Jowoong Ha

Low Temperature Solution-Processed InZnO Thin-Film Transistors

We prepared indium zinc oxide (IZO) semiconductors for low temperature solution-processed thin-film transistors (TFTs). The sol-gel derived IZO films, annealed at 300°C, are uniform and have smooth surface morphology (root-mean-square roughness of 0.27 nm). Both the composition and the film thickness need to be optimized for high performance TFTs. With the composition of In/Zn equal to 50/50 in mol percent, the IZO TFTs with a thickness of 10 nm exhibited the best performance for a clear switching behavior (on/off current ratio of 1.2 × 10 7 and output characteristics (drain current of 3.7 × 10 -4 A), with a relatively high field-effect mobility (0.54 cm 2 V -1 s -1 ) and a low threshold voltage (1.9 V). The nonpassivated IZO-TFT stably operates over a two-month period without any significant change in the on/off current ratio and the mobility.

Dielectric and Electromechanical Properties of Pb(Zr,Ti)O3 Thin Films for Piezo-Microelectromechanical System Devices

The dielectric and electromechanical properties of 1-µm-thick Pb(Zr,Ti)O3 (PZT) thin films with Zr/Ti ratio of 30/70, 52/48 and 70/30 are investigated. The magnitude of the effective longitudinal piezoelectric coefficient (d33) of these films is measured by a double-beam laser interferometric method. With increasing Ti-content, larger remanent polarization and higher coercive voltage are observed. The squareness of the polarization hysteresis loop is optimized in a Ti-rich composition. The remanent d33 values are maximized near a morphotropic phase boundary (MPB) composition, and the trend of d33 values strongly depends on the dielectric constant. It is observed that maximum dielectric constant is achieved near MPB composition, showing the same trend as bulk PZT ceramics. This result indicates that the piezoelectric response in 1-µm-thick films with composition (Zr to Ti ratio) is governed by extrinsic contribution such as that found in bulk PZT ceramics rather than intrinsic contribution.

Thickness effects on imprint in chemical-solution-derived (Pb, La)(Zr, Ti)O3 thin films

The film thickness-dependent imprinting behavior (voltage shift) of (Pb, La)(Zr, Ti)O3 capacitors was evaluated by a thermal stress process under a remanence bias. The remanent polarization (Pr) was found to be almost independent of the film thickness whereas in the 50–300 nm range the relative dielectric constant (er) increased linearly with the square root of the film thickness. It was found that the voltage shift, which was attributed to the accumulation of charged defects near the electrode interface, also increased linearly with increasing film thickness. In addition, the charge accumulated thickness varied with the square root of the film thickness. This was established from a simple assumption that the level of charge accumulation is determined by the product of the total amount of charged defects (total film thickness×charged defect density) and the internal field that is generated by the Pr. Therefore, the imprint is much more a bulk-related degradation phenomenon compared to the fatigue.

HIGHLY (200)-ORIENTED PT FILMS ON SIO2/SI SUBSTRATES BY SEED SELECTION THROUGH AMORPHIZATION AND CONTROLLED GRAIN GROWTH

Highly (200)-oriented Pt films on SiO 2 /Si substrates were successfully prepared by a combination of a dc magnetron sputtering using Ar/O 2 gas mixtures and subsequent controlled annealing. The intensity ratio of (200) to (111) planes ( I 200 / I 111 ) was over 200. The (200)-oriented Pt microcrystallites were less susceptible to amorphization due to their lower strain energy with oxygen incorporation than (111)-oriented ones. The controlled grain growth from the selected (200)-oriented seed microcrystallites during subsequent annealing provided a kinetic pathway where grain growth of the seed microcrystallites was predominant, while suppressing the nucleation of surface energy-driven, (111)-oriented seed microcrystallites and subsequent (111) preferred orientation.

MOCVD of PZT Thin Films with Different Precursor Solutions for Testing Mass-Production Compatibility

Metallorganic chemical vapor deposition (MOCVD) of thin ( 16 megabit) ferroelectric memory devices. The growth temperatures were set between 420 and 530°C to obtain a smooth surface morphology and to prevent damage to the underlying reaction barrier layer. A dome-type CVD chamber with a single- or double-cocktail precursor solution liquid delivery system was used for deposition. Four different precursor solution sets were investigated from the viewpoint of mass-production compatibility. The types of solvents had a great influence on the vaporizer lifetime and the level of oxygen incorporation into the PZT film that critically affected the electrical performance. The deposition behavior, including the within-wafer-thickness uniformity, was also dependent on the type of the precursor solutions. The oxygen injection method had a great influence on the Pb composition, the interfacial reaction between the film and the Ir electrode as well as the electrical properties. When nonpreheated oxygen was injected, the interfacial reaction became a serious problem resulting in the formation of IrO 2-x and Pb x Ir y O z layers without sufficient oxygen incorporation into the PZT film. Preheated oxygen was effective in incorporating Pb into the PZT films without serious interfacial reactions except for thin IrO 2-x layer formation.

Dependence of ferroelectric performance of sol-gel-derived Pb(Zr, Ti)O3 thin films on bottom-Pt-electrode thickness

Pb(Zr,Ti)O3 (PZT) thin films were deposited on Pt/Ti and Pt/IrO2 electrode stacks with various Pt thicknesses (30–200 nm) by a sol–gel process. The sputter-deposited Pt films showed a (111)-preferred texture irrespective of the thickness. However, a high-resolution x-ray diffraction study of the Pt films showed that the films were composed of three kinds of grains with slightly different lattice parameters. The grains with a bulk-like lattice parameter grew with increasing Pt thickness, which was accompanied with an improvement in the crystalline quality. Accordingly, the crystallization and ferroelectric behavior of the 100-nm-thick PZT films improved with increasing Pt film thickness. However, the PZT films on the Pt/IrO2 electrode showed a deteriorated ferroelectric performance due to the outward diffusion of the Ir (O) onto the Pt surface, which increases the depolarizing field and amount of charge injection by the formation of a conducting phase.

Preferred Orientation Controlled Giant Grain Growth of Platinum Thin Films on SiO2/Si Substrates

Platinum thin films were deposited by reactive magnetron sputtering on SiO2/Si substrates. Argon-oxygen sputtering gas mixtures were used to control the microstructure and the preferred orientation of platinum films. As the oxygen fraction in the sputtering gas increased, the preferred orientation of as-deposited film was changed from (111) to random orientation. Post-sputtering anneal was done at 750–1,000°C range in air ambient to study the effects of the incorporated oxygen on the grain growth behaviors of platinum films. After sputtering and anneal at optimum conditions, the 1-µm thick Pt films completely transformed to giant grains with sizes as large as several millimeters. Furthermore, the preferred orientation of the giant grains could be controlled to either (111) or (200).

Piezoelectric and Pyroelectric Properties of Pb(Zr,Ti)O3 Films for Micro-Sensors and Actuators

The piezoelectric and the pyroelectric properties of PZT films are systematically investigated for tetragonal (Zr/Ti = 30/70), morphotropic (52/48), and rhombohedral (70/30) compositions. The magnitude of the effective longitudinal piezoelectric coefficient (d33) and pyroelectric coefficient (p) of these films is measured by atomic force microscopy and Byer-Roundy method, respectively. All films are consistently highly textured (111) orientation and have dense microstructures. Slightly less degree of texture in higher Zr-rich composition is observed due to the lattice mismatch between PZT and Pt bottom electrode and higher activation energy for nucleation. Squareness of polarization hysteresis loops is optimized in tetragonal composition, which indicates the tetragonal PZT is closer to the ideal hysteresis behavior than other compositions. It is shown that the piezoelectric coefficient and the pyroelectric figure of merit are dependent on the dielectric properties of the films. The morphotropic PZT films with high dielectric constant and low pyroelectric figure of merit show the largest piezoelectric coefficient values, while the tetragonal PZT films with low dielectric constant and high remanent polarization values show the largest pyroelectric figure of merit compared to other compositions, which indicate the suitability for PIR sensor devices.

Effects of IrO2/Pt hybrid electrodes on the crystallization and ferroelectric performances of sol-gel-derived Pb(Zr, Ti)O3 thin film capacitors

The effects of IrO 2 /Pt layered hybrid bottom and/or top electrode structures on the leakage current density versus voltage (J-V), polarization versus voltage (P-V), ferroelectric imprint, and fatigue properties of chemical-solution-derived Pb[Zr x Ti 1 - x )O 3 (PZT, Zr/Ti = 35/65) thin films were investigated. The best P-V and J-V performances were obtained from a capacitor with nonhybrid electrodes (Pt/PZT/Pt capacitor). However, the poor fatigue performance of the capacitor required the adoption of hybrid electrode structures. A thin IrO 2 layer, as thin as 6 nm, which was inserted between top Pt electrode and PZT layer was sufficient for improving the fatigue performance without any degradation of the other ferroelectric properties. However, the same layer adopted on the bottom Pt electrode was not effective in improving the fatigue performance with degradation in P-V and J-V properties. This was ascribed to IrO 2 layer dissolution into the PZT layer during the crystallization annealing of the PZT thin film. A thicker IrO 2 layer resulted in more serious degradation.

Characterization of Platinum films Deposited by a Two-Step Magnetron Sputtering on SiO2/Si Substrates

In this study, defect-free Pt films with good adhesion were deposited on SiO 2 /Si substrates by a two-step magnetron sputtering. This method consists of the first sputtering step using Ar/O 2 gas mixture and the second step using Ar. After two-step deposition, an annealing process was followed at 600-1,000 °C in ambient atmosphere. In the first step, oxygen containing Pt films were deposited. Oxygen incorporated in the Pt films completely diffused out during the high temperature annealing. After the annealing process, the film became dense without catastrophic failures such as hillock, pinhole or buckling. Adhesion strength of films produced by this process was good enough to pass a tape test. It is believed that the good adhesion and the observed microstructural evolution are related to the oxygen in Pt films introduced during the first sputtering step. Adhesion, microstructural evolution and the role of oxygen in Pt films are briefly discussed.