Study the Structural Properties of Porous Silicon and their Applications as Thermal Sensors

impact of probe diameters


Introduction
One of the most crucial concerns in measuring technology is thermal sensing.Thermal transducers have been manufactured and are being made in abundance 1 .Anisotropic etching is widely used in silicon bulk micromachining to produce microstructures for diverse applications in the field of microelectromechanical systems (MEMS).Furthermore, it is most commonly employed for surface texturing in order to reduce light reflection and hence improve the efficiency of crystalline silicon (c-Si) solar cells 2 .This approach has been used in industry for many years now.It provides a lot of benefits, but there are also some serious disadvantages, so it's important to look into alternatives.Many amazing characteristics of porous silicon (pSi) have been explored in-depth 1,2 .The pSi has a number of intriguing characteristics in addition to its photoluminescent properties, such as a variable index of refraction, low visible absorption of light, substantial interior surface, customizable surface chemistry, or strong chemical activity 3 .All of these characteristics, as well as the simple production of material and the ability to create precisely regulated multilayer structures, make it suitable for usage in a variety of industries, including optics, Opto-and microelectronics, biomedical applications, and chemical sensing 4,5 .The thermal conductivity of bulk crystalline Si follows the temperature dependence of single crystal dielectric materials.It is often regulated by phonon scattering with other phonons, Si isotopes, and crystal boundaries 6 .
Paladiya and Kiani 7 , discussed the impact of probe diameters on sensing performance is explored

Abstract
The photo-electrochemical etching (PECE) method has been utilized to create pSi samples on n-type silicon wafers (Si).Using the etching time 12 and 22 min while maintaining the other parameters 10 mA/cm 2 current density and HF acid at 75% concentration..The capacitance and resistance variation were studied as the temperature increased and decreased for prepared samples at frequencies 10 and 20 kHz.Using scanning electron microscopy (SEM), the bore width, depth, and porosity % were validated.The formation of porous silicon was confirmed by x-ray diffraction (XRD) patterns, the crystal size was decreased, and photoluminescence (PL) spectra revealed that the emission peaks were centered at 2 of 28.5619° and 28.7644° for etching time 12 and 22 min, respectively.Studying the capacitance and resistivity during temperature increasing and decreasing for both itching times shows clearly that the prepared pSi as a thermal sensor is working better and in more selectivity for 20 min itching time.Chen and Zhang 9 reported that the silicon material with aligned distributed rectangular-shaped holes is proposed for the manufacture of semiconductor devices.A comprehensive understanding the heat conduction is of great significance to improve the efficiency of thermoelectrical materials.Baran et al. 10 discussed the effects of heat oxidation on the sensing characteristics of porous silicon.Electrochemical etching was used to create porous silicon substrates, which were then thermally oxidized at various temperatures.An EDS comparison reveals that porous surfaces oxidized at higher temperatures have greater oxygen-to-silicon ratios.This work aimed to prepare porous silicon (pSi) using the PECE method and study the structural properties of porous silicon and discussed the results.

Experimental Work
The Formation of pSi The materials and method used in the present study are n-type silicon (Si) wafer that is commercially available, 625  thick, with10 Ω. cm resistivities.The silicon wafer samples were cut as squares with dimensions of 1.51.5 cm 2 .The samples were washed with ethanol to remove the impurities.To remove the natural oxide layer, they were etched in diluted 10% hydrofluoric (HF) acid.The PECE process was used to create the pSi layer in an electrolyte solution that was a 0.75:1 combination of 75% HF and 1% C2H5OH.Typically, ethanol is used to prevent the accumulation of hydrogen bubbles.Teflon, which resists corrosion from the HF electrolyte exceptionally well, was used to construct the cell configuration.The sample is represented by the anode, which is situated halfway along the cell (Si).The cathode is shaped as a platinum (Pt) ring immersed in an HF electrolyte.The (HF) electrolyte was poured into the Teflon cell's top.The electrolyte must be present in sufficient amounts to both covers the Pt electrode and provide the required fluorine ions.The laser IR source 810 nm was used to execute the PECE for 12 and 22 min at a current density of 20 mA/cm 2 with intensities ranging from 15 to 20 mW/cm 2 .The typical schematic of the (PECE) system is illustrated in Fig. 1.

Scanning Thermal Microscopy
The scanning Thermal Microscopy (SThM) method is depicted in Fig. 2 as a thermal probe 12 .The thermosensitive Wollaston probe is used to perform thermal conductivity measurements.In order to precisely measure the probe's resistance, it is coupled to an electrical circuit using a Wheatstone bridge setup.The most popular way is to utilize a DC current to run the probe in what is known as the active mode 13,14 .The sample is placed far from the Wollaston wire, which, due to the Joule effect, is heated.After then, the electrical probe is powered

XRD analysis
Analysis of XRD patterns was performed in the 2θ range of 10°-80° for (12 and 22)min etching time as shown in Figs.The crystalline size (C.S) can be calculated by using the value of β from Figs. 2, 3 with the wavelength λ at broadening peak at the diffraction angle 28.5619 and 28.7644 for 12 min and 22 min, respectively.The crystalline size now can be determined by using equations 1 and 2 as 29.02 nm and 20.94 nm for 12 min and 22 min itching time, respectively.The microstrain (ε), dislocation density (), and number of crystallites per area (o) are calculated using Eqs.2, 3, 4, respectively.

Field-Emission Scanning Electron Microscopy (FESEM)
The surface and cross-section images of porous silicon at various etching times are shown in Figs. 4 and 5.The images reveal that a layer of tiny nanoscale spheres with a diameter of about 50 nm covers the area of the pores and the outer surface layer.
The sample etched for 12 min appears to develop grooves and pores down to a depth of 12.82 μm.The formation of hexagonal holes with a depth of 12.82 μm and walls dividing the pore spaces were caused by lengthening the etching duration to 22 minutes.These structures were then covered with a layer of spherical nanoparticles with a diameter of about 37.22 nm.
A group of PS is displayed in a homogeneous pattern and structural pores in (FESEM) pictures, which supports the creation of uniform porous structures on silicon.the effects of a 22 -minute growth period on surface morphology.
Field emission scanning electron microscopy (FESEM) images showed that porous Si etched using the 12 min itching time has a higher porosity and density than porous Si etched using 22 min itching time.The atomic force microscopy results supported the FESEM results showing that porous Si etched using 12 min itching time has the highest surface roughness relative to the samples produced using the 22 min itching time.High resolution Xray diffraction revealed that porous Si produced through 12 min itching time has the highest peak intensity out of the 22 min itching time suggesting an improvement in pore uniformity with better crystalline quality.
The pores in the P-Si are seen as dark dots in Fig. 3, the diameter of pSi increases with time in Fig. 4.This largeness of width increases in holes number on the surface of pSi with etching time.After irradiation, Fig. 4 shows a pore-like spherical shape elongated and the defect clusters of pSi samples were large than those of the samples before irradiation as in Fig. 3, also, pore width was increased with increasing etching time, since the radiation reduces the thin walls between neighbour pores as Fig. 4.
Fig. 6 depicts the EDX elemental analysis for porous silicon prepared at various times.In addition to the Au peak, their patterns show silicon and oxygen emission lines.The intensities of the peaks differ in their height depending on their presence ratio in the samples, with the oxygen peak growing while the silicon peak decreasing with increasing etching time, showing the oxidation of some silicon atoms during the etching process.

Thermal sensor
The response of the capacitance for the sensor shown in Fig. 7 for the 12 min etched samples were recorded during the increasing and decreasing of the measured temperature with constant frequency at 10 kHz.Panel (A) from Fig. 7 shows that the capacitance is decreasing linearly with increasing temperature and panel (B) shows that the capacitance is decreasing with decreasing temperature as curvature variation.This means that the prepared pSi sample is very sensitive to temperature changes.
The sensor resistance was also recorded and displayed in Fig. 8  The capacitance and resistance variation with time for the 22min etched sample was studied and plotted as a function of temperature as depicted in panels (A) and (B) of Figs. 9, 10.
The folk of the peaks around the measured temperature of 50 °C for the resistance while the measured temperature is decreasing means that the thermal sensor is more sensitive to the temperature of the sample etched at 22 min.

Conclusion
In this study, the porous silicon (pSi) is prepared by the photo-electrochemical etching (PECE) method.The etching time is taken 12 min and 22 min for the preparation of pSi.The XRD analysis shows that when the etching current density increased the photoluminescence peak shifted toward shorter wavelengths which can be attributed to the changes in porous morphology Photoluminescence study showed that the emission peaks centred at approximately 20-40 nm.The XRD analysis and SEM images and study of the resistivity and capacity variation with the measured temperature show that the porous silicon prepared with 22 min itching time is more adequate as a thermal sensor than the porous silicon prepared with 12 min itching time.

Figure 1 .
Figure 1.Schematic representation of the backside illumination electrochemical etching technique 11 .
2024, 21(3): 1086-1094 https://dx.doi.org/10.21123/bsj.2023.4097P-ISSN: 2078-8665 -E-ISSN: 2411-7986 Baghdad Science Journal away from contact and the Wheatstone bridge is balanced.The sample is then in touch with the tip while still being at room temperature.The sample receives heat from the tip, causing the tip temperature to decrease and the electrical resistance to alter accordingly.This decrease in temperature affects the output voltage of the Wheatstone bridge and is directly related to the substance under investigation's thermal conductivity.

Figure 2 .
Figure 2. The XRD pattern of the sample prepared at 12 min etching time.

Figure 3 .
Figure 3.The XRD pattern of the sample prepared at 22 min etching time.

Figure 6 .
Figure 6.EDX spectra for porous silicon samples prepared at different etching time.
with increasing and decreasing temperatures for the constant frequency at 20 KHz.The resistance is increasing steadily as the measured temperature is increasing as shown in panel (A) and also increases up to the temperature of wt%  O 18.4 0.0 Si 81.6 0.0 wt%  O 15.2 6.1 Si 84.8 6.1 2024, 21(3): 1086-1094 https://dx.doi.org/10.21123/bsj.2023.4097P-ISSN: 2078-8665 -E-ISSN: 2411-7986 Baghdad Science Journal 80 °C then steadily increases up to 100 °C and drops at the temperature of 110 °C.

Figure 7 .
Figure 7. Capacitance variation with (A) increasing temperature and (B) decreasing temperature, for 12 min etched sample.

Figure 8 .
Figure 8. Resistance variation with (A) increasing temperature and (B) decreasing temperature, for 12 min etched sample.

Figure 9 .
Figure 9. Capacitance variation with (A) increasing temperature and (B) decreasing temperature, for 22 min etched sample.

Figure 10 .
Figure 10.Resistance variation with (A) increasing temperature and (B) decreasing temperature, for 22 min etched sample.