Liquid Silicon Family Materials(2): SiC

Abstract

As the second part of liquid silicon family materials, we introduce SiC of which ink (precursor solution) can also be made from CPS. Amorphous silicon carbide (a-SiC) is an advanced material with high thermal conductivity, good chemical stability, and high mechanical strength. Many researchers have fabricated a-SiC by the thermal decomposition of polycarbosilane which is converted into a-SiC by heating [1]. The pyrolysis products of various other polysilanes, including poly(methylsilane), poly(silylenemethylene), and poly(silastyrene), have been investigated as starting materials for polycarbosilane [2–4]. Since most of these previous studies have focused on structural and mechanical properties of polymers and the resultant SiC [5], there has been very few works focusing to develop a semiconducting SiC by thermal decomposition of polycarbosilane.

Here we introduce deposition of amorphous silicon carbide (a-SiC) films via solution process using a polymeric precursor solution consisting of polydihydrosilane with pendant hexyl groups (PSH). Unlike conventional polymeric precursors, this polymer neither requires catalysts nor oxidation for its synthesis and cross-linkage, resulting in sufficient purity used for semiconducting a-SiC.

In Sect. 7.1, polymer-to-ceramic conversion is systematically investigated under various pyrolysis temperatures. The polymer primarily undergoes cross-linking at temperatures above 150 °C with increasing polymer fraction; this cross-linking is followed by incorporation of carbon atoms into an amorphous network at 380 °C. The incorporated carbon atoms in the film are predominantly in the sp³-bonding state with almost no amorphous graphite-like sp² C-C clusters, leading to marked changes in the film’s properties.

In Sect. 7.2, we investigated the correlation of Si/C stoichiometry between the polymeric precursor solution and the resultant a-SiC film. The structural, optical, and electrical properties of the films with various carbon contents were also explored. The results suggested that the excess carbon that did not participate in Si–C configurations was decomposed and was evaporated during polymer-to-SiC conversion. Consequently, the upper limit of the carbon in the resultant a-SiC film was less than 50 at%, i.e., silicon-rich a-SiC.

In Sect. 7.3, we introduce phosphorus-doped a-SiC films (n-type a-SiC), using a polymeric precursor synthesized from a mixture of cyclopentasilane, white phosphorus, and 1-hexyne. The effect of carbon and phosphorus concentrations on the structural, optical, and electrical properties of a-SiC films was studied. The valence and conduction states of these films were determined directly through the combination of inverse photoemission spectroscopy and photoelectron yield spectroscopy.

In Sect. 7.4, we present p-type a-SiC films prepared using a LVD (liquid vapor deposition) method which is described in Chap. 5. In this time, we used a simple chamber with a vaporized silicon ink consisting of cyclopentasilane, cyclohexene, and decaborane. The incorporation of carbon into the silicon network was induced by the addition of cyclohexene to the silicon ink.