Researchers at the University of California, Davis (UC Davis) are developing a strategy to boost the light absorption of thin silicon films. The team demonstrated silicon-based photodetectors with light-trapping micro- and nano-surface structures, achieving performance gains that rival that of gallium arsenide (GaAs) and other group III-V semiconductors. Traditionally, silicon has been the most prevalent semiconductor in the electronics industry. Unfortunately, silicon has a relatively weak light absorption coefficient in the near-infrared (NIR) spectrum compared to those of other semiconductors like GaAs. Accordingly, GaAs and related alloys thrive in photonic applications, but are incompatible with the traditional CMOS processes used in the production of most electronics. This leads to a drastic increase in manufacturing costs. Photon-trapping micro- and nano-size holes in silicon make normally incident light bend by almost 90°, making it propagate laterally along the plane and leading consequently to increased light absorption in the NIR band. Courtesy of Wayesh Qarony et al., doi 10.1117/1.APN.2.5.056001. The photodetectors proposed by the researchers consist of a micrometer-thick cylindrical silicon (SI) slab placed over an insulating substrate, with metallic “fingers” extending from the contact metals atop the slab in an interdigitated fashion. Importantly, the bulk silicon is filled with circular holes arranged in a periodic pattern that act as photon-trapping sites. The overall structure of the device causes normally incident light to bend by almost 90º upon hitting the surface, making it travel laterally along the silicon plane. These laterally propagating modes increase the propagation length of light and effectively slow it down, leading to more light-matter interaction and a consequent increase in absorption. The researchers also conducted optical simulations and theoretical analyses to better understand the effects of the photon-trapping structures and performed several experiments comparing photodetectors with and without them. They found that the photon trapping led to a significant increase in the absorption efficiency over a wide band in the NIR spectrum, staying above 68% and peaking at 86%. Notably, the observed absorption coefficient of the photon-trapping photodetector was several times higher than that of plain Si and exceeded that of GaAs in the NIR band. Further, although the proposed design was for a 1-μm-thick silicon slab, simulations of 30- and 100-nm silicon thin films compatible with CMOS electronics showed a similarly enhanced performance. According to the researchers, the findings of this study demonstrate a promising strategy to boost the performance of silicon-based photodetectors for emerging photonics applications. By achieving high absorption even in ultrathin silicon layers, the parasitic capacitance of the circuit can be kept low, which is crucial in high-speed systems. Further, the proposed approach is compatible with modern CMOS manufacturing processes and can potentially revolutionize the way in which optoelectronics are integrated into conventional circuits. In turn, this could pave the way for affordable ultrafast computer networks and substantial leaps in imaging technology. The research was published in Advanced Photonics Nexus (www.doi.org/10.1117/1.APN.2.5.056001).
