Jeong Hwan Han

Enhanced electrical properties of SrTiO3 thin films grown by atomic layer deposition at high temperature for dynamic random access memory applications

SrTiO3 (STO) thin films were deposited at 370°C by atomic layer deposition using H2O as the oxidant, and Ti(O–iPr)2(thd)2 and Sr(thd)2 as Ti, and Sr precursors, respectively. Denser STO films were produced at this deposition temperature. The saturated growth rate was 0.15A∕cycle. The adoption of a thin crystallized seed layer resulted in crystallized perovskite STO films at the as-deposited state without higher temperature post-annealing. A tox of 0.72nm (dielectric constant of 108) and a low leakage current density (∼10−7A∕cm2 at 0.8V) were obtained from a planar capacitor structure consisting of Pt∕20-nm-thick STO/Ru (bottom).

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.

Atomic Layer Deposition and Electrical Properties of SrTiO3 Thin Films Grown Using Sr ( C11H19O2 ) 2, Ti ( Oi-C3H7 ) 4, and H2O

Atomic layer deposited SrTiO 3 (STO) thin films were grown using Sr(C 11 H 19 O 2 ) 2 and Ti(Oi-C 3 H 7 ) 4 with a remote plasma activated or thermal H 2 O vapor as oxidant at growth temperatures ranging from 190 to 270°C. The as-grown films were amorphous and showed a low effective dielectric constant of -20 with a low leakage current density (< 10 -7 A/cm 2 at 1 V). The chemical binding status of the Sr ions varied with the degree of crystallization of the STO film. A reasonable film growth rate and stoichiometric cation composition were obtained when the vaporization temperature of Sr-precursor was <200°C with the thermal H 2 O vapor. The low density of the as-grown film induced a large shrinkage in the film thickness which caused microcracking of the crystallized films during postannealing, even with the increased H 2 O supply. Adoption of thin (∼5 nm) crystallized seed layer before the main layer STO growth improved the microstructure after the crystallization and the leakage current performance. As a result of the process optimization, the best electrical properties of an STO film grown on a Ru electrode were 0.45 nm for the equivalent oxide thickness and 1 X 10 -3 A/cm 2 for the leakage current density at 1 V.

A theoretical model for Schottky diodes for excluding the sneak current in cross bar array resistive memory

Kirchhoffs law was used to examine the electrical specifications of selection diodes, which are essential for suppressing the read interference problems in nano-scale resistive switching cross bar arrays with a high block density. The diode in the cross bar array with a 100 Mb block density should have a reverse/forward resistance ratio of > 10(8), and a forward current density of > 10(5) A cm(-2) for stable reading and writing operation. Whilst normal circuit simulators are heavily overloaded when the number of cells (m) connected to one bit and word line is larger (m >> 100), which is the desired range for high density cross bar arrays, the present model can provide a simple simulation. The validity of this new method was confirmed by a comparison with the previously reported method based on a voltage estimation.

Schottky diode with excellent performance for large integration density of crossbar resistive memory

A Schottky diode (SD) with Au/Pt/TiO2/Ti/Pt stacked structure were fabricated for its application to crossbar type resistive switching (RS) memory. The SDs showed a highly promising rectification ratio (∼2.4 × 106 @ ±2 V) between forward and reverse state currents and a high forward current density (∼3 × 105 A/cm2 @ 2 V), which is useful for highly integrated crossbar RS memory. The SD has local forward current conduction paths, which provides extremely scaled devices with an advantage. The minimization of interconnection line resistance is also important to provide sufficient current to achieve stable operation of RS memory.

Improvement in the leakage current characteristic of metal-insulator-metal capacitor by adopting RuO2 film as bottom electrode

The dielectric constant, equivalent oxide thickness (tox), and leakage current properties of Pt/(Al-doped)TiO2/RuO2 capacitors were examined in comparison with Pt/(Al-doped)TiO2/Ru capacitors. The Al-doped TiO2 and undoped TiO2 films grown on RuO2 showed high dielectric constants of 60 and 102, respectively. The minimum tox of these films were 0.46 nm and 0.56 nm, respectively, while still satisfying the dynamic random access memory leakage current density specification (< 1 × 10−7 Acm−2 at capacitor voltage of 0.8 V). These excellent electrical properties of (Al-doped) TiO2 on RuO2 were attributed to the high work function and the reduced interfacial effect on RuO2.

Influences of a Crystalline Seed Layer during Atomic Layer Deposition of SrTiO3 Thin Films Using Ti ( O-iPr ) 2 ( thd ) 2, Sr ( thd ) 2, and H2O

In situ crystallized ∼20 nm thick SrTiO 3 (STO) thin films were deposited by atomic layer deposition at a growth temperature of 370°C using a 3 or 5 nm thick crystalline STO seed layer before depositing the main layer. The influences of the annealing temperature used to crystallize the seed layer on the structure and electrical properties of the entire STO film were investigated. There was an optimum annealing temperature (650-700°C) for achieving the optimum electrical performance. When the annealing temperature was 750°C induced adverse chemical interactions between the seed STO and Ru electrode, which inhibited in situ crystallization of the main layer when the seed-layer thickness was 3 nm. A slightly thicker seed layer (5 nm) was necessary for achieving a well-crystallized STO film when the seed annealing temperature was 750°C.

The mechanism for the suppression of leakage current in high dielectric TiO2 thin films by adopting ultra-thin HfO2 films for memory application

The electrical leakage current of thin rutile structured TiO2 films deposited by atomic layer deposition on a Ru electrode was enormously reduced by depositing an extremely thin HfO2 (< 1 nm) on top. The sacrifice of the capacitance density by the HfO2 was minimized. The leakage mechanism analysis on the Pt/TiO2/Ru and Pt/HfO2/TiO2/Ru structures revealed that the improvement in leakage current was attributed to the reduction of defect (trap) density in the TiO2 film. The interfacial potential barrier height for electron transport in thinner (∼ 10 nm) TiO2 films was lower than that of thicker (∼ 20 nm) TiO2 films, which resulted in a higher leakage current in these films. The capping of ultra-thin (∼ 0.7 nm) HfO2 films effectively increased the potential barrier height, and the leakage current was decreased accordingly. The leakage current behavior was systematically analyzed from quantum mechanical transport simulations.

Electronic Conduction Mechanism of SrTiO3 Thin Film Grown on Ru Electrode by Atomic Layer Deposition

In situ crystallized 20 nm thick SrTiO 3 (STO) thin films were grown on a Ru substrate by atomic layer deposition. The leakage current-voltage characteristics of the film were examined from 313 to 403 K under electron injection conditions from the Ru electrode. The leakage current was dominated by Schottky emission in the low electric field ( 0.3 MV/cm) region. The estimated effective mass of the tunneling electron was ~0.1m 0 .

Controlling the initial growth behavior of SrTiO3 films by interposing Al2O3 layers between the film and the Ru substrate

The effects of thin Al2O3 layers interposed between atomic layer deposited SrTiO3 (STO) films and Ru substrates on the growth and properties of STO films were examined. Although a 3–4 nm thick TiO2 barrier layer was necessary to completely suppress the oxygen diffusion from the underlying Ru(O) layer, which could result in uncontrolled excessive Sr incorporation into the film, a 2–3 nm thick Al2O3 barrier layer was sufficient to achieve the saturated oxygen blocking effect. Therefore, only a 0.4 nm thick Al2O3 layer could effectively suppress uncontrollable initial excessive incorporation of Sr, and a 1 nm thick Al2O3 layer had a blocking effect that was equivalent to that of a 3 nm thick TiO2 layer. STO films were crystallized in situ by the assistance of crystallized 2–3 nm thick STO seed layers that were pre-deposited and annealed. The bulk dielectric constant of STO calculated by the slope of equivalent oxide thickness vs. physical thickness was 173 on the 1 nm thick Al2O3/Ru substrate. However, further research is necessary to reduce the adverse contribution of the interfacial layers to achieve a scaling of the equivalent oxide thickness to ≪0.5 nm.