Photostimulated luminescence of phosphors

Journal of Luminescence 9 (1974) 61-70. © North-Holland Publishing Company

PHOTOSTIMULATED

LUMINESCENCE

OF PHOSPHORS*

V.G. K R O N G A U Z and I.A. P A R F I A N O V I C H lrkutsk State University, Irkutsk, USSR

Received 22 August 1972 Revised manuscript received 20 April 1973 The published data on photostimulated luminescence and some new results obtained by the authors with the KC1-T1 phosphor are analyzed. It is shown that for phosphors with associated donor-acceptor pairs photo stimulated luminescence appears as a consequence of an inter-impurity electronic transition. The kinetic aspects of the luminescence depend upon the Coulomb attraction between a quasi-free electron and a charged donor. The character of the radiative recombination is determined by the interaction energy between the two partners an4 by the trap depth. This enables us to consider various well-known phosphor models from a general point of view. It is emphasized that the coexistence of various mechanisms in the same phosphor must be taken into account to obtain a correct interpretation of the luminescence characteristics. It is also shown that, for the mechanism involving free holes, an exciton must be created before the recombination energy can be transferred to the luminescent center.

1. I n t r o d u c t i o n Luminescence t h r o u g h optical de-excitation ( p h o t o s t i m u l a t e d luminescence PSL) was described by Becquerel [1]. This p h e n o m e n o n has been t h o r o u g h l y studied in the I I b - V I b (zinc sulphide type) and in the l a - V I I b (alkali halides) compounds. Also, it has been observed in m a n y other binary c o m p o u n d s [ 2 - 1 0 ] , as well as in a n u m b e r o f elemental s e m i c o n d u c t o r s such as silicon [11, 12] and diam o n d [13, 14]. PSL has been recently observed in a large group of o x y g e n dominated phosphors [ 15] and even in several organic phosphors [16, 17]. T h e r e f o r e , PSL is a rather general p h e n o m e n o n . Optical de-excitation can be p e r f o r m e d in a wide energy range ( 0 . 0 5 - 1 0 eV) [11, 18, 19]. A n u m b e r o f authors have investigated the kinetics o f PSL [ 2 0 - 2 8 ] . These works develop PSL as an effective m e t h o d for studying electronic processes and micro-defects in solids. The most valuable are the latest investigations. This paper is devoted to a brief discussion o f t h e m and some new data.

* Presented at International Conference on Luminescence, Leningrad, August 1972. 61

62

V.G. Krongauz, I.A. Parfianovieh, Photostimulated luminescence

2. Electron PSL In most cases PSL appears to be due to the stimulation of electron trapping centers. According to the Schoen-Klasens model it is accompanied by the transfer of electrons through the conductivity band (fig. 1). While stimulating only one kind of d-donors, according to ref. [20] we obtain for the PSL yield hole centers A of the r-kind: (1)

I = CB d r~arNr/(F + H ) ,

where C is a constant, B d is the absorbing part of the stimulating beam, a r is the recombination coefficient, N r is the concentration of the A r centers, F = Zj6j and H = ~'n a n N n are the trapping and recombination probabilities respectively for all kinds of donors and acceptors and r/is the quantum yield of ionization. When donor ionization can also occur thermally (fig. 1), then r/depends on temperature [29] and r/= [1 + q exp ( - E d , / k T ) ] - I

,

(2)

where q is a constant and E d , is the depth of the excited level. According to the classical phosphor band model the parameters of the centers do not depend on their spatial distribution and all typical interactions such as emission, quenching, sensiti-

Fig. 1. Band diagram of electronic PSL. Left: PSL due to the formation of conduction band electrons and right: tunnelling PSL. Notation of transitions: 1,1' - photostimulation; la - secondary thermostlmulation; 2,3 - radiative recombinations; 4 - non-stimulated direct radiative transition; 2a - tree electron transition to the excited acceptor level; 5 - tunnellingbetween donor and acceptor excited states.

V.G. Krongauz, I.A. Parfianovich, Photostimulated luminescence

63

(a)

3O

:

2 =

i X%{

,x> ,(',,4 x'Nf

20 - ~ . / / // //

10

),( " x ×x //

N Y~ x~

z~ I 150

! 200

I

5g5

OK

250

-o

~

Z z

z z/ /

Z

/

12

z~

z

2 I1

300

400

500

(c)

3,3'

Q

-

100

I

150

200

250

OK

o~

Fig. 2. Photostimulated flash luminescence of thallium emission in KC1, doped with 0.1 mol % T1. The phosphor was X-rayed at T x = 78°K (a), (b) 480°K and (c) 280°K. Excitation time t x = 10 rain except for the 1',2' and 3' curves (2c). In the latter case t x = 0.2 s. Probing light pulses of -2 -3 10 -10 s were used in all measurements. The reflected pulses of PSL are schematically shown in figs. 2a and 2b. (a) Irreversible rise of PSL stimulated by F light (E = 2.2 eV). The effect depends on increasing concentration of TI 2+ due to the thermal destruction of V x centers. (b) The reversible changes of kinetics and efficiency with temperature depending on intermediate trapping of electrons at the TI + sites (E = 2.2 eV). (c) The temperature dependence of PSL efficiency stimulated by light from F band (2,2'; E = 2.2 eV) and T1° bands ( 1 , 1 ' ; E = 0.75 eV and 3 , 3 ' ; E = 3.2 eV). c ~ - - ~ l = [1 + (2.9 X 10-S)T2] -1.

64

v.G. Krongauz, I.A. Parfianovich, Photostirnulated l u m i n e s c e n c e

zation, and retrapping are of a recombination character. PSL is successfully used not only for the investigation of kinetic parameters [30-33] but also for the examination of various types of interactions [34-53]. Thus, the thermal emptying of shallow acceptor levels usually leads to the increase of/, which is maintained during consequent cooling of the phosphor (fig. 2a). Further, with temperature variation the retrapping of electrons on shallow S-donors is revealed in reversible variations of both PSL kinetics and intensity (fig. 2b). The quantity A I / I 1 = 8 s / ( F + g - 8s)

(3)

characterizes the relative trapping probability in S-levels [41,42]. Effects shown for KC1-T1 (fig. 2) were observed in many phosphors [41-53]. These effects fit well in the conventional band picture though detailed examination of PSL indicates the insufficiency of this model; the enhancement of retrapping probability with increase of excitation time t x being a good example (fig. 3). This fact could be explained only on the basis of concepts which take into account the dependence o f F and H on spatial distribution of the centers. According to the diffusional theory [27] and the hypothesis of cascade capture [54], the recombination process should be monomolecular with decreasing distance R between partners until the magnitude of the Coulombic attraction energy becomes compatible with the magnitude of the thermal energy. The mean value of R decreases with weaker excitation [27]. Hence, diminution of A I / I 1 with smaller t x is determined by the increased sharing of close F and T12+ centers. The transition to the monomolecular kinetics is revealed also by the character of the electric field influence on PSL efficiency [55-58]. a~ 11 6

o o

4 o

O. 1

I

I

I

I

1

10

1 O0

1000

tx, sec

Fig. 3. The dependence of electron retrapping probability in KC1-T1on X-rayingtime, Tx = = 480°K, E = 2.2 eV. The meaning of ~xl/It = (I2 - 11 )/Ii is explained in the text and in fig. (2b).

td G. Krongauz, I.A. Parfianovich, Photostimulated luminescence

65

It is well known that the Coulombic attraction is capable of altering the energy parameters of centers [27, 59]. Thorough measurements have shown that for small t x the parameters of the ground state for deep donors do not change [51 ]. However, it is obvious that the size of the perturbation is proportional to Ei/Ed, where E i is the interaction energy and L"d the electron-binding energy. There are four stimulation bands for T10 centers in KC1-T1 w i t h e 1 = 0.75, E 2 = 1.1, E 3 = 1.6 and E 4 = = 3.2 eV. But only E 1 and E 2 correspond to transitions into the bound state [60]. The above data show that the binding energy of the T10 center both for the ground and first excited states is within the following limits: 1.1 > 1 it is easy to deduce I = (1 + C 1 T 2 ) - I ,

(4)

where C 1 is a constant. Our experimental data are well approximated by eq. (4), provided C 1 = 2.9 X 10 - 5 °K 2 (fig. 2c). Direct transitions in d o n o r - a c c e p t o r pairs are known to cause luminescence in a great number of phosphors [69]. This seems to exclude PSL. Nevertheless, the IRstimulability has been observed by a number o f authors [6, 9 - 1 2 , 70]. It has been shown that it induces the electron transition from the distant donor to the ionized d o n o r - a c c e p t o r pair [9, 10, 12]. We believe that PSL is a phenomenon characteristic of the associated pairs themselves. The probability of inter-impurity transitions can be written as W = Wma x exp [-(8mEd)l/2R/h ] ,

(5)

where m is the effective mass of the electron. The decrease o f E d with the donor transition to one o f the excited hydrogen-like levels results in an increase in W. The intermediate excited state within the pairs was postulated by Kallmann et al. [70] for the explanation of PSL features in zinc sulphide phosphors. Riehl seemed to have observed such PSL [19].

66

V. G. Krongauz, I.A. Parfianovich, Photostimulated luminescence

3. Hole PSL

Only a few cases of positive hole PSL are found, in contrast to that of the electron hole [71-75]. Thus, it is firmly established that the emission of T1+ and In + centers appears in X-rayed KI-T1 and KI In phosphors when illuminated by Vk light. However, under the same conditions there occurs non-activator luminescence in KC1-T1, KC1-Ag and NaC1-Ag phosphors with E = 2.9, 2.3, and 3.25 eV respectively. According to ref. [73] the pecularities of the hole PSL are stipulated by the fact that M° centers (T10, AgO, In 0) are large radius centers. Thus, instead of the direct hole recombination with M° electron tunnelling occurs from M° to the near V k center, resulting in the creation of a self-trapped exciton perturbed by the M+ field. Then, either its own emission (chlorides) or the energy transfer to the M+ center (iodides) is possible. In our opinion the non-equivalence between the hole and electron luminescence is of a more general character. Direct hole recombination means the transition of the electron trapped by the center to the valence band. The recombination energy is transmitted from the center to the lattice. Thus, for M+ emission, exciton creation and energy transfer must occur. Their realization depends not only on the effectiveness of the exciton mechanism but also on the value of the recombination energy. The importance of these conditions is underlined by the fact that radiative recombination in alkali halides and phosphors of the zinc sulphide type generally follows the Schoen-Klasens model, not the Lamb Klick model. The latter is possible under the indirect activation when the near-impurity exciton is an emitting center. However, the hole asymmetry can disappear if the recombination has "inner-center" character, i.e. the optical transitions correspond to electron transfer between the acceptor and donor components of the center. The donor-acceptor pair could be given as an example. Thus, irrespective of the sequence of electron and hole trapping, the state corresponding to the excited state of the charge-transfer complex is formed.

4. PSL and recombination mechanism Various types of electron PSL stimulation are shown in fig. 1. The process on the left corresponds to the recombination of a free charge carrier with a trapped one of opposite sign (fig. 1). The relationship between the luminescence and photoconductivity, [76, 77] along with other data [27], underlines the prevalence of such PSL [78-80]. It is worth mentioning that the stimulation of phosphors after their long storage confirms the Validity of such a recombination model [78-80]. The modification of the usual Schoen-Klasens model depends on the presence of an electronic interaction between the recombining partners (fig. 1, right) and also on the small meanfree path of carriers. The data obtained on PSL show that the critical-condition for occurrence of one of the recombination types is the relation be-

V.G. Krongauz, LA. Parfianovich, Photostimulated luminescence

67

tween the interaction energy of the charge carriers (El) and their binding energy (E d or EA). Under these circumstances it is possible to consider three important cases. 4.1. Case (1) weak, w h e n E i ~ k T

Localized partners (Ed, E A >~ k T ) are considered to be unbound. However, the interaction becomes noticeable if one of them is free and possesses an energy comparable with k T . 4.2. Case (2) intermediate, when E i ~ Ed. b u t E i ~ E d

The interaction between centers in the ground state has little influence on their properties. However, when one of them is excited, e.g. donor, not only is the probability of electron ejection to the conduction band changed but also tunnelling to the acceptor becomes possible; the above state is analogous to a chemical chargetransfer complex. This type of interaction may be quite important in recombination processes taking place in unassociated donor acceptor pairs. 4.3. Case (3) strong, w h e n E i ,~ E d

Here the donor-acceptor pair forms a charge-transfer complex. When bonding between the components is sufficiently strong, intra-complex storage of energy and stimulability should disappear. If one of the levels is emptied, PSL could exist as an external process in its relation to the complex. A case is known when such stimulation corresponds to the process of complex formation itself. ESR data indicate covalent bonding between the impurity (ns) 1 ions and their ligands in I VII and I1 V1 compounds [ 8 1 - 8 3 ] . We should also like to indicate a spectroscopic feature of charge-transfer complex creation: ionization of (ns) 2 ions like In +, Tl +, Ga + etc. gives rise to long-wavelength absorption and emission bands, e.g. 4.15 and 2.12 eV in K C I - I n . Absorption of photons with E = 4.15 eV by the [ln 2+ (ligands)] raises the complex into the excited state, i.e. [In + (ligands)+]. We note that the same state is achieved when the hole is captured by anions surrounding the In + center. Upon illumination with V k light one can observe PSL with E = 2.12 eV in K C I - l n [75]. These three interaction limits cover all contemporary modifications of the phosphor semiconductive model - diffusional theory, donor-acceptor theory, etc. The fundamental factor which determines the recombination mechanism is the electronic structure of the phosphor. Nevertheless, the distinguishing feature for all of these modifications is the dependence of the transition probability on the spatial distribution of charge carriers. Thus, it is not surprising that in a single phosphor different mechanisms can co-exist. The validity of this situation has been demonstrated here for classical KCI-T1 phosphors and is also, no doubt, true for many other ordinary phosphors.

68

V.G. Krongauz, LA. Parfianovich, Photostimulated luminescence

References [1] E. Becquerel, Lalumi~re 1 (1867). [2] P. Lenard, F. Schmidt and P. Tomaschek, Handbuch der experimental Physik, Bd. 23, Tell 1 (Leipzig, 1928). [3] V.V. Antonov-Romanobskii, V.L. Levshin, Z.L. Morgenshtern and Z.A. Trapeznikova, Dokl. Akad. SSSR 54 (1946) 19. [4] B. O'Brien, J. Opt. Soc. Amer. 36 (1946) 351,369. [5] V.M. Belous and N.G. Djachenko, Opt. Spectrosk. 10 (1961) 649. [6] E.F. Gross and D.S. Nedzvetskii, Dokl. Akad. Nauk SSSR 152 (1963) 1335. [7] C.M. Sunta, Phys. Stat. Sol. 37 (1970) K81. [8] L.A. Vinokurov and M.V. Fok, Opt. Spektrosk. 21 (1966) 588. [9] V.G. Gaivoron and V.I. Sidorov, Pisma v Zh. Eksper. Teor. Fiz. 8 (1968) 10, 534. [10] V.G. Gaivoron and V.I. Sidorov, FTP, 4 (1970) 702. [11] Sh. M. Kogan, T.M. Livshits and V.I. Sidorov, Pisma v Zh. Eksper. Teor. Fiz. 2 (1965) 365. [12] A.S. Kaminskii and Ja.E. Pokrovskii. Fiz. Tech. Polupr. 3 (1969) 1766. [13] C. Bull and F. Garlick, Proc. Phys. Soc. A63 (1950) 1283. [14] V.G. Krongauz, K.N. Pogadajev, E.S. Vilutis and V.S. Tatarinov, lzv. Akad. Nauk SSSR, Ser. Fiz. 33 (1969) 924. [15 ] A.T. Merzljakov and V.G. Krongauz, Luminescent Materials and Particular Clean Substances, 7. (Stavropol, 1972) p. 75. [16] N.M. McClain and A.C. Albrecht, J. Chem. Phys. 44 (1966) 1594. [17] A. D~roul~de, J. Luminscence, 3 (1971) 302. [18] G. Baur, J. Knobloch, N. Riehl and P. Thoma, Proc. Int. Conf. Luminescence, (1966) 1012. [19] N. Riehl, J. Luminescence 1, 2 (1970) 1. [20] R.T. Ellikson and W.L. Parker, Phys. Rev. 70 (1946) 290. [21] R.C. Herman, C.F. Meyer and H.S. Hopfield, J. Opt. Soc. Amer. 38 (1948) 999. [22] V.L. Levshin, Photoluminescence of Liquid and Solid (Moskva, Gostechteoretizdat, 1951 ). [23] Ch.B. Lushchik, Trudy IFA Akad. Nauk Estonian SSR 4 (1956) 42. [24] P. Br~unlich, Phys. Stat. Sol. 22 (1967) 391. [25] D. Curie, Luminescence in Crystals (Wiley, New York, 1963). [26] M.V. Fok, Introduction in Kinetics of Luminescence Crystal Phosphors (Nauka, Moskva, 1964). [27] V.V. Antonov-Romanovskii, Kinetics of Photoluminescence Crystal Phosphors (Nauka, Moskva, 1966). [28] D.E. Mason, Rev. Mod. Phys. 4 (1965) 37. [29] N. Mott and R. Genri, Electronic Processes in Ionic Crystals (Oxford, 1948). [30] L.A. Vinokurov and M.V. Fok, Opt. Spectrosk. 10 (1961) 374. [31] E. Bukke, L.A. Vinokurov and M.V. Fok, Opt. Spectrosk. 2l (1966) 449. [32] M.V. Fok, FTP4 (1970) 1009. [33] Vu Kuang and M.V. Fok, Zh. Prikl. Spectrosk. 16 (1972) 668. [34] R.G. Kaufman and W.B. Hadley, J. Chem. Phys. 47 (1967) 264. [35] I. Broser and H.-I. Schulz, J. Electrochem. Soc. 108 (1961) 545. [36] V.G. Krongauz and I.A. Parfianovich. Izv. VUZ'ov, Fiz. 2 (1971) 50. [37] V.G. Krongauz, lzv. Akad. Nauk SSSR, Set. Fiz. 30 (1966) 1420. [38] V.A. Gorjunov and V.L. Levshin, Zh. Prikl. Spectrosk. 4 (1966) 316. [39] V.A. Gorjunov, V.L. Levshin and A.V. Stankova, Izv. Akad. Nauk SSSR, Ser. Fiz. 30 (1966) 1549. [40] V.M. Belous, Zh. Prikl. Spectrosk. 5 (1966) 210. [41] V.G. Krongauz, P.N. Jarovoi and I.A. Parfianovich, Izv. Akad. Nauk SSSR, SeE Fiz. 33 (1969) 844.

V.G. Krongauz, LA. Parfianovich, Photostimulated luminescence

69

[42] V.G. Krongauz, I.A. Parfianovich, P.N. Jarovoi and B.I. Rogalev, Radiation Physics of Nonmetallic Crystals 3 (Kiev, Naukova Dumka, 1966) p. 66. [43] I.A. Parfianovich, V.G. Krongauz and E.I. Shuraleva, Izv. VUZ'ov, Fiz. 3 (1963) 66. [44] I.A. Parfianovich, V.G. Krongauz and E.I. Shuraleva, Izv. Akad. Nauk SSSR, Ser. Fiz. 29 (1965) 43. [45] I.A. Parfianovich and V.G. Krongauz, Opt. Spectrosk. 20 (1966) 1059. [46] I.A. Parfianovich, E.I. Shuraleva, E.E. Pensina, V.G. Krongauz and V.V. Pologrudov, Proc. Int. Conf. Luminescence, Budapest, (1966) p. 756. [47] M.M. Malley and P.M. Rentzepis, J. Chem. Phys. 52 (1970) 6443. [48] 1. Jaek, T.J. Laisaar, P.J. Raudsepp and T.I. Hjutt, Trudy IFA Akad. Nauk Estonian SSR 35 (1969) 88. [49] G.K. Zolotaryov, Ch.B. Lushchik, T.A. Soovik, I.V. Jaek and M.A. Elango. Izv. Akad. Nauk SSSR, Ser. Fiz. 29 (1965) 36. [50] A.I. Ryskin, N.A. Tolstoi and T.I. Halko, Opt. Spectrosk. 15 (1963) 659. [51] V.G. Krongauz and P.N. Jarovoi, Zh. Prikl. Spectrosk., in press. [52] I.A. Parfianovich and P.N. Jarovoi, lzv. VUZ'ov, l:iz. 6 (1970) 105. [53] V.L. Levshin and V.F. Tunitskaya, Opt. Spectrosk. 2 (1957) 350. [54] M. Lax, Phys. Rev. 119 (1960) 1502. [55] V.P. Denks and V.I. Leiman, Fiz. Tverd. Tela 10 (1968) 1812. [56] V.P. Denks, V.I. Leiman, N.L. Lukantsever and F.A. Savichin. Fiz. Tverd. Tela 12 (1970) 1455. [57] V.V. Pologrudov and E.N. Karnauchov, Izv. Akad. Nauk SSSR, Ser. Eiz. 35 (1971) 1320. [58] V.A. Grigoryev, V.K. Ljapidevskii and I.M. Obodovskii, Fiz. Tverd. Tela 11 (1969) 1042, 1996. [59] J.S. Prener and F.E. Williams, Phys. Rev. 101 (1956) 1427. [60] Ch.B. Lushchik, H.F. K~i~/mbre,Ju.L. Lukantsever, N.E. Lushchik, E.S. Tiisler and I.V. Jaek, Izv. Akad. Nauk SSSR, Ser. Fiz. 33 (1969) 863. [61] E.F. Martynovich, I.A. Parfianovich and Ju.B. Nemirovskii, Fiz. Tverd. Tela 19 (1972) 1240. [62] E.F. Martynovich and I.A. Parfianovich, Opt. Spectrosk. 32 (1972) 570. [63] I.A. Parfianovich and E.F. Martynovich, Izv. Akad. Nauk Latv. SSR, Ser. Fiz. Techn. 1 (1972) 39. [64] V.G. Chernjak and I.K. Pljavin, Izv. Akad. Nauk Latv. SSR, Ser. Fiz. Techn. 1 (1968) 8. [65] L.E. Nagli, Izv. Akad. Nauk Latv. SSR, Ser. Fiz. Techn. 3 (1971) 34. [66] L.E. Nagli, Izv. Akad. Nauk Latv. SSR, Ser. Fiz. Techn. 3 (1970) 52. [67] H.F. K~/~imbre, M.V. Fok and I.V. Jaek, Izv. Akad. Nauk SSSR, Set. Fiz. 30 (1966) 1452. [68] M. Okk, I. Jaek, Izv. Akad. Nauk SSSR, Ser. Fiz. 29 (1965) 46. [69] F. Williams, Phys. Star. Sol. 25 (1968) 493. [70] K. Luchner, H.P. Kallmann, B. Kramer and P. Wachter, Phys. Rev. 129 (1963) 593. [71 ] V.M. Belous, Opt. Spectrosk. 1. Luminescence. M.-L., lzd-vo Akad. Nauk SSSR (1963) 193. [72] G.K. Zolotaryov and Ch.B. Lushchik, Trudy IFA Akad. Nauk. Estonian SSR 31 (1966) 121. [731 M.F. Kink and I.V. Jaek, Trudy IFA Akad. Nauk. Estonian SSR 35 (1969) 244. [74] I. Jaek and M. Kink, Phys. Stat. Sol. 33 (1969) 905. [75] S. Zazubovich, Phys. Stat. Sol. (b) 50 (1972) 785. [76] D.B. Gurevich, N.A. Tolstoi and P.P. Feofilov, Dokl. Akad. Nauk. SSSR 71 (1950) 29. [77] I. Broser, H. Kallmann and R. Warminsky, Z. Naturforsch. 4a (1949) 631. [78] I.A. Parfianovich, Zh. Eksper. Teor. Fiz. 21 (1951) 314. [79] I.A. Parfianovich, Zh. Eksper. Teor. Fiz. 25 (1953) 614. [80] H.N. Hersch, J. Electrochem. Soc. 118 (1971) 144c.

70

V.G. Krongauz, LA. Parfianovich, Photostimulated luminescence

[81] N. Dreybrodt and D. Silver, Phys. Stat. Sol. 20 (1967) 337. [82] A. RSuber and I. Schneider, Phys. Star. Sol. 18 (1966) 125. [83] H. Watanabe, Phys. Rev. 149 (1966) 402.