New Trends in Wide Bandgap Semiconductors: Synthesis of Single Crystalline Silicon Carbide Layers by Low Pressure Chemical Vapor

Deposition Technique on P-Type Silicon (100 and/or 111) and their Characterization

F. Iqbal1, A. Ali1, A. Mehmood2, M. Yasin2, A. Raja3, A.S. Gerges3, S. Baang4, M. Asghar1,a and M.A. Hasan3

1 Semiconductor Physics Laboratory, The Islamia University of Bahawalpur 63100, Pakistan 2 NILOP, Pakistan Institute of Engineering and Applied Sciences, Islamabad, Pakistan

3 Department of Electrical and Computer Engineering and the Center for Optoelectronic and Optical Communications, University of North Carolina Charlotte, NC 28223

4 Department of Electronics Engineering, Hallym University, Chunchon 200-702, Korea aasghar_hashmi@yahoo.com

Keywords: X-ray diffraction; EDS; FTIR; Low pressure chemical vapor depostion; Atomic force microscopy; Scanning electron microscopy

Abstract. We report the growth of SiC layers on low cost p-type silicon (100 and/or 111) substrates maintained at constant temperature (1050 - 1350oC, ∆T=50oC) in a low pressure chemical vapor deposition reactor. Typical Fourier transform infrared spectrum showed a dominant peak at 800 cm-

1 due to Si-C bond excitation. Large area x-ray diffraction spectra revealed single crystalline cubic structures of 3C-SiC(111) and 3C-SiC(200) on Si(111) and Si(100) substrates, respectively. Cross-sectional views exposed by scanning electron microscopy display upto 104 µm thick SiC layer. Energy dispersive spectroscopy of the layers demonstrated stiochiometric growth of SiC. Surface roughness and morphology of the films were also checked with the help of atomic force microscopy. Resistivity of the as-grown layers increases with increasing substrate temperature due to decrease of isolated intrinsic defects such as silicon and/or carbon vacanies having activation energy 0.59 ±0.02 eV.

Introduction

Attractive properties of silicon carbide (SiC) have outclassed silicon (Si) and gallium arsenide (GaAs) in many applications and it is considered now as the material of choice for future electronic devices operating under harsh environment [1-3]. SiC based devices can operate under high temperature, high power, high radiation and aggressive environment [4-8]. Some other properties of SiC-devices include high electron velocity, high thermal conductivity and high breakdown electric field ~ 5000V [9, 10]. Most of these applications have been achieved from hexagonal 4H- and 6H-polytypes of SiC, yet, its high cost limits large-scale commercialization of this material [11]. One economical approach is to grow 3C-SiC on Si substrate [12-14]. Due to the smaller bandgap (2.2eV), 3C-SiC has many advantages, especially for metal oxide semiconductor (MOS) device applications over the other polytypes. In addition, the electron Hall mobility is isotropic and higher, compared with those of 4H- and 6H-polytypes, hence there is significant interest for synthesis of low-cost, large-size 3C-SiC wafers for microelectronic applications. Nevertheless, due to large differences in lattice constant (20%) and thermal expansion coefficient (8%) of the two materials, heteroepitaxy of SiC on Si substrate are reported to have high density of crystallographic structural defects such as stacking faults, microtwins, and inversion domain boundaries [15-18], however, the recent reports hint improvement of crystal quality by varying growth parameters such as substrate orientation and growth temperature etc, therefore the subject has got more interest amongst the researchers [19,20].

Key Engineering Materials Vol. 442 (2010) pp 195-201© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.442.195

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of thepublisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 152.15.21.40-24/03/10,14:35:02)

In this paper, we report the growth of crystalline 3C-SiC on various p-type templates including Si(100), Si(111) and porous-Si(111) by low pressure chemical vapor deposition technique (LP-CVD) using single gas source, trimethylsilane. A variety of characterization techniques have been employed to investigate the lattice structure, elemental composition (%age), film thickness, surface roughness, growth rate and resistivity of the as-grown layers of 3C-SiC. The study reveals that substrate orientation and quality have significant role on the crystal structure and lattice properties of the epilayers. Scheme of the paper is following: section 2 describes experimental details, results with discussion are explained in section 3 and conclusions are summarized in section 4.

Experimental details

SiC layers were grown on three different substrates of p-type silicon wafers: Si(100), Si(111) and porous Si(111), all of the substrates have resistivity (1-20 Ω-cm) and free carrier concentration, NA = 5.0×1017cm-3. The porous Si substrate was prepared by anodization method [21]. The growth of SiC layers was carried out in a low pressure chemical vapor deposition (CVD) reactor fitted with an RF heating source. Briefly, preparation scheme is described in the following: a piece of silicon wafer (1.5 × 2 cm2) was placed on the cylindrically shaped graphite susceptor in the CVD chamber. The chamber was pumped by a combination of a rotary and a turbo molecular pumps. Once the pressure inside the chamber approached ~ 10-6 torr, the graphite susceptor was heated by an RF-coil operated by DC power supply, and the temperature (1050 – 1350oC, ∆T = 50oC) was measured by infrared pyrometer. Next, precursor gas, trimethylsilane SiH(CH3)3 is allowed to enter using micrvalve into the chamber so that the chamber pressure was maintained at ~ 1×10-3 torr and thus growth started. A variety of techniques have been performed to ascertain the lattice properties of the as-grown films. Presence of Si-C bonds was established by FTIR using Thermo Nicolet Nexus 870 FTIR while crystal structure, composition, surface morphology, film thickness and surface roughness of the films were studied using x-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM), by means of Panalytical X'Pert Pro MRD x-ray diffractometer with CuKα radiation source, JEOL JSM-5910LV and Veeco SPM 3000 respectively. Resistivity measurement was performed by four probe collinear arrangement.

Results and Discussion

FTIR is a non-destructive optical technique, which gives reliable information about the chemical bonding, elemental composition in the material and its quality. Fig. 1 demonstrates the representative FTIR spectra (transmission mode) of SiC layers grown on three templates of p-type Si defined as Si(100), Si(111) and porous Si(111) (hereafter referred as A, B and C substrates). In all FTIR curves, an excitation at frequency appearing around 800 cm-1 is attributed to Si-C bond vibration [19] however, the same excitation shows up as broadened peak in the layers grown on B substrate but the excitation reaches its cut off limits for the layer grown on substrate C. Such cut off limits indicate that the thickness of the layer is beyond the measuring limit (which should be less than few hundred of nanometres) of the FTIR set up. Fig. 2 displays typical XRD spectra of SiC layers altogether with the responses from their substrates A, B and C (c.f. curves a-c respectively), where excluding Si peaks, we can see that the dominant peaks are in fact dependent on substrate orientation i.e. 3C-SiC(200) and 3C-SiC(111) grow on Si(100) and [Si(111) and porous Si(111)], respectively.

196 Advanced Materials XI

The intensities related to 3C-SiC layers in Fig. 2 correspond with the thickness of the respective layers; however, the absolute intensity of the peak is dependent on several parameters like growth time, substrate temperature, gas flow rate and chamber pressure therefore, the true picture will come

Fig. 1 Representative FTIR spectrs (transmission) of 3C-SiC on different templates: Si(100), Si(111) and porous Si(111) maintained at

substrate temperature of 1150, 1250 and 1050 oC, curves a-c, respectively.

Fig. 2 Typical XRD measurements of 3C-SiC layers grown on Si(100), Si (111) and porous Si (111) substrates (a-c),

respectively. Dominant XRD peaks of 3C-SiC are clearly visible therein.

Key Engineering Materials Vol. 442 197

out with the growth rate data, to be described elsewhere. It is also observed that 3C-SiC grown on substrate A has single crystalline structure and those on B and C substrates have polycrystalline-like structures because minor peaks associated with and [3C-SiC (220) and 3C-SiC (311)] and 3C-SiC (321) are seen on B and C substrates, respectively (c.f. curves b, c). Furthermore, two diffraction peaks from 8H-SiC (0010) and 8H-SiC (0013) also appear in XRD spectrum of the layer grown on B substrate (c.f. curve b), the reason will be described in the future correspondence.

Table 1 Details of useful lattice parameters of as-grown layers of 3C-SiC grown on various Si substrates by low pressure chemical vapor deposition technique. Deviation % with respect to the

theoretical value (4.3596Ǻ) of lattice constant of 3C-SiC is also depicted.

Substrate Peak I.D TSubstrate (oC) Crystallite Size (nm) ±5 Lattice Constant

(Å)

Si(100) 3C-SiC(200) 1150 59 4.3517±0.18%

Si(111)

Porous Si(111)

3C-SiC(400)

3C-SiC(111) 3C-SiC(220) 3C-SiC(311) 3C-SiC(222)

3C-SiC(111) 3C-SiC(321)

1250

1050

20

44 25 29 33

33 39

4.3448±0.34%

4.3710±0.26% 4.3564±0.07% 4.3568±0.06% 4.3603±0.01%

4.3687±0.21% 4.3805±0.47%

To explain the polycrystalline structure of SiC on substrates B and C, we refer to their (substrates) XRD spectra, where we can clearly see that either of the substrates no longer has single crystalline structure because of exhibiting a peak Si(221) and/or Si(222) –a disallowed plane in Si, as shown in the curves (b, c). We therefore, understand that the appearance of additional peaks of SiC is in fact, due to substrate orientation and/or surface quality in a similar perception as SiC(200) and SiC(111) peaks appear from Si(100) and Si(111) substrates, respectively. In literature, similar results regarding the quality of 3C-SiC/Si(100) and /Si(111) are found [20]. XRD measurements yield a number of lattice parameters of the tested samples such as Miller indices, crystallite size, lattice constant and etc., a few of them are listed in the Table 1 to support our arguments. Fig. 3 shows the cross sectional view of the SEM images of SiC layers grown on C substrate (upper), where we can see about 104µm thick 3C-SiC layer on porous-Si(111). The compositional ratio by atomic (%age) was worked out by using EDS mode of SEM set up (see lower part of Fig. 3). EDS data reveal that 3C-SiC layer grown on C substrate has Si:C At% as 44:56, this means about 6% Si sites have either been occupied by carbon (C) ions or left vacant and for the layer grown on porous-Si(111) substrate. The excess of vacancies and/or antisites contribute in the conductivity of the material and hence the resistivity of the material is affected, detail comes later. However, SiC grown on B substrate has the Si:C At% ratio 51:49, which is nearly a stoichiometric growth (not shown here). Fig. 4 is an AFM image that demonstrates the presence of voids in the 3C-SiC surface grown on substrate B. The structure of the void is strikingly congruent with the top view of the 3C-SiC unit cell [22]. This observation confirms our XRD results, described above.

198 Advanced Materials XI

Fig. 3 SEM micrographs representing the cross sectional view (upper) and EDAX measurements of

3C-SiC on Si(111) substrates (lower).

Fig.4 AFM image of 3C-SiC/Si (111) indicating the presence of voids/vacancies which are expected

to contribute towards the conductivity of the sample. Surface roughness (RMS) is estimated as 11.615 nm.

Fig. 5 illustrates the growth rate data of as-grown SiC layers as a function of substrate temperature, keeping chamber pressure constant (10-3 torr). The data indicate that growth rate of the SiC is lower if Tsubstrate5is less than 1200oC however; it increases with the polynomial of order 4 as given below.

410362 1069.51062.2005.046.334.993 TTTTY −− ×+×−+−= (1)

Element Wt% At% C 34.24 55.91 Si 65.76 44.09

Key Engineering Materials Vol. 442 199

The reason of this behavior is understandably linked with the adsorption and desorption rates occurring at the surface of the substrate. Briefly, the trimethylesilane (TMS) molecule adsorbs with the hot Si wafer surface and breaks into Si, C and H ions due to available (enough) thermal energy at the substrate, as a result, 3C-SiC molecule is built up and H ion is desorbed, otherwise larger amount of TMS or even Si-H or CH3 molecules could be desorbed. Under the scenario, it seems that Tsubstrate ≤ 1200oC is not enough to break the larger number of TMS molecules, and hence they are desorbed and low growth rate of 3C-SiC is achieved. Fig. 6 describes the resistivity data of as-grown 3C-SiC layers deposited at different substrate temperatures (1050 – 1350oC). It is observed that the resistivity decreases exponentially with the inverse of the Tsubstrate, the reason could be the increase of free carriers originated from the isolated intrinsic defects in the form of silicon and/or carbon vacancies. This result is in agreement with the theoretical explanation of the resistivity as well [23]. To strengthen this observation, we plotted the resistivity data on semilog scale as a function 1000/T shown as inset in Fig. 6, (Arrhenius plot), the activation energy calculated from the slope of the linear fit is found to be 0.59± 0.02eV, which is reported as the activation energy of carbon vacnacy [24,25], this result is consistent with our argument described earlier.

Fig. 5 Normalized growth rate of 3C-SiC as a function of substrate temperature. The

theoretical fit formula of the experimental data is displayed therein, which exhibits polynomial

of power 4 described in equation 1.

Fig. 6 Typical exponential behavior of resistivity of the as-grown 3C-SiC layers as a function of substrate temperature (1050–1350oC), here the inset displays the Arrhenius plot between log

(resistivity) versus 1000/T.

Conclusion

Stoichiometric 3C-SiC upto 104µm has been grown on low cost p-type Si substrate using (LP-CVD) technique. Three substrates of p-type Si having orientation along (100) and (111) and porous- (111) were used for the synthesis of SiC layers. XRD spectra due to the as-grown layers clearly demonstrated the dominant peaks of 3C-SiC, however, the phases of the 3C-SiC were dependent upon the substrate orientation. As a result, the single crystalline substrate (i.e Si(100)) helped in the growth of single crystalline 3C-SiC(200) on Si(100) and polycrystalline substrates (Si(111), Si(221) and Si(222)) assisted in the growth of 3C-SiC(111), 3C-SiC(220), 3C-SiC(321) and etc. The growth rate data reveal that the thicker (bulk) layer of the 3C-SiC is possible if the substrate temperature is greater than 1200oC and the resistivity data indicate that higher substrate temperature results in good qaulity or compact layers of 3C-SiC.

200 Advanced Materials XI

Acknowledgements

The authors are thankful to Fulbright Organization, USA, USEF-Pakistan for financial assistance, UNCC Charlotte Charlotte Research Institute and Department of Physics & Optical Sciences for research facilities. The authors are also grateful to Mr. John Hudak, Mr. Alec Martin, Dr. Robert Hudgins and Dr. Lou Duggezzman for technical assistance.

References

[1] G.-S. Chung and K.-S. Kim, Bull. Korean Chem. Soc. Vol. 28 (2007), p. 533. [2] M. Mehregany and C.A. Zorman, Thin Solid Films. Vol. 355 (1999), p. 518. [3] Sarro and P.M. Sen, Actuators A. Vol. 82 (2000), p. 210. [4] X.D. Chen, S. Fung, C.C. Ling, C.D. Beling, M. Gong, J. Appl. Phys. Vol. 94 (2003), p.

3004. [5] M. Reyes, M. Waits, S. Harvey, Y. Shishkin, B. Geil, J.T. Wolan, S.E. Saddow, Mater. Sci.

Forum. Vol. 527-529 (2006), p. 307. [6] A. Gupta, D. Paramanik, S.Varma ,C. Jacob, Bull. Mater. Sci. Vol. 27 (2004), p. 445. [7] A. Itoh, T. Kimoto, H. Matsunami, IEEE Electron Device Lett. Vol. 17 (1996), p. 139. [8] J.Y. Fan, X.L. Wu, H.X. Li, H.W. Liu, G.G. Siu, Paul K. Chu, Appl. Phys. Lett. Vol. 88

(2006), p. 041909. [9] H. Morkoç, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. Vol. 76

(1994), p. 1363. [10] Ch. Förster, V. Cimalla, K. Brückner, M. Hein, J. Pezoldt, O. Amabacher, Mater Sci. and

Engg C. Vol. 25 (2005), p. 804. [11] K. Semmelroth, M. Krieger, G. Pensl, H. Nagasawa, R. Puschl, M. Hundhausen, L. Ley, M.

Nerding, H.P. Strunk, Mater. Sci. Forum. Vol. 457-460 (2004), p. 15. [12] H. Nagasawa and Y. Yamaguchi, Springer Procededing in Physics, Springer-Verlag, Berlin

Vol. 71 (1992), p. 40. [13] A.J. Steckl, C. Yuan, J.P. Li, M.J. Loboda, Appl. Phys. Lett. Vol. 63 (1993), p. 3347. [14] Y.H. Seo, K.S. Nahm, E.K. Suh, H.J. Lee, Y.G. Huang, J. Vac. Sci. Technol. A Vol. 15

(1997), p. 2226. [15] Q. Wahab, A. Ellison, A. Henry, C. Hallin, J.D. Persio, R. Martinez and E. Janén, Appl.

Phys. Lett. Vol. 76 (2000), p. 2725. [16] Masashi Kato, Fumitaka Sobue, Masaya Ichimura, Eisuke Arai, Noboru Yamada, Yutaka

Tokuda and Tsugunori Okumura, Jpn. J. Appl. Phys. Vol. 41 (2001), p. 2983 [17] D. Purser, M. Jenkins, D. Lieu, F. Vaccaro, A. Faik, M.-A. Hasan, H. J. Leamy, C. Carlin,

M. R. Sardela Jr., Qingxiang Zhao, Magnus Willander, and M. Karlsteen, Proc. Inter. Conf on SiC and related Materilas-99 (ICSCRM-99), Oct. 10-15, 1999, Research Triangle Park, North Carolina, USA

[18] M.-A. Hasan, A. Faik, D. Purser, D. Lieu, Inter. Conf on SiC and related Materilas-2001 (ICSCRM-2001), Oct. 28-Nov. 2, 2001, Tsukuba, Japan.

[19] Z.C Feng, Microelectonic Engineering Vol. 83 (2006), p. 165. [20] A. Gupta, J. Sengupta, C. Jacob, Thin Solid Films Vol. 516 (2008), p. 1669. [21] L. N. Aleksandrov and P. L. Novikov, Thin Solid Films, Vol 330 (1998), p. 102. [22] Bechstedt F., Kackell P., Zywietz A., Karch K., Adolph B., Tenelsen K. and Furthmuller J.,

Phys. Stat. Solidi b. Vol. 35 (1997), p. 202. [23] Dieter K. Schroder, “SEMICONDUCTOR MATERIAL AND DEVICE

CHARACTERIZATOIN” 3rd ed. John Wiley & Sons, Inc, 2006. [24] T. Dilibor, G. Pensl, H. Matsunami, T. Kimoto, W.J. Choyke, A. Schöner and N. Nordell,

physs. stat.sol (a) Vol. 162 (1997), p. 199. [25] M. Asghar, I. Hussain, S.H.Noor, F. Iqbal, Q. Wahab and A.S. Bhatti, J. Appl. Phys. Vol.

101 (2007), p.073706.

Key Engineering Materials Vol. 442 201