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Room-temperature photoluminescence of amorphous hydrogenated silicon carbide doped with erbium
Journal of Non-Crystalline Solids 227–230 Ž1998. 488–492
Room-temperature photoluminescence of amorphous hydrogenated silicon carbide doped with erbium E.I. Terukov
a,)
, V.Kh. Kudoyarova a , A.N. Kuznetsov a , W. Fuhs b, G. Weiser c , H. Kuehne c
a
A.F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Politechnicheskaya 26, Russian Federation b Hahn-Meitner Institut, Rudower Chaussee 5, D-12489 Berlin, Germany c Fachbereich Physik Philipps-UniÕersitat Marburg, Renthof 5, D-35032 Marburg an der Lahn, Germany
Abstract Room temperature photoluminescence of Er ions at 1.54 m m corresponding to 4 I 13r2 y4 I 15r2 transition in the ion f-shell was observed in erbium-doped hydrogenated amorphous silicon carbide films Ža-Si 1yx C x :H²Er:.. Films of a-Si 1yx C x :H²Er: were prepared by co-sputtering of graphite and Er targets applying the magnetron-assisted silane-decomposition technique with mixtures of Ar and silane. The composition of films Ž x . was 0 F x F 0.29. The concentration of incorporated Er-ions was 6 = 10 19 cmy3. The excitation mechanism of the Er ions in amorphous hydrogenated silicon carbide doped with erbium is discussed within the framework of the defect related Auger excitation model. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Room temperature; Photoluminescence; Silicon
1. Introduction Recently the luminescent properties of erbiumdoped crystalline silicon Žc-Si:Er. have attracted much attention. The reason for this interest originates from the possible fabrication of LEDs which are integrable into silicon electronic devices and emit at a wavelength of 1.537 m m where the absorption of silica glass optical fibers is smallest. However, attempts to obtain efficient photoluminescence ŽPL. on the basis of erbium-doped crystalline silicon Žc-Si. have met serious difficulties connected with a strong temperature quenching of )
Corresponding author. Fax: q7-812 247 10 17; e-mail: [email protected],rssi.ru.
erbium luminescence in this material and segregation of rare earth impurities, limiting the highest concentration of erbium optical centers to ; 10 18 cmy3 . It should be noted that to achieve high emission intensity, c-Si²Er: the samples should be coimplanted with erbium and oxygen, and optimization requires extended high-temperature annealing w1x. These difficulties can be avoided if an amorphous but not crystalline semiconductor is doped with erbium impurity. In the case of an amorphous matrix, the disorder in the environment of the rare earth ion favors the mixing of the internal f-states of the rare earth ion which leads to an increased probability of radiative transition between these states and decreased response time of PL. Besides, the activation energy of defects involved in excitation of the rare
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 7 - 0
E.I. TerukoÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 488–492
earth ion is greater in amorphous as compared to crystalline material and it is believed that this fact is responsible for the absence of strong temperature quenching of erbium luminescence in amorphous silicon Ža-Si:H²Er:. w2x. It has been shown that the intensity of the Er emission depends on the band gap energy of the host semiconductor, mainly for the room temperature emission. To obtain an intense room temperature emission, a wide-gap semiconductor must be used w3,4x. The optical band gap of amorphous hydrogenated silicon carbide Ža-Si 1y x C x :H. depends on composition Ž x . and is larger than in amorphous hydrogenated Ža-Si:H. and crystalline silicon Žc-Si. w5x. In this paper, the results obtained on erbium-doped amorphous hydrogenated silicon Ža-Si:H²Er:. w2x are extended to erbium-doped amorphous hydrogenated silicon carbide Ža-Si 1y x C x :H²Er:. to increase the band gap energy limit. For the first time it is shown that a-Si 1y x C x :H²Er: exhibits room-temperature PL at 1.54 m m, which is assigned to the internal 4f-shell transition in Er ions.
2. Experiment We used the magnetron assisted silane decomposition technique Ž M A SD . for obtaining a-Si 1y x C x :H²Er: films. The reactor system used for MASD was described in detail in Ref. w6x. The magnetron target used was made of C and Er of 99.999% purity. The SirC ratio was varied by changing the SiH 4 percentage in the gas mixture of Ar q SiH 4 . The substrate temperature was 3008C. The use of a novel technology ŽMASD. permits insertion into the semiconductor matrix of Er ion concentrations two orders of magnitude larger than those in the case of a crystalline matrix. The composition of the films Ž x . and the presence of Er in the films were monitored by Rutherford back scattering ŽRBS., using RBS of 4 He 2q ions. The energy of 4 He 2q was 3.7 MeV, the angle of backscattering 1708 and the detector resolution energy 50 keV. Typical RBS spectra for a film with x s 0.29 and for crystalline silicon carbide c-6H SiC are shown in Fig. 1. The crystalline carbide, c-6H SiC, was used as a calibration sample. It has been shown that the
489
Fig. 1. Rutherford backscattering ŽRBS. spectra for 4 He 2q ions with energy Es 3.7 MeV backscattered at Si, C and Er as a function of the energy Ž channel number . for: Ž 1 . a-Si 0.71C 0.29 :H²Er:; Ž2. c-6H SiC Žcrystalline silicon carbide. Concentration of incorporated Er ions 6=10 19 cmy3 .
SirC ratio was 1.022 for c-6H SiC w7x. The content of C Ž x . was in the range 0 F x F 0.29. The concentration of incorporated Er ions was ; 6 = 10 19 cmy3 . The content of hydrogen was estimated at ; 10 at.% from the integrated IR absorption due to the Si–H wagging band Ž630 cmy1 . and C–H stretching band Ž2900 cmy1 .. The optical band gap Ž Egopt ., determined by photothermal deflection spectroscopy ŽPDS. as the energy where the absorption coefficient is 10 3 cmy1 , varied from 1.59 Ž x s 0. to 1.82 eV Ž x s 0.29.. The PL spectra were recorded with excitation by a krypton laser with l s 649.6 nm and an argon laser with l s 514.5 nm in the temperature range 77–400 K. The PL spectra were measured by means of a germanium detector cooled to the temperature of liquid nitrogen and a double monochromator ŽSPEX1403. with a resolution of 2 nm.
3. Results Fig. 2 shows the PL spectra of Er doped amorphous hydrogenated silicon carbide a-Si 1y x C x :H²Er: and amorphous hydrogenated silicon a-Si:H²Er: taken at 77 K. The PL spectra of a-Si 1y x C x :H²Er: and a-Si:H²Er: measured at 77 K exhibit emission from both the Er 3q center Ž0.81 eV. and the amorphous Si and Si–C matrix. The latter is a band with features at ; 1 eV and at ; 1.3 eV.
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E.I. TerukoÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 488–492
Fig. 2. PL spectra of Er doped: Ž1. a-Si:H²Er:; Ž2. aSi 0.71C 0.29 :H²Er: Žconcentration of incorporated Er ions 6=10 19 cmy3 . taken at 77 K excited with 514.5 nm Ž2.41 eV. at P s 50 mW.
Usually, the PL data Žintrinsic band. on a-Si 1y x C x :H alloys for x - 0.4 have been discussed in terms of the models put forward for a-Si:H. In undoped a-Si:H with a defect concentration Ždangling bond Si. ND²10 16 cmy3 a single broad band is observed at ; 1.3 eV with a typical width of 0.3 eV. This band is attributed to a transition between localized band tail states, radiative recombination occurring by tunneling. All reports on a-Si 1y x C x :H alloys
show only one PL spectrum whose width increases and whose maximum shifts to longer energy when x is increased w5x. In undoped a-Si:H samples we observed the intrinsic band Ž1.2 eV.. However, our investigation has shown that incorporation of erbium ions into the amorphous matrix is accompanied by the formation of dangling bond defects with concentration, ND ; 10 18 cmy3 . Usually such samples show a well resolved defect PL at 0.94 eV Žrecombination at Si dangling bond.. The defect PL in a-Si:H arises from the radiative capture of electrons by neutral dangling bonds, D 0 q e ™ Dy. The intrinsic PL Ž1.2 eV. involving holes in the valence band tail remains small ŽFig. 2, curve 1.. In the case of a-Si 1y x C x :H²Er: alloys Ž x - 0.4. the optical band gap Egopt increases with x, increasing from 1.59 " 0.02 eV Ž x s 0. to 1.82 " 0.02 eV Ž x s 0.29.. The increase of Egopt is accompanied by an increase in film disorder, which makes increases the density of localized states in the band tails and in the defect band due to dangling bonds. The total density of defects Ždangling bonds., ND , estimated from PDS data ranges from 3.5 = 10 18 cmy3 Ž x s 0. to 4.9 = 10 18 cmy3 Ž x s 0.29.. The luminescence bands of the alloy shift with increasing gap to larger energy, which enables better
Fig. 3. PL spectra of Er doped a-Si 0.78 C 0.22 :H²Er: at the different temperatures: 1–77 K, 2–300 K Žconcentration of incorporated Er ions 6 = 10 19 cmy3 . excited with 514.5 nm Ž2.41 eV. at P s 50 mW.
E.I. TerukoÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 488–492
491
Fig. 4. Temperature dependence of the peak intensity of the defect and Er 3q luminescence of a-Si 0.71C 0.29 :H²Er: Žconcentration of incorporated Er ions 6 = 10 19 cmy3 . excited with 514.5 nm Ž2.41 eV. at P s 50 mW.
separation from the Er PL at 1.53 m m. The defect luminescence band is observed at ; 1 eV and the intrinsic band at ; 1.3 eV ŽFig. 2, curve 2.. At 300 K the intrinsic emission from amorphous silicon carbide matrix Ž1.3 eV. is suppressed by temperature quenching and the remaining signal is due to Er 3q ions and defect PL ŽFig. 3, curve 2.. It should be noted that the various luminescence bands have differing temperature dependences. The intrinsic luminescence band decreases above 100 K. As in
the case of a-Si:H, the dangling bonds act as nonradiative centers, quenching the PL Žintrinsic band. by tunneling to the defect. The temperature quenching of luminescence above 100 K is explained by an increased mobility of carriers which diffuse to the recombination centers w5x. Temperature dependences of intensity of Er 3q PL and defect PL are displayed in Fig. 4. It can be seen that the temperature dependence of the defect luminescence is similar to that of the Er 3q emission.
Fig. 5. Comparison of the temperature dependence of Er 3q luminescence of a-Si 0.71C 0.29 :H²Er: Ž NEr s 6 = 10 19 cmy3 . Ž1. and a-Si:H²Er: Ž NEr s 1 = 10 20 cmy3 , N0 s 2 = 10 20 cmy3 . Ž2. excited with 514.5 nm at P s 50 mW. Data are normalized to the PL yield of this component at 77 K.
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E.I. TerukoÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 488–492
Fig. 5 shows the temperature dependence of the intensity of Er 3q emission in a-Si 1y x C x :H²Er: and in a-Si:H²Er:. We used the sample of a-Si:H²Er: in which the maximum intensity of PL is observed for comparison. As is seen in Fig. 5 the onset of temperature quenching of Er 3q emission in the case of a-Si 1y x C x :H²Er: is observed at higher temperatures than for a-Si:H²Er:.
4. Discussion The similar temperature dependences of defect and Er 3q luminescence in a-Si 1y x C x :H²Er: indicate that dangling bond defects play an important role in the excitation of Er ions. We assume that a-Si 1y x C x :H Ž x - 0.4. alloys are built by Si–Si bonds. Carbon contributes little to the formation of the structure and is incorporated into the Si structure as CH 3 and CH 2 groups w8x. Besides, carbon does induce dangling bonds, but does not change their properties. Consequently, the neutral and occupied Žby electron. dangling bond states, D8 and Dy, only adjust their position to a widening gap w9x. It is reasonable to assume that the mechanism of electronic excitation of Er ions in a-Si 1y x C x :H²Er: can be discussed in terms of the model previously proposed for a-Si:H²Er: w10x. In this model the mechanism of electronic excitation of Er ions is based on defect-related Auger excitation ŽDRAE.. This process involves the capture of mobile carries by dangling bond defects Ž D . in the amorphous matrix, connected with an Auger excitation of the Er 3q. In view of the densities of incorporated Er ions Ž10 20 cmy3 . and dangling bonds defects Ž10 18 cmy3 . we assume that a large concentration of closely spaced Er–defect ŽEr–D . pairs exists. When an electron is captured in a neutral defect Ž D8 q e ™ Dy. in such a close pair, part of the energy can be transferred to f-electrons of Er ions by Coulomb interaction. As a result, Er ion is excited from the ground state 4 I 15r2 to the excited state 4 I 13r2 , and this energy is transferred into the local vibration modes of the defect. In Ref. w10x the probabilities of the DRAE and the radiative recombination at defects Ž e q D8 ™ Dy. were calculated. It is shown that the probability of the DRAE process is greater by an order of magnitude than that of radiative defect
recombination. A correlation between the intensities of defect and erbium luminescence supports this mechanism. It should be noted that these are the first results and further studies are planned to obtain more specific information on this mechanism.
5. Conclusions Room temperature PL of Er ions at 1.54 m m corresponding to the 4 I 13r2 y4 I 15r2 transition in the ion f-shell was observed. We conclude that the excitation mechanism of the Er ions in amorphous hydrogenated silicon carbide doped is by the defect related Auger excitation ŽDRAE. model previously proposed for a-Si:H²Er:.
Acknowledgements This work was partly supported by the Volkswagen-Stiftung grant No. 1r71 646 and Russian Foundation for Basic Research through grant 96-0216931-a.
References w1x S. Coffa, G. Franzo, F. Priolo, A. Polman, R. Serna, Phys. Rev. B 49 Ž1994. 16313. w2 x M.S. Bresler, O.B. Gusev, V. Kh Kudoyarova, A.N. Kuznetsov, P.E. Pak, E.I. Terukov, I.N. Yassievich, B.P. Zakharchenya, W. Fuhs, A. Sturm, Appl. Phys. Lett. 67 Ž1995. 3599. w3x P.N. Favennec, H.L. Haridon, M. Salvi, D. Mountonnet, Y.LE. Guillou, Electron. Lett. 25 Ž1989. 718. w4x W.J. Choyke, L.L. Clemen, M. Yoganathan, G. Pensl, Ch. Hassler, Appl. Phys. Lett. 65 Ž1994. 1668. w5x J. Bullot, M.P. Schmidt, Phys. Stat. Sol. Žb. 143 Ž1987. 345. w6x V. Marachonov, N. Rogachev, J. Ishkalov, J. Marachonov, E. Terukov, V. Chelnokov, J. Non-Cryst. Solids 137–138 Ž1991. 817. w7x N.D. Sorokin, Yu.M. Tairov, V.F. Tsvetkov, M.A. Chernov, Kristalografia 28 Ž1993. 910. w8x J. Sotiropoulos, G. Weiser, J. Non-Cryst. Solids 92 Ž1987. 95. w9x H. Herremans, O. Oktu, W. Grevendok, G.J. Adriaenssens, J. Non-Cryst. Solids 137–138 Ž1991. 855. w10x I.N. Yassievich, O.B. Gusev, M.S. Bresler, W. Fuhs, A.N. Kuznetsov, V.F. Masterov, E.I. Terukov, B.P. Zakharchenya, Spring Meeting, April 8–12, San Francisco, Proc. Mater. Res. Soc., D7.3 Ž1996. 73.
Room-temperature photoluminescence of amorphous hydrogenated silicon carbide doped with erbium E.I. Terukov
a,)
, V.Kh. Kudoyarova a , A.N. Kuznetsov a , W. Fuhs b, G. Weiser c , H. Kuehne c
a
A.F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, Politechnicheskaya 26, Russian Federation b Hahn-Meitner Institut, Rudower Chaussee 5, D-12489 Berlin, Germany c Fachbereich Physik Philipps-UniÕersitat Marburg, Renthof 5, D-35032 Marburg an der Lahn, Germany
Abstract Room temperature photoluminescence of Er ions at 1.54 m m corresponding to 4 I 13r2 y4 I 15r2 transition in the ion f-shell was observed in erbium-doped hydrogenated amorphous silicon carbide films Ža-Si 1yx C x :H²Er:.. Films of a-Si 1yx C x :H²Er: were prepared by co-sputtering of graphite and Er targets applying the magnetron-assisted silane-decomposition technique with mixtures of Ar and silane. The composition of films Ž x . was 0 F x F 0.29. The concentration of incorporated Er-ions was 6 = 10 19 cmy3. The excitation mechanism of the Er ions in amorphous hydrogenated silicon carbide doped with erbium is discussed within the framework of the defect related Auger excitation model. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Room temperature; Photoluminescence; Silicon
1. Introduction Recently the luminescent properties of erbiumdoped crystalline silicon Žc-Si:Er. have attracted much attention. The reason for this interest originates from the possible fabrication of LEDs which are integrable into silicon electronic devices and emit at a wavelength of 1.537 m m where the absorption of silica glass optical fibers is smallest. However, attempts to obtain efficient photoluminescence ŽPL. on the basis of erbium-doped crystalline silicon Žc-Si. have met serious difficulties connected with a strong temperature quenching of )
Corresponding author. Fax: q7-812 247 10 17; e-mail: [email protected],rssi.ru.
erbium luminescence in this material and segregation of rare earth impurities, limiting the highest concentration of erbium optical centers to ; 10 18 cmy3 . It should be noted that to achieve high emission intensity, c-Si²Er: the samples should be coimplanted with erbium and oxygen, and optimization requires extended high-temperature annealing w1x. These difficulties can be avoided if an amorphous but not crystalline semiconductor is doped with erbium impurity. In the case of an amorphous matrix, the disorder in the environment of the rare earth ion favors the mixing of the internal f-states of the rare earth ion which leads to an increased probability of radiative transition between these states and decreased response time of PL. Besides, the activation energy of defects involved in excitation of the rare
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 7 - 0
E.I. TerukoÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 488–492
earth ion is greater in amorphous as compared to crystalline material and it is believed that this fact is responsible for the absence of strong temperature quenching of erbium luminescence in amorphous silicon Ža-Si:H²Er:. w2x. It has been shown that the intensity of the Er emission depends on the band gap energy of the host semiconductor, mainly for the room temperature emission. To obtain an intense room temperature emission, a wide-gap semiconductor must be used w3,4x. The optical band gap of amorphous hydrogenated silicon carbide Ža-Si 1y x C x :H. depends on composition Ž x . and is larger than in amorphous hydrogenated Ža-Si:H. and crystalline silicon Žc-Si. w5x. In this paper, the results obtained on erbium-doped amorphous hydrogenated silicon Ža-Si:H²Er:. w2x are extended to erbium-doped amorphous hydrogenated silicon carbide Ža-Si 1y x C x :H²Er:. to increase the band gap energy limit. For the first time it is shown that a-Si 1y x C x :H²Er: exhibits room-temperature PL at 1.54 m m, which is assigned to the internal 4f-shell transition in Er ions.
2. Experiment We used the magnetron assisted silane decomposition technique Ž M A SD . for obtaining a-Si 1y x C x :H²Er: films. The reactor system used for MASD was described in detail in Ref. w6x. The magnetron target used was made of C and Er of 99.999% purity. The SirC ratio was varied by changing the SiH 4 percentage in the gas mixture of Ar q SiH 4 . The substrate temperature was 3008C. The use of a novel technology ŽMASD. permits insertion into the semiconductor matrix of Er ion concentrations two orders of magnitude larger than those in the case of a crystalline matrix. The composition of the films Ž x . and the presence of Er in the films were monitored by Rutherford back scattering ŽRBS., using RBS of 4 He 2q ions. The energy of 4 He 2q was 3.7 MeV, the angle of backscattering 1708 and the detector resolution energy 50 keV. Typical RBS spectra for a film with x s 0.29 and for crystalline silicon carbide c-6H SiC are shown in Fig. 1. The crystalline carbide, c-6H SiC, was used as a calibration sample. It has been shown that the
489
Fig. 1. Rutherford backscattering ŽRBS. spectra for 4 He 2q ions with energy Es 3.7 MeV backscattered at Si, C and Er as a function of the energy Ž channel number . for: Ž 1 . a-Si 0.71C 0.29 :H²Er:; Ž2. c-6H SiC Žcrystalline silicon carbide. Concentration of incorporated Er ions 6=10 19 cmy3 .
SirC ratio was 1.022 for c-6H SiC w7x. The content of C Ž x . was in the range 0 F x F 0.29. The concentration of incorporated Er ions was ; 6 = 10 19 cmy3 . The content of hydrogen was estimated at ; 10 at.% from the integrated IR absorption due to the Si–H wagging band Ž630 cmy1 . and C–H stretching band Ž2900 cmy1 .. The optical band gap Ž Egopt ., determined by photothermal deflection spectroscopy ŽPDS. as the energy where the absorption coefficient is 10 3 cmy1 , varied from 1.59 Ž x s 0. to 1.82 eV Ž x s 0.29.. The PL spectra were recorded with excitation by a krypton laser with l s 649.6 nm and an argon laser with l s 514.5 nm in the temperature range 77–400 K. The PL spectra were measured by means of a germanium detector cooled to the temperature of liquid nitrogen and a double monochromator ŽSPEX1403. with a resolution of 2 nm.
3. Results Fig. 2 shows the PL spectra of Er doped amorphous hydrogenated silicon carbide a-Si 1y x C x :H²Er: and amorphous hydrogenated silicon a-Si:H²Er: taken at 77 K. The PL spectra of a-Si 1y x C x :H²Er: and a-Si:H²Er: measured at 77 K exhibit emission from both the Er 3q center Ž0.81 eV. and the amorphous Si and Si–C matrix. The latter is a band with features at ; 1 eV and at ; 1.3 eV.
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E.I. TerukoÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 488–492
Fig. 2. PL spectra of Er doped: Ž1. a-Si:H²Er:; Ž2. aSi 0.71C 0.29 :H²Er: Žconcentration of incorporated Er ions 6=10 19 cmy3 . taken at 77 K excited with 514.5 nm Ž2.41 eV. at P s 50 mW.
Usually, the PL data Žintrinsic band. on a-Si 1y x C x :H alloys for x - 0.4 have been discussed in terms of the models put forward for a-Si:H. In undoped a-Si:H with a defect concentration Ždangling bond Si. ND²10 16 cmy3 a single broad band is observed at ; 1.3 eV with a typical width of 0.3 eV. This band is attributed to a transition between localized band tail states, radiative recombination occurring by tunneling. All reports on a-Si 1y x C x :H alloys
show only one PL spectrum whose width increases and whose maximum shifts to longer energy when x is increased w5x. In undoped a-Si:H samples we observed the intrinsic band Ž1.2 eV.. However, our investigation has shown that incorporation of erbium ions into the amorphous matrix is accompanied by the formation of dangling bond defects with concentration, ND ; 10 18 cmy3 . Usually such samples show a well resolved defect PL at 0.94 eV Žrecombination at Si dangling bond.. The defect PL in a-Si:H arises from the radiative capture of electrons by neutral dangling bonds, D 0 q e ™ Dy. The intrinsic PL Ž1.2 eV. involving holes in the valence band tail remains small ŽFig. 2, curve 1.. In the case of a-Si 1y x C x :H²Er: alloys Ž x - 0.4. the optical band gap Egopt increases with x, increasing from 1.59 " 0.02 eV Ž x s 0. to 1.82 " 0.02 eV Ž x s 0.29.. The increase of Egopt is accompanied by an increase in film disorder, which makes increases the density of localized states in the band tails and in the defect band due to dangling bonds. The total density of defects Ždangling bonds., ND , estimated from PDS data ranges from 3.5 = 10 18 cmy3 Ž x s 0. to 4.9 = 10 18 cmy3 Ž x s 0.29.. The luminescence bands of the alloy shift with increasing gap to larger energy, which enables better
Fig. 3. PL spectra of Er doped a-Si 0.78 C 0.22 :H²Er: at the different temperatures: 1–77 K, 2–300 K Žconcentration of incorporated Er ions 6 = 10 19 cmy3 . excited with 514.5 nm Ž2.41 eV. at P s 50 mW.
E.I. TerukoÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 488–492
491
Fig. 4. Temperature dependence of the peak intensity of the defect and Er 3q luminescence of a-Si 0.71C 0.29 :H²Er: Žconcentration of incorporated Er ions 6 = 10 19 cmy3 . excited with 514.5 nm Ž2.41 eV. at P s 50 mW.
separation from the Er PL at 1.53 m m. The defect luminescence band is observed at ; 1 eV and the intrinsic band at ; 1.3 eV ŽFig. 2, curve 2.. At 300 K the intrinsic emission from amorphous silicon carbide matrix Ž1.3 eV. is suppressed by temperature quenching and the remaining signal is due to Er 3q ions and defect PL ŽFig. 3, curve 2.. It should be noted that the various luminescence bands have differing temperature dependences. The intrinsic luminescence band decreases above 100 K. As in
the case of a-Si:H, the dangling bonds act as nonradiative centers, quenching the PL Žintrinsic band. by tunneling to the defect. The temperature quenching of luminescence above 100 K is explained by an increased mobility of carriers which diffuse to the recombination centers w5x. Temperature dependences of intensity of Er 3q PL and defect PL are displayed in Fig. 4. It can be seen that the temperature dependence of the defect luminescence is similar to that of the Er 3q emission.
Fig. 5. Comparison of the temperature dependence of Er 3q luminescence of a-Si 0.71C 0.29 :H²Er: Ž NEr s 6 = 10 19 cmy3 . Ž1. and a-Si:H²Er: Ž NEr s 1 = 10 20 cmy3 , N0 s 2 = 10 20 cmy3 . Ž2. excited with 514.5 nm at P s 50 mW. Data are normalized to the PL yield of this component at 77 K.
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Fig. 5 shows the temperature dependence of the intensity of Er 3q emission in a-Si 1y x C x :H²Er: and in a-Si:H²Er:. We used the sample of a-Si:H²Er: in which the maximum intensity of PL is observed for comparison. As is seen in Fig. 5 the onset of temperature quenching of Er 3q emission in the case of a-Si 1y x C x :H²Er: is observed at higher temperatures than for a-Si:H²Er:.
4. Discussion The similar temperature dependences of defect and Er 3q luminescence in a-Si 1y x C x :H²Er: indicate that dangling bond defects play an important role in the excitation of Er ions. We assume that a-Si 1y x C x :H Ž x - 0.4. alloys are built by Si–Si bonds. Carbon contributes little to the formation of the structure and is incorporated into the Si structure as CH 3 and CH 2 groups w8x. Besides, carbon does induce dangling bonds, but does not change their properties. Consequently, the neutral and occupied Žby electron. dangling bond states, D8 and Dy, only adjust their position to a widening gap w9x. It is reasonable to assume that the mechanism of electronic excitation of Er ions in a-Si 1y x C x :H²Er: can be discussed in terms of the model previously proposed for a-Si:H²Er: w10x. In this model the mechanism of electronic excitation of Er ions is based on defect-related Auger excitation ŽDRAE.. This process involves the capture of mobile carries by dangling bond defects Ž D . in the amorphous matrix, connected with an Auger excitation of the Er 3q. In view of the densities of incorporated Er ions Ž10 20 cmy3 . and dangling bonds defects Ž10 18 cmy3 . we assume that a large concentration of closely spaced Er–defect ŽEr–D . pairs exists. When an electron is captured in a neutral defect Ž D8 q e ™ Dy. in such a close pair, part of the energy can be transferred to f-electrons of Er ions by Coulomb interaction. As a result, Er ion is excited from the ground state 4 I 15r2 to the excited state 4 I 13r2 , and this energy is transferred into the local vibration modes of the defect. In Ref. w10x the probabilities of the DRAE and the radiative recombination at defects Ž e q D8 ™ Dy. were calculated. It is shown that the probability of the DRAE process is greater by an order of magnitude than that of radiative defect
recombination. A correlation between the intensities of defect and erbium luminescence supports this mechanism. It should be noted that these are the first results and further studies are planned to obtain more specific information on this mechanism.
5. Conclusions Room temperature PL of Er ions at 1.54 m m corresponding to the 4 I 13r2 y4 I 15r2 transition in the ion f-shell was observed. We conclude that the excitation mechanism of the Er ions in amorphous hydrogenated silicon carbide doped is by the defect related Auger excitation ŽDRAE. model previously proposed for a-Si:H²Er:.
Acknowledgements This work was partly supported by the Volkswagen-Stiftung grant No. 1r71 646 and Russian Foundation for Basic Research through grant 96-0216931-a.
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