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

3.4 SiGe Buffer Layer Growth 
3.4.1 Multiple SiGe Buffer Growth Approach 
According to the previous discussion, it is desirable to grow SiGe/Ge quantum wells 
on a Ge-rich buffer to control the strain and its influence on material quality and band 
alignment. Fig 3.13 shows the prior methods used for SiGe buffer growth on Si: 
graded buffer method (REF), direct buffer method with single-growth-temperature 
(REF) and direct buffer method with two-growth-temperatures (REF). For the graded 
buffer method, Ge composition of SiGe layers increases continuously from zero at the 
Si substrate to the desired Ge composition. For the direct buffer method, films with a 
single SiGe composition or pure Ge are deposited on top of silicon substrates at low 
0
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Depth (µm)
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B

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50 
temperature. After growth, the wafers are annealed at higher temperatures. The 
procedure can be iterated several times. The two-temperature growth technique first 
deposits a thin layer of low temperature SiGe, anneals at high temperature and then 
grows thicker layers of SiGe at high temperature.
(a) (b) (c) 
Figure 3.13: Buffer growth methods: (a) graded buffer; (b) direct buffer with two cycles of low T 
growth and high T anneal; (c) direct buffer with two growth-temperatures. 
Table 3.2: Comparison of Ge-on-Si growth methods. [86-91] 
In the Ge-rich SiGe buffer growth, most studies were targeted toward the pure 
Ge-end for laser, photodetector, and electronics applications based on Ge-on-Si or