Photoelastic Waveguides In Sige/Si Heterostructures And Bulk Si
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ABSTRACT Photoelastic waveguides in bulk Si and SiGe/Si heterostructures have been modelled and characterised. The calculated transverse strain profiles of photoelastic waveguide structures in SiGe/Si heterostructures and bulk silicon are in good agreement with those obtained by microRaman experiments. The waveguide characteristics are also found to be in good agreement with those obtained from the strain modelling and demonstrate that low loss waveguides can be fabricated using these structures. INTRODUCTION SiGe is an interesting material for integrated optics as it highly stable and is low loss at near IR wavelengths [1]. In addition, it is suitable for monolithic integration with silicon VLSI circuits for true integration. Its one major drawback, however, is the lack of a suitable optical laser source. Recently progress has been mode on these devices and various laser structures have been demonstrated theoretically, including intersubband lasers and cascade lasers, and efficient experimental solutions are being worked on currently. There are a number of silicon waveguide technologies for the fabrication of waveguides, including mesa etching [2], thermal indiffusion of Ge [3] and photoelastic waveguides in bulk silicon[4], SiGe/Si heterostructures [5] and in silicon on insulator [6] structures. Photoelastic waveguide structures are simple to fabricate and are useful for the fabrication of a range of useful devices. However, it is important to determine the properties of various waveguide structure in SiGe/Si heterostructures and bulk Si. Photoelastic waveguides have been used in GaAs based structures for some time with useful results [7,8]. In this paper we discuss the characteristics of photoelastic waveguides in both bulk Si and SiGe/Si heterostructures. We also model the strain profile in the waveguide structures using ANSYS and compare these profile to those measured directly on samples using microRaman results. EXPERIMENTAL METHOD Photoelastic waveguides were fabricated by deposition of 0.6 gIm thick Si3N4 films by PECVD onto the surface of (100) silicon (n = 5 x 1015/cm) at a temperature of 300'C. The stressor stripes were defined by patterning the Si3N 4 films into 3 gim wide stripes using photolithography and wet chemical etching. Subsequently, the samples were cleaved and the facets polished on a Planopol-2 machine using 1 jIm diamond paste to produce low loss waveguide inputs. The sample used for the Raman measurements had a 6 jim wide Si3N 4 on the surface of the SiGe/Si heterostructure. The long dimension of the stripe is in the (110) 95 Mat. Res. Soc. Symp. Proc. Vol. 486 0 1998 Materials Research Society
crystallographic direction and the SiGe layer is 20 jim thick, with a germanium content of 1.3%. This sample was annealed at 600'C for 30 minutes. The waveguides were examined using end fire coupling with an expanded beam and a lOx lens; this system was used to obtain a sufficiently small numerical aperture as well as a small laser spot size to maximise the coupling into the waveguide. An IR came
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