Three-Dimensional Grids of Optimized Ti-Compounds on Si for Ultra-Wideband Optical Absorption

Typically, the optical applications of silicon (Si) are limited to wavelengths below ∼1100 nm. However, there is significant research on Si surface modification, which tries to extend the optical properties of Si further into the infrared (IR) region. In this work, we present an ultra-wideband complementary metal–oxide–semiconductor (CMOS)-biocompatible Si-based optical absorber with a hydrophobic surface. It consists of patterned three-dimensional grid-like structures of optimized compounds of titanium (Ti) on n-type Si (n-Si). Here, the Ti-compounds on Si were formed by subsequent deposition of patterned Ti and annealing. Moreover, we have shown that there are two possible Ti-compounds formed on Si, depending on the thickness of Ti deposited and the annealing time. The composition and the corresponding absorbance spectra for the two possibilities of Ti-compounds on n-Si, that is, Ti–O/Ti–O–Si/Ti–Si/n-Si (type 1) and Ti–O/Ti–O–Si/n-Si (type 2), were confirmed using an X-ray photoelectron spectroscopy depth profiler and ultraviolet–visible–near-infrared spectrometer. We also illustrate how type 1 improves the absorption of radiation in the IR region. Further, we experimentally demonstrate that our fabricated absorber has an average reflectance (R) of <25% and an average absorbance of approximately 60% for wavelengths ranging from 200 to 3300 nm. The average % R for wavelengths from 400 to 1500 nm is <10%. The surface hydrophobicity for the fabricated absorbers was confirmed using a water contact angle (WCA) measurement system with WCAs >100°, which makes the surface hydrophobic.