Understanding oxidation of layered chalcogenide semiconductors is important for device processing, as oxidation can be both an intentional and unintentional result of processing steps. Here, the authors investigate chemical and morphological changes in mechanically exfoliated few-layer MoS2 in oxidizing and inert environments using different microscopies (optical, scanning electron, and atomic force) and spectroscopy (Raman, x-ray photoelectron, and Auger electron) techniques. The environments studied were oxygen, oxygen and water vapor, argon, argon and water vapor, and ultraviolet-generated ozone at temperatures from 25 to 550 °C. Oxidation at low temperatures resulted in the formation of a condensed molybdenum oxide phase and sulfur trioxide gas. At sufficiently elevated temperatures, all the products of oxidation volatilize, resulting in a vapor-phase etch. The kinetics of oxidation and etching depended upon the annealing gas, temperature, time, and the number of layers of MoS2. Conditions can be selected to create isolated etch pits, smooth oxide layers, oxide islands, or flakes of reduced lateral dimensions (etching from the flakes' edges). These results can provide useful guidance for MoS2 device processing.

Surfaces of polycrystalline α-GeTe films were studied by X-ray photoelectron spectroscopy (XPS) after different treatments in an effort to understand the effect of premetallization surface treatments on the resistance of Ni-based contacts to GeTe. UV-O is often used to remove organic contaminants after lithography and prior to metallization; therefore, UV-O treatment was used first for 10 min prior to ex situ treatments, which led to oxidation of both Ge and Te to GeO (x < 2) and TeO, respectively. Then the oxides were removed by deionized (DI) HO, (NH)S, and HCl treatments. Additionally, in situ Ar ion etching was used to clean the GeTe surface without prior UV-O treatment. Ar ion etching, HO, and (NH)S treatments create a surface richer in Ge compared to the HCl treatment, after which the surface is Te-rich. However, (NH)S also oxidizes Ge and gradually etches the GeTe film. All treated surfaces showed poor stability upon prolonged exposure to air, revealing that even (NH)S does not passivate the GeTe surface. The refined transfer length method (RTLM) was used to measure the contact resistance (R) of as-deposited Ni-based contacts to GeTe as a function of premetallization surface preparation. HCl-treated samples had the highest R (0.036 ± 0.002 Ω·mm), which was more than twice that of the other surface treatments. This increase in R is attributed to formation of the NiTe phase at the Ni/GeTe interface due to an abundance of Te at the surface after HCl treatment. In general, treatments that resulted in Ge-rich surfaces offered lower R.

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.