Characterization of Undoped, N- and P-Type Hydrogenated Nanocrystalline Silicon Carbide Films Deposited by Hot-Wire Chemical Vapor Deposition at Low Temperatures

Abstract

Undoped, n- and p-type hydrogenated nanocrystalline cubic silicon carbide (nc-3C–SiC:H) films were successfully deposited on glass and silicon substrates at a low substrate temperature of about 300 °C by hot-wire chemical vapor deposition. The structural, optical, and electrical properties of the films were investigated by X-ray diffraction (XRD), Fourier transform infrared absorption (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry, photothermal deflection spectroscopy (PDS), and conductivity measurements. The XRD and FTIR measurements revealed a clear correlation between the average grain size and width of the SiC stretching mode vibration of the films. The dark conductivity of the films was increased from 5.8×10-11 to 6.2×10-6 S/cm with increasing the grain size from 6.4 to 16.6 nm. The detailed analysis of the dark conductivity indicates that the Fermi level position is affected by the grain size of the films. Spectroscopic ellipsometry measurements showed that the dielectric functions of the films are strongly affected by the grain size. The n- and p-type nc-3C–SiC:H films were also successfully deposited using phosphine, hexamethyldisilazane, and dimethylaluminumhydride as dopants. For the n- and p-type films, the dark conductivities of 5.32×10-0 and 7.67×10-4 S/cm were achieved, respectively. The optical absorption spectra of the doped films indicate that p-type doping significantly affects absorption coefficients above the bandgap of nc-3C–SiC:H compared with n-type doping. For the n-type films, the absorption coefficients below the bandgap are affected by free carrier absorption as well as by localized states within the bandgap.

New insight into the microstructure and doping of unintentionally n-type microcrystalline silicon carbide

“…In order to minimize a possible impact of the film thickness we adjusted the deposition time to obtain a thickness of approximately 200 nm for all samples in the study. The influence of F MMS on the material properties is comparable with the influence of an increase of hydrogen dilution, as reported by Finger et al 2 and Miyajima et al 6 They conclude that an increase of hydrogen dilution leads to a decrease of disorder in the microstructure. Similarly, we derived from the FWHM of the FTIR and XRD data that the reduction of F MMS leads to a decrease of disorder in the microstructure.…”

“…The deposition of lc-SiC:H using hot wire chemical vapor deposition (HWCVD) 5 or plasma-enhanced chemical vapor deposition (PECVD) has been described by several research groups in the past. 6,7 They reported on the n-type characteristic of the films, although no doping gas was actively added to a gas mixture of monomethylsilane (MMS) and molecular hydrogen. Nevertheless, intentional n-doping of lc-SiC:H with nitrogen 8 or phosphine 6 and intentional p-doping of lcSiC:H by overcompensation with aluminum 9,10 or boron 11 are also possible.…”

“…6,7 They reported on the n-type characteristic of the films, although no doping gas was actively added to a gas mixture of monomethylsilane (MMS) and molecular hydrogen. Nevertheless, intentional n-doping of lc-SiC:H with nitrogen 8 or phosphine 6 and intentional p-doping of lcSiC:H by overcompensation with aluminum 9,10 or boron 11 are also possible. Although, the electrical conductivity of the material can be enhanced by several orders of magnitude by the active doping, the highest optical transparency is obtained in lc-SiC:H without any active doping.…”

“…To date, it is known that dark conductivity increases with higher crystalline volume fraction. 2,6 However, K€ ohler et al 12 showed that the material is either crystalline or amorphous. Therefore, it is still unclear, how the structural properties of lc-SiC:H are related to the electrical properties.…”

“…5), while l only increases by less than 1 order of magnitude. In the literature, O and N impurities originating from contamination are discussed as possible candidates for the creation of donor-like states within the energy gap of lcSiC:H. 2,6,7 From c-SiC it is well known that N is a very good shallow donor impurity, due to its low ionization energy. 28 Unlike the case of unintentionally n-type doped microcrystalline silicon, where the use of gas purifiers during deposition decreased [O] and r d , 29 Finger et al 2 reported that the electrical properties of unintentionally n-type doped lcSiC:H were not affected by gas purifiers.…”