Kyu-Mann Lee

Enhanced Retention Characteristics of Pb(Zr, Ti)O3 Capacitors by Ozone Treatment

Effects of ozone treatment and charged defects on retention characteristics of Ir/IrO2/Pb(Zr, Ti)O3 (PZT)/Pt/IrO2/Ir capacitors were systematically investigated. For these purposes, PZT thin films were exposed to ozone environment to promote enhanced surface oxidation. After baking the Ir/IrO2/PZT/Pt/IrO2/Ir capacitors at 125°C for 500 h, degradation of Qnv (non-volatile charge) value of the ozone-treated capacitors was approximately 17.6%, that is less than one fifth of that of the untreated capacitors. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) studies showed that the amount of oxygen-vacancies near the PZT surface was dramatically decreased by the ozone treatment. The Schottky barrier height of the ozone-treated capacitors increased when compared to that of the untreated capacitors (the Schottky barrier height of the untreated and the ozone-treated capacitor was 0.29 eV and 0.43 eV, respectively). Therefore, one can conclude that the retention characteristics seem to be closely associated with oxygen related defects near the ferroelectric/electrode interface and the control of the interface properties of PZT thin film is a key technology to pursue reliable function characteristics of ferroelectric random access memory (FRAM) devices.

Temperature dependence of microstructure and strain evolution in strained ZnO films on Al2O3(0001)

We have studied the temperature dependence of the growth mode and microstructure evolution in highly mismatched sputter-grown ZnO/Al(2)O(3)(0001) heteroepitaxial films. The growth mode was studied by real-time synchrotron x-ray scattering. We find that the growth mode changes from a two-dimensional (2D) layer to a 3D island in the early growth stage with temperature (300-600 °C), in sharp contrast to the reported transition from three dimensions to two dimensions in metal-organic vapor phase epitaxy. At around 400 °C intermediate 2D platelets nucleate in the early stage, which act as nucleation cores of 3D islands and transform to a misaligned state during further growth. Meanwhile, at high temperature (above 500 °C), the spinel structure of ZnAl(2)O(4) grows in the early stage, and it undergoes a transition to wurtzite-ZnO (w-ZnO) with thickness. The spinel formation is presumably driven by high temperature and large incident energy of impacting atoms during sputtering. The results of the strain evolution as functions of temperature and thickness during growth suggest that the surface diffusion is a major factor determining the microstructural properties in the strained ZnO/Al(2)O(3)(0001) heteroepitaxy.

Integration of ferroelectric random access memory devices with Ir/IrO2/Pb(ZrxTi1-x)O3/Ir capacitors formed by metalorganic chemical vapor deposition-grown Pb(ZrxTi1-x)O3

Metal organic chemical vapor deposition (MOCVD) of Pb(ZrxTi1-x)O3 (PZT) and its capacitor module process were established for ferroelectric memory device integration. The 130 nm-thick PZT films were deposited on Ir layers at 530°C or 550°C. The remnant polarization of the Ir/IrO2/PZT/Ir capacitors is in the range of 15 to 21 µC/cm2, and their leakage current is 10-5 A/cm2 at 2.5 V without additional annealing. The degradation in their switching endurance is less than 5% after 1010 cycles, indicating that the interfaces formed between the PZT and Ir layers can be optimized to improve their fatigue properties. To evaluate the capacitors on the devices, the conventional backend process was performed after encapsulating the capacitors with AlOx/TiOx layers located on the poly-Si plug. High charge separation and fully functional bit activities were obtained, demonstrating that this MOCVD-PZT process is a reliable integration scheme for high-density ferroelectric memory devices.

Effect of the flow rate of oxygen and hydrogen gases on the characteristics of organic light emitting diodes with Al-doped ZnO anodes

We investigate the effect of the flow rate of oxygen and hydrogen gases on the characteristics of OLEDs (organic light emitting diodes) with Al-doped ZnO (AZO) anodes. The AZO thin films are deposited by radio frequency (RF) magnetron sputtering under different ambient gases (Ar, Ar + O2, and Ar + H2) at 300°C. To investigate the influences of the ambient gases, the flow rate of oxygen and hydrogen in argon was varied from 0.1 sccm to 5 sccm. The AZO thin films were preferentially oriented to the (002) direction regardless of the ambient gase used. The electrical resistivity of the AZO thin films increased with increasing O2 flow rates, whereas the electrical resistivity decreased sharply under an Ar + H2 atmosphere and was nearly the same regardless of the H2 flow rate used. The change of electrical resistivity with changes in the ambient gas composition was mainly interpreted in terms of the charge carrier concentration rather than the charge carrier mobility. All the films showed an average transmittance of over 85% in the visible range. The optical band gap of the AZO films increased with increasing H2 flow rates, whereas the optical band gap of the AZO films deposited in an O2 atmosphere decreased with increasing O2 flow rates. The current density and the luminance of the OLED devices with AZO thin films deposited in 5 sccm of H2 ambient gas were the highest among all the films. The optical band gap energy of AZO thin films plays a major role in OLED device performance, especially the current density and luminance.

Evaporation of Epitaxial ZnO Films During Post-Deposition Annealing

The effect of evaporation on the surface morphology of c-oriented epitaxial ZnO (40 nm thick)/Al2O3(0001) films during post-deposition annealing was examined. On the basis of real-time synchrotron X-ray scattering analysis and atomic force microscopy (AFM), we found that evaporation as well as grain growth played important roles on the surface morphology of the ZnO/Al2O3(0001) films. Facets with higher surface energies than the (0001) planes were formed during grain growth in the initial stage of annealing. By the preferential evaporation of the prism planes, the surface morphology of the ZnO film eventually evolves into a two-dimensional (2D) flat (0001) surface at 800 °C, which was confirmed by AFM. The real-time measurement of film thickness during annealing clearly supports the previous results that evaporation is directly related to surface morphology. The evaporation rate is high in the initial stage, which is caused by the preferential evaporation from high-energy facets but decreases after the transition to a flat (0001) surface.

Integration and Electrical Properties of Novel Ferroelectric Capacitors for 0.25 µm 1 Transistor 1 Capacitor Ferroelectric Random Access Memory (1T1C FRAM)

Since the space margin between capacitors has been greatly reduced in 32 Mb high-density ferroelectric random access memory (FRAM) with a 0.25 µm design rule, considering the limitation of current etching technology, the stack height of ferroelectric capacitors should be minimized for stable node separation. In this paper, novel capacitors with a total thickness of 4000 A were prepared using a seeding layer, low temperature processing, and optimal top electrode annealing. The 1000 A Pb(Zr1-xTix)O3 (PZT) films showed excellent structural and ferroelectric properties such as strong (111) orientation and large remanent polarization of 40 µC/cm2. The low stack capacitors were then implemented into 0.6 µm and prototype 0.25 µm FRAM. Compared to a conventional capacitor stack, the ferroelectric capacitors exhibited adequate sensing margin of 250 fC, thus giving rise to a fully working die of 4 Mb FRAM. Therefore, it was clearly demonstrated that the novel capacitors can enable the realization of a high-density 32 Mb FRAM device with a 0.25 µm design rule.

Effect of ultrathin buffer on the microstructure of highly mismatched epitaxial ZnO films on Al2O3 (0001)

The effect of the ultra thin (4nm) ZnO buffer layer grown at a low temperature of 300°C on the microstructural evolution of highly mismatched ZnO∕Al2O3(0001) films was investigated. Based on the real time synchrotron x-ray scattering, atomic force microscopy, and high resolution electron microscopy, it was shown that the ultrathin two-dimensional (2D) layers play a critical role for improving the ZnO layer quality by inducing 2D growth mode instead of three-dimensional mode at 500°C in early stage. The ZnO films grown on the ultrathin buffer exhibited structural coherence between the surface and the interface in the substrate normal direction in early stage. The great enhancement of the microstructural quality was attributed to the strain accommodation by the 2D ultrathin buffer.

Plasma-assisted dry etching of ferroelectric capacitor modules and application to a 32M ferroelectric random access memory devices with submicron feature sizes

In the manufacturing of a 32M ferroelectric random access memory (FRAM) device on the basis of 0.25 design rule (D/R), one of the most difficult processes is to pattern a submicron capacitor module while retaining good ferroelectric properties. In this paper, we report the ferroelectric property of patterned submicron capacitor modules with a stack height of 380 nm, where the 100 nm-thick Pb(Zr, Ti)O3 (PZT) films were prepared by the sol-gel method. After patterning, overall sidewall slope was approximately 70° and cell-to-cell node separation was made to be 80 nm to prevent possible twin-bit failure in the device. Finally, several heat treatment conditions were investigated to retain the ferroelectric property of the patterned capacitor. It was found that rapid thermal processing (RTP) treatment yields better properties than conventional furnace annealing. This result is directly related to the near-surface chemistry of the PZT films, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The resultant switching polarization value of the submicron capacitor was approximately 30 µC/cm2 measured at 3 V.

The Effect of Iridium Bottom Electrode on the Characteristics of Pb(Zr,Ti)O3 Films Grown by MOCVD Method

A novel method of metal-organic chemical vapor deposition (MOCVD) Pb(Zr,Ti)O3 (PZT) has been developed for use in high-density ferroelectric memory device. Well-aligned polycrystalline PZT films were grown onto Iridium bottom electrode by the MOCVD method with tmhd-family precursors in octane-based solvent under oxygen atmosphere at 550°C. Moreover, crystallinity of the PZT films on Ir bottom electrode was improved dramatically by inserting TiAlN barrier layer. It is also investigated the Iridium bottom electrode effect on the PZT in the ways of roughness, grain size, remnant polarization, fatigue and retention properties. Resultantly, highly reliable (111)-oriented PZT capacitors were obtained with 2Pr of 45 μC/cm2 and Vc of 0.8 V through MOCVD method and the interface engineering of the Iridium bottom electrode.

Doping amount dependence of phase formation and microstructure evolution in heavily Cu-doped Bi2Te3 films for thermoelectric applications

Heavily Cu-doped Bi2Te3 films with different Cu contents (∼47.9 at%) were prepared by co-sputtering with Cu and Bi2Te3 targets at room temperature and post-annealing at ∼500 °C. When the Cu content increased, the lattice parameters of the c-axis gradually increased with the increase of Cu intercalation into van der Waals gaps between the quintets of the Bi2Te3 layers. When a Cu content of above 43.7 at% was added, formation of a Cu2Te phase was detected along with the phase transition from Bi2Te3 to Te-deficient BixTey in the films annealed at 300 °C. The porous column structure of the pure Bi2Te3 film became gradually dense with the increase of Cu content. When the Cu content was increased, the thermoelectric power factors of heavily Cu-doped Bi2Te3 films were enhanced by the increase of carrier concentration.