High speed, low driving voltage vertical cavity germanium-silicon modulators for optical
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3.6 Selective Growth
In selective epitaxy, the substrate is covered by a dielectric masking layer everywhere except for the growth window. During selective epitaxy, growth occurs 57 only inside the growth window. In order to extend the selective growth technique to waveguide modulator applications, selective growth is studied in this work. Selective growth of bulk Ge on Si has been investigated for optical detector applications [89]. In our work, we extend the selective growth to a more complex structure: 10 pairs of 10nm compressively-strained Ge quantum wells with 18nm Si 0.15 Ge 0.85 barriers which are embedded in the intrinsic region of a vertical p-i-n Si 0.1 Ge 0.9 diode structure, for optical modulator applications. The entire p-i-n structure is grown at 395 o C on a pre-patterned (001) silicon substrate by reduced pressure chemical vapor deposition (RPCVD). Figure 3.18: (a) Schematic of selective growth. (b) SEM of actual growth Fig 3.18 (a) shows a schematic of the selective growth. The pre-patterned substrates have a 1.2µm thermal oxide, acting as a growth mask, with growth seeding windows of different widths. Boron and arsenic are incorporated in the p and n regions, respectively, through in-situ doping. Fig 3.18 (b) shows strong faceting within 1um in regions next to the growth window boundary. Planar and uniform quantum well structures are grown away from the faceting region. Si SiO 2 SiGe (a) (b) 58 Figure 3.19: Temperature dependence of the selective growth Fig 3.19 shows the selective growth for the same window size but with different growth temperatures. It can be seen that at different growth temperatures, not only is the growth rate changed, but the facet angle at the surface is different. At 380ºC, <211> (35º) facet growth dominates; at 400ºC, <111> and <311> facets can be observed; at 420ºC, <111> and <211> facets can be seen. There are several growth factors interacting with each other, leading to multi facet growth patterns. This is not favorable for future device fabrication, however, this problem can be solved by growing a thicker layer and using CMP for real device applications. Figure 3.20: SEM image of selective growth of SiGe on Si with different window opening sizes Fig 3.20 shows the selective growth of SiGe on Si with different window opening sizes. From the SEM picture, the grown thickness is independent of growth window size. That lack of dependence on growth window size leads to the conclusion that there is no loading effect in SiGe selective growth in this work. Also, measurement shows that there are no obvious discrepancies in facet angles with different window opening sizes under the same growth conditions. That reduces the complexity of future fabrication for different device lengths in waveguide modulator applications. One 10µm 5µm 2µm 1µm 59 interesting observation is the orientation of the quantum wells. It can be seen on the SEM picture that the quantum well was tilted at the edge of the oxide SiGe interface. |
