Composition-Controlled Atomic Layer Deposition of Phase-Change Memories and Ovonic Threshold Switches with High Performance.

Chalcogenide compounds are the main characters in a revolution in electronic memories. These materials are used to produce ultrafast ovonic threshold switches (OTSs) with good selectivity and moderate leakage current and phase-change memories (PCMs) with excellent endurance and short read/write times when compared with state-of-the-art flash-NANDs. The combination of these two electrical elements is used to fabricate nonvolatile memory arrays with a write/access time orders of magnitude shorter than that of state-of-the-art flash-NANDs. These devices have a pivotal role for the advancement of fields such as artificial intelligence, machine learning, and big-data. Chalcogenide films, at the moment, are deposited by using physical vapor deposition (PVD) techniques that allow for fine control over the stoichiometry of solid solutions but fail in providing the conformality required for developing large-memory-capacity integrated 3D structures. Here we present conformal ALD chalcogenide films with control over the composition of germanium, antimony, and tellurium (GST). By developing a technique to grow elemental Te we demonstrate the ability to deposit conformal, smooth, composition-controlled GST films. We present a thorough physical and chemical characterization of the solids and an in-depth electrical test. We demonstrate the ability to produce both OTS and PCM materials. GeTe4 OTSs exhibit fast switching times of ∼13 ns. Ge2Sb2Te5 ALD PCMs exhibit a wide memory window exceeding two orders of magnitude, short write times (∼100 ns), and a reset current density as low as ∼107 A/cm2-performance matching or improving upon state-of-the-art PVD PCM devices.

Investigation of the Changes in Electronic Properties of Nickel Oxide (NiOx) Due to UV/Ozone Treatment.

Drastic reduction in nickel oxide (NiOx) film resistivity and ionization potential is observed when subjected to ultraviolet (UV)/ozone (O3) treatment. X-ray photoemission spectroscopy suggests that UV/O3 treatment changes the film stoichiometry by introducing Ni vacancy defects. Oxygen-rich NiOx having Ni vacancy defects behaves as a p-type semiconductor. Therefore, in this work, a simple and effective technique to introduce doping in NiOx is shown. Angle-resolved XPS reveals that the effect of UV/O3 treatment does not only alter the film surface property but also introduces oxygen-rich stoichiometry throughout the depth of the film. Finally, simple metal/interlayer/semiconductor (MIS) contacts are fabricated on p-type Si using NiOx as the interlayer and different metals. Significant barrier height reduction is observed with respect to the control sample following UV/O3 treatment, which is in agreement with the observed reduction in film resistivity. From an energy band diagram point of view, the introduction of the UV/O3 treatment changes the defect state distribution, resulting in a change in the pinning of the Fermi level. Therefore, this work also shows that the Fermi level pinning property of NiOx can be controlled using UV/O3 treatment.

Engineering of Ferroelectric HfO2-ZrO2 Nanolaminates.

In this work, the ferroelectric properties of nanolaminates made of HfO2 and ZrO2 were studied as a function of the deposition temperature and the individual HfO2/ZrO2 layer thickness before and after electrical field cycling. The ferroelectric response was found to depend on the structure of the nanolaminates before any postdeposition annealing treatment. After annealing with a TiN cap, an "antiferroelectric-like" response was obtained from nanolaminates deposited in an amorphous state at a lower temperature, whereas a ferroelectric response was obtained from nanolaminates deposited at a higher temperature, where crystallites were detected in thick films before annealing. As the individual layer thicknesses were decreased, an increased lattice distortion and a concurrent increase in remanent polarization were observed from the nanolaminates deposited at high temperatures. After field cycling, nanolaminates deposited at lower temperatures exhibited an antiferroelectric-like to ferroelectric transition, whereas those deposited at higher temperatures exhibited a larger remanent polarization. Finally, we demonstrate that by leveraging the proper choice of process conditions and layer thickness, remanent polarizations exceeding those of the HfZrO4 solid solution can be obtained.

Microbead-separated thermionic energy converter with enhanced emission current.

The efficiency of thermionic energy converters is a strong function of the inter-electrode separation due to space-charge limitations. Here we demonstrate vacuum thermionic energy converters constructed using barium dispenser cathodes and thin film tungsten anodes, separated by size specific alumina microbeads for simple device fabrication and inter-electrode gap control. The current and device efficiency at the maximum power point are strongly dependent on the inter-electrode gap, with a maximum device efficiency of 0.61% observed for a gap on the order of 5 µm. Paths to further reductions in space charge and improved anode work function are outlined with potential for over an order of magnitude improvement in output power and efficiency.