Kwang Hwan Cho

High-temperature thermoelectric properties of nanostructured Ca3Co4O9 thin films

We prepared nanostructured Ca3Co4O9 (CCO) thin films by promoting localized epitaxial growth on polycrystalline Al2O3 substrates. The thermoelectric properties of the CCO films were studied in the temperature range 300 to 1023 K. We confirmed that localized epitaxial growth occurred on the seed grains that dominate the (006) plane. The nanostructured CCO thin films were found to have a maximum Seebeck coefficient of 206 μV/K and a power factor (at 920 K) of 0.514 mW/mK2. Moreover, the presence of nanostructure was found to reduce the thermal conductivity, and thus, should enhance the overall performance of CCO films in thermoelectric devices.

Influence of applied electric field annealing on the microwave properties of (Ba0.5Sr0.5)TiO3 thin films

The effect of heat treatment in electric field on the structure and dielectric properties at microwave range of rf magnetron sputtering derived (Ba0.5Sr0.5)TiO3 thin films have been studied. It has been demonstrated that postannealing in the proper electric field can increase the dielectric constant and the tunability. The increased out-of-plane lattice constant in the electric-annealed films indicated the formation of small polar regions with tetragonal structure, which are responsible for the increased dielectric constant and tunability. It was proposed that the segregation of Ti3+ ions caused by electric annealing could induce the formation of BaTiO3-like regions, which are ferroelectric at room temperature. And in dielectric loss, as the Ti–O bonding lengths increase, the energy scattering on the ferroelectric mode also increases. So, the value of dielectric loss is slightly increased.

Large in-plane permittivity of Ba0.6Sr0.4TiO3 thin films crystallized using excimer laser annealing at 300 °C

We demonstrated a way to fabricate the crystalline Ba0.6Sr0.4TiO3 (BST) thin films using excimer laser annealing technique on the amorphous BST thin films fabricated by sol-gel process. The grain size of the laser-annealed films is larger than that of the conventionally thermal-annealed films. However, an uncrystallized, amorphous layer was observed near the film/substrate interface due to the limited laser absorption depth. The uncrystallized layer has a critical influence on out-of-plane dielectric property of BST films. The significant difference of the relative dielectric permittivity (er) between in-plane (1383) and out-of-plane (184) directions is observed.

Titanium-substituted Bi1.5Zn1.0Nb1.5O 7 for high-density and low-temperature-coefficient-of-capacitance MIM capacitor by low-temperature process (300 °c)

A high-density metal-insulator-metal (MIM) capacitor at 300°C with a titanium-substituted Bi_1.5 Zn_1.0Nb_1.5O₇ (BZN) dielectric prepared by physical vapor deposition is presented for the first time. Improvements have been achieved in terms of both capacitance density and temperature coefficient of capacitance (TCC) for MIM capacitors. A 67-nm-thick (Bi_1.5 Zn_0.5)(Zn_0.4Nb_1.3Ti_0.3O₇) film has exhibited a high capacitance density of 14.8 fF/cm² at 100 kHz. The leakage current density is low, which is approximately 7.69 nA/cm² at 1 V. The values of linear voltage and TCC are approximately 156 ppm/V² and 98 ppm/°C at 100 kHz, respectively. All these make the Ti-substituted BZN capacitor very suitable for use in silicon RF and mixed-signal IC applications.

Structural Features and Microwave Properties of Ba0.5Sr0.5TiO3 Films Grown on Sapphire Substrates

The change in dielectric constant of ferroelectric materials as a function of electric field is the key to wide range of microwave application such as tunable filter, impedance matching network, and phase shifter. In this study, ferroelectric Ba0.5Sr0.5TiO3 (BST) films were grown on r-cut sapphire and polycrystalline sapphire (poly-sapphire) substrates by RF sputtering. The results of comprehensive structural diagnostics of the films are correlated with the dielectric constant and dielectric loss of a co-planar BST varactor, measured at a frequency range of 1~3 GHz. Textured BST films approximately 500 nm thick, grown on r-cut sapphire substrates, are characterized by high dielectric constant ≥ 650. However, polycrystalline BST films, grown on poly-sapphire substrates, are less strained, having dielectric constant range of 430 ~ 640.