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

4.2 Modulator DC Operation 
The cross section schematic of the DC PIN diode is shown in Fig. 4.2. It has a 400 nm 
relaxed boron-doped Si
0.1
Ge
0.9
p-type buffer grown on silicon, an intrinsic region 
containing 10 pairs of strained quantum wells (including 10 nm Ge well and 16 nm 

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62 
Si
0.15
Ge
0.85
barrier) two 100 nm Si
0.1
Ge
0.9
spacers, and a 200 nm arsenic-doped 
Si
0.1
Ge
0.9
n-type cap layer.
Figure 4.2: Schematic of the side view of PIN diode for DC measurements 
The photo-absorption current is measured at room temperature and shown in Fig 
4.3 below. The spectra show clear exciton absorption peaks at room temperature. In 
bulk Ge, an exciton absorption peak can be seen at low temperature [90], but such 
peaks are not usually clearly resolvable at room temperature. The appearance of room 
temperature peaks is characteristic of the quantum wells, and is explained by the 
increased carrier confinement which maintains the excitons. At zero applied voltage, 
there is also a clear shift of the direct optical absorption edge from its value in bulk, 
unstrained Ge (~0.8 eV at room temperature) to the lowest energy exciton peak 
position of ~0.88 eV. This shift can be explained as a combination of strain and 
quantum confinement. The peaks and the shift show empirically that, despite the 
lower-energy indirect conduction bands in the wells, there is strong quantum 
confinement at the zone center in the Ge conduction and valence bands. When an 
electric field is applied, there is a clear QCSE shift of the absorption edge to lower 
photon energies. Note also that the exciton peak width, 
∼8 meV half- width at 
half-maximum, has little or no apparent change with applied bias. 
Since the Ge wells are under compressive strain, the heavy hole band becomes the 
topmost valence band, and the band-edge absorption peak is related to the heavy-hole 
exciton. The high responsivity without any bias voltage indicates that the i-region of 
this p-i-n device is highly intrinsic with a low background doping level, and hence the 
built-in field depletes the whole i-region and sweeps all photo-generated carriers to the 

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63 
heavily doped regions where they are collected. This is also advantageous when these 
electroabsorption modulators are used as photodetectors. 
Figure 4.3: Optical absorption current measurement (DC) device size: 100×100μm 
The current under 0 V bias is low, because there is more leakage current under 
reverse bias. Also the current at 5V reverse bias is higher than that at 4V for the same 
reason. That means that the material quality is not optimized yet. Due to the dark 
current issue, we can see the best operating range is below 5V. 
In future applications, high contrast under a low voltage swing is required for 
the modulators. In this generation of devices, contrast ratio of almost 2 is achieved at 
1410nm as the reverse bias changes by 3 volts. Also the n doping level and doping 
activation are relatively low and the n contact is not as low resistance as desired. In 
order to improve the contrast ratio for future applications, we use the following 
techniques: (1) longer time purge of reacting gases after the p doping growth and 
before the intrinsic region growth; (2) eliminate the intrinsic spacer and grow more 
pairs of quantum wells; (3) increase the doping level of n region to form better contact; 
and (4) use 15 seconds anneal with RTA at 500°C after the growth to activate more 
n dopants. 
Fig 4.4 shows the optical absorption spectra after the growth and processing 
improvements. It can be seen that on the left side of the figure, after the short anneal, 

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64 
the material quality and the contacts have improved. We can see a clear trend of the 
exciton peaks becoming smaller and wider as the bias voltages increases due to 
reduced spatial overlap of the electron and hole states at higher electric fields. This is 
reflected in the decrease in absorption current as voltage increases. Most important, a 
contrast ratio of 2.4 is achieved with only a 1 V swing. This will be very useful for 
future applications. 
Figure 4.4: Optical absorption current measurement after the material and processing improvements