Electrical Properties of Materials EEE-3515 Ch# 1 (Crystal Structures) Part 2

1. Electrical Properties of MaterialsEEE-3515Ch# 1 (Crystal Structures)Part 2 Dr. (Ph.D) Mohammad Aminul Islam International Islamic University Chittagong Department of Electrical and Electronic Engineering

2. Book Reference 1. Solid state Electronic Device - Ben G. Stretman & Sanjay Kumar Banerjee 2.  Principles of Electronic Materials and Devices - S.O. Kasap M A Islam, EEE, IIUC

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4. Millar Indices Millar indices are symbolic vector representation for the orientation of an atomic plane in a crystal lattice and are defined as the reciprocals of the functional intercepts which the plane makes with the crystallographic axis. Millar indices for lattice direction of are represented by a set of 3 integer [1,1,1] Families of direction <a b c> Millar indices for crystallographic planes are represented by a set of 3 integer (1,1,1) M A Islam, EEE, IIUC

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12. Directions using the Miller system M A Islam, EEE, IIUC

13. Directions using Miller system M A Islam, EEE, IIUC

14. Directions using the Miller system M A Islam, EEE, IIUC

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18. Planes Indexing M A Islam, EEE, IIUC

19. Planes Indexing M A Islam, EEE, IIUC

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22. Crystallographic planes in cubic lattice M A Islam, EEE, IIUC

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27. Millar Indices for Three Types of cubic lattices M A Islam, EEE, IIUC

28. Various planes in cubic lattice z x y M A Islam, EEE, IIUC

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31. CRYSTAL DEFECTS • Metals are not perfect neither at the macro level and nor at the micro level • Contain a number of different types of crystalline defects (at the atomic level) • Defects are important in many processes • e.g. diffusion, deformation The TEM allowed experimental evidence to be collected that showed that the strength and ductility of metals are controlled by dislocations. M A Islam, EEE, IIUC

32. Type of Defects • Point Defects • Liner Defects • Planar Defects Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. M A Islam, EEE, IIUC

33. 1. POINT DEFECTS Point Defects due to lattice or distortion of few atomic distances. Point defect includes: a. lattice vacancies b. Self-interstitial defects c. Substitution impurity atoms (defects) d. Interstitial impurity atoms (defects) e. Scotty Defect f. Frenkel Defect M A Islam, EEE, IIUC

34. a. lattice vacancies Vacancies are empty spaces where an atom should be but missing. They are common, especially at high temperature where atoms are frequently and random change their position leveling behind empty lattice sites. In most cases diffusion can only occur because of vacancies. M A Islam, EEE, IIUC

35. b. Self-interstitial defects Occur only in low concentration in metal because they distort and highly stress the tightly packed lattice structure. 1 2 In pure metals 1. VACANCY 2. SELF-INTERSTITIAL M A Islam, EEE, IIUC

36. c. Substitution impurity defects Substitutional impurity atom is an atom of a different type than the bulk atoms, which has replaced one the bulk atoms in the lattice. Substitutional impurity atom are closely in size to the bulk atom. This atom are much smaller then the atom in bulk matrix d. Inferential impurity defects M A Islam, EEE, IIUC

37. 1. SUBSTITUTIONAL IMPURITY 2. INTERSTITIAL IMPURITY Important in strengthening M A Islam, EEE, IIUC

38. e. Scotty Defect Missing cations (Na+) and anions (Cl-) pair which may have migrated to the surface M A Islam, EEE, IIUC

39. f. Frenkel Defect • take cation out of position and • cram it into an interstitial site (void between normal atomic position). • Ag+ surrounded by 4Cl- stabilizes this defect. • tendency for vacancy and intersti-tial to form nearby pair. • also a stoichiometric defect • (vacancies = interstitials). M A Islam, EEE, IIUC

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41. 2. Linear defects: DISLOCATIONS • Dislocations are linear defects; they are lines through the crystal along which crystallographic registry is lost. • Dislocations also participate in the growth of crystals and in the structures of interfaces between crystals. • They act as electrical defects in optical materials and semiconductors, in which they are almost always undesirable. • Edge Dislocations • Screw Dislocations M A Islam, EEE, IIUC

42. Edge Dislocations The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. This line runs along the top of the extra half-plane. The inter-atomic bonds are significantly distorted only in the immediate vicinity of the dislocation line. M A Islam, EEE, IIUC

43. Screw Dislocations The motion of a screw dislocation is also a result of shear stress, but the defect line movement is perpendicular to direction of the stress and the atom displacement, rather than parallel. To visualize a screw dislocation, imagine a block of metal with a shear stress applied across one end so that the metal begins to rip. This is shown in the upper right image. The lower right image shows the plane of atoms just above the rip. M A Islam, EEE, IIUC

44. Screw Dislocations The atoms represented by the blue circles have not yet moved from their original position. The atoms represented by the red circles have moved to their new position in the lattice and have reestablished metallic bonds. The atoms represented by the green circles are in the process of moving. It can be seen that only a portion of the bonds are broke at any given time. As was the case with the edge dislocation, movement in this manner requires a much smaller force than breaking all the bonds across the middle plane simultaneously. M A Islam, EEE, IIUC

45. Planar Defects: two-dimensional defects • The two-dimensional defects that appear in crystals can be usefully • divided into three types: • free surfaces, which are the external surfaces at which the solid • terminates at a vapor or liquid. • intercrystalline boundaries, which separate grains or distinct phases • within the solid and • internal defects that disrupt the crystalline pattern over a surface • within a crystal. • All of these defects have two important characteristics. • First, since they are surfaces in a crystal they have particular atomic structures that depend on orientation. • Second, they have a positive energy. The energy per unit area is ordinarily equal to the surface tension, ß, of the interface. M A Islam, EEE, IIUC

46. Planar Defects: two-dimensional defects Fig. Solid surface showing segregated solutes and adsorbates Fig. intercrystalline boundary defects M A Islam, EEE, IIUC

47. Intercrystalline boundary defects: TEM image M A Islam, EEE, IIUC

48. Common semiconductor crystal structures • The most common crystal structure among frequently used semiconductors is the diamond lattice • Each atom in the diamond lattice has a covalent bond with four adjacent atoms, which together form a tetrahedron. • This lattice can also be formed from two face-centered-cubic lattices, which are displaced along the body diagonal of the larger cube in Figure by one quarter of that body diagonal. The diamond lattice therefore is a face-centered-cubic lattice with a basis containing two identical atoms. M A Islam, EEE, IIUC The diamond lattice of Si(silicon) and Ge(germanium)

49. zinc-blend crystal structure • Compound semiconductors such as GaAs and InP have a crystal structure that is similar to that of diamond. However, the lattice contains two different types of atoms. • Each atom still has four covalent bonds, but these are bonds to atoms of the other type. This structure is referred to as the zinc-blend lattice, named after zinc-blend crystal (ZnS) as shown in Figure The zinc-blend or FCC crystal structure of GaAs and InP. The cubic crystals are characterized by a single parameter, the lattice constant a, M A Islam, EEE, IIUC

50. Production : Starting material Si • Silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal, in an electric arc furnace using carbon electrodes. At temperatures over 1,900 ℃, the carbon reduces the silica to silicon according to the chemical equation • SiO2 + C → Si + CO2. • SiO2 + 2C → Si + 2CO. M A Islam, EEE, IIUC