Homoepitaxy of ZnTe on (100) oriented substrates: technology issues and MOVPE growth aspects

The metalorganic vapour phase epitaxy of ZnTe on single crystal (100)ZnTe:P wafers is reported. The technological steps to prepare a substrate surface suitable for the high quality homoepitaxy of ZnTe are identified and optimised in terms of structural and morphological properties of overgrown epilayers. Removal of ~7 µm of material from the ZnTe:P wafers by chemical etching in 1% Br2-methanol solution proved necessary to achieve a sufficiently smooth and homogeneous surface; in-situ H2 heat treatment of the wafers at 350°C immediately before growth ensures optimal desorption of residual oxides, allowing epilayer crystalline quality comparable to the substrate. However, the structure of epilayers degrades for growth temperatures (Tg) above 350°C due to the occurrence of stacking faults (SFs) within ~200-300 nm from the epilayer-substrate interface. Accordingly, the epilayer band-edge luminescence vanishes below 350 nm, indicating a worsening of the material radiative efficiency in very thin epilayers. The epilayer surface morphology is the result of a complex interplay between SF nucleation and Te:Zn ad-atom stoichiometry during growth. Almost featureless morphologies are obtained for growth at 350°C, i.e. under nearly stoichiometric surface conditions. Pyramid-like hillocks develop instead for TG≥360°C, corresponding to Te-rich surface conditions, their density rapidly increasing up to around 9×106 cm-2 at TG=400°C. Hillocks occur in close pairs on the epilayer surface, their nucleation being strongly reduced if a thin ZnTe buffer layer is grown at low (325°C) temperature, i.e. if SFs do not occur at the epilayer-substrate interface. This demonstrates that hillocks form as a result of three-dimensional growth around partial dislocations pairs bounding SFs, the phenomenon being driven by Te ad-atoms experiencing a Schwoebel potential barrier at the surface step edges around the dislocations.