Selective epitaxy grows single-crystal silicon only on exposed silicon surfaces while suppressing growth on oxide or nitride areas. Mechanism: Chemistry is tuned so precursor molecules preferentially nucleate and grow on clean silicon surfaces. Growth on dielectric surfaces is suppressed by adding HCl or Cl2 which etches nuclei on non-silicon surfaces. Chemistry: SiH2Cl2 + HCl + H2 is common selective epi chemistry. Chlorine provides selectivity. Temperature: 600-900 C typical. Higher temperature improves selectivity and crystalline quality. Applications: Source/drain epitaxy in FinFET and GAA transistors - grow SiGe (PMOS) or SiP/SiC (NMOS) selectively in recessed S/D regions. Raised S/D: Selective epi builds up S/D regions above original surface for reduced resistance. SiGe: Selective SiGe epitaxy for compressive channel stress in PMOS. Ge fraction 20-50%. Faceting: Crystal growth rate varies with orientation, creating faceted surfaces at feature edges. Loading effects: Growth rate depends on local pattern density. Isolated features grow faster than dense arrays. Defects: Must avoid defects at epi/substrate interface and in grown film. Pre-epi clean is critical. Equipment: Single-wafer epitaxy reactors (ASM, Applied Materials).