letters,35(ig75)1293.
LIGHT-INDUCED CHANGES IN ELECTRICAL PROPERTIES OF As3Se 2 THIN FILMS T.Botila,M.L.A!dea Institute
of Physics and Materials Technology Bucharest-Magurele,P.O.Box 5207 ROMANIA
We performed measurements of the photoconductivity (PC) and thermally stimulated depolarization currents (TSDC) before and after photodarkening (PD).Spectral onset of PC is shifted to lower energies after PD.TSDC display a low-temperature peak (0.23eV) independent on PD and a high-temperature peak (0.39eV) before PD and(O.54eV) after PD.The activation energy of the dark conductivity is unchanged by PD.The results could be explained by the presence of states with negative correlation energy of about l.leV. It is well known that some materials of the As-Se and As-S systems have photodarkening properties when irradiated with corresponding wave lengths.These materials do not imply a crystallisation phenomenon during exposure ~,2,3,4,5,8]. As it was experimentally observed,this photodarkening effect has both a reversible and an irreversible components.Photodarkening induces in these materials not only changes in optical properties (a shift of the absorption edge Towards lower energies and increase in the refractive index),but also in structural properties. De Neufville [I] considers that in molecular glasses (As~Seg,AsuSg) the irreversible photostructural Transformations are dfie ~o pho~opolimemization. The mechanism of The reversible shift of the absorption edge is not yet very well understood,although a few models based on photodecomposition[4] ,or on structural changes [~ ,or on Trapping of non-equilibrium carriers on deep levels [i] have been proposed. Gurevitch et al ~]performed photodarkening measurements on thin films of the system As-Se with As excess like AsSe and As~Se? and they shown that higher contrast and better reversibility Was-obtained for As3Se 2 thin films. The aim of this paper is the investigation of the light-induced changes in the Transport properties due to non-equilibrium carriers in As Se .The measurements were carried out on thin films (1.5-2~m) d~pos~te~ by flash evaporation on glass substrates.We used both c o planar and sadwich structures.0n coplanar structure the electrodes were both of aquadag and on sandwich structure the electrodes were either both electrodes of Al,or the bottom electrode of A1 and the top electrode of SnO?.0n these structures we performed The photoconductivity and Thermally stimulated depolarization current measurements before and after photodarkenin~.
1079
1080
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
PHOTOCONDUCTIVITY Because the photosignal on the coplanar structure was very small,we measured the photoconductivity with modulated light (8Hz).0n the sandwich structure we measured the photoconductivity with modulated light (SHz) as well as with continuos light. The results were practically the same. The measurements with modulated light were performed with a lock-in system (Brookdeal) and the measurements with continuous light with a vibrating reed electrometer (Cary 401).As spectral light source we used a low voltage tungsten lamp (100W) and a Carl Zeiss quartz monochromator. The spectral dependence O f the photoconductivity photodarkening is shown in fig.l and 2.
r
d
before
and after
obefore PD + Q f t e r PD
7 30
Figure 1 Spectral dependences of photoconduetivity (unnormalized units) before and after photodarkening
20
10
0
o.4
0.6 o.8
x, (,,u)
In fig.l the spectral dependences of photoconductivity before and after photodarkening are presented without normalization in order to put in evidence the increase of the photosignal after photodarkening In fig.2 simJlare curves for both the sandwich and the eoplanar structure are normalized to their maximum. The degree of photodarkening on the coplanar sample was higher than on sandwich sample and therefore the shift of the spectral dependence of the photoconductivity is more accentuated for the coplanar one.The highest value of this shift reached about 0.3eV compared with O.19eV found by Gurevitch at al[5]. In order to see if trappin~ phenomena could play an important role in photodarkening we investigated the influence of photodarkening on
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
the presence method.
and depth of traps
in As3Se 2 thin films,using
]081
the TSDC
1.0
~0.8
EJ c-
Q..
i
0.6
0.4
02
0 04
0 5
06
07 --
k~)
08
Figure 2 Spectral dependences of photoconductivity (normalized to their m a x i m u m ) % e - c o p l a n a r structure before PD;+-coplanar structure after P D ; A -sandwich structure before P D ; O -sandwich structure after PD TSDC MEASUREMENTS To perform the TSDC measurements,the sample had to be polarized.We polarized the sample in two ways:with light only [6,7] and with light and an external electric field simoultaneously. For the polarizaZion with light,the sample was cooled down to liquid nitrogen temperature and then irradiated with monochromatic light (l=0.5~m or ~=0.63~m) for 20 min. After switching off the light,the sample was short-circuited through an electrometer and heated up at a constant rate (0.1K/sec). Because the polarization with light only in the coplanar structure Was negligebly small,we polarized the sample with both light and an external electric field. The external electric field was applied at room temperature and the sample was cooled down to liquid nitrogen temperature under illumination. Upon heating,we got before photodarkening a TSDC curve which displayed two peaks,one situated at 26OK with an activation energy of (O.23±O.02)eV and another one situated at 298K with an activation energy of (0.39±O.02)eV.Similar curves
1082
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
obtained after photodarkening also displayed two peaks ,the peak situated at low Temperature (0.23eV) being unchanged and the second peak being now situated at 308K with an activation energy of (0.54 + 0. 02 )eV ,fig. 3.
, before PD sampte nr. 2. Uc~=340V
~< T
~=0.5~u
< O
t,.-
O
"-&
+ a f t e r PD sarnpte nr. 5 Ua=340V ~.=0.5 ~
10
2
220
2 70
-
T (K)
3 20
Figure 3 TSDC curves As for the sandwich structure,Al is known to give in almost all cases blocking contacts with chalkogenides.Therefore,in order to avoid complications in the case of polarization by an external electric field,we polarized the sample with light only. Indeed~TSDC measurements on sandwich structures display a very well shaped high-temperature peak whose position and activation energy are identical with those formed in coplanar structure before and after photodarkening. The low-temperature peak is present,but masked by strong residual currents and therefore difficult to evaluate it. DISCUSSION On the one hand fig.l shows that photoconductivity increases after photodarkening. On the other hand TSDC curves show that photodarkening deepens the high-temperature trapping level.We consider these
T. Botila, M.L. Aldea / Electrical Properties of As3Se 2 Thin Films
|083
two facts to be connected in the following way:if this deep level acts as a trapping level for minority carriers then after photodarkening this level being deeper,the ~hermal freeing probability for trapped carriers is decreased and this fact may lead to the increasing of the life time of the carriers which are responsible for the photoconductive process. Indeed,bleaching the sample tended to restore the initial situation.As we did not observe any change in the slope of the temperature dependence of the dark conductivity before and after photodarkening,we conclude that all photodarkening processes take place in such a way that the energy between the band which is responsible for the conductivity and the Fermi level remains unchanged. This and the fact That the activation energy of 0.23eV is unchanged after photodarkening could be explained by the presence of states with negative correlation energy[gJof about l.leV which pinn-up the Fermi level at (0.78±O.02)eV.These states can also act as trapes in TSDC measurements.The level of 0.4eV before photodarkening and 0.54eV after photodarkening is probably connected with the shifting band. AKNOWLEDGEMENTS We are very thanksful to prof. R.Grigorovici and T.Stoica for helpful discussions and to Dr. Liubin for samples. REFERENCES [i] De Neufville,J.P.,Moss,S.C.,0vshinsky,S.R.,Jrnl.of Non-Crystalline Solids,13,(Ig74) 191. [~ Tanaka,K.,Iizima,S.,Aoki,K.,Minomura,S.,Proc.6th Innt. Conf. Amorph. Liquid Semie.,Leningrad (1976) ,42. [4 Tanaka,K.,Kikuki,M.,Proc.5th Int. Conf. Amorph. Liquid Semic.,Garmisch Partenkirchen~Ig74) 439(Taylor and Francis LTD-London) [~ Berkes,J.S.,et al,Jrnl. Appl.Phys.,42 (1971) ~gOB. [~ Gurevitch,S.B.,et al,Golografia i obrabotka informatzii(in russian) (1978) 428. [~ BQ%ila~T. ,Thesis (in rom.) (1974). Botila,T.,Croitoru,N.,Phys. Stat. Sol.,(a),19 (1973) 357. E~Tanaka,K,Apll. Phys. Letters,26(Ig75)243. Street,R.A.,Mott,N.F.,Phys.Rev. Letters,35(ig75)1293.