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

6.2 Future Work 
The demonstration of high-speed optical modulation of SiGe based devices is just the 
beginning of research in the area of Si-based photonics for optical interconnects. This 

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opens the doors to novel design and fabrication of high-speed optical modulators that 
can be practical for system integration in on-chip optical interconnections. More work 
on device structures and relevant materials growth and device fabrication needs to be 
done. 
6.2.1 Material Growth Improvements 
As many problems emerged from our measurement results, a number of them came 
from the materials: (1) n type doping is ineffective. There is a need to have higher 
doping levels with a high activation rate to engineer the Fermi level. Better doping 
control with arsenic and phosphorous needs to be achieved. (2) High quality thin 
buffer layers. Practical applications in optical communications require single optical 
mode waveguide and cavity structures. The former needs thinner buffer layers for 
stronger coupling and the latter needs higher quality material with fewer dislocations 
to reduce absorption. Moreover, high quality quantum well material also improves 
the I-V characteristics and reduces signal to noise ratio (3) Better control over 
selective area growth.  
6.2.2 Cavity Modulator 
The Asymmetric Fabry-Perot (AFP) modulator cavity allows increased interaction 
between light and active material in a smaller form factor. After the Ge/SiGe QW 
QCSE electroabsorption, α, is experimentally determined for each sample, a range of 
acceptable front and back mirror reflectances, R
f
and R
b
, can be determined from the 
AFP design equation [98]. 
Currently, the DC characteristics of the fabricated device showed clear and high 
contrast with low voltage swing. The next step is to improve the monolithic device 
fabrication process and material deposition to match the cavity parameters. 

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