A method of switching a phase-change device (Device), including changing phase of the Device from a semiconducting 2H phase to a new 2Hd phase with a higher conductivity, the Device having an active material with a thickness including a phase transition material to thereby transition the Device from a high resistive state (HRS) to a low resistive state (LRS) by application of a set voltage and further to return the Device from the LRS back to the HRS by application of a reset voltage.
This invention relates to memory devices, in particular to resistive random-access memory (RRAM) cells. RRAM-based technology has gained attention of the semiconductor industry due to its potential scalability, high operation speed, high endurance and ease of process flow. RRAM devices are typically two-terminal cells whose operation are based on switching between a high resistive state (HRS that corresponds to a value of “0”) and a low resistive state (LRS, “1”) to thereby store information in a nonvolatile fashion by applying a sufficiently high voltage or by driving a large enough current through the cell. The existing RRAM devices utilize switching mechanisms that involve movement of atoms inside a cell. Accordingly, both reliability aspects and switching speed of such devices represent substantial challenges.
In our invention, a new switching mechanism is proposed that is based on a phase transition in the active material of the device from a semiconducting 2H phase (HRS) to a new 2Hd phase with a higher conductivity (LRS) by application of a set voltage. By reversing the electrical field, the cell can be transformed from the LRS back to the HRS, i. e., resetting the cell back to “0”. The active materials described in the present invention, transition metal dichalcogenides (TMDs) MoTe2 and Mo1-xWxTe2, belong to a class of two-dimensional layered materials which exhibit a structural phase transition.
The operation principle of the device can briefly be described as follows. As long as a critical forming voltage which depends on TMD thickness is not reached, the pristine metal-MoTe2-metal devices exhibit reproducible I-V curves. Progressing beyond this forming voltage results in formation of a conductive filament that brings the device into the resistive switching operation mode behavior, i.e. the RRAM behavior. Once a filament is formed, a voltage lower than the forming voltage (called a set voltage) is used to switch between the HRS and LRS of the cell. A transition back to the HRS occurs when a reverse polarity electric field is applied, due to a rupture of the conductive filaments.
Detailed scanning transmission electron microscopy (STEM) examination of cross-section of samples revealed that the crystal structure of the filaments, which we called 2Hd, differs from the original 2H phase of the Mo(W)Te2 active material. Instead of the well-aligned atomic columns as observed for the 2H structure of Mo(W)Te2, the atomic columns of the 2Hd structure show a distinct ‘splitting’ for both Te and Mo/W atomic columns, which suggests that the 2Hd phase is a distorted metallic modification of the 2H structure, i.e., some transient state with atoms displaced toward one of the lower symmetry structures, but still within the crystal symmetry of the 2H structure.
Thus, the present invention can result in a new class of the RRAM devices with advanced characteristics such as improved lifetime, lower energy consumption, higher operation speed. The RRAM devices based on phase-changing TMD materials may be used in fabrication of neuromorphic computing, in-memory computing or memristor-based nonvolatile logic circuits and devices.
Advantages of the proposed RRAM based on phase-changing TMD materials is that the phase transformation is reversible and does not involve a change from an amorphous to a crystalline state as typically observed in conventional phase change materials (PCMs). Instead, MoTe2 and Mo1-xWxTe2 remain crystalline when undergoing the local phase transition from a semiconducting 2H to a metallic 2Hd phase. In comparison to, e.g., VO2 that can also undergo an insulator-to-metal phase transition under an electric field but requires a hold current and exhibits a unipolar switching behavior, the MoTe2 phase transition reported here is bipolar, nonvolatile and it occurs at room-temperature.