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Ge nano-layer fabricated by high-fluence low-energy ion implantation
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 250 (2006) 183–187 www.elsevier.com/locate/nimb
Ge nano-layer fabricated by high-fluence low-energy ion implantation Tiecheng Lu a,b,*, Shaobo Dun a, Qiang Hu a, Songbao Zhang a, Zhu An c, Yanmin Duan c, Sha Zhu d, Qiangmin Wei d, Lumin Wang d a
c
Department of Physics, Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, China b International Center for Material Physics, Chinese Academy of Sciences, Shenyang 110015, China Institute of Nuclear Science and Technology, Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, China d Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA Available online 13 June 2006
Abstract A Ge nano-layer embedded in the surface layer of an amorphous SiO2 film was fabricated by high-fluence low-energy ion implantation. The component, phase, nano-structure and luminescence properties of the nano-layer were studied by means of Rutherford backscattering, glancing incident X-ray diffraction, laser Raman scattering, transmission electron microscopy and photoluminescence. The relation between nano-particle characteristics and ion fluence was also studied. The results indicate that nano-crystalline Ge and nano-amorphous Ge particles coexist in the nano-layer and the ratio of nano-crystalline Ge to nano-particle Ge increases with increasing ion fluence. The intensity of photoluminescence from the nano-layer increases with increasing ion fluence also. Prepared with certain ion fluences, high-density nano-layers composed of uniform-sized nano-particles can be observed. 2006 Elsevier B.V. All rights reserved. PACS: 78.67.Bf; 85.40.Ry; 87.64.Je; 78.55.m Keywords: Ge nano-layer; Ion implantation; Laser Raman scattering; Photoluminescence; Transmission electron microscopy; X-ray diffraction
1. Introduction Si/Ge nano-crystals embedded in an amorphous SiO2 film have attracted great interest, due to their potential application in novel optoelectronic and nano-electronic devices [1–21]. Preparing a high-density nano-layer, composed of nano-particles (especially nano-crystals), with high-intensity photoluminescence has been one of the principal challenges in this field. Ion implantation with subsequent annealing has been extensively investigated as a promising fabrication technique for forming nano-crystals *
Corresponding author. Address: Department of Physics, Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, China. Tel.: +86 28 85412031; fax: +86 28 85417106. E-mail address: [email protected] (T. Lu). 0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.04.105
in dielectric materials. This technique can enable the controlled depth distribution and the required density of implanted ions in matrix by changing ion energy and fluence. However, it is well known that the ion fluence of 1 · 1017 cm2 has greatly exceeded a solid surface atomic density (1 · 1015 cm2), so that the ion implantation with the fluence above 1 · 1017 cm2 was less carried out for nano-crystalline Ge (nc-Ge) preparation [1–21]. As a result, the density of nano-particles is low and the intensity of corresponding photoluminescence is weak. It has not been clarified whether high-density nano-particles with highintensity photoluminescence can be prepared by ion implantation with higher fluence than 1 · 1017 cm2. In addition, the subsequent annealing, which has been considered a necessary procedure for preparing nano-crystals, results in the enlarging of nc-Ge size and the widening of the size-distribution. Fortunately, low-energy ion
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implantation offers potential for control of nano-particle size in layered structures [1,13]. It may be used to prepare uniform-sized nano-particles (nano-crystals), without subsequent annealing, combining with high-fluence ion implantation. In this paper, in order to explore preparing a novel nano-structure with the high-density nano-particles (nano-crystals), high-fluence low-energy ion implantation technique was carried out. As a result, a high-density nano-layer with uniform-sized nano-particles and highintensity photoluminescence was obtained. 2. Experimental Amorphous SiO2 films of 300 nm thick were thermally grown on p-type Si(1 1 1) wafers by dry–wet–dry oxidation at 1180 C. The Ge atoms were ionized to Ge+ and Ge2+ ions with a ratio of 6:4 under arc discharge. The Ge+ and Ge2+ ions, with acceleration voltage of 40 kV, were implanted into SiO2 films simultaneously in a JZD-800 multi-functional ion-implanter without a magnetic analyzer. The projected fluences are 1 · 1016, 1 · 1017, 5 · 1017 and 1 · 1018 cm2, respectively and the flux of implantation is about 7 · 1013 cm2 s1. These samples are labeled as 1E16, 1E17, 5E17 and 1E18, respectively. During implantation, the samples were glued onto a copper plate (using silver dag) above flowing water and the implantation was paused 300 s after it ran for 300 s. The Rutherford backscattering (RBS) was used to quantify the elemental component of the samples; glancing incident X-ray scattering (GIXRD) and laser Raman scattering (LRS) were used to detect the existing phase of Ge atoms in the surface layer; and cross-sectional transmission electron microscopy (XTEM) and high-resolution transmission electron microscopy (HRTEM) were used to examine the morphology, size, phase and depth distribution of the nano-particles in the samples. The photoluminescence (PL) spectra were used to study the luminescence property of the nano-particles. RBS was conducted using a J2.5 electrostatic accelerator with a 1.98 MeV He+ ion beam and a detection angle of 150. GIXRD were carried out by means of PHILIPS X 0 Pert Pro MPD Apparatus with a working power of 40 kV · 30 mA. The incident angle of X-ray (0.154 nm) was 0.5. LRS was carried out with a RENISHAW RM-2000 microscopic confocal Raman spectrometer with 90 configuration and working power of 4.6 mW; the spectra was excited by 514.5 nm line of Ar+ laser. The samples were prepared in cross-section for TEM observation and were examined with a JEOL 2010 FEG transmission electron microscope. The PL spectra were measured by HITACHI fluorescence spectrometer and excited by 240 nm line of Xe lamp. 3. Results and discussion Fig. 1 shows RBS spectra of sample 1E16, 1E17 and 1E18. The amount of Ge atoms embedded in the SiO2 film were simulated from RBS results, shown in Table 1. From
Fig. 1. RBS spectra of sample 1E16, 1E17 and 1E18.
Table 1 Comparison of the projected Ge ion fluence and the measured ion fluence from RBS Projected fluence (cm2)
Measured fluence from RBS (cm2)
1 · 1016 1 · 1017 1 · 1018
1.4 · 1015 2.3 · 1016 4.3 · 1016
the RBS results, Ge atoms in sample 1E16 distribute in a very narrow zone. In sample 1E17 and 1E18, the distribution width increases obviously. The profile of sample 1E18 is similar to that of sample 1E17. Moreover, the amount of Ge atoms only increases slightly with increasing fluence. As for every fluence used in this study, the measured fluence is less than the projected fluence. It is suggested that when ion implantation is carried out at the surface layer of SiO2 film due to low-implantation energy, the sputtering loss of Ge ions is unavoidable; in addition, steady-state Ge profile is attained when fluence is increased to 1 · 1018 cm2 [22]. In order to investigate the phase of Ge nano-particles embedded in the SiO2 film, GIXRD spectra of four samples were measured, as shown in Fig. 2. In Fig. 2(a), there is no diffractive peak of crystalline Ge after ion implantation, which is in accord with other authors’ reports [14,6], i.e. there is no perceivable nc-Ge in as-implanted sample. In Fig. 2(b), a small crystalline peak of Ge (1 1 1) emerges. Fig. 2(c) shows that the crystalline peak of Ge (1 1 1) grows higher and other crystalline peaks (2 2 0), (3 1 1), (4 0 0), (3 3 1) and (4 4 2) appear, respectively. In Fig. 2(d), the peaks of crystalline Ge become more intense with increasing fluence. The results of GIXRD clearly indicate that nc-Ge can be fabricated by means of high-fluence lowenergy ion implantation. In order to verify the GIXRD results and confirm the nc-Ge directly formed by ion implantation, the LRS spectra of four samples were measured, as shown in Fig. 3. The
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Fig. 2. GIXRD spectra of sample 1E16, 1E17, 5E17 and 1E18.
Fig. 3. LRS spectra of sample 1E16, 1E17, 5E17 and 1E18.
wide diffuse peak centered at 275 cm1 belongs to nanoamorphous Ge (na-Ge) and the peak around 300 cm1 belongs to nc-Ge [15–17]. Fig. 3(b) shows that, when the fluence is increased to 1 · 1017 cm2, the nc-Ge appear in SiO2 films. In Fig. 3(c) and (d), the more the fluence is increased, the higher the crystalline peaks’ intensity becomes. The LRS result is consistent with GIXRD. In order to investigate the size and size distribution of nano-particles, as well as the ratio of nc-Ge to Ge nanoparticles in the nano-layer, XTEM and HRTEM images of samples were observed, as shown in Fig. 4. In the image of sample 1E16, there are not obvious nano-particles in the SiO2 film. About 40 nm-thick nano-layers, constructed by nano-particles, are obviously found in samples 1E17,
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5E17 and 1E18, respectively. The density of nano-particles in the nano-layer increases with increasing fluence from 1 · 1017 to 5 · 1017 cm2, then that saturates when fluence is increased higher. In sample 1E17 and 1E18, the size of the nano-particles in the nano-layer is almost uniform, but in sample 5E17, the size of nano-particles decreases with going deep into sample. That is to say, the size of nano-particles presents a gradient depth distribution. During observation, it can be found that in sample 1E17, most of the Ge nano-particles are amorphous, just 1% of Ge nano-particles is nano-crystals and the size of nano-crystals in the HRTEM image is about 2–3 nm. As for sample 1E18, the amount of nc-Ge increases obviously; about 30% of Ge nano-particles is nano-crystals. The size of the nano-crystals is more or less 4–5 nm. It seems that the nano-layer with uniform nano-particles, which contain nc-Ge, can be prepared by high-fluence low-energy ion implantation with certain fluence. The ratio of nc-Ge and the size of nano-particles (nano-crystals) increase with increasing fluence. The PL spectra of four samples with different fluences are shown in Fig. 5. A peak centered at 510 nm is a scattered peak of light source. It is found that three PL peaks, centered at 295, 400 and 575 nm, respectively, become higher with increasing fluence, but height change of their peaks of different samples is different, indicating that there is different PL origin among three peaks. Two PL peaks centered at 295 and 400 nm are related to an interface defect of Ge–SiO2 [19] and 575 nm PL peak is related to nc-Ge [10]. When fluence is increased, based on the TEM observation above, the concentrations of nc-Ge and Ge–SiO2 interface area increase, so the intensity of the PL peaks will increase. The possible formation mechanism of the Ge nano-layer may be discussed as follows based on self-organization theory. As for the nano-layer formation, nucleation and spinodal decomposition both exist during ion implantation [23,24]. When the samples are prepared by low-energy ion implantation with low-fluence of 1 · 1016 cm2, a surface sputtering effect induces a local high-temperature zone at the surface layer. The local high-temperature nucleates Ge ions into na-Ge due to the minimum energy principle, because the Gibbs free energy of a nano-particle aggregated from ions is much lower than the energy sum of these ions. But decomposition is preferred over nucleation; as a result, the size and the concentration of the nano-particles is too small to observe. When the fluence is increased from 1 · 1016 to 1 · 1017 cm2, the size and the concentration of nano-particles increase gradually. When the fluence is increased to 1 · 1017 cm2, the nucleation balances decomposition. As a result, the nano-particles become uniformsized; in particular, the nc-Ge start to separate form na-Ge. When the fluence is increased from 1 · 1017 to 5 · 1017 cm2, superfluous Ge ions bump on the nano-particles at the surface layer of the nano-layer; the balance between nucleation and decomposition is destroyed; nucleation is preferred over decomposition at the surface layer,
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Fig. 4. XTEM and HRTEM images of sample 1E16, 1E17, 5E17 and 1E18.
phous SiO2 film, can be directly fabricated with high-fluence low-energy ion implantation. The nc-Ge and na-Ge coexist in the nano-layer. The ratio of nc-Ge to nanoparticles increases with increasing fluence. The measured Ge atom distribution in the nano-layer is less than 5 · 1016 cm2, even if the projected fluence is increased up to 1 · 1018 cm2. Local high-temperature induced by the sputtering effect of superfluous Ge ions is contributed to na-Ge formation and nc-Ge separating from na-Ge and the size uniformity of nano-particles may be due to the balance between nucleation and decomposition. As a result, the intensity of PL in samples increases with increasing fluence also. Acknowledgements
Fig. 5. PL spectra of sample 1E16, 1E17, 5E17 and 1E18, excited by 240 nm UV-light.
thus the nano-particles at the surface layer grow to 10– 12 nm; and the size of deep-located nano-particles remains small. In addition, the local high-temperature caused by the superfluous ions sputtering makes more nc-Ge separate from na-Ge. When the fluence is increased to 1 · 1018 cm2, nucleation balances decomposition again due to reaching a steady-state Ge profile [22], and nanoparticles become uniform-sized again. Hence when the fluence is increased further, it is expected that the size of nano-particles will not change much. 4. Conclusions The high-density Ge nano-layer, with uniform-sized nano-particles, embedded in the surface layer of the amor-
This research was supported by a grant #3-405 from the Ministry of Science and Technology, Israel, within the Cooperation Program with the Ministry of Science and Technology, China and NSAF of NSFC-CAEP of China (no. 10376020), as well as Program for New Century Excellent Talents in University, China (no. NCET-04-0874). The authors also would like to thank Analytic and Testing Center, Sichuan University, for support on XRD and PL measurements of samples. References [1] A. Meidrum, R.F. Haglund, L.A. Boarter, C.W. White, Adv. Mater. 13 (2001) 1431. [2] C.W. White, J.D. Budai, S.P. Withrow, J.G. Zhu, S.J. Pennycook, R.A. Zuhr, D.M. Hembree, D.O. Henderson, R.H. Magruder, M.J. Yacaman, G. Mondragon, S. Prawer, Nucl. Instr. and Meth. B 127 (1997) 583. [3] Y.M. Yang, X.L. Wu, G.G. Siu, G.S. Huang, J.C. Shen, D.S. Hu, J. Appl. Phys. 96 (2004) 5239. [4] K.S. Min, K. Scheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 69 (1996) 2033.
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[15] S. Guha, M. Wall, L.L. Chase, Nucl. Instr. and Meth. B 147 (1999) 367. [16] D.C. Paine, C. Caragianis, Y. Shigesato, Appl. Phys. Lett. 60 (1992) 2886. [17] D.C. Paine, C. Caragianis, T.Y. Kim, Y. Shigesato, Appl. Phys. Lett. 62 (1993) 2842. [18] S.F. Ren, W. Cheng, P.Y. Yu, Phys. Rev. B 69 (2004) 235. [19] J.Y. Zhang, Y.H. Ye, X.L. Tan, X.M. Bao, J. Appl. Phys. 86 (1999) 6139. [20] J.K. Shen, X.L. Wu, R.K. Yuan, N. Tang, J.P. Zou, Y.F. Mei, C. Tan, X.M. Bao, G.G. Siu, Appl. Phys. Lett. 77 (1990) 3134. [21] M.C. Ridgway, G.M. Azevedo, R.G. Elliman, C.J. Glover, D.J. Llewellyn, R. Miller, W. Wesch, G.J. Foran, J. Hansen, A. Nylandsted-Larsen, Phys. Rev. B 71 (2005) 094107. [22] M. Nastasi, J.W. Mayer, J.K. Hirvonen, in: J.W. Rabalais (Ed.), Ion–Solid Interaction, Cambridge University Press, London, 1996, p. 228. [23] T. Muller, K.H. Heinig, W. Moller, Appl. Phys. Lett. 81 (1998) 3049. [24] T. Muller, K.H. Heinig, W. Moller, Appl. Phys. Lett. 85 (2004) 2373.
Nuclear Instruments and Methods in Physics Research B 250 (2006) 183–187 www.elsevier.com/locate/nimb
Ge nano-layer fabricated by high-fluence low-energy ion implantation Tiecheng Lu a,b,*, Shaobo Dun a, Qiang Hu a, Songbao Zhang a, Zhu An c, Yanmin Duan c, Sha Zhu d, Qiangmin Wei d, Lumin Wang d a
c
Department of Physics, Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, China b International Center for Material Physics, Chinese Academy of Sciences, Shenyang 110015, China Institute of Nuclear Science and Technology, Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, China d Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA Available online 13 June 2006
Abstract A Ge nano-layer embedded in the surface layer of an amorphous SiO2 film was fabricated by high-fluence low-energy ion implantation. The component, phase, nano-structure and luminescence properties of the nano-layer were studied by means of Rutherford backscattering, glancing incident X-ray diffraction, laser Raman scattering, transmission electron microscopy and photoluminescence. The relation between nano-particle characteristics and ion fluence was also studied. The results indicate that nano-crystalline Ge and nano-amorphous Ge particles coexist in the nano-layer and the ratio of nano-crystalline Ge to nano-particle Ge increases with increasing ion fluence. The intensity of photoluminescence from the nano-layer increases with increasing ion fluence also. Prepared with certain ion fluences, high-density nano-layers composed of uniform-sized nano-particles can be observed. 2006 Elsevier B.V. All rights reserved. PACS: 78.67.Bf; 85.40.Ry; 87.64.Je; 78.55.m Keywords: Ge nano-layer; Ion implantation; Laser Raman scattering; Photoluminescence; Transmission electron microscopy; X-ray diffraction
1. Introduction Si/Ge nano-crystals embedded in an amorphous SiO2 film have attracted great interest, due to their potential application in novel optoelectronic and nano-electronic devices [1–21]. Preparing a high-density nano-layer, composed of nano-particles (especially nano-crystals), with high-intensity photoluminescence has been one of the principal challenges in this field. Ion implantation with subsequent annealing has been extensively investigated as a promising fabrication technique for forming nano-crystals *
Corresponding author. Address: Department of Physics, Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, China. Tel.: +86 28 85412031; fax: +86 28 85417106. E-mail address: [email protected] (T. Lu). 0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.04.105
in dielectric materials. This technique can enable the controlled depth distribution and the required density of implanted ions in matrix by changing ion energy and fluence. However, it is well known that the ion fluence of 1 · 1017 cm2 has greatly exceeded a solid surface atomic density (1 · 1015 cm2), so that the ion implantation with the fluence above 1 · 1017 cm2 was less carried out for nano-crystalline Ge (nc-Ge) preparation [1–21]. As a result, the density of nano-particles is low and the intensity of corresponding photoluminescence is weak. It has not been clarified whether high-density nano-particles with highintensity photoluminescence can be prepared by ion implantation with higher fluence than 1 · 1017 cm2. In addition, the subsequent annealing, which has been considered a necessary procedure for preparing nano-crystals, results in the enlarging of nc-Ge size and the widening of the size-distribution. Fortunately, low-energy ion
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implantation offers potential for control of nano-particle size in layered structures [1,13]. It may be used to prepare uniform-sized nano-particles (nano-crystals), without subsequent annealing, combining with high-fluence ion implantation. In this paper, in order to explore preparing a novel nano-structure with the high-density nano-particles (nano-crystals), high-fluence low-energy ion implantation technique was carried out. As a result, a high-density nano-layer with uniform-sized nano-particles and highintensity photoluminescence was obtained. 2. Experimental Amorphous SiO2 films of 300 nm thick were thermally grown on p-type Si(1 1 1) wafers by dry–wet–dry oxidation at 1180 C. The Ge atoms were ionized to Ge+ and Ge2+ ions with a ratio of 6:4 under arc discharge. The Ge+ and Ge2+ ions, with acceleration voltage of 40 kV, were implanted into SiO2 films simultaneously in a JZD-800 multi-functional ion-implanter without a magnetic analyzer. The projected fluences are 1 · 1016, 1 · 1017, 5 · 1017 and 1 · 1018 cm2, respectively and the flux of implantation is about 7 · 1013 cm2 s1. These samples are labeled as 1E16, 1E17, 5E17 and 1E18, respectively. During implantation, the samples were glued onto a copper plate (using silver dag) above flowing water and the implantation was paused 300 s after it ran for 300 s. The Rutherford backscattering (RBS) was used to quantify the elemental component of the samples; glancing incident X-ray scattering (GIXRD) and laser Raman scattering (LRS) were used to detect the existing phase of Ge atoms in the surface layer; and cross-sectional transmission electron microscopy (XTEM) and high-resolution transmission electron microscopy (HRTEM) were used to examine the morphology, size, phase and depth distribution of the nano-particles in the samples. The photoluminescence (PL) spectra were used to study the luminescence property of the nano-particles. RBS was conducted using a J2.5 electrostatic accelerator with a 1.98 MeV He+ ion beam and a detection angle of 150. GIXRD were carried out by means of PHILIPS X 0 Pert Pro MPD Apparatus with a working power of 40 kV · 30 mA. The incident angle of X-ray (0.154 nm) was 0.5. LRS was carried out with a RENISHAW RM-2000 microscopic confocal Raman spectrometer with 90 configuration and working power of 4.6 mW; the spectra was excited by 514.5 nm line of Ar+ laser. The samples were prepared in cross-section for TEM observation and were examined with a JEOL 2010 FEG transmission electron microscope. The PL spectra were measured by HITACHI fluorescence spectrometer and excited by 240 nm line of Xe lamp. 3. Results and discussion Fig. 1 shows RBS spectra of sample 1E16, 1E17 and 1E18. The amount of Ge atoms embedded in the SiO2 film were simulated from RBS results, shown in Table 1. From
Fig. 1. RBS spectra of sample 1E16, 1E17 and 1E18.
Table 1 Comparison of the projected Ge ion fluence and the measured ion fluence from RBS Projected fluence (cm2)
Measured fluence from RBS (cm2)
1 · 1016 1 · 1017 1 · 1018
1.4 · 1015 2.3 · 1016 4.3 · 1016
the RBS results, Ge atoms in sample 1E16 distribute in a very narrow zone. In sample 1E17 and 1E18, the distribution width increases obviously. The profile of sample 1E18 is similar to that of sample 1E17. Moreover, the amount of Ge atoms only increases slightly with increasing fluence. As for every fluence used in this study, the measured fluence is less than the projected fluence. It is suggested that when ion implantation is carried out at the surface layer of SiO2 film due to low-implantation energy, the sputtering loss of Ge ions is unavoidable; in addition, steady-state Ge profile is attained when fluence is increased to 1 · 1018 cm2 [22]. In order to investigate the phase of Ge nano-particles embedded in the SiO2 film, GIXRD spectra of four samples were measured, as shown in Fig. 2. In Fig. 2(a), there is no diffractive peak of crystalline Ge after ion implantation, which is in accord with other authors’ reports [14,6], i.e. there is no perceivable nc-Ge in as-implanted sample. In Fig. 2(b), a small crystalline peak of Ge (1 1 1) emerges. Fig. 2(c) shows that the crystalline peak of Ge (1 1 1) grows higher and other crystalline peaks (2 2 0), (3 1 1), (4 0 0), (3 3 1) and (4 4 2) appear, respectively. In Fig. 2(d), the peaks of crystalline Ge become more intense with increasing fluence. The results of GIXRD clearly indicate that nc-Ge can be fabricated by means of high-fluence lowenergy ion implantation. In order to verify the GIXRD results and confirm the nc-Ge directly formed by ion implantation, the LRS spectra of four samples were measured, as shown in Fig. 3. The
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Fig. 2. GIXRD spectra of sample 1E16, 1E17, 5E17 and 1E18.
Fig. 3. LRS spectra of sample 1E16, 1E17, 5E17 and 1E18.
wide diffuse peak centered at 275 cm1 belongs to nanoamorphous Ge (na-Ge) and the peak around 300 cm1 belongs to nc-Ge [15–17]. Fig. 3(b) shows that, when the fluence is increased to 1 · 1017 cm2, the nc-Ge appear in SiO2 films. In Fig. 3(c) and (d), the more the fluence is increased, the higher the crystalline peaks’ intensity becomes. The LRS result is consistent with GIXRD. In order to investigate the size and size distribution of nano-particles, as well as the ratio of nc-Ge to Ge nanoparticles in the nano-layer, XTEM and HRTEM images of samples were observed, as shown in Fig. 4. In the image of sample 1E16, there are not obvious nano-particles in the SiO2 film. About 40 nm-thick nano-layers, constructed by nano-particles, are obviously found in samples 1E17,
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5E17 and 1E18, respectively. The density of nano-particles in the nano-layer increases with increasing fluence from 1 · 1017 to 5 · 1017 cm2, then that saturates when fluence is increased higher. In sample 1E17 and 1E18, the size of the nano-particles in the nano-layer is almost uniform, but in sample 5E17, the size of nano-particles decreases with going deep into sample. That is to say, the size of nano-particles presents a gradient depth distribution. During observation, it can be found that in sample 1E17, most of the Ge nano-particles are amorphous, just 1% of Ge nano-particles is nano-crystals and the size of nano-crystals in the HRTEM image is about 2–3 nm. As for sample 1E18, the amount of nc-Ge increases obviously; about 30% of Ge nano-particles is nano-crystals. The size of the nano-crystals is more or less 4–5 nm. It seems that the nano-layer with uniform nano-particles, which contain nc-Ge, can be prepared by high-fluence low-energy ion implantation with certain fluence. The ratio of nc-Ge and the size of nano-particles (nano-crystals) increase with increasing fluence. The PL spectra of four samples with different fluences are shown in Fig. 5. A peak centered at 510 nm is a scattered peak of light source. It is found that three PL peaks, centered at 295, 400 and 575 nm, respectively, become higher with increasing fluence, but height change of their peaks of different samples is different, indicating that there is different PL origin among three peaks. Two PL peaks centered at 295 and 400 nm are related to an interface defect of Ge–SiO2 [19] and 575 nm PL peak is related to nc-Ge [10]. When fluence is increased, based on the TEM observation above, the concentrations of nc-Ge and Ge–SiO2 interface area increase, so the intensity of the PL peaks will increase. The possible formation mechanism of the Ge nano-layer may be discussed as follows based on self-organization theory. As for the nano-layer formation, nucleation and spinodal decomposition both exist during ion implantation [23,24]. When the samples are prepared by low-energy ion implantation with low-fluence of 1 · 1016 cm2, a surface sputtering effect induces a local high-temperature zone at the surface layer. The local high-temperature nucleates Ge ions into na-Ge due to the minimum energy principle, because the Gibbs free energy of a nano-particle aggregated from ions is much lower than the energy sum of these ions. But decomposition is preferred over nucleation; as a result, the size and the concentration of the nano-particles is too small to observe. When the fluence is increased from 1 · 1016 to 1 · 1017 cm2, the size and the concentration of nano-particles increase gradually. When the fluence is increased to 1 · 1017 cm2, the nucleation balances decomposition. As a result, the nano-particles become uniformsized; in particular, the nc-Ge start to separate form na-Ge. When the fluence is increased from 1 · 1017 to 5 · 1017 cm2, superfluous Ge ions bump on the nano-particles at the surface layer of the nano-layer; the balance between nucleation and decomposition is destroyed; nucleation is preferred over decomposition at the surface layer,
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Fig. 4. XTEM and HRTEM images of sample 1E16, 1E17, 5E17 and 1E18.
phous SiO2 film, can be directly fabricated with high-fluence low-energy ion implantation. The nc-Ge and na-Ge coexist in the nano-layer. The ratio of nc-Ge to nanoparticles increases with increasing fluence. The measured Ge atom distribution in the nano-layer is less than 5 · 1016 cm2, even if the projected fluence is increased up to 1 · 1018 cm2. Local high-temperature induced by the sputtering effect of superfluous Ge ions is contributed to na-Ge formation and nc-Ge separating from na-Ge and the size uniformity of nano-particles may be due to the balance between nucleation and decomposition. As a result, the intensity of PL in samples increases with increasing fluence also. Acknowledgements
Fig. 5. PL spectra of sample 1E16, 1E17, 5E17 and 1E18, excited by 240 nm UV-light.
thus the nano-particles at the surface layer grow to 10– 12 nm; and the size of deep-located nano-particles remains small. In addition, the local high-temperature caused by the superfluous ions sputtering makes more nc-Ge separate from na-Ge. When the fluence is increased to 1 · 1018 cm2, nucleation balances decomposition again due to reaching a steady-state Ge profile [22], and nanoparticles become uniform-sized again. Hence when the fluence is increased further, it is expected that the size of nano-particles will not change much. 4. Conclusions The high-density Ge nano-layer, with uniform-sized nano-particles, embedded in the surface layer of the amor-
This research was supported by a grant #3-405 from the Ministry of Science and Technology, Israel, within the Cooperation Program with the Ministry of Science and Technology, China and NSAF of NSFC-CAEP of China (no. 10376020), as well as Program for New Century Excellent Talents in University, China (no. NCET-04-0874). The authors also would like to thank Analytic and Testing Center, Sichuan University, for support on XRD and PL measurements of samples. References [1] A. Meidrum, R.F. Haglund, L.A. Boarter, C.W. White, Adv. Mater. 13 (2001) 1431. [2] C.W. White, J.D. Budai, S.P. Withrow, J.G. Zhu, S.J. Pennycook, R.A. Zuhr, D.M. Hembree, D.O. Henderson, R.H. Magruder, M.J. Yacaman, G. Mondragon, S. Prawer, Nucl. Instr. and Meth. B 127 (1997) 583. [3] Y.M. Yang, X.L. Wu, G.G. Siu, G.S. Huang, J.C. Shen, D.S. Hu, J. Appl. Phys. 96 (2004) 5239. [4] K.S. Min, K. Scheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 69 (1996) 2033.
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