On December 7, Intel published world record results for a silicon-based photodetector in
Nature Photonics, and we wanted to explain the results in a little more detail in this blog. Before launching into the “in’s and out’s” of avalanche photodetectors (APDs) though, it is important to provide some context for the work.
Several companies are active in the field of silicon photonics because they believe that silicon has an advantage in making the very low cost optical parts needed for large markets. These potentially include a very diverse set of applications including supercomputing, data center communications, consumer electronics, automotive sensors, and medical diagnostics just to name a few. Up until now, only a few silicon photonics products have been commercialized as companies have been working through the latter stages of the development and qualification processes. This looks set to change over the next few (≤3) years as several devices exit this pipeline and go on the market.
Intel has been doing research in this area for more than 5 years and has already reported on silicon modulators, silicon Raman lasers, and hybrid InP-Si lasers (
view website). Last year we also published on a photodetector made from germanium and silicon that had a bandwidth of 31 GHz. The use of Ge is important because, unlike Si, it can efficiently detect light in the near infra-red which is the standard for communications. The drawback is that so much stress is developed in pure Ge films deposited on Si that defects are introduced near the Ge/Si interface. Careful design and processing is needed to minimize the impact of these defects on the electrical performance of the device, and this will be mentioned later.
We are now reporting on a different type of Ge/Si photodetector that has built-in amplification, which makes it much more useful in instances where very little light falls on the detector. It is called an
avalanche photodetector because an avalanche process occurs inside the device. First, a negative and a positive charge (electrons and holes in semiconductor terminology) are created when the light strikes the detector. The electron is accelerated by an electric field until it attains a high enough energy to slam into a silicon atom and create another pair of positive and negative charges. Each time this happens the number of total electrons doubles, until this “avalanche” of charges are collected by the detection electronics. Click on the image below to see how this works. This amplification effect (called gain) is the key to the device, and it serves as the motivation for why anyone would try to do this in silicon and not just continue to use traditional InP-based APDs. The materials properties of silicon inherently led to lower noise and better performance in this avalanche process. Another reason relates to this bit economic trivia; an individual 10 Gb/s InP APD can sell for more than $200 currently and has a semiconductor area of roughly 400x400 m2. Even the much cheaper 1-2 Gb/s APDs used in fiber to the home (FTTH) still sell for $3-5.
