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Three-dimensional (3D) cross-point (X-point) technology, including amorphous chalcogenide-based ovonic threshold switching (OTS) selectors, is bringing new changes to the memory hierarchy for high-performance computing systems. To prepare for future 3D X-point memory scaling, we studied the plasma-enhanced atomic layer deposition (PE-ALD) of Ge1−xSx amorphous chalcogenide alloy thin films, the selection of which was motivated by their high optical bandgap and wide amorphous forming regions. The PE-ALD Ge1−xSx thin films were synthesized using a GeCl4 precursor and H2S plasma reactant, and their self-limited growth characteristics were studied in detail as a function of the exposure time of the ALD steps, temperature, and plasma power. The PE-ALD GeS2 thin film showed an RMS roughness of 0.29 nm and good conformality in the vertical 3D structure. Moreover, the OTS behavior of GeS2 and Ge2S3 mushroom-type devices with a 50 nm bottom electrode contact (BEC) were investigated as well as the trade-off relationship between the threshold voltage (1.9–6.2 V) and the normalized off current (20–250 nA) based on scaling the film thickness down from 30 nm to 5 nm. In particular, the GeS2 device showed a higher threshold field (∼3.1 MV cm−1) and lower normalized off current characteristics than the Ge2S3 device due to the higher trap density (2.1 × 1021 cm−3), according to the modified Poole–Frenkel (PF) model. The results achieved by this PE-ALD research on this novel binary GeS2 amorphous chalcogenide for OTS applications will contribute to the development of future 3D cross-point memory scaling.

We developed an electro-thermal model for cross-point phase-change memory (X-PCM) and compared the calculation values with the experimental results. In order to simulate the electro-thermal phenomenon such as thermal disturbance (TDB) of victim cells and reset the current ( ${I}_{{\text {RESET}}}$ ) of aggressor cells in a fully confined cross-point structure, a three by three mini-array was constructed with the finite-elemental method. Unlike the conventional thermal model, which only shows the temperature gradient for TDB, our new model can predict the crystallization behavior of victim cells by combining the crystallization model for nucleation and growth. This makes it possible to compare the calculation results with the experimental ones through the crystalline fraction of the victim cell. The simulation results clearly reveal that our new model can closely estimate the victim cell’s crystalline fraction for TDB, with the variation of both pulse height (PH) and pulse width (PW) during the RESET operation. Applying the confirmed model for devices of smaller dimensions shows that TDB increases as the device scales down. The increase of TDB is particularly severe as cells become smaller than 16 nm. We suggest that TDB can be reduced not only by decreasing the thermal conductivity ( $\kappa $ ) of the interlayer dielectric (ILD), but also by minimizing the PW of the aggressor.

Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The pre...

Ge–Sb–Se–Te (GSST) quaternary films were prepared through atomic layer deposition (ALD) to ensure their amorphous stability for ovonic threshold switching (OTS) applications. Se, a typical phase-change material, was incorporated into Ge–Sb–Te (GST) films using Sb (OC2H5)3 and [(CH3)3Si]2Se precursors. The process produces highly conformal, uniform films consisting of stoichiometric binaries of GeTe2, Sb2Te3, and Sb2Se3. A detailed analysis of the atomic compositions revealed Te substitution for Se on the film surface through the ligand exchange reaction between the Te and Se precursors. Comparative experimental and simulation studies on the OTS behaviors showed that Ge and Se increase the optical bandgaps while Se decreases the density of the localized states. The amorphous stability of the Se-containing films enhances the cycling endurance of the OTS device up to 105 cycles. As conformality and atomic-level accuracy in terms of thickness and composition are prerequisites for three-dimensional crossbar-type passive arrays, the ALD process and the OTS performance presented in this work have a high potential utility for constructing a selector element in non-volatile high-density memories.

In this paper, a new atomic layer deposition (ALD) process for depositing binary GeTe and ternary Ge–Sb–Te thin films is reported, where HGeCl3 and ((CH3)3Si)2Te were used as Ge and Te precursors, respectively. The precursors reacted together to form the films at a low substrate temperature of 50–100 °C, without involving any additional reactive process gas. HCl elimination from the Ge precursor to form the divalent Ge intermediate, GeCl2, is proposed to explain the formation of 1:1 composition stoichiometric GeTe films. The GeTe films are promising for use in phase change memory applications. Ternary Ge–Sb–Te films were deposited by combining the GeTe ALD process with a previously developed ALD process for Sb2Te3 films, where Sb(OC2H5)3 and ((CH3)3Si)2Te were employed respectively as the Sb and Te precursors. However, the composition of the ternary GeSbTe films deviated slightly from the desired GeTe–Sb2Te3 pseudobinary composition suggesting that a certain unwanted reaction was involved between the prev...

In this report, a new method to deposit GeTe thin film by atomic layer deposition (ALD) is described. Ge(N(Si(CH3)3)2)2, in which Ge is in +2 oxidation state, and ((CH3)3Si)2Te were used as Ge and Te precursors, respectively. GeTe films were deposited at a low wafer temperature of 70–120 °C. To improve low reactivity of Ge(N(Si(CH3)3)2)2 toward ALD-type reaction with ((CH3)3Si)2Te, methanol vapor was coinjected with the Ge precursor and the Te precursor to form a reactive Ge intermediate having methoxy ligands and a Te intermediate having hydrogen ligands in the gas phase. This chemistry-specific ALD process deposited films various compositional GeTe alloys including the desired composition of Ge:Te = 1:1. Detailed mechanism study on the probable chemical reactions and film composition analysis revealed that the film growth could proceed via the formations of GeTe2, GeTe, and Ge2Te phases depending on the relative Ge-precursor concentration during the ALD process. The films showed feasibility for phase-ch...

The multilayered optical coating whose structure consists of optical cavity and multiple layers of ultrathin phase change material (PCM) film is presented. The color changing is enabled via transition between amorphous and crystalline phases of PCM, which is accompanied by high optical contrast. The phase transition of each PCM layer, separated by ultrathin oxide barrier, can be realized through various crystallization temperatures with respect to the different thickness of ultrathin (

The local bonding structures of GexTe1-x (x = 0.5, 0.6, and 0.7) films prepared through atomic layer deposition (ALD) with Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Te precursors were investigated using Ge K-edge X-ray absorption spectroscopy (XAS). The results of the X-ray absorption fine structure analyses show that for all of the compositions, the as-grown films were amorphous with a tetrahedral Ge coordination of a mixture of Ge-Te and Ge-Ge bonds but without any signature of Ge-GeTe decomposition. The compositional evolution in the valence band electronic structures probed through X-ray photoelectron spectroscopy suggests a substantial chemical influence of additional Ge on the nonstoichiometric GeTe. This implies that the ALD process can stabilize Ge-abundant bonding networks like -Te-Ge-Ge-Te- in amorphous GeTe. Meanwhile, the XAS results on the Ge-rich films that had undergone post-deposition annealing at 350 °C show that the parts of the crystalline Ge-rich GeTe became separated into Ge crystallites and rhombohedral GeTe in accordance with the bulk phase diagram, whereas the disordered GeTe domains still remained, consistent with the observations of transmission electron microscopy and Raman spectroscopy. Therefore, amorphousness in GeTe may be essential for the nonsegregated Ge-rich phases and the low growth temperature of the ALD enables the achievement of the structurally metastable phases.

SrTiO3 (STO) thin films, grown by atomic layer deposition (ALD), were studied for capacitors in dynamic random access memory. The STO ALD process consisted of two steps: the growth of seed layer followed by a rapid thermal annealing (RTA) process at 650 °C to crystallize it, and the deposition of the main layer on top of the seed layer at 370 °C to induce the in-situ crystallization. During single cooling process after the RTA of the seed layer, voids and nano-cracks were formed due to the thermal expansion mismatch between STO film and Si substrate. This problem was well mitigated by adopting the stepwise cooling process, wherein the holding time of 30 s at 500, 350, and 200 °C suppressed the defect formation in the seed layer. Therefore, the main layer grown on that seed layer showed an improved microstructure with a high bulk dielectric constant of 135. However, the increase in total annealing time degraded the interface quality between the STO and the bottom electrode, which finally worsened the insulating property. As a result, the minimum equivalent oxide thicknesses with low leakage current densities (< 10− 7 A/cm2 at 0.8 V) for the single and stepwise cooling processes were 0.39 nm and 0.46 nm, respectively.

The chemical interaction between the [(CH3)3Si]3Sb precursor and atomic layer deposited Sb2Te3 thin films was examined at temperatures ranging from 70 to 220 °C. The trimethylsilyl group [(CH3)3Si] displays greater affinity for Te than for Sb, and this drives replacement of Te in the film with Sb from the [(CH3)3Si]3Sb precursor, while eliminating volatile [(CH3)3Si]2Te, especially at elevated temperatures. The compositions of the resulting Sb–Te layers lie on the Sb2Te3–Sb tie line. The incorporation behavior of [(CH3)3Si]3Sb was explained in terms of a Lewis acid–base reaction. The exchange reactions occurred to relieve the unfavorable hard–soft Lewis acid–base pair between the trimethylsilyl group and Sb in [(CH3)3Si]3Sb. Such a reaction could be usefully adopted to control the chemical composition of ternary Ge–Sb–Te thin films.

The mechanism of bipolar resistive switching (BRS) of amorphous Ge2Sb2Te5 (GST) thin films sandwiched between inert electrodes (Ti and Pt) was examined. Typical bipolar resistive switching behavior with a high resistance ratio (∼103) and reliable switching characteristics was achieved. High-resolution transmission electron microscopy revealed the presence of a conductive Te-filament bridging between the top and bottom electrodes through an amorphous GST matrix. The conduction mechanism analysis showed that the low-resistance state was semiconducting and dominated by band transport, whereas Poole–Frenkel conduction governed the carrier transport in the high-resistance state. Thus, the BRS behavior can be attributed to the formation and rupture of the semiconducting conductive Te bridge through the migration of the Te ions in the amorphous GST matrix under a high electric field. The Te ions are provided by the thin (∼5 nm) Te-rich layer formed at the bottom electrode interface.

To replace or succeed the present NAND flash memory, resistive switching random access memory (ReRAM) should be implemented in the vertical-type crossbar array configuration. The ReRAM cell must have a highly reproducible resistive switching (RS) performance and an electroforming-free, self-rectifying, low-power-consumption, multilevel-switching, and easy fabrication process with a deep sub-μm(2) cell area. In this work, a Pt/Ta2O5/HfO2-x/TiN RS memory cell fabricated in the form of a vertical-type structure was presented as a feasible contender to meet the above requirements. While the fundamental RS characteristics of this material based on the electron trapping/detrapping mechanisms have been reported elsewhere, the influence of the cell scaling size to 0.34 μm(2) on the RS performance by adopting the vertical integration scheme was carefully examined in this work. The smaller cell area provided much better switching uniformity while all the other benefits of this specific material system were preserved. Using the overstressing technique, the nature of RS through the localized conducting path was further examined, which elucidated the fundamental difference between the present material system and the general ionic-motion-related bipolar RS mechanism.

The ovonic threshold switch (OTS) based on the voltage snapback of amorphous chalcogenides possesses several desirable characteristics: bidirectional switching, a controllable threshold voltage (V th) and processability for three-dimensional stackable devices. Among the materials that can be used as OTS, GeSe has a strong glass-forming ability (∼350 °C crystallization temperature), with a simple binary composition. Described herein is a new method of depositing GeSe films through atomic layer deposition (ALD), using HGeCl3 and [(CH3)3Si]2Se as Ge and Se precursors, respectively. The stoichiometric GeSe thin films were formed through a ligand exchange reaction between the two precursor molecules, without the adoption of an additional reaction gas, at low substrate temperatures ranging from 70 °C-150 °C. The pseudo-saturation behavior required a long time of Ge precursor injection to achieve the saturation growth rate. This was due to the adverse influence of the physisorbed precursor and byproduct molecules on the efficient chemical adsorption reaction between the precursors and reaction sites. To overcome the slow saturation and excessive use of the Ge precursor, the discrete feeding method (DFM), where HGeCl3 is supplied multiple times consecutively with subdivided pulse times, was adopted. DFM led to the saturation of the GeSe growth rate at a much shorter total injection time of the Ge precursor, and improved the film density and oxidation resistance properties. The GeSe film grown via DFM exhibited a short OTS time of ∼40 ns, a ∼107 ON/OFF current ratio, and ∼104 selectivity. The OTS behavior was consistent with the modified Poole-Frenkel mechanism in the OFF state. In contrast, the similar GeSe film grown through the conventional ALD showed a low density and high vulnerability to oxidation, which prevented the OTS performance. The ALD method of GeSe films introduced here will contribute to the fabrication of a three-dimensionally integrated memory as a selector device for preventing sneak current.

A double-layer-stacked 1 diode-1 resistor (1D1R) cross-bar array (CBA) resistance switching random access memory is fabricated. The TiO2-based Schottky diode and the unipolar resistance switching TiO2 comprise the cell selector and nonvolatile memory components, respectively. All the fabrication processes are performed near room temperature through physical vapor deposition processes, and the performance degradation by the thermal budget is well suppressed. As a result, a rectification ratio as high as 1.4 × 109 is achieved from the appropriately cycled diode, which can last up to 108 cycles in the integrated structure. Such highly promising performance is confirmed from both the upper and lower memory layers, which confirm the possible route for the multistacked memory structure. The two-bit-per-cell operation allows an effective minimum cell area of F2, which is confirmed from the randomly accessed cells to be completely free from adverse interference through the adoption of an extremely-high-performance diode. The present 1D1R integration technique will eventually allow the fabrication of an extremely high-integration-density (above 1 Gb per block) CBA with the help of the two-port sensing scheme. Such a high-performance CBA device is a feasible contender for memory-dominant computation.

A facile route to mitigate the Fermi-level pinning between a p-type GaSb semiconductor and an Al2O3 dielectric is proposed. Combination of the sulphurization of the GaSb surface and the subsequent rapid thermal annealing allowed for high quality GaSb/Al2O3 interfacial characteristics; the interfacial trap density value was ≤2.0 × 1012 cm−2 eV−1 in an energy range of 0.05 ≤ ET − Ev ≤ 0.45 eV for the PMOS capacitor via rapid thermal annealing at 575 °C. A physical rationale was given on the basis of the thermo-chemical conversion of Ga2O into Ga2O3 and the conformal elimination of Sb related elements and oxides on the GaSb surface.