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Competing thermal relaxation processes in response to intrinsic defects produced by exposing SiO2 to synchrotron radiation
Applied Surface Science 190 (2002) 26±29
Competing thermal relaxation processes in response to intrinsic defects produced by exposing SiO2 to synchrotron radiation Housei Akazawa* NTT Telecommunications Energy Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan
Abstract Irradiation of SiO2 with soft X-ray photons (hn > 100 eV) produces a variety of defects, of which E01 centers and neutral Si±Si bonds are mainly responsible for the dielectric response change. The thermal processes that modify the structures around the defect sites have been investigated by in situ spectroscopic ellipsometry. Annealing the irradiated SiO2 ®lm diminishes the number of defects which are assigned to E01 centers by about half. The competing channels for annihilation of E01 centers are the recovery of the Si±O±Si bonding con®guration and, in the opposite direction, the decomposition of the material into volatile products until the network is completely restructured. The other half of the defects are converted to Si±Si bond units and precipitates as nanocrystalline particles of Si. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SiOx; E01 center; Decomposition; nc-Si; Spectroscopic ellipsometry
1. Introduction Intrinsic defects in SiO2 are generally regarded as de®ciencies that give rise to leakage currents in metal-oxide±semiconductor transistors as well as to birefringence in optical lenses used for excimer-laser lithography. However, the light-induced structural changes in SiO2 has recently been utilized in fabricating optical waveguides, ®ber Bragg gratings, and lightemitting devices. Controlling the nature and number of defects is thus a basis for the exploitation of such applications in optoelectronics. Most irradiation experiments carried out so far have been under isothermal conditions, typically at room temperature. The stability of defects has, however, been shown to depend strongly on the temperature of irradiation and defects of some classes decay even at room temperature [1,2]. In the work reported here, the thermal stability of majority * Tel.: 81-46-240-2659; fax: 81-46-270-2315. E-mail address: [email protected] (H. Akazawa).
defects which are produced by irradiating SiO2 with soft X-ray photons has been investigated in terms of the material's dielectric property as analyzed by using in situ spectroscopic ellipsometry (SE). The thermal and photolytic contributions can be distinguished by employing a two-step process: low-temperature irradiation and post-annealing. Unlike ultraviolet lightinduced structural deformation, the changes caused by such high-energy photons is irreversible and an extensive range of qualities is affected. 2. Experimental An oxide ®lm was formed in the surface region of a Ê by H2O steam Si(1 0 0) wafer to a depth of 400 A oxidation at 1000 8C. The specimen was then UHVannealed at 800 8C to minimize the presence of OH species in the oxide ®lm, and irradiated with synchrotron radiation delivered by the ``super-ALIS'' electron storage ring [3]. The number of photons reaches its
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 8 7 5 - 3
H. Akazawa / Applied Surface Science 190 (2002) 26±29
maximum at 100 eVand decreases with photon energy up to the cut-off energy level of 1500 eV. The ¯ux of photons was 9:1 1015 s 1 mm 2 at a storage current of 400 mA and the irradiation times given in this report are normalized on this current. SE is used to measure the Fresnel-re¯ection-coef®cient ratio r between p-polarized (Rp) and s-polarized (Rs) light with respect to the solid surface, which de®nes the ellipsometric C and D angles as r Rp =Rs tan C exp iD. The pseudodielectric function hei he1 i ihe2 i is calculated from C and D. To monitor changes in the dielectric response of the system in real time, C and D were simultaneously recorded at 1.5, 2.3, 3.4, and 4.3 eV every 10 s. Cross-sectional transmission electron microscopy (XTEM) was used to identify crystalline structures in the oxide ®lm. 3. Results Fig. 1 depicts the C±D trajectories monitored at 3.4 eV during four cycles of irradiation at 400 8C and post-annealing at 900 8C. The following description refers to the second cycle (#2). After the irradiation
Fig. 1. C±D plots at 3.4 eV during four cycles (#1, #2, #3, and #4) of irradiation at 400 8C for 65 min (solid lines) and post-annealing at 900 8C (open circles). The dotted line shows the simulation curve obtained under the assumption of uniform etching of the SiO2 ®lm at 900 8C, while maintaining its composition as SiO2.
27
started, at room temperature, from point A, radiationinduced heating shifted the (C, D) point along trace A ! B, since the dielectric constant varies as a function of temperature. When the preset temperature of 400 8C was reached at point B, the subsequent slow movement along trace B ! C, which extends in the C direction, represents the modi®cation of the oxide. The con®guration of two adjacent Si atoms, as occurs with E01 centers (O3BSiSiBO3) with displaced oxygen atoms as well as with neutral Si±Si bond units (O3BSi±SiBO3), are responsible for this change in the material's dielectric property which originates from the polarization of chemical bonds. From the macroscopic point of view, the material's composition has been changed to nonstoichiometric SiOx with a certain loss of the ®lm's thickness [4]. Termination at point C suggests that the SiOx had become resistant to radiation. The explanation is that as the number of Si dangling bonds increases, the quenching rate of valence holes is enormously enhanced. The specimen was then subjected to rapid thermal annealing from room temperature (point D). When the preset temperature of 900 8C was reached at point E, the trajectory turned to move slowly along trace E ! F towards the upper-left of the ®gure. It is worth noting that the processes which correspond to trace E ! F and trace B ! C are discriminated from each other by the respective directions of the traces. The trajectory groups for the four irradiation-annealing cycles continue to be shifted upwards and to the left as the cycle number increases. They appear to be arranged along the simulation curve which was calculated under the assumption that the thickness of the SiO2 ®lm decreases uniformly at 900 8C, while the material's stoichiometric composition is maintained. The actual overall change is thus the decomposition of the oxide with some modi®cation of its composition. The time scale of a few minutes until convergence on the destination point F suggests that the outward diffusion and desorption of SiO molecules from the bulk material is the rate-limiting factor. The positions of the four trajectory sets at lower (C, D) levels than the simulation curve indicates the presence of defects or microstuctures in the SiOx ®lm. Fig. 2 is for comparison of the room-temperature pseudodielectric functions of the initial ®lm of SiO2, and of the ®lm after irradiation at 300 8C, and after subsequent annealing at 900 8C. The he2i amplitude,
28
H. Akazawa / Applied Surface Science 190 (2002) 26±29
Fig. 2. Real and imaginary parts of the room-temperature pseudodielectric functions of a non-irradiated ®lm of SiO2, the SiOx ®lm produced by irradiating the ®rst ®lm at 300 8C for 165 min, and the same ®lm after post-annealing at 900 8C.
Fig. 3. The proportion of SiO2 domains by volume and the layer's thickness under exposure to radiation at 400 8C (solid circles) and 300 8C (open circles) and post-annealing.
particularly above 4 eV, was intensi®ed after irradiation. Photoabsorption that corresponds to the deep valence levels became greater due to the formation of oxygen vacancy sites, which is similar to the result observed in SiO2 irradiated by a F2-excimer laser light [5]. To derive structural parameters from the SE data, the dielectric function of SiOx is approximated by a reference dielectric function, which is obtained, under the Bruggeman effective-medium approximation, by mixing the component dielectric functions of SiO2, which represents the Si±O bonds and of polycrystalline Si, which represents the con®guration of adjacent Si atoms [6]. Linear regression analysis determined the proportion of SiO2 domains by volume and the ®lm's thickness. Fig. 3 shows how the thickness and its proportion by volume of SiO2 domains changed in SiOx alternately exposed to radiation at 400 8C and post-annealing at 900 8C. Annealing diminished the proportion of defects by about half, while the other half remained in place. Along with the increases and decreases in the proportion of SiO2 by volume, the thickness decreased signi®cantly. The loss in the thickness is primarily ascribed to the desorption of SiO molecules, whereas the changes in the composition to SiOx is due to the
desorption of oxygen atoms. The similar slopes of the arrows that connect the points before and after annealing indicate that, during the thermal elimination of defects, the proportion of material required to produce a completely restructured network is being consumed to a certain depth below the surface. Fig. 4 shows an XTEM image of a SiOx ®lm that is the result of irradiation at 300 8C and post-annealing at 900 8C. Some nanocrystalline Si (nc-Si) particles that are embedded in the amorphous matrix are visible. Such nc-Si particles did not emerge by mere irradiation at 300 8C. Hence, annealing precipitates nc-Si from the Si±Si bond units which had been
Fig. 4. XTEM image of an SiOx ®lm produced from SiO2 by irradiation at 300 8C for 197 min and post-annealing at 900 8C. The nc-Si particles are indicated by arrows.
H. Akazawa / Applied Surface Science 190 (2002) 26±29
uniformly distributed in the SiOx ®lm. The rough ®lm/substrate interface may result from the solidphase epitaxy of Si precipitated in the vicinity of the interface. 4. Discussion The displacement of oxygen atoms into interstitial positions as a result of electronic excitation is independent of the temperature of irradiation. That annealing diminishes defects by about half indicates that the number of defects that appear during isothermal irradiation at elevated temperatures is controlled by the balance between the creation and thermal annihilation. The pair of Si atoms in an E01 center is separated by a longer distance than that in a neutral Si±Si bond. The vanished defects are assigned to E01 centers, and those that remain after annealing are then in the form of Si± Si bonds. Annealing makes thermodynamically opposite routes for the annihilation of E01 centers possible: one is the decomposition of the network into fragments from the E01 centers and the other is the recovery of a Si±O±Si network by returning the displaced oxygen atom. Once created, however, Si±Si bonds are stable and gather to form clusters and nanocrystals of Si. Since the proportion of SiO2 domains by volume recovers to a level that is similar to the level before irradiation, high-temperature annealing favors the decomposition rather than the precipitation of Si. This is why the stoichiometric composition is maintained for a suf®ciently long time during isothermal irradiation at elevated temperatures [4]. The signi®cant loss of material in the network restructuring of synchrotron radiation-irradiated SiO2 contrasts strikingly with the high-temperature
29
(>1000 8C) annealing behavior of SiOx produced by the sputtering of solid Si in an atmosphere of O2 or by the implantation of Si ions in a SiO2 ®lm. In such samples, annealing merely separates the SiOx into Si and SiO2 domains and there is negligible loss of the ®lm's thickness. Such difference is explained in terms of the different quantum ef®ciencies for the formation of isolated Si atoms. By ion implantation, all primary Si ions are incorporated in the SiO2 ®lm, i.e., the quantum ef®ciency for the production of interstitial Si atoms is unity. In contrast, until suf®cient number of defects is formed in synchrotron radiation-irradiated SiO2, the network is destroyed in many places. Decomposition into SiO molecules is hence predominant from the SiSi con®guration. 5. Conclusion Low-temperature irradiation and post-annealing of SiO2 have clari®ed that the responses to these processes are the annihilation of E01 centers, the decomposition of the network from E01 centers, and the conversion of E01 centers to Si±Si bond units, which are eventually precipitated as Si nanocrystals, as competing thermal processes. References [1] [2] [3] [4] [5]
D.L. Griscom, Non-Cryst. J. Solids 68 (1984) 301. R.A.B. Devine, Nucl. Instrum. Meth. B 1 (1984) 378. H. Akazawa, J. Takahashi, Rev. Sci. Instrum. 69 (1998) 265. H. Akazawa, Phys. Rev. B 52 (1995) 12386. Y. Ikuta, S. Kikugawa, M. Hirao, H. Hosono, J. Vac. Sci. Technol. B 18 (2000) 2891. [6] H. Akazawa, J. Vac. Sci. Technol. B 19 (2001) 649.
Competing thermal relaxation processes in response to intrinsic defects produced by exposing SiO2 to synchrotron radiation Housei Akazawa* NTT Telecommunications Energy Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan
Abstract Irradiation of SiO2 with soft X-ray photons (hn > 100 eV) produces a variety of defects, of which E01 centers and neutral Si±Si bonds are mainly responsible for the dielectric response change. The thermal processes that modify the structures around the defect sites have been investigated by in situ spectroscopic ellipsometry. Annealing the irradiated SiO2 ®lm diminishes the number of defects which are assigned to E01 centers by about half. The competing channels for annihilation of E01 centers are the recovery of the Si±O±Si bonding con®guration and, in the opposite direction, the decomposition of the material into volatile products until the network is completely restructured. The other half of the defects are converted to Si±Si bond units and precipitates as nanocrystalline particles of Si. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SiOx; E01 center; Decomposition; nc-Si; Spectroscopic ellipsometry
1. Introduction Intrinsic defects in SiO2 are generally regarded as de®ciencies that give rise to leakage currents in metal-oxide±semiconductor transistors as well as to birefringence in optical lenses used for excimer-laser lithography. However, the light-induced structural changes in SiO2 has recently been utilized in fabricating optical waveguides, ®ber Bragg gratings, and lightemitting devices. Controlling the nature and number of defects is thus a basis for the exploitation of such applications in optoelectronics. Most irradiation experiments carried out so far have been under isothermal conditions, typically at room temperature. The stability of defects has, however, been shown to depend strongly on the temperature of irradiation and defects of some classes decay even at room temperature [1,2]. In the work reported here, the thermal stability of majority * Tel.: 81-46-240-2659; fax: 81-46-270-2315. E-mail address: [email protected] (H. Akazawa).
defects which are produced by irradiating SiO2 with soft X-ray photons has been investigated in terms of the material's dielectric property as analyzed by using in situ spectroscopic ellipsometry (SE). The thermal and photolytic contributions can be distinguished by employing a two-step process: low-temperature irradiation and post-annealing. Unlike ultraviolet lightinduced structural deformation, the changes caused by such high-energy photons is irreversible and an extensive range of qualities is affected. 2. Experimental An oxide ®lm was formed in the surface region of a Ê by H2O steam Si(1 0 0) wafer to a depth of 400 A oxidation at 1000 8C. The specimen was then UHVannealed at 800 8C to minimize the presence of OH species in the oxide ®lm, and irradiated with synchrotron radiation delivered by the ``super-ALIS'' electron storage ring [3]. The number of photons reaches its
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 8 7 5 - 3
H. Akazawa / Applied Surface Science 190 (2002) 26±29
maximum at 100 eVand decreases with photon energy up to the cut-off energy level of 1500 eV. The ¯ux of photons was 9:1 1015 s 1 mm 2 at a storage current of 400 mA and the irradiation times given in this report are normalized on this current. SE is used to measure the Fresnel-re¯ection-coef®cient ratio r between p-polarized (Rp) and s-polarized (Rs) light with respect to the solid surface, which de®nes the ellipsometric C and D angles as r Rp =Rs tan C exp iD. The pseudodielectric function hei he1 i ihe2 i is calculated from C and D. To monitor changes in the dielectric response of the system in real time, C and D were simultaneously recorded at 1.5, 2.3, 3.4, and 4.3 eV every 10 s. Cross-sectional transmission electron microscopy (XTEM) was used to identify crystalline structures in the oxide ®lm. 3. Results Fig. 1 depicts the C±D trajectories monitored at 3.4 eV during four cycles of irradiation at 400 8C and post-annealing at 900 8C. The following description refers to the second cycle (#2). After the irradiation
Fig. 1. C±D plots at 3.4 eV during four cycles (#1, #2, #3, and #4) of irradiation at 400 8C for 65 min (solid lines) and post-annealing at 900 8C (open circles). The dotted line shows the simulation curve obtained under the assumption of uniform etching of the SiO2 ®lm at 900 8C, while maintaining its composition as SiO2.
27
started, at room temperature, from point A, radiationinduced heating shifted the (C, D) point along trace A ! B, since the dielectric constant varies as a function of temperature. When the preset temperature of 400 8C was reached at point B, the subsequent slow movement along trace B ! C, which extends in the C direction, represents the modi®cation of the oxide. The con®guration of two adjacent Si atoms, as occurs with E01 centers (O3BSiSiBO3) with displaced oxygen atoms as well as with neutral Si±Si bond units (O3BSi±SiBO3), are responsible for this change in the material's dielectric property which originates from the polarization of chemical bonds. From the macroscopic point of view, the material's composition has been changed to nonstoichiometric SiOx with a certain loss of the ®lm's thickness [4]. Termination at point C suggests that the SiOx had become resistant to radiation. The explanation is that as the number of Si dangling bonds increases, the quenching rate of valence holes is enormously enhanced. The specimen was then subjected to rapid thermal annealing from room temperature (point D). When the preset temperature of 900 8C was reached at point E, the trajectory turned to move slowly along trace E ! F towards the upper-left of the ®gure. It is worth noting that the processes which correspond to trace E ! F and trace B ! C are discriminated from each other by the respective directions of the traces. The trajectory groups for the four irradiation-annealing cycles continue to be shifted upwards and to the left as the cycle number increases. They appear to be arranged along the simulation curve which was calculated under the assumption that the thickness of the SiO2 ®lm decreases uniformly at 900 8C, while the material's stoichiometric composition is maintained. The actual overall change is thus the decomposition of the oxide with some modi®cation of its composition. The time scale of a few minutes until convergence on the destination point F suggests that the outward diffusion and desorption of SiO molecules from the bulk material is the rate-limiting factor. The positions of the four trajectory sets at lower (C, D) levels than the simulation curve indicates the presence of defects or microstuctures in the SiOx ®lm. Fig. 2 is for comparison of the room-temperature pseudodielectric functions of the initial ®lm of SiO2, and of the ®lm after irradiation at 300 8C, and after subsequent annealing at 900 8C. The he2i amplitude,
28
H. Akazawa / Applied Surface Science 190 (2002) 26±29
Fig. 2. Real and imaginary parts of the room-temperature pseudodielectric functions of a non-irradiated ®lm of SiO2, the SiOx ®lm produced by irradiating the ®rst ®lm at 300 8C for 165 min, and the same ®lm after post-annealing at 900 8C.
Fig. 3. The proportion of SiO2 domains by volume and the layer's thickness under exposure to radiation at 400 8C (solid circles) and 300 8C (open circles) and post-annealing.
particularly above 4 eV, was intensi®ed after irradiation. Photoabsorption that corresponds to the deep valence levels became greater due to the formation of oxygen vacancy sites, which is similar to the result observed in SiO2 irradiated by a F2-excimer laser light [5]. To derive structural parameters from the SE data, the dielectric function of SiOx is approximated by a reference dielectric function, which is obtained, under the Bruggeman effective-medium approximation, by mixing the component dielectric functions of SiO2, which represents the Si±O bonds and of polycrystalline Si, which represents the con®guration of adjacent Si atoms [6]. Linear regression analysis determined the proportion of SiO2 domains by volume and the ®lm's thickness. Fig. 3 shows how the thickness and its proportion by volume of SiO2 domains changed in SiOx alternately exposed to radiation at 400 8C and post-annealing at 900 8C. Annealing diminished the proportion of defects by about half, while the other half remained in place. Along with the increases and decreases in the proportion of SiO2 by volume, the thickness decreased signi®cantly. The loss in the thickness is primarily ascribed to the desorption of SiO molecules, whereas the changes in the composition to SiOx is due to the
desorption of oxygen atoms. The similar slopes of the arrows that connect the points before and after annealing indicate that, during the thermal elimination of defects, the proportion of material required to produce a completely restructured network is being consumed to a certain depth below the surface. Fig. 4 shows an XTEM image of a SiOx ®lm that is the result of irradiation at 300 8C and post-annealing at 900 8C. Some nanocrystalline Si (nc-Si) particles that are embedded in the amorphous matrix are visible. Such nc-Si particles did not emerge by mere irradiation at 300 8C. Hence, annealing precipitates nc-Si from the Si±Si bond units which had been
Fig. 4. XTEM image of an SiOx ®lm produced from SiO2 by irradiation at 300 8C for 197 min and post-annealing at 900 8C. The nc-Si particles are indicated by arrows.
H. Akazawa / Applied Surface Science 190 (2002) 26±29
uniformly distributed in the SiOx ®lm. The rough ®lm/substrate interface may result from the solidphase epitaxy of Si precipitated in the vicinity of the interface. 4. Discussion The displacement of oxygen atoms into interstitial positions as a result of electronic excitation is independent of the temperature of irradiation. That annealing diminishes defects by about half indicates that the number of defects that appear during isothermal irradiation at elevated temperatures is controlled by the balance between the creation and thermal annihilation. The pair of Si atoms in an E01 center is separated by a longer distance than that in a neutral Si±Si bond. The vanished defects are assigned to E01 centers, and those that remain after annealing are then in the form of Si± Si bonds. Annealing makes thermodynamically opposite routes for the annihilation of E01 centers possible: one is the decomposition of the network into fragments from the E01 centers and the other is the recovery of a Si±O±Si network by returning the displaced oxygen atom. Once created, however, Si±Si bonds are stable and gather to form clusters and nanocrystals of Si. Since the proportion of SiO2 domains by volume recovers to a level that is similar to the level before irradiation, high-temperature annealing favors the decomposition rather than the precipitation of Si. This is why the stoichiometric composition is maintained for a suf®ciently long time during isothermal irradiation at elevated temperatures [4]. The signi®cant loss of material in the network restructuring of synchrotron radiation-irradiated SiO2 contrasts strikingly with the high-temperature
29
(>1000 8C) annealing behavior of SiOx produced by the sputtering of solid Si in an atmosphere of O2 or by the implantation of Si ions in a SiO2 ®lm. In such samples, annealing merely separates the SiOx into Si and SiO2 domains and there is negligible loss of the ®lm's thickness. Such difference is explained in terms of the different quantum ef®ciencies for the formation of isolated Si atoms. By ion implantation, all primary Si ions are incorporated in the SiO2 ®lm, i.e., the quantum ef®ciency for the production of interstitial Si atoms is unity. In contrast, until suf®cient number of defects is formed in synchrotron radiation-irradiated SiO2, the network is destroyed in many places. Decomposition into SiO molecules is hence predominant from the SiSi con®guration. 5. Conclusion Low-temperature irradiation and post-annealing of SiO2 have clari®ed that the responses to these processes are the annihilation of E01 centers, the decomposition of the network from E01 centers, and the conversion of E01 centers to Si±Si bond units, which are eventually precipitated as Si nanocrystals, as competing thermal processes. References [1] [2] [3] [4] [5]
D.L. Griscom, Non-Cryst. J. Solids 68 (1984) 301. R.A.B. Devine, Nucl. Instrum. Meth. B 1 (1984) 378. H. Akazawa, J. Takahashi, Rev. Sci. Instrum. 69 (1998) 265. H. Akazawa, Phys. Rev. B 52 (1995) 12386. Y. Ikuta, S. Kikugawa, M. Hirao, H. Hosono, J. Vac. Sci. Technol. B 18 (2000) 2891. [6] H. Akazawa, J. Vac. Sci. Technol. B 19 (2001) 649.