Firing voltage reduction in thermally annealed Ge–As–Te thin film with ovonic threshold switching

Abstract

Recently, chalcogenide materials have exhibited ovonic threshold switching characteristics, improving their suitability as selector devices to effectively depress the sneak current in the cross-point array (CPA) structures. However, chalcogenides must be subjected to a firing process before they can exhibit the threshold switching behavior. The firing process causes operation problems with respect to the memory operation process in the case of the CPA structure. Although the firing process is expected to be related to Joule heating with a high electric current, the physical nature of the changes during the firing process remains unclear. In this study, selector devices are fabricated by sandwiching amorphous Ge–As–Te thin films between the TiN and W layers. Furthermore, the authors examine the microstructure of the Ge–As–Te thin films before and after the electric firing process using transmittance electron microscopy (TEM, JEOL JEM-F200). The TEM analysis of the pristine Ge–As–Te thin films denotes the uniform contrast of the image, which implies the uniform chemical composition of the film. However, the Ge–As–Te thin films exhibit nonuniform contrast due to the effects of Joule heating after the electric firing process. The Ge–As–Te thin films were thermally annealed at 150 and 250 °C for 1 min via the rapid thermal annealing process to verify the effects of thermal treatment on the firing process in chalcogenide thin films. The effect of thermal annealing on the threshold switching behavior was also investigated by studying the compositional stability. Their results showed that the thermal annealing process caused the uniform compositions on the pristine Ge–As–Te films to be fluctuated with decreasing firing voltage for threshold switching.

Screening Switching Materials with Low Leakage Current and High Thermal Stability for Neuromorphic Computing

“…The I OFF significantly degrades after the fire process, which is widely observed in the other OTSs. [71,77] As summarized in Figure 3f, the GaS-based selector not only shows extremely low OFF-state conduction as we expected, but also provides high J ON that is required to operate the coupled storage cell, like PCMs and resistance random access memories (RRAMs). Most importantly, the simple GaS OTS device can withstand the 400-450 °C thermal shock in the BEOL process like B-Te one, [21] namely, BEOL-compatible.…”

“…In general, OTS devices need a higher voltage to initiate the first sweep, called firing process. [71,72] Obviously, in the second sweep, the threshold switching phenomenon appears at lower voltage (threshold voltage V th ), ≈2 V, with an ON current (I ON ) of ≈0.7 mA, and the device switches off at the same V hold value. As shown in Figure 3c, the V th and I ON is device size-independent.…”

Rational engineering of a switching material for an Ovonic threshold switching (OTS) device with mitigated electroforming

“…The EF process requires V form a few times as high as V th and even lasts for several repetitions in some cases. [10][11][12] Since such an EF process not only imposes a burden on the peripheral circuits but is also detrimental to the life of the device due to the large surge current, 13 efforts have been made to clarify its origin and to make the OTS device free of the EF process. 10,14 Meanwhile, a similar phenomenon is well known in resistive random access memory (ReRAM) devices, 15 where the EF process is explained as the first-time building process of a conducting bridge.…”

“…[10][11][12] Since such an EF process not only imposes a burden on the peripheral circuits but is also detrimental to the life of the device due to the large surge current, 13 efforts have been made to clarify its origin and to make the OTS device free of the EF process. 10,14 Meanwhile, a similar phenomenon is well known in resistive random access memory (ReRAM) devices, 15 where the EF process is explained as the first-time building process of a conducting bridge. After the bridge is formed inside an active material, the successive on/off switching is performed by connecting/disconnecting a small part of the bridge, which requires much less energy than that required in the forming process.…”

“…The EF process requires V form a few times as high as V th and even lasts for several repetitions in some cases. 10–12 Since such an EF process not only imposes a burden on the peripheral circuits but is also detrimental to the life of the device due to the large surge current, 13 efforts have been made to clarify its origin and to make the OTS device free of the EF process. 10,14…”