Abstract
The structure, morphology, and luminescence of homoepitaxial ZnTe layers grown by metalorganic vapor phase epitaxy on (100)ZnTe:P wafers are reported as function of substrate surface treatment and growth conditions. Epilayers grown on substrates in situ H2 treated at temperatures above 240 °C exhibit long range crystalline perfection comparable to the substrate, but their structure rapidly degrades at lower temperatures, an effect ascribed to the incomplete removal of native oxides from the wafer surface. Instead, the material microscopic structure improves monotonically by annealing the wafer up to 350 °C. A nearly featureless surface morphology is obtained for epilayers grown within a narrow temperature interval around 350 °C, corresponding to the transition between surface kinetics and mass transport limited growth. Surface ridging along a 〈110〉 in-plane direction is observed at lower temperatures, while large pyramidlike hillocks occur randomly on the surface of samples grown above 350 °C. Besides the epilayer band-edge (BE) emission, 4.2 K photoluminescence (PL) of samples show recombination bands characteristic of the ZnTe:P wafer, their excitation being due to diffusion into the substrate of excitons photogenerated in the epilayer. Within the BE emission, both 1s- and 2s- state free exciton (at 2.3809 eV and 2.3904 eV), as well as a neutral donor (at 2.3776 eV) and acceptor bound (at 2.3748 eV, 2.3739 eV, and 2.3706 eV) exciton lines occur. Intense and narrow (1.7 meV) free-exciton emission is observed for samples grown at 350 °C and Te:Zn precursors molar flow ratio around 1, while bound-exciton lines dominate the PL spectra below 350 °C. A weak donor-acceptor pair (DAP) band, ascribed to a transition involving a donor substitutional on Zn site (DZn) and a CTe-DZn acceptor-type complex, appears between 2.16 and 2.27 eV in the PL spectra of epilayers grown below 350 °C or at low Zn precursor supply rates. The DAP band intensity well correlates with that of the 2.3776 eV donor and the 2.3706 eV acceptor bound exciton lines. At higher growth temperatures the epilayer crystallinity and radiative efficiency decrease.
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