Rapid Synthesis of Nonepitaxial Multilayer Silicene and Germanene Assisted By Plasma/Ion Implantation

Abstract

Owing to the zero-bandgap of graphene, its application for channel material of switching devices is almost totally limited even if there are some methods for the bandgap opening of graphene, which is unstable or temporary. Thus the synthesis and applications of transition metal dichalcogenides (TMDs) have been widely studied since they possess bandgaps with a transformation from indirect to direct as the monolayer is reached. Besides mechanical exfoliation, the chemical vapor deposition (CVD) is the most mature technique for the growth of TMDs so far. However, the as-grown TMD films on SiO2/Si are non-continuous, and the transfer step seriously deteriorating the quality of TMD films is still inevitable since the SiO2 corroded by S or Se during the CVD process loses the insulating capability. Moreover, almost all of the processes for device fabrication need to be altered in the future, if the TMD materials would be commercialized in the semiconductor industry. Hence there are lots of issues that should be overcome for the real applications of TMDs. In fact, Si and Ge that belong to group IV in periodic table as well have been commercialized in the semiconductor industry for a long while. Hence the 2D Si and Ge that would be compatible with the present process and technology became the newly topics in the material science. According to theoretical calculation, the crystal structure of 2D Si and Ge belongs to one kind of hexagonal lattice named lonsdaleite structure. Thus the 2D Si and Ge, so called silicene and germanene, are actually the single-layered lonsdaleite Si and Ge. It is worth to mention that although silicene and germanene are also with zero-band gap like graphene, it is easier to open the band gap by applying a vertical electric field due to the non-equivalent atomic positions or adopting surface adsorption; meanwhile, the doping effect could be accomplished by introducing the strain. However, the vast challenge in synthesis of them is the naturally lack of sp2 orbital hybridization in Si and Ge owing to the larger ionic radii than that of C, so that they may not be directly obtained from bulk by mechanical exfoliation. Recently, computational studies concerning the physical properties of silicene and germanene were performed by many groups. Silicene and germanene have been grown on Ag, Au, Al, Ir, ZrB2/Si, and Pt substrates using ultra-high vacuum (UHV) epitaxy systems. At present, the first issue in formation of silicene and germanene might be the inevitable transfer process, which would deteriorate the quality of it, for device applications in the future, if the metals are used as the substrates during growth. Up to now, all of the studies only used STM combined with a diffraction analysis to demonstrate the formation of silicene and germanene. Hence multiple analyses should be necessary to acquire more information for further identification of materials. In our first work for synthesis of 2D materials, the multilayer graphene has been successfully synthesized by the plasma-assisted process introducing nitrogen ions into a 4H-SiC substrate to cause the preferential silicon nitridation and simultaneously condense carbon atoms onto the surface during thermal treatment. Then we expanded the process to synthesize multilayer germanene on a SiGe/Si substrate. Furthermore, we modified the plasma-assisted process by replacing the N2 plasma immersion with the B ion implantation for direct synthesis of multilayer silicene on a 4H-SiC substrate.