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Photoluminescence of defects introduced by deuterium plasmas in silicon
MateriaLsScience and Engineering, B4 (1989) 461 465
461
Photoluminescence of Defects Introduced by Deuterium Plasmas in Silicon H. WEMAN Department o[ Physics and Measurement Technology, Link@ing Univers'ity, S-581 &?Link@ing (Sweden) J. L. LINDSTROM Swedish l)~JbnceResearch Establishrnent, P.O. Box 1165, S-581 l l, Link@ing (5'weden) G. S. OEHRLEIN IBM ThomasJ. WatxonResearch (enter, p.o. Box 218, Yorktoan tleights, NY 10595'(U.S.A.) (Received May 31), 1989)
Abstract Dejects" introduced by reactive-ion etching (R1E) and plasma etching (PE) using deuterium have been studied in boron-doped silicon with the photoluminescence (PL) technique. We have observed a set of broad luminescence bands" in the beh)w-bandgap range between 1.05 and 0.8 eV. These bands change in intensity as well as in photon energy with annealing. We attribute all these PL bands" to electron-hole recombination in heavily damaged regions, where electrons and holes can be localized in potential wells caused by the strain surrounding the microscopic hydrogen defects.
by TEM [1], as well as from electrical measurements showing the passivation effect of hydrogen on the dopants and other defects [2]. Low-temperature photoluminescence (PL) studies have shown that the damaged region gives rise to several luminescent lines and broad bands below the bandgap [3-6]. The purpose of this paper is to show the effect on these broad PL bands of different parameters in the dry etching process with deuterium as a plasma (RIE vs, PE, chamber pressure in RIE) and also to study the recovery effect a subsequent annealing has on the damage caused by the etching. A tentative model to explain the origin of the broad PL bands will also be given.
1. Introduction
2. Experimental conditions
Dry etching processes are of technological importance in the fabrication of integrated circuits. This will expose the semiconductor surface to bombardment by ions with energies ranging from tens of electronvolts to several hundreds of electronvolts from a plasma that often contains hydrogen. From transmission electron microscopy (TEM) measurements it is known that the effect of low energy bombardment on silicon by hydrogen and deuterium ions is a highly damaged semiconductor near the surface, originating both from the introduction of lattice damage by the high-energetic ions and from the incorporation of hydrogen. This has been documented in several papers, showing complex defects such as for example the {111} and {100} platelets and gas bubbles in heavily damaged regions as observed
Boron-doped, (100) oriented, 10-20 V2 cm, Czochralski zone (CZ)-silicon wafers have been used in this study. The wafers were placed on the water-cooled bottom electrode of a diode reactor, where the top and the bottom electrodes can be independently powered, thereby changing the experimental conditions from RIE to PE. For the deuterium plasma D2, the conditions were as follows: 13.56 MHz r.f. power of 200 W applied to the bottom electrode for RIE, and to the top electrode for the PE. Deuterium was used instead of hydrogen to enable a secondary ion mass spectrometry (SIMS) profiling on the same samples in an earlier study [1]. The deuterium pressure was varied from 2.6 to 250 mTorr in the case of RIE and was at 25 mTorr during the PE treatment with an exposure time of 10 rain.
0921-5107/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
462
Photoluminescence measurements were done at temperatures from 2 K to about 20 K, excited with the 6471 A line of a Kr + ion laser. The luminescence was dispersed with a SPEX 1404 0.85 m double grating monochromator with two 600 grooves mm-~ gratings blazed at 1.6/~m. For the detection a liquid nitrogen cooled North Coast EO-817 S germanium detector with a conventional lock-in technique was used. The excitation power on the sample was around 100 mW and the luminescence was detected in backscattering geometry.
3. Experimental results Typical PL spectra at 2 K directly after the RIE and PE treatments at 25 mTorr pressure are shown in Figs. l(a) and l(b) respectively. In the near-bandgap region the boron bound exciton (B(BE)) is observed with the no phonon recombination BNp at 1.151 eV, and the different phonon replicas, transverse acoustical BTA at 1.132 eV, transverse optical Bxo at 1.093 eV, two phonon B v o + O r at 1.021 eV (where O r is the zone centre optical phonon) and the two-hole transition Bvo + Bh* at 1.060 eV, as labelled in Fig. 1. The dominating PL line is the TO phonon replica of the B(BE) at 1.093 eV. The broad structure below 1 eV is due to the etching treatment of the samples. (The line structure around 0.9 eV is due to water-absorption of the broad PL band.) The broad band after the RIE treatment (Fig. l(a)) peaks at about 0.9 eV with a halfwidth of 100 meV (FWHM), while after the PE treat-
WAVELENGTH (pm) 1.7 1.6 1.5 1.4. 1.3 1.2 Si:B ~
T=2K
WAVELENGTH (pm) 1.1 1.7 1.5 1.3
z FZ
1.1
W
o z W o co W z
BT~
Z
ment (Fig. l(b)) the band peaks at about 0.92 e V with a halfwidth of 75 me"/. We have performed an isochronal annealing of the etched samples from 75 to 525 °C in steps of 50 °C each for 30 min, in addition to one treatment at 800 °C for 30 min. The evolution of the broad PL bands after some selected annealing treatments is shown for the RIE samples in Fig. 2. Up to 275 °C the intensity of the broad band increases relative to the B(BE) luminescence. From 325 °C and higher temperatures the peak position shifts to higher energies and the halfwidth decreases. For example at 375 °C the broad structure seems to be composed of three bands, with the main peak at about 0.95 eV with a halfwidth of about 50 meV, a shoulder below 0.9 eV and a band at about 1.(13 eV superimposed with the B r o + O r replica. Above 425 °C the broad bands decrease in intensity and have almost disappeared at 525 °C. However, the sample has not completely recovered after the annealing step at 800°C, where a broad band at 0.96 eV still remains.
BTO+Or I
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O 25rnTorr~,~.90eV"~,..~ w
13-
~ (a) RIE, ~ .
"1-
o..
~
i
)~ ~,
I
i
J
,
0.8 0.9 1.0 1.1 PHOTON ENERGY (eV)
,
Fig. 1. PL spectra at 2 K of CZ-silicon after R I E (a) a n d PE (b) t r e a t m e n t at 25 m T o r r pressure. (The different notations are explained in the text.)
0.8 0.9 1.0 1.1 PHOTON ENERGY (eV) Fig. 2. PL spectra at 2 K of CZ-silicon after RIE treatment at 25 m T o r r and a s u b s e q u e n t isochronal annealing for 30 min at 225 °C (a), 275 °C (b), 325 °C (c), 375 °C (d), 425 °C (e), 525 °C (f) and 800 °C (g).
463
Apart from the broad PL bands, in spectra taken at somewhat higher temperatures (greater than 10 K), we have observed the so-called "W"line at 1.018 eV in PIE samples annealed for 30 min between 175 and 425 °C [7]. This defect is trigonal and is suggested to be related to interstitials rather than vacancies [8]. The effect of different chamber pressures during RIE has been investigated and is shown in Fig. 3. The chamber pressure during the etching was at 2.6 mTorr in Fig. 3(a), 25 mTorr in Fig. 3(b) and 250 mTorr in Fig. 3(c), whereafter the samples were annealed at 425 °C for 30 min before the PL measurements. The general shape of the broad PL bands are quite similar; however, the relative intensity of the broad PL bands increase with decreasing chamber pressure. We have observed a shift in the energy position of the B(BE) line owing to the plasma treatment. In Fig. 4 PL spectra in the TO phonon replica region of the B(BE) (unperturbed position 1.0926 eV) with bound multiexciton lines bj (unperturbed position 1.0904 eV) and be (unperturbed position 1.0881 eV) and the electron hole droplet (EHD) emission (unperturbed position 1.083 eV) are shown after various PE treatments. The spectra were taken with the same laser excitation intensity, with a laser spot size of about 1 mm in diameter. The shifts varied after the different annealing steps and were dependent on the spatial position of the laser on the sample. In general the shifts were to the low energy side, with maxi-
WAVELENGTH (p.m) %- .71.6 1.5 1.4 1.3 1.2 'Si:IBT=2K ' ' /l~'B'to+OF~r~ Z L!J I-Z LU O Z LLI O CO IJJ Z
RIE at X mTorr~
+425~0, ,C
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.[
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fBTABh=.~
mum shifts of about 4 meV. The shifts are usually large directly after the dry etching and during the post annealing up to about 525°C, and quite small after the 800°C annealing (Fig. 4(c)). These shifts were larger in the PE samples than in the RIE samples, and larger at higher pressure (250 mTorr PIE) than at lower pressure (2.6 mTorr PIE). All PL bands caused by the dry etching disappeared after a chemical etch removing about 0.5/~m of the surface.
4. Discussion and conclusions
There seems to be no direct similarity in the shape and energy positions of the generally broad PL bands observed after plasma hydrogenation of silicon in the previous studies [3-5], in comparison to this study. Many parameters are of course different, e.g. the power and pressure of the microwave plasma, the concentration of hydrogen in the plasma, the etching time, in addition to the starting material (float-zone or Czochralski). However it seems to be conclusive that the plasma discharge species (hydrogen, hydrogen+ deuterium, helium) have no or very little influence on the shape of the broad PL bands, i.e. the radiative recombination is mainly caused by the introduction of extended defects from clustering of atoms [5]. A comparison of the effect of PIE (Fig. l(a)) vs. PE (Fig. l(b)) at the same chamber pressure
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PE +30 min at:
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o
tO
o_
13.. [
0.8 0.9 1.0 PHOTON ENERGY (eV)
Fig. 3. PL spectra at 2 K of CZ-silicon after RIE treatment at various chamber pressures X including a post-annealing at 425 °C for 30 min. The pressure was 2.6 mTorr in (a), 25 mTorr in (b) and 25(I mTorr in (c).
i
1.080
1.085
i
1.090
1.095
PHOTON ENERGY(eV} Fig. 4. PL spectra at 2 K of the 1"O phonon replica region of the boron bound exciton (B(BE)) and the electron hole droplet (EHD) after PE treatment and annealing [k)r 30 min at 225 °C (a), 375 °C (b) and 800 °C (c).
464
(25 mTorr) shows that the relative intensity of the PL bands are larger, the energy position is lower (about 20 meV) and the halfwidth is larger (about 25 meV) after the PIE treatment. This can be compared with the TEM pictures on the same samples in Figs. l(a) and l(b) in ref. 1, where the PIE treatment shows a heavily damaged top 15 nm layer with some {111 }planar defects while the PE sample shows a less damaged surface layer extending to about 5 rim. The maximum ion energy during the PIE treatment is about 0.5 keV, while during the PE treatment it is only about 30 eV [111. The isochronal annealing of the PIE samples (Fig. 2) shows that there are no distinct energy positions of the broad PL bandsl They gradually evolve to higher energies with somewhat different shapes and have almost disappeared at around 500 °C. This is connected to the recovery of the heavily damaged near-surface layer by the annealing, leaving some extended defects inside the silicon crystal, probably related to the 0.96 eV band observed after the 800°C annealing (Fig. 2(g)). The increase in the relative intensity of the broad PL bands with decreasing chamber pressures after the PIE treatment (Fig. 3) must be due to the difference in ion bombardment energy. The maximum ion energy is inversely proportional to the gas pressure and is about 204, 19.6 and 0.96 eV at 2.6, 25 and 250 mTorr, respectively. That the general shape of the broad PL bands is similar in Fig. 3 is consistent with the previous study by Jeng et al., where the T E M pictures showed very similar etched-induced damage after PIE treatments at different pressures [ 1 ]. We attribute these broad PL bands to electron-hole recombination in heavily damaged regions where electrons and holes can be localized in potential wells caused by the strain of the microscopic defects. This strain can cause a bending of the conduction and valence band so that an effective band gap narrowing occurs (Singh et al. have estimated a band gap narrowing of about 130 meV in their plasma etched samples from the observed shift of the acceptor (donor) bound exciton [5]). This type of strain-induced "intrinsic quantum well" (IQW) is similar to the charge-induced IQW that has been discussed previously as an explanation of some electron traps observed in oxygen precipitated CZ-silicon f9]. However, one important difference is that the
strain-induced IQW model carl accumulate both electrons and holes locally in quantum levels surrounding the precipitates, while the chargeinduced IQW model [911only accumulates carriers of one type. It therefore needs deep traps of the opposite type for recombination to occur in the energy range observed for the broad PL bands. which is not very likely in a hydrogen passivated surface region. Actually the broad PL bands observed here are very reminiscent of broad P1, bands observed in earlier work on long time annealed (oxygen precipitated) CZ-silicon [101 and annealed antimony implanted silicon (antimony precipitated)[11], again showing that the chemical species in the extended defects play a minor role. Since the strain can vary locally around the damaged region so will the energy of the electron-hole recombination energy, giving a very large halfwidth. This model is consistent with a linear dependence of the PL band vs'. laser intensity observed at lower power levels i7i. In this model more damage will cause more strain and PL bands at lower photon energies, which explains the PL band shifts to higher energies during the isochronal annealing when the strain in the damaged near-surface region relaxes, in addition to the observed energy shift between the "(l.90 eV" and "(t.92 eV" bands in the RIE and PE samples. The broad PL bands are only observed within the first half-micrometer, where SIMS analysis has shown that the deuterium concentration is higher than 10 ~7 cm 3 i l I. Since hydrogen (deuterium) is known to passivate dopants and other electrically active defects, the near-surface region will be passivated after the plasma treatments, including the boron atoms (about 10 ~5 cm ~). This will enhance the luminescence efficiency for the IQW recombination channel, and decrease the efficiency for other competing radiative in addition to non-radiative recombination channels. This is also the reason why the usual bombardment-induced excitonic lines, e.g. the Gand C-lines, are not observed in hydrogencontaining plasmas [4~. The B(BE) recombination observed in the spectra must arise from deeper regions (greater than 3 #m) where the passivation is negligible. Other recombination models suggested for the broad PL bands, including freeto-bound (FB), donor acceptor pair (DAP) and excitonic recombinations I4-61, should therefore also be ruled out.
465
The observed shifts of up to about 4 meV of the B(BE) line (Fig. 4) we attribute to the inhomogeneous long-range elastic strain fields from the damaged regions, due to deuterium-induced complexes and precipitates, that reaches the region of the unpassivated boron atoms (greater than 3/~m). In conclusion, the shape and energy positions of the broad PL bands observed after plasma etching are shown to be not unique, but related to the exact conditions of the etching treatment. The origin of the broad PL bands is suggested to be due to electron-hole recombination in straininduced potential wells surrounding the microscopic hydrogen defects.
Acknowledgment We want to thank Professor B. Monemar for useful discussions and interest in this work.
References 1 S. Jeng, G. S. Oehrlein and G. J. Scilla, Appl. Phys. Lett., 53(1988) 1735. 2 J. 1. Pankove, D. E. Carlson, J. E. Berkeyheiser and R. O. Wance, Phys. Rev. Lett., 51 (1983) 2224. 3 N. M. Johnson, F. A. Ponce, R, A. Street and R. J. Nemanich, l'hys. Rev. B. J5 (1987) 4166. 4 G. A. Northrop and G S. Oehrlein, in H. J. yon Bardeleben (ed.), l'roc. 14th Int. ('onJl on l)~li'cts in 5emiconductmw, Paris, t)ame, August 19k¢C~.Mater. Sci. For, m I0 /2(1986)1253. 5 M. Singh, J. Weber, T. Zundel, M. Konuma and H. Cerva, in G. Ferenczi, l'roc. 15th Int. (on/i on 1)elbcts in Semiconductors, Budapest, [hmgarv, August 1988, Mater. Sci. Forum, 3b'-41 (I 989) 1033. 6 I.-W. Wu, R. A. Slreet and J. C. Mikkelsen, Jr., ,I. Appl. l'hys., 63(1988) 1628. 7 H. Weman, J. L. Lindstr6m, G. S. Oehrlein and B. G. Svensson, unpublished. 8 G. Davies, E. C. Lightowlers and Z. E. Ciechanowska, J. l'hys. C, 20(1987) 191. 9 A. Henry, J. L. Pautrat, P. Vendange and K. Saminadayar, Appl. t'hys. Lett., 49 (1986) 1266. 10 H. Weman, B. Monemar and P. O. Holtz, Appl. I'hvs. Lett., 47(1985) 1110. 1 I J. Wagner, J. C. Gelpey and R. T. Hodgson, Appl. I'hvs. Lett., 45 (1984) 47.
461
Photoluminescence of Defects Introduced by Deuterium Plasmas in Silicon H. WEMAN Department o[ Physics and Measurement Technology, Link@ing Univers'ity, S-581 &?Link@ing (Sweden) J. L. LINDSTROM Swedish l)~JbnceResearch Establishrnent, P.O. Box 1165, S-581 l l, Link@ing (5'weden) G. S. OEHRLEIN IBM ThomasJ. WatxonResearch (enter, p.o. Box 218, Yorktoan tleights, NY 10595'(U.S.A.) (Received May 31), 1989)
Abstract Dejects" introduced by reactive-ion etching (R1E) and plasma etching (PE) using deuterium have been studied in boron-doped silicon with the photoluminescence (PL) technique. We have observed a set of broad luminescence bands" in the beh)w-bandgap range between 1.05 and 0.8 eV. These bands change in intensity as well as in photon energy with annealing. We attribute all these PL bands" to electron-hole recombination in heavily damaged regions, where electrons and holes can be localized in potential wells caused by the strain surrounding the microscopic hydrogen defects.
by TEM [1], as well as from electrical measurements showing the passivation effect of hydrogen on the dopants and other defects [2]. Low-temperature photoluminescence (PL) studies have shown that the damaged region gives rise to several luminescent lines and broad bands below the bandgap [3-6]. The purpose of this paper is to show the effect on these broad PL bands of different parameters in the dry etching process with deuterium as a plasma (RIE vs, PE, chamber pressure in RIE) and also to study the recovery effect a subsequent annealing has on the damage caused by the etching. A tentative model to explain the origin of the broad PL bands will also be given.
1. Introduction
2. Experimental conditions
Dry etching processes are of technological importance in the fabrication of integrated circuits. This will expose the semiconductor surface to bombardment by ions with energies ranging from tens of electronvolts to several hundreds of electronvolts from a plasma that often contains hydrogen. From transmission electron microscopy (TEM) measurements it is known that the effect of low energy bombardment on silicon by hydrogen and deuterium ions is a highly damaged semiconductor near the surface, originating both from the introduction of lattice damage by the high-energetic ions and from the incorporation of hydrogen. This has been documented in several papers, showing complex defects such as for example the {111} and {100} platelets and gas bubbles in heavily damaged regions as observed
Boron-doped, (100) oriented, 10-20 V2 cm, Czochralski zone (CZ)-silicon wafers have been used in this study. The wafers were placed on the water-cooled bottom electrode of a diode reactor, where the top and the bottom electrodes can be independently powered, thereby changing the experimental conditions from RIE to PE. For the deuterium plasma D2, the conditions were as follows: 13.56 MHz r.f. power of 200 W applied to the bottom electrode for RIE, and to the top electrode for the PE. Deuterium was used instead of hydrogen to enable a secondary ion mass spectrometry (SIMS) profiling on the same samples in an earlier study [1]. The deuterium pressure was varied from 2.6 to 250 mTorr in the case of RIE and was at 25 mTorr during the PE treatment with an exposure time of 10 rain.
0921-5107/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
462
Photoluminescence measurements were done at temperatures from 2 K to about 20 K, excited with the 6471 A line of a Kr + ion laser. The luminescence was dispersed with a SPEX 1404 0.85 m double grating monochromator with two 600 grooves mm-~ gratings blazed at 1.6/~m. For the detection a liquid nitrogen cooled North Coast EO-817 S germanium detector with a conventional lock-in technique was used. The excitation power on the sample was around 100 mW and the luminescence was detected in backscattering geometry.
3. Experimental results Typical PL spectra at 2 K directly after the RIE and PE treatments at 25 mTorr pressure are shown in Figs. l(a) and l(b) respectively. In the near-bandgap region the boron bound exciton (B(BE)) is observed with the no phonon recombination BNp at 1.151 eV, and the different phonon replicas, transverse acoustical BTA at 1.132 eV, transverse optical Bxo at 1.093 eV, two phonon B v o + O r at 1.021 eV (where O r is the zone centre optical phonon) and the two-hole transition Bvo + Bh* at 1.060 eV, as labelled in Fig. 1. The dominating PL line is the TO phonon replica of the B(BE) at 1.093 eV. The broad structure below 1 eV is due to the etching treatment of the samples. (The line structure around 0.9 eV is due to water-absorption of the broad PL band.) The broad band after the RIE treatment (Fig. l(a)) peaks at about 0.9 eV with a halfwidth of 100 meV (FWHM), while after the PE treat-
WAVELENGTH (pm) 1.7 1.6 1.5 1.4. 1.3 1.2 Si:B ~
T=2K
WAVELENGTH (pm) 1.1 1.7 1.5 1.3
z FZ
1.1
W
o z W o co W z
BT~
Z
ment (Fig. l(b)) the band peaks at about 0.92 e V with a halfwidth of 75 me"/. We have performed an isochronal annealing of the etched samples from 75 to 525 °C in steps of 50 °C each for 30 min, in addition to one treatment at 800 °C for 30 min. The evolution of the broad PL bands after some selected annealing treatments is shown for the RIE samples in Fig. 2. Up to 275 °C the intensity of the broad band increases relative to the B(BE) luminescence. From 325 °C and higher temperatures the peak position shifts to higher energies and the halfwidth decreases. For example at 375 °C the broad structure seems to be composed of three bands, with the main peak at about 0.95 eV with a halfwidth of about 50 meV, a shoulder below 0.9 eV and a band at about 1.(13 eV superimposed with the B r o + O r replica. Above 425 °C the broad bands decrease in intensity and have almost disappeared at 525 °C. However, the sample has not completely recovered after the annealing step at 800°C, where a broad band at 0.96 eV still remains.
BTO+Or I
_J
0F© 33
O 25rnTorr~,~.90eV"~,..~ w
13-
~ (a) RIE, ~ .
"1-
o..
~
i
)~ ~,
I
i
J
,
0.8 0.9 1.0 1.1 PHOTON ENERGY (eV)
,
Fig. 1. PL spectra at 2 K of CZ-silicon after R I E (a) a n d PE (b) t r e a t m e n t at 25 m T o r r pressure. (The different notations are explained in the text.)
0.8 0.9 1.0 1.1 PHOTON ENERGY (eV) Fig. 2. PL spectra at 2 K of CZ-silicon after RIE treatment at 25 m T o r r and a s u b s e q u e n t isochronal annealing for 30 min at 225 °C (a), 275 °C (b), 325 °C (c), 375 °C (d), 425 °C (e), 525 °C (f) and 800 °C (g).
463
Apart from the broad PL bands, in spectra taken at somewhat higher temperatures (greater than 10 K), we have observed the so-called "W"line at 1.018 eV in PIE samples annealed for 30 min between 175 and 425 °C [7]. This defect is trigonal and is suggested to be related to interstitials rather than vacancies [8]. The effect of different chamber pressures during RIE has been investigated and is shown in Fig. 3. The chamber pressure during the etching was at 2.6 mTorr in Fig. 3(a), 25 mTorr in Fig. 3(b) and 250 mTorr in Fig. 3(c), whereafter the samples were annealed at 425 °C for 30 min before the PL measurements. The general shape of the broad PL bands are quite similar; however, the relative intensity of the broad PL bands increase with decreasing chamber pressure. We have observed a shift in the energy position of the B(BE) line owing to the plasma treatment. In Fig. 4 PL spectra in the TO phonon replica region of the B(BE) (unperturbed position 1.0926 eV) with bound multiexciton lines bj (unperturbed position 1.0904 eV) and be (unperturbed position 1.0881 eV) and the electron hole droplet (EHD) emission (unperturbed position 1.083 eV) are shown after various PE treatments. The spectra were taken with the same laser excitation intensity, with a laser spot size of about 1 mm in diameter. The shifts varied after the different annealing steps and were dependent on the spatial position of the laser on the sample. In general the shifts were to the low energy side, with maxi-
WAVELENGTH (p.m) %- .71.6 1.5 1.4 1.3 1.2 'Si:IBT=2K ' ' /l~'B'to+OF~r~ Z L!J I-Z LU O Z LLI O CO IJJ Z
RIE at X mTorr~
+425~0, ,C
n
.[
\
Ill/
Will
fBTABh=.~
mum shifts of about 4 meV. The shifts are usually large directly after the dry etching and during the post annealing up to about 525°C, and quite small after the 800°C annealing (Fig. 4(c)). These shifts were larger in the PE samples than in the RIE samples, and larger at higher pressure (250 mTorr PIE) than at lower pressure (2.6 mTorr PIE). All PL bands caused by the dry etching disappeared after a chemical etch removing about 0.5/~m of the surface.
4. Discussion and conclusions
There seems to be no direct similarity in the shape and energy positions of the generally broad PL bands observed after plasma hydrogenation of silicon in the previous studies [3-5], in comparison to this study. Many parameters are of course different, e.g. the power and pressure of the microwave plasma, the concentration of hydrogen in the plasma, the etching time, in addition to the starting material (float-zone or Czochralski). However it seems to be conclusive that the plasma discharge species (hydrogen, hydrogen+ deuterium, helium) have no or very little influence on the shape of the broad PL bands, i.e. the radiative recombination is mainly caused by the introduction of extended defects from clustering of atoms [5]. A comparison of the effect of PIE (Fig. l(a)) vs. PE (Fig. l(b)) at the same chamber pressure
G"
EHDTO--;2~ O brl lo I aTo
PE +30 min at:
l/
Z L!J I-Z LU 0 Z LLI (D O9 III Z
3
oj o -.r
o
tO
o_
13.. [
0.8 0.9 1.0 PHOTON ENERGY (eV)
Fig. 3. PL spectra at 2 K of CZ-silicon after RIE treatment at various chamber pressures X including a post-annealing at 425 °C for 30 min. The pressure was 2.6 mTorr in (a), 25 mTorr in (b) and 25(I mTorr in (c).
i
1.080
1.085
i
1.090
1.095
PHOTON ENERGY(eV} Fig. 4. PL spectra at 2 K of the 1"O phonon replica region of the boron bound exciton (B(BE)) and the electron hole droplet (EHD) after PE treatment and annealing [k)r 30 min at 225 °C (a), 375 °C (b) and 800 °C (c).
464
(25 mTorr) shows that the relative intensity of the PL bands are larger, the energy position is lower (about 20 meV) and the halfwidth is larger (about 25 meV) after the PIE treatment. This can be compared with the TEM pictures on the same samples in Figs. l(a) and l(b) in ref. 1, where the PIE treatment shows a heavily damaged top 15 nm layer with some {111 }planar defects while the PE sample shows a less damaged surface layer extending to about 5 rim. The maximum ion energy during the PIE treatment is about 0.5 keV, while during the PE treatment it is only about 30 eV [111. The isochronal annealing of the PIE samples (Fig. 2) shows that there are no distinct energy positions of the broad PL bandsl They gradually evolve to higher energies with somewhat different shapes and have almost disappeared at around 500 °C. This is connected to the recovery of the heavily damaged near-surface layer by the annealing, leaving some extended defects inside the silicon crystal, probably related to the 0.96 eV band observed after the 800°C annealing (Fig. 2(g)). The increase in the relative intensity of the broad PL bands with decreasing chamber pressures after the PIE treatment (Fig. 3) must be due to the difference in ion bombardment energy. The maximum ion energy is inversely proportional to the gas pressure and is about 204, 19.6 and 0.96 eV at 2.6, 25 and 250 mTorr, respectively. That the general shape of the broad PL bands is similar in Fig. 3 is consistent with the previous study by Jeng et al., where the T E M pictures showed very similar etched-induced damage after PIE treatments at different pressures [ 1 ]. We attribute these broad PL bands to electron-hole recombination in heavily damaged regions where electrons and holes can be localized in potential wells caused by the strain of the microscopic defects. This strain can cause a bending of the conduction and valence band so that an effective band gap narrowing occurs (Singh et al. have estimated a band gap narrowing of about 130 meV in their plasma etched samples from the observed shift of the acceptor (donor) bound exciton [5]). This type of strain-induced "intrinsic quantum well" (IQW) is similar to the charge-induced IQW that has been discussed previously as an explanation of some electron traps observed in oxygen precipitated CZ-silicon f9]. However, one important difference is that the
strain-induced IQW model carl accumulate both electrons and holes locally in quantum levels surrounding the precipitates, while the chargeinduced IQW model [911only accumulates carriers of one type. It therefore needs deep traps of the opposite type for recombination to occur in the energy range observed for the broad PL bands. which is not very likely in a hydrogen passivated surface region. Actually the broad PL bands observed here are very reminiscent of broad P1, bands observed in earlier work on long time annealed (oxygen precipitated) CZ-silicon [101 and annealed antimony implanted silicon (antimony precipitated)[11], again showing that the chemical species in the extended defects play a minor role. Since the strain can vary locally around the damaged region so will the energy of the electron-hole recombination energy, giving a very large halfwidth. This model is consistent with a linear dependence of the PL band vs'. laser intensity observed at lower power levels i7i. In this model more damage will cause more strain and PL bands at lower photon energies, which explains the PL band shifts to higher energies during the isochronal annealing when the strain in the damaged near-surface region relaxes, in addition to the observed energy shift between the "(l.90 eV" and "(t.92 eV" bands in the RIE and PE samples. The broad PL bands are only observed within the first half-micrometer, where SIMS analysis has shown that the deuterium concentration is higher than 10 ~7 cm 3 i l I. Since hydrogen (deuterium) is known to passivate dopants and other electrically active defects, the near-surface region will be passivated after the plasma treatments, including the boron atoms (about 10 ~5 cm ~). This will enhance the luminescence efficiency for the IQW recombination channel, and decrease the efficiency for other competing radiative in addition to non-radiative recombination channels. This is also the reason why the usual bombardment-induced excitonic lines, e.g. the Gand C-lines, are not observed in hydrogencontaining plasmas [4~. The B(BE) recombination observed in the spectra must arise from deeper regions (greater than 3 #m) where the passivation is negligible. Other recombination models suggested for the broad PL bands, including freeto-bound (FB), donor acceptor pair (DAP) and excitonic recombinations I4-61, should therefore also be ruled out.
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The observed shifts of up to about 4 meV of the B(BE) line (Fig. 4) we attribute to the inhomogeneous long-range elastic strain fields from the damaged regions, due to deuterium-induced complexes and precipitates, that reaches the region of the unpassivated boron atoms (greater than 3/~m). In conclusion, the shape and energy positions of the broad PL bands observed after plasma etching are shown to be not unique, but related to the exact conditions of the etching treatment. The origin of the broad PL bands is suggested to be due to electron-hole recombination in straininduced potential wells surrounding the microscopic hydrogen defects.
Acknowledgment We want to thank Professor B. Monemar for useful discussions and interest in this work.
References 1 S. Jeng, G. S. Oehrlein and G. J. Scilla, Appl. Phys. Lett., 53(1988) 1735. 2 J. 1. Pankove, D. E. Carlson, J. E. Berkeyheiser and R. O. Wance, Phys. Rev. Lett., 51 (1983) 2224. 3 N. M. Johnson, F. A. Ponce, R, A. Street and R. J. Nemanich, l'hys. Rev. B. J5 (1987) 4166. 4 G. A. Northrop and G S. Oehrlein, in H. J. yon Bardeleben (ed.), l'roc. 14th Int. ('onJl on l)~li'cts in 5emiconductmw, Paris, t)ame, August 19k¢C~.Mater. Sci. For, m I0 /2(1986)1253. 5 M. Singh, J. Weber, T. Zundel, M. Konuma and H. Cerva, in G. Ferenczi, l'roc. 15th Int. (on/i on 1)elbcts in Semiconductors, Budapest, [hmgarv, August 1988, Mater. Sci. Forum, 3b'-41 (I 989) 1033. 6 I.-W. Wu, R. A. Slreet and J. C. Mikkelsen, Jr., ,I. Appl. l'hys., 63(1988) 1628. 7 H. Weman, J. L. Lindstr6m, G. S. Oehrlein and B. G. Svensson, unpublished. 8 G. Davies, E. C. Lightowlers and Z. E. Ciechanowska, J. l'hys. C, 20(1987) 191. 9 A. Henry, J. L. Pautrat, P. Vendange and K. Saminadayar, Appl. t'hys. Lett., 49 (1986) 1266. 10 H. Weman, B. Monemar and P. O. Holtz, Appl. I'hvs. Lett., 47(1985) 1110. 1 I J. Wagner, J. C. Gelpey and R. T. Hodgson, Appl. I'hvs. Lett., 45 (1984) 47.