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In situ optical investigations of diamond thin film growth on molybdenum substrates
@AMOND RELATED MATERIALS Dialnond and Related Materials 4 ( 1995t 750 753
ELSEVIER
In situ optical investigations
of diamond thin film growth on molybdenum substrates S. Moulin,
Laboratoire
A.M. Bonnot
d’Etudes des Propri&ds Electroniques des Solides, Centre Notional de 1~1Recherche Scient~fiyue, a.ssoci6 ir l’linirersit6 J. Fourier, BP 166, 38042 Grenoble Cet1e.x 9, France
Abstract In situ optical measurements have been performed during synthesis of diamond thin films by hot filament assisted chemical vapour deposition on molybdenum substrates. Monochromatic measurements of reflectivity and elastic scattered light intensity have enabled us to follow in real time the evolution of the diamond particle size. Hence, growth kinetic domains have been inferred. Spectroscopic measurements of differential reflectivity have evidenced diamond absorption processes during the diamond crystal growth. ~r)~ord.~
Heated
filament
CVD; In-situ characterization:
Nucleation
1. Introduction For several years, diamond films have been synthesized using CVD techniques. Up to now, whatever the synthesis techniques being used, the resulting films are always polycrystalline. Hence, the properties of diamond films never reach the perfection of these of bulk diamond. mostly concerning the transparency and the thermal conductivity. It is thus of crucial importance to obtain a better understanding of the nucleation and growth mechanisms so as to achieve monocrystalline film growth. We have developed in situ optical techniques of investigation of the first stages of diamond film formation. The synthesis was conducted using a hot filament assisted CVD (HFCVD) reactor. The in situ optical measurements were performed both monochromatically, reflectivity and elastic scattering of light [ 11, and spectroscopically, differential reflectivity [ 2,3]. Development of the scattered light intensity with deposition time is to be related to the evolution of the film surface roughness. The reflectivity is related to the transparency of the film which is affected by scattering processes. Spectroscopic differential reflectivity gives insight into surface optical property changes. In particular, diamond absorption processes can be evidenced by measurement of the differential reflectivity in the vicinity of the indirect band gap of bulk diamond (5.5 eV). The association of both a synthesis technique and in situ monochromatic optical 0925.9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI
0925.9635(94)05277-8
and growth:
Optical
properties
measurements has allowed us to follow in real time the diamond growth kinetics. None the less, in situ spectroscopic measurements also enables the presence of diamond particles to be revealed. In this paper we report on in situ optical measurements performed during diamond film deposition on molybdenum substrates. We compare these results with previous results obtained with films supported on silicon substrates [ 31.
2. Experimental
arrangement
The films were synthesized by HFCVD. The synthesis conditions were a 1 vol.% proportion of methane in hydrogen. a 20 standard cm3 min 1 total flow rate, a 30 mbar total pressure and a 700 ‘C substrate temperature 141. By scratching the substrates prior to the deposition, nucleation densities ranging from lo8 cmm2 to 10’ cm ~-’ were obtained. As far as the monochromatic measurements were concerned, laser diode radiation (1.85 eV) was focused on the substrate with a 45” incidence angle and with the electric field parallel to the substrate. A first photodiode was used to measure the reflectivity in the specular direction, and a second collected the scattered light emitted perpendicularly to the substrate. Reflectivity and scattered light were detected regardless of their polarization state.
S. Moulin, A. hf. BonnotjDiamond
For the spectroscopic measurements, the light emitted by a deuterium lamp (3-6 eV) was focused on the substrate with a 60” incidence angle. The electric field was in the plane of incidence. The specular reflected light was focused on the entrance slit of a dispersive monochromator. An optical multichannel analyser equipped with charge-coupled device detectors measured the reflectivity R,(E) vs. the energy E of the incident light for different deposition times t. Finally, a computer was used to calculate the differential reflectivity as
Hence, the (AR/R),,,(E) spectra reflect the changes in reflectivity in between the times t and t + At. The time interval At was chosen to reach the sensitivity threshold of the detection equipment: AR/R> 10P3.
3. Results and discussion 3.1. In situ monochromatic optical measurements: rejlectivity and elastic scattering of light Fig. 1 shows the development of the reflectivity and of the elastic scattered light intensity vs. deposition time t while depositing a diamond film on a molybdenum substrate. The intensities have been normalized to those measured with the bare substrate. At the end of the deposition, scanning electron microscopy observations revealed isolated diamond crystals with a nucleation density of the order of lo8 cm-2. During the first stages of the synthesis (t ~0-60 min), the elastic scattered light remains constant while the reflectivity decreases slowly. Thereafter (t ~60&00 min), the scattered light intensity increases strongly and monotonically while the reflectivity decreases. Finally (ta90 min), the scattered light intensity exhibits strong extrema, and the reflectivity continues to decrease until it reaches a minimum at approximately 140 min. The behaviours of the optical measurements for
0
’ 0
0 30
60 90 120 Deposition time (min)
150
180
Fig. 1. Development of the reflectivity (---) and of the elastic scattering of light (--) with deposition time for a diamond film deposited onto an MO substrate with a 10s cmm2 nucleation density.
and Related Materials
4 (1995) 750-753
751
diamond films supported by MO and Si substrates appear to be very similar [ 1,4]. This was as predicted by theoretical development [ 11. As long as the particle radii are inferior to A/4n,i, (where /z is the incident wavelength and ndia the index of refraction of diamond), the particles can be assimilated to dipoles. In the Rayleigh approximation, the elastic scattered light intensity is then proportional to the square of the volume I/ of the particles [S]. When the particle radii approach 2/4ndi,, the trajectories of light beams inside the particles can no longer be neglected and because diamond is transparent interference processes can be very effective. Two different interference phenomena can be considered. The first is due to an optical path difference between the incident light and the light which is reflected by the substrate. Both beams contribute to the local field which is applied to the particles. The second phenomenon is due to an optical path difference between the scattered light which is directly emitted along the direction of observation and the scattered light which is emitted toward the substrate and then reflected along the direction of observation. Since the particles develop conserving their shape, the variation in the light trajectory inside the growing particles only depends on their sizes. Assimilating these particles to spheres with mean radius a, the scattered light intensity I,, then satisfies the proportionality I,, cc I[ 1 + r,,(f3,) eisr] [ 1 + r,,(O”) eis=]12 where r,,(Q,) is the reflection coefficient at the airsubstrate interface for an incidence angle 8,, /?,= 47ta cos 8,//2 is the optical path difference between the incident light and the light which is reflected by the substrate, r,,(O”) is the reflection coefficient at the diamond&ubstrate interface at normal incidence, and /&c=4rcandi,/;l is the optical path difference between the scattered beams. From this model, which has been previously confirmed by electron microscopy observations [6], the dependence of the particle sizes on deposition time can be inferred. Three kinetics periods are observed. The slow variation in the optical measurements during the first stages of the deposition (t =O-60 min in Fig. 1) relates to an incubation period which precedes the growth of diamond. Thereafter, the monotonic increase in the scattered light intensity (t=60&80 min in Fig. 1) is related to the initiation of the diamond growth. At this stage, the Rayleigh scattered light intensity is proportional to the square of the volume 1/ of the particles. Whatever the substrate, Si or MO, v is proportional to t3” which denotes a limited growth period [ 71. Finally, from the extrema in the elastic scattered intensity, we find that for particles with radii a>60 nm (t ~90 min in Fig. l), I/ is proportional to t3. This relation indicates that the growth process is no longer limited. The development of the reflectivity with deposition
time is mainly related to scattering processes which are caused by film roughness changes [S]. Hence, the decrease in the reflectivity is attributed to the increase in the roughness due to the growth of diamond crystals. rejIectiz;itJl spectroscopy
3.2. D$ferential
To relate the differential reflectivity measurements to characteristic steps of the diamond film formation, monochromatic measurements have been undertaken simultaneously. Fig. 2 and Fig. 3 show the differential reflectivity spectra for various deposition times. According to Fig. 4, which shows the development of the scattered light intensity and of the reflectivity vs. deposition time t, the times in Fig. 2 correspond to the growth period, during which the scattered light intensity increases strongly with t. The times in Fig. 3 correspond to the incubation period, during which both the scattered light intensity and the reflectivity vary very slowly. At the end of the deposition, scanning electron microscopy observations shows that the nucleation density was of the order of lo9 cm-‘. differential reflectivity spectra Fig. 2 shows (AR/R),,,,(E) for a time interval At =30 s. According to the monochromatic measurements (Fig. 4) the mean crystal radii are about 30 nm for t=57 min and about 0
‘= 8
I min
84 min
-0.03 3
3.5
Fig. 2. Differential growth period.
reflectivity
4
4.5 5 Energy (cV)
spectra
5.5
(AT 130 s) during
6
the diamond
0
is
-0.05 3
3.5
4
Energy Fig. 3. Differential tion period.
reflectivity
spectra
5
5.5
6
(eV)
(At = 9 min) during
the incuba-
01 0
30
60 Deposition
120 90 time (min)
I so
0
w
) and of the elastic Fig. 4. Development of the reflectivity ( ) with deposition time for a diamond lilm scattering of light ( deposited onto an Mo substrate wth a IO’ cm ’ nucleation density. 50 nm for f= 84 min. During a time interval At= 30 s, the increase in the radii of the particles is approximately 1 nm. Between 3.3 and 5.8 eV, Fig. 2 evidences three energy domains. Firstly, at energy lower than 4 eV. (AR/R),,,,(E) decreases. The slope increases with increasing deposition time. Secondly, between 4 eV, and 5.5 eV. (AR/R),.,,(E) is almost constant when the particle radii are inferior to 50 nm (f= 57 min and t=63 min). When the particle radii are about 50 nm oscillations in (AR/R),,A,(E) are observed (t = 81 min and t = 84 min). Thirdly, for energies higher than 5.5 eV, (AR/R),,,,(E) increases strongly. Similar results have been obtained while depositing diamond on silicon substrates considering same experimental conditions [ 31. Two remarks should first be pointed out. Firstly. changes in reflectivity with deposition time are to be related to changes in scattered light intensity and to changes in absorption. Secondly, even though the optical properties of Si and MO in the 336 eV energy range are very different, the (AR/R),,,,(E) spectra which are observed for both substrates are very similar. Hence, the changes jn the reflectivity are to be mainly the consequence of the changes in the optical response of the growing diamond particles. For mean particle radii inferior to 60 nm, the Rayleigh scattered light intensity is found to be proportional to 1/‘E4 [S]. Consequently, if scattering processes dominate, an increase in the particle size will induce a decrease in the reflectivity and thus a negative value for AR/R. Moreover, (AR/R),,,,(E) will decrease with E, with the slope increasing with V. This is exactly what we observe for Et4 eV: in this domain (AR/R),,,,(E) is thus dominated by scattering of light by diamond particles. However. for E >4 eV, the constancy of (AR/R),,A,(E) followed by a strong increase cannot be related to scattering processes. Considering that the indirect optical band gap of diamond is 5.5 eV, we relate the slow evolution of (AR/R),,,,(E) in the 4-5.5 eV energy region to band gap state absorption. The oscillations which
S. Moulin, A.M. BonnotJDiumond
appear for particle radii of about 50 nm are due to interference phenomena inside the diamond crystals. Finally, the strong increase in (AR/R),,,,(E) for energies higher than 5.5 eV are related to absorption in the diamond particles. In Fig. 3, the deposition times correspond to the incubation period, according to Fig. 4. The differential reflectivity signal is much weaker than during the growth period and the time interval At has been increased to 9 min. Whatever the deposition time, (AR/R),,,,(E) decreases for E-c 5.5 eV and increases strongly for E >5.5 eV. The same results have also been obtained for diamond film synthesis on Si substrates [3]. As observed during the growth period the strong increase in (AR/R),,,,(E) takes place at energies higher than the bulk diamond indirect optical band gap (5.5 eV). This could indicate the existence of diamond particles in the very first stages of diamond synthesis. The amplitude of (AR/R),,,,(E) first increases (t=9 min and 18 min) and then decreases (t = 24 min) until diamond crystal growth actually takes place. We suggest that the decrease in the amplitude of (AR/R),,,,(E) relates to etching processes: the larger the particle surface, the more effective the etching processes [ 91.
4. Conclusion In situ monochromatic and spectroscopic optical measurements performed during diamond thin film synthesis on molybdenum and silicon substrates have allowed us to observe the evolution of the roughness size with deposition time and to detect the existence of diamond crystals in the first stages of the synthesis. By modelling the elastic scattered light, the development of the diamond crystal size with deposition time has been determined. Three kinetic periods were
and Related Materials
4 (1995) 750-753
753
observed: firstly, an incubation period which is preceding the diamond growth; second, a limited diamond growth period; finally, a non-limited diamond growth period. The development of the differential reflectivity with deposition time in the 3-6 eV energy range has given insights into scattering and absorption processes. During the growth period three energy domains are observed. At energies lower than 4 eV, the differential reflectivity is mainly affected by changes in the scattered light which are due to the growing diamond particles. At energies higher than 4 eV, the changes in the diamond particle absorption are predominant: at energies higher than the bulk diamond optical gap (5.5 eV) effective absorption is evidenced, while between 4 and 5.5 eV band gap state absorption is observed. During the incubation period which precedes the actual diamond growth, absorption phenomena are also observed and are suggested to be due to the existence of diamond nuclei. Atomic force microscopy observations are currently being undertaken to characterize closely the first steps of the deposition.
References 111 A.M. Bonnot, B.S. Mathis and S. Moulin, Appl. Phys. Lett., 63 (1993) 1754. 3 (1994) [21 S. Moulin and A.M. Bonnot, Diamond Relat. Mater., 577-580. World Ceramics 131 A.M. Bonnot and S. Moulin, Proc. 8th CIMTEC Congr. and Forum on New Materiuls, Florence, July 1994 in press. c41 B.S. Mathis and A.M. Bonnot, Diamond R&t. Muter. 2 (1993) 718. c51 H.C. Van de Hulst, Light Scattering by Small Particles, Dover Publications, New York, 1981. Thesis, University Joseph Fourier, Grenoble I, 161 B.S. Mathis, June 1993. c71 M.P. Everson and M.A. Tamor, J. Mater. Res., 7 (1992) 1438. H. Walcher, R. Kohl, 181 C. Wild, P. Koidl, W. Muller-Sebert, N. Herres, R. Lecher, R. Samlenski and R. Brenn, Diamond R&t. Mater., 2 (1993) 158. c91 J.L. Robins, Appl. Surf. Sci., 33-34 (1988) 379.
ELSEVIER
In situ optical investigations
of diamond thin film growth on molybdenum substrates S. Moulin,
Laboratoire
A.M. Bonnot
d’Etudes des Propri&ds Electroniques des Solides, Centre Notional de 1~1Recherche Scient~fiyue, a.ssoci6 ir l’linirersit6 J. Fourier, BP 166, 38042 Grenoble Cet1e.x 9, France
Abstract In situ optical measurements have been performed during synthesis of diamond thin films by hot filament assisted chemical vapour deposition on molybdenum substrates. Monochromatic measurements of reflectivity and elastic scattered light intensity have enabled us to follow in real time the evolution of the diamond particle size. Hence, growth kinetic domains have been inferred. Spectroscopic measurements of differential reflectivity have evidenced diamond absorption processes during the diamond crystal growth. ~r)~ord.~
Heated
filament
CVD; In-situ characterization:
Nucleation
1. Introduction For several years, diamond films have been synthesized using CVD techniques. Up to now, whatever the synthesis techniques being used, the resulting films are always polycrystalline. Hence, the properties of diamond films never reach the perfection of these of bulk diamond. mostly concerning the transparency and the thermal conductivity. It is thus of crucial importance to obtain a better understanding of the nucleation and growth mechanisms so as to achieve monocrystalline film growth. We have developed in situ optical techniques of investigation of the first stages of diamond film formation. The synthesis was conducted using a hot filament assisted CVD (HFCVD) reactor. The in situ optical measurements were performed both monochromatically, reflectivity and elastic scattering of light [ 11, and spectroscopically, differential reflectivity [ 2,3]. Development of the scattered light intensity with deposition time is to be related to the evolution of the film surface roughness. The reflectivity is related to the transparency of the film which is affected by scattering processes. Spectroscopic differential reflectivity gives insight into surface optical property changes. In particular, diamond absorption processes can be evidenced by measurement of the differential reflectivity in the vicinity of the indirect band gap of bulk diamond (5.5 eV). The association of both a synthesis technique and in situ monochromatic optical 0925.9635/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI
0925.9635(94)05277-8
and growth:
Optical
properties
measurements has allowed us to follow in real time the diamond growth kinetics. None the less, in situ spectroscopic measurements also enables the presence of diamond particles to be revealed. In this paper we report on in situ optical measurements performed during diamond film deposition on molybdenum substrates. We compare these results with previous results obtained with films supported on silicon substrates [ 31.
2. Experimental
arrangement
The films were synthesized by HFCVD. The synthesis conditions were a 1 vol.% proportion of methane in hydrogen. a 20 standard cm3 min 1 total flow rate, a 30 mbar total pressure and a 700 ‘C substrate temperature 141. By scratching the substrates prior to the deposition, nucleation densities ranging from lo8 cmm2 to 10’ cm ~-’ were obtained. As far as the monochromatic measurements were concerned, laser diode radiation (1.85 eV) was focused on the substrate with a 45” incidence angle and with the electric field parallel to the substrate. A first photodiode was used to measure the reflectivity in the specular direction, and a second collected the scattered light emitted perpendicularly to the substrate. Reflectivity and scattered light were detected regardless of their polarization state.
S. Moulin, A. hf. BonnotjDiamond
For the spectroscopic measurements, the light emitted by a deuterium lamp (3-6 eV) was focused on the substrate with a 60” incidence angle. The electric field was in the plane of incidence. The specular reflected light was focused on the entrance slit of a dispersive monochromator. An optical multichannel analyser equipped with charge-coupled device detectors measured the reflectivity R,(E) vs. the energy E of the incident light for different deposition times t. Finally, a computer was used to calculate the differential reflectivity as
Hence, the (AR/R),,,(E) spectra reflect the changes in reflectivity in between the times t and t + At. The time interval At was chosen to reach the sensitivity threshold of the detection equipment: AR/R> 10P3.
3. Results and discussion 3.1. In situ monochromatic optical measurements: rejlectivity and elastic scattering of light Fig. 1 shows the development of the reflectivity and of the elastic scattered light intensity vs. deposition time t while depositing a diamond film on a molybdenum substrate. The intensities have been normalized to those measured with the bare substrate. At the end of the deposition, scanning electron microscopy observations revealed isolated diamond crystals with a nucleation density of the order of lo8 cm-2. During the first stages of the synthesis (t ~0-60 min), the elastic scattered light remains constant while the reflectivity decreases slowly. Thereafter (t ~60&00 min), the scattered light intensity increases strongly and monotonically while the reflectivity decreases. Finally (ta90 min), the scattered light intensity exhibits strong extrema, and the reflectivity continues to decrease until it reaches a minimum at approximately 140 min. The behaviours of the optical measurements for
0
’ 0
0 30
60 90 120 Deposition time (min)
150
180
Fig. 1. Development of the reflectivity (---) and of the elastic scattering of light (--) with deposition time for a diamond film deposited onto an MO substrate with a 10s cmm2 nucleation density.
and Related Materials
4 (1995) 750-753
751
diamond films supported by MO and Si substrates appear to be very similar [ 1,4]. This was as predicted by theoretical development [ 11. As long as the particle radii are inferior to A/4n,i, (where /z is the incident wavelength and ndia the index of refraction of diamond), the particles can be assimilated to dipoles. In the Rayleigh approximation, the elastic scattered light intensity is then proportional to the square of the volume I/ of the particles [S]. When the particle radii approach 2/4ndi,, the trajectories of light beams inside the particles can no longer be neglected and because diamond is transparent interference processes can be very effective. Two different interference phenomena can be considered. The first is due to an optical path difference between the incident light and the light which is reflected by the substrate. Both beams contribute to the local field which is applied to the particles. The second phenomenon is due to an optical path difference between the scattered light which is directly emitted along the direction of observation and the scattered light which is emitted toward the substrate and then reflected along the direction of observation. Since the particles develop conserving their shape, the variation in the light trajectory inside the growing particles only depends on their sizes. Assimilating these particles to spheres with mean radius a, the scattered light intensity I,, then satisfies the proportionality I,, cc I[ 1 + r,,(f3,) eisr] [ 1 + r,,(O”) eis=]12 where r,,(Q,) is the reflection coefficient at the airsubstrate interface for an incidence angle 8,, /?,= 47ta cos 8,//2 is the optical path difference between the incident light and the light which is reflected by the substrate, r,,(O”) is the reflection coefficient at the diamond&ubstrate interface at normal incidence, and /&c=4rcandi,/;l is the optical path difference between the scattered beams. From this model, which has been previously confirmed by electron microscopy observations [6], the dependence of the particle sizes on deposition time can be inferred. Three kinetics periods are observed. The slow variation in the optical measurements during the first stages of the deposition (t =O-60 min in Fig. 1) relates to an incubation period which precedes the growth of diamond. Thereafter, the monotonic increase in the scattered light intensity (t=60&80 min in Fig. 1) is related to the initiation of the diamond growth. At this stage, the Rayleigh scattered light intensity is proportional to the square of the volume 1/ of the particles. Whatever the substrate, Si or MO, v is proportional to t3” which denotes a limited growth period [ 71. Finally, from the extrema in the elastic scattered intensity, we find that for particles with radii a>60 nm (t ~90 min in Fig. l), I/ is proportional to t3. This relation indicates that the growth process is no longer limited. The development of the reflectivity with deposition
time is mainly related to scattering processes which are caused by film roughness changes [S]. Hence, the decrease in the reflectivity is attributed to the increase in the roughness due to the growth of diamond crystals. rejIectiz;itJl spectroscopy
3.2. D$ferential
To relate the differential reflectivity measurements to characteristic steps of the diamond film formation, monochromatic measurements have been undertaken simultaneously. Fig. 2 and Fig. 3 show the differential reflectivity spectra for various deposition times. According to Fig. 4, which shows the development of the scattered light intensity and of the reflectivity vs. deposition time t, the times in Fig. 2 correspond to the growth period, during which the scattered light intensity increases strongly with t. The times in Fig. 3 correspond to the incubation period, during which both the scattered light intensity and the reflectivity vary very slowly. At the end of the deposition, scanning electron microscopy observations shows that the nucleation density was of the order of lo9 cm-‘. differential reflectivity spectra Fig. 2 shows (AR/R),,,,(E) for a time interval At =30 s. According to the monochromatic measurements (Fig. 4) the mean crystal radii are about 30 nm for t=57 min and about 0
‘= 8
I min
84 min
-0.03 3
3.5
Fig. 2. Differential growth period.
reflectivity
4
4.5 5 Energy (cV)
spectra
5.5
(AT 130 s) during
6
the diamond
0
is
-0.05 3
3.5
4
Energy Fig. 3. Differential tion period.
reflectivity
spectra
5
5.5
6
(eV)
(At = 9 min) during
the incuba-
01 0
30
60 Deposition
120 90 time (min)
I so
0
w
) and of the elastic Fig. 4. Development of the reflectivity ( ) with deposition time for a diamond lilm scattering of light ( deposited onto an Mo substrate wth a IO’ cm ’ nucleation density. 50 nm for f= 84 min. During a time interval At= 30 s, the increase in the radii of the particles is approximately 1 nm. Between 3.3 and 5.8 eV, Fig. 2 evidences three energy domains. Firstly, at energy lower than 4 eV. (AR/R),,,,(E) decreases. The slope increases with increasing deposition time. Secondly, between 4 eV, and 5.5 eV. (AR/R),.,,(E) is almost constant when the particle radii are inferior to 50 nm (f= 57 min and t=63 min). When the particle radii are about 50 nm oscillations in (AR/R),,A,(E) are observed (t = 81 min and t = 84 min). Thirdly, for energies higher than 5.5 eV, (AR/R),,,,(E) increases strongly. Similar results have been obtained while depositing diamond on silicon substrates considering same experimental conditions [ 31. Two remarks should first be pointed out. Firstly. changes in reflectivity with deposition time are to be related to changes in scattered light intensity and to changes in absorption. Secondly, even though the optical properties of Si and MO in the 336 eV energy range are very different, the (AR/R),,,,(E) spectra which are observed for both substrates are very similar. Hence, the changes jn the reflectivity are to be mainly the consequence of the changes in the optical response of the growing diamond particles. For mean particle radii inferior to 60 nm, the Rayleigh scattered light intensity is found to be proportional to 1/‘E4 [S]. Consequently, if scattering processes dominate, an increase in the particle size will induce a decrease in the reflectivity and thus a negative value for AR/R. Moreover, (AR/R),,,,(E) will decrease with E, with the slope increasing with V. This is exactly what we observe for Et4 eV: in this domain (AR/R),,,,(E) is thus dominated by scattering of light by diamond particles. However. for E >4 eV, the constancy of (AR/R),,A,(E) followed by a strong increase cannot be related to scattering processes. Considering that the indirect optical band gap of diamond is 5.5 eV, we relate the slow evolution of (AR/R),,,,(E) in the 4-5.5 eV energy region to band gap state absorption. The oscillations which
S. Moulin, A.M. BonnotJDiumond
appear for particle radii of about 50 nm are due to interference phenomena inside the diamond crystals. Finally, the strong increase in (AR/R),,,,(E) for energies higher than 5.5 eV are related to absorption in the diamond particles. In Fig. 3, the deposition times correspond to the incubation period, according to Fig. 4. The differential reflectivity signal is much weaker than during the growth period and the time interval At has been increased to 9 min. Whatever the deposition time, (AR/R),,,,(E) decreases for E-c 5.5 eV and increases strongly for E >5.5 eV. The same results have also been obtained for diamond film synthesis on Si substrates [3]. As observed during the growth period the strong increase in (AR/R),,,,(E) takes place at energies higher than the bulk diamond indirect optical band gap (5.5 eV). This could indicate the existence of diamond particles in the very first stages of diamond synthesis. The amplitude of (AR/R),,,,(E) first increases (t=9 min and 18 min) and then decreases (t = 24 min) until diamond crystal growth actually takes place. We suggest that the decrease in the amplitude of (AR/R),,,,(E) relates to etching processes: the larger the particle surface, the more effective the etching processes [ 91.
4. Conclusion In situ monochromatic and spectroscopic optical measurements performed during diamond thin film synthesis on molybdenum and silicon substrates have allowed us to observe the evolution of the roughness size with deposition time and to detect the existence of diamond crystals in the first stages of the synthesis. By modelling the elastic scattered light, the development of the diamond crystal size with deposition time has been determined. Three kinetic periods were
and Related Materials
4 (1995) 750-753
753
observed: firstly, an incubation period which is preceding the diamond growth; second, a limited diamond growth period; finally, a non-limited diamond growth period. The development of the differential reflectivity with deposition time in the 3-6 eV energy range has given insights into scattering and absorption processes. During the growth period three energy domains are observed. At energies lower than 4 eV, the differential reflectivity is mainly affected by changes in the scattered light which are due to the growing diamond particles. At energies higher than 4 eV, the changes in the diamond particle absorption are predominant: at energies higher than the bulk diamond optical gap (5.5 eV) effective absorption is evidenced, while between 4 and 5.5 eV band gap state absorption is observed. During the incubation period which precedes the actual diamond growth, absorption phenomena are also observed and are suggested to be due to the existence of diamond nuclei. Atomic force microscopy observations are currently being undertaken to characterize closely the first steps of the deposition.
References 111 A.M. Bonnot, B.S. Mathis and S. Moulin, Appl. Phys. Lett., 63 (1993) 1754. 3 (1994) [21 S. Moulin and A.M. Bonnot, Diamond Relat. Mater., 577-580. World Ceramics 131 A.M. Bonnot and S. Moulin, Proc. 8th CIMTEC Congr. and Forum on New Materiuls, Florence, July 1994 in press. c41 B.S. Mathis and A.M. Bonnot, Diamond R&t. Muter. 2 (1993) 718. c51 H.C. Van de Hulst, Light Scattering by Small Particles, Dover Publications, New York, 1981. Thesis, University Joseph Fourier, Grenoble I, 161 B.S. Mathis, June 1993. c71 M.P. Everson and M.A. Tamor, J. Mater. Res., 7 (1992) 1438. H. Walcher, R. Kohl, 181 C. Wild, P. Koidl, W. Muller-Sebert, N. Herres, R. Lecher, R. Samlenski and R. Brenn, Diamond R&t. Mater., 2 (1993) 158. c91 J.L. Robins, Appl. Surf. Sci., 33-34 (1988) 379.