The presence of silane gas in plasma-deposited hydrogenated amorphous silicon

Surface Science 222 (1989) L831-L836 North-Holland, Amsterdam

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S U R F A C E SCIENCE LETTERS T H E P R E S E N C E O F SILANE GAS IN P L A S M A - D E P O S I T E D H Y D R O G E N A T E D A M O R P H O U S SILICON T. BEKKAY, R. IZQUIERDO, M. ST-DENIS, E. SACHER and A. YELON Groupe des Couches Minces and Ddpartement de Gdnie Physique, Ecole Polytechnique de Montrdal, CP 6079, succursale A, Montrdal, Qudbec, Canada H3C 3A 7 Received 7 July 1989; accepted for publication 14 August 1989

We report here the finding that silane gas may be trapped in amorphous hydrogenated silicon under some circumstances. Its presence has been detected by X-ray photoelectron and infrared spectroscopies.

We have found that, under certain plasma deposition conditions, silane gas (SiH4) is trapped in the deposited hydrogenated amorphous silicon (a-Si : H). The presence of Sill 4 in a-Si : H has not been reported previously. Its presence was detected in the present study by both X-ray photoelectron spectroscopy (XPS) and by infrared (IR) spectroscopy. This is not the first time gases have been shown to be present in a-Si : H. It is well known [1] that Ar is implanted and trapped during Ar ion sputtering. If films are plasma deposited from reactant gases diluted with Ar, the same result is observed [2]. Molecular hydrogen has been detected in microvoids by low temperature 1H N M R T 1 measurements of a characteristic Pake doublet [3], calorimetric measurements of the o r t h o - p a r a transition [4,5] and IR measurements of characteristic vibrational and vibrational-rotational transitions in the 3900-5400 cm-1 range [6]. Similarly, SiF4 has been detected in a-Si : F and a-Si : (H, F). Ir spectra [7,8] exhibit peaks at 1010 and 828 cm -1 which are characteristic of SiF4 gas [9]. Silicon 2p XPS peaks also show the presence of SiF4 [10]. Because the same principles are used later in this paper, the XPS data will be covered in some detail. Consider an atom bonded tetrahedrally to four other identical atoms, such as Si in a crystalline matrix (the argument is the same for amorphous silicon which has a coordination number of 3.8 [11], rather than 4). Electronic interactions cause a uniform distribution of electrons among all the atoms so that an electron photoemitted from any atom should experience the same attractive-repulsive interactions with the matrix as an electron photoemitted 0039-6028/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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7~ Bekkav et al. / Silane gas in amorpho~ hydrogenated silicon

from the same level of any other atom. Thus, all electrons from a given level appear at the same binding energy, subject to lifetime, structural and instrumental broadening. If an atom to which the Si was found is replaced by an atom of a different electronegativity, for example, F, then the electron density at that Si atom changes (as, to a lesser extent, does the electron density at the adjacent Si atoms). This manifests itself in the photoelectron spectrum as a slight broadening and an increase in binding energy of the electron photoemitted from the Si bound to the F, due to the increased positive charge at that Si atom. This leads to a composite spectrum, one component being due to the unsubstituted Si and the other to the Si bonded to F; the areas give the relative concentrations. In the case of Si bound to additional F atoms, each new F causes an additional increase in binding energy. Gruntz and coworkers [10] found that each F substitution on a given Si caused a shift of the Si 2p level by 1.15 eV to higher energy. In this way, the Si 2p XPS envelope of a-Si : F was deconvoluted to show spectral shifts of 1.15 eV (Si-F), 2.30 eV (Si-F2), 3.45 eV ( S i - ~ ) and 4.60 eV; the latter is consistent with the shift expected for SiF4. Here we shall use the same principle to separate the components of the envelope of a-Si : H where it was previously shown [12] that each H substitution caused a shift of the Si 2p level by 0.335 eV to higher energy, the lower shift in the present case reflecting the lower electronegativity of H, compared to F (Allred-Rochow electronegativity values [13] for H and F are 2.20 and 4.10, respectively, while that for Si is 1.74). A series of samples was prepared in a plasma deposition unit described elsewhere [14], keeping all parameters except the RF power constant. It operates at 450 kHz. In the present study, pure Sill 4 was used as reactant, at a flow rate of 16 sccm and a pressure of 0.2 Torr. Samples were deposited onto c-Si held at 300 ° C, using power dissipation settings between 5 W (the lowest at which the plasma could be sustained) and 50 W. Such depositions, at powers of 10-50 W, produce high quality a - S i : H in our deposition systems [14,15]. Sample thicknesses were all 0.5 ~m. These samples were unavoidably exposed to atmosphere during the few minutes necessary to transfer them from the plasma deposition chamber to the XPS system. This contaminant was removed by exposing the sample to a few minutes of sputtering, using argon ions at 3 keV, after which the O l s XPS peak could not be detected. These conditions do not give rise to preferential sputtering, as shown by sputtering studies [16,17] and confirmed by our XPS and UPS measurements. X-ray photoelectron spectra were obtained on a Vacuum Generators ESCALAB 3 Mk II, using non-monochromated Mg K a radiation at 1253.6 eV, with a measured resolution of 0.7 eV. Curve fitting was carried out using programs developed in our laboratory, subsequent to background removal by Shirley's method [18]. I R spectroscopy was carried out on a B O M E M DA3 F T I R , using the programs supplied by the manufacturer.

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As mentioned earlier, Ley and co-workers [12] found that the various S i - H , (n = 1-3) structures present in a-Si : H manifest themselves as components of the Si 2p XPS peaks. They showed that each replacement of Si in the structure Si-(Sia) by hydrogen causes a binding energy shift of the Si2p doublet by 0.335 + 0.010 eV to higher binding energy. In the present case, we used the Si2s peak, for which our instrument has sufficient sensitivity; its use avoids any difficulties in 2pl/2-2p3/2 spin-orbit peak separation. Here, we found chemical shifts of 0.32 e V / H . A typical example is shown in fig. 1, for a-Si : H deposited at a power setting of 30 W. For all samples deposited at 10 W and higher, the XPS spectra could be analyzed completely, assuming peaks due to S i - H 1, S i - H 2 and S i - H 3, shifted by AE = 0.32, 0.64 and 0.96 eV, respectively, from the peak due to Si bonded to four Si neighbors. Their full width at half maximum (FWHM) values increased progressively from 2.00 eV for Si to 2.77 eV for Si-H3; such values are typical for data obtained on our instrument. These results are shown in fig. 2. However, for samples deposited at 5 W, an additional peak at AE = 1.28 eV was needed to fit the data. As shown in fig. 3, the higher asymmetry on the high energy side of the peak is completely accounted for by the additional component at AE = 1.28 eV, having a F W H M of 2.87 eV. As the films contain no trace of F or any other atom capable of producing such a large shift, the only explanation available is that Sill a is trapped in the film; its concentration at the surface, 11% of all the S i - H , (n --- 1-4) peaks, is far greater than that reported for H 2 trapped in a - S i : H [6], although SiF4 is trapped at similar concentrations in a-Si : F [7].

T. Bekkay et al. / Silane gas in amorphous hydrogenated silicon

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As with the trapped H 2 and SiF 4, I R spectroscopy m a y be used to demonstrate the presence of S i l l 4. Fig. 4 shows I R spectra in the S i - H , stretching region for samples deposited at 5 and 10 W. The S i - H , stretching peaks for n = 1 - 3 are seen in the c o m p o s i t e e n v e l o p e lying in the range 1 9 2 5 - 2 1 2 5 c m -1 [18]. Their peak m a x i m a are found near 1990 c m - t ( S i - H ) , 2040 c m -a ( S i - H 2 ) and 2090 c m a ( S i - H 3 ) . It is clear f r o m the figure that the sample deposited at 10 W has a significant S i - H 3 concentration and the I

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sample deposited at 5 W, a higher S i - H / S i - H z ratio. For the sample deposited at 10 W, the small peaks at 1880 cm -1 and below are due to deformation motions [19] of carbon-containing surface contaminants commonly found on a-Si : H [20] (a suggestion of such contamination is also seen for the 5 W sample). The peaks found at 2225 cm-1 and above are due to alkynyl stretching vibrations, indicating that the surface contaminants are partly composed of acetylenic hydrocarbons [19]. The 5 W sample, however, shows another peak, at 2189 cm -1, characteristic of Sill 4 [9,18]. This peak is absent in all spectra obtained at power settings greater than 5 W. Other chemical groups which might have been the source of this peak (nitrile, isocyanate) were shown by both IR and XPS to be absent. The concentrations of Si-H, Si-H 2 and Si-H 3 determined by IR are slightly lower than those determined by XPS. The Sill4 concentration, determined from IR peak areas relative to those of all the S i - H , (n = 1-4) peaks, is 2%, significantly lower than that obtained from fig. 2. These differences reflect the fact that IR is obtained in transmission while XPS probes the first 100 ,~ of the surface [21]. Clearly, the Sill 4 is trapped in higher concentration near the surface, consistent with our previous finding [22] that the surface is more porous than the bulk. It has been known for some time that films deposited at high RF power exhibit low photo-to-dark conductivity ratios and it is generally agreed [23] that this is due to columnar structure, present in films deposited at high rates. However, there have also been reports [24,25] that films deposited at very low RF power (5 W) show similar properties. It has been suggested [24] that this is

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T. Bekkay et al. / Silane gas in amorphous hydrogenated silicon

d u e to c r y s t a l l i z a t i o n in t h e s e films. It m a y b e t h a t t h e p r e s e n c e o f S i l l 4, r e p o r t e d h e r e , a l s o a f f e c t s film p r o p e r t i e s . T h e a u t h o r s w i s h to t h a n k t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f C a n a d a a n d t h e F o n d s p o u r la F o r m a t i o n d e C h e r c h e u r s et l ' A i d e ~, la R e c h e r c h e d u Q u 6 b e c f o r f u n d i n g ,

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