Investigation of UV and IR Laser Processing of Single- Crystalline 4H:SiC and Characterisation of Laser Grown Graphene Derivatives

The formation of graphene (G) on the surface of silicon carbide (SiC) has gathered interest over recent years as a potential component in high power nano and microdevices. However, it is still in the early stages of research, therefore there are many challenges to overcome. Among the existing problems, the formation of good quality graphene/SiC is one of the most critical factors that determine the behaviour of this heterostructure. Here we report a full study of the formation of graphene and its derivative structures on SiC using different laser systems with different controlled irradiation conditions.Laser ablation experiments on polished 4H-SiC wafers using a 193 nm ArF laser over a fluence range of 0.3Jcm−2–5Jcm−2 are reported. An onset of material modification was measured at a laser fluence of 925 ± 80 mJcm−2, and a concomitant etch rate of ∼200 pm per pulse. Laser ablation sites have been analysed using optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman microscopy and white light interferometry (WLI). Different surface modifications were observed. The influences of the laser fluence, number of pulses, and scanning velocity on the position of the microchannel are discussed. At a laser fluence in the region of 1.0 Jcm−2, the irradiated site removed material forming a uniform crater. At a higher laser fluence, in the region of 2.7 Jcm−2, nodule-like structures form on the base of the ablation crater. An increased fluence led to a smoother surface with higher etched depth and ripple formation. The dissociation of laser irradiated 4H-SiC was discussed. Graphene oxide (GO) and reduced graphene oxide (rGO) formed on the SiC surface by 193 nm laser- induced high-temperature thermal decomposition of the SiC substrate. The decomposition resulted in the presence of silicon (Si), especially on the edge of the irradiated site.Graphene formation on the 4H:SiC surface by high power CO2 laser. Two distinct ablation threshold energies of 4.3 mJ and 73 mJ were found. The etch rate was dependent on the applied pulse duration, laser power, the scanning velocity and the number of pulses. High temperature thermal decomposition of the SiC substrate was achieved with a CO2 laser over a power range of 1-30 W. The structure was different from the structure obtained from the UV laser irradiated samples. More rough surfaces were prepared with small islands of graphene, GO and rGO on SiC in addition to the ripples. Monolayer and Multilayer graphene was also achieved. The laser-induced surface decomposition of the SiC was controlled spatially. The processing was held at room temperature, and the operation carried out in either a vacuum chamber or at atmospheric pressure. A fast graphene growth rate was achieved. This method is achievable, scalable and compatible with semiconductors technology due to the onsite direct writing of graphitic structure formed by the laser. This method is cost-effective as it does not necessitate SiC pre- treatment, there is no need for a processing vacuum chamber, and it can be achieved on the nano/microsecond time scale.Analytical and Finite element simulations using COMSOLTM MetaphySiCs, 5.3 have been used to calculate laser-induced temperature rise of 4H-SiC as a function of laser fluence. The simulated temperature was always less the temperature anticipated analytically. The 193 nm laser fluence required to reach the melting points of silicon, silicon carbide, and carbon, have been calculated and correspond to ∼0.97, 1.95 and 2.6 Jcm−2, respectively. Extreme heating and cooling rates controlled the growing process of graphene and its derivatives. The CO2 laser-induced temperature rise was also estimated. The CO2 laser acted as a heat source for the SiC. High power was used to reach the high temperature needed to decompose the SiC. Pulse duration also played a significant role in controlling the temperature and the depth distribution inside the SiC.This work reports the graphene formation on the surface of SiC by laser-induced thermal decomposition for electrical characterisation. Current-voltage (I-V) measurements show a decrease of the electrical resistance per unit length by nine orders of magnitude. The lowest resistance per unit length was obtained using a laser fluence of ~1.5 Jcm-2, a pulse repetition frequency of 10 Hz and using a sample translation speed of 0.01 mms-1. Temperature simulations have been performed using the finite element method (FEM) to assist in understanding the dissociation mechanisms of SiC and hence optimise the experimental variables. 2D axis-symmetric FEM calculations predict a surface temperature of ~2500K at a laser fluence of 1.5 Jcm-2. Laser-irradiated 4H:SiC is an efficient and controllable method of producing highly reproducible electrically conducting tracks. An increase in the conductivity was observed when the graphitic structure was produced with the CO2 laser. However, the conductivity was less than the graphitic structure produced by the 193 nm laser. It is expected that the different graphene interfaces, including Ohmic contact and Schottky contact, was created