Diffusion of lithium and hydrogen in hydrogenated amorphous silicon

Journal of Non-Crystalline Solids 164-166 (1993) 289-292 North-Holland

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Diffusion of Lithium and Hydrogen in Hydrogenated Amorphous Silicon W. Beyer and U. Zastrow Institut fiir Schicht- mad Ionenteclmik (ISI-PV), Forschungszentrum Jfilich, D-52425 Jfilich, Germany

Both lithium and hydrogen diffusion processes in a-Si:H are found to be trap-limited. The time and concentration dependence of H diffusion is not observed for Li. While hydrogen probably diffuses in a neutral charge state and is trapped by intrinsic (Si bonding) sites, lithium diffuses largely in a positive charge state and is trapped mainly by extrinsic doping-related negatively charged ions or by oxygen.

1. INTRODUCTION In crystalline silicon quite similar diffusion behavior is found for lithium and hydrogen [1,2]. A study of lithium and hydrogen diffusion processes in amorphous silicon, therefore, may serve for a better understanding of impurity diffusion processes in amorphous semiconductors in general. Moreover, the stability and diffusion of lithium and hydrogen in amorphous silicon are of technical iraportance: the stability of lithium is decisive for application as an efficient interstitial donor [3]; that of hydrogen influences widely a-Si:H deposition processes, annealing effects in a-Si:H films and thermal stability of a-Si:H based devices, Here, we report results of a study of both lithium and hydrogen diffusion in doped and undoped a-Si:H films prepared under various deposition conditions. SIMS profiling of implanted lithium and deuterium as well as of hydrogen and deuterium in layered structures of hydrogenated and deuterated material at various annealing stages was used for obtaining the respective diffusion coefficients.

2. EXPERIMENTAL The majority of films investigated was prepared by plasma enhanced chemical vapor deposition, Silane ( S i l l4) , silane-dibonme (B H6) or silanephosphine (PH3)-mixtures were d2e~mposed in a capacitive reactor at an rf frequency of 13.5 MHz. Typical deposition conditions involved a substrate temperature T s of 200-300"C, a pressure of about

0.5 mbar, a silane flow of 2-5 sccm and an rf power of 5-10W resulting in deposition rates of 1-5 ]~/s. Film thickness was 0.5-3 lain. Part of the films was prepared in a (conventional) deposition system of 10-5 mbar base pressure while the other part was deposited in a system built from UHV eomponents with a typical base pressure < 10-8 mbar. Some samples were also prepared by low-pressure CVD. Lithium and deuterium ions were implanted at an energy of 30-120 keV using a mass separator. Fractions of the samples were annealed in vacuum at various temperatures and time intervals. For SIMS profiling an oxygen (0 2 +) sputtering beam at almost normal incidence with'an energy of 3 keV for Li profiling and 6-9 keV for deuterium profiling was used. A comparison with the implantation dose and (in the case of hydrogen) with hydrogen evolution data allowed an absolute calibration of SIMS signals for lithium and hydrogen (deuterium) concentrations.

3. RESULTS AND DISCUSSION The temperature dependence of lithium and hydrogen diffusion coefficients in a-Si:H films prepared in conventional vacuum shows great similarities (Fig. 1). In both cases similar diffusion activation energies for undoped samples and an enhanced diffusion for doped material are found. The diffusion energies for Li and H of 1.15 - 1.7 eV in a-Si:H are considerably higher than in crystalline Si where energies near 0.5 eV have been reported [ 1,2]. Likewise, the absolute values of Li

0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.

290

IV. Beyer, U. Zastrow / Diffusion of lithium and hydrogen in hydrogenated amorphous silicon

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and H diffusion coefficients at e.g. 300°C for undoped material are about 6-8 orders of magnitude below the respective (extrapolated) crystalline data. While the high diffusion coefficients of Li and H in c-Si have been associated with an interstitial diffusion process, we are dealing in the amorphous material apparently with a trap-limited interstitial diffusion for beth lithium and hydrogen. Beside these similarities between lithium and hydrogen diffusion, there are also significant differences. One is the time dependence of the diffusion coefficient which is well pronounced in case of hydrogen diffusion [4] but which we do not find for lithium diffusion in undoped a-S:H. Another difference shows up in the concentration dependence. While the Li diffusion coefficient is found to be essentially independent of the (implanted) lithium concentration, the hydrogen diffusion coefficient drops considerably when the hydrogen concentration is reduced. E.g. for a temperature of 400°C the hydrogen diffusion coeffieient decreases by about two orders of magnitude when the hydrogen concentration is lowered by a factor of 10. Fig. 2 shows the effect of hydrogen concentration both on the diffusion prefactor D O and on the diffusion energy E D. Both were deter-

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291

W. Beyer, U. Zastrow / Diffusion of lithium and hydrogen in hydrogenated amorphous silicon

mined for a fixed diffusion length 1= (D t) 1/2 = 300 - 1500/~. Although the data scatter appreciably due to the limited number of data points and due to extrapolation over a wide 1/T interval, a clear tendency is observed for both E D and D O to increase with falling hydrogen concentration CH. Note that a good agreement with previously published data by Shinar et al. [5] is obtained when we calculate from our data the diffusion coefficient at 325"C. We explain the concentration dependence of hydrogen diffusion in terms of an energy band model [6] by a wide energy distribution of Si-H states. As the CH decreases, the chemical potential lawill drop resulting in a higher diffusion energy, The concomitant increase of the diffusion prefactor may be due to a compensation law similar to that found for hydrogen desorption [7]. Another major difference between Li and hydrogen diffusion relates to the doping effect on the diffusion coefficient. In the case of hydrogen diffusion, the diffusion coefficient is clearly found to depend on the position of the Fermi level. This behavior was established e.g. by comparing the hydrogen diffusion in both singly doped and compensated a-Si:H films: the Fermi energy, not the presence of boron or phosphorus alone, determined the absolute value and the activation energy of hydrogen diffusion [8,9]. In the case of lithium diffusion, i

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the results of Fig.lb show already that this latter mechanism cannot be active. Since lithium is an efficient n-type dopant, the Fermi energy of Liimplanted undoped and P-doped material should be essentially the same, but the diffusion coefficient turns out to be quite different. Clearly, we find that the Li diffusion coefficient and Li solubility depends critically on the presence of various impurities. Besides boron impurities which lithium is found to decorate [10], oxygen influences strongly lithium solubility and diffusion. The effect of oxygen contamination on Li diffusion is demonstrated in Fig. 3. Two plasma deposited a-Si:H films, one deposited in a system built by UHV components, the other in a system with conventional vacuum, were implanted with a Li dose of 1015era-2 at an energy of 60 keV. By SIMS measurements, the oxygen concentration is estimated to about 1019/cm3 and 3x1020/cm3, respectively. The result of annealing at various temperatures (annealing time: 15 rain) is that for the material of higher oxygen content, Li penetrates the unimplanted material at a higher level and diffuses with a lower diffusion coefficient than in low oxygen material. Quantitatively, a factor of 30 higher oxygen content results in an increase of Li penetration level by about the same factor and decreases the Li diffusion coefficient (300°C) by

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292

IV. Beyer, U. Zastrow / Diffusion of lithium and hydrogen in hydrogenated amorphous silicon

about a factor of 20. These latter results can be explained in terms of oxygen providing traps for Li diffusion similar to what has been observed in crystalline Si [11,12]. For a high trap concentration N 0, the diffusion coefficient is reduced to D = D* C/N0 if D* is the diffusion coefficient without trapping and C is the equilibrium constant of the trapping reaction. Using the crystalline Si data for C by Poll [12] and the oxygen concentration from our SIMS data, we can calculate D* for our und o # material and arrive at a similar diffusion coefficient as in boron- and phosphorus-doped material. Thus the Li diffusion coefficient in a-Si:H would come out to be rather dopant independent. The mechanism of oxygen in causing traps for diffusing Li, however, is not clear. In crystalline Si, interstitial oxygen is considered to react with Li + to form L i e + complexes [ 11,12]. On the other hand, Pierz el al.[13] found from infrared absorption measurements that Li reacts rather with SiO2-type bonded oxygen than with interstitial oxygen. Note, however, that from our results in Fig. 3 only about one trap is formed for about ten oxygen atoms incorporated; thus the data by Pierz cannot exclude some interaction of Li with interstitial oxygen. In general, precipitation of Li near incorporated oxygen or trapping of Li + at negatively charged oxygen is also conceivable. No influence of oxygen incorporation on hydrogen diffusion has been observed so far. Likewise, the correspondence of diffusion coefficients determined by H/D interdiffusion and by H outdiffusion experiments for both doped and undoped a-Si:H [8] suggests that hydrogen is trapped in a neutral charge state. In contrast, in particular boron-doped a-Si:H shows an enhanced Li indiffusion and a retarded out-diffusion in agreement with the diffusion of positively charged Li [10]. Furthermore, the n-type interstitial doping effect of Li [3] is clear evidence for the presence of positive Li ions. Thus, while the trap depth for hydrogen diffusion is essentially given by the binding energy of hydrogen to silicon atoms, the trap depth of Li diffusion is likely given by the Coulomb attraction of positively charged Li ions to negatively charged impurity atoms, to negatively charged Si dangling bond (D-) states and/or to oxygen. A dependence of the Li trapping energy on Li concerttration, therefore, cannot necessarily be expected.

4. CONCLUSIONS The results show that H and Li diffusion processes are trap-limited with different trapping sites. While hydrogen is trapped in bonds to silicon, lithium is trapped by an ionic interaction with negatively charged ions and/or with oxygen. Differences result in the time, concentration and doping dependence of diffusion.

5. ACKNOWLEDGEMENTS The authors wish to thank M. Gebauer for the ion implantations and R. von de Berg, D. Drescher, F. Pennartz, R. Schmitz and H. Siekmann for technical assistance. The work was supported by the Bundesministerium fiir Forschung und Technologie.

REFERENCES 1. S.J. Pearton, J.W. Corbett and J.T. Borenstein, Physica B 170 (1991) 85. 2. B. Pratt and F. Friedman, J. Appl. Phys. 37 (1966) 1893. 3. W. Beyer and R. Fischer, Appl. Phys. Lett. 31 (1977) 850. 4. R.A. Street, C.C. Tsai, J. Kakalios, and W.B. Jackson, Philos. Mag. 56 (1987) 305. 5. R. Shinar, H. Jia, X.L. Wu and J. Shinar, MRS Symposium Proceedings 258(1992) 419. 6. W. Beyer, H.C. Weller and U. Zastrow, J. Non- Cryst. Solids 137-138 (1991) 37. 7. Y. Khait, R. Weil, R. Besermann, W. Beyer, H. Wagner, Phys. Rev.B 42 (1990) 9000. 8. W. Beyer, J. Herion, H. Moll and H. Wagner, MRS Symposium Proceedings 118(1988) 291. 9. W. Beyer, J. Herion and H. Wagner, J. NonCryst. Solids 114 (1989) 217. 10. W. Beyer, J. Herion and U.Zastrow, J. NonCryst. Solids 137-138 (1991) 111. 11. P. Siffert and A. Coche, in: Semiconductor Detectors, G. Bertolini and A. Coche, eds., North-Holland, Amsterdam, 1968, p. 27. 12. E.M. Pell, J. Appl. Phys. 32 (1961) 1048. 13. K. Pierz, M. Stutzanann, S. Zollner, W. Beyer and C. Billerty, J. Non-Cryst. Solids 137-138 (1991) 107.