Physics of Thin Films

PES 449 / PHYS 549

Structure of Solids: Bonds and electrons in solids

Ohring: Chapter 1, Section 1.4

Bonding in materials:

bonds lower the total energy of a system of atoms => solids are more stable than free atoms
bonds from electrostatic and quantum attraction and repulsion

electron structures around ion cores lead to different bond types and strengths

four basic types of bonds:

metallic
- relatively weak bond (1 -2 eV)
- complicated mixture of electrostatic and quantum attraction and repulsion
- no direction to bonds
  - => easy to deform
- examples: most metals, Fe, Ni, Cr . . .
ionic
- strong bonds (2 -10 eV)
- Coulomb attraction of oppositely charged ions
- Coulomb and quantum repulsions
- no direction to bonds
- examples: NaCl, MgF₂, ZnS
covalent
- strong bonds (2- 10 eV)
- "sharing" of electrons
- bonds are directional
  - => hard to deform
- examples: Si, Ge, GaAs
Van der Waals
- weak bonds (around 0.2 eV)
- often neglected
- from distortion of electron clouds
- examples: inert gasses, organics

Bonds: attractive and repulsive components

all bonds have attractive and repulsive components

exact details of repulsive and attractive parts give:

lattice parameters

binding energy

elastic modulus (from shape of well)

Surface bonds

surface atoms are less tightly bound than bulk atoms

Electrons in Solids

Metals:
some electrons bound to ion cores

other electrons are "free electrons" => conduction

f = work function = energy needed for electron to escape from metal

(important in thermionic emission, photoemission)

Insulators:

large gap between filled and empty states

=> electrons have no where to go => very poor conductors

Semiconductors:

similar to insulators except gap is smaller (around 1 eV)

=> can apply voltage or heat to excite electrons into conduction band

doped semiconductors create states in the gap => easier to excite electrons across gap

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