SEM analysis of ion implanted SiC

Abstract

SiC is a material used in two future energy production technologies, firstly as a photovoltaic layer to harness the UV spectrum in high efficient power solar cells, and secondly as a diffusion barrier material for radioactive fission products in the fuel elements of the next generation of nuclear power plants. For both applications, there is an interest in the implantation of reactive and nonreactive ions into SiC and their effects on the properties of the SiC. In this study 360 keV Ag + , I + and Xe + ions were separately implanted into 6H-SiC and in polycrystalline SiC at various substrate temperatures. The implanted samples were also annealed in vacuum at temperatures ranging from 900°C to 1600°C for various times. In recent years, there had been significant advances in scanning electron microscopy (SEM) with the introduction of an in-lens detector combined with field emission electron guns. This allows defects in solids, such as radiation damage created by the implanted ions, to be detected with SEM. Cross-sectional SEM images of 6H-SiC wafers implanted with 360 keV Ag + ions at room temperature and at 600°C and then vacuum annealed at different temperatures revealed the implanted layers and their thicknesses. A similar result is shown of 360 keV I + ions implanted at 600°C into 6H-SiC and annealed at 1600°C. The 6H-SiC is not amorphized but remained crystalline when implanting at 600°C. There are differences in the microstructure of 6H-SiC implanted with silver at the two temperatures as well as with reactive iodine ions. Voids (bubbles) are created in the implanted layers into which the precipitation of silver and iodine can occur after annealing of the samples. The crystallinity of the substrate via implantation temperature caused differences in the distribution and size of the voids. Implantation of xenon ions in polycrystalline SiC at 350°C does not amorphize the substrate as is the case with room temperature heavy ion bombardment. Subsequent annealing of the implanted polycrystalline samples leads to increased thermal etching effects such as grain boundary grooving. Damage due to channelling (or non-channelling) in the different crystallites resulted also in differences in thermal etching in the crystallites.

Scanning electron microscopy of the surfaces of ion implanted SiC

“…As was shown in ref. [19] the surface crystallites, similar to the ones in Figure 4, grow in size and shape with both increasing temperature and with increasing annealing time. This solid state crystal growth occurs according to the flow-step model of Burton et al [20] and Frank et al [21].…”

“…This solid state crystal growth occurs according to the flow-step model of Burton et al [20] and Frank et al [21]. Due to this growth process the surface becomes rough with cavities occurring at higher temperatures [19]. These two features are also visible in Figures 5 to 6.…”

“…This growth of the crystallites in the direction of their high energy surfaces, increase the area of their lower energy surfaces, thereby minimizing the total energy of the crystallites, occurs to satisfy Wulff's law [24]. The reason for the larger crystals on the iodine implanted surface is due to the beneficiation of adding small amounts of iodine to many materials, including SiC, to grow single and polycrystalline materials -see references [3] and [19] for an expanded discussion of this topic. An interesting question is what crystal type is formed inside and on the surface during crystallization of the ion bombardment-induced a-SiC layer on top of 6H-SiC (or any other polytype) after thermal annealing.…”

Synergistic effects of iodine and silver ions co-implanted in 6H–SiC

“…Previous studies into SiC have been known to form clusters after high temperature annealing cycles [12,18,23,38]. Although the XPS data suggests that both I 3d and Ag 3d spectra indicate the absence of AgI compound in the annealed sample, the binding energies of I 3d5/2 and Ag 3d5/2 for AgI in a host matrix like in SiC can be different from that of bulk AgI.…”

The effect of thermal annealing in a hydrogen atmosphere on tungsten deposited on 6HSiC

“…In HTGRs, SiC is used for fuel kernel encapsulation in tri-structural isotropic (TRISO) particles. In these TRISO particles, SiC is the main diffusion barrier for radioactive fission products (FPs) [6], [7]. Failure of the TRISO particle in retaining FPs will lead to the release of the FPs into the primary cooling circuit thus causing danger of contamination to personnel and environment.…”