- Email: [email protected]
Proton beam induced luminescence of silicon dioxide implanted with silicon
Nuclear Instruments and Methods in Physics Research B 267 (2009) 2579–2582
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Proton beam induced luminescence of silicon dioxide implanted with silicon Grzegorz Gawlik a,*, Jacek Jagielski a,b, Anna Stonert b, Renata Ratajczak b a b
Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warszawa, Poland The Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock/Swierk, Poland
a r t i c l e
i n f o
Article history: Available online 27 May 2009 PACS: 78.55.Qr 78.60.Fi 78.55.m 78.60.Hk 61.46.Hk 61.72.up Keywords: Ion implantation Light emission Silicon Silica Nanocrystals
a b s t r a c t Light emission from a silicon dioxide layer enriched with silicon has been studied. Samples used had structures made on thermally oxidized silicon substrate wafers. Excess silicon atoms were introduced into a 250-nm-thick silicon dioxide layer via implantation of 60 keV Si+ ions up to a fluence of 2 1017 cm2. A 15-nm-thick Au layer was used as a top semitransparent electrode. Continuous blue light emission was observed under DC polarization of the structure at 8–12 MV/cm. The blue light emission from the structures was also observed in an ionoluminescence experiment, in which the light emission was caused by irradiation with a H2+ ion beam of energy between 22 and 100 keV. In the case of H2+, on entering the material the ions dissociated into two protons, each carrying on average half of the incident ion energy. The spectra of the emitted light and the dependence of ionoluminescence on proton energy were analyzed and the results were correlated with the concentration profile of implanted silicon atoms. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon dioxide is commonly used in microelectronics technology as an insulator. The formation of silicon nanocrystals in a SiO2 matrix allows generation of new interesting luminescent properties, which make possible production of MOS-like silicon electroluminescent devices. Several experiments performed until now on Si-implanted SiO2 have shown that photoluminescence leads mainly to red light emission [1–6], whereas electroluminescence can also produce blue emission [2,7,8]. The aim of this work is to determine the role of nano-Si precipitates and the SiO2 matrix in the generation of light via an electroluminescence process in MOS (Metal Oxide Semiconductor) structures composed of SiO2 layers with embedded silicon nanocrystals. 2. Experimental MOS structures with silicon nanocrystals embedded in the SiO2 layer were used in this work. The structures were designed in such a way as to exhibit good photoluminescent, electroluminescent and ionoluminescent performance in the same sample. Silicon
* Corresponding author. Tel.: +48 604 618 009. E-mail address: [email protected] (G. Gawlik). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.046
dioxide layers with thickness of 250 nm were produced by thermal oxidation of silicon wafers in wet oxygen at a temperature of 1100 °C. Silicon atoms were introduced into the SiO2 layer by ion implantation using 60 keV silicon ions to a fluence of 2 1017 Si+/cm2. The energy of the silicon ions was chosen to produce a silicon-rich region that is thinner than the SiO2 layer itself and located close to the surface. Silicon nanocrystals were formed by annealing the implanted structures at temperature of 1100 oC for 4 h in a nitrogen atmosphere. A layer of Au of about 15 nm thickness was deposited on the SiO2 surface as a semitransparent top electrode. Such an electrode has a 2 mm 6 mm rectangular shape with rounded corners. Depth distribution of the implanted silicon atoms in the SiO2 layer was determined by Rutherford Backscattering (RBS). A typical RBS spectrum for 1.7 MeV He ions is shown in Fig. 1. Analysis confirmed that excess silicon atoms are located near the surface of the SiO2 layer, whereas deeper down the silica layer remained undoped. Formation of silicon nanocrystals has been verified by detection of the specific wide band photoluminescence (PL) centered about 785 nm when excited by the 325-nm UV laser [9]. Current–voltage (I–V) characteristics and electroluminescence (EL) spectra were measured using a typical laboratory electrical measurement setup, with light detection through an optical fiber entry placed in front of sample surface. MOS structures were polarized using a DC power supply up to about 12 MV/cm. I–V characteristics and EL spectra were measured simultaneously.
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Fig. 1. RBS spectrum of 250-nm-thick SiO2 layer with excess silicon ion-implanted at energy 60 keV and fluence 2 1017 Si+/cm2 and annealed in N2 atmosphere for 4 h at a temperature of 1100 oC.
Ionoluminescence (IL) was excited with a H2+ ion beam in the ion implanter target chamber equipped with a vacuum glass window and a vacuum fiber optic feedthrough. Irradiation was done with a H2+ beam of ranging in energy from 22 to 100 keV. The H2+ ions dissociated on entering the material into two protons, each carrying on average half of the incident ion energy [10]. The current density of the hydrogen ion beam was between 5 and 15 lA/cm2, while the sample was placed at an angle of 55o with respect to the direction of the ion beam direction. The optical axis of the light detection system was aligned normal to the beam direction. The IL intensity dependence on hydrogen ion energy was determined relative to the IL of the pure SiO2 layer. Both samples were irradiated with H2+ ion beam simultaneously. The light intensity was measured by a CCD digital camera through the glass vacuum window. Relative light intensity was defined as a ratio of signals recorded from Si-implanted and pure SiO2 layers. Light spectra were recorded using the Hamamatsu C10083CAH spectrometer equipped with a CCD image sensor, with spectral response range from 320 to 1000 nm. The light was collected and guided into the spectrometer entry slit through an optical fiber. 3. Results and discussion The current–voltage characteristics of the MOS structure with silicon nanocrystals embedded in the SiO2 layer are different from those of the current–voltage of MOS with pure SiO2. Electrons are injected from the surface electrode into the dielectric layer under the influence of a strong electric field applied to the MOS structure. A few of the mechanisms of such injection which are possible are, for example, Fowler–Nordheim tunneling through a thin potential barrier, or tunneling into localized states situated close to the surface [11]. The exponential dependence of DC current on applied voltage has been observed, as shown in Fig. 2. Since the electric current can be easy controlled by applied voltage, observation of steady electroluminescence under DC current flow through MOS structure containing nanocrystals are possible. PL spectra of the investigated samples excited with 325 nm UV light consist of a wide band with a maximum at about 785 nm (Fig. 3(a)). This well-known effect confirms the formation of Si nanocrystals in the SiO2 layer [9]. Green or blue light emission was not observed under UV 325 nm light excitation in this study. Visible light emission from the MOS structure containing nanoSi clusters was also observed under polarization of the structure with an electric field of 7–12 MV/cm (Fig. 3(b)). Strong electric field makes it possible to inject electrons from the surface electrode into
Fig. 2. Current–voltage characteristic of the MOS structure with silicon nanocrystals in the SiO2 layer.
the dielectric layer. The observed EL spectra differ significantly from those of the PL. A strong blue band luminescence (B), with a maximum at about 470 nm was observed, along with a green– yellow luminescence (G) with a maximum at about 560 nm. The latter band contained a long red tail extending down to about 800 nm. The difference between the PL and EL spectra suggests that EL generation is caused by different recombination centers than those of PL emission. Ionoluminescence was observed under hydrogen ion irradiation of Si-implanted and unimplanted SiO2 layers (Fig. 3(c)). In both cases the emission of a strong blue band, with maximum located at about 460 nm, was observed. IL spectra of pure the SiO2 layer are very similar to those reported in [12] and [13]. Apart from the main blue band, a weak red tail down to about 800 nm was also observed. In the case of Si-enriched silica, the shape of the IL band is strongly asymmetrical, with a distinct green/red tail complementing the main blue (460 nm) band. Clearly, the excess silicon induces an IL emission in a wide green–yellow–red band, ranging from about 460 to 660 nm (Fig. 3(c) – curve III). The spectral range of this nano-Si-related IL band is similar to that of the green–yellow band observed in electroluminescent experiments (Fig. 3(b)). This experiment clearly indicates that the hydrogen ion beam can excite the same recombination centers as those of the electric current in the EL experiments. Very similar light emission bands were observed by Trukhin et al. [14], Zatsepin et al. [15], Fitting et al. [16] and Salh et al. [17] in catodoluminescence (CL) experiments performed on SiO2 based materials like quartz, glass or SiOx layers on a Si substrate. Blue band luminescence centered at about 460 nm can be attributed to siliconrelated oxygen-deficient centers (SiODC) [14,15], whereas the red band luminescence centered at about 650 nm can be due to the non-bridging oxygen center (NBOHC) [14]. The mechanism of the green–yellow luminescence is not clear. The presence of such a band in SiO2 based materials has been reported by other authors [14–17]. Concerning the green luminescence, there are suggestions that it may be due to self-trapped excitons (STE) [16]. However this effect has only been observed at low temperatures. The yellow luminescence is usually associated with the higher silicon aggregates like hexamer rings. These are a first step in the formation of nano-Si precipitates in nonstoichiometric SiOx (x < 2) [17]. The relative intensity of the IL emission from Si-implanted and pure SiO2 layers depends on the hydrogen ion energy (Fig. 4). Intensity of the light emission from silicon-enriched SiO2 upon hydrogen ion bombardment at energies below 100 keV is essentially lower than that from pure silica, and decreases with decreasing hydrogen ion energy. The relative intensity of ionoluminescence from Si ion
G. Gawlik et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2579–2582
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Fig. 3. Photoluminescence (PL) spectrum excited with 325 nm UV laser light (a), electroluminescence (EL) spectrum excited with DC polarization of the MOS structure (b), ionoluminescence (IL) spectra of pure SiO2 (I) and SiO2 with Si nanocrystals (II) excited with 100 keV hydrogen ion beam (c). All spectra were measured using the same MOS sample with silicon ion-implanted SiO2 layer (Si+ ion energy E = 60 keV; fluence D = 21017 Si+/cm2; post-implantation annealing in N2 atmosphere for 4 h at temperature T = 1100 oC).
Fig. 4. Relative intensity of hydrogen ion induced ionoluminescence (defined as the ratio of ionoluminescence recorded for the Si ion implanted sample to the ionoluminescence recorded for the unimplanted sample) versus hydrogen ion (H2 þ ) energy.
implanted SiO2 increases at higher ion energy, and becomes equal to the IL from pure SiO2 at the highest hydrogen ion energy used, 100 keV. The projected range of H+ versus ion energy was simulated using SRIM 2006 code [18]. The ion beam incidence angle of 55°
was taken into account in the simulation. According to [10], on entering the target material H2 þ dissociates into two protons each with half of the ion energy. Hence, simulations were carried out for proton energies ranging from 11 to 50 keV, which corresponds to H2+ energies ranging from 22 up to 100 keV. The simulated projected range of 11 keV protons (at incidence angle 55°) corresponds to the thickness of the layer enriched with Si atoms, indicating that protons of this energy only excite the silicon-rich layer. Once the proton energy has been increased to 50 keV their projected range exceeds the thickness of the whole SiO2 layer, and the undoped part of the SiO2 layer becomes excited as well. Observed changes of IL intensity at different energies of hydrogen ions suggests that the silicon nanocrystals attenuate the ionoluminescence originating from the unmodified SiO2 layer. However, a small excess of silicon content above stoichiometry boosts the blue and green–yellow luminescence from SiOx (x < 2). 4. Conclusions Electroluminescence and ionoluminescence of SiO2 containing Si nanocrystals are apparently generated within the SiO2 matrix. The blue luminescence (460 nm) is most probably related to oxygendeficient centers (ODC) identified in the catodoluminescence experiments [13–16]. On the other hand, the green–yellow luminescence is likely to be related to the formation of larger silicon aggregates in
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nonstoichiometric silica [16]. Hydrogen ion bombardment effectively excites the blue luminescence from ODCs. However, the ion beam induced excitation of the green–yellow luminescence is less effective compared to the electroluminescence. Hydrogen ion irradiation does not lead to the red/infrared luminescence of Si nanocrystals, in contrast with the electroluminescence experiments. Moreover, the formation of Si nanocrystals attenuates the ionoluminescence of SiOx, while Si nanocrystals facilitate the formation of favorable conditions for field injection of electrons from the surface electrode into SiOx layer. A soft, exponential current–voltage characteristic is caused by the electrons injected through the nano-Si-related quantum well network located near the interface with the electrode. In the case of the pure dielectric SiO2 layer, the required electric field is that so high that it causes electric breakdown. Our study leads to the conclusion that electroluminescence of Si-implanted SiO2 is related to defects created within the SiO2 matrix, rather than to the presence of nanocrystals. However, the nanocrystal-rich region may strongly affect electrical properties of the Si-implanted SiO2 layer in the MOS structure. Acknowledgement This work has been supported by the Polish Ministry for Science and Higher Education through a Research Grant 3T11B 065 29.
References [1] A. Luna-Lopez, M. Aceves-Mijares, O. Malik, Sensors Actuators A 132 (2006) 278. [2] D. Muller, P. Knapek, J. Faure, B. Prevot, J.J. Grob, B. Hönerlage, I. Pelant, Nucl. Instr. and Meth. Phys. Res. B 148 (1999) 997. [3] J. Valenta, N. Lalic, J. Linross, Opt. Mater. 17 (2001) 45. [4] L. Dal Negro, L. Pavesi, G. Pucker, G. Franzo, F. Priolo, Opt. Mater. 17 (2001) 41. [5] J. De La Torre, A. Souifi, A. Poncet, C. Busseret, M. Lemiti, G. Bremond, G. Guillot, O. Gonzalez, B. Garrido, J.R. Morante, C. Bonafos, Physica E 16 (2003) 326. [6] B. Garrido, M. Lopez, A. Perez-Rodriguez, C. Garcia, P. Pellegrino, R. Ferrera, J.A. Moreno, J.R. Morante, C. Bonafos, M. Carrada, A. Claverie, J. De la Torre, A. Souifi, Nucl. Instr. and Meth. Phys. Res. B 216 (2004) 213. [7] G. Gawlik, J. Jagielski, Vacuum 81 (2007) 1371. [8] G. Gawlik, J. Jagielski, Nucl. Instr. and Meth. Phys. Res. B 266 (2008) 1307. [9] J. De La Torre, A. Soui, A. Poncet, C. Busseret, M. Lemiti, G. Bremond, G. Guillot, O. Gonzalez, B. Garrido, J.R. Morante, C. Bonafos, Physica E 16 (2003) 326. [10] R.L. Hines, Phys. Rev. 120 (1960) 1626. [11] L. Rebohle, J. von Boranyi, H. Fröb, T. Gebel, M. Helm, W. Skorupa, Nucl. Instr. and Meth. Phys. Res. B 188 (2002) 28. [12] S.I. Kononenko, O.V. Kalantaryan, V.I. Muratov, V.P. Zhurenko, Radiat. Meas. 42 (2007) 751. [13] S.I. Kononenko, O.V. Kalantaryan, V.I. Muratov, V.P. Zhurenko, Nucl. Instr. and Meth. Phys. Res. B 246 (2006) 340. [14] A.N. Trukhin, A. Janson, H.-J. Fitting, T. Barfels, B. Schmidt, J. Non-Cryst. Solids 331 (2003) 91. [15] A. Zatsepin, V.S. Kortov, H.-J. Fitting, J. Non-Cryst. Solids 351 (2005) 869. [16] H.-J. Fitting, T. Barfels, A.N. Trukhin, B. Schmidt, J. Non-Cryst. Solids 279 (2001) 51. [17] R. Salh, A. von Czarnowski, H.-J. Fitting, J. Non-Cryst. Solids 353 (2007) 546. [18] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Ranges of Ions in Solids, Vol. 1, Pergamon Press, 1985.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Proton beam induced luminescence of silicon dioxide implanted with silicon Grzegorz Gawlik a,*, Jacek Jagielski a,b, Anna Stonert b, Renata Ratajczak b a b
Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warszawa, Poland The Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock/Swierk, Poland
a r t i c l e
i n f o
Article history: Available online 27 May 2009 PACS: 78.55.Qr 78.60.Fi 78.55.m 78.60.Hk 61.46.Hk 61.72.up Keywords: Ion implantation Light emission Silicon Silica Nanocrystals
a b s t r a c t Light emission from a silicon dioxide layer enriched with silicon has been studied. Samples used had structures made on thermally oxidized silicon substrate wafers. Excess silicon atoms were introduced into a 250-nm-thick silicon dioxide layer via implantation of 60 keV Si+ ions up to a fluence of 2 1017 cm2. A 15-nm-thick Au layer was used as a top semitransparent electrode. Continuous blue light emission was observed under DC polarization of the structure at 8–12 MV/cm. The blue light emission from the structures was also observed in an ionoluminescence experiment, in which the light emission was caused by irradiation with a H2+ ion beam of energy between 22 and 100 keV. In the case of H2+, on entering the material the ions dissociated into two protons, each carrying on average half of the incident ion energy. The spectra of the emitted light and the dependence of ionoluminescence on proton energy were analyzed and the results were correlated with the concentration profile of implanted silicon atoms. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon dioxide is commonly used in microelectronics technology as an insulator. The formation of silicon nanocrystals in a SiO2 matrix allows generation of new interesting luminescent properties, which make possible production of MOS-like silicon electroluminescent devices. Several experiments performed until now on Si-implanted SiO2 have shown that photoluminescence leads mainly to red light emission [1–6], whereas electroluminescence can also produce blue emission [2,7,8]. The aim of this work is to determine the role of nano-Si precipitates and the SiO2 matrix in the generation of light via an electroluminescence process in MOS (Metal Oxide Semiconductor) structures composed of SiO2 layers with embedded silicon nanocrystals. 2. Experimental MOS structures with silicon nanocrystals embedded in the SiO2 layer were used in this work. The structures were designed in such a way as to exhibit good photoluminescent, electroluminescent and ionoluminescent performance in the same sample. Silicon
* Corresponding author. Tel.: +48 604 618 009. E-mail address: [email protected] (G. Gawlik). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.05.046
dioxide layers with thickness of 250 nm were produced by thermal oxidation of silicon wafers in wet oxygen at a temperature of 1100 °C. Silicon atoms were introduced into the SiO2 layer by ion implantation using 60 keV silicon ions to a fluence of 2 1017 Si+/cm2. The energy of the silicon ions was chosen to produce a silicon-rich region that is thinner than the SiO2 layer itself and located close to the surface. Silicon nanocrystals were formed by annealing the implanted structures at temperature of 1100 oC for 4 h in a nitrogen atmosphere. A layer of Au of about 15 nm thickness was deposited on the SiO2 surface as a semitransparent top electrode. Such an electrode has a 2 mm 6 mm rectangular shape with rounded corners. Depth distribution of the implanted silicon atoms in the SiO2 layer was determined by Rutherford Backscattering (RBS). A typical RBS spectrum for 1.7 MeV He ions is shown in Fig. 1. Analysis confirmed that excess silicon atoms are located near the surface of the SiO2 layer, whereas deeper down the silica layer remained undoped. Formation of silicon nanocrystals has been verified by detection of the specific wide band photoluminescence (PL) centered about 785 nm when excited by the 325-nm UV laser [9]. Current–voltage (I–V) characteristics and electroluminescence (EL) spectra were measured using a typical laboratory electrical measurement setup, with light detection through an optical fiber entry placed in front of sample surface. MOS structures were polarized using a DC power supply up to about 12 MV/cm. I–V characteristics and EL spectra were measured simultaneously.
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Fig. 1. RBS spectrum of 250-nm-thick SiO2 layer with excess silicon ion-implanted at energy 60 keV and fluence 2 1017 Si+/cm2 and annealed in N2 atmosphere for 4 h at a temperature of 1100 oC.
Ionoluminescence (IL) was excited with a H2+ ion beam in the ion implanter target chamber equipped with a vacuum glass window and a vacuum fiber optic feedthrough. Irradiation was done with a H2+ beam of ranging in energy from 22 to 100 keV. The H2+ ions dissociated on entering the material into two protons, each carrying on average half of the incident ion energy [10]. The current density of the hydrogen ion beam was between 5 and 15 lA/cm2, while the sample was placed at an angle of 55o with respect to the direction of the ion beam direction. The optical axis of the light detection system was aligned normal to the beam direction. The IL intensity dependence on hydrogen ion energy was determined relative to the IL of the pure SiO2 layer. Both samples were irradiated with H2+ ion beam simultaneously. The light intensity was measured by a CCD digital camera through the glass vacuum window. Relative light intensity was defined as a ratio of signals recorded from Si-implanted and pure SiO2 layers. Light spectra were recorded using the Hamamatsu C10083CAH spectrometer equipped with a CCD image sensor, with spectral response range from 320 to 1000 nm. The light was collected and guided into the spectrometer entry slit through an optical fiber. 3. Results and discussion The current–voltage characteristics of the MOS structure with silicon nanocrystals embedded in the SiO2 layer are different from those of the current–voltage of MOS with pure SiO2. Electrons are injected from the surface electrode into the dielectric layer under the influence of a strong electric field applied to the MOS structure. A few of the mechanisms of such injection which are possible are, for example, Fowler–Nordheim tunneling through a thin potential barrier, or tunneling into localized states situated close to the surface [11]. The exponential dependence of DC current on applied voltage has been observed, as shown in Fig. 2. Since the electric current can be easy controlled by applied voltage, observation of steady electroluminescence under DC current flow through MOS structure containing nanocrystals are possible. PL spectra of the investigated samples excited with 325 nm UV light consist of a wide band with a maximum at about 785 nm (Fig. 3(a)). This well-known effect confirms the formation of Si nanocrystals in the SiO2 layer [9]. Green or blue light emission was not observed under UV 325 nm light excitation in this study. Visible light emission from the MOS structure containing nanoSi clusters was also observed under polarization of the structure with an electric field of 7–12 MV/cm (Fig. 3(b)). Strong electric field makes it possible to inject electrons from the surface electrode into
Fig. 2. Current–voltage characteristic of the MOS structure with silicon nanocrystals in the SiO2 layer.
the dielectric layer. The observed EL spectra differ significantly from those of the PL. A strong blue band luminescence (B), with a maximum at about 470 nm was observed, along with a green– yellow luminescence (G) with a maximum at about 560 nm. The latter band contained a long red tail extending down to about 800 nm. The difference between the PL and EL spectra suggests that EL generation is caused by different recombination centers than those of PL emission. Ionoluminescence was observed under hydrogen ion irradiation of Si-implanted and unimplanted SiO2 layers (Fig. 3(c)). In both cases the emission of a strong blue band, with maximum located at about 460 nm, was observed. IL spectra of pure the SiO2 layer are very similar to those reported in [12] and [13]. Apart from the main blue band, a weak red tail down to about 800 nm was also observed. In the case of Si-enriched silica, the shape of the IL band is strongly asymmetrical, with a distinct green/red tail complementing the main blue (460 nm) band. Clearly, the excess silicon induces an IL emission in a wide green–yellow–red band, ranging from about 460 to 660 nm (Fig. 3(c) – curve III). The spectral range of this nano-Si-related IL band is similar to that of the green–yellow band observed in electroluminescent experiments (Fig. 3(b)). This experiment clearly indicates that the hydrogen ion beam can excite the same recombination centers as those of the electric current in the EL experiments. Very similar light emission bands were observed by Trukhin et al. [14], Zatsepin et al. [15], Fitting et al. [16] and Salh et al. [17] in catodoluminescence (CL) experiments performed on SiO2 based materials like quartz, glass or SiOx layers on a Si substrate. Blue band luminescence centered at about 460 nm can be attributed to siliconrelated oxygen-deficient centers (SiODC) [14,15], whereas the red band luminescence centered at about 650 nm can be due to the non-bridging oxygen center (NBOHC) [14]. The mechanism of the green–yellow luminescence is not clear. The presence of such a band in SiO2 based materials has been reported by other authors [14–17]. Concerning the green luminescence, there are suggestions that it may be due to self-trapped excitons (STE) [16]. However this effect has only been observed at low temperatures. The yellow luminescence is usually associated with the higher silicon aggregates like hexamer rings. These are a first step in the formation of nano-Si precipitates in nonstoichiometric SiOx (x < 2) [17]. The relative intensity of the IL emission from Si-implanted and pure SiO2 layers depends on the hydrogen ion energy (Fig. 4). Intensity of the light emission from silicon-enriched SiO2 upon hydrogen ion bombardment at energies below 100 keV is essentially lower than that from pure silica, and decreases with decreasing hydrogen ion energy. The relative intensity of ionoluminescence from Si ion
G. Gawlik et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2579–2582
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Fig. 3. Photoluminescence (PL) spectrum excited with 325 nm UV laser light (a), electroluminescence (EL) spectrum excited with DC polarization of the MOS structure (b), ionoluminescence (IL) spectra of pure SiO2 (I) and SiO2 with Si nanocrystals (II) excited with 100 keV hydrogen ion beam (c). All spectra were measured using the same MOS sample with silicon ion-implanted SiO2 layer (Si+ ion energy E = 60 keV; fluence D = 21017 Si+/cm2; post-implantation annealing in N2 atmosphere for 4 h at temperature T = 1100 oC).
Fig. 4. Relative intensity of hydrogen ion induced ionoluminescence (defined as the ratio of ionoluminescence recorded for the Si ion implanted sample to the ionoluminescence recorded for the unimplanted sample) versus hydrogen ion (H2 þ ) energy.
implanted SiO2 increases at higher ion energy, and becomes equal to the IL from pure SiO2 at the highest hydrogen ion energy used, 100 keV. The projected range of H+ versus ion energy was simulated using SRIM 2006 code [18]. The ion beam incidence angle of 55°
was taken into account in the simulation. According to [10], on entering the target material H2 þ dissociates into two protons each with half of the ion energy. Hence, simulations were carried out for proton energies ranging from 11 to 50 keV, which corresponds to H2+ energies ranging from 22 up to 100 keV. The simulated projected range of 11 keV protons (at incidence angle 55°) corresponds to the thickness of the layer enriched with Si atoms, indicating that protons of this energy only excite the silicon-rich layer. Once the proton energy has been increased to 50 keV their projected range exceeds the thickness of the whole SiO2 layer, and the undoped part of the SiO2 layer becomes excited as well. Observed changes of IL intensity at different energies of hydrogen ions suggests that the silicon nanocrystals attenuate the ionoluminescence originating from the unmodified SiO2 layer. However, a small excess of silicon content above stoichiometry boosts the blue and green–yellow luminescence from SiOx (x < 2). 4. Conclusions Electroluminescence and ionoluminescence of SiO2 containing Si nanocrystals are apparently generated within the SiO2 matrix. The blue luminescence (460 nm) is most probably related to oxygendeficient centers (ODC) identified in the catodoluminescence experiments [13–16]. On the other hand, the green–yellow luminescence is likely to be related to the formation of larger silicon aggregates in
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G. Gawlik et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 2579–2582
nonstoichiometric silica [16]. Hydrogen ion bombardment effectively excites the blue luminescence from ODCs. However, the ion beam induced excitation of the green–yellow luminescence is less effective compared to the electroluminescence. Hydrogen ion irradiation does not lead to the red/infrared luminescence of Si nanocrystals, in contrast with the electroluminescence experiments. Moreover, the formation of Si nanocrystals attenuates the ionoluminescence of SiOx, while Si nanocrystals facilitate the formation of favorable conditions for field injection of electrons from the surface electrode into SiOx layer. A soft, exponential current–voltage characteristic is caused by the electrons injected through the nano-Si-related quantum well network located near the interface with the electrode. In the case of the pure dielectric SiO2 layer, the required electric field is that so high that it causes electric breakdown. Our study leads to the conclusion that electroluminescence of Si-implanted SiO2 is related to defects created within the SiO2 matrix, rather than to the presence of nanocrystals. However, the nanocrystal-rich region may strongly affect electrical properties of the Si-implanted SiO2 layer in the MOS structure. Acknowledgement This work has been supported by the Polish Ministry for Science and Higher Education through a Research Grant 3T11B 065 29.
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