Atomic layer deposition of GeSe films using HGeCl₃and [(CH₃)₃Si]₂Se with the discrete feeding method for the ovonic threshold switch

Abstract

The ovonic threshold switch (OTS) based on the voltage snapback of amorphous chalcogenides possesses several desirable characteristics: bidirectional switching, a controllable threshold voltage (V ) 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 HGeCl and [(CH)Si]Se 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 HGeCl 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 ∼10 ON/OFF current ratio, and ∼10 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.

Refractive Uses of Layered and Two-Dimensional Materials for Integrated Photonics

“…ALD is a variation on MOCVD in which the film growth is kinetically limited by a precursor delivery sequence. ALD growth of LMs MoS 2 , WS 2 , MoSe 2 , WSe 2 , ZrS 2 , HfS 2 , TiS 2 , SnS, SnSe, GeS, and GeSe have been reported. − ALD offers excellent control over film morphology and layer number. However, the kinetic control inherent to ALD dictates rather low reaction temperature, and high-temperature postdeposition annealing is often required to improve film material quality.…”

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

“…This work presents the foundations for the future development of ultrahigh capacity 3D chalcogenide memory arrays. More elements could be easily added to the ALD GST system (e.g., selenium and arsenic) following similar approaches to further improve device performance, especially for ALD OTS selectors. ,− …”

ALD Heterojunction Ovonic Threshold Switches

“…An example of the impact of massively parallel fabrication of 3D memory architectures can be seen in vertical NAND Flash arrays, where the conformal deposition of electronic materials has transformed this technology increasing the number of layers into the hundreds . Unfortunately, while highly conformal high-performance films have been developed for PCM memories, − many attempts have failed in producing an actual OTS selector which shows stable, repeatable switching. , OTS selectors are two-terminal devices exhibiting high selectivity and a sharp on/off transition at a threshold voltage. The mechanism of conduction in the subthreshold region is fairly understood, being dominated by trap-assisted phenomena typical of amorphous solids; several models have instead been proposed for the switching mechanism such as field-assisted tunneling between trap states, , thermal induced threshold switching, the formation of a metastable filament, − or models bridging between the electronic and filamentary frameworks .…”

“…Despite this degree of uncertainty on the fundamental operation of OTSs, these selectors are currently used in commercially available products such as the Intel/Micron developed Optane where OTSs are used to minimize the currents, and therefore the power consumption, associated with inactive elements of the memory array. Previously, only binary ALD OTS films have been developed, , while the use of chalcogenide alloys comprising three or more elements is crucial for OTS performance even in PVD films. Such compositions are needed to address simultaneously a number of problems such as a suitable bandgap limiting the OTS subthreshold current , and electrical and thermal stability to guarantee device reliability. , The introduction of Arsenic into chalcogenide glasses, particularly, generated a breakthrough as this element was found to improve the thermal stability of the film and avoid crystallization; , incorporation of this element into OTS devices resulted in extremely stable selectors with high endurance. , Particularly, we posed our attention on Ge-Se-As-Te compositions .…”

“…We leveraged the combination of alkyl-silyl precursors with chlorinated species that successfully produced binary chalcogenides, ,, extending this methodology to arsenic based ALD films. Germanium selenide was grown following a previously reported process using trichlorogermane (HCl 3 Ge) and bis­(trimethylsilyl)-selenium (BTMS-Se). A characterization of this film, showing excellent film morphology, is shown in Figure S1 in the Supporting Information (SI).…”