  Layered oxysulfide LaCuSO (x = 0-0.03) was prepared to elucidate the effect
of Cu off-stoichiometry on their electrical and thermal transport properties.
Electrical resistivity drastically decreases down from ~10^5 ohmcm to ~10^-1
ohmcm as a result of Cu deficiency (x = 0.01) at 300 K. Thermal conductivity of
the samples at 300 K, which is dominated by lattice components, is estimated to
be 2.3(3) Wm^-1K^-1. Stoichiometric LaCuSO has an optical band gap of 3.1 eV,
while broad optical absorption at photon energies of approximately 2.1 eV was
observed for Cu-deficient samples. Density functional theory calculation
suggests that these broad absorption structures probably originate from the
in-gap states generated by the sulfur vacancies created to compensate the
charge imbalance due to Cu off-stoichiometry. These results clearly demonstrate
that Cu deficiency plays a crucial role in determining the electrical transport
properties of Cu-based p-type transparent semiconductors.
