Over years the study of potential applications like light emitting diodes or ultraviolet photodetectors of new optoelectronic materials has focused the attention of the materials science community. Among these materials, due to their huge physical properties and therefore their great potential as optoelectronic devices, II-VI oxides such as MgO, ZnO and CdO and its ternary alloys, namely Mg{1-x}Zn{x}O and Cd{1-x}Zn{x}O [Nieda2016, Wang2010, Mohanta2008, Sadofev2006], have been and are being widely studied. Regarding the alloy of CdO and MgO cubic binary oxides, in recent years, some theoretical papers were published showing the interesting properties that this alloy should present. Among them should be emphasised the energetic stability of the rock salt structure in the whole range of concentrations or the band gap energy tunability as a function of Mg content [Zhu2008,Paliwal2012,Amin2010,Joshi2013]. Since then, some experimental work has been carried out. Thus, isolated nanoparticles of the new ternary compound Cd{1-x}Mg{x}O in the whole range of Mg content (0 < x < 1) have been grown by using spray pyrolysis technique on r-sapphire substrates [Rajappan2014]. However, in the case of the growth of thin films, the whole range of concentrations is not covered yet. Looking at the literature we can find that by using the spray pyrolysis technique, CdO:Mg polycrystalline layers with a concentration up to 6% of Mg content have been synthesised on glass substrates [Atay2011,Vigneshwaran2012]. Furthermore, Guibin Chen et al. have achieved to deposit polycrystalline and random oriented Cd{1-x}Mg{x}O on microscope glass slides using a radio frequency sputtering system. They did estimate up to 28% of Mg content for the ternary alloy from the XRD peaks shift and using the Vegard's law [Chen2013a,Chen2013b]. Nevertheless, the region of high concentrations of Mg has been less studied in the current literature probably due to intrinsic difficulties in making compatible the different growth conditions for the two binary compounds. An alloy with a high concentration of Mg should show a big potential for technical applications due to the physical stability, high optical transparency and high secondary electron emission of the MgO [Thiandoume2009,Lee2003]. These properties should be present in the alloy in addition to the capabilities generated by the incorporation of Cd in the MgO matrix, e.g. a tuneable high band gap energy that could endow it with a great potential to be developed as a middle and far UV sensor. Thus, the main goal of this work is to grow layers of the alloy between cadmium and magnesium oxides trying to enlarge as far as possible the concentrations range. To do that we have used both, spray pyrolysis and MOCVD growth techniques to increase the amount of Mg introduced in the CdO structure and to synthesise the Mg rich phase of the alloy. It should be noted that we refer to the Cd-rich and to the Mg-rich phases of the alloy as Cd{1-x}Mg{x}O and Mg{1-x}Cd{x}O respectively whereas we refer generically to the alloy as CdMgO or MgCdO indistinctly. MgCdO GROWTH BY SPRAY PYROLYSIS - By using a fast and low-cost growth technique such as spray pyrolysis one, we can say that it is possible to synthesise the Cd{1-x}Mg{x}O alloy reaching a magnesium incorporation to the CdO lattice up to at. 20.5%, far away of what was achieved in the bibliography using this technique [Atay2011, Vigneshwaran2012]. - By using SEM we can observe the presence of two growth morphologies as a function of deposition conditions. At the CdO samples we have cubic nano-structures, whose size reaches up to 500 nm, and a layer of nano-dots of 10 nm of diameter. When we add Mg to the growth process the density of cubic nano-structures falls until they disappear in the growths with nominal concentrations of 30% of Mg while nano-dots diameter reaches 35 nm. - This behaviour could be related with the formation of new nucleation points in the growth processes with a high presence of cadmium due to the precipitation of pre-reaction particles during spray. Nano-structures would grow from this new nucleation points. The less cadmium precursor we have, the less prereactions occur and therefore the density of these cubic structures is lower. - By using different structural characterisation techniques such as XRD, electron diffraction and the acquisition of high-resolution TEM pictures, a progressive variation of the CdO lattice parameter with the Mg incorporation have been observed. Thus, theoretical works predictions [Amin2010, Paliwal2012] have been confirmed in the studied range. - Finally, the obtention of nano-structres of this alloy whose size reaches up to 500 nm, opens the door to the study of the fundamental properties of this new alloy. At this point in which both, the possibility of growing the ternary compound and the improvement of the experimental results from the bibliography were confirmed, the work was continued by the research group in two parallel lines. On the one hand the group worked on the growth of isolated nanoparticles of this material on R-sapphire substrates covering all the concentrations range using spray pyrolysis technique [Rajappan2014]. On the other hand, as shown below, the growth of layers of this alloy using MOCVD technique has also ben studied with the aim of synthesise the Mg-rich phase and expand the range reached in the bibliography. MgCdO GROWTH BY MOCVD In this section a chronological summary of the work carried out about the growth of MgCdO alloy using MOCVD technique is shown with the main conclusions from the data analysis of the morphological, structural and compositional characterisation. Thus, the studies about the synthesis of this alloy are grouped in different subsections. Study of the effect of precursors concentration - By using MOCVD technique the ternary alloy MgCdO has been deposited on C-sapphire substrates taking MgO growth conditions as a starting point. Precursors flows have been modified varying bubblers pressure and carrier gas flow in order to study the effect of its relative concentration in the morphology and structure of the samples. Growth temperature was set at 500 ºC. - In the XRD patterns we observe two different phases of the alloy when we have a high Cd precursor concentration. On the other hand at low concentrations the samples only have the Mg-rich phase. - With the SEM we can observe a compact layer and some non-defined shape structures that appear when Cd precursor concentration is increased. - EDX analysis shows us that these structures have a high Cd concentration while the compact layer is Mg-rich. This fact links with the XRD results: when we have these structures we also have a Cd-rich phase at the diffraction patterns while compact layer is related with the Mg{1-x}Cd{x}O phase. - When the Cd precursor concentration is increased, the Mg{1-x}Cd{x}O layer incorporates more Cd. This fact can be observed, apart from EDX measurements, in the XRD peaks shift and therefore in the rise of the lattice parameter. - The Willimason-Hall plot shows how the incorporation of Cd (or Mg) atoms to the MgO (or CdO) lattice increases the microstrain of the structure and therefore its crystalline quality. - Under the studied conditions up to 15.03% at. percentage of Cd determined by EDX has been incorporated to the Mg{1-x}Cd{x}O structure corresponding to a lattice parameter of 4.3220 Å. Study of the effect of growth temperature - MgCdO ternary alloy has been deposited on C-sapphire substrates using MOCVD technique. Growth temperature has been modified in order to study its effect on morphology, structure and composition of the samples. Metal-organic precursors ratio Cd/(Cd+Mg) was kept at 9.7% at. of Cd. XRD, SEM and EDX have been used to analyse the samples. -As a function of growth temperature we can observe a shift from a compact layer of Mg{1-x}Cd{x}O at high temperatures to a Cd{1-x}Mg{x}O layer at low temperatures. - At growth temperatures between 450 ºC and 500 ºC with XRD analysis we can observe that both phases coexist. Thus, as determined by SEM and EDX there is a Mg{1-x}Cd{x}O compact layer and some Cd{1-x}Mg{x}O structures without a defined shape. - This non-defined morphology of the Cd{1-x}Mg{x}O structures could be related with the reevaporation process of the CdO that happens when the growth is carried out at high temperatures. - Growth temperature let us, for a fixed MO precursors ratio, to get different concentrations of the alloy having the Mg-rich and the Cd-rich phases. - In the Mg-rich phase we can see that the more we reduce growth temperature, the more Cd is incorporated into the MgO lattice. Similar behaviour can be observed in the Cd-rich phase: the more we increase growth temperature, the more Mg is incorporated to the CdO lattice. This fact shows the influence that growth temperature has in the stability and solubility of Cd an Mg ions in the host lattice. - When both phases are present this behaviour is modified. It seems that there is a competition between them to incorporate more material. - This competition process between both phases can be explained due to reevaporation process of the CdO at temperatures beyond 400 ºC [Huerta2016]. Thus, at 450 ºC we have a Cd-rich phase with a significant reevaporation ratio and at 500 ºC with a higher one. This volatility favours the incorporation of Mg atoms and, to a lesser degree, Cd atoms, in the Mg{1-x}Cd{x}O phase. - In other words: - Mg{1-x}Cd{x}O phase incorporates more Cd atoms when the growth temperature is low due to the lower evaporation of Cd. At 500 ºC, despite the presence of the Cd{1-x}Mg{x}O phase, Cd atoms still incorporate to the Mg-rich phase. However, at 450 ºC growth conditions favour that Cd{1-x}Mg{x}O phase incorporates a significant amount of Cd reducing its incorporation to the Mg{1-x}Cd{x}O phase. - On the other hand, regarding Cd-rich phase, when we increase growth temperature, this phase incorporates more Mg atoms. Nevertheless, beyond 450 ºC this phase is not able to incorporate more Mg atoms so that they are incorporated to the Mg-rich phase. - Willimason-Hall plot shows that the vertical coherence length is high enough to not contribute to the widening of the diffraction peaks. However, microstrain is higher in both phases as the Mg (or Cd) atoms incorporate to the CdO (or MgO) lattice. - The great difference between growth conditions of the MgO and the CdO makes difficult the synthesis of the ternary alloy MgCdO. Thus, the limitation to grow layers with concentrations in the middle range of compositions has been evidenced because as a function of growth conditions what we obtain are the Cd-rich or the Mg-rich phases instead a phase in the middle range of concentrations. Study of the Mg{1-x}Cd{x}O and Cd{1-x}Mg{x}O phases - The evolution of the properties of the phases has been studied as a function of metal-organic precursors ratio. Mg-rich phase and Cd-rich phase have been obtained at growth temperatures of 600 ºC and 400 ºC respectively. - XRD measurements allow to stablish that at 400 ºC the main phase is Cd{1-x}Mg{x}O, while at 600 ºC we have the Mg{1-x}Cd{x}O phase. That is, under the studied growth conditions the formation of the different phases is determined by the growth temperature regardless the metal-organic precursors ratio. - Lattice parameters were calculated from the XRD peaks position. In both phases this parameter raises when the Cd precursor concentration is increased. - With the SEM it can be observed a layer of 115 nm of thickness for the Mg{1-x}Cd{x}O phase of the growths at 600 ºC. On the other hand, for the growths at 400 ºC we can see rougher layers that increase its thickness from 150 nm to 240 nm when Cd precursor concentration is increased. - EDX analysis links the XRD results showing how the Cd at. % rises in both phases when its precursor concentration is increased in the growth process. - Fitting the experimental data to a quadratic function we can see a curve above the Vegard's law like predicted in theoretical works [Amin2010, Joshi2013, Paliwal2012]. The experimental bowing parameter has been determined to be b=-0.4490 nm. Preliminary optical characterisation - An optical preliminary characterisation has been done on samples with the Cd{1-x}Mg{x}O phase. That is, the samples grown at low temperature. - By using spectrophotometry, transmittance patterns of the samples have been recorded from which the absorbance patterns have been calculated. In order to determine the optical band gap energy the maximum slope point of the absorbance patterns has been used. - Thus, the band gap energy rises with the Mg incorporation to the CdO lattice reaching a value up to 2.958 eV. The experimental points show, in the studied range, a linear behaviour. BIBLIOGRAPHY -Yoshiaki Nieda, Mari Suzuki, Atsushi Nakamura, Jiro Temmyo, Gema Tabares, Alejandro Kurtz, Manuel Lopez, Jos e Mar a Ulloa, Adrian Hierro, and El ías Muñoz. Journal of Crystal Growth, 449:27-34, 2016. -L.K. Wang, Z.G. Ju, C.X. Shan, J. Zheng, B.H. Li, Z.Z. Zhang, B. Yao, D.X. Zhao, D.Z. Shen, and J.Y. Zhang. Journal of Crystal Growth, 312(7):875-877, 2010. -A. Mohanta and R. K. Thareja. Journal of Applied Physics, 103(2):024901, 2008. -S. Sadofev, S. Blumstengel, J. Cui, J. Puls, S. Rogaschewski, P. Sch afer, and F. Henneberger. Applied Physics Letters, 89(20):201907, 2006. -Y. Z. Zhu, G. D. Chen, Honggang Ye, Aron Walsh, C. Y. Moon, and Su-Huai Wei. Phys. Rev. B, 77:245209, 2008. -U. Paliwal, T. Bredow, and K. B. Joshi. AIP Conference Proceedings, 1447(1):1037-1038, 2012. -Bin Amin, Iftikhar Ahmad, Muhammad Maqbool, Nazma Ikram, Yasir Saeed, Afaq Ahmad, and Suneela Arif. Journal of Alloys and Compounds, 493:212 - 218, 2010. -K.B. Joshi, U. Paliwal, K.L. Galav, D.K. Trivedi, and T. Bredow. Journal of Solid State Chemistry, 204:367 - 372, 2013. -Sreekumar Rajappan Achary, Said Agouram, Juan F. S anchez-Royo, M. Carmen Mart nez-Tom as, and Vicente Muñoz-Sanjos e. CrystEngComm, 16:8969-8976, 2014. -F. Atay, I. Akyuz, S. Kose, E. Ketenci, and V. Bilgin. Journal of Materials Science: Materials in Electronics, 22:492-498, 2011. -M. Vigneshwaran, R. Chandiramouli, B. G. Jeyaprakash, and D. Balamurugan. Journal of Applied Sciences, 12:1754-1757, 2012. -G. Chen, K. M. Yu, L. A. Reichertz, and W.Walukiewicz. 2013 IEEE 39th Photovoltaic Specialists Conference (PVSC), pages 1105-1109, 2013. -G. Chen, K. M. Yu, L. A. Reichertz, and W. Walukiewicz. Applied Physics Letters, 103(4), 2013. -C. Thiandoume, A. Lusson, P. Galtier, and V. Sallet. Journal of Crystal Growth, 311(19):4371 - 4373, 2009. -J. H. Lee, J. H. Eun, S. G. Kim, S. Y. Park, M. J. Lee, and H. J. Kim. Journal of Materials Research, 18:2895-2903, 2003. -Adelaida Huerta-Barber a, Llu s Manel Guia, Oleksii Klymov, Vicente Mari n-Borr as, Carmen Mart nez-Tom as, Julen Tamayo-Arriola, Alejandro Kurtz, Miguel Montes Bajo, El as Muñoz, Adri an Hierro, and Vicente Muñoz-Sanjos e. Applied Surface Science, 385:209 - 215, 2016.
