# US 7,613,369 B2 · Design of CMOS integrated germanium photodiodes

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- Number: US 7,613,369 B2
- Document ID: US7613369B2
- Status: Granted
- Kind: B2
- Application number: 11/735,251
- Filed: 2007-04-13
- Published: 2008-08-14
- Granted: 2009-11-03
- Earliest claimed: 2006-04-13
- Assignees: Luxtera, Inc.
- Inventors: Jeremy Witzens; Gianlorenzo Masini; Giovanni Capellini; Lawrence C. Gunn, III
- Web page: https://patentgrep.com/patent/US7613369B2

## CPC

- G02B 6/12007: {forming wavelength selective elements, e.g. multiplexer, demultiplexer}
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

A CMOS processing compatible germanium on silicon integrated waveguide photodiode. Positioning contacts in predicted low optical field regions, establishing side trenches in the silicon layer along the length of the photodiode reduces optical losses. Novel taper dimensions are selected based on the desirability of expected operational modes, reducing optical losses when light is injected from the silicon layer to the germanium layer. Reduced vertical mismatch systems have improved coupling between waveguide and photodiode. Light is coupled into and/or out of a novel silicon ring resonator and integrated waveguide photodiode system with reduced optical losses by careful design of the geometry of the optical path. An integrated waveguide photodiode with a reflector enables transmitted light to reflect back through the integrated waveguide photodiode, improving sensitivity. Careful selection of the dimensions of a novel integrated waveguide microdisk photodiode system results in reduced scattering. Improved sensitivity integrated waveguide photodiodes comprise integrated heaters.

## Claims

### Claim 1

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; and,
at least one electrode comprising at least one contact in electrical contact with the germanium layer, positioned in proximity to predicted low optical field regions in the region of the top surface of the germanium layer, thereby reducing optical losses associated with the contacts,
wherein the predicted low optical field regions are based on two dominant modes of the integrated waveguide photodiode substantially coupled to under operational conditions.

### Claim 2 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein the SOI substrate comprises an SOI substrate selected from the list of:
SSOI (strained silicon-on-insulator) and SOS (silicon on sapphire).

### Claim 3 (depends on 1)

The integrated waveguide photodiode system of claim 1 further comprising:
a transistor body disposed on the SOI substrate.

### Claim 4 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein
the predicted low optical field regions are at least in part based on
effective indices of at least the two dominant modes of the integrated waveguide photodiode substantially coupled to under operational conditions and
the coupling coefficients from the silicon waveguide into the integrated waveguide photodiode.

### Claim 5 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein:
the contacts are positioned in proximity to predicted low optical field regions in the region of the top surface of the germanium layer, thereby reducing optical scattering losses associated with the contacts.

### Claim 6 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein:
the contacts are positioned in proximity to predicted low optical field regions in the region of the top surface of the germanium layer, thereby reducing optical absorption losses associated with the contacts.

### Claim 7 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein:
the germanium layer has substantially constant thickness.

### Claim 8 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein:
the germanium layer is grown using a selective growth method.

### Claim 9 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein:
two or more redundant contacts are positioned at substantially the same longitudinal position.

### Claim 10 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein:
the electrode comprises at least one element selected from the list of:
an anode or a cathode.

### Claim 11 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein:
at least some portion of the germanium layer is tapered and;
at least one electrode comprising at least one contact is positioned in proximity to the onset of the taper.

### Claim 12 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein
the predicted low optical field regions are further based on and
two or more contacts positioned periodically in the longitudinal direction with a periodicity corresponding substantially to a beating length of the two dominant modes.

### Claim 13 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein
the predicted low optical field regions are further based on
two or more contacts positioned with a longitudinal spacing corresponding substantially to a beating length of the two dominant modes.

### Claim 14 (depends on 1)

The integrated waveguide photodiode system of claim 1 wherein
the predicted low optical field regions are further based on
a changing cross-section of the waveguide photodiode at different longitudinal positions,
effective indices of the two dominant modes varying along the integrated waveguide photodiode, and,
three or more contacts positioned with unequal spacing in the longitudinal direction corresponding substantially to a beating length of the two dominant modes.

### Claim 15 (depends on 1)

The integrated waveguide photodiode system of claim 1,
wherein beating between the two dominant modes causes the light to alternate between the silicon layer and the germanium layer as the light moves in the longitudinal direction in the integrated waveguide photodiode, and
wherein two or more contacts are positioned in the longitudinal direction at positions corresponding substantially to a beating length of the two dominant modes, in regions where the light is substantially in the silicon layer for operational conditions.

### Claim 16 (depends on 1)

The integrated waveguide photodiode system of claim 1,
wherein beating between the two dominant modes causes the lateral position of the low optical field regions to change along a length of the integrated waveguide photodiode, the lateral positions comprising:
a central position wherein a low optical field is disposed substantially in the center of a width of the integrated waveguide photodiode, in proximity to the region of the top surface of the germanium layer; and,
a peripheral position wherein low optical fields are disposed substantially along each side edge of the integrated waveguide photodiode, in proximity to the region of the top surface of the germanium layer;
wherein at least one contact is positioned in proximity to a low optical field region associated with a peripheral position.

### Claim 17 (depends on 1)

The integrated waveguide photodiode system of claim 1,
wherein beating between the two dominant modes causes the lateral position of the low optical field regions to change along a length of the integrated waveguide photodiode, the lateral positions comprising:
a central position wherein a low optical field is disposed substantially in the center of a width of the integrated waveguide photodiode, in proximity to the region of the top surface of the germanium layer; and,
a peripheral position wherein low optical fields are disposed substantially along each side edge of the integrated waveguide photodiode, in proximity to the region of the top surface of the germanium layer;
wherein at least one contact is positioned in proximity to a central position.

### Claim 18 (depends on 1)

The integrated waveguide photodiode system of claim 1 further comprising:
a second silicon waveguide formed from the top silicon layer of a silicon-on-insulator (SOI) substrate, and coupled to the second end of the integrated waveguide photodiode.

### Claim 19

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.

### Claim 20

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.

### Claim 21 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
the SOI substrate comprises an SOI substrate selected from the list of:
SSOI (strained silicon-on-insulator) and SOS (silicon on sapphire).

### Claim 22 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
an intermediate taper width wider than the characteristic width is selected to support the improvement of the germanium overlap and germanium confinement profile.

### Claim 23 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
one or more dimensions of the tapered portion are selected, at least in part, to support the improvement of the germanium overlap and germanium confinement profile, the dimensions selected from the list comprising:
the overall length of the tapered portion and the taper grade.

### Claim 24 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
the ground mode comprises a single lobed mode with high germanium overlap and good lateral confinement characteristics in the germanium layer.

### Claim 25 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
in the region of the onset of the tapered portion, the second mode comprises a mode with a vertical lobe structure and less overlap with the germanium layer than the ground mode.

### Claim 26 (depends on 25)

The integrated waveguide photodiode system of claim 25 wherein:
in the region of the onset of the tapered portion, the second mode comprises a mode with a horizontal lobe structure.

### Claim 27 (depends on 25)

The integrated waveguide photodiode system of claim 25 wherein:
the germanium layer further comprises a non-tapered portion.

### Claim 28 (depends on 27)

The integrated waveguide photodiode system of claim 27 wherein:
in the region of the onset of the tapered portion, the second mode comprises a mode with a horizontal lobe structure.

### Claim 29 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
less than eighty percent of the optical power is predicted to be coupled into the ground mode of the integrated waveguide photodiode system when under operation.

### Claim 30 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
more than twenty percent of the optical power is predicted to be coupled into the second mode when under operation.

### Claim 31 (depends on 20)

The integrated waveguide photodiode system of claim 20 further comprising:
a transistor body disposed on the SOI substrate.

### Claim 32 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
the germanium layer has substantially constant thickness.

### Claim 33 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
the germanium layer is grown using a selective growth method.

### Claim 34 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
the initial width of the tapered portion is large enough to accommodate a contact to the germanium layer.

### Claim 35 (depends on 20)

The integrated waveguide photodiode system of claim 20 wherein:
the integrated waveguide photodiode further comprises:
a second silicon waveguide formed from the top silicon layer of a silicon-on-insulator (SOI) substrate, and coupled to the second end of the integrated waveguide photodiode.

### Claim 36 (depends on 35)

The integrated waveguide photodiode system of claim 35 wherein:
the germanium layer comprises at least a second tapered portion disposed in proximity to the second end of the germanium layer.

### Claim 37 (depends on 35)

The integrated waveguide photodiode system of claim 35 wherein:
the germanium layer comprises at least a second tapered portion disposed at the second end of the germanium layer.

### Claim 38

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.