LATTICE LOCATION OF 5s-p IMPURITIES IMPLANTED IN TYPE-IV SEMICONDUCTORS

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LATTICE LOCATION OF 5s-p IMPURITIES

IMPLANTED IN TYPE-IV SEMICONDUCTORS

M. van Rossum, J. de Bruyn, G. Langouche, R. Coussement, P. Boolchand

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 37, Dkcembre 1976, page C6-889

LATTICE LOCATION OF

5s-p

IMPURITIES

IMPLANTED IN TYPE-IV SEMICONDUCTORS

M. VAN ROSSUM, J. DE BRUYN, G. LANGOUCHE (*) R. COUSSEMENT

and

P. BOOLCHAND (**)

Instituut voor Kern- en Stralingsfysika, University of Leuven, Belgium

RCsum6. - Nous avons utilise I'effet Mossbauer dans le 125Te, 129Xe et 133Cs afin d'etudier l'environnement des atomes de Sb, Te, I et Xe implant& dans le Ge, Si et le diamant au moyen d'un skparateur d'isotopes. Les rCsultats de ces experiences sont compares aux informations obtenues precedemment par effet Mossbauer et effet de Canalisation.

Abstract. - The Mossbauer Effect of 1 ZsTe, 129Xe and 1 3 3Cs has been used to study the lattice

location of Sb, Te, I and Xe ions implanted in Ge, Si and diamond lattices with an isotope separator. The results of these experiments are compared with previous Mossbauer Effect and channeling measurements.

I. Introduction. - The chemical state and lattice location of 5s-p impurities in Ge, Si and diamond has recently been the focus of considerable interest by 1291 and l19Sn Mossbauer spectroscopy [l, 21. The Mossbauer Effect (ME) also offers the possibility to study the implantation behaviour of Sb, Te and I by using the 35.6 keV transition in 125Te. In our continu- ing attempts to understand the details of this problem, we present in this paper a systematic study on 125Sb, lZ5"Te and 1251 implanted in type-IV semiconductors. Some preliminary results on the implantation of lZ9"Xe and 13,Xe in these semiconductors are also reported here.

2. Experimental procedure. - Slices of Ge and Si were cut off from n-doped single crystals perpendi- cular to the

<

111

>

axis. They were etched before the implantation using a mixture of HNO, and H F as an etchent. The diamond crystals (type Ia), which were obtained from Diamkaap N. V., Antwerp, were carefully etched using a procedure described in ref. 131. 12'1 and lZ5Sb activities were purchased from the National Institute of Radio-Elements (I. R. E.). 125mTe was obtained by neutron irradiation of enriched 124Te at the Mol reactor centre (S. C. K.). The implantations were performed at 70 keV using the Leuven Isotope Separator. A heating facility was built to suitably raise the temperature of the semiconductor host for hot implantations. The implanted doses were typically in

(*) Aangesteld Navorser N. F. W. 0.

(**) Permanent address : University of Cincinnati, Cincin- nati, Ohio 45221, U. S. A.

the range from 5 X 1013 to 5 X 1015 atoms/cm2 The lowest implanted dose corresponds to 1 2 5 ~ where

the highest specific activity was available. A conven- tional electromechanical driving system was used ;

in all experiments the source was moved. Both source and absorber were cooled to liquid helium temperature. The absorber consisted of ZnTe powder obtained by crushing of a ZnTe single crystal and had a thickness of 2 mg/cm2 of 125Te. A velocity calibration of the drive was recorded simultaneously with a 5 7 C o a source and an Fe metal foil. The detector we used was a Xe-filled proportional counter with the window setting on the K, and K/, escape peaks of the 35.5 keV y-ray. In some cases, a NaI(T1) scintillator of 0.2 mm thickness replaced the proportional counter.

3. Discussion of the experimental results. - The experimental results are shown in figures 1 t o 5. They have been appropriately least-squares fitted to one or two Lorentzians. The peak positions, linewidths and intensity ratios are listed in table I. The present ME data may be correlated with existing information on lattice location of Sb and Te implanted in Si from channeling measurements and from '19Sn and lZ9I ME experiments.

3 . 1 TELLURIUM IMPLANTATIONS. - Channeling experiments of Gyulai [4] have revealed that Te implanted in Si ends up with almost equal probability on two distinct sites, one of which is substitutional. Similar conclusions can be drawn from the 1291 [l] ME experiments which utilized implanted sources of 129mTe. Our own result on 1 2 5 " T e ~ e (Fig. 1) also

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C6-890 M. VAN ROSSUM, J. DE BRUYN, G. LANGOUCHE, R. COUSSEMENT AND P. BOOECHAND Impurity - 1251 l251 1251 1251 1 2 5 m ~ ~ Iz5Sb lZ5Sb Host - Ge Ge Si diam. Ge Ge Ge Implanted dose - 5 1013 5 X 1013 5 X 1013 1 X 1014 5 X 1015 I 1014 1 X 1014 Implantation temperature -

RT

200 OC

RT

R T

RT

200 OC

FIG. 1 . - Mossbauer spectrum of 12srnTe implanted in Ge. FIG. 2. - Mossbauer spectrum of 125Sb implanted in Ge.

shows two absorption lines which, in accordance with the previous results, we attribute to two different lattice positions of the Te atoms in the Ge host.

3.2 ANTIMONY IMPLANTATIONS. - In the spectrum obtained following implantation of lZ5Sb in Ge (Fig. 2) only one peak is observed, thus indicating the same surrounding for all implanted atoms. This result also appears to agree with the previous '19Sn ME experiments [2] which utilized implanted sources of

'19Sb in Si, and showed a single line.

3.3 IODINE IMPLANTATIONS. - The Mossbauer

+

spectrum of lz5IGe (Fig. 3) is quite similar to the one of _-+ lZ5"TeGe ; the peak positions and relative intensities of the lines are nearly the same for the two implantations. Therefore we conclude that I and Te show the same implantation behaviour in Ge. Moreover, only a slight difference in the ISs of the two lines was observed

between a

RT

and a hot (200 OC) implantation, thus suggesting that the 1 2 5 ~ e resonance is not quite sensi- tive to radiation damage effects.

3.4 FURTHER CONSIDERATIONS ON 12'Te EXPERI-

MENTS. - The identification of peaks in the lZ5Te and lZ51 spectra may be made by relating the observed peak positions (a1, 6,) to an IS scale made up of compounds

- 8 -i d i

-

1 v(mrn/s) FIG. 3. - Mossbauer spectrum of 1 2 5 1 implanted in Ge.

where the Te chemical state is better understood [S]. Bearing in mind that a positive velocity means that the source is moving towards the absorber, the positive velocity peak corresponds to a smaller y-ray energy. Peak 1, which then corresponds to a lower S-electron density and which falls in the general neighbourhood of the chemical compound

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LATTICE LOCATION OF 5s-p IMPURITIES l IMPLANTED IN TYPE-IV SEMICONDUCTORS C6-891

could thus be attributed to substitutionally located Te atoms. Indeed, K2Te0, is known to exhibit tetrahe- drally coordinated Te in roughly an sp3 configuration. The position of peak 2 is indicative of the largest elec- tronic density ever observed on l a S ~ e ; the corres- ponding chemical state of this interstitial site remains however unclear.

One expects Sb to implant substitutionally in Ge. No obvious confirmation of this point can however be extracted by direct comparison of the observed IS of 6 = - 0.32

+

0.05 mmls for the lz5sb implant in Ge, and the IS of the substitutional site

6 , =

+

2.22 -+_ 0.05 mm/s in the lZ5I implant in Ge. An unusual feature of the present data is revealed by comparing the IS difference <( S,

-

6, of the two Te

sites in spectra of implanted lZ5I sources in Ge, Si and diamond. For Si and Ge host, 6,

-

6, D was unam- biguously determined from the doublet spectra shown in figures 3 and 4. For the case of diamond (Fig. 5) where we observe a broad single line, we assume that there are two inequivalent sites, and obtained

<t d1

-

6, by fitting to two equally intense lines of width

r

= 8.0 mm/s. In figure 6, we see that

FIG. 4.

-

Mossbauer spectrum of 1251 implanted in Si.

FIG. 5.

-

Mossbauer spectrum of l2s.I implanted in dia-

mond.

FIG. 6. - I S of the lines in the Mossbauer spectra of 1251 implanted in Ge, Si and diamond, plotted as a function of host

bond length.

(( 6,

-

6, systematically decreases with decreasing

bond length cc d )) of the semiconductors host. This

trend of the present lz5Te ISs in semiconductors is just the opposite to the one previously observed in '"Sn and Ia9I ME experiments, when two sites are populated. If one accepts the identification of the two peaks mentioned earlier, one infers that on shortening of the host bonding length d ) ) , the S-electron density at the substitutional Te impurity increases, whereas the reverse behaviour is true for the interstitial Te impurity. These results on Te are puzzling since AntonEik has recently given theoretical justification for the observed decrease in the S-electron density at substitutional Sn or I impurity in these semiconductor hosts as a function of decreasing host binding length [6].

4. Preliminary results on Xe implantation.

-

We have implanted lZ9"'Xe and 133Xe in crystalline Ge and studied the 129Xe and 1 3 3 ~ s ME of these sources. In both cases, we observed a narrow single line indicating that Xe comes to rest at a unique site in Ge. The IS of this line in the 129mXe experiment (6 = 0.17 +_ 0.03 mm/s with respect to sodium perxenate) coincides exactly with the IS of Xe cla- thrate, thereby suggesting that Xe comes to rest in its atomic form. The IS of the single line in the 133Cs experiment was observed to be

with respect to the CsCl absorber used in these experiments ; this IS is near that of CS metal. This result seems to indicate that the chemical state of CS formed on P-decay of 133Xe corresponds to its atomic form CS (5s' 5p6 6s). Both these experiments suggest that Xe comes to rest in the Ge lattice at an interstitial location. This conclusion is also supported by channeling measurements of Xe in Si where the substitutional fraction was found to be low [7].

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C6-892 M. VAN ROSSUM, J. DE BRUYN, G. LANGOUCHE. R . COUSSEMENT AND P. BOOLCHAND trum of this source (Fig. 7) shows a rather well

resolved doublet structure. Clearly, a doublet spectrum

FIG. 7. - Mossbauer spectrum of 12gm Xe implanted in

diamond.

for a $ + $ transition can have one of the two inter- pretations : either the two equally intense peaks represent members of a quadrupole doublet or two chemically inequivalent Xe surroundings. Integral PAC measurements on the implanted sample are in progress to narrow down one of these possibilities. If one accepts that the 2 lines represent a quadrupole doublet, one has a QS :of 12.7 mm/s, which is un- usually large to expect for a tetrahedral coordination. On the other hand, if the two lines correspond to two chemically inequivalent sites, one is faced with an enormously large IS difference between these sites. This value 6,

-

6, is an order of magnitude larger than the range of 12'Xe IS encountered in various chemical compounds.

Acknowledgment.

-

The authors wish to thank

5. Odeurs, R. Vanautgaerden and H. Pattyn for the implantations performed at the Leuven Isotope Sepa- rator.

References

[l] HAFEMEISTER, D . W. and DE WAARD, H., Phys. Rev. B 7 [4] GYUALAI, J., MEYER, O., PASHLEY, R. D. and MAYER, J. W., (1973) 3014. Rad. Eff. 7 (1971) j 17.

[2] WEYER, G., DEUTCH, B. I., N - N ~ ~ T E D T - L ~ ~ ~ ~ , A., [51 BOOLCHAND, P., VAN ROSSUM, M., LANGOUCHE, G., ODEURS,

ANDERSEN, J. V. and NIELSEN, H. L., J. Physique J., PATTYN, H. and COUSSEMENT, R., to be published in Hyperfine Interactions.

Collog. 35 (1974) C6-297. _{161 ANTONCIK,}_E.,_{Hyp. Int. 1}_{(1976) 329.}

[3] DAVIDSON, L. A. CHOU, S., GIBBONS, J. F. and JOHNSON, [7] ERIKSSON, L., DAVIES, J. A., JOHANSSON, N . G. E. and