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ECE 371 – Chapter 1 Crystal Structure of Solids Classifying Materials on the Basis of Their Ability to Conduct Current

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ECE 371 – Chapter 1 Crystal Structure of Solids Classifying Materials on the Basis of Their Ability to Conduct Current ECE 371 – Chapter 1 Crystal Structure of solids Classifying materials on the basis of their ability to conduct current. Conductor – allows for flow of current ex: copper 2 . Insulator – prevents flow of current ex: rubber . Semiconductor - A semiconductor is a substance, usually a solid chemical element or compound, that can conduct electricity under some conditions but not others, making it a good medium for the control of electrical current. ECE 317 Chapter 1 Crystal structure of solids Classification of semiconductors . On the basis of the periodic chart 3 Group IV III-V II-VI Elemental Compound ECE 317 Chapter 1 Crystal structure of solids Group IV semiconductors . Consists of Carbon, Silicon and Germanium. 4 . Silicon is the dominant semiconductor material. Germanium has certain niche uses in high speed electronics, optoelectronics and photovoltaics. Carbon semiconductor research is currently being conducted with very promising results with carbon nanotube, diamond and graphene based semiconductors. ECE 317 Chapter 1 Crystal structure of solids III-V compound semiconductors . Consists of group III and group V elements. This class of material is considered as alloys. III-N also referred to as nitrides are the basis of 5 most visible light emitting diodes and lasers in the blue to green range. Ex: Blue-ray DVD players . III-P alloys are called phosphides – mainly used for red lasers and solar cells. III-As are referred to as arsenides used for a variety of near-IR opto-electronic and electronic Group V technologies. III-Sb alloys are called antimonides these are used for high speed electronics and mid-IR technologies like countermeasures lasers and Group III thermal cameras. ECE 317 Chapter 1 Crystal structure of solids CD Vs DVD Vs Blue-ray AlGaAs InGaP InGaN laser laser laser 6 ECE 317 Chapter 1 Crystal structure of solids II-VI semiconductors . Mainly used in detectors made of HgCdTe. These detectors are very useful for MWIR and LWIR applications such as 7 thermal sensing and night vision. Group VI Group II ECE 317 Chapter 1 Crystal structure of solids classification for compound semiconductors based on number of constituent elements . Binary: One group III and one group V. Simplistic model consists of one layer of group III and one layer of group V. Group III and V atomic site are mutually exclusive to their respective elements. Ex: GaAs, InP. 8 . Ternary: Three elements in all. Could be two group IIIs and one group V or vice-versa. Again group III sites and group V sites are exclusive thus in ternary with two group III species the group III atoms divide the spots up amongst themselves. Ex1: Al0.7Ga0.3As. Here 70% of the group III sites are occupied by Al and the rest by Ga and 100% of the group V sites are taken by As. Ex2: GaAs0.6P0.4. Here 100% of the group III sites are occupied by Ga and 60% of the group V sites are occupied by As and the rest and 40% of the group V sites are taken by P. ECE 317 Chapter 1 Crystal structure of solids Binaries and ternaries (cont.) GaAs – Binary alloy Group III Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Group V As As As As As As As As As As 9 Group III Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Group V As As As As As As As As As As Al0.7Ga0.3As – Ternary alloy Group III Al Ga Al Al Ga Al Al Ga Al Al Group V As As As As As As As As As As GaAs0.6P0.4 – Ternary alloy Group III Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Group V As P As As P As As P As P ECE 317 Chapter 1 Crystal structure of solids Quaternary alloys . Three group IIIs one group V Ex: Al0.3Ga0.3In0.4As . Two group IIIs and two group Vs Ex: Al0.4Ga0.6As0.2Sb0.8 10 . One group III and three group Vs. Ex: GaAs0.8Sb0.1P0.1 . Verify this yourself – in the above examples all the group III constituents add to give a 100% and all the group V constituents add to give 100%. Can you think of a quintinary (5 element) alloy? Is Al0.1Ga0.9In0.1As0.7Sb0.2 a valid composition? (hint: its not ). Feel free to change the compositions of this alloy to make it correct. ECE 317 Chapter 1 Crystal structure of solids Types of solids . Amorphous – no order in the atoms. Poly-crystalline – short range order. 11 . Single crystal – Long range order. See fig. 1.1 in neamen. ECE 317 Chapter 1 Crystal structure of solids Lattice and basis . The lattice is a periodic arrangement of points in space. Each point on the lattice is called a Lattice point. (duh!) 12 . The basis consists of the simplest arrangement of atoms which is repeated at every point in the lattice to build up the crystal structure. Translation to produce the lattice: Each lattice point can be translated by a1 in See fig. one direction and b1 in another non- 1.2 colinear direction. This results in a 2-D lattice. A third translation along another non-colinear direction results in a 3-D lattice. ECE 317 Chapter 1 Crystal structure of solids Unit Cell . Mathematical Definition (from P.K. Bhattacharya): A unit cell is the region of a crystal defined by vectors a, b and c and the angles α, β and γ such which when translated by integral multiples of those vectors reproduce a similar region of the crystal. OR A unit cell is a small volume of the crystal that can be used to reproduce the entire crystal. 13 . See fig. 1.3 . Translation property: r = ha + kb + lc a,b,c are basis vectors. r is the translational vector. a, b and c could be inter-atomic distances in which case they are called lattice-constants. Primitive Cell: A primitive cell is the smallest unit cell in volume that can be defined for a specific lattice. See fig. 1.4 ECE 317 Chapter 1 Crystal structure of solids Bravais Lattices . The number of ways in which lattice Auguste Bravais points can be specified in space while maintaining translational symmetry, is 14 limited. Auguste Bravais demonstrated 14 types of such point lattices in 1848. Nobody has come up with new ones since. ECE 317 Chapter 1 Crystal structure of solids The 14 bravais lattices 15 ECE 317 Chapter 1 Crystal structure of solids Cubic lattices . Simple cubic (SC) . Body-centered cubic (BCC) 16 . Face centered cubic (FCC) . See fig 1.5 in the text. ECE 317 Chapter 1 Crystal structure of solids Class problem #1 . Calculate the packing fraction of a BCC cell assuming spherical atoms. 17 . If the interatomic distance is 5 Å what is the density of atoms in the crystal. Do the same for . SC . FCC ECE 317 Chapter 1 Crystal structure of solids Defining planes (hkl) . See Fig. 1.6 for an example of a plane. Miller indices are an effective nomenclature for naming planes. 18 . Miller indices refer to the integers (hkl). Ex: (110), (111), (100) See fig. 1.7 . All parallel planes have the same indices and are equivalent to each other. So avoid planes through the origin. ECE 317 Chapter 1 Crystal structure of solids Class problems . Example 1.3, see fig. 1.8 . Problem #2: TYU E 1.3 Determine the distance between the nearest (110) planes in a SC lattice with a lattice constant of ao = 4.83 Å. 19 . Problem #3: TYU E 1.4 The lattice constant of a FCC structure is 4.75 Å. Calculate the surface density of atoms for (a) a (100) plane and (b) a (110) plane. ECE 317 Chapter 1 Crystal structure of solids Expressing directions . Fig. 1.9 . So (hkl) is the plane, [hkl] is the 20 direction. ECE 317 Chapter 1 Crystal structure of solids Diamond structure 21 ECE 317 Chapter 1 Crystal structure of solids GaAs - ZincBlende 22 ECE 317 Chapter 1 Crystal structure of solids Atomic bonding . Ionic bond: Na+Cl- . Covalent bond – sharing e- to complete 23 an octet . H need only one atom to complete the octet and therefore we only have H2. Silicon needs 4 e- and so can bond to four other Si atoms, forming a crystal. Metallic bond . Van der Waals ECE 317 Chapter 1 Crystal structure of solids Imperfections in solids . Lattice vibrations . Point defect 24 . Vacancy . Interstitial . Frenkel defect (vacancy-interstitial) . Line dislocation ECE 317 Chapter 1 Crystal structure of solids 25 ECE 317 Chapter 1 Crystal structure of solids Point defect 26 ECE 317 Chapter 1 Crystal structure of solids Impurities in solids . Substitution . Interstitial 27 . Doping ECE 317 Chapter 1 Crystal structure of solids Semiconductor growth 28 ECE 317 Chapter 1 Crystal structure of solids From a melt 29 ECE 317 Chapter 1 Crystal structure of solids Epitaxy - MOCVD 30 ECE 317 Chapter 1 Crystal structure of solids Epitaxy -MBE 31 ECE 317 Chapter 1 Crystal structure of solids .
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