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

4.4 Summary 
As discussed above, a p-i-n diode is fabricated on a SiGe quantum well structure 
and QCSE with high contrast ratio and clear exciton signal were observed. High-speed 
devices were fabricated based on the same processes which are CMOS compatible. 
High-speed optical measurement results will be discussed in the next chapter. 

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Chapter 5 High Speed 
Characterization 
5.1 High Speed E-O Response 
5.1.1 Measurement Setup 
To demonstrate high-speed modulation of the device, the setup up in Fig. 5.1 was 
used to measure the electro-optical response. All measurements were performed on 
wafer using a Cascade MicroTech Air Coplanar Ground-Signal-Ground (GSG) Wafer 
Probe and a Cascade Summit Probe station. The RF driving voltage was generated by 
a pulse generator, and it was sent to the device by the transmission line contact pattern 
fabricated on the device. Also, a reverse bias of about 3V was applied by the voltage 
source to keep the device within its working regime. A tunable laser with an operating 
range from 1260-1680nm was used. Light was coupled into the device through a 
single mode fiber with a lensed tip. The transmitted light was collected by an objective 
lens and coupled into a multimode fiber. The light was detected and processed by a 
digital communication analyzer (DCA). 
Broadband Tunable Laser
Single mode Fiber
Pulse Generator
DC Bias source
GSG probe station
Lense Fiber
Digital Communication analyzer
Multimode Fiber

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Figure 5.1: High speed measurement setup 
The AC response of different sized devices, ranging from 100μm to 6μm, was 
measured. The incident laser power was about 1mW. A high-frequency RF signal 
was sent onto the device through RF cables that operate up to 26GHz. The DCA 
sampling rate can reach 13GHz. We used two function generators, one of which can 
generate square wave pulses up to 3.5GHz; the second can generate sine waves up to 
20GHz.