It has been experimentally proven that forming a uniform Si layer on SiC via epitaxial techniques is difficult, due to the lattice mismatch between Si and SiC [38]. One molecular beam epitaxy experiment shows that the growth starts with many separated Si islands in different shapes and sizes, enlarging in three dimensions and merging into several bumpy continents (see Fig. C.1), if the epitaxy process is long enough [38]. This is ascribed to the weak affinity of Si with the SiC surface, and the atomic binding tends to occur between Si atoms mainly [38]. These Si-Si bonds only constitute a crystal lattice on a small scale and lack the homogeneity required for device fabrication.
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Figure C.1. Atomic force microscopy for the Si/SiC heterojunction layer by molecular beam epitaxy (MBE), designed to be 100 nm thick (left) and 1 µm thick (right) [38]
Wafer bonding, however, can transfer a monocrystalline Si layer onto a SiC substrate and the atomic interaction takes place exclusively between the two surfaces. Fig. C.2 demonstrates the pre and post-anneal directly-bonded Si/SiC samples, produced after a 3 inch on-axis semi-insulating (SI) 4H-SiC handle wafer and a 4 inch bulk-Si seed wafer having undergone the Smart CutTM process [13]. This process creates a hydrogen-rich region in the seed wafer by implantation, which defines the thickness of the transferred Si film (1 µm, here). This layer is then split away from the bulk Si during annealing as the hydrogen expands. The first anneal lasted 1.5 hours in an Argon environment at 450
⁰C in order to fracture the Si-Si bonds in the implanted region. This was followed by a 1000 ⁰C rapid thermal annealing (RTA) in Argon to enhance the Si/SiC interfacial bonds. The Si is a dark grey whereas the (SI) SiC is colourless and transparent in nature. It can be seen in Fig. C.2 that the wafer cleavage is only partially successful, with voids present in most of the surface areas.
Figure C.2. The 3 inch Si/SiC bonded samples before and after annealing (left & right). The Si seed wafer is treated with Smart CutTM process.
Fig. C.3 are the images for the post-anneal Si/SiC wafers captured by scanning electron microscopy (SEM) in different perspectives. Viewed in high angle, the edge of the bonded
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and unbonded region is visible and the Si area looks very rough. This is confirmed by the top view of the Si region and the surface morphology features flaking, which can be the trail of the H2 expansion. Though the layer transfer is functional only in some areas, the
cross-sectional shot clearly shows that the bonded Si is 1 µm thick uniformly. This indicates that the H+-induced micro crack can exfoliate the layer, and that the adhering
effect is achievable in the Si/SiC heterojunction. In [13], the yield of the Si/SiC bonded wafer by Smart CutTM is more than 80%, with the surface roughness of only 5.8 nm.
Therefore, the poor Si coverage on SiC in our samples can be due to the presence of alien particles that disrupt the binding. From a device fabrication point of view, this approach is deemed to be inadequate due to the low-yield bonded areas.
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Figure C.3. SEM images of the Si/SiC samples in high angle (top), of top view (middle) and side view (bottom)
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Apart from the Smart CutTM process, the thin Si film can be defined by transferring the device layer of a SOI substrate [130]. After bonding the top surfaces of a SOI wafer and a SiC substrate, the Si handle wafer and the BOX can be removed by grinding and etching so that only the device layer remains on the SiC wafer. Si/SiC wafers were developed for this project using this method by IceMOS Technology Ltd, Northern Ireland. These are shown in Fig. C.4, where the yield is satisfactory regardless of the Si layer thickness (1, 2 and 5 µm) [156]. The annealing temperature for these wafers is 1200
⁰C. The trenches are introduced prior to the wafer mating, as a solution to outgassing [156]. TEM images reveal that the Si/SiC heterojunction can be a sharp interface with and without an amorphous layer, or takes the form of an island-like defect. The dislocation region seems to exist mostly at the Si side and extends up to 8 nm. These results are in agreement with those by other research groups [14] [165]. Results qualifying these Si layers were not available at the beginning of this project and for the simulation, data was taken from other references with similar processing.
Figure C.4. (a) 100mm Si/SiC bonded wafers with a (1) 1-μm, (2) 2-μm or (3) 5-μm- thick Si film, as well as TEM views of the Si/SiC interfaces, showing (1) no interfacial layer, (2)&(3) presence of an amorphous layer and (4) an island-like
defect, respectively [156]
As such, it is expected that the dislocation of Si atoms close to the interface can degrade the device performance if a thin Si layer is used. However, Shinohara et al. proved that in their Si/SiC bonded wafers, with a 1 µm thick Si layer, the channel mobility of the fabricated MOSFETs is 575 cm2V-1S-1 at room temperature, higher than that of the bulk- Si counterpart (489 cm2V-1S-1)[14]. This is probably due to the presence of tensile strain in the Si film, which increases the electron mobility [85].In addition, other device
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characteristics are very similar between the Si/SiC and bulk-Si, meaning that the transferred Si retains the quality of the seed wafer and the physical damage after bonding is negligible. Lotfi et al. also found similarities in most of their results extracted from Si/poly-Si/poly-SiC and SOI LDMOSFETs with two main differences, namely the leakage current and breakdown voltage [130]. Specifically, the slightly higher leakage in the Si/SiC is likely to be caused by the use of the poly-Si interlayer and poly-SiC substrate. The lower breakdown voltage is attributed to the lack of RESURF effect in the Si/SiC architecture [130]. In summary, the Si layer in the Si/SiC wafer can be as good as those of the bulk-Si and SOI. This means that there is no need to change the parameters and physical behaviour of Si material in the TCAD models (e.g. carrier mobility), for modelling the Si/SiC devices.