MÖSSBAUER AND CHANNELING STUDIES ON 119Te, 119Sb AND 119Sn IMPLANTS IN GROUP-IV ELEMENTS

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MÖSSBAUER AND CHANNELING STUDIES ON 119Te, 119Sb AND 119Sn IMPLANTS IN GROUP-IV

ELEMENTS

G. Weyer, B. Deutch, A. Nylandsted-Larsen, J. Andersen, H. Nielsen

To cite this version:

G. Weyer, B. Deutch, A. Nylandsted-Larsen, J. Andersen, H. Nielsen. MÖSSBAUER AND CHAN-

NELING STUDIES ON 119Te, 119Sb AND 119Sn IMPLANTS IN GROUP-IV ELEMENTS. Journal

de Physique Colloques, 1974, 35 (C6), pp.C6-297-C6-300. �10.1051/jphyscol:1974646�. �jpa-00215803�

JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 35, De'cembre 1974, page C6-297

M&SBAUER AND CHANNELING STUDIES ON "9Te, I19Sb AND '19Sn IMPLANTS IN GROUP-IV ELEMENTS

G. WEYER (*), B. I. DEUTCH, A . NYLANDSTED-LARSEN, J. U. ANDERSEN and H. L. NIELSEN

Institute of Physics, University of Aarhus, Denmark

R&urn&. - Le processus d'implantation d'ions Te, Sb et Sn implantts a l'aide d'un stparateur d'isotopes dans des monocristaux des elements du groupe IV (diamant, silicium et germanium), a BtB etudie par spectroscopie Mossbauer a l'aide de la transition de 24 keV de

9Sn ainsi que par des mesures complCmentaires de canalisation. 1lgrnSn (245 j), llgrnTe (4,7 j) et ll9Sb (38 h) ont Ctb implantes a des energies de 60 keV, des doses de 1013 a 1017 atomeslcm2 et des temperatures de 20 a 450 O C . Une grande proportion des isotopes sont implantts en substitution dans tous les cas. On

discute la nature et la position possible d'un second site, observe dans certaines expbriences.

Abstract. - The implantation behaviour of isotope-separator implanted Te, Sb, and Sn in single crystals of group-IV elements (diamond, silicon, and germanium) has been studied in Mossbauer experiments on the 24-keV transition of IlgSn and in complementing channeling experiments. Radio- active 1lgrnSn (245 d), 1lgrnTe (4.7 d), and ll9Sb (38 h) was implanted at energies of 60 keV, doses of 1013-1017 atoms/cmz, and temperatures of 20-450 OC. Large substitutional fractions are iden- tified for all implantations. The nature and possible position of a second site observed in some implantations is discussed.

1. Introduction. - Hyperfine investigations with impurity atoms implanted in solids by means of beam methods (such as nuclear-reaction recoil techniques or isotope-separator implantations) are often hampered by lack of information regarding the location of the implanted atoms and the influence of radiation damage created in the implantation process. It is generally assumed that for ion implantations at energies E 2 10 keV, the final lattice location of the implanted atoms (in theirmal equilibrium) is not dominantly determined by dynamical events during the slowing- down process but rather by interactions in the host lattice afterwards. This assumption is supported by the observation that implanted atoms of the same mass number but different atomic number end up in different lattice sites. The implantation behaviour depends also on implantation temperature and dose, radiation damage, and on the presence of other impurity atoms in the host lattice [I]. For nuclear-reaction recoil implantations, where the hyperfine interactions are usually measured within picoseconds to milliseconds after implantation, thermal equilibrium of the sur- rounding distorted lattice may not be reached before the measurements are performed. However, in the case of isotope-separator implantations, where the measure- ment takes place hours or days after the implantation,

it is safe to assume that thermal equilibrium has been established.

It was the purpose of the experiments reported here to study the implantation behaviour of Te, Sb, and Sn in group-IV elements with diamond structure. The influence of radiation damage, which has heen investi- gated previously for '19Sn in silicon [2], was reduced by implanting at high temperatures and/or low doses.

2. Experimental procedure. - Implantations of

1 lgmsn

, l19Sb, and '19"Te were carried out at an energy of 60 keV and implantation temperatures of 20-450 OC with an electromagnetic isotope separator.

Single crystal samples of n-type silicon and germanium and a diamond (type IIB) (*) were implanted. The implantation dose was determined from the ion current of the separation and from the 2-MeV He+ backscatter- ing spectra [I]. Radioactive '19"Sn (245 d) in metallic form (specific activity -- 100 mCi/g) was chosen as source material. The '19"Te (4.7 d) and its daughter l19Sb (38 h) were obtained from bombardment of natural tin with 20-MeV cr-particles from the cyclotron at the Niels Bohr Institute in Copenhagen. The li9"Te activity was separated chemically from the Sn-target.

The l19Sb activity was milked from its ' I g m ~ e parent.

In the Mossbauer kxperiments, a resonance detec-

(*) Institut fiir Atom- und Festkorperphysik, Freie Universitat (*) We are grateful to Professor F. Sellshop for loaning us this

Berlin, D-1000 Berlin 33. diamond.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974646

C6-298 G. WEYER, B. I. DEUTCH, A. NYLANDSTED-LARSEN, J. U. ANDERSEN AND H. L. NIELSEN

tor (see ref. 121 and ref. cited therein) was employed.

This counter detects conversion electrons emitted after resonance absorption of incoming 24-keV y rays in a SnO, layer (enriched to 89.8 % in l19Sn). Either the resonance detector or the sources were vibrated on an electromechanical drive system synchronized to a multichannel analyser operating in the multiscaling mode.

The Aarhus University 2-MeV Van de Graaff- accelerator facilities were used for the channeling expe- riments. Energy spectra of 2-MeV He' ions back- scattered from implanted samples were measured with a silicon surface-barrier detector. From energy spectra for random and aligned incidence of the incoming He' beam relative to all major axial directions of the host crystals, the lattice location of the impurity atoms was deduced [I].

3. Experimental results. - The results from the Mossbauer and channeling measurements are sum- marized in table I.

3.1 IMPLANTATION IN SILICON. - The implantation behaviour of 'l9Sn in silicon has been studied in detail previously 121. Three typical results for high- and low-dose hot implantations and for a low-dose cold implantation are included in table I. Negligible radiation damage was observed for low-dose ( 5 5 x 1014 atoms/cm2), hot implanta- tions where the quality of the host crystal (as measured by the channeling effect) is barely affected. Under such conditions, substitutional fractions can be unambiguously determined from channeling experi- ments. For the other two 'l9Sn implantations given in table I, the channeling effect in the host crystals was somewhat reduced due to radiation damage. However, from the Mossbauer data it can be concluded that the microscopic surroundings of the impurity atoms are unaffected 121. Therefore, one must conclude that l19Sn ends on substitutional lattice sites in silicon under all the implantation conditions used here. The

Impl.

host impurity - C 119Sn C 119Te Si 119Sn Si 119Sn Si 119Sn Si 119Sb Si 119Te Si 119Te Ge 119Sn Ge ll9Sn Ge 119Te Ge 119Te

Impl.

dose atoms/cm2

-

5 5 x 1013

5 5

1013 1 x 1014

- ^{1 x 1017} - ^{1 X 1013} - ^{5 x 1014}

5 x 1014 6 x 1014 1 x 1014 1 x 1014 1 x 1015 2 x 1015

Impl.

temp.

"C

A

20 20 400 400 20 400 450 400 450 400 400 400

same Mossbauer result is obtained for 'l9Sb implanta- tion in silicon. This is in accordance with the channeling experiments reported by Eriksson et al. [3] for similar implantations. In the latter experiments, a substitutio- nal fraction of roughly 0.9 was measured.

The implantations of '19"Te in silicon were also done at high temperatures and low doses in order to avoid radiation-damage effects. However, the line widths observed in the Mossbauer spectra were larger (- 20 %) than for similar '19Sn experiments. A typical spectrum is shown in figure 1. Another difference between the l19Te and the l19Sn and 'l9Sb implanta- tions is the appearence of a second line indicating that

-.

-

2500' l l 9 ~ e IMPLANTED IN St Sn O2 SCATTERER

" 9

-

-

VELOCITY (rnrnls)

FIG. 1.

1luSn Mossbauer spectrum, source

in silicon, scatterer 119SnO2.

Te occupies a second lattice site. This second line was observed for all implantations. However, the intensity of this line compared to that from the substitutional site varied between the two extreme examples given in table I. Additional experiments tested possible causes of this variation. The Mossbauer measurements on the 24-keV transition of l19Sn are performed several days after implantation of 19"Te. The '19"Te nuclei decay during this time through "9Sb to l19Sn. Hence the variation of the measured amount of Te in the second

Substitutional fraction (from channeling)

-

Mossbauer measurements (room temperature) substitutional site second site rel. fract. isomer shift (*) rel. fract. isomer shift (*)

-

- -

0.95 1.60(4) - ^{0.05 4.90(20)}

0.95 1.58(6) - ^{0.05 4.94(25)}

1 .OO 1.84(4)

1 .OO 1.86(6)

-

- )(**,

1 .OO 1.86(6) -

1 .o ^{1.9 (1)}

^-

0.8 1.88(6) 0.2 4.10(15)

0.95 1.90(10) - ^{0.05 4.2 (2)}

1 .OO 1.90(6) -

1 .OO 1.97(6) -

0.96 1.97(8) - ^{0.04 3.4 (2)}

0.86 2.0 (6) 0.14 3.75(15)

(*) In mm/s relative to SnOz.

(**) See also reference [2].

MOSSBAUER AND CHANNELING STUDIES ON 119Te, 119Sb AND lr9Sn IMPLANTS C6-299

site to that in the first (substitutional) site might be due to later diffusional motion, possibly of the anti- mony. This process seems to be excluded, however. A number of Mossbauer experiments were undertaken in which the implanted sample was cooled to 77 K directly after implantation and warmed up to room tempera- ture several days later. No change in the intensity ratio of the two lines was observed at either of these tempe- ratures. This implies also that the temperature depen- dence of the Debye-Waller-factors for both sites is the same in this temperature range.

Another cause of the line-ratio variation may be due to the chemical-preparation procedure used to separate the '19Te activity from the irradiated source material.

Different amounts of '19Sn and '19Sb may be implant- ed together with "9Te in the different runs. This might change the implantation behaviour of Te (as observed for mixed implantations of Ga and Sb and T1 and Sb [I, page 1701). Diffe~ent amounts of inactive Sn, Sb, and Te together with radioactive '19Te were implanted to test this hypothesis, but the results are uncorrelated.

Unfortunately, no channeling experiments could be carried out on the '19Te-implanted samples because the interpretation of the experiments would have been confused by the unknown amounts of '19Sn implanted at the same time. In channeling experiments on silicon samples implanted under similar conditions with inactive 130Te, a substitutional fraction of 0.6 (see table I) was obtained. Similar fractions of 0.5-0.6 have been reported by Gyulai et al. [4].

A i

s h ~ f t

(re1 to

Sn021

4 5 -

- -

u

c

4 0 -

- P

-

a

FIG. 2. - Isomer shift dependence on bond-lengths of host crystals for substitutional and non-substitutional lattice sites (the isomer shift value for a-tin is included for comparison).

A

ther with the isomer shifts for the second sites (upper

3.2 IMPLANTATIONS IN GERMANIUM. - The results line) from ""Te implantations.

from implantations of '19Sn and '19"Te in germanium A strikingly similar dependence of the isomer shifts are qualitatively similar to those for silicon.

and the correlated s-electron densities at the nucleus

occupies substitutional sites while again with

1grnTe, a has been reported for the 28-keV transition of lZ91 by second site is observed. Hafemeister and de Waard [6] from implantations of 1 2 9 ~ e in group-IV elements. In these Mossbauer expe- 3 . 3 IMPLANTATIONS IN DIAMOND. - The implanta-

tions of ''9Sn and l19"Te in diamond could be done at room temperature only, therefore very low doses were implanted. Despite this precaution, considerable line- broadenings were observed (- 2 times the width for '19Sn in silicon) so that for these implantations, radia- tion damage may effect the results. The Mossbauer spectra for '19Sn and '19"Te are remarkably similar. A fraction of - 0.05 ends in a second site in both implan- tations, while the major fraction ends substitutionally.

The two sites are identified by the systematics of isomer shifts.

4. Discussion. - From the combination of Moss- bauer and channeling experiments on silicon and ger- manium crystals implanted with '19Sn, the isomer shifts for the substitutional sites in all host crystals could be identified. Furthermore, a quantitative understanding of the observed isomer shifts and Debye-Waller factors seems attainable for the isoelectronic impurity '19Sn in group-IV elements [5]. The dependence of the isomer shifts for the substitutional sites on the bond-length of the host crystals is shown in figure 2 (lower line) toge-

riments, two lines were also observed but with approxi- mately equal intensity. The two lines can be assigned to analogous substitutional and secoizd sites in the diamond-type lattice. For both Mossbauer nuclei Ii9sn and ?291, the s-electron densities for the substitu- tional sites dect ease with decreasing bond-length of the host atoms, while a strong increase in s-electron den- sities is characteristic foi the second site. The Moss- bauer experiments on '19Sn and '291 indicate a highly symmetric surrounding for this second site because no line broadening is observed when compared to the substitutional sites. The Debye-Waller facto~s are the same as for the substitutional sites. The observed s-electron densities are extremely high in both cases.

Therefore, it was suggested by Hafemeistei and de Waard that the second site might be the tetrahedral interstitial.

However, this possibility is excluded as a conse- quence of the channeling experiments on Te in silicon.

The non-substitutional fraction could not be located from simple channeling experiments along major axes.

On the other hand, the observed very high s-electron

densities are possibly due to a compression of the

C6-300 G. WEYER, B. I. DEUTCH, A. NYLANDSTED-LARSEN, J. U. ANDERSEN AND H. L. NIELSEN

impurity atoms, indicating that the surrounding host Acknowledgments. - We are grateful to the staff at lattice might be considerably distorted so that the the Niels Bohr Institute cyclotron for the many irra- determination of the lattice location from channeling diations they kindly carried out for us.

experiments may be complicated. Also the second site One of the authors (G. W.) would like to thank the may not be a regular interstitial site but a defect staff of the Institute of Physics, University of Aarhus, structure originating from the radiation damage of the for stimulation and cooperation during his stay at the

implantation process. institute.

References

[I] MAYER, J. W., ERIKSSON, L. and DAVIES, J. A., Zon Zmplan- tation in Semiconductors (Acad. Press, New York) 1970.

[2] WEYER, G., ANDERSEN, J. U., DEUTCH, B. I., GOLOV- CHENKO, J. A. and NYLANDSTED-LARSEN, A., to be published in Rad. Eff.

[3] ERIKSSON, L., DAVIES, J. A., JOHANSSON, N. G. E. and MAYER, J. W;, Appl. Phys. 40 (1969) 842.

[4] GYULAI, J., MEYER, O., PASHLEY, R. D. and MAYER, J. W., Rad. Eff. 7 (1971) 17.

[5] WEYER, G., DEUTCH, B. I., NYLANDSTED-LARSEN and ANDER- SEN, J. U., Proc. Internat. Conf. on Hypefine Interac- tion Studied in Nuclear Reactions and Decay, Uppsala, 1974.

[6] HAFEMEISTER, D. W. and de WAARD, H., Phys. Rev. B 7

(1973) 3014.