Electronic Coatings Lecture Slides PDF

Functional Coatings 4. Electronic coatings Prof. Dr. Tobias Kraus Wintersemester 2024/25 Lecture Functional Coatings 1 Electronic coatings Properties Analytics Materials Technology...

Functional Coatings 4. Electronic coatings Prof. Dr. Tobias Kraus Wintersemester 2024/25 Lecture Functional Coatings 1 Electronic coatings Properties Analytics Materials Technology Examples Organic light-emitting diodes www.licht-plattform.org Electrons and holes annihilate in a conducting polymer and emit light. Electrode layers must be optically transparent, electrically conductive, suitable in band structure. LG Wintersemester 2024/25 Lecture Functional Coatings 2 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Thin film solar cells Photons create charge carrier pairs at p-n junctions that are extracted by electrodes. Diode-type cell Bulk heterojunction Konarka Wintersemester 2024/25 Lecture Functional Coatings 3 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Sheet resistance The 2D analogy to the conventional resistance of a 1D wire. Sheet resistance is the ratio of (the potential gradient parallel to the current) to (the current density times the film thickness). 𝜀............ Field strength [V m-1] V............ Potential [V] L............ Length [m] j............ Current density [C m-2s-1] I............ Current [A] A............ Cross-sectional area [m2] ρ............ Resistivity [Ω m] R............ Resistance [Ω] Wintersemester 2024/25 Lecture Functional Coatings 4 Wintersemester 2024/25 Lecture Functional Coatings Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Sheet resistance Quantifies electrical conductivity of thin films with homogeneous thickness and no boundary effects. Can be directly obtained from 4-point probe measurements. R............ Resistance [Ω] ρ............ Resistivity [Ω m] L............ Length [m] A............ Cross-sectional area [m2] W............ Width [m] t............ Film thickness [m] direction of current R☐............ Sheet resistance [Ω] Wintersemester 2024/25 Lecture Functional Coatings 5 Electronic coatings Properties Analytics Materials Technology Examples Sheet resistance Quantifies electrical conductivity of thin films with homogeneous thickness and no boundary effects. Can be directly obtained from 4-point probe measurements. R............ Resistance [Ω] “Symbolic” units are often used to indicate that this ρ............ Resistivity [Ω m] resistance is defined for a thin conductive sheet L............ Length [m] geometry: A............ Cross-sectional area [m2] W............ Width [m] [Ω/□], [Ω/sq], [Ω/2], [ohm/sq], [Ω□], etc t............ Film thickness [m] R☐............ Sheet resistance [Ω] Wintersemester 2024/25 Lecture Functional Coatings 6 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Sheet resistance The classical free electron model indicates what sets resistivity, and therefore, sheet resistance, in metals: j............ Current density [C m-2s-1] ne............ Charge carrier density [m-3] ve............ Charge carrier velocity [m s-1] e............ Unit charge [C] me............ Electron mass [kg] vf............ Steady-state velocity [m s-1] 𝜀............ Field strength [V m-1] ρ............ Resistivity [Ω m] τ............ Relaxation time [s] Wintersemester 2024/25 Lecture Functional Coatings 7 Wintersemester 2024/25 Lecture Functional Coatings Wintersemester 2024/25 Lecture Functional Coatings Wintersemester 2024/25 Lecture Functional Coatings Wintersemester 2024/25 Lecture Functional Coatings Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Sheet resistance: measurement 2 point probe 4 point probe Simple geometry Normed method: ASTM F390 Constant current source, measure voltage Apply current outside Problems with highly conductive Measure potential difference inside layers (100 Ω typical limit) Wintersemester 2024/25 Lecture Functional Coatings 8 Electronic coatings Properties Analytics Materials Technology Examples Sheet resistance: measurement 4 point probe Works reliably between 10-2 Ω/□ to 104 Ω/□ Recommended: use toolmaker’s microscope to measure exact distance between electrodes (from their imprints) or a precision setup Geometrical correction necessary for finite sample sizes (that are on the order of the electrode spacing) Bridge Technology Bridge Technology Wintersemester 2024/25 Lecture Functional Coatings 9 Electronic coatings Properties Analytics Materials Technology Examples Contact resistance The electronic characteristics of an interface depend on the band structure of the contacting materials. Thin, unwanted (oxide) layers can govern the overall contact! Tunnel barrier contacts Barrier in both direction. Caused by thin insulating layers between (semi)conductors, e.g. air gaps from roughness or oxide layers on metals. Ohmic contacts No barrier in either direction. Form between clean metals and some metal-semiconductor pairings. Schottky barrier contacts It............ Tunnel current [A] κ............ Decay length [m-1] Asymmetric “rectifying” contacts. d............ Tunneling distance [m] Wintersemester 2024/25 Lecture Functional Coatings 10 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Contact resistance Ohmic contacts No barrier in either direction. Form between clean metals and some metal-semiconductor pairings. Schottky barrier contacts Asymmetric “rectifying” contacts. Form between certain metal-semiconductor pairings. In practice, also metal-doped insulators. Tell-tale sign: very asymmetric I-V curves. Wintersemester 2024/25 Lecture Functional Coatings 11 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Contact resistance Ohmic contacts No barrier in either direction. Form between clean metals and some metal-semiconductor pairings. Schottky barrier contacts Asymmetric “rectifying” contacts. Form between certain metal-semiconductor pairings. In practice, also metal-doped insulators. Tell-tale sign: very asymmetric I-V curves. Wintersemester 2024/25 Lecture Functional Coatings 12 Electronic coatings Properties Analytics Materials Technology Examples Alternative sheet conductivity measurements RF eddy current method Suitable for 0.1 Ω < R☐ < 100 kΩ Oscillating tank circuit (an RF coil and a capacitor in parallel) induces eddy currents in the sample that dissipate the electro-magnetic energy supplied by the RF current generator. The power absorbed in this process (assuming no flux leakage and negligible skin effects) is proportional to the sheet conductance of the sample. Olympus Absolute sheet conductance is obtained by calibration with a reference material. van der Pauw method Electrodes on the film are arranged at four corners of a rectangle or in a cloverleaf geometry. Two resistances are measured (along horizontal and vertical edges). The sheet resistance follows for arbitrary geometries. Tektronix Wintersemester 2024/25 Lecture Functional Coatings 13 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Thin-film strain gauge (Prof. Schultes, ZEMA) Cambridge University Measurement time in s Wintersemester 2024/25 Lecture Functional Coatings 14 Electronic coatings Properties Analytics Materials Technology Examples Thin-film strain gauge (Prof. Schultes, ZEMA) Wintersemester 2024/25 Lecture Functional Coatings 15 Electronic coatings Properties Analytics Materials Technology Examples Thin-film strain gauge (Prof. Schultes, ZEMA) New nanocomposite film material Ni:a-C:H that provides large strain sensitivity (gauge factors) and a small temperature coefficient. Ni clusters encapsulated by graphene sheets. Wintersemester 2024/25 Lecture Functional Coatings 16 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Temperature dependence of conductivity Metals Conductivity in metals is very sensitive to the microstructure. Temperature-dependant measurements show the contributions: Matthiessen‘s rule: T............ Temperature [K] α............ Temp. coefficient of resistivity [Ω m K-1] ρ............ Total resistivity [Ω m] ρth............ Thermally induced „ideal“ resistivity [Ω m] ρimp............ Resistivity from impurities [Ω m] ρdef............ Resistivity from defects [Ω m] [Hummel1985] ρres............ Residual resistivity [Ω m] Wintersemester 2024/25 Lecture Functional Coatings 17 Electronic coatings Properties Analytics Materials Technology Examples Temperature dependence of conductivity Semiconductors In doped semiconductors, contributions from charge carrier density and charge carrier mobilities: decrease of ρ due to the increasing amount of charge carriers high T: intrinsic conductivity dominates increase of ρ due to decreased mobility by phonon scattering Wintersemester 2024/25 Lecture Functional Coatings 18 Electronic coatings Properties Analytics Materials Technology Examples Pt100 resistive thermosensor (RTD) Wires or PVD-deposited thin films from (mainly) Pt with known linear temperature coefficients of resistivity (Pt100 ≙ 100 Ω at 0°C). Increasingly connected with 4-wire geometry! Wikimedia www.directindustry.de www.thermibel.de Wintersemester 2024/25 Lecture Functional Coatings 19  Insert Pixel micrographs here iPhone Huawei Wintersemester 2024/25 Lecture Functional Coatings Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Addressing pixels: passive and active matrix Displays, imaging detectors, touch-sensitive surfaces, and other devices need to address specific parts of a coating (“pixels”). Most straightforward is a “passive matrix”: a pattern of criss-crossing strip electrodes. Passive matrix: two layers of line electrodes, [Brotherton1995] pixels sit at crossings Wintersemester 2024/25 Lecture Functional Coatings 20 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Addressing pixels: passive and active matrix Passive matrices suffer from two limitations in performance: 1. Cross-talk between pixels that reduces contrast, 2. Slow switching due to capacitance (RC) that limits video rates. An “active matrix” with transistors can solve the problems: replace by FET: C R C R1 “Field effect transistor” “active” circuit FET R............ Resistance [Ω] R2 C............ Capacitance [F] τRC............ Characteristic time [s] Wintersemester 2024/25 Lecture Functional Coatings 21 Wintersemester 2024/25 Lecture Functional Coatings Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Addressing pixels: passive and active matrix Passive matrices suffer from two limitations in performance: 1. Cross-talk between pixels that reduces contrast, 2. Slow switching due to capacitance (RC) that limits video rates. An “active matrix” with transistors can solve the problems: “Passive matrix” “Active matrix” Wintersemester 2024/25 Lecture Functional Coatings 22 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Thin film transistors (TFT) Transistor-based “active matrix” arrangements hold the pixel states actively. Small or transparent thin film transistors are suitable for display applications. P.G. LeComber, W.E. Spear, A. Gaith, Electron. Lett. 15 (1979) 179 Wintersemester 2024/25 Lecture Functional Coatings 23 Wintersemester 2024/25 Lecture Functional Coatings Electronic coatings Properties Analytics Materials Technology Examples Thin film transistors (TFT) Modern TFT often contain αSi:H, ITO, and Si3N4. CdSe and p-Si are phasing out. All-transparent and organic TFT exist and are in development. [Kuo2013] [BraithwaiteWeaver1990] Wintersemester 2024/25 Lecture Functional Coatings 24 Electronic coatings Properties Analytics Materials Technology Examples Thin film transistors (TFT) The speed is set by the mobility of the charge carriers in the FET semiconductor, often a strong function of deposition parameters. Wintersemester 2024/25 Lecture Functional Coatings 25 Electronic coatings Properties Analytics Materials Technology Examples Indium tin oxide (ITO) A mixture of about 90% In2O3 and 10% SnO2 is the commercially prevalent transparent conductive oxide. It is an n-type semiconductor with a 4 eV band gap. Pale yellow to green-yellow as a powder 180 nm ITO ≤10Ω/⎕ (depending on SnO2 content) Excellent, flat films deposited by PVD and subsequent O2 annealing for stoichiometry Ceramic, brittle material Applications: displays, ohmic heating of windows, IR management on windows, sensors and detectors… Alternative coating: layers of ITO nanoparticles Wintersemester 2024/25 Lecture Functional Coatings 26 Electronic coatings Properties Analytics Materials Technology Examples Wet deposition of ITO (Sabine Heusing, INM) Approach: Development of ITO nanoparticles and a dispersion of ITO nanoparticles for an ITO ink to be deposited and patterned by gravure printing and annealed at low temperatures (< 130°C, e.g. by UV treatment). Principle: Use of crystalline, conducting ITO (In2O3:Sn) nanoparticles (10...30 nm) * Separation of crystallisation and film formation  Low temperature treatment/ UV curing possible  Low light scattering Use of a binder with the properties: „glueing“ of the nanoparticles and adhesion to the substrate Improved mechanical resistance and flexibility Wintersemester 2024/25 Lecture Functional Coatings 27 Electronic coatings Properties Analytics Materials Technology Examples Wet deposition of ITO Coating (e.g. spin UV or thermal coating, printing) treatment Wintersemester 2024/25 Lecture Functional Coatings 28 Electronic coatings Properties Analytics Materials Technology Examples Wet deposition of ITO Metal printing plate with different patterns: 110, 160 and 210 lines/cm (printing area: 12 × 13 cm2) Magnification of the printing plate: ink is taken up by diamond-engraved cells Wintersemester 2024/25 Lecture Functional Coatings 29 Electronic coatings Properties Analytics Materials Technology Examples Wet deposition of ITO 2 UV energy [J/cm ] Gravure printing of ITO ink on 4.7 9.4 23.4 14 18.7 PET and PEN foil: PEN-Foil  use of printing plates with 10 various line densities R/sq [k/sq]  Layer thickness increases with line density thickness decreasing line density: [lines/cm] [nm] 110 710 nm 710 nm (110 lines/cm) 160 210 470 nm 330 nm 470 nm (160 lines/cm) 1 330 nm (210 lines/cm) 0 2 4 6 8 10 UV treatment  R/sq decreases with increasing UV energy and increasing layer thickness  R/sq decreases after post treatment (forming gas N2/H2) at 120 °C and 180 °C Wintersemester 2024/25 Lecture Functional Coatings 30 Electronic coatings Properties Analytics Materials Technology Examples Wet deposition of ITO 100 100 UV VIS NIR PET foil 80 80 Transmission [%] Transmission [%] PET foil 60 60 ITO on PET foil 40 ITO on PET foil 40 (710 nm thick, (710 nm thick, 110 lines/cm) 20 20 110 lines/cm) 0 0 500 1000 1500 2000 2500 3000 300 400 500 600 700 800 wavelength [nm] wavelength [nm] Transmission of coating  High transmission in the divided by uncoated PET foil: visible range (380 – 780 nm) 400 nm: 95 %  Low transmission T in NIR range 550 nm: 99 % T1500 nm 780 nm: 95 % Wintersemester 2024/25 Lecture Functional Coatings 31 Electronic coatings Properties Analytics Materials Technology Examples Wet deposition of ITO Transmission in UV-VIS-NIR range* Reflectance in UV-VIS-NIR range 100 100 100 vacuum deposited ITO coating on PET foil 80 R/sq: 80 80 ITOvac: 0.250 kΩ/sq Transmission [%] printed ITO: 2.5 kΩ/sq Reflectance [%] 60 60 60 printed ITO coating on PET foil 40 40 40 printed ITO coating vacuum deposited ITO 20 20 coating on PET foil 20 0 0 0 500 1000 1500 2000 2500 3000 500 1000 1500 2000 2500 3000 wavelength [nm] wavelength [nm] * Transmission spectra were measured vs air as reference Wintersemester 2024/25 Lecture Functional Coatings 32 Electronic coatings Properties Analytics Materials Technology Examples Wet deposition of ITO White light interferometer (WLI) image of gravure printed ITO lines on PET foil. Minimal line width achieved: 100 µm J. Puetz et al., Thin Solid Films 516 (2008) 4495 Wintersemester 2024/25 Lecture Functional Coatings 33 Electronic coatings Properties Analytics Materials Technology Examples Other transparent conductive coatings Graphene A 2D crystal of sp2- hybridiced carbon, down to one atom thick Silver nanowires Silver wires with diameters below 100nm, layers of random wire arrays D. Langley et al., Flexible transparent conductive materials based on silver nanowire networks: a review, Nanotechnology, 2013, 24, 452001. Cambrios Wintersemester 2024/25 Lecture Functional Coatings 34 Electronic coatings Literature *[Hummel 2001] R. E. Hummel: Electronic properties of materials. New York, Berlin, Heidelberg: Springer 2001 A useful introduction and reference, available as PDF. [BraithwaiteWeaver1990] N. Braithwaite, G. Weaver, Electronic Materials, The Open University, 1990 An introductory-level textbook on some materials relevant for electronics, broad in scope. [Brotherton1995] S. D. Brotherton, Polycrystalline silicon thin film transistors. Semicond. Sci. Technol. 10 (1995) 721–738 An overview of the different TFT types. [Hilsum2010] C. Hilsum, Flat-panel electronics displays: a triumph of physics, chemistry and engineering. Phil. Trans. R. Soc. A 368 (2010) 1072–1082 More recent overview on the (not only material) challenges posed by modern displays, covering the problem of pixel addressing. With a strong focus on the history of the technology. [Kuo2013] Y. Kuo, Thin Film Transistor Technology—Past, Present, and Future. The Electrochemical Society Interface. Spring 2013, 55–61 Reviews the introduction of TFTs into display technology. Wintersemester 2024/25 Lecture Functional Coatings 35