SiGe QW excitonic absorption

Attention

This tutorial is under construction.

Input files:

1D_Ge_GeSi_QCSE_Lever2010_8kp_nnp_exciton.in

Scope of the tutorial:

In this tutorial, we show an approach how to model absorption spectrum in a quantum well. This tutorial reproduces results from [LeverJLT2010].

The most relevant keywords:

• contacts

• optics{ quantum_spectra{} }

• quantum{excitons}

Solvers:

• strain

• poisson

• quantum

• quantum_optics

Relevant output files:

bias_xxxxx\Optics\absorption_quantum_region_TE_eV.dat

Introduction

This tutorial shows how to model an absorption inside a quantum well — an active region of electro-absorption modulator. The tutorial reproduces results from [LeverJLT2010] with 9 nm Ge well with 12 nm Si _0.4 Ge _0.6 barrier grown on Si _0.3 Ge _0.7 substrate. The Ge concentration profile is smoothened by interdiffusion, which is modelled using analytic profile from [LeverJLT2010]. The Ge grown on the Si substrate is tensile strained, because the bulk thermal expansion coefficient of Ge is larger than of the Si substrate. In order to take in into account, 0.1% tensile residual strain is added to virtual substrate.

strain{
    residual_strain = 0.001
...
}

The figure Figure 2.4.399 shows the wave functions in conduction and valence bands.

Figure 2.4.399 The band edges (colored) and the wave function probabilities (gray) in the quantum well under 0 bias.

The bias sweep from 0 V to 0.5 V is specified in the input file in the contacts

$left_bias_start = 0
$left_bias_finish = 0.27
...
contacts{
    ohmic{ name = "left" bias = [$left_bias_start, $left_bias_finish] steps = 3}
    ohmic{ name = "right" bias = 0}
}

For each bias the absorption spectrum in the device is calculated. Due to the quantum confinement, the excitonic absorption is still observable at room temperatures. The excitonic correction is added; more details are explained in tutorial “Optical interband absorption in a quantum well including excitonic effects”. The absorption spectra at different biases is shows in the figure Figure 2.4.400.

Figure 2.4.400 Excitonic absorption spectra in the device. Labels indicate electric field in the middle of the quantum well.

The redshift of exciton peak is observed when bias is applied to the structure. At a given wavelength, the absorption increase is significant allowing for electro-optic absorption modulation. The modelling can be used to optimize the parameters of the device and to choose the optimal wavelength of the modulation for a given structure.

The position of exciton peaks are in a good agreement with simulation from [LeverJLT2010] — within 1 meV error for each bias. While the relative change of absorption spectra with applied bias also agrees with experimental data, the absolute value differs by a factor 1.4 – 1.6. The nextnano software is continuously improving to meet last criteria as well.

Acknowledgment

This tutorial is based on the nextnano GmbH collaboration in the scope of the SiPho-G Project aiming at development of ultrahigh-speed optical components for next-generation photonic integrated circuits, and it is funded by the European Union’s Horizon 2020 research and innovation program under grant agreement No 101017194.

Last update: nn/nn/nnnn