Light Absorbing Diamondfor PETE Application

CVD diamond has demonstrated high thermionic emission yield at relatively low temperatures (<800°C). The wide band gap of diamond (5.47 eV), on the other hand, makes it blind to visible and infrared radiation. Aiming at employing CVD diamond in solar energy conversion, specifically in photon enhanced thermionic emission (PETE) devices, the light absorption in solar spectrum range should be enhanced and so, a defect engineering strategy has to be applied. A laser-induced periodic surface structure (LIPSS) has been fabricated on polycrystalline diamond by an ultrashort Ti:Sapphire pulsed laser source (?=800 nm, P=3 mJ, 100 fs) in a high vacuum chamber (<10-7 mbar) in order to increase diamond absorption in the visible and infrared wavelength ranges. Horizontally polarized laser beam has been focused on the diamond surface, then the sample has been moved by an automated X-Y translational stage along the two directions orthogonal to the beam. Scanning electron microscopy (SEM) of samples reveals a LIPSS with a ripple period of about 170 nm, shorter than the laser wavelength. Raman spectra of processed sample do not point out any evident sp2 content and diamond peak presents a upward shift, indicating a compressive stress. The investigation of optical properties of fs-laser surface textured diamond will be reported. Spectral photometry in the range 200 ? 2000 nm wavelength shows a significant increase of visible and infrared absorption (more than 80%) compared to untreated specimens (less than 40%). The analysis of optical characterization data highlights a close relationship between fabricated LIPSS and absorption properties, confirming the optical effectiveness of such a treatment as a light trapping structure for diamond in the investigated wavelength range. I-V measurements have been performed in the -100 V-100 V range, showing a current increase of about 6 order of magnitude, from pA to µA range. Spectral photoconductivity measurements in the 190-1200 nm range denote an increase of the photocurrent for the treated sample: in the infrared and visible ranges an enhancement in photocurrent has been noticed of more than a 3 factor for low voltages applied. Optical and electronic properties of femtosecond laser treated diamond will be further investigated and results will be presented. Such properties, reported for the first time, together with diamond high thermionic emission yield at relatively low temperatures, will open the path for new application of CVD diamond in solar energy conversion for photon enhanced thermionic emission (PETE) devices.