Growth of polycrystalline silicon thin films on glass

Growth of polycrystalline silicon thin films on glass

ELSEVIER Thin Solid Films 296 (1997) 2-6 Growth of polycrystalline silicon thin films on glass Tetsuya Akasaka, Dyane He, Yusuke Miyamoto, Nobuaki K...

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ELSEVIER

Thin Solid Films 296 (1997) 2-6

Growth of polycrystalline silicon thin films on glass Tetsuya Akasaka, Dyane He, Yusuke Miyamoto, Nobuaki Kitazawa, Isamu Shimizu Tokyo Institute of Technology, The Graduate School, 4259 Nagatsuta Midori-ku, Yokohama-city226, Japan

Abstract Polycrystalline silicon (poly-Si) thin films were grown by plasma-enhance CVD from fluorinated precursors on glass substrate by twostep growth (TSG), i.e. ( 1) growth of seeds on gIass and (2) epitaxial-like growth on the seeds. The seed crystal with high crystallinity was grown by layer-by-layer technique. An optimum condition to promote epitaxial-like growth on the seeds having a certain texture was also found by varying the mixing ration of source gases, SiF4 and Ha, under the real time observation with a spectroscopic ellipsometer (SE). Smooth interface on atomic scale between the seeds and the Iayer grown epitaxial-Iike made by TSG was established by measurements with SE and SIMS. High spectral response obtained in a fully poly-Si p-i-n diode proves that high quality poly-Si is able to be grown on glass by TSG. © 1997 Elsevier Science S.A. Keywords: Polycrystalline silicon; Spectroscopic ellipsometry; Plasma-enhanced (PE)-CVD; Atomic hydrogen

1. Introduction Microcrystalline (/xc-) and polycrystalline (poly-) silicon thin films grown on glass substrate are of considerable interest owing to their wide application to electronic devices such as solar cells [ 1], thin film transistors (TFTs) allays for the display devices [ 2] or photosensors. Varieties of techniques have been proposed, so far, namely, laser annealing [3,4], solid phase recrystallization [ 5 ], rapid thermal annealing [ 6 ] and plasma-enhanced chemical vapor deposition (PE-CVD) [7]. High TFT mobility of 350 cm 2 Vs-1 was obtained for poly-Si grown on glass by laser annealing although its grain size was as small as 40 nm diameter [ 8]. In contrast, as far as mass-productions are concerned, PE-CVD has tremendous advantages among them, which have been established in fabrication of hydrogenated amorphous silicon (a-Si:H). In this study, we investigated the growth of poly-Si thin films on glass substrate by PE-CVD. Two-step growth (TSG), ( 1 ) growth of seed crystals on glass and (2) epitaxial-like growth of poly-Si on the seeds, is proposed as a novel technique which enables fabrication of the devices by stacking poly-Si layers on glass.

2. Experimental Fig. 1 shows a schematic representation of the preparation apparatus [9]. SiF4 and gaseous mixture of Ha + He are fed respectively through the inner stainless tube and the outer 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved

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quartz tube. The gaseous mixture, H2 + He, is excited and decomposed by microwave (2.45 GHz) plasma in the cavity, while SiF4 is decomposed at the outlet of the inner tube by weak plasma with the aid of atomic hydrogen [ 10]. Fluorinated radicals such as SiF2H and SiFHa are made by collision of chemically active species in gas phase, depending on the mixing ratio of gases (R: SiFJH2), microwave power (Pw watt), and the location of the substrate in respect to the end of the plasma [ 11,12]. The growing surface is monitored by a spectroscopic ellipsometer (SE: UVISEL Jovin Yvon). Volume fractions of crystal (f~%), amorphous phase (f~%) and void (fv%) together with surface roughness are estimated from the spectra of pseudo-dielectric function, < e2 > measured in a range of hv= (1.5-5.0 eV) using Bruggeman's effective medium approximation (EMA) [ 13]. Thereal time monitoring of the growing surface is performed by tracing < e2> at the photon's energies (hu: 3.4 and 4.2 eV) corresponding to the specific points of crystalline silicon (c-Si) [ 14]. Measurements of Raman scattering, X-ray diffraction (XRD) and TEM were made to evaluate the structure. The concentrations of the remaining F, H, O and P were measured by SIMS (CAMECA IMS-4f). The electrical conductivity in the dark and under light illumination (600 nm wavelength) was measured for the cells with a gap-type geometry using top surface electrodes separated by a 0.2 mm gap. Measurements of Hall effect were performed as well for P-doped poly-Si films using a Hall bar with six terminals fabricated by the conventional photo-lith-

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3. Results and discussion

3.1. Preparation of seeds on glass Si-network made from the fluorinated precursors depends strongly on the chemical species generated in plasma, the substrate temperature (Ts °C) and the surface topology of the substrate. There are two distinctive conditions given the terms of " A " and " B " depending on the mixing ratio, R = SiF4(sccm )/Hz(sccm) [ 11 ]. Fig. 2 shows the ellipsometric trajectories on ( kV',A) plane measured at hv = 3.2 eV in the early stage of growth on SiO2 coated c-Si substrate in "A: R = 9 0 / 5 " and "B: 3 0 / 5 " condition at Ts -- 360 °C. These trajectories were acquired at regular time intervals of 20 s by the thickness of 40 nm. The solid curves illustrate the best fitting trajectories calculated by assuming that the effective dielectric function is altered linearly with the thickness owing to an increase info, which is verified by TEM observation [ 12]. Thefo(40 n m ) = 23% and 61% are estimated for the films grown in " A " and " B " condition from the < e2> spectra. From these values, the fraction of the initial stage,f~ (0) ,fa (O) andfv(0) are predicted by extrapolating the thickness to zero. Amorphous-rich layer offa(0) = 55% with a smooth surface of rather smallfv(0) = 25% is made in " A " condition, while crystalline-rich layer [f~(0) = 30%] with a rough surface of large f v ( 0 ) = 5 5 % are made in " B " . Accordingly, neither " A " nor " B " are deduced to satisfy the requirements for the growth of seed crystals of rather large grains with high crystallinity on glass.

Layer-by-layer (LBL) technique in which the deposition of a very thin layer in " A " condition and the treatment with atomic hydrogen are alternately repeated is adopted to grow the seeds on glass. In Fig. 3, the changes in the < e2> at hu =4.2 eV during the growth of seeds on SiO2 coated c-Si in the optimal condition. Cyclic changes in < e2 > synchronized with the duration of the deposition (Td = 20 s) in " A " condition and the treatment with atomic hydrogen (T~ = 5 s). The < ea > value decreases smoothly during Td owing to the deposition of amorphous-rich layer but rises rapidly during Tt owing to the promotion of grain growth [ 15]. A thin layer showing high crystallinity overfc > 80% with rather smooth surface was consequently grown on glass, which gives a strong support to an idea that LBL offers a way for fabricating crystalline seeds on glass.

3.2. Epitaxial growth of poly-Si on c-Si(lO0) We attempted to compare the structures of films grown on c-Si by varying the flow of S i F 4 at Ts = 360 °C to reveal the 188

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T. Akasaka et al. / Thin Solid Fihns 296 (1997) 2--6

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depending on the structure. To the right of Fig. 4, the depth profiles of IF] for the films, ( a ) - ( d ) , are shown. The IF] in (d) is reduced down to 10 ~7 cm -3, whereas IF] of near 102o cm -3 remained in the top of film (a). In addition, a fairly large amount of [F] is piled up at the interface, corresponding to the dips in the < e2 > profiles at the onset of the growth. We will describe later on the interface. The surface topology of substrate plays an important role as well in making ordered structures because polycrystalline films are grown on c-Si(110) and (111 ) in the same condition [ 16]. The preferential attachment of precursor such as SiFzH on certain specific sites of Si(100) is a likely elucidation for the epitaxial growth similar to that of SIC12 into the follow bridge site in CVD [ 18].

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influences of surface topologies on the surface reaction. The c-Si(100), (110) and (111) used as the substrate were dipped in a HF solution (5%) before loading them into the reactor [16]. Fig. 4 (left part) shows the traces of < e2> at hu = 4.2 eV measured by varying the flow rate from 30 sccm (a) to 90 sccm (d). In the inset, the < Ca> spectra for the films grown are shown. High < e2 > value is kept constant during the growth in the condition " A " as shown in curve (d) except a dip at the onset. Single crystal of highly ordered structure was epitaxially grown in this condition. A sharp Raman TO band with the band-width of narrower than 4 c m - 1 (FWHM) and streak patterns with the Kikuchi-lines observed in RHEED offer a strong support to the epitaxial growth. In the other conditions ( a ) - ( c ) , on the contrary, the < if2> is markedly fallen with increasing the thickness. The slopes of the curves increase rapidly with decreasing flow rates of SiF4, which implies that the epitaxial growth on cSi(100) is markedly hindered by reduction in S i F 4. Poly-Si or/xc-Si:H were grown in ( a ) - ( c ) except for the initial stage. The concentration of fluorine IF cm -3] measured by SIMS provides powerful information for evaluation of the crystallinity since it changes by 5 orders of magnitudes

3.3. Epitaxial-like growth on seeds made by LBL To investigate simultaneously the structure and electric properties for the films grown on the seed crystal, the undoped layers were grown on the P-doped seeds deposited on Crcoated glass by LBL using the separated reactors to avoid the cross-contamination. It is worthwhile noticing that the quality of crystal is obviously degraded in the heavily P-doped films although the specific texture of (220) orientation is maintained. High electric conductivity of 50 S c m - 1 or higher is obtained for the heavily P-doped almost independent of temperature owing to rather low Hall mobility as low as 5-7 cm 2 Vs-1. In the slightly P-doped film, on the contrary, Hall mobility is improved up to 30 cm 2 Vs - i or higher owing to the improvement in its quality [ 17]. The undoped films was grown on the P-doped seeds made by LBL by varying the flow rates of SiF4, i.e. (a) 30 sccm, (b) 50 sccm, (c) 70 sccm and (d) 90 sccm, in a similar manner to the epitaxial growth on c-Si (100) described above. Fig. 5 shows traces of the < ea > at 4.2 eV during the growth in ( a ) - ( d ) . In the inset, the < E2> spectra of the seeds (top) and the undoped layers (bottom) grown by the thickness of about a half micron meter are illustrated. The < ea > values 20 (d) 15

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T. Akasaka et al. / Thin Solid Fihns 296 (1997) 2 - 6

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in both (a), (b) and (c) tend to fall slightly through the maximum but rises in (d) through the minimum. It is deduced from the < e2 > spectra that crystalline phase is dominant in (a), (b) and (c) but amorphous phase occurs on the top of layer (d). Accordingly, the increase in the < e2> observed in the trace (d) is explained in terms of the smoothing of the surface with amorphous phase. On the contrary, the increase in the surface roughness resulting from the grain growth is responsible for the reduction in the traces (a), (b) and (c). Fig. 6 shows the depth profiles of F, H and P measured by SIMS for films (a) and (d), which correspond to the condition " B " and " A " , respectively. The [HI is markedly increased by more than double in the layer (d), comparing with [H] = (2-3) × 10 a° cm -3 of the P-doped seeds. This obvious increase in [H] is responsible for the amorphous phase made preferentially in (d). On the contrary, low level of [H] equivalent to that in the P-doped seeds is kept in the undoped layer made in (a) and (b). IF] of the all undoped films is reduced markedly down to (2-4) × 1019 cm - 3, which are the one-third or one-fifth comparing with that of the Pdoped seed. Of particular interest in considering the texture of crystal is that the specific texture of (220) orientation is grown in (c) on the seeds, while the grains of random orientation are made on glass. These evidences lead us to a conclusion that the growth of crystals with the texture of (220) orientation is preferentially enhanced by the aid of surface topology as the analogous manner to the epitaxial growth in a certain condition such as (c) which corresponds to the condition between " A " and " B " . We consider that atomic hydrogen plays an important role to enhance the growth of grains having the specific texture, similarly to the growth ofpoly-Si by LBL [ 15 ]. Obvious dips of the traces of < e2> are observed at the beginning of growth of the undoped layer either on c-Si (100) or the P-doped seeds. The contaminators such as H, F, O and C are also piled up at the interfaces. To reveal the origin of contaminations piled up at the interface, we grew epitaxially the undoped layer in " A " condition on c-Si(100) substrate after various handling of the growing surface. Fig. 7 illus-

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trates the trace of < e2> at h u = 4 . 2 (top) and the depth profiles of 19F, 1H, 12C and 160 measured by SIMS (bottom). A deep and sharp dip is observed in the trace immediately after opening the shutter [open (1)] to start the growth on c-Si(100). After growing epitaxially for a certain period, the shutter was closed [close (1)] to interrupt the growth for about 5 rain and was opened again [open ( 2 ) ] . No marked changes in the < e2> are observed despite interruption of the growth for a certain period as far as the film is stored in the flow of gases. The small rectangular change in < e2 > during the interruption should be ignored owing to an accidental noise. On the contrary, a large dip equivalent to the initial one is piled up at the interface as well when the growth is started again [open (3)] on the layer after exposing the film to the air. A fairly large amount of contaminators, F, H, C and O are piled up at the interface by exposing the surface to the air. In contrast, no marked changes in SIMS signals are observed at the interface before and after the interruption of the growth as far as the films are stored in the reactor without breaking the vacuum. This fact leads us to an important conclusion that the epitaxial growth is smoothly progressed on the surface stored in the reactor. The epitaxial-like growth is prevented owing to island-like growth arising from the contamination of the surface. It is predicted that the growing surface is passivated with F and H during the growth but is altered to oxides or hydroxides by exposing the surface to air resulting from hydrolysis with water vapor [ 1 ]. 3.4. Fabrication of a PIN diode

Electrical conductivity of the undoped film made by LBL at room temperature is as low as (2-3) × 10 - 8 S cm - i owing

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Acknowledgements

This work was supported in part by NEDO and in part by a Grant-in-Aid for Scientific Research on Priority Areas of " F r e e Radical Science". The authors thank Professor C.R. Wronski and Professor C.M. Fortmann for their kind discussions.

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Poly-Si is epitaxial-like grown on the seeds having a specific texture of (220) orientation. (3) Smooth interface at the contact of poly-Si layer with the seed was made on atomic scale by the epitaxial-like growth, which enables fabrication of the devices consisting of the multi-layers as an analogous manner to the fabrication of a-Si:H devices.

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Fig. 8. Spectral response of a PIN diode fabricated by TSG under light illumination (AM1.5 I00 mW cm-:) The schematic representationof PIN diode is shown in the inset. to the Fermi-level located at the mid-gap [9]. The photoconductivity is about 10 . 6 S c m - 1 under light illumination (3 × 1015 photons cm - z s - 1,600 nm wavelength) which is equivalent to that of high quality a-Si:H. This high photoconductivity is owing to rather low defect density, less than 3 × 1016 cm -3 measured by ESR owing to well passivation with hydrogen [ 9 ]. W e fabricated a PIN diode consisting of the undoped polySi of 1 i~m thick and measured the photovoltaic performances under illumination of light (AM1.5 100 m W c m - 2 ) . Fig. 8 shows the spectral response measured for the poly-Si PIN diode under light illumination. The fact that the quantum efficiency over 80% is obtained at the maximum leads us to a conclusion that the undoped poly-Si grown by TSG on glass exhibits excellent photo-electric properties.

4. C o n c l u s i o n (1) W e proposed TSG, i.e. the epitaxial-like growth of poly-Si succeeding to the deposition of seed crystal on glass. (2) The seed crystal was grown on glass by repeating alternately the deposition of thin layer from fluorinated precursors and the treatment with atomic hydrogen ( L B L ) .

References

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