The First Commercial Simulator For Semiconductor Lasers

The first commercial software that simulates semiconductor lasers has just been released by Silvaco. This new software helps manufacturers of semiconductor lasers to develop technologies more efficiently. In this article we describe the new semiconductor laser simulation capabilities, and we provide examples of their use.

Introduction

Silvaco's 2D device simulation capabilities comprise the ATLAS II device simulation subframework and a set of modular products that use the framework. Basic device simulation capabilities are provided by S-Pisces for devices fabricated using silicon technologies; and by Blaze, for devices fabricated using arbitrary semiconductors and heterojunction combinations.

Several other ATLAS II products work with either S-Pisces or Blaze. Giga adds the ability to simulate the effects of lattice heating and heatsinks; Luminous adds comprehensive models of optoelectronic interactions, and includes sophisticated structure-resolving ray-tracing capabilities; TFT provides models associated with conduction in amorphous and polycrystalline materials; and MixedMode provides a SPICE-like circuit environment that can use numerically simulated devices as well as compact models. Capabilities for simulating semiconductor laser diodes have now been implemented as a new ATLAS II product called LASER, which works in conjunction with Blaze.

Physical Models

Several different approaches to simulating semiconductor lasers have been described in the literature, and have been implemented in research or proprietary codes. Rather than select one of these approaches for use in Laser, we chose to make available several different options that encompass virtually all the published approaches.

Laser therefore provides the following capabilities:

  • Solution of the Helmholtz equation, to calculate optical fields and photon densities
  • Calculation of carrier recombination due to light emission (i.e. stimulated emission)
  • Calculation of optical gain, including the dependence on optical frequency and quasi-Fermi level
  • Calculation of laser light output power
  • Calculation of the light intensity profile corresponding to the fundamental transverse mode
  • Calculation of light output and modal gain spectra for several longitudinal modes

 

The complex dielectric permittivity used in the solution of the Helmholtz equation is a function of frequency, local optical gain, and a line width broadening factor.

Two models for local optical gain are available. The first model is physically based, and takes into account frequency dependence. The second model is a simple but widely used empirical model that is valid only for the lasing frequency. This model can not be used for the calculation of longitudinal modes.

Carrier recombination due to stimulated light emission is modeled as a sum over modes. The contribution associated with each mode depends on the optical gain and the photon density of the mode, as well as the optical field magnitude. The photon density in each mode is calculated by solving a system of photon rate equations.

Linkage between the optical and electrical models is provided by the optical gain, which depends on quasi-Fermi levels and is used to calculate permittivity; and by the dependence of carrier recombination on photon

densities. Laser solves the electrical and optical equations self-consistently using one of two approaches. The first approach, which is used in most work in this area, is a single-frequency approach. The second approach accounts for multiple longitudinal modes. The first approach is faster, provides good results very reliably, and is recommended if the lasing spectrum is not the main focus of interest. The second approach involves significantly more computation, but allows users to determine the effect of the cavity length and other factors on the lasing spectrum.

 

Figure 1. The calculated light intensity distrubution
in a strip geometry GaAs/AlGaAs laser diode.

 

Example

Single-frequency and multi-mode simulation results for an InP/In GaAsP structure will be presented. The device structure is shown in Figure 2. The active lasing region is InGaAsP. All heterojunctions are modeled as abrupt.

Figure 2. The InP/InGaAsP laser diode structure.

 

The structure is simulated initially using the single frequency approach. The calculated forward IV characteristic is shown in Figure 3, and the predicted light power as a function of current is shown in Figure 4. The laser threshold current is calculated to be 1.4 mA. Figure 5 shows the light intensity distribution over the structure, i.e. the so-called near field pattern. (Figures 1 and 5 were produced using the monochrome mode of TonyPlot. Users also have access to the color mode, which provides more impressive visualization capabilities.)

 


Figure 3. Simulated IV characteristics

 


Figure 4. Light and output as a function of current.

 


Figure 5. Light intensity distribution.

The spectral characteristics of the InP/InGaAsP laser diode are investigated next. For a shorter cavity length, the energy spacing between adjacent longitudinal modes is wider, and fewer longitudinal modes actually lase. The cavity length for this example is set to 50µm which is short, but serves to highlight the effects. The total light output power is shown in Figure 6, along with the results calculated previously. The gain spectra for different diode currents are shown in Figure 7. The saturation effect after threshold is clearly visible. The light output spectra calculated at bias points below threshold and above threshold are shown in Figures 8 and 9. Only a few of the longitudinal modes are lasing, and the spectrum is practically unchanged once the lasing threshold has been reached.

Figure 6. Light output as a function of current.


Figure 7. Gain spectra above and below threshold.


Figure 8. Light output spectrum below laser threshold

Figure 9. Light output spectrum above laser threshold.


Conclusions

A new product called Laser has been added to the ATLAS II product range. Laser works in conjunction with Blaze to predict the behavior of semiconductor lasers. The combination of Blaze and Laser provides the first commercial capabilities for simulating semiconductor laser diodes.

Laser is available now. For more details about ATLAS II products, including Blaze and Laser, please contact your local Silvaco sales office.