>> H/D Isotope Effect

Hydrogen passivation of defects is a standard processing step in the production of metal-oxide-semiconductor (MOS) electronic devices. Devices treated with deuterium (D) have shown transconductance lifetimes 10-50 times longer than H-passivated devices, an effect that has been termed the “giant H/D isotope effect.”

Dissociation of Si-H and Si-D is believed to be caused by ineleastic electron scattering, which excites the bonds to either a dissociative electronic state or to excited vibrational states. The vibrational excitation mechanism has been described by the truncated harmonic oscillator (THO) model. This model describes the Si-H and Si-D bonds as harmonic oscillators, and assumes that the bonds dissociate at a rate R given by the rate of excitation from the highest bound vibrational state |Nmaxñ to the lowest unbound state |Nmax+1ñ. The THO model also provides a simple explanation for the H/D isotope effect. For instance, Nmax for Si-D is ~Ö2 times greater than for Si-H, which would lead to a great reduction in the dissociation rate. Moreover, Si-D modes are usually believed to have shorter lifetimes (T1’s) than the corresponding Si-H modes, which would make the isotope effect even greater.

Against conventional wisdom we find that lifetimes of Si-D modes typically are longer than for the corresponding Si-H modes. However, the inverted isotope dependence of T1 observed for HV·VH(110) implies that this defect and perhaps also Pb:H centers, exhibits much larger differences in Si-D and Si-H dissociation rates than most other H-related defects.