Amorphization of pure hafnium nanocontacts and continuous conductance control via phase transition treatment using nanosecond pulse voltage energization

Abstract

The amorphization of a pure metal nanocontacts (NCs) via pulse voltage energization was verified in pure hafnium (Hf) NCs with a hexagonal close-packed structure. The structural dynamics during amorphization was examined via in situ lattice imaging of transmission electron microscopy with simultaneous conductance measurements. The similarities and difference in the phase transition dynamics and conductance between Hf NCs and pure metal NCs having body-centered cubic structures were demonstrated. In Hf NCs, step-by-step crystallization from amorphous phases could be coordinated by the adjustment of the energization manner of nanosecond pulse voltages, i.e., the pulse height and the energization number. This led to gradual conductance control of the NCs; the conductance could be decreased down to 0.72 of the conductance of the crystalline phase.

Reversible phase-transition control in nanometer-sized zirconium wires via pulse-voltage impression

“…The average ratio of the conductance per a unit area at the initial crystalline states to that at the amorphized states was estimated to be 0.87 ( figure 8). This value is similar to the ratio for W and Ta NSWs (0.82 and 0.85, respectively) and larger than that for Mo and Hf NSWs (0.62 and 0.72, respectively) [21][22][23][24]. In this study, the amorphized region expanded gradually for every pulse-voltage impression ( figure 3), and the conductance of the NSWs decreased stepwise (figures 5 and 6).…”

“…For the conductive Ag filaments, the applied pulses with a width of hundreds of nanoseconds are beneficial to realize fast learning and computing [33], whereas for the present Zr NSWs, the conductance control was performed using the pulses of 4 ns-1 ms widths (figure 9). The amorphization voltage was 0.20±0.05 V for the NSWs having 8 nm width, much smaller than that for NSWs comprised of Ta, Mo, W, and Hf, in which amorphization were confirmed (0.5-2.5 V) [21,22,24,49]. In addition, the volume resistivity of Zr is the highest among the metals used for amorphous NSWs [58][59][60][61].…”

“…In NSWs of Mo, hafnium (Hf), and tungsten (W), the crystallization voltages of pulse waves are lower than their amorphization voltages; the crystallization occurs by weaker energization [19,[21][22][23]. In contrast, in tantalum (Ta) NSWs, the crystallization and amorphization occur via the same energization, i.e., the energization using pulse voltages of the same width and the same height [24].…”

“…Recently, the amorphization of pure metal NSWs via pulse-voltage impression has been investigated in connection with conductance variation [19][20][21][22][23][24]. The amorphous phases derived by this method are stable at ambient temperature, which is less common in amorphous metallic nanospheres prepared by spray freezing [25].…”