- Email: [email protected]
Selective dominance of photoluminescence lifetime in a-Si:H-based alloys
Journal of Non-Crystalline Solids 266±269 (2000) 578±582
www.elsevier.com/locate/jnoncrysol
Selective dominance of photoluminescence lifetime in a-Si:H-based alloys Hidetoshi Oheda * Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan
Abstract At 13 K, a distribution of photoluminescence (PL) lifetimes, G(s), for a-Si:H and wide-gap alloys containing C, N or O was dominated by the lifetime component peaked at around 1 ms, whereas, in a-SiGe:H with Ge > 20 at.%, G(s) was dominated by a lifetime component peaked at about 10 ls, which became evident in other alloys at elevated temperatures. The G(s) for alloys with reduced Ge < 10 at.% has been found to contain the 1-ms component even at 13 K, although the 10 ls component was still dominant. I suggest that those two speci®c photoluminescence lifetime components, which are observed commonly in G(s) for the a-Si:H-based alloys, come from a localized luminescent center associated with some structural unit which can have two con®gurations. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Owing to structural disorder, amorphous semiconductors show a featureless spectrum or a dispersive transient response in many properties. In contrast with those properties, a spectrum of a photoluminescence (PL) lifetime distribution, G(s), has been found to present distinct structures, although PL spectrum itself is very broad and featureless: two lifetime components peaked at 1 ms and 10 ls were observed in G(s) for a-Si:H and some of its alloys [1±3]. A ratio of magnitudes between the long-lifetime (LL) and short-lifetime (SL) components changes depending on experimental conditions, such as temperature, a combination between excitation and emission energies,
*
Tel.: +81-298 61 5420; fax: +81-298 61 5085. E-mail address: [email protected] (H. Oheda).
and on alloying elements [2]. Fig. 1 shows representative G(s) at 13 K for a-Si:H of device quality and several a-Si:H-based alloys, results of which were reported previously [2,3]. In wide-gap alloys containing C, N or O, which have the same optical gap of 2.2 eV, the LL component was predominant at 13 K as in undoped a-Si:H, whereas, in an alloy containing Ge > 20 at.%, the SL component was predominant even at 13 K. Such a short lifetime component could be observed in G(s) for aSi:H and the wide-gap alloys at temperatures above 50 K [2,3]. These results have posed a question of why G(s) diers depending on the alloys, although the LL and SL components seem to be observed commonly in G(s) irrespective of the materials. In this work, G(s) for alloys with reduced Ge content was examined, focusing especially on behavior of the LL component in a-SiGe:H. Based on results combined with previous results evaluated for many specimens having dierent properties,
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 0 5 2 - 1
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
579
and its intensity was held low enough to satisfy the condition of geminate-pair recombination [6].
3. Results G(s) at 13 K for the specimens J1 and J2 as well as previously obtained results for other specimens A and B [3], which contain Ge > 20 at.% and have optical gaps of 1.67 and 1.5 eV, respectively, are summarized in Fig. 2. G(s) for the specimens A and B were peaked at 5 10ÿ5 s. They seem to be dominated by the same lifetime component as the SL component which became obvious in G(s)
Fig. 1. Representative G(s) at 13 K for a-Si:H of device quality and its alloys [2,3]. Wide-gap alloys containing C, N or O had the same optical gap of 2.2 eV, whereas optical gap of an alloy containing Ge (specimen B) was 1.5 eV.
a PL mechanism at low temperatures is reconsidered.
2. Experimental Two a-SiGe:H specimens containing very slight Ge were prepared. They were labeled J1 and J2. Their Ge contents were estimated to be 5 2 and 10 at.%, and their optical gaps were 1.77 and 1.73 eV, respectively. G(s) for monochromatized PL at the wavelength where PL spectrum exhibited a maximum was evaluated by frequency-resolved spectroscopy [4,5]. An excitation was tuned slightly above the band gap energy of a specimen,
Fig. 2. G(s) at 13 K for a-Si:H and several Ge-containing alloys with dierent Ge contents. Optical gap of each alloy is indicated beside the respective sample name. G(s) for the specimen J1 is tried to simulate with two Gaussian lines centered at 1 ms and 52 ls, respectively. They are represented with broken lines and a sum of them with a solid line.
580
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
specimens A and B, whose G(s) was considered to be dominated mainly by the SL component, took place more rapidly. In the Ge-containing alloys, a way of temperature shift of the peak lifetime differed depending on the Ge content. When the Ge content was reduced as in J1 and J2, the peak lifetime was between those of the LL and SL components in a temperature range below 50 K, and merged with the same temperature dependence as that for the alloys containing large amounts of Ge above around 70 K. At ®rst sight, it was surprising that the alloys containing low amounts of Ge exhibited such a substantial peak shift, which was quite dierent from the shift observed in other cases. 4. Discussion
Fig. 3. Temperature shifts of the peak lifetimes observed in G(s) for all specimens presented in Figs. 1 and 2.
for a-Si:H and the wide-gap alloys at temperatures above 50 K [2,3], whereas, the peak lifetime in G(s) for the specimens J1 and J2 shifted towards a longer lifetime of 3 10ÿ4 s. Their G(s) became much broader than those for a-Si:H, the wide-gap alloys and even the specimens A and B, when the Ge content was reduced less than 10 at.%. However, although results are not shown here, a width of G(s) for the specimens J1 and J2 became narrower as temperature was increased as well as shifting peak lifetimes towards shorter times. Fig. 3 summarizes temperature shifts of the peak lifetimes observed in G(s) for all specimens presented in Figs. 1 and 2. In the wide-gap alloys, only the LL component was dominant at low temperatures as in undoped a-Si:H and its thermal stability was increased successively with increasing content of each alloying element [3], whereas, in the alloys with Ge, there was observed no temperature range where the LL component was predominant. Furthermore, although peak shift of the LL component with temperature was small, temperature shifts of the peak lifetime of the
Dierence in temperature shifts of the peak lifetimes depending on the LL or SL component should aect the shape of G(s), especially when both components have nearly the same magnitude as each other. In a-Si:H, a relative magnitude of the SL component became comparable with that of the LL component at around 50 K, where both peak lifetimes were separated enough to observe a double-peak structure in G(s) [2]. We have tried to simulate G(s) at 13 K for the specimens J1 and J2 with two Gaussian lines. By adjusting the peak position and width of one Gaussian line to obtain the same values as those for the specimens A and B, the main part of G(s) can be deconvoluted into two Gaussian lines which are centered at 1 ms and 52 ls, respectively, and have almost the same magnitudes as each other. An example for the case of J1 is demonstrated in Fig. 2: decomposed lifetime components are represented with broken lines and a sum of them with a solid line. We conclude that a broad, single-peak structure in G(s) at 13 K for the specimens J1 and J2 is a result of too close a mutual location of the two lifetime components and, furthermore, the substantially enhanced peak shifts as observed in Fig. 3 results from change in a ratio of magnitudes between the two lifetime components with temperature, which are observed more clearly in a-Si:H and the wide-gap alloys [2,3]. When a very small amount of Ge is alloyed
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
into a silicon network, the SL component appears even at 13 K, although the LL component is mixed to some extent. Now we reconsider the PL mechanism in these materials. Presence of the two speci®c lifetime components in G(s) for a-Si:H and its alloys having broad tail states distributions as well as the observation that a peak lifetime of the LL component depends very weakly on an emission energy [2] do not reconcile with the earlier proposal that the PL was due to tunneling recombination between carriers trapped at tail states after thermalization with the states [6]. Recently, it has been argued that there is a possibility of a triplet excitonic state as a cause for the long PL lifetime of 1 ms [7]. In this case, the SL component must correspond to a singlet state because of a close correlation between the two lifetime components, as inferred from the observation that in a-Si:H a change in relative magnitudes of the two lifetime components took place in a temperature range where integrated PL intensity remained almost constant [2]. Following this spin-dependent recombination model, a peak lifetime in G(s) is expected to shift continuously with temperature between two extreme lifetimes, since thermal activation of carriers from the triplet to singlet states limits recombination through the singlet state [8,9]. However, the peak shift took place discontinuously from 1 ms to 10 ls as temperature was increased [2,3]. As a consequence, the triplet-state model is not consistent with the data, either. Alternatively, I consider that those PL-lifetime components arise from a localized PL center associated with some structural unit which already exists in undoped a-Si:H. I suggest that the two speci®c lifetimes are due to a PL center with two structural con®gurations. Depending on the alloying elements, the structural environment around the PL center will be modi®ed so that it takes either of the allowed con®gurations. Consulting with systematic studies on the microscopic bonding con®guration of the alloys [10±18], it has been suggested that it is better to understand such a structural preference of the PL center in connection with a dierence in the nature of network structures depending on the alloys. The most crucial factors are dierences in electronegativity and
581
covalent-bonding radius between each alloying element and Si. C, N or O atoms have largely dierent electronegativities as compared with Si atom, so that each alloying element in the widegap alloys prefers bonding with Si. However, Si±X bonds (X¸C, N or O) have much shorter bond lengths than those of the Si±Si bonds [10]. As a result, a network structure of the wide-gap alloys has distorted bond angles which can be experimentally detected as increase of width of the silicon TO-like Raman band [11±16]. Such a distortion in the network structure causes the PL center to have a rather strongly localized structural con®guration with a PL lifetime of about 1 ms. The degree of localization of the PL center may be increased with increasing C, N or O content. However, when the structural distortion in the network is relaxed at elevated temperatures, the PL site will be able to take the other less localized con®guration. Ge atom has similar electronegativity and covalent-bonding radius as those of a Si atom [10]. Although high coordination of Ge seems to overconstrain a network structure, the width of the silicon TO-like Raman band was observed not to change with incorporation of Ge when the alloy was prepared under optimal conditions [17,18], indicating that the bond angle distortion in a network of the Ge-containing alloy is less than that of the wide-gap alloys. Furthermore, the Ge incorporation will reduce structural distortion still existing in the a-Si:H network, since angular constraints in the rigid Si-network is considered to be relaxed with the increasing Si±Ge or Ge±Ge bonds due to the more isotropic nature of Ge bonding orbitals or angular ¯exibility of Ge [18,19]. Then the PL center in a-SiGe:H is expected to have less localized structural con®guration with a PL lifetime of about 10 ls. 5. Summary PL-lifetime distributions were measured for the alloys containing Ge less than 10 at.%. In contrast with a-Si:H and the wide-gap alloys containing C, N or O, the 10-ls component was predominant even at 13 K, although the 1-ms component having
582
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
nearly the same magnitude co-existed. It has been found that the two lifetime components characterized each with speci®c peak lifetimes are commonly present in G(s) for all a-Si:H-based alloys studied here. This ®nding strongly suggests that PL in those materials at low temperatures comes from a localized PL center associated with some structural unit. I have considered at present that the selective dominance either of the two lifetime components in the a-Si:H-based alloys is closely related to the network structures of the alloys: structural con®guration of the PL site is aected by some dierence in the nature of the surrounding network structure, especially structural distortion.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Acknowledgements
[14]
The author thanks greatly Dr R. Carius for providing him with the alloys containing very slight Ge ( 6 10 at.%).
[15] [16] [17]
References [1] F. Boulitrop, D.J. Dunstan, J. Non-Cryst. Solids 77&78 (1985) 663.
[18] [19]
H. Oheda, Phys. Rev. B 52 (1995) 16530. H. Oheda, J. Non-Cryst. Solids 198±200 (1996) 284. P. Depinna, D.J. Dunstan, Philos. Mag. B 50 (1984) 579. R. Stachowitz, M. Schubert, W. Fuhs, Philos. Mag. B 70 (1994) 1219. R.A. Street, Adv. Phys. 30 (1981) 593. K. Morigaki, M. Yoshida, J. Non-Cryst. Solids 90 (1987) 139. L.H. Robins, M.A. Kastner, Philos. Mag. B 50 (1984) 29. M. Cazzanelli, C. Vinegoni, L. Pavesi, J. Appl. Phys. 85 (1999) 1760. S. Mobilio, A. Filipponi, J. Non-Cryst. Solids 97&98 (1987) 365. A. Morimoto, S. Oozora, M. Kumeda, T. Shimizu, Solid State Commun. 47 (1983) 773. E. Bustarret, E. Morgado, Solid State Commun. 63 (1987) 581. D.M. Bhusari, A.S. Kumbhar, S.T. Kshirsagar, J. Appl. Phys. 77 (1995) 54. B.G. Budaguan, D.A. Stryahilev, A.A. Aivazov, J. NonCryst. Solids 210 (1997) 267. F. Giorgis, C.F. Pirri, E. Tresso, V. Rigato, S. Zandolin, P. Rava, Physica B 229 (1997) 233. A.R. Zanatta, M. Mulato, I. Chambouleyron, J. Appl. Phys. 84 (1998) 5184. M.B. Schubert, H.D. Mohring, E. Lotter, G.H. Bauer, IEEE Trans. Electron Dev. 36 (1989) 2863. M.B. Schubert, K. Eberhardt, G.H. Bauer, Mater. Res. Symp. Proc. 192 (1990) 121. G.H. Bauer, C.E. Nebel, M.B. Schubert, G. Schumm, Mater. Res. Soc. Symp. Proc. 149 (1989) 485.
www.elsevier.com/locate/jnoncrysol
Selective dominance of photoluminescence lifetime in a-Si:H-based alloys Hidetoshi Oheda * Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan
Abstract At 13 K, a distribution of photoluminescence (PL) lifetimes, G(s), for a-Si:H and wide-gap alloys containing C, N or O was dominated by the lifetime component peaked at around 1 ms, whereas, in a-SiGe:H with Ge > 20 at.%, G(s) was dominated by a lifetime component peaked at about 10 ls, which became evident in other alloys at elevated temperatures. The G(s) for alloys with reduced Ge < 10 at.% has been found to contain the 1-ms component even at 13 K, although the 10 ls component was still dominant. I suggest that those two speci®c photoluminescence lifetime components, which are observed commonly in G(s) for the a-Si:H-based alloys, come from a localized luminescent center associated with some structural unit which can have two con®gurations. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Owing to structural disorder, amorphous semiconductors show a featureless spectrum or a dispersive transient response in many properties. In contrast with those properties, a spectrum of a photoluminescence (PL) lifetime distribution, G(s), has been found to present distinct structures, although PL spectrum itself is very broad and featureless: two lifetime components peaked at 1 ms and 10 ls were observed in G(s) for a-Si:H and some of its alloys [1±3]. A ratio of magnitudes between the long-lifetime (LL) and short-lifetime (SL) components changes depending on experimental conditions, such as temperature, a combination between excitation and emission energies,
*
Tel.: +81-298 61 5420; fax: +81-298 61 5085. E-mail address: [email protected] (H. Oheda).
and on alloying elements [2]. Fig. 1 shows representative G(s) at 13 K for a-Si:H of device quality and several a-Si:H-based alloys, results of which were reported previously [2,3]. In wide-gap alloys containing C, N or O, which have the same optical gap of 2.2 eV, the LL component was predominant at 13 K as in undoped a-Si:H, whereas, in an alloy containing Ge > 20 at.%, the SL component was predominant even at 13 K. Such a short lifetime component could be observed in G(s) for aSi:H and the wide-gap alloys at temperatures above 50 K [2,3]. These results have posed a question of why G(s) diers depending on the alloys, although the LL and SL components seem to be observed commonly in G(s) irrespective of the materials. In this work, G(s) for alloys with reduced Ge content was examined, focusing especially on behavior of the LL component in a-SiGe:H. Based on results combined with previous results evaluated for many specimens having dierent properties,
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 0 5 2 - 1
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
579
and its intensity was held low enough to satisfy the condition of geminate-pair recombination [6].
3. Results G(s) at 13 K for the specimens J1 and J2 as well as previously obtained results for other specimens A and B [3], which contain Ge > 20 at.% and have optical gaps of 1.67 and 1.5 eV, respectively, are summarized in Fig. 2. G(s) for the specimens A and B were peaked at 5 10ÿ5 s. They seem to be dominated by the same lifetime component as the SL component which became obvious in G(s)
Fig. 1. Representative G(s) at 13 K for a-Si:H of device quality and its alloys [2,3]. Wide-gap alloys containing C, N or O had the same optical gap of 2.2 eV, whereas optical gap of an alloy containing Ge (specimen B) was 1.5 eV.
a PL mechanism at low temperatures is reconsidered.
2. Experimental Two a-SiGe:H specimens containing very slight Ge were prepared. They were labeled J1 and J2. Their Ge contents were estimated to be 5 2 and 10 at.%, and their optical gaps were 1.77 and 1.73 eV, respectively. G(s) for monochromatized PL at the wavelength where PL spectrum exhibited a maximum was evaluated by frequency-resolved spectroscopy [4,5]. An excitation was tuned slightly above the band gap energy of a specimen,
Fig. 2. G(s) at 13 K for a-Si:H and several Ge-containing alloys with dierent Ge contents. Optical gap of each alloy is indicated beside the respective sample name. G(s) for the specimen J1 is tried to simulate with two Gaussian lines centered at 1 ms and 52 ls, respectively. They are represented with broken lines and a sum of them with a solid line.
580
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
specimens A and B, whose G(s) was considered to be dominated mainly by the SL component, took place more rapidly. In the Ge-containing alloys, a way of temperature shift of the peak lifetime differed depending on the Ge content. When the Ge content was reduced as in J1 and J2, the peak lifetime was between those of the LL and SL components in a temperature range below 50 K, and merged with the same temperature dependence as that for the alloys containing large amounts of Ge above around 70 K. At ®rst sight, it was surprising that the alloys containing low amounts of Ge exhibited such a substantial peak shift, which was quite dierent from the shift observed in other cases. 4. Discussion
Fig. 3. Temperature shifts of the peak lifetimes observed in G(s) for all specimens presented in Figs. 1 and 2.
for a-Si:H and the wide-gap alloys at temperatures above 50 K [2,3], whereas, the peak lifetime in G(s) for the specimens J1 and J2 shifted towards a longer lifetime of 3 10ÿ4 s. Their G(s) became much broader than those for a-Si:H, the wide-gap alloys and even the specimens A and B, when the Ge content was reduced less than 10 at.%. However, although results are not shown here, a width of G(s) for the specimens J1 and J2 became narrower as temperature was increased as well as shifting peak lifetimes towards shorter times. Fig. 3 summarizes temperature shifts of the peak lifetimes observed in G(s) for all specimens presented in Figs. 1 and 2. In the wide-gap alloys, only the LL component was dominant at low temperatures as in undoped a-Si:H and its thermal stability was increased successively with increasing content of each alloying element [3], whereas, in the alloys with Ge, there was observed no temperature range where the LL component was predominant. Furthermore, although peak shift of the LL component with temperature was small, temperature shifts of the peak lifetime of the
Dierence in temperature shifts of the peak lifetimes depending on the LL or SL component should aect the shape of G(s), especially when both components have nearly the same magnitude as each other. In a-Si:H, a relative magnitude of the SL component became comparable with that of the LL component at around 50 K, where both peak lifetimes were separated enough to observe a double-peak structure in G(s) [2]. We have tried to simulate G(s) at 13 K for the specimens J1 and J2 with two Gaussian lines. By adjusting the peak position and width of one Gaussian line to obtain the same values as those for the specimens A and B, the main part of G(s) can be deconvoluted into two Gaussian lines which are centered at 1 ms and 52 ls, respectively, and have almost the same magnitudes as each other. An example for the case of J1 is demonstrated in Fig. 2: decomposed lifetime components are represented with broken lines and a sum of them with a solid line. We conclude that a broad, single-peak structure in G(s) at 13 K for the specimens J1 and J2 is a result of too close a mutual location of the two lifetime components and, furthermore, the substantially enhanced peak shifts as observed in Fig. 3 results from change in a ratio of magnitudes between the two lifetime components with temperature, which are observed more clearly in a-Si:H and the wide-gap alloys [2,3]. When a very small amount of Ge is alloyed
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
into a silicon network, the SL component appears even at 13 K, although the LL component is mixed to some extent. Now we reconsider the PL mechanism in these materials. Presence of the two speci®c lifetime components in G(s) for a-Si:H and its alloys having broad tail states distributions as well as the observation that a peak lifetime of the LL component depends very weakly on an emission energy [2] do not reconcile with the earlier proposal that the PL was due to tunneling recombination between carriers trapped at tail states after thermalization with the states [6]. Recently, it has been argued that there is a possibility of a triplet excitonic state as a cause for the long PL lifetime of 1 ms [7]. In this case, the SL component must correspond to a singlet state because of a close correlation between the two lifetime components, as inferred from the observation that in a-Si:H a change in relative magnitudes of the two lifetime components took place in a temperature range where integrated PL intensity remained almost constant [2]. Following this spin-dependent recombination model, a peak lifetime in G(s) is expected to shift continuously with temperature between two extreme lifetimes, since thermal activation of carriers from the triplet to singlet states limits recombination through the singlet state [8,9]. However, the peak shift took place discontinuously from 1 ms to 10 ls as temperature was increased [2,3]. As a consequence, the triplet-state model is not consistent with the data, either. Alternatively, I consider that those PL-lifetime components arise from a localized PL center associated with some structural unit which already exists in undoped a-Si:H. I suggest that the two speci®c lifetimes are due to a PL center with two structural con®gurations. Depending on the alloying elements, the structural environment around the PL center will be modi®ed so that it takes either of the allowed con®gurations. Consulting with systematic studies on the microscopic bonding con®guration of the alloys [10±18], it has been suggested that it is better to understand such a structural preference of the PL center in connection with a dierence in the nature of network structures depending on the alloys. The most crucial factors are dierences in electronegativity and
581
covalent-bonding radius between each alloying element and Si. C, N or O atoms have largely dierent electronegativities as compared with Si atom, so that each alloying element in the widegap alloys prefers bonding with Si. However, Si±X bonds (X¸C, N or O) have much shorter bond lengths than those of the Si±Si bonds [10]. As a result, a network structure of the wide-gap alloys has distorted bond angles which can be experimentally detected as increase of width of the silicon TO-like Raman band [11±16]. Such a distortion in the network structure causes the PL center to have a rather strongly localized structural con®guration with a PL lifetime of about 1 ms. The degree of localization of the PL center may be increased with increasing C, N or O content. However, when the structural distortion in the network is relaxed at elevated temperatures, the PL site will be able to take the other less localized con®guration. Ge atom has similar electronegativity and covalent-bonding radius as those of a Si atom [10]. Although high coordination of Ge seems to overconstrain a network structure, the width of the silicon TO-like Raman band was observed not to change with incorporation of Ge when the alloy was prepared under optimal conditions [17,18], indicating that the bond angle distortion in a network of the Ge-containing alloy is less than that of the wide-gap alloys. Furthermore, the Ge incorporation will reduce structural distortion still existing in the a-Si:H network, since angular constraints in the rigid Si-network is considered to be relaxed with the increasing Si±Ge or Ge±Ge bonds due to the more isotropic nature of Ge bonding orbitals or angular ¯exibility of Ge [18,19]. Then the PL center in a-SiGe:H is expected to have less localized structural con®guration with a PL lifetime of about 10 ls. 5. Summary PL-lifetime distributions were measured for the alloys containing Ge less than 10 at.%. In contrast with a-Si:H and the wide-gap alloys containing C, N or O, the 10-ls component was predominant even at 13 K, although the 1-ms component having
582
H. Oheda / Journal of Non-Crystalline Solids 266±269 (2000) 578±582
nearly the same magnitude co-existed. It has been found that the two lifetime components characterized each with speci®c peak lifetimes are commonly present in G(s) for all a-Si:H-based alloys studied here. This ®nding strongly suggests that PL in those materials at low temperatures comes from a localized PL center associated with some structural unit. I have considered at present that the selective dominance either of the two lifetime components in the a-Si:H-based alloys is closely related to the network structures of the alloys: structural con®guration of the PL site is aected by some dierence in the nature of the surrounding network structure, especially structural distortion.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Acknowledgements
[14]
The author thanks greatly Dr R. Carius for providing him with the alloys containing very slight Ge ( 6 10 at.%).
[15] [16] [17]
References [1] F. Boulitrop, D.J. Dunstan, J. Non-Cryst. Solids 77&78 (1985) 663.
[18] [19]
H. Oheda, Phys. Rev. B 52 (1995) 16530. H. Oheda, J. Non-Cryst. Solids 198±200 (1996) 284. P. Depinna, D.J. Dunstan, Philos. Mag. B 50 (1984) 579. R. Stachowitz, M. Schubert, W. Fuhs, Philos. Mag. B 70 (1994) 1219. R.A. Street, Adv. Phys. 30 (1981) 593. K. Morigaki, M. Yoshida, J. Non-Cryst. Solids 90 (1987) 139. L.H. Robins, M.A. Kastner, Philos. Mag. B 50 (1984) 29. M. Cazzanelli, C. Vinegoni, L. Pavesi, J. Appl. Phys. 85 (1999) 1760. S. Mobilio, A. Filipponi, J. Non-Cryst. Solids 97&98 (1987) 365. A. Morimoto, S. Oozora, M. Kumeda, T. Shimizu, Solid State Commun. 47 (1983) 773. E. Bustarret, E. Morgado, Solid State Commun. 63 (1987) 581. D.M. Bhusari, A.S. Kumbhar, S.T. Kshirsagar, J. Appl. Phys. 77 (1995) 54. B.G. Budaguan, D.A. Stryahilev, A.A. Aivazov, J. NonCryst. Solids 210 (1997) 267. F. Giorgis, C.F. Pirri, E. Tresso, V. Rigato, S. Zandolin, P. Rava, Physica B 229 (1997) 233. A.R. Zanatta, M. Mulato, I. Chambouleyron, J. Appl. Phys. 84 (1998) 5184. M.B. Schubert, H.D. Mohring, E. Lotter, G.H. Bauer, IEEE Trans. Electron Dev. 36 (1989) 2863. M.B. Schubert, K. Eberhardt, G.H. Bauer, Mater. Res. Symp. Proc. 192 (1990) 121. G.H. Bauer, C.E. Nebel, M.B. Schubert, G. Schumm, Mater. Res. Soc. Symp. Proc. 149 (1989) 485.