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Low temperature plasma enhanced chemical vapour deposition boron nitride
Thin Solid Films 308–309 (1997) 219–222
Low temperature plasma enhanced chemical vapour deposition boron nitride M.N.P. Carren˜o*, J.P. Bottecchia, I. Pereyra LME, EPUSP, University of Sa˜o Paulo, CEP, CP 61548, 5424-970 Sa˜o Paulo, Brazil
Abstract In this paper we present our results concerning deposition of large area boron nitride films by the conventional plasma enhanced chemical vapor deposition (PECVD) technique, in a capacitively coupled reactor, from nitrogen (N2) and diborane (B2H6) gaseous mixtures and at pressures lower than 1 Torr and temperatures between 200 and 500°C. Series of samples were grown by changing the RF power, the substrate temperature and the (N2/B2H6) flow ratio. The characterization of the samples was carried out mainly by Fourier transform infrared spectroscopy (FTIR) and thickness measurements. The results have been very promising and samples grown in appropriate conditions show clearly the infrared spectra absorption bands characteristic of hexagonal BN material (h BN), without traces of hydrogen bonding, even at temperatures as low as 200°C. The material structure shows a strong dependence on the RF power and the diborane flow. For the lower studied B2H6 flows and the higher RF power, the onset of a cubic phase was observed while the h BN phase peaks decreased and shifted in frequency. However, for lower RF power values and higher B2H6 flows the in-plane hexagonal vibration increases and the out-of-plane peak decreases and shifts towards higher frequencies. The temperature did not show a very important effect on the films properties, except for the highest studied value (500°C) for which onset of cubic phase was also observed. 1997 Elsevier Science S.A. Keywords: Boron nitride; Plasma enhanced chemical vapor deposition; Hexagonal BN; Fourier transform infrared spectroscopy
1. Introduction Among the new wide gap semiconductor alloys recently studied, the boron nitride, BN, is one of the more interesting. Its particular properties [1], such as very high optical gap, hardness, chemical stability and high thermal conductivity, make this material a promising candidate for applications such as metallurgical coatings and in microelectronics and semiconductor device technology [2,3]. The production of BN crystals is, however, very difficult since it occurs at very high pressures and temperatures [4–6] and normally just small size specimens are obtained. Obtaining this material, therefore, at low temperatures and pressures and in large areas is a very important consideration as regards compatibility with the current semiconductor device technology processes. Different techniques have been utilized rather successfully to obtain BN films at low temperatures. These include RF sputtering [7], chemical vapor deposition (CVD) [8,9], ion-assisted pulsed laser deposition [10], ion-beam deposition [11] and glow discharge [12] utilizing a variety of gaseous mixtures containing N2, NH3, BF3, Borazine (B3N3H6) * Corresponding author.
0040-6090/97/$17.00 1997 Elsevier Science S.A. All rights reserved PII S0040-6090 (97 )0 0389-1
and diborane (B2H6). All those approaches have been utilized to obtain BN films for device applications, but further studies are necessary to clarify the films properties and their correlation with the deposition parameters. For example, the films obtained by these techniques seem to be a mixture of an amorphous phase with different hexagonal and cubic fractions [13] and it is desirable to control the relative concentrations of each of these phases. Also the methods that report cubic BN normally include high ion bombardment which is also responsible for smaller grain sizes. Obtaining the appropriated conditions for the deposition of cubic BN with a mild technique such as plasma enhanced chemical vapor deposition (PECVD) is therefore of real interest [14,15]. In this study we present our results on the deposition of boron nitride films by the conventional radio frequency PECVD technique from nitrogen (N2) and diborane (B2H6) mixtures at pressures lower than 1 Torr and temperatures lower than 500°C. The characterization was carried out mainly by Fourier transform infrared spectroscopy (FTIR) due to the efficacy of this technique in studying the chemical bonding in the films as well as the structure of the material [15], especially for very thin films where characterization of micro or poly-
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M.N.P. Carren˜o et al. / Thin Solid Films 308–309 (1997) 219–222
crystals by X-ray diffraction becomes troublesome. Even though the IR absorption spectra depend on the bonding vibration modes and thus mainly on the short range order, the vibration frequency as well as the band width at half maximum depend also on the long and medium range order. Also, the relative intensities of the different vibration modes give information on the material structure. Nevertheless, FTIR alone is not enough to determine material stoichiometry since non-bonded elements and bonds that do not possess dipole moment are not detected by IR absorption. As non-bonded elements represent a minor portion of the material and their presence does not affect the overall structure of the material, FTIR gives a good approximation in understanding the material properties and their dependence on the deposition parameters.
2. Experimental The boron nitride films were deposited by standard 13.56 MHz radio frequency PECVD technique in a capacitively coupled reactor (see Fig. 1), from appropriated gaseous mixtures of nitrogen (N2) and diborane (B2H6). The flow of gases is controlled by mass flow controllers and the deposition pressure monitored by a capacitance manometer (Baratron type). The gaseous exhaust and the pumping speed are controlled by an automatic throttle valve which was kept open to guarantee the lowest gas residence time, resulting in deposition pressures of about 30 mTorr. The sample holder (400 cm2) is resistively heated at the desired controlled temperature which can be varied from room temperature up to 600°C. The different sets of samples were obtained by varying the deposition time, the RF power, the gas flow, the sub-
Table 1 Deposition conditions for the boron nitride samples Sample
N2 (sccm)
B2H6 FR (sccm)
RF (mW/ cm2)
Temp (°C)
Deposition time (min)
nb2-200 nb2-300 nb2-400 nb2-500 1/2nb4* nb4* 2nb4* 4nb4* nb6030-200W nb6030 nb6030-9OW nb6030-60W nb6015-200W nb6015 (nb4*) nb6015-9OW nb6015-60W nb6007-200W nb6007- 120W nb6007-9OW nb6007-60W
60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
30 30 30 30 15 15 15 15 30 30 30 30 15 15 15 15 7 7 7 7
300 300 300 300 300 300 300 300 500 320 225 150 500 300 225 150 500 300 225 150
200 300 400 500 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400
90 90 90 90 50 100 200 400 100 100 100 100 100 100 100 100 100 100 100 100
2 2 2 2 4 4 4 2 2 2 2 4 4 4 4 8 8 8 8
strate temperature and the (N2)/(B2H6) flow ratio (FR). The deposition conditions for all the studied sets of samples are depicted in Table 1. The nitrogen and boron bonding in the films was analyzed by infrared spectroscopy. The films were deposited onto high resistivity poly-crystalline silicon substrates, polished on both sides. After deposition, the IR absorption peaks were measured in a Fourier transform spectrometer (FTIR), in the 400–4000 cm−1 range with a resolution of 2 cm−1 and a detection limit of about 1%. The thickness was measured on films deposited on glass substrates using a DekTak 3030 profile-meter.
3. Results
Fig. 1. Schematic of the Deposition System.
As we mentioned before, the deposition rate was very low for all the studied deposition conditions. The thickness of the samples varied between ~30 and 120 nm, the thicker one being the sample ‘4nb4*’, obtained after almost 7 h of deposition. Fig. 2 shows FTIR infrared spectra for a set of samples obtained as a function of deposition time. Here an evolution of the structure of the material with the deposition time is appreciated since, for the lowest time, only one peak at 1380 cm−1 (characteristic of the in-plane hexagonal BN vibration [l]) is observed and, for higher times, a vibration peak centered around 780 cm−1, characteristic of the out-of-plane hexagonal BN vibration, becomes apparent. Also, for higher times the ratio of the in-plane/out-of-plane relative intensities increases indicating an increase in structural ordering of the material [15].
M.N.P. Carren˜o et al. / Thin Solid Films 308–309 (1997) 219–222
Fig. 2. FTIR spectra for samples grown for different deposition times while keeping constant all the other deposition parameters (400°C; 300 mW/cm2 and FR = 4).
221
produce a decrease in the intensities of the hexagonal peaks, a decrease in the in-plane/out-of-plane ratio (R) and a shift of the out-of-plane vibration towards lower wave numbers. This transition occurs at lower RF power for higher FR values. In order to better analyze this behavior the deconvolution of the in-plane and out-of-plane peaks in gaussian peaks was performed for the sample set with FR = 2. It is observed that for all the spectra except the one corresponding to the sample deposited at the highest RF power density, the out-of-plane peak is the superposition of a band centered around 800 cm−1, which is characteristic of the out-of-plane hexagonal vibration of crystalline BN [15], with a wider one centered at 780 cm−1 (corresponding to amorphous material). The ratio of the relative intensities of these two peaks (A800/A780) is plotted together with the ratio R as a function of the RF power density in Fig. 5. It is observed that the crystalline fraction is higher for samples deposited with lower RF power, which also exhibit a higher R value. The onset of the cubic phase on the contrary, appears in samples with less hexagonal structural order deposited with higher RF power densities.
The substrate temperature dependence is shown in Fig. 3 for the samples grown with 300 mW/cm2 and FR = 4. A decrease in the intensity of the hexagonal peaks is observed for higher temperatures and also the appearance of a bump in the 1000 cm−1 region, which is related to the onset of a cubic phase. In Fig. 4a,b,c the dependence of the sample’s infrared absorption spectra on the RF power density for nitrogen to diborane flow ratios, FR = 2, 4 and 8, respectively, are shown. For the three cases, increasing RF power densities
Fig. 3. FTIR spectra for samples grown at different temperatures while keeping all the other deposition parameters constant (300 mW/cm2 and FR = 2).
Fig. 4. Dependence of the sample’s infrared absorption spectra with the RF power density for FR values of (a) 8, (b) 4 and (c) 2. For all these samples the substrate temperature was 400°C and the RF power shown; 60, 90, 120 and 200 W, correspond to 150,225, 300 and 500 mW/cm2, respectively.
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crystalline fraction, and for the highest values of these parameters as well as for the highest studied temperature, the onset of cubic phase was observed. The deposition rate was very low for all the studied deposition conditions (~ 3 nm/ min); this value must be increased in order to permit a more complete characterization of the films and even to perform hardness measurements. In order to achieve this goal, changes in the gaseous mixture will be tested. Also, Rutherford back scattering (RBS) measurements are due in order to evaluate precisely the stoichiometry of the films.
Acknowledgements The authors are grateful to the Brazilian Government agencies CNPq and FAPESP for financial support.
References
Fig. 5. In-plane/out-of-plane ratio (R) and A800/A780 ratio as a function of the RF power density for samples grown with FR = 2.
For the highest studied flow ratio a decrease in the hexagonal peaks accompanied by the appearance of a clear bump at the 1000 cm−l (see Fig. 4a) region is observed for the highest RF power density. This behavior is similar to that observed for higher temperatures.
4. Conclusions Boron nitride films with structures exhibiting a mixture of amorphous and hexagonal crystalline phases and even showing the onset of a cubic phase were obtained by the PECVD technique from gaseous (N2 + B2H6) mixtures at temperatures ranging from 200 to 500°C. The structure of the material showed a marked dependence on the RF power density and with the (N2)/(B2H6) flow ratio, FR. Increasing RF power densities and FR values decrease the hexagonal
[1] J. H. Edgar (ed.), Properties of Group III Nitrides, INSPEC, 1994. [2] L. Vel, G. Demazeau and J. Etourneau, Mater. Sci. Eng., B10 (1991) 149. [3] J.H. Edgar, J. Mater. Res., 7 (1992) 235. [4] R.H. Wentorf Jr., J. Chem. Phys., 26 (1957) 956. [5] R.H. Wentorf Jr., J. Chem. Phys., 34 (1961) 809. [6] O. Mishima, S. Yamaoka and O. Fukunaga, J. Appl. Phys., 61 (1987) 2822. [7] K. Bewilogua, J. Buth, H. Hu¨bsch and M. Grishke, Diamond Relat. Mater., 2 (1993) 1206. [8] R. Ishihara, O. Sugiura and M. Matsumura, Appl. Phys. Lett., 60 (1992) 3244. [9] J. Kouvetakis, V.V. Patel, C.W. Miller and D.B. Beach, J. Vac. Sci. Technol. A., 8 (1990) 3929. [10] P.B. Mirkarimi, D.L. Medlin, K.F. McCarty and J.C. Barbour, Appl. Phys. Lett., 66 (1995) 2813. [11] C. Weissmantel, J. Vac. Sci. Technol., 18 (1991) 179. [12] W. Dworschak, K. Jung and H. Ehrhardt, Diamond Relat. Mater., 3 (1994) 337. [13] S.H. Lin and B.J. Feldman, in R.W. Collins, C.C. Tsai, M. Hirose, F. Koch and L. Brus (eds.), Microcrystalline and Nanocrystalline Semiconductors, Mater. Res. Soc. Proc. 358, Pittsburgh, PA, 1995, pp. 817. [14] T. Yoshida, Diamond Relat. Mater., 5 (1996) 501. [15] M. Kuhr, S. Reinke, and W. Kulisch, Diamond Relat. Mater., 4 (1995) 375.
Low temperature plasma enhanced chemical vapour deposition boron nitride M.N.P. Carren˜o*, J.P. Bottecchia, I. Pereyra LME, EPUSP, University of Sa˜o Paulo, CEP, CP 61548, 5424-970 Sa˜o Paulo, Brazil
Abstract In this paper we present our results concerning deposition of large area boron nitride films by the conventional plasma enhanced chemical vapor deposition (PECVD) technique, in a capacitively coupled reactor, from nitrogen (N2) and diborane (B2H6) gaseous mixtures and at pressures lower than 1 Torr and temperatures between 200 and 500°C. Series of samples were grown by changing the RF power, the substrate temperature and the (N2/B2H6) flow ratio. The characterization of the samples was carried out mainly by Fourier transform infrared spectroscopy (FTIR) and thickness measurements. The results have been very promising and samples grown in appropriate conditions show clearly the infrared spectra absorption bands characteristic of hexagonal BN material (h BN), without traces of hydrogen bonding, even at temperatures as low as 200°C. The material structure shows a strong dependence on the RF power and the diborane flow. For the lower studied B2H6 flows and the higher RF power, the onset of a cubic phase was observed while the h BN phase peaks decreased and shifted in frequency. However, for lower RF power values and higher B2H6 flows the in-plane hexagonal vibration increases and the out-of-plane peak decreases and shifts towards higher frequencies. The temperature did not show a very important effect on the films properties, except for the highest studied value (500°C) for which onset of cubic phase was also observed. 1997 Elsevier Science S.A. Keywords: Boron nitride; Plasma enhanced chemical vapor deposition; Hexagonal BN; Fourier transform infrared spectroscopy
1. Introduction Among the new wide gap semiconductor alloys recently studied, the boron nitride, BN, is one of the more interesting. Its particular properties [1], such as very high optical gap, hardness, chemical stability and high thermal conductivity, make this material a promising candidate for applications such as metallurgical coatings and in microelectronics and semiconductor device technology [2,3]. The production of BN crystals is, however, very difficult since it occurs at very high pressures and temperatures [4–6] and normally just small size specimens are obtained. Obtaining this material, therefore, at low temperatures and pressures and in large areas is a very important consideration as regards compatibility with the current semiconductor device technology processes. Different techniques have been utilized rather successfully to obtain BN films at low temperatures. These include RF sputtering [7], chemical vapor deposition (CVD) [8,9], ion-assisted pulsed laser deposition [10], ion-beam deposition [11] and glow discharge [12] utilizing a variety of gaseous mixtures containing N2, NH3, BF3, Borazine (B3N3H6) * Corresponding author.
0040-6090/97/$17.00 1997 Elsevier Science S.A. All rights reserved PII S0040-6090 (97 )0 0389-1
and diborane (B2H6). All those approaches have been utilized to obtain BN films for device applications, but further studies are necessary to clarify the films properties and their correlation with the deposition parameters. For example, the films obtained by these techniques seem to be a mixture of an amorphous phase with different hexagonal and cubic fractions [13] and it is desirable to control the relative concentrations of each of these phases. Also the methods that report cubic BN normally include high ion bombardment which is also responsible for smaller grain sizes. Obtaining the appropriated conditions for the deposition of cubic BN with a mild technique such as plasma enhanced chemical vapor deposition (PECVD) is therefore of real interest [14,15]. In this study we present our results on the deposition of boron nitride films by the conventional radio frequency PECVD technique from nitrogen (N2) and diborane (B2H6) mixtures at pressures lower than 1 Torr and temperatures lower than 500°C. The characterization was carried out mainly by Fourier transform infrared spectroscopy (FTIR) due to the efficacy of this technique in studying the chemical bonding in the films as well as the structure of the material [15], especially for very thin films where characterization of micro or poly-
220
M.N.P. Carren˜o et al. / Thin Solid Films 308–309 (1997) 219–222
crystals by X-ray diffraction becomes troublesome. Even though the IR absorption spectra depend on the bonding vibration modes and thus mainly on the short range order, the vibration frequency as well as the band width at half maximum depend also on the long and medium range order. Also, the relative intensities of the different vibration modes give information on the material structure. Nevertheless, FTIR alone is not enough to determine material stoichiometry since non-bonded elements and bonds that do not possess dipole moment are not detected by IR absorption. As non-bonded elements represent a minor portion of the material and their presence does not affect the overall structure of the material, FTIR gives a good approximation in understanding the material properties and their dependence on the deposition parameters.
2. Experimental The boron nitride films were deposited by standard 13.56 MHz radio frequency PECVD technique in a capacitively coupled reactor (see Fig. 1), from appropriated gaseous mixtures of nitrogen (N2) and diborane (B2H6). The flow of gases is controlled by mass flow controllers and the deposition pressure monitored by a capacitance manometer (Baratron type). The gaseous exhaust and the pumping speed are controlled by an automatic throttle valve which was kept open to guarantee the lowest gas residence time, resulting in deposition pressures of about 30 mTorr. The sample holder (400 cm2) is resistively heated at the desired controlled temperature which can be varied from room temperature up to 600°C. The different sets of samples were obtained by varying the deposition time, the RF power, the gas flow, the sub-
Table 1 Deposition conditions for the boron nitride samples Sample
N2 (sccm)
B2H6 FR (sccm)
RF (mW/ cm2)
Temp (°C)
Deposition time (min)
nb2-200 nb2-300 nb2-400 nb2-500 1/2nb4* nb4* 2nb4* 4nb4* nb6030-200W nb6030 nb6030-9OW nb6030-60W nb6015-200W nb6015 (nb4*) nb6015-9OW nb6015-60W nb6007-200W nb6007- 120W nb6007-9OW nb6007-60W
60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
30 30 30 30 15 15 15 15 30 30 30 30 15 15 15 15 7 7 7 7
300 300 300 300 300 300 300 300 500 320 225 150 500 300 225 150 500 300 225 150
200 300 400 500 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400
90 90 90 90 50 100 200 400 100 100 100 100 100 100 100 100 100 100 100 100
2 2 2 2 4 4 4 2 2 2 2 4 4 4 4 8 8 8 8
strate temperature and the (N2)/(B2H6) flow ratio (FR). The deposition conditions for all the studied sets of samples are depicted in Table 1. The nitrogen and boron bonding in the films was analyzed by infrared spectroscopy. The films were deposited onto high resistivity poly-crystalline silicon substrates, polished on both sides. After deposition, the IR absorption peaks were measured in a Fourier transform spectrometer (FTIR), in the 400–4000 cm−1 range with a resolution of 2 cm−1 and a detection limit of about 1%. The thickness was measured on films deposited on glass substrates using a DekTak 3030 profile-meter.
3. Results
Fig. 1. Schematic of the Deposition System.
As we mentioned before, the deposition rate was very low for all the studied deposition conditions. The thickness of the samples varied between ~30 and 120 nm, the thicker one being the sample ‘4nb4*’, obtained after almost 7 h of deposition. Fig. 2 shows FTIR infrared spectra for a set of samples obtained as a function of deposition time. Here an evolution of the structure of the material with the deposition time is appreciated since, for the lowest time, only one peak at 1380 cm−1 (characteristic of the in-plane hexagonal BN vibration [l]) is observed and, for higher times, a vibration peak centered around 780 cm−1, characteristic of the out-of-plane hexagonal BN vibration, becomes apparent. Also, for higher times the ratio of the in-plane/out-of-plane relative intensities increases indicating an increase in structural ordering of the material [15].
M.N.P. Carren˜o et al. / Thin Solid Films 308–309 (1997) 219–222
Fig. 2. FTIR spectra for samples grown for different deposition times while keeping constant all the other deposition parameters (400°C; 300 mW/cm2 and FR = 4).
221
produce a decrease in the intensities of the hexagonal peaks, a decrease in the in-plane/out-of-plane ratio (R) and a shift of the out-of-plane vibration towards lower wave numbers. This transition occurs at lower RF power for higher FR values. In order to better analyze this behavior the deconvolution of the in-plane and out-of-plane peaks in gaussian peaks was performed for the sample set with FR = 2. It is observed that for all the spectra except the one corresponding to the sample deposited at the highest RF power density, the out-of-plane peak is the superposition of a band centered around 800 cm−1, which is characteristic of the out-of-plane hexagonal vibration of crystalline BN [15], with a wider one centered at 780 cm−1 (corresponding to amorphous material). The ratio of the relative intensities of these two peaks (A800/A780) is plotted together with the ratio R as a function of the RF power density in Fig. 5. It is observed that the crystalline fraction is higher for samples deposited with lower RF power, which also exhibit a higher R value. The onset of the cubic phase on the contrary, appears in samples with less hexagonal structural order deposited with higher RF power densities.
The substrate temperature dependence is shown in Fig. 3 for the samples grown with 300 mW/cm2 and FR = 4. A decrease in the intensity of the hexagonal peaks is observed for higher temperatures and also the appearance of a bump in the 1000 cm−1 region, which is related to the onset of a cubic phase. In Fig. 4a,b,c the dependence of the sample’s infrared absorption spectra on the RF power density for nitrogen to diborane flow ratios, FR = 2, 4 and 8, respectively, are shown. For the three cases, increasing RF power densities
Fig. 3. FTIR spectra for samples grown at different temperatures while keeping all the other deposition parameters constant (300 mW/cm2 and FR = 2).
Fig. 4. Dependence of the sample’s infrared absorption spectra with the RF power density for FR values of (a) 8, (b) 4 and (c) 2. For all these samples the substrate temperature was 400°C and the RF power shown; 60, 90, 120 and 200 W, correspond to 150,225, 300 and 500 mW/cm2, respectively.
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M.N.P. Carren˜o et al. / Thin Solid Films 308–309 (1997) 219–222
crystalline fraction, and for the highest values of these parameters as well as for the highest studied temperature, the onset of cubic phase was observed. The deposition rate was very low for all the studied deposition conditions (~ 3 nm/ min); this value must be increased in order to permit a more complete characterization of the films and even to perform hardness measurements. In order to achieve this goal, changes in the gaseous mixture will be tested. Also, Rutherford back scattering (RBS) measurements are due in order to evaluate precisely the stoichiometry of the films.
Acknowledgements The authors are grateful to the Brazilian Government agencies CNPq and FAPESP for financial support.
References
Fig. 5. In-plane/out-of-plane ratio (R) and A800/A780 ratio as a function of the RF power density for samples grown with FR = 2.
For the highest studied flow ratio a decrease in the hexagonal peaks accompanied by the appearance of a clear bump at the 1000 cm−l (see Fig. 4a) region is observed for the highest RF power density. This behavior is similar to that observed for higher temperatures.
4. Conclusions Boron nitride films with structures exhibiting a mixture of amorphous and hexagonal crystalline phases and even showing the onset of a cubic phase were obtained by the PECVD technique from gaseous (N2 + B2H6) mixtures at temperatures ranging from 200 to 500°C. The structure of the material showed a marked dependence on the RF power density and with the (N2)/(B2H6) flow ratio, FR. Increasing RF power densities and FR values decrease the hexagonal
[1] J. H. Edgar (ed.), Properties of Group III Nitrides, INSPEC, 1994. [2] L. Vel, G. Demazeau and J. Etourneau, Mater. Sci. Eng., B10 (1991) 149. [3] J.H. Edgar, J. Mater. Res., 7 (1992) 235. [4] R.H. Wentorf Jr., J. Chem. Phys., 26 (1957) 956. [5] R.H. Wentorf Jr., J. Chem. Phys., 34 (1961) 809. [6] O. Mishima, S. Yamaoka and O. Fukunaga, J. Appl. Phys., 61 (1987) 2822. [7] K. Bewilogua, J. Buth, H. Hu¨bsch and M. Grishke, Diamond Relat. Mater., 2 (1993) 1206. [8] R. Ishihara, O. Sugiura and M. Matsumura, Appl. Phys. Lett., 60 (1992) 3244. [9] J. Kouvetakis, V.V. Patel, C.W. Miller and D.B. Beach, J. Vac. Sci. Technol. A., 8 (1990) 3929. [10] P.B. Mirkarimi, D.L. Medlin, K.F. McCarty and J.C. Barbour, Appl. Phys. Lett., 66 (1995) 2813. [11] C. Weissmantel, J. Vac. Sci. Technol., 18 (1991) 179. [12] W. Dworschak, K. Jung and H. Ehrhardt, Diamond Relat. Mater., 3 (1994) 337. [13] S.H. Lin and B.J. Feldman, in R.W. Collins, C.C. Tsai, M. Hirose, F. Koch and L. Brus (eds.), Microcrystalline and Nanocrystalline Semiconductors, Mater. Res. Soc. Proc. 358, Pittsburgh, PA, 1995, pp. 817. [14] T. Yoshida, Diamond Relat. Mater., 5 (1996) 501. [15] M. Kuhr, S. Reinke, and W. Kulisch, Diamond Relat. Mater., 4 (1995) 375.