Fabrication and Characterization of PZN-4.5PT Inorganic Perovskites Nanoparticles Thin Films Deposited on P-Type Silicon Substrate

This work involves an investigation of nanostructures, microelectronic properties and domain engineering of nanoparticles thin layers of Pb(Zn1/ 3Nb2/3)O3-PbTiO3 (PZN-PT) ferroelectric single crystals deposited on nanostructured silicon substrate. In this study, devices made from PZN-4.5PT nanoparticles thin films successfully deposited on silicon substrate have been studied and discussed. SEM images show the formation of local black circles and hexagonal shapes probably due to the nucleation of a new Si-gel component or phase induced by annealing. Micro Xray Fluorescence mapping shows that the high values of Si and B atoms (≅7 and 4 normalized unit respectively) can be explained by the fact that the substrate is p-type silicon. The most interesting result of optical measurements is the very good absorption for all the thin films in UV, Visible and NIR regions with values from 70% to 90% in UV, from 75% to 93% in Visible and NIR. Tauc plots present particularities (rarely encountered behavior) with different segments or absorption changes showing the presence of multiple band gaps coming from the heterogeneity of the thin films (nanowires, gel and nanoparticles). Their values are 1.9 and 2.8 eV for DKRN-Gel, 2.1 and 3.1 eV for DKRN-UD and 2.1 and 3.2 eV for DKRN-D) corresponding respectively to the band gap of nanowires and that of the gel while the last ones correspond to the undoped and doped nanoparticles (3.1 and 3.2 eV respectively).


Introduction
Among various everlasting desires of all the materials scientists, the most important is the search of materials with as maximum functionalities as possible [1] [2]. The transition metals have very rich chemistry due to variable oxidation state, which offer multiple properties, if selected judicially. A huge number of metal oxides has been investigated and is being studied for various technological applications [3] [4] [5] [6]. As the global concern for the financial and environmental costs of traditional energy resources increases, research on renewable energy, most notably solar energy, has taken center stage. Many alternative photovoltaic (PV) technologies for "the next generation solar cell" have been extensively studied. The novel photovoltaic mechanism for the next generation solar cells using silicon nanostructures and ferroelectric nanoparticles were investigated.
However, the greatest difficulty to use such single crystals on electronic devices is to achieve them in thin layers form because of their incongruent melting property. The integration of such materials as thin films has attracted considerable research attention thanks to their outstanding performances that allow to consider new features for the realization of photovoltaic device.
In this paper, to integrate them into silicon nanostructures, we realized PZN-PT nanoparticles deposition, already synthesized by the so-called solution flux method [25], as thin film on p-type nanoporous silicon substrate. Structural, morphological, optical and electrical properties of such nanoparticles and their as deposited thin film were investigated.

Experimental Procedure
A boron doped p-type (100) silicon wafer of 600 -650 µm of thickness with resistivity of 1 -5 Ω•cm of the one polished face was purchased from BT Electronics.

PZN-PT Nanoparticles Thin Film Fabrication
The grounded powder was dispersed in a gel fabricated in the laboratory LCPM in Assane Seck University of Ziguinchor. To obtain a homogeneous film, spin coating process was carried out at room temperature using a spin coater Midas

Surface Characterization
The as-synthesized powders were characterized by TEM analysis using a FEI CM10 microscope (Philips) by depositing a drop of nanoparticles suspension on carbon coated copper grids placed on a filter paper. Surface morphology and composition of thin films samples were observed respectively by using a

Optical Properties
Optical properties of thin films were investigated using an UV-Visible spectrophotometer with an integrating sphere model LAMBDA 950S in IM2NP (Marseille).        The signal intensity in Figure 3(a) shows the high content of Si compared to B and P, which represent the substrate dopants. The ratio between the dopants and silicon intensities is in agreement with these elements doping concentration (1016 at/cm 3 for B and 1019 at/cm 3 for P).

TEM and SEM Images
These high values of Si and B (around 7 and 4 counts/s respectively) can be explained by the fact that it is a p-type silicon substrate (Figure 3(a) and Figure   3(c)). The low value of phosphorus concentration (around 0.2 count/s) compared to boron is surprisingly, however this could be explained by the possibility of P atoms to be diffused more deeply inside nanowires and the bottom of these latters than on their top surface (Figure 3(b)) creating less concentration at the layer surface.  (Figure 4(a)) and the image of the atomic force microscopy (Figure 4(b)). The root mean square roughness (RMS) measured by the mechanical profilometer and the atomic force microscopy is 1.4 µm. We have a high roughness, which causes the phenomenon of diffusion of the incident light which results in a decrease in transmittance in infrared region [28]. Figure 5 shows UV-Visible reflectance and absorbance curves for Si p-type influence on the reflectance value and is in agreement with the literature (5% -15%) [29]. It decreases from over 45% for the p-type Si substrate in the wavelength range of 250 to 1500 nm to less than 10% for the nanostructured silicon.

Reflectance and Absorbance
For DKRN-UD and DKRN-D, reflectance decreases from 45 to less than 20% ( Figure 5(a)). However, their values increased compared to DKRN-Gel, which has the lowest reflectance value (around 10%) in the whole measurement range.
This behavior of the thin film from perovskite nanoparticles could be explained by the high reflectance values of PZN-PT single crystals (20% -25%) [30]. The

Determination of Refractive Index and Band Gap
The values of the refractive indices are determined from the reflection spectra of where R min is the minima taken from the spectrum, n 1 is the reflective index of the layer, n 0 (n 0 = 1) and n 2 (n 2 = 3.4777) are respectively the refractive index of air and Silicon substrate. The layer's refractive index n 1 was determined from the following relation: ( ) From reflection spectra of Figure 6  The Si band gap gets transformed from an indirect into a pseudo-direct one resulting in photoluminescence (PL) and electroluminescence (EL) of the Silicon nanowires as a consequence of quantum confinement effects [33]. The quantum confinement clearly led to a widening of the gap. In these nanostructures, small by definition, the charges remain close to each other. The recombination probability is increased and the radiative lifetime decreases and then increases the ra- In addition, the presence of the gel and the confinement effects could also explain the increasing for the deposited films compared to the nanowires band gap.

Conclusion
PZN-4.5PT nanoparticles thin layers were successfully prepared using a gel fabricated in our laboratory. Silicon p-type nanostructured was used to deposit this thin film. SEM images revealed homogeneous distribution of the gel inside the nanowires. Optical properties investigation shows very good absorption for all the thin films in UV, Visible and NIR regions with values from 70% to 90% in UV, from 75% to 93% in Visible and NIR. Refractive index is measured and shows that their values confirm the possible use of such materials as antireflective and passivation thin layers. The Tauc plots show a behavior rarely encountered in materials like presence of more than one band gaps absorption energy (different changes) confirming the heterogeneity of the thin layers. These results show good optical properties favorable to the solar cell development.