High speed, low driving voltage vertical cavity germanium-silicon modulators for optical

6.2.3 Waveguide Device 
One of the natural evolutions of modulator device design is a waveguide structure. 
It has several advantages: (1) Small dimension and potentially high speed (2) Easy to 
integrate (3) Easy to control the absorption length and get high contrast ratio There 
are several designs for low operating voltage, high-speed waveguide modulators. 
Currently we are working on selective growth based waveguide structures [93]. 
Figure 6.1: Ge quantum well waveguide modulator with SOI bus waveguide 
The schematic of a newly proposed device is illustrated in Fig 6.1. The input and 
output passive waveguides are single mode SOI waveguides, 500nm wide and 300nm 
high. Between the input and output SOI waveguides is the active modulator section. In 
this region, the top Si layer and the buried oxide layer (BOX) of the SOI substrate are 
removed completely. Si is grown selectively from the bottom handle substrate of the 
SOI wafer to fill the BOX region and act as the bottom cladding for the waveguide 
modulator. The active waveguide modulator core in our design consists of 10 pairs of 
Ge/SiGe quantum wells sandwiched in the intrinsic region of a vertical p-i-n structure. 
Currently the optical properties of the structure have been simulated, and the 
material growth has been calibrated as well. Further fabrication and measurement 
efforts need to be carried out. 
6.2.4 Group IV Laser Integration 
An off-chip laser is the solution for today’s optical interconnect. However, this 
will eventually be a cost barrier and performance limitation for optical interconnect 

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technology development. Searching for the solution of efficient light emission from 
group IV based materials is still one of the utmost important missions. Among the 
multiple possible solutions, we believe that Ge-rich, tensily strained GeSn alloy is the 
most promising path to pursue based on theoretical calculations and experimental 
results [100-104]. Most of the challenges in GeSn alloy material are the low solubility 
of Sn in Ge and thermodynamic instability. The lattice mismatch between Sn and Ge 
is even larger than that of Ge and Si. Our effort is to grow a Ge-rich GeSn alloy on a 
Si substrate with low dislocation density relaxed structure. If we can make that type of 
structure direct bandgap, it will truly open the door to group-IV based integrated 
optical interconnects. 

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