Electronic mechanism of orthorhombic phase formation in someAIVBVIsemiconductors

Abstract

The electronic mechanism of phase transformation in $A^{\mathrm{IV}{B}^{\mathrm{VI}}}$ semiconductors from the NaCl-type structure to the orthorhombic black-phosphorus-type structure is proposed. The case of GeTe is considered most thoroughly. Concept of pseudospin and pseudospin static wave was proposed to describe a distribution of strong and weak bonds in an $A^{\mathrm{IV}{B}^{\mathrm{VI}}}$ compound. The origin of the two optical static displacement waves that together generate the orthorhombic modification, the transverse wave with the wave vector ${\mathrm{q}}_0$=(2π/a)(0,0,1/2), and polarization along the [110] direction of the NaCl-type structure and the longitudinal one with the wave vector ${\mathrm{q}}_1$=(2π/a)(0,0,1), is explained. The phase transformation is discussed in terms of L-Σ intervalley-coupling model and the maximum-amplitude principle. The change in band structure and the energy gain at the phase transition are calculated on the basis of the $L_v$-${\Sigma }_v$ coupling scheme. The longitudinal wave is accounted for by abnormally strong anharmonic interaction with the transverse wave. Onset of the orthorhombic γ phase in GeTe at high concentrations of nonstoichiometric Ge vacancies (or holes) is due to an increase in the $L_v$-${\Sigma }_v$ interaction on one hand, and to a decrease in the lattice parameter on the other hand. An attempt is made to extend these considerations to the orthorhombic phases of arsenic and phosphorus and thus account for stabilization of the orthorhombic modification in the presence of mercury.