Study of the effect of rapid thermal annealing in high Cd content Zn1-XCdXO nanowires grown by MOCVD.
Résumé
Zn1-XCdXO alloys offer the possibility to cover a wide spectral range by controlling the Cd concentration. However, the growth of this alloy, especially with high Cd contents, can lead to phase separation and to segregation of Cd due to its high vapour pressure. This problem may be circumvented by using nanocolumnar structures, where high Cd incorporation may be achieved [1]. However, in order to have electrical access to the ZnCdO nanowires necessary for any device, ohmic contacts must be deposited on the nanocolumns. This requires the use of a post-growth thermal cycle which may be higher than the nanowire growth temperature, and thus can also affect its optical, electrical and structural properties. In this study, we have performed an optical (cw-PL), structural (HRTEM, XRD, and micro-EDX), and electrical analysis of a series of ZnCdO nanowires grown by MOCVD on sapphire as a function of rapid thermal annealing (RTA), spanning from below (200ºC) to above (550ºC) the growth temperature (300ºC). The nanowires were typically 1-3 µm-long and 100-200 nm-wide, and contain different Cd contents (x=0, 0.14, 0.27 and 0.45). Through HRTEM and micro-EDX of single as-grown nanowires the crystal structure has been established to be wurtzite, with measured Cd contents equivalent to the nominal values, and with no indication of phase segregation or formation of cubic clusters, contrary to what has been reported by other groups at such high contents. Annealing at temperatures higher than the growth temperature produces a large increase in the PL intensity and a narrowing of the linewidth of the nanowires, especially at the highest temperatures. This effect is found to be the result of two mechanisms. First, annealing induces an increase of the free-electron concentration, especially in the reference ZnO nanowires where the as-grown electron carrier concentration is low. This in turn causes a decrease of the surface-related depletion region and thus the total nanowire volume contributing to PL is increased. Second, a decrease in the non-radiative recombination through defects is also observed, effect especially large in the Cd-containing nanowires, where the change in the surface-related depletion region is found to be negligible. However, the PL improvement also carries a blue-shift of the peak energy, which is as large as 130 meV for 550ºC annealings. HRTEM and micro-EDX of annealed single nanowires indicate that the wurtzite crystal structure is preserved even at the highest annealing cycles, and again no evidence of Cd-rich cluster formation is observed. Rather, the overall Cd content is found to decrease over the entire nanowire, between 3-5 % for the highest annealed temperature, in agreement with the change in lattice parameter observed by XRD in the ensemble of annealed nanowires. The optimum RTA cycle producing a large enhancement of the PL and minimum blue-shift., i.e. loss of Cd, is therefore found to be 450ºC, which is sufficiently high to allow the formation of ohmic contacts on the nanowires. References: [1] K. Yamamoto, T. Tsuboi, T. Ohashi, T. Tawara, H. Gotoh, A. Nakamura, and J. Temmyo, J. Cryst. Growth 312, 10 (2010).