- Email: [email protected]
Study of the radiative and nonradiative processes of rare earth implanted semiconductors at low temperatures
Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 286±291
www.elsevier.nl/locate/nimb
Study of the radiative and nonradiative processes of rare earth implanted semiconductors at low temperatures T. Kimura *, H. Toda, T. Ishida, H. Isshiki, R. Saito Department of Electronic Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, 182-8585 Tokyo, Japan
Abstract Factors determining the low temperature ¯uorescent transition rate of the luminescence of rare earth implanted semiconductors are studied. Photocarrier induced Auger deexcitation has been used to separate the radiative transition rate from the ¯uorescent one for Er-, Er and Ne-, Er and O-implanted Si and Ho-implanted GaAs. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The ¯uorescent lifetime of the rare earth related luminescence from rare earth doped semiconductors shows usually a large temperature quenching, whereas it is nearly independent of temperature at low temperatures (<20 K) [1] and ranges around 1±3 ms for most rare earth ions in semiconductors. These low temperature ¯uorescent lifetimes are considered to include no nonradiative transitions. However, much longer [2] or shorter [3] ¯uorescent lifetimes have also been reported for rare earth doped semiconductors. For example, a decay time in the submillisecond region is observed for Erimplanted silicon with insucient post-implantation thermal anneal [3]. The shorter values are in general explained in terms of the additional nonradiative radiation in terms of concentration quenching, up-conversion or crystal defects.
Coa et al. [4] and our group [5] have observed the Er±O related 1.54 lm luminescence with a ¯uorescent lifetime shorter than 1 ms from Er and O coimplanted Si. We ®nd that it is dicult to explain the result in terms of the increased nonradiative transition and that there is a possibility of enhanced radiative transition rate due to coimplantation. In this paper, the free-carrier Auger deexcitation of the rare earth related luminescence is used to study the factors determining the ¯uorescent transition rate at low temperatures to evaluate separately the radiative and nonradiative transition rates, using Er or Ho-implanted semiconductors (Si and GaAs). 2. Experimental 2.1. Sample preparation
*
Corresponding author. Tel.: +81-424-43-5143; fax: +81424-43-5210. E-mail address: [email protected] (T. Kimura).
Samples used in this study are Er-implanted Si (Si:Er), Er and O coimplanted Si (Si:Er±O), Er and
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 5 6 8 - 1
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
287
Table 1 Samples Si host Er O Ne GaAs host Ho
p-Si(100) CZ, q 1±2 X cm 180 keV, 1:0 1013 cm 2 ; Rp 60 nm; Np 2 1018 cm 3 30 keV, 2 1013 cm 2 2 1015 cm 40 keV, 2 1013 cm 2 2 1015 cm
2 2
SI n-GaAs, q 1 107 X cm 180 keV, 1014 cm 2 ; Rp 50 nm; Np 2:5 1019 cm 3 Fig. 1. Auger deexcitation measurement system.
Ne coimplanted Si(Si:Er±Ne), and Ho-implanted GaAs (GaAs:Ho). Ne coimplantation is done to study the eect of implantation induced defects on the Er-related luminescence characteristics and to compare it with the peculiar eect of O coimplantation. The implantation conditions are listed in Table 1. The peak Er concentration (Np 2 1018 cm 3 ) is suciently below the critical concentration (1020 cm 3 ) for the concentration quenching or up-conversion. 2.2. Photoluminescence and Auger deexcitation measurements Photoluminescence spectra are measured using a chopped Ar ion laser line k 488 nm as the excitation light source. The luminescence is dispersed with a single-grating monochromator (Jobin-Yvon HR320) and a cooled germanium pin photodiode. The decay characteristics are measured by exciting samples with an N2 pulse laser line (k 337 nm, pulse width tp 0:35 ns, pulse energy Ep 60 lJ/pulse (1014 photons)). The freecarrier Auger deexcitation is measured by the addition of the Ar ion 488 nm cw laser as shown in Fig. 1. The time resolution of the system used is 10 ls. Sample temperature is below 20 K. The peak intensity Ip of the decay curve is aected by the energy transfer eciency g and is independent of the nonradiative transition of the rare earth, since the pulse width of the N2 laser is extremely short. Note that the peak intensity does not suer any N2 pulse laser induced free-carrier Auger deexcitation due to the time delay for the appearance of the luminescence peak.
3. Results and discussion 3.1. Er-related 1.54 lm emission of Er-implanted Si 3.1.1. Eects of the ion implantation induced defects Fig. 2 shows the post-implantation annealing temperature dependence (annealing time is 1 min) of the N2 laser pulse response of the 1.54 lm Errelated luminescence at 20 K from Er-implanted Si. The luminescence begins to be observable at 600°C. The peak intensity Ip increases and the decay rate decreases with increasing the annealing temperature. The peak intensity Ip which is related to the energy transfer eciency from host to Er is greatly reduced when the e±h recombination rate wrec is increased due to defects. On the other hand, the ¯uorescent decay rate wfl of the 1.54 lm luminescence re¯ects directly the relaxation rate of the excited Er ions from 4 I13=2 ! 4 I15=2 : wfl wr wnr defects:
1
Here, wr denotes the radiative transition rate and wnr (defects) the nonradiative transition rate due to defects. Defect-related nonradiative transition may be a kind of Auger deexcitation due to electrons bound on defect levels. Because of high Er doses 1:0 1013 cm 2 , we expect a very high density of defects and this causes a large amount of the nonradiative transitions, though the Auger energy backtransfer rate to electrons bound on deep traps is expected to be small. At the annealing temperature of 900°C for 1 min, the defect density seems to become too low to aect the the decay of the Er-related luminescence.
288
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Fig. 2. Decay curves of the 1.54 lm luminescence of the Erimplanted Si as functions of post-implantation annealing temperature. Annealing time is 1 min.
3.1.2. Eects of Ne coimplantation induced defects In order to separate the peculiar eects of O coimplantation (which will be presented in the next section) from implantation induced crystalline defects, Ne ions are coimplanted together with Er into Si. The post-implantation anneal is at 1000°C for 1 min. Ne coimplantation is found to give neither new Er-related luminescence peaks in the photoluminescence (Fig. 3) nor any change in the decay rate (Fig. 4) for the Ne peak density below < 1019 cm 3 (Ne/Er 10). 3.1.3. Eect of O ion coimplantation O ion coimplantation reveals a peculiar eect on the Er-related luminescence. As shown in Fig. 4, a new peak with a decay time less than 1 ms was observed to grow after O coimplantation, though the original decay of the luminescence from Si:Er is not aected. The new peak with a fast
Fig. 3. Photoluminescence spectra of the Si:Er, Si:Er±Ne and Si:Er±O. The samples were annealed at 1000°C for 1 min.
decay was ®rst reported by Coa et al. [4], and also by our group [5]. For O concentration below 1019 cm 3 (where the implantation induced defects are annealed out and do not aect the luminescence characteristics), the peak intensity Ip of this new emission increases with O concentration. The ¯uorescent lifetime sfl ( 1=wfl ) of this new emission is below 1 ms. In comparison with the Ne coimplanted samples, the increase in Ip with O dose is due to the increase of O-related Er luminescence centers. In order to study whether the fast decay of the O-related Er luminescence observed here is due to the appearance of some nonradiative transition or due to the increased radiative transition, the freecarrier Auger deexcitation is measured. After Langer's theory [6], the free-carrier Auger deexcitation rate is given by PA;free CA n wr
n ; n0
2
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Fig. 4. Decay curves of the 1.54 lm luminescence of the Si:Er, Si:Er±Ne and Si:Er±O samples. The samples were annealed at 1000°C for 1 min.
where wr is the radiative transition rate of rare earth ions, n is the free-carrier concentration, and n0 is the critical free-carrier concentration. For the 1:54 lm luminescence of Er-doped Si, n0 is 1015 cm 3 [7]. The eect of the Ar cw irradiation on the decay curves of the 1.54 lm luminescence of Si:Er and the fast component of Si:Er±O is shown in Fig. 5. The results for Si:Er±Ne and for the slow component of Si:Er±O are almost the same as that of Si:Er. The Ne and O concentrations are below 1019 cm 3 . It is found that the slow decay (sfl 2 ms) of Si:Er, Si:Er±Ne and Si:Er±O samples shows nearly the same free-carrier Auger deexcitation. However, the fast component of Si:Er±O is much more sensitive to the Ar cw irradiation. Fig. 6 shows the ¯uorescent transition rate wfl as a function of p the square root ofpthe Ar cw irradiation power P (theoretically P / n). The gradient of the plots is proportional to the Auger coecient CA wr =n0 . CA for the fast decay
289
Fig. 5. Eects of the Ar cw irradiation on the decay curves of the 1.54 lm luminescence of the Si:Er±O sample.
component of Si:Er±O is found to be about 5 times larger than that for the slow decay component of the Si:Er, Si:Er±Ne and Si:Er±O samples. This ratio is nearly the same as the ratio of the ¯uorescent decay rate of the two without Ar cw irradiation. This clearly indicates that the observed ¯uorescent lifetimes of the above samples at 20 K are purely the radiative lifetime of the 1.54 lm Er luminescence, and no nonradiative transitions are involved in both the slow and fast components. This also means that the O coimplantation into Si:Er samples forms new 1.54 lm luminescence centers whose radiative lifetime is shorter than the hitherto accepted value of 1±2 ms. 3.2. Ho-implanted GaAs Ho-implanted GaAs shows the Ho-related photoluminescence at 1.19 lm 5 I6 ! 5 I8 at 20 K (Fig. 7). This band shows the ¯uorescent lifetime
290
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Fig. 7. Photoluminescence spectrum of the GaAs:Ho sample.
Fig. 6. Fluorescent transition rate as a function of the square root of the Ar cw irradiation power for the Si:Er, Si:Er±O (fast component) and GaAs:Ho.
ranging between 200 and 400 ls. The enhancement of the ¯uorescent transition rate due to the free-carrier Auger eect is of the same order as the Si:Er for the same Ar cw 488 nm power (Fig. 6). Judging from the fact that the steady-state free-carrier concentration is expected to be much smaller than that in Si due to the shorter recombination time of free- carriers in GaAs, the Auger coecient of the Ho-related luminescence is considered to be much larger than that of Er in Si. Therefore, the observed submillisecond ¯uorescent lifetime is almost the radiative lifetime itself. 3.3. Hints for the reduction of free-carrier Auger deexcitation Recently, our group has reported a strong enhancement of the Er-related luminescence from Er doped porous silicon, when the host porous silicon is slightly pre-oxidized before the Er incorporation
[8]. A similar result is also reported by Shin et al. [9] for Si=SiO2 (2 nm)/SiO2 ±Er superlattices structure. In both cases, the 2 nm thick SiO2 buer layer is very eective to improve the room temperature 1.54 lm Er-related luminescence. These results indicate that the Er ions can be effectively excited even if they are separated by a few nanometers, and at the same time this separation reduces the energy backtransfer due to free-carrier Auger as well as trap-assisted energy backtransfer drastically.
4. Conclusion The radiative transition rates of the Er-implanted Si and Ho-implanted GaAs at low temperatures are evaluated using the free-carrier Auger deexcitation measurements. Coimplantation of O into Si:Er is found to form a new luminescence peak whose radiative transition rate is about 5 times larger than that of Si:Er. Ho-related 1.19 lm luminescence from Ho-implanted GaAs shows that the ¯uorescent lifetime ranges from 200 ls to 400 ls, and this is also considered to be the value of the radiative lifetime.
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Acknowledgements This work is partly supported by the grant in aid (no. 12450008) by the Ministry of Education, Science, Sports and Culture. The authors thank Prof. A. Polman of FOM Institute for his valuable discussion. We also thank Dr. H. Yasuhara and Dr. A. Sato of Fujikura Co. Ltd. for their support of this research. References [1] H. Ennen, G. Pomrenke, A. Axman, K. Eisele, W. Haydl, J. Schneider, Appl. Phys. Lett. (1985) 381.
291
[2] J.F. Suyver, P.G. Kik, T. Kimura, A. Polman, G. Franz o, S. Coa, Nucl. Instr. and Meth. B 148 (1999) 497. [3] T. Nakanose, T. Kimura, H. Isshiki, S. Yugo, R. Saito, Nucl. Instr. and Meth. B 161±164 (2000) 1080. [4] S. Coa, G. Franz o, F. Priolo, A. Polman, R. Serna, Phys. Rev. B 49 (1994) 16313. [5] T. Kimura, T. Nakanose, W. Wang, H. Isshiki, R. Saito, Nucl. Instr. and Meth. B 148 (1999) 486. [6] A. Suchocki, J.M. Langer, Phys. Rev. B 39 (1989) 7905. [7] F. Priolo, G. Franz o, S. Coa, A. Polman, S. Libertino, R. Barklie, D. Carey, J. Appl. Phys. 78 (1995) 3874. [8] S. Ide, H. Isshiki, S. Yugo, R. Saito, T. Kimura, Extended Abstracts (The 47th Spring Meeting, 2000); The Japan Society of Applied Physics and Related Societies. [9] J.H. Shin, J.-H. Jhe, S.-Y. Seo, Appl. Phys. Lett. 76 (2000) 3567.
www.elsevier.nl/locate/nimb
Study of the radiative and nonradiative processes of rare earth implanted semiconductors at low temperatures T. Kimura *, H. Toda, T. Ishida, H. Isshiki, R. Saito Department of Electronic Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, 182-8585 Tokyo, Japan
Abstract Factors determining the low temperature ¯uorescent transition rate of the luminescence of rare earth implanted semiconductors are studied. Photocarrier induced Auger deexcitation has been used to separate the radiative transition rate from the ¯uorescent one for Er-, Er and Ne-, Er and O-implanted Si and Ho-implanted GaAs. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The ¯uorescent lifetime of the rare earth related luminescence from rare earth doped semiconductors shows usually a large temperature quenching, whereas it is nearly independent of temperature at low temperatures (<20 K) [1] and ranges around 1±3 ms for most rare earth ions in semiconductors. These low temperature ¯uorescent lifetimes are considered to include no nonradiative transitions. However, much longer [2] or shorter [3] ¯uorescent lifetimes have also been reported for rare earth doped semiconductors. For example, a decay time in the submillisecond region is observed for Erimplanted silicon with insucient post-implantation thermal anneal [3]. The shorter values are in general explained in terms of the additional nonradiative radiation in terms of concentration quenching, up-conversion or crystal defects.
Coa et al. [4] and our group [5] have observed the Er±O related 1.54 lm luminescence with a ¯uorescent lifetime shorter than 1 ms from Er and O coimplanted Si. We ®nd that it is dicult to explain the result in terms of the increased nonradiative transition and that there is a possibility of enhanced radiative transition rate due to coimplantation. In this paper, the free-carrier Auger deexcitation of the rare earth related luminescence is used to study the factors determining the ¯uorescent transition rate at low temperatures to evaluate separately the radiative and nonradiative transition rates, using Er or Ho-implanted semiconductors (Si and GaAs). 2. Experimental 2.1. Sample preparation
*
Corresponding author. Tel.: +81-424-43-5143; fax: +81424-43-5210. E-mail address: [email protected] (T. Kimura).
Samples used in this study are Er-implanted Si (Si:Er), Er and O coimplanted Si (Si:Er±O), Er and
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 5 6 8 - 1
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
287
Table 1 Samples Si host Er O Ne GaAs host Ho
p-Si(100) CZ, q 1±2 X cm 180 keV, 1:0 1013 cm 2 ; Rp 60 nm; Np 2 1018 cm 3 30 keV, 2 1013 cm 2 2 1015 cm 40 keV, 2 1013 cm 2 2 1015 cm
2 2
SI n-GaAs, q 1 107 X cm 180 keV, 1014 cm 2 ; Rp 50 nm; Np 2:5 1019 cm 3 Fig. 1. Auger deexcitation measurement system.
Ne coimplanted Si(Si:Er±Ne), and Ho-implanted GaAs (GaAs:Ho). Ne coimplantation is done to study the eect of implantation induced defects on the Er-related luminescence characteristics and to compare it with the peculiar eect of O coimplantation. The implantation conditions are listed in Table 1. The peak Er concentration (Np 2 1018 cm 3 ) is suciently below the critical concentration (1020 cm 3 ) for the concentration quenching or up-conversion. 2.2. Photoluminescence and Auger deexcitation measurements Photoluminescence spectra are measured using a chopped Ar ion laser line k 488 nm as the excitation light source. The luminescence is dispersed with a single-grating monochromator (Jobin-Yvon HR320) and a cooled germanium pin photodiode. The decay characteristics are measured by exciting samples with an N2 pulse laser line (k 337 nm, pulse width tp 0:35 ns, pulse energy Ep 60 lJ/pulse (1014 photons)). The freecarrier Auger deexcitation is measured by the addition of the Ar ion 488 nm cw laser as shown in Fig. 1. The time resolution of the system used is 10 ls. Sample temperature is below 20 K. The peak intensity Ip of the decay curve is aected by the energy transfer eciency g and is independent of the nonradiative transition of the rare earth, since the pulse width of the N2 laser is extremely short. Note that the peak intensity does not suer any N2 pulse laser induced free-carrier Auger deexcitation due to the time delay for the appearance of the luminescence peak.
3. Results and discussion 3.1. Er-related 1.54 lm emission of Er-implanted Si 3.1.1. Eects of the ion implantation induced defects Fig. 2 shows the post-implantation annealing temperature dependence (annealing time is 1 min) of the N2 laser pulse response of the 1.54 lm Errelated luminescence at 20 K from Er-implanted Si. The luminescence begins to be observable at 600°C. The peak intensity Ip increases and the decay rate decreases with increasing the annealing temperature. The peak intensity Ip which is related to the energy transfer eciency from host to Er is greatly reduced when the e±h recombination rate wrec is increased due to defects. On the other hand, the ¯uorescent decay rate wfl of the 1.54 lm luminescence re¯ects directly the relaxation rate of the excited Er ions from 4 I13=2 ! 4 I15=2 : wfl wr wnr defects:
1
Here, wr denotes the radiative transition rate and wnr (defects) the nonradiative transition rate due to defects. Defect-related nonradiative transition may be a kind of Auger deexcitation due to electrons bound on defect levels. Because of high Er doses 1:0 1013 cm 2 , we expect a very high density of defects and this causes a large amount of the nonradiative transitions, though the Auger energy backtransfer rate to electrons bound on deep traps is expected to be small. At the annealing temperature of 900°C for 1 min, the defect density seems to become too low to aect the the decay of the Er-related luminescence.
288
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Fig. 2. Decay curves of the 1.54 lm luminescence of the Erimplanted Si as functions of post-implantation annealing temperature. Annealing time is 1 min.
3.1.2. Eects of Ne coimplantation induced defects In order to separate the peculiar eects of O coimplantation (which will be presented in the next section) from implantation induced crystalline defects, Ne ions are coimplanted together with Er into Si. The post-implantation anneal is at 1000°C for 1 min. Ne coimplantation is found to give neither new Er-related luminescence peaks in the photoluminescence (Fig. 3) nor any change in the decay rate (Fig. 4) for the Ne peak density below < 1019 cm 3 (Ne/Er 10). 3.1.3. Eect of O ion coimplantation O ion coimplantation reveals a peculiar eect on the Er-related luminescence. As shown in Fig. 4, a new peak with a decay time less than 1 ms was observed to grow after O coimplantation, though the original decay of the luminescence from Si:Er is not aected. The new peak with a fast
Fig. 3. Photoluminescence spectra of the Si:Er, Si:Er±Ne and Si:Er±O. The samples were annealed at 1000°C for 1 min.
decay was ®rst reported by Coa et al. [4], and also by our group [5]. For O concentration below 1019 cm 3 (where the implantation induced defects are annealed out and do not aect the luminescence characteristics), the peak intensity Ip of this new emission increases with O concentration. The ¯uorescent lifetime sfl ( 1=wfl ) of this new emission is below 1 ms. In comparison with the Ne coimplanted samples, the increase in Ip with O dose is due to the increase of O-related Er luminescence centers. In order to study whether the fast decay of the O-related Er luminescence observed here is due to the appearance of some nonradiative transition or due to the increased radiative transition, the freecarrier Auger deexcitation is measured. After Langer's theory [6], the free-carrier Auger deexcitation rate is given by PA;free CA n wr
n ; n0
2
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Fig. 4. Decay curves of the 1.54 lm luminescence of the Si:Er, Si:Er±Ne and Si:Er±O samples. The samples were annealed at 1000°C for 1 min.
where wr is the radiative transition rate of rare earth ions, n is the free-carrier concentration, and n0 is the critical free-carrier concentration. For the 1:54 lm luminescence of Er-doped Si, n0 is 1015 cm 3 [7]. The eect of the Ar cw irradiation on the decay curves of the 1.54 lm luminescence of Si:Er and the fast component of Si:Er±O is shown in Fig. 5. The results for Si:Er±Ne and for the slow component of Si:Er±O are almost the same as that of Si:Er. The Ne and O concentrations are below 1019 cm 3 . It is found that the slow decay (sfl 2 ms) of Si:Er, Si:Er±Ne and Si:Er±O samples shows nearly the same free-carrier Auger deexcitation. However, the fast component of Si:Er±O is much more sensitive to the Ar cw irradiation. Fig. 6 shows the ¯uorescent transition rate wfl as a function of p the square root ofpthe Ar cw irradiation power P (theoretically P / n). The gradient of the plots is proportional to the Auger coecient CA wr =n0 . CA for the fast decay
289
Fig. 5. Eects of the Ar cw irradiation on the decay curves of the 1.54 lm luminescence of the Si:Er±O sample.
component of Si:Er±O is found to be about 5 times larger than that for the slow decay component of the Si:Er, Si:Er±Ne and Si:Er±O samples. This ratio is nearly the same as the ratio of the ¯uorescent decay rate of the two without Ar cw irradiation. This clearly indicates that the observed ¯uorescent lifetimes of the above samples at 20 K are purely the radiative lifetime of the 1.54 lm Er luminescence, and no nonradiative transitions are involved in both the slow and fast components. This also means that the O coimplantation into Si:Er samples forms new 1.54 lm luminescence centers whose radiative lifetime is shorter than the hitherto accepted value of 1±2 ms. 3.2. Ho-implanted GaAs Ho-implanted GaAs shows the Ho-related photoluminescence at 1.19 lm 5 I6 ! 5 I8 at 20 K (Fig. 7). This band shows the ¯uorescent lifetime
290
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Fig. 7. Photoluminescence spectrum of the GaAs:Ho sample.
Fig. 6. Fluorescent transition rate as a function of the square root of the Ar cw irradiation power for the Si:Er, Si:Er±O (fast component) and GaAs:Ho.
ranging between 200 and 400 ls. The enhancement of the ¯uorescent transition rate due to the free-carrier Auger eect is of the same order as the Si:Er for the same Ar cw 488 nm power (Fig. 6). Judging from the fact that the steady-state free-carrier concentration is expected to be much smaller than that in Si due to the shorter recombination time of free- carriers in GaAs, the Auger coecient of the Ho-related luminescence is considered to be much larger than that of Er in Si. Therefore, the observed submillisecond ¯uorescent lifetime is almost the radiative lifetime itself. 3.3. Hints for the reduction of free-carrier Auger deexcitation Recently, our group has reported a strong enhancement of the Er-related luminescence from Er doped porous silicon, when the host porous silicon is slightly pre-oxidized before the Er incorporation
[8]. A similar result is also reported by Shin et al. [9] for Si=SiO2 (2 nm)/SiO2 ±Er superlattices structure. In both cases, the 2 nm thick SiO2 buer layer is very eective to improve the room temperature 1.54 lm Er-related luminescence. These results indicate that the Er ions can be effectively excited even if they are separated by a few nanometers, and at the same time this separation reduces the energy backtransfer due to free-carrier Auger as well as trap-assisted energy backtransfer drastically.
4. Conclusion The radiative transition rates of the Er-implanted Si and Ho-implanted GaAs at low temperatures are evaluated using the free-carrier Auger deexcitation measurements. Coimplantation of O into Si:Er is found to form a new luminescence peak whose radiative transition rate is about 5 times larger than that of Si:Er. Ho-related 1.19 lm luminescence from Ho-implanted GaAs shows that the ¯uorescent lifetime ranges from 200 ls to 400 ls, and this is also considered to be the value of the radiative lifetime.
T. Kimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 286±291
Acknowledgements This work is partly supported by the grant in aid (no. 12450008) by the Ministry of Education, Science, Sports and Culture. The authors thank Prof. A. Polman of FOM Institute for his valuable discussion. We also thank Dr. H. Yasuhara and Dr. A. Sato of Fujikura Co. Ltd. for their support of this research. References [1] H. Ennen, G. Pomrenke, A. Axman, K. Eisele, W. Haydl, J. Schneider, Appl. Phys. Lett. (1985) 381.
291
[2] J.F. Suyver, P.G. Kik, T. Kimura, A. Polman, G. Franz o, S. Coa, Nucl. Instr. and Meth. B 148 (1999) 497. [3] T. Nakanose, T. Kimura, H. Isshiki, S. Yugo, R. Saito, Nucl. Instr. and Meth. B 161±164 (2000) 1080. [4] S. Coa, G. Franz o, F. Priolo, A. Polman, R. Serna, Phys. Rev. B 49 (1994) 16313. [5] T. Kimura, T. Nakanose, W. Wang, H. Isshiki, R. Saito, Nucl. Instr. and Meth. B 148 (1999) 486. [6] A. Suchocki, J.M. Langer, Phys. Rev. B 39 (1989) 7905. [7] F. Priolo, G. Franz o, S. Coa, A. Polman, S. Libertino, R. Barklie, D. Carey, J. Appl. Phys. 78 (1995) 3874. [8] S. Ide, H. Isshiki, S. Yugo, R. Saito, T. Kimura, Extended Abstracts (The 47th Spring Meeting, 2000); The Japan Society of Applied Physics and Related Societies. [9] J.H. Shin, J.-H. Jhe, S.-Y. Seo, Appl. Phys. Lett. 76 (2000) 3567.