Full Length ArticleEffect of controlled Mn doping on transition of oxygen vacancies in Bi2Ti2O7 thin films: An electrochemical study
Introduction
Metal-insulator-metal (MIM) capacitors are among the most important passive devices in radio-frequency (RF) and mixed-signal integrated circuits. The International Technology Roadmap for Semiconductors (ITRS) demands that next generation MIM capacitors have a high capacitance density, high quality factor (Q-value), high voltage linearity and low leakage current for RF or analog/mixed applications; MIM capacitors should have capacitance densities of >12 fF/um2, leakage current densities of <10−9 A/cm2, and voltage coefficients of capacitance (VCC) of <100 ppm/V2 [1]. In addition to these competing performance requirements, fabrication at low temperatures (≤400 °C) is essential to dielectric thin films used for MIM capacitors in semiconductor devices due to limitations in very large scale integration (VLSI) back end of line (BEOL) processes.
Recently, Bi-based dielectric thin films such as Bi6Ti5TeO22 [2], Bi5Nb3O15 (B5N3) [3], [4], Bi1.5Zn1.0Nb1.5O7 (BZN) [5], [6], [7], Bi2Ti2O7 (B2T2) [8], [9] and Bi4Ti3O12 (B4T3) [10] have been studied extensively because of not only their dielectric properties but also low deposition temperatures. In particular, specimens with pyrochlore structures containing polarizable A-cations (in the case of the Bi-based structure, the Bi ion belongs to an A-cation) such as BZN and B2T2 have attracted considerable interest by virtue of their high dielectric constants (, low dielectric loss (tan δ), and satisfactory temperature coefficients of capacitance which can be tuned by composition [11], [12]; The A2B2O7 formula, which is typical way of describing pyrochlores, is sometimes expressed as to indicate the interpenetrating networks of a cuprite-like tetrahedral net with hexagonal tungsten bronze sheets that share an octahedral corner composed of . With regard to the dielectric behavior of Bi-pyrochlore materials, S. Kamba et al. demonstrated BZN ceramics having a distinguished cubic pyrochlore structure and found that many more polar modes were detected in BZN than the seven expected for an ideal pyrochlore arrangement, and attributed the glass-like dielectric behavior to disorder in the network [13], [14]. As mentioned above, polycrystalline thin films of B2T2 composition with a pyrochlore structure show high and low leakage current. They have been considered as alternatives to the B5N3 and B4T3 dielectric thin films used in MIM capacitors or gate insulators that are critical to advanced metal-oxide-semiconductor (MOS) transistors [15], [16]. Most of these studies, however, were done with a particular focus on the electrical properties of B2T2 films, and exclude further details regarding improvement of electrical properties and investigation of growth and/or electrochemical mechanisms.
We have previously demonstrated crystalline pyrochlore B2T2 thin films grown at 300 °C under various oxygen partial pressures, even though a B4T3 target was used [9]. In the previous work, a crystalline B2T2 phase with a high value (∼67.2) was well developed on a Pt/Ti/SiO2/Si(100) substrate at a low temperature via pulsed laser deposition. Further, Mn doping was performed to improve the electrical properties of the B2T2 thin films. These results showed that Mn doped B2T2 dielectric thin films might be used as high performance MIM capacitors. In view of these promising applications, further distinct studies of the Mn doping effect on the behavior of the oxygen vacancy and leakage current mechanisms of Mn:B2T2 films are necessary.
Here, we elucidate changes in the behavior of oxygen vacancies and electrical properties such as I–V characteristics and the work function at the interface between Pt electrodes and dielectric Mn:B2T2 films.
Section snippets
Experimental
A Bi4Ti3O12 + 10 mol% Mn (Mn:B4T3) ceramic target with a 3 inch diameter was fabricated by a conventional solid-state sintering process. In order to produce the stoichiometric composition of Mn:B4T3, Bi2O3 and TiO2 (High Purity Chemicals, Osaka, Japan) powders were ball-milled for 24 h, and then calcined at 800 °C for 10 h. Afterward, as-prepared powders were re-milled with the MnO2 (High Purity Chemicals, Osaka, Japan) additive, followed by sintering at 1000 °C for 4 h. A Nd-YAG laser beam (NL303HT,
Results and discussion
To determine the valences of Mn ions in the Bi2Ti2O7 (B2T2) phase, XPS analysis was executed on the 10 mol% Mn added Bi2Ti2O7 (Mn:B2T2) film, as shown in Fig. 1. In general, Mn occurs in three oxidation states such as Mn2+, Mn3+ and Mn4+, which are determined by process conditions. The XPS spectrum shows that most of the Mn ions exist as Mn2+ in the Mn:B2T2 film which was grown at 300 °C under 740 mTorr of oxygen partial pressure(OPP), as well as sparingly coexistence of Mn3+ and Mn4+ ions were
Conclusions
Crystalline pyrochlore Bi2Ti2O7 (B2T2) thin films were well grown at a relatively low temperature of 300 °C under 740 mTorr of oxygen partial pressure. The complex impedances of pure B2T2 and 10 mol% Mn doped Bi2Ti2O7 (Mn:B2T2) were examined to determine their resistances. Mn doping increases the resistance of the B2T2 film from 178.2 to 381.6 GΩ. Since the equilibrium concentration of the total oxygen vacancies should be constant in the crystalline B2T2 phase, the number of intrinsic oxygen
Acknowledgments
This work is supported by the R&D Center for Valuable Recycling (Global–Top Environmental Technology Development Program) and funded by the Ministry of the Environment of Republic of Korea (Project no.: RE201606123)
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