- Email: [email protected]
Direct observation of localized unoccupied states by synchrotron radiation two-color resonant photoemission
Applied Surface Science 212–213 (2003) 73–77
Direct observation of localized unoccupied states by synchrotron radiation two-color resonant photoemission H.L. Hsiaoa,*, A.B. Yanga, H.L. Hwangb a
b
Department of Physics, Tunghai University, Taichung 407, Taiwan Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
Abstract Observation of localized unoccupied state in luminescent amorphous silicon-rich nitride films was reported by using the synchrotron radiation two-color resonant photoemission technique. The energy separation between the localized unoccupied state and top valence band was observed to shift from 2.4 to 1.8 eV while increasing the silicon richness. This observation is in agreement with the previously reported photoluminescence red-shift results. Moreover, it is found that the localized unoccupied states are strongly correlated with the Si–N bonding configurations. Considering the microstructures (silicon platelets embedded into the amorphous silicon nitride environments) of these samples, it is believed that the localized unoccupied states would possible come from the surface region of silicon platelets. # 2003 Elsevier Science B.V. All rights reserved. PACS: 73.22.-f; 78.60.-b; 79.60.-i Keywords: Silicon-rich nitride; Two-color resonant photoemission; Photoluminescence
1. Introduction A quantum confinement of carriers in crystalline silicon wires was proposed as the origin of the luminescence in the first publication reporting strong, visible, room temperature emission from porous silicon [1]. Numerous models [2,3] have been put forward as alternative explanations for this phenomenon. Recent experimental [4–6] and theoretical results [7–9] strongly suggest that the band gap opening by quantum confinement and surface passivation, including oxygen and oxyhydride [10,11], plays an important role.
*
Corresponding author. Tel.: þ886-4-23500896; fax: þ886-4-23594643. E-mail address: [email protected] (H.L. Hsiao).
The large discrepancy between the optical gap and photoluminescence peak energies in luminescent silicon-rich nitride [17] samples demonstrates the possible existence of localized gap states. Probing and understanding the characteristics of these gap states would be helpful for further identifying the luminescence mechanism. However, direct probing these gap states was not an easy task and most of the developed techniques are concentrated on the measurement of defects. Moreover, the poor electrical conductivity of our samples limits the applications of this instrumentation to our system. Although, optical characterization can supply some information about these states, but the evidences are usually not strong enough to be accepted due to the unknown and complicated optical transitions involved in the optical characterization processes. In our case, the localized gap states might
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00371-4
74
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
be the quantum confinement induced metastable states, or weakly localized surface states, and it is very difficult to clarify them. Therefore, in this study, we present a novel technique to recognize the localized gap states.
2. Experimental Photoelectron spectroscopy has been developed over the last 30 years as the most powerful method for investigating the electronic states in solids, both in the bulk and on the surface. Photoemission coupled with the synchrotron radiation light source (an energy tunable excitation source) provides an unique and possible site-specific technique. It is especially suited for studying the occupied states. Obviously, the energy range between the Fermi level and the vacuum level is inaccessible by this method. Inverse photoemission has been applied extensively to investigate the bulk and surface states below and above the vacuum level [12–14]. An alternative to inverse photoemission is the two-photon photoemission [15,16]. In contrast to the conventional onephoton photoemission, it involves, in addition to an initial and a final state, a ‘‘real’’ intermediate state. The specially emphasized word ‘‘real’’ is used to distinguish from the ‘‘virtual’’ state in the conventional two-photon process. Therefore, it yields information about the intermediate state involved in the process. The intermediate state of a two-photon photoemission process is (n 1) electron system and an electron–hole pair. If two photons with different energy are employed, it is possible to obtain additional information. However, it is subjected to some limitations. First, the lifetime of the intermediate state has to be sufficiently long (>1012 s). Another limitation is that the intermediate states are limited to the range between Fermi level (Ef) and the vacuum level (Evac). The wide-range tunability of the synchrotron radiation source provides the extreme utility for photoemission. In addition to the conventional energy distribution curve (EDC), we can sweep the surface Nðho; TÞ along a line defined by T ho ¼ const:, where T is the final-state energy (kinetic energy) of the excited photoelectrons and ho is the synchrotron radiation photon energy. The quantity T ho, which
is the final-state energy minus the photon energy, gives the energy (measured from the vacuum level) of the initial state for primary photoelectrons of the photoemission process. The obtained curves, which are called constant-initial-state (CIS) spectra, can provide useful information about the interaction cross-section of the photon and the samples. By choosing the suitable photon energy, it is possible to enhance the signal from some specific initial electronic states and to provide element-specific and bonding-specific information. This mode of photoemission spectroscopy is called resonant photoemission spectroscopy. If the luminescence is originated from the radiation recombination of localized excitons, or trapped electron–hole pairs, there unoccupied localized state should exist in the gap. The lifetime of the luminescent state in porous silicon was extensively studied by using time-resolved technique and estimated to be approximately 1–100 ms at room temperature. The long room temperature decay time in porous Si as compared to the nanosecond radiative lifetimes observed in direct gap semiconductors suggests that the main reason for the high quantum efficiency in porous Si is not a reduction in the radiative lifetime, but rather a strong suppression of non-radiative processes. Therefore, it is possible to probe the unoccupied localized state by using ‘‘two-color resonant photoemission method’’. Here the word ‘‘two-color’’, which represents two photons with different energy, is used to distinguish from the conventional two-photon photoemission. The experiments were performed at the 6 mLSGM beamline of the Synchrotron Radiation Research Center (SRRC) in Hsinchu, Taiwan. The tunable photon energy of this beamline ranges from 20 to 200 eV. All the measurements were performed in an ultra-high vacuum (UHV) chamber system with a base pressure less than 7 1011 Torr. The photoelectrons were collected and analyzed by the VSW EA-125 hemi-spherical analyzer (HSA). Prior to the measurement, Ar-ion sputtering was carried out to remove surface contamination. The CIS data were normalized to the photon flux measured by a nickel mesh located in front of the sample. Conventionally, an aluminum foil is used to filter out the higher order photon flux above 72 eV while measuring valence band density of states. However, in our resonant
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
photoemission measurement, the Si 2p core-states were used to resonantly enhance the optical absorption. Thus, the aluminum foil must be removed from the beamline. The photon flux of the second order and the third order in the beamline were estimated to be approximately 20 and 10%, respectively. The secondand the third-order photons energies were used as an excitation source to excite the Si-related bonding core-electrons to the unoccupied intermediate state in the two-color resonant photoemission process while the primary (first-order) photons excite electrons from the intermediate state to the final state above Evac. To our knowledge, this is the only possible way to perform two-color resonant photoemission using two different synchrotron radiation photon energies. Although, it is possible to use the synchrotron and laser sources, but it is very difficult to concurrently focus the two photons, which are
75
emitted from two different sources onto the same area of the sample surface.
3. Results and discussion Fig. 1a and b show the photoemission spectrum of silicon-rich nitride thin films with G ¼ 1:2 which were excited by 150 and 50 eV photon energy. It is noted that the feature of the valence band, which contains no fine structures, exhibits amorphous characteristics. However, the slowly decreased density of states around the top valence band indicates the existence of tail states in the samples. The reported photoemission experimental and theoretical spectra [13,14] agree in that N 2s electrons do not mix with the other N and Si valence electrons and form the peak at 20 eV from Fermi level (Ef). It is also found that
Fig. 1. Photoemission spectrum of G ¼ 1:2 a-SiNx:H thin films excited by (a) 150 eV and (b) 50 eV photon energy. (c) CIS spectra.
76
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
the binding energy of Si 2p core-level is about 102.5 eV. The CIS technique was applied to identify the characters of the valence band density of state, in particular the top valence band. Based on Fig. 1b, the initial state was selected to be 7.5 eV below the top valence band (VB band) and 2.5 eV above the top valence band (UL band). The UL band was chosen according to the PL peak energy of this sample. Fig. 1c shows the CIS spectra for these two initial states. Although the CIS spectra, taken at various initial states, show significant differences, they all show prominent features. The feature in the CIS spectrum of the VB band exhibits an absorption character at about 47.5 eV. This absorption feature was believed to correlate with the resonant behavior of Si 2p corelevel. However, the CIS spectrum of UL band displays quite different characters with the conventional resonant photoemission and they were identified to be the core-localized unoccupied state transitions. This identification was based on the fact that the sharp features in the CIS spectrum could not be explained by the conventional one-photon resonant photoemission concepts. In the one-photon resonant photoemission process, the incidence of photon with the energy equal to the core-to-conduction band minimum transition would result in large photon absorption. This kind of absorption process is the same as that observed from the X-ray absorption fine structure spectroscopy. The spectra exhibit a whiteline feature followed by an absorption threshold (edge-jump). It was found that the sharp feature appeared in the CIS spectrum of the UL band is similar to the transition between the two highly localized states. Because the core-levels are localized states, the other state should be also a localized state located above the Fermi level and to provide empty states for the transition. Therefore, the only possible state must be localized and located above the Fermi level as an unoccupied localized gap state. The photoemission spectra recorded at various excitation photon energies are shown in Fig. 2. It was found that an additional peak appears located above the top of the valence band while the sample was excited by 35.0 and 52.5 eV photon energies. The 105 eV photon energy (second- and third-order line of 52.5 and 35 eV) could probably excite the Si 2p coreelectrons to the localized unoccupied states, while
Fig. 2. Photoemission spectra of G ¼ 1:2 a-SiNx:H films excited by using various photon energies.
the primary photons carrying an energy of 35 or 52.5 eV excite electrons in the localized states. Fig. 3 shows photoemission spectra of silicon-rich nitride thin film with different values of G excited by 52.5 eV photons. It is noted that the top valence band exhibits almost the same feature, except that the localized unoccupied states move toward the top valence band edge as the silicon contents increase. Moreover, it is found that the optimal (the largest
Fig. 3. Two-color resonant photoemission spectra of a-SiNx:H thin films excited by a 52.5 eV synchrotron radiation light.
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
counts of UL band) excitation energies decrease from 52.5 to 52.1 eV and the peak positions of UL bands are shifted from 2.4 to about 1.8 eV while decreasing the silicon richness. It means that the excited core-levels are almost located at about 102.5 eV, irrespective of the silicon richness. It seems to indicate that the localized unoccupied states are strongly correlated with Si–N bonding sites. Moreover, it is demonstrated that the transition between the top valence band tail state (or the localized occupied state close to the top valence band) and the localized unoccupied state could be responsible for the strong radiative recombination.
4. Conclusions Observation of localized unoccupied states in luminescent amorphous silicon-rich nitride films was reported by using the synchrotron radiation two-color resonant photoemission technique. The energy separation between the localized unoccupied state and the top valence band was observed to shift from 2.4 to 1.8 eV while increasing the silicon richness. Moreover, it also indicated that the luminescence centers (localized unoccupied states) are related with the Si–N bonding configurations. It is noted that the surface of silicon platelets embedded into the amorphous silicon nitride environments would be the most possible sites. The anomalous and anisotropic characters of surface bonding configurations exhibit a strong dipole property. It is believed that the strong dipole character accounts for the surprisingly high luminescence efficiency.
77
Acknowledgements We would like to thank the Hsinchu Synchrotron Radiation Research Center (SRRC) for their technical assistance. This work was supported by the ROC National Science Council (Grant No. NSC 90-2212M-029-005) and the Ministry of Education (Grant No. 91-E-FA04-1-4).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. V. Lehman, U. Gosele, Appl. Phys. Lett. 58 (1991) 865. S.M. Prokes, Appl. Phys. Lett. 62 (1993) 3244. M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 82 (1999) 197. S. Schuppler, S.L. Friedman, M.A. Marcus, D.L. Adler, Y.H. Xie, F.M. Ross, Phys. Rev. B 52 (1995) 4910. F. Bentosela, P. Exner, V.A. Zagrebnov, Phys. Rev. B 57 (1998) 1382. L. Dongoni, O. Bisi, F. Bernardini, S. Ossicini, Phys. Rev. B 53 (1996) 4557. L.W. Wang, A. Zunger, J. Chem. Phys. 98 (1994) 2158. M. Nakamura, Y. Mochizuki, K. Usami, J. Electrochem. Soc. 132 (1985) 482. C.S. Yang, C.J. Lin, P.Y. Kuei, S.F. Horng, C.C.H. Hsu, M.C. Liaw, Appl. Surf. Sci. 113/114 (1997) 116. D.B. Geohegan, A.A. Puretzky, G. Duscher, S.J. Pennycook, Appl. Phys. Lett. 73 (1998) 438. J.F. Tong, H.L. Hsiao, H.L. Hwang, Appl. Phys. Lett. 74 (1999) 2316. V. Dose, Surf. Sci. Rep. 5 (1985) 337. N.V. Smith, D.P. Woodruff, Prog. Surf. Sci. 21 (1986) 295. G. Borstel, G. Thorner, Surf. Sci. Rep. 8 (1988) 1. W. Steinmann, Appl. Phys. A 49 (1989) 365. B. Quiniou, V. Bulovic, R.M. Osgood, Phys. Rev. B 47 (1993) 15890.
Direct observation of localized unoccupied states by synchrotron radiation two-color resonant photoemission H.L. Hsiaoa,*, A.B. Yanga, H.L. Hwangb a
b
Department of Physics, Tunghai University, Taichung 407, Taiwan Department of Electrical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
Abstract Observation of localized unoccupied state in luminescent amorphous silicon-rich nitride films was reported by using the synchrotron radiation two-color resonant photoemission technique. The energy separation between the localized unoccupied state and top valence band was observed to shift from 2.4 to 1.8 eV while increasing the silicon richness. This observation is in agreement with the previously reported photoluminescence red-shift results. Moreover, it is found that the localized unoccupied states are strongly correlated with the Si–N bonding configurations. Considering the microstructures (silicon platelets embedded into the amorphous silicon nitride environments) of these samples, it is believed that the localized unoccupied states would possible come from the surface region of silicon platelets. # 2003 Elsevier Science B.V. All rights reserved. PACS: 73.22.-f; 78.60.-b; 79.60.-i Keywords: Silicon-rich nitride; Two-color resonant photoemission; Photoluminescence
1. Introduction A quantum confinement of carriers in crystalline silicon wires was proposed as the origin of the luminescence in the first publication reporting strong, visible, room temperature emission from porous silicon [1]. Numerous models [2,3] have been put forward as alternative explanations for this phenomenon. Recent experimental [4–6] and theoretical results [7–9] strongly suggest that the band gap opening by quantum confinement and surface passivation, including oxygen and oxyhydride [10,11], plays an important role.
*
Corresponding author. Tel.: þ886-4-23500896; fax: þ886-4-23594643. E-mail address: [email protected] (H.L. Hsiao).
The large discrepancy between the optical gap and photoluminescence peak energies in luminescent silicon-rich nitride [17] samples demonstrates the possible existence of localized gap states. Probing and understanding the characteristics of these gap states would be helpful for further identifying the luminescence mechanism. However, direct probing these gap states was not an easy task and most of the developed techniques are concentrated on the measurement of defects. Moreover, the poor electrical conductivity of our samples limits the applications of this instrumentation to our system. Although, optical characterization can supply some information about these states, but the evidences are usually not strong enough to be accepted due to the unknown and complicated optical transitions involved in the optical characterization processes. In our case, the localized gap states might
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00371-4
74
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
be the quantum confinement induced metastable states, or weakly localized surface states, and it is very difficult to clarify them. Therefore, in this study, we present a novel technique to recognize the localized gap states.
2. Experimental Photoelectron spectroscopy has been developed over the last 30 years as the most powerful method for investigating the electronic states in solids, both in the bulk and on the surface. Photoemission coupled with the synchrotron radiation light source (an energy tunable excitation source) provides an unique and possible site-specific technique. It is especially suited for studying the occupied states. Obviously, the energy range between the Fermi level and the vacuum level is inaccessible by this method. Inverse photoemission has been applied extensively to investigate the bulk and surface states below and above the vacuum level [12–14]. An alternative to inverse photoemission is the two-photon photoemission [15,16]. In contrast to the conventional onephoton photoemission, it involves, in addition to an initial and a final state, a ‘‘real’’ intermediate state. The specially emphasized word ‘‘real’’ is used to distinguish from the ‘‘virtual’’ state in the conventional two-photon process. Therefore, it yields information about the intermediate state involved in the process. The intermediate state of a two-photon photoemission process is (n 1) electron system and an electron–hole pair. If two photons with different energy are employed, it is possible to obtain additional information. However, it is subjected to some limitations. First, the lifetime of the intermediate state has to be sufficiently long (>1012 s). Another limitation is that the intermediate states are limited to the range between Fermi level (Ef) and the vacuum level (Evac). The wide-range tunability of the synchrotron radiation source provides the extreme utility for photoemission. In addition to the conventional energy distribution curve (EDC), we can sweep the surface Nðho; TÞ along a line defined by T ho ¼ const:, where T is the final-state energy (kinetic energy) of the excited photoelectrons and ho is the synchrotron radiation photon energy. The quantity T ho, which
is the final-state energy minus the photon energy, gives the energy (measured from the vacuum level) of the initial state for primary photoelectrons of the photoemission process. The obtained curves, which are called constant-initial-state (CIS) spectra, can provide useful information about the interaction cross-section of the photon and the samples. By choosing the suitable photon energy, it is possible to enhance the signal from some specific initial electronic states and to provide element-specific and bonding-specific information. This mode of photoemission spectroscopy is called resonant photoemission spectroscopy. If the luminescence is originated from the radiation recombination of localized excitons, or trapped electron–hole pairs, there unoccupied localized state should exist in the gap. The lifetime of the luminescent state in porous silicon was extensively studied by using time-resolved technique and estimated to be approximately 1–100 ms at room temperature. The long room temperature decay time in porous Si as compared to the nanosecond radiative lifetimes observed in direct gap semiconductors suggests that the main reason for the high quantum efficiency in porous Si is not a reduction in the radiative lifetime, but rather a strong suppression of non-radiative processes. Therefore, it is possible to probe the unoccupied localized state by using ‘‘two-color resonant photoemission method’’. Here the word ‘‘two-color’’, which represents two photons with different energy, is used to distinguish from the conventional two-photon photoemission. The experiments were performed at the 6 mLSGM beamline of the Synchrotron Radiation Research Center (SRRC) in Hsinchu, Taiwan. The tunable photon energy of this beamline ranges from 20 to 200 eV. All the measurements were performed in an ultra-high vacuum (UHV) chamber system with a base pressure less than 7 1011 Torr. The photoelectrons were collected and analyzed by the VSW EA-125 hemi-spherical analyzer (HSA). Prior to the measurement, Ar-ion sputtering was carried out to remove surface contamination. The CIS data were normalized to the photon flux measured by a nickel mesh located in front of the sample. Conventionally, an aluminum foil is used to filter out the higher order photon flux above 72 eV while measuring valence band density of states. However, in our resonant
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
photoemission measurement, the Si 2p core-states were used to resonantly enhance the optical absorption. Thus, the aluminum foil must be removed from the beamline. The photon flux of the second order and the third order in the beamline were estimated to be approximately 20 and 10%, respectively. The secondand the third-order photons energies were used as an excitation source to excite the Si-related bonding core-electrons to the unoccupied intermediate state in the two-color resonant photoemission process while the primary (first-order) photons excite electrons from the intermediate state to the final state above Evac. To our knowledge, this is the only possible way to perform two-color resonant photoemission using two different synchrotron radiation photon energies. Although, it is possible to use the synchrotron and laser sources, but it is very difficult to concurrently focus the two photons, which are
75
emitted from two different sources onto the same area of the sample surface.
3. Results and discussion Fig. 1a and b show the photoemission spectrum of silicon-rich nitride thin films with G ¼ 1:2 which were excited by 150 and 50 eV photon energy. It is noted that the feature of the valence band, which contains no fine structures, exhibits amorphous characteristics. However, the slowly decreased density of states around the top valence band indicates the existence of tail states in the samples. The reported photoemission experimental and theoretical spectra [13,14] agree in that N 2s electrons do not mix with the other N and Si valence electrons and form the peak at 20 eV from Fermi level (Ef). It is also found that
Fig. 1. Photoemission spectrum of G ¼ 1:2 a-SiNx:H thin films excited by (a) 150 eV and (b) 50 eV photon energy. (c) CIS spectra.
76
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
the binding energy of Si 2p core-level is about 102.5 eV. The CIS technique was applied to identify the characters of the valence band density of state, in particular the top valence band. Based on Fig. 1b, the initial state was selected to be 7.5 eV below the top valence band (VB band) and 2.5 eV above the top valence band (UL band). The UL band was chosen according to the PL peak energy of this sample. Fig. 1c shows the CIS spectra for these two initial states. Although the CIS spectra, taken at various initial states, show significant differences, they all show prominent features. The feature in the CIS spectrum of the VB band exhibits an absorption character at about 47.5 eV. This absorption feature was believed to correlate with the resonant behavior of Si 2p corelevel. However, the CIS spectrum of UL band displays quite different characters with the conventional resonant photoemission and they were identified to be the core-localized unoccupied state transitions. This identification was based on the fact that the sharp features in the CIS spectrum could not be explained by the conventional one-photon resonant photoemission concepts. In the one-photon resonant photoemission process, the incidence of photon with the energy equal to the core-to-conduction band minimum transition would result in large photon absorption. This kind of absorption process is the same as that observed from the X-ray absorption fine structure spectroscopy. The spectra exhibit a whiteline feature followed by an absorption threshold (edge-jump). It was found that the sharp feature appeared in the CIS spectrum of the UL band is similar to the transition between the two highly localized states. Because the core-levels are localized states, the other state should be also a localized state located above the Fermi level and to provide empty states for the transition. Therefore, the only possible state must be localized and located above the Fermi level as an unoccupied localized gap state. The photoemission spectra recorded at various excitation photon energies are shown in Fig. 2. It was found that an additional peak appears located above the top of the valence band while the sample was excited by 35.0 and 52.5 eV photon energies. The 105 eV photon energy (second- and third-order line of 52.5 and 35 eV) could probably excite the Si 2p coreelectrons to the localized unoccupied states, while
Fig. 2. Photoemission spectra of G ¼ 1:2 a-SiNx:H films excited by using various photon energies.
the primary photons carrying an energy of 35 or 52.5 eV excite electrons in the localized states. Fig. 3 shows photoemission spectra of silicon-rich nitride thin film with different values of G excited by 52.5 eV photons. It is noted that the top valence band exhibits almost the same feature, except that the localized unoccupied states move toward the top valence band edge as the silicon contents increase. Moreover, it is found that the optimal (the largest
Fig. 3. Two-color resonant photoemission spectra of a-SiNx:H thin films excited by a 52.5 eV synchrotron radiation light.
H.L. Hsiao et al. / Applied Surface Science 212–213 (2003) 73–77
counts of UL band) excitation energies decrease from 52.5 to 52.1 eV and the peak positions of UL bands are shifted from 2.4 to about 1.8 eV while decreasing the silicon richness. It means that the excited core-levels are almost located at about 102.5 eV, irrespective of the silicon richness. It seems to indicate that the localized unoccupied states are strongly correlated with Si–N bonding sites. Moreover, it is demonstrated that the transition between the top valence band tail state (or the localized occupied state close to the top valence band) and the localized unoccupied state could be responsible for the strong radiative recombination.
4. Conclusions Observation of localized unoccupied states in luminescent amorphous silicon-rich nitride films was reported by using the synchrotron radiation two-color resonant photoemission technique. The energy separation between the localized unoccupied state and the top valence band was observed to shift from 2.4 to 1.8 eV while increasing the silicon richness. Moreover, it also indicated that the luminescence centers (localized unoccupied states) are related with the Si–N bonding configurations. It is noted that the surface of silicon platelets embedded into the amorphous silicon nitride environments would be the most possible sites. The anomalous and anisotropic characters of surface bonding configurations exhibit a strong dipole property. It is believed that the strong dipole character accounts for the surprisingly high luminescence efficiency.
77
Acknowledgements We would like to thank the Hsinchu Synchrotron Radiation Research Center (SRRC) for their technical assistance. This work was supported by the ROC National Science Council (Grant No. NSC 90-2212M-029-005) and the Ministry of Education (Grant No. 91-E-FA04-1-4).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. V. Lehman, U. Gosele, Appl. Phys. Lett. 58 (1991) 865. S.M. Prokes, Appl. Phys. Lett. 62 (1993) 3244. M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 82 (1999) 197. S. Schuppler, S.L. Friedman, M.A. Marcus, D.L. Adler, Y.H. Xie, F.M. Ross, Phys. Rev. B 52 (1995) 4910. F. Bentosela, P. Exner, V.A. Zagrebnov, Phys. Rev. B 57 (1998) 1382. L. Dongoni, O. Bisi, F. Bernardini, S. Ossicini, Phys. Rev. B 53 (1996) 4557. L.W. Wang, A. Zunger, J. Chem. Phys. 98 (1994) 2158. M. Nakamura, Y. Mochizuki, K. Usami, J. Electrochem. Soc. 132 (1985) 482. C.S. Yang, C.J. Lin, P.Y. Kuei, S.F. Horng, C.C.H. Hsu, M.C. Liaw, Appl. Surf. Sci. 113/114 (1997) 116. D.B. Geohegan, A.A. Puretzky, G. Duscher, S.J. Pennycook, Appl. Phys. Lett. 73 (1998) 438. J.F. Tong, H.L. Hsiao, H.L. Hwang, Appl. Phys. Lett. 74 (1999) 2316. V. Dose, Surf. Sci. Rep. 5 (1985) 337. N.V. Smith, D.P. Woodruff, Prog. Surf. Sci. 21 (1986) 295. G. Borstel, G. Thorner, Surf. Sci. Rep. 8 (1988) 1. W. Steinmann, Appl. Phys. A 49 (1989) 365. B. Quiniou, V. Bulovic, R.M. Osgood, Phys. Rev. B 47 (1993) 15890.