Preparation and Some Properties of Metal Organic Chemical Vapour Deposited Al-Doped ZnO Thin Films Using Single Solid Precursors

Zinc Oxide (ZnO) and Aluminium doped ZnO (AZO) thin films were deposited on soda lime glass by Metal Organic Chemical Vapour deposition technique (MOCVD), using prepared compound mixtures of Zinc Acetate di-hydrate (Zn(CH3COO)2·2H2O; ZAD) and Aluminium Acetyl-Acetonate (Al(C5H702)3; AAA) precursors at a temperature of 420 ̊C. Effects of the varying mole percent concentrations of AAA precursor additives on the Al dopant concentrations in ZnO were systematically studied. The observations were made via investigations carried out on the morphological, optical, electrical and compositional properties of the deposited thin films. The thin films morphology was found to be strongly dependent on the varying concentration of AAA in the precursor mixtures. The average optical transmittance of the thin films in the uv-visible region was over 85% except 5 mol.% Al. While the energy band gaps were found to be in range of 3.27 3.36 eV. There is a blue-shift of the energy band edge observed between 0 and 5 mol.% AAA, which may be due to Burstein-Moss’ band gap widening effect and an opposing band gap renormalization effect at 10 mol.% AAA along with an extra band gap stabilization effect (Roth’s effect) at 15 mol.% AAA in rather quasi-sinusoidal or anomalous behaviour. The optical transmittance and electrical conductivity of ZnO were enhanced with addition of Al dopants. The RBS confirm the presence of Al, Zn and O, and evidence that Al dopants were successfully incorporated into the ZnO. How to cite this paper: Akinwunmi, O.O., Ogundeji, J.A.O., Famojuro, A.T., Akinwumi, O.A., Ilori, O.O., Fadodun, O.G. and Ajayi, E.O.B. (2018) Preparation and Some Properties of Metal Organic Chemical Vapour Deposited Al-Doped ZnO Thin Films Using Single Solid Precursors. Journal of Modern Physics, 9, 2073-2089. https://doi.org/10.4236/jmp.2018.911130 Received: July 13, 2018 Accepted: September 25, 2018 Published: September 28, 2018 Copyright © 2018 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
Within the present decade, the rate of development of optoelectronic materials applications has rapidly increased, with current research in transparent conducting oxides (TCO's) being directed towards choice materials capable of combining economical and environmentally sustainable superior performance. TCO's are generally classified as materials with combined high average optical transparency > 80%, high electrical conductivity, and low resistivity of crust when compared to Indium (In), eco-friendly, has high thermal, chemical and radiation stability in hydrogen plasmas and other harsh environments [4].
Moreover, there is a general ongoing trend in the TCO industry to move towards alternative solutions for more advanced applications [5]. In the recent time, AZO thin films were been deposited on flexible substrates and are attracting both research and technology's attention [6] [7].
TCOs' are majorly compound semiconductors, generally classified as either n-type or p-type in line with their electrical properties. Their classifications are further subdivided into binary, ternary, quaternary compound and doped types.

Metal organic chemical vapour deposition (MOCVD), which is a variant of
Chemical method is an attractive technique for the fabrication of TCO's. This is due to its large-area to volume and conformal deposition capability even with less sophisticated apparatus, as well as close to ambient temperature deposition in contrast to PVD techniques. This paper is a report of MOCVD AZO thin films deposited on soda lime glass substrate, using single solid source precursors and the trends observed in its properties with varying Al dopant concentration.

Experimental Details
ZnO and AZO thin films were prepared by MOCVD technique. A schematic of the experimental set-up is shown in Figure 1

Results and Discussion
A pyrolytic process is known to occur close to a heated glass substrate when aerosol droplets in a jet or cloud arrive at the substrate, so that a highly adherent thin films of ZnO and AZO were formed on the substrate.

Precursor Mixture Composition
FTIR spectrum of the final single solid source precursor used and the starting solid materials AAA and ZAD were shown in Figure 1 Figure  2(c), show a uniformly granular polycrystalline morphology having almost the same sizes, with well-defined grain boundaries and an inhomogeneous surface with polygon-like grains that may efficiently scatter incident photons. The as-deposited 15 mol.% AZO is polycrystalline with grains and obvious grain boundaries. This observation may be as a result of the an Al rich thin film, resulting in a degenerately doped layer of AZO and hence creating a highly distinct segregation of grains with possible accumulation of excess Al ions at the grain boundaries thus preventing further enlargement of the grains and grain boundaries. Al 3+ as dopant substitute Zn 2+ in AZO thereby increasing the nucleation site number [13]. In a degenerately doped ZnO, there is a tendency for heavy and localized nucleation of grains to occur, such as observed in Figure  2(d).

Optical Properties
The   following a heavy dopant concentration [20]. These effects are typical of the optical band gap of nanocrystalline ZnO thin films. According to Srikant and Clarke, the Burstein-Moss effect is valid for materials having low effective mass of electrons and holes. That is, materials in which quantum confinement lead to an initial rise in the optical band gap [21]. This appears contradictory with the results of this study, in which the AZO thin films are not only degenerately n-type doped, but thicknesses are way more than that permissible in quantum confinement studies. Yet, an initial band gap widening offset partially by a band-gap renormalization was observed. However, recent studies through density-functional band-structure theory demonstrate that the band-gap renormalization is related to the non-parabolic nature of the host conduction band (ZnO), created as a result of the dopant impurities, and not because of a rigid shift of the band edges as studies have earlier assumed [22]. As a result, the carrier dependence of the Burstein-Moss' band-gap widening in this study is rather highly sensitive to the electronic states of the dopant ion (Al 3+ ) which in turn can be intrinsically involved in the complete reconstruction of the conduction band, thus lowering it. This effect is better observed in Figure 3(e), the band-gap widening from 0 to 5 mol.% Al dopant concentration is as a result of the occupation of the AZO conduction band, inducing optical transitions at higher energies than the minimum-energy of the fundamental electronic gap (undoped ZnO). Meanwhile, and on the other hand, increase in free carrier screening will result in the decrease in the binding energy of the optical transitions associated with both free and bound excitons. So that, in the undoped ZnO and the AZO thin films, both the free and bound exciton transitions show a quasi-sinusoidal blue-shift and red-shift as the exciting wavelength of the UV-Visible radiation is decreased and intensity maintained with increasing dopant concentration [23]. The renormalization effect results in a red-shift of the optical transitions, and therefore a reduced free-carrier screening effect of photons resulting in increased average transmittance as a result of the reduction in the band-gap energy. The energies of the optical transitions are therefore the resultants of the blue-shift due to screening, and the red-shift due to the renormalization effect. The entire energy interplay weakens as the dopant concentration increases (up to15 mol.%), so that a stability effect (Roth's effect) converges both the average transmittance and the band-gap to a stable value. The energy positions are determinable by the aggregated effects of screening and band-gap renormalization resulting from the added free electrons. In this study the exact energy position was not determined but rather we try to establish the trend for the optical transition energies.

Electrical Properties
Linear current-voltage (I-V) characteristics are observed in the 5 and 10 mol.% AZO thin films as shown in Figure 4(a) and Figure 4 where s R is the sheet resistance in ohm/square, ρ is the resistivity in ohm-cm and t is the thickness of the thin film.
A decrease in the sheet resistances of the AZO thin films is observed for thickness ranges from 487 to 634 nm. Therefore, a corresponding decrease in the resistivity is partly due to the presence of defects and majorly to Al dopants located as interstitial atoms and hence substitute Zn, and filling Oxygen vacancies [24]. This may be due to the donor states created as a result of the Al 3+ ions impurity introduced in the ZnO lattice structure, producing in all cases an n-type semiconductor [4].
where T (%) is the optical transmittance at a specified wavelenght λ = 550 nm, and R s (Ω/sq) is the sheet resistance. Figure 4(e) shows that Φ TC values of the thin films decrease from 0 mol.% to 5 mol.% with a slight decrease of the thin film thickness from 634 to 612 nm, but increase sharply beyond this range. The value of Φ TC increased abruptly to a high level at 10 mol.% due to the low sheet resistance and the high transmission of the AZO layer combined with a decreased thickness of 487 nm. Φ TC decreased sharply below the 10 mol.% value to a value comparable to the 0 mol.% value at 15 mol.%. This is due to a high sheet resistance, though with the same transmission as the 10 mol.% AZO thin film, but at a higher thickness of 509 nm. While Figure 4(f) shows an opposite trend with Figure 4(d) in the sheet resistance values with the band gap, Figure 4(g) shows a one to one correlation between the sheet resistance and resistivity giving an almost linear correlation in the values of the thickness as compared with the large magnitude of the sheet resistances.    [28]. This is evidence in observed highest sheet resistance obtained in the undoped ZnO layer with the highest O concentration in Figure 5( Al is no longer as effective [29]. The study therefore shows Al 3+ solubility factor of 15 below the solubility limit, implying the possibility of further incorporation of Al 3+ in the ZnO lattice given the advantages of the favorable process conditions used. The RBS spectrum in Figure 5

Conclusions
In this work, AZO thin films were deposited using MOCVD technique using single solid precursor. The factorial decrease in the Zn/Al ratio and the corresponding increase in the Al dopant concentrations of the AZO thin films play an important role in the properties of the AZO thin films, i.e. in the sheet resistance, morphology, transmittance and energy band gap after the thin films growth.
We have used a range of characterization techniques to study an ensemble of AZO thin films which allowed us to show that ZnO thin films can be effectively doped under improvable but yet economic processing conditions at relatively low temperature (420˚C). Particularly, 10 mol.% AAA AZO thin film exhibited a much promising lower resistivity of 19.88 Ω-cm with a corresponding average transmittance of 89% and 91% at λ = 550 nm. This value of resistivity establishes a remarkable improvement by a factor of 957 over that of the undoped ZnO and an improvement by a factor of 53 over that of 5 mol.% AAA. Thus, our results that the doping methodology adopted for deposition of AZO thin films in this work are suitable for potential applications in high performance optoelectronics devices. These results also establish a route to achieve adequately controllable doping efficiency of ZnO via the incorporation of Al dopants within its solubility limit.