12: band structure calculations - durham university
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The Nuts and Bolts of First-Principles Simulation
Durham, 6th-13th December 2001
12: Band Structure Calculations
CASTEP Developers’ Group
with support from the ESF ψk Network
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Outline
r Introduction to band structures
r Calculating band structures using Castep
r Examples results
r Some applications
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Band Structuresr Bloch’s theorem introduces a wavevector k.
r It can always be confined to the 1st BZ (any k outside the 1st BZ can be mapped back into it).
r The band index appears in Bloch’s theorem because for each kthere are many solutions.
knknkn EH ,,. Ψ=Ψ
This leads to a description of the energy levels of electrons in a
periodic potential in terms of a family of continuous functions En,k.
This is the band structure of the solid.
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Simple Methods
Free electrons give
parabolic bands
Using a basic basis set for the
wavefunctions improves results slightly
In Castep we use and accurate plane wave basis set.
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Calculating Band Structuresr After calculating the electronic structure of a material we have the
charge density n(r) , the potential V(r) and hence the complete Hamiltonian:
)]([)]([)]([2
1 2 rnVrnVrnVHxceiee +++∇−= −−
n(r) V(r)⇒
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Algorithmr Calculate self-consistent density and potential.
r For a chosen k-point in the band structure calculate it’s eigenvalues and eigenvectors (wavefunctions) as follows:
r Start from random wavefunction.
r For the first band, calculate the search direction using self-consistent potential.
r Assuming parabolic energy curve, step wavefunction coefficients to bottom in given direction.
r Repeat previous two steps (using orthogonal search direction) until no change occurs.
r For subsequent bands at this k-point, perform similar process, ensuring the bands remain orthogonal to all lower bands.
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Silicon Band Structure
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Conventional Cell?r Although the full set of levels can be described with restricted k,
it is often useful to allow k to range over more of k-space.
r The set of wavefunctions and energy levels for 2 values of kdiffering by a reciprocal lattice vector K are identical.
knKkn
knKkn
EE
rr
,,
,, )()(
=
Ψ=Ψ
+
+
By not using a primitive cell, the band structure contains redundant
information.
For each of the k=k+K we can map them back into the 1st Brillouin zone of the primitive cell.
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Non-primitive Cell
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Density of Statesr Many electronic properties depend on the electronic structure
throughout the whole Brillouin zone.
r A band structure usually shows the electronic states along lines of high symmetry.
r Instead, we need to sample the whole Brillouin zone in a method similar to the SCF.
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Example of DOS Calculation
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Magnetic Materials
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Applications – Band Offsetsr Heterojunctions are formed when two different types of
semiconductor are joined together.
r Heterostructures are used extensively in the electronic (e.g. transistors) and optoelectronic (e.g. LED’s) industry.
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Calculating Band Offsets
r Knowledge of the band structures of the two individual materialsis not sufficient to determine the band offset.
r Band energies are determined with respect to the average potential in the solid.
r It is also necessary to perform a supercell calculation to determine how the potentials are lines up with respect to each other.
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The Average Potentialr The potential can be averaged across the x-y planes
(junction normal to z).
r This is the microscopic average potential.
r The macroscopic average potential is calculated over the period over the microscopic potential.
r The band structures are measured relative to the macroscopic average.
∫ ∫+
−
+
−
=2/
2/
2/''
2/''
''')''('
1)(
λ
λ
λ
λλλ
z
z
z
z
tottot dzdzzVzV
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Band Line-up Using Potential
( ) ( )vbmRtotRvbmLtotLv EVEVE +−+=∆
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An Example: FeSi2 on Si
Offset is 0.4eV – useful for LED’s and near-IR detectors
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Applications - Surfacesr We can also use the supercell method to calculate the band
structures of surfaces.
r The wavefunctions at surfaces decay exponentially into the vacuum region.
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Details of GaAs Surface
Surface supercell
There is reconstruction of the surface –
this is calculated first.
A band structure of the supercell is then used for the surface band structure.
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The GaAs Band Structurer K-points are chosen using the
surface Brillouin zone.
r Many regions have a continuous energy spectrum, while gaps still exist.
r The details of the surface band structure depend on the details of the surface reconstruction.
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Applications - Orbitals
r In addition to the energy eigenvalues for a given material, a band structure calculation also gives the eigenvectors (wavefunctions) for any point in the Brillouin zone.
r It should be noted that no proof exists which confirms that the DFT single particle wavefunctions generated here correspond to the many particle wavefunction.
r However, the orbitals generated can lead to useful physics and give further insight in the the nature of the bonding in materials.
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Example of DFT Orbitals
r Instead of summing up the squares of all the wavefunctions from
each electron, we can look at each one individually.
r Each one will be the charge density for a Kohn-Sham orbital.
r This gives a chemistry point of view!
r We can examine the electronic structure electron by electron.
r Note: Kohn-Sham orbitals/one electron orbitals: meaning is not necessarily well defined!
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Example of Orbitals
Example shown is for LaMnO3 – a magnetic material.