Ultrafast second-harmonic generation spectroscopy of GaN thin films on sapphire

Abstract

We have performed ultrafast second-harmonic generation spectroscopy of ${\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3.$ A formalism was developed to calculate the nonlinear response of thin nonlinear films excited by an ultrashort laser source $\left({\mathrm{T}\mathrm{i}:\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\right),$ and then used to extract ${\chi }_{\mathrm{zxx}}^{\mathrm{}\left(2\right)\mathrm{}}(\omega =2\mathrm{}{\omega }_o)$ and ${\chi }_{\mathrm{xzx}}^{\mathrm{}\left(2\right)\mathrm{}}(\omega =2\mathrm{}{\omega }_o)$ from our SHG measurements over a two-photon energy range of 2.6–3.4 eV. The spectra are compared to theory [J. L. P. Hughes, Y. Wang, and J. E. Sipe, Phys. Rev. B 55, 13 630 (1997)]. A weak sub-band-gap enhancement of ${\chi }_{\mathrm{zxx}}^{\mathrm{}\left(2\right)\mathrm{}}(\omega =2\mathrm{}{\omega }_o)$ was observed at a two-photon energy of 2.80 eV; it was not present in ${\chi }_{\mathrm{xzx}}^{\mathrm{}\left(2\right)\mathrm{}}(\omega =2\mathrm{}{\omega }_o).\mathrm{}$ The enhancement, which may result from a three-photon process involving a midgap defect state, was independent of the carrier concentration, intentional doping, and the presence of the “yellow luminescence band” defects. In addition, we determined sample miscuts by rotational SHG; the miscuts did not generate observable strain induced nonlinearities. The linear optical properties of GaN from 1.38 to 3.35 eV were also determined.