Lecture 7 - Optical Properties of Materials PDF

Lecture 7 – Optical properties of materials Prof. Monica Morales-Masis Advanced Materials – M6 December 14 2023 0  Why do we study optical properties of materials? Optical properties of materials:...

Lecture 7 – Optical properties of materials Prof. Monica Morales-Masis Advanced Materials – M6 December 14 2023 0  Why do we study optical properties of materials? Optical properties of materials: Materials response to interaction with electromagnetic radiation Display Lighting Energy Harvesting Touch screens OLEDs TFTs Low e-windows Photovoltaics 1 A white light beam experiences both refraction and dispersion as it passes through the triangular glass prism. Refraction occurs when the direction of the light beam is bent at both glass-air prism interfaces (i.e., as it passes into and out of the prism). Dispersion (chromatic) occurs when the degree of bending depends on wavelength (i.e., the beam is separated into its component colours). 2 Contents of this Lecture Electromagnetic Radiation Light Interaction with Solids Transmission Reflection Absorption Optical Coefficients Absorption coefficient Lambert Beer Law Refractive Index Refraction and Reflection of Light Optical Materials (examples) Absorption of Light and Band Gap Summary 3 The content of this lecture is based on: Chapter 1 (and part of Ch.3) of Optical Properties of Solids – Second edition Mark Fox https://www.vitalsource.com/referral?term=9780191576720 Chapter 14 (and part of Ch.11) of Understanding Solids – Second edition Richard J. D. Tilley 4  Electromagnetic radiation Wavelike: consisting of electric and magnetic field components perpendicular to each other and in the direction of propagation All electromagnetic radiation travels at the same velocity, the velocity (speed) of light: c Photon energy Photons: light as a particle with specific energy 5 The electromagnetic spectrum The visible part is just a small part of the whole spectrum 6  Light Interaction with Solids at the Macroscopic level Photons incident on the surface of a material will be either: Reflected, Propagated, or Transmitted The phenomena that can occur while light propagates are: refraction absorption luminescence scattering 7 The phenomena that can occur while light propagates are: Refraction: light propagates with smaller velocity than in free space. This leads to bending of light rays as described by Snell’s law of refraction Absorption: occurs if the frequency of the light is resonant with the transition frequency of the atoms in the medium. In this way the beam is attenuated as it progresses. Selected absorption is responsible for the colouration of many optical materials. Scattering: phenomenon in which light changes direction and possible also its frequency as it interacts with the medium. Elastic scattering: frequency of the scattered light is unchanged Inelastic scattering: frequency changes in the process 8  Light Interaction with Solids at the Macroscopic level Photons incident on the surface of a material will be either: Reflected, Propagated, or Transmitted How to describe this The phenomena that can occur while light with Optical propagates are: Coefficients? refraction absorption luminescence scattering 9 Light Interaction with Solids at the Macroscopic level T+R+A=1 (matter) T + R + A = 100% IR T: transmittance R: reflectance IT A: absorptance I0 (light) IA 10  Optical Coefficients: Absorption coefficient The absorption of light by a material is quantified by its absorption coefficient ‘a’. ‘a’ is defined as the fraction of the intensity of light absorbed in a unit length of the material (e.g. a semiconductor) which can be integrated to obtain: Beer-Lambert law z z=0 Beer-Lambert law shows that the intensiy of light in absorbing medium decays exponentially 11 Beer-Lambert Law Important: a depend on frequency or wavelength! But how do we use it in terms of transmitance (T) and reflectance (R)? 12  Beer-Lambert Law, Transmission and Reflection (plate, thickness l) The intensity of the absorbed radiation (light) depends on the IR material and the thickness, Coherent light (common case for thin films) following B-L law: IT l 0 or a -> 0 , therefore the ñ ~ n. n is 1.77 for sapphire 3. The dip in the transmission in the IR (~ 3um) and the sharp drop at ~ 6 um is caused by vibrational absorption (phonon absorption or lattice absorption) 4. The sharp drop in transmission in the UV region (l < 200 nm) is the fundamental absorption edge. It is determined by the band gap of the insulator (or semiconductor). 33 Optical Materials Case I: Crystalline Insulators and Semiconductors Transparent in the visible range Transparent in the NIR - IR What properties of CdSe can we extract from the T spectra? 34 Absorption of Light 35  Light Interaction with Solids at the Atomic Level Light interaction in solids involve interaction with atoms, ions and/or electrons Two of the most important interactions are Electronic Polarization and Electron Transitions Electronic Polarization Results from distortion of an atomic electron cloud by an electric field Consequences of it: 1. Absorption of radiation energy 2. Decrease in velocity of light waves as they pass through the medium (refraction) 36  Light Interaction with Solids at the Atomic Level Electron Transitions Absorption and emission of electromagnetic radiation may involve electron transitions from one energy state to another. The example below considers an isolated atom. An electrom may be excited from one ocupied energy state to a vacant and higher energy state by absorption of a photon Photon energy 37 Optical Properties of Non-Metals vs Metals 38 Optical Properties of Semiconductors Interband absorption Ei: energy of the electron in the lower band Ef: energy of the electron in the upper band : photon energy Eg: band gap Note: the minimum value of Ef - Ei is Eg. Interband transitions are not possible unless the photon energy (. ) is higher than Eg 39 Optical Properties of Semiconductors Interband absorption For a material with a band gap above 3 eV, no visible light is absorp Condition for absorption of a photon: photon energy higher than the Eg Eg = band gap 40 Optical Properties of Semiconductors Interband absorption Minimum possible Eg energy for absorption of visible light: Eg = band gap Materials with Eg < 1.8 eV are opaque (all visible light is absorbed) Materials with 1.8 eV < Eg < 3 eV, some part of the visible light is absorbed, i.e. the materials are semitransparent (and colored). Materials with Eg > 3 eV are transparent 41  Optical Properties of Semiconductors Interband absorption DIRECT BAND GAP INDIRECT BAND GAP 42  Optical Properties of Semiconductors Band edge absorption DIRECT BAND GAP INDIRECT BAND GAP Phonon energy 43 Tauc plot to extract the optical band gap from absorption coefficient Example for a direct band gap semiconductor  Optical Properties of Metals (absorption) Metals are opaque because the incident radiation having frequencies within the visible range excites electrons into unocupied energy states above the Fermi energy, resulting in absorption of the incident radiation. 45 Optical Properties of Metals (absorption) Total absorption is within a very thin outer layer, usually very few µm 46 Light Absorption in crystalline silicon (example) Wavelength l (µm) 1.24 0.62 0.41 0.35 α = absorption coefficient, and dp = penetration depth in crystalline silicon a (cm-1) 5 10 4 300K Infrared 10 Absorption coefficient 1 10 3 λ = 1.05 μm α = 10 cm-1 dp = = 1000 μm α 2 Violet 10 1 10 λ = 0.41 μm α = 105 cm-1 dp = = 0.1 μm α 1 -1 10 FIG 048.1 1.0 1.5 2.0 2.3 2.5 3.5 Photon energy (eV) 47 Absorption depends on the color blue green red IR 48  Summary The propogation of light through a medium is quantified by the complex refractive index (ñ) The real part of the refractive index determines la velocity of light in the medium The imaginary part determines the absorption coefficient Lamberts Beer’s law shows that the intensity of light in absorbing medium decays exponentially The transmission of a sample can be determined by the reflectivities of the surfaces and the absorption coefficient Interband transitions occur when electrons jump to an excited state band by absorption of photons. The absorption process may be considered as the creation of electron-hole pairs Interband absorption is only possible if the photon energy exceeds the band gap energy 49 Exercises Tilley (Chapter 14 second edition/Chapter 12 third edition): 14.10, 14.11, 14.16, 14.22, 14.25, 14.27, 14.31, 14.34 50 Solar Cells Electrical Properties of Materials Determined by Band Structure Insulator Degenerate Semiconductor Metal Semiconductor EF Transparent Non-Transparent We use them all to make solar cells!  The Solar Cell ‘Light-Absorber’ Semiconductor Sunlight (Eph = hv) 1 electrons holes semiconductor Solar spectrum on the earth surface AM 1.5G = 1.5 air mass (AM) global (G) Absorption of a photon (with energy Eph = hv) in a semiconductor https://www.pvlighthouse.com.au with bandgap (EG) leads to the generation of an electron-hole pair. Figure from Book: Solar Energy, the physics and engineering of photovoltaic conversion technologies and systems. Smets, A., Jäger, K. et al. Cambridge UIT  The working principle of a solar cell sunlight 3 1 3 electrons holes 2 5 electrons holes 4 1. Absorption of a photon leads to the generation of an electron-hole pair. 2. Electrons and holes will recombine. 3. With semipermeable membranes (n-type and p-type materials) the electrons and holes can be separated. 4. Separated electrons are used to drive an electric circuit. 5. After electrons have passed through the circuit they will recombine with holes. Reference: Solar Energy, the physics and engineering of photovoltaic conversion technologies and systems. Smets, A., Jäger, K. et al. Cambridge UIT  Absorption spectra of thin film materials for Solar Cells Emerging Photovoltaic Technologies, Joel Jean and Patrick Richard Brown. IOP. 2020 Transparent Conductive Oxides (TCOs) Transparent Conductive Oxides (TCOs) EF Transparent Conductive Oxides (TCOs): Eg > 3 eV Degenerate semiconductors TCOs application example: touch-screens TCO functionality Capacitive detection: need for conductive layer. Source:http://www.digikey.com The light should go out of the screen: need for transparent layer.