Light emission and propagation in 1D and 2D Si photonic crystals
V. Calvo, B. Cluzel, Th. Charvolin, E. Hadji, E. Picard, D. Sotta, M. Zelsmann, M. Heitzmann, H. Moriceau (LETI-DTS), P. Ferrand, R. Romestain (LSP/CNRS)

Low cost Si technology makes this material potentially interesting for optical applications operating in the 1-1.5 µm wavelength range like telecommunications or optical interconnections within microelectronic chips. But light emitters are still difficult to obtain from silicon due to its indirect band gap. Nevertheless recent progress showed the possibility of strong light emission from doped silicon material at room temperature. These results open the route to practical Si based microphotonic devices if one is able to control the propagation of photons in the material and even more if one is able to enhance their emission. To improve the emission properties we propose in this work to change the emission rate and/or extraction of photons from the Si layer by the use one dimensional (1D) and or two dimensional (2D) photonic bad gap (PBG) structures [1],[2]. A first structure for photon confinement is the microcavity. This system consists of two mirrors surrounding a cavity material which optical thickness should equals the propagating wavelength. By this way one realise the confinement of photons in one direction (i.e. perpendicular to the plane of the layers) and select the optical modes which can propagate inside the cavity.

 

Low cost Si technology makes this material potentially interesting for optical applications operating in the 1-1.5 µm wavelength range like telecommunications or optical interconnections within microelectronic chips. But light emitters are still difficult to obtain from silicon due to its indirect band gap. Nevertheless recent progress showed the possibility of strong light emission from doped silicon material at room temperature. These results open the route to practical Si based microphotonic devices if one is able to control the propagation of photons in the material and even more if one is able to enhance their emission. To improve the emission properties we propose in this work to change the emission rate and/or extraction of photons from the Si layer by the use one dimensional (1D) and or two dimensional (2D) photonic bad gap (PBG) structures [1],[2]. A first structure for photon confinement is the microcavity. This system consists of two mirrors surrounding a cavity material which optical thickness should equals the propagating wavelength. By this way one realise the confinement of photons in one direction (i.e. perpendicular to the plane of the layers) and select the optical modes which can propagate inside the cavity. An other way to get photon confinement is to use the natural light confinement of the high index Si material in the vertical direction and to structure the material in the two other in-plane directions. This can be done for instance by etching a waveguide in the Si layer and then etching different arrays of holes to get a photonic bandgap effect in the waveguide.

 

One example of a waveguide integrated 2D PBG microcavity is presented here. We started with a SOI substrate having a 0.4 µm thick mono-crystalline Si layer on a 0.7 µm buried SiO2. We first fabricated several 8 µm wide optical waveguides in the SOI substrate. Then a triangular lattice of holes (period = 390 nm, hole diameter = 240 nm) was fabricated using e-beam lithography and plasma etching in one waveguide. This structure was designed to have its PBG centred at 1.3 µm. Two similar PBG mirrors were then fabricated in an other waveguide with a lambda spacing between them to form a resonant cavity. Optical characterisation was performed on a very wide wavelength range (1.1 to 1.7 µm). This was allowed by the coupling of a white light source in each waveguide through a cleaved edge of the sample and collecting the light from the other side of the waveguide. The measurements performed on the waveguides containing the PBG structures were normalized by the one made on the waveguide without PBG structure to get the absolute transmission of the PBG mirrors and cavities. When the waveguide contains only one mirror, the transmission measurement shows a clear photonic band gap in the TE-polarization. In the case were the waveguide contains a complete microcavity structure with two mirrors, the resonant cavity modes are clearly seen within the bandgap. We then performed 2D finite difference time domain (FDTD) and 3D plane-wave based frequency domain calculations taking into account the PBG parameters (diameter, period) measured on scanning electron microscope (SEM) pictures. All these calculations appear to be in very good agreement with the measurements.

 

With the capability to control the propagation of photons in the plane of the structure, we can now try to change the emission rate and/or extraction of photons in the two dimensions of the Si layer. Different structures have been developed : hexagonal cavities (H5) and defect-less structures.

 

In both cases we have observed a strong light extraction enhancement as compared to SOI films without photonic crystals. The H5 cavity is working in the bandgap regime of the photonic crystal surrounding the cavity. In this case, the 15 time intensity enhancement is attributed to the DOS change obtained by the resonance effect in the plane of the structure. In the photonic bandgap, the photons which are concentrated at resonant wavelengths cannot escape in the plane of the structure. At the same time they can couple to the continuum of optical modes in the air above the semiconductor. For the defect-less structure we have also observed a strong increase (70 fold) of light emission in the vertical direction. While the previous effect was wavelength sensitive due to the resonance inside the cavity, the defect-less extraction is a broadband process. The structure has been designed to obtain optical modes with a low group velocity feature at the wavelength band corresponding to Si emission. The light emitted by the Si layer can then couple to these modes located in the photonic conduction band. Since these modes have a very low group velocities around the gamma point of the band diagram, light can efficiently radiate in the air.

 

These preliminary results on PBG structures open the route to the integration of functional devices like filters, modulators or resonant detectors within silicon chips. They might also be a route towards CMOS compatible silicon-based light emitters by controlling in 3D the emission of photons from Si based nanostructures. Moreover they may be a very good candidate to demonstrate Purcell effect in a small volume Si based strongly confined PBG cavities.

 

[1] D. Sotta, E. Hadji, N. Magnea, E. Delamadeleine, P. Besson, P. Renard, H. Moriceau, Journ. Appl. Phys. 92 (4) 2207 (2002). [2] M. Zelsmann, E. Picard, T. Charvolin, E. Hadji, B.Dal’zotto M.E. Nier, C. Seassal, P. Rojo-Romeo, and X. Letartre, Appl. Phys. Lett. 81 (13) 2340 (2002).

 

Maj : 27/09/2016 (71)

 

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