Methods for forming carbon silicon alloy (CSA) and structures thereof are disclosed. The method provides improvement in substitutionality and deposition rate of carbon in epitaxially grown carbon silicon alloy layers (i.e., substituted carbon in Si lattice). In one embodiment of the disclosed method, a carbon silicon alloy layer is epitaxially grown on a substrate at an intermediate temperature with a silicon precursor, a carbon (C) precursor in the presence of an etchant and a trace amount of germanium material (e.g., germane (GeH4)). The intermediate temperature increases the percentage of substitutional carbon in epitaxially grown CSA layer and avoids any tendency for silicon carbide to form. The presence of the trace amount of germanium material, of approximately less than 1% to approximately 5%, in the resulting epitaxial layer, has an effect of stabilizing and enhancing deposition/growth rate without compromising the tensile stress of CSA layer formed thereby.