Fast electronic resistance switching involving hidden charge density wave states

Abstract

The functionality of computer memory elements is currently based on multi-stability, driven either by locally manipulating the density of electrons in transistors or by switching magnetic or ferroelectric order. Another possibility is switching between metallic and insulating phases by the motion of ions, but their speed is limited by slow nucleation and inhomogeneous percolative growth. Here we demonstrate fast resistance switching in a charge density wave system caused by pulsed current injection. As a charge pulse travels through the material, it converts a commensurately ordered polaronic Mott insulating state in 1T-TaS2 to a metastable electronic state with textured domain walls, accompanied with a conversion of polarons to band states, and concurrent rapid switching from an insulator to a metal. The large resistance change, high switching speed (30 ps) and ultralow energy per bit opens the way to new concepts in non-volatile memory devices manipulating all-electronic states.

Figure 1. The equilibrium phases of 1T–TaS₂ and demonstration of switching behaviour.

(a) An illustration of the different CDW states in 1T–TaS₂ at different temperatures. (b) Temporal behaviour of switching from HI to LO resistance in response to a pulse from a current source. The extrapolated voltage in the absence of switching is shown by the dashed lines. The time–response is determined by τ_RC=RC where R is the two-terminal circuit resistance and changes from 53 to 44 ns. Fits of the relaxation times show that the difference between τ after switching and τ_RC at the end of the pulse is<0.3 ns, setting a limit on the intrinsic speed of the device. The insert shows the measuring circuit. The small superimposed oscillations are due to ringing of the circuit. Scale bar, 2 μm.

Figure 2. Switching speed and threshold behaviour.

(a) Switching resistance ratio measured with a single pulse above threshold I_T, each time starting from the HI resistance state (red circles). Black circles show resistivity after current pulse. (b) Erasing with 1-s pulses, each time starting from the LO resistance state (red circles). Black circles show resistivity after applying erase pulse. The data were obtained by alternating write/erase cycles and increasing the pulse length. (c) The circuit used for the 30-ps measurement with an metal–semiconductor–metal (MSM) device as the current source. (d) The voltage V measured during each pulse, first incrementally increasing and then decreasing the pulse current I, shown by paths (1) and (2), respectively. Above the threshold current I_T, V abruptly drops between two consecutive current values. (e) Same as in d, but starting from the low resistance state increasing I_pulse along path (1) and then reducing I_pulse along (2). In all cases T=20 K and the pulse length was 10 μs. Scale bar, 2 μm.

Figure 3. The T-dependence of switching.

(a) The pulsed V–I characteristic as in Fig. 2, measured at different temperatures and at 10 K intervals with τ_pulse=50 μs. The V–I curve is measured in pulsed mode, by increasing the current incrementally and measuring the voltage at each point. The resistance drops sharply in a few consecutive measurements at the critical value of I_T. The sample was reset in between each V–I curve by heating it above 310 K and then cooling down to the indicated temperature for the next measurement. (b) I_T and V₀ as a function of temperature. Error bars are obtained from fitting the data with the exponential function.

Figure 4. An illustration of carrier trapping and DW formation after carrier injection.

(a) The injected charges are trapped in the C structure causing the formation of a textured glassy electronic state. The charge density ρ is depleted at the leading edge of the pulse as it travels through the sample. (b) The free energy as a function of CDW wavevector based on the model of Nakanishi and Shiba. q_C and q_I are the q-vectors of the undoped commensurate and IC states, respectively. The q-vector in the H state q_H is determined by the density of carriers forming the DWs. (c) A depiction of the charge injection at the electrodes in the C state and (d) the band structure after the Mott-to-band state conversion process.