The prospects of using ferroelectric thin films for the fabrication and development of frequency and phase agile microwave devices have increased in past few years due to improvements in ferroelectric thin film processing techniques as well as innovative circuit design. In microwave devices, ferroelectric thin film is used as tuning layer through the nonlinear electric field dependence of the relative dielectric constant.
In this study, we report on an investigation of the doping effects of several dopants in $Ba_{0.6}Sr_{0.4}TiO_3$ as a significant factor affecting the dielectric properties and discuss the dependence of tunability and dielectric loss on the characteristics of dopants at low frequency(100kHz).
Firstly, to examine the effect of LSCO buffer layer on BST tunability, two types of capacitors such as Pt/BST/Pt and Pt/BST/LSCO/Pt were compared. Compared with the crystallinity of BST film on Pt electrode, BST film grown on LSCO layer showed an improved (110) phase formation. LSCO buffer layer can promote the growth of BST films due to the structural match between BST and LSCO. At the applied voltage of 5V, the BST film deposited on LSCO/Pt shows a tunability of 62.8%. It is noted that the tunability of the BST film grown on LSCO buffer layer is considerably higher than that (12.6%) obtained at the same deposition condition by using Pt electrode.
Secondly, in this work, Ni doped BST/LSCO/Pt films were fabricated to investigate the acceptor doping effect on dielectric properties of BST films, such as tunability and dielectric loss.
Both dielectric loss and tunability were found to decrease as a function of increasing Ni concentration. This decrease in dielectric tuning with addition of acceptor dopants is not surprising and has been reported in other studies for doped BST thin films. (16,37) The decreased dielectric loss of Ni doped BST/LSCO/Pt film is attributed to suppression of the concentration of oxygen vacancies and enlagement of grain boundary potential barrier. But, as compromising factor, figure of merit, which is defined as the ratio of tunability to dielectric loss, was highest in the case of 1% Ni doping.
Thirdly, 1%Ce doped BST/LSCO/Pt stack was fabricated to examine the donor doping effect on the dielectric properties of BST. By capacitance-voltage measurement, we have observed that 1%Ce doped BST film with LSCO buffer layer had higher dielectric constant(1725) than that(955.6) of undoped BST/LSCO/Pt film. But as trade-off, dielectric loss(0.0528) of 1%Ce doped BST was more increased than that (0.0275)of undoped BST film. This may be concerned with the BST Curie temperarure increase due to donor Ce addition.
Fourthly, to search the acceptor Ni and donor Ce co-doping effect on the dielectric properties of BST thin film, 1%Ni-1%Ce co-doped BST was deposited on LSCO/Pt electrode. The dielectric loss (0.018) of co-doped BST film was similar to that(0.0183) of 1%Ni doped BST/LSCO/Pt, but the tunability (9.3%)of the former was more significantly decreased than pure 1%Ni (54%) or 1%Ce (58%) doped cases. The considerably decreased dielectric tuning of Ni-Ce co-doped BST/LSCO/Pt film is attributed to the significantly reduced BST phase formation or second phase formation in the co-doped BST target shown as x-ray diffraction patterns.
From the above results, we observed that 1mol%Ni doped BST film to be more suitable for high performance tunable device applications.
And then, the microwave properties of ferroelectric BST thin films were being investigated. Interdigital capacitors fabricated on ferroelectric thin films of $Ba_{0.6}Sr_{0.4}TiO_3$ were characterized at room temperature from 50MHz to 10GHz. Interdigital capacitor using 1%Ni doped BST, which had better figure of merit property at low frequency measurement was also fabricated and characterized. The capacitance and resistance of ferroelectric thin films were then extracted using parallel resistor and capacitor model. It was found that the tunability and resistance decreased obviously with 1%Ni doping, and 1%Ni doped BST thin films had a tunabibty of 0.3% at 30volts and resistance of 3.493 at 6GHz. But, significantly low tunability obtained both BST thin films(0% and 1%Ni doped) may be related to large finger gap size(low electric field ) and thin conductor thickness(high conductor loss).
$(Ba,Sr)TiO_3$ (BSTO)와 같은 유전 박막은 인가전압에 따라 유전상수가 비선형적으로 변화하는 가변 캐패시터 특성을 보이므로 이를 공진기(Resonator), 필터(Filter), 위상 천이기(Phase Shifter)와 같은 마이크로파 동조 디바이스에 적용하려는 연구가 근래에 활발히 진행되고 있다. 이러한 유전 박막을 이용한 마이크로파 디바이스는 기존의 동조 디바이스(Pin diode, Varactor, and Ferrites)와 비교하여 인가전압에 따른 정전 용량의 변화 속도가 더욱 빠르고, 사용 가능한 마이크로파 전력이 더 크며, 작고 간단한 구조로 제작될 수 있다는 등의 장점이 있다.
본 연구에서는 먼저 산화물 전극인$La_{0.5}Sr_{0.5}CoO_3(LSCO)$ buffer layer에 따른 BST 박막의 저주파(100kHz)에서의 유전 특성에 미치는 영향을 알아보기 위하여 Pt/BST/Pt와 Pt/BST/LSCO/Pt의 캐패시터를 제조하였다. LSCO/Pt위에 성장된 BST 박막의 경우 Pt위에 성장된 BST보다 (110) 배향성이 향상된 것을 확인할 수 있었다. 5V의 인가전압에서 Pt와 LSCO 전극위에 성장된 BST 박막의 동조성은 각각 12.6% 와 62.8%로 측정되었다. 즉, LSCO위에 성장된 BST 박막의 경우 동조성이 더욱 향상되었음을 확인할 수 있었다.
다음으로, acceptor인 Ni과 donor인 Ce을 BST에 각각 도핑하여 BST 박막의 유전 특성에 미치는 영향을 저주파(100kHz)에서 알아보았다. Ni이 도핑된 BST/LSCO/Pt구조에서 도핑의 양이 늘어날수록 동조성, 유전상수, 유전 손실값 등이 작아지는 것을 확인할 수 있었다. 1%Ce이 도핑된 BST/LSCO/Pt 캐패시터에서는 동조성, 유전상수, 유전 손실값 모두 증가하였다. 1%Ni과 1%Ce을 함께 BST 박막에 도핑하였을 경우, 유전 손실값은 순수한 1%Ni 도핑의 수준으로 감소하였으나 유전상수와 동조성 모두 심각하게 작아졌음을 알 수 있었다. Ni과 Ce을 함께 도핑한 BST 분말에서는 이차상이 형성되었으며, 박막에서는 상형성이 현저하게 감소한 것을 XRD 분석을 통해 알 수 있었다.
BST박막의 마이크로파에서의 유전 특성을 알아 보기위해, 본 연구에서는 MgO 단결정 위에 BST 박막을 증착한 후, 그 위에 interdigital capacitor(IDT)를 제조하였다. 앞서 저주파에서 가장 좋은 유전특성을 보여준 1%Ni이 도핑된 BST박막에서도 IDT를 제조하여 Ni 도핑에 따른 BST 마이크로파 특성에 미치는 영향도 아울러 살펴 보았다. 6GHz의 주파수에서 도핑이 안된 BST와 1%Ni 도핑된 BST는 30V의 전압에서 1.2%, 0.3%의 동조성을 갖는 것으로 확인할 수 있었다. 1%Ni이 도핑된 BST는 도핑이 안된 BST(4.1 Ω) 보다 더 낮은 3.5 Ω의 저항값을 갖았다. 이는 Ni이 도핑됨으로써, 동조성과 유전 손실값이 작아지는 것을 보여주며, 이는 저주파에서의 경향과 유사하였다. 하지만 마이크로파 주파수에서 얻어진 동조성이 저주파에서 보다 심각하게 작아졌는데, 이는 제조된 IDT에서 finger gap(12μm)이 너무 커서 가해진 유효 전기장이 작았거나 전극의 두께(0.55 μm)가 얇아서 전극을 통한 마이크로파 손실값이 현저히 큰 것에 기인한 것으로 판단된다.
