Design of CMOS integrated germanium photodiodes

The process of designing an integrated waveguide photodiode system, the process comprising the steps of:

• selecting an integrated waveguide photodiode system comprising:
• a silicon waveguide formed from the top silicon layer of a silicon-on-insulator (SOI) substrate;
• and,
• an integrated waveguide photodiode, disposed on the substrate and optically coupled to the silicon waveguide wherein the integrated waveguide photodiode comprises:
• a silicon layer comprising substantially silicon, formed from the top silicon layer of a silicon-on-insulator (SOI) substrate, at least partially contiguous with the silicon waveguide;
• and
• a germanium layer comprising substantially germanium for conducting light, disposed on top of the silicon layer;
• predicting low optical field regions at the top surface of the germanium layer;
• and,
• configuring the position of at least one electrode comprising at least one contact in electrical contact with the germanium layer, in proximity to predicted low optical field regions at the top surface of the germanium layer, thereby reducing optical losses associated with the contacts,
• wherein at least some portion of the germanium layer is tapered, and
• wherein at least one electrode comprising at least one contact is positioned in proximity to the onset of the taper.

An integrated waveguide photodiode system comprising:

• a silicon waveguide formed from the top silicon layer of a silicon-on-insulator (SOI) substrate;
• an integrated waveguide photodiode, disposed on the substrate and optically coupled to the silicon waveguide wherein the integrated waveguide photodiode comprises:
• a silicon layer comprising substantially silicon, formed from the top silicon layer of a silicon-on-insulator (SOI) substrate, at least partially contiguous with the silicon waveguide;
• and
• a germanium layer comprising substantially germanium for conducting light, disposed on top of the silicon layer, the germanium layer comprising at least one tapered portion wherein:
• the configuration of the silicon waveguide and the integrated waveguide photodiode is predicted to couple light from the silicon waveguide into at least two predominant modes of the integrated waveguide photodiode, the two dominant modes comprising:
• a ground mode corresponding to the mode with highest refractive index of modes of the integrated waveguide photodiode;
• and,
• a second mode with a germanium overlap which improves for germanium layers wider than a characteristic width;
• wherein:
• the initial width of the taper is chosen to be smaller than the characteristic width.

A method for designing an improved integrated waveguide photodiode system comprising the steps of:

• selecting an integrated waveguide photodiode system comprising:
• a silicon waveguide formed from the top silicon layer of a silicon-on-insulator (SOI) substrate;
• and,
• an integrated waveguide photodiode, disposed on the substrate and optically coupled to the silicon waveguide wherein the integrated waveguide photodiode comprises:
• a silicon layer comprising substantially silicon, formed from the top silicon layer of a silicon-on-insulator (SOI) substrate, at least partially contiguous with the silicon waveguide;
• and
• a germanium layer comprising substantially germanium for conducting light, disposed on top of the silicon layer;
• designing the shape of the germanium layer, wherein the shape of the germanium layer comprises a tapered portion and wherein the silicon waveguide is coupled to the integrated waveguide photodiode in the region of the tapered portion, the step of designing comprising the step of:
• selecting the initial width of the tapered portion germanium layer, at least in part, to excite at least two predominant modes when under operation, the two predominant modes comprising:
• a ground mode corresponding to the mode with highest refractive index of modes of the integrated waveguide photodiode;
• and,
• a second mode with a germanium overlap which improves for germanium layers wider than a characteristic width.