Suppression of slow capacitance relaxation phenomenon in Pt/Ba_0.3Sr_0.7TiO₃/Pt thin film ferroelectric structures by annealing in oxygen atmosphere

Ferroelectrics (FE) and, in particular, Ba_xSr_1−xTiO₃ (BSTO) thin films in the paraelectric state are promising for use in microwave electronics. The nonlinearity in FE dielectric permittivity (ε) under applied electrical field enables the development of a variety of tunable devices.^1–4 The main parameter of such devices is their switching time under short control voltage pulses with duration between (10⁻³–1) μs. In the paraelectric state under harmonic microwave E-fields there is no frequency dispersion for nonlinear response of the BSTO permittivity up to ∼100 GHz. However, when a unipolar voltage pulse is applied, the response of the FE film based metal/FE/metal (M/FE/M) capacitor structures become ambiguous. This is because, besides the fast response process (soft mode response, <1 ns), there are slow relaxation processes, which cause up to 20% of the capacitance value to relax (i.e., return to initial state after the pulse duration) for a time τ ∼ 10³ s.^5–8 The oscillogram (Fig. 1) demonstrates the typical response of a FE varactor on a rectangular voltage pulse. One of the main parameters determining the slow relaxation process is the residual capacitance (ΔC), which depends on the pulse amplitude, pulse duration, and repetition frequency. This residual capacitance phenomenon has a threshold nature (see, for example, Ref. 8), which appears only when the applied pulsed voltage is bigger than a threshold voltage level (V > V_th; at V < V_th, the ΔC = 0). This threshold voltage level depends on the FE films and electrode material, their quality, and fabrication method used.^8,9 The nature of switching kinetics in ferroelectric structures at temperatures lower than the Curie temperature (T < T_c) is rather well determined,^10–12 however, the explanation of slow relaxation processes at T > T_c is not entirely clear.

Investigations of a slow relaxation nature for M/FE/M structures in paraelectric state showed that the main reason for this phenomenon is the redistribution of the space charge formed in the ferroelectric film by trapped charge carriers injected from electrodes under control voltages.^8,13,14 It has been demonstrated that the slow relaxation processes in BSTO structures are connected with oxygen vacancies, which commonly exist in oxide films.^7,8,13 So, it can be concluded that one way to suppress the slow relaxation process in these films is to decrease the concentration of oxygen vacancies by annealing in an oxygen atmosphere. Nowadays, a high-temperature annealing operation is often used to increase the structure quality and improve the electrical properties of FE films.^15–20 Despite the large number of publications about the influence of the high-temperature annealing on structure and properties of ferroelectric films, the impact of this operation on the slow relaxation process of capacitance has not been investigated. The high temperature annealing operation is usually carried out immediately after the ferroelectric film deposition. However, the subsequent technological operations (e.g., electrodes' deposition, patterning, etc.) can diminish the effect of annealing. For example, during the deposition of the metal electrodes (usually performed in vacuum or in inert atmosphere), the substrate temperature could reach up to 400 °C (if no active cooling is used). This could lead to formation of additional oxygen vacancies in surface layer that in turn could worsen the slow relaxation characteristics of the FE material. Another cause for the FE material properties' degradation is their exposure to photoresist developer, which in the most common cases is a base solution.

In this paper, we report the properties of the M/FE/M multilayer structure based devices, where the annealing operation was performed after the electrodes' deposition and patterning.

Sandwich ferroelectric varactors on the base of Pt/Ba_0.3Sr_0.7TiO₃/Pt structures were investigated. To form the structures, BSTO film (thickness 0.45 μm) was deposited on prepatterned platinized 0.5 mm thick single crystal R-cut Al₂O₃ substrate by magnetron sputtering. The platinum electrodes were formed by magnetron sputtering deposition, followed by photolithography and Ar ion milling. The sputtering conditions and the composition of BSTO films were chosen to provide the optimum performance for microwave application.²¹ The X-ray analysis of the ferroelectric films showed preferential (111) orientation with a small amount of (110) orientation. The compositional uniformity of the BSTO films was investigated by secondary ion mass spectroscopy that demonstrated a uniform depth distribution of Ba, Sr, and Ti with not more than 5% variation. The patterning procedure (as above) was followed to form the top Pt electrodes. The measured electrical properties of the capacitor structure were as follows: total microwave losses tan δ (1 GHz) = 0.02–0.03; dc bias tunability n_dc = C(0 V)/C(15 V) = 1.75.

As fabricated capacitor structures were annealed for 3 h at 750 °C in flowing oxygen at atmosphere pressure.

The comparison of the capacitor's properties before and after the annealing is presented in Fig. 2. As one can note, the capacitance value and its voltage tunability increased, which is a typical result of such a treatment, caused by the thickness reduction of the amorphous interface (so called “dead”) layers.¹⁵ 

Unknown earlier effects of the annealing process to the FE capacitor structure were also observed: suppression of slow relaxation phenomena and increase of the voltage threshold values. We explain this improvement with reference to Fig. 3, where the voltage dependence of the slow relaxation (ΔC/C₀) before and after the annealing are presented. The horizontal, dashed line in Fig. 3 marks the level of slow relaxation ΔC/C₀ < 1%, which is considered as suitable for most fast switching microwave devices. Taking into account the results of Fig. 2, one can note the increase of the capacitor's fast acting tunability from n_dc ∼ 1.06 (at 2.5 V) before annealing to n_dc ∼ 1.7 (at 12.5 V) after annealing. This result could be also presented as a suppression of the slow relaxation (ΔC/C₀(−12.5 V)) from = 14% down to below 1% and the increase of the threshold voltage from V_th = 2 V before, to V_th = 7.5 V after annealing.

The observed effect can be attributed to the decrease of oxygen vacancies concentration in the BSTO film and to the increase of the barrier heights for electrons injection from Pt into BSTO.²¹ The improvement of the BSTO film microstructure after the high temperature annealing in oxygen atmosphere due to decrease of oxygen vacancy concentration provides both: the weakening the bulk trapping of electron injected¹³ and the depression of oxygen vacancy migration, giving rise the appearance of p-n junction, the electric field of which slow relaxes after the voltage switching-off.⁷ 

The asymmetry observed in the voltage dependence of the residual capacitance after annealing could be explained with the slight properties variation of the top and bottom interfaces of the ferroelectric film.

Thus, the impact of oxygen annealing on the switching time of ferroelectric thin film capacitor structures Pt/Ba_0.3Sr_0.7TiO₃/Pt was investigated. It was demonstrated that the annealing results in suppression of the capacitance slow relaxation processes and increase of the threshold control voltages making therefore this structure attractive for fabrication of fast acting microwave devices.

This study was partially supported by the Ministry of Science and Education of the Russian Federation and by RFBR, research Project No. 13-02-12096 ofi-m. P.K. Petrov and N. Alford acknowledge the financial support from the EPSRC, UK (grants EP/G060940/1 and EP/H023003/1) and the King Abdullah University for Science and Technology (KAUST) Global Collaborative Research Academic Excellence Alliance (AEA) and Academic Partnership Programs (APP).