ECE6450L2-Introduction Chap 1 and 2-continued

ECE6450L2-Introduction Chap 1 and 2-continued - Lecture 2...

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ECE 6450 - Dr. Alan Doolittle Georgia Tech Lecture 2 Semiconductor Substrates Reading: Chapter 2
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ECE 6450 - Dr. Alan Doolittle Georgia Tech Phase Diagrams Only certain compositions of materials are allowed at a given temperature and pressure when the material is in thermodynamic equilibrium. These compositions can be described by “Phase Diagrams”. These allowed compositions effect semiconductor growth, and dictate the stability and electrical properties of metal- semiconductor contacts. A phase diagram contains a “map” of a materials state including liquid only, solid only, coexisting as liquid with solid, or even various crystal structures of solid phases verses changes in material temperature, composition or pressure. We will only examine phase diagrams at a single pressure.
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ECE 6450 - Dr. Alan Doolittle Georgia Tech Solid Liquid Liquid+ Solid Consider the SiGe material system. Things to know about phase diagrams: 1.) Note the melting points of pure Ge and pure Si. 2.) Identify the Solid, Liquid and combination regions as well as the solidus (S/L+S) and liquidus (L/L+S) lines. 3.) Assume a 40% atomic Si composition of the starting material (thoroughly mixed powder). 4.) As the material is heated from room temperature it starts in a “single phase field” (I.e, solid phase) and the composition remains the same, 40% atomic Si. However, as the temperature is elevated, the material begins melting at ~1070 degrees entering into a “two phase field” containing part liquid and part solid. When the material reaches ~1230 degrees, the material is completely melted and enters a “single phase field” (liquid) again. When in the solid or liquid regions, the composition is identical to the original composition (40%). 5.) When the 1070<Temperature<1230, both liquid and solid phases coexist. However, the composition of the liquid and solid materials can differ from the original composition. For example at 1200 C, draw a horizontal “tie line” (shown as green and purple) from the liquidus line to the solidus line. Read off the compositions of the liquid, 34%at Si , and the solid, 67% at Si . Note that the compositions of the solid and liquid are different. 6.)
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ECE6450L2-Introduction Chap 1 and 2-continued - Lecture 2...

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