Apparent bipolarity and Seebeck sign inversion in a layered semiconductor: LaZnOP

The optoelectronic properties of a layered mixed-anion compound, LaZnOP, which is expected to be a wide gap n-type semiconductor, are examined. LaZnOP shows electronic conduction with conductivities up to 5.2 S/cm at room temperature by Cu doping. The signs of Seebeck coefficients suggest that undoped LaZnOP is typically an n-type semiconductor, but Cu-doped LaZnOP is a p-type. However, the Seebeck coefficients are very small compared to those observed for isostructural p-type semiconductors, LaCuOCh (Ch=S,Se), and show a sign inversion as the temperature is varied in the undoped and Zr-doped LaZnOP. The optical band gap is 1.3-1.7 eV, which is much smaller than those of related compounds such as LaCuOS (3.1 eV) and ZnO (3.3 eV). These properties can be understood by the electronic structure obtained using photoemission spectroscopy (PES) and ab initio band calculations. The results indicate that the synthesized LaZnOP has subgap states that pin the Fermi level, and these subgap states form a defect band, which gives the abnormally small Seebeck coefficients and their inversion with temperature. The band calculations suggest that anion defects may be the origin of the defect band implied by the PES measurements and control the observed electrical characteristics.