- Email: [email protected]
Experimental determination of positronic and electronic characteristics of 3C-SiC
surface s c i e n c e ELSEVIER
Applied Surface Science 116 (1997) 19-22
Experimental determination of positronic and electronic characteristics of 3C-SiC G. Brauer a,*, W. Anwand a E.-M. Nicht a P.G. Coleman b N. Wagner c H. Wirth d, W. Skorupa d A rbeitsgruppe Positronen-Annihilations-Spektroskopie der Technischen Uniuersit~t Dresden, c / o Forschungszentrum Rossendorf PosOCach510119, D-01314 Dresden, Germany b School of Physics, Universi~."of East Anglia, Norwich NR4 7TJ, UK " Fachbereich Physik, Martin-Luther-Universiti~t Halle-Wittenberg, Friedemann-Bach-Platz 6, D-06108 Halle/Saale, Germany d lm'titut./~r lonenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf PosOrach 510119, D-01314 Dresden, Germany Received 2 June 1996; accepted 15 July 1996
Abstract
The positron diffusion length L+ and electron and positron work functions (4,_ and ~+) of the same sample of 3C-SiC epitaxially grown on a Si(001) substrate have been experimentally determined, together with re-emitted slow positron yields. The results allow conclusions on the quality and thickness of the 3C-SiC epitaxial layer as well as an assessment of the accuracy of earlier first-principles calculations of positron-related materials properties. PACS: 61.70.By; 71.60.+ z; 79.90.+ b Kevwords: Silicon carbide; Work function; Diffusion length
1. Introduction
There is much current interest in silicon carbide (SIC), which exists in many different crystallographic structures (polytypes), as a new semiconductor for device technology. In order to improve the performance of m o d e m planar devices, and wafer die yields, it is necessary to characterize thoroughly the starting material, and to establish a microscopic understanding of defects [1,2].
Corresponding author. Tel.: +49-351-2602117; fax: +49351-2602595; e-mail: [email protected].
Recently it has been demonstrated that by positron annihilation spectroscopy in combination with electronic structure calculations it is possible to monitor and to characterize unambiguously the vacancy-type damage caused by ion implantation in 6H-SiC [3,4]. Furthermore, observation of copious positron reemission from crystalline 6H-SiC, with no pre-treatment and without the need for ultra-high vacuum conditions, suggests that this material may form the basis of an important new moderator for the production of monoenergetic positrons [5]. In the present paper the results of measurements of some basic positron-related characteristics of 3CSiC are presented and discussed.
0169-4332/07/$17.00 Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Pll S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 9 6 7 - 1
20
G. Brauer et al. /Applied Surface Science 116 (1997) 19-22
2. Sample preparation A 3C-SiC specimen of about 5 / z m thickness was prepared by chemical vapor deposition on a Si(001) substrate. X-ray diffraction has shown that the crystallographic orientations of the cubic lattices of Si and the single crystalline SiC are the same. However, the coexistence of polycrystalline (cubic) SiC was indicated at a low but unquantifiable level. The dominant single-crystalline SiC is characterized by a mosaic structure having a FWHM of 1.6°. The variation of the lattice constant was found not to exceed 0.7%.
3. Electron work function measurement The electronic properties of the 3C-SiC specimen were determined by use of the contact potential (CP) method and photoacoustic spectroscopy [6], which have already been successfully applied to TiC [7] and 6H-SiC [4]. The CP between the tungsten cathode ( + 4.50 eV [7]) and the SiC anode ( - 0.67 + 0.02 eV) gives the electron affinity of SiC, Xsic = + 3.83 + 0.02 eV.
In determining the energy gap, Eg, photoacoustic spectroscopy (PAS) has an advantage over optical spectroscopy in that the indirect band transition Flsv-Xlc can be observed without any error caused by light scattering. The phase sensitive lock-in amplifier signal was monitored with a chopper frequency of 135 Hz which was chosen according to the specimen thickness in order to give the best signal-to-noise ratio. The PAS data (thermoelastic mode) presented in Fig. 1 lead to the result Eg = + 2.35 + 0.06 eV. The value of Eg depends on the polytype; in the case of 6H-SiC values of Eg between 2.9 eV and 3.08 eV have been reported [8,9], whereas for 4H-SiC a value of 3.2 eV has been quoted [8]. The value found here for 3C-SiC shows that a very homogeneous specimen has been produced and is in excellent agreement with the literature [10,11] where values of 2.34 and 2.37 eV have been given for this polytype. Taking the experimental results for Xsic and Eg together, the electron work function ~b is given by ~b_= +6.18 +__0.08 eV. This value is 0.33 eV lower than that estimated for 6H-SiC recently [4].
4. Positron work function measurement 1,0
0,8 -q
•~ 0,6
'< 0,4
0,2 i
I
i
i
t
i
1,0
1,5
2,0
2,5
3,0
3,5
PhotonEnergy(eV) Fig. 1. Photoacoustic signal amplitude (PZT-transducer) of the 3C-SiC interface at a chopper frequency of 135 Hz.
The yield of slow positrons re-emitted from the specimen was measured using the computer-controlled magnetic-transport positron beam at UEA Norwich [12]. A dc beam 4 mm in diameter and of controllable energy is transported to the sample, to which a bias potential ~ is applied. Annihilation gamma rays from the sample are detected by a 72% efficiency Ge detector which views the sample through a 10 mm wide slit in a lead block, so that only annihilation events at the sample surface are observed [13]. Backscattered positrons are deflected by E × B plates so that they cannot be reflected back on to the sample. Integral spectra of the type shown in Fig. 2 for the 3C-SiC specimen and for 6H-SiC were accumulated by recording the Ge detector pulse count rate (511 keV photopeak only) as was ramped from - 5 to + 5 V. It is clear from Fig. 2 that the data for 3C-SiC are very similar to those for 6H-SiC. Extraction of the positron work function ~b+ from such data is, however, not straightforward. There appear to be two
G. Brauer et al. /Applied Surface Science 116 (1997) 19-22 7 6'
~
w
5
W
o oo
0 1
-5
I
I
L
~
I
I
I
-4
-3
-2
-1
0
1
2
3
SAMPLE BIAS (V) Fig. 2. Integral count rate profiles for 3C-SiC (full circles) and 6H-SiC (open circles) (incident positron energy E = 1.4 keV).
sections to the signal distribution; a relatively narrow spectrum extends to 2.0 _+ 0.2 eV, and a high-energy tail, comprising about 10% of the total signal intensity, extending to 4.2 _+ 0.2 eV. As the shape of the integral spectrum appears to remain constant for higher incident positron energies it is reasonable to attribute the latter value to the work function ~b+, but one must always bear in mind the possibility of higher-energy positron emission due to the band gap of the material. The present work on 3C-SiC and earlier work on 6H-SiC [4] shows that the local density approximation, widely used in electronic structure calculations, usually yields large errors for electron and positron energy levels for systems having an energy gap [14,15]. To overcome this problem in the future, an improved approach to obtain reasonable gap and positron level values from first principles has to be used. This requires the inclusion of self-interaction correction terms as well modification of the electron-positron correlation potential; such work is already underway and the results will be published elsewhere [16].
portional to the non-backscattered incident flux. The count rate for V, = + 5 V is proportional to the incident flux minus those positrons which are reemitted at low (eV) energies. In both cases the count rates include a contribution from short-lived parapositronium (p-Ps) which self-annihilates just above the surface (and hence in sight of the Ge detector). The V, = - 2 5 V measurement includes any epithermai positrons emitted at incident energies below 1 keV. If one assumes that the branching ratio for Ps formation is the same for both incident and returned slow positrons then these extra 'events' will increase both asymptotic count rates by the same fraction, and one can therefore (to first order) ignore their effect. The re-emitted positron fraction is therefore computed directly from the two count rates and is plotted in Fig. 3 as a tunction of incident positron energy. Also plotted is the result for ion-implanted (i.e.. highly defected) 6H-SiC for comparison. The solid line in Fig. 3 is a fit which assumes a Gaussian derivative implantation profile and diffusion characterized by the length L + = 20 + 3 rim. Although this length is a factor of three shorter than that measured previously for bulk 6H-SiC, workfunction emission of thermalized positrons still occurs, albeit at relatively low incident energies, as can be seen from the evidence of Fig. 2 and the comparison with the energy dependence of epithermal positron emission from defected SiC in Fig. 3. The short L+ for 3C-SiC may reflect the structure of the polytype; it is, however, probable that diffusion is hampered by defects in the deposited 3C-SiC layer. 0.6 0.5
0.4
-
8 o.aL >" 0.2
•
0.0 1
5. Positron diffusion length measurement Following the method described in Section 4 the asymptotic count rate for VS = - 2 5 V was also measured and, after background subtraction, is pro-
21
t& 2
3
4
5
INCIDENT POSITRON ENERGY (keV) Fig. 3. Re-emitted positron fractions as a function of incident positron energy E: Full circles - 3C-SiC. Triangles - ion-implanted 6H-SiC. Solid line - implantation/diffusion model fit to 3C-SiC data, yielding a diffusion length of 20-t-3 nm.
22
G. Brauer et al./ Applied Surface Science 116 (1997) 19-22
6. Conclusions The positron diffusion length was extracted from measurements of the Doppler broadened annihilation lineshapes from variable energy positrons in 3C-SiC and compared to the diffusion length for 6H-SiC. The new measurements also allowed the quality of the 3C-SiC epilayer on Si(001) substrate to be characterized. An experimental estimation of the electron work function of 3C-SiC, combined with independent positron work function measurements on the same specimen, allows the evaluation of the positron affinity for this sample. Although its comparison with the theoretical value shows less than satisfactory agreement, this result forms another part of the needed experimental basis for further improvements in the theoretical calculations.
Acknowledgements The authors are grateful to the Daimler-Benz AG (Forschung und Technik, Frankfurt/M) for supplying us with the 3C-SiC material used in this work.
References [1] G. Pensl and R. Helbig, in: Festk~Srperprobleme/Advances in Solid State Physics, Ed. U. R~Sssler, Vol. 30 (Vieweg, Braunsehweig, 1990)p. 133.
[2] G. Pensl and W. Choyke, Physica B 185 (1993) 264. [3] G. Brauer, W. Anwand, P.G. Coleman, A.P. Knights, F. Plazaola, Y. Pacaud, W. Skorupa, J. St~Srmerand P. Willutzki, Phys. Rev. B 54 (1996) 3084. [4] G. Brauer, W, Anwand, E.-M. Nicht, J. Kuriplach, M. Sob, N. Wagner, P.G. Coleman, M.J. Puska and T. Korhonen, Phys. Rev. B 54 (1996) 2512. [5] J. Sttirmer, A. Goodyear, W. Anwand, G. Brauer, P.G. Coleman and W. Triftsh~iuser, J. Phys. Condens. Matter 8 (1996) L89. [6] N. Wagner, in: Proc. Autumn School Mikrosonde (University of Halle, 1990) p. 57. [7] G. Brauer, W. Anwand, E.-M. Nicht, P.G. Coleman, A.P. Knights, H. Schut, G. KiSgel and N. Wagner, J. Phys.: Condens. Matter 7 (1995) 9091. [8] J.A. Powell and L.G. Matus, in: Springer Proc. in Physics, Eds. G.L. Harris and C.Y.-W. Yang, Vol, 34 (Springer, Berlin, 1989) p. 2. [9] H. Matsunami, K. Shibakara, N. Kuroda, W. Yoo and S. Niskina, in: Springer Proc. in Physics, Eds. G.L. Harris and C.Y.-W. Yang, Vol. 34 (Springer, Berlin, 1989) p. 34. [10] W.J. Choyke, Mater. Res. Bull. 4 (1969) 141. [11] N.M. Ravindra and V.K. Srivastava, Phys. Chem. Solids 40 (1979) 791. [12] S.M. Hutchins, M.A. Alam, P.G. Coleman and R.N. West, in: Positron Annihilation, Eds. R.M. Singru, P.C. Jain and K.P. Gopinathan (World Scientific, Singapore, 1985) p. 983. [13] P.G. Coleman, L. Albrecht, A.B. Walker and K.O. Jensen, J. Phys.: Condens. Matter 4 (1992) 10311. [14] J.P. Perdew and A. Zunger, Phys. Rev. B 23 (1981) 5048. [15] M.J. Puska and R.M. Nieminen, Phys. Rev. B 46 (1992) 1278. [16] J. Kuriplach, M. Sob, M.J. Puska, G. Brauer, W. Anwand, E.-M. Nicht, P.G. Coleman and N. Wagner, submitted (1996).
Applied Surface Science 116 (1997) 19-22
Experimental determination of positronic and electronic characteristics of 3C-SiC G. Brauer a,*, W. Anwand a E.-M. Nicht a P.G. Coleman b N. Wagner c H. Wirth d, W. Skorupa d A rbeitsgruppe Positronen-Annihilations-Spektroskopie der Technischen Uniuersit~t Dresden, c / o Forschungszentrum Rossendorf PosOCach510119, D-01314 Dresden, Germany b School of Physics, Universi~."of East Anglia, Norwich NR4 7TJ, UK " Fachbereich Physik, Martin-Luther-Universiti~t Halle-Wittenberg, Friedemann-Bach-Platz 6, D-06108 Halle/Saale, Germany d lm'titut./~r lonenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf PosOrach 510119, D-01314 Dresden, Germany Received 2 June 1996; accepted 15 July 1996
Abstract
The positron diffusion length L+ and electron and positron work functions (4,_ and ~+) of the same sample of 3C-SiC epitaxially grown on a Si(001) substrate have been experimentally determined, together with re-emitted slow positron yields. The results allow conclusions on the quality and thickness of the 3C-SiC epitaxial layer as well as an assessment of the accuracy of earlier first-principles calculations of positron-related materials properties. PACS: 61.70.By; 71.60.+ z; 79.90.+ b Kevwords: Silicon carbide; Work function; Diffusion length
1. Introduction
There is much current interest in silicon carbide (SIC), which exists in many different crystallographic structures (polytypes), as a new semiconductor for device technology. In order to improve the performance of m o d e m planar devices, and wafer die yields, it is necessary to characterize thoroughly the starting material, and to establish a microscopic understanding of defects [1,2].
Corresponding author. Tel.: +49-351-2602117; fax: +49351-2602595; e-mail: [email protected].
Recently it has been demonstrated that by positron annihilation spectroscopy in combination with electronic structure calculations it is possible to monitor and to characterize unambiguously the vacancy-type damage caused by ion implantation in 6H-SiC [3,4]. Furthermore, observation of copious positron reemission from crystalline 6H-SiC, with no pre-treatment and without the need for ultra-high vacuum conditions, suggests that this material may form the basis of an important new moderator for the production of monoenergetic positrons [5]. In the present paper the results of measurements of some basic positron-related characteristics of 3CSiC are presented and discussed.
0169-4332/07/$17.00 Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Pll S 0 1 6 9 - 4 3 3 2 ( 9 6 ) 0 0 9 6 7 - 1
20
G. Brauer et al. /Applied Surface Science 116 (1997) 19-22
2. Sample preparation A 3C-SiC specimen of about 5 / z m thickness was prepared by chemical vapor deposition on a Si(001) substrate. X-ray diffraction has shown that the crystallographic orientations of the cubic lattices of Si and the single crystalline SiC are the same. However, the coexistence of polycrystalline (cubic) SiC was indicated at a low but unquantifiable level. The dominant single-crystalline SiC is characterized by a mosaic structure having a FWHM of 1.6°. The variation of the lattice constant was found not to exceed 0.7%.
3. Electron work function measurement The electronic properties of the 3C-SiC specimen were determined by use of the contact potential (CP) method and photoacoustic spectroscopy [6], which have already been successfully applied to TiC [7] and 6H-SiC [4]. The CP between the tungsten cathode ( + 4.50 eV [7]) and the SiC anode ( - 0.67 + 0.02 eV) gives the electron affinity of SiC, Xsic = + 3.83 + 0.02 eV.
In determining the energy gap, Eg, photoacoustic spectroscopy (PAS) has an advantage over optical spectroscopy in that the indirect band transition Flsv-Xlc can be observed without any error caused by light scattering. The phase sensitive lock-in amplifier signal was monitored with a chopper frequency of 135 Hz which was chosen according to the specimen thickness in order to give the best signal-to-noise ratio. The PAS data (thermoelastic mode) presented in Fig. 1 lead to the result Eg = + 2.35 + 0.06 eV. The value of Eg depends on the polytype; in the case of 6H-SiC values of Eg between 2.9 eV and 3.08 eV have been reported [8,9], whereas for 4H-SiC a value of 3.2 eV has been quoted [8]. The value found here for 3C-SiC shows that a very homogeneous specimen has been produced and is in excellent agreement with the literature [10,11] where values of 2.34 and 2.37 eV have been given for this polytype. Taking the experimental results for Xsic and Eg together, the electron work function ~b is given by ~b_= +6.18 +__0.08 eV. This value is 0.33 eV lower than that estimated for 6H-SiC recently [4].
4. Positron work function measurement 1,0
0,8 -q
•~ 0,6
'< 0,4
0,2 i
I
i
i
t
i
1,0
1,5
2,0
2,5
3,0
3,5
PhotonEnergy(eV) Fig. 1. Photoacoustic signal amplitude (PZT-transducer) of the 3C-SiC interface at a chopper frequency of 135 Hz.
The yield of slow positrons re-emitted from the specimen was measured using the computer-controlled magnetic-transport positron beam at UEA Norwich [12]. A dc beam 4 mm in diameter and of controllable energy is transported to the sample, to which a bias potential ~ is applied. Annihilation gamma rays from the sample are detected by a 72% efficiency Ge detector which views the sample through a 10 mm wide slit in a lead block, so that only annihilation events at the sample surface are observed [13]. Backscattered positrons are deflected by E × B plates so that they cannot be reflected back on to the sample. Integral spectra of the type shown in Fig. 2 for the 3C-SiC specimen and for 6H-SiC were accumulated by recording the Ge detector pulse count rate (511 keV photopeak only) as was ramped from - 5 to + 5 V. It is clear from Fig. 2 that the data for 3C-SiC are very similar to those for 6H-SiC. Extraction of the positron work function ~b+ from such data is, however, not straightforward. There appear to be two
G. Brauer et al. /Applied Surface Science 116 (1997) 19-22 7 6'
~
w
5
W
o oo
0 1
-5
I
I
L
~
I
I
I
-4
-3
-2
-1
0
1
2
3
SAMPLE BIAS (V) Fig. 2. Integral count rate profiles for 3C-SiC (full circles) and 6H-SiC (open circles) (incident positron energy E = 1.4 keV).
sections to the signal distribution; a relatively narrow spectrum extends to 2.0 _+ 0.2 eV, and a high-energy tail, comprising about 10% of the total signal intensity, extending to 4.2 _+ 0.2 eV. As the shape of the integral spectrum appears to remain constant for higher incident positron energies it is reasonable to attribute the latter value to the work function ~b+, but one must always bear in mind the possibility of higher-energy positron emission due to the band gap of the material. The present work on 3C-SiC and earlier work on 6H-SiC [4] shows that the local density approximation, widely used in electronic structure calculations, usually yields large errors for electron and positron energy levels for systems having an energy gap [14,15]. To overcome this problem in the future, an improved approach to obtain reasonable gap and positron level values from first principles has to be used. This requires the inclusion of self-interaction correction terms as well modification of the electron-positron correlation potential; such work is already underway and the results will be published elsewhere [16].
portional to the non-backscattered incident flux. The count rate for V, = + 5 V is proportional to the incident flux minus those positrons which are reemitted at low (eV) energies. In both cases the count rates include a contribution from short-lived parapositronium (p-Ps) which self-annihilates just above the surface (and hence in sight of the Ge detector). The V, = - 2 5 V measurement includes any epithermai positrons emitted at incident energies below 1 keV. If one assumes that the branching ratio for Ps formation is the same for both incident and returned slow positrons then these extra 'events' will increase both asymptotic count rates by the same fraction, and one can therefore (to first order) ignore their effect. The re-emitted positron fraction is therefore computed directly from the two count rates and is plotted in Fig. 3 as a tunction of incident positron energy. Also plotted is the result for ion-implanted (i.e.. highly defected) 6H-SiC for comparison. The solid line in Fig. 3 is a fit which assumes a Gaussian derivative implantation profile and diffusion characterized by the length L + = 20 + 3 rim. Although this length is a factor of three shorter than that measured previously for bulk 6H-SiC, workfunction emission of thermalized positrons still occurs, albeit at relatively low incident energies, as can be seen from the evidence of Fig. 2 and the comparison with the energy dependence of epithermal positron emission from defected SiC in Fig. 3. The short L+ for 3C-SiC may reflect the structure of the polytype; it is, however, probable that diffusion is hampered by defects in the deposited 3C-SiC layer. 0.6 0.5
0.4
-
8 o.aL >" 0.2
•
0.0 1
5. Positron diffusion length measurement Following the method described in Section 4 the asymptotic count rate for VS = - 2 5 V was also measured and, after background subtraction, is pro-
21
t& 2
3
4
5
INCIDENT POSITRON ENERGY (keV) Fig. 3. Re-emitted positron fractions as a function of incident positron energy E: Full circles - 3C-SiC. Triangles - ion-implanted 6H-SiC. Solid line - implantation/diffusion model fit to 3C-SiC data, yielding a diffusion length of 20-t-3 nm.
22
G. Brauer et al./ Applied Surface Science 116 (1997) 19-22
6. Conclusions The positron diffusion length was extracted from measurements of the Doppler broadened annihilation lineshapes from variable energy positrons in 3C-SiC and compared to the diffusion length for 6H-SiC. The new measurements also allowed the quality of the 3C-SiC epilayer on Si(001) substrate to be characterized. An experimental estimation of the electron work function of 3C-SiC, combined with independent positron work function measurements on the same specimen, allows the evaluation of the positron affinity for this sample. Although its comparison with the theoretical value shows less than satisfactory agreement, this result forms another part of the needed experimental basis for further improvements in the theoretical calculations.
Acknowledgements The authors are grateful to the Daimler-Benz AG (Forschung und Technik, Frankfurt/M) for supplying us with the 3C-SiC material used in this work.
References [1] G. Pensl and R. Helbig, in: Festk~Srperprobleme/Advances in Solid State Physics, Ed. U. R~Sssler, Vol. 30 (Vieweg, Braunsehweig, 1990)p. 133.
[2] G. Pensl and W. Choyke, Physica B 185 (1993) 264. [3] G. Brauer, W. Anwand, P.G. Coleman, A.P. Knights, F. Plazaola, Y. Pacaud, W. Skorupa, J. St~Srmerand P. Willutzki, Phys. Rev. B 54 (1996) 3084. [4] G. Brauer, W, Anwand, E.-M. Nicht, J. Kuriplach, M. Sob, N. Wagner, P.G. Coleman, M.J. Puska and T. Korhonen, Phys. Rev. B 54 (1996) 2512. [5] J. Sttirmer, A. Goodyear, W. Anwand, G. Brauer, P.G. Coleman and W. Triftsh~iuser, J. Phys. Condens. Matter 8 (1996) L89. [6] N. Wagner, in: Proc. Autumn School Mikrosonde (University of Halle, 1990) p. 57. [7] G. Brauer, W. Anwand, E.-M. Nicht, P.G. Coleman, A.P. Knights, H. Schut, G. KiSgel and N. Wagner, J. Phys.: Condens. Matter 7 (1995) 9091. [8] J.A. Powell and L.G. Matus, in: Springer Proc. in Physics, Eds. G.L. Harris and C.Y.-W. Yang, Vol, 34 (Springer, Berlin, 1989) p. 2. [9] H. Matsunami, K. Shibakara, N. Kuroda, W. Yoo and S. Niskina, in: Springer Proc. in Physics, Eds. G.L. Harris and C.Y.-W. Yang, Vol. 34 (Springer, Berlin, 1989) p. 34. [10] W.J. Choyke, Mater. Res. Bull. 4 (1969) 141. [11] N.M. Ravindra and V.K. Srivastava, Phys. Chem. Solids 40 (1979) 791. [12] S.M. Hutchins, M.A. Alam, P.G. Coleman and R.N. West, in: Positron Annihilation, Eds. R.M. Singru, P.C. Jain and K.P. Gopinathan (World Scientific, Singapore, 1985) p. 983. [13] P.G. Coleman, L. Albrecht, A.B. Walker and K.O. Jensen, J. Phys.: Condens. Matter 4 (1992) 10311. [14] J.P. Perdew and A. Zunger, Phys. Rev. B 23 (1981) 5048. [15] M.J. Puska and R.M. Nieminen, Phys. Rev. B 46 (1992) 1278. [16] J. Kuriplach, M. Sob, M.J. Puska, G. Brauer, W. Anwand, E.-M. Nicht, P.G. Coleman and N. Wagner, submitted (1996).