Optical characterization of high-quality ZnO (0002) / Cu (111) epilayers grown by electrodeposition

Wurtzite ZnO thin films were grown on Cu substrates of three orientations – (111), (100), and (130), respectively – by a simple electrodeposition technique. Among them, the films deposited on the (111) and (130) substrates were epilayers with different crystallinity. Optical characterizations were therefore conducted for the (0002)_ZnO/(111)_Cu epilayers showing promising crystallinity while having different morphologies. Photoluminescence (PL) spectra of epilayers show a sharp UV near band edge (NBE) emission band, owing to excitonic transitions in ZnO layer, whereas a wide feature in visible spectral region is related to deep defect (O-vacancy) states in ZnO. The strong excitonic ZnO band (NBE) suppresses the defect (DEF) band intensity by ∼100 times, indicating good optical quality of ZnO epilayers. Using the Arrhenius-type equation, activation energies were determined as E_a = 23 meV and 27 meV, which appeared to be related with the Stokes shift between emission and absorption spectra. From the excitation-dependent (80–700 kW/cm²) PL measurements at 3 K, a linear behavior of ratio NBE:DEF was deduced, which indicated that excitonic- and defect-related radiative recombination paths are independent. Furthermore, from the dependence of the PL intensity on the laser power density, characterized with exponential factor of k ≃ 1.4, the donor-bound nature of the excitonic transitions (D^oX) in the ZnO/Cu epilayers is suggested. Corroborative structural and optical findings with the bandgap values estimated (3.362 eV and 3.371 eV) in a very good agreement with the literature data for ZnO single crystal, confirm high-quality of the studied ZnO/Cu (111) epilayers.