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Effect of As doping on the photoluminescence of nanocrystalline 74Ge embedded in SiO2 matrix
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Journal of Luminescence 128 (2008) 1363–1368 www.elsevier.com/locate/jlumin
Effect of As doping on the photoluminescence of nanocrystalline embedded in SiO2 matrix
74
Ge
Shaobo Duna, Tiecheng Lua,, Youwen Hua, Qiang Hua, Liuqi Yua, Zheng Lia, Ningkang Huanga, Songbao Zhangb, Bin Tangb, Junlong Daib, Lev Resnickc, Issai Shlimakc a
Department of Physics and Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, PR China b Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, PR China c Minerva Center and Jack and Pearl Resnick Institute of Advanced Technology, Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel Received 15 May 2007; received in revised form 9 December 2007; accepted 4 January 2008 Available online 10 January 2008
Abstract Samples of nanocrystalline 74Ge embedded in amorphous SiO2 film were prepared by 74Ge ion implantation and subsequent primary thermal annealing. These samples were irradiated by neutron flux in a nuclear reactor then the second annealing followed. Irradiation with thermal neutrons leads to doping of nanocrystalline 74Ge with As impurities due to nuclear transmutation of isotope 74Ge into 75As. Transmission electron microscope, X-ray fluorescence, X-ray photoelectron spectroscopy, laser Raman scattering and photoluminescence of the obtained samples were measured. It was observed that with the increase in As-donors concentration, photoluminescence intensity first increased but then significantly decreased. An explanatory model of this non-monotonic behavior was discussed. r 2008 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Quenching; Nanocrystals; Doping
1. Introduction Recently, nanocrystalline Si and Ge (nc-Si, nc-Ge) embedded in silicon dioxide (SiO2) matrix have been widely studied for their luminescence and charge retention properties [1–4]. Application of those to commercial devices requires deeper knowledge of their energy band structures; in particular, the information related to the electronic properties of the impurity atoms. Up to now, some methods, for example, colloidal synthesis, cosputtering, sol–gel, ion implantation, have been reported to dope semiconductor nanostructures [5–10]. Despite some success, many attempts have failed. For example, Mn can be incorporated into nanocrystals of CdSe and ZnSe, but it cannot be doped into CdSe [9]. It is reported that acceptor (B) or donor (P) impurities doping can decrease luminescence intensity of nc-Si or porous Si; and B and P codoping can increase the luminescence intensity Corresponding author. Tel./fax: +86 28 85412031.
E-mail addresses: [email protected] (S. Dun), [email protected] (T. Lu). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.01.005
of nc-Si [5–8]. Up to now, studies about doping Ge nanostructures are few. In our studies, nc-Ge samples are doped by the neutron transmutation doping (NTD) method, which is an important technique for doping bulk semiconductors. The doping progress, microstructures, and optical properties of donor-doped nc-Ge are studied.
2. Experimental details Natural Ge consists of five stable isotopes: 70Ge (20.55%), 72Ge (27.37%), 73Ge (7.67%), 74Ge (36.74%) and 76Ge (7.67%). The main feature of the NTD process is that the Ge atoms create both the acceptor (transmutation of 70Ge into 71Ga) and the donor impurities (transmutation of 74Ge into 75As) [11,12]. In our experiment, samples of isotope nanocrystalline 74Ge samples were prepared by isotopic 74Ge ion implantation, followed by the thermal annealing. Ion implantation was performed in LC-4 highenergy ion implanter in 105 Torr vacuum atmosphere. Gas GeH4 was ionized into Ge+ and Ge2+ by arc
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discharging, isotope 74Ge+ ions accelerated to 150 keV were selected by magnetism analyses equipment (the mass resolution of magnet m/Dm=120), and implanted into 640 nm thick amorphous SiO2 film. The implanted dose of 74 Ge+ ions is 3 1016 and 1 1017 cm2. After implantation, samples were annealed at 800 1C for 0.5 h in forming gas (10% H2 and 90% Ar) atmosphere (labeled as asprepared nc-Ge). Neutron irradiation was performed in a nuclear reactor, with integral thermal neutron fluence of 1 1019 and 1 1020 cm2, respectively (corresponding samples were labeled as doped nc-Ge and heavily doped nc-Ge). When as-prepared nc-Ge sample is irradiated by thermal neutrons, some nuclei of isotope 74Ge atoms are transmuted into nuclei of 75As atoms, which are donor impurities in crystal Ge: 74
Geðn; gÞ75 Ge!75 As þ b .
The net concentration of NTD-introduced As impurities (NAs) can be calculated by equation NAs=NGedj, where NGe is concentration of Ge atoms (cm2), j is the intensity of the thermal neutron fluence (neutrons/cm2), d is the thermal neutron capture cross-section (for 74Ge, d=0.5 1024 cm2). There are two different thermal neutrons fluence: 1 1019 and 1 1020 neutrons/cm2; the dimensionless ratio N=NAs/NGe of samples is: 5 106 and 5 105. When the concentrations of Ge ions is 1 1017 cm2, the concentration of As atoms is 5 1012 and 5 1013 cm2, respectively. After nuclear reaction, the introduced As atoms are usually not in their lattice positions but displaced in interstitial positions due to the recoil produced by the g- and b-particles in the nuclear reactions. Besides, fast neutrons produce the radiation damages to the crystalline structure. Therefore, irradiated samples were subjected to the second annealing at 800 1C for 0.5 h to eliminate irradiated defects and re-crystallize nanocrystals. Cross-section and high-resolution transmission electron microscope (TEM) images were examined by JEOL 2010 FEG transmission electron microscope. Energy disperse Xray fluorescence (XRF) spectroscopy was measured by CANBERRA X-ray fluorescence spectrum analyzer with Si(Li) detector. X-ray photoelectron spectroscopy (XPS) spectra were measured by Ultra-DLD XPS, with mono-Al source operated at a vacuum of 3 109 Torr. For depth profile, the sample was eroded by 4 keV Ar ions for 80 min. Laser Raman scattering (LRS) and photoluminescence (PL) spectra were exited by 514.5 nm Ar+ laser and analyzed by a RENISHAW-1000 fluorescence spectrometer at room temperature.
Fig. 1. TEM images of as-prepared nc-Ge (a) and heavily doped nc-Ge (b) samples, insert images are the high-resolution TEM of one nanocrystal.
Fig. 2. XRF spectrum of heavily doped nc-Ge sample.
3. Results Fig. 1 shows the cross-section and high-resolution TEM images of the as-prepared nc-Ge and heavily doped nc-Ge samples (Ge ion dose 1 1017 cm2). In Fig. 1(a), nc-Ge particles well disperse in the zone near the surface of SiO2
film. We can clearly see the lattice fringes corresponding to the (1 1 1) planes of nanocrystals with the diamond structure. In Fig. 1(b), size and distribution of doped nanocrystals are similar to those of the undoped sample.
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Furthermore, twin defects are observed in part of nanocrystals, as shown in inset of HRTEM image. Fig. 2 shows the XRF spectrum of heavily doped nc-Ge sample (Ge ion dose 1 1017 cm2). The XRF spectrum shows the special energy peaks centered at 9.89, 10.54, and 10.98 keV. Based on the table of characteristic X-ray energy and absorption limit of elements [13], these three
Fig. 3. XPS spectrum of heavily doped nc-Ge sample.
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peaks are corresponding to the Ge ka, As ka, and Ge kb spectral lines, respectively. The result indicates that As impurities appear after neutron irradiation. Fig. 3 shows the low-energy regions of XPS spectrum of the heavily doped nc-Ge sample (Ge ion dose 1 1017 cm2). There are two broad bands in the spectrum, from 23 to 33 eV and 40 to 50 eV, which are related to Ge, SiO2, and As impurities. The positions of the peaks are: 25.3 eV for O 2s from SiO2, 29.1 eV for Ge 3d from Ge, and 33.2 eV for Ge 3d from GeO2, 40.7 eV for As 3d from As, 44.8 for As 3d from As2O3 and 46.0 eV for As 3d from As2O5 [14,15]. Fig. 4 shows the LRS spectra of as-prepared nc-Ge, doped nc-Ge and heavily doped nc-Ge samples (Ge ion dose 1 1017 cm2). The Raman spectrum of as-prepared nc-Ge consists mainly of two bands at 220–280 and 300 cm1, corresponding respectively to amorphous Gerelated components and nc-Ge [16,17]. In the LRS spectrum of the doped samples, it is found that the relative intensity of the nc-Ge peak (300 cm1) decreased, while the relative intensity of the low-frequency tail of amorphous Ge increased. Moreover, a high-frequency tail (from 320 to 400 cm1) was observed. This phenomenon is attributed to the fast neutron irradiation, which leads to the formation of radiation damages and lattice distortion in part of
Fig. 4. LRS spectra of as-prepared nc-Ge, doped nc-Ge and heavily doped nc-Ge samples.
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Fig. 5. Room temperature PL spectra, excited by 514.5 nm Ar ions laser: (a) heavily doped nc-Ge (Ge ion dose 1 1017 cm2), (b) doped nc-Ge (Ge ion dose 1 1017 cm2), (c) as-prepared nc-Ge (Ge ion dose 1 1017 cm2), (d) heavily doped nc-Ge (Ge ion dose 3 1016 cm2), (e) doped nc-Ge (Ge ion dose 3 1016 cm2), and (f) as-prepared doped nc-Ge (Ge ion dose 3 1016 cm2).
nc-Ge. Parts of the disordered Ge clusters are not recrystallized even after the second annealing. There are twin crystal defects observed in part of nanocrystals in the TEM image (Fig. 1b). The TEM and LRS results prove that thermostable radiation defects remain even after annealing at 800 1C. Fig. 5 shows the room-temperature PL spectra of the samples excited by 514.5 nm Ar ions laser. The scaling factors are shown (a larger factor corresponding to a larger PL intensity). The as-prepared nc-Ge samples show broad visible luminescence band peaks at 630 nm (sample of Ge ion dose 1 1017 cm2) and 600 nm (sample of Ge ion dose 3 1016 cm2). In our previous work, we demonstrated that the PL peak is sensitive to the size of nanocrystals. From the observed size dependence, we attribute the peak to the recombination of electrons and holes confined in the nc-Ge [18–20]. Moreover, in low-energy side of PL, nc-Ge shows a weak peak at 780 nm, which does not shift as the size of nc-Ge decreases. It is proved by XPS result that Ge and GeO2 coexit in ncGe. The peak at 780 nm is related to GeO color centers in the interface of nc-Ge and SiO2 matrix [21]. With increasing neutron dose from 1019 to 1020 cm2, the PL intensity of doped samples first increases and then decreases dramatically. And the band-edge PL peak shifts to high-energy side with the increase in As-donor concentration.
4. Discussions Light emissions in the near-infrared and visible regions have been observed in a variety of Si and Ge nanostructures. Owing to the large surface area of nanostructures, proper surface passivation is essential in obtaining efficient luminescence. Surface passivation controls the luminescence intensity through the elimination of competing nonradiative carrier relaxation pathways. Passivation by hydrogen and oxygen is reported to be effective in improving the luminescence efficiency. It is reported that dangling bond at the interface of oxidized nanocrystals and SiO2 matrices (Pb centers) is one of the important defects. At room temperature, the Pb centers act as a non-radiative recombination center, therefore decreasing the band-edge luminescence efficiency. Therefore, by decreasing the density of Pb centers, the intensity of PL can be enhanced in P doping nc-Si or nc-SiGe [7,22,25]. It is the same case for the increasing of PL in As-doped nc-Ge. Free electrons from donor impurities (As atoms) can passivate nonradiative centers and enhance the intensity of band-edge luminescence. On the other hand, one can assume that the increase of the number of free electrons may reduce the radiative efficiency because a new channel appears by non-radiative recombination of generated electron–hole pairs. This channel is connected with possibility of Auger-like process
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where recombination energy is transferred to a free electron followed by its thermal relaxation via emission of phonons [23–25]. The As atoms located inside a nanocrystal can provide an extra electron. This carrier would quench the PL by the annihilation of an optically generated exciton through Auger recombination. These opposite tendencies in our opinion determine the observed non-monotonic dependence of PL on the level of doping: at comparable small concentration of NTD-introduced As donors, the donor electrons are mainly trapped by non-radiative centers and the mechanism of dominate passivation, while at high concentration of As-donor electrons remain free and significantly reduce the radiative efficiency of electron–hole pairs in nc-Ge. When a uniform concentration of dopant is introduced into a sample containing nanocrystals, the probability of doping a nanocrystal depends on the dopant concentration and the size of the nanocrystals. From the TEM pictures, the size distribution of doped nanocrystals is similar to undoped nanocrystals. So the shift of PL peak in heavily doped nc-Ge sample is related to doping, not to size change. In doped nc-Ge sample, only a few nanocrystals have dopants because of low As concentration; so the quenching of PL is not obvious. In heavily doped nc-Ge sample, larger nanocrystals are doped and hence should be optically inactive, while most of the smaller ones remain undoped, and hence this majority can still emit light. So, it is expected that the doping of nanocrystals will cause a decrease in intensity combined with an overall blue shift of PL emission spectrum [8]. Otherwise, a lot of irradiated defects were fabricated when irradiating in nuclear reactor, there are still some defects stay even after second annealing at 800 1C, especially in heavily doped nc-Ge sample, which is proved by Raman data and HRTEM image. These defects also act as non-radiative centers in the nanocrystals. The theoretical estimations show that a single non-radiative recombination center is sufficient to suppress luminescence of one nanocrystal [23]. That is another reason for the quench of PL in heavily doped nc-Ge sample, besides the Auger effect. 5. Conclusions In conclusion, donor impurities were introduced into ncGe embedded in SiO2 matrix by neuron irradiation and subsequent annealing. Some irradiation defects still remain in nc-Ge after second annealing from TEM and Raman results. It is found that intensity of PL increases at relatively low concentration of As donors and decreases dramatically in heavily doped nc-Ge sample. This nonmonotonic behavior can be explained by the following model: at low concentration, the radiative efficiency is increased because that the Pb non-radiative centers are trapped by donor electrons from As, while at high concentration, donor electrons remain free and reduce
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significantly the radiative efficiency due to the appearance of new non-radiative Auger-like recombination channel. Besides, irradiated defects acting as non-radiative centers can quench the luminescence of nanocrystals. Acknowledgments 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 NASF of NSFC-CAEP of China (no. 10376020), as well as Program for New Century Excellent Talents in University, China (no. NCET-040874). The authors thank A.N. Ionov and I. Lazebnik for their help in sample irradiation with a neutron flux in a research nuclear reactor; also thank Kai Sun and Lumin Wang for their help in XPS and TEM measurements and results discussion. L.R. and I.S. are thankful to the Erick and Sheila Samson Chair of Semiconductor Technology for the financial support. References [1] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408 (2000) 440. [2] Y. Maeda, Phys. Rev. B 51 (1995) 1658. [3] Sandip Tiwari, Farhan Rana, Hussein Hanafi, Allan Hartstein, Emmanuel F. Crabbe´, Kevin Chan, Appl. Phys. Lett. 68 (1996) 1377. [4] Q. Wan, N.L. Zhang, W.L. Liu, C.L. Lin, T.H. Wang, Appl. Phys. Lett. 83 (2003) 138. [5] Minoru Fujii, Shinji Hayashi, Keiichi Yamamoto, J. Appl. Phys. 83 (1998) 7953. [6] S. Sen, J. Siejka, A. Savtchouk, J. Lagowski, Appl. Phys. Lett. 70 (1997) 2253. [7] Minoru Fujii, Atsushi Mimura, Shinji Hayashi, Keiichi Yamamoto, Chika Urakawa, Hitoshi Ohta, J. Appl. Phys. 87 (2000) 1855. [8] Atsushi Mimura, Minoru Fujii, Shinji Hayashi, Dmitri Kovalev, Frederick Koch, Phys. Rev. B 62 (2000) 12625. [9] Steven C. Erwin, Linjun Zu, Michael I. Haftel, Alexander L. Efros, Thomas A. Kenned, David J. Norris, Nature 436 (2005) 91. [10] G.A. Kachurin, S.G. Cherkova, V.A. Volodin, V.G. Kesler, A.K. Gutakovsky, A.G. Cherkov, A.V. Bublikov, D.I. Tetelbaum, Nucl. Instrum. Methods B 222 (2004) 497. [11] I.S. Shlimak, Phys. Solid State 41 (1999) 716. [12] E.E. Haller, J. Appl. Phys. 77 (1995) 2857. [13] Rene E. Van Grieken, A. Markowicz, Handbook of X-ray Spectrometry, Marcel Dekker, New York, 2001, p. 47. [14] A.K. Dutta, Appl. Phys. Lett. 68 (1996) 1189. [15] N. Xu, Y. Xu, L. Li, Y. Shen, T. Zhang, J. Wu, J. Sun, Z. Ying, J. Vac. Sci. Technol. A 24 (2006) 517. [16] X.L. Wu, T. Gao, X.M. Bao, F. Yan, S.S. Jiang, D. Feng, J. Appl. Phys. 82 (1997) 2704. [17] C.E. Bottani, C. Mantini, P. Milani, M. Manfredini, Appl. Phys. Lett. 69 (1996) 2409. [18] P.K. Giri, R. Kesavamoorthy, B.K. Panigrahi, K.G.M. Nair, Solid State Commun. 133 (2005) 229. [19] X.L. Wu, T. Gao, G.G. Siu, S. Tong, X.M. Bao, Appl. Phys. Lett. 74 (1999) 2420. [20] P.K. Giri, R. Kesavamoorthy, B.K. Panigrahi, K.G.M. Nair, Solid State Commun. 133–134 (2005) 229. [21] J.K. Shen, X.L. Wu, R.K. Yuan, N. Tang, J.P. Zou, Y.F. Mei, C. Tan, X.M. Bao, Appl. Phys. Lett. 77 (1990) 3134.
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[22] Kimiaki Toshikiyo, Masakazu Tokunaga, Shinji Takeoka, Minoru Fujii, Shinji Hayashi, Kazuyuki Moriwaki, J. Appl. Phys. 90 (2001) 5147. [23] M. Lannoo, C. Delerue, G. Allan, J. Lumin. 70 (1996) 170.
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Journal of Luminescence 128 (2008) 1363–1368 www.elsevier.com/locate/jlumin
Effect of As doping on the photoluminescence of nanocrystalline embedded in SiO2 matrix
74
Ge
Shaobo Duna, Tiecheng Lua,, Youwen Hua, Qiang Hua, Liuqi Yua, Zheng Lia, Ningkang Huanga, Songbao Zhangb, Bin Tangb, Junlong Daib, Lev Resnickc, Issai Shlimakc a
Department of Physics and Key Laboratory for Radiation Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, PR China b Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, PR China c Minerva Center and Jack and Pearl Resnick Institute of Advanced Technology, Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel Received 15 May 2007; received in revised form 9 December 2007; accepted 4 January 2008 Available online 10 January 2008
Abstract Samples of nanocrystalline 74Ge embedded in amorphous SiO2 film were prepared by 74Ge ion implantation and subsequent primary thermal annealing. These samples were irradiated by neutron flux in a nuclear reactor then the second annealing followed. Irradiation with thermal neutrons leads to doping of nanocrystalline 74Ge with As impurities due to nuclear transmutation of isotope 74Ge into 75As. Transmission electron microscope, X-ray fluorescence, X-ray photoelectron spectroscopy, laser Raman scattering and photoluminescence of the obtained samples were measured. It was observed that with the increase in As-donors concentration, photoluminescence intensity first increased but then significantly decreased. An explanatory model of this non-monotonic behavior was discussed. r 2008 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Quenching; Nanocrystals; Doping
1. Introduction Recently, nanocrystalline Si and Ge (nc-Si, nc-Ge) embedded in silicon dioxide (SiO2) matrix have been widely studied for their luminescence and charge retention properties [1–4]. Application of those to commercial devices requires deeper knowledge of their energy band structures; in particular, the information related to the electronic properties of the impurity atoms. Up to now, some methods, for example, colloidal synthesis, cosputtering, sol–gel, ion implantation, have been reported to dope semiconductor nanostructures [5–10]. Despite some success, many attempts have failed. For example, Mn can be incorporated into nanocrystals of CdSe and ZnSe, but it cannot be doped into CdSe [9]. It is reported that acceptor (B) or donor (P) impurities doping can decrease luminescence intensity of nc-Si or porous Si; and B and P codoping can increase the luminescence intensity Corresponding author. Tel./fax: +86 28 85412031.
E-mail addresses: [email protected] (S. Dun), [email protected] (T. Lu). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.01.005
of nc-Si [5–8]. Up to now, studies about doping Ge nanostructures are few. In our studies, nc-Ge samples are doped by the neutron transmutation doping (NTD) method, which is an important technique for doping bulk semiconductors. The doping progress, microstructures, and optical properties of donor-doped nc-Ge are studied.
2. Experimental details Natural Ge consists of five stable isotopes: 70Ge (20.55%), 72Ge (27.37%), 73Ge (7.67%), 74Ge (36.74%) and 76Ge (7.67%). The main feature of the NTD process is that the Ge atoms create both the acceptor (transmutation of 70Ge into 71Ga) and the donor impurities (transmutation of 74Ge into 75As) [11,12]. In our experiment, samples of isotope nanocrystalline 74Ge samples were prepared by isotopic 74Ge ion implantation, followed by the thermal annealing. Ion implantation was performed in LC-4 highenergy ion implanter in 105 Torr vacuum atmosphere. Gas GeH4 was ionized into Ge+ and Ge2+ by arc
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discharging, isotope 74Ge+ ions accelerated to 150 keV were selected by magnetism analyses equipment (the mass resolution of magnet m/Dm=120), and implanted into 640 nm thick amorphous SiO2 film. The implanted dose of 74 Ge+ ions is 3 1016 and 1 1017 cm2. After implantation, samples were annealed at 800 1C for 0.5 h in forming gas (10% H2 and 90% Ar) atmosphere (labeled as asprepared nc-Ge). Neutron irradiation was performed in a nuclear reactor, with integral thermal neutron fluence of 1 1019 and 1 1020 cm2, respectively (corresponding samples were labeled as doped nc-Ge and heavily doped nc-Ge). When as-prepared nc-Ge sample is irradiated by thermal neutrons, some nuclei of isotope 74Ge atoms are transmuted into nuclei of 75As atoms, which are donor impurities in crystal Ge: 74
Geðn; gÞ75 Ge!75 As þ b .
The net concentration of NTD-introduced As impurities (NAs) can be calculated by equation NAs=NGedj, where NGe is concentration of Ge atoms (cm2), j is the intensity of the thermal neutron fluence (neutrons/cm2), d is the thermal neutron capture cross-section (for 74Ge, d=0.5 1024 cm2). There are two different thermal neutrons fluence: 1 1019 and 1 1020 neutrons/cm2; the dimensionless ratio N=NAs/NGe of samples is: 5 106 and 5 105. When the concentrations of Ge ions is 1 1017 cm2, the concentration of As atoms is 5 1012 and 5 1013 cm2, respectively. After nuclear reaction, the introduced As atoms are usually not in their lattice positions but displaced in interstitial positions due to the recoil produced by the g- and b-particles in the nuclear reactions. Besides, fast neutrons produce the radiation damages to the crystalline structure. Therefore, irradiated samples were subjected to the second annealing at 800 1C for 0.5 h to eliminate irradiated defects and re-crystallize nanocrystals. Cross-section and high-resolution transmission electron microscope (TEM) images were examined by JEOL 2010 FEG transmission electron microscope. Energy disperse Xray fluorescence (XRF) spectroscopy was measured by CANBERRA X-ray fluorescence spectrum analyzer with Si(Li) detector. X-ray photoelectron spectroscopy (XPS) spectra were measured by Ultra-DLD XPS, with mono-Al source operated at a vacuum of 3 109 Torr. For depth profile, the sample was eroded by 4 keV Ar ions for 80 min. Laser Raman scattering (LRS) and photoluminescence (PL) spectra were exited by 514.5 nm Ar+ laser and analyzed by a RENISHAW-1000 fluorescence spectrometer at room temperature.
Fig. 1. TEM images of as-prepared nc-Ge (a) and heavily doped nc-Ge (b) samples, insert images are the high-resolution TEM of one nanocrystal.
Fig. 2. XRF spectrum of heavily doped nc-Ge sample.
3. Results Fig. 1 shows the cross-section and high-resolution TEM images of the as-prepared nc-Ge and heavily doped nc-Ge samples (Ge ion dose 1 1017 cm2). In Fig. 1(a), nc-Ge particles well disperse in the zone near the surface of SiO2
film. We can clearly see the lattice fringes corresponding to the (1 1 1) planes of nanocrystals with the diamond structure. In Fig. 1(b), size and distribution of doped nanocrystals are similar to those of the undoped sample.
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Furthermore, twin defects are observed in part of nanocrystals, as shown in inset of HRTEM image. Fig. 2 shows the XRF spectrum of heavily doped nc-Ge sample (Ge ion dose 1 1017 cm2). The XRF spectrum shows the special energy peaks centered at 9.89, 10.54, and 10.98 keV. Based on the table of characteristic X-ray energy and absorption limit of elements [13], these three
Fig. 3. XPS spectrum of heavily doped nc-Ge sample.
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peaks are corresponding to the Ge ka, As ka, and Ge kb spectral lines, respectively. The result indicates that As impurities appear after neutron irradiation. Fig. 3 shows the low-energy regions of XPS spectrum of the heavily doped nc-Ge sample (Ge ion dose 1 1017 cm2). There are two broad bands in the spectrum, from 23 to 33 eV and 40 to 50 eV, which are related to Ge, SiO2, and As impurities. The positions of the peaks are: 25.3 eV for O 2s from SiO2, 29.1 eV for Ge 3d from Ge, and 33.2 eV for Ge 3d from GeO2, 40.7 eV for As 3d from As, 44.8 for As 3d from As2O3 and 46.0 eV for As 3d from As2O5 [14,15]. Fig. 4 shows the LRS spectra of as-prepared nc-Ge, doped nc-Ge and heavily doped nc-Ge samples (Ge ion dose 1 1017 cm2). The Raman spectrum of as-prepared nc-Ge consists mainly of two bands at 220–280 and 300 cm1, corresponding respectively to amorphous Gerelated components and nc-Ge [16,17]. In the LRS spectrum of the doped samples, it is found that the relative intensity of the nc-Ge peak (300 cm1) decreased, while the relative intensity of the low-frequency tail of amorphous Ge increased. Moreover, a high-frequency tail (from 320 to 400 cm1) was observed. This phenomenon is attributed to the fast neutron irradiation, which leads to the formation of radiation damages and lattice distortion in part of
Fig. 4. LRS spectra of as-prepared nc-Ge, doped nc-Ge and heavily doped nc-Ge samples.
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Fig. 5. Room temperature PL spectra, excited by 514.5 nm Ar ions laser: (a) heavily doped nc-Ge (Ge ion dose 1 1017 cm2), (b) doped nc-Ge (Ge ion dose 1 1017 cm2), (c) as-prepared nc-Ge (Ge ion dose 1 1017 cm2), (d) heavily doped nc-Ge (Ge ion dose 3 1016 cm2), (e) doped nc-Ge (Ge ion dose 3 1016 cm2), and (f) as-prepared doped nc-Ge (Ge ion dose 3 1016 cm2).
nc-Ge. Parts of the disordered Ge clusters are not recrystallized even after the second annealing. There are twin crystal defects observed in part of nanocrystals in the TEM image (Fig. 1b). The TEM and LRS results prove that thermostable radiation defects remain even after annealing at 800 1C. Fig. 5 shows the room-temperature PL spectra of the samples excited by 514.5 nm Ar ions laser. The scaling factors are shown (a larger factor corresponding to a larger PL intensity). The as-prepared nc-Ge samples show broad visible luminescence band peaks at 630 nm (sample of Ge ion dose 1 1017 cm2) and 600 nm (sample of Ge ion dose 3 1016 cm2). In our previous work, we demonstrated that the PL peak is sensitive to the size of nanocrystals. From the observed size dependence, we attribute the peak to the recombination of electrons and holes confined in the nc-Ge [18–20]. Moreover, in low-energy side of PL, nc-Ge shows a weak peak at 780 nm, which does not shift as the size of nc-Ge decreases. It is proved by XPS result that Ge and GeO2 coexit in ncGe. The peak at 780 nm is related to GeO color centers in the interface of nc-Ge and SiO2 matrix [21]. With increasing neutron dose from 1019 to 1020 cm2, the PL intensity of doped samples first increases and then decreases dramatically. And the band-edge PL peak shifts to high-energy side with the increase in As-donor concentration.
4. Discussions Light emissions in the near-infrared and visible regions have been observed in a variety of Si and Ge nanostructures. Owing to the large surface area of nanostructures, proper surface passivation is essential in obtaining efficient luminescence. Surface passivation controls the luminescence intensity through the elimination of competing nonradiative carrier relaxation pathways. Passivation by hydrogen and oxygen is reported to be effective in improving the luminescence efficiency. It is reported that dangling bond at the interface of oxidized nanocrystals and SiO2 matrices (Pb centers) is one of the important defects. At room temperature, the Pb centers act as a non-radiative recombination center, therefore decreasing the band-edge luminescence efficiency. Therefore, by decreasing the density of Pb centers, the intensity of PL can be enhanced in P doping nc-Si or nc-SiGe [7,22,25]. It is the same case for the increasing of PL in As-doped nc-Ge. Free electrons from donor impurities (As atoms) can passivate nonradiative centers and enhance the intensity of band-edge luminescence. On the other hand, one can assume that the increase of the number of free electrons may reduce the radiative efficiency because a new channel appears by non-radiative recombination of generated electron–hole pairs. This channel is connected with possibility of Auger-like process
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where recombination energy is transferred to a free electron followed by its thermal relaxation via emission of phonons [23–25]. The As atoms located inside a nanocrystal can provide an extra electron. This carrier would quench the PL by the annihilation of an optically generated exciton through Auger recombination. These opposite tendencies in our opinion determine the observed non-monotonic dependence of PL on the level of doping: at comparable small concentration of NTD-introduced As donors, the donor electrons are mainly trapped by non-radiative centers and the mechanism of dominate passivation, while at high concentration of As-donor electrons remain free and significantly reduce the radiative efficiency of electron–hole pairs in nc-Ge. When a uniform concentration of dopant is introduced into a sample containing nanocrystals, the probability of doping a nanocrystal depends on the dopant concentration and the size of the nanocrystals. From the TEM pictures, the size distribution of doped nanocrystals is similar to undoped nanocrystals. So the shift of PL peak in heavily doped nc-Ge sample is related to doping, not to size change. In doped nc-Ge sample, only a few nanocrystals have dopants because of low As concentration; so the quenching of PL is not obvious. In heavily doped nc-Ge sample, larger nanocrystals are doped and hence should be optically inactive, while most of the smaller ones remain undoped, and hence this majority can still emit light. So, it is expected that the doping of nanocrystals will cause a decrease in intensity combined with an overall blue shift of PL emission spectrum [8]. Otherwise, a lot of irradiated defects were fabricated when irradiating in nuclear reactor, there are still some defects stay even after second annealing at 800 1C, especially in heavily doped nc-Ge sample, which is proved by Raman data and HRTEM image. These defects also act as non-radiative centers in the nanocrystals. The theoretical estimations show that a single non-radiative recombination center is sufficient to suppress luminescence of one nanocrystal [23]. That is another reason for the quench of PL in heavily doped nc-Ge sample, besides the Auger effect. 5. Conclusions In conclusion, donor impurities were introduced into ncGe embedded in SiO2 matrix by neuron irradiation and subsequent annealing. Some irradiation defects still remain in nc-Ge after second annealing from TEM and Raman results. It is found that intensity of PL increases at relatively low concentration of As donors and decreases dramatically in heavily doped nc-Ge sample. This nonmonotonic behavior can be explained by the following model: at low concentration, the radiative efficiency is increased because that the Pb non-radiative centers are trapped by donor electrons from As, while at high concentration, donor electrons remain free and reduce
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significantly the radiative efficiency due to the appearance of new non-radiative Auger-like recombination channel. Besides, irradiated defects acting as non-radiative centers can quench the luminescence of nanocrystals. Acknowledgments 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 NASF of NSFC-CAEP of China (no. 10376020), as well as Program for New Century Excellent Talents in University, China (no. NCET-040874). The authors thank A.N. Ionov and I. Lazebnik for their help in sample irradiation with a neutron flux in a research nuclear reactor; also thank Kai Sun and Lumin Wang for their help in XPS and TEM measurements and results discussion. L.R. and I.S. are thankful to the Erick and Sheila Samson Chair of Semiconductor Technology for the financial support. References [1] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzo, F. Priolo, Nature 408 (2000) 440. [2] Y. Maeda, Phys. Rev. B 51 (1995) 1658. [3] Sandip Tiwari, Farhan Rana, Hussein Hanafi, Allan Hartstein, Emmanuel F. Crabbe´, Kevin Chan, Appl. Phys. Lett. 68 (1996) 1377. [4] Q. Wan, N.L. Zhang, W.L. Liu, C.L. Lin, T.H. Wang, Appl. Phys. Lett. 83 (2003) 138. [5] Minoru Fujii, Shinji Hayashi, Keiichi Yamamoto, J. Appl. Phys. 83 (1998) 7953. [6] S. Sen, J. Siejka, A. Savtchouk, J. Lagowski, Appl. Phys. Lett. 70 (1997) 2253. [7] Minoru Fujii, Atsushi Mimura, Shinji Hayashi, Keiichi Yamamoto, Chika Urakawa, Hitoshi Ohta, J. Appl. Phys. 87 (2000) 1855. [8] Atsushi Mimura, Minoru Fujii, Shinji Hayashi, Dmitri Kovalev, Frederick Koch, Phys. Rev. B 62 (2000) 12625. [9] Steven C. Erwin, Linjun Zu, Michael I. Haftel, Alexander L. Efros, Thomas A. Kenned, David J. Norris, Nature 436 (2005) 91. [10] G.A. Kachurin, S.G. Cherkova, V.A. Volodin, V.G. Kesler, A.K. Gutakovsky, A.G. Cherkov, A.V. Bublikov, D.I. Tetelbaum, Nucl. Instrum. Methods B 222 (2004) 497. [11] I.S. Shlimak, Phys. Solid State 41 (1999) 716. [12] E.E. Haller, J. Appl. Phys. 77 (1995) 2857. [13] Rene E. Van Grieken, A. Markowicz, Handbook of X-ray Spectrometry, Marcel Dekker, New York, 2001, p. 47. [14] A.K. Dutta, Appl. Phys. Lett. 68 (1996) 1189. [15] N. Xu, Y. Xu, L. Li, Y. Shen, T. Zhang, J. Wu, J. Sun, Z. Ying, J. Vac. Sci. Technol. A 24 (2006) 517. [16] X.L. Wu, T. Gao, X.M. Bao, F. Yan, S.S. Jiang, D. Feng, J. Appl. Phys. 82 (1997) 2704. [17] C.E. Bottani, C. Mantini, P. Milani, M. Manfredini, Appl. Phys. Lett. 69 (1996) 2409. [18] P.K. Giri, R. Kesavamoorthy, B.K. Panigrahi, K.G.M. Nair, Solid State Commun. 133 (2005) 229. [19] X.L. Wu, T. Gao, G.G. Siu, S. Tong, X.M. Bao, Appl. Phys. Lett. 74 (1999) 2420. [20] P.K. Giri, R. Kesavamoorthy, B.K. Panigrahi, K.G.M. Nair, Solid State Commun. 133–134 (2005) 229. [21] J.K. Shen, X.L. Wu, R.K. Yuan, N. Tang, J.P. Zou, Y.F. Mei, C. Tan, X.M. Bao, Appl. Phys. Lett. 77 (1990) 3134.
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