Halbleiter Prof. Yong Lei Prof. Thomas Hannappel

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Halbleiter Prof. Yong Lei Prof. Thomas Hannappel yong.lei@tu-ilmenau.

1 Halbleiter Prof. Yong Lei Prof. Thomas Hannappel

2 Solid State Structure of Semiconductor

3 Semiconductor manufacturing techniques - Czochralski Method - Bridgman-Stockbarger Technique - Zone Melting Method - Flame Fusion Method (Verneuil Method) - Epitaxial Growth - Atomic Layer Deposition (ALD) Technique Single Crystal The properties of semiconductors are determined to a large extent by single-crystal lattice structure.

4 Classification of semiconducting materials In order to build electronic devices, we have to understand the charge transport in material. The electric properties of a material are determined not only by the chemical composition but also by the structural properties of material. Based on the structural properties of material, different classes of materials can be classified as amorphous, polycrystalline & monocrystalline (single crystal):

5 Cross section of a MOSFET polycrystalline amorphous crystalline

6 The most important semiconductors Monocrystalline (or single-crystal) semiconductors Elemental semiconductors: Si (silicon), Ge (germanium), etc. Compound semiconductors: GaAs (gallium arsenide), GaAsP (gallium arsenide phosphide), InGaAsP (indium gallium arsenide phosphide), etc. They are used to create LEDs. Amorphous semiconductors: mainly amorphous Si TFTs and solar cells are made of them. Organic semiconductors: small molecules and polymers OLEDs, PLEDs, OFETs, etc.

7 Silicon Crystal Structure Silicon crystallizes in the same pattern as diamond. A silicon atom has four electrons which it can share in covalent bonds with its neighbors.

Crystal = Lattice + Basis A crystal is a regular ordered arrangement of atoms over large scale. Atoms could be a single type or repetition of a complex arrangement of many different atoms.

8 Crystal = Lattice + Basis A crystal is a regular ordered arrangement of atoms over large scale. Atoms could be a single type or repetition of a complex arrangement of many different atoms. A crystal can be thought of as two separate parts: lattice and basis: Lattice: an ordered arrangement of points in space; basis consists of simplest arrangement of atoms which is repeated at every point in lattice to build up crystal structure. A good analogy is patterned wallpaper: basis like a motif on wallpaper; lattice like periodic pattern of points on which motif is repeated.

9 Unit Cell - a small volume of crystal that can be used to reproduce the entire crystal Two-dimensional representation of a single-crystal lattice shows various possibility of unit cells. The entire two-dimensional lattice can be constructed by using either of these unit cells.

10 Primitive unit cell - is the smallest unit cell that can be repeated to form a lattice A generalized primitive unit cell

11 Classfication of Crystal

12 Fourteen Barvais lattices, illustrsting all the possible 3D crystal lattices Auguste Bravais ( )

13 Face centered and Body centered

14 3 basic lattices based on cubic unit cells in semiconductors Simple cubic unit cell (sc) How many atoms per unit cell? Body centered cubic unit cell (bcc) Face centered cubic unit cell (fcc)

15 Simple Cubic Lattice

16 Body Centered Cubic (bcc) Lattice

17 Face Centered Cubic (fcc) Lattice

18 Diamond Lattice (Si, Ge, a-sn) 2 interlaced fcc lattices

19 Zincblende Lattice (GaAs, AlAs, InP, CdTe) - 2 interlaced fcc lattices Most III-V semiconductors crystallize in zincblende lattice, which is identical to a diamond lattice except that one of face center cubic sub-lattices has column III (gallium) atom and the other has column V (arsenic) atoms.

20 Rock-salt Lattice (fcc) and Wurtzite lattice (bcc) rock-salt lattice can be considered as 2 interpenetrating facecentered cubic lattices, each atom has six nearest neighbors. wurtzite lattice can be considered as 2 interpenetrating hexagonal close-packed lattices, with a tetrahedral arrangement of four equidistant nearest neighbors.

21 Crystal Structure of Typical Semiconductors Page 21

22 Directions and planes in crystals: Miller indices Crystals along different planes are different (anisotropy) and thus electrical, thermal and mechanical properties are dependent on crystal orientation. Miller Indices are used to identify planes of atoms within a crystal structure. (100) Si vs. (110) Si

23 Definition of Miller indices (hkl) Three steps to define Miller indices of a plane: 1. Find the intercepts of the plane on the three coordinates in terms of lattice constant. 2. Take the reciprocals of these numbers and reduce them to the smallest 3 integers with the same ratio. 3. Enclose the result in parentheses (hkl) as Miller indices for a single plane.

24 Miller indices in cubic lattice If the intercept is at infinity, then the plane is parallel to that axis and the Miller index is zero. Negative numbers are represented by placing a bar over the top of digit.

25 Imperfections in crystal structure vacancy defect: an atom is missing from a particular lattice site interstitial defect: an atom is located between lattice sites Result: Imperfections tend to alter the electrical properties of a material and, in some cases, electrical parameters can be dominated by these defects.

26 Intrinsic & Extrinsic Semiconductor

Charge Carriers in Semiconductor Si as example: at T absolute zero, some electrons are excited across band gap into conduction band - producing current.

27 Charge Carriers in Semiconductor Si as example: at T absolute zero, some electrons are excited across band gap into conduction band - producing current. When electron in pure silicon crosses gap, it leaves behind an electron vacancy ( hole ) on valence band, thus create an electron-hole pair in silicon crystal. Under an external voltage, both electron and hole can move across the material.

28 Charge Carriers in Semiconductor

29 Current flow in a semiconductor

charge carriers in intrinsic semiconductor Electron-hole pairs (EPHs) in covalent bonding model in Si crystal.

30 Intrinsic Semiconductor A perfect semiconductor crystal with no impurities/lattice defects: intrinsic semiconductor. At T = 0 K: No charge carriers Valence band is filled with electrons Conduction band is empty At T > 0 K: Electron-hole pairs generated Electron-hole pairs are the only charge carriers in intrinsic semiconductor Electron-hole pairs (EPHs) in covalent bonding model in Si crystal. Since electron and holes are created in pairs (EHPs) electron concentration in conduction band, n (electron/cm 3 ) is equal to hole concentration in valence band, p (holes/cm 3 ). These intrinsic carrier concentrations is denoted as n i. Thus for intrinsic semiconductor: n = p = n i

31 Increasing conductivity by temperature As temperature increases, the number of free electrons and holes created increases exponentially. Conductivity of semiconductor increases when temperature increases - heat makes some electrons in valence band to move to conduction band. More applied heat - higher the number of electrons with required energy to make conduction band transition and become available as charge carriers. This is how temperature affects the carrier concentration.

Intrinsic Semiconductor For a pure semiconductor like Si, crystal lattice structure forms excellent insulator at room-t, since all atoms are bound to one another and not free for current flow.

32 Intrinsic Semiconductor For a pure semiconductor like Si, crystal lattice structure forms excellent insulator at room-t, since all atoms are bound to one another and not free for current flow. Good insulating semiconductor material is referred to an intrinsic semiconductor. In intrinsic silicon, the outer valence electrons of each atoms are tightly bound together with one another, electrons are difficult for current flow. To be useful in electronic devices, pure semiconductors must be altered so as to greatly increase its conducting properties. Covalent Bonds: Shared electrons to fill orbital Intrinsic Silicon: Poor conductor: No free electrons to carry current Need to engineer electrical properties (conduction)

Enhancing Current flow - Doping Electronic and optical properties of semiconductors are strongly affected by impurities, being added in precisely controlled amounts (impurity concentration of one

33 Enhancing Current flow - Doping Electronic and optical properties of semiconductors are strongly affected by impurities, being added in precisely controlled amounts (impurity concentration of one part per million (ppm) changes a Si sample from a poor conductor to a good conductor), called Doping. The purpose of doping is to increase either the number of free electrons or holes in the semiconductor crystal. Doping to add electrons is called ntype (negative) doping Doping to remove electrons is called p-type (positive) doping

The Doping of Semiconductors: Extrinsic Semiconductors Adding a small percentage of foreign atoms in crystal lattice of Si or Ge produces dramatic changes in electrical properties n-type & p-type

34 The Doping of Semiconductors: Extrinsic Semiconductors Adding a small percentage of foreign atoms in crystal lattice of Si or Ge produces dramatic changes in electrical properties n-type & p-type semiconductors. Penta-valent impurities Impurity atoms with 5 valence electrons produce n-type semiconductors by contributing extra electrons. Tri-valent impurities Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a "hole" or electron deficiency.

N-Type Semiconductor Adding penta-valent impurities like antimony, arsenic or phosphorous contributes free electrons, greatly increasing conductivity of intrinsic

N-Type Band Structure Addition of donor impurities makes extra electron energy levels high in semiconductor band gap - electrons are easily excited into conduction

35 N-Type Semiconductor Adding penta-valent impurities like antimony, arsenic or phosphorous contributes free electrons, greatly increasing conductivity of intrinsic semiconductor. N-Type Band Structure Addition of donor impurities makes extra electron energy levels high in semiconductor band gap - electrons are easily excited into conduction band. This shifts the effective Fermi level to a point about halfway between the donor levels and the conduction band. Electrons can be elevated to conduction band with the energy provided by an applied voltage and move through material. Electrons are termed as "majority carriers" for current flow in n-type semiconductors.

36 Current Flow in N-type Semiconductors DC voltage source has a positive terminal that attracts free electrons in semiconductor and pulls them away from their atoms leaving atoms charged positively. Electrons from negative terminal of supply enter semiconductor and are attracted by the positive charge of atoms missing one of their electrons. Electrons flows from negative terminal to positive terminal.

P-Type Semiconductor P-Type Band Structure Addition of trivalent impurities like boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons, called

Addition of acceptor impurities contributes hole levels low in semiconductor band gap so that electrons are easily excited from valence band into these levels, leaving mobile holes in

37 P-Type Semiconductor P-Type Band Structure Addition of trivalent impurities like boron, aluminum or gallium to an intrinsic semiconductor creates deficiencies of valence electrons, called "holes". Addition of acceptor impurities contributes hole levels low in semiconductor band gap so that electrons are easily excited from valence band into these levels, leaving mobile holes in valence band. This shifts effective Fermi level to a point about halfway between acceptor levels and valence band. Electrons can be elevated from valence band to hole levels in band gap with the energy provided by an applied voltage. Since electrons can be exchanged between holes, the holes are termed as "majority carriers" for current flow in a p-type semiconductor.

38 Current Flow in P-type Semiconductors Electrons from negative supply terminal are attracted to positive holes and fill them. While positive terminal pulls electrons from holes - leaving the holes to attract more electrons. Inside the semiconductor, current flow is actually by the movement of holes from positive to negative.

39 Bands for Doped Semiconductors Bands of n-type and p-type semiconductors shows extra levels by impurities. In n-type material, there are electron energy levels near the top of band gap so that they can be easily excited into the conduction band. In p-type material, extra hole levels in the band gap allow excitation of valence band electrons, leaving mobile holes in the valence band. Fermi level: For n-type material, Fermi level moves to higher energy, for p- type material, Fermi level moves to lower energy.

Doping methods: (1) Diffusion Diffusion Phosphorous added into Si by diffusion of phosphine gas (PH 3 ). Diborane gas (B 2 H 6 ) is used to diffuse boron into Si.

40 Doping methods: (1) Diffusion Diffusion Phosphorous added into Si by diffusion of phosphine gas (PH 3 ). Diborane gas (B 2 H 6 ) is used to diffuse boron into Si. Impurity diffusion occurs when a semiconductor crystal is placed in a high T (~ 1000 ºC) gas atmosphere containing desired impurity atom. At this high T, many crystal atoms randomly move in and out of their single-crystal lattice sites. Vacancies are created so that impurity atoms can move through lattice by hopping from one vacancy to another. Impurity diffusion is a process of impurity particles move from a region of high concentration near the surface to a region of lower concentration within the crystal. When T decreases, the impurity atoms become permanently frozen into the lattice sites. Diffusion of various impurities into selected regions of a semiconductor allows us to fabricate complex electronic circuits in a single semiconductor crystal.

41 Doping methods: (2) Ion Implantation Ion Implantation Ion implantation generally takes place at a lower T than diffusion. A beam of impurity ions accelerated to kinetic energies of 50 kev or greater and then directed to semiconductor surface. High-energy impurity ions enter crystal and rest at some average depth from the surface. Advantage: controlled numbers of impurity atoms can be introduced into specific regions of crystal. Disadvantage: incident impurity atoms collide with crystal atoms, causing lattice-displacement damage. However, most lattice damages can be removed by thermal annealing, in which the T of crystal is raised for a short time. Thermal annealing is a required step after implantation.

42 Diffusion vs. Ion Implantation Diffusion Advantages No damage created by doping Batch fabrication Problems Usually limited to solid solubility Low surface concentration very difficult Low dose very difficult Ion Implantation Room temperature mask Precise dose control ( atoms cm -2 doses) Accurate depth control Implant damage enhances diffusion Dislocations caused by damage may cause junction leakage Implant channeling may affect profile

44 Carrier concentrations (n i ) at a given temperature Intrinsic (undoped) semiconductor At low T: carrier density ~ 0. At higher T: carriers are excited. Extrinsic (doped) semiconductor At low T: carrier density increases as dopant carriers are thermally excited. At intermediate T, carriers density ~ constant (= doping density). At high T, intrinsic carrier excitation dominates.

Conductivity and Resistivity Measurement

45 Conductivity and Resistivity Measurement Four-point probe method V I w CF cm where CF is a well-documented correction factor. The correction factor depends on the ratio of d/s, where s is the probe spacing. When d/s > 20, the correction factor approaches 4.54.

46 Conductivity of Intrinsic Semiconductors

47 Conductivity of Extrinsic Semiconductors Si (300 K) intrinsic carrier density n i : /m 3, μ n : m 2 /Vs, μ p : m 2 /Vs; Extrinsic Si doped with As typical concentration atoms/m 3 : Majority carriers n 0 = e/m 3 ; Mass action law: n i 2 = n 0 p 0 Minority carriers: p 0 = ( ) 2 /10 21 = holes/m 3 Conductivity: Majority carriers: n = (e/m 3 )(m 2 /Vs)(A s C) =0.216 ( cm) -1 Minority carriers: p = = ( cm) -1 Conductivity total total = n + p ( cm) -1

48 The Hall Effect The carrier concentration in a semiconductor may be different from the impurity concentration, because the ionized impurity density depends on the temperature and the impurity energy level. To measure the carrier concentration directly, the most commonly used method is the Hall effect. Hall measurement is also one of the most convincing methods to give directly the carrier type. Basic setup to measure carrier concentration using the Hall effect: an electric field applied along the x- axis and a magnetic field applied along the z-axis.

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

Semiconductor Very Basics

Semiconductor Very Basics Material (mostly) from Semiconductor Devices, Physics & Technology, S.M. Sze, John Wiley & Sons Semiconductor Detectors, H. Spieler (notes) July 3, 2003 Conductors, Semi-Conductors,