PRODUCTION OF OXYGEN VACANCIES BY ELASTIC COLLISIONS IN ALKALINE EARTH OXIDES

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PRODUCTION OF OXYGEN VACANCIES BY ELASTIC COLLISIONS IN ALKALINE EARTH

OXIDES

A. Hughes

To cite this version:

A. Hughes. PRODUCTION OF OXYGEN VACANCIES BY ELASTIC COLLISIONS IN AL- KALINE EARTH OXIDES. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-515-C9-518.

�10.1051/jphyscol:1973987�. �jpa-00215463�

JOURNAL DE PHYSIQUE

C O / / O ~ U ~ C9, ~u/)p/Pt11etlt f f u

1 1 - 12, Totne 34, Nouembre-Dkcembre 1973, page 0 - 5 1 5

PRODUCTION OF OXYGEN VACANCIES BY ELASTIC COLLISIONS IN ALKALINE EARTH OXIDES

A. E. H U G H E S

Materials Development Division, AERE Harwell, Didcot, Berks, U. K.

RCsumC.

La production des lacunes d'oxygene dans MgO et CaO par des protons de 400 keV et

MeV a ete mesuree a -

^{K , 77 K et 300 K . La} vitesse de production de defaut stable est nettement infkrieure a celle prevue par une simple theorie de defaut d'irradiation par choc, et est seulement environ deux fois plus grande aux basses temperatures qu'a la temperature ambiante.

I1 n'y a pas de recuit des lacunes Iorsque I'echantillon est porte a la temperature ambiante apres irradiation a basse temperature, m@me en presence d'irradiation. I1 est suggerk que le depart de l'interstitiel hors d'une region criticlue de recornbinaison autour de sa lacuneest I'etape de contrhle pour les defauts stables, et que la probabilite de depart est superieure a basse temperature.

Abstract.

The production of oxygen vacancies in MgO and CaO by 400 keV and

MeV protons has been measured at - ^{15 K, 77 K and 300 K.} The stable defect production rate is substan- tially lower than expected according to siniple knock-on radiation damage theory, and is only about a factor of two higher a t low temperatures than at room temperature. N o annealing of vacancies occurs when the sample is warmed up to room temperature after low temperature irra- diation, even in the presence of irradiation. I t is suggested that escape of the interstitial from a critical recombination region around its vacancy is the controlling step for stable damage, aud that the escape probability is highest at low temperatures.

1. Introduction. - T h e identification o f optical a b s o r p t i o n b a n d s associated with simple vacancy defects i n tlie alkaline earth oxides has n o w been firmly established, a s a result of detailed c o l o u r cent[-e measurements (e. g. Hughes a n d Henderson. 1972).

111

particular, oxygen vacancies m a y t r a p o n e o r t w o electrons t o become F i o r F centres respectively.

T h e F+ a n d F optical absorption b a n d s may therefore b e used t o s t u d y point defect production in these materials. M o s t radiation d a m a g e studies have been m a d e o n M g O a t r o o m temperature ( H e n d e r s o n and King, 1966, Sibley a n d C h e n , 1967, C h e n et d., 1970, H e n d e r s o n a n d Bowen, 1971. Tench a n d Duck, 1973) where t h e n u m b e r o f defects produced is a b o u t a n o r d e r o f magnitude o r more fewer t h a n expected on t h e basis o f simple knock-on d a m a g e theory.

However, C h e n a n d Sibley (1967) have s l ~ o w n that t h e daniage mechanism is indeed caused by elastic collisions a n d not rndiolysis, a n d C h e n ct crl. (1970) have established that the displaccmcnt energy o f the oxygen ion in M g O is 60 eV (see also Sondes and Sibley, 1972). T h e M g displacenient energy has been estiinated a s (64 1 2) eV ( S h a r p a n d Rumsby, 1972).

Sibley a n d C h e n also irr:tdiatcd MgO with 1.6 MeV electrons a t 80 K, atid report t11;lt there seemed t o be n o defect annealing bclwccll SO K ;lnd roo111 tempe- rature. In view o f thc belia\iour of irr:\di;lted metals a n d alkali halides, o n e \\,auld expect that isolated interstitials would be more stahlc at lo\\, tempcraturcc, resulting i n higher dcf'ect prc~rluction r:ttcs ( i . e. less

r e c o m b i n ~ ~ t i o n ) followed by annealing stages d u r i n g warm-up. T o investigate this situation further a n d t o improve d a t a available in the oxides a t low tem- peratures, this paper reports t h e results of measure- ments o n M g O a n d C a O between a b o u t 15 K a n d soon1 temperature. Ideally, electron irradiaticn a t a few MeV provides t h e cleanest irr;tdiation tool, since the displacements a r e mainly d u e t o primary collisions. However. the low defect production rate at practical currents

I pA .CIII-') ~ n e ; l n s very long irradiation runs. making low temperature mea- surenients difficult. especially if the cryostat has t o be removed from the accelerator f o r optical measu- rement. Proton irradiation is a convenient c o m p r o - mise : the number of secondary collisions per primary displaced a t o m is only a few, tlius avoiding [lie large collision ciiscades inevitable with neutron o r heavy particle daniage, but nevertheless the defect production rate is fairly high.

2. Esperiniental details.

Single crystals o f M g O a n d C a O obtained from W. a n d C. Spices Ltd.

have bcen irrr~diated with 400 keV a n d 3 MeV p r o - tons froin t w o diffcrcnt Van de GraalT accelerators.

Tlie current densities employed were between 0.1 p ~ . c r n - ' and 1.0 l ~ A . c m - ' , arranged t o keep the incident power below - 0.5 W.cm-'. For runs a t l i q ~ t i d nitrogen a n d liquid helium temperatures the s:~mples wcrc held o n tlie c o p p e r cold-finger o f a cryostat fillcd with the a p p r o p r i a t e coolant. Tlie

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973987

C9-5 16 A. E. HUGHES

temperature of tlie sample was monitored witli two calibrated Allen-Bradley resistors, one in contact witli tlie cold finger and one mounted on the sample itself. With the irradiation geometry used, the latter thermometer should register a temperature rather higher than tlie irradiated face of the crystal, which was in direct contact with the cold finger. The tempe- rature registered by the sample resistor during irra- diation at liquid Iielium temperatures (e. g. 0.1 PA. c m W 2 at 3 MeV, 0.35 p A . c m - 2 at 400 keV) was 12-15 K.

3. Theory.

The number of defects expected from knock-on damage has been calculated using the Kinchin-Pease (1955) theory, assuming the Ruther- ford cross section for elastic collisions between the protons and the ions in the solid (e. g. Corbett, 1966).

Allowance was made for the energy loss of protons as they penetrate the crystal, and threshold displa- cement energies of 60 eV were used. Results are collected in table I .

Tlleoretical tiuinber

of o.uygen displacet~lerlts per proton (*)

MgO CaO

Proton Primaries Secondaries Primaries Secondaries

energy only included only included

- - - - -

400 keV 1.5 3.5 1 .O 2.9

3 MeV 2.4 8.1 1.6 6.9

( * )

Calculated using a stopping power dE/dr

I/E above

400 keV and independent of E below 400 keV. Stopping powers and ranges were extrapolated from data given in the

of

The oxygen vacancy concentration was calculated from tlie optical density of the F + and F bands using tlie integrated area under the bands. Tlie oscillator strength of tlie F+ band was taken as 0.8 in MgO (Henderson and King, 1966) and 0.76 in CaO. The latter value was obtained by comparing the optical density with the number of centres determined by EPR ( M . J . Duck, private communication). In CaO (where the F and F+ bands are well separated) it was clear that most of the oxygen vacancies produced are in tlie form of F + centres.

4. Results. - Figures 1 and 2 show 3 MeV growth curves for MgO and CaO a t - ¹⁵ K, 77 K and 300 K. Over the limited range of beam currents used tlie number of defects produced was independent of dose rate. Results for both 3 MeV and 400 keV irradiations are given in table I I in terms of the number of vacancies per incident proton, as measured from the initial slopes of growth curves such as in figures 1 and 2.

Although tlie production efficiency is higher at low temperatures, so that a larger number of defects

FIG.

Growth curves of oxygen vacancies in MgO under 3 MeV proton irradiation.

FIG. 2.

Growth curves of oxygen vacancies in CaO under

Nutnber of oxygen vacancies produced per incident proton

Proton MgO CaO

energy 1 5 K 7 7 K 3 0 0 K 1 5 K 7 7 K 300K

- - - - - - -

400keV 0.25 0.15 0.10 0.14 0.11 0.06

0 77 0.50 0.45 0.80 0.58 0.36

are produced for a given dose, no defects are lost if the sai?ip/e is subsequei7t/jq ~ ~ a m e d up to 300 K.

This is in contrast to the annealing stages observed for metals and alkali halides (e. g. Corbett 1966, Jtoh et a/., 1965, Belir

a/., 1967). In fact, if anything there is a very small ^{( <} 10 0/;',) i17cl~ease in the number of F and F t centres, altliougli this may be due t o a systematic error is measuring the number of centres from the absorption bands. Tlie lack of annealing is illustrated by figure 3, which shows tlie effect of changing the irradiation te~uperature in the middle of a run. Although the production rate changes, the excess defects produced at the lower temperature remain stable.

A selection of sanlples were X-irradiated at room

temperature to a dose of 10 Mrad following proton

PRODUCTION O F OXYGEN VACA .NCIES BY ELASTIC COLLISIONS C9-5 1 7

FIG. 3.

Growth curve of oxygen vacancies in CaO under 350 keV proton irradiation, in which the temperature was changed from - ^{15 K to 300 K .} The dashed curve shows a typical growth curve carried out entirely at 300 K under similar

conditions.

irradiation. N o change in the total ( F f + F) optical absorption could be detected, although in CaO about 20 % of the F' centres were converted temporarily to F centres. There is thus good reason to believe that all the isolated oxygen vacancies are being moni- tored in these experiments. No significant absorption which might be due to F aggregate centres was observed (Hughes and Henderson, 1972).

Annealing samples irradiated at - ^{15 K, 77 K}

and 300 K above room temperature produced results shown in figure 4 for CaO. The general annealing

C O O 3 M c V P R O T O N S

A N N E A L I N G TEMPERATURE ( O C )

FIG. 4.

Annealing of the total optical density in the F and F bands of CaO, following irradiation at -

^{K, 77 K and}

300 K. The samples were held for

h at each teniperature.

trend follows results obtained on neutron and ion- beam irradiated samples of CaO. Results for MgO agreed well with those obtained by Chen

rrl.

1969).

being close to their results for ncutron irri~diated crystals. Figure 4 indicates that there may be an initial drop in the oxygen vacancy concentration at - 100 OC for samples irradiated at low lempcratilre.

A similar result was obtained for MgO irradiated at 77 K, but 15 K irradiation produced too wide a scatter on the experimental points (due to high absorp- tion background from Fe3+ impurities) to tell if the effect was present or not. Further experiments will be required to check this point.

5. Discussion.

It is clear from tables I and I 1 that the number of vacancies produced by proton irradiation, even at low temperatures, falls well below that expected on simple damage theories.

The fact that the experimental number is even well below the number expected for primary displacements shows that the error is not simply in the Kinchin- Pease model of the secondary cascade, and it is unli- kely that the approximations used in calculating the numbers in table I are wholly responsible for the discrepancy. The obvious explanation is that only a small fraction of the displaced ions result in stable vacancy-interstitial pairs, although it is worth noting that to reduce the itiitial slope of the growth curves any recombination must be correlated, at least within a particular collision cascade (i. e. an interstitial recombines with a vacancy produced in the same cascade). Purely random recombination does not lead to a reduction in initial growth rate. We are therefore led to suppose that an interstitial is only stabilized if i t escapes from some critical region around the vacancy. It is therefore not sufficient (on average) just to displace an ion : it must be given enough energy to escape from the critical region.

The higher growth rate at low temperatures may thus result from one or other of the following mecha- nisms.

( i ) The critical region is larger at 300 K than at low temperatures. In this case one would expect some defect annealing to occur on wiirming from low lem- peratures, at least during the presence of irradiation (the defect recombination inside the critical region may be radiation-enhanced). The data in figure 3 d o not support this view.

(ii) At low temperatures more interstitids escape from the critical region. This picture is consistent with all the data, i n that one would not then expect any annealing on warming if an escaped interstitial is trapped at a stable site. The greater escape probability at low temperatures may come about because of, say, increased channelling at low temperatures, but further speculation is not i n order.

The results reported in this paper offer some new data on low temperature knock-on damage in insu- lating materials. Although no detailed explanation of all the features is forthcoming at present. the results sliow clearly t h i ~ t defect I-ecombination events are crucial i n establishing the level of stable defect struc- ture produccd by particle irradiation.

Acknowledgments.

I should like to acknow-

Ieclge the expcriment:11 assistance of Mr. R . K . Dawson.

C9-5 1 8 A. E. HUGHES

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