Influence of Nd and Ce doping on the structural, optical and electrical properties of V2O5 thin films

Nano-structural of vanadium pentoxide (V2O5) thin films weredeposited by chemical spray pyrolysis technique (CSPT). Nd and Cedoped vanadium oxide films were prepared, adding Neodymiumchloride (NdCl3) and ceric sulfate (Ce(SO4)2) of 3% in separatesolution. These precursor solutions were used to deposit un-dopedV2O5 and doped with Nd and Ce films on the p-type Si (111) andglass substrate at 250°C. The structural, optical and electricalproperties were investigated. The X-ray diffraction study revealed apolycrystalline nature of the orthorhombic structure with thepreferred orientation of (010) with nano-grains. Atomic forcemicroscopy (AFM) was used to characterize the morphology of thefilms. Un-doped V2O5 and doped with 3% concentration of Nd andCe films have direct allowed transition band gap. The mechanisms ofdc-conductivity of un-doped V2O5 and doped with Nd and Ce filmsat the range 303 K to 473 K have been discussed.


Introduction
One goal of today's technology is the miniaturization of the electronic, actuating, sensing, and optical devices and their components; hence, nanotechnology is an advanced technology that has received a lot of attention from the worlds of the science and industry for its ability to make use of the unique properties of nanosized materials. Nanotechnology is capable of manipulating and controlling material structures at the nano level (a nanometer is equal to one millionth of a millimeter) and offering unprecedented functions and excellent material properties [1]. Vanadium oxide is of enormous research interest because of its multivalent nature. The vanadium oxides exist in the V 2+ , V 3+ , V 4+ and V 5+ oxidation states and form the VO, V 2 O 3 , VO 2 and V 2 O 5 materials [2]. Among these, vanadium pentoxide (V 2 O 5 ) has drawn significant interest over the past decades owing to its wide range of applications. Its multivalency, layered structure, wide optical band gap, good chemical and thermal stability, excellent thermoelectric property, etc., are the characteristics that make vanadium pentoxide (V 2 O 5 ) a promising material for applications in microelectronics, and for electrochemical and optoelectronic devices. It can be used as a catalyst, gas sensors, a window for solar cell and electrochromic devices as well as electronic and optical switches [3]. V 2 O 5 is the most stable oxide and show semiconductor property with an energy gap of ~2.2 eV at room temperature, and displays electrochromic properties with varying color from blue to green and yellow within two seconds upon charging/discharging [4]. Vanadium pentoxide films have been prepared using various physical and chemical techniques such as, spray pyrolysis [5][6][7], electron beam evaporation [8], thermal evaporation [9], pulsed laser deposition [10], and sol-gel [11] methods. Different literature reviews were added to study the structural, optical and electrical properties of Vanadium pentoxide thin films. Structure and semiconducting properties of amorphous vanadium pentoxide obtained by splat cooling [12]. Structural and optical studies for V 2 O 5 thin films. The films gave two-step electrochromism, yellow to green and then green to blue [3]. Amorphous and crystalline of V 2 O 5 thin films growled onto glass substrates with different concentrations from 0.1M to 0.5M Optical analyses showed the absorption coefficient shifts towards lower energies [13]. Developed a method of a facile synthesis for preparing nano-sized of V 2 O 5 for high-rate lithium batteries using vanadyl oxalate in air [14]. In the present paper, un-doped vanadium pentoxide and doped with Nd and Ce thin films have been prepared by (CSPT) to produce large area and uniform coating [15].

Experimental procedure
Before starting the deposition, the solutions according to the films components was mixed then put it on the magnetic stirrer for about 15 minutes to be sure that the mixture solutions are mixed properly. Prior to deposition, Si substrates (for studying the structural properties) and glass substrates (for studying the optical and electrical properties) were cleaned and places on the flat plate heater surface, which it is an electrically controlled, and leaves them for about 10 minutes so as to allow their temperature to reach the set temperature at (250 ± 5)°C. After that, can start the deposition process within deposition time of 5 sec, and then stop this process for 10 sec. In the spray system, compressed and purified air was used  [2,17,18,19]. The presents a preferential orientation of the film was along the plane (010) at diffraction angle of 2 =20.36°, d 4.35 nm and lattice-parameter values of a=11.4734 Å, b=4.35809 Å and c=3.5533 Å. It is very close to the result obtained in [17,20] The average crystallite size was equal to 26.29 nm. It was estimated with the Debye-Scherrer formula for the (010) reflection as follow: where  is the wavelength of XRD photons which equal to 0.154 nm, is the full-width at half maximum (FWHM) and  is the Bragg diffraction angle in degrees.
There was increasing in the intensity of peaks diffraction with doping of both Nd and Ce. Not to appearance of new phases for a new compound which returns to the doping material in XRD diagram at these ratios. That may be due to the low proportion of neodymium and cerium that were doped, so it is difficult to be discovered in the examination of (XRD). The structure of prepared films are still as polycrystalline after doping with Nd and Ce, in addition to that increase in the (010) peak intensity may be attributed to the formation of new nucleating centers due to the dopant atoms resulting from the decrease of nucleation energy barrier. We can observe from Fig.1 [27]. The D.C. activation energies calculated from the plot of ln(σ d.c ) versus 1000/ T, then founded the slope and multiple it by k β as follow: E a = k β . slope (4) Two stages of conductivity throughout the heating temperature range are noted, first activation energy (E a1 ) occurs at low temperatures near Fermi level within the range (303-383) K, while the second activation energy (E a2 ) occurs at high temperatures within localized states at the range (383-473) K. This result is in agreement with [28].
Electrical conductivity was increased with Nd and Ce doping concentration which resulting from the increase in the concentration of charge carriers because of the presence of donor levels in the energy gap. While there was decreasing in activation energy with doping concentration at low temperature region. This drop in activation energy may be increased in oxygen vacancies created upon Nd and Ce doping into the V 2 O 5 lattice. The low activation energy may be due to the large percentage of highly disordered interfaces. Doping likely increases this disorder, as well as creating more oxygen vacancies for material transport during material synthesis [29]. Higher activation energy at higher temperatures, this is likely due to the segregation of Nd and Ce out of the V 2 O 5 lattice structure and into the grain boundaries.
The variation of carriers concentration (n H ) and Hall mobility (μ H ) of undoped V 2 O 5 and doped with Nd and Ce at 3% doping concentration thin films are shown in Table 3. Hall measurements show that all these films have a negative Hall coefficient (n-type charge carriers), this result was agreement with [5,30]. This is attributed to following two reasons: [31] i) The number of electrons excited above the conduction band mobility edge is larger than the number of holes excited below the valance band mobility edge.
ii) The life time of free electrons excited from negative defect state is higher than the life time of free holes excited from positive defect state. It's clear from Table 3 that the carrier concentration increases with dopant ratio of both Nd and Ce while there was decreasing in carrier mobility (μ H ) with Nd and Ce dopant concentration and the doping process did not affect on the type of the charge carriers. The increasing in carrier density with Nd and Ce doping leads to decrease in the resistivity of doped V 2 O 5 thin films. It is due to decrease the disorder of the crystal lattice, which causes decreases in phonon scattering and ionized impurity scattering and results in a decrease in mobility [32]. In other words, the reduction in carrier mobility with Nd and Ce doping ratio because of decreasing in crystallite size, as stated in the measurements of X-ray, which in turn leads to an increase in grain boundary and will thus decreasing mobility [33]. Conclusions XRD measurement showed that the films to be polycrystalline with orthorhombic phase and with preferred orientation of (010). Analysis of the absorption curves revealed allowed direct transition with optical energy gap 2.34eV. Also, the absorption edge shifts towards higher energies. The absorption edge showed a blue shift, and the optical band gap of the thin films revealed allowed direct transition with optical energy gap 2.2eV. Optical energy gap decreased with doping of both Nd and Ce and with same ratio. The optical transmission of the films increased with doping concentration, which provides a satisfactory optical window for optoelectronic applications. Carrier concentration increases with dopant ratio of both Nd and Ce while there was decreasing in carrier mobility (μ H ) with Nd and Ce dopant concentration and the doping process did not affect on the type of the charge carriers.