Optical properties of ZnO homoepitaxial thin films grown by an rf‐magnetron sputtering method

Abstract

We have investigated optical properties of ZnO homoepitaxial thin films grown by an rf-magnetron sputtering method. The surface morphology observed by an atomic force microscope shows that the surface roughness is within 1.5 nm. In the reflection spectrum at 10 K, fine structures of excitonic transitions of the A and B excitons are observed. In the photoluminescence (PL) spectrum, the free exciton band is observed in addition to very sharp bound-exciton bands, while the defect-related band is negligibly weak, reflecting high crystallinity of the film. Furthermore, under high-density excitation conditions, a PL band originated from the biexciton is observed. ; however, the growth of ZnO thin films with high quality by the sputtering method has remained as a key subject. In conventional sputtering processes, thin films of ZnO have been grown on Al 2 O 3 substrates at low temperatures between room temperature and ~200 ºC. Such a low growth temperature is one of major factors for degrading crystallinity of films. Furthermore, the thin films usually contain a large residual strain because of a large lattice misfit between ZnO and Al 2 O 3 . We previously reported that the introduction of a low-temperature buffer layer and the high growth temperature of 600 ºC are essential in the growth of high-quality ZnO thin films on the Al 2 O 3 substrate by an rf-magnetron sputtering method [3,4].In the present work, we have performed homoepitaxial growth of ZnO thin films by the rf-magnetron sputtering method in order to further improve the crystalline quality and investigated their excitonic properties. The point of view in this work is to reveal the potential of the rf-magnetron sputtering method in the growth of a high-quality ZnO crystalline film, so we performed homoepitaxy of ZnO. In the reflection spectrum, fine structures of A-and B-exciton transitions are observed. In the photoluminescence (PL) spectrum, the free-exciton PL is observed in addition to very sharp bound-exciton PL, while the defect-related PL is negligibly weak. These results demonstrate high crystallinity of the films. Furthermore, we observe a PL band due to the biexciton under high-density excitation conditions.

Photoluminescence decay profiles of exciton‐exciton scattering in a ZnO thin film

“…However, in the energy region higher than 3.332 eV, the fitted result deviates from the inverse of the P-emission decay time. In this energy region, the biexciton emission is observed as shown in our previous report [11]. So, the observed decay time is affected by the coexistence biexciton component which has a long lifetime.…”