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Luminescence properties of Ge implanted SiO2:Ge and GeO2:Ge films
Applied Surface Science 256 (2009) 954–957
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Luminescence properties of Ge implanted SiO2:Ge and GeO2:Ge films Nobutoshi Arai a,b,*, Hiroshi Tsuji a, Masashi Hattori a, Masayuki Ohsaki a, Hiroshi Kotaki b, Toyotsugu Ishibashi a, Yasuhito Gotoh a, Junzo Ishikawa a a b
Department of Electronic Science and Engineering, Kyoto University, KyotoDaigakuKatsura, Nishikyo-ku, Kyoto 615-8510, Japan Advanced Technology Research Laboratories, Sharp Corporation, 2613-1, Ichinomoto-cho, Tenri 632-8567, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 30 May 2009
We have investigated cathodeluminescence (CL) of Ge implanted SiO2:Ge and GeO2:Ge films. The GeO2 films were grown by oxidation of Ge substrate at 550 8C for 3 h in O2 gas flow. The GeO2 films on Ge substrate and SiO2 films on Si substrate were implanted with Ge-negative ions. The implanted Ge atom concentrations in the films were ranging from 0.1 to 6.0 at%. To produce Ge nanoparticles the SiO2:Ge films were thermally annealed at various temperatures of 600–900 8C for 1 h in N2 gas flow. An XPS analysis has shown that the implanted Ge atoms were partly oxidized. CL was observed at wavelengths around 400 nm from the GeO2 films before and after Ge -implantation as well as from SiO2:Ge films. After Ge -implantation of about 0.5 at% the CL intensity has increased by about four times. However, the CL intensity from the GeO2:Ge films was several orders of magnitude smaller than the intensity from the 800 8C-annealed SiO2:Ge films with 0.5 at% of Ge atomic concentration. These results suggested that the luminescence was generated due to oxidation of Ge nanoparticles in the SiO2:Ge films. ß 2009 Elsevier B.V. All rights reserved.
PACS: 61.72.Tt 41.75.Cn 61.72.Ww 81.07. b 81.40.Gh 78.60.Hf Keywords: Ion implantation Heat treatment Germanium Cathodeluminescence
1. Introduction Semiconductor is attractive material for light-emitting devices. Especially semiconductor materials compatible with the standard Si technology are desired for the development of light emission source for communication in a LSI chip. For example, silicon and germanium in silicon-dioxide layers are expected to apply to electroluminescence source in LSI chips, since 500-nm-thick SiO2 films implanted with Ge+ ions were reported to show blue and violet electroluminescence by applying a voltage of 350 V [1]. However, it is not only required to decrease the operation voltage [2] but also required to increase the luminescence intensity for versatile application. Moreover, it is not clear whether the quantum confinement effect or oxygen defect causes the luminescence. In order to meet this requirement, we have tried to implant Ge ions into a shallow region in SiO2 layer on Si substrate by using a negative-ion implantation method with multi-energy. Moreover, we studied the cathodeluminescence (CL) of thermally grown GeO2 films and of Ge implanted SiO2 films
* Corresponding author at: Advanced Technology Research Laboratories, Sharp Corporation, 2613-1, Ichinomoto-cho, Tenri 632-8567, Japan Tel.: +81 743 65 2383; fax: +81 743 65 2393. E-mail addresses: [email protected], [email protected], [email protected] (N. Arai). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.062
(SiO2:Ge). Negative-ion implantation has an advantage of almost ‘‘charge-up free’’ feature for insulators and isolated electrodes [3]. Therefore, the penetration depth of ions can be controlled correctly with a relatively low energy implantation into a thin insulator such as silicon-dioxide. Moreover, the technique is suitable for careful examination about implanted atoms in thin dielectric films such as SiO2 films, because the charge up can make accidental dispersion of implanted atoms and the breakdown can make accidental defects. 2. Samples and methods Germanium negative ions (Ge ) were implanted at room temperature into SiO2 films with thickness of 100 nm thermally grown on Silicon substrate. We used a multi-energy implantation technique to make a relatively flat profile of Ge concentrations in some regions of the SiO2:Ge films. In the multi-energy implantation, the Ge were implanted three times into the same sample by changing the energies of 50, 20 and 10 keV. Maximum Ge atom concentrations in the SiO2:Ge films were ranging from 0.1 to 6.0 at%. For example, the concentration profiles for Ge atoms about 6 at% at peak concentration are shown in Fig. 1(a). The profiles were calculated by using the Transport of Ions in Matter (TRIMDYN) program [4]. The total concentration curve in Fig. 1(a) expects the formation of a layer with Ge concentration of about 6 at% from 10 to 40 nm in depth. The oxygen atoms in the SiO2:Ge
N. Arai et al. / Applied Surface Science 256 (2009) 954–957
955
Fig. 1. Calculated depth profile of Ge atoms by multi-energy implantation in SiO2 layer (a) and Ge and excess oxygen balance in the SiO2 layer (b).
films are expected to be scattered by the collisions of Ge atoms and be recoiled due to their lightweight comparing to the weights of Ge and Si atoms. We calculated a lack or excess of O atoms from the stoichiometric value of the SiO2. The O balance distribution in depth direction is shown in Fig. 1(b). We expected O lacking in the surface region and O excess in the deep region around the end of range (EOR) of 40–100 nm. The gas pressure during the implantation was kept less than 1 10 4 Pa. We expected to obtain SiO2:Ge films with about 30-nm-thick Ge implanted region by the multienergy implantation. The Ge implanted SiO2:Ge films were then annealed for 1 h by an electrical oven at various temperatures of 600, 800 and 900 8C in a quartz tube with a N2 gas flow (50 ml/min) under low vacuum condition by a rotary pump (400 l/min). The GeO2 films were grown by oxidation of Ge substrates at 550 8C for 3 h in O2 gas flow. A GeO2 films with thickness of about 100 nm were formed on the Ge substrates. Fig. 2 shows reflectance spectra obtained from the Ge substrate before and after thermal oxidization. The reflectance is obviously decreased after thermal oxidization. The reduction of reflectance indicates that germanium atoms near the surface were oxidized. The GeO2 films were then implanted with Ge ions to prepare GeO2:Ge films.
The depth distribution of Ge atoms in the SiO2:Ge films and fractions of unoxidized and oxidized Ge atoms were measured by X-ray-induced photoelectron spectroscopy (XPS) with Ar+ etching. We measured survey spectra after every etching for 300 s by Ar+ irradiation at 4 keV. CL of the SiO2:Ge and GeO2:Ge films were investigated with an electron beam at 4–5 keV. 3. Oxidation of Ge atoms in SiO2 film Ge atomic fractions in the SiO2:Ge and GeO2:Ge films were obtained by the comparison of peak areas from detecting photoelectrons of Ge 3d, O 2 s and Si 2p. As for fractions of unoxidized and oxidized Ge atoms, a chemical shift due to oxidation of Ge atoms was about 2.3 and 3.7 eV to high binding energy for GeO and GeO2, respectively. We divided the detected Ge 3d peak into three Gaussian distributions from non-oxide (Ge–Ge), mono-oxide (Ge– O) and di-oxide (Ge=O2) bonding by the peak fitting. Then, each ratio of Ge bonding was calculated. Fig. 3 shows each percentage of Ge atoms as a function of depth for the SiO2:Ge films. After annealing at 900 8C, Ge atoms in the surface region were oxidized. This was considered to be due to penetration of oxygen from the residual gas to the surface region during the annealing. On the contrary, the asimplanted and 600 8C-annealed SiO2:Ge films showed that the higher oxidation degree of Ge atoms in the deep region of SiO2:Ge films than in the surface region. This oxidation at deep region is considered to be due to the local excess of oxygen atoms as shown in Fig. 1(b). In Fig. 3(a), the average oxidations of Ge atoms in the asimplanted, 600 8C-annealed and 900 8C-annealed SiO2:Ge films were about 26, 30 and 60%, respectively. After annealing at 800 8C in N2 and in Air as shown in Fig. 3(b), the average oxidations of Ge atoms in SiO2:Ge films were about 50 and 85%, respectively. Fig. 4 shows each percentage of Ge atoms in the GeO2 films before Ge implantation. In Fig. 4(a), the average mono-oxidations and dioxidations of Ge atoms in GeO2 films were about 10 and 90%, respectively. It was found that almost all Ge atoms in the surface region of the GeO2 film were oxidized. After Ge -implantation, the fractions of GeO2 and GeO, decreased and increased, respectively, as shown in Fig. 4(b). The di-oxide (Ge=O2) bonding in GeO2 film is considered to be dissociated by the Ge -implantation. 4. Cathodeluminescence
Fig. 2. Optical reflectance spectra for germanium substrate before and after thermal annealing.
Cathodeluminescence (CL) of the Ge implanted SiO2:Ge films after annealing at 800 8C in N2 was investigated by using an electron beam at 5 keV with 11 mA. Strongly emitted luminescence
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N. Arai et al. / Applied Surface Science 256 (2009) 954–957
Fig. 3. Ratio of unoxidized Ge atoms as a function of depth after annealing at various temperatures (a) and in various atmospheres (b).
Fig. 4. Ratio of un-oxidized, mono-oxidized and di-oxidized Ge atoms as a function of etching time corresponded to depth before (a) and after Ge -implantation (b).
Fig. 5. CL spectra of SiO2:Ge film in comparison to unimplanted SiO2 film (a) and for different implanted Ge concentrations (b).
was expected at about 50% of the oxidized Ge. Fig. 5(a) shows CL spectra of the Ge implanted SiO2:Ge film at 6 at% after annealing at 800 8C and from an unimplanted SiO2 film at room temperature measurement. The unimplanted SiO2 film showed only a broad luminescence peak at around 460 nm, but the Ge implanted SiO2:Ge film showed a large sharp luminescence peak at around 400 nm. This luminescence peak is well agreed with the CL spectra reported by Rebohle et al. [1] and Fitting et al. [5]. Therefore, the peak around 400 nm is considered to be due to oxygen deficient center (ODC) attributing to twofold coordinated Ge atoms. CL spectra of SiO2:Ge films prepared under other conditions were also measured to investigate the dependence of the luminescence on the implanted Ge concentration in SiO2:Ge films. The SiO2:Ge films
were prepared in the Ge concentration range of 0.05–6 at% and annealed at 800 8C. As shown in Fig. 5(b), the CL intensity increased with the Ge concentration from 0.05 to 0.5 at% and it decreased at the Ge concentration more than 1.4 at%. It is considered that these inverse proportions above 0.5 at% were caused by lack of oxygen, density of large Ge nanoparticle and/or rich of defects in SiO2. The lack of oxygen and excess of Ge resulted in high unoxidized Ge concentration, and therefore large population and/or large size of Ge nanoparticles were formed. Because the nanoparticles in the shallow region of SiO2:Ge film blocked out the luminescence from the source in the deep region, the number of effective luminescence source, those emitting light reached surface of the film, did not increase. In addition, many implanted Ge ions into SiO2:Ge
N. Arai et al. / Applied Surface Science 256 (2009) 954–957
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Fig. 6. CL spectra of thermally grown GeO2 film before Ge -implantation in comparison to Ge substrate (a) and in comparison to GeO2:Ge film after Ge -implantation (b).
films caused many defects which could trap electrons for the luminescence. For lower than 0.5 at% of implanted Ge concentration, it is considered that Ge nanoparticles in the SiO2:Ge films were small enough in size and in number so that they did not intercept the luminescence, and so that they could be easily oxidized to increase ODC of the luminescence sources. Therefore, the luminescence showed the maximum intensity at 0.5 at% of implanted Ge concentration under the condition of sample preparation. These discussions may lead to the strong luminescence that can be developed by tuning condition of Ge implantation and subsequence annealing. Fig. 6(a) shows CL spectra of the thermally grown GeO2 films on Ge substrate in comparison to Ge substrate before oxidation at room temperature measurement. The Ge substrate before oxidation could scarcely show luminescence in the range of 250–500 nm, but the GeO2 film showed clearly a weak luminescence peak at around 400 nm. Fig. 6(b) shows CL spectra of the Ge implanted GeO2:Ge films. The CL intensity increased about four times after the Ge -implantation at the Ge concentration of about 0.5 at%. This result suggested that the CL intensity was not due to the amount of Ge or GeO2. These luminescence peaks seem corresponding to above CL peak in Fig. 5. Although total amounts of mono-oxidized Ge atoms and di-oxidized Ge atoms in the thermally grown GeO2 films were much more than those in the Ge implanted SiO2:Ge films, the intensity of luminescence from the GeO2 films was obviously weaker than that from the Ge implanted SiO2:Ge films. The CL intensity from the GeO2:Ge films was several orders of magnitude smaller than the intensity from the 800 8C-annealed SiO2:Ge films with 0.5 at% of Ge atom concentration. These results suggested that the strong luminescence was generated due to oxidation of Ge nanoparticles in the SiO2:Ge films. It might indicate that not only Ge-oxidation but also Ge nanoparticles formation was important for strong light emission source with Ge-ODC. Nanoparticles were probably suitable for generation and/or stabilization of the ODC.
5. Conclusions Flat profiles of Ge atom concentration in some regions of SiO2 films can be formed by using a multi-energy implantation technique. In the multi-energy implantation, Ge ions were implanted three times into the same SiO2 films by changing the energies of 50, 20 and 10 keV. XPS analysis revealed that the implanted Ge atoms into the SiO2 films were oxidized not only after annealing at above 600 8C but also just after the implantation without subsequence annealing. The CL spectra of the Ge implanted SiO2:Ge films showed peaks around 400 nm in wavelength. The peak position was independent of Ge -implantation fluence, annealing temperature and measurement temperature. These independencies suggested that the luminescence was not due to quantum confinement effects of Ge nanoparticles. It is considered that the luminescence is due to oxygen deficiency centers of oxidized Ge atoms. The CL intensities were varied with Ge -implantation fluence. These results suggested that size, density and oxidation degree of Ge nanoparticle affected the effective intensity of the luminescence. We also studied the thermally grown GeO2 films. Although it showed luminescence peak around 400 nm in wavelength corresponding to the luminescence peak from the SiO2:Ge films, the intensity of luminescence from the thermally grown GeO2 films was obviously weaker than that from the SiO2:Ge films. These results can provide guidance in strong light emission in UV-blue region from SiO2:Ge films. References [1] L. Rebohle, J. Von Borany, R.A. Yankov, W. Skorupa, I.E. Tyschenko, H. Froeb, K. Leo, Appl. Phys. Lett. 71 (19) (1997) 2809–2811. [2] J.M.J. Lopes, F.C. Zawislak, M. Behar, P.F.P. Fichtner, L. Rebohle, W. Skarupa, J. Appl. Phys. 94 (9) (2003) 6059–6064. [3] J. Ishikawa, H. Tsuji, Y. Toyota, Y. Gotoh, K. Matsuda, M. Tanho, S. Sakai, Nucl. Instr. Meth. B96 (1995) 7–12. [4] J.P. Biersack, Nucl. Instr. Meth. B 27 (1987) 21. [5] H.J. Fitting, T. Barfels, A.N. Trukhin, B. Schmidt, A. Gulans, A. von Czarnowski, J. NonCryst. Solids 303 (2002) 218–231.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Luminescence properties of Ge implanted SiO2:Ge and GeO2:Ge films Nobutoshi Arai a,b,*, Hiroshi Tsuji a, Masashi Hattori a, Masayuki Ohsaki a, Hiroshi Kotaki b, Toyotsugu Ishibashi a, Yasuhito Gotoh a, Junzo Ishikawa a a b
Department of Electronic Science and Engineering, Kyoto University, KyotoDaigakuKatsura, Nishikyo-ku, Kyoto 615-8510, Japan Advanced Technology Research Laboratories, Sharp Corporation, 2613-1, Ichinomoto-cho, Tenri 632-8567, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 30 May 2009
We have investigated cathodeluminescence (CL) of Ge implanted SiO2:Ge and GeO2:Ge films. The GeO2 films were grown by oxidation of Ge substrate at 550 8C for 3 h in O2 gas flow. The GeO2 films on Ge substrate and SiO2 films on Si substrate were implanted with Ge-negative ions. The implanted Ge atom concentrations in the films were ranging from 0.1 to 6.0 at%. To produce Ge nanoparticles the SiO2:Ge films were thermally annealed at various temperatures of 600–900 8C for 1 h in N2 gas flow. An XPS analysis has shown that the implanted Ge atoms were partly oxidized. CL was observed at wavelengths around 400 nm from the GeO2 films before and after Ge -implantation as well as from SiO2:Ge films. After Ge -implantation of about 0.5 at% the CL intensity has increased by about four times. However, the CL intensity from the GeO2:Ge films was several orders of magnitude smaller than the intensity from the 800 8C-annealed SiO2:Ge films with 0.5 at% of Ge atomic concentration. These results suggested that the luminescence was generated due to oxidation of Ge nanoparticles in the SiO2:Ge films. ß 2009 Elsevier B.V. All rights reserved.
PACS: 61.72.Tt 41.75.Cn 61.72.Ww 81.07. b 81.40.Gh 78.60.Hf Keywords: Ion implantation Heat treatment Germanium Cathodeluminescence
1. Introduction Semiconductor is attractive material for light-emitting devices. Especially semiconductor materials compatible with the standard Si technology are desired for the development of light emission source for communication in a LSI chip. For example, silicon and germanium in silicon-dioxide layers are expected to apply to electroluminescence source in LSI chips, since 500-nm-thick SiO2 films implanted with Ge+ ions were reported to show blue and violet electroluminescence by applying a voltage of 350 V [1]. However, it is not only required to decrease the operation voltage [2] but also required to increase the luminescence intensity for versatile application. Moreover, it is not clear whether the quantum confinement effect or oxygen defect causes the luminescence. In order to meet this requirement, we have tried to implant Ge ions into a shallow region in SiO2 layer on Si substrate by using a negative-ion implantation method with multi-energy. Moreover, we studied the cathodeluminescence (CL) of thermally grown GeO2 films and of Ge implanted SiO2 films
* Corresponding author at: Advanced Technology Research Laboratories, Sharp Corporation, 2613-1, Ichinomoto-cho, Tenri 632-8567, Japan Tel.: +81 743 65 2383; fax: +81 743 65 2393. E-mail addresses: [email protected], [email protected], [email protected] (N. Arai). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.05.062
(SiO2:Ge). Negative-ion implantation has an advantage of almost ‘‘charge-up free’’ feature for insulators and isolated electrodes [3]. Therefore, the penetration depth of ions can be controlled correctly with a relatively low energy implantation into a thin insulator such as silicon-dioxide. Moreover, the technique is suitable for careful examination about implanted atoms in thin dielectric films such as SiO2 films, because the charge up can make accidental dispersion of implanted atoms and the breakdown can make accidental defects. 2. Samples and methods Germanium negative ions (Ge ) were implanted at room temperature into SiO2 films with thickness of 100 nm thermally grown on Silicon substrate. We used a multi-energy implantation technique to make a relatively flat profile of Ge concentrations in some regions of the SiO2:Ge films. In the multi-energy implantation, the Ge were implanted three times into the same sample by changing the energies of 50, 20 and 10 keV. Maximum Ge atom concentrations in the SiO2:Ge films were ranging from 0.1 to 6.0 at%. For example, the concentration profiles for Ge atoms about 6 at% at peak concentration are shown in Fig. 1(a). The profiles were calculated by using the Transport of Ions in Matter (TRIMDYN) program [4]. The total concentration curve in Fig. 1(a) expects the formation of a layer with Ge concentration of about 6 at% from 10 to 40 nm in depth. The oxygen atoms in the SiO2:Ge
N. Arai et al. / Applied Surface Science 256 (2009) 954–957
955
Fig. 1. Calculated depth profile of Ge atoms by multi-energy implantation in SiO2 layer (a) and Ge and excess oxygen balance in the SiO2 layer (b).
films are expected to be scattered by the collisions of Ge atoms and be recoiled due to their lightweight comparing to the weights of Ge and Si atoms. We calculated a lack or excess of O atoms from the stoichiometric value of the SiO2. The O balance distribution in depth direction is shown in Fig. 1(b). We expected O lacking in the surface region and O excess in the deep region around the end of range (EOR) of 40–100 nm. The gas pressure during the implantation was kept less than 1 10 4 Pa. We expected to obtain SiO2:Ge films with about 30-nm-thick Ge implanted region by the multienergy implantation. The Ge implanted SiO2:Ge films were then annealed for 1 h by an electrical oven at various temperatures of 600, 800 and 900 8C in a quartz tube with a N2 gas flow (50 ml/min) under low vacuum condition by a rotary pump (400 l/min). The GeO2 films were grown by oxidation of Ge substrates at 550 8C for 3 h in O2 gas flow. A GeO2 films with thickness of about 100 nm were formed on the Ge substrates. Fig. 2 shows reflectance spectra obtained from the Ge substrate before and after thermal oxidization. The reflectance is obviously decreased after thermal oxidization. The reduction of reflectance indicates that germanium atoms near the surface were oxidized. The GeO2 films were then implanted with Ge ions to prepare GeO2:Ge films.
The depth distribution of Ge atoms in the SiO2:Ge films and fractions of unoxidized and oxidized Ge atoms were measured by X-ray-induced photoelectron spectroscopy (XPS) with Ar+ etching. We measured survey spectra after every etching for 300 s by Ar+ irradiation at 4 keV. CL of the SiO2:Ge and GeO2:Ge films were investigated with an electron beam at 4–5 keV. 3. Oxidation of Ge atoms in SiO2 film Ge atomic fractions in the SiO2:Ge and GeO2:Ge films were obtained by the comparison of peak areas from detecting photoelectrons of Ge 3d, O 2 s and Si 2p. As for fractions of unoxidized and oxidized Ge atoms, a chemical shift due to oxidation of Ge atoms was about 2.3 and 3.7 eV to high binding energy for GeO and GeO2, respectively. We divided the detected Ge 3d peak into three Gaussian distributions from non-oxide (Ge–Ge), mono-oxide (Ge– O) and di-oxide (Ge=O2) bonding by the peak fitting. Then, each ratio of Ge bonding was calculated. Fig. 3 shows each percentage of Ge atoms as a function of depth for the SiO2:Ge films. After annealing at 900 8C, Ge atoms in the surface region were oxidized. This was considered to be due to penetration of oxygen from the residual gas to the surface region during the annealing. On the contrary, the asimplanted and 600 8C-annealed SiO2:Ge films showed that the higher oxidation degree of Ge atoms in the deep region of SiO2:Ge films than in the surface region. This oxidation at deep region is considered to be due to the local excess of oxygen atoms as shown in Fig. 1(b). In Fig. 3(a), the average oxidations of Ge atoms in the asimplanted, 600 8C-annealed and 900 8C-annealed SiO2:Ge films were about 26, 30 and 60%, respectively. After annealing at 800 8C in N2 and in Air as shown in Fig. 3(b), the average oxidations of Ge atoms in SiO2:Ge films were about 50 and 85%, respectively. Fig. 4 shows each percentage of Ge atoms in the GeO2 films before Ge implantation. In Fig. 4(a), the average mono-oxidations and dioxidations of Ge atoms in GeO2 films were about 10 and 90%, respectively. It was found that almost all Ge atoms in the surface region of the GeO2 film were oxidized. After Ge -implantation, the fractions of GeO2 and GeO, decreased and increased, respectively, as shown in Fig. 4(b). The di-oxide (Ge=O2) bonding in GeO2 film is considered to be dissociated by the Ge -implantation. 4. Cathodeluminescence
Fig. 2. Optical reflectance spectra for germanium substrate before and after thermal annealing.
Cathodeluminescence (CL) of the Ge implanted SiO2:Ge films after annealing at 800 8C in N2 was investigated by using an electron beam at 5 keV with 11 mA. Strongly emitted luminescence
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Fig. 3. Ratio of unoxidized Ge atoms as a function of depth after annealing at various temperatures (a) and in various atmospheres (b).
Fig. 4. Ratio of un-oxidized, mono-oxidized and di-oxidized Ge atoms as a function of etching time corresponded to depth before (a) and after Ge -implantation (b).
Fig. 5. CL spectra of SiO2:Ge film in comparison to unimplanted SiO2 film (a) and for different implanted Ge concentrations (b).
was expected at about 50% of the oxidized Ge. Fig. 5(a) shows CL spectra of the Ge implanted SiO2:Ge film at 6 at% after annealing at 800 8C and from an unimplanted SiO2 film at room temperature measurement. The unimplanted SiO2 film showed only a broad luminescence peak at around 460 nm, but the Ge implanted SiO2:Ge film showed a large sharp luminescence peak at around 400 nm. This luminescence peak is well agreed with the CL spectra reported by Rebohle et al. [1] and Fitting et al. [5]. Therefore, the peak around 400 nm is considered to be due to oxygen deficient center (ODC) attributing to twofold coordinated Ge atoms. CL spectra of SiO2:Ge films prepared under other conditions were also measured to investigate the dependence of the luminescence on the implanted Ge concentration in SiO2:Ge films. The SiO2:Ge films
were prepared in the Ge concentration range of 0.05–6 at% and annealed at 800 8C. As shown in Fig. 5(b), the CL intensity increased with the Ge concentration from 0.05 to 0.5 at% and it decreased at the Ge concentration more than 1.4 at%. It is considered that these inverse proportions above 0.5 at% were caused by lack of oxygen, density of large Ge nanoparticle and/or rich of defects in SiO2. The lack of oxygen and excess of Ge resulted in high unoxidized Ge concentration, and therefore large population and/or large size of Ge nanoparticles were formed. Because the nanoparticles in the shallow region of SiO2:Ge film blocked out the luminescence from the source in the deep region, the number of effective luminescence source, those emitting light reached surface of the film, did not increase. In addition, many implanted Ge ions into SiO2:Ge
N. Arai et al. / Applied Surface Science 256 (2009) 954–957
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Fig. 6. CL spectra of thermally grown GeO2 film before Ge -implantation in comparison to Ge substrate (a) and in comparison to GeO2:Ge film after Ge -implantation (b).
films caused many defects which could trap electrons for the luminescence. For lower than 0.5 at% of implanted Ge concentration, it is considered that Ge nanoparticles in the SiO2:Ge films were small enough in size and in number so that they did not intercept the luminescence, and so that they could be easily oxidized to increase ODC of the luminescence sources. Therefore, the luminescence showed the maximum intensity at 0.5 at% of implanted Ge concentration under the condition of sample preparation. These discussions may lead to the strong luminescence that can be developed by tuning condition of Ge implantation and subsequence annealing. Fig. 6(a) shows CL spectra of the thermally grown GeO2 films on Ge substrate in comparison to Ge substrate before oxidation at room temperature measurement. The Ge substrate before oxidation could scarcely show luminescence in the range of 250–500 nm, but the GeO2 film showed clearly a weak luminescence peak at around 400 nm. Fig. 6(b) shows CL spectra of the Ge implanted GeO2:Ge films. The CL intensity increased about four times after the Ge -implantation at the Ge concentration of about 0.5 at%. This result suggested that the CL intensity was not due to the amount of Ge or GeO2. These luminescence peaks seem corresponding to above CL peak in Fig. 5. Although total amounts of mono-oxidized Ge atoms and di-oxidized Ge atoms in the thermally grown GeO2 films were much more than those in the Ge implanted SiO2:Ge films, the intensity of luminescence from the GeO2 films was obviously weaker than that from the Ge implanted SiO2:Ge films. The CL intensity from the GeO2:Ge films was several orders of magnitude smaller than the intensity from the 800 8C-annealed SiO2:Ge films with 0.5 at% of Ge atom concentration. These results suggested that the strong luminescence was generated due to oxidation of Ge nanoparticles in the SiO2:Ge films. It might indicate that not only Ge-oxidation but also Ge nanoparticles formation was important for strong light emission source with Ge-ODC. Nanoparticles were probably suitable for generation and/or stabilization of the ODC.
5. Conclusions Flat profiles of Ge atom concentration in some regions of SiO2 films can be formed by using a multi-energy implantation technique. In the multi-energy implantation, Ge ions were implanted three times into the same SiO2 films by changing the energies of 50, 20 and 10 keV. XPS analysis revealed that the implanted Ge atoms into the SiO2 films were oxidized not only after annealing at above 600 8C but also just after the implantation without subsequence annealing. The CL spectra of the Ge implanted SiO2:Ge films showed peaks around 400 nm in wavelength. The peak position was independent of Ge -implantation fluence, annealing temperature and measurement temperature. These independencies suggested that the luminescence was not due to quantum confinement effects of Ge nanoparticles. It is considered that the luminescence is due to oxygen deficiency centers of oxidized Ge atoms. The CL intensities were varied with Ge -implantation fluence. These results suggested that size, density and oxidation degree of Ge nanoparticle affected the effective intensity of the luminescence. We also studied the thermally grown GeO2 films. Although it showed luminescence peak around 400 nm in wavelength corresponding to the luminescence peak from the SiO2:Ge films, the intensity of luminescence from the thermally grown GeO2 films was obviously weaker than that from the SiO2:Ge films. These results can provide guidance in strong light emission in UV-blue region from SiO2:Ge films. References [1] L. Rebohle, J. Von Borany, R.A. Yankov, W. Skorupa, I.E. Tyschenko, H. Froeb, K. Leo, Appl. Phys. Lett. 71 (19) (1997) 2809–2811. [2] J.M.J. Lopes, F.C. Zawislak, M. Behar, P.F.P. Fichtner, L. Rebohle, W. Skarupa, J. Appl. Phys. 94 (9) (2003) 6059–6064. [3] J. Ishikawa, H. Tsuji, Y. Toyota, Y. Gotoh, K. Matsuda, M. Tanho, S. Sakai, Nucl. Instr. Meth. B96 (1995) 7–12. [4] J.P. Biersack, Nucl. Instr. Meth. B 27 (1987) 21. [5] H.J. Fitting, T. Barfels, A.N. Trukhin, B. Schmidt, A. Gulans, A. von Czarnowski, J. NonCryst. Solids 303 (2002) 218–231.