Presentation 2 (crystal structure and defect)

Presentation 2 (crystal structure and defect) - Types Types...

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Unformatted text preview: Types Types of Semiconductors Lattice Lattice & Unit Cells The line of dots is called the latticeand each lattice point (dot) must have identical surroundings. A unit cell is an imaginary parallelepiped from which the entire crystal can be built from which the entire crystal can be built by simple displacement operations only. Unit cell fit perfectly without any empty cell fit perfectly without any empty space. Unit cells may be chosen in a variety of ways. Lattice Lattice & Unit Cells Crystal Crystal Systems Cubic BCC FCC Atomic Atomic Density BCC Crystallographic Crystallographic Planes The heavy arrows in each case illustrate crystal directions, designated [hkl] The red area are the corresponding crystal planes, red area are the corresponding crystal planes designated (hkl) Crystallographic Crystallographic Planes Crystallographic Crystallographic Planes Crystallographic Crystallographic Planes Crystallographic Crystallographic Planes Crystallographic Crystallographic Planes Silicon Silicon Crystal http://jas.eng.buffalo.edu/education/solid/unitCell/home.html Silicon Crystal Single FCC unit cell Two FCC unit cells offset by a/4 Silicon Silicon Crystal Standard Standard Flat Orientation for Different Si Wafers Ni = N0 exp (-Ea/kT) N0 : # density of atoms in the crystal lattice density of atoms in the crystal lattice (#/cm3) Ni : # density of intrinsic carrier (#/cm3) Ea : active energy associated wit the energy associated wit the formation of a vacancy (eV) k : Boltzmann constant (8.617×10-5 eV/K) T : temperature(K) Si: Ni = 5.02×1022×exp(-2.6eV/kT) Ni = 1.4×1010/cm3 for Si at RT Crystal Crystal Defects Point Point Defects Intrinsic defects defects Vacancy: vacancy is a lattice site without an atom Self-Interstitial: a same atom is not on a lattice site, but in the space between the lattice positions th Frenkel defect: an interstitial and its associated vacancy extrinsic defects defects Substitution impurity: an impurity atom at an intersititial site or at a lattice site Line Defects A missing line or additional line of atoms is called a dislocation. Line defects can be classified into edge dislocation and dislocation loop loop Line Line Defects Edge dislocation Extra plane is terminated on one end by the crystal edge one end by the crystal edge Dislocation loop Extra plane is completely contained (a closed loop) contained (a closed loop) The term, “threading”, describes specific cases in which the dislocation threads cases in which the dislocation “threads” through a grown layer (i.e. starts at or near the substrate and ends on the surface). Dislocation Dislocation Moving Climb The growth (shrinkage) of a dislocation line from point defects caused by point defects agglomeration by point defects agglomeration Glide Movement of a dislocation in a direction other than along the line caused by shear stress Area Defects stacking fault An extra plane of atoms formed and the pattern is disrupted in 2D and only regular in the third. Often happens with an insertion or removal of a {111} plane Bulk Defects crystalline precipitate Bulk Defects crystalline precipitate irregular all 3D. Impurity precipitate Area Area Defects stacking fault Bulk Bulk Defects crystalline precipitate Precipitates are three dimensional defects that have a different chemical makeup from the host lattice. They can result from an impurity exceeding the maximum solubility of the crystal. Generally, these defects are harmful, but they do have some technological value oxygen gettering form denuded zone do have some technological value - oxygen gettering to form a denuded zone and As precipitates for high speed optical devices. Internal Gettering Using internal defects to trap impurities (e.g., O) away from the O) th active region of the wafer. External Gettering Roughing the backside of the wafer to provide a low energy “sink” for impurities. ...
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