Investigation of micro-strain in ZnO/(Cd,Zn)O multiple quantum well nanowires grown on Si by MBE

Abstract

The present work is dedicated to ZnO/CdO and ZnO/ZnCdO multiple quantum well (MQW) nanowires grown by molecular beam epitaxy (MBE) on Si substrates. The samples were investigated by means of micro-Raman (μ-Raman) spectroscopy and X-ray diffraction (XRD) for the presence of strain as well as verification of structural properties. For micro-Raman experiments two excitation wavelengths of 514.5 nm and 325 nm were used. The non-resonant Raman spectra show phonon modes originating from the Si substrate as well as ZnO, whereas the resonant Raman measurements exhibit multiphonon modes characteristic only for ZnO. Both non-resonant and resonant Raman spectra of ZnO/ZnCdO and ZnO/CdO MQW heterostructures reveal a red-shift of Raman peaks position with respect to that of bulk ZnO. Moreover, in the case of ZnO/CdO sample a significant broadening of the A1LO ZnO line with comparison to the one observed for pure ZnO has been observed. Both effects, the red-shift and a broadening of ZnO-related phonon modes indicate a tensile type of strain occurring in analyzed samples. The XRD signals having its source in ZnO buffer and MQWs are also significantly broadened. Such an effect has been assigned to the micro-strain in nanowires as it affects the XRD peak profile. Thus, based on μ-Raman and XRD measurements the value of micro-strain has been estimated for the studied samples and obtained results are consistent. Application of both complementary methods, μ-Raman spectroscopy and XRD, allowed to study the nanowire structures as well as MQWs in the analyzed samples.