Novel Sn-based Contact Structure for GeTe Phase Change Materials.

Germanium telluride (GeTe) is a phase change material (PCM) that has gained recent attention due to its incorporation as an active material for radio frequency (RF) switches, as well as memory and novel optoelectronic devices. Considering PCM-based RF switches, parasitic resistances from Ohmic contacts can be a limiting factor in device performance. Reduction of the contact resistance (Rc) is therefore critical for reducing the ON-state resistance in order to meet the requirements of high-frequency RF applications. To engineer the Schottky barrier between the metal contact and GeTe, Sn was tested as an interesting candidate to alter the composition of the semiconductor near its surface, potentially forming narrow bandgap (0.2 eV) SnTe or a graded alloy with SnTe in GeTe. For this purpose, a novel contact stack of Sn/Fe/Au was employed and compared to a conventional Ti/Pt/Au stack. Two different pre-metallization surface treatments of HCl and deionized (DI) H2O were employed to make the Te-rich and Ge-rich interface, respectively. Contact resistance values were extracted using the refined transfer length method (RTLM). The best results were obtained with DI H2O for the Sn-based contacts but HCl treatment for the Ti/Pt/Au contacts. The as-deposited contacts had Rc (ρc) of 0.006 Ω.mm (8 × 10-9 Ω-cm2) for Sn/Fe/Au and 0.010 Ω.mm (3 × 10-8 Ω-cm2) for Ti/Pt/Au. However, the Sn/Fe/Au contacts were stable and their resistance decreased further to 0.004 Ω.mm (4 × 10-9 Ω-cm2) after annealing at 200 oC. In contrast, the contact resistance of the Ti/Pt/Au stack increased to 0.012 Ω.mm (4 × 10-8 Ω-cm2). Transmission electron microscopy (TEM) was used to characterize the interfacial reactions between the metals and GeTe. It was found that formation of SnTe at the interface, in addition to Fe-diffusion (doping) into GeTe, are likely responsible for the superior performance of Sn/Fe/Au contacts, resulting in the lowest reported contact resistance on GeTe.