Showing papers by "Suzanne E. Mohney published in 2020"

Uncovering the Effects of Metal Contacts on Monolayer MoS2

Abstract: Metal contacts are a key limiter to the electronic performance of two-dimensional (2D) semiconductor devices. Here, we present a comprehensive study of contact interfaces between seven metals (Y, S...

Achieving Minimal Heat Conductivity by Ballistic Confinement in Phononic Metalattices

Abstract: Controlling the thermal conductivity of semiconductors is of interest to the design of thermoelectric and phononic devices. The insertion of inclusions of nanometer size in a semiconductor is an e...

Reactivity of contact metals on monolayer WS2

Abstract: Incorporating two-dimensional transition metal dichalcogenides (TMDs) into electronic and optoelectronic applications requires a fundamental understanding of metal/TMD interactions. This work applies a fast and easy approach to observe reactivity between metal contacts and monolayer (1L) WS2 via Raman spectroscopy using both destructive and non-destructive methods. We compare findings from Raman spectra collected via a backside geometry and also from mechanically exfoliated metal/WS2 films after annealing with our previously published thermodynamic predictions for reactivity of bulk materials. The disappearance of the Raman-active phonon modes for WS2 suggests the consumption of WS2 through reactions with the continuous metal film, as observed completely for Ti upon deposition and nearly completely for Al after annealing at and above 100 °C. On the other hand, the persistence of multiple Raman-active phonon modes for WS2 confirms that Au, Cu, and Pd are unreactive with WS2 upon deposition and after cumulatively annealing for 1 h at 100, 200, and 300 °C, even though unreactive metal overlayers can shift some of the peaks in the spectrum. The metal/WS2 reactivity observed in this study is in excellent agreement with predictions from bulk thermodynamics, which can provide good guidance for studies of other metal/TMD systems. In addition, using a backside geometry for collecting Raman spectra can aid in fundamental studies of interfaces with TMDs.

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.

Enhanced thermoelectric efficiency in nanocrystalline bismuth telluride nanotubes.

Abstract: We report on the thermal and thermoelectric properties of individual nanocrystalline Bi2 Te3 nanotubes synthesized by the solution phase method using 3ω method and a microfabricated testbench. Measurements show that the nanotubes offer improved ZT compared to bulk Bi2Te3 near room temperature due to an enhanced Seebeck coefficient and suppressed thermal conductivity. This improvement in ZT originates from the nanocrystalline nature and low dimensionality of the nanotubes. Domain boundary filtering of low-energy electrons provides an enhanced Seebeck coefficient. The scattering of phonons at the surface of the nanotube leads to suppressed thermal conductivity. These have been theoretically analyzed using the Boltzmann equation based on the relaxation time approximation and Landauer approach. This work clearly demonstrates the possibility of achieving enhancement in thermoelectric efficiency by combining nanocrystalline and low-dimensional systems.

First-principles study and experimental characterization of metal incorporation in germanium telluride

Abstract: Germanium telluride is a well-known phase change material (PCM) used in non-volatile memory cells and radio frequency switches. Controlling the properties of GeTe for improved PCM device performance has sometimes been achieved by doping and/or alloying with metals, often at concentrations greater than 10 at. % and using non-equilibrium methods. Since switching PCMs between the low-resistance crystalline and high-resistance amorphous states requires a heating cycle, the stability of metal-incorporated GeTe ( Ge 0.5 − x M x Te 0.5) films is also critical to practical implementation of these materials in electronic and optoelectronic devices. In this work, we use both density-functional theory and experimental characterization methods to probe the solubility and critical properties of Ge 0.5 − x M x Te 0.5 films. Using first-principles calculations, we determine the enthalpy of formation for GeTe with 2.08, 4.17, and 6.25 at. % of Cu, Fe, Mn, Mo, and Ti and show trends between the stability of the Ge 0.5 − x M x Te 0.5 systems and the atomic position, composition, and distribution of the metal atoms in the GeTe matrix. Out of all the studied systems, Mo was the only metal to cluster within GeTe. Analysis of the Ge–Te bond lengths and volumes of the Ge 0.5 − x M x Te 0.5 supercells shows that increasing the atomic concentration (2.08, 4.17, 6.25 at. %) of the different metals causes varied distortions of the crystal structure of GeTe that are accompanied by significant changes in the projected density of states. Computational predictions concerning metal solubility and the effect of metal incorporation on critical properties of GeTe are compared to experimental results in the literature (Cu, Mn, Mo, and Ti) and to transmission electron microscopy and transport data from newly characterized co-sputtered Ge 0.5 − x Fe x Te 0.5 films. The computational predictions of decreasing solubility (Mn > Cu, Fe > Ti, Mo) shows good agreement with experimental observations (Mn, Cu > Fe > Ti, Mo), and Ge 0.5 − x Fe x Te 0.5 films exhibited increased crystallization temperatures from pure GeTe.

Precision Wet Etching of ZnO Using Buffer Solutions

Abstract: Zinc oxide (ZnO) is a metal oxide semiconductor of interest for a wide range of electronic and optoelectronic device applications. Many devices require etching of ZnO structures and there have been many investigations of ZnO wet-etching processes. However, most reported etches have problems with reproducibility and especially control of vertical and lateral (or undercut) etching uniformity. In this work, we report new buffer solutions that provide controlled vertical and lateral etching of ZnO. We compare the ZnO etch characteristics of buffer etchants to a previously reported ammonium chloride etch. Using buffer etchants of suitable composition we find reaction-rate-limited, uniform, reproducible etching, and similar vertical and lateral etch rates. [2020-0009]

Quantum transport in three-dimensional metalattices of platinum featuring an unprecedentedly large surface area to volume ratio

Abstract: Three-dimensional (3D) electronic nanomaterials are less explored than their counterparts in the lower dimensions because of the limited techniques for the preparation of high-quality materials. Here we report characterization of and quantum transport measurement on 3D metalattices of Pt grown in the voids of self-assembled templates of silica nanospheres by confined chemical fluid deposition. These Pt metalattices featuring an unprecedentedly large surface area to volume ratio were found to show positive magnetoresistance (MR) at low temperatures, changing from positive to negative in low magnetic fields within a narrow temperature window as the temperature was raised. The low-field MR was attributed to the effect of quantum interference of electronic waves in the diffusive regime in three dimensions, processes known as weak localization and antilocalization. We argue that the presence of the large surface area to volume ratio results in such a strong enhancement in the electron-phonon scatterings that a change in the sign in the MR is enabled.

Vapor phase passivation of (100) germanium surfaces with HBr

Abstract: Hydrobromic acid (HBr) vapor was used to remove native oxide from an undoped (100) germanium (Ge) wafer with n-type conductivity and to passivate its surface against reoxidation. The Ge surfaces, examined by x-ray photoelectron spectroscopy, were suspended above a 48% HBr solution for 1, 10, 20, 60 min, and 24 h. A steady decrease in oxide thickness was observed for up to 60 min of HBr exposure time. Beyond this time, little or no difference was observed, and the wafers maintained subnanometer levels of oxidation. The stability of the passivated surface was then measured with each treated sample exposed to ambient air for 19 h. The samples exposed to HBr for 60 min or less reoxidized when exposed to air, while the sample previously exposed to HBr for 24 h showed levels of oxidation similar to the freshly passivated wafer, thereby demonstrating HBr vapor-phase treatment as a simple method for passivating Ge.

Electron beam evaporated Au islands as a nanoscale etch mask on few-layer MoS2 and fabrication of top-edge hybrid contacts for field-effect transistors.

Abstract: Metal contacts to two-dimensional layered semiconductors are crucial to the performance of field-effect transistors (FETs) and other applications of layered materials in nanoelectronics and beyond. In this work, the wetting behavior of very thin Au films on exfoliated MoS2 flakes was studied and evaluated as a nanoscale, self-assembled dry etch mask. Etching nanoscale pits into MoS2 flakes prior to metallization from the top of the flake forms edge sites that contribute some fraction of edge contacts in addition to top contacts for additional carrier injection and lower contact resistance. The morphology and thickness of Au islands and MoS2 were studied with scanning electron microscopy and atomic force microscopy before and after etching with low-power plasmas. A Cl2 plasma etch of 10 s with a Au island mask of 6 nm (nominal) showed the best resulting morphology among the plasma conditions studied. Back-gated MoS2-based FETs on SiO2/p +-Si with Ti/Au contacts were fabricated using a Cl2 etch of only the contact regions, and they yielded devices with ON currents of 100s µA/µm, ON/OFF ratios ⩾106, and contact resistance <10 kΩ µm. The best set of devices had a very low contact resistance of ∼1 kΩ µm with almost no dependence of contact resistance on gating. Using nanoscale etch masks made from metal islands could be highly customizable and shows promise for engineering FETs with low contact resistance.

Scanning Transmission Electron Tomography and Electron Energy Loss Spectroscopy of Silicon Metalattices

Abstract: Transmission electron microscopy, scanning transmission electron tomography, and electron energy loss spectroscopy were used to characterize three-dimensional artificial Si nanostructures called "metalattices", focusing on Si metalattices synthesized by high-pressure confined chemical vapor deposition in 30-nm colloidal silica templates with ~7 and ~12 nm "meta-atoms" and ~2 nm "meta-bonds". The "meta-atoms" closely replicate the shape of the tetrahedral and octahedral interstitial sites of the face-entered cubic colloidal silica template. Composed of either amorphous or nanocrystalline silicon, the metalattice exhibits long-range order and interconnectivity in two-dimensional micrographs and three-dimensional reconstructions. Electron energy loss spectroscopy provides information on local electronic structure. The Si L2,3 core-loss edge is blue-shifted compared to the onset for bulk Si, with the meta-bonds displaying a larger shift (0.55 eV) than the two types of meta-atoms (0.30 and 0.17 eV). Local density of state calculations using an empirical tight binding method are in reasonable agreement.