ECE 6450 - Dr. Alan DoolittleGeorgia TechLecture 2Semiconductor SubstratesReading: Chapter 2ECE 6450 - Dr. Alan DoolittleGeorgia TechPhase DiagramsOnly 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.ECE 6450 - Dr. Alan DoolittleGeorgia TechSolidLiquidLiquid+ SolidConsider 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.) The amount of the melt that is solid and the amount that isliquid can be determined by the “Tie line” between the solidus line, and the liquidus line.%2.1810034673440%100%=−−=−−=xSolidxXXXXSolidLSLo%8.8110034674067%100%=−−=−−=xLiquidxXXXXLiquidLSoSXSXLXoDetermine the Composition of the Liquid and solidECE 6450 - Dr. Alan DoolittleGeorgia TechConsider the Si-Al material system.Weight Percent SiliconAtomic Percent SiliconTemperature (C)99.9714001200100080060099.96 99.98 99.9999.9799.96 99.98 99.99140012001000800600577 (C)(Si)(Al)+(Si)L-(Si)LL-(Si)LL-(Al)L-(Al)Things to know about phase diagrams:7.) In certain material systems a Horizontal, isothermal boundary exists that indicates the existence of phase transformations involving three phases. If α, β and γ are solid phases, we can classify the transformations as a:a.) Eutectic: L--> α + βb.) Eutectiod: γ--> α + βc.) Peritectic: L + α −−> γSi-Al experiences a “Eutectic” transformation at 577 C. Note this is lower than the melting point of Al (660 C) and Si (1412 C). At this temperature, a liquid containing 11.3% Si will transform to two solid phases, (Al with 1.59%Si) + (Si + minimal fraction of Al-I.e. Al doped)Si containing Al (Al Doped)Al containing SiECE 6450 - Dr. Alan DoolittleGeorgia TechConsider the Si-Al material system.Si-Al “Eutectic” transformation at 577 C. Note this is lower than the melting point of Al (660 C) and Si (1412 C). At this temperature, a liquid containing 11.3% Si will transform to two solid phases, (Al with 1.59%Si) + (Si + minimal fraction of Al-I.e. Al doped)Chunks/Flakes of Si in the Al-Si mix.Figures from and a great explanation of phase diagrams in more detail than we have discussed here can be found at:http://www.soton.ac.uk/~pasr1/index.htmECE 6450 - Dr. Alan DoolittleGeorgia TechCrystalline defects can be classified as: 1.) Point defects2.) Line defects3.) Planar defects4.) Volume defects.1.) Point Defects: Some include Vacancies (Schottky defect), interstitials, substitutional, and impurity-vacancy complexes (Frenkel defect, SiI- V, is shown).Point Defects dictate most diffusion mechanisms, and thus, determine the impurity profile.Defects in SemiconductorsECE 6450 - Dr. Alan DoolittleGeorgia TechNormal BondingRearrangement of Bonds at a VacancyPoint Defects: VacanciesCharge neutrality must exist in the crystal. When a vacancy is created, 1.) it can cleanly brake all four bonds ==> Neutral vacancy, Vo, neutral Interstitial,2.) n electrons may stay at the vacancy ==> V-n+ I+n, 3.) n electrons may go with the interstitial==> V+n+ I-nThe number of neutral vacancies is thermodynamically determined by,where Nois the density of Atoms/cm3and Ea is the activation energy for the formation of the vacancy (in silicon, No=5.02e22 cm-3 and Ea=2.6 eV)Singularly and doubly charged vacancies have concentrations that depend on the number of carriers present,[]kTEOaeNV−=0[][]()TnfnnVVi,0=∝−[][]()TnfnnVVi,220=⎥⎦⎤⎢⎣⎡∝−−[][]()TpfnpVVi,0=∝+[][]()TpfnpVVi,220=⎥⎦⎤⎢⎣⎡∝++ECE 6450 - Dr. Alan DoolittleGeorgia TechPoint Defects: InterstitialsAtoms not residing on lattice sites are called interstitials. They can be foreign, unwanted impurities, intentionally introduced impurities, or “misplaced” host atoms. Dopant atoms diffuse through the semiconductor faster as interstitials, but we need to place them in substitutional sites to make use of them.Oxygen InterstitialEquivalent Oxygen SitesECE 6450 - Dr. Alan DoolittleGeorgia TechPoint Defects: SubstitutionalImpurities can replace a host atom in the lattice site. They can be foreign, unwanted impurities, or intentionally introduced impurities. You may want a dopant impurity to be on a substitutional site, but you may not want a heavy metal atom or other unwanted impurity to be on a substitutional site (harder to remove).Si LatticeCarbonECE 6450 - Dr. Alan DoolittleGeorgia TechLine Defects: DislocationsThe term, “threading”, describes specific cases in which the dislocation “threads” through a grown layer (i.e. starts at or near the substrate and ends on the surface).A missing line or additional line of atoms is called a dislocation. Dislocations are either pure edge, pure screw or a combination of both type called