Clara
Sanchez-Perez
*ab,
Sriluxmi
Srimurugananthan
a,
Carlos
Sotelo-Vazquez
c,
Sanjayan
Sathasivam
ad,
Mingyue
Wang
a,
Javier
Marugán
c,
Ivan P.
Parkin
a and
Claire J.
Carmalt
*a
aUniversity College London, Department of Chemistry, 20 Gordon St, London WC1H 0AJ, UK. E-mail: c.j.carmalt@ucl.ac.uk
bInstituto de Energía Solar, Universidad Politécnica de Madrid, 28040 Madrid, Spain. E-mail: c.sanchez.perez@upm.es
cDepartment of Chemical and Environmental Technology (ESCET), Universidad Rey Juan Carlos, C/Tulipán s/n, Móstoles, 28933 Madrid, Spain
dSchool of Engineering, London South Bank University, London, SE1 0AA, UK
First published on 7th August 2023
ZnO thin film coatings were synthesised by aerosol-assisted chemical vapour deposition (AACVD) from molecular precursors with different Zn:
O ratios, from which synthetic parameters that promote self-texturing were analysed. To that end, the effect of the Zn environment and temperature on the morphology of the resulting ZnO coatings was evaluated for a wide range of synthetic conditions. With the aim to overcome the issues of commercial ZnO precursors, a non-pyrophoric molecular precursor has been investigated to produce 002-oriented, compact, transparent ZnO thin coatings with minimal carbon contamination at 300 °C and fast growth rates (>40 nm min−1). The deposition described herein is a new approach to rapidly achieve coatings at ambient pressure with optimal conditions for optoelectronic applications, and their potential as structural templates to epitaxially grow ZnO in a fast tandem configuration. The results presented in this work provide important insight on the influence of the chemical precursor properties and synthesis conditions to deliver functional ZnO coatings via solution-based methods, while obtaining morphologies only achievable by physical vapour deposition techniques.
Optoelectronic applications demand coatings with flat surfaces and uniformly distributed and densely packed particles, which requires precise control over film thickness, conformity and morphology.11,17 Such compact near-to-monocrystalline microstructures are obtained when small crystallites were grown in a single direction,18 which limits the span of suitable synthetic methodologies. Physical deposition methods can produce said coatings19,20 as they have negligible carbon intake and the Zn:
O ratio can be precisely controlled,17,21 but they require vacuum conditions, are energy intensive and hard to scale up. In contrast, chemical deposition methodologies are generally faster and can be carried out at atmospheric pressure. Of these, solution-based methods such as sol–gel can use carbon-free reagents, but require stabilising ligands and annealing processes to form crystalline films.15,22 Therefore, high temperatures are needed to remove the stabilising ligands, which can alter their surface polarity and optoelectronic properties.6,23 Vapour-phase methods, such as metalorganic chemical vapour deposition (MOCVD), are potentially more advantageous because highly pure and crystalline ZnO coatings are attainable at reasonable reactor temperature ranges during a single step process24,25 without the need of annealing steps that may trigger surface reconstruction.6 Still, it should be noted that the choice of precursor and solvent can greatly affect the crystal quality and purity of the resulting film.26,27 Good-performing precursors for MOCVD of ZnO are currently restricted to volatile but highly corrosive dialkiyl zinc species such as diethyl zinc (DEZ),28 which require adapted instrumentation and are a fire risk.29 Thus, the development of non-hazardous and environmentally friendly precursors is therefore of great importance from a manufacturing perspective.
The aerosol-assisted chemical vapour deposition (AACVD) route, in which a mist is created from a precursor solution using a piezoelectric device, widens the range of potential precursors as they need to be soluble rather than volatile.30 When designing larger precursors for use in processes like AACVD, it is important to consider their decomposition profile to minimise carbon contamination.31 Additionally, the supersaturation of solvent droplets can lead to multiple nucleation and growth regimes, which further complicates the control of self-texturing and the achievement of uniformly thin coatings.32 However, by carefully tailoring the design of precursors, it should be possible to overcome these challenges and obtain homogeneously thin coatings through AACVD. Several non-volatile precursors with saturated Zn centres have been investigated due to their high stability and their ability to undergo non-violent thermal decomposition, the most relevant being [Zn(acac)2].33–36 However, optoelectronic properties of undoped ZnO coatings are still far from those of PVD-grown coatings,37 and the effect of dopants is insufficient to compete with other transparent conducting oxides (TCOs).35 This can be explained due to the large amount of carbon content in [Zn(acac)2] that is not easily volatilised upon polymerisation,38 and the fact that they are partially self-textured with a high amount of grain boundaries. To overcome this issue, directional ZnO coatings can be deposited epitaxially over a seed-layer and/or a template such as c-silicon or c-Al2O3,39–42 however these templates are opaque and/or expensive, and therefore unsuitable for certain applications and large-scale production. It is then of great technological interest to find alternative precursors suitable for AACVD of fully self-textured high purity ZnO thin film coatings without crystallographic templates. To achieve our objective, we investigated the synthetic process and microstructure of ZnO thin films. These films were prepared using solutions containing a pre-organised precursor of zinc alkoxide, which undergoes thermal decomposition to form stable and volatile by-products. Previous reports have suggested that this method is capable of producing thick ZnO coatings that are both crystalline and pure, with a self-textured structure, using AACVD.30
Alkyl zinc alkoxides are highly soluble and stable in most aprotic dry organic solvents and, although less reactive than alkyl zinc reagents, they can produce ZnO through a similar path from thermal activation or reaction with water.43 Their “Zn4O4” cubane core is considered as “preorganised” ZnO16,39 and decompose via a polymerisation route that promotes polar growth,44,45 which determines the shape, size and preferred orientation of the ZnO particles. The ZnO formation rate is correlated to the nature of the alkoxide group, the reaction temperature and the concentration of the solution containing the chemical precursor.46 However, there is limited insight regarding the nature of the early stages of ZnO nucleation from precursors with pre-organised Zn4O4 moieties47 an how it differs from other common ZnO precursors. Henceforth, understanding nucleation and growth limitations in films deposited from solutions of molecular precursors with different nature is therefore key to design thin film coatings with an intended morphology. In the present work, AACVD was used to study the processes involved in the nucleation and growth of ZnO thin film coatings from Zn(acac)2, DEZ and alkyl zinc alkoxide precursor solutions. Furthermore, the relationship between parameters that enable O-rich or O-poor conditions and precursor is established. It is shown how the use of ethylzinc isopropoxide (EZI) provide great advantages to deliver ZnO coatings with a high degree of grain coalescence, avoiding hazardous alkyl zinc precursors and/or Zn-based precursors with high hydrocarbon content. Through AACVD of EZI, a very thin flat and homogeneous 002-oriented ZnO coating can be deposited even in non-epitaxial substrates, with features comparable to those obtained by PVD techniques.
Precursor | Zn![]() ![]() |
AACVD configuration | Hydrocarbon | Alcohol | Flow | Time | Average growth rate | ||
---|---|---|---|---|---|---|---|---|---|
Solvent | (mL) | Solvent | (mL) | (L min−1) | (min) | (nm min−1) | |||
Zn(acac)2 | 1![]() ![]() |
Single-inlet | – | MeOH | 100 | 1.2 | 100 | 6 | |
DEZ | 1![]() ![]() |
Dual-inlet | Hexane | 30 | MeOH | 35 | 2 × 0.8 | 20 | 30 |
Toluene | 20 | MeOH | 35 | 2 × 0.8 | 30 | 20 | |||
Hexane | 30 | IPA | 15 | 2 × 0.8 | 10 | 60 | |||
Toluene | 20 | IPA | 15 | 2 × 0.8 | 20 | 30 | |||
EZM | 1![]() ![]() |
Single-inlet | Toluene | 10 | MeOH | 15 | 1.8 | — | — |
EZI | 1![]() ![]() |
Single-inlet | Hexane | 15 | IPA | 15 | 1.8 | 15 | 40 |
Toluene | 10 | IPA | 15 | 1.8 | 15 | 40 |
On the contrary, the isopropyl group in EZM (Fig. 1c) enables a β-H elimination process that favours cluster aggregation (step A) and formation of volatile organic by-products that can be carried out through the exhaust.45 Notably, ZnO formation in helium entails a two-step process triggered by thermal decomposition with complete ZnO crystalline formation below 400 °C (step B), whilst in air EZI suffers additional reaction with O-containing species (moisture). Regardless of pathway, for both EZM and EZI the structural reconfiguration occurs below 400 °C, which suggests that it could be related to diffusion-controlled oxidation of graphitic carbon adsorbed in the thin film53 rather than the carbonate formation linked to Zn(acac)2 decomposition.
In AACVD the precursor is carried by a solvent to the reactor chamber, typically a mixture of alcohol and hydrocarbon for DEZ and alkoxide solutions.30 It is noteworthy that the EZM precursor was only stable in toluene/MeOH mixtures at low temperature (−78 °C) and partially decomposed in the bubbler, producing thin films with very low crystallinity (Fig. S2, ESI†). The Zn centres in EZI are more sterically protected than in EZM, so the precursor was stable at RT and could be isolated and stored in an inert atmosphere. Partial decomposition was observed in EZI solutions in anhydrous methanol (MeOH) but was stable in anhydrous isopropanol (IPA). With this palette of precursors and solvent combinations, we have analysed their effect on the Zn:
O ratios for Zn coordination sphere during growth and nucleation in ZnO coatings grown in O-poor (Zn
:
O > 1) and O-rich (Zn
:
O < 1) conditions.
Crystallographic analysis of the as-synthesised samples shows that all planes in the range 20–66° belong to the 0001 and 20 families of planes in the hexagonal crystal system of Wurtzite (P63mc). The only visible growth direction from the first family appears due to polar growth along the (002) polar plane along the c-axis (002) of the cubic system, whilst the second family appears due to growth in the respectively non-polar [110] and [10n] of the cubic system (Fig. 2a). GIXRD results showed that molecular precursors with pre-formed polar Zn–O bonds led to c-axis preferential growth (Fig. 2), which was consistent with the existence of O-rich growth conditions.21 The use of DEZ/hydrocarbon and alcohol solutions in a dual-inlet system led to ZnO thin films with mainly non-polar plane growth when MeOH was employed, which is reported for O-poor synthetic conditions.21 Interestingly, coatings with mainly polar growth were obtained in similar conditions when IPA was used, suggesting that O-rich conditions are met during the synthesis of the thin film coatings. This effect can be explained by the stabilisation of the EZI intermediate (Scheme 1) that enables ZnO growth via small cluster aggregation prior to polymerisation.45
This statement is supported by the fact that GIXRD patterns of ZnO films grown from isolated EZI solutions were identical to those of films grown from dual-source setups (DEZ in hexane or toluene/IPA) in analogous solvent and temperature conditions (Fig. 2e and f). It is worth noting that for films grown at 500 °C from DEZ in hexane/IPA, the GIXRD pattern indicates polycrystalline nature, which is in good agreement with the μm-sized features observed in Fig. 3. Finally, X-ray diffraction data of coatings deposited from solutions of isolated EZM intermediate also exhibited preferential polar growth (Fig. S2, ESI†), whilst dual-source depositions of DEZ in analogous temperature and solvent conditions exhibited clear non-polar preferred growth (Fig. 2c and d). This implies that the synthetic path for ZnO formation in oxygen-deficient conditions does not involve stabilisation of the tetrameric intermediate at high temperature (Scheme 1), as GIXRD patterns from DEZ/MeOH and EZM solutions with the same co-solvent and deposition temperature were not the same.
Scanning electron microscopy (SEM) was used to study the different morphology shown by the as-deposited ZnO thin film coatings. The different morphology features are due the relative nucleation and growth rates. Micrographs showed a self-texturing effect (Fig. 3), where the surface morphology of the ZnO films can be directly related to their degree of crystallinity and preferential growth. The relative orientation of crystalline planes for the ZnO wurtzite structure is illustrated in Fig. 2a where the (002) polar plane is parallel to the substrate, whilst planes (100) and (110) are perpendicular to it, and planes in the [10n] direction are oblique to it. Thin film coatings synthesised at low temperature exhibited small and irregular particles with grainy or small bubble-shaped surface features (Fig. 3). Conversely, high reaction temperatures promoted the formation of larger particles with morphologies connected to their respective preferential growth (Fig. 2a). It was observed that thin films synthesised in O-deficient conditions exhibited shallow wedge-like surface crystallites (Fig. 3 (DEZ in Hex/MeOH and DEZ in Tol/MeOH)), a morphology product of the combination of non-polar planes. Instead, thin films gown in O-rich environments promoted the growth of ZnO towards the polar c-axis upon lattice stabilisation due to the filling of oxygen vacancies, forming large blocks and plates parallel to the substrate (Fig. 3 (DEZ in Hex/IPA and DEZ in Tol/IPA)).
SEM micrographs of films grown at 500 °C from DEZ in hexane/IPA exhibited μm-sized features (Fig. 3) which explains the lack of self-texturing (Fig. 2e). Self-texturing above 450 °C was not observed in ZnO coatings prepared from Zn(acac)2 solutions, likely due to adsorption of decomposition by-products causing significant carbon contamination (Fig. S3, ESI†)38 and resulting in thin films with a bubbly and/or wedgy appearance (Fig. 3). The observed correlation between texturing and thin film morphology in this study provides evidence supporting the idea that the chemical environment at the surface plays a crucial role in stabilising and determining the atomic structure of surface reconstruction.7 Therefore, it is evident that self-texturing is influenced not only by the deposition temperature of the film,54 but also by the specific characteristics of the precursor solution used.55
It is well-reported that thermal decomposition of precursors with pre-formed Zn–O bonds (O-rich conditions) generate supersaturated proto-nuclei clusters at a large nucleation rate, which are larger than the critical size to evolve as crystallites.56 This effect is clearly observed in films grown from Zn(acac)2 in which large nucleation sites form and evolve into large and 002-oriented crystallites (Fig. 4). Despite the fast nucleation rate, AACVD of Zn(acac)2 features a slow growth rate, which explains the observed slow film growth. It can be inferred from SEM images that the growth rate is controlled by nucleation at low temperature, and ZnO films are defined by the formation of nano-particular crystallites with no preferred orientation. These synthetic conditions explain the formation of non-equiaxed columnar growth due to the deposition of a partial amorphous phase that led to random nucleation (Fig. 2b). Then, with increasing deposition temperature, crystallite size and crystallinity also increase, and at reaction temperatures slightly above the decarbonisation temperature highly crystalline thin films with equiaxed columnar growth were observed (Fig. 4). Despite this acute self-texturing, organic non-volatile by-products are significantly entrapped in these films, and such contamination hinders their applicability in optoelectronics. Although carbon contamination can be substantially reduced by a short annealing process at 400 °C under O-rich conditions,53 this has been proved to negatively modify the morphological and optoelectronic features of the as-synthesised ZnO films.57,58
In contrast to the previous observation, under O-poor conditions (e.g. dual-source CVD of DEZ/MeOH) precursors undergo immediate reaction upon entering the reaction chamber. As a result, the formation of random nucleation sites without significant nuclei proto-clusters is anticipated. Indeed, top-down view and cross-sectional SEM analyses (Fig. 3 and 4) of these thin films show compact coatings without defined crystal boundaries and with high crystallite coalescence, which is consistent with the random formation of small proto-nuclei. The preferred orientation in these films is then expectedly ruled mainly by grain growth towards non-polar directions, and in good accordance with literature.59 Therefore, in O-poor conditions crystal growth occurs at high supersaturation and not through the formation of a stable intermediate, so it is dominated by slow nucleation rates.
The general morphology of ZnO thin films synthesised from DEZ/IPA is strikingly similar to that of DEZ/MeOH, but their high crystallinity and (002)-preferential growth can only emanate from the formation of small but highly polar proto-nuclei clusters. This effect can be explained by the formation of the stable EZI tetrameric intermediate with Zn–O pre-formed bonds in a cubane-type arrangement. This structure can undergo polymerisation activated through either β-hydrogen elimination or β-hydride transfer, in both cases evolving into c-axis oriented proto-nuclei.45 However, while β-hydrogen elimination produces stoichiometric ZnO, the β-hydride transfer yields oxygen-deficient ZnO1−x, which leads to the formation of smaller nucleation sites. Hence, crystal growth occurs at intermediate supersaturation conditions where crystal nucleation and growth have similar rates, which is consistent with the high degree of crystallite coalescence (Fig. S4, ESI†). Both processes produce highly volatile and stable by-products, which is the driving force for high rate, and the minimal adsorption of these by-products leads to formation of very crystalline nucleation sites.
The optical properties of the as-deposited ZnO thin films were evaluated using UV-Visible spectroscopy. ZnO thin films are optically transparent materials along the solar spectrum with an absorption edge at ∼3.3 eV, although their optical properties are affected by the thickness11 and their crystallinity.60 The formation of partially amorphous ZnO films can be influenced by the specific conditions employed.61 This is evident from the observation of an additional blue-shifted optical bandgap in the resulting films.62,63Fig. 5 shows how the optical properties of ZnO film coatings vary with synthetic conditions, where the bandgap values were calculated using Tauc's formula for direct-bandgap semiconductors.64
![]() | ||
Fig. 5 Transmittance (a)–(e) and band gap calculation using a Tauc64 plot (f)–(j) of ZnO thin films at reaction temperatures 300 °C (light green), 350 °C (olive), 400 °C (light blue), 450 °C (dark blue) and 500 °C (purple). Dotted black lines represent transmittance of the glass substrate (a)–(e), red arrows represent the bandgap of crystalline ZnO film coatings, red dashed arrows represent the bandgap of amorphous ZnO film coatings and dotted black arrow represents the bandgap of the glass substrate. |
ZnO coatings grown from Zn(acac)2/MeOH solutions at reaction temperatures below 400 °C exhibit partially amorphous nature, while films deposited above that temperature are crystalline but have low transparency linked to severe carbon contamination (Fig. 5a). Hence, although the use of Zn(acac)2 solutions is convenient, the resulting films have unsuitable optical and electrical properties due to either low crystallinity or low purity. In regard to films grown from DEZ solutions, generally lower transparency was observed in coatings deposited from hexane-containing solutions at high temperature (Fig. 5b and d), making toluene the optimal co-solvent. ZnO coatings obtained from DEZ/MeOH solutions were fully crystalline only above 450 °C, exhibiting high transparency considering they are undoped and their thickness (500–600 nm). Despite the disadvantages associated with the use of DEZ – pyrophoric, low deposition rates –, these synthetic conditions have largely been considered optimal for AACVD of ZnO for optoelectronic applications.52,59,65 However, the high temperature needed to obtain fully crystalline films hamper their use in numerous advanced functional materials based on heterostructures.66 Remarkably, coatings deposited from DEZ toluene/IPA solutions were found to be highly transparent and fully crystalline below 350 °C, a substantially lower temperature range than previously reported (Fig. 5e and j). As a result, it has been discovered that fully polar, self-texturing, and crystalline ZnO films can be achieved at temperatures significantly lower (by 100 °C) than those previously reported for AACVD. This finding holds great significance for the advancement of ZnO-based crystallographic templates and heterostructures, opening up new possibilities for their development.66
The general conception for ZnO coatings formed via AACVD is that dual-source deposition using DEZ/toluene and MeOH produces films with the lowest resistivity,59 which are grown in O-poor conditions that favour non-polar growth and have high purity.21 Deposition using Zn(acac)2 or other O-rich single-source precursor produces films with large grains and a mainly polar structure with many facets, which are more resistive and less pure so their functionality is better applied in other fields such as photocatalysis.13 The cubane-type precursor EZI does fall in between those two conditions, as it is a single-source precursor that provides an O-rich growth environment (β-hydrogen elimination) but can also enable O-poor growth environment (β-hydride transfer). Although it produces polar films that are structurally slightly more resistive than non-polar ones,21 its decomposition process leads to equally compact films with higher purity than even those from DEZ (Fig. S5, ESI†). Then, the use of ZnO-based materials as TCOs requires adequate microstructure and high purity – both provided in films grown by AACVD of EZI/toluene solutions – as well as doping to boost both optical and electronic materials. In fact, the ability of EZI to produce GZO coatings with enhanced optoelectronic properties was recently reported in the AACVD report published by some of the authors of this work,45 where films were grown for 15 minutes on glass substrates with prime optoelectronic properties (ρ = 4.7 × 10−4 Ω cm; Rsh = 9.4 Ω). It this study it was shown that Ga at 3% doping produces self-texturing of ZnO towards the (002) direction, but the formation of exclusively c-axis oriented in undoped ZnO coatings is limited to 350 °C (Fig. 2f).
The use of EZI precursor solution offers several advantages, including its non-pyrophoric nature and highly solubility. These properties enable the use of high flow rates, resulting in significantly shorter deposition timeframes. The similar timescale of nucleation and growth allows for the deposition of homogeneous and compact films, which are highly relevant in technological applications such as heterojunctions or as continuous crystallographic template layers. To test the suitability of our synthetic procedure, we conducted a tandem AACVD process using EZI/toluene/IPA as the precursor solution. In this process, a thin fully self-textured layer of crystalline ZnO was first deposited at 350 °C as a “template”, followed by a deposition at 450 °C. The growth rates were measured at 30–40 nm min−1 (Fig. 6a). Initially, we evaluated the feasibility of forming a very thin and uniform layer of ZnO at 350 °C, which is uncommon in solution-processed thin film fabrication.30Fig. 6b and c showed the transparent and compact nature of the ZnO template layer, with a homogeneous thickness below 200 nm. When comparing ZnO grown directly on glass at 450 °C to ZnO deposited over the ZnO template layer at the same temperature, it was observed that the latter exhibited full self-texturing towards the c-axis (Fig. 6b) without noticeable grain boundaries or interfaces between layers (Fig. 6d). The double-layer ZnO coating also exhibited high transmittance considering it is undoped, although slightly lower due to its increased thickness (Fig. 6e). These results demonstrate the potential of our thin layers as templates for homoepitaxial deposition of ZnO. Moreover, this approach can be extrapolated to enable oriented growth of non-continuous layers as well.
![]() | ||
Fig. 6 (a) Schematic representation of the tandem AACVD process proposed for the rapid fabrication of ZnO thin film coatings. (b) GIXRD data of the template layer (bottom) and the ZnO thin film (top) showing uniaxial polar growth and (c) transmittance spectra and band gap calculation using a Tauc64 plot of the template layer and ZnO thin film made through the tandem deposition process. The transmittance of the substrate glass is included as reference (dotted black line). Cross-sectional SEM images of (d) the template layer grown at 350 °C (184 nm thickness) and (e) the tandem ZnO thin film grown at 450 °C (502 nm thickness). X-ray photoelectron spectroscopy (XPS) spectra of the O 2p environment (f), the Zn 2p environment (g) and the C 1s environment (h) of the ZnO double layer. |
Finally, compositional analysis of selected ZnO coatings was performed using X-ray photoelectron spectroscopy (XPS) after etching (200 s under Ar) to evaluate the bulk film characteristics and obtain a qualitative evaluation of the carbon intake during ZnO formation. For that, the presence and chemical nature of Zn, O and C on ZnO double layers deposited from EZI/IPA/toluene solutions was compared to that of ZnO coatings deposited from precursor solutions of Zn(acac)2 and DEZ in MeOH/toluene at 450 °C. The oxygen 2p environments could be deconvoluted into three separate peaks, OI and OII, with binding energies of 529.9 eV (±0.2 eV) and 531.6 eV (±0.2 eV), respectively (Fig. 6f). The most intense peak (OI) can be attributed to O2− ions in ZnO. OII can be attributed to O2− ions located in oxygen deficient regions in ZnO.67,68 The binding energies of the zinc 2p3/2 and 2p1/2 environments were found to be 1021.1 eV (±0.2 eV) and 1044.3 eV (±0.2 eV), respectively, which can be attributed to the Zn2+ in ZnO (Fig. 6g).69,70 Analysis of C1s environment of the ZnO coatings showed that the precursors with unsaturated small ligands with H in β-positions minimised intake of carbon-containing species into the coating (Fig. S5, ESI†). Extremely high levels of carbon contamination (∼40%) were found in ZnO coatings from Zn(acac)2 solutions at 450 °C and DEZ solutions, which was not unexpected from mere visual inspection (Fig. S3b, ESI†). Carbon signals amounted to ∼4% in films grown from DEZ whilst in the tandem EZI deposition were found within the 1–2% instrumental error of XPS (Fig. 6h). These results confirm that the highly volatile organic by-products produced from decomposition of EZI are better desorbed at lower temperature, favouring minimal contamination during deposition. Thus, we can conclude that our proposed EZI/IPA/toluene precursor solution can successfully generate ZnO coatings with properties suitable for high-end applications71 through a fast and non-toxic process that requires minimal equipment, showing enormous potential to become the go-to route for fabrication of functional CVD-grown ZnO thin film coatings for TCO applications.
Overall, the novel route to synthesise 002-oriented crystalline template layers with excellent substrate coverage presented herein represents a considerable technological improvement in terms of instrumental resources and time when compared with non-solvent-based methodologies prevalent in industry. Ultimately, the results obtained in this study open an avenue to generate both nanostructure and flat/compact oriented ZnO coatings through an affordable and safely scalable solution-based route.
Although ZnO thin films formed in O-poor conditions generally show optimal features for optoelectronic applications, their fabrication through solution-based methods is extremely slow and entails dangers associated to the pyrophoric nature of DEZ. Synthesis of thin film coatings in O-rich conditions is industrially more appealing, however, typical acetylacetonate-based precursors produce ZnO coatings with unsuitable properties for optoelectronic applications, such as low purity and non-compact structures with large crystallites. In this work, the synthesis of ZnO film coatings with similar nucleation and growth rates was achieved from a chemical intermediate of the reaction of DEZ/toluene with IPA. The EZI precursor is not pyrophoric and is stable in inert conditions, and it is highly soluble in hydrocarbon/IPA mixtures, which allows the use of a single-inlet AACVD configuration resulting in a fast and exothermic but non-violent reaction pathway. The synthetic route present in this work therefore avoids blockage issues, resulting in the formation of c-axis oriented ZnO thin films with a degree of purity, grain coalescence and transparency comparable to those achieved by PVD methodologies.
This work presents a new synthetic route to produce homogeneous and very thin ZnO film coatings in non-epitaxial substrates through a fast, cost-effective and safely scalable methodology. Importantly, these coatings can further act as structural templates to epitaxially grow ZnO film coatings in a tandem configuration, allowing to rapidly produce prime quality ZnO thin films for optoelectronic applications at ambient pressure.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ma00468f |
This journal is © The Royal Society of Chemistry 2023 |