A layer of phase change material with silicon or another semiconductor, or a silicon-based or other semiconductor-based additive, is formed using a composite sputter target including the silicon or other semiconductor, and the phase change material. The concentration of silicon or other semiconductor is more than five times greater than the specified concentration of silicon or other semiconductor in the layer being formed. For silicon-based additive in GST-type phase change materials, sputter target may comprise more than 40 at % silicon. Silicon-based or other semiconductor-based additives can be formed using the composite sputter target with a flow of reactive gases, such as oxygen or nitrogen, in the sputter chamber during the deposition.