Specific Contact Resistance of Phase Change Materials to Metal Electrodes

Abstract

For phase change random access memory (PCRAM) cells, it is important to know the contact resistance of phase change materials (PCMs) to metal electrodes at the contacts. In this letter, we report the systematic determination of the specific contact resistance (ρ c ) of doped Sb 2 Te and Ge 2 Sb 2 Te 5 to TiW metal electrodes. These data are reported for both the amorphous and the crystalline states of these PCMs. The temperature and voltage dependences of ρ c are also studied. A detailed understanding of these contacts is essential for the scaling, design, device modeling, and optimization of PCRAM cells. Index Terms-Cross bridge Kelvin resistor (CBKR), doped Sb 2 Te, Ge 2 Sb 2 Te 5 , phase change materials (PCMs), specific contact resistance (ρ c ).

Phase change materials and phase change memory

“…16 This is benefi cial for scaling down of voltage and power, although contact resistance effects may become dominant for the smallest devices. 57 The resistivity ratio of phase change materials does not appear to decrease in experiments on nanoscale devices, 58 , 59 although simulations mentioned previously predict such effects may occur at dimensions below a dozen atomic layers. 52 This means there would be a large enough window for multi-level cell programming in PCM even at diminished dimensions.…”

Phonon and electron transport through Ge2Sb2Te5 films and interfaces bounded by metals

“…33 The equation shows that electron transport strongly depends on the carrier concentration (log R b,e $ N À1/2 ) and the barrier height (log R b,e $ / b ). The barrier height for crystalline Ge 2 Sb 2 Te 5 does not depend on the metal work function, 31 possibly due to Fermi level pinning and the potential presence of a large trap density. Therefore, our data may suggest that the electron contribution to boundary thermal transport may generally be negligible between Ge 2 Sb 2 Te 5 and all of the likely metals with which it might be combined.…”

“…18,30 The electron transport across the interface of Ge 2 Sb 2 Te 5 films increases by more than five orders of magnitude after crystallization and slightly improves with higher annealing temperatures. 31 The specific contact resistance measured by the CBKR and TLM structures (see Fig. 2) for fcc Ge 2 Sb 2 Te 5 prepared by annealing at 200 C is 84 6 8 X-lm 2 and for hcp Ge 2 Sb 2 Te 5 prepared by annealing at 300 C is 7 6 2 X-lm 2 .…”

Density functional study of the TiN/Ge2Sb2Te5 interface

“…The properties of the TiN/GST interface and in particular the band lineup between the two materials affect the contact resistance and thus the programming current of the device. Although measurements of the Schottky barrier and contact resistance at the interface between GST and several metals have been reported 7, 8, little information is available on the properties of the GST/TiN interface.…”