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Pulsed Laser Deposition of Thin Films of Functional Materials

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Graduate School at MC2 2016 course: Micro- and Nanoprocessing Technologies lecture: Pulsed laser deposition of thin films of functional materials Lecturer: Andrei Vorobiev Course responsible: Ulf Södervall 1 Objectives You will learn about: • concept of pulsed laser deposition PLD of thin films • functional materials • features of PLD of functional materials Literature: 1) H. M. Christen and G. Eres, Recent advances in pulsed-laser deposition of complex oxides, J. Phys.: Condens. Matter 20 (2008) 264005 (16pp) 2) Pulsed laser deposition of thin films: applications-led growth of functional materials / edited by Robert Eason, N.J., Wiley, cop. 2007 3) Pulsed laser deposition of thin films / edited by Douglas B. Chrisey and Graham K. Hubler, New York, Wiley, cop. 1994 2 Outline • History and fundamentals of PLD • PLD of functional materials • Equipment • Advantages and limitations • Mechanisms of pulsed Laser sputtering • Film nucleation and growth in PLD • Splashing and forward peaking • PLD of HTS and ferroelectric thin films • Summary 3 Definitions Laser is an electronic-optical device that produces coherent radiation. The term is acronym for Light Amplification by Stimulated Emission of Radiation. Pulsed laser deposition (PLD) is a physical vapor deposition technique where a high power pulsed laser beam is focused to strike a target of the desired composition. Material is then vaporized and deposited as a thin film on a substrate facing the target. This process can occur in ultra high vacuum or in the presence of a background gas, such as oxygen when depositing films of oxides. Functional materials are materials having physical properties sensitive to the external effects (temperature, electric and magnetic fields, pressure etc.). 4 Evolution of laser technology and its applications Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 2. 5 Historical Development of PLD 1917 - Albert Einstein postulated the stimulated emission process. 1960 - Maiman constructed the first ruby laser. 1962 - Breech and Cross used ruby laser to vaporize and excite atoms from a solid. 1965 - Smith and Turner used ruby laser to deposit thin films. 1970s - i) reliable electronic Q-switches for generating very short pulses; ii) high-efficiency second harmonic generators for shorter wavelength. 1987 - PLD in Bellcore group used successfully to grow HTS YBCO. 1990 - PLD growth of ferroelectric Bi based perovskite oxide films in Ramesh group. 1990s - PLD production related issues concerning reproducibility and large-area scale up have begun to be addressed. 2000s - Numerous device applications based on PLD films of functional materials (YBCO, BSTO etc.) are being explored. 6 Functional materials function of Functional material External parameters: properties: • temperature • permittivity • pressure • permeability • electric field • resistivity • magnetic field • optical wavelength • refractivity • absorbed gas • sound velocity • pH value • … • … • Utilizing a functional material offers higher functionality of a system. • Science and technology rely heavily on the development of functional materials. 7 Mobile Convergence Portable Media MP3 PDA Cellular Smart Phone Phones Convergent devices DSC Gaming Mobile Video Imaging 8 Adapted from Philips Functional materials • Ferroelectric………………………….. BaxSr1-xTiO3 • High temperature superconductor….YBa2Cu3O7 • Magnetic field sensor……………….. La1-xCaxMnO3 • Surface acoustic wave sensor………LiNbO3 • Liquid petroleum gas sensor………...Pd-doped SnO2 • High temperature piezoelectric………Ta2O5 • Fast-ion conductor…………………….Y2(SnyTi1-y)2O7 • … • A wide range of functional materials are complex oxides. • A key requirement in preparations is to control compositional evolution. • A unique feature of PLD is stoichiometric preservation of composition. 9 PLD of Functional materials x Pulsed laser deposition of thin films / edited by Douglas B. Chrisey and Graham K. Hubler, New York, Wiley, 1994 • PLD and S are the most appropriate techniques for deposition of complex oxides. • PLD reproduces target stoichiometry in an oxidizing ambient. 10 Concept of PLD PLD stages: • laser ablation of target • dynamic of plasma • film nucleation and growth Chrisey, D. B., and G. K. Hubler (1994), Pulsed Laser Deposition of Thin Films, Wiley, New York, p. 3. • The laser-target interaction: electromagnetic energy is converted into electronic excitation and then into thermal/mechanical energy to cause ablation. • A plume: atoms, molecules, electrons, ions, clusters, particles, and molten globules. • The plume expands with hydrodynamic flow characteristics. 11 Growth of BSTO films by PLD Vacuum chamber 0.4 mbar O2 Laser BSTO target Calas PLD System - MC2ProcessLab, Chalmers 1 Hz Heater at 650C 12 Advantages-Disadvantages of PLD Advantages Disadvantages • versatile method (any material) • splashing of micron-sized particulates • congruent evaporation • small area of uniformity (1 cm2) • high deposition rates (10s nm/min) • non-conformal coverage • clean process • extremely complex models hinder • plume at high energy theory based improvements • reactive gases (oxygen) • broad range of gas pressures • To a large extent the two first problems have been solved. • Congruent (stoichiometric) evaporation – main advantage for PLD of films of functional materials. 13 Laser basics Light Amplification by Stimulated Emission of Radiation • When perturbed by a photon matter may create another photon. • The second photon has the same direction, frequency, phase and polarization. • The first photon is not destroyed (no absorption) - light amplification. 14 Laser basics (cont.) laser major parts Pump source Gain medium 1) The pump source provides energy to the gain medium. 2) The gain medium transfers energy into the laser beam amplified by stimulated emission. 3) The optical resonator provides a narrow, low-divergence beam. 15 Wavelengths of common lasers Laser Type Wavelength (mm) Argon fluoride (Excimer-UV) 0.193 Helium neon (yellow) 0.594 Krypton chloride (Excimer-UV) 0.222 Helium neon (orange) 0.610 Krypton fluoride (Excimer-UV) 0.248 Gold vapor (red) 0.627 Xenon chloride (Excimer-UV) 0.308 Helium neon (red) 0.633 Xenon fluoride (Excimer-UV) 0.351 Krypton (red) 0.647 Helium cadmium (UV) 0.325 Rohodamine 6G dye (tunable) 0.570-0.650 Nitrogen (UV) 0.337 Ruby (CrAlO3) (red) 0.694 Helium cadmium (violet) 0.441 Gallium arsenide (diode-NIR) 0.840 Krypton (blue) 0.476 Nd:YAG (NIR) 1.064 Argon (blue) 0.488 Helium neon (NIR) 1.15 Copper vapor (green) 0.510 Erbium (NIR) 1.504 Argon (green) 0.514 Helium neon (NIR) 3.39 Krypton (green) 0.528 Hydrogen fluoride (NIR) 2.70 Frequency doubled 0.532 Carbon dioxide (FIR) 9.6 Nd YAG (green) Carbon dioxide (FIR) 10.6 Helium neon (green) 0.543 Krypton (yellow) 0.568 Copper vapor (yellow) 0.570 • The PLD range is 200 - 400 nm (absorption by matter is strong enough). • Excimer laser uses “excited dimer” pseudo-molecules for the gain media. 16 Excimer laser basics KrF electronic potential excimers formation reactions + + Ar + 2Ar Ar2 + Ar + + charge-transfer Ar2 + Kr Kr + 2Ar Kr* + eˉ Kr + eˉ excitation Kr+ + 2 eˉ Kr* + F2 (KrF)* + F harpooning Kr+ + Fˉ + He (KrF)* + He recombination • Excimers are only stable in excited states. • If excimers are generated, the medium is automatically in population inversion with the unstable ground state. • Technical implementation: gas mixtures in high-voltage gas discharge 17 Commercial excimer laser design cross sectional view of excimer laser tube • Pulsed mode provides non-equilibrium vaporization (10000 K in 10 nm surface layer). 2 • KrF Lambda Physik, Inc. typical parameters: =248 nm, =30 ns, r=10 Hz, E0=1.5 J/cm 18 Advantages of PLD Advantages Versatile method • versatile method (any material) 1/2 E = (2Φ/c0n) • congruent evaporation • high deposition rates (10s nm/min) Φ – power density (109 W/cm2) • clean process c – velocity of light • plume at high energy 0 – permittivity of vacuum • reactive gases (oxygen) n – refractive index (1.5) 6 • broad range of gas pressures E – electric field (10 V/cm) • The electric field inside the material (106 V/cm) is sufficiently high to cause dielectric breakdown. • Thus, any material will be transformed to form a plasma. 19 Philosophy of multicomponent film deposition location 1 location 2 source of adequate transport coated elements of elements substrate (target) energy a pure film of the correct composition “…This process transports elements from one location to another by supplying energy to elements in a source, causing them to be transported to a surface to be coated. Ideally, such a process coats the surface with a pure film of the correct composition.” T. Venkatesan and Steven M. Green, The Industrial Physicist, p. 22 (1996) 20 Decomposition by equilibrium Ba Ti 1700 C BaTiO source x Ti -2 Pe =110 torr Ba 2 equilibrium heater Pe =510 torr 2 Ba Ba Pe J Pe 4 J Ti Ti 510 2kTm J Pe • Equilibrium heaters: resistive, e-beam, inductive systems. • The vapour and deposited films initially are almost pure Ba. • The composition of deposited films would slowly drift enriching by Ti. 21 Congruent evaporation by PLD Criterion for congruent evaporation laser beam L ≤ dev 1 L 2(D ) 2 - heated thickness evaporants dev ls F dev ln - evaporated thickness 2 Fth L nonequilibrium heating SrTiO3 ablation by pulse =30 ns: L ≈ 0.3 µm target dev ≈ 0.2 µm E. G. Gamaly et al., Physics of Plasmas, 9 (2002) 949 Nonequilibrium heating by pulsed laser beam produces a flash of evaporants that deposit on a substrate as a film
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