- Email: [email protected]
Multimodal electronic–vibronic spectra of luminescence in ion-implanted silica layers
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 743–746 www.elsevier.com/locate/jlumin
Multimodal electronic–vibronic spectra of luminescence in ion-implanted silica layers Hans-Joachim Fittinga,, Roushdey Salha, Bernd Schmidtb a
Institute of Physics, University of Rostock, Universita¨tsplatz 3, D-18051 Rostock, Germany Research Center Rossendorf, Institute of Ion Beam Physics and Materials Research, P.O. Box 510119, D-01314 Dresden, Germany
b
Available online 24 March 2006
Abstract Thermally oxidized SiO2 layers of 100 and 500 nm thickness have been implanted by oxygen and sulfur ions with a dose of 3 1016 and 5 1016 ions/cm2, respectively, leading to an atomic dopant fraction of about 4 at.% at the half depth of the SiO2 layers. The cathodoluminescence spectra of oxygen and sulfur implanted SiO2 layers show besides characteristic bands a sharp and intensive multimodal structure beginning in the green region at 500 nm over the yellow-red region extending to the near IR measured up to 820 nm. The energy step differences of the sublevels amount on average 120 meV and indicate vibration associated electronic states, probably of O2-interstitial molecules, as we could demonstrate by a respective configuration coordinate model. r 2006 Elsevier B.V. All rights reserved. PACS: 61.72.Cc; 68.55.Ln; 78.60.Hk; 82.30.Rs Keywords: Cathodoluminescence; Defect luminescence; Vibronic Spectra; Photonic Crystals
1. Introduction Cathodoluminescence (CL) spectroscopy in a scanning electron microscope (SEM) is a powerful technique enabling high sensitivity and high spatial resolution detection of defect centers in dielectric and insulating materials, especially in thin layers [1,2]. The main luminescent centers in SiO2 are the red (R) luminescence (650 nm, 1.9 eV) of the non-bridging oxygen hole center (NBOHC), the blue (B) band (460 nm, 2.7 eV) and the ultraviolet (UV) luminescence (290 nm, 4.3 eV) both related commonly to oxygen-deficient centers (ODC), [3]. In the present work, we will either enhance or replace the second constituent oxygen isoelectronically by additional oxygen or sulfur implantation. Analogous to oxygen surplus in the former work [2], we expect a strong influence on the R luminescence band of the NBOHC. The results obtained are very surprising, having encouraged us to a Corresponding author. Tel.: +49 381 498 6760; fax: +49 381 498 6802.
E-mail address: hans-joachim.fi[email protected] (H.-J. Fitting). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.276
first rapid communication [4] to look for further discussion. One contribution in Ref. [5] is on similar reflectance and photoluminescence spectra of porous silicon (PS) layers and microcavities, but is discussed as photonic crystals. However, these PS layers possess a high internal surface area of native oxide coating Silicon-di-oxide (SiO2), a resemblance to the SiO2 layers in the present paper. 2. Experimental details The CL spectra recording and measurements were performed in a digital SEM (Zeiss DSM 960), via a parabolic mirror collector, a spectrograph (Spex-270 M) and a CCD camera (Princenton Instr., EEV 1024 256), (see Ref. [2]). CL spectra ranging from 1.5 to 6.5 eV were accumulated in single shot mode within a time of 1 s and with a spectral resolution of 4 nm. A cooling and heating temperature stage provides sample temperatures between 80 and 670 K. We have used amorphous, thermally grown SiO2 layers, 100 nm thick dry oxidized at 1000 1C as samples for oxygen ion (O+) implantation with energy E þ 0 ¼ 20 keV and a
ARTICLE IN PRESS 744
H.-J. Fitting et al. / Journal of Luminescence 122–123 (2007) 743–746
Fig. 1. Electron beam excitation densities in SiO2 layers for different beam energies Eo. Here a sulfur profile S+ is implanted with an ion energy Eþ s ¼ 150 keV in the mean projected range Rp ¼ 190 nm.
dose 3 1016 cm2, and 500 nm thick wet oxidized at 1100 1C as samples for sulfur ion (S+) implantation with 16 2 energy E þ s ¼ 150 keV and a dose 5 10 cm . These implantation energies and doses led to an atomic dopand fraction of about 4 at. % at nearly the half depth of the oxide layers (see Fig. 1). The dry oxide for oxygen implantation was chosen to avoid the presence of hydrogen, and formation of radiolytic water and bounding of oxygen. Afterwards, a post-implantation thermal annealing has been performed for 30 min in dry nitrogen at a temperature T a ¼ ð90021000Þ1C [2]. The CL excitation was performed with an electron beam energy of E o ¼ 10 keV (see Fig. 1) and a beam current density of j o 5 103 A=cm2 , also (see Ref. [2]). 3. Results and discussion First of all, in Fig. 2 (above), we see the typical CL spectra of wet oxidized SiO2 for liquid nitrogen temperature (LNT) and room temperature (RT). Besides the main bands: red R (660 nm, 1.9 eV), blue B (460 nm, 2.7 eV), and UV (290 nm, 4.4 eV), we recognize a yellow band Y (570 nm, 2.2 eV) decaying at LNT very rapidly at the beginning of the electron beam irradiation and CL excitation, but appearing and increasing at RT after a long time of irradiation. Then in Fig. 2 (middle), the spectra of additional implanted oxygen in SiO2 are shown. In order to avoid water formation and binding of oxygen, we have chosen dry oxidized SiO2 layers. Because of slower oxidation, they are of less thickness d ox ¼ 100 nm. Thus, the overall CL intensity is about one order of magnitude lower than in 500 nm thick oxides. But, the surprisingly new feature of the spectra appears in its multiple regular structure from
Fig. 2. CL spectra of wet non-implanted SiO2 at room temperature RT and liquid nitrogen temperature LNT (above) together with O+ (middle) and S+ (bottom) implanted SiO2 layers showing a sharp multiplet structure in the green–yellow–red–near IR region at LNT in dependence on irradiation time; dox: oxide layer thickness , Ta: annealing temperature.
ARTICLE IN PRESS H.-J. Fitting et al. / Journal of Luminescence 122–123 (2007) 743–746
the green (G) over the yellow (Y) and red (R) regions into near infrared (IR). An identical multimodal structure, but still more sharp and regular, is found in sulfur (S+) implanted layers, as shown in Fig. 2 (bottom). There, the CL spectra of S+doped SiO2 are presented for LNT and RT, as well as with their time dependence. Obviously, the high violet (V) intensity at 410 nm is assigned to S+ implantation. Moreover, the sharp and intensive multi-step emission spectra in the green-yellow-red-near IR (500–820 nm) region is observed for these layers as in O+ implanted layers. The exact band positions in wavelengths are given in Fig. 2 (middle). They correspond to almost equidistance energy steps of 140 meV in the G region, and then they slightly decrease to 110 meV in the IR region, already shown in Ref. [4]. Thus, the mean energy step width is about 120 meV. Further on, we observe the UV band (290 nm) at the same position in all samples; pure SiO2 as well as in oxygen and sulfur doped silica SiO2:O and SiO2:S, respectively. Moreover, after longer irradiation of about 1 min, i.e. an electron beam dose of 0.3 As/cm2, the multiple structure disappears (probably destroyed by the electron beam) and the characteristic red band R (660 nm) of the NBOHC in SiO2 becomes visible besides remaining components at the blue band B (460 nm) position and in the Y region at 560 nm and 590 nm. On the other hand, the sulfur associated violet band V (410 nm) remains still visible. Hence, the mean step width amounts about 120 meV. This energy difference may correspond to a series of almost equidistant vibronic levels of non-saturated sulfur radicals Si–Sd or Si–O–Sd formed during implantation and thermal annealing analogous to the R band center of the NBOHC Si–Od in pure SiO2. Even the vibronic step widths are decreasing with lower photon energy, beginning with DE ¼ 140 meV at hn ¼ 2.48 eV and are dropping to DE ¼ 110 meV at hn ¼ 1.51 eV, indicating a widened (subquadratic) potential curve of the luminescence ground states with compressed higher vibronic levels in terms of an adiabatic configuration coordinate model. But contrary to Ref. [4], here we find exactly the same multimodal structure in oxygen implanted dry SiO2 as before in sulfur implanted SiO2 layers. It leads us to the conclusion that not sulfur but oxygen seems to be the source of these multiple spectra. We know already that oxygen is responsible for the R luminescence in SiO2 (see above and Refs. [2,3]). Looking to literature, [6], we found excitation [7] and emission spectra [8] of the negatively charged molecule O 2 on interstitial sites in alkali halide crystals. Especially in the paper of Ewig and Tellinghuisen [8], the ground electronic state and several low-lying excited states of the superoxide ion O 2 have been studied by multiconfiguration self-consistence fields (MCSCF) (see Fig. 3). For comparison, also the ground state of the neutral O2 molecule was considered. Parallel computation was carried out for the species in vacuo and in a simulated
745
Fig. 3. Comparison of MCSCF in vacuo (triangles) and in lattio (circles) energies with Morse curves derived from analysis of absorption, excitation and luminescence spectra of O 2 in NaCl, Ref. [8]. Respective vibronic levels are shown by bars.
KCl crystal lattice (in lattio). Computed spectroscopic parameters are in good agreement with experiment for the ground (X) and excited (A) states of O 2 in vacuo. In Fig. 3 there is also substantial agreement between the computed energy curves for both X and A states in a point-charge lattice and those measured in alkali halide lattices. Further, the spectroscopic parameters of the electron scattering resonance states in vacuo agree well with those of the analogous lattice-stabilized excited electronic states in the solid. There is a typical absorption from the X to the A of about hn ¼ 5.1 eV, corresponding to the R luminescence excitation associated with the NBOHC, [3]. Moreover, the related luminescent transition from A-X shows the R luminescence at about hn ¼ 2 eV. Looking to vibronic levels within the potential configuration curves, we see levels of about 120 meV, step-widened towards higher potential energies, i.e., towards the R luminescence transition region. All is in good agreement with our CL spectra of SiO2:O and SiO2:S layers. Thus we favor the interstitial O 2 model for the multimodal CL structure against the concept of a photonic crystal structure, as the geometry of the oxygen implanted layers with a thickness of d ¼ 100 nm and the sulfur implanted layers with thickness 500 nm, are fully different, see Fig. 1. Also, their implantation profiles differ from each other in depth and width, as well as in species kind of implanted ions: O+ and S+. Thus, only oxygen and its interstitial molecules remain as candidates for the electronic–vibronic multiple spectra. In case of sulfur implantation, the oxygen is likely released from the SiO2 network, and probably, substituted by sulfur atoms, leading to free oxygen and finally to O 2 molecules on interstitial sites.
ARTICLE IN PRESS 746
H.-J. Fitting et al. / Journal of Luminescence 122–123 (2007) 743–746
References [1] M.A. Stevens-Kalceff, Phys. Rev. B 57 (1998) 5674. [2] H.-J. Fitting, T. Barfels, A.N. Trukhin, B. Schmidt, A. Gulans, A. von Czarnowski, J. Non-Cryst. Solids 303 (2002) 218. [3] L. Skuja, J. Non-Cryst. Solids 167 (1994) 229.
[4] R. Salh, B. Schmidt, H.-J. Fitting, Phys. Stat. Sol. (a) 202 (2005) R53. [5] R.C. Bailey, M. Parpia, J.T. Hupp, Material Today 8 (4) (2005) 46. [6] R.A. Weeks, Private Communication (November 2001), giving the hint to O 2 and Ref. [8] [7] J. Rolfe, J. Chem. Phys. 70 (1979) 2463. [8] C.S. Ewig, J. Tellinghuisen, J. Chem. Phys. 95 (1991) 1097.
Journal of Luminescence 122–123 (2007) 743–746 www.elsevier.com/locate/jlumin
Multimodal electronic–vibronic spectra of luminescence in ion-implanted silica layers Hans-Joachim Fittinga,, Roushdey Salha, Bernd Schmidtb a
Institute of Physics, University of Rostock, Universita¨tsplatz 3, D-18051 Rostock, Germany Research Center Rossendorf, Institute of Ion Beam Physics and Materials Research, P.O. Box 510119, D-01314 Dresden, Germany
b
Available online 24 March 2006
Abstract Thermally oxidized SiO2 layers of 100 and 500 nm thickness have been implanted by oxygen and sulfur ions with a dose of 3 1016 and 5 1016 ions/cm2, respectively, leading to an atomic dopant fraction of about 4 at.% at the half depth of the SiO2 layers. The cathodoluminescence spectra of oxygen and sulfur implanted SiO2 layers show besides characteristic bands a sharp and intensive multimodal structure beginning in the green region at 500 nm over the yellow-red region extending to the near IR measured up to 820 nm. The energy step differences of the sublevels amount on average 120 meV and indicate vibration associated electronic states, probably of O2-interstitial molecules, as we could demonstrate by a respective configuration coordinate model. r 2006 Elsevier B.V. All rights reserved. PACS: 61.72.Cc; 68.55.Ln; 78.60.Hk; 82.30.Rs Keywords: Cathodoluminescence; Defect luminescence; Vibronic Spectra; Photonic Crystals
1. Introduction Cathodoluminescence (CL) spectroscopy in a scanning electron microscope (SEM) is a powerful technique enabling high sensitivity and high spatial resolution detection of defect centers in dielectric and insulating materials, especially in thin layers [1,2]. The main luminescent centers in SiO2 are the red (R) luminescence (650 nm, 1.9 eV) of the non-bridging oxygen hole center (NBOHC), the blue (B) band (460 nm, 2.7 eV) and the ultraviolet (UV) luminescence (290 nm, 4.3 eV) both related commonly to oxygen-deficient centers (ODC), [3]. In the present work, we will either enhance or replace the second constituent oxygen isoelectronically by additional oxygen or sulfur implantation. Analogous to oxygen surplus in the former work [2], we expect a strong influence on the R luminescence band of the NBOHC. The results obtained are very surprising, having encouraged us to a Corresponding author. Tel.: +49 381 498 6760; fax: +49 381 498 6802.
E-mail address: hans-joachim.fi[email protected] (H.-J. Fitting). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.276
first rapid communication [4] to look for further discussion. One contribution in Ref. [5] is on similar reflectance and photoluminescence spectra of porous silicon (PS) layers and microcavities, but is discussed as photonic crystals. However, these PS layers possess a high internal surface area of native oxide coating Silicon-di-oxide (SiO2), a resemblance to the SiO2 layers in the present paper. 2. Experimental details The CL spectra recording and measurements were performed in a digital SEM (Zeiss DSM 960), via a parabolic mirror collector, a spectrograph (Spex-270 M) and a CCD camera (Princenton Instr., EEV 1024 256), (see Ref. [2]). CL spectra ranging from 1.5 to 6.5 eV were accumulated in single shot mode within a time of 1 s and with a spectral resolution of 4 nm. A cooling and heating temperature stage provides sample temperatures between 80 and 670 K. We have used amorphous, thermally grown SiO2 layers, 100 nm thick dry oxidized at 1000 1C as samples for oxygen ion (O+) implantation with energy E þ 0 ¼ 20 keV and a
ARTICLE IN PRESS 744
H.-J. Fitting et al. / Journal of Luminescence 122–123 (2007) 743–746
Fig. 1. Electron beam excitation densities in SiO2 layers for different beam energies Eo. Here a sulfur profile S+ is implanted with an ion energy Eþ s ¼ 150 keV in the mean projected range Rp ¼ 190 nm.
dose 3 1016 cm2, and 500 nm thick wet oxidized at 1100 1C as samples for sulfur ion (S+) implantation with 16 2 energy E þ s ¼ 150 keV and a dose 5 10 cm . These implantation energies and doses led to an atomic dopand fraction of about 4 at. % at nearly the half depth of the oxide layers (see Fig. 1). The dry oxide for oxygen implantation was chosen to avoid the presence of hydrogen, and formation of radiolytic water and bounding of oxygen. Afterwards, a post-implantation thermal annealing has been performed for 30 min in dry nitrogen at a temperature T a ¼ ð90021000Þ1C [2]. The CL excitation was performed with an electron beam energy of E o ¼ 10 keV (see Fig. 1) and a beam current density of j o 5 103 A=cm2 , also (see Ref. [2]). 3. Results and discussion First of all, in Fig. 2 (above), we see the typical CL spectra of wet oxidized SiO2 for liquid nitrogen temperature (LNT) and room temperature (RT). Besides the main bands: red R (660 nm, 1.9 eV), blue B (460 nm, 2.7 eV), and UV (290 nm, 4.4 eV), we recognize a yellow band Y (570 nm, 2.2 eV) decaying at LNT very rapidly at the beginning of the electron beam irradiation and CL excitation, but appearing and increasing at RT after a long time of irradiation. Then in Fig. 2 (middle), the spectra of additional implanted oxygen in SiO2 are shown. In order to avoid water formation and binding of oxygen, we have chosen dry oxidized SiO2 layers. Because of slower oxidation, they are of less thickness d ox ¼ 100 nm. Thus, the overall CL intensity is about one order of magnitude lower than in 500 nm thick oxides. But, the surprisingly new feature of the spectra appears in its multiple regular structure from
Fig. 2. CL spectra of wet non-implanted SiO2 at room temperature RT and liquid nitrogen temperature LNT (above) together with O+ (middle) and S+ (bottom) implanted SiO2 layers showing a sharp multiplet structure in the green–yellow–red–near IR region at LNT in dependence on irradiation time; dox: oxide layer thickness , Ta: annealing temperature.
ARTICLE IN PRESS H.-J. Fitting et al. / Journal of Luminescence 122–123 (2007) 743–746
the green (G) over the yellow (Y) and red (R) regions into near infrared (IR). An identical multimodal structure, but still more sharp and regular, is found in sulfur (S+) implanted layers, as shown in Fig. 2 (bottom). There, the CL spectra of S+doped SiO2 are presented for LNT and RT, as well as with their time dependence. Obviously, the high violet (V) intensity at 410 nm is assigned to S+ implantation. Moreover, the sharp and intensive multi-step emission spectra in the green-yellow-red-near IR (500–820 nm) region is observed for these layers as in O+ implanted layers. The exact band positions in wavelengths are given in Fig. 2 (middle). They correspond to almost equidistance energy steps of 140 meV in the G region, and then they slightly decrease to 110 meV in the IR region, already shown in Ref. [4]. Thus, the mean energy step width is about 120 meV. Further on, we observe the UV band (290 nm) at the same position in all samples; pure SiO2 as well as in oxygen and sulfur doped silica SiO2:O and SiO2:S, respectively. Moreover, after longer irradiation of about 1 min, i.e. an electron beam dose of 0.3 As/cm2, the multiple structure disappears (probably destroyed by the electron beam) and the characteristic red band R (660 nm) of the NBOHC in SiO2 becomes visible besides remaining components at the blue band B (460 nm) position and in the Y region at 560 nm and 590 nm. On the other hand, the sulfur associated violet band V (410 nm) remains still visible. Hence, the mean step width amounts about 120 meV. This energy difference may correspond to a series of almost equidistant vibronic levels of non-saturated sulfur radicals Si–Sd or Si–O–Sd formed during implantation and thermal annealing analogous to the R band center of the NBOHC Si–Od in pure SiO2. Even the vibronic step widths are decreasing with lower photon energy, beginning with DE ¼ 140 meV at hn ¼ 2.48 eV and are dropping to DE ¼ 110 meV at hn ¼ 1.51 eV, indicating a widened (subquadratic) potential curve of the luminescence ground states with compressed higher vibronic levels in terms of an adiabatic configuration coordinate model. But contrary to Ref. [4], here we find exactly the same multimodal structure in oxygen implanted dry SiO2 as before in sulfur implanted SiO2 layers. It leads us to the conclusion that not sulfur but oxygen seems to be the source of these multiple spectra. We know already that oxygen is responsible for the R luminescence in SiO2 (see above and Refs. [2,3]). Looking to literature, [6], we found excitation [7] and emission spectra [8] of the negatively charged molecule O 2 on interstitial sites in alkali halide crystals. Especially in the paper of Ewig and Tellinghuisen [8], the ground electronic state and several low-lying excited states of the superoxide ion O 2 have been studied by multiconfiguration self-consistence fields (MCSCF) (see Fig. 3). For comparison, also the ground state of the neutral O2 molecule was considered. Parallel computation was carried out for the species in vacuo and in a simulated
745
Fig. 3. Comparison of MCSCF in vacuo (triangles) and in lattio (circles) energies with Morse curves derived from analysis of absorption, excitation and luminescence spectra of O 2 in NaCl, Ref. [8]. Respective vibronic levels are shown by bars.
KCl crystal lattice (in lattio). Computed spectroscopic parameters are in good agreement with experiment for the ground (X) and excited (A) states of O 2 in vacuo. In Fig. 3 there is also substantial agreement between the computed energy curves for both X and A states in a point-charge lattice and those measured in alkali halide lattices. Further, the spectroscopic parameters of the electron scattering resonance states in vacuo agree well with those of the analogous lattice-stabilized excited electronic states in the solid. There is a typical absorption from the X to the A of about hn ¼ 5.1 eV, corresponding to the R luminescence excitation associated with the NBOHC, [3]. Moreover, the related luminescent transition from A-X shows the R luminescence at about hn ¼ 2 eV. Looking to vibronic levels within the potential configuration curves, we see levels of about 120 meV, step-widened towards higher potential energies, i.e., towards the R luminescence transition region. All is in good agreement with our CL spectra of SiO2:O and SiO2:S layers. Thus we favor the interstitial O 2 model for the multimodal CL structure against the concept of a photonic crystal structure, as the geometry of the oxygen implanted layers with a thickness of d ¼ 100 nm and the sulfur implanted layers with thickness 500 nm, are fully different, see Fig. 1. Also, their implantation profiles differ from each other in depth and width, as well as in species kind of implanted ions: O+ and S+. Thus, only oxygen and its interstitial molecules remain as candidates for the electronic–vibronic multiple spectra. In case of sulfur implantation, the oxygen is likely released from the SiO2 network, and probably, substituted by sulfur atoms, leading to free oxygen and finally to O 2 molecules on interstitial sites.
ARTICLE IN PRESS 746
H.-J. Fitting et al. / Journal of Luminescence 122–123 (2007) 743–746
References [1] M.A. Stevens-Kalceff, Phys. Rev. B 57 (1998) 5674. [2] H.-J. Fitting, T. Barfels, A.N. Trukhin, B. Schmidt, A. Gulans, A. von Czarnowski, J. Non-Cryst. Solids 303 (2002) 218. [3] L. Skuja, J. Non-Cryst. Solids 167 (1994) 229.
[4] R. Salh, B. Schmidt, H.-J. Fitting, Phys. Stat. Sol. (a) 202 (2005) R53. [5] R.C. Bailey, M. Parpia, J.T. Hupp, Material Today 8 (4) (2005) 46. [6] R.A. Weeks, Private Communication (November 2001), giving the hint to O 2 and Ref. [8] [7] J. Rolfe, J. Chem. Phys. 70 (1979) 2463. [8] C.S. Ewig, J. Tellinghuisen, J. Chem. Phys. 95 (1991) 1097.