ISOMER-SHIFT SYSTEMATICS OF 57Co IMPLANTED IN GROUP IV SEMICONDUCTORS

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ISOMER-SHIFT SYSTEMATICS OF 57Co

IMPLANTED IN GROUP IV SEMICONDUCTORS

G. Weyer, G. Grebe, A. Kettschau, B. Deutch, A. Nylandsted Larsen, O.

Holck

To cite this version:

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ISOMER-SHIFT SYSTEMATICS OF

CO

IMPLANTED IN GROUP IV SEMICONDUCTORS

G. WEYER

(i),

G. GREBE, and A. KETTSCHAU

Institut fiir Atom- und Festkorperphysik, Freie Universitat Berlin, Germany B. I. DEUTCH and A. NYLANDSTED LARSEN

Institute of Physics, University of Aarhus, Denmark

0. HOLCK

The Niels Bohr Institute, Copenhagen, Denmark

RbumB. - On a implante du 57Co radioactif dans du diamant, silicium, germanium et etain cc

au moyen d'un separateur d'isotopes. Les spectres Mossbauer de haute resolution des atomes d'impurete, obtenus par une technique de comptage par resonance, montre une structure complexe

B raies multiples. Cette structure se decompose en deux groupes de raies. Differentes d6pendances presque lineaires des dkplacements isomkriques de ces groupes sur la distance du plus proche voisin des cristaux implant& sont observkes. A partir des valeurs de densit6 6lectronique et B partir des similarites d'autres atomes Mossbauer implantks dans des Blkments du groupe IV, on suggkre que les deux groupes de raies proviennent des impuretks situkes sur des sites (reguliers ou irreguliers) substitutionels et interstitiels, respectivement.

Abstract.

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Radioactive 57C0 has been implanted in diamond, silicon, germanium, and R-tin by means of an isotope separator. High-resolution Mossbauer spectra of the impurity atoms obtained by resonance-counting technique show a complicated multiline structure. This structure decomposes into two groups of lines. Different nearly linear dependences of the isomer shifts of these groups on the nearest-neighbour distance of the host crystals are observed. From the inferred electron-density values and from similarities to other Mossbauer atoms implanted in group IV

elements, it is suggested that the two groups of lines originate from impurity atoms on regular or disturbed substitutional and interstitial sites, respectively.

1. Introduction.

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Ion implantation of Mossbauer isotopes has been demonstrated t o be a powerful method for studies of impurity atoms in regular and defect sites in solids. This implantation method allows systematic studies of impurity atoms in families of materials unrestricted by limits from diffusion cons- tants or solubility, e. g., diffusion of impurity atoms is hardly possible in diamond or a-tin. The drawback of the method is that, due to the radiation damage intro- duced by the implantation process, complicated damage structures can be produced, the nature of which is largely unknown.

Several attempts to determine the lattice location and electronic structure of implanted iron or cobalt atoms in group IV elements by means of Mossbauer spectroscopy have been reported. Coulomb-excitation- recoil implantation of 57Fe in diamond, silicon, and germanium was investigated by the Stanford group

[l, 21. Isotope-separator-implantation technique was applied to implant 57Co [3, 41 and 57Fe [5]. In all

(T) Present address : Institute of Physics, University of Aarhus.

spectra, two broad groups of lines were observed, the interpretation of which remained doubtful [4].

Here, a partial report is given on systematic inves- tigations of isotope-separator-implanted 57Co in group IV elements with diamond structure, including for the first time a-tin. Compared to earlier experiments, better experimental resolution and statistical accuracy have been obtained.

2. Experiments.

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The Niels Bohr Institute isotope separator has been used to implant radioactive 57Co at energies of 50 and 60 keV. Diamond, silicon, and germanium single crystals and polycrystalline a-tin were implanted a t room temperature. Throughout all cobalt separations, a contaminating iron beam was observed. Thus, simultaneously with radioactive 57Co, stable 57Fe has been implanted in all samples. The 57Fe dose was determined from the measured ion current of the 59Co carrier and its ratio measured to 56Fe, using the natural isotopic-abundance ratio for 5 6 ~ e and 57Fe. As the implanted iron doses are small (the iron concentration is

5

10m3 in the implanted

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C6-894 G. WEYER et al.

volume), no direct interaction of the iron contamina- tion with implanted 57Co atoms is expected. The implanted 57Co doses, as determined from the radio- activity, are about two orders of magnitude smaller than those for 57Fe.

Mossbauer spectra of implanted samples were measured at room temperature, using resonance counters of the parallel-plate avalanche-counter type

[6]. Layers of 57Fe enriched K,Fe(CN), . 3 H,O and a stainless-steel foil were used as absorber materials. The measuring times were of the order of days for reasonable statistical accuracy of spectra obtained from sources with activities of some pCi.

resonance counter). Two groups of lines can be well separated in the spectra of diamond, silicon, and germanium. The relatively large widths of these groups suggest that each group may contain several unresol- ved lines. The spectra as shown in figure 1 are fitted with four Lorentzian lines, which gives satisfactory results within the statistical accuracy of the data ;

however, the individual lines are still broadened and may consist of more than one line. Figure 2 demonstrates that a fit with only two Lorentzian

3. Experimental Results and Discussion.

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Figure 1

shows for comparison typical Mossbauer spectra of

FIG. 2.

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Comparison of a two- and four-lines fit for 57C0 in germanium.

FIG. 1. - Mossbauer spectra of 57C0 implanted at room-temperature in diamond, silicium, germanium and a-tin. All spectra are fitted with four lines. The velocities are given relative to a

K4Fe(CN)6. 3 H20 absorber for all spectra ; the diamond and a-tin spectra have been measured with a stainless-steel resonance counter. Positive velocity for approach of source and

absorber.

57Co in the four group IV elements with diamond structure. The experimental resolutions as measured for a reference source 57CoRh were FWHM = 0.27 mm/s for the silicon and germanium spectra (K,Fe

(CN), . 3 H,O resonance counter) and FWHM = 0.34 mm/s for the diamond and a-tin spectra (stainless-steel

lines is insufficient. For the case of "CO in silicon, in a detailed study on the implantation temperature and dose dependence of the spectra, more than four individual lines have been revealed. Several of these lines are in the isomershift region of the two groups of lines in the silicon spectrum of figure 1. The population of these lines is strongly temperature dependent, but nearly independent of dose. The result of these studies will be given in a forthcoming paper. A strong tempe-

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Results from 2 and 4 lines Jits of the spectra

1

Group A

I

IS

1

width

I

ampl.

I

IS

1

width

1

ampl.

Group B

IS

1

width

1

ampl,

I

IS

l

width

l

ampl.

The values of the IS and line widths are given in mmls. Si and Ge data are measured with PFC absorber, diamond and a-tin with SS absorber, the IS is related to PFC. Amplitudes are given in arbitrary units ; the maximum-line amplitude is normalized to one. Implanted 57Fe doses z 1 0 1 4 atoms/cmz for diamond, germanium and a-tin ; 1 0 1 5 atorns/cm2 for silicon.

structure of the electronic properties, while individual lines (or their distributions) originate from different individual surroundings (or distribution of surroundings) of the impurity atoms. A variety of surroundings can be expected to occur from the defects loca- lized in the damage cascades produced by the implantation. These defects are likely to form more complicated structures than simple point defects. Therefore, the individual lines may be due to slightly different defect structures in the neighbourhood of impurity atoms, which cause slightly different isomer shifts.

The results from four-line fits to the experimental data are listed in Table I. The mean values for the isomer shifts of groups A and B are in agreement with those measured by Latshaw [2] for 57Fe implanted in diamond, silicon, and germanium by the Coulomb- excitation-recoil implantation method. Although in the measurements on isotope-separator-implanted 57Co the relevant electronic structure is that for iron, the local environment determining the hyperfine structure is characteristic of the implantation behaviour and interactions of the implanted cobalt atoms. Thus the implantation properties of iron and cobalt can be con- cluted to be roughly similar, and no influence of cr after- effects>> from the EC decay of 57Co is indicated. Figure 3 displays the dependence of the isomer

shifts of groups A and B on the nearest-neighbour distance of the host crystals. Latshaw's data [2] are included for comparison. Obviously there exists a nearly linear dependence between the electron densities at the nucleus for both groups. It emerges from the plot in addition to the large differences in electron densities the slopes of the straight lines are a feature

Isomer s h ~ f t lmrnlsl nearest ne~ghbour d~stonce ( A ) I I 15 2 0 2 5 30 d~arnond 51 Ge a- Sn

FIG. 3. - Plot of the mean values of the isomer shifts for the two groups of lines, A and B, versus nearest-neighbour distance of the host crystals. Crosses are from different samples (Si, Ge) or different measurements on the same sample (cc-tin). Filled points are from a silicon sample deviating in the position of line A. Open circles are data from Ref. [2] for comparison. IS

relative to K4Fe(CN)6.3 Hz0.

distinguishing the two groups.

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C6-896 G. WEYER et al.

tional as well as defect sites of unknown geometrical configurations. The magnitude and dependence of the electron density at the nucleus on substitutional sites could be explained semiquantitatively on the basis of a tight-binding approximation for the band structure of the host crystals [10, 111. As pointed out by Antoncik [Ill, it is a characteristic feature of these substitutional impurity atoms to redistribute their

S- and p-valence electrons according to the dehybridization of the covalent bonding of the host crystals when going from diamond to a-tin. This results in an increase in S-electron density at the nucleus with increas- ing nearest-neighbour distance due to an increase in the s character of the bond from diamond to a-tin. Although this model neglects the possibility of loca- lized states representing quite different electronic structures, it may be adequate for the gross electronic structure of the majority of impurity atoms in slightly disturbed surroundings where the interaction with nearest-neighbour atoms is the most important one. For iron impurity atoms no p-valence electrons are available ; however, a similar effect may be expected for the redistribution of the 4s and 3d electrons involved in the covalent bonding. It is tempting to assume that also here, the effect of the dehybridization on the s electrons at least should not be overwhelmed by the changes in the number of d electrons.

On the other hand, a steep rise in electron density with decreasing lattice constant has been found to be characteristic for tellurium atoms in interstitial defect sites of high electron density [7-91. This is attributed to a compression of the large interstitial impurity atoms by the host lattice. Less compression might be expected for interstitial iron atoms. However, the extremely high electron density measured for group B in diamond,

which is above the value for atomic iron in rare-gas- matrix isolation

(-

-

0.72 mm/s [12]), hardly allows for interpretations other than that it is due to compression. The isomer-shift value reached for group B in a-tin is approximately that for metallic iron, 3d74s1. The radii of iron in atomic and metallic configurations

(- 1.3

A)

are (except for a-tin) larger than the nearest- neighbour distances in tetrahedal interstitial sites where the distance is equal to that for a substitutional site. For a hexagonal interstitial site even less space is available.

As a result of these qualitative considerations, the two groups of lines, A and B, can be tentatively associated with (regular or disturbed) substitutional and interstitial sites, respectively. The isomer shift for the substitutional sites, which is about equal for all host materials (- 1 mm/s), is somewhat higher than what has been recorded for covalent iron compounds ; however, this does not exclude the above interpretation. I t should be mentioned that an isomer shift of 0.54 mm/s has been assigned to the substitutional line from measurements on diffused 57Co in silicon [13]. This value is lower than the mean value from the implanti- tions. However, for a single sample (indicated by the filled points in figure 3), this line has also been observed. The difference might be due to defects associated with substitutional impurities. An analogous effect has been found for substitutional tellurium in silicon [g]. The slight difference in isomer shift observed for interstitial impurities for implanted (- 0.24 mm/s) or diffused

(-

0.01 mm/s [13]) "CO in silicon may also be due to radiation damage.

This work has been supported by the Deutsche Forschungsgemeinschaft, SFB 161, and by the Danish State Research Foundation.

References

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1949. on the Applications of the Mossbauer EEect, Bendor

[2] LATSHAW, G. L., Ph. D. thesis, Stanford University (1971). 1974, J. Physique Colloq. 35 -

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(1974) C6-297.

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DELL-ONDRUSZ, E., ~ h y s . Sol. (b) 57 (1973) K143. _{[l11 ANTONCIK,}_E.,_{Hyp. Znt. 1}_{(1976) 329.} [6] WEYER, G., Mossbauer Effect Method., (eds. Gruvermann,

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