The prospects of using ferroelectric thin films for the fabrication and development of frequency and phase agile microwave devices have increased in the past few years due to improvements in ferroelectric thin film processing techniques as well as innovative circuit designs. In microwave devices, ferroelectric thin film is used as a tuning layer through the nonlinear electric field dependence of relative dielectric constant.
In the thesis, we report on an investigation of the Mn doping effects in $Ba_{0.6}Sr_{0.4}TiO_3$ affecting the dielectric properties and discuss the dependence of tunability and dielectric loss on the characteristics of dopants at low frequency(100 kHz) and microwave frequency(～GHz). Firstly, Mn doped BST(1～5mol%)/Pt/$TiO_2$ films were fabricated to investigate the acceptor doping effect on dielectric properties of BST films, such as tunability and dielectric loss. We have observed that Mn doped BST thin films have (100)-preferred orientation and smoother surface than undoped BST film. The dielectric loss was found to decrease from 0.0191 to 0.0158 as a function of increasing Mn concentration. The decreased dielectric loss of Mn doped BST films is attributed to a suppression of the concentration of mobile carriers. The dielectric constants of undoped BST films are 901 as its maximum value and the dielectric constants of Mn doped BST films decreased upto 711. At the applied electric field of 150kV/cm, the tunability of undoped and Mn-doped BST films is ranged from 46% to 49% and is the high value. The figure of merit(FOM), ratio of tunability to dielectric loss, was the highest in the case of 3% Mn doped BST thin film and the FOM value is 30. From the above results, we concluded that 3 mol% Mn doped BST film is more suitable for high performance tunable device applications.
Secondly, to investigate the microwave properties of BST thin films, we fabricated the interdigital capacitors ( IDC ) on the epitaxial BST film grown on MgO(100) substrate and characterized at room temperature from 0.5 GHz to 5GHz using HP 8510C network analyzer system. 3% Mn doped BST film(3% BST), which had better figure of merit property at low frequency, was also fabricated and characterized. The capacitance and Q-factor of BST films were then extracted from the $S_{11}$ using parallel resistor and capacitor model. It was found that 3% BST film has a smaller capacitance than that of 0% BST film but has larger tunablilty(27.4%) than that of 0% BST film(11.6%) at 3.5 GHz. Q-factor of 0% BST and 3% BST decreased obviously as frequency increased from 0.5GHz to 5GHz.
And Q-factor of 3% BST is larger than that of 0% BST at 3.5 GHz.
From the above results, we concluded that the BST films are a promising material for the application of tunable microwave device at room temperature and 3 mol% Mn doped BST film is more suitable for high performance tunable device applications.
