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Boundary conditions for formation of Er–O optical centers in Er- and O-coimplanted Si
Nuclear Instruments and Methods in Physics Research B 148 (1999) 517±522
Boundary conditions for formation of Er±O optical centers in Erand O-coimplanted Si Kenshiro Nakashima a b
a,* ,
Osamu Eryu a, Akikazu Saito a, Toshitake Nakata b, Masanori Watanabe b
Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan Ion Engineering Institute Corporation, Tsuda, Hirakata, Osaka, Japan
Abstract Photoluminescence (PL) of a variety of samples co-implanted with erbium and oxygen in FZ Si has been investigated to determine boundary conditions for the formation of Er-luminescent centers. Within a limited range of annealing temperatures, the formation of these centers can be controlled by the relative concentration of Er and O. As the annealing temperature increases from 600°C to 900°C for samples implanted with 1 ´ 1019 Er/cm3 , the minimum concentration ratio of O/Er required for obtaining a PL intensity of the main line at 0.8068 eV from Er-centers increases from 0.1 at 700°C annealing to 10 at 900°C. We give evidence that oxygen migration is involved in enhancing the PL intensity of Er-optical centers. Preliminary Rutherford Backscattering spectroscopy and channeling (RBS/C) measurements give an experimental result on the lattice location of the Er3 ions in samples dominated with cubic symmetry optical centers. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 66.39.Jt; 78.55.Ap; 81.05.Cy Keywords: Erbium; Oxygen; Ion-implantation; Photoluminescence; Annealing
1. Introduction Understandings on optical properties of Erdoped Si have been much progressed since the ®rst observation of its characteristic luminescence lines by Ennen et al. [1]. Generally, two types of optical centers with characteristic luminescence spectra have been observed in Si co-implanted with Er and O (Si:Er,O) [2±4]. One is a so called cubic site
* Corresponding author. Tel.: +81 52 735 5418; fax: 81 52 735 5585; e-mail: [email protected]
symmetry center with ®ve characteristic luminescence lines (Er-1 center, hereafter), and the other one is a non-cubic symmetry center with seven spectral lines (Er-2 center, hereafter). Our previous papers have shown that these two kinds of centers could be formed in a controllable way by selecting the appropriate concentration ratio of O to Er for speci®c annealing condition [5]. A similar result has been presented by Priolo et al. [6]. The Er-1 center becomes dominant in samples with O/Er ratio of around 1 to 2. On the contrary, the Er-2 center are favorably formed in the range of O/Er ratio larger than 5. In this paper, we will show that
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 8 2 - 4
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K. Nakashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 517±522
the above behavior is true in samples co-implanted with 1 ´ 1018 to 1 ´ 1019 Er/cm3 in peak concentrations and with O atoms in the variable range. Annealing temperature is another important factor not only to optimize PL intensity in Si:Er,O, but also to determine spectral shapes re¯ecting the symmetry of surroundings of Er ions. Hardly any well-de®ned PL spectra could be obtained in Si samples doped with the above conditions, if annealed above 1000°C or below 600°C. The purpose of this paper is to determine the boundary conditions for creation of Er-optical center in Si:Er, O. We report on the ®rst RBS and channeling measurements made on Si:Er,O samples showing ®ve well-de®ned PL lines from Er-1 centers. On the basis of RBS/C measurements we will give some discussions on the lattice locations of Er-1 optical centers formed by both optimum selections of Er and O concentrations, and annealing temperature. 2. Experimental Samples of n-type FZ (1 1 1) wafers (P-doped, 15 X cm) were implanted with both Er (300 keV) and O (40 keV) ions to obtain the same projected range. Er ion doses ranged from 7 ´ 1012 to 7 ´ 1013 cmÿ2 , corresponding to peak concentrations of 1 ´ 1018 to 1 ´ 1019 cmÿ3 , and O ion doses from 1 ´ 1013 to 5 ´ 1015 cmÿ2 , the peak concentrations of which ranged from 1 ´ 1018 to 5 ´ 1020 cmÿ3 . The projected range and straggling of implanted ions are estimated to be 105 and 29 nm for Er, and 95 and 37 nm for O, respectively. The samples were annealed in a gold image furnace evacuated to 2 ´ 10ÿ6 Torr at temperature from 550°C to 1000°C for 30 min. The PL was excited with the 514.5 nm line from an Ar ion laser (100 mW at the sample surface), and detected at both 4.2 and 77 K with a cooled Ge diode. The scattering in the reproducibility of the PL intensity lies within a few percent at each run. He ion RBS and channeling measurements were performed using a three-axis goniometer with the 0.005° angle resolution at a scattering angle of 170°. The incident ion energy was adjusted at 2.969 MeV, the resonant scattering condition for O, to enhance the scattering probability of O. Our previous paper has shown that the
remarkable recovery of the crystal quality is brought about by annealing of Er-doped samples co-implanted with O [5]. 3. Results and discussion 3.1. Boundary conditions of Er-optical center formation Two typical PL spectra frequently observed in Si:Er,O are shown in Fig. 1(a) and (b). Fig. 1(a), spectrum showing 4 of 5 characteristic luminescence lines from Er-1 centers with so called Tdinterstitial site symmetry, was obtained in a sample implanted with both Er (1 ´ 1019 cmÿ3 ) and O (2 ´ 1019 cmÿ3 ), and annealed at 800°C for 30 min. PL was measured at 4.2 K with a resolution of 0.6 nm. Photon energy of each emission line agrees well with the theoretical prediction within an experimental error [3]. When the concentration of Er
Fig. 1. Photoluminescence spectrum of samples implanted (a) with both Er 1 ´ 1019 cmÿ3 and O 2 ´ 1019 cmÿ3 (b) with both Er 1 ´ 1018 cmÿ3 and O 5 ´ 1019 cmÿ3 after annealing at 800°C for 30 min. Measured at 4.2 K with a wavelength resolution of 0.6 nm.
K. Nakashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 517±522
is decreased by one order magnitude (Er 1 ´ 1018 cmÿ3 ), keeping other conditions except the O concentration (O 5 ´ 1019 cmÿ3 in this case) unchanged, another type of an Er-center (Er-2) becomes dominant instead of Er-1 centers. The PL spectrum of Er-2 centers is similar to that due to non-cubic symmetric centers [3] or are suggested to be due to the axial symmetric centers [4] emitting seven characteristic PL lines as shown in Fig. 1(b). Examinations of several samples with varying O concentrations (1 ´ 1019 to 2 ´ 1020 cmÿ3 ) and a constant Er concentration (1 ´ 1018 cmÿ3 ) revealed that both the PL spectral shape and intensity remained substantially unchanged among them, that is, an Er-2 center is favorably formed under the excess oxygen condition. Moreover, once an Er-2 center is formed, the supply of extra oxygen has no in¯uence on the center formation. This means that not only the absolute concentration of Er and O, but also their relative concentration plays an important role for determining the symmetry of surroundings around an Er ion [5]. In order to establish a guiding principle to create selectively Er-optical centers in Si in a controllable manner, we have prepared several Erimplanted samples at three levels of Er concentration; 1 ´ 1018 , 5 ´ 1018 , and 1 ´ 1019 cmÿ3 , respectively. The O concentration was varied from 1 ´ 1018 to 5 ´ 1020 cmÿ3 , resulting in O/Er ratios of 0.1 to 500. Examples of PL results for the Er concentration of 1 ´ 1019 cmÿ3 are shown in Fig. 2, in which the relative luminescence intensity of the main transition line at 0.8068 eV due to Er-1 centers are plotted against combinations of the O/ Er ratio and the annealing temperature. The diameter of each circle represents the relative PL intensity, respectively. The PL spectra were measured at 77 K with a wave length resolution of 1.4 nm. The hatched area indicates the region in which the PL spectrum due to Er-1 centers is below the detection limit. This ®gure shows that the Er-1 center can only be formed in a certain growthwindow, a region bounded with both the O/Er ratio and the annealing temperature. It must be mentioned here that, since the detection limit depends on the experimental setup, the growth-window will become larger with more sensitive measurements, or vice versa. At a ®xed O/Er ratio,
519
Fig. 2. The relative PL intensity of the main transition line at 0.8068 eV from Er-1 centers for combinations of the value of O/ Er ratio and the annealing temperature. The diameter of circles is proportional to the PL intensity. Measured at 77 K with a wavelength resolution of 1.4 nm.
annealing below 600°C can hardly form Er-1 centers, and the PL intensity of the main line (0.8068 eV) decreases at the higher annealing temperature. When the annealing temperature is ®xed, the PL intensity of the main line becomes detectable at a certain minimum value of the O/Er ratio, which becomes larger as the annealing temperature is elevated. In the oxygen-rich region beyond certain values of the O/Er ratio, 4±20
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dependent on the annealing temperature, Er-2 centers shown in Fig. 1(b) become dominant instead of Er-1 centers. In Fig. 3 the PL intensity of the main line shown in Fig. 2 is plotted as a function of the O/Er ratio, taking the annealing temperature as a parameter. Initially the PL intensity grows linearly or super linearly with the increasing O/Er ratio, and then saturates at the O/Er ratio of 2±5. Data points after annealing at 800°C, obtained in samples implanted with 1 ´ 1018 Er/cm3 , are also plotted for comparison in Fig. 3. In this case the PL intensity reaches a maximum at the O/Er ratio of two, followed by a rapid decrease in intensity at O/Er 5. On the contrary, it continues to increase by two times in samples implanted with 1 ´ 1019 Er/cm3 till O/Er reaches 4. If the PL intensity is taken to be proportional to the concentration of Er-1 centers, the maximum concentration of Er-1 centers in samples implanted with 1 ´ 1018 Er/cm3 should be equal to or less than the concentration of implanted Er ions. From these considerations the saturation concentration of Er-1 centers should be at most around 1±2 ´ 1018 cmÿ3 in all the samples investigated in the present experiment.
From Fig. 2, the minimum values of the O/Er ratio necessary for obtaining any detectable intensity of the main line of Er-1 centers are plotted as a function of the reciprocal annealing temperature as shown in Fig. 4. The ordinate means the average number of O atoms per one Er ion required in order to promote the PL due to Er-1 centers during the annealing period. Increasing the annealing temperature, larger numbers of O atoms are necessary for the detectable PL intensity due to Er-1 centers. Data plots are represented approximately with a straight line, from the slope of which the activation energy is estimated to be about 2.2 eV. This value is close to the activation energy 2.54 eV for migration of interstitial oxygen in Si [7].
Fig. 3. PL intensity of the main transition line at 0.8068 eV as a function of the O/Er ratio taking the annealing temperature as a parameter.
Fig. 4. Arrhenius plot of the minimum values of the O/Er ratio necessary for obtaining the detectable intensity of the main line from Er-1 centers vs. the reciprocal annealing temperature.
K. Nakashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 517±522
The formation process of the Er-1 center is possibly proceeded by such competitive processes due to the oxygen migration as one eventually resulting in formation of the Er-1 center, or the other to escape from the strain ®eld of Er ions leading to, for instance, oxygen aggregations. Erbium ions in excess of the solubility limit of the Er-1 center, probably 1±2 ´ 1018 cmÿ3 in the present case, seem to be transformed into non-radiative centers such as ErSi2 -type point defects [8,9]. The increase in the PL intensity due to the increasing O/Er ratios seems to re¯ect the increase in the concentration of Er-1 optical centers, as a result of the increasing probability for an Er ion to encounter diusing O atoms. This argument seems to be consistent with the increased solubility of Er ions in Si co-doped with Er and O [5,10]. An Er ion at tetrahedral interstitial sites surrounded with O atoms may be a possible structure of the Er-1 center. As for the Er2 center, discussions are beyond the scope of this paper. 3.2. RBS and channeling measurements Samples implanted with both 5 ´ 1018 Er/cm3 and 1 ´ 1019 O/cm3 , annealed at 750°C for 30 min, have been analyzed with RBS/C measurements to get insight on the lattice location of Er ions responsible for the optical centers. Characteristic PL lines due to Er-1 centers, similar to Fig. 1(a), are observed in these samples. The á1 0 0ñ, á1 1 0ñ, and á1 1 1ñ aligned and the random spectra were measured (the total charge 60 lC). The á1 0 0ñ aligned and the corresponding random spectra are shown in Fig. 5. A vmin value of 3% for the Si substrate was obtained, indicating that the crystal quality of the implanted region is nearly perfect after annealing. The Er random pro®les for both as-implanted and annealed samples and aligned pro®les are shown in Fig. 6 for the á1 0 0ñ, á1 1 0ñ, and á1 1 1ñ directions. The slight redistribution of Er ions was found towards the inward direction. A uniform decrease of the RBS yield by the amount of 40% is found along both the á1 1 1ñ and á1 0 0ñ directions except the surface region. The RBS yield at the peak position along the á1 1 0ñ direction is larger by 20% than that along the á1 1 1ñ or á1 0 0ñ direction. The tetrahedral interstitial
521
Fig. 5. Random and á1 0 0ñ aligned RBS spectra of Si samples implanted with both Er 5 ´ 1018 cmÿ3 and O 1 ´ 1019 cmÿ3 , annealed at 750°C for 30 min. vmin 3%.
positions are shadowed by the á1 1 1ñ and á1 0 0ñ atomic rows, but they scatter the ions incident along the á1 1 0ñ direction. Though angular scan experiments are required for the precise determination of the location of Er ions, we can draw a preliminary conclusion that a fraction of implanted Er ions is located on the well-de®ned interstitial sites. When we assume an Er ion on the
Fig. 6. Random Er pro®les of both as-implanted and annealed samples, and aligned Er pro®les of annealed samples along the á1 0 0ñ, á1 1 0ñ and á1 1 1ñ axis, respectively.
522
K. Nakashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 517±522
hexagonal interstitial site, a ¯ux peaking of the scattering yield should be expected along the á1 1 0ñ direction. It is not the case in the present experiments. Results of Fig. 6 lead us to discussion that some parts of Er ions might reside on substitutional sites. The precise determination of locations of Er ions is under way.
4. Conclusions We have shown boundary conditions for the formation of Er-optical centers, which are limited not only by the relative concentration of Er and O, but by the annealing temperature. It seems that the saturation concentration of Er-1 centers lies at or below 1±2 ´ 1018 cmÿ3 under the present experimental conditions. Under the excess oxygen condition (the O/Er ratio is larger than 5±10 for example), instead of Er-1 centers, another Er-optical center (Er-2) with seven characteristic PL lines becomes dominant, the location of which is not still clear. Er ions on the well-de®ned interstitial sites have been detected
in samples dominated with cubic symmetry Eroptical centers. References [1] H. Ennen, J. Wagner, H.D. Muller, R.S. Smith, J. Appl. Phys. 61 (1987) 4877. [2] J. Michel, J.L. Benton, R.F. Ferrante, D.C. Jacobson, D.J. Eaglesham, E.A. Fitzgerald, Y.-H. Xie, J.M. Poate, L.C. Kimerling, J. Appl. Phys. 70 (1991) 2672. [3] Y.S. Tang, K.C. Heasman, W.P. Gillin, B.J. Sealy, Appl. Phys. Lett. 55 (1989) 432. [4] H. Przybylinska, G. Hendorfer, M. Bruckner, L. Palmetshofer, W. Jantsch, Appl. Phys. Lett. 66 (1995) 490. [5] K. Nakashima, O. Eryu, O. Iioka, T. Nakata, M. Watanabe, Nucl. Instr. and Meth. B 127/128 (1997) 425. [6] F. Priolo, G. Franzo, S. Coa, A. Polman, S. Libertino, R. Barklie, D. Carey, J. Appl. Phys. 78 (1995) 3874. [7] M. Stavola, J.R. Patel, L.C. Kimerling, P.E. Freeland, Appl. Phys. Lett. 42 (1983) 73. [8] D.A. Adler, D.C. Jacobson, D.J. Eaglesham, M.A. Marcus, J.L. Benton, J.M. Poate, P.H. Citrin, Appl. Phys. Lett. 61 (1992) 2181. [9] A. Kozanecki, R.J. Wilson, B.J. Sealy, J. Kaczanowski, L. Nowicki, Appl. Phys. Lett. 67 (1995) 1847. [10] A. Polman, G.N. van den Hoven, J.S. Custer, J.H. Shin, R. Serna, J. Appl. Phys. 77 (1995) 1256.
Boundary conditions for formation of Er±O optical centers in Erand O-coimplanted Si Kenshiro Nakashima a b
a,* ,
Osamu Eryu a, Akikazu Saito a, Toshitake Nakata b, Masanori Watanabe b
Nagoya Institute of Technology, Showa-ku, Nagoya 466-8555, Japan Ion Engineering Institute Corporation, Tsuda, Hirakata, Osaka, Japan
Abstract Photoluminescence (PL) of a variety of samples co-implanted with erbium and oxygen in FZ Si has been investigated to determine boundary conditions for the formation of Er-luminescent centers. Within a limited range of annealing temperatures, the formation of these centers can be controlled by the relative concentration of Er and O. As the annealing temperature increases from 600°C to 900°C for samples implanted with 1 ´ 1019 Er/cm3 , the minimum concentration ratio of O/Er required for obtaining a PL intensity of the main line at 0.8068 eV from Er-centers increases from 0.1 at 700°C annealing to 10 at 900°C. We give evidence that oxygen migration is involved in enhancing the PL intensity of Er-optical centers. Preliminary Rutherford Backscattering spectroscopy and channeling (RBS/C) measurements give an experimental result on the lattice location of the Er3 ions in samples dominated with cubic symmetry optical centers. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 66.39.Jt; 78.55.Ap; 81.05.Cy Keywords: Erbium; Oxygen; Ion-implantation; Photoluminescence; Annealing
1. Introduction Understandings on optical properties of Erdoped Si have been much progressed since the ®rst observation of its characteristic luminescence lines by Ennen et al. [1]. Generally, two types of optical centers with characteristic luminescence spectra have been observed in Si co-implanted with Er and O (Si:Er,O) [2±4]. One is a so called cubic site
* Corresponding author. Tel.: +81 52 735 5418; fax: 81 52 735 5585; e-mail: [email protected]
symmetry center with ®ve characteristic luminescence lines (Er-1 center, hereafter), and the other one is a non-cubic symmetry center with seven spectral lines (Er-2 center, hereafter). Our previous papers have shown that these two kinds of centers could be formed in a controllable way by selecting the appropriate concentration ratio of O to Er for speci®c annealing condition [5]. A similar result has been presented by Priolo et al. [6]. The Er-1 center becomes dominant in samples with O/Er ratio of around 1 to 2. On the contrary, the Er-2 center are favorably formed in the range of O/Er ratio larger than 5. In this paper, we will show that
0168-583X/98/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 7 8 2 - 4
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K. Nakashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 517±522
the above behavior is true in samples co-implanted with 1 ´ 1018 to 1 ´ 1019 Er/cm3 in peak concentrations and with O atoms in the variable range. Annealing temperature is another important factor not only to optimize PL intensity in Si:Er,O, but also to determine spectral shapes re¯ecting the symmetry of surroundings of Er ions. Hardly any well-de®ned PL spectra could be obtained in Si samples doped with the above conditions, if annealed above 1000°C or below 600°C. The purpose of this paper is to determine the boundary conditions for creation of Er-optical center in Si:Er, O. We report on the ®rst RBS and channeling measurements made on Si:Er,O samples showing ®ve well-de®ned PL lines from Er-1 centers. On the basis of RBS/C measurements we will give some discussions on the lattice locations of Er-1 optical centers formed by both optimum selections of Er and O concentrations, and annealing temperature. 2. Experimental Samples of n-type FZ (1 1 1) wafers (P-doped, 15 X cm) were implanted with both Er (300 keV) and O (40 keV) ions to obtain the same projected range. Er ion doses ranged from 7 ´ 1012 to 7 ´ 1013 cmÿ2 , corresponding to peak concentrations of 1 ´ 1018 to 1 ´ 1019 cmÿ3 , and O ion doses from 1 ´ 1013 to 5 ´ 1015 cmÿ2 , the peak concentrations of which ranged from 1 ´ 1018 to 5 ´ 1020 cmÿ3 . The projected range and straggling of implanted ions are estimated to be 105 and 29 nm for Er, and 95 and 37 nm for O, respectively. The samples were annealed in a gold image furnace evacuated to 2 ´ 10ÿ6 Torr at temperature from 550°C to 1000°C for 30 min. The PL was excited with the 514.5 nm line from an Ar ion laser (100 mW at the sample surface), and detected at both 4.2 and 77 K with a cooled Ge diode. The scattering in the reproducibility of the PL intensity lies within a few percent at each run. He ion RBS and channeling measurements were performed using a three-axis goniometer with the 0.005° angle resolution at a scattering angle of 170°. The incident ion energy was adjusted at 2.969 MeV, the resonant scattering condition for O, to enhance the scattering probability of O. Our previous paper has shown that the
remarkable recovery of the crystal quality is brought about by annealing of Er-doped samples co-implanted with O [5]. 3. Results and discussion 3.1. Boundary conditions of Er-optical center formation Two typical PL spectra frequently observed in Si:Er,O are shown in Fig. 1(a) and (b). Fig. 1(a), spectrum showing 4 of 5 characteristic luminescence lines from Er-1 centers with so called Tdinterstitial site symmetry, was obtained in a sample implanted with both Er (1 ´ 1019 cmÿ3 ) and O (2 ´ 1019 cmÿ3 ), and annealed at 800°C for 30 min. PL was measured at 4.2 K with a resolution of 0.6 nm. Photon energy of each emission line agrees well with the theoretical prediction within an experimental error [3]. When the concentration of Er
Fig. 1. Photoluminescence spectrum of samples implanted (a) with both Er 1 ´ 1019 cmÿ3 and O 2 ´ 1019 cmÿ3 (b) with both Er 1 ´ 1018 cmÿ3 and O 5 ´ 1019 cmÿ3 after annealing at 800°C for 30 min. Measured at 4.2 K with a wavelength resolution of 0.6 nm.
K. Nakashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 517±522
is decreased by one order magnitude (Er 1 ´ 1018 cmÿ3 ), keeping other conditions except the O concentration (O 5 ´ 1019 cmÿ3 in this case) unchanged, another type of an Er-center (Er-2) becomes dominant instead of Er-1 centers. The PL spectrum of Er-2 centers is similar to that due to non-cubic symmetric centers [3] or are suggested to be due to the axial symmetric centers [4] emitting seven characteristic PL lines as shown in Fig. 1(b). Examinations of several samples with varying O concentrations (1 ´ 1019 to 2 ´ 1020 cmÿ3 ) and a constant Er concentration (1 ´ 1018 cmÿ3 ) revealed that both the PL spectral shape and intensity remained substantially unchanged among them, that is, an Er-2 center is favorably formed under the excess oxygen condition. Moreover, once an Er-2 center is formed, the supply of extra oxygen has no in¯uence on the center formation. This means that not only the absolute concentration of Er and O, but also their relative concentration plays an important role for determining the symmetry of surroundings around an Er ion [5]. In order to establish a guiding principle to create selectively Er-optical centers in Si in a controllable manner, we have prepared several Erimplanted samples at three levels of Er concentration; 1 ´ 1018 , 5 ´ 1018 , and 1 ´ 1019 cmÿ3 , respectively. The O concentration was varied from 1 ´ 1018 to 5 ´ 1020 cmÿ3 , resulting in O/Er ratios of 0.1 to 500. Examples of PL results for the Er concentration of 1 ´ 1019 cmÿ3 are shown in Fig. 2, in which the relative luminescence intensity of the main transition line at 0.8068 eV due to Er-1 centers are plotted against combinations of the O/ Er ratio and the annealing temperature. The diameter of each circle represents the relative PL intensity, respectively. The PL spectra were measured at 77 K with a wave length resolution of 1.4 nm. The hatched area indicates the region in which the PL spectrum due to Er-1 centers is below the detection limit. This ®gure shows that the Er-1 center can only be formed in a certain growthwindow, a region bounded with both the O/Er ratio and the annealing temperature. It must be mentioned here that, since the detection limit depends on the experimental setup, the growth-window will become larger with more sensitive measurements, or vice versa. At a ®xed O/Er ratio,
519
Fig. 2. The relative PL intensity of the main transition line at 0.8068 eV from Er-1 centers for combinations of the value of O/ Er ratio and the annealing temperature. The diameter of circles is proportional to the PL intensity. Measured at 77 K with a wavelength resolution of 1.4 nm.
annealing below 600°C can hardly form Er-1 centers, and the PL intensity of the main line (0.8068 eV) decreases at the higher annealing temperature. When the annealing temperature is ®xed, the PL intensity of the main line becomes detectable at a certain minimum value of the O/Er ratio, which becomes larger as the annealing temperature is elevated. In the oxygen-rich region beyond certain values of the O/Er ratio, 4±20
520
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dependent on the annealing temperature, Er-2 centers shown in Fig. 1(b) become dominant instead of Er-1 centers. In Fig. 3 the PL intensity of the main line shown in Fig. 2 is plotted as a function of the O/Er ratio, taking the annealing temperature as a parameter. Initially the PL intensity grows linearly or super linearly with the increasing O/Er ratio, and then saturates at the O/Er ratio of 2±5. Data points after annealing at 800°C, obtained in samples implanted with 1 ´ 1018 Er/cm3 , are also plotted for comparison in Fig. 3. In this case the PL intensity reaches a maximum at the O/Er ratio of two, followed by a rapid decrease in intensity at O/Er 5. On the contrary, it continues to increase by two times in samples implanted with 1 ´ 1019 Er/cm3 till O/Er reaches 4. If the PL intensity is taken to be proportional to the concentration of Er-1 centers, the maximum concentration of Er-1 centers in samples implanted with 1 ´ 1018 Er/cm3 should be equal to or less than the concentration of implanted Er ions. From these considerations the saturation concentration of Er-1 centers should be at most around 1±2 ´ 1018 cmÿ3 in all the samples investigated in the present experiment.
From Fig. 2, the minimum values of the O/Er ratio necessary for obtaining any detectable intensity of the main line of Er-1 centers are plotted as a function of the reciprocal annealing temperature as shown in Fig. 4. The ordinate means the average number of O atoms per one Er ion required in order to promote the PL due to Er-1 centers during the annealing period. Increasing the annealing temperature, larger numbers of O atoms are necessary for the detectable PL intensity due to Er-1 centers. Data plots are represented approximately with a straight line, from the slope of which the activation energy is estimated to be about 2.2 eV. This value is close to the activation energy 2.54 eV for migration of interstitial oxygen in Si [7].
Fig. 3. PL intensity of the main transition line at 0.8068 eV as a function of the O/Er ratio taking the annealing temperature as a parameter.
Fig. 4. Arrhenius plot of the minimum values of the O/Er ratio necessary for obtaining the detectable intensity of the main line from Er-1 centers vs. the reciprocal annealing temperature.
K. Nakashima et al. / Nucl. Instr. and Meth. in Phys. Res. B 148 (1999) 517±522
The formation process of the Er-1 center is possibly proceeded by such competitive processes due to the oxygen migration as one eventually resulting in formation of the Er-1 center, or the other to escape from the strain ®eld of Er ions leading to, for instance, oxygen aggregations. Erbium ions in excess of the solubility limit of the Er-1 center, probably 1±2 ´ 1018 cmÿ3 in the present case, seem to be transformed into non-radiative centers such as ErSi2 -type point defects [8,9]. The increase in the PL intensity due to the increasing O/Er ratios seems to re¯ect the increase in the concentration of Er-1 optical centers, as a result of the increasing probability for an Er ion to encounter diusing O atoms. This argument seems to be consistent with the increased solubility of Er ions in Si co-doped with Er and O [5,10]. An Er ion at tetrahedral interstitial sites surrounded with O atoms may be a possible structure of the Er-1 center. As for the Er2 center, discussions are beyond the scope of this paper. 3.2. RBS and channeling measurements Samples implanted with both 5 ´ 1018 Er/cm3 and 1 ´ 1019 O/cm3 , annealed at 750°C for 30 min, have been analyzed with RBS/C measurements to get insight on the lattice location of Er ions responsible for the optical centers. Characteristic PL lines due to Er-1 centers, similar to Fig. 1(a), are observed in these samples. The á1 0 0ñ, á1 1 0ñ, and á1 1 1ñ aligned and the random spectra were measured (the total charge 60 lC). The á1 0 0ñ aligned and the corresponding random spectra are shown in Fig. 5. A vmin value of 3% for the Si substrate was obtained, indicating that the crystal quality of the implanted region is nearly perfect after annealing. The Er random pro®les for both as-implanted and annealed samples and aligned pro®les are shown in Fig. 6 for the á1 0 0ñ, á1 1 0ñ, and á1 1 1ñ directions. The slight redistribution of Er ions was found towards the inward direction. A uniform decrease of the RBS yield by the amount of 40% is found along both the á1 1 1ñ and á1 0 0ñ directions except the surface region. The RBS yield at the peak position along the á1 1 0ñ direction is larger by 20% than that along the á1 1 1ñ or á1 0 0ñ direction. The tetrahedral interstitial
521
Fig. 5. Random and á1 0 0ñ aligned RBS spectra of Si samples implanted with both Er 5 ´ 1018 cmÿ3 and O 1 ´ 1019 cmÿ3 , annealed at 750°C for 30 min. vmin 3%.
positions are shadowed by the á1 1 1ñ and á1 0 0ñ atomic rows, but they scatter the ions incident along the á1 1 0ñ direction. Though angular scan experiments are required for the precise determination of the location of Er ions, we can draw a preliminary conclusion that a fraction of implanted Er ions is located on the well-de®ned interstitial sites. When we assume an Er ion on the
Fig. 6. Random Er pro®les of both as-implanted and annealed samples, and aligned Er pro®les of annealed samples along the á1 0 0ñ, á1 1 0ñ and á1 1 1ñ axis, respectively.
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hexagonal interstitial site, a ¯ux peaking of the scattering yield should be expected along the á1 1 0ñ direction. It is not the case in the present experiments. Results of Fig. 6 lead us to discussion that some parts of Er ions might reside on substitutional sites. The precise determination of locations of Er ions is under way.
4. Conclusions We have shown boundary conditions for the formation of Er-optical centers, which are limited not only by the relative concentration of Er and O, but by the annealing temperature. It seems that the saturation concentration of Er-1 centers lies at or below 1±2 ´ 1018 cmÿ3 under the present experimental conditions. Under the excess oxygen condition (the O/Er ratio is larger than 5±10 for example), instead of Er-1 centers, another Er-optical center (Er-2) with seven characteristic PL lines becomes dominant, the location of which is not still clear. Er ions on the well-de®ned interstitial sites have been detected
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