*Corresponding author. Tel.: #81-424-43-5143; fax: #81-424-43-5210.

E-mail address: t-kimura@ee.uec.ac.jp (T. Kimura)1Present address: High Reliability Components Corporation,

Shiba Daimon, Minato-ku, Tokyo 105-0012, Japan.

Journal of Luminescence 87}89 (2000) 319}322

Site of the Er3` optical centers of the 1.54 lmroom-temperature emission in Er-doped porous silicon

and the excitation mechanism

Wei Wang1, Hideo Isshiki, Shigemi Yugo, Riichiro Saito, Tadamasa Kimura*

Department of Electronic Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan

Abstract

Sites of the Er3` luminescent centers in Er-doped porous silicon (PS : Er) formed by immersion are studied in order tomake clear the cause of the strong room temperature luminescence at 1.54 lm due to the 4f intra-transition of Er3` ions.The luminescence spectra and the temperature quenching of the intensity and the #uorescence lifetime are comparedbetween PS : Er samples formed by immersion in an ErCl

3/alcohol solution and Er-implanted PS, using the same PS

hosts. PS : Er samples formed by immersion show a small temperature quenching in the intensity and the #uorescencelifetime, resulting in a strong luminescence at RT. On the other hand, PS : Er samples formed by Er ion implantation intoPS shows almost the same strong temperature quenching as the Er-implanted crystalline Si. These results indicate thatthe sites of Er ions responsible for the strong RT 1.54 lm luminescence in PS : Er formed by immersion is not inside Sinanocrystals but on the surface of Si nanocrystals. ( 2000 Elsevier Science B.V. All rights reserved.

Keywords: Er; Porous silicon; Excitation mechanism

1. Introduction

The 1.54 lm emission of Er3`-doped silicon is of greatconcern in the Si-based optoelectronic devices [1}4]. Theproblem is its very weak luminescence intensity at roomtemperature when Er3` 4f electrons are excited by therecombination energy of electrons and holes in hostsilicon. This is explained in terms of energy backtransfer(a reverse process to carrier-mediated excitation) [5,6].The temperature quenching is mitigated when wide-gapsemiconductors are used as hosts [7].

Our group reported for the "rst time that Er-dopedporous silicon (PS) showed a strong Er-related 1.54 lmphotoluminescence at RT, when Er3` ions were incorp-

orated into pores of PS and thereafter annealed at hightemperatures [8]. This "nding triggered lots of studies onthe luminescence of Er-doped PS [9]. We have alsoobserved the Yb-related 1.0 lm peak and the Nd-related0.92 and 1.06 lm peaks [10]. The widening of the ban-dgap of porous silicon is speculated to be responsible forboth the small temperature quenching of the Er-relatedemission as well as the luminescence of rare-earth ions atenergies larger than the bandgap of c-Si. From the photo-luminescence excitation (PLE) experiment conducted byZhao et al. [11], it was con"rmed that Er3` 4f electronswere excited by the recombination energy of electronsand holes which were generated in PS. The question hereis where in PS are located Er ions which are responsiblefor the strong RT emission, when PS consists of variouscrystalline phases (Si nanocrystals, SiO

x(x"0}2)

amorphous layers surrounding the Si nanocrystals andtheir interface region), and how is the e}h recombinationenergy transferred from PS hosts to Er3` 4f electrons.

In this paper, we make an attempt to "nd the sites ofEr3` ions which cause strong RT emission in PS, bycomparing PS : Er samples formed by immersion with

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0022-2313/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 4 2 - 7

Fig. 1. Conceptual diagrams of the structures of the three typesof Er-doped silicon samples prepared in this study. (a) PS :Er(immersed), (b) PS : Er (implanted), and (c) c-Si : Er (implanted).

Fig. 2. Er-related luminescence spectra of the three types ofEr-doped silicon samples at 20 K.

those formed by ion implantation. From the results ob-tained, the excitation mechanism is discussed to obtainsome hints for the improvement of the luminescencee$ciency to a practical use level.

2. Experimental

PS layers 1}10 lm thick are formed by anodic etchingof p-type c-Si (o: 10}20 ) cm) in a HF :C

2H

5OH (1 : 1)

solution at a current density 5}15 mA/cm2 for 10}20 min.The photoluminescence (PL) peak of the PS is located at0.83}0.85 lm. In the immersion method (PS :Er (immer-sed) samples), PS is immersed in an ErCl

3/alcohol solu-

tion for 2}3 h. Er ions are expected to penetrate intopores of PS layers. Then the samples are annealed at10003C for 2 min in Ar. This process, necessary for theemission of the 1.54 lm peak, is considered to enhanceEr ions to react with the surface states of the Si nanocrys-tals and to di!use into Si nanocrystals (the di!usionlength of several nanometers is expected at 10003C for2 min [12]).

Ion implantation doping of PS with Er (PS : Er (im-planted) samples) is carried out at 180 keV at a dose of1]1014 cm~2. The projected range R

1of Er in c-Si at

180 keV is &70 nm with a deviation of &50 nm. PSlayers of 8}10 lm are used to avoid implantation of Erions directly into the c-Si bulk region through pores ofPS layers. Er ions are expected to be incorporated into Sinanocrystals directly as well as in the surroundingSiO

x(x"0}2) amorphous layers. Er-implanted c-Si (c-

Si : Er samples) is also prepared for comparison (the sameenergy and the same dose as in the case of PS : Er (im-planted)). After the implantation, both implanted sam-ples are annealed at 10003C for 2 min in Ar.

Fig. 1 shows conceptual diagrams of three types ofEr-doped Si samples prepared in this study. PS layers areconsiderd to be composed of small Si crystallites or Sinanocrystals, SiO

xlayers covering the Si nanocrystals

and pores of small diameter. In PS :Er (immersed), sitesof Er ions may be possible in every crystalline phase ofPS. Er ions are considered to di!use at least severalnanometers into Si during the annealing process at10003C for 2 min. In PS : Er (implanted), a large portion

of Er ions are considered to be incorporated into Sinanocrystals, judging from R

1"&70 nm in c-Si.

3. Results and discussion

Fig. 2 compares the Er-related luminescence spectra ofthree types of Er-doped Si samples. PS : Er (immersed)shows an Er-related main luminescent peak at 1.535 lm,while the other two show a main peak at 1.539 lm. Notethat the PS-related 0.8 lm band is observed in PS : Er(immersed) samples while it is not observed in the PS : Er(implanted) samples.

As shown in Fig. 3, the 1.535 lm main peak of PS : Er(immersed) disappears by HF etch, while the 1.539 lmpeak of PS : Er (implanted) remains almost unchanged.This result shows that the Er luminescent centers arelocated on the surface of Si nanocrystals or in SiO

xamorphous layers in the PS : Er (immersed) samples,while they are located in Si nanocrystals in the PS : Er(implanted) samples.

Fig. 4 compares the temperautre dependence of the PLdecay time for the three types of samples. The strongroom temperature luminescence of the PS : Er (immersed)samples is directly related with the small temperaturequenching of the decay time (#uorescence lifetime).PS : Er (implanted) and c-Si : Er both show a large tem-perature quenching in the decay time above &100 K.This result suggests that Er ions implanted into the PSlayers are located in the c-Si nanocrystals whose bandgapis almost the same as that of bulk c-Si. In other words, thenanocrystals in PS used in this study are not smallenough to increase the bandgap due to the quantum sizee!ect. Therefore, the site of Er ions responsible for the

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320 W. Wang et al. / Journal of Luminescence 87}89 (2000) 319}322

Fig. 3. Er-related luminescence spectra of PS :Er (immersed)and PS : Er (implanted) before and after HF etching at 20 K.

Fig. 4. Temperature dependence of the PL decay time for thethree types of Er-doped silicon samples.

strong RT 1.54 lm luminescence in PS : Er (immersed) isnot inside nanocrystals, but is on the surface of Si nanoc-rystals, since PLE measurements of PS : Er (immersed)have shown that Er ions are excited by e}h recombina-tion energy. This result also indicates that the lumines-cence of PS itself is related to the surface states of Sinanocrystals.

Putting these results together, we can deduce the fol-lowing concluding remarks. The large temperaturequenching of the 1.54 lm Er-related luminscence (bothintensity and #uorescent lifetime) in PS : Er (implanted)

indicates that the Si nanocrystals of PS used in this studyare not small enough to show quantum size e!ects(widening of the bandgap). Therefore, the luminescenceof PS itself is related with the surface states of Sinanocrystals. In PS : Er (immersed) samples, Er ions arechemically combined with the surface states of PS byannealing at high temperatures after immersion and areexcited by the recombination energy of electrons andholes which are generated in the surface region of siliconnanocrystals.

4. Conclusion

The 1.54 lm photoluminescence characteristics ofEr-doped porous silicon formed by immersion and ionimplantation are compared. The former shows a smalltemperature quenching and illuminates strongly at roomtemperature, while the latter shows a strong temperaturequenching similar to the Er-implanted crystalline silicon.From these results, the site of Er3` ions which showa strong RT emission in Er-doped porous silicon formedby immersion is on the surface of Si nanocrystals, and notin Si nanocrystals. The electron}hole recombination en-ergy of &1.5 eV (&0.8 lm) is transferred to Er 4felectrons via the surface states.

Another important "nding of this study is that theluminescent centers (0.8 lm) of the porous silicon used inthis study may be related to the surface states of nanoc-rystals, and not due to quantum size e!ects. The size ofthe nanocrystals of the porous silicon studied here is notsmall enough to increase the band gap.

Acknowledgements

This work is partly supported by the grant in aid (no.07555135) by the Ministry of Education, Science, Sportsand Culture. We thank Dr. H. Yasuhara and Dr. A. Satoof Fujikura Co. Ltd. for their support of this research. Wethank also Miss. S. Ide for her technical help.

References

[1] H. Ennen, J. Schneider, G. Pomrenke, A. Axmann, Appl.Phys. Lett. 43 (1983) 943.

[2] H. Ennen, G. Pomrenke, A. Axmann, K. Eisele, W. Haydl,J. Schneider, Appl. Phys. Lett. 46 (1985) 381.

[3] J. Michel, J.L. Beneton, R.F. Ferrante, D.C. Jacobson,D.J. Eaglesham, E.A. Fitzgerald, Y.-H. Xie, J.M. Poate,L.C. Kimmerling, J. Appl. Phys. 70 (1991) 2672.

[4] G. FranzoH , F. Priolo, S. Co!a, A. Polman, A. Canera,Appl. Phys. Lett. 64 (1994) 2235.

[5] A. Taguchi, H. Nakagome, K. Takahei, J. Appl. Phys. 70(1991) 5604.

Lumin=5082=Rohini=VVC=BG

W. Wang et al. / Journal of Luminescence 87}89 (2000) 319}322 321

[6] A. Taguchi, K. Takahei, J. Appl. Phys. 83 (1998) 2800.[7] P.N. Favennec, H. L'Haridon, D. Moutonnet, M. Salvi,

M. Gauneau, Jpn. J. Appl. Phys. 29 (1990) L521.[8] T. Kimura, A. Yokoi, H. Horiguchi, R. Saito, T. Ikoma,

A. Sato, Appl. Phys. Lett. 65 (1994) 983.[9] J.H. Shin, G.N. van den Hoven, A. Polman, Appl. Phys.

Lett. 66 (1995) 2379.

[10] T. Kimura, A. Yokoi, Y. Nishida, R. Saito, S. Yugo,T. Ikoma, Appl. Phys. Lett. 67 (1995) 2687.

[11] X. Zhao, S. Komuro, S. Maruyama, H. Isshiki, Y. Aoyagi,T. Sugano, Rare Earth Doped Semiconductors II Sympo-sium, 1996, p. 143.

[12] D.E. Nazyrov, G.S. Kulikov, R.Sh. Malkovich, Fiz. Tekh.Poluprovodn. (Russia) 25 (1991) 1653.

Lumin=5082=Rohini=VVC=BG

322 W. Wang et al. / Journal of Luminescence 87}89 (2000) 319}322