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Development of high resolution position sensitive UV detector based on highly oriented polycrystalline diamond
Diamond & Related Materials 14 (2005) 2035 – 2038 www.elsevier.com/locate/diamond
Development of high resolution position sensitive UV detector based on highly oriented polycrystalline diamond K. Tsuji a,*, K. Hayashi b, J.H. Kaneko a, F. Fujita a, A. Homma a, Y. Oshiki a, T. Sawamura a, M. Furusaka a a
Division of Quantum Science and Engineering, Hokkaido University, North 13 West 8, Sapporo, Hokkaido, 060-8628, Japan b Kobe Steel Ltd., Japan Available online 31 August 2005
Abstract UV position-sensitive sensor using a polycrystalline highly-oriented diamond film with a sensitive area of 2 4.5 mm2 developed, and a proof-of-concept study was conducted. The charge division method was employed to establish the position when the light hits the sensitive area. A fifth higher harmonic of a Nd:YAG laser with pulse width of 100 ps was used as a light source. The position resolution was 0.25 mm, and there was good position linearity throughout the sensitive area. Considering the possibility of light-spot broadening due to diffraction by the slit, the sensor may have a better resolution than that indicated by the result. But the electric-field strength of some parts of the crystal was insufficient, the response time was relatively slow, approximately 0.5 ms. D 2005 Elsevier B.V. All rights reserved. Keywords: UV detector; Highly oriented diamond; Position sensitive detector; 213 nm UV laser
1. Introduction Recently, intensive short wavelength ultraviolet radiation is being used in industry, and the demand for highly accurate and stable position sensors is growing. Silicon, gallium nitride, and photoelectric cells have been used as UV detection elements but these are not suitable for very intensive short wavelength UV radiations because of inferior light resistance [1,2]. Diamond has been considered able to overcome this problem, and has attracted growing attention. Diamond is insensitive to visible light having a wide band gap, has light, heat resistant. Diamond UV sensors are commercially available [3– 5], but the demands on position sensors are expected to increase with developments in microfabrication of future generations of LSI and acceleration of chemical reaction processes. To meet this demand Salvatori et al. developed a diamond UV position sensor but due to the measure-
* Corresponding author. Tel.: +81 11 706 6678; fax: +81 11 706 6678. E-mail address: [email protected] (K. Tsuji). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.07.022
ment principle, the sensor is only useful with UV radiations whose the light spot exceeds a certain size [5]. The sensor proposed by Lansley et al. has a very complicated circuit as it measures the signal of each electrode separately [6]. The high resolution position sensitive UV sensor developed and tested here has no limitation on light spot size and works based on a simple charge divider principle using highly oriented polycrystalline diamond. Highly oriented diamond consists of azimuthally oriented (001) facets and contain a lower density of grain boundaries than polycrystalline diamond films, and as a result these is less charge trapping than with polycrystalline diamond film. The proof of principle tests with this diamond was success.
2. Fabrication of the sensor The photograph of the sensor is shown in Fig. 1. A highly oriented 10 Am thin diamond film was formed on a low-resistibility Si substrate, 15 6 mm2 in area and 500 Am in thickness using the bias-enhanced nucleation techni-
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K. Tsuji et al. / Diamond & Related Materials 14 (2005) 2035 – 2038
boron-doped diamond covered a part of the highly oriented diamond film [9]. Next, the graphite on the surface of the diamond was removed by soaking the sample in a heated chromic acid solution, and oxygen terminations were made. The chrome adhering to the surface from the previous stage was removed by nitro hydrochloric acid, and RCA cleansing was also performed as well. The electrode pattern was created by photolithography, and platinum strip electrodes were evaporated onto the diamond film by sputtering. Finally, patterning was performed by the lift-off method. The width of each strip electrode as well as the separation between two electrodes was 200 Am, and the sensitive area was 2 4.5 mm.
Extraction electrode
Sensitive area
2 mm
3. Principles of measurements Fig. 2 shows a diagram of the detection principles. When UV radiation, with an energy exceeding the forbidden band gap of diamond (5.47 eV), enters the sensor, electron hole pairs are produced. Since there is an electric potential between the front and back of the sensor, holes move towards the back side while electrons move to the strip electrodes on the front side. The electric current is produced as an induced charge, and flows in the direction that counteracts the charge in the diamond. Assuming that a sensor with five electrodes and a sensitive area 4a + 4b wide, and let the produced charge be 4(a + b)q when UV radiation is irradiated at 3a + 2b from the left edge. As the resistance of the boron doped diamond is lower than that of intrinsic diamond, the currents flowing to the adjoining electrodes sandwiching the light spot are divided in
4.5mm Fig. 1. Photo of detector. Sensitive area was 2 4.5 mm2.
que [7,8]. After the formation of the diamond film, selectedarea deposition was applied, namely, photoresist was patterned by photolithography on the area on which boron-doped-diamond should be formed. Then, a SiO2 film was formed on the whole area of the diamond film. The resist was removed by sousing the sample in detaching liquid. The whole sample except the area where B-doped diamond film was deposited was covered by the photoresit. Finally, the SiO2 film was removed by HF solution after formation of 1 Am boron-doped diamond film. At this stage,
Terminal 1
Terminal 2 Resistance by boron doped diamond
R
R
R a
aq+2bq
2
R b
3aq+2bq
1
4 bq 4aq aR'
bR'
Laser spot d1 = 3a + 2b
d 2 = a + 2b
Fig. 2. Principles of the measurements. Let the produced charge be (a + b)q when UV radiation is irradiated at the distance of 3a + 2b from the left edge. The induced charge is divided in accordance with the resistances (distances), and flow to the adjoining electrodes sandwiching the light spot. Thus, charge ! become 4aq and charge " is 4bq, respectively. Then charge again, is divided in accordance with the resistance of boron doped diamond. And the charge flowing to terminal 1 becomes aq + 2bq, to terminal 2 it is 3aq + 2bq. The ratio between the distances from a edge of the sensor to the light spot is equivalent to the inverse ratio of the charges that flow to terminals 1 and 2.
K. Tsuji et al. / Diamond & Related Materials 14 (2005) 2035 – 2038
2037
1.0
ortec142A Ratio of Current flowing to terminal 1 to total charge
0.9
80V Charge sensitive preamplifier ortec142A
Terminal 1
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Ch2
Ch1 Terminal 2
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Distance from the left edge of sensitive area[mm] Fig. 5. Relation between the light spot position and the ratio of the charge flowing to terminal 1 to the total charge flowing to terminal 1 and 2. Error bars are covered by the plots.
Oscilloscope
Laser
detected. To develop a two-dimensional position sensor for soft X radiations, we adopted a configuration in which charge carriers move in the bulk.
Boron doped diamond
Highly oriented diamond
4. Experiment Fig. 3. Outline of the detector and measurement system.
accordance with the resistances, as determined by the distances to the electrodes. Thus, a charge of 4bq flows to the left electrode while 4aq flows to the right. The current flowing to each electrode reaches terminals 1 or 2, respectively, through the boron doped diamond. At this stage, the charge is again divided in accordance with the distances to the terminals. And the result are given by the Eq. (1). Q1 aq þ 2bq a þ 2b d2 ¼ ¼ ¼ 3aq þ 2bq 3a þ 2b Q2 d1
ð1Þ
Where Q 1 and Q 2 are the charges that reach terminal, 1 and 2, and d 1 and d 2 are the distances from terminals 1 and 2 to light spot, respectively. From this principle, the ratio of the distances from each edge of the sensor to the light spot is equivalent to the inverse ratio of the charges that flow to terminals 1 and 2, and the position of UV light spot can be
5. Experimental result and discussion The output signal from the charge-sensitive preamplifier was observed by a digital oscilloscope. Measurements were carried out ten times for each light spot position, and the position error was estimated. As shown in Fig. 4(a)¨(c), the
CH1 CH2
5 4 3 2 1 0 -1 0.000
0.002 Time[s]
0.004
(c) 7 6 5 4 3 2 1 0 -1
CH1 CH2
0.000 Time[s]
0.005
Preamplifier output signal [V]
(b) Preamplifier output signal [V]
(a) Preamplifier output signal [V]
An operational test of the sensor was conducted. A schematic diagram of the experimental set up is shown in Fig. 3. An 80V DC bias voltage was applied to the sensor, and the fifth harmonic of Nd : YAG laser with a pulse width of 100 ps, frequency 5 Hz and energy 1.8 mW was used as the light source. The light spot was collimated up to 0.3 0.3 mm2 with a slit. Since the signal from the sensor was minute, a charge sensitive preamplifier (Ortec142A), input capacitance 1 pF, and time constant 500 As, which is often used in radiation measurement, was also utilised.
3
CH1 CH2
2 1 0
-1 -0.002
0.000
0.002
0.004
Time[s]
Fig. 4. Preamplifier output signals (a) Light spot at X = 1 mm (distance from left edge of sensitive area to light spot is 1 mm ), (b) X = 1.5 mm, (c) X = 2.75 mm.
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K. Tsuji et al. / Diamond & Related Materials 14 (2005) 2035 – 2038
ratio of the charges that were detected by each terminal varied as the light-spot position changed. Fig. 5 shows the relation between light-spot position, the ratio of the charge signal appearing at terminal 1 and the total charge detected by terminal 1 and 2. The calculations have used the peak values of each output signal from the preamplifiers were taken. Fig. 5 shows good position linearity throughout the sensor, and from this result, it is reasonable to conclude that the quality of diamond crystal was satisfactory and uniform throughout the sensitive area. The position resolution was estimated to be 0.25 mm, and the statistical error bars in Fig. 5 are hidden by the markers. In the experiment, a slit was used to collimate the light, and broadening of the light spot due to diffraction may have occurred. Due to this, it is likely that the position resolution of the sensor was better than 0.25 mm. A UV position sensor with very good linearity and the position resolution of 0.25 mm was developed. In fact the sensor previously proposed has much poorer resolution than 0.25 mm. It must be noted that the rise time of the sensor was slow, exceeding 0.5 ms, as shown in Fig. 4. The separation of the electrodes and the widths between electrodes on the front side were 200 Am, which is much wider than the thickness of the sensitive layer of 10 Am, resulted in the low elctricfield strength in the sensitive layer. This is the reason for the slow response time of the sensor. There are two measures that could improve this problem. The first to thin the strip electrodes and increase the total area of electrode. The second one restructure the sensor, and let the charge carriers move on the surface of the diamond while the charge move within the bulk in the present system. Another problem is that a charge sensitive preamplifier is needed because the charge caused by laser irradiation was as low as 10 pC. This problem may be attributed to the insufficient electric
properties of the diamond. Especially, there is possibility that there are numerous grain boundaries at silicon side boundary, and further improvement of the crystal is needed. Estimating of electric property of diamond was carried out using radiation measurement technique, and the deposition technology has progressed further based on the data [10]. By making use of better diamond, further improvement to the sensor would be possible. A part of this work was supported by KAKENHI (15360498).
References [1] R.A. Al-Wazzan, T. Morrw, H.F.J. Cormican, D.J. Denvir, Nucl. Instrum. Methods Phys. A 364 (1995) 166. [2] Uwe Arp, Ping-Shine Shaw, Rajeev Gupta, Keith R. Lykke, J. Electron Spectrosc. Relat. Phenom. 144-147 (2005) 1039. [3] K. Hayashi, Y. Yokota, T. Tachibana, K. Kobashi, J. Achard, A. Gicquel, C. Olivero, M. Castex, A. Treshchalov, Diamond Relat. Mater. 10 (2001) 1794. [4] Michael D. Whitfield, Stuart P. Lansley, Oliver Gaudin, Robert D. Mckeag, Nadeem Rizvi, Richard B. Jackman, Diamond Relat. Mater. 10 (2001) 693. [5] S. Salvatori, G. Mazzeo, G. Conte, M.C. Rossi, V. Ralchenko, Diamond Relat. Mater. 13 (2004) 948. [6] Stuart P. Lansley, Olivier Gaudin, Haitao Ye, Nadeem Rizvi, Michael D. Whitfield, Robert D. Mckeag, Richard B. Jackman, Diamond Relat. Mater. 11 (2002) 433. [7] S. Yugo, T. Kanai, T. Kimura, T. Muto, Appl. Phys. Lett. 58 (1991) 1036. [8] T. Tachibana, K. Hayashi, K. Kobashi, Appl. Phys. Lett. 68 (1996) 1491. [9] T. Inoue, H. Tachibana, K. Kumagai, K. Miyata, K. Nishimura, K. Kobashi, A. Nakaue, J. Appl. Phys. 67 (1990) 7329. [10] Y. Oshiki, Junichi H. Kaneko, K. Hayashi, K. Meguro, F. Fujita, A. Homma,Y. Yokota, Y. Yamamoto, K. Kobashi,T Imai, T. Sawamura, M. Furusaka, Diamond relat mater (in preparation).
Development of high resolution position sensitive UV detector based on highly oriented polycrystalline diamond K. Tsuji a,*, K. Hayashi b, J.H. Kaneko a, F. Fujita a, A. Homma a, Y. Oshiki a, T. Sawamura a, M. Furusaka a a
Division of Quantum Science and Engineering, Hokkaido University, North 13 West 8, Sapporo, Hokkaido, 060-8628, Japan b Kobe Steel Ltd., Japan Available online 31 August 2005
Abstract UV position-sensitive sensor using a polycrystalline highly-oriented diamond film with a sensitive area of 2 4.5 mm2 developed, and a proof-of-concept study was conducted. The charge division method was employed to establish the position when the light hits the sensitive area. A fifth higher harmonic of a Nd:YAG laser with pulse width of 100 ps was used as a light source. The position resolution was 0.25 mm, and there was good position linearity throughout the sensitive area. Considering the possibility of light-spot broadening due to diffraction by the slit, the sensor may have a better resolution than that indicated by the result. But the electric-field strength of some parts of the crystal was insufficient, the response time was relatively slow, approximately 0.5 ms. D 2005 Elsevier B.V. All rights reserved. Keywords: UV detector; Highly oriented diamond; Position sensitive detector; 213 nm UV laser
1. Introduction Recently, intensive short wavelength ultraviolet radiation is being used in industry, and the demand for highly accurate and stable position sensors is growing. Silicon, gallium nitride, and photoelectric cells have been used as UV detection elements but these are not suitable for very intensive short wavelength UV radiations because of inferior light resistance [1,2]. Diamond has been considered able to overcome this problem, and has attracted growing attention. Diamond is insensitive to visible light having a wide band gap, has light, heat resistant. Diamond UV sensors are commercially available [3– 5], but the demands on position sensors are expected to increase with developments in microfabrication of future generations of LSI and acceleration of chemical reaction processes. To meet this demand Salvatori et al. developed a diamond UV position sensor but due to the measure-
* Corresponding author. Tel.: +81 11 706 6678; fax: +81 11 706 6678. E-mail address: [email protected] (K. Tsuji). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.07.022
ment principle, the sensor is only useful with UV radiations whose the light spot exceeds a certain size [5]. The sensor proposed by Lansley et al. has a very complicated circuit as it measures the signal of each electrode separately [6]. The high resolution position sensitive UV sensor developed and tested here has no limitation on light spot size and works based on a simple charge divider principle using highly oriented polycrystalline diamond. Highly oriented diamond consists of azimuthally oriented (001) facets and contain a lower density of grain boundaries than polycrystalline diamond films, and as a result these is less charge trapping than with polycrystalline diamond film. The proof of principle tests with this diamond was success.
2. Fabrication of the sensor The photograph of the sensor is shown in Fig. 1. A highly oriented 10 Am thin diamond film was formed on a low-resistibility Si substrate, 15 6 mm2 in area and 500 Am in thickness using the bias-enhanced nucleation techni-
2036
K. Tsuji et al. / Diamond & Related Materials 14 (2005) 2035 – 2038
boron-doped diamond covered a part of the highly oriented diamond film [9]. Next, the graphite on the surface of the diamond was removed by soaking the sample in a heated chromic acid solution, and oxygen terminations were made. The chrome adhering to the surface from the previous stage was removed by nitro hydrochloric acid, and RCA cleansing was also performed as well. The electrode pattern was created by photolithography, and platinum strip electrodes were evaporated onto the diamond film by sputtering. Finally, patterning was performed by the lift-off method. The width of each strip electrode as well as the separation between two electrodes was 200 Am, and the sensitive area was 2 4.5 mm.
Extraction electrode
Sensitive area
2 mm
3. Principles of measurements Fig. 2 shows a diagram of the detection principles. When UV radiation, with an energy exceeding the forbidden band gap of diamond (5.47 eV), enters the sensor, electron hole pairs are produced. Since there is an electric potential between the front and back of the sensor, holes move towards the back side while electrons move to the strip electrodes on the front side. The electric current is produced as an induced charge, and flows in the direction that counteracts the charge in the diamond. Assuming that a sensor with five electrodes and a sensitive area 4a + 4b wide, and let the produced charge be 4(a + b)q when UV radiation is irradiated at 3a + 2b from the left edge. As the resistance of the boron doped diamond is lower than that of intrinsic diamond, the currents flowing to the adjoining electrodes sandwiching the light spot are divided in
4.5mm Fig. 1. Photo of detector. Sensitive area was 2 4.5 mm2.
que [7,8]. After the formation of the diamond film, selectedarea deposition was applied, namely, photoresist was patterned by photolithography on the area on which boron-doped-diamond should be formed. Then, a SiO2 film was formed on the whole area of the diamond film. The resist was removed by sousing the sample in detaching liquid. The whole sample except the area where B-doped diamond film was deposited was covered by the photoresit. Finally, the SiO2 film was removed by HF solution after formation of 1 Am boron-doped diamond film. At this stage,
Terminal 1
Terminal 2 Resistance by boron doped diamond
R
R
R a
aq+2bq
2
R b
3aq+2bq
1
4 bq 4aq aR'
bR'
Laser spot d1 = 3a + 2b
d 2 = a + 2b
Fig. 2. Principles of the measurements. Let the produced charge be (a + b)q when UV radiation is irradiated at the distance of 3a + 2b from the left edge. The induced charge is divided in accordance with the resistances (distances), and flow to the adjoining electrodes sandwiching the light spot. Thus, charge ! become 4aq and charge " is 4bq, respectively. Then charge again, is divided in accordance with the resistance of boron doped diamond. And the charge flowing to terminal 1 becomes aq + 2bq, to terminal 2 it is 3aq + 2bq. The ratio between the distances from a edge of the sensor to the light spot is equivalent to the inverse ratio of the charges that flow to terminals 1 and 2.
K. Tsuji et al. / Diamond & Related Materials 14 (2005) 2035 – 2038
2037
1.0
ortec142A Ratio of Current flowing to terminal 1 to total charge
0.9
80V Charge sensitive preamplifier ortec142A
Terminal 1
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Ch2
Ch1 Terminal 2
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Distance from the left edge of sensitive area[mm] Fig. 5. Relation between the light spot position and the ratio of the charge flowing to terminal 1 to the total charge flowing to terminal 1 and 2. Error bars are covered by the plots.
Oscilloscope
Laser
detected. To develop a two-dimensional position sensor for soft X radiations, we adopted a configuration in which charge carriers move in the bulk.
Boron doped diamond
Highly oriented diamond
4. Experiment Fig. 3. Outline of the detector and measurement system.
accordance with the resistances, as determined by the distances to the electrodes. Thus, a charge of 4bq flows to the left electrode while 4aq flows to the right. The current flowing to each electrode reaches terminals 1 or 2, respectively, through the boron doped diamond. At this stage, the charge is again divided in accordance with the distances to the terminals. And the result are given by the Eq. (1). Q1 aq þ 2bq a þ 2b d2 ¼ ¼ ¼ 3aq þ 2bq 3a þ 2b Q2 d1
ð1Þ
Where Q 1 and Q 2 are the charges that reach terminal, 1 and 2, and d 1 and d 2 are the distances from terminals 1 and 2 to light spot, respectively. From this principle, the ratio of the distances from each edge of the sensor to the light spot is equivalent to the inverse ratio of the charges that flow to terminals 1 and 2, and the position of UV light spot can be
5. Experimental result and discussion The output signal from the charge-sensitive preamplifier was observed by a digital oscilloscope. Measurements were carried out ten times for each light spot position, and the position error was estimated. As shown in Fig. 4(a)¨(c), the
CH1 CH2
5 4 3 2 1 0 -1 0.000
0.002 Time[s]
0.004
(c) 7 6 5 4 3 2 1 0 -1
CH1 CH2
0.000 Time[s]
0.005
Preamplifier output signal [V]
(b) Preamplifier output signal [V]
(a) Preamplifier output signal [V]
An operational test of the sensor was conducted. A schematic diagram of the experimental set up is shown in Fig. 3. An 80V DC bias voltage was applied to the sensor, and the fifth harmonic of Nd : YAG laser with a pulse width of 100 ps, frequency 5 Hz and energy 1.8 mW was used as the light source. The light spot was collimated up to 0.3 0.3 mm2 with a slit. Since the signal from the sensor was minute, a charge sensitive preamplifier (Ortec142A), input capacitance 1 pF, and time constant 500 As, which is often used in radiation measurement, was also utilised.
3
CH1 CH2
2 1 0
-1 -0.002
0.000
0.002
0.004
Time[s]
Fig. 4. Preamplifier output signals (a) Light spot at X = 1 mm (distance from left edge of sensitive area to light spot is 1 mm ), (b) X = 1.5 mm, (c) X = 2.75 mm.
2038
K. Tsuji et al. / Diamond & Related Materials 14 (2005) 2035 – 2038
ratio of the charges that were detected by each terminal varied as the light-spot position changed. Fig. 5 shows the relation between light-spot position, the ratio of the charge signal appearing at terminal 1 and the total charge detected by terminal 1 and 2. The calculations have used the peak values of each output signal from the preamplifiers were taken. Fig. 5 shows good position linearity throughout the sensor, and from this result, it is reasonable to conclude that the quality of diamond crystal was satisfactory and uniform throughout the sensitive area. The position resolution was estimated to be 0.25 mm, and the statistical error bars in Fig. 5 are hidden by the markers. In the experiment, a slit was used to collimate the light, and broadening of the light spot due to diffraction may have occurred. Due to this, it is likely that the position resolution of the sensor was better than 0.25 mm. A UV position sensor with very good linearity and the position resolution of 0.25 mm was developed. In fact the sensor previously proposed has much poorer resolution than 0.25 mm. It must be noted that the rise time of the sensor was slow, exceeding 0.5 ms, as shown in Fig. 4. The separation of the electrodes and the widths between electrodes on the front side were 200 Am, which is much wider than the thickness of the sensitive layer of 10 Am, resulted in the low elctricfield strength in the sensitive layer. This is the reason for the slow response time of the sensor. There are two measures that could improve this problem. The first to thin the strip electrodes and increase the total area of electrode. The second one restructure the sensor, and let the charge carriers move on the surface of the diamond while the charge move within the bulk in the present system. Another problem is that a charge sensitive preamplifier is needed because the charge caused by laser irradiation was as low as 10 pC. This problem may be attributed to the insufficient electric
properties of the diamond. Especially, there is possibility that there are numerous grain boundaries at silicon side boundary, and further improvement of the crystal is needed. Estimating of electric property of diamond was carried out using radiation measurement technique, and the deposition technology has progressed further based on the data [10]. By making use of better diamond, further improvement to the sensor would be possible. A part of this work was supported by KAKENHI (15360498).
References [1] R.A. Al-Wazzan, T. Morrw, H.F.J. Cormican, D.J. Denvir, Nucl. Instrum. Methods Phys. A 364 (1995) 166. [2] Uwe Arp, Ping-Shine Shaw, Rajeev Gupta, Keith R. Lykke, J. Electron Spectrosc. Relat. Phenom. 144-147 (2005) 1039. [3] K. Hayashi, Y. Yokota, T. Tachibana, K. Kobashi, J. Achard, A. Gicquel, C. Olivero, M. Castex, A. Treshchalov, Diamond Relat. Mater. 10 (2001) 1794. [4] Michael D. Whitfield, Stuart P. Lansley, Oliver Gaudin, Robert D. Mckeag, Nadeem Rizvi, Richard B. Jackman, Diamond Relat. Mater. 10 (2001) 693. [5] S. Salvatori, G. Mazzeo, G. Conte, M.C. Rossi, V. Ralchenko, Diamond Relat. Mater. 13 (2004) 948. [6] Stuart P. Lansley, Olivier Gaudin, Haitao Ye, Nadeem Rizvi, Michael D. Whitfield, Robert D. Mckeag, Richard B. Jackman, Diamond Relat. Mater. 11 (2002) 433. [7] S. Yugo, T. Kanai, T. Kimura, T. Muto, Appl. Phys. Lett. 58 (1991) 1036. [8] T. Tachibana, K. Hayashi, K. Kobashi, Appl. Phys. Lett. 68 (1996) 1491. [9] T. Inoue, H. Tachibana, K. Kumagai, K. Miyata, K. Nishimura, K. Kobashi, A. Nakaue, J. Appl. Phys. 67 (1990) 7329. [10] Y. Oshiki, Junichi H. Kaneko, K. Hayashi, K. Meguro, F. Fujita, A. Homma,Y. Yokota, Y. Yamamoto, K. Kobashi,T Imai, T. Sawamura, M. Furusaka, Diamond relat mater (in preparation).