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3CSiC Heteroepitaxial Growth by VaporLiquidSolid Mechanism on Patterned 4HSiC Substrate Using SiGe Melt J. Lorenzzi,† M. Lazar,‡ D. Tournier,‡ N. Jegenyes,† D. Carole,† F. Cauwet,† and G. Ferro†,* †
Universite Claude Bernard Lyon 1, CNRS, UMR 5615, Laboratoire des Multimateriaux et Interfaces 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France ‡ Laboratoire Ampere, UMR-CNRS 5005, B^at. Leonard de Vinci, 21 avenue Jean Capelle, F 69621 Villeurbanne cedex ABSTRACT: In this work, we report on the use of patterned 4HSiC(0001) substrates for the heteroepitaxial growth of 3CSiC by vaporliquidsolid (VLS) mechanism using Ge50Si50 melt. Mesas structures of various size and shape were obtained by standard photolithography and dry etching processes. On the temperature range investigated 13001450 C, 3CSiC deposit was obtained on top and outside the mesas. Some lateral enlargement of these mesas was observed, but it was systematically homoepitaxial. The lateral growth rate was found rather low compared to other techniques like chemical vapor deposition, with a maximum value of ∼12 μm/h. In addition, elimination of twin boundaries (TBs) inside the 3CSiC deposit on top of the mesas was observed in the temperature range of 14001450 C and for specific mesa shape or orientation of the sidewalls. The best case for eliminating these TBs was found to be with initially circular mesas, which spontaneously form well orientated hexagonal facets and then lead to TB-free deposit on top after VLS growth.
1. INTRODUCTION Among the various attempts made to grow high quality heteroepitaxial 3CSiC layers, the best results were probably obtained by Neudeck et al.1 using the step free mesa approach on off-axis R-SiC substrate. Surprisingly, this method was not studied by other groups probably because of tricky growth conditions and yield limited by the screw dislocations density inside the substrate. On the other hand, growth on patterned substrate is very interesting for having more fundamental insight of the growth process used. One can then evaluate the lateral growth rates, the growth rate anisotropy or the evolution of crystalline defects. For the specific case of silicon carbide, this was already done using chemical vapor deposition (CVD),24 sublimation,5 and liquid phase epitaxy (LPE)6 techniques. In the latter case, only Si melt was used, which means temperatures higher than 1400 C and thus homoepitaxial growth was obtained. Using vaporliquidsolid (VLS) mechanism, one can grow SiC at lower temperature.7 Furthermore, depending on the melt composition or temperature, VLS mechanism can lead to either 3CSiC or homoepitaxial layers. So, the use of patterned substrate for VLS growth can give fundamental insight on the growth from the liquid phase on various unstudied issues like the growth at low temperature or the lateral enlargement of 3CSiC polytype. This is done in the present work using GeSi melts down to 1300 C. In addition, since the mesa patterns were originally used by Neudeck et al.1 for growing twin-free 3CSiC layers by CVD, we have followed the evolution of the twin boundaries (TBs) during VLS growth on similarly patterned surfaces. r 2011 American Chemical Society
2. EXPERIMENTAL DETAILS In this work, on-axis 4HSiC(0001) Si-face substrates were patterned using simple lithography and dry etching processes. It led to the formation of flat plateaux, also called mesas, of 2 μm height above the substrate surface and with different sizes and shapes (see Figure 1). After such surface processing, the resulting substrates were cut into pieces of 1 cm.2 The VLS growth experiments were carried out in a homemade vertical water-cooled reactor specifically dedicated to such experiments. It works under atmospheric pressure with high purity Ar as carrier gas. The temperature of the RF-heated graphite susceptor is regulated and controlled by an optical pyrometer. The samples were stuck in the center of a 20 mm inner diameter graphite crucible, which was subsequently stuck on top of the graphite susceptor. The sources of the GeSi alloys were pieces of Si (6N) and Ge (6N) which were stacked inside the crucible without prealloying. The relative amount of each element was calculated to form a liquid upon melting with the targeted molar composition of Si50Ge50. Afterward, the entire system “suceptor þ crucicle þ sample þ alloy” was placed inside the reactor. Before the growth, 3 cycles of 5 min with alternated evacuation and Ar filling were done. High-purity propane (5% diluted in H2) was used as carbon source. Initial heating was done under Ar with a ramp of 163.3 K/min, and when the growth temperature was reached, 5 sccm of propane was introduced inside the reactor to start the VLS growth. At the end of experiment, propane was removed to stop the growth and the liquid was extracted using an aspiration system before cooling. After they were cooled, the remaining alloys inside the crucible were dissolved by wet Received: November 9, 2010 Revised: April 8, 2011 Published: April 13, 2011 2177
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Figure 1. Optical micrographs displaying some examples of the asfabricated mesa structures on 4H-SiC substrates.
Table 1. Experimental Codnitions Used for the VLS Growth on Patterned Substrates and Values Deduced from These Experiments (C)
lateral growth vertical growth
time (min) rate (μm/h)
/^
)
temperature growth sample
rate (μm/h) anisotropy
Mesa-1
1300
60
∼0
4.2
∼0
Mesa-6
1350
60
∼0
4.3
∼0
Mesa-4
1400
60
∼5
4.5
∼1
Mesa-5
1450
30
∼12
4.8
∼2.5
chemical etching in HF/HNO3 with 1:1 volume ratios. A summary of the different sets of experiment with the main growth conditions, for all samples investigated in this work, are listed in Table 1. Note that growth duration of sample mesa-5 was limited to 30 min instead of 1 h because of the higher growth temperature of 1450 C which induces important liquid loss by evaporation. The surface morphology of as-grown layers and the lateral enlargement of the mesas were characterized by Nomarski optical microscopy. μ-Raman spectroscopy was used for polytype identification of the films. The HeNe laser beam (λ = 633 nm) was focused down to a spot of a few micrometers squared in a confocal mode configuration. The vertical growth rate was deduced from interference fringes of FTIR (Fourier Transform infrared) reflectance spectra.8
3. RESULTS The surface morphologies obtained after VLS growth at various temperatures are shown in Figures 2 and 3. The growth temperature has obviously a significant effect on the results. At low temperature (1300 C, Figure 2a), the mesas have almost identical dimension as before growth which means that the lateral growth is almost negligible. Some slight faceting can be seen at the mesa tips. Note that the layer presents a high density of defects (dark lines) located in and outside the mesas. From our experience these lines are TBs forming a network of dark lines.9 Confirmation was obtained by electron backscatter diffraction (EBSD) which clearly shows that these lines separates two crystals with different in plane orientation (Figure 3). Note that the 3C surrounding the mesa has the same in plane orientation as the main 3C orientation on top of the mesa. EBSD mapping
Figure 2. Surface morphologies of the 3CSiC layers grown in this study at (a) and (b) 1300 C (mesa-1), (c and d) 1350 C (mesa-6), (e and f) 1400 C (mesa-4), and (g and h) 1450 C (mesa-5).
performed on twin free mesa (not shown here) confirms this point (same in plane orientation of the 3C deposit in and out of the mesa). Upon temperature increase up to 1450 C, one can see that the mean density of TBs decreases gradually. Interestingly, at 1400 and 1450 C, these defects were completely eliminated on top of triangular mesas having specific orientations of their sidewalls. This is illustrated in Figure 2e and 2g (dashed circles) and also in Figure 4f. Such TB elimination was also observed on differently shaped mesas such as hexagons (Figure 2h). Around the mesas, some lateral growth could be detected (see Figure 4) with mean rate values of 5 and 12 μm/h for 1400 and 1450 C, respectively. Note in Figure 4b that some layer-by layer growth mode is occurring on top of a triangular mesa with 2D nucleation in the middle. Local measurements by μ-IR reflectance spectroscopy allowed to determine that the deposit inside and outside the mesas is of the same thickness. μ-Raman analyses were performed in order to obtain local information on the polytype. On samples grown from 1300 to 1400 C, 3CSiC polytype was always found on the entire surface. This is not the case on sample grown at 1450 C (mesa5) where 4H polytype was systematically detected on the places corresponding to lateral enlargement of the mesas (number 4 in Figure 4a). 3CSiC was found on areas noted 1 and 2 in Figure 4 while in area noted 3, corresponding to the defected zone surrounding the mesa, a mixture of 3C4H polytypes was detected. This was typically found for all the mesas of this particular sample. It means that the area where layer-by-layer growth is occurring corresponds to 3CSiC. 2178
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Figure 3. (a) Surface morphology of a triangular mesa after growth at 1450 C (mesa-5), see in the inset the line crossing the mesa parallel to one side, and (b) EBSD phase mapping image of a similar mesa showing a clear contrast each side of this crossing line. This demonstrates that the 3C deposit on each side of this line is rotated (60) one to each other and thus that this line is a TB. Note that the mesa sidewalls are oriented differently as the ones leading to TB elimination (like in Figure 2g).
In the case of circular mesas, one can see in Figure 4c and 4d that the lateral enlargement induces some faceting of the growth front. The circles have become hexagons, even if for bigger circles the shape transformation is apparently not completed yet (Figure 4c). Note also that this shape transformation from circle to hexagon is always accompanied with extra lateral enlargement forming petal-like areas on the flat parts, unlike the case of originally hexagonal mesas where the enlargement remains flat (Figure 4e). These petal-like enlarged areas are of the 4H polytype. For the triangular mesas, one can see that the faceting of the lateral growth depends strongly on the sidewall orientations. When the sidewalls are parallel to crystallographic directions of growth (Figure 4b), the lateral growth remains parallel to the mesa side. But when the sidewalls are intermediately oriented compared to crystallographic directions, then the lateral growth gets jagged (Figure 4a). Interestingly, TBs elimination was more often observed for mesas with mean dimension e200 μm and on hexagonal mesas (either originally circular or hexagonal, see Figure 4d and 4e. Furthermore, concerning the hexagonal mesas displaying the presence of TBs, these defects are very often running along the same three opposite mesa edges among the six possible ones (Figure 4c and 5). On other mesa shapes, one needs apparently a 3-fold symmetry (such as triangles, Figure 4f) with specific orientation of the sidewalls for TB elimination while on mesas without any 3-fold symmetry (Figures 2g and 2h and 4g), TBs are almost always found.
Figure 4. Various examples of morphologies obtained on sample mesa5 grown at 1450 C: (a, b and f) triangular mesas; (c and d) originally circular mesas; (e) originally hexagonal mesa; (g) elongated mesa. Note on f that all the triangular mesas are smooth and without TBs. Insert in b is a zoom of the steps forming inside twin-free triangle.
4. DISCUSSION Before going further in the discussion, it is worth reminding that the present experimental conditions used in this study are commonly leading to 3CSiC growth on a bare R-SiC(0001) substrate.10 The 3CSiC nucleation mechanism has been detailed in ref 11. Briefly, it involves a slight dissolution of the SiC seed in contact with almost pure liquid germanium in the early stage of heating. It is then followed by a fast precipitation of 3C islands due to the progressive dissolution of silicon inside the melt upon further heating. It is thus an out-of-equilibrium dissolutionprecipitation process which seems to be specific to VLS growth. This process is so fast that 3CSiC is always form in this way, independently on the presence of step edges on the surface, even around screw dislocations crossing the substrate surface. 4.1. Lateral Enlargement and Faceting. The experimental results of VLS growth on patterned substrate clearly show a competition between homoepitaxial enlargement and 3CSiC heteroepitaxial one. The easiest to detect and see is homoepitaxial enlargement from the sidewalls of the mesas for temperature g1400 C. At lower temperature, such homoepitaxial enlargement 2179
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Figure 5. Morphology of originally hexagonal mesas after VLS growth at 1450 C (mesa-5). Note the presence of TBs along three opposite sidewalls only among the six possible.
)
)
Figure 6. Visualization of the crystalline directions after faceting of a circular mesa (sample mesa-5) for (a) hexagonal and (b) cubic symmetry.
found.6 But when CVD technique is used, the typical values of / ^ anisotropy are in the 102103 range at 1600 C.2 This is because of the very high lateral growth rates (>400 μm/h). Note that when the CVD temperature is decreased down to 1200 C, the anisotropy is apparently reduced down to values below 100, which is still far above the results of the present study. But we observe the same trend of growth anisotropy decrease when decreasing temperature. As for CVD, this may be due to the decreases of the lateral growth rate at low temperatures, owing to the decrease in the surface diffusion length of adatoms. 4.2. Twin Boundary Elimination. Before going further in the discussion about TB elimination, it is worth mentioning that, because of the peculiar nucleation mechanism of 3CSiC during VLS growth,11 TBs are most probably always formed in the early stage of growth. It means that the observation of mesas without TB after VLS growth is the result of a mechanism promoting TB elimination. It was shown that TB-free 3CSiC deposit was not obtained on all the mesas, while for specific shape, size, and sidewalls orientations of these mesas, TBs were eliminated. According to our observations, the conditions favoring elimination of this particular defect are the following: (i) lateral growth rate in the 512 μm/h range (set by the temperature), (ii) 3-fold symmetry mesas with sidewalls perfectly orientated toward specific crystallographic directions, (iii) hexagonal or circular mesas, and (iv) mesas with lateral dimension e200 μm. All these information suggest a strong interaction between appearance/propagation of TBs and the sidewalls. For instance, these sidewalls may act as sink for TBs only for proper crystallographic orientations. And due to the 3-fold symmetry of 3C-SiC, there should be three possible planes for TB sinking. That is why in the case of hexagonal (circular) mesas, half of the sidewalls are perfectly orientated and interact with TBs, while the other half does not interact with TBs so that they run along these sidewalls (Figure 4). This is schematized in Figure 7. )
cannot be excluded but the growth runs were probably too short for such observation. Substrate polytype replication is surely because the vertical walls of the mesas expose the crystalline planes perpendicular to (0001) direction. These are the planes where the polytype information is kept so that any lateral growth starting from these walls should be homoepitaxial with the substrate. This also means that the initial 3CSiC island nucleation using Ge-based melts (as detailed in ref 11) does not occur on the sidewalls. On top of the mesas, a layer-by-layer growth mode was observed with 2D nucleation in the center (figure 4b). This is because the mesas have limited size (with initially few steps on it) while growth from a liquid phase induces high step bunching and thus large step width. Rapidly, the mesas are covered with a unique terrace and growth can only continue by 2D nucleation. Such growth mechanism does not take place outside the mesa because then the steps are not in finite number and they just advance by step flow. An interesting effect observed with the homoepitaxial lateral growth is the faceting of the circular mesas toward hexagonal shape because of in plane anisotropy of growth rate. It was shown that lateral step velocity (υ) toward [1120] and [1100] directions are different, with υ [1120] > υ [1100] for temperature < ∼1600 C.2,12 This is probably what is seen here: the family plane [1120] disappear because v[1120] is higher. The stable planes are the ones with the smallest velocity, that is, of the [1100] family (see Figure 6). That is why when mesa sidewalls are initially oriented parallel to [1120] planes, the surface become jagged after VLS growth by the decomposition of the step front into pieces of [1100] family planes. In the case of originally circular mesas, petal-like extra enlargement was observed. Such kind of enlargement, which does not follow the simple approach of in-plane anisotropy of growth rate, is still not understood yet. Note also in the case of these originally circular mesas, that the 3C deposit became also hexagonal in shape. This is rather surprising since the free-fold symmetry of this polytype should lead naturally to triangular shape (Figure 6b). It is believed that the 3C layer, which is on top of the mesas, just follows the shape evolution of the mesa underneath so that this is the R-SiC anisotropic enlargement which fixes the final shape of the 3C deposit. Knowing the layer thickness, one can calculate the in-plane vs normal growth rate anisotropy (also noted /^). Typical values of this /^ anisotropy calculated in this work are very small, below 3 in the 14001450 C range and close to 0 at lower temperature (see Table 1). If we now compare these values with the ones that can be found in the literature for the epitaxial growth of SiC, one can see that it is similar with the case of SiC growth from the melt using pure Si where values close to 1 are
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sidewalls. The only way of obtaining TB-free 3CSiC deposit using CVD is not to form this defect at all during the nucleation. For reaching this goal, one needs to grow on top of step-free mesas to promote one kind of 3C orientation among the two possible.1 This is not an easy task because of the presence of screw dislocations in the substrates acting as infinite sources of steps. In the case of VLS mechanism, one needs only to play with controllable parameters, which are mainly the temperature, the propane flux, the shape, and the size of the mesas.
Figure 7. Schematic evolution of circular mesas during VLS growth: the circle evolves to hexagon due to in-plane anisotropy of growth rate. The TBs on top of the mesas get confined along one of the two possible 3-fold symmetry family of sidewalls. In the present drawing, 3C(I) orientation is the main orientation of the deposit since its natural triangle faceting is parallel to the sidewalls without twin.
In fact, the ideal case seems to be the circular mesa with diameter of ∼100 μm since the homoepitaxial lateral enlargement will spontaneously create the perfectly orientated sidewalls which will then help eliminating the TBs inside the growing 3CSiC deposit. If the original circle is bigger, the faceting toward hexagon will take longer so that TB elimination will be less efficient. For mesa mean dimension > ∼100 μm, original hexagonal shape should be preferred but in this case the mesa sidewalls have to be perfectly aligned with the crystallographic orientations of the substrate. Increasing mesa dimension above 200 μm will reduce the probability of sidewall interaction by moving away the center from these sidewalls. Basically, observation of a TB-free mesa means that the 3CSiC deposit on its top presents only one of the two possible 60 rotated in-plane orientations. According to the present results, the same family of 3-fold symmetry sidewalls lead to TB elimination. It means that, everywhere on the sample, the same orientation on the 3C deposit is obtained for TB-free mesas. This is puzzling since both orientations of the 3CSiC deposit have the same 50% probability of formation. For instance, if sidewalls only were at the origin of TB elimination, one would expect on Figure 2e and 2g all the triangle to be TBfree since they all present sidewalls ideally orientated toward one family of 3C in-plane orientation. Obviously, this is not the case so that another parameter is playing also an essential role. This is probably the same parameter which leads to the formation of the same orientation in and outside the mesas. This fundamental point needs further investigation. Finally, for the TB elimination mechanism, it is worth comparing the present results on 3CSiC heteroepitaxial growth with the ones obtained using CVD.1,13 Indeed, the conditions promoting TB elimination during VLS growth involve lateral growth rates in the 512 μm/h range which is not very high. It is even not excluded that lower growth rates, which could be achieved by reducing propane flux at similar growth temperatures of 14001450 C, could lead also to TB elimination. As mentioned previously, CVD is known to promote much higher lateral growth rates but, in this case, it is very difficult to avoid TB formation or elimination during growth even on mesas, with some success though.1,13,14 In additions, these experiments do not show any evidence of interaction between TBs and mesa
5. CONCLUSION VLS growth on patterned substrate using Si50Ge50 melt allowed obtaining various fundamental information on the 3CSiC heteroepitaxial growth on 4HSiC substrate. Mesa enlargement from the sidewalls was homoepitaxial while 3CSiC was successfully deposited on top of all the mesas within the conditions studied. The major identified parameters allowing TBs elimination inside the 3CSiC deposit formed on the mesas are (i) a lateral growth rate in the 512 μm range (set by the temperature) and (ii) proper mesa size and orientation of the sidewalls. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: þ33 472 431 039. Fax: þ33 472 440 618.
’ ACKNOWLEDGMENT The authors acknowledge the European Union (Marie-Curie Research and Training network “MANSiC”, Project Nb: MRTNCT-2006-035735) for financial support. The authors would like to thank P. Chaudou€et for EBSD characterization. ’ REFERENCES (1) Neudeck, P. G.; Truneck, A. J.; Spry, D. J.; Powell, J. A.; Du, H.; Skowronski, M.; Huang, X. R.; Dudley., M. Chem. Vap. Deposition 2006, 12, 531–540. (2) Kimoto, T.; Matsunami, H. J. Appl. Phys. 1994, 76 (ll), 7322–7327. (3) Neudeck, P.G.; Powell, J.A.; Truneck, A.J.; Spry, D.J.; Beheim, G. M.; Benavage, E.; Abel, P.; Vetter, W.M.; Dudley, M. Materials Science Forum; Trans Tech publications Ltd.: Switzerland, Vols 389393; 2002; pp. 251254. (4) Trunek, A.J.; Neudeck, P.G.; Spry, D.J. Materials Science Forum; Trans Tech publications Ltd.: Switzerland, Vols 556557; 2007; pp.117120. (5) Saparin, G. V.; Obyden, S. K.; Mokhov, E. N.; Roenkov, A. D.; Ahmedov, B. A. Scanning 1994, 16, 21. (6) Nikolaev, A. E.; Ivantsov, V. A.; Rendakova, S. V.; Blashenkov, M. N.; Dmitriev, V. A. J. Cryst. Growth 1996, 166, 607–611. (7) Ferro, G.; Soueidan, M.; O. Kim-Hak, Cauwet, F.; Monteil, Y. Materials Science Forum; Trans Tech publications Ltd.: Switzerland, Vols. 556557; 2007; pp. 4146. (8) Soueidan, M.; Ferro, G.; Nsouli, B.; Roumie, M.; Polychroniadis, E.; Kazan, M.; Juillaguet, S.; Chaussende, D.; Habka, N.; Stoemenos, J.; Camassel, J.; Monteil, Y. Cryst. Growth Des. 2006, 6 (n11), 2598–2602. (9) Soueidan, M.; Ferro, G. Adv. Funct. Mater. 2006, 16 (7), 975–979. (10) Soueidan, M.; Ferro, G.; Kim-Hak, O.; Cauwet, F.; Nsouli, B. Crystal Growth Des. 2008, 8 (3), 1044–1050. (11) Soueidan, M.; Ferro, G.; Kim-Hak, O.; Robaut, F.; Dezellus, O.; Dazord, J.; Cauwet, F.; Viala, J. C.; Nsouli, B. Acta Mater. 2007, 55, 6873–6880. 2181
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