Extremely Low Leakage Threshold Switch with Enhanced Characteristics via Ag Doping on Polycrystalline ZnO Fabricated by Facile Electrochemical Deposition for an X-Point Selector

Leakage current, that causes interferences in the read/write operation, arising from neighboring unselected or half-selected memory cells is considered as one of the main hurdles to be overcome to increase density of cross-point memory arrays. In this work, the common drawbacks for a Ag-based steep-slope threshold switching selector, threshold voltage variability, and poor cycling endurance have been mended. This is achieved by lightly doping the switching layer with Ag instead of implementing the Ag active electrode that acts as a reservoir, which provides unlimited access of Ag to the selector medium. Here, we doped polycrystalline ZnO with Ag, fabricated by facile electrochemical deposition, making a prototypical candidate for the crystalline switching layer. When the amount of Ag is limited by doping, switching characteristics, that is, threshold voltage variability and cycling endurance, are improved. Lastly, different mechanisms causing a threshold switching device to fail are also discussed for the two different test vehicles. It has been found that an unlimited Ag source causes the devices to fail in a short-circuited manner, and a limited Ag source results in devices to fail in an open-circuited manner, after repeated measurements.11Nsciescopu

Highly Reliable Selection Behavior with Controlled Ag Doping of Nano-polycrystalline ZnO Layer for 3D X-Point Framework

IEEEIn this letter, a threshold switching (TS) selector with Ag doping-based nano-polycrystalline ZnO switching layer (SL) having (002) preferred orientation has been manifested, without incorporating an active Ag metal layer, using a facile co-sputtering deposition technique. The TS selectors with extremely controlled doping of ~0.14 at. % Ag concentration showed remarkable electroforming (EF)-free selection behavior such as gigantic selectivity (~1011), extreme-low off-current (~10 fA), high on-current density (~1.6 MA/cm2), ultra-steep switching slope (~0.8 mV/decade), satisfactory endurance (>106), fast switch-on speed (~38 ns) and relaxation speed (~64 ns), and high device yield (~90%). Furthermore, selector devices showed reproducible selection behavior with stable threshold voltage (Vth) having merely 8% variances.11Nsciescopu

Nano-polycrystalline Ag-doped ZnO layer for steep-slope threshold switching selectors

In this work, a nano-polycrystalline Ag-doped ZnO-based threshold switching (TS) selector via a facile co-sputtering technique is investigated without using an Ag active metal layer. The effects of the Ag concentration with respect to OFF-state leakage current (I-off) were studied, and the results demonstrate that by regulating the Ag doping concentration in the switching layer (SL), an electroforming-free switching with an I-on/I-off ratio of & SIM;10(8) could be achieved, having an extremely low I-off value of & SIM;10(-13) A. Furthermore, cycling endurance can also be improved as the formation of a laterally thick and stable filament does not happen promptly with consequent measurements when there is a limited amount of Ag in the SL. The selector device performance enhancement is attributed to the doping-based polycrystalline structure that facilitates enhanced control on filament formation due to the restricted availability and anisotropic diffusion of Ag ions in the polycrystalline ZnO SL, thereby trimming down the overall stochasticity during metallic filament growth. The present study demonstrates that a doping-based polycrystalline SL structure can be implemented in a selector device to augment TS characteristics, i.e., device variances and cycling endurance for adoption in ultra-high density memory applications.11Nsciescopu

A Comprehensive Study on the Effect of TiN Top and Bottom Electrodes on Atomic Layer Deposited Ferroelectric Hf0.5Zr0.5O2 Thin Films

The discovery of ferroelectricity in HfO2-based materials in 2011 provided new research directions and opportunities. In particular, for atomic layer deposited Hf0.5Zr0.5O2 (HZO) films, it is possible to obtain homogenous thin films with satisfactory ferroelectric properties at a low thermal budget process. Based on experiment demonstrations over the past 10 years, it is well known that HZO films show excellent ferroelectricity when sandwiched between TiN top and bottom electrodes. This work reports a comprehensive study on the effect of TiN top and bottom electrodes on the ferroelectric properties of HZO thin films (10 nm). Investigations showed that during HZO crystallization, the TiN bottom electrode promoted ferroelectric phase formation (by oxygen scavenging) and the TiN top electrode inhibited non-ferroelectric phase formation (by stress-induced crystallization). In addition, it was confirmed that the TiN top and bottom electrodes acted as a barrier layer to hydrogen diffusion into the HZO thin film during annealing in a hydrogen-containing atmosphere. These features make the TiN electrodes a useful strategy for improving and preserving the ferroelectric properties of HZO thin films for next-generation memory applications

Low Temperature Thermal Atomic Layer Deposition of Aluminum Nitride Using Hydrazine as the Nitrogen Source

Aluminum nitride (AlN) thin films were grown using thermal atomic layer deposition in the temperature range of 175–350 °C. The thin films were deposited using trimethyl aluminum (TMA) and hydrazine (N2H4) as a metal precursor and nitrogen source, respectively. Highly reactive N2H4, compared to its conventionally used counterpart, ammonia (NH3), provides a higher growth per cycle (GPC), which is approximately 2.3 times higher at a deposition temperature of 300 °C and, also exhibits a low impurity concentration in as-deposited films. Low temperature AlN films deposited at 225 °C with a capping layer had an Al to N composition ratio of 1:1.1, a close to ideal composition ratio, with a low oxygen content (7.5%) while exhibiting a GPC of 0.16 nm/cycle. We suggest that N2H4 as a replacement for NH3 is a good alternative due to its stringent thermal budget

Hollow Cathode Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride Using Pentachlorodisilane

In this work, a novel chlorodisilane precursor, pentachlorodisilane (PCDS, HSi₂Cl₅), was investigated for the growth of silicon nitride (SiN_<i>x</i>) via hollow cathode plasma-enhanced atomic layer deposition (PEALD). A well-defined self-limiting growth behavior was successfully demonstrated over the growth temperature range of 270–360 °C. At identical process conditions, PCDS not only demonstrated approximately >20% higher growth per cycle than that of a commercially available chlorodisilane precursor, hexachlorodisilane (Si₂Cl₆), but also delivered a better or at least comparable film quality determined by characterizing the refractive index, wet etch rate, and density of the films. The composition of the SiN_<i>x</i> films grown at 360 °C using PCDS, as determined by X-ray photoelectron spectroscopy, showed low O content (∼2 at. %) and Cl content (<1 at. %; below the detection limit). Fourier transform infrared spectroscopy spectra suggested that N–H bonds were the dominant hydrogen-containing bonds in the SiN_<i>x</i> films without a significant amount of Si–H bonds originating from the precursor molecules. The possible surface reaction pathways of the PEALD SiN_<i>x</i> using PCDS on the surface terminated with amine groups (−NH₂ and −NH−) are proposed. The PEALD SiN_<i>x</i> films grown using PCDS also exhibited a leakage current density as low as 1–2 nA/cm² at 2 MV/cm and a breakdown electric field as high as ∼12 MV/cm