DOI: 10.1021/cg1005743
Chloride-Based SiC Epitaxial Growth toward Low Temperature Bulk Growth
2010, Vol. 10 3743–3751
Stefano Leone,* Franziska C. Beyer, Anne Henry, Carl Hemmingsson, Olof Kordina, and Erik Janzen Department of Physics, Chemistry and Biology, Link€ oping University, SE-58 183 Link€ oping, Sweden Received April 29, 2010; Revised Manuscript Received June 3, 2010
ABSTRACT: In this study, chloride-based chemical vapor deposition (CVD) of SiC is used either to grow epitaxial layers at high growth rate and to facilitate homopolytypic growth on on-axis substrates or to grow bulk material at temperatures lower than 2000 °C. A vertical reactor configuration with an inlet of gas flow placed at the bottom of the reactor chamber and the exhaust at the top of it has been used. The chlorinated precursors have helped to eliminate or greatly reduce cluster formation, thereby allowing the deposition of thick SiC epilayers at growth rates exceeding 300 μm/h at 1700-1900 °C. Up to 1.5 mm thick homoepitaxial layers have been grown on up to 75 mm diameter 4H- or 6H-SiC wafers. Both on-axis and off-axis, Si-face and C-face polarities have been used. Our results show great promise for the realization of a high growth rate epitaxial process suitable for bulk growth at temperatures lower than those typically used. Such a process is interesting on account of the higher quality material and lower operating cost.
1. Introduction Silicon carbide has established itself in the semiconductor industry for its superior qualities for operations at high temperature, high power, high frequency, and harsh environmental conditions. However, SiC power devices have not yet had their commercial breakthrough in part attributed to the still very high cost of the bulk material (calculated to be 50% of the final cost of a SiC Schottky barrier diode1) and to the quality of the material. Though the commercially available wafers have low micropipe densities, they still have a high density of dislocations (below 104 cm-2).2 Most commercially available wafers come from SiC crystals that are grown by seeded physical vapor transport (PVT),3 and others are produced by the high temperature CVD (HTCVD) technique.4 Promising experimental results have been obtained by halide CVD (HCVD) growth,5 which is a chlorinated gas-based technique in many ways similar to the HTCVD technique. These processes have a common feature: they are performed at temperatures above 2000 °C. PVT is usually performed between 2200 and 2500 °C,3,7 while HCVD and HTCVD are between 2000 and 2300 °C.4,5 Such high operating temperatures have several disadvantages: it is expensive to heat and maintain the reactor at such high temperatures; the lifetime of the graphite susceptor is short considering that in most cases parts of it have to be replaced after a single growth process even if it is coated with protective coatings such as TaC;6 it is difficult to establish and maintain a uniform radial temperature distribution, which is crucial in order to obtain a high quality crystal;7 and finally, micropipes, dislocations, and other crystallographic defects (such as vacancies and interstitials) are easily and abundantly formed at these temperatures, and they can have enough energy to replicate and propagate (coalesce or glide) through the whole crystal.8 Outstanding material quality and high growth rates have been obtained from the CVD processes developed for the *To whom correspondence should be addressed. E-mail:
[email protected]. r 2010 American Chemical Society
growth of thick epitaxial layers.9-12 This can be done if Si cluster formation is properly addressed. In one of these techniques (performed in a reactor referred to as the chimney reactor), the clusters were controlled using a combination of temperature and pressure. It should be noted that clusters were abundantly formed in this technique and that they were in fact required in order to obtain efficient material transport. The clusters evaporated thermally as they propagated through the hot zone and released Si, which could be incorporated in the growing layer. The substrates were attached on the walls of the chimney with their surface parallel to the gas stream. In the chimney reactor, growth rates up to 50 μm/h were achieved at a process temperature of 1650-1850 °C, the quality of material grown at 25 μm/h was comparable to that of the epitaxial layers grown at a much lower temperature, but the results were less reproducible, and the gain in growth rate was not worth using a more expensive growth process. The technique was abandoned on account of the difficulty in obtaining good uniformity of thickness and doping over larger areas. Another way to address the clusters is by chemical means, which has been done with great success using the chloride-based CVD process, performed in horizontal hot-wall CVD reactors, which have been developed by several groups in the past few years.9,13-15 In our group, we have demonstrated growth rates as high as 170 μm/h and epilayer thicknesses exceeding 200 μm on 4H-SiC 8° off substrates.16 We have also investigated other benefits of chloride-based epi, such as the homoepitaxial growth of 4H-SiC on on-axis Si-face substrates17 and the improved morphology of epilayers grown at 28 μm/h on 4H-SiC 4° off substrates.18 A bulk growth process at temperatures lower than 2000 °C is not yet available, though it would be very desirable as it would facilitate the growth of SiC crystals with improved quality. Correspondingly, it would also be a less expensive and more reproducible process, which would be beneficial for manufacturing as well as for research and development. Combining the proven concepts of the vertical reactor and of the chloride-based CVD growth should provide the settings Published on Web 06/18/2010
pubs.acs.org/crystal
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Crystal Growth & Design, Vol. 10, No. 8, 2010
Leone et al.
for the growth of thick crystals at high growth rates even on on-axis substrates, which ultimately could be used for SiC bulk growth. The objective of this study is to investigate the possibility of a low temperature, chloride-based process in a vertical reactor suitable for bulk growth.
using a three window technique to enhance the resolution.34 Minority carrier transient spectroscopy (MCTS) was used to probe hole trapping and emission. Charge carriers were excited by an UV laser pulse (Arþ ion laser at about 351-355 nm) during 200 ms under constant reverse bias conditions.
2. Experimental Procedures
Some preliminary growth experiments were done by changing the gas carrier flow, the pressure, or the sample position inside the reactor, in order to find an optimal gas speed to run the following experiments. Once the best values of temperature, Cl/Si ratio, and C/Si ratio for the different substrates (different off-angles and polarities) were found, a further set of experiments was done to find the best gas speed for the optimized process. The precursors used were always SiH4 þ C2H4 þ HCl, except for a last series of experiments where SiCl4 was used in combination with C2H4. 3.1. Temperature and Cl/Si Ratio. A matrix of experiments was done by changing the temperature in a range between 1600 and 1900 °C and the Cl/Si ratio from 5 to 20 by varying the HCl flow. All of these experiments were run by keeping a gas carrier flow of 20 slm, a pressure of 400 Torr, and a C/Si ratio of 0.5. The substrates used were mainly 6H-SiC Si-face on-axis (off-angle between 0.15 and 0.6°, as measured by XRD), although a few experiments were done also on offaxis (3.5°) substrates. Similar to what we have recently reported for a horizontal hot-wall reactor,17 an increase in temperature always helps to decrease the percentage of 3C-SiC inclusions in favor of homopolytypic growth. Keeping a Cl/Si ratio of 10, several temperatures were tried: at 1600 °C, the samples were 100% 3C-SiC (Figure 1a); by increasing the temperature, there was a proportional decrease of 3C inclusions as seen at 1800 °C (Figure 1b), which completely disappeared at higher temperature, such as 1850 °C (Figure 1c). The growth rate increased with temperature, but it decreased over 1850 °C because of an enhancement of etching due to the high amount of HCl in the gas mixture (Figure 2a). A temperature of 1850 °C was found to be the best in terms of high growth rate (250 μm/h) and absence of 3C inclusions. Subsequently, at a fixed temperature of 1800 °C, the Cl/Si ratio was varied from 5 to 20, and similar to the temperature effect, the same trend was found in the epitaxial growth on on-axis substrates in horizontal CVD reactors: at very low Cl/Si ratio, lower or equal to 10, the percentage of 3C was high (similar to that shown in Figure 1a), while for higher values of the Cl/Si ratio, the inclusions disappeared completely, albeit at the expense of a decreased growth rate on account of the etching effects (Figure 2b). For other conditions of temperature and Cl/Si ratios, similar trends were observed: at higher temperatures, a lower Cl/Si ratio could be used still avoiding 3C-inclusions and vice versa. For example, several growth experiments were done at 1850 °C with a Cl/Si ratio as low as 7 without resulting in any undesired polytype inclusions, which was henceforth considered to be the best combination of temperature and Cl/Si ratio for fast homoepitaxial growth. 3.2. C/Si Ratio. A few experiments were done by changing the C/Si ratio when using 6H-SiC on-axis substrates (offangle between 0.15 and 0.6°); it was found that with a temperature in the range 1800-1900 °C and a Cl/Si ratio in the range 7-15, a C/Si ratio between 0.4 and 0.6 did not markedly affect the material morphology. However, the C/Si ratio played a noticeable role on 3.5° off-angle 6H-SiC
Seventy-five millimeter diameter or smaller 6H and 4H-SiC wafers of different polarities (Silicon- and Carbon-face), different off-angles (
