- Email: [email protected]
An effect of Si nanoparticles on enhancing Er3+ electroluminescence in Si-rich SiO2:Er films
PERGAMON
Solid State Communications 118 (2001) 599±602
www.elsevier.com/locate/ssc
An effect of Si nanoparticles on enhancing Er 31 electroluminescence in Si-rich SiO2:Er ®lms G.Z. Ran a, Y. Chen a, F.C. Yuan a, Y.P. Qiao a, J.S. Fu a, Z.C. Ma b, W.H. Zong b, G.G. Qin a,c,* a
Department of Physics, Peking University, Beijing 100871, People's Republic of China National Key Laboratory for ASIC, HSRI, Shijiazhuang 050051, People's Republic of China c International Center for Materials Physics, Academia Sinica, Shenyang 110015, People's Republic of China b
Received 18 February 2001; accepted 16 March 2001 by Z.Z. Gan
Abstract Er-doped Si-rich SiO2 (SRSO:Er) ®lms have been deposited on n 1-Si substrates by the magnetron sputtering technique, and both photoluminescence (PL) and electroluminescence (EL) at 1.54 mm have been observed from the ®lms at room temperature. Dependence of PL and EL intensities on the excess-Si content and annealing temperature has been studied. It is found that proper Si content and annealing temperature can evidently enhance EL intensity. An SRSO:Er ®lm with 20% excess Si (area ratio of the Si target to the whole target) had more intense EL than a SiO2:Er ®lm without excess Si, both annealed at 8008C, by a factor of 5. This fact clearly demonstrates that energy coupling between Si nanometer particles and Er 31 ions also exists in the EL process as well as in the PL process. Experimental results also indicate that crystallization is not a prerequisite for NSPs enhancing luminescence in SRSO:Er ®lms. The PL and EL spectra of the SRSO:Er ®lms have much broader full widths at half maximum (FWHM, ,60 nm) than those of the other Er-doped materials reported. This wide FWHM can perhaps be used in wavelength division multiplexing in optical communication in future. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 73.61.Tm; 78.55.2m; 78.60.Fi; 71.20.Eh Keywords: A. Thin ®lms; A. Nanostructures; D. Recombination and trapping; E. Luminescence
The realization of practical Si-based light emission including laser, attracts much interest due to its potential application in integrated optoelectronics [1,2]. Many good methods of materials engineering have been employed in attempt to overcome the inherent de®ciency of weak light emission from silicon [3], among which Er-doping into Si is considered to be a promising way [4±8]. Luminescence at 1.54 mm in the Er-doped materials resulting from 4f optical transition 4I13/2 ! 4I15/2 in Er 31 coincides with the absorption minimum window of silica-based optical ®bers. In latest years, silicon-rich SiO2 (SRSO) has been found very suitable as a host material of erbium [8±12], because it contains suf®cient oxygen natively, bene®cial to optical * Corresponding author. Address: Department of Physics, Peking University, Beijing 100871, People's Republic of China. Tel.: 1861-250-1732; fax: 186-1-627-51615. E-mail address: [email protected] (G.G. Qin).
activation of Er; a large amount of nanometer silicon particles (NSPs), bene®cial to excitation of Er 31; and has a wide energy gap, bene®cial to suppression of energy backtransfer. NSPs in an SRSO:Er materials have been shown to be very effective in enhancing photoluminescence (PL) intensity, but to our knowledge the effect of NSPs on enhancing electroluminescence (EL) intensity has not been demonstrated so far. In this study, we did such a demonstration and con®rmed that the NSP concentration and annealing temperature are the crucial factors for enhancing intensity of Er 31 EL from the SRSO:Er ®lms. The substrates used were (100)-oriented, 10 22 V cm n 1type Si wafers. The magnetron sputtering (ION Tech INC MPS-3000 FC) was performed under a base vacuum of ,10 25 Pa at 2008C. A SiO2 ±Si±Er composite target was employed in depositing SRSO:Er ®lms. In different magnetron sputtering processes, the percentage area of the Si target in the composite SiO2 ±Si±Er target was changed from 0 to
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00180-6
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G.Z. Ran et al. / Solid State Communications 118 (2001) 599±602
Fig. 1. I±V characteristics of the Au/SiO2:Er/n 1-Si and Au/SRSO (20% excess Si): Er/n 1-Si structures with the active ®lms unannealed or annealed at 8008C.
10, 20 and 30%, while the percentage area of the Er target was ®xed at 1%. An X-ray photoelectron spectroscopy study demonstrated that the excess-Si content increased with increasing Si-target area proportionally [13]. Hereafter, the excess-Si content is described using the percentage area of Si target in the composite target. The thickness of the SRSO: Er ®lms, controlled by deposition time, were 12 nm. Deposition rates of SiO2 and Si were determined by separately depositing thick SiO2 and Si layers and measuring their thickness using a surface pro®le meter. The above ®lms were annealed separately at 600, 700, 800, 900, and 10008C for 30 min in N2. A spectrometer consisted of a single-grating monochromator, a liquid-nitrogen-cooled Ge detector, and a lock-in ampli®er. PL was excited by the 488 nm line of an Ar laser, chopped at 200 Hz. For EL measurements, Al ohmic contacts on the backside of the
Fig. 2. PL spectra of SRSO:Er ®lms containing 1% Er and 0, 10, 20 and 30% excess Si, annealed at 8008C.
Fig. 3. EL spectra of the Au/SRSO:Er/n 1-Si structures, with the SRSO:Er ®lms containing 1% Er and 0, 10, 20 and 30% excess Si, annealed at 8008C.
n 1-Si wafers and the Au ®lms upon the SRSO:Er ®lms were formed, where a 1.0 mm £ 1.0 mm hole in the Al ®lm had been left for light-collection. The Au/SRSO:Er/ n 1-Si diodes were biased by square voltage pulses with a frequency of 200 Hz, and a duty cycle of 1:2. Fig. 1 represents the typical I±V characteristics of the Au/ SiO2:Er/n 1-Si and Au/SRSO:Er/n 1-Si diodes unannealed and annealed at 8008C. A reverse bias means that the n 1Si substrate was biased positive. The threshold voltage for current increasing sharply under a forward bias was only slightly smaller than that under a reverse bias, that is, little sign of rectifying has been observed. An Au/SiO2:Er/n 1-Si diode had a smaller current than an Au/SRSO:Er/n 1-Si diode did under an identical bias. Both the forward and reverse currents of the two types of diodes decreased a little after an 8008C annealing. Figs. 2 and 3 show, respectively, room temperature PL and EL spectra of the SRSO:Er ®lms containing excess-Si contents of 0, 10, 20 and 30% and in all the ®lms the Er concentration was 1% (area ratio of the Er target to the whole target). All the EL spectra were measured at reverse biases of 6 V. Quite strong Er 31 PL and EL at 1.54 mm have been observed. As shown in Fig. 2, PL intensity of the SRSO:Er ®lm containing 20% excess Si is most intense and about 15 times more intense than that of the SiO2:Er ®lm. As for EL, the intensity of the SRSO:Er ®lm containing 20% excess Si is also most intense and about ®ve times more intense than that of the SiO2:Er ®lm. From Figs. 2 and 3, we can also ®nd that the full widths at half maximum (FWHM) of the spectra are ,60 nm, which are broader than those of the other Er-doped materials reported [4±8]. Fig. 4a,b show, respectively, PL and EL intensities of the samples containing different excess-Si contents as functions of annealing temperature. For each sample shown in Fig. 4a, there is an optimum annealing temperature leading to the
G.Z. Ran et al. / Solid State Communications 118 (2001) 599±602
Fig. 4. (a) PL and (b) EL intensities as functions of annealing temperature for the SRSO:Er ®lms containing 1% Er and 0, 10, 20 or 30% excess Si.
maximum of PL intensity. The optimum annealing temperatures are 700, 800, 900, and 9008C for the SRSO:Er ®lm containing excess Si of 30, 20, 10, and 0%, respectively. As can be seen, the more the Si content, the lower will be the optimum annealing temperature. As for EL, there is a similar annealing behavior. A minor difference lies in that for the SRSO:Er ®lm containing 30% excess Si the EL
Fig. 5. Schematic representation of the possible NSP-related Er 31 excitation mechanism in the EL process of an Au/SRSO:Er/n 1-Si structure under a reverse bias.
601
intensity decreases much more rapid with increasing annealing temperature than the PL intensity does above 8008C. As mentioned above, the EL intensity of the SRSO:Er ®lm containing 20% excess Si is about 5 times more intense than that of the SiO2:Er ®lms without excess Si, both annealed at 8008C. We consider, it should be attributed to the fact, a lot of NSPs exist in the SRSO:Er ®lm rather than in the SiO2:Er ®lm. Because of the thin thickness of the SRSO:Er and SiO2:Er ®lms (12 nm), the average electrical ®elds in the ®lms under a bias of 6 V reached values as high as 5 £ 10 6 V/cm, so the SRSO ®lm may start to breakdown on some points due to nonuniformity and/or defectiveness of the ®lms. The electrons and holes generated in the breakdown process relax from the conduction and valence band of the SiO2 to the unoccupied molecular orbits and the occupied molecular orbits of the NSPs, respectively, then can be trapped by various defects, in the SiO2:Er and SRSO:Er ®lms to form bound excitons. The energies dissipated in the nonradiative recombination processes of these excitons can excite Er 31, as shown schematically in Fig. 5. Based on this supposition, we can tentatively explain why the EL and PL spectrum of the SRSO:Er ®lm had a wide FWHM: due to variety of defects that exist in SiO2:Er and SRSO:Er ®lms, so various bound excitons with different recombination energies can form. Thus, Er 31 can be excited to different excited states, leading to the luminescence at different wavelengths around 1.54 mm. However, it seems that NSPs are less effective in enhancing EL intensity than enhancing PL intensity. The different roles of the NSPs in EL and PL processes may be responsible for this fact. In the Er 31 PL process of SRSO:Er ®lms, almost all the photogenerated carriers come from the NSPs, so the existence of NSPs affects PL intensity strongly. But in the Er 31 EL process of SRSO:Er ®lms, the NSP-related Er 31 excitation shown in Fig. 5 is just one of the various Er 31 excitation paths. There are some Er 31 excitation processes, which are nothing to do with NSPs, for example: (1) the electrons and holes injected outside the NSPs can also be captured by the defects in Si oxide to form bound excitons, and their nonradiative recombination can excite Er 31; (2) the hot carriers generated in the breakdown process of the SRSO ®lms impact Er 31 and excite them directly. All these Er 31 excitation processes compete with the NSP-related Er 31 excitation process. Thus, the effect of NSPs on enhancing EL intensity is weaker than on enhancing PL intensity. Although the NSPs play an important role in both Er 31 PL and EL processes, as can be seen, however, we cannot conclude that the more excess-Si content, the more intense would be the luminescence. For the Er 31 PL, we consider, the intensity of an SRSO ®lm is mainly determined by two factors: concentration of NSPs and optically activated Er 31. Raising excess Si content in the SRSO:Er ®lm can increase the former, nevertheless, decrease the latter because more excess Si means that oxygen are relatively de®cient. Twenty per cent excess Si may represent a compromise between
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G.Z. Ran et al. / Solid State Communications 118 (2001) 599±602
these two opposite effects. In addition, more excess Si in the SRSO:Er ®lm results in larger-sized NSPs, and too large NSPs, which have too small energy gaps, are less effective in enhancing Er 31 luminescence [9]. Such an explanation may hold in the Er 31 EL process. As for the annealing behavior of the Er 31 PL and EL intensities, an obvious characteristic is that the more excess Si, the lower would be the optimum annealing temperature. We consider that the optimum temperature for Er 31 luminescence is determined by the annealing behavior of both the NSPs and Er-O luminescence centers [12]. For the SiO2: Er ®lms with few NSPs, because the Er-O luminescence centers increase with increasing annealing temperature below 9008C and begin to dissociate above 9008C [12], the optimum annealing temperature is about 9008C. For the SRSO:Er ®lms containing NSPs, the effect of the NSPs should be taken into account. The NSPs in the SRSO:Er ®lm containing more excess Si are easier to grow up in size with increasing annealing temperature and too large NSPs are ineffective in excitation of Er 31 [9], therefore, the more excess Si, the lower would be the optimum annealing temperature. As far as we know, crystalline NSPs in SRSO ®lms have not been observed when the annealing temperature is below 9008C [13,14]. The fact that the optimum annealing temperatures for the SRSO:Er ®lms containing 20 and 30% excess Si are below 9008C means that crystallization of NSPs is not a prerequisite for enhancement of luminescence. In summary, we have deposited Er-doped Si-rich SiO2 ®lms on n 1 substrates using the magnetron sputtering technique, and room temperature PL and EL at 1.54 mm have been observed. PL and EL intensities are strongly dependent on the excess-Si content and annealing temperature. It was found that the sample containing 20% excess Si annealed at 8008C, produces the most intense Er 31 PL and EL. The experimental results demonstrate that the existence of NSPs can improve intensity of EL from the SRSO:Er ®lms, however, the improvement effect is not so great as in the PL case. Experimental results indicate that crystallization is not a prerequisite for NSPs enhancing lumi-
nescence in SRSO:Er ®lms. The wide FWHM of PL and EL spectra of the SRSO:Er ®lms can perhaps be used in wavelength division multiplexing in optical communication in future.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the State Key Laboratory on Integrated Optoelectronics.
References [1] L. Canham, Nature 408 (2000) 411. [2] L. Pavesi, L. Dal Negro, C. Mazzileni, G. FranzoÁ, F. Priolo, Nature 408 (2000) 440. [3] Light emission from Si, Progress towards Si-based optoelectronics, E-MRS Spring Conference, June 1998. [4] H. Ennen, G. Pomrenke, A. Axmann, K. Eisele, W. Haydl, J. Schneider, Appl. Phys. Lett. 46 (1985) 381. [5] B. Zheng, J. Michel, F.Y.G. Ren, L.C. Kimerling, D.C. Jacobson, J.M. Poate, Appl. Phys. Lett. 64 (21) (1994) 2842. [6] S. Lombardo, S.U. Campisano, G.N. van den Hoven, A. Polman, J. Appl. Phys. 77 (2) (1995) 6504. [7] J. Stimmer, A. Reittinger, J.F. NuÈtzel, G. Abstreiter, H. Holzbrecher, Ch. Buchal, Appl. Phys. Lett. 68 (1996) 3290. Á , V. Vincigurerra, F. Priolo, Appl. Phys. A, [8] G. FranzO published online: 2 June 1999. [9] M. Fujji, M. Yoshida, S. Hayashi, K. Yamamoto, J. Appl. Lett. 84 (8) (1998) 4525. [10] C.E. Chryssou, A.J. Kenyon, T.S. Iwayama, C.W. Pitt, D.E. Hole, Appl. Phys. Lett. 75 (4) (1999) 2011. [11] L. Tsybeskov, S.P. Duttagupta, K.D. Hirschman, P.M. Fauchet, K.L. Moore, D.G. Hall, Appl. Phys. Lett. 70 (14) (1997) 1790. [12] H.-S. Han, S.-y. Seo, J.H. Shin, D.-s. Kim, J. Appl. Phys. 88 (4) (2000) 2160. [13] L.P. You, C.L. Heng, Z.C. Ma, W.H. Zong, Z.L. Wu, S.Y. Ma, G.G. Qin, J. Cryst. Growth 212 (2000) 109±114. [14] M. Cheong, W. Paul, S.P. Withrow, J.G. Zhu, J.D. Budai, C.M. White, D.G. Hembree, Appl. Phys. Lett. 68 (1996) 78.
Solid State Communications 118 (2001) 599±602
www.elsevier.com/locate/ssc
An effect of Si nanoparticles on enhancing Er 31 electroluminescence in Si-rich SiO2:Er ®lms G.Z. Ran a, Y. Chen a, F.C. Yuan a, Y.P. Qiao a, J.S. Fu a, Z.C. Ma b, W.H. Zong b, G.G. Qin a,c,* a
Department of Physics, Peking University, Beijing 100871, People's Republic of China National Key Laboratory for ASIC, HSRI, Shijiazhuang 050051, People's Republic of China c International Center for Materials Physics, Academia Sinica, Shenyang 110015, People's Republic of China b
Received 18 February 2001; accepted 16 March 2001 by Z.Z. Gan
Abstract Er-doped Si-rich SiO2 (SRSO:Er) ®lms have been deposited on n 1-Si substrates by the magnetron sputtering technique, and both photoluminescence (PL) and electroluminescence (EL) at 1.54 mm have been observed from the ®lms at room temperature. Dependence of PL and EL intensities on the excess-Si content and annealing temperature has been studied. It is found that proper Si content and annealing temperature can evidently enhance EL intensity. An SRSO:Er ®lm with 20% excess Si (area ratio of the Si target to the whole target) had more intense EL than a SiO2:Er ®lm without excess Si, both annealed at 8008C, by a factor of 5. This fact clearly demonstrates that energy coupling between Si nanometer particles and Er 31 ions also exists in the EL process as well as in the PL process. Experimental results also indicate that crystallization is not a prerequisite for NSPs enhancing luminescence in SRSO:Er ®lms. The PL and EL spectra of the SRSO:Er ®lms have much broader full widths at half maximum (FWHM, ,60 nm) than those of the other Er-doped materials reported. This wide FWHM can perhaps be used in wavelength division multiplexing in optical communication in future. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 73.61.Tm; 78.55.2m; 78.60.Fi; 71.20.Eh Keywords: A. Thin ®lms; A. Nanostructures; D. Recombination and trapping; E. Luminescence
The realization of practical Si-based light emission including laser, attracts much interest due to its potential application in integrated optoelectronics [1,2]. Many good methods of materials engineering have been employed in attempt to overcome the inherent de®ciency of weak light emission from silicon [3], among which Er-doping into Si is considered to be a promising way [4±8]. Luminescence at 1.54 mm in the Er-doped materials resulting from 4f optical transition 4I13/2 ! 4I15/2 in Er 31 coincides with the absorption minimum window of silica-based optical ®bers. In latest years, silicon-rich SiO2 (SRSO) has been found very suitable as a host material of erbium [8±12], because it contains suf®cient oxygen natively, bene®cial to optical * Corresponding author. Address: Department of Physics, Peking University, Beijing 100871, People's Republic of China. Tel.: 1861-250-1732; fax: 186-1-627-51615. E-mail address: [email protected] (G.G. Qin).
activation of Er; a large amount of nanometer silicon particles (NSPs), bene®cial to excitation of Er 31; and has a wide energy gap, bene®cial to suppression of energy backtransfer. NSPs in an SRSO:Er materials have been shown to be very effective in enhancing photoluminescence (PL) intensity, but to our knowledge the effect of NSPs on enhancing electroluminescence (EL) intensity has not been demonstrated so far. In this study, we did such a demonstration and con®rmed that the NSP concentration and annealing temperature are the crucial factors for enhancing intensity of Er 31 EL from the SRSO:Er ®lms. The substrates used were (100)-oriented, 10 22 V cm n 1type Si wafers. The magnetron sputtering (ION Tech INC MPS-3000 FC) was performed under a base vacuum of ,10 25 Pa at 2008C. A SiO2 ±Si±Er composite target was employed in depositing SRSO:Er ®lms. In different magnetron sputtering processes, the percentage area of the Si target in the composite SiO2 ±Si±Er target was changed from 0 to
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00180-6
600
G.Z. Ran et al. / Solid State Communications 118 (2001) 599±602
Fig. 1. I±V characteristics of the Au/SiO2:Er/n 1-Si and Au/SRSO (20% excess Si): Er/n 1-Si structures with the active ®lms unannealed or annealed at 8008C.
10, 20 and 30%, while the percentage area of the Er target was ®xed at 1%. An X-ray photoelectron spectroscopy study demonstrated that the excess-Si content increased with increasing Si-target area proportionally [13]. Hereafter, the excess-Si content is described using the percentage area of Si target in the composite target. The thickness of the SRSO: Er ®lms, controlled by deposition time, were 12 nm. Deposition rates of SiO2 and Si were determined by separately depositing thick SiO2 and Si layers and measuring their thickness using a surface pro®le meter. The above ®lms were annealed separately at 600, 700, 800, 900, and 10008C for 30 min in N2. A spectrometer consisted of a single-grating monochromator, a liquid-nitrogen-cooled Ge detector, and a lock-in ampli®er. PL was excited by the 488 nm line of an Ar laser, chopped at 200 Hz. For EL measurements, Al ohmic contacts on the backside of the
Fig. 2. PL spectra of SRSO:Er ®lms containing 1% Er and 0, 10, 20 and 30% excess Si, annealed at 8008C.
Fig. 3. EL spectra of the Au/SRSO:Er/n 1-Si structures, with the SRSO:Er ®lms containing 1% Er and 0, 10, 20 and 30% excess Si, annealed at 8008C.
n 1-Si wafers and the Au ®lms upon the SRSO:Er ®lms were formed, where a 1.0 mm £ 1.0 mm hole in the Al ®lm had been left for light-collection. The Au/SRSO:Er/ n 1-Si diodes were biased by square voltage pulses with a frequency of 200 Hz, and a duty cycle of 1:2. Fig. 1 represents the typical I±V characteristics of the Au/ SiO2:Er/n 1-Si and Au/SRSO:Er/n 1-Si diodes unannealed and annealed at 8008C. A reverse bias means that the n 1Si substrate was biased positive. The threshold voltage for current increasing sharply under a forward bias was only slightly smaller than that under a reverse bias, that is, little sign of rectifying has been observed. An Au/SiO2:Er/n 1-Si diode had a smaller current than an Au/SRSO:Er/n 1-Si diode did under an identical bias. Both the forward and reverse currents of the two types of diodes decreased a little after an 8008C annealing. Figs. 2 and 3 show, respectively, room temperature PL and EL spectra of the SRSO:Er ®lms containing excess-Si contents of 0, 10, 20 and 30% and in all the ®lms the Er concentration was 1% (area ratio of the Er target to the whole target). All the EL spectra were measured at reverse biases of 6 V. Quite strong Er 31 PL and EL at 1.54 mm have been observed. As shown in Fig. 2, PL intensity of the SRSO:Er ®lm containing 20% excess Si is most intense and about 15 times more intense than that of the SiO2:Er ®lm. As for EL, the intensity of the SRSO:Er ®lm containing 20% excess Si is also most intense and about ®ve times more intense than that of the SiO2:Er ®lm. From Figs. 2 and 3, we can also ®nd that the full widths at half maximum (FWHM) of the spectra are ,60 nm, which are broader than those of the other Er-doped materials reported [4±8]. Fig. 4a,b show, respectively, PL and EL intensities of the samples containing different excess-Si contents as functions of annealing temperature. For each sample shown in Fig. 4a, there is an optimum annealing temperature leading to the
G.Z. Ran et al. / Solid State Communications 118 (2001) 599±602
Fig. 4. (a) PL and (b) EL intensities as functions of annealing temperature for the SRSO:Er ®lms containing 1% Er and 0, 10, 20 or 30% excess Si.
maximum of PL intensity. The optimum annealing temperatures are 700, 800, 900, and 9008C for the SRSO:Er ®lm containing excess Si of 30, 20, 10, and 0%, respectively. As can be seen, the more the Si content, the lower will be the optimum annealing temperature. As for EL, there is a similar annealing behavior. A minor difference lies in that for the SRSO:Er ®lm containing 30% excess Si the EL
Fig. 5. Schematic representation of the possible NSP-related Er 31 excitation mechanism in the EL process of an Au/SRSO:Er/n 1-Si structure under a reverse bias.
601
intensity decreases much more rapid with increasing annealing temperature than the PL intensity does above 8008C. As mentioned above, the EL intensity of the SRSO:Er ®lm containing 20% excess Si is about 5 times more intense than that of the SiO2:Er ®lms without excess Si, both annealed at 8008C. We consider, it should be attributed to the fact, a lot of NSPs exist in the SRSO:Er ®lm rather than in the SiO2:Er ®lm. Because of the thin thickness of the SRSO:Er and SiO2:Er ®lms (12 nm), the average electrical ®elds in the ®lms under a bias of 6 V reached values as high as 5 £ 10 6 V/cm, so the SRSO ®lm may start to breakdown on some points due to nonuniformity and/or defectiveness of the ®lms. The electrons and holes generated in the breakdown process relax from the conduction and valence band of the SiO2 to the unoccupied molecular orbits and the occupied molecular orbits of the NSPs, respectively, then can be trapped by various defects, in the SiO2:Er and SRSO:Er ®lms to form bound excitons. The energies dissipated in the nonradiative recombination processes of these excitons can excite Er 31, as shown schematically in Fig. 5. Based on this supposition, we can tentatively explain why the EL and PL spectrum of the SRSO:Er ®lm had a wide FWHM: due to variety of defects that exist in SiO2:Er and SRSO:Er ®lms, so various bound excitons with different recombination energies can form. Thus, Er 31 can be excited to different excited states, leading to the luminescence at different wavelengths around 1.54 mm. However, it seems that NSPs are less effective in enhancing EL intensity than enhancing PL intensity. The different roles of the NSPs in EL and PL processes may be responsible for this fact. In the Er 31 PL process of SRSO:Er ®lms, almost all the photogenerated carriers come from the NSPs, so the existence of NSPs affects PL intensity strongly. But in the Er 31 EL process of SRSO:Er ®lms, the NSP-related Er 31 excitation shown in Fig. 5 is just one of the various Er 31 excitation paths. There are some Er 31 excitation processes, which are nothing to do with NSPs, for example: (1) the electrons and holes injected outside the NSPs can also be captured by the defects in Si oxide to form bound excitons, and their nonradiative recombination can excite Er 31; (2) the hot carriers generated in the breakdown process of the SRSO ®lms impact Er 31 and excite them directly. All these Er 31 excitation processes compete with the NSP-related Er 31 excitation process. Thus, the effect of NSPs on enhancing EL intensity is weaker than on enhancing PL intensity. Although the NSPs play an important role in both Er 31 PL and EL processes, as can be seen, however, we cannot conclude that the more excess-Si content, the more intense would be the luminescence. For the Er 31 PL, we consider, the intensity of an SRSO ®lm is mainly determined by two factors: concentration of NSPs and optically activated Er 31. Raising excess Si content in the SRSO:Er ®lm can increase the former, nevertheless, decrease the latter because more excess Si means that oxygen are relatively de®cient. Twenty per cent excess Si may represent a compromise between
602
G.Z. Ran et al. / Solid State Communications 118 (2001) 599±602
these two opposite effects. In addition, more excess Si in the SRSO:Er ®lm results in larger-sized NSPs, and too large NSPs, which have too small energy gaps, are less effective in enhancing Er 31 luminescence [9]. Such an explanation may hold in the Er 31 EL process. As for the annealing behavior of the Er 31 PL and EL intensities, an obvious characteristic is that the more excess Si, the lower would be the optimum annealing temperature. We consider that the optimum temperature for Er 31 luminescence is determined by the annealing behavior of both the NSPs and Er-O luminescence centers [12]. For the SiO2: Er ®lms with few NSPs, because the Er-O luminescence centers increase with increasing annealing temperature below 9008C and begin to dissociate above 9008C [12], the optimum annealing temperature is about 9008C. For the SRSO:Er ®lms containing NSPs, the effect of the NSPs should be taken into account. The NSPs in the SRSO:Er ®lm containing more excess Si are easier to grow up in size with increasing annealing temperature and too large NSPs are ineffective in excitation of Er 31 [9], therefore, the more excess Si, the lower would be the optimum annealing temperature. As far as we know, crystalline NSPs in SRSO ®lms have not been observed when the annealing temperature is below 9008C [13,14]. The fact that the optimum annealing temperatures for the SRSO:Er ®lms containing 20 and 30% excess Si are below 9008C means that crystallization of NSPs is not a prerequisite for enhancement of luminescence. In summary, we have deposited Er-doped Si-rich SiO2 ®lms on n 1 substrates using the magnetron sputtering technique, and room temperature PL and EL at 1.54 mm have been observed. PL and EL intensities are strongly dependent on the excess-Si content and annealing temperature. It was found that the sample containing 20% excess Si annealed at 8008C, produces the most intense Er 31 PL and EL. The experimental results demonstrate that the existence of NSPs can improve intensity of EL from the SRSO:Er ®lms, however, the improvement effect is not so great as in the PL case. Experimental results indicate that crystallization is not a prerequisite for NSPs enhancing lumi-
nescence in SRSO:Er ®lms. The wide FWHM of PL and EL spectra of the SRSO:Er ®lms can perhaps be used in wavelength division multiplexing in optical communication in future.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the State Key Laboratory on Integrated Optoelectronics.
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