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DIRECT LUMINESCENCE AND THERMALLY
STIMULATED LUMINESCENCE OF 4H CdI2
C. Ronda, J. van der Meer, A. van Heuzen, C. Haas
To cite this version:
DIRECT LUMINESCENCE AND THERMALLY STIMULATED LUMINESCENCE OF 4H CdIZ
C.R. Ronda, J.H. van der Meer, A.A. van Heuzen and C. Haas
Laboratory of Inorganic Chemistry, Materials Science Center of the University, Nijenborgh 16, 9747 AG Groningen, The NetherZands
Résumé
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Nous présentons les spectres de luminescence directe (LD) du 4H Cd12 dans la région 1.8 à 3.4 eV, aux températures entre 9.3 et 215 K, ainsi que les spectres de luminescence thermiquement stimulée (LTS) du même composé. Les spectres de LD sont composgs de plusieurs larges bandes d'émission qui se recouvrent. Ces bandes sont dues à la recombinaison des électrons locali- sés avec des trous localisés. Grâce à une théorie simple, qui tient compte de l'interaction électrostatique entre électron et trou nous avons pu expliquer quantitativement la position des bandes spectrales d'émission. Les spectres de LTS montrent des,bandes d'émission à plusieurs températures. Ces bandes, sont dues à la recombinaison des porteurs de r-harges initialement piègés. L'énergie d'activation des pièges est calculée en considérant les interactions électrostatiques entre porteurs de charges piègés.Abstract
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Direct luminescence (DL) spectra of 4H Cd12 in the spectral region 1.8-3.4 eV at temperatures between 9.3 and 215 K and thermally stimulated luminescence (TSL) spectra are reported. The DL spectra consist of several overlapping broad emission bands. These bands are due to radiative recombina- tion of localized (trapped) electrons with localized (trapped) holes. With a simple theory, which takes into account the electrostatic interaction between electron and hole it was possible to explain the position of the observed emission bands quantitatively. The TSL spectra show emission bands at several temperatures. These bands are due to the recombination of initially trapped charge carriers. The trap depths can be calculated, by taking into account the electrostatic interaction between the trapped charge carriers.1 - INTRODUCTION
Cd12 is a layer compound. The most stable modification (4H Cd12) crystallizes in spacegroup P63rnc (fig. 1.). The Cd ions occupy slightly trigonally distorted octa- hedral sites.
Fig. 1. The hexagonal u n i t ce22 of
/---di"
4H Cd12 and the (210) section. The squa- res are the octahedral i n t e r s t i t i a lI s i t e s i n the van der Waals gap.
! O
'
1The direct luminescence (DL) of Cd12 in the spectral region 1.8-3.4 eV has been extensively studied by Matsumoto et al. [ 1-41
.
The spectrum shows sharp emission lines at 3.38, 3.23 and 3.12 eV. The emission line at 3.38 eV originates £rom the emission of the free exciton in Cd12 and the emission lines at 3.23 and 3.12 eV have been interpreted as due to Pb impurities [ 5 ] .C7-464 JOURNAL DE PHYSIQUE
The spectrum shows broad emission bands below 3.1 eV with a large Stokes shift. The broad emission bands have been interpreted by Matsumoto et al. [Il as due to localized telaxed excitonic states, belonging to excited states of the molecular ion (Cd1 ) in a Dgd crystal field. These authors distinguish two relaxed excitonic states, fine consisting of a hole in the Cd 4d level and an electron in the Cd 5s level and the other consisting of a hole in 1 5p level and an electron in the Cd 5s level
.
It follows £rom a recent study [61 that the energy difference between the Cd 4d level and the Cd 5s level is about 13 eV. This means that the DL cannot find its origin in an excitonic state involving the Cd 4d levels.
We report measurements of the DL of Cd12 as a function of temperature and show that the broad DL bands of Cd12 are due to pair luminescence.
We also report thermally stimulated luminescence (TSL) measurements of Cd12. The observed trap depths confirm the proposed model.
II
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EXPERIMENTAL METHODSCd1 was prepared from the high purity elements. Single crystals were grown with the ~ri3gman technique. In order to investigate the dependence of the DL characteristics
on the way Cd12 had been prepared, we also prepared crystals of Cd12 from a solution of Cd12 in H20 at 7S0C and in C2H50H at 50°C.
The DL spectra were measured using a home-built equipment. The light of a 1000 W Xe arc (Oriel 6269) is dispersed by means of a Jobin-Yvon H.20 UV monochromator. The eniitted light is dispersed by means of a Jobin-Yvon H.20 VIS monochromator. The spectra were recorded with a resolution of about 0.02 eV. The sample is mounted in an Oxford Instruments MB 4 cryostat. The temperature of the sample may be varied between 2-300 K.
The TSL spectra were measured using the same equipment by illuminating Cd12 during 15 minutes at 5 K. Thereafter the compound was heated at a constant rate. We used band pass filters instead of a monochromator to determine the spectral region where
the TSL occurred because the TSL was very weak. III
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EXPERIMENTAL RESULTSThe DL spectra of Cd12 as a function of the temperature are given in fig. 2. The excitation energy was 3.54 eV. The DL characteristics such as position, band shape and intensity of the bands do not depend on the way the Cd12 crystals had
been prepared.
The TSL spectrum of Cd12 (irradiated with the same excitation energy) at a heating rate of 3.14"C/min. is given in fig. 3. The TSL occurs mainly between 2.0 and 2.5 eV. IV
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ANALYSIS OF THE DL AND THE TSL IN TERMS OF PAIR LUMINESCENCEWe ascribe the various observed luminescence bands of Cd12 to the pair recombination of localized (trapped) electrons with localized (trapped) holes. The several DL bands correspond to different distances between electron and hole according to the following expression valid for a uniaxial compound [71:
m
d i c a t e s t h a t any d e f e c t s I I involved i n t h e recombination p r o c e s s must b e i n t r i n s i c . We assume t h a t t h e recombining e l e c t r o n s and h o l e s a r e l o c a t e d on i n t e r s t i t i a l o c t a h e d r a l s i t e s i n t h e van d e r Waals
-
gap, and on Cd s i t e s , 3 r e s p e c t i v e l y . This completely m-
A determines t h e p o s s i b l e ~1 p a i r d i s t a n c e s i n t h e l a t t i c e ; + t h e f i r s t 7 p a i r c o o r d i n a t e s a r e given i n t a b l e 1. The v a l u e s of x 2 , y2 and z2 a r e o b t a i n e d £rom t h e l a t t i c e I I parameters of 4H Cd12 (a- O 50 T I K ) 100 150 a x i s 4.248A,
c-axis 13.72A ) .
The f i r s t DL maximum ( a t 2.16Fig. 3. TSL spectmm of 4H CdIZ recorded a t a heu- e ~ ) i s n o t c l e a r l y r e s o l v e d
ting rate of 3.14'C/min. i n t h e s p e c t r a . Matsumoto e t a l . , who measured t h e luminescence of Cd12 a s a f u n c t i o n of time r e s o l v e d t h i s maximum c l e a r l y LI-31. The DL bands a r e broad, t h i s makes i t d i f f i c u l t t o determine t h e p o s i t i o n of t h e DL maxima accurately. I n t a b l e 1. t h e observed DL maxima and t h e temperature a t which t h e y a r e observed a r e given and compared w i t h t h e c a l c u l a t e d DL maxima. The o v e r a l l agreement i s q u i t e good. I n a coming p u b l i c a t i o n , we w i l l p r e s e n t a more ge- n e r a l t h e o r y w i t h which we a r e a b l e t o c a l c u l a t e t h e l i n e s h a p e of t h e DL and t h e observed p a r t i a l p o l a r i z a t i o n of t h e emission bands 131.
The t r a p d e p t h s fromwhich t h e TSL o r i g i n a t e s have been c a l c u l a t e d , u s i n g t h e f u l l p r o f i l e of t h e TSL spectrum. I f one neglects t h e r e t r a p p i n g p r o b a b i l i t y , t h e TSL of a given t r a p i s given by
-E /kT
C7-466
JOURNAL
DE
PHYSIQUE
in which B is the heating rate, no is the number of initially trapped charge car- riers, A is the recombination cross section and ET is the trap depth.
The thermal excitation energy of an electron from a trap (or hole; we cannot deter- mine whether the electron or the hole is thermally excited) in the presence of a center with the same charge is given by
in which E is the trap depth and Eo isthe ionization energy of the isolated trap. s0 and s0 :re the static dielectric constants (cO = 5.4 and
EI
= 12.9[fol).
In d b l e 11'the temperatures of the TSL maxima and the values of the Coulomb destabili- zation energy are given for the first 5 distances together with the calculated trap depths. It follows that the ionization energy of an isolated trap is 0.31 eV, consistent with results obtained in a study of the photoconductivity of Cd1 [Il].2 Finally, we remark that the origin of the localization of electrons and holes after the photo excitation is not known. A possibility is a localization (self-trapping) of electrons and holes due to a very strong electron-phonon interaction. A recom- bination of self-trapped electrons with self-trapped holes indeed has a Coulomb
stabilization of the initial state, as observed. It is also possible that the trap- ping involves defects induced optically; photo excitation of the crystal produces free excitons, which relax to self-trapped excitonic States by the displacement of ions in the crystal.
ACKNOWLEDGEMENTS
This investigation was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO)
.
Table 1. Comparison of observed and calcmated maxmma of pair DL peaks in Cd12. The calculations are for E = 3.04 eV, E// = 4.3, = 4.6 and z = (1 14) c.
pair Observed maximum (eV) Calculated maximum (eV)
Table II. Calculation of the ionization energy the observed trap depths in the TSL experiment ci = 5.4, = 12.9 and z = (114) c.
pair trap depths (eV) O 0.01 (0.01) (13.5 K) (1/3)aJ3 0.02 (0.01) (24.1 K) a 0.03 (0.02) (48.3 K) (2/3)aJ3 0.05 (0.02) (75.2 K) (1/3)aJ21 0.15 (0.05) (106.3 K) REFERENCE S
of the isolated trap in 4H Cd12 using
.
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