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Ne–He bubble formation in co-implanted Si(111) substrates
Thin Solid Films 548 (2013) 465–469
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Ne–He bubble formation in co-implanted Si(111) substrates L.G. Matos, R.M.S. dos Reis, R.L. Maltez ⁎ Instituto de Física, Universidade Federal do Rio Grande do Sul, C.P. 15051, 91501-970 Porto Alegre, RS, Brazil
a r t i c l e
i n f o
Article history: Received 20 August 2012 Received in revised form 24 September 2013 Accepted 25 September 2013 Available online 2 October 2013 Keywords: Ne ion implantation He ion implantation Hybrid bubble system Silicon substrate Rutherford backscattering spectrometry/ channeling Transmission electron microscopy
a b s t r a c t We have studied Si(111) substrates co-implanted with Ne and He, and also separately implanted with these ions, as a function of the annealing temperature. Ne implantations were performed up to fluences of 1 × 1015 and 5 × 1015 cm−2 while keeping the substrate at 350 °C temperature. He implantations were performed at room temperature up to fluences of 5 × 1015 and 1 × 1016 cm−2. The co-implanted samples were first implanted by Ne and then by He ions. These samples were submitted to rapid thermal annealing with temperatures ranging from 350 to 1000 °C. Rutherford backscattering spectrometry/channeling measurements have demonstrated temperature dependent beam dechanneling starting at the implanted ion region. The co-implanted system with 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 shows an improved dechanneling stability in the range of 400–800 °C. Transmission electron microscopy has demonstrated a bubble morphology of Ne–He similar to the sample implanted only with Ne, even for a Ne:He co-implantation ratio of 1:10. We have concluded that the observed dechanneling is mainly due to Ne residual implantation damage and that the annealing behavior of such systems is very different from the He implanted samples. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Some current major difficulties for GaN growth on Si(111) [1–6] are the large difference in thermal expansion coefficients [2,4] and the substantial lattice mismatch (17%) between these two materials. Lateral overgrowth (similar to the one on sapphire [7–9]) has also been applied [5,6] to reduce the observed high density of dislocations [3,4,6]. More recently, a different approach was developed for improving the structural quality of overgrown GaN on Si [10]. The procedure was based on He bubble formation in the subsurface of Si(111) substrate. He was implanted into the substrate before the growth of GaN/AlN, then the AlN layer was grown at the required temperature for He bubble formation, and finally the GaN layer at a temperature about 50 °C higher was grown. Transmission electron microscopy (TEM) and Rutherford backscattering spectrometry/channeling (RBS/C) results showed that He bubbles are formed below the Si surface and that misfit dislocations interact with the stress field around these bubbles. In this way, threading dislocations are redirected into the substrate instead of propagating into the epi-layer. Such threading dislocation redirection has also been observed for the growth of Si–Ge layers [11–13] on Si. In the case of the SiGe growth, He (or H) was implanted into Si substrates through a previously grown pseudomorphic Si–Ge layer (up to 30% Ge). Relaxed and defect free Si–Ge layers were obtained after annealing [11–13]. However, a similar pseudomorphic growth of GaN on Si is impossible due to the large lattice mismatch between these two materials and this procedure does not apply for growth of GaN/AlN on Si. ⁎ Corresponding author. Tel.: +55 51 33086541; fax: +55 51 33087286. E-mail address: [email protected] (R.L. Maltez). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.09.076
GaN growth is usually performed at about 1000 °C [4–9]. However, earlier studies show that GaN growth can be achieved at about 630 °C with the He bubble approach [10]. Such low temperature is required since helium bubbles begin to disappear at higher temperatures [10,14]. For such reason, an over-pressurized bubble system with higher temperature stability may offer even better results. In this work, we have investigated hybrid bubble systems containing Ne, since Ne bubbles have demonstrated much higher thermal stability [15,16] than the He ones. Here we carried out a systematic study of Si(111) implanted with Ne, He, and also co-implanted with Ne and He, aiming to understand the differences and similarities of these systems as compared to the pure He implantation ones. 2. Experimental details Two sets of bare Si 111 samples were implanted with 75 keV-Ne ions up to fluences of 1 and 5 × 1015 cm−2. They were kept at 350 °C during the Ne implantation procedure to minimize the implantation damage. Subsequently, pieces of these two sets were further and individually implanted at room temperature (RT) with 15 keV-He ions up to fluences of 5 × 1015 and 1 × 1016 cm−2, giving rise to four more sets of samples. All implantations were performed using the 500 kV HVEE-ion implanter of the Physics Institute at UFRGS. Both the Ne (75 keV) and the He (15 keV) depth distributions simulated by SRIM 2003 [17] are Gaussian-like profiles with a projected range of about 160 nm and with a full width at half maximum of about 55 nm. Different pieces from all these six sets of samples were subjected to Rapid Thermal Annealing (RTA) for 120 s at 400, 600, 700, 800, 900 and 1000 °C temperatures under N2 flux. The purpose of these annealings
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was to characterize the thermal stability h and i morphology of the formed bubbles. RBS/C measurements along 111 sample direction were carried out with a 1.2 MeV-He+ beam produced by the 3 MV HVEETandem accelerator of the Physics Institute at UFRGS. As-implanted samples from each set were also characterized. Some key samples, selected by RBS/C studies, were characterized by TEM using the JEOL JEM-2010 transmission electron microscope of the Center for Electron Microscopy at UFRGS operated at 200 kV acceleration voltage. TEM specimens were prepared by mechanical polishing and dimpling, followed by ion polishing at shallow angles (~6°). 3. Results and discussions Fig. 1 shows results from the 75 keV-Ne sample implanted up to the fluence of 1 × 1015 cm−2. The sample was kept at 350 °C during implantation. The figure displays channeling spectra for several annealing temperatures. The spectra show a dechanneling plateau whose onset is at about the channel 320 of the multichannel analyzer, which corresponds to the sample region where the Ne ions were implanted. This observed dechanneling plateau could be due to ion implantation damage or bubble formation. The latter would be related either to damage generated in the matrix by the formed bubbles or to channel deformation due to bubble-stress field of over-pressurized bubbles. Both possibilities would result in the same dechanneling profiles as a function of annealing temperature, i.e., there would be a maximum dechanneling rate at the optimum temperature for bubble formation. However, it can be observed in Fig. 1 that the spectrum from the asimplanted sample is the highest one, suggesting that the dechanneling is a consequence of implantation damage rather than bubble formation. Some unclear oscillations of dechanneling intensities at intermediate annealing temperatures can also be observed in Fig. 1. They may be a consequence of factors such as defect annealing in the surface region but negligible change in the “end of range” defect region, or some second order influence of Ne bubble formation. Only the 1000 °C spectrum shows a clear difference regarding the other annealing temperatures. It shows a low yield and smoother raising curve, instead of the characteristic plateau step. The 1000 °C spectrum is only higher than the virgin Si one. The samples implanted up to the fluence of 5 × 1015 Ne/cm2 (not shown), show thermal evolutions which are about the same. Fig. 2(a) and (b) shows spectra from the 15 keV-He implanted samples up to fluences of 5 × 1015 and 1 × 1016 cm−2 after 2 min of RTA in the temperature range of 350–1000 °C. This implantation energy for He results in an ion depth distribution equivalent to the 75 keV-Ne one. As already reported in the literature [10,14], the dechanneling thermal behavior is essentially a consequence of crystal distortions due to Hebubble formation — the low helium mass and implantation fluence
h i Fig. 1. Channeling spectra along the 111 channel from 75 keV-Ne implanted samples up 15 −2 to fluence of 1 × 10 cm . The figure shows spectra as a function of the RTA annealing temperature (see legend) and also the channeled spectrum from non-implanted Si 111 wafer.
Fig. 2. Panels (a) and (b) show spectra from 15 keV-He implanted samples up to fluences of 5 × 1015 and 1 × 1016 cm−2, respectively. This implantation energy results in an equivalent depth distribution to the 75 keV-Ne one at the as-implanted condition. Annealed spectra are shown after RTA in the temperature range of 350–1000 °C for 2 min (see legend).
produces low implantation damage even when it is performed at room temperature. Compare, for example, the as-implanted spectrum in Fig. 2(a) and the 350 °C-annealed one in Fig. 2(b) [full circles in both] to the as-implanted Ne spectrum shown in Fig. 1 (all the spectra shown in this work were normalized by the same integrated current of the RBS incident beam, thus their intensities can be directly compared to each other). The yield of the channeling spectra for the He implanted samples is still about a 2.5 factor lower than the one for the Ne implanted sample. This indicates that keeping the temperature at 350 °C during Ne implantation was not enough to reduce the Ne residual implantation damage to a level as low as the one for He implanted at RT. The shape and intensity behavior of channeling spectra from He implanted samples are very different in the low temperature annealing range (below 700 °C) as compared to the Ne implanted ones. As the annealing temperature increases up to 700 °C, dechanneling also increases for He-implanted samples — see 450 °C [open triangles in Fig. 2(b)], 600 °C [open squares in Fig. 2(a) and (b)] and 700 °C [full triangles in Fig. 2(a)] spectra. The maximum channeling yield for the He implantation is obtained after 450 °C annealing, and it stays at this level up to about 700 °C. This thermal behavior is consistent with a bubble coalescence regime, in which the bubbles increase in size leading to a larger crystal distortion. Ne implantation, however, has not shown dechanneling increase but rather an opposite tendency for this temperature range, as already observed in Fig. 1. Another difference is that, for the He case, the highest fluence (1 × 1016 cm−2) corresponds to the highest dechanneling rate, which is consistent with larger overpressurized bubbles due to an increased He content. This effect was not observed for Ne implantation that has shown about the same dechanneling level, regardless of the fluence. For temperatures higher than 700 °C, the He system shows an excellent channeling quality with the lowest yield for all depths — see 750 °C and 800 °C [crosses in Fig. 2(b) and full squares in Fig. 2(a), respectively], and 1000 °C [full squares in Fig. 2(b)] spectra. This lower dechanneling at higher temperatures is much more pronounced than the one observed for the Ne-implanted sample annealed at 1000 °C. It indicates a much better
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crystal quality of He implanted samples due to bubble dissolution and annihilation. The distinct dechanneling thermal behavior of Ne-implanted samples suggests a decrease of the implanted damage level with annealing. Therefore, the dechanneling is a consequence of residual implantation damage rather than over-pressurized Ne bubbles. However, it does not necessarily mean that we have not formed Ne bubbles. On the contrary, our TEM analysis demonstrates that Ne bubbles are also present in the annealed samples. Fig. 3(a) and (b) are cross-sectional TEM micrographs from the 1 × 1015 Ne/cm2 implanted sample annealed at 800 °C, where the sample surface is indicated by the arrow. These images were taken in an underfocus kinematical condition, which is better to visualize bubbles, and in an in-focus weak-beam condition to visualize damaged regions of the sample. Fig. 3(a) demonstrates a 100 nm wide band of Ne bubbles centered at a depth of about 150 nm, which is in agreement with the depth distribution simulated by SRIM 2003 [17]. The Ne bubbles have a spherical shape with about 5 nm in diameter. The dark areas observed in Fig. 3(b) at about the same region where the Ne bubbles are formed indicate damaged regions in the sample. However, there is no correlation between the bubbles and the damaged area locations, since the latter can also be observed deeper in the sample, where no bubbles are formed. These damaged areas could be attributed to some point-defect clusters located at end of range regions. At this point, we can conclude that Ne implantation temperature of 350 °C is not sufficient to completely annihilate the ion implantation damage, since damaged areas are still observed even in the sample annealed at 800 °C. The present TEM results also indicate that the RBS/C thermal behavior presented in Fig. 1 is dominated by Ne residual implantation damage. The tendency of the Ne implanted system to improve the channeling quality as the annealing temperature increases may also indicate that bubbles in the Ne-implanted samples are less pressurized than the ones formed by He implantation. Fig. 4 shows the results from Ne and He co-implanted sample with 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 ions (1:10 Ne:He ratio with the lowest Ne implantation fluence we have employed in this study). It presents the RBS/C spectra for several RTA temperatures and shows a different thermal behavior from the previously analyzed Ne-implanted samples (see Fig. 1). In the co-implanted case, there is a dechanneling peak (at about channel 310) which is higher for the annealing temperatures of 400 °C and 600 °C than the as-implanted one. The spectra from the samples annealed at 700 and 800 °C have yields as high as the
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h i Fig. 4. Channeling spectra along the 111 channel from Ne and He co-implanted samples 15 2 up to fluencies of 1 × 10 Ne/cm and 1 × 1016 He/cm2, respectively. The figures show spectra as a function of the RTA temperature (see legend). The channeled annealing spectrum from non-implanted Si 111 wafer was also inserted.
as-implanted sample for the lower channels (below channel 275), but the peak has practically vanished. The spectra from the samples annealed at 900 °C and 1000 °C have the lowest yields but still three times higher than the non-implanted Si sample. This thermal evolution and the peak at about channel 310 were unique. All the other Ne plus He co-implanted samples have shown annealing evolution and shape of RBS/C spectra (not shown) very similar to the ones implanted only with Ne — as displayed in Fig. 1. It is interesting to analyze the stability of the hybrid bubble systems at temperatures higher than 700 °C, since this is about the temperature at which helium bubbles begin to be unstable and start to annihilate. For this reason, in Fig. 5 we compare the 800 °C RBS/C spectra from different implantation conditions. The 800 °C annealing temperature was chosen because the RBS/C yields for the next higher annealing temperature, i.e., 900 °C, are systematically lower. We also show in this figure the RBS random spectrum to give a more precise idea about the dechanneling rate. As can be seen in Fig. 5, the 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample [open triangles] has also shown the highest dechanneling. We have also inserted the spectrum from the 1 × 1016 He/cm2-implanted sample annealed at 600 °C (open circles) as reference, since, for this implantation condition, dislocation redirection
Fig. 3. Panels (a) and (b) are cross-sectional TEM micrographs from 1 × 1015 Ne/cm2 implanted sample after annealed at 800 °C, where the surface position is indicated by arrows at the top; (a) underfocus image under kinematical condition, which is better to visualize bubbles, and (b) an in-focus weak-beam image, good to visualize damaged regions in the sample.
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Fig. 5. RBS/C spectra from different Ne–He co-implanted samples and with post-implantation annealing at 800 °C (see legend). Additional spectra for reference: RBS-random spectrum (continuous line) and the spectrum from a sample implanted with 1 × 1016 He/cm2 and annealed at 600 °C (open circles).
was observed, when the AlN/GaN was grown on such substrate [10]. The 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample shows a yield about twice higher than the one for the He-implanted sample (the reference spectrum). However, in the He implantation case, dechanneling is an exclusive consequence of over-pressurized He-bubbles. In the Ne implantations, residual implantation damage is also present in the samples and seems to be the main factor for the observed dechanneling. In Fig. 6(a)–(e) we present TEM of selected samples in order to understand the role of the implantation damage and bubble pressure on dechanneling. Fig. 6(a) and (b) are cross-sectional TEM micrographs from the 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample after 600 °C annealing. They are, respectively, an underfocus image under kinematical condition, which is better to visualize bubbles, and an in-focus weak-beam condition, better to visualize damaged regions in the sample. These co-implantation fluences and this annealing temperature were chosen because they have shown the highest dechanneling rate of all. In addition, as demonstrated by the channeling results in Fig. 2, 600 °C temperature was an optimum temperature for He-bubble formation and then we would expect to have some hybrid morphology for this co-implanted system. However, we have just observed spherical-shaped bubbles, characteristic of the Ne pure system (compare with the TEM image shown in Fig. 3(a)). For comparison sake, Fig. 6(e) presents a micrograph from a He-implanted sample up to the fluence of 1 × 1016 He/cm2 and annealed at 600 °C. The sample surface is indicated by the arrow. This demonstrates that annealed Heimplanted Si-samples show formation of He-filled platelet-like disks [14] with a clear indication of the strain field around. Some smaller bubbles around these platelets are also visible. These strained platelet bubbles were not observed in any Ne co-implanted samples. We then speculate that the pressure is lower in the Ne co-implanted samples. It is remarkable that the bubble morphology of the co-implanted samples does not show any similarity with the He system, even though the co-implanted system has 10 times more He than Ne. In Fig. 6(c) and (d) we present cross-sectional TEM micrographs from the same 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample annealed at 1000 °C. They are an underfocus image under kinematical condition and an in-focus weak-beam image, respectively. As we can see in Fig. 6(c), the spherical bubbles are still visible after annealing at 1000 °C with much larger diameter (15 nm) in comparison with the samples annealed at 600 °C [see Fig. 6(a)]. The decreasing of bubble density is consistent with Ostwald ripening and/or He out-diffusion. TEM images cannot distinguish which atoms fill the bubbles in the annealed Ne–He co-implanted samples. However, TEM images of He-implanted and 1000 °C-annealed samples (not shown) demonstrate a complete annihilation of He bubbles leading to a defect-
Fig. 6. Panels (a)–(d) are cross-sectional TEM micrographs from (1 × 1015 Ne/cm2 and 1 × 1016 He/cm2) co-implanted samples: (a) and (b) are from a sample annealed at 600 °C for 2 min and, (c) and (d), are from a sample annealed at 1000 °C for 2 min. Surface position is marked by arrows. Images at the left-hand side [(a) and (c)] are underfocus images, and at the right-hand side [(b) and (d)] are in-focus weak-beam images; (e) cross-sectional TEM micrograph from a 1 × 1016 He/cm2 implanted sample after annealed at 600 °C.
free Si crystal. Therefore, the bubbles in the Ne–He co-implanted samples annealed at 1000 °C [Fig. 6(c)] are expected to be mainly filled with Ne atoms. Fig. 6 is very important to understand the observed dechanneling in our RBS/C spectra. Fig. 6(a) and (c) reveal an increase in the bubble size as the annealing temperature increases, whereas RBS/C spectra show the lowest yield for annealing at 1000 °C, as previously shown in Fig. 4. This indicates a bubble system with less pressure than the one obtained with He, where dechanneling was a sole consequence of the strain field formed around the over-pressurized He bubbles. This conclusion is also corroborated by the fact that we have not observed, in TEM images from the co-implanted system [Fig. 6(c)], rounded black lobs around the large bubbles. Black lobs would indicate strained Si material due to over-pressurized bubbles. Thus, the bubbles in the co-implanted system are not the main factor to produce dechanneling in the substrate. Therefore, the residual Ne ion implantation damage is the main cause for the observed dechanneling in our co-implanted samples. Comparing the residual damage of the sample annealed at 600 °C [Fig. 6(b)] to the one at
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1000 °C [Fig. 6(d)], we conclude that the latter leads to a good recovery of the residual damage. The observed improvement of the channeling quality has, therefore, correlation with the significant reduction of the damage and not with the increase in bubble size.
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to further understand the role of Ne-implantation damage on the morphology of He bubbles, and also to determine if there is change in the He retention for the hybrid system. Acknowledgments
4. Conclusions In this work, we have shown that bubbles are always present in Ne– He hybrid system, even after annealing at 1000 °C. However, while bubbles increase in size with increasing annealing temperature, the channeling quality of the spectra significantly improves. This fact demonstrates that the dechanneling is not due to over-pressurized bubbles, since in this case the channeling quality of the spectra should decrease with increasing temperature. Furthermore, we do not observe correlation between dechanneling and bubble formation for all the investigated annealing temperature ranges. Among all analyzed samples, only co-implanted 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 showed a slightly different channeling behavior with the annealing temperature. In particular, for the annealing temperatures of 400 and 600 °C, RBS/C spectra have higher yields than the ones from the as-implanted sample. However, typically the annealing decreases the dechanneling with respect to the as-implanted sample. In addition, this specific co-implantation set and temperatures also presented a dechanneling peak (at about channel 310) which resembles the one for implantation with only He. On the contrary, we have observed a consistent correlation between dechanneling decrease and implantation damage reduction, which suggests that defects are the main cause of the observed dechanneling for the systems containing Ne. From TEM analysis it was also clear that bubbles in the hybrid system are all spherical and do not recall any similarity to the He system, even if the Ne:He ratio is as small as 1:10. Thus, from a morphologic point of view, the He–Ne hybrid system and the one implanted only with Ne are very much alike. Work is in progress
We acknowledge financial support from FAPERGS, CNPq and Capes. References [1] C.A. Tran, A. Osinski, R.F. Karlicek, I. Berishev, Appl. Phys. Lett. 75 (1999) 1494. [2] A. Dadgar, M. Poschenrieder, J. Blasing, K. Fehse, A. Diez, A. Krost, Appl. Phys. Lett. 80 (2002) 3670. [3] A. Krost, A. Dadgar, Mater. Sci. Eng. B93 (2002) 77. [4] A. Dadgar, P. Veit, F. Schulze, J. Bläsing, A. Krtschil, H. Witte, A. Diez, T. Hempel, J. Christen, R. Clos, A. Krost, Thin Solid Films 515 (2007) 4356. [5] P. Kung, D. Walker, N. Hamilton, J. Diaz, M. Razeghi, Appl. Phys. Lett. 74 (1999) 570. [6] S. Tanaka, Y. Kawaguchi, N. Sawaki, M. Hibino, K. Hiramatsu, Appl. Phys. Lett. 76 (1999) 2701. [7] S. Nakamura, M. Senoh, S. Nagahara, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Uemoto, M. Sano, K. Chocho, Appl. Phys. Lett. 72 (1998) 211. [8] A. Sakai, H. Sunakawa, A. Usui, Appl. Phys. Lett. 71 (1997) 2259. [9] Y. Kato, S. Kitamura, K. Hiramatsu, N. Sawaki, J. Cryst. Growth 144 (1994) 133. [10] Z. Liliental-Weber, R.L. Maltez, J. Xie, H. Morkoc, J. Cryst. Growth 310 (2008) 3917. [11] B. Hollander, S. Mantl, R. Liedtke, S. Mesters, H.J. Herzog, H. Kibbel, T.B. Hackbarth, Nucl. Instrum. Methods Phys. Res. B148 (1999) 200. [12] H. Trinkaus, B. Holander, St. Rongen, S. Mantl, H.J. Herzog, J. Kuchenbeckern, T. Hackbarth, Appl. Phys. Lett. 76 (2000) 3552. [13] M. Luysberg, D. Kirch, H. Trnkaus, B. Hollander, St. Lenk, S. Mantl, H.J. Hertzog, T. Hackbarth, P.F.P. Fichtner, J. Appl. Phys. 92 (2002) 4290. [14] P.F.P. Fichtner, J.R. Kaschny, A. Kling, H. Trinkhaus, R.A. Yankov, A. Mcklich, W. Skorupa, F.C. Zawislak, L. Amaral, M.F. da Silva, J.C. Soares, Nucl. Instrum. Methods Phys. Res. B136–138 (1998) 460. [15] E. Oliviero, S. Peripolli, P.F.P. Fichtner, L. Amaral, Mater. Sci. Eng. B112 (2004) 111. [16] E. Oliviero, S. Peripolli, L. Amaral, P.F.P. Fichtner, M.F. Beaufort, J.F. Barbot, S.E. Donnelly, J. Appl. Phys. 100 (2006) 043505. [17] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, vol. 1, Pergamon Press, Oxford, 1985.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Ne–He bubble formation in co-implanted Si(111) substrates L.G. Matos, R.M.S. dos Reis, R.L. Maltez ⁎ Instituto de Física, Universidade Federal do Rio Grande do Sul, C.P. 15051, 91501-970 Porto Alegre, RS, Brazil
a r t i c l e
i n f o
Article history: Received 20 August 2012 Received in revised form 24 September 2013 Accepted 25 September 2013 Available online 2 October 2013 Keywords: Ne ion implantation He ion implantation Hybrid bubble system Silicon substrate Rutherford backscattering spectrometry/ channeling Transmission electron microscopy
a b s t r a c t We have studied Si(111) substrates co-implanted with Ne and He, and also separately implanted with these ions, as a function of the annealing temperature. Ne implantations were performed up to fluences of 1 × 1015 and 5 × 1015 cm−2 while keeping the substrate at 350 °C temperature. He implantations were performed at room temperature up to fluences of 5 × 1015 and 1 × 1016 cm−2. The co-implanted samples were first implanted by Ne and then by He ions. These samples were submitted to rapid thermal annealing with temperatures ranging from 350 to 1000 °C. Rutherford backscattering spectrometry/channeling measurements have demonstrated temperature dependent beam dechanneling starting at the implanted ion region. The co-implanted system with 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 shows an improved dechanneling stability in the range of 400–800 °C. Transmission electron microscopy has demonstrated a bubble morphology of Ne–He similar to the sample implanted only with Ne, even for a Ne:He co-implantation ratio of 1:10. We have concluded that the observed dechanneling is mainly due to Ne residual implantation damage and that the annealing behavior of such systems is very different from the He implanted samples. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Some current major difficulties for GaN growth on Si(111) [1–6] are the large difference in thermal expansion coefficients [2,4] and the substantial lattice mismatch (17%) between these two materials. Lateral overgrowth (similar to the one on sapphire [7–9]) has also been applied [5,6] to reduce the observed high density of dislocations [3,4,6]. More recently, a different approach was developed for improving the structural quality of overgrown GaN on Si [10]. The procedure was based on He bubble formation in the subsurface of Si(111) substrate. He was implanted into the substrate before the growth of GaN/AlN, then the AlN layer was grown at the required temperature for He bubble formation, and finally the GaN layer at a temperature about 50 °C higher was grown. Transmission electron microscopy (TEM) and Rutherford backscattering spectrometry/channeling (RBS/C) results showed that He bubbles are formed below the Si surface and that misfit dislocations interact with the stress field around these bubbles. In this way, threading dislocations are redirected into the substrate instead of propagating into the epi-layer. Such threading dislocation redirection has also been observed for the growth of Si–Ge layers [11–13] on Si. In the case of the SiGe growth, He (or H) was implanted into Si substrates through a previously grown pseudomorphic Si–Ge layer (up to 30% Ge). Relaxed and defect free Si–Ge layers were obtained after annealing [11–13]. However, a similar pseudomorphic growth of GaN on Si is impossible due to the large lattice mismatch between these two materials and this procedure does not apply for growth of GaN/AlN on Si. ⁎ Corresponding author. Tel.: +55 51 33086541; fax: +55 51 33087286. E-mail address: [email protected] (R.L. Maltez). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.09.076
GaN growth is usually performed at about 1000 °C [4–9]. However, earlier studies show that GaN growth can be achieved at about 630 °C with the He bubble approach [10]. Such low temperature is required since helium bubbles begin to disappear at higher temperatures [10,14]. For such reason, an over-pressurized bubble system with higher temperature stability may offer even better results. In this work, we have investigated hybrid bubble systems containing Ne, since Ne bubbles have demonstrated much higher thermal stability [15,16] than the He ones. Here we carried out a systematic study of Si(111) implanted with Ne, He, and also co-implanted with Ne and He, aiming to understand the differences and similarities of these systems as compared to the pure He implantation ones. 2. Experimental details Two sets of bare Si 111 samples were implanted with 75 keV-Ne ions up to fluences of 1 and 5 × 1015 cm−2. They were kept at 350 °C during the Ne implantation procedure to minimize the implantation damage. Subsequently, pieces of these two sets were further and individually implanted at room temperature (RT) with 15 keV-He ions up to fluences of 5 × 1015 and 1 × 1016 cm−2, giving rise to four more sets of samples. All implantations were performed using the 500 kV HVEE-ion implanter of the Physics Institute at UFRGS. Both the Ne (75 keV) and the He (15 keV) depth distributions simulated by SRIM 2003 [17] are Gaussian-like profiles with a projected range of about 160 nm and with a full width at half maximum of about 55 nm. Different pieces from all these six sets of samples were subjected to Rapid Thermal Annealing (RTA) for 120 s at 400, 600, 700, 800, 900 and 1000 °C temperatures under N2 flux. The purpose of these annealings
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was to characterize the thermal stability h and i morphology of the formed bubbles. RBS/C measurements along 111 sample direction were carried out with a 1.2 MeV-He+ beam produced by the 3 MV HVEETandem accelerator of the Physics Institute at UFRGS. As-implanted samples from each set were also characterized. Some key samples, selected by RBS/C studies, were characterized by TEM using the JEOL JEM-2010 transmission electron microscope of the Center for Electron Microscopy at UFRGS operated at 200 kV acceleration voltage. TEM specimens were prepared by mechanical polishing and dimpling, followed by ion polishing at shallow angles (~6°). 3. Results and discussions Fig. 1 shows results from the 75 keV-Ne sample implanted up to the fluence of 1 × 1015 cm−2. The sample was kept at 350 °C during implantation. The figure displays channeling spectra for several annealing temperatures. The spectra show a dechanneling plateau whose onset is at about the channel 320 of the multichannel analyzer, which corresponds to the sample region where the Ne ions were implanted. This observed dechanneling plateau could be due to ion implantation damage or bubble formation. The latter would be related either to damage generated in the matrix by the formed bubbles or to channel deformation due to bubble-stress field of over-pressurized bubbles. Both possibilities would result in the same dechanneling profiles as a function of annealing temperature, i.e., there would be a maximum dechanneling rate at the optimum temperature for bubble formation. However, it can be observed in Fig. 1 that the spectrum from the asimplanted sample is the highest one, suggesting that the dechanneling is a consequence of implantation damage rather than bubble formation. Some unclear oscillations of dechanneling intensities at intermediate annealing temperatures can also be observed in Fig. 1. They may be a consequence of factors such as defect annealing in the surface region but negligible change in the “end of range” defect region, or some second order influence of Ne bubble formation. Only the 1000 °C spectrum shows a clear difference regarding the other annealing temperatures. It shows a low yield and smoother raising curve, instead of the characteristic plateau step. The 1000 °C spectrum is only higher than the virgin Si one. The samples implanted up to the fluence of 5 × 1015 Ne/cm2 (not shown), show thermal evolutions which are about the same. Fig. 2(a) and (b) shows spectra from the 15 keV-He implanted samples up to fluences of 5 × 1015 and 1 × 1016 cm−2 after 2 min of RTA in the temperature range of 350–1000 °C. This implantation energy for He results in an ion depth distribution equivalent to the 75 keV-Ne one. As already reported in the literature [10,14], the dechanneling thermal behavior is essentially a consequence of crystal distortions due to Hebubble formation — the low helium mass and implantation fluence
h i Fig. 1. Channeling spectra along the 111 channel from 75 keV-Ne implanted samples up 15 −2 to fluence of 1 × 10 cm . The figure shows spectra as a function of the RTA annealing temperature (see legend) and also the channeled spectrum from non-implanted Si 111 wafer.
Fig. 2. Panels (a) and (b) show spectra from 15 keV-He implanted samples up to fluences of 5 × 1015 and 1 × 1016 cm−2, respectively. This implantation energy results in an equivalent depth distribution to the 75 keV-Ne one at the as-implanted condition. Annealed spectra are shown after RTA in the temperature range of 350–1000 °C for 2 min (see legend).
produces low implantation damage even when it is performed at room temperature. Compare, for example, the as-implanted spectrum in Fig. 2(a) and the 350 °C-annealed one in Fig. 2(b) [full circles in both] to the as-implanted Ne spectrum shown in Fig. 1 (all the spectra shown in this work were normalized by the same integrated current of the RBS incident beam, thus their intensities can be directly compared to each other). The yield of the channeling spectra for the He implanted samples is still about a 2.5 factor lower than the one for the Ne implanted sample. This indicates that keeping the temperature at 350 °C during Ne implantation was not enough to reduce the Ne residual implantation damage to a level as low as the one for He implanted at RT. The shape and intensity behavior of channeling spectra from He implanted samples are very different in the low temperature annealing range (below 700 °C) as compared to the Ne implanted ones. As the annealing temperature increases up to 700 °C, dechanneling also increases for He-implanted samples — see 450 °C [open triangles in Fig. 2(b)], 600 °C [open squares in Fig. 2(a) and (b)] and 700 °C [full triangles in Fig. 2(a)] spectra. The maximum channeling yield for the He implantation is obtained after 450 °C annealing, and it stays at this level up to about 700 °C. This thermal behavior is consistent with a bubble coalescence regime, in which the bubbles increase in size leading to a larger crystal distortion. Ne implantation, however, has not shown dechanneling increase but rather an opposite tendency for this temperature range, as already observed in Fig. 1. Another difference is that, for the He case, the highest fluence (1 × 1016 cm−2) corresponds to the highest dechanneling rate, which is consistent with larger overpressurized bubbles due to an increased He content. This effect was not observed for Ne implantation that has shown about the same dechanneling level, regardless of the fluence. For temperatures higher than 700 °C, the He system shows an excellent channeling quality with the lowest yield for all depths — see 750 °C and 800 °C [crosses in Fig. 2(b) and full squares in Fig. 2(a), respectively], and 1000 °C [full squares in Fig. 2(b)] spectra. This lower dechanneling at higher temperatures is much more pronounced than the one observed for the Ne-implanted sample annealed at 1000 °C. It indicates a much better
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crystal quality of He implanted samples due to bubble dissolution and annihilation. The distinct dechanneling thermal behavior of Ne-implanted samples suggests a decrease of the implanted damage level with annealing. Therefore, the dechanneling is a consequence of residual implantation damage rather than over-pressurized Ne bubbles. However, it does not necessarily mean that we have not formed Ne bubbles. On the contrary, our TEM analysis demonstrates that Ne bubbles are also present in the annealed samples. Fig. 3(a) and (b) are cross-sectional TEM micrographs from the 1 × 1015 Ne/cm2 implanted sample annealed at 800 °C, where the sample surface is indicated by the arrow. These images were taken in an underfocus kinematical condition, which is better to visualize bubbles, and in an in-focus weak-beam condition to visualize damaged regions of the sample. Fig. 3(a) demonstrates a 100 nm wide band of Ne bubbles centered at a depth of about 150 nm, which is in agreement with the depth distribution simulated by SRIM 2003 [17]. The Ne bubbles have a spherical shape with about 5 nm in diameter. The dark areas observed in Fig. 3(b) at about the same region where the Ne bubbles are formed indicate damaged regions in the sample. However, there is no correlation between the bubbles and the damaged area locations, since the latter can also be observed deeper in the sample, where no bubbles are formed. These damaged areas could be attributed to some point-defect clusters located at end of range regions. At this point, we can conclude that Ne implantation temperature of 350 °C is not sufficient to completely annihilate the ion implantation damage, since damaged areas are still observed even in the sample annealed at 800 °C. The present TEM results also indicate that the RBS/C thermal behavior presented in Fig. 1 is dominated by Ne residual implantation damage. The tendency of the Ne implanted system to improve the channeling quality as the annealing temperature increases may also indicate that bubbles in the Ne-implanted samples are less pressurized than the ones formed by He implantation. Fig. 4 shows the results from Ne and He co-implanted sample with 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 ions (1:10 Ne:He ratio with the lowest Ne implantation fluence we have employed in this study). It presents the RBS/C spectra for several RTA temperatures and shows a different thermal behavior from the previously analyzed Ne-implanted samples (see Fig. 1). In the co-implanted case, there is a dechanneling peak (at about channel 310) which is higher for the annealing temperatures of 400 °C and 600 °C than the as-implanted one. The spectra from the samples annealed at 700 and 800 °C have yields as high as the
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h i Fig. 4. Channeling spectra along the 111 channel from Ne and He co-implanted samples 15 2 up to fluencies of 1 × 10 Ne/cm and 1 × 1016 He/cm2, respectively. The figures show spectra as a function of the RTA temperature (see legend). The channeled annealing spectrum from non-implanted Si 111 wafer was also inserted.
as-implanted sample for the lower channels (below channel 275), but the peak has practically vanished. The spectra from the samples annealed at 900 °C and 1000 °C have the lowest yields but still three times higher than the non-implanted Si sample. This thermal evolution and the peak at about channel 310 were unique. All the other Ne plus He co-implanted samples have shown annealing evolution and shape of RBS/C spectra (not shown) very similar to the ones implanted only with Ne — as displayed in Fig. 1. It is interesting to analyze the stability of the hybrid bubble systems at temperatures higher than 700 °C, since this is about the temperature at which helium bubbles begin to be unstable and start to annihilate. For this reason, in Fig. 5 we compare the 800 °C RBS/C spectra from different implantation conditions. The 800 °C annealing temperature was chosen because the RBS/C yields for the next higher annealing temperature, i.e., 900 °C, are systematically lower. We also show in this figure the RBS random spectrum to give a more precise idea about the dechanneling rate. As can be seen in Fig. 5, the 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample [open triangles] has also shown the highest dechanneling. We have also inserted the spectrum from the 1 × 1016 He/cm2-implanted sample annealed at 600 °C (open circles) as reference, since, for this implantation condition, dislocation redirection
Fig. 3. Panels (a) and (b) are cross-sectional TEM micrographs from 1 × 1015 Ne/cm2 implanted sample after annealed at 800 °C, where the surface position is indicated by arrows at the top; (a) underfocus image under kinematical condition, which is better to visualize bubbles, and (b) an in-focus weak-beam image, good to visualize damaged regions in the sample.
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Fig. 5. RBS/C spectra from different Ne–He co-implanted samples and with post-implantation annealing at 800 °C (see legend). Additional spectra for reference: RBS-random spectrum (continuous line) and the spectrum from a sample implanted with 1 × 1016 He/cm2 and annealed at 600 °C (open circles).
was observed, when the AlN/GaN was grown on such substrate [10]. The 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample shows a yield about twice higher than the one for the He-implanted sample (the reference spectrum). However, in the He implantation case, dechanneling is an exclusive consequence of over-pressurized He-bubbles. In the Ne implantations, residual implantation damage is also present in the samples and seems to be the main factor for the observed dechanneling. In Fig. 6(a)–(e) we present TEM of selected samples in order to understand the role of the implantation damage and bubble pressure on dechanneling. Fig. 6(a) and (b) are cross-sectional TEM micrographs from the 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample after 600 °C annealing. They are, respectively, an underfocus image under kinematical condition, which is better to visualize bubbles, and an in-focus weak-beam condition, better to visualize damaged regions in the sample. These co-implantation fluences and this annealing temperature were chosen because they have shown the highest dechanneling rate of all. In addition, as demonstrated by the channeling results in Fig. 2, 600 °C temperature was an optimum temperature for He-bubble formation and then we would expect to have some hybrid morphology for this co-implanted system. However, we have just observed spherical-shaped bubbles, characteristic of the Ne pure system (compare with the TEM image shown in Fig. 3(a)). For comparison sake, Fig. 6(e) presents a micrograph from a He-implanted sample up to the fluence of 1 × 1016 He/cm2 and annealed at 600 °C. The sample surface is indicated by the arrow. This demonstrates that annealed Heimplanted Si-samples show formation of He-filled platelet-like disks [14] with a clear indication of the strain field around. Some smaller bubbles around these platelets are also visible. These strained platelet bubbles were not observed in any Ne co-implanted samples. We then speculate that the pressure is lower in the Ne co-implanted samples. It is remarkable that the bubble morphology of the co-implanted samples does not show any similarity with the He system, even though the co-implanted system has 10 times more He than Ne. In Fig. 6(c) and (d) we present cross-sectional TEM micrographs from the same 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 co-implanted sample annealed at 1000 °C. They are an underfocus image under kinematical condition and an in-focus weak-beam image, respectively. As we can see in Fig. 6(c), the spherical bubbles are still visible after annealing at 1000 °C with much larger diameter (15 nm) in comparison with the samples annealed at 600 °C [see Fig. 6(a)]. The decreasing of bubble density is consistent with Ostwald ripening and/or He out-diffusion. TEM images cannot distinguish which atoms fill the bubbles in the annealed Ne–He co-implanted samples. However, TEM images of He-implanted and 1000 °C-annealed samples (not shown) demonstrate a complete annihilation of He bubbles leading to a defect-
Fig. 6. Panels (a)–(d) are cross-sectional TEM micrographs from (1 × 1015 Ne/cm2 and 1 × 1016 He/cm2) co-implanted samples: (a) and (b) are from a sample annealed at 600 °C for 2 min and, (c) and (d), are from a sample annealed at 1000 °C for 2 min. Surface position is marked by arrows. Images at the left-hand side [(a) and (c)] are underfocus images, and at the right-hand side [(b) and (d)] are in-focus weak-beam images; (e) cross-sectional TEM micrograph from a 1 × 1016 He/cm2 implanted sample after annealed at 600 °C.
free Si crystal. Therefore, the bubbles in the Ne–He co-implanted samples annealed at 1000 °C [Fig. 6(c)] are expected to be mainly filled with Ne atoms. Fig. 6 is very important to understand the observed dechanneling in our RBS/C spectra. Fig. 6(a) and (c) reveal an increase in the bubble size as the annealing temperature increases, whereas RBS/C spectra show the lowest yield for annealing at 1000 °C, as previously shown in Fig. 4. This indicates a bubble system with less pressure than the one obtained with He, where dechanneling was a sole consequence of the strain field formed around the over-pressurized He bubbles. This conclusion is also corroborated by the fact that we have not observed, in TEM images from the co-implanted system [Fig. 6(c)], rounded black lobs around the large bubbles. Black lobs would indicate strained Si material due to over-pressurized bubbles. Thus, the bubbles in the co-implanted system are not the main factor to produce dechanneling in the substrate. Therefore, the residual Ne ion implantation damage is the main cause for the observed dechanneling in our co-implanted samples. Comparing the residual damage of the sample annealed at 600 °C [Fig. 6(b)] to the one at
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1000 °C [Fig. 6(d)], we conclude that the latter leads to a good recovery of the residual damage. The observed improvement of the channeling quality has, therefore, correlation with the significant reduction of the damage and not with the increase in bubble size.
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to further understand the role of Ne-implantation damage on the morphology of He bubbles, and also to determine if there is change in the He retention for the hybrid system. Acknowledgments
4. Conclusions In this work, we have shown that bubbles are always present in Ne– He hybrid system, even after annealing at 1000 °C. However, while bubbles increase in size with increasing annealing temperature, the channeling quality of the spectra significantly improves. This fact demonstrates that the dechanneling is not due to over-pressurized bubbles, since in this case the channeling quality of the spectra should decrease with increasing temperature. Furthermore, we do not observe correlation between dechanneling and bubble formation for all the investigated annealing temperature ranges. Among all analyzed samples, only co-implanted 1 × 1015 Ne/cm2 and 1 × 1016 He/cm2 showed a slightly different channeling behavior with the annealing temperature. In particular, for the annealing temperatures of 400 and 600 °C, RBS/C spectra have higher yields than the ones from the as-implanted sample. However, typically the annealing decreases the dechanneling with respect to the as-implanted sample. In addition, this specific co-implantation set and temperatures also presented a dechanneling peak (at about channel 310) which resembles the one for implantation with only He. On the contrary, we have observed a consistent correlation between dechanneling decrease and implantation damage reduction, which suggests that defects are the main cause of the observed dechanneling for the systems containing Ne. From TEM analysis it was also clear that bubbles in the hybrid system are all spherical and do not recall any similarity to the He system, even if the Ne:He ratio is as small as 1:10. Thus, from a morphologic point of view, the He–Ne hybrid system and the one implanted only with Ne are very much alike. Work is in progress
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