SiC Homoepitaxial Growth at Low Temperature by Vapor-Liquid-Solid Mechanism in Al-Si Melt Christophe Jacquier,† Gabriel Ferro,*,† Franc¸ ois Cauwet,† D. Chaussende,‡ and Yves Monteil† Laboratoire des Multimate´ riaux et Interfaces (UMR 56-15), Universite´ Claude Bernard Lyon 1, 43 Boulevard du 11 novembre 1918, 69622 Villeurbanne (France), and NOVASiC, Savoie Technolac, BP267, 73375 Le Bourget Cedex, France Received November 6, 2002;
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 3 285-287
Revised Manuscript Received February 18, 2003
ABSTRACT: Homoepitaxial growth of SiC was successfully performed at a temperature as low as 1100 °C via a vaporliquid-solid (VLS) mechanism where propane feeds an Al-Si droplet. This approach has several advantages compared to the conventional liquid-phase epitaxy (LPE) such as an easier mastering of the growth as no thermal gradient (vertical or radial) needs to be controlled. We observed however the formation at the surface of small crystals during the cooling. Some small nonwetted zones are also seen, but they occupy less than 1% of the sample area. Both defects were also present in LPE configuration. Growing good quality epitaxial layers of SiC at low temperature is a challenging problem that could help reduce the costs and increase the safety of the process. Toward this aim, growth from the melt is an interesting approach due to the low supersaturation conditions at the liquid-solid interface. However, carbon solubility in the melt at low temperature is the limiting parameter. The growth in Si-based melts with well selected metal additives could enable both to reduce the growth temperature and increase the C solubility. Among several metals studied, Al seems to be the most advantageous as it can dissolve as much carbon at 1100 °C as liquid Si at 1600 °C. Moreover, the Al-Si alloys melting point is as low as 577 °C. Growth of homoepitaxial SiC layers by liquid phase epitaxy (LPE) in Al-based melts has successfully been reported for temperature below 1200 °C.1,2 These layers displayed higher level of p-type doping than Al implanted layers. This is very promising to reduce the contact resistance on the peripheral protection of some power devices.3 However, our own experience of the LPE of SiC in Al-Si melts showed that several difficulties have to be solved such as the high reactivity of Al alloys with graphite at high temperature or the temperature gradient value and homogeneity.4 Recently, we reported an alternative approach to grow SiC epitaxial layers by vapor-liquid-solid (VLS) mechanism in liquid Si.5 Propane was flowed on a Si droplet which was placed on a SiC seed. Even if a high growth rate were obtained, the temperature was in the 1500-1600 °C range. In the present paper, we will combine these two approaches, i.e., VLS growth at low temperature in Al-Si melts. A comparable study, in which propane was flowed on top of Al-Si melts with Ga or Sn addition, was already reported but with a different and more complex configuration.6 Furthermore, the layers were not homoepitaxial as 3C-SiC was grown on 6H-SiC substrate. The experimental apparatus was composed of a vertical water-cooled cold wall reactor made of quartz. The rotating graphite susceptor was RF heated, and the temperature was measured by optical pyrometry. The VLS and LPE configurations used in this study are detailed in Figure 1. High purity H2, Ar, C3H8 (5% diluted in H2), and SiH4 (1% diluted in H2) were used as gases. The Al and Si sources for the melts were 4N and electronic grade (pieces of Si * Corresponding author:
[email protected]. Tel +33 4 72 43 16 07; Fax + 33 4 72 44 06 18. † Universite´ Claude Bernard Lyon 1. ‡ NOVASiC, Savoie Technolac.
Figure 1. (a) VLS configuration: (1) Al-Si droplet; (2) SiC substrate; (3) SiC plate; (4) graphite susceptor; (5) water-cooled quartz reactor; (6) RF coils. (b) LPE configuration: (1) graphite rod; (2) SiC substrate; (3) Al-Si melt; (4) graphite crucible.
wafers), respectively. The Al and Si pieces were ultrasonic cleaned in methanol and the Si pieces were desoxidized in HF just before use. The seeds were 8° off oriented 4HSiC(0001) single crystals. After ultrasonic cleaning of the sample in methanol, the SiC substrate was introduced in the reactor and heated at 1600 °C under a mixture of H2 + C3H8 for surface preparation. Then the temperature was reduced to 1000 °C at which time C3H8 was replaced by SiH4 to grow 3 µm thick silicon layer. The Si layer thickness was measured by IR reflectance spectrometry taking advantage of the refractive index difference between SiC and Si which creates interference fringes. Using the same formula as in ref 7 and a value of 3.42 for the refractive index of Si, the thickness can be deduced from the interference fringe period. The Si layer had two roles: (1) to protect the seed from air exposure when removed from the reactor; (2) to improve the wettability of the seed by the Al-Si melt.4 The substrate was then removed from the reactor to place a stack of Si and Al pieces on top of it. After introduction again in the reactor, the chamber was evacuated for 15 min and filled with argon. All the VLS experiments were performed with Ar as carrier gas. A previous study has shown that the best conditions for SiC crystallization in Al-Si melts are a temperature of 1100 °C and a Si content of 30 at. %.8 We thus used these conditions for the VLS growth together with a propane flow rate of 1-3 sccm. A polycrystalline SiC plate was placed between the seed and the susceptor to protect the susceptor from the aggressive Al-Si melt and for easier removing of the sample. After the experiments, the samples were wet etched in hot HCl and HF-HNO3 mixture to remove the Al-Si alloy. At the melting, the Al-Si alloy remains on top of the
10.1021/cg0256069 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/01/2003
286
Crystal Growth & Design, Vol. 3, No. 3, 2003
Figure 2. Optical microscopy image showing the surface morphology of a SiC layer grown at 1100 °C by VLS in Al-Si melt. The numerous small black points are SiC crystal grown from the melt upon cooling. The white circular print corresponds to area where no growth occurred.
SiC seed, taking the shape of a droplet of few millimeters in height. After 1 h of VLS growth under propane at 1100 °C, the seed surface displayed the morphology shown in Figure 2. It is clear that a two-dimensional SiC growth occurred with pronounced step bunching typical of growth from the melt. Some small black spots of few micrometers in size can be seen on the surface. As they do not seem to disturb the growth front, we believe that they do not form during the growth but rather at the end of the experiment at the cooling. Indeed, the Al-Si alloy is carbon saturated at 1100 °C under the propane flow and during the cooling the carbon solubility decreases provoking its precipitation under the form of SiC crystals. Note that these crystals were also present when growth was performed in LPE configuration. With the aim of avoiding the formation of these crystals, we modified the cooling step in several ways: (1) removing of the propane 5 min before cooling down to be sure that these crystals do not come from some propane remaining in the chamber at the cooling; (2) high cooling ramp rate to provoke the crystallization inside the melt and not on the seed surface; (3) slow cooling ramp rate to enhance the crystallization on the cool (top) part of the droplet. Unfortunately, no significant reduction of the black spots on the surface of the grown layer was noticed. A way to avoid this cooling crystallization would be to remove the liquid before cooling. This is under progress. In some places, we found some flat round-like features of a few tens of micrometers in diameter (Figure 2). Careful investigations by optical microscopy showed that the surface inside the circles is always at a lower altitude than the average altitude of the sample surface. The difference in altitude was enough to be detected by changing the optical focus but too small to be accurately estimated. In our experience, it is in the micrometer range. In addition to this altitude difference, no step bunching morphology could be seen inside these circles. It seems that no growth occurs inside the circles. These defects were also found when we performed SiC growth by LPE in the same solvent.4 However, to our knowledge, no articles from other authors mentioned similar circular prints when using Albased solvent for growing SiC. Syva¨rja¨rvy et al. reported the formation of similar pits during the LPE growth of SiC in Sc-Si melts.9 They suggest that these pits originate from graphite particles blocking the growth. In our case, there cannot be any graphite contamination as the carbon source is propane and not solid carbon. The appearance of these defects are obviously not related to micropipes as they are not located at the same place. They could come from gas trapping at the SiC/liquid interface. They could also be related to wetting properties and kinetics of the liquid Al-Si/SiC system. The reported
Communications contact angle between molten Al-Si and SiC is as low as 30° at 1100 °C,10 which means good wetting. However, this wetting is time dependent and, at 1100 °C, it takes a few tens of minutes to reach the final value, starting from an initial angle higher than 90°. So, these circle prints can come from poor initial wetting of the seed by the melt leading to local absences of growth. Fortunately, it creates only small nonwetted areas which represent less than 1% of the total area of the samples. It is important to note that the growth in the VLS configuration is under a forced regime as the temperature of the SiC seed is higher than the one of the liquid. That means that the propane cracks and the carbon dissolves in the cooler part of the liquid and the SiC condenses at the hotter part. This growth regime is opposed to the one used in conventional LPE (Figure 1b) where the seed is at the cooler part. In the case of VLS growth at 1500-1600 °C in pure liquid Si under H2 + C3H8 and with identical configuration (seed hotter), a columnar-like growth was evidenced.5 This is clearly not the case in Al-Si melts. Two main differences may explain this discrepancy: the temperature and the carbon solubility in the liquid. Concerning the latter one, if the carbon solubility in molten Al or Si is known,11,12 this is not the case in Al-Si melts. So, it is difficult to speculate on this parameter. On the other hand, the effect of temperature is easier to estimate. Indeed, for identical carrier gas and flow rate, the lower the temperature, the lower the thermal gradient inside the droplet. Furthermore, in this study we used Ar as vector gas which is known to have a lower cooling efficiency than H2. So, we believe that, in these experiments, the thermal gradient inside the Al-Si droplet was lower than inside the Si one. We performed some experiments under both silane and propane to compensate the possible Si loss in the alloy during the growth, mainly due to conversion in SiC. After the growth, the alloys were not etched but cut and polished for cross sectional observation by optical microscopy. The quantity of Si in the alloys can be roughly estimated (( 5 at. %) by the amount of Si crystals formed at the cooling. We did not notice any change in Si quantity for alloys treated under propane alone or propane + silane. So there is no need of using Si gaseous species in the VLS growth as the initial Si amount in the liquid is enough. When comparing the VLS configuration with the standard LPE one, the advantage of the former is obvious. Indeed, this technique is easier to control as, for a given temperature, the growth is only dependent on the propane flux and not on a thermal gradient (radial or vertical) which is difficult to master. There is no need to fix or glue the seed as it is simply posed on the susceptor. Furthermore, we noticed a smaller size of the zones of nonwetting in VLS experiments, which means a better wetting of the seed by the liquid. This is probably due to a combination of two factors: (1) in VLS, the contact interface SiC/liquid is the hotter part of the liquid; (2) the seed is not just dipped in the liquid but the liquid presses on it by gravity. In summary, we demonstrated the possibility to grow homoepitaxial layers of SiC at a temperature as low as 1100 °C with a VLS configuration where propane feeds an Al-Si droplet. This approach, which does not work in pure liquid Si, is very promising when Al-based melts are used. It has several advantages compared to the conventional LPE, but still more work needs to be done to avoid the appearance of the crystals at the cooling and to improve the wettability of the liquid on the seed. Acknowledgment. The authors would like to thank the French De´le´gation Ge´ne´rale de l’Armement (DGA) and the Re´gion Rhoˆne Alpes (FITT) for financial support.
Communications
Crystal Growth & Design, Vol. 3, No. 3, 2003 287
References (1) Rendakova, S.; Ivantsov V.; Dmitriev V. Mater. Sci. Forum 1998, 264-268, 163-166. (2) Dmitriev V. A.; Elfimov L. B.; Il’inskaya N. D.; Rendakova S. V. Springer Proceedings in Physics, Amorphous and Crystalline Silicon Carbide III 1992, 56, 307-311. (3) Syrkin A.; Dmitriev V.; Kovalenkov O.; Bauman D.; Crofton J. Mater. Sci. Forum 2002, 389-393, 291-294. (4) Jacquier C.; Ferro G.; Cauwet F.; Viala J. C.; Monteil Y. Mater. Sci. Forum 2003, accepted for publication. (5) Ferro G.; Chaussende D.; Cauwet F.; Monteil Y. Mater. Sci. Forum 2002, 389-393, 287-290. (6) Tanaka A.; Shiozaki N.; Katsuno H. J. Cryst. Growth 2002, 237-239 (part 2), 1202-1205.
(7) Neyret E. Thesis, Universite´ Montpellier II, 2000. (8) Jacquier C.; Chaussende D.; Ferro G.; Viala J. C.; Cauwet F.; Monteil Y. J. Mater. Sci. 2002, 37, 3299-3306. (9) Syva¨rja¨rvi M.; Yakimova R.; Radamson H. H.; Son N. T.; Wahab Q.; Ivanov I. G.; Janze´n E. J. Cryst. Growth 1999, 197, 147-154. (10) Ferro A. C.; Derby B. Acta Metall. Mater. 1995, 43 (8), 30613073. (11) Oden L. L.; McCune R. A. Metall. Trans. 1987, 18A(12), 2005-2014. (12) Simensen C. J. Metall. Trans. 1989, 20A, 191.
CG0256069
