Investigations of Ar ion irradiation effects on nanocrystalline SiC thin films

Highlights

• Thin polycrystalline SiC films grown by the pulsed laser deposition technique were irradiated by 800 keV Ar ions at a dose of 2.6 × 10¹⁴ at/cm².

• The SiC films hardness and Young modulus values significantly decreased after irradiation.

• Glancing X-ray diffraction investigations showed a partial transformation of the SiC hexagonal phase into the cubic phase.

• Smooth PLD grown thin films are excellent for radiation effects investigations using XRR, GIXRD and nanoindentation techniques.

Abstract

The effects of 800 keV Ar ion irradiation on thin nanocrystalline SiC films grown on (100) Si substrates using the pulsed laser deposition (PLD) technique were investigated. On such PLD grown films, which were very dense, flat and smooth, X-ray reflectivity, glancing incidence X-ray diffraction and nanoindentation investigations were easily performed to evaluate changes induced by irradiation on the density, surface roughness, crystalline structure, and mechanical properties. Results indicated that the SiC films retained their crystalline nature, the cubic phase partially transforming into the hexagonal phase, which had a slightly higher lattice parameter then the as-deposited films. Simulations of X-ray reflectivity curves indicated a 3% decrease of the films density after irradiation. Nanoindentation results showed a significant decrease of the hardness and Young's modulus values with respect to those measured on as-deposited films. Raman and X-ray photoelectron spectroscopy investigations found an increase of the CC bonds and a corresponding decrease of the SiC bonds in the irradiated area, which could explain the degradation of mechanical properties.

Introduction

Due to its excellent mechanical, optical, thermochemical, electronic, and electrical properties coupled with a low neutron absorption cross section, SiC has been extensively investigated for potential uses in environments where there is a high level of radiation [1], [2]. Such thin films have been used for applications in the nuclear industry as components in composite materials [3], encapsulating coatings for nuclear fuel particles used in the next generation reactors [4], [5] or as a wall material in fusion experiments [6]. The effect of radiation on the structure, mechanical and electronic properties of SiC has been investigated in detail for single crystals that are mainly used for microelectronics applications [7], [8], [9]. However, the deposited films used as coatings for the encapsulation and protection of nuclear fuel or on the walls are either amorphous or polycrystalline, containing many polytypes and a high density of defects. The effect of radiation on such coatings has been only recently investigated [10], [11], [12]. It was reported that some results were quite different from those observed for single crystalline SiC samples. For example, the amorphization fluence threshold was found to be higher for nanocristalline SiC than its value measured for single crystals [10], [11]. Since the depth of the radiation damaged region for medium energy ions is of the order of hundreds of nm, nanoindentation investigations have been recently used to assess changes of the mechanical properties induced by radiation [13], [14]. However, nanoindentation investigations of polycrystalline SiC films have not been yet reported, because the errors caused by the high surface roughness of the investigated films were too large [15]. Using the pulsed laser deposition (PLD) technique, SiC films were deposited on Si substrates and their structure and properties investigated [10], [16], [17], [18], [19]. Results showed that PLD grown films were dense (around 3.20 g/cm³, similar to single crystal values), nanocrystalline, contained low O concentrations and possessed smooth surfaces (rms values below 1 nm) [19]. Because of its ability to manufacture very smooth thin films, the PLD technique offers then the unique opportunity to perform nano-indentation and to probe the damaged area with the glancing incidence X-ray diffraction (GIXRD) and X-ray reflectivity (XRR) techniques and extract information on the mechanical properties, structure and the microstructure of irradiated samples [20], [21], [22], [23]. Even if the surface morphology of PLD grown film could be quite different from that of films deposited using other techniques, the combination of nano-indentation, XRR and GIXRD investigations allows for the observation of the structural (unit cell parameter) and the microstructural (strain field) evolution of SiC samples versus the irradiation fluence and their influence on the mechanical properties. To illustrate these advantages, PLD grown SiC samples were irradiated by 800 keV Ar ions and the changes of the mechanical properties, surface morphology and structure were investigated.