Cyclic Voltammetry and Micro-Raman Study of Graphene Oxide-Coated Silicon Substrates

Abstract

This work presents the improvement of the electro-optical response of n-type crystalline silicon via dip-coated graphene oxide (GO) thin films. GO was deposited on Si/SiO₂ by immersion, and the resulting heterostructures were characterized by cyclic voltammetry measurements and Raman spectroscopy. Raman analysis revealed a slight but measurable broadening (~0.7 cm⁻¹) of the Si TO phonon mode at 514 cm⁻¹, indicating local interfacial strain. Cyclic voltammetry measurements showed a substantial increase in photocurrent in comparison to pristine silicon substrates. These effects are attributed to a GO-induced p-type inversion layer and enhanced interfacial charge transfer. The results suggest that GO can serve as a functional interfacial layer for improving silicon-based optoelectronic and photoelectrochemical devices.

1. Introduction

Conventional semiconductor technology primarily relies on crystalline silicon, a material known for its poor light emission due to its indirect band structure [1].

As silicon-based electronics make some limited improvements in efficiency and scaling potential when devices are miniaturized, graphene, a monolayer of carbon atoms structured in a hexagonal pattern, has attracted widespread interest in the semiconductor field [2].

Due to its extremely high carrier mobility, wide-spectrum light absorption, and a linearly dispersive energy band profile, graphene is suitable for diverse uses in electronic and optoelectronic fields [3,4].

An additional benefit of graphene [5] is the possibility to fabricate it as ultrathin sheets, which can then be moved onto appropriate substrates [6]. Graphene sheets can be synthesized through mechanical separation from graphite or generated via chemical vapor deposition (CVD) [7,8]. The prevalent method to exfoliate graphite into GO involves treatment with potent oxidizing chemicals [9], then a method of GO reduction [10] is used to obtain graphene, for example, thermal annealing [11], chemical reduction [12], and green reduction methods [13]. GO contains a higher density of oxygenated functional groups and structural irregularities compared to graphene obtained through mechanical or CVD processes, which is known to improve photodetector functionality [14]. In addition, prior research demonstrates that GO thin films offer high transparency in the visible range, making them suitable for transparent electrodes and protective optical layers, including in solar technologies [15].

Although graphene exhibits exceptional physical and electrical characteristics, its lack of a bandgap leads to a limited on/off current ratio, which makes it unsuitable as a full substitute for silicon [13]. Nonetheless, it has been proposed that integrating graphene with silicon technology could enhance broadband functionalities in photonic and optoelectronic systems, enabling improved performance in devices such as photodetectors, modulators, and polarizing elements [16]. Within this framework, significant progress has been made in graphene–silicon-based junctions [17,18], waveguides [19], and photodiodes [20].

Previous research has investigated how silicon doping influences graphene’s properties [21,22], for which the silicon doping opens the band gap of graphene and increases graphene's optical conductivity. However, a less studied aspect is the enhancement of silicon performances due to the interaction with graphene-related materials. This work wants to fill this gap.

In this communication, the impact of GO layers deposited via dip coating on n-doped Si/SiO₂ is reported. These results show a substantial enhancement of the photocurrent of n-type doped silicon attributed to the interplay with GO thin layers, opening new potentialities for optoelectronic device applications.

2. Materials and Methods

N-type Si (native SiO₂ layer of approximately 2 nm) substrates were treated in a mixture of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) to remove surface contaminants. GO was dip-coated on Si/SiO₂ at a speed of 0.33 mm/s, using GO (2 g/L dispersion in H₂O, Punto Quantico).

Scanning electron microscopy (SEM) images were acquired by means of a FEI Quanta FEG 400 ESEM microscope (Fei, Eindhoven, The Netherlands).

The SiO₂ layer was selectively removed at both ends of the silicon strip by brief immersion in a dilute hydrogen fluoride (HF) solution to enable electrical contact. This selective HF treatment removed the native SiO₂ layer at the edges of the substrate, allowing direct contact between the copper and the exposed silicon. For cyclic voltammetry measurements, copper wires with a thickness of approximately 80–100 μm were applied onto the HF-etched regions at the ends of the silicon strip. A PVC insulating layer was applied to the backside of the silicon wafer to prevent the formation of metal–semiconductor junctions between the electrical contact clips and the unpolished silicon surface. The sample scheme is shown in Figure 1a. The area coated with GO is approximately (45 ± 1) mm², while the illuminated region is circular with an area of (20 ± 4) mm². This configuration ensured that the electrical measurements reflected only the behavior of the GO-coated region. A SEM figure of a large single GO sheet on Si/SiO₂ substrate is reported in Figure 1b.

Micro-Raman measurements (Horiba, Darmstadt, Germany) were performed on n-Si/SiO₂ and n-Si/SiO₂/GO samples using a Horiba-Jobin Yvon apparatus, model LabRam HR, consisting of a single grating spectrograph, a 1800 lines/mm holographic grating, an edge filter, and a 100× Mplan Olympus objective with a NA of 0.90. Spectra were excited by a source laser (632 nm) with a typical power of 50 mW, focused on a spot of about 1 μm diameter.

Cyclic voltammetry [23] was performed using a Keithley 6212a source meter (Tektronix, Cleveland, OH, USA) in conditions of dark conditions and white light illumination (4450 lux). Moreover, the measurements were carried out under 400 nm, 500 nm, 600 nm, 800 nm, and 1000 nm illumination light.

3. Results and Discussion

3.1. Micro-Raman Measurements

Figure 2a displays the Raman spectrum acquired from a GO flake deposited on crystalline silicon, as visualized in the optical image in Figure 2b.

The strong and narrow peak at 514 cm⁻¹ corresponds to the transverse optical (TO) phonon mode of crystalline silicon, which is commonly observed due to the Raman-active substrate beneath the GO layer. A band observed at approximately 952 cm⁻¹ is attributed to the Si–O–Si bending/stretching modes of the thin native oxide (SiO₂) layer present on the silicon substrate [24]. Two broad features are observed at 1326 cm⁻¹ and 1586 cm⁻¹, assigned to the D and G bands of GO, respectively. The D band arises from breathing modes of sp² carbon rings and is activated by structural disorder or edge defects, while the G band corresponds to the E₂g phonon mode of sp² carbon atoms. The presence of the D band indicates a high degree of disorder in the GO structure due to the disruption of the sp² carbon lattice by defects, edge sites, and oxygen-containing functional groups such as epoxy and hydroxyl moieties [25]. The D band position at ~1326 cm⁻¹ and the G band shift to ~1586 cm⁻¹ are consistent with partially oxidized or defective GO, as reported in [26]. The continuous signal superimposed on the peaks at 1326 and 1586 cm⁻¹ is due to fluorescence from the GO film, which arises due to electronic transitions involving defect sites and oxygen-containing functional groups within the carbon lattice [27].

The FWHM (full width at half maximum) of n-type silicon/GO is found to be ~4.5 cm⁻¹ while the FWHM of the n-type crystalline silicon peak is ~3.8 cm⁻¹. The peak is well fitted by a sharp and symmetric Lorentz curve. The FWHMs of n-type silicon/GO peaks are thus broadened by about 0.7 cm⁻¹ more in comparison with n-type silicon peaks. Although the FWHM of the silicon Raman peak increases by only ~0.7 cm⁻¹ upon GO deposition, such a change supports the presence of local strain or interfacial disorder [28].

3.2. Cyclic Voltammetry Measurements

In Figure 3, cyclic voltammetry measurements for n-silicon (black lines) and for n-silicon/GO (red lines) samples under white illumination (a) and dark (b) conditions are reported. Moreover, the measurements are shown for specific wavelengths such as 400 nm, 500 nm, 600 nm, 800 nm, and 1000 nm (Figure 4 and Figure 5).

The I-V characteristics show a great enhancement of photocurrent on n-silicon/GO samples with respect to n-silicon samples under white light illumination.

Graphene oxide (GO) typically exhibits p-type behavior due to the presence of numerous electron-withdrawing oxygen-containing functional groups, such as hydroxyl, epoxy, and carboxyl moieties [29]. When GO is deposited on n-type crystalline silicon, a charge redistribution takes place in order to align the Fermi levels of the two materials.

Because GO has a higher electron affinity than silicon, electrons are transferred from the silicon conduction band to the acceptor-like states in the GO layer. This charge transfer induces band bending near the silicon surface: the conduction and valence bands bend upwards, shifting the surface Fermi level closer to the valence band. As a result, a p-type inversion layer is formed, meaning that holes become the majority carriers at the surface, even though the bulk remains n-type [30]. The thin native SiO₂ layer (~2 nm) present between GO and silicon does not prevent the inversion from occurring but acts as a dielectric spacer, similar to the gate oxide in a metal-oxide-semiconductor capacitor, allowing the electric field generated by the GO layer to modulate the band structure of silicon [31].

The Si + GO curves show higher current values, suggesting that the GO layer enhances the intrinsic conductivity of the silicon surface. This behavior, in addition to the broadening of the FWHM of n-type Si/GO with respect to n-type Si, is consistent with the hypothesis of GO acting as a dopant or facilitating improved charge transfer at the interface. However, additional research is needed to support this claim. The current enhancement is more pronounced under illumination, indicating a clear photoelectrochemical effect, likely due to increased carrier generation in the GO–Si system and more efficient charge separation. The increase in sensitivity is observed for all measured wavelengths, but it becomes more pronounced from 600 nm onward. This behavior can be explained by the fact that GO absorbs more strongly in the blue and UV regions.

In conclusion, the data provide strong evidence that GO coating significantly improves the photo-responsiveness of silicon, making GO/Si heterostructures promising candidates for light-assisted electrochemical applications, such as sensors, capacitive storage, or photo electrocatalysis.

4. Conclusions

Cyclic voltammetry performed on n-type silicon samples coated with GO by immersion revealed a significant enhancement in photocurrent performance. Supporting evidence from Raman spectroscopy shows a modest but consistent broadening of the FWHM (~0.7 cm⁻¹) of the silicon TO phonon peak near 514 cm⁻¹, indicating interfacial strain or disorder introduced by the GO layer. These findings suggest that GO coatings hold promise for modulating interfacial charge dynamics in silicon-based devices. Ongoing work is focused on comparing these effects across intrinsic and extrinsic Si substrates, including thickness-dependent GO responses.