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

3.3.3 Growth Rate Design 
There are five growth parameters, including the germane flux, silane flux, carrier 
gas (H
2
) flux, chamber pressure, and growth temperature, which determine the SiGe 

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composition and growth rate. Some of these parameters affect both the composition 
and the growth rate. 
(a) 
(b) 
Figure 3.9: (a) Temperature dependence of Si and Ge growth. (b) Temperature dependence of Ge 
composition in SiGe layers 
Fig 3.9 (a) schematically shows the temperature dependence of Si and Ge growth. 
Changing the temperature not only alters the growth rate, but also changes the Ge 
composition. That’s because the Si and Ge precursors have different activation 
energies. According to Fig 3.3, in order to get a smooth surface for Ge rich SiGe 
layers, the growth temperature has to be lower than 450ºC. 
Previous work tried several growth temperatures from 350ºC to 450ºC[12]. It 
was found that when the germane and silane fluxes as well as the growth temperature 
are fixed, the growth rate of SiGe (with 5% Si content or more) and pure Ge are both 
inversely proportional to the carrier gas flow; however, the former is almost 
independent of the chamber pressure while the later is proportional to it. Moreover, for 

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45 
Ge-rich SiGe, the X
Si
/X
Ge
composition ratio is proportional to the silane - germane flux 
ratio and is independent of the temperatures. Also, the overall growth rate is 
proportional to the Ge composition at a given growth temperature. 
A series of SiGe samples was grown at different temperatures (300, 325, 350, 
375, 400, 425 and 450 º
C). The germane flux, hydrogen flux, and chamber pressure 
were 40 sccm, 30 slpm, and 40 Torr, respectively, and the silane flux was varied from 
10 to 70 sccm. By considering the needed growth rate at the desired Ge composition, 
375 º
C is chosen to be the growth temperature. 
(a) 
(b) 
Figure 3.10: (a) SiGe concentration ratio vs Si/Ge flux ratio. (b) Growth rate vs.Ge composition. 
Fig 3.10 shows the growth result under the above conditions. Fig 3.10 (a) clearly 
indicates that under these conditions, the Si/Ge concentration ratio is proportional to 
80
85
90
95
100
3.5
3.7
3.9
4.1
4.3
4.5
4.7
4.9
5.1
5.3
5.5
Li
n
ea
r 
gr
o
w
th
ra
te
Ge composition

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46 
the Si/Ge flux ratio. Fig 3.10 (b) shows that the growth rate of SiGe is linearly 
dependent on Ge composition.