(Invited) Ge-Cu-Te Phase Change Material for Pcram Application
Authors: Sutou Yuji , Saito Yuta , Shindo Satoshi , Koike Junichi 
Published: February 2020

Phase Change Random Access Memory (PCRAM) is a promising candidate as next generation non-volatile memory. The principle of PCRAM relies on the resistance contrast between crystalline and amorphous states of phase change material (PCM). Generally, the crystalline phase shows a low electrical resistance (set state), while the amorphous phase shows a high electrical resistance (reset state). The phase transition of PCM is achieved by way of Joule heating induced by an electrical pulse. Currently, Ge-Sb-Te films, namely Ge2Sb2Te5 (GST) compound film, are widely studied for PCRAM application because they show fast phase change speed and excellent reversibility of phase transition. However, GST has a low crystallization temperature (~160°C), which limits high temperature data retention capability. In addition, GST possesses a relatively high melting point (~630°C), which leads to a high power consumption for reset operation. Moreover, with further scaling down of PCRAM memory cell, the thermal disturbance between cells becomes serious problem. To overcome those weakness and problem, it is desired to develop new PCMs with high crystallization temperature and low melting point.

 From the above background, we have studied the effect of doping element, X on the crystallization temperature in Ge-Te-based PCMs and found that Al, Si and Cu are very effective to improve the thermal stability of Ge-Te amorphous film, based on the total bonding enthalpy of amorphous Ge-Te-X. Among them, Ge-Cu-Te ternary film has been found to have various advantages for PCRAM application. According to Ge-Cu-Te ternary phase diagram, there is a GeCu2Te3 (GCT) ternary compound which shows a melting point of around 500°C. Moreover, Cu2Te-Ge33.3Te66.7 pseudobinary phase diagram indicates that the liquidus line deeply decreases toward the compound composition like eutectic-type phase diagram, which suggests that the amorphous phase can be easily obtained around the compound composition. In this study, the phase transition characteristics of GeTe-CuTe pseudobinary films were investigated. GeTe-CuTe pseudobinary films, including GeCu2Te3 compound film, with a thickness of 200 nm were fabricated by co-sputtering of GeTe and CuTe targets on SiO2/Si substrate. Moreover, to evaluate the memory operation characteristics, simple memory devices with GCT layer were fabricated using a conventional photolithography technique.

 It was found that in GeTe-CuTe pseudobinary film, the crystallization temperature increases with increasing Cu content in the range of Cu < 15 at.% and the maximum value of about 250°C is obtained, and then the crystallization temperature decreases with further increasing Cu content. The GCT amorphous was found to show the crystallization temperature of about 230°C. It was confirmed that the GCT memory cell exhibits a reversible phase transition between amorphous and crystalline states and shows a lower power consumption for reset operation than GST memory cell. In addition, it was also found by static laser testing that the phase change speed of the GCT film is as fast as that of GST film. Furthermore, it was found that the GCT shows unique phase transition characteristics, namely, a small volume expansion and reflectance decrease by crystallization. Such changes upon phase transition for the GCT are opposite to those for conventional PCMs. It was suggested from AIMD simulation that such unique phase transition characteristics may be caused by unusual structural features of GCT amorphous, such as short Cu bond lengths, threefold rings, and dense Cu-rich regions. In this presentation, the phase transition characteristics of Ge-Cu-Te ternary films and the memory device characteristics will be discussed.