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Micro-ERDA studies of hydrogen in polycrystalline CVD diamond windows
Nuclear Instruments and Methods in Physics Research B 190 (2002) 324–328 www.elsevier.com/locate/nimb
Micro-ERDA studies of hydrogen in polycrystalline CVD diamond windows R. Samlenski a, Ch. Haug a, R. Delto a, Ch. Wild b, R. Brenn a
a,*
Department of Physics and Materials Research Center, University of Freiburg, Hermann-Herderstrasse 3, D-79104 Freiburg/Br, Germany b Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany
Abstract The incorporation of hydrogen in thick polycrystalline chemical vapour deposition diamond plates has been studied by elastic recoil detection with a 2.8 MeV Heþ microbeam. Hydrogen-rich inclusions of lateral dimensions 50–100 lm have been found and analysed quantitatively by depth resolved hydrogen area maps and line scans. The laterally averaged hydrogen content and its depth profile were also characterized by the 1 H(15 N, ac)12 C reaction. The observed hydrogen-rich inclusions correlate with features visible in optical microscopy but not with the optically visible crystallite topography. Ó 2002 Published by Elsevier Science B.V. PACS: 61.16.Ch; 61.18.Bn; 61.72.Mm; 61.72.Ss; 81.05.Tp Keywords: Hydrogen; Polycrystalline CVD diamond; Nuclear microprobe; Ion beam analysis
1. Introduction Megaelectron volt ion beam analysis methods, e.g. helium or heavy ion (HI) elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA) by 15 N induced gamma emission have been used for many years to study the incorporation of hydrogen into chemical vapour deposition (CVD) diamond from the plasma. It was found that the bulk hydrogen concentration in homoepitaxially grown CVD diamond depends on growth direction, being typically in the range of 0.1 at.% for (1 0 0) grown material and in the 1% * Corresponding author. Tel.: +49-761-203-5768; fax: +49761-203-5873. E-mail addresses: [email protected], ruediger.brenn@ physics.uni-freiburg.de (R. Brenn).
range for (1 1 1) growth [1,2]. The difference was interpreted as due to a dominant incorporation of hydrogen at structural defect sites, which are more abundant in (1 1 1) material because of twinning. Other, more indirect, evidence for a defect related H incorporation is the failure to observe a welldefined lattice site in recent channeling experiments [3]. This can be taken as evidence for an incorporation at non-oriented structural defect sites, and can be expected to be also valid for extended defect regions or, possibly, for grain boundaries in polycrystalline material. A decrease of the average hydrogen concentration with increasing film thickness and with the corresponding increase of crystallite size, which supports this conclusion, was observed for the early growth stages of polycrystalline growth, both by MeV ion beam analysis [4] and Raman spectroscopy [5].
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 5 4 - 8
R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
The recent availability of thick polycrystalline CVD diamond plates with grain sizes above 100 lm prompted the present experiment to use MeV ion microprobe techniques to look for correlations between lateral and depth distributions of hydrogen and visible structural defect sectors or grain boundaries between the crystallites. Hydrogen content measurements in diamond plates are also of practical interest because a high H concentration may be detrimental to their use as high radiation power windows in the IR or microwave regions. Micro-ERDA (l-ERDA) studies in natural diamond using He or heavy ion ERDA for laterally and depth resolved analysis of heavier elements have been published before [6,7]. In this contribution we focus on a hydrogen analysis of specific spotlike defect features in CVD grown polycrystalline diamond films.
2. Experimental The experiments were performed at FreNIMP, the University of Freiburg Nuclear Ion Microprobe facility, recently installed in the 1958 High Voltage Engineering Corporation 7.5 MV model CN Van de Graaff laboratory. The lenses, magnetic scanning system and data acquisition software (MARCO, University of Melbourne), the object and aperture slits (Fischer, Darmstadt, Germany) and the 2000 scattering chamber with a stepping motor controlled target station with x-y-z motions are briefly described in Ref. [8]. We used 2.8 MeV Heþ beams focussed to 5–10 lm diameter and beam currents of 50–100 pA for ERDA analysis. The samples, polished CVD diamond plates, were mounted at grazing angles, with the surface plane typically at 15° to the incoming beam direction. The diamond films were grown by microwave assisted CVD in the large scale growth facilities at Fraunhofer Institute for Solid State Physics at Freiburg, Germany [9]. A tightly collimated Si particle detector was positioned at 30° forward angle to register the ejected protons from the diamond sample. A 12.5 lm mylar foil absorbed the primary He particles scattered off the diamond into the forward detector and the ejected carbon ions. The detector res-
325
olution of 20 keV and the energy straggling of the ejected protons in the absorber foil resulted in a near surface depth resolution of 20 nm perpendicular to the diamond surface. At 2.8 MeV incident energy and for our very shallow target angle a maximum layer of about 150 nm thickness perpendicular to the sample surface could be analysed. A second detector at 110° backward angle was used for RBS and beam normalization. For additional, area averaged hydrogen depth profiling we used the 1 H(15 N, ac)12 C reaction with a millimeter sized 15 N2þ beam of 6.4–7 MeV at 20 nA beam current and a BGO bore hole detector for near 4p c-ray detection solid angle. Because of limitations in the strength of the focussing microbeam lenses and in the 15 N2þ output current from the HF ion source no microbeam analysis of hydrogen could be performed with the 15 N reaction. In Fig. 1 we show an ERDA spectrum for a 2 mm thick polycrystalline diamond plate accumulated at 30° forward angle, together with a schematic view of ion beam, detector and target geometry. The increased hydrogen concentration of 1–2% at 0–50 nm depth below the surface arises from a hydrocarbon layer on the surface and from the bound hydrogen saturating the surface bonds. The 0.1–0.2% average H content below the hydrogen-rich surface layer is the bulk H concentration. For these measurements the Heþ ion beam
Fig. 1. Total ERDA spectrum and schematics of experimental scattering geometry.
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R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
was scanned accross a 1:55 1:70 mm2 area. A micrograph of the analysed area is shown in Fig. 2(a) with an enlarged portion (Fig. 2(b)) of the dotlike structures to be discussed below. Fig. 3 shows a hydrogen area map accumulated by scanning He l-ERDA, generated from the stored single events file for the surface (channels 350–470) and bulk (channels 100–325) regions, respectively. The maps show distinct spotlike areas
of strongly increased hydrogen concentration, which correspond to the spotlike features in Fig. 2. In Fig. 4 line scans accross selected features of Fig. 2 are shown which were performed in order to investigate, at better statistics, the hydrogen content of the visually observed defect features, which have diameters of about 50 to over 100 lm. The observed concentration increase in these hydrogen-rich spots was found to be over 100-fold compared to the uniform bulk background level. In order to investigate the hydrogen depth profile of the observed defect features ERDA spectra for a selected spot area (area 1 in Fig. 2(b)), normalized to unit area, are displayed in Fig. 5 together with equally normalized H depth spectra for the total area. We observe a considerably increased hydrogen concentration, reaching maximum values of 40%, extending through the analysis depth and decreasing towards the deeper bulk.
3. Discussion and summary
Fig. 2. (a) Micrograph of the scan area including the line scan positions 1–4 and the position of the enlarged detail section shown in (b). (b) Enlarged section showing defect spot (area 1, used in Fig. 5). The triangular, black features, not visible in the hydrogen maps, are reflections or shadowing caused by the lateral illumination incident on internal reflecting surfaces between crystallites.
Depending on CVD growth conditions the polycrystalline diamond windows have a grayish to clear appearance. The large crystallites are optically visible in all diamond plates, but the dotlike inclusions were only observed for the grayish samples, which appear to represent non-optimum growth conditions. By applying l-ERDA with 2.8 MeV He beams we have been able to correlate the hydrogen-rich spots found in H maps on the grayish samples with these optically visible features and to characterize them as hydrogen-rich inclusions of 50–100 lm diameter, extending from the surface about 150 nm into the bulk. The observed hydrogen-rich inclusions may be highly defective diamond phases or amorphous sections consisting of hydrocarbons or a-C:H, all of which could can be expected to show a high hydrogen concentration. Additional micro-Raman measurements are in progress to study a possible crystalline nature of these defects more closely. We were so far unsuccessful in finding lateral hydrogen contrast correlated with the visible crystallite features, as might be expected for hydrogen incorporation in structurally defective
R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
327
Fig. 3. Hydrogen area maps for bulk and surface layer.
Fig. 4. Hydrogen line scans 1–4 (see Fig. 2(a)) summed over total analysis depth.
sections between the crystallites. This indicates that for these large thicknesses and crystallite sizes no strong hydrogen accumulation on a lateral scale of a few microns is observed in the area fractions between the crystallites. This agrees with the expectation that for optimum growth conditions the grain boundaries can be of atomic dimensions between the slightly tilted crystallites,
too narrow to be detected by the presently reached beam sizes in He l-ERDA. The total hydrogen concentration in the bulk, as measured by both ERDA and 15 N NRA, is found to be 0.1–0.2%, similar to our earlier measurements on homoepitaxially grown (1 0 0) samples [1,2] and much below our earlier data on 10–100 lm thick polycrystalline films with
328
R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
cally interesting and important devices. We have, however, not been able to detect hydrogen line structures correlated with the topography of the crystallites.
Acknowledgements The authors thank J. F€ urderer and P. Winterhalter for the efficient running of the accelerator. Financial assistance by Deutsche Forschungsgemeinschaft and by the University of Freiburg Interdisciplinary Research Funds are gratefully acknowledged. Fig. 5. ERDA spectra for selected features (area 1 in Fig. 2(b)) and for total scan area. Both spectra are normalized to the same area.
References
crystallite dimensions up to a few microns [4]. This indicates again that a high fraction of these diamond plates is taken up by structurally almost perfect large-sized crystallites. Because of the small analysis depth of the He l-ERDA technique we are not able to quote bulk values for greater depths. Measurements in side view, extending accross the full plate thickness, are in preparation to look for the expected decrease in total hydrogen content with growth thickness and crystallite size. In conclusion, we have demonstrated that l-ERDA can be successfully applied to study laterally and depth resolved hydrogen distributions in coarse grained CVD diamond plates and to provide useful information towards an optimization of the growth processes for these technologi-
[1] R. Samlenski, J. Schm€alzlin, R. Brenn, Ch. Wild, W. M€ uller-Sebert, P. Koidl, Diamond Relat. Mater. 4 (1995) 503. [2] G. Dollinger, A. Bergmaier, C.M. Frey, M. Roesler, H. Verhoeven, Diamond Relat. Mater. 4 (1995) 591. [3] Ch. Haug, H. G€artner, J. Portmann, R. Samlenski, Ch. Wild, R. Brenn, Diamond Relat. Mater. 10 (2001) 411. [4] R. Samlenski, Doctoral Dissertation, University of Freiburg, 1996, unpublished. [5] J. Wagner, F. Rauch, R. Haubner, B. Lux, Thin Solid Films 207 (1992) 24. [6] S.H. Connell, E. Sideras-Haddad, C.G. Smallman, J.P.F. Sellschop, I.Z. Machi, K. Bharuth-Ram, Nucl. Instr. and Meth. B 118 (1996) 332. [7] R.J. Sweeney, V.M. Prozesky, K.S. Viljoen, S. Connell, Nucl. Instr. and Meth. B 158 (1999) 582. [8] R. Brenn, Ch. Haug, U. Klar, S. Zander, K.W. Alt, D.N. Jamieson, K.K. Lee, H. Schutkowski, Nucl. Instr. and Meth. B 158 (1999) 270. [9] M. F€ uner, Ch. Wild, P. Koidl, Surf. Coat. Technol. 116–119 (1999) 853.
Micro-ERDA studies of hydrogen in polycrystalline CVD diamond windows R. Samlenski a, Ch. Haug a, R. Delto a, Ch. Wild b, R. Brenn a
a,*
Department of Physics and Materials Research Center, University of Freiburg, Hermann-Herderstrasse 3, D-79104 Freiburg/Br, Germany b Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany
Abstract The incorporation of hydrogen in thick polycrystalline chemical vapour deposition diamond plates has been studied by elastic recoil detection with a 2.8 MeV Heþ microbeam. Hydrogen-rich inclusions of lateral dimensions 50–100 lm have been found and analysed quantitatively by depth resolved hydrogen area maps and line scans. The laterally averaged hydrogen content and its depth profile were also characterized by the 1 H(15 N, ac)12 C reaction. The observed hydrogen-rich inclusions correlate with features visible in optical microscopy but not with the optically visible crystallite topography. Ó 2002 Published by Elsevier Science B.V. PACS: 61.16.Ch; 61.18.Bn; 61.72.Mm; 61.72.Ss; 81.05.Tp Keywords: Hydrogen; Polycrystalline CVD diamond; Nuclear microprobe; Ion beam analysis
1. Introduction Megaelectron volt ion beam analysis methods, e.g. helium or heavy ion (HI) elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA) by 15 N induced gamma emission have been used for many years to study the incorporation of hydrogen into chemical vapour deposition (CVD) diamond from the plasma. It was found that the bulk hydrogen concentration in homoepitaxially grown CVD diamond depends on growth direction, being typically in the range of 0.1 at.% for (1 0 0) grown material and in the 1% * Corresponding author. Tel.: +49-761-203-5768; fax: +49761-203-5873. E-mail addresses: [email protected], ruediger.brenn@ physics.uni-freiburg.de (R. Brenn).
range for (1 1 1) growth [1,2]. The difference was interpreted as due to a dominant incorporation of hydrogen at structural defect sites, which are more abundant in (1 1 1) material because of twinning. Other, more indirect, evidence for a defect related H incorporation is the failure to observe a welldefined lattice site in recent channeling experiments [3]. This can be taken as evidence for an incorporation at non-oriented structural defect sites, and can be expected to be also valid for extended defect regions or, possibly, for grain boundaries in polycrystalline material. A decrease of the average hydrogen concentration with increasing film thickness and with the corresponding increase of crystallite size, which supports this conclusion, was observed for the early growth stages of polycrystalline growth, both by MeV ion beam analysis [4] and Raman spectroscopy [5].
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 5 4 - 8
R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
The recent availability of thick polycrystalline CVD diamond plates with grain sizes above 100 lm prompted the present experiment to use MeV ion microprobe techniques to look for correlations between lateral and depth distributions of hydrogen and visible structural defect sectors or grain boundaries between the crystallites. Hydrogen content measurements in diamond plates are also of practical interest because a high H concentration may be detrimental to their use as high radiation power windows in the IR or microwave regions. Micro-ERDA (l-ERDA) studies in natural diamond using He or heavy ion ERDA for laterally and depth resolved analysis of heavier elements have been published before [6,7]. In this contribution we focus on a hydrogen analysis of specific spotlike defect features in CVD grown polycrystalline diamond films.
2. Experimental The experiments were performed at FreNIMP, the University of Freiburg Nuclear Ion Microprobe facility, recently installed in the 1958 High Voltage Engineering Corporation 7.5 MV model CN Van de Graaff laboratory. The lenses, magnetic scanning system and data acquisition software (MARCO, University of Melbourne), the object and aperture slits (Fischer, Darmstadt, Germany) and the 2000 scattering chamber with a stepping motor controlled target station with x-y-z motions are briefly described in Ref. [8]. We used 2.8 MeV Heþ beams focussed to 5–10 lm diameter and beam currents of 50–100 pA for ERDA analysis. The samples, polished CVD diamond plates, were mounted at grazing angles, with the surface plane typically at 15° to the incoming beam direction. The diamond films were grown by microwave assisted CVD in the large scale growth facilities at Fraunhofer Institute for Solid State Physics at Freiburg, Germany [9]. A tightly collimated Si particle detector was positioned at 30° forward angle to register the ejected protons from the diamond sample. A 12.5 lm mylar foil absorbed the primary He particles scattered off the diamond into the forward detector and the ejected carbon ions. The detector res-
325
olution of 20 keV and the energy straggling of the ejected protons in the absorber foil resulted in a near surface depth resolution of 20 nm perpendicular to the diamond surface. At 2.8 MeV incident energy and for our very shallow target angle a maximum layer of about 150 nm thickness perpendicular to the sample surface could be analysed. A second detector at 110° backward angle was used for RBS and beam normalization. For additional, area averaged hydrogen depth profiling we used the 1 H(15 N, ac)12 C reaction with a millimeter sized 15 N2þ beam of 6.4–7 MeV at 20 nA beam current and a BGO bore hole detector for near 4p c-ray detection solid angle. Because of limitations in the strength of the focussing microbeam lenses and in the 15 N2þ output current from the HF ion source no microbeam analysis of hydrogen could be performed with the 15 N reaction. In Fig. 1 we show an ERDA spectrum for a 2 mm thick polycrystalline diamond plate accumulated at 30° forward angle, together with a schematic view of ion beam, detector and target geometry. The increased hydrogen concentration of 1–2% at 0–50 nm depth below the surface arises from a hydrocarbon layer on the surface and from the bound hydrogen saturating the surface bonds. The 0.1–0.2% average H content below the hydrogen-rich surface layer is the bulk H concentration. For these measurements the Heþ ion beam
Fig. 1. Total ERDA spectrum and schematics of experimental scattering geometry.
326
R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
was scanned accross a 1:55 1:70 mm2 area. A micrograph of the analysed area is shown in Fig. 2(a) with an enlarged portion (Fig. 2(b)) of the dotlike structures to be discussed below. Fig. 3 shows a hydrogen area map accumulated by scanning He l-ERDA, generated from the stored single events file for the surface (channels 350–470) and bulk (channels 100–325) regions, respectively. The maps show distinct spotlike areas
of strongly increased hydrogen concentration, which correspond to the spotlike features in Fig. 2. In Fig. 4 line scans accross selected features of Fig. 2 are shown which were performed in order to investigate, at better statistics, the hydrogen content of the visually observed defect features, which have diameters of about 50 to over 100 lm. The observed concentration increase in these hydrogen-rich spots was found to be over 100-fold compared to the uniform bulk background level. In order to investigate the hydrogen depth profile of the observed defect features ERDA spectra for a selected spot area (area 1 in Fig. 2(b)), normalized to unit area, are displayed in Fig. 5 together with equally normalized H depth spectra for the total area. We observe a considerably increased hydrogen concentration, reaching maximum values of 40%, extending through the analysis depth and decreasing towards the deeper bulk.
3. Discussion and summary
Fig. 2. (a) Micrograph of the scan area including the line scan positions 1–4 and the position of the enlarged detail section shown in (b). (b) Enlarged section showing defect spot (area 1, used in Fig. 5). The triangular, black features, not visible in the hydrogen maps, are reflections or shadowing caused by the lateral illumination incident on internal reflecting surfaces between crystallites.
Depending on CVD growth conditions the polycrystalline diamond windows have a grayish to clear appearance. The large crystallites are optically visible in all diamond plates, but the dotlike inclusions were only observed for the grayish samples, which appear to represent non-optimum growth conditions. By applying l-ERDA with 2.8 MeV He beams we have been able to correlate the hydrogen-rich spots found in H maps on the grayish samples with these optically visible features and to characterize them as hydrogen-rich inclusions of 50–100 lm diameter, extending from the surface about 150 nm into the bulk. The observed hydrogen-rich inclusions may be highly defective diamond phases or amorphous sections consisting of hydrocarbons or a-C:H, all of which could can be expected to show a high hydrogen concentration. Additional micro-Raman measurements are in progress to study a possible crystalline nature of these defects more closely. We were so far unsuccessful in finding lateral hydrogen contrast correlated with the visible crystallite features, as might be expected for hydrogen incorporation in structurally defective
R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
327
Fig. 3. Hydrogen area maps for bulk and surface layer.
Fig. 4. Hydrogen line scans 1–4 (see Fig. 2(a)) summed over total analysis depth.
sections between the crystallites. This indicates that for these large thicknesses and crystallite sizes no strong hydrogen accumulation on a lateral scale of a few microns is observed in the area fractions between the crystallites. This agrees with the expectation that for optimum growth conditions the grain boundaries can be of atomic dimensions between the slightly tilted crystallites,
too narrow to be detected by the presently reached beam sizes in He l-ERDA. The total hydrogen concentration in the bulk, as measured by both ERDA and 15 N NRA, is found to be 0.1–0.2%, similar to our earlier measurements on homoepitaxially grown (1 0 0) samples [1,2] and much below our earlier data on 10–100 lm thick polycrystalline films with
328
R. Samlenski et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 324–328
cally interesting and important devices. We have, however, not been able to detect hydrogen line structures correlated with the topography of the crystallites.
Acknowledgements The authors thank J. F€ urderer and P. Winterhalter for the efficient running of the accelerator. Financial assistance by Deutsche Forschungsgemeinschaft and by the University of Freiburg Interdisciplinary Research Funds are gratefully acknowledged. Fig. 5. ERDA spectra for selected features (area 1 in Fig. 2(b)) and for total scan area. Both spectra are normalized to the same area.
References
crystallite dimensions up to a few microns [4]. This indicates again that a high fraction of these diamond plates is taken up by structurally almost perfect large-sized crystallites. Because of the small analysis depth of the He l-ERDA technique we are not able to quote bulk values for greater depths. Measurements in side view, extending accross the full plate thickness, are in preparation to look for the expected decrease in total hydrogen content with growth thickness and crystallite size. In conclusion, we have demonstrated that l-ERDA can be successfully applied to study laterally and depth resolved hydrogen distributions in coarse grained CVD diamond plates and to provide useful information towards an optimization of the growth processes for these technologi-
[1] R. Samlenski, J. Schm€alzlin, R. Brenn, Ch. Wild, W. M€ uller-Sebert, P. Koidl, Diamond Relat. Mater. 4 (1995) 503. [2] G. Dollinger, A. Bergmaier, C.M. Frey, M. Roesler, H. Verhoeven, Diamond Relat. Mater. 4 (1995) 591. [3] Ch. Haug, H. G€artner, J. Portmann, R. Samlenski, Ch. Wild, R. Brenn, Diamond Relat. Mater. 10 (2001) 411. [4] R. Samlenski, Doctoral Dissertation, University of Freiburg, 1996, unpublished. [5] J. Wagner, F. Rauch, R. Haubner, B. Lux, Thin Solid Films 207 (1992) 24. [6] S.H. Connell, E. Sideras-Haddad, C.G. Smallman, J.P.F. Sellschop, I.Z. Machi, K. Bharuth-Ram, Nucl. Instr. and Meth. B 118 (1996) 332. [7] R.J. Sweeney, V.M. Prozesky, K.S. Viljoen, S. Connell, Nucl. Instr. and Meth. B 158 (1999) 582. [8] R. Brenn, Ch. Haug, U. Klar, S. Zander, K.W. Alt, D.N. Jamieson, K.K. Lee, H. Schutkowski, Nucl. Instr. and Meth. B 158 (1999) 270. [9] M. F€ uner, Ch. Wild, P. Koidl, Surf. Coat. Technol. 116–119 (1999) 853.