Substrate treatment and precursor stoichiometry effects on the homoepitaxy of CdTe grown by MOVPE on detector-grade (111)B-CdTe crystals

We report the metalorganic vapour phase epitaxy (MOVPE) of CdTe layers on detector-grade travelling heater method (THM)-grown (111)B-CdTe substrates, a technological step towards the fabrication of homoepitaxial p-i-n diodes as nuclear radiation detectors. CdTe layers were grown at 330 C using dimethylcadmium (Me2Cd) and di-isopropyltelluride. A quantitative analysis of the substrate X-ray surface reflectivity after a sequence of treatment steps, i.e. (i) Br2-methanol etching, (ii) in situ H2 heat cleaning at 350 C, and (iii) H2 heat cleaning and annealing in H2+Me2Cd atmosphere, demonstrated that the rms roughness of the (111)B surface steadily decreases after each treatment, the smoothest surface being obtained after annealing in H2+Me2Cd. The growth of (111)-oriented homoepitaxial layers depends critically on a combination of in situ substrate treatment and precursor stoichiometry during the MOVPE process. CdTe layers grown under a 1:1 molar flow ratio between Te and Cd precursors on H2 heat cleaned substrates show a polycrystalline structure and a rough surface morphology, an effect ascribed to poor material nucleation on the Te-rich (111)B surface, as left upon Br2-methanol etching of the THM-grown crystal. Further annealing of the substrates in H2+Me2Cd shifts the Te:Cd stoichiometry of the crystal surface closer to the ideal 1:1 bulk value. Layers grown on such Cd-annealed substrates show a more (111)-oriented crystalline texture and substantially improved surface morphologies, but also the occurrence of a tetragonal secondary phase. Fully epitaxial (111)-oriented layers were instead obtained on H2 heat cleaned substrates by growing under Cd-rich vapour conditions. (orig.)