Research Express@NCKU - Articles Digest

Nanoparticles have been widely studied in the fabrication and

characterization because of their specific properties for quantum confinement effects and the promising application. The silicon carbide (SiC) material of wide band gap has attracted much attention due to its broad potential applications in radiation sensors, high-power electronics and optoelectronic devices. Also, the SiC nanoparticles (np-SiC) or films have been fabricated in a variety of technologies for the formation of SiC particles or films, including nanopowder synthesis of CO2 laser pyrolysis, chemical vapor deposition (CVD) nanopowder synthesis or physical vapor deposition (PVD). The wafer-level deposition, instead of nanopowder synthesis, will be important for the future application of devices together with integrated circuit. In this paper, a novel approach to SiC nanoparticles formation is proposed by vacuum thermal annealing of the Si/C/Si multilayer deposited on the Si (100) wafer using ultra high vacuum ion beam sputtering technology. The annealing temperature significantly affected the size, density, and distribution of np-SiC. The higher the annealing temperature was, the larger the nanoparticle size, and the higher the density. The higher superheating at 900 ºC increased the amount of stable nuclei, and resulted in a higher particle density compared to that at 700 ºC. Thermal energy is needed for the interdiffusion between Si and C layers for the formation of SiC phase to reduce the interfacial energy. The higher temperature gives rise to the higher mobility or rapid diffusion of the atoms together with the larger driving force of superheating for the formation of SiC. The density of large submicron particles with uniform distribution is about 1.37x10⁸ cm^-2 at an annealing temperature of 900 ºC.

Figure 1 FESEM micrographs of np-SiC formed on the three-layer Si/C/Si composite films: annealed at (a) 500 °C, (b) 700 °C and (c) 900 °C for 1.0 hour, respectively.

Figs. 1(a)-(c) show the SEM micrographs of the three-layer Si/C/Si multilayers annealed at 500 °C, 700 °C and 900 °C for 1.0 hour, respectively. There are no particles shown up at 500 ºC (Fig. 1(a)) and few particles are present at 700 ºC (Fig. 1(b)). Many SiC nanoparticles (np-SiC) appear on the surface of Si/C/Si multilayers annealed at 900 ºC (Fig. 1(c)). The higher the annealing temperature was, the larger the nanoparticle size became, and the higher the density was. Due to the high surface energy of SiC film with strong Si-C bonding, SiC nanoparticles, instead of films, were preferred to form on the surface in order to reduce the surface energy of film during post annealing. More thermal energy or higher atomic mobility at high temperature of 900 ºC enhances the particle growth to be a larger size.

Figure 2 (a) A magnified HR-FESEM micrograph of np-SiC with small particles surrounding a large one on the surface of Si/C/Si multilayers annealed at 900 °C for 1.0 hour; (b) an Energy Dispersive X-ray (EDX) spectrum for the measurement of Si/C composition ration in np-SiC area. The atomic composition ratio of Si/C is about 53/47.

Small C atoms with radius of 0.077 nm diffuse into the Si layer along grain boundaries much faster than large Si atoms with radius of 0.117 nm which diffuse out to the C layer to result in formation of a new crystalline SiC phase at 900 °C. Figs 2(a) and (b) show the magnified HR-FESEM micrograph and energy dispersive X-ray (EDX) spectrum of a SiC particle formed at 900 °C annealing for one hour, respectively. The formation of paricles during growth is tended to be the shape of layer-by-layer structures (Fig. 2(a)), instead of spherical one. It implies the defects of stacking faults or disorder occurring at the stage of SiC particle formation with the mixed stacking sequence of hexagonal close-packed (HCP) and face-centered cubic (FCC) structure along the normal direction of surface. This stacking defect will induce extra interfacial free energy to the particle phase transformation. So, the increasing stacking defect energy limits the size of large particle growth together with some small particles of tens of nanometers surrounding a large one without shrink to dissipation.

Figure 3 GIXRD spectra of the three-layer Si/C/Si films: (a) as-deposition at room temperature, and annealed at (b) 500 °C, (c) 700 °C and (d) 900 °C for 1.0 hour, respectively.

The phase identification and crystallinity of the three-layer Si/C/Si structure were characterized by GIXRD. Figs. 3(a)- (d) show the GIXRD spectra of the as-deposited three-layer Si/C/Si films and annealed at 500, 700 and 900 °C for 1.0 hr, respectively. It is stable as annealed at 500 °C and becomes polycrystalline with obvious diffracted peaks of Si(111), (220) and (311) as annealed at 700 and 900 °C. Another obvious SiC(111) diffracted peak appears at 900 °C annealing. It indicates that the SiC phase is crystalline and primarily grows along the [111] direction, the normal direction of close-pack (111) plane.

Fig. 4 shows the schematic diagram of the proposed np-SiC formation mechanism via inter-diffusion and reaction of C and Si in the a-Si/C/a-Si multilayer by means of thermal annealing. As annealing is performed at proper temperature of 700 °C, crystallization of two a-Si layers first takes place to form polycrystalline ones with many grain boundaries which are fast paths for interdiffusion between Si and C. Then crystalline SiC with high surface energy follows to form. The SiC particles are preferred to appear on the surface in order to reduce the total free energy of film. The grain growth of polycrystalline Si and interdiffusion between Si and C become more rapid at higher temperature of 900 °C than 700 °C owing to the higher thermal energy and atomic mobility. The particle density increases with the increasing temperature due to larger superheating at 900 ºC with the more number of nuclei.

Figure 4 The schematic diagram of a proposed np-SiC formation mechanism via interdiffusion and reaction of C and Si in the a-Si/C/a-Si multilayer by means of thermal annealing.

The larger superheating at 900 ºC increases the nucleation rate to result in the higher particle density. The particle growth at higher temperature is to reduce the total surface energy of small particles. The radius of particle above critical radius will grow while it will shrink to dissipate below the critical radius from the theory of nucleation and growth of phase transformation in material science. The critical nucleus radius (r^*) is related to the surface free energy (γ) (or interface energy or phase boundary energy, in positive value) and the change of volume free energy (∆G_v) (in negative value) and expressed in the following equation (1). A critical free energy (∆G^*) occurs at the critical radius, corresponding to an activation free energy for the requirement of a stable nucleus, and is expressed in the following equation (2)

………(1)

………(2)
A novel wafer-level approach to form np-SiC from three-layer a-Si/C/a-Si structure by post vacuum annealing has been demonstrated. Different characterization methods of SEM, EDX, GIXRD and AES depth profile are used to investigate the particle size, density, morphology, composition, crystalline phase and interdiffusion behavior of np-SiC, respectively. An interesting phenomenon of some small SiC nanoparticles surrounding a large one has been observed as annealed at 900 °C for one hour. The higher the annealing temperature, the larger the nanoparticle size, and the more the density. The formation of SiC particle, instead of film, is attributed to the high surface energy of SiC during post annealing. The larger superheating at 900 ºC than 700 °C increases the number of stable nuclei for higher particle density. Because of the higher atomic mobility and growth rate at 900 ºC, the particle will grow larger to reduce the total surface energy of small particles. The growth of large SiC particles at the dissipation of small particles with a radius below the critical size are accompanied by the defects of stacking faults or disorder. This induced extra free energy of stacking defects will limit the size of large particle during growth as well as many small particles of 10-50 nm retained to surround large ones. A mechanism of np-SiC formation in the a-Si/C/a-Si multilayer using thermal annealing is also described.