Excitonic emission in highly excited lead iodine

Solid State Communications, Vol. 18, pp. 9—12, 1976.

Pergamon Press.

Printed in Great Britain

EXCITONIC EMISSION IN HIGHLY EXCITED LEAD IODIDE* F. Adduci, A. Cingolani, M. Ferrara, M. Lugarà and A. Minafra Istituto di Fisica dell’Università, Ban, Italy (Received 5 June 1975 by F. Bassani)

Excitonic luminescence in Pb12 has been investigated in the temperature range between 77 and 300°Kunder high excitation by a nitrogen laser. A single emission band is observed which, at low pumping, is centered at the free exciton energy. When the excitation intensity is increased, the band broadens and shifts towards lower energies. These facts are interpreted on the basis of an exciton—electron interaction.

1. INTRODUCTION

2. EXPERIMENTAL PROCEDURE

THE PHOTOLUMINESCENCE of Pb12 at low temperature and low excitation 2 intensities has been extensively investigated.”

Absorption and luminescence measurements which are reported large samples of Pb1here have been carned out on 2 cleaved from a Bridgman-grown monocrystal ingot 50—80 mm in length and 6—10 mm in diameter, with the c-axis normal to the excited surface.

These works give evidence of free and bound exciton emission and of their phonon replicas. The line strongly polarized at about 4meV above the free exciton has been2attributed to the extraordinary On the other hand,only one expolariton mode. periment is available on the luminescence of Pb1 2 3 The observed induced by nitrogen laser excitation. shift and broadening of the emission band is tentatively ascribed to some type of excitonic interaction, such as the recombination of an excitonic molecule.

exciting source2 nsec was apulse pulsed nitrogen1 MW laser (1w =The 3.678 eV.) with duration, peak powerwas andattenuated repetition by ratea up to of 100neutral Hz. The laser beam stack den. sity filters and focused on the sample. The maximum power density inpinging on the surface of the crystal was limited to 4MW/cm2. Several excitation-collection configurations were used, to reduce self- absorption effects for the radia-

In the present work the near edge luminescence of Pb1 2 has been investigated at high excitation intensities and at temperatures in the range between 77 and 300°K.

tion of energy close to the fundamental optical edge. A mercury lamp was also utilized to measure the emission under low power excitation. All the luminescence spectra were recorded by a 300mm grating double monochromator with photoelectric detection.

We conclude from the experimental results that the lineshape modification of the exciton recombination band could be connected with an exciton— electron scattering process.

Absorption measurements were made by using a 650W tungsten lamp and analyzing the transmitted light with the same apparatus.

______________

*

Work partially supported by C.N.R. 9

10

EXCITONIC EMISSION IN HIGHLY EXCITED LEAD IODIDE

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Vol. 18, No. 1

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linear subsequently (Fig. the The dependence samethese effects becomes are obtained slightly 2, at and subternthen rections do 2). not modify pends quadratically on I~up tostatements. 0.2 MW/cm peratures above 77°K,up to room temperature. How-

FIG. 1. Luminescence spectra of PbI

2 at 77°Kat vanous pumping intensities (nitrogen laser). The dashed spectrum is excited by the mercury light. The insert shows the optical density of the same sample at 77°K.

3. RESULTS AND DISCUSSION Figure 1 shows the luminescence spectra of Pb12 at 77°Kexcited by the Hg-lamp (dashed line) and by the nitrogen laser at various intensities. The optical density spectrum of the same sample is reported iii the insert, By comparing absorption and emission spectra at the weakest excitation it is observed that they are coincident in energy i.e. at 2.494 eV, the n = I free exciton line. By increasing the exciting intensity J~,the following facts are observed concerning the excitonic emission: the energy of the peak gradually shifts towards lower energies; simultaneously the low energy tail enhances, while the high energy one stays almost constant.

ever, the broadening effects are not so evident as at the lower temperature. Sample heating was ruled out by using different duty cycles in the excitation source. Since the temperature increase would not only shift the emission peak, but also increase its hal1~ width, which are the main effects under study, we were very careful in ruling out this possibility. In fact, when we try to match the high intensity spectra at low temperatures with the low intensity ones. hut at high temperatures, allowing for the band gap thermal reduction, they never coincide. The shift z~Eof the peak energy of the emission band is plotted vs J~in Fig. 3 and may be fitted by a ~ j~3dependence. The shift particularly pro2 , i.e.is in the intensity nounced at i~> 0.2 MW/cm region in which the integrated emission is nearly linear with excitation. Incidentally, we note that the thermal shift of the luminescence between 77 and 300°Kis —0.25 meV/°K,in good agreement with the thermal

Vol. 18, No. 1

EXCITONIC EMISSION IN HIGHLY EXCITED LEAD IODIDE

11

explain the slope variation with I~(perhaps, interaction with more than one free carrier may be

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integrated emission £ ~ NexNe, where Nex and Ne are the free taken into exciton account). and The free electron same equation concentrations. gives for the

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3. Red-shift of the band peak as function of nitrogen laser intensity.

shift of the free exciton measured at low excitation.1 Measurements of optical gain, by the technique of the unidimensional optical amplifier,4 showed that the light emission of Fig. 1 is mainly spontaneous.

On the other hand, by considering the rate equations which governs the kinetic of the process,5 at relatively high excitation Nex, Ne jo~while at the highest pumping Nex, Ne vui. This is consistent with the observed dependence of Fig. 2. ‘~

The high energy tail is independent of the excitation intensity, while it depends on temperature and, exponentially, on the photon energy. This means that the tail mirrors the Maxwellian distribution of the exciton kinetic energy, provided that the system can be considered nearly at thermal equilibrium. Qualitatively, the peak shift towards lower ener-

If we plot the spectra of PbI 2 between 77 and 300°Kon a semi-logarithmic scale the following facts are seen. First of all, the tails of both sides are well represented by an exponential function of the photon energy. Moreover the slope of the high energy side depends mainly on temperature. pointed out thatatthe bandItathas lowbeen pumping is located theluminescence free exciton energy. On this basis several characteristics of the luminescence can be explained. The significant increase of the low energy tail is consistent with an exciton—electron interaction. A free exciton, with momentum Kex scatters a free electron, transferring its momentum and a fraction of its energy to the electron. After scattering, the exciton reaches another exciton state, of energy Fex from C. which it decays giving a phonon —

Since the scattered electron can take off a wide range of energies, the resulting low energy tail is continuous. Benoit a la Guillaume et a!. ~ have derived an analytic expression for the low energy tail. This formula can explain the exponential form of the tail and its change with temperature; however it cannot

gies, with increasing I,~,can be partly explained with an increase of the interaction energy, which, on the mean, shifts the center of gravity of the emission band. However, if it is assumed that at high excitation intensity the electron gas is degenerate, according to the recent calculation of Klingshirn et al. ,6 the energy of the peak is: 3

hi.’

=

and then, for Ne Fig. 3 is obtained.

—

(EgEex)kN~.’

~

a dependence of the type of

Three other models which could explain the energy shift of the excitonic emission of Pbl 2 have been discarded for the following reasons: (1) exciton— exciton inelastic scattering, because this mechanism 7 (2) elastic does not explain the energy position; exciton—exciton collision, because in this case the tail of the emission band should be at the high energy side;8 (3) many body and hot phonon effects,9 because this model does not explain the quadratic dependence on

Acknowledgements The authors wish to thank Prof. F. Bassani for helpful discussions. Prof. E. Mooser kindly supplied the samples of Pb1 2. —

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EXCITONIC EMISSION IN HIGHLY EXCITED LEAD IODIDE

Vol. 18, No. I

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2.

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KLINGSHIRN C., OSTERTAG E. & LEVY R.,Solid State Commun. 15, 883 (1974).

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CATALANO I.M., CINGOLANI A., FERRARA M. & MINAFRA A., Phys. Status Solidi (b) 68, 341 (1975). and references cited therein.

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LELYAKOV A.V..Sov. Phys. Solid State 15, 891 (1973).

9.

MENESES E.A., JANNUZZI N., RAMOS J.G.P., LUZZI R. & LEITE R.C.C.. Pkys. Rev. BI 1. 2213 (1975).