Investigating Quantum Confinement and Enhanced Luminescence in Nanoporous Silicon: A Photoelectrochemical Etching Approach Using Multispectral Laser Irradiation

Abstract

This study explores electrochemical etching to form porous silicon (PS), which has diverse biomedical and energy applications. Our objective is to gain new insights and drive significant scientific and technological advancements. Specifically, we study the effect of electrochemical etching of P-type silicon using laser irradiation in a hydrofluoric acid (HF) solution. The formation of the nanoscale PS structure can be successfully controlled by incorporating laser irradiation into the electrochemical etching process. The wavelength and power of the laser influence the formation of nanoporous silicon (NPS) on the surface during the electrochemical etching process. The luminous flux is monitored with the help of a customized integrating sphere system and an LED-based excitation source to find the light flux values distributed across the P-type nanolayer PS wafers. Analysis of the NPS and luminescence characteristics shows that the laser bandwidth controls the band gap energy absorption (BEA) phenomenon during the electrothermal reaction. It is demonstrated that formation of the NPS layer can be controlled in this combined laser irradiation and electrochemical etching technique by adjusting the range of the laser wavelength. This also allows for further precise control of the numerical trend of the luminous flux.

1. Introduction

Porous silicon was first discovered by Uhlir in 1956 [1]. In typical experiments, when hydrofluoric acid (HF) was used to electrolytically polish silicon wafers, an unexpected black layer was produced on the surface of the wafer. The thin film which formed on the surface has been identified as porous silicon (PS). In subsequent decades, PS has been extensively studied, with various applications being found, such as in MEMS [2], semiconductor devices [3], biosensors [4], light-emitting diodes [5], and fuel cells [6]. In the traditional approach, PS is formed by electrochemical anodization (ECA) etching in an HF solution. The ECA method is still used by many researchers today, but the structural properties of the PS layer formed depend on the electrolyte, current density, and etching time. However, controlling the thickness and porosity of the PS layer is extremely difficult because there are so many process variables which can affect the formation of large areas of nanoscale PS [7]. One important application is surface-emitting diodes, whose active areas contain PS formed by nanocrystalline porous silicon (NC-PS) dots in a tree-like network [8,9].

The effects of diode laser irradiation at varying wavelengths on the formation of P-type PS has attracted the interest of many researchers. The simultaneous application of laser beam irradiation during the anodic oxidation etching process along with the concentration and temperature of the etching solution may induce a band gap energy absorption (BEA) [10] effect. The accumulation of photogenerated electrons on the P-type silicon surface forms a region of electron-hole depletion [10,11,12] which inhibits electrothermal etching and slows down the formation of nanoporous silicon (NPS). This NPS has found numerous applications and been continuously studied in recent years.

In an earlier work [13], we reported on the feasibility of using laser irradiation combined with electrochemical etching to produce NPS on P-type silicon. The formation of NPS on a silicon surface by laser irradiation during electrochemical etching is influenced by the laser power and wavelength. The formation mechanism of NPS can be explained by BEA, which controls the process during electrothermal reactions. Analytical techniques have been employed to analyze the physical and chemical properties of the NPS. For example, an integrating sphere system can be utilized to measure the light flux emitted by the PS layer on the P-type wafer, along with the structural characteristics, such as pore size and pore distribution. These characteristics affect the scattering and reflection of photons within the PS, in turn influencing the resulting photoluminescence [14,15,16]. This study aims to control the distribution curve of the light flux values emitted by the PS by adjusting the laser wavelength and determine the optimal values for obtaining the desired PS luminescence characteristics.

Herein, the effects of the electrochemical etching of P-type silicon wafers with the application of different output laser wavelengths and etching times are reported. The etching rate changes due to the different absorption gap energies used during the photoelectrochemical process. When the energy of the incident laser light is high, the etching reaction is more difficult to control, and a nanoscale PS layer can form. The suppression of the formation of PS by laser radiation energy is explained by the reversal in conductivity from P-type silicon to N-type silicon and the simultaneous BEA reaction which occurs during the reaction process. In the experiments, an electrochemical etching tank was employed, and a simple laser holder was set up. During electrochemical etching, laser irradiation was performed on the surface of a P-type silicon wafer using four different laser wavelengths: 633, 830, 1064, and 1310 nm [13,17,18,19]. This study primarily investigates the influence of laser power on the formation of PS nanostructures. When the anodizing process is carried out in combination with laser irradiation, the electrochemical etching solution produces a low-concentration electrothermal reaction, causing a BEA reaction. The formation of general-level and micron-level PS structures is strongly inhibited under laser irradiation. Thus, laser-generated electrons accumulate on the P-type surface, forming an area of electron-hole depletion which completely inhibits electrothermal etching and slows down the formation of NPS. Analysis was then carried out to determine the trends in the luminous flux effects. The goal was to overcome the shortcomings of the traditional electrochemical etching process, specifically the difficulty in controlling the size range of the PS structures. The BEA reaction caused during the electrothermal reaction helps optimize the preparation process of light-assisted NC-PS to achieve precise modulation of the light flux characteristics. This new method allows for fast and accurate control of the photoluminescence intensity and luminous flux intensity of the PS. Looking to the future, the role of PS diode components is expected to be further developed and the number of applications expanded. It is critical, however, to enhance the photoluminescent properties of nanoscale PS for use in future PS diode devices [20].

2. Experimental Process

The photoelectrochemical etching set-up devised in this study combines two major systems: the electrochemical etching system and the laser source with optical path erection system. Before the etching process began, each test piece was cleaned as follows. First, a two-inch, boron-doped, P-type single-crystal double-side polished (DSP) silicon wafer (100) with a resistance of 0.001–0.005 Ω-cm (B = 1.34 − 12.7 × 10¹⁶/cm³) was chosen as the test piece. The dimensions were two inches to match the size of the electrochemical etching tank. The wafer surface was then cleaned to prevent it from being affected by contaminants during etching. Since contamination by dust, oil, metal ions, etc. can affect the stability of the experimental process, leading to uneven etching and distortion of the data, reducing external variables by cleaning the wafer surface is essential. The single-crystal silicon surface was first cleaned with a Piranha solution in a sulfuric peroxide mixture (SPM) and then with boiling solutions of RCA-1 (NH₄OH:H₂O₂ = 1:1:5 for 240 s) and RCA-2 (HCl:H₂O₂ = 1:1:6 for 300 s). Finally, the native surface oxides and residual contaminants were removed by immersing the wafer in HF for 10 s. It was then dried with N₂ gas and stored in a filtered vacuum desiccator (model 550, Kartell^®; Noviglio, Italy). Furthermore, the electrochemical etching equipment used in this study included two major systems: the power supply and the electrochemical etching tank. The power supply system (model ABM-PR8363) (ABM, New Taipei, Taiwan)allowed the required current or voltage to be set. Electrochemical anodization was performed using a mixed solution of 49.5% HF and 99.5% ethanol (mixed at a ratio of 1:1.5), which was stored in a polytetrafluoroethylene (PTFE) container for later experiments. In the electrochemical method, a gold (Au) (purity: 99%) cathode was utilized for the electrode part during the etching process. Au was chosen to achieve the desired surface impedance or specific chemical reactions during the etching process. The maximum electrolyte capacity in the PTFE tank was 500 mL, and the sample etching area was 6.4516 × π cm².

The process for the formation of porous silicon involved two steps. First, a laser with a specific wavelength was used. Electrothermal etching was inhibited throughout the process, slowing the formation of PS. It should be noted that diode lasers were used because they are typically small and therefore easy to integrate into various devices and systems. They also have the advantages of being efficient at converting the input energy into output light energy with relatively low power consumption and heat generation. This eliminates thermal interference and promotes PS formation during etching, facilitating BEA. Therefore, in this work, lasers with wavelengths of 633, 830, 1064, and 1310 nm with a power range of 1–20 mW were utilized for the formation of PS nanocrystals. In addition, the laser mechanism platform was also equipped with new polarizers (high-energy Glan-laser polarizers, wavelength range: 350–2300 nm (Unice 2-GL-3522-3) (Edmund Optics (EO), NJ, USA). The laser energy could be precisely controlled by the polarizer. A model PM100D laser power meter (THORLABS; Newton, NJ, USA) was used to meet the required process conditions. This photoelectrochemical etching system was optimized. The wafer holder controlled the etching range during the photoelectrochemical anode treatment process. Additionally, the traditional step of immersion in a hydrogen fluoride electrolyte for etching, which causes the current to concentrate at the tip, resulting in uneven etching, was replaced by the use of optical lenses to eliminate the Gaussian distribution phenomenon of the laser beam and improve the overall wafer uniformity, as shown in Figure 1.

This study employs precisely controlled electrochemical etching parameters to optimize the current density and etching time, which in turn controls the size and distribution of the PS. Ensuring uniform and moderate pore sizes enhances the quantum confinement effect, thereby increasing the photoluminescence (PL) efficiency. High-purity, low-resistivity P-type silicon wafers were used as the starting material, which not only reduce impurities and defects but also improve the emission efficiency. A surface passivation treatment was carried out to minimize surface defects by soaking the sample in a chemical solution. A 325 nm LED external light source was used to excite the P-type PS nanodots and enhance the PL efficiency. By integrating these methods, the aim was to optimize the optical and electrical properties of the PS, thereby improving the overall luminous flux efficiency.

First, laser powers ranging from 1 to 20 mW were applied for various etching times to form the nanoscale PS layer. In the second step, electrochemical etching was performed, and the etching times were adjusted to achieve the desired structure. The Si wafers were electrochemically etched for 1–30 min with a current density of 100 mA/cm². After the formation of PS, the wafers were rinsed with deionized water, dried at room temperature, and placed in a filtered vacuum dryer (box) for storage. Further analysis of the luminous flux values and luminous efficiency of the NPS samples was then conducted.

The surface morphology of the NPS formed by photoelectrochemical etching was studied using a field emission-scanning electron microscope (FE-SEM). In addition, the NPS sample was subjected to excitation by 325 nm ultraviolet light in a sphere system inside the integrator, as shown in Figure 2b. The NPS layer produced green light irradiation, as shown in Figure 2a.

The size of the PS quantum dots could be precisely tuned by adjusting the laser power and the parameters of the photoelectrochemical process, allowing for optimization of their emission characteristics. For this reason, we used a low-power laser for photoelectrochemical etching. The wavelength of the green light was approximately 536 nm with an intensity of 5106 a.u., as determined from the PL spectra and as shown in Figure 2a. The size of the quantum dots is shown in Figure 3 and Figure 4, which include the SEM image and the size distribution curve. This precise control is essential for meeting the application requirements in the preliminary fabrication process for PS diode components.

3. Results and Discussion

Electron-hole pairs are created as silicon absorbs photons. Laser light with a short wavelength is absorbed within a short distance, while longer wavelengths penetrate to greater depths. The generation of electron holes by light diffusion and drift is identified as transfer in the depletion region [9,21]. In this work, the photoelectrochemical etching process accelerates the flow of the electric field through the etching solution, encouraging an BEA reaction [22]. When the electron holes encounter the energy level barrier, they diffuse onto the surface of the silicon wafer. The closer the energy level barrier is to the laser irradiation point, the more reactive BEA etching will occur. For P-type semiconductors, the photocurrent may be several orders of magnitude larger than the dark current [23]. This phenomenon, which suppresses photoelectrochemical etching in this region, can be ignored. Long-wavelength laser light can be transmitted much deeper, generating electron-hole pairs which spread from the surface to the corresponding deeper areas.

When long-wavelength laser light is used for photoelectrochemical etching, it is the energy level barrier of silicon that is the factor in the BEA process which explains the formation of different dimensions of PS particles. The irradiated area on the silicon surface appeared to be black (as marked by the yellow line in Figure 3). This morphology confirms that the intensity of the laser was sufficient to inhibit subtractive electrolysis on the silicon wafer, which facilitates etching, while the non-irradiated area exhibited an anti-etching phenomenon, fully demonstrating the BEA effect [10]. However, the higher the power density of the long-wavelength laser, the more it could suppress the early occurrence of the BEA effect. The NPS structures produced in the laser-irradiated area were smaller than 10 nm (Figure 3).

The NPS particles produced on the samples had a uniform distribution with an average size ranging from 10 to 20 nm (Figure 4). The NPS distribution was calculated using the ImageJ analysis tool, and the distribution was graphically represented as a histogram with a superimposed normal distribution curve. This uniformity of distribution is critical because it influences the physical properties of the NPS, which is crucial for various applications. In summary, the size distribution of the NPS particles and the associated physical phenomena, such as the quantum confinement effect and the BEA, had a significant influence on the properties of the resultant material and the potential applications. The enhanced uniformity in particle size improved the reliability and performance of the NPS, leading to technological advancement.

Free carrier absorption [18] leads to energy accumulation when boron is excited by photons. In addition, it has been shown that long laser wavelengths can completely penetrate silicon. This allows for aggregation of boron-silicon bonds (B-Si) in both shallow and deeper layers due to the irradiation of laser energy. Some of the silicon atoms around the boron are protected, which has the effect of inhibiting etching. The electrochemical etching equations for NPS formation are presented below. The electrons in the silicon combine with the holes to form a depletion layer [24], and the photoelectrochemical etching reaction produces the BEA effect:

$$\mathrm{S}\mathrm{i}+2\mathrm{H}\mathrm{F}+(2-\mathrm{n})\to \mathrm{S}\mathrm{i}{\mathrm{F}}_2+2{\mathrm{H}}^{+}+\mathrm{n}{\mathrm{e}}^{-};$$

(1)

$$\mathrm{S}\mathrm{i}{\mathrm{F}}_2+2\mathrm{H}\mathrm{F}\to \mathrm{S}\mathrm{i}{\mathrm{F}}_4\to \mathrm{S}\mathrm{i}{\mathrm{F}}_4+{\mathrm{H}}_2;$$

(2)

$$\mathrm{S}\mathrm{i}{\mathrm{F}}_4+2\mathrm{H}\mathrm{F}\to \mathrm{S}\mathrm{i}{\mathrm{F}}_6^{2-}+2{\mathrm{H}}^{+}.$$

(3)

During the process of anodization under laser irradiation, the photoelectrochemical etching solution produces an electrothermal reaction at a low concentration, causing a BEA reaction. Boron becomes energy-saturated when laser photons are excited, leading to the BEA phenomenon. After being isolated through diffusion, the boron atoms are surrounded by silicon atoms so they cannot jump to a higher energy level. Consequently, their energy diffuses to the surrounding silicon atoms, causing the surrounding boron and silicon electrons to be attracted to each other. This attraction prevents the B-Si bond in the inner layer from being destroyed by the HF. This mutual pull between the inner boron and silicon electrons (B-e- tow Si-e-) affects the outer Si-Si bonds, causing it to be influenced by SiF₂. The electrons are then released, leading to electrochemical etching. A schematic diagram of the reaction mechanism is shown in Figure 5.

The trends of the luminous flux on photoelectrochemical PS were measured using laser wavelengths of 633, 830, 1064, and 1310 nm (Figure 6). A comparison of the maximum and minimum luminous flux with the etching time for all the wavelengths is presented in

Table 1. Calculated luminous flux for different etching times produced with different wavelengths.

Laser Wavelength (nm)	Highest Luminous Flux (lm)	Etching Time (min)	Lowest Luminous Flux (lm)	Etching Time (min)
633	123	5	55	30
830	66	15	25	30
1064	67	10	42	30
1310	62	10	19	30

.

The maximum etching time was 30 min, proving that the BEA phenomenon occurs during the photoelectrochemical reaction and that the laser suppression time is the key influencing factor to extrinsic absorption and free carrier absorption. It can be seen that the highest luminous flux values of the 1064 nm and 1310 nm lasers were both found at 10 min. However, under 30 min etching conditions, the luminous flux of the 1310 nm laser dropped to 19 lm, while for the 1064 nm laser, it was 42 lm. The luminous flux measurements confirm the interaction of the free carrier absorption effect with the BEA process. The silicon atoms aggregated when the boron atoms absorbed photon energy. As the outer silicon was etched by the HF, trapped holes formed around the silicon, inhibiting and reducing the effectiveness of the process. However, suppression by the 1064 nm laser involved a comprehensive phenomenon of extrinsic absorption and free carrier absorption [25]. The suppression of photoelectrochemical etching in the area was better than that with the 1310 nm laser, resulting in the formation of a larger number of nanocrystals in the polysilicon. The number of nanocrystals in the PS affected the luminous flux value.

The relationship between the luminous flux value and luminous efficiency under different laser energy irradiation powers was examined. The luminous flux value and luminous efficiency of NC-Si were influenced by the laser powers. We observed distinct trends in the luminous flux values during the process of photoelectrochemical etching using lasers with wavelengths of 633, 830, 1064, and 1310 nm. The highest luminous flux values, ranging from 100 to 120 lm, were achieved with the 633 nm laser, with a corresponding luminous efficiency between 30 and 40 lm/W (Figure 7a). This result indicates that shorter wavelengths (633 nm) are more effective at producing greater luminous flux and improved efficiency. This efficiency is attributed to the optimal interaction of shorter wavelength photons with silicon, enhancing the quantum confinement effect and the boron-enhanced absorption phenomenon. The quantum confinement effect occurs when the size of the silicon nanocrystals is comparable to or smaller than the exciton Bohr radius. This results in discrete energy levels and an increased bandgap, which can enhance luminous efficiency. The electrons and holes generated by light diffuse and drift, resulting in a transfer phenomenon within the depletion region. For the 830 nm laser (Figure 7b), the luminous flux ranged from 50 to 80 lm, with efficiencies between 15 and 25 lm/W. The 1064 nm laser (Figure 7c) demonstrated a wider range of luminous flux from 55 to 85 lm, with efficiencies from 20 to 30 lm/W, indicative of its potential for applications requiring varied luminous outputs. Last but not least, the 1310 nm laser (Figure 7d) produced luminous flux values between 60 and 90 lm and efficiencies from 20 to 30 lm/W. Despite its longer wavelength, the 1310 nm laser still achieved a relatively high luminous flux and efficiency, although not as high as the 633 nm laser. This was due to the effective suppression of non-radiative recombination processes at this wavelength to maintain reasonable efficiency levels.

The method described above not only enhances the luminous flux but also maintains a narrow energy dispersion, optimizing the luminescence stability of the resultant NPS. We studied the physical and chemical characteristics for the formation of NPS layers through laser irradiation and electrochemical etching. It was found that stronger laser powers produced PS with higher luminous flux values, positively correlating with the BEA reaction. Our results indicate that stability of the NPS luminescence was achieved based on the flux characteristics of the nanoscale PS layer produced through the electrochemical etching and laser suppression processes. The 1064 nm laser showed a comprehensive absorption phenomenon of extrinsic absorption and free carrier absorption, leading to the formation of a significant number of nanocrystals in the polysilicon compared with the 1310 nm laser. This method of integrating PS with lasers to control the etching process demonstrates good potential for the production of silicon-based microelectronic devices and offers a promising development to improve the performance of NC-Si diodes and other silicon-based components.

These results illustrate the influence of different laser wavelengths on the luminous flux and efficiency of NC-Si. The high luminous flux and efficiency achieved with the 633 nm laser can be attributed to its optimal interaction with the material, which enhanced the quantum confinement effect and the boron-enhanced BEA phenomenon. This effect was more pronounced at shorter wavelengths, such as 633 nm. At this wavelength, the energy of the incident photons was sufficient to excite electrons to higher energy states within the confined nanostructures, thus enhancing the overall luminous efficiency.

Optimizing the photoelectrochemical etching process is crucial for enhancing silicon-based microelectronic devices and for NC-Si diode applications. By leveraging the quantum confinement effect and understanding the role of BEA, we can develop more efficient and stable luminescent materials for advanced technologies.

Figure 8 illustrates the physical characterization of quantum confinement effects in photoelectrochemically etched NPS structures. The figure shows an NPS sample under laser illumination, highlighting the luminescent properties of the etched silicon (Figure 8a). This luminescence was a direct consequence of the quantum confinement effect, where the reduction in particle size led to discrete energy levels and an increased bandgap, resulting in enhanced photoluminescence.

The size distribution results indicate that most pores ranged in size from 10 to 20 nm (Figure 8b). As can be seen in the distribution graph, approximately 15% of the pores were ~10 nm. This size distribution is critical for the quantum confinement effect, as the smaller pore sizes fall within the regime where quantum confinement significantly alters the electronic and optical properties of the silicon nanocrystals.

The quantum confinement effect is particularly evident in the photoluminescence of NPS. When the size of the silicon nanocrystal approaches the exciton Bohr radius, quantum confinement leads to the quantization of energy levels. This results in a blue shift of the emission spectrum and an increase in the recombination efficiency of the electron-hole pairs, enhancing the luminescent properties of the material. The data in the panel in Figure 8b support this, showing a high proportion of small-sized pores, which are essential for achieving strong quantum confinement.

In this context, the observed laser-induced photoluminescence (Figure 8a) can be attributed to the efficient recombination of electron-hole pairs within these quantum-confined nanocrystals. This is further corroborated by the porous distribution, which suggests that most pores were of an optimal size to exhibit quantum confinement effects.

In summary, the luminescent properties and the size distribution of the NPS structures demonstrate the significant role of the quantum confinement effect due to photoelectrochemical etching. The resultant data underscore the importance of optimizing the etching parameters to achieve the desired nanocrystal sizes, thereby enhancing the photoluminescent efficiency and expanding the potential applications of NPS in advanced technological fields [26].

4. Conclusions

This study delved into the effects of photoelectrochemical etching techniques on the formation of NPS and examined the influence of different laser wavelengths on the process. It was found that simultaneous laser irradiation and electrochemical etching could effectively form NPS layers. Physical and chemical characterization of the NPS identified the variation in the luminous flux values of the samples with different laser wavelengths. Higher laser power produced PS with higher luminous flux values and a positive correlation with BEA. Thus, laser excitation facilitated NPS BEA reactions based on photothermal etching. Using this technique, we determined the flux characteristics of the nanoscale PS layer needed to achieve the most stable luminescence during the photoelectrochemical etching and laser suppression processes. This approach effectively reduces the loss caused by the reduction of quasi-ballistic electron emission. It optimizes the uniformity of the structure of electrical NPS produced through the photoelectrochemical etching method by minimizing scattering loss. The peak energy emitted maintains a narrow energy dispersion, meeting the optimization goals. These results pave the way for the future production of NC-Si diode components.