Effect of precursors stoichiometry on morphology, crystallinity and electrical properties of ZnTe epilayers grown on (100)GaAs by MOVPE

The effect of precursors vapour stoichiometry on the morphological, structural and electrical properties of nominally undoped ZnTe grown on (100)GaAs by metalorganic vapour phase epitaxy is reported. The epilayers were grown at 350 degrees C using dimethylzinc (MeZn) and di-isopropyltelluride, varying their molar flow rate ratios (MFRs) between 0.17 and 3.10. Growth in nearly stoichiometric (MFR=1.03) conditions results in best surface morphology, while samples grown in Te-rich conditions (MFR > 1.7) showed micron-size hollow defects (with surface densities up to similar to 10(6) cm(-2)) elongated in one of the < 011 > in-plane directions. The defects are associated to a local structural disorder of the material, ascribed to the formation of a Ga2Te3 extrinsic phase at the ZnTe/GaAs interface. Ohmic contacts to p-ZnTe epilayers were prepared by tungsten evaporation and annealing at 350 degrees C. The RT hole concentration in the epilayers varies almost linearly with Me2Zn molar flow between 2x10(15) cm(-3) and 5x10(16) cm(-3). Temperature-dependent Hall measurements performed on samples grown at MFR <= 1.03 demonstrate that the material p-type conductivity originates from a single acceptor centre with an ionisation energy between 94.7 meV and 118 meV, its concentration being in the 10(16) cm(-3) range and slowly increasing with Me2Zn flow. We ascribe this acceptor to a complex formed by a substitutional carbon atom on a Te site and a donor on a nearest neighbor site (C-Te-D-Zn), the donor impurity being further identified as Ga diffusing from the substrate.