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Ionisation and trapping of hydrogen at SiO2 interfaces
Materials Science and Engineering B58 (1999) 56 – 59
Ionisation and trapping of hydrogen at SiO2 interfaces V.V. Afanas’ev *, A. Stesmans Department of Physics, Uni6ersity of Leu6en, Celestijnenlaan 200D, 3001 Leu6en, Belgium
Abstract Annealing of interfaces between SiO2 and (111)Si, (100)Si in H2 in the temperature range 450 – 800°C is found to introduce a considerable density (up to 1013 cm − 2) of positively charged centres. There is no comparable density of dangling bonds initially present nor generated during the annealing at the Si/SiO2 interfaces or in the SiO2 layer that could account for the observed hydrogen bonding. Therefore, the hydrogen is suggested to be trapped in the positively charged valence alternation state 3-fold coordinated oxygen resembling the well known hydronium ion (H3O) + . © 1999 Elsevier Science S.A. All rights reserved. Keywords: Silicon; Hydrogen; Interfaces; Surface ionization; Fixed charge
1. Introduction
2. Experimental
Interaction of hydrogen with silicon interfaces, including that of the Si/SiO2 unit, vital for Si electronic devices, is routinely pictured as passivation/depassivation of dangling bonds of the surface Si atoms. Along this scheme, the hydrogen is chemically bonded in the Si3/Si–H configuration, i.e. an electrically neutral diamagnetic state. This Si – H bond can be thermally dissociated with activation energy of dissociation of about 2.6 eV [1]. The Si-dangling bonds left are the well known Pb-centres (Pb in (111)Si/SiO2; Pb0 and Pb1 in (100)Si/SiO2), electrically neutral when paramagnetic. Thus, along this scheme, hydrogen interacts with Si/ SiO2 interfaces chemically, without affecting electrical neutrality. In the present work we will provide experimental evidence for another, physical type of interaction. The hydrogen is found to form positively charged diamagnetic states at the interfaces of SiO2 with Si, without correlation with the density of available dangling bonds. The latter refers to H bonding in an overcoordinated configuration to an interfacial oxygen atom, resembling that of the hydronium ion (H3O) + . The activation energy of dissociation of this state is found to be remarkably high (:2.4 eV).
The studied samples were prepared by oxidation of n- and p-type (111) and (100)Si wafers. These were subsequently annealed in H2 (99.9999%, 1h, 1.1 atm) in the temperature range 400–800°C. The metal-oxidesemiconductor (MOS) structures were prepared by evaporation of 15-nm thick Au or Al electrodes onto the oxide. The charge density at the Si/SiO2 interfaces was monitored using 1 MHz capacitance–voltage (C– V) characteristics. Next, upon de-hydrogenation of the sample in vacuum at 620°C, the density of Si/SiO2 interface dangling bond defects was determined from the interface state density measurements and electron spin resonance (ESR) spectroscopy. More details regarding sample preparation and measurement procedures were published elsewhere [2–4].
* Corresponding author. Tel.: + 32-16-327167; fax: + 32-16327987; e-mail: [email protected].
3. Results The treatment in hydrogen at T\ 475°C was found to introduce positive charge (Qf) at the Si/SiO2 interface. The Qf density observed at the (111)Si/SiO2 and (100)Si/SiO2 interfaces is shown as a function of the anneal temperature in Fig. 1. The density of interface states after H2 anneal was found to be below 5×1010 cm − 2, suggesting nearly complete passivation of the Pb centres. In spite of the high density of the positively charged centres (up to 1013 cm − 2) no ESR signal could
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 2 7 5 - X
V.V. Afanas’e6, A. Stesmans / Materials Science and Engineering B58 (1999) 56–59
be traced attributable to them. Apparently then, the H-produced Qf states are diamagnetic. The charge density is seen to rise from 450°C onward, showing a two peak-like structure, and decreases above 670°C. The (111)Si/SiO2 and (100)Si/SiO2 structures show a similar charging behaviour, except for the T \670°C range where the charge density in (111)Si/SiO2 is always higher than in (100)Si/SiO2. The latter points towards the interfacial nature of the H-induced charge. This conclusion is further supported by the observed absence of any SiO2 thickness dependence of Qf down to ultra˚ thick), and by the resistance to thin oxides (50 A neutralisation by electrons injected from silicon [2–4]. The experiments on internal photoemission of electrons from Si into SiO2 reveal the Coulomb-like perturbation of the image force barrier after H2 anneal. The corresponding locally reduced potential barrier for electrons is found to be dependant on the strength of the electric field in SiO2. From the analysis of the barrier lowering with increasing electric field strength in SiO2, the mean distance between the positively charged centre and the ˚ , which physically Si surface was estimated as 2 91 A means location within the first atomic bilayer of the ˚ [5]). oxide (the Si–O bond length in SiO2 is 1.63 A Subsequent dehydrogenation of the H2-annealed samples in vacuum at 620°C was found to eliminate the positive charge. The observed reversibility of the charging (in H2) and discharging (in vacuum) points towards some bonded state of H as the origin of the positive charge. The isochronal discharging annealing experiments (results not shown) allowed to determine the binding energy of the positively bonded hydrogen: 2.39 and 2.33 eV for the (111)Si/SiO2, and (100)Si/SiO2 interfaces, respectively [2 – 4]. The relationship of Qf to bonded H at the Si/SiO2 interfaces is also supported by the observed strong reduction of Qf at 400°C by atomic H in contrast to molecular H2: The charge is annealed in structures with an Al overlayer on the oxide, but not in the samples with bare SiO2.
Fig. 1. Density of H2-annealing induced positive charge in various Si/SiO2 structures as a function of annealing temperature. Symbols refer to (111)Si/SiO2 with dox = 72 nm (O) and (100)Si/SiO2 with dox = 55 nm ( ). Filled and open symbols correspond to n-, and p-type Si, respectively.
57
Fig. 2. Fraction of the positive charge, introduced by annealing in H2 at the indicated temperature (cf. Fig. 1), remaining at the (111)Si/SiO2 (O) and (100)Si/SiO2 ( ) interface after an additional 30 min anneal in H2 at 450°C. Filled and open symbols correspond to n-, and p-type Si, respectively. Lines are guides to the eye.
An important observation regarding the stability of the H-induced positive charge is its sensitivity to the crystallographic orientation of the Si substrate. In Fig. 2 is shown the fraction of Qf remaining in (111)Si/SiO2 and (100)Si/SiO2 structures, first annealed in H2 at different temperatures, after subsequent annealing in H2 at 450°C for 1 h. Within experimental accuracy, the remaining portion of the charge is independent on the annealing temperature at the (111)Si/SiO2 interface. In contrast, in (100)Si/SiO2 this fraction decreases for TH670°C. The reduced stability of Qf may explain the difference in the charge density in (100)Si/SiO2 than in (111)Si/SiO2. The densities of the Si-dangling bond defects (Pb-centres) observed in the H2-annealed samples after dehydrogenation as electrically active interface states are shown in Fig. 3 for the n- and p-type samples prepared on (111)Si and (100)Si. It remains nearly unchanged up to : 550°C, then gradually increases, and shows a distinct increase above 670°C. While the ESR experiments in (111)Si/SiO2 (cf. Fig. 1 in [4]) show a trend similar to electrical measurements, the density of ESR active centres in (100)Si/SiO2 appears considerably
Fig. 3. Interface state density in various Si – SiO2 structures as a function of H2 annealing temperature, measured after subsequent depassivation in vacuum (620°C, 1 h.). Symbols refer to (111)Si–SiO2 with dox =72 nm (O) and (100)Si/SiO2 with dox =55 nm ( ). Filled and open symbols correspond to n-, and p-type Si, respectively.
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V.V. Afanas’e6, A. Stesmans / Materials Science and Engineering B58 (1999) 56–59
higher than the density of electrically active interface traps (cf. Figs. 1 and 3). The latter is related to the fact that one of the Si-dangling bond defects in (100)Si/SiO2 (Pb1) does not have energy levels in the Si bandgap [7]. Another one (Pb0) is detected electrically, and, as shown in Fig. 3, exhibits essentially the same behaviour as the Pb centre in (111)Si/SiO2. The latter observation supports the suggestion that the Pb0 centre in (100)Si/SiO2 is similar to the Pb centre in (111)Si/SiO2. Comparison between the positive charge density (Fig. 1), and the density of Si-dangling bond states (Fig. 3), indicates the absence of any correlation between them. For instance, Qf rises in the temperature range 450–550°C to :1 ×1013 q cm − 2 without any measurable variation in the dangling bond density: The latter remains considerably lower, i.e. (4.89 0.5) ×1012 and (2.49 0.3) ×1012 cm − 2 in (111)Si/ SiO2, and (100)Si/SiO2, respectively, as determined from the ESR measurements. Therefore, one can exclude a direct relationship between the H-induced positive charge and dangling bonds at the Si surface. To this should be added that no correlation between Qf and the Si-dangling bonds in the oxide (E% centres) could be found either [2 – 4]. Therefore, it appears that the bonding of hydrogen in a positively charged state does not require any kind of bond break. Instead, the difference in the charge densities observed after H2 anneal between Si/SiO2 and SiC/SiO2 structures suggests the electron transfer from the trapped H to the semiconductor substrate to be involved in the rate limiting step [4].
4. Discussion In short, the experimental data suggest that hydrogen may be bonded at the Si – SiO2 interfaces in an ionised (positively charged) state. This bonding occurs in the first atomic layers of the oxide adjacent to the interface without direct participation of either dangling bond defects located at the Si surface nor in SiO2. Taking into account the large binding energy of H in the charged state and the involvement of the electron transfer in its formation, we suggest the observed state to be similar to the hydronium ion (H3O) + , in which strong proton bonding is known to occur [8]. At the Si – SiO2 interface the hydronium-like bonding configuration can be pictured as Si3 Si– (OH) + –Si O3, i.e. as an over-coordinated oxygen atom with two Si neighbours and an H + bonded by its lone-pair electrons. This configuration was suggested previously to be generically related to the interfacial dangling bond defects (Pb centres) [9 – 12]. The present results, however, reveal the absence of such a relationship. Contrary to the previous expectations,
the density of the charge shows a trend to decrease above :550 and : 670°C on (Fig. 1), while the density of Pb-related interface states exhibits an increase starting from these temperatures onward. As a possible explanation of the coinciding temperature of Qf decrease and increasing Pb density we suggest the destruction of some positively charged centres leading to the generation of Si-dangling bonds: The 3-fold coordinated oxygen atom returns to the 2-fold neutral configuration, but with one of the bonds saturated with H instead of the surface Si atom (bond switching): O3/Si— O— Si/Si3 + HO3/Si—(OH) + — Si/Si3 + e O3/Si— O— H + — Si/Si3 In support of this suggestion one could refer to the fact that, in terms of numbers, the variation in density of H-induced positive charge and Pb centres in (111)Si/SiO2 have close values, i.e. (2–3)× 1012 cm − 2 at T\ 550°C and :1× 1013 cm − 2 at T\670°C. Worth mentioning here also is the observation that no Pb generation is observed at TB 640°C for annealing done in high vacuum [6]. Clearly then, the generation of Pb centres during H2 annealing from : 550°C on is due to the chemical action of hydrogen and may proceed as a result of the above described bond switching. The sensitivity of the H-induced positive charge to the crystallographic orientation of the Si points to the importance of the particular c–Si–oxide connectivity for bonding and ionisation of H. Worth noticing here is that the positively charged (donor-like) interface states generated in (100)Si/SiO2 by atomic H at room temperature are unstable in time and anneal nearly completely at T\ 150°C [13]. By contrast, the H-induced charge observed in the present study is found remarkably stable, which suggests the involvement of some interfacial network rearrangement in the formation of a stable charged state. Theoretical calculations predict an increase in the binding energy of atomic H to a bridging O atom in SiO2 with increasing Si–O–Si angle [5]. Apparently then, the H bonding may be strengthened if rotation of SiO4 tetrahedra is allowed. Regarding the very first interfacial layer of the oxide, this would require a long-range atomic rearrangement involving both the Si substrate and SiO2 without bond break. Such flipping of the SiO4 tetrahedra is known to occur during the phase transition between a and b-quartz near T= 846 K [14]. The correlation function of this transition shows a steep increase from T=750 K onward, which is close to the initiation temperature of the H-induced positive charging. Therefore, we hypothesise that the charging is triggered by the rearrangement of the near-interfacial oxide in a way allowing oxygen to form the stable over-coordinated state with hydrogen.
V.V. Afanas’e6, A. Stesmans / Materials Science and Engineering B58 (1999) 56–59
59
5. Conclusion
References
The observed positively-charged bonded state of hydrogen at the interfaces of Si with SiO2 exposes several novel aspects of hydrogen interaction with the interface. First, the high binding energy of a proton in the overcoordinated oxygen complex (: 2.4 eV) gives allowance to cracking of H2 molecules in a similar way as it occurs at Pb centres at the Si/SiO2 interface. Thus, the generation of atomic H at elevated temperatures (\ 500°C) will occur at the interface independent of the defects present. Second, the formed charged states may account for the well known oxidation-induced fixed charge in Si/SiO2 structures: There is a good correlation between their behaviour [15]. Third, the built-in electric field at the Si/SiO2 interfaces may promote electrochemical reactions which would accompany the conventional chemical ones. In a broader view, the observed H bonding in an ionised state exposes the interface as a possible site for physical impurity uptake followed by a charge transfer, thus providing a solid state phenomenon similar to the well known surface ionisation effect.
[1] E.H. Poindexter, J. Non-Cryst. Solids 187 (1995) 257. [2] V.V. Afanas’ev, A. Stesmans, J. Phys. Condens. Matter 10 (1998) 89. [3] V.V. Afanas’ev, A. Stesmans, Appl. Phys. Lett. 72 (1998) 79. [4] V.V. Afanas’ev, A. Stesmans, Phys. Rev. Lett. 80 (1998) 5176. [5] A.H. Edwards, G. Germann, Nucl. Instr. Methods B 32 (1988) 238. [6] A. Stesmans, V.V. Afanas’ev, Appl. Phys. Lett. 72 (1998) 2271. [7] A. Stesmans, V.V. Afanas’ev, Phys. Rev. B 57 (1998) 10 030. [8] B.E. Conway, Ionic Hydration in Chemistry and Biophysics, Elsevier, Amsterdam, 1981. [9] A.H. Edwards, Phys. Rev. B 44 (1991) 1832. [10] G.J. Gerardi, E.H. Poindexter, M. Harlatz, W.L. Warren, E.H. Nicollian, A.H. Edwards, J. Electrochem. Soc. 138 (1991) 3765. [11] Z. Jing, G. Lucovsky, J.L. Whitten, J. Vac. Sci. Technol. B 13 (1995) 1613. [12] W.L. Warren, K. Vanheusden, D.M. Fleetwood, J.R. Schwank, M.R. Shaneyfelt, P.S. Winokur, R.A.B. Devine, IEEE Trans. Nucl. Sci. 43 (1996) 2617. [13] J.M.M. de Nijs, K.G. Druijf, V.V. Afanas’ev, E. van der Drift, P. Balk, Appl. Phys. Lett. 65 (1994) 2428. [14] S. Tsuneyuki, H. Aoki, M. Tsukada, Y. Matsui, Phys. Rev. Lett. 64 (1990) 776. [15] B.E. Deal, J. Electrochem. Soc. 121 (1974) 199C.
.
Ionisation and trapping of hydrogen at SiO2 interfaces V.V. Afanas’ev *, A. Stesmans Department of Physics, Uni6ersity of Leu6en, Celestijnenlaan 200D, 3001 Leu6en, Belgium
Abstract Annealing of interfaces between SiO2 and (111)Si, (100)Si in H2 in the temperature range 450 – 800°C is found to introduce a considerable density (up to 1013 cm − 2) of positively charged centres. There is no comparable density of dangling bonds initially present nor generated during the annealing at the Si/SiO2 interfaces or in the SiO2 layer that could account for the observed hydrogen bonding. Therefore, the hydrogen is suggested to be trapped in the positively charged valence alternation state 3-fold coordinated oxygen resembling the well known hydronium ion (H3O) + . © 1999 Elsevier Science S.A. All rights reserved. Keywords: Silicon; Hydrogen; Interfaces; Surface ionization; Fixed charge
1. Introduction
2. Experimental
Interaction of hydrogen with silicon interfaces, including that of the Si/SiO2 unit, vital for Si electronic devices, is routinely pictured as passivation/depassivation of dangling bonds of the surface Si atoms. Along this scheme, the hydrogen is chemically bonded in the Si3/Si–H configuration, i.e. an electrically neutral diamagnetic state. This Si – H bond can be thermally dissociated with activation energy of dissociation of about 2.6 eV [1]. The Si-dangling bonds left are the well known Pb-centres (Pb in (111)Si/SiO2; Pb0 and Pb1 in (100)Si/SiO2), electrically neutral when paramagnetic. Thus, along this scheme, hydrogen interacts with Si/ SiO2 interfaces chemically, without affecting electrical neutrality. In the present work we will provide experimental evidence for another, physical type of interaction. The hydrogen is found to form positively charged diamagnetic states at the interfaces of SiO2 with Si, without correlation with the density of available dangling bonds. The latter refers to H bonding in an overcoordinated configuration to an interfacial oxygen atom, resembling that of the hydronium ion (H3O) + . The activation energy of dissociation of this state is found to be remarkably high (:2.4 eV).
The studied samples were prepared by oxidation of n- and p-type (111) and (100)Si wafers. These were subsequently annealed in H2 (99.9999%, 1h, 1.1 atm) in the temperature range 400–800°C. The metal-oxidesemiconductor (MOS) structures were prepared by evaporation of 15-nm thick Au or Al electrodes onto the oxide. The charge density at the Si/SiO2 interfaces was monitored using 1 MHz capacitance–voltage (C– V) characteristics. Next, upon de-hydrogenation of the sample in vacuum at 620°C, the density of Si/SiO2 interface dangling bond defects was determined from the interface state density measurements and electron spin resonance (ESR) spectroscopy. More details regarding sample preparation and measurement procedures were published elsewhere [2–4].
* Corresponding author. Tel.: + 32-16-327167; fax: + 32-16327987; e-mail: [email protected].
3. Results The treatment in hydrogen at T\ 475°C was found to introduce positive charge (Qf) at the Si/SiO2 interface. The Qf density observed at the (111)Si/SiO2 and (100)Si/SiO2 interfaces is shown as a function of the anneal temperature in Fig. 1. The density of interface states after H2 anneal was found to be below 5×1010 cm − 2, suggesting nearly complete passivation of the Pb centres. In spite of the high density of the positively charged centres (up to 1013 cm − 2) no ESR signal could
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 2 7 5 - X
V.V. Afanas’e6, A. Stesmans / Materials Science and Engineering B58 (1999) 56–59
be traced attributable to them. Apparently then, the H-produced Qf states are diamagnetic. The charge density is seen to rise from 450°C onward, showing a two peak-like structure, and decreases above 670°C. The (111)Si/SiO2 and (100)Si/SiO2 structures show a similar charging behaviour, except for the T \670°C range where the charge density in (111)Si/SiO2 is always higher than in (100)Si/SiO2. The latter points towards the interfacial nature of the H-induced charge. This conclusion is further supported by the observed absence of any SiO2 thickness dependence of Qf down to ultra˚ thick), and by the resistance to thin oxides (50 A neutralisation by electrons injected from silicon [2–4]. The experiments on internal photoemission of electrons from Si into SiO2 reveal the Coulomb-like perturbation of the image force barrier after H2 anneal. The corresponding locally reduced potential barrier for electrons is found to be dependant on the strength of the electric field in SiO2. From the analysis of the barrier lowering with increasing electric field strength in SiO2, the mean distance between the positively charged centre and the ˚ , which physically Si surface was estimated as 2 91 A means location within the first atomic bilayer of the ˚ [5]). oxide (the Si–O bond length in SiO2 is 1.63 A Subsequent dehydrogenation of the H2-annealed samples in vacuum at 620°C was found to eliminate the positive charge. The observed reversibility of the charging (in H2) and discharging (in vacuum) points towards some bonded state of H as the origin of the positive charge. The isochronal discharging annealing experiments (results not shown) allowed to determine the binding energy of the positively bonded hydrogen: 2.39 and 2.33 eV for the (111)Si/SiO2, and (100)Si/SiO2 interfaces, respectively [2 – 4]. The relationship of Qf to bonded H at the Si/SiO2 interfaces is also supported by the observed strong reduction of Qf at 400°C by atomic H in contrast to molecular H2: The charge is annealed in structures with an Al overlayer on the oxide, but not in the samples with bare SiO2.
Fig. 1. Density of H2-annealing induced positive charge in various Si/SiO2 structures as a function of annealing temperature. Symbols refer to (111)Si/SiO2 with dox = 72 nm (O) and (100)Si/SiO2 with dox = 55 nm ( ). Filled and open symbols correspond to n-, and p-type Si, respectively.
57
Fig. 2. Fraction of the positive charge, introduced by annealing in H2 at the indicated temperature (cf. Fig. 1), remaining at the (111)Si/SiO2 (O) and (100)Si/SiO2 ( ) interface after an additional 30 min anneal in H2 at 450°C. Filled and open symbols correspond to n-, and p-type Si, respectively. Lines are guides to the eye.
An important observation regarding the stability of the H-induced positive charge is its sensitivity to the crystallographic orientation of the Si substrate. In Fig. 2 is shown the fraction of Qf remaining in (111)Si/SiO2 and (100)Si/SiO2 structures, first annealed in H2 at different temperatures, after subsequent annealing in H2 at 450°C for 1 h. Within experimental accuracy, the remaining portion of the charge is independent on the annealing temperature at the (111)Si/SiO2 interface. In contrast, in (100)Si/SiO2 this fraction decreases for TH670°C. The reduced stability of Qf may explain the difference in the charge density in (100)Si/SiO2 than in (111)Si/SiO2. The densities of the Si-dangling bond defects (Pb-centres) observed in the H2-annealed samples after dehydrogenation as electrically active interface states are shown in Fig. 3 for the n- and p-type samples prepared on (111)Si and (100)Si. It remains nearly unchanged up to : 550°C, then gradually increases, and shows a distinct increase above 670°C. While the ESR experiments in (111)Si/SiO2 (cf. Fig. 1 in [4]) show a trend similar to electrical measurements, the density of ESR active centres in (100)Si/SiO2 appears considerably
Fig. 3. Interface state density in various Si – SiO2 structures as a function of H2 annealing temperature, measured after subsequent depassivation in vacuum (620°C, 1 h.). Symbols refer to (111)Si–SiO2 with dox =72 nm (O) and (100)Si/SiO2 with dox =55 nm ( ). Filled and open symbols correspond to n-, and p-type Si, respectively.
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V.V. Afanas’e6, A. Stesmans / Materials Science and Engineering B58 (1999) 56–59
higher than the density of electrically active interface traps (cf. Figs. 1 and 3). The latter is related to the fact that one of the Si-dangling bond defects in (100)Si/SiO2 (Pb1) does not have energy levels in the Si bandgap [7]. Another one (Pb0) is detected electrically, and, as shown in Fig. 3, exhibits essentially the same behaviour as the Pb centre in (111)Si/SiO2. The latter observation supports the suggestion that the Pb0 centre in (100)Si/SiO2 is similar to the Pb centre in (111)Si/SiO2. Comparison between the positive charge density (Fig. 1), and the density of Si-dangling bond states (Fig. 3), indicates the absence of any correlation between them. For instance, Qf rises in the temperature range 450–550°C to :1 ×1013 q cm − 2 without any measurable variation in the dangling bond density: The latter remains considerably lower, i.e. (4.89 0.5) ×1012 and (2.49 0.3) ×1012 cm − 2 in (111)Si/ SiO2, and (100)Si/SiO2, respectively, as determined from the ESR measurements. Therefore, one can exclude a direct relationship between the H-induced positive charge and dangling bonds at the Si surface. To this should be added that no correlation between Qf and the Si-dangling bonds in the oxide (E% centres) could be found either [2 – 4]. Therefore, it appears that the bonding of hydrogen in a positively charged state does not require any kind of bond break. Instead, the difference in the charge densities observed after H2 anneal between Si/SiO2 and SiC/SiO2 structures suggests the electron transfer from the trapped H to the semiconductor substrate to be involved in the rate limiting step [4].
4. Discussion In short, the experimental data suggest that hydrogen may be bonded at the Si – SiO2 interfaces in an ionised (positively charged) state. This bonding occurs in the first atomic layers of the oxide adjacent to the interface without direct participation of either dangling bond defects located at the Si surface nor in SiO2. Taking into account the large binding energy of H in the charged state and the involvement of the electron transfer in its formation, we suggest the observed state to be similar to the hydronium ion (H3O) + , in which strong proton bonding is known to occur [8]. At the Si – SiO2 interface the hydronium-like bonding configuration can be pictured as Si3 Si– (OH) + –Si O3, i.e. as an over-coordinated oxygen atom with two Si neighbours and an H + bonded by its lone-pair electrons. This configuration was suggested previously to be generically related to the interfacial dangling bond defects (Pb centres) [9 – 12]. The present results, however, reveal the absence of such a relationship. Contrary to the previous expectations,
the density of the charge shows a trend to decrease above :550 and : 670°C on (Fig. 1), while the density of Pb-related interface states exhibits an increase starting from these temperatures onward. As a possible explanation of the coinciding temperature of Qf decrease and increasing Pb density we suggest the destruction of some positively charged centres leading to the generation of Si-dangling bonds: The 3-fold coordinated oxygen atom returns to the 2-fold neutral configuration, but with one of the bonds saturated with H instead of the surface Si atom (bond switching): O3/Si— O— Si/Si3 + HO3/Si—(OH) + — Si/Si3 + e O3/Si— O— H + — Si/Si3 In support of this suggestion one could refer to the fact that, in terms of numbers, the variation in density of H-induced positive charge and Pb centres in (111)Si/SiO2 have close values, i.e. (2–3)× 1012 cm − 2 at T\ 550°C and :1× 1013 cm − 2 at T\670°C. Worth mentioning here also is the observation that no Pb generation is observed at TB 640°C for annealing done in high vacuum [6]. Clearly then, the generation of Pb centres during H2 annealing from : 550°C on is due to the chemical action of hydrogen and may proceed as a result of the above described bond switching. The sensitivity of the H-induced positive charge to the crystallographic orientation of the Si points to the importance of the particular c–Si–oxide connectivity for bonding and ionisation of H. Worth noticing here is that the positively charged (donor-like) interface states generated in (100)Si/SiO2 by atomic H at room temperature are unstable in time and anneal nearly completely at T\ 150°C [13]. By contrast, the H-induced charge observed in the present study is found remarkably stable, which suggests the involvement of some interfacial network rearrangement in the formation of a stable charged state. Theoretical calculations predict an increase in the binding energy of atomic H to a bridging O atom in SiO2 with increasing Si–O–Si angle [5]. Apparently then, the H bonding may be strengthened if rotation of SiO4 tetrahedra is allowed. Regarding the very first interfacial layer of the oxide, this would require a long-range atomic rearrangement involving both the Si substrate and SiO2 without bond break. Such flipping of the SiO4 tetrahedra is known to occur during the phase transition between a and b-quartz near T= 846 K [14]. The correlation function of this transition shows a steep increase from T=750 K onward, which is close to the initiation temperature of the H-induced positive charging. Therefore, we hypothesise that the charging is triggered by the rearrangement of the near-interfacial oxide in a way allowing oxygen to form the stable over-coordinated state with hydrogen.
V.V. Afanas’e6, A. Stesmans / Materials Science and Engineering B58 (1999) 56–59
59
5. Conclusion
References
The observed positively-charged bonded state of hydrogen at the interfaces of Si with SiO2 exposes several novel aspects of hydrogen interaction with the interface. First, the high binding energy of a proton in the overcoordinated oxygen complex (: 2.4 eV) gives allowance to cracking of H2 molecules in a similar way as it occurs at Pb centres at the Si/SiO2 interface. Thus, the generation of atomic H at elevated temperatures (\ 500°C) will occur at the interface independent of the defects present. Second, the formed charged states may account for the well known oxidation-induced fixed charge in Si/SiO2 structures: There is a good correlation between their behaviour [15]. Third, the built-in electric field at the Si/SiO2 interfaces may promote electrochemical reactions which would accompany the conventional chemical ones. In a broader view, the observed H bonding in an ionised state exposes the interface as a possible site for physical impurity uptake followed by a charge transfer, thus providing a solid state phenomenon similar to the well known surface ionisation effect.
[1] E.H. Poindexter, J. Non-Cryst. Solids 187 (1995) 257. [2] V.V. Afanas’ev, A. Stesmans, J. Phys. Condens. Matter 10 (1998) 89. [3] V.V. Afanas’ev, A. Stesmans, Appl. Phys. Lett. 72 (1998) 79. [4] V.V. Afanas’ev, A. Stesmans, Phys. Rev. Lett. 80 (1998) 5176. [5] A.H. Edwards, G. Germann, Nucl. Instr. Methods B 32 (1988) 238. [6] A. Stesmans, V.V. Afanas’ev, Appl. Phys. Lett. 72 (1998) 2271. [7] A. Stesmans, V.V. Afanas’ev, Phys. Rev. B 57 (1998) 10 030. [8] B.E. Conway, Ionic Hydration in Chemistry and Biophysics, Elsevier, Amsterdam, 1981. [9] A.H. Edwards, Phys. Rev. B 44 (1991) 1832. [10] G.J. Gerardi, E.H. Poindexter, M. Harlatz, W.L. Warren, E.H. Nicollian, A.H. Edwards, J. Electrochem. Soc. 138 (1991) 3765. [11] Z. Jing, G. Lucovsky, J.L. Whitten, J. Vac. Sci. Technol. B 13 (1995) 1613. [12] W.L. Warren, K. Vanheusden, D.M. Fleetwood, J.R. Schwank, M.R. Shaneyfelt, P.S. Winokur, R.A.B. Devine, IEEE Trans. Nucl. Sci. 43 (1996) 2617. [13] J.M.M. de Nijs, K.G. Druijf, V.V. Afanas’ev, E. van der Drift, P. Balk, Appl. Phys. Lett. 65 (1994) 2428. [14] S. Tsuneyuki, H. Aoki, M. Tsukada, Y. Matsui, Phys. Rev. Lett. 64 (1990) 776. [15] B.E. Deal, J. Electrochem. Soc. 121 (1974) 199C.
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