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

5.1.2 Large Signal Measurements 
Fig.5.2 shows the optical eye diagram of the modulator at 3.125GHz. The laser 
output power is 1mW, at a wavelength of 1408nm. The operating characteristics and 
wavelength response vary from device to device on the wafer, mostly due to 
non-uniformity of the device structure and materials introduced during thin-film 
deposition and device fabrication. The diode dark current is at a level of 10nA at low 
reverse bias. At -5V, the device shows soft breakdown and the dark current increases 
to ~1μA for the 100μm device, the dark current gradually decreases to about~800nA 
as the device size decreases to 100μm. However, the optical absorption current is 
always at ~60μA, from 100 μm to 10μm. With 1mW input power, the dark current is 
almost two orders of magnitude lower than the optical absorption current and thus can 
be neglected in the measurement. The modulation depth is roughly 12% and the period 
is 320ps. 

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70 
Figure 5.2: Optical eye diagram of the 40

m device at 3.125GHz (Measured in HP Labs, Palo Alto, 
CA) 
Fig 5.3 shows the 10GHz E-O response of 40μm device. It can be seen that the 
period is 100ps, and the modulation depth is lower than 2%. It can be seen that the 
noise level is very high, most likely due to n-type contact resistance problems in the 
fabrication. 10GHz E-O response test was performed on devices with different mesa 
sizes; it is observed from the readings of the DCA that the modulation amplitude has 
little variation from 10μm to 50μm and decays significantly when the mesa size is 
larger than 50μm.
Figure 5.3: High Speed Electrical-Optical Response at 10GHz for 40

m device (Measurement 
reference) 
5.1.3 Small Signal Measurements 
Accurate modeling of the equivalent circuit of the fabricated devices will help us 
to understand the physical limitations of the high-speed operation. By combining the 
modeling with real data we can reach conclusions that allow future improvement in 
material deposition and device fabrication.