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COLOUIR CENTRES IN OXIDESOrigin of the luminescence excited in the F absorption band in MgO
P. Edel
To cite this version:
P. Edel. COLOUIR CENTRES IN OXIDESOrigin of the luminescence excited in the F absorption band in MgO. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-531-C6-532.
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COLOUR CENTRES IN OXIDES.
Origin of the luminescence excited in the F absorption band in MgO
Abstract. — The F and F+ centres are known to have almost coincident absorption bands at 250 nm in MgO.
Exciting in this absorption band results in two luminescence bands at 400 nm and 520 nm. The former has been attributed for a long time to the F+ luminescence and the latter to the F luminescence. Optically detected magnetic resonance, stress effects and lifetime measurements invalidate this last statement. It will be shown that the 520 nm luminescence originates in a recombination process, similar to donor-acceptor recombination in semi-conductors.
The recombining entities are the F + and the F ~ centres.
The observation of the 400 nm and (or) the 520 nm bands is very sensitive to the wavelength of the exciting light, showing a complicated photoconversion process between F and F+ centres.
F+ and F centres (respectively one and two elec- trons trapped at an anion vacancy) are known to have almost coincident absorption bands at 250 nm in magnesium oxide MgO. The F+ centre has been seen in neutron irradiated crystals and in lightly additively colored crystals ; the absorption results in a lumines- cence at 400 nm. The F centre was assumed to give rise to the luminescence peaking at 520 nm excited in more heavily additively colored crystals; it was also thought that only a careful choice of the concentration of anion vacancies could allow to have both lumines- cences with notable amplitude. A recent review of the properties of the excited states of F centres in oxides has been given by Henderson and O'Connell [1].
We will show that :
i) even at helium temperatures a complicated photoconversion process between F and F+ centres exists and allows a balance between the two lumines- cences ;
ii) the luminescence at 520 nm is not an intrinsic F luminescence and should likely be attributed to a recombination process; the two recombining entities are shown to be the F+ centre and probably the F~
centre.
{') Laboratoire associe au C.N.R.S.
1. F and F+ luminescences. — Figure 1 shows the luminescence spectra of a crystal containing a total concentration of 1 01 8c m- 3 for different excitation wavelengths. We obtain notable intensity only for the 520 nm (respectively the 400 nm) luminescence band if we excite at wavelengths higher (respectively lower) than 267 nm. The transition from one luminescence to the other is very sharp and one has to select carefully
Fig. 1. —Luminescence spectra at 4.2 K of an MgO additively colored crystal (total anion vacancies concentration of 1018 cm-3) at different exciting wavelength, a) 260 nm ; V) 267 nm ; c) 270 nm;
d) 280 nm.
35
JOURNAL D E PHYSIQUE Colloque C6, supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-531
P. Edel (*), B. Henderson (**), Y. Merle d'Aubigné (*) and R. Romestain (*) (*) Laboratoire de Spectrométne Physique ('),
Université Scientifique et Médicale de Grenoble, B.P. 53 X, 38041 Grenoble-Cedex, France (**) Physical Laboratory, Trinity Collège, Dublin 2, Ireland
Résumé. — Les bandes d'absorption des centres F et F+ sont pratiquement confondues à 250 nm dans l'oxyde de magnésium MgO. En excitant dans cette bande d'absorption, on observe deux bandes de fluorescence à 400 nm et 520 nm. La première bande est attribuée depuis longtemps à la luminescence du centre F+ ; la seconde était, jusqu'à présent, considérée comme provenant de la luminescence du centre F. Des expériences de détection optique de la résonance magnétique, de contraintes uniaxiales et de durée de vie conduisent à rejeter cette dernière affirma- tion. Nous montrons que la luminescence à 520 nm provient d'un processus de recombinaison analogue à la recombinaison donneur-accepteur dans les semi-conducteurs. Les deux espèces qui se recombinent sont le centre F+ et le centre F " .
L'observation de la bande à 400 nm et (ou) de celle à 520 nm dépend très fortement de la longueur d'onde de la lumière excitatrice et montre l'existence d'un processus compliqué de photoconversion entre les centres F et F+.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806139
C6-532 P. EDEL, B. HENDERSON, Y. MERLE D'AUBIGNE AND R. ROMESTAIN
the wavelength (267 nm) in order to observe both which cannot be assigned to any known g value.
.bands with large amplitudes. The excitation spectra of the two luminescence bands, very sharp near 267 nm, are obviously not a direct reflection of the absorption spectrum. A tentative explanation (to be published elsewhere) which requires the presence of traps will also take into account that, a t given excita- tion wavelength, both luminescence bands grow linearly with the excitation intensity. This last expe- rimental fact discounts the simplest model of double excitation (an F centre that would be ionized, leaving an F+ centre that can be photoexcited).
2. Nature of the 520 nm band. - Analysis of optically detected magnetic resonance (O.D.M.R.) and stress effects on the emission shows that the 520 nm band is not an intrinsic F luminescence but originates probably in a donor-acceptor like recombi- nation process. Detailed analysis is given elsewhere [2]
and we will just recall here the main results, obtained at helium temperatures. In MgO, uniaxial stress measurements (that give information on the orbital nature of the luminescent state) can be done by polarization analysis of the emitted light, the lumi- nescence band, broad and structureless preventing us from performing stress induced splitting measure- ments. If the luminescent state was orbitally degene- rate one would expect a strong linear polarization ; in the limit of experimental error this is not the case, either for [loo] or [I101 applied stress.
An O.D.M.R. experiment has been performed at 9 GHz and shows an increase of the emitted light in the presence of resonant microwaves; one observes :
i) a narrow (3 G) isotropic line at g = 2.003 iden- tified as the F+ resonance line ;
.ii) a broader (25 G) isotropic line at g = 1.996, value
[I] HENDERSON, B., O'CONNELL, D. C., Semicond. Znsulat. 3 (1978) 299.
[2] EDEL, P., HENDERSON, B., MERLE D'AUBIGNE, Y., ROMESTAIN, R
.
J. Phys. C 12 (1979) 5245.
The isotropy of the lines and the closeness of the g value to the free spin value agree well with the stress effect measurements, showing that the luminescent state is not orbitally degenerate. The main point is that the F+ resonance is detected though the F + lumines- cence is not excited (Aexci, = 280 nm, see Fig. 1) and that the relative amplitude of both lines is the same whatever the anion vacancies concentration (ranging from l o t 7 to 6 x 10"). We propose therefore that the 520 nm luminescence is due to recombination between the transiently photocreated Ff and F- centres, an F- centre consisting of three electrons trapped at an anion vacancy. This luminescence looks then very like donor-acceptor recombination luminescence in semiconductors, the difference being that here the initial state consists of the charged centres, the
contrary of donor-acceptor recombination.
Two other points .support this assignment. - The highly non exponential behaviour of the luminescence when the excitation is switched off : at long times (some seconds) the luminescence behaviour looks much more like a l / t function than an e-"' function.
This behaviour is depicted in donor-acceptor recom- bination case [3, 41 where the lifetime is strongly dependent on the pair separation. The dynamics is also faster in more concentrated crystaIs. this being due to the fact that the average separation is shorter in more concentrated crystals.
- The second point is that we observe also a non resonant light enhancement when applying micro- waves ; this is commonly the case in donor-acceptor recombination 151 and, though not understood at now, has probably to be associated with the electric field present in the microwave cavity and its interac- tion with electrons in the conduction band.
[3] THOMAS, D. G., HOPFIELD, J. J., AUGUSTINIAK, W. M., Phys.
Rev. 140 (1965) A 202.
[4] DELBECQ, C. J., DEXTER, D. L., YUSTER, P. H., Phys. Rev. B 17 (1978) 4765.
[S] DUNSTAN, D. J., DAVIES, J. J., J. Phys. C 12 (1979) 2927.
