Semiconductor Contacts for Nanoelectronics: SnS Nanoribbons, SnS Thin Films, and Silver Nanowire-Based Transparent Conductors
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Metal-semiconductor contacts are ubiquitous in electronic and optoelectronic devices. However, control over the properties and performance of metal-semiconductor contacts is often difficult to achieve. Reductions in dimensions and the need for low-cost materials for optoelectronic applications have introduced new semiconductors to which contact behavior must be understood. The bulk of this thesis concerns the behavior of contacts to tin(II) sulfide (SnS), a layered semiconductor that is of interest for two-dimensional electronics and as an earthabundant, low-toxic absorber material for thin film solar cells. Contact metals with a range of work functions were characterized on two forms of SnS: (1) individual, solution-synthesized, ptype SnS nanoribbons and (2) electron-beam evaporated, polycrystalline SnS thin films. Lower work function metals (Cr/Au and Ti/Au) formed Schottky contacts on SnS nanoribbons, whereas higher work function metals (Ni/Au and Pd/Au) formed ohmic or semiohmic contacts. Schottky barrier heights and ideality factors of Cr/Au and Ti/Au contacts were calculated by fitting current-voltage measurements to a back-to-back Schottky diode model. Specific contact resistance values for Ni/Au (≤10−4 Ω cm2) and Pd/Au (≤10−3 Ω cm2) were calculated from TLM and contact end resistance measurements on individual nanoribbons. The calculated Schottky barriers for Cr/Au (~0.39 eV) and Ti/Au (~0.50 eV) and the ohmic behavior of Ni/Au and Pd/Au correspond well with behavior predicted by Schottky–Mott theory. Nanocrystalline SnS thin films were deposited by electron-beam evaporation. A substrate temperature of 300°C followed by a 300°C anneal produced the α-SnS phase, whereas Raman spectroscopy and x-ray diffraction indicated a mixture of π -SnS and α-SnS phases in films deposited at 100°C and 200°C. Ti/Au, Ru/Au, Ni/Au, and Au as-deposited metallizations formed ohmic contacts to α-SnS thin films. Average specific contact resistances measured with circular transfer length method (CTLM) structures were found to decrease with increasing metal work function from Ti/Au, Ru/Au, Ni/Au, to Au. After annealing the contacts at 350°C in argon, Ru/Au had the lowest average specific contact resistance of 1.9 x 10-3 Ω-cm2 . Ni/Au and Ti/Au contacts were found to be unstable after annealing. Solution-processed, silver nanowire (Ag NW)-polymer composites were also investigated as alternatives to contacts based on transparent conducting oxides. Solution-processed Ag NWs dispersed in polymers such as poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) demonstrated improved film uniformity relative to those dispersed in isopropanol. Whereas PEDOT:PSS reduced the transmittance of the films, two non-conductive, transparent polymers (polystyrene sulfonate (PSS) and polyvinyl alcohol (PVA)) resulted in NW films with improved figures of merit for transmittance and conductance. The Ag NWs formed rectifying contacts to Si prior to annealing. The lowest interfacial resistance of 90 nm-diameter Ag NWs on Si was observed after annealing at 650°C for 15 sec. Fragmentation at Ag NW junctions occurred at temperatures as low as 200°C for 35-nm diameter Ag NWs and precluded the formation of ohmic contacts.