Seung Eon Moon

Microwave properties of compositionally graded (Ba, Sr)TiO3 thin films according to the direction of the composition gradient for tunable microwave applications

Compositionally graded (Bax, Sr1−x)TiO3 (BST) thin films were deposited on MgO substrates by pulsed laser ablation. The microwave properties of the graded BST thin films were investigated at microwave frequencies with coplanar waveguide (CPW) meander-line phase shifters as a function of the direction of the composition gradient with respect to the substrate at room temperature. CPW phase shifters using graded BaTiO3(BT)→SrTiO3(ST) and ST→BT thin films exhibited a differential phase shift of 73° and 22° at 10 GHz with a dc bias voltage of 150 V, respectively. The figure of merit of the phase shifters at 10 GHz was 14.6 deg/dB for the graded BT→ST film, and 10 deg/dB for the graded ST→BT film. The graded BT→ST thin films showed a larger phase tuning than the ST→BT thin films. The microwave properties of the graded BST films depended strongly on the crystalline structure and the direction of the composition gradient with respect to the substrate.

Electro-optic characteristics of (001)-oriented Ba0.6Sr0.4TiO3 thin films

Ba0.6Sr0.4TiO3 thin films were grown on MgO(001) substrates using pulsed-laser deposition. The films were highly oriented along the (001) direction, and showed a high electro-optic response to an external electric field. The quadratic electro-optic coefficient Rc was 1.0×10−14 m2/V2, and the birefringence variation was as large as 0.09. Various electro-optic applications as well as the microwave applications of barium strontium titanate thin films are expected.Ba0.6Sr0.4TiO3 thin films were grown on MgO(001) substrates using pulsed-laser deposition. The films were highly oriented along the (001) direction, and showed a high electro-optic response to an external electric field. The quadratic electro-optic coefficient Rc was 1.0×10−14 m2/V2, and the birefringence variation was as large as 0.09. Various electro-optic applications as well as the microwave applications of barium strontium titanate thin films are expected.

Study of Transport and Dielectric of Resistive Memory States in NiO Thin Film

We have measured the DC resistance R(T) and AC dielectric constant e(ω) for the bistable high-R and low-R states of NiO thin film. The high-R state shows thermally activated resistance and Debye relaxation of e(ω). In the low-R state, R(T) exhibits a metallic temperature dependence of R(300 K)/R(5 K)=1.6. The value of e(ω) is drastically different from that of the high-R state, and we interpret it in terms of the free-carrier Drude dielectric response. The plasma frequency ωp2 in the metallic low-R state is estimated to be 1.2×109/cm3.

Ferroelectric Ba 0.6 Sr 0.4 TiO 3 Phase Shifters Under High Power Microwave

High power microwave properties of the ferroelectric phase shifters have been studied to reduce the effect of the nonlinear response of ferroelectrics under high power microwave with an applied dc bias field of 0 - 40 kV/cm. The Ba 0.6 Sr 0.4 TiO 3 ferroelectric films were prepared on (001) MgO single crystals by pulsed laser deposition. Coplanar waveguide (CPW) phase shifters were fabricated from 2 w m thick Au layer to reduce metal loss. The fabricated CPW phase shifters exhibited more than 100° of differential phase at 10 GHz with a 40 kV/cm dc bias field between ground and center conductors. Furthermore, the CPW phase shifter showed a constant differential phase shift with microwave power range from m 10 to +30 dBm. Microwave power and/or dc bias field dependant responses of phase shifters are presented in the paper.

DIELECTRIC PROPERTIES OF PARAELECTRIC Ba(Zr,Ti)O 3 THIN FILMS FOR TUNABLE MICROWAVE APPLICATIONS

ABSTRACT Paraelectric Ba(Zr,Ti)O3 [BZT] thin films were prepared on MgO substrates by pulsed laser ablation and their dielectric characteristics and tunabiltiy were investigated as a function of the Zr content. Dielectric properties of BZT thin films were measured under dc bias using planar interdigitated capacitor. The excellent tunable dielectric characteristics were achieved for x = 0.25 at room temperature; tunability ∼74.5%, tanδ ∼0.0535 at 100 kHz, indicating that it is a promising canditate for voltage tunable dielectric working at room temperature.

Microwave Properties of Mn Doped (Ba1 - x,Srx)TiO3Thin Films for Tunable Phase Shifter

Ferroelectric Mn doped Ba0.5Sr0.5TiO3 (Mn-BST) films with/without BaTiO3 (BT) buffer layer have been grown on (001) MgO substrates by a pulsed laser deposition to investigate electrical tunability at microwave frequencies. Structural properties and surface morphologies of the films were investigated using an X-ray diffractometer and a scanning electron microscope, respectively. Microwave dielectric properties of Mn-BST thin films with BT buffer were studied for reduction of dielectric loss and improvement of electrical tunability. Distributed analog phase shifters have been designed and fabricated on Mn-BST films with/without BT buffer layer to understand microwave dielectric properties. The differential phase shift of the phase shifter fabricated on Mn-BST film was 22° at 10 GHz with 80 V of applied dc bias voltage. In comparison, phase shifter fabricated on Mn-BST/BT multilayers exhibit 41° of differential phase shift at the same condition. This suggests that a BT buffer layer is for microwave tunable device applications. The phase shifter fabricated on Mn-BST/BT multilayers exhibit a low insertion loss (S21) of −1.1 dB, and a low return loss (S11) of −14 dB with a bias voltage of 80 V.

A K-Band Distributed Analog Phase Shifter Using Etched Ba0.6Sr0.4TiO3 Thin Films

This work presents the design, fabrication and microwave performance of distributed analog phase shifter (DAPS) fabricated on etched Ba0.6Sr0.4TiO3 (BST) thin films for K-band applications. Single phase Ba0.6Sr0.4TiO3 thin films for interdigitated capacitors (IDC) were deposited by the pulsed laser deposition (PLD) on (001) MgO substrates. The DAPS design consists of periodically loading high impedance coplanar waveguide (CPW) with tunable BST IDC, and the phase shift of the loaded line can be controlled by varying the applied bias voltage to the BST thin films. BST thin films except IDC areas were etched, which means that BST layers locally exist beneath IDC and are removed from the CPW by photolithography and RF-ion milling, to reduce the insertion loss of DAPS and to eliminate the alteration of unloaded CPW properties according to an applied dc bias voltage. The DAPS can be accurately and easily modeled using a combination of full-wave electromagnetic and microwave circuit analysis. Experimental results are shown to agree very well with the simulated ones at the frequencies of interest. The fabricated DAPS showed that the return loss was better than -13 dB through the whole K-band frequency range. The measured differential phase shift based on BST thin films was 179° and the insertion loss is -5.6 dB–-2.1 dB with increasing the bias voltage from 0 to 200 V at 20 GHz.

Orientational Dependence of Electro-optic Properties of SrBi2Ta2O9 Ferroelectric Thin Films

A-/b- and c-axis oriented SrBi2Ta2O9 thin films were grown on the MgO(110) and MgO(100) substrates, respectively, by rf magnetron sputtering deposition method. Optical anisotropy was shown in the infrared reflectance spectra for the SrBi2Ta2O9 thin films. Quadratic electro-optic effects were observed only in the a-/b-axis oriented films. The effective quadratic electro-optic coefficient was about 3.8 ×10-17 m2/V2.

ENHANCED DIELECTRIC PROPERTIES OF (Ba, Sr)TiO3 THIN FILMS FOR HIGH-PERFORMANCE MICROWAVE PHASE SHIFTERS

ABSTRACT Barium strontium titanate (BST) thin films have been intensively studied as a frequency and phase agile materials for the tunable microwave devices such as tunable filter, phase shifter and phased array antennas. The dielectric properties of BST thin films, including the dielectric constant, dielectric loss and tunability of capacitance are affected by many factors, such as Ba/Sr ratio, growth temperature, crystallographic orientation, grain size, defect chemistry, oxygen vacancies, strain and stress, and dopants. In this work, we report on the enhanced dielectric properties of (Ba0.6, Sr0.4)TiO3 (BST) thin films for high-performance microwave tunable device applications. Epitaxial BST thin films were deposited on MgO substrates by the insertion of thin BaTiO3 [BT] seed layers with intermediate lattice mismatch between the BST films and the MgO substrate using a pulsed laser ablation. Dielectric measurements were carried out using planar interdigitated (IDT) capacitor in ranges of RF/microwave frequency. IDT capacitor based on epitaxial BST film exhibited the large capacitance tunability of 77% at a frequency of 100 kHz and applied electric field of 80 kV/cm. CPW phase shifter showed the large phase tuning of 153° at 10 GHz and 40 V. These results indicate that it is a promising candidate for high-performance microwave phase shifters at room temperature. †Originally presented at 2004 ISIF, Kyung Ju, Korea, April 5-8, 2004.

Microwave Performance of Distributed Analog Phase Shifter Using Ferroelectric (Ba,Sr)TiO3 Thin Films

This work presents the design, fabrication and microwave performance of distributed analog phase shifter (DAPS) fabricated on (Ba,Sr)TiO3 (BST) thin films for X-band applications. Ferroelectric BST thin films were deposited on MgO substrates by pulsed laser deposition. The DAPS consists of high impedance coplanar waveguide (CPW) and periodically loaded tunable BST interdigitated capacitors (IDC). In order to reduce the insertion loss of DAPS and to remove the alteration of unloaded CPW properties according to an applied dc bias voltage, BST layer under transmission lines were removed by photolithography and RF-ion milling. The measured results are in good agreement with the simulated results at the frequencies of interest. The measured differential phase shift based on BST thin films was 24° and the insertion loss decreased from −1.1 dB to −0.7 dB with increasing the bias voltage from 0 to 40 V at 10 GHz.