An investigation of photo_electrical properties of silicon nanoparticles/PSi/p-Si hetero structures

In this work, two forms of nanocrystalline, were used (as-prepared macroPorous silicon and silicon nanoparticle) to synthesize hybrid structures for photodetectors applications. The fabrication pathway was carried out through two 2-steps processes. The 1-step was formation of as-prepared silicon nanoparticle (SiNPs), while and the 2-step was creation of low spam of macroPSi size substrate via electrochemical etching process in HF solutions. Specific features of SiNPs and low spam of macroPSi size substrate were explored using scanning electron microscopy SEM, energy-dispersive x-ray (EDX), Atomic Force Microscope (AFM), and photoluminescence (PL) spectroscopy respectively. Dark and photo current characteristics and spectral responsively of photodetectors were investigated for the macPSi layer and the hybrid structure. The performance of the hybrid configuration shows an improvement in the sensitivity of about 0.75 A/W with appearance of new additional peak at 450 nm as compared with the PSi photodetector of about 0.93 A/W. The achieved improvement is related with the appearance of double Heterojunction device between PSi/si and SiNPs/PSi. Also this improvement may be related with the reduction of the reflectd light from the hybrid structure due to the multiple reflection between SiNPs/PSi. The quantum efficiency η of the photodetector in the spectral range 450–700 nm was found to be 65%.


Introduction
In recent years, UV photodetectors have obtained wide regard from researchers around the world due to presence many industrial applications, such as space communications, semiconductor devices processing, Nuclear reactor and ozone monitor, missile pillar detection. The UV detector can be passed out to a wide-ranging of the physical, chemical, electrochemical, and optical principles (Ding M et al. 2015). Advantages of low-cost UV detection sensors and the potential for integral with optoelectronics applications Elhouichet et al. 2002;Alwan et al.2018a, b). Porous silicon (PSi) photodetector has been used to detect ultraviolet rays. It has many advantages compared to other materials. It has many advantages, such as a high absorption coefficient of the ultraviolet area. Also, it does not require anti-reflective coating, and on the one hand, the cost of being Low and needs simple manufacturing technology. However, for optoelectronics applications, electroluminescent materials have great attractiveness. However, this significance is not enough for practical used Dheyab et al. 2020;Xiang-Liang et al. 2015;Narayan et al. 2000). Device structures were made using a new method of (PSi) forming called metal-assisted chemical etching, as this type of structure can be used as a radiation photodetector. (Wail et al.2018). The Al/p-Si/cSi/Al photodetector was made without any oxidation or post-annealing. Has been the effected of PSi prepared conditions on the performance of the photo-detector is underway studied (Khodami et al. 2008). The main target of this work, the effect of incorporating SiNPs on macro Psi substrate was investigated to develop the performance of photo sensing properties in UV region.The principles of increasing the energy bandgap within the hetrostructures devices of nanoparticles/PSi/p-Si heterojunction was employed successfully.

Materials and methods
Fundamental aspects of the production of porous silicon and nano-structures by electrochemical etching process, and this basic process were 2-steps as in Fig. 1 shows the etching set-up. P-type monocrystalline silicon substrate of (111) orientation and (3.5-8) Ω.cm resistivity was employed for the preparation of the SiNPs and macro-Porous silicon layer. The doping concentration can be calculated from the relation (S. M. SZE.1981): where (σ) is the conductivity (Ω cm −1 ), ( e ) is the electron mobility and equals is the electron mobility and equals (1500 cm 2 /V.s), (N A ) is the density of carrier, and (q) is the charge on the electron. The increase in resistivity will decrease the conductivity according to the following relation (S. M. SZE.1981): (1) p = q p N A Fig. 1 A 1-step. Schematic diagram of the etching process for porous Silicon This would give a doping carrier concentration of donor impurities (4.1 × 10 14 ) and for accepted impurities is about (1.25 × 10 15 ). The substrates were shaped to (2 × 2) cm 2 area and cleaned in dilute HF of concentration about 15% with the ultrasonic bath for 20 min to remove dirt residuals and native oxide layer. The backside of the wafers was coated with 0.2 μm thick high purity aluminum film by advanced device. The etching process was carried out with current density 8 mA/cm 2 , and hydrofluoric acid (HF) of 40% concentration, was used and diluted with 99.999% absolute ethanol (HF: C 2 H 5 OH) with a mixing ratio of (1:1) to prepare 20% HF concentration as the etching solution. The etching way was used in the teflon-cell with externally applied voltage for a period of (15) min and the area was fixed to about the diameter of about 1 cm. So an internal etching current was passed from the upper silicon surface to the lower silicon as in Fig. 1-1 setup.
The aqueous solutions of SiNPs were prepared using an electrochemical etching process in the regime of electro polishing by using high etching current density of about 100 mA/ cm 2 for 10 min. The suspension of colloidal SiNp to synthesize Si was deposited on the macroPSi by using drop casting process, this pathway of incorporating SiNps on porous structures is a very simple and rapid dipping method (Amer et al. 2018): Characterization of Si nanoparticle prepared by electrochemical etching method. In the process of preparing previously prepared silicon nanoparticles recurring and at fixed conditions, the etching process is collected and left for a period of time for a solution to evaporate HF, after that, the sample is taken and immersed in a solution to the process of incorporating the SiNPs with macro-porous silicon as in Fig. 2-2 step this may be called a process free-standing.
The mechanism of etching silicon nanoparticles and porous silicon is similar to the etching of silicon wafers, and includes the following reactions: In this reaction, silicon donates electrons to reduce SiNPs, and simultaneously becomes oxidized and etched away by F-(Nam-Ki Min et al. 2001).
Structural, morphological, and optical properties of porous silicon and Si nanoparticles were examined by, To reveal the features of as-prepared SiNPs and macro-Psi structures were explored using scanning electron microscopy (Angstrom Advanced type (AIS2300C) made in (USA) with EDS analysis, X-ray diffraction XRD pattern type (XRD-6000, Shemadzue).Angstrom AA 300 atomic force microscopy, and Shimadzu UV-Vis spectrophotometer. The dark and photo I-V characteristics of photodetectors were investigated; the photocurrent of the photodetectors was estimated under white light and UV source (365 nm) illumination. The spectral the responsivity (R λ ) of the photodetectors before and after adding SiNPs and measured in the range of 400-900 nm using a monochromator. For monochromatic calibration, a Sanwa silicon power meter was used for this. Figure 3a, b shows the SEM image of low spam of macroPSi size. After incorporating Si nanoparticles, it is clear that the pores were uniformly distributed and had a different shapes. Porosity was estimated and was about 85%.

Results and discussion
(2) = 1 Fig. 3a, the as-prepared low spam of macroPSi size has a pore-like structure with nearly cylindrical, rectangular, and random distributed formed pores. The histogram of the pores dimensions displays that the pore size distribution is non-symmetrical. The pore dimensions vary from 125 to 250 nm and the maximum of the histogram is positioned at 125 nm. Figure 3b displays the morphology of SiNPs / macro-PSi, the incorporation process of silicon nanoparticles occurs at the outer boundaries of pores rather than inside the pores itself and no aggregation behavior of SiNPs was observed. The histogram of SiNPs size vary from 30 to 90 nm and the maximum of the histogram is positioned at 40, and 50 nm, the SiNPs size is much smaller than that of the pore dimensions.
The incorporation of metallic nanoparticles on porous structure via ion reduction process during the dipping process follows the Volmert-Weber mechanism due to a weak interaction between metal ions and the semiconductor (Beaumont et al.2001). In order to have an idea of the chemical composition of the si nanoparticles, EDX analysis was carried out and the profile is shown in Fig. 4. The elements SiNPs are attributed to the porous silicon. The presence of O is evidence of biomolecular attachments. On the other hand, the occurrence of SiNPs element confirmed the successful synthesis of Si NPs over the porous silicon surface.
The 3D AFM of the morphology surface of PSi layer structure and SiNPs layers are given in Fig. 5a  The morphology of SiNPs studied using the AFM technique is shown in Fig. 5b; we have seen that all the particles were distributed inside a pore and on wall silicon. The middle particle size evaluated from this measure was approximately 50 nm. The surface morphology of the porous layer was significantly changed after adding SiNPs, as shown in Fig. 5b, and this confirms that the pores have relatively different shapes and sizes. The valued average particles' size of the porous layer before and after adding was 30 and 90 nm, respectively. Figure 6 displays the absorption spectrum of SiNPs. It is clear that the peak is located at 380 nm due to the quantum size effect and increased energy gap, which is blue-shifted with respect to that of the Si bulk due to the nanometer-size effect. This result is in good agreement with reported results (Monroy et al. 2003). The importance of this peak in the application of UV photodetection. The high absorption peak value of the surface plasmon resonance (SPR) band indicates that nano-particles are able to efficiently gain a very great extent of energy when irradiated with light at a suitable wavelength. Figure 7 illustrates photoluminescence (PL) emission and the nanocrystallite size contributions to its spectrum. Before adding nanoparticles it was a single peak emission at 680 nm was due to PSi nanocrystalline between pores. And after adding nanoparticles, two emission peaks (double) appeared at 680 nm (it's the same as PSi) and 450 nm of Si nanoparticles sitting on pores (bonds on pores).
The structural properties of SiNPs & macro-PSi structures were examined through the analysis of XRD pattern as presented in Fig. 8a, b.
From this Fig. 8, The XRD peaks located at 2 diffraction angle vary from 28.6°, 47.5°, 56.9°, 69.4°, and 76.2° respectively, which naturally located between peaks (reference:JCPDS card) at plane index as the (111) The (S) is a physical property and a much important feature of nanostructure material, and can be computed via using the following equation (Monroy et al. 2003): (4) L = 0.9 ∕βcosθ  where, density of monometalic NPs is the density of the Silicon nanoparticles. The results can be summarized and put into a Table 1. In Table 1, it is clear that the minmum Silicon Particles grain size (23.1 nm) and Specific surface area (S) at (13.24 m 2 / gm). And it is clear that the maximum Silicon Particles grain size (2.1 nm) and Specific surface area (S) at (182m 2 /gm).

Current density-voltage measurements
The schematic drawing of fabricated proto-type heterojunction UV photodetector is shown in Fig. 9. The ohmic contact of Al/SiNPs/PSi/Si/Al. The effects of deposition of SiNPs inside the macroPSi layer and the effects of macroporous silicon on the electrical properties of the fabricated photodetectors in the presence of the illumination were investigated. Figures 10 and 11, illustrate the electrical and photoelectrical properties of p-PSi based device sandwiches (Al/Si NPs/ macro-PSi/p-Si/Al) were studied processes. The measurements were carried out at dark and photo. Electrical properties of the fabricated p-type macro-PSi UV device double-heterojunction-based SiN/PSi and PSi/cSi are based on dark current density-Voltage. All measurements were taken in the range from (0 to 5 V) in dark conditions. The curves typically showed rectifying behavior for both of PSi and after embedding of SiNPs samples at the dark as in Fig. 10. The dark current (I d ) was decrease by 60% due to the increasing the resistance after incorporating SiNPs. This is one of the (5) S.S.A. = 6000 grain size of nanoparticles * density of monometalic NPs  . 9 the schematic of double junction device main advantages due to the fact that reducing the I d with decrease the noise current in the fabrication detector. Which this adding led to an increase in both the dielectric constant of the macroPSi layer and the mobility of the charge carrier. Fig. 11 Demonstrates the J-V characteristics of Al/macroPSi/p-Si/Al and Al/SiNPs/ macroPSi/p-Si/Al structures sandwich. The two junctions have the same polarity this mean increasing the sensitive region depletion layer of J 1 (junction) and J 2 (junction) Which, we can notice that the current passing in the Al/SiNPs/macroPSi/p-Si/Al structures was valied according to the morphologies of the low spam of macroPSi size substrate and the morphology of the SiNPs layer. And due to the increasing density of the available free charge carriers at the wall of the pore and over the porous surface due to the incorporation of Si nanoparticles. Which, improves the rectification properties by increasing the forward current at applied voltage due to the presence of double-heterojunction and decreasing the resistivity of the porous layer. And also, the forward current increases with the increase of the deactivation process of the charge trapping centers (at the porous channels) due to the existence of double-heterojunction-based SiN/PSi and PSi/cSi devices Fig. 10 Dark I-V of P-type porous silicon UV detector prepared at as prepared and after adding Si nanoparticles Fig. 11 The I-V characteristics of (Al/SiNPs/ macro-PSi/p-Si/Al) UV detector contain as-prepared p-PSi layers prepared and after adding Si nanoparticle Figure 12 demonstrates the responsivity (R λ ) of as prepared and after adding SiNPs structures. Where two peaks are observed and located at 450 nm and 700 nm. The first response peak indicates that a wavelength of 450 nm is absorbed within PSi layer. The second response peak is a result of absorption of 700 nm within the depletion region between PSi and Crystalline-Si contact since the bandgap of SiNPs is 3.5 eV, ∼3.82 eV for PSi, and 1.12 eV for cSi at room temperature. An increase in the indirect bandgap, the smaller the nanoparticle diameter, the larger the indirect bandgap, which can be attributed to the quantum confinement due to the size-dependent potential. The energy band gap of nanoscale silicon (E confi ) is larger than that of the crystalline silicon due to double-heterojunction-based SiN/PSi and PSi/cSi and the quantum confinement (E conf ) [Yamaguchi et al. 2003;Jung et al.2011;Rashid et al. 2022;Akram et al. 2021;Narasimhan et al. 2012). It is given as: where me* and mv* are the effective masses of conduction and valence bands, respectively,E bulk is energy band-gap of crystalline silicon, Lx, Ly and Lz are the dimensions of confined region.
From the sensitivity plot, and according to the following equation (Rashid et al. 2022): The quantum efficiency (η) of the photo-detector in the spectral range 450-700 nm was found to be 65%. Table 2 demonstrates the responsivity (R λ ) of as prepared and after adding SiNPs structures. Where, one peaks are observed and located at 700 nm with sensitivities of about 0.28 A/W as prepared and two peaks are observed and located at 450 nm and 700 nm with sensitivities of 0.75 A/W and 0.93A/W after adding SiNPs photodetectors. These values are higher than that for the standard p-n Si photodetector due to band-gap double-heterojunction-based SiN/PSi and PSi/cSi.

Conclusion
In this work, a significant improvement was satisfied in both of sensitivity and the dark current were achieved by incorporation SiNPs with the porous matrix. The concepts of double-heterojunction devices was employed to enhance the performance of the fabricated photodiode. The partially enclosing of porous with SiNPs will enhance the performance in addition to the band gaps of SiNPs. This research reflects a new toends of combination multi-forms nano same native material (porous and SiNPs) in fabrication of an effect optoelectronic devices.