Vibrational Raman and infrared studies of ordering in epitaxialZnSnP2

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 ${\Gamma }_1$ and ${\Gamma }_3$ modes on tetrahedral distortion of chalcopyrite structure. For the ordered ${\mathrm{ZnSnP}}_2$ films $(\approx -0.01\%$ lattice mismatch with the substrate) we observed several narrow $(2\mathrm{}–5{\mathrm{cm}}^{-1})$ Raman and IR lines in the $290–320{\mathrm{cm}}^{-1}$ range. Using Raman and IR selection rules for c orientation of the growth axis and lattice-dynamical calculations, we identify ${\Gamma }_1,$ ${\Gamma }_3,$ $2{\Gamma }_{4L},$ and $3{\Gamma }_{5T,L}$ modes. For the disordered sphalerite ${\mathrm{ZnSnP}}_2$ films $(\approx +0.1\%$ lattice mismatch with the substrate) we observed much broader features $(10\mathrm{}–25{\mathrm{cm}}^{-1})$ consisting of two IR-active polar ${\Gamma }_{15}$ modes and four nonpolar ${\Gamma }_1$ modes. We assign the nonpolar modes to allowed $({\Gamma }_1,{\Gamma }_3)$ and forbidden $\left(2\mathrm{}{\Gamma }_2\right)$ vibrations of chalcopyrite-type phase (nanocrystals) in a disordered matrix. Using spatial correlation model the average nanocrystal size is estimated to be $\approx 70$ nm. The spectra of the films of intermediate lattice mismatch are well described by a superposition of the spectra of the ordered and disordered ${\mathrm{ZnSnP}}_2$.