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Growth of CVD heteroepitaxial diamond on silicon (001) and its electronic properties
Diamond and Related Materials 9 Ž2000. 1626᎐1631
Growth of CVD heteroepitaxial diamond on silicon ž 001/ and its electronic properties U
X.C. Hea, , H.S. Shena , Z.M. Zhanga , X.J. Hua , Y.Z. Wana , T. Shenb a
State Key Lab of MMCM’s, Shanghai Jiaotong Uni¨ ersity, Shanghai 200030, PR China b Hirata Precision Products Co. Ltd, Shanghai 200137, PR China
Abstract Microwave CVD heteroepitaxial diamond film on a 4⬚ off-axis SiŽ100. substrate is obtained by two stages. The first one is to grow oriented 3c-SiC layers on SiŽ100. using a non-toxic and non-inflammable ŽCH3 .6 Si2 NH organic compound carried by hydrogen. The following stage is to grow oriented diamond films on them under the atmosphere of CH4 and H2 . In each stage there are bias and growth processions. The micro-Raman and micro-Auger analyses prove that there is a perfect orientation relationship between the film and substrate as following: diamond ²001:rr3c-SiC²001:rrSi²001:. The Hall effect indicates that the film is a P type, whose resistivity is 9.4= 10y3 ⍀ cm, the Hall coefficient is 2.9 cm3rQ, the hole mobility is 309 cm2rV s and the carrier concentration reaches 2.2= 1018 cmy3. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Microwave CVD; Diamond films heteroepitaxy; 3c-SiC; Interface
1. Introduction The heteroepitaxial diamond chemical vapor deposition ŽCVD. is a process of considerable scientific and technological importance. Many applications of diamond, especially in the area of electronic devices are awaiting the larger area and higher quality heteroepitaxy of diamond, further enhancement of which is needed for the achievement of comparable electronic properties to those observed in natural diamond. A major goal of diamond technology is to achieve good nucleation and heteroepitaxial growth on a low cost substrate such as Si. However, its very poor intrinsic nucleation density on Si is due to: Ž1. the large surface energy of diamond Ži.e. 5.3᎐9.2 Jrm2 for diamond, 1.4s 6 Jrm2 for silicon, and 3.0 Jrm2 for 3c-SiC.; Ž2. the large lattice mismatch of 1:1.52 of diamond and Si, which prevents diamond from forming oriented, two-dimensional nuclei on a foreign substrate surface; Ž3. the low mobility of growth species w1x. U
Corresponding author.
For practical use, the deposition of diamond with high crystalline quality onto substrate candidates of silicon and silicon carbide has been widely investigated. A novel technique, bias enhanced nucleation, has been proved to be extraordinarily useful for the in situ promotion of diamond or 3c-SiC nucleation and enabled the growth of heteroepitaxial diamond film on them. However, the mechanism of bias-enhanced nucleation and the nature of the interface between the heteroepitaxial diamond film and substrate are not well understood to date. Numerous studies about the nucleation process imply the importance of the formation of an amorphous carbon and SiC layer prior to and during the nucleation w2᎐4x. Presently, the diamond heteroepitaxial growth has been successful on Si or SiC by negative bias pretreatment, but there is a lot of defects in the film due to the mis-orientation and lattice mismatch between diamond film and substrate. This limits an approach to the electronic applications of diamond films. To achieve the device quality, diamond should satisfy the following conditions:
0925-9635r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 3 1 7 - 4
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
1. atomically flat surface, which improves junction properties of each contact and allows high integration of these devices; and 2. low defect density and low residual impurities which ensure the designed device operation w5x. Since the larger lattice mismatch between diamond and silicon and the fact that SiC is easily formed as an interface layer during CVD diamond growth on Si, it has been suggested that epitaxial diamond growth occurs on a cubic silicon carbide interlayer w6x. In a common way, the bias-enhanced nucleation ŽBEN. method is usually used in a microwave plasma-assisted CVD system. The BEN method consists of two steps. The first step is carbonization, in which single crystals of 3c-SiC, which plays a role as a buffer layer between Si and diamond thin film, has grown on Si substrate with gaseous hydrocarbons at elevated temperatures is essential for growth of epitaxial single crystal 3c-SiC w7x. The second step is negative bias treatment, in which diamond has been nucleated on the 3c-SiC layer. After the BEN method, diamond growth has been carried out w8x. However, there exist both lattice mismatch and the mis-orientation since the larger lattice mismatch of 3c-SiC and Si Žapprox. 20%., or diamond and 3c-SiC Žapprox. 18%.. The solution of these obstacles requires an advanced procession of heteroepitaxial diamond growth on Si. In this work we use double stages, first to grow heteroepitaxial 3c-SiC on Si Ž001. and subsequently to deposit heteroepitaxial diamond on 3c-SiC. In every stage, there are two processions of bias treatment and CVD growth. The plasma conditions are generally adjusted to facilitate the 3c-SiC or diamond in both bias treatment and epitaxial growth sequence.
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Table 1 Deposition conditions for 3c-SiC and diamond film
Bias voltage ŽV. Gas flow ŽSCCM. C source concentration Pressure ŽPa. Temperature Ž⬚C.
Deposition for 3c-SiC
Deposition for diamond
y150 80 - 0.5% 266 850
y100 100 1.0% 2000 800
The images of the films and the surface morphologies were examined by atomic force microscopy ŽAFM., optical microscopy ŽOM. and scanning electronic microscopy ŽSEM., respectively. Details of the component and structure information was investigated by Fourier transform infrared spectroscopy ŽFTIR., Scanning Auger microprobe ŽMicro-Lab 310F. and micro-Raman spectrum ŽSuper Lab Raman. for CVD growing particles. The beam dimension of micro-Auger or microRaman is approximately 70 nm to ensure that the test is performed only for one particle. Finally, Hall measurement has been done to get the electrical properties of the heteroepitaxial diamond films. In order to reduce the effect of leakage currents through the substrate on the Hall data, we choose the silicon substrates with larger resistivity of approximately 5000 ⍀ cm. The diamond films were cleaned in a solution of CrO3 and H2 SO4 at 200⬚C followed by rinsing in a solution of H2 O2 and NH4 OH. Four Indium contacts were on the every edge center of the sample surface, which showed ohmic behavior of the contacts. The van der Pauw geometry was performed to get the electrical properties.
3. Results and discussion 2. Experimental Growth was carried out in a microwave plasma CVD in a 850-W Ž2.45 GHz. microwave CVD system. The substrates used in this study were mirror polished silicon Ž001. with 4⬚ off toward the w011x direction. Before treatment, the samples were dipped in hydrofluoric acid and subjected into H plasma to clean the surface and to remove surface oxide. Growth could be separated into two stages, one is to deposit oriented 3c-SiC on Si Ž001. by using ŽCH3 .6 Si2 NH as a carbon source carried by hydrogen, instead of toxic and inflammable SiH4 , and the following is to perform diamond-oriented growth on 3c-SiC in a CH4rH2 atmosphere for 8᎐10 h. In addition, a small amount of oxygen was added into the reactant gases. The deposition conditions are different for the two stages. In every stage, there were 15᎐20 min bias treatment and CVD growth followed, as listed in Table 1.
We have provided an experimental study of the oriented nucleation density measured as a function of the conditions during bias treatment, which was described in detail separately by He et al. w9x. The essential effect of negative bias treatment for both 3c-SiC and diamond is to enhance the grained energies of the radical species. The ion energy must be limited to a narrow area to just promote oriented particles nucleation. The formation of epitaxial 3c-SiC on Si could be performed in various atmospheres. The most commonly applied agent is SiH4 . The source we have chosen is non-toxic and non-inflammable hexamethyl disilazane ŽCH3 .6 Si2 NH, which has higher C content and Si᎐C bonding which is beneficial to SiC growth on Si at low deposition temperature of 850⬚C and have higher nucleation density of 108 cmy2 in our experimental condition. There has been no report to choose
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X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
this organic source as a source to grow 3c-SiC on Si. However, in some reports w10x the silicon surface can be converted to 3c-SiC by a relatively low temperature plasma CVD technique called carbonization, and this
layer thickness could be several micrometers, but it is rough with a low crystalline texture. It is noticed that the heteroepitaxial SiC layer surface is more smooth and has higher orientation by using organic sources for
Fig. 1. AFM images Ža. clean 4⬚ off Si Ž001. surface; Žb. growing 3c-SiC surface.
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
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Fig. 2. FTIR spectroscopy of 3c-SiC.
SiC growth on Si, such as SiH2 C2rC2 H2 and C7 H20 Si2 w11,12x. Unfortunately, there is still several degrees of misorientation of this layer with the silicon substrate. Fig. 1a shows the surface morphology of the 4⬚ off the Si Ž001. substrate after polishing and surface cleaning by atomic force microscopy ŽAFM.. The surface ranges uniform rising and falling in the distance. This is due to the surface roughness and the 4⬚ off orientation of the substrate. Fig. 1b is the AFM image of the surface for the first stage of CVD 3c-SiC growth. The surface is covered by regular cuboid pillars without obvious grooves on it. The film is continuous and uniform in the scanning area of the substrate surface. This positively indicates that it is the layer growth mode of the film merely during the first stage procession of 3c-SiC on Si Ž001.. Fig. 2 shows the Fourier transform infrared spectroscopy ŽFTIR. in the SiC phonon band regime at approximately 800 cm over the substrate surface. Since a very thin nanometer lever thickness is easily observable by IR spectroscopy, the presence of SiC is observable even below 3 nm without thickness determination. In our case, the growing SiC
layer thickness is approximately 5 nm. Since the IR spectroscopy has no structural evidence to indicate whether the silicon carbide corresponding to the absorption information is 3c-SiC, we also do not know whether the silicon carbide layers are continuous. Consequently, the process has been optimized by microAuger and micro-Raman spectrums. The film surface is composed of cubic Ž001. growth facets in the view of micro-Auger, which orientation is strongly coincident with the Si Ž001. substrate. Fig. 3a is the micro-Auger microscopy for one growing particle. The growing particles at this stage on Si were composed of Si and C proved by micro-Auger and have no obvious content difference between these particles. The atomic ratio of Si and C is approximately 1, which means that their chemical composition is stoichiometrical. The structure analysis by micro-Raman spectroscopy shows a typical peak at approximately 965 cm that is related to 3c-SiC ŽFig. 4a.. At the second stage, we used CH4 , as the carbon source instead of ŽCH3 .6 Si2 NH and changed the deposition condition to grow diamond on surface. The
Fig. 3. Micro-Auger spectroscopy: Ža. 3c-SiC; and Žb. diamond.
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
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Fig. 4. Micro-Raman spectroscopy: Ža. 3c-SiC; and Žb. diamond.
growing particles are mainly composed of C, which was proved by micro-Auger spectroscopy in Fig. 3b. If we stop this growth after 1 h, there are two different sizes of the particles: for the smaller one, its content is Si and C, but for the bigger one, it is only C. The related micro-Raman spectroscopy has been carried out after the second growth stage, as shown in Fig. 4b. There is a peak at 1334 cmy1 corresponding to the diamond phase. This means that the bigger particle structure is diamond. It is clear that the diamond is grown on 3c-SiC, and with the growth of diamond particles, the 3c-SiC layers became thinner and thinner, which could be examined by FTIR absorption. The determination of the orientation of growing particles clearly indicated that diamond particles align to both the 3c-SiC interlayer and SŽ001.. Fig. 5 shows the surface morphology of diamond film. These are covered by regular particles on the surface. The appearance of Ž001. facets on the film surface shows that it is strong coincident with the substrate SiŽ001. also. It is very clear that the related orientation is diamond Ž001.rr3c-SiCŽ001.rrSiŽ001.. Finally, the electronic measurement has been done by Hall measurement. In our experiment, the film thickness is 0.35 m, size of 5 = 5 mm, p type. The Hall coefficient RH is RH s EyrJx Bz
Ž1.
where Ey is the electric field intensity, Jx is the current density and Bz is the magnetic induction. Also,
Fig. 5. Surface morphology of diamond film.
RH s y3r8 pe
Ž2.
here, p is the hole density per unit volume and e is the electron charge. The Hall mobility H is: H s < RH pep <
Ž3.
Here, p is the hole mobility. The results are listed in Table 2. For quite low resistivity and quite high carrier density per unit volume of the diamond film, this could be expressed by the existence of the higher conduction layer close to the film surface during H-plasma CVD deposition. This electrical property could be attributed to a doping lever caused by the hydrogen close to the film surface. It is believed that the hydrogeneted sample is ionized with impurities to generate free holes w12x.
Table 2 Electrical properties of the heteroepitaxial diamond films Resistivity Ž ⍀ cm.
Hall coefficient R H Žcm3rQ.
Hall mobility H Žcm2 rV⭈ s.
Carrier concentration Žcmy3 .
9.4= 10y3
2.9
309
2.2= 10 18
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
4. Conclusion The heteroepitaxial diamond film was deposited on 3c-SiC, which was heteroepitaxialy grown on Si Ž001.. The diamond ²001:, 3c-SiC ²001: and Si ²001: keep perfect oriented coincidence. There are double growth stages, each of them includes both bias and growth processions. It is critical to choose deposition conditions and to perform the oriented nucleation of 3c-SiC as well as diamond particles on Si Ž001.. ŽCH3 .6 Si2 NH is an effective agent to deposit the oriented 3c-SiC on Si at a lower temperature. The surface analyses by micro-Auger and micro-Raman spectrums show that 3c-SiC particles are formed at the first growth stage. The 3c-SiC layers became thinner when the diamond particles grow on them at the second growth stage. These particles have the same orientation with silicon substrate ²001: directions. The Hall measurement indicates that the heteroepitaxial diamond films have lower resistivity and higher carrier mobility due to the surface conduction layer during H-plasma CVD deposition.
Acknowledgements This work is supported by the Chinese National Natural Foundation, No. 59682001.
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References w1x Gerber, J. Roberson, S. Sattal, H. Ehrbardt, Diamond Relat. Mater., 5 Ž1996. 261. w2x B.R. Stoner, G.H.M. Ma, S.D. Wolter, J.T. Glass, Phys. Rev. B 45 Ž1992. 11067᎐11084. w3x F. Arezzo, N. Zacchetti, W. Zhu, J. Appl. Phys. 75 Ž1994. 5375᎐5391. w4x H. Yagi, K. Hoshina, A. Hatta, T. Ito, T. Sasaki, A. Huaki, Jpn. J. Appl. Phys. 36 Ž1977. L507. w5x K. Hayashi, S. Yamanaka, H. Watanabe, T. Sekiguchi, H. Okushi, K. Kajimura, J. Cryst. Growth 183 Ž1998. 338᎐346. w6x W. Zhu, X.H Wang, B.R. Stoner et al., Phys. Rev. B 47 Ž1993. 6529᎐6542. w7x T. Kusunoki, M. Hiroi, T. Sato, Y. Igari, S. Tomoda, Appl. Surf. Sci. 45 Ž1990. 171᎐187. w8x T. Yamamoto, T. Maki, T. Kobayashi, Appl. Surf. Sci. 117r118 Ž1997. 582᎐586. w9x X.C. He, H.S. Shen, Y.Z. Wan, Z.M. Zhang, T. Shen, Experimental characterization of bias enhanced nucleation of oriented diamond on SiŽ100. substrate, Asian Conference on Chemical Vapor Deposition, May 10᎐13, Shanghai, China, 1999, pp. 2a᎐11. w10x S.D. Wolter, B.R. Stoner, J.T. Glass, Appl. Phys. Lett. 62 Ž11. Ž1993. 1215᎐1217. w11x M. Friedrich, S. Morley, B. Mainz, H.J. Hinneberg, D.R.T. Zahn, Diamond Relat. Mater. 4 Ž1995. 944. w12x K. Hayashi, S. Yamanka, H. Okushi, K. Kajimura, Appl. Phys. Lett. 68 Ž3. Ž1996. 376᎐378.
Growth of CVD heteroepitaxial diamond on silicon ž 001/ and its electronic properties U
X.C. Hea, , H.S. Shena , Z.M. Zhanga , X.J. Hua , Y.Z. Wana , T. Shenb a
State Key Lab of MMCM’s, Shanghai Jiaotong Uni¨ ersity, Shanghai 200030, PR China b Hirata Precision Products Co. Ltd, Shanghai 200137, PR China
Abstract Microwave CVD heteroepitaxial diamond film on a 4⬚ off-axis SiŽ100. substrate is obtained by two stages. The first one is to grow oriented 3c-SiC layers on SiŽ100. using a non-toxic and non-inflammable ŽCH3 .6 Si2 NH organic compound carried by hydrogen. The following stage is to grow oriented diamond films on them under the atmosphere of CH4 and H2 . In each stage there are bias and growth processions. The micro-Raman and micro-Auger analyses prove that there is a perfect orientation relationship between the film and substrate as following: diamond ²001:rr3c-SiC²001:rrSi²001:. The Hall effect indicates that the film is a P type, whose resistivity is 9.4= 10y3 ⍀ cm, the Hall coefficient is 2.9 cm3rQ, the hole mobility is 309 cm2rV s and the carrier concentration reaches 2.2= 1018 cmy3. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Microwave CVD; Diamond films heteroepitaxy; 3c-SiC; Interface
1. Introduction The heteroepitaxial diamond chemical vapor deposition ŽCVD. is a process of considerable scientific and technological importance. Many applications of diamond, especially in the area of electronic devices are awaiting the larger area and higher quality heteroepitaxy of diamond, further enhancement of which is needed for the achievement of comparable electronic properties to those observed in natural diamond. A major goal of diamond technology is to achieve good nucleation and heteroepitaxial growth on a low cost substrate such as Si. However, its very poor intrinsic nucleation density on Si is due to: Ž1. the large surface energy of diamond Ži.e. 5.3᎐9.2 Jrm2 for diamond, 1.4s 6 Jrm2 for silicon, and 3.0 Jrm2 for 3c-SiC.; Ž2. the large lattice mismatch of 1:1.52 of diamond and Si, which prevents diamond from forming oriented, two-dimensional nuclei on a foreign substrate surface; Ž3. the low mobility of growth species w1x. U
Corresponding author.
For practical use, the deposition of diamond with high crystalline quality onto substrate candidates of silicon and silicon carbide has been widely investigated. A novel technique, bias enhanced nucleation, has been proved to be extraordinarily useful for the in situ promotion of diamond or 3c-SiC nucleation and enabled the growth of heteroepitaxial diamond film on them. However, the mechanism of bias-enhanced nucleation and the nature of the interface between the heteroepitaxial diamond film and substrate are not well understood to date. Numerous studies about the nucleation process imply the importance of the formation of an amorphous carbon and SiC layer prior to and during the nucleation w2᎐4x. Presently, the diamond heteroepitaxial growth has been successful on Si or SiC by negative bias pretreatment, but there is a lot of defects in the film due to the mis-orientation and lattice mismatch between diamond film and substrate. This limits an approach to the electronic applications of diamond films. To achieve the device quality, diamond should satisfy the following conditions:
0925-9635r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 3 1 7 - 4
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
1. atomically flat surface, which improves junction properties of each contact and allows high integration of these devices; and 2. low defect density and low residual impurities which ensure the designed device operation w5x. Since the larger lattice mismatch between diamond and silicon and the fact that SiC is easily formed as an interface layer during CVD diamond growth on Si, it has been suggested that epitaxial diamond growth occurs on a cubic silicon carbide interlayer w6x. In a common way, the bias-enhanced nucleation ŽBEN. method is usually used in a microwave plasma-assisted CVD system. The BEN method consists of two steps. The first step is carbonization, in which single crystals of 3c-SiC, which plays a role as a buffer layer between Si and diamond thin film, has grown on Si substrate with gaseous hydrocarbons at elevated temperatures is essential for growth of epitaxial single crystal 3c-SiC w7x. The second step is negative bias treatment, in which diamond has been nucleated on the 3c-SiC layer. After the BEN method, diamond growth has been carried out w8x. However, there exist both lattice mismatch and the mis-orientation since the larger lattice mismatch of 3c-SiC and Si Žapprox. 20%., or diamond and 3c-SiC Žapprox. 18%.. The solution of these obstacles requires an advanced procession of heteroepitaxial diamond growth on Si. In this work we use double stages, first to grow heteroepitaxial 3c-SiC on Si Ž001. and subsequently to deposit heteroepitaxial diamond on 3c-SiC. In every stage, there are two processions of bias treatment and CVD growth. The plasma conditions are generally adjusted to facilitate the 3c-SiC or diamond in both bias treatment and epitaxial growth sequence.
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Table 1 Deposition conditions for 3c-SiC and diamond film
Bias voltage ŽV. Gas flow ŽSCCM. C source concentration Pressure ŽPa. Temperature Ž⬚C.
Deposition for 3c-SiC
Deposition for diamond
y150 80 - 0.5% 266 850
y100 100 1.0% 2000 800
The images of the films and the surface morphologies were examined by atomic force microscopy ŽAFM., optical microscopy ŽOM. and scanning electronic microscopy ŽSEM., respectively. Details of the component and structure information was investigated by Fourier transform infrared spectroscopy ŽFTIR., Scanning Auger microprobe ŽMicro-Lab 310F. and micro-Raman spectrum ŽSuper Lab Raman. for CVD growing particles. The beam dimension of micro-Auger or microRaman is approximately 70 nm to ensure that the test is performed only for one particle. Finally, Hall measurement has been done to get the electrical properties of the heteroepitaxial diamond films. In order to reduce the effect of leakage currents through the substrate on the Hall data, we choose the silicon substrates with larger resistivity of approximately 5000 ⍀ cm. The diamond films were cleaned in a solution of CrO3 and H2 SO4 at 200⬚C followed by rinsing in a solution of H2 O2 and NH4 OH. Four Indium contacts were on the every edge center of the sample surface, which showed ohmic behavior of the contacts. The van der Pauw geometry was performed to get the electrical properties.
3. Results and discussion 2. Experimental Growth was carried out in a microwave plasma CVD in a 850-W Ž2.45 GHz. microwave CVD system. The substrates used in this study were mirror polished silicon Ž001. with 4⬚ off toward the w011x direction. Before treatment, the samples were dipped in hydrofluoric acid and subjected into H plasma to clean the surface and to remove surface oxide. Growth could be separated into two stages, one is to deposit oriented 3c-SiC on Si Ž001. by using ŽCH3 .6 Si2 NH as a carbon source carried by hydrogen, instead of toxic and inflammable SiH4 , and the following is to perform diamond-oriented growth on 3c-SiC in a CH4rH2 atmosphere for 8᎐10 h. In addition, a small amount of oxygen was added into the reactant gases. The deposition conditions are different for the two stages. In every stage, there were 15᎐20 min bias treatment and CVD growth followed, as listed in Table 1.
We have provided an experimental study of the oriented nucleation density measured as a function of the conditions during bias treatment, which was described in detail separately by He et al. w9x. The essential effect of negative bias treatment for both 3c-SiC and diamond is to enhance the grained energies of the radical species. The ion energy must be limited to a narrow area to just promote oriented particles nucleation. The formation of epitaxial 3c-SiC on Si could be performed in various atmospheres. The most commonly applied agent is SiH4 . The source we have chosen is non-toxic and non-inflammable hexamethyl disilazane ŽCH3 .6 Si2 NH, which has higher C content and Si᎐C bonding which is beneficial to SiC growth on Si at low deposition temperature of 850⬚C and have higher nucleation density of 108 cmy2 in our experimental condition. There has been no report to choose
1628
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
this organic source as a source to grow 3c-SiC on Si. However, in some reports w10x the silicon surface can be converted to 3c-SiC by a relatively low temperature plasma CVD technique called carbonization, and this
layer thickness could be several micrometers, but it is rough with a low crystalline texture. It is noticed that the heteroepitaxial SiC layer surface is more smooth and has higher orientation by using organic sources for
Fig. 1. AFM images Ža. clean 4⬚ off Si Ž001. surface; Žb. growing 3c-SiC surface.
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
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Fig. 2. FTIR spectroscopy of 3c-SiC.
SiC growth on Si, such as SiH2 C2rC2 H2 and C7 H20 Si2 w11,12x. Unfortunately, there is still several degrees of misorientation of this layer with the silicon substrate. Fig. 1a shows the surface morphology of the 4⬚ off the Si Ž001. substrate after polishing and surface cleaning by atomic force microscopy ŽAFM.. The surface ranges uniform rising and falling in the distance. This is due to the surface roughness and the 4⬚ off orientation of the substrate. Fig. 1b is the AFM image of the surface for the first stage of CVD 3c-SiC growth. The surface is covered by regular cuboid pillars without obvious grooves on it. The film is continuous and uniform in the scanning area of the substrate surface. This positively indicates that it is the layer growth mode of the film merely during the first stage procession of 3c-SiC on Si Ž001.. Fig. 2 shows the Fourier transform infrared spectroscopy ŽFTIR. in the SiC phonon band regime at approximately 800 cm over the substrate surface. Since a very thin nanometer lever thickness is easily observable by IR spectroscopy, the presence of SiC is observable even below 3 nm without thickness determination. In our case, the growing SiC
layer thickness is approximately 5 nm. Since the IR spectroscopy has no structural evidence to indicate whether the silicon carbide corresponding to the absorption information is 3c-SiC, we also do not know whether the silicon carbide layers are continuous. Consequently, the process has been optimized by microAuger and micro-Raman spectrums. The film surface is composed of cubic Ž001. growth facets in the view of micro-Auger, which orientation is strongly coincident with the Si Ž001. substrate. Fig. 3a is the micro-Auger microscopy for one growing particle. The growing particles at this stage on Si were composed of Si and C proved by micro-Auger and have no obvious content difference between these particles. The atomic ratio of Si and C is approximately 1, which means that their chemical composition is stoichiometrical. The structure analysis by micro-Raman spectroscopy shows a typical peak at approximately 965 cm that is related to 3c-SiC ŽFig. 4a.. At the second stage, we used CH4 , as the carbon source instead of ŽCH3 .6 Si2 NH and changed the deposition condition to grow diamond on surface. The
Fig. 3. Micro-Auger spectroscopy: Ža. 3c-SiC; and Žb. diamond.
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
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Fig. 4. Micro-Raman spectroscopy: Ža. 3c-SiC; and Žb. diamond.
growing particles are mainly composed of C, which was proved by micro-Auger spectroscopy in Fig. 3b. If we stop this growth after 1 h, there are two different sizes of the particles: for the smaller one, its content is Si and C, but for the bigger one, it is only C. The related micro-Raman spectroscopy has been carried out after the second growth stage, as shown in Fig. 4b. There is a peak at 1334 cmy1 corresponding to the diamond phase. This means that the bigger particle structure is diamond. It is clear that the diamond is grown on 3c-SiC, and with the growth of diamond particles, the 3c-SiC layers became thinner and thinner, which could be examined by FTIR absorption. The determination of the orientation of growing particles clearly indicated that diamond particles align to both the 3c-SiC interlayer and SŽ001.. Fig. 5 shows the surface morphology of diamond film. These are covered by regular particles on the surface. The appearance of Ž001. facets on the film surface shows that it is strong coincident with the substrate SiŽ001. also. It is very clear that the related orientation is diamond Ž001.rr3c-SiCŽ001.rrSiŽ001.. Finally, the electronic measurement has been done by Hall measurement. In our experiment, the film thickness is 0.35 m, size of 5 = 5 mm, p type. The Hall coefficient RH is RH s EyrJx Bz
Ž1.
where Ey is the electric field intensity, Jx is the current density and Bz is the magnetic induction. Also,
Fig. 5. Surface morphology of diamond film.
RH s y3r8 pe
Ž2.
here, p is the hole density per unit volume and e is the electron charge. The Hall mobility H is: H s < RH pep <
Ž3.
Here, p is the hole mobility. The results are listed in Table 2. For quite low resistivity and quite high carrier density per unit volume of the diamond film, this could be expressed by the existence of the higher conduction layer close to the film surface during H-plasma CVD deposition. This electrical property could be attributed to a doping lever caused by the hydrogen close to the film surface. It is believed that the hydrogeneted sample is ionized with impurities to generate free holes w12x.
Table 2 Electrical properties of the heteroepitaxial diamond films Resistivity Ž ⍀ cm.
Hall coefficient R H Žcm3rQ.
Hall mobility H Žcm2 rV⭈ s.
Carrier concentration Žcmy3 .
9.4= 10y3
2.9
309
2.2= 10 18
X.C. He et al. r Diamond and Related Materials 9 (2000) 1626᎐1631
4. Conclusion The heteroepitaxial diamond film was deposited on 3c-SiC, which was heteroepitaxialy grown on Si Ž001.. The diamond ²001:, 3c-SiC ²001: and Si ²001: keep perfect oriented coincidence. There are double growth stages, each of them includes both bias and growth processions. It is critical to choose deposition conditions and to perform the oriented nucleation of 3c-SiC as well as diamond particles on Si Ž001.. ŽCH3 .6 Si2 NH is an effective agent to deposit the oriented 3c-SiC on Si at a lower temperature. The surface analyses by micro-Auger and micro-Raman spectrums show that 3c-SiC particles are formed at the first growth stage. The 3c-SiC layers became thinner when the diamond particles grow on them at the second growth stage. These particles have the same orientation with silicon substrate ²001: directions. The Hall measurement indicates that the heteroepitaxial diamond films have lower resistivity and higher carrier mobility due to the surface conduction layer during H-plasma CVD deposition.
Acknowledgements This work is supported by the Chinese National Natural Foundation, No. 59682001.
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