  Structural defects in the p-ZnTe/i-CdTe/n-CdTe single-crystalline
heterojunctions designed for photovoltaic applications have been investigated
by transmission electron microscopy (TEM) and deep-level transient spectroscopy
(DLTS). Lattice parameters and misfit strain in the undoped CdTe absorber
layers of the heterojunctions, grown by the molecular-beam epitaxy technique on
two different substrates, GaAs and CdTe, have been determined with
high-resolution X-ray diffractometry. A dense network of misfit dislocations at
the lattice-mismatched CdTe/GaAs and ZnTe/CdTe interfaces and numerous
threading dislocations and stacking faults have been shown by the
cross-sectional TEM imaging of the heterojunctions. The DLTS measurements
revealed five deep-level traps in the heterojunctions grown on the GaAs
substrates and only three of them in the heterojunctions grown on CdTe. One of
the traps, showing the exponential capture kinetics of charge carriers, has
been identified as associated with the double acceptor level of Cd vacancies in
the CdTe absorber layers. All the other traps have been attributed to the
electronic states of extended defects, presumably dislocations, on the grounds
of their logarithmic capture kinetics. Two of these traps, displaying the
largest values of their capture cross-section and the properties characteristic
of bandlike electronic states, have been ascribed to the core states of
dislocations. It is argued that they are most likely responsible for decreased
lifetime of photo-excited carriers resulting in a low energy conversion
efficiency of solar cells based on similarly grown heterojunctions.
