Host isotope effect on the local vibration modes of VH₂ and VOH₂ defects in isotopically enriched²⁸Si, ²⁹Si and³⁰Si single crystals

Local vibrational modes of a vacancy with two hydrogen atoms (VH ₂) and of a vacancy with one oxygen and two-hydrogen atoms (VOH ₂) in silicon have been investigated using isotopically enriched ²⁸Si, ²⁹Si, and ³⁰Si single crystals. Infrared absorption spectroscopy revealed shifts in the Si-H stretch frequencies of the two defects when the mass of the silicon host atoms was changed. The observed stretch frequencies can for each defect be accounted for with a simple vibrational model based on two coupled Morse oscillators. The anharmonic contribution to the local vibrational mode frequencies of these two defects is evaluated.