HAL Id: jpa-00216841
https://hal.archives-ouvertes.fr/jpa-00216841
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.
COLLOID ABSORPTION BANDS ASSOCIATED
WITH IMPLANTED ALKALI IONS IN MgO SINGLE
CRYSTALS
P. Thevenard
To cite this version:
C7-526 JOURNAL DE PHYSIQUE Colloque C7, supple'ment au no 12, Tome 37, De'cembre 1976
COLLOID ABSORPTION BANDS ASSOCIATED WITH IMPLANTED
ALKALI IONS IN MgO SINGLE CRYSTALS
P. THEVENARD
Dkpartement de Physique des MatCriaux (*)
Universitk Claude Bernard Lyon I 43, boulevard du 11 novembre 191 8
69621 Villeurbanne, France
R6sum6. - Les defauts cri.6~ dans MgO par implantation d'ions alcalins d'energie de l'ordre de 0,5 MeV ont et6 etudies par des mesures d'absorption optique. Les defauts du type F, F+ et V sont observks aprks bombardement a 77 ou 300 K. De nouvelles bandes d'absorption ont et6 obtenues apres traitement thermique et ont ete attribuees aux ions implantes. Les positions des bandes d'absorption correspondantes dependent de la nature de l'ion implant6 et ont et6 obtenues a 2,53 eV pour le sodium et 1,38 eV pour le potassium. La position et la largeur a mi-hauteur de ces bandes Bvoluent comme celles des bandes d'absorption de collofdes dans les halog6nures alcalins. Les oscil- lations de plasma de petites particules metalliques dans un milieu donnent theoriquement une absorption optique a 2,3 eV et 1,48 eV pour le sodium et le potassium respectivement dans MgO. Les resultats expkrimentaux sont en bon accord avec ces valeurs.
Abstract. - Defects induced in MgO with implanted alkali ions in the 0.5 MeV range have been investigated by optical absorption measurements. Vacancies defect (F+ or F) and V centers are observed after bombardment at 77 or 300 K. New absorption bands have been obtained after thermal annealings and have been attributed to implanted ion centres. The corresponding absorp tion band positions depend on the nature of the implanted ions and have been obtained at 2.53 eV for sodium and 1.38 eV for potassium. The position and half-width of these bands evolve like these of colloid absorption bands in alkali halides. Plasma oscillations of small metallic particles in a medium give theoretically optical absorptions at 2.3 and 1.48 eV for sodium and potassium respec- tively in MgO. Experimental results are in good agreement with these values.
1. Introduction. - Intrinsic defects in MgO creat- ed by irradiation have been studied extensively during the past decade [l]. Both Van de Graaff electron irradiation and fast neutron irradiation have been used essentially to produce defects like F+, F or V centres [2-41. On the other hand, few results concern the creation of defects with high energy particles [5-81.
In alkali halides, the implantation of alkali ions allows one t o produce small aggregates of implanted par- ticles [9, 101 and t o study the properties of these colloids. The study of these colloids with electron microscopy is not easy in alkali halides [ll], because the observation time is limited by the high concen- tration of defects created in the alkali halide by the electron bombardment [12]. To observe colloids and t o follow easily their evolution in size by using optical absorption and electron microscopy, it was necessary to obtain colloids of alkali ions in a medium which had a low rate of defect creation under electron bombardment. Magnesium oxide has been chosen for these properties and the purpose of this preli- minary work is to report the formation of sodium and potassium colloids in implanted MgO.
2. Experimental procedure. - The MgO samples used were obtained from Spicer single blocks of 3 N o r 4 N purity. The samples cleaved along
<
100>
had an area of 1 cm2 and a thickness of about 0.1 cm.- The implantations were performed at 77 or 300 K using the Van de Graaff accelerator of the Institut
de Physique Nucliaire de Lyon. The energy was 470 keV for sodium and potassium ions. The alkali ion sources allowed good beam conditions for particle fluxes in the range 5 X 10'' to 1013 i o n ~ . c m - ~ . s - ' , and
doses in the range 1017 to 2 X 1017 ions.cm-'.
The values of the mean projected range R, of sodium and potassium in MgO can be calculated using Northcliffe and Schilling's tables [13]. The calculated penetration depths are 5 500
A
and 3 200A,
respecti- vely for 470 keV sodium and potassium in MgO. This value of R, in the case of potassium is in good agreement with the experimental value obtained from Rutherford basckscattering of alpha particles [8].The depth profile of implanted ions is nearly gaussian and the concentration of implanted ions N(x) at a distance X from the surface of the sample is given by :
N 1 X - R ,
N ( x ) = exp
- - -
J2
AR, 2 AR,COLLOIDS ASSOCIATED WITH IMPLANTED ALKALI IONS IN MgO C7-527 where N is the concentration of implanted ions
per cm2,
and AR, the standard deviation in projected range. The maximum of concentration N(R,) corresponds for the doses used to a concentration of about 10
%.
-
Optical absorption measurements during the implantations were performed in situ using a spec- trophotometer Beckmann DK 2A. The sliding sample holder could be cooled at 77 K (LNT) and could be located alternatively in front of the ion beam or in the optical beam for analysis. The F+ centre concen- trations per cm2 were calculated using Dexter's modification of Smakula's formula [14].-
During thermal annealings the samples were placed in a platinium box. The implanted crystals were annealed for 30 minutes in 50 OC steps and optical spectra were taken at room temperature with a Cary 17 spectrophotometer.3. Experimental results.
-
3.1 SODIUM IMPLANTA- TION IN MgO AND THERMAL ANNEALINGS. - 3.1 . l Ki- netics of intrinsic defect creation. - Sodium implan- tation in an MgO crystal (4 N) has been performed at LNT. The optical spectra were recorded at LNT during the irradation.Three optical absorption bands are observed princi- paly at 250 nm, 575 nm and 350 nm during the implan- tation. The 250 nm band associated with F + and F centres, has an evolution, with the dose of implanted ions, represented in figure 1. The 575 nm band evolu- tion js also reported in this figure.
For higher doses, the creation yield of F+ centres decreases.
The depth distribution of F+ centres is unknown, but a mean value of the F' concentration in the irradiated zone can be estimated by assuming an homogeneous depth distribution of F+ centres in the volume R,. 1 cm2. For exemple, the average concen- tration F+ centres is of the order of 4 X 1020 per cm3 for a dose of 7 X 1016 ions.cm-2. Of course, in the
region corresponding to the maximum of the nuclear energy loss, the concentration is expected to be greater than the average.
The 575 nm band has an amplitude greater than those observed by Evans et al. [5] in 3 MeV neon irradiations.
3.1 .2 Thermal annealing of a 2 X 1017 Na implanted crystal.
-
Figure 2 shows the results of the optical studies before and after heat treatment at 350 OC, 600 OC and 900 OC. The 250 nm and 575 nm bands decrease after the 300 O C anneal. A band located at470 nm (2.63 eV) can be clearly resolved in the spectrum taken after the 300 O C anneal. The width of this band
is quite large of the order of 1.5 eV.
FIG. l . - Optical density evolution a t 250 nm (a) and 575 nm (b) with the dose of 470 keV sodium implanted in MgO at
77 K.
For doses lower than 2 X 1015 i o n ~ . c m - ~ , the F+
centre concentration is proportional to the square root of the implanted ions dose N :
FIG. 2. - Absorption spectrum evolution with heat treatment of a MgO sample implanted with 2 X 1017 Na ions. cm-2 before thermal treatment (a) and after 350 OC (b), 600 OC (c) and 900 OC (d) anneals (isochronaf treatment 30 minutes
in steps of 50 OC).
C7-528 P. THEVENARD
FIG. 3.
-
Evolution with thermal annealings of the position of the absorption band associated with sodium in MgO (isochronaltreatment : 30 minutes in steps of 50 OC).
stays at 650 nm (1.92 eV), the width increases and the amplitude decreases.
This absorption band, which has the same charac- teristics than colloid absorption band in alkali halides [l 5, 161, is associated with the presence of small metal- lic aggregates of Na in MgO. This band is denoted Another absorption band at 382 nm (3.24 eV) can be seen clearly resolved in the spectrum taken after the 600 OC anneal. This band, denoted X::', increases in amplitude after treatment at higher temperatures. Its position does not depend of the annealing tempera- ture ; its width is of the order of 0.5 eV. The
X:?
band could be correlated with the high concentration of colloids in the implanted region.
3 . 2 POTASSIUM IMPLANTATION IN MgO AND THER- MAL ANNEALINGS.
-
3.2.1 Kinetics of intrinsic defectFIG. 4.
-
Optical density evolution at 77 K of the 250 nm absorp- tion band with the dose of 470 keV implanted potassium inMgO samples of purity 4 N (a) and 3 N (b).
creation in MgO.
-
Figure 4 shows the 250 nm band evolution with the dose of implanted potassium at 77 K for two crystals of purity 3 Nand 4 N. By taking into account the differences between the penetration depths of 470 keV Na and K in MgO, the 250 nm evo- lution is quite similar in the case of 4 N crystals implanted with Na or K.In the case of implanted samples of purity 3
I?,
the first stage of F+ centre creation is not observed and a saturation is observed at a lower dose than in the case of 4 N samples. This result indicates the great influ- ence of impurities on the defect creation in MgO. The 575 nm band has an amplitude which is much lower than in sodium implantation3 . 2 . 2 Thermal treatment of a crystal implanted with
1 0 1 7 p ~ t a ~ ~ i u m p e r cm2.
-
Figure 5 shows the results of the optical studies before and after heat treatment at 450 O C , 510 O C and 800 OC.5 0 0 l 0 0 0 1 5 0 0
X
lnm1
FIG. 5. - Optical spectrum evolution with heat treatment of a MgO sample implanted with 1017 K ions.cm-2 at 77 K : before thermal treatment (a) and after 450 OC (b), 510 OC (c) and
800 OC (d) anneals.
An absorption band, located at about 875 nm (1.42 eV) can be seen clearly resolved after the 450 O C
anneal ; the width of this band is approximately 0.9 eV. This band, denoted
CY,
shifts towards higher wavelengths with treatment at higher temperatures, while the width decreases. After the 800 QC anneal, the band is located at 900 nm (1.38 eV) and its width equal to 0.56 eV.The C P O absorption band has the characteristics of colloid absorption bands and it can be associated with the presence of small colloids of potassium in MgO.
COLLOIDS ASSOCIATED WITH IMPLANTED ALKALI IONS IN MgO C7-529
4. Discussion. - Two stages appear in F+ (or F) centre creation when pure MgO crystals (4 N) are irradiated with 470 keV sodium and potassium.
In the first stage, corresponding to doses lower than 1015 i ~ n s . c m - ~ the F+ centre concentration is pro- portional to the square root of the implanted ion dose. So the generation of F+ centres in MgO can be explain- ed with the help of a recombination model of free interstitials with F+ centres [l. 171.
The second stage of F + centre creation corresponds to a saturation dating from a dose which depends on the implanted ions. Even for high doses like 2 X 10i7 a
decrease in F + centre concentration is not observed. For these high doses the F' centre concentration is five times higher than those obtained by Evans et al. 151
with neon irradiation of MgO. The saturation is strongly dependent on the crystal purity as is shown in figure 4. This effect of impurities can explain the diffe- rences observed between our results and those obtained by Evans et al. (5).
- The effect of heat treatment on the implanted samples allows the observation of optical absorption bands associated with the implanted ions. The posi- tion of these bands depends on the nature of the implanted particles. The shifts of these bands towards higher wavelengths with thermal annealings and the decreases of their widhts are related to the presence of Na or K colloids in MgO.
The position and the width of the and bands are independent of the observation temperature (77 or 300 K).
The wavelength, corresponding to the absorption of very small aggregates of alkali metals in a surrounding medium of refractive index no, is related to the wave- length
A,
associated with the plasma frequency of electrons in the bulk metal by the formulaThis formula is obtained from Doyle's theory [l51 with the assumptions of spherical aggregates being smaller than 100
A
and the mean free path of electrons in a colloid equal to the radius r of the colloid. The last assumption implies that the width of the colloid absorption band AE depends on the colloid radius r by the relationwhere v, is the electron velocity at the Fermi level. By using critical wavelengths
A,
for the onset of ultraviolet transparency of the alkali metal, 209 nm for sodium and 318 nm for potassium, the theoretical wavelengths for absorption of small colloids are respectively 550 nm for sodium and 840 nm for potassium. The results concerning the appearance of the andC P O
bands (position A, width AE) arereported on the table I. The size of the colloids in the initial stage of formation is deduced from AE, and the number N of atoms in the smallest colloid is estimated from this size.
c,",gO
-
cpO
-
I (nm) theoretical 550 840 A (nm) observed 470 875 AE (eV) 1.5 0.9 N (atomes) 13 17The minimum number of atoms necessary to have an aggregate with metallic properties would be of the order of 15 atomes. However, we have to point out that a size distribution of colloids has been neglected in this calculation. But without electron microscopy measu- rements of this size distribution it is difficult to have another hypothesis.
The presence of satellite absorption bands X ~ : O (382 nm) and X?' (500 nm) is not understood. These bands could be due to the high concentration of colloids in the medium and perhaps to the shape of colloids [18]. The high concentration of colloids which can be obtained only in the case of implanted samples could permit an understanding of the non-observation of these bands in the case of colloids created in additi- vely colored alkali halides.
5. Conclusion. - The metallic aggregates of sodium and potassium in MgO observed from optical measu- rements have absorption bands located near the values predicted from electron plasma oscilla'ions in the colloids. In the beginning of the aggregation, the width of the colloid absorption bands is higher than those observed in alkali halides for colloids of the same nature. Assuming only one spherical size distri- bution for the colloids, the minimum number of atoms in an aggregate having metallic properties is of the order of 15 atomes for sodium or potassium.
The studies nowly realised by Treilleux, Chassagne and Hobbs [l91 in electron microscopy on these sam- ples will permit verification of the correlation between the size of colloids and the width of the absorption band.
A new absorption band is observed on the short wavelength side of the colloid absorption band. This could be connected with the shape and the high concentration of colloids. Further investigations are necessary to explain the presence of these satellite bands.
P. THEVENARD
References [l] SONDER, E. and SIBLEY, W. A., Defects in Solids, edited
by Crawford J. H. and Slifkin L. (Plenum, New York). [2] KAPPERS, L. A., KROES, R. L. and HENSLEY, E. B., Phys.
Rev. B 1 (1970) 4151.
[3] CHEN, Y., WILLIAMS, R. T. and SIBLEY, W. A., Phys. Rev.
182 (1969) 960.
[4] HENDERSON, B. and KING, R. D., Phil. Mag. 13 (1966) 1149.
[5] EVANS, B. D., COMAS, J. and MALMBERG, P. R., Phys. Rev.
B 6 (1 972) 2453.
[6] HENDERSON, B., Report A. E. R. E. HARWELL, unpu- blished.
[7] HUGUES, A. E. and PELLS, G. P,, Phys. Status Solidi (a) 25 (1974) 437.
[g] THEVENARD, P., DUPIN, J. P., CACHARD, A. and DAVE- NAS, J., Colloque de MCtallurgie, Saclay (1976).
[9] DAVENAS, J., PEREZ, A., THEVENARD, P. and DUPW, C. H. S.,
Phys. Status Solidi (a) 19 (1973) 679. 1101 THEVENARD, P., These Lyon (1974). [l1 l CHASSAGNE, G., Thhe Lyon (1976).
[l21 HOBBS, L. W., The Chemical Society, London : Edited by Roberts M. W. and Thomas J. M., 4 (1975) 152. [l31 NORTHCLIFFE, L. C. and SCHILLING, R. F., Nuclear Data
A 7 (1970) 233.
[l41 DEXTER, D. L., Sol. Stat. Phys. 6 (1958) 353. 1151 DOYLE, W. T., Phys. Rev. 111 (1958) 1067.
[l61 SMITHARD, M. A. and TRAN, M. Q., Helv. Phys. Acta 46
(1974) 869.
[l71 DURAND, P., FARGE, Y. and LAMBERT, M., J. Phys. & Chem. Solids 30 (1969) 1353.
[l81 F ~ C H S , R., Phys. Rev. B 11 (1975) 1732.
[l91 HOBBS, L., TREILLEUX, M., CHASSAGNE, G. and THEVE- NARD, P., to be published in.
DISCUSSION
