Nanoscale evidence of erbium clustering in Er-doped silicon-rich silica
© Talbot et al.; licensee Springer. 2013
Received: 4 October 2012
Accepted: 19 November 2012
Published: 21 January 2013
Photoluminescence spectroscopy and atom probe tomography were used to explore the optical activity and microstructure of Er3+-doped Si-rich SiO2 thin films fabricated by radio-frequency magnetron sputtering. The effect of post-fabrication annealing treatment on the properties of the films was investigated. The evolution of the nanoscale structure upon an annealing treatment was found to control the interrelation between the radiative recombination of the carriers via Si clusters and via 4f shell transitions in Er3+ ions. The most efficient 1.53-μ m Er3+ photoluminescence was observed from the films submitted to low-temperature treatment ranging from 600°C to 900°C. An annealing treatment at 1,100°C, used often to form Si nanocrystallites, favors an intense emission in visible spectral range with the maximum peak at about 740 nm. Along with this, a drastic decrease of 1.53-μ m Er3+ photoluminescence emission was detected. The atom probe results demonstrated that the clustering of Er3+ ions upon such high-temperature annealing treatment was the main reason. The diffusion parameters of Si and Er3+ ions as well as a chemical composition of different clusters were also obtained. The films annealed at 1,100°C contain pure spherical Si nanocrystallites, ErSi3O6 clusters, and free Er3+ ions embedded in SiO2 host. The mean size and the density of Si nanocrystallites were found to be 1.3± 0.3 nm and (3.1± 0.2)×1018 Si nanocrystallites·cm−3, respectively. The density of ErSi3O6 clusters was estimated to be (2.0± 0.2)×1018 clusters·cm−3, keeping about 30% of the total Er3+ amount. These Er-rich clusters had a mean radius of about 1.5 nm and demonstrated preferable formation in the vicinity of Si nanocrystallites.
KeywordsErbium Silicon Nanocrystallites Nanoclusters Sputtering Atom probe tomography Photoluminescence
Silicon-based photonics is a fast growing field of semiconductor nanoscience. A part of this area focuses on the realization of integrated optoelectronic devices (such as light planar waveguide amplifier, light-emitting diodes, lasers, ..) to overcome the interconnect bottleneck for Si-based integrated circuits. In this regard, the use of optical interconnection is the most promising. Among the different strategies, the most considered for Si-based telecommunication are (1) doping of silica fibers with Er3+ ions which offered the emission at the standard telecommunication wavelength (1.53 μ m) and (2) incorporation of quantum-confined Si nanoclusters (Si-ncs) or nanocrystallites (Si-NCs) in such doped fibers, favoring an enhancement of Er-effective excitation cross section. Both these approaches fully exploit the individual properties of Si-ncs (Si-NCs) and rare-earth ions [1, 2].
It was already demonstrated that Si-nc/SiO2 interface affects significantly not only the properties of the Si-ncs themselves, but also the optical activity of Er3+ ions coupled with Si-ncs [1, 3, 4]. It was shown that a thin 0.8-nm sub-stoichiometric interface between the Si-nc and the SiO2 host plays a critical role in the Si-nc emission [5, 6]. Furthermore, numerous studies allowed the determination of the main mechanism of the interaction between the Si-ncs and the neighboring Er3+ ions [1, 2, 7]. Along with the effect of structural environment of both Er3+ ions and Si-ncs on their individual properties, it has also been observed that very small Si-ncs, even amorphous, allow an efficient sensitizing effect towards Er3+ ions. However, the efficiency of this process depends on the separating distance between Si-ncs and rare-earth ions [7–9]. Critical interaction distances were found to be about 0.5 nm [7, 9, 10].
In spite of the significant progress in the investigation of the excitation processes in Er-doped Si-rich SiO2 materials, some issues are still debatable, such as the spatial location of optically active Er3+ ions with regard to Si-ncs. Another aspect, which may control the optical properties, is the distribution of Er dopants in the film, i.e., either these ions are uniformly distributed or they form some agglomerates . Thus, mapping the Si and Er3+ distributions in Er-doped Si-rich SiO2 films as well as the investigation of the evolution of these distributions versus fabrication conditions and post-fabrication processing are the key issues to manage the required light-emitting properties of such systems.
Up to now, high-resolution and energy-filtered transmission electron microscopies were the only techniques offered a direct visualization of Si and Er distributions [11–13]. Nevertheless, other indirect techniques, such as fluorescence-extended X-ray absorption fine-structure spectroscopy [14–16] or X-ray photoelectron spectroscopy , have evidenced that the amount of Er clusters in Er-doped Si-rich SiO2 films depends strongly on the preparation conditions or annealing temperature. We have recently demonstrated the feasibility of atom probe tomography (APT) analysis of Si-rich SiO2 systems, giving its atomic insight [18, 19]. With the benefit of this expertise, the purpose of this paper is to perform a deep analysis of Er-doped Si-rich SiO2 thin films by means of APT experiments to understand the link between the nanoscale structure of the films and their optical properties. The distributions of Si and Er3+ ions in as-grown films were investigated. The evolutions of chemical composition of the films upon annealing treatment, the formation of Si-ncs, and the redistribution of Er3+ ions were studied with the aim of finding the way to control the microstructure at the atomic scale and to optimize light-emitting properties of the Er-doped Si-rich SiO2 system.
Er-doped Si-rich SiO2 (Er-SRSO) layers were grown by radio-frequency (RF) magnetron-sputtering technique. For the APT experiments, the deposition was performed on an array of p-doped Si(100) posts (5 μ m in diameter and 100 μ m in height). This method, already used in previous works, allows a simple procedure for atom probe sample preparation . For optical experiments, the layers were grown on standard p-type (100) Si wafers in the same deposition run. The film fabrication approach comprises the co-sputtering of Er2O3, SiO2, and Si targets in pure argon plasma on substrate kept at 500°C. The Er content and the Si excess were independently controlled through the RF power applied on the corresponding cathode. More details on the fabrication processes can be found in other works [12, 21]. The thickness of the Er-SRSO layer was 200 nm. The concentration of Er3+ ions in the sample was 1×1021at./cm3, while the Si excess was about 5 at.% . To study the effect of post-fabrication treatment on structural and optical properties of the layers, each sample was divided into several parts. One of them was kept as a reference for the ‘as-deposited’ state. The others were submitted to an annealing treatment in conventional furnace in constant nitrogen flow to study the phase separation, the Si-nc formation, the recovering of the defects, and thus, the enhancement of Er emission. The samples were annealed at 600°C for 10 h, 900°C for 1 h, and 1,100°C for 1 h. The annealing time for each temperature corresponds to optimal conditions, giving rise to the highest photoluminescence of the Er3+ ions.
Atom probe tomography
Among the various analytical techniques, atom probe tomography is one of the most promising when atomic scale resolution, three-dimension reconstruction, and quantitative chemical characterization are required [22, 23]. The recent improvement of this technique with the implementation of femtosecond laser pulses  allowed to enlarge the variety of materials to be studied. Thus, an atomic observation of photonic, solar cells, magnetic semiconductor, or nanoelectronic devices is now available [18, 19, 25–28]. The Er-SRSO film with the shape of a tiny needle, required for APT analyses, was prepared using a focused ion beam annular milling procedure. The details of this standard procedure are reported in another work . In order to prevent the layer of interest from Ga damages and/or amorphization during the sample processing, a 300-nm-thick layer of Cr was pre-deposited on the top of the sample. Films were then ion-milled into sharp tips with an end radius close to 30 nm. A low-accelerating voltage (2 kV) was used for the final stage in order to avoid Ga implantation and sample amorphization. The APT used in this work is the CAMECA (CAMECA SAS, Gennevilliers Cedex, France) laser-assisted wide-angle tomographic atom probe. The experiments were performed with samples cooled down to 80 K, with a vacuum of (2 to 3)×10−10 mbar in the analysis chamber and with ultraviolet (λ=343 nm) femtosecond (350 fs) laser pulses. The laser energy was fixed at 50 nJ/pulse focused onto an approximately 0.01-mm2 spot.
To identify the clusters, the algorithm described hereafter was applied. Each step of this identification comprises the placement of a sphere (sampling volume) over one atom of the volume investigated and the estimation of the local composition of the selected elements by counting atoms within this sphere. If the composition exceeds a given threshold, the atom at the center of the sphere is associated to a cluster. If the composition is lower than the threshold, the atom at the center of the sphere belongs to the matrix. The sphere is then moved to the next atom, and this procedure is applied again to estimate the composition and to compare it with the threshold value. This approach was used for all the atoms of the volume to identify those belonging either to the clusters or to the matrix. In this paper, a threshold of 75% of Si and 5% of Er was used to identify pure Si nanoclusters and Er-rich regions with a sphere radius of 1 nm.
The photoluminescence (PL) properties of the samples were examined using the 476-nm excitation line delivered by an Innova 90C coherent Ar+ laser (Coherent Inc., Santa Clara, CA, USA). The pumping at 476 nm, which is nonresonant for Er3+ ions, was always used to ensure that Er3+ excitation was mediated by the Si-based sensitizers. The Er3+ PL spectra in the 1.3- to 1.7-μ m spectral range were measured at room temperature by means of a Jobin Yvon (HORIBA Jobin Yvon Inc., Edison, NJ, USA) 1-m single-grating monochromator coupled to a North Coast germanium detector (North Coast Scientific Co., Santa Rosa, CA, USA) cooled with liquid nitrogen. The Si-nc PL properties were investigated in the 550- to 1,150-nm spectral range using a Triax 180 Jobin Yvon monochromator with an R5108 Hamamatsu PMT (HAMAMATSU PHOTONICS DEUTSCHLAND GmbH, Herrsching am Ammersee, Germany). The PL signal was recorded in both cases through an SRS lock-in amplifier (SP830 DPS; Stanford Research Systems, Inc., Sunnyvale, CA, USA) referenced to the chopping frequency of light of 9.6 Hz. All PL spectra were corrected on the spectral response of experimental setup.
Results and discussion
Atom probe experiments
APT compositions of the Er-doped SRSO layer in the as-deposited and 1,100°C 1-h annealed state
Annealed at 1,100°C
35.1 ± 0.4
35.0 ± 0.4
63.2 ± 0.4
63.1 ± 0.4
1.7 ± 0.4
1.9 ± 0.4
1.1 × 1021
1.3 × 1021
Si excess (at.%)
Approximately 3.6 %
This result shows that thermal treatment at 1,100°C leads to a formation of a three-phase system: silica matrix, Si-ncs, and Er-rich clusters. The formation of such Er clusters is accompanied by the enlargement of the distance between Si-ncs, and it explains why annealing at 1,100°C quenches the PL emission with respect to the lower annealing treatments. Although the formation of Si-ncs increases the probability of absorbing excitation light, the total number of Si sensitizers decreases due to the merging of several small Si sensitizers along with the increase of Si-to-Er distance.
where , , and are the compositions of Si in the Si-pure clusters, in the whole sample and in the matrix, respectively. The compositions have been extracted from the concentration (in at.%) using the density of pure Si (dSi=5.0×1022 at./cm3) and pure silica (dSiO2=6.6×1022 at./cm3); % is obtained from Equation 1.
The 3D chemical maps also indicate that the Er-rich clusters are likely formed in the vicinity of Si-ncs upon an annealing stage. This fact can be attributed to a preferential segregation of Er atoms at the Si-ncs/matrix interface during the phase separation process, similar to the results reported by Crowe et al. . However, this hypothesis is not supported by the results of Pellegrino et al. , who concluded to a preferential segregation of Er in poor Si-nc region. In their paper, a double-implantation annealing process was applied to fabricate an Er-doped SRSO layer. This double process may stimulate Er diffusion explaining the segregation of Er and Si during the different implantation stages, which is contrary to our case.
Based on the hypothesis of spherical radius and on the determination of an amount of Er, Si, and O atoms in Er-rich clusters detected by APT method, the mean Er-rich cluster radius is estimated to be 1.4 ± 0.3 nm in the sample annealed at 1,100°C (< ρ >=5.1 nm and t=3,600 s). Erbium diffusion coefficient in the SRSO layer has been deduced using the Einstein equation of self-diffusivity. It has been found to be DEr≈1.2×10−17cm2· s −1 at 1,100°C. This value is about one order of magnitude lower than that reported by Lu et al. (4.3×10−16cm2· s −1)  which has been measured in SiO2. This difference could be attributed to the presence of Si excess in the film.
The formation of Er-rich clusters explains the evolution of the optical properties of Er-doped layers upon high-temperature annealing treatment applied [12, 13, 29]. It is worth to note that the fabrication approach, chemical composition, and microstructure of initial samples define strongly the effect of post-annealing processing.
In this paper, the first investigation by APT, to our knowledge, of the nanostructure of Er-doped silicon-rich silica layer was performed at the atomic level and correlated with photoluminescence properties. The phase separation process between Si excess and the surrounding matrix was studied, and a formation of Si-rich or Er-rich phases was observed for samples annealed at high-temperature (1,100°C). The Si excess atoms precipitate in the form of pure Si nanoclusters in the silica matrix. Simultaneously, Er atoms form Er-rich clusters (about 30% of total amount), whereas 70% of the total Er atoms are free-dispersed in the host, demonstrating a super-saturation state but with an increase of the Si-ncs-to-Er distances. The Er-rich clusters have complex shape and composition. They are localized at the Si-nc/matrix interface or in poor Si-nc regions, indicating a complicated precipitation mechanism. Diffusion coefficients for Si and Er have been deduced from APT experiments. We have directly evidenced the clustering of rare-earth ions upon high-temperature annealing in Er-doped Si-rich SiO2 films. This process has been often expected but, to our knowledge, never observed and demonstrated directly for these materials fabricated by different techniques. These results evidence the critical point to monitor the microstructure of Er-doped SRSO layers for the required inversion of 50% of the Er concentration to achieve a net gain in future Er-doped amplifier device.
This work was supported by the Upper Normandy Region and the French Ministry of Research in the framework of Research Networks of Upper Normandy.
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