High Power VCSELs with Multi-Junctions
Motivation
Harold’s VCSEL model supports the simulation of multi-junctions separated by tunnel junctions. This helps create VCSELs with higher output power limits. With a single junction, increasing the current forced through the resistive DBR stacks of VCSELs causes increased I2R heating. It also introduces large thermal gradients across the grating stack causing detuning in the DBR; Harold’s self-consistent 3D optical/electric/thermal model simulates all of this in a rigorous self-consistent manner.
Multi-Junctions re-use the same applied current, passing through tunnel junctions to increase the power output with a cost of a larger terminal voltage over the full design.
Results
Solid lines show LI curves for VCSEL designs with 1, 2, 3, and 4 pn junctions. With more junctions we see a greater power output for a given current. The additional voltage required for each junction is shown in dashed lines increasing with more junctions.
Harold clearly shows that adding more junctions eventually reduces efficiency as the losses in the tunnel junction add up and problems relating to the increased total active layer thickness mount. Harold’s heating model includes the Joule heating, Non-Radiative Recombination, and Free Carrier Absorption.
Band Diagram
Harold’s VCSEL model includes a 3D drift-diffusion model based on Fermi-Dirac statistics to solve the energy band structure of the VCSEL (on axis profile shown).
Thermal Model
The temperature gradients due to applied current are simulated in the VCSEL model which inform the gain.
Optical Model
The optical mode confined by DBR gratings is simulated with a full 3D rigorous model. The overlap in the optical profile with the gain spectra simulated is used to simulate the output power. Given this optical mode, the effects of spatial hole burning can also be accounted for.
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Optical Model (cont)
Optical mode shown graphically marking the locations of tunnel junctions.