- Email: [email protected]
Ion implantation-caused damage in SiC measured by spectroscopic ellipsometry
Thin Solid Films 455 – 456 (2004) 239–243
Ion implantation-caused damage in SiC measured by spectroscopic ellipsometry P. Petrika,*, E.R. Shaabana,b, T. Lohnera, G. Battistiga, M. Frieda, J. Garcia Lopezc, Y. Morillac, ´ a, J. Gyulaia O. Polgar a
´ ut ´ 29-33, Hungary Research Institute for Technical Physics and Materials Science, H-1121 Budapest, Konkoly Thege Miklos b Physics Department, Faculty of Science, Al-Azhar University, Asuit, 71111, Egypt c Centro Nacional de Aceleradores, 41092 Sevilla, Spain
Abstract Since ion implantation-caused damage changes the complex refractive index of SiC significantly, optical methods can be used to measure the sample properties sensitively, non-destructively and quickly. In the present work the damage created by ion implantation into SiC and its change upon annealing was characterized by spectroscopic ellipsometry (SE). 4H SiC samples were implanted with 150 keV Al using doses between 4=l014 and 2=l015 cmy2 with current densities from 0.4 to 2.5 mA cmy2. They were subsequently annealed at 1100 8C in Ar for 1 h. SE measurements were made before and after annealing. The relative damage was measured using the Bruggeman effective medium approximation combining the dielectric function of singlecrystalline and totally amorphized SiC. Therefore, the prerequisite of the proper interpretation of SE data measured on partially damaged SiC is the knowledge of the complex dielectric function of single-crystalline and completely ion implantation amorphized SiC. The complex dielectric function of ion implantation-amorphized SiC was determined from a high-dose implant. Different optical models and the influence of experimental conditions on the damage were investigated. The results were crosschecked by Rutherford backscattering spectrometry. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Ellipsometry; SiC; Ion implantation; Optical properties
1. Introduction SiC has a great potential for a wide range of device applications involving high power, high temperature or high frequency. Devices are formed by doping SiC with different elements. It is important to note that SiC is the only compound semiconductor, whose native oxide is silicon dioxide. Aluminum is an outstanding acceptor, however, the use of traditional thermal diffusion techniques is not practical because of the low diffusivity of dopants. Ion implantation provides an excellent way of bringing dopants to well-defined positions without thermodynamic constraints, however, this method creates lattice disorder, the level of which depends on the experimental conditions like dose or ion current density. Thermal annealing has to be performed subsequently to *Corresponding author. Tel.: q36-1-392-2222y1693; fax: q36-1392-2226. E-mail address: [email protected] (P. Petrik).
remove ion implantation-caused damage and to electrically activate the dopants. Therefore, the fundamental understanding of creation and recovery of damage is of great importance for the fabrication of future SiC devices. Because ion implantation changes the refractive index of SiC, optical techniques have been successfully applied for the characterization of SiC w1–4x. 2. Experimental details 4H SiC (c-SiC) samples were implanted with 150 keV Al using doses between 4=1014 and 2=l015 Alq cmy2 with current densities from 0.4 to 2.5 mA cmy2. The substrates were kept at room temperature during ion implantation. A post implantation annealing was performed at 1100 8C in Ar for 1 h. The samples were measured with a SOPRA ES4G rotating polarizer spectroscopic ellipsometer in the wavelength range of 280–840 nm using an angle of incidence of 75.18. Rutherford backscattering spectrom-
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.01.009
240
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243
etry (RBS) and channeling techniques (RBSyC) w5x were used for cross-checking and to construct realistic optical models. The energy of the Heq ions was 1.5 MeV to avoid high ion beam density effect on nearsurface damage induced in SiC w6x. In the scattering chamber the vacuum was better than 1=10y4 Pa using liquid N2 cooled traps along the beam path and around the wall of the chamber. Backscattered Heq ions were detected using an ORTEC surface barrier detector mounted in Cornell geometry at scattering angle of 1658. In some cases, to increase the depth resolution at the surface a glancing detection angle of 978 was used. To reduce the damage created by the analyzing beam itself low current of 5 nA was used during the measurements and monitored by a transmission Faraday cup w7x. Heq beam of 3.5 MeV energy was used taking the advantage that the cross section of carbon at this energy at 1658 detection angle is about six times larger than the Rutherford type one. This enhanced cross section allows us to investigate the lattice disorder in the carbon sublattice with greater sensitivity. RBS spectra were evaluated using the software RBX w8x. 3. Results and discussion SE measurements were made before and after annealing for the samples implanted using different doses and current densities (Fig. 1). There is a systematic change with changing doses (cosD decreases with increasing dose in the wavelength range of approximately 300– 600 nm, the extinction coefficient increases with increasing dose as seen in Fig. 2), even the different current densities have a significant effect. There is a good agreement between the virgin spectra and the simulation using a surface oxide thickness of 6.5 nm. This rather thick layer may include an eventual surface roughness w9x. The surface roughness can be considered to be a density deficient overlayer in an ellipsometric measurement. In our case, using only an oxide layer, this results in an (virtual) increase in the layer thickness. The measured spectra of the annealed samples get close to the virgin one, especially for lower doses. The spectra of the samples implanted with a higher dose and subsequently annealed have interference fringes below 500 nm. This indicates that a thin amorphous (partially or totally) layer remains on the surface. The pseudo refractive indices show systematic changes as well. The extinction coefficient (k) increases with increasing dose, as expected (Fig. 2). The relative damage was measured using the Bruggeman effective medium approximation combining the dielectric function of c-SiC w1x and ion implantation amorphized SiC (i-a-SiC). Therefore, the prerequisite of the proper interpretation of SE data measured on partially damaged SiC is the knowledge of the complex dielectric function of c-SiC and i-a-SiC. We calculated
Fig. 1. Measured SE spectra for all doses at the angle of incidence of 75.18. The simulation with the optical model of 6.5 nm SiO2 on top of a 4H SiC w3x is also shown.
reference refractive indices of i-a-SiC using the sample implanted with the highest dose (using a current density of 0.4 mA cmy2). This sample was considered to be totally amorphous. The refractive index was determined in the wavelength range, where the damaged layer is opaque, i.e. it can be regarded as bulk. A surface oxide was taken into account in the calculation. Its thickness was determined from the measurement on a virgin sample. The refractive indices calculated using this method are plotted in Fig. 3 together with other references. There is a large effect compared to the c-SiC reference data and a significant discrepancy from the reference measured by Shaaban et al. w2x. The singlewavelength data of Musumeci et al. w4x seem to be in a good agreement with our data, although our curves stop at 450 nm. Fitted spectra of as-implanted and annealed samples are plotted in Fig. 4 using the same wavelength range, where the i-a-SiC reference was calculated from the measurement on the totally amorphized sample (sample implanted with the highest dose with a current density of 0.4 mA cmy2). This allows a simple optical model
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243
241
es compared to the SE measurements (Table 1). While SE revealed a systematic change of the relative damage from 60 to 100% as a function of the as-implanted dose, RBS measured the relative damage to be 100% for all as-implanted samples. While SE revealed the same damage recovery after annealing for all doses (11– 13%), RBS measured the relative damages to change systematically between 9 and 95% after annealing. Note that even the raw measurement data and the pseudo n-k values determined by SE show high and systematic change as a function of dose for the as-implanted samples, and all the spectra get very close to the virgin ones after annealing. This may indicate that the change of relative damage measured by SE is a real physical effect, not an error of the evaluation or a result of a possible non-realistic optical model. The reason for the difference between the relative damage determined by SE and RBS may be the following. From channeling point of view, amorphous behavior means, that there is no long distance ordering in the material. During post implantation annealing of the highly damaged material, the short distance order of the material is recovered, but the long distance order is lost.
Fig. 2. Pseudo n-k values of as-implanted and annealed samples. The ion current was 2.5 mA cmy2 in each cases.
with a thin surface oxide and a bulk with components i-a-SiC and 4H c-SiC. The relative damage is defined then as the ratio of i-a-SiCyc-SiC similar to that for ion implanted single-crystalline silicon w10x. Using this model, the measured spectra of the as-implanted, such as the annealed samples were well fitted, as shown in Fig. 4. For the highest dose samples, for that a thin amorphous (or polycrystalline w11x) layer remains after annealing, even the thickness of this layer can be determined using an optical model with a surface oxide layer, a thin layer with a mixture of i-a-SiC and c-SiC, and a c-SiC bulk. The relative damage was measured using RBSyC as well. RBS revealed total amorphization for all doses, even for the lowest dose samples (Fig. 5). After annealing an almost perfect recrystallization was measured by RBS (Fig. 5). For the samples implanted with a dose of 1=1015 Alq cmy2 (not shown in the figure) a total amorphization and a partial recovery of the crystalline structure after annealing is shown by RBS. The highest dose samples were measured to be totally amorphous after ion implantation, and a slight damage recovery occurred after annealing. These are significant differenc-
Fig. 3. Refractive indices of different crystalline and amorphous SiC samples.
242
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243 Table 1 Relative damage determined by SE and RBS. The ion current was 2.5 mA cmy2 in each cases Dose (=1015 Al cmy2)
0.4 0.4 1.0 1.0 2.0 2.0
Annealing
no yes no yes no yes
Relative damage (%) SE
RBS
60 11 76 13 100 12
100 9 100 76 100 95
Grains of different polytypes may be formed w11x and the alignments of these grains may vary along the original direction. The mis-aligned grains are seen by RBS as if they were totally amorphous, while SE sees this structure as polycrystalline. If the lattice disorder is small enough, the original crystalline structure may be recovered. Hence further structural investigations are needed. Cross-sectional TEM may give information about the crystal structure of ion implanted and annealed SiC. 4. Conclusions
Fig. 4. Measured (symbols) and fitted (solid lines) spectra of asimplanted and annealed samples. The ion current was 2.5 mA cmy2. Closed symbols denote the annealed samples.
It was demonstrated that SE is sensitive to ion implantation-created disorder and its annealing in SiC. Accurate reference data are needed for implantation amorphized SiC (i-a-SiC) in a broad spectral range. There may be different amorphous states in SiC as revealed in c-Si, so special care has to be taken when determining and using reference data for as-implanted and annealed samples. There is a significant difference between the relative damage measured by SE and RBS. A possible explanation might be that the micro-crystalline regions in the highly damaged layer get out of orientation (RBS determines them as amorphous from channeling measurements). The surface overlayer was thicker than expected. The reason may be a possible surface roughness, which has to be checked using comparative techniques. Acknowledgments
Fig. 5. RBS measurement on as-implanted and annealed samples using the smallest dose.
Support from the Hungarian Science Research Foundation (OTKA grant Nos. D34594, T030441, T033072, T43704, T034332) and from project EU5 Center of Excellence ICAI-CT-200-70029 is greatly appreciated. ´ P.P and M.F. are grantees of the Bolyai Janos Scholarship. The implantation was performed in the Institute Tecnologico e Nuclear, Lisboa, Portugal. J. Garcia Lopez acknowledges the Spanish McyT for financial support through the ‘Ramon y Cajal’ program and the project MAT2002-02843.
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243
References w1x S. Zollner, J.G. Chen, E. Duda, T. Wetteroth, S.R. Wilson, J.N. Hilfiker, J. Appl. Phys. 85 (1999) 8353. w2x E.R. Shaaban, T. Lohner, P. Petrik, N.Q. Khanh, ´ M. Fried, O. ´ J. Gyulai., Phys. Status Solidi (a) 195(2003). Polgar, w3x Z.C. Feng, F. Yan, W.Y. Chang, J.H. Zhao, J. Lin, Mater. Sci. Forum 389–393 (2002) 647. w4x P. Musumeci, L. Calcagno, M.G. Grimaldi, G. Foti, Appl. Phys. Lett. 69 (1996) 468. w5x Z. Zolnai, N.Q. Khanh, ´ ´ ´ E. Szilagyi, E. Kotai, A. Ster, M. Posselt, T. Lohner, J. Gyulai, Diamond Relat. Mater. 11 (2002) 1239.
243
w6x N.Q. Khanh, ´ ´ Z. Zolnai, T. Lohner, L. Toth, L. Dobos, J. Gyulai, Nucl. Instrum. Methods B 161–163 (2000) 424. w7x F. Paszti, ´ A. Manuaba, C. Hajdu, A.A. Melo, M.F. Da Silva, Nucl. Instrum. Methods Phys. Res. B 47 (1990) 187. w8x E. Kotai, ´ Nucl. Instrum. Methods Phys. Res. B 85 (1994) 588. w9x L. Ottaviani, M. Lazar, M.-L. Locatelly, D. Planson, J.-P. Chante, C. Dubois, Mater. Sci. Eng. B 90 (2002) 301. w10x P. Petrik, O. Polgar, ´ M. Fried, T. Lohner, N.Q. Khanh, ´ J. Gyulai, J. Appl. Phys. 93 (4) (2003) 1987. w11x R. Nipoti, A. Parisini, Philos. Mag. B 80 (4) (2000) 647.
Ion implantation-caused damage in SiC measured by spectroscopic ellipsometry P. Petrika,*, E.R. Shaabana,b, T. Lohnera, G. Battistiga, M. Frieda, J. Garcia Lopezc, Y. Morillac, ´ a, J. Gyulaia O. Polgar a
´ ut ´ 29-33, Hungary Research Institute for Technical Physics and Materials Science, H-1121 Budapest, Konkoly Thege Miklos b Physics Department, Faculty of Science, Al-Azhar University, Asuit, 71111, Egypt c Centro Nacional de Aceleradores, 41092 Sevilla, Spain
Abstract Since ion implantation-caused damage changes the complex refractive index of SiC significantly, optical methods can be used to measure the sample properties sensitively, non-destructively and quickly. In the present work the damage created by ion implantation into SiC and its change upon annealing was characterized by spectroscopic ellipsometry (SE). 4H SiC samples were implanted with 150 keV Al using doses between 4=l014 and 2=l015 cmy2 with current densities from 0.4 to 2.5 mA cmy2. They were subsequently annealed at 1100 8C in Ar for 1 h. SE measurements were made before and after annealing. The relative damage was measured using the Bruggeman effective medium approximation combining the dielectric function of singlecrystalline and totally amorphized SiC. Therefore, the prerequisite of the proper interpretation of SE data measured on partially damaged SiC is the knowledge of the complex dielectric function of single-crystalline and completely ion implantation amorphized SiC. The complex dielectric function of ion implantation-amorphized SiC was determined from a high-dose implant. Different optical models and the influence of experimental conditions on the damage were investigated. The results were crosschecked by Rutherford backscattering spectrometry. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Ellipsometry; SiC; Ion implantation; Optical properties
1. Introduction SiC has a great potential for a wide range of device applications involving high power, high temperature or high frequency. Devices are formed by doping SiC with different elements. It is important to note that SiC is the only compound semiconductor, whose native oxide is silicon dioxide. Aluminum is an outstanding acceptor, however, the use of traditional thermal diffusion techniques is not practical because of the low diffusivity of dopants. Ion implantation provides an excellent way of bringing dopants to well-defined positions without thermodynamic constraints, however, this method creates lattice disorder, the level of which depends on the experimental conditions like dose or ion current density. Thermal annealing has to be performed subsequently to *Corresponding author. Tel.: q36-1-392-2222y1693; fax: q36-1392-2226. E-mail address: [email protected] (P. Petrik).
remove ion implantation-caused damage and to electrically activate the dopants. Therefore, the fundamental understanding of creation and recovery of damage is of great importance for the fabrication of future SiC devices. Because ion implantation changes the refractive index of SiC, optical techniques have been successfully applied for the characterization of SiC w1–4x. 2. Experimental details 4H SiC (c-SiC) samples were implanted with 150 keV Al using doses between 4=1014 and 2=l015 Alq cmy2 with current densities from 0.4 to 2.5 mA cmy2. The substrates were kept at room temperature during ion implantation. A post implantation annealing was performed at 1100 8C in Ar for 1 h. The samples were measured with a SOPRA ES4G rotating polarizer spectroscopic ellipsometer in the wavelength range of 280–840 nm using an angle of incidence of 75.18. Rutherford backscattering spectrom-
0040-6090/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.01.009
240
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243
etry (RBS) and channeling techniques (RBSyC) w5x were used for cross-checking and to construct realistic optical models. The energy of the Heq ions was 1.5 MeV to avoid high ion beam density effect on nearsurface damage induced in SiC w6x. In the scattering chamber the vacuum was better than 1=10y4 Pa using liquid N2 cooled traps along the beam path and around the wall of the chamber. Backscattered Heq ions were detected using an ORTEC surface barrier detector mounted in Cornell geometry at scattering angle of 1658. In some cases, to increase the depth resolution at the surface a glancing detection angle of 978 was used. To reduce the damage created by the analyzing beam itself low current of 5 nA was used during the measurements and monitored by a transmission Faraday cup w7x. Heq beam of 3.5 MeV energy was used taking the advantage that the cross section of carbon at this energy at 1658 detection angle is about six times larger than the Rutherford type one. This enhanced cross section allows us to investigate the lattice disorder in the carbon sublattice with greater sensitivity. RBS spectra were evaluated using the software RBX w8x. 3. Results and discussion SE measurements were made before and after annealing for the samples implanted using different doses and current densities (Fig. 1). There is a systematic change with changing doses (cosD decreases with increasing dose in the wavelength range of approximately 300– 600 nm, the extinction coefficient increases with increasing dose as seen in Fig. 2), even the different current densities have a significant effect. There is a good agreement between the virgin spectra and the simulation using a surface oxide thickness of 6.5 nm. This rather thick layer may include an eventual surface roughness w9x. The surface roughness can be considered to be a density deficient overlayer in an ellipsometric measurement. In our case, using only an oxide layer, this results in an (virtual) increase in the layer thickness. The measured spectra of the annealed samples get close to the virgin one, especially for lower doses. The spectra of the samples implanted with a higher dose and subsequently annealed have interference fringes below 500 nm. This indicates that a thin amorphous (partially or totally) layer remains on the surface. The pseudo refractive indices show systematic changes as well. The extinction coefficient (k) increases with increasing dose, as expected (Fig. 2). The relative damage was measured using the Bruggeman effective medium approximation combining the dielectric function of c-SiC w1x and ion implantation amorphized SiC (i-a-SiC). Therefore, the prerequisite of the proper interpretation of SE data measured on partially damaged SiC is the knowledge of the complex dielectric function of c-SiC and i-a-SiC. We calculated
Fig. 1. Measured SE spectra for all doses at the angle of incidence of 75.18. The simulation with the optical model of 6.5 nm SiO2 on top of a 4H SiC w3x is also shown.
reference refractive indices of i-a-SiC using the sample implanted with the highest dose (using a current density of 0.4 mA cmy2). This sample was considered to be totally amorphous. The refractive index was determined in the wavelength range, where the damaged layer is opaque, i.e. it can be regarded as bulk. A surface oxide was taken into account in the calculation. Its thickness was determined from the measurement on a virgin sample. The refractive indices calculated using this method are plotted in Fig. 3 together with other references. There is a large effect compared to the c-SiC reference data and a significant discrepancy from the reference measured by Shaaban et al. w2x. The singlewavelength data of Musumeci et al. w4x seem to be in a good agreement with our data, although our curves stop at 450 nm. Fitted spectra of as-implanted and annealed samples are plotted in Fig. 4 using the same wavelength range, where the i-a-SiC reference was calculated from the measurement on the totally amorphized sample (sample implanted with the highest dose with a current density of 0.4 mA cmy2). This allows a simple optical model
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243
241
es compared to the SE measurements (Table 1). While SE revealed a systematic change of the relative damage from 60 to 100% as a function of the as-implanted dose, RBS measured the relative damage to be 100% for all as-implanted samples. While SE revealed the same damage recovery after annealing for all doses (11– 13%), RBS measured the relative damages to change systematically between 9 and 95% after annealing. Note that even the raw measurement data and the pseudo n-k values determined by SE show high and systematic change as a function of dose for the as-implanted samples, and all the spectra get very close to the virgin ones after annealing. This may indicate that the change of relative damage measured by SE is a real physical effect, not an error of the evaluation or a result of a possible non-realistic optical model. The reason for the difference between the relative damage determined by SE and RBS may be the following. From channeling point of view, amorphous behavior means, that there is no long distance ordering in the material. During post implantation annealing of the highly damaged material, the short distance order of the material is recovered, but the long distance order is lost.
Fig. 2. Pseudo n-k values of as-implanted and annealed samples. The ion current was 2.5 mA cmy2 in each cases.
with a thin surface oxide and a bulk with components i-a-SiC and 4H c-SiC. The relative damage is defined then as the ratio of i-a-SiCyc-SiC similar to that for ion implanted single-crystalline silicon w10x. Using this model, the measured spectra of the as-implanted, such as the annealed samples were well fitted, as shown in Fig. 4. For the highest dose samples, for that a thin amorphous (or polycrystalline w11x) layer remains after annealing, even the thickness of this layer can be determined using an optical model with a surface oxide layer, a thin layer with a mixture of i-a-SiC and c-SiC, and a c-SiC bulk. The relative damage was measured using RBSyC as well. RBS revealed total amorphization for all doses, even for the lowest dose samples (Fig. 5). After annealing an almost perfect recrystallization was measured by RBS (Fig. 5). For the samples implanted with a dose of 1=1015 Alq cmy2 (not shown in the figure) a total amorphization and a partial recovery of the crystalline structure after annealing is shown by RBS. The highest dose samples were measured to be totally amorphous after ion implantation, and a slight damage recovery occurred after annealing. These are significant differenc-
Fig. 3. Refractive indices of different crystalline and amorphous SiC samples.
242
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243 Table 1 Relative damage determined by SE and RBS. The ion current was 2.5 mA cmy2 in each cases Dose (=1015 Al cmy2)
0.4 0.4 1.0 1.0 2.0 2.0
Annealing
no yes no yes no yes
Relative damage (%) SE
RBS
60 11 76 13 100 12
100 9 100 76 100 95
Grains of different polytypes may be formed w11x and the alignments of these grains may vary along the original direction. The mis-aligned grains are seen by RBS as if they were totally amorphous, while SE sees this structure as polycrystalline. If the lattice disorder is small enough, the original crystalline structure may be recovered. Hence further structural investigations are needed. Cross-sectional TEM may give information about the crystal structure of ion implanted and annealed SiC. 4. Conclusions
Fig. 4. Measured (symbols) and fitted (solid lines) spectra of asimplanted and annealed samples. The ion current was 2.5 mA cmy2. Closed symbols denote the annealed samples.
It was demonstrated that SE is sensitive to ion implantation-created disorder and its annealing in SiC. Accurate reference data are needed for implantation amorphized SiC (i-a-SiC) in a broad spectral range. There may be different amorphous states in SiC as revealed in c-Si, so special care has to be taken when determining and using reference data for as-implanted and annealed samples. There is a significant difference between the relative damage measured by SE and RBS. A possible explanation might be that the micro-crystalline regions in the highly damaged layer get out of orientation (RBS determines them as amorphous from channeling measurements). The surface overlayer was thicker than expected. The reason may be a possible surface roughness, which has to be checked using comparative techniques. Acknowledgments
Fig. 5. RBS measurement on as-implanted and annealed samples using the smallest dose.
Support from the Hungarian Science Research Foundation (OTKA grant Nos. D34594, T030441, T033072, T43704, T034332) and from project EU5 Center of Excellence ICAI-CT-200-70029 is greatly appreciated. ´ P.P and M.F. are grantees of the Bolyai Janos Scholarship. The implantation was performed in the Institute Tecnologico e Nuclear, Lisboa, Portugal. J. Garcia Lopez acknowledges the Spanish McyT for financial support through the ‘Ramon y Cajal’ program and the project MAT2002-02843.
P. Petrik et al. / Thin Solid Films 455 – 456 (2004) 239–243
References w1x S. Zollner, J.G. Chen, E. Duda, T. Wetteroth, S.R. Wilson, J.N. Hilfiker, J. Appl. Phys. 85 (1999) 8353. w2x E.R. Shaaban, T. Lohner, P. Petrik, N.Q. Khanh, ´ M. Fried, O. ´ J. Gyulai., Phys. Status Solidi (a) 195(2003). Polgar, w3x Z.C. Feng, F. Yan, W.Y. Chang, J.H. Zhao, J. Lin, Mater. Sci. Forum 389–393 (2002) 647. w4x P. Musumeci, L. Calcagno, M.G. Grimaldi, G. Foti, Appl. Phys. Lett. 69 (1996) 468. w5x Z. Zolnai, N.Q. Khanh, ´ ´ ´ E. Szilagyi, E. Kotai, A. Ster, M. Posselt, T. Lohner, J. Gyulai, Diamond Relat. Mater. 11 (2002) 1239.
243
w6x N.Q. Khanh, ´ ´ Z. Zolnai, T. Lohner, L. Toth, L. Dobos, J. Gyulai, Nucl. Instrum. Methods B 161–163 (2000) 424. w7x F. Paszti, ´ A. Manuaba, C. Hajdu, A.A. Melo, M.F. Da Silva, Nucl. Instrum. Methods Phys. Res. B 47 (1990) 187. w8x E. Kotai, ´ Nucl. Instrum. Methods Phys. Res. B 85 (1994) 588. w9x L. Ottaviani, M. Lazar, M.-L. Locatelly, D. Planson, J.-P. Chante, C. Dubois, Mater. Sci. Eng. B 90 (2002) 301. w10x P. Petrik, O. Polgar, ´ M. Fried, T. Lohner, N.Q. Khanh, ´ J. Gyulai, J. Appl. Phys. 93 (4) (2003) 1987. w11x R. Nipoti, A. Parisini, Philos. Mag. B 80 (4) (2000) 647.