Laser Truncation of Silicon Nanowires Fabricated by Ag-Assisted Chemical Etching for Reliable Electrode Deposition in Solar Cells

Abstract

Silicon nanowires (SiNWs) fabricated by Ag-assisted metal-assisted chemical etching (MACE) exhibit excellent light-trapping performance, yet their fragile high-aspect-ratio morphology severely limits reliable metallization in photovoltaic devices. Conventional electrode deposition methods often fail on dense SiNW arrays due to poor mechanical stability of the nanowire tips, leading to delamination, inhomogeneous coverage, and high contact resistance. In this work, we introduce a maskless laser-based truncation technique that selectively shortens MACE-derived SiNWs to controlled residual heights of 300–500 nm exclusively within the regions intended for electrode formation, while preserving the full nanowire morphology in active areas. A detailed parametric study of laser power, scanning speed, and pulse repetition frequency allowed the identification of an optimal processing window enabling controlled tip melting without damaging the nanowire roots or the crystalline silicon substrate. High-resolution SEM imaging confirms uniform planarization, well-preserved structural integrity, and the absence of subsurface defects in the laser-processed tracks. Optical reflectance measurements further demonstrate that introducing 2% and 5% truncated surface fractions—corresponding to the minimum and maximum metallized front-grid coverage in industrial Si solar cells—results in only a minimal reflectance increase, preserving the advantageous the light-trapping behavior of the SiNW texture. The proposed laser truncation approach provides a clean, scalable, and industrially compatible route toward creating electrode-ready surfaces on nanostructured silicon, enabling reliable metallization while maintaining optical performance. This method offers strong potential for integration into silicon photovoltaics, photodetectors, and nanoscale electronic and sensing devices.

1. Introduction

Silicon nanowires (SiNWs) produced by metal-assisted chemical etching (MACE, also known as MacEtch) have emerged as a versatile top-down route to high-aspect-ratio, large-area nanostructured silicon surfaces with strong light-trapping and anti-reflective properties. These properties make SiNW arrays attractive for front-side texturing in crystalline silicon photovoltaics and for creating highly absorptive surfaces in various optoelectronic devices [1,2,3,4,5].

Recent reviews on emerging nanostructured materials for high-efficiency photovoltaics further highlight the importance of advanced surface texturing strategies for improving light management and device performance [6]. In this broader context, SiNW-based architectures represent one of the promising routes toward enhanced optical absorption and interface engineering in next-generation PV devices. The MACE family of methods offers tunable control over nanowire length, diameter, density and morphology through selection of metal catalyst, etchant composition, temperature and substrate parameters. Reviews and comparative studies document reproducible formation of dense SiNW forests and discuss the interplay between catalyst morphology and etching kinetics that dictates final nanowire geometry and surface porosity [7,8,9].

Despite clear optical advantages, MACE-derived SiNWs typically present porous, fragile, and mechanically weak top regions that complicate subsequent device processing —notably the formation of stable, low-resistance electrical contacts. Standard metallization routes (screen printing, inkjet or paste deposition, sputtering) can fail on dense nanowire surfaces because metal pastes or evaporated films do not wet or mechanically engage with the fragile tips, leading to delamination, high contact resistance, or inhomogeneous coverage. Addressing these processability issues is therefore essential for the practical integration of SiNW texturing into scalable photovoltaic manufacturing [10,11].

A number of strategies have been explored to reconcile robust electrical contact formation with nanostructured topographies. Process-level approaches include filling or planarizing the nanowire forest with conductive pastes, conformal thin-film seed layers (e.g., evaporation or atomic layer deposition), localized annealing or silicidation to form low-resistance metal–silicon interfaces, and selective plating after seed-layer deposition. Each route offers tradeoffs in series resistance, optical penalty (from planarization), and process complexity. For example, local silicidation or electrodeposited interlayers have been shown to improve contact properties on non-planar Si features, but often require careful thermal or current control to avoid damaging delicate nanostructures [12,13,14].

Some advances have shown that femtosecond- and nanopulse-laser processing can produce black silicon and nanostructured Si surfaces with excellent light-trapping performance; however, these approaches are still largely global or non-selective and do not meet the technological requirement of planarizing only the electrode contact regions while preserving high-aspect-ratio nanowires in the active area. Techniques such as large-scale femtosecond-laser-induced black silicon [15] or controlled femtosecond modification of Si surfaces [16] achieve strong absorption improvement but lack spatial selectivity for contact region processing. Furthermore, recent work on ultrafast-laser modified Si [17] highlights challenges such as surface melting, debris redeposition and pulse overlap instabilities under high repetition rate processing. Building on this context, our study introduces a maskless, locally selective laser truncation method capable of shortening MACE-derived SiNWs to controlled residual heights while preserving the nanowire base integrity and enabling the formation of metallization-compatible planar tracks.

Recent work has begun to converge on integrated approaches that combine MACE texturing with selective post-processing to balance optical and electrical requirements. Reported strategies include laser-assisted reshaping and local planarization, seed-layer engineering for electroplating, and hybrid workflows that maintain active nanostructures while forming metallization-friendly pads. However, a systematic demonstration that (i) laser irradiation can reproducibly truncate SiNWs to a controlled length, (ii) the underlying substrate and nanowire roots remain mechanically and electrically intact, and (iii) the resulting flattened regions are compatible with standard large-scale metallization techniques, is still limited in the literature [10,18,19]. The present work addresses this gap by establishing a reproducible laser-processing window for local contact region planarization while preserving the light-trapping SiNW morphology in unmodified active areas.

Laser-based surface engineering provides an alternative route for locally modifying SiNW morphology while preserving the optical functionality of adjacent regions. Pulsed and continuous-wave lasers have been used for annealing, localized melting, ablation, and crystallinity modification in Si and SiNW systems; controlled laser irradiation can remove or reshape top segments, recrystallize amorphous shells, or create smooth, flattened patches suitable for further processing without globally altering the substrate. Prior demonstrations include polarized laser annealing of individual SiNWs and laser sintering of printed metal inks to improve conductivity and adhesion. These studies indicate that laser treatments can be spatially selective and compatible with downstream metallization steps [20,21].

From the device perspective, reliable metallization of nanostructured silicon has implications across a range of applications beyond photovoltaics. In silicon electronics and nanodevices, controlled metal–semiconductor contacts and silicidation strategies are central to contact resistance, device scaling and thermal stability. In nanoscale sensors, SiNW field-effect transistors exhibit exceptional sensitivity to surface charge and molecular binding, but require well-defined, low-resistance electrodes for stable readout; fabrication routes that preserve nanowire sensing surfaces while providing robust contacting are therefore highly desirable [22,23].

Nanowire-based photodetectors likewise benefit from engineered contacts and selective surface modification. SiNW photodetectors and radial-junction devices demonstrate high responsivity and broadband absorption when the nanowire geometry is optimized; however, electrode placement and contact morphology significantly affect dark current, speed, and noise. Thus, methods that enable localized planar regions for contact formation —while maintaining high-aspect-ratio arrays in active areas—would help to realize high-performance SiNW photodetectors compatible with wafer-scale processing [24,25,26].

In this work we address that gap by investigating a laser truncation approach applied to SiNWs fabricated by Ag-assisted MACE. We aim to demonstrate controlled removal of the fragile top segments (target length 300–500 nm) only in areas allocated for metallization, producing smooth, mechanically robust contact pads while preserving the high-aspect-ratio texture in active regions. We characterize morphological outcomes using SEM and evaluate structural integrity and process compatibility with standard metallization strategies reported in the literature. The approach seeks to combine the light-trapping advantages of SiNW arrays with the processability required for reliable electrode deposition in photovoltaic, sensing and photodetection applications.

2. Materials and Methods

2.1. Materials

The study utilized p-type monocrystalline silicon wafers characterized by a (100) crystallographic orientation and a resistivity ranging from 2.5 to 6 Ω·cm. The wafers were sectioned by laser into square specimens with a surface area of 25 cm² and an average thickness close to 200 µm. Before further processing, each sample was subjected to a multi-stage cleaning protocol involving mechanical rinsing in analytical-grade acetone (CHEMPUR, Piekary Śląskie, Poland) and isopropanol (99.7%, POCH, Gliwice, Poland), followed by ultrasonic treatment in both solvents at elevated temperatures to ensure complete removal of organic contaminants from the surface. Vertically aligned silicon nanowires were then fabricated via the metal-assisted chemical etching (MACE) method. The process employed an aqueous mixture of hydrofluoric acid (HF, 40%) and silver nitrate (AgNO₃, 99.9%, Sigma-Aldrich, Dramstadt, Germany) as the catalytic oxidizing medium. The Ag⁺ ions facilitated localized oxidation of silicon and its preferential dissolution along the ⟨100⟩ crystallographic direction, enabling the formation of densely packed, high-aspect-ratio nanowire arrays. The morphology of the resulting structures was governed by the concentration of reagents and etching time. The adopted procedure builds upon methodologies developed in our earlier research, in which we explored the optimization of MACE parameters, the resulting morphology and surface chemistry of SiNWs, and their integration into photovoltaic and sensing structures [27,28,29,30].

2.2. Methods

The morphological characterization of the processed samples was performed using three different scanning electron microscopes. On the (SEM) JSM-5000LV (JEOL Ltd., Tokyo, Japan) the analyses were carried out under high-vacuum conditions, employing a working distance of 10 mm in secondary electron imaging mode. Magnifications ranging from 100× to 100,000× were applied, depending on the observed surface features and the scale of interest. SEM micrographs were obtained for both surface topographies and cross-sectional views of the examined samples. Particular attention was paid to the identification of structural details resulting from the laser modification and chemical etching processes. The obtained images clearly revealed the presence of the targeted surface morphologies and nanostructured features, confirming the reproducibility of the fabrication procedure and the effectiveness of the applied processing parameters. The laser contribution on SiNWs and structure morphology was imaged by Phenom ProX (Thermo Scientific, Waltham, MA, USA) operating at an acceleration voltage of 10 kV. Prior to SEM analysis, the samples were placed on aluminum holders and then coated with a 5 nm layer of gold using the diffusion method (Leica EM ACE 200 low-vacuum coater Leica, Wetzlar, Germany). HR SEM surface morphology was investigated using QUANTA 200 3D FEI Europe B.v. (Eindhoven, The Netherlands) dual beam system scanning electron microscope. High-resolution imaging was performed under high-vacuum conditions with an accelerating voltage of 13 kV and a probe current of 16.00 nA. The secondary electron mode was used to acquire micrographs at magnifications up to 65,000×.

Reflection spectra were acquired with a Lambda 950S Perkin Elmer (Shelton, CT, USA) spectrometer halogen–deuterium light source with 150 mm integrating sphere in the wavelength range 250–1500 nm.

2.3. Laser Processing

Laser surface modification was performed using a fiber laser system Raycus JPT MOPA M7 (Hilden, Germany) with a maximum output power of 30 W and a wavelength of 1054 nm. The laser beam was directed through a galvanometric scanning head equipped with an F-theta lens, providing a working field of 200 × 200 mm. The MOPA (Master Oscillator Power Amplifier) configuration of the laser enabled precise control over pulse energy, frequency, and scanning speed, allowing the adjustment of fluence and interaction time in a wide range of processing parameters. The laser operated in a pulsed mode with variable repetition frequencies and scanning speeds, to investigate their influence on the surface morphology and degree of structural modification. The beam was focused on the sample surface under normal incidence, with the focal plane adjusted to ensure uniform energy distribution across the irradiated area. All experiments were conducted under ambient conditions. Following irradiation, the samples were subjected to SEM analysis (Section 2.2) to evaluate the morphological response of the silicon surfaces and nanowire layers. The comparison of laser parameters and resulting surface features allowed identification of characteristic processing regimes corresponding to under-, optimal, and over-exposed conditions.

3. Results

Reliable metallization of nanostructured silicon surfaces remains one of the key technological challenges in integrating silicon nanowires into photovoltaic, electronic, and optoelectronic devices. While vertically aligned SiNWs fabricated by Ag-assisted chemical etching (MACE) provide exceptional light-trapping and anti-reflective properties, their highly porous, mechanically fragile morphology creates severe difficulties during subsequent electrode formation. In conventional screen-printed solar cells, photodetectors, or chemical sensors, conductive pastes or evaporated metals must form stable, continuous contact with the underlying semiconductor. However, when such electrodes are deposited directly on dense SiNW arrays, several detrimental effects typically occur:

(i)

the metal paste or ink does not fully penetrate through the porous nanowire forest, leading to poor electrical contact with the silicon base;

(ii)

weak mechanical anchoring causes delamination or peeling of the electrode together with the detached nanowires;

(iii)

inhomogeneous metal coverage results in non-uniform current paths and degraded device reliability.

These limitations have become a critical bottleneck preventing large-scale application of SiNW-based architectures in industrial photovoltaics and nanoelectronics. Thus, achieving a selective planarization of nanowire regions—limited only to metallization areas while maintaining optical benefits elsewhere—is of paramount importance for practical device integration. Figure 1 illustrates the concept and effects of laser truncation of silicon nanowires designed to facilitate reliable electrode deposition.

The left panel Figure 1a shows a top-view schematic of the processed silicon surface, where the laser beam is scanned along predefined linear tracks corresponding to the future metal grid pattern used in crystalline silicon solar cells. The processed stripes represent the flattened regions where the laser has selectively shortened the nanowires. These areas will serve as the electrode contact zones, while the unirradiated regions between them preserve their high-aspect-ratio SiNW structure to maximize optical absorption and minimize surface reflection. The outer frame marks the perimeter of the processed wafer area. The right-hand panels Figure 1b–d present cross-sectional profiles of the nanowire layer after laser processing at different fluence levels and exposure conditions. In Figure 1b, the insufficient truncation regime is shown: the laser fluence is too low or the scanning overlap too small, leading to only partial shortening of the nanowires. The residual tall, flexible nanowires still form a porous layer that prevents complete electrical contact and can result in poor electrode adhesion. In Figure 1c, the optimal truncation regime is achieved. Here, the laser fluence and scan parameters are precisely tuned to remove only the upper portions of the nanowires, leaving mechanically stable, shortened stubs (approximately 300–500 nm in height) firmly attached to the silicon substrate. This configuration creates a quasi-planar surface that supports uniform electrode spreading and strong adhesion during screen-printing or evaporation, while maintaining partial nanostructuring beneficial for light management and charge collection. Panel Figure 1d represents the over-truncation regime, where excessive laser fluence or prolonged exposure leads to thermal overprocessing, including melting of the nanowire bases or even minor ablation of the crystalline silicon substrate. Such damage degrades both mechanical integrity and electrical conductivity of the contact region, and may also affect adjacent active areas by introducing surface defects or microcracks. Overall, the schematic highlights the critical importance of precise laser process control—particularly fluence, pulse overlap, and exposure time—to ensure that the nanowire layer is only partially removed where metal contacts will later be formed.

3.1. Subsubsection Laser Process Parameters and Energy Deposition Model

Precise control of laser–matter interaction is crucial for achieving reproducible and selective truncation of silicon nanowires. The key process parameters—laser power (P), pulse repetition frequency (f), and scanning speed (v)—determine the energy density (fluence) and pulse overlap, which directly influence the extent of nanowire shortening, thermal modification, and the quality of the resulting surface.

3.1.1. Energy per Pulse and Fluence

For a pulsed laser, the average power P_avg and repetition frequency f define the energy per pulse:

$$E_{pulse}={\displaystyle \frac{P_{avg}}{f}},$$

(1)

where E_pulse is the single pulse energy in joules (J).

The fluence (F), i.e., the energy delivered per unit area during one pulse, is given by:

$$F={\displaystyle \frac{E_{pulse}}{A_{beam}}}={\displaystyle \frac{P_{avg}}{f·A_{beam}}},$$

(2)

where A_beam = πr² is the effective beam spot area, and r is the 1/e² radius of the laser beam [31,32].

In practical processing, the laser fluence must exceed the ablation threshold of the nanowire material but remain below the melting threshold of crystalline silicon to avoid substrate damage [33,34].

3.1.2. Pulse Overlap and Scanning Speed

During surface scanning, consecutive laser pulses partially overlap depending on the scanning velocity and repetition rate. The pulse overlap ratio (O) along the scanning direction is defined as:

$$O=1-{\displaystyle \frac{v}{f·d}},$$

(3)

where d is the beam diameter on the surface [35,36].

When O approaches 0, pulses are separated, resulting in discrete ablation spots.

For O > 0.7, adjacent pulses merge, forming continuous modification tracks.

Excessive overlap (O > 0.9) may lead to localized overheating, excessive melting, or debris redeposition, while too low overlap (<0.4) produces non-uniform truncation and residual nanowire height variations [37,38].

3.1.3. Effective Exposure and Pulse Accumulation

The total number of pulses N_p incident on a single point during scanning can be expressed as:

$$N_p=1-{\displaystyle \frac{d·f}{v}},$$

(4)

The cumulative effective fluence F_eff (or accumulated energy density) at a given surface location is then:

$$E_{eff}=N_p·F={\displaystyle \frac{P_{avg}·d}{v·A_{beam}}}$$

(5)

This accumulated energy determines the effective truncation depth of SiNWs.

For Si nanowires fabricated by metal-assisted chemical etching, the optimal regime typically occurs when F_eff is sufficient to induce melting and reflow of the upper wire sections (300–500 nm) without damaging the roots or substrate [39,40].

3.1.4. Practical Relationship Between Process Parameters and Morphology

Experimentally, the relationship between laser control parameters and resulting surface morphology can be summarized as follows:

The parameters listed in

Table 1. Laser control parameters vs. resulting surface morphology.

Parameter	Increase Effect	Physical Result	Reference
Power (P)	↑	Increases fluence → deeper truncation, possible melting	[32,41]
Frequency (f)	↑	Reduces energy per pulse, increases overlap → smoother but shallower ablation	[35,37]
Speed (v)	↑	Reduces overlap and energy deposition → incomplete truncation	[38,42]
Spot size (d)	↓	Increases fluence at constant power → enhances local heating	[31,43]

represent the typical variable settings of the laser system, including laser power, scanning speed, and pulse repetition frequency. These parameters directly determine the key physical quantities such as fluence, pulse overlap, and exposure time that govern the interaction between the laser beam and the silicon surface. In the following sections of this manuscript, these adjustable laser settings will be used as the primary variables in the analysis, since they have the most direct and measurable influence on the resulting microstructural modifications of the silicon surface. By systematically varying these operational parameters, the correlation between laser energy input and the observed morphological transformations—such as nanowire truncation, surface melting, or planarization—can be clearly established and discussed.

These correlations allow the definition of three characteristic regimes consistent with the schematic shown in Figure 1: under-truncated regime (low power, high speed, low frequency) incomplete removal of nanowire tops, poor contact area formation (b), optimal regime (balanced power, frequency and speed) uniform flattening and stable electrode adhesion (c), over-truncated regime (high power, low speed, high frequency) substrate melting, Si recrystallization defects (d).

3.1.5. Relevance to Device Fabrication

Maintaining control over these parameters is essential for achieving reproducible, localized planarization of nanostructured silicon surfaces. In solar cells, this ensures reliable screen-printed metallization without delamination or series resistance increase [44,45]. In photodetectors and sensors, controlled truncation allows formation of low-resistance contacts while preserving the high surface-to-volume ratio crucial for light absorption or analyte binding [7,46]. Thus, the interplay between fluence, overlap, and scanning speed represents a key process window for scalable integration of SiNWs into functional electronic and optoelectronic devices.

This selective laser planarization provides a practical and scalable pathway to integrate SiNW-textured surfaces with conventional metallization processes, not only in silicon solar cells, but also in Si-based photodetectors, chemical sensors, and nanoelectronic devices where localized and robust electrical connections are essential.

3.2. SEM Characterization and Morphological Analysis

To fully understand the influence of laser processing parameters on the structural modification of silicon nanowires, detailed morphological characterization was performed using scanning electron microscopy (SEM). Figure 2 presents plane-view SEM images of two distinct surface morphologies of crystalline silicon: (a) a macroscopic view of chemically polished photovoltaic-grade wafer and (b) a microscopic silicon nanowire array fabricated by Ag-assisted chemical etching. The comparison highlights the fundamental structural differences that directly influence both the optical performance and the mechanical behavior of the surfaces during subsequent device processing steps.

The chemically polished silicon wafer (Figure 2a) exhibits a macroscopically smooth and uniform surface obtained after precision slicing of the ingot with a diamond wire saw and subsequent chemical polishing to remove residual saw marks and surface damage. The resulting morphology is continuous and flat, occasionally intersected by crystallographic squares characteristic of Czochralski-grown silicon. In contrast, the surface shown in Figure 2b represents the same chemically polished wafer after MACE, which introduces a nanoscale wire-like morphology on the previously flat substrate. The etching process transforms the macroscopically smooth surface into a dense array of vertically aligned silicon nanowires with typical lengths of 300–500 nm. Under SEM observation, the surface reveals a fine, porous texture that microscopically follows the underlying crystallographic pattern of the wafer, including the square-shaped features imprinted by the original wafer orientation and polishing geometry. This nanowire architecture dramatically reduces optical reflectance through multiple scattering and light trapping within the high-aspect-ratio structures.

While laser power, scanning speed and frequency define the energy deposition on the surface, it is the resulting micro- and nanoscale morphology that ultimately determines the feasibility of subsequent device integration. Silicon nanowires typically exhibit high aspect ratios and mechanical fragility, and laser interaction can cause a wide range of structural effects—from partial tip melting and reflow to complete truncation or even substrate ablation. Therefore, establishing the correlation between laser parameter regimes and the resulting nanowire morphology is critical for identifying the optimal processing window. From a technological standpoint, SEM characterization plays a dual role. First, it confirms the selective removal of the upper SiNW segments within the areas intended for metallization, while ensuring that the remaining nanowire roots and the crystalline silicon substrate remain intact. Second, it verifies the mechanical integrity and surface uniformity of the laser-truncated regions, which are essential for achieving stable and low-resistance electrode deposition in subsequent screen printing or sputtering steps.

In the following section, the influence of varying laser processing parameters on the surface morphology is examined in detail through SEM analysis. The presented micrographs illustrate how gradual changes in laser power, scanning speed, and repetition frequency—corresponding to different regimes of fluence and pulse overlap—affect the micro- and nanoscale structure of the silicon nanowire arrays. By systematically comparing SEM images obtained under different laser conditions, the progressive transition from under-truncated to optimally truncated and finally to over-truncated surfaces can be clearly observed. These morphological variations provide direct visual evidence of how the controlled adjustment of laser operational parameters governs the extent of nanowire removal, surface planarization, and potential damage to the underlying silicon substrate.

3.2.1. Morphological Response to Laser Power Variation

To establish the baseline interaction between the laser beam and bulk crystalline silicon, preliminary experiments were performed on a chemically polished wafer surface prior to processing SiNW-textured samples. The morphological response of the smooth silicon surface to decreasing laser power is presented in Figure 3 with corresponding process parameters listed in

Table 2. Laser control parameters versus the resulting surface morphology at constant scanning speed (1000 mm/s) and pulse repetition frequency (35 kHz).

Laser Power  [%]	Pulse Repetition Frequency  [kHZ]	Scanning Speed  [mm/s]	Indication in Figure 3
100	35	1000	(a)
90	35	1000	(b)
80	35	1000	(c)
70	35	1000	(d)
60	35	1000	(e)
50	35	1000	(f)
40	35	1000	(g)

.

At high power levels (100–90%), the laser irradiation induced strong melting and re-solidification, forming smooth, flattened regions and spherical droplets of reflowed silicon. A moderate reduction in power (around 80%) produced uniform, continuous modification without excessive melting or cracking, indicating an optimal energy density for controlled surface restructuring. Further power decrease (70–50%) resulted in progressively weaker interaction, characterized by shallow grooves, partial reflow, and scattered redeposited particles, while at 40% no measurable morphological change was observed. This behavior clearly demonstrates the transition from the over-processed to the optimal and finally under-processed regimes as the laser fluence decreases, providing a reference for subsequent SiNW truncation experiments.

3.2.2. Morphological Response to Laser Scanning Speed Variation

To further examine the effect of scanning parameters on surface modification, experiments were performed at a constant laser power of 90% and pulse repetition frequency of 35 kHz, while the scanning speed was varied from 1000 mm/s to 200 mm/s. The corresponding process parameters are listed in

Table 3. Laser control parameters versus the resulting surface morphology at constant laser power (90%) and pulse repetition frequency (35 kHz).

Laser Power  [%]	Pulse Repetition Frequency  [kHZ]	Scanning Speed  [mm/s]	Indication in Figure 4
90	35	1000	(a)
90	35	900	(b)
90	35	800	(c)
90	35	700	(d)
90	35	600	(e)
90	35	500	(f)
90	35	400	(g)
90	35	300	(h)
90	35	200	(i)

, and the resulting surface morphologies are shown in Figure 4.

At high scanning speeds (1000–800 mm/s), the laser interaction with the silicon surface was weak due to the limited pulse overlap and reduced effective fluence per unit area. The resulting modification was shallow, with discontinuous traces and incomplete surface restructuring (Figure 4a–c). As the scanning speed decreased to the range of 700–500 mm/s, the cumulative energy input increased, leading to more uniform melting and smooth, continuous laser tracks indicative of the optimal processing regime (Figure 4d–f). Further reduction in scanning speed below 400 mm/s caused excessive local heating and material reflow, producing irregular, roughened tracks and redeposited droplets along the scan paths (Figure 4g–i). This trend clearly illustrates that the degree of surface modification is inversely proportional to the scanning speed. The transition from under-processed to optimally modified and finally over-processed morphologies with decreasing scanning speed confirms the strong dependence of local energy accumulation on pulse overlap and dwell time. These findings provide an essential reference for optimizing laser processing parameters in subsequent SiNW truncation experiments.

3.2.3. Morphological Response to Laser Pulse Repetition Frequency Variation

To analyze the influence of pulse repetition frequency on the laser–silicon interaction, experiments were performed at a constant laser power of 90% and a fixed scanning speed of 1000 mm/s, while the pulse repetition frequency was varied from 5 kHz to 50 kHz. The corresponding process parameters are summarized in

Table 4. Laser control parameters versus the resulting surface morphology at constant laser power (90%) and scanning speed (1000 mm/s).

Laser Power  [%]	Pulse Repetition Frequency  [kHZ]	Scanning Speed  [mm/s]	Indication in Figure 5
90	5	1000	(a)
90	10	1000	(b)
90	15	1000	(c)
90	20	1000	(d)
90	25	1000	(e)
90	30	1000	(f)
90	35	1000	(g)
90	40	1000	(h)
90	45	1000	(i)
90	50	1000	(j)

, and the resulting morphological evolution of the silicon surface is shown in Figure 5.

At low repetition frequencies (5–15 kHz), individual pulses were spatially separated with minimal cumulative heating, leading to discrete ablation spots and discontinuous surface modification (Figure 5a–c). As the frequency increased to the range of 20–35 kHz, the pulse overlap became sufficient to produce continuous, uniform laser tracks with moderate melting and re-solidification—representing the optimal processing regime (Figure 5d–g). Further increase in repetition frequency above 40 kHz caused excessive local heat accumulation and reflow, resulting in broadened and irregular melt zones accompanied by redeposited droplets and microcracks (Figure 5h–j).

This systematic evolution of surface morphology with increasing repetition frequency demonstrates the strong effect of pulse overlap and thermal accumulation on energy coupling efficiency. The results indicate that intermediate frequencies (25–35 kHz) offer the best balance between surface continuity and controlled modification, providing stable reference conditions for laser truncation of SiNW arrays.

3.3. Morphological Response of Silicon Nanowires to Laser Power Variation

After optimizing the laser parameters on polished crystalline silicon, the process was applied to silicon nanowire arrays fabricated by Ag-assisted chemical etching. The structural characteristics of the as-prepared nanowires are shown in Figure 6, where the average height of the SiNWs ranges from approximately 420 to 460 nm. This nanowire length was intentionally chosen to match the targeted truncation depth identified during preliminary surface studies, providing a balance between optical light trapping and mechanical stability.

The optimized laser conditions derived from the planar silicon experiments—namely moderate power (70–90%), scanning speed of 1000 mm/s, and repetition frequency of 15 kHz—were selected for nanowire processing. These settings correspond to the optimal energy window determined earlier, ensuring sufficient fluence to induce partial melting of the nanowire tips without damaging the underlying crystalline substrate. Lower repetition frequencies had produced discrete ablation spots, while higher frequencies led to excessive thermal accumulation and substrate melting; therefore, an intermediate frequency of 15 kHz was adopted as the most controllable regime for SiNW truncation.

The laser power was then systematically varied from 90% to 30%, while maintaining constant frequency and scanning speed. The detailed process parameters are summarized in

Table 5. Laser control parameters versus the resulting surface morphology of SiNW samples.

Laser Power  [%]	Pulse Repetition Frequency  [kHZ]	Scanning Speed  [mm/s]	Indication in Figure 7
90	15	1000	(a)
80	15	1000	(b)
70	15	1000	(c)
60	15	1000	(d)
50	15	1000	(e)
40	15	1000	(f)
30	15	1000	(g)

, and the resulting surface morphologies are shown in Figure 7.

At high laser power (90–70%), the top portions of the SiNWs were completely removed, leaving smooth, planar regions indicative of strong melting and reflow (Figure 7a–c). As the power decreased to 60–50%, partial truncation occurred: the upper segments of the nanowires were shortened uniformly to approximately 300–500 nm, producing mechanically stable stubs attached to the substrate (Figure 7d,e). This range of parameters was identified as the optimal truncation regime, combining effective planarization with preservation of the nanowire roots. Further reduction in laser power (40–30%) resulted in incomplete modification, with the SiNW forest largely retained and only superficial morphological changes visible (Figure 7f,g).

These results confirm that the process window defined from bulk-silicon experiments successfully translates to nanostructured surfaces. The intermediate laser fluence range ensures localized energy absorption and tip melting without delamination or substrate damage, validating the feasibility of the laser truncation approach for reliable metallization of SiNW-based surfaces.

Following the evaluation of the laser power dependence, additional optimization was performed to refine the processing conditions for patterned electrode formation on SiNW-textured surfaces. The objective was to produce continuous linear laser traces suitable for subsequent metallization rather than isolated modification spots. Based on the results presented in Figure 7, a laser power of 50% was identified as optimal for achieving partial truncation of SiNWs without damaging the substrate or inducing excessive melting. To transform the discrete laser-irradiated spots into a continuous track, the pulse repetition frequency was increased from 15 kHz to 25 kHz, while maintaining the scanning speed constant at 1000 mm/s. This adjustment increased the pulse overlap along the scan direction, effectively densifying the irradiated points and allowing the laser to form a uniform line corresponding to the future electrode path. The SEM image presented in Figure 8 confirms the successful formation of well-defined, overlapping melt zones with diameters in the range of 50–55 µm, producing a continuous linear feature across the silicon nanowire surface.

The observed concentric melt rings indicate controlled local melting and resolidification, consistent with stable thermal accumulation at the chosen energy density. These results validate the selected parameter combination—50% laser power, 25 kHz repetition frequency, and 1000 mm/s scanning speed—as optimal for generating continuous, uniform, and reproducible tracks suitable for reliable electrode deposition in subsequent device fabrication steps.

A detailed examination of the laser-modified area was performed at higher magnification to assess the structural integrity of silicon nanowires within a single laser-irradiated cylinder. The representative SEM image presented in Figure 9 reveals distinct morphological zones corresponding to different degrees of laser-induced modification across the beam profile.

The area marked in red represents the region of unaffected nanowires, where the SiNW array retains its original high-aspect-ratio morphology. This outermost zone lies beyond the effective laser footprint and shows no signs of thermal melting or structural deformation. The region marked in yellow corresponds to slightly affected nanowires, which remain clearly visible but display minor rounding and partial surface smoothing due to limited heat transfer from the adjacent irradiated zone. These nanowires preserve their vertical alignment and continuity, indicating that the local fluence remained below the melting threshold. In contrast, the cyan zone illustrates the remnant roots of nanowires, where the upper sections were completely truncated by the laser pulse overlap. The residual short stubs—typically 300–500 nm in height—form a continuous, quasi-planar surface suitable for subsequent metallization. Finally, the magenta region represents the fully truncated area, where the nanowire layer was removed down to the substrate level. The surface appears smooth and uniform, confirming that the laser fluence in this zone was sufficient to induce complete tip melting and reflow without damaging the underlying crystalline silicon.

It is also important to note that the bright, randomly distributed white dots visible across all regions are not related to laser processing but correspond to residual silver nanoparticles remaining from the Ag-assisted chemical etching process. The authors acknowledge that incomplete removal of these Ag nanospheres during post-etch cleaning may locally affect the contrast and texture observed in SEM images but does not influence the structural conclusions drawn from the presented analysis.

This multi-zone morphology clearly demonstrates the precision of the applied laser parameters (50% power, 25 kHz repetition frequency, and 1000 mm/s scanning speed) and confirms that controlled truncation can selectively remove the upper nanowire segments while preserving the surrounding structure, ensuring both mechanical stability and process compatibility for subsequent electrode deposition.

To further verify the structural effects of laser truncation and confirm the uniformity of the processed regions, cross-sectional SEM analyses were performed on the SiNW samples after laser treatment. Representative cross-sections are presented in Figure 10 and Figure 11.

The image in Figure 10 shows a continuous, slightly concave laser-modified path with an overall width of approximately 54 μm. This curvature reflects the Gaussian energy distribution of the laser beam, where the highest fluence is concentrated at the center of the scan line, gradually decreasing toward the edges. Despite the curvature, the cross-section remains smooth and free of cracks or delamination, indicating stable thermal processing conditions and effective control of the melting–reflow dynamics.

High-magnification cross-sectional views shown in Figure 11 provide further insight into the nanostructural transformation within the irradiated zone. The uppermost layer exhibits a uniform, consolidated surface corresponding to the truncated SiNW region, while beneath it a well-defined interface separates the modified zone from the unaffected crystalline substrate. The preserved crystalline order and absence of voids or amorphous interlayers confirm that the selected process parameters (50% power, 25 kHz repetition frequency, and 1000 mm/s scanning speed) allow precise removal of the nanowire tops without inducing subsurface damage.

Additionally, the gradual transition visible in the peripheral region of the cross-section illustrates the smooth redistribution of heat during irradiation, ensuring continuous and homogeneous surface formation along the entire processed track. Together, these observations verify that the optimized laser parameters produce consistent and reproducible planarization of the SiNW layer—an essential condition for subsequent metal electrode deposition.

3.4. Optical Characterization of Laser-Truncated SiNW Surfaces

Optical reflectance measurements provide a direct and sensitive method for evaluating how laser-induced truncation of SiNWs affects the overall light-management performance of the textured silicon surface. Since high-aspect-ratio SiNW arrays owe their superior photovoltaic potential to strong light trapping, low reflectance, and broadband absorption enhancement, any surface modification—especially partial planarization in electrode contact regions—must be assessed in terms of its impact on these optical properties. Reflectance spectroscopy therefore serves as a key diagnostic tool to verify whether the proposed selective laser truncation preserves the global antireflective function of the nanowire array despite local morphological changes.

The selected 2% and 5% truncated area fractions correspond to the minimum and maximum surface coverage typically introduced by front-side metallization in standard screen-printed crystalline silicon solar cells (AIKO and JinkoSolar technical datasheet). These metallized regions completely occlude the underlying active area and therefore directly reduce the effective light-absorbing surface of the device. Evaluating reflectance for these specific area fractions provides a realistic representation of how laser-processed contact regions influence the optical performance of SiNW-textured silicon within practical photovoltaic architectures.

Figure 12 shows the reflectance spectra of silicon nanowire arrays with different fractions of laser-truncated surface area (0%, 2%, and 5%), together with a polished silicon reference. The unprocessed SiNW sample exhibits the characteristic low-reflectance behavior over the entire UV–NIR range, confirming the strong light trapping capability of the high-aspect-ratio MACE-etched nanowires. Introducing laser-processed regions covering 2% and 5% of the surface results in only a minor increase in reflectance, most notably in the infrared region (λ > 1000 nm), while the overall suppression of reflection remains substantially stronger than for polished silicon.

The 2% truncated sample shows reflectance curves similar to the unprocessed nanowire reference, indicating that a small degree of laser planarization—equivalent to the minimal metallized area of typical front contacts—does not meaningfully affect the optical response. Even for the 5% truncated surface, corresponding to the upper limit of front-grid metallization coverage, reflectance remains below approximately 7% across the visible range. This demonstrates that localized nanowire removal required for forming electrode pathways maintains the desirable antireflective properties of the SiNW architecture while enabling the controlled fabrication of contact-ready regions.

4. Discussion

The results of this study demonstrate that the proposed laser-based truncation method enables controlled and spatially selective modification of MACE-derived silicon nanowires, addressing one of the key technological limitations in integrating SiNW-textured surfaces with conventional metallization processes. The systematic parametric evaluation performed on both polished silicon and nanowire-covered substrates revealed stable correlations between laser fluence, pulse overlap, and the resulting morphological regimes, which enabled identification of a reproducible processing window suitable for localized electrode contact formation.

Experiments performed on planar crystalline silicon established three characteristic regimes of laser–matter interaction—under-processed, optimal, and over-processed—each defined by distinct energy density conditions. Importantly, these regimes translated consistently to nanowire-covered samples. The established optimal interval (≈50% power, 25 kHz repetition frequency, and 1000 mm/s scanning speed) produced sufficient energy to melt and remove only the upper nanowire segments, while avoiding subsurface damage or excessive thermal accumulation. High-resolution SEM imaging confirmed that nanowires were shortened to controlled heights of 300–500 nm, leaving mechanically stable residual stubs firmly attached to the substrate. These stubs provide a quasi-planar surface able to support uniformly deposited metal contacts, overcoming the poor adhesion and electrical discontinuity characteristic of unmodified SiNW arrays.

Furthermore, the multi-zone morphology observed within individual laser-processed spots—ranging from unmodified SiNWs at the periphery to fully truncated regions at the center—demonstrates the spatial precision of the process and reflects the Gaussian energy distribution typical of laser beams. The ability to remove nanowires only in predefined linear paths, while fully preserving the nanowire architecture in the surrounding active surface, represents a major functional advantage over global planarization or chemical flattening methods, which inherently sacrifice optical performance.

Optical reflectance measurements complement the morphological findings and confirm the technological viability of the laser truncation strategy. For truncated-area fractions of 2% and 5%—representing the lower and upper bounds of front-electrode coverage in industrial crystalline silicon solar cells—the reflectance of the modified surfaces remains only slightly higher than that of pristine SiNW arrays across the UV–NIR spectral range. This demonstrates that local planarization for electrode placement introduces negligible optical penalty and does not compromise the global light trapping capability of the nanowire texture. Consequently, the laser-processed regions can be incorporated into photovoltaic device architectures without degrading optical performance.

Taken together, the morphological, structural, and optical results confirm that the developed laser truncation method provides a clean, maskless, and industrially scalable solution for producing electrode-ready surfaces on nanostructured silicon. The process allows the mechanical and electrical compatibility required for reliable metallization while preserving the optical benefits of SiNW texturing in unmodified regions. These capabilities open pathways not only for silicon photovoltaics but also for SiNW-based photodetectors, sensors, and nanoelectronic devices, which similarly require robust and spatially precise contact formation integrated with functional nanostructures.

Future work will focus on integrating laser-processed SiNWs into complete device structures, including junction formation and electrical characterization (e.g., contact resistivity, PL/EL imaging, I–V response). Such studies will further validate the practical applicability of the method and allow deeper insight into long-term stability and metallization compatibility in functional devices.

5. Conclusions

This work demonstrates a maskless and locally selective laser-based method for truncating MACE-derived silicon nanowires (SiNWs) to controlled residual heights suitable for metallization, without compromising the mechanical stability of the nanowire bases or the optical functionality of the surrounding active regions. Through systematic variation in laser power, repetition rate, and scanning velocity, a reproducible processing window was identified that enables uniform removal of the upper SiNW segments while preventing substrate damage, excessive melting, or thermal accumulation effects. High-resolution SEM imaging confirmed the formation of smooth, continuous planarized tracks with well-preserved nanowire roots, providing mechanically robust surfaces compatible with standard electrode deposition processes.

Optical reflectance measurements further revealed that laser-processed regions occupying 2% and 5% of the total surface—corresponding to the minimum and maximum metallized front-grid coverage in industrial crystalline silicon solar cells—introduce only a minimal reflectance increase across the UV–NIR spectrum. These findings verify that localized laser planarization maintains the strong light trapping performance of the SiNW texture while enabling reliable electrode contact formation.

Overall, the proposed laser truncation strategy offers a clean, scalable, and industrially compatible route for integrating high-aspect-ratio SiNWs with conventional metallization schemes. The method holds significant potential for application not only in silicon photovoltaics but also in SiNW-based photodetectors, sensors, and nanoelectronic devices requiring spatially precise contact regions. Future work will focus on incorporating laser-processed SiNWs into full device structures and evaluating their electrical performance, including contact resistivity, junction behavior, and complete photovoltaic metrics.