Pulsed-laser induced graphitization of single crystal silicon carbide

Abstract

In this thesis, graphitization mechanism of single crystal silicon carbide (SiC) is investigated by pulsed laser irradiation, which is widely used in GaN lift-off and low temperature polycrystalline silicon (LTPS). Such a combination of laser annealing and SiC material makes graphene, a monolayer of carbon atoms arranged to form a two-dimensional honeycomb lattice, on insulating substrate without needs for transfer process. In addition, high fluence of laser can supply enough thermal energy to synthesize graphene within nanosecond timescales. Through systematical experiments and analysis, new growth mechanisms are discovered that each laser pulse leads to melting of SiC surface. In the course of this thesis, two different mechanisms are identified with phase separation of carbon and silicon layer and amorphous phase transition via quenching.

Systematical experiments and analysis were performed with a variety of laser energy densities and pulses to explore growth mechanism of pulsed-laser induced graphene on SiC surface. Time-resolved transient reflectivity (TR) measurements were employed to directly observe and understand melting of SiC surface during laser irradiation. The first irradiation leads to separation of graphitic carbon and poly-crystal silicon layer on top surface. Also, thin 3C-SiC is formed at the same time between separated layer and 4H-SiC substrate. Based on the melting signal of TR, we propose that new surface is developed to decrease surface energy. Graphite presents upper than silicon layer because it has the lowest surface energy from binary compound of carbon and silicon. Experimental result of the second laser pulse shows a vanishment of silicon layer and results in 2-3 layer graphene. This is definitely different growth mechanism compared to epitaxial graphene by thermal decomposition of SiC. Classical mechanism of graphitization is known as a sublimation of Si atoms due to its high vapor pressure. Therefore, melting of SiC cannot be occurred in equilibrium condition. Additional laser pulses make an incremental increase of graphene layer by supplying carbon sources from 3C-SiC layer. Therefore, this work demonstrates very easy and good controllability of graphene layers by increasing laser pulses.

Another experimental conditions show definitely different growth mechanism via metastable amorphous SiC. Low energy of laser beam leads to amorphization of 4H-SiC surface. By increasing of laser pulses, sublimation of Si atoms from amorphous SiC layer was observed by HRTEM, EELS, and EDS mapping analysis. Interestingly, interface layer of 3C-SiC was also observed likewise experimental results with high energy density. Carbon nucleation was occurred on 3C-SiC and characterized by Raman analysis. Numerical simulation was carried out to understand temperature history with three different condition of SiC surface. After the formation of amorphous layer, surface can be melting with even low energy density due to low thermal conductivity of amorphous material. This work shows feasibility of new growth method using amorphous SiC thin film-laser interaction.

Finally, for application in tuning the material properties by using pulsed laser annealing, synthesis of solid-phase doped graphene on SiC surface was studied with highly nitrogen doped substrate. This method provides the direct growth of doped graphene on an insulating substrate without an additional dopant supply. The XPS analysis confirms that the C-N bonding conformation of the N-doped graphene was pyridinic-N type. Systematic analysis of the G band shift in the Raman spectra suggests that solid-phase doping can provide precise controllability of doping concentration by simply changing the dopant concentration of SiC. This work is expected to provide a solid-phase doping strategy with excellent controllability which is primarily used in advanced Si CMOS technology.