Bipolar switching in chalcogenide phase change memory

Abstract

Phase change materials based on chalcogenides are key enabling technologies for optical storage, such as rewritable CD and DVD, and recently also electrical nonvolatile memory, named phase change memory (PCM). In a PCM, the amorphous or crystalline phase affects the material band structure, hence the device resistance. Although phase transformation is extremely fast and repeatable, the amorphous phase suffers structural relaxation and crystallization at relatively low temperatures, which may affect the temperature stability of PCM state. To improve the time/temperature stability of the PCM, novel operation modes of the device should be identified. Here, we present bipolar switching operation of PCM, which is interpreted by ion migration in the solid state induced by elevated temperature and electric field similar to the bipolar switching in metal oxides. The temperature stability of the high resistance state is demonstrated and explained based on the local depletion of chemical species from the electrode region.

Towards a 3D GeSbTe phase change memory with integrated selector by non-aqueous electrodeposition

“…This behaviour is shown in Fig 12b. Bipolar resistive switching has been demonstrated previously in Ge2Sb2Te5 and is shown to be caused by ionic migration leading to local depletion and species separation at bottom electrode interface. 21 Electrical characterisation on the 10 x 10 passive matrix resulted in resistance measurements consistent with those taken from individual devices on 1 x 10 arrays. However, as the matrices currently do not contain a selector and non-linear element sneak path currents are significant.…”

High‐Speed Bipolar Switching of Sputtered Ge–Te/Sb–Te Superlattice iPCM with Enhanced Cyclability

“…While very common in resistive switching memory oxide‐based and similar devices, bipolar switching in PCRAM has rarely been reported. There are many reasons for this, for example, the switching may not have been reproducible, might exhibit low switching speeds or relatively poor endurance or reliability . These negative aspects reflect the character of the underlying bipolar switching mechanism which is generally associated with either void or filament formation resulting from ionic flow within the active area of a device .…”

“…There are many reasons for this, for example, the switching may not have been reproducible, might exhibit low switching speeds or relatively poor endurance or reliability . These negative aspects reflect the character of the underlying bipolar switching mechanism which is generally associated with either void or filament formation resulting from ionic flow within the active area of a device . In principle, such switching tends to be much slower than the conventional unipolar melt‐quench switching in PCRAM and also unavoidably leads to additional stress in the device, hastening eventual switching failure.…”

“…In this work, this was accomplished by using Ge–Te/Sb–Te SL‐based iPCM. It is shown that using a SL structure one can obtain bipolar switching that is 1000 times faster and has significantly better endurance than found for bipolar switching of Ge‐Sb‐Te alloy‐based devices . It has been suggested that the unique structure of the SL allows bipolar switching to occur via a crystal‐crystal phase transition owing to the asymmetry of the SL unit cell.…”

Electrical Transport in Crystalline and Amorphous Chalcogenide

“…Fig . 7 shows the measured current-voltage (I-V) curves for a PCM device with confined bottom electrode [10,39]. In the measured device, the GST film was deposited by RF sputtering [40].…”