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MAGNETO - OPTICAL EFFECTS IN THE IMPURITY SPECTRAL REGION OF CdCr2Se4
P. Hlídek, M. Zvára, V. Prosser, S. Unterricker
To cite this version:
P. Hlídek, M. Zvára, V. Prosser, S. Unterricker. MAGNETO - OPTICAL EFFECTS IN THE IMPU-
RITY SPECTRAL REGION OF CdCr2Se4. Journal de Physique Colloques, 1980, 41 (C5), pp.C5-
335-C5-337. �10.1051/jphyscol:1980556�. �jpa-00219991�
JOURNAL DE PHYSIQUE Colloque CS, supplPment au n O. 6, Tome 41, juin 1980, page C5-335
POS TERS.
MAGNETO
-
OPTICAL EFFECTS IN THE IMPURITY SPECTRAL REGION OF CdCr2Se4 P. ~lidek, M. Zvira, V. Prosser and S. ~nterricker'I n s t i t u t e of Physics, CharZes University, 121 16 Prague 2, Ke KarZovu 5 , CSSR.
+ Department o f Physics, Mining Academy, 92 Freiberg/Sa., GDR.
Rdsum5.- Nous avons mesure l'absorption optique et le dichroxsme circulaire magngtique sur les monocristaux de CdCr Se4. On compare,lleffet des traitements thermiques soit dans le vide soit dans $a vapeur de selenium avec celui qu'on observe dans les spectres des cristaux dop6s a l'argent ou irradies par les deuterons.
Abstract.- Optical absorption and magnetic circular didhroPsm measurements in CdCr2Se4 single crystals are reported. We compare the samples annealed in vacuum or in selenium vapour with the silver doped ones and with the samples irradiated by deuterons.
1. Introduction.- In spite of the great influence of various defects on semiconduc- ting and magnetic properties of CdCr2Se4, the optical properties in the impurity region have been investigated only exceptio- nally /1/. The present paper brings our measurements of the optical absorption and magnetic circular dichroYsm /MCD/ on the following samples :
No. 1. Ag-doped, Cdo.ggAgo.01Cr2Se4 as a starting material.
No.2,3. The crystals annealed 24 hours and 6 hours, respectively, in vacuum at 600°C, then ground adpolished.
No. 4. The ground plate of CdCr2Se4 annealed like No.3.
No. 5. The ground plate of CdCr2Se4
annealed 6 hours in a closed quartz ampoule in selenium vapour
,
T (sample)=680°Cr T(Se source) = 640°C.
N0.6~7. The ground plates of CdCr2Se4 were irradiated by 13.5 MeV deuterons with total dose about 1pA hr cm-2.
N0.8~9. The same, except the total dose was about lOpA hr
The deuteron irradiated samples were conse- cutively annealed for 15 minutes in closed evacuated ampoules with an additional CdCr2Se4 material in order to investigate recovery of the physical properties.
2. Experimental.- Both the starting material and the all measured samples were found to be p-type. Some of our results are shown in
figures 1 to 3.
Fig. 1, Absorption spectra of the Ag-doped sample (No. 1)
,
the vacuum annealed sample(No.4)
,
the selenium annealed sample (No. 5) and by the deuterons irradiated samples(No.6,8,9), T gives the temperature of the annealing: n. a. denotes non-annealed sample. Because of the unknown distribution of defects through the irradiated samples, relative values of the optical density are plotted.
An absorption peak near 0.2 eV and a structure above 0.6 eV which is related to a relatively high MCD can be seen in the spectra of Ag-doped samples. The absorption spectra of the vacuum annealed samples reveal a broad structure above 0.3 eV, their spectra of MCD differ from those of Ag- doped samples.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980556
JOURNAL DE PHYSIQUE
The absorption and MCD of the samples irradiated by the lower dose of deuterons are similar to the spectra of the vacuum annealed samples. The samples irradiated by the higher dose without any annealing were not transparent in the region above 0.5 eV. The decrease of the absorption of the irradiated samples was reached by a consecutive annealing up to 450°C, the annealing above 500°C led to a new increase of the absorption and MCD, especially in the regions below 0.3 eV and near 0.8 eV with a shape similar to the Ag-doped samples.
Fig. 2
.
MCD spectra of the Ag-doped sample (No.1) and the effect of annealing on the MCD spectra of the deuteron irradiated sample (NO. 8).
According to the preliminary results, the well-pronounced temperature shift of a structde consisting of three MCD peaks has been observed on the sample No.9 after annealing between 350°C and 480°C.
3. Discussion.- It is impossible at present to give a consistent microscopic interpre- tation of the observed effects. Comparing the spectra, some comments can be made only.
Fig. 3 , MCD spectra of the vacuum annealed samples (No. 2,3,4)
,
of the selenium annealed sample (No. 5) and of the sample irradiated by deuterons (Mo.~), no annealing.The similarity of the vacuum annealed samples to the irradiated ones and a great tendency of CdCr2Se4 to p-type conductivity disagree with the usually accepted assurrp-ticc, that anion vacancies (donors) are the basic type of native defects which are introduced with the most probability by vacuum annea- ling as well. The effect of vacuum annealing is obviously more complicated and creation of acceptor levels (cadmium vacancies) is very important, too.
According to our estimation of thecross sections for Rutherford collisions, we can expect the concentration of selenium
vacancies and interstitials in the irradia- ted samples to be higher than the concentra- tion of cadmium and chromium ones in the ratio Se : Cd : Cr = 3.0 : 1.1 : 0.5 /2/.
The effect of heat treatment on the spectra of these samples indicates a consecutive recombination of some radiative deffects at the lower annealing temperatures. At the temperatures above 500°C, the strongly damaged areas (dijplacement spikes) could
serve as a source of various types of
defects migrating through the sample. The / 3 / ~lidek, P., zvgra, M., Prosser,
v.,
similarity to the Ag-doped samples could Phys. Status Solidi (b) 84 (1977) K 119.
show that acceptor-type defects are produced with a high probability in such a case.
The main component of the absorption and of MCD has no observable temperature shift and it may be related to transitions from the complex valence band to defect levels. The preliminary observed anomalous temperature dependence of a structure in
blCD on the sample No.9 may be caused by a contribution of transitions from defect levels to the energy band which is due to the red-shift of the absorption edge.
The fact that the spectra of the Ag- doped samples, of the irradiated samples after the annealing at high temperatures and the earlier reported plboto-induced snec
-
tra Fn as-grown ssrn>les /3/ differ remarkably from those of the vacuum annealed samples and of the samples measured directly after irradiation may indicate important diffe- rences between the two groups in the nature and/or in the population of the responsible defects.
In conclusion, we can tentatively suggest that selenium vacancies and/or their complexes with other defects play an important role in the high energy part of our spectral region, whereas cadmium
vacancies and/or their complexes dominate at the lower energies. Nevertheless, a wide variety of defects should be expected in the real case and a further investigation of different treatments of the crystals is necessary to elucidate this problem.
Acknowledgements.- The authors would like gratefully acknowledge Dr. P. Hbschl and Mr. V. Kubla for kindly supplying crystals and for technical assistance.
References
/1/ Miyatani, K., Okamoto, F., Baltzer, P.K., Osaka, S., Oka, T., AIP conf.
Proc. 5 (1972) 285.
/2/ Kinchin, G.H., Pease, R.S., in Reports on Progress in Physics XVIII, A.C.
Stiokland, editor, (London) 1955, p. 1.
