(Zn, Mg) interdiffusion in vapour transport grown ZnO/MgO core/shell nanowires and its influence on the optical properties

G. GRINBLAT; L. J. BORRERO-GONZÁLEZ; L. A. O. NUNES; M. TIRADO; D. COMEDI

Simposio; 21st International Symposium on Metastable, Amorphous and Nanostructured Materials; 2014

Many years of research have shown that the effective passivation of nanowires (NW) walls to reduce density of surface states is crucial for applications in optoelectronics due to the large specific surface area of these nanostructures. Surface trap states shorten carrier lifetimes and are believed to be responsible for the well-known defect related green luminescence [1, 2]. The MgO coating of the ZnO NW has been proved to reduce lasing thresholds and enhance excitonic photoluminescence (PL) [3]. However, there is still a need to establish growth conditions that lead to effective carrier confinement and ZnO surface trap passivation leading to optimum excitonic emission.In this work, we apply a two-step vapour transport deposition method for the growth of ZnO/MgO (core/shell) NWs, using different conditions for the MgO shell growth over the ZnO NWs. We discuss interdiffusion across the ZnO/MgO interfaces in terms of the MgO deposition parameters (temperature and time) and its influence on the PL and its temperature dependence.The samples are examined by x-ray diffraction, photoluminescence excitation and temperature (10-300 K) dependent PL. We present MgO deposition conditions that yield to a successful enhancement of the excitonic emission over the defect related luminescence. In these samples, a strong reduction of the PL thermal quenching is concomitantly observed, implying excellent passivation of surface defect states and photocarrier confinement. When the MgO deposition is carried out under conditions favouring interdiffusion between the oxides, these beneficial effects are partially inhibited. We quantify these effects using a recombination model which, in addition, enables us to determine non-radiative lifetime activation energies.These results should be important in defining NW growth conditions of high quality surface-passivated ZnO NWs for various optoelectronic and photonic applications. References: [1] Y. Yang, X.W Sun, B.K. Tay, P.H.T. Cao, J.X. Wang, X.H. Zhang; J. Appl. Phys. 103 (2008) 064307; [2] G. Grinblat, M.G. Capeluto, M. Tirado, A.V. Bragas, D. Comedi; Appl. Phys. Lett. 100 (2012) 233116; [3] Y. Wu, W. Wu, X.M. Zou, L. Xu, J.C. Li; Mater. Lett. 84 (2012) 147.