J. PRAKASH: Electronic Excited State and the Phenomena of PIP and PID 61

phys. stat. sol. (b) 114, 61 (1982)

Subject classification: 10.2; 14.4; 22.5.2

Department of Physics, University of Gorakhpur’)

Electronic Excited State and the Phenomena of Photo-Induced Polarization and Depolarization

BY J. PEAKASH

That the activation energy for the orientation of impurity-vacancy dipoles in the electronic excited state (ees) should be less than that required in the electronic ground state (egs) is not the sufficient condition for the occurrence of PIP and PID. It is observed that the activation energy is not the only parameter which controls the occurrence of PIP and PID. The fundamental relaxa- tion time also participates in deciding the situations in which PIP and PID take place. The neces- mry condition found, is that the product of the activation energy and the fundamental relaxation time in ees should be less than the corresponding value in egs.

DaB die Aktivierungsenergie fiir die Orientierung von Storstellen-Leerstellendipolen im angeregten elektronischen Zustand (ees) kleiner sein sol1 als die fur den elektrischen Grundzustand (egs), ist keine hinreichende Bedingung fur das Auftreten von PIP und PID. Es wird beobachtet, daB die Aktivierungsenergie nicht der einzige Parameter ist, der dns Auftreten von PIP und PID steuert. Die Grundgitterrelaxationszeit hat ebenfalls Anteil bei der Entscheidung der Situationen in denen PIP und PID stattfindet. Die gefundene notwendige Bedingung ist, daB das Produkt der Akti- vierungsenergie und der Grundgitterrelaxationszeit in ees kleiner als der entsprechende Wert in egs sein muB.

1. Introduction

Select-ive irradiation in the presencelabsence of an electric field influences the orien- tation of impurity-vacancy (IV) dipoles [l to 111. It has been observed [2 to 41 that the irradiation in the presence of an electric field creates a situation in which more dipoles are aligned along the electric field direction whereas such an irradiation in the absence of the electric field destroys preferred orientation of the frozen-in polarized dipoles. These phenomena are named photo-induced polarization (PIP) and photo- induced depolarization (PID), respectively. It has been proposed [2 to 111 that PIP and PID take place via electronic excited state (ees) where the activation energy for the orientation of IV dipoles is less than that required in the electronic ground state (egs). PIP and PID have been studied in detail in KI:S-- [2 to 41, K1:Se-- [12], and KBr:S-- [I31 systems as a function of irradiation temperature, irradiation time, irradiation wavelength, and electric field strength. PIP and PID have also been observed for Suzuki phase in Pb++-doped KCI by Capelletti et al. [7] and Benci et al. [S]. These experimental results can largely be explained [14] by the following mecha- nisms: (i) thermally activated orientation of IV dipoles via ees in accordance with the bistable model [15] and (ii) electro-optically stimulated exchange [I61 between divalent impurity anion and the anion vacancy. However, the results presented in Pig. 4 of [12] in K1:Se-- could not be explained with the help of these mechanisms. It was inferred [12] that the potential energy curve in the ees in KI:Se-- is somewhat different from that in KI:S-- and KBr:S--. This led us to reconsider the respective

273001 Gorakhpur, India.

62 J. ~ A K A S R

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values of the activation energy in ees and egs. I n the present paper the effective parameters of the ees controlling the occurrence of PIP and P ID are discussed. It has been observed that the activation energy in ees is not the only parameter which con- trols the occurrence of PIP and PID. The fundamental relaxation time also partici- pates in deciding the situations in which PIP and PID take place. The necessary condition for the occurrence of PIP and PID has been found to be that the product of the activation energy and the fundamental relaxation time in ees should be less than the corresponding value in egs.

Fig. 1. Variation of I n t with 1/T in egs and ees

2. Parameters of Electronic Excited State Dielectric relaxation parameters in egs are related through the Arrhenius relation

z =to exp (z), where z is the relaxation time a t T , zo the fundamental relaxation time, and E, the activation energy for the orientation of IV dipoles. It is apparent from (1) that, a plot of In against 1/T will be a straight line and the slope gives the value of E,. The value of z at different temperatures, required for the above plot, can be computed from the depolarization curve observed in ionic thermocurrent measurements as suggested by Bucci et al. [17]. Inz as a function of 1/T in egs is shown in Fig. 1. The relaxation time in ees (t') is similarly controlled by

where zb and E; are the corresponding fundamental relaxation time and act,ivation energy in ees, respectively. It has been established [I51 that EL should be less than E, for the possibility of dipole reorientation through ees during PIP and PID. Conse- quently, In z versus 1/T curves in ees and egs will be as shown in Fig. 1. With decreas- ing temperature, z as well as t' increase exponentially according to (1) and ( 2 ) , respec- tively. Let us suppose that a t a particular temperature TI, the corresponding values of z in egs and ees are zl and z; respectively, as shown in Fig. 1. Since z; is less than z,, the I V dipole may cross over the potential energy barrier [15] in ees or may polarize along the electric field direction through ees, if given a chance. With decreasing

temperature, i t may happen that z, becomes suffici- ently large so that the I V dipoleis unable to cross over the potential energy barrier in the egs whereas i t can cross over the barrier easily in the ees since z;' < zl. Thus, the polarization ultimately increases or decreases through the ees during PIP or PID, respectively. E: - much smaller than E,, proposed by Fischer [I81 for the occurrence of PIP and PID, does not seem to be

Electronic Excited State and the Phenomena of PIP and PID 63

the necessary condition as apparent from Fig. 1. The curves presented in Fig. 1, are capable of explaining the results of PIP and PID collected in KI : S-- [2 to 41, KBr : S-- 1131, and also in KCl:Pb++ [7, 91. However, the results presented in Fig. 4 of [12] in K1:Se-- could not be explained with the help of Fig. 1. This initiated to reexamine the validity and generality of the proposed assumption, i.e., is i t necessary that EL be less than En for the occurrence of PIP or P I D ? While considering l n t versus 1/T curves coresponding to egs and ees of Fig. 1, the values of to and t b also ascertain the inclination of these curves with respect to each other. It is obvious that with different values of to and t b , the situation shall be quite different from that shown in Fig. 1. The possible combinations of E,, E,, to, and t b are: (i) E; < E, and zb < to (corres- ponding to Fig. l ) , (ii) E, < En and t b = to, (iii) E; < E, and z; > to, (iv) E; = E, and t; < to, (v) EA = E, and 7; = to, (vi) E; = E , and t; >to, (vii) E; > En and t; < to, (viii) EL > En and t; = to, and (ix) E; > E , and ti > zo. Now, in cases (i), (ii), and (iv), PIP and PID do take place and l n z versus 1/T is similar to that shown in Fig. 1. It is obvious from case (iv) that EL leas than E , is not a necessary condition as proposed earlier by Prakash and Fischer [2 to 41, Prakash [15], Fischer [lS], Capel- letti et al. [7], and Benci et al. [9]. Thus, the occurrence of PIP and PID cannot be generalized simply by the magnitude of the activation energy. It can be seen that one obtains Eit; < Eato in cases (i), (ii), and (iv). The results of PIP and PID in K1:S-- [2 to 41 and KBr:S-- [13] can be explained following case (i) or also with the help of cases (ii) and (iv). It can be seen from the corresponding In t versus 1/T plots, that t’ happens to be greater than or equal t o t in cases (v), (vi), (viii), and (ix). Consequently, no change is expected to take place via ees during PIP or PID in these cases. Moreover, one obtains ELth 2 Eato in such situations. It is suggested, therefore, that the favour- able condition for the occurrence of PIP and PID happens to be

whereas no change during PIP or PID is observed when

Obviously, EL < En is not the sufficient condition for the occurrence of PIP or PID. For example in case (iii), although EL is less than E, (see Fig. 2), no thermal orienta-

E;& < Ento , (3)

(4) , , E,zo 2 Eato.

l / T - I/T-

Fig. 2 Fig. 3 Fig. 2. Variation of In t with 1/T when EL < E, and 7; > to

Fig. 3. Variation of Int with 1/T when EL > E, and ti < T,,

64 J. PRAKASH

tion (disorientation) will take place via ees a t higher temperatures, whereas i t may take place a t lower temperatures. It should be mentioned, however, that the thermal orientations a t lower temperatures may be restricted (frozen). It is apparent from Fig. 2 that T, is the temperature below which PIP and PID take place. After giving tentative values to E,, E;, z,, and t; corresponding to case (iii), it can be established that Elz; < E,z, happens to be the favourable condition for the occurrence of PIP and PID. Thus, E; < E, or E6 sufficiently smaller than E, is essentially not needed for the occurrence of PIP and PID, rather it should be in accordance with the values of to and z; such that ELz; < Eiz,,. It can further be realized that in case (iii) with increasing value of zh in comparison to'z,, the curve corresponding to ees of Fig. 2 will shift more and more upward leading to still lower values of T, . A stage comes ultimately when PIP and PID do not take place even if EL < E,. It can be seen that such a situation corresponds to EAT; > E,z,. Thus, the occurrence of PIP and PID is con- trolled by (3) and (4).

Let us now consider case (vii), which has not been discussed so far. I n this case EL > E, and t; < to. Consequently, In z versus 1/T curves in egs and in ees will be as shown in Fig. 3. These curves, namely AB and CD, cut each other a t E corresponding t o a temperature T*. Following results are expected to be observed according to Fig. 3:

a) When the specimen is polarized a t temperatures higher than T* and irradiated a t a lower temperature up to T*, PIP would take place.

b) When the specimen is polarized a t temperatures higher than T* and irradiated a t temperatures below T*, PIP would still take place. I n this case the system instead of following the path CD in the ees follows path CEB. Due to intersection a t E, i t accepts only portion CE of the path CD and then proceeds along EB due to the pre- sence of the electric field during PIP. Obviously, the percentage increase during PIP will not be as high as observed in the case of Fig. 1.

c) When the specimen is polarized and irradiated simultaneously a t temperatures above T*, PIP would be observed.

d) When the specimen is polarized and irradiated simultaneously a t T*, the experi- mental conditions between egs and ees are unaltered and the system is unable to distinguish between egs and ees. Thus, no change would be observed when the specimen is polarized and irradiated simultaneously a t T *.

e) When the specimen is polarized and irradiated siinultaneously below T*, one should observe a decrease during PIP. I n this case the polarization temperature and irradiation temperature both are on the right side of E and the system follows path EB in egs and path ED in ees. Due to the fact that the point of intersection a t E corres- ponds to T*, one expects an increase a t temperatures above T* and a decrease a t temperatures below T* during PIP. Thus, one would observe during PIP an increase for T > T* and a decrease for T < T*.

With the help of this expected experimental behaviour a) to e), the results of PIP in K I : Se-- [12] can be successfully explained. If the above-mentioned statements happen to be t*he responsible mechanism during PIP in X I : Se--, they should as well explain the PID results. It has been mentioned in b) that the system follows path CEB during PIP instead of proceeding along CD. However, the system during PID fol- lows the path CD in the ees since the frozen-in polarized dipoles are irradiated in the absence of the electric field. Consequently, the lower temperature limits ob- served through PIP and PID, below which the thermal orientations in ees are frozen, should not be the same. It can be seen from Fig. 3 that the temperature limit through PIP will be lower than that observed through PID. This statement is in accordance with the experimental results collected on K1:Se-- [12]. It should be

Electronic Excited State and the Phenomena, of PIP and PID 65

mentioned, however, that Fig. 1 suggests same value of the temperature limit through PIP and PID as has also been experimentally observed in K1:S-- [4]. It can be established after giving tentative values to E,, E;, to, and t; corresponding to case (vii), that ELTA < Eato happens to be the required condition for the occurrence of PIP and PID. I n situations EdtA 2 Eato, the phenomena of PIP and P ID donot take place.

PIP and PID results in KI:S-- [2 to 41 and KBr:S-- [13] can be successfully ex- plained by the following mechanisms: (i) Thermally activated orientation of IV dipoles via ees in accordance with the bist,able model where E; is less than E, and (ii) electro- optically stimulated exchange between divalent impurity anion and anion vacancy. Mechanism (ii) has been found to be responsible for the constant increase in the percentage polarization during PIP a t lower temperatures. PIP results in K I : Se-- [12] at lower temperatures can, thus, be explained following mechanism (ii). However, PIP results collected in K1:Se-- [12] a t higher temperatures could not be explained with the help of mechanism (i). This necessitated to reconsider the effective para- meters in ees active during PIP and PID. Obviously, El < E, is not the sufficient condition for the orientation (disorientation) of IV dipoles via ees. It has been establish- ed in the preceding section that EitA < Eato is the required condition for the occurrence of PIP and PID.

It has been observed in K1:Se-- [12] that the simultaneous irradiation and polari- zation a t 205 K is ineffective in producing any change during PIP, whereas the simul- taneous irradiation and polarization a t temperatures above 205 K show an increment during PIP. Also, a decrease has been observed during PIP for the simultaneous polarization and irradiation a t temperatures below 205 K. This experimental be- haviour presented in Fig. 4 of [12] can now be explained with the help of case (vii). T* of Fig. 3 corresponds to 205 K observed in the K I : Se-- system. Thus, the observed decrease during PIP a t temperatures below 205 K and an increase a t temperatures above 205 K are in accordance with t,he expectations of Fig. 3. Incorporating the statements c) to e), t,he results presented in Fig. 4 of [12] can, thus, be explained. It has also been experimentally observed that the polarization a t temperatures above 205 K and irradiation at temperatures below 205 K during PIP results in an increase as shown in Fig. 2 of [12]. This behaviour is similar to that observed in KI:S-- [2 to 41 and KBr:S-- [13] and can be explained similarly. However, since we take the help of case (vii) in explaining the results of PIP inK1:Se--, i t is logical to explain the results presented in Fig. 2 of [12] also with the help of case (vii). Statements a) and b) explain successfully the results presented in Fig. 2 of [12]. Furthermore, the observ- ed percentage increase during PIP in K1:Se-- is not as large as in the case of K I : S--, which is well in accordance with statement b). If case (vii)happens to be the responsible mechanism for PIP in KI : Se--, it should explain equally well the PID results. In case (vii) corresponding to Fig. 3, the system during PIP follows the path CEB in the ees instead of CD, whereas it follows the path CD during PID. Con- sequently, t,he lower temperaturelimit, when the thermal orientations in ees is frozen, observed through PIP and PID should not be the same.Thisdifference isdueto thefact that the system follows different paths in ees during PIP and PID. It is obvious from Fig. 3 that the temperature limit in PIP will be lower than that inPID. Thevalues of the observed lower temperature limits in K I : See- through PIP and P ID meas- urements [12] are 110 and 160 K, respectively. Thus, the P ID results inKI:Se-- can also be explained with the help of case (vii). It is obvious, therefore, that the phe- nomena of PIP and PID in K1:Se-- occur under the conditions E’ > E, and t6 < < to such that E;tA < E,T,. However, the qualitative arguments proposed here for the occurrence of PIP and PID, await further justification through other studies.

3. PIP and PID Results

5 physica (b) l l 4 / l

66 J. PRAKASH: Electronic Excited State and the Phenomena of PIP and PID

4. Conclusion

Phenomena of PIP and PID take place via ees.2) It has been observed that EL < E, is not the sufficient condition for the orientation (disorientation) of IV dipoles via ees. EizA < Eato has been found to be the required condition for the occurrence of PIP and PID. Neither PIP nor PID would be observed when E;rA 2 Earo. Thus, the activation energy is not the only parameter which controls the occurrence of PIP and PID. The fundamental relaxation time also participates in deciding the situations in which PIP and PID take place. Just to have an idea of the magnitude of E; in comparison to E,, following permutations are theoretically possible provided EitA <

(i) E; may be sufficiently smaller than E, (as proposed by Fischer [IS]) if zi is in

(ii) E i should be smaller than E,. (iii) EL may be equal to E, if to > t;. (iv) E; may be slightly greater than E, if tA is in close neighbourhood of to such that

(v) EL may be sufficiently greater than E, provided t o is fairly smaller than to. Thus with the help of the proposed model, various results of PIP and PID collected

< E a t 0 :

close neighbourhood of zo such that to > zi.

to >tA, and

in K I : S--, K I : Se--, KBr : S--, and KCl : Pb++ can be qualitatively explained. Ach.nowledgements

The author is thankful to Prof. F. Fischer (University Munster, FRG) for private communications and valuable suggestions. He is also thankful to Prof. N. K. Sanyal and Prof. S. Chandra for helpful discussions.

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[Z] J. PRAKASH and F. FISCHER, J. Physique 37, C7-167 (1976). [3] J. PRAKASH and F. FISCHER, phys. stat. sol. (a) 39, 499 (1977). [4] J. PRAKASH, phys. stat. sol. (a) 54, 681 (1979). [5] J. PRAKASH, Indian J. pure appl. Phys. 16, 995 (1978). [6] J. F’RAEASH, Japan. J. appl. Phys. 17, 1067 (1978). [7] R. CAPELLETTI, R. FIESCHI, and E. OKUNO, Internat. Conf. Color Centers in Ionic Crystals,

[8] R. CAPELLETTI, F. FERMI, F. LEONI, and E. OEUNO, Internat. Symp. Electrets and Dielec-

[9] S. BENCI, R. CAPELLETTI, F. FERMI, and M. MANFREDI, J. Physique 37, C7-138 (1976).

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Sendai (Japan) 1974 (p. 6136).

trics, SLo Carlos (Brazil), Sept. 1975 (p. A22).

[lo] R. CAPELLETTI, F. FERMI, F. LEONI, and E. OKUNO, Internat. Symp. Electrets and Diel-

[ll] R. CAPELLETTI and R. FIESCHI, ibid. (p. 131). [12] J. PRAKASH, to be published. [ 131 J. PRARASH, unpublished. [14] J. PRAKASH, Indian J. pure appl. Phys. 20, 434 (1982). [15] J. PRAKASH, phys. stat. 801. (a) 70, 747 (1982). [16] J. PRAKASH, phys. stat. sol. (a) 57, 775 (1980). [17] C. BUCCI, R. FIESCHI, and G. GUIDI, Phys. Rev. 148, 816 (1966). [18] F. FISCHER, phys. stat. sol. (a) 62, 189 (1979). [19] J. VANDERSCHUWEN and J. GASIOT, in: Topics in Applied Physics, Vol. 37, Chap. 4, Ed.

(Received May 4, 1982)

*) In addition to El], our work on PID in the KI: S-- system has also been quoted in Fig. 4.53

P. BRAUNLICH, Springer-Verlag, Berlin 1979 (p. 212).

of [19].