A method includes forming a fin on a semiconductor substrate and forming recesses on sidewalls of the fin. A silicon alloy material is formed in the recesses. A thermal process is performed to define a silicon alloy fin portion from the silicon alloy material and the fin. A semiconductor device includes a substrate, a fin defined on the substrate and an isolation structure disposed adjacent the fin. A first portion of the fin extending above the isolation structure has a substantially vertical sidewall and a different material composition than a second portion of the fin not extending above the isolation structure.