Sol–Gel Spin-Coated Multilayer nc-Si Thin Films on Silicon Substrates: Controlled Growth, Composition, and Electronic Structure

1. Introduction

Multilayer silicon thin films have attracted much attention since they can be applied for many kinds of optoelectronic devices, such as the next generation of solar cells technology and light emitting devices [1]. The structural properties of nc-Si layers and the quality of buffer layers have a great impact on the device’s performance. For higher efficiency, high quality textured silicon thin films are needed. To obtain it, the analysis of grain size, structure and morphology of the thin layers are crucial. Efforts have been taken recently to fabricate high efficiency solar cells using improved amorphous and microcrystalline silicon [2,3,4,5,6,7]. The main challenge for the solar industry is to reduce the usage of materials and the deposition cost, and to increase the efficiency of photovoltaic devices. Silicon (Si) as an optically active layer appeared to be impossible to utilize properly because of its indirect band gap.

However, in recent years, nc-Si materials are showing many revolutions of photonic functions and the quantum confinement effect (QCE) of charge excitons in the silicon nanostructure is leading to a quasi-direct transition. In particular, the nc-Si band gap energy (Eg) can be elongated to shift down the valence states and to shifting up of the conduction states once the small nanometric size approaches the size of its Bohr exciton radius. The band gap modification due to the quantum confinement will be given in a simple effective mass approximation as follows: ∆Eg∝1/(m.a² ) where m is an effective isotropic mass in the confinement direction, and a is a nanoparticle size.

Various conventional deposition techniques such as plasma enhanced chemical vapor deposition (PECVD)[8], RF magnetron sputtering [9], Ion implantation [10] and so on have been used to deposit nc-Si:H thin films which are compatible with the standard Si technology. However, it is very difficult in achieving concentrations high enough to obtain efficient optical properties. Many techniques mentioned above have been found to be expensive and time-consuming because of the production under high vacuum and/or annealing processes. A sol-gel method is quite inexpensive and easy to fabricate such Si nanocrystals embedded into its dielectric matrix. The sol-gel method by centrifugal processing has been recommended as a technique to be achieved in this objective [11]. The sol-gel was used as a viscous medium through Si crystallites settle. Nevertheless, centrifugal processing might produce bulk material which has such a functional constraint for thin film optoelectronic devices.

In this study, four multilayer nc-Si/Si thin-film samples were prepared using an identical sol–gel spin-coating process, followed by annealing at 100 °C, 200 °C, 300 °C, and 400 °C. The resulting silicon nanostructures consisted of nano-Si powders dispersed within a continuous SiO₂ matrix. Boron-doped p-type (100) Si wafers were finely ground to serve as the nano-Si powder precursor. A sol–gel–derived SiO₂ buffer layer, synthesized using the buffer solution described below, served as a critical component of the multilayer architecture.

Compositional analysis and the chemical bonding environment of boron were examined using FTIR spectroscopy. The structural, optical, and electronic properties of the ML nc-Si/Si thin films were characterized using Raman spectroscopy, glancing-incidence XRD, ellipsometry, four-point-probe measurements, SEM, and UV spectroscopy.

2. Materials and Methods

2.1. Substrates Preparation

Multilayer nanocrystalline silicon (ML nc-Si/Si) thin films were deposited on boron-doped p-type silicon substrates. Prior to deposition, the substrates were ultrasonically cleaned for 15 minutes each in acetone, ethanol, and deionized (DI) water.

2.2. Nano-Si Powders Preparation

The nano-Si powders were produced from (100) p-type monocrystalline Si wafers by a prolonged grinding process lasting 4–10 hours. The wafers contained boron impurities corresponding to a resistivity of 1–20 Ω·cm. The resulting Si powders were then mixed with absolute ethanol and passed through a 20 µm pore-size sieve for filtration.

2.3. Preparation of SiO₂ Buffer Layer

Tetraethylorthosilicate, Si(OC2H5)4 (TEOS, 98% Fluka) and ethanol absolute, (C2H5OH, 99% BDH) (EtOH)) were used in this experiment, as a silicon-oxide precursor. In the fabrication of Si oxide buffer layer, the dielectric sol-gel (buffer solution) was first prepared as follows: 1 mole of TEOS and 2 moles of EtOH were mixed and then stirred for 15 min at the room temperature. Cetyl Trimethyl Ammonium Bromide, C19H42BrN (CTAB, 99% Sigma Aldrich) was used as surfactant. In addition, 0.0013M CTAB and 0.1M HCl catalysts in water were subsequently added dropwise to the solution until the water to TEOS molar ratio of 2. The condensation of TEOS at about pH = 2 was controlled by adding HCl catalyst. The proper solutions were then stirred at room temperature for 60 minutes. To obtain the suspension uniformity, sol-suspension was prepared from the mixture of nano-scaled Si powders (0.10 g) with TEOS (buffer solution) of 5ml under a magnetic stirrer for 30 min [12]. The conditions of substrate cleaning and film deposition are summarized in Table 1 and Table 2.

[Si(OC2H5)4 + C2H5OH] + [C19H42BrN + HCl] → SiO2 Buffer Solution

(1)

2.4. Synthesis of nc-Si/Si Thin Films Layers

Figure 1 shows the spin-coating setup used in this experiment. Nitrogen (N2) gas cylinder as back pressure is connected to the liquid dispenser. Sol-gel liquid holder is also connected to the liquid dispenser with a stand just above the spin coater disk. Adjusting the back pressure, the amount of precursor liquid (0.1 ml) can be disseminated on the mounted p-type Si (100) substrate in the spin coater.

After aging of the gel for 8-day, 0.2 ml of the prepared TEOS gel as a si-oxide precursor was spun at 11,800 rpm for 10 s on silicon substrate. The acquired Si-oxide is an important buffer layer to have a good coherence between its surface and the sol-suspension for the next step. To inhibit the crack of the buffer layer structure, the first Si-oxide film was suitably dried for 2 hours on a hot plate in air ambient at 50^oC. Afterward, 0.2ml of sol-suspension was released from the sol-gel holder located just above the spin coater on the dried oxide buffer layer and nc-Si thin film deposition was performed by spinning the coater for 10 s with same rpm of 11,800 as above. By repeating the above proper sequence processes, two layers of nc-Si thin films were prepared in a continuous oxide dielectric phase in this work.

2.5. Stoichiometry of SiOx Buffer Layer

The stoichiometry of as-deposited SiO_x was estimated by 2 (SiO₂) from shifts of the asymmetric Si-O-Si stretching peak with adjacent O-atoms in the IR absorption spectra. The change in chemical bonding state of Si oxide film was also analyzed by using Fourier transform infrared spectroscopy, FT-IR (Perkin-Elmer System 2000) with a wavenumber resolution of 2 cm⁻¹ as the following equation:

[ψ] = 978.72 + 30.63x

(2)

where [ψ] is a position of peak frequency (cm⁻¹) and x is stoichiometry of SiO_x film [12].

3. Results

3.1. Raman Scattering

The transition of thin film structure and the degree of MLnc-Si/Si crystallinity have been investigated by Raman scattering measurements. The Raman spectra of the films were recorded using a portable iRaman (B&W TeK) with the argon ion laser having an excitation wavelength of 514 nm was used and the power was less than 5 mW.

Figure 2 shows (a) the Raman spectra of as-synthesized silicon (p-type, B-doped, 1-20 Ω-cm) powders and (b) the Raman spectra of MLnc-Si/Si thin films as a function of annealing temperatures. Sharp peak centered at ~519.23 cm⁻¹ is clearly indicating the crystalline phase (Figure 2a).

Figure 2b shows the Raman spectra of ML nc-Si/Si thin films as a function of annealing temperature. It is observed that all films have shown a strong narrow peak at 520 cm-1 and the Raman intensity is gradually enhanced with increasing the annealing temperature which is indicating the enhancement of crystallization.

Figure 3 shows the crystallite size and Full Width Half Maximum (FWHM) as a function of annealing temperature. With increasing annealing temperature, crystallite size estimated using Eq. 3, goes up and has the maximum of 10.98 nm at 400oC (Fig.3). On the other hand, FWHM showed the opposite relationship. It seems that the increment of annealing temperature is helpful for promoting crystallization. Results indicate that with increasing the annealing temperature, the crystallites size of formed MLnc-Si is increased and becomes more ordered which may be related to reduce the strain in the films. The Raman crystallites size was estimated from FWHM using the equation as follows [13]

Δω = - A(β/L)^γ

(3)

where A = 47.41 cm⁻¹, γ = 1.44, β = 0.543 nm), L is silicon dot diameter, and ∆ω is the FWHM.

With increasing annealing temperature, the films show enhanced crystallinity, reduced defects, enlarged and better-ordered nanocrystals, and improved Raman scattering efficiency, resulting in a stronger 520 cm⁻¹ Raman peak. The multilayer architecture amplifies the annealing effects by providing more nucleation sites for faster crystallization, a confined geometry for controlled nanocrystal growth, interfaces that relax defects, improved phonon transport that strengthens the 520 cm⁻¹ Raman peak, and enhanced Si diffusion that promotes better crystal ordering.

3.2. X-Ray Diffraction (XRD)

The structural properties were investigated using an XRD instrument (SIEMENS D5000 X-ray diffractometer, λ = 1.54 Å). The XRD spectra were recorded in the 2θ range from 20o to 80o at a fixed grazing angle of 5o and a scan rate of 0.02o/s. In the present study, any difference in the film thickness was corrected using the X-ray absorption coefficient for Si. Thus, the XRD intensities observed for different films can be compared. The average crystallite size, [δ], was estimated from the width of the XRD spectra using Scherrer’s formula as follows:

[δ] = kλ/βcosθ

(4)

where k, λ, β and θ are a constant, the wavelength of X-ray (1.54 Å), the full width at half maximum (FWHM) and Bragg angle of the diffraction peak respectively.

Figure 4 shows the Glancing angle XRD (GXRD) spectra of MLnc-Si/Si thin films as a function of annealing temperature. Strong diffraction peaks of (111) and (220) planes were observed for all the samples (Fig. 4). Films show a (111) preferential orientation. An increase in the relative intensity of the (111) and (220) XRD spectra were found as a function of annealing temperature corresponds well with an increase in the intensity of 520 cm-1 component (crystalline phase) in Raman spectra.

Figure 5 shows the (111) and (110) XRD grain sizes ([δ]) as a function of annealing temperature. Both (111) and (110), [δ] were found to be increased with increasing annealing temperature. Maximum [δ] were found to be <111> ~ 12 nm and (110) ~ 11 nm at 400oC. XRD results are well consistent with Raman results.

Results show both Raman and XRD confirm the same structural evolution: the multilayer design enhances crystallization and reduces disorder during annealing.

3.3. Fourier Transform Infra-Red (FT-IR)

Figure 6 shows the IR absorption spectra over the range of 400-4000 cm-1 for films deposited with different annealing temperatures. The IR absorption was measured by a Thermo- Scientific, Nicolet iS10 FTIR spectroscopy with the wavenumber range of 400 to 4,000 cm-1 and resolution of 2 cm-1. The spectra at 747.97 cm-1, 902.49 cm-1 and 1541.66 cm-1 are assigned to Si-O Rocking, Si-O bending, Si-O stretching and adjacent O-atom out of phase absorption bands, respectively. These bands are attributed to the in-phase motion of the oxygen atoms [14].

In multilayer nc-Si/Si films, the repeated Si/SiOₓ interfaces create well-defined Si–O–Si structural units, and these interfaces enhance the coordinated (in-phase) vibrational motions of oxygen atoms. As a result, the characteristic Si–O rocking, bending, and stretching bands become more pronounced in the IR spectra, because the multilayer architecture provides more ordered and repeated Si–O bonding environments that strengthen these vibrational modes.

3.4. Scanning Electron Microscope (SEM)

The topological images were taken by scanning electron microscopy (SEM, 1450VP, Phenom Pure). Figure 7 shows the SEM micro graphs of ML nc-Si/Si thin films as a function of annealing temperature, (a) Cross-sectional view of nc-Si top-layer and (b) Topographic view. It is a branch of microscopy that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about both the sample’s surface morphology, cross-sectional information and the composition as well. As shown in Figure 7a, average film thickness was estimated ~1 micrometer. As shown in Figure 7b, ununiformed larger island-type of micro-structured was observed for the film deposited at 100oC. However, at 400oC, uniformed grains distributions were observed, which also showing the crack-free uniform surface with the highest crystallinity as shown above [15].

3.5. Optical Measurement and Band Gap Energy

The optical band gap energy of ML nc-Si/Si thin film is calculated by Tauc’s equation as below:

(αhν)=A(hν-Eg)ⁿ

(5)

where hν is photon energy and n is equal to ½ and 2 respectively. The linear variation of (αhν) 2 vs. hν at absorption edge. The Extrapolating the straight-line portion of the plot (αhν)2 versus hν for zero absorption coefficient values gives the optical band gap (Eg). Figure 8 shows the optical transmittance spectra of ML nc-Si/Si thin films as a function of annealing temperature. The transmittance spectrum revealed a high transparency above 90% in the near-infrared region providing a potential application for infrared optoelectronic devices.

However, the transmittance was improved in the visible region with the wavelength less than 650 nm generally due to the optical absorption characteristics of the ML nc-Si/Si thin films which are near their optical band gap region as the annealing temperature increased due to the lower band gap energy.

Figure 9 shows the optical band gap energy (Eg) of ML nc-Si/Si as a function of annealing temperature. Optical band gap energy decreased with increasing the annealing temperature having the lowest value, Eg = 1.3 eV at 400oC. The decrease in optical band gap energy with increasing annealing temperature can be attributed to structural relaxation and increased crystallinity within the nc-Si films. At higher annealing temperatures, nanocrystalline grains tend to grow, and the degree of disorder or amorphous fractions within the film are reduced. As the crystallite size increases, the quantum confinement effect becomes weaker.

In nanocrystalline or partially amorphous silicon, stronger confinement in smaller grains produces a wider band gap. When annealing promotes grain growth and reduces disorder, the electronic states become more delocalized, narrowing the band gap. This results in the lowest band gap value (Eg ≈ 1.3 eV at 400 °C).

Additionally, high-temperature annealing can reduce defect-related localized states within the band gap. The reduction of these defect states shifts the optical absorption edge toward lower energies, further contributing to a decreased optical band gap.

3.6. Ellipsometry

Thickness of the films was obtained by using an ellipsometer (Rudolph Research, Ellipsometer Auto EL-III) with a wavelength of 632.8 nm. Figure 10 shows the influence of annealing temperature of the ML nc-Si/Si thin film on the thickness and refractive index values. By increasing the annealing temperature, the average value of the refractive index exhibits a slight increase from 3.32-3.59. It might contribute to start of pore removal and densification as a function of annealing temperature. Moreover, the film thickness gradually shrinks from 950 nm to 689 nm.

It is mainly due to the shrinkage of the gels during drying, and it is forced by capillary pressure of the small pore liquid [16]. Using annealing temperature, the surface tension between liquid and vapor plays a very important role. Therefore, the less shrinkage of the film annealed at 400oC can lead to the less crack or crack free. With increasing the annealing temperature, the thickness was found to be decreased because of more gel viscosity. Moreover, thickness and refractive index were showing the opposite relationship to each other due to that EtOH as a solvent evaporates, causing shrinkage of the gel network.

3.7. Electrical Measurement

The resistivity [μ] of ML nc-Si/Si thin films was measured using four-point probes by the following equation:

[μ] = π/ln2 V/I

(6)

where v and I are voltage and current respectively.

Figure 11 shows the variation of hall mobility and the resistivity of nc-Si thin films as a function of annealing temperature. Mobility and resistivity show opposite relationships to each other having the minimum mobility of ~37.5 cm2/v.s and the maximum resistivity of ~7.35 Ω-cm at higher annealing temperature, respectively.

The observed opposite trends of Hall mobility and resistivity with increasing annealing temperature can be explained by changes in the microstructure and defect density of the nc-Si thin films. As the annealing temperature increases, structural disorder, grain boundary defects, or impurity scattering may become more pronounced. These defects act as scattering centers that hinder charge carrier movement, resulting in reduced mobility (down to ~37.5 cm²/V·s at the highest annealing temperature).

At the same time, an increase in defect density or the formation of insulating phases can decrease effective carrier concentration. Since resistivity (ρ) is inversely related to both carrier concentration (n) and mobility (μ) through the relation ρ = 1/(n·q·μ), a reduction in either parameter leads to higher resistivity. Consequently, the films exhibit maximum resistivity (~7.35 Ω·cm) at higher annealing temperatures.

Thus, the opposite behavior of mobility and resistivity arises because increased thermal treatment degrades charge transport pathways by introducing or enlarging defect-related barriers within the nanocrystalline silicon network.

4. Discussion

Multilayer nanocrystalline silicon (nc-Si/Si) thin films were fabricated on crystalline silicon substrates using a spin-coating approach, followed by thermal annealing at progressively higher temperatures. The annealing treatment played a central role in governing the structural, electrical, and optical properties of the films. As the annealing temperature increased, the films exhibited a clear enhancement in crystallinity, reflected by the systematic growth of nanocrystalline domains and improved orientation. X-ray diffraction analysis revealed that the average grain sizes associated with the <111> and <110> orientations were approximately 11 nm and 12 nm, respectively, indicating the formation of relatively uniform and well-developed nanocrystals. The increase in grain size at elevated annealing temperatures suggests enhanced atomic mobility and more complete crystallization within the multilayer structure.

Electrical characterization further demonstrated the strong influence of annealing. At the highest annealing temperatures, the films displayed a minimum charge-carrier mobility of ~37.5 cm²/V·s accompanied by a maximum resistivity of ~7.35 Ω·cm. This inverse relationship between mobility and resistivity is consistent with changes in microstructure: while improved crystallinity generally favors charge transport, the presence of grain boundaries, defect states, and possible depletion regions can counteract mobility gains. The observed trends imply that although crystallization is enhanced at higher temperatures, scattering at enlarged grain boundaries or residual defects may still limit mobility.

Optical measurements showed a continuous decrease in bandgap energy with increasing annealing temperature, reaching its lowest value of approximately 1.3 eV at 400 °C. This reduction in optical bandgap aligns with the transition from a more amorphous to a more crystalline state, where the weakening of quantum confinement and the reduction of localized defect states both contribute to bandgap narrowing. The evolution of the bandgap, therefore, supports the structural findings and confirms the enhancement of long-range order in the material.

Raman spectroscopy provided additional evidence of structural improvement. With higher annealing temperatures, the intensity of the characteristic crystalline Si Raman peak at ~520 cm⁻¹ increased significantly. This enhancement is attributed to reduced disorder, improved phonon coherence, and the enlargement of crystalline regions. The strengthened 520 cm⁻¹ peak confirms that the films developed higher crystallinity and lower defect density as annealing progressed.

The multilayer architecture of the films further amplifies these annealing-induced improvements. The repeated stacking of thin layers creates abundant nucleation sites that promote rapid crystallization during thermal treatment. The confined geometry of each layer restricts crystal growth in a controlled manner, promoting uniform nanocrystal sizes and minimizing the formation of extended defects. Additionally, the interfaces between layers help relieve structural strain and reduce defect densities, contributing to improved microstructural quality. Enhanced phonon transport across well-ordered multilayers also increases Raman scattering efficiency, giving rise to the more intense 520 cm⁻¹ Raman signature. Moreover, the multilayer design facilitates interlayer silicon diffusion during annealing, which further smooths structural discontinuities and promotes the formation of highly oriented nanocrystals.

Overall, the results collectively demonstrate that thermal annealing-combined with the advantages of a multilayer configuration-significantly improves crystallinity, optical properties, and structural quality in nc-Si thin films. These improvements highlight the effectiveness of multilayer spin-coated architectures for tailoring the structural and functional properties of silicon-based nanomaterials.

5. Conclusions

Multilayer nanocrystalline silicon thin films were deposited on silicon substrates using a spin-coating technique and subsequently annealed at various temperatures. The crystallinity and grain size of the films increased progressively with higher annealing temperatures. The average grain sizes for the <111> and <110> orientations were approximately 11 nm and 12 nm, respectively. At elevated annealing temperatures, the films exhibited a minimum mobility of ~37.5 cm²/V·s and a maximum resistivity of ~7.35 Ω·cm. The optical bandgap decreased with increasing annealing temperature, consistent with enhanced structural ordering. Higher annealing temperatures resulted in improved crystallinity, reduced defect density, enlarged and better-oriented nanocrystals, and enhanced Raman scattering efficiency, leading to a more intense 520 cm⁻¹ Raman peak. The multilayer architecture further amplifies these annealing effects by providing additional nucleation sites for rapid crystallization, a confined geometry for controlled nanocrystal growth, interfaces that help relax structural defects, improved phonon transport that strengthens the 520 cm⁻¹ Raman mode and facilitated silicon diffusion that promotes superior crystal ordering.

ML	Multilayer
QCE	Quantum confinement effect
XRD	X-Ray Diffraction
FT-IR	Fourier transform infrared