An alternative route to improve the epitaxial growth of 3C-SiC (100) on Si(100) was developed. It consists in covering the silicon wafers with germanium prior to the carbonization step of the silicon substrate. Transmission electron microscopy and μ -Raman investigations revealed an improvement in the residual strain and crystalline quality of the grown 3C-SiC layers comparable to or better than in the case of 3C-SiC grown on silicon on insulator substrates. These beneficial effects were reached by using a Ge coverage in the range of 0.5-1 monolayer

Ambacher, O.

Förster, Ch.

Masri, P.

McNeil, L.E.

Morales, F.M.

Pezoldt, J.

Stauden, Th.

Zgheib, Ch.

Carolina Digital Repository

Ge-modified Si„100… substrates for the growth of 3C-SiC„100…Ch. ZgheibaDepartment of Electrical Engineering, Notre Dame University, P.O. Box 72, Zouk Mikhayel,Zouk Mosbeh 2211 4501, LebanonL. E. McNeilDepartment of Physics and Astronomy, University of North Carolina at Chapel Hill,Chapel Hill, North Carolina 27599-3255P. MasriGroupe d’Etude des Semi-conducteurs, CNRS-UMR 5650, GES, Université de Montpellier II,cc 074, 34095 Montpellier, FranceCh. Förster, F. M. Morales, Th. Stauden, O. Ambacher, and J. PezoldtDepartment of Nanotechnology, Center of Micro- and Nanotechnologies, TU Ilmenau,PF 100565, 98684 Ilmenau, GermanyReceived 27 February 2006; accepted 26 April 2006; published online 25 May 2006An alternative route to improve the epitaxial growth of 3C-SiC100 on Si100 was developed. Itconsists in covering the silicon wafers with germanium prior to the carbonization step of the siliconsubstrate. Transmission electron microscopy and -Raman investigations revealed an improvementin the residual strain and crystalline quality of the grown 3C-SiC layers comparable to or better thanin the case of 3C-SiC grown on silicon on insulator substrates. These beneficial effects were reachedby using a Ge coverage in the range of 0.5–1 monolayer. © 2006 American Institute of Physics.DOI: 10.1063/1.2206558The increasing demands to integrate diverse electronicand optoelectronic devices operating at high temperaturesand harsh environments1 where silicon Si-based technolo-gies cannot operate have motivated great improvements inalternative heteroepitaxial systems. Silicon carbide SiCcompared to silicon exhibits a larger band gap, a higherbreakdown field, and a higher thermal conductivity. Theseproperties make it profitable to replace Si with SiC in theserestrictive applications. Besides the advantages describedabove, SiC presents disadvantages when compared to Si, i.e.,the high cost of SiC substrates and the smaller size of com-mercialized wafers, which prevent its integration in the de-vice market. The challenge is to combine the favorablephysical properties of SiC with the low-cost, mature Si tech-nology; and the cubic phase, 3C-SiC, appears as the mostplausible candidate.However, due to the large lattice mismatch 20% and the high difference in thermal expansion coefficients8% , it has been difficult to obtain a coherent interfacebetween the two cubic materials. The standard depositionprocess to achieve single-crystalline SiC on Si consists in atwo-step epitaxial process and was originally published byNishino et al. in 1983.2 The first process step consists in theformation of a thin 3C-SiC layer by converting the Si in thenear-surface region into SiC at 1400 °C, while the secondstep consists in the subsequent epitaxial growth, at the sametemperature, on the formed SiC pseudosubstrate. The ob-tained 3C-SiC layers are normally tensile strained, whichaffects the device properties. Up to now, different methodshave been developed in order to modify the strain in thegrown 3C-SiC layers. They consist in the tuning of the crys-talline properties3 or the carbonization conditions,4,5 or in theuse of different types of compliant substrates.6–9Another consequence of the standard growth method isthe development of voids in the silicon substrate just beneaththe SiC layer due to Si out-diffusion. The void formationcan be stopped by adjusting the conversion conditions andselecting the Si-to-C ratio during the subsequent epitaxialgrowth.10–12 Both effects, the strain and related defect forma-tion as well as the voids, deteriorate the electrical propertiesof the grown SiC.13 Furthermore, since usually the tempera-ture required for growing good SiC approaches the meltingpoint of Si, low temperature growth is preferred even at thecost of a lower 3C-SiC crystalline quality.14 The previousarguments reduced crystalline quality, high strain inside3C-SiC and the presence of voids have prevented the devel-opment of a large scale 3C-SiC integration on Si substrates.With the aim to obtain better 3C-SiC/Si heteroepitaxy,our approach consists in the deposition of Ge prior to thecarbonization step in order to create a modulated transition atthe interface. In the case of Si111 it has already been shownthat Ge plays the role of a diffusion barrier for Si out-diffusion from the substrate through the growing SiC layersuppressing void formation.15In the present study it will be demonstrated that Ge pre-deposition prior to the conversion process of Si100 into3C-SiC100, combined with 3C-SiC epitaxial growth bychemical vapor deposition CVD, reduces the residual strainin the heteroepitaxial 3C-SiC/Si system up to values previ-ously achieved only by using silicon on insulator SOIsubstrates.Si100 10  cm p-type boron doped wafers were usedas substrates. In the first step, the wafers were subjected to aRadio Corporation of America RCA cleaning with a finish-ing dip in buffered HF. Subsequently, the wafers were intro-duced in an UMS 500 Balzers molecular beam epitaxychamber. After annealing steps of 1 h at 400 and 750 °CaAuthor to whom correspondence should be addressed; electronic mail:czgheib@ndu.edu.lbAPPLIED PHYSICS LETTERS 88, 211909 20060003-6951/2006/8821/211909/3/$23.00 © 2006 American Institute of Physics88, 211909-1leading to a clean 21-Si 001Si reconstructed surface,the temperature was set to 530 °C. After temperature stabi-lization Ge was deposited on the 21-Si reconstructedsurface in amounts varying from 0 to 4 ML ML are mono-layers with respect to the Si surface. The Ge-modified wa-fers were transferred to an ultra high vacuum chemical vapordeposition UHVCVD chamber where the carbonizationand 3C-SiC epitaxial overgrowth were carried out. Thegrowth temperature was 1050 °C. The detailed growth pro-cedure is described elsewhere.16 The 3C-SiC layers werechosen to be thin around 120 nm to gain insight into theimpact of the substrate modification on the early growthstages.To extract the values of residual strain inside the3C-SiC layers, -Raman measurements were performed inbackscattering geometry with an Ar-ion laser operating at488 nm and a spot size of 4 m. The crystalline quality ofthe structures was also assessed by transmission electron mi-croscopy TEM in both image and diffraction modes, usinga TECNAI 20S electron microscope.Figure 1a shows a bright-field TEM micrograph of asample with 1 ML of Ge presenting a very flat surface. Fig-ure 1b displays a selected-area electron diffraction SAEDpattern taken at the interface region of the same sample onthe 110 zone axis. The perfect alignment of both Si and SiCrelated reflection spots gives an idea of the good epitaxialrelationship and crystalline quality of the grown SiC layer.For comparison, a bright field TEM image of the referencesample, i.e., without Ge deposition, was recorded. Figure1c illustrates a surface with distorted terraces and highersurface roughness, indicating the improved crystallinity dueto the modification of the silicon substrate by Ge. This ob-served beneficial effect of the Ge predeposition correspondsto the effect observed in the case of Si111 substrates and3C-SiC layers grown by molecular beam epitaxy.15In order to extract the strain values inside the 3C-SiClayers and to evaluate its variation with the Ge coverageQGe prior to the conversion process of Si into SiC,-Raman measurements were carried out. In the Ramanspectra the LO and TO modes were evident. According to theselection rule for the 100 surface of a zinc-blende crystal,the LO mode is allowed but the TO mode is forbidden. Theappearance of the TO mode could be due to a certain degreeof disorder, i.e., the defect densities, in this epitaxial filmsand also partially due to a non-perfect backscattering geom-etry due to the finite focusing angle of the objective of themicroscope in the measurements.5,17 The strain was deter-mined using the method developed by Feng et al.18 after theextraction of the transverse and longitudinal optical phononmodes TO and LO by using a Lorentzian fit. Figure 2shows the effect of the Ge coverage on the strain inside thegrown 3C-SiC layers.Two strain regions, I and II, are clearly observed. Thestrain state inside the 3C-SiC layers is always tensile anddecreases with increasing Ge coverages, if the predepositedGe amount is below 1 ML region I. If the Ge predepositionexceeds 1 ML region II, the strain increases again. At pre-coverages above 2 ML Ge region II the residual strain inthe epitaxial 3C-SiC layer grown on Si100 exceeds thevalue obtained on substrates without Ge predeposition. Thestrain reduction reaches a minimum in the 0.5–1 ML precov-erage region. The strain value obtained for the referencesample, i.e., without Ge predeposition, is comparable to thevalues obtained for thicker layers grown at temperaturesabove 1300 °C,6 where the strain approaches 210−3. In thecase of Ge precoverages around the optimum value, the re-sidual strain is lower than in the case of thick 3C-SiC layersgrown on SOI substrates,6 where values between 5.010−4and 810−4 in dependence on the used SOI substrate werefound.Strain reduction is normally connected to dislocationgeneration, i.e., higher defects densities. An increase in thedefects densities normally lowers the electronic and opticalproperties of the grown layers. In TEM investigations in theupper part of the 3C-SiC layer a substantial change in thedefect densities could not be observed with respect to thereference sample, i.e., without Ge. In the interface region thedefect densities were too high to formulate a reliable conclu-sion. For this reason another parameter, which gives accessto the overall crystal quality and therefore to the electronicand optical properties, was used: the intensity ratio of the LOand TO phonon modes ILO/ ITO.5,17 This ratio increaseswith the improvement of the crystalline quality. Figure 3shows the ILO/ ITO intensity ratio as a function of the Gecoverage. The intensity ratio increases up to a Ge predepo-sition of 2 ML where it reaches a maximum improvementregime. If this Ge precoverage is exceeded, the 3C-SiCquality decreases deterioration regime; see Fig. 3. There-fore, it can be concluded that the strain reduction is not dueFIG. 1. TEM micrographs of samples with 1 ML of Ge a and without Gec, and SAED pattern b of the interface region shown in a.FIG. 2. Strain variation vs Ge coverage QGe. The solid line shows the borderbetween the two strain regions of strain decrease and increase. The dashedline is a guide for the eyes.211909-2 Zgheib et al. Appl. Phys. Lett. 88, 211909 2006to a deterioration of the overall crystalline quality.Two components of the strain reduction must be consid-ered: i extrinsic, i.e., thermal strain caused by the differ-ence in thermal expansion coefficients between Si and3C-SiC, and ii intrinsic, also known as the growth strain.19The extrinsic part of the residual strain is lowered in thecurrent growth method by lowering the growth temperatureto around 1000 °C. The intrinsic part of the residual stresscan be and is indeed affected by the presence of a bufferlayer between the grown layer and the substrate which re-duces the stress in the early stages of the heteroepitaxy. Sucha layer can effectively improve the crystalline quality of thegrown layer20 and is affected by the Ge predeposited prior tothe conversion of Si into SiC. In the case of Si111 andmolecular beam epitaxy MBE growth conditions, it wasshown that during the conversion process an ultra thin bufferlayerlike structure is formed.21 This buffer layer and the Geincorporation into the Si substrate improve the interfaceproperties22 and the residual stress.23 So, the heteroeptaxialrelationship in a pair of materials with extremely large latticeand thermal expansion mismatch can be improved throughthe matching of the elasticity-related properties at the inter-faces by introducing a Ge-containing buffer layer. This effectwas predicted theoretically in an earlier publication.24Based on Fig. 2, it can be concluded that the optimumstrain and crystalline quality is reached by using 1ML of Ge.If this amount is exceeded, the nucleation conditions tend toroughen the surface and interface of the conversion layer dueto a reduced nucleation density and a pronounced three-dimensional nucleation of the 3C-SiC on Si.25 These effectsin turn lead to an increase of the intrinsic stress and to thereduction of the ILO/ ITO ratio.In conclusion, an alternative type of substrate for theheteroepitaxy of 3C-SiC on Si100 has been developed.These substrates consist of a Si100 wafer with a chemicallyGe-modified surface. It allows a reduction of the stress andan improvement of the crystalline quality of the grown3C-SiC layer comparable or better than achieved by usingSOI substrates, if the Ge coverage is in the range of 0.5–1ML. These results can provide a promising route towards theoptimization of the heteroepitaxial growth of 3C-SiC on sili-con substrates.The authors would like to acknowledge the financialsupport of this research by the European Union under con-tract Growth 2001 Project No. GRD 1-2001-40466. One ofthe authors L.E.M. acknowledges the support of a WISCTravel Grant from the American Association for the Ad-vancement of Science. Another author F.M.M. would liketo acknowledge the Alexander von Humboldt Foundation forthe financial support SPA/1114640STP.1P. Djemia, Y. Roussigné, G. F. Diras, and K. M. Jackson, J. Appl. Phys.95, 2324 2004.2S. Nishino, J. A. Powel, and H. A. Wills, Appl. Phys. Lett. 42, 4601983.3X. A. Fu, R. Jezeski, Ch. A. Zorman, and M. Mehregany, Appl. Phys.Lett. 84, 341 2004.4T. Chassagne, G. Ferro, C. Gourbeyre, M. Le Berre, D. Barbier, and Y.Monteil, Mater. Sci. Forum 353-356, 155 2001.5S. Veprek, Th. Kunstmann, D. Volm, and B. K. Meyer, J. Vac. Sci. Tech-nol. A 15, 10 1997.6J. Camassel, J. Vac. Sci. Technol. B 16, 1648 1998.7J.-H. Park, J.-H. Kim, Y. Kim, B.-T. Lee, S.-J. Jang, C.-K. Moon, and H.-J.Song, Appl. Phys. Lett. 83, 1989 2003.8Z. C. Zhang, Y. H. Chen, D. B. Li, F. Q. Zhang, S. Y. Yang, B. S. Ma, G.S. Sun, Z. G. Wang, and X. P. Zhang, J. Cryst. Growth 257, 321 2003.9Y. Irokawa, M. Kodama, and T. Kachi, Jpn. J. Appl. Phys., Part 1 40, 59072001.10T. Fuyuki, T. Hatayama, and H. Matsunami, Phys. Status Solidi B 202,359 1997.11V. Cimalla, W. Attenberger, J. K. N. Lindner, B. Stritzker, and J. Pezoldt,Mater. Sci. Forum 338-342, 285 2000.12B. Burkland, Z. Y. Xie, J. H. Edgar, M. Ervin, J. Chaudhuri, and S.Farsinivas, J. Electrochem. Soc. 149, G550 2002.13T. T. Cheng, P. Pirouz, and J. A. Powel, Mater. Res. Soc. Symp. Proc. 148,229 1998.14Y. Gao, J. H. Edgar, J. Chaudhuri, S. N. Cheema, M. V. Sidorov, and D. N.Braski, J. Cryst. Growth 191, 439 1998.15F. M. Morales, Ch. Zgheib, S. I. Molina, D. Araújo, R. García, C.Fernández, A. Sanz-Hervás, P. Masri, P. Weih, Th. Stauden, V. Cimalla, O.Ambacher, and J. Pezoldt, Phys. Status Solidi C 1, 341 2004.16Ch. Förster, V. Cimalla, O. Ambacher, and J. Pezoldt, Mater. Sci. Forum483-485, 173 2005.17Z. C. Feng, C. C. Tin, R. Hu, and J. Williams, Thin Solid Films 266, 11995.18Z. C. Feng, W. J. Choyke, and J. A. Powell, J. Appl. Phys. 64, 68271988.19S. Hearne, E. Chason, J. Han, J. A. Floro, J. Figiel, J. Hunter, H. Amano,and I. S. T. Tsong, Appl. Phys. Lett. 74, 356 1999.20P. Masri, Phys. Rev. B 52, 16627 1995.21J. Pezoldt, P. Masri, M. R. Laridjani, M. Averous, T. Wöhner, J. A.Schaefer, Th. Stauden, G. Ecke, R. Pieterwas, and L. Spieß, Mater. Sci.Forum 338-342, 289 2000.22J. Pezoldt, Ch. Zgheib, P. Masri, M. Averous, F. M. Morales, R. Kosiba, G.Ecke, P. Weih, and O. Ambacher, Surf. Interface Anal. 36, 969 2004.23Ch. Zgheib, P. Masri, P. Weih, O. Ambacher, and J. Pezoldt, Mater. Sci.Forum 457-460, 301 2004.24P. Masri, Th. Stauden, J. Pezoldt, and M. Averous, Phys. Status Solidi A187, 439 2001.25V. Cimalla and K. Zekentes, Mater. Sci. Forum 389-393, 747 2002.FIG. 3. Dependence of the LO/TO intensity ratio vs Ge coverage QGe. Thesolid line shows the border between the regions of crystallinity improvementand deterioration. The dashed line is a guide for the eyes.211909-3 Zgheib et al. Appl. Phys. Lett. 88, 211909 2006

Ge-modified Si(100) substrates for the growth of 3C-SiC (100)

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Ge-modified Si(100) substrates for the growth of 3C-SiC (100)

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