Femtosecond Laser Nanostructuring of Semiconductors and Metals

We have developed a unique technique to change the optoelectronic properties of many materials through hyperdoping and texturing [1]. By irradiating materials, such as silicon and titanium dioxide, with a train of amplified femtosecond (fs) laser pulses in the presence of a wide variety of dopant precursors, we can introduce dopants above the solubility limit while producing surface structures that have excellent anti-reflection and light-trapping properties.

Femtosecond-laser texturing originates from the formation of laser induced period surface structures (LIPSS) and consists of semi- periodic nanometer- and micrometer-scaled structures. Recently, we identified laser parameters for independently tuning the hyperdoping and texturing processes.

Using femtosecond-laser hyperdoping techniques, we endow silicon with remarkable optoelectronic properties, such as near unity absorption and a strong sensitivity to infrared wavelengths [2]. Laser hyperdoping creates defect- and bandgap engineered semiconductors, and laser texturing further enhances the optical density through light trapping. Hyperdoped silicon devices represent the fruit of a novel fabrication technique for Earth-abundant, semiconductor- based solar energy harvesters with the potential for both low-cost and high-photoconversion efficiency.

We recently extended the femtosecond-laser hyperdoping technique to titanium dioxide (TiO2), created by the femtosecond-laser irradiation of titanium metal in the presence of oxygen and other dopants. TiO2 is a wide bandgap semiconductor (Egap = 3.0 eV) that can be used as the photoanode in photoelectrochemical cells for hydrogen production from water. In order to extend TiO2’s optical absorption range to include visible light (and thus increase its photoconversion efficiency limit under the solar spectrum), we fabricated hyperdoped TiO2 using femtosecond -laser processing. Preliminary results show that after femtosecond-laser irradiation, the titanium-based substrates exhibited enhanced surface area and incorporation of nitrogen [3] and chromium dopants.

In conclusion, we use femtosecond -laser pulses to fabricate hyperdoped and textured semiconductors and metals, transforming their optoelectronic properties and improving their energy harvesting efficiency.

[1] M.J. Sher, M.T. Winkler, and E. Mazur, Pulsed-laser hyperdoping and surface texturing for photovoltaics. MRS Bulletin, vol. 36 (6), pp. 439-445, (2011)
[2] C.H. Crouch, J.E. Carey, M. Shen, E. Mazur, and F.Y. Genin, Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation, Applied Physics A-Materials Science & Processing, vol. 79 (7), pp. 1635-1641, (2004).
[3] E.C. Landis, K.C. Phillips, E. Mazur, and C. Friend, Formation of nanostructured TiO2 by femtosecond laser irradiation of titanium. Publication submitted.