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Wafer scale on-axis homoepitaxial growth of 4HSiC(0001) for high power devices: Influence of different gas phase chemistries and growth rate limitations Jawad Ul Hassan, Robin Karhu, Louise Lilja, and Erik Janzén Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00141 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 21, 2019
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Crystal Growth & Design
Wafer scale on-axis homoepitaxial growth of 4H-SiC(0001) for high power devices: Influence of different gas phase chemistries and growth rate limitations Jawad Ul Hassan, Robin Karhu, Louise Lilja, and Erik Janzén Department of Physics, Chemistry and Biology. IFM Linköping University, SE-581 83 Linköping, Sweden
Abstract On-axis homoepitaxy of 4H-SiC has the advantage of producing epilayers that are free of basal plane dislocations. Such layers can be highly beneficial for SiC-based high-power bipolar electronic devices which otherwise suffer from bipolar degradation phenomena related to basal plane dislocations in epilayers. In this study, we have developed on-axis homoepitaxy on the Si-face of 100 mm diameter 4H-SiC wafers with only 4H polytype in the epilayer excluding the edges of the wafer. We have also compared standard and chloride-based growth, the influence of different ambient on surface preparation of the substrate, the influence of the histories of different growth-cells and the geometry of the susceptors regarding 4H-polytype stability in the epilayer. Substrate surface preparation, growth temperature, C/Si ratio, and Si/H ratio are found to be the most influential parameters to achieve homoepitaxy. On-axis homoepitaxial growth rate is limited to a very low value of 100 m) and it is not feasible to grow such thick layers using standard on-axis epitaxial growth process which has a low growth rate of 100 µm/h) in off-cut epilayers through just increasing the flow rates of precursors25-28. In this study, we have also made a detailed investigation if similar growth rates can be achieved for on-axis homoepitaxy while maintaining 4H polytype in the epilayers. 2. Experimental Epitaxial growth was performed on the CMP polished Si-face nominally on-axis 4H-SiC(0001) substrates. The substrates are either (16x16) mm2 pieces cut out from a 100 mm diameter wafer or full wafers and have an unintentional off-cut in the range of 0.01-0.05°. The growth and insitu surface preparation of nominally on-axis substrates were performed in a horizontal hot2 ACS Paragon Plus Environment
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Crystal Growth & Design
wall CVD reactor with gas foil rotation. The susceptor is coated with TaC and has the capacity of a 100 mm diameter single wafer. Two separate growth-cells, a standard-cell (silane+propane) and a chlorinated-cell (trichlorosilane; TCS, HCl, and propane), with exact same susceptor design have been used to compare the evolution of substrate’s surface morphology during insitu surface preparation. Standard and chloride-based chemistries are compared regarding the polytype stability in the epilayer and their impact on the growth rate. The evolution of the substrate’s surface step structure and the growth process are also compared with another susceptor that has a different design. Propane (C3H8) gas is used as a source of C while silane (SiH4) or trichlorosilane (TCS; Cl3SiH) are used as sources of Si for standard and chloridebased chemistry, respectively. Additional HCl gas was used to further increase the Cl/Si ratio above 3, which is the natural Cl/Si ratio using TCS. H2 gas was used as the carrier gas. Growth was performed in a temperature range of 1610-1740° C and at a fixed pressure of 100 mbar. The influence of C/Si ratio and Si/H ratio on 4H-SiC polytype stability in epilayers was studied in the range of 0.8-1.4 and 0.026%-0.068%, respectively. In-situ surface preparation of substrates prior to the growth was performed in standard- and chlorinated-cell under similar ambient conditions to study the surface-step evolution. Surface morphology and step-structure were analyzed using optical microscopy with Nomarski contrast and atomic force microscope (AFM) in tapping mode, respectively. Identification of 3C- and 4H-SiC regions in the epilayer was made through low-temperature photoluminescence (LTPL) spectroscopy and optical microscopy. 3. Results and discussion 3.1
Surface-step evolution during in-situ surface preparation
Epitaxial growth on off-cut substrates has the advantage of substrate’s polytype replication into the epilayer. This is due to the presence of high-density of atomic steps on the surface of the off-cut substrate (4° in [112̅0] direction). The steps reveal the stacking sequence of the polytype of the substrate on the surface, provide unique nucleation sites for the add-atoms, and help to recreate the same polytype in the epilayer during growth. In the case of on-axis substrates, natural steps are not available on the surface and epitaxial growth on such surface leads to the formation of 3C-SiC polytype inclusion in the epilayer29. This is mainly because the crystal structure of the basal plane of the 4H polytype is identical to that of the (111) planes of the 3C polytype. In addition, thermodynamically, the formation of 3C polytype is more favorable under the growth conditions used for 4H-SiC epitaxy. Nevertheless, steps on the surface of an on-axis substrate can be created through surface etching at high temperature in H230. The main source of steps is threading screw dislocations (TSDs) intersecting the surface. At high temperature, under H2 ambient, preferential etching around TSDs leads to the formation of shallow pits that are covered with 0.5-1 nm high steps which spread-out over the entire surface of the sample. These steps act as nucleation sites for the adatoms and assist the substrate’s polytype replication into the epilayer14. Another source of the formation of steps on the on-axis surface is small unintentional off-cut (which is usually directions. Optical profilometry of epilayer (images given at the bottom of Fig.7) revealed the height of hillock and star-like features in the range of 1-5 µm. Etching of the epilayer in KOH revealed only the etch pits related to threading edge and screw dislocations. No BPD related oval shaped etch pits are observed on the surface and no stacking faults related partial dislocations are revealed on the surface. The epilayer is completely free of BPDs and other epi-defects that are usually observed in off-cut epilayers. 13 ACS Paragon Plus Environment
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Fig. 8 (a) Optical image taken from a region of a 100 µm thick on-axis homoepitaxial layer dominated with spiral growth. AFM image take from (b) the center of hillock showing spiral growth, all facets in < 112̅0 > and < 011̅0 > directions corresponding to the hexagonal crystal structure of 4H-SiC are clearly visible, (c) the central region of the spiral marked in (b), (d) two neighboring hillocks intersecting hillocks without forming any defect at the interface (e) high magnification image taken from a marked region in (d). 3.4.1 Growth mechanism On-axis epitaxial growth is driven through the incorporation of adatoms at the step-edges originating from TSDs during in-situ surface preparation. A high density of steps in the vicinity of the core of the dislocation leads to locally high growth rate and results in the formation of large hillocks. With the increasing growth time, hillocks grow both vertically and horizontally leading to the formation of a hillock-valley structure on the surface, Fig. 8a. The surface of hillocks is covered with TSDs related spirals which continue to supply 0.5 nm high steps (Fig. 8b) throughout the growth. A high magnification AFM image taken around the core of TSD is given in Fig. 8c. The sign and magnitude of the Burger vector can also be obtained from the winding direction of the spiral and the height of steps originating from the core. In the case of Fig. 8c, the Burger vector is -1C. In some other local regions of the samples, step-flow growth has also been observed. The surface morphology of thick on-axis epilayer becomes dominated by large hillocks, deep valleys, and large steps. The formation of large steps through surface step bunching and the formation of hillocks/valley structure through spiral growth in on-axis 14 ACS Paragon Plus Environment
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Crystal Growth & Design
epilayers significantly degrade the surface morphology and increase surface roughness. The major reason for the formation of such features is a lack of control over the growth mechanism. The growth mechanism can be different in different regions of the wafer depending on the local off-cut and local density of TSDs. The diameter of hillocks, to a large extent, depends on the C/Si ratio during growth and has shown to expand laterally in < 112̅0 > under C limited growth24. However, large hillocks coalesce with the neighboring hillocks as shown in Fig. 8d. The neighboring hillocks coalesce without the formation of any structural defects at the interface, as shown in the AFM image in Fig. 8e taken from the marked region in Fig. 8d. Large steps, grown through step-flow growth, have also not shown any sign of the formation of defects or polytype instability at step-edges or on wide terraces. This is mainly because on large terraces, at the microscopic level, the growth mechanism is still dominated by the spiral-growth and micro-steps originated from TSDs continue supplying polytypic information in the growing epilayer. We have also observed that the surface is more homogeneous, and roughness is relatively low in the regions of epilayer grown through step-flow growth. A relatively small step-height in regions of epilayer grown through step-flow growth impose that if we can suppress the formation of large hillocks/valley structure and promote the step-flow growth, more homogeneous surface morphology with low roughness may be obtained. 4. Conclusions We have studied the influence of ambient conditions on the evolution of the surface step structure of the Si-face of 4H-SiC on-axis substrates during in-situ etching. We have also compared the influence of two growth-cells with the history of standard- and chloride-based chemistry to understand how the memory effect of Cl influences during in-situ etching. This led us to the conclusion that chloride-based chemistry during growth does not have any negative influence on homoepitaxy. The formation of 3C inclusions during chloride-based epitaxy is mainly due to the Cl memory effect which leads to more aggressive etching of the substrate and brings in more Si from the upstream side of the susceptor. Susceptor design also significantly influences the gas phase chemistry during in-situ etching and results in polytype instability. C/Si ratio, Si/H ratio, and growth temperature are found to be the key parameters to achieve pure homoepitaxy. On-axis homoepitaxial growth rate is limited to a low growth rate of
