The photosensitivity of silicon is inherently very low in the visible electromagnetic spectrum, and it drops rapidly beyond 800 nm in near-infrared wavelengths. We have experimentally demonstrated a technique utilizing photon-trapping surface structures to show a prodigious improvement of photoabsorption in 1-μm-thin silicon, surpassing the inherent absorption efficiency of gallium arsenide for a broad spectrum. The photon-trapping structures allow the bending of normally incident light by almost 90 deg to transform into laterally propagating modes along the silicon plane. Consequently, the propagation length of light increases, contributing to more than one order of magnitude improvement in absorption efficiency in photodetectors. This high-absorption phenomenon is explained by finite-difference time-domain analysis, where we show an enhanced photon density of states while substantially reducing the optical group velocity of light compared to silicon without photon-trapping structures, leading to significantly enhanced light–matter interactions. Our simulations also predict an enhanced absorption efficiency of photodetectors designed using 30- and 100-nm silicon thin films that are compatible with CMOS electronics. Despite a very thin absorption layer, such photon-trapping structures can enable high-efficiency and high-speed photodetectors needed in ultrafast computer networks, data communication, and imaging systems, with the potential to revolutionize on-chip logic and optoelectronic integration.

In this paper, high-speed surface-illuminated Ge-on-Si pin photodiodes with improved efficiency are demonstrated. With photon-trapping microhole features, the external quantum efficiency (EQE) of the Ge-on-Si pin diode is >80% at 1300 nm and 73% at 1550 nm with an intrinsic Ge layer of only 2 μm thickness, showing much improvement compared to one without microholes. More than threefold EQE improvement is also observed at longer wavelengths beyond 1550 nm. These results make the microhole-enabled Ge-on-Si photodiodes promising to cover both the existing C and L bands, as well as a new data transmission window (1620–1700 nm), which can be used to enhance the capacity of conventional standard single-mode fiber cables. These photodiodes have potential for many applications, such as inter-/intra-datacenters, passive optical networks, metro and long-haul dense wavelength division multiplexing systems, eye-safe lidar systems, and quantum communications. The CMOS and BiCMOS monolithic integration compatibility of this work is also attractive for Ge CMOS, near-infrared sensing, and communication integration.