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p-Si structure
Nuclear Instruments and Methods in Physics Research B 183 (2001) 305±310
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Eects of Si, Ge and Ar ion-implantation on EL from Au/Si-rich SiO2 /p-Si structure Y. Chen a, G.Z. Ran a, Y.K. Sun a, Y.B. Wang a, J.S. Fu a, Wen-tai Chen b, Yi-yuan Gong b, De-Xin Wu b, Z.C. Ma c, W.H. Zong c, G.G. Qin a,d,* a
b
Department of Physics, Peking University, Beijing 100871, China Microelectronics Research and Development Center, The Chinese Academy of Sciences, Beijing 100029, China c National Key Laboratory of ASIC, HSRI, Shijiazhuang 050051, China d International Center for Materials Physics, Academia Sinica, Shenyang 110015, China Received 13 March 2001
Abstract Si-rich SiO2 ®lms were deposited on p-Si substrates using the magnetron sputtering technique and then implanted by Si, Ge or Ar ions. Electroluminescence (EL) was observed from the semitransparent Au ®lm/ion-implanted Si-rich SiO2 / p-Si diodes with the ion-implanted Si-rich SiO2 /p-Si annealed at 1050°C. In comparison with the Au/non-implanted Si-rich SiO2 /p-Si diode, whose EL spectrum has a main peak at 1.8 eV and a shoulder at 2.4 eV, the Au/Si-implanted Si-rich SiO2 /p-Si diode has an EL spectrum with the 1.8 and 2.4 eV peaks enhanced in intensity by factors of 2 and 8, respectively. Both EL spectra of Au/Ge-implanted Si-rich SiO2 /p-Si diode Au/Ar-implanted Si-rich SiO2 /p-Si diode have new strong peaks at 2.2 eV. The mechanisms for EL intensity enhancement and appearance of new EL peaks caused by ion-implantation are discussed. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Recently, the investigation of nanoscale-Si-embedded SiO2 has received considerable attention [1±6]. Among various methods preparing nanoscale-Si-embedded SiO2 , ion-implantation has been shown to be one of the promising processes. Some authors have carried out studies on the luminescence from the ion-implanted Si-based
*
Corresponding author. Tel.: +86-10-6275-1743; fax: +8610-6275-1615. E-mail address: [email protected] (G.G. Qin).
nanostructures [7±12]. But so far the luminescence intensity and stability of these nanostructures remain to be enhanced and their luminescence mechanisms to be further explored. In this work, an attempt has been made to enhance the electroluminescence (EL) intensity and eciency using ion-implantation into the SiO2 layer. And through a comparative study on the EL spectra of Au/ion-implanted Si-rich SiO2 (SRSO)/p-Si diodes with implanted ions of Si, Ge and Ar and of Au/ non-implanted SRSO/p-Si diodes, we try to get a further understanding of the functions of ion-implantation and the EL mechanisms.
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 7 0 2 - 9
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Y. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 305±310
2. Experiment The substrates used in this experiment were (1 0 0)-oriented, 6±9 X cm p-Si wafers. The SRSO layers were deposited using a magnetron sputtering system MPS-3000FC with a Si±SiO2 composite target having a one-fourth area ratio of Si to SiO2 . For a comparison purpose, thermal-grown SiO2 ®lms with 100 and 200 nm thicknesses were grown at 1050°C, and the reaction gases used were chloroethylene (TCA, 150 ml/min) and oxygen (500 ml/ min), and hydrogen (81 ml/min) and oxygen (51 ml/min), respectively. To obtain a wider implantation distribution, the SRSO/p-Si and thermalgrown SiO2 /p-Si samples with both the SRSO and SiO2 layers of 200 nm thicknesses were implanted twice with Si ions: an energy of 50 keV with a dose of 1 1016 cm 2 at ®rst, and then an energy of 90 keV with a dose of 1:6 1016 cm 2 . The SRSO/pSi and thermal-grown SiO2 /p-Si samples with both the SRSO and SiO2 layers of 100 nm thicknesses were divided into two groups. One group was implanted with Ge ions with an energy of 80 keV and a dose of 1 1016 cm 2 , and the other group was implanted with Ar ions with an energy of 50 keV and a dose of 5 1016 cm 2 . The implanted ®lms were annealed at 550°C, 650°C, 750°C, 850°C, 950°C and 1050°C for 30 min in a N2 ambience. Subsequently, ohmic contacts were formed by evaporating Al ®lms onto the backside of all the implanted and non-implanted samples and then annealed at 530°C for 7 min in a N2 ambience. Semitransparent circular Au dots with diameters of 3 mm were deposited using a mask onto the front surface of the samples at last. All I±V and EL measurements were performed at room temperature. 3. Results Fig. 1 shows the I±V characteristic curves of the Au/Si-implanted SRSO/p-Si, Au/Ge-implanted SRSO/p-Si and Au/Ar-implanted SRSO/p-Si diodes, along with those of the Au/non-implanted SRSO/p-Si diodes with the SRSO ®lms of 100 and 200 nm. Under the same bias, no matter forward bias (the p-Si substrate was biased positive) or
Fig. 1. I±V curves of Au/SRSO/p-Si diodes with the SRSO ®lms: (a) of 200 nm thickness and non-implanted, (b) of 100 nm thickness and non-implanted, (c) of 200 nm thickness and Siimplanted, (d) of 100 nm thickness and Ar-implanted, (e) of 100 nm thickness and Ge-implanted.
inverse bias, the currents of the implanted diodes were evidently larger than those of the non-implanted diode. Fig. 2 shows the I±V characteristic curves of the Au/non-implanted thermal grown SiO2 /p-Si and Au/Si-implanted thermal grown SiO2 /p-Si diodes. The current of the latter was much larger than that of the former, however, still about three orders of magnitude smaller than the current of the Au/Siimplanted SRSO/p-Si diode under the same bias. The Au/ion-implanted SRSO/p-Si diodes with implanted ions of Si, Ge and Ar showed EL only when the ion-implanted SRSO/p-Si samples annealed at 1050°C, the highest annealing temperature used, before Au ®lms deposition. Therefore, all the EL spectra shown in Figs. 3±5 for the Au/ ion-implanted SRSO/p-Si diodes are those with the ion-implanted SRSO/p-Si samples annealing at 1050°C. The curves a and b in Fig. 3 show the EL spectra of the Au/non-implanted SRSO (200 nm)/ p-Si and Au/Si-implanted SRSO (200 nm)/p-Si diodes, respectively, under a forward bias of 22 V. The former EL spectrum had a peak at about 1.8
Y. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 305±310
Fig. 2. I±V curves of Au/thermal-grown SiO2 /p-Si diodes with the thermal-grown SiO2 ®lms: (a) of 200 nm thickness and nonimplanted (b) of 200 nm thickness and Si-implanted.
Fig. 3. EL spectra of (a) Au/non-implanted SRSO (200 nm)/pSi. (b) Au/Si-implanted SRSO (200 nm)/p-Si diodes under a forward bias of 22 V, where the dotted lines are the Gaussian ®tting curves.
eV and a shoulder at about 2.4 eV. As shown in Fig. 3, after implantation, the EL intensities of the 1.8 and 2.4 eV peaks enhanced by factors of about
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Fig. 4. EL spectra of (a) Au/non-implanted SRSO (100 nm)/pSi. (b) Au/Ge-implanted SRSO (100 nm)/p-Si diodes under a forward bias of 20 V, where the dotted lines are the Gaussian ®tting curves.
2 and 8, respectively, and the main peak shifted from 1.8 to 2.4 eV. The curves a and b in Fig. 4 show the EL spectra of Au/non-implanted SRSO (100 nm)/p-Si and Au/Ge-implanted SRSO (100 nm)/p-Si diodes, respectively, under a forward bias of 20 V. The former EL spectrum had a shape almost the same as that of the Au/non-implanted SRSO (200 nm)/ p-Si diode shown in Fig. 3 except that the diode with a thinner SiO2 ®lm had a little stronger intensity. However, dierent from Si-implantation, Ge-implantation induced a new strong peak at about 2.2 eV, which made the 1.8 eV peak, the main peak of the non-implanted diode, become a shoulder. Fig. 5 shows the EL spectra of Au/Ar-implanted SRSO (100 nm)/p-Si and Au/non-implanted SRSO (100 nm)/p-Si diodes. Identical to the Ge-implantation, Ar-implantation induced a new peak at 2.2 eV, which became the major peak of the EL spectrum. Both Ge and Ar ion-implantation evidently enhanced the integral EL intensity, while Ge ionimplantation more prominently.
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Fig. 5. EL spectra of (a) Au/non-implanted SRSO (100 nm)/pSi. (b) Au/Ar-implanted SRSO (100 nm)/p-Si diodes under a forward bias of 20 V, where the dotted lines are the Gaussian ®tting curves.
In Figs. 3±5 dotted lines are the Gaussian ®tting curves.
4. Discussion Although extensive studies have been carried out, there is a long debate about the EL mechanisms of nanometer Si materials, such as porous Si, SRSO ®lms and ion-implanted SiO2 , etc. The quantum-con®nement model [13] claimed that EL is from the band to band recombination of the electron±hole pairs in the nanoscale Si particles. According to this model, the energies of the photons emitted are determined by the sizes of the Si particles, and appearance of a new EL peak implies the appearance of new Si particles with a size dierent from those of the original Si particles. Using this model to explain the appearance of a new EL peak at 2.2 eV after Ge (or Ar)-implantation and annealing, we have to suppose that implanted Ge (or Ar) ions precipitate to form new nanoscale Ge (or Ar) particles. However, Ar is an inert gas, and it is hardly to conceive that Ar ion-
implantation can induce any new nanoscale particles, and they have the same luminescence energy as the nanoscale Ge particles introduced by Ge ion-implantation do. Thus, the fact that both Ge and Ar ion-implantation induced luminescence peaks with the same energy of 2.2 eV awaits another explanation. Ion implantation has two eects: inducing defects as well as ions in the implanted materials. We consider that the 2.2 eV EL peaks presenting in Au/Ge-implanted SRSO/p-Si and Au/Ar-implanted SRSO/p-Si diodes may be relevant to a common type of defects introduced into SiO2 by ion-implantation. Qin et al. suggested an EL model for an Au/SRSO/p-Si structure as follows: EL comes mainly from electronic±hole radiative recombination in luminescence centers (LCs) rather than nanoscale Si particles in the SRSO layer, i.e. under a forward bias, electrons and holes are injected, respectively, from the Au electrode and the p-Si substrate and tunnel into the LCs, which are some types of defects or impurities, in the SRSO layer and radiative recombine there, and the nanoscale Si particles play a role of increasing tunneling probability of carriers [14,15]. We consider this model as suitable to explain the EL processes presented in this paper. Because the SRSO ®lms are relatively thick (100 or 200 nm), the possibility that an electron from the Au electrode and a hole from the p-Si substrate tunnel directly into the same LC in Si oxide layer is very small, and the dominant EL process may be as follows: the electrons from the Au electrode and holes from the p-Si substrate tunnel against and along, respectively, the electrical ®eld, from the Au electrode and the p-Si substrate into the nearby nanoscale Si (or Ge) particles ®rst, then to the neighboring Si (or Ge) particles continuously, and ®nally tunnel into the same LC and radiative recombine there. Because Si (or Ge) is a kind of indirect forbidden-band semiconductor, the possibility that electrons and holes recombine in nanoscale Si (or Ge) particles is small, therefore the radiatively recombination via nanoscale Si (or Ge) particles is a minor process. We consider that there are at least two kinds of LCs with the luminescent energies of 2.4 and 1.8 eV in the non-implanted SRSO layers. The inten-
Y. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 305±310
sities of the 2.4 and 1.8 eV peaks are determined by the densities of the two types of LCs as well as the radiative recombination rates of the electron±hole pairs in them. Thus, the fact that the Au/non-implanted SRSO/p-Si diode had an EL spectrum with a 1.8 eV peak and 2.4 eV shoulder can be qualitatively explained. The Si-implantation and annealing induced not only the nanoscale Si particles but also new defects, including both LCs and non-radiative centers. The fact that the main peak energy shifted from 1.8 to 2.4 eV after implantation can be explained as: the induced density of the 2.4 eV LCs is more than that of the 1.8 eV ones in the Si-implantation process, or the luminescence eciency of the 2.4 eV LCs is higher than that of the 1.8 eV ones, or both cases are true, and then, after Si-implantation, the intensity increase of the 2.4 eV luminescence peak is much larger than that of 1.8 eV luminescence peak. Dierent from the Si-implantation, Ge and Ar ion-implantation induced a new type of LC with luminescence energy of 2.2 eV, which can also be attributed to a defect in the Si oxide. A question arises why the Ge and Ar ion-implantation can induce such defects while the Si ion-implantation cannot. A possible reason is that the types of defects produced in an ion-implantation process are critically dependent on the mass of the ions, and the implantation of the heavier ions of Ge and Ar rather than lighter ions of Si can induce the defects responsible for the 2.2 eV EL peak. As discussed above, ion-implantation can induce defects, which is also exhibited in the I±V characteristic curves for ion-implanted diodes as shown in Fig. 1. In fact, all the three types of ionimplantation can induce defects, which can play roles as recombination centers, leading to increase of current. As for the three types of LCs with luminescence energies of 1.8, 2.2 and 2.4 eV, it is a very dicult problem to explore their nature and attribution. Tentatively, we consider that the corresponding LC for the 1.8 eV band may be ascribed to the non-bridging oxygen hole center, an important LC in Si oxide, which has been reported having a light emission energy around 1.9 eV [16]. The LC being responsible for the 2.2 eV band is probably the oxygen-surplus-type defects [17], or the E0d center
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[18] which have light emission energies of about 2.25 or 2.2 eV. Robertson [19] has attributed the 2.4±2.6 eV-photoluminescence band to the radiative recombination at the Si dangling-bond center, and the 2.4 eV band reported in this paper could have the same origin. Dierent from the diodes based on SRSO, neither the non-implanted nor implanted diodes based on thermal-grown SiO2 showed EL. The cause can be ascribed to the very dierent I±V characteristics of the two types of diodes as shown in Figs. 1 and 2. Because thermal-grown SiO2 was more compact and purer than the SRSO, especially, the former is a stoichiometric one, while the latter is a Si-rich one, the current ¯owing through a diode based on the former is much smaller than that through a diode based on the latter, as a result, the EL from the diode based on the former is much weaker in intensity than that based on the latter and cannot be detected. 5. Conclusion Three types of ions, Si, Ge and Ar, have been implanted into the SRSO and the EL spectra from the Au/SRSO/p-Si structure before and after ion implantation have been compared and investigated. The Si-implantation greatly enhanced the 2.4 eV shoulder and resulted in an evident doublepeak (2.4 and 1.8 eV) EL spectrum, while both the Ge and Ar ion-implantation induced new EL bands at 2.2 eV. It is considered that all the EL peaks at 1.8, 2.2 and 2.4 eV originate from the LCs (defects) in Si oxide. In other words, electrons from the Au electrode and holes from the p-Si substrate tunnel through the nanoscale Si (or Ge) particles to three types of LCs with luminescence energies of 1.8, 2.2 and 2.4 eV, and recombine radiatively there to result in the EL spectra observed. Acknowledgements This work was supported by the National Science Foundation of China and the State Key Laboratory on integrated optoelectronics.
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References [1] T. Kawaguchi, S. Miyamiza, Jpn. J. Appl. Phys. 32 (1993) L215. [2] S.H. Risbud, L.C. Liu, J.F. Shackelford, Appl. Phys. Lett. 63 (1993) 1648. [3] X. Zhao, O. Schoenfeld, J. Kusano, Y. Aoyagi, T. Sugano, Jpn. J. Appl. Phys. 33 (1994) L649. [4] Q. Zhang, S.C. Bayliss, R.A. Hutt, Appl. Phys. Lett. 66 (1995) 1997. [5] K.D. Hirchman, L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Nature 384 (1996) 338. [6] B. Gelloz, T. Nakagawa, N. Koshida, Appl. Phys. Lett. 73 (1998) 2021. [7] T. Shimizu-lwayama, K. Fujita, S. Nakato, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [8] L.S. Liao, X.M. Bao, N.S. Li, X.Q. Zheng, N.B. Min, Solid State Commun. 97 (1996) 1039. [9] K.S. Min, K.V. Shcheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 69 (1996) 2033.
[10] L. Rebohole, J. VonBorany, R.A. Yankov, W. Skorupa, Appl. Phys. Lett. 71 (1997) 2809. [11] T. Shimizu-Iwayama, N. Kurumado, D.E. Hole, P.D. Townsend, J. Appl. Phys. 83 (1998) 6018. [12] I.E. Tyschenko, L. Rebohole, R.A. Yankov, W. Skorupa, A. Misiluk, Appl. Phys. Lett. 73 (1998) 1418. [13] D.J. Dimaria, J.R. Kirtley, E.J. Pakulis, D.W. Dong, T.S. Kuan, F.L. Pesavento, T.N. Theis, J.A. Cutro, S.D. Brorson, J. Appl. Phys. 56 (1984) 401. [14] G.G. Qin, A.P. Li, B.R. Zhang, B.C. Li, J. Appl. Phys. 78 (1995) 2006. [15] G.G. Qin, A.P. Li, Y.X. Zhang, Phys. Rev. B 54 (1996) 11122. [16] N. Skuja, A.R. Silin, Physica Status Solidi A 56 (1979) K11. [17] Y. Sakurai, K. Nagasawa, J. Appl. Phys. 86 (1999) 1377. [18] S-T. Chou, J-H. Tsai, B-C. Sheu, J. Appl. Phys. 83 (1998) 5394. [19] J. Robertson, Mater. Res. Soc. Symp. Proc. 284 (1993) 65.
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Eects of Si, Ge and Ar ion-implantation on EL from Au/Si-rich SiO2 /p-Si structure Y. Chen a, G.Z. Ran a, Y.K. Sun a, Y.B. Wang a, J.S. Fu a, Wen-tai Chen b, Yi-yuan Gong b, De-Xin Wu b, Z.C. Ma c, W.H. Zong c, G.G. Qin a,d,* a
b
Department of Physics, Peking University, Beijing 100871, China Microelectronics Research and Development Center, The Chinese Academy of Sciences, Beijing 100029, China c National Key Laboratory of ASIC, HSRI, Shijiazhuang 050051, China d International Center for Materials Physics, Academia Sinica, Shenyang 110015, China Received 13 March 2001
Abstract Si-rich SiO2 ®lms were deposited on p-Si substrates using the magnetron sputtering technique and then implanted by Si, Ge or Ar ions. Electroluminescence (EL) was observed from the semitransparent Au ®lm/ion-implanted Si-rich SiO2 / p-Si diodes with the ion-implanted Si-rich SiO2 /p-Si annealed at 1050°C. In comparison with the Au/non-implanted Si-rich SiO2 /p-Si diode, whose EL spectrum has a main peak at 1.8 eV and a shoulder at 2.4 eV, the Au/Si-implanted Si-rich SiO2 /p-Si diode has an EL spectrum with the 1.8 and 2.4 eV peaks enhanced in intensity by factors of 2 and 8, respectively. Both EL spectra of Au/Ge-implanted Si-rich SiO2 /p-Si diode Au/Ar-implanted Si-rich SiO2 /p-Si diode have new strong peaks at 2.2 eV. The mechanisms for EL intensity enhancement and appearance of new EL peaks caused by ion-implantation are discussed. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Recently, the investigation of nanoscale-Si-embedded SiO2 has received considerable attention [1±6]. Among various methods preparing nanoscale-Si-embedded SiO2 , ion-implantation has been shown to be one of the promising processes. Some authors have carried out studies on the luminescence from the ion-implanted Si-based
*
Corresponding author. Tel.: +86-10-6275-1743; fax: +8610-6275-1615. E-mail address: [email protected] (G.G. Qin).
nanostructures [7±12]. But so far the luminescence intensity and stability of these nanostructures remain to be enhanced and their luminescence mechanisms to be further explored. In this work, an attempt has been made to enhance the electroluminescence (EL) intensity and eciency using ion-implantation into the SiO2 layer. And through a comparative study on the EL spectra of Au/ion-implanted Si-rich SiO2 (SRSO)/p-Si diodes with implanted ions of Si, Ge and Ar and of Au/ non-implanted SRSO/p-Si diodes, we try to get a further understanding of the functions of ion-implantation and the EL mechanisms.
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 7 0 2 - 9
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Y. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 305±310
2. Experiment The substrates used in this experiment were (1 0 0)-oriented, 6±9 X cm p-Si wafers. The SRSO layers were deposited using a magnetron sputtering system MPS-3000FC with a Si±SiO2 composite target having a one-fourth area ratio of Si to SiO2 . For a comparison purpose, thermal-grown SiO2 ®lms with 100 and 200 nm thicknesses were grown at 1050°C, and the reaction gases used were chloroethylene (TCA, 150 ml/min) and oxygen (500 ml/ min), and hydrogen (81 ml/min) and oxygen (51 ml/min), respectively. To obtain a wider implantation distribution, the SRSO/p-Si and thermalgrown SiO2 /p-Si samples with both the SRSO and SiO2 layers of 200 nm thicknesses were implanted twice with Si ions: an energy of 50 keV with a dose of 1 1016 cm 2 at ®rst, and then an energy of 90 keV with a dose of 1:6 1016 cm 2 . The SRSO/pSi and thermal-grown SiO2 /p-Si samples with both the SRSO and SiO2 layers of 100 nm thicknesses were divided into two groups. One group was implanted with Ge ions with an energy of 80 keV and a dose of 1 1016 cm 2 , and the other group was implanted with Ar ions with an energy of 50 keV and a dose of 5 1016 cm 2 . The implanted ®lms were annealed at 550°C, 650°C, 750°C, 850°C, 950°C and 1050°C for 30 min in a N2 ambience. Subsequently, ohmic contacts were formed by evaporating Al ®lms onto the backside of all the implanted and non-implanted samples and then annealed at 530°C for 7 min in a N2 ambience. Semitransparent circular Au dots with diameters of 3 mm were deposited using a mask onto the front surface of the samples at last. All I±V and EL measurements were performed at room temperature. 3. Results Fig. 1 shows the I±V characteristic curves of the Au/Si-implanted SRSO/p-Si, Au/Ge-implanted SRSO/p-Si and Au/Ar-implanted SRSO/p-Si diodes, along with those of the Au/non-implanted SRSO/p-Si diodes with the SRSO ®lms of 100 and 200 nm. Under the same bias, no matter forward bias (the p-Si substrate was biased positive) or
Fig. 1. I±V curves of Au/SRSO/p-Si diodes with the SRSO ®lms: (a) of 200 nm thickness and non-implanted, (b) of 100 nm thickness and non-implanted, (c) of 200 nm thickness and Siimplanted, (d) of 100 nm thickness and Ar-implanted, (e) of 100 nm thickness and Ge-implanted.
inverse bias, the currents of the implanted diodes were evidently larger than those of the non-implanted diode. Fig. 2 shows the I±V characteristic curves of the Au/non-implanted thermal grown SiO2 /p-Si and Au/Si-implanted thermal grown SiO2 /p-Si diodes. The current of the latter was much larger than that of the former, however, still about three orders of magnitude smaller than the current of the Au/Siimplanted SRSO/p-Si diode under the same bias. The Au/ion-implanted SRSO/p-Si diodes with implanted ions of Si, Ge and Ar showed EL only when the ion-implanted SRSO/p-Si samples annealed at 1050°C, the highest annealing temperature used, before Au ®lms deposition. Therefore, all the EL spectra shown in Figs. 3±5 for the Au/ ion-implanted SRSO/p-Si diodes are those with the ion-implanted SRSO/p-Si samples annealing at 1050°C. The curves a and b in Fig. 3 show the EL spectra of the Au/non-implanted SRSO (200 nm)/ p-Si and Au/Si-implanted SRSO (200 nm)/p-Si diodes, respectively, under a forward bias of 22 V. The former EL spectrum had a peak at about 1.8
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Fig. 2. I±V curves of Au/thermal-grown SiO2 /p-Si diodes with the thermal-grown SiO2 ®lms: (a) of 200 nm thickness and nonimplanted (b) of 200 nm thickness and Si-implanted.
Fig. 3. EL spectra of (a) Au/non-implanted SRSO (200 nm)/pSi. (b) Au/Si-implanted SRSO (200 nm)/p-Si diodes under a forward bias of 22 V, where the dotted lines are the Gaussian ®tting curves.
eV and a shoulder at about 2.4 eV. As shown in Fig. 3, after implantation, the EL intensities of the 1.8 and 2.4 eV peaks enhanced by factors of about
307
Fig. 4. EL spectra of (a) Au/non-implanted SRSO (100 nm)/pSi. (b) Au/Ge-implanted SRSO (100 nm)/p-Si diodes under a forward bias of 20 V, where the dotted lines are the Gaussian ®tting curves.
2 and 8, respectively, and the main peak shifted from 1.8 to 2.4 eV. The curves a and b in Fig. 4 show the EL spectra of Au/non-implanted SRSO (100 nm)/p-Si and Au/Ge-implanted SRSO (100 nm)/p-Si diodes, respectively, under a forward bias of 20 V. The former EL spectrum had a shape almost the same as that of the Au/non-implanted SRSO (200 nm)/ p-Si diode shown in Fig. 3 except that the diode with a thinner SiO2 ®lm had a little stronger intensity. However, dierent from Si-implantation, Ge-implantation induced a new strong peak at about 2.2 eV, which made the 1.8 eV peak, the main peak of the non-implanted diode, become a shoulder. Fig. 5 shows the EL spectra of Au/Ar-implanted SRSO (100 nm)/p-Si and Au/non-implanted SRSO (100 nm)/p-Si diodes. Identical to the Ge-implantation, Ar-implantation induced a new peak at 2.2 eV, which became the major peak of the EL spectrum. Both Ge and Ar ion-implantation evidently enhanced the integral EL intensity, while Ge ionimplantation more prominently.
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Fig. 5. EL spectra of (a) Au/non-implanted SRSO (100 nm)/pSi. (b) Au/Ar-implanted SRSO (100 nm)/p-Si diodes under a forward bias of 20 V, where the dotted lines are the Gaussian ®tting curves.
In Figs. 3±5 dotted lines are the Gaussian ®tting curves.
4. Discussion Although extensive studies have been carried out, there is a long debate about the EL mechanisms of nanometer Si materials, such as porous Si, SRSO ®lms and ion-implanted SiO2 , etc. The quantum-con®nement model [13] claimed that EL is from the band to band recombination of the electron±hole pairs in the nanoscale Si particles. According to this model, the energies of the photons emitted are determined by the sizes of the Si particles, and appearance of a new EL peak implies the appearance of new Si particles with a size dierent from those of the original Si particles. Using this model to explain the appearance of a new EL peak at 2.2 eV after Ge (or Ar)-implantation and annealing, we have to suppose that implanted Ge (or Ar) ions precipitate to form new nanoscale Ge (or Ar) particles. However, Ar is an inert gas, and it is hardly to conceive that Ar ion-
implantation can induce any new nanoscale particles, and they have the same luminescence energy as the nanoscale Ge particles introduced by Ge ion-implantation do. Thus, the fact that both Ge and Ar ion-implantation induced luminescence peaks with the same energy of 2.2 eV awaits another explanation. Ion implantation has two eects: inducing defects as well as ions in the implanted materials. We consider that the 2.2 eV EL peaks presenting in Au/Ge-implanted SRSO/p-Si and Au/Ar-implanted SRSO/p-Si diodes may be relevant to a common type of defects introduced into SiO2 by ion-implantation. Qin et al. suggested an EL model for an Au/SRSO/p-Si structure as follows: EL comes mainly from electronic±hole radiative recombination in luminescence centers (LCs) rather than nanoscale Si particles in the SRSO layer, i.e. under a forward bias, electrons and holes are injected, respectively, from the Au electrode and the p-Si substrate and tunnel into the LCs, which are some types of defects or impurities, in the SRSO layer and radiative recombine there, and the nanoscale Si particles play a role of increasing tunneling probability of carriers [14,15]. We consider this model as suitable to explain the EL processes presented in this paper. Because the SRSO ®lms are relatively thick (100 or 200 nm), the possibility that an electron from the Au electrode and a hole from the p-Si substrate tunnel directly into the same LC in Si oxide layer is very small, and the dominant EL process may be as follows: the electrons from the Au electrode and holes from the p-Si substrate tunnel against and along, respectively, the electrical ®eld, from the Au electrode and the p-Si substrate into the nearby nanoscale Si (or Ge) particles ®rst, then to the neighboring Si (or Ge) particles continuously, and ®nally tunnel into the same LC and radiative recombine there. Because Si (or Ge) is a kind of indirect forbidden-band semiconductor, the possibility that electrons and holes recombine in nanoscale Si (or Ge) particles is small, therefore the radiatively recombination via nanoscale Si (or Ge) particles is a minor process. We consider that there are at least two kinds of LCs with the luminescent energies of 2.4 and 1.8 eV in the non-implanted SRSO layers. The inten-
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sities of the 2.4 and 1.8 eV peaks are determined by the densities of the two types of LCs as well as the radiative recombination rates of the electron±hole pairs in them. Thus, the fact that the Au/non-implanted SRSO/p-Si diode had an EL spectrum with a 1.8 eV peak and 2.4 eV shoulder can be qualitatively explained. The Si-implantation and annealing induced not only the nanoscale Si particles but also new defects, including both LCs and non-radiative centers. The fact that the main peak energy shifted from 1.8 to 2.4 eV after implantation can be explained as: the induced density of the 2.4 eV LCs is more than that of the 1.8 eV ones in the Si-implantation process, or the luminescence eciency of the 2.4 eV LCs is higher than that of the 1.8 eV ones, or both cases are true, and then, after Si-implantation, the intensity increase of the 2.4 eV luminescence peak is much larger than that of 1.8 eV luminescence peak. Dierent from the Si-implantation, Ge and Ar ion-implantation induced a new type of LC with luminescence energy of 2.2 eV, which can also be attributed to a defect in the Si oxide. A question arises why the Ge and Ar ion-implantation can induce such defects while the Si ion-implantation cannot. A possible reason is that the types of defects produced in an ion-implantation process are critically dependent on the mass of the ions, and the implantation of the heavier ions of Ge and Ar rather than lighter ions of Si can induce the defects responsible for the 2.2 eV EL peak. As discussed above, ion-implantation can induce defects, which is also exhibited in the I±V characteristic curves for ion-implanted diodes as shown in Fig. 1. In fact, all the three types of ionimplantation can induce defects, which can play roles as recombination centers, leading to increase of current. As for the three types of LCs with luminescence energies of 1.8, 2.2 and 2.4 eV, it is a very dicult problem to explore their nature and attribution. Tentatively, we consider that the corresponding LC for the 1.8 eV band may be ascribed to the non-bridging oxygen hole center, an important LC in Si oxide, which has been reported having a light emission energy around 1.9 eV [16]. The LC being responsible for the 2.2 eV band is probably the oxygen-surplus-type defects [17], or the E0d center
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[18] which have light emission energies of about 2.25 or 2.2 eV. Robertson [19] has attributed the 2.4±2.6 eV-photoluminescence band to the radiative recombination at the Si dangling-bond center, and the 2.4 eV band reported in this paper could have the same origin. Dierent from the diodes based on SRSO, neither the non-implanted nor implanted diodes based on thermal-grown SiO2 showed EL. The cause can be ascribed to the very dierent I±V characteristics of the two types of diodes as shown in Figs. 1 and 2. Because thermal-grown SiO2 was more compact and purer than the SRSO, especially, the former is a stoichiometric one, while the latter is a Si-rich one, the current ¯owing through a diode based on the former is much smaller than that through a diode based on the latter, as a result, the EL from the diode based on the former is much weaker in intensity than that based on the latter and cannot be detected. 5. Conclusion Three types of ions, Si, Ge and Ar, have been implanted into the SRSO and the EL spectra from the Au/SRSO/p-Si structure before and after ion implantation have been compared and investigated. The Si-implantation greatly enhanced the 2.4 eV shoulder and resulted in an evident doublepeak (2.4 and 1.8 eV) EL spectrum, while both the Ge and Ar ion-implantation induced new EL bands at 2.2 eV. It is considered that all the EL peaks at 1.8, 2.2 and 2.4 eV originate from the LCs (defects) in Si oxide. In other words, electrons from the Au electrode and holes from the p-Si substrate tunnel through the nanoscale Si (or Ge) particles to three types of LCs with luminescence energies of 1.8, 2.2 and 2.4 eV, and recombine radiatively there to result in the EL spectra observed. Acknowledgements This work was supported by the National Science Foundation of China and the State Key Laboratory on integrated optoelectronics.
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