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The origin of photoluminescence in Ge-implanted SiO2 layers
Journal of Luminescence 80 (1999) 281—284
The origin of photoluminescence in Ge-implanted SiO layers H.B. Kim , K.H. Chae , C.N. Whang *, J.Y. Jeong, M.S. Oh, S. Im, J.H. Song Department of Physics, Atomic-scale Surface Science Research Center, Yonsei University, Seoul 120-749, South Korea Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, South Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 130-650, South Korea
Abstract Ge ions were implanted at 100 keV with 3;10 cm\ into a 300 nm thick SiO layer on Si. Visible photoluminescence (PL) around 2.1 eV from an as-implanted sample is observed, and faded out by subsequent annealing at 900°C for 2 h. However, PL shows up again after annealing above 900°C at the same peak position. Compared with the as-implanted sample, significant increase of Ge—Ge bonds is measured in X-ray photoelectron spectroscopy, and the formation of Ge nanocrystals with a diameter of 5 nm are observed in transmission electron microscopy from the sample annealed at 1100°C. We conclude that the PL peak from the sample annealed above 900°C is caused by the quantum confinement effects from Ge nanocrystals, while the luminescence from the as-implanted sample is due to some radiative defects formed by Ge implantation. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 78.55.Ap; 78.66.Db Keywords: Ge; SiO; Implantation; Quantum confinement; Radiative defect
1. Introduction Semiconductor nanocrystals (Si, Ge) obtained by various techniques [1—3] emit luminescence that usually does not appear in the bulk materials. Light emission from semiconductor nanocrystals embedded in SiO is becoming an expanding field of interest because of their potential as optoelectronic emission devices directly coupled with Si integrated circuits. For a fabrication technique of nanocrystals, ion implantation is a good candidate in that a given number of ions can be placed in a control-
* Corresponding author. Tel.: #82 2 361 2613; fax: #82 2 312 7090; e-mail: [email protected].
led depth by changing ion fluences and acceleration energies, and it is extensively used in semiconductor technology [4]. Many researchers [3,5—7] explained the origin of photoluminescence from the Si-implanted SiO layers with two mechanisms. One is the defectrelated mechanism that appears mainly from asimplanted samples. The other is the quantum confinement effect arisen from nanocrystals formed by subsequent annealing at a high temperature. Few studies of photoluminescence from the Geimplanted SiO layer have been reported and the origin of PL is still in debate. Min et al. [8] reported that the visible PL is primarily due to a luminescent defect center in Ge-implanted SiO layers.
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 1 1 2 - 4
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In this paper, we present the PL results from the Ge implanted SiO layer and discuss luminescence mechanisms in terms of defect-related luminescence center and quantum confinement model.
2. Experiments SiO layers with a thickness of 300 nm were thermally grown by wet oxidation of Si(1 0 0). Ge ions were implanted into SiO layer at room tem perature (RT) with an energy of 100 keV. The employed dose of Ge ions was 3;10 ions/cm. After implantation, the samples were annealed in nitrogen ambient for 2 h at various temperatures. Photoluminescence spectra were taken at RT in a conventional way. An Ar-ion laser (457.9 nm) was used as an excitation source and the luminescence was detected by a cooled photomultiplier tube employing the photon-counting technique with a cutoff filter to pass only long waves above 530 nm in front of the detector. In order to confirm the formation of the Ge nanocrystals, XPS measurements were performed using a standard Al K (1486.7 eV) excitation source in an electron a spectrometer ESCA 5700 (PHI Ldt.) at a residual gas pressure of &2;10\ torr. The photoelectrons were detected by a hemispherical analyzer with a pass energy of 23.5 eV. All XPS spectra were corrected by fixing Si—O binding energy at 103.3 eV. The samples for cross-section TEM were prepared in a standard way of mechanical polishing and ion milling step. High-resolution transmission electron microscopy (HRTEM) was performed at 200 keV to observe the Ge nanocrystalline precipitates in the SiO layer by using a JEOL 2010 system.
3. Results and discussion Fig. 1 shows the PL spectra of an as-implanted sample and samples annealed for 2 h at 900°C, 1000°C, and 1100°C. A PL peak around 2.1 eV was observed from the as-implanted sample. After annealing at 900°C in nitrogen ambient for 2 h, this peak disappears. This means that the luminescence from the as-implanted sample is related to some
Fig. 1. PL spectra of the as-implanted sample (——) and postannealed samples at 900°C (- - -), 1000°C (2), and 1100°C (— z —) for 2 h.
radiative defects formed by Ge implantation because implantation induced defects are annealed out by subsequent high temperature annealing. Similar luminescence around 2.1 eV from asimplanted sample was reported for Si-implanted samples [5,9—11]. According to the result of Shimizu-Iwayama et al. [5] the luminescence band in the as-implanted sample is attributed to Si excess defects. The Ge 3d XPS spectra are shown in Fig. 2 for as-implanted and a sample annealed at 1100°C for 2 h. The spectra were taken near the projected range of 100 keV Ge ion implanted in SiO layer. As can be seen in Fig. 2a, the XPS spectrum taken from the as-implanted sample shows that Ge—O bonds, consisting of a mixture of GeO (32.6 eV) [12] and GeO (30.7 eV) [13], was dominant with only a small amount of Ge—Ge bonds (29.3 eV) [12]. Since metastable phases can be created by ion irradiation, Ge implantation produces not only the stable GeO phase but also the metastable GeO phase in an SiO layer. As a consequence, some of Si atoms contribute to the formation of Si excess
H.B. Kim et al. / Journal of Luminescence 80 (1999) 281—284
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Fig. 3. XPS depth profiles of Ge for (a) as-implanted and (b) post-annealed sample at 1100°C for 2 h. These spectra were corrected by fixing Si—O binding energy at 103.3 eV. The relative concentrations were obtained from the curve fitting of XPS spectra. Total Ge indicates the sum of Ge—Ge bonds, GeO, and GeO .
Fig. 2. Ge 3d XPS spectra for (a) as-implanted and (b) sample annealed at 1100°C for 2 h. The spectra were measured near the projected range of 100 keV Ge in SiO layer. Inset in (b) is HRTEM image of sample annealed at 1100°C for 2 h. The diameter of Ge nanocrystals are found to be about 5 nm.
defects because of the coalescence between the implanted Ge ions and the oxygen in SiO . Therefore, we suggest that Si excess defects play a role of a main luminescence center in Ge-implanted samples as they do in Si-implanted SiO . After an nealing at temperatures higher than 900°C, the luminescence with the same peak position shows up again, and its intensity increases with annealing temperature as shown in Fig. 1. Also, the XPS spectrum taken from a sample annealed at 1100°C in Fig. 2b shows remarkable increase of Ge—Ge bonds with a decrease of Ge—O bonds when compared with the XPS spectrum of the as-implanted sample.
In order to trace the recurrence of luminescence, the XPS depth profiles of Ge for the as-implanted and a sample annealed at 1100°C for 2 h were analyzed, as shown in Fig. 3a and 3b. The relative concentrations were obtained from the curve fitting of XPS spectra as in Fig. 2. In case of the asimplanted sample, the shape of the Ge depth profile is almost Gaussian. In addition, Ge—O bonds (GeO and GeO phase) are dominantly distributed around the projected range of 100 keV Ge as the sputtering time is converted into depth. However, the conspicuous increase of Ge—Ge bonds relative to Ge—O bonds is observed from the XPS depth profile of the annealed sample in Fig. 3b, and the distribution of GeO phase is similar to that in the as-implanted sample except for a little decrease of amount. It is known that germanium oxide is thermodynamically less stable than SiO [14]. From the fact, we can see that the unstable phase of germanium oxides (GeO ) changes into germanium V and excess Si in as-implanted sample reacts with
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the oxygen atom in GeO . Therefore, the following V reaction is expected to occur during high temperature annealing in nitrogen ambient: SiO #GeO PSiO #Ge. V V This result supports that the implanted Ge ions form nanocrystals after annealing at temperatures above 900°C. In order to confirm the presence of nanocrystals, cross-sectional high-resolution transmission electron microscopy was performed for an annealed sample at 1100°C for 2 h. Ge nanocrystals of about 5 nm were observed as shown in the inset in Fig. 2b. The peak position, 2.1 eV, agrees well with a theoretical calculation based on the quantum confinement model [12,15]. The quantum confinement effect is obvious when the nanocrystals size is smaller than the exciton effective Bohr radius (about 24 nm in case of Ge nanocrystals) [12]. Since Ge has smaller electron and hole effective masses and a larger dielectric constant than Si, the effective Bohr radius of the excitons in Ge is larger than that in Si. This includes the fact that Ge nanocrystals show a larger shift of an optical band gap than Si nanocrystals [12]. It was reported that a PL peak centered at 1.7 eV is observed from Si nanocrystals formed in an Si-implanted SiO layer. We suggest that the PL peak around 2.1 eV from the sample annealed at temperature above 900°C originates from quantum confinement effects of the Ge nanocrystals. Hence, the PL from the annealed sample at temperatures above 900°C should be regarded as luminescence emitted from the Ge nanocrystals.
4. Conclusions The PL peak around 2.1 eV for the as-implanted sample is attributed to radiative defects generated by Ge ion implantation. Those defects are annealed out by the annealing in nitrogen ambient at 900°C for 2 h. Ge agglomerates around the projected range observed by the XPS measurements, and Ge
nanocrystals are formed upon high-temperature annealing, producing the same luminescence as the radiative defects emit. We, thus, conclude that there exist two kinds of luminescent origins in Geimplanted SiO , radiative defects and Ge nano crystals.
Acknowledgements This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) through the ASSRC at Yonsei University, basic research fund of KIST (2E15540), and the grants from KOSEF (981-0209-035-2).
References [1] K. Kohno, Y. Osaka, F. Toyomura, H. Katayama, Jpn. J. Appl. Phys. Lett. 33 (1994) 6616. [2] H. Tagaki, H. Owada, Y. Yamazaki, A. Ishizaki, T. Nakagiri, Appl. Phys. Lett. 56 (1990) 2379. [3] S. Guha, M.D. Pace, D.N. Dunn, I.L. Singer, Appl. Phys. Lett. 70 (1997) 1207. [4] E. Rimini, Ion Implantation: Basic to Device Fabrication, Kluwer Academic Publishers, Dordrecht, 1995, p. 19. [5] T. Shimizu-Iwayama, S. Nakao, K. Saitoh, Nucl. Instr. Meth. B 121 (1997) 450. [6] A.D. Lan, B.X. Liu, X.D. Bai, J. Appl. Phys. 82 (1997) 5144. [7] G.A. Kachurin, K.S. Zhuravlev, N.A. Pazdnikov, A.F. Leier, I.E. Tyschenko, V.A. Volodin, W. Skorupa, R.A. Yankov, Nucl. Instr. Meth. B 127/128 (1997) 583. [8] K.S. Min, K.V. Shcheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 68 (1996) 2511. [9] L. Liao, X. Bao, N. Li, X. Zheng, N. Min, J. Lumin. 68 (1996) 199. [10] A.D. Lan, B.X. Liu, X.D. Bai, J. Appl. Phys. 82 (1997) 5144. [11] J.Y. Jeong, S. Im, M.S. Oh, H.B. Kim, K.H. Chae, C.N. Whang, J.H. Song, European Mater. Res. Soc. Spring Meeting, 1998. [12] Y. Maeda, N. Tsukamoto, Y. Yazawa, Y. Kanemitsu, Y. Masumoto, Appl. Phys. Lett. 59 (1991) 3168. [13] K. Prabhakaran, T. Ogino, Surf. Sci. 325 (1995) 263. [14] F.K. LeGoues, R. Rosenberg, T. Nguyen, F. Himpsel, B.S. Meyerson, J. Appl. Phys. 65 (1989) 1724. [15] T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15 578.
The origin of photoluminescence in Ge-implanted SiO layers H.B. Kim , K.H. Chae , C.N. Whang *, J.Y. Jeong, M.S. Oh, S. Im, J.H. Song Department of Physics, Atomic-scale Surface Science Research Center, Yonsei University, Seoul 120-749, South Korea Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, South Korea Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 130-650, South Korea
Abstract Ge ions were implanted at 100 keV with 3;10 cm\ into a 300 nm thick SiO layer on Si. Visible photoluminescence (PL) around 2.1 eV from an as-implanted sample is observed, and faded out by subsequent annealing at 900°C for 2 h. However, PL shows up again after annealing above 900°C at the same peak position. Compared with the as-implanted sample, significant increase of Ge—Ge bonds is measured in X-ray photoelectron spectroscopy, and the formation of Ge nanocrystals with a diameter of 5 nm are observed in transmission electron microscopy from the sample annealed at 1100°C. We conclude that the PL peak from the sample annealed above 900°C is caused by the quantum confinement effects from Ge nanocrystals, while the luminescence from the as-implanted sample is due to some radiative defects formed by Ge implantation. 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80.Jh; 78.55.Ap; 78.66.Db Keywords: Ge; SiO; Implantation; Quantum confinement; Radiative defect
1. Introduction Semiconductor nanocrystals (Si, Ge) obtained by various techniques [1—3] emit luminescence that usually does not appear in the bulk materials. Light emission from semiconductor nanocrystals embedded in SiO is becoming an expanding field of interest because of their potential as optoelectronic emission devices directly coupled with Si integrated circuits. For a fabrication technique of nanocrystals, ion implantation is a good candidate in that a given number of ions can be placed in a control-
* Corresponding author. Tel.: #82 2 361 2613; fax: #82 2 312 7090; e-mail: [email protected].
led depth by changing ion fluences and acceleration energies, and it is extensively used in semiconductor technology [4]. Many researchers [3,5—7] explained the origin of photoluminescence from the Si-implanted SiO layers with two mechanisms. One is the defectrelated mechanism that appears mainly from asimplanted samples. The other is the quantum confinement effect arisen from nanocrystals formed by subsequent annealing at a high temperature. Few studies of photoluminescence from the Geimplanted SiO layer have been reported and the origin of PL is still in debate. Min et al. [8] reported that the visible PL is primarily due to a luminescent defect center in Ge-implanted SiO layers.
0022-2313/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 1 1 2 - 4
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H.B. Kim et al. / Journal of Luminescence 80 (1999) 281—284
In this paper, we present the PL results from the Ge implanted SiO layer and discuss luminescence mechanisms in terms of defect-related luminescence center and quantum confinement model.
2. Experiments SiO layers with a thickness of 300 nm were thermally grown by wet oxidation of Si(1 0 0). Ge ions were implanted into SiO layer at room tem perature (RT) with an energy of 100 keV. The employed dose of Ge ions was 3;10 ions/cm. After implantation, the samples were annealed in nitrogen ambient for 2 h at various temperatures. Photoluminescence spectra were taken at RT in a conventional way. An Ar-ion laser (457.9 nm) was used as an excitation source and the luminescence was detected by a cooled photomultiplier tube employing the photon-counting technique with a cutoff filter to pass only long waves above 530 nm in front of the detector. In order to confirm the formation of the Ge nanocrystals, XPS measurements were performed using a standard Al K (1486.7 eV) excitation source in an electron a spectrometer ESCA 5700 (PHI Ldt.) at a residual gas pressure of &2;10\ torr. The photoelectrons were detected by a hemispherical analyzer with a pass energy of 23.5 eV. All XPS spectra were corrected by fixing Si—O binding energy at 103.3 eV. The samples for cross-section TEM were prepared in a standard way of mechanical polishing and ion milling step. High-resolution transmission electron microscopy (HRTEM) was performed at 200 keV to observe the Ge nanocrystalline precipitates in the SiO layer by using a JEOL 2010 system.
3. Results and discussion Fig. 1 shows the PL spectra of an as-implanted sample and samples annealed for 2 h at 900°C, 1000°C, and 1100°C. A PL peak around 2.1 eV was observed from the as-implanted sample. After annealing at 900°C in nitrogen ambient for 2 h, this peak disappears. This means that the luminescence from the as-implanted sample is related to some
Fig. 1. PL spectra of the as-implanted sample (——) and postannealed samples at 900°C (- - -), 1000°C (2), and 1100°C (— z —) for 2 h.
radiative defects formed by Ge implantation because implantation induced defects are annealed out by subsequent high temperature annealing. Similar luminescence around 2.1 eV from asimplanted sample was reported for Si-implanted samples [5,9—11]. According to the result of Shimizu-Iwayama et al. [5] the luminescence band in the as-implanted sample is attributed to Si excess defects. The Ge 3d XPS spectra are shown in Fig. 2 for as-implanted and a sample annealed at 1100°C for 2 h. The spectra were taken near the projected range of 100 keV Ge ion implanted in SiO layer. As can be seen in Fig. 2a, the XPS spectrum taken from the as-implanted sample shows that Ge—O bonds, consisting of a mixture of GeO (32.6 eV) [12] and GeO (30.7 eV) [13], was dominant with only a small amount of Ge—Ge bonds (29.3 eV) [12]. Since metastable phases can be created by ion irradiation, Ge implantation produces not only the stable GeO phase but also the metastable GeO phase in an SiO layer. As a consequence, some of Si atoms contribute to the formation of Si excess
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Fig. 3. XPS depth profiles of Ge for (a) as-implanted and (b) post-annealed sample at 1100°C for 2 h. These spectra were corrected by fixing Si—O binding energy at 103.3 eV. The relative concentrations were obtained from the curve fitting of XPS spectra. Total Ge indicates the sum of Ge—Ge bonds, GeO, and GeO .
Fig. 2. Ge 3d XPS spectra for (a) as-implanted and (b) sample annealed at 1100°C for 2 h. The spectra were measured near the projected range of 100 keV Ge in SiO layer. Inset in (b) is HRTEM image of sample annealed at 1100°C for 2 h. The diameter of Ge nanocrystals are found to be about 5 nm.
defects because of the coalescence between the implanted Ge ions and the oxygen in SiO . Therefore, we suggest that Si excess defects play a role of a main luminescence center in Ge-implanted samples as they do in Si-implanted SiO . After an nealing at temperatures higher than 900°C, the luminescence with the same peak position shows up again, and its intensity increases with annealing temperature as shown in Fig. 1. Also, the XPS spectrum taken from a sample annealed at 1100°C in Fig. 2b shows remarkable increase of Ge—Ge bonds with a decrease of Ge—O bonds when compared with the XPS spectrum of the as-implanted sample.
In order to trace the recurrence of luminescence, the XPS depth profiles of Ge for the as-implanted and a sample annealed at 1100°C for 2 h were analyzed, as shown in Fig. 3a and 3b. The relative concentrations were obtained from the curve fitting of XPS spectra as in Fig. 2. In case of the asimplanted sample, the shape of the Ge depth profile is almost Gaussian. In addition, Ge—O bonds (GeO and GeO phase) are dominantly distributed around the projected range of 100 keV Ge as the sputtering time is converted into depth. However, the conspicuous increase of Ge—Ge bonds relative to Ge—O bonds is observed from the XPS depth profile of the annealed sample in Fig. 3b, and the distribution of GeO phase is similar to that in the as-implanted sample except for a little decrease of amount. It is known that germanium oxide is thermodynamically less stable than SiO [14]. From the fact, we can see that the unstable phase of germanium oxides (GeO ) changes into germanium V and excess Si in as-implanted sample reacts with
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the oxygen atom in GeO . Therefore, the following V reaction is expected to occur during high temperature annealing in nitrogen ambient: SiO #GeO PSiO #Ge. V V This result supports that the implanted Ge ions form nanocrystals after annealing at temperatures above 900°C. In order to confirm the presence of nanocrystals, cross-sectional high-resolution transmission electron microscopy was performed for an annealed sample at 1100°C for 2 h. Ge nanocrystals of about 5 nm were observed as shown in the inset in Fig. 2b. The peak position, 2.1 eV, agrees well with a theoretical calculation based on the quantum confinement model [12,15]. The quantum confinement effect is obvious when the nanocrystals size is smaller than the exciton effective Bohr radius (about 24 nm in case of Ge nanocrystals) [12]. Since Ge has smaller electron and hole effective masses and a larger dielectric constant than Si, the effective Bohr radius of the excitons in Ge is larger than that in Si. This includes the fact that Ge nanocrystals show a larger shift of an optical band gap than Si nanocrystals [12]. It was reported that a PL peak centered at 1.7 eV is observed from Si nanocrystals formed in an Si-implanted SiO layer. We suggest that the PL peak around 2.1 eV from the sample annealed at temperature above 900°C originates from quantum confinement effects of the Ge nanocrystals. Hence, the PL from the annealed sample at temperatures above 900°C should be regarded as luminescence emitted from the Ge nanocrystals.
4. Conclusions The PL peak around 2.1 eV for the as-implanted sample is attributed to radiative defects generated by Ge ion implantation. Those defects are annealed out by the annealing in nitrogen ambient at 900°C for 2 h. Ge agglomerates around the projected range observed by the XPS measurements, and Ge
nanocrystals are formed upon high-temperature annealing, producing the same luminescence as the radiative defects emit. We, thus, conclude that there exist two kinds of luminescent origins in Geimplanted SiO , radiative defects and Ge nano crystals.
Acknowledgements This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) through the ASSRC at Yonsei University, basic research fund of KIST (2E15540), and the grants from KOSEF (981-0209-035-2).
References [1] K. Kohno, Y. Osaka, F. Toyomura, H. Katayama, Jpn. J. Appl. Phys. Lett. 33 (1994) 6616. [2] H. Tagaki, H. Owada, Y. Yamazaki, A. Ishizaki, T. Nakagiri, Appl. Phys. Lett. 56 (1990) 2379. [3] S. Guha, M.D. Pace, D.N. Dunn, I.L. Singer, Appl. Phys. Lett. 70 (1997) 1207. [4] E. Rimini, Ion Implantation: Basic to Device Fabrication, Kluwer Academic Publishers, Dordrecht, 1995, p. 19. [5] T. Shimizu-Iwayama, S. Nakao, K. Saitoh, Nucl. Instr. Meth. B 121 (1997) 450. [6] A.D. Lan, B.X. Liu, X.D. Bai, J. Appl. Phys. 82 (1997) 5144. [7] G.A. Kachurin, K.S. Zhuravlev, N.A. Pazdnikov, A.F. Leier, I.E. Tyschenko, V.A. Volodin, W. Skorupa, R.A. Yankov, Nucl. Instr. Meth. B 127/128 (1997) 583. [8] K.S. Min, K.V. Shcheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 68 (1996) 2511. [9] L. Liao, X. Bao, N. Li, X. Zheng, N. Min, J. Lumin. 68 (1996) 199. [10] A.D. Lan, B.X. Liu, X.D. Bai, J. Appl. Phys. 82 (1997) 5144. [11] J.Y. Jeong, S. Im, M.S. Oh, H.B. Kim, K.H. Chae, C.N. Whang, J.H. Song, European Mater. Res. Soc. Spring Meeting, 1998. [12] Y. Maeda, N. Tsukamoto, Y. Yazawa, Y. Kanemitsu, Y. Masumoto, Appl. Phys. Lett. 59 (1991) 3168. [13] K. Prabhakaran, T. Ogino, Surf. Sci. 325 (1995) 263. [14] F.K. LeGoues, R. Rosenberg, T. Nguyen, F. Himpsel, B.S. Meyerson, J. Appl. Phys. 65 (1989) 1724. [15] T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15 578.