Damage profiles in MgO after ion implantation

Nuclear

Instruments

and Methods

in Physics Research

Nuclear instruments & Methods in Physics Research Sk.< ~IOP B

BhS (1002) 287-290

North-flolland

Damage profiles in MgO after ion implantation E. Friedland and M. Hayes

Damage a-particle

depths

channeling

were performed

in MgO

1.

after

iit room temperature.

ionic crystals. However, pronounced

(100)

in a backscattering

implantation geometry.

Even at the highest ion dose only partial

in all cases damaye

with increasing

of IS0 keV argon, krypton and xenon ions were analyzed by means of ’ and all implantations

Ion flucnces ranged between 4 x IO”’ to I x IO’” at. cm

depths exceeded

the projected

lattice disorder was observed, which is typical for

ion ranges si~nifi~ntl~f.

This effect becomes

more

ion mass.

Introduction

The stopping of cncrgctic ions in solids leads to radiation damage due to atomic collisions and clcctronic excitations. Initially the resulting primary damage in a crystal lattice consists of point defects. The lift time of a particular point defect depends critically on many parameters of which tctnpcraturc, local defect density and lattice structure arc the more important ones. According to cnvir(~nm~ntal circumstan~cs point defects might cithcr migrate, form aggregates with other defects or be annihilated with time constants varying between extrcmcly small and very large. In the case of heavy ion implantation most of the damage is produced in collision cascades, where the defect density is generally so high that spontaneous collapse of the entire core of the cascade takes place [I]. This leads either to the formation of amorphized regions or dislocation loops. Amorphization is normally found in covalent and ionocovalcnt materials, whilst extended dcfccts are typically for metals and ionic crystals. However, independent of the final defect structure, one would expect that defect c~~ncent~ti(~n profiles are similar to the original point defect distributions. This seems to be the case for all semiconductors and other covalent materials, where damage depths more or less coincide with the projected ion ranges. However, in metals damage depths were found, which in some cases were up to an order of magnitude larger than the projected ranges of the implanted ions [Z-7]. Huge damage ranges were especially observed in fee metals, indicating that dislocation propagation mechanisms might be largely responsible for the final damage profiles in these materials [8,9]. If dislocation dynamics is indeed responsible for the enhanced damage ranges, it would bc reasonable to expect that deep radiation damage should also bc observed in certain non-metals. Natural candidates Elsevier Science Publishers B.V.

would bc crystalline materials where initial defect processes evolve along similar lines as in metals, provided the dislocation movement resisting Pcicrls force is not too high. This is probably true for ionic oxides. as in thcsc materials defects are mainly created by atomic collisions [lO,ll] with a final dcfcct structure prcdominantly exhibiting dislocation loops [ 121. Damage depths exceeding theoretical projected ranges by approximately 50% were rcportcd for MgO after implantation of 2 MeV oxygen ions and 4 McV iron ions [13]. However, the distribution of the implanted iron ions was also found dccpcr than cxpcctcd, whilst the oxygen profile could not bc analyzed. This might hc a result of inaccuracies of stopping theories at high energies [14]. As furthermore no special care was taken to avoid beam ali~limcnt with a major crystal axis, the observed deep damage might also be due to ion channeling. On the other hand a number of implantation profile analysts for a variety of metal ions with energies in the region of 100 kcV showed good agreement with theoretical range calculations [IS171. In order to test for deep damage it was therefore dccidcd to measure damage depths in MgO single crystals after implantation of chemically inactive ions at medium encrgics.

2. Experimental

method

Single crystals of magnesium oxide with thickncsscs of I mm were implanted at room temperature with 150 keV argon, krypton and xenon ions with fluences ranging from 5 x 1OlJ to 10” at. cm ‘. To avoid target heating dose rates were kept at approximately 1013 at. s ‘. The target was furthermore tilted by 7” cm-’ relative to the (100) axis in order to limit ion channeling as far as possible. One crystal was also irradiated V.

OXIDES/CERAMICS/CARBIDES

with a 12.5 kcV electron pulse of 300 ns duration and an cncrgy dcnsitity of 0.X.3 .I cm ‘. Samples wuc anafyzcd before and after ion hornbardmcnt by cu-particlc charmcling in a backscattcring gcomctry. For this purpose crystals wcrc mounted on a three-axis goniomctcr in a scattering chamber which was pumped down to 10 ” Torr. The goniomctcr was cquippcd with clcctr~)nic~llly read digit&s rigidly fixed to its axes. allowing a rcscttability of hcttcr than 0.05”. To suppress secondary clcctrons a negative voltage of 300 V was applied to a ring-shaped clcctrodc in front of the target holder. The analyzing beam of a-particles was obtained from the 2.5 MV Van de Graaff acccfcrator of the University of Pretoria. Beam divcrgcnce was less than 0.04” with a spot size of approximately 1 mm and a hcam current of about 20 nA, which was directly measured on the target. Scattcrcd particles wcrc dctcctcd by a surface barrier dctcctor at an angle of 165” with an acceptance angle of 2”. The energy resolution of the detection system was about 13 kcV. Aligned spectra were obtained for the (100) oricntation with beam encrgics of IS and I.8 McV. Thcsc spectra were normalized to random spectra obtained by rotating the target during data acquisition about an axis t&cd by 5” with respect to the aligned orientation. The cncrgy scale was convcrtcd to a depth scale by using the clcmcntal stopping power data of Zicglcr [IX] in conjmlction with Bragg’s rule for compounds. These stopping power data arc strictly valid for ~lnchannclcd particles only and conscqucntly slightly ovcrcstimate the cncrgy losses for the aligned orientations. This will lead to a slight undcrcstimation of the depths.

3. Results and discussion Before implantati~~n the aligned spectra along the (100) orientation gave minimum yields near the surfact of approximately 5% for all specimens, indicating rclativcly good single crystal qualities. After implantation with flucnccs above 10’” at.cm ’ a well dcfincd damage peak dcvcloped, which grew rclativcly fast with increasing dose but reached a saturation value significantly below random yield at a flucncc somewhere in tho region of IOn’ at.cm-‘. Backscattering spectra before and after implant~~ti~)n of 2 X IO” Xc’ cm ’ arc shown in fig. 1 for the aligned and random oricntations. According to electron microscopic investigations 112,131, damage due to iron implantation in MgO consists mainly of dislocation n&works. The dcchanneling cross section after platinum implantation reveals a square root dcpcndence on cncrgy [IY], which also points to dislocations as the dominant defect structure. If a similar defect structure is assumed after implantation of rare gas ions, the appearance of a damage peak

Energy

(MeV)

0.5

1 .o

1.5

-”

) MgO (‘00)

Pig. I. Aligned and after implantation

random of

150

1

Channel backscattering spectra hefore and keV xenon ions into MgO (IOO)

with 3 fluttnce of ?X lOI at. cm- ‘: Q) virgin, @ implanted, @ random. E,, = 1.5 Me\‘: H = I65 O.

in the aligned spectrum is somewhat uncxpectcd. During passage through an cxtcndcd dislocation, a-particles arc gradually stcercd away from the original alignment by corrclatcd small angle scattcrings. As a result. the analyzing beam should become progressively more disoriented with rcspcct to the undisturbed crystal lattice, leading to an increasing dechanncling cross section with increasing depth. This situation is typically found in ion implanted metals, which gcncrally show an increasing dcch~~nncling cross section thr(~ugh(~ut the damaged region with a knee at the end of the disturbed lattice. The fact that this is not true in this cast. points to a dcfcct structure consisting of regions which deviate grossly from the original crystal lattice. This may be due to partial amorphization, polycrystallinc regions or scvcrc lattice distortions in the ncighhourhood of rare gas precipitates. The minimum yield for the magnesium sublatticc at the center of the damage peak after impiantati~~n of 150 kcV argon ions is shown in fig. 2 as a function of fluencc. Saturation cffccts above a dose of approximatcly 10” Ar+ cm-’ arc quite obvious. The normalized dcch~~nlleling of approximately 70% at high flucnccs agrees well with similar observations after implantation of iron [15,20], gold [16] and platinum [ IY] into MgO. This is a further confirmation of previous observations, that the lattice structure of MgO is nut totally destroyed by high dose implantations. The thickness of the disordered zone, which extends from the surface into the bulk, will be referred to as damage depth. This depth is directly determined from the half-width of the damage peak in the aligned spectrum. Experimental damage depths Rex,, for flu-

150

80 -

keV Ar+

---)

MgO(100)

G? m .c a, : 0 _c

,A 60-

J”

-”

40-

: 0

1 :

/ 20 -

if’ 1

#d

1 /“‘zll’

0. 1o14

1o16

1o15

Fluence Fig. 2. Normalized MgO

(100)

dechanneling

as a function

10”

(cm-Z) after argon implantation

of tluence

at the center

into

of the

damage peak. The solid line is drawn to guide the eye.

of 2 x 10” at. cm ’ arc given in the second column of table 1. The quoted errors were cstimatcd from a statistical analysis of results obtained from different cxperimcntal mcasuremcnts. For comparison theoretical damage depths R,,, which wcrc obtained from the Monte Carlo simulation code TRIM [21] by assuming a displacement energy of 60 eV [22], arc given in the third column. In this context the thcoretical damage depth is defined as the distance from the surface at which the computed vacancy density has dropped to 50% of the maximum value. Theoretical projected ion ranges R, are given in the fourth column. In the last column the minimum yield is given as a pcrcentagc of the random yield at the position of the damage peak. This value is a direct measure of the crystal lattice disorder in this region. The results show that experimental damage depths exceed the thcoretical values appreciably. This effect increases with increasing ion mass. The ratios between experimental and theoretical damage depths arc approximately 1.4 for argon, 2.2 for krypton and 2.8 for xenon. Contrary to the above results, the depth distributions of the implanted ions as determined from the same backscattering spectra agreed reasonably well with the thcoretical projected ranges. The enhanced damage depths can therefore not be cxplaincd by channeling effects. Damage depths approximately 7% deeper than expected enccs

from LSS theory were also reported for 3 McV neon implantations by ref. [IO]. From the shape of the dcchanncling spectra one has to conclude that the dominant defect structure consists of severely disturbed regions in the crystal lattice. A large proportion of these regions arc probably rcmnants of collision cascades. This would imply a final damage depth in accordance with the primary defect distribution and could not explain the cnhanccd damage ranges. Howcvcr. as total amorphization is not achieved. it is ohvious that thcsc regions do not cvcntually overlap complctcly at high doses. This is an indication of a relatively efficient ion beam induced recrystallization process. which prcvcnts a complete destruction of the crystal lattice. Naturally, these rccrystallizcd zones arc not defect free and probably contain a fair amount of dislocation loops. The observed deep damage may hc due to free charge carriers, which are created copiously during electronic stopping of the ions. The energies of clectrons due to ionization proccsscs arc, howcvcr, far below the threshold energy of 330 kcV necessary for displacement of an atom from its lattice position in a binary collision. Therefore. free electrons can only effect damage by subthreshold proccsscs, which have been observed in some materials. In order to look for those processes. one of the samples was irradiated with 12.5 keV electrons with an energy density of 0.83 J cm-’ and a pulse duration of approximately 300 ns. This corresponds to a fluencc of approximately 4 x 10” c: cm ‘, which is more than four orders of magnitude smaller than the minimum dose at which optical absorption spectroscopy reveals any F-type centers in MgO after low intensity electron irradiations at suh-

Energy 0.4

600

0.6

I

I

Target:

. .. .:,

,,; -.. ; . ..

cn

73 PI

I

Comparison Ion

of experimental

lLi,

[nml

and theoretical

R,,, [nml

X [%I

Ar+

140+ 14

102

94

S8

Kr+

114*

I4

s3

54

53

Xe+

94+

10

34

3Y

49

1

Irradiation

MgO (100) V =

12.5

keV

E =

0.83

J/cm-2

‘:: 1 0 \ ‘Y,,. zoo ~..:..;,.:,, ” :,‘T.’ .-...{. .,,, .h<.“.. : ‘..;. @ -., ..I: :.. ,_ ..,-x .. ,.

: ,’ .._... _‘.,Z,

....~~~‘,i.~,~__,.,.. _._0 -..:.. ,. _... ~~._.,+...‘..~.*-. 0 I I I 250 100 150 200

damage depths

R, [nml

Beam

1.2

300 -

100 -

Table

1.o I

I

Electron

500 400 -

(MeV)

0.6

I 350

I 300

400

Channel Fig. 3. Aligned after

and random backscattering

pulsed electron

beam

ated. @ random.

irradiation: E,, =

spectra hefore and 0

virgin, @

I .5 MeV; B =

165 O

V. OXIDES/CERAMICS/CARBIDES

irradi-

threshold cnergics [22]. In fig. 3 aligned and random hackscattcring spectra are shown before and after pulsed electron beam irradiation. The dcchannehng rate reveals a damage level comparable with dcfcct densities due to heavy ion implantations. The electron beam intensity during the pulse is approximately tight orders of magnitude higher than in the low intensity experiments discussed above. This lcads to transient steep temperature gradients similar to those cxpcctcd during the thermal spike phase in a collision cascade. The observed dakage may therefore bc a result of stress fields due to the accompanying density fluctuations. Similar effects may also be rcsponsiblc for the deep damage observed after ion implantation. Dislocation loops in the vicinity of ion tracks could possibly act as sources of dislocation multiplication [23] under the influcncc of strong time-dcpendcnt stress field gradients as discussed in ref. [24] and could furthcrmorc lead to dislocation propagation beyond the theoretically expected defect distribution.

Acknowledgements

The authors the

implantations

would done

like to thank at

the

Dr. J.F. Prins for

Wits-CSIR

Sciences. grateful to the Foundation of Research for partially funding this work. Research

Centrc

for

Nuclear

Schonland

We are also Development

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