Cheol Hyun An

Structure and Electrical Properties of Al-Doped HfO2 and ZrO2 Films Grown via Atomic Layer Deposition on Mo Electrodes

The effects of Al doping in atomic-layer-deposited HfO2 (AHO) and ZrO2 (AZO) films on the evolutions of their crystallographic phases, grain sizes, and electric properties, such as their dielectric constants and leakage current densities, were examined for their applications in high-voltage devices. The film thickness and Al-doping concentration were varied in the ranges of 60-75 nm and 0.5-9.7%, respectively, for AHO and 55-90 nm and 1.0-10.3%, respectively, for AZO. The top and bottom electrodes were sputtered Mo films. The detailed structural and electrical property variations were examined as functions of the Al concentration and film thickness. The AHO films showed a transition from the monoclinic phase (Al concentration up to 1.4%) to the tetragonal/cubic phase (Al concentration 2.0-3.5%), and finally, to the amorphous phase (Al concentration >4.7%), whereas the AZO films remained in the tetragonal/cubic phase up to the Al concentration of 6.4%. For both the AHO and AZO films, the monoclinic and amorphous phases had dielectric constants of 20-25, and the tetragonal/cubic phases had dielectric constants of 30-35. The highest electrical performance levels for the application to the high-voltage charge storage capacitors in flat panel displays were achieved with the 4.7-9.7% Al-doped AHO films and the 2.6% Al-doped AZO films.

Time-Dependent Negative Capacitance Effects in Al2O3/BaTiO3 Bilayers

The negative capacitance (NC) effects in ferroelectric materials have emerged as the possible solution to low-power transistor devices and high-charge-density capacitors. Although the steep switching characteristic (subthreshold swing < sub-60 mV/dec) has been demonstrated in various devices combining the conventional transistors with ferroelectric gates, the actual applications of the NC effects are still some way off owing to the inherent hysteresis problem. This work reinterpreted the hysteretic properties of the NC effects within the time domain and demonstrated that capacitance (charge) boosting could be achieved without the hysteresis from the Al2O3/BaTiO3 bilayer capacitors through short-pulse charging. This work revealed that the hysteresis phenomenon in NC devices originated from the dielectric leakage of the dielectric layer. The suppression of charge injection via the dielectric leakage, which usually takes time, inhibits complete ferroelectric polarization switching during a short pulse time. It was demonstrated that a nonhysteretic NC effect can be achieved only within certain limited time and voltage ranges, but that these are sufficient for critical device applications.

Controlling the Al-Doping Profile and Accompanying Electrical Properties of Rutile-Phased TiO2 Thin Films

The role of Al dopant in rutile-phased TiO2 films in the evaluation of the mechanism of leakage current reduction in Al-doped TiO2 (ATO) was studied in detail. The leakage current of the ATO film was strongly affected by the Al concentration at the interface between the ATO film and the RuO2 electrode. The conduction band offset of the interface increased with the increase in the Al dopant concentration in the rutile TiO2, which reduced the leakage current in the voltage region pertinent to the next-generation dynamic random access memory application. However, the Al doping in the anatase TiO2 did not notably increase the conduction band offset even with a higher Al concentration. The detailed analyses of the leakage conduction mechanism based on the quantum mechanical transfer-matrix method showed that Schottky emission and Fowler-Nordheim tunneling was the dominant leakage conduction mechanism in the lower and higher voltage regions, respectively. The chemical analyses using X-ray photoelectron spectroscopy corroborated the electrical test results.

Evaluating the Top Electrode Material for Achieving an Equivalent Oxide Thickness Smaller than 0.4 nm from an Al-Doped TiO2 Film

The effects of Pt and RuO2 top electrodes on the electrical properties of capacitors with Al-doped TiO2 (ATO) films grown on the RuO2 bottom electrode by an atomic layer deposition method were examined. The rutile phase ATO films with high bulk dielectric constant (>80) were well-grown because of the local epitaxial relationship with the rutile structured RuO2 bottom electrode. However, the interface between top electrode and ATO was damaged during the sputtering process of the top electrode, resulting in the decrease in the dielectric constant. Postmetallization annealing at 400 °C was performed to mitigate the sputtering damage. During the postmetallization annealing, the ATO layer near the RuO2 top electrode/ATO interface was well-crystallized because of the structural compatibility between RuO2 and rutile ATO, while the ATO layer near the Pt top electrode/ATO interface still exhibited an amorphous-like structure. Despite the same thickness of the ATO films, therefore, the capacitors with RuO2 top electrodes showed higher capacitance compared to the capacitors with Pt top electrodes. Eventually, an extremely low equivalent oxide thickness of 0.37 nm with low enough leakage current density (<1 × 10(-7) A/cm(2) at 0.8 V) and physical thickness of 8.7 nm for the next-generation dynamic random access memory was achieved from ATO films with RuO2 top electrodes.

Ta-Doped SnO2 as a reduction–resistant oxide electrode for DRAM capacitors

Noble metal oxides, such as RuO2, have received attention as capacitor electrodes in dynamic random access memories (DRAMs). Noble metal oxides generally have a high work function compared to noble metals and enhance the crystallinity of dielectric materials grown on them, resulting in a lower leakage current and higher dielectric constants. Despite these advantages, noble metal oxides are easily reduced during the dielectric film, such as TiO2, growth on top or by annealing under a forming gas atmosphere, degrading the capacitor performance. In this work, Ta-doped SnO2 is suggested as a potential capacitor electrode for DRAMs. Ta-Doped SnO2 films have a high work function, comparable to that of RuO2, and induce the formation of a high-temperature phase with a high dielectric constant, namely rutile TiO2, at low temperatures. More importantly, the Ta-doped SnO2 films show suitable structural and chemical stabilities, even after annealing at 400 °C under a forming gas atmosphere. RuO2 films, on the other hand, turn into a mixture of RuO2 and Ru after annealing under the same conditions. These findings suggest that Ta-doped SnO2 could serve as capacitor electrodes in next-generation DRAMs.

Chemistry of active oxygen in RuOx and its influence on the atomic layer deposition of TiO2 films

Rutile structured TiO2 films have received great attention as dielectric materials in capacitors of the next-generation dynamic random access memory (DRAM) due to their high dielectric constant (80–150). Ru or RuO2, which is one of the most promising electrode materials in DRAM capacitors, is indispensable to form the rutile structure. In this work, a series of the Ru-related layers with compositions ranging from Ru to RuO2via RuOx (x: ∼1.12) was used as a bottom electrode for the ALD growth of TiO2 films. It was found that the growth per cycle of TiO2 at the initial growth stage was drastically increased on RuOx (RuO2/Ru mixture) compared to Ru and RuO2. This is attributed to the drastic increase in the chemical activity of oxygen in the mixture film of RuO2/Ru. The catalytic decomposition of RuO2 with the help of Ru in the film played the crucial role for the increase in the active oxygen. Although RuO2 and Ru mostly retained their structures during the ALD of TiO2 or chemical etching using O3 gas, the RuOx film, which was composed of 56% RuO2 and 44% Ru, drastically changed its phase composition during the ALD of TiO2 at 250 °C and changed almost to Ru. Other chemical effects depending on the chemical composition and phase structure were also examined in detail.

Balancing the Source and Sink of Oxygen Vacancies for the Resistive Switching Memory

The high nonuniformity and low endurance of the resistive switching random access memory (RRAM) are the two major remaining hurdles at the device level for mass production. Incremental step pulse programming (ISPP) can be a viable solution to the former problem, but the latter problem requires material level innovation. In valence change RRAM, electrodes have usually been regarded as inert (e.g., Pt or TiN) or oxygen vacancy (VO) sources (e.g., Ta), but different electrode materials can serve as a sink of VO. In this work, an RRAM using a 1.5 nm-thick Ta2O5 switching layer is presented, where one of the electrodes was VO-supplying Ta and the other was either inert TiN or VO-sinking RuO2. Whereas TiN could not remove the excessive VO in the memory cell, RuO2 absorbed the unnecessary VO. By carefully tuning (balancing) the capabilities of VO-supplying Ta and VO-sinking RuO2 electrodes, an almost invariant ISPP voltage and a greatly enhanced endurance performance can be achieved.

Quantitative Analysis of the Incorporation Behaviors of Sr and Ti Atoms During the Atomic Layer Deposition of SrTiO3 Thin Films

The atomic layer deposition (ALD) of multication oxide films is complicated because the deposition behaviors of the component oxides are not independent of one another. In this study, the Ti and Sr atom incorporation behaviors during the ALD of SrTiO3 films were quantitatively examined via the carefully designed ALD process sequences. H2O and O3 were adopted as the oxygen sources of the SrO subcycles, whereas only O3 was used for the TiO2 ALD subcycles. Apart from the general conjecture on the roles of the different types of oxygen sources, the oxygen source that was adopted for the subcycles of the other component oxide had almost complete control of the metal atom incorporation behaviors. This means that the first half-cycle of ALD played a dominant role in determining the metal incorporation rate, which revealed the critical role of the steric hindrance effect during the metal precursor injection for the ALD rate. O3 had almost doubled its reactivity toward the Ti and Sr precursors compared with H2O. Although these are the expected results from the common knowledge on ALD, the quantitative analysis of the incorporation behaviors of each metal atom provided insightful viewpoints for the ALD process of this technically important oxide material. Furthermore, the SrTiO3 films with a bulk dielectric constant as high as 236 were obtained by the Ru-SrTiO3-RuO2 capacitor structure.