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SIMOX structures
Applied Surface Science 184 (2001) 178–182
Non-Rutherford backscattering studies of SiC/SIMOX structures K.W. Chena, Y.H. Yua,*, Y.M. Leia, L.L. Chenga, B. Sundaravalb, E.Z. Luob, S.P. Wongb, I.H. Wilsonb, L.Z. Chena, C.X. Rena, S.C. Zoua a
Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, China b Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, NT, Hong Kong
Abstract SiC films were deposited on SIMOX (Separation by IMplanted OXygen) substrates reactive by DC magnetron sputtering. A 4 in. magnetron silicon target was sputtered in an Ar/CH4 mixed DC magnetron glow discharge plasma. Non-Rutherford backscattering spectra were obtained using different incident Helium ion beam energies. The oxygen in the buried oxide layer formed by ion implantation and post anneal was detected using an incident energy of about 3.3 MeV, with a much higher backscattering yield than Rutherford backscattering. The composition of the buried layer was thus determined. Using incident energy of 4.3 MeV, which greatly enhance the intensity of carbon backscattering signals, the SiC films prepared were found stoichiometric by non-Rutherford backscattering. Furthermore, atomic force microscopy (AFM) studies found that the deposited SiC film had a rather smooth surface morphology. IR reflectance measurements showed that there existed a reststrahlen peak around 800 cm1 which is characteristic for SiC. No IR reflectance peaks related to bonded hydrogen in the film were identified. Spreading resistance probe (SRP) measurements were taken and found layered structure with quite different resistance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Non-Rutherford backscattering; SIMOX substrates; DC magnetic sputtering
1. Introduction As a wide band gap semiconductor, SiC is a promising material for high temperature electronics and sensors because of its excellent physical and chemical properties such as high breakdown field, high electron saturation velocity, high radiation resistivity, high chemical inertness, etc. [1]. While growth of bulk SiC and homoepitaxial growth is under intensive investigation, the heteroepitaxial growth of SiC on silicon substrates is also of great interests observed that it provides a material system combining the high *
Corresponding author. Tel.: þ86-21-62511070, ext. 8303; fax: þ86-21-62513510. E-mail address: [email protected] (Y.H. Yu).
temperature capabilities of SiC and the micromachining possibilities of Si [2]. However, the SiC/Si heterojunction starts leaking at temperatures higher than about 500 K, resulting in a current flow through the silicon substrate. To solve this problem, the SiC on SOI system [3] has been proposed because SOI substrates are well known for their low leakage and high temperature capabilities. This time another problem emerges: the high deposition temperature used in CVD of SiC films causes damages in the buried oxide layer. This damage may cause failure in the devices (piezoresistive pressure sensors as reported by Ziermann et al. [3]) manufactured. Therefore, we employed reactive DC magnetron sputtering to deposit SiC films on SOI substrates at relatively lower temperatures and report our initial results here.
0169-4332/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 9 9 - 8
K.W. Chen et al. / Applied Surface Science 184 (2001) 178–182
2. Experiments In our studies, the SiC film was reactively sputtered on SIMOX (Separation by IMplanted OXygen) substrate. The substrates were RCA cleaned and dipped into buffered HF etchant before loaded into the deposition chamber. The apparatus and the deposition process have been described in detail elsewhere, here we put it in a few words. The base pressure was about 3 104 Pa first, then the substrate was heated to the 900 8C, finally the mixed Ar/CH4 was inlet and the plasma ignited. The voltage and current were kept at 430 V and about 550 mA during deposition. The sample we studied in this paper was prepared with methane partial pressure of 0.0665 Pa and total pressure of 0.4 Pa. Non-Rutherford elastic scattering was done on a system capable of accelerating ion beams to energy higher than 4 MeV. In the experiments, 4He ion beam with energies ranging from 2.0 to 4.3 MeV was used and the backscattered particles were detected at y ¼ 170 . The infrared reflectance spectra were measured by a Perkin-Elmer 983 double beam spectrometer for the range 400–4000 cm1 using near-normal incidence. The measured reflectance was calibrated using a high quality aluminum mirror to obtain the absolute reflectance. Spreading resistance probe (SRP) analysis was made on an ASR-100C/2 spreading resistance probing system.
3. Results and discussion Structural properties of the SiC films prepared by DC sputtering as described above have been characterized and reported elsewhere [4]. In those studies, we found that textured polycrystalline 3C–SiC films were successfully formed on silicon substrates. NonRutherford backscattering is presented here because it not only gives out compositional but also depth distribution information of the SiC/SOI structure. Rutherford backscattering has several drawbacks such as not suitable for probing light atoms on or in heavier substrates. To cope with such conditions, nonRutherford backscattering can be employed. Accelerated 4He ion beam was employed in our experiments. Fig. 1(a) shows a conventional RBS spectrum using 2 MeV incident beam, no observable signals for C or O
179
can be seen in this spectrum. Fig. 1(b) shows a spectrum obtained with higher incident ion energy of 2.9 MeV, and the oxygen in the buried oxide layer could be identified in the spectrum. In Fig. 1(c), there is the spectrum measured with incident beam energy of 3.3 MeV. In this case, there are clear evidences of both oxygen and carbon. The peak starts at channel 900 represents the ion backscattered from the surface silicon. The next peak, around channel 700, is caused by the Si overlayer of the SOI structure. This silicon overlayer is relatively thin. The thickness determined from the analysis is about 1200 atoms/cm2. Next around channel 650, the counts falls down, which is the result of the buried oxide layer where the atomic density of Si is lower. After penetrating this layer, the ion beam reached the bulk silicon substrate. As the cross-section of carbon just below 3.3 MeV decreases with the decreasing beam energy, in the spectrum, the counts related to carbon decreases with the decrease in detected energy. Finally, the oxygen signal (corresponding to the buried oxide) has been greatly enhanced. This is due to the greatly increased backscattering crosssection for oxygen when the incident beam energy is close to 3.03 MeV [5] (in our experiment, the beam energy decreased to this level when the ions reached the oxide layer through the interactions with the upper layers). In order to detect oxygen content in the uppermost SiC layer, one should use incident beam energy of just a bit higher than 3.03 MeV. We have also done so to check if the SiC film contains oxygen. The detection limit of non-Rutherford backscattering on detecting oxygen content is 1%. The result shows that there is no evidence for the existence of oxygen in the top layer. The explanation may be that in our deposition process, the argon ions and other charged particles (such as CHm þ ) were accelerated toward the target by the DC electric field between the plasma and the silicon target. The energetic bombarding of the sputtered particles (about 400 eV) on the deposited film made it dense and void-free thus eliminated the oxidation enhancing effect that could be caused by porous structures [6]. Therefore, we attribute the absence of oxygen in the films to the energetic bombarding of the film. The spectrum shown in Fig. 1(d) is measured using 4.27 MeV 4He ion beam as incident beam, which exhibits a greatly enhanced peak for carbon.
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Fig. 1. Non-Rutherford backscattering spectra for the SiC film deposited on SOI using different beam energies: (a) 2 MeV; (b) 2.9 MeV; (c) 3.3 MeV; (d) 4.27 MeV.
From above analysis and with the help of the software developed by Matej Mayer [7], we determined the compositional and structural parameters of our SiC on SOI samples as shown in Fig. 2. The uppermost layer is SiC with composition of Si:C ¼ 1:1 and a thickness of 6:8 1018 atoms=cm2 ; the second is the silicon overlayer with a thickness of 1:2 1018 atom= cm2 as mentioned above; the third is the BOX (buried
Fig. 2. Cross-section of SiC on SOI structure (not to scale).
oxide) with a composition of Si:O ¼ 1:1.2 and thickness of 2:0 1018 atoms=cm2 . If we take 3.21 g/cm3 as the density of the SiC film, then the thickness can be estimated to be about 0.7 mm. The SiC film deposited on SOI substrates was studied by using atomic force microscopy (AFM), which can reveal the detail of the morphology of the films. Fig. 3 shows the AFM image of the SiC/SOI surface. The root-mean-square (Rrms) roughness is given by the standard deviation over all height values within the surface area interested. The nominal overall Rrms roughness of SiC/SOI surface is 1.436 nm. From this graph, it can be seen that the surface of the SiC film deposited on SOI substrate is rather smooth. IR reflectance spectrum has been employed to probe the bonding status of the SiC film prepared on the SOI substrate in our experiments. Fig. 4(a) shows the infrared spectrum of the SOI substrate prior to the deposition of the SiC films. There are two peaks
K.W. Chen et al. / Applied Surface Science 184 (2001) 178–182
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Fig. 3. AFM image of SiC film deposited on SOI substrate.
in the spectrum corresponding to the SiOx rocking mode (R) at 480 cm1 and SiOx asymmetric stretching (AS) mode at about 1120 cm1. The peaks in the spectrum are centered at about 455 and 1105 cm1; the peak reflectances are 36.21 and 41.07%, respectively. The shift of the peaks to lower wavenumbers may be caused by the relatively poor quality of the oxide formed by implantation of oxygen and subsequent high temperature annealing. Fig. 4(b) shows the IR reflectance of the SiC/SOI materials system. A reststrahlen peak in the range of around 800–1000 cm1 is observed in this curve, which stems from the transverse optical phonon of SiC at 794 cm1
[8]. It is reported that Si–H has an oscillating mode (stretching vibration) around 2080 cm1, another (Si–H rocking vibration) around 640 cm1, and the wagging vibration of C–H2 is around 1000 cm1 [9]. However, no peak corresponding to hydrogenated silicon or carbon is identified in our measured IR reflectance spectrum. From the interference peaks in the higher wavenumber range (higher frequency range), the thickness of the SiC film is determined using the following equation d ¼ 12 nf Do
Fig. 4. Infrared reflectance spectrum for: (a) original SOI substrate; (b) with SiC film deposited on it.
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K.W. Chen et al. / Applied Surface Science 184 (2001) 178–182
4. Summary SiC films deposited on SIMOX substrates by DC reactive sputtering were characterized by nonRutherford backscattering, infrared reflectance, and spreading resistance probing. The non-Rutherford backscattering results show that the film was stoichiometric and reveal the composition of the buried oxide layer. IR studies show that the film is composed of mainly Si–C bonds. Spreading resistance probing reveals layered conducting–insulating–conducting sandwich structure. Fig. 5. Resistivity of SiC/SOI measured by spreading resistance probing.
Acknowledgements where Do is the frequency difference between two consecutive fringe maxima (or minima), and nf the refractive index of the film. To improve the accuracy of this method, d can be calculated using a Do containing several complete cycles. As our film contains only one cycle in the high frequency region, the result may have relatively large aberration. The thickness thus obtained is about 1.06 mm, which is consistent with the value given by NBS. Spreading resistance probing is widely used in the study of layered structures of semiconductor. Fig. 5 shows the SRP result of our SiC/SOI sample. The resistivity of the top SiC film is about 10,000 O; the lower resistive silicon layer may have been missed by the instrument because of its small thickness. Then the resistivity rises quickly to higher than 9 108 O, and then falls to the normal value (3000 O) of the silicon substrate. The SiC layer is about 850 nm according to this result, which agrees with that of the non-Rutherford backscattering and the IR reflectance spectrum.
This work is supported by Shanghai Municipal Board of Sciences and Technology under Grant No. 98QME1403. References [1] M.N. Yoder, IEEE Trans. Electron. Dev. 43 (1996) 1633. [2] G. Muller, G. Krotz, E. Niemann, Sens. Actuat. A 43 (1994) 259. [3] R. Ziermann, J. von Berg, W. Reichert, E. Obermeier, M. Eickhoff, G. Kro¨ tz, in: Proceedings of the 1997 International Conference on Solid-state Sensors and Actuators, 1997, p. 1411. [4] Y.M. Lei, Y.H. Yu, et al., Thin Solid Films 365 (2000) 53. [5] Y. Feng, Z. Zhou, Y. Zhou, G. Zhou, Nucl. Instrum. Meth. B 86 (1994) 225. [6] Y.H. Yu, S.P. Wong, I.H. Wilson, Mater. Sci. Eng. B 52 (1998) 55. [7] Matej Mayer, SIMNRA Ver. 4.4, Institute of Plasma Physics, Germany. [8] D. Olego, M. Cardona, P. Vogl, Phys. Rev. B 25 (1982) 3878. [9] M.A. El Khakani, M. Chaker, A. Jean, S. Boily, H. Pepin, J.C. Kieffer, S.C. Gujrathi, J. Appl. Phys. 74 (1993) 2834.
Non-Rutherford backscattering studies of SiC/SIMOX structures K.W. Chena, Y.H. Yua,*, Y.M. Leia, L.L. Chenga, B. Sundaravalb, E.Z. Luob, S.P. Wongb, I.H. Wilsonb, L.Z. Chena, C.X. Rena, S.C. Zoua a
Ion Beam Laboratory, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, China b Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, NT, Hong Kong
Abstract SiC films were deposited on SIMOX (Separation by IMplanted OXygen) substrates reactive by DC magnetron sputtering. A 4 in. magnetron silicon target was sputtered in an Ar/CH4 mixed DC magnetron glow discharge plasma. Non-Rutherford backscattering spectra were obtained using different incident Helium ion beam energies. The oxygen in the buried oxide layer formed by ion implantation and post anneal was detected using an incident energy of about 3.3 MeV, with a much higher backscattering yield than Rutherford backscattering. The composition of the buried layer was thus determined. Using incident energy of 4.3 MeV, which greatly enhance the intensity of carbon backscattering signals, the SiC films prepared were found stoichiometric by non-Rutherford backscattering. Furthermore, atomic force microscopy (AFM) studies found that the deposited SiC film had a rather smooth surface morphology. IR reflectance measurements showed that there existed a reststrahlen peak around 800 cm1 which is characteristic for SiC. No IR reflectance peaks related to bonded hydrogen in the film were identified. Spreading resistance probe (SRP) measurements were taken and found layered structure with quite different resistance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Non-Rutherford backscattering; SIMOX substrates; DC magnetic sputtering
1. Introduction As a wide band gap semiconductor, SiC is a promising material for high temperature electronics and sensors because of its excellent physical and chemical properties such as high breakdown field, high electron saturation velocity, high radiation resistivity, high chemical inertness, etc. [1]. While growth of bulk SiC and homoepitaxial growth is under intensive investigation, the heteroepitaxial growth of SiC on silicon substrates is also of great interests observed that it provides a material system combining the high *
Corresponding author. Tel.: þ86-21-62511070, ext. 8303; fax: þ86-21-62513510. E-mail address: [email protected] (Y.H. Yu).
temperature capabilities of SiC and the micromachining possibilities of Si [2]. However, the SiC/Si heterojunction starts leaking at temperatures higher than about 500 K, resulting in a current flow through the silicon substrate. To solve this problem, the SiC on SOI system [3] has been proposed because SOI substrates are well known for their low leakage and high temperature capabilities. This time another problem emerges: the high deposition temperature used in CVD of SiC films causes damages in the buried oxide layer. This damage may cause failure in the devices (piezoresistive pressure sensors as reported by Ziermann et al. [3]) manufactured. Therefore, we employed reactive DC magnetron sputtering to deposit SiC films on SOI substrates at relatively lower temperatures and report our initial results here.
0169-4332/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 9 9 - 8
K.W. Chen et al. / Applied Surface Science 184 (2001) 178–182
2. Experiments In our studies, the SiC film was reactively sputtered on SIMOX (Separation by IMplanted OXygen) substrate. The substrates were RCA cleaned and dipped into buffered HF etchant before loaded into the deposition chamber. The apparatus and the deposition process have been described in detail elsewhere, here we put it in a few words. The base pressure was about 3 104 Pa first, then the substrate was heated to the 900 8C, finally the mixed Ar/CH4 was inlet and the plasma ignited. The voltage and current were kept at 430 V and about 550 mA during deposition. The sample we studied in this paper was prepared with methane partial pressure of 0.0665 Pa and total pressure of 0.4 Pa. Non-Rutherford elastic scattering was done on a system capable of accelerating ion beams to energy higher than 4 MeV. In the experiments, 4He ion beam with energies ranging from 2.0 to 4.3 MeV was used and the backscattered particles were detected at y ¼ 170 . The infrared reflectance spectra were measured by a Perkin-Elmer 983 double beam spectrometer for the range 400–4000 cm1 using near-normal incidence. The measured reflectance was calibrated using a high quality aluminum mirror to obtain the absolute reflectance. Spreading resistance probe (SRP) analysis was made on an ASR-100C/2 spreading resistance probing system.
3. Results and discussion Structural properties of the SiC films prepared by DC sputtering as described above have been characterized and reported elsewhere [4]. In those studies, we found that textured polycrystalline 3C–SiC films were successfully formed on silicon substrates. NonRutherford backscattering is presented here because it not only gives out compositional but also depth distribution information of the SiC/SOI structure. Rutherford backscattering has several drawbacks such as not suitable for probing light atoms on or in heavier substrates. To cope with such conditions, nonRutherford backscattering can be employed. Accelerated 4He ion beam was employed in our experiments. Fig. 1(a) shows a conventional RBS spectrum using 2 MeV incident beam, no observable signals for C or O
179
can be seen in this spectrum. Fig. 1(b) shows a spectrum obtained with higher incident ion energy of 2.9 MeV, and the oxygen in the buried oxide layer could be identified in the spectrum. In Fig. 1(c), there is the spectrum measured with incident beam energy of 3.3 MeV. In this case, there are clear evidences of both oxygen and carbon. The peak starts at channel 900 represents the ion backscattered from the surface silicon. The next peak, around channel 700, is caused by the Si overlayer of the SOI structure. This silicon overlayer is relatively thin. The thickness determined from the analysis is about 1200 atoms/cm2. Next around channel 650, the counts falls down, which is the result of the buried oxide layer where the atomic density of Si is lower. After penetrating this layer, the ion beam reached the bulk silicon substrate. As the cross-section of carbon just below 3.3 MeV decreases with the decreasing beam energy, in the spectrum, the counts related to carbon decreases with the decrease in detected energy. Finally, the oxygen signal (corresponding to the buried oxide) has been greatly enhanced. This is due to the greatly increased backscattering crosssection for oxygen when the incident beam energy is close to 3.03 MeV [5] (in our experiment, the beam energy decreased to this level when the ions reached the oxide layer through the interactions with the upper layers). In order to detect oxygen content in the uppermost SiC layer, one should use incident beam energy of just a bit higher than 3.03 MeV. We have also done so to check if the SiC film contains oxygen. The detection limit of non-Rutherford backscattering on detecting oxygen content is 1%. The result shows that there is no evidence for the existence of oxygen in the top layer. The explanation may be that in our deposition process, the argon ions and other charged particles (such as CHm þ ) were accelerated toward the target by the DC electric field between the plasma and the silicon target. The energetic bombarding of the sputtered particles (about 400 eV) on the deposited film made it dense and void-free thus eliminated the oxidation enhancing effect that could be caused by porous structures [6]. Therefore, we attribute the absence of oxygen in the films to the energetic bombarding of the film. The spectrum shown in Fig. 1(d) is measured using 4.27 MeV 4He ion beam as incident beam, which exhibits a greatly enhanced peak for carbon.
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Fig. 1. Non-Rutherford backscattering spectra for the SiC film deposited on SOI using different beam energies: (a) 2 MeV; (b) 2.9 MeV; (c) 3.3 MeV; (d) 4.27 MeV.
From above analysis and with the help of the software developed by Matej Mayer [7], we determined the compositional and structural parameters of our SiC on SOI samples as shown in Fig. 2. The uppermost layer is SiC with composition of Si:C ¼ 1:1 and a thickness of 6:8 1018 atoms=cm2 ; the second is the silicon overlayer with a thickness of 1:2 1018 atom= cm2 as mentioned above; the third is the BOX (buried
Fig. 2. Cross-section of SiC on SOI structure (not to scale).
oxide) with a composition of Si:O ¼ 1:1.2 and thickness of 2:0 1018 atoms=cm2 . If we take 3.21 g/cm3 as the density of the SiC film, then the thickness can be estimated to be about 0.7 mm. The SiC film deposited on SOI substrates was studied by using atomic force microscopy (AFM), which can reveal the detail of the morphology of the films. Fig. 3 shows the AFM image of the SiC/SOI surface. The root-mean-square (Rrms) roughness is given by the standard deviation over all height values within the surface area interested. The nominal overall Rrms roughness of SiC/SOI surface is 1.436 nm. From this graph, it can be seen that the surface of the SiC film deposited on SOI substrate is rather smooth. IR reflectance spectrum has been employed to probe the bonding status of the SiC film prepared on the SOI substrate in our experiments. Fig. 4(a) shows the infrared spectrum of the SOI substrate prior to the deposition of the SiC films. There are two peaks
K.W. Chen et al. / Applied Surface Science 184 (2001) 178–182
181
Fig. 3. AFM image of SiC film deposited on SOI substrate.
in the spectrum corresponding to the SiOx rocking mode (R) at 480 cm1 and SiOx asymmetric stretching (AS) mode at about 1120 cm1. The peaks in the spectrum are centered at about 455 and 1105 cm1; the peak reflectances are 36.21 and 41.07%, respectively. The shift of the peaks to lower wavenumbers may be caused by the relatively poor quality of the oxide formed by implantation of oxygen and subsequent high temperature annealing. Fig. 4(b) shows the IR reflectance of the SiC/SOI materials system. A reststrahlen peak in the range of around 800–1000 cm1 is observed in this curve, which stems from the transverse optical phonon of SiC at 794 cm1
[8]. It is reported that Si–H has an oscillating mode (stretching vibration) around 2080 cm1, another (Si–H rocking vibration) around 640 cm1, and the wagging vibration of C–H2 is around 1000 cm1 [9]. However, no peak corresponding to hydrogenated silicon or carbon is identified in our measured IR reflectance spectrum. From the interference peaks in the higher wavenumber range (higher frequency range), the thickness of the SiC film is determined using the following equation d ¼ 12 nf Do
Fig. 4. Infrared reflectance spectrum for: (a) original SOI substrate; (b) with SiC film deposited on it.
(1)
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K.W. Chen et al. / Applied Surface Science 184 (2001) 178–182
4. Summary SiC films deposited on SIMOX substrates by DC reactive sputtering were characterized by nonRutherford backscattering, infrared reflectance, and spreading resistance probing. The non-Rutherford backscattering results show that the film was stoichiometric and reveal the composition of the buried oxide layer. IR studies show that the film is composed of mainly Si–C bonds. Spreading resistance probing reveals layered conducting–insulating–conducting sandwich structure. Fig. 5. Resistivity of SiC/SOI measured by spreading resistance probing.
Acknowledgements where Do is the frequency difference between two consecutive fringe maxima (or minima), and nf the refractive index of the film. To improve the accuracy of this method, d can be calculated using a Do containing several complete cycles. As our film contains only one cycle in the high frequency region, the result may have relatively large aberration. The thickness thus obtained is about 1.06 mm, which is consistent with the value given by NBS. Spreading resistance probing is widely used in the study of layered structures of semiconductor. Fig. 5 shows the SRP result of our SiC/SOI sample. The resistivity of the top SiC film is about 10,000 O; the lower resistive silicon layer may have been missed by the instrument because of its small thickness. Then the resistivity rises quickly to higher than 9 108 O, and then falls to the normal value (3000 O) of the silicon substrate. The SiC layer is about 850 nm according to this result, which agrees with that of the non-Rutherford backscattering and the IR reflectance spectrum.
This work is supported by Shanghai Municipal Board of Sciences and Technology under Grant No. 98QME1403. References [1] M.N. Yoder, IEEE Trans. Electron. Dev. 43 (1996) 1633. [2] G. Muller, G. Krotz, E. Niemann, Sens. Actuat. A 43 (1994) 259. [3] R. Ziermann, J. von Berg, W. Reichert, E. Obermeier, M. Eickhoff, G. Kro¨ tz, in: Proceedings of the 1997 International Conference on Solid-state Sensors and Actuators, 1997, p. 1411. [4] Y.M. Lei, Y.H. Yu, et al., Thin Solid Films 365 (2000) 53. [5] Y. Feng, Z. Zhou, Y. Zhou, G. Zhou, Nucl. Instrum. Meth. B 86 (1994) 225. [6] Y.H. Yu, S.P. Wong, I.H. Wilson, Mater. Sci. Eng. B 52 (1998) 55. [7] Matej Mayer, SIMNRA Ver. 4.4, Institute of Plasma Physics, Germany. [8] D. Olego, M. Cardona, P. Vogl, Phys. Rev. B 25 (1982) 3878. [9] M.A. El Khakani, M. Chaker, A. Jean, S. Boily, H. Pepin, J.C. Kieffer, S.C. Gujrathi, J. Appl. Phys. 74 (1993) 2834.