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Tuned photoluminescence from Si+-implanted SiO2 films with rapid and conventional thermal annealing
Vacuum 86 (2012) 1983e1987
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Tuned photoluminescence from Siþ-implanted SiO2 films with rapid and conventional thermal annealing Jen-Hwan Tsai* Department of Mathematics and Physics, Air Force Academy, No. Sisou 1, Jieshou W. Road, Kangshan, Kaohsiung 820, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2011 Received in revised form 4 April 2012 Accepted 4 April 2012
In this study, by using a conventional thermal annealing (CTA), the obviously near-infrared shift and intensity amplification of room-temperature photoluminescence (PL) spectrum could be observed from the 3 1016 cm2 Siþ-implanted 400-nm-thick SiO2 films after rapid thermal annealing (RTA) at 1150 C in dry nitrogen. For isothermal RTA durations 20 s at the heating rate of 100 C/s, the PL peaks from the only RTA-treated films were detected around 1.7 eV and, for 1050 C CTA durations between 1 and 3 h, no significant PL could be found from the only CTA-treated films. However, when annealing the RTA-treated films with the CTA for only 1 h, then, we varied the terminal PL-peak from 1.7 to 1.5 eV and obviously increased their respective intensities from the films. These results are attributed to the variation of silicon nano-crystals embedded in SiO2 film. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Photoluminescence Siþ-implanted SiO2 films Rapid thermal annealing Silicon nano-crystals
1. Introduction In view of the potential of the CMOS-compatible photonic devices into integrated circuits, the use of high temperature thermal annealing to produce silicon compatible light source in the structures consisting of silicon nano-crystals (Si NCs) embedded in SiO2 once caused considerable attention [1e4]. In previous studies, we have demonstrated that varying the heating rate and holding time (duration) of rapid thermal annealing (RTA) reveals a broadband shift of room-temperature photoluminescence (PL) in 3 1016 cm2 Siþ-implanted 400-nm-thick SiO2 films after RTA at 1150 C in dry nitrogen [5e7]. At a heating rate of 100 C/s, the PL peaks shift from blue to near-infrared band for isothermal RTA durations from 1 to 20 s [6,7]. However, their emitted intensities are all much smaller than those from the optical device of IIIeV compounds and this method could not provide significant nearinfrared emission at less than 1.5 eV (w0.85 mm). In addition, no significant PL could be detected from 3 1016 cm2 Siþ-implanted SiO2 films after dry-N2 conventional thermal annealing (CTA) (batch furnace) at 1150 C for the duration between 1 and 3 h. Obviously, the method using only silicon implantation along with RTA or CTA is not suitable for applications in optoelectronics. For these years, that the study from Si-rich SiO2 films with the subsequent RTA after rare-earth ions doping could produce emissions with effective
* Tel.: þ886 7 6268846; fax: þ886 7 6250699. E-mail address: [email protected]. 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.04.011
intensity and broad near-infrared bands was in vogue. But, sacrificing the luminescence at visible band or near-infrared band at around 0.85 mm is still a pity [8e10]. In line with the observation by Iwayama et al. [2], we have also observed that the PL intensity from the RTA-treated film can be amplified by subsequent CTA and the amplified intensity depends on the holding temperature, time (duration), and heating rate of RTA and CTA. However, Iwayama et al. emphasized CTA tuning and were unable obviously shift the amplified PL-peak away from the 1.7 eV band. In contrast to their study, our study concentrates on tuning RTA parameters and finds the obviously near-infrared shift of the PL-peak from the initially RTA-treated and terminally CTA-treated film. In this study, we use a CTA to re-anneal 3 1016 cm2 Siþimplanted 400-nm-thick SiO2 films that have been rapidly thermally annealed at 1150 C in dry nitrogen and find out that for isothermal RTA durations 20 s at the heating rate of 100 C/s, the PL peaks from the films alone after the RTA are nearly fixed at 1.7 eV and the PL intensities rapidly decrease with the increase of RTA durations. Furthermore, no PL sign can be detected when the duration surpasses 120 s. However, if we re-annealed the RTAtreated films with the CTA at 1050 C for only 1hr, then, the terminal PL-peak from the film with the RTA duration of 20 s would vary from 1.7 to 1.5 eV. In addition, when assigning the RTA durations between 20 and 120 s, their respective intensities from the films would all enlarge and increase with the increase of RTA durations. These attractive features are attributed to the presence of silicon nano-crystallizations (Si NCs) embedded in SiO2 films and are closely related to the size and quantity of these nano-crystals.
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J.-H. Tsai / Vacuum 86 (2012) 1983e1987
2. Experimental procedure Samples were prepared by implantation of Siþ onto a 400-nmthick SiO2 layers that were thermally grown on (100)-oriented pdoped Si substrates. The doses for Siþ-implantation were 3 1016 cm2. The temperature of the samples during ion implantation was kept at liquid nitrogen temperature. The acceleration energy of w160 keV was selected so that the maximum concentration was at a depth of w250 nm below the surface and the standard deviation of the implanted region was 60 nm. These samples were subjected to RTA at substrate temperature of 1150 C under dry-N2. In addition, the heating rates of 100 C/s and the average cooling rate of about 100 C/min were used for the RTA system. Although the average cooling rate was about 100 C/min, samples were cooled down to below 1000 C within 4 s initially. On the other hand, in order to improve and adjust the PL spectra from the samples, we annealed the samples again at temperatures of 1050 C with the heating rate of about 10 C/min by using a conventional furnace. To detect the PL spectra, a HeeCd laser (3.8 eV) was used as the excitation, and the lock-in technique was employed to improve the signal-to-noise ratio, together with a monochromator and cooled photomultiplier tube. Moreover, FTIR measurements were performed to examine the SieOeSi bonding quality of the SiO2 films in the samples according to the absorption peak position at w1100 cm1 in the spectra. The absorption peak position was assigned to the anti-symmetric stretching mode (TO3 mode) of SieOeSi units [11e14]. With the reference being the same silicon plate as the sample, the spectra were measured at room temperature in N2 atmosphere at a 1 cm1 resolution with 100 scan accumulations. Finally, the sample preparation for the plan-view HRTEM used the carbon-film-replica-removal (CFRR) method [6], which was described below. A sacrificial oxide (w1 nm) was etched off by HF (5%) dip to expose Si particles in the annealed Siþimplanted SiO2 film, then a carbon layer was deposited on the etched film at the pressure of 50 103 torr in a cold sputtering chamber. The specimens were then etched by HF (48%) to remove all the oxide films, thus leaving the Si particles absorbed on the carbon layer. The layer floating on the HF (48%) solution was dredged up and steeped in DI water to expand the area, and then it was dried up by spraying nitrogen. The thickness of the carbon layer was about 20 nm and the HRTEM observation was performed with JEDL 2000EX operating at 200 keV. 3. Results and discussion In our previous study, we have found that when a HeeCd laser (3.8 eV) operated at a power of w5 mW was used as the excitation source to detect the PL spectra, no PL sign could be found in the 3 1016 cm2 Siþ-implanted 400-nm-thick SiO2 films without RTA or alone with dry-N2 CTA at 1150 C for 3 h. Moreover, when the heating rate of RTA was kept at 100 C/s, the PL- peak from the asimplanted films after RTA at 1150 C for the durations of 20 s was mainly attributed to the growth of Si NCs [8]. Recently, we further found that, when the duration surpasses 20 s, the PL-peak showed no obvious shift and were all around 1.7 eV but the PL intensities rapidly decrease with the increase of RTA durations. Fig. 1 shows that the heights of PL spectra change as a function of RTA duration even though the bands of PL peaks are nearly fixed at 1.7 eV. The PL spectra of Fig. 1(a)e(c) were obtained initially from the Siþimplanted SiO2 films after the 100 C/s RTA at 1150 C for the duration of 20 s, 40 s, and 60 s, respectively. As shown in Fig. 1(a) and (b), the intensity of the PL-peak still exists at about 30% during the RTA duration, from 20 to 40 s. Then, while the duration continues increasing toward the value of 60 s, the intensity of the PL-peak only remains at about 15%, as shown in Fig. 1(c). Finally, no
Fig. 1. PL spectra of Siþ-implanted SiO2 film after RTA at the heating rate of 100 C/s and at 1150 C for (a) 20 s, (b) 40 s, and (c) 60 s.
sign of PL could be seen in the films at the RTA duration of 120 s. For our study, the spectrum area of PL intensity e photo energy, as shown in the paper’s figures, always increases with the increase of the PL-peak intensity; the PL-peak intensity can represent the spectrum area that is proportional to the number of PL mechanism in the film. Hence, the number of the PL mechanism also decreases with the increase of RTA duration between 20 and 120 s. On the other hand, the position of FTIR peak in the w1100 cm1 range shows the anti-asymmetric stretching vibration of a SieOeSi unit in a SiO2 film, which can accurately reflect the situation of SieO bonds in the film. The shift of the FTIR peak to higher wavenumbers, as observed from the as-implanted SiO2 films after RTA, represents the reconstruction of broken bonds and displaced atoms, causing the relaxation of the SieO bonding network in the films and the improved SiO2 structures [12e14]. Fig. 2 shows that the FTIR-peak position and its respective PL-peak intensity changes as a function of RTA duration. The FTIR-peak positions at w1100 cm1 are also observed from the 3 1016 cm2 Siþimplanted 400-nm-thick SiO2 films after the 100 C/s RTA at 1150 C for different durations. As shown in Fig. 2(a) and (b), when the wavenumber of the SieOeSi stretching mode from the asimplanted SiO2 film increases from 1085 to 1093 cm1 with RTA duration change from 20 to 80 s, the PL-peak intensity in the film remains at 10%. When the duration greater than 120 s, the wavenumber from the films went through no obvious changes around 1095 cm1 and no PL sign could be seen in the films. According to the result, the PL-peak intensity decreases with the decrease of the rate (v) at which FTIR-peak position (x) is changing with RTA duration (t) at a given instant; that is, v is the derivative of x with respect to t and v at any instant is the slop of the positioneduration curve at the point representing that instant, too. Besides of determining the PL mechanism number in the as-implanted film, the
J.-H. Tsai / Vacuum 86 (2012) 1983e1987
Fig. 2. Change in the FTIR-peak positions around 1100 cm1 and in the respective PLpeak intensity from Siþ-implanted SiO2 film after 100 C/s RTA at 1150 C as a function of the RTA duration.
Fig. 3. Plan-view HRTEM image taken by the CFRR method and from the Siþimplanted SiO2 films after 100 C/s RTA at 1150 C (a) for 20 s, and (b) for 120 s. In the micrograph, Si NCs are shown as black spots on the carbon layer. Inset: electron diffraction pattern from the Si NCs in the film after the RTA for 20 s.
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magnitude of v displays the speed of the film’s reconstruction. Therefore, Fig. 2 also implies that the speed reduces clearly from the duration 20 se120 s and go through no obvious changes after 120 s. Fig. 3(a) and (b) shows two plan-view HRTEM images taken by the CFRR method and images of the respective samples treated by the RTA for the duration of 20 s and 120 s. The presence of Si NCs in the former sample is evidenced by the characteristic {200}, {220}, and {113} electron diffraction rings shown in the inset of Fig. 3(a). However, the special rings are never observed from the clusters in the plane of Fig. 3(b) and so no Si NCs can be identified in the latter sample. In addition, the average dimension of Si clusters on the carbon layer from the 1150 C RTA-treated sample with the duration of 20 s (the size less than 4 nm) is greater than that from the isothermal RTA-treated sample with the duration of 120 s (the size of less than 2 nm). Noticeably, because all of the Si clusters were not absorbed on the carbon layer in the CFRR method, the precise density of the Si clusters in the Siþ-implanted SiO2 film could not be obtained. However, the opportunity for absorption increases with the Si-cluster size. The results still show that, not like the former sample, the latter sample does not aggregate enough silicon to grow the Si NCs. Therefore, although the RTA allows the implanted silicon to diffuse and segregate due to the rapid thermal expansion and quickly helps the film to improve itself, the increasing duration from 20 s gradually reduces the number of crystallization in the as-implanted film, After increasing the duration up to 120 s, the Si NCs never occur in the film and no PL sign can be detected. Together with the previous discussion about the speed of the film’s reconstruction, we find that, when the RTA
Fig. 4. PL spectra of Siþ-implanted SiO2 film after 100 C/s RTA at 1150 C for (a) 20 s, (b) 40 s, and (c) 60 s prior to CTA at 1050 C for 1 h. Inset: PL spectra of the RTA-treated film for 20 s (a) without CTA and (b) with the subsequent CTA.
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J.-H. Tsai / Vacuum 86 (2012) 1983e1987
duration increases from 20 s, the deceleration of the film’s reconstruction reduces the probability of Si collision and aggregation during the duration and of final crystallization after quick cooling; for the duration 120 s, the reconstructive speed is already closed to zero and so the cluster of Si aggregation is not enough to crystallize in the film after RTA. Fig. 4 shows the PL spectra from the Si-implanted SiO2 films after RTA at 1150 C for the duration between 20 and 60 s and subsequent CTA at 1050 C for 1 h. The spectra from the RTA-treated as-implanted films without CTA are shown in Fig. 1 and their PLpeak positions have no obvious shift around 1.7 eV. With the subsequent CTA treatment, the positions from the films shift large scale from 1.5 to 1.67 eV for the RTA duration from 20 to 60 s, respectively. Then, Fig. 5 also shows the PL spectra from the Siimplanted SiO2 films with the subsequent CTA after RTA. However, the RTA treatment of the films at 1150 C takes from duration 60e120 s and the respective PL-peak positions from these films have weak shift from 1.67 to 1.7 eV at the range of RTA duration. The results imply that the only RTA-treated film emitting stronger PL intensity would produce the lower-band PL mechanism in the film with the subsequent CTA treatment after RTA. On the other hand, the insets (a) and (b) of Fig. 4 show that the maximum value of PLpeak intensity at w1.7 eV from the film without CTA after RTA for 20 s is weaker than the value of PL-peak intensity at w1.5 eV from the film with the subsequent CTA after the RTA treatment. Moreover, in comparison with the PL-peak intensity at w1.7 eV (in the inset (a) of Fig. 4), the PL-peak intensity from the initially RTAtreated and terminally CTA-treated film with the prior RTA duration of 60 s, as shown in Fig. 4(c) or Fig. 5(a), enlarges about three times. When the RTA duration increased up to 120 s, the PL-peak
Fig. 5. PL spectra of Siþ-implanted SiO2 film after 100 C/s RTA at 1150 C for (a) 60 s, (b) 80 s, (c) 100 s, and (d) 120 s prior to CTA at 1050 C for 1 h.
intensity from the initially RTA-treated and terminally CTA-treated film, as shown in Fig. 5(d), has enlarged about seven times. But, since the duration greater than 120 s, the PL intensity from the film has stopped the increase. These imply that using the CTA to reanneal the RTA-treated film can enhance the PL intensity from the film with the increase of the prior RTA duration and the intensity approximates a constant value from the duration 120 s. Finally, Fig. 6(a) and (b) shows the plan-view HRTEM images from the samples originated from the Siþ-implanted SiO2 films after RTA at 1150 C for 20 s and 120 s, respectively, and subsequent CTA at 1050 C for 1 h. Like the image of Fig. 3(a), in the only RTA-treated as-implanted film emitting the 1.7 eV PL-peak, the presences of Si NCs are both evidenced by the characteristic {200}, {220}, and {113} electron diffraction rings. As shown in Fig. 6(a) and (b), the average dimension of Si NCs on the carbon layer (the size less than 5 nm) from the former sample with the 1.5 eV PL-peak is greater than that from the latter sample with the 1.7 eV PL-peak. The results imply that the above initially RTA-treated and terminally CTA-treated SiO2 film with the smaller RTA duration produces larger Si NCs and so the respective PL-peak shifts to a lower near-infrared band. In summary, the above near-infrared PL spectra are related closely to the size and quantity of the Si NCs embedded in SiO2 films. The Si NCs in the as-implanted SiO2 film without CTA after RTA can be a new nucleation point and can again aggregate the segregated Si to grow new larger NCs in the film with the subsequent CTA treatment after RTA; thus, the only RTA-treated film with the bigger quantity of Si NCs, producing the stronger PL intensity, would have larger average-size Si NCs in the film with the
Fig. 6. Plan-view HRTEM image taken by the CFRR method and from the Siþimplanted SiO2 films after 100 C/s RTA at 1150 C (a) for 20 s, and (b) for 120 s prior to CTA at 1050 C for 1 h. In the micrograph, Si NCs are shown as black spots on the carbon layer.
J.-H. Tsai / Vacuum 86 (2012) 1983e1987
subsequent CTA treatment after RTA. Moreover, the larger averagesize Si NCs embedded in the film produce the lower near-infrared PL band; thus, the only RTA-treated film emitting the PL spectrum of stronger intensity (in Fig. 1(a)) transfers to emit the PL spectrum of the lower near-infrared band (in Fig. 4(a)) from the film with the subsequent CTA after RTA. On the other hand, when increasing the RTA duration from 20 to 120 s, the PL-intensity decrease (in Fig. 1) also implies that the number of small Si aggregates increases with the decrease of Si NCs number in the only RTA-treated film until the RTA duration of 120 s. In addition, the quantity of Si clusters including Si NCs and the smaller aggregates increases with the Si NCs decrease and the smaller Si aggregates increase in the only RTA-treated film. Not only the NCs but all the clusters play the role of new nucleation points and continue to aggregate the segregated silicon to crystallize in the film with the subsequent CTA after RTA. These findings indicate that more clusters in the only RTA-treated film grow more new-Si-NCs embedded in the film with the subsequent CTA after RTA. Since the duration 120 s, the number of the clusters started at the maximum value in the only RTA-treated film; the number of new-Si-NCs never increases in the film with the subsequent CTA after RTA. Therefore, although the PL intensity from the only RTA-treated films obviously decreases since the duration >20 s, as shown in Figs. 1 and 2, the results shown in Figs. 4 and 5 both imply that using the CTA to reanneal the RTA-treated film can enhance the PL intensity from the film with the prior RTA duration increase until the duration 120 s. 4. Conclusions With the subsequent CTA, obvious PL-peak near-infrared shift and PL-intensity amplification can be observed in the 3 1016 cm1 Siþ/-implanted 400-nm-thick SiO2 film after RTA. When the initial RTA treatment at 1150 C increases the duration from 20 to 120 s and the terminally CTA treatment is kept at 1050 C for only 1 h, the intensity of the PL-peak can be amplified from 1.5 to 7 times and the PL-peak of w1.5 eV can be observed at the RTA duration of 20 s. However, when the RTA duration >120 s, the PL-peak from these films is nearly fixed at 1.7 eV and the corresponding intensity no longer increases. The Si NCs in the as-implanted SiO2 film without CTA after RTA can be a new nucleation point and can again aggregate the segregated Si to grow new larger NCs in the film with the subsequent CTA treatment after RTA; thus, the above PL-peak shift
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from 1.7 eV to 1.5 eV are attributed to the average-size increase of Si NCs embedded in the film. On the other hand, the quantity of Si clusters in the only RTA-treated film increases with the RTA duration from 20 to 120 s. In addition to Si NCs, the clusters belonged to the smaller aggregates also play the role of new nucleation points and continue to aggregate the segregated silicon to crystallize in the film with the subsequent CTA after RTA; thus, the above PLintensity amplification are attributed to the quantity increase of Si NCs embedded in the film. References [1] Fiory AT. Rapid thermal processing for silicon nanoelectronics applications. JOM 2005;57(6):21e7. [2] Iwayama TS, Hama T, Hole DE, Boyd IW. Enhanced luminescence from encapsulated silicon nanocrystals in SiO2 with rapid thermal anneal. Vacuum 2006;81(2):179e85. [3] Kachurin GA, Cherkova SG, Marin DV, Yankov RA, Deutschmann M. Formation of light-emitting Si nanostructures in SiO2 by pulsed anneals. Nanotechnology 2008;19(35):355305. [4] Silalahi STH, Yang HY, Pita K, Mingbinc Y. Rapid thermal annealing of sputtered silicon-rich oxide/SiO2 superlattice structure. Electrochem Solid-State Lett 2009;12(4):K29e32. [5] Fu MY, Tsai JH, Yang CF, Liao CH. Shift in room-temperature photoluminescence of low-fluence Siþ-implanted SiO2 films subjected to rapid thermal annealing. Sci Technol Adv Mater 2008;9(4):045001. [6] Tsai JH, Fu MY. Improvement of photoluminescence mechanism of CTAtreated Siþ-implanted SiO2 films by using RTA. J Luminescence 2010;130: 1680e6. [7] Fu MY, Tsai JH, Yang CF. Formation of a broad photoluminescence band from Siþ-implanted SiO2 films by varying the heating rate of rapid thermal annealing. Opt Eng 2010;49(7):073801. [8] Franzo G, Vinciquerra V, Priolo F. The excitation mechanism of rare-earth ions in silicon nanocrystals. Appl Phys A 1999;69(3). [9] Savchyn O, Kik PG. Effect of hydrogen passivation on luminescence-centermediated Er excitation in Si-rich SiO2 with and without nanocrystals. Phys Rev B 2008;77:205438. [10] Savchyn O, Todi RM, Coffey KR, Kik PG. High-temperature optical properties of sensitized Er3þ in Si-rich SiO2-implications for gain performance. Opt Mater 2010;32:1274. [11] Nakamura M, Kanzawa R, Sakai K. Stress and density effects on infrared absorption spectra of silicate glass films. J Electrochem Soc 1986;133(6): 1167e71. [12] Sen PN, Thorpe MF. Phonons in AX2 glasses: from molecular to band-like modes. Phys Rev B 1976;15(8):4030e8. [13] Sano N, Sekiya M, Hara M, Kohno A, Sameshima T. Improvement of SiO2/Si interface by low-temperature annealing in wet atmosphere. Appl Phys Lett 1995;66(16):2107e9. [14] Garrido Fernanedz B, Lopez M, Garcia C, Perez-Rodriguez A, Morante JR. Influence of average size and interface passivation on the spectral emission of Si nanocrystals embedded in SiO2. J Appl Phys 2002;91(2):798e807.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Tuned photoluminescence from Siþ-implanted SiO2 films with rapid and conventional thermal annealing Jen-Hwan Tsai* Department of Mathematics and Physics, Air Force Academy, No. Sisou 1, Jieshou W. Road, Kangshan, Kaohsiung 820, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2011 Received in revised form 4 April 2012 Accepted 4 April 2012
In this study, by using a conventional thermal annealing (CTA), the obviously near-infrared shift and intensity amplification of room-temperature photoluminescence (PL) spectrum could be observed from the 3 1016 cm2 Siþ-implanted 400-nm-thick SiO2 films after rapid thermal annealing (RTA) at 1150 C in dry nitrogen. For isothermal RTA durations 20 s at the heating rate of 100 C/s, the PL peaks from the only RTA-treated films were detected around 1.7 eV and, for 1050 C CTA durations between 1 and 3 h, no significant PL could be found from the only CTA-treated films. However, when annealing the RTA-treated films with the CTA for only 1 h, then, we varied the terminal PL-peak from 1.7 to 1.5 eV and obviously increased their respective intensities from the films. These results are attributed to the variation of silicon nano-crystals embedded in SiO2 film. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Photoluminescence Siþ-implanted SiO2 films Rapid thermal annealing Silicon nano-crystals
1. Introduction In view of the potential of the CMOS-compatible photonic devices into integrated circuits, the use of high temperature thermal annealing to produce silicon compatible light source in the structures consisting of silicon nano-crystals (Si NCs) embedded in SiO2 once caused considerable attention [1e4]. In previous studies, we have demonstrated that varying the heating rate and holding time (duration) of rapid thermal annealing (RTA) reveals a broadband shift of room-temperature photoluminescence (PL) in 3 1016 cm2 Siþ-implanted 400-nm-thick SiO2 films after RTA at 1150 C in dry nitrogen [5e7]. At a heating rate of 100 C/s, the PL peaks shift from blue to near-infrared band for isothermal RTA durations from 1 to 20 s [6,7]. However, their emitted intensities are all much smaller than those from the optical device of IIIeV compounds and this method could not provide significant nearinfrared emission at less than 1.5 eV (w0.85 mm). In addition, no significant PL could be detected from 3 1016 cm2 Siþ-implanted SiO2 films after dry-N2 conventional thermal annealing (CTA) (batch furnace) at 1150 C for the duration between 1 and 3 h. Obviously, the method using only silicon implantation along with RTA or CTA is not suitable for applications in optoelectronics. For these years, that the study from Si-rich SiO2 films with the subsequent RTA after rare-earth ions doping could produce emissions with effective
* Tel.: þ886 7 6268846; fax: þ886 7 6250699. E-mail address: [email protected]. 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.04.011
intensity and broad near-infrared bands was in vogue. But, sacrificing the luminescence at visible band or near-infrared band at around 0.85 mm is still a pity [8e10]. In line with the observation by Iwayama et al. [2], we have also observed that the PL intensity from the RTA-treated film can be amplified by subsequent CTA and the amplified intensity depends on the holding temperature, time (duration), and heating rate of RTA and CTA. However, Iwayama et al. emphasized CTA tuning and were unable obviously shift the amplified PL-peak away from the 1.7 eV band. In contrast to their study, our study concentrates on tuning RTA parameters and finds the obviously near-infrared shift of the PL-peak from the initially RTA-treated and terminally CTA-treated film. In this study, we use a CTA to re-anneal 3 1016 cm2 Siþimplanted 400-nm-thick SiO2 films that have been rapidly thermally annealed at 1150 C in dry nitrogen and find out that for isothermal RTA durations 20 s at the heating rate of 100 C/s, the PL peaks from the films alone after the RTA are nearly fixed at 1.7 eV and the PL intensities rapidly decrease with the increase of RTA durations. Furthermore, no PL sign can be detected when the duration surpasses 120 s. However, if we re-annealed the RTAtreated films with the CTA at 1050 C for only 1hr, then, the terminal PL-peak from the film with the RTA duration of 20 s would vary from 1.7 to 1.5 eV. In addition, when assigning the RTA durations between 20 and 120 s, their respective intensities from the films would all enlarge and increase with the increase of RTA durations. These attractive features are attributed to the presence of silicon nano-crystallizations (Si NCs) embedded in SiO2 films and are closely related to the size and quantity of these nano-crystals.
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2. Experimental procedure Samples were prepared by implantation of Siþ onto a 400-nmthick SiO2 layers that were thermally grown on (100)-oriented pdoped Si substrates. The doses for Siþ-implantation were 3 1016 cm2. The temperature of the samples during ion implantation was kept at liquid nitrogen temperature. The acceleration energy of w160 keV was selected so that the maximum concentration was at a depth of w250 nm below the surface and the standard deviation of the implanted region was 60 nm. These samples were subjected to RTA at substrate temperature of 1150 C under dry-N2. In addition, the heating rates of 100 C/s and the average cooling rate of about 100 C/min were used for the RTA system. Although the average cooling rate was about 100 C/min, samples were cooled down to below 1000 C within 4 s initially. On the other hand, in order to improve and adjust the PL spectra from the samples, we annealed the samples again at temperatures of 1050 C with the heating rate of about 10 C/min by using a conventional furnace. To detect the PL spectra, a HeeCd laser (3.8 eV) was used as the excitation, and the lock-in technique was employed to improve the signal-to-noise ratio, together with a monochromator and cooled photomultiplier tube. Moreover, FTIR measurements were performed to examine the SieOeSi bonding quality of the SiO2 films in the samples according to the absorption peak position at w1100 cm1 in the spectra. The absorption peak position was assigned to the anti-symmetric stretching mode (TO3 mode) of SieOeSi units [11e14]. With the reference being the same silicon plate as the sample, the spectra were measured at room temperature in N2 atmosphere at a 1 cm1 resolution with 100 scan accumulations. Finally, the sample preparation for the plan-view HRTEM used the carbon-film-replica-removal (CFRR) method [6], which was described below. A sacrificial oxide (w1 nm) was etched off by HF (5%) dip to expose Si particles in the annealed Siþimplanted SiO2 film, then a carbon layer was deposited on the etched film at the pressure of 50 103 torr in a cold sputtering chamber. The specimens were then etched by HF (48%) to remove all the oxide films, thus leaving the Si particles absorbed on the carbon layer. The layer floating on the HF (48%) solution was dredged up and steeped in DI water to expand the area, and then it was dried up by spraying nitrogen. The thickness of the carbon layer was about 20 nm and the HRTEM observation was performed with JEDL 2000EX operating at 200 keV. 3. Results and discussion In our previous study, we have found that when a HeeCd laser (3.8 eV) operated at a power of w5 mW was used as the excitation source to detect the PL spectra, no PL sign could be found in the 3 1016 cm2 Siþ-implanted 400-nm-thick SiO2 films without RTA or alone with dry-N2 CTA at 1150 C for 3 h. Moreover, when the heating rate of RTA was kept at 100 C/s, the PL- peak from the asimplanted films after RTA at 1150 C for the durations of 20 s was mainly attributed to the growth of Si NCs [8]. Recently, we further found that, when the duration surpasses 20 s, the PL-peak showed no obvious shift and were all around 1.7 eV but the PL intensities rapidly decrease with the increase of RTA durations. Fig. 1 shows that the heights of PL spectra change as a function of RTA duration even though the bands of PL peaks are nearly fixed at 1.7 eV. The PL spectra of Fig. 1(a)e(c) were obtained initially from the Siþimplanted SiO2 films after the 100 C/s RTA at 1150 C for the duration of 20 s, 40 s, and 60 s, respectively. As shown in Fig. 1(a) and (b), the intensity of the PL-peak still exists at about 30% during the RTA duration, from 20 to 40 s. Then, while the duration continues increasing toward the value of 60 s, the intensity of the PL-peak only remains at about 15%, as shown in Fig. 1(c). Finally, no
Fig. 1. PL spectra of Siþ-implanted SiO2 film after RTA at the heating rate of 100 C/s and at 1150 C for (a) 20 s, (b) 40 s, and (c) 60 s.
sign of PL could be seen in the films at the RTA duration of 120 s. For our study, the spectrum area of PL intensity e photo energy, as shown in the paper’s figures, always increases with the increase of the PL-peak intensity; the PL-peak intensity can represent the spectrum area that is proportional to the number of PL mechanism in the film. Hence, the number of the PL mechanism also decreases with the increase of RTA duration between 20 and 120 s. On the other hand, the position of FTIR peak in the w1100 cm1 range shows the anti-asymmetric stretching vibration of a SieOeSi unit in a SiO2 film, which can accurately reflect the situation of SieO bonds in the film. The shift of the FTIR peak to higher wavenumbers, as observed from the as-implanted SiO2 films after RTA, represents the reconstruction of broken bonds and displaced atoms, causing the relaxation of the SieO bonding network in the films and the improved SiO2 structures [12e14]. Fig. 2 shows that the FTIR-peak position and its respective PL-peak intensity changes as a function of RTA duration. The FTIR-peak positions at w1100 cm1 are also observed from the 3 1016 cm2 Siþimplanted 400-nm-thick SiO2 films after the 100 C/s RTA at 1150 C for different durations. As shown in Fig. 2(a) and (b), when the wavenumber of the SieOeSi stretching mode from the asimplanted SiO2 film increases from 1085 to 1093 cm1 with RTA duration change from 20 to 80 s, the PL-peak intensity in the film remains at 10%. When the duration greater than 120 s, the wavenumber from the films went through no obvious changes around 1095 cm1 and no PL sign could be seen in the films. According to the result, the PL-peak intensity decreases with the decrease of the rate (v) at which FTIR-peak position (x) is changing with RTA duration (t) at a given instant; that is, v is the derivative of x with respect to t and v at any instant is the slop of the positioneduration curve at the point representing that instant, too. Besides of determining the PL mechanism number in the as-implanted film, the
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Fig. 2. Change in the FTIR-peak positions around 1100 cm1 and in the respective PLpeak intensity from Siþ-implanted SiO2 film after 100 C/s RTA at 1150 C as a function of the RTA duration.
Fig. 3. Plan-view HRTEM image taken by the CFRR method and from the Siþimplanted SiO2 films after 100 C/s RTA at 1150 C (a) for 20 s, and (b) for 120 s. In the micrograph, Si NCs are shown as black spots on the carbon layer. Inset: electron diffraction pattern from the Si NCs in the film after the RTA for 20 s.
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magnitude of v displays the speed of the film’s reconstruction. Therefore, Fig. 2 also implies that the speed reduces clearly from the duration 20 se120 s and go through no obvious changes after 120 s. Fig. 3(a) and (b) shows two plan-view HRTEM images taken by the CFRR method and images of the respective samples treated by the RTA for the duration of 20 s and 120 s. The presence of Si NCs in the former sample is evidenced by the characteristic {200}, {220}, and {113} electron diffraction rings shown in the inset of Fig. 3(a). However, the special rings are never observed from the clusters in the plane of Fig. 3(b) and so no Si NCs can be identified in the latter sample. In addition, the average dimension of Si clusters on the carbon layer from the 1150 C RTA-treated sample with the duration of 20 s (the size less than 4 nm) is greater than that from the isothermal RTA-treated sample with the duration of 120 s (the size of less than 2 nm). Noticeably, because all of the Si clusters were not absorbed on the carbon layer in the CFRR method, the precise density of the Si clusters in the Siþ-implanted SiO2 film could not be obtained. However, the opportunity for absorption increases with the Si-cluster size. The results still show that, not like the former sample, the latter sample does not aggregate enough silicon to grow the Si NCs. Therefore, although the RTA allows the implanted silicon to diffuse and segregate due to the rapid thermal expansion and quickly helps the film to improve itself, the increasing duration from 20 s gradually reduces the number of crystallization in the as-implanted film, After increasing the duration up to 120 s, the Si NCs never occur in the film and no PL sign can be detected. Together with the previous discussion about the speed of the film’s reconstruction, we find that, when the RTA
Fig. 4. PL spectra of Siþ-implanted SiO2 film after 100 C/s RTA at 1150 C for (a) 20 s, (b) 40 s, and (c) 60 s prior to CTA at 1050 C for 1 h. Inset: PL spectra of the RTA-treated film for 20 s (a) without CTA and (b) with the subsequent CTA.
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duration increases from 20 s, the deceleration of the film’s reconstruction reduces the probability of Si collision and aggregation during the duration and of final crystallization after quick cooling; for the duration 120 s, the reconstructive speed is already closed to zero and so the cluster of Si aggregation is not enough to crystallize in the film after RTA. Fig. 4 shows the PL spectra from the Si-implanted SiO2 films after RTA at 1150 C for the duration between 20 and 60 s and subsequent CTA at 1050 C for 1 h. The spectra from the RTA-treated as-implanted films without CTA are shown in Fig. 1 and their PLpeak positions have no obvious shift around 1.7 eV. With the subsequent CTA treatment, the positions from the films shift large scale from 1.5 to 1.67 eV for the RTA duration from 20 to 60 s, respectively. Then, Fig. 5 also shows the PL spectra from the Siimplanted SiO2 films with the subsequent CTA after RTA. However, the RTA treatment of the films at 1150 C takes from duration 60e120 s and the respective PL-peak positions from these films have weak shift from 1.67 to 1.7 eV at the range of RTA duration. The results imply that the only RTA-treated film emitting stronger PL intensity would produce the lower-band PL mechanism in the film with the subsequent CTA treatment after RTA. On the other hand, the insets (a) and (b) of Fig. 4 show that the maximum value of PLpeak intensity at w1.7 eV from the film without CTA after RTA for 20 s is weaker than the value of PL-peak intensity at w1.5 eV from the film with the subsequent CTA after the RTA treatment. Moreover, in comparison with the PL-peak intensity at w1.7 eV (in the inset (a) of Fig. 4), the PL-peak intensity from the initially RTAtreated and terminally CTA-treated film with the prior RTA duration of 60 s, as shown in Fig. 4(c) or Fig. 5(a), enlarges about three times. When the RTA duration increased up to 120 s, the PL-peak
Fig. 5. PL spectra of Siþ-implanted SiO2 film after 100 C/s RTA at 1150 C for (a) 60 s, (b) 80 s, (c) 100 s, and (d) 120 s prior to CTA at 1050 C for 1 h.
intensity from the initially RTA-treated and terminally CTA-treated film, as shown in Fig. 5(d), has enlarged about seven times. But, since the duration greater than 120 s, the PL intensity from the film has stopped the increase. These imply that using the CTA to reanneal the RTA-treated film can enhance the PL intensity from the film with the increase of the prior RTA duration and the intensity approximates a constant value from the duration 120 s. Finally, Fig. 6(a) and (b) shows the plan-view HRTEM images from the samples originated from the Siþ-implanted SiO2 films after RTA at 1150 C for 20 s and 120 s, respectively, and subsequent CTA at 1050 C for 1 h. Like the image of Fig. 3(a), in the only RTA-treated as-implanted film emitting the 1.7 eV PL-peak, the presences of Si NCs are both evidenced by the characteristic {200}, {220}, and {113} electron diffraction rings. As shown in Fig. 6(a) and (b), the average dimension of Si NCs on the carbon layer (the size less than 5 nm) from the former sample with the 1.5 eV PL-peak is greater than that from the latter sample with the 1.7 eV PL-peak. The results imply that the above initially RTA-treated and terminally CTA-treated SiO2 film with the smaller RTA duration produces larger Si NCs and so the respective PL-peak shifts to a lower near-infrared band. In summary, the above near-infrared PL spectra are related closely to the size and quantity of the Si NCs embedded in SiO2 films. The Si NCs in the as-implanted SiO2 film without CTA after RTA can be a new nucleation point and can again aggregate the segregated Si to grow new larger NCs in the film with the subsequent CTA treatment after RTA; thus, the only RTA-treated film with the bigger quantity of Si NCs, producing the stronger PL intensity, would have larger average-size Si NCs in the film with the
Fig. 6. Plan-view HRTEM image taken by the CFRR method and from the Siþimplanted SiO2 films after 100 C/s RTA at 1150 C (a) for 20 s, and (b) for 120 s prior to CTA at 1050 C for 1 h. In the micrograph, Si NCs are shown as black spots on the carbon layer.
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subsequent CTA treatment after RTA. Moreover, the larger averagesize Si NCs embedded in the film produce the lower near-infrared PL band; thus, the only RTA-treated film emitting the PL spectrum of stronger intensity (in Fig. 1(a)) transfers to emit the PL spectrum of the lower near-infrared band (in Fig. 4(a)) from the film with the subsequent CTA after RTA. On the other hand, when increasing the RTA duration from 20 to 120 s, the PL-intensity decrease (in Fig. 1) also implies that the number of small Si aggregates increases with the decrease of Si NCs number in the only RTA-treated film until the RTA duration of 120 s. In addition, the quantity of Si clusters including Si NCs and the smaller aggregates increases with the Si NCs decrease and the smaller Si aggregates increase in the only RTA-treated film. Not only the NCs but all the clusters play the role of new nucleation points and continue to aggregate the segregated silicon to crystallize in the film with the subsequent CTA after RTA. These findings indicate that more clusters in the only RTA-treated film grow more new-Si-NCs embedded in the film with the subsequent CTA after RTA. Since the duration 120 s, the number of the clusters started at the maximum value in the only RTA-treated film; the number of new-Si-NCs never increases in the film with the subsequent CTA after RTA. Therefore, although the PL intensity from the only RTA-treated films obviously decreases since the duration >20 s, as shown in Figs. 1 and 2, the results shown in Figs. 4 and 5 both imply that using the CTA to reanneal the RTA-treated film can enhance the PL intensity from the film with the prior RTA duration increase until the duration 120 s. 4. Conclusions With the subsequent CTA, obvious PL-peak near-infrared shift and PL-intensity amplification can be observed in the 3 1016 cm1 Siþ/-implanted 400-nm-thick SiO2 film after RTA. When the initial RTA treatment at 1150 C increases the duration from 20 to 120 s and the terminally CTA treatment is kept at 1050 C for only 1 h, the intensity of the PL-peak can be amplified from 1.5 to 7 times and the PL-peak of w1.5 eV can be observed at the RTA duration of 20 s. However, when the RTA duration >120 s, the PL-peak from these films is nearly fixed at 1.7 eV and the corresponding intensity no longer increases. The Si NCs in the as-implanted SiO2 film without CTA after RTA can be a new nucleation point and can again aggregate the segregated Si to grow new larger NCs in the film with the subsequent CTA treatment after RTA; thus, the above PL-peak shift
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from 1.7 eV to 1.5 eV are attributed to the average-size increase of Si NCs embedded in the film. On the other hand, the quantity of Si clusters in the only RTA-treated film increases with the RTA duration from 20 to 120 s. In addition to Si NCs, the clusters belonged to the smaller aggregates also play the role of new nucleation points and continue to aggregate the segregated silicon to crystallize in the film with the subsequent CTA after RTA; thus, the above PLintensity amplification are attributed to the quantity increase of Si NCs embedded in the film. References [1] Fiory AT. Rapid thermal processing for silicon nanoelectronics applications. JOM 2005;57(6):21e7. [2] Iwayama TS, Hama T, Hole DE, Boyd IW. Enhanced luminescence from encapsulated silicon nanocrystals in SiO2 with rapid thermal anneal. Vacuum 2006;81(2):179e85. [3] Kachurin GA, Cherkova SG, Marin DV, Yankov RA, Deutschmann M. Formation of light-emitting Si nanostructures in SiO2 by pulsed anneals. Nanotechnology 2008;19(35):355305. [4] Silalahi STH, Yang HY, Pita K, Mingbinc Y. Rapid thermal annealing of sputtered silicon-rich oxide/SiO2 superlattice structure. Electrochem Solid-State Lett 2009;12(4):K29e32. [5] Fu MY, Tsai JH, Yang CF, Liao CH. Shift in room-temperature photoluminescence of low-fluence Siþ-implanted SiO2 films subjected to rapid thermal annealing. Sci Technol Adv Mater 2008;9(4):045001. [6] Tsai JH, Fu MY. Improvement of photoluminescence mechanism of CTAtreated Siþ-implanted SiO2 films by using RTA. J Luminescence 2010;130: 1680e6. [7] Fu MY, Tsai JH, Yang CF. Formation of a broad photoluminescence band from Siþ-implanted SiO2 films by varying the heating rate of rapid thermal annealing. Opt Eng 2010;49(7):073801. [8] Franzo G, Vinciquerra V, Priolo F. The excitation mechanism of rare-earth ions in silicon nanocrystals. Appl Phys A 1999;69(3). [9] Savchyn O, Kik PG. Effect of hydrogen passivation on luminescence-centermediated Er excitation in Si-rich SiO2 with and without nanocrystals. Phys Rev B 2008;77:205438. [10] Savchyn O, Todi RM, Coffey KR, Kik PG. High-temperature optical properties of sensitized Er3þ in Si-rich SiO2-implications for gain performance. Opt Mater 2010;32:1274. [11] Nakamura M, Kanzawa R, Sakai K. Stress and density effects on infrared absorption spectra of silicate glass films. J Electrochem Soc 1986;133(6): 1167e71. [12] Sen PN, Thorpe MF. Phonons in AX2 glasses: from molecular to band-like modes. Phys Rev B 1976;15(8):4030e8. [13] Sano N, Sekiya M, Hara M, Kohno A, Sameshima T. Improvement of SiO2/Si interface by low-temperature annealing in wet atmosphere. Appl Phys Lett 1995;66(16):2107e9. [14] Garrido Fernanedz B, Lopez M, Garcia C, Perez-Rodriguez A, Morante JR. Influence of average size and interface passivation on the spectral emission of Si nanocrystals embedded in SiO2. J Appl Phys 2002;91(2):798e807.