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Real and apparent grain sizes in chemical vapor deposited diamond
Materials Letters 57 (2003) 3690 – 3693 www.elsevier.com/locate/matlet
Real and apparent grain sizes in chemical vapor deposited diamond S.J. Charles a, J.W. Steeds a,*, D.J.F. Evans b, J.E. Butler c a
H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK b DTC Research Centre, Maidenhead, Berks, SL6 6XW, UK c Naval Research Laboratory, Washington, DC 20375, USA
Received 9 December 2002; received in revised form 4 February 2003; accepted 4 February 2003
Abstract A boron-doped, chemical vapor deposited (CVD) diamond, with a doping level of approximately 1018 cm 3, is used to demonstrate the very different grain sizes deduced from the application of various scanning electron microscopy (SEM) techniques to a polished polycrystalline surface. The boron-doped sample was chosen for this investigation because of the very different boron up-take on the {111} and {100} growth sectors and the consequent changes on the physical properties of the underlying crystal. It is shown that SEM contrast resulting from electron channeling gives a gross over-estimate of the grain size, while luminescence microscopy and surface enhanced secondary emission give a better indication, although they can give slight underestimates. The results are consistent with recently published transmission electron microscopy (TEM) observations which show that the grains in thick polycrystalline CVD diamond form clusters with common growth directions [Philos. Mag., A 82 (2002) 1741]. The individual grains within a cluster are related by various twinning operations. This behavior is characteristic of columnar CVD growth and does not depend on boron doping. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Grain boundaries; Surfaces; Microstructure; Characterization methods; Boron-doped diamond; Grain size
As-grown, thick, polycrystalline, CVD diamond has a highly faceted surface which, when studied in a scanning electron microscope (SEM), can give a reasonable indication of the grain size of the material. Once polished, however, these obvious features are removed and different methods are required to show the grain structure. Transmission electron microscopy (TEM) can be used for this purpose and has revealed an important distinction between single grains and
* Corresponding author. Tel.: +44-117-928-8730, +44-117-9288712; fax: +44-117-928-5624. E-mail address: [email protected] (J.W. Steeds).
grain clusters [1]. However, the sample preparation for TEM investigation is time consuming, the technique is destructive, and relatively small thin areas are obtained. It is therefore useful to have techniques that provide similar information without these disadvantages. Boron-doped diamonds are particularly useful in this regard since different growth sectors, having different crystallographic orientations, have very different boron incorporation rates and the resulting boron doping level can be detected using several different techniques. It is widely reported that {111} growth sectors incorporate boron preferentially, while {100} sectors have much lower levels of boron incorporation [2,3]. It has also been reported that
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00152-6
S.J. Charles et al. / Materials Letters 57 (2003) 3690–3693
boron is desegregated at grain boundaries [4]. The CVD diamond sample used in this work was grown on a molybdenum substrate and was very thick, of the order of 400 Am in thickness, and was polished to a flat surface. Several different techniques have been used to examine the polished growth surface of this material, and the results are described. A low-voltage field-emission scanning electron microscope (FEGSEM) was used to examine the samples to investigate the grain structure of the diamonds. Fig. 1a shows an image taken using this microscope with an accelerating voltage of 1 keV and there can clearly be seen structures, with a wide range of dimensions down to f 1 Am in size. These structures cannot be seen when the sample is studied by standard optical microscopy, however some of the features can be seen in Nomarski differential interference contrast (DIC) microscopy after careful matching of the images. The reason that the Nomarski DIC shows some of these features is that different surface facets polish at different rates, therefore there are small height differences on the sample that can be picked out. A room temperature cathodoluminescence (CL) topograph of the same area of this sample is shown in Fig. 1b. It is obvious that the areas with different grayscale can be matched in detail to the 1 keV FEGSEM image. The image recorded is actually colored green, blue and yellow. Dark regions in Fig. 1b correspond to green, mid-gray regions correspond to blue and light gray regions to yellow. The sample was then investigated in Cambridge Instruments Stereoscan 360 SEM with a LaB6 cathode. This machine was set up to measure the ultraviolet cathodoluminescence produced by the incident electron beam. This cathodoluminescence is caused by free excitons and by excitons bound to the boron acceptors. The ratio of the free to bound-exciton intensity has been shown to depend on the boron concentration [5], and it can be used to determine
Fig. 1. Comparison of microscopy techniques for studying a bulk polished sample of boron doped CVD diamond. The images are of the same area of a h110i growth. (a) Low voltage (1 keV) secondary electron image, (b) room temperature cathodoluminescence topograph, (c) low temperature cathodoluminescence image, passing the free and boron-bound excitons, (d) high voltage (15 keV) SEM image, the contrast produced is due to differences in the electron backscatter intensities.
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concentrations up to a value of 6 1018 cm 3. An image of the UV cathodoluminescence from the surface of this sample (Fig. 1c) shows very clear distinctions between high- and low-boron areas, with the areas with high boron concentration dark and low concentration light. The contrast obtained correlates in detail with that of Fig. 1a and b. It follows that the blue (mid-gray) areas in the CL topograph have low boron concentration whereas the yellow (light gray) and green (dark gray) areas have progressively higher boron concentrations. This inference is in agreement with the work of Freitas et al. [6] who deduced that as boron concentration increased the luminescence became progressively more green. They attributed this behavior to donor –acceptor pair luminescence. The boron levels deduced from the measured ratio of the free exciton to the bound exciton luminescence are as follows: 7 ppm in the green (dark gray) areas and 0.7 ppm in the blue (mid gray) areas, with the yellow (light gray) areas intermediate in concentration. Since several authors [7,8] have reported that boron concentrations of approximately 7 ppm produce greatly enhanced secondary electron emission from borondoped diamond, the consistency of these conclusions with the contrast in Fig. 1a is established, bright areas corresponding to regions of high secondary electron emission. A quite different result was obtained after prolonged examination of the sample in the FEGSEM. When the electron accelerating voltage was increased from 1 keV to above 5 keV a dramatic transformation of the contrast was observed. Instead of the fine, sharp structure found previously, a new, much less bright, structure was seen, which had much larger length scales, between 50 and 200 Am in size. The results are shown in Fig. 1d for exactly the same area of the specimen as in Fig. 1a– c: the arrow marks an identifying feature that is dark in Fig. 1c, but bright in Fig. 1d. Note, in particular, that the square-faceted grain, marked F in Fig. 1a, and apparent in Fig. 1b and c, is not visible in this final image. When the electron beam voltage was again reduced back to 1 keV this contrast was unchanged, and even if the sample was left for a week in the FEGSEM, under vacuum, this less bright, larger-scale contrast remained. It was only when the sample had been exposed to atmosphere and then examined again at 1 keV that the original, sharp, bright contrast returned.
In order to investigate the origin of the contrast induced by exposure to the 5 keV electron beam, the sample was tilted. Then different areas became bright, while those that were the brightest became darker, indicating that the contrast was caused by a channeling effect, producing different rates of secondary electron production. This conclusion was confirmed by electron back-scattered diffraction experiments. Each bright area of Fig. 1d corresponds to a region of identical h110i orientation with internal twinning preserving the common h110i direction. These results are in agreement with TEM investigation of this and other polycrystalline CVD samples [1]. Steeds et al. [1] showed that in a large area there are many smaller grains with a common growth direction grouped together forming a ‘‘grain cluster’’. These grain clusters have the approximate size and shape of the large, dark structures shown in the backscattered electron image, while the smaller grains within this area correspond to the structure shown by the low voltage FEGSEM image, the CL maps, and hence the single grain size. We now discuss the reason for the sudden change of SEM contrast after exposure to 5 keV electrons. Shih et al. [7] carried out studies on CVD diamonds, doped with either nitrogen or boron, and showed that the boron doped diamonds had much greater secondary electron emission. They also demonstrated that the presence of surface adsorbates and hydrogenation of the surface also led to higher secondary electron emission. Hydrocarbon molecules are present on the surface of air-exposed diamonds, which desorb completely at temperatures of 400 jC (as seen by Thoms et al. [9]). It is possible that, under exposure to the electron beam, these molecules may decompose to form a hydrogen termination layer on the sample surface. Miller and Brandes [8] and Malta et al. [10] also demonstrated that the hydrogenation of the surface increased the secondary electron emission and that the removal of this layer caused the emission to decrease. Hopman et al. [11] went further to say that there were three regimes in the secondary electron emission from the diamond surface. Firstly, that there is high emission, which then, after a short time where internal electric fields are set up, the emission increases. Eventually, the desorption of the hydrogen from the surface dramatically reduces the yield. We see a similar phenomenon here, where, after time, the
S.J. Charles et al. / Materials Letters 57 (2003) 3690–3693
high secondary electron emission decreases leaving the much darker back-scattered image. The cause of this apparent reduction of the secondary electron emission could either be caused by the removal of either the hydrogen termination layer (produced by the decomposition of hydrocarbon molecules on the sample surface) or the surface adsorbates from the sample. The hydrogen termination layer or adsorbate is removed from the sample surface by the higher energy electron beam, but is rapidly replenished by exposure to the air. This relates to work carried out on the surface conductivity of diamond, as shown in papers by Williams et al. [12], Maier et al. [13] and Ristein et al. [14]. To get the higher surface conductivity they report that both hydrogen termination and a surface adsorbate layer are required. The hydrogen termination layer produces a hole accumulation layer near the surface of the sample and the surface adsorbate layer provides an electron sink to help the formation of this layer. Comparing this with the secondary electron emission case that we have here, it is possible to see that the adsorbate can operate in the same way here, bringing electrons out of the bulk of the sample, allowing them to be emitted more easily by the electron beam. It is unknown whether complete hydrogen termination would increase the yield further, but it would appear to be less of a requirement in the case of boron doped diamond, which already has a ptype character. The two different regimes that are seen in the SEM correspond with the single grains and the commonly diffracting ‘grain clusters’ seen be Steeds et al. [1]. The fact that the boron has been preferentially taken up in one grain allows these different techniques to show the smaller grain sizes, which are very difficult to see in undoped material (the undoped material will only show the larger back-scattered, grain cluster contrast when examined in the SEM). It is therefore of great importance when describing the grains in CVD diamond, that are examined in the SEM, to determine whether it is the grain cluster or the single grain contrast that is being seen. The contrast shown in Fig. 1a, b and c are not necessarily only due to single grains (although the majority are) as it is
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possible that the contrast is due to different growth sectors on a single crystal, such as a crystal with a h100i growth direction, which will have four h111i facets surrounding a central h100i facet. These differences in grain size may have an important bearing on the charge collection distances of CVD diamond when used as a radiation detector [15] and the differences of boron incorporation will influence the use of diamond as an electronic material.
Acknowledgements S.J. Charles would like to thank de Beers Industrial Diamonds, in particular Simon Lawson, Garry Stewart and Rob Caveney, for funding and support throughout the course of his PhD.
References [1] J.W. Steeds, A.E. Mora, J.E. Butler, K.M. Bussmann, Philos. Mag., A 82 (2002) 1741. [2] R. Samlenski, C. Haug, R. Brenn, C. Wild, R. Locher, P. Koidl, Diamond Relat. Mater. 5 (1996) 947. [3] P. Wurzinger, P. Pongratz, P. Hartmann, R. Haubner, B. Lux, Diamond Relat. Mater. 6 (1997) 763. [4] J.T. Huang, C.S. Hu, J. Hwang, H. Chang, L.J. Lee, Appl. Phys. Lett. 67 (1995) 2382. [5] H. Kawarada, H. Matsuyama, Y. Yokota, T. Sogi, A. Yamaguchi, A. Hiraki, Phys. Rev., B 47 (1993) 3633. [6] J.A. Freitas, P.B. Klein, A.T. Collins, Appl. Phys. Lett. 64 (1994) 2136. [7] A. Shih, J. Yater, P. Pehrsson, J. Butler, C. Hor, R. Abrams, J. Appl. Phys. 82 (1997) 1860. [8] J.B. Miller, G.R. Brandes, J. Appl. Phys. 82 (1997) 4538. [9] B.D. Thoms, P.E. Pehrsson, J.E. Butler, J. Appl. Phys. 75 (1994) 1804. [10] D.P. Malta, J.B. Posthill, T.P. Humphreys, M.J. Mantini, R.J. Markunas, Mater. Res. Soc. Symp. Proc. 416 (1996) 311. [11] H.J. Hopman, J. Verhoeven, P.K. Bachmann, H. Wilson, R. Kroon, Diamond Relat. Mater. 8 (1999) 1033. [12] O.A. Williams, M.D. Whitfield, R.B. Jackman, J.S. Foord, J.E. Butler, C.E. Nebel, Appl. Phys. Lett. 78 (2001) 3460. [13] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472. [14] J. Ristein, M. Riedel, F. Maier, B. Mantel, M. Stammler, L. Ley, J. Phys. Condens. Mater. 13 (2001) 8979. [15] E. Vittone, A. Lo Giuduce, C. Paolini, F. Fizzotti, C. Manfredotti, R. Barrett, Diamond Relat. Mater. 11 (2002) 1472.
Real and apparent grain sizes in chemical vapor deposited diamond S.J. Charles a, J.W. Steeds a,*, D.J.F. Evans b, J.E. Butler c a
H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK b DTC Research Centre, Maidenhead, Berks, SL6 6XW, UK c Naval Research Laboratory, Washington, DC 20375, USA
Received 9 December 2002; received in revised form 4 February 2003; accepted 4 February 2003
Abstract A boron-doped, chemical vapor deposited (CVD) diamond, with a doping level of approximately 1018 cm 3, is used to demonstrate the very different grain sizes deduced from the application of various scanning electron microscopy (SEM) techniques to a polished polycrystalline surface. The boron-doped sample was chosen for this investigation because of the very different boron up-take on the {111} and {100} growth sectors and the consequent changes on the physical properties of the underlying crystal. It is shown that SEM contrast resulting from electron channeling gives a gross over-estimate of the grain size, while luminescence microscopy and surface enhanced secondary emission give a better indication, although they can give slight underestimates. The results are consistent with recently published transmission electron microscopy (TEM) observations which show that the grains in thick polycrystalline CVD diamond form clusters with common growth directions [Philos. Mag., A 82 (2002) 1741]. The individual grains within a cluster are related by various twinning operations. This behavior is characteristic of columnar CVD growth and does not depend on boron doping. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Grain boundaries; Surfaces; Microstructure; Characterization methods; Boron-doped diamond; Grain size
As-grown, thick, polycrystalline, CVD diamond has a highly faceted surface which, when studied in a scanning electron microscope (SEM), can give a reasonable indication of the grain size of the material. Once polished, however, these obvious features are removed and different methods are required to show the grain structure. Transmission electron microscopy (TEM) can be used for this purpose and has revealed an important distinction between single grains and
* Corresponding author. Tel.: +44-117-928-8730, +44-117-9288712; fax: +44-117-928-5624. E-mail address: [email protected] (J.W. Steeds).
grain clusters [1]. However, the sample preparation for TEM investigation is time consuming, the technique is destructive, and relatively small thin areas are obtained. It is therefore useful to have techniques that provide similar information without these disadvantages. Boron-doped diamonds are particularly useful in this regard since different growth sectors, having different crystallographic orientations, have very different boron incorporation rates and the resulting boron doping level can be detected using several different techniques. It is widely reported that {111} growth sectors incorporate boron preferentially, while {100} sectors have much lower levels of boron incorporation [2,3]. It has also been reported that
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00152-6
S.J. Charles et al. / Materials Letters 57 (2003) 3690–3693
boron is desegregated at grain boundaries [4]. The CVD diamond sample used in this work was grown on a molybdenum substrate and was very thick, of the order of 400 Am in thickness, and was polished to a flat surface. Several different techniques have been used to examine the polished growth surface of this material, and the results are described. A low-voltage field-emission scanning electron microscope (FEGSEM) was used to examine the samples to investigate the grain structure of the diamonds. Fig. 1a shows an image taken using this microscope with an accelerating voltage of 1 keV and there can clearly be seen structures, with a wide range of dimensions down to f 1 Am in size. These structures cannot be seen when the sample is studied by standard optical microscopy, however some of the features can be seen in Nomarski differential interference contrast (DIC) microscopy after careful matching of the images. The reason that the Nomarski DIC shows some of these features is that different surface facets polish at different rates, therefore there are small height differences on the sample that can be picked out. A room temperature cathodoluminescence (CL) topograph of the same area of this sample is shown in Fig. 1b. It is obvious that the areas with different grayscale can be matched in detail to the 1 keV FEGSEM image. The image recorded is actually colored green, blue and yellow. Dark regions in Fig. 1b correspond to green, mid-gray regions correspond to blue and light gray regions to yellow. The sample was then investigated in Cambridge Instruments Stereoscan 360 SEM with a LaB6 cathode. This machine was set up to measure the ultraviolet cathodoluminescence produced by the incident electron beam. This cathodoluminescence is caused by free excitons and by excitons bound to the boron acceptors. The ratio of the free to bound-exciton intensity has been shown to depend on the boron concentration [5], and it can be used to determine
Fig. 1. Comparison of microscopy techniques for studying a bulk polished sample of boron doped CVD diamond. The images are of the same area of a h110i growth. (a) Low voltage (1 keV) secondary electron image, (b) room temperature cathodoluminescence topograph, (c) low temperature cathodoluminescence image, passing the free and boron-bound excitons, (d) high voltage (15 keV) SEM image, the contrast produced is due to differences in the electron backscatter intensities.
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concentrations up to a value of 6 1018 cm 3. An image of the UV cathodoluminescence from the surface of this sample (Fig. 1c) shows very clear distinctions between high- and low-boron areas, with the areas with high boron concentration dark and low concentration light. The contrast obtained correlates in detail with that of Fig. 1a and b. It follows that the blue (mid-gray) areas in the CL topograph have low boron concentration whereas the yellow (light gray) and green (dark gray) areas have progressively higher boron concentrations. This inference is in agreement with the work of Freitas et al. [6] who deduced that as boron concentration increased the luminescence became progressively more green. They attributed this behavior to donor –acceptor pair luminescence. The boron levels deduced from the measured ratio of the free exciton to the bound exciton luminescence are as follows: 7 ppm in the green (dark gray) areas and 0.7 ppm in the blue (mid gray) areas, with the yellow (light gray) areas intermediate in concentration. Since several authors [7,8] have reported that boron concentrations of approximately 7 ppm produce greatly enhanced secondary electron emission from borondoped diamond, the consistency of these conclusions with the contrast in Fig. 1a is established, bright areas corresponding to regions of high secondary electron emission. A quite different result was obtained after prolonged examination of the sample in the FEGSEM. When the electron accelerating voltage was increased from 1 keV to above 5 keV a dramatic transformation of the contrast was observed. Instead of the fine, sharp structure found previously, a new, much less bright, structure was seen, which had much larger length scales, between 50 and 200 Am in size. The results are shown in Fig. 1d for exactly the same area of the specimen as in Fig. 1a– c: the arrow marks an identifying feature that is dark in Fig. 1c, but bright in Fig. 1d. Note, in particular, that the square-faceted grain, marked F in Fig. 1a, and apparent in Fig. 1b and c, is not visible in this final image. When the electron beam voltage was again reduced back to 1 keV this contrast was unchanged, and even if the sample was left for a week in the FEGSEM, under vacuum, this less bright, larger-scale contrast remained. It was only when the sample had been exposed to atmosphere and then examined again at 1 keV that the original, sharp, bright contrast returned.
In order to investigate the origin of the contrast induced by exposure to the 5 keV electron beam, the sample was tilted. Then different areas became bright, while those that were the brightest became darker, indicating that the contrast was caused by a channeling effect, producing different rates of secondary electron production. This conclusion was confirmed by electron back-scattered diffraction experiments. Each bright area of Fig. 1d corresponds to a region of identical h110i orientation with internal twinning preserving the common h110i direction. These results are in agreement with TEM investigation of this and other polycrystalline CVD samples [1]. Steeds et al. [1] showed that in a large area there are many smaller grains with a common growth direction grouped together forming a ‘‘grain cluster’’. These grain clusters have the approximate size and shape of the large, dark structures shown in the backscattered electron image, while the smaller grains within this area correspond to the structure shown by the low voltage FEGSEM image, the CL maps, and hence the single grain size. We now discuss the reason for the sudden change of SEM contrast after exposure to 5 keV electrons. Shih et al. [7] carried out studies on CVD diamonds, doped with either nitrogen or boron, and showed that the boron doped diamonds had much greater secondary electron emission. They also demonstrated that the presence of surface adsorbates and hydrogenation of the surface also led to higher secondary electron emission. Hydrocarbon molecules are present on the surface of air-exposed diamonds, which desorb completely at temperatures of 400 jC (as seen by Thoms et al. [9]). It is possible that, under exposure to the electron beam, these molecules may decompose to form a hydrogen termination layer on the sample surface. Miller and Brandes [8] and Malta et al. [10] also demonstrated that the hydrogenation of the surface increased the secondary electron emission and that the removal of this layer caused the emission to decrease. Hopman et al. [11] went further to say that there were three regimes in the secondary electron emission from the diamond surface. Firstly, that there is high emission, which then, after a short time where internal electric fields are set up, the emission increases. Eventually, the desorption of the hydrogen from the surface dramatically reduces the yield. We see a similar phenomenon here, where, after time, the
S.J. Charles et al. / Materials Letters 57 (2003) 3690–3693
high secondary electron emission decreases leaving the much darker back-scattered image. The cause of this apparent reduction of the secondary electron emission could either be caused by the removal of either the hydrogen termination layer (produced by the decomposition of hydrocarbon molecules on the sample surface) or the surface adsorbates from the sample. The hydrogen termination layer or adsorbate is removed from the sample surface by the higher energy electron beam, but is rapidly replenished by exposure to the air. This relates to work carried out on the surface conductivity of diamond, as shown in papers by Williams et al. [12], Maier et al. [13] and Ristein et al. [14]. To get the higher surface conductivity they report that both hydrogen termination and a surface adsorbate layer are required. The hydrogen termination layer produces a hole accumulation layer near the surface of the sample and the surface adsorbate layer provides an electron sink to help the formation of this layer. Comparing this with the secondary electron emission case that we have here, it is possible to see that the adsorbate can operate in the same way here, bringing electrons out of the bulk of the sample, allowing them to be emitted more easily by the electron beam. It is unknown whether complete hydrogen termination would increase the yield further, but it would appear to be less of a requirement in the case of boron doped diamond, which already has a ptype character. The two different regimes that are seen in the SEM correspond with the single grains and the commonly diffracting ‘grain clusters’ seen be Steeds et al. [1]. The fact that the boron has been preferentially taken up in one grain allows these different techniques to show the smaller grain sizes, which are very difficult to see in undoped material (the undoped material will only show the larger back-scattered, grain cluster contrast when examined in the SEM). It is therefore of great importance when describing the grains in CVD diamond, that are examined in the SEM, to determine whether it is the grain cluster or the single grain contrast that is being seen. The contrast shown in Fig. 1a, b and c are not necessarily only due to single grains (although the majority are) as it is
3693
possible that the contrast is due to different growth sectors on a single crystal, such as a crystal with a h100i growth direction, which will have four h111i facets surrounding a central h100i facet. These differences in grain size may have an important bearing on the charge collection distances of CVD diamond when used as a radiation detector [15] and the differences of boron incorporation will influence the use of diamond as an electronic material.
Acknowledgements S.J. Charles would like to thank de Beers Industrial Diamonds, in particular Simon Lawson, Garry Stewart and Rob Caveney, for funding and support throughout the course of his PhD.
References [1] J.W. Steeds, A.E. Mora, J.E. Butler, K.M. Bussmann, Philos. Mag., A 82 (2002) 1741. [2] R. Samlenski, C. Haug, R. Brenn, C. Wild, R. Locher, P. Koidl, Diamond Relat. Mater. 5 (1996) 947. [3] P. Wurzinger, P. Pongratz, P. Hartmann, R. Haubner, B. Lux, Diamond Relat. Mater. 6 (1997) 763. [4] J.T. Huang, C.S. Hu, J. Hwang, H. Chang, L.J. Lee, Appl. Phys. Lett. 67 (1995) 2382. [5] H. Kawarada, H. Matsuyama, Y. Yokota, T. Sogi, A. Yamaguchi, A. Hiraki, Phys. Rev., B 47 (1993) 3633. [6] J.A. Freitas, P.B. Klein, A.T. Collins, Appl. Phys. Lett. 64 (1994) 2136. [7] A. Shih, J. Yater, P. Pehrsson, J. Butler, C. Hor, R. Abrams, J. Appl. Phys. 82 (1997) 1860. [8] J.B. Miller, G.R. Brandes, J. Appl. Phys. 82 (1997) 4538. [9] B.D. Thoms, P.E. Pehrsson, J.E. Butler, J. Appl. Phys. 75 (1994) 1804. [10] D.P. Malta, J.B. Posthill, T.P. Humphreys, M.J. Mantini, R.J. Markunas, Mater. Res. Soc. Symp. Proc. 416 (1996) 311. [11] H.J. Hopman, J. Verhoeven, P.K. Bachmann, H. Wilson, R. Kroon, Diamond Relat. Mater. 8 (1999) 1033. [12] O.A. Williams, M.D. Whitfield, R.B. Jackman, J.S. Foord, J.E. Butler, C.E. Nebel, Appl. Phys. Lett. 78 (2001) 3460. [13] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472. [14] J. Ristein, M. Riedel, F. Maier, B. Mantel, M. Stammler, L. Ley, J. Phys. Condens. Mater. 13 (2001) 8979. [15] E. Vittone, A. Lo Giuduce, C. Paolini, F. Fizzotti, C. Manfredotti, R. Barrett, Diamond Relat. Mater. 11 (2002) 1472.