RSoft Enewsletter

June 2017

Co-Simulation of Integrated Photonic Devices Using Sentaurus TCAD and RSoft

Most photonic devices, including lasers, photodiodes, and modulators, are optoelectronic devices. To completely capture the physics of these devices in a computer simulation, both a photonic/optic and an electronic simulation are needed.

Synopsys’ RSoft and Sentaurus TCAD products are the gold standard tools for photonic and electronic process simulations. Synopsys’ RSoft-TCAD interface allows users to seamlessly combine the individual modeling capabilities of RSoft and TCAD to achieve fast, rigorous optoelectronic simulations. When using the interface, data is transferred bidirectionally between RSoft and TCAD in native format without interpolation and extrapolation. All geometric and material data can be loaded directly from TCAD into RSoft, and users do not need to generate the structure twice. This enews highlights the RSoft-TCAD interface and demonstrates its use for simulating a SiGe waveguide photodetector.

Following are a few key features of RSoft and TCAD.

RSoft:

  • Market leader in PDA (Photonic Design Automation); provides full spectrum of simulation tools at different levels
    • Device Simulation:
      • Models detailed behavior of photonics components, with the ability to generate, modify, transmit and detect light
      • Utilizes multiple rigorous and semi-rigorous algorithms to describe and simulate the innumerable variety of photonic components
      • Considers full 3D spatial behavior of the EM field and/or electron device characteristics
    • Circuit Simulation: Models interaction of device components on a photonic chip
    • System Simulation: Models discrete components together to form a system for data transmission

Sentaurus TCAD:

  • Production-proven 3D TCAD simulation technology
  • Most accurate results through calibrated process and device models for the most advanced processes
  • Technical and market leadership across all technologies: Logic, Memory, Analog, Power, Optoelectronics

The SiGe waveguide photodetector (PD) that will be simulated is shown in Figure 1. This photodetector is a Ge photodetector integrated on top of the Si waveguide, separated by a thin SiON layer. A taper is used to convert the optical mode from Si to Ge. Approximately 30 tungsten (W) pillars are alternately connected to one of two bias contacts. The structure is surrounded by SiO2, and a bias of 3V is applied between the contacts.

SiGe waveguide photodetector

Figure 1: A schematic of the SiGe waveguide photodetector studied in this tutorial

The SiGe PD structure is described in detail in the following reference:
Assefa, Solomon, et al., "CMOS-integrated high-speed MSM germanium waveguide photodetector", Optics Express 18 No. 5 (2010), 4986-4999.

To simulate the PD, we will use Sentaurus TCAD for the layout and the electronic simulation. We will use RSoft products to calculate the optical absorption; either RSoft FullWAVE or BeamPROP tools can be used for this purpose.

The simulation is run in Sentaurus Workbench (SWB), and broadly consists of three main steps:

  1. Create the geometry in SDE
  2. Run the optical simulation in RSoft
  3. Use S-Device and Inspect for the electronic simulation and the data visualization

Figure 2: The SWB work flow (left to right)
(Click image for a larger image)

1. Create Photodetector Geometry with SDE

SDE is used to create the structure displayed below. We will not describe the exact steps for creating this structure in SDE as it is beyond the scope of this enews.

SiGe Photodetector structure,

Figure 3: 3D SiGe Photodetector structure, as created in SDE, displayed without oxide cover

2. Use RSoft for the Optical Simulation

Using RSoft in SWB requires two inputs: an RSoft CAD design file and an RSoft simulation script. The RSoft CAD design file is imported from Sentaurus TCAD. The script is a standard RSoft bash script that will be called from Sentaurus. Figure 4 shows part of the simulation script.

Photodetector structure imported from Sentaurus TCAD to the RSoft CAD

Figure 4: Photodetector structure imported from Sentaurus TCAD to the RSoft CAD (top). Part of the RSoft simulation script (bottom).

    Either RSoft FullWAVE FDTD or BeamPROP BPM simulation tools can be used to rigorously model the EM propagation in the photodetector and calculate the absorption profile for use in the TCAD electronic simulation. For this simulation, we will use BeamPROP since it is more efficient (see Figure 7 for additional details).

    The optical results are shown below.

 Input Mode (left) and  absorption profile (right) of photodetector structure

 Figure 5: Input Mode (left) and absorption profile (right) of photodetector structure

3. Import absorption data into TCAD and calculate the photodetector’s electrical aspects

Sentaurus Device uses the absorption data from the RSoft step and the underlying geometry from SDE. The resulting output by Inspect shows the I-V curves generated by SDevice for 3 different incident powers, including the dark-current condition.

Photodetector optical absorption profile

Figure 6: Photodetector optical absorption profile as seen in Sentaurus TCAD (left) IV curves for various optical input powers (right)

As mentioned earlier, for this structure, we have a choice of using FullWAVE or BeamPROP for the optical simulation. BeamPROP, based on the BPM algorithm, is the more efficient tool and is ~50x faster than FDTD with comparable results.

BPM vs FDTD comparison

Figure 7: BPM vs FDTD comparison. Note that BPM is ~50x faster than FDTD for this simulation

In conclusion, the RSoft component tools are fully integrated into Sentaurus WorkBench (SWB), allowing you to easily combine photonic and electronic simulation to rigorously model an optoelectronic device. In this enews, we demonstrated the combined electronic and photonic simulation of a waveguide photodetector; however, in principle any optoelectronic device can be simulated using the RSoft/TCAD interface.

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