Silicon optical modulator using asymmetric shallow waveguide and the method to make the same
The invention provides methods of forming an optical device, in particular, a silicon optical modulator using shallow rib waveguide structure. According to the embodiments of the present invention, the silicon optical waveguide modulator includes a shallow rib waveguide with asymmetric shoulder heights disposed on a surface of a substrate; one side terminated by the waveguide edge and the other side terminated by a second laterally oriented PN junction, a first vertically oriented PN junction is positioned inside the light propagation region of the waveguide; and higher doping regions with the same type of doping type of the adjoining regions are positioned on the asymmetric shoulders outside the light propagation regions in electrical contact with metal contacts.
1. An optical device comprising:
a substrate having an insulating layer;
an optical waveguide structure on top of the insulating layer, formed of an asymmetric rib structure, which includes a center region, and a higher slab region and a lower slab region at opposite sides of the rib structure, the higher slab region having a higher height than the lower slab region;
a vertically-oriented PN diode formed inside the asymmetric rib structure of the optical waveguide structure, comprising a first doping layer and a second doping layer disposed on top of the first doping layer with opposite doping type, the first and second doping layers adjoining to form a vertical PN junction oriented substantially parallel to a top surface of the substrate, wherein the vertically-oriented PN junction is terminated on one side by an edge of the optical waveguide structure;
a first higher doping region located in the lower slab region outside a light propagation region of the optical waveguide structure, the first higher doping region extending vertically to a surface of the substrate, the first higher doping region having a same doping type as and a higher doping concentration than the first doping layer;
a second higher doping region located in the higher slab region outside the light propagation region of the optical waveguide structure, the second higher doping region directly contacting both the first doping layer and the second doping layer and penetrating the vertically oriented PN junction and extending vertically to directly contact the surface of the insulating layer of the substrate, the second higher doping region having a same doping type as and a higher doping concentration than the second doping layer;
a first metal contact being positioned in electrical contact with the first higher doping region; and
a second metal contact being positioned in electrical contact with the second higher doping region.
2. The optical device of claim 1 , wherein the first doping layer partially fills the asymmetric rib structure and extends to the higher and lower slab regions and substantially fills the lower slab region and partially fills the higher slab region, wherein the second doping layer partially fills a remaining portion of the asymmetric rib structure and extends horizontally to a remaining portion of the higher slab region and is terminated by the edge of the optical waveguide structure on the lower slab side.
3. The optical device of claim 2 , wherein a height of the first doping layer is larger than a height of the lower slab region such that the vertically-oriented PN junction is terminated by the edge of the optical waveguide structure and no PN junction exists in the lower slab region.
4. The optical device of claim 2 , wherein a height of the first doping layer is smaller than a height of the higher slab region such that a PN junction exists in the asymmetric rib structure and extends horizontally across the higher slab region and is terminated by a second lateral PN junction.
5. The optical device of claim 1 , wherein a second laterally oriented PN junction exists at an interface of the first doping layer and the second higher doping region outside the propagation region.
6. The optical device of claim 1 , wherein the first doping layer is P-type and the second doping layer is N-type, or the first doping layer is N-type and the second doping layer is P-type.
7. The optical device of claim 6 , wherein a first depletion region exists in a vicinity of the vertically oriented PN junction and a second depletion region exists in a vicinity of the second laterally oriented PN junction when no driving voltage is applied to the first and second metal contacts.
8. The optical device of claim 7 , wherein the first depletion region is located inside the light propagation region and the second depletion region is located outside the light propagation region.
9. The optical device of claim 7 , wherein when a driving voltage is applied to the first and second metal contacts, the first depletion region extends perpendicularly to the vertically oriented PN-junction interface to fill more light propagation region; and wherein when the highest light power density is present, the extended depletion region overlaps the light propagation region.
10. The optical device of claim 1 , wherein the substrate is silicon-on-insulator and the optical waveguide is formed of silicon.
11. The optical device of claim 1 , wherein the center region of the optical waveguide structure has a higher height than the higher slab region.