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Planar MeV ion channeling on strained buried nanofilms
Nuclear Instruments and Methods in Physics Research B 190 (2002) 570–573 www.elsevier.com/locate/nimb
Planar MeV ion channeling on strained buried nanofilms L.J.M. Selen a, L.J. van IJzendoorn a
a,*
, P.J.M. Smulders b, M.J.A de Voigt
a
Department of Applied Physics, Research School CPS and COBRA, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b Materials Science Center, Groningen University, Nijenborgh 4, 9747 A6 Groningen, The Netherlands
Abstract Planar MeV ion channeling experiments have been used to characterize a buried Si1x Gex film with a thickness of 2.2 nm in a silicon host crystal. The tetragonal deformation in the film shows up as a translation of the {0 1 1} planes across the film, which was measured as a step in the yield of the planar channeled Rutherford backscattering spectra. The flux distribution between the planes acts as a probe for the position of the planes. The angular dependence of the step height has been measured for different incident ion energies and interpreted with Monte Carlo calculations. Simulations only resemble the measured spectra when the Hartree–Fock potential is used for the ion–atom interaction. The translation of the planes can be measured with an accuracy of approximately 10%, which is comparable to results obtained with axial channeling experiments. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.85.þp; 68.55.a Keywords: Channeling; Monte Carlo simulations; Strain; Thin films
1. Introduction The analysis of lattice deformation in ultrathin (<5 nm) buried films is of general interest for materials research programs needed to develop state of the art semiconductor devices [1]. Tetragonal distortion is traditionally measured by the shift of an angular scan of the strained film with respect to the angular scan of the host crystal. This procedure requires a decrease of the scattering probability by shadow cone formation in the thin film and is not applicable for buried films with a thickness less than 5 nm. *
Corresponding author. E-mail address: [email protected] (L.J. van IJzendoorn).
Recently the analysis of lattice deformation in these so-called nanofilms was demonstrated with Rutherford backscattering spectrometry (RBS) ion channeling using the flux distribution in the h0 1 1i channel as a probe for the translation of atomic strings below the nanofilm with respect to their position above the film [2]. In this paper, the application of planar channeling is investigated to probe the displacement of the planes across a nanofilm. For planar channeling the flux distribution is characterized by coherency of the ion trajectories up to large depths (typically P 200 nm) and a relatively small critical angle for channeling. Consequently, a non-equilibrium flux distribution is expected at the depth of the nanofilm, which might enhance the sensitivity for lattice deformations.
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 5 1 - 2
L.J.M. Selen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 570–573
571
2. Experimental A buried Si1x Gex nanofilm in Si has been grown by chemical vapour deposition at atmospheric pressure on a 6 in. [0 0 1] oriented Si wafer at Philips Research Laboratories Eindhoven. At first the wafer was cleaned with H2 at a temperature of 1100 °C and then a 5 nm thick Si buffer layer was grown at 625 °C. Subsequently, a 2.2 nm thick Si1x Gex film was grown by deposition from GeH4 and SiH2 Cl2 at 625 °C. Finally a 280 nm Si capping layer was deposited on top of the thin film. The Ge areal density was determined by RBS: ð4:6 0:3Þ 1015 Ge/cm2 and the thickness of the layer was found to be 2:2 0:2 nm measured with high resolution secondary ion mass spectrometry and transmission electron microscopy. The corresponding Ge concentration in the thin film is 45% (x ¼ 0:45) and the tetragonal distortion in the film according to elasticity theory is eT ¼ 0:033. The thickness of the Si capping layer was determined with X-ray diffraction and found to be 291 2 nm. The ion channeling experiments have been performed at the 2–30 MeV AVF Cyclotron at Eindhoven University of Technology. The beam divergence was controlled by two sets of slits in the beam guiding system and set to 0.07° FWHM. Backscattered ions were detected with a 100 mm2 PIPS detector (Canberra) at a backscattering angle of 130°. For the planar channeling experiments the beam was tilted 5° to the [0 1 1] axis into the {0 1 1} plane and series of channeling spectra were obtained by varying the angle w between the incoming beam and the {0 1 1} plane. The sign convention for the tilt angle is positive towards the [0 0 1] axis oriented along the surface normal. The translation of the {0 1 1} plane of the substrate with respect to the capping layer is calculated to be 0.052 nm and the rotation angle of the {0 1 1} plane at the interface with the nanofilm is 0.95°. Strain relaxation induced by ion bombardment during analysis was not observed.
Fig. 1. Planar channeled RBS spectrum measured with 2 MeV He ions. The angle between the incoming beam and the {0 1 1} plane is 0.05°.
scattering yield at the depth of the strained nanofilm. The measured step height is considerably larger than the one measured for ions channeled along the [0 1 1] axis. (see e.g. Fig. 2 of Ref. [2]). Apparently the flux distribution at the depth of the nanofilm is effectively probing the translated planes and leads to a high scattering probability. More information was obtained by varying the angle w between the incident beam and the {0 1 1} plane for different ion energies. The step height was quantified as the difference between the average yield in the depth intervals 500–700 and 150– 360 nm. The depth intervals in the spectra were estimated on the basis of the random stopping power of silicon. Fig. 2 shows the step height as a function of angle for different incident ion energies. The asymmetry of the measured curves with respect to w ¼ 0 for 2.5 and 3.5 MeV ions shows that the flux distribution is not at equilibrium at the depth of the nanofilm. Apparently a node in the oscillating particle trajectories is at or near the nanofilm at these energies.
3. Results
3.1. The ion–atom potential
Fig. 1 shows a typical planar channeled RBS spectrum of a Si1x Gex film with a step in the
A detailed interpretation of the measured step height curves can be obtained by Monte Carlo
572
L.J.M. Selen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 570–573
Littmark (ZBL) potential [4], the Moliere potential [5], and a Hartree–Fock (HF) potential based on solid state charge densities as tabulated in [4]. The commonly applied ZBL potential does not show the measured asymmetry and only the use of a HF potential resembles the measured curves. This result is at present the most clear example in literature [6] that shows that a HF potential is needed to describe the ion–atom potential in trajectory calculations. 3.2. Analysis of lattice deformation
simulations with FLUX7, which is an improved and extended version of FLUX [3]. The measured step in the scattering yield shows up in the simulations as a sudden increase in the nuclear encounter probability (NEP) and the difference between the average normalized NEP in the depth intervals 483–722 and 145–385 nm was taken to calculate the step height. Fig. 3 shows the measured step height curve for 3.5 MeV He ions and three simulations in which different ion–atom potentials have been used: the Ziegler–Biersack–
Obviously, the use of the step height curves to interpret lattice deformation of buried nanofilms is only possible with the application of the HF potential in the simulations. In order to judge the analytical relevance of the measured step height curves, it is interesting to investigate whether the simulated curves are sensitive to the translation of the atomic planes over the nanofilm. A set of simulations has been carried out for 2 MeV He ions in which the translation of the {0 1 1} plane in the substrate with respect to the the {0 1 1} plane in the capping layer was varied from 0.041 to 0.063 nm. The results of the simulations are depicted in Fig. 4 together with the measured step height curve. The calculated curves show that an asymmetry appears for larger translations. From Fig. 4 the translation can be determined with an accuracy of approxi-
Fig. 3. Measured and simulated normalized step height curves.
Fig. 4. Calculated step height curves for different translations of the {0 1 1} plane.
Fig. 2. Step height curves for four different ion energies. The stepsize (vertical axis) is normalised on the random height. The horizontal axis has been scaled with the characteristic angle for planar channeling.
L.J.M. Selen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 570–573
mately 10% which is comparable to the accuracy reached for axial channeling [2]. 4. Conclusions Lattice deformation in a buried Si1x Gex film of only 2.2 nm leads to a step in the RBS channeling yield of the host crystal at the depth of the nanofilm which is significantly larger than the step found for axial channeling. The sudden increase in yield is also found in the NEP calculated with the Monte Carlo program FLUX7. Comparing the angular dependence of the step height between measurements and simulations shows that only the HF potential is able to give close resemblance between measured and simulated curves. In spite of the high selectivity in the simulations, the accuracy for measuring the lattice deformation is at present not better than the accuracy reached with axial channeling.
573
Acknowledgements The authors are indebted to Dr. M.J.J. Theunissen (Philips Research Laboratories Eindhoven) for kindly providing the samples.
References [1] P. Mushini, K.P. Roenker, Sol. State Electron. 44 (2000) 2239. [2] L.J.M. Selen, F.J.J. Janssen, L.J. van IJzendoorn, M.J.J. Theunissen, P.J.M. Smulders, M.J.A. de Voigt, Nucl. Instr. and Meth. B 161–163 (2000) 492. [3] P.J.M. Smulders, D.O. Boerma, Nucl. Instr. and Meth. B 29 (1987) 471. [4] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [5] G. Moliere, Z. Naturforsch. 2A (1947) 133. [6] P.J.M. Smulders, A. Dygo, D.O. Boerma, Nucl. Instr. and Meth. B 67 (1992) 185.
Planar MeV ion channeling on strained buried nanofilms L.J.M. Selen a, L.J. van IJzendoorn a
a,*
, P.J.M. Smulders b, M.J.A de Voigt
a
Department of Applied Physics, Research School CPS and COBRA, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b Materials Science Center, Groningen University, Nijenborgh 4, 9747 A6 Groningen, The Netherlands
Abstract Planar MeV ion channeling experiments have been used to characterize a buried Si1x Gex film with a thickness of 2.2 nm in a silicon host crystal. The tetragonal deformation in the film shows up as a translation of the {0 1 1} planes across the film, which was measured as a step in the yield of the planar channeled Rutherford backscattering spectra. The flux distribution between the planes acts as a probe for the position of the planes. The angular dependence of the step height has been measured for different incident ion energies and interpreted with Monte Carlo calculations. Simulations only resemble the measured spectra when the Hartree–Fock potential is used for the ion–atom interaction. The translation of the planes can be measured with an accuracy of approximately 10%, which is comparable to results obtained with axial channeling experiments. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.85.þp; 68.55.a Keywords: Channeling; Monte Carlo simulations; Strain; Thin films
1. Introduction The analysis of lattice deformation in ultrathin (<5 nm) buried films is of general interest for materials research programs needed to develop state of the art semiconductor devices [1]. Tetragonal distortion is traditionally measured by the shift of an angular scan of the strained film with respect to the angular scan of the host crystal. This procedure requires a decrease of the scattering probability by shadow cone formation in the thin film and is not applicable for buried films with a thickness less than 5 nm. *
Corresponding author. E-mail address: [email protected] (L.J. van IJzendoorn).
Recently the analysis of lattice deformation in these so-called nanofilms was demonstrated with Rutherford backscattering spectrometry (RBS) ion channeling using the flux distribution in the h0 1 1i channel as a probe for the translation of atomic strings below the nanofilm with respect to their position above the film [2]. In this paper, the application of planar channeling is investigated to probe the displacement of the planes across a nanofilm. For planar channeling the flux distribution is characterized by coherency of the ion trajectories up to large depths (typically P 200 nm) and a relatively small critical angle for channeling. Consequently, a non-equilibrium flux distribution is expected at the depth of the nanofilm, which might enhance the sensitivity for lattice deformations.
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 5 1 - 2
L.J.M. Selen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 570–573
571
2. Experimental A buried Si1x Gex nanofilm in Si has been grown by chemical vapour deposition at atmospheric pressure on a 6 in. [0 0 1] oriented Si wafer at Philips Research Laboratories Eindhoven. At first the wafer was cleaned with H2 at a temperature of 1100 °C and then a 5 nm thick Si buffer layer was grown at 625 °C. Subsequently, a 2.2 nm thick Si1x Gex film was grown by deposition from GeH4 and SiH2 Cl2 at 625 °C. Finally a 280 nm Si capping layer was deposited on top of the thin film. The Ge areal density was determined by RBS: ð4:6 0:3Þ 1015 Ge/cm2 and the thickness of the layer was found to be 2:2 0:2 nm measured with high resolution secondary ion mass spectrometry and transmission electron microscopy. The corresponding Ge concentration in the thin film is 45% (x ¼ 0:45) and the tetragonal distortion in the film according to elasticity theory is eT ¼ 0:033. The thickness of the Si capping layer was determined with X-ray diffraction and found to be 291 2 nm. The ion channeling experiments have been performed at the 2–30 MeV AVF Cyclotron at Eindhoven University of Technology. The beam divergence was controlled by two sets of slits in the beam guiding system and set to 0.07° FWHM. Backscattered ions were detected with a 100 mm2 PIPS detector (Canberra) at a backscattering angle of 130°. For the planar channeling experiments the beam was tilted 5° to the [0 1 1] axis into the {0 1 1} plane and series of channeling spectra were obtained by varying the angle w between the incoming beam and the {0 1 1} plane. The sign convention for the tilt angle is positive towards the [0 0 1] axis oriented along the surface normal. The translation of the {0 1 1} plane of the substrate with respect to the capping layer is calculated to be 0.052 nm and the rotation angle of the {0 1 1} plane at the interface with the nanofilm is 0.95°. Strain relaxation induced by ion bombardment during analysis was not observed.
Fig. 1. Planar channeled RBS spectrum measured with 2 MeV He ions. The angle between the incoming beam and the {0 1 1} plane is 0.05°.
scattering yield at the depth of the strained nanofilm. The measured step height is considerably larger than the one measured for ions channeled along the [0 1 1] axis. (see e.g. Fig. 2 of Ref. [2]). Apparently the flux distribution at the depth of the nanofilm is effectively probing the translated planes and leads to a high scattering probability. More information was obtained by varying the angle w between the incident beam and the {0 1 1} plane for different ion energies. The step height was quantified as the difference between the average yield in the depth intervals 500–700 and 150– 360 nm. The depth intervals in the spectra were estimated on the basis of the random stopping power of silicon. Fig. 2 shows the step height as a function of angle for different incident ion energies. The asymmetry of the measured curves with respect to w ¼ 0 for 2.5 and 3.5 MeV ions shows that the flux distribution is not at equilibrium at the depth of the nanofilm. Apparently a node in the oscillating particle trajectories is at or near the nanofilm at these energies.
3. Results
3.1. The ion–atom potential
Fig. 1 shows a typical planar channeled RBS spectrum of a Si1x Gex film with a step in the
A detailed interpretation of the measured step height curves can be obtained by Monte Carlo
572
L.J.M. Selen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 570–573
Littmark (ZBL) potential [4], the Moliere potential [5], and a Hartree–Fock (HF) potential based on solid state charge densities as tabulated in [4]. The commonly applied ZBL potential does not show the measured asymmetry and only the use of a HF potential resembles the measured curves. This result is at present the most clear example in literature [6] that shows that a HF potential is needed to describe the ion–atom potential in trajectory calculations. 3.2. Analysis of lattice deformation
simulations with FLUX7, which is an improved and extended version of FLUX [3]. The measured step in the scattering yield shows up in the simulations as a sudden increase in the nuclear encounter probability (NEP) and the difference between the average normalized NEP in the depth intervals 483–722 and 145–385 nm was taken to calculate the step height. Fig. 3 shows the measured step height curve for 3.5 MeV He ions and three simulations in which different ion–atom potentials have been used: the Ziegler–Biersack–
Obviously, the use of the step height curves to interpret lattice deformation of buried nanofilms is only possible with the application of the HF potential in the simulations. In order to judge the analytical relevance of the measured step height curves, it is interesting to investigate whether the simulated curves are sensitive to the translation of the atomic planes over the nanofilm. A set of simulations has been carried out for 2 MeV He ions in which the translation of the {0 1 1} plane in the substrate with respect to the the {0 1 1} plane in the capping layer was varied from 0.041 to 0.063 nm. The results of the simulations are depicted in Fig. 4 together with the measured step height curve. The calculated curves show that an asymmetry appears for larger translations. From Fig. 4 the translation can be determined with an accuracy of approxi-
Fig. 3. Measured and simulated normalized step height curves.
Fig. 4. Calculated step height curves for different translations of the {0 1 1} plane.
Fig. 2. Step height curves for four different ion energies. The stepsize (vertical axis) is normalised on the random height. The horizontal axis has been scaled with the characteristic angle for planar channeling.
L.J.M. Selen et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 570–573
mately 10% which is comparable to the accuracy reached for axial channeling [2]. 4. Conclusions Lattice deformation in a buried Si1x Gex film of only 2.2 nm leads to a step in the RBS channeling yield of the host crystal at the depth of the nanofilm which is significantly larger than the step found for axial channeling. The sudden increase in yield is also found in the NEP calculated with the Monte Carlo program FLUX7. Comparing the angular dependence of the step height between measurements and simulations shows that only the HF potential is able to give close resemblance between measured and simulated curves. In spite of the high selectivity in the simulations, the accuracy for measuring the lattice deformation is at present not better than the accuracy reached with axial channeling.
573
Acknowledgements The authors are indebted to Dr. M.J.J. Theunissen (Philips Research Laboratories Eindhoven) for kindly providing the samples.
References [1] P. Mushini, K.P. Roenker, Sol. State Electron. 44 (2000) 2239. [2] L.J.M. Selen, F.J.J. Janssen, L.J. van IJzendoorn, M.J.J. Theunissen, P.J.M. Smulders, M.J.A. de Voigt, Nucl. Instr. and Meth. B 161–163 (2000) 492. [3] P.J.M. Smulders, D.O. Boerma, Nucl. Instr. and Meth. B 29 (1987) 471. [4] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [5] G. Moliere, Z. Naturforsch. 2A (1947) 133. [6] P.J.M. Smulders, A. Dygo, D.O. Boerma, Nucl. Instr. and Meth. B 67 (1992) 185.