nextnano.MSB

Material database

The material parameters that are used by the nextnano.MSB software are stored in a file called materials.xml.

This XML file can be edited by the users to modify material parameters or to add further materials.

If you run nextnano.MSB via the nextnanomat GUI, you can choose to read in a customized material database as follows:

nextnanomat ==> Tools ==> Options ==> Expert settings ==> Additional arguments passed to the executable ==> Command line ==> -database "E:\nextnano.MSB\Release-007\Materials_no_Varshni_THz_QCL_NovelDesignPeterGreck.xml"

There are entries for binary compounds like GaAs, AlAs, InP, ..., as well as for ternary compounds like AlGaAs, InGaAs, ...

<!-- binary compound -->
   <Material>

      <Name>GaAs</Name>

      <ConductionBandOffset  Unit = "eV"     >   2.979     </ConductionBandOffset>
      <ValenceBandOffset     Unit = "eV"     >   1.346     </ValenceBandOffset>

      <BandGap               Unit = "eV"     >   1.519     </BandGap>
      <BandGapAlpha          Unit = "eV/K"   >   0.5405e-3 </BandGapAlpha>
      <BandGapBeta           Unit = "K"      > 204         </BandGapBeta>

      <ElectronMass          Unit = "m0"     >   0.067     </ElectronMass>

      <EpsStatic                             >  12.93      </EpsStatic>
      <EpsOptic                              >  10.10      </EpsOptic>

      <LOPhononEnergy        Unit = "eV"     >  35e-3      </LOPhononEnergy>
      <LOPhononWidth         Unit = "eV"     >   3e-3      </LOPhononWidth>

      <DeformationPotential  Unit = "eV"     >  -9.36      </DeformationPotential>
      <MaterialDensity       Unit = "kg/m^3" >   5.3616e3  </MaterialDensity>
      <VelocityOfSound       Unit = "m/s"    >   4.73e3    </VelocityOfSound>
      <AcousticPhononEnergy  Unit = "eV"     >   5e-3      </AcousticPhononEnergy>

      <Lattice_a             Unit="nm"       >  0.56611    </Lattice_a>
      <Elastic_c11           Unit="GPa"      > 12.5        </Elastic_c11>
      <Elastic_c12           Unit="GPa"      >  5.34       </Elastic_c12>
      <Elastic_c44           Unit="GPa"      >  5.42       </Elastic_c44>
      <Piezo_e14             Unit="C/m^2"    > -0.015      </Piezo_e14>

   </Material>

ConductionBandOffset  [eV]      energy value that defines the position of the conduction band edges on an absolute energy scale (The zero point of energy is arbitrary.)
                                It can be used to define a conduction band offset between two different materials.

ValenceBandOffset     [eV]      energy value that defines the position of the average valence band edge energy E_v,av on an absolute energy scale (The zero point of energy is arbitrary.)
                                It can be used to define a valence band offset between two different materials.
                                average valence band edge energy: E_v,av = ( E_hh + E_lh + E_so ) / 3

BandGap               [eV]      band gap at the Gamma point given for T = 0 K
                                If the band gap is specified here for another temperature, the Varshni parameters alpha and beta should be set to zero.
BandGapAlpha          [eV/K]    Varshni parameter alpha to allow for temperature dependent band gap
BandGapBeta           [K]       Varshni parameter beta  to allow for temperature dependent band gap

                                BandGap, BandGapAlpha, BandGapBeta are not used inside the calculation. They are just needed to output the valence band edge (which is not used either.)

ElectronMass          [m0]      isotropic effective electron mass of the Gamma conduction band

EpsStatic             []        static dielectric constant, low frequency dielectric constant epsilon(0)
EpsOptic              []        optical dielectric constant, high frequency dielectric constant epsilon(infinity)

LOPhononEnergy        [eV]      longitudinal optical (LO) phonon energy E_OP
                                (This parameter must not be set to zero as there will be a divison by zero in this case, see p. 44 of PhD thesis of Peter Greck: N_OP = 1 / [ exp(E_OP/(k_BT)) - 1 ]... = 1 / (1 - 1) = NAN ("not a number"))
                                N_OP is the phonon distribution and a prefactor of the equation (eq. (7.5)) where the LO phonon scattering strength is calculated, i.e. if N_OP << 1, then the LO phonon scattering is rather small.
                                N_OP = 8 * 10^-45 for GaAs at T =     4 K (E_OP = 35 meV in GaAs)
                                N_OP = 1.3 * 10^-6 for GaAs at T =  30 K
                                N_OP = 0.000297 for GaAs at T =   50 K
                                N_OP = 0.017524 for GaAs at T = 100 K
                                N_OP = 0.151055 for GaAs at T = 200 K
                                N_OP = 0.348148 for GaAs at T = 300 K

LOPhononWidth         [eV]      This is a numerical value that avoids reducing the coupling strength to a delta function: E + E_OP ==> E + E_OP +- Delta E/2, where Delta E = LOPhononWidth

The following variables are only relevant for acoustic phonon scattering.
DeformationPotential  [eV]      scalar deformation potential - It is used for acoustic phonon scattering.
MaterialDensity       [kg/m^3]  material density or mass density
VelocityOfSound       [m/s]     sound velocity
AcousticPhononEnergy  [eV]      acoustic phonon energy

The following variables are relevant for strain calculations.
Lattice_a             [nm]      lattice constant a
Elastic_c11           [GPa]     elastic constant c₁₁
Elastic_c12           [GPa]     elastic constant c₁₂
Elastic_c44           [GPa]     elastic constant c₄₄
Piezo_e14             [C/m^2]   piezoelectric constant e₁₄
 

For ternary compounds like Al_xGa_1-xAs, we have to specify bowing parameters.
The material parameters in many ternary alloys (A_xB_1-xC or CA_xB_1-x) can be approximated in the form of the usual quadratic function

T_ABC = xB_AC + (1-x) B_BC - x(1-x)C_ABC

where C_ABC is the bowing parameter.

<!-- ternary compounds -->
   <Material>

      <Name>In(x)Ga(1-x)As</Name>

      <Alloy>InAs(x)</Alloy>
      <Alloy>GaAs(1-x)</Alloy>
      <ValenceBandOffset     Unit = "eV"     > -0.38  </ValenceBandOffset>
      <BandGap               Unit = "eV"     >  0.477 </BandGap>
      <BandGapAlpha          Unit = "eV/K"   >  0     </BandGapAlpha>  Currently, the Varshni parameters alpha and beta are interpolated. It is probably better to interpolate the band gap instead.
      <BandGapBeta           Unit = "K"      >  0     </BandGapBeta>
      <ElectronMass          Unit = "m0"     >  0.0091</ElectronMass>
      <EpsStatic                             >  0     </EpsStatic>
      <EpsOptic                              >  0     </EpsOptic>
      <DeformationPotential  Unit = "eV"     >  2.61  </DeformationPotential>
      <MaterialDensity       Unit = "kg/m^3" >  0     </MaterialDensity>
      <VelocityOfSound       Unit = "m/s"    >  0     </VelocityOfSound>
      <LOPhononEnergy        Unit = "eV"     >  0     </LOPhononEnergy>
      <LOPhononWidth         Unit = "eV"     >  0     </LOPhononWidth>
      <AcousticPhononEnergy  Unit = "eV"     >  0     </AcousticPhononEnergy>

   </Material>
 

<!-- quaternary compounds -->
   <Material>

      <Name>In(x)Ga(y)Al(1-x-y)As</Name>

      <Alloy>InAs(x)</Alloy>
      <Alloy>GaAs(y)</Alloy>
      <Alloy>AlAs(1-x-y)</Alloy>
  </Material>
 

It is recommended to use position dependent material parameters, i.e. for parameters like LO phonon energy, deformation potential, sound velocity, material density and acoustic phonon energy.
Obviously, the Büttiker probes B(x) depend on position.
But in fact, the parameters for the wells are the most important ones.
The parameters in the barriers have only a minor influence.
One can include them in the calculation but the Büttiker probes in the barriers should not have any significant influence on the final result.

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