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SiO2 films
Applied Surface Science 233 (2004) 288–293
The defect-related photoluminescence from Si ion-beam-mixed SiO2/Si/SiO2 films J.H. Sona, H.B. Kima, C.N. Whanga, K.H. Chaeb,* a
Atomic-scale Surface Science Research Center and Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, South Korea b Materials Science and Technology Division, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Sungbuk-gu, Seoul 136-791, South Korea Received in revised form 22 March 2004; accepted 23 March 2004 Available online 27 April 2004
Abstract We have studied the photoluminescence characteristics associated with the defects produced by Siþ ion-beam mixing of SiO2/ Si/SiO2 films. The photoluminescence peaks show a strong dependence on annealing process before ion-beam mixing. Using electron spin resonance and X-ray photoelectron spectroscopy, the relationship between the photoluminescence peaks and the defects is discussed. # 2004 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 78.55.Ap; 78.66.Db Keywords: Ion-beam mixing; Photoluminescence; Radiative defects
1. Introduction Photoluminescence (PL) from silicon-based materials consisting of nanoscale crystallites in an oxide environment has been under active investigation due to its potential application in Si-based optoelectronic devices [1,2]. The ion implantation technique has been employed to make Si nanocrystals in SiO2 matrix by many researchers [3–8]. However, the high concentration of Si nanocrystals is hardly achieved by ion implantation because of the sputtering effect. That is, the sputtered atoms are typically balanced with the incident ones about 10 at.% of doping concentration * Corresponding author. Fax: þ82-2-958-5409. E-mail address: [email protected] (K.H. Chae).
[9]. On the other hand, ion-beam mixing is a powerful technique to form the Si nanocrystals with high concentration, keeping most useful advantages of ion implantation. Recently, Chae et al. [10] reported the fabrication of the Si nanocrystals by the Ar ion-beam mixing and subsequent high temperature annealing of the SiO2/Si/ SiO2 sandwiched structure made by ion sputter deposition method, in which the embedded Si layer has a few nanometer thicknesses. Kim et al. [11] reported the PL characteristics of Si ion-beam-mixed SiO2/Si/SiO2 sandwiched structure made by ion sputter deposition method. Their results revealed that the ion-beam-mixed SiO2/Si/SiO2 layered system showed a higher PL intensity around 700 nm than that from the Si ion-implanted SiO2 system. This means that the
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.234
J.H. Son et al. / Applied Surface Science 233 (2004) 288–293
ion-beam mixing of the SiO2/Si/SiO2 layered structure with the subsequent high-temperature annealing can be used for the fabrication of the Si nanocrystals in SiO2 matrix. The researches for ion-beam-mixed SiO2/Si/SiO2 layered structure have been focused mainly on the formation of Si nanocrystals. However, since many kinds of defects were created in the ion-beam-mixed samples by energetic ions, the research for the properties and/or effects of the defects is indispensable in the samples made by ion-beam mixing process. For this study, electron-beam evaporation was used for the fabrication of the SiO2/Si/SiO2 sandwiched system with nanometer-thick Si layer because this method is a relatively clean process than the ion sputter deposition one. In order to clarify the PL characteristics related to defects produced by Si ion-beam mixing, electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) were used.
2. Experiments Amorphous SiO2 (50 nm)/amorphous Si (3 nm)/ SiO2 (100 nm) films were deposited on Si (1 0 0) substrate by electron-beam evaporation at room temperature (RT). In these processes, the amorphous Si ˚ /s, layer was deposited with a deposition rate of 0.1 A and the amorphous SiO2 layer was deposited with a ˚ /s. The chamber pressure deposition rate of 0.5 A during the deposition was 5 108 Torr. After deposition, some of the samples were annealed in N2 ambient at 1100 8C for 2 h (pre-annealing). For the ion-beam mixing, Siþ ions with an energy of 55 keV were irradiated into the samples with a dose of 1:5 1016 ions/cm2 at RT. The energy of Siþ ion is chosen so that the mean damage depth (RD) is match with the depth of embedded Si layer, since the total number of intermixed atoms at the interface reaches maximum value when the depth of interface is approximately coincident with the RD ðRD ’ ð2=3ÞRp Þ [12]. According to TRIM code [13], the mean projected range (Rp) of 55 keV Si ion in SiO2 is 74 nm. After Siþ ion-beam mixing, some of samples were annealed in N2 ambient at 1100 8C for 2 h to promote the formation of Si nanocrystals. The detailed treatments for the samples are listed in Table 1.
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Table 1 The detailed explanation of treatments for each samples Sample
Sample Sample Sample Sample Sample
Treatments
A B C D E
Pre-annealing
Ion-beam mixing
Post-annealing
No Yes No No Yes
No No Yes Yes Yes
No No No Yes No
At first, the photoluminescence measurements are performed for the samples prepared with Si ion-beam mixing and/or high-temperature annealing to see the luminescence characteristics of the samples. For PL measurements, an Ar ion laser (351 nm) was used as an excitation source and a cooled photomultiplier tube employing the photon counting technique was used as a detector for luminescence. To block the light scattered from the source, a high pass filter that cuts off the wavelengths shorter than 375 nm was used. ESR measurements were performed to quantitatively evaluate the amounts of paramagnetic defects in the samples. The ESR signal was measured at RT using Bruker ESP-300 X-band spectrometer operating at a microwave power of 5.024 mW and modulation amplitude of 5.213 G. In order to confirm the existence of the Si layer between SiO2 layers and to study the chemical state of the SiO2 layers, O 1s and Si 2p core level X-ray photoelectron spectroscopy measurements were performed using a standard Al Ka (1486.7 eV) excitation source in an electron spectrometer ESCA 5700 (PHI Ltd.) at a residual gas pressure of 2 1010 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 of 103.3 eV at RT [14].
3. Results and discussion Fig. 1 shows PL spectra from as-deposited sample (Sample A), pre-annealed (Sample B), ion-beammixed (Sample C), post-annealed after ion-beam mixing (Sample D), and ion-beam-mixed after pre-annealing at 1100 8C for 2 h (Sample E). A very weak PL
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PL intensity (arb. units)
400 Sample A Sample B Sample C Sample D Sample E
300
200
cutoff filter
100
0 300
400
500 600 Wavelength (nm)
700
800
Fig. 1. PL spectra observed from the as-deposited (Sample A), preannealed (Sample B), Si ion-beam-mixed (Sample C), postannealed after Si ion-beam mixing (Sample D), and Si ion-beammixed SiO2/Si/SiO2 samples after pre-annealing (Sample E).
peak around 430 nm is shown from Sample A having the structure of 3 nm Si layer sandwiched by SiO2 layers. This PL peak is hardly seen after the annealing at 1100 8C for 2 h as shown in Fig. 1 (Sample B). It may be that the PL peak from Sample A is due to the radiative defects formed during the sample preparation, and the quantity of the defects related to the 430 nm PL peak is relatively small. The Si ion-beam mixing process is performed to modify the embedded Si layer to make proper-sized Si nanocrystals after high-temperature post-annealing. One can see that the Sample C shows a broad PL characteristics with two peaks. One of the PL peak positions for the Sample C is located around 450 nm and its intensity is increased. According to Liao et al.’s report [15], this peak is related to the B2 band (O3BSi–SiBO3). We can infer from this fact that the Si layer embedded between the SiO2 layers can be broken into pieces and mixed with SiO2 by Si ion-beam mixing, and B2 bands should be increased due to the increase of Si-rich SiO2 region caused by ion-beam mixing. Thus, the intensity of the PL peak is increased after Si ion-beam mixing. Another PL peak position for the Sample C is located around 550 nm, which will be discussed later in more detail. The PL measurement after high-temperature post-annealing after ion-beam mixing (Sample D) shows that the PL peak at 450 nm diminished. This means that the PL peak of Sample C is associated with defects and the defects are cured during the annealing
at 1100 8C for 2 h. An interesting point on the PL spectrum of Sample D is that it does not show any PL peak around 700 nm associated with the Si nanocrystals. This contrasts with Kim et al.’s results on the similar films deposited by the ion sputter deposition, an intense 700 nm PL peak was observed at RT after Si ion-beam mixing and subsequent high-temperature annealing [16]. It is known that ion sputter deposited films are denser than electron-beam evaporated ones, since the deposited atoms have more energy in ion sputter deposition than electron-beam evaporation. Therefore, we can infer from these results that the formation conditions of Si nanocrystals are affected by the sample preparation method. Thus, we performed the pre-annealing process before the ion-beam mixing to give conditions similar to those of the ion sputter deposition. Pre-annealing is preformed in N2 ambient at 1100 8C for 2 h. In the case of Sample E, the Si ionbeam-mixed sample after the pre-annealing at high temperature, the PL peak appeared at different position around 600 nm. This peak is related to radiative defects such as non-bridging oxygen hole center [17]. The PL spectra shown in Fig. 1 are related to radiative defects. However, we cannot obtain lots of information for the relationship between the PL spectra and the defects created by ion-beam mixing, because it is almost impossible to measure the defects in the sample directly with existing techniques in spite of the technical efforts of many scientists up to now. Since the quantity of paramagnetic defects can be measured using ESR measurement, this result can give us some clues that the difference of PL properties of each sample can be explained by the existence and the quantity of the paramagnetic defects. Fig. 2 shows the ESR spectra from Samples A, B, C, and E. As-deposited sample showed a strong signal with the zero-crossing g-value of 2.0042 as shown in Fig. 2. According to Stesmans et al. [18] the zerocrossing g-value of the Pb center, depicted as SiBSi and known as a nonradiative defect, vary from 2.0015 to 2.0087. This implies that as-deposited sample contains the Pb center formed during sample deposition. After high-temperature annealing (Sample B), the ESR signal cannot be observed within our experimental extent. We can see from the results of PL and ESR measurements that the defects are cured by hightemperature annealing irrespective of the kind of defects.
J.H. Son et al. / Applied Surface Science 233 (2004) 288–293
291
400
g = 2.0018
Sample C
PL intensity (arb. units)
ESR signal (arb. units)
Sample E
g = 2.0023
3250
200
400
500 Wavelength (nm)
600
700
Fig. 3. PL spectrum of Si ion-beam-mixed SiO2/Si/SiO2 samples (Sample C). The peak deconvolution is carried out with two Gaussian peaks whose positions are 450 and 550 nm, respectively.
g = 2.0042
3350 3450 3550 3650 Magnetic field (gauss)
cutoff filter
100
0 300
Sample B
Sample A
300
3750
Fig. 2. ESR spectra measured from the as-deposited (Sample A), pre-annealed (Sample B), Si ion-beam-mixed (Sample C), and Si ion-beam-mixed SiO2/Si/SiO2 samples after pre-annealing (Sample E).
The intensity of ESR signal diminished in ionbeam-mixed sample (Sample C) compared with Sample A. The ESR signals and the PL spectra from Sample C imply that the relative quantity of nonradiative defects is decreased and the relative quantity of radiative defects is increased by Si ion irradiation. In general, many kinds of defects are created in the sample by ion irradiation and the quantity of defects increases after ion irradiation. We can find a very interesting phenomenon from PL spectra and ESR signals for Sample C. This exciting result can be explained as follows. The Pb centers exist mainly at the interface between Si layer and SiO2 layer and in Si substrate. The Si layer embedded between SiO2 layers plays an important role in the formation of the Pb centers in the deposited layers on the substrate. The Si layer between the SiO2 layers can be broken into pieces and mixed with SiO2 by Si ion-beam mixing. During this process, the bond between Si atoms of the Pb centers are broken, and the Si atoms in the Si layer can be bonded easily with the O atoms in the SiO2 layers since the binding energy of Si–O is larger than that of Si–Si. Thus, the quantity of nonradiative
defects decreases, and the intensity of ESR signal decreases. On the contrary, the augmentation of Si– O bond brings on the increase of radiative defects such as B2 bands and/or non-bridging oxygen hole centers. Thus, a broad PL spectrum with the peak position around 450 nm and the shoulder around 550 nm was observed from the Sample C. Fig. 3 shows the peak deconvolution of PL spectrum measured from Sample C. The PL spectrum from Sample C can be expressed by the superposition of two Gaussian functions whose peak positions are 450 and 550 nm, respectively. According to Liao et al.’s report [15], the peak around 450 nm is ascribed with the B2 band. However, no one has reported what kinds defects correspond to the PL peak around 550 nm under our knowledge. Kim et al. reported only that the defect-related PL peak position from Si ionimplanted SiO2 layers varied from 470 to 600 nm with the dose of implanted Si ions without any assignment of PL peak positions [19]. In case of the last process at high-temperature post-annealing (Sample D), the ESR signals disappear and cannot be observed within our experimental limit (not shown here). The zero-crossing g-value of Sample E was 2.0018. Sample E is a Si ion-beam-mixed sample after hightemperature pre-annealing. The defects created during the deposition of Si layer and SiO2 layers are cured by the pre-annealing at high temperature. Besides, Jeong et al. reported the zero-crossing g-value of 2.0017 from Si ion-implanted SiO2 layers [20]. According to Warren et al. [21] the zero-crossing g-value for E0 defects, depicted as O3BSi , is ranged from 1.9990 to
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Concentraion ratio (O/Si)
4
3
2
1 Si Layer 0
0
10
20 30 40 Sputter time (min.)
50
60
Fig. 4. Concentration ratio of O to Si obtained from XPS depth profile of O 1s and Si 2p core levels for the SiO2/Si/SiO2 sample pre-annealed at high temperature (Sample B).
2.0018. Since the zero-crossing g-value of 2.0018 is contained the region of E0 defects as well as Pb centers, the defect type corresponding to this value cannot be distinguished. It is known that E0 defects are more likely to occur in oxygen-rich SiO2 rather than in silicon-rich SiO2. Fig. 4 shows the concentration ratio of O to Si obtained from XPS depth profile of O 1s and Si 2p core levels in Sample B to confirm the existence of Si layer between SiO2 layers after high-temperature preannealing. The region indicated by an arrow in Fig. 4 represents the Si layer embedded between SiO2 layers. This means that the Si layer remains even after high-temperature pre-annealing. The ratios of O to Si in the bottom SiO2 layer of an as-deposited and a pre-annealed sample are same (not shown here). But the ratio of O to Si in the top SiO2 layer increased a little bit after high-temperature annealing. The values are 1.95 and 2.11 for an as-deposited and a preannealed sample, respectively. At a view point in the structure of Pb and E0 defects, E0 defect can be formed more easily in oxygen-rich SiO2 layer rather than Pb centers. Moreover, Prokes et al.’s study shows that the E0 defect (g-value of 2.0002 in ESR) is related to oxygen-related center through oxidation/ annealing step [22]. Therefore, we can propose from these facts that the zero-crossing g-value of the ESR signal for Sample E corresponds to that of E0 defects. After annealing of Sample E at high temperature, the PL peak around 700 nm corresponding to Si
nanocrystals was observed (not shown here). Interesting phenomenon is that the sample deposited by electron-beam evaporation requires a pre-annealing process before the Si ion irradiation in order to show the PL peak around 700 nm. Thus, in order to get the PL peak related to the Si nanocrystals from the sample deposited by electron-beam evaporation, a pre-annealing process is required before Si ion-beam mixing. This result differs from our previous paper [23]. This could be related with the thermal budget of the sample to create nucleation seeds forming Si nanocrystals. The pre-annealing before ion-beam mixing and postannealing compensate the difference of thermal budget according to the deposition methods, since the energy of deposited Si atoms in the electron-beam evaporation is smaller than that in the ion beam sputtering deposition. However, it is not easy to prove a definitive relationship between the thermal budget and nucleation seed in our samples.
4. Conclusions SiO2/Si/SiO2 films were deposited by electron-beam evaporation to clarify the PL peaks associated with the defects created by Si ion-beam mixing. A PL peak was observed around 450 nm with a shoulder around 550 nm from Si ion-beam-mixed SiO2/Si/SiO2 sample without pre-annealing. On the contrary, the PL peak was measured around 600 nm from Si ion-beam-mixed SiO2/Si/SiO2 sample after pre-annealing. The zerocrossing g-values of the samples without and with pre-annealing are 2.0023 and 2.0018, respectively.
Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Atomic-scale Surface Science Research Center (ASSRC) at Yonsei University.
References [1] M. Lopez, B. Garrido, C. Garcia, P. Pellegrino, A. PerezRodriguez, C. Bonafos, M. Carrada, A. Claverie, Appl. Phys. Lett. 80 (2002) 1637.
J.H. Son et al. / Applied Surface Science 233 (2004) 288–293 [2] P. Bettotti, M. Cazzanelli, L.D. Negro, B. Danese, C.J. Oton, G.V. Prakash, L. Pavesi, J. Phys. Condens. Matter 14 (2002) 8253. [3] T. Shimizu-Iwayama, K. Fuhita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [4] P. Mutti, G. Ghislotti, S. Bertoni, L. Bonoldi, G.F. Cerofolini, L. Meda, E. Grilli, M. Guzzi, Appl. Phys. Lett. 66 (1995) 851. [5] W. Skorupa, R.A. Yankov, I.E. Tyschenko, H. Frob, T. Bohme, K. Leo, Appl. Phys. Lett. 68 (1996) 2410. [6] S. Guha, M.D. Pace, D.N. Dunn, I.L. Singer, Appl. Phys. Lett. 70 (1997) 1207. [7] S. Zhang, W. Zhang, J. Yuan, Thin Solid Films 326 (1998) 92. [8] H.B. Kim, T.G. Kim, K.H. Chae, C.N. Whang, J.Y. Jeong, M.S. Oh, S. Im, J.H. Song, J. Korean Phys. Soc. 35 (1999) S588. [9] C.M. Preece, J.K. Hiroven, Ion Implantation Metallurgy, Metallurgic Society of AIME, Warrendale, PA, 1979, p. 38. [10] K.H. Chae, J.H. Son, G.S. Chang, H.B. Kim, J.Y. Jeong, S. Im, J.H. Song, K.J. Kim, H.K. Kim, C.N. Whang, Nanostruct. Mater. 11 (1999) 1239. [11] H.B. Kim, J.H. Son, K.H. Chae, J.Y. Jeong, W.S. Lee, S. Im, J.H. Song, C.N. Whang, Mater. Sci. Eng. B 69/70 (2000) 401. [12] J.H. Kim, H.J. Kang, K.H. Chae, J.H. Song, J.J. Woo, C.N. Whang, H.K. Kim, D.W. Moon, Nucl. Instrum. Methods B 71 (1992) 271.
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[13] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, vol. 1, Pergamon Press, New York, 1985. [14] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, in: J. Chasttain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Eden Prairie, MN, 1992, p. 57. [15] L.S. Liao, X.M. Bao, X.Q. Zheng, N.S. Li, N.B. Min, Appl. Phys. Lett. 68 (1996) 850. [16] H.B. Kim, J.H. Son, C.N. Whang, K.H. Chae, W.S. Lee, S. Im, S.O. Kim, J.J. Woo, J.H. Song, J. Korean Phys. Soc. 37 (2000) 466. [17] H.J. Fitting, T. Barfels, A. von Czarnowski, A.N. runkhin, Mater. Sci. Eng. B 71 (2000) 109. [18] A. Stesmans, J. Braet, J. Witters, R.F. Dekerrsmaecker, Surf. Sci. 141 (1984) 255. [19] H.B. Kim, T.G. Kim, J.H. Son, C.N. Whang, K.H. Chae, W.S. Lee, S. Im, J.H. Song, J. Appl. Phys. 88 (2000) 1851. [20] J.Y. Jeong, S. Im, M.S. Oh, H.B. Kim, K.H. Chae, C.N. Whang, J.H. Song, J. Lumin. 80 (1999) 285. [21] W.L. Warren, E.H. Poindexter, M. Offenberg, W. MullerWarmuth, J. Electrochem. Soc. 139 (1992) 872. [22] S.M. Prokes, W.E. Carlos, S. Veprek, Ch. Ossadnik, Phys. Rev. B 58 (1998) 15632. [23] J.H. Son, T.G. Kim, S.W. Shin, H.B. Kim, W.S. Lee, S. Im, J.H. Song, C.N. Whang, K.H. Chae, Opt. Mater. 17 (2001) 125.
The defect-related photoluminescence from Si ion-beam-mixed SiO2/Si/SiO2 films J.H. Sona, H.B. Kima, C.N. Whanga, K.H. Chaeb,* a
Atomic-scale Surface Science Research Center and Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, South Korea b Materials Science and Technology Division, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Sungbuk-gu, Seoul 136-791, South Korea Received in revised form 22 March 2004; accepted 23 March 2004 Available online 27 April 2004
Abstract We have studied the photoluminescence characteristics associated with the defects produced by Siþ ion-beam mixing of SiO2/ Si/SiO2 films. The photoluminescence peaks show a strong dependence on annealing process before ion-beam mixing. Using electron spin resonance and X-ray photoelectron spectroscopy, the relationship between the photoluminescence peaks and the defects is discussed. # 2004 Elsevier B.V. All rights reserved. PACS: 61.80.Jh; 78.55.Ap; 78.66.Db Keywords: Ion-beam mixing; Photoluminescence; Radiative defects
1. Introduction Photoluminescence (PL) from silicon-based materials consisting of nanoscale crystallites in an oxide environment has been under active investigation due to its potential application in Si-based optoelectronic devices [1,2]. The ion implantation technique has been employed to make Si nanocrystals in SiO2 matrix by many researchers [3–8]. However, the high concentration of Si nanocrystals is hardly achieved by ion implantation because of the sputtering effect. That is, the sputtered atoms are typically balanced with the incident ones about 10 at.% of doping concentration * Corresponding author. Fax: þ82-2-958-5409. E-mail address: [email protected] (K.H. Chae).
[9]. On the other hand, ion-beam mixing is a powerful technique to form the Si nanocrystals with high concentration, keeping most useful advantages of ion implantation. Recently, Chae et al. [10] reported the fabrication of the Si nanocrystals by the Ar ion-beam mixing and subsequent high temperature annealing of the SiO2/Si/ SiO2 sandwiched structure made by ion sputter deposition method, in which the embedded Si layer has a few nanometer thicknesses. Kim et al. [11] reported the PL characteristics of Si ion-beam-mixed SiO2/Si/SiO2 sandwiched structure made by ion sputter deposition method. Their results revealed that the ion-beam-mixed SiO2/Si/SiO2 layered system showed a higher PL intensity around 700 nm than that from the Si ion-implanted SiO2 system. This means that the
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.234
J.H. Son et al. / Applied Surface Science 233 (2004) 288–293
ion-beam mixing of the SiO2/Si/SiO2 layered structure with the subsequent high-temperature annealing can be used for the fabrication of the Si nanocrystals in SiO2 matrix. The researches for ion-beam-mixed SiO2/Si/SiO2 layered structure have been focused mainly on the formation of Si nanocrystals. However, since many kinds of defects were created in the ion-beam-mixed samples by energetic ions, the research for the properties and/or effects of the defects is indispensable in the samples made by ion-beam mixing process. For this study, electron-beam evaporation was used for the fabrication of the SiO2/Si/SiO2 sandwiched system with nanometer-thick Si layer because this method is a relatively clean process than the ion sputter deposition one. In order to clarify the PL characteristics related to defects produced by Si ion-beam mixing, electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) were used.
2. Experiments Amorphous SiO2 (50 nm)/amorphous Si (3 nm)/ SiO2 (100 nm) films were deposited on Si (1 0 0) substrate by electron-beam evaporation at room temperature (RT). In these processes, the amorphous Si ˚ /s, layer was deposited with a deposition rate of 0.1 A and the amorphous SiO2 layer was deposited with a ˚ /s. The chamber pressure deposition rate of 0.5 A during the deposition was 5 108 Torr. After deposition, some of the samples were annealed in N2 ambient at 1100 8C for 2 h (pre-annealing). For the ion-beam mixing, Siþ ions with an energy of 55 keV were irradiated into the samples with a dose of 1:5 1016 ions/cm2 at RT. The energy of Siþ ion is chosen so that the mean damage depth (RD) is match with the depth of embedded Si layer, since the total number of intermixed atoms at the interface reaches maximum value when the depth of interface is approximately coincident with the RD ðRD ’ ð2=3ÞRp Þ [12]. According to TRIM code [13], the mean projected range (Rp) of 55 keV Si ion in SiO2 is 74 nm. After Siþ ion-beam mixing, some of samples were annealed in N2 ambient at 1100 8C for 2 h to promote the formation of Si nanocrystals. The detailed treatments for the samples are listed in Table 1.
289
Table 1 The detailed explanation of treatments for each samples Sample
Sample Sample Sample Sample Sample
Treatments
A B C D E
Pre-annealing
Ion-beam mixing
Post-annealing
No Yes No No Yes
No No Yes Yes Yes
No No No Yes No
At first, the photoluminescence measurements are performed for the samples prepared with Si ion-beam mixing and/or high-temperature annealing to see the luminescence characteristics of the samples. For PL measurements, an Ar ion laser (351 nm) was used as an excitation source and a cooled photomultiplier tube employing the photon counting technique was used as a detector for luminescence. To block the light scattered from the source, a high pass filter that cuts off the wavelengths shorter than 375 nm was used. ESR measurements were performed to quantitatively evaluate the amounts of paramagnetic defects in the samples. The ESR signal was measured at RT using Bruker ESP-300 X-band spectrometer operating at a microwave power of 5.024 mW and modulation amplitude of 5.213 G. In order to confirm the existence of the Si layer between SiO2 layers and to study the chemical state of the SiO2 layers, O 1s and Si 2p core level X-ray photoelectron spectroscopy measurements were performed using a standard Al Ka (1486.7 eV) excitation source in an electron spectrometer ESCA 5700 (PHI Ltd.) at a residual gas pressure of 2 1010 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 of 103.3 eV at RT [14].
3. Results and discussion Fig. 1 shows PL spectra from as-deposited sample (Sample A), pre-annealed (Sample B), ion-beammixed (Sample C), post-annealed after ion-beam mixing (Sample D), and ion-beam-mixed after pre-annealing at 1100 8C for 2 h (Sample E). A very weak PL
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J.H. Son et al. / Applied Surface Science 233 (2004) 288–293
PL intensity (arb. units)
400 Sample A Sample B Sample C Sample D Sample E
300
200
cutoff filter
100
0 300
400
500 600 Wavelength (nm)
700
800
Fig. 1. PL spectra observed from the as-deposited (Sample A), preannealed (Sample B), Si ion-beam-mixed (Sample C), postannealed after Si ion-beam mixing (Sample D), and Si ion-beammixed SiO2/Si/SiO2 samples after pre-annealing (Sample E).
peak around 430 nm is shown from Sample A having the structure of 3 nm Si layer sandwiched by SiO2 layers. This PL peak is hardly seen after the annealing at 1100 8C for 2 h as shown in Fig. 1 (Sample B). It may be that the PL peak from Sample A is due to the radiative defects formed during the sample preparation, and the quantity of the defects related to the 430 nm PL peak is relatively small. The Si ion-beam mixing process is performed to modify the embedded Si layer to make proper-sized Si nanocrystals after high-temperature post-annealing. One can see that the Sample C shows a broad PL characteristics with two peaks. One of the PL peak positions for the Sample C is located around 450 nm and its intensity is increased. According to Liao et al.’s report [15], this peak is related to the B2 band (O3BSi–SiBO3). We can infer from this fact that the Si layer embedded between the SiO2 layers can be broken into pieces and mixed with SiO2 by Si ion-beam mixing, and B2 bands should be increased due to the increase of Si-rich SiO2 region caused by ion-beam mixing. Thus, the intensity of the PL peak is increased after Si ion-beam mixing. Another PL peak position for the Sample C is located around 550 nm, which will be discussed later in more detail. The PL measurement after high-temperature post-annealing after ion-beam mixing (Sample D) shows that the PL peak at 450 nm diminished. This means that the PL peak of Sample C is associated with defects and the defects are cured during the annealing
at 1100 8C for 2 h. An interesting point on the PL spectrum of Sample D is that it does not show any PL peak around 700 nm associated with the Si nanocrystals. This contrasts with Kim et al.’s results on the similar films deposited by the ion sputter deposition, an intense 700 nm PL peak was observed at RT after Si ion-beam mixing and subsequent high-temperature annealing [16]. It is known that ion sputter deposited films are denser than electron-beam evaporated ones, since the deposited atoms have more energy in ion sputter deposition than electron-beam evaporation. Therefore, we can infer from these results that the formation conditions of Si nanocrystals are affected by the sample preparation method. Thus, we performed the pre-annealing process before the ion-beam mixing to give conditions similar to those of the ion sputter deposition. Pre-annealing is preformed in N2 ambient at 1100 8C for 2 h. In the case of Sample E, the Si ionbeam-mixed sample after the pre-annealing at high temperature, the PL peak appeared at different position around 600 nm. This peak is related to radiative defects such as non-bridging oxygen hole center [17]. The PL spectra shown in Fig. 1 are related to radiative defects. However, we cannot obtain lots of information for the relationship between the PL spectra and the defects created by ion-beam mixing, because it is almost impossible to measure the defects in the sample directly with existing techniques in spite of the technical efforts of many scientists up to now. Since the quantity of paramagnetic defects can be measured using ESR measurement, this result can give us some clues that the difference of PL properties of each sample can be explained by the existence and the quantity of the paramagnetic defects. Fig. 2 shows the ESR spectra from Samples A, B, C, and E. As-deposited sample showed a strong signal with the zero-crossing g-value of 2.0042 as shown in Fig. 2. According to Stesmans et al. [18] the zerocrossing g-value of the Pb center, depicted as SiBSi and known as a nonradiative defect, vary from 2.0015 to 2.0087. This implies that as-deposited sample contains the Pb center formed during sample deposition. After high-temperature annealing (Sample B), the ESR signal cannot be observed within our experimental extent. We can see from the results of PL and ESR measurements that the defects are cured by hightemperature annealing irrespective of the kind of defects.
J.H. Son et al. / Applied Surface Science 233 (2004) 288–293
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g = 2.0018
Sample C
PL intensity (arb. units)
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Sample E
g = 2.0023
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400
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Fig. 3. PL spectrum of Si ion-beam-mixed SiO2/Si/SiO2 samples (Sample C). The peak deconvolution is carried out with two Gaussian peaks whose positions are 450 and 550 nm, respectively.
g = 2.0042
3350 3450 3550 3650 Magnetic field (gauss)
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Fig. 2. ESR spectra measured from the as-deposited (Sample A), pre-annealed (Sample B), Si ion-beam-mixed (Sample C), and Si ion-beam-mixed SiO2/Si/SiO2 samples after pre-annealing (Sample E).
The intensity of ESR signal diminished in ionbeam-mixed sample (Sample C) compared with Sample A. The ESR signals and the PL spectra from Sample C imply that the relative quantity of nonradiative defects is decreased and the relative quantity of radiative defects is increased by Si ion irradiation. In general, many kinds of defects are created in the sample by ion irradiation and the quantity of defects increases after ion irradiation. We can find a very interesting phenomenon from PL spectra and ESR signals for Sample C. This exciting result can be explained as follows. The Pb centers exist mainly at the interface between Si layer and SiO2 layer and in Si substrate. The Si layer embedded between SiO2 layers plays an important role in the formation of the Pb centers in the deposited layers on the substrate. The Si layer between the SiO2 layers can be broken into pieces and mixed with SiO2 by Si ion-beam mixing. During this process, the bond between Si atoms of the Pb centers are broken, and the Si atoms in the Si layer can be bonded easily with the O atoms in the SiO2 layers since the binding energy of Si–O is larger than that of Si–Si. Thus, the quantity of nonradiative
defects decreases, and the intensity of ESR signal decreases. On the contrary, the augmentation of Si– O bond brings on the increase of radiative defects such as B2 bands and/or non-bridging oxygen hole centers. Thus, a broad PL spectrum with the peak position around 450 nm and the shoulder around 550 nm was observed from the Sample C. Fig. 3 shows the peak deconvolution of PL spectrum measured from Sample C. The PL spectrum from Sample C can be expressed by the superposition of two Gaussian functions whose peak positions are 450 and 550 nm, respectively. According to Liao et al.’s report [15], the peak around 450 nm is ascribed with the B2 band. However, no one has reported what kinds defects correspond to the PL peak around 550 nm under our knowledge. Kim et al. reported only that the defect-related PL peak position from Si ionimplanted SiO2 layers varied from 470 to 600 nm with the dose of implanted Si ions without any assignment of PL peak positions [19]. In case of the last process at high-temperature post-annealing (Sample D), the ESR signals disappear and cannot be observed within our experimental limit (not shown here). The zero-crossing g-value of Sample E was 2.0018. Sample E is a Si ion-beam-mixed sample after hightemperature pre-annealing. The defects created during the deposition of Si layer and SiO2 layers are cured by the pre-annealing at high temperature. Besides, Jeong et al. reported the zero-crossing g-value of 2.0017 from Si ion-implanted SiO2 layers [20]. According to Warren et al. [21] the zero-crossing g-value for E0 defects, depicted as O3BSi , is ranged from 1.9990 to
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Concentraion ratio (O/Si)
4
3
2
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0
10
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Fig. 4. Concentration ratio of O to Si obtained from XPS depth profile of O 1s and Si 2p core levels for the SiO2/Si/SiO2 sample pre-annealed at high temperature (Sample B).
2.0018. Since the zero-crossing g-value of 2.0018 is contained the region of E0 defects as well as Pb centers, the defect type corresponding to this value cannot be distinguished. It is known that E0 defects are more likely to occur in oxygen-rich SiO2 rather than in silicon-rich SiO2. Fig. 4 shows the concentration ratio of O to Si obtained from XPS depth profile of O 1s and Si 2p core levels in Sample B to confirm the existence of Si layer between SiO2 layers after high-temperature preannealing. The region indicated by an arrow in Fig. 4 represents the Si layer embedded between SiO2 layers. This means that the Si layer remains even after high-temperature pre-annealing. The ratios of O to Si in the bottom SiO2 layer of an as-deposited and a pre-annealed sample are same (not shown here). But the ratio of O to Si in the top SiO2 layer increased a little bit after high-temperature annealing. The values are 1.95 and 2.11 for an as-deposited and a preannealed sample, respectively. At a view point in the structure of Pb and E0 defects, E0 defect can be formed more easily in oxygen-rich SiO2 layer rather than Pb centers. Moreover, Prokes et al.’s study shows that the E0 defect (g-value of 2.0002 in ESR) is related to oxygen-related center through oxidation/ annealing step [22]. Therefore, we can propose from these facts that the zero-crossing g-value of the ESR signal for Sample E corresponds to that of E0 defects. After annealing of Sample E at high temperature, the PL peak around 700 nm corresponding to Si
nanocrystals was observed (not shown here). Interesting phenomenon is that the sample deposited by electron-beam evaporation requires a pre-annealing process before the Si ion irradiation in order to show the PL peak around 700 nm. Thus, in order to get the PL peak related to the Si nanocrystals from the sample deposited by electron-beam evaporation, a pre-annealing process is required before Si ion-beam mixing. This result differs from our previous paper [23]. This could be related with the thermal budget of the sample to create nucleation seeds forming Si nanocrystals. The pre-annealing before ion-beam mixing and postannealing compensate the difference of thermal budget according to the deposition methods, since the energy of deposited Si atoms in the electron-beam evaporation is smaller than that in the ion beam sputtering deposition. However, it is not easy to prove a definitive relationship between the thermal budget and nucleation seed in our samples.
4. Conclusions SiO2/Si/SiO2 films were deposited by electron-beam evaporation to clarify the PL peaks associated with the defects created by Si ion-beam mixing. A PL peak was observed around 450 nm with a shoulder around 550 nm from Si ion-beam-mixed SiO2/Si/SiO2 sample without pre-annealing. On the contrary, the PL peak was measured around 600 nm from Si ion-beam-mixed SiO2/Si/SiO2 sample after pre-annealing. The zerocrossing g-values of the samples without and with pre-annealing are 2.0023 and 2.0018, respectively.
Acknowledgements This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Atomic-scale Surface Science Research Center (ASSRC) at Yonsei University.
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