Modeling - Introduction

Modeling is one of the decisive points for successful device fabrication. Since semiconductor technology is costly and time consuming, a trial-and-error approach would not be feasible and adequate device simulations are absolutely essential.

Opitcal, electrical and thermal simulations are of potential interest for the device design. But also band structure calculations as well as computations on quantization effects (e.g. of quantum wells) are being carried out.

Various commercially available simulation packages as well as custom-made simulation software is employed in our group. On the following pages, examples will be given for some of the aforementioned topics.

 

Modeling - Electrical Simulations

In the area of millimeter wave devices rf-circuits are developed which typically consist of waveguide, microstrip and coplanar components. The design and the optimisation of the circuits is carried out by the help of simulation programmes such as HFSS (High Frequency Structure Simulator) and ADS (Advanced Design System). HFSS is used for calculating the frequency dependent scattering parameters of passive structures using the finite elements method. ADS is based on the method of moments and enables the simulation and characterisation of analogue and digital devices and rf-circuits also considering scattering parameters calculated with HFSS.

The figure below for example shows the field distribution of a waveguide to microstrip transition simulated with HFSS at a frequency of 77 GHz. An antenna is used to couple the incoming wave from the WR-15 waveguide to the microstrip.

 

Modeling - Optical Simulations

Sophisticated optoelectronic devices, as they are being fabricated in our group, require of course an adequate optical design. Points of interest are for example:

  • Waveguiding properties of multilayer stacks
  • Reflection properties of distributed Bragg reflectors (DBRs)
  • Reflection properties of various grating types: distributed feedback (DFB) gratings, sampled gratings (SGs), superstructure gratings (SSGs)
  • Reflection properties anti-reflection (AR) and high-reflection (HR) coatings
  • Cavity modes of DFB-type lasers

Most of these properties cannot be calculated straightforwardly and require numerical simulations.

The figures below show simulations of the reflection spectra (lower figures) of a sampled grating (SG) and a superstructure grating (SSG). Also the corresponding refractive index profiles are schematically depicted (upper figures). In a SG, typically about 90% of one sampling period are uncorrugated while only about 10% contain a grating with uniform pitch. On the other hand, in a SSG the whole superperiod is corrugated. However, in this case the grating pitch is not uniform but is varied either continuously or in discrete steps. Both of these gratings provide a comb like reflection spectrum and are commonly employed in widely tunable laser diodes.

 

Modeling - Thermal Simulations

Heat management is crucial for high-perfomance optoelectronic devices. As most of the device characteristics deteriote exponentially with increasing temperature, efficient heat removal from the active layers of laser diodes or light emitting diodes has to be ensured by an appropriate device design. Detailed analysis of the thermal behavior of devices is carried out using the Quickfield™ simulation package, which is based on the finite-elements method.

Shown below is a simulation of the thermal behavior of a GaSb-based light emitting diode, as schematically depicted in the right hand image. Assuming a heat generation of 10 kW/cm² in the InAsSb active layer, the temperature in the active layer will rise 20.4 K above the heat sink temperature during operation.

Fig. 2: Concept of an injectorless QC laser, based on a five-level staircase.

 

 

 

TUM Technische Universität München TUM Technische Universität München Physik Department Elektrotechnik und Informationstechnik TUM Technische Universität München
 

Events & News

17 Jan 2018

ERC Consolidator Grant for Gregor Koblmüller   more

16 Jan 2018

Light-steering of spin-polarized currents in topological insulators   more

10 Aug 2017

Best Poster Awards for Ganpath Veerabathran and Alexander Andrejew at iNOW 2017   more

27 Jun 2017

Best Poster Award at Nanowire Week for Jochen Bissinger   more

15 Mar 2017

Dr. Kai Müller admitted to the “Junges Kolleg” of the Bavarian Academy of Sciences   more

Seminars

March 12, 2018

Two-dimensional coherent spectroscopy of a semiconductor microcavity   more

March 05, 2018

Diamond-organic photovoltaics   more