- Email: [email protected]
Cathodoluminescence of diamond films grown on pretreated Si (001) substrates by microwave plasma chemical vapour deposition
Diamond and Related Materials 8 (1999) 712–716
Cathodoluminescence of diamond films grown on pretreated Si (001) substrates by microwave plasma chemical vapour deposition Do-Geun Kim a,b, Tae-Yeon Seong a,*, Young-Joon Baik b, M.A. Stevens Kalceff c, M.R. Phillips c a Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 506-712, South Korea b Thin Film Technology Research Centre, Korea Institute of Science and Technology, Seoul 136-791, South Korea c Microstructural Analysis Unit, Faculty of Science, University of Technology, Sydney, NSW2007, Australia Received 22 July 1998; accepted 17 September 1998
Abstract Diamond films were grown on a.c. bias-enhanced nucleated Si(001) wafers using different CH concentrations by microwave 4 plasma chemical vapour deposition. Cathodoluminescence (CL) spectra from the films exhibit emission components which are associated with defects such as neutral atomic vacancies, nitrogen-vacancy complexes and structural defects such as dislocations. The luminescence intensities of the related peaks were found to depend on the BEN and CH concentrations. Comparison of the 4 CL and SEM images indicates that a nitrogen-associated defect is primarily distributed in the {001} growth facets of the diamond grains. However, the structural defect-related centres are found to be located mainly near grain boundaries and {111} growth facets. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cathodoluminescence; Defects; Diamond films; Impurities
1. Introduction Diamond films are of considerable research interest because of their potential technological applications, in particular in the fabrication of high-temperature electronic and optical devices [1,2]. Diamond films grown by chemical vapour deposition (CVD) usually contain a high density of crystallographic defects such as twins, stacking faults, dislocations and grain boundaries. These structural defects can significantly influence the intrinsic properties of diamond films. Therefore, an understanding of the character and behaviour of such defects is important for the optimisation of the performance of devices fabricated from these materials. The cathodoluminescence (CL) technique is recognised as a powerful tool for assessing the optical properties of wide-bandgap materials such as diamond films, since electron-beam excitation of solids produces greater carrier generation rates than typical optical excitation [3]. CL spectra obtained from CVD diamond films exhibit not only various components related to point-
* Corresponding author. Fax: +82 62 9702304; e-mail: [email protected]
defect associated features [4–9], but also broad bands associated with the presence of dislocations [10] and donor–acceptor pairs [11]. CL study of CVD diamond films has shown that the distribution of particular types of defects, i.e. structural defects or impurities, is dependent on growth sectors [12,13]. In this work, CL spectroscopy and imaging in a scanning electron microscope (SEM ) were used to investigate optically active defects and impurities in diamond films grown on differently pretreated Si(100) wafers using microwave plasma CVD. It is shown that nitrogen impurities are mainly distributed in diamond grains with {100} facets, while structural defect-related centres are located primarily in the grain boundaries and the {111} growth sectors.
2. Experimental The bias-enhanced nucleation (BEN ) pretreatment and growth of diamond films were performed in a microwave plasma CVD system (ASTeX@) described previously [14,15]. The substrates used were mirrorpolished p-type (001) Si wafers, which were cleaned in
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 27 9 - 9
D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
situ by H plasma etching. The pretreatment and growth 2 conditions were as follows. The nucleation parameters were: CH concentrations, 4 and 8% in H ; a.c. bias 4 2 voltage, 200 V (60 Hz); microwave power, 1 kW; RMS pressure, 20 Torr; substrate temperature ~850 °C; time, 20, 25 and 40 min (termed here ‘‘20 BEN wafer’’, etc.). The growth parameters were: CH concentration, 2% 4 in H ; microwave power, 1.5 kW; pressure, 30 Torr; 2 substrate temperatures, 780 and 820 °C; time, 20 h. The CL experiments were performed in a JEOL JSM 35C SEM equipped for maximum versatility and sensitivity with Oxford Instruments liquid helium (LHe) cryogenic stages, and an Oxford Instruments MonoCL2 cathodoluminescence imaging and spectral analysis system. The CL was excited using a continuous electron beam (E =5–25 keV and I =10 nA) at normal inci0 B dence, and measured using a retractable parabolic mirror collector. Spectra measured over the wavelength range 350–850 nm with a 5 nm bandpass were collected by photon counting using a Hamamatsu R943-02 Peltier cooled high-sensitivity photomultiplier tube (HSPMT ) with a 1200 lines per mm grating, blazed at 500 nm. The total instrument response was a smoothly varying function between ~50 and 900 nm, peaking at ~500 nm. The CL spectra, collected as a function of wavelength l(nm), were not corrected for total instrument response, as the diamond emissions of interest for imaging were well resolved.
3. Results and discussion In Fig. 1a and b we show SEM secondary electron (SE) images obtained from diamond films deposited at 820 °C on wafers which were bias-nucleated with 4% CH for 25 and 40 min, respectively. The alignment of 4 the diamond grains is strongly affected by the BEN time. Growth on the 25 BEN wafer leads to a (100)oriented diamond film (Fig. 1a), whereas an ill-defined
713
Fig. 2. CL spectra at 295 K observed in (a) (100)-oriented and (b) randomly oriented films.
surface morphology is obtained in the film deposited on the 40 BEN wafer, which completely lost the alignment of the grains (Fig. 1b). The effect of the BEN time on the alignment could be related to the presence of heteroepitaxially oriented diamond crystallites which were nucleated during the BEN process [14,15]. Fig. 2a and b show CL spectra observed at 295 K on the (100)-oriented (Fig. 1a) and randomly oriented ( Fig. 1b) films, respectively. The CL spectrum for the (001)-oriented film consists of three prominent components. First, there is a broad approximately Gaussian line with a peak at 435 nm (2.85 eV ) and a full width at half maximum ( FWHM ) of 0.42 eV. Second, there is a broad red-to-yellow component with a peak at 575 nm (2.16 eV ) and a plateau from 590 to 603 nm (2.09–2.06 eV ). Third, there is an intense peak at 740 nm (1.676 eV ) with a shoulder on the low-energy side of the main peak. The spectrum for the randomly oriented film is composed of two prominent components. First, there is a main emission peak located at 435 nm (2.85 eV ). It can be seen that there is a broad shoulder on the low-energy side of the peak. Second, there is a weak peak at 740 nm
Fig. 1. SEM secondary-electron images obtained from diamond films deposited at 820 °C on wafers which were bias-nucleated with 4% CH for 4 (a) 25 and (b) 40 min.
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D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
Fig. 3. CL spectra at 295 K observed in the {001} and {111} sectors of a (001)-oriented film (areas not shown in Fig. 1(a)).
(1.676 eV ). The relative intensity of the 435 nm (2.85 eV ) peak is significantly enhanced when compared with the (100)-oriented film, whereas the intensity of the 740 nm peak is decreased. Furthermore, the broad red-to-yellow component observed in the (001)-oriented film is absent. Comparison of our results and those reported in the literature indicates that the characteristic CL components observed in this work arise from different types of defects. The 435 nm (2.85 eV ) peak is attributed to dislocation-related centres [10,16 ]. The broad red-toyellow component is somewhat similar to that of sintered polycrystalline diamonds [4]. Comparison indicates that the component is a vibronic spectrum with a zerophonon line at 575 nm (2.156 eV ) [4,13]. Residual stresses were suggested to cause significant broadening of the vibronic structure [4]. The 575 nm (2.156 eV ) peak is thought to be related to nitrogen-vacancy complexes [5,6,13]. The 740 nm (1.676 eV ) peak has been assigned to two different centres: (1) neutral carbon vacancy (GR centre) which results in a zero-phonon line (the GR1 line) at 741 nm (1.673 eV ) [9,13] and (2) a centre containing Si interstitials at 738 nm (1.685 eV ) [7,8]. Robins et al. [13], investigating defects in diamond films deposited by hot-filament CVD, reported that internal stress produced peak shift. However, it was shown that the silicon-related centre was essentially unaffected by stress [7,13]. In the present work, depos-
ition was carried out in a stainless-steel reactor, so Si impurties do not result from plasma etching of the reactor walls [17] but could arise from the diffusion of Si from the substrate during growth. Thus, at this stage it is not possible to assign the 740 nm (1.676 eV ) peak, and so both or either centre may be present in these diamond films. Fig. 3 shows CL spectra observed at 295 K in the {001} and {111} sectors of the (001)-oriented film (areas not shown in Fig. 1a). There is a distinct difference between the two spectra. The dislocation-related component for the {001} sector is significantly reduced. However, the nitrogen-related component for the {111} sector is greatly decreased. This indicates that the different growth sectors contain different types of defects, i.e. dislocations are located mainly near grain boundaries and {111} facets, whilst the nitrogen-vacancy complexes are distributed on the {001} facets. CL spectra (not shown) at 295 K obtained from diamond films deposited at 780 and 820 °C on (001) Si wafers which were bias-nucleated with 8% CH for 4 20 min are similar to those observed in Fig. 3. Fig. 4a shows an SEM SE image of a film deposited at 780 °C on (001) Si wafers which were bias-nucleated with 8% CH for 20 min. The growth habit of the crystal 4 grains is dominantly octahedral with {111} facets. The monochromatic 435 nm CL image of the same region of the film ( Fig. 4b) shows that the luminescence arises mainly from the {111} facets, although non-uniform and ill-defined. This is consistent with the CL spectral results shown in Figs. 2 and 3. Fig. 5a shows an SEM SE image of a film grown at 820 °C on (001) Si wafers which were bias-nucleated with 8% CH for 20 min. The image reveals rectangular 4 {001} facets bounded by {111} facets. In Fig. 5b–d we show CL images taken using the emission peaks at 435, 575 and 740 nm, respectively. It can be seen in the 435 nm image ( Fig. 5b) that the {111} facets are more intensely luminescent than the (001) facets. However, in the 575 and 740 nm images (Fig. 5b and c), the luminescence seems to arise primarily from the smooth rectangular regions of the (001) facets. There is a difference in the size and CL intensity of the luminescent
Fig. 4. (a) SEM secondary-electron image of a film deposited at 780 °C. (b) CL image of the same region of the film shown in (a).
D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
715
Fig. 5. (a) SEM secondary-electron image of a film grown at 820 °C, revealing rectangular {001} facets bounded by {111} facets. CL images taken using the emission peaks of (b) 435 nm, (c) 575 nm and (d) 740 nm.
regions in the two images: the details of the rectangular surface geometry are better defined in the 575 nm image than in the 740 nm image. This difference may indicate that the defects responsible for the 575 nm CL are distributed closer to the surface than the defects responsible for the 740 nm CL. CL studies of CVD diamond films showed that defects such as nitrogen-vacancy complexes and dislocations were mainly located in the near-surface regions adjacent to the {100} growth faces [12,13]. On the other hand, Raman and X-ray diffraction studies of diamond films showed that {111} faces contain more planar defects such as stacking faults and twins than {100} faces [18]. Transmission electron microscopy ( TEM ) studies of highly oriented diamond films deposited by microwave plasma CVD showed that structural defects such as dislocations and twins were located in the {111} growth sectors, whereas the {100} faces were nearly dislocationfree [19]. In the present work, the luminescent centres associated with dislocations were distributed primarily in the {111} growth sectors, whilst the nitrogen-related luminescent centres were located in the {100} growth facets. CL spectral features are known to be strongly dependent on the deposition conditions and methods. In our previous work [14,15], selected-area electron diffraction and TEM results showed that the epitaxial relation between the diamond crystallites and Si substrate was best defined in a 25 BEN (4% CH ) wafer, 4 in which growth of (100)-oriented diamond films was achieved (Fig. 1a). However, the diamond crystallites were randomly oriented ( Fig. 1b) in the 40 BEN (4% CH ) wafer. Thus, the different CL spectral features in 4
this work may be associated with the difference in the BEN time and the growth temperature, which result in different initial growth behaviour and hence different defect behaviour.
4. Summary and conclusion CL spectroscopy and SEM imaging were used to investigate crystallinity and luminescent defects and impurities in (100)-oriented and randomly oriented diamond films deposited on differently BEN Si wafers at 780 and 820 °C. It was shown that the luminescent centres were related to atomic neutral vacancies/Si interstitial centres, nitrogen-vacancy complexes and structural defects such as dislocations. The luminescence intensities of the associated peaks were found to depend on the alignment and growth sectors of the diamond films. The CL intensity features and the monochromatic images of different growth sectors indicate that the dislocation-related luminescent centres are located mainly near grain boundaries and {111} growth sectors, whereas the nitrogen impurities are distributed in the {100} growth facets.
Acknowledgement This work was supported in part by the Ministry of Science and Technology ( Korea) (Chucheon Programme).
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D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
References [1] W. Zhu, B.R. Stoner, B.E. Williams, J.T. Glass, Proc. IEEE. 79 (1991) 621. [2] E. Rohrer, C.F.O. Graeff, R. Janssen, C.E. Nebel, M. Stutzmann, J. Guttler, R. Zachai, Phys. Rev. B 54 (1996) 7874. [3] B.G. Yacobi, D.B. Holt, J. Appl. Phys. 59 (1986) R1. [4] A.T. Collins, S.H. Robertson, J. Mater. Sci. Lett. 4 (1985) 681. [5] A.T. Collins, M. Kamo, Y. Sato, J. Phys. D 22 (1989) 1402. [6 ] A.T. Collins, S.C. Lawson, J. Phys. Condens. Matter 1 (1989) 6929. [7] V.S. Vavilov, A.A. Gippius, A.M. Zaitsev, B.V. Deryaguin, B.V. Spitsyn, A.E. Aleksenko, Sov. Phys. Semicond. 14 (1980) 1078. [8] A.T. Collins, M. Kamo, Y. Sato, J. Mater. Res. 5 (1990) 2507. [9] G. Davies, Proc. R. Soc. London Ser. A 336 (1974) 507. [10] R.J. Graham, T.D. Moustakas, M.M. Disco, J. Appl. Phys. 69 (1991) 3212.
[11] P.J. Dean, Phys. Rev. A 139 (1957) 588. [12] H. Kawarada, Y. Yokota, Y. Mori, K. Nishimura, A. Hiraki, J. Appl. Phys. 67 (1990) 983. [13] L.H. Robins, L.P. Cook, E.N. Farabaugh, A. Feldman, Phys. Rev. B 39 (1989) 13367. [14] T.-Y. Seong, D.G. Kim, K.K. Choi, Y.J. Baik, Appl. Phys. Lett. 70 (1997) 3368. [15] D.G. Kim, T.-Y. Seong, Y.J. Baik, J. Electrochem. Soc. 145 (1998) 2095. [16 ] J. Ruan, W.J. Choyke, W.D. Partlow, J. Appl. Phys. 69 (1991) 6632. [17] B.G. Yacobi, A.R. Badzian, T. Badzian, J. Appl. Phys. 69 (1991) 1643. [18] A.R. Badzian, T. Badzian, L. Pilione, Int. J. Refract. Met. Hard Mater. 9 (1990) 92. [19] N. Bozzolo, S. Barrat, I. Dieguez, E. Bauer-Grosse, Diamond Relat. Mater. 6 (1997) 41.
Cathodoluminescence of diamond films grown on pretreated Si (001) substrates by microwave plasma chemical vapour deposition Do-Geun Kim a,b, Tae-Yeon Seong a,*, Young-Joon Baik b, M.A. Stevens Kalceff c, M.R. Phillips c a Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 506-712, South Korea b Thin Film Technology Research Centre, Korea Institute of Science and Technology, Seoul 136-791, South Korea c Microstructural Analysis Unit, Faculty of Science, University of Technology, Sydney, NSW2007, Australia Received 22 July 1998; accepted 17 September 1998
Abstract Diamond films were grown on a.c. bias-enhanced nucleated Si(001) wafers using different CH concentrations by microwave 4 plasma chemical vapour deposition. Cathodoluminescence (CL) spectra from the films exhibit emission components which are associated with defects such as neutral atomic vacancies, nitrogen-vacancy complexes and structural defects such as dislocations. The luminescence intensities of the related peaks were found to depend on the BEN and CH concentrations. Comparison of the 4 CL and SEM images indicates that a nitrogen-associated defect is primarily distributed in the {001} growth facets of the diamond grains. However, the structural defect-related centres are found to be located mainly near grain boundaries and {111} growth facets. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cathodoluminescence; Defects; Diamond films; Impurities
1. Introduction Diamond films are of considerable research interest because of their potential technological applications, in particular in the fabrication of high-temperature electronic and optical devices [1,2]. Diamond films grown by chemical vapour deposition (CVD) usually contain a high density of crystallographic defects such as twins, stacking faults, dislocations and grain boundaries. These structural defects can significantly influence the intrinsic properties of diamond films. Therefore, an understanding of the character and behaviour of such defects is important for the optimisation of the performance of devices fabricated from these materials. The cathodoluminescence (CL) technique is recognised as a powerful tool for assessing the optical properties of wide-bandgap materials such as diamond films, since electron-beam excitation of solids produces greater carrier generation rates than typical optical excitation [3]. CL spectra obtained from CVD diamond films exhibit not only various components related to point-
* Corresponding author. Fax: +82 62 9702304; e-mail: [email protected]
defect associated features [4–9], but also broad bands associated with the presence of dislocations [10] and donor–acceptor pairs [11]. CL study of CVD diamond films has shown that the distribution of particular types of defects, i.e. structural defects or impurities, is dependent on growth sectors [12,13]. In this work, CL spectroscopy and imaging in a scanning electron microscope (SEM ) were used to investigate optically active defects and impurities in diamond films grown on differently pretreated Si(100) wafers using microwave plasma CVD. It is shown that nitrogen impurities are mainly distributed in diamond grains with {100} facets, while structural defect-related centres are located primarily in the grain boundaries and the {111} growth sectors.
2. Experimental The bias-enhanced nucleation (BEN ) pretreatment and growth of diamond films were performed in a microwave plasma CVD system (ASTeX@) described previously [14,15]. The substrates used were mirrorpolished p-type (001) Si wafers, which were cleaned in
0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 9 2 5- 9 6 3 5 ( 9 8 ) 0 0 27 9 - 9
D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
situ by H plasma etching. The pretreatment and growth 2 conditions were as follows. The nucleation parameters were: CH concentrations, 4 and 8% in H ; a.c. bias 4 2 voltage, 200 V (60 Hz); microwave power, 1 kW; RMS pressure, 20 Torr; substrate temperature ~850 °C; time, 20, 25 and 40 min (termed here ‘‘20 BEN wafer’’, etc.). The growth parameters were: CH concentration, 2% 4 in H ; microwave power, 1.5 kW; pressure, 30 Torr; 2 substrate temperatures, 780 and 820 °C; time, 20 h. The CL experiments were performed in a JEOL JSM 35C SEM equipped for maximum versatility and sensitivity with Oxford Instruments liquid helium (LHe) cryogenic stages, and an Oxford Instruments MonoCL2 cathodoluminescence imaging and spectral analysis system. The CL was excited using a continuous electron beam (E =5–25 keV and I =10 nA) at normal inci0 B dence, and measured using a retractable parabolic mirror collector. Spectra measured over the wavelength range 350–850 nm with a 5 nm bandpass were collected by photon counting using a Hamamatsu R943-02 Peltier cooled high-sensitivity photomultiplier tube (HSPMT ) with a 1200 lines per mm grating, blazed at 500 nm. The total instrument response was a smoothly varying function between ~50 and 900 nm, peaking at ~500 nm. The CL spectra, collected as a function of wavelength l(nm), were not corrected for total instrument response, as the diamond emissions of interest for imaging were well resolved.
3. Results and discussion In Fig. 1a and b we show SEM secondary electron (SE) images obtained from diamond films deposited at 820 °C on wafers which were bias-nucleated with 4% CH for 25 and 40 min, respectively. The alignment of 4 the diamond grains is strongly affected by the BEN time. Growth on the 25 BEN wafer leads to a (100)oriented diamond film (Fig. 1a), whereas an ill-defined
713
Fig. 2. CL spectra at 295 K observed in (a) (100)-oriented and (b) randomly oriented films.
surface morphology is obtained in the film deposited on the 40 BEN wafer, which completely lost the alignment of the grains (Fig. 1b). The effect of the BEN time on the alignment could be related to the presence of heteroepitaxially oriented diamond crystallites which were nucleated during the BEN process [14,15]. Fig. 2a and b show CL spectra observed at 295 K on the (100)-oriented (Fig. 1a) and randomly oriented ( Fig. 1b) films, respectively. The CL spectrum for the (001)-oriented film consists of three prominent components. First, there is a broad approximately Gaussian line with a peak at 435 nm (2.85 eV ) and a full width at half maximum ( FWHM ) of 0.42 eV. Second, there is a broad red-to-yellow component with a peak at 575 nm (2.16 eV ) and a plateau from 590 to 603 nm (2.09–2.06 eV ). Third, there is an intense peak at 740 nm (1.676 eV ) with a shoulder on the low-energy side of the main peak. The spectrum for the randomly oriented film is composed of two prominent components. First, there is a main emission peak located at 435 nm (2.85 eV ). It can be seen that there is a broad shoulder on the low-energy side of the peak. Second, there is a weak peak at 740 nm
Fig. 1. SEM secondary-electron images obtained from diamond films deposited at 820 °C on wafers which were bias-nucleated with 4% CH for 4 (a) 25 and (b) 40 min.
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D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
Fig. 3. CL spectra at 295 K observed in the {001} and {111} sectors of a (001)-oriented film (areas not shown in Fig. 1(a)).
(1.676 eV ). The relative intensity of the 435 nm (2.85 eV ) peak is significantly enhanced when compared with the (100)-oriented film, whereas the intensity of the 740 nm peak is decreased. Furthermore, the broad red-to-yellow component observed in the (001)-oriented film is absent. Comparison of our results and those reported in the literature indicates that the characteristic CL components observed in this work arise from different types of defects. The 435 nm (2.85 eV ) peak is attributed to dislocation-related centres [10,16 ]. The broad red-toyellow component is somewhat similar to that of sintered polycrystalline diamonds [4]. Comparison indicates that the component is a vibronic spectrum with a zerophonon line at 575 nm (2.156 eV ) [4,13]. Residual stresses were suggested to cause significant broadening of the vibronic structure [4]. The 575 nm (2.156 eV ) peak is thought to be related to nitrogen-vacancy complexes [5,6,13]. The 740 nm (1.676 eV ) peak has been assigned to two different centres: (1) neutral carbon vacancy (GR centre) which results in a zero-phonon line (the GR1 line) at 741 nm (1.673 eV ) [9,13] and (2) a centre containing Si interstitials at 738 nm (1.685 eV ) [7,8]. Robins et al. [13], investigating defects in diamond films deposited by hot-filament CVD, reported that internal stress produced peak shift. However, it was shown that the silicon-related centre was essentially unaffected by stress [7,13]. In the present work, depos-
ition was carried out in a stainless-steel reactor, so Si impurties do not result from plasma etching of the reactor walls [17] but could arise from the diffusion of Si from the substrate during growth. Thus, at this stage it is not possible to assign the 740 nm (1.676 eV ) peak, and so both or either centre may be present in these diamond films. Fig. 3 shows CL spectra observed at 295 K in the {001} and {111} sectors of the (001)-oriented film (areas not shown in Fig. 1a). There is a distinct difference between the two spectra. The dislocation-related component for the {001} sector is significantly reduced. However, the nitrogen-related component for the {111} sector is greatly decreased. This indicates that the different growth sectors contain different types of defects, i.e. dislocations are located mainly near grain boundaries and {111} facets, whilst the nitrogen-vacancy complexes are distributed on the {001} facets. CL spectra (not shown) at 295 K obtained from diamond films deposited at 780 and 820 °C on (001) Si wafers which were bias-nucleated with 8% CH for 4 20 min are similar to those observed in Fig. 3. Fig. 4a shows an SEM SE image of a film deposited at 780 °C on (001) Si wafers which were bias-nucleated with 8% CH for 20 min. The growth habit of the crystal 4 grains is dominantly octahedral with {111} facets. The monochromatic 435 nm CL image of the same region of the film ( Fig. 4b) shows that the luminescence arises mainly from the {111} facets, although non-uniform and ill-defined. This is consistent with the CL spectral results shown in Figs. 2 and 3. Fig. 5a shows an SEM SE image of a film grown at 820 °C on (001) Si wafers which were bias-nucleated with 8% CH for 20 min. The image reveals rectangular 4 {001} facets bounded by {111} facets. In Fig. 5b–d we show CL images taken using the emission peaks at 435, 575 and 740 nm, respectively. It can be seen in the 435 nm image ( Fig. 5b) that the {111} facets are more intensely luminescent than the (001) facets. However, in the 575 and 740 nm images (Fig. 5b and c), the luminescence seems to arise primarily from the smooth rectangular regions of the (001) facets. There is a difference in the size and CL intensity of the luminescent
Fig. 4. (a) SEM secondary-electron image of a film deposited at 780 °C. (b) CL image of the same region of the film shown in (a).
D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
715
Fig. 5. (a) SEM secondary-electron image of a film grown at 820 °C, revealing rectangular {001} facets bounded by {111} facets. CL images taken using the emission peaks of (b) 435 nm, (c) 575 nm and (d) 740 nm.
regions in the two images: the details of the rectangular surface geometry are better defined in the 575 nm image than in the 740 nm image. This difference may indicate that the defects responsible for the 575 nm CL are distributed closer to the surface than the defects responsible for the 740 nm CL. CL studies of CVD diamond films showed that defects such as nitrogen-vacancy complexes and dislocations were mainly located in the near-surface regions adjacent to the {100} growth faces [12,13]. On the other hand, Raman and X-ray diffraction studies of diamond films showed that {111} faces contain more planar defects such as stacking faults and twins than {100} faces [18]. Transmission electron microscopy ( TEM ) studies of highly oriented diamond films deposited by microwave plasma CVD showed that structural defects such as dislocations and twins were located in the {111} growth sectors, whereas the {100} faces were nearly dislocationfree [19]. In the present work, the luminescent centres associated with dislocations were distributed primarily in the {111} growth sectors, whilst the nitrogen-related luminescent centres were located in the {100} growth facets. CL spectral features are known to be strongly dependent on the deposition conditions and methods. In our previous work [14,15], selected-area electron diffraction and TEM results showed that the epitaxial relation between the diamond crystallites and Si substrate was best defined in a 25 BEN (4% CH ) wafer, 4 in which growth of (100)-oriented diamond films was achieved (Fig. 1a). However, the diamond crystallites were randomly oriented ( Fig. 1b) in the 40 BEN (4% CH ) wafer. Thus, the different CL spectral features in 4
this work may be associated with the difference in the BEN time and the growth temperature, which result in different initial growth behaviour and hence different defect behaviour.
4. Summary and conclusion CL spectroscopy and SEM imaging were used to investigate crystallinity and luminescent defects and impurities in (100)-oriented and randomly oriented diamond films deposited on differently BEN Si wafers at 780 and 820 °C. It was shown that the luminescent centres were related to atomic neutral vacancies/Si interstitial centres, nitrogen-vacancy complexes and structural defects such as dislocations. The luminescence intensities of the associated peaks were found to depend on the alignment and growth sectors of the diamond films. The CL intensity features and the monochromatic images of different growth sectors indicate that the dislocation-related luminescent centres are located mainly near grain boundaries and {111} growth sectors, whereas the nitrogen impurities are distributed in the {100} growth facets.
Acknowledgement This work was supported in part by the Ministry of Science and Technology ( Korea) (Chucheon Programme).
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D.-G. Kim et al. / Diamond and Related Materials 8 (1999) 712–716
References [1] W. Zhu, B.R. Stoner, B.E. Williams, J.T. Glass, Proc. IEEE. 79 (1991) 621. [2] E. Rohrer, C.F.O. Graeff, R. Janssen, C.E. Nebel, M. Stutzmann, J. Guttler, R. Zachai, Phys. Rev. B 54 (1996) 7874. [3] B.G. Yacobi, D.B. Holt, J. Appl. Phys. 59 (1986) R1. [4] A.T. Collins, S.H. Robertson, J. Mater. Sci. Lett. 4 (1985) 681. [5] A.T. Collins, M. Kamo, Y. Sato, J. Phys. D 22 (1989) 1402. [6 ] A.T. Collins, S.C. Lawson, J. Phys. Condens. Matter 1 (1989) 6929. [7] V.S. Vavilov, A.A. Gippius, A.M. Zaitsev, B.V. Deryaguin, B.V. Spitsyn, A.E. Aleksenko, Sov. Phys. Semicond. 14 (1980) 1078. [8] A.T. Collins, M. Kamo, Y. Sato, J. Mater. Res. 5 (1990) 2507. [9] G. Davies, Proc. R. Soc. London Ser. A 336 (1974) 507. [10] R.J. Graham, T.D. Moustakas, M.M. Disco, J. Appl. Phys. 69 (1991) 3212.
[11] P.J. Dean, Phys. Rev. A 139 (1957) 588. [12] H. Kawarada, Y. Yokota, Y. Mori, K. Nishimura, A. Hiraki, J. Appl. Phys. 67 (1990) 983. [13] L.H. Robins, L.P. Cook, E.N. Farabaugh, A. Feldman, Phys. Rev. B 39 (1989) 13367. [14] T.-Y. Seong, D.G. Kim, K.K. Choi, Y.J. Baik, Appl. Phys. Lett. 70 (1997) 3368. [15] D.G. Kim, T.-Y. Seong, Y.J. Baik, J. Electrochem. Soc. 145 (1998) 2095. [16 ] J. Ruan, W.J. Choyke, W.D. Partlow, J. Appl. Phys. 69 (1991) 6632. [17] B.G. Yacobi, A.R. Badzian, T. Badzian, J. Appl. Phys. 69 (1991) 1643. [18] A.R. Badzian, T. Badzian, L. Pilione, Int. J. Refract. Met. Hard Mater. 9 (1990) 92. [19] N. Bozzolo, S. Barrat, I. Dieguez, E. Bauer-Grosse, Diamond Relat. Mater. 6 (1997) 41.