region-clusters

Regions can be clustered to bigger objects by the keyword $region-cluster. Any region must be assigned to a cluster which is labeled by its cluster-number.
cluster-number = integer  (>=1)

Also the so-called default region must be assigned to a cluster. The default region is the rest of the simulation domain which is not filled out by defined regions. It might happen, that the default region is an empty region. Nevertheless, it must be assigned to a cluster.
In any case:
           default region number = maximum region number defined + 1

The regions which have to be clustered to one cluster are specified by their numbers with
region-number = regnum1 regnum2 regnum3 ...

!----------------------------------------------------------!
$region-cluster                                  required  !
 cluster-number                 integer          required  !
 region-numbers                 integer_array    required  !
 apply-constant-el-Fermi-level  double           optional  !
 apply-constant-hl-Fermi-level  double           optional  !
$end_region-cluster                              required  !
!----------------------------------------------------------!

Syntax

cluster-number
An integer number to refer to clustered geometry element.

region-numbers
Region numbers, belonging to this cluster.

apply-constant-el-Fermi-level = 0.2  ! [eV]
apply-constant-hl-Fermi-level = 0.2  ! [eV]
Applies a constant electron and/or hole Fermi level to this cluster.

Example

We have defined 5 regions under keyword $regions, region 2 and 4 are clustered to form cluster-number 2 (i.e. region 2 and 4 now will consist of the same material as a material kind is assigned to a cluster and not to a region).
Region-number 6 is an undefined default region (Assumption: We have defined only 5 regions.) and must be the default rest. Now there are two possibilities:

Possibility 1:

!----------------------------------------------------------!
$region-cluster                                            !
 cluster-number = 1      region-numbers = 1                !
 cluster-number = 2      region-numbers = 2  4             !
 cluster-number = 3      region-numbers = 3                !
 cluster-number = 4      region-numbers = 5  6             !
$end_region-cluster                                        !
!----------------------------------------------------------!

Here, the default region has the same material properties as cluster-number 4. The default region can consist of some undefined region (the remaining part of the simulation area which is not filled yet) or it can be "empty" if the whole simulation area is already filled with other objects.

Possibility 2:

!----------------------------------------------------------!
$region-cluster                                            !
 cluster-number = 1      region-numbers = 1                !
 cluster-number = 2      region-numbers = 2  4             !
 cluster-number = 3      region-numbers = 3                !
 cluster-number = 4      region-numbers = 5                !
 cluster-number = 5      region-numbers = 6                !
$end_region-cluster                                        !
!----------------------------------------------------------!

Here, the default region has its own material properties which have to be defined later under keyword $material. Again, the default region can consist of some undefined region (the remaining part of the simulation area which is not filled yet) or it can be "empty" if the whole simulation area is already filled with other objects.
Application: If one has a complicated geometry which is surrounded by a GaAs cap layer, one can define the complicated geometry as usual and for the (complicated) surrounding material one just uses the "default" option.

Note: Be careful in 3D when you specify for strain calculation "strain-minimization" (see $strain-minimization-model) to define your substrate as a separate cluster.