Effect of Li-doping on Photoluminescence of Screen-printed Zinc Oxide Films
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Effect of Li-doping on Photoluminescence of Screen-printed Zinc Oxide Films L. Khomenkova1, V. Kushnirenko1, M. Osipenok1, K. Avramenko1, Y. Polishchuk1, I. Markevich1, V. Strelchuk1, V. Kladko1, L. Borkovska1 and T. Kryshtab2 1
V. Lashkaryov Institute of Semiconductor Physics of NAS of Ukraine, Pr. Nauky 41, 03028 Kyiv, Ukraine E-mail: [email protected] 2
IPN – ESFM, Av. IPN, Edificio 9 U.P.A.L.M. C.P. 07738 México E-mail: [email protected]
ABSTRACT Undoped and Li-doped ZnO films were fabricated by screen printing approach on sapphire substrate. The effect of Li doping and annealing temperature on the luminescent, optical, electrical and structural properties of the films has been investigated by the photoluminescence (PL), Raman scattering, conductivity, Atomic Force microscopy and X-ray diffraction (XRD) methods. The XRD study revealed that the films have polycrystalline wurtzite structure with grain sizes ranging from 26 to 38 nm. In the undoped ZnO films, the increase of annealing temperature from 800 to 1000 °C resulted in the increase of the grain sizes, film conductivity and the intensity of the ultraviolet PL. The introduction of Li of low concentration of 0.003 wt % at 800 °C or 900 °C allows producing the low-resistive films with enhanced ultraviolet PL and reduced density of crystalline defects. Highly doped films (with 0.3 wt % of Li) were found to be semi-insulating with deteriorated PL properties irrespectively of the annealing temperature. It is shown that introduction of Li in the ZnO films affects their PL spectra mainly via the evolution of the film crystallinity and the density of intrinsic defects.
INTRODUCTION Over the past decades, ZnO-based materials have been attracted considerable attention for possible application in optoelectronic devices, especially to ultraviolet (UV) light emitters [1]. The structural, optical and electrical properties of ZnO films can be governed by dopants, deposition parameters and post-treatments. Doped ZnO films are of great interest for their applications in transparent conducting electrodes for solar cells and displays [2, 3] and insulating or ferroelectric layers for optical memory devices [4]. However, hardly achievable p-type conductivity of ZnO-related materials remains the drawback for their wide applications. The large efforts were applied to obtain p-type ZnO by doping with elements of groups I (Li, Na, K) or V (N, P, As) as well as by their co-doping [1]. Among these, lithium (Li) has been considered as a promising dopant for ZnO [5-8] in spite of rather contradictory viewpoints on realization of stable p-type activity under equilibrium conditions.
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As a rule, Li-doping occurs as follows [9]:
Li2O ZnO o LiZn Lii OO
(1)
Where LiZn represents lithium on zinc lattice site, Lii is a lithium in interstitial position, and OO is an oxygen on its lattice site. The most of experimental reports supposed that LiZn is a deep acceptor with a binding energy of about 800 meV responsible for luminescence band peaked at 2.0-2.2 eV [10-13]. How
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