DOI: 10.1021/cg901189k
Growth of Epitaxial 3C-SiC Films on Si(100) via Low Temperature SiC Buffer Layer
2010, Vol. 10 36–39
Wei-Cheng Lien,†,§ Nicola Ferralis,§ Carlo Carraro,†,‡,§ and Roya Maboudian*,†,‡,§ †
Berkeley Sensor & Actuator Center, University of California, California 94720, ‡INM-Leibniz Institute for New Materials, Campus D2 2, 66125 Saarbruecken, Germany, and §Department of Chemical Engineering, University of California, Berkeley, California 94720 Received September 27, 2009; Revised Manuscript Received November 19, 2009
ABSTRACT: The epitaxial growth of 3C-SiC films on Si(100) substrates is demonstrated using a two-step chemical vapor deposition (CVD) process. A thin (50 nm) SiC buffer layer grown at 925 C using 1,3-disilabutane is shown to enable the growth of a high crystalline quality epitaxial 3C-SiC film using methyltrichlorosilane at 1200 C. The ability to deposit high-quality epitaxial film is traced to the suppression of void defects and to the improvement in film adhesion obtained by the deposition of the buffer layer at low temperature. Silicon carbide thin film is regarded as the material of choice for high temperature, high power, high frequency, and radiationresistant electronic and micro-electromechanical systems applications in a harsh environment.1,2 Its wide bandgap makes it suitable for electronic device operation in high temperature environments.1 Additionally, its high breakdown electric field strength (3-5 MV/cm, 10 times that of silicon)3 is perhaps the most important factor for its use in high power applications. When employed in high-frequency devices, the saturated electron drift velocity of SiC (2-2.5 107 cm/s)3 is twice that of silicon, thus enabling microwave devices to reach high channel currents.1 Although commercial 4H and 6H-SiC wafers are available, 3CSiC is often considered because it is the only polytype that can be grown heteroepitaxially on Si substrates, and it has one of the largest electron mobilities (∼800 cm2 V-1 s-1) and saturated drift velocities (∼2.5 107 cm/s) of all polytypes despite having the smallest bandgap.4,5 The large lattice mismatch (20%) and difference in thermal expansion coefficient (8%) make SiC heteroepitaxy on Si difficult.6 Several approaches have been developed.6-10 Of particular relevance to this paper, the single precursor 1,3-disilabutane (DSB, CH3SiH2CH2SiH3) has been shown in a high-vacuum chemical vapor deposition (CVD) system to yield epitaxial 3CSiC on Si(100) substrate for deposition temperatures above 900 C.7 However, in low-pressure CVD reactors, which are preferable for large-scale production, DSB is found to suffer from high depletion rates along the reactor tube for deposition temperatures above 850 C.11 This is understood to be a consequence of the production of reactive species with high sticking probabilities, which in turn result in nonuniform film growth.12 Methyltrichlorosilane (MTS, CH3SiCl3) single precursor has been used at deposition temperatures above 1200 C for SiC epitaxy.6,13,14 Typically higher deposition temperatures are needed with MTS, due to the strength of the Si-Cl bond. However, the presence of voids with inverse pyramid shape at the SiC/Si interface caused by Si outdiffusion at such high temperature15 leads to poor adhesion of the deposited film.16 For the deposition of epitaxial SiC films on Si substrates, the conventional method to increase the crystallinity and reduce voids consists of a precarbonization process at temperatures above 1400 C,17 which still causes Si outdiffusion. To reduce the strain at the interface due to lattice mismatch, it has been suggested that the growth of epitaxial SiC films without carbonization *To whom correspondence should be addressed. E-mail: maboudia@ berkeley.edu. pubs.acs.org/crystal
Published on Web 12/01/2009
can be achieved through a modified temperature program. By an ultrafast heating rate (600 C/min) under MTS flow, the lattice mismatch can be released at the interface via the growth of a thin SiC layer to achieve the epitaxial SiC growth on Si(100).6 Unfortunately, this method, involving such high heating rates, is not feasible with the current state of the CVD reactor technology. In this paper, we present a novel growth method to obtain high quality epitaxial SiC films on Si(100), by means of a stratified, sequential, multilayer film growth using both DSB and MTS as precursors. The selection of DSB precursor for the growth of the buffer film is motivated by the fact that with this precursor, films grown above 900 C have a nearly epitaxial relationship with the Si(100) substrate.7 The thin SiC “buffer” film grown using DSB is shown to act as an adherent seed layer for the growth of a thicker SiC film using MTS. The results show that the crystallinity of the epitaxial film is preserved and enhanced in the presence of the buffer film, as a result of the inhibition of void defects and the improvement in film adhesion. Si(100) samples, p-doped with a resistivity of 2 Ω 3 cm and size of 1 1 cm2, are used as substrates. The cleaning procedure consists of repeated cycles of UV ozone treatments and wet etching in concentrated hydrofluoric acid. Next, the sample is introduced to a hot-wall low-pressure CVD reactor (Tekvac, CVD-300-M), on which a SiC buffer film is grown using 1,3disilabutane (Gelest Inc., 98%). Here, the temperature is increased to 925 C at a heating rate of 35 C/min, and then the deposition is initiated with the introduction of DSB at 5 sccm under a reactor pressure of ∼50 mTorr. After the growth of the buffer layer (also referred to as low-temperature film), a second, thicker layer (referred to as high-temperature film) is grown using the MTS precursor in a high-temperature LPCVD reactor (Thermo Electron Corporation, Lindberg/BlueM, HTF55122A). Here, the temperature is increased to 1200 C at a heating rate of approximately 90 C/min under a flow of H2 (Praxair Inc., 99.9% þ). Upon reaching the synthesis temperature, the deposition is initiated with the introduction of MTS (Sigma-Aldrich Inc., 99%) for a specific deposition time. At the end of the deposition, the reactor is cooled down to room temperature under hydrogen gas flow with a cooling rate of 12 C/min. The crystal structure and composition of the films is monitored with X-ray diffraction (XRD) (Siemens D5000 with the instrumental resolution of ( 0.05) and transmission electron microscopy (TEM) (JEOL JEM-2100F, operating at 200 kV). Film crystalline quality is probed by Raman spectroscopy (JYHoriba LabRam, excitation provided by a He-Ne laser, 633 nm). Cross sectional morphology of films is monitored by field-emission scanning electron r 2009 American Chemical Society
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Figure 2. Dependence of the full-width-at-half-maximum (fwhm) of (a) SiC (200) and (400) Bragg reflections (b) Raman 3C-SiC TO mode on the thickness of the high-temperature film. Synthesis condition: temperature of 1200 C and H2/MTS flow rate ratio of 32. The thickness of the low-temperature buffer film is 50 nm. Raman spectrum of 4.2 μm epitaxial film is shown in the inset.
Figure 1. XRD patterns of (a) polycrystalline 3C-SiC film with thickness of 2.3 μm using 100 nm SiC buffer layer; and (b) epitaxial 3C-SiC films with thickness of 4.2 μm using 50 nm SiC buffer layer. Deposition temperature = 1200 C and H2/MTS = 32.
microscopy (SEM) (LEO 1550). The surface morphology of SiC film is examined by a commercial atomic force microscope (AFM) (Digital Instruments Nanoscope IIIa) in tapping mode. In order to determine the optimal growth conditions, several high-temperature films are grown on buffer layers with varied thicknesses. The high-temperature films are found to be polycrystalline film if the low-temperature buffer layer is thicker than 50 nm (Figure 1a). Therefore, the thickness of the buffer layer is chosen not to exceed 50 nm. In this paper, the buffer layer is set at 50 nm thickness. The epitaxial high-temperature film is achieved with an MTS flow rate of 2.5 sccm and a H2 flow rate of 80 sccm, while the reactor pressure is maintained at approximately 1.35 Torr. The XRD spectrum of a 4.2 μm film grown with MTS (deposition time of 85 min) shows only the (200) and (400) 3C-SiC peaks located at 2θ = 41.45 and 89.99, respectively, along with the (200) and (400) peaks from the Si substrate (Figure 1b). The data clearly indicate that the 3C-SiC film grown employing the buffer layer is heteroepitaxial with the Si(100) substrate. The epitaxial film exhibits a high degree of crystallinity, as indicated by the presence of a sharp SiC Raman transverse optical (TO) phonon peak around 794.4 cm-1 and longitudinal optical (LO) phonon peak around 969.0 cm-1 as shown in the inset of Figure 2. If the Gruneisen parameters for the SiC films are taken to be 1.56 for LO and 1.55 for TO,18 the biaxial strain estimated from the shifts of Raman peaks is 0.13%.19 This value is comparable to the value of 0.15% reported by Rohmfeld et al.,20 for a 2.5 μm film using atmospheric pressure CVD at 1280 C with a precarbonization process. If we assume C11 = 540 GPa, C22 = 180 GPa and C44= 250 GPa obtained from ref 21, the residual stress estimated from the shifts of Raman peaks is 727.5 ( 103.9 MPa.22 Investigating a wide range of growth conditions indicates that the epitaxial film is obtained with the growth rate of the hightemperature film in the range of 50-65 nm/min. With high growth rates, a polycrystalline silicon carbide film is deposited, most likely due to fast nucleation of SiC, depositing on the buffer layer in the environment of high precursor flux.23
Figure 3. AFM 2D images (20 μm 20 μm, z-scale 250 nm) of (a) 0.64 μm, (b) 1 μm, (c) 1.7 μm, (d) 2.5 μm, (e) 3.5 μm, (f) 4.2 μm high-temperature epitaxial SiC films. Synthesis condition: deposition temperature of 1200 C and a H2/MTS flow rate ratio of 32. The thickness of the low-temperature buffer film is 50 nm.
Figure 2 shows the dependence of the full-width-at-half-maximum (FWHM) of the SiC (200) and (400) Bragg reflections and of the SiC Raman TO mode on the thickness of the hightemperature epitaxial SiC films. As the thickness of the hightemperature film increases, the decrease in FWHM implies that the crystalline quality is improved. Figure 3 shows AFM images of the high-temperature epitaxial SiC films for varied thicknesses, depicting the evolution in the surface morphology. Initially, the smaller grain size implies that the nucleation phase follows a three-dimensional growth mode (Figure 3a,b). However, when the SiC film is thicker than 1.7 μm, the grain size becomes larger and the surface morphology is transferred from domed topography to the flat-top topography (Figure 3c-e). The Ostwald ripening theory might provide an explanation for the observed transformation of grain size; namely, recrystallization of epitaxial SiC films occurs due to atom transfer through the grain boundary from the smaller grain to the larger one, thus eliminating the small grain.16 At the same time, compared with the growth rate (25 nm/ min) of the low-temperature SiC buffer layer, the relatively higher growth rate (>50 nm/min) with higher lateral growth velocity of islands during high-temperature SiC film deposition results in quasi-two-dimensional growth, similar to what has been proposed for the growth of epitaxial gallium nitride via a low temperature aluminum nitride buffer layer.24 Cross-sectional SEM is used to investigate the distribution of voids as shown in Figure 4. As a comparison, an epitaxial film was
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Figure 4. Cross-sectional SEM images of the 3C-SiC epitaxial films (a) by using a modified temperature program deposited at 1200 C and H2/MTS = 100; film thickness 1 μm; and (b) deposited at 1200 C and H2/MTS = 32; film thickness 4.2 μm with 50 nm SiC buffer layer. Scale bar: 2 μm.
deposited (Figure 4a) using a modified temperature program similar to the one reported in ref 6, by introducing the MTS precursor at 600 C while raising the temperature to 1200 C at a heating rate of 90 C/min. In this case and in the absence of the buffer layer, the presence of voids with inverse pyramid shapes at the SiC/Si interface is clearly visible. In addition, the film thickness is limited to 1 μm due to poor adhesion caused by the voids. Conversely, for a 4.2-μm SiC film deposited on a 50 nm buffer layer (Figure 4b), the distribution of voids at the SiC/Si interface is significantly reduced by the incorporation of the SiC buffer film. Figure 5a shows the electron diffraction (ED) pattern along the [110] zone axis of the SiC/Si interface. The area selected for the diffraction pattern (∼100 nm size) comprises the low-temperature SiC layer and Si substrate. Unlike the ED pattern of the single crystal SiC/Si interface obtained with the conventional precarbonization method (which shows the discrete ED spots of combination of Si and 3C-SiC structure14,25,26), the low-temperature SiC/Si interface exhibits discrete spots corresponding to the SiC film toward the Æ111æ direction, very weak spots toward the Æ220æ direction, and lack of visible spots toward the Æ200æ direction. The weak intensity of the SiC spots in the ED are attributed to the presence of twin defects along the {111} plane of the lowtemperature SiC film. The twin defects are introduced to release the large lattice mismatch between SiC and Si23 as shown in high resolution TEM (HRTEM) images of SiC/Si interface (Figure 5b). The twin defects, originated at the interface between the low-temperature SiC film and Si substrate, are observed to propagate into the high-temperature film which might be caused by the large lattice mismatch and different thermal expansion coefficient of SiC and Si as the sample is cooled down from the synthesis temperature of 1200 C to room temperature.13,23 The lattice constant of the high temperature 3C-SiC film measured from the (111), (200), and (220) reflections in the ED pattern (Figure 5c) are 4.35, 4.36, and 4.33 ( 0.08 A˚, respectively. The average value is 4.35 ( 0.08 A˚, which is consistent, within experimental error, with the value extracted from the (200) peak
Lien et al.
Figure 5. (a) Electron diffraction pattern along the [110] zone axis and (b) HRTEM images of low-temperature SiC buffer film on Si(100). (c) ED pattern along the [110] zone axis. Synthesis condition: temperature of 1200 C and H2/MTS flow rate ratio of 32. The thickness of the low-temperature buffer film is 50 nm. The film thickness of high-temperature epitaxial SiC films is 4.2 μm.
position in the XRD spectrum (4.353 ( 0.005 A˚), and the tabulated value for single crystalline 3C-SiC (4.3596 A˚27). The results here presented show that depositing the thin SiC buffer film at 925 C using DSB can effectively reduce Si outdiffusion from the substrate during the deposition of the hightemperature film, due to the high thermal stability of SiC (sublimation temperature is 2830 C4). Films subsequently deposited at 1200 C using MTS exhibit strong crystallinity with a relatively high growth rate. This is in clear contrast with SiC films grown directly using MTS without precarbonization17 or modified temperature programs6 which result in polycrystalline porous films at 1200 C.28 The results presented here show that a precise control over the thickness of the low-temperature buffer layer enables the transfer of the crystallographic order of substrate through the buffer film.10 For thicker buffer layers (>50 nm), however, the epitaxial order is further reduced or prevented, leading to SiC films exhibiting polycrystalline structure. In summary, heteroepitaxial SiC films with enhanced adhesion to the Si substrate and with a significant reduction in voids at the interface are achieved by depositing SiC buffer film at low temperature first with DSB. The thickness of the SiC buffer layer and the concentration of the MTS precursor for high temperature film are considered to be the main factors to grow void free, adherent epitaxial 3C-SiC films on Si(100) at 1200 C. Acknowledgment. The authors would like to thank Dr. C.-H. Lien from SFI Electronic Technology Inc. and Dr. Chen from Integrated Service Technology Inc. for assistance with TEM acquisition. Support of DARPA MTO through the HERMiT Program (Grant No. NBCH1050002) and Center for Interfacial Engineering of MEMS, and of the National Science Foundation
Communication
Crystal Growth & Design, Vol. 10, No. 1, 2010
(Grant Nos. EEC-0425914 and CMMI-0825531) is gratefully acknowledged.
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