Spectroscopic Characterization

As the optical transmission properties (optical absorption below and above bandgap) will be discussed in the next section, we concentrate here on only the structure-related spectroscopies. As already stated, nanocrystallines prepared by PECVD and HWCVD have complex structures. Combined data of X-ray diffraction (XRD), Raman spectroscopy (RS), and transmission electron microscopy (TEM) can be useful to analyze the structure of nanosilicon [4]. Raman spectroscopy and XRD are used to estimate crystalline volume fraction, in which the intensity ratio of the different signal contributions is calculated from a deconvolution of the spectra. A popular method to estimate the grain size is the application of the Scherror formula on XRD peaks.

It is noted that RS and XRD are not sensitive for low crystalline volume fraction Xc (10-20%) and are not sensitive to the spatial distribution of crystallites (or amorphous phase), although TEM imaging still reveals isolated grains in amorphous tissue for low Xc. TEM imaging in cross section is important to know the spatial inhomogeneity. It is noted also that the crystalline volume fraction Xc estimated from Raman spectroscopy is always smaller than that estimated from XRD [4]. In spite of this difficulty of exact determination of Xc, RS or XRD is an easy technique and hence these techniques are often used as a measure of crystalline volume fraction.

The other spectroscopic methods, such as electron spin resonance (ESR) study, produce information on defect structure [7, 8, 21]. The ESR signals at g-values, 2.0052 ± 0.004, 2.0043 ± 0.002, 1.996 < g < 1.998 (depending on doping and temperature), and 2.1, have been found in nano-crystalline silicon films and each can be attributed to Si dangling bonds (2.0052), dangling bonds in oxygen-rich regions (2.0043), electrons in conduction band, conduction band tails or donor states (1.996-1.998) which is called the conduction electron (CE) resonance, and acceptor states (2.1). The spin densities of g = 2.0052 and 2.0043 resonances remain around 1016 cm-3 for undoped and doped samples, while the spin density of g = 1.996-1.998 increases with P-doping up to around 1018 cm-3. The CE resonance increases with the dc conductivity, which will be discussed also in Section 4, and the density of CE is reported to be almost the same as that of the donor density.

The number of Si dangling bonds (g = 2.0052) is smaller compared with polycrystalline Si, which may be due to an excellent hydrogen passivation during the PECVD process. The majority of Si dangling bonds are expected to locate at the boundaries between crystalline column and disordered (amorphous) region.

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