Micro-Raman Studies of Li Doped and Undoped ZnO Needle Crystals

Abstract

The interest in ZnO structures has increased drastically in recent years as it is a wide band gap (3.4 ev) II-VI compound semiconductor, with a stable wurtzite structure with lattice spacing a = 0.325 and c = 0.521 nm. A prominent feature of ZnO is its large exciton binding energy (~60mev) at room temperature which results in extreme stability of exciton [Khan,2005]. ZnO has been used as transparent conductors in solar cells, UV light emitters, as components in high power electronics and gas and chemical sensors. ZnO nanostructure has attracted attention for possible applications in optoelectronic and spintronic devices, such as high-emitting and laser diodes with polarized output, spin based memory and logic [Vladimir,2006]. The topic of p-doping is especially difficult and undoped ZnO exhibits n-type conductivity, and resists being doped p-type. This technological issue pulls up the use of ZnO for optoelectronics. p-type ZnO can be hypothetically achieved by doping with either Group-I or Group-V elements. Doping with Group-I elements is possibly more effective than doping with Group-V elements because of more shallow acceptors[Yamomoto,1999].It was observed that doping with Group-I elements increases donor concentration. This is attributed to tendency of Group-I dopants to occupy the interstitial sites, partly due to their small atomic radii [Park ,2002].The Group-V elements have low solubility in ZnO due to the mismatch in ionic radii. Several works on p-type ZnO doping have been made and however the results are not reproducible or questionable [Look ,2002, Look,2004]. It is believed that large difference in ionic radii between the host Zn (0.74A) and the dopant Li (0.60A) is very important for the appearance of Ferro electricity in Li-doped ZnO. The electrical resistivity due to carriers can be improved by the introduction of Li ions [Wang, 2003].The interstice impurity may result in lattice distortion. Therefore the resistivity of the ZnO sample will increase. If the annealing temperature is high, the oxygen vacancy increases which produces more electrons and hence the resistivity of Li-doped ZnO decreases [Min-Rui,2005]. Raman spectroscopy is a non destructive characterization method of choice for many recent studies of the vibrational properties of ZnO nanostructures [Khan,2005, Vladimir,2006, Harish Kumar,2007]. In the present work we have carried out a comprehensive MicroRaman scattering study of the phonons in Li doped and undoped ZnO needle crystals have been grown using flux growth to ensure good quality and an effective incorporation of the