Optical properties of non-polar ZnO using Terahertz time domain spectroscopy

The m-plane ZnO thin films grown on m-plane sapphire substrates by atomic layer deposition were investigated with the terahertz time domain spectroscopy. The terahertz emission was generated by exciting a LT-GaAs antenna with laser pulses from Ti:sapphire at the wavelength of 800 nm. One of the ZnO samples was thermally annealed with a rapid thermal annealing system with O$_{\mathrm{2}}$ at a temperature of 700 $^{\mathrm{o}}$C. The other ZnO sample was studied without annealing. The refractive indices and extinction coefficients of m-plane ZnO along c- and a-axis were derived and found significant different. For both samples, the extinction coefficient and refractive index decreases monotonically with frequency. While the mobility along a-axis was found about the same after annealing, the mobility along c-axis has been improved significantly due to annealing. The annealing treatment has shown its impact on reducing the carrier concentration of 4.5 x 10$^{\mathrm{19}}$ to 3.5 x 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}}$ for unannealed and annealed ZnO, respectively.