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H2 molecules in crystalline silicon
Materials Science and Engineering B58 (1999) 1 – 5
H2 molecules in crystalline silicon R.E. Pritchard *, M.J. Ashwin, R.C. Newman, J.H. Tucker Interdisciplinary Research Centre for Semiconductor Materials, Blackett Laboratory, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BZ, UK
Abstract Infrared spectra from low-doped, hydrogenated silicon have revealed very weak absorption from hydrogen molecules. In Czochralski silicon, vibrational modes from molecules paired with interstitial oxygen atoms (OiH2) have been identified, together with a vibrational mode (n3HH) from molecules trapped at a second site. A low temperature annealing study (T B 200°C) has now led to the proposal that this second site is an interstitial lattice site with the axes of the isolated molecules aligned along either 111 or 110 to account for their IR activity. The n3HH mode is also detected in hydrogenated, boron-doped float zone (FZ) Si, together with the stretch mode of HB pairs. An estimate of the molecular concentration indicates that this is the source of so-called ‘hidden hydrogen’ that is observed for boron-doped Si. This identification is confirmed in the present sample by a second low temperature annealing study. Further work is in progress to establish the dissociation mechanism of H2 molecules and the subsequent formation of HB pairs. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Silicon; Hydrogen; Vibrational mode
1. Introduction The presence of interstitial hydrogen molecules in crystalline silicon was first predicted by theory [1–3] but, until recently, there were no direct experimental observations to support this proposal. The first direct evidence was obtained from Raman scattering measurements of silicon samples that had been hydrogenated using the plasma technique [4]. The observed vibrational frequency was 4158 cm − 1, close to that of gaseous H2 [5], and it was presumed that the molecules giving rise to this line were located at interstitial lattice sites. Calculations using the ab initio Hartree–Fock method [6] predicted a vibrational frequency of : 4500 cm − 1 in support of this interpretation. However, other groups have performed ab initio calculations using density–functional theory and predicted much reduced frequencies in the range 3000 – 3600 cm − 1 [7 – 9]. It was then argued that, alternatively, the molecules giving rise to the Raman signal are located in voids created by the plasma treatment of the samples [8,10]. This latter interpretation could explain the correlated Raman signals from hydrogen-passivated Si dangling bonds [10]. * Corresponding author. Tel.: +44-171-5946682; fax: + 44-1715813817; e-mail: [email protected].
Soon after, an infrared (IR) absorption study by our group revealed weak absorption from H2 molecules [11] in hydrogenated silicon. This rather surprising result was obtained using long path-length samples (17 mm) of low-doped Si that had been heat treated in hydrogen gas at 1300°C and then cooled rapidly to room temperature. In Czochralski (CZ) silicon samples, absorption from perturbed Oi atoms with a peak at 1075.1 cm − 1 and a shoulder at 1075.8 cm − 1 was assigned to modes of OiH2 complexes with two different configurations. Weak but sharp (D= 0.2 cm − 1) lines detected at n1HH = 3789 and n2HH = 3731 cm − 1 were assigned to the vibrational modes of the molecules present in these centres and their dipole moments per unit displacement, h, were estimated to be :0.1e [11]. An additional line, also due to interstitial H2 molecules, was detected at n3HH = 3618 cm − 1 (D=0.2 cm − 1). This mode had an integrated absorption coefficient (IA) similar to those of n1HH and n2HH and was also detected in hydrogenated float zone (FZ) Si. This implies that the molecules in the n3HH centre are not associated with Oi atoms and it was originally suggested that they might be trapped by an unknown defect or impurity centre [11]. At this time isolated interstitial H2 molecules were thought to be IR-inactive but recent theoretical analysis [8] has shown that molecules with a 110 or 111 alignment have a dipole moment of h: 0.1e.
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R.E. Pritchard et al. / Materials Science and Engineering B58 (1999) 1–5
We now present the results of a low temperature annealing study of hydrogenated CZ silicon that provides information about the lattice location of H2 molecules in the n3HH centre. The data are analysed with the assumption that the molecules released upon dissociation of (OiH2) are all converted to molecules at n3HH sites. Using a two-centre statistical model, the value determined for the number density of n3HH sites accessible to H2 molecules is found to be essentially equal to the number density of interstitial silicon lattice sites (5× 1022 cm − 3). This leads to the proposal that the centres giving rise to the n3HH mode are isolated hydrogen molecules located at tetrahedral lattice sites. The above proposal has initiated a second annealing study of hydrogenated boron-doped FZ silicon to determine whether or not isolated molecules are present in such samples. It should be recalled that hydrogenation of boron-doped silicon leads to the formation of (HB) pairs[12]. These centres give rise to a hydrogen stretch mode at 1904 cm − 1 and their concentration can be estimated from IA (HB) using an established calibration [12]. A comparison of this measurement with the results of secondary ion mass spectrometry (SIMS), indicated that not all of the hydrogen was detected by the IR technique [13]. That is, hydrogen must have been incorporated in other centre(s) that were not revealed by IR spectroscopy. Hydrogen present in this form was therefore termed ‘hidden’ hydrogen. On annealing such samples with [B] =1017 cm − 3, additional HB pairs were formed, indicating that ‘hidden’ hydrogen acts as a source of atomic hydrogen [14]. The maximum value of [HB] corresponded closely to the H(D) concentration deduced by SIMS. It was suggested [14] that ‘hidden’ hydrogen centres were hydrogen molecules but as the thickness of samples used in this earlier work was only 1 mm, vibrational modes of hydrogen molecules could not have been detected. We now present annealing data from thick samples (17 mm) that confirms this earlier proposal.
or 3×1016 cm − 3. The two samples were hydrogenated using the technique described above and isothermally annealed in an oil-bath at 160°C. For both samples, [Oi]= 3× 1015 cm − 3 and the carbon concentrations were below 1016 cm − 3. The low oxygen concentration implies that the concentration of OiH2 centres should be very low, as confirmed by IR absorption measurements. Samples with [B]: 1017 cm − 3 were not examined by IR spectroscopy as they were opaque at high frequencies due to the intense free-carrier absorption and their long path-length. The samples used in both studies were examined by IR absorption spectroscopy in their as-grown state and immediately after the annealing treatments. Spectra were obtained using a Bruker IFS113v interferometer operated at a resolution of 0.1 cm − 1 and with samples cooled to 10 K. Scan times were extended up to 16 h to achieve very low noise spectra.
3. Hydrogen molecules in low-doped Czochralski silicon Fig. 1a shows vibrational absorption from the asquenched, hydrogenated CZ silicon sample. The absorption from OiH2 centres comprises a peak, PH, at 1075.1 cm − 1 and a shoulder, SH, at 1075.8 cm − 1 that have relative absorption strengths close to 3:1. The total IA is equal to 0.149 cm − 2. The two features to lower wave number occur because Oi atoms occupy bond-centred sites (Si 16OiSi bonding) and so these satellites correspond to 28Si16Oi29Si and 28Si16Oi30Si complexes [11]. The associated H2 modes of the (OiH2) centres have IAs of 1.1× 10 − 3 (n1HH) and 1.7×10 − 3 cm − 2 (n2HH). To minimise the error in the evaluation of [OiH2] (see below), we have used the IA of the 1075 cm − 1 absorption profile rather than the appropriate
2. Experimental details A long path-length sample (17 mm) of CZ silicon was used for the first annealing study. The sample was lightly-doped n-type with phosphorus to 5×1014 cm − 3, with [Oi]= 1018 cm − 3 and a carbon concentration below 1015 cm − 3. For hydrogenation, the as-grown sample was heated in a quartz tube at 1300°C for 60 min in flowing hydrogen gas and was then cooled rapidly by plunging the tube into water while the hydrogen flow was maintained. The sample was then subjected to sequential anneals in the temperature range 35–130°C in an oil-bath for periods of 30 – 60 min. For the second annealing study, long path-length samples (17 mm) of boron-doped FZ silicon were used with [B]=1× 1016
Fig. 2. IR spectra showing n3HH for (a) as-quenched sample and (b) (c) following anneals at 50 and 130°C, respectively. Lines either side of n3HH are from water vapour in the interferometer.
R.E. Pritchard et al. / Materials Science and Engineering B58 (1999) 1–5
Fig. 1. IR spectra showing absorption from OiH2 centres in (a) as-quenched sample and (b) (c) and (d) following anneals at 60, 70, and 130°C, respectively.
sum of the much smaller IAs of n1HH and n2HH. The only other hydrogen-related mode that is detected is the n3HH line at 3618 cm − 1 with IA =2.3 ×10 − 3 cm − 2. Weak modes from passivated Si dangling bonds that occur in the spectral range 1800 – 2300 cm − 1 [15] are sometimes detected in hydrogenated samples, presumably as a result of the introduction of point defects into the crystal during the quenching process. However, these lines were not detected in the present sample during any stage of the annealing treatment. As the temperature of the anneal is increased sequentially, the strength of the 1075 cm − 1 absorption decreases monotonically (Fig. 1). However, the relative strength of the peak and shoulder components show no detectable change demonstrating that the binding energies of the two types of OiH2 centres must be essentially the same. As the 1075 cm − 1 absorption decreases there are corresponding increases in the strength of the n3HH line (Fig. 2). There is, in fact, a linear anti-correlation (Fig. 3) implying that a fixed fraction of molecules
Fig. 3. IA of the n3HH mode against IA of the 1075 cm − 1 absorption, showing their linear anti-correlation.
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released by dissociation of (OiH2) diffuse to form n3HH centres. These are re6ersible cycles that occur in the temperature range 23–160°C, but the period of the anneal must be sufficiently long to achieve equilibrium (30 min for T\ 50°C). By extrapolating the line shown in Fig. 3 and taking the calibration for the 1075 cm − 1 absorption feature to be the same as that for isolated Oi atoms [11,16], the minimum dipole moment for the n3HH centre is estimated to be h3 : 0.08e, comparable to the values estimated for the H2 modes of the OiH2 centres (n1HH and n2HH). We can now determine the density of states available to the H2 molecule in the n3HH centre with the assumption that the molecules are either trapped at Oi atoms or at n3HH sites. This two-centre statistical model is based on the reversibility in absorption strengths of the anti-correlated (OiH2) and n3HH modes upon annealing. Since both centres are known to incorporate H2, this reversibility implies diffusion of the molecule between the two sites. If a third trap is present, it would have to have identical properties to the n3HH site, that is, similar density of states and similar binding energies so that reversibility is maintained. We write the ratio of the concentration of OiH2 centres to n3HH centres at temperature, T, as, [OiH2]T /[n3HH]T = (g1/g2) exp(+DE/kT) where DE is the difference in energy of H2 bound to an Oi atom and H2 present as the n3HH centre, and g1 and g2 are the number densities of sites accessible to the molecule in the two complexes. For the OiH2 complex, we take g1 = 6× 1018 cm − 3 (there are six equivalent interstitial sites around a 111 SiOiSi axis). An Arrhenius plot of [OiH2]T /[n3HH]T allows us to determine DE from the gradient and a value of g1/g2 from the intercept. We obtain DE = 0.2690.02 eV and g2 = 1.2(+ 1.0, − 0.7)×1023 cm − 3. The latter value is close to the concentration of interstitial silicon lattice sites (5× 1022 cm − 3) and so it is proposed that n3HH sites should be identified with these lattice locations. Consideration of all possible orientations of the H2 molecules in the two types of complexes leads to minimum and maximum values for the number density of n3HH sites of 1× 1022 and 6 × 1022 cm − 3 [16]. Since it is proposed that the n3HH line is a mode of an isolated IR-active molecule, we now consider the possible orientations of the molecule that can account for its dipole moment. Molecules aligned along 100 directions have a zero dipole moment because of their D2d symmetry and so this alignment can be ruled out. However, alignments along 110 and 111 are expected to have small but non-zero dipole moments because of their lower C2v and C3v symmetries respectively, and ab initio theory [8] has indicated that h:0.1e in both cases. Therefore, we conclude that the isolated molecules giving rise to the n3HH mode have their axes aligned along a 111 or 110 direction.
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R.E. Pritchard et al. / Materials Science and Engineering B58 (1999) 1–5
Fig. 4. IR spectra showing (a) the vibrational mode from (HB) pairs and (b) the n3HH mode. The bottom spectra are for the as-quenched [B]= 1 × 1016 cm − 3 sample and, in ascending order, following anneals at 160°C for 7, 22, and 36 h.
4. Hydrogen molecules in boron-doped float zone silicon For the as-quenched [B] =1016 cm − 3 sample, IA of the (HB) stretch mode was 0.16 cm − 2 indicating that only 5% of the boron atoms were passivated. The n3HH mode was also detected with IA =0.006 cm − 2. Anneals led to a progressive reduction in the IA of the n3HH line and an increase in IA(HB) (Fig. 4). These changes are irre6ersible and continued until :50% of the boron atoms had formed pairs: longer anneals produced no further increase in [HB] even though the n3HH line could still be detected. For the as-quenched [B]= 3× 1016 cm − 3 sample, IA of the (HB) stretch mode was 0.71 cm − 1, indicating that : 7% of the boron atoms were passivated. The n3HH mode was also detected with an IA =0.006 cm − 2. Anneals again led to a progressive reduction in this latter value and an increase in IA(HB). This process continued until :40% of the boron atoms were passivated when [HB]max = 1.2× 1016 cm − 3, and the n3HH line could no longer be detected. Longer anneals produced no further increase in the concentration of (HB) pairs. Using the calibrations estimated for the n3HH mode and the (HB) stretch mode [14], we can estimate the concentration of these two centres at each stage of the anneal. By summing these two contributions, it is found that the total concentration of hydrogen is constant and equal to 1.1 ×1016 cm − 3 for the low-doped sample (Fig. 5). For the high-doped sample, the total hydrogen concentration was also constant with a value of 1.2× 1016 cm − 3. The former value should be increased slightly since very weak, defect-related HSi modes were detected for the low-doped sample but these lines showed no measurable change with annealing for times up to 60 h. These results demonstrate that the hydrogen atoms released upon dissociation of molecules are subsequently captured by isolated boron atoms to form additional HB pairs. We conclude that so called ‘hid-
den hydrogen’ is indeed in the form of hydrogen molecules that are present as n3HH centres. We now consider the possible mechanisms by which isolated H2 molecules dissociate to form H atoms that are then converted to HB pairs. One possibility is that molecules diffuse to and are captured by B − acceptors and that there is a subsequent reaction leading to (HB)0 + H + . The rapidly diffusing H + atom [17] would then be captured by another unpaired boron acceptor. In this case, we expect the initial rate of dissociation of the molecules to depend on the boron concentration. The only other simple mechanism that could occur is the spontaneous dissociation of isolated H2 molecules to 2H + with the subsequent capture of the rapidly diffusing H + by B − . Clearly, for this process, the rate at which the molecules dissociate should not depend on the boron concentration. Since the measured values of d[H2]/dt = − 3× 1010 cm − 3 s − 1
Fig. 5. [HB] concentration (full circles), concentration of H atoms in n3HH centres (open triangles) and the sum of these concentrations (full squares) as a function of anneal time for the 1 × 1016 cm − 3 sample.
R.E. Pritchard et al. / Materials Science and Engineering B58 (1999) 1–5
([B]=1× 1016 cm − 3) (Fig. 5) and d[H2]/dt = −10× 1010 cm − 3 s − 1 ([B] =3 ×1016 cm − 3) are in the same ratio as the boron concentrations in the as-quenched samples, the annealing data lend support for the former mechanism.
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Acknowledgements The authors thank the Engineering and Physical Sciences Research Council for their financial support on grant number GR/K96977 and Professor E.C. Lightowlers for hydrogenation of samples.
5. Conclusions
References
An analysis using a two-centre statistical model leads to an estimate of the number of available sites for hydrogen molecules to form n3HH centres in the range 1022 –1023 cm − 3. It is therefore proposed that this centre should be identified with molecules located at isolated interstitial sites of the silicon lattice and either 110 or 111 alignments to account for their IR activity. Its low vibrational frequency (3618 cm − 1) compared with that of the free molecule (4200 cm − 1) is due to interactions between the molecule and the surrounding cage of Si atoms that must occur in order that the molecule acquires a dipole moment. It has also been demonstrated that the ‘hidden hydrogen’ observed in hydrogenated boron-doped silicon should be identified with isolated molecules, as suggested in previous work [14]. During annealing, the molecules dissociate to form additional (HB) pairs. By comparing the rate of this process for two different boron concentrations, it is implied that H2 molecules diffuse to and are captured by B − acceptors and that these molecules subsequently dissociate leading to the formation of (HB) pairs. Further work is required to confirm this reaction process and also to understand the mechanism for eventual dissociation of (HB) pairs for very long anneal times (\60 h at 160 °C).
[1] A. Mainwood, A.M. Stoneham, Physica B116 (1983) 101. [2] J.W. Corbett, S.N. Sahu, T.S. Shi, L.C. Snyder, Phys. Lett. A93 (1983) 303. [3] R. Jones, Physica B170 (1991) 181. [4] K. Murakami, N. Fukata, S. Sasaki, et al., Phys. Rev. Lett. 77 (1996) 3161. [5] G. Herzberg, Infrared and Raman Spectra, Reinhold, New York, 1945. [6] K.G. Nakamura, M. Kitajima, Jpn. J. Appl. Phys. Part 1 36 (1997) 2004. [7] Y. Okamoto, M. Saito, A. Oshiyama, Phys. Rev. B56 (R10) (1997) 016. [8] B. Hourahine, R. Jones, S. O8 berg, R.C. Newman, P.R. Briddon, E. Roduner, Phys. Rev. B 57 (1998) 2666. [9] C.G. Van de Walle, Phys. Rev. Lett. 80 (1998) 2177. [10] A.W.R. Leitch, V. Alex, J. Weber, Mater. Sci. Forum 258–263 (1997) 241. [11] R.E. Pritchard, M.J. Ashwin, R.C. Newman, et al., Phys. Rev. B56 (1997) 13118. [12] S.A. McQuaid, R.C. Newman, J.H. Tucker, E.C. Lightowlers, R.A.A. Kubiak, M. Goulding, Appl. Phys. Lett. 58 (1991) 2933. [13] S.A. McQuaid, M.J. Binns, R.C. Newman, E.C. Lightowlers, J.B. Clegg, Appl. Phys. Lett. 62 (1993) 1612. [14] M.J. Binns, S.A. McQuaid, R.C. Newman, E.C. Lightowlers, Semicond. Sci. Technol. 8 (1993) 1908. [15] B. Bech Nielsen, L. Hoffmann, M. Budde, R. Jones, J. Goss, S. O8 berg, Mater. Sci. Forum 196 – 201 (1995) 933. [16] R.E. Pritchard, M.J. Ashwin, J.H. Tucker, R.C. Newman, Phys. Rev. B 57 (1998) R15048. [17] A. Van Wieringen, W. Warmoltz, Physica 22 (1956) 849.
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H2 molecules in crystalline silicon R.E. Pritchard *, M.J. Ashwin, R.C. Newman, J.H. Tucker Interdisciplinary Research Centre for Semiconductor Materials, Blackett Laboratory, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BZ, UK
Abstract Infrared spectra from low-doped, hydrogenated silicon have revealed very weak absorption from hydrogen molecules. In Czochralski silicon, vibrational modes from molecules paired with interstitial oxygen atoms (OiH2) have been identified, together with a vibrational mode (n3HH) from molecules trapped at a second site. A low temperature annealing study (T B 200°C) has now led to the proposal that this second site is an interstitial lattice site with the axes of the isolated molecules aligned along either 111 or 110 to account for their IR activity. The n3HH mode is also detected in hydrogenated, boron-doped float zone (FZ) Si, together with the stretch mode of HB pairs. An estimate of the molecular concentration indicates that this is the source of so-called ‘hidden hydrogen’ that is observed for boron-doped Si. This identification is confirmed in the present sample by a second low temperature annealing study. Further work is in progress to establish the dissociation mechanism of H2 molecules and the subsequent formation of HB pairs. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Silicon; Hydrogen; Vibrational mode
1. Introduction The presence of interstitial hydrogen molecules in crystalline silicon was first predicted by theory [1–3] but, until recently, there were no direct experimental observations to support this proposal. The first direct evidence was obtained from Raman scattering measurements of silicon samples that had been hydrogenated using the plasma technique [4]. The observed vibrational frequency was 4158 cm − 1, close to that of gaseous H2 [5], and it was presumed that the molecules giving rise to this line were located at interstitial lattice sites. Calculations using the ab initio Hartree–Fock method [6] predicted a vibrational frequency of : 4500 cm − 1 in support of this interpretation. However, other groups have performed ab initio calculations using density–functional theory and predicted much reduced frequencies in the range 3000 – 3600 cm − 1 [7 – 9]. It was then argued that, alternatively, the molecules giving rise to the Raman signal are located in voids created by the plasma treatment of the samples [8,10]. This latter interpretation could explain the correlated Raman signals from hydrogen-passivated Si dangling bonds [10]. * Corresponding author. Tel.: +44-171-5946682; fax: + 44-1715813817; e-mail: [email protected].
Soon after, an infrared (IR) absorption study by our group revealed weak absorption from H2 molecules [11] in hydrogenated silicon. This rather surprising result was obtained using long path-length samples (17 mm) of low-doped Si that had been heat treated in hydrogen gas at 1300°C and then cooled rapidly to room temperature. In Czochralski (CZ) silicon samples, absorption from perturbed Oi atoms with a peak at 1075.1 cm − 1 and a shoulder at 1075.8 cm − 1 was assigned to modes of OiH2 complexes with two different configurations. Weak but sharp (D= 0.2 cm − 1) lines detected at n1HH = 3789 and n2HH = 3731 cm − 1 were assigned to the vibrational modes of the molecules present in these centres and their dipole moments per unit displacement, h, were estimated to be :0.1e [11]. An additional line, also due to interstitial H2 molecules, was detected at n3HH = 3618 cm − 1 (D=0.2 cm − 1). This mode had an integrated absorption coefficient (IA) similar to those of n1HH and n2HH and was also detected in hydrogenated float zone (FZ) Si. This implies that the molecules in the n3HH centre are not associated with Oi atoms and it was originally suggested that they might be trapped by an unknown defect or impurity centre [11]. At this time isolated interstitial H2 molecules were thought to be IR-inactive but recent theoretical analysis [8] has shown that molecules with a 110 or 111 alignment have a dipole moment of h: 0.1e.
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R.E. Pritchard et al. / Materials Science and Engineering B58 (1999) 1–5
We now present the results of a low temperature annealing study of hydrogenated CZ silicon that provides information about the lattice location of H2 molecules in the n3HH centre. The data are analysed with the assumption that the molecules released upon dissociation of (OiH2) are all converted to molecules at n3HH sites. Using a two-centre statistical model, the value determined for the number density of n3HH sites accessible to H2 molecules is found to be essentially equal to the number density of interstitial silicon lattice sites (5× 1022 cm − 3). This leads to the proposal that the centres giving rise to the n3HH mode are isolated hydrogen molecules located at tetrahedral lattice sites. The above proposal has initiated a second annealing study of hydrogenated boron-doped FZ silicon to determine whether or not isolated molecules are present in such samples. It should be recalled that hydrogenation of boron-doped silicon leads to the formation of (HB) pairs[12]. These centres give rise to a hydrogen stretch mode at 1904 cm − 1 and their concentration can be estimated from IA (HB) using an established calibration [12]. A comparison of this measurement with the results of secondary ion mass spectrometry (SIMS), indicated that not all of the hydrogen was detected by the IR technique [13]. That is, hydrogen must have been incorporated in other centre(s) that were not revealed by IR spectroscopy. Hydrogen present in this form was therefore termed ‘hidden’ hydrogen. On annealing such samples with [B] =1017 cm − 3, additional HB pairs were formed, indicating that ‘hidden’ hydrogen acts as a source of atomic hydrogen [14]. The maximum value of [HB] corresponded closely to the H(D) concentration deduced by SIMS. It was suggested [14] that ‘hidden’ hydrogen centres were hydrogen molecules but as the thickness of samples used in this earlier work was only 1 mm, vibrational modes of hydrogen molecules could not have been detected. We now present annealing data from thick samples (17 mm) that confirms this earlier proposal.
or 3×1016 cm − 3. The two samples were hydrogenated using the technique described above and isothermally annealed in an oil-bath at 160°C. For both samples, [Oi]= 3× 1015 cm − 3 and the carbon concentrations were below 1016 cm − 3. The low oxygen concentration implies that the concentration of OiH2 centres should be very low, as confirmed by IR absorption measurements. Samples with [B]: 1017 cm − 3 were not examined by IR spectroscopy as they were opaque at high frequencies due to the intense free-carrier absorption and their long path-length. The samples used in both studies were examined by IR absorption spectroscopy in their as-grown state and immediately after the annealing treatments. Spectra were obtained using a Bruker IFS113v interferometer operated at a resolution of 0.1 cm − 1 and with samples cooled to 10 K. Scan times were extended up to 16 h to achieve very low noise spectra.
3. Hydrogen molecules in low-doped Czochralski silicon Fig. 1a shows vibrational absorption from the asquenched, hydrogenated CZ silicon sample. The absorption from OiH2 centres comprises a peak, PH, at 1075.1 cm − 1 and a shoulder, SH, at 1075.8 cm − 1 that have relative absorption strengths close to 3:1. The total IA is equal to 0.149 cm − 2. The two features to lower wave number occur because Oi atoms occupy bond-centred sites (Si 16OiSi bonding) and so these satellites correspond to 28Si16Oi29Si and 28Si16Oi30Si complexes [11]. The associated H2 modes of the (OiH2) centres have IAs of 1.1× 10 − 3 (n1HH) and 1.7×10 − 3 cm − 2 (n2HH). To minimise the error in the evaluation of [OiH2] (see below), we have used the IA of the 1075 cm − 1 absorption profile rather than the appropriate
2. Experimental details A long path-length sample (17 mm) of CZ silicon was used for the first annealing study. The sample was lightly-doped n-type with phosphorus to 5×1014 cm − 3, with [Oi]= 1018 cm − 3 and a carbon concentration below 1015 cm − 3. For hydrogenation, the as-grown sample was heated in a quartz tube at 1300°C for 60 min in flowing hydrogen gas and was then cooled rapidly by plunging the tube into water while the hydrogen flow was maintained. The sample was then subjected to sequential anneals in the temperature range 35–130°C in an oil-bath for periods of 30 – 60 min. For the second annealing study, long path-length samples (17 mm) of boron-doped FZ silicon were used with [B]=1× 1016
Fig. 2. IR spectra showing n3HH for (a) as-quenched sample and (b) (c) following anneals at 50 and 130°C, respectively. Lines either side of n3HH are from water vapour in the interferometer.
R.E. Pritchard et al. / Materials Science and Engineering B58 (1999) 1–5
Fig. 1. IR spectra showing absorption from OiH2 centres in (a) as-quenched sample and (b) (c) and (d) following anneals at 60, 70, and 130°C, respectively.
sum of the much smaller IAs of n1HH and n2HH. The only other hydrogen-related mode that is detected is the n3HH line at 3618 cm − 1 with IA =2.3 ×10 − 3 cm − 2. Weak modes from passivated Si dangling bonds that occur in the spectral range 1800 – 2300 cm − 1 [15] are sometimes detected in hydrogenated samples, presumably as a result of the introduction of point defects into the crystal during the quenching process. However, these lines were not detected in the present sample during any stage of the annealing treatment. As the temperature of the anneal is increased sequentially, the strength of the 1075 cm − 1 absorption decreases monotonically (Fig. 1). However, the relative strength of the peak and shoulder components show no detectable change demonstrating that the binding energies of the two types of OiH2 centres must be essentially the same. As the 1075 cm − 1 absorption decreases there are corresponding increases in the strength of the n3HH line (Fig. 2). There is, in fact, a linear anti-correlation (Fig. 3) implying that a fixed fraction of molecules
Fig. 3. IA of the n3HH mode against IA of the 1075 cm − 1 absorption, showing their linear anti-correlation.
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released by dissociation of (OiH2) diffuse to form n3HH centres. These are re6ersible cycles that occur in the temperature range 23–160°C, but the period of the anneal must be sufficiently long to achieve equilibrium (30 min for T\ 50°C). By extrapolating the line shown in Fig. 3 and taking the calibration for the 1075 cm − 1 absorption feature to be the same as that for isolated Oi atoms [11,16], the minimum dipole moment for the n3HH centre is estimated to be h3 : 0.08e, comparable to the values estimated for the H2 modes of the OiH2 centres (n1HH and n2HH). We can now determine the density of states available to the H2 molecule in the n3HH centre with the assumption that the molecules are either trapped at Oi atoms or at n3HH sites. This two-centre statistical model is based on the reversibility in absorption strengths of the anti-correlated (OiH2) and n3HH modes upon annealing. Since both centres are known to incorporate H2, this reversibility implies diffusion of the molecule between the two sites. If a third trap is present, it would have to have identical properties to the n3HH site, that is, similar density of states and similar binding energies so that reversibility is maintained. We write the ratio of the concentration of OiH2 centres to n3HH centres at temperature, T, as, [OiH2]T /[n3HH]T = (g1/g2) exp(+DE/kT) where DE is the difference in energy of H2 bound to an Oi atom and H2 present as the n3HH centre, and g1 and g2 are the number densities of sites accessible to the molecule in the two complexes. For the OiH2 complex, we take g1 = 6× 1018 cm − 3 (there are six equivalent interstitial sites around a 111 SiOiSi axis). An Arrhenius plot of [OiH2]T /[n3HH]T allows us to determine DE from the gradient and a value of g1/g2 from the intercept. We obtain DE = 0.2690.02 eV and g2 = 1.2(+ 1.0, − 0.7)×1023 cm − 3. The latter value is close to the concentration of interstitial silicon lattice sites (5× 1022 cm − 3) and so it is proposed that n3HH sites should be identified with these lattice locations. Consideration of all possible orientations of the H2 molecules in the two types of complexes leads to minimum and maximum values for the number density of n3HH sites of 1× 1022 and 6 × 1022 cm − 3 [16]. Since it is proposed that the n3HH line is a mode of an isolated IR-active molecule, we now consider the possible orientations of the molecule that can account for its dipole moment. Molecules aligned along 100 directions have a zero dipole moment because of their D2d symmetry and so this alignment can be ruled out. However, alignments along 110 and 111 are expected to have small but non-zero dipole moments because of their lower C2v and C3v symmetries respectively, and ab initio theory [8] has indicated that h:0.1e in both cases. Therefore, we conclude that the isolated molecules giving rise to the n3HH mode have their axes aligned along a 111 or 110 direction.
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Fig. 4. IR spectra showing (a) the vibrational mode from (HB) pairs and (b) the n3HH mode. The bottom spectra are for the as-quenched [B]= 1 × 1016 cm − 3 sample and, in ascending order, following anneals at 160°C for 7, 22, and 36 h.
4. Hydrogen molecules in boron-doped float zone silicon For the as-quenched [B] =1016 cm − 3 sample, IA of the (HB) stretch mode was 0.16 cm − 2 indicating that only 5% of the boron atoms were passivated. The n3HH mode was also detected with IA =0.006 cm − 2. Anneals led to a progressive reduction in the IA of the n3HH line and an increase in IA(HB) (Fig. 4). These changes are irre6ersible and continued until :50% of the boron atoms had formed pairs: longer anneals produced no further increase in [HB] even though the n3HH line could still be detected. For the as-quenched [B]= 3× 1016 cm − 3 sample, IA of the (HB) stretch mode was 0.71 cm − 1, indicating that : 7% of the boron atoms were passivated. The n3HH mode was also detected with an IA =0.006 cm − 2. Anneals again led to a progressive reduction in this latter value and an increase in IA(HB). This process continued until :40% of the boron atoms were passivated when [HB]max = 1.2× 1016 cm − 3, and the n3HH line could no longer be detected. Longer anneals produced no further increase in the concentration of (HB) pairs. Using the calibrations estimated for the n3HH mode and the (HB) stretch mode [14], we can estimate the concentration of these two centres at each stage of the anneal. By summing these two contributions, it is found that the total concentration of hydrogen is constant and equal to 1.1 ×1016 cm − 3 for the low-doped sample (Fig. 5). For the high-doped sample, the total hydrogen concentration was also constant with a value of 1.2× 1016 cm − 3. The former value should be increased slightly since very weak, defect-related HSi modes were detected for the low-doped sample but these lines showed no measurable change with annealing for times up to 60 h. These results demonstrate that the hydrogen atoms released upon dissociation of molecules are subsequently captured by isolated boron atoms to form additional HB pairs. We conclude that so called ‘hid-
den hydrogen’ is indeed in the form of hydrogen molecules that are present as n3HH centres. We now consider the possible mechanisms by which isolated H2 molecules dissociate to form H atoms that are then converted to HB pairs. One possibility is that molecules diffuse to and are captured by B − acceptors and that there is a subsequent reaction leading to (HB)0 + H + . The rapidly diffusing H + atom [17] would then be captured by another unpaired boron acceptor. In this case, we expect the initial rate of dissociation of the molecules to depend on the boron concentration. The only other simple mechanism that could occur is the spontaneous dissociation of isolated H2 molecules to 2H + with the subsequent capture of the rapidly diffusing H + by B − . Clearly, for this process, the rate at which the molecules dissociate should not depend on the boron concentration. Since the measured values of d[H2]/dt = − 3× 1010 cm − 3 s − 1
Fig. 5. [HB] concentration (full circles), concentration of H atoms in n3HH centres (open triangles) and the sum of these concentrations (full squares) as a function of anneal time for the 1 × 1016 cm − 3 sample.
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([B]=1× 1016 cm − 3) (Fig. 5) and d[H2]/dt = −10× 1010 cm − 3 s − 1 ([B] =3 ×1016 cm − 3) are in the same ratio as the boron concentrations in the as-quenched samples, the annealing data lend support for the former mechanism.
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Acknowledgements The authors thank the Engineering and Physical Sciences Research Council for their financial support on grant number GR/K96977 and Professor E.C. Lightowlers for hydrogenation of samples.
5. Conclusions
References
An analysis using a two-centre statistical model leads to an estimate of the number of available sites for hydrogen molecules to form n3HH centres in the range 1022 –1023 cm − 3. It is therefore proposed that this centre should be identified with molecules located at isolated interstitial sites of the silicon lattice and either 110 or 111 alignments to account for their IR activity. Its low vibrational frequency (3618 cm − 1) compared with that of the free molecule (4200 cm − 1) is due to interactions between the molecule and the surrounding cage of Si atoms that must occur in order that the molecule acquires a dipole moment. It has also been demonstrated that the ‘hidden hydrogen’ observed in hydrogenated boron-doped silicon should be identified with isolated molecules, as suggested in previous work [14]. During annealing, the molecules dissociate to form additional (HB) pairs. By comparing the rate of this process for two different boron concentrations, it is implied that H2 molecules diffuse to and are captured by B − acceptors and that these molecules subsequently dissociate leading to the formation of (HB) pairs. Further work is required to confirm this reaction process and also to understand the mechanism for eventual dissociation of (HB) pairs for very long anneal times (\60 h at 160 °C).
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