Mg-induced Enhancement of ZnO Optical Properties via Electrochemical Processing
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Mg-induced Enhancement of ZnO Optical Properties via Electrochemical Processing Hongtao Shi, Kalie R. Barrera, Timothy L. Hessong, and Cristhyan F. Alfaro Department of Physics and Astronomy, Sonoma State University, Rohnert Park, CA 94928, U.S.A.
ABSTRACT Electrochemical deposition was used to fabricate polycrystalline ZnO thin films in solutions containing zinc nitrate and hexamethylenetetramine. All samples showed intense UV photoluminescence (PL) near the band edge in addition to weak broad bands due to defects. When the source solution was slightly doped with Mg2+ ions, the defect induced emission was significantly suppressed while the UV peak position and intensity remained the same. Auger electron spectroscopy revealed no Mg contents in the films within the detection limit. A possible growth mechanism was proposed, based on the chemical reactivity of Mg and Zn, to interpret the observed PL data, which is supported by samples grown in Ca-doped solutions. INTRODUCTION Zinc oxide (ZnO) has recently drawn much attention due to its outstanding physical properties and potential applications in fields such as ultraviolet (UV) optoelectronics and photonics [1]. It is generally accepted that the UV photoluminescence (PL) from ZnO is due to the excitonic recombination, whereas a broad band PL involves different types of defects, presenting an obstacle to the realization of practical devices. While most sample fabrication techniques require a growth temperature of 350 oC or higher, a low temperature solution route has indeed demonstrated wafer-scale production of ZnO nanowires [2,3], and possible room temperature ferromagnetism [4]. Moreover, MgxZn1-xO alloys have been demonstrated to have a tunable band gap from 3.3 eV to as high as 7.8 eV, which depends linearly on the Mg content in the alloy and could be used in deep UV applications [5,6,7]. While the band gap enlargement is well established in MgxZn1-xO, few reports have concentrated on the effects of Mg as a dopant on the formation of deep level defects, in particular in a low-temperature solution process, as these defects can significantly affect the optical and electrical performances of ZnO-based devices. In this paper, we report on the growth of ZnO thin films on aluminum (Al) foils using electrochemical deposition (ECD). Samples, grown in solutions containing Mg2+ or Ca2+ ions, show much enhanced optical properties, which is attributed to the chemical reactivity of these elements. EXPERIMENTAL DETAILS ZnO thin films were grown in a Virgin Teflon cell on 0.2 mm thick, 99.9995% pure Al foils, which were electropolished in a perchloric acid and ethanol mixture (1:4, v/v). In a typical cycle, a solution of 40 ml, containing 20 millimolar (mM) zinc nitrate hexahydrate [Zn(NO3)2•6H2O]
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and 20 mM hexamethylenetetramine (C6H12N4 or HMT), was used as electrolyte with the latter providing a basic environment, resulting in crystals with well-defined smooth surfaces [8]. A pure ZnO sample (referred to as ZnO:Pure hereinafter) was grown at 80 oC for 3 hours under a c
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