The Ancient “Periodic Table”

1. The Ancient “Periodic Table”

2. A Quick Survey of thePeriodic TableConsider the possible compounds formed by combining atoms from different columns of the periodic table.Ask the question: Which of these compounds are semiconductors?

3. Group IVCrystalline Materials Elemental Semiconductors formed from atoms in Column IV C (carbon):Different Crystalline Phases Diamond Structure:Diamond! An insulator or semiconductor. Graphite:Metallic! The most common carbon solid. Fullerenes: Based on Buckminsterfullerene: “Bucky Balls”, Nanotubes, Insulators, Semiconductors, or Metals depending on preparation. Clathrates:Possible new forms of C solids? Semiconductors or Semimetals, Compounds, Recent Research!!

4. Si (silicon): Different Crystalline Phases Diamond Structure:A Semiconductor. The most common Si solid. Clathrates:“New” forms of Si solids. Semiconductors, Semimetals, Compounds, Recent Research Ge (germanium): Different Crystalline Phases Diamond Structure:A Semiconductor. The most common Ge solid. Clathrates:“New” forms of Ge solids. Semiconductors, Semimetals, Compounds, Recent Research

5. Sn (tin): Different Crystal Phases Diamond Structure: Gray tin or α-Sn. A Semimetal! Body Centered Tetragonal Structure: White tin or β-Sn. A Metal. The most common Sn solid. Clathrates:“New” forms of Sn solids. Semiconductors, Semimetals, Compounds, Recent Research Pb (lead): Face Centered Cubic Structure: A Metal.

6. Group IVMaterialsA Chemical Trend– Material Bandgap as a function of Near-Neighbor Distance for Diamond Structure Solids Decreasing BandgapEgcorrelates with Increasing Nearest-Neighbor Bond Lengthd Atom Eg (eV) d (Å) C 6.0 2.07 Si 1.1 2.35 Ge 0.7 2.44 Sn (a semimetal) 0.0 2.80 Pb(a metal) 0.0 1.63 Not the diamond structure!

7. Elemental Semiconductors • Mainly, these are from the Column IVelements C (diamond), Si,Ge, Sn(gray tin or α-Sn) The atoms are tetrahedrally bonded in the diamond crystal structureand each atom has 4 nearest-neighbors. Bonding:sp3covalent bonds. • Some Column V & Column VI elements are semiconductors: P - A 3-fold coordinated lattice. S, Se, Te5-fold coordinated lattices.

8. III-V CompoundsPeriodic Table Columns III & V Column III Column V B N Al P Ga As In Sb Tl not used Bi Some possible compounds which are semiconductors are: BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN GaP, GaAs, GaSb, InP, InAs, InSb,….

9. Some Applications of III-V Materials IR detectors, LED’s, solid state lasers, switches, …. BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN GaP, GaAs, GaSb; InP, InAs, InSb,…. A Chemical Trend The bandgap decreases& the interatomic distance increasesgoing down the periodic table. There is tetrahedral coordinationof the atoms. Many III-V compounds have the zincblende crystal structure. Some (B compounds & Ncompounds) have the wurtzite crystal structure. Interatomic Bonding:The bonds are not purely covalent! The charge separation due to the valence differences leads to Partially Ionic bonds.

10. II-VI CompoundsPeriodic Table Columns II & VI Column II Column VI Zn O Cd S Hg Se Mn  sometimes Te not used Po Some possible compounds which are semiconductors or semimetalsare: ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS HgSe, HgTe,… + some compounds with Mn….

11. Some Applications of II-VI Materials IR detectors, LED’s, switches ZnO, ZnS, ZnSe, ZnTe; CdS, CdSe, CdTe, HgS HgSe, HgTe(semimetals) + some compounds with Mn A Chemical Trend The bandgap decreases& the interatomic distance increasesgoing down the periodic table. There is tetrahedral coordinationof the atoms. Except for the Hg compounds, which are semimetals with zero gaps, the II-VI materials have large bandgapscompared to the Column IV and the III-V materials. Some of these materials have a zincblende crystal stucture & some have wurtzite crystal structures. Interatomic Bonding:The charge separation due to the valence difference is large.  The bonds are more ionic than covalent!

12. IV- IV CompoundsPeriodic Table Column IV Column IV Binary combinations of C, Si, Ge, Sn  SiC Other compounds: GeC, SnC, SiGe, SiSn, GeSn,.. Cannot be made or cannot be made without species segregation or are not semiconductors. Two common crystalline phases forSiCare zincblende (a semiconductor), & hexagonal close packed (a large gap insulator). There are also MANY other crystal structures for SiC!

13. IV- VI CompoundsPeriodic Table Columns IV & VI Column IV Column VI C O Si S Ge Se Sn Te Pb Some possible compounds which are semiconductorsare: PbS, PbTe, PbSe, SnS . Other compounds:SnTe, GeSe, .. can’t be made, can’t be made without segregation, or aren’t binary compounds, or aren’t semiconductors.

14. Some Applications of IV-VI Materials: IR detectors, switches,… PbS, PbTehave the zincblende crystal structure Most others have 6-fold coordinated lattices. The bonding is ~ 100% ionic These materials have very small bandgaps, which makes them very useful as IR detectors

15. I-VII Compounds Periodic Table Columns I & VII • These materials are mostly Ionic Insulators: NaCl, KCl, CsCl, … • Their lattices do not have tetrahedral coordination. Most of them are 6- or 8-fold coordinated and have theNaClorCsClcrystal structures (discussed in any elementary Solid State Physics book). The bonding is ~ 100% ionic Their bandgaps are large (which is why they are insulators!)

16. Oxide CompoundsThese are a category all their own • Most of these materials are good insulatorswith large bandgaps. • A few are Semiconductors: CuO, Cu2O, ZnO • Many of their properties are not very well understood. Partially as a result of this there are relatively few applications. An exception to this is ZnO, which has wide use in ultrasonic transducers. • At low T, some oxides are superconductors Many “high”Tcsuperconductors are based on La2CuO4 (Tc~ 135K)

17. Some OtherSemiconductor Materials • “Alloy” mixturesof elemental materials(binary alloys): SixGe1-x ,... (0 ≤ x ≤ 1) • “Alloy” mixtures of binary compounds (ternary alloys): Ga1-xAlxAs, GaAs1-xPx,… (0 ≤ x ≤ 1) • “Alloy” mixturesof binary compounds with mixtures on both sublattices (quaternary alloys): Ga1-xAlxAs1-yPy, .., (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) • In the growth process, x & y can be varied, which varies the material bandgap & other properties. “BANDGAP ENGINEERING!”

18. “Exotic” Semiconductors Layered Compounds:PbI2, MoS2, PbCl2, … • These materials have strong Covalent Bondingwithin each layer & weak Van Der Waals bonding between layers. • This means that they are effectively “2 dimensional solids” • That is, their electronic & vibrational properties have a ~ 2 dimensional character. Organic Semiconductors:Polyacetyline(CH2)nand other polymers • “These materials show great promise for future applications” (I’ve heard this for  35 years!) Many of these materials are not well understood

19. Other Semiconductors Magnetic Semiconductors • Compounds with Mn and/or Eu(& other magnetic ions) • These are simultaneously semiconducting & magnetic EuS, CdxMn1-xTe, Optical modulators,… Others (see YC, p 4) I-II-(VI)2 & II-IV-(V)2compounds AgGaS2, ZnSiP2,…., Tetrahedral bonding V2-(VI)3compounds As2Se3….