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ECE 371 – Chapter 1

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 Insulator – prevents flow of current ex: rubber

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ECE 371 – Chapter 1

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  1. ECE 371 – Chapter 1 Crystal Structure of solids

  2. Classifying materials on the basis of their ability to conduct current. ECE 317 Chapter 1 Crystal structure of solids Conductor – allows for flow of current ex: copper 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.

  3. Classification of semiconductors Group IV III-V II-VI Elemental Compound ECE 317 Chapter 1 Crystal structure of solids On the basis of the periodic chart

  4. Group IV semiconductors ECE 317 Chapter 1 Crystal structure of solids Consists of Carbon, Silicon and Germanium. 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.

  5. III-V compound semiconductors Group III Group V ECE 317 Chapter 1 Crystal structure of solids 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 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 technologies. III-Sb alloys are called antimonides these are used for high speed electronics and mid-IR technologies like countermeasures lasers and thermal cameras.

  6. CD Vs DVD Vs Blue-ray AlGaAs laser InGaP laser InGaN laser ECE 317 Chapter 1 Crystal structure of solids

  7. II-VI semiconductors Group II Group VI ECE 317 Chapter 1 Crystal structure of solids Mainly used in detectors made of HgCdTe. These detectors are very useful for MWIR and LWIR applications such as thermal sensing and night vision.

  8. classification for compound semiconductors based on number of constituent elements ECE 317 Chapter 1 Crystal structure of solids 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. 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.

  9. 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 Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Group III As As As As As As As As As As Group V 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 Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga Group III As P As As P As As P As P Group V ECE 317 Chapter 1 Crystal structure of solids

  10. Quaternary alloys ECE 317 Chapter 1 Crystal structure of solids Three group IIIs one group V Ex: Al0.3Ga0.3In0.4As Two group IIIs and two group Vs Ex: Al0.4Ga0.6As0.2Sb0.8 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.

  11. Types of solids ECE 317 Chapter 1 Crystal structure of solids Amorphous – no order in the atoms. Poly-crystalline – short range order. Single crystal – Long range order. See fig. 1.1 in neamen.

  12. Lattice and basis See fig. 1.2 ECE 317 Chapter 1 Crystal structure of solids The lattice is a periodic arrangement of points in space. Each point on the lattice is called a Lattice point. (duh!) 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 one direction and b1 in another non-colinear direction. This results in a 2-D lattice. A third translation along another non-colinear direction results in a 3-D lattice.

  13. Unit Cell ECE 317 Chapter 1 Crystal structure of solids 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. 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

  14. Bravais Lattices Auguste Bravais ECE 317 Chapter 1 Crystal structure of solids The number of ways in which lattice points can be specified in space while maintaining translational symmetry, is limited. Auguste Bravais demonstrated 14 types of such point lattices in 1848. Nobody has come up with new ones since.

  15. The 14 bravais lattices ECE 317 Chapter 1 Crystal structure of solids

  16. Cubic lattices ECE 317 Chapter 1 Crystal structure of solids Simple cubic (SC) Body-centered cubic (BCC) Face centered cubic (FCC) See fig 1.5 in the text.

  17. Class problem #1 ECE 317 Chapter 1 Crystal structure of solids • Calculate the packing fraction of a BCC cell assuming spherical atoms. • If the interatomic distance is 5 Å what is the density of atoms in the crystal. • Do the same for • SC • FCC

  18. Defining planes (hkl) ECE 317 Chapter 1 Crystal structure of solids See Fig. 1.6 for an example of a plane. Miller indices are an effective nomenclature for naming planes. 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.

  19. Class problems ECE 317 Chapter 1 Crystal structure of solids 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. 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.

  20. Expressing directions ECE 317 Chapter 1 Crystal structure of solids Fig. 1.9 So (hkl) is the plane, [hkl] is the direction.

  21. Diamond structure ECE 317 Chapter 1 Crystal structure of solids

  22. GaAs - ZincBlende ECE 317 Chapter 1 Crystal structure of solids

  23. Atomic bonding ECE 317 Chapter 1 Crystal structure of solids • Ionic bond: Na+Cl- • Covalent bond – sharing e- to complete 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

  24. Imperfections in solids ECE 317 Chapter 1 Crystal structure of solids • Lattice vibrations • Point defect • Vacancy • Interstitial • Frenkel defect (vacancy-interstitial) • Line dislocation

  25. ECE 317 Chapter 1 Crystal structure of solids

  26. Point defect ECE 317 Chapter 1 Crystal structure of solids

  27. Impurities in solids ECE 317 Chapter 1 Crystal structure of solids Substitution Interstitial Doping

  28. Semiconductor growth ECE 317 Chapter 1 Crystal structure of solids

  29. From a melt ECE 317 Chapter 1 Crystal structure of solids

  30. Epitaxy - MOCVD ECE 317 Chapter 1 Crystal structure of solids

  31. Epitaxy -MBE ECE 317 Chapter 1 Crystal structure of solids

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