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Electron exo-emission study of PECVD and thermal CVD silicon rich silicon oxide
SSC 4773
PERGAMON
Solid State Communications 111 (1999) 431–435
Electron exo-emission study of PECVD and thermal CVD silicon rich silicon oxide S. Dusane a, T. Bhave b, S. Hullavard b, S.V. Bhoraskar b,*, S. Lokhare c a
Society for Applied Microwave Electronics Engineering and Research (SAMEER), IIT Campus, Powai, Bombay 400 076, India b Department of Physics, University of Poona, Pune 411 007, India c Tata Institute of Fundamental Research, Bombay 400 005, India Accepted 6 May 1999 by R.T. Phillips
Abstract Silicon oxide films have been deposited by the PECVD and Thermal CVD methods under different deposition conditions and using different source gases. The FTIR spectra of these films reveal varying microstructure in terms of the Si–O stoichiometry for films prepared under different sets of process conditions. The room temperature photoluminescence spectra of the as deposited films show a luminescence band around 400 nm. The 400 nm band remains as it is, while the 580 nm band arises after high temperature rapid thermal anneal. These bands have been detected even earlier and are attributed to defects in the Si–O network and from silicon nano-crystals present in the SiO2 host matrix, respectively. We studied the exo-emission of electrons as a function of temperature of the as deposited and the annealed films. The exo-emission data reveal four narrow bands at different temperatures. We try to explain these on the basis of tunneling of electrons with different energies across the potential barriers present at the silicon nanoparticle–SiO2 matrix interface. q 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; A. Nanostructures; B. Chemical synthesis; E. Luminescence; E. Electron emission spectroscopies
1. Introduction Silicon nano-size particles formed and stabilized in oxide host matrix such as SiO2 form a topic of interest [1–3] due to the possibility of these being efficient emitters [4,5] of light in the visible region. This possibility opens up opportunities for such materials to be used in high-density optical data storage. Numerous papers published so far [6–7] have demonstrated that the wavelength of the emitted light depends upon the crystallite size, an effect of quantum confinement on the band gap of silicon. The advantage of this material over porous silicon is that the surrounding oxide matrix stabilizes the crystallites and so avoids degra* Corresponding author.
dation in the luminescent properties, a major obstacle for porous silicon technology advancement. In this paper we look at an important aspect of silicon nano-particles present in deposited SiO2 films pertaining to the interface between these particles and the host SiO2 matrix by using a surface sensitive gap state spectroscopic technique namely Thermally Stimulated Exo-Electron emission (TSEE). The method involves the detection of electrons de-localized from the defect states, under the influence of a thermal ramp. During the process of detection by TSEE the sample gets heated to around 2508C. The technique is essentially a surface sensitive defect related spectroscopy, since the energies of emitted electrons are very low. Thus, this technique reveals the energetics of surface defects and has been
0038-1098/99/$ - see front matter q 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00219-7
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S. Dusane et al. / Solid State Communications 111 (1999) 431–435
[10,11] and metal induced gap states in silicon. In this paper we have carried out TSEE measurements for as deposited and high temperature rapid thermal annealed SiO2 films. The results show four distinct narrow exo-emission bands at different temperatures, which increase in height after rapid thermal annealing. We think these are due to different interface defects present at the interface of Si nano-particles and SiO2 host matrix. The PL spectra clearly show room temperature luminescence arising from such nano-crystallites.
2. Experimental Fig. 1. Typical FTIR spectra of the Plasma CVD deposited SiO films with silane 1 nitrous oxide (—), silane 1 oxygen (– – –) and thermal CVD films with silane 1 oxygen (– · – · –).
effectively used for the study of hydrogenated amorphous silicon (a-Si:H) deposited by PECVD [8]. In our earlier publications we have used TSEE for studying surface states in gallium arsenide [9], passivation of the surface states in GaAs by plasma polymer
Silicon oxide films were deposited on crystalline silicon wafers and corning 7059 glass substrates by radio frequency plasma enhanced chemical vapor deposition (PECVD) as well as thermal CVD. In order to obtain silicon rich silicon-oxide films and study the role of silicon crystallites in silicon oxide matrix pertaining to luminescence and defect properties, films were deposited at process parameters, which are different from those which yield
Fig. 2. Room temperature photoluminescence spectra of different SiO samples silane 1 oxygen 1 argon (—), silane 1 nitrous oxide (– · – · –), silane 1 oxygen (– – –), and thermal CVD films with silane 1 oxygen (· · ·).
S. Dusane et al. / Solid State Communications 111 (1999) 431–435
433
Fig. 3. TSEE spectra of as deposited SiO films: silane 1 nitrous oxide (a); thermal silane 1 oxygen 1 argon (c); silane 1 oxygen (d); and thermal CVD films with silane 1 oxygen (b).
electronically good silicon-dioxide. Silicon oxide films were deposited with different silane to nitrous oxide gas ratios as well as silane (the gases were of Semiconductor purity from Matheson) to oxygen gas ratios. Silane flow was deliberately kept towards higher side. During deposition substrate temperature was held at 3008C. The power density of glow discharge was 0.16 W/cm 2, gas pressure 0.6– 0.88 Torr. Prior to deposition, the chamber was evacuated to 6 × 10 27 Torr using a turbo molecular-rotary pump combination (Alcatel, France). Silane to nitrous oxide (or oxygen) ratio was varied from 0.1 to 0.5. Another set of silicon oxide films was deposited by thermal CVD in the same chamber using the gas reaction of silane and oxygen at 4258C. It has been reported that SiO2 deposited under such conditions is structurally different from those deposited at higher substrate temperatures (,6508C). Films were characterized for electrical resistivity, refractive index (single wavelength ellipsometry), chemical composition (FTIR, Nicolet-570), photo luminescence (PL) and TSEE. The room temperature PL spectra were recorded with fluorescence photo-spectrometer with a photomultiplier tube using 250 nm as the excitation
Fig. 4. TSEE spectra of rapid thermally annealed samples: silane 1 nitrous oxide (a); silane 1 oxygen 1 argon (c); silane 1 oxygen (d); and thermal CVD films with silane 1 oxygen (b).
wavelength from a Xenon arc lamp. The TSEE experimental details are described in Ref. [8]. The films were then rapid-thermally annealed at 8508C and again characterized.
3. Results The resistivity varied from sample to sample and was within 10 8 –10 10 V cm for all the samples studied. The refractive index determined by ellipsometry was in the range of 1.5–1.6. These values of resistivity and the refractive index indicate the silicon rich nature of the films [11]. Ideally for stoichiometric SiO2 these values are in the range of 10 14 and 1.45 V cm, respectively. Fig. 1 shows the FTIR spectra of typical films deposited under different deposition conditions and using different source gas mixtures. Fig. 2 shows room temperature PL spectra with one broad peak centered at 400 nm for as deposited samples. Figs. 3 and 5 show the TSEE spectra of the as deposited and
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S. Dusane et al. / Solid State Communications 111 (1999) 431–435
Table 1 Table showing the peak temperatures and the corresponding energies of the different peaks that occur in the as deposited and annealed samples No
1 2 3 4
Peak temperature (K)
Energy (eV)
As deposited
Annealed
As deposited
Annealed
302.5 325 416 461
325 390 400 446
0.65 0.70 0.89 0.99
0.7 0.84 0.86 0.96
RTA annealed samples. The common features in the spectra for as deposited samples include sharp lines superimposed on broad spectral peaks. The nature of the spectra for all the as deposited SiO2 samples are seen to be more or less similar. However, after annealing significant differences occur. Thus, the broad peaks have disappeared and the spikes have become more prominent with a flat background. Intensity of the lines increases roughly by about two times as compared to the as deposited samples. The positions of the spikes in TSEE are related to energy of defect levels below the conduction band edge by the approximate relation (11), Ec 2 Et 25 kT, where Ec is the
Fig. 5. Room temperature PL spectra of rapid thermally annealed samples; silane 1 oxygen 1 argon (—), silane 1 nitrous oxide (· · ·), silane 1 oxygen (– – –), and thermal CVD films with silane 1 oxygen (– · – · –).
bottom of conduction Band, Et the trap level, k the Boltzmann constant and T the peak temperature. Typical energy values of defect levels calculated from this relation for the as deposited and annealed samples are given in Table 1. Fig. 4 shows room temperature PL spectra of annealed samples, which again have broad peaks around 400 and 580 nm.
4. Discussion The refractive index and the resistivity values indicate the silicon rich nature of these films [12]. From the FTIR data we see that the films made with SiH4 1 N2O gas mixture and those made from SiH4 1 O2 mixture have different IR signatures. The IR spectrum of the former shows an Si–O related absorb band at 1025 cm 21 with a shoulder at 1120 cm 21, Si–H stretching band shifted to 2250 cm 21 due to an oxygen environment, N–H absorption at 3350 cm 21, bending modes of SiH2 near 840 and 880 cm 21. One significant difference that we have observed in these films is the distinct shoulder at 1120 cm 21. Coming to IR spectra of the films from SiH4 1 O2 mixture (Plasma CVD as well as thermal CVD) we see that the band at 2250 cm 21 corresponding to Si–H vibrations is absent. Secondly, the shoulder at 1120 cm 21 reduces greatly in strength and shifts towards higher wavelength. In addition there is a broad band around 3400 cm 21, which could arise due to overlapped N– H vibrations at 3350 cm 21 and H–O–H vibrations of the water vapor at 3500 cm 21. The peak around 3500 cm 21 is completely absent in the films deposited with silane 1 nitrous oxide gas mixture. This peak in the films deposited with silane 1 oxygen gas mixture may be due to unintentional impurities. However this band is consistently present in all these types of films. The low purity oxygen gas could be a source of these impurities. An absorption band around 820 cm 21 is also present in the plasma CVD deposited film with silane 1 oxygen combination and thermal CVD samples. This band is in doublet form in thermal CVD samples. Thus different samples have a different structural and chemical microstructure. However all these films exhibit a strong room temperature PL at around 400 nm wavelength, which is as high as porous silicon. This peak is consistently seen in all the samples and also remains unaffected after RTA
S. Dusane et al. / Solid State Communications 111 (1999) 431–435
annealing. All these films also exhibit an additional room temperature PL at 580 nm after RTA anneals. Thus these films exhibit a PL behavior similar to material reported by earlier workers [13]. The results of the TSEE shown in Fig. 3 reveal more information about the surface defects of the samples. The broad TSEE peaks are representative of continuous density of defect states in the energy gap whereas the sharp spikes represent the concentration of defect levels at discrete positions in the band gap. One of the possible explanations for the observed effects in the TSEE spectra of Fig. 3 may arise from the continuous distribution of defects in silicon rich SiO2 arising from the large varieties of defects resulting from the stresses in the non-stoichiometric lattice. The discrete defects may be most probably arising from the dangling bonds of silicon at the Si/SiO2 interface. A similar feature was obtained in the TSEE spectra of porous silicon [14]. The disappearance of the background indicates that the defects related to the lattice stresses are annealed out when the samples are heated at 8508C. On the contrary, the defects arising from the interface show increase in number, in the annealed samples, on account of the larger extent of the interface of Si/SiO2 resulting from the formation of silicon clusters. The peak at 580 nm observed in the PL spectra though appearing only in the annealed films may be present even in the as deposited films but gets suppressed due to the stress related network defects in the as deposited films. 5. Conclusions TSEE is employed for the first time to study silicon rich silicon-oxide films deposited by PECVD and thermal CVD. This technique is found to be very useful to identify structural changes pertaining to defects. From the width of these peaks it appears
435
that the structural configuration of the present films is somewhere between that of amorphous silicon and porous silicon [11,13]. Presence of distinct peak like features in TSEE spectra in as deposited as well as annealed silicon oxide and presence of strong room temperature PL at around 580 nm after annealing, suggest that as deposited samples may also contain silicon nano-crystals embedded in amorphous phase of silicon oxide matrix. Such a mixed phase appears more defective from the TSEE results. In this technologically important material, rapid thermal annealing plays an important role to improve room temperature luminescence efficiency. References [1] H.Z. Song, X.M. Bao, N.S. Lee, X.L. Wu, Appl. Phys. Lett. 72 (1998) 3157. [2] A. Lan, B. Liv, X. Bai, Japn, J. Appl. Phys. 36 (1997) L1019. [3] C.H. Lin, S.C. Lee, Y.F. Chen, Appl. Phys. Lett. 63 (1993) 902. [4] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [5] N. Harron, J.C. Celabrese, W.E. Farneth, Y. Wang, Science 259 (1993) 1426. [6] M.W. Cole, J.F. Harvey, R.A. Lux, D.W. EcKart, R. Tsu, Appl. Phys. Lett. 60 (1992) 2800. [7] Z. Sui, P.P. Leong, I.P. Herman, Appl. Phys. Lett. 60 (1992) 2086. [8] R.S. Bhide, V. Manorama, S.M. Babras, S.V. Bhoraskar, V.G. Bhide, Appl. Phys. Lett. 57 (1990) 1528. [9] H. Hullavarad Shiva, S.V. Bhoraskar, J. Appl. Phys. 82 (1997) 1. [10] T.A. Railkar, R.S. Bhide, S.V. Bhoraskar, V. Manorama, V.J. Rao, J. Appl. Phys. 72 (1992) 155. [11] T.A. Railkar, S.V. Bhoraskar, Appl. Phys. Lett. 66 (1995) 974. [12] M. Shokrani, V.J. Kapoor, Plasma properties, deposition and etching, in: J.J. Ponch, S.A. Alterovitz (Eds.), Materials Science Forum, 140–142, , 1992, pp. 185. [13] J.T. Randall, M.H.F. Wilkins, R. Proc, Soc. Lond. Ser. A 184 (1995) 366. [14] S.V. Bhoraskar, Bhave Tejashree, T.A. Railkar, Bull. Mat. Sci. 17 (1994) 523.
PERGAMON
Solid State Communications 111 (1999) 431–435
Electron exo-emission study of PECVD and thermal CVD silicon rich silicon oxide S. Dusane a, T. Bhave b, S. Hullavard b, S.V. Bhoraskar b,*, S. Lokhare c a
Society for Applied Microwave Electronics Engineering and Research (SAMEER), IIT Campus, Powai, Bombay 400 076, India b Department of Physics, University of Poona, Pune 411 007, India c Tata Institute of Fundamental Research, Bombay 400 005, India Accepted 6 May 1999 by R.T. Phillips
Abstract Silicon oxide films have been deposited by the PECVD and Thermal CVD methods under different deposition conditions and using different source gases. The FTIR spectra of these films reveal varying microstructure in terms of the Si–O stoichiometry for films prepared under different sets of process conditions. The room temperature photoluminescence spectra of the as deposited films show a luminescence band around 400 nm. The 400 nm band remains as it is, while the 580 nm band arises after high temperature rapid thermal anneal. These bands have been detected even earlier and are attributed to defects in the Si–O network and from silicon nano-crystals present in the SiO2 host matrix, respectively. We studied the exo-emission of electrons as a function of temperature of the as deposited and the annealed films. The exo-emission data reveal four narrow bands at different temperatures. We try to explain these on the basis of tunneling of electrons with different energies across the potential barriers present at the silicon nanoparticle–SiO2 matrix interface. q 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: A. Thin films; A. Nanostructures; B. Chemical synthesis; E. Luminescence; E. Electron emission spectroscopies
1. Introduction Silicon nano-size particles formed and stabilized in oxide host matrix such as SiO2 form a topic of interest [1–3] due to the possibility of these being efficient emitters [4,5] of light in the visible region. This possibility opens up opportunities for such materials to be used in high-density optical data storage. Numerous papers published so far [6–7] have demonstrated that the wavelength of the emitted light depends upon the crystallite size, an effect of quantum confinement on the band gap of silicon. The advantage of this material over porous silicon is that the surrounding oxide matrix stabilizes the crystallites and so avoids degra* Corresponding author.
dation in the luminescent properties, a major obstacle for porous silicon technology advancement. In this paper we look at an important aspect of silicon nano-particles present in deposited SiO2 films pertaining to the interface between these particles and the host SiO2 matrix by using a surface sensitive gap state spectroscopic technique namely Thermally Stimulated Exo-Electron emission (TSEE). The method involves the detection of electrons de-localized from the defect states, under the influence of a thermal ramp. During the process of detection by TSEE the sample gets heated to around 2508C. The technique is essentially a surface sensitive defect related spectroscopy, since the energies of emitted electrons are very low. Thus, this technique reveals the energetics of surface defects and has been
0038-1098/99/$ - see front matter q 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00219-7
432
S. Dusane et al. / Solid State Communications 111 (1999) 431–435
[10,11] and metal induced gap states in silicon. In this paper we have carried out TSEE measurements for as deposited and high temperature rapid thermal annealed SiO2 films. The results show four distinct narrow exo-emission bands at different temperatures, which increase in height after rapid thermal annealing. We think these are due to different interface defects present at the interface of Si nano-particles and SiO2 host matrix. The PL spectra clearly show room temperature luminescence arising from such nano-crystallites.
2. Experimental Fig. 1. Typical FTIR spectra of the Plasma CVD deposited SiO films with silane 1 nitrous oxide (—), silane 1 oxygen (– – –) and thermal CVD films with silane 1 oxygen (– · – · –).
effectively used for the study of hydrogenated amorphous silicon (a-Si:H) deposited by PECVD [8]. In our earlier publications we have used TSEE for studying surface states in gallium arsenide [9], passivation of the surface states in GaAs by plasma polymer
Silicon oxide films were deposited on crystalline silicon wafers and corning 7059 glass substrates by radio frequency plasma enhanced chemical vapor deposition (PECVD) as well as thermal CVD. In order to obtain silicon rich silicon-oxide films and study the role of silicon crystallites in silicon oxide matrix pertaining to luminescence and defect properties, films were deposited at process parameters, which are different from those which yield
Fig. 2. Room temperature photoluminescence spectra of different SiO samples silane 1 oxygen 1 argon (—), silane 1 nitrous oxide (– · – · –), silane 1 oxygen (– – –), and thermal CVD films with silane 1 oxygen (· · ·).
S. Dusane et al. / Solid State Communications 111 (1999) 431–435
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Fig. 3. TSEE spectra of as deposited SiO films: silane 1 nitrous oxide (a); thermal silane 1 oxygen 1 argon (c); silane 1 oxygen (d); and thermal CVD films with silane 1 oxygen (b).
electronically good silicon-dioxide. Silicon oxide films were deposited with different silane to nitrous oxide gas ratios as well as silane (the gases were of Semiconductor purity from Matheson) to oxygen gas ratios. Silane flow was deliberately kept towards higher side. During deposition substrate temperature was held at 3008C. The power density of glow discharge was 0.16 W/cm 2, gas pressure 0.6– 0.88 Torr. Prior to deposition, the chamber was evacuated to 6 × 10 27 Torr using a turbo molecular-rotary pump combination (Alcatel, France). Silane to nitrous oxide (or oxygen) ratio was varied from 0.1 to 0.5. Another set of silicon oxide films was deposited by thermal CVD in the same chamber using the gas reaction of silane and oxygen at 4258C. It has been reported that SiO2 deposited under such conditions is structurally different from those deposited at higher substrate temperatures (,6508C). Films were characterized for electrical resistivity, refractive index (single wavelength ellipsometry), chemical composition (FTIR, Nicolet-570), photo luminescence (PL) and TSEE. The room temperature PL spectra were recorded with fluorescence photo-spectrometer with a photomultiplier tube using 250 nm as the excitation
Fig. 4. TSEE spectra of rapid thermally annealed samples: silane 1 nitrous oxide (a); silane 1 oxygen 1 argon (c); silane 1 oxygen (d); and thermal CVD films with silane 1 oxygen (b).
wavelength from a Xenon arc lamp. The TSEE experimental details are described in Ref. [8]. The films were then rapid-thermally annealed at 8508C and again characterized.
3. Results The resistivity varied from sample to sample and was within 10 8 –10 10 V cm for all the samples studied. The refractive index determined by ellipsometry was in the range of 1.5–1.6. These values of resistivity and the refractive index indicate the silicon rich nature of the films [11]. Ideally for stoichiometric SiO2 these values are in the range of 10 14 and 1.45 V cm, respectively. Fig. 1 shows the FTIR spectra of typical films deposited under different deposition conditions and using different source gas mixtures. Fig. 2 shows room temperature PL spectra with one broad peak centered at 400 nm for as deposited samples. Figs. 3 and 5 show the TSEE spectra of the as deposited and
434
S. Dusane et al. / Solid State Communications 111 (1999) 431–435
Table 1 Table showing the peak temperatures and the corresponding energies of the different peaks that occur in the as deposited and annealed samples No
1 2 3 4
Peak temperature (K)
Energy (eV)
As deposited
Annealed
As deposited
Annealed
302.5 325 416 461
325 390 400 446
0.65 0.70 0.89 0.99
0.7 0.84 0.86 0.96
RTA annealed samples. The common features in the spectra for as deposited samples include sharp lines superimposed on broad spectral peaks. The nature of the spectra for all the as deposited SiO2 samples are seen to be more or less similar. However, after annealing significant differences occur. Thus, the broad peaks have disappeared and the spikes have become more prominent with a flat background. Intensity of the lines increases roughly by about two times as compared to the as deposited samples. The positions of the spikes in TSEE are related to energy of defect levels below the conduction band edge by the approximate relation (11), Ec 2 Et 25 kT, where Ec is the
Fig. 5. Room temperature PL spectra of rapid thermally annealed samples; silane 1 oxygen 1 argon (—), silane 1 nitrous oxide (· · ·), silane 1 oxygen (– – –), and thermal CVD films with silane 1 oxygen (– · – · –).
bottom of conduction Band, Et the trap level, k the Boltzmann constant and T the peak temperature. Typical energy values of defect levels calculated from this relation for the as deposited and annealed samples are given in Table 1. Fig. 4 shows room temperature PL spectra of annealed samples, which again have broad peaks around 400 and 580 nm.
4. Discussion The refractive index and the resistivity values indicate the silicon rich nature of these films [12]. From the FTIR data we see that the films made with SiH4 1 N2O gas mixture and those made from SiH4 1 O2 mixture have different IR signatures. The IR spectrum of the former shows an Si–O related absorb band at 1025 cm 21 with a shoulder at 1120 cm 21, Si–H stretching band shifted to 2250 cm 21 due to an oxygen environment, N–H absorption at 3350 cm 21, bending modes of SiH2 near 840 and 880 cm 21. One significant difference that we have observed in these films is the distinct shoulder at 1120 cm 21. Coming to IR spectra of the films from SiH4 1 O2 mixture (Plasma CVD as well as thermal CVD) we see that the band at 2250 cm 21 corresponding to Si–H vibrations is absent. Secondly, the shoulder at 1120 cm 21 reduces greatly in strength and shifts towards higher wavelength. In addition there is a broad band around 3400 cm 21, which could arise due to overlapped N– H vibrations at 3350 cm 21 and H–O–H vibrations of the water vapor at 3500 cm 21. The peak around 3500 cm 21 is completely absent in the films deposited with silane 1 nitrous oxide gas mixture. This peak in the films deposited with silane 1 oxygen gas mixture may be due to unintentional impurities. However this band is consistently present in all these types of films. The low purity oxygen gas could be a source of these impurities. An absorption band around 820 cm 21 is also present in the plasma CVD deposited film with silane 1 oxygen combination and thermal CVD samples. This band is in doublet form in thermal CVD samples. Thus different samples have a different structural and chemical microstructure. However all these films exhibit a strong room temperature PL at around 400 nm wavelength, which is as high as porous silicon. This peak is consistently seen in all the samples and also remains unaffected after RTA
S. Dusane et al. / Solid State Communications 111 (1999) 431–435
annealing. All these films also exhibit an additional room temperature PL at 580 nm after RTA anneals. Thus these films exhibit a PL behavior similar to material reported by earlier workers [13]. The results of the TSEE shown in Fig. 3 reveal more information about the surface defects of the samples. The broad TSEE peaks are representative of continuous density of defect states in the energy gap whereas the sharp spikes represent the concentration of defect levels at discrete positions in the band gap. One of the possible explanations for the observed effects in the TSEE spectra of Fig. 3 may arise from the continuous distribution of defects in silicon rich SiO2 arising from the large varieties of defects resulting from the stresses in the non-stoichiometric lattice. The discrete defects may be most probably arising from the dangling bonds of silicon at the Si/SiO2 interface. A similar feature was obtained in the TSEE spectra of porous silicon [14]. The disappearance of the background indicates that the defects related to the lattice stresses are annealed out when the samples are heated at 8508C. On the contrary, the defects arising from the interface show increase in number, in the annealed samples, on account of the larger extent of the interface of Si/SiO2 resulting from the formation of silicon clusters. The peak at 580 nm observed in the PL spectra though appearing only in the annealed films may be present even in the as deposited films but gets suppressed due to the stress related network defects in the as deposited films. 5. Conclusions TSEE is employed for the first time to study silicon rich silicon-oxide films deposited by PECVD and thermal CVD. This technique is found to be very useful to identify structural changes pertaining to defects. From the width of these peaks it appears
435
that the structural configuration of the present films is somewhere between that of amorphous silicon and porous silicon [11,13]. Presence of distinct peak like features in TSEE spectra in as deposited as well as annealed silicon oxide and presence of strong room temperature PL at around 580 nm after annealing, suggest that as deposited samples may also contain silicon nano-crystals embedded in amorphous phase of silicon oxide matrix. Such a mixed phase appears more defective from the TSEE results. In this technologically important material, rapid thermal annealing plays an important role to improve room temperature luminescence efficiency. References [1] H.Z. Song, X.M. Bao, N.S. Lee, X.L. Wu, Appl. Phys. Lett. 72 (1998) 3157. [2] A. Lan, B. Liv, X. Bai, Japn, J. Appl. Phys. 36 (1997) L1019. [3] C.H. Lin, S.C. Lee, Y.F. Chen, Appl. Phys. Lett. 63 (1993) 902. [4] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [5] N. Harron, J.C. Celabrese, W.E. Farneth, Y. Wang, Science 259 (1993) 1426. [6] M.W. Cole, J.F. Harvey, R.A. Lux, D.W. EcKart, R. Tsu, Appl. Phys. Lett. 60 (1992) 2800. [7] Z. Sui, P.P. Leong, I.P. Herman, Appl. Phys. Lett. 60 (1992) 2086. [8] R.S. Bhide, V. Manorama, S.M. Babras, S.V. Bhoraskar, V.G. Bhide, Appl. Phys. Lett. 57 (1990) 1528. [9] H. Hullavarad Shiva, S.V. Bhoraskar, J. Appl. Phys. 82 (1997) 1. [10] T.A. Railkar, R.S. Bhide, S.V. Bhoraskar, V. Manorama, V.J. Rao, J. Appl. Phys. 72 (1992) 155. [11] T.A. Railkar, S.V. Bhoraskar, Appl. Phys. Lett. 66 (1995) 974. [12] M. Shokrani, V.J. Kapoor, Plasma properties, deposition and etching, in: J.J. Ponch, S.A. Alterovitz (Eds.), Materials Science Forum, 140–142, , 1992, pp. 185. [13] J.T. Randall, M.H.F. Wilkins, R. Proc, Soc. Lond. Ser. A 184 (1995) 366. [14] S.V. Bhoraskar, Bhave Tejashree, T.A. Railkar, Bull. Mat. Sci. 17 (1994) 523.