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Micro-Raman Mapping of 3C-SiC Thin Films Grown by Solid–Gas Phase Epitaxy on Si (111)
© The Author(s) 2010
Received: 5 May 2010
Accepted: 7 June 2010
Published: 20 June 2010
A series of 3C-SiC films have been grown by a novel method of solid–gas phase epitaxy and studied by Raman scattering and scanning electron microscopy (SEM). It is shown that during the epitaxial growth in an atmosphere of CO, 3C-SiC films of high crystalline quality, with a thickness of 20 nm up to few hundreds nanometers can be formed on a (111) Si wafer, with a simultaneous growth of voids in the silicon substrate under the SiC film. The presence of these voids has been confirmed by SEM and micro-Raman line-mapping experiments. A significant enhancement of the Raman signal was observed in SiC films grown above the voids, and the mechanisms responsible for this enhancement are discussed.
Silicon carbide (SiC) is a very attractive material for the fabrication of microelectronic and optoelectronic devices due to its wide bandgap, high thermal conductivity, excellent thermal and chemical stability and its resistance to radiation damage and electrical breakdown . SiC has over 170 different polytypes . The most common forms are 4H, 6H, known as the hexagonal (α-SiC) types, and the cubic 3C-SiC type [2–5]. Among the various polytypes, the 3C-SiC variety possesses unique properties, including a high electron mobility up to 1000 cm2/Vs and a consequent high saturation drift velocity. 3C-SiC can be used as a buffer layer for the subsequent heteroepitaxial growth of gallium nitride and other group III-nitrides . Because of the small lattice mismatch between SiC and gallium nitride (GaN), 6H-SiC can also act as a substrate for the epitaxial growth of GaN , which has application in blue and violet light-emitting diodes and lasers. Therefore, reproducible growth of SiC on silicon wafers is a very important issue for the semiconductor and MEMS industry.
A low-pressure CVD system with a vertical cold-wall reactor made from sapphire, with a diameter of 40 mm and length of 50 mm, in which the central zone was heated, was used for SiC film deposition. The silicon wafer was placed on a graphite holder, with a thermocouple attached to the end. The sapphire tube was connected to a high vacuum system, consisting of diffusion and turbo-molecular pumps. Initially, the system was pumped down to a pressure of 10−5–10−6 Torr. For SiC deposition, a 2′ (111)-orientated Si substrate with a thickness of 300 μm and a tilt of 4° was used. Growth of SiC films on Si(111) was achieved using the chemical reaction of monocrystalline silicon and CO gas, supplied at a rate of 1–10 ncm3/min and a pressure of 0.1–10 Torr. Growth occurs in the temperature range 1100–1350°C, and growth durations of 10–60 min were used. Due to the fabrication procedure, the SiC samples obtained are mainly lightly doped with nitrogen at a level of 1014 cm−3.
Raman spectroscopy is a powerful technique for the characterisation of SiC structures in particular, since it allows the identification of various polytypes [2–5]. The Raman efficiency of SiC is sufficiently high because of the strong covalent bonds in the material. In addition, Raman spectral parameters such as peak position, intensity, linewidth and polarisation provide useful information on the crystal quality . Raman spectra were registered in a backscattering geometry using a RENISHAW 1000 micro-Raman system equipped with a CCD camera and a Leica microscope. Two types of measurements were performed: single-spot measurements from both a void area and outside the void area in the SiC layers, and line-mapping measurements conducted along the voids with nanoscale depth profile of the void varied from 30 nm up to 2000 nm (at the centre of void). For single measurements, an Ar+ laser at 457 nm with a power of 10 mW was used as the excitation source, while for line mapping an excitation wavelength of 633 nm from a HeNe laser with a laser power of 10 mW was used. Line mapping was performed at a distance, x, ranging from 0 to 13 μm with an in-plane step size of 300 nm, where zero corresponds to the starting point of the measurements. Laser radiation was focused onto the sample using a 100× microscope objective with a short-focus working distance, providing a spot size of ~600 nm. Cross-sectional morphologies of the SiC films were characterised with a Tescan Mira SEM.
Results and Discussions
A large enhancement of the Raman peak intensity, by up to 40 times for some samples, for both TO and LO modes, is observed at the void area. This enhancement enables the acquisition of a reasonably good Raman spectrum from ultra-thin SiC layers, as shown in Fig. 2a. Three mechanisms can contribute to the observed enhancement of Raman signal: (a) multiple reflection of the incident light inside the void, (b) multiple reflection of the Raman signal in the SiC layer on top of the void and (c) the presence of additional SiC material grown on the (110) Si ribs of the pyramid inside the voids [9, 10, 18]. The first mechanism is also responsible for the moderate enhancement of the Si second-order peak, by approximately 4 times, from the Si ribs. A somewhat similar effect was discussed for porous Si and SiC in Refs. [19, 20]. For the second mechanism, the enhancement of the Raman signal in thin films, surrounded by media with low refractive indices, was discussed recently in Ref.  for graphene. We use a similar approach for the estimation of the effect of multiple reflections of the Raman signal on the peak intensity from the thin film using a three-layer model consisting of SiC–air–silicon. This model will be discussed together with the line-mapping results in the next paragraph.
The strong enhancement of the Raman peak intensity of the SiC-TO mode, by a factor of 20, inside the cavities is confirmed by the line-mapping measurements presented in Fig. 3c. It can be seen that the enhancement is significantly larger at the centre of the voids, corresponding to a larger cavity depth or a thicker air layer (see Fig. 1). By considering the multiple reflection of the Raman signal based on Fresnel’s equation , and by varying the thickness of the air layer from 0 to 2000 nm and the thickness of the SiC layer between 0 and 800 nm, we estimated the Raman enhancement at the centre of the void to be approximately 10 times larger than that at the edge of the void for a SiC layer with a thickness of about 120 nm (details of these calculations will be published elsewhere ). An increase in the layer thickness to 800 nm reduces the Raman signal enhancement by a factor of ~5. This was confirmed experimentally by Raman line-mapping measurements for the sample with an ~800-nm-thick SiC layer, where enhancement of the Raman signal by a factor of only two was detected at the void centre.
In summary, the presence of voids during the growth of thin SiC layers by solid–gas phase epitaxy has been confirmed experimentally by scanning electron microscopy and micro-Raman spectroscopy. The Raman line-mapping experiments presented in this work confirm that the voids formed in the Si substrate under the SiC layer cause relaxation of the elastic stress caused by lattice mismatch between the SiC and Si. It is shown that the SiC layers investigated here are composed mainly of the cubic polytype of SiC, with small amounts of 6H-SiC. It is worth mentioning that in accordance with Ref. , the quality of GaN layers grown on SiC layers consisting of a mixture of the cubic and hexagonal polytype is better than that for GaN layers grown on a single SiC polytype. A strong enhancement in the peak intensity of the TO and LO modes is observed for the Raman signal measured at the voids.
J. Wasyluk would like to acknowledge the financial support of the IRCSET Ireland (Postgraduate Award) and ICGEE Programme. The study has been performed with financial support from the Russian Foundation for Fundamental Research (grants 07-08-00542, 09-03-00596 and 08-08-12116-ofi) and the RAS Program: «Basis of Fundamental Research in Nanotechnologies and Nanomaterials». S. Dyakov is acknowledged for performing the calculation of Raman enhancement.
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