Optical loss by surface transfer doping in silicon waveguides

TL;DR

This paper investigates how surface transfer doping causes free-carrier absorption in silicon waveguides, affecting optical losses, and suggests methods to minimize these losses for improved silicon photonics.

Contribution

It demonstrates that surface transfer doping significantly impacts optical losses in silicon waveguides and proposes strategies to reduce propagation losses without gate voltage application.

Findings

Free-carrier absorption causes up to 1.8 dB/cm loss.

Higher sheet resistances correlate with lower losses.

Surface transfer doping is a key factor in free-carrier generation.

Abstract

We show that undoped silicon waveguides may suffer of up to 1.8 dB/cm free-carrier absorption caused by improper surface passivation. To verify the effects of free-carriers we apply a gate field to the waveguides. Smallest losses correspond to higher electrical sheet resistances and are generally obtained with non-zero gate fields. The presence of free carriers for zero gate field is attributed to surface transfer doping. These results open new perspectives for minimizing propagation losses in silicon waveguides and for obtaining low-loss and highly conductive silicon films without applying a gate voltage.