Piezoelectric photothermal spectra of Co doped ZnO semiconductor

Piezoelectric photothermal spectroscopy (PPTS) is a powerful tool for investigating optical properties of semiconductors. Temperature dependence of the PPT signal intensity of the Co-doped ZnO semiconductor was measured from 4.2 to 300 K. The Co concentration dependence of the PPT signal intensity was also studied to clarify the effect of doping with transition metals. In the case of doping at a low level, Co atoms form deep impurity levels in the ZnO matrices in the spectral region from 1.2 to 1.7 eV. However, for samples doped at higher levels, many crystal field split-off levels from localized Co2+ ions appeared at approximately 0.7-1.2 and 1.7-2.4 eV and these two bands became dominant in the spectra. The doping effect was investigated from the point of view of nonradiative transition for the first time.