- Email: [email protected]
Investigation of diamond growth at high pressure by microwave plasma chemical vapor deposition
Diamond and Related Materials 13 (2004) 604–609
Investigation of diamond growth at high pressure by microwave plasma chemical vapor deposition V. Mortet, A. Kromka, R. Kravets, J. Rosa*, V. Vorlicek, J. Zemek, M. Vanecek Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ-16253 Prague 6, Czech Republic
Abstract Polycrystalline diamond thin films were grown on 2-inch silicon wafers at high pressures, up to 250 mbar, in high power microwave plasma CVD rotational ellipsoidal reactor. Influence of the gas pressure and the gas mixture on the diamond growth was investigated. High growth rates, up to 4.5 mmyh, were obtained at high pressure and low methane concentration. Hydrogen desorption from the diamond surface under atomic hydrogen flux were found to limit diamond deposition rate and the activation energy of this process is 0.2 eV. During deposition, the growth temperature is monitored with a two-colors pyrometer. The observed variation of the apparent temperature was related to the diamond film thickness and the instantaneous deposition rate. Diamond layers were characterized by Raman spectroscopy, scanning electron microscopy and X-ray photoemission spectroscopy. New absorption peaks were observed in the spectra of Fourier transform photocurrent spectroscopy. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Hydrogen desorption; Columnar growth; Photocurrent spectroscopy
1. Introduction
2. Experimental
Diamond affords a plethora of potential applications based on its abundant extreme properties. The ability to coat a large area on a variety of substrate materials by chemical vapor deposition (CVD) techniques vastly expands the potential application areas of diamond. For these applications, diamond films grown at high growth rate on economically viable substrates are preferred. In this article, a study of diamond growth on 2-inche diameter silicon substrates at high pressure and high power (6 kW) in a recently developed microwave plasma reactor with an ellipsoidal cavity w1,2x is reported. The diamond layers were characterized by scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and Raman and photoluminescence spectroscopy. Film thickness and instant deposition rate were determined in-situ from the apparent temperature measured with a two colors pyrometer. In addition, the diamond films were characterized by Fourier transform photocurrent spectroscopy (FTPS) and new photocurrent peaks have been observed in the FTPS spectra.
Diamond films were grown on 5-mm thick (100) oriented silicon, 2-inches in diameter, by microwave plasma enhanced chemical vapor deposition (MWPECVD) in the ellipsoidal cavity reactor w1,2x. Prior to deposition process, the silicon substrates were mechanically seeded with a submicron diamond powder. Before introduction of methane into the deposition chamber, the silicon substrates are pre-treated in hydrogen plasma for 6 min. Diamond growth at high power was investigated at various gas mixture of H2 yCH4, in the range 0.5–3% of CH4 in hydrogen, and at various total gas pressure, up to 250 mbar. During deposition, the substrate temperature is monitored by a two-colors pyrometer (CHINO). Raman and photoluminescence spectra of the deposited layers have been recorded with a Renishaw Raman Micro-Spectrometer Ramascope 1000 using a 514.5 nm wavelength laser excitation. The nearsurface composition of the diamond layers was studied by XPS using an ADES 400 angular-resolved photoelectron spectrometer (VG Scientific, UK) equipped with a twin anode X-ray source with the standard AlyMg anodes and with a hemispherical analyzer. The halfcone acceptance angle of the analyzer was set to 4.18. Spectra were recorded using Mg Ka source operated at
*Corresponding author. Tel.: q420-220-318-437; fax: q420-233343 184. E-mail address: [email protected] (J. Rosa).
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.034
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
605
a power of 200 W at the constant pass energy of 100 eV. Static sample charging of the spectra was corrected with respect to the C 1s peak. Diamond film surface and cross section morphologies have been observed by scanning electron microscopy (electron microprobe JXA-733 from JEOL). A Nicolet Nexus FTIR spectrometer with an external beam output and an external detector option was used for the FTPS measurements. Coplanar electrodes (approx. 3-mm long with a typical spacing of 3 mm) were drawn by colloidal graphite to collect a photocurrent signal from the illuminated samples w3–5x. 3. Results and discussion In Fig. 1, the measured deposition rates as a function of the methane concentration and the total pressure are presented. The deposition rate was simply calculated from the deposited mass and the total deposition time. The apparent final temperature for the different deposition conditions is presented in Fig. 1c and d. In the CH4 concentration series (Fig. 1a), a linear increase of the deposition rate with methane concentration up to 1%, followed by the saturation was observed. Fig. 1b shows a nearly linear increase of the diamond deposition rate with the total pressure at a methane concentration of 0.5%. The deposition increases from 0.7 mmyh at low pressure (125 mbar) to 4.5 mmyh at 250 mbar. One can see on Fig. 1 the similar trends of the deposition rate and the deposition temperature as a function of the investigated growth parameters. The
Fig. 2. Diamond deposition rate as a function of reverse absolute temperature (Arrhenius plot) for our data (s) and the data from T.H. Chein (h) w6x.
observed variation of the deposition rate has a strong correlation with the deposition temperature. This is in agreement with the high grow rates obtained at high power, high methane concentration high temperature and pressure up to 200 mbar reported by Chein et al. w6x. The deposition rate was found to follow an Arrhenius’s law as a function of the temperature with an activation energy of ;0.2 eV, as shown in Fig. 2. If we plot the data of Ref. w6x we will get the same activation energy (Fig. 2). This activation energy exactly corresponds to
Fig. 1. Diamond growth rate on silicon substrate and diamond underlayer as function of the methane concentration at 175mbar of total pressure (a) and the total pressure at 0,5% of methane in hydrogen (b). Apparent substrate temperature as function of the methane concentration (c) and the total pressure (d).
606
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
Fig. 3. (a) Schematic representation of the in-situ temperature measurement setup with a two-colors pyrometer with t, r1 , r2 the Fresnel’s transmission at the diamond-air interface and reflections coefficient at the diamond-air interface (1) and diamond–silicon interface (2), St and Sint the transmission and internal scattering factors at the diamond–air interface. (b) Variation of the apparent substrate temperature during diamond growth process. Variation of the diamond film thickness (c) and the diamond deposition rate (d) as a function of time.
the activation energy of hydrogen extraction from monohydride diamond surface under low energy hydrogen atom irradiation, as calculated by Kanai et al. by ab initio molecular dynamics method w7x. This brings the direct evidence that hydrogen desorption from diamond surface under atomic hydrogen flux is limiting the deposition rate in a high-pressure region and the activation energy of this process is 0.2 eV. Next, Fig. 1a and c indicate that the deposition rate calculated for the diamond growth on the silicon substrate is always lower than that for the diamond growth on CVD diamond underlayer. This phenomenon cannot be explained only through the incubation time of the diamond growth over the non-diamond substrates. To study this effect quantitatively, we monitored the diamond thickness and the instantaneous deposition rate from the dependence of the apparent substrate temperature, measured with two-colors pyrometer. The pyrometer, working at the infrared wavelengths l1s1.55 mm and l2s1.35 mm, is located 37 cm from the substrate at the detection angle of 638 (Fig. 3a). Fig. 3b represents the variation of the substrate temperature during diamond deposition. In this case, the average deposition rate was 3.1 mmyh as calculated from the deposited mass. Generally, in this figure, several oscillations of the temperature are observed and the extrema’s ampli-
tude decreases with the deposition time. These observed oscillations are explained by interference effect of infrared light being emitted by the hot silicon substrate in diamond thin film w8x. The amplitude of the interference fringes decreases due to the light scattering at rough surface (surface roughness is developing with the growth time) and due to non-sufficient spectral resolution of the pyrometer or thickness variation for thicker layers w9,10x. Measured apparent temperature is determined by the true substrate temperature, by transmittance of the diamond film at both wavelengths and by the Plank’s law. The pyrometer calculates the measured temperature from the ratio R(T) of the measured intensities emitted at the two wavelengths I(l1) and I(l2) The intensity of emitted light is described by the Plank’s law. The ratio R(T) is independent of the sample’s emissivity (´) in the grey body approximation (i.e. ´(l1)s´(l2)). RŽT.f
l25 l51
yl2 EE IŽl1. FFs l1l2 GG IŽl2.
B hk B l
expC D
B
C
kT D
1
(1)
where h is the Plank’s constant, kB is the Boltzmann’s constant and c is the velocity of light. During the deposition, the interference effect in the thin diamond layer modulates the emitted light intensity for both wavelengths. The equation of the intensity for both p
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
Fig. 4. (a) Schematic representation of the ‘evolutionary selection growth’ process, reprinted from w13x. Cross-section scanning electron microscopy image of the grain size of a CVD diamond layer.
and s polarization (Ip(l) and Is(l)) collected by the pyrometer for one wavelength can be determined from the Fresnel’s coefficients and the scattering factors (scalar scattering theory) for the rough diamond surface w11,12x. I(l)sŽTp(l)qTs(l). B
C
s Sq D
1q
calculated the diamond thickness corresponding to the measured, ‘apparent’ temperature extrema. The variation of the film thickness and the deposition rate as a function of the deposition time have been determined assuming nDs2.35 (see Fig. 3b and c). It is clearly observed that the deposition rate is not constant during deposition. At the beginning the deposition rate increases with time and saturates for process time of 3000 s and higher. This result explains the differences between the deposition rate of diamond deposited on silicon or on thick polycrystalline diamond substrate. This phenomenon agrees with the evolutionary selection growth mode of diamond w13x. Diamond grains with the highest growth rate survive (Fig. 4a). This is well illustrated by the diamond grain size evolution observed by scanning electron microscopy cross-section (Fig. 4b). Typical Raman spectra of films deposited under the different deposition are presented in Fig. 5, together with photoluminescence spectra excited by the Argon laser. The Raman spectrum exhibits a strong narrow band (FWHMs4.3 cmy1) of diamond, centred at 1332.6 cmy1, without presence of additional bands due to non-diamond carbon phases. Photoluminescence spectra exhibit mainly an intense and broad band centred at ;1.8 eV and some other peaks attributed to nitrogen and silicon. The origin of this broad photoluminescence band is probably related to a continuous distribution of gap states characteristic of amorphous material located at the grain boundaries w14–16x. The band centred at 2265 cmy1 (1.681 eV) is attributed to silicon vacancy complex w17x. The band centred at 4249 cmy1 (1.945 eV) corresponds to nitrogen vacancy complex w18x. The band centred at 3756 cmy1 (1.95 eV) reflects a substitutional nitrogen atom bonded to the nearest vacancy filled with one electron, which can be negatively charged w19,20x. The defect centred at 2054 cmy1 (2.15 eV) is
I0(l) 2
ŽStt.2
2 t
607
E
F
Sintr1r2 cos(w)qS2intr21r22 G 2
I0(l) 2
(2)
with ws(4pnDecos a)y l and nD: refractive index of diamond. It is worth to note that in the use configuration, the p polarized light is fully transmitted at the diamondy air interface. The measured ratio (Rexp(T)), which gives the apparent temperature, is written as: RexpŽT.s
T(l1) RŽT. T(l2)
(3)
Based on the above-described theory, we numerically
Fig. 5. Photoluminescence spectrum of a diamond film deposited at 200 mbar and a methane concentration of 0.5%.
608
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
Fig. 6. Spectral dependence of photocurrent, measured by Fourier transform photocurrent spectroscopy at 77 K. Three diamond wafers deposited under the different deposition conditions was investigated.
attributed to the nitrogen vacancy or substitutional nitrogen in neutral state w21,22x. Diamond films were also characterized by X-ray photoemission spectroscopy. Diamond samples exhibit a clearly dominating carbon signal. An oxygen signal is recognized in wide-range survey spectra. This is probably due to air-exposition of the sample surface. Further analyses were carried on following the surface cleaning by ion bombardment. Silicon, nitrogen and molybdenum traces were found in a long time acquisition spectra. Si and N results are in agreement with the photoluminescence spectroscopy. Surprising was detection of molybdenum in some diamond layers, on the level of XPS detection limit, approximately 0.1–0.2%. We suppose that this contamination come from the excentric position of plasma ball, shifted from Si wafer to Mo ring, which is surrounding the substrate. Detection of Mo was complemented by observation of the new absorption lines in the optical (photocurrent) spectrum obtained by Fourier transform photocurrent spectroscopy. In samples where Mo was detected by XPS, a new, distinct absorption peaks at 0.513, 0.705 andyor 0.746 eV were seen, as shown in Fig. 6. Weaker peaks at 0.623 and 1.14 eV were observed too. No such peaks have been observed so far w23x. 4. Conclusion In this paper, diamond growth by microwave plasma enhanced chemical vapor deposition in the ellipsoidal cavity reactor was studied at various gas mixture of H2 y CH4 in the range 0.5–3% of CH4 in hydrogen and at 125–250 mbar of total gas pressure. We demonstrated the possibility of diamond deposition rate up to 4.5 mmyh, homogeneously over 2-inch silicon substrates. Hydrogen desorption from diamond surface under atom-
ic hydrogen flux is limiting the deposition rate in a high pressure region and the activation energy of this process is 0.2 eV, in accord with a theoretical calculations w7x. The instantaneous deposition rate was determined from the observed fringes of the apparent substrate temperature measured with a two-colors pyrometer. Deposition rate increases during first 30–60 min of deposition. This phenomenon can be explained by the theory of an evolutionary selection growth mode of diamond. Diamond grains with the highest growth rate survive. Silicon, nitrogen (in agreement with the photoluminescence analysis) and surprisingly molybdenum traces were found by XPS analysis in some diamond layers. In the layers with molybdenum impurity, the new absorption peaks located between 0.5 and 1.2 eV were observed by Fourier transform photocurrent spectroscopy. Acknowledgments This work was supported by the Research Training Network of EC, project DoDDS, contract HPRN-CT1999-00139, by the Grant Agency of the Czech Republic, contract 202y02y0218 and NOA015, MSMT CR. References w1x M. Funer, C. Wild, P. Koidl, Surf. Coat. Technol. 116–119 (1999) 853. w2x M. Funer, C. Wild, P. Koidl, Appl. Phys. Lett. 72 (1998) 1149. w3x M. Nesladek, L.M. Stals, A. Stesmans, K. Iakoubovskij, G.J. Adriaenssens, J. Rosa, et al., Appl. Phys. Lett. 72 (1998) 3306. w4x R. Kravets, V. Ogorodniks, A. Poruba, P. Moravec, M. Nesladek, J. Rosa, et al., Phys. Stat. Sol. (a) 193 (2002) 502. w5x M. Vanecek, A. Poruba, Appl. Phys. Lett. 80 (2002) 719. w6x T.-H. Chein, J. Wei, Y. Tzeng, Diamond. Relat. Mater. 8 (1999) 1686. w7x C. Kanai, K. Watanabe, Y. Takakuwa, Appl. Surf. Sci. 159160 (2000) 599. w8x A.J. Spring Thorpe, T.P. Humphreys, A. Majeed, W.T. Moore, Appl. Phys. Lett. 55 (1989) 2138. w9x Z. Remes, Ph. D. Thesis, Charles University, Prague, 1999, http:yywww.fzu.czy;remes. w10x R. Swanepoel, J. Phys. E: Sci. Instrum. 7 (1984) 896. w11x Z. Yin, H.S. Tam, F.W. Smith, Diamond Relat. Mater. 5 (1996) 1490. w12x A. Poruba, A. Fejfar, Z. Remes, J. Springer, M. Vanecek, J. Kocka, et al., J. Appl. Phys. 88 (2000) 148. w13x S.K. Han, M.T. McClure, C.A. Wolden, B. Vlahovic, A. Soldi, S. Sitar, Diamond Relat. Mater. 9 (2000) 1008. w14x A.E. Mora, J.W. Steeds, J.E. Butler, Diamond Relat. Mater. 12 (2003) 310. w15x L. Bergmann, B.R. Stoner, K.F. Turner, J.T. Glass, R.J. Nemanich, J. Appl. Phys. 73 (1993) 3951. w16x M.G. Donato, G. Faggio, M. Marinelli, G. Messina, E. Milani, A. Paoletti, et al., Eur. Phys. J.B 20 (2001) 133. w17x J. Ruan, W.J. Choyke, W.D. Parlow, Appl. Phys. Lett. 58 (1991) 295.
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609 w18x G. Dollinger, A. Bergmaier, C.M. Frey, M. Roesler, H. Verhoeven, Diamond Relat. Mater. 4 (1995) 591. w19x J.H.N. Loubser, J.A. Wyl, Rep. Prog. Phys. 41 (1978) 1210. w20x Mainwood, Phys. Rev. 49 (1994) 746. w21x Y. Mita, Phys. Rev. B 53 (1996) 11360.
609
w22x S.C. Lawson, D. Fisher, D.C. Hunt, M.E Newton, J. Phys. Condes. Matter. 10 (1998) 9171. w23x A.M. Zaitsev, Optical Properties of Diamond, Springer, Berlin, 2001.
Investigation of diamond growth at high pressure by microwave plasma chemical vapor deposition V. Mortet, A. Kromka, R. Kravets, J. Rosa*, V. Vorlicek, J. Zemek, M. Vanecek Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ-16253 Prague 6, Czech Republic
Abstract Polycrystalline diamond thin films were grown on 2-inch silicon wafers at high pressures, up to 250 mbar, in high power microwave plasma CVD rotational ellipsoidal reactor. Influence of the gas pressure and the gas mixture on the diamond growth was investigated. High growth rates, up to 4.5 mmyh, were obtained at high pressure and low methane concentration. Hydrogen desorption from the diamond surface under atomic hydrogen flux were found to limit diamond deposition rate and the activation energy of this process is 0.2 eV. During deposition, the growth temperature is monitored with a two-colors pyrometer. The observed variation of the apparent temperature was related to the diamond film thickness and the instantaneous deposition rate. Diamond layers were characterized by Raman spectroscopy, scanning electron microscopy and X-ray photoemission spectroscopy. New absorption peaks were observed in the spectra of Fourier transform photocurrent spectroscopy. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Hydrogen desorption; Columnar growth; Photocurrent spectroscopy
1. Introduction
2. Experimental
Diamond affords a plethora of potential applications based on its abundant extreme properties. The ability to coat a large area on a variety of substrate materials by chemical vapor deposition (CVD) techniques vastly expands the potential application areas of diamond. For these applications, diamond films grown at high growth rate on economically viable substrates are preferred. In this article, a study of diamond growth on 2-inche diameter silicon substrates at high pressure and high power (6 kW) in a recently developed microwave plasma reactor with an ellipsoidal cavity w1,2x is reported. The diamond layers were characterized by scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and Raman and photoluminescence spectroscopy. Film thickness and instant deposition rate were determined in-situ from the apparent temperature measured with a two colors pyrometer. In addition, the diamond films were characterized by Fourier transform photocurrent spectroscopy (FTPS) and new photocurrent peaks have been observed in the FTPS spectra.
Diamond films were grown on 5-mm thick (100) oriented silicon, 2-inches in diameter, by microwave plasma enhanced chemical vapor deposition (MWPECVD) in the ellipsoidal cavity reactor w1,2x. Prior to deposition process, the silicon substrates were mechanically seeded with a submicron diamond powder. Before introduction of methane into the deposition chamber, the silicon substrates are pre-treated in hydrogen plasma for 6 min. Diamond growth at high power was investigated at various gas mixture of H2 yCH4, in the range 0.5–3% of CH4 in hydrogen, and at various total gas pressure, up to 250 mbar. During deposition, the substrate temperature is monitored by a two-colors pyrometer (CHINO). Raman and photoluminescence spectra of the deposited layers have been recorded with a Renishaw Raman Micro-Spectrometer Ramascope 1000 using a 514.5 nm wavelength laser excitation. The nearsurface composition of the diamond layers was studied by XPS using an ADES 400 angular-resolved photoelectron spectrometer (VG Scientific, UK) equipped with a twin anode X-ray source with the standard AlyMg anodes and with a hemispherical analyzer. The halfcone acceptance angle of the analyzer was set to 4.18. Spectra were recorded using Mg Ka source operated at
*Corresponding author. Tel.: q420-220-318-437; fax: q420-233343 184. E-mail address: [email protected] (J. Rosa).
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.034
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
605
a power of 200 W at the constant pass energy of 100 eV. Static sample charging of the spectra was corrected with respect to the C 1s peak. Diamond film surface and cross section morphologies have been observed by scanning electron microscopy (electron microprobe JXA-733 from JEOL). A Nicolet Nexus FTIR spectrometer with an external beam output and an external detector option was used for the FTPS measurements. Coplanar electrodes (approx. 3-mm long with a typical spacing of 3 mm) were drawn by colloidal graphite to collect a photocurrent signal from the illuminated samples w3–5x. 3. Results and discussion In Fig. 1, the measured deposition rates as a function of the methane concentration and the total pressure are presented. The deposition rate was simply calculated from the deposited mass and the total deposition time. The apparent final temperature for the different deposition conditions is presented in Fig. 1c and d. In the CH4 concentration series (Fig. 1a), a linear increase of the deposition rate with methane concentration up to 1%, followed by the saturation was observed. Fig. 1b shows a nearly linear increase of the diamond deposition rate with the total pressure at a methane concentration of 0.5%. The deposition increases from 0.7 mmyh at low pressure (125 mbar) to 4.5 mmyh at 250 mbar. One can see on Fig. 1 the similar trends of the deposition rate and the deposition temperature as a function of the investigated growth parameters. The
Fig. 2. Diamond deposition rate as a function of reverse absolute temperature (Arrhenius plot) for our data (s) and the data from T.H. Chein (h) w6x.
observed variation of the deposition rate has a strong correlation with the deposition temperature. This is in agreement with the high grow rates obtained at high power, high methane concentration high temperature and pressure up to 200 mbar reported by Chein et al. w6x. The deposition rate was found to follow an Arrhenius’s law as a function of the temperature with an activation energy of ;0.2 eV, as shown in Fig. 2. If we plot the data of Ref. w6x we will get the same activation energy (Fig. 2). This activation energy exactly corresponds to
Fig. 1. Diamond growth rate on silicon substrate and diamond underlayer as function of the methane concentration at 175mbar of total pressure (a) and the total pressure at 0,5% of methane in hydrogen (b). Apparent substrate temperature as function of the methane concentration (c) and the total pressure (d).
606
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
Fig. 3. (a) Schematic representation of the in-situ temperature measurement setup with a two-colors pyrometer with t, r1 , r2 the Fresnel’s transmission at the diamond-air interface and reflections coefficient at the diamond-air interface (1) and diamond–silicon interface (2), St and Sint the transmission and internal scattering factors at the diamond–air interface. (b) Variation of the apparent substrate temperature during diamond growth process. Variation of the diamond film thickness (c) and the diamond deposition rate (d) as a function of time.
the activation energy of hydrogen extraction from monohydride diamond surface under low energy hydrogen atom irradiation, as calculated by Kanai et al. by ab initio molecular dynamics method w7x. This brings the direct evidence that hydrogen desorption from diamond surface under atomic hydrogen flux is limiting the deposition rate in a high-pressure region and the activation energy of this process is 0.2 eV. Next, Fig. 1a and c indicate that the deposition rate calculated for the diamond growth on the silicon substrate is always lower than that for the diamond growth on CVD diamond underlayer. This phenomenon cannot be explained only through the incubation time of the diamond growth over the non-diamond substrates. To study this effect quantitatively, we monitored the diamond thickness and the instantaneous deposition rate from the dependence of the apparent substrate temperature, measured with two-colors pyrometer. The pyrometer, working at the infrared wavelengths l1s1.55 mm and l2s1.35 mm, is located 37 cm from the substrate at the detection angle of 638 (Fig. 3a). Fig. 3b represents the variation of the substrate temperature during diamond deposition. In this case, the average deposition rate was 3.1 mmyh as calculated from the deposited mass. Generally, in this figure, several oscillations of the temperature are observed and the extrema’s ampli-
tude decreases with the deposition time. These observed oscillations are explained by interference effect of infrared light being emitted by the hot silicon substrate in diamond thin film w8x. The amplitude of the interference fringes decreases due to the light scattering at rough surface (surface roughness is developing with the growth time) and due to non-sufficient spectral resolution of the pyrometer or thickness variation for thicker layers w9,10x. Measured apparent temperature is determined by the true substrate temperature, by transmittance of the diamond film at both wavelengths and by the Plank’s law. The pyrometer calculates the measured temperature from the ratio R(T) of the measured intensities emitted at the two wavelengths I(l1) and I(l2) The intensity of emitted light is described by the Plank’s law. The ratio R(T) is independent of the sample’s emissivity (´) in the grey body approximation (i.e. ´(l1)s´(l2)). RŽT.f
l25 l51
yl2 EE IŽl1. FFs l1l2 GG IŽl2.
B hk B l
expC D
B
C
kT D
1
(1)
where h is the Plank’s constant, kB is the Boltzmann’s constant and c is the velocity of light. During the deposition, the interference effect in the thin diamond layer modulates the emitted light intensity for both wavelengths. The equation of the intensity for both p
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
Fig. 4. (a) Schematic representation of the ‘evolutionary selection growth’ process, reprinted from w13x. Cross-section scanning electron microscopy image of the grain size of a CVD diamond layer.
and s polarization (Ip(l) and Is(l)) collected by the pyrometer for one wavelength can be determined from the Fresnel’s coefficients and the scattering factors (scalar scattering theory) for the rough diamond surface w11,12x. I(l)sŽTp(l)qTs(l). B
C
s Sq D
1q
calculated the diamond thickness corresponding to the measured, ‘apparent’ temperature extrema. The variation of the film thickness and the deposition rate as a function of the deposition time have been determined assuming nDs2.35 (see Fig. 3b and c). It is clearly observed that the deposition rate is not constant during deposition. At the beginning the deposition rate increases with time and saturates for process time of 3000 s and higher. This result explains the differences between the deposition rate of diamond deposited on silicon or on thick polycrystalline diamond substrate. This phenomenon agrees with the evolutionary selection growth mode of diamond w13x. Diamond grains with the highest growth rate survive (Fig. 4a). This is well illustrated by the diamond grain size evolution observed by scanning electron microscopy cross-section (Fig. 4b). Typical Raman spectra of films deposited under the different deposition are presented in Fig. 5, together with photoluminescence spectra excited by the Argon laser. The Raman spectrum exhibits a strong narrow band (FWHMs4.3 cmy1) of diamond, centred at 1332.6 cmy1, without presence of additional bands due to non-diamond carbon phases. Photoluminescence spectra exhibit mainly an intense and broad band centred at ;1.8 eV and some other peaks attributed to nitrogen and silicon. The origin of this broad photoluminescence band is probably related to a continuous distribution of gap states characteristic of amorphous material located at the grain boundaries w14–16x. The band centred at 2265 cmy1 (1.681 eV) is attributed to silicon vacancy complex w17x. The band centred at 4249 cmy1 (1.945 eV) corresponds to nitrogen vacancy complex w18x. The band centred at 3756 cmy1 (1.95 eV) reflects a substitutional nitrogen atom bonded to the nearest vacancy filled with one electron, which can be negatively charged w19,20x. The defect centred at 2054 cmy1 (2.15 eV) is
I0(l) 2
ŽStt.2
2 t
607
E
F
Sintr1r2 cos(w)qS2intr21r22 G 2
I0(l) 2
(2)
with ws(4pnDecos a)y l and nD: refractive index of diamond. It is worth to note that in the use configuration, the p polarized light is fully transmitted at the diamondy air interface. The measured ratio (Rexp(T)), which gives the apparent temperature, is written as: RexpŽT.s
T(l1) RŽT. T(l2)
(3)
Based on the above-described theory, we numerically
Fig. 5. Photoluminescence spectrum of a diamond film deposited at 200 mbar and a methane concentration of 0.5%.
608
V. Mortet et al. / Diamond and Related Materials 13 (2004) 604–609
Fig. 6. Spectral dependence of photocurrent, measured by Fourier transform photocurrent spectroscopy at 77 K. Three diamond wafers deposited under the different deposition conditions was investigated.
attributed to the nitrogen vacancy or substitutional nitrogen in neutral state w21,22x. Diamond films were also characterized by X-ray photoemission spectroscopy. Diamond samples exhibit a clearly dominating carbon signal. An oxygen signal is recognized in wide-range survey spectra. This is probably due to air-exposition of the sample surface. Further analyses were carried on following the surface cleaning by ion bombardment. Silicon, nitrogen and molybdenum traces were found in a long time acquisition spectra. Si and N results are in agreement with the photoluminescence spectroscopy. Surprising was detection of molybdenum in some diamond layers, on the level of XPS detection limit, approximately 0.1–0.2%. We suppose that this contamination come from the excentric position of plasma ball, shifted from Si wafer to Mo ring, which is surrounding the substrate. Detection of Mo was complemented by observation of the new absorption lines in the optical (photocurrent) spectrum obtained by Fourier transform photocurrent spectroscopy. In samples where Mo was detected by XPS, a new, distinct absorption peaks at 0.513, 0.705 andyor 0.746 eV were seen, as shown in Fig. 6. Weaker peaks at 0.623 and 1.14 eV were observed too. No such peaks have been observed so far w23x. 4. Conclusion In this paper, diamond growth by microwave plasma enhanced chemical vapor deposition in the ellipsoidal cavity reactor was studied at various gas mixture of H2 y CH4 in the range 0.5–3% of CH4 in hydrogen and at 125–250 mbar of total gas pressure. We demonstrated the possibility of diamond deposition rate up to 4.5 mmyh, homogeneously over 2-inch silicon substrates. Hydrogen desorption from diamond surface under atom-
ic hydrogen flux is limiting the deposition rate in a high pressure region and the activation energy of this process is 0.2 eV, in accord with a theoretical calculations w7x. The instantaneous deposition rate was determined from the observed fringes of the apparent substrate temperature measured with a two-colors pyrometer. Deposition rate increases during first 30–60 min of deposition. This phenomenon can be explained by the theory of an evolutionary selection growth mode of diamond. Diamond grains with the highest growth rate survive. Silicon, nitrogen (in agreement with the photoluminescence analysis) and surprisingly molybdenum traces were found by XPS analysis in some diamond layers. In the layers with molybdenum impurity, the new absorption peaks located between 0.5 and 1.2 eV were observed by Fourier transform photocurrent spectroscopy. Acknowledgments This work was supported by the Research Training Network of EC, project DoDDS, contract HPRN-CT1999-00139, by the Grant Agency of the Czech Republic, contract 202y02y0218 and NOA015, MSMT CR. References w1x M. Funer, C. Wild, P. Koidl, Surf. Coat. Technol. 116–119 (1999) 853. w2x M. Funer, C. Wild, P. Koidl, Appl. Phys. Lett. 72 (1998) 1149. w3x M. Nesladek, L.M. Stals, A. Stesmans, K. Iakoubovskij, G.J. Adriaenssens, J. Rosa, et al., Appl. Phys. Lett. 72 (1998) 3306. w4x R. Kravets, V. Ogorodniks, A. Poruba, P. Moravec, M. Nesladek, J. Rosa, et al., Phys. Stat. Sol. (a) 193 (2002) 502. w5x M. Vanecek, A. Poruba, Appl. Phys. Lett. 80 (2002) 719. w6x T.-H. Chein, J. Wei, Y. Tzeng, Diamond. Relat. Mater. 8 (1999) 1686. w7x C. Kanai, K. Watanabe, Y. Takakuwa, Appl. Surf. Sci. 159160 (2000) 599. w8x A.J. Spring Thorpe, T.P. Humphreys, A. Majeed, W.T. Moore, Appl. Phys. Lett. 55 (1989) 2138. w9x Z. Remes, Ph. D. Thesis, Charles University, Prague, 1999, http:yywww.fzu.czy;remes. w10x R. Swanepoel, J. Phys. E: Sci. Instrum. 7 (1984) 896. w11x Z. Yin, H.S. Tam, F.W. Smith, Diamond Relat. Mater. 5 (1996) 1490. w12x A. Poruba, A. Fejfar, Z. Remes, J. Springer, M. Vanecek, J. Kocka, et al., J. Appl. Phys. 88 (2000) 148. w13x S.K. Han, M.T. McClure, C.A. Wolden, B. Vlahovic, A. Soldi, S. Sitar, Diamond Relat. Mater. 9 (2000) 1008. w14x A.E. Mora, J.W. Steeds, J.E. Butler, Diamond Relat. Mater. 12 (2003) 310. w15x L. Bergmann, B.R. Stoner, K.F. Turner, J.T. Glass, R.J. Nemanich, J. Appl. Phys. 73 (1993) 3951. w16x M.G. Donato, G. Faggio, M. Marinelli, G. Messina, E. Milani, A. Paoletti, et al., Eur. Phys. J.B 20 (2001) 133. w17x J. Ruan, W.J. Choyke, W.D. Parlow, Appl. Phys. Lett. 58 (1991) 295.
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