Structural and optical impact of transition metal implantation into zinc oxide single crystals and nanowires

Abstract

Transition metal alloyed zinc oxide (ZnO) materials are promising candidates for spin-based electronics (spintronics) and optoelectronic applications. The interaction between the ZnO host lattice and the incorporated transition metal ions form the basis for these applications and can induce a ferromagnetic alignment of the transition metal spins with a Curie temperature above room temperature. On the other hand, the electronic structure of the 3d-shell of the transition metal ions is affected by the crystal field, spin-orbit and Jahn-Teller interaction with the ZnO host matrix. As a result, optical transitions between the 3d-shell states become partly allowed. The control of the transition metal concentration as well as the formation of secondary phases are the major issues not only for the preparation of transition metal alloyed ZnO single crystals and thin films but also for nanowires. Commonly ZnO nanowires are grown via the vapour liquid solid mechanism, which is discussed for ZnO nanowires with respect to the issues of the transition metal incorporation. Ion implantation after the growth of single crystals, thin films and nanowires is an alternative method for the controlled incorporation of transition metals into ZnO. However, ion implantation generates crystal defects, which are observed by an intensity increase of the A1(LO) phonon disorder band of as implanted ZnO. Annealing under air ambient at different temperature stages leads to an reduction of a major part of these defects. High dose transition metal implanted ZnO samples show after annealing some additional Raman modes and Bragg diffractions, which can not be assigned to ZnO. A detailed analysis of this signals reveals the formation of several transition metal rich segregations. On the other hand, photoluminescence and cathodoluminescence measurements show two new deep luminescence bands after transition metal ion implantation and annealing. The red-yellow luminescence band is assigned to interstitial oxygen defects with tetrahedral surroundings. The structured green luminescence band is commonly assigned to Cu impurities. However, the configuration coordinate model is used to explain the structured green luminescence band, whereby the structured green luminescence band is a superposition of two donor-acceptor transitions with the same deep acceptor and two different shallow donors. The chemical nature of the deep acceptor is assigned to interstitial oxygen on an octahedral site, and the two shallow donors are assigned to interstitial zinc on octahedral as well as tetrahedral sites. Co and Fe implanted ZnO samples show additional sharp luminescence lines, which are assigned to intra-3d-shell transitions of Co2+ and Fe3+ ions in the ZnO host lattice. The intensity of intra-shell luminescence increases almost linearly with increasing excitation power and exceeds the linear behaviour for excitation power densities above 300 mW/cm2, which suggest a transfer from spontaneous emission to stimulated emission.