Abstract

Silicon is an attractive material for monolithic integration of optical and electronic components for wavelengths above 1.1 μm. In this concept even laser diodes may be integrated by means of heteroepitaxy of III–V compounds on silicon, and detectors can be fabricated by heteroepitaxial growth of germanium¹, whereas electronic components can be realized by standard techniques. Rib waveguides have been realized in Ref. ² for an n/n⁺ silicon and proposed in Ref. ³ for a Ge-doped epilayer grown on an undoped substrate. The basic idea of our technique for integrated-optical waveguides in silicon is to indiffuse an element with a larger refractive index than that of silicon into a lightly n-doped epilayer, which is grown on a heavily doped n⁺ substrate. Because losses increase with an increase of free carriers, the element for indiffusion must belong to group IV of the periodic system. Both conditions are met by Ge that is deposited as a GeSi alloy onto the epilayer, either by quasi-simultaneous evaporation of Ge_xSi_(1-x) using a jumping E-beam or by evaporating a system of thin alternating Ge and Si layers. The channel waveguides are formed through standard lift-off techniques (Figs, 1a–1c). To avoid evaporation during diffusion, a SiO₂-overlay is sputtered onto the epilayer (Fig. 1d). After diffusion at 1200C for 69 h (Fig. 1e) the SiO₂ overlay is removed by etching (Fig. 1f). A typical depth profile of Ge is shown in Fig. 2, yielding a diffusion depth of d = 2.1 μm and a diffusion constant of D = 1.6 × 10^-2 μm²/h. Various waveguides with different stripe widths, alloy ratios, and layer thicknesses have been fabricated, and the spot sizes have been measured to be in the range of 4–8 μm for the vertical and 6–12 μm for the horizontal spot. Fitting the measured vertical spot sizes to theoryyields the maximum index enhancement as a function of Ge content, which is shown in Fig. 3. The Ge content before indiffusion is given in terms of the product of the deposited layer thickness τ and the alloy ratio x from which the maximum Ge concentration at the surface after indiffusion can be calculated if we assume that the material is completely indiffused.

© 1990 Optical Society of America