Study of Some Properties of PbI 2 Deposited on Porous Silicon Using Thermal Evaporation Technique for Many Applications

The present work is a study of some properties of PbI 2 deposited on porous silicon (n-PSi) by using the thermal evaporation technique. X-ray diffraction, scanning electron microscopy, UV–Vis spectrophotometer, and FTIR analysis were used to characterize the structural, optical, and morphological properties of n-Psi. X-ray diffraction showed that the PbI 2 film has a hexagonal polycrystalline structure, while FE-SEM images showed porous silicone in Photoelectrochemical etching, the pore distribution is irregular and the pore refers to the increased surface area of the silicon. SEM images of pbI 2 film showed that particles were scattered and resembled gravel in size. The estimated optical energy value of thin films of PbI 2 was 2.6 eV. PbI 2 film has lower transmittance values at short wavelengths, but as the wavelength increases, the transmittance values gradually increased. The greatest transmittance value was 0.88. From FTIR analysis, chemical bonds were determined between porous silicon and PbI 2 .


Introduction
Lead Iodide (pbI2) has emerged as a promising material with broad technical applications, including Perovskite photovoltaic solar cells, X-ray, and γ-ray detectors at room temperature [1][2][3][4][5][6]. Lead-iodide is an important ingredient in the fabrication of solar perovskite cells, which have been extensively explored by numerous researchers across the world and have recently achieved a Power Conversion Efficiency (PCE) of more than 20% [4]. Lead iodide is a semi-conductive type P with a high energy gap of 2.3-2.6 eV depending on the deposition process, and a high atomic number with iodine having atomic number = 82 and lead atomic number = 52, making it useful as an ionizing radiation detector [3], it also can be utilized in medical imaging, nuclear detection and photosensitivity semiconductor metal applications [7]. Generally, PbI2 can be crystallized by a hexagonal lattice layered form. Its hexagonal closed-pack (HCP) crystal is made up of covalently bound layers of I-Pb-I that are piled on top of one another by weak van der Waals bonds perpendicular to the crystal c-axis [00l]. The anisotropic features of pbI2 crystals are due to this layered structure, which may lead to the formation of numerous polytypic structural changes. [8]. Porous silicon (PS) has recently been a matter of significant investigation, owing to its photoluminescence features and possible uses in photovoltaic devices, chemical sensors, and biological sensors [9,10]. The idea of etching silicon surfaces has gained a lot of importance in semiconductors and solar cells since it helps to improve and produce devices that have a wide range of applications. The etching process has progressed to nanotechnology, where the material acquires new properties as it reaches atomic dimensions and is governed by quantum rather than classical rules [11]. Porous silicon (PSi) is made up of a network of silicon wires and voids that are nanoscale in size. It is made in a variety of ways, including photoelectrochemical etching of the surface of crystalline silicon in an aqueous solution of (HF)acid, with carefully controlling the preparation conditions (etching time -slice specifications -acid concentration and current density) to obtain a suitable crystal structure from porous silicon with various layers of porosity [12,13]. This work aims to prepare lead iodide films deposited on porous silicon by a vacuum evaporation method, it also studies some properties of this material to present preliminary results for a variety of applications. In (2018), Rana Kadhim Abid alnabia, Malek A.H. Muhi et al have prepared lead iodide that was applied to glass bases by a thermal evaporation process, they also studied optical and electrical properties. The film showed a hexagonal crystalline shape. The value of the energy gap for a sample of 200 nm thickness is 2.9051 eV, the intensity is at the plane (003), it equals 12.5 nm. The electrical properties were studied for the measurements of electrical conductivity, mobility, carrier concentration, and finally the Hall coefficient. The results are (1.038*10-5, 0.6727*10+2, 1.009*1012 and 2.631*106), respectively [14].

Experimental Procedures 2.1. Preparation of Porous Silicon
The substrate was a crystalline wafer of n-type Silicon with a resistivity of (5) Ω.cm, a thickness of 500 m, and an orientation of 100. The substrates were cut with (1.5 x 1.5cm 2 ) areas. After chemical treatment, 0.1μ m thick Al layers were formed on the backsides of the wafer using an evaporation method. Photoelectrochemical etching was then carried out at room temperature by employing a platinum electrode in a mixture (1:1) of HF (45%) and Ethanol (99.99%). The light source was a Halogen lamp with a light intensity of 20mW/cm 2 to ensure homogeneity of the etch layer, current of 14mA/cm 2 was applied for 8 minutes as shown in Figure 1. The etched area was (0.785 cm 2 ) as shown in Figure 2.

Preparation of PbI2thin Film
PbI2 films of 200 nm thickness were deposited on a cleaned glass and PSi substrate by the thermal evaporation method at a 10-5 Torr vacuum using a high vacuum coating apparatus (Edwards type E306A). The distance between the source and the substrate was roughly 18 cm inside the vacuum chamber, where a molybdenum boat was utilized to transport PbI2 powder. Figure 3 a & b shows the thermal evaporation procedure and the PbI2 thin film after it has been deposited on the PSi substrate, while Figure 4 shows the cross-section image of the heterojunction that has been prepared.

Characterization
PbI2 and PSi film were characterized by utilizing characterization techniques, namely: X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV-Vis, and Fourier transform infrared spectroscopy (FTIR).  Figure 5 shows X-ray diffraction of crystalline Si and n-PSi materials. Blue plots for n-Si and red drawings for PSi were produced by anodization with a current density of 14 mA/cm 2 and an etching period of 8 minutes. When compared to the peak of n-type porous silicon (n-PSi), the crystalline Si peak has a higher intensity value. Even after the etching procedure, the etched silicon retains its single crystal structure, but due to strain, it slightly moved to a small diffraction angle (2θ 69.428 o and 69.381 o ), orientated exclusively along the 400 direction, resulting in a modest expansion in the lattice parameter [17,18]. The Cu-K target with wavelength 1.54060°A was used to achieve X-ray diffraction of PbI2 film that was produced on PSi substrate (using XRD Analysis XRD-6000). Figure 6 reveals the principle diffraction peaks at corresponding planes 001, 101, 002, 202, 003, 210, and 004 at the diffraction angles 12.73°, 23.98°, 25.58°, 28, 38.65°, 39.7, and 52.13°, respectively. Similarly, the figure reveals the sharp and narrow peak at the angle of 69.8 o for the porous silicon layer with orientation (004). PbI2 thin film is polycrystalline and has a hexagonal structure, it was indicated in the JCDPS card about [14]. Scherer's Formula (Eq.1) was then used to calculate the average crystalline size that was around (18.28) nm [14][15][16][17][18][19][20] when θ, and in radian angle, and k is a shape factor that equals 0.9.

=
(1) D: crystalline size, ( ): wavelength for x-ray (1.5406), : is the full Width at half maximum, and : is a degree of diffraction [21]. Figure 7a shows  The optical properties were recorded by using a UV-Vis spectrophotometer (Metertech SP8001). Figure 8 shows the transmission spectra of PbI2 films deposited on a glass substrate using thermal evaporation as a function of wavelength (200-1000) nm. The film's structure, preparation, film thickness, and deposition circumstances all have a significant impact on transmission. PbI2 film has lower transmittance values at shorter wavelengths, but as the wavelength increases, the transmittance values gradually increase. The greatest transmittance value was 0.88, which is ideal for optoelectronic devices, particularly solar cell window layers. Furthermore, a dramatic drop at the band's edge refers to PbI2 film crystallinity, which is consistent with XRD data.

Figure 8:
The transmission spectrum of PbI2 films deposited on glass substrate by thermal evaporation Tauc plot was used to analyze the optical band gap. Figure 9 depicts a linear relationship between (αhv) 2 and photon energy. This behavior indicates that a direct permitted transition is possible in the PbI2 film. From the linear part of the curve to the photon energy axis, the optical band gap can be extracted [15]. For PbI2 thin film, the extrapolation yielded a band gap of 2.65 eV. The energy gap was calculated by using Eq. (2):  FTIR technique is a potent tool for determining the chemical species present in the substance. This approach measures how much radiation can be absorbed by chemical bonds in the substance as the wavelength of the radiation changes in the infrared range. Different chemical bonds absorb different frequencies of radiation [22]. The best way to determine the chemical composition of the PSi surface is to use Fourier Transform Infrared (FTIR) spectroscopy. Because PSi has a substantially bigger specific area than bulk Si, the FTIR signal is larger and easier. The surface of the manufactured PSi layer oxidizes spontaneously after a few hours in ambient air [23]. For original impurities such as hydrogen and fluorine, which are residuals from the electrolyte, the pore surface has a high density of dangling Si bonds. Figure10a shows that the FTIR spectra can be measured from samples at current density (14 mA/cm 2 ), and etching time (8) minutes. The peaks at around 617 cm-1, 663 cm-1, and 891 cm -1 are from Si-Si, Si-H, and Si-O, respectively. The transmittance peak at 1423-1465 cm -1 and 1519 cm -1 isdue to C-H. The peak at 1708 and 2380 cm -1 is due to C-O, and peaks at 3614 and 3745 cm -1 are due to O-H and Si-OH [24][25][26][27][28]. Figure 10b shows IR spectra of the PbI2 sample that was deposited on the PSi substrate. The peak at frequency 1627 is referred to as the band Pb-I [29]. The peaks around 3412 and 3383cm-1 are due to asymmetric and symmetric stretching vibrations of the Pb-I cluster [30]. It can be noted that the appearance of weak peaks of PbI2, which perhaps can be attributed to the PSi substrate.

Conclusions
We have prepared and characterized the nanocrystalline porous silicon layer and PbI2 thin film by thermal evaporation technique to study some of its properties. X-ray diffraction showed that the PbI2 film has a hexagonal polycrystalline structure. The band gap energy for PbI2 was estimated to be 2.6 eV. FE-SEM images showed porous silicone via using the photoelectrochemical etching method, the pore distribution is irregular. SEM images of PbI2 film revealed that particles were scattered and resembled gravel in size. The results indicated that PbI2 can be used in physical applications such as solar cells.