- Email: [email protected]
Comparative study of band-A cathodoluminescence and Raman spectroscopy in CVD diamond films
Diamond and Related Materials 8 (1999) 640–644
Comparative study of band-A cathodoluminescence and Raman spectroscopy in CVD diamond films G. Faggio a, M. Marinelli b, G. Messina a,*, E. Milani b, A. Paoletti b, S. Santangelo a, A. Tucciarone b, G. Verona Rinati b a INFM, Facolta` di Ingegneria dell’Universita`, Localita` Feo di Vito, I-89060 Reggio Calabria, Italy b INFM, Dipartimento di Scienze e Tecnologie Fisiche ed Energetiche, Universita` di Roma Tor Vergata, Via di Tor Vergata, I-00133 Rome, Italy Received 27 July 1998; accepted 17 September 1998
Abstract A set of diamond films was grown by microwave plasma enhanced chemical vapour deposition using a CO –CH gas mixture. 2 4 Film morphology, preferential orientation and crystal quality were systematically changed by varying the CH concentration and 4 substrate temperature in the ranges 47–52% and 750–850 °C, respectively. The resulting films were characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and cathodoluminescence (CL). The crystalline quality of the films, as assessed by Raman spectroscopy, increases at lower substrate temperatures (T =750 °C ) and when moving from (110) towards s (100) texturing. Independently of the substrate temperature, a strong decrease of the band-A cathodoluminescence at 435 nm is found as the film preferential orientation goes from (110) to (100). A clear correlation between the width of the diamond Raman line and the band-A emission is observed, giving insight into the nature of this band. In particular, this result is consistent with the attribution of band-A CL to the presence of dislocations. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Cathodoluminescence; CVD diamond; Raman spectroscopy; Texture
1. Introduction Thanks to recent progress achieved in microwave plasma enhanced chemical vapour deposition (MWPECVD), synthetic diamond films with properties approaching those of natural diamond can be obtained in a reproducible and controlled environment. However, the polycrystalline nature of CVD diamond films and the inevitable inclusion of impurities and defects during the growth process may have dramatic effects on the optical and electronic properties of the synthesised films. The production of high-quality films requires the development of growth processes which can drastically reduce the density of undesired inclusions. In this context, a crucial role is played by characterisation studies which, giving insight into the nature of impurities and defects, allow us to control and limit their formation. Clearly, the combined use of many characterisation techniques has several advantages: when giving comple* Corresponding author. Fax: +39 96 5875801; e-mail: [email protected]
mentary information, they increase overall knowledge of the properties of the material, while when they give correlated information this can be used both to select the most convenient technique to be used for a particular study and to give insight into the kind of defects which are likely to influence the response of the measurements performed. In the case of synthetic diamond, Raman and luminescence spectroscopies are among the most important techniques by which to assess sample quality. The information which can be obtained ranges from phase purity to crystal perfection, to presence of impurities and extended defects. Structural defects and impurities reduce the crystalline perfection of diamond films and introduce states within the diamond band gap which give rise to luminescence in the visible spectral region. In particular, the broad ‘‘band A’’ at 435 nm in the cathodoluminescence spectra seems to be related to growth-induced dislocations [1,2]. Thus, the combined use of both Raman spectroscopy and cathodoluminescence (CL) may be very effective to determine the quality of diamond films [3]. In this work, an extensive investigation of the proper-
0925-9635/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 27 3 - 8
G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
ties of diamond films grown by MWPECVD is carried out by means of complementary techniques, i.e. Raman spectroscopy, cathodoluminescence, X-ray diffraction ( XRD) and scanning electron microscopy (SEM ). Since growth conditions such as the deposition temperature and plasma composition strongly influence the phase purity, morphological and crystallographic properties of the synthesised films, finding a correlation between crystalline quality, preferential orientation, cathodoluminescence and growth conditions may be very important to improve the deposition process and to determine the nature of defects. In particular, it is shown that band-A emission is correlated to the width of the Raman line, giving support to the attribution of band-A luminescence to the presence of dislocations.
2. Experimental Diamond films were deposited on 5 mm×5 mm polished p-type Si(100) substrates by MWPECVD using a CH –CO gas mixture. The conventional scratching 4 2 procedure was adopted in order to promote diamond nucleation on the Si surface. Two sets of samples were grown at different substrate temperatures of T =750 °C and T =850 °C. Within s s each set, the CH content was systematically varied in 4 the nominal composition range 47–52% in order to systematically change the film morphology, preferential orientation and crystal quality. The sample thickness, measured by SEM analysis of the cross-section, was kept in the range 20–30 mm by correct choice of the growth times. The preferential orientation of each sample was checked by XRD by means of a Bragg–Brentano diffractometer, using the Cu Ka line as incident radiation. The surface morphology was examined by SEM using a Cambridge Stereoscan 260 instrument. The Raman spectra were recorded on an Instrument S.A. Ramanor U1000 equipped with a microscope (Olympus BX40) for micro-Raman sampling. Excitation was provided by the 514.5 nm line of an argon-ion laser. Using a ×100 objective, the laser beam was focused to a diameter of about 1 mm. Room-temperature CL measurements were performed in the 1.55–6.20 eV range (200–800 nm). The electron beam of the scanning electron microscope was used to excite the luminescence. In particular, the acceleration voltage was fixed at 30 keV with a probe current of about 10−7 A. The custom-built detection system was based on an optical fibre, a monochromator with a 150 grooves per mm grating and a linear UV-enhanced silicon diode array.
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3. Results and discussion The crystalline quality and the phase purity of the different diamond films were assessed by measuring, respectively, the full widths at half maximum ( FWHM ) of the diamond Raman line at 1332 cm−1 and the ratios of diamond to non-diamond components in the Raman spectrum. Fig. 1 shows the Raman spectra of two diamond films deposited at T =750 °C and T =850 °C at the same s s methane concentration (CH =48.1%). For a better com4 parison, the diamond peak is magnified in the inset. The change in the substrate temperature from T =850 °C s to T =750 °C results in a narrowing of the diamond s linewidth from 8.7 to 4.7 cm−1 and a reduction in the large band around 1500 cm−1 due to non-diamond carbon contamination. A similar behaviour is observed at all methane concentrations used in this study. This indicates that better crystalline quality and higher phase purity are obtained at lower substrate temperatures. Fig. 2 shows the XRD patterns of two diamond films grown at the same substrate temperature (T =850 °C ) s using two different CH concentrations. The film grown 4 at CH =47.4% shows an intense (220) diffraction peak 4 ( Fig. 2(a)). A peak intensity ratio I /I greater than 220 111 2 indicates that a well-defined (110) preferential orientation is obtained. The X-ray spectrum of the sample grown at CH =50.0% exhibits a clear (400) diffraction 4 peak ( Fig. 2(b)), indicating a good (100) texturing. A similar evolution of the texturing level with CH content 4 is observed for the samples grown at lower substrate temperatures [4]. However, definitively lower values of the peak intensity ratio are obtained; the (100) preferential orientation is achieved only using a higher methane concentration (CH =52%). 4
Fig. 1. Raman spectra of films deposited at fixed CH concentration 4 (CH =48.1%) and different substrate temperatures (T =750 °C and 4 s T =850 °C ). s
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G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
Fig. 2. X-ray spectra of diamond films grown at a substrate temperature of 850°C and different CH concentrations: (a) CH =47.4%, (b) 4 4 CH =50.0%. 4
Fig. 3 compares the Raman spectra of the same samples as Fig. 2. As the film preferential orientation moves from (110) texturing (at CH =47.4%) towards 4 (100) texturing (at CH =50.0%), the FWHM of the 4 diamond Raman line narrows from about 7.6 cm−1 to about 4.6 cm−1, indicating that higher crystal perfection is obtained in (100)-textured films. The diamond linewidth as a function of the CH 4 content at T =750 °C and T =850 °C is reported in s s Fig. 4. At both temperatures, the FWHM remains almost constant, within the experimental error, as long as the preferential orientation is (110), while it decreases strongly as the preferential orientation goes to (100). Ever narrower linewidths are obtained at lower substrate temperatures. An FWHM of only 2.4 cm−1 was mea-
Fig. 3. Raman spectra of diamond films grown at a substrate temperature of 850 °C and different CH concentrations of CH =47.4 and 4 4 50.0%.
Fig. 4. Dependence of the FWHM of the diamond Raman line on the CH concentration. 4
sured in the film grown at T =750 °C and CH =52%, s 4 which exhibits a good (100) texture. The cathodoluminescence spectra of CVD diamond films are often extremely complex, especially when measured at low temperatures: they carry a huge amount of information concerning the sample analysed. However, this richness of information and the possible superposition of spectral features with different origins make their interpretation difficult. In this paper we will focus on the well-known band-A emission, a broad peak having a maximum at about 435 nm. The reason for this choice is that the origin of this band has been debated in the past, but there is now increasing evidence that it may be related to growth-induced dislocations [1,2]. If this is true, it can be expected that the crystalline quality, as measured through the Raman linewidth, should be affected at the same time. Varying the CH concentration 4 in the growth plasma changes the preferential orientation of CVD diamond films and the dislocation concentration, and it is possible to observe how this affects both band-A CL and the Raman linewidth. In Fig. 5 the room-temperature CL spectra of films grown at different CH concentrations and T =850 °C 4 s are reported, evidencing the drop of the band-A CL at high CH values. Since the preferential orientation of 4 the deposited films is strictly related to the CH content 4 in the plasma, the correlation between band-A CL and film texturing is straightforward. High band-A emission is observed in (110)-textured films (CH =47.4 and 4 48.1%), while a very weak CL emission is found in the (100)-oriented sample (CH =50.0%). These results indi4 cate that the density of the structural defects (or of the related impurities) from which the band-A emission originates is lower in (100) than in (110) crystal sectors. The same qualitative trend of the band-A CL as a function of the CH concentration is observed in the 4
G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
Fig. 5. CL spectra of diamond films deposited at a substrate temperature T =850 °C and different CH concentrations. s 4
samples grown at T =750 °C, although the much weaker s band-A emission leads to substantially higher relative measurement errors, and the crystal quality is good for the lowest methane concentrations. These results, although obtained with a CH –CO mixture, should 4 2 hold for all hydrocarbon-containing growth plasmas [5,6 ]. The general conclusion is that a lower density of crystalline defects is found in samples grown at lower substrate temperatures and with plasma compositions leading to a preferential (100) orientation. This is the same behaviour found for the Raman linewidth. In Fig. 6 the integrated intensity of the band-A emission is plotted as a function of the CH content both 4 at T =750 °C and at T =850 °C. Comparison with s s Fig. 4 shows a clear correlation between width of the diamond Raman line and band-A CL. In particular, at both substrate temperatures, high linewidths are mea-
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sured in samples exhibiting high band-A emission, while low Raman linewidths, indicating high crystalline quality, are associated with weak cathodoluminescence. This is in agreement with the results of Kawarada et al. [7], showing that no band-A emission can be detected from high-quality isolated particles. These results suggest a common origin of Raman line broadening and band-A CL, consistent with the attribution of band-A CL to dislocations. An analogous correlation between Raman linewidth and photoluminescence has been recently observed by Pickard et al. [8]. In Ref. [3] it was reported that higher band-A CL intensities are obtained for samples possessing narrow Raman lines, in contrast to what is found in this paper. However, the results of Ref. [3] refer to samples grown by a hot filament, with Raman linewidths well in excess of 6 cm−1 and a very pronounced scatter of the data. In Ref. [9] it is stated that higher quality CVD films have more intense CL emission. In this case, however, the quality of films grown using a CH –H gas mixture was 4 2 determined from the ratio of the diamond to nondiamond components. Clearly, this is connected to the phase purity of the films rather than to their crystalline quality. Since the phase purity increases at lower methane concentrations in the plasma, the more intense CL emission in these samples is consistent with our data.
4. Conclusions The effect of the change in the deposition temperature from 850 to 750 °C and the methane concentration on morphology, preferential orientation and crystal quality of CVD diamond films grown using a CH –CO gas 4 2 mixture has been studied by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and cathodoluminescence. Raman spectroscopy shows that films of higher crystalline quality and phase purity are obtained at lower substrate temperatures. An improvement of the crystalline quality is also observed as the film orientation moves from (110) to (100). The evolution of the band-A cathodoluminescence has been studied as a function of the crystallographic properties of the film. It transpires that irrespective of the substrate temperature, the band-A emission, which is high in (110)-textured films, lessens drastically as the (100) texturing is achieved. A clear correlation between film quality, as assessed by the FWHM of the diamond Raman line, and band-A cathodoluminescence has been observed. These results support the attribution of band-A emission to the presence of a high concentration of dislocations.
References Fig. 6. Integrated intensity of the band-A emission as a function of the CH concentration at T =750 °C and T =850 °C. 4 s s
[1] R.J. Graham, T.D. Moustakas, M.M. Disko, J. Appl. Phys. 69 (1991) 3212–3218.
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G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
[2] M. Marinelli, A. Paoletti, A. Tucciarone, A. Hatta, T. Ito, A. Hiraki, T. Nishino, Diamond Relat. Mater. 6 (1997) 717–720. [3] L.-T.S. Lin, G. Popovici, Y. Mori, A. Hiraki, M.A. Prelas, B.V. Spitsyn, S. Khasawinah, T. Sung, Diamond Relat. Mater. 5 (1996) 1236–1245. [4] M. Marinelli, E. Milani, A. Paoletti, A. Tucciarone, G. Verona Rinati, N. Randazzo, R. Potenza, M. Pillon, M. Angelone, Diamond Relat. Mater. 7 (1998) 519–522. [5] M. Marinelli, E. Milani, M. Montuori, A. Paoletti, P. Paroli, J. Thomas, Appl. Phys. Lett. 65 (1994) 2839–2841.
[6 ] M. Marinelli, E. Milani, M. Montuori, A. Paoletti, A. Tebano, G. Balestrino, J. Appl. Phys. 76 (1994) 5702–5705. [7] H. Kawarada, T. Tsutsumi, H. Hirayama, A. Yamaguchi, Appl. Phys. Lett. 64 (1994) 451–453. [8] C.D.O. Pickard, T.J. Davis, W.N. Wang, J.W. Steeds, Diamond Relat. Mater. 7 (1998) 238–242. [9] W.D. Partlow, J. Ruan, R.E. Witkowski, W.J. Choyke, D.S. Knight, J. Appl. Phys. 73 (1990) 7019–7025.
Comparative study of band-A cathodoluminescence and Raman spectroscopy in CVD diamond films G. Faggio a, M. Marinelli b, G. Messina a,*, E. Milani b, A. Paoletti b, S. Santangelo a, A. Tucciarone b, G. Verona Rinati b a INFM, Facolta` di Ingegneria dell’Universita`, Localita` Feo di Vito, I-89060 Reggio Calabria, Italy b INFM, Dipartimento di Scienze e Tecnologie Fisiche ed Energetiche, Universita` di Roma Tor Vergata, Via di Tor Vergata, I-00133 Rome, Italy Received 27 July 1998; accepted 17 September 1998
Abstract A set of diamond films was grown by microwave plasma enhanced chemical vapour deposition using a CO –CH gas mixture. 2 4 Film morphology, preferential orientation and crystal quality were systematically changed by varying the CH concentration and 4 substrate temperature in the ranges 47–52% and 750–850 °C, respectively. The resulting films were characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and cathodoluminescence (CL). The crystalline quality of the films, as assessed by Raman spectroscopy, increases at lower substrate temperatures (T =750 °C ) and when moving from (110) towards s (100) texturing. Independently of the substrate temperature, a strong decrease of the band-A cathodoluminescence at 435 nm is found as the film preferential orientation goes from (110) to (100). A clear correlation between the width of the diamond Raman line and the band-A emission is observed, giving insight into the nature of this band. In particular, this result is consistent with the attribution of band-A CL to the presence of dislocations. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Cathodoluminescence; CVD diamond; Raman spectroscopy; Texture
1. Introduction Thanks to recent progress achieved in microwave plasma enhanced chemical vapour deposition (MWPECVD), synthetic diamond films with properties approaching those of natural diamond can be obtained in a reproducible and controlled environment. However, the polycrystalline nature of CVD diamond films and the inevitable inclusion of impurities and defects during the growth process may have dramatic effects on the optical and electronic properties of the synthesised films. The production of high-quality films requires the development of growth processes which can drastically reduce the density of undesired inclusions. In this context, a crucial role is played by characterisation studies which, giving insight into the nature of impurities and defects, allow us to control and limit their formation. Clearly, the combined use of many characterisation techniques has several advantages: when giving comple* Corresponding author. Fax: +39 96 5875801; e-mail: [email protected]
mentary information, they increase overall knowledge of the properties of the material, while when they give correlated information this can be used both to select the most convenient technique to be used for a particular study and to give insight into the kind of defects which are likely to influence the response of the measurements performed. In the case of synthetic diamond, Raman and luminescence spectroscopies are among the most important techniques by which to assess sample quality. The information which can be obtained ranges from phase purity to crystal perfection, to presence of impurities and extended defects. Structural defects and impurities reduce the crystalline perfection of diamond films and introduce states within the diamond band gap which give rise to luminescence in the visible spectral region. In particular, the broad ‘‘band A’’ at 435 nm in the cathodoluminescence spectra seems to be related to growth-induced dislocations [1,2]. Thus, the combined use of both Raman spectroscopy and cathodoluminescence (CL) may be very effective to determine the quality of diamond films [3]. In this work, an extensive investigation of the proper-
0925-9635/99/$ – see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 27 3 - 8
G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
ties of diamond films grown by MWPECVD is carried out by means of complementary techniques, i.e. Raman spectroscopy, cathodoluminescence, X-ray diffraction ( XRD) and scanning electron microscopy (SEM ). Since growth conditions such as the deposition temperature and plasma composition strongly influence the phase purity, morphological and crystallographic properties of the synthesised films, finding a correlation between crystalline quality, preferential orientation, cathodoluminescence and growth conditions may be very important to improve the deposition process and to determine the nature of defects. In particular, it is shown that band-A emission is correlated to the width of the Raman line, giving support to the attribution of band-A luminescence to the presence of dislocations.
2. Experimental Diamond films were deposited on 5 mm×5 mm polished p-type Si(100) substrates by MWPECVD using a CH –CO gas mixture. The conventional scratching 4 2 procedure was adopted in order to promote diamond nucleation on the Si surface. Two sets of samples were grown at different substrate temperatures of T =750 °C and T =850 °C. Within s s each set, the CH content was systematically varied in 4 the nominal composition range 47–52% in order to systematically change the film morphology, preferential orientation and crystal quality. The sample thickness, measured by SEM analysis of the cross-section, was kept in the range 20–30 mm by correct choice of the growth times. The preferential orientation of each sample was checked by XRD by means of a Bragg–Brentano diffractometer, using the Cu Ka line as incident radiation. The surface morphology was examined by SEM using a Cambridge Stereoscan 260 instrument. The Raman spectra were recorded on an Instrument S.A. Ramanor U1000 equipped with a microscope (Olympus BX40) for micro-Raman sampling. Excitation was provided by the 514.5 nm line of an argon-ion laser. Using a ×100 objective, the laser beam was focused to a diameter of about 1 mm. Room-temperature CL measurements were performed in the 1.55–6.20 eV range (200–800 nm). The electron beam of the scanning electron microscope was used to excite the luminescence. In particular, the acceleration voltage was fixed at 30 keV with a probe current of about 10−7 A. The custom-built detection system was based on an optical fibre, a monochromator with a 150 grooves per mm grating and a linear UV-enhanced silicon diode array.
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3. Results and discussion The crystalline quality and the phase purity of the different diamond films were assessed by measuring, respectively, the full widths at half maximum ( FWHM ) of the diamond Raman line at 1332 cm−1 and the ratios of diamond to non-diamond components in the Raman spectrum. Fig. 1 shows the Raman spectra of two diamond films deposited at T =750 °C and T =850 °C at the same s s methane concentration (CH =48.1%). For a better com4 parison, the diamond peak is magnified in the inset. The change in the substrate temperature from T =850 °C s to T =750 °C results in a narrowing of the diamond s linewidth from 8.7 to 4.7 cm−1 and a reduction in the large band around 1500 cm−1 due to non-diamond carbon contamination. A similar behaviour is observed at all methane concentrations used in this study. This indicates that better crystalline quality and higher phase purity are obtained at lower substrate temperatures. Fig. 2 shows the XRD patterns of two diamond films grown at the same substrate temperature (T =850 °C ) s using two different CH concentrations. The film grown 4 at CH =47.4% shows an intense (220) diffraction peak 4 ( Fig. 2(a)). A peak intensity ratio I /I greater than 220 111 2 indicates that a well-defined (110) preferential orientation is obtained. The X-ray spectrum of the sample grown at CH =50.0% exhibits a clear (400) diffraction 4 peak ( Fig. 2(b)), indicating a good (100) texturing. A similar evolution of the texturing level with CH content 4 is observed for the samples grown at lower substrate temperatures [4]. However, definitively lower values of the peak intensity ratio are obtained; the (100) preferential orientation is achieved only using a higher methane concentration (CH =52%). 4
Fig. 1. Raman spectra of films deposited at fixed CH concentration 4 (CH =48.1%) and different substrate temperatures (T =750 °C and 4 s T =850 °C ). s
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G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
Fig. 2. X-ray spectra of diamond films grown at a substrate temperature of 850°C and different CH concentrations: (a) CH =47.4%, (b) 4 4 CH =50.0%. 4
Fig. 3 compares the Raman spectra of the same samples as Fig. 2. As the film preferential orientation moves from (110) texturing (at CH =47.4%) towards 4 (100) texturing (at CH =50.0%), the FWHM of the 4 diamond Raman line narrows from about 7.6 cm−1 to about 4.6 cm−1, indicating that higher crystal perfection is obtained in (100)-textured films. The diamond linewidth as a function of the CH 4 content at T =750 °C and T =850 °C is reported in s s Fig. 4. At both temperatures, the FWHM remains almost constant, within the experimental error, as long as the preferential orientation is (110), while it decreases strongly as the preferential orientation goes to (100). Ever narrower linewidths are obtained at lower substrate temperatures. An FWHM of only 2.4 cm−1 was mea-
Fig. 3. Raman spectra of diamond films grown at a substrate temperature of 850 °C and different CH concentrations of CH =47.4 and 4 4 50.0%.
Fig. 4. Dependence of the FWHM of the diamond Raman line on the CH concentration. 4
sured in the film grown at T =750 °C and CH =52%, s 4 which exhibits a good (100) texture. The cathodoluminescence spectra of CVD diamond films are often extremely complex, especially when measured at low temperatures: they carry a huge amount of information concerning the sample analysed. However, this richness of information and the possible superposition of spectral features with different origins make their interpretation difficult. In this paper we will focus on the well-known band-A emission, a broad peak having a maximum at about 435 nm. The reason for this choice is that the origin of this band has been debated in the past, but there is now increasing evidence that it may be related to growth-induced dislocations [1,2]. If this is true, it can be expected that the crystalline quality, as measured through the Raman linewidth, should be affected at the same time. Varying the CH concentration 4 in the growth plasma changes the preferential orientation of CVD diamond films and the dislocation concentration, and it is possible to observe how this affects both band-A CL and the Raman linewidth. In Fig. 5 the room-temperature CL spectra of films grown at different CH concentrations and T =850 °C 4 s are reported, evidencing the drop of the band-A CL at high CH values. Since the preferential orientation of 4 the deposited films is strictly related to the CH content 4 in the plasma, the correlation between band-A CL and film texturing is straightforward. High band-A emission is observed in (110)-textured films (CH =47.4 and 4 48.1%), while a very weak CL emission is found in the (100)-oriented sample (CH =50.0%). These results indi4 cate that the density of the structural defects (or of the related impurities) from which the band-A emission originates is lower in (100) than in (110) crystal sectors. The same qualitative trend of the band-A CL as a function of the CH concentration is observed in the 4
G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
Fig. 5. CL spectra of diamond films deposited at a substrate temperature T =850 °C and different CH concentrations. s 4
samples grown at T =750 °C, although the much weaker s band-A emission leads to substantially higher relative measurement errors, and the crystal quality is good for the lowest methane concentrations. These results, although obtained with a CH –CO mixture, should 4 2 hold for all hydrocarbon-containing growth plasmas [5,6 ]. The general conclusion is that a lower density of crystalline defects is found in samples grown at lower substrate temperatures and with plasma compositions leading to a preferential (100) orientation. This is the same behaviour found for the Raman linewidth. In Fig. 6 the integrated intensity of the band-A emission is plotted as a function of the CH content both 4 at T =750 °C and at T =850 °C. Comparison with s s Fig. 4 shows a clear correlation between width of the diamond Raman line and band-A CL. In particular, at both substrate temperatures, high linewidths are mea-
643
sured in samples exhibiting high band-A emission, while low Raman linewidths, indicating high crystalline quality, are associated with weak cathodoluminescence. This is in agreement with the results of Kawarada et al. [7], showing that no band-A emission can be detected from high-quality isolated particles. These results suggest a common origin of Raman line broadening and band-A CL, consistent with the attribution of band-A CL to dislocations. An analogous correlation between Raman linewidth and photoluminescence has been recently observed by Pickard et al. [8]. In Ref. [3] it was reported that higher band-A CL intensities are obtained for samples possessing narrow Raman lines, in contrast to what is found in this paper. However, the results of Ref. [3] refer to samples grown by a hot filament, with Raman linewidths well in excess of 6 cm−1 and a very pronounced scatter of the data. In Ref. [9] it is stated that higher quality CVD films have more intense CL emission. In this case, however, the quality of films grown using a CH –H gas mixture was 4 2 determined from the ratio of the diamond to nondiamond components. Clearly, this is connected to the phase purity of the films rather than to their crystalline quality. Since the phase purity increases at lower methane concentrations in the plasma, the more intense CL emission in these samples is consistent with our data.
4. Conclusions The effect of the change in the deposition temperature from 850 to 750 °C and the methane concentration on morphology, preferential orientation and crystal quality of CVD diamond films grown using a CH –CO gas 4 2 mixture has been studied by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and cathodoluminescence. Raman spectroscopy shows that films of higher crystalline quality and phase purity are obtained at lower substrate temperatures. An improvement of the crystalline quality is also observed as the film orientation moves from (110) to (100). The evolution of the band-A cathodoluminescence has been studied as a function of the crystallographic properties of the film. It transpires that irrespective of the substrate temperature, the band-A emission, which is high in (110)-textured films, lessens drastically as the (100) texturing is achieved. A clear correlation between film quality, as assessed by the FWHM of the diamond Raman line, and band-A cathodoluminescence has been observed. These results support the attribution of band-A emission to the presence of a high concentration of dislocations.
References Fig. 6. Integrated intensity of the band-A emission as a function of the CH concentration at T =750 °C and T =850 °C. 4 s s
[1] R.J. Graham, T.D. Moustakas, M.M. Disko, J. Appl. Phys. 69 (1991) 3212–3218.
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G. Faggio et al. / Diamond and Related Materials 8 (1999) 640–644
[2] M. Marinelli, A. Paoletti, A. Tucciarone, A. Hatta, T. Ito, A. Hiraki, T. Nishino, Diamond Relat. Mater. 6 (1997) 717–720. [3] L.-T.S. Lin, G. Popovici, Y. Mori, A. Hiraki, M.A. Prelas, B.V. Spitsyn, S. Khasawinah, T. Sung, Diamond Relat. Mater. 5 (1996) 1236–1245. [4] M. Marinelli, E. Milani, A. Paoletti, A. Tucciarone, G. Verona Rinati, N. Randazzo, R. Potenza, M. Pillon, M. Angelone, Diamond Relat. Mater. 7 (1998) 519–522. [5] M. Marinelli, E. Milani, M. Montuori, A. Paoletti, P. Paroli, J. Thomas, Appl. Phys. Lett. 65 (1994) 2839–2841.
[6 ] M. Marinelli, E. Milani, M. Montuori, A. Paoletti, A. Tebano, G. Balestrino, J. Appl. Phys. 76 (1994) 5702–5705. [7] H. Kawarada, T. Tsutsumi, H. Hirayama, A. Yamaguchi, Appl. Phys. Lett. 64 (1994) 451–453. [8] C.D.O. Pickard, T.J. Davis, W.N. Wang, J.W. Steeds, Diamond Relat. Mater. 7 (1998) 238–242. [9] W.D. Partlow, J. Ruan, R.E. Witkowski, W.J. Choyke, D.S. Knight, J. Appl. Phys. 73 (1990) 7019–7025.