Carbon and oxygen isotope effects in the 0.79 eV defect photoluminescence spectrum in irradiated silicon

Solid State Communications, Printed in Great Britain.

CARBON

AND

Vol.51,No.3,

OXYGEN

PHOTOLUMINESCENCE

pp.127-130,

ISOTOPE

EFFECTS

SPECTRUM

IN

1984.

IN

0038-I098/84 $3.00 + .00 Pergamon Press Ltd.

THE

IRRADIATED

0.79

eV

DEFECT

SILICON

K. Thonke , G. D. Watkins $ and R. Sauer Physikalisches Institut (Tell 4) der Universit~t Stuttgart D-7000 Stuttgart 80, Pfaffenwaldring 57 Federal Republic of Germany (Received 6 April 1984 by M. Cardona) Abstract The 0.79 eV photoluminescence spectrum known to emerge in oxygen-rlch irradiated sillcon is studied in 13C and 180 enriched crystals. The principal no-phonon transition at 0.79 eV splits into a doublet in the 13C enriched sample in a way dlrectly demonstrating that one carbon atom per center is optically active. In contrast~ no isotope effects are observed in the local mode replicas. Doping the silicon with 180 slightly influences two local mode replicas giving the first direct evidence that oxygen is involved in the defect.

When pulled silicon is irradiated by highenergy electrons (1.5 - 2 MeV), neutrons, or y rays, a prominent photoluminescence spectrum arises with a no-phonon (NP) electronic ground state-to-ground state transition at -0.79 eV (frequently labelled the C llne) and a number of associated electronic - vlbronic sidebands. I-9 All previous workers have concluded that the optical center is oxygen-dependent since the spectrum has not been observed in oxygen-lean floatln~ zone material. In addition, Kirkpatrick et al. ° noted that the 0.79 eV llne is stronger in samples doped with both carbon and oxygen than in samples doped with oxygen alone 5, suggesting the possible involvement of carbon as well. In an absorption experiment, Foy I0 has established the symmetry of the center to be monoclinic I confirming previous tentative classifications 3,11. In a recent photoluminescence excitation study 12, four sharp local mode replicas of the 0.79 eV NP transition have been identified with associated vibrational quantum energies of 65.5, 72.5, 138.1 and 145.3 meV. Possible correlations of the center with EPR3, 5-8- and IR6-active defects have been discussed in the literature. However, no positive identification of the center has emerged. In fact, no direct evidence has been presented that either carbon or oxygen are actually incorporated in the center.

In this Communication, we present results in 13C and 180 enriched samples. We observe an isotope shift of the no-phonon llne due to carbon, providing the first direct and unambiguous evidence for the presence of a carbon atom in the defect. Evidence for weak isotope shifts in some of the local mode satellites due to oxygen is also presented confirming the direct incorporation of oxygen as well. The 13C enriched sample was a vacuum-floating-zone sample obtained from K.L. Brower 13 into which we diffused normal isotopic abundance oxygen prior to electron irradiation. The relative isotopic concentration was determined to be [13C]:[12C] ffi 1.3 : i by monitoring the relative intensities of the 13C and 12C local mode vibrational satellites on the 0.97 eV (G) luminescencel4, 15, also present in the sample. The 180enriched sample was origlnally obtained from Rrostowski and Alder 16 who quoted concentration ratios [180] : [170] : [160] ffi 12 : I : 87 for a sample from the same boule. In the present work we have Independently confirmed the relative concentrations of 180 and 160 by IR absorption of the 9 ~m band in our sample at 4.2 K. These samples along with several reference samples containing O and C isotopes in natural abundances were irradiated by 2 MeV electrons at room temperature and annealed between room temperature and 300 °C. The photolumlnescence was excited by the 647 nm line of a Kr + laser (~1.5 W output power). The emitted light was dispersed by a grating monochromator of Im focal length, detected by a cooled Ge-detector in conJunction with metal shielding and spikes suppression electronlcs 17 and processed by conventional lock-ln technique.

Humboldt US Senior Awardee with the MaxPlanck-lnstitut fur Festk~rperforschung and the Physikalisches Institut der Universlt~t~ Stuttgart; on leave from Lehigh University, Bethlehem, Pa 18015, USA 127

0.79 eV DEFECT PHOTOLUMINESCENCE

] 28

Fig. I shows the carbon isotope effect on the 0.79 eV NP llne. The 13C enriched sample shows structure which, as shown, can be decomposed into two identically shaped components of 1.3 : I intensity and separated by 0.084 meV, with the low energy unit intensity component at the spectral position of the 12C single isotope llne in the unenriched sample. The same doublet structure though not equally well resolved was observed in a photolumlnescence excitation experiment. 18 We conclude therefore that the structure arises from an isotope shift due to a single carbon atom. Normally such spectral positional accuracy would not be possible in comparing the spectra from two different samples because of the resettability error (~ ±0.6 A with our 600 grooves/mm grating) of the monochromator. We have avoided this error, however, by recording the spectra from the two samples of Fig. 1 simultaneously: The exciting laser beam was split into two beams

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' 12C+13C

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==

12C

Q_ 1.5694

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1

The 0.79 eV no-phonon llne in silicon containing natural abundant carbon isotopes (lower spectrum) and in 13C enriched silicon to concentrations [12C]:[13C] = 1:1.3 (upper spectrum). Spectral resolution is Ak=0.48~ (AE=0.024 meV). Dashed and full lines show the deconvolutlon of the experimental doublet based on the llneshape of the simultaneously recorded 12C single llne using a halfwidth scaling factor of 0.65 (see text).

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of equal intensities. The beams were independently chopped at different frequencies, one illuminating the 13C enriched sample, the other illuminating the unenriched sample, which were mounted on the crystal holder one above the other. The luminescence from the two samples was then simultaneously recorded in one scan and the detector signal was processed by two lock-in amplifiers separating the luminescence channels. Cross-coupling was smaller than 1/40 in each channel. The exact coincidende of the 12C components in both samples provides evidence that only a single carbon atom is involved in the center. If a second inequlvalent carbon atom were also involved, shifts and broadenlngs of the undlsplaced component might be expected in analogy to that observed for the local mode satellites of the 0.97 eV G llne, known to contain two carbon atoms. 19 In the course of this accurate double beam study we discovered that the position of the NP lines is sensitive to temperature and, at high excitation powers~ differences in the sample thicknesses, sizes, etc produced an apparent shift of the NP lines between the two samples being studied. The spectra of Fig. I were taken at low excitation levels to avoid this. We point this out to alert others to this possible pitfall in making such studles. 20 The halfwidth of the NP lines deserves special attention: In a study of several different, non enriched samples we find that the width is sample dependent with measured values from 0.09 meV to 0.05 meV (note in Fig. I that the width in the unenriched sample is greater than that for the components in the enriched sample). Most interesting is the observation that the overall shape of the lines appears identical with the same asymmetric tall to the high energy side (Fig. I) but simply scaled by the halfwldth. This has bearing on a recent interpretation by Foy 21 where the asymmetry was interpreted as evidence of 29Si and 30Si isotope effects with two Si atoms per optical center being suggested. The halfwldth in Foys" studies was ~ 0.I meV. Our results cast doubt on this interpretation as one would expect sample dependent shapes and even resolved Si isotope components in our sample exhibiting the smallest halfwldth of 0.05 meV. A careful study of the shape of the NP llne in the 180 enriched material revealed no measurable difference from that for the unenriched samples. We detect therefore no isotope effect in the no-phonon llne due to oxygen. In Fig. 2 we plot the region of the spectrum where the local mode vibrational replicas L[ and L2 of the 0.79 eV line show up. The corresponding quantum energies of the vibrations are 65.5 meV (LI) and 72.5 meV (L2). 12 These lines previously often labelled B and A and ascribed to no-phonon, purely electronic transitions at the 0.79 eV center 3-5 were recently demonstrated by Wagner et al. 12 to be local mode sidebands. In high-

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0.79 eV DEFECT PHOTOLUMINESCENCE

Photon 0.72 I

E n e r g y (eV)

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Wavelength k (~m) Tl~. 2 P o r t i o n o f the v i b r o n i c s i d e b a n d spectrum a s s o c i a t e d w i t h the 0.79 eV NP l i n e at 5 K and 20 K. L1 and L2 (labelled A and B in references 3,5), and LI" and L2" are local mode replicas of the NP llne. Lines labelled with asterisks are local mode replicas of an excited state electronic transition.

SPECTRUM IN IRRADIATION SILICON

129

further local mode replica with vibration energy of 30.9 meV. This rich vibronlc spectrum of the 0.79 eV line is not affected to a detectable degree in the 13C enriched samples. We observe neither a shift or broadening of the local mode lines nor an effect in the broad associated phonon band. It is clear that we should have seen isotope effects due to 13C in all modes if they existed. Line shifts would have been detectable down to ±0.15 meV in LI and ±0. I0 meV in L2. We conclude therefore that although carbon is clearly involved in the defect, it is not participating significantly in any of the resolved local modes. In Fig. 3 we compare the L1 and L2 local modes in 180 enriched and unenriched samples. Evidence of an isotope shift due to the 12% 180 is seen in both lines. (The weaker intensities of the other local modes precludes the possibillty of detecting similar shifts for them). This provides the first direct confirmation of the presence of oxygen in the defect. The fractional isotopic shift is only -0.5 meV, however, compared to =4 meV corresponding to the ideal predicted relative shift of (I- / ~ 7 ~ ) for a single vibrating oxygen atom or ~6.45 meV typically seen for the 9 ~m band for isolated interstitial oxygen 16. The oxygen atom therefore also appears to play only a small role in these local mode vibrations. Both oxygen and carbon are lighter than sillcon and would normally be expected to give local modes associated with these vibrations. The failure to detect isotope effects for carbon and only weak isotope shifts for oxygen is therefore a very surprising result. (The vibrational modes

Rel. Energy ( 2 m e V / D i v )

l

i

// ,

i

0.79 eV [ine ' local modes resolution, we detect four additional lines which are related to the same center. LI" and L2" are observed independent o£ temperature and we interpret them as additional local mode replicas with vibration quantum energies of 61.6 meV (LI') and 67.9 meV (L2"). The two lines labelled with asterisks emerge when the temperature is raised. They have equal spacings from L1 or L2, (**) - L1 = (*) - L2 = 5.2 meV. This energy is in turn equal to the spacing between the 0.79 eV ground state-to-ground state NP transition and a NP llne originating from an upper excited state (the lines 00 and C[, respectively, in the notation of Wagner et al.12). Therefore, the lines (**) and (*) are local mode replicas of the excited state-llne C[ which correspond to L1 and L2 of the C O transitlon. 22 We note that there are two other local mode replicas at much higher vibration quantum energies of 138.1 meY and 145.3 meV (cf. also Ref. 12). A line at 1.634 ~m (unlabelled in Ref. 12) is also correlated with the 0.79 eV line and is most probable due to a

, =

I

11

Rel. W a v e l e n g t h

x

i k (50~/Div]

Fi~. 3 Lineshapes of the L1 and L2 local mode replicas in the 180 enriched sample (original traces) and in several reference samples containing natural abundant oxygen (dashed lines).

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0.79 eV DEFECT PhOTOLUMINESCENCE

other than LI and L2, and LI" and L2", could involve significant oxygen isotope shifts). The existence of a carbon isotope shift in the nophonon llne requires a significant change in vibrational frequency between ground and excited state. This suggests an environement for carbon with a soft electronically determined force constant which could lead to resonant undetected modes for its vibration. The small LI and L2 oxygen shifts imply that oxygen motion is also playing a small role in these modes. Since no other impurities are expected to be involved, this implies that the primary motion involves silicon atoms, the local modes arising therefore by stiffened force constants for the silicon. We note t h a t isolated interstitial oxygen produces an infrared band in this frequency range (64.1 meV ~ 517 cm -I) which also displays no oxygen

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Vol. 51, No. 3

isotope effect. 23 These considerations provide hints as to the composition of the defect but provide no detailed model at present. If a sample were available with higher 180 enrichment, the oxygen isotope shifts on the other local modes could be studied. Such study might provide crucial information. Acknowledgements We thank J. Weber (Sherman Fairchild Laboratory, Lehigh University, Bethlehem, PA) for the irradiation of the 180 enriched sample and A. Breltschwerdt (Max-Planck-lnstitut fur Festkorperforschung, Stuttgart) for performing the IR absorption on the 9 ~m band. The financial support of the Deutsche Forschungsgemeinschaft under contract no. Sa 311/2-4 is gratefully acknowledged.

References I. A.V. Yukhnevich, V.D. Tkachev, and M.V. Bortnlk, Soy. Phys. - Solid State 8, 2571 (1967) - [Fizika Tverdogo Tela 8, 3213 (1966)] 2. R.J. Spry and W.D. Compton, Phys. Rev. 175, I010 (1968) 3. C.E. Jones and W.D. Compton, Radlat. Elf. 9, 83 (1971) 4. A.P.G. Hare, G. Davies, and A.T. Collins, J. Phys. C5, 1265 (1972) 5. C.E. Jones, E.S. Johnson, W.D. Compton, J.R. Noonan, and B.G. Streetman, J. Appl. Phys. 44, 5402 (1973) 6. J.R. Noonan, C.G. Kirkpatrlck, and B.G. Streetman, J. Appl. Phys. 47, 3010 (1976) 7. C.G. Kirkpatrick, J.R. Noonan, and B.G. Streetman, Radlat. Elf. 30, 97 (1976) 8. C.G. Kirkpatrick, D.R. Myers, and B.G. Streetman, Radlat. Eff. 31, 175 (1977) 9. V.D. Tkachev and A.V. Mudryi, Inst. Phys. Conf. Set. No 31, 231 (1977) [Proc. Int. Conf. Radlat. Effects Semiconductors, Dubrovnik, 1976)] I0. C.P. Foy, J. Phys. C15, 2059 (1982) II. J. Walker, J. Appl. Phys. 45, 4653 (1974) 12. J. Wagner, K. Thonke, and R. Sauer, Phys. Rev. (submitted) 13. K.L. Brower, Phys. Rev. B_9, 2607 (1974)

14. G. Davies and M.C. do Carmo, J. Phys. C14, L687 (1981) 15. K. Thonke, H. Klemisch, J. Weber, and R. Sauer, Phys. Rev. B24, 5874 (1981) 16. H.J. Hrostowskl and B.J. Alder, J. Chem. Phys. 33, 980 (1960); H.J. Hrostowski and R.H. Kaiser, Phys. Rev. 107, 966 (1957) 17. A.T. Collins and T. Jeffries, J. Phys. El5, 712 (1982) 18. J. Wagner, private communication 19. K.P. O'Donnell, K.M. Lee, and G.D. Watklns, Physlca II6B, 258 (1983); G. Davies, E.C. Lightowlers, and M. do Carmo, J. Phys. C16, 5503 (1983) 20. A slight change of the relative line intensities in the doublet from 1:1.3 to I:i.I was observed for temperatures below 4.2 K. This effect is under further investigation. 21. C.P. Foy, Physica 116B,276 (1983) 22. The line (*) was earlier observed and given the same interpretation by Hare et al. (Ref. 4) who did not resolve the L2" flnestructure. The combined LI', (**) structure was obviously also seen (cf. Fig. I of Ref. 4) but not discussed. 23. See, for references, R.C. Newman, "Infrared Studies of Crystal Defects" (Taylor and Francis, London, 1973)