Photoluminescence from Er-doped silicon oxide microcavities

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Abstract

Er-doped silicon oxide (SiO:Er) thin films with Er concentrations ranging from 0.04 to ∼2.5 at.% were deposited on SiO2 and Si substrates by co-evaporation of SiO and Er2O3 under high vacuum. Electron energy filtered imaging and elemental mapping confirm the presence of amorphous Si nanoclusters surrounded by a SiO2 matrix. Steady-state and time-resolved photoluminescence indicate that the 1.54 μm emission is highest for a 0.20-at.%-Er specimen annealed at 500 °C in 95% N2 + 5% H2, yielding effective excitation cross-sections in the range of 10−16 cm2 for 476 nm excitation. To narrow and tune the Er emission, we have incorporated SiO:Er into planar microcavities with metal mirrors. The low thermal processing temperatures permitted the demonstration of simple-to-fabricate optical microcavities with intensified and directional emission in the 1480–1610 nm range.

Introduction

Much of the recent interest in rare-earth doped silicon nanocrystals (nc-Si) has been driven by their potential use in silicon-based integrated optoelectronics [1]. The intense 1.54 μm emission from the 4I13/2  4I15/2 intra-4f transition of Er3+ has made erbium-doped nc-Si (nc-Si:Er) particularly interesting from an integrated optics standpoint, as this corresponds to the wavelength of minimum attenuation in conventional silica optical fibers. While Er-doped SiO2 suffers from ion clustering effects and low luminescence efficiency, co-doping with Si nanoparticles can increase the 1.54 μm luminescence efficiency by as much as two orders of magnitude [2]. Thus, Er-doped Si nanocomposites comprise a potentially attractive class of materials for application in optical waveguide amplifiers.

Most recent studies of Er-doped nc-Si have relied on thermal processing on the order of 1000 °C or more to induce phase separation in silicon oxides containing excess Si, precipitating it into nanocrystals surrounded by a SiO2 matrix e.g. Refs. [3], [4], [5]. While high-quality, well-passivated nanocrystals may thus be produced [6], the high annealing temperatures are incompatible with standard complementary metal oxide semiconductor (CMOS) fabrication processes [7]. This poses a significant barrier to the monolithic integration of nc-Si:Er-based photonics components with CMOS driving circuitry for optoelectronic applications. Low-temperature methods of producing Er-doped Si nanocomposites with comparable luminescent properties are therefore of practical interest. For example, erbium-doped semi-insulating crystalline and amorphous Si (SIPOS) [8], [9], and silicon monoxide [10] can exhibit intense 1.54 μm emission after annealing at temperatures on the order of 400–600 °C. Relatively recently, the optical properties of such Er-doped silicon-rich oxides have received renewed interest e.g. Refs. [11], [12], [13].

In this study, we investigate the photoluminescence from Er-doped SiO produced via standard thin film deposition techniques. The microstructure of the SiO:Er, and its effect on the emission mechanism, is examined. We optimize the 1.54 μm luminescence with respect to annealing temperature and Er concentration. To further illustrate the value of this low-temperature-anneal, we have constructed planar Fabry–Perot microcavities with metal mirrors to spectrally narrow and tune the emission, using fabrication steps that should be compatible with the post-metal processing stages of CMOS production. The resonant wavelength of these cavities is tunable across the entire 1.54 μm Er3+ emission band simply by varying the SiO:Er layer thickness.

Section snippets

Experimental

Five 150-nm-thick films of silicon oxide with varying Er doping concentrations were deposited under high vacuum on high-purity SiO2 substrates via co-evaporation of SiO and Er2O3 at a base pressure of ∼1.5 × 10−4 Pa. The silicon monoxide was thermally evaporated using a baffled box source, whereas electron-beam evaporation was used for the erbium oxide. The samples were subsequently annealed for 1 h in flowing N2, Ar, or forming gas (95% N2 + 5% H2), at temperatures ranging from 300 to 1000 °C.

Compositional analysis of SiO:Er films

The microstructure of the SiO films was investigated using TEM techniques, including high-resolution electron microscopy (HREM), selected area electron diffraction, and energy filtered TEM (EFTEM). The results illustrated in Fig. 1 are for an undoped SiO film annealed at 500 °C in forming gas for 1 h. The HREM image and diffraction pattern shown in Fig. 1(a) and (b), respectively, confirm that the SiO (when annealed at such low temperatures) is fully amorphous with a cluster/matrix combination.

Conclusions

Er-doped silicon nanocomposite thin films fabricated by co-evaporation of SiO and Er2O3 have been optimized for 1.54 μm emission with respect to Er concentration, annealing temperature, and process gas. A 0.20-at.%-Er film annealed at 500 °C for 1 h exhibited the most intense Er3+ emission, compatible with standard CMOS fabrication. This makes SiO:Er a candidate for monolithically integrated optoelectronic applications. TEM analysis indicated the presence of amorphous Si nanoclusters 2–3 nm in

Acknowledgements

Financial support for this work is due to the PRF, NSERC, and iCORE. Ken Marsh and Sergei Matveev are gratefully acknowledged for technical assistance.

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