Mössbauer study of the amorphous layer in ion implanted diamond

Nuclear Instruments and Met.hods 182/183 (1981) 407-411 © North-Holland Publishing Company

407

MOSSBAUER STUDY OF THE AMORPHOUS LAYER IN ION IMPLANTED DIAMOND M. van ROSSUM, G. LANGOUCHE *, J. de BRUYN, M. de POTTER and R. COUSSEMENT Instituut voor Kern- en Stralingsfysika, Leuven University, 3030 Leuven, Belgium

Tlie structure of the amorphous layer in ion implanted diamond has been investigated by M6ssbaucr spectroscopy, using the nuclear probes 133Cs' 125Te and 1291. A strong resemblance is found in the spectra of ion implanted diamond and graphite, which leads to the conclusion that the C coordination in amorphous diamond is predominantly sp2-1ike. Debye-Waller factor measurements of 133Cs in diamond and graphite allow us to point out some characteristic differences between both lattice surroundings. It turns out that "amorphous" diamond is a more compact lattice than graphite in pyrolytic or polycrystaUine form.

1. Introduction Using diamond as a material for device fabrication requires the possibility of changing its electrical properties by implanting suitable impurities [1]. Unfortunately, the defect production associated with implantation is known to affect the structure of the implanted volume in a drastic way. With increasing ion dose, a disordered or so-caUed amorphous diamond zone is formed, which does not retain the same shortrange order as its crystalline counterpart. Indeed, several experiments have pointed out that the lattice of amorphous diamond exhibits many features of the graphite sp2-1ike coordination. For doses exceeding the amorphization threshold (roughly 1014 atoms/cm 2 for low energy heavy ions), a striking similarity was found between the Raman spectra of ion implanted diamond and graphite [2,3]. The amorphous layer is formed inside the bulk at the maximum of the damage distribution [4]. At very high doses however, this zone may eventually reach the diamond surface and blacken its colour. In a previous paper, we have shown how the amorphization process o f ion implanted diamond could be followed by M6ssbauer spectroscopy on inaplanted 133Cs [5]. In this paper, we want to describe how the same method can yield structural information on the disordered layer. For this purpose, we have used 133Cs as well as two other M6ssbauer nuclides, namely 12STe and 129I. Although the last two isotopes were akeady used for M6ssbauer spectros-

* Authorized researcher N.F.W.O.

copy on diamond [6,7], no attention was paid to the amorphization process at that time. In the present work, the same M6ssbauer probes have been implanted in diamond and graphite under similar conditions, in order to permit a structural comparison between both lattices.

2. Experimental Parent activities of 12STe and 129Te were prepared by the (n, 3') reaction at the BR-2 reactor of the SCK-Mol (Belgium). Xe gas enriched with l a3 Xe (the parent nucleus of 133Cs) was commercially purchased from IRE, Fleums (Belgium). The implantations were carried out at 300 K with the Leuven Isotope Separator at an accelerating voltage of 85 kV. The implanted dose on the various samples varied between 2 × 1014 and 2 × 10 is at/era2; the beam spot was swept homogeneously over the implantation area by vertical and horizontal electrostatic deflection plates. The diamond targets consisted o f natural diamond stones o f type IA obtained from Drukker and Zonen N.V., Antwerp. For the graphite implantations we used some commercially purchased pyrolytic and polycrystalline platelets (Le Carbone, Brussels). Unless otherwise stated, all implantations took place at room temperature. The samples were carefully degreased prior to implantations but they did not undergo any special etching procedure. M6ssbauer absorbers were CsCt for la3Cs, ZnTe for 12STe and CuI for 129I. The spectra were recorded with a conventional electromechanical transducer moving the source; both source and absorber were III. NON-EQUILIBRIUM PHASES

408

M. van Rossum et aL / Anwrphous layer in ion implanted diamond

kept at liquid helium temperature during standard measurements. In order to measure recoil-free fractions of 133Cs, the ~33Xe source was located in a separate vacuum chamber, which allowed raising the source temperature up to 110 K, while keeping the absorber in liquid helium. As radiation detector we used a NaI(TI) scintillator for the X33Cs and 129I gamma rays, and a Xe-filled proportional counter for the ~2STe source.

3. Experimental

results

3.1. Experiments with 133Cs

From the strong dose dependence of the spectrum of 133Cs in diamond we deduced in a previous paper that this probe was able to sense the amorphization of its surrounding [5]. Above the amorphization limit ('~2 × 1014 at./cm 2) the M6ssbauer spectrum is dominated by an unsplit resonance (a-line)with isomer shift 5 = ( - 0 . 2 0 +0.05) mm/s and linewidth 1"= (1.3-+ 0.1) mm/s. This spectrum bears a strong similarity with the one of ~33Cs implanted in graphite, consisting of a single line with 6 = (+0.03--0.05) mm/s and F = (1.0-+ 0.1) mm/s (fig. 1). In spite of their general resemblance, the two spectra still differ in some respects. First, the diamond spectrum also contains side wings next to the main peak; from a comparison with the earlier low-dose results

[5], we identify these with Cs atoms still in a crystalline diamond-like surrounding. Secondly, there exists a small, but well reproducible difference between the isomer shift of the resonance in a-diamond and graphite. The latter coincides with the value measured by Campbell et al. [8] for 133Cs in the intercalate compound CsC2a, where the Cs atoms are supposed to be in completely ionized Cs÷ state. On the other hand, the isomer shift of 133Cs in amorphized diamond corresponds to a formal occupation number n s = 1.24 for the 6s-shell, which could point towards an atomic Cs° state with some additional compression of the outer s-orbital by the surroundings. Finally, a large difference is found in the absorption depth of both spectra: 3.3% in a-diamond vs. 1.1% in graphite. In order to obtain a better quantitative picture of this discrepancy, we have performed 3"-factor (or Debye-Waller factor) measurements on 1SaCs in a-diamond and in pyrolytic and polycrystalline graphite. Relative areas of the resonance were measured at different source temperatures between 4.2 K and 1 10 K. The peak area is directly proportional to the f-factor at that temperature, and for low temperatures f ~ exp (_ ER

kO M

7r2ER

kO~ T2

)

'

T<<0M '

(1)

where ER is the recoil energy of the nucleus, T the temperature of the sample and 0M the "effective" Debye temperature of the M6ssbauer impurity in the lattice. It follows from eq. (1) that I n f should scale linearly against T 2. From the slope of this line, values for 0M of 13aCs in the different lattices were obtained and have been tabulated in table 1. Since the a-diamond spectrum contains contributions from the crystalline as well as from the amorphous region, care was taken to separate the latter components by a suitable fitting procedure. One notices the strong difference between 0 M of X33Cs in a-diamond and in both graphite species, which yield

Table 1 Effective Debye temperature of 133Cs in various host lattices. Host

0M

(K) VELOCI/Y(mm/s) Fig. 1. M6ssbauer spectrum of 133Cs implanted in (a) graphite, (b) diamond.

a-diamond Pyrolyt. graphite Polycryst. graphite

286 -+ 16 160 +-12 166 +-20

409

M. van Rossum et al. / Amorphous layer in ion implanted diamond

almost identical results. The M6ssbauer temperature of la3Cs in graphite is close to the value obtained by Campbell and Perlow of 133Cs in CsCs, where 0M = 145 K was found.

[ I

~-~

o

o .o.o

600 * c

"~'~'-. v~

3.2. l';xperiments with 12STe and 1291

.o,-oo

The suspected similarity between the short-range order of a-diamond and graphite is reinforced by the results of 12StaTe and 129mTe implantations. In pyrolytic graphite, the spectra of 12STe and 129I (figs. 2 and 3) show large splittings which we interpret as arising from a strong quadrupole interaction at the Te and I sites, respectively. Nearly identical splittings are found as the main component of the diamond spectra; however, a small distribution (-+10%) of quadrupole splitting values has to be introduced in order to obtain an adequate fitting

,Vf MX rx'/" v ~"

•

•

•-

-

GRAPHITE

•

.7"

curve.

In addition to the main component, the diamond spectra contain at least one more unsplit resonance (table 2). In a search for the origin of these lines, we have performed a hot implantation of t29mTe in diamond, keeping the target at 600°C. EPR and M6ssbauer studies have shown that under these circumstances, the formation of the amorphous layer is strongly inhibited, even for doses exceeding 1014 at/era 2 [9,5]. lndeed the spectrum resulting from this hot

Z 0

e e

~- e~'~.~% Z <

-

•

eee•~ .

• •

~,

.

DIAMOND

~

•

•

rr I--

•

.,'..

.

-10

-5

0

5

10

VELOCITYImm/s) Fig. 3. M6ssbauer spectrum of 1291implanted in (a) graphite, (b) diamond, (c) diamond kept at 600°C during implantation.

implant (fig. 3c) is very different from the previous one. It essentially displays a strong single line with a small sidepeak, which could be interpreted either as an unsplit resonance or as part of a small residual quadrupole interaction. The main peak could be representative of I atoms situated in the crystalline diamond lattice. Moreover, it turns out that the specTable 2 Fitting parameters for the spectra of 125Te and 1291 in diam o n d and graphite. Isomer shift (6), quadrupole splitting (~) and linewidth ( r ) o f the different c o m p o n e n t s are given.

125Te

diamond

129I

diamond

graphite

-1;

-10

-;

(I

;

10

1'5

VELOC1T Y (mm/s) graphite Fig. 2. M6ssbauer spectrum o f 12STe implanted in (a) graphite, Co) diamond.

a

A

r

(mm/s)

(mm/s)

(mm/s)

- 0 . 3 2 -+ 0.05 +0.04 ± 0.4 - 0 . 2 0 ± 0.05 --0.87 ± 0.1 +0.74 ± 0.2 - 0 . 6 4 ± 0.05

8.97 ± 0.1 9.14 ± 0.1 13.4

~: 0.2 a)

13.3

-+ 0.2

5.9 ± 0.5 5.9 ± 0.5 6.8 ± 0.2 1.7 ± 0.1 1.7 ± 0.1 2.0 -* 0.2

a) A distribution o f ± 10% was included in the fit. III. NON-EQUILIBRIUM PHASES

410

M. van Rossum et al. / Amorphous layer in ion implanted diamond

trum of the room temperature implant (fig. 3b) can be fitted quite satisfactorily by adding the single line of fig. 3c to the smeared-out quadrupole split component. This component, which we identify with l atoms in the a-diamond layer, accounts for about 85% of the total spectral intensity. Although no hot implant of 12StaTe in diamond has been performed up to now, we suppose that a similar type of analysis will also be valid for the spectrum of fig. 2b.

4. Discussion

The experiments presented in this paper were intended to compare the structure of ion implanted diamond and graphite. It is seen that, above the diamond amorphization limit, both spectra are essentially similar, whatever the M6ssbauer probe element used (Te, I or Cs). We therefore think that the lattice surrounding around the M6ssbauer impurities is essentially the same, and thus consists of threefold coordinated C atoms in sp z configuration. Our observation of graphite bonds in amorphized diamond is in agreement with similar conclusions drawn from Raman scattering measurements [2,3]. The question may be raised whether this graphitic zone is only present around the landing place of the implanted atom, or whether it is characteristic for the bulk of the implanted volume. Our earlier measurements showing the growth of the graphitic a-line in the diamond spectra as a function of ion dose favors the second hypothesis, since in the first case the presence of a graphitic environment around the ion should be essentially dose independent. However, all diamond spectra reveal the presence of a small (<20%) non-graphitic component probably due to M6ssbauer atoms in a crystalline diamond-type (sp 3) lattice environment. In view of the just described considerations, the most plausible explanation would be to attribute this component to probe atoms located in the tails of the impurity distribution, where the defect density is too low to induce graphitization. This implies that, above 2X 1014 at/cm 2, the bulk of the implanted volume would be homogeneously sp2-like. The discrepancy between the characteristic M6ssbauer temperature in a-diamond and graphite is a puzzling result. This discrepancy was particularly evident from the f-factor measurements of ~33Cs, but it

is noticeable in the spectra of XZSTe and 1291 as well. However, by combining this information with the other hyperfine parameters, the following picture may be proposed. The isomer shift values of the 133Cs probe atoms consistently point towards a higher electronic density in a-diamond than in graphite. Again, this effect is especially pronounced in the case of 13aCs, which has the largest atomic radius. It can therefore be supposed that the space available for the impurities is smaller in a-diamond than in graphite. In this host, the Cs atoms are intercalated between the C layers, as they are in CsCs and CSC24 [10]. The bonding between the layers is of the weak Van der Waals type, whereas the C atoms are strongly covalently bound in the planes. It is also known that the interlayer distance is not rigidly constant, but that it may be affected in various ways by defects or intercalated impurities [11]. When intercalating Cs atoms in pyrolytic graphite, the interlayer distance is known to increase from 3.35 to 5.95 A in CSC24 [12]. Our experiments tend to suggest that, in the highly disordered diamond region, the distance between the sp 2 coordinated C networks is on the average smaller than in pyrolytic or polycrystalline graphite. The squeezing of the implanted impurities which results from this situation should increase the electronic density at the nucleus and simultaneously reduce their mean vibrational amplitude, hereby explaining the larger isomer shifts and the higher f-values. Finally, we would like to add a few comments on the quadrupole interaction which is present in the taSTe and 129I spectra, but not in the 1SaCs results. In the intercalation compounds CsCa and CsC24 a quadrupole splitting has been observed. However, lattice sum calculations have shown that this should most probably arise from Cs+ ions distributed in the C lattice on a regular geometrical pattern [8]; quite evidently, this situation is not to be expected in an implanted source and therefore no field gradient of lattice origin should be detected here. On the other hand, Te and I atoms possess unfilled p-shells; when polarized by non-cubic surroundings, these valence p-electrons may create large field gradients at the Te or I nucleus. A more quantitative analysis of the observed quadrupole interaction would certainly yield useful information on the lattice site occupied by both impurities, but this analysis lies beyond the scope of the present work.

M. van Rossum etal./Amorphous layer in ion implanted diamond

411

5. Conclusion

References

In this work, we have tried to develop a novel approach for the study of the amorphous layer in ion implanted diamond, by using implanted M6ssbauer atoms as microscopic probes for the characterization of their surroundings. A strong resemblance was found between the M6ssbauer spectra o f 1~3Cs, 12STe and 1291 in ion implanted diamond and graphite, which led to the conclusion that these impurities are predominantly situated in an environment of sp2-coor dinated C atoms. However, the observed discrepancies between the isomer shifts and Debye-Waller factors in both lattices seem to indicate that the lattice is m o r e closely packed in graphitized diamond than in graphite. We expect that a systematic application of this method will provide a better understanding of the complex phenomena connected with ion implantation in diamond.

[1] V.S. Vavilov, Rad. Eft. 37 (1978) 229. [2] J.F. Morhange, R. Beserman and J.C. Bourgoin, Jap. J. Appl. Phys. 14 (1975) 544. [3] M.H. Brodsky and M. Cardona, J. Non-Cryst. Sol. 31 (1978) 81. [4] A. Talmi, R. Beserman, G. Braunstein, R. Bernstein and R. Kalish, Defects and radiation effects in semiconductors 1978 (Int. Phys. Conf. Scr. 46, ed. J.H. Albany) p. 399. [5] M. van Rossum, J. de Bruyn, G. Langouche, M. de Potter and R. Cousement, Phys. Lett. 73A (1979) 127. [6] D.W. Hafemeister and H. de Waard, Phys. Rev. B7 (1973) 3014. [7] M. van Rossum, J. de Bruyn, G. Langouche, R. Coussement and P. Boolchand, J. Physique 37 (1976) C6-889. [8] L.E. Campbell, G.L. Montet and G.J. Perlow, Phys. Rev. B15 (1977) 3318. [9] Y.M. Lee, P.R. Brosious and J.W. CoIbett, Phys. Stat. Sol. 50 (1978) 237. [10] I. D6zsi et al., to be published. [11] V.A. Nicolaenko and P.A. Platonov, Rad. Effects 45 (1980) 185. [12] W. Rudorff and E. Schulze, Z. Anorg. Chem. 277 (1954) 156.

The authors wish to thank 1t. Pattyn and R. Vanautgaerden for the ion implantations. They also thank the S.C.K,-K.U.L. association for the neutron irradiations and the I.I.K.W. for f'mancial support.

III. NON-EQUILIBRIUM PHASES