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

1.3.1.3 Electroabsorption Effect 
The absorption coefficient in a material can be changed when the electric field 
applied across it is changed. This is called the electroabsorption effect, which includes 
the Franz-Keldysh effect [30-31] and the quantum-confined Stark effect (QCSE) 
[32-33]. The QCSE is widely used in high-speed optical modulation applications 
today [34-35]. Much work has been done in III-V based materials before the first 
QCSE in SiGe was observed in 2005 [36-41]. 
1.3.2 Previous work on high-speed Si based modulator 
A Mach-Zehnder interferometer using the electro-optic effect was developed by 
Intel in 2005. Recently, Intel has demonstrated high-speed performance with a 10Gbps 

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eye diagram and small signal modulation with a 30GHz 3dB bandwidth [42]. The 
contrast ratio is higher compared to prior devices, and the transverse size is small, 
leading to even higher-speed possibilities. These devices are also completely CMOS 
compatible. The major issue for these devices is that they are too long (~11mm), 
leading to high power consumption, and it is very hard to bring a high-speed RF signal 
onto the device. Device design and fabrication are thus very complicated. 
The ring structure [29] based on SOI has a smaller size at roughly ~12 μm in 
diameter. This is three orders of magnitude smaller than the MZ modulator. A ring 
resonator modulator operating at 1.5GHz has been demonstrated with high contrast 
ratio. The disadvantages of the structure are the large power consumption, high 
voltage requirement, narrow bandwidth and thermal sensitivity. Because the 
modulator needs a high quality-factor (Q ~ tens of thousands) resonator, the result is a 
very narrow optical bandwidth and severe thermal instability.
A key question is therefore whether germanium could produce strong, 
high-speed modulation using the QCSE. Germanium, like silicon, is an indirect gap 
semiconductor, but unlike silicon, it has a useful direct band gap only slightly higher 
in energy than the indirect band gap. However, modest QCSE was been observed in 
indirect gap AlGaAs [37-38].
Previous efforts to find mechanisms such as the QCSE in Si/Ge structures were 
largely unsuccessful. SiGe/Si quantum wells with type-I alignment (electron and hole 
minima in the same material layer) show either no QCSE [39] or relatively inefficient 
effects [40-41]. Strained SiGe/Si quantum wells on relaxed SiGe buffers and Ge/Si 
quantum dots on Si substrates, with type-II band alignment (electron and hole energy 
minima in different layers), can exhibit large shifts of optical transitions with electric 
field, but have relatively low absorption associated with the shifted transitions. 
Another approach to SiGe waveguide modulators is based upon the Franz-Keldysh 
effect [44]. Waveguide structures using the Franz-Keldysh effect provide an easy 
means to control the extinction ratio and because the device has small dimension, it 
has low power consumption. Using SiGe material, the device is compatible with 

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CMOS processes. This device has so far achieved a limited small signal 3dD 
bandwidth of 1.2GHz without large signal measurements being done. A relative 
serious limitation is that a rather high bias voltage (>10V) was needed to achieve the 
required band edge shift.