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

3.3.5 Doping Control 
Four different diluted dopant sources are available, including 1% B
2
H
6 
(for high 
p-doping), 100 ppm B
2
H
6 
(for low p-doping)
, 
1% PH
3 
(for high n-doping), and 100 
ppm AsH
3 
(for low n-doping). Their growth mechanisms are similar to those of Si and 
Ge hydride. High-level PH
3
or B
2
H
6
doping for SiGe growth can change the growth 
rate, but its influence on SiGe composition is less than 1% [12].
In this work, the buffer layer and the cap layer are doped. The intrinsic region 
needs to have very low doping. The sharpness of the doping profile and the 
consistency of SiGe composition are very important to device operation. Therefore 
low-level 100 ppm, 100 ppm B
2
H
6
and 100 ppm AsH
3 
are used. 
Fig 3.12 shows the measured doping profile in a SiGe p-i-n diode grown on a Si 
substrate. The total thickness is 2.8 μm, with 0.4μm of P region, 0.4μm of n region and 
2μm of intrinsic region. The average composition of the entire structure is Si
0.1
Ge
0.9
. 
The red line is the Ge composition, and the green line is the Si composition. The 
yellow line is the B doping profile. The P region doping reaches 4e18 cm
-3
, which falls 
into our doping expectations. In the i-region and n-region, since the B concentration is 
much lower than 1e16cm
-3
, the lower limit of the SIMS characterization, the 
measurement result is noisy. That’s mostly due to the auto doping in the chamber. 
When the gas sources are switched, there are still residues of previous gases in the 

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49 
chamber that will affect doping concentration. This is also true for the As 
concentration measurement in the p region and i regions. In the n region, the As 
concentration is lower than 1e18 cm
-3
. This is lower than we expected. And the doping 
consistency over the depth is not very stable.
Figure 3.12: Doping profile: SIMS measurement of a SiGe p-i-n test diode. Red line is the Ge 
composition, green line is the Si composition. Blue line is the As doping level and yellow line is the B 
doping level.