- Email: [email protected]
A study of the charge collection properties of polycrystalline CVD diamond with synchrotron radiation
Diamond & Related Materials 20 (2011) 398–402
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
A study of the charge collection properties of polycrystalline CVD diamond with synchrotron radiationii Alexander Oh a,⁎, Thorsten Wengler b, Mahfuza Ahmed a, Cinzia Da Via a, Stephen Watts a a b
University of Manchester, Manchester, UK CERN, Geneva, Switzerland
a r t i c l e
i n f o
Available online 1 February 2011 Keywords: Detector Synchrotron-radiation Induced-current Grain-boundaries
a b s t r a c t Polycrystalline CVD diamond samples have been prepared with different electrode configurations, allowing to produce an electric field parallel and perpendicular to the direction of the grain boundaries. A photon beam with an energy of 15 keV was used to study the response with a spatial resolution of about 7 μm. Results on the influence of the field direction on the signal, the pumping effect, and the signal response as a function of the film thickness were obtained. © 2011 Elsevier B.V. All rights reserved.
1. Introduction CVD diamond has established itself as a sensor material for particle detection and has found many applications, among others in accelerator based experiments, synchrotrons and dosimetry applications. A key property of diamond is its excellent radiation resistance [1], which is why it is being used and proposed for detector applications [2] in harsh radiation environments. Previous studies have investigated the charge collection properties with synchrotron radiation [3], the novelty of this study is the comparison of the signal response with respect to the direction and orientation of the bulk electric field. The comparison of an electric field direction perpendicular and parallel to the CVD diamond film growth direction allows to estimate the effect of grain boundaries on the charge transport, since the crystallites constituting the film have a columnar structure. Because of this charge carriers drifting parallel to the growth direction will encounter on average less grain boundaries than when drifting orthogonal to the growth direction. This work is aimed at the realization of a novel electrode geometry where the electrodes are reaching into the bulk of the detector material. This concept has been proven to increase the radiation resistance of silicon [4] and should also be applicable to CVD diamond if the grain boundaries are sufficiently transparent to the charge transport. This paper presents measurements of the signal response for both electric field configurations with varying field strength. A direct measurement of the diamond quality as a function of the diamond film thickness was obtained by measuring along the growth direction
ii Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. E-mail address: [email protected] (A. Oh). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.01.042
of the sample. Additionally, the pumping effect [5] was measured spatially resolved and parameterized. 2. Material and methods 2.1. Material Samples with different electrode configurations were prepared producing an electric field parallel and perpendicular to the direction of film growth. Polycrystalline CVD diamond samples were cut from a wafer grown to an initial thickness of 1.2 mm, and thinned by about 100 μm from growth side and substrate side to reach a final thickness of 1 mm. The cuboid shaped samples with dimensions of 1 mm by 1 mm in cross section and 3 mm wide were polished on all sides and Ti–Pt–Au contacts were applied. Two electrode configurations were fabricated, (A) electrodes on the growth and substrate sides, and (B) electrodes on the sides along the growth direction. The electrode dimensions are about 0.9 mm by 2.9 mm. The quality of the contacts was verified by measuring IV curves and requiring the leakage current to be less then 1 nA at 500 V. The samples were cut out from the same main wafer and are therefore of similar structure in terms of grain sizes and are of similar quality. The collection distance [1] is expected to be typically 250 μm [6], corresponding to a collection efficiency of 25% when measured with minimum ionizing particles and growth– substrate side electrode configuration (A). 2.2. Experimental set-up The beam-line B16 [7] at the Diamond Light Source Ltd. in Didcot was used for this study. The beam-line was set-up to provide a monochromatic beam of photons with an energy of 15 keV. The absorption length in the diamond of photons at this energy is about 4 mm [8], consequently the ionization density produced by the
A. Oh et al. / Diamond & Related Materials 20 (2011) 398–402
photon beam is in good approximation uniform over the full length of the trajectory within the sample (1 mm). Compound refractive lenses were used to obtain a focussed beam. The dimensions of the beam were determined by scanning a 200 μm Au wire. The FWHM was measured to be 2.1 μm in the vertical and 3.0 μm in the horizontal plane. The range of electrons in diamond with a kinetic energy of 15 keV is about 1.7 μm [9], resulting in an effective ionization width of the beam of less than 7 μm. The photon flux of the beam was determined with an ionization chamber to be between 4 ⋅ 107 and 10 ⋅ 107 photons per second depending on the chosen absorber settings in the beam-line. The variations due to current changes in the synchrotron ring were measured to be less then 0.4%. The samples were mounted in a metal box with aluminum foil entrance windows to shield against light and electromagnetic noise. The box was mounted on a 5-circle Huber Diffractometer with an XYZ sample stage to scan the sample surface. The accuracy of the spatial translation was about 1 μm. The samples were scanned in x and y for area scans of about 1 mm by 1 mm. Additionally, various line scans were performed for different bias voltages. The detector response was studied by measuring the beam induced current, IBI, as a function of the position of the beam on the
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sample. A current amplifier was used (Keithley 428) to measure the signal on the electrode to ground, while positive and negative bias voltages were applied to the opposite electrode. A data acquisition system was recording the current measurements of the device under test, the beam position, the current of the ionization chamber, and the ring current of the accelerator. 2.3. Results The signal response of the samples to the focussed photon beam was measured with position scans on the electrode side. The scanned area was approximately 1 mm2, with a step size of 10 μm. The area scans obtained with sample (A) and (B) are shown in Fig. 1. The scans show a non-uniform response to the photon beam due to the polycrystalline nature of the material. Sample (A) shows unordered patches with different response heights, while sample (B) exhibits a dependence of the signal with position in x. The substrate side at about x = − 1.5 mm has a lower response, which is increasing towards the growth side at x = − 0.5 mm. Columnar structured patches, due to the columnar structure of the crystallites are visible. Furthermore a clear dependence on the field direction can be
Fig. 1. Area scans of the response of a diamond detector to 15 keV photons. The beam direction is orthogonal to the electrodes. The scan area is about 1 mm by 1 mm. In (a) above y ≈ − 9mm no measurements were made. Plotted is the signal response in nA vs. the absolute position of the beam in mm. (a) and (b) show scans of sample (B) for negative and positive field (|UB| = 1000 V), respectively. (c) shows the response of sample (A) for UB = − 500 V. The beam intensity was a factor 2.6 higher for sample (A) than for sample (B).
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observed. The structural features of the response change significantly, which can be attributed to the different contributions of electrons and holes to the signal. The structures have feature sizes comparable to the grains sizes and are on a scale of about 100 μm. The graphs in Fig. 2 show the response to the beam when parallel to the electrodes for sample B. It is observed that the charge signal is about a factor 5 lower when compared to the signal obtained with a configuration where the beam is orthogonal to the electrodes as shown in Fig. 1a and b. The response shown in Fig. 2 is not uniform and changes significantly with the field direction. It is known that the quality of polycrystalline CVD diamond improves with the thickness of the material, however the actual improvement is difficult to measure, as samples have to be thinned down successively, electrodes have to be reapplied and the samples have to be characterized [10]. In this experiment we obtain a direct measurement of the film quality as a function of the thickness by projecting out the response along the growth direction. This allows to directly measure the average signal response as a function of the film thickness as shown in Fig. 3. The data show an increase of the signal response with film thickness in qualitative agreement with measurements from thinning experiments [10]. The evolution of the signal response has been measured for different field strengths for positive and negative bias voltage. Fig. 4(a) shows the signal as a function of position along a line across the sample. The data shows a similar scaling of the signal with voltage across the sample, while the absolute signal is varying by about a factor 4 as was observed in the area scans. As for the area scans it is observed that the structure changes with the bias voltage polarity, indicating the difference in electron and hole contribution to the signal. To compare the signal response of sample (A) and (B) the data of line scans were averaged for different values for UB. The averages are shown in Fig. 4(b). For sample (A) only data up to UB = 300V is included in the comparison as for higher bias voltages the signal-to-noise ratio deteriorated due to increasing leakage current. The leakage current is subtracted from the signal for the comparison. The leakage current was estimated by measuring the signal with the beam outside of the sample area. The leakage current was about 0.4 nA at UB = 300 V for sample A, and below 0.01 nA at UB = 1000 V for sample B. The signal of sample (A) is scaled by a factor f to account for the difference in beam intensity when the samples were characterized. Two curves for the signal
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response near the growth and the substrate side are shown for sample (B). From the comparison at Ub = 300 V it can be seen that the charge collection of sample (B) is between 40% (growth side) to 70% (substrate side), and on average about 50% lower than for sample (A). To estimate the charge created by the photo-electric absorption an ionization energy of 13 eV [11] was assumed and a photo-electric absorption coefficient of 0.56 cm2g− 1 [8] neglecting the small contribution from Compton scattering to the charge creation. For a 100% efficient charge collection a signal of about 1.2 nA for the normalized beam induced current f ⋅ IIB is expected. The observed values of f ⋅ IIB for sample (A) at Ub = 300 V is 0.18 nA. The collection distance of samples from this wafer reach typically 250 μm at Ub = 1000 V [6], which would correspond to an expected signal of 0.3 nA. The signal response is known to improve with irradiation of the diamond. This effect is called ‘priming’ or ‘pumping’ and has been observed with charged particles [5]. This effect is also observed in this experiment with a photon beam. The novelty is that in this set-up it is possible to measure the priming effect spatially resolved.
Fig. 2. Area scans of the response of sample (B) to 15 keV photons for negative (a) and positive (b) electric field. The beam direction is parallel to the electrodes. The scan area is about 1 mm by 1 mm. The axis gives the absolute position of the beam in mm, the scale on the right gives the signal response in nA.
A. Oh et al. / Diamond & Related Materials 20 (2011) 398–402
(a) Signal for different bias voltages, sample (B).
(b) Average signal of sample (A) and (B). Sample A, f = 1/2.6 Sample B, f = 1.0 Sample B, growth side, f = 1.0 Sample B, substrate side, f = 1.0
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Fig. 4. (a) shows the response signal as a function of position along a line across sample (B) for different field strengths for positive and negative bias voltage. (b) shows the averaged response signal as a function of the bias voltage for samples (A) and (B). The signal of sample (A) is scaled by a factor f to account for the difference in beam intensity when the samples were characterized. The curves for the signal response near the growth and the substrate side, as well as the average are shown for sample (B).
Fig. 5(a) shows successive line-scans of the same area. A total of 41 scans were made. The exposure time per scan was about 1 s. The priming effect is clearly visible as an increase of the signal. Furthermore the spatially resolved measurement shows that the priming is effective over the entire scanned area, albeit with different strength. The sample areas used for the priming scans were not exposed directly to the beam prior to the measurements. To parametrize the priming effect a function n f ðnÞ = a + b⋅ 1−e−c [12] was fitted to the successive measurements n at a fixed position in x, with a the signal before pumping, b the gain in signal, and c a constant proportional to the specific “priming” dose. The fit describes the data well except for the first measurement point in the series, which is systematically too low for a good fit with the model introduced in [12]. To quantify this deviation a further term 0n ⋅ p0 is added to the fit function f(n), which effectively allows the first point of the measurement series to float. For the fit procedure an uncorrelated error of 0.0003 nA is assigned to each data point. This value corresponds to the RMS of the differences observed between the last and the last but one scan (see Fig. 5(a)), which represents an upper limit on the reproducibility of an individual line scan. The uncertainties on the fit parameters shown below are extracted from fits including this uncertainty on the individual data points. The data and the fitted function (including the float term 0n ⋅ p0) are shown in Fig. 5(b) for two measurement locations. The distribution of the float parameter p0 for the first measurement points has a mean of 10.2% with an RMS of 4.7%. Fig. 5(c) shows the value of b normalized to the signal at n = 0, corresponding to the relative increase of the signal, as a function of position x. A mean relative increase of about 16% is observed with a large scatter of about 30% RMS. No clear correlation to the signal response is seen. In Fig. 5(d) the value of c is plotted as a function of x. Also here no clear correlation with the signal response is observed. The mean value of c is 7.9 with an RMS of about 54%. 3. Conclusions CVD diamond samples with electrode configurations perpendicular (A) and parallel (B) to the direction of growth were investigated with a micro-focussed 15 keV photon beam. Spatial scans of the response revealed non-uniform response patterns with structure sizes of the order
of 100 μm, with a clear difference in the structure of (A) and (B) due to the electrode configuration and the columnar CVD diamond film growth. The lower signal response in configuration (A) compared to (B) can be interpreted as the occurrence of an internal polarization field generated by the trapping of charge carriers in the bulk, which partly compensates the external field and thus lowers the signal response. An enhanced signal is observed near the electrodes at x = 0.45 mm and x = − 0.55 mm which is due to a weakening of the polarization effect in the immediate vicinity of the electrodes as here only one carrier type effectively contributes to polarization. The polarization effect is not as pronounced when the beam is orthogonal to the electrodes in configuration (B) as the ionization density is uniformly distributed between the two electrodes. The average response of configuration (B) was found to be 40% to 70% lower than for configuration (A). This can be attributed to the multiple crossing of grain boundaries of the charge carriers. Further studies with different sample widths are needed to quantify the effect of the grain boundaries, however it is evident that the crossing has a detrimental effect on the signal response. The electrode configuration (B) allowed to directly measure the signal response as a function of the film thickness. In agreement with thinning experiments an increase of the response signal with film thickness is observed. The priming effect, known to increase the response signal under exposure to radiation, was studied for the first time spatially resolved. The priming was parameterized in terms of relative signal increase and specific “priming” dose, and both parameters were found to vary significantly as a function of the position on the sample, showing that the priming effect is non-uniform and dependent on the sample location. The priming parameters and the signal response exhibit different structures, no clear correlation was found between them, indicating that the grain structure has a different effect on the processes of priming and the signal generation. Acknowledgements We would like to thank Kevin Oliver and Arnaldo Galbiati from Diamond Detectors Ltd. for the preparation of the diamond samples,
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Fig. 5. (a) shows the change of the signal of sample B with exposure to the photon beam. The same area was scanned multiple times. In (b) two measurements at different positions with their pumping characteristics are shown, together with a function fitted to the data. (c) shows the relative increase of the signal and (d) shows the characteristic pumping dose c as a function of position.
and Kawal Sawhney and Manoj Tiwari from Diamond Light Source Ltd. for the support at the beam-line B16.
References [1] H. Kagan, NIM A 546 (1) (2005) 222. [2] M et al. Barbero. Development of diamond tracking detectors for high luminosity experiments at lhc. Technical Report LHCC-RD-012. CERN-LHCC-2007-002, CERN, Geneva, Jan 2007. [3] M.J. Guerrero, et al., phys. stat. sol. 201 (11) (2004) 2529.
[4] A. Kok, G. Anelli, C. DaVia, J. Hasi, P. Jarron, C. Kenney, J. Morse, S. Parker, J. Segal, S. Watts, E. Westbrook, Nucl. Instrum. Methods Phys. Res. Sect. A 560 (1) (2006) 1278 Proceedings of the 13th International Workshop on Vertex Detectors — VERTEX 2004. [5] A. Oh, et al., Diamond Relat. Mater. 7 (1998) 1553. [6] Kevin Oliver. private communication. 2010. [7] K.J.S. Sawhney, I.P. Dolbnya, M.K. Tiwari, L. Alianelli, S.M. Scott, G.M. Preece, U.K. Pedersen, R.D. Walton, AIP Conf. Proc. 1234 (1) (2010) 387. [8] M.J. Berger et al. Xcom: Photon cross sections database. 2010. [9] D.K. Brice, Nucl. Instrum. Methods Phys. Res. Sect. B 1 (12) (1984) 187. [10] S. Zhao, et al., Appl. Phys. Lett. 64 (2) (2010) 193. [11] M. Pomorski, et al., phys. stat. sol. 203 (a) (2006) 3152. [12] Oh. Alexander, Michael Moll, Albrecht Wagner, Wolfram Zeuner, Diamond Relat. Mater. 9 (11) (2010) 1897.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
A study of the charge collection properties of polycrystalline CVD diamond with synchrotron radiationii Alexander Oh a,⁎, Thorsten Wengler b, Mahfuza Ahmed a, Cinzia Da Via a, Stephen Watts a a b
University of Manchester, Manchester, UK CERN, Geneva, Switzerland
a r t i c l e
i n f o
Available online 1 February 2011 Keywords: Detector Synchrotron-radiation Induced-current Grain-boundaries
a b s t r a c t Polycrystalline CVD diamond samples have been prepared with different electrode configurations, allowing to produce an electric field parallel and perpendicular to the direction of the grain boundaries. A photon beam with an energy of 15 keV was used to study the response with a spatial resolution of about 7 μm. Results on the influence of the field direction on the signal, the pumping effect, and the signal response as a function of the film thickness were obtained. © 2011 Elsevier B.V. All rights reserved.
1. Introduction CVD diamond has established itself as a sensor material for particle detection and has found many applications, among others in accelerator based experiments, synchrotrons and dosimetry applications. A key property of diamond is its excellent radiation resistance [1], which is why it is being used and proposed for detector applications [2] in harsh radiation environments. Previous studies have investigated the charge collection properties with synchrotron radiation [3], the novelty of this study is the comparison of the signal response with respect to the direction and orientation of the bulk electric field. The comparison of an electric field direction perpendicular and parallel to the CVD diamond film growth direction allows to estimate the effect of grain boundaries on the charge transport, since the crystallites constituting the film have a columnar structure. Because of this charge carriers drifting parallel to the growth direction will encounter on average less grain boundaries than when drifting orthogonal to the growth direction. This work is aimed at the realization of a novel electrode geometry where the electrodes are reaching into the bulk of the detector material. This concept has been proven to increase the radiation resistance of silicon [4] and should also be applicable to CVD diamond if the grain boundaries are sufficiently transparent to the charge transport. This paper presents measurements of the signal response for both electric field configurations with varying field strength. A direct measurement of the diamond quality as a function of the diamond film thickness was obtained by measuring along the growth direction
ii Presented at the Diamond 2010, 21st European Conference on Diamond, Diamond- Like Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. E-mail address: [email protected] (A. Oh). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.01.042
of the sample. Additionally, the pumping effect [5] was measured spatially resolved and parameterized. 2. Material and methods 2.1. Material Samples with different electrode configurations were prepared producing an electric field parallel and perpendicular to the direction of film growth. Polycrystalline CVD diamond samples were cut from a wafer grown to an initial thickness of 1.2 mm, and thinned by about 100 μm from growth side and substrate side to reach a final thickness of 1 mm. The cuboid shaped samples with dimensions of 1 mm by 1 mm in cross section and 3 mm wide were polished on all sides and Ti–Pt–Au contacts were applied. Two electrode configurations were fabricated, (A) electrodes on the growth and substrate sides, and (B) electrodes on the sides along the growth direction. The electrode dimensions are about 0.9 mm by 2.9 mm. The quality of the contacts was verified by measuring IV curves and requiring the leakage current to be less then 1 nA at 500 V. The samples were cut out from the same main wafer and are therefore of similar structure in terms of grain sizes and are of similar quality. The collection distance [1] is expected to be typically 250 μm [6], corresponding to a collection efficiency of 25% when measured with minimum ionizing particles and growth– substrate side electrode configuration (A). 2.2. Experimental set-up The beam-line B16 [7] at the Diamond Light Source Ltd. in Didcot was used for this study. The beam-line was set-up to provide a monochromatic beam of photons with an energy of 15 keV. The absorption length in the diamond of photons at this energy is about 4 mm [8], consequently the ionization density produced by the
A. Oh et al. / Diamond & Related Materials 20 (2011) 398–402
photon beam is in good approximation uniform over the full length of the trajectory within the sample (1 mm). Compound refractive lenses were used to obtain a focussed beam. The dimensions of the beam were determined by scanning a 200 μm Au wire. The FWHM was measured to be 2.1 μm in the vertical and 3.0 μm in the horizontal plane. The range of electrons in diamond with a kinetic energy of 15 keV is about 1.7 μm [9], resulting in an effective ionization width of the beam of less than 7 μm. The photon flux of the beam was determined with an ionization chamber to be between 4 ⋅ 107 and 10 ⋅ 107 photons per second depending on the chosen absorber settings in the beam-line. The variations due to current changes in the synchrotron ring were measured to be less then 0.4%. The samples were mounted in a metal box with aluminum foil entrance windows to shield against light and electromagnetic noise. The box was mounted on a 5-circle Huber Diffractometer with an XYZ sample stage to scan the sample surface. The accuracy of the spatial translation was about 1 μm. The samples were scanned in x and y for area scans of about 1 mm by 1 mm. Additionally, various line scans were performed for different bias voltages. The detector response was studied by measuring the beam induced current, IBI, as a function of the position of the beam on the
399
sample. A current amplifier was used (Keithley 428) to measure the signal on the electrode to ground, while positive and negative bias voltages were applied to the opposite electrode. A data acquisition system was recording the current measurements of the device under test, the beam position, the current of the ionization chamber, and the ring current of the accelerator. 2.3. Results The signal response of the samples to the focussed photon beam was measured with position scans on the electrode side. The scanned area was approximately 1 mm2, with a step size of 10 μm. The area scans obtained with sample (A) and (B) are shown in Fig. 1. The scans show a non-uniform response to the photon beam due to the polycrystalline nature of the material. Sample (A) shows unordered patches with different response heights, while sample (B) exhibits a dependence of the signal with position in x. The substrate side at about x = − 1.5 mm has a lower response, which is increasing towards the growth side at x = − 0.5 mm. Columnar structured patches, due to the columnar structure of the crystallites are visible. Furthermore a clear dependence on the field direction can be
Fig. 1. Area scans of the response of a diamond detector to 15 keV photons. The beam direction is orthogonal to the electrodes. The scan area is about 1 mm by 1 mm. In (a) above y ≈ − 9mm no measurements were made. Plotted is the signal response in nA vs. the absolute position of the beam in mm. (a) and (b) show scans of sample (B) for negative and positive field (|UB| = 1000 V), respectively. (c) shows the response of sample (A) for UB = − 500 V. The beam intensity was a factor 2.6 higher for sample (A) than for sample (B).
A. Oh et al. / Diamond & Related Materials 20 (2011) 398–402
observed. The structural features of the response change significantly, which can be attributed to the different contributions of electrons and holes to the signal. The structures have feature sizes comparable to the grains sizes and are on a scale of about 100 μm. The graphs in Fig. 2 show the response to the beam when parallel to the electrodes for sample B. It is observed that the charge signal is about a factor 5 lower when compared to the signal obtained with a configuration where the beam is orthogonal to the electrodes as shown in Fig. 1a and b. The response shown in Fig. 2 is not uniform and changes significantly with the field direction. It is known that the quality of polycrystalline CVD diamond improves with the thickness of the material, however the actual improvement is difficult to measure, as samples have to be thinned down successively, electrodes have to be reapplied and the samples have to be characterized [10]. In this experiment we obtain a direct measurement of the film quality as a function of the thickness by projecting out the response along the growth direction. This allows to directly measure the average signal response as a function of the film thickness as shown in Fig. 3. The data show an increase of the signal response with film thickness in qualitative agreement with measurements from thinning experiments [10]. The evolution of the signal response has been measured for different field strengths for positive and negative bias voltage. Fig. 4(a) shows the signal as a function of position along a line across the sample. The data shows a similar scaling of the signal with voltage across the sample, while the absolute signal is varying by about a factor 4 as was observed in the area scans. As for the area scans it is observed that the structure changes with the bias voltage polarity, indicating the difference in electron and hole contribution to the signal. To compare the signal response of sample (A) and (B) the data of line scans were averaged for different values for UB. The averages are shown in Fig. 4(b). For sample (A) only data up to UB = 300V is included in the comparison as for higher bias voltages the signal-to-noise ratio deteriorated due to increasing leakage current. The leakage current is subtracted from the signal for the comparison. The leakage current was estimated by measuring the signal with the beam outside of the sample area. The leakage current was about 0.4 nA at UB = 300 V for sample A, and below 0.01 nA at UB = 1000 V for sample B. The signal of sample (A) is scaled by a factor f to account for the difference in beam intensity when the samples were characterized. Two curves for the signal
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response near the growth and the substrate side are shown for sample (B). From the comparison at Ub = 300 V it can be seen that the charge collection of sample (B) is between 40% (growth side) to 70% (substrate side), and on average about 50% lower than for sample (A). To estimate the charge created by the photo-electric absorption an ionization energy of 13 eV [11] was assumed and a photo-electric absorption coefficient of 0.56 cm2g− 1 [8] neglecting the small contribution from Compton scattering to the charge creation. For a 100% efficient charge collection a signal of about 1.2 nA for the normalized beam induced current f ⋅ IIB is expected. The observed values of f ⋅ IIB for sample (A) at Ub = 300 V is 0.18 nA. The collection distance of samples from this wafer reach typically 250 μm at Ub = 1000 V [6], which would correspond to an expected signal of 0.3 nA. The signal response is known to improve with irradiation of the diamond. This effect is called ‘priming’ or ‘pumping’ and has been observed with charged particles [5]. This effect is also observed in this experiment with a photon beam. The novelty is that in this set-up it is possible to measure the priming effect spatially resolved.
Fig. 2. Area scans of the response of sample (B) to 15 keV photons for negative (a) and positive (b) electric field. The beam direction is parallel to the electrodes. The scan area is about 1 mm by 1 mm. The axis gives the absolute position of the beam in mm, the scale on the right gives the signal response in nA.
A. Oh et al. / Diamond & Related Materials 20 (2011) 398–402
(a) Signal for different bias voltages, sample (B).
(b) Average signal of sample (A) and (B). Sample A, f = 1/2.6 Sample B, f = 1.0 Sample B, growth side, f = 1.0 Sample B, substrate side, f = 1.0
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Fig. 4. (a) shows the response signal as a function of position along a line across sample (B) for different field strengths for positive and negative bias voltage. (b) shows the averaged response signal as a function of the bias voltage for samples (A) and (B). The signal of sample (A) is scaled by a factor f to account for the difference in beam intensity when the samples were characterized. The curves for the signal response near the growth and the substrate side, as well as the average are shown for sample (B).
Fig. 5(a) shows successive line-scans of the same area. A total of 41 scans were made. The exposure time per scan was about 1 s. The priming effect is clearly visible as an increase of the signal. Furthermore the spatially resolved measurement shows that the priming is effective over the entire scanned area, albeit with different strength. The sample areas used for the priming scans were not exposed directly to the beam prior to the measurements. To parametrize the priming effect a function n f ðnÞ = a + b⋅ 1−e−c [12] was fitted to the successive measurements n at a fixed position in x, with a the signal before pumping, b the gain in signal, and c a constant proportional to the specific “priming” dose. The fit describes the data well except for the first measurement point in the series, which is systematically too low for a good fit with the model introduced in [12]. To quantify this deviation a further term 0n ⋅ p0 is added to the fit function f(n), which effectively allows the first point of the measurement series to float. For the fit procedure an uncorrelated error of 0.0003 nA is assigned to each data point. This value corresponds to the RMS of the differences observed between the last and the last but one scan (see Fig. 5(a)), which represents an upper limit on the reproducibility of an individual line scan. The uncertainties on the fit parameters shown below are extracted from fits including this uncertainty on the individual data points. The data and the fitted function (including the float term 0n ⋅ p0) are shown in Fig. 5(b) for two measurement locations. The distribution of the float parameter p0 for the first measurement points has a mean of 10.2% with an RMS of 4.7%. Fig. 5(c) shows the value of b normalized to the signal at n = 0, corresponding to the relative increase of the signal, as a function of position x. A mean relative increase of about 16% is observed with a large scatter of about 30% RMS. No clear correlation to the signal response is seen. In Fig. 5(d) the value of c is plotted as a function of x. Also here no clear correlation with the signal response is observed. The mean value of c is 7.9 with an RMS of about 54%. 3. Conclusions CVD diamond samples with electrode configurations perpendicular (A) and parallel (B) to the direction of growth were investigated with a micro-focussed 15 keV photon beam. Spatial scans of the response revealed non-uniform response patterns with structure sizes of the order
of 100 μm, with a clear difference in the structure of (A) and (B) due to the electrode configuration and the columnar CVD diamond film growth. The lower signal response in configuration (A) compared to (B) can be interpreted as the occurrence of an internal polarization field generated by the trapping of charge carriers in the bulk, which partly compensates the external field and thus lowers the signal response. An enhanced signal is observed near the electrodes at x = 0.45 mm and x = − 0.55 mm which is due to a weakening of the polarization effect in the immediate vicinity of the electrodes as here only one carrier type effectively contributes to polarization. The polarization effect is not as pronounced when the beam is orthogonal to the electrodes in configuration (B) as the ionization density is uniformly distributed between the two electrodes. The average response of configuration (B) was found to be 40% to 70% lower than for configuration (A). This can be attributed to the multiple crossing of grain boundaries of the charge carriers. Further studies with different sample widths are needed to quantify the effect of the grain boundaries, however it is evident that the crossing has a detrimental effect on the signal response. The electrode configuration (B) allowed to directly measure the signal response as a function of the film thickness. In agreement with thinning experiments an increase of the response signal with film thickness is observed. The priming effect, known to increase the response signal under exposure to radiation, was studied for the first time spatially resolved. The priming was parameterized in terms of relative signal increase and specific “priming” dose, and both parameters were found to vary significantly as a function of the position on the sample, showing that the priming effect is non-uniform and dependent on the sample location. The priming parameters and the signal response exhibit different structures, no clear correlation was found between them, indicating that the grain structure has a different effect on the processes of priming and the signal generation. Acknowledgements We would like to thank Kevin Oliver and Arnaldo Galbiati from Diamond Detectors Ltd. for the preparation of the diamond samples,
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Fig. 5. (a) shows the change of the signal of sample B with exposure to the photon beam. The same area was scanned multiple times. In (b) two measurements at different positions with their pumping characteristics are shown, together with a function fitted to the data. (c) shows the relative increase of the signal and (d) shows the characteristic pumping dose c as a function of position.
and Kawal Sawhney and Manoj Tiwari from Diamond Light Source Ltd. for the support at the beam-line B16.
References [1] H. Kagan, NIM A 546 (1) (2005) 222. [2] M et al. Barbero. Development of diamond tracking detectors for high luminosity experiments at lhc. Technical Report LHCC-RD-012. CERN-LHCC-2007-002, CERN, Geneva, Jan 2007. [3] M.J. Guerrero, et al., phys. stat. sol. 201 (11) (2004) 2529.
[4] A. Kok, G. Anelli, C. DaVia, J. Hasi, P. Jarron, C. Kenney, J. Morse, S. Parker, J. Segal, S. Watts, E. Westbrook, Nucl. Instrum. Methods Phys. Res. Sect. A 560 (1) (2006) 1278 Proceedings of the 13th International Workshop on Vertex Detectors — VERTEX 2004. [5] A. Oh, et al., Diamond Relat. Mater. 7 (1998) 1553. [6] Kevin Oliver. private communication. 2010. [7] K.J.S. Sawhney, I.P. Dolbnya, M.K. Tiwari, L. Alianelli, S.M. Scott, G.M. Preece, U.K. Pedersen, R.D. Walton, AIP Conf. Proc. 1234 (1) (2010) 387. [8] M.J. Berger et al. Xcom: Photon cross sections database. 2010. [9] D.K. Brice, Nucl. Instrum. Methods Phys. Res. Sect. B 1 (12) (1984) 187. [10] S. Zhao, et al., Appl. Phys. Lett. 64 (2) (2010) 193. [11] M. Pomorski, et al., phys. stat. sol. 203 (a) (2006) 3152. [12] Oh. Alexander, Michael Moll, Albrecht Wagner, Wolfram Zeuner, Diamond Relat. Mater. 9 (11) (2010) 1897.