Prof. Yong Lei Prof. Thomas Hannappel [email protected] thomas.hannappel@tu-ilmenau · 2018-01-15 · A conductor is a material that easily conducts electrical current. The best

Halbleiter

Prof. Yong Lei

Prof. Thomas Hannappel

[email protected]

[email protected]

mailto:[email protected]






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.

"for their researches on semiconductors and their discovery of the

transistor effect"

The Nobel Prize in Physics 1956


"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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"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"


"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".

http://www.tu-ilmenau.de/nanostruk/

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"for groundbreaking achievements

concerning the transmission of light

in fibers for optical communication"

"for the invention of an imaging

semiconductor circuit - the CCD sensor"



“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)


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”


Atomic structure

The element in periodic table are arranged

according to its atomic number.

Atomic number = number of electrons in nucleus

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


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

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.

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.

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/

03.11.2017 Page 24

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/

Semiconductors manufacturing techniques

- Czochralski Method

- Bridgman-Stockbarger Technique

- Zone Melting Method

- Flame Fusion Method (Verneuil Method)

- Epitaxial Growth

- Atomic Layer Deposition (ALD) Technique

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.

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.


• Simple and popular way to producing semiconductor crystals GaAs, InP, and CdTe. http://www.tu-ilmenau.de/nanostruk/

03.11.2017 Page 32

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.


Preparing high purity semiconductors for manufacturing transistors. http://www.tu-ilmenau.de/nanostruk/

03.11.2017 Page 33

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.


Growing crystals of metal oxides with high melting points, such as gemstones (ruby, sapphire). http://www.tu-ilmenau.de/nanostruk/

03.11.2017 Page 34

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/

03.11.2017 Page 36

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/

Atomic Layer Deposition (ALD) technique

• Amorphous film

• Metallic oxides, metallic nitrides, sulfides (ZnS, CdS),

phosphides (GaP, InP),

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 nm to >100 μm Configuration diagram of the PAMs

Template-based techniques to prepare functional nanostructures

Templates with large-scale (1 mm2) perfect rectangular pore arrays without defect

2010

Templates with large-scale (1 mm2) perfect rectangle pore arrays without defect

TiO2 nanotubes grown in the template

(Before removing template)

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.

03.11.2017 www.tu-ilmenau.de/nanostruk Page 45

A B

03.11.2017 www.tu-ilmenau.de/nanostruk Page 46

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)

Binary nanowire arrays realized by electrodeposition via template

TiO2/Au TiO2/Ag TiO2/Ni

Halbleiter Thank you !!!

Prof. Yong Lei

Prof. Thomas Hannappel

[email protected]

[email protected]







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Prof. Yong Lei Prof. Thomas Hannappel [email protected] thomas.hannappel@tu-ilmenau · 2018-01-15 · A conductor is a material that easily conducts electrical current. The best