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Resonant excitation of interfacial Si–O: possibility of nonthermal processing
Microelectronic Engineering 59 (2001) 291–294 www.elsevier.com / locate / mee
Resonant excitation of interfacial Si–O: possibility of nonthermal processing J. Diener*, D. Kovalev, F. Koch ¨ Munchen ¨ , James-Franck Str. 1, 85747 Garching, Germany Physik-Department E16, Technische Universitat Abstract Studying the heating of oxidized porous Si excited by IR-pulses from a CO 2 laser we observe a time delay of a fraction of a ms for some infrared frequencies. For 1030 cm 21 the transfer of energy is nearly instantaneous. We discuss the implications of these results for the nonthermal processing. The surface Si–O layer can be resonantly excited above the temperature of the adjoining Si. Specific vibration modes of the surface layer (1076 cm 21 , 1030 cm 21 ) give sharp resonances in the light generation. 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Heat is conventionally used to grow surface oxides on Si, to anneal surface-related structural defects and activate chemical processes at the Si–SiO 2 interface. In view of unwanted side effects, such as diffusion of impurity distributions near the surface, thermal processing must be done deliberately and with great care. Effort is devoted to develop methods of rapid thermal processing and in general to limit the time–temperature product (thermal budget) in order to keep the undesired effects of heating to a minimum. In this connection we search for ways to optically excite selected vibrational modes of the surface layer without heating the underlying silicon structures. In previous work on oxidized porous Si we have come across an effect that merits attention with the perspective of nonthermal processing in mind [1]. There has recently appeared a paper by other authors that relates to our effect and also begs for more detailed understanding [2].
2. Experiments on oxidized porous silicon The experiments involve a pulsed CO 2 -laser with grating–reflector tuned laser lines in the 922–1090 cm 21 range. These excite specific modes of the Si–O polar stretching vibrations. The * Corresponding author. Tel.: 149-89-2891-2336; fax: 149-89-2891-2317. E-mail address: [email protected] (J. Diener). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00612-8
2001 Elsevier Science B.V. All rights reserved.
292
J. Diener et al. / Microelectronic Engineering 59 (2001) 291 – 294
sample is oxidized porous Si with its huge internal surface and nanometer sized Si core structures. The freshly prepared porous Si is initially covered by hydride. Storing such samples in air causes them to acquire a natural oxide on the surface. In order to achieve the desired degree of surface oxidation we have tempered the porous Si at 2008C in air for various lengths of time. Fig. 1 shows the effect of oxidation on the infrared vibration spectrum. The Si–O stretching vibration band (1000– 1250 cm 21 ) grows with time. Along with this increase, there occur distinct changes in the Si–H vibrations. The Si–H stretching mode near 2100 cm 21 decreases in strength. In its place, and anticorrelated in intensity, appears a band at |2260 cm 21 . The latter is known as the Si–H stretching mode when back-bonded oxygen is present. Analysis of the relative intensities of the vibrations shows that the oxide coverage for such samples ranges up to a monolayer. Hydrogen is present in about equal concentration. The surface is an oxyhydride of Si. The exact chemical composition is relevant for the sharp resonances to be discussed later. From Fig. 1 it is clear that the absorption of energy from a CO 2 laser pulse that falls into the spectral range of the Si–O stretch vibration band is via the surface oxide. Since we are dealing with | 3 nm particles and submonolayer oxide coverage the surface represents something like 1 / 3 of the number of atoms. It will have about 1 / 3 of the specific heat of the total particle. We have devised a measurement of the temperature rise DT that takes place as a function of time when the IR laser pulse is applied. The experiment makes use of the T-dependent absorption of He–Ne laser light (1.96 eV) in porous Si [3]. In this way we can quantitatively follow the temperature as it evolves with time. The CO 2 -laser pulse has a complicated time structure. It peaks sharply during the first 0.2 ms followed by a tail that extends over nearly 2 ms. We record in Fig. 2 the temperature rise DT(t) for laser pulses at 1076 cm 21 and 1030 cm 21 . Intensities have been adjusted to account for differences in the absorbance (Fig. 1). Why these particular lines of the laser have been chosen will become evident below. The final DT is identical but there is a distinct difference in the variation with time. There is a delay of a fraction of a ms for the 1076 cm 21 excitation. While the fast 1030 cm 21 signal approximately follows the integrated intensity of the laser, the slower response lags by a few tenths of a ms. The time delay was unexpected and implies that the energy is accumulated in the surface layer before it equilibrates in the nm-sized particle. The magnitude of DT increases linearly with the input power as in the insert of Fig. 2. The choice of the two IR modes was motivated by work on the light emission mechanism in porous
Fig. 1. Infrared transmission spectra typical of porous Si samples used in these experiments. Emission lines of the CO 2 laser overlap with the Si–O stretching vibration band.
J. Diener et al. / Microelectronic Engineering 59 (2001) 291 – 294
293
Fig. 2. Heating of the porous Si (DT ) versus time for the IR pulse at two different spectral energies, 1076 cm 21 and 1030 cm 21 ). The insert shows the magnitude of DT increasing linearly with the input power.
Si. The high power IR-pulses cause the porous Si-layer to luminescence but in a manner that is very different from the standard photoexcited luminescence peak. The spectrum is a featureless linear decay on an exponential scale. The intensity is strongly nonlinear with the energy of the laser pulse [1]. Even more dramatic is the spectral characteristic of the intensity of the visible light emission versus the frequency of the IR-laser irradiation. Fig. 3 shows the light emission at 1.55 eV as the excitation is tuned line by line through the accessible range with a constant pulse energy. Two sharp maxima emerge in this figure at the lines previously mentioned in the time response experiments. Their half-widths are only a few cm 21 . The peak at 1076 cm 21 is much stronger than the 1030 cm 21 maximum.
3. Discussion: Implications for nonthermal processing The time-delayed temperature rise found for the 1076 cm 21 line means that the surface vibration is excited to a characteristic energy above the Si particle temperature. For the estimated ratio of specific heats of the surface and the whole nm-sized particle this could be many times the measured few
Fig. 3. Visible light intensity at 1.55 eV versus the wavenumber of the excitation within the Si–O absorption band.
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J. Diener et al. / Microelectronic Engineering 59 (2001) 291 – 294
hundred 8C of the porous Si-layer. The sharp spectral response in Fig. 3 shows that 1076 cm 21 mode is essentially decoupled from the neighbouring vibrations. It appears like a local mode of the oxyhydride surface. Experiments reported in [1] showed that the sharp spectral response exists only in the monolayer coverage range. It disappears when the sample is heavily oxidized and the hydrogen vibration bands in Fig. 1 no longer exist. The strong visible luminescence is understandable as arising from a highly excited molecular species weakly coupled to both the surrounding surface layer and the Si substrate (i.e. the nanoparticle). This is an ideal situation for nonthermal processing. Energy is available in a specific surface vibration without heating the remainder of the structure. Experiments have shown that the time delay also persists outside the 1076 cm 21 mode peak and over most of the spectral range with the exception of a band of frequencies around 1030 cm 21 . It appears that the fast energy transfer near 1030 cm 21 is the result of special physical processes that apply at this frequency and make possible the energy exchange between Si and its surface layer. We may expect a two-phonon process which peaks at twice the transverse optical phonon frequency of 521 cm 21 . In nanoparticles the phonon energy is reduced to make an even better match to the experiment. To explain the visible emission peak at 1030 cm 21 requires that the vibration does not strongly couple to other surface modes. Compared to the maximum at 1076 cm 21 its strength is weaker because of the energy transfer to the core of the Si particle. In the introduction we had cited experiments [2] in which the influence of visible illumination on the surface vibrations was studied in a time resolved manner. The authors find vibration modes near those examined here that react to the presence of the extra energy supplied by pulses of visible light energy. This is in a sense the inverse of the present experiments. We conclude that pulsed IR excitation of Si–O vibrations is possible such that a surface layer on Si is ‘hot’ for times of ms. It is necessary to avoid efficient energy transfer to bulk Si by the proper choice of the IR frequency. Since the infrared radiation penetrates into the Si it could be used for local heating of oxides buried inside a Si substrate (bonded wafers for example). References [1] J. Diener, M. Ben-Chorin, D. Kovalev, S. Ganichev, F. Koch, Phys. Rev. B 52 (1995) R8617. [2] J. Wang, L. Song, B. Zou, M. El-Sayed, Phys. Rev. B 59 (1999) 5026. [3] D. Kovalev, G. Polisski, M. Ben-Chorin, J. Diener, F. Koch, J. Appl. Phys. 80 (1996) 5978.
Resonant excitation of interfacial Si–O: possibility of nonthermal processing J. Diener*, D. Kovalev, F. Koch ¨ Munchen ¨ , James-Franck Str. 1, 85747 Garching, Germany Physik-Department E16, Technische Universitat Abstract Studying the heating of oxidized porous Si excited by IR-pulses from a CO 2 laser we observe a time delay of a fraction of a ms for some infrared frequencies. For 1030 cm 21 the transfer of energy is nearly instantaneous. We discuss the implications of these results for the nonthermal processing. The surface Si–O layer can be resonantly excited above the temperature of the adjoining Si. Specific vibration modes of the surface layer (1076 cm 21 , 1030 cm 21 ) give sharp resonances in the light generation. 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Heat is conventionally used to grow surface oxides on Si, to anneal surface-related structural defects and activate chemical processes at the Si–SiO 2 interface. In view of unwanted side effects, such as diffusion of impurity distributions near the surface, thermal processing must be done deliberately and with great care. Effort is devoted to develop methods of rapid thermal processing and in general to limit the time–temperature product (thermal budget) in order to keep the undesired effects of heating to a minimum. In this connection we search for ways to optically excite selected vibrational modes of the surface layer without heating the underlying silicon structures. In previous work on oxidized porous Si we have come across an effect that merits attention with the perspective of nonthermal processing in mind [1]. There has recently appeared a paper by other authors that relates to our effect and also begs for more detailed understanding [2].
2. Experiments on oxidized porous silicon The experiments involve a pulsed CO 2 -laser with grating–reflector tuned laser lines in the 922–1090 cm 21 range. These excite specific modes of the Si–O polar stretching vibrations. The * Corresponding author. Tel.: 149-89-2891-2336; fax: 149-89-2891-2317. E-mail address: [email protected] (J. Diener). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00612-8
2001 Elsevier Science B.V. All rights reserved.
292
J. Diener et al. / Microelectronic Engineering 59 (2001) 291 – 294
sample is oxidized porous Si with its huge internal surface and nanometer sized Si core structures. The freshly prepared porous Si is initially covered by hydride. Storing such samples in air causes them to acquire a natural oxide on the surface. In order to achieve the desired degree of surface oxidation we have tempered the porous Si at 2008C in air for various lengths of time. Fig. 1 shows the effect of oxidation on the infrared vibration spectrum. The Si–O stretching vibration band (1000– 1250 cm 21 ) grows with time. Along with this increase, there occur distinct changes in the Si–H vibrations. The Si–H stretching mode near 2100 cm 21 decreases in strength. In its place, and anticorrelated in intensity, appears a band at |2260 cm 21 . The latter is known as the Si–H stretching mode when back-bonded oxygen is present. Analysis of the relative intensities of the vibrations shows that the oxide coverage for such samples ranges up to a monolayer. Hydrogen is present in about equal concentration. The surface is an oxyhydride of Si. The exact chemical composition is relevant for the sharp resonances to be discussed later. From Fig. 1 it is clear that the absorption of energy from a CO 2 laser pulse that falls into the spectral range of the Si–O stretch vibration band is via the surface oxide. Since we are dealing with | 3 nm particles and submonolayer oxide coverage the surface represents something like 1 / 3 of the number of atoms. It will have about 1 / 3 of the specific heat of the total particle. We have devised a measurement of the temperature rise DT that takes place as a function of time when the IR laser pulse is applied. The experiment makes use of the T-dependent absorption of He–Ne laser light (1.96 eV) in porous Si [3]. In this way we can quantitatively follow the temperature as it evolves with time. The CO 2 -laser pulse has a complicated time structure. It peaks sharply during the first 0.2 ms followed by a tail that extends over nearly 2 ms. We record in Fig. 2 the temperature rise DT(t) for laser pulses at 1076 cm 21 and 1030 cm 21 . Intensities have been adjusted to account for differences in the absorbance (Fig. 1). Why these particular lines of the laser have been chosen will become evident below. The final DT is identical but there is a distinct difference in the variation with time. There is a delay of a fraction of a ms for the 1076 cm 21 excitation. While the fast 1030 cm 21 signal approximately follows the integrated intensity of the laser, the slower response lags by a few tenths of a ms. The time delay was unexpected and implies that the energy is accumulated in the surface layer before it equilibrates in the nm-sized particle. The magnitude of DT increases linearly with the input power as in the insert of Fig. 2. The choice of the two IR modes was motivated by work on the light emission mechanism in porous
Fig. 1. Infrared transmission spectra typical of porous Si samples used in these experiments. Emission lines of the CO 2 laser overlap with the Si–O stretching vibration band.
J. Diener et al. / Microelectronic Engineering 59 (2001) 291 – 294
293
Fig. 2. Heating of the porous Si (DT ) versus time for the IR pulse at two different spectral energies, 1076 cm 21 and 1030 cm 21 ). The insert shows the magnitude of DT increasing linearly with the input power.
Si. The high power IR-pulses cause the porous Si-layer to luminescence but in a manner that is very different from the standard photoexcited luminescence peak. The spectrum is a featureless linear decay on an exponential scale. The intensity is strongly nonlinear with the energy of the laser pulse [1]. Even more dramatic is the spectral characteristic of the intensity of the visible light emission versus the frequency of the IR-laser irradiation. Fig. 3 shows the light emission at 1.55 eV as the excitation is tuned line by line through the accessible range with a constant pulse energy. Two sharp maxima emerge in this figure at the lines previously mentioned in the time response experiments. Their half-widths are only a few cm 21 . The peak at 1076 cm 21 is much stronger than the 1030 cm 21 maximum.
3. Discussion: Implications for nonthermal processing The time-delayed temperature rise found for the 1076 cm 21 line means that the surface vibration is excited to a characteristic energy above the Si particle temperature. For the estimated ratio of specific heats of the surface and the whole nm-sized particle this could be many times the measured few
Fig. 3. Visible light intensity at 1.55 eV versus the wavenumber of the excitation within the Si–O absorption band.
294
J. Diener et al. / Microelectronic Engineering 59 (2001) 291 – 294
hundred 8C of the porous Si-layer. The sharp spectral response in Fig. 3 shows that 1076 cm 21 mode is essentially decoupled from the neighbouring vibrations. It appears like a local mode of the oxyhydride surface. Experiments reported in [1] showed that the sharp spectral response exists only in the monolayer coverage range. It disappears when the sample is heavily oxidized and the hydrogen vibration bands in Fig. 1 no longer exist. The strong visible luminescence is understandable as arising from a highly excited molecular species weakly coupled to both the surrounding surface layer and the Si substrate (i.e. the nanoparticle). This is an ideal situation for nonthermal processing. Energy is available in a specific surface vibration without heating the remainder of the structure. Experiments have shown that the time delay also persists outside the 1076 cm 21 mode peak and over most of the spectral range with the exception of a band of frequencies around 1030 cm 21 . It appears that the fast energy transfer near 1030 cm 21 is the result of special physical processes that apply at this frequency and make possible the energy exchange between Si and its surface layer. We may expect a two-phonon process which peaks at twice the transverse optical phonon frequency of 521 cm 21 . In nanoparticles the phonon energy is reduced to make an even better match to the experiment. To explain the visible emission peak at 1030 cm 21 requires that the vibration does not strongly couple to other surface modes. Compared to the maximum at 1076 cm 21 its strength is weaker because of the energy transfer to the core of the Si particle. In the introduction we had cited experiments [2] in which the influence of visible illumination on the surface vibrations was studied in a time resolved manner. The authors find vibration modes near those examined here that react to the presence of the extra energy supplied by pulses of visible light energy. This is in a sense the inverse of the present experiments. We conclude that pulsed IR excitation of Si–O vibrations is possible such that a surface layer on Si is ‘hot’ for times of ms. It is necessary to avoid efficient energy transfer to bulk Si by the proper choice of the IR frequency. Since the infrared radiation penetrates into the Si it could be used for local heating of oxides buried inside a Si substrate (bonded wafers for example). References [1] J. Diener, M. Ben-Chorin, D. Kovalev, S. Ganichev, F. Koch, Phys. Rev. B 52 (1995) R8617. [2] J. Wang, L. Song, B. Zou, M. El-Sayed, Phys. Rev. B 59 (1999) 5026. [3] D. Kovalev, G. Polisski, M. Ben-Chorin, J. Diener, F. Koch, J. Appl. Phys. 80 (1996) 5978.