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Photoluminescent characteristics of ion beam synthesized Ge nanoparticles in thermally grown SiO2 films
Nuclear Instruments and Methods in Physics Research B 307 (2013) 171–176
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Photoluminescent characteristics of ion beam synthesized Ge nanoparticles in thermally grown SiO2 films C.F. Yu a, D.S. Chao b,⇑, Y.-F. Chen a, J.H. Liang a,c a
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 300, Taiwan, ROC c Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 300, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 30 September 2012 Received in revised form 21 December 2012 Accepted 23 December 2012 Available online 18 January 2013 Keywords: Photoluminescence (PL) Ion beam synthesis Ge nanoparticles Oxygen-deficiency-related defects Quantum-confinement effects
a b s t r a c t Prospects of developing into numerous silicon-based optoelectronic applications have prompted many studies on the optical properties of Ge nanoparticles within a silicon oxide (SiO2) matrix. Even with such abundant studies, the fundamental mechanism underlying the Ge nanoparticle-induced photoluminescence (PL) is still an open question. In order to elucidate the mechanism, we dedicate this study to investigating the correlation between the PL properties and microstructure of the Ge nanoparticles synthesized in thermally grown SiO2 films. Our spectral data show that the peak position, at 3.1 eV or 400 nm, of the PL band arising from the Ge nanoparticles was essentially unchanged under different Ge implantation fluences and the temperatures of the following annealing process, whereas the sample preparation parameters modified or even fluctuated (in the case of the annealing temperature) the peak intensity considerably. Given the microscopically observed correlation between the nanoparticle structure and the sample preparation parameters, this phenomenon is consistent with the mechanism in which the oxygen-deficiency-related defects in the Ge/SiO2 interface act as the major luminescence centers; this mechanism also successfully explains the peak intensity fluctuation with the annealing temperature. Moreover, our FTIR data indicate the formation of GeOx upon ion implantation. Since decreasing of the oxygen-related defects by the GeOx formation is expected to be correlated with the annealing temperature, presence of the GeOx renders further experimental support to the oxygen defect mechanism. This understanding may assist the designing of the manufacturing process to optimize the Ge nanoparticle-based PL materials for different technological applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Efficient luminescent characteristics induced by nanoscale semiconductor materials have received growing interest in developing silicon-based optoelectronic applications. In recent years, a significant amount of research has been devoted to forming nanoparticles of group-IV elements (such as Si, Ge, and Sn) in silicon oxide (SiO2) layer by ion beam synthesis because of its great compatibility with Si technology and excellent control of implanted species in the ion implantation process. In particular, the optical properties of Ge-implanted oxide film are of special interest due to their strong photoluminescence (PL) and electroluminescence (EL) in the ultraviolet (UV)-blue spectral range as compared to other elements [1–3]. Thus far several studies have showed that the defects at the interface between Ge nanoparticles and SiO2 matrix are the ⇑ Corresponding author. Tel.: +886 3 5742866; fax: +886 3 5713849. E-mail address: [email protected] (D.S. Chao). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.12.116
primary luminescence centers for the corresponding emission bands. However, very diverse PL emission energy levels ranging from 2.2 to 3.2 eV (560–380 nm) have been reported [2–6]. In addition, a few studies have also claimed that Ge nanoparticle-induced luminescence could be attributed to quantum confinement effects originating from radiative recombination of excitons confined in Ge nanoparticles [7–9]. All of these indicate that the mechanisms behind the Ge nanoparticle-induced luminescence are indefinite and still in debate. In order to clarify the origin of PL emission resulting from Ge nanoparticles, this study thus aims to investigate the effects of ion implantation and post-annealing conditions on the PL characteristics induced by Ge nanoparticles. For accurate experimental supports, a thorough characteristic analysis using secondary ion mass spectrometry (SIMS), cross-section transmission electron microscopy (XTEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy is also conducted to find the correlation between optical properties of Ge nanoparticles and postannealing treatments.
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2. Experimental procedures In the present study, SiO2 films with a thickness of 100 nm were thermally grown on 4-inch (1 0 0)-oriented n-type Si substrates in wet oxide ambient at 1000 °C. The SiO2/Si substrates were roomtemperature implanted with 50 keV Ge ions to fluences of 3 1015, 1 1016, and 3 1016 cm 2. The Monte-Carlo Stopping and Range of Ions in Matter (SRIM) code was also used to simulate the Ge ion distribution, ensuring that the implanted Ge ions can adequately distribute over the SiO2 film and have distributions roughly peaked at the center of the SiO2 film. After ion implantation, the implanted substrates were cut into smaller specimens of about 1 1 cm2 in order to perform the post-annealing treatments. The specimens were annealed using conventional furnace annealing (FA) method in vacuum chamber (about 1 10 6 torr) with various temperatures ranging from 450 to 1000 °C for a fixed time of 1 h. In conducting the characteristic analysis of the synthesized Ge nanoparticles embedded in SiO2 film, PL spectra of Ge-implanted SiO2 thin films were characterized at room temperature using a Hitachi F-7000 Fluorescence Spectrophotometer with a fixed PMT voltage and excitation wavelength of 400 V and 260 nm, respectively. SIMS was employed to measure the depth profiles of Ge ions in the as-implanted and as-annealed specimens. Ge nanoparticle distribution and microstructure were examined by a high-resolution JEOL 2000FXII XTEM with an acceleration voltage of 200 kV. Compositional bonding configurations of the Ge-implanted SiO2 films that underwent various post-annealing temperatures were also carried out by a Bomem DA-8.3 FTIR. Infrared transmittance measurements of various specimens were performed at room temperature by scanning the wavenumber ranging from 400 to 4000 cm 1 with a spectral resolution of 2 cm 1. Furthermore, Raman spectra which can be used to identify the formation of Ge nanoparticles in SiO2 matrix were measured using a T-64000 Jobin–Yvon spectrometer having a spectral resolution of 1 cm 1. All samples were recorded in the black-scattering mode at room temperature, and the integration time was 20 min.
Fig. 1. (a) SRIM-simulated depth profile of 50 keV Ge ions implanted into a SiO2 layer of 100 nm. (b) SIMS-measured depth profiles of 50 keV, 3 1015 cm 2 Ge ions implanted into a SiO2 layer of 100 nm in the as-implanted specimen and asannealed ones that underwent annealing of 400 and 800 °C for 1 h.
3. Results and discussion In order to characterize the Ge ion distribution, the depth profiles of the implanted Ge ions were predicted by SRIM code and measured using SIMS analysis as shown in Fig. 1a and b, respectively. As can be seen, the measured peak Ge concentration in the as-implanted specimen locates at a depth of about 44 nm, which is in good agreement with the simulated projected range (Rp) of 51 nm. Comparison between the depth profiles in the as-implanted specimen and as-annealed ones that underwent annealing of 400 and 800 °C for 1 h is also shown in Fig. 1b. It is clear that the Ge ions in the 800 °C-annealed specimen have a broader distribution when compared to the other ones. Increasing post-annealing temperature drives the implanted Ge ions diffusing toward the SiO2/Si interface, thus resulting in a reduction in the Ge peak concentration. PL spectra of the Ge-implanted samples have been preliminarily investigated for the possibility of light emission. Fig. 2a shows the measured PL spectra of the Ge-implanted SiO2 films in the as-implanted specimen and as-annealed ones that underwent various post-annealing temperatures. As shown in Fig. 2a, all the spectra exhibit a UV-blue PL peak around 3.1 eV (400 nm). The peak positions are almost constant with the increase of post-annealing temperature. Notably, the emission band can be also found in the as-implanted specimen without undergoing any post-annealing treatment. The peak PL intensity as a function of post-annealing temperature is also inset in Fig. 2a. The PL intensity fluctuates with
post-annealing temperature and shows a maximum PL intensity at an annealing temperature of 450 °C. When the temperature increases, an unusual reduction in the PL intensity occurs at 600 °C. Then the PL intensity again increases as the temperature is further raised to 800 °C. Finally the PL disappears at the temperature of 1000 °C. The trend of PL intensity versus post-annealing temperature is also similar in other specimens implanted with various doses. The variation in PL intensity implies that the microstructure or physical properties of the synthesized Ge nanoparticles are incessantly changed during the post-annealing process. Although the luminescent characteristics induced by Ge-doped silicon glass have been extensively studied, the origin of the emission band induced by Ge nanoparticles still remains controversial. It has been reported in several literatures that the emission band in the vicinity of 400 nm can be attributed to the defect states located at the interface between nanoparticles and SiO2 matrix, and it is not unique to the presence of Ge [5,11]. Some of them have also claimed that oxygen-deficiency-related defects originating from Ge-related particles could be the recombination centers responsible for this emission [4,10–12]. In addition, quantum confinement effects which closely correlate with the size of Ge nanoparticles have also been believed to be another possible mechanism involved in quantum-assisted photonic luminescence [7–9]. These statements can be good reference in establishing a logical correlation between optical characteristics and processing conditions of the Ge nanoparticles synthesized in this study. Due to the fact that
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Fig. 2. (a) Measured PL spectra of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h. The inset shows the trend of PL intensity versus post-annealing temperature (lines are only for a guide). (b) Measured PL spectra of 800 °C, 1 h-annealed specimens as a function of implantation fluence. The inset shows the trend of PL intensity versus implantation fluence (line is only for a guide).
no shift in PL peak position can be observed in the specimens with various post-annealing temperatures, the possible mechanism which dominates PL emission in Ge-implanted SiO2 film should be the interfacial oxygen-deficiency-related defects existing at the interface between Ge nanoparticles and SiO2 matrix. Based on the above-mentioned statement, the dependence of the PL spectra on post-annealing temperature can be explained below. It is supposed that the implanted Ge ions are uniformly distributed over the SiO2 film. At first the isolated Ge ions start clustering to form Ge nanoparticles as temperature increases. At a high temperature such as 600 °C, Ostwald ripening effect enhances the growth of Ge nanoparticles, thus minimizing the number density as well as the total interfacial area of Ge nanoparticles. Since the luminescence centers are strongly related to the interfacial defects, a decrease in the total interfacial area thus reduces the luminescent intensity. When the temperature exceeds 600 °C, the PL intensity again increases and then maximizes at 800 °C, suggesting that significant transformation in Ge nanoparticles may occur during this stage of annealing. The possible reason why this transformation takes place will be discussed later according to
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the results obtained from XTEM, Raman, and FTIR. As the postannealing temperature further increases to 1000 °C, the PL is nearly undetectable and the intensity is even lower than that in the as-implanted case. Due to the fact that bulk Ge has a low melting point (900 °C), a high-temperature annealing of 1000 °C would lead to bond breaking and dissolution of Ge nanoparticles embedded in SiO2 matrix. As a result, the luminescence centers vanish. The relationship between PL intensity and implantation dose was also investigated as shown in Fig. 2b, which compares the PL spectra of the 800 °C-annealed specimens having Ge ion fluences of 3 1015, 1 1016, and 3 1016 cm 2. It is clear that a same PL peak at about 3.1 eV can be observed in the specimens with various implantation fluences. However, the PL intensity decreases with the increase of Ge concentration. Ge concentration makes a great impact on PL intensity showing that the maximum PL intensity at implantation dose of 3 1015 cm 2 is approximately 10 times higher than that at 3 1016 cm 2. Since the PL peak position has no shift with the implanted Ge ion fluence, oxygen-deficiency-related defects at the Ge/SiO2 interface are still considered to be a major medium for luminescence centers. Thus the inverse effect of Ge concentration on PL intensity can be explained by the fact that high Ge concentration leads to larger Ge nanoparticles, and thus reduces the amount of oxygen-deficiency-related defects available for luminescence. In addition, high-fluence Ge implantation may generate more non-radiative defects which could hinder electrons from carrier recombination [11]. The presence of interfacial oxygen-deficiency-related defects is considered to be the origin of luminescence centers in Ge-implanted SiO2 film, but it cannot still account for why a sudden drop in PL intensity that occurs at 600 °C. In order to clarify the temperature-dependent luminescent characteristics, further investigation using XTEM was carried out to verify the thermal evolution of the microstructure of Ge nanoparticles embedded in SiO2 matrix. Fig. 3 displays the XTEM-examined image of the SiO2 layer implanted with 50 keV, 3 1015 cm 2 Ge ions and annealed at 450 °C for 1 h. As can be seen, the Ge nanoparticles are formed in a nearly spherical shape with a mean diameter of 7 nm, and they are densely distributed within the SiO2 thin film. The inset of Fig. 3 also reveals a high-resolution lattice pattern showing that most of the Ge nanoparticles are essentially in a crystalline state. Fig. 4a–d also reveals the microstructure of the Ge-implanted SiO2 film as a function of post-annealing temperature. In contrast to the case of 450 °C, a noticeable increase in particle size from 7 to 16 nm can be observed in the 600 °C-annealed specimen. When the temperature reaches to 800 °C, larger particles are replaced by smaller ones having a diameter less than 3 nm. According to the theory of Ostwald ripening, it is expected that increasing temperature would result in particle coarsening at the expense of aggregating smaller ones in time. Nevertheless, a variation in particle size which is totally different from theory prediction was found in the XTEM results. In addition to a dramatic increase in particle size found at 600 °C, it is also unusual to again appear tiny particles at 800 °C. It is thus speculated that such a microstructure change may be caused by a significant transformation in these nanoparticles, and it may be a possible cause for a sudden drop in PL intensity at 600 °C, as shown in Fig. 2a. For further verification, Raman spectra and FTIR transmittance spectra were also characterized to identify the chemical bonding status of the synthesized nanoparticles in Ge-implanted SiO2 film. In regard to the Raman spectra as shown in Fig. 5, a distinct peak at about 300 cm 1 which is associated with Ge–Ge bonding was discovered in the as-implanted and the as-annealed specimens, indicating that Ge nanoparticles were indeed synthesized in the Ge-implanted SiO2 film. In particular, the Raman intensity associated with Ge–Ge bonds in the as-implanted and 1000 °C-annealed specimens is much weaker than that in other as-annealed ones. In
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Fig. 3. XTEM image of 50 keV, 3 1015 cm pattern of Ge nanoparticles.
2
Ge-implanted SiO2 film annealed at 450 °C for 1 h. The inset shows a high-resolution image which clearly reveals the lattice
addition, FTIR transmittance spectra of the specimens underwent various post-annealing temperatures are shown in Fig. 6. It reveals two well defined peaks around 600 and 800 cm 1, which are associated with Ge–O–Ge vibration mode and Si–O bending mode, respectively. It is worth noting that the peak around 600 cm 1 due to Ge–O–Ge vibration mode exists in all specimens even for that without conducting subsequent post-annealing treatment. FTIR measurements evidence the presence of germanium oxide (GeOx) in the Ge-implanted SiO2 film. The source of oxygen in GeOx may originate from implantation-induced recoiled oxygen defects in SiO2 matrix and residual oxygen in vacuum FA chamber. Previous studies have reported that Ge-implanted SiO2 film followed by a high-temperature annealing leads to GeOx formation [11– 16]. Some of them have also shown that GeOx formation is a necessary intermediate process for the creation of Ge nanoparticles. A displacement reaction that is initiated by Si and O atoms together with GeOx results in Ge precipitation [13]. Moreover, due to the fact that Ge implantation induces lot of recoiled O atoms in SiO2 matrix, these unattached O atoms provide many dangling bonds and thus enhance a spontaneous oxidation reaction before annealing. Therefore, the results of FTIR spectra imply that GeOx formation should play a crucial role in determining the optical properties of Ge-implanted SiO2 film. It can be expected that GeOx formation would affect the amount of oxygen-deficiency-related defects at the interface between Ge nanoparticles and SiO2 matrix as well as the intensity of the UV-blue luminescence around 400 nm, as shown in Fig. 2a and b. Since the PL peak at about 400 nm is associated with oxygendeficiency-related defects, the intensity strongly depends on the amount of oxygen-deficiency-related defects available in SiO2 matrix. As for the as-implanted specimen, a spontaneous interaction between implanted Ge ions and local excess of Si and O atoms induced by implantation may proceed during Ge ion implantation. As a result, oxygen vacancy defects would be produced in SiO2 film after Ge ion implantation. This thus explains why the PL emission can be found in the as-implanted specimen. When the specimen is annealed by 450 °C, Ge ions start clustering to form Ge nanoparticles as shown in Fig. 3. The formation of Ge nanoparticles increases the total interfacial area as well as the amount of oxygen-deficiency-related defects at the interface between Ge nanoparticles and SiO2 matrix, thus allowing PL intensity maximizing at 450 °C.
When the temperature comes to 600 °C, smaller Ge nanoparticles evolve into larger ones simply relying on typical coarsening theory. In addition, as stated earlier, a high-temperature annealing enhances a process of Ge oxidation in Ge-implanted SiO2 film. GeOx reaction would first occur at the interface between Ge nanoparticles and SiO2 matrix due to the fact that plenty of Ge dangling bonds exist over there and they can easily react with O atoms. If annealing treatment supplies enough energy, more O atoms are expected to diffuse into Ge nanoparticles for GeOx reaction to proceed. Consequently, Ge nanoparticles are gradually oxidized from surface and also its particle size increases due to GeOx formation. However, GeOx formation reduces oxygen vacancies near the interface and is thus unfavorable for PL emission. Except for the particle coarsening induced by Ostwald ripening effect, another possible reason for a PL intensity reduction and a noticeable increase in particle size which occur in the 600 °C-annealed specimen is that portion of Ge nanoparticles were oxidized and transferred to GeOx during post-annealing treatment. When considering the 800 °C-annealed specimen, very small particles again spread out the SiO2 film by replacing larger oxidized particles. Due to the fact that GeOx is thermodynamically instable, Ge–O bonds would be broken under a high-temperature annealing. At the same time, the broken Ge atoms from GeOx redistribute and then precipitate to form new Ge nanoparticles, thus increasing interfacial oxygen-deficiency-related defects and enhancing PL intensity. This explains the elimination of larger oxidized particles and the increase of PL intensity which were found in the 800 °C-annealed specimen. In fact, this transformation of Ge–O phase into Ge precipitations has been also reported by Mestanza et al. [16] showing that a reduction in GeOx results in an increase in PL intensity, which is well in agreement with our inference. 4. Summary The dependence of luminescent characteristics in Ge-implanted SiO2 film on post-annealing treatments was thoroughly explored in this study. The PL spectra of the Ge-implanted SiO2 film show a UV-blue emission band at around 3.1 eV. No shift in PL peak position can be observed in the specimens implanted with various fluences and annealed by various temperatures, implying that the emission band is neither due to quantum confinement effects nor
C.F. Yu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 171–176
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as-implanted
Ta=600oC
Fig. 5. Raman spectra of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h.
Ta=800oC
Ta=1000oC
Fig. 4. XTEM images of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h.
donor accepter pair of Ge nanoparticles. Oxygen-deficiency-related defects at the interface between Ge nanoparticles and SiO2 matrix are considered to be possible luminescence centers in Ge-implanted SiO2 film. The PL intensity was also found to be closely related to Ge implantation dose and post-annealing temperature. The
Fig. 6. FTIR transmission spectra of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h.
fact that PL intensity increases as decreasing Ge concentration suggests that high-fluence Ge implantation results in larger Ge nanoparticles and may induce more non-radiative defects. In addition, high-temperature annealing enhances particle coarsening effect as well as germanium oxidation process, both of which determine the amount of luminescence centers and thus lead to a fluctuation in PL intensity with post-annealing temperature. Particle coarsening effect minimizes the total interfacial area and reduces the amount of oxygen-deficiency-related defects. Also, germanium oxidation process reduces oxygen vacancies near the interface and is thus unfavorable for PL emission. Furthermore, the thermal evolution of the microstructure of Ge nanoparticles embedded in SiO2 film was found to be highly correlative to the variation in PL intensity. The formation of Ge nanoparticles results in an increase in PL intensity at 450 °C. The particle growth and GeOx formation lead to a dramatic increase in particle size and a sudden PL reduction at 600 °C. When the temperature comes to 800 °C, the oxidized GeOx tends to dissolve and the excess Ge atoms then precipitate to form new Ge nanoparticles,
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consequently again maximizing the PL intensity. Therefore, the results in this study can be useful guidance in optimizing light emission in Ge-implanted SiO2 film. Acknowledgements The authors would like to thank Prof. J. Li (Institute of Semiconductors, Chinese Academy of Sciences, People’s Republic of China) and Mr. C.H. Wang (National Tsing Hua University, Republic of China) for their excellent assistance with Ge ion implantation and SIMS measurements, respectively. Especially thanks Prof. Y.C. Hung at Institute of Photonics Technologies in National Tsing Hua University for the facility support of the fluorescence spectrophotometer. References [1] J.V. Borany, L. Rebohle, W. Skorupa, K.H. Heinig, in: IECON ‘99 Proceedings, vol. 1, 1999, pp. 62–67. [2] T.V. Torchynska, J. Aguilar-Hernandez, L.S. Hernández, G. Polupan, Y. Goldstein, A. Many, J. Jedrzejewski, A. Kolobov, Microelectron. Eng. 66 (2003) 83–90.
[3] R. Salh, L. Kourkoutis, M.V. Zamoryanskaya, B. Schmidt, H.-J. Fitting, J. NonCryst. Solids 355 (2009) 1107–1110. [4] J.M.J. Lopes, F.C. Zawislak, M. Behar, P.F.P. Fichtner, L. Rebohle, W. Skorupa, J. Appl. Phys. 94 (2003) 6059–6064. [5] P.K. Giri, S. Bhattacharyya, R. Kesavamoorthy, B.K. Panigrahi, K.G.M. Nair, J. Nanosci. Nanotechnol. 9 (2009) 1–7. [6] T.V. Torchynsk, G. Polupan, J.P. Gomez, A.V. Kolobov, Microelectron. J. 34 (2003) 541–543. [7] J.E. Chang, P.H. Liao, C.Y. Chien, J.C. Hsu, M.T. Hung, H.T. Chang, S.W. Lee, W.Y. Chen, T.M. Hsu, T. George, P.W. Li, J. Phys. D: Appl. Phys. 45 (2012) 105303. [8] C. Bostedt, T.V. Buuren, T.M. Willey, N. Franco, L.J. Terminello, C. Heske, T. Möller, Appl. Phys. Lett. 84 (2004) 4056–4058. [9] Y. Maeda, Phys. Rev. B 51 (1995) 1658–1670. [10] P.K. Giri, S. Bhattacharyya, S. Kumari, K. Das, S.K. Ray, J. Appl. Phys. 103 (2008) 103534. [11] N. Arai, H. Tsuji, H. Nakatsuka, K. Kojima, K. Adachi, H. Kotaki, T. Ishibashi, Y. Gotoh, J. Ishikawa, Mater. Sci. Eng. B 147 (2008) 230–234. [12] N. Arai, H. Tsuji, M. Hattori, M. Ohsaki, H. Kotaki, T. Ishibashi, Y. Gotoh, J. Ishikawa, Appl. Surf. Sci. 256 (2009) 954–957. [13] H. Krzyz´anowska, H. Bubert, J. Z´uk, W. Skorupa, J. Non-Cryst. Solids 354 (2008) 4363–4366. [14] S.N.M. Mestanza, I. Doi, J.W. Swart, N.C. Frateschi, J. Mater. Sci. 42 (2007) 7757–7761. [15] M. Klimenkov, J.V. Borany, W. Matz, R. Grötzschel, F. Herrmann, J. Appl. Phys. 91 (2002) 10062. [16] S.N.M. Mestanza, E. Rodriguez, N.C. Frateschi, Nanotechnology 17 (2006) 4548–4553.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Photoluminescent characteristics of ion beam synthesized Ge nanoparticles in thermally grown SiO2 films C.F. Yu a, D.S. Chao b,⇑, Y.-F. Chen a, J.H. Liang a,c a
Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 300, Taiwan, ROC c Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 300, Taiwan, ROC b
a r t i c l e
i n f o
Article history: Received 30 September 2012 Received in revised form 21 December 2012 Accepted 23 December 2012 Available online 18 January 2013 Keywords: Photoluminescence (PL) Ion beam synthesis Ge nanoparticles Oxygen-deficiency-related defects Quantum-confinement effects
a b s t r a c t Prospects of developing into numerous silicon-based optoelectronic applications have prompted many studies on the optical properties of Ge nanoparticles within a silicon oxide (SiO2) matrix. Even with such abundant studies, the fundamental mechanism underlying the Ge nanoparticle-induced photoluminescence (PL) is still an open question. In order to elucidate the mechanism, we dedicate this study to investigating the correlation between the PL properties and microstructure of the Ge nanoparticles synthesized in thermally grown SiO2 films. Our spectral data show that the peak position, at 3.1 eV or 400 nm, of the PL band arising from the Ge nanoparticles was essentially unchanged under different Ge implantation fluences and the temperatures of the following annealing process, whereas the sample preparation parameters modified or even fluctuated (in the case of the annealing temperature) the peak intensity considerably. Given the microscopically observed correlation between the nanoparticle structure and the sample preparation parameters, this phenomenon is consistent with the mechanism in which the oxygen-deficiency-related defects in the Ge/SiO2 interface act as the major luminescence centers; this mechanism also successfully explains the peak intensity fluctuation with the annealing temperature. Moreover, our FTIR data indicate the formation of GeOx upon ion implantation. Since decreasing of the oxygen-related defects by the GeOx formation is expected to be correlated with the annealing temperature, presence of the GeOx renders further experimental support to the oxygen defect mechanism. This understanding may assist the designing of the manufacturing process to optimize the Ge nanoparticle-based PL materials for different technological applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Efficient luminescent characteristics induced by nanoscale semiconductor materials have received growing interest in developing silicon-based optoelectronic applications. In recent years, a significant amount of research has been devoted to forming nanoparticles of group-IV elements (such as Si, Ge, and Sn) in silicon oxide (SiO2) layer by ion beam synthesis because of its great compatibility with Si technology and excellent control of implanted species in the ion implantation process. In particular, the optical properties of Ge-implanted oxide film are of special interest due to their strong photoluminescence (PL) and electroluminescence (EL) in the ultraviolet (UV)-blue spectral range as compared to other elements [1–3]. Thus far several studies have showed that the defects at the interface between Ge nanoparticles and SiO2 matrix are the ⇑ Corresponding author. Tel.: +886 3 5742866; fax: +886 3 5713849. E-mail address: [email protected] (D.S. Chao). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.12.116
primary luminescence centers for the corresponding emission bands. However, very diverse PL emission energy levels ranging from 2.2 to 3.2 eV (560–380 nm) have been reported [2–6]. In addition, a few studies have also claimed that Ge nanoparticle-induced luminescence could be attributed to quantum confinement effects originating from radiative recombination of excitons confined in Ge nanoparticles [7–9]. All of these indicate that the mechanisms behind the Ge nanoparticle-induced luminescence are indefinite and still in debate. In order to clarify the origin of PL emission resulting from Ge nanoparticles, this study thus aims to investigate the effects of ion implantation and post-annealing conditions on the PL characteristics induced by Ge nanoparticles. For accurate experimental supports, a thorough characteristic analysis using secondary ion mass spectrometry (SIMS), cross-section transmission electron microscopy (XTEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy is also conducted to find the correlation between optical properties of Ge nanoparticles and postannealing treatments.
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2. Experimental procedures In the present study, SiO2 films with a thickness of 100 nm were thermally grown on 4-inch (1 0 0)-oriented n-type Si substrates in wet oxide ambient at 1000 °C. The SiO2/Si substrates were roomtemperature implanted with 50 keV Ge ions to fluences of 3 1015, 1 1016, and 3 1016 cm 2. The Monte-Carlo Stopping and Range of Ions in Matter (SRIM) code was also used to simulate the Ge ion distribution, ensuring that the implanted Ge ions can adequately distribute over the SiO2 film and have distributions roughly peaked at the center of the SiO2 film. After ion implantation, the implanted substrates were cut into smaller specimens of about 1 1 cm2 in order to perform the post-annealing treatments. The specimens were annealed using conventional furnace annealing (FA) method in vacuum chamber (about 1 10 6 torr) with various temperatures ranging from 450 to 1000 °C for a fixed time of 1 h. In conducting the characteristic analysis of the synthesized Ge nanoparticles embedded in SiO2 film, PL spectra of Ge-implanted SiO2 thin films were characterized at room temperature using a Hitachi F-7000 Fluorescence Spectrophotometer with a fixed PMT voltage and excitation wavelength of 400 V and 260 nm, respectively. SIMS was employed to measure the depth profiles of Ge ions in the as-implanted and as-annealed specimens. Ge nanoparticle distribution and microstructure were examined by a high-resolution JEOL 2000FXII XTEM with an acceleration voltage of 200 kV. Compositional bonding configurations of the Ge-implanted SiO2 films that underwent various post-annealing temperatures were also carried out by a Bomem DA-8.3 FTIR. Infrared transmittance measurements of various specimens were performed at room temperature by scanning the wavenumber ranging from 400 to 4000 cm 1 with a spectral resolution of 2 cm 1. Furthermore, Raman spectra which can be used to identify the formation of Ge nanoparticles in SiO2 matrix were measured using a T-64000 Jobin–Yvon spectrometer having a spectral resolution of 1 cm 1. All samples were recorded in the black-scattering mode at room temperature, and the integration time was 20 min.
Fig. 1. (a) SRIM-simulated depth profile of 50 keV Ge ions implanted into a SiO2 layer of 100 nm. (b) SIMS-measured depth profiles of 50 keV, 3 1015 cm 2 Ge ions implanted into a SiO2 layer of 100 nm in the as-implanted specimen and asannealed ones that underwent annealing of 400 and 800 °C for 1 h.
3. Results and discussion In order to characterize the Ge ion distribution, the depth profiles of the implanted Ge ions were predicted by SRIM code and measured using SIMS analysis as shown in Fig. 1a and b, respectively. As can be seen, the measured peak Ge concentration in the as-implanted specimen locates at a depth of about 44 nm, which is in good agreement with the simulated projected range (Rp) of 51 nm. Comparison between the depth profiles in the as-implanted specimen and as-annealed ones that underwent annealing of 400 and 800 °C for 1 h is also shown in Fig. 1b. It is clear that the Ge ions in the 800 °C-annealed specimen have a broader distribution when compared to the other ones. Increasing post-annealing temperature drives the implanted Ge ions diffusing toward the SiO2/Si interface, thus resulting in a reduction in the Ge peak concentration. PL spectra of the Ge-implanted samples have been preliminarily investigated for the possibility of light emission. Fig. 2a shows the measured PL spectra of the Ge-implanted SiO2 films in the as-implanted specimen and as-annealed ones that underwent various post-annealing temperatures. As shown in Fig. 2a, all the spectra exhibit a UV-blue PL peak around 3.1 eV (400 nm). The peak positions are almost constant with the increase of post-annealing temperature. Notably, the emission band can be also found in the as-implanted specimen without undergoing any post-annealing treatment. The peak PL intensity as a function of post-annealing temperature is also inset in Fig. 2a. The PL intensity fluctuates with
post-annealing temperature and shows a maximum PL intensity at an annealing temperature of 450 °C. When the temperature increases, an unusual reduction in the PL intensity occurs at 600 °C. Then the PL intensity again increases as the temperature is further raised to 800 °C. Finally the PL disappears at the temperature of 1000 °C. The trend of PL intensity versus post-annealing temperature is also similar in other specimens implanted with various doses. The variation in PL intensity implies that the microstructure or physical properties of the synthesized Ge nanoparticles are incessantly changed during the post-annealing process. Although the luminescent characteristics induced by Ge-doped silicon glass have been extensively studied, the origin of the emission band induced by Ge nanoparticles still remains controversial. It has been reported in several literatures that the emission band in the vicinity of 400 nm can be attributed to the defect states located at the interface between nanoparticles and SiO2 matrix, and it is not unique to the presence of Ge [5,11]. Some of them have also claimed that oxygen-deficiency-related defects originating from Ge-related particles could be the recombination centers responsible for this emission [4,10–12]. In addition, quantum confinement effects which closely correlate with the size of Ge nanoparticles have also been believed to be another possible mechanism involved in quantum-assisted photonic luminescence [7–9]. These statements can be good reference in establishing a logical correlation between optical characteristics and processing conditions of the Ge nanoparticles synthesized in this study. Due to the fact that
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Fig. 2. (a) Measured PL spectra of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h. The inset shows the trend of PL intensity versus post-annealing temperature (lines are only for a guide). (b) Measured PL spectra of 800 °C, 1 h-annealed specimens as a function of implantation fluence. The inset shows the trend of PL intensity versus implantation fluence (line is only for a guide).
no shift in PL peak position can be observed in the specimens with various post-annealing temperatures, the possible mechanism which dominates PL emission in Ge-implanted SiO2 film should be the interfacial oxygen-deficiency-related defects existing at the interface between Ge nanoparticles and SiO2 matrix. Based on the above-mentioned statement, the dependence of the PL spectra on post-annealing temperature can be explained below. It is supposed that the implanted Ge ions are uniformly distributed over the SiO2 film. At first the isolated Ge ions start clustering to form Ge nanoparticles as temperature increases. At a high temperature such as 600 °C, Ostwald ripening effect enhances the growth of Ge nanoparticles, thus minimizing the number density as well as the total interfacial area of Ge nanoparticles. Since the luminescence centers are strongly related to the interfacial defects, a decrease in the total interfacial area thus reduces the luminescent intensity. When the temperature exceeds 600 °C, the PL intensity again increases and then maximizes at 800 °C, suggesting that significant transformation in Ge nanoparticles may occur during this stage of annealing. The possible reason why this transformation takes place will be discussed later according to
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the results obtained from XTEM, Raman, and FTIR. As the postannealing temperature further increases to 1000 °C, the PL is nearly undetectable and the intensity is even lower than that in the as-implanted case. Due to the fact that bulk Ge has a low melting point (900 °C), a high-temperature annealing of 1000 °C would lead to bond breaking and dissolution of Ge nanoparticles embedded in SiO2 matrix. As a result, the luminescence centers vanish. The relationship between PL intensity and implantation dose was also investigated as shown in Fig. 2b, which compares the PL spectra of the 800 °C-annealed specimens having Ge ion fluences of 3 1015, 1 1016, and 3 1016 cm 2. It is clear that a same PL peak at about 3.1 eV can be observed in the specimens with various implantation fluences. However, the PL intensity decreases with the increase of Ge concentration. Ge concentration makes a great impact on PL intensity showing that the maximum PL intensity at implantation dose of 3 1015 cm 2 is approximately 10 times higher than that at 3 1016 cm 2. Since the PL peak position has no shift with the implanted Ge ion fluence, oxygen-deficiency-related defects at the Ge/SiO2 interface are still considered to be a major medium for luminescence centers. Thus the inverse effect of Ge concentration on PL intensity can be explained by the fact that high Ge concentration leads to larger Ge nanoparticles, and thus reduces the amount of oxygen-deficiency-related defects available for luminescence. In addition, high-fluence Ge implantation may generate more non-radiative defects which could hinder electrons from carrier recombination [11]. The presence of interfacial oxygen-deficiency-related defects is considered to be the origin of luminescence centers in Ge-implanted SiO2 film, but it cannot still account for why a sudden drop in PL intensity that occurs at 600 °C. In order to clarify the temperature-dependent luminescent characteristics, further investigation using XTEM was carried out to verify the thermal evolution of the microstructure of Ge nanoparticles embedded in SiO2 matrix. Fig. 3 displays the XTEM-examined image of the SiO2 layer implanted with 50 keV, 3 1015 cm 2 Ge ions and annealed at 450 °C for 1 h. As can be seen, the Ge nanoparticles are formed in a nearly spherical shape with a mean diameter of 7 nm, and they are densely distributed within the SiO2 thin film. The inset of Fig. 3 also reveals a high-resolution lattice pattern showing that most of the Ge nanoparticles are essentially in a crystalline state. Fig. 4a–d also reveals the microstructure of the Ge-implanted SiO2 film as a function of post-annealing temperature. In contrast to the case of 450 °C, a noticeable increase in particle size from 7 to 16 nm can be observed in the 600 °C-annealed specimen. When the temperature reaches to 800 °C, larger particles are replaced by smaller ones having a diameter less than 3 nm. According to the theory of Ostwald ripening, it is expected that increasing temperature would result in particle coarsening at the expense of aggregating smaller ones in time. Nevertheless, a variation in particle size which is totally different from theory prediction was found in the XTEM results. In addition to a dramatic increase in particle size found at 600 °C, it is also unusual to again appear tiny particles at 800 °C. It is thus speculated that such a microstructure change may be caused by a significant transformation in these nanoparticles, and it may be a possible cause for a sudden drop in PL intensity at 600 °C, as shown in Fig. 2a. For further verification, Raman spectra and FTIR transmittance spectra were also characterized to identify the chemical bonding status of the synthesized nanoparticles in Ge-implanted SiO2 film. In regard to the Raman spectra as shown in Fig. 5, a distinct peak at about 300 cm 1 which is associated with Ge–Ge bonding was discovered in the as-implanted and the as-annealed specimens, indicating that Ge nanoparticles were indeed synthesized in the Ge-implanted SiO2 film. In particular, the Raman intensity associated with Ge–Ge bonds in the as-implanted and 1000 °C-annealed specimens is much weaker than that in other as-annealed ones. In
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Fig. 3. XTEM image of 50 keV, 3 1015 cm pattern of Ge nanoparticles.
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Ge-implanted SiO2 film annealed at 450 °C for 1 h. The inset shows a high-resolution image which clearly reveals the lattice
addition, FTIR transmittance spectra of the specimens underwent various post-annealing temperatures are shown in Fig. 6. It reveals two well defined peaks around 600 and 800 cm 1, which are associated with Ge–O–Ge vibration mode and Si–O bending mode, respectively. It is worth noting that the peak around 600 cm 1 due to Ge–O–Ge vibration mode exists in all specimens even for that without conducting subsequent post-annealing treatment. FTIR measurements evidence the presence of germanium oxide (GeOx) in the Ge-implanted SiO2 film. The source of oxygen in GeOx may originate from implantation-induced recoiled oxygen defects in SiO2 matrix and residual oxygen in vacuum FA chamber. Previous studies have reported that Ge-implanted SiO2 film followed by a high-temperature annealing leads to GeOx formation [11– 16]. Some of them have also shown that GeOx formation is a necessary intermediate process for the creation of Ge nanoparticles. A displacement reaction that is initiated by Si and O atoms together with GeOx results in Ge precipitation [13]. Moreover, due to the fact that Ge implantation induces lot of recoiled O atoms in SiO2 matrix, these unattached O atoms provide many dangling bonds and thus enhance a spontaneous oxidation reaction before annealing. Therefore, the results of FTIR spectra imply that GeOx formation should play a crucial role in determining the optical properties of Ge-implanted SiO2 film. It can be expected that GeOx formation would affect the amount of oxygen-deficiency-related defects at the interface between Ge nanoparticles and SiO2 matrix as well as the intensity of the UV-blue luminescence around 400 nm, as shown in Fig. 2a and b. Since the PL peak at about 400 nm is associated with oxygendeficiency-related defects, the intensity strongly depends on the amount of oxygen-deficiency-related defects available in SiO2 matrix. As for the as-implanted specimen, a spontaneous interaction between implanted Ge ions and local excess of Si and O atoms induced by implantation may proceed during Ge ion implantation. As a result, oxygen vacancy defects would be produced in SiO2 film after Ge ion implantation. This thus explains why the PL emission can be found in the as-implanted specimen. When the specimen is annealed by 450 °C, Ge ions start clustering to form Ge nanoparticles as shown in Fig. 3. The formation of Ge nanoparticles increases the total interfacial area as well as the amount of oxygen-deficiency-related defects at the interface between Ge nanoparticles and SiO2 matrix, thus allowing PL intensity maximizing at 450 °C.
When the temperature comes to 600 °C, smaller Ge nanoparticles evolve into larger ones simply relying on typical coarsening theory. In addition, as stated earlier, a high-temperature annealing enhances a process of Ge oxidation in Ge-implanted SiO2 film. GeOx reaction would first occur at the interface between Ge nanoparticles and SiO2 matrix due to the fact that plenty of Ge dangling bonds exist over there and they can easily react with O atoms. If annealing treatment supplies enough energy, more O atoms are expected to diffuse into Ge nanoparticles for GeOx reaction to proceed. Consequently, Ge nanoparticles are gradually oxidized from surface and also its particle size increases due to GeOx formation. However, GeOx formation reduces oxygen vacancies near the interface and is thus unfavorable for PL emission. Except for the particle coarsening induced by Ostwald ripening effect, another possible reason for a PL intensity reduction and a noticeable increase in particle size which occur in the 600 °C-annealed specimen is that portion of Ge nanoparticles were oxidized and transferred to GeOx during post-annealing treatment. When considering the 800 °C-annealed specimen, very small particles again spread out the SiO2 film by replacing larger oxidized particles. Due to the fact that GeOx is thermodynamically instable, Ge–O bonds would be broken under a high-temperature annealing. At the same time, the broken Ge atoms from GeOx redistribute and then precipitate to form new Ge nanoparticles, thus increasing interfacial oxygen-deficiency-related defects and enhancing PL intensity. This explains the elimination of larger oxidized particles and the increase of PL intensity which were found in the 800 °C-annealed specimen. In fact, this transformation of Ge–O phase into Ge precipitations has been also reported by Mestanza et al. [16] showing that a reduction in GeOx results in an increase in PL intensity, which is well in agreement with our inference. 4. Summary The dependence of luminescent characteristics in Ge-implanted SiO2 film on post-annealing treatments was thoroughly explored in this study. The PL spectra of the Ge-implanted SiO2 film show a UV-blue emission band at around 3.1 eV. No shift in PL peak position can be observed in the specimens implanted with various fluences and annealed by various temperatures, implying that the emission band is neither due to quantum confinement effects nor
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as-implanted
Ta=600oC
Fig. 5. Raman spectra of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h.
Ta=800oC
Ta=1000oC
Fig. 4. XTEM images of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h.
donor accepter pair of Ge nanoparticles. Oxygen-deficiency-related defects at the interface between Ge nanoparticles and SiO2 matrix are considered to be possible luminescence centers in Ge-implanted SiO2 film. The PL intensity was also found to be closely related to Ge implantation dose and post-annealing temperature. The
Fig. 6. FTIR transmission spectra of 50 keV, 3 1015 cm 2 Ge-implanted SiO2 films as a function of post-annealing temperature with a post-annealing time of 1 h.
fact that PL intensity increases as decreasing Ge concentration suggests that high-fluence Ge implantation results in larger Ge nanoparticles and may induce more non-radiative defects. In addition, high-temperature annealing enhances particle coarsening effect as well as germanium oxidation process, both of which determine the amount of luminescence centers and thus lead to a fluctuation in PL intensity with post-annealing temperature. Particle coarsening effect minimizes the total interfacial area and reduces the amount of oxygen-deficiency-related defects. Also, germanium oxidation process reduces oxygen vacancies near the interface and is thus unfavorable for PL emission. Furthermore, the thermal evolution of the microstructure of Ge nanoparticles embedded in SiO2 film was found to be highly correlative to the variation in PL intensity. The formation of Ge nanoparticles results in an increase in PL intensity at 450 °C. The particle growth and GeOx formation lead to a dramatic increase in particle size and a sudden PL reduction at 600 °C. When the temperature comes to 800 °C, the oxidized GeOx tends to dissolve and the excess Ge atoms then precipitate to form new Ge nanoparticles,
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consequently again maximizing the PL intensity. Therefore, the results in this study can be useful guidance in optimizing light emission in Ge-implanted SiO2 film. Acknowledgements The authors would like to thank Prof. J. Li (Institute of Semiconductors, Chinese Academy of Sciences, People’s Republic of China) and Mr. C.H. Wang (National Tsing Hua University, Republic of China) for their excellent assistance with Ge ion implantation and SIMS measurements, respectively. Especially thanks Prof. Y.C. Hung at Institute of Photonics Technologies in National Tsing Hua University for the facility support of the fluorescence spectrophotometer. References [1] J.V. Borany, L. Rebohle, W. Skorupa, K.H. Heinig, in: IECON ‘99 Proceedings, vol. 1, 1999, pp. 62–67. [2] T.V. Torchynska, J. Aguilar-Hernandez, L.S. Hernández, G. Polupan, Y. Goldstein, A. Many, J. Jedrzejewski, A. Kolobov, Microelectron. Eng. 66 (2003) 83–90.
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