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Defect structure in nitrogen-rich amorphous silicon nitride films
Journal of Non-Crystalline Solids 227–230 Ž1998. 528–532
Defect structure in nitrogen-rich amorphous silicon nitride films Baojie Yan, J.H. Dias da Silva 1, P.C. Taylor
)
Department of Physics, UniÕersity of Utah, Salt Lake City, UT 84112, USA
Abstract Electron spin resonance and photoluminescence measurements were carried out on nitrogen-rich, hydrogenated amorphous silicon-nitride films. Paramagnetic Si dangling bonds ŽK 0 centers. are found in as-deposited films only after UV illumination. High temperature post-deposition annealing, followed by UV illumination, creates ESR-active, two-fold coordinated nitrogen dangling bonds ŽN20 centers.. These two types of spin center are normally observed at room temperature. An extra component appears on the ESR spectrum measured at low temperature Ž6 K. for the as-deposited samples after UV illumination, at which temperature the line of the K 0 center is saturated by microwave power. This component is tentatively attributed to N20 centers that are partially saturated. The photoluminescence efficiency is significantly decreased by the high temperature annealing. q 1998 Elsevier Science B.V. All rights reserved. Keywords: a-SiN x :H films; K center; N20 center
1. Introduction Nitrogen-rich, hydrogenated amorphous silicon nitride Ža-SiN x :H, x G 4r3. is an important dielectric material in electronic applications. The deep trap states in the gap of this material can affect the performance of devices. In the last two decades, studies have identified several important defects in a-SiN x :H films. Electron spin resonance ŽESR. measurements have identified the dominant defect in as-deposited or UV-illuminated, nitrogen-rich a-SiN x :H as a dangling bond on a silicon atom that is back bonded to three nitrogen atoms, termed the K center w1–3x. A two-fold coordinated nitrogen center
)
Corresponding author. Fax: q1-801 581 4246 or 4801; e-mail: [email protected]. 1 Permanent address: Depto. de Fisica-FC, Campus Unesp. CEP17033-360, Bauru-Sp, Brazil.
has been observed in samples annealed at a temperature higher than the deposition temperature, followed by illumination with UV-light ŽN20 center. w4–9x. Kumeda et al. w10x and Yan et al. w11x found a complicated ESR spectrum in stoichiometric a-SiN x :H films Ž x s 4r3. prepared at higher deposition rates. These authors ascribed this complicated spectrum to an ‘N-pair’ defect. The K center is assumed to be the principle defect in as-deposited or UV-illuminated, nitrogen-rich a-SiN x :H films. The K center has three charged states Ky, K 0 and Kq, corresponding to occupancy by two electrons, one electron and no electrons, respectively. The creation of the ESR-active K 0 center during UV illumination is sometimes assumed to result from charge conversion, Kqq Kyq hn ™ 2 K 0 , a reaction that requires a negative electron–electron correlation energy ŽUeff - 0. for this defect w12x. However, a theoretical calculation suggests a positive Ueff for the K center w13,14x. A second possibil-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 0 9 1 - X
B. Yan et al.r Journal of Non-Crystalline Solids 227–230 (1998) 528–532
ity that allows a positive Ueff for the K center, but is consistent with the increase in K 0 after UV illumination, is that there exists a compensating defect. For example, one could postulate that the Kq center and a two-fold coordinated nitrogen defect, Ny 2 , co-exist in the nitrogen-rich a-SiN x :H films w15x. In this situation the UV-induced paramagnetic defect creation corresponds to charge transfer according to the y 0 0 reaction, Kq 3 q N2 q h n ™ K 3 q N2 . An immediate consequence of this assumption is that one should observe both the creation of the K 0 center and the N20 center in equal numbers, after UV illumination of the a-SiN x :H films. At ESR measurement temperatures above about 40 K, the N20 center has only been observed in samples after annealing, followed by UV-illumination w5–8x or after UV-illumination at low Ž- 100 K. temperature w9x. Recently, in ESR measurements at high Ž) 10 mW. microwave power and at temperatures below 40 K, Chen et al. w15x discovered another paramagnetic center in as-deposited a-SiN1.6 :H films. The spin density of this center is essentially the same as that of the K 0 center in a-SiN1.6 :H. Because the ESR lineshape of this new center is greater than that of the K 0 center, it is only observed at low temperatures Ž; 6 K. at higher microwave powers that saturate the K 0 ESR response. These authors suggested that the new center is also a two-fold-coordinated N center ŽN20 .. An obvious difficulty with this assignment is that the ESR lineshape of this new center is different from the one attributed to the N20 center in annealed samples Ž) 4008C., followed by UV illumination. In this paper, we report ESR and photoluminescence ŽPL. measurements for a-SiN1.6 :H films. In addition to the K 0 center, we also find a broad ESR line in the samples that have been UV-illuminated. High temperature annealing creates extra nitrogen defects and decreases the PL efficiency.
2. Experimental procedure The samples were prepared by plasma-enhanced chemical vapor deposition ŽPECVD. from an NH 3 and SiH 4 gas mixture. Samples were deposited onto fused-quartz substrates held at 4008C. Details of the sample preparation are available elsewhere w16x. Substrates were roughened for PL samples to reduce the
529
influence of multiple internal reflections Žthe fringing effect. on the PL spectra. A broad-band Hg lamp Ž100 W. was used to illuminate the sample with UV light. The average light intensity on the sample was about 50 mWrcm2 . The high temperature Žhigher than 4008C. anneals were performed in a furnace at different temperatures prior to UV illumination. Heating and cooling rates were controlled at about 18Crmin. The ESR measurements were made on an X-band Ž9.4 GHz. spectrometer ŽBruker ER200-SRC. at temperatures from 5 K to room temperature. The PL spectra were measured at 80 K with excitation at 3.4 eV from an Arq laser.
3. Results Fig. 1 shows the ESR spectra of the a-SiN1.6 :H film after various treatments Žtop three spectra. and a simulation of the ESR spectrum of the N20 center
Fig. 1. ESR spectra of a-SiN1.6 :H films after various treatments. The three experimental curves correspond from top to bottom to the as-deposited sample, the sample annealed at 5508C and the sample annealed at 7008C, respectively. All these spectra were taken after 160 min of UV-illumination. The measurements were made at room temperature with 0.2 mW microwave power and 4 Gauss modulation amplitude. The bottom curve is a simulation for the N20 center with g H s 2.0078, g 5 s 2.0035, A H s6.5 gauss, A 5 s 36 gauss and a Gaussian convolution function of 6.5 gauss width.
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B. Yan et al.r Journal of Non-Crystalline Solids 227–230 (1998) 528–532
Fig. 2. Low temperature ESR derivative spectra of the as-deposited a-SiN1.6 :H films after 160 min of UV-illumination. The measurements were made at 6 K with 4 gauss modulation amplitude.
Žbottom spectrum.. The three experimental curves correspond to the sample in its as-deposited state, after annealing at 5508C for one hour, and after annealing at 7008C for one hour, respectively. All these spectra were taken after 160 min of UV illumination. No detectable ESR signals were observed before UV illumination even for the sample annealed at the higher temperatures. The ESR lineshapes differ for the as-deposited film and the film after annealing at 7008C. A single ESR lineshape with g s 2.003 2 and the width D Hpp f 13.5 gauss appears in the as-deposited a-SiN1.6 :H film after UV illumination. This lineshape has been identified as the K 0 center w3,6x. On the other hand, the ESR lineshape after annealing at 7008C and after UV illumination consists of three features that have been identified as due to the N20 center w5,6x. The three features result from a hyperfine interaction with the 14 N nucleus whose spin is I s 1. The bottom curve of Fig. 1 is a simulation of the ESR spectrum of the N20 center with spin-Hamiltonian parameters similar to those used by Warren et al. w5x. This simulation reproduces the essential experimental features. The sample annealed at 5508C shows an ESR signal that consists of a mixture of the K 0 and N20 lineshapes. All of the
2 g s hn r b H where h, n , and b have their customary definitions and H is the magnetic field at which the maximum amplitude of a spectral component is observed.
above results agree well with those reported previously w1–9x. Fig. 2 shows the low temperature Ž6 K. ESR spectra of the as-deposited film after 160 min UV illumination, where the dashed and solid curves are measured with microwave powers - 0.1 mW and ) 0.1 mW. With the increase of microwave power a broad component appears, whose amplitude increases more than the central line Žthe K 0 center.. This phenomenon was first found by Chen et al. w15x in as-deposited a-SiN1.6 :H films without UV illumination. Chen et al. w15x suggested that the broad component results from a N20 center. Because the lineshape was different from that of the N20 center observed in annealed films of a-SiN1.6 :H w7–9x, Chen et al. suggested that the center they attributed to N20 was somehow different from that observed at higher temperatures. Fig. 3 shows the lineshape change as a function of microwave power of the low temperature Ž6 K. ESR spectrum of the a-SiN1.6 :H film annealed at 7008C, followed by UV illumination. At a low microwave power the ESR lineshape is essentially the same as
Fig. 3. Low temperature ESR derivative spectra of the a-SiN1.6 :H film annealed at 7008C, followed by 160 min UV illumination. The measurements were made at 6 K with 4 gauss modulation amplitude. The curves are normalized to a similar magnitude for clarity.
B. Yan et al.r Journal of Non-Crystalline Solids 227–230 (1998) 528–532
the one measured at room temperature, i.e., the lineshape of the N20 center. With increasing microwave power, the lineshape starts to change. The minima in the derivative spectrum labeled E and F appear to fill in and the result is a lineshape with fewer resolved features. The change of the lineshape is associated with the saturation of the ESR signal. Fig. 4 is a log–log plot of the double integral of the ESR spectrum vs. the microwave power for both the as-deposited sample after UV illumination and the sample annealed at 7008C for 1 h, followed by UV illumination. It is worth noting that the ESR signal for the as-deposited sample after UV illumination is more easily saturated than the one for the sample annealed at 7008C, followed by UV illumination. Also, the ESR lineshape starts to change when the total intensity Ždouble integral. of the ESR signal begins to depart from a dependence proportional to the square root of microwave power, as indicated by the dashed line in Fig. 4. The higher temperature annealing decreases the luminescence efficiency as illustrated in Fig. 5. For
531
Fig. 5. PL spectra of a-SiN1.6 :H film. v: as-deposited film; B: annealed at 5278C for one hour; ': annealed at 6278C for one hour. The solid lines are guides to the eye. Errors are less than "10% in all cases except for the sharp spike near 3.1 eV which is an artifact of the measurement system.
the annealed samples the measurements were made immediately after annealing at 5278C and 6278C for one hour without subsequent UV illumination Žthe middle and bottom curves, respectively.. As a comparison, the PL spectrum of the as-deposited sample is also plotted in the figure Žthe top curve.. The PL spectra were measured using the 3.4 eV line from an Arq laser. No detectable ESR signal was created by the 3.4 eV illumination even though this illumination results in a fatigue of the PL band similar to that which occurs with broad-band UV illumination ŽJ.H. Dias da Silva and P.C. Taylor, unpublished.. The details of the PL results will be reported elsewhere.
4. Discussion Fig. 4. Intensity of the ESR signal Ždouble integral of the ESR derivative spectra. vs. the microwave power P. The measurements were made at 6 K with 4 gauss modulation amplitude. The symbols, B and v represent the data of the as-deposited sample and the sample annealed at 7008C for 1 h, respectively. Both samples were UV light-soaked for 160 min. Errors are in all cases less than the sizes of the data points. The dashed line, which is an aid to the eye, indicates the square-root dependence of the signal intensity on the microwave power.
The room temperature ESR results are generally consistent with the previous reports of Refs. w1–9x. The K 0 center is the dominant ESR-active center in nitrogen-rich, as-deposited a-SiN x :H films; the high temperature annealing creates a two-fold coordinated nitrogen defect and decreases the relative concentration of the K 0 center.
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B. Yan et al.r Journal of Non-Crystalline Solids 227–230 (1998) 528–532
The low temperature Ž6 K. ESR spectrum has a broader component ŽFig. 2. in the as-deposited a-SiN1.6 :H film for microwave powers at which the K 0 center is saturated. Since this spectrum is too broad to be associated with the K 0 center, we suggest, following the earlier interpretation of Chen et al. w15x, that the new component results from a center that is less easily saturated than the K 0 center and is probably associated with nitrogen. The most likely candidate is a type of N20 center. The difference in the lineshape between this new N20 center and the ‘traditional’ N20 center Žas shown in Fig. 1. may be due to microwave broadening as occurs in Fig. 3. The concentration of the N20 center in the sample annealed at higher temperature Ž7008C., followed by UV illumination, is much larger than that in the as-deposited sample. Consequently, the ESR signal of the N20 would be more easily saturated in the as-deposited sample than in the high-temperature-annealed sample. Therefore, we speculate that in the as-deposited sample, we cannot observe the ESR spectrum at microwave power low enough such that the undistorted three line structure of the N20 center is apparent. The lineshapes of the low temperature ESR spectra for the as-deposited sample and the sample annealed at high temperature are similar at higher microwave power. This similarity lends support to the assignment of the broad component of these two spectra to the same N20 center. Although the above interpretation is self-consistent, one still cannot rule out the possibility of the existence of other types of spin centers, such as the nitrogen-pair center w10,11x or a four-fold-coordinated nitrogen center w17,18x.
5. Conclusion The K 0 center is normally found in as-deposited a-SiN x :H Ž x ) 4r3. films. Low temperature ESR measurements reveal that the N20 center may also exist in the as-deposited films. High temperature
annealing creates more N20 defects and decreases the relative concentration of the K 0 centers. Acknowledgements The authors gratefully acknowledge J. Kanicki for supplying the a-SiN1.6 :H samples. This work was supported by NREL under subcontract No. XAD3121142 and by ONR under grant No. N00014410941. One of the authors ŽJHDS. acknowledges Fapesp Žproject 95r0271-0. and CNPq Žproject 450696r97.. References w1x D.T. Krick, P.M. Lenahan, J. Kanicki, J. Appl. Phys. 64 Ž1988. 3558. w2x D. Jousse, J. Kanicki, Appl. Phys. Lett. 55 Ž1989. 1112. w3x W.L. Warren, P.M. Lenahan, Phys. Rev. B 42 Ž1990. 1773. w4x V.A. Nadolinnyi, V.V. Vasilev, I.P. Mikhailovskii, Phys. Status Solidi 116 Ž1989. k105. w5x W.L. Warren, P.M. Lenahan, S.E. Curry, Phys. Rev. Lett. 65 Ž1990. 207. w6x W.L. Warren, F.C. Rong, E.H. Poindexter, G.J. Gerardi, J. Kanicki, J. Appl. Phys. 70 Ž1991. 346. w7x W.L. Warren, P.M. Lenahan, J. Kanicki, J. Appl. Phys. 70 Ž1991. 2220. w8x W. L Warren, J. Kanicki, J. Robertson, E.H. Poindexter, Mater. Res. Soc. Symp. Proc. 284 Ž1993. 101. w9x W.L. Warren, J. Robertson, J. Kanicki, Appl. Phys. Lett. 63 Ž1993. 2685. w10x M. Kumeda, N. Awaki, H. Yan, A. Morimoto, T. Shimizu, J. Non-Cryst. Solids 137 Ž138. Ž1991. 887. w11x H. Yan, M. Kumeda, N. Ishh, T. Shimizu, Jpn. J. Appl. Phys. 32 Ž1993. 876. w12x S.E. Curry, P.M. Lenahan, D.T. Krick, J. Kanicki, C.T. Kirk, Appl. Phys. Lett. 56 Ž1990. 1359. w13x J. Robertson, M.J. Powell, Appl. Phys. Lett. 44 Ž1984. 415. w14x J. Robertson, Mater. Res. Soc. Symp. Proc. 284 Ž1993. 65. w15x D.Q. Chen, J.M. Viner, P.C. Taylor, Solid State Commun. 98 Ž1996. 745. w16x D.Q. Chen, J.M. Viner, P.C. Taylor, J. Kanicki, Phys. Rev. B 49 Ž1994. 13420. w17x T.E. Tsai, D.L. Griscom, E.J. Friebele, Phys. Rev. B 38 Ž1988. 2140. w18x K.L. Brower, Phys. Rev. Lett. 44 Ž1980. 1627.
Defect structure in nitrogen-rich amorphous silicon nitride films Baojie Yan, J.H. Dias da Silva 1, P.C. Taylor
)
Department of Physics, UniÕersity of Utah, Salt Lake City, UT 84112, USA
Abstract Electron spin resonance and photoluminescence measurements were carried out on nitrogen-rich, hydrogenated amorphous silicon-nitride films. Paramagnetic Si dangling bonds ŽK 0 centers. are found in as-deposited films only after UV illumination. High temperature post-deposition annealing, followed by UV illumination, creates ESR-active, two-fold coordinated nitrogen dangling bonds ŽN20 centers.. These two types of spin center are normally observed at room temperature. An extra component appears on the ESR spectrum measured at low temperature Ž6 K. for the as-deposited samples after UV illumination, at which temperature the line of the K 0 center is saturated by microwave power. This component is tentatively attributed to N20 centers that are partially saturated. The photoluminescence efficiency is significantly decreased by the high temperature annealing. q 1998 Elsevier Science B.V. All rights reserved. Keywords: a-SiN x :H films; K center; N20 center
1. Introduction Nitrogen-rich, hydrogenated amorphous silicon nitride Ža-SiN x :H, x G 4r3. is an important dielectric material in electronic applications. The deep trap states in the gap of this material can affect the performance of devices. In the last two decades, studies have identified several important defects in a-SiN x :H films. Electron spin resonance ŽESR. measurements have identified the dominant defect in as-deposited or UV-illuminated, nitrogen-rich a-SiN x :H as a dangling bond on a silicon atom that is back bonded to three nitrogen atoms, termed the K center w1–3x. A two-fold coordinated nitrogen center
)
Corresponding author. Fax: q1-801 581 4246 or 4801; e-mail: [email protected]. 1 Permanent address: Depto. de Fisica-FC, Campus Unesp. CEP17033-360, Bauru-Sp, Brazil.
has been observed in samples annealed at a temperature higher than the deposition temperature, followed by illumination with UV-light ŽN20 center. w4–9x. Kumeda et al. w10x and Yan et al. w11x found a complicated ESR spectrum in stoichiometric a-SiN x :H films Ž x s 4r3. prepared at higher deposition rates. These authors ascribed this complicated spectrum to an ‘N-pair’ defect. The K center is assumed to be the principle defect in as-deposited or UV-illuminated, nitrogen-rich a-SiN x :H films. The K center has three charged states Ky, K 0 and Kq, corresponding to occupancy by two electrons, one electron and no electrons, respectively. The creation of the ESR-active K 0 center during UV illumination is sometimes assumed to result from charge conversion, Kqq Kyq hn ™ 2 K 0 , a reaction that requires a negative electron–electron correlation energy ŽUeff - 0. for this defect w12x. However, a theoretical calculation suggests a positive Ueff for the K center w13,14x. A second possibil-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 0 9 1 - X
B. Yan et al.r Journal of Non-Crystalline Solids 227–230 (1998) 528–532
ity that allows a positive Ueff for the K center, but is consistent with the increase in K 0 after UV illumination, is that there exists a compensating defect. For example, one could postulate that the Kq center and a two-fold coordinated nitrogen defect, Ny 2 , co-exist in the nitrogen-rich a-SiN x :H films w15x. In this situation the UV-induced paramagnetic defect creation corresponds to charge transfer according to the y 0 0 reaction, Kq 3 q N2 q h n ™ K 3 q N2 . An immediate consequence of this assumption is that one should observe both the creation of the K 0 center and the N20 center in equal numbers, after UV illumination of the a-SiN x :H films. At ESR measurement temperatures above about 40 K, the N20 center has only been observed in samples after annealing, followed by UV-illumination w5–8x or after UV-illumination at low Ž- 100 K. temperature w9x. Recently, in ESR measurements at high Ž) 10 mW. microwave power and at temperatures below 40 K, Chen et al. w15x discovered another paramagnetic center in as-deposited a-SiN1.6 :H films. The spin density of this center is essentially the same as that of the K 0 center in a-SiN1.6 :H. Because the ESR lineshape of this new center is greater than that of the K 0 center, it is only observed at low temperatures Ž; 6 K. at higher microwave powers that saturate the K 0 ESR response. These authors suggested that the new center is also a two-fold-coordinated N center ŽN20 .. An obvious difficulty with this assignment is that the ESR lineshape of this new center is different from the one attributed to the N20 center in annealed samples Ž) 4008C., followed by UV illumination. In this paper, we report ESR and photoluminescence ŽPL. measurements for a-SiN1.6 :H films. In addition to the K 0 center, we also find a broad ESR line in the samples that have been UV-illuminated. High temperature annealing creates extra nitrogen defects and decreases the PL efficiency.
2. Experimental procedure The samples were prepared by plasma-enhanced chemical vapor deposition ŽPECVD. from an NH 3 and SiH 4 gas mixture. Samples were deposited onto fused-quartz substrates held at 4008C. Details of the sample preparation are available elsewhere w16x. Substrates were roughened for PL samples to reduce the
529
influence of multiple internal reflections Žthe fringing effect. on the PL spectra. A broad-band Hg lamp Ž100 W. was used to illuminate the sample with UV light. The average light intensity on the sample was about 50 mWrcm2 . The high temperature Žhigher than 4008C. anneals were performed in a furnace at different temperatures prior to UV illumination. Heating and cooling rates were controlled at about 18Crmin. The ESR measurements were made on an X-band Ž9.4 GHz. spectrometer ŽBruker ER200-SRC. at temperatures from 5 K to room temperature. The PL spectra were measured at 80 K with excitation at 3.4 eV from an Arq laser.
3. Results Fig. 1 shows the ESR spectra of the a-SiN1.6 :H film after various treatments Žtop three spectra. and a simulation of the ESR spectrum of the N20 center
Fig. 1. ESR spectra of a-SiN1.6 :H films after various treatments. The three experimental curves correspond from top to bottom to the as-deposited sample, the sample annealed at 5508C and the sample annealed at 7008C, respectively. All these spectra were taken after 160 min of UV-illumination. The measurements were made at room temperature with 0.2 mW microwave power and 4 Gauss modulation amplitude. The bottom curve is a simulation for the N20 center with g H s 2.0078, g 5 s 2.0035, A H s6.5 gauss, A 5 s 36 gauss and a Gaussian convolution function of 6.5 gauss width.
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B. Yan et al.r Journal of Non-Crystalline Solids 227–230 (1998) 528–532
Fig. 2. Low temperature ESR derivative spectra of the as-deposited a-SiN1.6 :H films after 160 min of UV-illumination. The measurements were made at 6 K with 4 gauss modulation amplitude.
Žbottom spectrum.. The three experimental curves correspond to the sample in its as-deposited state, after annealing at 5508C for one hour, and after annealing at 7008C for one hour, respectively. All these spectra were taken after 160 min of UV illumination. No detectable ESR signals were observed before UV illumination even for the sample annealed at the higher temperatures. The ESR lineshapes differ for the as-deposited film and the film after annealing at 7008C. A single ESR lineshape with g s 2.003 2 and the width D Hpp f 13.5 gauss appears in the as-deposited a-SiN1.6 :H film after UV illumination. This lineshape has been identified as the K 0 center w3,6x. On the other hand, the ESR lineshape after annealing at 7008C and after UV illumination consists of three features that have been identified as due to the N20 center w5,6x. The three features result from a hyperfine interaction with the 14 N nucleus whose spin is I s 1. The bottom curve of Fig. 1 is a simulation of the ESR spectrum of the N20 center with spin-Hamiltonian parameters similar to those used by Warren et al. w5x. This simulation reproduces the essential experimental features. The sample annealed at 5508C shows an ESR signal that consists of a mixture of the K 0 and N20 lineshapes. All of the
2 g s hn r b H where h, n , and b have their customary definitions and H is the magnetic field at which the maximum amplitude of a spectral component is observed.
above results agree well with those reported previously w1–9x. Fig. 2 shows the low temperature Ž6 K. ESR spectra of the as-deposited film after 160 min UV illumination, where the dashed and solid curves are measured with microwave powers - 0.1 mW and ) 0.1 mW. With the increase of microwave power a broad component appears, whose amplitude increases more than the central line Žthe K 0 center.. This phenomenon was first found by Chen et al. w15x in as-deposited a-SiN1.6 :H films without UV illumination. Chen et al. w15x suggested that the broad component results from a N20 center. Because the lineshape was different from that of the N20 center observed in annealed films of a-SiN1.6 :H w7–9x, Chen et al. suggested that the center they attributed to N20 was somehow different from that observed at higher temperatures. Fig. 3 shows the lineshape change as a function of microwave power of the low temperature Ž6 K. ESR spectrum of the a-SiN1.6 :H film annealed at 7008C, followed by UV illumination. At a low microwave power the ESR lineshape is essentially the same as
Fig. 3. Low temperature ESR derivative spectra of the a-SiN1.6 :H film annealed at 7008C, followed by 160 min UV illumination. The measurements were made at 6 K with 4 gauss modulation amplitude. The curves are normalized to a similar magnitude for clarity.
B. Yan et al.r Journal of Non-Crystalline Solids 227–230 (1998) 528–532
the one measured at room temperature, i.e., the lineshape of the N20 center. With increasing microwave power, the lineshape starts to change. The minima in the derivative spectrum labeled E and F appear to fill in and the result is a lineshape with fewer resolved features. The change of the lineshape is associated with the saturation of the ESR signal. Fig. 4 is a log–log plot of the double integral of the ESR spectrum vs. the microwave power for both the as-deposited sample after UV illumination and the sample annealed at 7008C for 1 h, followed by UV illumination. It is worth noting that the ESR signal for the as-deposited sample after UV illumination is more easily saturated than the one for the sample annealed at 7008C, followed by UV illumination. Also, the ESR lineshape starts to change when the total intensity Ždouble integral. of the ESR signal begins to depart from a dependence proportional to the square root of microwave power, as indicated by the dashed line in Fig. 4. The higher temperature annealing decreases the luminescence efficiency as illustrated in Fig. 5. For
531
Fig. 5. PL spectra of a-SiN1.6 :H film. v: as-deposited film; B: annealed at 5278C for one hour; ': annealed at 6278C for one hour. The solid lines are guides to the eye. Errors are less than "10% in all cases except for the sharp spike near 3.1 eV which is an artifact of the measurement system.
the annealed samples the measurements were made immediately after annealing at 5278C and 6278C for one hour without subsequent UV illumination Žthe middle and bottom curves, respectively.. As a comparison, the PL spectrum of the as-deposited sample is also plotted in the figure Žthe top curve.. The PL spectra were measured using the 3.4 eV line from an Arq laser. No detectable ESR signal was created by the 3.4 eV illumination even though this illumination results in a fatigue of the PL band similar to that which occurs with broad-band UV illumination ŽJ.H. Dias da Silva and P.C. Taylor, unpublished.. The details of the PL results will be reported elsewhere.
4. Discussion Fig. 4. Intensity of the ESR signal Ždouble integral of the ESR derivative spectra. vs. the microwave power P. The measurements were made at 6 K with 4 gauss modulation amplitude. The symbols, B and v represent the data of the as-deposited sample and the sample annealed at 7008C for 1 h, respectively. Both samples were UV light-soaked for 160 min. Errors are in all cases less than the sizes of the data points. The dashed line, which is an aid to the eye, indicates the square-root dependence of the signal intensity on the microwave power.
The room temperature ESR results are generally consistent with the previous reports of Refs. w1–9x. The K 0 center is the dominant ESR-active center in nitrogen-rich, as-deposited a-SiN x :H films; the high temperature annealing creates a two-fold coordinated nitrogen defect and decreases the relative concentration of the K 0 center.
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The low temperature Ž6 K. ESR spectrum has a broader component ŽFig. 2. in the as-deposited a-SiN1.6 :H film for microwave powers at which the K 0 center is saturated. Since this spectrum is too broad to be associated with the K 0 center, we suggest, following the earlier interpretation of Chen et al. w15x, that the new component results from a center that is less easily saturated than the K 0 center and is probably associated with nitrogen. The most likely candidate is a type of N20 center. The difference in the lineshape between this new N20 center and the ‘traditional’ N20 center Žas shown in Fig. 1. may be due to microwave broadening as occurs in Fig. 3. The concentration of the N20 center in the sample annealed at higher temperature Ž7008C., followed by UV illumination, is much larger than that in the as-deposited sample. Consequently, the ESR signal of the N20 would be more easily saturated in the as-deposited sample than in the high-temperature-annealed sample. Therefore, we speculate that in the as-deposited sample, we cannot observe the ESR spectrum at microwave power low enough such that the undistorted three line structure of the N20 center is apparent. The lineshapes of the low temperature ESR spectra for the as-deposited sample and the sample annealed at high temperature are similar at higher microwave power. This similarity lends support to the assignment of the broad component of these two spectra to the same N20 center. Although the above interpretation is self-consistent, one still cannot rule out the possibility of the existence of other types of spin centers, such as the nitrogen-pair center w10,11x or a four-fold-coordinated nitrogen center w17,18x.
5. Conclusion The K 0 center is normally found in as-deposited a-SiN x :H Ž x ) 4r3. films. Low temperature ESR measurements reveal that the N20 center may also exist in the as-deposited films. High temperature
annealing creates more N20 defects and decreases the relative concentration of the K 0 centers. Acknowledgements The authors gratefully acknowledge J. Kanicki for supplying the a-SiN1.6 :H samples. This work was supported by NREL under subcontract No. XAD3121142 and by ONR under grant No. N00014410941. One of the authors ŽJHDS. acknowledges Fapesp Žproject 95r0271-0. and CNPq Žproject 450696r97.. References w1x D.T. Krick, P.M. Lenahan, J. Kanicki, J. Appl. Phys. 64 Ž1988. 3558. w2x D. Jousse, J. Kanicki, Appl. Phys. Lett. 55 Ž1989. 1112. w3x W.L. Warren, P.M. Lenahan, Phys. Rev. B 42 Ž1990. 1773. w4x V.A. Nadolinnyi, V.V. Vasilev, I.P. Mikhailovskii, Phys. Status Solidi 116 Ž1989. k105. w5x W.L. Warren, P.M. Lenahan, S.E. Curry, Phys. Rev. Lett. 65 Ž1990. 207. w6x W.L. Warren, F.C. Rong, E.H. Poindexter, G.J. Gerardi, J. Kanicki, J. Appl. Phys. 70 Ž1991. 346. w7x W.L. Warren, P.M. Lenahan, J. Kanicki, J. Appl. Phys. 70 Ž1991. 2220. w8x W. L Warren, J. Kanicki, J. Robertson, E.H. Poindexter, Mater. Res. Soc. Symp. Proc. 284 Ž1993. 101. w9x W.L. Warren, J. Robertson, J. Kanicki, Appl. Phys. Lett. 63 Ž1993. 2685. w10x M. Kumeda, N. Awaki, H. Yan, A. Morimoto, T. Shimizu, J. Non-Cryst. Solids 137 Ž138. Ž1991. 887. w11x H. Yan, M. Kumeda, N. Ishh, T. Shimizu, Jpn. J. Appl. Phys. 32 Ž1993. 876. w12x S.E. Curry, P.M. Lenahan, D.T. Krick, J. Kanicki, C.T. Kirk, Appl. Phys. Lett. 56 Ž1990. 1359. w13x J. Robertson, M.J. Powell, Appl. Phys. Lett. 44 Ž1984. 415. w14x J. Robertson, Mater. Res. Soc. Symp. Proc. 284 Ž1993. 65. w15x D.Q. Chen, J.M. Viner, P.C. Taylor, Solid State Commun. 98 Ž1996. 745. w16x D.Q. Chen, J.M. Viner, P.C. Taylor, J. Kanicki, Phys. Rev. B 49 Ž1994. 13420. w17x T.E. Tsai, D.L. Griscom, E.J. Friebele, Phys. Rev. B 38 Ž1988. 2140. w18x K.L. Brower, Phys. Rev. Lett. 44 Ž1980. 1627.