impurity segregation n.
Skip this Video
Loading SlideShow in 5 Seconds..
Impurity Segregation PowerPoint Presentation
Download Presentation
Impurity Segregation

Impurity Segregation

139 Views Download Presentation
Download Presentation

Impurity Segregation

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Impurity Segregation Where Co is the initial concentration of th impurity in the melt

  2. Float Zone

  3. Impurity Segregation Where Co is the initial concentration of the impurity in the solid and L is the width of the melted region within RF coil

  4. Impurity Segregation

  5. Bridgeman • Used for some compound semiconductors • Particularly those that have a high vapor pressure • Produced “D” shaped boules

  6. Crystalline Defects • Point Defects • Vacancies • Impurities • Antisite Defects • Line Defects • Dislocations • Edge • Loop • Volume Defects • Voids • Screw Dislocations

  7. Edge Dislocation

  8. Screw Dislocation

  9. Strain induced Dislocations • The temperature profile across the diameter of a boule is not constant as the boule cools • the outer surface of the boule contracts at a different rate than the internal region • Thermal expansion differences produces edge dislocations within the boule • Typical pattern is a “W”

  10. Strain due to Impurities • An impurity induces strain in the crystal because of differences in • ionic radius as compared to the atom it replaced • Compressive strain if the ionic radius is larger • Tensile strain if the ionic radius is smaller • local distortions because of Coulombic interactions • Both cause local modifications to Eg

  11. Dislocation Count • When you purchase a wafer, one of the specifications is the EPD, Etch Pit Density • Dislocations etch more rapidly in acid than crystalline material • Values for EPD can run from essentially zero (FZ grown under microgravity conditions) to 106 cm-2 for some materials that are extremely difficult to grow. • Note that EPD of 106 cm-2 means that there is a dislocation approximately every 10mms.

  12. Wafer Manufacturing • Boules are polished into cylinders • Aligned using an x-ray diffraction system • Cut into slices using a diamond edged saw • Slices are then polished smooth using a colloidal grit • Mechanical damage from sawing causes point defects that can coalesce into edge dislocations if not removed


  14. Epitaxial Material Growth • Liquid Phase Epitaxy (LPE) • Vapor Phase Epitaxy (VPE) • Molecular Beam Epitaxy (MBE) • Atomic Layer Deposition (ALD) or Atomic Layer Epitaxy (ALE) • Metal Organic Chemical Vapor Deposition (MOCVD) or Organometallic Vapor Phase Epitaxy (OMVPE)

  15. MBE • Wafer is moved into the chamber using a magnetically coupled transfer rod • Evaporation and sublimation of source material under ultralow pressure conditions (10-10 torr) • Shutters in front of evaporation ovens allow vapor to enter chamber, temperature of oven determines vapor pressure • Condensation of material on to a heated wafer • Heat allows the atoms to move to appropriate sites to form a crystal

  16. Schematic View


  18. Advantages • Slow growth rates • In-situ monitoring of growth • Extremely easy to prevent introduction of impurities

  19. Disadvantages • Slow growth rates • Difficult to evaporate/sublimate some materials and hard to prevent the evaporation/sublimation of others • Hard to scale up for multiple wafers • Expensive

  20. MOCVD • Growths are performed at room pressure or low pressure (10 mtorr-100 torr) • Wafers may rotate or be placed at a slant to the direction of gas flow • Inductive heating (RF coil) or conductive heating • Reactants are gases carried by N2 or H2 into chamber • If original source was a liquid, the carrier gas is bubbled through it to pick up vapor • Flow rates determines ratio of gas at wafer surface

  21. Schematic of MOCVD System


  23. Advantages • Less expensive to operate • Growth rates are fast • Gas sources are inexpensive • Easy to scale up to multiple wafers

  24. Disadvantages • Gas sources pose a potential health and safety hazard • A number are pyrophoric and AsH3 and PH3 are highly toxic • Difficult to grow hyperabrupt layers • Residual gases in chamber • Higher background impurity concentrations in grown layers

  25. Misfit Dislocations • Occur when the difference between the lattice constant of the substrate and the epitaxial layers is larger than the critical thickness.

  26. Carrier Mobility and Velocity • Mobility - the ease at which a carrier (electron or hole) moves in a semiconductor • Symbol: mn for electrons and mp for holes • Drift velocity – the speed at which a carrier moves in a crystal when an electric field is present • For electrons: vd = mn E • For holes: vd = mp E

  27. L H W Va Va

  28. Resistance

  29. Resistivity and Conductivity • Fundamental material properties

  30. Resistivity n-type semiconductor p-type semiconductor

  31. Drift Currents

  32. Diffusion • When there are changes in the concentration of electrons and/or holes along a piece of semiconductor • the Coulombic repulsion of the carriers force the carriers to flow towards the region with a lower concentration.

  33. Diffusion Currents

  34. Relationship between Diffusivity and Mobility

  35. Mobility vs. Dopant Concentration in Silicon

  36. Wafer Characterization • X-ray Diffraction • Crystal Orientation • Van der Pauw or Hall Measurements • Resistivity • Mobility • Four Point Probe • Resisitivity • Hot Point Probe • n or p-type material

  37. Van der Pauw • Four equidistant Ohmic contacts • Contacts are small in area • Current is injected across the diagonal • Voltage is measured across the other diagonal Top view of Van der Pauw sample

  38. Calculation • Resistance is determined with and without a magnetic field applied perpendicular to the sample. F is a correction factor that takes into account the geometric shape of the sample.

  39. Hall Measurement • See for a more complete explanation

  40. Calculation • Measurement of resistance is made while a magnetic field is applied perpendicular to the surface of the Hall sample. • The force applied causes a build-up of carriers along the sidewall of the sample • The magnitude of this buildup is also a function of the mobility of the carriers where A is the cross-sectional area.

  41. Four Point Probe • Probe tips must make an Ohmic contact • Useful for Si • Not most compound semiconductors

  42. Hot Point Probe • Simple method to determine whether material is n-type or p-type • Note that the sign of the Hall voltage, VH, and on D R13,24 in the Van der Pauw measurement also provide information on doping.