 
nextnano^{3}  Tutorial
next generation 3D nano device simulator
1D Tutorial
k.p dispersion in bulk unstrained ZnS, CdS, CdSe and ZnO (wurtzite)
Author:
Stefan Birner
If you want to obtain the input files that are used within this tutorial, please contact stefan.birner@nextnano.de.
> bulk_6x6kp_dispersion_ZnS_nn3.in
> bulk_6x6kp_dispersion_CdS_nn3.in
> bulk_6x6kp_dispersion_CdSe_nn3.in
> bulk_6x6kp_dispersion_ZnO_nn3.in / *_nnp.in  input file for nextnano^{3}
/ nextnano++ software
k.p dispersion in bulk unstrained ZnS, CdS, CdSe and ZnO (wurtzite)
This tutorial is based on
Valence band parameters of wurtzite materials
J.B. Jeon, Yu.M. Sirenko, K.W. Kim, M.A. Littlejohn, M.A. Stroscio
Solid State Communications 99, 423 (1996)
 We want to calculate the dispersion E(k) from k=0 [1/nm] to k=2.0
[1/nm] along the
following directions in k space:
 [000] to [0001], i.e. parallel to the c axis (Note: The c axis is parallel
to the z axis.)
 [000] to [110], i.e. perpendicular to the c axis (Note: The (x,y) plane is
perpendicular to the c axis.)
We compare 6band k.p theory results vs. singleband (effectivemass)
results.
 We calculate E(k) for bulk ZnS, CdS and CdSe (unstrained).
Bulk dispersion along [0001] and [110]
$outputkpdata
destinationdirectory = kp/
bulkkpdispersion = yes
gridposition =
5d0
! in units of [nm]
!
! Dispersion along [001] direction, i.e.
parallel to c=[0001] axis in wurtzite
! Dispersion along [110] direction, i.e.
perpendicular to c=[0001] axis in wurtzite
! maximum k vector = 2.0 [1/nm]
!
kdirectionfromkpoint = 0d0
0d0 2.0d0 !
kdirection and range for dispersion plot [1/nm]
kdirectiontokpoint = 1.41421356d0
1.41421356d0 0d0 ! kdirection and range for
dispersion plot [1/nm]
! The dispersion is calculated from the k point 'kdirectionfromkpoint '
to Gamma, and then from the Gamma point to 'kdirectiontokpoint '.
numberofkpoints = 100 !
number of k points to be calculated (resolution)
$end_outputkpdata
 We calculate the pure bulk dispersion at
gridposition=5d0 ,
i.e. for the material located at the grid point at 5 nm. In our case this is
ZnS but it could be any strained alloy.
In the latter case, the k.p
BirPikus strain Hamiltonian will be diagonalized.
The grid point at gridposition must be located inside a quantum cluster.
shiftholestozero = yes forces the
top of the valence band to be located at 0 eV.
How often the bulk k.p Hamiltonian should be solved can be specified
via numberofkpoints . To increase the resolution, just increase
this number.
 The maximum value
of k is 2.0 [1/nm].
Note that for values of k larger than 2.0
[1/nm],
k.p theory might not
be a good
approximation any more. This depends on the material system, of course.
 Start the calculation.
The results can be found in:
kp_bulk/bulk_6x6kp_dispersion_as_in_inputfile_kxkykz_000_kxkykz.dat
(6band k.p)
kp_bulk/bulk_sg_dispersion.dat (singleband approximation)
bulk_6x6kp_dispersion_as_in_inputfile_kxkykz_000_kxkykz.dat :
The first column contains the k vector in units of [1/nm], the next six
columns the six eigenvalues of the 6band k.p Hamiltonian for this
k=(k_{x},k_{y},k_{z}) point.
The resulting energy dispersion is usually discussed in terms of a
nonparabolic and anisotropric energy dispersion of heavy, light and
splitoff holes, including valence band mixing.
bulk_sg_dispersion.dat :
The first column contains the k vector in units of [1/nm], the next
three columns the energy for heavy (A), light (B) and crystalfield
splitoff (C) hole for this k=(k_{x},k_{y},k_{z})
point.
The singleband effective mass dispersion is parabolic and depends on a
single parameter: The effective mass m*.
Note that in wurtzite materials, the mass tensor is usually anisotropic with
a mass m_{zz} parallel to the c axis, and two masses perpendicular
to it m_{xx}=m_{yy}.
Results
 Here we visualize the results.
The final figures will look like this (left: dispersion along [0001], right:
dispersion along [110]):
 These three figures are in excellent agreement to Figure 1 of the paper by [Jeon].
 The dispersion along the hexagonal c axis is substantially different than
the dispersion in the plane perpendicular to the c axis.
The effective mass approximation is indicated by the dashed, grey lines.
For the heavy holes (A), the effective mass approximation is very good
for the dispersion along the c axis, even at large k vectors.
 For comparison, the singleband (effectivemass) dispersion is
also shown. For ZnS, it corresponds to the following effective hole masses:
valencebandmasses = 0.35d0 0.35d0
2.23d0 ! [m0] heavy hole A
(2.23 along c axis)
0.485d0 0.485d0 0.53d0 ! [m0]
light hole B
(0.53 along c axis)
0.75d0 0.75d0 0.32d0 ! [m0]
crystal
hole C (0.32 along c axis)
The effective mass approximation is a simple parabolic dispersion which is
anisotropic if the mass tensor is anisotropic (i.e. it also depends on the
k
vector direction).
One can
see that for k < 0.5 [1/nm] the singleband approximation is in
excellent agreement with 6band k.p but
differs at larger k values substantially.
 Plotting E(k) in three dimensions
Alternatively one can print out the 3D data field of the bulk E(k) =
E(k_{x},k_{y},k_{z}) dispersion.
$outputkpdata ... bulkkpdispersion3D =
yes
! ! maximum k vector =
2.0 [1/nm] ! kdirectiontokpoint =
0d0 0d0 2.0d0 !
kdirection
and range for dispersion plot [1/nm] numberofkpoints =
40
!
number of k points to calculated (resolution)
The meaning of numberofkpoints =
41
is the following: 40 k points from ' maximum k vector'
to zero (plus the Gamma point) and 40 k points from zero to '+ maximum k vector'
(plus the Gamma point)
along all three directions, i.e. the whole 3D volume then contains 81 * 81 * 81 = 531441
k points.
k.p dispersion in bulk unstrained ZnO
The following figure shows the bulk 6band k.p energy dispersion for ZnO.
The gray lines are the dispersions assuming a parabolic effective mass.
The following files are plotted:
 kp_bulk/bulk_6x6kp_dispersion_as_in_inputfile_kxkykz_000_kxkykz.dat
 kp_bulk/bulk_sg_dispersion.dat
The files
 bulk_6x6kp_dispersion_axis_100_000_100.dat and
 bulk_6x6kp_dispersion_diagonal_110_000_110.dat
contain the
same data because for a wurtzite crystal, due to symmetry, the dispersion in the
plane perpendicular to the k_{z} direction (corresponding to
[0001]) is isotropic.
 Please help us to improve our tutorial! Send comments to
support
[at] nextnano.com .
