Electrical Transport and Optical Properties of Hydrogen-Deficient YH ₂ Films

We have investigated electron transport characteristics of a hydrogen-deficient dihydride of yttrium YH_2-3 (-0.3 < & δ < 0.05) focusing particularly on the effect of hydrogen deficiency on carrier relaxation time. At room- and low-temperatures, dc-specific resistivities obtained from electrical measurements show minimum values when hydrogen-deficient quantity δ is approximately -0.1. Optical conductivities were obtained from reflectance measurements by the Kramers-Knonig transformation. When & is changed, the carrier scattering rate determined from the optical conductivity shows a behavior similar to those of dc resistivities. Not only the carrier scattering rate but also a band-gap energy E₀ associated with a Γ₂-Γ₂₅ transition is influenced by the hydrogen deficiency; E₀ shows a maximum value as δ is about -0.1. Hall coefficients are found to be positive but fairly small (∼10^-11 m³/C), indicating that electrons as well as holes presumably contribute to electric conduction in YH_2+δ.