Halbleiter. Prof. Yong Lei. Prof. Thomas Hannappel.

Transcription

1 Halbleiter Prof. Yong Lei Prof. Thomas Hannappel

2 Important Events in Semiconductors History 1833 Michael Faraday discovered temperature-dependent conductivity of silver sulfide Wi. Smith discovered photoconductivity of selenium F. Braun discovered that point contacts on metal sulfides are rectifying J. Bardeen, W. Brattain, and W. Shockley invented the transistor, and this work was awarded Nobel Prize in physics in 1956.

3 The Nobel Prize in Physics 1956 "for their researches on semiconductors and their discovery of the transistor effect"

4 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".

5 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" Page 5

6 The Nobel Prize in Physics 2000 "for basic work on information and communication technology" "for developing semiconductor heterostructures used in highspeed- and opto-electronics" "for his part in the invention of the integrated circuit" Page 6

7 The Nobel Prize in Chemistry 2000 "for the discovery and development of conductive polymers" Page 7

8 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" Page 8

9 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"

10 Definition of conductor, insulator, and semiconductor Conductivity (or resistivity) Atomic structure (valence electron) Energy band structure (band-gap) Page 10

11 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.

12 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 Page 12

13 Atomic structure The element in periodic table are arranged according to its atomic number. Atomic number = number of electrons in nucleus

14 Element Periodic Table

15 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 Page 15

16 Bohr model of an atom Page 16

17 Silicon vs. Copper The atomic number of silicon is 14. A silicon atom has 4 electrons in its valence shell. This makes it a semiconductor. It takes 2n 2 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 2n 2 electrons or in this case 32 electrons to fill the valence shell.

18 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.

19 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.

20 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.

21 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. Page 21

22 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 Page 22

23 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 Page 23

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 Page 24

25 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 Page 25

26 Short summary Page 26

27 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 Page 27 are concerns.

28 Semiconductors manufacturing techniques - Czochralski Method - Bridgman-Stockbarger Technique - Zone Melting Method - Flame Fusion Method (Verneuil Method) - Epitaxial Growth - Atomic Layer Deposition (ALD) Technique

29 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.

30 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.

31 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 Page 31 Simple and popular way to producing semiconductor crystals GaAs, InP, and CdTe.

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. Usages & Applications Page 32 Preparing high purity semiconductors for manufacturing transistors.

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. Usages & Applications Growing crystals of metal oxides with high melting points, such as gemstones (ruby, sapphire). Page 33

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" Page 34

35 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. Page 35

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 Page 36

37 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 Page 37

38 Atomic Layer Deposition (ALD) technique Amorphous film Metallic oxides, metallic nitrides, sulfides (ZnS, CdS), phosphides (GaP, InP),

39 Template-based techniques to prepare functional nanostructures Porous Anodic Aluminum Oxide (AAO) Templates Configuration diagram of the PAMs Interesting and useful features: Ordered pore arrays + large area Nanometer-sized pores High aspect ratio Controllable diameter ( nm) Length <100 nm to >100 μm

40 Templates with large-scale (1 mm 2 ) perfect rectangular pore arrays without defect 2010

41 Templates with large-scale (1 mm 2 ) perfect rectangle pore arrays without defect

42 TiO 2 nanotubes grown in the template (Before removing template)

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44 Sb Ni Ni-TiO 2 L. Liang, Y. Lei, et al. Energy & Environmental Science, 2015, 8, 2954; Y. Xu, Y. Lei, et al. Chemistry of Materials, 2015, 27, 4274.

45 Page 45 A B

46 (a) (e) 200 nm 200 nm (b) Ni Ag C o Al P Ag Ni L Ag L 200 nm 200 nm (c) (f) 200 nm 200 nm (d) S K Cd L Ni L Ti K Page nm 200 nm

47 Binary nanowire arrays realized by electrodeposition via template TiO2/Ag TiO2/Au TiO2/Ni

48 Halbleiter Thank you!!! Prof. Yong Lei Prof. Thomas Hannappel