Sang-Ouk Ryu

Sb-Se-based phase-change memory device with lower power and higher speed operations

A phase-change material of Sb/sub 65/Se/sub 35/ was newly proposed for the nonvolatile memory applications. The fabricated phase-change memory device using Sb/sub 65/Se/sub 35/ showed a good electrical threshold switching characteristic in the dc current-voltage (I-V) measurement. The programming time for set operation of the memory device decreased from 1 /spl mu/s to 250 ns when Sb/sub 65/Se/sub 35/ was introduced in place of the conventionally employed Ge/sub 2/Sb/sub 2/Te/sub 5/ (GST). The reset current of Sb/sub 65/Se/sub 35/ device also dramatically reduced from 15 mA to 1.6 mA, compared with that of GST device. These results are attributed to the low melting temperature and high crystallization speed of Sb/sub 65/Se/sub 35/ and will contribute to lower power and higher speed operations of a phase-change nonvolatile memory.

Crystallization Behavior and Physical Properties of Sb-Excess Ge2Sb2 + x Te5 Thin Films for Phase Change Memory (PCM) Devices

The crystallization behavior of antimony(Sb)-excess Ge 2 Sb 2+x Te 5 was examined. Sb-excess GST showed crystallization (T c ) and melting (T M ) temperatures of 205 and 550°C, respectively, slightly higher T C and lower T M values than stoichiometric Ge 2 Sb 2 Te 5 compounds. It also showed a substantially different crystallization behavior compared to the stoichiometric Ge 2 Sb 2 Te 5 composition. The resulting Sb-excess GeSbTe thin film showed a grain growth dominated crystallization behavior.

SrTa2O6 Thin Films Deposited by Plasma-Enhanced Atomic Layer Deposition

SrTa2O6 (STO) thin films were prepared by plasma-enhanced atomic layer deposition (PEALD) with an alternating supply of reactant sources, Sr[Ta(C2H5O)5(C4H10NO)]2 {strontium bis-[tantalum penta-ethoxide dimethyllaminoethoxide]; Sr(Ta(OEt)5dmae)2} and O2 plasma. It was observed that the uniform and conformal STO thin films were successfully deposited using PEALD and the film thickness per cycle was saturated at approximately 0.8 A at 300°C. Electrical properties of STO thin films prepared on Pt/SiO2/Si substrates at various annealing temperatures were investigated. While the grain size and dielectric constant of STO films increased with increasing annealing temperature, the leakage current characteristics of STO films slightly deteriorated. The leakage current density of a 40 nm STO film annealed at 600°C was approximately 5×10-8 A/cm2 at 3 V.

Polycrystalline silicon-germanium heating layer for phase-change memory applications

This letter reports on the performance improvement of phase-change memory (PCM) cells by applying silicon-germanium (SiGe) alloys as resistive heating layers. The in situ doped polycrystalline Si0.75Ge0.25 films, lying under holes filled with a Ge2Sb2Te5 (GST) phase-change material in a pore-style configuration, promoted the temperature rise and phase transition in the GST. The SiGe heating layer caused drastic reduction in both set and reset currents compared to a conventional TiN heater material. The threshold voltages of the PCM cells were almost uniform irrespective of the kind of heating layers. It is considered that this beneficial effect of the SiGe heating layer originates from the high electrical resistivity and low thermal conductivity of a SiGe alloy.

Characterization of silver-saturated Ge–Te chalcogenide thin films for nonvolatile random access memory

Programmable metallization cell structure with a device diameter of 2μm composed of Ag–Ge–Te chalcogenide films was prepared by a cosputtering technique at room temperature. The device with a 150nm thick Ag30(Ge0.3Te0.7)70 electrolyte switches at 0.2V from an “off” state resistance Roff close to 106Ω to an “on” resistance state Ron more than four orders of magnitude lower for this programming current. The switching characteristics were closely related with a diffusion of silver anode into silver-saturated GeTe electrolyte films.

Crystallographic orientations and electrical properties of Bi3.47La0.85Ti3O12 thin films on Pt/Ti/SiO2/Si and Pt/SiO2/Si substrates

We report on the crystallization and electrical properties of Bi3.47La0.85Ti3O12 (BLT) thin films for possible ferroelectric non-volatile memory applications. The film properties were found to be strongly dependent on process conditions especially on the intermediate heat treatment conditions. The crystallographic orientation of the films showed sharp changes at the intermediate rapid thermal annealing (RTA) temperature of 450°C. Below 450°C, BLT thin films have orientation while they have preffered c-axis orientation above 450°C. We found that RTA conditions of the first coating layer play a major role in determining the entire crystallographic orientation of the films. The films also showed of ferroelectric hysterisis behavior strongly dependent on RTA treatment. In fact, the remanent polarization of Bi3.465La0.85Ti3O12 thin films having preferred crystallographic orientation showed much smaller value compared to those having orientation. The BLT films also exhibited good fatigue characteristics under bipolar stress up to 1011 cycles regardless of their intermediate thermal treatments.

New High-k SrTa2O6 Gate Dielectrics Prepared by Plasma-Enhanced Atomic Layer Chemical Vapor Deposition

We report on the electrical characterization of SrTa2O6 (STA) films as alternative gate dielectrics deposited directly on Si by plasma-enhanced atomic layer chemical vapor deposition. The dielectric constant extracted from the accumulation capacitance and the fixed charge of the interface state for STA films annealed in O2 ambient are about 20 and 6.54×10-8 C/cm2, respectively. The interface trap density Dit at the midgap obtained from a quasistatic capacitance-voltage technique is estimated approximately to be in the low 1011 cm-2eV-1 range and the leakage current is approximately 2.98×10-9 A/cm2 at 1 V bias in accumulation. The films annealed in N2 ambient exhibit a relatively larger fixed charge and interface trap density than those annealed in O2 ambient. These electrical properties support the possibility of STA oxide application to a new high-k gate dielectric.

Etching Characteristics of Ge2Sb2Te5 Using High-Density Helicon Plasma for the Nonvolatile Phase-Change Memory Applications

We investigated the etching behaviors of GST, which has been mainly employed for the realization of phase-change type nonvolatile memory devices, using high-density helicon plasma etching system under the various etching gas conditions. Our results provide the etch rates of GST thin films as a function of gas mixing ratio when the gas mixtures of Ar/Cl2 and Ar/CHF3 were applied. It was found that the etch selectivities of GST to SiO2 and to TiN were optimized at Ar/Cl2 = 90/10 and Ar/CHF3 = 80/20, respectively. It was also confirmed that there is no significant change in composition of GST after the etching process. Using the obtained results, we can design the etching process in a systematic way for the fabrication of GST-loaded phase-change type memory devices.

Characterization of in situ diffusion of silver in Ge–Te amorphous films for programmable metallization cell memory applications

Silver-rich GeTe solid electrolytes for use in programmable metallization cell (PMC) memory devices were prepared by the in situ diffusion of silver into the GeTe films during the deposition of silver by rf sputtering on GeTe chalcogenide glass films. The concentration of silver in the silver-rich GeTe films was controlled by adjusting the concentration of Te in the GeTe films. A PMC memory device with a Ag(300nm)∕Agx(Ge45Te55)1−x(200nm)∕TiW(100nm) structure and a device diameter of 0.5μm showed reproducible memory characteristics based on resistive switching at low voltage with high Roff∕Ron ratios. The PMC memory device exhibited good switching characteristics up to 100cycles at an amplitude of ±2V and a pulse width of 1μs.

Low Voltage Switching Characteristics of 60 nm Thick SrBi2Ta2 O 9 Thin Films Deposited by Plasma-Enhanced ALD

60 nm thick SrBi 2 Ta 2 O 9 (SBT) thin films were fabricated using plasma-enhanced atomic layer deposition (ALD) with an alternating supply of single cocktail source and oxygen plasma. The linear relationship between the number of cycles and film thickness and a constant deposition rate with source pulse time suggests that a self-limiting process that has distinct characteristics of ALD was achieved. The 60 nm thick SrBi 2 Ta 2 O 9 films that were annealed at 750°C completely crystallized to the layer-structured perovskite phase. The SBT capacitors showed excellent ferroelectric switching properties. Low voltage switching below 1.5 V was achieved successfully in the 60 nm SBT capacitor.