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.

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.

Effect of Growth Temperature during the Atomic Layer Deposition of the SrTiO3 Seed Layer on the Properties of RuO2/SrTiO3/Ru Capacitors for Dynamic Random Access Memory Applications.

The atomic layer deposition process of SrTiO3 (STO) films at 230 °C was studied with Sr(iPr3Cp)2 and Ti(CpMe5)(OMe)3 (Pr, Cp, and Me are propyl, cyclopentadienyl, and methyl groups, respectively) on Ru substrates. The growth behavior and properties of STO films grown at 230 °C were compared with those deposited at 370 °C. With the limited over-reaction of the Sr precursor during the initial growth stage at a lower temperature, the cation composition was more controllable, and the surface morphology after crystallization annealing at 650 °C had more uniform grains with fewer defects. Here, the excess reaction of the Sr precursor means the chemical-vapor-deposition-like growth of the SrO component mediated through the thermal decomposition of the adsorbed Sr precursor molecules. It was by the reaction of the Sr precursor with the oxygen supplied from the partly oxidized Ru substrate. The second STO was grown at 370 °C (main layer) on the annealed first STO layer (crystallized seed layer) to lead to the in situ crystallization of the main layer. Due to the improved microstructure of STO films induced by the seed layer deposited at 230 °C, the bulk dielectric constant of 167 was obtained for the main layer, which was higher than the value of 101 where the seed layer was deposited at 370 °C, even though the crystallization annealing condition of the seed layer and the deposition condition of the main layer were consistent. The seed layer grown at 230 °C, however, had a lower dielectric constant of only ∼49, whereas the high-temperature seed layer had a dielectric constant of ∼106. Therefore, the low-temperature seed layer posed a severe limitation in acquiring an advanced capacitor property with the involvement of a low-dielectric interfacial layer.

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.

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.

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.

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.