raman scattering studies in phosphorus implanted and laser annealed boron doped si

Solid State Communications, Vol.53,No.|0, pp.861-865, 1985. 0038-I098/85 $3.00 + .00 Printed in Great Britain. Pergamon Press Ltd.

RAMAN SCATTERING STUDIES IN PHOSPHORUS IMPLANTED AND LASER ANNEALED BORON DOPED Si

G. Contreras +, M. Cardona,

Max-Planck-Institut ffir Festk6rperforschung Heisenbergstrasse I, D-7OOO Stuttgart 80

Federal Republic of Germany

and

A. Axmann,

Fraunhofer-Institut ffir Angewandte Festk6rperphysik Eckerstrasse 4, 7800 Freiburg Federal Republic of Germany

Received: December 14, 1984, by Manuel Cardona

Implantation of P in B bulk doped Si (hole concentration = 2.1xlO 2° cm -3) followed by multiple pulse laser annealing with a XeCI laser has produced the incorporation of P in substitutio- nal sites of the matrix and also charge carrier compensation. We report in this paper light scattering studies of these samples, in which a decrease of the hole-induced softening of the Si-Ra- man line is observed. This softening depends on the type and concentration of free carriers and thus, in our case, on the degree of compensation. This softening is discussed in terms of the interaction of the continuum of free carrier excitations with the Raman phonon. We also report the observation of the vibrations of I~B-3~F pairs.

INTRODUCTION

Bulk Si doped with boron offers the possibility of studying the hole-induced softening of the Raman phonon and the vibrational local modes (VLM's) of B. 1-s The VLM's originate from the fact that B has a lighter mass than Si. A lack of site inversion symmetry makes these VLM's observable both in Raman a and ir 6 ab- sorption experiments. From the latter it was found that the ZZB and its ~°B isotope VLM frequencies are 620 cm -I and 644 cm , respectivelg for a B concentration of 5xlO ~8 cm-3~ The presence of free holes produces a continuum of electronic transitions between the valence bands near the F-point of the Si Bril- louin zone. This continuum of inter-valence-band transitions overlaps the Ra- man phonon and the B-VLM and hence a dis- crete-continuum Fano-type interaction~be - tween the one-phonon scattering and one electron excitations takes place. This interaction accounts for a softening in the frequency of the Raman phonon and of the B-VLM which depends on the hole con-

+DAAD Fellow, on leave from E.S.F.M.-I.P.N., Edif. 6, U.P.A.L.M., Deleg. G.A. Madero, 07300 M~xico, D.F., MEXICO

centration. Softening of the Raman phonon is also observed in n-type Si. 8'9 In this case the phonon interaction takes place with the continuum of the intervalley electronic transitions between the k I and k S conduction bands. In this paper, we are cbncerned with the B+P compensated Si system. Earlier studies I° with this same system were directed to the (B+P)-VLM's. Here we extend these studies to the phonon softening of the Si zone-center optic phonon and its dependence on compensation.

EXPERIMENT

B doped Si slabs with a hole concentration of 2.lxlO 2° cm -3 were P-implanted using three different fluxes, 3x10 Is cm -2, 4xlO rs cm -z and IxlO I~ cm -2. This implantation was followed by a multiple pulse laser annealing with a XeCI laser. Epi- taxial regrowth was achieved, yielding compensated layers of a thickness of about 2000 ~. The type of charge carrier and the carrier concentration were determined by four-probe resistivity measurements and ir reflectivity. Raman measurements

861

862

were performed at room temperature, using a Im Spex double monochromator by the photon counting technique. A~ the exciting light we used the 4067 A Ar laser line. This line was chosen in order to probe only the implanted film and thus to avoid the signal from the substrate.

RESULTS

Some typical spectra of the Raman phonon of Si are shown in Fig. 1. Some features to be noted in this figure are a shift to higher frequencies of the phonon peak, a narrowing, and an increase in the strength of the peak in the spectra of the implanted samples as compared with that of the unimplanted one. Figure 2 shows 11B and 1°B vibrational local modes for each of the sam~$es m~asured. We note that for the 4xiO cm- P implantation dose a vibrational local mode cor- responding to the ~air 11B-31P appears clearly at 608 cm- . According to Ref. 12 this mode vibrates perpendicular to the axis of the pair and has the E symmetry (double degenerate); the parallel mode, with A I symmetry, should be located at

628 cm -~ ~2 but is too weak to be observed in our spectra, being hidden under the wing of the unpaired B-modes. Both E and A~ modes of the ~°B-3~P pairs seem to be t~o weak for unambiguous identifi- cation. Table I lists the carrier concentration of each sample, the frequencies of the Si-Raman phonons, VLM's of ~B, i°B, and the ~B-3~P pair and the corres- ponding softenings. The data with super- scripts are theoretical and experimental values are also given for comparison. From the table we see that as the implantation doses get higher, electrons become the ma- jority carriers in the crystal. Softening

Si:B÷P impionted

-- 517 5cm q

"-T

O

>-

Z ~uB= 619cm -1 LLI

z

z ~-- B :2xlO16/cm 2

c~

400 5OO 6OO RAMAN SHIFT(cm q)

RAMAN SCATTERING STUDIES IN PHOSPHORUS

Spectra of the Raman phonon of each of the P implanted Si samples. The dashed line gives the frequency for undoped Si.

700

Vol. 53, No. 10

Si ( B, 2.1x102°/cm 3 ). ! P implanted

P - dose

O 0

°i- 0 490 550

RAMAN S H I F T ( c m -1)

Fig. 2: Spectra of the 11B, 1°B, and ~T~vibrational local modes for each of the P-implanted samples.

of the zone center optic phonon is ex- plained in terms of the interaction of the continuum of the electronic transitions with the Raman phonon; this continuum is related to the maximum of the valence band (F-point) or the minimum of the conduction band (X-point), depending on whether the material is p- or n-type, respectively. Our experimental data are in good agreement with the theoretical work of Lawaetz for p-Si and with Ref. 9 for n-Si. We note that for the laser line at 4067 ~ (~ 3.O5 eV) no Fano-type fitting was done. Since this line lies near the E I resonance, the asymmetry para- meter q bedomes large and the asymmetry is small. The frequency of the B-VLM approaches 620 cm as the type of the charge carriers switches from p to n. The VLM softening should also be due to the interaction with the continuum of electronic transitions in the conduction or valence band, respectively. The position of the ~°B-VLM frequency agrees with earlier measurements. ~'" The frequency of the IIB-31P-VLM (last columnziTable I) shows a mean value of 607.5 cm , in agreement with measurements by M. Jouanne et al. ~° and also with IR measurements. ~2

Vol. 53, No. I O RAMAN SCATTERING STUDIES IN PHOSPHORUS 863

Im ~ I r I ~mI~ ; ~; /\ I o

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864 RAMAN SCATTERING STUDIES IN PHOSPHORUS Vol. 53, No. 10

We have also measured the Raman spectra of a pure Si sample implanted with both species, 11B and 31p~ using an implantation flux of 2xIO 16 cm 2 for both impurities and an implantation energy of 30 kV and 1OO kV, respectively, followed by laser annealing. From the IR reflectivity spectra, an electron concen-

pulsed laser annealing. These samples allowed observations of the zone center optic phonon of Si and of the local vibrations of the iZB-31P pair. Disappea- rance of the phonon softening of the heavily doped bulk p-type sample upon compensation confirms its assignment to interaction of the phonon with electronic

Si (B,2.1x1020/cm3): P imp|anted

d

l -

z U.J I-. Z

Z .< X .<

550 600 700 RAMAN SHIFT (¢rn4)

Fig. 3: Raman spectra of the 31 iI P+ B implanted sample, showing the vibrational local mode of ~:B at 619 cm -x.

tration of ].4xiO cm was obtained. Figure 3 shows the spectra obtained for this sample. We observe a softening of the Si-Raman phonon of -2.5 cm -~ and a frequency of 619 cm -~ for the ~B-VLM. No evidence of the IIB-31P-VLM pair was observed.

CONCLUSION

High concentrations of P in bulk doped Si were achieved by means of implantation and epitaxial regrowth by

excitations within the valence or conduction band. Correspondingly, the softening of the unpaired B local mode also decrea- ses upon compensation and thus must also be related to electronic interband excitations.

ACKNOWLEDGEMENTS

We wish to thank M. Siemers, P. Wur- ster, and H. Hirt for technical assistance and also H. Breitschwerdt for measuring the infrared reflection spectra.

Vol. 53, No. I0 RAMAN SCATTERING STUDIES IN PHOSPHORUS

REFERENCES

865

I. CERDEIRA, F., FJELDLY, T.A., and CARDONA, M., Phys. Rev. B 8, 4734 (1973) .

2. CERDEIRA, F., FJELDLY, T.A., and CARDONA, M., Phys. Rev. B 9, 4344 [1974) .

3. CHANDRASEKHAR, M., CHANDRASEKHAR, H.R., GRIMSDITCH, M., and CARDONA, M., Phys. Rev. B 2~, 4825 (1980).

4. SZABO, Z., Ph.D. Thesis (unpublished, University of Stuttgart, 1983).

5. LAWAETZ, P., Ph.D. Thesis (unpublished, Technical University of Den- mark, 1978).

6. ANGRESS, J.F., GOODWIN, A.R., and SMITH, S.D., Proc. Roy. Soc. 287, 64 (196S).

7. FANO, U., Phys. Rev. 124, 1866 (1961) 8. CHANDRASEKHAR, M., REN~CI, J.B.,

and CARDONA, M., Phys. Rev. B 17, 1623 (1978).

9. CONTRERAS, G., SOOD, A.K., CARDONA, M., and COMPAAN, A., Solid State Commun. 49, 303 (1984).

10. JOUANNE,-~., MORHANGE, J.E., and BAL- KANSKI, M., Phys. Stat. Sol. 9~, 2SS (1979).

I I . COMPAAN, A. , CONTRERAS, G., CARDONA, M., and AXMANN, A. , Journa l de Phy- s ique 44, C5-197 (1983).

12. NEWMAN, R.C. and SMITH, R.S. , Loca- l i z e d E x c i t a t i o n s i n So l ids (R.F. Wa l l i s , ed. Plenum Press, 1968), p. 177.

raman scattering studies in phosphorus implanted and laser annealed boron doped si

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