Fabrication of Amorphous Silicon-Carbon Hybrid Films Using Single-Source Precursors

Abstract

The aim of this study was the preparation of different amorphous silicon-carbon hybrid thin-layer materials according to the liquid phase deposition (LPD) process using single-source precursors. In our study, 2-methyl-2-silyltrisilane (methylisotetrasilane; 2), 1,1,1-trimethyl-2,2-disilyltrisilane (trimethylsilylisotetrasilane; 3), 2-phenyl-2-silyltrisilane (phenylisotetrasilane; 4), and 1,1,2,2,4,4,5,5-octamethyl-3,3,6,6-tetrasilylcyclohexasilane (cyclohexasilane; 5) were utilized as precursor materials and compared with the parent compound 2,2-disilyltrisilane (neopentasilane; 1). Compounds 2-5 were successfully oligomerized at λ = 365 nm with catalytic amounts of the neopentasilane oligomer (NPO). These oligomeric mixtures (NPO and 6-9) were used for the preparation of thin-layer materials. Optimum solution and spin coating conditions were investigated, and amorphous silicon-carbon films were obtained. All thin-layer materials were characterized via UV/vis spectroscopy, light microscopy, spectroscopic ellipsometry, XPS, SEM, and SEM/EDX. Our results show that the carbon content and especially the bandgap can be easily tuned using these single-source precursors via LPD.

Chart 1. Currently Used Precursors for LPD Processing of Silicon Films

Scheme 1. Synthesis of Precursor Molecules 2–4

Scheme 2. Synthesis of Precursor 5

Figure 1

ORTEP representation of 5. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms except for the hydrides are omitted. Selected bond lengths (Å) and bond angles (deg) with estimated standard deviations: Si(1)–Si(2) 2.3450(3), Si(1)–Si(3) 2.3473(3), Si(1)–Si(4) 2.3393(3), Si(1)–Si(5) 2.3348(3), Si(2)–Si(3) 2.3417(3), Si(2)–Si(1)–Si(3) 113.099(10), Si(4)–Si(1)–Si(2) 112.887(10), Si(4)–Si(1)–Si(3) 109.711(10), Si(5)–Si(1)–Si(2) 105.836(10).

Figure 2

(a) UV/vis spectra of compounds 1–3 and 5 (c = 1 × 10^–4 M; solvent = n-hexane) and of compound 4 (c = 1 × 10^–5 M; solvent = n-hexane), (b) orbitals involved in the first transition for compound 4 (with a contour value of 0.04 au), and (c) orbitals involved in the first transition for compound 5 (with a contour value of 0.04 au).

Scheme 3. Oligomerization of Compound 1 at λ = 365 nm to Form NPO (Left); Photolysis Reactor Used for the Oligomerization at λ = 365 nm of Precursor Materials (Right)

Figure 3

UV/vis spectra of compound 1 (neat).

Scheme 4. Oligomerization of Compound 2–5 at λ = 365 nm to Form the Oligomeric Mixtures 6–9

Figure 4

¹H NMR spectra of the oligomerization process of compounds 3 to 7 (TMSISO) (top: before photolysis at λ = 365 nm; middle: photolysis for 10 h at λ = 365 nm; bottom: photolysis for 20 h at λ = 365 nm).

Figure 5

General procedure of the LPD process of oligomeric mixtures 6–9 and NPO.

Figure 6

Picture of the successfully formed thin layer of 50 wt % NPO (scratches on the surface for analytical measurements) and light microscopy images at 200× magnification (top right: 30 wt % of NPO in toluene; bottom left: 50 wt % of NPO in 40 wt % toluene and 10 wt % cyclooctane; bottom right: 50 wt % of NPO in toluene).

Figure 7

Picture of the successfully formed thin layer of 50 wt % 6 (MeISO) and light microscopy images of 200× magnification (top right: 50 wt % of 6 in toluene at −30 °C (high oligomeric/polymeric compounds not filtrated); bottom left: 50 wt % of 6 in toluene at −30 °C (filtrated); bottom right: 70 wt % of 6 in cyclooctane at −30 °C).

Figure 8

Picture of the successfully formed thin layer of 50 wt % 7 (TMSISO) in 40 wt % toluene and 10 wt % cyclooctane and light microscopy images of the optimized SiC layer at 200× magnification.

Figure 9

Picture of the successfully formed thin layer of 50 wt % 8 (PhISO) in 40 wt % toluene and 10 wt % cyclooctane and light microscopy images of an optimized SiC layer at 200× magnification.

Figure 10

Picture of the formed layer of 15 wt % 9 (CHSO) in THF and light microscopy images of 200× magnification (homogeneous (top right) and inhomogeneous (bottom right) parts of the thin-layer material).

Figure 11

UV/vis spectra of the optimized thin-layer material of NPO and 6–9.

Figure 12

Raman spectra of the solution-processed a-Si/H films formed from 6 to 8.

Figure 13

Optical constants n and k of the investigated thin films were determined by spectroscopic ellipsometry.

Figure 14

SEM images of MeISO (6) (left: 40,000× magnification; right: layer thickness measurement at 80,000× magnification).