Amorphous Silicon Based Waveguides And Light Modulators For Silicon Low-Cost Photonic Integrated Circuits
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dellacor@irecel .irece.na.cnr.it •* Ente per le Nuove Tecnologie, l'Energia e l'Ambiente - Centro di Portici (ENEA-CRP), Via Vecchio Macello, 1-80055 Portici (Naples), Italy () also with University of Calabria - Electronic Engineering Dept., 1-87036 Rende (CS), Italy
ABSTRACT This paper reports about the fabrication and experimental test of an interferometric light intensity modulator integrated in a low loss (0.7 dB/cm), amorphous silicon based waveguide. It measures approximately 1 mm in length, while its cross section is 30-ptm-wide and 3-pim-high. The device, which exploits the strong thermo-optic effect in thin film a-Si for its operation, is designed for application at the infrared wavelengths of 1.3 and 1.55 jim. The measured maximum operating on-off switching frequency of the device is 600 kHz. The very simple fabrication technology involves maximum process temperatures of 230 'C, and is therefore compatible with the standard microelectronic technology. This offers a new opportunity for the integration of optical and electronic functions on the same substrate. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) and its related alloys, like amorphous silicon
carbon (a-SiC:H) and silicon germanium (a-SiGe:H), have been recently considered for the fabrication of new active and passive optoelectronic devices. In fact, visible and infrared photodetectors [1,2], light emitting diodes [3,4], optocouplers [5], low loss optical waveguides [6] have been successfully fabricated with these materials. The main advantages offered by the aSi:H are the simple and inexpensive technology, which is basically the one developed for the wide-area, low-cost, thin film solar cells industry, and the low deposition temperature that makes it compatible with the VLSI technology. Another important benefit is the possibility of changing its optical gap. In fact, by mixing the main process gas, i.e. SiH 4 , with other components like CH 4, CO 2 or NO 2 during the plasma discharge, it is possible to obtain alloys with a wider gap, while the opposite behavior is achieved adding GeH 4. Using this technology, in this paper we present preliminary results on the realization and characterization of new light amplitude modulators designed for optical digital communication purposes in the II and III fiber windows. The devices consist of a planar Fabry-Perot interferometric cavity, integrated in a rib waveguide fabricated onto a crystalline silicon wafer. The three-dimensional waveguiding structure has been defined by photolithography out of an aSi:H/a-SiC:H stack, grown by Plasma Enhanced Chemical Vapor Deposition (PECVD). The step-wise refractive index structure produces a strong confinement of the radiation in the a-Si:H core layer, while the high electronic quality of the material confers low absorption at the near infrared wavelengths. The modulators exploit the strong thermo-optic effect in a-Si:H to operate. A variation in the longitudinal modes of the resonant cavity is induced by a rapid change of the 113 Mat. Res. Soc. Symp. Proc. Vol.
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