Review of electrical contacts to phase change materials and an unexpected trend between metal work function and contact resistance to germanium telluride

Abstract

Devices based on the unique phase transitions of phase change materials (PCMs) like GeTe and Ge2Sb2Te5 (GST) require low-resistance and thermally stable Ohmic contacts. This work reviews the literature on electrical contacts to GeTe, GST, GeCu2Te3 (GCuT), and Ge2Cr2Te6 (GCrT), especially GeTe due to the greater number of studies. We briefly review how the method used to measure the contact resistance (Rc) and specific contact resistance (ρc) can influence the values extracted, since measurements of low contact resistances are susceptible to artifacts, and we include a direct comparison of Au-, Pt-, Ni-, Mo-, Cr-, Sn-, and Ti-based contacts using a systematic approach. Premetallization surface treatment of GeTe, using ex situ or in situ approaches, is critical for minimizing contact resistance (Rc). Transmission electron microscopy reveals that interfacial reactions often occur and also clearly influence Rc. The lowest Rc values (∼0.004 ± 0.001 Ω mm) from the direct comparison were achieved with as-deposited Mo/Ti/Pt/Au (Ar+ plasma treatment) contacts and annealed Sn/Fe/Au (de-ionized H2O premetallization treatment). In the case of Sn-based contacts, low Rc was attributed, in part, to the formation of SnTe at the contact interface; however, for Mo-based contacts, no such interfacial reaction was observed. Comparing all contact metals tested beneath a cap of at least 100 nm of Au, Mo/Ti/Pt/Au offered the lowest contact resistance as-deposited, even though the work function of Mo is only 4.6 eV, and the low contact resistance remained stable even after annealing at 200 °C for 30 min. This trend is surprising, as high work function metals, like Ni and Pt, would be expected to provide lower Rc values when they are in contact with a p-type semiconductor like GeTe. Through materials’ characterization, an inverse relationship between the metal work function and Rc for higher work function metals can be attributed to the reactivity of many of the metals with GeTe. Studies of contacts to GST in the literature involve only a small number of contact materials (Ti, TiN, TiW, W, Pt, and graphene) and employ varied geometries for extracting contact resistance. For hexagonal GST, TiW is reported to provide the lowest ρc of ∼2 × 10−7 Ω cm2, while TiN provided the lowest reported ρc of ∼3 × 10−7 Ω cm2 to cubic GST. For the ternary PCMs GCuT and GCrT, contact resistance studies in the literature are also limited, with W being the only metal studied. While more extensive work is necessary to draw wider conclusions about trends in current transport at metal/GST, metal/GCuT, and metal/GCrT interfaces, reduction of Rc and high thermal stability are critical to engineering more efficient and reliable devices based on these materials.