Energy dependence of concentration of implants and its chemical consequences

446

Nuclear

ENERGY DEPENDENCE OF CONCENTRATION AND ITS CHEMICAL CONSEQUENCES K. ROESSLER Instrtut

Instruments

and Methods

in Physics Research B32 (1988) 446-452 North-Holland, Amsterdam

OF IMPLANTS

and G. EICH

ftirChemie 1 (Nuklearchemie), Kernforschungsanlage Jidich GmbH, Postfach 1913, D-5I 70 Jiihch, FRG

Collision cascades induced by H, He, C, N and 0 projectiles with kinetic energies from 10 eV to 250 keV m H,O-ice at 77 K and SO, at 298 K were calculated by the computer program MARLOWE. ‘Ibe concentration of implants was determined from the penetration distribution within the half-width of the peak maximum. Due to increase of longitudinal straggling, the concentration decreases drastically by almost three orders of magnitude in the energy range studied. The implantation of 1015 cm-’ carbon ions with 10 eV into H,O-ice results in a specific concentration of about 69 mole ‘%,whereas the same number at 250 keV gives about 0.16 mole $ only. This dependence is important for the chemical consequences of ion implantation, in particular for combination of stopped projectiles to dimers, multimers and larger molecules at lower temperature when far reaching diffusion is impeded.

1. Introduction The chemical effects of ion implantation are in general considered to consist of the reactions of the projectiles with the matrix or those of activated matrix atoms or molecules with each other. Only little work has been devoted to the combination reactions of the implants themselves [l]. Among the products studied are C,, C;, CN-, CH,CHO, CH,COCH, and CHO formed by C+ and N+ implantation in KC1 [l-4] and frozen H,O [5-81, and metal colloids formed by alkali and indium ion implantation in alkali halides and rutile [9-131. The formation of these molecules was thought to proceed at high implantation doses. It is the aim of this paper to show that this it not necessarily true. The change of combination of implants depends primarily on their local concentration, in particular at lower temperatures when far reaching diffusion is impeded. The concentration depends on the number of implants (integrated dose, fluence) and the implantation profile (range distribution). Many methods can be applied to determine the profiles such as, e.g., Rutherford-backscattering, energy dependence of recoil atoms induced by nuclear reactions, radioactivity measurement after layerwise etching of materials labelled with radionuclides, measurement of coloration parallel to the surface, etc. Besides these experimental approaches, calculation via analytical formulae and computer simulation of collision cascades is frequently used to evaluate ranges and profiles [14-251. Tabulated values can be found in [26-321. It is well known that longitudinal straggling leads to a broader distribution with increasing primary energy of the implants, cf. e.g. refs [29,32]. For an evaluation of the local concentration of implants a detailed knowledge of the penetration (projected 0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

range) distribution in multielemental targets is necessary, such as given e.g. by the computer program MARLOWE from M.T. Robinson, ORNL [16,19]. The profiles of some light to medium heavy projectiles with kinetic energies ranging from 10 eV to 250 keV were calculated in frozen H,O (77 K) and in SiOz (298 K) as typical model substances. Similar combinations of projectiles and targets have been used in preceding studies on secondary knock-on particles in matter relevant for space science [20,33].

2. Computer program and parameters The computer code MARLOWE for cascades in the binary collision approximation was used in its version 11. Classical potentials were supplied by the default mode of the program: Thomas-Fermi-Moliere for the elastic, and Firsov with LSS parameters for the inelastic part. Only the primary projectiles were followed, the secondary cascade branches were truncated. H, He, C, N and 0 projectiles with primary energies ranging from 10 eV to 250 keV were studied. lo4 cascades were averaged for energies up to 100 eV, 1000-5000 cascades for energies up to 3000 eV, and 100 to 500 for energies up to 250 keV. The calculation was terminated when the energy of the projectile fell below 2 eV. The other energy parameters were derived from the next nearest neighbor distances in the lattice structure and amounted to those reported in [20]. Search radii for next collision partners were 0.258 nm and 0.213 nm in H,O-ice and SiO,, respectively. The model lattices were Hz0 (77 K) with a density of p = 0.87 g cmm3 with an antifluorite arrangement with a,, = 0.516 nm and SiO, (298 K) with p = 2.635 g cmp3 (quartz) with a fluorite lattice type

447

K. Roessler, G. Elch / EnergV dependence of concentratron of Implants

with aa = 0.533 nm. The lattices were supplied in the mode “POLY = T” which simulates a highly polycrystalline arrangement and prevents channelons. For the sake of simplicity and in order to avoid specific surface effects the program was operated as if it would have to calculate recoils from an in situ site with random distributions. The values for penetration (projected range from a virtual surface) allowed to use the data for ion implantation from a polycrystalline surface. The penetration profiles were plotted and the number of projectiles was determined within a f 1.18 u range (one half-width) around the distribution maximum. The concentration was calculated for the implantation of 1015 projectiles into a 1 cm-’ target in mole 5%on the base of the target molecules in that area. The values given in figs. 5 and 6 are mean concentrations near the distribution maximum and include (73 f 7)% of the projectiles. This individual procedure takes into account the deviation from classical Gauss curves. In the present calculation only the longitudinal distribution had to be considered, since for the implanted area of 1 cmp2 the transverse straggling could be neglected.

lOe5,

lOe2

lOe1

lOe3

primary

kinetic

lOe4

energy,

I

I

lOe5

lOe6

eV

Fig. 2. Dependence of mean penetration (projected range) of five projectiles in SiO, (298 K) on the primary kinetic energy as calculated by MARLOWE.

publication [34]. Figs. 3 and 4 show as an example the projected range profiles of 10 eV to 250 keV H in SiO, and of 3 X lo3 to 2.5 x lo5 eV C, N and 0 projectiles

3. Results The mean penetration for the set of five projectiles in H,O-ice and SiO, is displayed in figs. 1 and 2. The agreement with the values for SiO, from the code TRIM/PRAL by Biersack [32] is reasonable when taken into account the lower density of SiO, (p = 2.27 g cme3) used in the latter calculations. The digital values of all curves of this work are given in a separate

1Oe5,

1 ~ lOeO+ lOe1

lOe2

primary

lOe3

kinetic

I

/

I

lOe4

lOe5

lOe6

energy,

eV

Fig. 1. Dependence of mean penetration (projected range) of five projectiles in H,O-ice (77 K) on the primary kinetic energy as calculated by MARLOWE.

151

,x

I

0

3

lo3

6 lo3

9 10'

1

/

12 loL

15 10‘

ronge.S

Fig. 3. Dependence of penetration (projected range) distribution of Hf projectiles in SiO, on the primary kinetic energy as calculated by MARLOWE. VIII. DEVELOPING

TRENDS

K. Roes&r, G. Elch / Energy dependence ofconcentration of rmplanrs

448

olyyA,

~,~,

2

0

6

4

10

8

range,lO'

,

12

14

W

Fig. 4. Dependence of penetration (projected range) distribution of C+, N+ and O+ projectiles in H,O-ice at 77 K on the primary kinetic energy as calculated by MARLOWE.

in H,O-ice. The con~ntrati~ns of 1015 cm-’ H+ and He’ in H,O-ice and SiOa are displayed in fig. 5 as a function of primary energy; fig. 6 shows the corresponding for C”, N+ and O+. It can be seen, that the concentration decreases drastically with increasing

lOe3

-w +

9 a, z E

0e2

H/HZO-its He/W20-ice

+

H/SO?.

8

Ha/SiOZ

primary energy, e.g. from 69 mole % for 10 eV carbon to 0.16 mole % for 250 keV carbons. The concentrations of the light particles in fig. 5 show a steep decrease at lower energies, finally reaching a steady state at about lo4 eV primary energy. The concentrations of the heavier

lOe3 1

lOe-1 lOe1

10eZ

lOe3

10e4

lOe5

10e6

primary kinetic energy, eV

Fig. 5. Dependence of implant concentration (mole W)near the maximum of the distribution curve for Hf and He+ projectiles in H,O-ice (77 K) and SiOz (298 K) on the primary kinetic energy as calculated by MARLOWE.

lOe1

10e2

lOe3

lOe4

lOe5

lOe6

primary kinetic energy, eV

Fig. 6. Dependence of implant concentration (mole %) near the maximum of the distribution curve for C+, N’ and 0” projectiles in H,O-ice (77 K) and SiO, (298 K) on the primary kinetic energy as calculated by MARLOWE.

449

K. Roessler, G. Etch / Energy dependence of concentration of implants

ones in fig. 6 stay somewhat constant for lower energies, show the steep fall between 100 eV and 4 X lo4 eV and finally reach a steady state likewise. The reason for this behaviour is, among other collisional peculiarities of the interaction of atoms with different masses, in particular the different contributions of inelastic energy transfer in the different systems and for the variation of energies. Figs. 7 and 8 display the inelastic loss of the projectiles primary energy in percent of the total energy distributed. The light particles show a steep increase in the inelastic loss fraction already at lower energies than the heavier ones. It is the transition from preferential elastic loss modes to the inelastic ones which is responsible for the dynamics of the concentration in figs. 5 and 6. At medium energies relatively large direction changes due to the still intense elastic collisions are coupled with the increase in total range due to inelastic energy transfer. When inelastic processes prevail at higher primary energies, the trajectories become more aligned in the primary direction and the ratio of longitudinal straggling to projected range decreases slightly, cf. also ref. [30]. This could be the reason for the levelling off at higher energies. The distribution curves obtained by the calculations are somewhat simplified. In reality, the overlap of cascades will result in knock-on processes of the projectiles with already thermalized implants. For a dose of lOi ions cm-* in a homogeneous distribution, each particle is injected into a new 10 A* area of the surface, which may lead to few implant-implant collisions. Under the conditions assumed for the calculations reported above, no significant change of the concentration pattern may be expected. For higher doses, however, this

1Oel

lOe2

lOe3

primary

lOe4

energy,

lOe5

lOe6

eV

Fig. 8. Dependence of inelastic loss (electronic energy transfer) of five projectiles in SiO, (298 K) on the primary kinetic energy as calculated by MARLOWE.

effect, in particular with implants at lower ranges, will lead to an asymmetric form of the distribution curve and to an increase of the concentration near distribution maximum. Furthermore, it has to be emphasized that the distributions are those “as implanted”. No thermally activated diffusion, nor fast motions such as recombination with vacancies have been considered here.

4. The effect of sputtering

r 90 !? 2 80 al 0 70 ’

60

&

50

6 2 .u

40

t; 0 u .f

+---I *H

30

1 He

+C

20

+N

10

+O

0

1Oel

1Oe2

lOe3

primary

lOe5

1Oe4

energy,

lOe6

eV

Fig. 7. Dependence of inelastic loss (electronic energy transfer) of five projectiles in H,O-ice (77 K) on the primary kinetic energy as calculated by MARLOWE.

The concentration dependence in figs. 5 and 6 does not include the effect of a possible recession of the surface due to sputtering, such as considered e.g. for high fluence oxygen and nitrogen implantation into metals [35] or that of carbon and neon ions into frozen NH, [36]. For SiO, sufficient sputtering data exist in the literature [38] to evaluate their effect on the implant’s concentration. In a first simple approach it is assumed that the recession of the surface adds numerically to the half-width of the distribution curve. The increase of the half-width leads to a proportional decrease of the concentration in %. Table 1 shows the effect for SiO,. The sputtering data for H and He were taken from [39], those for C+ implants were extrapolated from [38,39]. The half-widths were those of the projected range distribution from the MARLOWE calculations. It can be seen, that the relative increase of the distribution is negligibly small for lOi cm-* H+ and He+ (< l%), but reaches values of several % for C+. This is in particular the case at medium energies where substantial sputtering occurs, and the projectiles show only a low penetraVIII. DEVELOPING

TRENDS

450

K. Roessler, G. Ewh / Energy dependence of concentration

ojrmplants

Table 1 Relative increase Ax of distribution half-width by the recession of surface due to sputtering of SiO, with lOI ions cm-* E

H/SiO,

He/SiO,

C/SiO,

(ev)

S ‘) (mols/ion)

Ax (%)

S b, (mols/ion)

Ax (%)

S (mols/ion)

Ax (%)

102 3x10* 10s 3x103 104 3x104 10s

0.0065 0.016 0.022 0.016 0.0045

0.12 0.11 0.05 0.02 0.003

0.006 0.06 0.15 0.14 0.07

0.16 0.67 0.59 0.21 0.05

0.05 0.3 0.5 1.0 2.0 2.0 2.5

4.1 9.1 4.8 3.8 2.4 1.4 1.1

a) From [39]. b, Extrapolated from [38,39].

Table 2 Relative increase Ax of distribution half-width by the recession of surface due to sputtering of H,O-ice (77 K) with lOI ions cm-* E

H/H,0

(77 K)

He/H,0

(77 K)

C/H,0

eV

S (mols/ion)

Ax (%)

S (mols/ion)

Ax (%)

3 x103 104 3 x104 lo5 2.5 x lo5

0.6 1.2 3 3 1

0.6 0.7 1.1 1.2 0.3

2.8 7 14 18

1.4 3.9 3.5 3.7

[43] [43] 140,431 1431 [41,431

[42] 140,421

[42] [42]

tion. The receding surface, however, will even in those cases not touch the halfwidth region of distribution. For the sputtering of H,O-ice much less data are known for the lower energy range. Table 2 lists the increase of distribution half-width in H,O-ice. For H+ and He+ the changes are relatively small in the order of a few %. However, they are remarkably higher for heavier implants. 250 keV C+ ions are diluted by 25% with respect to the values given in fig. 6. These evaluations show that for low and medium doses and lighter particles the concentration patterns of figs. 5 and 6 are basically correct. For heavier particles and for higher doses and, in particular, for matrices with binding energies even lower than those for H,O,

(77 K)

S (mols/ion)

10

Ax (%)

[40]

= 10 * [44,45]

4.2 = 25

corrections are necessary. The decrease of the curves will be somewhat steeper, the levelling off will occur at higher primary energies and at lower concentrations. Proper corrections can, however, only be made if the corresponding sputtering data are known.

5. Chemical implications of implant concentration The local concentration of the projectiles at the end of the cascade such as calculated in this work is certainly not the only parameter influencing the formation of molecules containing two or more of the implants. Diffusion processes, in particular upon thermal anneal-

Table 3 Some examples of multimer formation by implants Matrix

Implant

Dose (ions cm-*)

Product

Implant concentration (mole %)

Reference

KC1 (5 K)

250 keV iZC+

1.2-3 x lOi

c,, c;

1.5-4.0

[2,31

KC1 (298 K)

100 keV isc+ +14N+

2 -8x10”

CN-

0.8-2.0

141

7-247 eV WC+

0.5-5 x lOi

2 to 4 carbon containing molecules

20-200

17,461

451

K. Roessler, G. Eich / Energy dependence of concentratron of implants

ing, the interaction of intrinsic or radiation induced defects, the binding energy of the multimers, the stabilization on lattice and interstitial sites, the free space available and the compressibility of the matrix, and many parameters more are likewise important for the combination reactions. However, at low temperatures and for dimers or trimers, the concentration will be the most important parameter. Three examples are given: The formation of CT by 250 keV carbon ion implantation into alkali halides increases suddenly at fluences of 2-5 mCb cm-* [2,3]. Extrapolating from SiO, to KC1 (p = 1.98 g. cme3) and correcting for sputtering effects (cf. also ref. [34]) one might expect for the corresponding dose of 1.2-3 X 1Or6 ions cm-* a concentration of 1.5 to 4 mole 8. This would easily explain the formation of the observed C, (C;) dimers on site, via few diffusion steps or by a kind of ion beam mixing. Similar is true for the formation of CN- upon 100 keV C+ and N+ implantation into KC1 at ambient temperature [4]. The doses ranged from 2 to 8 X 10” ions cm-* which should result in a concentration of both implants of about 0.7 to 4 mole %. A very interesting case is that of 14C+ implantation into frozen Hz0 at 77 K with doses of 5 X 1Or4 to 5 X 1Or5 ions cm-* [7,46]. Since the energies ranges from 7 to 247 eV only, the concentration extrapolated from fig. 6 should amount to 20-200 mole %. This can easily explain the formation of CH,CHO, CH,COCH, and CH,O-CH,-CH,-OCH, species which were experimentally detected in relatively high yields. Table 3 gives an overview on the three systems discussed above.

6. Conclusions The MARLOWE calculations of range distribution profiles in two model systems revealed the effect of a drastic decrease of the local concentration of implants at medium energies depending in particular on the ratio of elastic to inelastic collisional loss processes. Corrections for sputtering effects at higher doses of heavier implants were quantitized. The energy dependence of the distribution and local concentration of implants is of importance for the formation of dimers and multimers, or larger molecules containing more than one of the implants.

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TRENDS

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[34] [35] 1361 [37] [38]

[39]

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