Vibrational Raman and infrared studies of ordering in epitaxialZnSnP2

Abstract

We report a Raman-scattering and infrared (IR) reflectivity investigation of chalcopyrite (ordered) and sphalerite (disordered) ${\mathrm{ZnSnP}}_{2}$ grown by gas-source molecular-beam epitaxy on (001) GaAs substrates. Variation in the ${\mathrm{ZnSnP}}_{2}$ structure was obtained by controlling the Sn/Zn flux ratio during growth. Experimental results are compared with extensive calculations of the vibrational structure for the ordered ${\mathrm{ZnSnP}}_{2}$. The rigid-ion model is used to obtain lattice-dynamical properties, and the bond polarizability model was used for calculation of Raman tensor components. The model predicts a strong dependence of Raman intensity of ${\ensuremath{\Gamma}}_{1}$ and ${\ensuremath{\Gamma}}_{3}$ modes on tetrahedral distortion of chalcopyrite structure. For the ordered ${\mathrm{ZnSnP}}_{2}$ films $(\ensuremath{\approx}\ensuremath{-}0.01%$ lattice mismatch with the substrate) we observed several narrow $(2--5{\mathrm{cm}}^{\ensuremath{-}1})$ Raman and IR lines in the $290--320{\mathrm{cm}}^{\ensuremath{-}1}$ range. Using Raman and IR selection rules for c orientation of the growth axis and lattice-dynamical calculations, we identify ${\ensuremath{\Gamma}}_{1},$ ${\ensuremath{\Gamma}}_{3},$ $2{\ensuremath{\Gamma}}_{4L},$ and $3{\ensuremath{\Gamma}}_{5T,L}$ modes. For the disordered sphalerite ${\mathrm{ZnSnP}}_{2}$ films $(\ensuremath{\approx}+0.1%$ lattice mismatch with the substrate) we observed much broader features $(10--25{\mathrm{cm}}^{\ensuremath{-}1})$ consisting of two IR-active polar ${\ensuremath{\Gamma}}_{15}$ modes and four nonpolar ${\ensuremath{\Gamma}}_{1}$ modes. We assign the nonpolar modes to allowed $({\ensuremath{\Gamma}}_{1},{\ensuremath{\Gamma}}_{3})$ and forbidden $(2{\ensuremath{\Gamma}}_{2})$ vibrations of chalcopyrite-type phase (nanocrystals) in a disordered matrix. Using spatial correlation model the average nanocrystal size is estimated to be $\ensuremath{\approx}70$ nm. The spectra of the films of intermediate lattice mismatch are well described by a superposition of the spectra of the ordered and disordered ${\mathrm{ZnSnP}}_{2}$.