Luminescence of PbI2: Cu+

JOURNAL OF

LUMINESCENCE ELSJzYIER

Journal of Luminescence 63 (1995) 309-316

Luminescence of PbI,: Cd* I. Baltog”v*, S. Lefrantb, L. Mihut”, R.P. Mondescua,l aInstitute of Atomic Physics, IFTM-Lab 130, P.O. Box MG-6, R-76900, Bucharest-Magurele, Romania b Institut des Materiaux de Nantes. 2 Rue de la Houssini&e, F-44702 Nantes Cedex 03. France

Received 5 May 1994, revised 17 October 1994, accepted 5 November 1994

Abstract Under band-to-band irradiation, the PbI,:Cu’ crystals are strongly luminescent at 77 K. In the emission spectrum one can notice, besides the characteristic excitonic band E at 2.5 eV, three additional bands denoted D, C and M, with maxima located at 2.42,2.06 and 1.76 eV respectively. Correlated studies of luminescence decay time, photoconductivity, excitation spectra and thermostimulated currents have revealed the presence of different luminescence centres with and without appearance of Cu+ ions. In the case of C emission (2.06 eV) which increases with increasing concentration of impurities, a dependence of the decay time versus excitation wavelength has been observed. This can be interpreted as the result of the coexistence of two types of recombination mechanisms: one fast, bimolecular, of the Schiin-Klasens type, which is dominant at excitation in the fundamental absorption band and the other slow, dominant at excitation in the excitonic band which seems to be due to tunnelling donor-acceptor recombination. The M emission (1.76 eV) is generated through the prior band-to-band irradiation of the sample and is related to the formation of a complex luminescent centre consisting of Cut and Pb+ ions, the latter resulting from products of the photodecomposition of Pbl, under band-to-band irradiation. We also consider that the recombination of free holes wth trapped electrons around the defects in the sample is the main process for the origin of the D emission that is observed in both pure and doped crystals.

1. Introduction

Pb12 crystals with a layered structure show interesting electrical and optical properties which depend both on their varied polytype structure and on the type and concentration of lattice defects [ 11. *This work was performed under the frame of the Protocole de Cooptration Scientifique between Laboratoire de Physique Cristalline (Institut des Materiaux de Nantes) and the Laboratory of Optics and Spectroscopy in the Institute of Physics Bucharest. *Corresponding author.

’ Present address: Department

of Physics and Astronomy, Hasbrouck Labs 411, University of Massachusetts, Amherst, MA 01003, USA.

0022-2313/95/$09.50 SSDl 0022-231

More interest has been shown in studying the influence of the polytype structure upon the optical properties of the crystals, with noticeable results obtained using Raman spectroscopy and luminescence methods [2-81. For crystals with a layered structure, the coexistence of ionic and covalent bonds ensures properties similar to those of semiconductor compounds. As in the case of doped semiconductors (ZnS type) we noticed that PbIz crystals containing impurities of elements from the I-b group exhibit an intense luminescence under UV irradiation at low temperatures. Some published results discuss only the PbI,:Ag+ crystal [9, lo], where the most interesting experimental feature is the variation of

0 1995 - Elsevier Science B.V. All rights reserved 3(94)00072-7

I. Balrog et al. /Journal of Luminescence63 (1995) 309-316

310

the decay time of the characteristic lumiunescene when the excitation radiation is tuned across the band gap [lo]. The lack of any information regarding the luminescence of PbI,:Cu+ crystals triggered our interest in these studies, the results obtained being presented in this paper.

excitation. For the continuous luminescence spectra, an Ar ion laser and RAMANOR double monochromator system were used. A tunable dye laser in the range of 475-515 nm, pumped by a pulsed nitrogen laser (with 5 ns pulse duration and less than 5% peak power fluctuation) was used as the excitation source for the measurements of the luminescence decay time. The recording of the luminescence decay was performed using a fast photomultiplier EM1 9818 B and a boxcar averager. A CARRY vibrating reed electrometer was employed in photoconductivity (PC) and thermostimulated current (TSC) measurements.

2. Experimental The PbIz :Cu+ samples were cleaved from 4H polytype single crystals grown by the Bridgman method. The results reported in this work were obtained on ingots doped with Cu+ at a weight concentration of cl = 3.3 x 10e3 and c2 = 3.3 x 10m4. Because in the Bridgman-grown Pb12 crystals the most frequently observed polytypes were 2H and 4H, previous to luminescence measurements each ingot was submitted to an inter-polytype irreversible transformation (2H + 4H) by thermal annealing at 420 K. This inter-polytype transformation was monitored by the Raman lines at 15 and 76cm-‘, which attain their maximum for the 4H polytype [3,7]. The luminescence spectra were recorded in reflection at a right angle, with the C-crystalline axis in the incident plane under cotinuous and pulsed

255

25

245

2L -

235

3. Results 3.1. Luminescence

and excitation spectra

The typical luminescence spectrum of PbI,:Cu+ sample at 77 K under continuous excitation in the fundamental absorption band (Jexcit = 488 nm) is shown in Fig. 1 curve a. One can remark that besides the characteristic excitonic emission band E at 2.5 eV three additional bands denoted D, C and M, with maxima located at 2.42, 2.06 and 1.76 eV respectively, are present. Analyzing the

22

20

18

1.6

photon energy /eVl

Fig. 1. Luminescence spectrum of PbI,:Cu+ crystals at 77 K with continuous excitation b IcXci, = 501.7 nm. Curve 2 inside insert was obtained after band-to-band irradiation

using an Ar ion laser. Curves: a ,I_, of the sample.

= 488 nm;

I. Baltog

et al. / Journal

of Luminescence

dependence of this spectrum upon the excitation wavelength, impurity concentration and sample temperature, the following conclusions can be drawn: 1) The presence of Cu + ions in Pb12 crystals engenders the appearance of the luminescence bands C and M. Their intensities increase with increasing impurity concentration. For the appearance of the M band in the luminescence spectrum, a prior band-to-band irradiation of the sample is also required. This is depicted in the insert in Fig. 1 that shows the relative change in the intensity of the C and M bands for the same sample, before and after band-to-band excitation, at 77 K. The M band thus created can be destroyed by heating the sample close to room temperature. Upon returning to LNT the value of the intensity ratio of the C and M bands is restored to within + 5% of the ratio for the freshly cleaved sample. 2) The smaller gain in generating C emission excitation in the under excitonic band 501.7 nm) than the gain obtained by pump(j’excit = ing in the fundamental absorption band (3Lexcit= 488 nm) is revealed by spectra a and b in Fig. 1, obtained for the same density of photoexcitation. This already suggests that a luminescence mechanism based on the diffusion of the excitons and their trapping on impurities is less probable. 3) Observed in both pure and doped crystals, the D band is independent of the impurity concentration and its intensity depends on the concentration of surface defects produced either by cleavage or by pulse annealing. 4) A significant result concerning the temperature dependence of the PbI,:Cu+ luminescence is presented in Fig. 2. Here it can be seen that like the variation of the band gap energy, the maxima corresponding to the E and D bands shift to lower energies when the temperature increases. For the C emission maximum, the shift is toward higher energies in the temperature range 77-100K and toward lower energies for temperatures higher than lOOK. 5) A distinctive feature regarding the photogeneration of the C and M emission bands is shown in their excitation spectra in Fig. 3. For the C band the excitation spectrum (Fig. 3, curve a) corresponds to the fundamental absorption band which

311

63 (1995) 309-316

80

100

90

T IKI

Fig. 2. Shift of the E, D and temperature.

110

I

720

130

_-

C emission

peaks

function

of

2.5 _

Photon

energy

IeVi

Fig. 3. The excitation spectra of the C and M emission 77 K, curves a and b, respectively.

bands at

includes both the absorption at the limit of the fundamental band at 2.53 eV and a marked peak for the absorption in the excitonic band at 2.5 eV.

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I. Baltog et al. / Journal of Luminescence

63 (1995) 309-316

In contrast with C band, the excitation spectrum of the M band (Fig. 3., curve b) shows that this band is generated when excitation occurs in the fundamental band and also in a lower energy range, i.e. in a band with the maximum located at 2.44 eV.

than when excitation is done in the fundamental band (1encit= 485 nm) even at the same density of optical excitation. The measured curves of decay can be quite well reproduced using the relation:

3.2. Luminescence

Z(t) z i,,exp

decay

-k (

As concerning the dependence of the C emission actually the most important one in PbI,:Cu+ crystals - on the resonant excitation in the fundamental absorption band or in the excitonic band, this is best characterized by the luminescence decay. A meaningful element depicted in Fig. 4 is that the decay of the C emission band becomes slower when it is pumped in the excitonic band (Aexcit= 500 nm)

aml 0

1

1

5

10

20

25

x8.1 ti

5.0

2.5 2 _

0

G

-2.5 -5.d

I 0

5

10

15 frnw/s/

20

25

(1)

2’

>

1’; (

I 15

i0,

+

>

which corresponds to a somewhat simplified formalism describing the decay as a superposition of monomolecular (io, exp[ - t/rJ) and bimolecular (io,/[l + t/r212) processes with different decay times r2 < ti. The coexistence of two types of processes, mono and bimolecular, in the C emission is supported by the specific modification of the decay curves with variation in the excitation intensity. Increasing its value leads to a linear and quadratic increase in the iO, and io, intensities, respectively. When tracing the effect of a change in excitation intensity upon pi and r2, we note only a decrease of z2 with increasing excitation intensity, a typical behaviour for kinetics of bimolecular character. When the excitation radiation is tuned in the 485-500 nm range, the weights of the components io, and iol, are changing continuously between the limits observed for the two resonant excitations, as in Table 1. Using the values for the io, and ioZ, z1 and z2 from Table 1 in expression (l), one can reproduce the experimental decay curves from Fig. 4(a), as shown in Fig. 4(b) where the relative s(t) = 100[lexP(t) - zco”p(t)]/z~xp(t) deviation is plotted as a function of time. The temperature dependence of the C decay curves obtained by excitation at 490 nm; intermediate in wavelength between the two limiting values mentioned in Table I, shows a gradual decrease in the weight of the slower component with respect to the faster one, without a sensible change in the

x8.10-6

Fig. 4 (a) Semilogarithmic plot of the C luminescence intensity versus time at 77 K under resonant excitation at the edge of the fundamental absorption band (Acrci, = 485 nm) and in the excitonic band (Asrsi, = 500 nm), curves 1 and 2, respectively. (b) The relative deviation s(t) = 100IIc”p(t) - IcOmp(t)]/f’“~(t) between the experimental and theoretical decay curves, computed by using expression (1) and Table 1.

Table 1 Luminescence

decay parameters

Leil Cnml

i0,

71

485 500

0.16 0.52

1 x10-6 1 x 10-b

Is1

according

to relation

(1)

Is1

i0,

52

0.84 0.48

1.5 X 10-1 1.5 x 10-7

I. Baltog et al. / Journal of Luminescence43 (1995) 309-316

313

Table2 Temperature dependence of the luminescence decay parameters. i,cxc,,= 490 nm Temperature

[sl

io,

ri

0.30 0.25 0.20 0.12

3.0 x 3.0 x 2.8 x 2.8 x

[sl

i0,

52

0.70 0.75 0.80 0.88 1.oo 1.00 1.00 1.00

2.60 x lo-’ 2.50 x lo-’ 2.50 x lo-’ 2.40 x lo-’ 2.10 x lo-’ 0.90 x lo-’ 0.35 x lo-’ 0.15 x lo-’

WI II 83 90 100 124 157 203 240

1o-6 1o-6 1O-6 10-e

0’

2.6

2.5 -

values of ri and r2, as noticed in the range 77-100 K (Table 2). At temperatures higher than lOOK, the decay curve can be entirely associated with the fast component, with a decay time constant (z2) displaying a temperature dependence like T; ' - exp( - AE/kT). This functional form allows for an evaluation of the depth of the capture level related to the respective luminescent process: E = 0.08 + 0.02 eV. The measurements related to the M emission band and carried out at 77K resulted only in the appearance of a slow decay component entirely monomolecular, with a decay time constant close to the value of z1 corresponding to the C band.

2.4 23 pholcm energy

.

Fig. 5. Photoconductivity spectra of PbI,:Cu+ at 77 K before and after band-to-band irradiation, curves I and 2, respectively.

3.3. Photoconductivity

and thermostimulated current measurements

The results of the photoconductivity (PC) and thermostimulated current (TSC) measurements, carried out along the C crystallographic axis, are presented in Figs. 5 and 6, respectively. A previously known observation [l] also revealed in our experiments, is that the photoconduction in the layer plane is approximately lo* times higher than that along the C crystallographic axis. From Fig. 5 one should keep in mind that in the PC spectrum at 77 K, beside the characteristic maxima at 2.54 and 2.5 eV corresponding to the fundamental absorption edge and to the excitonic band, respectively, a shoulder at 2.44 eV, associated with the M band in the luminescence spectrum, is manifest.

TIKI

-

Fig. 6. TSC curve for PbI,:Cu+

The TSC measurements (Fig. 6) performed with a heating rate of 0.5 deg/ at 800 V/cm exhibited two maxima, located at 95 and 162 K. Their activation energies AE of 0.1 and 0.32 eV, respectively, I - exp were computed using the relation [ - AE/fkT]. 4. Discussion and conclusions

As for Z&type phosphors doped with Cu, for Pb12 crystals, Cu+ substitues for Pb*+ forming an

314

I.Baltog et al. 1 Journal of Luminescence

acceptor centre tightly associated with the I- vacancy that plays the role of charge compensation. Such an acceptor centre localized at 0.32 eV above the valence band is identified in Fig. 6 as the maximum arising in TSC measurements at 162 K. Taking into account the gap energy Egap = 2.54 eV and the C emission peaking at Eemiss= 2.06 eV, the optical trap depth (determining the position of the emission band) becomes Egap - Eemiss= 0.48 eV, which is close enough to EA = 0.32 eV to suggest for the C emission band a Schon-Klasens recombination mechanism involving an acceptor centre. The fact that the optical trap depth is larger than the thermal trap depth EA = 0.32 eV can be considered as a consequence of the Franck-Condon principle. Now let us look at the most important attribute of the C emission, the variation of the luminescence decay with the wavelength of the excitation light. Under excitation in the fundamental absorption band, when an intense photoconductivity is also seen, the free carriers (holes and electrons) that are generated have typically a kinetic energy higher than kT and they diffuse independently through the lattice, such that the luminescence can be thought of as appearing from their recombination as nonmutual electron-hole pairs. For this kind of process the type of recombination kinetic is bimolecular and it is characterized by a fast decay time constant [ll]. We believe that this mechanism could explain the origin of the faster component of the C emission, which is of a bimolecular type and it is dominant under excitation in the fundamental absorption band. Let us apply the same kind of reasoning for the generation of the C emission by pumping in the excitonic band. In this case the photogenerated free carriers have a kinetic energy slightly lower than kT, which makes their drift through the lattice short before being trapped. The luminescence can be regarded now as being due to the mutual electron-hole recombination involving electrons trapped in the nearest neighbour sites of acceptor centres, so the recombination is described by a monomolecular kinetics. Although this mechanism could explain qualitatively the increase in the weight of the slow component - of a monomolecular type - under excitation in the excitonic band,

63 (1995) 309-316

certain, characteristics of the C emission arising with a temperature variation demand further comments: 1) If the slower component has been generated through the monomolecular type recombination mechanism previously mentioned, then it would be necessary to consider that the capture level for electrons is located at AE under the conduction band. As the temperature increases, the thermal entrapping of the electrons entered in the radiative recombination must also lead to a decrease in the value of zi (here the lifetime of the trapped electrons) following a dependence r-i - exp [ - AE/kT]. Experimentally, this fact is not manifest. When the temperature is raised only a decrease in the weight of the slow component iO, occurs, without a perceptible change in rr. 2) If the Schon-Klasens mechanism, as a superposition of both mutual and nonmutual electron-hole recombination when electrons are moving from the bottom of the conduction band to the level of the acceptor centre, would exclusively be responsible for producing the C emission, then a shift of the C band maximum toward lower energies should take place when the temperature increases, as a result of the adjustment of the depth of EA level corresponding to the variation of the band gap. In Fig. 2 this kind of shift is noticed for the peaks E and D but not for the C maximum. In the temperature range 77-100 K where the slow component persists in the C emission, the C peak shifts toward higher energies with increasing temperature but the direction is reversed to lower energies for temperatures higher than 100 K when the C luminescence is entirely of a bimolecular type. Considering the above described properties concerning the slow component of the C luminescence band we believe it is more accurate to consider that this results from a tunnelling transition of a trapped electron to the acceptor centre. In this circumstance the probability of recombination per unit time p(R) should be considered as a function of the spatial separation R of the randomly distributed donor-acceptor pairs [ 121: p(R)--e

1 z

-aR

.

I. Baltog et al. / Journal of Luminescence

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63 (1995) 309.-316

Denoting with no the initial concentration of the two kinds of centres with the distance between them lying within (R, R + AR) and using the expression dn(R,t) = noexp[ - p(R)t]4~R’ dR, the intensity is proportional to:

= with the main contribution around region the

coming from a narrow point of maximum

p(~)evC - p(Rbl C131.

At a critical separation distance R,(t) for which p(R)t = 1, the previous equation allows us to approximate the time evolution of the luminescence intensity as:

l(t) - f [8xR2

C %dR=R,(r)

=t'

where C is a constant. This relation shows that when a tunnelling recombination mechanism is acting the quantity Z(t)t remains constant during quite a long period of time, when R,(t) is slightly varying. Such a representation for the experimental data is given, for the C emission, in Fig. 7. Here the behaviour of the product J(t)t as a function of time constant) can be inspected, at relatively long (times after the excitation pulse when only the slow component is present in the C luminescence band. Its observed decay, closely resembling an exponential one, is supported by the small range of variation of the R,(t) value. Inflexions on curves a and b from Fig. 7 could be interpreted as the temporal limits of separation between the two recombination processes coexistent in the C emission, the fast one, bimolecular, of the Schon-Klasens type, and the slow one, of a donor-acceptor tunnelling type, respectively. Regarding the donor level implied by this luminescent process, we consider that this must be associated with the Pb+ ions as an intermediate product of photolysis of Pb12 under band-to-band irradiation [ 141. The release, by thermal excitation, of the trapped electrons forming the Pb+ ions is correlated with the maximum appearing at 95 K on the TSC curve in Fig. 6, which corresponds to a 0.1 eV level below the conduction band.

Fig. 7. Decay

curves corresponding

to Fig. 4(a) represented

as

I(t) t,iC 0%. (4)).

Tightly connected to the presence of Pb+ ions, usually produced near the dislocations and defects of the lattice when the Pb12 crystals undergo a band-to-band irradiation [14], is the appearance of the D emission band in the luminescence spectrum. This explains the variation in the intensity of the D band, even for samples cut from the same ingot, due to changes in the density of dislocations generated by cleaving D emission band is present in both pure and doped crystals, it is not relaed to the presence of impurities and is considerably enhanced by pulse annealing [ 151. For the D luminescence, considering its peak shift with increasing temperature presented in Fig. 2, it seems correct to consider that this appears as a result of the recombination of free holes with the trapped electrons which form the Pb+ ions. In the case of the M band, its excitation spectrum in Fig. 3, curve b, shows that this emission band appears both with band-to-band irradiaion and at lower energies (2.44 eV). We assume that this band must be related to a complex centre of a molecular type that involves on one side Cu+ and on the other Pb+ as an intermediate product of the photodecomposition. Now one can consider that the M band appears as a transition from the excited state to the ground state originating from a donorlike level to an acceptor-like level. For such a luminescence centre a configurational diagram can be imagined, where Eabs = 2.44 eV and Eemiss = 1.76 eV.

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I. Baltog et al. / Journal of Luminescence 63 (1995) 309-316

Acknowledgements

The authors would like to express their gratitude to the referees for their useful comments and suggestions.

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