1. The Ancient “Periodic Table”

2. 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?

3. 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!!

4. 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
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Ge (germanium): Different Crystalline Phases
The most common Ge solid.
2. Clathrates: “New” forms of Ge solids.
Semiconductors, Semimetals, Compounds, Recent Research

5. 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
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Pb (lead): Face Centered Cubic Structure:
A Metal.

6. 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 Si Ge
1 Sn (semimetal) l
Pb (metal)
Not the diamond structure!

7. 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.

8. 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,….

9. 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.

10. 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….

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 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!

12. 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!

13. 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.

14. 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

15. 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!)

16. 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)

17. 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!”

18. “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

19. 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….