Femtosecond laser reconstruction of graphene field effect transistor

Possibility of femtosecond laser pulses to affect the materials properties arises the interest in ultrafast processes based research and technology. In the case of graphene surface modification and functionalization using femtosecond laser, there are several effects appear, such as ablation, covalent bonding of different chemical groups, re-crystallization in three-dimensional shapes. CVD grown graphene was transferred on Si/SiO2. Through several lithography steps, graphene-based field-effect transistors were formed with Cr/Au source-drain electrodes and Si back gate electrode. For graphene modification we used 100 fs 80 MHz laser with 780 nm wavelength with different irradiation doses. Exposure of graphene to a femtosecond laser pulse is determined by the prevalence of physical or chemical effects during exposure to a laser pulse. The range of laser exposure was narrowed down to values causing the formation of atomic defects in the carbon lattice, which makes it possible to form nanopores in graphene and these doses are below the graphene ablation. The main tool for studying the effect of femtosecond laser irradiation was Raman spectroscopy. By evaluating the intensity ratio of certain peaks, namely the G-band (similar to 1600 cm(-1)) and D-band (similar to 1350 cm(-1)), the degree of functionalization, or amorphization of graphene, was estimated. It was found that the ablation threshold starts from 18 mW at the beam speed in the range of 400-500 mu m/s. Just below this range, both graphene functionalization and a change in the graphene surface roughness were observed. Despite the change in the morphology of graphene, the graphene resistance fell by only similar to 4 times, and the transfer current-voltage curves of the graphene transistor did not change much, showing a shift towards higher voltages. With a decrease in the slope of the transfer current-voltage characteristics, the resistance of the structure also decreases with an increase in the dose of laser exposure, since the number of defects and functional groups in graphene increases. In addition, we found the effect of the laser polarization on the modification of graphene. The difference in parameters between the samples modified with different polarization directions along the direction of the beam motion can be explained as the interference interaction of the electron density in graphene. A beam passing over the graphene region excites hot electrons, which partially cause the graphene modification. After passing by the laser, the electron density does not have time to relax, and the next beam of photons affects the already excited electrons, increasing the total dose of laser radiation.