Mechanical stress in InP and GaAs ridges formed by reactive ion etching

The mechanical deformation induced by reactive ion etching (RIE) of rectangular ridge waveguides in GaAs and InP has been investigated by photoluminescence and cathodoluminescence techniques. Several trends were identified and are discussed. First, it is concluded that the RIE process itself is the source of the mechanical deformation. A compressive volume change occurs mainly within the ridge (with a maximum close to the vertical etched sidewalls), extending outside the ridges, up to several micrometers below the bottom etched surface. An anisotropic deformation also appears, again mainly close to the etched sidewalls and below the bottom etched surface. A narrow area under tensile stress was also identified, localized outside the ridges and in a shallow region below the bottom etched surface. Cumulative, overlapping effects are seen inside the ridges where the compressive stress fields originating at the vertical etched sidewalls contribute to an overall compression inside the ridge which increases as the ridge width decreases. In addition, a tensile stress is also observed outside the ridge, strongly enhanced by the presence of neighboring ridges. These conclusions are significant for the design of photonic structures. Because of the photoelastic effect, which is important in GaAs and InP, the properties of devices such as waveguides might be affected by the mechanical stress described herein.