LETTER pubs.acs.org/NanoLett
Epitaxial Graphene Nucleation on C-Face Silicon Carbide Jennifer K. Hite,* Mark E. Twigg, Joseph L. Tedesco, Adam L. Friedman, Rachael L. Myers-Ward, Charles R. Eddy, Jr., and D. Kurt Gaskill U.S. Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, D.C. 20375, United States ABSTRACT: The initial stages of epitaxial graphene growth were studied by characterization of graphene formed in localized areas on C-face 6HSiC substrates. The graphene areas were determined to lie below the level of the surrounding substrate and showed different morphologies based on size. Employing electron channeling contrast imaging, the presence of threading screw dislocations was indicated near the centers of each of these areas. After the graphene was removed, these dislocations were revealed to lie within the SiC substrate. These observations suggest that screw dislocations act as preferred nucleation sites for graphene growth on C-face SiC. KEYWORDS: Graphene, nucleation, dislocation, electron channeling contrast imaging
G
raphene has garnered enormous interest lately due to its unusual properties, including high free-carrier mobility and compatibility with conventional semiconductor processing.1-3 Moreover, epitaxial graphene (EG), produced by the sublimation of Si from SiC substrates, extends the promise of graphene to large scale, uniform production.4,5 Another distinct advantage to epitaxial growth on SiC is that a semi-insulating substrate can be used, eliminating the need to exfoliate or transfer the graphene to another substrate for device processing. Further heightening expectations, EG radio frequency (rf) field effect transistors have recently been demonstrated.5-7 However, the growth mechanism of this material remains poorly understood. Current rf device work has focused on EG grown on the Si-face of basal plane oriented semi-insulating SiC substrates,5-8 as the EG on this face mainly consists of a monolayer of graphene.4 In contrast, growth on the C-face consists of up to a dozen or more graphene layers and has a significantly rougher topography.4,9 Yet, there is significant interest in obtaining few layer, smooth EG on the C-face of SiC due to its superior electrical properties as compared to EG on the Si-face.10 Recently, it was shown that an argon ambient slows the growth rate of EG on the C-face, resulting in only localized growth of the graphene under certain conditions.11 Such conditions present an opportunity to investigate the initial stages of graphene growth on C-face SiC. In this work, scanning electron microscopy (SEM) and electron channeling contrast imaging (ECCI), which has proven productive in determining defects in wide band gap semiconductors such as SiC and GaN,12-15 are employed to investigate the initial stages of EG growth. The EG samples used in this study were grown on the C-face of 16 16 mm2 6H semi-insulating SiC substrates (II-VI, Inc.) that were nominally basal plane oriented and were cut from 76.2 mm diameter parent wafers. All growth was carried out in a commercially available hot-wall Aixtron VP508 chemical vapor deposition reactor. Prior to graphene growth, the substrates were etched in situ in a H2 ambient at 1620 °C. This etching produces a controlled starting surface that is dominated by SiC surface r 2011 American Chemical Society
steps with heights of roughly 1.5 nm. After the H2 etching step, the ambient was switched to Ar with a transition period of 2 min during which pressures varied by (50% around 100 mbar. The subsequent 60 min graphene growth process was conducted under a flowing Ar ambient of 20 standard L min-1 at 100 (200) mbar (200), with a growth temperature of 1550 (1650)°C. The growth conditions chosen created localized pockets of graphene growth.16 A majority of the ECCI investigation is taken from the first conditions; however the localized graphene areas resulting from either conditions were morphologically similar. In general, nucleation becomes continuous for reduced pressures and is suppressed for increased pressure. For reduced temperatures, nucleation becomes suppressed whereas for increased temperatures nucleation becomes continuous. Additional details on sample preparation and growth have been published previously.11,16 The localized graphene growth was morphologically investigated via SEM and ECCI. The structural characteristics were also revealed with ECCI, using a FEI Nova 600 NanoLab SEM and hkl Technology Nordlys electron backscatter diffraction (EBSD) detector equipped with forescattered detectors. This nondestructive characterization method has been shown to pinpoint dislocations and distinguish between edge and screw/mixed dislocations as effectively as transmission electron microscopy (TEM) in both GaN and SiC materials.14,15 Since ECCI is diffraction-based, it only responds to morphological changes and lattice distortions in the material. For morphological variations, ECCI can detect step-flow edges in both SiC and GaN, making it sensitive to height variations of a few angstroms.17,18 In comparison, SEM imaging of graphene is sensitive to contrast between atomic species and suffers from large charging effects, especially when performed on semi-insulating SiC. Hence, while graphene attracts charge and becomes extremely dark in SEM Received: November 29, 2010 Revised: February 4, 2011 Published: February 15, 2011 1190
dx.doi.org/10.1021/nl104072y | Nano Lett. 2011, 11, 1190–1194
Nano Letters
LETTER
Figure 2. (a) ECCI image of a small GCB with spiraling growth highlighted by the dotted line. (b) Magnified image of the GCB center, revealing a dislocation, imaged as a light-to-dark contrast. The arrow indicates the direction of the transition of light-to-dark contrast.
Figure 1. (a) SEM images of a small GCB with a roughly hexagonal shape. Notice the central ridges (giraffe stripes) circling the interior. (b) SEM image of an expanded GCB demonstrating expansion along SiC surface step edges. (c) Large GCB consisting of coalesced small GCBs and lacking any crystallographic symmetry. Step edge direction is indicated by arrows.
images, in ECCI this charge contrast is not visible, facilitating the morphological study of graphene. The initial graphene growth regions were originally described as “islands,” since they appeared as pockets of graphene on a sea of SiC.11,19 However, by examining shadowing in the samples at high tilt angles, ECCI images reveal that these areas actually exist at a lower level than the surrounding surface of the SiC substrate; in effect, they are sunken into the surface and will be referred to here as graphene-covered basins (GCBs). This configuration results because graphene synthesis on SiC substrates requires the sublimation of Si from about three bilayers of SiC to form a monolayer of graphene on the new substrate surface.20 Thus the graphene layer lies below the original substrate surface. These GCBs vary in size but can be classified into three categories: small, intermediate, and large GCBs. The first of these, small GCBs, consists of GCBs with small diameters (e20 μm), which show roughly hexagonal symmetry, as can be seen in Figure 1a. Additionally, for these small GCBs, the highly distinctive graphene ridges, or “giraffe stripes”,9,21 emerge as a dominant ring around the interior of the basin, with more stripes radiating out from the ring to the SiC wall surrounding the GCB. The second category (intermediate GCBs) comes about as the GCBs expand (diameters of 20-80 μm), where a more circular shape begins to materialize as shown in Figure 1b. In this case, the original hexagonal GCB is generally still visible in the center of these expanded GCBs and appears darker than the expanded regions. This is indicative of the graphene in the center being thicker than the graphene in the expanded regions.22 In these expanded GCBs, tendrils of graphene can be seen beginning to creep along the SiC step edge. Further growth leads to coalescence of the GCBs, resulting in a third category of large GCBs which is comprised of GCBs with diameters over 100 μm that are stretched along the SiC step direction, illustrated in Figure 1c. For these large, coalesced GCBs, the giraffe stripes appear similar to those typically observed in complete films of C-face graphene.9 As the GCBs expand, the graphene expands along the stepbunched down step region as well as in the down step direction. During expansion, when encounters with other GCBs occur, the
region between the GCBs erodes but the bottom of adjacent basins may be at different levels due to differing nucleation rates. This result, in part, may be the reason for the rough morphology (apart from ridges) reported for continuous graphene films.16 This morphological progression demonstrates that the initial stage of graphene growth consists of small, hexagonal GCBs, from which expansion and coalescence along SiC surface steps result in larger GCBs which possess the same morphology as fully coalesced films. Consequently, the small GCBs provide an excellent opportunity to study the initial stages of graphene growth. Employing ECCI to study the small GCBs reveals many important aspects concerning initial graphene growth. Most strikingly, from a survey of over 30 GCBs, evidence of a single dislocation was found at the center of every GCB. An example of this is found in Figure 2. From the lower magnification view in Figure 2a, a faint spiral can be seen starting from the center of the GCB, which is highlighted by a dotted line in the image. Further magnifying the GCB center (Figure 2b) unveils a pinpoint of light/dark contrast in the center of the GCB. This pinpoint, associated with the spiral, suggests the presence of a threading screw dislocation (TSD). Twigg and Picard have discussed the behavior of TSDs in ECCI imaging.18,23 In their papers, it is demonstrated that the light/dark contrast originating from a TSD reverses as the deviation from the Bragg angle changes sign. Therefore, if the feature being imaged is a TSD, then tilting the sample through the Bragg conditions by small increments should change the contrast from dark-to-light to light-to-dark. This method was applied to several GCBs, an example of which is shown in Figure 3. While changing the deviation from Bragg by (0.2°, the ECCI images show that the contrast reverses directions, dark-tolight (Figure 3a) switching to light-to-dark (Figure 3b), confirming the presence of a screw dislocation. Each image recorded on opposing sides of the Bragg angle is shown with an inset ECCI image of the entire GCB. The same light/dark contrast can be discerned in the center of the inserts, but it is much clearer in the magnified images. In addition, a loose spiral can be observed originating at the screw dislocation, traveling up and to the left and then curving clockwise. Imaging the same GCB using the secondary electron detector instead of the forescattered detectors used in ECCI imaging allows the comparison of pure morphology to the diffraction-sensitive imaging in ECCI. Comparing the inserts of Figure 3a and 3b with the secondary electron detector image of the same island (Figure 3c) lends additional verification that the ECCI is revealing the presence of a dislocation, as there is no evidence of a surface feature at the location of the TSD. Furthermore, TSD behavior discussed by Twigg and Picard 1191
dx.doi.org/10.1021/nl104072y |Nano Lett. 2011, 11, 1190–1194
Nano Letters
LETTER
Figure 3. (a) Magnified ECCI image of a screw dislocation in the center of a GCB, with an arrow indicating the direction of the dark-to-light contrast. The inset shows an image of the entire GCB. (b) Same GCB, only tilted to the opposite side of the Bragg angle (0.4° difference)—behavior that confirms the nature of the threading screw dislocation. The arrow indicates that the dark-to-light contrast has changed direction. (c) Secondary electron detector image of the same GCB shows giraffe stripes and no features where the TSD is located (circled), proving that the signal at the TSD location is not a morphological feature.
Figure 4. (a) ECCI image of the center of a GCB showing the dislocation with the graphene intact. Inset shows SEM image at lower magnification. (b) Same GCB, with the EG removed in the central portion of the GCB. ECCI image confirms that the TSD remains and is therefore in the SiC substrate. Inset shows SEM image of the same GCB.
Figure 5. (a) Secondary electron detector image and (b) ECCI image of a GCB with triangular graphene patch remaining in the middle of a CGB where the majority of the EG had been removed. The triangular patch is located over the TSD. Location of the TSD is circled.
predicts that at large deviations from Bragg conditions, the contrast should fade and disappear, a trend that dislocations centered in the GCBs were also seen to follow (not shown). In this survey of small GCBs, every GCB that was imaged included a TSD. Only one GCB showed indications of several TSDs. Other than TSDs, no additional dislocations were discovered via ECCI. It is also noteworthy that the giraffe stripes are not highly contrasted at the diffraction conditions conducive to seeing the TSDs, although they are easily observed when using secondary electron beam imaging (Figure 3c). Although the presence of TSDs was detected with ECCI, it is not apparent whether these TSDs were in the graphene or if they reside in the underlying SiC. Since the graphene layers were thinner than the electron penetration depth for ECCI (at least tens of nanometers),18 it was reasonable to conjecture that the TSDs are in the underlying SiC substrate. The absence of giraffe stripes at the diffraction conditions for observing the TSDs is also consistent with the claim that the primary source of the TSD ECCI signal is the SiC substrate. To further investigate the source of the ECCI TSD intensity profile, samples were imaged after the graphene was removed using several methods. The graphene was removed by both exfoliating the graphene via the Scotch tape method and mechanical abrasion by swiping the surface with cotton swabs. Panels a and b of Figure 4 contain images of a GCB before and after graphene removal, respectively. The insets in this figure portray SEM images of the GCB, while the large images present ECCI images from the center of the GCB. The SEM images show a wide strip of graphene removed from the center of the GCB. The ECCI images illustrate that the TSD is present both before and after graphene removal. Additionally, the signal from the TSD is sharper and more intense once the graphene has been
removed. This finding was confirmed across all basins investigated and demonstrates direct evidence of a TSD under each small GCB, which represent the initial stage of graphene growth. Another interesting observation from the removal study came from basins with only partial graphene removal in the center. Over a third of these basins retained a triangular piece of graphene in the middle of the basin after the mechanical removal process, directly over the TSD location. An example of this is displayed in panels a and b of Figure 5, showing a secondary electron detector image and ECCI image of the basin, respectively. This suggests that the SiC was further recessed at the location of the TSD, so that as the graphene was either exfoliated or abraded, the graphene in the deeper recess is left undisturbed. Such an observation can be explained considering the work of Camara et al., which states that direct sublimation of subsurface Si atoms is highly improbable. Instead, an assisting defect would be necessary to enhance Si sublimation from the SiC interior.24 The work presented here shows not only the presence of a TSD but also a deeper recess into the dislocation which is filled with graphene, demonstrating that the dislocation is acting as a low-energy surface site that enhances silicon sublimation enabling and/or accelerating graphene growth. Other potential graphene nucleation sites initiating from SiC include micropipes and threading edge dislocations.25 However, in this work, all but one GCB were found to originate over a TSD, with the outlier forming around a micropipe. The morphology of the depression left after the graphene removal matches its morphology prior to the removal, indicating the consumption of SiC during the formation of graphene. It also confirms that the graphene growth was conformal to the SiC. Further attempts to remove all traces of graphene using an oxygen plasma etch resulted in significant roughening of the surface, thereby making ECCI impossible. 1192
dx.doi.org/10.1021/nl104072y |Nano Lett. 2011, 11, 1190–1194
Nano Letters
LETTER
substrate. A single TSD was found within each of these small GCBs. It was determined that the TSD actually resides in the SiC. Not only did every single GCB show indisputable indications of threading screw dislocations, but in many cases an additional depression was discovered at the location of the dislocation. This corroborative evidence of TSDs under the GCBs lends credence to the idea that they are acting as nucleation sites for graphene growth, allowing a direct path for Si sublimation from the SiC.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Figure 6. The center of a large GCB shown at high magnification. Location of the TSD is barely discernible due to increased graphene thickness and rotational disorder.
As larger GCBs appear to be due to either expansion of small GCBs or coalescence of multiple small GCBs studied herein, it is expected that the same ECCI results would be found. However, attempting ECCI on the large GCBs involves many difficulties. The large GCBs exhibit significantly greater morphological complexity than the small GCBs, making the search for dislocations in these large GCBs difficult. Additionally, dislocations imaged in large GCBs are characterized by a much weaker ECCI signal than those observed for small GCBs (Figure 6). As the graphene layer grows, it undoubtedly increases in thickness, and as the dislocations are acting as both Si and C pumps, the graphene is expected to grow thicker in the immediate vicinity of the dislocation. The presence of increasingly thick graphene layers would be expected to give rise to diffraction effects that may mask the ECCI signal associated with the underlying SiC TSD—a problem that is only enhanced by the shifts and rotations inherent in multilayer graphene.4,26 Therefore, the fact that large GCBs are characterized by weak or nonexistent ECCI TSD images is consistent with the complex structure of multilayer graphene. Finally, analysis of large-scale images of the GCB-covered SiC surface reveals a total GCB density of (4-6) 103 cm-2. In high-quality SiC substrates, a TSD density on the order of 104 cm-2 would be expected.27 The GCB density is slightly lower than this value. However, when considering that many of the larger coalesced GCBs consist of several small GCBs, each of which sits over a TSD, then a GCB density lower by a factor of 2 or more compared to TSD density would be expected. Thus, this analysis supports the conclusion that EG growth on the C-face of SiC nucleates at the location where threading screw dislocations in the SiC intersect the surface. It is possible that graphene nucleates by other mechanisms, but we are unaware of these at this time. However, the results imply that a reduction of TSDs should lead to more uniform graphene growth across the sample. As for the effect of GCBs on transport properties, it has recently been observed that step bunching results in additional resistance for EG grown on the Si-face.28 From this, it seems reasonable that an increase in step bunching concentrations locally due to a screw dislocation would also occur for EG on the C-face. This would result in an additional resistance (decrease in mobility) for that local area. In summary, this study shows direct evidence that C-face EG growth is initiated at threading screw dislocations in the SiC substrate. A morphological study on GCBs revealed the initial growth begins with small, hexagonal GCBs recessed in the SiC
’ ACKNOWLEDGMENT Work at the U.S. Naval Research Laboratory is supported by the Office of Naval Research. J.K.H., J.L.T., and A.L.F. acknowledge the support of the American Society for Engineering Education Naval Research Laboratory Postdoctoral Fellowship Program. ’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Nature 2005, 438, 197. (3) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Science 2008, 321, 385. (4) Geim, A. K. Science 2009, 3246, 1530. (5) Berger, C.; Song, Z.; Li, X.; Wu, X.; Brown, N.; Naud, C.; Mayou, D.; Li, T.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191. (6) Moon, J. S.; Curtis, C.; Bui, S.; Hu, M.; Gaskill, D. K.; Tedesco, J. L.; Asbeck, P.; Jernigan, G. G.; VanMil, B. L.; Myers-Ward, R. L.; Eddy, C. R.; Campbell, P. M.; Weng, X. IEEE Electron Device Lett. 2010, 31, 260. (7) Lin, Y. M.; Dimitrakopoulos, C.; Jenkins, K. A.; Farmer, D. B.; Chiu, H.-Y.; Grill, A.; Avouris, Ph. Science 2010, 327, 662. (8) Moon, J. S.; Curtis, D.; Bui, S.; Hu, M.; Gaskill, D. K.; Tedesco, J. L..; Asbeck, P.; Jernigan, G. G.; VanMil, B. L.; Myers-Ward, R. L.; Eddy, C. R.; Campbell, P. M..; Weng, X. IEEE Electron Device. Dev. Lett. 2010, 31 (68), 260. (9) Jernigan, G. G.; VanMil, B. L.; Tedesco, J. L.; Tischler, J. G.; Glaser, E. R.; Davidson, A., III; Campbell, P. M.; Gaskill, D. K. Nano Lett. 2009, 9, 2605. (10) Tedesco, J. L.; VanMil, B. L.; Myers-Ward, R. L.; McCrate, J. M.; Kitt, S. A.; Campbell, P. M.; Jernigan, G. G.; Culbertson, J. C.; Eddy, C. R.; Gaskill, D. K. Appl. Phys. Lett. 2009, 95, No. 122102. (11) Tedesco, J. L.; Jernigan, G. G.; Culbertson, J. C.; Hite, J. K.; Yang, Y; Daniels, K. M.; Myers-Ward, R. L.; Eddy, C. R.; Robinson, J. A.; Trumbull, K. A.; Wetherington, M. T.; Campbell, P. M.; Gaskill, D. K. Appl. Phys. Lett. 2010, 96, No. 222103. (12) Picard, Y. N.; Twigg, M. E.; Caldwell, J. D.; Eddy, C. R., Jr.; Mastro, M. A.; Holm, R. T. Scr. Mater. 2007, 61, 773. (13) Hite, J. K.; Mastro, M. A.; Eddy, C. R., Jr. J. Cryst. Growth 2010, 312, 3143. (14) Picard, Y. N.; Caldwell, J. D.; Twigg, M. E.; Eddy, C. R., Jr; Mastro, M. A.; Henry, R. L.; Holms, R. T. Appl. Phys. Lett. 2007, 91, No. 094106. (15) Picard, Y. N.; Twigg, M. E.; Caldwell, J. D.; Eddy, C. R., Jr; Neudeck, P. G.; Trunek, A. J.; Powell, J. A. Appl. Phys. Lett. 2007, 90, No. 234101. (16) Tedesco, J. L.; VanMil, B. L.; Myers-Ward, R. L.; Culbertson, J. C.; Jernigan, G. G.; Campbell, P. M.; McCrate, J. M.; Kitt, S. A.; Eddy, C. R., Jr.; Gaskill, D. K. ECS Trans. 2009, 19, 137. 1193
dx.doi.org/10.1021/nl104072y |Nano Lett. 2011, 11, 1190–1194
Nano Letters
LETTER
(17) Trager-Cowan, C.; Sweeny, F.; Trimby, P. W.; Day, A. P.; Gholinia, A.; Schmidt, N.-H.; Parbrook, P. J.; Wilkinson, A. J.; Watson, I. M. Phys. Rev. B 2007, 75, No. 085301. (18) Twigg, M. E.; Picard, Y. N. J. Appl. Phys. 2009, 105, No. 0943250. (19) Rutter, G. M.; Guisinger, N. P.; Crain, J. N.; First, P. N.; Stroscio, J. A. Phys. Rev. B 2010, 81, No. 245408. (20) Lauffer, P.; Emtsev, K. V.; Graupner, R.; Seyller, Th.; Ley, L.; Reshanov, S. A.; Weber, H. B. Phys. Rev. B 2008, 77, No. 155426. (21) Luxmi, P. J.; Fisher, N.; Srivastava, R. M.; Feenstra, Y.; Sun, J.; Kedzierski, P.; Healey; Gu, G. Appl. Phys. Lett. 2009, 95, No. 073101. (22) Ilyin, A.; Guseinov, N.; Nikitin, A.; Tsyganov, I. Physica E 2010, 42, 2078. (23) Picard, Y. N.; Twigg, M. E. J. Appl. Phys. 2008, 104, No. 3042224. (24) Camara, N.; Rius, G.; Huntzinger, J.-R.; Tiberj, A.; Magaud, L.; Mestres, N.; Godignon, P.; Camassel, J. Appl. Phys. Lett. 2008, 93, No. 263102. (25) Ha, S.; Mieszkowski, P.; Skowronski, M.; Rowland, L. B. J. Cryst. Growth 2002, 244, 257. (26) Hass, J.; de Heer, W. A.; Conrad, E. H. J. Phys.: Condens. Matter 2008, 20, No. 323202. (27) Hasan, J.; Bergman, J. P.; Henry, A.; Janzen, E. J. Cryst. Growth 2004, 310, 4430. (28) Yakes, M. K.; Gunlycke, D.; Tedesco, J. L.; Campbell, P. M.; Myers-Ward, R. L.; Eddy, C. R., Jr.; Gaskill, D. K.; Sheehan, P. E.; Laracuente, A. R. Nano Lett. 2010, 10, 1559.
1194
dx.doi.org/10.1021/nl104072y |Nano Lett. 2011, 11, 1190–1194
