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Tin(ii) monosulfide (SnS) is a layered, anisotropic material that is of interest as a two-dimensional semiconductor for opto-electronic, thermoelectric, and piezoelectric applications. In this study, the effect of work function on contact behavior was investigated. Ni/Au, Pd/Au, Cr/Au, and Ti/Au contacts were fabricated onto individual, solution-synthesized, p-type SnS nanoribbons. The lower work function metals (Cr and Ti) formed Schottky contacts, whereas the higher work function metals (Ni and Pd) formed ohmic or semi-ohmic contacts. Of the ohmic contacts, Ni was found to have a lower contact resistance (∼10 Ω cm or lower) than Pd (∼10 Ω cm or lower). Both the calculated Schottky barriers (0.39 and 0.50 eV) for Cr and Ti, respectively, and the ohmic behavior for Ni and Pd agree with behavior predicted by Schottky-Mott theory. The results indicate that high work function metals should be considered to form low resistance contacts to SnS multilayers.

Solution-processed silver nanowire (Ag-NW) network structures are candidate materials for application as flexible transparent conductors in a wide range of flexible electronics. However, challenges remain in fabricating these materials in an efficient and scalable manner in addition to making them mechanically robust. Here, a method based on the direct processing of Ag-NW network structures from aqueous polymer solutions is presented that significantly increases the uniformity of the NW networks and lowers the percolation threshold, thus resulting in an increased figure of merit (percolative FoM > 40) of Ag-NW based transparent conductors. Direct polymer solution processing also reduces the batch-to-batch variability of optical and electrical properties of the NW networks to <5% of the mean (as compared to 10-20% variability for pristine Ag-NW networks), and significantly improves their resistance to mechanical deformation. Our results further indicate that semicrystalline polymers such as poly (vinyl alcohol) (PVA) could serve as viable and benign alternatives to conducting polymer matrices (such as PEDOT:PSS) that pose a challenge to device fabrication due to their corrosive properties.Transparent conductors (TCs) are ubiquitous in a host of civil and military applications, including transparent electrical contacts in photovoltaic devices, antireflection coatings, heated glass for aircraft and automobile windows, heat reflecting mirrors for glass and incandescent bulbs, electrochromic devices and smart windows, electrodes for LCD displays, to name a few. Degenerately doped oxides of indium, tin and zinc (TCOs), providing resistivities <10 −4 cm and optical transmittances >95%, 1,2 constitute the mainstream TC material for commercial applications. However, high process temperatures, expensive instrumentation, high interfacial contact resistances and limited supply of raw materials have become barriers to the application of this class of materials. 3 In the emerging field of "plastic electronics", brittleness and cost are additional limitations of TCOs.The limitations of established TCOs have thus fueled research in alternate material systems for use in such components. Prominent candidate materials include carbon nanotube (CNT) films, 4,5 graphene, 6,7 metallic microstructures 8-10 and random networks of metal nanowires (NWs), particularly those of Ag and Cu, 11-13 and polymer conductors like poly(ethylene dioxythiophene):poly(styrene sulfonate) (PE-DOT:PSS), derivatives of poly-pyrrole (PPy), poly-fluorene (PF), poly-aniline (PANi), and similar conjugated systems. [14][15][16] While each of the aforementioned material systems has its advantages, no single "perfect solution" has been identified. For example, in the case of CNT films and graphene, the balance between electrical resistivity and optical transparency is generally seen to be a challenge, due to grain boundaries and high contact resistances. 17 Metal gratings or nanowire array structures can be precisely fabricated using lithographic patterning, but p...

The ultra-wide bandgap semiconductor gallium oxide (Ga2O3) offers substantial promise to significantly advance power electronic devices as a result of its high breakdown electric field and maturing substrate technology. A key remaining challenge is the ability to grow electronic-grade epitaxial layers at rates consistent with 20–40 μm thick drift regions needed for 20 kV and above technologies. This work reports on extensive characterization of epitaxial layers grown in a novel metalorganic chemical vapor deposition tool that permits growth rates of 1.0–4.0 μm h−1. Specifically, optical, structural and electrical properties of epilayers grown at ∼1 μm h−1 are reported, including employment in an operating MOSFET. The films demonstrate relatively smooth surfaces with a high degree of structural order, limited point defectivity (Nd − Na ≈ 5 × 1015 cm−3) and an optical bandgap of 4.50 eV. Further, when employed in a MOSFET test structure with an n+ doped channel, a record high mobility for a transistor structure with a doped channel of 170 cm2 V−1 s−1 was measured via the Hall technique at room temperature. This work reports for the first time a β-Ga2O3 MOSFET grown using Agnitron Technology’s high growth rate MOCVD homoepitaxial process. These results clearly establish a significant improvement in epilayer quality at growth rates that can support future high voltage power device technologies.

As part of a Special Issue in Honor of 30 years of the American Vacuum Society’s Nellie Yeoh Whetten Award, this Invited Perspective discusses results and trends from the authors’ and other published research on metal contacts to β-Ga2O3, (4H and 6H)-SiC, nanocrystalline diamond (NCD), and nanocrystalline thin films and single-crystalline nanoribbons of α-SnS. The paper is not a comprehensive review of research on contacts to each of these semiconductors; it is instead a perspective that focuses on Schottky barrier height (Φb) measurements and factors that affect Φb, such as metal work function (Φm) and crystallographic surface plane. Metals and the associated processing conditions that form ohmic or Schottky contacts to each of these semiconductors are also described. Estimates of the index of interface behavior, S, which measures the dependence of Φb on Φm, show large variations both among different semiconductors (e.g., S ∼ 0.3 for NCD and S ∼ 1.0 for SnS nanoribbons) and between different surface planes of the same semiconductor [e.g., (2¯01) vs (100) Ga2O3]. The results indicate that Φb is strongly affected by the nature of the semiconductor surface and near-surface region and suggest that the sharp distinction between covalent and ionic semiconductors as described in seminal theories can be adjustable.

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Nanocrystalline tin sulfide (SnS) thin films were deposited by electron-beam evaporation at growth temperatures ranging from room temperature to 300 °C and characterized prior to and after annealing at 300 °C in high vacuum. X-ray diffraction and Raman spectroscopy results indicated that SnS films deposited at 100 and 200 °C contained predominately a mixture of orthorhombic α-SnS and cubic π-SnS phases, whereas only α-SnS was detected in SnS films deposited at 300 °C. Contacts with a range of work functions were deposited onto p-type α-SnS films. All of the contacts investigated (Ti/Au, Ru/Au, Ni/Au, and Au) were ohmic as-deposited and yielded average specific contact resistance values that decreased with increasing metal work function, suggesting that the barrier height has at least a partial dependence on the work functions of the metals. Annealing at 350 °C for 5 min in Ar reduced the specific contact resistance value for Ru/Au contacts, resulting in the lowest value (1.9 × 10−3 Ω cm2) of contacts investigated to SnS thin films.

Electron emission from quasi-freestanding bilayer epitaxial graphene (QEG) on a silicon carbide substrate is reported, demonstrating emission currents as high as 8.5 µA, at ~200 °C, under 0.3 Torr vacuum. Given the significantly low turn-on temperature of these QEG devices, ~150°C, the electron emission is explained by phonon-assisted electron emission, where the acoustic and optical phonons of QEG causes carrier acceleration and emission. Devices of differing dimensions and shapes are fabricated via a simple and scalable fabrication procedure and tested. Variations in device morphology increase the density of dangling bonds, which can act as electron emission sites. Devices exhibit emission enhancement at increased temperatures, attributed to greater phonon densities. Devices exhibit emission under various test conditions, and a superior design and operating methodology are identified.