1. ECE 371 – Chapter 1 Crystal Structure of solids

2. 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  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 2

3. Classification of semiconductors  On the basis of the periodic chart ECE 317 Chapter 1 Crystal structure of solids 3 Group IVIII-VII-VI ElementalCompound

4. Group IV semiconductors  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. ECE 317 Chapter 1 Crystal structure of solids 4

5. 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 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. ECE 317 Chapter 1 Crystal structure of solids 5 Group III Group V

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

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

8. 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.  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: Al 0.7 Ga 0.3 As. 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: GaAs 0.6 P 0.4. 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 8

9. Binaries and ternaries (cont.) ECE 317 Chapter 1 Crystal structure of solids 9 Group III Group V Ga Ga Ga Ga Ga As As As As As Ga Ga Ga Ga Ga As As As As As Group III Group V GaAs – Binary alloy Group III Group V Al Ga Al Al Ga Al Al Ga Al Al As As As As As Al 0.7 Ga 0.3 As – Ternary alloy Ga Ga Ga Ga Ga As P As As P As As P As P Group III Group V GaAs 0.6 P 0.4 – Ternary alloy

10. Quaternary alloys  Three group IIIs one group V Ex: Al 0.3 Ga 0.3 In 0.4 As  Two group IIIs and two group Vs Ex: Al 0.4 Ga 0.6 As 0.2 Sb 0.8  One group III and three group Vs. Ex: GaAs 0.8 Sb 0.1 P 0.1  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 Al 0.1 Ga 0.9 In 0.1 As 0.7 Sb 0.2 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 10

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

12. Lattice and basis  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 a 1 in one direction and b 1 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. ECE 317 Chapter 1 Crystal structure of solids 12 See fig. 1.2

13. 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.  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 13

14. Bravais Lattices  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. ECE 317 Chapter 1 Crystal structure of solids 14 Auguste Bravais

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

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

17. Class problem #1  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 ECE 317 Chapter 1 Crystal structure of solids 17

18. Defining planes (hkl)  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. ECE 317 Chapter 1 Crystal structure of solids 18

19. 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 a o = 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. ECE 317 Chapter 1 Crystal structure of solids 19

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

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

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

23. Atomic bonding  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 H 2.  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 23

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

25. ECE 317 Chapter 1 Crystal structure of solids 25

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

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

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

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

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

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