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Study of defects in CVD and ultradisperse diamond
Diamond and Related Materials 8 (1999) 1476–1479 www.elsevier.com/locate/diamond
Study of defects in CVD and ultradisperse diamond K. Iakoubovskii a, *, G.J. Adriaenssens a, K. Meykens b, M. Nesladek b, A.Y. Vul c, V.Y. Osipov c a Laboratorium voor Halfgeleiderfysica, K.U. Leuven, Celestijnenlaan 200 D, B-3001 Heverlee-Leuven, Belgium b Institute for Materials Research, Limburg University, Wetenschapspark 1, B-3590 Diepenbeek, Belgium c Ioffe Physico-Technical Institute, Polytechnicheskaya 26, St. Petersburg 194021, Russia Accepted 11 December 1998
Abstract Characterization of defects in chemical vapor deposition (CVD) and detonation synthesis ultradisperse diamond ( UDD) is reported. Electron Spin Resonance, Raman, and photothermal deflection spectroscopies show that sp2-bonded carbon is a dominant defect in UDD diamond. Although UDD was made from trinitrotoluene, no substitutional nitrogen was detected. Photoluminescence (PL) from CVD films showed narrow lines at 1.68, 1.945 and 2.156 eV, in addition to broad red and green bands, while only a blue band was observed in UDD samples. On the basis of PL excitation measurements, the green band in CVD diamond is attributed to donor–acceptor pair recombination. On the basis of a spatial variation of PL intensity in CVD films, the incorporation mechanism for silicon, nitrogen, and boron atoms is discussed. The Manuscript Prime Novelty statement: It is shown that sp2 carbon is a dominant defect in diamond obtained by detonation technique from trinitrotoluene, while no substitutional nitrogen was detected. The green band in CVD diamond is proved to originate from donor–acceptor pair recombination. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Chemical vapor deposition (CVD); Nitrogen; Photoluminescence; sp2 bonding
1. Introduction Photoluminescence (PL) has been widely used for characterizing defects in diamond [1]. Exciton luminescence and emission from defect centers are well-identified and are used to check the quality of samples and to monitor the presence of specific defects in a material [1]. Some broad bands (blue, green and red bands) are also observed. It is believed that the green band results from the donor–acceptor (D–A) pair recombination [2,3], while the blue band originates from dislocations [4]. We will discuss the origin of the Si- and N-related 1.68, 2.156 and 1.945 eV lines and red band in a separate paper [5]. Here we concentrate mainly on the green band and the spatial distributions of all those features in our chemical vapor deposition (CVD) samples. Recently, a type of diamond, called ultradisperse diamond ( UDD), was obtained by detonating carboncontaining explosives [6 ]. Study of defects in UDD is * Corresponding author. Fax: +32 1632 7987. E-mail address: [email protected] ( K. Iakoubovskii)
stimulated by its small crystallite sizes (a few nanometers) and short synthesis time. For example, quantumsize effect in Raman scattering from UDD is reported [7]. The present paper reports a comprehensive study of defects in UDD by PL, ESR, and photothermal deflection spectroscopies (PDS), along with PL results from CVD diamond films.
2. Experimental Diamond films were grown from CH in an ASTEX 4 PDS-17 microwave plasma reactor at the Institute for Materials Research, Diepenbeek, Belgium [8]. Nitrogen doping was performed by adding N to methane. Low2 resistive p-type Si wafers were used as substrates for diamond growth. Ultradisperse diamond was obtained by commercial detonation synthesis from a 60:40 mixture of trinitrotoluene and hexogene at the Ioffe Institute, St. Petersburg, Russia [9]. The UDD was separated by oxidation of the non-diamond component of the detonation carbon, using aqueous nitric acid at
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 9 ) 0 0 07 1 - 0
K. Iakoubovskii et al. / Diamond and Related Materials 8 (1999) 1476–1479
elevated temperature in a continuous-flow system. The matter obtained after this purification process comprised a suspension of UDD in 30% nitric acid with a diamond concentration of around 3 wt%. Further treatment of the suspension involved multiple dilution with distilled water, mixing, and removal of the sediments. As a result, a 0.04% suspension of UDD in water with a settling time of about 20 days was obtained. Layers of ultradisperse diamond of about 200 nm in thickness were prepared by deposition of UDD particles from an aqueous suspension onto a fused quartz substrate by evaporating off the water. Steady-state PL and PL excitation (PLE ) experiments were performed at room temperature using the following excitation sources: CW Xe lamp with double monochromator, or with single monochromator and interference filters; CW Ar+ laser (different lines in the range 2.41 to −2.71 eV ); and nitrogen laser (0.3 mJ, 3 ns pulse at 3.68 eV ). PL relaxation was measured at room temperature in the time range 0–50 ns using a cavity-dumped dye laser (Spectra Physics 375B) excited by a modelocked Ar+ laser, providing 50 ps pulses at 320 nm. ESR measurements were performed in a K-band home-built spectrometer at 4.2 K. Special care was taken to avoid saturation of the spectra. Details about PDS experimental setup are published elsewhere [8].
3. Results and discussion 3.1. CVD films PL measurements on CVD films have revealed narrow lines at 1.68, 1.945 and 2.156 eV, in addition to broad red and green bands. It is worthwhile to note that a classification of broad bands by color is not certain. PL spectra presented in Fig. 1 illustrate this effect: Curve 1
Fig. 1. PL and PLE spectra of the green band for nitrogen-doped CVD film: (1, 2) PL spectra under 3.68 eV (same for 4.9 eV ) (1), and 2.54 eV (2) excitation; (3, 4) PLE spectra measured at 2.3 eV (3) and 1.8 eV (4) (PLE positions are shown by arrows with numbers); and (5) PLE spectrum for the red band from Ref. [5]. Individual graphs are offset for clarity.
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shows a green PL band following CW excitation at either 3.7 or 4.9 eV, while curve 2 shows an ‘orangered’ PL from the same sample when the excitation energy is lowered to 2.54 eV. While the latter can be interpreted as just the low-energy tail of the former, examination of PLE spectra shows that the situation is more complex. Indeed, the PLE spectrum at 1.8 eV (curve 3) shows features from both the traditional greenband PLE (shown by curve 4 for the present sample) and from the PLE spectrum (curve 5) that is seen in samples where a red band rather than a green is observed [5]. In other words: lowering the excitation energy results in the excitation of centers that are not addressed by the higher energy, a circumstance, which the PLE does reveal. The green band is known to be strongly enhanced by boron doping [10]. At the same time, results of ODMR experiment [11] show that this band is related to the P1 center, which is a substitutional nitrogen donor. PL and PLE spectra of the green band in a nitrogen-doped sample are shown in Fig. 2. The sudden rise at about 3.2 eV in the PLE spectrum is characteristic for a transition between a single level and a band. This feature lends strong support to the notion of D–A recombination as an origin of the green band: transitions from the valence band provide electrons to the donors and create holes in the valence band, which are trapped by the acceptors. There is also a maximum at 2.85 eV in PLE spectrum in Fig. 2, which we attribute to D–A absorption transitions. The energy difference between the maximum and the sharp rise in Fig. 2, 3.2−2.85=0.35 eV agrees with the literature value for the boron activation energy. The PL spectrum of the green band in Fig. 2 shows some sharp lines (they are not seen in PLE spectrum as a result of lower spectral resolution). Those lines are coincident with the ones observed in [3]. In that work, these lines (and the whole green band) were unambiguously assigned to transitions between donors and acceptors sitting at definite lattice sites: assuming
Fig. 2. PL and PLE spectra of the green band for nitrogen-doped CVD film. PL was excited at 3.68 eV, PLE was measured at 2.43 eV. Arrows indicate D–A recombination peaks.
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boron as the acceptor, the ionization energy (E ) of the D donor was determined to be E −3.2 eV. This energy C differs from the activation energy of substitutional nitrogen in diamond (E −1.7 eV [1]). Also, the energy C difference between the substitutional nitrogen and boron levels, 5.5−1.7−0.37=3.43 eV, is to high for the green band. Both mentioned above facts suggest that a center, different from P1 should be considered as the donor in a D–A pair. To check for a spatial distribution of PL features, we measured PL using 5.6 eV excitation at the substrate and front sides of a 240 mm thick CVD film. 5.6 eV photons are strongly absorbed by CVD diamond (typical absorption coefficient is 1 mm−1 [12]) and allow the selective excitation of PL from the near surface part of a film. Results are presented in Fig. 3. It is seen that concentration of the 1.68 eV centers, which are believed to originate from Si-vacancy (Si-V ) complexes [13], is much larger at the substrate side than at the front side. This result shows that Si atoms diffuse to the diamond film from the Si substrate and do not penetrate deeply into the diamond film. From Fig. 3 it also seen that the concentration of the 2.156 eV centers, which are due to nitrogen-vacancy (N-V ) complexes [1], is larger at the front side of a film, while the concentration of the nitrogen–boron (N–B) pairs, which are responsible for the green band, is almost the same at both sides. The presence of boron in our films is probably due to the p-Si substrate, since we do not observe the green band in samples grown on IIa diamond. Similar information about defect distribution in CVD films was obtained from another experiment: PL measurements in films with different thickness. Thin (1–5 mm) films show only the green band and the 1.68 eV line, but with the increasing film thickness the red band gradually appears in the PL spectrum. A similar increase of the intensity of the red band relative to that of the green band is seen with increasing doping level in nitrogen-doped samples. Differences in N, B and Si concentrations can be
Fig. 3. PL spectra excited at 5.6 eV at the substrate and front side of the CVD film.
explained as following: the amount of the N–B pairs is limited by the small boron concentration, which is almost the same on both sides of the film. Even a small amount of nitrogen is enough to form D–A pairs with all boron atoms. When the N–B pair concentration is saturated (as a result of a limited amount of boron atoms), concentration of the N-V complex follows the nitrogen profile, which, according to Ref. [14], increases from the substrate to the front side. This could be as a result of diminishing competition between Si and N in forming Si-V and N-V complexes, as the Si concentration decreases. Examples of such a competition for vacancies between B, N and Si are known: it was already reported that boron doping leads to a decrease in the concentration of N-V [15] and Si-V [16 ] complexes, probably because boron traps vacancies more effectively than nitrogen and silicon. 3.2. UDD Raman measurements on UDD samples [9] show a narrow peak at 1322 cm−1, attributed to crystalline diamond, in addition to broad bands at about 1240 cm−1 and 1600 cm−1, which indicate the presence of an amorphous carbon component. The shift of the diamond peak from its normal position (1332 cm−1) is attributed to quantum confinement owing to the small ˚ , as determined from X-Ray scattering size (about 40 A [9]) of diamond crystallites [7,9]. PDS measurements on UDD shown a featureless spectrum with a threshold at about 1 eV and slow increase towards a higher photon energy, which is typical for amorphous carbon or sp2 carbon in CVD diamond [8]. ESR spectra reveal a broad (11 G), single, Lorentzian shape band with a g-factor of 2.0026 and spin concentration of 1.1×1020 cm−3. This band is well known in CVD diamond and normally consists of two Lorentzian bands, with widths about 3 and 8 G, that are attributed to amorphous carbon in diamond and non-diamond phases, respectively [17]. Only the second one of those bands, having a large width, probably as a result of the relatively high concentration, is observed in our measurements. This result correlates with Raman and PDS measurements, and shows that amorphous carbon is a main paramagnetic defect in UDD. Although UDD was obtained from nitrogen-containing explosive, no paramagnetic nitrogen (<1016 spins cm−3) was detected. This could be due to the short synthesis time of UDD: nitrogen atoms (and probably other impurities) do not have enough time to diffuse into the diamond crystallites; they possibly remain at the grain boundaries or in a graphitic component that is removed in the preparation procedure. PL and PLE spectra for UDD are presented in Fig. 4. PL reveals only a blue band with a relaxation time of
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trinitrotoluene and hexogene, no substitutional nitrogen was detected. On the basis of surface PL measurements at both sides of a CVD film, and results on nitrogen doping, we conclude the following: concentration of boron is almost the same at both sides of CVD film; concentration of Si atoms is higher at the substrate side; concentration of nitrogen increases from the substrate side to the front side.
Acknowledgements Fig. 4. Normalized PL and PLE spectra for UDD sample: (1) PL spectrum under 3.68 eV (same for 4.9 eV ) excitation; and (2–5) PLE spectra measured at 2.23, 2.36, 2.51 and 2.71 eV (positions for the corresponding curve are shown by arrows with numbers).
Authors are indebted to Andre Stesmans for ESR measurements, and to the FWO (grant G.0014.96) for financial support.
References about 1 ns. No nitrogen-related bands, such as the green band, or the 2.156, 1.945 eV lines, were observed in PL, confirming the above conclusion about a small nitrogen content in the material. The position of the PLE maximum for this blue band depends on the chosen wavelength in the PL band. This is probably because the PL band is composed of emission from different levels, distributed over a wide range within the forbidden gap. The blue PL band in UDD, as in natural and CVD diamond [4], can be attributed to dislocations, however, additional experiments are required to reveal the microscopic nature of the emitting centers.
4. Summary and conclusions Results of PL, PLE, PDS and ESR measurements in CVD and detonation synthesis diamonds are reported. PL from CVD films showed narrow lines at 1.68, 1.945 and 2.156 eV, in addition to broad red and green bands, while only a blue band was observed in UDD samples. On the basis of PLE measurements, the green band in CVD diamond is attributed to donor–acceptor pair recombination. ESR, PDS and Raman spectroscopies have shown that sp2-bonded carbon is a dominant defect in UDD. Although UDD was made from a mixture of
[1] A.T. Collins, Diamond Relat. Mater. 1 (1992) 457. [2] P.B. Klein, M.D. Crossfield, J.A. Freitas Jr., A.T. Collins, Phys. Rev. B 51 (1995) 9634. [3] B. Dischler, W. Rothemud, C. Wild, R. Locher, H. Biebl, P. Koidl, Phys. Rev. B 49 (1994) 1685. [4] R.J. Graham, K.V. Ravi, Appl. Phys. Lett. 60 (1992) 1310. [5] K. Iakoubovskii, G.J. Adriaenssens, M. Nesladek, L.M. Stals, Diamond. Relat. Mater. 8 (1999) 717. [6 ] N.R. Greiner, D.S. Phillips, J.D. Johnson, F. Volk, Nature 333 (1988) 440. [7] M. Yoshikawa, Y. Mori, H. Obata, M. Maegawa, G. Katagiri, H. Ishida, A. Ishitani, Appl. Phys. Lett. 67 (1995) 694. [8] M. Nesladek, M. Vanecek, L.M. Stals, Phys. St. Sol. (a) 154 (1996) 283. [9] A.E. Alexensky, M.V. Baidakova, A.Y. Vul’, V.Y. Davydov, Y.A. Pevtsova, Phys. Solid State 39 (1997) 1007. [10] J. Ruan, K. Kobashi, W.J. Choyke, Appl. Phys. Lett. 60 (1992) 3138. [11] M.N. Nazare, P.W. Mason, G.D. Watkins, H. Kanda, Phys. Rev. B 51 (1995) 16741. [12] Z. Yin, H.S. Tan, F.W. Smith, Diamond Relat. Mater. 5 (1996) 1490. [13] C.D. Clark, C.B. Dickerson, Surf. Sci. Technol. 47 (1991) 336. [14] D.F. Talbotponsoby, M.E. Newton, J. Appl. Phys. 82 (1997) 1201. [15] J.A. Freitas Jr., K. Doverspike, P.B. Klein, Y.L. Khong, A.T. Collins, Diamond Relat. Mater. 3 (1994) 821. [16 ] X.H. Wang, G.H.M. Maa, W. Zhu, J.T. Glass, L. Bergman, K.F. Turner, R.J. Nemanich, Diamond Relat. Mater. 1 (1992) 828. [17] Y. Show, Y. Nakamura, T. Izumi, M. Deguchi, M. Kitabake, T. Hirao, Y. Mori, A. Hatta, T. Ito, A. Hiraki, Thin Soild Films 281/282 (1996) 275.
Study of defects in CVD and ultradisperse diamond K. Iakoubovskii a, *, G.J. Adriaenssens a, K. Meykens b, M. Nesladek b, A.Y. Vul c, V.Y. Osipov c a Laboratorium voor Halfgeleiderfysica, K.U. Leuven, Celestijnenlaan 200 D, B-3001 Heverlee-Leuven, Belgium b Institute for Materials Research, Limburg University, Wetenschapspark 1, B-3590 Diepenbeek, Belgium c Ioffe Physico-Technical Institute, Polytechnicheskaya 26, St. Petersburg 194021, Russia Accepted 11 December 1998
Abstract Characterization of defects in chemical vapor deposition (CVD) and detonation synthesis ultradisperse diamond ( UDD) is reported. Electron Spin Resonance, Raman, and photothermal deflection spectroscopies show that sp2-bonded carbon is a dominant defect in UDD diamond. Although UDD was made from trinitrotoluene, no substitutional nitrogen was detected. Photoluminescence (PL) from CVD films showed narrow lines at 1.68, 1.945 and 2.156 eV, in addition to broad red and green bands, while only a blue band was observed in UDD samples. On the basis of PL excitation measurements, the green band in CVD diamond is attributed to donor–acceptor pair recombination. On the basis of a spatial variation of PL intensity in CVD films, the incorporation mechanism for silicon, nitrogen, and boron atoms is discussed. The Manuscript Prime Novelty statement: It is shown that sp2 carbon is a dominant defect in diamond obtained by detonation technique from trinitrotoluene, while no substitutional nitrogen was detected. The green band in CVD diamond is proved to originate from donor–acceptor pair recombination. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Chemical vapor deposition (CVD); Nitrogen; Photoluminescence; sp2 bonding
1. Introduction Photoluminescence (PL) has been widely used for characterizing defects in diamond [1]. Exciton luminescence and emission from defect centers are well-identified and are used to check the quality of samples and to monitor the presence of specific defects in a material [1]. Some broad bands (blue, green and red bands) are also observed. It is believed that the green band results from the donor–acceptor (D–A) pair recombination [2,3], while the blue band originates from dislocations [4]. We will discuss the origin of the Si- and N-related 1.68, 2.156 and 1.945 eV lines and red band in a separate paper [5]. Here we concentrate mainly on the green band and the spatial distributions of all those features in our chemical vapor deposition (CVD) samples. Recently, a type of diamond, called ultradisperse diamond ( UDD), was obtained by detonating carboncontaining explosives [6 ]. Study of defects in UDD is * Corresponding author. Fax: +32 1632 7987. E-mail address: [email protected] ( K. Iakoubovskii)
stimulated by its small crystallite sizes (a few nanometers) and short synthesis time. For example, quantumsize effect in Raman scattering from UDD is reported [7]. The present paper reports a comprehensive study of defects in UDD by PL, ESR, and photothermal deflection spectroscopies (PDS), along with PL results from CVD diamond films.
2. Experimental Diamond films were grown from CH in an ASTEX 4 PDS-17 microwave plasma reactor at the Institute for Materials Research, Diepenbeek, Belgium [8]. Nitrogen doping was performed by adding N to methane. Low2 resistive p-type Si wafers were used as substrates for diamond growth. Ultradisperse diamond was obtained by commercial detonation synthesis from a 60:40 mixture of trinitrotoluene and hexogene at the Ioffe Institute, St. Petersburg, Russia [9]. The UDD was separated by oxidation of the non-diamond component of the detonation carbon, using aqueous nitric acid at
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 9 ) 0 0 07 1 - 0
K. Iakoubovskii et al. / Diamond and Related Materials 8 (1999) 1476–1479
elevated temperature in a continuous-flow system. The matter obtained after this purification process comprised a suspension of UDD in 30% nitric acid with a diamond concentration of around 3 wt%. Further treatment of the suspension involved multiple dilution with distilled water, mixing, and removal of the sediments. As a result, a 0.04% suspension of UDD in water with a settling time of about 20 days was obtained. Layers of ultradisperse diamond of about 200 nm in thickness were prepared by deposition of UDD particles from an aqueous suspension onto a fused quartz substrate by evaporating off the water. Steady-state PL and PL excitation (PLE ) experiments were performed at room temperature using the following excitation sources: CW Xe lamp with double monochromator, or with single monochromator and interference filters; CW Ar+ laser (different lines in the range 2.41 to −2.71 eV ); and nitrogen laser (0.3 mJ, 3 ns pulse at 3.68 eV ). PL relaxation was measured at room temperature in the time range 0–50 ns using a cavity-dumped dye laser (Spectra Physics 375B) excited by a modelocked Ar+ laser, providing 50 ps pulses at 320 nm. ESR measurements were performed in a K-band home-built spectrometer at 4.2 K. Special care was taken to avoid saturation of the spectra. Details about PDS experimental setup are published elsewhere [8].
3. Results and discussion 3.1. CVD films PL measurements on CVD films have revealed narrow lines at 1.68, 1.945 and 2.156 eV, in addition to broad red and green bands. It is worthwhile to note that a classification of broad bands by color is not certain. PL spectra presented in Fig. 1 illustrate this effect: Curve 1
Fig. 1. PL and PLE spectra of the green band for nitrogen-doped CVD film: (1, 2) PL spectra under 3.68 eV (same for 4.9 eV ) (1), and 2.54 eV (2) excitation; (3, 4) PLE spectra measured at 2.3 eV (3) and 1.8 eV (4) (PLE positions are shown by arrows with numbers); and (5) PLE spectrum for the red band from Ref. [5]. Individual graphs are offset for clarity.
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shows a green PL band following CW excitation at either 3.7 or 4.9 eV, while curve 2 shows an ‘orangered’ PL from the same sample when the excitation energy is lowered to 2.54 eV. While the latter can be interpreted as just the low-energy tail of the former, examination of PLE spectra shows that the situation is more complex. Indeed, the PLE spectrum at 1.8 eV (curve 3) shows features from both the traditional greenband PLE (shown by curve 4 for the present sample) and from the PLE spectrum (curve 5) that is seen in samples where a red band rather than a green is observed [5]. In other words: lowering the excitation energy results in the excitation of centers that are not addressed by the higher energy, a circumstance, which the PLE does reveal. The green band is known to be strongly enhanced by boron doping [10]. At the same time, results of ODMR experiment [11] show that this band is related to the P1 center, which is a substitutional nitrogen donor. PL and PLE spectra of the green band in a nitrogen-doped sample are shown in Fig. 2. The sudden rise at about 3.2 eV in the PLE spectrum is characteristic for a transition between a single level and a band. This feature lends strong support to the notion of D–A recombination as an origin of the green band: transitions from the valence band provide electrons to the donors and create holes in the valence band, which are trapped by the acceptors. There is also a maximum at 2.85 eV in PLE spectrum in Fig. 2, which we attribute to D–A absorption transitions. The energy difference between the maximum and the sharp rise in Fig. 2, 3.2−2.85=0.35 eV agrees with the literature value for the boron activation energy. The PL spectrum of the green band in Fig. 2 shows some sharp lines (they are not seen in PLE spectrum as a result of lower spectral resolution). Those lines are coincident with the ones observed in [3]. In that work, these lines (and the whole green band) were unambiguously assigned to transitions between donors and acceptors sitting at definite lattice sites: assuming
Fig. 2. PL and PLE spectra of the green band for nitrogen-doped CVD film. PL was excited at 3.68 eV, PLE was measured at 2.43 eV. Arrows indicate D–A recombination peaks.
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boron as the acceptor, the ionization energy (E ) of the D donor was determined to be E −3.2 eV. This energy C differs from the activation energy of substitutional nitrogen in diamond (E −1.7 eV [1]). Also, the energy C difference between the substitutional nitrogen and boron levels, 5.5−1.7−0.37=3.43 eV, is to high for the green band. Both mentioned above facts suggest that a center, different from P1 should be considered as the donor in a D–A pair. To check for a spatial distribution of PL features, we measured PL using 5.6 eV excitation at the substrate and front sides of a 240 mm thick CVD film. 5.6 eV photons are strongly absorbed by CVD diamond (typical absorption coefficient is 1 mm−1 [12]) and allow the selective excitation of PL from the near surface part of a film. Results are presented in Fig. 3. It is seen that concentration of the 1.68 eV centers, which are believed to originate from Si-vacancy (Si-V ) complexes [13], is much larger at the substrate side than at the front side. This result shows that Si atoms diffuse to the diamond film from the Si substrate and do not penetrate deeply into the diamond film. From Fig. 3 it also seen that the concentration of the 2.156 eV centers, which are due to nitrogen-vacancy (N-V ) complexes [1], is larger at the front side of a film, while the concentration of the nitrogen–boron (N–B) pairs, which are responsible for the green band, is almost the same at both sides. The presence of boron in our films is probably due to the p-Si substrate, since we do not observe the green band in samples grown on IIa diamond. Similar information about defect distribution in CVD films was obtained from another experiment: PL measurements in films with different thickness. Thin (1–5 mm) films show only the green band and the 1.68 eV line, but with the increasing film thickness the red band gradually appears in the PL spectrum. A similar increase of the intensity of the red band relative to that of the green band is seen with increasing doping level in nitrogen-doped samples. Differences in N, B and Si concentrations can be
Fig. 3. PL spectra excited at 5.6 eV at the substrate and front side of the CVD film.
explained as following: the amount of the N–B pairs is limited by the small boron concentration, which is almost the same on both sides of the film. Even a small amount of nitrogen is enough to form D–A pairs with all boron atoms. When the N–B pair concentration is saturated (as a result of a limited amount of boron atoms), concentration of the N-V complex follows the nitrogen profile, which, according to Ref. [14], increases from the substrate to the front side. This could be as a result of diminishing competition between Si and N in forming Si-V and N-V complexes, as the Si concentration decreases. Examples of such a competition for vacancies between B, N and Si are known: it was already reported that boron doping leads to a decrease in the concentration of N-V [15] and Si-V [16 ] complexes, probably because boron traps vacancies more effectively than nitrogen and silicon. 3.2. UDD Raman measurements on UDD samples [9] show a narrow peak at 1322 cm−1, attributed to crystalline diamond, in addition to broad bands at about 1240 cm−1 and 1600 cm−1, which indicate the presence of an amorphous carbon component. The shift of the diamond peak from its normal position (1332 cm−1) is attributed to quantum confinement owing to the small ˚ , as determined from X-Ray scattering size (about 40 A [9]) of diamond crystallites [7,9]. PDS measurements on UDD shown a featureless spectrum with a threshold at about 1 eV and slow increase towards a higher photon energy, which is typical for amorphous carbon or sp2 carbon in CVD diamond [8]. ESR spectra reveal a broad (11 G), single, Lorentzian shape band with a g-factor of 2.0026 and spin concentration of 1.1×1020 cm−3. This band is well known in CVD diamond and normally consists of two Lorentzian bands, with widths about 3 and 8 G, that are attributed to amorphous carbon in diamond and non-diamond phases, respectively [17]. Only the second one of those bands, having a large width, probably as a result of the relatively high concentration, is observed in our measurements. This result correlates with Raman and PDS measurements, and shows that amorphous carbon is a main paramagnetic defect in UDD. Although UDD was obtained from nitrogen-containing explosive, no paramagnetic nitrogen (<1016 spins cm−3) was detected. This could be due to the short synthesis time of UDD: nitrogen atoms (and probably other impurities) do not have enough time to diffuse into the diamond crystallites; they possibly remain at the grain boundaries or in a graphitic component that is removed in the preparation procedure. PL and PLE spectra for UDD are presented in Fig. 4. PL reveals only a blue band with a relaxation time of
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trinitrotoluene and hexogene, no substitutional nitrogen was detected. On the basis of surface PL measurements at both sides of a CVD film, and results on nitrogen doping, we conclude the following: concentration of boron is almost the same at both sides of CVD film; concentration of Si atoms is higher at the substrate side; concentration of nitrogen increases from the substrate side to the front side.
Acknowledgements Fig. 4. Normalized PL and PLE spectra for UDD sample: (1) PL spectrum under 3.68 eV (same for 4.9 eV ) excitation; and (2–5) PLE spectra measured at 2.23, 2.36, 2.51 and 2.71 eV (positions for the corresponding curve are shown by arrows with numbers).
Authors are indebted to Andre Stesmans for ESR measurements, and to the FWO (grant G.0014.96) for financial support.
References about 1 ns. No nitrogen-related bands, such as the green band, or the 2.156, 1.945 eV lines, were observed in PL, confirming the above conclusion about a small nitrogen content in the material. The position of the PLE maximum for this blue band depends on the chosen wavelength in the PL band. This is probably because the PL band is composed of emission from different levels, distributed over a wide range within the forbidden gap. The blue PL band in UDD, as in natural and CVD diamond [4], can be attributed to dislocations, however, additional experiments are required to reveal the microscopic nature of the emitting centers.
4. Summary and conclusions Results of PL, PLE, PDS and ESR measurements in CVD and detonation synthesis diamonds are reported. PL from CVD films showed narrow lines at 1.68, 1.945 and 2.156 eV, in addition to broad red and green bands, while only a blue band was observed in UDD samples. On the basis of PLE measurements, the green band in CVD diamond is attributed to donor–acceptor pair recombination. ESR, PDS and Raman spectroscopies have shown that sp2-bonded carbon is a dominant defect in UDD. Although UDD was made from a mixture of
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