The Ancient “Periodic Table”

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

C (carbon): Different Crystalline Phases Group IV Crystalline Materials Elemental Semiconductors formed from atoms in Column IV C (carbon): Different Crystalline Phases 1. Diamond Structure = Diamond! An insulator or semiconductor. 2. Graphite: Metallic! The most common carbon solid. 3. Fullerenes: Based on Buckminsterfullerene. “Bucky Balls” C60, etc. Nanotubes, Insulators, Semiconductors, or Metals depending on preparation. 4. Clathrates: Possible new forms of C solids? Semiconductors or Semimetals, Compounds, Recent Research!!

Si (silicon): Different Crystalline Phases 1. Diamond Structure: A Semiconductor. The most common Si solid. 2. Clathrates: “New” forms of Si solids. Semiconductors, Semimetals, Compounds, Recent Research ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ Ge (germanium): Different Crystalline Phases The most common Ge solid. 2. Clathrates: “New” forms of Ge solids. Semiconductors, Semimetals, Compounds, Recent Research

Sn (tin): Different Crystalline Phases White tin or β-Sn. A Metal. 1. Diamond Structure: Gray Tin or α-Sn. A Semimetal! 2. Body Centered Tetragonal Structure: White tin or β-Sn. A Metal. The most common Sn solid. 3. Clathrates:“New” forms of Sn solids. Semiconductors, Semimetals, Compounds, Recent Research ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ Pb (lead): Face Centered Cubic Structure: A Metal.

An Increasing Lattice Constant a Not the diamond structure! Group IV Materials Chemical Trend: Fundamental Band Gap as a function of Lattice Constant for Diamond Structure Solids A Decreasing Gap Eg correlates with An Increasing Lattice Constant a Atom Eg (eV) a (Å) C 6.0 3.567 Si 1.1 5.431 Ge 0.7 5.646 1 Sn (semimetal) 0.0 6.489 l Pb (metal) 0.0 4.95 Not the diamond structure!

Elemental Semiconductors Mainly, these are from the Column IV elements C (diamond), Si,Ge, Sn (gray tin or α-Sn) The atoms are tetrahedrally bonded in the diamond crystal structure and each atom has 4 nearest-neighbors. Bonding: sp3 covalent bonds. Some Column V & Column VI elements are semiconductors: P - A 3-fold coordinated lattice. S, Se, Te 5-fold coordinated lattices.

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

BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN 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 increases going down the periodic table. There is tetrahedral coordination of the atoms. Many III-V compounds have the zincblende crystal structure. Some (B compounds & N compounds) 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.

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

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 increases going down the periodic table. There is tetrahedral coordination of the atoms. Except for the Hg compounds, which are semimetals with zero gaps, the II VI materials have large bandgaps compared to the Column IV & 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!

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

IV- VI Compounds Periodic Table Columns IV & VI Column IV Column VI C O Si S Ge Se Sn Te Pb Some compounds which are semiconductors are: 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.

Some Applications of IV-VI Materials: IR detectors, switches,… PbS, PbTe have 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

Periodic Table Columns I & VII Their bandgaps are large 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 the NaCl or CsCl crystal structures (discussed in elementary Solid State Physics books). The bonding is ~ 100% ionic Their bandgaps are large (which is why they are insulators!)

Oxide Compounds These are a category all their own Most of these materials are good insulators with 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” Tc superconductors are based on La2CuO4 (Tc~ 135K)

Some Other Semiconductor Materials “Alloy” mixtures of elemental materials (binary alloys): SixGe1-x ,... (0 ≤ x ≤ 1) “Alloy” mixtures of binary compounds (ternary alloys): Ga1-xAlxAs, GaAs1-xPx,… (0 ≤ x ≤ 1) “Alloy” mixtures of 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!”

“Exotic” Semiconductors Layered Compounds: PbI2, MoS2, PbCl2, … These materials have strong Covalent Bonding within 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)n and other polymers “These materials show great promise for future applications” (I’ve heard this for ~> 40 years!) Many of these materials are not well understood

Magnetic Semiconductors EuS, CdxMn1-xTe, Optical modulators,… 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)2 compounds: AgGaS2, ZnSiP2, Tetrahedral bonding V2-(VI)3 compounds: As2Se3….