LASER ANNEALING OF57Co SOURCES IMPLANTED IN Si AND Ge

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LASER ANNEALING OF57Co SOURCES IMPLANTED IN Si AND Ge

G. Langouche, M. de Potter, J. de Bruyn, M. van Rossum, R. Coussement, I.

Dézsi

To cite this version:

G. Langouche, M. de Potter, J. de Bruyn, M. van Rossum, R. Coussement, et al.. LASER ANNEAL-

ING OF57Co SOURCES IMPLANTED IN Si AND Ge. Journal de Physique Colloques, 1980, 41

(C1), pp.C1-421-C1-422. �10.1051/jphyscol:19801163�. �jpa-00219650�

JOURNAL DE PHYSIQUE Colloque C1, supplPment au n ^O 1 , Tome 41, janvier 1980, page C1-421

LASER ANNEALING OF 5 7 ~ o SOURCES IMPLANTED I N Si _AND Ge

G. Langouche, M. de Potter, J. de Bruyn, M. Van Rossum, R. Coussement and I. ~ d z s i ~ I n s t i t u u t voor Kern- en Stralingsfysika, Leuven University, B-3030 Leuven, Belgiwn.

x Central Research I n s t i t u t e for Physics, H-1525 Budapest, Hungary.

Recently /I-2/ it has been shown that the MESS- bauer spectrum of 5 7 ~ e in Si, after implantation at room temperature with an energy of about 70 keV, and a dose higher than l0I4 atoms/cm 2

,

le doublet which can be associated with an amorphous surrounding of the Fe impurity atom. It was also demonstrated /3/ that the same quadrupole interac- tion doublet is present in Coulomb-excitation re- coil-implantation experiments into Ge. It is the goal of this study to examine the Lattice location of the Fe atom after the amorphous layer has been recrystallized, or when its formation was inhibited.

Experiments. Four different methods were used : low dose implantation, thermal annealing above the recrystallization temperature, implantation into heated samples and laser annealing. Typical spectra are shown in Figures 1 and 2. Except for the low dose implant all samples were implanted with 5 7 ~ o to a dose of l0I4 atoms/cm2, with about 70 kV acce- lerator voltage. A 0.5 mg/cm2 57Fe in potassiumfer- rocyanide absorber was used in all experiments.

A common feature in all the spectra obtained is that the large quadrupole doublet (about 1 m/s) is not present any more, or at least greatly reduced in intensity in favor of other components. This adds confidence to the interpretation that this large doublet is correlated with the presence of the a- morphous layer. It is also obvious from Fig. 1 and 2 that the different techniques do not result in the same Mijssbauer spectrum, and that consequently dif- ferent final Fe sites are formed.

Low dose impZantations result in a dominating single line near zero velocity, which we believe are Fe atoms in an undamaged lattice surrounding _{/ 4 / .}

Y'hermal annealing of both low dose and high dose implants at 400'~ for three hours in inert gas at- mosphere results in a doublet with a smaller split- ting of 0.60+0.05 m / s and an isomer shift of -(0.25tO.O5)mm/s and -(0.33 *O.OS)mm/s for Si and Ge respectively. An external field measurement showed that this doublet is also due to quadrupole interaction.

IntpZantation i n t o heated S i and Ge samples results

in Miissbauer spectra which are essentially the same as after thermal annealing to the same temperature.

Laser annealing. It is well known by now that the amorphous layer due to ion implantation recrystal- lizes upon flashing with laser light, provided that the laser energy is larger than the recrystalliza- tion threshold of the crystal, below which the re- crystalization is incomplete, and lower than the distortion threshold, above which the crystal sur- face is visibly damaged. These thresholds depend on the absorption cross section of the crystal for the laser light, which is governed by the nature of the crystal surface, the implanted species, dose and energy, and of course on the laser wavelength, in- tensity and pulse duration.

Both a ruby and a neodymium glass laser were used to flash the Si and Ge surfaces with a typical pulse duration of 20 ns. No change was observed in the

spectrum of 5 7 ~ e in Si when flashed with the Nd la- ser, but after the ruby laser flash (2.7 J/cm 2 ) the

spectrum shown in Fig. 1 was observed. On the other hand, for Ge both the ruby and Nd laser resulted in spectra as shown in Fig. 2 for the Nd laser with 1 J/cm 2

.

The Nd and ruby laser photon energies are 1.17 eV and 1.79 eV respectively, while the bandgaps in crystalline Si and Ge are 1.14 eV and 0.67 eV re- spectively. The fact that the Nd laserlight just barely overlaps the Si bandgap might be responsible for the fact that Nd laser annealing is very sensi- tive to the implanted species and dose / S f , which alter the Si and band structure. This might explain why no change was observed in the MEssbauer spectrum of 5 7 ~ e in Si when flashed with the Nd laser.

Biscussion. 5 7 ~ o in non-amorphous Si and Ge sta- bilizes in at least two different sites, character- ized by a single line,orbyasnall (0.6 mm/s) qua- drupole doublet. The lattice site characterized by the small quadrupole doublet is probably related to the lattice site of the larger doublet, which be- longs to the Co atoms in the amorphized layer, as the isomer shifts are the same /2/ within error li-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801163

(21-422 JOURNAL DE PHYSIQUE

mits. The only difference is merely a contribution to the electric field gradient which originates or vanishes upon recrystallization.

Two distinct sites can be observed which are cha- racterized by a single line with nearly the same isomer shift. The first one is the one observed in the low dose implantation, which therefore could be- long to Fe in an undamaged surrounding. This single line disappears completely upon heating the sample above the recrystallization temperature. On the other hand, upon further heating the Si sample to higher temperatures, the spectrum is gradually do- minated by a broad single line, as shown in Ref. 2 . It thus is highly probable that this is not the same, although the isomer shift is very similar. As the mobility of Co in Si is very high at these tem- peratures these might be clusters or Co-Si precipi- tates / 6 / .

After laser annealing of Fe implanted Si, a se- gregation of the Fe atoms to the surface has been observed in backscattering experiments / 7 / , No da- ta are available yet for Co impurities. In Fig. I and 2 we see that the Mzssbauer spectrum of 5 7 ~ e in Si and Ge after laser annealing shows the small dou- blet for Si and the single li.ne for Ge. This stri- kingly different behaviour is not yet understood.

Channeling studies are planned to further study the lattice sites involved. A first paper on this

VELOCITY ( m m k ) Figure 2 : 5 7 ~ 0 i n Ge has recently been published /8/.

The authors greatfuZZy acknowZedge the heZp of Prof. J . D'OZiesZage~ with h i s Zaser set-up.

References.

/I/ Sawicka, B.D., Sawicki, J.A., Phys.Lett.A&

(1977) 311.

/ 2 / Langouche, G., DLzsi, I., Van Rossum, M., De bruyn, J., Coussement, R., Phys.Stat.So1.E

(1978)K17.

-

/3/ Langouche ,G. ,Dixon,N. S., Gettner,~. ,~anna, S.

s'. ,

this conference.

/ 4 / Langouche,G.,Dbzsi,I.,Van Rossum,M.,De bruyn,J.,

Coussement,R., Phys.Stat.Sol.b93(1979)K.

/ 5 / Khaibullin,I.B.,Shtyrkov,E.I.,Zaripov,M.M.,B;iya- zitov,R.M.,Galjautdinov,M.F.,Rad.Eff.~(l978)225.

/6/ DLzsi,I.,Coussement,R.,Langouche,G.,Molnar,B., Nagy,D.L., this conference.

/ 7 / App1et0n~B.R. ,White,C.W. ,Larson,B.C. ,Wilson,S.R., Narayan,J.,IEEE Trans.Nucl.Sc.26<1979)1686,

/ 8 / Kotai,E.,Lohner,T.,Manuaba,A.,Mezey,G.,Cousse- ment,R.,DBzsi,I.,Langouc,he,G., Rad-Eff., to be published.