HAL Id: jpa-00216867
https://hal.archives-ouvertes.fr/jpa-00216867
Submitted on 1 Jan 1976HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
THE EFFECT OF DISLOCATION ELECTRIC FIELD
ON PHOTOELECTRIC PROPERTIES OF IONIC
CRYSTALS
G. Turchányi, I. Földvári, I. Tarján
To cite this version:
JOURNAL DE PHYSIQUE Colloque C7, suppl6ment au no 12, Tome 37, Dkcembre 1976, page C7-607
THE EFFECT OF DISLOCATION ELECTRIC FIELD
ON PHOTOELECTRIC PROPERTIES OF IONIC CRYSTALS
G.
TURCHANYI,
I.FOLDVARI
and I.TARJAN
Research Laboratory for Crystal Physics, Hungarian Academy of Sciences, Budapest, Hungary
RBsume. - Nous avons cherche la relation entre 1'6mission optiquement stimulQ des exoklec- trons et le champ Clectrique interne qui est derive du deplacement des dislocations dans des halo- genures alcalins. Nous avons constate que l'efficacite est grande, le mouvernent des exoelectrons est orient6 par le champ Blectrique des dislocations. Nos experiences donnent peut-Etre une nou- velle demonstration de la production d'un champ Blectrique dans la couche de surface des cris- taux ioniques deformes.
Abstract. - We had studied the relation of optically stimulated exoelectron-emission to the internal electric field produced by dislocation movement in alkali halide crystals. We stated that the efficiency is high, the movement of exoelectrons is directed by the dislocation electric field. Our experiences give probably a new proof of the production of an electric field in the surface layer of deformed ionic crystals.
1 . Introduction.
-
In the dislocation photoconduc- This voltage, produced by the movement of edge dislo- tion of ionic crystals studied by us [l, 2, 31 the exo- cations, was in the case of KC1, KBr and NaCl negative electron-emission of internal surfaces very probably and for LiF positive ( l ) .plays some role. Therefore it appeared necessary to us to study with a slight alteration of our experimental technique applied in dislocation photoconduction the photoelectric phenomenon of ionic crystals, namely their optically stimulated exoelectron-emission.
2. Experimental procedure. - A hydrogen lamp
without window and a vacuum monochromator were used as a light source for experiments in the region of 90-300 nm. The LiF, KBr and NaCl crystals used as specimens were nominally pure, while the KC1 was extremely pure and OH- free, though the original purity of the specimen surfaces could not be maintained during the mounting of the crystals.
In order to decrease the surface impurities, and to avoid the fast, photoelectrically not affectable processes the very strong dark current after X-irradiation, the specimens had been kept in vacuum before the measu- rement for about 20 hours. For the purpose of mount- ing crystals of 14 X 4 X 2 mm3 size were cut into two halves perpendicularly to their 2 mm side, a metal wire- net was glued between the two halves and the specimens thus obtained were glued by their 2 X 4 mm2 surface
to plexi cubes of 10 X 8 X 8 mm3 sizes. The wire-net
was connected to a vibrating reed electrometer in the open position. The knife edges serving for the fourpoint bending of the specimen were put to the plexi cubes, the light had fallen on the 14 X 4 mm2 surface of the
crystal (Fig. 1). The specimens had been bent to pro- duce a voltage of about 300 mV on the electrometer.
Our measurements were carried out in the following way.
Under the effect of various wandering charges the vibrating reed condenser in the time unit chosen by us (30 S) became charged to about 0
+
0.5 mV. This dark current was always taken into account in our data.Illuminating the specimen the vibrating reed conden- ser gradually became charged. This process in the
(1) It should be mentioned here that we never experienced
that after X-ray coloration the voltage in the case of LiF had changed its sign. Dupuy's results 141 referred to also in a review paper [5] we attribute to the previous treatment of the speci- mens i. e. more exactly to the phenomenon expectable in this
case analysed in detail by Whitworth and Glen [6].
beginning usually showed a linear time dependence. Illumination was kept on only for 5-10 S and its value
was extrapolated to the mentioned time unit.
The voltages developed and corrected for the dark current and converted for unit incident enevgy (denoted by c( a B in the figures) were compared in various expe- rimental conditions.
In figure 2 the spectrum of a LiF crystal is shown before (dotted line) and after deformation (continuous line). At the measurement we proceeded from 90 nm to longer wavelengths. The applied slit width was 0.1 mm.
,,a" voltage larb. unitsl 5.0 - 4.0 - ..a" voltage I"'
T"
. - - - - . - - p - - - . - - -.---.-.
..
l 2 3 L 5Spectral energy larh un~tsl
FIG. 3a.
-
cc a >> voltages obtained for LiF with increasingilluminating energies and normalized to unit incident ener- gies. 0 O o
.
3
0.-/V * 'CA X S P-*, f!"
.$; "{:&!l"-
f 2 2 o -2
P 9 0 -In figure 3a (( a values are shown, obtained in the
case of LiF illuminating the crystal always with the same 121.55 nm light and varying its energy by varying the slit width before (dotted line) and after deforma- tion (continuous line).
In figure 3b these same data can be seen in the case of NaCl.
In figure 4 the decrease of the quantity a is shown in dependence of the illumination time after the defor- mation of an X-ray coloured NaCl crystal illuminating it continuously with light of 121.55 nm.
L V V V V V o o o o 7 O m V : I.
I/ 0 0 0 0 v 1 . x ~ ~ ~ -360 mV: lL,/lL, /L!
111. ' V//. A A A A -110 mV: I/, V/.;VII., VNL IV 0 0 0 0 Vlll " * * C
Spectrol energy larb unitsl
I 2 3 4 5
0 0 v 0 0 v 0 WR
a-,
- 1 - v v v
FIG. 36. - cc a voltages obtained for NaCl with increasing
(+) and decreasing (c) illuminating energies and normalized
to unit incident energies. I) before deformation ; 11-IV) after
deformation to a voltage of
-
360 mV ; V-VIII) after deforma-tion to a voltage of - 410 mV.
In figure 5 the (( a values obtained on the surface of
EFFECT OF DISLOCATION ELECTRIC FIELD ON PHOTOELECTRIC PROPERTIES C7-609
I
-
fime larb. units1
FIG. 4.
millimeter slit in 28 steps. The quick disappearance of the local maximum obtained about the end of the crystal is shown by the points below the peak belonging to the measurements during the following time units.
3. Discussion. - We note that the described phe- nomena were not changed by the fact whether the electric voltage produced by deformation was present on the vibrating reed condenser or was extinguished by the momentary grounding of the electrometer. After the illumination of the specimen the decrease of the
electric field inside it produced by the movement of edge dislocations takes a much longer time even in case of X-irradiated crystals then the duration of our presently shown measurements.
It should be also noted that though we did not measure the deformation of our specimens, in the present cases it was so slight that it was not observable. Comparing the developed electric voltages with the measurements on dislocation photoconduction [l]
when the F-centres present in a quantity of about 1016/cm3 were excited with a light intensity of about 5 pW/cm2, we concluded that presently when the intensity of the exciting light is at least by two orders of magnitude lower we are dealing with excited electrons in a quantity higher by two orders of magnitude, or it may be assumed that in these processes not the number of the electrons is great but their excitation probability. Another possibility is that the excited electrons are moving to a much greater distance, than in the case of dislocation photoconduction. So considering the fact that as it is shown by figure 4 we experienced the steep decrease of the effect of illumination it is possible that the electrons could move to less and less distance. Considering the fact that we did not use any external electric voltage source in our measurements the assumption is that in the case of LiF the positive electric sign at illumination after deformation should be attri- buted to electrons emerging from the surface of the crystal i. e. moving away from the measuring electrode. In the case of NaCl we have to assume an opposite phenomenon, i. e. electrons moving toward the measur- ing electrode.
We do not think that the charge itself on edge dislo- cations (in LiF a resultant positive majority, in NaC1, KCl, KBr a negative majority) plays the decisive role, since if in the case of NaCl type crystals too the electric voltage produced by bending was positive (2), under
the effect of illumination a positive signal was produced in the same way as in the case of LiF.
The results shown in figure 5 indicate that along the illuminated surface too take place some very fast processes not detected by our measuring apparatus. There are other reasons as well to indicate that we are confronted by a more complex phenomenon. Whitworth [7] assumed, Dupuy [4] made it experi- mentally probable by simple xerography and we by another method
[Z]
that due to deformation an electric field of opposite sign too is produced on the surface of the crystals. Since in our present experiment the electric voltage produced by the bending is increased by illumi- nation, we have to assume, that the electrons in ques- tion may play some role in the compensation of the surface electric field and thus our present experiences give new justification of the ideas of Whitworth [7].References
[l] TURCHANYI, G., M ~ T R A I , M. and TARJAN, I., Revue Row- [4] DUPW, C. H. S., Thesis, Strasbourg, 1965.
maine de Physique 13 (1968) 59. [5] WHITWORTH, R. W., Adv. Phys. (G. B.) 24 (1975) 203- [2] T U R C ~ N Y I , G., MATRAI, M., TAWAN, I., Kristallografia 13 304.
(1968) 717. [6] WHITWORTH, R. W. and GLEN, I. W., Trans. Brit. Cevam. 131 TWRC~NYI, G., MATRAI, M., JANSZKY, J. and TARJAN, I., Soc. 62 (1963) 731.
