- Email: [email protected]
Local structure of gold impurities in silicon determined by EXAFS
ARTICLE IN PRESS
Physica B 376–377 (2006) 57–60 www.elsevier.com/locate/physb
Local structure of gold impurities in silicon determined by EXAFS J. Bollmanna,, T. Leiseganga, D.C. Meyera, J. Webera, H.-E. Mahnkeb a
Technische Universita¨t Dresden, D-01062 Dresden, Germany Hahn-Meitner-Institut Berlin GmbH, D-14109 Berlin, Germany
b
Abstract The incorporation of Au atoms within the silicon lattice was determined from X-ray absorption fine structure (XAFS). A detection limit of about 1014 cm2 doses equivalent Au atoms in silicon was achieved by grazing incidence of the X-rays and fluorescence detection. Our results are (i) after Au implantation (as-implanted state) single Au atoms occupy regular high symmetric substitutional lattice sites in silicon, (ii) after thermal treatments some of the Au-atoms remain substitutional other diffuse to the sample surface. For the Au atoms near the surface very similar short range parameters as for metallic gold are detected and X-ray reflectrometry gives evidence for a nearsurface segregation of gold atoms. r 2005 Elsevier B.V. All rights reserved. Keywords: Au; Deep impurity; Local structure; XAFS; Silicon
1. Introduction The detection of X-ray absorption fine structure (XAFS) near the absorption edge of inner atomic shells is well suited to give detailed information about the microscopic structure of defects in semiconductor hosts. From the modeling of the scattering process the number of neighbouring atoms and their distance are determined. However, the X-ray absorption on highly diluted defects is disturbed by the high absorption background from elastic scattering and from photoionisation of other elements and/or higher atomic shells. The improvement in synchroton sources as well as in detection techniques allows one to go down to about 1018 cm3 in defect concentration and even lower under optimal conditions. For various dopants in silicon XAFS results were reported for concentrations a few times 1018 cm3 [1,2]. A further increase in sensitivity to reach lower concentrations is however highly desirable. Gold impurities were one of the first deep centers studied in Si [3,4]. Despite the technological interest the electrical and optical properties remained controversial for a long time. Interstitial, substitutional and many Au-complexes Corresponding author. Tel.: +49 351 46335622;
fax: +49 351 46337060. E-mail address: [email protected] (J. Bollmann). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.016
are known to exist [5]. Substitutional Au was found to introduce a deep donor level at E v þ 0:35 eV and a deep acceptor state at E v þ 0:62 eV (mid gap level) [5]. Watkins and coworkers [6] verified that both states belong to the same substitutional Au center. From the Zeeman splitting of the effective mass like donor and acceptor transitions, the ground state g-values for neutral Au0 were determined. Au0 has a g? 0, which explains the missing EPR activity of this Au species. The ground state of Au0 was found to have a tetragonal distortion with a fast reorientation between the equivalent distortions even below 2 K. The established properties of Au along with the optical transitions seen in absorption and photoluminescence make substitutional Au a unique system. The motivation of this paper was to enhance the sensitivity of XAFS and use it to study directly the local symmetry of Au in Si. We were able to determine the lattice position of highly dissolved isolated Au-atoms and could correlate our results with known data on isolated substitutional gold. 2. Experimental Phosphorous-doped, n-type, float zone refined and dislocation-free silicon crystals were doped with Au atoms by ion implantation (60 keV, 1015 cm2 ). The implantation
ARTICLE IN PRESS 58
J. Bollmann et al. / Physica B 376–377 (2006) 57–60
energy of 60 keV leads to a projected range of 30 nm and a straggling of 7 nm. Thermal treatments at 550 or 950 C for 20 min were carried out under He-atmosphere in sealed quartz ampoules to anneal the implantation damage. The EXAFS experiments were carried out using synchrotron radiation at beamline C1 (CEMO) of the ‘Hamburger Synchrotronstrahlungslabor DESY-HASYLAB’ [7]. Energy dependent monochromatisation was performed by means of a Si-[2 2 0] double crystal monochromator. The Au-LIII fluorescence yield was detected by a thermoelectrically cooled silicon solid state detector (KEVEX). The X-ray polarisation vector of the synchrotron radiation was parallel to the sample surface. To increase the signal-to-noise ratio the sensitive area of the fluorescence detector is mounted perpendicular to the electric field vector of the exciting X-rays. The penetration depth of the X-rays was optimized for the implantation depth by using an incidence angle of the X-rays close to total reflection. With an incident angle of 0:17 we achieve with our set up a detection limit of about 1015 cm3 gold atoms. The energy range between 11 822 and 12 500 eV covers the Au-LIII absorption edge (11919 eV) and was scanned with a step width of 3 eV. Integration time for each step was in the order of 6 min.
Fig. 1. Background corrected and normalized Au-La XAFS intensities. (dotted: as-implanted state, solid: after 950 C anneal).
3. Fluorescence EXAFS analysis We have analyzed the fluorescence spectra from the AuLIII edge by the standard XAFS spectra evaluation procedures using the FEFF8 and FEFFIT code from the University of Washington software package [8]. In the program, the backscattering amplitudes and phase shifts are calculated corresponding to a given atomic arrangement. Structural parameters such as interatomic distances, as well as mean square displacements, and, if not known, co-ordination numbers can be extracted from fits to the experimental data. For details of the EXAFS method the reader is referred to Sayers et al. [9] and Lytle [10]. In Fig. 1, the normalized Au-La XAFS intensities for an as-implanted and annealed ð950 CÞ sample are represented. The differences in the near edge region are significant and indicate a change of electron state, whereas the ‘phase shift’ expresses different short-range arrangements of the Au atoms. Details will be discussed later.
(a)
(b)
4. Results and discussion From the measured absorption spectra a smoothly varying atomic like background is subtracted following standard XAFS spectra evaluation procedures and by transformation into the k-space of the emitted wave vector the complex fine structure function wðkÞ is received. All structural analysis is performed on the function wðkÞ. In Fig. 2, we have calculated the scattering contributions of the nearest and next nearest Si neighbour shell around an
(c) Fig. 2. XAFS function wðkÞ for the Au-LIII edge in silicon from interstitial (a) and substitutional (b) Au atoms in silicon. (Both situations differ clearly (bold lines) if contributions from nearest (solid) and next-nearest neighbours (dotted) are included.) (c) wðkÞ for Au in silicon (solid) and in crystalline gold (dotted).
ARTICLE IN PRESS J. Bollmann et al. / Physica B 376–377 (2006) 57–60
interstitial (Fig. 2(a)) or a substitutional (Fig. 2(b)) Au atom. The function wðkÞ is the sum over both shell contributions. The calculations reveal that only substitutional Au atoms will give distinct XAFS oscillations, whereas the interstitial positions lead to shell contributions with a phase shift of about 180 which results in a strong damping of the resulting XAFS oscillation. In Fig. 2(c), the contribution of the four next nearest Si neighbours at a distance of 0.23507 nm to the substitutional Au-atom are calculated (diamond-type structure, space group Fd-3m (2 2 7), lattice parameter 0.54305 nm) (solid line, same as (b) corresponds to first neighbour shell). The simulated result (dotted line) for a gold crystal is shown for comparison (Cu-type structure, space group Fm-3m (2 2 5), lattice parameter 0.40789 nm). It is obvious that the XAFS analysis allows the separation of structural short-range order for both kinds of arrangements. The experimental results on our three samples are presented as dotted curves in Fig. 3. The as implanted
(a)
59
sample (Fig. 3(a)) shows clear oscillations dominated by just one frequency. This indicates that all Au atoms occupy indentical environments. The simulation of the XAFS function for a substitutional Au atom by using only the next Si neighbours results in a qualitatively and quantitatively good correspondence with our measurements (see Fig. 3(a)). The XAFS signal is dominated by the contribution of the four nearest Si neighbours, the reduced scattering from the next nearest and further neighbours can be explained by the structural distortion due to the implantation induced damage. A simulation which includes the six most significant scattering paths results in a Au–Si distance of 0.241(7) nm, which is about 0.006 nm larger than the distance in an unstrained silicon lattice. After annealing Au partly segregates and to the Au–Si XAFS spectrum Au–Au correlations have to be added. The measured oscillations are fitted by superposition of both contributions (see Fig. 2(c)). A good simulation for the sample annealed at 550 C is achieved with a superposition of 70ð20Þ% of the Au–Si spectra and 30ð20Þ% of the Au–Au spectra. The gold segregates in near-surface clusters and contributes after 950 C annealing more than 60ð20Þ% to the spectrum. We have measured the low temperature photoluminescence of all our samples. The only PL signal in the regime below the band edge luminescence was the 792 meV line attributed to the Au donor in Si [6]. Further results from DLTS- and PL-measurements are needed to support the correlation of the substitutional Au center with the Au donor state. Additional EXAFS measurements will be useful to unravel the low symmetry distortion of substitutional Au and the fast reorientation.
5. Summary
(b)
(c) Fig. 3. XAFS function wðkÞ for the Au-LIII edge in silicon at 300 K (a) after implantation (as-implanted state); (b) after annealing (550 C, 20 min); (c) after annealing (950 C, 20 min) (symbols: experiment; line: simulation).
With an increased sensitivity of fluorescence detection of EXAFS we were able to analyse XAFS oscillations from highly diluted Au atoms in silicon. Under grazing incidence Au-La fluorescence gives a detection limit of less than 1014 Au-atoms, whereas for 1015 Au-atoms a quantitative analysis of the spectra was possible. After implantation at room temperature (as-implanted state) gold atoms occupy regular substitutional lattice sites in silicon. The interatomic distance between Au and the four nearest Si atoms is increased by about 2%. After thermal treatment part of the Au atoms diffuse into the samples but keep their lattice position while another part segregates at near-surface regions as gold clusters. After annealing at 950 C up to 60% of the Au is bound in clusters. The substitutional Au signal is linked to the Au donor state by the observed low temperature photoluminescence of the 792 meV line detected in both the as-implanted state and after annealing.
ARTICLE IN PRESS 60
J. Bollmann et al. / Physica B 376–377 (2006) 57–60
Acknowledgements This work was funded by the BMBF under contract 05 KK 40D1/4. We thank HASYLAB for the support of our proposal. References [1] S. Wei, H. Oyanagi, H. Kawanami, K. Sakamoto, T. Sakamoto, N.L. Saini, J. Synchroton Rad. 6 (1999) 573. [2] V. Koteski, N. Ivanovic, H. Haas, E. Holub-Krappe, H.-E. Mahnke, Nucl. Instr. Meth. B 200 (2003) 60.
[3] E.A. Taft, F.H. Horn, Phys. Rev. 93 (1954) 64. [4] C.B. Collins, R.O. Carlson, C.J. Gallagher, Phys. Rev. 105 (1957) 1168. [5] W. Schro¨ter, M. Seibt, Properties of Crystalline Silicon, in: R. Hull (Ed.), EMIS Data Rev. Ser., vol. 20, Short Run Press Ltd., Exeter, UK, 1999. [6] G.D. Watkins, M. Kleverman, A. Thilderkvist, H.G. Grimmeiss, Phys. Rev. Lett. 67 (1991) 1149. [7] For details see hwww-hasylab.desy.dei. [8] J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky, R.C. Albers, J. Am. Chem. Soc. 113 (1991) 5135. [9] D.E. Sayers, E.A. Stern, F.W. Lytle, Phys. Rev. Lett. 27 (1971) 1204. [10] F.W. Lytle, J. Synchrotron Rad. 6 (1999) 123.
Physica B 376–377 (2006) 57–60 www.elsevier.com/locate/physb
Local structure of gold impurities in silicon determined by EXAFS J. Bollmanna,, T. Leiseganga, D.C. Meyera, J. Webera, H.-E. Mahnkeb a
Technische Universita¨t Dresden, D-01062 Dresden, Germany Hahn-Meitner-Institut Berlin GmbH, D-14109 Berlin, Germany
b
Abstract The incorporation of Au atoms within the silicon lattice was determined from X-ray absorption fine structure (XAFS). A detection limit of about 1014 cm2 doses equivalent Au atoms in silicon was achieved by grazing incidence of the X-rays and fluorescence detection. Our results are (i) after Au implantation (as-implanted state) single Au atoms occupy regular high symmetric substitutional lattice sites in silicon, (ii) after thermal treatments some of the Au-atoms remain substitutional other diffuse to the sample surface. For the Au atoms near the surface very similar short range parameters as for metallic gold are detected and X-ray reflectrometry gives evidence for a nearsurface segregation of gold atoms. r 2005 Elsevier B.V. All rights reserved. Keywords: Au; Deep impurity; Local structure; XAFS; Silicon
1. Introduction The detection of X-ray absorption fine structure (XAFS) near the absorption edge of inner atomic shells is well suited to give detailed information about the microscopic structure of defects in semiconductor hosts. From the modeling of the scattering process the number of neighbouring atoms and their distance are determined. However, the X-ray absorption on highly diluted defects is disturbed by the high absorption background from elastic scattering and from photoionisation of other elements and/or higher atomic shells. The improvement in synchroton sources as well as in detection techniques allows one to go down to about 1018 cm3 in defect concentration and even lower under optimal conditions. For various dopants in silicon XAFS results were reported for concentrations a few times 1018 cm3 [1,2]. A further increase in sensitivity to reach lower concentrations is however highly desirable. Gold impurities were one of the first deep centers studied in Si [3,4]. Despite the technological interest the electrical and optical properties remained controversial for a long time. Interstitial, substitutional and many Au-complexes Corresponding author. Tel.: +49 351 46335622;
fax: +49 351 46337060. E-mail address: [email protected] (J. Bollmann). 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.12.016
are known to exist [5]. Substitutional Au was found to introduce a deep donor level at E v þ 0:35 eV and a deep acceptor state at E v þ 0:62 eV (mid gap level) [5]. Watkins and coworkers [6] verified that both states belong to the same substitutional Au center. From the Zeeman splitting of the effective mass like donor and acceptor transitions, the ground state g-values for neutral Au0 were determined. Au0 has a g? 0, which explains the missing EPR activity of this Au species. The ground state of Au0 was found to have a tetragonal distortion with a fast reorientation between the equivalent distortions even below 2 K. The established properties of Au along with the optical transitions seen in absorption and photoluminescence make substitutional Au a unique system. The motivation of this paper was to enhance the sensitivity of XAFS and use it to study directly the local symmetry of Au in Si. We were able to determine the lattice position of highly dissolved isolated Au-atoms and could correlate our results with known data on isolated substitutional gold. 2. Experimental Phosphorous-doped, n-type, float zone refined and dislocation-free silicon crystals were doped with Au atoms by ion implantation (60 keV, 1015 cm2 ). The implantation
ARTICLE IN PRESS 58
J. Bollmann et al. / Physica B 376–377 (2006) 57–60
energy of 60 keV leads to a projected range of 30 nm and a straggling of 7 nm. Thermal treatments at 550 or 950 C for 20 min were carried out under He-atmosphere in sealed quartz ampoules to anneal the implantation damage. The EXAFS experiments were carried out using synchrotron radiation at beamline C1 (CEMO) of the ‘Hamburger Synchrotronstrahlungslabor DESY-HASYLAB’ [7]. Energy dependent monochromatisation was performed by means of a Si-[2 2 0] double crystal monochromator. The Au-LIII fluorescence yield was detected by a thermoelectrically cooled silicon solid state detector (KEVEX). The X-ray polarisation vector of the synchrotron radiation was parallel to the sample surface. To increase the signal-to-noise ratio the sensitive area of the fluorescence detector is mounted perpendicular to the electric field vector of the exciting X-rays. The penetration depth of the X-rays was optimized for the implantation depth by using an incidence angle of the X-rays close to total reflection. With an incident angle of 0:17 we achieve with our set up a detection limit of about 1015 cm3 gold atoms. The energy range between 11 822 and 12 500 eV covers the Au-LIII absorption edge (11919 eV) and was scanned with a step width of 3 eV. Integration time for each step was in the order of 6 min.
Fig. 1. Background corrected and normalized Au-La XAFS intensities. (dotted: as-implanted state, solid: after 950 C anneal).
3. Fluorescence EXAFS analysis We have analyzed the fluorescence spectra from the AuLIII edge by the standard XAFS spectra evaluation procedures using the FEFF8 and FEFFIT code from the University of Washington software package [8]. In the program, the backscattering amplitudes and phase shifts are calculated corresponding to a given atomic arrangement. Structural parameters such as interatomic distances, as well as mean square displacements, and, if not known, co-ordination numbers can be extracted from fits to the experimental data. For details of the EXAFS method the reader is referred to Sayers et al. [9] and Lytle [10]. In Fig. 1, the normalized Au-La XAFS intensities for an as-implanted and annealed ð950 CÞ sample are represented. The differences in the near edge region are significant and indicate a change of electron state, whereas the ‘phase shift’ expresses different short-range arrangements of the Au atoms. Details will be discussed later.
(a)
(b)
4. Results and discussion From the measured absorption spectra a smoothly varying atomic like background is subtracted following standard XAFS spectra evaluation procedures and by transformation into the k-space of the emitted wave vector the complex fine structure function wðkÞ is received. All structural analysis is performed on the function wðkÞ. In Fig. 2, we have calculated the scattering contributions of the nearest and next nearest Si neighbour shell around an
(c) Fig. 2. XAFS function wðkÞ for the Au-LIII edge in silicon from interstitial (a) and substitutional (b) Au atoms in silicon. (Both situations differ clearly (bold lines) if contributions from nearest (solid) and next-nearest neighbours (dotted) are included.) (c) wðkÞ for Au in silicon (solid) and in crystalline gold (dotted).
ARTICLE IN PRESS J. Bollmann et al. / Physica B 376–377 (2006) 57–60
interstitial (Fig. 2(a)) or a substitutional (Fig. 2(b)) Au atom. The function wðkÞ is the sum over both shell contributions. The calculations reveal that only substitutional Au atoms will give distinct XAFS oscillations, whereas the interstitial positions lead to shell contributions with a phase shift of about 180 which results in a strong damping of the resulting XAFS oscillation. In Fig. 2(c), the contribution of the four next nearest Si neighbours at a distance of 0.23507 nm to the substitutional Au-atom are calculated (diamond-type structure, space group Fd-3m (2 2 7), lattice parameter 0.54305 nm) (solid line, same as (b) corresponds to first neighbour shell). The simulated result (dotted line) for a gold crystal is shown for comparison (Cu-type structure, space group Fm-3m (2 2 5), lattice parameter 0.40789 nm). It is obvious that the XAFS analysis allows the separation of structural short-range order for both kinds of arrangements. The experimental results on our three samples are presented as dotted curves in Fig. 3. The as implanted
(a)
59
sample (Fig. 3(a)) shows clear oscillations dominated by just one frequency. This indicates that all Au atoms occupy indentical environments. The simulation of the XAFS function for a substitutional Au atom by using only the next Si neighbours results in a qualitatively and quantitatively good correspondence with our measurements (see Fig. 3(a)). The XAFS signal is dominated by the contribution of the four nearest Si neighbours, the reduced scattering from the next nearest and further neighbours can be explained by the structural distortion due to the implantation induced damage. A simulation which includes the six most significant scattering paths results in a Au–Si distance of 0.241(7) nm, which is about 0.006 nm larger than the distance in an unstrained silicon lattice. After annealing Au partly segregates and to the Au–Si XAFS spectrum Au–Au correlations have to be added. The measured oscillations are fitted by superposition of both contributions (see Fig. 2(c)). A good simulation for the sample annealed at 550 C is achieved with a superposition of 70ð20Þ% of the Au–Si spectra and 30ð20Þ% of the Au–Au spectra. The gold segregates in near-surface clusters and contributes after 950 C annealing more than 60ð20Þ% to the spectrum. We have measured the low temperature photoluminescence of all our samples. The only PL signal in the regime below the band edge luminescence was the 792 meV line attributed to the Au donor in Si [6]. Further results from DLTS- and PL-measurements are needed to support the correlation of the substitutional Au center with the Au donor state. Additional EXAFS measurements will be useful to unravel the low symmetry distortion of substitutional Au and the fast reorientation.
5. Summary
(b)
(c) Fig. 3. XAFS function wðkÞ for the Au-LIII edge in silicon at 300 K (a) after implantation (as-implanted state); (b) after annealing (550 C, 20 min); (c) after annealing (950 C, 20 min) (symbols: experiment; line: simulation).
With an increased sensitivity of fluorescence detection of EXAFS we were able to analyse XAFS oscillations from highly diluted Au atoms in silicon. Under grazing incidence Au-La fluorescence gives a detection limit of less than 1014 Au-atoms, whereas for 1015 Au-atoms a quantitative analysis of the spectra was possible. After implantation at room temperature (as-implanted state) gold atoms occupy regular substitutional lattice sites in silicon. The interatomic distance between Au and the four nearest Si atoms is increased by about 2%. After thermal treatment part of the Au atoms diffuse into the samples but keep their lattice position while another part segregates at near-surface regions as gold clusters. After annealing at 950 C up to 60% of the Au is bound in clusters. The substitutional Au signal is linked to the Au donor state by the observed low temperature photoluminescence of the 792 meV line detected in both the as-implanted state and after annealing.
ARTICLE IN PRESS 60
J. Bollmann et al. / Physica B 376–377 (2006) 57–60
Acknowledgements This work was funded by the BMBF under contract 05 KK 40D1/4. We thank HASYLAB for the support of our proposal. References [1] S. Wei, H. Oyanagi, H. Kawanami, K. Sakamoto, T. Sakamoto, N.L. Saini, J. Synchroton Rad. 6 (1999) 573. [2] V. Koteski, N. Ivanovic, H. Haas, E. Holub-Krappe, H.-E. Mahnke, Nucl. Instr. Meth. B 200 (2003) 60.
[3] E.A. Taft, F.H. Horn, Phys. Rev. 93 (1954) 64. [4] C.B. Collins, R.O. Carlson, C.J. Gallagher, Phys. Rev. 105 (1957) 1168. [5] W. Schro¨ter, M. Seibt, Properties of Crystalline Silicon, in: R. Hull (Ed.), EMIS Data Rev. Ser., vol. 20, Short Run Press Ltd., Exeter, UK, 1999. [6] G.D. Watkins, M. Kleverman, A. Thilderkvist, H.G. Grimmeiss, Phys. Rev. Lett. 67 (1991) 1149. [7] For details see hwww-hasylab.desy.dei. [8] J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky, R.C. Albers, J. Am. Chem. Soc. 113 (1991) 5135. [9] D.E. Sayers, E.A. Stern, F.W. Lytle, Phys. Rev. Lett. 27 (1971) 1204. [10] F.W. Lytle, J. Synchrotron Rad. 6 (1999) 123.