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An inelastic neutron scattering study of the dynamics of hydrogenated and deuterated amorphous silicon
Physica A 201 (1993) 395-401 North-Holland SDZ: 037%4371(93)EO275-J
An inelastic neutron scattering study of the dynamics of hydrogenated and deuterated amorphous silicon Adrian C. Wright”, Thierry M. Bruniera, Carlo A. Guy”, N. Sinclairb, Alex C. Hannonc and Frank Jansend,’
Roger
aJ.J. Thomson Physical Laboratory, Reading University, Whiteknights, Reading RG6 2AF, UK b AEA Technology, Harwell Laboratory, Didcot OX11 ORA, UK ’ ISIS Science Division, Rutherford Appleton Laboratory, Didcot OX11 OQX, UK d Xerox Webster Research Center, Webster, NY 14580, USA
A short summary is presented of inelastic neutron scattering studies of hydrogenated and deuterated amorphous silicon with particular emphasis on the structural role played by the hydrogen (deuterium) atoms. It is concluded that the data are consistent with the existence of molecular H, (D,) trapped within the cages of the amorphous covalent network.
1. Introduction A series of inelastic neutron scattering experiments has been performed on hydrogenated and deuterated amorphous silicon as part of a comprehensive neutron scattering investigation of this important, but extremely complex, technological material. Despite its widespread use, for example in xerography, solar cells and flat panel liquid crystal displays, its structure and dynamics are very poorly understood, especially in respect of the structural role played by the hydrogen atoms. This paper, therefore, will particularly address the structural information concerning the environment of the H atoms that can be obtained from inelastic neutron scattering experiments, with special reference to the results of NMR and IR studies which suggest that a significant fraction of the hydrogen is present as H, molecules trapped within the cages of the amorphous covalent network [ 11.
1 Present address: Airco Coating Technology, Concord, CA 94524, USA. 037%4371/93/%06.00
0
1993 - Elsevier Science Publishers B.V. All rights reserved
396
A.C.
Wright et al. I Dynamics of amorphous
silicon
2. Results and discussion Due to the very large incoherent neutron scattering cross section for hydrogen, the inelastic scattering from hydrogenated amorphous silicon is dominated by the hydrogen modes and hence yields very little information concerning the dynamics of the underlying silicon network. For this reason, measurements have been performed on both hydrogenated and deuterated samples which were prepared by the glow discharge technique and contain 22 at% hydrogen (deuterium). The samples have also been studied by a number of other techniques, including neutron diffraction [2,3] and small angle scattering (SANS) [4]. 2.1. Neutron weighted density of vibrational states High resolution measurements of the inelastic scattering are required to investigate the vibrational modes of the system to check the interpretation of IR and Raman spectra and to look for modes which may be missed in the optical spectra because of “selection rules”. These have been undertaken with the HET spectrometer at the ISIS pulsed neutron source using two different incident neutron energies to allow the optimum energy transfer resolution to be obtained for selected regions of the spectra. Fig. 1 compares the uncorrected scattering functions, S(Q, E), obtained with incident energies of 129.9 and 292.5 meV, for the hydrogenated and deuterated samples at 20 K. The stretching vibrations for =SiH, and =SiH groups are observed at 258 and 247 meV, respectively, and it is interesting that these modes are significantly broader than the resolution of the instrument (FWHM = 3.3 meV) at this energy transfer - i.e., the stretch modes are not sharp and well defined. The =SiH, bending mode is observed at 110 meV, which is contrary to the results of Kamitakahara et al. [5] who failed to observe this mode in a previous neutron experiment and deduced that published conclusions from infrared spectroscopy were wrong because of an abnormally large matrix element. The present spectra can be analysed using theoretical neutron cross sections [6] and show that approximately equal numbers of hydrogen atoms are contained in qSiH and =!SiH, groupings. There are no indications of excitations corresponding to -SiH, groups. The average reduction in excitation energy introduced by deuteration is slightly less than the factor of 1.414 expected for a hydrogen atom connected to an infinite mass, indicating the small but observable influence of the silicon matrix. A large peak at 14.5 meV is observed for the hydrogenous sample which has not been previously reported. Two model calculations have predicted a peak at this energy: the first [6] involves a rotational mode of a -SiH, grouping and the second [7] a surface mode on large internal surfaces. There is no evidence in
A.C.
Wright et al. I Dynamics of amorphous
397
silicon
I.0
(4
2
0.8
-
0.6
-
0.4
:
c
z
5 ”
0.2
0.0
-
x
0.8
-
E 2
0.6
-
.%
C
0.4
0.2
-
0.0
1 0
_I 20
60
40
Energy
80
Transfer
120
100
(meV)
0.0 0
50
100
Energy
150
Transfer
200
250
(meV)
Fig. 1. The inelastic neutron scattering from hydrogenated (lower curves) and deuterated (upper curves) amorphous silicon measured at 20K using the ISIS HET spectrometer and incident energies of (a) 129.9 and (b) 292.5 meV.
the full density of states for -SiH, groupings in this sample and the surface mode suggestion is not compatible with the observed isotopic shift (cf. section 2.2). 2.2. Low energy excitations In order to further investigate the low energy excitations, and in particular the mode occurring at 14.5 meV for the hydrogenous sample, higher resolution
398
A.C.
Wright et al. I Dynamics
of amorphous
silicon
0.0016
7
0.0004
-c E
0.0000
2 x
0.0016
0.0012
O.OOOE
0.0004
0.0000 0
10
20
30
40
50
Energy Transfer (meV) Fig. 2. The effective vibrational density of states for hydrogenated (bottom) and deuterated (top) amorphous silicon at 100 K as measured with the IN6 spectrometer at ILL. The energy scale has been doubled for the deuterated sample.
measurements were carried out at both 100 and 300 K, using the IN6 spectometer at the Institut Laue-Langevin (ILL) with an incident energy of 3.12 meV. Fig. 2 shows the form of the vibrational densities of states at 100 K. The energy transfer scale for the deuterated sample has been doubled. Neither density of states shows the pure Debye variation (E*) as E tends to zero, although corrections have not yet been made for multiphonon effects. These are, however, believed to be small, since the measurements were performed at low temperature and at low Q. As in the HET experiment, a strong peak is observed for the hydrogenated sample at 14.5 meV, while for the deuterated sample, the corresponding mode appears as a bulge on the side of the silicon density of states at about 7 meV. The Q dependence of the inelastic structure factor at this energy indicates that the mode is not associated with sound propagation and is of a localised nature, although experiments using a higher value of E, are strictly required to cover a larger range of Q, both for this mode and the rest of the low energy transfer region, so that unambiguous conclusions can be drawn. The isotopic shift of a factor of about 2 is consistent with an assignment as a rotation of molecular hydrogen, but the breadth of the mode and the absence of higher order transitions are indications that the rotation is constrained [S].
A.C.
Wright et al. I Dynamics of amorphous
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399
Further neutron scattering evidence for the existence of H, (D,) molecules in the present samples is afforded by the neutron diffraction data for the deuterated sample, which exhibit a small feature at the expected intramolecular D-D distance, and SANS experiments, which indicate the presence of hydrogen (deuterium) filled voids. A search has also been made to detect both the rotational diffusion of the hydrogen molecules, by quasielastic neutron scattering, and the intramolecular H-H stretching vibration at very high energy transfers, as discussed in the following two sections. 2.3. Quasielastic neutron scattering Quasielastic neutron scattering experiments allow both translational and rotational hydrogen diffusion to be examined and were undertaken with an energy resolution (FWHM) of 1.23 PeV, using the IN10 instrument at the ILL reactor and an incident energy of 2.08 meV The scattering in the region of the elastic line was recorded at 4 K and 302 K but no increase in width or change in peak shape was observed, as may be seen from fig. 3a, which compares the two elastic peak profiles after internormalisation to the same maximum intensity. In addition, the profile at both temperatures is in excellent agreement with the instrumental resolution function as determined from a vanadium standard. A further experiment was performed to measure the temperature dependence of the Debye-Waller factor for the hydrogen atoms between 2K and 302 K by recording the scattering in a 1.23 p,eV window at the elastic energy. The temperature dependence of the logarithm of the ratio of the integrated elastic peak intensity at zero kelvin to that at temperature T is shown in fig. 3b
Fig. 3. Quasi-elastic neutron scattering data for hydrogenated amorphous silicon from the IN10 spectrometer (ILL). (a) The quasi-elastic peak profiles for the sample at 4 K (0) and 302 K (0). together with their difference (bottom curve). (b) The temperature dependence of the logarithm of the ratio of the integrated elastic peak intensity at zero kelvin, I,, to that at temperature T, I,.
400
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Wright et al. I Dynamics
of amorphous
silicon
and appears to start to increase significantly from its zero kelvin value at - 30 K. This is extremely interesting in the light of the solid-to-thud phase transition, which has been reported [9] at around 30 K for molecular hydrogen occluded at high pressure in hydrogenated amorphous silicon. (Note that at normal pressure this mode is not IR active.) Evidence that any H, molecules in the present samples are trapped at high pressure is provided by the width of the 14.5 meV peak and by the SANS measurements [4]. Further experiments are, however, required to observe the motion of the different hydrogen groups, as the temperature is raised, to ascertain the temperature at which each type of hydrogen atom structural environment allows diffusion to commence. 2.4. High energy excitations Experiments to look for the H-H stretch mode have been undertaken using the ISIS MAR1 spectrometer with the sample at 22, 33 and 42 K and an incident energy of 908 meV. These temperatures were chosen because of the solid-to-fluid phase transition mentioned in section 2.3. The observed spectra did not change over this temperature range but this may just reflect the relatively low resolution of the neutron technique compared to that of IR absorption spectra in this energy range. Summed data for the combined measurements are plotted in fig. 4. The peak at 420 meV is the fifth overtone of the group of SiH, bending modes around 80 meV while the strong peak at 510 meV is the second overtone of the SiH, stretch group at 250 meV The peak at 570 meV can be assigned to the expected molecular hydrogen H-H stretch mode since its intensity is too large to be an overtone in the known sequences.
a0 ,
Fig. 4. The high energy transfer inelastic neutron scattering from hydrogenated amorphous silicon measured at 30 K with the MAR1 spectrometer (ISIS) and an incident neutron energy of 908 meV.
A.C.
Wright et al. I Dynamics of amorphous
silicon
401
3. Conclusions
It may thus be concluded that the present series of inelastic neutron scattering experiments are consistent with the existence of high pressure molecular H, trapped in the voids of the amorphous network. However, to positively correlate the peak at 14.5 meV with a rotational mode of the hydrogen molecules, it will be necessary to record the peak shape and intensity as a function of temperature through the region of the suggested solid-to-fluid phase transition at - 30 K.
Acknowledgements
This work was supported by the Corporate Research Programme of AEA Technology and by the SERC (UK) using facilities at the Institut LaueLangevin and the Rutherford-Appleton Laboratory.
References [l] S.R. Elliott, Adv. Phys. 38 (1989) 1. [2] R. Bellisent, A. Menelle, W.S. Howells, A.C. Wright, T.M. Brunier, R.N. Sinclair and F. Jansen, Physica B 156-157 (1989) 217. (31 T.M. Brunier, Ph.D. Thesis (Reading University, 1990). [4] C.A. Guy, Ph.D. Thesis (Reading University, 1992). [5] W.A. Kamitakahara, H.R. Shanks, J.F. McClelland, U. Buchenau, F. Gompf and L. Pintschovius, Phys. Rev. Lett. 52 (1984) 644. [6] R. Barrio, R.J. Elliott and M.F. Thorpe, J. Phys. C 16 (1983) 3425. [7] E. Martinez and F. Yndurain, Physica B 117-118 (1983) 935. [8] W.J. Stead, P. Meehan and J.W. White, J. Chem. Sot. Faraday Trans. II 84 (1988) 1655. [9] Y.J. Chabal and C.K.N. Patel, Phys. Rev. Lett. 53 (1984) 1771.
An inelastic neutron scattering study of the dynamics of hydrogenated and deuterated amorphous silicon Adrian C. Wright”, Thierry M. Bruniera, Carlo A. Guy”, N. Sinclairb, Alex C. Hannonc and Frank Jansend,’
Roger
aJ.J. Thomson Physical Laboratory, Reading University, Whiteknights, Reading RG6 2AF, UK b AEA Technology, Harwell Laboratory, Didcot OX11 ORA, UK ’ ISIS Science Division, Rutherford Appleton Laboratory, Didcot OX11 OQX, UK d Xerox Webster Research Center, Webster, NY 14580, USA
A short summary is presented of inelastic neutron scattering studies of hydrogenated and deuterated amorphous silicon with particular emphasis on the structural role played by the hydrogen (deuterium) atoms. It is concluded that the data are consistent with the existence of molecular H, (D,) trapped within the cages of the amorphous covalent network.
1. Introduction A series of inelastic neutron scattering experiments has been performed on hydrogenated and deuterated amorphous silicon as part of a comprehensive neutron scattering investigation of this important, but extremely complex, technological material. Despite its widespread use, for example in xerography, solar cells and flat panel liquid crystal displays, its structure and dynamics are very poorly understood, especially in respect of the structural role played by the hydrogen atoms. This paper, therefore, will particularly address the structural information concerning the environment of the H atoms that can be obtained from inelastic neutron scattering experiments, with special reference to the results of NMR and IR studies which suggest that a significant fraction of the hydrogen is present as H, molecules trapped within the cages of the amorphous covalent network [ 11.
1 Present address: Airco Coating Technology, Concord, CA 94524, USA. 037%4371/93/%06.00
0
1993 - Elsevier Science Publishers B.V. All rights reserved
396
A.C.
Wright et al. I Dynamics of amorphous
silicon
2. Results and discussion Due to the very large incoherent neutron scattering cross section for hydrogen, the inelastic scattering from hydrogenated amorphous silicon is dominated by the hydrogen modes and hence yields very little information concerning the dynamics of the underlying silicon network. For this reason, measurements have been performed on both hydrogenated and deuterated samples which were prepared by the glow discharge technique and contain 22 at% hydrogen (deuterium). The samples have also been studied by a number of other techniques, including neutron diffraction [2,3] and small angle scattering (SANS) [4]. 2.1. Neutron weighted density of vibrational states High resolution measurements of the inelastic scattering are required to investigate the vibrational modes of the system to check the interpretation of IR and Raman spectra and to look for modes which may be missed in the optical spectra because of “selection rules”. These have been undertaken with the HET spectrometer at the ISIS pulsed neutron source using two different incident neutron energies to allow the optimum energy transfer resolution to be obtained for selected regions of the spectra. Fig. 1 compares the uncorrected scattering functions, S(Q, E), obtained with incident energies of 129.9 and 292.5 meV, for the hydrogenated and deuterated samples at 20 K. The stretching vibrations for =SiH, and =SiH groups are observed at 258 and 247 meV, respectively, and it is interesting that these modes are significantly broader than the resolution of the instrument (FWHM = 3.3 meV) at this energy transfer - i.e., the stretch modes are not sharp and well defined. The =SiH, bending mode is observed at 110 meV, which is contrary to the results of Kamitakahara et al. [5] who failed to observe this mode in a previous neutron experiment and deduced that published conclusions from infrared spectroscopy were wrong because of an abnormally large matrix element. The present spectra can be analysed using theoretical neutron cross sections [6] and show that approximately equal numbers of hydrogen atoms are contained in qSiH and =!SiH, groupings. There are no indications of excitations corresponding to -SiH, groups. The average reduction in excitation energy introduced by deuteration is slightly less than the factor of 1.414 expected for a hydrogen atom connected to an infinite mass, indicating the small but observable influence of the silicon matrix. A large peak at 14.5 meV is observed for the hydrogenous sample which has not been previously reported. Two model calculations have predicted a peak at this energy: the first [6] involves a rotational mode of a -SiH, grouping and the second [7] a surface mode on large internal surfaces. There is no evidence in
A.C.
Wright et al. I Dynamics of amorphous
397
silicon
I.0
(4
2
0.8
-
0.6
-
0.4
:
c
z
5 ”
0.2
0.0
-
x
0.8
-
E 2
0.6
-
.%
C
0.4
0.2
-
0.0
1 0
_I 20
60
40
Energy
80
Transfer
120
100
(meV)
0.0 0
50
100
Energy
150
Transfer
200
250
(meV)
Fig. 1. The inelastic neutron scattering from hydrogenated (lower curves) and deuterated (upper curves) amorphous silicon measured at 20K using the ISIS HET spectrometer and incident energies of (a) 129.9 and (b) 292.5 meV.
the full density of states for -SiH, groupings in this sample and the surface mode suggestion is not compatible with the observed isotopic shift (cf. section 2.2). 2.2. Low energy excitations In order to further investigate the low energy excitations, and in particular the mode occurring at 14.5 meV for the hydrogenous sample, higher resolution
398
A.C.
Wright et al. I Dynamics
of amorphous
silicon
0.0016
7
0.0004
-c E
0.0000
2 x
0.0016
0.0012
O.OOOE
0.0004
0.0000 0
10
20
30
40
50
Energy Transfer (meV) Fig. 2. The effective vibrational density of states for hydrogenated (bottom) and deuterated (top) amorphous silicon at 100 K as measured with the IN6 spectrometer at ILL. The energy scale has been doubled for the deuterated sample.
measurements were carried out at both 100 and 300 K, using the IN6 spectometer at the Institut Laue-Langevin (ILL) with an incident energy of 3.12 meV. Fig. 2 shows the form of the vibrational densities of states at 100 K. The energy transfer scale for the deuterated sample has been doubled. Neither density of states shows the pure Debye variation (E*) as E tends to zero, although corrections have not yet been made for multiphonon effects. These are, however, believed to be small, since the measurements were performed at low temperature and at low Q. As in the HET experiment, a strong peak is observed for the hydrogenated sample at 14.5 meV, while for the deuterated sample, the corresponding mode appears as a bulge on the side of the silicon density of states at about 7 meV. The Q dependence of the inelastic structure factor at this energy indicates that the mode is not associated with sound propagation and is of a localised nature, although experiments using a higher value of E, are strictly required to cover a larger range of Q, both for this mode and the rest of the low energy transfer region, so that unambiguous conclusions can be drawn. The isotopic shift of a factor of about 2 is consistent with an assignment as a rotation of molecular hydrogen, but the breadth of the mode and the absence of higher order transitions are indications that the rotation is constrained [S].
A.C.
Wright et al. I Dynamics of amorphous
silicon
399
Further neutron scattering evidence for the existence of H, (D,) molecules in the present samples is afforded by the neutron diffraction data for the deuterated sample, which exhibit a small feature at the expected intramolecular D-D distance, and SANS experiments, which indicate the presence of hydrogen (deuterium) filled voids. A search has also been made to detect both the rotational diffusion of the hydrogen molecules, by quasielastic neutron scattering, and the intramolecular H-H stretching vibration at very high energy transfers, as discussed in the following two sections. 2.3. Quasielastic neutron scattering Quasielastic neutron scattering experiments allow both translational and rotational hydrogen diffusion to be examined and were undertaken with an energy resolution (FWHM) of 1.23 PeV, using the IN10 instrument at the ILL reactor and an incident energy of 2.08 meV The scattering in the region of the elastic line was recorded at 4 K and 302 K but no increase in width or change in peak shape was observed, as may be seen from fig. 3a, which compares the two elastic peak profiles after internormalisation to the same maximum intensity. In addition, the profile at both temperatures is in excellent agreement with the instrumental resolution function as determined from a vanadium standard. A further experiment was performed to measure the temperature dependence of the Debye-Waller factor for the hydrogen atoms between 2K and 302 K by recording the scattering in a 1.23 p,eV window at the elastic energy. The temperature dependence of the logarithm of the ratio of the integrated elastic peak intensity at zero kelvin to that at temperature T is shown in fig. 3b
Fig. 3. Quasi-elastic neutron scattering data for hydrogenated amorphous silicon from the IN10 spectrometer (ILL). (a) The quasi-elastic peak profiles for the sample at 4 K (0) and 302 K (0). together with their difference (bottom curve). (b) The temperature dependence of the logarithm of the ratio of the integrated elastic peak intensity at zero kelvin, I,, to that at temperature T, I,.
400
A.C.
Wright et al. I Dynamics
of amorphous
silicon
and appears to start to increase significantly from its zero kelvin value at - 30 K. This is extremely interesting in the light of the solid-to-thud phase transition, which has been reported [9] at around 30 K for molecular hydrogen occluded at high pressure in hydrogenated amorphous silicon. (Note that at normal pressure this mode is not IR active.) Evidence that any H, molecules in the present samples are trapped at high pressure is provided by the width of the 14.5 meV peak and by the SANS measurements [4]. Further experiments are, however, required to observe the motion of the different hydrogen groups, as the temperature is raised, to ascertain the temperature at which each type of hydrogen atom structural environment allows diffusion to commence. 2.4. High energy excitations Experiments to look for the H-H stretch mode have been undertaken using the ISIS MAR1 spectrometer with the sample at 22, 33 and 42 K and an incident energy of 908 meV. These temperatures were chosen because of the solid-to-fluid phase transition mentioned in section 2.3. The observed spectra did not change over this temperature range but this may just reflect the relatively low resolution of the neutron technique compared to that of IR absorption spectra in this energy range. Summed data for the combined measurements are plotted in fig. 4. The peak at 420 meV is the fifth overtone of the group of SiH, bending modes around 80 meV while the strong peak at 510 meV is the second overtone of the SiH, stretch group at 250 meV The peak at 570 meV can be assigned to the expected molecular hydrogen H-H stretch mode since its intensity is too large to be an overtone in the known sequences.
a0 ,
Fig. 4. The high energy transfer inelastic neutron scattering from hydrogenated amorphous silicon measured at 30 K with the MAR1 spectrometer (ISIS) and an incident neutron energy of 908 meV.
A.C.
Wright et al. I Dynamics of amorphous
silicon
401
3. Conclusions
It may thus be concluded that the present series of inelastic neutron scattering experiments are consistent with the existence of high pressure molecular H, trapped in the voids of the amorphous network. However, to positively correlate the peak at 14.5 meV with a rotational mode of the hydrogen molecules, it will be necessary to record the peak shape and intensity as a function of temperature through the region of the suggested solid-to-fluid phase transition at - 30 K.
Acknowledgements
This work was supported by the Corporate Research Programme of AEA Technology and by the SERC (UK) using facilities at the Institut LaueLangevin and the Rutherford-Appleton Laboratory.
References [l] S.R. Elliott, Adv. Phys. 38 (1989) 1. [2] R. Bellisent, A. Menelle, W.S. Howells, A.C. Wright, T.M. Brunier, R.N. Sinclair and F. Jansen, Physica B 156-157 (1989) 217. (31 T.M. Brunier, Ph.D. Thesis (Reading University, 1990). [4] C.A. Guy, Ph.D. Thesis (Reading University, 1992). [5] W.A. Kamitakahara, H.R. Shanks, J.F. McClelland, U. Buchenau, F. Gompf and L. Pintschovius, Phys. Rev. Lett. 52 (1984) 644. [6] R. Barrio, R.J. Elliott and M.F. Thorpe, J. Phys. C 16 (1983) 3425. [7] E. Martinez and F. Yndurain, Physica B 117-118 (1983) 935. [8] W.J. Stead, P. Meehan and J.W. White, J. Chem. Sot. Faraday Trans. II 84 (1988) 1655. [9] Y.J. Chabal and C.K.N. Patel, Phys. Rev. Lett. 53 (1984) 1771.