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?

Group IV Crystalline 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!!

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

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.

Group IV Materials A Chemical Trend – Material Bandgap as a function of Near-Neighbor Distance for Diamond Structure Solids Decreasing Bandgap E g correlates with Increasing Nearest-Neighbor Bond Length d AtomE g (eV) d (Å) C Si Ge Sn (a semimetal) Pb (a metal) 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: sp 3 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 possible compounds which are semiconductors are: BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN GaP, GaAs, GaSb, InP, InAs, InSb,….

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 possible 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 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!

IV- IV Compounds Periodic 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 for SiC are zincblende (a semiconductor), & hexagonal close packed (a large gap insulator). There are also MANY other crystal structures for SiC !

Column IV Column VI C O Si S Ge Se Sn Te Pb Some possible 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. IV- VI Compounds Periodic Table Columns IV & VI

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

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 any elementary Solid State Physics book). The bonding is ~ 100% ionic Their bandgaps are large (which is why they are insulators!) I-VII Compounds Periodic Table Columns I & VII

Oxide Compounds These are a category all their own Most of these materials are good insulators with large bandgaps. A few are Semiconductors: CuO, Cu 2 O, 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” T c superconductors are based on La 2 CuO 4 (T c ~ 135K)

Some Other Semiconductor Materials “Alloy” mixtures of elemental materials (binary alloys): Si x Ge 1-x,... (0 ≤ x ≤ 1) “Alloy” mixtures of binary compounds (ternary alloys): Ga 1-x Al x As, GaAs 1-x P x,… (0 ≤ x ≤ 1) “ Alloy” mixtures of binary compounds with mixtures on both sublattices (quaternary alloys): Ga 1-x Al x As 1-y P y,.., (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: PbI 2, MoS 2, PbCl 2, … 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 (CH 2 ) n and 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

Magnetic Semiconductors Compounds with Mn and/or Eu (& other magnetic ions) These are simultaneously semiconducting & magnetic EuS, Cd x Mn 1-x Te, Optical modulators,… Others (see YC, p 4) I-II-(VI) 2 & II-IV-(V) 2 compounds AgGaS 2, ZnSiP 2, …., Tetrahedral bonding V 2 -(VI) 3 compounds As 2 Se 3 …. Other Semiconductors