Mse phd lecture
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Materials Science & Engineering (MSE) PhD lecture. Advanced Materials Synthesis. Thin Films. By Toru Hara, PhD Oct. 23, 2014 1
Transcript of Mse phd lecture
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Materials Science & Engineering (MSE) PhD lecture.
Advanced Materials Synthesis.
Thin Films.
By Toru Hara, PhD
Oct. 23, 2014
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Research history of the lecturer.
1. Thin film capacitor Sputtering Chemical Solution Deposition (CSD) Chemical Vapor Deposition (CVD)
Electric Current(From top electrode)
Electric Current(From botom electrode)
Dielectric material
Magnetic Ffux
Solder ball
+ +--
2. Thin film oxygen sensor Electron-Beam Evaporation Sputtering Pulsed Laser Deposition (PLD) Atomic Layer Deposition (ALD)
O
Sr
Sr
O
O
Sr
Sr
O
Sr
Sr
O
O
O e
Ti
O
Ti
O
3. Novel batteries (in progress) Plating Electrospray Deposition
Polaron diameter
Larger polaron diameter:Higher conductance(Lower resistance)
Smaller polaron diameter:Lower conductance(Higher resistance)
Film thickness
TiO6 unit cell
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Why thin film?
Flexible Solar Cell
Smaller Electronic Circuit
C, Capacitance; ε0, vacuum permittivity;εr, relative dielectric constant of a material;A, electrode area;d: thickness of a dielectric material.The thinner the dielectric layer, the higher the capacitance even small capacitors can give great enough capacitance.
Polaron diameter
Larger polaron diameter:Higher conductance(Lower resistance)
Smaller polaron diameter:Lower conductance(Higher resistance)
Film thickness
TiO6 unit cell
New functional device
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Contents.
Definition of thin films.
General applications & Required theory.
Brief introduction on each application.
Thin film deposition technique as an advanced materials synthesis method
Research topics.
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Definition of thin films.
General applications & Required theory.
Brief introduction on each application.
Thin film deposition technique as an advanced materials synthesis method
Research topics.
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Definition of thin films.
A thin film is a layer of material ranging from fractions of a sub-nanometer to several micrometers in thickness.
Help for imagining.
Bulk: 1022-1023 atoms exist in 1.0*1.0*1.0 cm3
Thin Film: 1015-1016 atoms exist in 1.0*1.0 cm2
(one-atom-thick thin film)
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Definition of thin films.
General applications & Required theory.
Brief introduction on each application.
Thin film deposition technique as an advanced materials synthesis method
Research topics.
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General applications & Required theory.
1. Electronic semiconductor devicesBand engineering
2. Optical coatingsRefractive indices engineering
3. Optoelectronic semiconductor devicesBand engineering &
Refractive indices engineering4. Quantum devices
Quantum dynamics/design
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1. Electronic semiconductor devicesBand engineering
It uses semiconductor alloys, in which a wide-gap semiconductor and a narrow-gap semiconductor are combined to give a substance with a desired intermediate band gap.
The left figure shows the band gaps and lattice parameters for some of the more common elemental and
binary semiconductors.
*The practical band engineering involves making ternary alloys such as (Ga,Al)As and (Hg,Cd)Te.
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2. Optical coatingsRefractive indices engineering
Optical transmission/reflection can be modulated by refractive indices engineering (destructive/constructive interferences).
http://www.intechopen.com/books/electromagnetic-waves/propagation-of-electromagnetic-waves-in-thin-dielectric-and-metallic-films
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3. Optoelectronic semiconductor devicesBand engineering & Refractive indices engineering
Band gap modulation causes the refractive indices modulation at the same time, resulting in “optical guide.”
http://www.tf.uni-kiel.de/matwis/amat/semitech_en/kap_2/backbone/r2_3_1.htmlhttp://optipedia.info/lsource-index/fiberlaser-index/fiber/parameters/numerical_aperture/
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4. Quantum devices Quantum dynamics/design
(L) Quantum well structure is made by the combination of a layer of a wider gap material such as (Ga,Al)As, and a very thin layer of a narrower gap material such as (GaAs). The narrower-gap material forms a one-dimensional potential well in the conduction and valence bands; thus, the electron and hole levels are bound states of the well, known as subbands. The well will contain three sets of subbands, the electron subbands, the light hole subbands and the heavy hole subbands. By intersubband absorption/emission of light realizes the use of long-wavelength light. The merits of quantum well structure are (1) all of the transitions are excitonic (i.e. sharp features at a well-defined energy, rather than broad edges), even at 300 K; (2) electrons and holes are held in close proximity, to encourage more efficient recombination in lasers and LEDs.
(R) A superlattice contains a set of quantum wells which are sufficiently closely spaced for the carriers to tunnel between wells. The subbands will broaden out to form minibands with minigaps, resulting in the longer-wavelength (far-infrared) light response.
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Definition of thin films.
General applications & Required theory.
Brief introduction on each application.
Thin film deposition technique as an advanced materials synthesis method
Research topics.
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Brief introduction on each application.1. Electronic semiconductor devices
Schematic diagrams of a GaAs metal-semiconductor field effect transistor (MESFET). Si3N4 is deposited by using Chemical Vapor deposition (CVD) in order to passivate the single-crystal-based GaAs electronic device.
A. R. Barron, CVD of Nonmetals, Ed. W. S. Rees, Jr., Wiley, NY (1996).
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Brief introduction on each application.2. Optical coatings
Optical coating films are deposited onto the substrate, which is a usually optically transparent glass, in order to modulate the optical reflection. Electron beam physical vapor deposition is often used in order to modulate the optical reflection.
http://www.calctool.org/CALC/phys/optics/thin_film
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Brief introduction on each application.3. Optoelectronic semiconductor devices
One of the most famous application is probably Light Emitting Diodes (LEDs) application. Each layer is deposited by Chemical Vapor Deposition (CVD).
http://creativentechno.wordpress.com/2012/01/07/how-led-works/
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Brief introduction on each application.
4. Quantum devicesOne of the important
concept in order to realize quantum devices is the concept, “Superlattice.” This is a periodic structure of layers of two or more materials; the thickness of one layer is typically several nanometers. Molecular beam epitaxy (MBE) is a popular deposition technique in order to make superlattice structures.http://www.semiconductor-today.com/news_items/2011/FEB/MEIJO_020211.htm
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Definition of thin films.
General applications & Required theory.
Brief introduction on each application.
Thin film deposition technique as an advanced materials synthesis method
Research topics.
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Thin film deposition technique as an advanced materials synthesis method
1. Chemical deposition
2. Physical deposition
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Thin film deposition technique as an advanced materials synthesis method
1. Chemical deposition
Fluid/Gas precursor undergoes a chemical change at a solid surface, leaving a solid layer.
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Thin film deposition technique as an advanced materials synthesis method
Various types of chemical deposition technique
- Plating- Chemical Solution Deposition (CSD)- Chemical Vapor Deposition (CVD)- Atomic Layer Deposition (ALD)
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Plating
(1)Liquid precursors ( a solution of water with a salt of the metal to be deposited) are used.
(2) Some plating processes are driven by reagents, e.g., reducing agents, in the solution.(3) Electroplating is done through the use of an external electron supply. Electroplating is a process in which a metal or two or more metals is/are reductively deposited onto the surface of a negatively biased electrode.
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http://worldofsecrets.org/en/2014/06/batterie-von-bagdad/
This is the world oldest electroplating equipment that found in an ancient tomb in Bagdad in 1936. It consists of a 14-centimeter-high egg-shaped clay jar with an asphalt stopper. An iron rod protruding out of the asphalt is the anode, which is surrounded by a copper cylinder used as the cathode. Filled with vinegar as an electrolytic solution.
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http://www.sparknotes.com/chemistry/electrochemistry/electrolytic/section1.rhtml
This is the setup for gold electroplating. At the cathode surface, Au3+ ion will be reduced into metal Au. Thanks to this technique, we can use golden spoons at an inexpensive cost.
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In today's nanoscience researches, electroplating is quite popular. This is the picture of electroelectroplated patterned surface.
P. Kim et al., Nano Lett. ,12 (2012) 527–533.
5 μm
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Electroplating can be used for novel battery research. At the conductive carbon fiber surface, nano-sized electrochemically active materials are directly plated: highly durable (expected life = 25 years) battery will be realized.
http://toruhara.page.tl/_-Research-plan-1.htmFor this purpose, nano-crystals are
preferred rather than thin films.
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Chemical Solution Deposition (CSD)Chemical Bath Deposition (CBD)
(1)Liquid precursor, usually a solution of organometallic powders dissolved in an organic solvent, are used in order to gain thin films.
(2)This is a relatively inexpensive, simple thin film process that is able to produce stoichiometrically accurate crystalline phases.
(3)This technique is also known as the sol-gel method because the 'sol' (or solution) gradually evolves towards the formation of a gel-like system.
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K Sarkar et al., J. Mater. Chem. A, 2 (2014) 6945-6951.
This is a schematic of the sol-gel process for zinc-oxide-based solar cell. Onto the silicon substrate, a solution is spin-coated; then, dehydrated; and finally, calcinated.
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Chemical vapor deposition (CVD)
It uses a gas-phase precursor, often a halide or hydride of the element to be deposited. In the case of MOCVD, an organometallic gas is used. Commercial techniques often use very low pressures of precursor gas.
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This shows the schematic mechanism of CVD. Precursors deposit onto the substrate surface; then, surface reaction takes place; and finally, reaction products form thin films.
http://cnx.org/contents/[email protected]:35
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Atomic layer deposition (ALD)
It uses gaseous precursor to deposit conformal thin films one layer at a time. The process is split up into two half reactions, run in sequence and repeated for each layer, in order to ensure total layer saturation before beginning the next layer. Therefore, one reactant is deposited first, and then the second reactant is deposited, during which a chemical reaction occurs on the substrate, forming the desired composition.
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This shows ALD process of Strontium Titanate thin film, (20-nm-thick) which is used for novel oxygen sensor [T. Hara et al., Sens. Actuators B 136 (2009) 489.].
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Thin film deposition technique as an advanced materials synthesis method
2. Physical deposition
Mechanical, electromechanical or thermodynamic energy is used in order to produce a thin film of solid. Since most engineering materials are held together by relatively high energies, and chemical reactions are not used to store these energies, commercial physical deposition systems tend to require a low-pressure vapor environment to function properly; most can be classified as physical vapor deposition (PVD).
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Thin film deposition technique as an advanced materials synthesis method
Various types of physical deposition technique
- Thermal Evaporation- Electron-Beam Evaporation- Molecular Beam Epitaxy (MBE)- Sputtering- Pulsed Laser Deposition (PLD)- Cathodic Arc Deposition- Electrospray Deposition
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Thermal Evaporation
It uses an electric resistance heater to melt the material and raise its vapor pressure to a useful range. This is done in a high vacuum, both to allow the vapor to reach the substrate without reacting with or scattering against other gas-phase atoms in the chamber, and reduce the incorporation of impurities from the residual gas in the vacuum chamber.
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This is the schematic of thermal evaporation equipment. Under the high vacuum atmosphere, heated material will evaporate and deposit onto the substrate.
http://www.ece.utep.edu/research/cdte/Fabrication/index.htm
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Electron-Beam Evaporation
It fires a high-energy beam from an electron gun to boil a small spot of material; since the heating is not uniform, lower vapor pressure materials can be deposited. The beam is usually bent through an angle of 270° in order to ensure that the gun filament is not directly exposed to the evaporant flux.
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This is the schematic of electron-beam evaporation equipment. Under the high vacuum atmosphere, heated material will evaporate and deposit onto the substrate. The difference between the thermal evaporation and electron-beam evaporation is the heating method.
http://www.optics.rochester.edu/workgroups/cml/opt307/spr13/greg/
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Molecular Beam Epitaxy (MBE)
Slow streams of an element can be directed at the substrate, so that material deposits one atomic layer at a time. Compounds such as gallium arsenide are usually deposited by repeatedly applying a layer of one element (i.e., gallium), then a layer of the other (i.e., arsenic), so that the process is chemical, as well as physical. The beam of material can be generated by either physical means (that is, by a furnace) or by a chemical reaction (chemical beam epitaxy).
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This is the schematic of molecular beam epitaxy equipment.
http://en.rusnano.com/portfolio/companies/semiteq
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Sputtering
It relies on a plasma (usually a noble gas, such as argon) to knock material from a "target" a few atoms at a time. The target can be kept at a relatively low temperature, since the process is not one of evaporation, making this one of the most flexible deposition techniques. It is especially useful for compounds or mixtures, where different components would otherwise tend to evaporate at different rates.
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This is the schematic of sputtering equipment. Argon atom is ionized into argon cation under static and/or alternative electric field; argon cation knocks the target surface; knocked atoms will be deposited onto the substrte.
http://en.wikipedia.org/wiki/File:Sputtering.gif
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Pulsed Laser Deposition
This systems work by an ablation process. Pulses of focused laser light vaporize the surface of the target material and convert it to plasma; this plasma usually reverts to a gas before it reaches the substrate.
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This is the schematic of pulsed laser deposition equipment. Target materials are knocked by laser irraduiation, then, deposited onto the substrte.
http://groups.ist.utl.pt/rschwarz/rschwarzgroup_files/PLD_files/PLD.htm
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Cathodic Arc Deposition
It is a kind of ion beam deposition where an electrical arc is created that literally blasts ions from the cathode. The arc has an extremely high power density resulting in a high level of ionization (30–100%), multiply charged ions, neutral particles, clusters and macro-particles (droplets).
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This is the schematic of pulsed laser deposition equipment. Target materials are set as a part of electrode, and evaporated by a plasma-induced excitation, then deposited onto the substrate.
http://groups.ist.utl.pt/rschwarz/rschwarzgroup_files/PLD_files/PLD.htm
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Electrospray deposition
The liquid to be deposited, either in the form of nano-particle solution or simply a solution, is fed to a small capillary nozzle (usually metallic) which is connected to a high voltage. The substrate on which the film has to be deposited is connected to ground. Through the influence of electric field, the liquid coming out of the nozzle takes a conical shape and at the apex of the cone a thin jet emanates which disintegrates into very fine and small positively charged droplets. The droplets keep getting smaller and smaller and ultimately get deposited on the substrate as a uniform thin layer.
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This is the schematic of electrospray deposition equipment. (1) Precursors are fed to the nozzle. (2) The substrate is connected to ground. (3) Through the influence of electric field, the liquid coming out of the nozzle. (4) The liquid disintegrates into very fine and small positively charged droplets. The droplets keep getting smaller and smaller and (5) ultimately get deposited on the substrate as a uniform thin layer.
http://www.nanowerk.com/spotlight/spotid=9685.php(1)
(2)
(3)(4)
(5)
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Definition of thin films.
General applications & Required theory.
Brief introduction on each application.
Thin film deposition technique as an advanced materials synthesis method
Research topics.
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Research Topics.
L. Bovo et al., “Restoration of the third law in spin ice thin films,” Nat. Commun. DOI: 10.1038/ncomms4439.Spin liquid found in the pyrochlore structure is the apparent violation of the third law of thermodynamics. The authors found that even in a pyrochlore compound the third law can be restored by fabricating thin epitaxial films: this restoration results from strain-induced (the lattice constant difference between the substrate and the film deposited onto the substrate) ordering of the spins. showing how the physics of frustrated pyrochlore magnets such as spin ice may be significantly modified in thin-film samples.
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http://phys.org/news204201232.html
The third law of thermodynamics is sometimes stated as follows, regarding the properties of systems in equilibrium at absolute zero temperature: the entropy of a perfect crystal, at absolute zero (zero kelvins), is exactly equal to zero. However, the large quantum fluctuation of the magnetic spins (see, it is tetrahedral) melt the spin ice structure and form the spin liquid (or spin liquid-crystal).
Research Topics.
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Research Topics.
T. Hara et al., “Effect of Oxygen Adsorption on Polaron Conduction in Nanometer-Scale Nb5+-, Fe3+-, and Cr3+-Doped SrTiO3 Thin Films,” Jpn. J. Appl. Phys., 50 (2011) 065807.The polaron diameter in SrTiO3-based epitaxial thin films decreases owing to oxygen adsorption, resulting in the decrease in electronic conductivity; this can be understood by assuming that oxygen adsorbates induce local distortions of TiO6 unit cells at which conduction electrons are frequently trapped.
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Research Topics.
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Research Topics.
What was new?
Although SrTiO3 is
paraelectric,
near the surface of STO
adsorbed oxygen can
induce the
ferroelectric-like
Ionic displacement.
O
Sr
Sr
O
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Sr
Sr
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Sr
Sr
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O e
Ti
O
Ti
O
