Raman spectroscopy of V and Co doped ZnO ceramics and thin films
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Raman spectroscopy of V and Co doped ZnO ceramics and thin films K. Samanta, N. Awasthi, B. Sundarakannan, P. Bhattacharya, and R.S. Katiyar Department of Physics, University of Puerto Rico, San Juan, PR 00931-3343.
ABSTRACT Lattice dynamical and electronic transition changes due to V and Co doped ZnO have been investigated using optical techniques. Vanadium and Co doped ZnO pellets were prepared using conventional ceramic processing route and thin films were fabricated by pulsed laser deposition. Raman spectra of Zn1-xVxO targets showed many additional peaks in the range of 230 to 350 cm-1 and 750 to 900 cm-1. Integrated intensities of these additional modes decreased with increase of temperature as similar to the host ZnO modes, which precludes electronic Raman scattering to be the origin. Raman peaks for stoichiometric Zn3(VO4)2 and Zn2V2O7 compounds also had additional peaks that can be attributed to the secondary phases formed in the compositions of Zn1-xVxO. Raman spectra of Zn1-xCoxO showed no additional modes besides ZnO modes, however, the intensity of the second order peak at 540 cm-1 was increased due to Co doping. Thin films of Zn1-xCoxO exhibited highly c-axis orientation deposited on (001)Al2O3 substrates. The optical absorption of the films showed that the band gap decreased with increase of Co concentrations at room temperature along with the sub bandgap absorptions due to d-d transitions of Co2+. INTRODUCTION: Electronic devices based on spin current controlled by internal magnetic fields are expected to play a dominant role in the development of novel magneto-electronic devices. Long spin relaxation time, large spin transport distances in semiconductors and various device structures have already been demonstrated [1,2]. There is a constant surge to modify ZnO and related wide band gap semiconductors for applications in shortwavelength semiconductor diode lasers [3-4]. It has also been experimentally demonstrated that the 3d transition metal atoms are soluble up to several mole fraction (~0.35) in ZnO host, which made it a promising candidate for fabrication of dilute magnetic semiconductor (DMS) with a high Curie temperature. Mn-doped ZnO can be ferromagnetic with a very high Curie temperature (>300K) as predicted by Dietl et al, if doped into a p-type semiconductor [5]. However, Norton et al. [6] reported ferromagnetism that was obtained into n-type Sn doped ZnO single crystals with Mn implantation. Several theoretical predictions have raised the possibility of ferromagnetism above room temperature in 3d transition metal (TM) doped ZnO [7]. It is expected to exhibit a strong magnetic interaction between the mobile carriers and the localized magnetic ions due to its large electron mass of 0.3 me. One of the major challenges for the introduction of magnetic impurity is clustering of the transition metal related secondary phase in DMS. Solid solubility of transition metal (Mn, Fe,Co,V etc.) is not so stringent for II-VI semiconductor compared to III-V materials. However, depending upon the
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