Dimensional transformation of chemical bonding during crystallization in a layered chalcogenide material

Abstract

Two-dimensional (2D) van der Waals (vdW) materials possess a crystal structure in which a covalently-bonded few atomic-layer motif forms a single unit with individual motifs being weakly bound to each other by vdW forces. Cr2Ge2Te6 is known as a 2D vdW ferromagnetic insulator as well as a potential phase change material for non-volatile memory applications. Here, we provide evidence for a dimensional transformation in the chemical bonding from a randomly bonded three-dimensional (3D) disordered amorphous phase to a 2D bonded vdW crystalline phase. A counterintuitive metastable “quasi-layered” state during crystallization that exhibits both “long-range order and short-range disorder” with respect to atomic alignment clearly distinguishes the system from conventional materials. This unusual behavior is thought to originate from the 2D nature of the crystalline phase. These observations provide insight into the crystallization mechanism of layered materials in general, and consequently, will be useful for the realization of 2D vdW material-based functional nanoelectronic device applications.

Amorphous-to-Crystal Transition in Quasi-Two-Dimensional MoS2: Implications for 2D Electronic Devices

“…The buckling of the layers may be attributable to the positional and compositional disorder that still exists during the initial stages of crystallization into a 2D layered structure. We note here that a similar process, namely, the formation of the layered structure with the remaining local disorder, was also observed during the crystallization of a different 2D chalcogenide Cr 2 Ge 2 Te 6 , , suggesting that this may be a typical feature of a 3D–2D crystallization process.…”

Understanding the Origin of Low-Energy Operation Characteristics for Cr2Ge2Te6 Phase-Change Material: Enhancement of Thermal Efficiency in the High-Scaled Memory Device

“…The crystallization speed for amorphous CrGT is as fast as that of GST, but there is the possibility that the time duration of 40 ns is not sufficient for the molten CrGT to completely transform to high-resistive crystalline phase, leading to a residual metastable phase in the set state. Amorphous CrGT has been reported to crystallize to a high-resistive crystalline phase via the low-resistive metastable phase. , Recent studies have revealed that the metastable phase shows long-range order and short-range disorder. , Because of these unique structural features, the physical properties of the metastable phase have not been sufficiently investigated. To incorporate the influence of the metastable phase, the temperature dependence of σ was measured for the 290 °C-annealed film, which is reported as metastable CrGT. , To verify this influence, we assumed that the metastable phase is formed above the amorphous region with a thickness t of 7 nm, which is equivalent to the range between the amorphous surface and T max point or 77 nm, which is equivalent to the distance between the amorphous surface and the farthest point reaching T m from the electrode (Figure S5).…”

“…Amorphous CrGT has been reported to crystallize to a high-resistive crystalline phase via the low-resistive metastable phase. , Recent studies have revealed that the metastable phase shows long-range order and short-range disorder. , Because of these unique structural features, the physical properties of the metastable phase have not been sufficiently investigated. To incorporate the influence of the metastable phase, the temperature dependence of σ was measured for the 290 °C-annealed film, which is reported as metastable CrGT. , To verify this influence, we assumed that the metastable phase is formed above the amorphous region with a thickness t of 7 nm, which is equivalent to the range between the amorphous surface and T max point or 77 nm, which is equivalent to the distance between the amorphous surface and the farthest point reaching T m from the electrode (Figure S5). Since the metastable phase exhibits structural features similar to both amorphous and crystalline phases, , we chose κ values of 0.15 and 0.20 W/mK, which are intermediate values between 0.13 and 0.24 W/mK.…”

“…To incorporate the influence of the metastable phase, the temperature dependence of σ was measured for the 290 °C-annealed film, which is reported as metastable CrGT. , To verify this influence, we assumed that the metastable phase is formed above the amorphous region with a thickness t of 7 nm, which is equivalent to the range between the amorphous surface and T max point or 77 nm, which is equivalent to the distance between the amorphous surface and the farthest point reaching T m from the electrode (Figure S5). Since the metastable phase exhibits structural features similar to both amorphous and crystalline phases, , we chose κ values of 0.15 and 0.20 W/mK, which are intermediate values between 0.13 and 0.24 W/mK. The simulated set energy with metastable state is shown in Figure b.…”

Phase control of sputter-grown large-area MoTe2 films by preferential sublimation of Te: amorphous, 1T′ and 2H phases

“…Such highly oriented features can be often observed in other layered tellurides. 43,44 A fast Fourier transformed (FFT) image shows spots along the out-of-plane direction, and the spot location was found to correspond to the (002) lattice spacing. These TEM results strongly support the potential of the large area growth of a MoTe 2 film by sputtering followed by optimized annealing.…”