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

2.3 Band Engineering 
Earlier work done in Stanford University related to QCSE on a SiGe platform has 
focused on finding the right band alignment of SiGe heterostructures. Most 
investigations can be divided into two categories based on quantum well alignment: (1) 
type-I system (2) type-II system. All of them were based on Si-rich alloys, which leads 
to weak indirect band absorption. 
Strained 
Si
1-x
Ge
x
Relaxed
Si
1-y
Ge
y
∆E
c
∆E
v
y
substrate
x
layer
E
c
E
v

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2.3.1 Type-I Aligned Quantum Well 
The type-I aligned quantum well has both the conduction band minimum and the 
valence band maximum in the same layer. Si-rich SiGe quantum well structures were 
grown on Si [35-37]. Their absorption is very weak due to two effects: (1) the 
absorption is based on indirect band transitions; (2) the electron confinement is very 
weak due to the small conduction band offset discussed in the previous section.
2.3.2 Type-II Aligned Quantum Well 
The type-II aligned quantum system has the conduction band minimum and the 
valence band maximum in different layers. Different structures, such as relaxed SiGe 
buffers [38] or higly strained Ge quantum dots on Si substrates [39] have been 
characterized. In this scenario, holes and electrons are not strongly confined.
Figure 2.11: QCSE in a type-II aligned quantum well. Both blue and red shifts occur in the transitions 
under an electric field. 
Fig 2.11 shows the transitions in a type-II aligned quantum well with and without 
an electric field. The electrons are confined in the Si-rich material and holes are 
confined in the Ge-rich material. As can be seen in Fig 2.11, electrons and holes are 
not confined in the same layer. When an electric field is applied, the transition energy 
on one side of the quantum well decreases and on the other side of the quantum well, 
the electron energy increases. That leads to large absorption shifts. However, since the 
coupling of carriers is weak, the absorption is very small and not practical for 
modulator applications. 
Ec
Ev
No E-field
E-field
Ec
Ev
No E-field
E-field

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