Photoacoustic Spectra of ZnO-CoO Alloy Semiconductors

Photoacoustic spectra (PA) of Zn1-x Co x O alloy semiconductors sintered as powdered samples were measured to estimate the optical absorption. Photoacoustic spectroscopy (PAS) is insensitive to the light scattering effects on the samples. CoO molecular percentages in the alloy were changed from 0 to 20 mol%. The samples were sintered at 600 or 950°C for 24 h in a quartz glass tube. From the PA spectra in the short wavelength region, we plotted (ρh ν)2 vs h ν and estimated the band-gap energy (E g) of the alloy semiconductors, where ρ is the PA signal intensity and h ν is the excitation light photon energy. E g of ZnO-CoO alloy decreases as the CoO content increase and reaches to 2.19 eV in the 20 mol% CoO sample. Although the band-gap variation in the lower-CoO-content samples (CoO content below 10 mol%) is large, it becomes small in the high CoO concentration samples. This result concurred with the result of dissolving CoO in ZnO as obtained by X-ray diffraction measurement.