Changes in structures and electrical conduction mechanisms of chemical vapor deposited Ta 2 O 5 thin films by annealing
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The electrical conduction mechanisms of approximately 8-nm-thick Ta2O5 films grown by metalorganic chemical vapor deposition were investigated by measuring the current density–voltage characteristics at various temperatures. The Ta2O5 films were grown in two steps with or without intermittent annealing at 450 °C under an O3 atmosphere with ultraviolet light radiation (UV-O3 treatment). High-resolution transmission electron microscopy of the films after post-deposition annealing at 750 °C under an O2 atmosphere showed that the intermittent UV-O3 treatment improved the crystallization of the film during post-annealing. Auger electron spectroscopy of the variously treated samples showed that the improvement in crystallization was due to the increase in the oxygen concentration of the Ta2O5 films by the UV-O3 treatment. The Ta2O5 film without the UV-O3 treatment mostly exhibited a Poole–Frenkel conduction behavior with the electron trap level of 0.62 eV from the conduction band edge. The whole layer UV-O3 treated Ta2O5 films also showed a Poole–Frenkel conduction behavior with an almost identical electron trap level and a reduced density. The partially UV-O3 treated Ta2O5 films exhibited a direct tunneling behavior in a relatively low voltage region by the tunneling through the thin (∼3.8 nm) UV-O3 treated surface layer. However, these films showed a Poole–Frenkel conduction behavior in the high-voltage region. In general, the UV-O3 treatment was an efficient method to reduce the leakage current of the high-dielectric Ta2O5 films. I. INTRODUCTION
High-dielectric tantalum pentoxide (Ta2O5) thin films have attracted a great deal of interest for use as a capacitor dielectric for dynamic random access memory (DRAM) devices with design rules 10−7 A/cm2) at the operation voltage of the device (∼1 V) as a result of a large density of defects that are mostly oxygen vacancies.4,5 Therefore, post-deposition annealing at high temperatures (∼750 °C) under an oxygen or O3 atmosphere is generally performed to reduce the oxygen vacancy concentration and to crystallize the films.6–8 Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2004.0203 a)
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http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 5, May 2004 Downloaded: 16 Mar 2015
However, the high temperature post-annealing oxidizes the polycrystalline-Si surface too much, which reduces the capacitance density even though the interfacial oxidation certainly helps improving the leakage current performance. Hence, a two step deposition process of the Ta2O5 films using the metalorganic chemical vapor deposition (MOCVD) has been used where a thin (2–3 nm) first deposited Ta2O5 layer is annealed at a lower temperature (∼450 °C) under an O3 atmosphere with ultraviolet light radiation (UV-O3 treatment), which is followed by depositing the main thickness of the Ta2O5 layer (∼10 nm) with a final UV-O3 treatment. This two-step process has been regarded as a viable method to improve the quality and red
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