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Electroluminescence properties of SiOx layers implanted with rare earth ions
Nuclear Instruments and Methods in Physics Research B 216 (2004) 222–227 www.elsevier.com/locate/nimb
Electroluminescence properties of SiOx layers implanted with rare earth ions a, F. Priolo a, A. Irrera a, M. Miritello a, D. Pacifici a, G. Franzo F. Iacona b,*, D. Sanfilippo c, G. Di Stefano c, P.G. Fallica c a
INFM and Dipartimento di Fisica e Astronomia, Universita di Catania, Via S. Sofia 64, I-95123 Catania, Italy b CNR-IMM, Sezione di Catania, Stradale Primosole 50, I-95121 Catania, Italy c STMicroelectronics, Stradale Primosole 50, I-95121 Catania, Italy
Abstract In this work we have studied the structural, electrical and optical properties of MOS devices, where the dielectric layer consists of a SiOx (x < 2) thin film prepared by plasma enhanced chemical vapor deposition and implanted with rare earth ions. As deposited SiOx films were annealed at high temperature (>1000 C) to induce the separation of the Si and SiO2 phases with the formation of Si nanocrystals embedded in the insulating matrix. Devices based on this system present a strong light emission at room temperature at a wavelength of about 900 nm. Devices emitting at different wavelengths have been fabricated by implanting SiOx films with Er or Tm. Devices based on Er-doped Si nanoclusters film exhibit an intense 1.54 lm room temperature electroluminescence (EL). We have calculated the excitation crosssection for Er ions in presence of Si nanoclusters under electrical pumping and the value is 1 · 1014 cm2 , comparable to the value found for the electrical excitation of undoped Si nanocrystals. Finally, devices based on Tm-doped Si nanoclusters exhibit two EL peaks at 0.78 and 1.7 lm. 2003 Elsevier B.V. All rights reserved. PACS: 78.60.Fi; 78.67.Bf; 76.30.Kg Keywords: Si nanocrystals; Electroluminescence; Rare earth; Optoelectronic devices
1. Introduction Silicon has for a long time been considered unsuitable for optoelectronic applications. Due to the indirect nature of its energy band gap, bulk silicon is indeed a highly inefficient light source. Many efforts have been devoted towards the
*
Corresponding author: Tel.: +39-095-591212; fax: +39-0957139154. E-mail address: [email protected] (F. Iacona).
research of different systems compatible with Si technology and able to act as light emitters [1]. Quantum confinement in Si nanostructures and rare earth (RE) doping of silicon have dominated the scientific scenario of silicon-based microphotonics. In particular, due to their promising optical properties, different kinds of silicon nanostructures, i.e. porous Si [2,3], Si nanocrystals (nc) embedded in SiO2 [4–12] and Si/SiO2 superlattices [13–15], have been widely studied. Si nc dispersed in SiO2 have attracted a great attention, due to their high stability and to their full compatibility
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.038
A. Irrera et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 222–227
with Si technology. Si nc are characterized by an energy band gap which is enlarged with respect to bulk Si, and an intense room temperature photoluminescence (PL) in the visible-near IR range can be obtained [12]. Si nc embedded in SiO2 are produced with several different techniques, such as ion implantation [5,7], chemical vapor deposition [12,16], sputter deposition and laser ablation [17]. Recently, the interest towards this material is greatly increased due to the first observation of light amplification in Si nanostructures [18]. Moreover by implanting this system with RE, for instance with erbium or thulium ions, it is possible to obtain a wide range of materials potentially suitable for future near-infrared light sources and optical amplifiers. Indeed it has been demonstrated that Si nanoclusters act as efficient sensitizers for the rare earth ions [16,19], since excited Si nanoclusters promptly transfer their energy to the nearby RE ions, which can then decay radiatively emitting photons, at 1.54 lm in the case of Er ions and at 0.78 lm and 1.7 lm in the case Tm ions. We have used these systems based on REimplanted Si nanoclusters to fabricate electroluminescent devices operating at room temperature. In this work we report the results of the structural and optical characterization of the devices and their implications for optoelectronic applications.
2. Experimental Substoichiometric SiOx (x < 2) films, 75 nm thick, with different silicon concentrations (39, 42 and 46 at.%) were deposited on top of low resistivity p-type Si substrates by plasma enhanced chemical vapor deposition. After deposition the films were annealed at high temperature (in the range 1100–1250 C) to induce the separation of the Si and SiO2 phases with the formation of Si nc embedded in the insulating matrix. The annealed SiOx films have been used as the active layer in light emitting devices [20,21]. To fabricate electroluminescent devices operating at different wavelengths, as deposited SiOx films (46 at.% Si) were also implanted with Er or Tm ions to a dose of respectively 7 · 1014 cm2 and 1 · 1015 cm2 ; the energy was chosen in order to locate the rare earth
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profile in the middle of the dielectric layer. After the implantation step, the samples were annealed at 900 C for 1 h in N2 atmosphere in order to activate the implanted ions and to remove the damage due to the implant. This annealing temperature induces the formation of Si nc with a mean radius lower than 1 nm or amorphous Si clusters. The RE-implanted SiOx films were then used as dielectric layers in metal-oxide-semiconductor (MOS) structures [22]. Note finally that, as a consequence of the use of N2 O as precursor gas, in the SiOx films there is a nitrogen content of 10 at.%. Even if a detailed study is still in progress, we expect, on the basis of the comparison with Si nc synthesized by Si ion implantation in thermally grown SiO2 (i.e. in a nitrogen-free system), that nitrogen does not influence in an appreciable way the luminescence properties of the system. The structural properties of these devices have been studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM analyses were carried out with a Stereoscan 250 Mk3 Cambridge Instrument. The TEM analyses were carried out with a 200 kV Jeol 2010 FX microscope. Electroluminescence (EL) spectra were taken by biasing the device with a square pulse at a frequency of 55 Hz (for Si nc) or 11 Hz (for RE) by using a fast Agilent Pulse Generator. EL signals were analyzed by a single grating monochromator and detected by a photomultiplier tube or by a liquid N2 -cooled Ge detector. Spectra were recorded with a lock-in amplifier. All spectra have been corrected for the detector response. Low-temperature measurements were performed by using a closed-cycle liquid He cooler system with the sample kept in vacuum.
3. Results and discussion The structure of a MOS device based on Si nc is shown in the SEM micrograph of Fig. 1. Aluminium-based contacts, defined as rings, were made to a n-type polysilicon film and a p-type Si substrate acting as conducting layers. The inset (a) is a cross-sectional TEM micrograph obtained in the zone of the contact to the polysilicon layer and
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and 46 at.% Si (annealed at 1100 C) [20]. From the analysis of the emission coming from the different devices under a wide range of excitation conditions it is possible to conclude that the device exhibiting the best performance (i.e. the highest EL intensity and the lowest operating voltage) is based on a SiOx matrix containing 46 at.% of Si and annealed at 1100 C [20]. This careful balance between Si concentration and annealing temperature implies the formation of a very dense distribution of Si nc having the proper size (about 1 nm) for light emission; this situation also strongly contributes to a high electrical conduction in the SiOx layer, and therefore to a good electrical injection in Si nc. In Fig. 2 the room temperature EL spectrum measured by forward biasing with a current density of 6.5 A/cm2 a device based on a SiOx layer with 46 at.% of Si and annealed at 1100 C is reported. Note that the emission is centered at about 850 nm; this relatively high wavelength means that we are mainly exciting the largest Si nc (characterized by a smaller bandgap) present in the sample. In fact, to excite the Si nc with a smaller size (i.e. larger bandgap) and then to observe a blueshift of the EL peak it is necessary to increase the energy of hot electrons whose impact excites
Fig. 1. Scanning electron microscopy (SEM) image of a device based on Si nc. Inset (a) cross-sectional TEM view of the zone of the device with the contact to the polysilicon layer; inset (b) cross-sectional TEM view of the metal-free central area.
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the layers sequence is clearly visible. From the top we observe in sequence the metallization layer, the poly-Si film, the SiOx active region and the Si substrate. In Fig. 1 is also visible a metal-free circular central area for the exit of the light; the sequence of the layers in this region from the top (poly-Si, SiOx film, Si substrate) is reported in the cross-section TEM micrograph shown in the inset (b). As a first step, we have optimized the EL performances of the devices based on Si nc, by changing the Si concentration in the SiOx layer and the annealing temperature. Most of the investigations have been done on SiOx films containing 39 and 42 at.% Si (annealed at 1250 C)
EL intensity (a.u.)
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Fig. 2. Room temperature EL spectrum of a Si nc MOS device obtained with a current density of 6.5 A/cm2 .
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density is reported. The EL signal increases less than linearly and tends to a saturation by increasing the current; this suggests that we are exciting almost all the Er ions. We have studied also the excitation and deexcitation properties of this system. In Fig. 4(a), the time evolution of the EL signal at the device switch on is reported for a current density of 0.15 A/cm2 . At time t ¼ 0 the voltage is switched on and the signal is seen to saturate in a characteristic time son of 77 ls. Fig. 4(b) reports the time-decay of the 1.54 lm EL signal at room temperature and for the same current density of 0.15 A/cm2 . After the voltage switch-off, the EL signal goes to zero with a lifetime s of 660 ls. We have measured also the lifetime of the PL signal at 1.54 lm, by illuminating the device with the 488 nm line of an Ar laser, by finding a value of 660 ls [22]; this suggests that the excited sites and the de-excitation mechanisms are not dependent on the kind of pumping. By using the son and s values reported in Fig. 4 it is possible to calculate the excitation cross-section r under electrical pumping for Er ions in presence of Si nanoclusters. In fact, the rate 1 of excitation R ¼ s1 can be estimated to be on s 1 11 470 s and since R ¼ r/e , being /e the electron flux, it is possible to estimate for the excitation cross-section r a value of 1 · 1014 cm2 . This value is comparable to the value found for the
1.5 1.6 Wavelength (µm)
1.7
Fig. 3. Room temperature EL spectrum of Er-doped Si nanoclusters MOS device obtained with a current density of 20 A/cm2 . In the inset is reported the EL intensity at 1.54 lm as a function of the current density at room temperature.
Normalized EL intensity (a.u.)
EL intensity (a.u.)
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300 K 20 A/cm 2
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the nc and this is possible only by increasing the applied voltage [20]. By using ion implantation it is possible to change the emission wavelength of these devices. In fact in RE-implanted Si nc a very efficient energy transfer from the excited nc to the RE occurs, and, as a result, a strong photoluminescence at wavelengths corresponding to the de-excitation of the RE ions is observed [19]. To explore the potentialities of RE-doped SiOx layers as systems for optoelectronic applications, we have studied the EL properties of Er and Tm-doped Si nanoclusters. In Fig. 3 we have reported the room temperature EL spectrum obtained by forward biasing a device based on Er-doped Si nanoclusters with a current density of 20 A/cm2 . The spectrum shows the typical features of Er emission with a main peak centered at 1.54 lm due to the transitions from the Er first excited multiplet 4 I13=2 to the ground state 4 I15=2 . No relevant signals coming from Si nanoclusters or due to the matrix have been observed after Er implantation. It should be noted that these devices are very stable and can work for several days without deterioration. In the inset of Fig. 3 the room temperature EL intensity at 1.54 lm as a function of the current
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(b)
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Fig. 4. (a) Time evolution of the room temperature EL signal at 1.54 lm at the device switch on for a current density of 0.15 A/cm2 . (b) Room temperature EL time decay curve at 1.54 lm measured at a current density of 0.15 A/cm2 .
A. Irrera et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 222–227
electrical excitation of Si nc alone, which is 4 · 1014 cm2 [21] and this constitutes a strong indication that Si nc play an active role in Er ions excitation also under electrical pumping. Furthermore the value of r under electrical pumping is two orders of magnitude higher than the effective excitation cross-section of Er ions through Si nc under optical pumping at 488 nm [16], and therefore is possible to conclude that the electrical pumping is more efficient than optical pumping. Even if Er is the most studied RE in Si-based materials, due to the outstanding importance of its emission at 1.54 lm for the telecommunication field, it has been recently demonstrated that strong PL signal can be observed also by doping Si nc with different RE, such as Tm, Yb, Nd, Tb [19,23]. We have therefore investigated the EL properties of Tm-doped Si nanoclusters. Devices based on Tm-doped Si nanoclusters exhibit two different EL peaks at 0.78 and 1.7 lm, due respectively to the transitions 3 F4 fi 3 H6 and 3 H4 fi 3 H6 . In Fig. 5 we have reported the EL peak at 0.78 lm as a function of temperature in the range 50–300 K and at a fixed current density of 15 A/cm2 . The 0.78 lm signal intensity increases with decreasing temperature and the EL signal at 300 K is about a factor of 3 smaller than the value at 50 K; this suggests
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0.1 λ rev = 0.78 µm
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Fig. 6. EL intensity of Tm-doped Si nanoclusters as a function of the current density at a temperature of 50 K.
that weak non-radiative de-excitation processes are present in this system. Fig. 6 shows the EL intensity at 0.78 lm, as a function of the current density measured at 50 K. It is possible to note that the EL signal at 0.78 lm increases by increasing the current density. The behaviour is not linear and tends to saturate by increasing the current. The excitation mechanism of Tm ions is probably due to impact by energetic carriers. Tm can be excited directly by hot carrier impact or through energy transfer from Si nanoclusters excited by hot carriers; the two above situations are not distinguishable. In both cases, however, Si nanoclusters play an active role in the RE excitation process, since they give a strong contribution to the high electrical conduction of this system.
4. Conclusions
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750 800 Wavelength (nm)
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Fig. 5. EL spectra of Tm-doped Si nanoclusters device at different temperatures (50–300 K) for a fixed current density of 15 A/cm2 .
In conclusion, we have studied the structural and optical properties of MOS devices where the dielectric layer consists of a substoichiometric SiOx (x < 2) thin film grown by plasma enhanced chemical vapor deposition. We have demonstrated that the devices with silicon nanocrystals are efficient light sources at room temperature. It is necessary to reach a compromise between a high Si
A. Irrera et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 222–227
content, which is favourable for the electrical properties, and the Si nc mean size, which determines the optical properties. We have used the SiOx films with the best EL properties to fabricate also RE-implanted silicon nanoclusters devices. We have studied the EL properties of light emitting devices based on Er-implanted Si nanoclusters. The devices are very stable and exhibit a strong EL signal at 1.54 lm at room temperature. Finally, we have also studied the EL properties of Tm-implanted Si nanoclusters. With this system it is possible to change the emission wavelength of the devices, in fact we have observed at room temperature two EL peaks respectively at 0.78 and 1.7 lm. These results open the way to new interesting applications in optoelectronics, such as electrically driven optical amplifiers or light sources.
Acknowledgements The authors want to thank A. La Mantia, N. Marino, A. Spada, A. Marino and S. Pannitteri for their expert technical collaboration. This work is supported by the EU project SINERGIA and by the project FIRB financed by MIUR.
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[5] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [6] J.G. Zhu, C.W. White, J.D. Budai, S.P. Withrow, Y. Chen, J. Appl. Phys. 78 (1995) 4386. [7] K.S. Min, K.V. Shcheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 69 (1996) 2033. [8] M.L. Brongersma, A. Polman, K.S. Min, E. Boer, T. Tambo, H.A. Atwater, Appl. Phys. Lett. 72 (1998) 2577. [9] V.I. Klimov, Ch. Schwarz, D. McBranch, C.W. White, Appl. Phys. Lett. 73 (1998) 2603. [10] T. Shimizu-Iwayama, N. Kunumado, D.E. Hole, P. Townsend, J. Appl. Phys. 83 (1998) 6018. [11] J. Linnros, N. Lalic, A. Galeckas, V. Grivickas, J. Appl. Phys. 86 (1999) 6128. [12] F. Iacona, G. Franz o, C. Spinella, J. Appl. Phys. 87 (2000) 1295. [13] Z.H. Lu, D.J. Lockwood, J.-M. Baribeau, Nature 378 (1995) 258; D.J. Lockwood, Z.H. Lu, J.M. Baribeau, Phys. Rev. Lett. 96 (1996) 539. [14] V. Vinciguerra, G. Franz o, F. Priolo, F. Iacona, C. Spinella, J. Appl. Phys. 87 (2000) 8165. [15] P. Photopoulos, A.G. Nassiopoulou, D.N. Kouvatsos, A. Travlos, J. Appl. Phys. 76 (2000) 3588. [16] F. Priolo, G. Franz o, D. Pacifici, V. Vinciguerra, F. Iacona, A. Irrera, J. Appl. Phys. 89 (2001) 264. [17] E. Werwa, A.A. Seraphin, L.A. Chin, C. Zhou, K.D. Kolenbrander, Appl. Phys. Lett. 64 (1994) 1821. [18] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franz o, F. Priolo, Nature 408 (2000) 440. [19] G. Franz o, V. Vinciguerra, F. Priolo, Appl. Phys. A 69 (1999) 3. [20] G. Franz o, A. Irrera, E.C. Moreira, M. Miritello, F. Iacona, D. Sanfilippo, G. Di Stefano, P.G. Fallica, F. Priolo, Appl. Phys. A 74 (2002) 1. [21] A. Irrera, D. Pacifici, M. Miritello, G. Franz o, F. Priolo, F. Iacona, D. Sanfilippo, G. DiStefano, P.G. Fallica, Appl. Phys. Lett. 81 (2002) 1866. [22] F. Iacona, D. Pacifici, A. Irrera, M. Miritello, G. Franz o, F. Priolo, D. Sanfilippo, G. DiStefano, P.G. Fallica, Appl. Phys. Lett. 81 (2002) 3242. [23] K. Watanabe, H. Tamaoka, M. Fujii, S. Hayashi, J. Appl. Phys. 92 (2002) 4001.
Electroluminescence properties of SiOx layers implanted with rare earth ions a, F. Priolo a, A. Irrera a, M. Miritello a, D. Pacifici a, G. Franzo F. Iacona b,*, D. Sanfilippo c, G. Di Stefano c, P.G. Fallica c a
INFM and Dipartimento di Fisica e Astronomia, Universita di Catania, Via S. Sofia 64, I-95123 Catania, Italy b CNR-IMM, Sezione di Catania, Stradale Primosole 50, I-95121 Catania, Italy c STMicroelectronics, Stradale Primosole 50, I-95121 Catania, Italy
Abstract In this work we have studied the structural, electrical and optical properties of MOS devices, where the dielectric layer consists of a SiOx (x < 2) thin film prepared by plasma enhanced chemical vapor deposition and implanted with rare earth ions. As deposited SiOx films were annealed at high temperature (>1000 C) to induce the separation of the Si and SiO2 phases with the formation of Si nanocrystals embedded in the insulating matrix. Devices based on this system present a strong light emission at room temperature at a wavelength of about 900 nm. Devices emitting at different wavelengths have been fabricated by implanting SiOx films with Er or Tm. Devices based on Er-doped Si nanoclusters film exhibit an intense 1.54 lm room temperature electroluminescence (EL). We have calculated the excitation crosssection for Er ions in presence of Si nanoclusters under electrical pumping and the value is 1 · 1014 cm2 , comparable to the value found for the electrical excitation of undoped Si nanocrystals. Finally, devices based on Tm-doped Si nanoclusters exhibit two EL peaks at 0.78 and 1.7 lm. 2003 Elsevier B.V. All rights reserved. PACS: 78.60.Fi; 78.67.Bf; 76.30.Kg Keywords: Si nanocrystals; Electroluminescence; Rare earth; Optoelectronic devices
1. Introduction Silicon has for a long time been considered unsuitable for optoelectronic applications. Due to the indirect nature of its energy band gap, bulk silicon is indeed a highly inefficient light source. Many efforts have been devoted towards the
*
Corresponding author: Tel.: +39-095-591212; fax: +39-0957139154. E-mail address: [email protected] (F. Iacona).
research of different systems compatible with Si technology and able to act as light emitters [1]. Quantum confinement in Si nanostructures and rare earth (RE) doping of silicon have dominated the scientific scenario of silicon-based microphotonics. In particular, due to their promising optical properties, different kinds of silicon nanostructures, i.e. porous Si [2,3], Si nanocrystals (nc) embedded in SiO2 [4–12] and Si/SiO2 superlattices [13–15], have been widely studied. Si nc dispersed in SiO2 have attracted a great attention, due to their high stability and to their full compatibility
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.038
A. Irrera et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 222–227
with Si technology. Si nc are characterized by an energy band gap which is enlarged with respect to bulk Si, and an intense room temperature photoluminescence (PL) in the visible-near IR range can be obtained [12]. Si nc embedded in SiO2 are produced with several different techniques, such as ion implantation [5,7], chemical vapor deposition [12,16], sputter deposition and laser ablation [17]. Recently, the interest towards this material is greatly increased due to the first observation of light amplification in Si nanostructures [18]. Moreover by implanting this system with RE, for instance with erbium or thulium ions, it is possible to obtain a wide range of materials potentially suitable for future near-infrared light sources and optical amplifiers. Indeed it has been demonstrated that Si nanoclusters act as efficient sensitizers for the rare earth ions [16,19], since excited Si nanoclusters promptly transfer their energy to the nearby RE ions, which can then decay radiatively emitting photons, at 1.54 lm in the case of Er ions and at 0.78 lm and 1.7 lm in the case Tm ions. We have used these systems based on REimplanted Si nanoclusters to fabricate electroluminescent devices operating at room temperature. In this work we report the results of the structural and optical characterization of the devices and their implications for optoelectronic applications.
2. Experimental Substoichiometric SiOx (x < 2) films, 75 nm thick, with different silicon concentrations (39, 42 and 46 at.%) were deposited on top of low resistivity p-type Si substrates by plasma enhanced chemical vapor deposition. After deposition the films were annealed at high temperature (in the range 1100–1250 C) to induce the separation of the Si and SiO2 phases with the formation of Si nc embedded in the insulating matrix. The annealed SiOx films have been used as the active layer in light emitting devices [20,21]. To fabricate electroluminescent devices operating at different wavelengths, as deposited SiOx films (46 at.% Si) were also implanted with Er or Tm ions to a dose of respectively 7 · 1014 cm2 and 1 · 1015 cm2 ; the energy was chosen in order to locate the rare earth
223
profile in the middle of the dielectric layer. After the implantation step, the samples were annealed at 900 C for 1 h in N2 atmosphere in order to activate the implanted ions and to remove the damage due to the implant. This annealing temperature induces the formation of Si nc with a mean radius lower than 1 nm or amorphous Si clusters. The RE-implanted SiOx films were then used as dielectric layers in metal-oxide-semiconductor (MOS) structures [22]. Note finally that, as a consequence of the use of N2 O as precursor gas, in the SiOx films there is a nitrogen content of 10 at.%. Even if a detailed study is still in progress, we expect, on the basis of the comparison with Si nc synthesized by Si ion implantation in thermally grown SiO2 (i.e. in a nitrogen-free system), that nitrogen does not influence in an appreciable way the luminescence properties of the system. The structural properties of these devices have been studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM analyses were carried out with a Stereoscan 250 Mk3 Cambridge Instrument. The TEM analyses were carried out with a 200 kV Jeol 2010 FX microscope. Electroluminescence (EL) spectra were taken by biasing the device with a square pulse at a frequency of 55 Hz (for Si nc) or 11 Hz (for RE) by using a fast Agilent Pulse Generator. EL signals were analyzed by a single grating monochromator and detected by a photomultiplier tube or by a liquid N2 -cooled Ge detector. Spectra were recorded with a lock-in amplifier. All spectra have been corrected for the detector response. Low-temperature measurements were performed by using a closed-cycle liquid He cooler system with the sample kept in vacuum.
3. Results and discussion The structure of a MOS device based on Si nc is shown in the SEM micrograph of Fig. 1. Aluminium-based contacts, defined as rings, were made to a n-type polysilicon film and a p-type Si substrate acting as conducting layers. The inset (a) is a cross-sectional TEM micrograph obtained in the zone of the contact to the polysilicon layer and
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and 46 at.% Si (annealed at 1100 C) [20]. From the analysis of the emission coming from the different devices under a wide range of excitation conditions it is possible to conclude that the device exhibiting the best performance (i.e. the highest EL intensity and the lowest operating voltage) is based on a SiOx matrix containing 46 at.% of Si and annealed at 1100 C [20]. This careful balance between Si concentration and annealing temperature implies the formation of a very dense distribution of Si nc having the proper size (about 1 nm) for light emission; this situation also strongly contributes to a high electrical conduction in the SiOx layer, and therefore to a good electrical injection in Si nc. In Fig. 2 the room temperature EL spectrum measured by forward biasing with a current density of 6.5 A/cm2 a device based on a SiOx layer with 46 at.% of Si and annealed at 1100 C is reported. Note that the emission is centered at about 850 nm; this relatively high wavelength means that we are mainly exciting the largest Si nc (characterized by a smaller bandgap) present in the sample. In fact, to excite the Si nc with a smaller size (i.e. larger bandgap) and then to observe a blueshift of the EL peak it is necessary to increase the energy of hot electrons whose impact excites
Fig. 1. Scanning electron microscopy (SEM) image of a device based on Si nc. Inset (a) cross-sectional TEM view of the zone of the device with the contact to the polysilicon layer; inset (b) cross-sectional TEM view of the metal-free central area.
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the layers sequence is clearly visible. From the top we observe in sequence the metallization layer, the poly-Si film, the SiOx active region and the Si substrate. In Fig. 1 is also visible a metal-free circular central area for the exit of the light; the sequence of the layers in this region from the top (poly-Si, SiOx film, Si substrate) is reported in the cross-section TEM micrograph shown in the inset (b). As a first step, we have optimized the EL performances of the devices based on Si nc, by changing the Si concentration in the SiOx layer and the annealing temperature. Most of the investigations have been done on SiOx films containing 39 and 42 at.% Si (annealed at 1250 C)
EL intensity (a.u.)
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800 900 Wavelength (nm)
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Fig. 2. Room temperature EL spectrum of a Si nc MOS device obtained with a current density of 6.5 A/cm2 .
A. Irrera et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 222–227
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100
0 1.4
density is reported. The EL signal increases less than linearly and tends to a saturation by increasing the current; this suggests that we are exciting almost all the Er ions. We have studied also the excitation and deexcitation properties of this system. In Fig. 4(a), the time evolution of the EL signal at the device switch on is reported for a current density of 0.15 A/cm2 . At time t ¼ 0 the voltage is switched on and the signal is seen to saturate in a characteristic time son of 77 ls. Fig. 4(b) reports the time-decay of the 1.54 lm EL signal at room temperature and for the same current density of 0.15 A/cm2 . After the voltage switch-off, the EL signal goes to zero with a lifetime s of 660 ls. We have measured also the lifetime of the PL signal at 1.54 lm, by illuminating the device with the 488 nm line of an Ar laser, by finding a value of 660 ls [22]; this suggests that the excited sites and the de-excitation mechanisms are not dependent on the kind of pumping. By using the son and s values reported in Fig. 4 it is possible to calculate the excitation cross-section r under electrical pumping for Er ions in presence of Si nanoclusters. In fact, the rate 1 of excitation R ¼ s1 can be estimated to be on s 1 11 470 s and since R ¼ r/e , being /e the electron flux, it is possible to estimate for the excitation cross-section r a value of 1 · 1014 cm2 . This value is comparable to the value found for the
1.5 1.6 Wavelength (µm)
1.7
Fig. 3. Room temperature EL spectrum of Er-doped Si nanoclusters MOS device obtained with a current density of 20 A/cm2 . In the inset is reported the EL intensity at 1.54 lm as a function of the current density at room temperature.
Normalized EL intensity (a.u.)
EL intensity (a.u.)
300
300 K 20 A/cm 2
EL intensity (a.u.)
the nc and this is possible only by increasing the applied voltage [20]. By using ion implantation it is possible to change the emission wavelength of these devices. In fact in RE-implanted Si nc a very efficient energy transfer from the excited nc to the RE occurs, and, as a result, a strong photoluminescence at wavelengths corresponding to the de-excitation of the RE ions is observed [19]. To explore the potentialities of RE-doped SiOx layers as systems for optoelectronic applications, we have studied the EL properties of Er and Tm-doped Si nanoclusters. In Fig. 3 we have reported the room temperature EL spectrum obtained by forward biasing a device based on Er-doped Si nanoclusters with a current density of 20 A/cm2 . The spectrum shows the typical features of Er emission with a main peak centered at 1.54 lm due to the transitions from the Er first excited multiplet 4 I13=2 to the ground state 4 I15=2 . No relevant signals coming from Si nanoclusters or due to the matrix have been observed after Er implantation. It should be noted that these devices are very stable and can work for several days without deterioration. In the inset of Fig. 3 the room temperature EL intensity at 1.54 lm as a function of the current
225
(b)
1.0
10
0
0.8 0.6
λ rev= 1.54 µm 0.15 A/cm2 RT
0.4
-1
10
0.2 τ on= 77 µs
0.0
0
1
τ = 660 µs
2
0 Time (ms)
1
2
3
Fig. 4. (a) Time evolution of the room temperature EL signal at 1.54 lm at the device switch on for a current density of 0.15 A/cm2 . (b) Room temperature EL time decay curve at 1.54 lm measured at a current density of 0.15 A/cm2 .
A. Irrera et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 222–227
electrical excitation of Si nc alone, which is 4 · 1014 cm2 [21] and this constitutes a strong indication that Si nc play an active role in Er ions excitation also under electrical pumping. Furthermore the value of r under electrical pumping is two orders of magnitude higher than the effective excitation cross-section of Er ions through Si nc under optical pumping at 488 nm [16], and therefore is possible to conclude that the electrical pumping is more efficient than optical pumping. Even if Er is the most studied RE in Si-based materials, due to the outstanding importance of its emission at 1.54 lm for the telecommunication field, it has been recently demonstrated that strong PL signal can be observed also by doping Si nc with different RE, such as Tm, Yb, Nd, Tb [19,23]. We have therefore investigated the EL properties of Tm-doped Si nanoclusters. Devices based on Tm-doped Si nanoclusters exhibit two different EL peaks at 0.78 and 1.7 lm, due respectively to the transitions 3 F4 fi 3 H6 and 3 H4 fi 3 H6 . In Fig. 5 we have reported the EL peak at 0.78 lm as a function of temperature in the range 50–300 K and at a fixed current density of 15 A/cm2 . The 0.78 lm signal intensity increases with decreasing temperature and the EL signal at 300 K is about a factor of 3 smaller than the value at 50 K; this suggests
300 K
15 A/cm 2
250 K 200 K
EL intensity (a.u.)
150 K 50 K
50 K 1 EL intensity (a.u.)
226
0.1 λ rev = 0.78 µm
0
3
6 9 12 Current density (A/cm2)
15
Fig. 6. EL intensity of Tm-doped Si nanoclusters as a function of the current density at a temperature of 50 K.
that weak non-radiative de-excitation processes are present in this system. Fig. 6 shows the EL intensity at 0.78 lm, as a function of the current density measured at 50 K. It is possible to note that the EL signal at 0.78 lm increases by increasing the current density. The behaviour is not linear and tends to saturate by increasing the current. The excitation mechanism of Tm ions is probably due to impact by energetic carriers. Tm can be excited directly by hot carrier impact or through energy transfer from Si nanoclusters excited by hot carriers; the two above situations are not distinguishable. In both cases, however, Si nanoclusters play an active role in the RE excitation process, since they give a strong contribution to the high electrical conduction of this system.
4. Conclusions
700
750 800 Wavelength (nm)
850
900
Fig. 5. EL spectra of Tm-doped Si nanoclusters device at different temperatures (50–300 K) for a fixed current density of 15 A/cm2 .
In conclusion, we have studied the structural and optical properties of MOS devices where the dielectric layer consists of a substoichiometric SiOx (x < 2) thin film grown by plasma enhanced chemical vapor deposition. We have demonstrated that the devices with silicon nanocrystals are efficient light sources at room temperature. It is necessary to reach a compromise between a high Si
A. Irrera et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 222–227
content, which is favourable for the electrical properties, and the Si nc mean size, which determines the optical properties. We have used the SiOx films with the best EL properties to fabricate also RE-implanted silicon nanoclusters devices. We have studied the EL properties of light emitting devices based on Er-implanted Si nanoclusters. The devices are very stable and exhibit a strong EL signal at 1.54 lm at room temperature. Finally, we have also studied the EL properties of Tm-implanted Si nanoclusters. With this system it is possible to change the emission wavelength of the devices, in fact we have observed at room temperature two EL peaks respectively at 0.78 and 1.7 lm. These results open the way to new interesting applications in optoelectronics, such as electrically driven optical amplifiers or light sources.
Acknowledgements The authors want to thank A. La Mantia, N. Marino, A. Spada, A. Marino and S. Pannitteri for their expert technical collaboration. This work is supported by the EU project SINERGIA and by the project FIRB financed by MIUR.
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