- Email: [email protected]
Visible photoluminescence of Ge enriched SiOx layers
Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 619–622
Visible photoluminescence of Ge enriched SiOx layers T.V. Torchynska a,∗ , A.Vivas Hernandez a , A.V. Kolobov b , Y. Goldstein c , E. Savir c , J. Jedrzejewski c a
b
ESFM—National Polytechnic Institute, Mexico D.F. 07738, Mexico LAOTECH—National Institute for Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562, Japan c Racah Institute of Physics, Hebrew University, 91904 Jerusalem, Israel Available online 19 March 2004
Abstract This paper concentrates on results of photoluminescence (PL), Raman scattering and extended X-ray absorption fine structure (EXAFS) investigations of silicon oxide films enriched by Ge. All the spectra were analyzed both for “as prepared” silicon oxide films enriched by Ge (without Ge-quantum dots, QD) and for films after high temperature annealing in inert atmosphere with the aim of creating Ge-nano-crystallites (nc)-quantum dots. Five PL bands were discovered in Ge–SiOx systems. It is shown that the visible PL bands peaked at 1.61–1.65 eV (2), 1.75–1.80 eV (3), 2.00–2.06 eV (4) and 2.25–2.30 eV (5) do not correlate with the presence of Ge atoms or Ge-nc in the samples. The nature of the infrared PL band peaked at 1.43–1.52 eV is not clear. The intensity of this PL band increases in thermally annealed samples with increasing Ge concentration. © 2004 Elsevier B.V. All rights reserved. Keywords: Visible photoluminescence; Ge–SiOx layers; Ge-quantum dots
1. Introduction Semiconductor low-dimensional structures such as quantum wires and dots based on Ge and Si nano-crystallites are attracting great attention because of the expectation that the radiation efficiency of indirect optical transitions may be significantly increased if the size of the semiconductor crystallites is in the nanometer-scale range [1,2]. The photoluminescence (PL) from Ge and Si nano-crystallites (nc) embedded in SiO2 in the spectral range of 1.5–2.4 eV was observed in a number of scientific laboratories [2–12]. The mechanism of this photoluminescence has been discussed extensively but it is not clear up to now. These optical transitions are attributed to: radiation recombination inside Si (Ge) nano-crystallites via quantum-confined states [3–5], defect-related emission in silicon oxide [6,7,10,11] or at the Si/SiOx interface [5,8,10,11], as well as to exciton recombination at the Si/SiOx interface [12]. Thus, the origin of visible PL of Si or Ge-nc embedded in silicon oxide still remains to be clarified.
∗ Corresponding author. Tel.: +52-572-96000/55031; Tel.: +52-572-96000/55003. E-mail address: [email protected] (T.V. Torchynska).
0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.02.039
This paper concentrates on results of photoluminescence, Raman scattering and extended X-ray absorption fine structure (EXAFS) investigations of silicon oxide films enriched by Ge. All the spectra were analyzed both for “as prepared” silicon oxide films enriched by Ge (without Ge-quantum dots) and for films after high temperature annealing in inert atmosphere with the aim of creating Ge-nc-quantum dots (QD).
2. Experimental techniques Silicon oxide films enriched by Ge were prepared by co-deposition of Ge and silicon oxide onto quartz substrates by radio frequency magnetron sputtering. The Ge concentration varied from 25 to 60 mol%. After deposition, some of the samples were isothermally annealed for 1 h at 800 ◦ C in an argon atmosphere in order to produce nano-crystals with a characteristic size of 6–10 nm [3]. PL and Raman spectra have been excited in our samples with an Ar laser tuned to a wavelength of 4880 Å. The laser power was 80 mW. Raman signals were measured with a model 1403 spectrometer in a backscattering geometry at room temperature. The PL measurements were carried out as well at room temperature using a Control Development
620
T.V. Torchynska et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 619–622
Inc. Spectrometer with CCD detectors. The EXAFS measurements were performed at beam line 13B at the Photon Factory [13].
PL intensity [arb. units]
10
3. Experimental results
6 4
2
2
3
PL intensity [arb. units]
0 1
8
(b)
6 4
2
2
3
0 1.4
1.6
1.8
2.0
2.2
2.4
Emission energy (eV) Fig. 1. PL spectra of the Ge–SiO2 system in as-prepared state (a) and after thermal annealing (b) for concentrations of Ge, 25 mol% (1), 42 mol% (2) and 60 mol% (3).
3.5 PL intensity [arb. units]
Fig. 1 presents typical PL spectra of silicon oxide films enriched by different molar concentrations of Ge in as-prepared state (a) and after thermal annealing (b). In both cases, very wide PL spectra were observed. Each of these PL spectra can be decomposed into at least five elementary PL bands peaked at: 1.43–1.52 eV (band 1), 1.61–1.65 eV (2), 1.75–1.90 eV (3), 2.00–2.06 eV (4) and 2.25–2.35 eV (5) with the full width at half maximum (FWHM) equal to 100, 150, 200, 225 and 280 meV, respectively, as shown in Fig. 2. With increasing Ge molar concentration in the silicon oxide the intensities of all PL bands generally decrease, see Fig. 1 and Table 1. The intensities of 2–4 PL bands remain unchanged with thermal annealing, in all samples (Fig. 1, Table 1). The intensities of 1 and 5 PL bands decreased (or do not changed) for samples with low Ge concentration (25 and 42 mol%) and increased for samples with 60 mol% Ge concentration (Fig. 1). At the same time the peak position of IR PL band shifts to lower energies (Fig. 1b). Raman scattering studies have shown that Ge–SiO2 films in as-prepared state are amorphous with the broad feature characteristic of Ge–Ge vibrations. This is shown in Fig. 3 for a Ge–SiO2 sample with 60 mol% Ge. Upon annealing, a crystalline peak centered at ≈290 cm−1 appears in the samples with 60 mol% Ge, see Fig. 3, and in some samples with 42 mol% Ge concentration (not shown here). Raman scattering in samples with lower Ge concentration (25 mol%) did not detect Ge–Ge bond vibrations. Fourier transformed EXAFS spectra have been used to characterize the local structure of Ge atoms and are shown in Fig. 4. The two peaks corresponding, apparently, to Ge–O and Ge–Ge bonds were detected in the samples with 60 mol% Ge concentration. Both these peaks are complex: the Ge–O (1.7 Å) peak includes, probably, the Si–O bond (1.58 Å) and the Ge–Ge (2.44 Å) peak, apparently, includes also the Si–Si bond (2.33 Å). The Ge–O bonds were detected for all the investigated samples, while the Ge–Ge bonds have been detected only for the 60 mol% Ge concentration. Upon annealing, the intensities of these two peaks increase
(a)
1 8
5
3.0
4
2.5 2
1
2.0
3 1.5 1.0 0.5 0.0 1.4
1.6
1.8
2.0
2.2
2.4
Emission energy (eV)
Fig. 2. PL spectrum decomposition into five elementary PL bands for Ge–SiO2 sample with Ge equal to 42 mol% in the state after thermal annealing.
Table 1 Dependence of PL band intensities (II ) on Ge molar concentrations and thermal treatment N
Treatment
Ge (mol%)
I1 (arbitrary units)
I2 (arbitrary units)
I3 (arbitrary units)
I4 (arbitrary units)
I5 (arbitrary units)
1 2 3 4 5 6
As deposed As deposed As deposed 800 ◦ C/1 h 800 ◦ C/1 h 800 ◦ C/1 h
25 42 60 25 42 60
2.88 2.01 1.51 3.21 1.86 1.83
3.21 1.72 0.81 2.80 1.94 0.92
4.83 2.08 1.07 5.10 1.55 1.02
7.69 2.97 1.49 8.06 2.73 1.36
6.22 2.33 1.10 5.20 3.01 2.73
T.V. Torchynska et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 619–622
Fig. 3. Raman scattering spectra of the Ge–SiO2 system for concentrations of Ge equal to 60 mol% before (1) and after (2) thermal annealing.
Fig. 4. Fourier transformed EXAFS spectra for as-deposited and annealed samples with 60 mol% Ge.
and, in addition, two new peaks at 3.85 and 4.60 Å appear. These new peaks can be attributed to second and third shell neighbors (such as Ge–Ge–Ge or Ge–Ge–Ge–Ge), and are evidence for the formation of Ge nano-crystallite clusters.
621
intensities of these PL bands decrease with the Ge concentration increasing from 25 to 60 mol% and do not depend on thermal treatment (Fig. 1, Table 1). The maximum intensity of these PL bands is observed for samples with 25 mol% Ge concentration. In these samples no Ge-nc were found either by Raman scattering or by EXAFS investigations. So, we can conclude, that these PL bands in Ge–SiO2 films are connected with silicon-oxide related defects. In support of our above conclusion we wish to point out that PL bands at the same position as reported here, have been observed by us in SiOx films enriched by Si [14,18], by Song et al. in Si-ion-implanted SiO2 films [19] and by several workers in porous silicon [20–23]. The 2.00–2.06 eV PL band was observed also in silica optical fibers [15,16] and in ion-implanted SiO2 films [17]. It was shown earlier that the intensity of the 2.0–2.2 eV PL band strongly correlates with the E center concentration in silicon oxide [24]. This PL band is annihilated when the E center concentration decreases during thermal treatment. Note that 1.75–1.80 eV and 2.00–2.06 eV PL bands in porous silicon were attributed to non-bridging oxygen hole centers (NBOHC) in silicon oxide [20,21]. The nature of the IR PL band peaked at 1.43–1.52 eV in the investigated Ge–SiOx system is not clear. The intensity of this PL band either decreases in as-prepared Ge–SiOx samples or increases in thermally annealed samples with increasing Ge concentration (Fig. 1). It is possible that this PL band is connected with the Ge QDs (nano-crystallites) and is the result of carrier recombination between quantum-confinement states. Indeed, with decreasing Ge concentration in the SiO2 films, when the size of the Ge-nc should decrease, this PL band peak shifts to higher energies (Fig. 1). But in this case it is not clear why we have seen this PL band also in as-prepared samples (Fig. 1a), where Ge-nc have not been detected. A more detailed investigation of the PL band peaked at 1.43–1.52 eV is necessary for final conclusion concerning its nature.
4. Discussion Acknowledgements From Raman scattering and EXAFS investigations it is possible to conclude that thermal treatment of Ge–SiO2 films in inert atmosphere at 800 ◦ C for 1 h creates Ge-nc in films with Ge concentration equal to 60 mol% and in some samples with Ge concentration equal to 42%. For films with lower Ge concentration (25 mol%) the situation with Ge-nc is not clear. We could not detect any Ge–Ge bonds by the EXAFS method, but maybe this is due to the limit of the experimental accuracy (5%). Thus, it is possible to investigate the influence of Ge-nc content on the PL spectra of the Ge–SiO2 films with 60 mol% Ge and for some samples with 42 mol% Ge concentrations. Our results show that the visible PL bands peaked at 1.61–1.65 eV (2), 1.75–1.80 eV (3), 2.00–2.06 eV (4) and 2.25–2.30 eV (5) in Ge–SiO2 films do not correlate with the presence of Ge atoms or Ge-nc in the samples. Actually, the
This work was supported by CONACYT (project 33427-U and International Cooperation Project, Mexico-Israel), as well as by CGPI-IPN Mexico. References [1] [2] [3] [4]
T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15578. S. Okada, Y. Kanemitsu, Phys. Rev. B 54 (1996) 16421. Y. Maeda, Phys. Rev. B 51 (1995) 1658. W. Wu, X.F. Huang, K.J. Chen, J.B. Xu, X. Gao, J. Xu, W. Li, J. Vac. Sci. Technol. A 17 (1) (1999) 159. [5] J. Linnros, N. Lalic, A. Galeckas, V. Grivickas, J. Appl. Phys. 86 (1999) 6128. [6] S. Charvet, R. Madelon, R. Rizk, B. Garrido, O. Gonzalez-Varona, M. Lopez, A. Perez-Rodriguez, J.R. Morante, J. Lumines. 80 (1999) 241.
622
T.V. Torchynska et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 619–622
[7] N.E. Korsunskaya, T.V. Torchynskaya, B.R. Dzhumaev, L.Yu. Khomenkova, Semiconductors 31 (8) (1997) 773. [8] G.G. Qin, A.P. Li, Y.X. Zhang, Phys. Rev. B 54 (1996) 11122. [9] S.M. Prokes, W.E. Carlos, S. Veprek, Ch. Ossadnik, Phys. Rev. B 58 (1998) 15632. [10] L. Tsybeskov, G.F. Grom, P.M. Fauchet, J.P. McCaffrey, J.M. Baribeau, G.I. Sproule, D.J. Lockwood, Appl. Phys. Lett. 75 (1999) 2265. [11] A.D. Lan, B.X. Liu, X.D. Bai, J. Appl. Phys. 82 (10) (1997) 5144. [12] Y. Kanemitsu, T. Ogawa, K. Shiraishi, K. Takeda, Phys. Rev. B 48 (1993) 4883. [13] A.V. Kolobov, Y. Maeda, K. Tanaka, J. Appl. Phys. 88 (2000) 3285. [14] L. Khomenkova, N. Korsunska, V. Yukhimchuk, B. Jumaev, T. Torchynska, A. Vivas Hernandez, A. Many, Y. Goldstein, E. Savir, J. Jedrzejewski, J. Lumin., 102–103 (2003) 705–711. [15] L.N. Skuja, A.R. Silin, Phys. Stat. Solid A 56 (1979) K11. [16] K. Nagasawa, Y. Ohki, Y. Hama, Jpn. J. Appl. Phys. 26 (1987) L1009.
[17] V.Ya. Bratus, V.A. Yukhinchuk, L.I. Berezhinsky, M.Ya. Valakh, I.P. Vorona, I.Z. Indutnyy, T.T. Petrenko, P.E. Shepeliavyi, I.B. Yanchuk, Semiconductors 35 (2001) 763. [18] T. Torchynska, J. Aguilar-Hernandez, A.I. Diaz Cano, G. ContrerasPuente, F.G. Becerril Espinoza, Yu.V. Vorobiev, Y. Goldstein, A. Many, J. Jedrzejewski, B.M. Bulakh, L.V. Scherbina, Physica B 308–310 (2001) 1108. [19] H.Z. Song, X.M. Bao, Phys. Rev. B. 55 (1997) 6988. [20] S.M. Prokes, W.E. Carlos, J. Appl. Phys. 78 (1995) 2671. [21] T.V. Torchynska, N.E. Korsunska, L.Yu. Khomekova, S.M. Prokes, Microelectron. Eng. 51–52 (2000) 485. [22] V.M. Dubin, J.-N. Chazalviel, F. Ozaman, J. Lumin. 57 (1993) 61. [23] T.V. Torchynska, M. Morales Rodriguez, F.G. Becerril-Espinoza, N.E. Korsunskaya, L.Yu. Khomenkova, L.V. Scherbina, Phys. Rev. B 65 (2002) 115313. [24] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779.
Visible photoluminescence of Ge enriched SiOx layers T.V. Torchynska a,∗ , A.Vivas Hernandez a , A.V. Kolobov b , Y. Goldstein c , E. Savir c , J. Jedrzejewski c a
b
ESFM—National Polytechnic Institute, Mexico D.F. 07738, Mexico LAOTECH—National Institute for Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562, Japan c Racah Institute of Physics, Hebrew University, 91904 Jerusalem, Israel Available online 19 March 2004
Abstract This paper concentrates on results of photoluminescence (PL), Raman scattering and extended X-ray absorption fine structure (EXAFS) investigations of silicon oxide films enriched by Ge. All the spectra were analyzed both for “as prepared” silicon oxide films enriched by Ge (without Ge-quantum dots, QD) and for films after high temperature annealing in inert atmosphere with the aim of creating Ge-nano-crystallites (nc)-quantum dots. Five PL bands were discovered in Ge–SiOx systems. It is shown that the visible PL bands peaked at 1.61–1.65 eV (2), 1.75–1.80 eV (3), 2.00–2.06 eV (4) and 2.25–2.30 eV (5) do not correlate with the presence of Ge atoms or Ge-nc in the samples. The nature of the infrared PL band peaked at 1.43–1.52 eV is not clear. The intensity of this PL band increases in thermally annealed samples with increasing Ge concentration. © 2004 Elsevier B.V. All rights reserved. Keywords: Visible photoluminescence; Ge–SiOx layers; Ge-quantum dots
1. Introduction Semiconductor low-dimensional structures such as quantum wires and dots based on Ge and Si nano-crystallites are attracting great attention because of the expectation that the radiation efficiency of indirect optical transitions may be significantly increased if the size of the semiconductor crystallites is in the nanometer-scale range [1,2]. The photoluminescence (PL) from Ge and Si nano-crystallites (nc) embedded in SiO2 in the spectral range of 1.5–2.4 eV was observed in a number of scientific laboratories [2–12]. The mechanism of this photoluminescence has been discussed extensively but it is not clear up to now. These optical transitions are attributed to: radiation recombination inside Si (Ge) nano-crystallites via quantum-confined states [3–5], defect-related emission in silicon oxide [6,7,10,11] or at the Si/SiOx interface [5,8,10,11], as well as to exciton recombination at the Si/SiOx interface [12]. Thus, the origin of visible PL of Si or Ge-nc embedded in silicon oxide still remains to be clarified.
∗ Corresponding author. Tel.: +52-572-96000/55031; Tel.: +52-572-96000/55003. E-mail address: [email protected] (T.V. Torchynska).
0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.02.039
This paper concentrates on results of photoluminescence, Raman scattering and extended X-ray absorption fine structure (EXAFS) investigations of silicon oxide films enriched by Ge. All the spectra were analyzed both for “as prepared” silicon oxide films enriched by Ge (without Ge-quantum dots) and for films after high temperature annealing in inert atmosphere with the aim of creating Ge-nc-quantum dots (QD).
2. Experimental techniques Silicon oxide films enriched by Ge were prepared by co-deposition of Ge and silicon oxide onto quartz substrates by radio frequency magnetron sputtering. The Ge concentration varied from 25 to 60 mol%. After deposition, some of the samples were isothermally annealed for 1 h at 800 ◦ C in an argon atmosphere in order to produce nano-crystals with a characteristic size of 6–10 nm [3]. PL and Raman spectra have been excited in our samples with an Ar laser tuned to a wavelength of 4880 Å. The laser power was 80 mW. Raman signals were measured with a model 1403 spectrometer in a backscattering geometry at room temperature. The PL measurements were carried out as well at room temperature using a Control Development
620
T.V. Torchynska et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 619–622
Inc. Spectrometer with CCD detectors. The EXAFS measurements were performed at beam line 13B at the Photon Factory [13].
PL intensity [arb. units]
10
3. Experimental results
6 4
2
2
3
PL intensity [arb. units]
0 1
8
(b)
6 4
2
2
3
0 1.4
1.6
1.8
2.0
2.2
2.4
Emission energy (eV) Fig. 1. PL spectra of the Ge–SiO2 system in as-prepared state (a) and after thermal annealing (b) for concentrations of Ge, 25 mol% (1), 42 mol% (2) and 60 mol% (3).
3.5 PL intensity [arb. units]
Fig. 1 presents typical PL spectra of silicon oxide films enriched by different molar concentrations of Ge in as-prepared state (a) and after thermal annealing (b). In both cases, very wide PL spectra were observed. Each of these PL spectra can be decomposed into at least five elementary PL bands peaked at: 1.43–1.52 eV (band 1), 1.61–1.65 eV (2), 1.75–1.90 eV (3), 2.00–2.06 eV (4) and 2.25–2.35 eV (5) with the full width at half maximum (FWHM) equal to 100, 150, 200, 225 and 280 meV, respectively, as shown in Fig. 2. With increasing Ge molar concentration in the silicon oxide the intensities of all PL bands generally decrease, see Fig. 1 and Table 1. The intensities of 2–4 PL bands remain unchanged with thermal annealing, in all samples (Fig. 1, Table 1). The intensities of 1 and 5 PL bands decreased (or do not changed) for samples with low Ge concentration (25 and 42 mol%) and increased for samples with 60 mol% Ge concentration (Fig. 1). At the same time the peak position of IR PL band shifts to lower energies (Fig. 1b). Raman scattering studies have shown that Ge–SiO2 films in as-prepared state are amorphous with the broad feature characteristic of Ge–Ge vibrations. This is shown in Fig. 3 for a Ge–SiO2 sample with 60 mol% Ge. Upon annealing, a crystalline peak centered at ≈290 cm−1 appears in the samples with 60 mol% Ge, see Fig. 3, and in some samples with 42 mol% Ge concentration (not shown here). Raman scattering in samples with lower Ge concentration (25 mol%) did not detect Ge–Ge bond vibrations. Fourier transformed EXAFS spectra have been used to characterize the local structure of Ge atoms and are shown in Fig. 4. The two peaks corresponding, apparently, to Ge–O and Ge–Ge bonds were detected in the samples with 60 mol% Ge concentration. Both these peaks are complex: the Ge–O (1.7 Å) peak includes, probably, the Si–O bond (1.58 Å) and the Ge–Ge (2.44 Å) peak, apparently, includes also the Si–Si bond (2.33 Å). The Ge–O bonds were detected for all the investigated samples, while the Ge–Ge bonds have been detected only for the 60 mol% Ge concentration. Upon annealing, the intensities of these two peaks increase
(a)
1 8
5
3.0
4
2.5 2
1
2.0
3 1.5 1.0 0.5 0.0 1.4
1.6
1.8
2.0
2.2
2.4
Emission energy (eV)
Fig. 2. PL spectrum decomposition into five elementary PL bands for Ge–SiO2 sample with Ge equal to 42 mol% in the state after thermal annealing.
Table 1 Dependence of PL band intensities (II ) on Ge molar concentrations and thermal treatment N
Treatment
Ge (mol%)
I1 (arbitrary units)
I2 (arbitrary units)
I3 (arbitrary units)
I4 (arbitrary units)
I5 (arbitrary units)
1 2 3 4 5 6
As deposed As deposed As deposed 800 ◦ C/1 h 800 ◦ C/1 h 800 ◦ C/1 h
25 42 60 25 42 60
2.88 2.01 1.51 3.21 1.86 1.83
3.21 1.72 0.81 2.80 1.94 0.92
4.83 2.08 1.07 5.10 1.55 1.02
7.69 2.97 1.49 8.06 2.73 1.36
6.22 2.33 1.10 5.20 3.01 2.73
T.V. Torchynska et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 619–622
Fig. 3. Raman scattering spectra of the Ge–SiO2 system for concentrations of Ge equal to 60 mol% before (1) and after (2) thermal annealing.
Fig. 4. Fourier transformed EXAFS spectra for as-deposited and annealed samples with 60 mol% Ge.
and, in addition, two new peaks at 3.85 and 4.60 Å appear. These new peaks can be attributed to second and third shell neighbors (such as Ge–Ge–Ge or Ge–Ge–Ge–Ge), and are evidence for the formation of Ge nano-crystallite clusters.
621
intensities of these PL bands decrease with the Ge concentration increasing from 25 to 60 mol% and do not depend on thermal treatment (Fig. 1, Table 1). The maximum intensity of these PL bands is observed for samples with 25 mol% Ge concentration. In these samples no Ge-nc were found either by Raman scattering or by EXAFS investigations. So, we can conclude, that these PL bands in Ge–SiO2 films are connected with silicon-oxide related defects. In support of our above conclusion we wish to point out that PL bands at the same position as reported here, have been observed by us in SiOx films enriched by Si [14,18], by Song et al. in Si-ion-implanted SiO2 films [19] and by several workers in porous silicon [20–23]. The 2.00–2.06 eV PL band was observed also in silica optical fibers [15,16] and in ion-implanted SiO2 films [17]. It was shown earlier that the intensity of the 2.0–2.2 eV PL band strongly correlates with the E center concentration in silicon oxide [24]. This PL band is annihilated when the E center concentration decreases during thermal treatment. Note that 1.75–1.80 eV and 2.00–2.06 eV PL bands in porous silicon were attributed to non-bridging oxygen hole centers (NBOHC) in silicon oxide [20,21]. The nature of the IR PL band peaked at 1.43–1.52 eV in the investigated Ge–SiOx system is not clear. The intensity of this PL band either decreases in as-prepared Ge–SiOx samples or increases in thermally annealed samples with increasing Ge concentration (Fig. 1). It is possible that this PL band is connected with the Ge QDs (nano-crystallites) and is the result of carrier recombination between quantum-confinement states. Indeed, with decreasing Ge concentration in the SiO2 films, when the size of the Ge-nc should decrease, this PL band peak shifts to higher energies (Fig. 1). But in this case it is not clear why we have seen this PL band also in as-prepared samples (Fig. 1a), where Ge-nc have not been detected. A more detailed investigation of the PL band peaked at 1.43–1.52 eV is necessary for final conclusion concerning its nature.
4. Discussion Acknowledgements From Raman scattering and EXAFS investigations it is possible to conclude that thermal treatment of Ge–SiO2 films in inert atmosphere at 800 ◦ C for 1 h creates Ge-nc in films with Ge concentration equal to 60 mol% and in some samples with Ge concentration equal to 42%. For films with lower Ge concentration (25 mol%) the situation with Ge-nc is not clear. We could not detect any Ge–Ge bonds by the EXAFS method, but maybe this is due to the limit of the experimental accuracy (5%). Thus, it is possible to investigate the influence of Ge-nc content on the PL spectra of the Ge–SiO2 films with 60 mol% Ge and for some samples with 42 mol% Ge concentrations. Our results show that the visible PL bands peaked at 1.61–1.65 eV (2), 1.75–1.80 eV (3), 2.00–2.06 eV (4) and 2.25–2.30 eV (5) in Ge–SiO2 films do not correlate with the presence of Ge atoms or Ge-nc in the samples. Actually, the
This work was supported by CONACYT (project 33427-U and International Cooperation Project, Mexico-Israel), as well as by CGPI-IPN Mexico. References [1] [2] [3] [4]
T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15578. S. Okada, Y. Kanemitsu, Phys. Rev. B 54 (1996) 16421. Y. Maeda, Phys. Rev. B 51 (1995) 1658. W. Wu, X.F. Huang, K.J. Chen, J.B. Xu, X. Gao, J. Xu, W. Li, J. Vac. Sci. Technol. A 17 (1) (1999) 159. [5] J. Linnros, N. Lalic, A. Galeckas, V. Grivickas, J. Appl. Phys. 86 (1999) 6128. [6] S. Charvet, R. Madelon, R. Rizk, B. Garrido, O. Gonzalez-Varona, M. Lopez, A. Perez-Rodriguez, J.R. Morante, J. Lumines. 80 (1999) 241.
622
T.V. Torchynska et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 619–622
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