Sequential two-stage displacive transformation from β to α via β′ phase in polymorphic MnTe film

• Main Authors: Shunsuke Mori, Daisuke Ando, Yuji Sutou
• Format: Article
• Language: English
• Published: Elsevier 2020-11-01
• Series: Materials & Design
• Subjects: Sputtering; Thin film; Compound semiconductors; Phase transformation; Displacive transformation
• Online Access: http://www.sciencedirect.com/science/article/pii/S0264127520306766

MnTe films exhibit a remarkable change in physical properties upon polymorphic transformation from β (wurtzite-type) to α (nickeline-type) phase. In this study, the mechanism of the β to α transformation in MnTe films capped with a tungsten layer during isothermal annealing at 500 °C was investigated. X-ray diffraction analyses revealed that the out-of-plane c-axis orientation was maintained during the polymorphic transformation. High-resolution transmission electron microscopy revealed that the β-phase transformed to the α-phase via a β′-phase which has a wurtzite-type structure as the β-phase but with a slight difference in the coordinates of the Te atoms. The β → β′ transformation was induced by the puckering process during which Mn- and Te-atomic planes shift in opposite directions along the c-axis, whereas the β′ → α transformation was induced by a buckling process during which pairs of Mn- and Te-atomic planes alternately move to the opposite directions along the 210 direction. Meanwhile, in the MnTe film without the tungsten capping layer, the out-of-plane c-axis orientation was not retained during the polymorphic transformation. These results imply that the sequential two-stage polymorphic transformation maintaining the out-of-plane c-axis orientation is caused by the constraint on the MnTe film induced by the tungsten capping layer.