Excitonic Energy Shifts in Isotopically Controlled $I-III-VI_2$ Chalcopyrites: $CuGaS_2$ and $AgGaS_2$

$CuGaS_2$ and $AgGaS_2$ tetrahedrally co-ordinated chalcopyrites are ``genealogically related" to $II-VI$ semiconductors like ZnS. We have investigated the shifts in their excitonic signatures by controlling the isotopic mass of the $I$, $III$ or $VI_2$ constituent in the crystals grown by physical vapor deposition. The excitonic signatures are observed in wavelength modulated reflectivity employing a high S/N, LED based technique.\footnote{J. S. Bhosale, Rev. Sci. Instrum. 82, 093103 (2011)} For example it reveals a 3.9 meV shift for the A exciton in $Ag^{71}GaS_2$ with respect to that of natural $AgGaS_2$; a smaller increase occurs in ZnS\footnote{M. Cardona and M.L.W. Thewalt, Rev. Mod. Phys. 77, 575 (2002)}. These effects have been related to electron-phonon interaction caused by the zero-point vibrations. Similar effects, but with an opposite sign, have been observed for Cu-isotopes in $CuGaS_2$ as well as in the Cu-monohalides CuCl, CuBr, and CuI\footnote{D.Olguin et al., Solid State Commun. 122, 575 (2002)}; their origin is receiving considerable attention at present though not yet understood. In this context the excitonic temperature dependence\footnote{Cardona, op. cit}$^,$\footnote{H.Alawadhi et al., Phys. Rev.B 75, 205207 (2007)} will be discussed.