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Phase diagram Only certain compositions of materials are allowed at a

given temperature and pressure when the material is inthermodynamic equilibrium. These compositions can bedescribed by Phase Diagrams. These allowedcompositions effect semiconductor growth, and dictatethe stability and electrical properties of metalsemiconductor contacts.

A phase diagram contains a map of a materials stateincluding liquid only, solid only, coexisting as liquid withsolid, or even various crystal structures of solid phases

verses changes in material temperature, composition or pressure. We will only examine phase diagrams at asingle pressure.

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solidus

liquidus

Determine the Composition ofthe Liquid and solid

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Things to know about phasediagrams:

1.) Note the melting points ofpure Ge and pure Si.

2.) Identify the Solid, Liquid andcombination regions as wellas the solidus (S/L+S) andliquidus (L/L+S) lines.

3.) Assume a 40% atomic Sicomposition of thestarting material(thoroughly mixed powder).

Determine the Composition of the Liquid and solid

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4.) As the material is heated fromroom temperature it starts in asingle phase field ( I.e, solidphase) and the compositionremains the same, 40% atomic Si.However, as the temperature iselevated, the material beginsmelting at ~1070 degreesentering into a two phase fieldcontaining part liquid and partsolid. When the material reaches~1230 degrees, the material iscompletely melted and enters asingle phase field (liquid) again.

When in the solid or liquidregions, the composition isidentical to the originalcomposition (40%).

Determine the Composition ofthe Liquid and solid

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5.) When the1070

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Crystal Defects

Crystalline defects can be classified as:1.) Point defects

2.) Line defects3.) Planar defects4.) Volume defects

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Point defects Point defects are where an atom is missing or

is in an irregular place in the lattice structure.

Types of point defects: self interstitial atoms: A self interstitial atom

is an extra atom that has crowded its wayinto an interstitial void in the crystalstructure.

Substitutional atoms: A substitutionalimpurity atom is an atom of a different typethan the bulk atoms, which has replaced oneof the bulk atoms in the lattice

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Point defects Interstitial impurity atoms:

Interstitial impurity atoms are much

smaller than the atoms in the bulkmatrix. Interstitial impurity atomsfit into the open space between thebulk atoms of the lattice structure.

Vacancies: Vacancies are emptyspaces where an atom should be,but is missing. They are common,especially at high temperatures

when atoms are frequently andrandomly change their positionsleaving behind empty lattice sites.In most cases diffusion (masstransport by atomic motion) can

only occur because of vacancies. impurity-vacancy complexes(Frenkel defect, SiI V)

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Linear Defects - Dislocations

Dislocations are another type of defect in crystals. Dislocations areareas were the atoms are out of position in the crystal structure.Dislocations are generated and move when a stress is applied. Themotion of dislocations allows slip plastic deformation to occur.

2 types of dislocations:1. Edge Dislocations2. Screw Dislocations

edge and screw dislocations are just extreme forms of the possible

dislocation structures that can occur. Most dislocations are probably a hybrid of the edge and screw

forms but this discussion will be limited to these two types.

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Edge Dislocations The edge defect can be easily

visualized as an extra half-planeof atoms in a lattice.

The dislocation is called a line

defect because the locus ofdefective points produced in thelattice by the dislocation lie alonga line.

This line runs along the top ofthe extra half-plane. The inter-atomic bonds are significantlydistorted only in the immediate

vicinity of the dislocation line.http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Graphics/EdgeDislocation1.jpg

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Screw Dislocations The motion of a screw dislocation

is also a result of shear stress, butthe defect line movement isperpendicular to direction of thestress and the atom displacement,rather than parallel.

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Two dimensional defects include:1.) Grain boundaries in polycrystalline materials2.) Stacking faults in crystalline material.

Planar Defects

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A grain boundary is an array ofdislocations that line up to forma plane that forms a boundarybetween two crystalline regions(grains) that are misorientedrelative to oneanother.

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Planar Defects: Stacking Faults A stacking fault is a disruption in the stacking of layers in the

crystal.

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Volume Defects: Precipitates

Precipitates are three dimensional defectsthat have a different chemical makeupfrom the host lattice. They can result froman impurity exceeding the maximumsolubility of the crystal (much likesupersaturation of sugar in water).

Generally, these defects are harmful, butthey do have some technological value -oxygen gettering to form a denuded zoneand As precipitates for high speed opticaldevices.

External Gettering: Roughing thebackside of the wafer to provide a lowenergy sink for impurities.

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Volume Defects: Precipitates Internal gettering: Using internal

defects to trap impurities movesimpurities away from the activeregion of the wafer, were transistorsare to be formed. Oxygen precipitatesare the gettering sites. 15 to 20 ppm

Oxygen wafers are required. Less than15 ppm precipitate density is toosparse to be an effective getterer.Greater than 20 ppm, wafers tend to

warp during the high temperatureprocess. Note: devices that use theentire wafer as the active region (solar

cells, thyristors, power diodes, etc...)can not use this technique, but canuse extrinsic gettering. Today, most

wafer manufactures perform this task.

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Volume Defects: Oxygen Precipitates A given denuded zone may

need to have minimal Oxygen(to prevent precipitation inthe active regions of devicesduring later thermal cycles)and be of a required depth(enough clean material toform the active deviceregion).

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