The effect of post-hydrogenation on the equilibrium and metastable properties of hydrogenated amorphous silicon

Journal of Non-Crystalline Solids 164-166(1993) 281-284 North-Holland

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The effect of post-hydrogenation on the equilibrium and metastable properties of hydrogenated amorphous silicon N. H. Nickel and W. B. Jackson Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, California 94304, USA Hydrogenated amorphous silicon films were exposed to monatomic deuterium at 350°C up to 8 hours. The defect densities in the annealed state, after illumination and after deuteration were determined using CPM measurements following each exposure sequence. We find that an increase of the concentration of Si-H bonds by as much as 3 × 102icm-3 changes neither the defect density, the weak Si-Si bond density nor the defect metastability. This suggests that the w e a k SiSi bond density as well as the dangling bond density is determined by equilibration with strong Si-Si bonds through the interchange of H. The implications of these results for H bonding are discussed. 1. I N T R O D U C T I O N Hydrogenated amorphous silicon (a-Si:H) contains about 8-10 at.% hydrogen atoms most of which are bonded to silicon. The low defect densities found in a-Si:H are due to H passivation of Si dangling bonds. However, a serious limiting factor for device applications is metastability. The underlying physical mechanism is believed to be hydrogen migration [1], because H diffuses relatively rapidly through the amorphous network at low temperatures [2], and illumination [3] and doping [4] which cause defects also enhance H migration. Higher deposition temperatures have been suggested as an approach to create more stable a-Si:H films. The increased stability is attributed to a reduction in the H content [5]. An improved thermal stability of a-Si:H films deposited in a remote-hydrogen-plasma reactor at high temperatures was attributed to a change in H transport and bonding configuration since the H concentration was maintained at 10 at.% [6]. While the effects of hydrogen introduced during film growth on the equilibrium defect density, metastability and the Si-Si bond disorder in a-Si:H have been widely studied, little is known about the influence of additional hydrogen introduced from a remote plasma after the sample preparation. In this paper we Elsevier Science Publishers B.V.

examined the effect of post-deuteration on the equilibrium and metastable properties of hydrogenated amorphous silicon. Despite changing the total H + D concentration by as much as 3 X 1021cm-3, we found very few changes in any of the properties of a-Si:H. The results will be discussed in terms of current models for hydrogen bonding in a-Si:H. 2. E X P E R I M E N T Hydrogenated amorphous silicon films were post-deuterated in an optically isolated remote D plasma. The native oxide layer of the a-Si:H films was removed in dilute H F prior to the deuteration to avoid a barrier for the deuterium incorporation. Two sets of a-Si:H films were deuterated. The samples of the first set were deuterated through a sequence of 1 hour exposures at TD =350°C. The concentration profiles of hydrogen and deuterium were measured simultaneously by secondary ion mass spectrometry (SIMS). Before and after each deuteration the defect density N D in the annealed state A and after an intense exposure to white light (5 W cm-2) for 15.5 hrs was measured, using the constant photocurrent method (CPM). The samples of the second set were deuterated at TD=200°C for 30min and the concentration dependence of the diffusion ac-

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tivation energy was determined in an evolution experiment, 3. R E S U L T S A N D D I S C U S S I O N The exposure of a-Si:H to monatomic deuterium increases the total hydrogen and deuterium concentration from 5 × 1021cm-3 to 8 X 102icm-3 [7]. Raman back-scattering measurements revealed that the indiffused D is bonded to Si atoms and that the Si-D bond density increases consistent with SIMS results [7]. One would expect that the increase of the Si-D bond density has a direct influence on the optical band gap, the defect density, the weak Si-Si bond density and the metastability ofa-Si:H, The incorporation of H in amorphous silicon is known to increase the optical gap. From optical transmission measurements the Taucgap of a-Si:H films was determined before and after an exposure to monatomic deuterium for 8 hours. The increase of the H + D concentration by 3 X 102icm-3 increased the Tauc-gap by only about 4meV. Similar small changes of the optical gap for high H concentrations have also been observed by ellipsometry studies [8].

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Fig. i shows CPM spectra measured in the annealed state and after intense illumination with white light for 15.5 hrs. The solid curves represent the unexposed sample. The squares and crosses were obtained after 3 and 6 hrs post-deuteration at 350°C, respectively. The CPM spectra are normalized to the saturation value at 2eV where the curves are approximately flat. The normalized spectra of the post-deuterated samples were then adjusted to the band tail region of the unexposed film, since the increase of the Tauc-gap due to an increase of the H concentration is small and within the error bars of the CPM spectra. Thus, the CPM signal of the post-deuterated samples increased by a factor of 2 at hv _ 2eV presumably due to a change in the surface band bending of the film. This interpretation is consistent with the variation of the Fermi level with tD [7]. The interesting result of Fig. 1, however, is that in both the annealed and the light soaked states, neither the Urbach-slope nor the defect region (hv = 0.8 to 1.3eV) changes with an increase of the total H + D concentration. This indicates that the additional D neither increases the defect density in state A,

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N.H. Nickel, W.B. Jackson / Effect of post-hydrogenation passivates preexisting Si dangling bonds nor decreases the weak bond density. Light soaking increases the defect density up to a factor of 3 indicating that an increase of the total H + D concentration does not alter the stability of the amorphous network, Deuteration yields a total H + D concentration that exceeds the concentration of dan . gling bonds ( - 1017cm -3) by 5 orders of magnitude. The incorporation of D as interstitial molecular D2 can be ruled out by R a m a n measurements and nuclear magnetic resonance measurements performed on annealed samples and single crystal silicon [9]. Therefore, the in-diffused additional D increases the Sill bond density by 3 × 1021cm -3. Hence, the added D atoms are bonded in locations where dangling bonds exist neither before nor after deuteration. A simple trap filling model cannot account for these results. With increasing H content, the trap density should decrease the defect density. Nor are the results consistent with preferential bonding in weak Si-Si bonds, One would expect that the exposure to monatomic D at 350°C raises the hydrogen chemical potential, /ZH such that the additional in-diffused deuterium changes the SiSi bond distribution by preferentially breaking weak bonds resulting in a decrease in the weak Si-Si bond concentration (dotted line in Fig. 2). This should decrease the Urbach-slope because the added D concentration exceeds the weak Si bond density by a factor of 30. This decrease is not observed. The weak and dangling bond densities remain essentially unchanged (Fig. 1) indicating that the net el'fect of D bonding is an overall reduction of all Si-Si bonds, irrespective of their energy, to SiD bonds (dashed line). Because single occupancy of weak Si-Si bonds should give rise to midgap states,/z H cannot move upwards with increasing H . / z H a p p e a r s to be independent of the total H + D concentration, This is corroborated through measurement of the H bonding energies obtained by determination of the diffusion activation energy for various H concentrations employing a modified evolution experiment. A series of lin-

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density of states distribution. Strong Si-H bonds are located below the hydrogen chemical potential, /zH, whereas the strong and weak Si-Si bonds are above/zH. The dotted line indicates a change of the H density of states d u e t o a n i n c r e a s e o f / z H t o / z , H d e c r e a s " ing the weak bond density and resulting in a steeper Urbach tail. The dashed-dotted line shows the change in the density of states if /ZH is pinned and all Si-Si bonds are removed with equal probability. ear temperature increases and decreases was performed while the evolution of H2, D2 and HD was measured. For each t e m p e r a t u r e cycle the maximum temperature, Trnax was increased by about 30°C. The activation energy of the evolution reflects EA of the diffusion for a decreasing H concentration [10]. In Fig. 3 EA is plotted as a function of the peak temperature. At low Tmax, EA reflecting the energy between/z H and the transport level remains between 1.5 and 2eV independent of concentration. At 360°C EA increases abruptly to 2.5eV due to the decreasing H concentration. Only the deeply bound H with EA = 2.5eV characteristic of strongly bound H remains. Therefore, diffusion energies less than 2.5eV are not determined by the release of H from a

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N.H. Nickel, W.B. Jackson / Effect of post-hydrogenation

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Tmax (°C) Figure 3: Activation energy of the HD evolution as a function of the peak temperature, single dangling bond but rather from higher energy configurations which include H2* and larger H clusters, Our results strongly support the idea of a negative U system for H and D. In a negative U system singly occupied sites reside above/z H and doubly occupied sites are located below PH. Hence, an increase of the H content will preferentially increase the concentration of doubly occupied sites and therefore not increase the number of dangling bonds. Since the additional H does not decrease the concentration of weak Si-Si bonds, we suggest that the density of weak bonds is determined by equilibration with strong Si-Si bonds through the interaction of H. The effect on the hydrogen density of states distribution is shown by the dashed-dotted line in Fig. 2. H is either deeply bound to Si atoms passivating preexisting dangling bonds (EA = 2.5eV) or it is accommodated in clusters ( E A = 1.4-1.7eV). Post-deuteration increases the Si-H bond density without changing the defect density or the weak bond density indicating that either existing clusters grow or new clusters nucleate in response to the added D. These Clusters form at concentrations above 1020cm-3 pinning/~H. The structure of these clusters is assumed to be similar to the platelet structure appearing in hydrogenated c-Si. The interior Si-Si bonds are broken by H

and relax. Around the perimeter Si-Si bonds are strained due to the platelet. Mobile D atoms from the plasma preferentially break the strained bonds near the perimeter causing the next neighbor strong Si-Si bond to become strained. Hence, the number of strained Si-Si bonds remains nearly the same, increasing very slowly compared to the number of H atoms added. In summary, we have shown that an increase of the Si-H bond density by up to 3)< 1021cm-3 neither changes the annealed defect density, the weak bond density nor the metastability. Our results suggest that the density of weak Si-Si bonds is determined by equilibration between strong Si-Si bonds and weak bonds. The results support the idea of a negative U system for H and are consistent with the clustering model [11]. One of the authors, NHN, is pleased to acknowledge partial support from the Alexander yon Humboldt Foundation, Federal Republic of Germany. REFERENCES 1. H. Dersch, J. Stuke, and J. Beichler, Appl. Phys. Lett. 38 (1980)456. 2. D.E. Carlson, and C.W. Magee, Appl. Phys. Lett. 33 (1978) 81. 3. P.V. Santos, N.M. Johnson, and R.A. Street, Phys. Rev. Lett. 67 (1991) 2686. 4. W. Beyer, J. Herion, and H. Wagner, J. Non-cryst. Sol. l14 (1989) 217. 5. A.H. Mahan, and M. Vanecek, AIP Proceedings Vol. 234, Denver 1991, p. 195. 6. N.M. Johnson, C.E. Nebel, P.V. Santos, W.B. Jackson, R.A. Street, K.S. Stevens, and J. Walker, Appl. Phys. Lett. 59 (1991) 1443. 7. N.H. Nickel, W.B. Jackson, and C.C. Tsai, Mat. Res. Soc. Proc. Amorphous Silicon Technology (San Francisco 1993) in print. 8. R.W. Collins, private communications.. 9. J.B. Boyce, N.M. Johnson, S.E. Ready, and J. Walker, Phys. Rev. B 46(1992) 4308. 10. W.B. Jackson unpublished data. 11. W.B. Jackson, P.V. Santos, and C.C. Tsai, Phys. Rev. B 47 (1993) 9993.