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Slow electron induced defects in alkali halides
Volume 26A, number
PHYSICS
4
15 January 1968
LETTERS
quantities; they reduce to -(X/U)1 and (l-X/U)1 respectively when both Fg and Bg vanish.
given previously [l] for the case in which VpO= 0; then FO = 0, corresponding to the absence of static body forces other than BI-J. With Fg = 0, V’ and V” each reduce to V; also L = I so that M’ = M and K = I + M, K then being the relative permittivity dyadic defined as for a cold magnetoplasma. Another special case is that in which the magnetostatic field vanishes: then p = 0, M = -(X/U)1 and L-l = I- Fg W/NolJo2, while M’ = -(X/lJ)L and K= I-(X/U)L. Thus M’ and K are still, in general, essentially tensor
The work
in this
letter
suggests
that the dy-
L might be an important quantity in the theory of wave propagation in warm plasmas with static pressure gradients. adic
References 1. R.Burman, 2. R.Burman,
Proc. Proc.
IEEE 55 (1967) 723. IEEE 55 (1967) 1528.
*****
SLOW
ELECTRON
INDUCED
DEFECTS
IN ALKALI
HALIDES
P. D. TOWNSEND and J. C. KELLY * School of Mathematical and Physical Sciences, University of Sussex, England Received
15 December
1967
Low energy electron bombardment of alkali halides produces colour centres and sputtering. processes have been considered in the light of various theories of defect formation.
When alkali halides are subject to ionizing radiation the principal defects produced are F centres, electrons trapped in negative ion vacancies. Many theories have been advanced [l-5] to explain how such vacancies are produced by radiation which is not sufficiently energetic to cause direct ionic displacement. Some of the theories predict a different temperature dependence and the usual experimental method of investigating this problem is to measure the rate of formation of F centres as a function of temperature [6]. Such measurements may be complicated by the presence of additional, or alternative, types of colour centres. Several of the proposed mechanisms involve mass transport by a replacement collision chain along the (110) directions and this has suggested to us a more direct method of studying the basic .mechanisms involved. If such collision chains should be initiated near the surface, it is probable that they would cause ejection of surface ions in a sputtering like manner. We have therefore bombarded alkali halide crystals with low * S. R. C. Senior Visiting Fellow, School of Physics, University Kensington, Australia.
138
Permanent address: of New South Wales,
These
energy electrons and used a mass spectrometer to seek evidence of such an ejection process. Each of the alkali halides, NaCl, KCl, KI and LiF, when bombarded with electrons with energies from 100 to 600 eV, were found to eject halogen ions with a high efficiency. Easily visible craters, coloured at the sides and edges, appeared in the crystals in a matter of minutes. Electrons in this energy range do not penetrate far below the surface and lack, by orders of magnitude, the energy to produce displacements of direct collision with ions. They are therefore producing ejection by one of the indirect mechanisms relevant to F centre production. A study of this ejection rate as a function of temperature should enable a choice to be made between the various proposed F centre production mechanisms. The rate of ejection of chlorine ions and of surface erosion was found to be strongly dependent on the bombarding electron current density (fig. 1). NaCl obeyed the empirical relation. (Ejected chlorine)O.6 0~electron beam current density. No intensity dependence on voltage with-
Volume 26A, number 4
1'
I 1 10 CNLURINE WA1 , ARBIIRARV UNITS
PHYSICS
100
Fig. 1. Chlorine yield from NaCl bombardedwith electrons as a function of electron flux. in the range 100-600 eV was observed. At current densities less than 1 mA/cma only the normal halogen ion mass peaks were recorded. At higher fluxes, in the region of 3 mA/cm2, a more complex peak structure was observed. Lithium fluoride gave very erratic F- signals and produced whisker like structures, presumably reformed from ejected ionic material, in the electron bombarded regions. This contrasts with the clean simple holes produced in NaCl and KI. This may be related to a difference in surface potential of the LiF compared with NaCl and KI. We have found that this dose dependent potential can be considerable [7]. At room temperature the ejection process is extremely efficient. Typically, for sodium chloride one chlorine ion is removed for every five 240 volt electrons striking the surface. That the process is not a simple surface boiling has been shown by control experiments with glass, magnesium oxide and potassium azide. None of these showed any detectable ion ejection, even though the azide thermally decomposes at 2500C. Moreover, if a sufficiently high surface temperature were reached to boil off material, the colour centres would be annealed out, and they are not.
LETTERS
15 January 1968
NaCl and KI have been bombarded with slow electrons at 77oK. Both were eroded, but at a reduced rate. The KI rate of erosion being more effected by the reduction in temperature. In NaCl visible fluorescence was produced. These results should be regarded as qualitative as there are still problems relating to non-uniform surface charge and its effect on incident electron current density and ion ejection. Work is continuing, to overcome these difficulties and extend the measurements down to the theoretically more interesting region between 77OK and 4oK. The results obtained so far support the Pooley-Hersh defect production mechanism which predicts a strong temperature sensitivity and an antocorrelation between luminescence and defect production. The ease with which slow electrons can erode an alkali halide surface is a factor which must also be taken into account in epitaxy studies. The electron current, to which an alkali halide substrate is subjected, prior to the evaporation of the epitaxial material, must seriously modify the structure of the surface and hence the initial nucleation and subsequent epitaxial growth. Such effects have been detected by several authors [8,9]. We wish to thank the Science Research Council for support of this project.
References 1. F.Seitz, Rev. Mod. Phys. 26 (1954) 7. 2. J.H.O.Varley, J. Phys. Chem. Solids 23 (1962) 985. 3. C.C.Klick, Phys. Rev. 120 (1960) 760. 4. D. Pooley, Proc. Phys. Sot. 87 (1966) 245. 5. H. N. Hersch, Phys. Rev. 148 (1966) 928. 6. T.P.P.Hall, D.Pooley, W.A.RuncimanandP.T. Wedepohl, Proc. Phys. Sot. 84 (1964) 719. 7. P. D. Townsendand J. C. Kelly, Physics Letters 25A (1967) 673. 8. D. J. Stirland, Appl. Phys. Letters 8 (1966) 326. 9. P. W. Palmberg, T. N.Rhodin and C. J. Todd, Appl. Phys. Letters 10 (1967) 122.
*****
139
PHYSICS
4
15 January 1968
LETTERS
quantities; they reduce to -(X/U)1 and (l-X/U)1 respectively when both Fg and Bg vanish.
given previously [l] for the case in which VpO= 0; then FO = 0, corresponding to the absence of static body forces other than BI-J. With Fg = 0, V’ and V” each reduce to V; also L = I so that M’ = M and K = I + M, K then being the relative permittivity dyadic defined as for a cold magnetoplasma. Another special case is that in which the magnetostatic field vanishes: then p = 0, M = -(X/U)1 and L-l = I- Fg W/NolJo2, while M’ = -(X/lJ)L and K= I-(X/U)L. Thus M’ and K are still, in general, essentially tensor
The work
in this
letter
suggests
that the dy-
L might be an important quantity in the theory of wave propagation in warm plasmas with static pressure gradients. adic
References 1. R.Burman, 2. R.Burman,
Proc. Proc.
IEEE 55 (1967) 723. IEEE 55 (1967) 1528.
*****
SLOW
ELECTRON
INDUCED
DEFECTS
IN ALKALI
HALIDES
P. D. TOWNSEND and J. C. KELLY * School of Mathematical and Physical Sciences, University of Sussex, England Received
15 December
1967
Low energy electron bombardment of alkali halides produces colour centres and sputtering. processes have been considered in the light of various theories of defect formation.
When alkali halides are subject to ionizing radiation the principal defects produced are F centres, electrons trapped in negative ion vacancies. Many theories have been advanced [l-5] to explain how such vacancies are produced by radiation which is not sufficiently energetic to cause direct ionic displacement. Some of the theories predict a different temperature dependence and the usual experimental method of investigating this problem is to measure the rate of formation of F centres as a function of temperature [6]. Such measurements may be complicated by the presence of additional, or alternative, types of colour centres. Several of the proposed mechanisms involve mass transport by a replacement collision chain along the (110) directions and this has suggested to us a more direct method of studying the basic .mechanisms involved. If such collision chains should be initiated near the surface, it is probable that they would cause ejection of surface ions in a sputtering like manner. We have therefore bombarded alkali halide crystals with low * S. R. C. Senior Visiting Fellow, School of Physics, University Kensington, Australia.
138
Permanent address: of New South Wales,
These
energy electrons and used a mass spectrometer to seek evidence of such an ejection process. Each of the alkali halides, NaCl, KCl, KI and LiF, when bombarded with electrons with energies from 100 to 600 eV, were found to eject halogen ions with a high efficiency. Easily visible craters, coloured at the sides and edges, appeared in the crystals in a matter of minutes. Electrons in this energy range do not penetrate far below the surface and lack, by orders of magnitude, the energy to produce displacements of direct collision with ions. They are therefore producing ejection by one of the indirect mechanisms relevant to F centre production. A study of this ejection rate as a function of temperature should enable a choice to be made between the various proposed F centre production mechanisms. The rate of ejection of chlorine ions and of surface erosion was found to be strongly dependent on the bombarding electron current density (fig. 1). NaCl obeyed the empirical relation. (Ejected chlorine)O.6 0~electron beam current density. No intensity dependence on voltage with-
Volume 26A, number 4
1'
I 1 10 CNLURINE WA1 , ARBIIRARV UNITS
PHYSICS
100
Fig. 1. Chlorine yield from NaCl bombardedwith electrons as a function of electron flux. in the range 100-600 eV was observed. At current densities less than 1 mA/cma only the normal halogen ion mass peaks were recorded. At higher fluxes, in the region of 3 mA/cm2, a more complex peak structure was observed. Lithium fluoride gave very erratic F- signals and produced whisker like structures, presumably reformed from ejected ionic material, in the electron bombarded regions. This contrasts with the clean simple holes produced in NaCl and KI. This may be related to a difference in surface potential of the LiF compared with NaCl and KI. We have found that this dose dependent potential can be considerable [7]. At room temperature the ejection process is extremely efficient. Typically, for sodium chloride one chlorine ion is removed for every five 240 volt electrons striking the surface. That the process is not a simple surface boiling has been shown by control experiments with glass, magnesium oxide and potassium azide. None of these showed any detectable ion ejection, even though the azide thermally decomposes at 2500C. Moreover, if a sufficiently high surface temperature were reached to boil off material, the colour centres would be annealed out, and they are not.
LETTERS
15 January 1968
NaCl and KI have been bombarded with slow electrons at 77oK. Both were eroded, but at a reduced rate. The KI rate of erosion being more effected by the reduction in temperature. In NaCl visible fluorescence was produced. These results should be regarded as qualitative as there are still problems relating to non-uniform surface charge and its effect on incident electron current density and ion ejection. Work is continuing, to overcome these difficulties and extend the measurements down to the theoretically more interesting region between 77OK and 4oK. The results obtained so far support the Pooley-Hersh defect production mechanism which predicts a strong temperature sensitivity and an antocorrelation between luminescence and defect production. The ease with which slow electrons can erode an alkali halide surface is a factor which must also be taken into account in epitaxy studies. The electron current, to which an alkali halide substrate is subjected, prior to the evaporation of the epitaxial material, must seriously modify the structure of the surface and hence the initial nucleation and subsequent epitaxial growth. Such effects have been detected by several authors [8,9]. We wish to thank the Science Research Council for support of this project.
References 1. F.Seitz, Rev. Mod. Phys. 26 (1954) 7. 2. J.H.O.Varley, J. Phys. Chem. Solids 23 (1962) 985. 3. C.C.Klick, Phys. Rev. 120 (1960) 760. 4. D. Pooley, Proc. Phys. Sot. 87 (1966) 245. 5. H. N. Hersch, Phys. Rev. 148 (1966) 928. 6. T.P.P.Hall, D.Pooley, W.A.RuncimanandP.T. Wedepohl, Proc. Phys. Sot. 84 (1964) 719. 7. P. D. Townsendand J. C. Kelly, Physics Letters 25A (1967) 673. 8. D. J. Stirland, Appl. Phys. Letters 8 (1966) 326. 9. P. W. Palmberg, T. N.Rhodin and C. J. Todd, Appl. Phys. Letters 10 (1967) 122.
*****
139