Seol Choi

Anode-interface localized filamentary mechanism in resistive switching of TiO2 thin films

The filamentary resistance switching mechanism of a Pt∕40nm TiO2∕Pt capacitor structure in voltage sweep mode was investigated. It was unambiguously found that the conducting filaments propagate from the cathode interface and that the resistance switching is induced by the rupture and recovery of the filaments in the localized region (3–10nm thick) near the anode. The electrical conduction behavior in the high resistance state was well explained by the space charge limited current (SCLC) mechanism that occurs in the filament-free region. The various parameters extracted from the SCLC fitting supported the localized rupture and formation of filaments near the anode.

Identification of a determining parameter for resistive switching of TiO2 thin films

Electric-pulse-induced resistive switching of 43nm thick TiO2 thin films grown by metalorganic chemical vapor deposition was studied by current-voltage (I-V) and constant voltage-time measurements. The resistance ratio between the two stable states of the film constitutes approximately 1000. The allowed current level and voltage step width during the sweep mode I-V measurements influenced switching parameters, such as the switching voltage, time before switching, and resistance values. However, it was clearly observed that the power imparted to the film controlled mainly switching. The required power for successful switching was almost invariant irrespective of other measurement variables.

Study on the resistive switching time of TiO2 thin films

The required time for voltage-pulse-induced resistive switching of 40-nm-thick TiO2 thin films integrated in a contact-type structure (Pt top and TiN bottom contact, contact area ∼0.07μm2) was studied as a function of pulse voltage. For off→on switching at least 2V was necessary and the minimum switching times were ∼20ns at 2V and ∼10ns at 3V. For on→off switching, a minimum switching time of 5μs was obtained at 2.5V. The resistance of the on-state device was also dependent on the switching voltage and time.

Resistive Switching in Pt ∕ Al2O3 ∕ TiO2 ∕ Ru Stacked Structures

The electric-pulse-induced resistive switching properties of TiO 2 , Al 2 O 3 , Al 2 O 3 /TiO 2 , and Al 2 O 3 /TiO 2 /Al 2 O 3 thin films were studied by current-voltage (I-V) measurements using Pt/insulator/Ru structures and conductive atomic force microscopy. The switching parameters of the TiO 2 film were stable, whereas those of the Al 2 O 3 films show random variations during repeated I-V measurements. Both films show resistive switching by a filamentary switching mechanism with linear conduction behavior in the low V region. The stacked film shows a bias polarity-dependent switching behavior. This suggests that the nucleation of the conducting filaments occurs at the interface where the electrons are injected.

(In,Sn)2O3∕TiO2∕Pt Schottky-type diode switch for the TiO2 resistive switching memory array

A Schottky-type diode switch consisting of a Pt∕(In,Sn)2O3∕TiO2∕Pt stack was fabricated for applications to cross-bar type resistive-switching memory arrays. The high (0.55eV) and low potential barrier at the TiO2∕Pt and TiO2∕(In,Sn)2O3 junctions, respectively, constitute the rectifying properties of the stacked structure. The forward/reverse current ratio was as high as ∼1.6×104 at an applied voltage of ∼1V. When Pt∕TiO2∕Pt memory was connected to this diode in series, there was an insignificant interference on the memory function from the diode under the forward bias and virtually no resistive switching under a reverse bias.

Cyclic PECVD of Ge2Sb2Te5 Films Using Metallorganic Sources

Ge 2 Sb 2 Te 5 (GST) thin films were deposited on SiO 2 /Si and TiN/Si substrates by cyclic metallorganic chemical vapor deposition using Ge(i-C 4 H 9 ) 4 , Sb(i-C 3 H 7 ) 3 , Te(i-C 3 H 7 ) 2 as Ge, Sb, and Te precursors, respectively, with the help of Ar + H 2 plasma at temperatures ranging from 180 to 290°C. The application of plasma power was essential in obtaining a high growth rate and stoichiometric GST thin films. The chemical composition of the films was properly controlled by the cycling ratio and sequence of each precursor pulse. The stoichiometric films grown at 200°C showed a smooth surface morphology, highest density, and lowest impurity concentration. GST film was selectively grown inside the contact hole having a TiN/W plug.

Phase change memory cell using Ge2Sb2Te5 and softly broken-down TiO2 films for multilevel operation

A phase change memory cell was fabricated by stacking plasma-enhanced cyclic chemical-vapor-deposited Ge2Sb2Te5 (GST) and atomic layer deposited TiO2 thin films. Different pairs of resistance states were obtained by controlling the current flow, which can be used to achieve higher memory density by multilevel operation. The multiresistance states of the stacked cell were explained by the resistance switching phenomena of TiO2 and the thermoelectric phase change properties of GST. The phase change characteristics of GST could be altered by controlling the degree of filament formation in the TiO2 layer, which eventually changed the phase change volume in the GST.

Ge2Sb2Te5 Charge Trapping Nanoislands with High-k Blocking Oxides for Charge Trap Memory

Charge trap memory capacitors using Ge 2 Sb 2 Te 5 (GST) nanoislands as charge storage media were fabricated. The GST nanoislands were prepared by plasma-enhanced cyclic chemical vapor deposition on a 6 nm thick tunneling SiO 2 layer. A 16 nm thick A1 2 O 3 or 40 nm thick HfO 2 film was used as the blocking oxide (BO). A shift in the flatband voltage in the capacitance-voltage test was achieved when the GST nanoislands were interposed between the SiO 2 and BOs, highlighting the feasibility of memory applications. The charges were trapped at the interface between the GST and BOs. Stable charge retention up to 10 4 s was observed.

Dynamic threshold switching behavior of Ge2Sb2Te5 and Sb-doped Ge2Sb2Te5 thin films using scanning electrical nanoprobe

The dynamic oscillation in the current-time characteristics of Ge 2 Sb 2 Te 5 and Sb-doped Ge 2 Sb 2 Te 5 thin-film materials was investigated using a scanning electrical nanoprobe. The dynamic oscillation was attributed to the threshold switching of the amorphousphase Ge 2 Sb 2 Te 5 and doped Ge 2 Sb 2 Te 5 thin films. The microscopic mechanism of the threshold switching behavior was also studied. X-ray diffraction showed that the excess Sb improved the crystallization of Ge 2 Sb 2 Te 5 during postannealing at 300°C. Sb-doping also increased the threshold switching speed.