Impedance analysis of amorphous and polycrystalline tantalum oxide sputtered films
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S.S.N. Bharadwaja and Jing Li Center for Dielectric Studies, Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 20 August 2010; accepted 5 October 2010)
Impedance spectroscopy studies were conducted on amorphous tantalum oxide thin films prepared using pulsed-DC reactive sputtering, which were post-annealed to crystallize the films. X-ray diffraction results showed that crystallization to Ta2O5 b phase occurs for samples annealed above 650 °C, with a crystallite size of ;40 nm. The film microstructure was studied by electron microscopy, and remnants of the columnar amorphous microstructure were found in the polycrystalline films. Complex impedance analyses revealed significant differences in dielectric behavior between the amorphous and crystalline films. Lumped circuit models were conducted on the films using resistors, capacitors, and constant phase elements. Amorphous films exhibited a single relaxation with Arrhenius activation energy of 1.1–1.3 eV. Crystallized films exhibited two relaxations with activation energies equal to 1.1 6 0.08 and 0.6 6 0.03 eV. The relative permittivity of the bulk crystalline grain in tantalum oxide films is close to the established permittivity of the b phase (er 5 40) of Ta2O5.
I. INTRODUCTION
Numerous articles have reported on growth of tantalum pentoxide (Ta2O5) thin films and their charge transport mechanisms in metal–insulator–semiconductor (MIS)1–4 and metal–insulator–metal (MIM)5,6 configurations. In all these articles, the Ta2O5 films have been fabricated with various techniques such as chemical vapor deposition, physical vapor deposition, and anodization. Tantalum oxide films in both MIS and MIM configurations have defects that influence the electrical properties of the films. For example, they have variations in the interfacial charge densities at the Si/Ta2O5 interface and high dielectric loss and leakage current density due to Si diffusion and Si oxidation at high temperatures.7–9 Thin Ta2O5 MIM films processed under different gas-ambient conditions10,11 demonstrate increase in relative permittivity with increasing annealing temperature. Ezhilvalavan et al.12 and Pignolet et al.13 measured relative permittivity .50, which exceeds the theoretically predicted value of 40 for b tantalum oxide.14–16 Such high relative permittivity was attributed to the movement of tantalum cations across the sample17 analogous to tantalum anodization under high fields.18 Other defects such as oxygen vacancies19–20 and their accumulation near grain boundaries and film– a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.77 J. Mater. Res., Vol. 26, No. 6, Mar 28, 2011
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electrode interfaces may also be responsible for such high polarization response. A comprehensive understanding of the point defect and microstructural origins of high permittivity and dielectric loss values in tantalum oxide films is still lacking. Microstructural features such as gra
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