CHARACTERIZATION OF SINGLE-CRYSTAL 3C-SIC ...

CHARACTERIZATION OF SINGLE-CRYSTAL 3C-SIC PITAXIAL LAYERS ON SI SUBSTRATES S. E. Saddow,"l M. E. Okhusyen,a) M. S. Mazzola,ol M. Dudley,bl X.R. Huang,b) W. Huang,blH. Su,b)M. Shamsuzzoha,"l and Y. H. LodJ a) Emerging Materials Research Laboratory, Dept of ECE, Missis ippi State University, Miss. State, MS 39762 b) Department of Materials Science, SUNY-Stony Brook, Stony rook, NY cJ School of Mines & Energy Development, Dept of Metallurgic & Materials El).gineering, University of Alabama, Tuscaloosa, AL dJ School of Electrical Engineering, Cornell University, Ithaca, 14853

ABSTRACT Jn this paper we discuss the growth and characterization o 3C-SiC epitaxial layers grown s a typical three step on both a Si substrate as well as on a novel substrate. The growth process. First an etch of the Si surface is perfonned, second the su ace of the Si is carbonired and third 3C-SiC is grown on the carbonized surface. Several ch terization techniques were used to verify the quality of the 3C-SiC film. Microscopy was use to investigate the surface morphology, X-ray and electron diffraction were used to detennin crystal structure, cross section IBM was used to verify crystal structure and highlight twi ·ng, and x-ray topography was used to measure the strain fields induced in Si substrate at the 3C-SiC/Si interface. INTRODUCTION Commercially available SiC substrates are made using them dified Lely method 1 and are available in both the 4H and 6H SiC polytypes. To date numerous evices have been demonstrated on both 4H and 6H substrates. However, a major co cem is the fact that these substrates have a high density of defects such as micro pipes (- 2 per cm 2 ) along with other crystallographic defects that can lead to device fai!ures.2 Although further materials processing can be used to improve these substrates such as a method being re arched to fill in micropipes, 3.4 it is doubtful that SiC substrates will approach the q ality of Si wafers in the near future. The 3C polytype of SiC is the only polytype that can be gro on Si wafers because the growth temperature for the hexagonal polytypes is above the melti g point of Si (1410°C at STP). Clearly the development of a device epilayer process that c take advantage of inexpensive yet mature high-quality Si technology is advantageous if the tremendous potential of SiC is to be utilized in devices and systems in the near future. This work was therefore undertaken to assess the state-of-the-art of3C-SiC epi on Si which "11 Jead to the establishment of novel heteroepitaxy techniques to realize 3C-SiC device layers large-area, low-cost, highquality Si substrates. 3C-SiC on Si GROWTH EXPERIMENTS · Heteroepitaxy of 3C-SiC on Si is believed to require three dis inct steps: first any native oxide is removed using a in-situ hydrogen etch, second a carboniza ·on step is performed to form the initial layer of SiC, and third the 3C-SiC film is grown. 5·6 Grow of SiC films on (100) Si substrates were conducted in an radio frequency (rf) induction hea d horizontal APCVD reactor

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Mat. Res. SOc. Symp. Proc. Vol. 535 e>1999 Materials Res

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using the process schedule shown in Figure I. After a RCA c an the Si substrates were placed was evacuated of all residual on a SiC coated graphite susceptor. After the reaction cham gases a hydrogen carrier gas flow of 3 SLM was established. he substrate was then heated to 1000° Cover a 3 minute interval and a 10 minute native oxid etch then performed. At the end of the 10 minute in-situ etch the rf power is turned off, and the s sceptor is allowed to cool for 5 minutes to approximately room temperature. The Si substrate is now ready for the carbonization step. With the RF energy still switched off, 50 seem of prop e (3% C3H 8 in ultra-high purity hydrogen) is added to the hydrogen carrier gas. After a 1 min te delay to allow the propane to stabilize in the reactor, the RF power is turned on and the su eptor temperature ramped to 890° C in 3 minutes. The temperatul"f'. is 1beri maintained at 890° C for 5 minntes after which the substrate temperature is ramped to 1335 "C in 1 minute while the propane is maintained at 50 seem for 1.5 minutes. This completes the carbonization proce ure. Next the 3C-SiC epilayer growth is begun by reducing the propane flow to 11 seem wh le silane (SiH 4 3% in ultra-high purity hydrogen) is simultaneously introduced into the reacti chamber at a flow rate of typically 15 seem. The growth process may be continued for y length of time but usually was performed for 120 minute~ resulting in a 3C-SiC layer of app oximately 4 µm (typical growth rate is - 2 µm/hour). To terminate the growth process the s· flow is discontinued and after a 1 minute delay the hydrogen carrier gas and the propane is disc tinued, and the reaction chamber is purged with argon. After a 5 minute argon purge at the gro th temperature of 1335 "C, the susceptor is allowed to cool to approximately room temperat re. This completes the 3C-SiC on Si baseline process that was developed during this research. T(°C}

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Figure l 3C-SiC on Si Baseline Growth Schedule. The oxid etch, carbonization and 3C growth are indicated as I, 2, and 3 resp tively. 3C-SiC EPITAXIAL LA YER CHARACTERIZATION Since the primary limitation of device layers grown fro crystal morphology, numerous crystallographic characterizati the 3C-SiC on Si epitaxial layers developed during this resear electron microscopy (SEM), transmission electron microscop diffraction, Synchrotron White Beam X-Ray Topography (S LEO S360 SEM was used in the secondary electron mode to The advantage of SEM is that it provides a good measure of particularly important for 3C-SiC on Si because SiC is transp

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3C-SiC on Si pertains to the n tools were used to characterize h. These are namely scanning (TEM), X-Ray and electron XT) and optical microscopy. A e the micrograph in Figure 2. e surface morphology. This is nt to visible light, while

electrons at the accelerating voltage used for these micrographs o ly penetrate a short distance into the 3C-SiC film (less than .01 µm). The micrograph taken on he optimized baseline process described above show that the surface of the 3C-SiC film is smoo with some shallow depressions and a few deep depressions; however, no polycrystal · defects were seen and the film texture appeard to be single crystalline. X-ray diffraction {XRD) data was taken at SUNY Stony Bro k on the baseline 3C-SiC material. Two-theta scans displayed peaks associated with both th Si substrate (100) and 3CSiC (200) film but no peaks were observed for 3C-SiC (111) or (2 0) orientations. This snggests a single crystalline epitaxial 3C-SiC film on the (100) Si substrate omparable to that reported by Goleck et al. 7 Rocking curve data indicate that the 3C film is like! of reasonable quality due to the well-defined peak shape observed. Electron diffraction and TEM experiments were performed the University of Alabama using a 200 keV system by Hitachi. In Figure 3(a) is selected area lectron diffraction pattern indicating that the 3C-SiC crystal structure is orientated parallel to the Si (100) substrate. Figure 3(b) is a cross-section TEM micrograph showing the 3C-SiC/Si in rface and well-defined twin boundaries in the 3C film. Samples must be thinned to electron tr sparency to be characterized by TEM to allow the electrons to be transmitted through the sampl . Therefore the electrons must be accelerated by a large voltage to perfonn TEM measure nts; a beam acceleration voltage as high as 200 kV was used to take these micrographs. Voids at the 3C-SiC/Si interface were observed with optical icroscopy and were noted to appear as spots in the synchrotron white beam x-ray topograph (S XT) in Figure 4. Therefore these voids do indeed affect the Si substrate crystal structure since WBXT data displays a change in contrast whenever there is any disturbance in the atomic eparation (i.e., a change in the lattice constant at a localized region in the crystal). Strain lines ue to dislocation groups caused by the lattice mismatch between 3C-SiC and Si are clearly vi dent in the SWBXT data. An extended area of polycrystalline material in the SiC film (see Fi . 4, top of photo) appears as an area with no strain lines, but dark in color. The total penetration epth of the X-rays was approximately 50 µm for the crystal orientation used in this experi ent, while the SiC film is only approximately 4 µm thick. In addition the extinction length is onger in SiC than for Si. Therefore, most of the contrast in the SWBXT is due to the Si subs ate and not the 3C-SiC film. Since contrast in the SWBXT is caused by any change in the inter omic spacing of the Si substrate, localized stress at the 3C-SiC /Si interface is clearly evid nt in the data. Data shown tional Labs, Beam Line Xwas taken at the National Synchrotron Light Source, Brookhaven I9C.

Figure 2 SEM micrograph of 3C-SiC film on (100) Si. Note the s ootn suna= minimmn evidence of polycrystalline defects resent.

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Figure 4 SWBXT data showing the strain field present in th Si substrate resulting rrom tue .:.CSiC epi layer. The cross-hatched pattern is caused by dis ation groups at the 3C-SiC/Si interface. Sample size is approx. I x I cm. The films discussed above show that one can grow sing e crystal "cube-on-cube" 3C-SiC on Si with reasonable morphology. These films are far from' device-quality" and some method for relaxing the stress caused by the 20% lattice mismatch m st be developed to achieve device grade films. 3C-SIC GROWTH ON A Si CU SUBSTRATE Compliant Universal (CU) Substrates have been propos by numerous researchers with et al. 8 The technique developed impressive results in 111-V compounds recently reported by by Lo consists of a thin layer of bonded semiconductor mate al at an angle with respect to a thicker supporting substrate. In principle this arrangement all ws the 1hin "twist bonded" layer to absorb the strain created by growing a subsequent film that h a lattice mismatch with the

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original substrate. The thickness and twist angle must be enginee carefully. Ejeckam et al discussed how to choose the thickness and twist angle of the CU l er with respect to the support substrate as well as the bonding mechanism of the CU layer to the upport substrate.~ Ejeckam et al have demonstrated the benefits of this technique with the gro of an InSb film on a GaAs CU substrate. 10 lnSb and GaAs have about a 15 o/o lattice constant ·smatch thus this work appears to be promising for 3C-SiC on Si which has about a 22 % "smatch. For this work a Si CU substrate was fabricated by Cornell U iversity by twist bonding a (100) Si SOI layer at45° with respect to a (100) Si substrate. The i0 2 layer was then removed and the Si film thinned to a thickness of approximately 100 A. 11 A C-SiC film was then grown on this Si CU substrate using the 3C-SiC baseline process describe earlier. The growth time was 120 minutes with a growth temperature of 1335°C. Cross-section TEM data on the 3C-SiC epi film was taken wi a result similar to the baseline data shown in Fig. 2. Again, microtwins were evident in t e 3C-SiC layer, which would indicate that the lattice mismatch has not been accomodated at the i terface by the CU layer. However, the spacing between the microtwins was farther apart for the 3C layer grown on the CU Si substrate, indicating that a modest improvement in defect st cture was achieved. In addition, SEM micrographs on this sample also showed a slight im rovement in surface morphology. In the optical micrograph shown in Figure 5(a) the JC SiC film is oriented 45° from the support substrate (bottom right-hand comer) which was th twist angle for the CU layer. A large circle appears in the center of the SiC film (Fig 5(a), pper left corner) which was caused by thickness uniformity variations in the twist-bonded CU l yer. Figure 5(b) shows the corresponding SWBXT data for this sample. As before for the base ine 3C-SiC on Si process, strain lines associated with dislocation groups caused by the lattice ismatch are evident in the SWBXT data. The strain Jines are parallel to the (100) and (010) di ections in the bulk Si substrate thus suggesting that the CU layer may have relaxed to the nderlying Si support substrate orientation. An extended area of polycrystalline material i the SiC film appears as an area with no strain lines as was the case for the baseline topographs (upper left and lower right corners in the figure).

Figure 5 SWBXT image of the 3C-SiC on CU film. Lack of contra ·tin bulk Si material (upper left hand and lower right hand corners) due to polyccystalline 3C-Si growth. Single-ccystal 3C film in center region where dislocation groups are clearly evident nd along substrate (100). Sample siz.e approx. 5 mm x S mm.

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CONCLUSIONS

A three step process was used to grow 3C-SiC on (100) Si substrates. First an etch was performed, second the Si surface was carbonized and third 3 -SiC was grown on the carbonized surlace. Several characterization techniques were used to ve fy that the film grown was of high quality. SEM was used to demonstrate that the surface was ooth and lacking of polycrystalline defects. XRD (2-theta scans) and RHEED suggested a single rystalline film that is oriented "cube on cube" with respect to the substrate. Cross section M confirmed that the film was single crystalline with twinning being the dominant defect p nt in the epitaxial layer. SWBXT highlighted dislocation groups at the 3C-SiC/Si interface. A 3C-SiC epi layer was grown on a Si substrate using e baseline process described above. The SWBXT data indicate that the CU layer appeared to have relaxed to the orientation of the support substrate; however, further characterization is quired 10 determine the exact orientation of the CU layer. If CU film relaxation did indeed ccur, this is likely due to the 3CSiC setpoint temperature of 1335° C being too close to the Si melting point of 1410°. Lo at Cornell also had problems with the CU substrate relaxing at oderate temperatures during the Si CU substrate fabrication process so this is likely the explanat on for the result observed in this work. Clearly further development of this promising approa his needed. REFERENCES 1 Philip G. Neudeck, "Recent Progress in Silicon Carbide Semiconductor Electronics Technology," Http:/fwww .lerc.nasa.gov!WWW/SiC.SiCReview.html 2 Neudeck, P. G., Wei Huang, and M. Dudley, "Study of Bulk and Elem tary Screw Dislocation Assisted Reverse Breakdown in Low-Voltage (