Woojin Jeon

Highly Improved Uniformity in the Resistive Switching Parameters of TiO2 Thin Films by Inserting Ru Nanodots

Limiting the location where electron injection occurs at the cathode interface to a narrower region is the key factor for achieving a highly improved RS performance, which can be achieved by including Ru Nanodots. The development of a memory cell structure truly at the nanoscale with such a limiting factor for the electric-field distribution can solve the non-uniformity issue of future ReRAM.

Grain size engineering for ferroelectric Hf0.5Zr0.5O2 films by an insertion of Al2O3 interlayer

The degradation of ferroelectric (FE) properties of atomic layer deposited Hf0.5Zr0.5O2 films with increasing thickness was mitigated by inserting 1 nm-thick Al2O3 interlayer at middle position of the thickness of the FE film. The large Pr of 10 μC/cm2, which is 11 times larger than that of single layer Hf0.5Zr0.5O2 film with equivalent thickness, was achieved from the films as thick as 40 nm. The Al2O3 interlayer could interrupt the continual growth of Hf0.5Zr0.5O2 films, and the resulting decrease of grain size prevented the formation of non-ferroelectric monoclinic phase. The Al2O3 interlayer also largely decreased the leakage current of the Hf0.5Zr0.5O2 films.

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.

Frustration of Negative Capacitance in Al2O3/BaTiO3 Bilayer Structure.

Enhancement of capacitance by negative capacitance (NC) effect in a dielectric/ferroelectric (DE/FE) stacked film is gaining a greater interest. While the previous theory on NC effect was based on the Landau-Ginzburg-Devonshire theory, this work adopted a modified formalism to incorporate the depolarization effect to describe the energy of the general DE/FE system. The model predicted that the SrTiO3/BaTiO3 system will show a capacitance boost effect. It was also predicted that the 5 nm-thick Al2O3/150 nm-thick BaTiO3 system shows the capacitance boost effect with no FE-like hysteresis behavior, which was inconsistent with the experimental results; the amorphous-Al2O3/epitaxial-BaTiO3 system showed a typical FE-like hysteresis loop in the polarization – voltage test. This was due to the involvement of the trapped charges at the DE/FE interface, originating from the very high field across the thin Al2O3 layer when the BaTiO3 layer played a role as the NC layer. Therefore, the NC effect in the Al2O3/BaTiO3 system was frustrated by the involvement of reversible interface charge; the highly stored charge by the NC effect of the BaTiO3 during the charging period could not be retrieved during the discharging process because integral part of the polarization charge was retained within the system as a remanent polarization.

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.

Optimization of chemical structure of Schottky-type selection diode for crossbar resistive memory.

The electrical performances of Pt/TiO(2)/Ti/Pt stacked Schottky-type diode (SD) was systematically examined, and this performance is dependent on the chemical structures of the each layer and their interfaces. The Ti layers containing a tolerable amount of oxygen showed metallic electrical conduction characteristics, which was confirmed by sheet resistance measurement with elevating the temperature, transmission line measurement (TLM), and Auger electron spectroscopy (AES) analysis. However, the chemical structure of SD stack and resulting electrical properties were crucially affected by the dissolved oxygen concentration in the Ti layers. The lower oxidation potential of the Ti layer with initially higher oxygen concentration suppressed the oxygen deficiency of the overlying TiO(2) layer induced by consumption of the oxygen from TiO(2) layer. This structure results in the lower reverse current of SDs without significant degradation of forward-state current. Conductive atomic force microscopy (CAFM) analysis showed the current conduction through the local conduction paths in the presented SDs, which guarantees a sufficient forward-current density as a selection device for highly integrated crossbar array resistive memory.

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.

Interfacial charge-induced polarization switching in Al2O3/Pb(Zr,Ti)O3 bi-layer

Detailed polarization switching behavior of an Al2O3/Pb(Zr,Ti)O3 (AO/PZT) structure is examined by comparing the phenomenological thermodynamic model to the experimental polarization–voltage (P-V) results. Amorphous AO films with various thicknesses (2–10 nm) were deposited on the polycrystalline 150-nm-thick PZT film. The thermodynamic calculation showed that the transition from the ferroelectric-like state to the paraelectric-like state with increasing AO thickness occurs at ∼3 nm thickness. This paraelectric-like state should have exhibited a negative capacitance effect without permanent polarization switching if no other adverse effects are involved. However, experiments showed typical ferroelectric-like hysteresis loops where the coercive voltage increased with the increasing AO thickness, which could be explained by the carrier injection through the thin AO layer and trapping of the carriers at the AO/PZT interface. The fitting of the experimental P-V loops using the thermodynamic model considering the depolarization energy effect showed that trapped charge density was ∼±0.1 Cm−2 and critical electric field at the Pt electrode/AO interface, at which the carrier transport occurs, was ∼±10 MV/cm irrespective of the AO thickness. Energy band model at each electrostatic state along the P-V loop was provided to elucidate correlation between macroscopic polarization and internal charge state of the stacked films.

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.

Variation in the threshold voltage of amorphous-In2Ga2ZnO7 thin-film transistors by ultrathin Al2O3 passivation layer

The variations in the performance of amorphous In2Ga2ZnO7 thin-film transistors with ultrathin Al2O3 passivation layers deposited by atomic layer deposition (ALD) were examined. As the ALD Al2O3 deposition cycle number increased, the threshold voltage shifted to the negative voltage direction, while the saturation mobility was invariant. These variations are attributed to the removal of electronegative species such as OH− groups on back channel surface, while the bulk properties of the channel were hardly affected during the ALD. The ALD may not influence the oxygen vacancy concentration in the amorphous In2Ga2ZnO7 channel. The OH− groups on the Al2O3 surface further influenced the threshold voltage through capacitive coupling. The shifted properties recover the initial values after long-term exposure to air (100 days), by diffusion of OH− to the Al2O3/In2Ga2ZnO7 interface. These findings were further confirmed by spectroscopic ellipsometry, x-ray photoelectron spectroscopy, and electrical characterizatio...