Fabrication and Characterization of Copper Oxide Nanoparticles/PSi Heterodiode

. Nanocrystalline porous silicon (PSi) is prepared by Photoelectrochemical etching (PECE). PSi was characterized by the measurement of X-ray diffraction(XRD), Fourier transform infrared spectrophotometer (FTIR) and atomic force microscopy(AFM). The FTIR analyses indicate that Si dangling bonds of the as-prepared PSi layer has large amount of Hydrogen,forming a weak Si–H bonds. The structural, morphological, optical, and electrical properties of CuO NPs have been studied. X-ray diffraction measurement confirms that the CuO NPs were tetragonal crystal structure. AFM reveals that produced CuO NPs have a spherical shape. The energy band gap of CuO NPs prepared was found to be about (2.61eV). The effect of CuO NP s diffused on PSi heterodiode was reported.


INTRODUCTION
Copper oxides are one of the metal oxides, that has been studied for several reasons such as the nature and their reasonably good electrical and optical properties by Cu 2 O Copper formed two known oxides: cupric (CuO) and curprous (Cu 2 O) were p-type with a band gap energy 1.21 to 1.51 eV and 2.1 to 2.6 eV respectively [1][2][3]. On the other hand CuO with different nano shape has been synthesized by different methods in many research papers [4][5][6][7][8][9][10] In the synthesis of metal oxide nanoparticles, polymers are used to stabilize the aggregation of metal atoms. Polyvinylpyrrolidone (PVP) is the most commonly used polymer in the preparation of metal oxides because of its distinct shape, dissolved metal salts, and transport facility. In addition, PVP can be kinetically and thermodynamically controlled. Zhang et al. [11] used PVP as a capping agent to synthesize Cu 2 O nanocubes. Park et al. [12] utilized PVP to fabricate Cu 2 O nanocubes and CuO nanoparticles. The aim of this study was focused on the preparing CuO NPs utilizing the chemical reaction technique and studies the structural, topographical and optical properties in order to reach the optimum condition in fabricating the heterodiode.

prepared of CuO Nanoparticles by chemical reaction:
Re-distilled water was used throughout the experiment. In a typical procedure, 1.5 g of Cu(NO 3 ) 2 .H 2 O (BDH Chemicals Ltd Pool England) was dissolved in 50 mL of PVP (Sigma Aldrich USA) 1 WT. %. The solution was added into a round-bottom flask with stirring. The color of the mixture was blue. About 15 ml of NaOH (1M) was rapidly added to the mixture, and a nanopowder suspension was formed. The suspension was kept at 75 °C for 1 h. A large amount of black precipitate was produced. After cooling to room temperature, the particles were separated by centrifugation and were washed with distilled water to remove any contaminations. The particles were then dried in an oven at 80 °C.

Fabrication of porous silicon
The simplest cell which can be used to anodize silicon is shown in figure (1). Crystalline wafer of n-type Silicon (n-Si) with resistivity (1-4.5) Ω.cm 500 µm thickness and (100) orientation were used as substrate, which cut into rectangles with areas of (1.00 x 1.50) cm. A thick aluminum layers were deposited by using evaporation method on the backsides of the wafer. Photolectrochemically dipped into the mixture (1:1) HF (40%)-Ethanol (99.99%) and used gold electrode as in figure (2).15 min etching time and 15 mA/cm 2 current density with etched area 0.785 cm 2

Thin film deposition by drop casting method
Glass slides of (1.00 x1.50) cm 2 area, were used as a substrate. They were cleaned with alcohol in an ultrasonic bath in order to remove the impurities and residuals from their surface. 5 drops of the colloidal were used in preparing the CuO thin films on glass by drop casting method..The structural properties of the deposited thin films at room temperature were studied by using X-ray diffractometer (XRD-6000,Shimadzu X-ray Diffractometer). The optical absorption of the colloidal CuO NPs was measured using spectrophotometer (CARY, 100 CONC plus, UV-Vis-NIR, Splitbeam Optics, Dual detectors) in the range of (200-900nm), using quartz vessel. The shape and size of CuO nanoparticles were investigated by using AFM (AA 3000 Scanning Probe Microscope).

CuO NPs thin film studies.
Figure (2) shows a freshly CuO colloidal nanoparticles NPs prepared by quick chemical precipitation method. The colloidal CuO NPs have black color. CuO nanocrystals are a visually engaging way to demonstrate quantum effects in chemistry [13].

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ILCPA Volume 57 The XRD diffraction patterns of synthesized CuO nano-particle film prepared by quick chemical precipitation method is shown in Figure (2). The XRD patterns of CuO contain two peaks at a diffraction angle of 36.6 o , and 49.7 o corresponds to (002), and (202) planes. All the diffraction peaks are indexed to the cubic structure, and The d-values of nanocrystalline CuO are given in Table 1. The crystallite size D was calculated by using Scherrer formula [14] .The strain (ŋ) value and dislocation density (δ) are calculated and listed in Table 1. Figure (4) reveals the (3-D) AFM images and distribution chart of CuO NPs film. AFM images prove that the grains are uniformly distributed within the scanning area (2000x2000nm) with individual columnar grains extending upwards. This surface topography is important for many applications such as responsivity of photodetector and catalysts [15].

Fig. 4: Shows 3-D image and granularity cumulating distribution chart of CuO
The CuO NPs have spherical shaped with good dispensability, homogeneous grains aligned vertically. The estimated values of root mean square (RMS) of surface roughness average and average grain size are listed in Table (2). Electrical conductivity (σ) for CuO films was measured within temperature range (300-473 K). In general It has been noticed in all films, that the electrical conductivity increases as the temperature increased exponentially, and this represents common semiconductor's property, which is related to an increase in the charge carrier's concentration. We observe that CuO film is conducted at room temperature (300 K) around (1.1 x 10 -5 ) (Ohm. cm) -1 , and reach (2.369 x10 -2 ) (Ohm.cm) -1 when temperature increase to (473 K). The electrical activation energy of CuO films was calculated from ln σ versus ) / 1 ( T ) plot as shown in figure (5) .Since the activation energy Ea can be expressed by :

International Letters of Chemistry, Physics and Astronomy Vol. 57
Where K B is Boltzmann constant and q is the charge of electron, From above calculations, it was found that the activation energy of CuO is ( 0.45 eV), which closely agree with the results obtained by Kim  (Ω) ln σ 1/T K -1

Fig. 5: The variation of ln R vs. T -1 of CuO NPs film
The results of Hall effect are shown in Table. 3 revealed that CuO NPs film is (p-type ), which is in good agreement with the kim et al. [16].

Fig. 6: Optical absorbance of CuO NPs.
Figure (7) shows that the reflectance varies between 0.1 to 0.2 and the maximum value was 475 nm wavelength and the refractive index (n) which was estimated from reflectance (R) data using the following equation [14]: It is clear from figure (7) that the maximum value of refractive index was at 475 nm then it decrease sharply with wavelength up to 475 nm. Furthermore, the refractive index of CuO NPs is found to decrease as the wavelength, this might due to the effect of particle size.

Fig. 7: Reflectance and Reflective index of CuO NPs.
The energy band gap of CuO nanoparticles was estimated by plotting the square of (αhv) 2 versus (hv) as shown in figure (8). The value of optical band gap of CuO NPs is about 2.61 eV.
International Letters of Chemistry, Physics and Astronomy Vol. 57

Fig. 8: (αhv) 2 versus photon energy gap of CuO NPs.
PL emission spectra of CuO NPs prepared by Quick chemical method has been recorded at room temperature with an excitation source of wavelength of 485 nm as shown in figure (9). A single sharp broad emission peak centered at the 475 nm (2.61 eV). The PL spectra of CuO NPs have Gaussian-shape, and this may be due to an expected photo-physical result of the band measurement of NPs at room temperature arise from inhomogeneous broadening due to size and shape distribution within NPs and homogeneous broadening due to thermal energy (26 meV at room temperature ) [18]

porous silicon(n-ps)studies
X-ray diffraction (XRD) spectra shows a distinct variation between the fresh silicon surface and PSi surfaces formed at different etching time. A strong peak of (PSi) at 5min etching time shows a very sharp peak at 2θ = 69.7° oriented along the (400) direction is observed confirming the monocrystalline structure of the PSi layer which belongs to the (400) reflecting plane of Si of cubic structure. The broadening in the diffracted peaks is due to the thickness increase in pore walls, and upward shifts are due to relaxation of strain in the porous structure [19]. XRD spectra shows the formation of porous silicon. The structure was amorphous at 15 mA/cm 2 current density and 15 min etching time as shown in figure (10).
The mean crystallite size D of strong (400) diffraction was determined using Debye -Scherrer formula (XRD line broadening) [14] and listed in table 1 D = 0.9 λ / β cosθ (4) Where λ is the wavelength of x-ray , θ is the diffraction angle and β is the FWHM. The strong and narrow peaks may be ascribed to the preferential growth along (400) planes of CuO

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ILCPA Volume 57 crystallites.the strain value () and the dislocation density ( ) can be evaluated by using the following relations [20], see Table 4: The results revealed that the strain and dislocation density are decreasing with the increase of the grain size  From FTIR data as shown in Figure ( and 2924 is due to the plane C-H angle deformation.It can easily replace a silicon atom, leading to the presence of carbon in the porous structure, since carbon is located in the same column of the periodic table as silicon [21].Upon anodization in air, new chemical bonds appear on the surface as a wide transmission band due to different Si-H and Si-O chemical bond configurations in the IR spectra.
Also note that if a molecule is so symmetrical that the stretching of a bond does not produce any change in dipole moment, then no IR peak will be found in the spectrum [22].

CuO/PSi/Si Hetrodiodes characterization:
The CuO/PSi/Si Heterodiode was fabricated from thick Al metal (0.5 µm) by thermal evaporation on front of Si substrate then the CuO colloids absorbed on the internal pores within PSi and were then prepared by dipping the PSi in CuO NPs (0.6 mg) at room temperature for 30 min. A schematic diagram for this diode was illustrated on fig (13).

Fig. 13: CuO/Psi/nSi diode structure
Dark I-V measurement was done by using Keithley electrometer automatic system. The illuminated I-V characteristics were measured under a tungsten-halogen lamp. The spectral responsivity was measured by means of 1200 lines/mm diffraction grating to monochromator. This system was calibrated with (0.5cm 2 ) commercial silicon photodetector . Figure (14) shows the results obtained from I-V measurements. It can be seen that the formation of the pores is strongly related to the I-V characteristics of the CuO/PSi/Si. Also and that higher resistivity is a result of carriers trapped in the pore walls.

Fig. 14: I-V characteristic under forward reverse bias of the CuO/ n -PSi/Si
Figure (15) shows that the reversed I-V measurements of diode structure under 10 mW/cm 2 light intensity. It can be seen that the reverse current value of a given voltage for CuO /PSi/n-Si diode structure under illumination is higher than dark   Figure (17) shows the specific detectivity as a function as a function of wavelength from 400 nm to 800 nm for CuO/PSi/n-Si. The max. value of detectivity is around ≈ 13 ˟ 10 12 W -1. cm.Hz -1 at wavelength 780 nm .

CONCLUSIONS
The synthesized CuO NP S were in nanosized 94 nm prepared by chemical reaction method. The optical properties revealed that the band gap of CuO NP S indicated by the effect of quantum size. X-ray diffraction (XRD) measurement disclosed that the CuO NP S are polycrystalline and has tetragonal crystal structure and no other phases were noticed.
Deposition of CuO NP S onto porous silicon (PSi), enhanced the properties porous photodetectors. The spectral responsivity (R  ) of Al/CuO/Psi/Si/Al photodetector was around 0.8 A/W at  780 nm wavelength due to the absorption edge of silicon and around 0.6 A/W at  650nm wavelength owing to the absorption edge of CuO NP S . The maximum value of The specific detectivity (D * ) found to be 13 10 12 W -1 .cm.Hz -1 , located at 780 nm wavelength for Al/CuO/PSi/Si/Al photodetector.