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qcl:faq
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qcl:faq [2018/04/19 14:26] thomas.grange |
qcl:faq [2022/10/12 09:27] (current) thomas.grange |
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| We recommend 16 GB RAM. 8 GB is sufficient for some input files. The code becomes incredibly slow if there is not sufficient memory is available. | We recommend 16 GB RAM. 8 GB is sufficient for some input files. The code becomes incredibly slow if there is not sufficient memory is available. | ||
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| + | === I get an error message when I launch a simulation === | ||
| + | |||
| + | Check that .NET framework version 4.6 or later is installed on your system. | ||
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| === Where should I start my layer sequence in the input file? === | === Where should I start my layer sequence in the input file? === | ||
| - | The results of the calculation should not depend on which material layer the sequence starts, i.e. a cyclic permutation in the material layer sequence should not change the simulation results. | + | The results of the calculation should not depend on which material layer the sequence starts, i.e. a cyclic permutation in the material layer sequence should not change the simulation results (if not the case, it means that the convergence factors are not chosen to be accurate enough). |
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| $\Delta_{\rm so}$ is the spin-orbit split-off energy. | $\Delta_{\rm so}$ is the spin-orbit split-off energy. | ||
| - | * Option a) Specify conduction band offset (CBO) $E_{\rm c}$\\ ''<UseConductionBandOffset>true</UseConductionBandOffset>'' | + | These two different options have different consequences in how the temperature dependence of the bandgap is accounted. Indeed: |
| + | |||
| + | * Option a) Specify conduction band offset (CBO) $E_{\rm c}$\\ ''<UseConductionBandOffset>yes</UseConductionBandOffset>'' | ||
| + | |||
| + | As a consequence, the band offset of the light hole becomes temperature dependent: | ||
| $$E_{\rm hh}(T) = E_{\rm c} - E_{\rm gap}(T)$$ | $$E_{\rm hh}(T) = E_{\rm c} - E_{\rm gap}(T)$$ | ||
| - | * Option b) Specify valence band offset (VBO) $E_{\rm v,av}$\\ The conduction band edge $E_{\rm c}$ is calculated and depends on temperature.\\ ''<UseConductionBandOffset>false</UseConductionBandOffset>'' (default) | + | |
| + | * Option b) Specify valence band offset (VBO) $E_{\rm v,av}$\\ The conduction band edge $E_{\rm c}$ is calculated and depends on temperature.\\ ''<UseConductionBandOffset>no</UseConductionBandOffset>'' (default) | ||
| \begin{align*} | \begin{align*} | ||
| E_{\rm hh} & = E_{\rm v,av}+\frac{1}{3}\Delta_{\rm so}\\ | E_{\rm hh} & = E_{\rm v,av}+\frac{1}{3}\Delta_{\rm so}\\ | ||
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| ---- | ---- | ||
| - | === How shall I choose the grid spacing ''<Spatial_grid_spacing unit=%%"%%nm%%"%%>''? === | + | === How shall I choose the grid spacing ''<Spatial_grid_spacing unit=nm>''? === |
| Usually the layers in a QCL have thicknesses of around 1 nm, e. g. 1.3 nm and 1.7 nm. | Usually the layers in a QCL have thicknesses of around 1 nm, e. g. 1.3 nm and 1.7 nm. | ||
| Therefore, in this case, does a grid spacing of 0.2 nm and 0.3 nm make a big difference in the results? | Therefore, in this case, does a grid spacing of 0.2 nm and 0.3 nm make a big difference in the results? | ||
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| ---- | ---- | ||
| - | **Q:** In the gain simulation, some energy points are missing. | + | === In the gain simulation, some energy points are missing. === |
| In the input file, the energy interval, i.e. the energy spacing between two photon energies (''<dE_Phot unit=%%"%%meV%%"%%>'' or ''<dE_Phot_Self_Consistent unit=%%"%%meV%%"%%>''), has been set to be 2 meV. | In the input file, the energy interval, i.e. the energy spacing between two photon energies (''<dE_Phot unit=%%"%%meV%%"%%>'' or ''<dE_Phot_Self_Consistent unit=%%"%%meV%%"%%>''), has been set to be 2 meV. | ||
| However, for the final results, the interval is 4 meV. | However, for the final results, the interval is 4 meV. | ||
| - | **A:** The energy interval for the gain calculation will always be at least the energy grid spacing ''<Energy_grid_spacing unit=%%"%%meV%%"%%>''. | + | The energy interval for the gain calculation will always be at least the energy grid spacing ''<Energy_grid_spacing unit=%%"%%meV%%"%%>''. |
| ---- | ---- | ||
| - | **Q:** The simulation results contain a ''Gain/'' folder but it is empty. | + | === The simulation results contain a ''Gain/'' folder but it is empty.=== |
| - | **A:** Note that the gain output is only done for the voltages specified in the input file.\\ | + | Note that the gain output is only done for the voltages specified in the input file.\\ |
| <code> | <code> | ||
| <!-- Calculate gain only between the following values of | <!-- Calculate gain only between the following values of | ||
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| ---- | ---- | ||
| - | **Q:** What is the difference between the output files ''RealSpaceModes.dat'' and ''Wannier-Stark_levels.dat''? How are they related to the usual approach of calculating the eigenstates in a given conduction band edge profile (single-band Schrödinger equation)? | + | === What is the difference between the output files ''RealSpaceModes.dat'' and ''Wannier-Stark_levels.dat''? How are they related to the usual approach of calculating the eigenstates in a given conduction band edge profile (single-band Schrödinger equation)? === |
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| + | The output file ''Wannier-Stark_levels.dat'' gives the usual eigenstates of the conduction band profile for the periodic heterostructure, by solving the single-band Schrödinger equation (with/without nonparabolicity). The output file ''RealSpaceModes.dat'' gives the position eigenstates within the subspace obtained after applying the axial cut-off energy. These position eigenstates are used as a basis in the NEGF calculation. Note that these states depend on the axial cut-off energy: the larger the axial energy cut-off is, the more localized they are. | ||
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| + | === The self-consistent gain and semi-classical gain show maximum at different photon energies. Which one to trust more? === | ||
| + | |||
| + | The semi-classical calculation is made in the Wannier-Stark states, so it is expected to give maximum photon energy around the same energy as the Wannier-Stark transition energies (though it can be slightly offset as in general multiple peaks are added). | ||
| + | |||
| + | On the other hand, the self-consistent (fully quantum) simulation does not consider any preferred basis and accounts more accurately on broadening. If the broadening (induced by scattering processes are small), semi-classical and self-consistent calcualtions should give the same result. | ||
| + | However, as broadening becomes important, there will be a red shift with respect to the bare transition energies. This shift will depend on the scattering processes. So then the question of which one to trust more is also related to the question whether the parameters for scattering (interface roughness, Coulomb scattering...) matches the reality. And it should be kept in mind there are some underlying assumptions in the NEGF model (in particular the self-consistent Born approximation) which could lead to deviation with respect to reality (such as an overestimate of the is red-shifting effect of transition energy with broadening). | ||
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| + | === At zero bias, when the current asymptotically approaches 0, the current convergence factor does not converge to zero. Is this ok? === | ||
| - | **A:** The output file ''Wannier-Stark_levels.dat'' gives the usual eigenstates of the conduction band profile for the periodic heterostructure, by solving the single-band Schrödinger equation (with/without nonparabolicity). The output file ''RealSpaceModes.dat'' gives the position eigenstates within the subspace obtained after applying the axial cut-off energy. These position eigenstates are used as a basis in the NEGF calculation. Note that these states depend on the axial cut-off energy: the larger the axial energy cut-off is, the more localized they are. | + | When the current approaches 0, it is indeed normal that the current convergence factor does not goes to 0. In this case, the convergence should be checked accordingly to the other convergence factor which is based on the lesser Green’s function. |
qcl/faq.1524147971.txt.gz · Last modified: 2018/04/19 14:26 by thomas.grange
