Photoluminescent Enhancement by Effect of Incorporation Nickel in ZnO Films Grown

DOI: http://dx.doi.org/10.24018/ejers.2021.6.5.2550 Vol 6 | Issue 5 | August 2021 177 Abstract — Microstructured films of undoped zinc oxide (ZnO) and ZnO doped with nickel (ZnO:Ni) were grown by hot filament chemical vapor deposition (HFCVD) technique on Si (100) substrates at 500 °C. Pellets of ZnO and ZnO:NiO as oxidant agenst were used. A shift to the right around 0.17 degree of the X-Ray Diffraction pattern of the ZnO:Ni film was observed with respect to undoped ZnO films. Morphologically by Scanning Electron Microscopy was noticed a Core-Shell type growth in ZnO undoped and a nanostructured type (Nano-wire) in ZnO doped with Ni. Photoluminescence measurements showed an increase in the intensity of the green emission band of ZnO:Ni. It was attributed to defects of oxygen vacancies (VO), zinc vacancies (VZn), zinc interstitials (Zni), oxygen interstitials (Oi), and oxygen vacancies complex (VO complex) in the structure of the film. The incorporation of Ni atoms in the ZnO structure stresses the crystal lattice, leaving behind a large number of surface defects that increase the emission of PL.

by doping ZnO with different elements of the periodic table and with different deposition techniques. For example, by pulse laser deposition (PLD), metalorganic molecular-bean epitaxy (MOMBE), radiofrequency magnetron sputtering (RF Magnetron Sputtering), and metalorganic chemical vapor deposition (MOCVD); ZnO was doped with Arsenic [6], Nitrogen [6], Phosphorus [7], and Nitrogen [7], respectively. However, these results have been difficult to replicate in order to achieve the desired homojunction. By another hand, different morphological growths have been reported, such as microwires [8], microribbons [9], microspheres [9], and even nanometric structures as nanoparticles [10], nanorods [11], and nanowires [12] of ZnO with the aim of manufacture different types of devices as gas sensors [13], light-emitting diodes [14] photodetectors [15], high electron mobility transistors (HEMT) [16], [17], field-effect transistors (FET) [18] and thin-film transistors (TFT) [19]. In this work, microsphere and sea urchin-type structures were found; the last structure is obtained when ZnO films are doped with Nickel. The obtaining process consists of the interactions of Ho with ZnO:NiO pellets for the generation of gaseous precursors and their subsequent deposit on silicon substrate using the HFCVD technique. This with the aim of improving the photoluminescence of ZnO for a possible application as light-emitting devices.

II. MATERIALS AND METHODS
ZnO films doped with Ni were grown from ZnO:NiO pellets with a mass ratio of 1:1, such pellets were used as a solid source which was obtained by compressing powders of ZnO (Mallinckrodt Chemical CAS 1314-13-2, St. Louis, MO, USA), and NiO (ALDRICH CAS 1313-99-1, St. Louis, MO, USA) at a pressure of 600 Kgf/cm. For the deposited films of undoped ZnO, the compressed powders of ZnO were used as solid sources. For deposits of the films, p-type silicon substrates of 1.5 cm 2 , with a resistivity of 5-10 Ω-cm and <100> oriented were used. Films of ZnO:Ni were grown by the HFCVD method. Such technique involves heating up to 2000 °C a tungsten filament. The substrate is exposed to a region where H2 species have higher kinetic energy because H2 is thermally activated in the filament. This configuration can lead to greater interaction between the growth of ZnO and the H2 species.
The following chemical reaction is proposed: @ @ @ @ @ @ @ @ @ @ Photoluminescent Enhancement by Effect of Incorporation Nickel in ZnO Films Grown Delfino R. Gutiérrez-Olvera, Tomás Díaz-Becerril, Godofredo García-Salgado, Antonio Coyopol-Solís, Enrique Rosendo-Andrés, Reina Galeazzi-Isasmendi, Ramón Peña-Sierra, Crisóforo Morales-Ruiz, Roman Romano-Trujillo, and Fabiola G. Nieto-Caballero where MO is a Metal Oxide (ZnO or Nickel Oxide (NiO)), H2 is molecular hydrogen, M is a Metal and, H2O water molecule [20]. According to equation (1), it was expected that both metal Nickel (Ni) and Zinc (Zn) with lower melting points (mp) (1455 and 420 °C, respectively) than NiO and ZnO (1955 and 1974 °C, respectively) can be easily sublimed. Into the gaseous phase and under an oxidant atmosphere, NiO and ZnO films are expected to be grown over the substrate. The distance between the filament and the source was calibrated to reach a temperature of 500 °C on the substrate surface. The growth process was carried out as a result of the reactivity of the atmosphere into the HFCVD reactor which increases and promotes the precursor gas formation. In the experiment, a flow of hydrogen gas at a constant rate of 50 sccm through the filament for 60 s was used [21]. The hydrogen gaseous atmosphere used was of chromatographic grade supplied by INFRA. H2 reacted with a ZnO: NiO source generates gas precursors. To study the effect of doping in ZnO with Nickel, experiments were performed under the same conditions using an undoped ZnO pellet. The X-ray diffraction measurements were carried out on a Bruker AXS D8 Discovery (Billerica, MA, USA) diffractometer. Copper radiation with a wavelength of 1.54 Å, was used. PL spectra was obtained using a Fluoro max 3 system (HORIBA, Ltd., Kyoto, Japan), with an excitation wavelength of 330 nm. Morphological characterization of the microstructures was made with an AURIGA 3916-FESEM scanning electron microscope (Zeiss, Jena, Germany).

III. RESULTS AND DISCUSSION
A. X-ray Diffraction Analysis  In XRD patterns from undoped ZnO film (labeled as S0) and ZnO:Ni fils (labeled S1) the hexagonal phase of ZnO and Zn were observed. The analysis of the patterns showed a small shift to the left in the peaks of ZnO:Ni with respect to the diffraction peaks of the ZnO film. The effect is probably caused by a compression-type micro-stress attributed to the Ni atoms, that substitute Zn atoms inside the ZnO structure [22] or to the fact that the ionic radius of Ni (0.68 Å) is slightly bigger than the ionic radius of Zn (0.60 Å) [23].

B. Scanning Electron Microscopy (SEM)
In the SEM micrograph of undoped ZnO film Fig. 3(a), microsphere morphology on the surface of this structure a chain-like framework is observed. In the same way, this growth is shown in the deep of the microstructure. In Fig.  3(b), of the sample of ZnO:Ni, the morphology of microsphere type is shown. On the surface of the microstructure, a nano-wires type structure can be seen over all the surfaces of the film. In both processes the growth mechanism for these microstructures is as follows, first, the Zn atoms are dispersed on the substrate surface which was heated at 500 °C. The hot filament simultaneously heats the substrate and the source maintaining a temperature gradient between them, which facilitates the transport of the material. The Zn powders mp = 420 °C are the first to melt and aggregate to form droplets on the surface of the substrate. The decomposition of the surface is generated an atmosphere rich in oxygen inside the reaction chamber [24]. During the nucleation, the oxygen bound with Zn droplet forming Zn and ZnO Core-Shell. The difference between these processes consists of the function of the Ni atoms, that act as a catalyst, favoring the growth of ZnO nano-wire on the surface of the microsphere, forming shapes like a sea urchin [25] (see Fig 3(b)).

C. Energy Dispersive Spectroscopy (EDS)Analysis
EDS spectrum has been determined for different regions of the S0 and S1 films, see Table II. According to the analysis of the results, for undoped ZnO (S0) film the amount of Zn is bigger than the O. (on average 80 at% and 20 at%, respectively). The ZnO:Ni (S1) film can see a decrease in Zn and an increase in Ni (on average 80 at% and 0.3 at %, respectively). EDS analysis was performed inside the microsphere, S0 and S1 films rich in Zn were observed, (on average 80 at%). This ratifies the aforementioned.  Fig. 4(a), are shown the PL-spectrum for the films of S0 and S1 at room temperature. The most notable effect of the incorporation of Ni in the ZnO matrix is the increase in the PL intensity emission band located at 410 and 700 nm (1.77 eV-3.02 eV). In addition, the maximum emission shifts to higher energy in S1. This could mean that there is an increase and/or introduction of Ni radiative centers in the ZnO. In Fig. 4(b), the deconvolution of the spectrum S1 was performed in order to obtain the bands that compose such spectrum. It was observed that the maximum emission is centered in the energy range of the green emission band, around 509-520 nm (2.45-2.39 eV). This optical behavior of ZnO is usually associated with the presence of several types of defects as VO, VZn, Zni, Oi, OZn [26] or some metals transitions [24][25][26], generation recombination centers inside the bandgap (Eg) of the structure.
Hwang et al. [29] and Fedorov et al. [5], associate the green emission band to undoped ZnO with the presence of VO and VZn [27], [30] that introduces deep acceptors in the Eg. Green luminescence is because of the electronic transition between shallow donors and deep acceptors.
The material Zn-rich guarantees the existence of VO. The transition between those states and VO (deep acceptor states) might be the origin of the increase in the green emission band. Additionally, a blue luminescence band emerges around 450 nm (2.21 eV) in the ZnO PL spectrum. Recently Zeng et al. [31] and Cao et al. [32], obtained visible dichromatic photoluminescence emitting in blue/violet, and green regions, introducing Zni defects in the ZnO structure.

IV. CONCLUSION
ZnO and ZnO:Ni films were grown using the HFCVD technique and pellets of ZnO and ZnO:NiO as oxidant agents. Probably Ni atoms modify the crystalline structure of ZnO since it causes a shift in the position of the diffraction peaks with respect to undoped ZnO. By SEM, shapes like a sea urchin due to the doping of ZnO were observed. The growth of these nanostructures resulted in a larger surface area, increasing the number of defects on the surface of the ZnO:Ni film compared to the undoped ZnO structures. The green-yellow emission band is due to defects in Zni, VO, Oi, OZn, and some VO complexes. This work shows the possibility of obtained ZnO:Ni films in a short time process that implies a lower cost for the manufacturing of the optoelectronic device.