Hopping and Drift Mechanisms of Photoconductivity in ZnO:Li Films R. K. Hovsepyan, N. R. Aghamalyan, and S. I. Petrosyan Institute for Physical Research, NAS of Armenia, Ashtarak, Armenia Received June 12, 2008 Abstract⎯The mechanisms of dark- and photoconductivity in ZnO:Li films are investigated. The obtained results are interpreted on the basis of hopping mechanism of charge carriers transport for the dark conductivity and the hopping or drift mechanism for the photoconductivity depending on the energy of an exciting photon. For photons with the energy more than the forbidden band gap the drift mechanism of carriers transport takes place, while for photons with the energy less than the forbidden band gap the hopping mechanism takes place. PACS numbers: 72.20.Ee, 72.20.Jv DOI: 10.3103/S106833720901006X Key words: ZnO:Li films, photoconductivity, charge carriers, hopping and drift mechanisms 1. INTRODUCTION High energy-gap semiconductor zinc oxide (ZnO), possessing many interesting properties, is of interest in application as light-emitting diodes and lasers working in the blue–green and UV regions, as transparent displays, UV photodetectors, gas sensors, transparent electrodes, and in converters of solar energy. Introduction of acceptor or donor impurities allows one to control the magnitude and type of the conductivity of these films and to create p–n-junctions on their base. Introduction of an acceptor Li impurity gives an opportunity to decrease the conductivity and obtain a compensated semiconductor with the n-type conductivity and high photosensitivity in the UV region of the spectrum. In high energy-band semiconductors, as a rule, three basic mechanisms of charge carriers exist [5, 6]: 1) the drift transport of charge carriers excited into delocalized states in the conduction band. For such carriers the frequency dependence of the conductivity ( ) σ ω is described by the Drude formula ( ) ( ) ( ) 2 0 , 1 σ σω= + ωτ (1) where ω is the cyclic frequency ( ) 2 f ω= π and τ is the relaxation time of charge carriers; 2) the hopping transport of charge carriers excited into localized states near the conduction band bottom. These carriers obey the statistics of a nondegenerate electron gas; 3) the hopping transport of charge carriers along localized states near the Fermi level in the impurity conduction band. These carriers obey the statistics of a degenerate electron gas. In the case of hopping mechanisms (2 and 3) the following frequency dependence of the conductivity takes place [5]: ( ) ( ) 0 , s A σ ω =σ + ω (2) where 0.8 s = and A is some constant. The transport of photoexcited charge carriers for the mechanisms 1 and 2 is realized in the conduction band. In the first case the drift photoconductivity is observed at the interband photoexcitation with the photon energy g h E ν > ( g E is the forbidden band gap). In the second case the hopping photoconductivity in observed at the interband photoexcitation with the photon energy . g h E ν= It should be noted that the hopping photoconductivity exists in the same temperature interval as the equilibrium dark hopping conductivity. In high energy-gap semiconductors containing the attachment and recombination centers, where the charge carriers transport occurs at the expense of the drift mechanism, the relaxation is caused by several processes. In this case a nonexponential decay of the photoconductivity currnet is observed. In ZnO:Li films with the hopping mechanism of the photocarriers transport the nonexponential relaxation of the ISSN 1068–3372, Journal of Contemporary Physics (Armenian Academy of Sciences), 2009, Vol. 44, No. 1, pp. 29–35. © Allerton Press, Inc., 2009. Original Russian Text © R.K. Hovsepyan, N.R. Aghamalyan, S.I. Petrosyan, 2009, published in Izvestiya NAN Armenii, Fizika, 2009, Vol. 44, No. 1, pp. 44–53. 29