Electrical Contact Property of GeCu2Te3 Phase Change Material to Electrode
Authors: Shindo Satoshi , Sutou Yuji , Ando Daisuke , Koike Junichi , Saito Yuta , Song Yun-Heub 
Published: February 2020

To overcome the scaling limits of flash memory, new phase change materials has been intensively studied for phase change random access memory (PCRAM). Currently, Ge-Sb-Te (GST) film has been widely studied for commercial PCRAM. However, GST shows a poor thermal stability of amorphous phase because of its low crystallization temperature (~160 °C). Consequently, GST PCRAM shows a poor data retention property in high temperature and is suppressed to show a low thermal disturbance resistance in high density array. Recently, Sutou et al. have found that GeCu2Te3 (GCT) shows higher crystallization temperature (~230 °C) than GST. Moreover, they demonstrated that the reset process in GCT memory cell exhibits lower power consumption than that in GST memory cell, which is due to a low melting point of GCT (~500 °C). Since GCT shows phase change speed as fast as GST, it is promising PCM for future PCRAM.The memory cell resistance in PCRAM is very essential factor for accuracy in data reading. As the memory cell size is scaled down, the resistance of the memory cell becomes dominated by the contact resistance between the phase change material (PCM) and electrode. Nevertheless, studies of contact resistance in PCRAM are still limited. To understand the contact resistivity and its contrast between amorphous and crystalline states is more essential for future PCRAM. Therefore, in this study, the contact resistivity of GCT to electrodes was investigated using the circular transfer length method (CTLM).

A CTLM pattern was fabricated by photolithography in order to measure the contact resistivity. GCT amorphous film was deposited on SiO2/Si substrates by co-sputtering of GeTe and CuTe targets. For comparison, GST amorphous film was also prepared. GCT crystalline film was obtained by heating the as-deposited amorphous film up to 300 °C in an Ar atmosphere. After reverse sputtering of the surface of the GCT layer, a metal electrode was deposited on the GCT films in the same chamber to prevent the surface oxidation effects. Electrical resistance of the CTLM patterned samples were measured by four-point probe method. Based on the obtained results, the contact resistivity of GCT to electrode was calculated.

Current (I) - Voltage (V) characteristics of GCT CTLM samples showed an Ohmic behavior in both amorphous and crystalline states. The contact resistivity could be successfully obtained from GCT CTLM samples in both states since the relationship between the total resistance of CTLM samples and the gap spacing showed a liner relation. The contact resistivity of W/amorphous GCT (as-deposited) contact was calculated to be 3.9×10-2 Ω cm2, while that of W/crystalline GCT (annealed at 300 °C) contact was found to be 4.8 ×10-6 Ω cm2. The ρ
c of W/amorphous GST (as-deposited) was 3.2 ×10-2 Ω cm2, whereas that of W/crystalline GST (annealed at 300 °C) was 1.4×10-5 Ω cm2. The resistivity contrast in GCT between amorphous and crystalline state was about one order of magnitude smaller than that in GST. Meanwhile, the contact resistivity contrast in GCT between amorphous and crystalline state was slightly larger than that in GST.

Based on the obtained results, we calculated the total resistance of simple memory cell model. When the PCM layer thickness, t is over 10-2 m, the resistance contrast of the GCT memory cell was 17 times smaller than that of the GST cell. This is because the cell resistance is dominated by the ρ of PCM. Meanwhile, the resistance contrast of the GCT cell becomes 4 times larger than that of the GST cell at t < 10 nm because the cell resistance is dominated by the ρ
c
of PCM to electrode. Therefore, GCT is expected to show not only better thermal stability but also better accuracy in data readout than GST. In the presentation, we will also discuss the electrode dependence of the contact resistivity in GCT film.