Interfacial Microstructure and Thermal Property of Diamond/Si and 3C-SiC/Si film

Efficient thermal transport and effective heat dissipation pose significant challenges for the material application in modern electronic devices due to their accelerated miniaturization [1–4]. Diamond and SiC are attractive choices of materials for next-generation high-performance device design and fabrication due to their exceptional thermal conductivities of 2000 W/(mK) [5] and 346 W/(mK) [6], respectively. However, their prohibitive cost and/or inaccessibility of structurally controllable high-quality large single crystals has limited their application potentials. Alternatively, significant interest is, therefore, attracted to the research of diamond and SiC thin films [7,8]. In this study, we evaluated the microstructural impacts of polycrystalline diamond and epitaxial 3C-SiC thin films and interfaces on the thermal conductivity and interfacial thermal conductance. We used a polycrystalline diamond film that was 0.63 μm thick grown on Si (100) of 1” diameter and a 3C-SiC film of 2.2 μm thick epitaxially grown on Si (100) of 10 mm× 10 mm (MTI corporation, Richmond, CA, USA). A focused ion beam (FIB)/field emission scanning electron microscope (SEM), Zeiss Auriga® 60, was used to evaluate the surface morphology of the diamond film and the FIB was used to prepare transmission electron microscopy (TEM) specimens. The interfacial microstructure of the films was characterized using a JEM-2010F TEM operating at 200 kV. To measure the thermal properties of the thin films and interfaces of diamond/Si and SiC/Si, we used time-domain thermoreflectance (TDTR) with a Ti:Sapphire laser that generated a series of pulses of <200 fs with a frequency of 76 MHz and a central wavelength of 785 nm. Diamond film was first observed in SEM and the surface morphology of diamond grains are clearly identified in Figure 1. The diamond film has an average grain size of about 0.5 μm at the top surface, as estimated from SEM images. The thickness of the diamond film was confirmed to be 0.63 μm by TEM, as shown in a typical image in Figure 2a. The inset in Figure 2a shows a selected area electron diffraction pattern of the diamond film, suggesting the polycrystalline nature. High-resolution transmission electron microscopy (HRTEM) of the cross-sectional diamond/Si interfacial region suggests the existence of an amorphous transition region with a thickness of 5 nm, as shown in Figure 2b and 2c, which can potentially dramatically decrease the heat transport across the interface. Figure 3a shows a low magnification TEM image of the 3C-SiC thin film deposited on Si (100) wafer. Considerable stacking faults are visible in the film, as indicated by arrows in Figure 3a. The area close to the SiC surface has a lower stacking fault density, whereas the SiC/Si interface region has a much higher stacking fault density. The high defect content can significantly reduce the phonon mean free path (MFP) in 3C-SiC. Figure 3b shows the region of the SiC/Si heteroboundary. No amorphous transition layer at the interface was observed. The thermal property of diamond/Si(100) and 3C-SiC/Si(100) was determined by TDTR. The thermal conductivities (k) of polycrystalline diamond film and 3C-SiC deposited on Si (100) were found to be 240.93 W/(mK) and 297.20 W/(mK), respectively. Compared to the k of 3C-SiC, the reduction of k in diamond film is attributed to the grain boundary scattering and a film thickness shorter than the MFP of phonons in diamond. The interfacial thermal conductance (G) values for diamond/Si(100) and 3C-SiC/Si(100) were found to be 0.018 GW/(m2K) and 0.077 GW/(m2K), respectively. The low G between diamond and Si may be due to the presence of an amorphous transition layer and the material lattice mismatch. In summary, the diamond film with a grain size of 0.5 μm at the top surface has an amorphous transition region with a thickness of 5 nm at the interface between diamond and Si. Nano-crystalline diamond grains are also observed close to the interface. The low interfacial thermal conductance of diamond/Si(100) appears attributable to the significant amorphous layer and the lattice mismatch between diamond and Si. For the 3C-SiC film, the region close to the SiC surface is relatively defect free, whereas close to the SiC/Si interface, there are significant stacking faults detected in the SiC. The high defect content can considerably affect the effective phonon mean free path (MFP) in 3C-SiC and, thus, the thermal conductivity of 3C-SiC [9]. SEM image of the diamond film. (a) Low magnification TEM image of cross-section diamond film with a thickness of about 0.63 μm; (b) and (c) HRTEM images of the interfacial area with an amorphous transition region with a thickness of 5 nm. TEM images of the cross-section of 3C-SiC/Si thin film: (a) Low magnification TEM image; (b) HRTEM image of the 3C-SiC/Si interface.