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Electron and hole trapping in thermal oxides that have been ion implanted
Microelectronic Engineering 59 (2001) 285–289 www.elsevier.com / locate / mee
Electron and hole trapping in thermal oxides that have been ion implanted a, a b c B.J. Mrstik *, H.L. Hughes , P.J. McMarr , P. Gouker a
Naval Research Laboratory, Washington, DC 20375, USA b SFA, Inc., Landover, MD 20785, USA c Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02420, USA Abstract Photo-assisted charge injection techniques in conjunction with capacitance–voltage measurements have been used to study electron and hole trapping in thermal oxides implanted with up to 1 3 10 16 cm 22 Si or Ar. Defects having large cross sections for electron trapping and photoionization are found in oxides implanted with large doses of Si, but not of Ar, suggesting the formation of Si clusters. It is also found that the magnitude of the shift in the flatband voltage, DVfb , resulting from hole trapping is increased in oxides implanted with up to 1310 15 cm 22 Si or Ar. In oxides implanted with higher doses of Si, however, DVfb is decreased. An explanation is proposed involving trapping of radiolytic hydrogen at Si clusters. 2001 Elsevier Science B.V. All rights reserved. Keywords: Hole traps; Electron traps; Silicon nanocrystals; Ion implanted SiO 2
1. Introduction Electron and hole trapping in ion implanted thermal oxides have been studied for many years. Most of these studies have involved implant doses of 1310 15 ions / cm 2 or less, and have found that the implantation creates additional hole traps and small capture cross section (s , 2 3 10 215 cm 2 ) electron traps [1–3]. Recently Mrstik et al. [4] studied charge trapping in thermal oxides implanted with up to 5310 15 cm 22 of Al, Si, and P. They found that at the highest implant doses electron traps having large capture cross section (s | 1 3 10 213 cm 2 at a field of 1 MV/ cm) are formed, and that the shift in the flatband voltage, DVfb , following hole injection is decreased, indicating a reduction of hole trapping near the Si–SiO 2 interface. They attributed the formation of the large capture cross section electron traps to chemical aspects of the implant species, and the decrease in DVfb following hole injection to implant related damage that impedes the transport of radiolytic hydrogen to the Si–SiO 2 interface. * Corresponding author. Tel.: 11-202-404-4850; fax: 11-202-404-7194. E-mail address: [email protected] (B.J. Mrstik). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00611-6
2001 Elsevier Science B.V. All rights reserved.
286
B. J. Mrstik et al. / Microelectronic Engineering 59 (2001) 285 – 289
In this work the effect of high doses of implanted Si and Ar on charge trapping in thermal oxides is studied. Since Si and Ar have similar masses, they create similar amounts of damage during implantation into SiO 2 . But since Ar is inert, it will not interact chemically with injected charge. Comparison of electron and hole trapping in Si and Ar implanted oxides should therefore reveal whether chemical or structural damage effects are responsible for the formation of large capture cross section electron traps and the reduction of DVfb following hole injection. 2. Experimental Thermal oxides |209 nm thick were grown on p-type (100) Si substrates at 9258C in dry oxygen. The oxides were implanted with either 43 keV Si or 58 keV Ar ions so as to place the peak of the implant 60 nm below the oxide surface (149 nm from the Si–SiO 2 interface). Implant doses of 1310 13 , 1310 14 , 1310 15 , or 1310 16 cm 22 were used. The implanted oxides, along with unimplanted control oxides, were then annealed for 30 min in Ar at 700, 900, or 10508C. Following the anneal, 0.005-cm 2 capacitors were made by evaporating semitransparent (15-nm thick) Au gates on the oxide. Charge was injected into the oxide using photo-assisted injection techniques. During charge injection, the gate electrode was biased at 120.9 V so as to apply a field of 1 MV/ cm across the oxide. For electron injection, the semitransparent gate electrode was illuminated with ultraviolet (UV) light (hn , 6 eV), resulting in the photoemission of electrons from the Si substrate into the oxide. The injected electrons then moved through the oxide to the gate electrode under the influence of the applied field. For hole injection, the gate electrode was illuminated with vacuum UV light (hn 5 10.0 eV). This light is absorbed in the oxide within |10 nm of the gate electrode by forming electron–hole pairs. Because of the positive gate bias, the holes are swept through the oxide, and the electrons return to the gate electrode. During both types of charge injection, the injected dose was determined by measuring the amount of charge transported across the oxide. The extent of charge trapping in the oxide was characterized by DVfb , the shift in the flatband voltage resulting from the charge injection. The flatband voltage was determined before and after injection using standard high frequency (1 MHz) capacitance–voltage measurements. For electron trapping measurements, DVfb was determined after injecting 0.3310 14 electrons / cm 2 into the oxide. At this dose all electron traps with capture cross sections larger than |1310 213 cm 2 should be filled. Hole trapping was characterized by the value of DVfb obtained after injecting 1310 14 holes / cm 2 . Since the dominant hole trap in unimplanted oxides has a capture cross section of |3310 213 cm 2 , these traps should all be filled at this dose.
3. Results and discussion Significant values of DVfb following electron injection were found only in oxides that had been implanted with very high doses of Si, in agreement with results of Ref. [4]. As shown in Fig. 1, DVfb resulting from electron injection into oxides implanted with 1310 16 Si / cm 2 is strongly dependent on the post-implant anneal temperature. For oxides annealed at 700 or 9008C, DVfb is positive, indicating that net negative charge is trapped in the oxide. For oxides annealed at 10508C, however, DVfb is
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Fig. 1. DVfb versus number of injected electrons for thermal oxides implanted with 1310 16 Si 1 / cm 2 , then annealed in Ar at 700, 900, or 10508C for 30 min in Ar.
negative. Afanas’ev et al. [5] noted a similar negative shift in Vfb following UV photo-injection into some oxides fabricated by the separation by implantation of oxygen (SIMOX) process, and attributed the effect to the presence of small Si clusters in the oxide. These clusters are amphoteric, i.e. they can become negatively charged by capturing an electron, or positively charged by emitting an electron when photoionized by a photon having an energy of at least 4.3 eV. Since UV light and electrons are both present during the electron injection, DVfb is determined by the relative amounts of photoionization and electron trapping / neutralization. High temperature annealing is known to precipitate cluster formation after implantation [6], and therefore would be expected to affect DVfb as shown in Fig. 1. Fig. 2 shows DVfb after injecting 1310 14 holes / cm 2 as a function of implant dose and anneal temperature. For all oxides except those implanted with the highest dose of Si, the implant results in larger values of DVfb , in agreement with earlier results of Nietzert et al. [3]. For oxides implanted with Si, however, Fig. 2 shows that increasing the implant dose from 1310 15 to 1310 16 Si / cm 2 drastically reduces DVfb to a value even smaller than in the unimplanted oxide. A similar reduction in 15 22 DVfb was reported in Ref. [4] for oxides implanted with 5310 cm Al, Si, or P. As pointed out in Ref. [4], the observation that oxides implanted with a high dose of Si have a smaller DVfb than unimplanted oxides cannot be explained in terms of models in which the primary hole traps are assumed to be oxygen vacancies (E9 centers) located near the Si–SiO 2 interface, since there is no reason to suspect that implantation of additional Si |149 nm from the Si–SiO 2 interface would reduce the number of oxygen vacancies near the Si–SiO 2 interface. Afanas’ev and Stesmans [7,8] have recently discussed several additional problems with these types of E9-related models. They show that the dominant hole trap is related to defects formed by radiolytic hydrogen. The reduced values of DVfb observed here in oxides that have been implanted with high doses of Si can be understood in terms of the results of Refs. [7] and [8] if it is assumed that the implantation obstructs the movement of radiolytic hydrogen to the Si–SiO 2 interface, and therefore affects where the injected charge is trapped. Fig. 3 shows that the shape of the DVfb vs. injected hole dose curve gradually changes as the implant dose is increased, and is consistent with this hypothesis. For unimplanted oxides DVfb initially changes quickly, then saturates because the movement of hydrogen
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Fig. 2. DVfb versus implanted dose of either Si or Ar ions. After implantation oxides were annealed in Ar at 700, 900, or 10508C for 30 min. DVfb is measured after injecting 1310 14 holes / cm 2 into oxide. Top panel, Si implants; lower panel, Ar implants.
to the interface is unimpeded, so most charge is trapped near the interface. As the implant dose is increased, however, more hydrogen is temporarily trapped in the implanted region. Although this hydrogen can trap injected holes, the effect on DVfb is decreased because of the larger distance from the interface to the trapped charge. At the highest implant dose, most of the hydrogen is trapped in the
Fig. 3. Normalized DVfb versus injected hole dose for oxides implanted with various doses of Si then annealed in Ar at 10508C for 30 min. DVfb is normalized so that 21 corresponds to DVfb obtained after injecting 1310 14 holes / cm 2 .
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implanted region and is only gradually released during subsequent charge injection. Once released, it can diffuse to the interface where it again traps holes, further increasing DVfb . 4. Conclusion The results presented here for Si implanted oxides are in agreement with those presented in Ref. [4], and again suggest that high dose Si implantation prevents injected holes from being trapped near the Si–SiO 2 interface. In Ref. [4] this was attributed to hydrogen trapping by implant damage to the SiO 2 structure. The results presented here, however, show that high dose Ar implantation does not similarly reduce DVfb , indicating that the reduction comes about from chemical aspects of the implant. The presence of silicon nanoclusters in high dose Si implanted oxides, shown by the data in Fig. 1, suggests that it is the presence of silicon clusters in the oxide, not structural effects, that is responsible for trapping hydrogen (and therefore injected charge) in the oxide bulk rather than at the Si–SiO 2 interface in oxides implanted with large doses of Si. Acknowledgements This work was supported by the Defense Threat Reduction Agency. References [1] D.R. Young et al., Characterization of electron traps in aluminum-implanted SiO 2 , IBM J. Res. Develop. 22 (1978) 285–288. [2] M. Offenberg, P. Balk, Ion implantation induced stoichiometric imbalance in SiO 2 , Appl. Surf. Sci. 30 (1987) 265–271. [3] H. Neitzert et al., Hole capture in SiO 2 after ion implantation, Appl. Surf. Sci. 30 (1987) 272–277. [4] B.J. Mrstik et al., Hole and electron trapping in ion implanted thermal oxides and SIMOX, IEEE Trans. Nucl. Sci. 47 (2000) 2189–2195. [5] V.V. Afanas’ev et al., Confinement phenomena in buried oxides of SIMOX structures as affected by processing, J. Electrochem. Soc. 143 (1996) 695–700. [6] T. Shimizu-Iwayama et al., Optical properties of silicon nanoclusters fabricated by ion implantation, J. Appl. Phys. 83 (1998) 6018–6022. [7] V.V. Afanas’ev, A. Stesmans, Charge state of paramagnetic E9 centre in thermal SiO 2 layers on silicon, J. Phys.: Condens. Matter. 12 (2000) 2285–2290. [8] V.V. Afanas’ev, A. Stesmans, Proton nature of radiation-induced positive charge in SiO 2 layers on Si, Europhys. Lett. 53 (2001) 233–239.
Electron and hole trapping in thermal oxides that have been ion implanted a, a b c B.J. Mrstik *, H.L. Hughes , P.J. McMarr , P. Gouker a
Naval Research Laboratory, Washington, DC 20375, USA b SFA, Inc., Landover, MD 20785, USA c Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02420, USA Abstract Photo-assisted charge injection techniques in conjunction with capacitance–voltage measurements have been used to study electron and hole trapping in thermal oxides implanted with up to 1 3 10 16 cm 22 Si or Ar. Defects having large cross sections for electron trapping and photoionization are found in oxides implanted with large doses of Si, but not of Ar, suggesting the formation of Si clusters. It is also found that the magnitude of the shift in the flatband voltage, DVfb , resulting from hole trapping is increased in oxides implanted with up to 1310 15 cm 22 Si or Ar. In oxides implanted with higher doses of Si, however, DVfb is decreased. An explanation is proposed involving trapping of radiolytic hydrogen at Si clusters. 2001 Elsevier Science B.V. All rights reserved. Keywords: Hole traps; Electron traps; Silicon nanocrystals; Ion implanted SiO 2
1. Introduction Electron and hole trapping in ion implanted thermal oxides have been studied for many years. Most of these studies have involved implant doses of 1310 15 ions / cm 2 or less, and have found that the implantation creates additional hole traps and small capture cross section (s , 2 3 10 215 cm 2 ) electron traps [1–3]. Recently Mrstik et al. [4] studied charge trapping in thermal oxides implanted with up to 5310 15 cm 22 of Al, Si, and P. They found that at the highest implant doses electron traps having large capture cross section (s | 1 3 10 213 cm 2 at a field of 1 MV/ cm) are formed, and that the shift in the flatband voltage, DVfb , following hole injection is decreased, indicating a reduction of hole trapping near the Si–SiO 2 interface. They attributed the formation of the large capture cross section electron traps to chemical aspects of the implant species, and the decrease in DVfb following hole injection to implant related damage that impedes the transport of radiolytic hydrogen to the Si–SiO 2 interface. * Corresponding author. Tel.: 11-202-404-4850; fax: 11-202-404-7194. E-mail address: [email protected] (B.J. Mrstik). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00611-6
2001 Elsevier Science B.V. All rights reserved.
286
B. J. Mrstik et al. / Microelectronic Engineering 59 (2001) 285 – 289
In this work the effect of high doses of implanted Si and Ar on charge trapping in thermal oxides is studied. Since Si and Ar have similar masses, they create similar amounts of damage during implantation into SiO 2 . But since Ar is inert, it will not interact chemically with injected charge. Comparison of electron and hole trapping in Si and Ar implanted oxides should therefore reveal whether chemical or structural damage effects are responsible for the formation of large capture cross section electron traps and the reduction of DVfb following hole injection. 2. Experimental Thermal oxides |209 nm thick were grown on p-type (100) Si substrates at 9258C in dry oxygen. The oxides were implanted with either 43 keV Si or 58 keV Ar ions so as to place the peak of the implant 60 nm below the oxide surface (149 nm from the Si–SiO 2 interface). Implant doses of 1310 13 , 1310 14 , 1310 15 , or 1310 16 cm 22 were used. The implanted oxides, along with unimplanted control oxides, were then annealed for 30 min in Ar at 700, 900, or 10508C. Following the anneal, 0.005-cm 2 capacitors were made by evaporating semitransparent (15-nm thick) Au gates on the oxide. Charge was injected into the oxide using photo-assisted injection techniques. During charge injection, the gate electrode was biased at 120.9 V so as to apply a field of 1 MV/ cm across the oxide. For electron injection, the semitransparent gate electrode was illuminated with ultraviolet (UV) light (hn , 6 eV), resulting in the photoemission of electrons from the Si substrate into the oxide. The injected electrons then moved through the oxide to the gate electrode under the influence of the applied field. For hole injection, the gate electrode was illuminated with vacuum UV light (hn 5 10.0 eV). This light is absorbed in the oxide within |10 nm of the gate electrode by forming electron–hole pairs. Because of the positive gate bias, the holes are swept through the oxide, and the electrons return to the gate electrode. During both types of charge injection, the injected dose was determined by measuring the amount of charge transported across the oxide. The extent of charge trapping in the oxide was characterized by DVfb , the shift in the flatband voltage resulting from the charge injection. The flatband voltage was determined before and after injection using standard high frequency (1 MHz) capacitance–voltage measurements. For electron trapping measurements, DVfb was determined after injecting 0.3310 14 electrons / cm 2 into the oxide. At this dose all electron traps with capture cross sections larger than |1310 213 cm 2 should be filled. Hole trapping was characterized by the value of DVfb obtained after injecting 1310 14 holes / cm 2 . Since the dominant hole trap in unimplanted oxides has a capture cross section of |3310 213 cm 2 , these traps should all be filled at this dose.
3. Results and discussion Significant values of DVfb following electron injection were found only in oxides that had been implanted with very high doses of Si, in agreement with results of Ref. [4]. As shown in Fig. 1, DVfb resulting from electron injection into oxides implanted with 1310 16 Si / cm 2 is strongly dependent on the post-implant anneal temperature. For oxides annealed at 700 or 9008C, DVfb is positive, indicating that net negative charge is trapped in the oxide. For oxides annealed at 10508C, however, DVfb is
B. J. Mrstik et al. / Microelectronic Engineering 59 (2001) 285 – 289
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Fig. 1. DVfb versus number of injected electrons for thermal oxides implanted with 1310 16 Si 1 / cm 2 , then annealed in Ar at 700, 900, or 10508C for 30 min in Ar.
negative. Afanas’ev et al. [5] noted a similar negative shift in Vfb following UV photo-injection into some oxides fabricated by the separation by implantation of oxygen (SIMOX) process, and attributed the effect to the presence of small Si clusters in the oxide. These clusters are amphoteric, i.e. they can become negatively charged by capturing an electron, or positively charged by emitting an electron when photoionized by a photon having an energy of at least 4.3 eV. Since UV light and electrons are both present during the electron injection, DVfb is determined by the relative amounts of photoionization and electron trapping / neutralization. High temperature annealing is known to precipitate cluster formation after implantation [6], and therefore would be expected to affect DVfb as shown in Fig. 1. Fig. 2 shows DVfb after injecting 1310 14 holes / cm 2 as a function of implant dose and anneal temperature. For all oxides except those implanted with the highest dose of Si, the implant results in larger values of DVfb , in agreement with earlier results of Nietzert et al. [3]. For oxides implanted with Si, however, Fig. 2 shows that increasing the implant dose from 1310 15 to 1310 16 Si / cm 2 drastically reduces DVfb to a value even smaller than in the unimplanted oxide. A similar reduction in 15 22 DVfb was reported in Ref. [4] for oxides implanted with 5310 cm Al, Si, or P. As pointed out in Ref. [4], the observation that oxides implanted with a high dose of Si have a smaller DVfb than unimplanted oxides cannot be explained in terms of models in which the primary hole traps are assumed to be oxygen vacancies (E9 centers) located near the Si–SiO 2 interface, since there is no reason to suspect that implantation of additional Si |149 nm from the Si–SiO 2 interface would reduce the number of oxygen vacancies near the Si–SiO 2 interface. Afanas’ev and Stesmans [7,8] have recently discussed several additional problems with these types of E9-related models. They show that the dominant hole trap is related to defects formed by radiolytic hydrogen. The reduced values of DVfb observed here in oxides that have been implanted with high doses of Si can be understood in terms of the results of Refs. [7] and [8] if it is assumed that the implantation obstructs the movement of radiolytic hydrogen to the Si–SiO 2 interface, and therefore affects where the injected charge is trapped. Fig. 3 shows that the shape of the DVfb vs. injected hole dose curve gradually changes as the implant dose is increased, and is consistent with this hypothesis. For unimplanted oxides DVfb initially changes quickly, then saturates because the movement of hydrogen
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Fig. 2. DVfb versus implanted dose of either Si or Ar ions. After implantation oxides were annealed in Ar at 700, 900, or 10508C for 30 min. DVfb is measured after injecting 1310 14 holes / cm 2 into oxide. Top panel, Si implants; lower panel, Ar implants.
to the interface is unimpeded, so most charge is trapped near the interface. As the implant dose is increased, however, more hydrogen is temporarily trapped in the implanted region. Although this hydrogen can trap injected holes, the effect on DVfb is decreased because of the larger distance from the interface to the trapped charge. At the highest implant dose, most of the hydrogen is trapped in the
Fig. 3. Normalized DVfb versus injected hole dose for oxides implanted with various doses of Si then annealed in Ar at 10508C for 30 min. DVfb is normalized so that 21 corresponds to DVfb obtained after injecting 1310 14 holes / cm 2 .
B. J. Mrstik et al. / Microelectronic Engineering 59 (2001) 285 – 289
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implanted region and is only gradually released during subsequent charge injection. Once released, it can diffuse to the interface where it again traps holes, further increasing DVfb . 4. Conclusion The results presented here for Si implanted oxides are in agreement with those presented in Ref. [4], and again suggest that high dose Si implantation prevents injected holes from being trapped near the Si–SiO 2 interface. In Ref. [4] this was attributed to hydrogen trapping by implant damage to the SiO 2 structure. The results presented here, however, show that high dose Ar implantation does not similarly reduce DVfb , indicating that the reduction comes about from chemical aspects of the implant. The presence of silicon nanoclusters in high dose Si implanted oxides, shown by the data in Fig. 1, suggests that it is the presence of silicon clusters in the oxide, not structural effects, that is responsible for trapping hydrogen (and therefore injected charge) in the oxide bulk rather than at the Si–SiO 2 interface in oxides implanted with large doses of Si. Acknowledgements This work was supported by the Defense Threat Reduction Agency. References [1] D.R. Young et al., Characterization of electron traps in aluminum-implanted SiO 2 , IBM J. Res. Develop. 22 (1978) 285–288. [2] M. Offenberg, P. Balk, Ion implantation induced stoichiometric imbalance in SiO 2 , Appl. Surf. Sci. 30 (1987) 265–271. [3] H. Neitzert et al., Hole capture in SiO 2 after ion implantation, Appl. Surf. Sci. 30 (1987) 272–277. [4] B.J. Mrstik et al., Hole and electron trapping in ion implanted thermal oxides and SIMOX, IEEE Trans. Nucl. Sci. 47 (2000) 2189–2195. [5] V.V. Afanas’ev et al., Confinement phenomena in buried oxides of SIMOX structures as affected by processing, J. Electrochem. Soc. 143 (1996) 695–700. [6] T. Shimizu-Iwayama et al., Optical properties of silicon nanoclusters fabricated by ion implantation, J. Appl. Phys. 83 (1998) 6018–6022. [7] V.V. Afanas’ev, A. Stesmans, Charge state of paramagnetic E9 centre in thermal SiO 2 layers on silicon, J. Phys.: Condens. Matter. 12 (2000) 2285–2290. [8] V.V. Afanas’ev, A. Stesmans, Proton nature of radiation-induced positive charge in SiO 2 layers on Si, Europhys. Lett. 53 (2001) 233–239.