Presented at the 3rd World Conference on Photovoltaic Energy Conversion; Osaka, Japan; May 11-18, 2003.A promising method to introduce H into Si solar cells in order to passivate bulk defects is by the post-deposition
annealing of an H-rich, SiN(x) surface layer. It previously has been difficult to characterize the small concentration
of H that is introduced by this method. IR spectroscopy has been used together with marker impurities in the Si to determine the concentration and depth of H introduced into Si from an annealed SiN(x) film

Atwater, H.

Holt, J.

Jiang, Fan

Kalejs, Juris P.

Kim, Dong Seop

Rohatgi, Ajeet

Stavola, Michael

Scholarly Materials And Research @ Georgia Tech

    HYDROGENATION OF Si FROM SiNx:H FILMS: HOW MUCH HYDROGEN IS REALLY IN THE Si?  Michael Stavola1, Fan Jiang1, A. Rohatgi2, D. Kim2, J. Holt3, H. Atwater3, and J. Kalejs4 1. Department of Physics, Lehigh University, Bethlehem, Pennsylvania  18015 2. School of Electrical Engineering, Georgia Institute of Technology, Atlanta, Georgia  30332 3. Department of Applied Physics, California Institute of Technology, Pasadena, California  91125 4. RWE Schott Solar, Inc., Billerica, Massachusetts  01821     ABSTRACT  A promising method to introduce H into Si solar cells in order to passivate bulk defects is by the post-deposition annealing of an H-rich, SiNx surface layer.  It previously has been difficult to characterize the small concentration of H that is introduced by this method.  IR spectroscopy has been used together with marker impurities in the Si to determine the concentration and depth of H introduced into Si from an annealed SiNx film.   1. INTRODUCTION   Hydrogen is commonly introduced into Si solar cells to reduce the deleterious effects of defects and increase the minority carrier lifetime [1-7].  A promising method to introduce H is from a hydrogen-rich (~20 at.%) layer of SiNx that is deposited onto the Si to act as an antireflection coating [5-7].  A post-deposition anneal is used to diffuse H from the SiNx layer into the Si in order to hydrogenate defects in the bulk Si.  Unfortunately, it has been difficult to detect directly the H that is introduced because of its small concentration.  Therefore, the effectiveness of the hydrogenation that results from the postdeposition annealing of SiNx layers has remained controversial.  For example, there are model calculations that suggest that H concentrations near 5 x 1016 cm-3 can be introduced into bulk Si from a SiNx layer [8], while other recent studies question whether bulk hydrogenation from SiNx occurs at all [9].  Furthermore, the indiffusion of H into Si is often limited by trapping [10], making the appropriate effective diffusion constant to describe the penetration depth of H uncertain.   In the present paper, a novel method that combines IR spectroscopy with marker impurities that can trap H in Si is used to determine the concentration and depth of H that is introduced into Si from a SiNx layer.  Pt impurities are used as model traps for H because the Pt-H complexes in Si are thermally stable (up to 650°C) and their H vibrational absorption lines have been identified [11].  Furthermore, the intensities of the PtH IR lines have been calibrated so that the concentration of H in the Si sample can be estimated [12].   2. EXPERIMENT    To increase the sensitivity of vibrational spectroscopy for the detection of small concentrations of H-containing defects, IR absorption measurements have been made for samples in a multiple-internal-reflection (MIR) geometry.  Samples with dimensions 15 x 18 mm2 and a thickness of 1.5 mm were made from lightly doped Si grown by the floating-zone method.  The ends of the samples were beveled at 45º.  The probing light was introduced through one of the beveled ends so as to be multiply reflected many times from the internal surfaces as it passes through the sample (Fig. 1).   Pt impurities, to be used as traps for H, were diffused into the Si samples at a temperature of 1225°C.  The resulting Pt concentration is estimated to be ~1017 cm-3 from solubility data in the literature [14].  SiNx layers were deposited onto the Si samples either by plasma-enhanced chemical vapor deposition (PECVD) [6] or by hot-wire chemical vapor deposition (HWCVD) [14].  The substrate temperature for the depositions was ~300°C and the nitride layer thicknesses were ~80 to 100 nm.  Samples were annealed in a tube furnace in an ambient of flowing N2.  IR absorption measurements were made with a Bomem DA3 FTIR spectrometer equipped with a KBr beamsplitter and an InSb detector.  Samples were cooled for IR measurements to near 4.2K with a Helitran, continuous-flow cryostat.                  Figure 1.  Multiple-internal-reflection geometry used for IR measurements of Si samples hydrogenated from a SiNx surface layer.   3. EXPERIMENTAL RESULTS   IR spectra are shown in Fig. 2 for a Si:Pt sample onto which a SiNx layer had been deposited by HWCVD.  The                        Figure 2.  IR spectra for a bulk Si:Pt sample with a SiNx layer deposited on its surface by hot-wire CVD.  The sample was annealed (10 min.) at the indicated temperatures to introduce H into the Si. Vibrational lines assigned to Si-H bonds in the SiNx layer and to PtH complexes in the Si are shown.   broad band centered at 2160 cm-1 is due to Si-H bonds in the H-rich SiNx layer [15].  For anneals performed at successively higher temperatures, the Si-H band from the SiNx decreases in intensity and a H vibrational line at 1880 cm-1, assigned previously to the PtH complex in Si [8], grows in intensity.  No other IR lines due to H-containing defects in the Si bulk were detected in our experiments.  These results show unambiguously that H from the SiNx surface layer diffuses into the Si substrate during a post-deposition anneal and that this H forms complexes with defects in the bulk.  The intensity of the PtH IR line at 1880 cm-1 provides quantitative information about the concentration of H in the Si sample.  From the calibration of the intensity of the PtH line reported in ref. (12), the concentration of PtH complexes is related to the area of the 1880 cm-1 absorbance line by,     [PtH] (cm-3) = 2.6 x 1016 cm-1 )dA (∫ ν  / deff. (1)  Here, ν  is the frequency of the light in wave numbers (cm-1) and A is the absorbance (which is related to the absorption coefficient by A = α deff log10 e).  For the MIR geometry shown in Fig. 1, the optical path length, deff, for the probing light is proportional to the thickness d of the hydrogenated layer and is given by,    deff = N d sec θ  = (L cot θ / w) d sec θ. (2)  Here, L is the length of the MIR sample, w is its thickness, θ is the bevel angle, and N is the number of passes the light makes through the absorbing layer.  For our samples, with L = 18 mm, w = 1.5 mm, and θ = 45º, the MIR geometry, when compared with a single pass at normal incidence, typically increases the optical path length by a factor of N sec θ ≈ 17.  Initially, the thickness d of the absorbing layer is unknown.  In this case, the product of the defect concentration times the layer thickness, or the areal density, can be determined from the area of the absorbance line [16].  Fig. 3(a) shows the PtH IR line for a sample that had been hydrogenated by a post-deposition anneal (5 min. at 600ºC) of a SiNx layer deposited by PECVD.  To determine the depth of H penetration, hydrogenated samples were mechanically thinned in successive steps in which ~50 to 100 µm were removed from the surface onto which the SiNx had been deposited.  Figs. 3(a) and (b) show that the areal density of PtH centers is reduced as layers are removed successively from the sample.  The linear decrease of the IR intensity with the increasing thickness of the removed layer shows that the concentration of PtH centers for these annealing conditions is approximately constant up to a maximum penetration depth of 500 µm into the sample.  The concentration of PtH complexes can be determined from their areal density once the thickness of the hydrogenated layer has been determined.  The concentration of PtH complexes for the sample whose spectra are shown in Fig. 3 is 1 x 1013 cm-3.  The sample whose spectra are shown in Fig. 2 was thinned and studied similarly by IR spectroscopy (following the anneal at 700ºC, 10 min).  In this case, the penetration depth of H was estimated to be 1 x 103 µm and the concentration of PtH complexes was determined to be 5 x 1013 cm-3.  Our previous experience with the trapping of H by Pt impurities in Si indicates that the majority of the H introduced into the Si will be trapped by Pt to form Pt-H complexes because the concentration of Pt (~1017 cm-3) is much greater than the concentration of H [12].  Therefore, the concentration of PtH complexes reflects the total concentration of H introduced into the Si from the SiNx layer [17].    The concentration of H introduced into Si from a SiNx layer has been found to be modest in the experiments performed here, i.e., from near 1 to 5x1013 cm-3.  The H that diffuses into the Si bulk corresponds to the order of only ~ a tenth of a percent of the total H that is liberated from the SiNx coating by an anneal at near 600°C.  For example, for the data shown in Fig. 2, the 600°C anneal reduces the area of the 2160 cm-1 Si-H IR band by 5%.  If we estimate that the 80 nm-thick SiNx layer contains a H concentration of ~1x1022 cm-3, this corresponds to an areal density of released H of ~4 x 1015 cm-2 [18].  The intensity of the 1880 cm-1 PtH line following the anneal at 600°C leads to an areal density of PtH complexes in the Si of 6 x 1012 cm-2, or only 0.15% of the total H released from the SiNx.  While the concentration of H introduced into Si is small, it is, nonetheless, sufficient to affect the electrical activity of dislocations and impurities like the transition metals that are highly effective lifetime killers.  Our experiments suggest that hydrogenation of Si from a SiNx layer yields a H concentration that lies between optimistic estimates of the amount of H that can be introduced into the Si bulk [8] and the negative viewpoint taken by Boehme and Lucovsky that bulk hydrogenation does not occur [9].  The modest H concentration introduced by a post-deposition anneal of a SiNx film is consistent with the sensitivity of the effectiveness of hydrogenation treatments to processing methods [4-8].  That is, one is not in the situation where there is an excess of H available to effectively passivate all f the defects that might be present.  Therefore, change  in the H concentration that  1875 1880 1885 2000 2200 24000.000.020.040.060.08SiNx: Si-HSi: PtH0400oC500oC600oC700oC  AbsorbanceFrequency (cm-1)os                        Figure 3.  (a) IR spectra showing the vibrational line of the PtH complex in a Si:Pt sample that was hydrogenated by the post-deposition annealing (5 min at 600°C) of a PECVD SiNx film.  Spectra were measured after a surface layer of the indicated thickness had been removed from the sample.  (b) The areal density of PtH complexes remaining in the sample vs. the thickness of the layer removed from the surface, derived from the data shown in (a).    result from differences in processing methods can have a pronounced effect.   Experiments are currently underway to investigate the synergistic effect that cofiring the SiNx AR coating and Al back contact layers has on the improvement of the minority carrier lifetime of solar cells [7,19], and whether this processing synergy might result from the introduction of a greater H concentration into the solar cell.  The results presented here provide information about the diffusivity of H during hydrogenation from SiNx.  There has been considerable interest in the penetration depth of H into Si because of its importance in the design of processes that will optimize the hydrogen passivation of solar cells [3,20].  The diffusivity of H in Si was measured at high temperature in an early study by Van Wieringen and Warmoltz [21] and gives the diffusion constant,     D =  9.4 x 10-3  exp(-0.48 eV / kT)  cm2/s. (3)  Subsequent studies have found that the D given by Eq. (3) is valid for the diffusion of H+, the dominant charge state of H in Si at elevated temperature [22], for situations where H does not interact with other defects and where relatively immobile H2 molecules are not formed [11].  In many experimental situations, the diffusion of H is limited by trapping, and an effective diffusion constant is found that is typically two to three orders of magnitude smaller than Eq. (3) would predict [11].  Therefore, it becomes important to obtain data on the indiffusion depth of H that are valid for specific hydrogenation conditions.  A compilation of results for the diffusivity of H in Si is shown in Fig. 4.  The data in Fig. 3 show that for a 5 min. anneal at 600ºC, the thickness of the hydrogenated layer that results is ~500 µm.  We take this penetration depth to be ≈  tDeff  to obtain a diffusivity of Deff(600ºC) = 0.8 x 10-5 cm2/s.                         Figure 4.  Diffusion constant for H in Si.  The straight-line corresponds to the diffusion constant given in Eq. (3), first determined by Van Wieringen and Warmoltz (♦), [21].  Values of Deff determined in the present paper are shown by open circles at 600 and 700°C and are marked by an arrow.  Data from Seeger et al. (●) [23] and Gorelkinskii and Nevinnyi (not shown) [24] are consistent with Eq. (3) and have been attributed to the diffusivity of isolated H+ [22].  Values for Deff determined in several other studies are also shown.  These values have been attributed to an effective diffusivity for H that is limited by trapping [10]; see Ichimiya & Furuichi (▼) [25], Pearton (∆) [26], Mogro-Campero et al. (x) [27], Newman et al. (◊)[28], Zundel & Weber (□) [29], and John on and Herring (+,∇) [30,31]. 1878 1881 18840.0000.0010.0020.003 (a)PtH0500 µm350 µm163 µm0 µm6000C (5 min)  AbsorbanceFrequency (cm-1)0 100 200 300 400 500012345 (b)PtH areal density  (1011cm-2)thickness removed (µm)  s   Similarly, results for the sample annealed at 700ºC give a diffusivity of Deff(700ºC) = 1.8 x 10-5 cm2/s.  These values are plotted in Fig. 4 and show that the effective diffusion constant in our experiments is roughly within a factor of two of the value extrapolated from the results of Van Wieringen and Warmoltz [21].  This is a surprising result, given that one might expect the H penetration depth to be limited by trapping in our experiments and to be greatly reduced from the value predicted from Eq. (3).  Nonetheless, the rapid indiffusion observed here is consistent with the effectiveness of typical annealing treatments of a few minutes near 750ºC used to passivate Si solar cells and suggests that such anneals drive H deep into the Si bulk.     4. CONCLUSION   Vibrational spectroscopy, coupled with the use of Pt marker impurities in Si, has been used to probe the H that is introduced into Si by the post-deposition annealing of an H-rich SiNx AR coating.  Our results show unambiguously that H diffuses from the SiNx layer into the Si where it forms complexes with Pt impurities in the Si bulk.  The H concentration has been found to be modest in these first experiments, less than ~1014 cm-3.  The thickness of the hydrogenated layer is consistent with an H diffusion constant that is a factor of only ~2 smaller than an extrapolation of the classic results of Van Wieringen and Warmoltz [21] would predict.     5. ACKNOWLEDGMENTS   We thank Mark Rosenblum for his assistance with our experiments.   Work performed at Lehigh University was supported by NREL Contract No. AAT-1-31606-04 and NSF Grant No. DMR-0108914.   REFERENCES  [1] C. H. Seager, D. S. Ginley, and J. D. Zook, Appl. Phys. Lett. 36, 831 (1980). [2] J.I. Hanoka, C.H. Seager, D.J. Sharp, and J.K.G. Panitz, Appl. Phys. Lett. 42, 618 (1983). [3]  B.L. Sopori, X. Den, J.P. Benner, A. Rohatgi, P. Sana, S.K. Estreicher, Y.K. Park, and M.A. Roberson, Solar Energy Materials and Solar Cells 41/42, 159 (1996). [4] R. Lüdemann, Mater. Sci. Eng. B 58, 86 (1999).  [5] A. Rohatgi, P. Doshi, J. Moschner, T. Lauinger, A.G. Aberle, and D.S. Ruby, IEEE Trans. Electron. Dev. 47, 987 (2000). [6] A. G. Aberle, Sol. Energy Mater. Sol. 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Goodwin, A.M. Gabor, F. Jiang, M. Stavola, and H.A. Atwater, Thin Solid Films, to be published. [15] A. Fadini and F.-M. Schnepel, Vibrational Spectroscopy (Wiley, New York, 1989). p. 41. [16] The integrated absorption coefficient is proportional to the concentration of absorbing centers.  However, the absorbance, A, determined by experiment, is proportional to the product of the absorption coefficient times the thickness of the absorbing layer.  The integrated absorbance, therefore, is proportional to the concentration of centers times the thickness of the absorbing layer, that is, to the areal density. [17] If H2 molecules were introduced into the Si, they would not be easily seen by IR spectroscopy because of the weakness of their vibrational absorption.  We have annealed a few samples at 200ºC where the H2 molecule would be dissociated and give rise to impurity-H complexes.  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Radiat. Isotopes 19, 573 (1968). [26] S. J. Pearton, J. Electron. Mater. 14a, 737 (1985). [27] A. Mogro-Campero, R. P. Love, R. Schubert, J. Electrochem. Soc. 132, 2006 (1985). [28] R. C. Newman, J. H. Tucker, A. R. Brown, and S. A. McQuaid, J. Appl. Phys. 70, 3061 (1991). [29] T. Zundel and J. Weber, Phys. Rev. B 46, 2071 (1992). [30] N. M. Johnson and C. Herring, Phys. Rev. B 46, 15554 (1992). [31] N. M. Johnson and C. Herring, Mater. Sci. Forum 143-147, 867 (1994).  

Hydrogenation of Si from SiN(x):H Films: How Much Hydrogen Is Really in the Si?

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Hydrogenation of Si from SiN(x):H Films: How Much Hydrogen Is Really in the Si?

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