Pn-junction phase modulator in a large silicon waveguide platform

Abstract

A modulator. In some embodiments, the modulator includes a portion of an optical waveguide, the waveguide including a rib extending upwards from a surrounding slab. The rib may have a first sidewall, and a second sidewall parallel to the first sidewall. The rib may include a first region of a first conductivity type, and a second region of a second conductivity type different from the first conductivity type. The second region may have a first portion parallel to and extending to the first sidewall, and a second portion parallel to the second sidewall. The first region may extend between the first portion of the second region and the second portion of the second region.

What is claimed is: 
     
         1 . A modulator, comprising:
 a portion of an optical waveguide, the optical waveguide comprising a rib extending upwards from a surrounding slab;   the rib having a first sidewall, and a second sidewall parallel to the first sidewall;   the rib including a first region of a first conductivity type, a second region of a second conductivity type different from the first conductivity type, and a third region of the first conductivity type;   the second region having:
 a first portion parallel to and extending to the first sidewall, and 
 a second portion parallel to the second sidewall, 
   the third region being parallel to and extending to the second sidewall;   the first region extending between the first portion of the second region and the second portion of the second region; and   the second portion of the second region being between the first region and the third region.   
     
     
         2 . The modulator of  claim 1 , wherein the ratio of the height of the rib to the width of the rib is greater than 2 and less than 6. 
     
     
         3 . The modulator of  claim 1 , wherein the first region has a keyhole shape including a rounded upper portion having a width exceeding, by at least 5%, a width of a narrower lower portion. 
     
     
         4 . The modulator of  claim 1 , wherein an interface between the first region and the second region includes two parallel vertical portions, the modulator being configured to impose a phase shift on light propagating through it, in response to a reverse-bias voltage applied being applied across the first region and the second region, at least 90% of the phase shift being due to interaction of the light with the two parallel vertical portions. 
     
     
         5 . The modulator of  claim 1 , further comprising:
 a first metal contact, and   a second metal contact,   the first metal contact being connected to the first region through a conductive path traversing, in a direction from the first metal contact to the first region:
 first, a heavily doped region of the first conductivity type, and 
 second, a doped region of the first conductivity type, with a doping level between that of the heavily doped region of the first conductivity type and that of the first region; and 
   the second metal contact being connected to the second region through a conductive path traversing, in a direction from the second metal contact to the second region:
 first, a heavily doped region of the second conductivity type, and 
 second, a doped region of the second conductivity type, with a doping level between that of the heavily doped region of the second conductivity type and that of the second region. 
   
     
     
         6 . The modulator of  claim 1 , wherein the first conductivity type is p-type and the second conductivity type is n-type. 
     
     
         7 . The modulator of  claim 1 , wherein the rib has a height of at least 1.8 microns and less than 4 microns and a width of at least 0.5 microns and less than 1.5 microns. 
     
     
         8 . The modulator of  claim 1 , wherein the rib is composed of crystalline silicon or of crystalline silicon germanium. 
     
     
         9 . A method for fabricating a modulator on a semiconductor wafer, the method comprising:
 performing a first ion implantation operation on a rib, the rib extending upwards from an upper surface of the semiconductor wafer and having a first sidewall, and a second sidewall parallel to the first sidewall; and   performing a second ion implantation operation on the rib,   wherein:
 the implantation angle of the first ion implantation operation is greater than 45 degrees, 
 the implantation angle of the second ion implantation operation is greater than 45 degrees, and 
 the azimuth of the direction of the first ion implantation operation differs from the azimuth of the direction of the second ion implantation operation by between 150 and 210 degrees. 
   
     
     
         10 . The method of  claim 9 , further comprising:
 performing a third ion implantation operation on the rib, and   performing a fourth ion implantation operation on the rib, wherein:   the implantation angle of the third ion implantation operation is greater than 45 degrees,   the implantation angle of the fourth ion implantation operation is greater than 45 degrees, and   the azimuth of the direction of the first ion implantation operation differs from the azimuth of the direction of the second ion implantation operation by between 150 and 210 degrees.   
     
     
         11 . The method of  claim 10 , wherein:
 both the first and second ion implantation operations are performed before the third ion implantation operation and before the fourth ion implantation operation,   the first and second ion implantation operations implant dopants of a first conductivity type, and   the third and fourth ion implantation operations implant dopants of a second conductivity type, different from the first conductivity type.   
     
     
         12 . The method of  claim 11 , wherein the first conductivity type is p-type and the second conductivity type is n-type. 
     
     
         13 . The method of  claim 11 , further comprising forming a barrier, before performing the third ion implantation operation and before performing the fourth ion implantation operation,
 the barrier being configured to at least partially shade at least a lower portion of a sidewall of the rib from ions during the third ion implantation operation.   
     
     
         14 . The method of  claim 13 , wherein the fourth ion implantation operation is performed before the
 third ion implantation operation.   
     
     
         15 . The method of  claim 13 , wherein the barrier is a layer of photoresist, separated from the rib by a gap. 
     
     
         16 . The method of  claim 15 , wherein the thickness of the layer of photoresist is greater than 0.6 times the height of the rib, and less than 2.5 times the height of the rib. 
     
     
         17 . The method of  claim 15 , wherein the width of the gap is greater than 0.4 times the thickness of the layer of photoresist and less than 1.2 times the thickness of the layer of photoresist. 
     
     
         18 . The method of  claim 10 , further comprising performing a fifth ion implantation operation on the rib, wherein:
 the first, second, and fifth ion implantation operations implant dopants of a first conductivity type, and   the fifth ion implantation operation is performed at a lower implantation energy than the first ion implantation operation.