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Electrically detected magnetic resonance of a-Si:H at low magnetic fields: the influence of hydrogen on the dangling bond resonance
Journal of Non-Crystalline Solids 227–230 Ž1998. 343–347
Electrically detected magnetic resonance of a-Si:H at low magnetic fields: the influence of hydrogen on the dangling bond resonance M.S. Brandt
a,)
, M.W. Bayerl a , M. Stutzmann a , C.F.O. Graeff
b
a
b
Walter Schottky Institut, Technische UniÕersitat D 85748 Garching, Germany ¨ Munchen, ¨ Departamento de Fısica e Matematica, FFCLRP-USP, AÕ. Bandeirantes 3900, 14040-91 Riberao ´ ´ ˜ Preto, Brazil
Abstract The hyperfine interaction between the dangling bonds and the hydrogen atoms in hydrogenated amorphous silicon Ža-Si:H. is observed using low-field electrically detected magnetic resonance ŽEDMR.. In deuterated amorphous silicon Ža-Si:D., a peak-to-peak linewidth of 1.4 G is found at 434 MHz. Distant hyperfine involving the background hydrogen present in the material is found to broaden the resonance in a-Si:H. The broadening is proportional to the square-root of the hydrogen concentration and reaches 1 G at 10 at.% hydrogen. The lineshape is more Lorentz-like in the degraded than in the as-grown or annealed states. An identical EDMR signal intensity D srs is observed for a-Si:H using spectrometers working at 434 MHz, 9 and 34 GHz, indicating that the signal intensity is independent of the Zeeman splitting. q 1998 Elsevier Science B.V. All rights reserved. Keywords: a-Si:H; Hydrogen; Dangling bond resonance
1. Introduction Since the first observation of light-induced metastability in amorphous hydrogenated silicon ŽaSi:H. by Staebler and Wronski w1x, the microscopic origin of this effect was subject to intensive investigation. The notorious decrease in the photoconductivity of undoped a-Si:H has been correlated to an increase in the density of paramagnetic dangling bond defects Ždb.. Various microscopic models have been proposed to account for the metastable creation )
Corresponding author. Tel.: q49-89 2891 2758; fax: q49 89 2891 2737; e-mail: [email protected].
of dangling bonds under illumination: weak bonddangling bond conversion, Si–Si bond-switching, charge exchange between diamagnetic defects, Si–H bond-switching or dissociation of H 2)-complexes. However, while electron spin resonance experiments have provided detailed information on the structure of the dangling bond orbital, no decisive information on the structure of the surrounding network was obtained. In particular, no difference in the resonance line shape of the dangling bonds in annealed and degraded a-Si:H was found, indicating that the metastable dangling bonds are spatially separated. In addition, no hyperfine interaction of the dangling bond with the large nuclear magnetic moment of
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 7 3 - 8
344
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
hydrogen was observed Žthe resonances in a-Si:H and a-Si:D also being identical in conventional Xband ESR., excluding a well defined db–H complex ˚ somewhat with an interatomic distance of F 3 A, larger than the Si–Si bond length. Electron spin-echo envelope modulation experiments ŽESEEM. on aSi:D have further shown that no deuterium is present ˚ of the dangling bond, the dangling bond, within 4 A therefore, being in a deuterium Žor hydrogen. denuded zone w2x. Here, we will show that hyperfine interaction of hydrogen with dangling bonds can indeed be observed in a-Si:H. Due to the small size of the interaction to be expected from the X-band results, low-field electrically detected magnetic resonance ŽEDMR. was used. In contrast to conventional ESR, EDMR on a-Si:H has a sensitivity independent of the Zeeman splitting, as will be shown below. However, only distant hyperfine interaction between the ensembles of dangling bond defects and hydrogen nuclei was detected. No resolved ligand hyperfine structure was found.
2. Experimental The magnetic resonance experiments were performed in a low-field spectrometer using a radio frequency of 434 MHz. Ham radio equipment ŽYaesu FT 790 transceiver and Tokyo High Power amplifier. provided up to 25 W of rf power to a helical resonator with a quality factor of Q f 10. For the detection of the dangling bond resonance at g s 2.0055, a magnetic field of 154.6 G is needed, which was supplied by a pair of Helmholtz coils. A second, smaller pair of Helmholtz coils supplied additional magnetic field modulation. Photoconductivity was excited by illumination with red light Ž630 nm F l F 700 nm. only. The resonant change of the photoconductivity was measured with lock-in detection. Various deuterated and hydrogenated a-Si samples have been investigated for this study. Results obtained on standard glow discharge a-Si:D with a deuterium concentration of 10 at.%, hot-wire a-Si:H Ž2 at.% H., standard glow-discharge a-Si:H Ž10 at.% H. and on a-Si:H grown at low substrate temperature Ž18 at.% H. are shown here. The hydrogen concentration was determined using both FTIR and effusion measurements.
3. Results Fig. 1 shows the comparison of the central parts of the room temperature EDMR spectra of the four typical samples discussed. All samples are in the degraded state Ž Ns f 10 17 cmy3 as determined from conventional X-band ESR., which leads to the observation of the pure g s 2.0055 dangling bond resonance in EDMR. Using a maximum modulation amplitude of Hmod s 0.7 G and a rf power of 300 mW corresponding to a rf magnetic field of H1 s 80 mG, care has been taken not to distort the resonance lines by experimental conditions. A clear broadening of the db resonance is observed as a function of the hydrogen content, increasing the linewidth from D Hpp s 1.4 G in a-Si:D to D Hpp s 2.7 G in a-Si:H containing 18 at.% hydrogen. Due to the significantly smaller nuclear g-factor g n of deuterium Ž g n s 0.857. compared to hydrogen Ž g n s 5.585., hyperfine interaction involving D is expected to be negligible in a-Si:D. Since the g-factor anisotropy dominating the linewidth in X-band spectroscopy Ž g 5 s 2.004 and g H s 2.008 w3x. would give rise to a powder pattern with a width of only 0.3 G at 434 MHz, the linewidth observed for a-Si:D is very near the natural linewidth of the dangling bonds in amorphous silicon. Taking a-Si:D as a reference for the additional broadening induced by the hyperfine interaction with hydrogen in a-Si:H, an increase in linewidth D Hpp –D Hpp Ža-Si:D. is found as shown in Fig. 2.
Fig. 1. Comparison of the electrically detected magnetic resonance of undoped a-Si:D and a-Si:H films with different hydrogen concentration. The broadening is caused by hyperfine interaction between dangling bonds and the ‘background gas’ of hydrogen.
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
Fig. 2. Increase of the peak-to-peak linewidth of the EDMR signal in a-Si:H with respect to a-Si:D as a function of the hydrogen content f. A f 0.55 dependence fits the data.
The interaction of the ensembles of two nuclear magnetic species has been studied in magnetically diluted crystals by van Vleck w4x as well as by Kittel and Abrahams w5x. They have found that in the case of high concentrations of magnetic species, the linewidth increases as
(
D Hpp A ² D H 2 : A
( S Ž S q 1. r3h
2
f
where f is the hydrogen content. Only at very small concentrations, the dependence changes to D Hpp A f, as expected for simple dipolar broadening. Indeed, the increase of the db resonance follows the predicted square-root dependence as seen in Fig. 2. The effect of the distant hyperfine interaction with 29 Si nuclei Ž4.7% natural abundance, g n s y1.11. can be estimated using the results above to account for a further broadening of about 0.13 G. In addition to this distant–hyperfine interaction of the dangling bond with the ‘background gas’ of hydrogen present in a-Si:H, no ligand–hyperfine interaction caused by a structurally well defined db–H complex is observed. Assuming that such a hyperfine interaction is hidden underneath the observed resonance, the measured linewidth can be taken as an upper bound for the size of such a ligand–hyperfine interaction. As noted above, the 9 GHz-linewidth of 8 G has been used to estimate a ˚ Since D H A 1rr 3, minimum db–H distance r of 3 A. the reduced linewidth of 2 G observed at 434 MHz would allow to increase this estimate to about r G 4.5 ˚ in accordance with the ESEEM results w2x. HowA, ever, due to the amorphous network, it must be expected that db–H complexes of such an interatomic distance exhibit a structural variation leading
345
to a broadening of hyperfine lines, which would make this estimation somewhat inconclusive. The exact lineshape of the EDMR resonances at 434 MHz is shown in Fig. 3 using standard glow– discharge a-Si:H with 10 at.% hydrogen as an example. The figure shows a comparison of the lineshapes observed in the as-grown, degraded and annealed states. The size of the resonant current change D srs increases proportionally to the enhanced spin density after degradation as expected. In addition, the line shape, which shows a notable deviation from the ideal Lorentzian shape around 150 and 160 G in as-grown and annealed material, becomes more Lorentz-like after degradation, which is not due to the slight change in D Hpp . To fully understand the line shape, both the remaining g-factor anisotropy, as well the deviations from a Lorentzian line shape predicted for distant–hyperfine interaction w5x must be taken into account. However, the increased defect density after pulsed-light degradation w6x leads to db–db interaction, which should indeed result in the observed exchange narrowing of the resonance. Finally, we briefly address the question of the sensitivity of EDMR as a function of the size of the Zeeman splitting induced by the external magnetic field. Various models have been proposed to quantitatively describe the EDMR signal intensity. The two limiting cases are the Lepine-model, which invokes the interaction of the polarisations of the two spin ensembles, and the Kaplan–Solomon–Mott model
Fig. 3. Comparison of the electrically detected magnetic resonance of a-Si:H Ž10 at.% H. as a function of the degradation. The resonance becomes somewhat more Lorentzian after degradation with pulsed light.
346
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
ŽKSM., which assumes the formation of a spin-pair prior to the actual recombination w7,8x. The two models differ in particular in their predictions for the dependence of the EDMR signal D srs on the Zeeman energy hy and the temperature T. The Lepine-model predicts a very strong dependence D srs A Ž hyrT . 2 , while the KSM-model predicts that D srs is independent of these experimental conditions. Fig. 4 shows the EDMR signal intensity D srs of undoped a-Si:H Ž10 at.% H. at the center of resonance as a function of the microwave or rf field H1 for three different microwave frequencies: 434 MHz, 9 and 34 GHz. Although it is known that the signal intensity D srs is nearly independent of the photocarrier generation rate wM.S. Brandt, unpublishedx, care has been taken to use the identical light intensities in all three experiments, keeping the positions of the quasi-Fermi levels constant. Below H1 s 0.1 G, all signal intensities are identical within the accuracy of the H1 determination. In this regime, D srs is proportional to H12 . For higher powers, two different behaviours are observed: In the case of the low-frequency EDMR, immediate saturation oc-
Fig. 4. EDMR signal intensity of undoped a-Si:H at 434 MHz, 9 and 34 GHz. Magnetic field overmodulation has been used in the first case, microwave power modulation in the two latter cases. The microwave magnetic field has been determined via line broadening at 434 MHz and by calculation for the X- and Q-band cavities. The estimated error in the determination of H1 is about a factor of 2.
curs with a maximum D srs f 10y5 . In the inhomogeneously broadened cases at higher frequencies, a different, weak saturation regime is observed, with D srs A H1. At 9 GHz, using a particular thinfilm–transistor structure which led to an additional enhancement of the microwave field, the EDMR intensity dependence could even be observed for microwave fields greater than the inhomogeneous linewidth w9x. In this case, again strong saturation at H1 ) D Hpp is found, as in the case of the homogeneous line.
4. Discussion The three different regimes observed at 9 GHz have been discussed in detail by Kawachi et al. w9x, who have demonstrated that for an inhomogeneous line in the weak saturation regime Žg 2 H12 T1T2 ) 1 and ŽT2rT1 .1r2 ) H1 g T2) ., the EDMR signal intensity increases as D srs A H1 , while the linewidth remains constant. In contrast, in the strong saturation regime Ž H1 g T2) ) ŽT2rT1 .1r2 ., D srs s const. and D Hpp A H1 is found. The latter behaviour also holds for a homogeneous line under saturation. Correcting the plotted EDMR signal intensities for the increasin g lin e w id th b y m u ltip ly in g w ith Ž D Hpp Ž H1 .rD Hpp Ž H1 ™ 0.. eliminates this apparent saturation. In this case, all curves in Fig. 2 show the same H1-dependence within the experimental error. The EDMR signal intensity in a-Si:H, therefore, does not depend on the size of the Zeeman level splitting induced. It should, however, be noted that this only applies because the relaxation times T1 and T2 do not depend on the Zeeman splitting, as shown by the values obtained for ŽT1T2 .1r2 included in Fig. 4. Two experimental observations have, for a long time, provided evidence against the applicability of the Lepine-model to EDMR of amorphous silicon. First, the room temperature signal intensity easily surpasses the maximum value of about D srs s 3 = 10y6 predicted by Lepine. Second, no significant temperature dependence of D srs was found at least down to 100 K. Here, the independence of D srs on the size of the microwave frequency or the magnetic field, is a further argument against the Lepine-model in a-Si:H.
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
5. Conclusion and outlook Comparing the signal intensities of electrically detected magnetic resonance at microwave frequencies Žand magnetic fields. differing by two orders of magnitude, we have shown that indeed EDMR of amorphous silicon does not suffer the loss of sensitivity at low microwave frequencies typical for conventional ESR. Using 434 MHz EDMR of undoped a-Si:D and a-Si:H with varying hydrogen concentration, we have identified a distant–hyperfine interaction between the paramagnetic dangling bonds and the background hydrogen ‘gas’, leading to a broadening of the natural linewidth. Extension of this work to study the EDMR at low temperatures, e.g., in search of a possible B-hyperfine line, and to study a-Ge:H, as well as microcrystalline silicon, are currently underway. The 434 MHz-linewidth of annealed a-Si:D at 180 K appears unchanged, indicating that either the natural linewidths of the conduction and valence band tail states are smaller than that of the dangling bond, or that a similar hyperfine broadening affects these states. Finally, first experi-
347
ments on a-Ge:H indicate that the low-frequency linewidth in these materials is D Hpp s 9.5 G. Acknowledgements The authors are indebted to R. Janssen for the growth of the 18 at.% sample and the analysis of the effusion experiments. References w1x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 32 Ž1977. 292. w2x J. Isoya, S. Yamasaki, A. Matsuda, K. Tanaka, Philos. Mag. B 69 Ž1994. 263. w3x M. Stutzmann, D.K. Biegelsen, Phys. Rev. B 40 Ž1989. 9834. w4x J.H. van Vleck, Phys. Rev. B 74 Ž1948. 1168. w5x C. Kittel, E. Abrahams, Phys. Rev. 90 Ž1953. 238. w6x M. Stutzmann, M.C. Rossi, M.S. Brandt, Phys. Rev. B 50 Ž1994. 11592. w7x D.J. Lepine, Phys. Rev. B 6 Ž1972. 436. w8x D. Kaplan, I. Solomon, N.F. Mott, J. Phys. ŽParis. 39 Ž1978. L51. w9x G. Kawachi, C.F.O. Graeff, M.S. Brandt, M. Stutzmann, Jpn. J. Appl. Phys. 36 Ž1997. 121.
Electrically detected magnetic resonance of a-Si:H at low magnetic fields: the influence of hydrogen on the dangling bond resonance M.S. Brandt
a,)
, M.W. Bayerl a , M. Stutzmann a , C.F.O. Graeff
b
a
b
Walter Schottky Institut, Technische UniÕersitat D 85748 Garching, Germany ¨ Munchen, ¨ Departamento de Fısica e Matematica, FFCLRP-USP, AÕ. Bandeirantes 3900, 14040-91 Riberao ´ ´ ˜ Preto, Brazil
Abstract The hyperfine interaction between the dangling bonds and the hydrogen atoms in hydrogenated amorphous silicon Ža-Si:H. is observed using low-field electrically detected magnetic resonance ŽEDMR.. In deuterated amorphous silicon Ža-Si:D., a peak-to-peak linewidth of 1.4 G is found at 434 MHz. Distant hyperfine involving the background hydrogen present in the material is found to broaden the resonance in a-Si:H. The broadening is proportional to the square-root of the hydrogen concentration and reaches 1 G at 10 at.% hydrogen. The lineshape is more Lorentz-like in the degraded than in the as-grown or annealed states. An identical EDMR signal intensity D srs is observed for a-Si:H using spectrometers working at 434 MHz, 9 and 34 GHz, indicating that the signal intensity is independent of the Zeeman splitting. q 1998 Elsevier Science B.V. All rights reserved. Keywords: a-Si:H; Hydrogen; Dangling bond resonance
1. Introduction Since the first observation of light-induced metastability in amorphous hydrogenated silicon ŽaSi:H. by Staebler and Wronski w1x, the microscopic origin of this effect was subject to intensive investigation. The notorious decrease in the photoconductivity of undoped a-Si:H has been correlated to an increase in the density of paramagnetic dangling bond defects Ždb.. Various microscopic models have been proposed to account for the metastable creation )
Corresponding author. Tel.: q49-89 2891 2758; fax: q49 89 2891 2737; e-mail: [email protected].
of dangling bonds under illumination: weak bonddangling bond conversion, Si–Si bond-switching, charge exchange between diamagnetic defects, Si–H bond-switching or dissociation of H 2)-complexes. However, while electron spin resonance experiments have provided detailed information on the structure of the dangling bond orbital, no decisive information on the structure of the surrounding network was obtained. In particular, no difference in the resonance line shape of the dangling bonds in annealed and degraded a-Si:H was found, indicating that the metastable dangling bonds are spatially separated. In addition, no hyperfine interaction of the dangling bond with the large nuclear magnetic moment of
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 7 3 - 8
344
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
hydrogen was observed Žthe resonances in a-Si:H and a-Si:D also being identical in conventional Xband ESR., excluding a well defined db–H complex ˚ somewhat with an interatomic distance of F 3 A, larger than the Si–Si bond length. Electron spin-echo envelope modulation experiments ŽESEEM. on aSi:D have further shown that no deuterium is present ˚ of the dangling bond, the dangling bond, within 4 A therefore, being in a deuterium Žor hydrogen. denuded zone w2x. Here, we will show that hyperfine interaction of hydrogen with dangling bonds can indeed be observed in a-Si:H. Due to the small size of the interaction to be expected from the X-band results, low-field electrically detected magnetic resonance ŽEDMR. was used. In contrast to conventional ESR, EDMR on a-Si:H has a sensitivity independent of the Zeeman splitting, as will be shown below. However, only distant hyperfine interaction between the ensembles of dangling bond defects and hydrogen nuclei was detected. No resolved ligand hyperfine structure was found.
2. Experimental The magnetic resonance experiments were performed in a low-field spectrometer using a radio frequency of 434 MHz. Ham radio equipment ŽYaesu FT 790 transceiver and Tokyo High Power amplifier. provided up to 25 W of rf power to a helical resonator with a quality factor of Q f 10. For the detection of the dangling bond resonance at g s 2.0055, a magnetic field of 154.6 G is needed, which was supplied by a pair of Helmholtz coils. A second, smaller pair of Helmholtz coils supplied additional magnetic field modulation. Photoconductivity was excited by illumination with red light Ž630 nm F l F 700 nm. only. The resonant change of the photoconductivity was measured with lock-in detection. Various deuterated and hydrogenated a-Si samples have been investigated for this study. Results obtained on standard glow discharge a-Si:D with a deuterium concentration of 10 at.%, hot-wire a-Si:H Ž2 at.% H., standard glow-discharge a-Si:H Ž10 at.% H. and on a-Si:H grown at low substrate temperature Ž18 at.% H. are shown here. The hydrogen concentration was determined using both FTIR and effusion measurements.
3. Results Fig. 1 shows the comparison of the central parts of the room temperature EDMR spectra of the four typical samples discussed. All samples are in the degraded state Ž Ns f 10 17 cmy3 as determined from conventional X-band ESR., which leads to the observation of the pure g s 2.0055 dangling bond resonance in EDMR. Using a maximum modulation amplitude of Hmod s 0.7 G and a rf power of 300 mW corresponding to a rf magnetic field of H1 s 80 mG, care has been taken not to distort the resonance lines by experimental conditions. A clear broadening of the db resonance is observed as a function of the hydrogen content, increasing the linewidth from D Hpp s 1.4 G in a-Si:D to D Hpp s 2.7 G in a-Si:H containing 18 at.% hydrogen. Due to the significantly smaller nuclear g-factor g n of deuterium Ž g n s 0.857. compared to hydrogen Ž g n s 5.585., hyperfine interaction involving D is expected to be negligible in a-Si:D. Since the g-factor anisotropy dominating the linewidth in X-band spectroscopy Ž g 5 s 2.004 and g H s 2.008 w3x. would give rise to a powder pattern with a width of only 0.3 G at 434 MHz, the linewidth observed for a-Si:D is very near the natural linewidth of the dangling bonds in amorphous silicon. Taking a-Si:D as a reference for the additional broadening induced by the hyperfine interaction with hydrogen in a-Si:H, an increase in linewidth D Hpp –D Hpp Ža-Si:D. is found as shown in Fig. 2.
Fig. 1. Comparison of the electrically detected magnetic resonance of undoped a-Si:D and a-Si:H films with different hydrogen concentration. The broadening is caused by hyperfine interaction between dangling bonds and the ‘background gas’ of hydrogen.
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
Fig. 2. Increase of the peak-to-peak linewidth of the EDMR signal in a-Si:H with respect to a-Si:D as a function of the hydrogen content f. A f 0.55 dependence fits the data.
The interaction of the ensembles of two nuclear magnetic species has been studied in magnetically diluted crystals by van Vleck w4x as well as by Kittel and Abrahams w5x. They have found that in the case of high concentrations of magnetic species, the linewidth increases as
(
D Hpp A ² D H 2 : A
( S Ž S q 1. r3h
2
f
where f is the hydrogen content. Only at very small concentrations, the dependence changes to D Hpp A f, as expected for simple dipolar broadening. Indeed, the increase of the db resonance follows the predicted square-root dependence as seen in Fig. 2. The effect of the distant hyperfine interaction with 29 Si nuclei Ž4.7% natural abundance, g n s y1.11. can be estimated using the results above to account for a further broadening of about 0.13 G. In addition to this distant–hyperfine interaction of the dangling bond with the ‘background gas’ of hydrogen present in a-Si:H, no ligand–hyperfine interaction caused by a structurally well defined db–H complex is observed. Assuming that such a hyperfine interaction is hidden underneath the observed resonance, the measured linewidth can be taken as an upper bound for the size of such a ligand–hyperfine interaction. As noted above, the 9 GHz-linewidth of 8 G has been used to estimate a ˚ Since D H A 1rr 3, minimum db–H distance r of 3 A. the reduced linewidth of 2 G observed at 434 MHz would allow to increase this estimate to about r G 4.5 ˚ in accordance with the ESEEM results w2x. HowA, ever, due to the amorphous network, it must be expected that db–H complexes of such an interatomic distance exhibit a structural variation leading
345
to a broadening of hyperfine lines, which would make this estimation somewhat inconclusive. The exact lineshape of the EDMR resonances at 434 MHz is shown in Fig. 3 using standard glow– discharge a-Si:H with 10 at.% hydrogen as an example. The figure shows a comparison of the lineshapes observed in the as-grown, degraded and annealed states. The size of the resonant current change D srs increases proportionally to the enhanced spin density after degradation as expected. In addition, the line shape, which shows a notable deviation from the ideal Lorentzian shape around 150 and 160 G in as-grown and annealed material, becomes more Lorentz-like after degradation, which is not due to the slight change in D Hpp . To fully understand the line shape, both the remaining g-factor anisotropy, as well the deviations from a Lorentzian line shape predicted for distant–hyperfine interaction w5x must be taken into account. However, the increased defect density after pulsed-light degradation w6x leads to db–db interaction, which should indeed result in the observed exchange narrowing of the resonance. Finally, we briefly address the question of the sensitivity of EDMR as a function of the size of the Zeeman splitting induced by the external magnetic field. Various models have been proposed to quantitatively describe the EDMR signal intensity. The two limiting cases are the Lepine-model, which invokes the interaction of the polarisations of the two spin ensembles, and the Kaplan–Solomon–Mott model
Fig. 3. Comparison of the electrically detected magnetic resonance of a-Si:H Ž10 at.% H. as a function of the degradation. The resonance becomes somewhat more Lorentzian after degradation with pulsed light.
346
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
ŽKSM., which assumes the formation of a spin-pair prior to the actual recombination w7,8x. The two models differ in particular in their predictions for the dependence of the EDMR signal D srs on the Zeeman energy hy and the temperature T. The Lepine-model predicts a very strong dependence D srs A Ž hyrT . 2 , while the KSM-model predicts that D srs is independent of these experimental conditions. Fig. 4 shows the EDMR signal intensity D srs of undoped a-Si:H Ž10 at.% H. at the center of resonance as a function of the microwave or rf field H1 for three different microwave frequencies: 434 MHz, 9 and 34 GHz. Although it is known that the signal intensity D srs is nearly independent of the photocarrier generation rate wM.S. Brandt, unpublishedx, care has been taken to use the identical light intensities in all three experiments, keeping the positions of the quasi-Fermi levels constant. Below H1 s 0.1 G, all signal intensities are identical within the accuracy of the H1 determination. In this regime, D srs is proportional to H12 . For higher powers, two different behaviours are observed: In the case of the low-frequency EDMR, immediate saturation oc-
Fig. 4. EDMR signal intensity of undoped a-Si:H at 434 MHz, 9 and 34 GHz. Magnetic field overmodulation has been used in the first case, microwave power modulation in the two latter cases. The microwave magnetic field has been determined via line broadening at 434 MHz and by calculation for the X- and Q-band cavities. The estimated error in the determination of H1 is about a factor of 2.
curs with a maximum D srs f 10y5 . In the inhomogeneously broadened cases at higher frequencies, a different, weak saturation regime is observed, with D srs A H1. At 9 GHz, using a particular thinfilm–transistor structure which led to an additional enhancement of the microwave field, the EDMR intensity dependence could even be observed for microwave fields greater than the inhomogeneous linewidth w9x. In this case, again strong saturation at H1 ) D Hpp is found, as in the case of the homogeneous line.
4. Discussion The three different regimes observed at 9 GHz have been discussed in detail by Kawachi et al. w9x, who have demonstrated that for an inhomogeneous line in the weak saturation regime Žg 2 H12 T1T2 ) 1 and ŽT2rT1 .1r2 ) H1 g T2) ., the EDMR signal intensity increases as D srs A H1 , while the linewidth remains constant. In contrast, in the strong saturation regime Ž H1 g T2) ) ŽT2rT1 .1r2 ., D srs s const. and D Hpp A H1 is found. The latter behaviour also holds for a homogeneous line under saturation. Correcting the plotted EDMR signal intensities for the increasin g lin e w id th b y m u ltip ly in g w ith Ž D Hpp Ž H1 .rD Hpp Ž H1 ™ 0.. eliminates this apparent saturation. In this case, all curves in Fig. 2 show the same H1-dependence within the experimental error. The EDMR signal intensity in a-Si:H, therefore, does not depend on the size of the Zeeman level splitting induced. It should, however, be noted that this only applies because the relaxation times T1 and T2 do not depend on the Zeeman splitting, as shown by the values obtained for ŽT1T2 .1r2 included in Fig. 4. Two experimental observations have, for a long time, provided evidence against the applicability of the Lepine-model to EDMR of amorphous silicon. First, the room temperature signal intensity easily surpasses the maximum value of about D srs s 3 = 10y6 predicted by Lepine. Second, no significant temperature dependence of D srs was found at least down to 100 K. Here, the independence of D srs on the size of the microwave frequency or the magnetic field, is a further argument against the Lepine-model in a-Si:H.
M.S. Brandt et al.r Journal of Non-Crystalline Solids 227–230 (1998) 343–347
5. Conclusion and outlook Comparing the signal intensities of electrically detected magnetic resonance at microwave frequencies Žand magnetic fields. differing by two orders of magnitude, we have shown that indeed EDMR of amorphous silicon does not suffer the loss of sensitivity at low microwave frequencies typical for conventional ESR. Using 434 MHz EDMR of undoped a-Si:D and a-Si:H with varying hydrogen concentration, we have identified a distant–hyperfine interaction between the paramagnetic dangling bonds and the background hydrogen ‘gas’, leading to a broadening of the natural linewidth. Extension of this work to study the EDMR at low temperatures, e.g., in search of a possible B-hyperfine line, and to study a-Ge:H, as well as microcrystalline silicon, are currently underway. The 434 MHz-linewidth of annealed a-Si:D at 180 K appears unchanged, indicating that either the natural linewidths of the conduction and valence band tail states are smaller than that of the dangling bond, or that a similar hyperfine broadening affects these states. Finally, first experi-
347
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