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SURFACE AND SPACE CHARGESElectrostatic charge effects on surface defects in alkali halide crystals
M. Yacamán, J. Hirth
To cite this version:
M. Yacamán, J. Hirth. SURFACE AND SPACE CHARGESElectrostatic charge effects on surface defects in alkali halide crystals. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-485-C6-487.
�10.1051/jphyscol:19806126�. �jpa-00220034�
SURFACE AND SPACE CHARGES.
Electrostatic charge effects on surface defects in alkali halide crystals
Abstract. — The presence of a surface charge and a compensating Debye-Hiickel layer of charged point defects in ionic crystals was suggested by Lehovec [1]. The surface charge was later associated [2, 3] with charged surface entities. Indirect evidence from sublimation measurements supported such a view, but direct evidence was lacking.
In the present work, the direct effect of an externally applied electrostatic field on the topography of monomolecular ledges was determined for sublimated alkali halide surfaces decorated with gold. The changes of shape of the ledges produced by the field show the presence of charged kinks on the surface.
1. Introduction. — The presence of a surface charge and a compensating surface charge of point defects was first suggested by Lehovec [1]. Later the nature of the surface charge was clarified by the work of Poepel and Blakely [2] and Short et al.[3]. This charge was associated with charged surface entities (ledges, kinks, adions ..., etc.). Some indirect evidence obtained by Munir and Hirth [4] showed that surface charges might have an strong influence on sublimation process. In the present work we show the first direct evidence of existence of charge along steps. We show by electron microscopy that when an external electric field is applied during the sublimation the topography of monomolecular ledges changes very dramatically.
As an aid for further discussion the charge of different surface defects is shown in table I.
When an alkali halide surface is heated at the proper temperature ( ~ 400 °C for NaCl), sublimation takes Table I. — Various defects at solid surfaces and its charge.
Type of defect Charge Kinks on steps + e/2
Vacancies on surfaces + e Divalent impurity on a surface + e Divalent impurity at ledge + e Divalent impurity at kink 3 e/2
Monovalent impurities at surface e
place by the advance of spiral and close steps of mono and bi-atomic height. Those steps can be revealed by gold decoration technique of Bassett [5].
The evaporation features has been described in several papers [6, 7]. The main step structures which are observed in the dislocation controlled regime are; rounded spirals which step height is equal to a/2 (a = lattice parameter), square spirals which step height is a, rounded concentric steps which step height is a, and double spirals. These structures are produced on the emergence point of different dislocations at the surface, as it has been fully dis- cussed by Betghe and co-workers [6].
2. Experimental. — NaCl and KC1 crystals were used for the experiments. The crystals were grown at the crystal growth facility of the National Univer- sity of Mexico. Impurity levels in those crystals are reported to be below 1 ppm. Gold decoration was made by deposition of a 5 A layer of gold on a vacuum cleaved surface in a vacuum of 1 0_ 4 Pa. The resulted surfaces were sublimated at various temperatures with and without the application of an electric field.
Temperatures were measured with two thermocouples located at the sublimated surface and in contact with the lower surface respectively. The electric field of 0.2 MV/m was imposed by the application of a 1.5 kV potential across two capacitor plates positioned next to the sample and perpendicular to the sublimating surface.
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-485
M. .1. Y a c a m â n (*) and J. P. Hirth (**)
(*) Instituto de Fisica Universidad Nacional Autônoma de Mexico, Mexico, D.F.
(**) Metallurgical Engineering Department, Ohio State University, Colombus, Ohio, U.S.A.
Résumé. — Il a été signalé par Lehovec [1] la présence d'une charge superficielle ainsi qu'une couche de compen- sation Debye-Hiickel de défauts ponctuels chargés dans les cristaux ioniques. La charge superficielle a été associée avec des entités de surface chargées [2, 3], ce qui a été démontré par les mesures de sublimation, bien qu'il n'est pas confirmé par des mesures directes. Dans ce papier nous déterminons l'effet direct d'un champ électrostatique extérieur sur les marches monomoléculaires pour les surfaces des halures alkalins décorés à l'or. Le changement de la forme des marches produit par le champ montre la présence d'arrêts sur la surface.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806126
3. Effects of the electrical field on step structure. -
When evaporation takes place under the influence of an electric field the most striking feature is that evaporation structures become skewed in some direc- tion as shown in figure 1. The skewing direction is
Fig. 2. - Surface structure produced at same conditions of figure 1 , showing skewing in two different directions. Direction of skewing is indicated by dotted lines.
Fig. 1. - Evaporation structure produced by heating a NaCl by 1 hr a t 400 oC. An electric field of 1 .S keV was applied along the 100 direcbon.
out of a low index direction. In some evaporation structures which are skewed in two different direc- tions both of them out of a low index direction.
These effects can be explained in terms of the charge kink model. Let us consider that an electric field is applied in a direction perpendicular to one of the sides of a closed square step. In one of the perpendi- cular sides will be an increased tendency to nucleate positive kinks whereas in the other perpendicular side negative kinks will be nucleated. The increase number of kinks will tend to produce a rough step giving rise to a serrated structure with portions of steps along low index planes. Alternatively the step can be skewed out to high index direction.
It seems that the second alternative as the one pre- sented under our experimental conditions.
We have measured that the step distance in a12 structures do not change significantly with respect to the values reported for field free evaporation [7].
It was found that the step distance value was relatively independent of the field intensity up to some limit value of about 2 keV. For larger field the evaporation rate was largely increased. In fact for a temperature of 400 O C with a field of 2.5 keV, the same step struc- tures that are produced at 480 O C in free field evapo-
Fig. 3 . -Step structure obtained a t 400 OC during 1 hr of heating but with a field of 1 keV. Formation of two dimensional nuclei between the spiral steps can be observed.
ration were observed. This indicated an increase on the velocity of the step. Increasing the field was then equivalent to increasing the temperature.
In the case of a low field
-
1 keV the step struc- ture of figure 3 was found. In the figure can be observed that two dimensional nuclei produced by clustering of vacancies are formed. It appears that while the evaporation rate is about the same for the spirals, there is an enhanced two dimensional nucleation and disck shaped pits are stabilized bet ween steps.This fact is also consistent with the charged kink model.
We conclude that the electron microscope evidence on sublimation under electric field gives direct evidence of the charged nature of the surface steps in ionic crystals, as predicted by theoretical models.
ELECTROST.ATIC CHARGE EFFECTS ON SURFACE DEFECTS
DISCUSSION
Question. - L. SLIFKIN.
The ionic conductivity of the crystal relaxes the externally applied field, but the field remains at full strength outside the crystal (e.g, above the surface).
Therefore there must develop a large component of field which is perpendicular to the surface. What is the likely role of this component in the effects you des- cribe ?
Reply. - M . J . YACAMAN.
The steps move along the surface during the subli- mation process and therefore a perpendicular compo- nent of the field is not likely to play an important role on the kinetics. On the other hand the relaxation of the field has to be consistent with the double-charge
layer that is set under equilibrium and therefore a more complicated situation can arise.
Question. - M . GEORGIEV.
Could you describe in greater detail the symmetry involved in your calculations ?
Reply. - M . J . YACAMAN.
We calculated the potential profile around the charged step using the gauss Law. The charge itself was calculated from the Debye-Hiickel theory for steps as reported by Short et al. [I] and a full discus- sion of the symmetries can be found there.
[I] SHORT, D. W., R.IPP, R. A. and PHILIP, J., J. Chem.
Phys. 57 (1972) 1381.
References
[I] LEHOVEC, K. J., Chem. Phys. 21 (1969) 1123. [5] BASSETT, G. A., Phil. Mag. 3 (1958) 1042.
[2] POEPPEL, R. B. and BLAKELY, J. M., Surf: Sci 15 (1969) 507. [6] BETGHE, H. and KELLER, J. Cryst. Growfk 23 (1974) 105.
[3] SHORT, D. W., RAPP, R. A. and HIRTH, J. P., .I.Chem. Pkys. [7] Jose YACAMAN, M., OCAKTA, T., SONNEMANN, A. and
57 (1972) 1381. HIRTH, J. P., J. Crysr. Growth 37 (1977) 37.
[4] MUNIR, Z. A. and HIRTH, J. P., J. Appl. Phys. 41 (1970) 2697.
