Vertical optical path structure for infrared photodetection

DETAILED DESCRIPTION The present invention SiGe optical path structure absorbs IR wavelength light that is normal to a silicon substrate surface and parallel to the SiGe/Si heterojunction interface, increasing the length of the optical path.
Therefore, a two-dimensional IR image detection can be realized with a thin SiGe film thickness.
Because of the relatively poor quantum efficiencies associated with SiGe, the IR absorption length of SiGe must be long, and conventionally a thick SiGe layer is needed to absorb high amounts of IR energy.
However, it is very difficult to grow defect-free thick SiGe film on Si substrate because of the lattice mismatch between these two materials.
The present invention eliminates the need for a thick SiGe film.
SiGe film is grown on the sidewall of a Si substrate trench or pillar, forming a relatively long optical path for light normal to the substrate surface.
The present invention's use of relatively thin SiGe films permits a SiGe IR photodetector to be easily integrated with Si CMOS devices.
As a result of the SiGe only being grown on the sidewalls, a better SiGe crystalline structure is obtained, improving the performance of the IR detector.
Accordingly, a method is provided for selectively forming a SiGe optical path normal structure for IR photodetection.
The method comprises: forming a Si substrate surface; forming a Si feature, normal with respect to the Si substrate surface, such as a trench, via, or pillar; and, selectively forming a SiGe optical path overlying the Si normal feature.
In some aspects, the Si substrate surface is formed in a first plane and the Si normal feature has walls (sidewalls), normal with respect to the Si substrate surface, and a surface in a second plane, parallel to the first plane.
Then, selectively forming a SiGe optical path overlying the Si normal feature includes forming a SiGe vertical optical path overlying the normal feature walls.
In some aspects, the Si substrate surface is associated with a silicon-on-insulator (SOI) material including the Si substrate, a buried oxide (BOX) layer overlying the Si substrate, and a top Si layer overlying the BOX