Il-Doo Kim

Low-voltage ZnO thin-film transistors with high-KBi1.5Zn1.0Nb1.5O7 gate insulator for transparent and flexible electronics

We report on the fabrication of field-effect transistors with transparent oxide semiconductor ZnO serving as the electron channel and high-KBi1.5Zn1.0Nb1.5O7 (BZN) as the gate insulator. The devices exhibited very low operation voltages ( 80% for wavelength >400nm), low-temperature processing, and low operation voltage of ZnO-based thin-film transistors with integrated BZN dielectric offer a promising route for the development of transparent and flexible electronics.

High tunability (Ba, Sr)TiO3 thin films grown on atomic layer deposited TiO2 and Ta2O5 buffer layers

In this letter, we report on increased tunability of Ba0.6Sr0.4TiO3 (BST) thin films by use of Ta2O5 and TiO2 films as buffer layers between BST and Si substrates. Ta2O5 and TiO2 buffer layers were grown by atomic layer deposition (ALD) onto Si substrates followed by pulsed laser deposition of Ba0.6Sr0.4TiO3 thin films onto the buffer layers. The randomly oriented BST films deposited on TiO2∕Si substrates exhibited a broader grain size distribution than the (110) textured BST films grown on Ta2O5∕Si substrates. At an applied voltage of 10 V, the BST films grown on Ta2O5∕Si and TiO2∕Si substrates showed much enhanced tunability values of 53.1% and 72.9%, respectively, as compared to the 20.7% value obtained with BST films grown on MgO single crystal substrates. Successful integration of BST low voltage microwave tunable devices onto Si substrates thus appears possible with the aid of ALD grown Ta2O5 or TiO2 buffer layers.

Direct current bias effects on grain boundary Schottky barriers in CaCu3Ti4O12

CaCu3Ti4O12 exhibits an unusually high dielectric constant on the order of 105 and highly nonlinear I-V characteristics. Impedance spectroscopy measurements carried out in this work point to the crucial role played by grain boundary barriers in controlling the electrical properties of this material. Under dc bias, the grain boundary resistance decreases, followed by a precipitous breakdown at higher applied voltages. The barrier height is estimated to be ∼0.82eV. The grain conductivity shows a transition from a negative temperature coefficient of resistance with activation energy of ∼0.08eV to a positive temperature coefficient of resistance at 280u2009°C suggesting a transition from impurity ionization to scattering controlled mobility in the carrier saturation region.

Study of orientation effect on nanoscale polarization in BaTiO3 thin films using piezoresponse force microscopy

We have investigated the effect of texture on in-plane (IPP) and out- of plane (OPP) polarizations of pulsed-laser-deposited BaTiO3 thin films grown on Pt and La0.5Sr0.5CoO3 (LSCO) buffered Pt electrodes. The OPP and IPP polarizations were observed by piezoresponse force microscopy (PFM) for three-dimensional polarization analyses in conjunction with conventional diffraction methods using x-ray diffraction and reflection high energy electron diffraction measurements. BaTiO3 films grown on Pt electrodes exhibited highly (101) preferred orientation with higher IPP component whereas BaTiO3 film grown on LSCO/Pt electrodes showed (001) and (101) orientations with higher OPP component. Measured effective d(33) values of BaTiO3 films deposited on Pt and LSCO/ Pt electrodes were 14.3 and 54.0 pm/ V, respectively. Local piezoelectric strain loops obtained by OPP and IPP-PFM showed that piezoelectric properties were strongly related to film orientation.

Dielectric Properties of Mn-Doped BST Thin Films for Microwave Application

In this letter, we report on the effect of Mn doping on the tunability and dielectric loss of Ba0.6Sr0.4TiO3 thin films at low frequency (100 kHz) and at microwave frequency (3.5 GHz), respectively. Perovskite Ba0.6Sr0.4TiO3 thin films were deposited on Pt/TiO2/SiO2/Si and MgO single crystal substrates by pulsed laser deposition. At an applied electric field of 150 kV/cm, the BST films (0, 1, 3, 5 mol% Mn doped) grown on Pt/TiO2/SiO2/Si substrate showed the tunability value within the range from 46% to 49%. With 3% Mn doped BST thin films, results gave a tunability of 49% and a loss tangent as low as 0.0158 and the highest figure of merit of about 31. The microwave properties of BST films grown on MgO substrate were measured using interdigital capacitors (IDC) structure at 3.5 GHz. 3 mol% Mn doped BST also showed better tunability (27.4%) compared to that (11.6%) of undoped BST film. It is suggested that 3 mol% Mn doped BST film is effective candidate for high performance tunable device applications.

Ridge waveguide using highly oriented BaTiO3 thin films for electro-optic application

Abstract In this work, 750 nm-thick BaTiO3 thin films with highly (0 0 1) preferred orientation were grown on single crystal MgO substrates by RF-sputtering. Hydrogen silsesquioxane (HSQ) resist material based ridge waveguides, which were fabricated on BaTiO3 thin films, were formed by e-beam lithography. Au electrodes were deposited on top of the BaTiO3 films beside the waveguide. Propagation losses of the BaTiO3 ridge waveguide were 3–5 dB/cm in transverse electric polarization. The measured electrooptic coefficient value (r51) was 110 pm/V, which is three times larger than the electrooptic coefficient (r33 = 30.8 pm/V) of single crystal LiNbO3. SiO2 strip waveguide formed by HSQ exhibited light propagation with loss lower than 5 dB/cm. This result demonstrates potential possibility of creating highly oriented and/or epitaxially grown BaTiO3 waveguides and optical components on oxide substrates.