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Enhanced emission of Er3+ from alternately Er doped Si-rich Al2O3 multilayer film with Si nanocrystals as broadband sensitizers
Applied Surface Science 258 (2012) 1896–1901
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhanced emission of Er3+ from alternately Er doped Si-rich Al2 O3 multilayer film with Si nanocrystals as broadband sensitizers Xiao Wang a , Zuimin Jiang a , Fei Xu b,c,∗ , Zhongquan Ma b , Run Xu d , Bin Yu b , Mingzhu Li a , Lingling Zheng a , Yongliang Fan a , Jian Huang d , Fang Lu a a
Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai 200433, China SHU-SolarE R&D Lab, Department of Physics, College of Sciences, Key Laboratory for Material Microstructures, Shanghai University, Shanghai 200444, China c Instituto de Óptica, CSIC, Serrano 121, 28006 Madrid, Spain d Department of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China b
a r t i c l e
i n f o
Article history: Available online 4 August 2011 PACS: 78.20.−e 78.55.−m 78.67.Bf 81.15.Cd Keywords: Erbium doping Silicon rich Al2 O3 Silicon nanocrystals
a b s t r a c t Alternately Er doped Si-rich Al2 O3 (Er:SRA) multilayer film, consisting of alternate Er–Si-codoped Al2 O3 (Er:Si:Al2 O3 ) and Si-doped Al2 O3 (Si:Al2 O3 ) sublayers, has been synthesized by co-sputtering from separated Er, Si, and Al2 O3 targets. The dependence of Er3+ related photoluminescence (PL) properties on annealing temperatures over 700–1100 ◦ C was studied. The maximum intensity of Er3+ PL, about 10 times higher than that of the monolayer film, was obtained from the multilayer film annealed at 950 ◦ C. The enhancement of Er3+ PL intensity is attributed to the energy transfer from the silicon nanocrystals in the Si:Al2 O3 sublayers to the neighboring Er3+ ions in the Er:Si:Al2 O3 sublayers. The PL intensity exhibits a nonmonotonic temperature dependence: with increasing temperature, the integrated intensity almost remains constant from 14 to 50 K, then reaches maximum at 225 K, and slightly increases again at higher temperatures. Meanwhile, the PL integrated intensity at room temperature is about 30% higher than that at 14 K. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Er-doped materials are of great interest in optical communication technology, because they can serve as the gain medium in optical amplifiers operating at the standard telecommunication wavelength of 1.53 m. The fabrication of efficient light-emitting devices operating at room temperature (RT) from Er-doped Si appeared to be impractical due to their low luminescence efficiency [1]. Due to the low absorption cross-section of Er3+ and the temperature quenching behavior in Si, the host matrix has been changed from Si to SiO2 -based materials [2]. Meanwhile, in order to increase the effective excitation of Er3+ ions, sensitizers such as Si nanocrystals (Si-NCs) and Yb3+ ions in SiO2 materials have been adopted and investigated [2,3]. However, to further improve Er3+ emission at 1.53 m, a deeper understanding of the mechanism of the energy transfer (ET) from Si-NCs to Er3+ ions as well as the optimization of size-controlled Si-NCs distribution in SiO2 matrix is required [4–6]. For instance, in order to control the distri-
∗ Corresponding author at: SHU-SolarE R&D Lab, Department of Physics, College of Sciences, Key Laboratory for Material Microstructures, Shanghai University, Shanghai 200444, China. E-mail address: [email protected] (F. Xu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.064
butions of Er and Si, using molecular beam epitaxy, Xu et al. [7] synthesized the erbium–oxygen-codoped silicon multilayer film consisting of alternate erbium–oxygen-codoped silicon (Er:O:Si) and oxygen-doped silicon (O:Si) layers. Recently Gourbilleau et al. [8,9] employed rf reactive magnetron sputtering to fabricate Erdoped SRSO multilayer films consisting in the stacking of SRSO/SiO2 sublayers in which the erbium has been incorporated closer to the Si-NCs, resulting in the enhancement of 1.53 m emission. Al2 O3 is an interesting host material for optical doping because waveguide fabrication technology based on this material has been well developed [10,11]. The waveguides formed by relatively high index Al2 O3 confine light efficiently, making compact device structures possible. Furthermore, the similarity of valency and lattice constant between Al2 O3 and Er2 O3 may allow for incorporation of high concentrations of Er into Al2 O3 . In fact light emission from Er-doped Al2 O3 at RT has been demonstrated [10,12,13]. As high concentrations of Er can be incorporated into Al2 O3, and Si-NCs as sensitizers can transfer energy to Er effectively, Er-doped Si-rich Al2 O3 (Er:SRA), specially Er:SRA multilayer film, may be a promising material. Nevertheless, it was rarely studied [14–16]. In this work, we present an approach to fabricate alternately Er doped Si-rich Al2 O3 multilayer film by magnetron sputtering. Photoluminescence (PL) spectra of Er3+ in the multilayer films with different annealing temperatures over the range of 700–1100 ◦ C
X. Wang et al. / Applied Surface Science 258 (2012) 1896–1901
Fig. 1. Schematic diagram of (a) Er:SRA multilayer film and (b) Er:SRA monolayer film.
were measured. The PL intensity of Er3+ in the multilayer film was found much higher than that in the monolayer film and exhibited a nonmonotonic temperature dependence. Moreover, at RT the intensity of Er3+ PL from the annealed multilayer film was found higher than that at 14 K. 2. Experimental details The substrates used were Czochralski (CZ) grown p-type Si (1 0 0) single crystals with the resistivity of 3–5 cm. Er:SRA multilayer film was deposited on it by magnetron sputtering from separated Er, Si and Al2 O3 target. The multilayer film, consisting of 20 layers of alternate Er–Si-codoped Al2 O3 (Er:Si:Al2 O3 ) and Sidoped Al2 O3 (Si:Al2 O3 ). Each Er:Si:Al2 O3 sublayer was deposited for 9 min, followed by 3 min deposition of Si:Al2 O3 layer. The average thicknesses of Er:Si:Al2 O3 and Si:Al2 O3 sublayers were about 6 nm and 2 nm, respectively. The background vacuum was better than 6 × 10−4 Pa before the deposition. During the deposition, the substrate temperature was kept at 150 ◦ C, the sputtering pressure of Ar gas was controlled as ∼3 Pa and its flux was about 70–80 sccm. For comparison, the Er:SRA monolayer film, consisting of only one single Er:Si:Al2 O3 sublayer and one single Si:Al2 O3 sublayer, was also prepared. The monolayer film contained the same Er concentration as the multilayer film and the thicknesses of Er:Si:Al2 O3 sublayer and Si:Al2 O3 sublayer equal to the sum of the thickness of all Er:Si:Al2 O3 sublayers and the sum of the thickness of all Si:Al2 O3 sublayers in the multilayer film, respectively. Finally, in order to activate Er3+ ions and form Si-NCs [17,18], the as-deposited samples were annealed for 30 min at different temperatures over the range of 700–1100 ◦ C under N2 ambient. The schematic diagram of Er:SRA multilayer film and monolayer film is shown in Fig. 1. The doping profiles of RE concentrations in the Er:SRA films are investigated by using Rutherford backscattering (RBS). The energy of He+ ion beam in RBS was 2 MeV. The backscattered ions were detected by an Au/Si surface barrier detector placed at a scattering angle of 165◦ . Raman spectra were measured by Raman microscope (Jobin Yvon LabRam HR800) with 514 nm Ar+ laser. Cross section was characterized using a Philips CM200FEG high-resolution transmission electron microscopy (HRTEM) with 0.10 nm line resolution. The phase structures were investigated by a D/MAX-III C X-ray diffractometer (XRD) using Cu K␣ radiation ( = 0.154 nm). PL spectra were detected by an InGaAs detector in near-infrared region. 5 spectral lines from Ar+ laser were chosen as pumping sources. The excitation power was 12.5 mW. Decay curves were recorded by 50 MHz digital storage oscilloscope. The ambient temperatures of
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Fig. 2. RBS spectra of the Er:SRA films before and after annealing at 950 ◦ C.
the sample for PL measurements were varied from 14 to RT (298 K) by a closed-cycle helium cryostat. 3. Results and discussion Firstly, the compositions and structures of the Er:SRA films are determined by RBS, Raman and HRTEM. Fig. 2 shows the RBS spectra of the Er:SRA films before and after annealing at 950 ◦ C. The arrows in the figure indicate RBS energies corresponding to Er, Al, Si and O located at the sample surfaces. All signals from different elements and layers were separated from the RBS spectra by using SIMNRA60 program [19]. This is estimated roughly that the average Si content in Er:SRA films is ∼5 at.% and the average Er content is about 2 at.%. Moreover, they are almost no changes in the profiles of different elements in the Er:SRA films before and after annealing at 950 ◦ C. This suggests that the significant diffusion of the elements did not occur in the Er:SRA films upon annealing at 950 ◦ C. Until the samples were annealed at the temperature higher than 1100 ◦ C, the diffusion of the elements became significant (not shown). Fig. 3 shows the Raman spectra of the Er:SRA multilayer film annealed at different temperatures. The multilayer film annealed at temperature below 900 ◦ C exhibits two broad bands located at 150 and 475 cm−1 (not shown), which are attributed to the transverse acoustic (TA) and transverse optical (TO) modes of amorphous
Fig. 3. Raman spectra of the Er:SRA multilayer films annealed at different temperatures: (a) 900 ◦ C, (b) 950 ◦ C, (c) 1100 ◦ C.
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Fig. 5. Near-infrared RT PL spectra of the Er:SRA multilayer films annealed at different temperatures. The inset shows the annealing temperature dependence of the peak intensity of Er3+ PL from the multilayer and monolayer films, respectively.
Fig. 4. Cross-section HRTEM images of the Er:SRA multilayer films annealed at 950 ◦ C.
silicon (␣-Si), respectively. Usually few ␣-Si clusters exist in asdeposited Si-rich SiO2 film and the phase separation of Si and SiO2 starts at 900 ◦ C, while Si nanoparticles (Si-NPs) including Si-NCs and well-defined ␣-Si clusters form above 1000 ◦ C [20–22]. However, upon annealing at 950 ◦ C, instead of the two broad ␣-Si bands, a sharp band centered at 516 cm−1 appears, corresponding to the TO mode of Si-NCs. It indicates that Si-NCs in Er:Si:Al2 O3 and Si:Al2 O3 sublayers have been formed via phase separation of Si and Al2 O3 . With increasing the annealing temperature up to 1100 ◦ C, the TO mode signal of Si-NCs increases and narrowers as a result of the enhancement of crystallization. To further investigate the structures of the multilayer films and check the existence of Si-NCs in Al2 O3 matrix, the cross-section HRTEM observations were performed. Fig. 4 shows the crosssection HRTEM images of the Er:SRA multilayer films annealed at 950 ◦ C. It can be seen that a dense array of nearly spherical Si-NCs is embedded in the Al2 O3 matrix. The size distribution of Si-NCs identified on TEM images gives a mean diameter of ∼6 nm. The inset shows the electron diffraction pattern taken from the area of the annealed multilayer film. This result shows the well-distributed diffraction circles of Si polycrystals and no existence of the diffraction circles or spots of Er segregation and Er precipitates. It suggests that Si atoms are accumulated to form Si clusters and then Si crystal particles such as Si-NCs are formed and grow up during annealing. These results are consistent with those by Raman spectroscopy. It also indicates that the Er:SRA Al2 O3 multilayer system is powerful to obtain size-controlled Si-NCs given by building sample structure and controlling sample preparation. Fig. 5 shows the near-infrared RT PL spectra of the Er:SRA multilayer films annealed at different temperatures. With increasing annealing temperature, the intensity of Er3+ PL (originating from the transition of Er3+ : 4 I13/2 → 4 I15/2 ) from the multilayer film first increases to its maximum at 950 ◦ C, then drops sharply. The inset shows the annealing temperature dependence of the peak intensity of Er3+ PL of the Er:SRA multilayer and monolayer films, respectively. It can be seen that the peak intensity of Er3+ PL from the multilayer film annealed at 950 ◦ C is about 10 times higher than that of the monolayer film annealed at 1000 ◦ C. This clearly indicates
that the enhancement of Er3+ PL intensity in the multilayer film is due to the presence of Si:Al2 O3 layers without dopant Er, similar to the enhancement of Er3+ PL intensity in the Er–Si-codoped multilayer film due to the presence of Si–O-codoped sublayers without dopant Er [7,23]. The dependence of the Er3+ PL intensity, the lifetime and their ratio of the Er:SRA multilayer films on annealingtemperature is shown in Fig. 6a–c, respectively. Different from the above-mentioned dependence of the Er3+ PL intensity on annealing-temperature, the Er3+ lifetime of 0.1 ms order of magnitude is almost a constant, except a slight increase at 950 ◦ C. Upon annealing below 950 ◦ C, the lifetimes observed are similar to the
Fig. 6. Annealing-temperature dependence of (a) peak intensity, (b) lifetime and (c) their ratio of Er3+ PL from the Er:SRA multilayer films, respectively.
X. Wang et al. / Applied Surface Science 258 (2012) 1896–1901
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Fig. 7. Excitation energy dependence of the peak intensity of Er3+ PL from the multilayer film annealed at 950 ◦ C.
values reported in the Er-doped Si-rich SiO2 (Er:SRSO) system by Savchyn et al. [22], which was explained by the nonradiative decay channels that may involved defects or concentration quenching at high Er concentrations etc. [10]. The Er3+ lifetime is found to increase slightly for the sample annealed at 950 ◦ C, which is generally attributed to the removal of the defects due to annealing. However, upon annealing at 1000 ◦ C, the lifetime is not found to further increase, different to the value reported in the Er-doped Sirich SiO2 system [22]. It could be considered to be caused by the phase transition in the Er:SRA multilayer film that might occur at 1000 ◦ C, which will be discussed later. In the linear pump regime for a fixed pump power, assuming a constant Er3+ radiative lifetime, the total Er3+ PL intensity is written [22] ∗ Rexc,Er dec,Er = dec,Er dec,Er IEr ∞NEr
(1)
∗ is the density of Er3+ ions that are excited indirectly by where NEr sensitizers, dec,Er is the Er3+ measured lifetime of, Rexc,Er is the Er3+ excitation rate, and the Er3+ excitation density dec,Er , defined as the number of Er3+ excitation events per second per unit volume, is proportional to the ratio of the Er3+ intensity to the measured Er3+ lifetime IEr / dec,Er . It can be seen that the Er3+ excitation density reaches maximum at 950 ◦ C. To investigate the excitation mechanism of Er3+ , the dependence of the excitation energy (exc ) on the peak intensity of Er3+ PL was studied. Fig. 7 shows the Er3+ PL peak intensity as a function of excitation energy for the Er:SRA multilayer film annealed at 950 ◦ C. It shows that the PL intensity of the annealed multilayer film is independent of the excitation energy. It clearly indicates that the excitation of Er3+ could be attributed to the broad absorption by Si-NCs rather than the direct absorption by Er3+ . The temperature dependence of PL spectra of the Er:SRA multilayer film annealed at 950 ◦ C is shown in Fig. 8. The inset shows the temperature dependence of the full width at halfmaximum (FWHM) of Er3+ PL. The PL peak position of the Er3+ emission at 1532 nm remains unchanged over the temperature range of 14–298 K. As a result of the thermal redistribution over the Stark levels, the FWHM increases from 45 to 68 nm with temperature increasing up to 270 K, and then it is approximately constant. It is noted that the temperature dependence of Er3+ PL is quite complicated. For comparison, Er:SRSO film containing SiNCs was also synthesized by metal vapor vacuum arc ion source implantation. The detailed descriptions of sample preparation and characterizations have been given otherwhere [24]. The temperature dependence of the integrated PL intensity of the Er:SRA multilayer film and Er:SRSO film is shown in Fig. 9. The PL intensity
Fig. 8. Temperature dependence of the near-infrared PL spectra of the Er:SRA multilayer film annealed at 950 ◦ C. The inset shows the temperature dependence of the FWHM of Er3+ PL.
of Er3+ exhibits a nonmonotonic temperature dependence (Fig. 9a), which is different from that of the Er:SRSO film (Fig. 9b). With temperature increasing the integrated intensity remains constant from 14 to 50 K and then gradually reaches the maximum at 225 K, finally increases again at higher temperatures. For the Er:SRSO film, the PL integrated intensity at RT is ∼40% lower than that at 14 K. Interestingly, the PL integrated intensity from the Er:SRA multilayer film at RT is ∼30% higher than that at 14 K. The reason that the PL response of Er3+ in the multilayer film annealed at 950 ◦ C reaches the maximum is suggested in the following: at 950 ◦ C ␣-Si clusters in the Er:Si:Al2 O3 and Si:Al2 O3 sublayers have been crystallized into Si-NCs with the largest numbers. Meanwhile, it occurred to optically activate Er3+ ions in the Er:Si:Al2 O3 layers [17,18]. Since the interfaces between Si:Al2 O3 layers and Er:Si:Al2 O3 layers in the multilayer film are not smooth but rough enough as observed by HRTEM, it is possible that the Er3+ ions responsible for the strong Er3+ PL are located around Si-NCs in the Er:Si:Al2 O3 sublayers and Si:Al2 O3 sublayers although a few Er3+ ions may exist in Si-NCs [21]. As the excitation rate is almost independent of the annealing temperature, the Er3+ excitation density is proportional to the density of Er3+ ions excited by Si-NP sensitizers therefore to the density of Si-NPs. In fact, it seems that a great
Fig. 9. Temperature dependence of the integrated PL intensity of (a) the Er:SRA multilayer film and (b) Er:SRSO film, respectively.
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number of Si-NPs exist at 950 ◦ C [20,21]. Therefore we propose that the indirect broad excitation of Er3+ is Si-NP-mediated excitation. Thus, the enhancement of Er3+ PL in the multilayer film annealed at 950 ◦ C is thought to be a result of the ET from Si-NCs in the Si:Al2 O3 sublayers to the neighboring Er3+ ions in Er:Si:Al2 O3 sublayers and then energy migration to distant Er3+ ions in Er:Si:Al2 O3 sublayers. It should be noted that the ET between Si-NCs and Er3+ ions could enhance the Er3+ sensitizing efficiency as its distance between them was much shorter than the critical radius for the ET. Recently Shin et al. [23] found that the Si-NC/Er effective ET distance was constant at ∼0.3 nm in Er-doped silicon nitride and the Er3+ effective sensitization distance via Si-NC could be as large as ∼1.3 nm or more if Er–Er migration is significant so that more Er3+ ions could be excited by Si-NCs. Gourbilleau et al. [8,9] thought that in the Er-doped SRSO the characteristic interaction distances were 0.4 ± 0.1 nm for the amorphous Si and 2.6 ± 0.4 nm for Si-NC. Serna et al. [16] believed that the characteristic distance for ET from Si-NPs to Er3+ ions in Al2 O3 could be attained to be in the 1 nm range. Toudert et al. reported that the distance, for the ET in the Si/Er:Al2 O3 /Si film, could be as long as 8 nm [15], at the resonance absorption (4 I15/2 ∼ 4 F7/2 ) with the excitation of 488 nm, the direct excitation of Er3+ ions is dominant in their Si/Er:Al2 O3 /Si samples. In our alternately Er:SRA multilayer film, the distance between Er3+ ions and Si-NCs may be much smaller than 2 nm. The reason that the observed PL intensity from the multilayer film is higher than that of the monolayer film could be explained as follows. Although the total areal densities of Si both in the multilayer film and monolayer film are equal to each other, the effect of the ET in the multilayer film between Si-NCs in Si:Al2 O3 sublayers and Er3+ ions in Er:Si:Al2 O3 sublayers is more significant because of the short distance between Si-NCs and Er3+ ions due to many interfaces between Si:Al2 O3 sublayers and Er:Si:Al2 O3 sublayers. It is noted that the probability of the ET from Si-NCs to Er3+ ions will decrease with increasing the distances between them [25]. In this case, the indirect excitation of Er3+ ions may still be dominant even with the resonance excitation of 488 nm, thus no peak being observed in photoluminescence excitation spectrum as shown in Fig. 7. Moreover, the density of Si-NCs as broadband sensitizers should also be considered. It seems likely that the formation of Si-NCs mainly depends on the sample preparation, sample structure and annealing condition. Therefore, in order to achieve efficient ET from Si-NCs to Er3+ ions, to control the distance between Er3+ ions and Si-NCs and the numbers of optically activated Er3+ ions and Si-NCs is important. On the other hand, there might be two reasons for why the PL in the multilayer decreases so drastically upon annealing at 1000 ◦ C. Firstly, due to gradual crystallization the mean size of Si-NCs increases with the annealing temperature, resulting in the decrease of the number of Si-NCs. Secondly, the phase transition in the multilayer film might occur at 1000 ◦ C so that the number of optically activated Er3+ ions would decrease. For example, in Erdoped Al2 O3 film, the different phase structures, including Er–Al–O phases, were obtained at 1000 ◦ C [26]. In fact, upon annealing at 1000 ◦ C, it was found to form the Er2 O3 and Er4 Al2 O9 phases in our alternately Er:SRA multilayer film by XRD. The phase structures would effectively influence the PL response of Er3+ . Intriguingly, the Er:SRA multilayer film exhibits a nonmonotonic temperature dependence of Er3+ PL in the temperature range of 14–298 K, which seems different from that of Er-doped Si–SiO2 system [2,5,7,27,28], but partly similar to that of Si-NCs [5,29,30]. Furthermore, it is noteworthy that the PL intensity of the annealed multilayer film at RT is not lower but higher than that at 14 K. In Er-doped Si–SiO2 , the small thermal quenching behavior has been observed and explained by a model of isolated luminescence-
center-mediated Er3+ excitation [22]. However, this model seems not suitable to explain present experimental results of the temperature dependence of Er3+ PL. This might be caused by the competition between quasidirect nonphonon optical transition and the phononassisted transition [6,31]. Since Er-doped Si–Al2 O3 system is more complex than Er-doped Si–SiO2 system in both structure and chemical composition, the further investigation is expected to reveal the PL mechanism of Er-doped SRA. 4. Conclusions In summary, the Er:SRA multilayer film has been synthesized by magnetron sputtering. The intensity of Er3+ PL from the multilayer film is much higher than that of the monolayer film. The enhancement of Er3+ PL might be due to the ET from Si-NCs to Er3+ ions located around Si-NCs in the Er:Si:Al2 O3 layers and Si:Al2 O3 layers. Meanwhile, the multilayer film exhibits a nonmonotonic temperature dependence. Furthermore, the PL intensity of the multilayer film at RT is higher than that at 14 K. These properties of the Er3+ PL from the Er:SRA multilayer structures might provide good perspectives for the development of Er-doped waveguide amplifiers operating at high temperature. Acknowledgements This work was supported by the National Natural Science Foundation (Nos. 50602029, 60425411 and 10621063) of China and the Ministry of Science and Innovation of Spain (SB2005-003), and also supported by the special funds for Major State Basic Research Project No. 2011CB925601 of China, Shanghai Municipal Education Commission, Shanghai Science and Technology Commission and Shanghai Leading Academic Discipline Project (No. S30105). We acknowledge Prof. Dr. Rosalía Serna at the Instituto de Óptica in Spain for many beneficial discussions and PL measurements. Measurements were partly supported by Instrumental Analysis & Research Center of Shanghai University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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Enhanced emission of Er3+ from alternately Er doped Si-rich Al2 O3 multilayer film with Si nanocrystals as broadband sensitizers Xiao Wang a , Zuimin Jiang a , Fei Xu b,c,∗ , Zhongquan Ma b , Run Xu d , Bin Yu b , Mingzhu Li a , Lingling Zheng a , Yongliang Fan a , Jian Huang d , Fang Lu a a
Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai 200433, China SHU-SolarE R&D Lab, Department of Physics, College of Sciences, Key Laboratory for Material Microstructures, Shanghai University, Shanghai 200444, China c Instituto de Óptica, CSIC, Serrano 121, 28006 Madrid, Spain d Department of Electronic Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China b
a r t i c l e
i n f o
Article history: Available online 4 August 2011 PACS: 78.20.−e 78.55.−m 78.67.Bf 81.15.Cd Keywords: Erbium doping Silicon rich Al2 O3 Silicon nanocrystals
a b s t r a c t Alternately Er doped Si-rich Al2 O3 (Er:SRA) multilayer film, consisting of alternate Er–Si-codoped Al2 O3 (Er:Si:Al2 O3 ) and Si-doped Al2 O3 (Si:Al2 O3 ) sublayers, has been synthesized by co-sputtering from separated Er, Si, and Al2 O3 targets. The dependence of Er3+ related photoluminescence (PL) properties on annealing temperatures over 700–1100 ◦ C was studied. The maximum intensity of Er3+ PL, about 10 times higher than that of the monolayer film, was obtained from the multilayer film annealed at 950 ◦ C. The enhancement of Er3+ PL intensity is attributed to the energy transfer from the silicon nanocrystals in the Si:Al2 O3 sublayers to the neighboring Er3+ ions in the Er:Si:Al2 O3 sublayers. The PL intensity exhibits a nonmonotonic temperature dependence: with increasing temperature, the integrated intensity almost remains constant from 14 to 50 K, then reaches maximum at 225 K, and slightly increases again at higher temperatures. Meanwhile, the PL integrated intensity at room temperature is about 30% higher than that at 14 K. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Er-doped materials are of great interest in optical communication technology, because they can serve as the gain medium in optical amplifiers operating at the standard telecommunication wavelength of 1.53 m. The fabrication of efficient light-emitting devices operating at room temperature (RT) from Er-doped Si appeared to be impractical due to their low luminescence efficiency [1]. Due to the low absorption cross-section of Er3+ and the temperature quenching behavior in Si, the host matrix has been changed from Si to SiO2 -based materials [2]. Meanwhile, in order to increase the effective excitation of Er3+ ions, sensitizers such as Si nanocrystals (Si-NCs) and Yb3+ ions in SiO2 materials have been adopted and investigated [2,3]. However, to further improve Er3+ emission at 1.53 m, a deeper understanding of the mechanism of the energy transfer (ET) from Si-NCs to Er3+ ions as well as the optimization of size-controlled Si-NCs distribution in SiO2 matrix is required [4–6]. For instance, in order to control the distri-
∗ Corresponding author at: SHU-SolarE R&D Lab, Department of Physics, College of Sciences, Key Laboratory for Material Microstructures, Shanghai University, Shanghai 200444, China. E-mail address: [email protected] (F. Xu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.064
butions of Er and Si, using molecular beam epitaxy, Xu et al. [7] synthesized the erbium–oxygen-codoped silicon multilayer film consisting of alternate erbium–oxygen-codoped silicon (Er:O:Si) and oxygen-doped silicon (O:Si) layers. Recently Gourbilleau et al. [8,9] employed rf reactive magnetron sputtering to fabricate Erdoped SRSO multilayer films consisting in the stacking of SRSO/SiO2 sublayers in which the erbium has been incorporated closer to the Si-NCs, resulting in the enhancement of 1.53 m emission. Al2 O3 is an interesting host material for optical doping because waveguide fabrication technology based on this material has been well developed [10,11]. The waveguides formed by relatively high index Al2 O3 confine light efficiently, making compact device structures possible. Furthermore, the similarity of valency and lattice constant between Al2 O3 and Er2 O3 may allow for incorporation of high concentrations of Er into Al2 O3 . In fact light emission from Er-doped Al2 O3 at RT has been demonstrated [10,12,13]. As high concentrations of Er can be incorporated into Al2 O3, and Si-NCs as sensitizers can transfer energy to Er effectively, Er-doped Si-rich Al2 O3 (Er:SRA), specially Er:SRA multilayer film, may be a promising material. Nevertheless, it was rarely studied [14–16]. In this work, we present an approach to fabricate alternately Er doped Si-rich Al2 O3 multilayer film by magnetron sputtering. Photoluminescence (PL) spectra of Er3+ in the multilayer films with different annealing temperatures over the range of 700–1100 ◦ C
X. Wang et al. / Applied Surface Science 258 (2012) 1896–1901
Fig. 1. Schematic diagram of (a) Er:SRA multilayer film and (b) Er:SRA monolayer film.
were measured. The PL intensity of Er3+ in the multilayer film was found much higher than that in the monolayer film and exhibited a nonmonotonic temperature dependence. Moreover, at RT the intensity of Er3+ PL from the annealed multilayer film was found higher than that at 14 K. 2. Experimental details The substrates used were Czochralski (CZ) grown p-type Si (1 0 0) single crystals with the resistivity of 3–5 cm. Er:SRA multilayer film was deposited on it by magnetron sputtering from separated Er, Si and Al2 O3 target. The multilayer film, consisting of 20 layers of alternate Er–Si-codoped Al2 O3 (Er:Si:Al2 O3 ) and Sidoped Al2 O3 (Si:Al2 O3 ). Each Er:Si:Al2 O3 sublayer was deposited for 9 min, followed by 3 min deposition of Si:Al2 O3 layer. The average thicknesses of Er:Si:Al2 O3 and Si:Al2 O3 sublayers were about 6 nm and 2 nm, respectively. The background vacuum was better than 6 × 10−4 Pa before the deposition. During the deposition, the substrate temperature was kept at 150 ◦ C, the sputtering pressure of Ar gas was controlled as ∼3 Pa and its flux was about 70–80 sccm. For comparison, the Er:SRA monolayer film, consisting of only one single Er:Si:Al2 O3 sublayer and one single Si:Al2 O3 sublayer, was also prepared. The monolayer film contained the same Er concentration as the multilayer film and the thicknesses of Er:Si:Al2 O3 sublayer and Si:Al2 O3 sublayer equal to the sum of the thickness of all Er:Si:Al2 O3 sublayers and the sum of the thickness of all Si:Al2 O3 sublayers in the multilayer film, respectively. Finally, in order to activate Er3+ ions and form Si-NCs [17,18], the as-deposited samples were annealed for 30 min at different temperatures over the range of 700–1100 ◦ C under N2 ambient. The schematic diagram of Er:SRA multilayer film and monolayer film is shown in Fig. 1. The doping profiles of RE concentrations in the Er:SRA films are investigated by using Rutherford backscattering (RBS). The energy of He+ ion beam in RBS was 2 MeV. The backscattered ions were detected by an Au/Si surface barrier detector placed at a scattering angle of 165◦ . Raman spectra were measured by Raman microscope (Jobin Yvon LabRam HR800) with 514 nm Ar+ laser. Cross section was characterized using a Philips CM200FEG high-resolution transmission electron microscopy (HRTEM) with 0.10 nm line resolution. The phase structures were investigated by a D/MAX-III C X-ray diffractometer (XRD) using Cu K␣ radiation ( = 0.154 nm). PL spectra were detected by an InGaAs detector in near-infrared region. 5 spectral lines from Ar+ laser were chosen as pumping sources. The excitation power was 12.5 mW. Decay curves were recorded by 50 MHz digital storage oscilloscope. The ambient temperatures of
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Fig. 2. RBS spectra of the Er:SRA films before and after annealing at 950 ◦ C.
the sample for PL measurements were varied from 14 to RT (298 K) by a closed-cycle helium cryostat. 3. Results and discussion Firstly, the compositions and structures of the Er:SRA films are determined by RBS, Raman and HRTEM. Fig. 2 shows the RBS spectra of the Er:SRA films before and after annealing at 950 ◦ C. The arrows in the figure indicate RBS energies corresponding to Er, Al, Si and O located at the sample surfaces. All signals from different elements and layers were separated from the RBS spectra by using SIMNRA60 program [19]. This is estimated roughly that the average Si content in Er:SRA films is ∼5 at.% and the average Er content is about 2 at.%. Moreover, they are almost no changes in the profiles of different elements in the Er:SRA films before and after annealing at 950 ◦ C. This suggests that the significant diffusion of the elements did not occur in the Er:SRA films upon annealing at 950 ◦ C. Until the samples were annealed at the temperature higher than 1100 ◦ C, the diffusion of the elements became significant (not shown). Fig. 3 shows the Raman spectra of the Er:SRA multilayer film annealed at different temperatures. The multilayer film annealed at temperature below 900 ◦ C exhibits two broad bands located at 150 and 475 cm−1 (not shown), which are attributed to the transverse acoustic (TA) and transverse optical (TO) modes of amorphous
Fig. 3. Raman spectra of the Er:SRA multilayer films annealed at different temperatures: (a) 900 ◦ C, (b) 950 ◦ C, (c) 1100 ◦ C.
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Fig. 5. Near-infrared RT PL spectra of the Er:SRA multilayer films annealed at different temperatures. The inset shows the annealing temperature dependence of the peak intensity of Er3+ PL from the multilayer and monolayer films, respectively.
Fig. 4. Cross-section HRTEM images of the Er:SRA multilayer films annealed at 950 ◦ C.
silicon (␣-Si), respectively. Usually few ␣-Si clusters exist in asdeposited Si-rich SiO2 film and the phase separation of Si and SiO2 starts at 900 ◦ C, while Si nanoparticles (Si-NPs) including Si-NCs and well-defined ␣-Si clusters form above 1000 ◦ C [20–22]. However, upon annealing at 950 ◦ C, instead of the two broad ␣-Si bands, a sharp band centered at 516 cm−1 appears, corresponding to the TO mode of Si-NCs. It indicates that Si-NCs in Er:Si:Al2 O3 and Si:Al2 O3 sublayers have been formed via phase separation of Si and Al2 O3 . With increasing the annealing temperature up to 1100 ◦ C, the TO mode signal of Si-NCs increases and narrowers as a result of the enhancement of crystallization. To further investigate the structures of the multilayer films and check the existence of Si-NCs in Al2 O3 matrix, the cross-section HRTEM observations were performed. Fig. 4 shows the crosssection HRTEM images of the Er:SRA multilayer films annealed at 950 ◦ C. It can be seen that a dense array of nearly spherical Si-NCs is embedded in the Al2 O3 matrix. The size distribution of Si-NCs identified on TEM images gives a mean diameter of ∼6 nm. The inset shows the electron diffraction pattern taken from the area of the annealed multilayer film. This result shows the well-distributed diffraction circles of Si polycrystals and no existence of the diffraction circles or spots of Er segregation and Er precipitates. It suggests that Si atoms are accumulated to form Si clusters and then Si crystal particles such as Si-NCs are formed and grow up during annealing. These results are consistent with those by Raman spectroscopy. It also indicates that the Er:SRA Al2 O3 multilayer system is powerful to obtain size-controlled Si-NCs given by building sample structure and controlling sample preparation. Fig. 5 shows the near-infrared RT PL spectra of the Er:SRA multilayer films annealed at different temperatures. With increasing annealing temperature, the intensity of Er3+ PL (originating from the transition of Er3+ : 4 I13/2 → 4 I15/2 ) from the multilayer film first increases to its maximum at 950 ◦ C, then drops sharply. The inset shows the annealing temperature dependence of the peak intensity of Er3+ PL of the Er:SRA multilayer and monolayer films, respectively. It can be seen that the peak intensity of Er3+ PL from the multilayer film annealed at 950 ◦ C is about 10 times higher than that of the monolayer film annealed at 1000 ◦ C. This clearly indicates
that the enhancement of Er3+ PL intensity in the multilayer film is due to the presence of Si:Al2 O3 layers without dopant Er, similar to the enhancement of Er3+ PL intensity in the Er–Si-codoped multilayer film due to the presence of Si–O-codoped sublayers without dopant Er [7,23]. The dependence of the Er3+ PL intensity, the lifetime and their ratio of the Er:SRA multilayer films on annealingtemperature is shown in Fig. 6a–c, respectively. Different from the above-mentioned dependence of the Er3+ PL intensity on annealing-temperature, the Er3+ lifetime of 0.1 ms order of magnitude is almost a constant, except a slight increase at 950 ◦ C. Upon annealing below 950 ◦ C, the lifetimes observed are similar to the
Fig. 6. Annealing-temperature dependence of (a) peak intensity, (b) lifetime and (c) their ratio of Er3+ PL from the Er:SRA multilayer films, respectively.
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Fig. 7. Excitation energy dependence of the peak intensity of Er3+ PL from the multilayer film annealed at 950 ◦ C.
values reported in the Er-doped Si-rich SiO2 (Er:SRSO) system by Savchyn et al. [22], which was explained by the nonradiative decay channels that may involved defects or concentration quenching at high Er concentrations etc. [10]. The Er3+ lifetime is found to increase slightly for the sample annealed at 950 ◦ C, which is generally attributed to the removal of the defects due to annealing. However, upon annealing at 1000 ◦ C, the lifetime is not found to further increase, different to the value reported in the Er-doped Sirich SiO2 system [22]. It could be considered to be caused by the phase transition in the Er:SRA multilayer film that might occur at 1000 ◦ C, which will be discussed later. In the linear pump regime for a fixed pump power, assuming a constant Er3+ radiative lifetime, the total Er3+ PL intensity is written [22] ∗ Rexc,Er dec,Er = dec,Er dec,Er IEr ∞NEr
(1)
∗ is the density of Er3+ ions that are excited indirectly by where NEr sensitizers, dec,Er is the Er3+ measured lifetime of, Rexc,Er is the Er3+ excitation rate, and the Er3+ excitation density dec,Er , defined as the number of Er3+ excitation events per second per unit volume, is proportional to the ratio of the Er3+ intensity to the measured Er3+ lifetime IEr / dec,Er . It can be seen that the Er3+ excitation density reaches maximum at 950 ◦ C. To investigate the excitation mechanism of Er3+ , the dependence of the excitation energy (exc ) on the peak intensity of Er3+ PL was studied. Fig. 7 shows the Er3+ PL peak intensity as a function of excitation energy for the Er:SRA multilayer film annealed at 950 ◦ C. It shows that the PL intensity of the annealed multilayer film is independent of the excitation energy. It clearly indicates that the excitation of Er3+ could be attributed to the broad absorption by Si-NCs rather than the direct absorption by Er3+ . The temperature dependence of PL spectra of the Er:SRA multilayer film annealed at 950 ◦ C is shown in Fig. 8. The inset shows the temperature dependence of the full width at halfmaximum (FWHM) of Er3+ PL. The PL peak position of the Er3+ emission at 1532 nm remains unchanged over the temperature range of 14–298 K. As a result of the thermal redistribution over the Stark levels, the FWHM increases from 45 to 68 nm with temperature increasing up to 270 K, and then it is approximately constant. It is noted that the temperature dependence of Er3+ PL is quite complicated. For comparison, Er:SRSO film containing SiNCs was also synthesized by metal vapor vacuum arc ion source implantation. The detailed descriptions of sample preparation and characterizations have been given otherwhere [24]. The temperature dependence of the integrated PL intensity of the Er:SRA multilayer film and Er:SRSO film is shown in Fig. 9. The PL intensity
Fig. 8. Temperature dependence of the near-infrared PL spectra of the Er:SRA multilayer film annealed at 950 ◦ C. The inset shows the temperature dependence of the FWHM of Er3+ PL.
of Er3+ exhibits a nonmonotonic temperature dependence (Fig. 9a), which is different from that of the Er:SRSO film (Fig. 9b). With temperature increasing the integrated intensity remains constant from 14 to 50 K and then gradually reaches the maximum at 225 K, finally increases again at higher temperatures. For the Er:SRSO film, the PL integrated intensity at RT is ∼40% lower than that at 14 K. Interestingly, the PL integrated intensity from the Er:SRA multilayer film at RT is ∼30% higher than that at 14 K. The reason that the PL response of Er3+ in the multilayer film annealed at 950 ◦ C reaches the maximum is suggested in the following: at 950 ◦ C ␣-Si clusters in the Er:Si:Al2 O3 and Si:Al2 O3 sublayers have been crystallized into Si-NCs with the largest numbers. Meanwhile, it occurred to optically activate Er3+ ions in the Er:Si:Al2 O3 layers [17,18]. Since the interfaces between Si:Al2 O3 layers and Er:Si:Al2 O3 layers in the multilayer film are not smooth but rough enough as observed by HRTEM, it is possible that the Er3+ ions responsible for the strong Er3+ PL are located around Si-NCs in the Er:Si:Al2 O3 sublayers and Si:Al2 O3 sublayers although a few Er3+ ions may exist in Si-NCs [21]. As the excitation rate is almost independent of the annealing temperature, the Er3+ excitation density is proportional to the density of Er3+ ions excited by Si-NP sensitizers therefore to the density of Si-NPs. In fact, it seems that a great
Fig. 9. Temperature dependence of the integrated PL intensity of (a) the Er:SRA multilayer film and (b) Er:SRSO film, respectively.
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number of Si-NPs exist at 950 ◦ C [20,21]. Therefore we propose that the indirect broad excitation of Er3+ is Si-NP-mediated excitation. Thus, the enhancement of Er3+ PL in the multilayer film annealed at 950 ◦ C is thought to be a result of the ET from Si-NCs in the Si:Al2 O3 sublayers to the neighboring Er3+ ions in Er:Si:Al2 O3 sublayers and then energy migration to distant Er3+ ions in Er:Si:Al2 O3 sublayers. It should be noted that the ET between Si-NCs and Er3+ ions could enhance the Er3+ sensitizing efficiency as its distance between them was much shorter than the critical radius for the ET. Recently Shin et al. [23] found that the Si-NC/Er effective ET distance was constant at ∼0.3 nm in Er-doped silicon nitride and the Er3+ effective sensitization distance via Si-NC could be as large as ∼1.3 nm or more if Er–Er migration is significant so that more Er3+ ions could be excited by Si-NCs. Gourbilleau et al. [8,9] thought that in the Er-doped SRSO the characteristic interaction distances were 0.4 ± 0.1 nm for the amorphous Si and 2.6 ± 0.4 nm for Si-NC. Serna et al. [16] believed that the characteristic distance for ET from Si-NPs to Er3+ ions in Al2 O3 could be attained to be in the 1 nm range. Toudert et al. reported that the distance, for the ET in the Si/Er:Al2 O3 /Si film, could be as long as 8 nm [15], at the resonance absorption (4 I15/2 ∼ 4 F7/2 ) with the excitation of 488 nm, the direct excitation of Er3+ ions is dominant in their Si/Er:Al2 O3 /Si samples. In our alternately Er:SRA multilayer film, the distance between Er3+ ions and Si-NCs may be much smaller than 2 nm. The reason that the observed PL intensity from the multilayer film is higher than that of the monolayer film could be explained as follows. Although the total areal densities of Si both in the multilayer film and monolayer film are equal to each other, the effect of the ET in the multilayer film between Si-NCs in Si:Al2 O3 sublayers and Er3+ ions in Er:Si:Al2 O3 sublayers is more significant because of the short distance between Si-NCs and Er3+ ions due to many interfaces between Si:Al2 O3 sublayers and Er:Si:Al2 O3 sublayers. It is noted that the probability of the ET from Si-NCs to Er3+ ions will decrease with increasing the distances between them [25]. In this case, the indirect excitation of Er3+ ions may still be dominant even with the resonance excitation of 488 nm, thus no peak being observed in photoluminescence excitation spectrum as shown in Fig. 7. Moreover, the density of Si-NCs as broadband sensitizers should also be considered. It seems likely that the formation of Si-NCs mainly depends on the sample preparation, sample structure and annealing condition. Therefore, in order to achieve efficient ET from Si-NCs to Er3+ ions, to control the distance between Er3+ ions and Si-NCs and the numbers of optically activated Er3+ ions and Si-NCs is important. On the other hand, there might be two reasons for why the PL in the multilayer decreases so drastically upon annealing at 1000 ◦ C. Firstly, due to gradual crystallization the mean size of Si-NCs increases with the annealing temperature, resulting in the decrease of the number of Si-NCs. Secondly, the phase transition in the multilayer film might occur at 1000 ◦ C so that the number of optically activated Er3+ ions would decrease. For example, in Erdoped Al2 O3 film, the different phase structures, including Er–Al–O phases, were obtained at 1000 ◦ C [26]. In fact, upon annealing at 1000 ◦ C, it was found to form the Er2 O3 and Er4 Al2 O9 phases in our alternately Er:SRA multilayer film by XRD. The phase structures would effectively influence the PL response of Er3+ . Intriguingly, the Er:SRA multilayer film exhibits a nonmonotonic temperature dependence of Er3+ PL in the temperature range of 14–298 K, which seems different from that of Er-doped Si–SiO2 system [2,5,7,27,28], but partly similar to that of Si-NCs [5,29,30]. Furthermore, it is noteworthy that the PL intensity of the annealed multilayer film at RT is not lower but higher than that at 14 K. In Er-doped Si–SiO2 , the small thermal quenching behavior has been observed and explained by a model of isolated luminescence-
center-mediated Er3+ excitation [22]. However, this model seems not suitable to explain present experimental results of the temperature dependence of Er3+ PL. This might be caused by the competition between quasidirect nonphonon optical transition and the phononassisted transition [6,31]. Since Er-doped Si–Al2 O3 system is more complex than Er-doped Si–SiO2 system in both structure and chemical composition, the further investigation is expected to reveal the PL mechanism of Er-doped SRA. 4. Conclusions In summary, the Er:SRA multilayer film has been synthesized by magnetron sputtering. The intensity of Er3+ PL from the multilayer film is much higher than that of the monolayer film. The enhancement of Er3+ PL might be due to the ET from Si-NCs to Er3+ ions located around Si-NCs in the Er:Si:Al2 O3 layers and Si:Al2 O3 layers. Meanwhile, the multilayer film exhibits a nonmonotonic temperature dependence. Furthermore, the PL intensity of the multilayer film at RT is higher than that at 14 K. These properties of the Er3+ PL from the Er:SRA multilayer structures might provide good perspectives for the development of Er-doped waveguide amplifiers operating at high temperature. Acknowledgements This work was supported by the National Natural Science Foundation (Nos. 50602029, 60425411 and 10621063) of China and the Ministry of Science and Innovation of Spain (SB2005-003), and also supported by the special funds for Major State Basic Research Project No. 2011CB925601 of China, Shanghai Municipal Education Commission, Shanghai Science and Technology Commission and Shanghai Leading Academic Discipline Project (No. S30105). We acknowledge Prof. Dr. Rosalía Serna at the Instituto de Óptica in Spain for many beneficial discussions and PL measurements. Measurements were partly supported by Instrumental Analysis & Research Center of Shanghai University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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