Models for (001) epitaxial interfaces between CdTe and ZnO

Epitaxial interfaces between ZnO and CdTe appear difficult to achieve given their different crystal structures (CdTe is zinc blende with conventional lattice constant $a$ = 6.482 {\AA}, ZnO is hexagonal wurtzite with $a$ = 3.253 {\AA} and $c$ = 5.213 {\AA}.) However, ZnO also occurs in a metastable zinc-blende structure with an fcc primitive lattice constant close to the hexagonal $a$ value. Since this value is close to half of the CdTe conventional (simple cubic) lattice constant, (001)-oriented cubic ZnO films might grow epitaxially on a CdTe (001) surface in an R45\r{ } $\surd $2$\times \surd $2 configuration. Many alignments of the interfacial layers are possible, and we describe density-functional calculations on several of these to identify the most likely, and to predict valence-band offsets between CdTe and ZnO for each. Growth of ZnO on Te-terminated CdTe (001) is predicted to produce small or even negative (CdTe below ZnO) valence band offsets, resulting in a Type I band alignment. Growth on Cd-terminated CdTe is predicted to produce large positive offsets for a type II alignment as needed, for example, in solar cells. Calculations with the GGA + U method (with U = 7.5 eV for Zn 3d states) gave a valence band offset of +1.8 eV while a hybrid HSE06 + U calculation gave +2.6 eV. An experimental measurement on a ZnO film grown on CdTe (001) yielded a value of +2.2 eV for the valence band offset.