3C-SiC growth on Si substrates via CVD: An introduction - Mansic

<strong>3C</strong>-<strong>SiC</strong> <strong>growth</strong> on Si substrates via CVD:
An introduction
Abstract
Written by Jessica Eid
LMGP/INPG, France, jessica.eid@inpg.fr
and Irina Georgiana Galben
LMGP/INPG, France, irina.galben@inpg.fr
based on the lecture of Prof. Stephen Saddow
University of South Florida, USA
In this work, <strong>growth</strong> and characterization of <strong>3C</strong>-<strong>SiC</strong> thin films have been studied.
Chemical vapor deposition (CVD) of <strong>3C</strong>-<strong>SiC</strong> thin films on silicon substrates using a
horizontal hot-wall CVD reactor has been described. A <strong>growth</strong> rate of 30 µm/h
was achieved at 1380°C. A comparison in the quality of crystals grown by Low
Pressure (LP) and Atmospheric Pressure (AP) CVD was made. The results suggest
that the LPCVD process leads to thinner films with a smoother surface and better
crystal quality.
1. Introduction
Silicon carbide (<strong>SiC</strong>) possesses electrical and mechanical properties that make it a
very promising semiconductor superior to silicon for high environments and high
power applications. It is well known that there are a large number of polytypes
for the <strong>SiC</strong> crystal. The most common are hexagonal (2H, 4H and 6H-<strong>SiC</strong>),
rhombohedral (15R and 21R-<strong>SiC</strong>) and cubic (<strong>3C</strong>-<strong>SiC</strong>).
The only pure cubic polytype, <strong>3C</strong>-<strong>SiC</strong>, has many advantages for MOS device
applications over the other polytypes due to the smaller band gap. In addition,
the electron Hall mobility is isotropic and higher compared with these of 4H and
6H polytypes [1]. Most important is that this polytype can be grown on silicon
substrates hence there is significant interest for low-cost, large-size <strong>3C</strong>-<strong>SiC</strong>
wafers for microelectronic applications.
<strong>SiC</strong> <strong>growth</strong> from a stoichiometric melt is not feasible due to thermodynamic
reasons because it sublimes at T > 2830°C, therefore vapor phase epitaxy is
needed for film <strong>growth</strong>. The chemical vapor deposition (CVD) method is the
preferred technique to grow <strong>3C</strong>-<strong>SiC</strong> films on Si substrates. However, due to the
significant difference of the thermal expansion coefficients and the 20% misfit
between Si and <strong>3C</strong>-<strong>SiC</strong> the defect density in these films is high. Progress has
been achieved recently in defect reduction at the interface by studying the
temperature of the <strong>growth</strong> process, the Si substrate orientation, etc.
In this paper, the basics of the CVD process are given followed by a brief
summary of important achievements in CVD <strong>growth</strong> of cubic silicon carbide. The
films quality (structural and electronic defects) was observed mainly by X-Ray
diffraction (XRD), transmission electron microscopy (TEM) and low temperature
photoluminescence (LTPL). One of the key <strong>growth</strong> experimental parameters
(pressure) is investigated to grow <strong>3C</strong>-<strong>SiC</strong> on (100) Si substrates. Finally some
applications using these <strong>3C</strong>-<strong>SiC</strong> films are demonstrated.
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2. CVD <strong>growth</strong> basics
CVD is a materials synthesis process in which one or more volatile precursors
react in the vapor phase near or on heated substrate to form a solid thin film. The
CVD technology combines several scientific and engineering disciplines including
thermodynamics, kinetics, fluid dynamics, plasma physics (for plasma enhanced
CVD) and of course the chemistry. Several steps that must happen in every CVD
process can be listed as follows (Figure 1): reactant gases are transported with
carrier gas to the reactor, reactant species diffuse through the boundary layer
above the <strong>growth</strong> surface, the species migrate to the reaction site on the surface
via surface mobility, reaction (adsorption or chemisorption) takes place on the
surface, gaseous byproducts are transported away from the surface, and finally
diffused away through the boundary layer.
Figure 1. Schematic illustrating the fundamental steps in chemical vapor deposition
Depending on the type of source supplying the required activation energy, there
are various CVD processes: hot and cold wall CVD and plasma enhanced CVD,
etc. Pressure dependent CVD are normally of two types: atmospheric pressure
CVD (APCVD) and low pressure CVD (LCVD).
The CVD process is a viable single crystal production method with satisfactory
doping behavior. As discussed in the section dealing with the fundamentals of
CVD, the precursor gases are transported using a carrier gas, usually hydrogen,
into a reactor where a substrate is heated to specific temperature where chemical
reactions occur on the surface of the substrate. This produces a solid deposit on
the substrate, and volatile byproducts to be pumped away from the system.
Through chemical vapor deposition thin films of <strong>SiC</strong> can be grown homoepitaxially
(<strong>SiC</strong> on the same polytype substrate) and heteroepitaxially (on different
substrates e.g., silicon.
The advantages of using silicon as a substrate in heteroepitaxy of <strong>3C</strong>-<strong>SiC</strong> are the
large size, high quality, low price, and good polytype control. The equipment used
in <strong>SiC</strong> heteroepitaxy is the same as for homoepitaxy. Thin films of <strong>3C</strong>-<strong>SiC</strong> on 6H-
<strong>SiC</strong> <strong>growth</strong> at temperatures of 1450 °C with deposition rates of 4 µm.h -1 and
thicknesses up to 12 µm were reported by Powell et al. [2]. Compared with <strong>3C</strong>-
<strong>SiC</strong> deposition on Si, these samples exhibited fewer interface defects. The initial
experiments with deposition of <strong>SiC</strong> on Si [3,4] illustrated that the difference in
both the lattice constant (almost 20%) and the thermal expansion coefficient
Physics of Advanced Materials Winter School 2008 2

(almost 8%) between <strong>SiC</strong> and Si produced a high concentration of defects at the
<strong>SiC</strong>-Si interface and, consequently, this process of crystal <strong>growth</strong> turned out to be
problematic. Nishino et al. [5] solved this problem by growing films with the so
called "Buffer layer" technique where the film is deposited at atmospheric
pressure with specific gases (SiH4/C3H8/H2) in a quartz reactor with a graphite
susceptor, and an inductive heating system.
The production of epitaxial films with high quality depends on a number of
factors. In particular, the substrate orientation has a large influence on the
epitaxy process [6] and it has been shown that for the (100) and (111)
orientations the best results are achieved. On the other hand, Si (111) substrates
frequently show cracks in the film during the cooling process because higher
mechanical stresses result for this substrate orientation compared to (100)
substrates. Exhaustive studies show that the deposition rate is connected with
the quality of the film, i.e. by using lower deposition rates, the quality is
improved. In addition, when separate precursors are used for Si and C, the
proportion of Si and C (i.e., the Si/C ratio) in the gaseous phase changes the film
properties. Other factors such as deposition pressure, temperature, carbonization
temperature, carbonization duration and gaseous purity influence the <strong>3C</strong>-<strong>SiC</strong> film
quality [7]. The Buffer-Layer-Process at temperatures higher than 1300°C leads
to the formation of several film crystal defects. In order to reduce the defects
density at the interface between the Si substrate and the <strong>3C</strong>-<strong>SiC</strong> film, Nagasawa
et al. [8] have developed a CVD process using a cold-wall low pressure CVD
reactor at 1350°C. The film was deposited on an undulant Si substrate.
One of the experimental parameters which affect the process is the pressure. The
pressure changes the gas velocity and, therefore, the thickness of the boundary
layer (BL). When a low pressure is used, the boundary layer is thin and the
reactant diffusion is faster. While a high pressure results in a thick boundary layer
and increased residence time as the Figure 2 shows.
3. Experimental
Figure 2. Schematic illustrating the AP and LP CVD
The CVD system consists of a horizontal hot wall reaction chamber. The precursor
gases silane (SiH4) and propane (C3H8) as Si and C sources respectively and
hydrogen as carrier gas had been used in this work. The carbonization process
was carried out at specific temperature reached in less than 2 minutes. For
carbonization a mixture of hydrogen and propane gases is introduced on the
heated substrate (T substrate = 1175°C). After the carbonization step, the
substrate was heated to the required <strong>growth</strong> temperature (1385°C) while a
mixture of silane, propane and hydrogen was introduced into the reactor. After
completion of the <strong>growth</strong> process, the system was cooled down from <strong>growth</strong>
temperature to room temperature in about 30 minutes. The complete CVD <strong>growth</strong>
process used is demonstrated in Figure 3.
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Figure 3. Time vs. temperature graph of CVD process
4. Results and Characterizations
4.1. <strong>3C</strong>-<strong>SiC</strong> on Si hetero-defects
The defects formation in the <strong>3C</strong>-<strong>SiC</strong> layers grown by CVD on silicon is due to the
large lattice mismatch and the difference in thermal expansion coefficient
between Si and <strong>SiC</strong>. The Figure 4 illustrates the affect of the lattice mismatch at
the interface between the <strong>3C</strong>-<strong>SiC</strong> layers and the Si substrates.
Figure 4. Illustration of the affect of lattice mismatch in heteroepitaxy. The – symbol
denotes the location of a missing row of atoms which is known as a line defect.
Note the stretched and compressed covalent bonds at the interface resulting
from the lattice mismatch between the two crystals.
The most common defects in the <strong>3C</strong>-<strong>SiC</strong> layers are stacking faults (SFs),
microtwins (MTs) and anti-phase boundaries (APBs).
4.2. X-ray diffraction (XRD)
4.2.1. Powder diffraction
The crystalline quality of the grown layers is analyzed by x-rays diffraction in
Bragg-Brentano mode with Cu-Kα monochromatic source. In the Figure 5, XRD
spectra of <strong>SiC</strong> layers grown on Si (001) substrates with different <strong>growth</strong> rates of
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(a) 18 µm/h and (b) 30 µm/h are shown. For both samples, the peak at 33° is
the substrate Si (200) peak. Along with the substrate peaks a pronounced peak
at 41.3° attributed to the diffraction due to <strong>3C</strong>-<strong>SiC</strong> (200) planes was observed.
For both samples, the peak at about 36° attributed to the <strong>3C</strong>-<strong>SiC</strong> (111) plane did
not appeared. From the spectra it could be concluded that the layers were grown
epitaxially with preferred orientation and highly aligned with that of the
substrates.
4.2.2. Rocking curve
Figure 5. XRD spectra for <strong>3C</strong>-<strong>SiC</strong> grown on Si (001)
The XRD rocking curve data of the 10 µm thick <strong>3C</strong>-<strong>SiC</strong> epitaxial film grown at a
rate of about 30 µm/h shows that the full width at half maximum (FWHM) of the
(200) peak was 300 arcsec.
The <strong>growth</strong> of <strong>3C</strong>-<strong>SiC</strong> thin films on Si(001), Si(011) and Si(111) substrates using
silane and propane precursors in LPCVD (400 Torr) at carbonization
temperatures 1135°C and <strong>growth</strong> temperature 1380°C was investigated.
Different from APCVD technique, the LPCVD method produces homogeneous films
with a smoother surface but thinner films (i.e., lower <strong>growth</strong> rate). The rocking
curve data shows a narrower FWMH and therefore a better quality is obtained
when a LPCVD process is used, except for the films grown on Si (110) substrate.
4.3. Transmission electron microscopy (TEM)
A plane view TEM micrograph (Figure 6) from the <strong>3C</strong>-<strong>SiC</strong> film shows the presence
of the stacking faults and the anti-phase boundaries.
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Figure 6. Plane view TEM micrograph
The cross-section TEM shows excellent quality and perfect alignment of films with
substrate. Micro-twins and stacking faults are typical defects for <strong>3C</strong>-<strong>SiC</strong>/Si
heteroepitaxy system.
Figure 7. Cross-section TEM (X-TEM) micrographs showing the <strong>SiC</strong>/Si interface: a) Low
Pressure (LP) process, b) Atmospheric Pressure (LP) process on Si (100)
substrate and the associated selected area electron diffraction images.
The thickness affects strongly the size of the defect-free domains. As Figure 7.b
shows, in the APCVD carbonization, a higher density of defects in initial stage of
<strong>growth</strong> was observed. The stacking faults (SFs) along the (111) directions
detected (by plan view) on the surface with a characteristic cross-shaped
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diffraction of the (200) <strong>3C</strong>-<strong>SiC</strong> spots. The micro-twins appear in the initial stage
of <strong>growth</strong> leading to the exposure of the (111) plane on the surface during
<strong>growth</strong>.
4.4. Low Temperature photoluminescence (LTPL)
The LTPL measurements were performed on different layers grown with different
<strong>growth</strong> rate. The samples were irradiated with 40 mW He-Cd laser (325nm) at 2K.
The <strong>3C</strong>-<strong>SiC</strong> penetration is 2,9 µm. In Figure 9 we see a global picture of the <strong>3C</strong>-
<strong>SiC</strong> LTPL spectrum. It shows that the nitrogen no-phonon line (N0) at 5210A° is
absent. The peak 2 is an intrinsic defect whose no-phonon line is at 5373A°. TA,
LA, TO, LO are the first order phonon replicas of N0.
5. Summary
Figure 9. 2K spectrum of the <strong>3C</strong>-<strong>SiC</strong> film grown by CVD
A process suitable to produce high-quality <strong>3C</strong>-<strong>SiC</strong> hetero-epitaxial films of single
crystal morphology has been developed in a hot-wall CVD reactor. Epitaxial film
deposition on planar (100) Si substrates was performed at a <strong>growth</strong> rate ranging
between 15 and 30 µm/h.
The density of different defects still needs to be reduced in order to realize
electronic devices. The carbonization step and compliant substrates are under
investigation. Finally, <strong>3C</strong>-<strong>SiC</strong> membranes for MEMS applications are under
development.
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7. References
[1] E. Polychroniadis, M. Syväjärvi, R. Yakimova, J. Stoemenos.
Microstructural characterization of very thick freestanding <strong>3C</strong>-<strong>SiC</strong>
wafers. Journal of Crystal Growth 263 (2004), p. 68-75
[2] J.A. Powell, D.J. Jarkin, L.G. Matus, W.J. Choyke, J.L. Bradshaw, L.
Henderson, M. Yoganathan, J. Yang, P. Pirouz. Growth of improved
quality <strong>3C</strong>-<strong>SiC</strong> films on 6H-<strong>SiC</strong> substrates. Applied Physics Letters 56
(1990), p. 1353
[3] S. Nishino, Y. Hazuki, H. Matsunami, T. Tanaka. Chemical Vapor
Deposition of Single Crystalline β-<strong>SiC</strong> Films on Silicon Substrate with
Sputtered <strong>SiC</strong> Intermediate Layer. Journal of Electrochemical Society
127 (1980), p. 2674
[4] H. Matsunami, S. Nishino, T. Tanaka. Heteroepitaxial <strong>growth</strong> of β-<strong>SiC</strong> on
silicon substrate using <strong>SiC</strong>l4-C3H8-H2 system. Journal of Crystal
Growth 45 (1978), p. 138.
[5] S. Nishino, J.A. Powell, H.A. Will. Production of Large-area Singlecrystal
Wafers of Cubic <strong>SiC</strong> for Semiconductors. Applied Physics Letters
42 (1983), p. 460-462.
[6] C.A. Zorman, A.J. Fleischman, A.S. Dewa, M. Mehregany, C. Jacob, S.
Nishino, P. Pirouz. Epitaxial <strong>growth</strong> of <strong>3C</strong>-<strong>SiC</strong> films on 4 in. diam (100)
silicon wafers by atmospheric pressure chemical vapor deposition.
Journal of Applied Physics 78 (1995), p. 5136
[7] K. Ikoma, M. Yamanaka, H. Yamaguchi, Y. Shichi. Heteroepitaxial Growth
of β-<strong>SiC</strong> on Si(111) by CVD using a CH<strong>3C</strong>l-SiH4-H2 Gas System.
Journal of Electrochemical Society 138 (1991), p. 3028
[8] H. Nagasawa, K. Yagi, T. Kawahara. <strong>3C</strong>-<strong>SiC</strong> Hetero-epitaxial <strong>growth</strong> on
undulant Si(001) substrate. Journal of Crystal Growth 237 (2002), p.
1244
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