Reflectivity of Gray Tin Single Crystals in the Fundamental Absorption Region

Single crystals of gray tin have been grown by the method of Ewald from a mercury solution. The reflection coefficient of a surface of growth has been measured in the fundamental absorption region. The reflectivity shows two maxima as in other semiconductors with diamond and zinc-blende structure. The lower energy maximum exhibits a doublet structure. The energy at which this maximum occurs is interpreted as the energy gap at the $L$ point of the Brillouin zone. The energy separation of the doublet components gives the spin-orbit splitting of the valence band at the $L$ point. From the measured value of the $L$ gap the transverse effective mass at the bottom of the conduction band is estimated to be $0.06 m$. Several general conclusions about the systematics of the band structure in all semiconductors with diamond and zinc-blende structure are presented.