Low temperature photoconductance defect spectroscopy in ZnOLi Microwires

B. STRAUBE; G. BRIDOUX; I. LORITE; P. ESQUINAZI; M. VILLAFUERTE; C. E. RODRIGUEZ TORRES; S. P. HELUANI

Rio de Janeiro

Conferencia; XIV Brazil Meeting; 2015

Due to their interesting physics the subject of defect induced properties of widegap semiconductors have attracted much attention in the last five years [1, 2]. Thementioned properties ranging from enhanced electrical conductivity of semiconductors,changes the color and luminescence of certain crystals and recently to introducemagnetic order. In this later subject, there is now a consent that the originof the magnetically ordered state in diluted oxide semiconductors is related to latticedefects and not necessarily magnetic ions, a phenomenon called defect-inducedmagnetism (DIM).[2, 3]. However, in spite the effort of several researches, the reproducible generation of defects or introduction of dopant, to the aim of obtain magneticsemiconductors, is a goal not attained yet.Recently we report the existence of magnetic order at room temperature in 3%Li-doped ZnO microwires after low energy H+ implantation [4]. Combining spectroscopytechniques, the influence of the electronic structure and local environmentof Zn, O and Li and their vacancies on the magnetic response were described. Ourresults suggest that the origin of Room Temperature Ferromagnetism (RTF) is theO-2p spin polarization induced by the proximity to Zinc vacancies. Although ouroverall results indicate that low-energy proton implantation is an effective methodto produce the necessary amount of Zn vacancies, it is necessary to investigate whichis the critical Lithium concentration that stabilizes Zn vacancies avoiding compensationwithout formation of second phases. To this aim we prepare a new set ofsamples of ZnO microwires with Lithium concentration above and below of 3%. Theconcentration of Li ranged between 0 and 5%.In this works, low temperature photoconductance spectroscopy temerature dependenceof resistance and magnetoresistance is investigated at ZnOLi microwires.Using transport data added to luminescence measurements we successfully probethe stabilizing of Zn vacancies and the formation of shallow acceptor. Our resultsshows a pronounced decreasing of photoconductivity in Li doped samples. In samplesdoped with Li the conductivity is activated at an energy of 2.8 eV , this energycorrespond at an acceptor energy level of Zn vacancies. The pure and doped sampleshow a peak in the photoconductance at 3.4 eV , in correspondence with the gap energyof ZnO. Also the appears of a conducting metastable phase was detected usingparticularly measurements protocol below some critical temperature.1] J. M. D. Coey, P. Stamenov, R. D. Gunning, M. Venkatesan, and K. Paul, njp 12, 1 (2010).[2] P. Esquinazi, W. Hergert, D. Spemann, A. Setzer, and A. Ernst, IEEE TRANSACTIONS ONMAGNETICS 49, 4668 (2013).[3] C. Guglieri, E. Cspedes, A. Espinosa, M. n. Laguna-Marco, N. Carmona, Y. Takeda, T. Okane, T. Nakamura, M. Garca-Hernndez, M. n. Garca, et al., Advanced Functional Materials 24,2094 (2014), ISSN 1616-3028, URL http://dx.doi.org/10.1002/adfm.201303087.[4] I. Lorite, B. Straube, H. Ohldag, P. Kumar, M. Villafuerte, P. Esquinazi, C. E. Rodrguez Torres,S. Perez de Heluani, V. N. Antonov, L. V. Bekenov, et al., Applied Physics Letters106, 082406 (2015), URL http://scitation.aip.org/content/aip/journal/apl/106/8/10.1063/1.4913763.