Techniques for forming contacts comprising at least one crystal on source and drain (S/D) regions of semiconductor devices are described. Crystalline S/D contacts can be formed so as to conform to some or all of the top and side surfaces of the S/D regions. Crystalline S/D contacts of the present disclosure are formed by selectively depositing precursor on an exposed portion of one or more S/D regions. The precursor are then reacted in situ on the exposed portion of the S/D region. This reaction forms the conductive, crystalline S/D contact that conforms to the surface of the S/D regions.