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Influence of growth temperature on the microcrystallinity and native defect structure of hydrogenated amorphous silicon
Journal of Non-Crystalline Solids 299–302 (2002) 103–107 www.elsevier.com/locate/jnoncrysol
Influence of growth temperature on the microcrystallinity and native defect structure of hydrogenated amorphous silicon M. H€ arting a,*, D.T. Britton a, R. Bucher a, E. Minani a, A. Hempel a, M. Hempel b, T.P. Ntsoane a,b, C. Arendse c, D. Knoesen c a
Department of Physics, University of Cape Town, Rondebosch 7701, South Africa Materials Research Group, National Accelerator Centre, Faure 7131, South Africa Department of Physics, University of the Western Cape, Bellville 7530, South Africa
b c
Abstract The microstructure of hydrogenated amorphous silicon grown by hot-wire chemical vapour deposition (HW-CVD) on glass substrates, at different substrate temperatures ranging from 300 to 500 °C, has been studied using X-ray diffraction and positron annihilation techniques. In previous studies it has been shown that recrystallization is accompanied by a relaxation of the defect structure with an increase in the free volume at the positron annihilation site. The object of this work is to relate the initial defect configuration to the degree of order in the structure, which has been characterized through its radial density function giving accurate estimates of the nearest-neighbour separation and bond angles. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.43.Dq; 61.10.Nz; 78.70.Bj; 61.72.Ji; 61.72.Qg
1. Introduction Hydrogenated amorphous silicon (a-Si:H) exhibits short-range order with a high proportion of equal nearest-neighbour separations and almost equal bond angles classified by a continuous random network [1]. The general feature of a-Si, produced by any deposition processes, is the presence of point defects and larger defects in the form of voids. This was already presumed in 1967 by Moss and Graczyk [2] for evaporated a-Si.
*
Corresponding author. Tel.: +27-21 650 3359; fax: +27-21 650 3342. E-mail address: [email protected] (M. H€arting).
A variety of techniques [1], such as Raman scattering, EXAFS, Si NMR, and diffraction using X-rays, electrons and neutrons, has been widely applied to investigate the atomic scale structure of a-Si. For the diffraction, the scattering intensity I as a function of scattering angle 2h, using monochromatic radiation is recorded. The scattering intensity I is related to the scattering vector k, where k ¼ ð4p=kÞ sin h. The transform of IðkÞ from (momentum) k-space to real-space is achieved by Fourier transformation, leading to a radial distribution function (RDF) [3], which contains information on preferred neighbour distances and mean bond angles, as well as the co-ordination number of the first-neighbour peak [4].
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 5 - 1
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M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107
The presumed presence of open-volume defects, mentioned above [2], was confirmed in more recent work using positron annihilation techniques [5–7]. Electron-momentum spectroscopy, in the form of Doppler-broadening, measures the momentum imparted to the annihilation quanta by the participating electron via an energy shift due to the centre-of-mass motion of the annihilating pair. The annihilation lineshape is characterized in terms of the fraction of annihilations with different electron distributions. Firstly, the low-momentum (valence) electron fraction (S parameter), depends mainly on the open-volume size and concentration of defects. Secondly, the high-momentum (core) electron fraction (W parameter) is more sensitive than S to the electronic structure at the annihilation site. For a specific spectrometer, the pair of values, S and W, present a unique signature of the defect. For example in sputtered [5] and PE-CVDgrown [6] a-Si:H, a higher microvoid concentration is found than in hot-wire chemical vapour deposition (HW-CVD) material [7], where the positrons predominantly annihilate at danglingbond defects [8]. High-resolution RDF investigation [4,9] related the structural relaxation of ion-implanted a-Si films on thermal annealing, to the removal of point self-defects. In other studies [7] it has been shown that crystallization is accompanied by a relaxation of the defect structure with an increase in free volume at positron annihilation sites. The object of this work is a combined study of X-ray diffraction and positron annihilation to investigate the microstructural differences in the initial state of a-Si:H grown by HW-CVD on glass at different substrate temperatures.
2. Samples and experiment All the samples studied (Table 1) were produced by HW-CVD on barium borosilicate glass (Corning 7059) substrates, in an atmosphere of pure silane. Except for the change in substrate temperature from 300 to 500 °C in steps of 50 °C, the deposition conditions were kept constant. The temperature of the tantalum filament was always in the region of 1600 °C. The substrates were cleaned by washing in acetone and methanol. The thickness of the layers was subsequently determined by profilometry, and the total hydrogen content by FTIR spectroscopy. The X-ray diffraction studies were carried out using a Bruker AXS D8 Advance diffractometer at the regional X-ray facility, Faure, South Africa. The measurements were performed in Bragg– Brentano geometry using a secondary monochromator for Cu Ka radiation. The diffraction pattern was recorded over a 2h range from 20° to 70°, with a step size of 0.04° and minimum measuring time of 100 s per step. A polystyrene sample holder was used to eliminate any spurious crystalline peaks from the spectra. The contributions from the substrate and sample holder were subtracted by measurements on the reverse side of the sample. After stripping the substrate pattern from the data, the patterns were smoothed. The RDF was then calculated by the numerical sine transform of the scattering factor as a function of scattering vector [3]. For the positron annihilation studies, electronmomentum spectroscopy was performed using the continuous positron beam at the University of Cape Town [10]. The positron beam energy was
Table 1 Characterization of the a-Si:H layers grown by HW-CVD of glass substrates at different temperatures ) T (°C) d ðlmÞ H (at.%) R (A 300 350 400 450 500
1.98 2.383 2.288 3.367 1.241
10.2 5.37 0.77 0.5 10.53
2.36 0.01 2.33 0.01 2.33 0.01 2.31 0.01 2.3 0.01
a (°) 111:0 0:9 111:4 0:9 113:2 1:0 114:7 1:0 119:3 1:1
T: thickness d determined by profilometry; total hydrogen concentration H determined by FTIR; bond length R and angle a determined from the RDF (Fig. 1).
M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107
105
continuously variable, up to a maximum of 18 keV, at which energy the positrons sample a broad distribution around a depth of 1:8 0:9 lm [7]. This was sufficient in all cases to allow a separation of the a-Si:H and substrate contributions.
3. Results The radial densities of the five samples are shown in Fig. 1. The first peak, corresponding to the nearest-neighbour separation, is poorly resolved, only appearing as a shoulder in some cases because of the limited range of scattering vectors sampled. For Cu Ka radiation the maximum scattering angle of 1 70° corresponds to a scattering . This also leads to a limit on vector of only 5 A [4], the spatial resolution of approximately 0.6 A although the peak position, which is also determined by counting statistics, can be determined more accurately. Several trends are apparent in Fig. 1. Firstly, with the exception of the layer grown at 350 °C, the peaks in the RDF become more pronounced with increasing growth temperature. There is also a general decrease in the nearest-neighbour separation, and an increase in the second-nearestneighbour separation, with growth temperature. These are both indications of a widening of the average bond angle and a shortening of the bond length. Quantitative estimates for these parameters, taken from the peak positions in Fig. 1, are
Fig. 2. Low-electron-momentum fraction S for positrons annihilating in the a-Si:H layers as a function of growth temperature. (Inset: correlation between low-momentum fraction S and high-momentum fraction W.)
listed in Table 1. For different deposition temperatures between 300 and 500 °C, the mean bond length decreases by 2.5% from 2:36 0:2 to 2:30 , and the angle increases from 111 0:9° to 0:2 A 119:3 1:1°. Depth profiling of the positron annihilation characteristics shows all the layers, with the exception of the layer grown at 350 °C, to be uniform. The parameters shown in Fig. 2 for the layers are averages over a constant depth range, which corresponded to annihilation in the layer. For the 350 °C sample this region covered only the top 0:3 lm. Fig. 2 shows the dependence of the low-electron momentum fraction S on deposition temperature. There is a large increase in S, and therefore free-volume defects, in the layer, for a 50 °C increase in growth temperature. This is followed by a slow monotonic decrease with increasing growth temperature.
4. Discussion
Fig. 1. RDF for a-Si:H layers grown at five different temperatures by HW-CVD.
The reduction of defect free volume for higher growth temperatures indicated by Fig. 2 is consistent with the increased ordering seen by X-ray diffraction, as well as the reduction in bond length shown in Fig. 3. This variation is approximately linear with deposition temperature, indicating a
106
M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107
Fig. 3. Dependence of the nearest-neighbour separation R on deposition temperature in HW-CVD grown a-Si:H. (Inset: correlation with positron annihilation lineshape parameter S.)
the exception of the layer grown at 300 °C, all the points cluster in the same region of the graph. This suggests that, for high-temperature growth, there are no qualitative differences in the defect structure, only a relaxation of its size. On the other hand, for low-temperature growth the defect structure is fundamentally different. A possible explanation is given by the hydrogen content (Table 1), which decreases rapidly with growth temperature between 300 and 400 °C. It is likely that material grown at low temperature, contains larger microvoids which are decorated with hydrogen. The effect of this is to mask the defect signal by reducing the effective free volume seen by the positron [11].
5. Conclusions continual relaxation of the network, and is tracked at higher growth temperatures by the positron annihilation parameter (inset). In this range the positrons almost exclusively sample vacancy-like dangling-bond defects [8]. The reduction in the low-momentum fraction with deposition temperature is indicative of a reduction in the average size of this defect, allowing more annihilation with higher momentum electrons, resulting from the increased order and shortening of the bond length. It could therefore be supposed that the very low value of S for the layer grown at 300 °C can be attributed to an even higher degree of ordering and smaller, or fewer, defects. This, however, is not seen in either the RDF (Fig. 1) or the dependence of the nearest-neighbour separation (Fig. 3). At the lowest deposition temperature, the layer has the least order and the longest average bond length. Furthermore, from Fig. 3, the bond length appears to agree with the natural sequence of the others grown at higher temperatures. We are therefore left with the conclusion that the reduced low-electron-momentum fraction for 300 °C growth results from an increased defect size or concentration. For this to be the case, there must be a change in the nature of the dominant defect, and not just its size, when the growth temperature is increased above 300–350 °C. This supposition is born out by the combined behaviour of the two positron annihilation fractions (inset Fig. 2). With
We have used a combined study of X-ray diffraction and positron annihilation to investigate the microstructural differences in the initial state of a-Si:H grown by HW-CVD on glass substrates at different substrate temperatures, under otherwise identical conditions. Both techniques indicate a relaxation of the network with increasing growth temperature, leading to a higher degree of ordering, shorter bond lengths, and a reduction in the average size of the defects. For low-temperature (300 °C) growth, the layers are the most disordered, and highly defective. The dominant defects are probably microvoids, decorated with hydrogen, in contrast to the smaller dangling-bond defects seen in high-temperature grown sample.
References [1] A. Madan, M.P. Shaw, The Physics and Application of Amorphous Semiconductors, Academic Press, London, 1988. [2] S.C. Moss, J.F. Graczyk, Phys. Rev. Lett. 23 (1969) 116. [3] R.W. James, in: Optical Principles of the Diffraction of Xrays, Bell, London, 1948, p. 477 (Chapter 9). [4] K. Laaziri, S. Kycia, S. Roorda, M. Chicione, J.L. Robertson, J. Wang, S.C. Moss, Phys. Rev. B 60 (1999) 1352. [5] R. Haakvoort, A. van Veen, H. Schut, M.J. van den Boogaard, A.J.M. Berntsen, S. Roorda, P.A. Stolk, A.H. Reader, AIP Conf. Proc. 303 (1994) 48.
M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107 [6] X. Zou, Y.C. Chan, D.P. Webb, Y.W. Lam, Y.F. Hu, C.D. Beling, S. Fung, H.M. Weng, Phys. Rev. Lett. 84 (2000) 769. [7] D.T. Britton, A. Hempel, M. H€arting, G. K€ ogel, P. Sperr, W. Triftsh€ auser, C. Arendse, D. Knoesen, Phys. Rev. B 64 (2001) 075403. [8] D.T. Britton, M. H€ arting, A. Hempel, G. K€ ogel, P. Sperr, W. Triftsh€ auser, M. Hempel, D. Knoesen, these Proceedings, p. 249.
107
[9] L. Laazari, S. Kycia, S. Roorda, M. Chicoine, J.L. Robertson, J. Wang, S.C. Moss, Phys. Rev. Lett. 82 (1999) 3460. [10] D.T. Britton, M. H€arting, M.B.R. Teemane, S. Mills, F.M. Nortier, T.N. van der Walt, Appl. Surf. Sci. 116 (1997) 53. [11] P. Hautoj€arvi, C. Corbel, in: A. Dupasquier, A.P. Mills jr. (Eds.), Positrons Spectroscopy of Solids, IOS, Amsterdam, 1995, p. 491.
Influence of growth temperature on the microcrystallinity and native defect structure of hydrogenated amorphous silicon M. H€ arting a,*, D.T. Britton a, R. Bucher a, E. Minani a, A. Hempel a, M. Hempel b, T.P. Ntsoane a,b, C. Arendse c, D. Knoesen c a
Department of Physics, University of Cape Town, Rondebosch 7701, South Africa Materials Research Group, National Accelerator Centre, Faure 7131, South Africa Department of Physics, University of the Western Cape, Bellville 7530, South Africa
b c
Abstract The microstructure of hydrogenated amorphous silicon grown by hot-wire chemical vapour deposition (HW-CVD) on glass substrates, at different substrate temperatures ranging from 300 to 500 °C, has been studied using X-ray diffraction and positron annihilation techniques. In previous studies it has been shown that recrystallization is accompanied by a relaxation of the defect structure with an increase in the free volume at the positron annihilation site. The object of this work is to relate the initial defect configuration to the degree of order in the structure, which has been characterized through its radial density function giving accurate estimates of the nearest-neighbour separation and bond angles. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 61.43.Dq; 61.10.Nz; 78.70.Bj; 61.72.Ji; 61.72.Qg
1. Introduction Hydrogenated amorphous silicon (a-Si:H) exhibits short-range order with a high proportion of equal nearest-neighbour separations and almost equal bond angles classified by a continuous random network [1]. The general feature of a-Si, produced by any deposition processes, is the presence of point defects and larger defects in the form of voids. This was already presumed in 1967 by Moss and Graczyk [2] for evaporated a-Si.
*
Corresponding author. Tel.: +27-21 650 3359; fax: +27-21 650 3342. E-mail address: [email protected] (M. H€arting).
A variety of techniques [1], such as Raman scattering, EXAFS, Si NMR, and diffraction using X-rays, electrons and neutrons, has been widely applied to investigate the atomic scale structure of a-Si. For the diffraction, the scattering intensity I as a function of scattering angle 2h, using monochromatic radiation is recorded. The scattering intensity I is related to the scattering vector k, where k ¼ ð4p=kÞ sin h. The transform of IðkÞ from (momentum) k-space to real-space is achieved by Fourier transformation, leading to a radial distribution function (RDF) [3], which contains information on preferred neighbour distances and mean bond angles, as well as the co-ordination number of the first-neighbour peak [4].
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 8 5 - 1
104
M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107
The presumed presence of open-volume defects, mentioned above [2], was confirmed in more recent work using positron annihilation techniques [5–7]. Electron-momentum spectroscopy, in the form of Doppler-broadening, measures the momentum imparted to the annihilation quanta by the participating electron via an energy shift due to the centre-of-mass motion of the annihilating pair. The annihilation lineshape is characterized in terms of the fraction of annihilations with different electron distributions. Firstly, the low-momentum (valence) electron fraction (S parameter), depends mainly on the open-volume size and concentration of defects. Secondly, the high-momentum (core) electron fraction (W parameter) is more sensitive than S to the electronic structure at the annihilation site. For a specific spectrometer, the pair of values, S and W, present a unique signature of the defect. For example in sputtered [5] and PE-CVDgrown [6] a-Si:H, a higher microvoid concentration is found than in hot-wire chemical vapour deposition (HW-CVD) material [7], where the positrons predominantly annihilate at danglingbond defects [8]. High-resolution RDF investigation [4,9] related the structural relaxation of ion-implanted a-Si films on thermal annealing, to the removal of point self-defects. In other studies [7] it has been shown that crystallization is accompanied by a relaxation of the defect structure with an increase in free volume at positron annihilation sites. The object of this work is a combined study of X-ray diffraction and positron annihilation to investigate the microstructural differences in the initial state of a-Si:H grown by HW-CVD on glass at different substrate temperatures.
2. Samples and experiment All the samples studied (Table 1) were produced by HW-CVD on barium borosilicate glass (Corning 7059) substrates, in an atmosphere of pure silane. Except for the change in substrate temperature from 300 to 500 °C in steps of 50 °C, the deposition conditions were kept constant. The temperature of the tantalum filament was always in the region of 1600 °C. The substrates were cleaned by washing in acetone and methanol. The thickness of the layers was subsequently determined by profilometry, and the total hydrogen content by FTIR spectroscopy. The X-ray diffraction studies were carried out using a Bruker AXS D8 Advance diffractometer at the regional X-ray facility, Faure, South Africa. The measurements were performed in Bragg– Brentano geometry using a secondary monochromator for Cu Ka radiation. The diffraction pattern was recorded over a 2h range from 20° to 70°, with a step size of 0.04° and minimum measuring time of 100 s per step. A polystyrene sample holder was used to eliminate any spurious crystalline peaks from the spectra. The contributions from the substrate and sample holder were subtracted by measurements on the reverse side of the sample. After stripping the substrate pattern from the data, the patterns were smoothed. The RDF was then calculated by the numerical sine transform of the scattering factor as a function of scattering vector [3]. For the positron annihilation studies, electronmomentum spectroscopy was performed using the continuous positron beam at the University of Cape Town [10]. The positron beam energy was
Table 1 Characterization of the a-Si:H layers grown by HW-CVD of glass substrates at different temperatures ) T (°C) d ðlmÞ H (at.%) R (A 300 350 400 450 500
1.98 2.383 2.288 3.367 1.241
10.2 5.37 0.77 0.5 10.53
2.36 0.01 2.33 0.01 2.33 0.01 2.31 0.01 2.3 0.01
a (°) 111:0 0:9 111:4 0:9 113:2 1:0 114:7 1:0 119:3 1:1
T: thickness d determined by profilometry; total hydrogen concentration H determined by FTIR; bond length R and angle a determined from the RDF (Fig. 1).
M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107
105
continuously variable, up to a maximum of 18 keV, at which energy the positrons sample a broad distribution around a depth of 1:8 0:9 lm [7]. This was sufficient in all cases to allow a separation of the a-Si:H and substrate contributions.
3. Results The radial densities of the five samples are shown in Fig. 1. The first peak, corresponding to the nearest-neighbour separation, is poorly resolved, only appearing as a shoulder in some cases because of the limited range of scattering vectors sampled. For Cu Ka radiation the maximum scattering angle of 1 70° corresponds to a scattering . This also leads to a limit on vector of only 5 A [4], the spatial resolution of approximately 0.6 A although the peak position, which is also determined by counting statistics, can be determined more accurately. Several trends are apparent in Fig. 1. Firstly, with the exception of the layer grown at 350 °C, the peaks in the RDF become more pronounced with increasing growth temperature. There is also a general decrease in the nearest-neighbour separation, and an increase in the second-nearestneighbour separation, with growth temperature. These are both indications of a widening of the average bond angle and a shortening of the bond length. Quantitative estimates for these parameters, taken from the peak positions in Fig. 1, are
Fig. 2. Low-electron-momentum fraction S for positrons annihilating in the a-Si:H layers as a function of growth temperature. (Inset: correlation between low-momentum fraction S and high-momentum fraction W.)
listed in Table 1. For different deposition temperatures between 300 and 500 °C, the mean bond length decreases by 2.5% from 2:36 0:2 to 2:30 , and the angle increases from 111 0:9° to 0:2 A 119:3 1:1°. Depth profiling of the positron annihilation characteristics shows all the layers, with the exception of the layer grown at 350 °C, to be uniform. The parameters shown in Fig. 2 for the layers are averages over a constant depth range, which corresponded to annihilation in the layer. For the 350 °C sample this region covered only the top 0:3 lm. Fig. 2 shows the dependence of the low-electron momentum fraction S on deposition temperature. There is a large increase in S, and therefore free-volume defects, in the layer, for a 50 °C increase in growth temperature. This is followed by a slow monotonic decrease with increasing growth temperature.
4. Discussion
Fig. 1. RDF for a-Si:H layers grown at five different temperatures by HW-CVD.
The reduction of defect free volume for higher growth temperatures indicated by Fig. 2 is consistent with the increased ordering seen by X-ray diffraction, as well as the reduction in bond length shown in Fig. 3. This variation is approximately linear with deposition temperature, indicating a
106
M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107
Fig. 3. Dependence of the nearest-neighbour separation R on deposition temperature in HW-CVD grown a-Si:H. (Inset: correlation with positron annihilation lineshape parameter S.)
the exception of the layer grown at 300 °C, all the points cluster in the same region of the graph. This suggests that, for high-temperature growth, there are no qualitative differences in the defect structure, only a relaxation of its size. On the other hand, for low-temperature growth the defect structure is fundamentally different. A possible explanation is given by the hydrogen content (Table 1), which decreases rapidly with growth temperature between 300 and 400 °C. It is likely that material grown at low temperature, contains larger microvoids which are decorated with hydrogen. The effect of this is to mask the defect signal by reducing the effective free volume seen by the positron [11].
5. Conclusions continual relaxation of the network, and is tracked at higher growth temperatures by the positron annihilation parameter (inset). In this range the positrons almost exclusively sample vacancy-like dangling-bond defects [8]. The reduction in the low-momentum fraction with deposition temperature is indicative of a reduction in the average size of this defect, allowing more annihilation with higher momentum electrons, resulting from the increased order and shortening of the bond length. It could therefore be supposed that the very low value of S for the layer grown at 300 °C can be attributed to an even higher degree of ordering and smaller, or fewer, defects. This, however, is not seen in either the RDF (Fig. 1) or the dependence of the nearest-neighbour separation (Fig. 3). At the lowest deposition temperature, the layer has the least order and the longest average bond length. Furthermore, from Fig. 3, the bond length appears to agree with the natural sequence of the others grown at higher temperatures. We are therefore left with the conclusion that the reduced low-electron-momentum fraction for 300 °C growth results from an increased defect size or concentration. For this to be the case, there must be a change in the nature of the dominant defect, and not just its size, when the growth temperature is increased above 300–350 °C. This supposition is born out by the combined behaviour of the two positron annihilation fractions (inset Fig. 2). With
We have used a combined study of X-ray diffraction and positron annihilation to investigate the microstructural differences in the initial state of a-Si:H grown by HW-CVD on glass substrates at different substrate temperatures, under otherwise identical conditions. Both techniques indicate a relaxation of the network with increasing growth temperature, leading to a higher degree of ordering, shorter bond lengths, and a reduction in the average size of the defects. For low-temperature (300 °C) growth, the layers are the most disordered, and highly defective. The dominant defects are probably microvoids, decorated with hydrogen, in contrast to the smaller dangling-bond defects seen in high-temperature grown sample.
References [1] A. Madan, M.P. Shaw, The Physics and Application of Amorphous Semiconductors, Academic Press, London, 1988. [2] S.C. Moss, J.F. Graczyk, Phys. Rev. Lett. 23 (1969) 116. [3] R.W. James, in: Optical Principles of the Diffraction of Xrays, Bell, London, 1948, p. 477 (Chapter 9). [4] K. Laaziri, S. Kycia, S. Roorda, M. Chicione, J.L. Robertson, J. Wang, S.C. Moss, Phys. Rev. B 60 (1999) 1352. [5] R. Haakvoort, A. van Veen, H. Schut, M.J. van den Boogaard, A.J.M. Berntsen, S. Roorda, P.A. Stolk, A.H. Reader, AIP Conf. Proc. 303 (1994) 48.
M. H€arting et al. / Journal of Non-Crystalline Solids 299–302 (2002) 103–107 [6] X. Zou, Y.C. Chan, D.P. Webb, Y.W. Lam, Y.F. Hu, C.D. Beling, S. Fung, H.M. Weng, Phys. Rev. Lett. 84 (2000) 769. [7] D.T. Britton, A. Hempel, M. H€arting, G. K€ ogel, P. Sperr, W. Triftsh€ auser, C. Arendse, D. Knoesen, Phys. Rev. B 64 (2001) 075403. [8] D.T. Britton, M. H€ arting, A. Hempel, G. K€ ogel, P. Sperr, W. Triftsh€ auser, M. Hempel, D. Knoesen, these Proceedings, p. 249.
107
[9] L. Laazari, S. Kycia, S. Roorda, M. Chicoine, J.L. Robertson, J. Wang, S.C. Moss, Phys. Rev. Lett. 82 (1999) 3460. [10] D.T. Britton, M. H€arting, M.B.R. Teemane, S. Mills, F.M. Nortier, T.N. van der Walt, Appl. Surf. Sci. 116 (1997) 53. [11] P. Hautoj€arvi, C. Corbel, in: A. Dupasquier, A.P. Mills jr. (Eds.), Positrons Spectroscopy of Solids, IOS, Amsterdam, 1995, p. 491.