CONVERSION ELECTRON MÖSSBAUER SPECTROSCOPY OF 57Fe IMPLANTS IN Si AND Ge

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CONVERSION ELECTRON MÖSSBAUER

SPECTROSCOPY OF 57Fe IMPLANTS IN Si AND Ge

B. Sawicka, J. Sawicki, J. Stanek

To cite this version:

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RADIA

TION

DAMAGE AND DEFECT STRUCTURE.

CONVERSION ELECTRON MOSSBAUER

SPECTROSCOPY

OF s7Fe IMPLANTS IN

Si AND Ge

B. D. SAWICKA

Institute of Nuclear Physics, Cracow, Poland and

J. SAWICKI, J. STANEK

Institute of Physics, Jagiellonian University, Cracow, Poland

Rksumk. - On a ktudik du Si et Ge implant& dans du 57Fe avec une energie de 70 keV et des doses de 5 X 1014 h 1016/cm2. Tous les spectres obtenus reprksentent des doublets symktriques,

que I'on interprkte comtne dus h une interaction quadrupolaire. Le dkdoublement quadrupolaire depend de I'augmentation de la concentration en fer en fonction de la dkcroissance de la dose de fer.

Abstract. - The study of Si and Ge implanted with 57Fe at the energy 70 keV and doses from

5 X 1014 to 1016/cm2 was performed. All spectra measured represent symmetric doublets interpreted as caused by a quadrupole interaction. The quadrupole splitting depends on iron concentration increasing with the lowering of iron dose.

1. Introduction. - Ion implantation has proved highly successful for fabrication semiconductor devices [l], but the local state of an implant in a lattice is still not clear. The possibilities of Mijssbauer spectroscopy in this area have not yet been exploited. For instance, the nature of such deep donors as iron and cobalt, and possible multiple charge forma- tion 121 could be studied. Unfortunately, the Moss- bauer data on Fe or CO impurities in silicon and germanium are inconsistent and inconclusive. As both elements are hardly soluble and easily precipitate in Si and Ge hosts, it is very difficult to prepare good samples. The only practical way for getting solid solutions of Fe or CO in Si and Ge is via the ion implantation.

The present work is a continuation of our earlier study of the microscopic nature of Fe implants in silicon [3, 41. In the case of close packed metallic hosts the situation of Fe implants, e. g. of Fe in d-metals [5] or aluminium [6], seems quite clear. In the case of silicon and germanium, which have loosely packed lattices of a diamond type, the final location and the electronic state of iron implants

is not easy to describe.

2. Experiment and results. - The samples were prepared by implanting stable isotope 57Fe with a mass separator, according to the method presented elsewhere [3, 61. The energies of Fe' ions varied

from 10 to 70 keV, the doses from 5 X 1014 to 1016

ions s7Fe/cm2. Monocrystalline

<

111

>

cut plates of Si and Ge were used as targets. All implantations were performed at room temperature. The samples were not thermally annealed.

In figure 1 the parameters describing the implantation profile, i. e. the projected range R, and range straggling AR,, calculated with LSS theory for Si and Ge targets are presented. The average atomic concentration of implants, defined as

-

X = dose/4. AR,. N,

(where N, is a number of host atoms per cc), is plotted

for the dose 1016/cm2.

In all samples studied the iron concentration was higher than the solubility limit of Fe in both Si and Ge, which equals about 1 ppm at 1300oC. The concentration i? varied from 0.1 to 2.6

%

for Si samples, and from 0.2 to 3.2

%

for Ge ones.

The Mossbauer spectra were measured by means of the conversion electron counting method described elsewhere [7]. The 50 mCi 57Co : Pd source was used. The velocity scale is given with respect to a source in a metallic iron. The results presented were obtained at room temperature.

The spectra are very similar for all the samples, they represent weil-separated symmetric doublets (Figs. 2 and 3). The Lorentzian line fit parameters are compiled in the tabe 11.

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C6-880 B. D. SAWICKA, J. SAWICKI AND J. STANEK

The distance between the lines strongly depends on the concentration of iron impurities, as indicated

[HI in figure 4. In this figure the point for

Z

= 17

%,

measured for

57Fe

: Si in transmission experiment [3]

- 600 - "Fe S, 70 LoV

...

1 0 0 5 - E,LkeVl (a)

-

300 -10

-

200 E, CkeVl

FIG. 1. - Projected range RP, range straggling ARp, and ave- VELOCITY crnm/.l

rage concentration 2 of F e atoms in Si and Ge versus ion

energy. FIG. 2. - Mossbauer spectra of 57Fe in Si.

The Lorentzian line fit parameters : Q S = quadvupole splitting, IS = isomer shift with respect to source in a

metallic iron,

r

= line widtlz, S , / S , = intensity ratio. D, E, Jc = dose, energy, and average atomic concentration

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CONVERSION ELECTRON MOSSBAUER SPECTROSCOPY OF 57Fe IMPLANTS IN Si AND Ge _C6-881

VELOCITY [rnrn/rl

FIG. 3.

-

Mossbauer spectra of 57Fe in Ge.

FIG. 4. - The quadrupole splitting versus concentration of Fe implants in Si and Ge.

-

.

E U "3 0 0 09 08 0 7 - 06-

has also been included. The distances between two lines in the spectra measured for 57Co implanted samples by Weyer et al. [8] (dose 1014 57Co/cm2 at 60 keV) are represented by open circles.

r

J

f

"Fe Ge - -

6

f I

3. Discussion.

-

We believe that the present data confirm the earlier prediction [3, 41 that the doublet observed for 57Fe : Si and 57Fe : Ge absorbers is of quadrupole origin. The concept that the two lines are caused by iron occupying two different lattice sites, e. g. interstitial and substitutional ones 181, cannot be accepted in view of following arguments :

0 1 t 10

P [%I

1) The intensities of both lines are equal within the limits of error. There is no obvious reason that two different lattice sites have the same recoilless factor and are equally occupied in a wide range of impurity concentrations.

2) The centroid position of the spectra is nearly constant, while the positions of the separate lines depend on the concentration. The values of the isomer shifts for iron implanted Si and Ge (IS = 0.15 mm/s and IS = 0.30 mm/s, respectively) follow the tendency of the dependence of the electric density at Fe impurity nuclei on the number of outer electrons in the host elements. Both IS values lie in a region of those for iron-silicon and iron-germanium compounds. If two single lines are assumed the shifts are unusual.

3) The widths of both lines are equal and inde- pendent on dose. The average line-width of about 0.5 mm/s, reduced to 0.4 mm/s after sample anneal- ing [4], shows the homogeneity of iron local states. The indications shown above enable us to conclude that Fe in Si and Ge lattices is influenced by an electric field gradient, unique for all iron atoms. The origin of this efg and the reason of its dependence on iron concentration is not clear.

There is no electric field gradient on substitutional and interstitial sites in a perfect diamond lattice. However, in a disturbed diamond host, various effects which could cause an efg at the Fe nuclei are possible. For instance, according to De Barros et al. [g], the equilibrium position of an 57Fe implant in diamond is shifted along one of C 11 1 > axes from the center of the tetrahedral interstitial site. Another type of defect can be the impurity-vacancy pair, which in semiconductors can be formed and be stable at relatively high temperatures. In both cases a large axial field gradient can be expected.

In order to explain the nature of the iron deep donor states in silicon and germanium further exper- iments are necessary. In particular, measurements in high magnetic fields and at low temperatures will be helpful.

Conversion electron counting method at low temperatures was developed [l01 and measurements for Fe : Si and Fe : Ge are started now,

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B. D . SAWICKA, J. SAWICKI AND 5. STANEK

References

[l] DEARNALEY, G., Nature 256 (1975) 701. [7] STANEK, J., SAWICKI, J., SAWICKA, B. D., Nucl. Instrum.

[2] -DANE, F. D. M. and ANDERSON, P. W., Phys. Rev. Meth. 130 (1975) 613.

13 (1976) 2553.

SAWICKI, J., SAWICKA, B. D., LAZARSKI, S. and MAY- DELL-ONDRUSZ, E., Phys. Stat. Sol. (b) 57 (1973) K143. SAWICKI, J., SAWICKA, B. D., STANEK, J., KOWALSKI, J.,

Phys. Stat. Sol. (b) 77 (1976).

SAWICKA, B. D., SAWICKI, J., STANEK, J., Phys. Lett. 59 A (1976) 59.

SAWICKA, B. D., SAWICKI, J., STANEK, J., DRWIEGA, M., Report INP No 935/PS Cracow (1976).

[8] WEYER, G., DEUTCH, B. I., NYLANDSTED-LARSEN, A., HOLCK, I., Proc. Int. Conf. on Mossbauer Spectroscopy Cracow 1975, Vol. 1, p. 213.

[9] DE BARROS, F., ~ F E M E I S T E R , D., VICARO, J. P., J. Chem. Phys. 52 (1970) 2865.