Elsevier

Thin Solid Films

Volume 536, 1 June 2013, Pages 196-201
Thin Solid Films

Erbium enhanced formation and growth of photoluminescent Er/Si nanocrystals

https://doi.org/10.1016/j.tsf.2013.03.027Get rights and content

Highlights

  • Photolumininescent Er/Si-nanocrystal growth in co-sputtered Er/SiOx thin films.

  • Upon annealing Er enhances SiOx disproportionation, nanocrystal nucleation and growth.

  • Er3 + coordination increase with increasing SiOx disproportionation.

Abstract

Photoluminescent Er/Si-nanocrystal composites were obtained after annealing Er doped silicon suboxide (SiOx) thin films. The films were prepared by reactive sputtering (Ar/O2 atmosphere) with a pure silicon target partially covered with metallic Er. The presence of Er in the resulting films strongly influences Si nanocrystal nucleation and growth during thermal treatment at temperatures between 300 and 1300 °C. A correlation between Er photoluminescence (PL) spectra, Er speciation and Si nanocrystal properties indicated that PL bands and their intensity are directly influenced by the nanocrystal size and density, and their vicinity to the Er3 + centers. This correlation is explained by considering Er centers as promotor for SiOx disproportionation, locally increasing Si0 concentration which leads to formation of Si nanocrystals in the vicinity of Er.

Introduction

Erbium doped silicon nanocrystals provide a promising route to obtain energy-efficient photoluminescence (PL) at 1.54 μm (emission due to 4I13/2  4I15/2 transition in Er3 +) [1], [2], [3], [4], [5], [6]. Due to the minimal signal attenuation in commercial optical fibers, 1.54 μm light is of high interest for photonic communication. In contrast with the direct excitation of Er in most materials, this wavelength can be generated very specifically and much more efficiently using Er doped Si nanocrystal assemblies. In this case, the transfer of energy from the Si nanocrystals to Er compensates the small cross section (10- 21 cm2) of Er ions [7].

One of the more popular techniques to grow pure Si nanocrystals is thermal treatment of sputtered Si suboxide (SiOx) thin films [8], [9], [10]. Alternative techniques are, co-sputtering from Si/SiO2 targets [11], deposition of SiO/SiO2 multilayers [12] and sputtering from a SiO2 target in a reducing atmosphere [13].

Er dopage of the Si nanocrystals is most often achieved by ion implantation into pre-formed Si nanocrystals after thermal treatment [14], [15]. Although this technique requires a secondary thermal treatment to recover implantation damage suffered by the nanocrystals, it is preferred over Er/SiOx deposition by Plasma Enhanced Chemical Vapor Deposition (PECVD), where difficulties are encountered due to the low vapor pressure of erbium compounds [16]. To overcome the problems associated with ion implantation and PECVD, an alternative method for the preparation of Er doped thin films of SiOx has been presented a few years ago. Co-sputtering of Er and Si atoms using a silicon target covered with pieces of metallic erbium in a reactive atmosphere results in thin films of Er doped SiOx. Upon annealing, these films directly yield Er/Si nanocrystal assemblies [17].

Er3 + luminescence at 1.54 μm is strongly dependent on the local coordination of Er [18], [19]. In the case of isolated ions, the luminescence intensity is very low because the electric dipole transitions between the initial and final states are forbidden. When the ions occupy non-symmetric positions in a framework, the local environment of the ions not only influences the shape of the spectrum, but also the PL intensity because the transitions become partially allowed due to mixing of states of opposite parity [20].

This work describes and explains the influence of co-sputtered Er in SiOx thin films on Si nanocrystal nucleation and growth during thermal treatment and the self organization into a highly efficient Si-nanocrystal/Er photoluminescent composite.

Section snippets

Experimental section

Er doped SiOx thin films (~ 1.0 μm thickness) were prepared by RF-sputtering in a modified Leybold 400 system. The single target consisted of a 75 mm diameter 99.999% Si block partially covered with pieces of metallic Er (~ 4 mm2, thickness ~ 1000 μm). During sputtering the sputtering chamber was flushed with an Ar + O2 electronic grade (99.999%) gas mixture. The base pressure of the system was always kept below 2.5 × 10 4 Pa using a turbo-molecular pump. While the Ar flux was controlled to keep the

Results and discussion

The area of the Er sheets mounted on the silicon target during co-sputtering determines the final concentration of Er in the resulting silicon suboxide thin films (see Fig. 1). Fig. 2 shows HRTEM images representative for three samples sputtered on Si < 111 > using 5.5 × 10 3 Pa of partial oxygen pressure with different Er concentration and annealed at 1150 °C. Based on the selected area electron diffraction patterns the observed nanocrystals can be identified as Si crystallites with different

Conclusions

In summary, this report demonstrates the feasibility of producing silicon nanocrystal/Er assemblies by reactive RF-sputtering using a single Si target covered with small pieces of metallic Er. The nanocrystal size and density after annealing are highly correlated to the Er concentration in the original suboxide thin films, indicating Er centers promote the nucleation of Si nanocrystals. At annealing temperatures below 1100 °C, the uniformly distributed Er centers scavenge mobile oxygen atoms,

Acknowledgements

The authors acknowledge Prof. Manfredo Tabakniks for help with the RBS measurements, Prof. Horst Strunk and Dr. Ines Häusler for support with HRTEM. This work was partially supported by FAPESP, PROBRAL CAPES/DAAD and LNLS – National Synchrotron Light Laboratory, Brazil. E.B. acknowledges a fellowship as Postdoctoraal onderzoeker van FWO-Vlaanderen. C.E.A.K and J.A.M acknowledge the Flemish Government for long-term structural funding (Methusalem).

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