||This work studied the electric and luminescent properties of Cu-doped ZnO epilayer films and epilayer that have been grown on LAO (LiAlO2) and MgO substrates, respectively, by plasma-assisted molecular beam epitaxy. The doping quantity was controlled by regulating Cu Kundsen cell temperature.The crystallinity of the Cu-doped ZnO films and epilayer were characterized by high resolution X-ray diffraction (HRXRD). The surface morphology was observed by scanning electron microscopy (SEM). The chemical state and elemental compositions was analyzed by X-ray photoelectron spectroscopy (XPS). Finally, the near-band-edge emission (NBE) and the defect-related emission (DLE) were analyzed by photoluminescence (PL) spectroscopy. The electric properties were determined by Hall measurement and four-point probe measurement.|
The experimental results revealed ZnO films grown on LAO substrates were polycrystalline and have a //ND textured ZnO with a wavy surface. PL spectra exhibited extremely low intensity of NBE peaks. On the other hand, ZnO was grown epitaxially on MgO substrate with (101 ̅0)ZnO//(100)MgO. The full width at half maximum (FWHM) of the (101 ̅0)ZnO X-ray rocking curve were in the range of 0.7o~1.4o. The m-plane ZnO epilayers exhibit a typical stripe morphology
The intensity of the NBE peak of the epitaxial ZnO is much higher than that of the polycrystalline ZnO. Furthermore, the NBE peak of the epitaxial Cu-doped ZnO can be deconvoluted into three sub-peaks: a free exciton emission (FX) at 3.29 eV, a basal plane stacking fault emission (I1 BSF) at 3.19 eV, and a third peak at 3.25 eV. The third peak might be related to an acceptor-bound excitons (AoX) due to the doping of copper.
The XPS analysis showed the copper atoms have a monovalent state in the ZnO layers grown at Cu Kundsen cell temperatures lower than 900oC. The amount of Cu incorporation increased from 2x1019 to 3x1020 atoms/cm3.
Hall measurement indicated that both pure and Cu-doped ZnO epilayers are n-type semiconductor. The electron concentration of pure ZnO epilayer is 1x1018 cm-3. Whereas that of the Cu-doped ZnO epilayers are in the range of 5x1015~5x1016 cm-3. Obviously, the electron concentration of ZnO is compensated by the substitution of Cu+ to Zn2+ which generates acceptor levels. However, the electron concentration of the Cu-doped ZnO epilayers rose to the order of magnitudes 1018 cm-3 after annealed in oxygen or hygrogen, indicating the acceptor states were eliminated. Moreover, in this study, copper doped into ZnO does not increase of the yellow-green luminescence.
Four-point probe measurement given the resistivity of Cu-doped ZnO epilayers which grown at Cu Kundsen cell temperature 400oC~800oC were in the range of 10-1~100 Ωcm, corresponding to an electron mobility of ~50 cm2V-1s-1.