The structural characterisation of HWCVD-deposited nanocrystalline silicon films

Introduction Nanocrystalline silicon (nc-Si:H) prepared as thin films by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature has attracted much attention because of its potential application in low-cost, large-area electronic devices, such as flat panel displays and solar cells. Hydrogen dilution of silane strongly affects the microstructure of silicon films so deposited. Hydrogen-diluted nc-Si reportedly exhibits high stability against light soaking, increases crystallite size and enhances mobility compared with amorphous silicon. Silane dilution can lead to a phase transition of silicon from an amorphous to a nanocrystalline and polycrystalline state. However, macroscopic inhomogeneity in the growth direction, as well as in the growth plane of nc-Si, has been observed in HWCVD films. Another aspect of nc-Si or poly-Si growth on glass and a-Si:H on graphite substrates in HWCVD processes is that nucleation of nanocrystalline material does not occur before the film thickness reaches 5 nm. This means that the initial growth of the films, during the deposition of the first few nanometres, is structurally amorphous. Methods based on scattering phenomena are useful for studying material which is heterogeneous at a nanoscale, especially those exhibiting nanocrystalline features. To calculate grain size and other characteristics of the crystallite interface, the following assumptions were made: (a) the scattering particles and their surrounding medium have uniform but different electron densities, (b) the volume fraction of the particles is low, and (c) the scattering particles are of an approximately known shape. Bingqing et al. analysed the influence of structural diversity and the size of spherical particles on the calculated size of distributions. The interpretation of the scattering effect from complex, disordered materials is notably simplified when fractal geometry is invoked to describe their structure. Much attention has been paid to the structural properties of nc-Si of HWCVD-deposited films derived from small-angle X-ray (SAXS) measurements, but few papers describe the structural and interfacial nature of nanocrystalline silicon. This article reports on structural and interfacial studies of nanocrystalline silicon using both SAXS as well as X-ray diffraction (XRD) and Raman spectroscopy.


Introduction
Nanocrystalline silicon (nc-Si:H) prepared as thin films by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature has attracted much attention because of its potential application in low-cost, large-area electronic devices, such as flat panel displays and solar cells. 1 Hydrogen dilution of silane strongly affects the microstructure of silicon films so deposited. 2,3ydrogen-diluted nc-Si reportedly exhibits high stability against light soaking, 4,5 increases crystallite size and enhances mobility compared with amorphous silicon.Silane dilution can lead to a phase transition of silicon from an amorphous to a nanocrystalline and polycrystalline state. 4,5However, macroscopic inhomogeneity in the growth direction, as well as in the growth plane of nc-Si, has been observed in HWCVD films. 6,7nother aspect of nc-Si or poly-Si growth on glass and a-Si:H on graphite substrates in HWCVD processes is that nucleation of nanocrystalline material does not occur before the film thickness reaches 5 nm. 8This means that the initial growth of the films, during the deposition of the first few nanometres, is structurally amorphous.Methods based on scattering phenomena are useful for studying material which is heterogeneous at a nanoscale, 9,10 especially those exhibiting nanocrystalline features.To calculate grain size and other characteristics of the crystallite interface, the following assumptions were made: (a) the scattering particles and their surrounding medium have uniform but different electron densities, (b) the volume fraction of the particles is low, and (c) the scattering particles are of an approximately known shape.Bingqing et al. analysed the influence of structural diversity and the size of spherical particles on the calculated size of distributions. 11he interpretation of the scattering effect from complex, disordered materials is notably simplified when fractal geometry is invoked to describe their structure.Much attention has been paid to the structural properties of nc-Si of HWCVD-deposited films derived from small-angle X-ray (SAXS) measurements, 8,12 but few papers describe the structural and interfacial nature of nanocrystalline silicon.This article reports on structural and interfacial studies of nanocrystalline silicon using both SAXS as well as X-ray diffraction (XRD) and Raman spectroscopy.

Experimental details
Nc-Si:H films were formed on glass and crystalline Si substrates by hot-wire chemical vapour deposition under various H 2 dilution conditions in silane.Deposition parameters are shown in Table 1.XRD samples were studied with a vertical Philips X-pert (PANalytical) Model 3040/60 X-ray diffractometer.Raman spectra were recorded using a Labram-1 (Dilor-Jobin Yvon-Spex) spectrometer in backscattering mode with a spectral resolution of 2 cm -1 .The 488.0 nm (approximately 2.54 eV) line of an argon laser was used for excitation and the Raman scattered light was analysed using a charge-coupled device (CCD) camera for multi-channel detection.SAXS data were collected from a SAXSess Anton Parr, Model 3040/60 X-ray generator: PW 3830 PANalytical in-line collimating geometry using the transmission method at a wavelength of 0.154 nm.The experimental samples were deposited on Si(100) wafers, which were folded into two layers for signal enhancement.The X-ray intensity scattered at an angle of 2θ relative to the incident beam passing through the sample was obtained with a highly collimated beam.(The angle 2θ is usually converted into a scattering vector in terms of h = 4π(sin θ)/λ.)The X-ray intensity measured as a function of the scattering angle 2θ was corrected for absorption and scattering from the substrate.It was normalised by incident beam intensity I o and film thickness to yield the normalised SAXS intensity, I(h).After correction for background, data were analysed by Guinier plot, Porod plot, scaling behaviour and correlation distance.

Interpretation of SAXS results
Figure 1 shows SAXS intensities I(q) obtained from nc-Si nanocrystals as a function of wavenumber (q = (4π/λ)sinθ).The scattering profiles have quantitatively similar features and can be divided into three regions on the q-axis by shape profile.The low q region (Region I; q > 0.001) gives an idea of grain size and shape of the nanocrystalline material.The sufficient condition for measuring radius of gyration is q max R g < 1.The Guinier approximation to determine radius of gyration is I(q) = I o exp (q 2 R g 2 /3).In the interface, the electron inhomogeneities are   different from those in a crystalline or purely amorphous network.The scattering vector corresponding to Region II (0.05 < q < 0.15) provides information about the interface and interfacial inhomogeneities of crystalline materials.Scaling behaviour and the Debye law are applicable to this region, where scattering exhibits self-similarity, based on fractals.Scaling law, the Power law or the presence of fractals gives a further idea of the distribution of nanocrystalline materials in a deposit, depending largely on cluster mobility.A Power plot is defined by the equation I(q) = Aq -α , where A is a constant related to the structural configuration of the nc-Si.α represents the fractal dimension and is equal to three for perfectly ordered materials.The material is considered to be mass fractal if 3 < α < 0 and surface fractal if 4 > α > 3. Debye theory provides a general description of an inhomogeneous system, from which scattering measurements can be used to indicate average electron density and the degree of correlation between two fluctuations as a function of their distance of separation.refs 13,14).

Results and discussion
The XRD spectra of silicon films prepared under different H 2 dilution conditions are shown in Figure 2.With increasing H 2 dilution, the appearance of three prominent peaks at about 28.4°, 47.28°and 56.8°indicates progressive crystallisation in the <111>, <220> and <311> directions, respectively.Figure 2 also shows that the full width at half maximum (FWHM) height of the XRD peak pattern decreased as the hydrogen dilution ratio increased.This indicated that the intermediate range order improves with H 2 flow rate.The average grain size of the experimental films is therefore estimated by the Debye-Scherrer formula to be 20-35 nm.The intensities, the positions and the FWHM of different XRD peaks of the experimental films are listed in Table 2.For clarity, the intensities were normalised to the (220) peak intensity.In the XRD data, the peak intensities, I(hkl), of nc-Si:H films listed in Table 2 show that the I (111) /I (220) ratio was in the range 0.68-2.11,whereas the I (111) /I (311) ratio was in the range 4-5.9.
Figure 3(a) shows a typical Raman spectrum from a nc-Si:H film, which can be identified as four regions corresponding to two kinds of phonon modes-a longitudinal optical (LO) branch with a peak at ~400 cm -1 and a transverse optical (TO) mode.
Further, the TO band can be further divided into three parts: a transverse optical branch (TO 1 ) with a peak at 480 cm -1 from the amorphous Si contribution, a transverse optical mode (TO 1 ) at about 510 cm -1 from the contribution of Si interfacial nanocrystals, and another transverse optical mode (TO 2 ) at 520 cm -1 from the contribution of Si core nanocrystal.These modes are depicted in Figure 3(a), from which the LO and a-TO, TO 1 and TO 2 branches can be seen to fit well to a Gaussian distribution.This implies that every fitted line can be approximately repre-     indicates surface fractal characteristics.Table 3 shows the scaling factor for films deposited at different H 2 flow rates.This result suggests that this particular crystallite network consists of fractal clusters (D m = 2.95 ± 0.02) of particles with smooth surfaces.Figure 5(b) is a Guinier plot, in which the slope is taken over the range where R g q < 1. R g , the radius of gyration of the clusters, varies from 9.8 nm to 12.9 nm as H 2 dilution increases from 15 to 35 sccm.Assuming the presence of uniform spheres, the corresponding cluster diameter is equal to 2R g (5/3). 0.5 Grain size increases from 25 to 33 nm and corresponds with crystallite size determined by XRD. Figure 5(c) shows a Debye plot as a measure of the mean inhomogeneity in the experimental materials.The inhomogeneity distance is approximately 2 nm for a H 2 flow rate in the range 15-35 sccm.

Conclusion
These various studies based on SAXS, XRD and Raman spectra of nc-Si:H films deposited by HWCVD techniques under varying hydrogen dilution conditions, indicate that crystallite size and crystalline fraction increased from 20 nm to 35 nm and from 40% to 49%, respectively.Crystallites of nanocrystalline silicon displayed a mass fractal nature and interfacial inhomogeneities were of the order of 2 nm in size.The relative effect of grain boundaries in relation to grain size declined in films deposited with reduced H 2 flow rate.
Nanocrystalline silicon (nc-Si) films were deposited by hot-wire chemical vapour deposition (HWCVD) in the presence of varying H 2 concentrations and their structural and interfacial character investigated by X-ray diffraction, small-angle X-ray scattering (SAXS) and Raman spectroscopy.The crystalline fraction was around 30-50% and the nc-Si crystallite size was in the range 20-35 nm.The SAXS results were analysed by Guinier plot, scaling factor, and correlation distance.The nc-Si grains displayed a mass fractal appearance, and the interfacial inhomogeneity distance was ~2 nm.: HWCVD, SAXS, Raman, XRD

Fig. 1 .
Fig. 1.SAXS plot of various nanocrystalline materials and structural information in relation to regions I-III.

Fig. 2 .
Fig. 2. XRD scans of different nc-Si films as a function of H 2 dilution.

Fig. 4 .
Fig. 4. SAXS plots of different nc-Si films as a function of H 2 flow rate.

Fig. 5 .
Fig. 5.(a) ln(I) versus ln(q) for scaling behaviour; (b) ln(I) versus q 2 as a Guinier plot; (c) I -2/3 versus q 2 as a Debye plot.The sample of nanocrystalline silicon in each case was deposited at a H 2 flow rate of 15 sccm.

Table 2 .
Details of XRD results.

Table 3 .
SAXS structural parameters of nc-Si:H films.