Silicon (Si) is one of the most popular semiconductors and is used extensively in the fields of energy, photoelectronic imaging, and remote sensing owing to its abundant reserves, low cost, and compatibility with standard complementary metal-oxide-semiconductor (CMOS) technology. To meet current needs, crystalline Si with high absorption coefficients across a broad range of wavelengths and sub-bandgap photon sensitivity is required, which can potentially fulfill the rising demand for higher photovoltaic conversion efficiency in solar cells, as well as elevated photoelectric conversion efficiency in photodetectors.

In order to realize this, promising methods such as pulsed laser irradiation and ion implantation have been applied to achieve hyperdoping of impurities far beyond the solubility limit of a semiconductor and enhance its spectral responsivity over a wide spectral range. Among these methods, black Si (b-Si) processed using ultrafast laser irradiation has emerged as a compelling all-silicon material owing to its microstructured and hyperdoped surface, leading to an antireflection effect and the formation of intermediate levels in Si. As a result, b-Si demonstrates exceptional optical and electronic properties, making it a promising material for applications in silicon photonics and silicon optoelectronics.

Based on the above background, researchers from Hefei University of Technology and Nankai University selected Ti as the doping element, and employed Density Functional Theory (DFT) and femtosecond laser irradiation to investigate the electronic and optical properties of Ti-hyperdoped Si. Ti-hyperdoped black silicon (b-Si:Ti) with high absorption and sub-bandgap sensitivity were developed, and the source of sub-bandgap absorption was analyzed. The relevant results were published in Chinese Optics Letters, Vol. 22, No. 11, 2024. "Song Huang, Anmin Wu, Guanting Song, Jiaxin Cao, Jianghong Yao, Qiang Wu, Weiqing Gao, Jingjun Xu, "Titanium hyperdoped black silicon prepared by femtosecond laser irradiation: first-principle calculations and experimental verification," Chin. Opt. Lett. 22, 113801 (2024)"

Fig. 1(a) The simulated results of Ti hyperdoped silicon, (b) The surface morphology and absorptance of femtosecond laser Ti-hyperdoped black silicon.

The simulated results indicated that interstitial Ti with low formation energy could introduce a broad intermediate band within the bandgap of Si, which probably resulted in the observed stable sub-bandgap absorption, as shown in Fig. 1(a). According to the simulated results, b-Si:Ti was experimentally fabricated through Ti film deposition followed by femtosecond laser irradiation. The fabricated b-Si:Ti samples exhibited broad spectral absorption ranging from visible to infrared wavelengths (400-2500 nm). The absorptance can exceed 90% for visible light and 60% for sub-bandgap wavelengths. Additionally, it also offered stable sub-bandgap absorption after undergoing an optimized rapid thermal annealing process to improve the lattice quality, indicating a stable deep-level impurity of Ti in silicon. Based on the advantages of high absorption over a broadband spectrum and good thermal stability, this enhancement demonstrates an effective hyperdoping of Ti in Si. The Raman spectra results showed that the lattice quality of b-Si:Ti could be effective repaired by a suitable rapid thermal annealing treatment. Based on the advantages of high absorption over a broadband spectrum and good thermal stability, this enhancement demonstrates an effective hyperdoping of Ti in Si.

The experimental findings align well with the simulated results, providing insight into the underlying physical mechanisms of Ti-hyperdoped Si and thus promoting the future application of b-Si:Ti in Si photonics.