Growth of 3C-SiC on Si(111) using the four-step non-cooling process

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Growth of 3C

–SiC on Si(111) using the four-step non-cooling process

Jui-Min Liu

a

_{, Wei-Yu Chen}

a

_{, J. Hwang}

a,

⁎

_{, C.-F. Huang}

b

_{, Wei-Lin Wang}

c

_{, Li Chang}

c

a

Department of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu City, Taiwan, ROC

b

Department of Electrical Engineering, National Tsing Hua University, Hsin-Chu City, Taiwan, ROC

c_{Department of Materials Science and Engineering, National Chiao Tung University, Hsin-Chu City, Taiwan, ROC}

a b s t r a c t

a r t i c l e i n f o

Available online 12 May 2010 Keywords:

Silicon carbide

Low pressure chemical vapor deposition X-ray diffraction

Crystal microstructure

A modified four-step method was applied to grow a 3C–SiC thin film of high quality on the off-axis 1.5° Si(111) substrate in a mixed gas of C3H8, SiH4and H2using low pressure chemical vapor deposition. The modified

four-step method adds a diffusion four-step after the carburization four-step and removes the cooling from the traditional three-step method (clean, carburization, and growth). The X-ray intensity of the 3C–SiC(111) peak is enhanced from 5 × 104_{counts/s (the modi}

ﬁed three steps) to 1.1×105_{counts/s (the modi}

fied four steps). The better crystal quality of 3C–SiC is confirmed by the X-ray rocking curves of 3C–SiC(111). 3C–SiC is epitaxially grown on Si(111) supported by the selected area electron diffraction patterns taken at the 3C–SiC/Si(111) interface. Some {111} stacking faults and twins appear inside the 3C–SiC, which may result from the stress induced in the 3C–SiC thin film due to lattice mismatch. The diffusion step plays roles in enhancing the formation of Si–C bonds and in reducing the void density at the 3C–SiC/Si(111) interface.

1. Introduction

The growth of 3C–SiC on either Si(100) or Si(111) has attracted much attention due to both fundamental and practical reasons. The physical properties of SiC such as high thermal conductivity, high breakdownﬁeld and high saturation velocity[1,2]increase the potential for the applications of high power and high frequency electronic devices

[3]. The drives to grow 3C–SiC on Si(100) or Si(111) in the past decades are due to lower production cost.

The major breakthrough of the growth of 3C–SiC on Si was achieved by Nishino and Powell in 1983[4]. The difficulties of large lattice mismatch (~20%) and large difference in thermal expansion coefficients (~8%) between Si and SiC were partially overcome by introducing a carburization process. The carburization process is inserted before the growth of SiC using a three-step method (clean, carburization and growth). A carbon containing gas can be decomposed and to deposit amorphous carbon on Si at the carburization step such that an ultra-thin SiCfilm can be formed for further growth of SiC at the growth step. However, there still exist defects such as dislocations, twins, stacking faults and anti-phase boundaries near the 3C–SiC/Si interface[5–9].

The concept of carburization has been widely used by other research groups since then[10–14]. In the conventional three-step method, the full process time is long since it is required to cool the samples back to room temperature or lower temperature (e.g. ~500 °C) between each step in order to obtain good crystal quality. In 2008, Chen et al.

devel-oped the modiﬁed four-step method (clean, carburization, diffusion and growth) by inserting a diffusion step and removing the cooling step in the conventional three-step method[15]. The temperature versus time for the modiﬁed four-step method is sketched inFig. 1. The diffusion step, after the carburization step, is performed at 1350 °C such that the formation of SiC in the as-carburized layer on Si(100) is enhanced in a short time. The quick formation of an ultra-thin SiC layer on top of Si (100) enables the formation of void-free 3C–SiC/Si(100) interface[10]. A better crystal quality of 3C–SiC can be grown on Si(100) in a mixed gas of propane (C3H8), silane (SiH4) and hydrogen (H2), supported by the X-ray rocking curve data.

In this article, we attempt to apply the modiﬁed four-step method to grow 3C_{–SiC on Si(111). The optimum experimental parameters are} different from those used for the growth of 3C–SiC on Si(100) since the atomic arrangements in Si(111) and Si(100) are different[15]. The crystal quality of 3C_{–SiC on Si(111) is characterized. The effect of} diffusion step on the growth of 3C–SiC on Si(111) is emphasized. 2. Experimental details

The experimental procedures for the growth of 3C–SiC(111) on Si (111) are similar to those used in our previous work[15] and are described in the following. 3C–SiC ﬁlms were grown on off-axis 1.5° p-type Si(111) substrates by low pressure chemical vapor deposition (LPCVD). The LPCVD system is a horizontal type of a cold wall quartz tube that has been illustrated by other groups[16_–18]. The Si(111) substrates were etched in a solution of 1% HF for 1 min in order to remove the native oxide and then rinsed with deionized water. The as-rinsed Si(111) substrates were put on a SiC coated graphite susceptor in

Thin Solid Films 518 (2010) 5700–5703

⁎ Corresponding author. 101, Sec. II, Kuang-Fu Road, Hsin-Chu, Taiwan, ROC. Tel.: + 886 3 5722577.

E-mail address:jch@mx.nthu.edu.tw(J. Hwang).

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the reactor. A mixed gas of C3H8(99.99%) and SiH4(5% diluted in H2) was carried by H2(99.999%) of 1 slm into the LPCVD system for the growth of 3C–SiC ﬁlms at a base pressure of ~0.266 Pa. The substrate

temperature was ramped up at the rate of 11 °C/s, monitored by either a thermocouple or an infrared pyrometer. The optimum experimental parameters of the modi_{ﬁed four-step process for the growth of high} quality of 3C–SiC ﬁlms on Si(111) are tabulated in Table 1. The concentration of C3H8in the carburization step and diffusion step are 1% and 0.3%, respectively. The carburization temperature should be high enough to dissociate C3H8. The optimum carburization temperature is 1250 °C, in such a condition the voids at the 3C–SiC/Si(111) interface can be minimized. The role of diffusion step is to transform the as-carburized layer fully into SiC by transporting Si atoms out of the Si(111) substrate at high temperature. The optimum diffusion temperature is 1350 °C.

The crystal qualities of 3C–SiC ﬁlms were characterized using a Rigaku D/MAX2000 X-ray diffractometer (XRD) with Cu Kα radiation (1.545 Å ) (operation voltage: 30 kV, currents: 20 mA) and a Philips TECNAI 20 Transmission Electron Microscope (TEM) (operation voltage: 200 kV). The 3C–SiC/Si(111) cross-sectional samples for TEM studies were prepared using a focused ion beam facility (FIB; FEI NOVA 200). The Ga+_{ions in FIB are accelerated and incident onto the 3C}

–SiC/Si (111) sample at various angles in preparation for TEM cross-sectional samples. The surface morphologies of 3C–SiC films were investigated by a JEOL JSM-6500F Field-Emission Scanning Electron Microscope (FESEM) operated at 15 kV. An X-ray photoemission spectroscopy (XPS; PHI ESCA 1600) was used to characterize the bonding character-istics of the grown 3C–SiC films. All the samples were slightly sputtered to clean their surfaces by Ar+_{ions before XPS measurements. C 1s} spectra are plotted in a binding energy scale, which were excited using Mg Kα (1253.6 eV; 0.3A/cm2_{). The}_{fitting software PeakFit was used to} de-convolute the C 1s spectra using a Voigt function that is a Gaussian

Table 1

Optimum experimental parameters for the growth of high quality 3C–SiC ﬁlms on Si (111) using the modiﬁed four-step process.

Clean Carburization Diffusion Growth

H2(sccm) 1000 1000 1000 1000 C3H8(sccm) 0 11 0 3 SiH4(sccm) 0 0 0 20 Temperature (°C) 900 1250 1350 1395 Pressure (Torr) 10 2 2 0.8 Time (min) 5 1.5 5 30

Fig. 1. Schematic showing the four-step (clean, carburization, diffusion, and growth) process for the growth of 3C–SiC on the 1.5° off-axis Si (111) towards [11̅0].

Fig. 2. (a) X-ray diffraction patterns of the 3C–SiC grown on the 1.5° off-axis Si(111) towards [11̅0]. The prominent peak at 2θ=35.7° is diffracted from 3C–SiC (111). The X-ray rocking curve of 3C–SiC(111) is inserted in the upper-right corner. (b) Bright ﬁeld image of the 3C–SiC/Si(111). There exist crystal defects such as stacking faults (SFs) and twins in the SiC layer near the interface. (c) The selective area electron diffraction (SAED) patterns of SiCﬁlm along the zone axis [11̅0]. (d) The selective area electron diffraction (SAED) patterns of the 3C–SiC/Si(111) along the zone axis [11̅0].

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function convoluted with a Lorentzian function. The background func-tion was represented in a power-law form. The C 1s peak widths of de-convoluted components are constrained to be the same in the peak_ﬁt analysis.

3. Results and discussion

Fig. 2a shows the XRD patterns of 3C–SiC films grown on the off-axis 1.5° Si(111) substrates using the three-step (without diffusion) and the modified four-step (with diffusion) methods. The three-step method is a modified three-step one consisting of three steps (clean, carburization and growth) without cooling between each step that is used for comparison. The diffracted peak at 2_{θ=35.7° is from the} (111) plane of 3C–SiC according to the database of joint committee on powder diffraction standards (JCPDS). No other peaks corresponding to the (200) or (220) planes of 3C_{–SiC are detected, indicating that the} as-grown 3C–SiC films are in either epitaxial (single crystalline) or highly oriented (textured) form[19]. The crystal quality of 3C–SiC is improved by adding the diffusion step since the (111) peak intensity of 3C–SiC increases from 5×104_{counts/s (the modi}_{fied three steps) to} 1.1 × 105_{counts/s (the modi}_{fied four steps). The improvement of 3C–} SiC crystal quality is further confirmed by the X-ray rocking curve of the (111) peak of 3C–SiC as shown in the inset ofFig. 2a. The full width at half maximum (FWHM) of 3C–SiC(111) reduces from 0.89 to 0.78 when the diffusion step is added. This supports that the crystal quality of 3C–SiC is improved by adding the diffusion step. In comparison with 3C–SiC film grown on Si(100) where the intensity of (200) plane at 2θ=41.5° is about 3×104_counts/s_[15]_{, the intensity of (111) plane of} 3C_{–SiC grown on Si(111) is higher by about 2.7 times. This implies that} the crystal quality of 3C–SiC on Si(111) is better than that on Si(100). The crystal quality of 3C–SiC on Si(111) is further revealed in the bright_{field image taken at the 3C–SiC/Si(111) interface shown in}

Fig. 2b. There exist crystal defects such as stacking faults and twins in the 3C–SiC layer[6,8]. The selective area electron diffraction (SAED) pattern inFig. 2c was taken from the 3C–SiC along the zone axis [11̅0]. Two pairs ofb111N type SAED spots are indexed to be twins of 3C–SiC. This is in good agreement with the observation of the twin image marked in the brightﬁeld image inFig. 2b. The crystal orientations of SiC and Si at the interface can be revealed from the SAED pattern taken at the 3C–SiC/Si(111) interface along the zone axis [11̅0]. The SAED spots in the SAED pattern are indexed inFig. 2d, where the [111] (or [002]) spots of 3C_{–SiC are in parallel with those of Si. This supports that} 3C–SiC[111] is lined up with Si[111]. In other words, 3C–SiC is epitaxially grown on Si(111). Note that some weak SAED spots arising

from stacking faults (SFs) and twins marked with _{“A” and “B,”}

respectively, also appear in Fig. 2d [9]. This agrees well with the observation of stacking faults and twins in the brightﬁeld image in

Fig. 2b. The existence of twins and stacking faults implies the hetero-epitaxial layer of 3C–SiC on Si(111) is under stress due to large mismatch at the interface[20,21]. This can also explain why the crystal quality of 3C–SiC becomes better far away from the interface according to the brightﬁeld image.

The chemical bonding information in the carburized and as-diffused 3C–SiC layers can be extracted from the curve-ﬁts of C 1s XPS spectra inFig. 3a and b. Similar to the work of 3C–SiC on Si(100)[15], the C 1s curve of the as-carburized Si (111) (after the 2nd step) in

Fig. 3a exhibits a prominent C–C and a Si–C component approximately at a binding energy of 284.5 eV and 283.2 eV, respectively. This indicates that not all the C–C bonds are transformed into Si–C bonds in the as-carburized layer at 1250 °C for 90 s during the carburization step. With the aid of the diffusion step at 1350 °C for 5 min, the C–C bonds greatly reduce associated with the increase of Si–C bonds, as clearly supported by the curve-ﬁt result inFig. 3b. Considering the SAED patterns taken from 3C–SiC and the 3C–SiC/Si(111) interface in

Fig. 2c and d, both SiC is in a form of crystal. No crystalline structures other than SiC were detected in the SAED patterns. The C–C bonds

extracted from the curve-ﬁt in Fig. 3thus result from the residual carbon in a non-crystalline form, in good agreement with the results of Sun et al.[22]. The crystal quality of SiC in the as-diffused layer enables the growth of a SiCﬁlm of good crystal quality during the growth step (4th step). This is supported by the higher intensity and smaller FWHM of the 3C–SiC(111) peak inFig. 2a.

The main difference in the growth of 3C–SiC on Si(111) and Si(100) is the easier formation of voids at the 3C_{–SiC/Si(111) interface. It has} been proven that diffusion of Si atoms out of Si(111) occurs at a temperature higher than a critical temperature (approximately 850 °C)

[13,23,24]. The carburization step is kept at approximately 1250 °C such that C3H8can be decomposed and a large amount of carbon can be deposited onto Si(111) in order to form an ultra-thin SiC layer. The formation of voids at the 3C–SiC/Si(111) interface has been attributed to the out-diffusion of Si atoms during the formation of the ultra-thin SiC layer at the carburization step[10,23–25].Fig. 4a and b are the SEM images showing the voids at the 3C–SiC/Si(111) interfaces prepared by the three-step (without diffusion) and the modiﬁed four-step (with diffusion) methods, respectively. Voids of 0.2–0.5 μm in width and ~0.2μm in depth reside at both interfaces as shown in the insets in

Fig. 4a and b. The density of voids becomes less at the 3C–SiC/Si(111) interface prepared by the modiﬁed four-step method. This indicates that the diffusion step can reduce the density of voids. The void formation has been attributed to the out-diffusion of Si atoms at the carburization step in previous publications[10,23–25]. If the void formation were completed at the carburization step, the diffusion step at higher temperature (1350 °C) would not affect the density of voids. However, it is not the case. The reduction of void density in the modiﬁed four-step method can be well explained by the incomplete void formation at the

Fig. 3. C 1s XPS spectra of (a) as-carburized Si(111) (after the 2nd step) and (b) as-diffused 3C–SiC/Si(111) (after the 3rd step).

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carburization step. In the three-step method, voids continue to form at the 3C–SiC/Si(111) interface during the growth step at 1395 °C. The diffusion step, in the modiﬁed four-step method, plays a major role in enhancing the formation of Si–C bonds and also a minor role in reducing the void formation due to lower temperature (1350 °C). The argument above and the data inFig. 4b thus support that void formation is not completed at the carburization step. The temperature at the diffusion step is lower than that at the growth step, which is the physical reason for the reduction of density of voids.

4. Conclusions

A high quality of 3C_{–SiC can be grown on Si(111) using the}

modiﬁed four-step method in LPCVD. The optimum experimental

parameters for the growth of 3C–SiC on Si(111) are different from those on Si(100). The diffusion step followed by the carburization step can transform most residual C–C bonds into the Si–C bonds in the as-carburized layer. The formation of Si–C bonds helps the epitaxial growth of 3C–SiC on Si(111) at the growth step. The diffusion step also reduces the void density at the 3C–SiC/Si(111) interface. However, stacking faults and twins are generated near the 3C–SiC/Si(111) interface due to large lattice mismatch.

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