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Electronic recombinations and ionic transport in BPSG layers
Microelectronic Engineering 55 (2001) 59–64 www.elsevier.nl / locate / mee
Electronic recombinations and ionic transport in BPSG layers a, b a a a a a A. Vedda *, E. Carollo , S. Croci , M. Martini , A. Morbiato , G. Spinolo , M. Vitali , L. Zanotti c a
INFM and Dipartimento di Scienza dei Materiali-Universita` di Milano-Bicocca, Via Cozzi 53, 20125 Milan, Italy b Dipartimento di Fisica dell’ Universita` di Padova, Via Marzolo 8, 35100 Padova, Italy c ST Microelectronics, Via C. Olivetti 2, 20041 Agrate Brianza, Italy
Abstract Thermally stimulated luminescence (TSL) and current (TSC) measurements above room temperature were performed on 1150 nm borophosphosilicate glass films obtained by sub-atmospheric chemical vapour deposition. Several concentrations of B and P ions were considered, in the range 2–5% and 4–9% in weight, respectively. A TSC peak of ionic character at 908C was observed without prior X-irradiation, followed by a monotonically growing signal. Upon X-irradiation, broad TSL and TSC signals extending from 50 to 3008C were detected: the intensities of both the ionic peak and the radiation induced TSL and TSC signals increase by increasing the phosphorus content, while they decrease on boron increase. The TSL emission spectrum features two bands peaking at 2.5 and 2.85 eV and a minor component at around 3.2 eV. On the basis of these data, the dynamics of ionic carriers and electronic traps in BPSG layers are discussed. 2001 Elsevier Science B.V. All rights reserved. Keywords: BPSG layers; Thermoluminescence; Point defects; Ionic transport
1. Introduction Borophosphosilicate glass (BPSG) thin films are used in ULSI semiconductor manufacturing as premetal dielectrics for insulation between metal interconnections and underlying polysilicon or silicide gate structures. The main required film properties, such as conformal step coverage or gap filling capability (design rules # 0.25 mm), effective barrier to alkali ions, low reflow temperature ( , 9508C) and high planarization degree, are nowadays mostly provided by layers deposited in atmospheric or subatmospheric chemical vapour deposition (APCVD, SACVD) systems using tetraethylorthosilicate (TEOS), as silicon source, and ozone based chemistry. The macroscopic physical and chemical properties of these films (as density, refractive index, wet etch rate, planarization, defect formation and film stability, IR absorption) were largely studied in the *Corresponding author. Tel.: 139-02-6448-5162; fax: 139-02-6448-5400. E-mail address: [email protected] (A. Vedda). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 00 )00429-9
2001 Elsevier Science B.V. All rights reserved.
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past [1]; however, very little is known about point defects and mobile charges in these films, which can affect the electrical characteristics of active devices [2], and of their dependence upon B and P concentration. To investigate these points, thermally stimulated luminescence (TSL) and current (TSC) measurements have been performed on BPSG layers grown by SACVD with different B and P contents. The results provide evidence for the existence of both point defects acting as charge traps, and of mobile ionic carriers. The dependence of TSL and TSC signals upon B and P content is presented, and the possible nature of the observed defects is discussed.
2. Experimental BPSG film samples (1150 nm thick) with different B and P concentrations were prepared on k100l Czochralski silicon wafers, boron doped, with 1.7–2.5 V ? cm resistivity. The BPSG films were deposited in a single chamber SACVD system at 200 Torr and 5508C and then annealed by rapid thermal processing (RTP) at 10508C in N 2 lasting 30 s. Dopant concentrations were in the range 2–5% and 4–9% in weight for B and P, respectively. For TSC measurements, Al dots (f 5 3 mm) were patterned on the BPSG layers after RTP; a thermal treatment at 4308C lasting 30 min in forming gas ambient, was finally performed to assure a good adhesion of the Al dots. Samples of about 10310 mm were cleaved. TSL measurements were performed from room temperature (RT) up to 4008C with a linear heating rate of 18C / s using two different apparatus. In the first one the total emitted light was detected as a function of temperature by photon counting using an EMI 9635 QB photomultiplier tube. By this apparatus, simultaneous TSC measurements were also performed by using a Keithley 617 electrometer with applied voltages in the range 1–80 V; the sensitivity of the TSC measurements was of the order of 10 214 A. The second apparatus used was a laboratory-made high sensitivity TSL spectrometer measuring the TSL intensity as a function of both temperature and emission wavelength. The detector was a double stage Microchannel plate followed by a 512 diode array; the dispersive element was a 140 lines / mm holographic grating, the detection range being 200–800 nm. Irradiations were performed at room temperature by a Machlett OEG50 X-ray tube operated at 32 kV; the dose was (10 3 Gy.
3. Results and discussion TSC measurements were performed both without prior irradiation and following X-irradiation at RT. A TSC measurement performed without prior X-irradiation is displayed in Fig. 1, curve (A), in the case of a sample with 2%B and 9%P: a principal peak at around 908C is observed, followed by a shoulder at around 2008C superimposed to a monotonically growing signal. Moreover, a decreasing current signal very close to RT is also evidenced. Signals of similar shape were observed in all the other cases, so that the TSC curve is representative of all the considered samples. The TSC signal was also recorded after a polarization procedure: namely, after the first TSC run displayed in Fig. 1, curve (A), the sample was cooled at RT while keeping the applied electric field, and then heated again up to 4008C (second TSC run): in the second TSC curve (Fig. 1, curve (B) the 908C peak is completely
A. Vedda et al. / Microelectronic Engineering 55 (2001) 59 – 64
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Fig. 1. TSC curves without prior irradiation of a sample with 2%B and 9%P. (A) First TSC run; (B) second TSC run (the cooling cycle after the first TSC run was performed by keeping the applied voltage). Applied voltage580 V.
suppressed while the current signal close to RT and the signal above 2008C are still observable. Both the 908C peak and the monotonic growth observed at higher temperatures can be preliminarily assigned to drift of ionic carriers present in the films in different configurations. The 908C structure can be attributed to ionic charges which are thermally freed from trapping sites and undergo strong polarization upon the heating / cooling cycle with the applied field, possibly due to accumulation near to interfaces: this leads to a typical ‘space charge’ effect [3] evidenced by the comparison between curves (A) and (B) of Fig. 1. On the other hand, the monotonic growth can be associated to ionic carriers more deeply bound, only partially freed at higher temperatures. An opposite effect on the TSC intensity, and particularly of the 908C peak, has been observed by varying the concentration of B and P ions (Fig. 2(a) and (b)): namely, the signals increase by increasing the phosphorus content, while they decrease on boron increase. The nature of the ionic carriers responsible for the TSC structures is an open question. In this respect, it may be suggested that alkali ions present as trace impurities could be related with the monotonic current growth observed at T . 2008C, in analogy with what observed in bulk silica and in thermal SiO 2 films [4]. Moreover, alkali ions in a less deeply bound configuration could give rise to the TSC peak at 908C as well: the increase of the intensity of this peak on phosphorus increase calls for a direct participation of such dopant in the trapping site, and is in agreement with the well known role of phosphorus in gettering alkali ions in a stable form at room temperature [5]. On the other hand, the opposite behaviour of the 908C peak on the boron increase could be explained by suggesting that this ion introduces other ionic traps, which can compete with P-related ones in the capture of alkalis. As no stable gettering of alkalis by boron was ever observed, these boron-related traps should be more shallow with respect to P-related ones, and this could justify the lack of a TSC signal correlated with the presence of B in our measurements. In reality, a decreasing current is detected just above RT, which is however too close to the irradiation temperature to be resolved. We emphasize the preliminary character of the present interpretation which needs further experimental investigation, for example the extension of TSC measurements to lower temperatures and the direct evaluation of the alkali content in the layers. No TSL signal was observed without prior irradiation. Upon X-irradiation at RT, both TSL and
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Fig. 2. (a) TSC curves without prior X-irradiation of BPSG layers with 3% B and different P concentrations. Applied voltage580 V; (b) TSC curves without prior X-irradiation of BPSG layers with 7% P and different B concentrations. Applied voltage580 V.
TSC signals were detected in all samples. In Fig. 3 the TSL and TSC measurements are presented in a representative case, relative to the sample with 3%B and 7% P. A broad TSL signal extending from 50 to 3008C is detected, indicating the existence of a distribution of electronic trap levels [6]. The TSC signal is significantly increased after irradiation, and a new structure peaking at around 2008C is detected superimposed on a lower temperature shoulder and on a monotonically increasing signal at higher temperatures, already detected without prior irradiation. All samples displayed a similar qualitative behaviour: however, as in the case of the TSC data without irradiation, higher TSL and TSC signals were detected by increasing the P concentration while the opposite behaviour was observed on B increase: such a behaviour suggests that P doping increases the concentration of electronic traps and / or recombination centres. The radiation induced TSC structure at around 2008C can be attributed to electronic carriers freed from traps and at least partially undergoing radiative recombination at luminescent sites giving rise to TSL signal. The different shape of the TSL and TSC curves can be explained by taking into account
A. Vedda et al. / Microelectronic Engineering 55 (2001) 59 – 64
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Fig. 3. TSL and TSC curves of (3%B)(7%P)SG layer after X irradiation. (A) TSL (left ordinate scale); (B) TSC (right ordinate scale). Applied voltage580 V.
(i) the existence of ionic processes in the TSC pattern as demonstrated by measurements performed without irradiation, and (ii) the occurrence of trap-to-centre recombinations not involving the conduction band at temperatures lower than 1508C. Finally, the spectral composition of the TSL was investigated: a typical spectrum is displayed in Fig. 4 in the case of (2%B)(9%P)SG. Two emission bands are detected at 2.5 and 2.85 eV followed by a shoulder at around 3.2 eV, indicating the existence of different luminescent centres in the TSL recombination process; no significant variations were observed by varying the B and P concentrations. The highest energy band is similar to the ‘beta’ emission previously found in the photoluminescence of bulk undoped silica and associated to the presence of impurities, most notably Ge [7]. On the other hand, no emissions at 2.5 and 2.85 eV were found by photoluminescence studies in bulk silica, so that their appearance in these layers seems to be associated to the presence of dopants. Hole centres related
Fig. 4. Emission spectrum of the TSL signal of (2%B)(9%P)SG integrated in the 20–3008C temperature region. Filled circles, experimental data; continuous line, numerical fit; dashed lines, spectral components.
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to boron and phosphorus were found by electron paramagnetic resonance in thin PBSG layers [8] as well as in bulk doped silica [9]; in particular, several variants of phosphorus–oxygen hole centers (POHC) were recognized, and relations between EPR signals and optical absorption bands were established [9]. The possible correlation between P and B related hole centers and the emission bands here observed will be the object of a future investigation. In conclusion, the influence of P and B doping on the dynamics of ionic carriers, as well as on the radiation induced electronic traps and recombination centres in BPSG layers has been evidenced. A TSC peak at 908C of ionic nature has been detected, which appears related to phosphorus doping; an increase of the intensity of radiation induced TSL and TSC signals has been noticed by increasing the P content of the layers, suggesting that P doping increases the concentration of electronic traps and / or recombination centres. Finally, the spectral composition of the TSL emitted light has revealed a composite pattern, which can be further investigated by taking into account the existence of radiation induced B- and P related hole centres. References [1] S. Rojas, R. Gomarasca, L. Zanotti, A. Borghesi, A. Sassella, G. Ottaviani, L. Moro, P. Lazzeri, J. Vacuum Sci. Technol.B 10 (2) (1992) 633. [2] G. Crisenza, G. Ghidini, S. Manzini, A. Modelli, M. Tosi, in: Proceedings of the International Electron Device Meeting, San Francisco, CA, IEEE, New York, 1990, p. 107. [3] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon Press, Oxford, 1981. [4] D. Del Frate, S. Quilici, G. Spinolo, A. Vedda, Phys. Rev. B 59 (1999) 9741. [5] J.M. Eldridge, D.R. Kerr, J. Electrochem. Soc.: Solid State Sci. 118 (1971) 986. [6] M. Martini, F. Meinardi, E. Rosetta, G. Spinolo, A. Vedda, J.L. Leray, P. Paillet, J.L. Autran, R.A.B. Devine, IEEE Trans. Nucl. Sci. 43 (1996) 845. [7] A. Corazza, B. Crivelli, M. Martini, G. Spinolo, J. Phys. Cond. Matter 7 (1995) 6739. [8] W.L. Warren, M.R. Shaneyfelt, D.M. Fleetwood, P.S. Winokur, S. Montague, IEEE Trans. Nucl. Sci. 42 (1995) 1731. [9] D.L. Griscom, E.J. Friebele, K.J. Long, J. Appl. Phys. 54 (1983) 3743.
Electronic recombinations and ionic transport in BPSG layers a, b a a a a a A. Vedda *, E. Carollo , S. Croci , M. Martini , A. Morbiato , G. Spinolo , M. Vitali , L. Zanotti c a
INFM and Dipartimento di Scienza dei Materiali-Universita` di Milano-Bicocca, Via Cozzi 53, 20125 Milan, Italy b Dipartimento di Fisica dell’ Universita` di Padova, Via Marzolo 8, 35100 Padova, Italy c ST Microelectronics, Via C. Olivetti 2, 20041 Agrate Brianza, Italy
Abstract Thermally stimulated luminescence (TSL) and current (TSC) measurements above room temperature were performed on 1150 nm borophosphosilicate glass films obtained by sub-atmospheric chemical vapour deposition. Several concentrations of B and P ions were considered, in the range 2–5% and 4–9% in weight, respectively. A TSC peak of ionic character at 908C was observed without prior X-irradiation, followed by a monotonically growing signal. Upon X-irradiation, broad TSL and TSC signals extending from 50 to 3008C were detected: the intensities of both the ionic peak and the radiation induced TSL and TSC signals increase by increasing the phosphorus content, while they decrease on boron increase. The TSL emission spectrum features two bands peaking at 2.5 and 2.85 eV and a minor component at around 3.2 eV. On the basis of these data, the dynamics of ionic carriers and electronic traps in BPSG layers are discussed. 2001 Elsevier Science B.V. All rights reserved. Keywords: BPSG layers; Thermoluminescence; Point defects; Ionic transport
1. Introduction Borophosphosilicate glass (BPSG) thin films are used in ULSI semiconductor manufacturing as premetal dielectrics for insulation between metal interconnections and underlying polysilicon or silicide gate structures. The main required film properties, such as conformal step coverage or gap filling capability (design rules # 0.25 mm), effective barrier to alkali ions, low reflow temperature ( , 9508C) and high planarization degree, are nowadays mostly provided by layers deposited in atmospheric or subatmospheric chemical vapour deposition (APCVD, SACVD) systems using tetraethylorthosilicate (TEOS), as silicon source, and ozone based chemistry. The macroscopic physical and chemical properties of these films (as density, refractive index, wet etch rate, planarization, defect formation and film stability, IR absorption) were largely studied in the *Corresponding author. Tel.: 139-02-6448-5162; fax: 139-02-6448-5400. E-mail address: [email protected] (A. Vedda). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 00 )00429-9
2001 Elsevier Science B.V. All rights reserved.
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A. Vedda et al. / Microelectronic Engineering 55 (2001) 59 – 64
past [1]; however, very little is known about point defects and mobile charges in these films, which can affect the electrical characteristics of active devices [2], and of their dependence upon B and P concentration. To investigate these points, thermally stimulated luminescence (TSL) and current (TSC) measurements have been performed on BPSG layers grown by SACVD with different B and P contents. The results provide evidence for the existence of both point defects acting as charge traps, and of mobile ionic carriers. The dependence of TSL and TSC signals upon B and P content is presented, and the possible nature of the observed defects is discussed.
2. Experimental BPSG film samples (1150 nm thick) with different B and P concentrations were prepared on k100l Czochralski silicon wafers, boron doped, with 1.7–2.5 V ? cm resistivity. The BPSG films were deposited in a single chamber SACVD system at 200 Torr and 5508C and then annealed by rapid thermal processing (RTP) at 10508C in N 2 lasting 30 s. Dopant concentrations were in the range 2–5% and 4–9% in weight for B and P, respectively. For TSC measurements, Al dots (f 5 3 mm) were patterned on the BPSG layers after RTP; a thermal treatment at 4308C lasting 30 min in forming gas ambient, was finally performed to assure a good adhesion of the Al dots. Samples of about 10310 mm were cleaved. TSL measurements were performed from room temperature (RT) up to 4008C with a linear heating rate of 18C / s using two different apparatus. In the first one the total emitted light was detected as a function of temperature by photon counting using an EMI 9635 QB photomultiplier tube. By this apparatus, simultaneous TSC measurements were also performed by using a Keithley 617 electrometer with applied voltages in the range 1–80 V; the sensitivity of the TSC measurements was of the order of 10 214 A. The second apparatus used was a laboratory-made high sensitivity TSL spectrometer measuring the TSL intensity as a function of both temperature and emission wavelength. The detector was a double stage Microchannel plate followed by a 512 diode array; the dispersive element was a 140 lines / mm holographic grating, the detection range being 200–800 nm. Irradiations were performed at room temperature by a Machlett OEG50 X-ray tube operated at 32 kV; the dose was (10 3 Gy.
3. Results and discussion TSC measurements were performed both without prior irradiation and following X-irradiation at RT. A TSC measurement performed without prior X-irradiation is displayed in Fig. 1, curve (A), in the case of a sample with 2%B and 9%P: a principal peak at around 908C is observed, followed by a shoulder at around 2008C superimposed to a monotonically growing signal. Moreover, a decreasing current signal very close to RT is also evidenced. Signals of similar shape were observed in all the other cases, so that the TSC curve is representative of all the considered samples. The TSC signal was also recorded after a polarization procedure: namely, after the first TSC run displayed in Fig. 1, curve (A), the sample was cooled at RT while keeping the applied electric field, and then heated again up to 4008C (second TSC run): in the second TSC curve (Fig. 1, curve (B) the 908C peak is completely
A. Vedda et al. / Microelectronic Engineering 55 (2001) 59 – 64
61
Fig. 1. TSC curves without prior irradiation of a sample with 2%B and 9%P. (A) First TSC run; (B) second TSC run (the cooling cycle after the first TSC run was performed by keeping the applied voltage). Applied voltage580 V.
suppressed while the current signal close to RT and the signal above 2008C are still observable. Both the 908C peak and the monotonic growth observed at higher temperatures can be preliminarily assigned to drift of ionic carriers present in the films in different configurations. The 908C structure can be attributed to ionic charges which are thermally freed from trapping sites and undergo strong polarization upon the heating / cooling cycle with the applied field, possibly due to accumulation near to interfaces: this leads to a typical ‘space charge’ effect [3] evidenced by the comparison between curves (A) and (B) of Fig. 1. On the other hand, the monotonic growth can be associated to ionic carriers more deeply bound, only partially freed at higher temperatures. An opposite effect on the TSC intensity, and particularly of the 908C peak, has been observed by varying the concentration of B and P ions (Fig. 2(a) and (b)): namely, the signals increase by increasing the phosphorus content, while they decrease on boron increase. The nature of the ionic carriers responsible for the TSC structures is an open question. In this respect, it may be suggested that alkali ions present as trace impurities could be related with the monotonic current growth observed at T . 2008C, in analogy with what observed in bulk silica and in thermal SiO 2 films [4]. Moreover, alkali ions in a less deeply bound configuration could give rise to the TSC peak at 908C as well: the increase of the intensity of this peak on phosphorus increase calls for a direct participation of such dopant in the trapping site, and is in agreement with the well known role of phosphorus in gettering alkali ions in a stable form at room temperature [5]. On the other hand, the opposite behaviour of the 908C peak on the boron increase could be explained by suggesting that this ion introduces other ionic traps, which can compete with P-related ones in the capture of alkalis. As no stable gettering of alkalis by boron was ever observed, these boron-related traps should be more shallow with respect to P-related ones, and this could justify the lack of a TSC signal correlated with the presence of B in our measurements. In reality, a decreasing current is detected just above RT, which is however too close to the irradiation temperature to be resolved. We emphasize the preliminary character of the present interpretation which needs further experimental investigation, for example the extension of TSC measurements to lower temperatures and the direct evaluation of the alkali content in the layers. No TSL signal was observed without prior irradiation. Upon X-irradiation at RT, both TSL and
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Fig. 2. (a) TSC curves without prior X-irradiation of BPSG layers with 3% B and different P concentrations. Applied voltage580 V; (b) TSC curves without prior X-irradiation of BPSG layers with 7% P and different B concentrations. Applied voltage580 V.
TSC signals were detected in all samples. In Fig. 3 the TSL and TSC measurements are presented in a representative case, relative to the sample with 3%B and 7% P. A broad TSL signal extending from 50 to 3008C is detected, indicating the existence of a distribution of electronic trap levels [6]. The TSC signal is significantly increased after irradiation, and a new structure peaking at around 2008C is detected superimposed on a lower temperature shoulder and on a monotonically increasing signal at higher temperatures, already detected without prior irradiation. All samples displayed a similar qualitative behaviour: however, as in the case of the TSC data without irradiation, higher TSL and TSC signals were detected by increasing the P concentration while the opposite behaviour was observed on B increase: such a behaviour suggests that P doping increases the concentration of electronic traps and / or recombination centres. The radiation induced TSC structure at around 2008C can be attributed to electronic carriers freed from traps and at least partially undergoing radiative recombination at luminescent sites giving rise to TSL signal. The different shape of the TSL and TSC curves can be explained by taking into account
A. Vedda et al. / Microelectronic Engineering 55 (2001) 59 – 64
63
Fig. 3. TSL and TSC curves of (3%B)(7%P)SG layer after X irradiation. (A) TSL (left ordinate scale); (B) TSC (right ordinate scale). Applied voltage580 V.
(i) the existence of ionic processes in the TSC pattern as demonstrated by measurements performed without irradiation, and (ii) the occurrence of trap-to-centre recombinations not involving the conduction band at temperatures lower than 1508C. Finally, the spectral composition of the TSL was investigated: a typical spectrum is displayed in Fig. 4 in the case of (2%B)(9%P)SG. Two emission bands are detected at 2.5 and 2.85 eV followed by a shoulder at around 3.2 eV, indicating the existence of different luminescent centres in the TSL recombination process; no significant variations were observed by varying the B and P concentrations. The highest energy band is similar to the ‘beta’ emission previously found in the photoluminescence of bulk undoped silica and associated to the presence of impurities, most notably Ge [7]. On the other hand, no emissions at 2.5 and 2.85 eV were found by photoluminescence studies in bulk silica, so that their appearance in these layers seems to be associated to the presence of dopants. Hole centres related
Fig. 4. Emission spectrum of the TSL signal of (2%B)(9%P)SG integrated in the 20–3008C temperature region. Filled circles, experimental data; continuous line, numerical fit; dashed lines, spectral components.
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to boron and phosphorus were found by electron paramagnetic resonance in thin PBSG layers [8] as well as in bulk doped silica [9]; in particular, several variants of phosphorus–oxygen hole centers (POHC) were recognized, and relations between EPR signals and optical absorption bands were established [9]. The possible correlation between P and B related hole centers and the emission bands here observed will be the object of a future investigation. In conclusion, the influence of P and B doping on the dynamics of ionic carriers, as well as on the radiation induced electronic traps and recombination centres in BPSG layers has been evidenced. A TSC peak at 908C of ionic nature has been detected, which appears related to phosphorus doping; an increase of the intensity of radiation induced TSL and TSC signals has been noticed by increasing the P content of the layers, suggesting that P doping increases the concentration of electronic traps and / or recombination centres. Finally, the spectral composition of the TSL emitted light has revealed a composite pattern, which can be further investigated by taking into account the existence of radiation induced B- and P related hole centres. References [1] S. Rojas, R. Gomarasca, L. Zanotti, A. Borghesi, A. Sassella, G. Ottaviani, L. Moro, P. Lazzeri, J. Vacuum Sci. Technol.B 10 (2) (1992) 633. [2] G. Crisenza, G. Ghidini, S. Manzini, A. Modelli, M. Tosi, in: Proceedings of the International Electron Device Meeting, San Francisco, CA, IEEE, New York, 1990, p. 107. [3] R. Chen, Y. Kirsh, Analysis of Thermally Stimulated Processes, Pergamon Press, Oxford, 1981. [4] D. Del Frate, S. Quilici, G. Spinolo, A. Vedda, Phys. Rev. B 59 (1999) 9741. [5] J.M. Eldridge, D.R. Kerr, J. Electrochem. Soc.: Solid State Sci. 118 (1971) 986. [6] M. Martini, F. Meinardi, E. Rosetta, G. Spinolo, A. Vedda, J.L. Leray, P. Paillet, J.L. Autran, R.A.B. Devine, IEEE Trans. Nucl. Sci. 43 (1996) 845. [7] A. Corazza, B. Crivelli, M. Martini, G. Spinolo, J. Phys. Cond. Matter 7 (1995) 6739. [8] W.L. Warren, M.R. Shaneyfelt, D.M. Fleetwood, P.S. Winokur, S. Montague, IEEE Trans. Nucl. Sci. 42 (1995) 1731. [9] D.L. Griscom, E.J. Friebele, K.J. Long, J. Appl. Phys. 54 (1983) 3743.