Byoung Taek Lee

A comparative study on the electrical conduction mechanisms of (Ba0.5Sr0.5)TiO3 thin films on Pt and IrO2 electrodes

Electrical conduction mechanisms for Pt/(Ba0.5Sr0.5)TiO3 (BST)/Pt, IrO2/BST/IrO2, and Pt/BST/IrO2 capacitors were studied. The Pt/BST/Pt capacitor shows a Schottky emission behavior with interface potential barrier heights of about 1.5–1.6 eV. The barrier height is largely determined by the surface electron trap states of the BST. The IrO2/BST interface shows an ohmic contact nature due to the elimination of the surface trap states as the result of the formation of strong chemical bonds between the IrO2 and BST which results in the Poole–Frenkel emission conduction mechanism. Pt/BST/IrO2 capacitor shows Schottky emission behavior and a positive temperature coefficient of resistivity (PTCR) effect depending on the bias polarity. The electron trap states at the Pt/BST interface and the positive space charges within the carrier depletion layer result in the PTCR effect.

Depletion layer thickness and Schottky type carrier injection at the interface between Pt electrodes and (Ba, Sr)TiO3 thin films

The electrical conduction properties of rf sputter-deposited (Ba, Sr)TiO3 (BST) films on Pt and IrO2 electrodes and metalorganic chemical vapor deposited (MOCVD) BST films on a Pt electrode were investigated and a new energy band model that satisfactorily explains the observed leakage current characteristics and film thickness dependent dielectric properties is proposed. The BST and Pt junction constituted a blocking contact with interface potential barrier heights of 1.6–1.7 eV and 1.2 eV for the sputtered and MOCVD films, respectively. Schottky emission behavior was observed at measurement temperatures higher than 120 °C and tunneling related conduction behavior appeared below that temperature for a film thickness of 40 nm. A partial depletion model with a very thin (about 1 nm) layer devoid of space charge at the interface with the Pt electrode is proposed to explain the V1/2 dependent variation of ln(Jo) as well as the decreasing dielectric constant with decreasing film thickness.

Influences of interfacial intrinsic low-dielectric layers on the dielectric properties of sputtered (Ba,Sr)TiO3 thin films

The influences of low-dielectric interfacial layers on the dielectric properties of Pt/(Ba,Sr)TiO3/Pt capacitors were investigated before and after postannealing. The interfacial layer is believed to be the intrinsic dead layer (low-dielectric layer) due to the termination of chemical bonds of the (Ba,Sr)TiO3 (BST) material at the interfaces. The dielectric constant of the capacitor decreases with decreasing BST film thickness owing to the low dielectric constant of the dead layer. The dead-layer capacitance varies with processes such as film deposition temperature, and postannealing. Higher deposition temperatures result in a larger dead-layer capacitance and a higher bulk dielectric constant. Although annealing under a N2 atmosphere is less effective in reducing the dead-layer effect than under an O2 atmosphere, it is more effective in increasing the bulk dielectric constant. Therefore, a N2, rather than an O2 atmosphere, results in a larger increase in the overall dielectric constant.

Preparation and characterization of iridium oxide thin films grown by DC reactive sputtering

Iridium oxide ( IrO2) thin films were successfully grown by a DC magnetron reactive sputtering method. It was found that the crystalline nature and morphology of IrO2 films were strongly dependent on the oxygen partial pressure, total pressure and growth temperature. The growth of IrO2 is well explained by the generic curve for the total pressure as a function of O2 content. The films showed good barrier performance between Pt and poly-Si up to 750° C. A 40-nm-thick Ba0.5Sr0.5TiO3 film was grown by RF magnetron sputtering on the Pt/IrO2/poly-Si electrode. The leakage current density and dielectric constant of a Pt/Ba0.5Sr0.5TiO3/Pt capacitor on the IrO2/poly-Si electrode were comparable to those of the capacitor on a SiO2/Si substrate. However, an additional ohmic layer was required to prevent the formation of a SiO2 layer between the IrO2 and poly-Si.

Preparation and Electrical Properties of SrTiO3 Thin Films Deposited by Liquid Source Metal-Organic Chemical Vapor Deposition (MOCVD)

SrTiO 3 thin films with thicknesses ranging from 30 nm to 60 nm were grown on 6-inch-diameter, platinized Si wafers by liquid source metal-organic chemical vapor deposition (MOCVD). The crystalline quality and cation concentrations of the films are strongly dependent on the deposition temperature with optimum temperatures ranging from 500°C to 550°C. Semi-conformal deposition on submicron-sized storage node patterns is obtained but further improvements in conformality and reproducibility are required. The dielectric constant is about 210 irrespective of the film thickness and leakage current densities are sufficiently small for the dynamic random access memory (DRAM) applications. SiO 2 equivalent thickness (T ox ) of the 30-nm-thick STO film is 0.51 nm. The finding that the leakage current density and dielectric constant are independent of the film thickness can be explained by a fully depleted model of the STO film.

Deposition Characteristics of (Ba, Sr)TiO3 Thin Films by Liquid Source Metal-Organic Chemical Vapor Deposition at Low Substrate Temperatures

The deposition characteristics of (Ba, Sr)TiO3 (BST) thin films using a liquid source metal-organic chemical vapor deposition on a 6-inch-diameter Pt/SiO2/Si wafer were investigated. Ba(DPM)2 tetraglyme, Sr(DPM)2 tetraglyme and Ti(DPM)2(O-i-Pr)2, dissolved in n-butyl acetate, were used as the sources of Ba, Sr and Ti, respectively. Step coverage, within-wafer uniformities in composition and thickness of the BST films were investigated as a function of substrate temperatures ranging from 420° C to 570° C. As the substrate temperature decreased, the step coverage improved, whereas the within-wafer uniformities degraded. From BST films deposited in a temperature range from 450° C to 480° C, good step coverage (>80%), as well as good within-wafer uniformity were obtained. However, in that temperature range, hazy deposition was observed due to many humps on the surface. The humps are an agglomeration of crystalline grains and are a few hundred A in diameter, which have a Ti-rich composition compared to flat film region.

IMPROVEMENT OF LEAKAGE CURRENT CHARACTERISTICS OF BA0.5SR0.5TIO3 FILMS BY N2O PLASMA SURFACE TREATMENT

The effects of plasma surface treatment, using N2O gas, of Ba0.5Sr0.5TiO3 (BST) film on the leakage current characteristic of a Pt/BST/Pt capacitor were investigated. As a result of exposure of BST film to the plasma, the leakage current density of the BST capacitor decreased by two orders of magnitude in the high voltage region, and higher onset voltage of an abrupt increase in leakage current was observed. The improvement of leakage properties of BST films can be attributed to the elimination of the bulged curve in the leakage current characteristics. Thermal desorption spectroscopy showed that the elimination was closely related to the reduction of carbon content in the BST film.

A positive temperature coefficient of resistivity effect from a paraelectric Pt/(Ba0.5,Sr0.5)TiO3/IrO2 thin-film capacitor

A (Ba0.5,Sr0.5)TiO3 (BST) thin film was deposited on IrO2 thin-film electrode by a rf magnetron sputtering method. Top Pt electrode was deposited on the BST film to make a planar structured capacitor. The BST thin film showed paraelectric behavior at room temperature. A positive temperature coefficient of resistivity (PTCR) effect was observed when the conduction electrons were injected from the IrO2 electrode to BST, while Schottky emission behavior was observed when they were injected from the Pt electrode to BST. The electrical double layer formed at the Pt/BST interface results in the PTCR effect. A model that can explain the asymmetrical conduction behavior with respect to the bias polarity is suggested based on the energy band configurations at the interfaces with the electrodes.

Interface potential barrier height and leakage current behavior of Pt/(Ba, Sr)TiO3/Pt capacitors fabricated by sputtering process

Abstract Variations of the leakage current behaviors and interface potential barrier height (φ B ) of rf-sputter deposited (Ba, Sr)TiO3 (BST) thin films, with thickness ranging from 20nm to 150 nm are investigated as a function of the thickness and bias voltages. The top and bottom electrodes are dc-sputter-deposited Pt films. φ B critically depends on the BST film deposition temperature, postannealing atmosphere and time after the annealing. The postannealing under N2 atmosphere results in a high interface potential barrier height and low leakage current. Maintaining the BST capacitor in air for a long time reduces the φ B from about 2.4 eV to 1.6eV due to the oxidation. φ B is not so dependent on the film thickness in this experimental range. The leakage conduction mechanism is very dependent on the BST film thickness; the 20nm thick film shows tunneling current, 30 and 40 nm thick films show Schottky emission current and the thicker films show a mixed characteristics and bulk and interface limited curr...

Structural and electrical properties of Ba0.5Sr0.5TiO3 films on Ir and IrO2 electrodes

High-dielectric Ba0.5Sr0.5TiO3 (BST) thin films are grown on Ir and IrO2 electrodes by a rf sputtering method. The structural and electrical properties of BST films are investigated. After postannealing at 750° C for 30 min in N2 atmosphere, the interface between BST and IrO2 remains flat and the IrO2 does not show any structural change. On the other hand, a thin IrO2 layer is formed at the interface between the BST and Ir after the same post-annealing. A larger grain size of BST film is obtained on the Ir electrode compared to that of BST film on the IrO2 electrode. Dielectric constants of 36-nm-thick BST films on Ir and IrO2 are 338 and 290, respectively. The leakage current densities of both the Pt/BST/Ir and Pt/BST/IrO2 capacitors are about 20 nA/cm2 at ±1.5 V, which is sufficiently small for application to dynamic random access memory devices.