A method of forming a semiconductor structure is provided. The method includes forming a gate structure over an active region of a substrate, forming an epitaxial layer comprising first dopants of a first conductivity type over portions of the active region on opposite sides of the gate structure, applying a cleaning solution comprising ozone and deionized water to the epitaxial layer, thereby forming an oxide layer on the epitaxial layer, forming a patterned photoresist layer over the oxide layer and the gate structure to expose a portion of the oxide layer, forming a contact region including second dopants of a second conductivity type opposite the first conductivity type in the portion of the epitaxial layer not covered by the patterned photoresist layer, and forming a contact overlying the contact region.