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Submitted on 1 Jan 1976

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MÖSSBAUER STUDIES OF 129I ATOMS

IMPLANTED IN α- AND β-TIN

H. de Waard, G. Kemerink

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, DPcembre 1976, page C6-897

M~SSBAUER STUDIES OF' lZ9I

ATOMS IMPLANTED IN

a-

AND

fl-TIN

H. DE WAARD and G. J. KEMERINK

Laboratorium voor Algemene Natuurkunde, University of Groningen, The Netherlands

Rbumk. - Les deux raies du spectre Mossbauer de 1 2 9 1 implant6 dans le a-Sn ont un dkpla-

cement isomkrique qui se place bien sur La loi linkaire reliant le dkplacement isom6rique au para- m6tre du reseau cristallin et trouvk precedernment pour les autres semi-conducteurs du groupe IV.

Les raies correspondant A un deplacement isomerique positif sont attribuees B des impuretks en positions interstitielles tandis que les raies ayant des deplacements isomeriques nkgatifs seraient dues B des impuretks substitutionnelles. L'ensemble peut 6tre compris sur la base d'un modde simple. L'implantation de 1 2 9 1 dans B-Sn conduit B un spectre B 2 raies identiques A celui obtenu

partir de a-Sn transform6 en 8-Sn par chauffage. Des changements de phases successifs montrent que le positionnement des impuretks est reversible au cours de transformation de a

--

/3 -+ a ou 6 + a +D.

Abstract. - The two single line components of the Mossbauer spectrum of 1 2 9 1 implanted

in a-Sn have isomer shifts that form a linear continuation of the isomer shift vs lattice spacing behaviour earlier found for 1 2 9 1 impurities in the other group IV semiconductors.

The components with positive shift are interpreted as belonging to impurities in interstitial sites and those with negative 'shift to impurities in substitutional sites. All shifts can be understood on the basis of a simple model. Implants of 1 2 9 1 in /3 tin yield two line spectra identical to those

found for implants in a tin converted to /3 tin by heating. Repeated phase transitions show that the impurity location is reversible after an a + /3 + a or /3 + a + /3 change.

We have extended our earlier measurements [l] on 12'1 implanted in semiconductors to tin. As in diamond, silicon and germanium, we find two distinct single line components, both in a- and P-tin. These components have been interpreted before [l] to be due to atoms in two different lattice sites, the substitutional site and an interstitial site of not precisely defined character. For the case of a-Sn (diamond structure) the regular dependence of the isomer shifts of both spectral components on lattice spacing, previously found for 1291 in C, Si and Ge, is continued. This is shown in the figure.

The trend of the isomer shift with lattice spacing can be quantitatively understood. The positive shifts, corresponding to impurities in interstitial sites, are reproduced quite well by a simple model in which the isomer shift caused by overlap of the electron charge distributions of host and impurity atoms is calculated. The negative shifts, corresponding to impurities in substitutional sites, are thought to be the sum of two contributions of opposite sign : a positive shift due to overlap, which is taken equal to that found for the interstitial impurities and an even larger negative contribution caused by a reduction of the number of impurity valence S-electrons due to hybridization. This model also accounts for the shifts observed for '19Sn implanted in group IV semiconductors [2] and it removes a discrepancy in the work of Antoncik [3].

We have also investigated what happens to implanted ' 2 9 m ~ e impurities under a

z

p

phase conditions. Every observed spectrum was fitted with only two Lorentzian

NEAREST NEIGHBOUR SPACING IN

a

FIG. 1. - Isomer shifts for the Mossbauer lines of 1291 in dia- mond, Si, Ge and Sn vs. nearest-neighbour spacing. components of which positions (d), widths

(r)

and absorption depths (13 were free parameters in the fits. The values of the experimental parameters are compiled in the table. The main results are :

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C6-898 H. D E WAARD A N D G . J. KEMERINK

Isomer shifts ((6, and 6,) W. r. t. I - ion*, line widths (T1 and T2) and intensity ratios (A,/A2) for the substitutio-

nal (1) and interstitial (2) components of Mossbauer spectra of lZ9I in a- and P-tin. Source and absorber at 4.2K

Source 61

rl

62 r2

Phase Nr. implant Treatment

-

-

-

(mm/s)

-

-

A 1 /A2 a-Sn

1.

129m% a - ~ n

-

0.18 (2) 1.72 (10) 1.77 (4) 2.89 (12) 0.45 (3) 3. 1 2 9 6 P-sn l10 h,

-

0.25 (4) 1.49 (12) 1.96 (6) 2.59 (20) 0.57 (4)

-

40 OC 1.

129m6

cc-~n 70 h, 0.22(4) 2.01(8) 1.64(4) 2.13(8) 0.86(7) 110oc 2.

129mc

P - ~ n 70 h 0.29(6) 1.89(16) 1.73(6) 1.84(14) 0.94(7) 110 OC

* 6 = (- d

+

0.13) mm/s ; d is source line position W. r. t. Cu17-91 absorber.

+- (i) The a +

P

transition, induced in a lZ9"Te a-Sn

source by heating to 110 OC for 70 h, causes a change of the values of d,

r

and A =

zr

of the two compo- nents. All of these parameters have then become close to those found for a source of lZ9"Te implanted in P-Sn and equally heated to 110 OC for 70 h.

-+

(ii) If in a lz9Te P-Sn source a

P

-+ a-phase transi- tion is produced by cooling to

-

40 OC for 110 h, the values of d7 d , and A become close to those found for the

source originally implanted in a-Sn. From these two observations we conclude that any change of location of the Te impurities is reversible under an a-+P+a phase transition.

(iii) The a +

P

transition causes a positive change of isomer shift Ad, = 0.44(4) mm/s, corresponding to an increase of the 5s density for the substitutional lZ9I impurities. Such a change is plausible because the 5s shell occupation of

P

tin is higher than that of a tin [4] and the impurity will follow this trend. However, a quantitative theoretical estimate is hard to make. (iv) The widths and intensity ratios of the two components have changed considerably after a phase transition. We feel that this is due to our restriction to only two Lorentzian lines in the analysis. The large line width, especially for component 2 (interstitial)

probably implies the existence of more than one frac- tion of atoms, combined with a residual quadrupole splitting. In particular, association of substitutional impurities with vacancies would give rise to a spectral component with a different isomer shift. Removal of these vacancies by the 110 OC anneal would then cause an increase of the intensity of line 1, attributed to substitutional impurities.

The increase of the width of line 1 after the a +

phase transition may indicate the presence of some quadrupole interaction, to be expected for the tetra- gonal

P

phase but not for the tetrahedral a phase. From a fit of this line to a quadrupole split spectrum with minimum width components we can only give an upper limit of the interaction strength :

I

eZ qQ127/h

I

= 137 MHz.

(v) The a +

p

transition causes a small decrease,

A62 =

-

0.18(6) mm/s, in the isomer shift of the interstitial lZ9I impurities. Since the interstitial spaces in P-Sn are much smaller than in a-Sn, one would expect a large increase of 6, instead. Perhaps, vacancies are created around the Te-impurity during the a +

P

phase transition to accomodate the large atom and thus provide a space about as large as in a-Sn. It is also possible that the a + ,l? phase transition is hindered close to the impurity.

References

[l] HAFEMEISTER, D . W. and DE WAARD, H., Phys. Rev. B7

(1973) 3014.

[2] WEYER, G., NYLAND-LARSEN, A., DEUTCH, B. I., ANDER-

SEN, J. U. and ANTONCIK, E., Hyperfne Interactions 1

(1975) 93.

[3] ANTONCIK, E., Hyperjine Interactions 1 (1976) 329.

[4] FREEMAN, R. M., WATSON, R. E., HUDIS, J. and PERLMAN, M.

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