Rietveld analysis of CaCu3Ti4O12 thin films obtained by RF-sputtering

Abstract

Calcium copper titanate, CaCu 3 Ti 4 O 12 , CCTO, thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO 2 /Si (100) substrates at a room temperature followed by annealing at 600°C for 2 h in a conventional furnace. The crystalline structure and the surface morphology of the films were markedly affected by the growth conditions. Rietveld analysis reveal a CCTO film with 100 % pure perovskite belonging to a space group Im3 and pseudo-cubic structure. The XPS spectroscopy reveal that the in a reducing N 2 atmosphere a lower Cu/Ca and Ti/Ca ratio were detected, while the O 2 treatment led to an excess of Cu, due to Cu segregation of the surface forming copper oxide crystals. The film present frequency-independent dielectric properties in the temperature range evaluated, which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 600-nm-thick CCTO films annealed at 600°C at 1 kHz was found to be 70. The leakage current of the MFS capacitor structure was governed by the Schottky barrier conduction mechanism and the leakage current density was lower than 10-7 A/cm 2 at a 1.0 V. The current-voltage measurements on MFS capacitors established good switching characteristics.

Origin of Abnormal Dielectric Behavior and Chemical States in Amorphous CaCu3Ti4O12 Thin Films on a Flexible Polymer Substrate

“…30 The decrease in the number of oxygen vacancies might contribute largely to minimizing the charge imbalance toward the stoichiometric CCTO structure in the amorphous structure and, thus, to influencing chemical states of cations. 31,32 As another example, Figure 2c also shows the Ti 2p 3/2 states of the XPS spectra for the films deposited at 4.3 mTorr pressure. The spectra could be divided into the contributions from Ti 3+ and Ti 4+ as a result of the identical curve fitting.…”

“…Oxygen vacancies were reported to exist in crystalline CCTO films prepared by rf sputtering under low oxygen pressures . The decrease in the number of oxygen vacancies might contribute largely to minimizing the charge imbalance toward the stoichiometric CCTO structure in the amorphous structure and, thus, to influencing chemical states of cations. , …”

“…In addition, the atomic concentration and the ratio of Ca:​Cu:​Ti:​O are presented in the inserted table of Figure . The values were estimated from the relative area of each element’s curve in the XPS spectra according to different oxygen partial pressures as reported elsewhere . The result suggests that the increase of oxygen partial pressure approaches the stoichiometric ratio (Ca:​Cu:​Ti:​O = 1:3:​4:​12) of CCTO even in the amorphous state, particularly with the gradual increase of oxygen concentration.…”

Evidence of ferroelectric behaviour in CaCu3Ti4O12 thin films deposited by RF-sputtering

“…In our previous work, CCTO thin films were obtained by RF-sputtering from the CCTO target with relatively high values of the profile R-factors which reflects the good statistics of the data; consistently, the Le Bail fit with identical values of profile coefficients yielded a space group Im3, a = 7.39612(3) Å, Z = 2, χ 2 = 2.64, R p = 12.0%, R wp = 16.5%, R B = 4.35%, R F = 3.84% [23]. Refinement was limited to isotropic ADPs -anisotropic refinement did not yield any significant improvement in the fit quality and components of ADP tensors were refined with large uncertainties.…”

“…The CCTO targets were prepared by conventional solid-state reaction. Calcium carbonate, copper oxide and titanium dioxide (>99% purity and <1 µm) were mixed by ball milling in ethanol for 12 h. After drying, the powder was calcined at 900°C for 12 h. The calcined powder was mixed with a binder (PVA) and uniaxially pressed into discs of approximately 50 mm in diameter and 3 mm thickness and sintered at 1100°C for 3 h. From these targets, nanostructured CaCu 3 Ti 4 O 12 films deposited on Pt/Ti/SiO 2 /Si substrates were obtained by RF-sputtering at 300°C as previously published elsewhere [23]. Deposition of CCTO films were carried out by sputtering a CCTO target in pure argon and oxygen at 2 × 10 −2 mbar and 150 W RF power.…”