Electrical- Compound Semiconductors Flashcards
Properties of materials that are iso-electronic with Si
They have the same number of MOs, the same number of electrons, so they should exhibit similar properties
What combination of groups of elements should make a semiconductor material?
III/V
II/VI
And possibly I/VII
Is an isovalent series of compounds like Ge, GaAs, ZnSe and CuBr, the bond length hardly changes but why does the band gap increase?
There is a difference in bonding. The difference in electronegativity also has a significant influence in orbital splitting (like bond length does). Some of these compounds do not share the valence electrons equally in the bonds. Along this series there is increasing ionic character.
What is heteropolar bonding?
Where solids have bonding which is intermediate between the simple ionic and covalent extremes
How do the orbital energies compare for bonding between atoms of different electronegativity?
A is atom with lower electronegativity and B is atom with higher. The AO energy of A is greater than B (EA>EB). The bonding MO has lower energy than both and antibonding MO has higher energy than both. Ionic contribution to energy gap is Ei=EA-EB. Ec is covalent part of the splitting and is magnitude of the gap in a homonuclear case where Ei=0. Energy gap between MOs is ΔE=(Ei-Ec)^1/2.
Which atom has the larger contribution to the bonding orbital?
The more electronegative one
Describe the bonding, CB and VB in SrO insulator
The bonding is ionic and 2 electrons are transferred from the Sr to the O to form Sr2+ and O2- ions in the rocksalt structure. The CB is empty and cationic in character and composed mainly from the empty valence orbitals on the cation. The VB is full and anionic in character and composed mainly from the filled orbitals on the anion. The band gap is large (>5eV)
How can electronic insulators become semiconductors or metals?
By suitable chemical doping. Especially true for transition metal oxides because the transition metal cations can exhibit variable oxidation states. The mixed valency of the TM cation can induce mobile holes or electrons in such solids
Oxidation and reduction in NiO and TiO
NiO: there is oxidation of the Ni cation due to oxygen gain
NiO1+δ where there is a mix of Ni2+ and Ni3+
TiO: there is reduction of the Ti cation due to oxygen loss
TiO2-δ where there is a mix of Ti4+ and Ti3+
Two examples of doping for transition metal oxides
Li+ doping of NiO (p-type) where Li+ replaces Ni2+ to be an acceptor.
La3+ doping of BaTiO (n-type) where La3+ replaces Ba2+ to be a donor.
What happens when NiO is heated in air at temperatures greater than 1100°C?
Goes from pale green insulator to black semiconductor. NiO picks up O2 molecules which adsorb on the surface, dissociate and pick up 2e to form O2-. These electrons come from oxidation of the Ni2+ to Ni3+ (d8 to d7). To partially balance the extra layer of O2- ions at surface, Ni2+ ions migrate leaving cation vacancies throughout the crystal. Each extra O2- generated creates one Ni vacancy and two Ni3+ ions. General formula Ni1-xO becomes Ni2+(1-3x)Ni3+(2x)O
How does conduction work for black NiO?
It is a p-type hoping semiconductor because the electrons how from Ni2+ to an adjacent Ni3+. Electrons are localised on the Ni2+ so concept of CB and VB doesn’t apply. Main conduction process is:
Ni2+->Ni3+ + e-
An activation energy is needed to get the electrons to hop. Number of carriers=[Ni3+]. For lnσ vs 1/T graph gradient is -E/k. Difficult to control degree of oxidation so n varies meaning so does σ
How to control oxidation of NiO
Dope with Li+ to obtain Ni1-xLixO. 2Ni2+ becomes Ni3+ and Li+ and compound becomes Ni2+(1-2x)Ni3+(x)Li+(x)O. Means [Ni3+] controlled by x (Li+). Li+ + h+ -> Ni2+
Ni1-xLixO is a solid solution with rock salt structure where Li+ distributed at random over the crystal octahedral sites. When x fixed, at a fixed field E, σ is controlled. Can be used as a temperature control for a thermistor.
