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

Float Zone /crystalgrowth.htm

Impurity Segregation Where C o is the initial concentration of the impurity in the solid and L is the width of the melted region within RF coil

Impurity Segregation Atom CuAgAuCGeSnAs koko 4 · 10 –4 10 –6 2.5 · 10 –5 7 · 10 –2 3.3 · 10 –2 1.6 · 10 –2 0.3 Atom OBGaFeCoNiSb koko · 10 –3 8 · 10 –6 4 · 10 –4 2.3 · 10 –2

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

Crystalline Defects Point Defects Point Defects –Vacancies –Impurities –Antisite Defects Line Defects Line Defects –Dislocations  Edge  Loop Volume Defects Volume Defects –Voids –Screw Dislocations

Edge Dislocation

Screw Dislocation

Strain induced Dislocations The temperature profile across the diameter of a boule is not constant as the boule cools 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”

Strain due to Impurities An impurity induces strain in the crystal because of differences in 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 Both cause local modifications to Eg

Dislocation Count When you purchase a wafer, one of the specifications is the EPD, Etch Pit Density 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 10 6 cm -2 for some materials that are extremely difficult to grow.  Note that EPD of 10 6 cm -2 means that there is a dislocation approximately every 10  ms.

Wafer Manufacturing Boules are polished into cylinders Boules are polished into cylinders Aligned using an x-ray diffraction system Aligned using an x-ray diffraction system Cut into slices using a diamond edged saw 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

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

MBE Wafer is moved into the chamber using a magnetically coupled transfer rod Wafer is moved into the chamber using a magnetically coupled transfer rod Evaporation and sublimation of source material under ultralow pressure conditions ( torr) Evaporation and sublimation of source material under ultralow pressure conditions ( 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 Condensation of material on to a heated wafer –Heat allows the atoms to move to appropriate sites to form a crystal

Schematic View

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

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

MOCVD Growths are performed at room pressure or low pressure (10 mtorr-100 torr) 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 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 N 2 or H 2 into chamber Reactants are gases carried by N 2 or H 2 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

Schematic of MOCVD System

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

Disadvantages Gas sources pose a potential health and safety hazard Gas sources pose a potential health and safety hazard –A number are pyrophoric and AsH 3 and PH 3 are highly toxic Difficult to grow hyperabrupt layers Difficult to grow hyperabrupt layers –Residual gases in chamber Higher background impurity concentrations in grown layers Higher background impurity concentrations in grown layers

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

Carrier Mobility and Velocity Mobility - the ease at which a carrier (electron or hole) moves in a semiconductor Mobility - the ease at which a carrier (electron or hole) moves in a semiconductor –Symbol:  n for electrons and  p for holes Drift velocity – the speed at which a carrier moves in a crystal when an electric field is present Drift velocity – the speed at which a carrier moves in a crystal when an electric field is present –For electrons: v d =  n E –For holes: v d =  p E

H L W VaVa VaVa

Resistance

Resistivity and Conductivity Fundamental material properties Fundamental material properties

Resistivity n-type semiconductor p-type semiconductor

Drift Currents

Diffusion When there are changes in the concentration of electrons and/or holes along a piece of semiconductor 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.

Diffusion Currents

Relationship between Diffusivity and Mobility

Mobility vs. Dopant Concentration in Silicon

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

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

Calculation Resistance is determined with and without a magnetic field applied perpendicular to the sample. 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.

Hall Measurement See for a more complete explanation See for a more complete explanationhttp://

Calculation Measurement of resistance is made while a magnetic field is applied perpendicular to the surface of the Hall sample. 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.

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

Hot Point Probe Simple method to determine whether material is n-type or p-type Simple method to determine whether material is n-type or p-type –Note that the sign of the Hall voltage, V H, and on  R 13,24 in the Van der Pauw measurement also provide information on doping.