New Prof. Yong Lei Prof. Thomas Hannappel [email protected] … · 2017. 11. 3. · The atomic...
Transcript of New Prof. Yong Lei Prof. Thomas Hannappel [email protected] … · 2017. 11. 3. · The atomic...
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Halbleiter
Prof. Yong Lei
Prof. Thomas Hannappel
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]
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Important Events in Semiconductors History
1833 Michael Faraday discovered temperature-dependent
conductivity of silver sulfide.
1873 Wi. Smith discovered photoconductivity of selenium.
1874 F. Braun discovered that point contacts on metal
sulfides are rectifying.
1947 J. Bardeen, W. Brattain, and W. Shockley invented
the transistor, and this work was awarded Nobel Prize in
physics in 1956.
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"for their researches on semiconductors and their discovery of the
transistor effect"
The Nobel Prize in Physics 1956
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The Nobel Prize in Physics 1964
"for fundamental work in the field of quantum electronics, which
has led to the construction of oscillators and amplifiers (IC) based
on the maser-laser principle".
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The Nobel Prize in Physics 1973
"for their experimental discoveries
regarding tunneling phenomena in
semiconductors & superconductors,
respectively"
"theoretical predictions of
properties of a supercurrent
through a tunnel barrier, in
particular those phenomena
which are generally known as
Josephson effects"
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"for developing semiconductor
heterostructures used in high-
speed- and opto-electronics"
"for his part in the invention of
the integrated circuit"
The Nobel Prize in Physics 2000
"for basic work on information and communication technology"
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The Nobel Prize in Chemistry 2000
"for the discovery and development of conductive polymers".
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The Nobel Prize in Physics 2009
"for groundbreaking achievements
concerning the transmission of light
in fibers for optical communication"
"for the invention of an imaging
semiconductor circuit - the CCD sensor"
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The Nobel Prize in Physics 2014
“for the invention of efficient blue light-emitting diodes which
has enabled bright and energy-saving white light sources"
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Definition of conductor, insulator, and semiconductor
Conductivity (or resistivity)
Atomic structure (valence electron)
Energy band structure (band-gap)
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Electrical conduction the movement of electrically charged particles through a transmission medium. The movement can form an electric current in response to an electric field. The underlying mechanism for this movement depends on the material.
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Classification of materials in terms of their conductivity (or resistivity) • High conductivity (low resistivity) => “Conductor”
• Low conductivity (high resistivity) => “Insulator”
• Intermediate conductivity (intermediate resistivity) => “Semiconductor”
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Atomic structure
The element in periodic table are arranged
according to its atomic number.
Atomic number = number of electrons in nucleus
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Element Periodic Table
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Bohr model of an atom
This model was proposed by Niels Bohr in 1915: electron circles the nucleus in orbit
and around the nucleus. The “tails” on the electrons indicate the motion. Generally,
atomic structure of a material determines it’s ability to conduct or insulate.
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Bohr model of an atom
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The atomic number of silicon is 14.
A silicon atom has 4 electrons in its
valence shell. This makes it a
semiconductor. It takes 2n2
electrons or in this case 18 electrons
to fill the valence shell.
The atomic number of copper is 29. A
copper atom has only 1 electron in it’s
valence shell. This makes it a good
conductor. It takes 2n2 electrons or in
this case 32 electrons to fill the
valence shell.
Silicon vs. Copper
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Definition of Conductors, Insulators and Semiconductors based on atomic structure
A conductor is a material that easily conducts electrical
current. The best conductors are single-element material, such
as copper, gold and aluminum, which are normally
characterized by atoms with only one valence electron very
loosely bound to the atom.
An insulator is a material that does not conduct electrical
current under normal conditions. Valence electrons are
tightly bound to the atoms.
A semiconductor is a material that is between conductors and
insulators in its ability to conduct electrical current. The most
common single–element semiconductors are silicon,
germanium and carbon.
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Band and Band-gap
In solid-state physics, electronic band structure (or
band structure) of a solid describes range of energies
that an electron within solid may have (called energy
bands, or simply bands) and ranges of energy that it
may not have (called band gaps or forbidden bands).
A band-gap (energy gap) is an energy range in a solid
where no electron states can exist. In graphs of
electronic band structure of solid, band gap generally
refers to energy difference (in electron volts) between
the top of the valence band and the bottom of the
conduction band in insulators and semiconductors. It
is the energy required to promote a valence electron
bound to an atom to become a conduction electron,
which is free to move within the crystal lattice and
serve as a charge carrier to conduct electric current.
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Band Theory of Solids
A useful way to show the difference between conductors, insulators and
semiconductors is to plot the available energies for electrons in the materials. the
available energy states form bands. Crucial to the conduction process is whether or not
there are electrons in the conduction band.
• In insulators, the electrons in the valence band are separated by a large gap
from the conduction band.
• In conductors like metals, the valence band overlaps the conduction band.
• In semiconductors there is a small enough gap between the valence and
conduction bands that thermal or other excitations can bridge the gap.
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Insulator Energy Bands
There is a large forbidden gap between the energies of the
valence electrons and the conduction band).
Glass is an insulating material which is transparent to
visible light - closely correlated with its nature as an
electrical insulator. The visible light photons do not have
enough quantum energy to bridge the band gap and get the
electrons up to an available energy level in the conduction
band. The visible properties of glass can also give some
insight into the effects of "doping" on the properties of
solids. A very small percentage of impurity atoms in the
glass can give it color by providing specific available
energy levels which absorb certain colors of visible light.
While the doping of insulators can dramatically change
their optical properties, it is not enough to overcome the
large band gap to make them good conductors of electricity. http://www.tu-ilmenau.de/nanostruk/
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Conductor Energy Bands
In terms of the band theory of solids,
metals are unique as good conductors of
electricity. This can be seen to be a result
of their valence electrons being
essentially free. In the band theory, this is
depicted as an overlap of the valence
band and the conduction band so that at
least a fraction of the valence electrons
can move through the material.
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Semiconductor Energy Bands
For intrinsic semiconductors like Si and Ge, Fermi
level is essentially halfway between the valence and
conduction bands. Although no conduction occurs at 0
K, at higher temperatures a certain number of electrons
can reach conduction band and provide some current.
In doped semiconductors, extra energy levels are added.
At certain temperatures, the number of electrons which
reach conduction band and contribute to current can be
modeled by the Fermi function.
Silicon Energy Bands Germanium Energy Bands http://www.tu-ilmenau.de/nanostruk/
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Fermi Level
"Fermi level" is the term used to describe
the top of the collection of electron
energy levels at absolute zero temperature.
This concept comes from Fermi-Dirac
statistics. Electrons are fermions and by
the Pauli exclusion principle cannot exist
in identical energy states. So at absolute
zero they pack into the lowest available
energy states and build up a "Fermi sea"
of electron energy states. The Fermi level
is the surface of that sea at absolute zero
where no electrons will have enough
energy to rise above the surface. http://www.tu-ilmenau.de/nanostruk/
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Fermi function for the electrical conductivity of a semiconductor
The position of the Fermi level with the relation to the conduction band is a
crucial factor in determining electrical properties.
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Short summary
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Semiconductor Materials
Elemental semiconductors – single species of atoms – Si and Ge (column IV of periodic
table).
Compound semiconductors – more than one specie of atoms – combinations of atoms of
group III and group V; some atoms from group II and group VI, and some atoms from group
IV (SiC, SiGe). (combination of two atoms results in binary compounds).
There are also three-elements (ternary) compounds (GaAsP), four-elements (quaternary)
compounds (InGaAsP), and even five-elements (penternary) compounds (GaInPSbAs).
Not all combinations are possible: lattice mismatch, room temperature instability, etc.
are concerns. http://www.tu-ilmenau.de/nanostruk/
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Semiconductors manufacturing techniques
- Czochralski Method
- Bridgman-Stockbarger Technique
- Zone Melting Method
- Flame Fusion Method (Verneuil Method)
- Epitaxial Growth
- Atomic Layer Deposition (ALD) Technique
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Czochralski method – Single Crystal Silicon
The crystal growth process is that a solid seed crystal is rotated and slowly
extracted from a pool of molten silicon.
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Czochralski method
Principle & Process: crystal growth method to obtain semiconductors (e.g. Si, Ge,
GaAs) and metals (e.g. Pd, Pt, Ag, Au)
Characteristics
• Rod-shaped single crystal is obtained from a melt of the same composition of melt.
• Very large crystal is obtained at once (e.g. 50 kg silicon rod with the size of ~2 m
and width of 30 cm)
• Extremely little impurities. (< 0.01 ppb)
• Drawback: materials with high vapor pressure cannot be grown.
Usages & Applications
• Production of highly pure semiconductors, metals, salts, and gemstones.
• Mass production of silicon wafers.
• Dopants can be added to make p-type or n-type semiconductors.
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Bridgman-Stockbarger Technique
Principle & Process: Heating polycrystalline material above its melting point and
slowly cooling it from one end of its container, where a seed crystal is located.
Stockbarger method: a pulling method like Czochralski method, boat pulled out
through temperature gradient.
Bridgman method: Melt is inside a temperature gradient furnace.
Characteristics
• The shape of the crystal is defined by the container
• Drawback: materials is constantly in contact with sample boat, which introduces
mechanical stress that possibly changes ideal crystal structure.
Usages & Applications
• Simple and popular way to producing semiconductor crystals GaAs, InP, and CdTe. http://www.tu-ilmenau.de/nanostruk/
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Zone Melting Method
Principle & Process:
• Method for purifying crystals: impurities concentrate in the melt, and move to
one end of container.
• Molten zone melts impure solid at its forward edge, and purer material is
solidified behind it.
Characteristics
• Pure solid can be obtained in a sample manner.
• Drawback: materials with high vapor pressure cannot be grown.
Usages & Applications
Preparing high purity semiconductors for manufacturing transistors. http://www.tu-ilmenau.de/nanostruk/
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Flame Fusion Method (Verneuil Method)
Principle & Process:
• Precursor pass through flame and then melted into liquid.
• Melted droplets fall on surface and crystal grows on it.
Characteristics
• Rod-shaped gemstone crystal is obtained
• Useful for materials with high melting points.
• Drawback: excess oxygen induces gas bubble which includes imperfection of solids.
Usages & Applications
Growing crystals of metal oxides with high melting points, such as gemstones (ruby, sapphire). http://www.tu-ilmenau.de/nanostruk/
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Epitaxial Growth
• Epitaxy refers to the method of depositing a monocrystalline film on a
monocrystalline (single crystal) substrate.
• The deposited film is denoted as epitaxial film or epitaxial layer. The term
epitaxy comes from the Greek roots epi, meaning "above", and taxis, meaning "in
ordered manner". It can be translated "to arrange upon".
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Molecular Beam Epitaxy (MBE)
• Molecular beam epitaxy takes place in high vacuum or ultra high
vacuum (10−8 Pa).
• The most important aspect of MBE is the slow deposition rate
(typically less than 1000 nm per hour), which allows the films to
grow epitaxially.
• The slow deposition rates require better vacuum to achieve the same
impurity levels as other deposition techniques. http://www.tu-ilmenau.de/nanostruk/
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Band gap engineering by Epitaxy
• Repeating a crystalline structure by: atom by atom addition.
• Chemistry controls the epitaxy to insure that, Ga bonds only to N
and not Ga-Ga or N-N bonds.
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Metalorganic Vapor Phase Exitaxy
For epitaxy of materials and
compound semiconductors:
combinations of Group III and
Group V, Group II and Group
VI, Group IV, or Group IV, V
and VI elements. http://www.tu-ilmenau.de/nanostruk/
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Atomic Layer Deposition (ALD) technique
• Amorphous film
• Metallic oxides, metallic nitrides, sulfides (ZnS, CdS),
phosphides (GaP, InP),
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Porous Anodic Aluminum Oxide (AAO) Templates
Interesting and useful features:
• Ordered pore arrays + large area
• Nanometer-sized pores
• High aspect ratio
• Controllable diameter (10 – 400 nm)
• Length 100 μm Configuration diagram of the PAMs
Template-based techniques to prepare functional nanostructures
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Templates with large-scale (1 mm2) perfect rectangular pore arrays without defect
2010
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Templates with large-scale (1 mm2) perfect rectangle pore arrays without defect
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TiO2 nanotubes grown in the template
(Before removing template)
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Sb Ni Ni-TiO2
L. Liang, Y. Lei, et al. Energy & Environmental Science, 2015, 8, 2954;
Y. Xu, Y. Lei, et al. Chemistry of Materials, 2015, 27, 4274.
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A B
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Ni L
Cd LS K
Ti K
Ag LNi L
o
C P
Al
Ag
Ag
Ni
200 nm200 nm
200 nm 200 nm
200 nm
200 nm 200 nm
200 nm
(a)
(b)
(c)
(d)
(e)
(f)
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Binary nanowire arrays realized by electrodeposition via template
TiO2/Au TiO2/Ag TiO2/Ni
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Halbleiter Thank you !!!
Prof. Yong Lei
Prof. Thomas Hannappel
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]