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. GeTe 4 OTSs exhibit fast switching times of ∼13 ns. Ge 2 Sb 2 Te 5 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 ∼10 7 A/cm 2 performance matching or improving upon state-of-the-art PVD PCM devices.
Chalcogenide compounds are leading a revolution in the electronic memories space. Phase-change-memory (PCM) elements and ovonic threshold switches (OTSs) combined in the cross-point (X-point) architecture produce memory arrays with access and write times orders of magnitude faster than state-of-the-art flash nands and also provide nonvolatile storage, a larger scale of integration compared to traditional memory arrays, and the opportunity to develop beyond von Neumann architectures to support computationally demanding applications such as artificial intelligence. The commercial success of chalcogenide X-point arrays will depend on the ability to integrate chalcogenide films into sophisticated three-dimensional architectures such as vertical structures for economical manufacturing. To do so, highly conformal deposition techniques are required such as atomic layer deposition (ALD). State-of-the-art chalcogenide cross-point devices are currently fabricated using PVD, which fails to provide any film conformality. ALD PCMs with performance comparable to their PVD counterparts have been demonstrated; however, fabricating OTS selectors using ALD remains a challenge. Here, we present an approach to deposit ALD ternary germanium-selenium-tellurium (Ge-Se-Te) spanning a wide range of compositions. The ALD Ge-Se-Te films show excellent conformality, low surface roughness, and good compositional homogeneity. We fabricated OTS devices and demonstrated the ability to produce low leakage selectors with threshold voltage tuning achieved by control over the film composition.
Conformal atomic layer deposition (ALD) of chalcogenide films would enable aggressive 3D integration of nonvolatile memories; unfortunately, the fabrication of high-performance ALD OTSs remains an unsolved problem. Here we present an ALD process to incorporate As in a quaternary GeAsSeTe (GAST) film showing excellent conformality, controllable thickness, and composition homogeneity. The films were used to produce a GeSe/GAST heterojunction-OTS selector. We demonstrate low leakage current (∼10–9 A over a 0.01 μm2 area measured at VTH/2), a VTH ∼ 3 V, a switching time of ∼2 nsand an outstanding endurance exceeding 109 cycleson par with the current OTS state-of-the-art.
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