EFFECT OF DEGRADATION ON THE QUALITY AND PERFORMANCE OF ALUMINIUM-DOPED ZINC OXIDE (ZNO) THIN FILMS

International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN (P): 2249–6890; ISSN (E): 2249–8001 Vol. 11, Issue 2, Apr 2021, 283-294 © TJPRC Pvt. Ltd.

EFFECT OF DEGRADATION ON THE QUALITY AND PERFORMANCE OF ALUMINIUM-DOPED ZINC OXIDE (ZNO) THIN FILMS DEPOSITED ON FLAT GLASS GILBERT OSAYEMWENRE & FREDDIE INAMBAO* Department of Mechanical of Engineering, University of KwaZulu-Natal, Durban, South Africa ABSTRACT Four Aluminium doped zinc oxide (ZnO:Al) thin-film layers on three flat glasses and a nanostructured substrate were subjected to long term degradation conditions for two years. The thin films were stored under normal laboratory conditions in a plastic bag to induce slight heat damping at times to accelerate the physical and chemical degradation process. Low temperature fabricated transparent conductive oxide (TCO) aluminium doped Zinc oxide (AZO) has attracted much interest in solar cell application due to its opto-electrical properties. However, its application is limited because of instability to temperature, fast degradation, and low reliability in the presence of an acidic medium. These parameters (temperature, degradation and acidic medium) can be used as a gauge for assessing the stability of TCO

of AZO thin film. The long-term effect of the doping substance in mitigating the effect of AZO degradation was investigated in this work, by exploiting the interplay between surface morphology and defects. The roughness/degradation of the different thin films was evaluated using scanning probe microscopy (SPM) to demonstrate degradation. The roughness analysis was compared to the initial roughness after fabrication of the thin films. The analysis showed that different dope substances offer different mitigation levels, and that AZO/Car had less morphological degradation. Studies by scanning probe microscope showed that surface roughness and structural changes were due to the degradation of AZO films, and the low RMS roughness of AZO:Al/Car after degradation reflected the stability degradation of AZO films in an underwater electrolysis environment. The low RMS roughness of AZO:Al/Car reflected the higher stability of this thin film evident from the low degradation of AZO:Al/Car thin film.

KEYWORDS: Aluminium doped zinc oxide (ZnO:Al), aluminium doped zinc oxide (AZO), transparent conductive oxide (TCO), surface roughness, morphological degradation, RMS roughness.

Received: Jan 18 2021; Accepted: Feb 08, 2021; Published: Mar 15, 2021; Paper Id.: IJMPERDAPR202121

1. INTRODUCTION In the optoelectronic industries, three types of material are currently being used as a transparent conductive oxide layer. Transparent conductive oxides (TCOs) serve as components in optoelectronic devices [1-3]. TCOs can combine two vital features of optical transmission of about 80 % under visible region and very low electrical resistivity of about 10-3 ohm-cm to 10-4 ohm-cm [2-3] in optoelectronic devices. Indium tin oxide (ITC) exhibits the above named two features above [4-5]. ITC are costly to manufacture and time consuming due to its very process states, beside it is limited in nature. The high production cost and short supply nature is the reason the researchers engaged in research to find an alternative material that can replace ITO. ZnO, which has been demonstrated as a possible replacement, is an n-type material; therefore, it needs to be doped with metal from group III like gallium and aluminium. This doping helps to increase the carrier’s concentration which indirectly lowers the material

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devices. Various AZOs of different doping material were used to demonstrate how degradation influences the roughness

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resistivity. The reasons for the choice of ZnO includes its optical band gap of 3.3 eV, good electrical and optical properties, low level of toxicity and high chemical stability [1, 3-4].

Zinc oxide (ZnO) research attracts attention because of its unique ability to be modified for better quality. ZnO in research has become significant in the last decade because of its potential applications. The material is currently being used in various industries such as medical science, ceramics, cosmetics, organic solar cells, and concrete manufacturing to mention just a few [5]. In the electronics industry, ZnO is being used as a transparent electrode in touch screens, LCDs, plasma display panels (PDPs), and solar cells [5, 6, 7]. Previously ITO was most commonly used as an optoelectronic material and still is the most prominently used thin film in some organic light-emitting diodes, antistatic coatings, and solar cells. However, due to its scarcity in nature and huge production costs, ITO has been replaced with the more economic transparent conducting oxides (TCO). One of the best candidate materials that fits perfectly as a TCO is ZnO. This is because of its abundance in nature and cheap production costs. At the moment ZnO is less attractive as a conductive oxide because it is highly prone to degradation compared to ITO. To correct this, an impurity is introduced into ZnO to form impurity-doped ZnO to improve its stability. This is important because the better the stability of TCO the more widely it can be applied. Recently an impurity-doped ZnO [7-8] has been produced which shows better electrical and optical features for TCO and is believed to be a suitable candidate for replacement of ITO. Previous work by [5, 11] shows that ntype ZnO exhibits high binding energy of the order of 60 meV coupled with good chemical and thermal stability. Based on this, ZnO can be doped to increase its properties such as stability, optical and electronic properties. A doped Al with ZnO (AZO) has shown an increase in free charge carriers of larger optical bandgap [12] compared to an undoped ZnO. The low-temperature production of ZnO has created many options for its processing; TCO thin films are currently being produced by several techniques which include spray pyrolysis [13], pulsed laser deposition [14] sputtering [12], laser ablation [13], atomic layer deposition [15], magnetron sputtering [16] plasma-enhanced CVD (PECVD) [14], single-source CVD (SSCVD) [17], sol-gel technique [18] metal-organic CVD (MOCVD) [19], pulsed laser deposition (PLD) [19], molecular beam epitaxy (MBE) [20], thermal oxidation of Zn or ZnS [20-22] and atomic layer deposition (ALD) [23].

Magnetron sputtering stands out due to its low temperature, large/flat area deposition of thin-film and great crystalline intrinsic thin film [24-26]. However, the sol-gel technique is economical and allows for the control of its chemical composition. In respect of the method adopted for the processing of the transparent conductive oxide thin films, the stability of the material is of utmost importance due to long-term degradation of thin-film devices.

This study aimed to investigate the effects of degradation on the morphological structure and the quality of transparent AZO thin film. The investigation of degradation on the surface of TCO is essential to achieve a better understanding of its variation in performance over a long period. As degradation continues, it affects the roughness and thickness of the AZO thin film, and this may lead to inhomogeneity in its morphology. Previous studies have shown that the thickness of AZO is related to its stability [7, 12]; hence a change in its thickness is detrimental to its performance.

2. INITIAL CHARACTERISATION AND SAMPLE PREPARATION After fabricating the thin films, a series of analytical tools were employed to characterise their properties. The structural properties, namely, film roughness, interface roughness, thickness deposited, and thickness measured by XRR, were investigated. After two weeks, scanning probe microscopy measures were conducted to confirm the film roughness and interface roughness. The thickness and roughness of the different transparent conductive (AZO) thin film used in this study

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are presented in Table 1. This table serves as baseline readings for future use regarding the results of degradation analysis.

Deposition thickness (nm) AZO/Car-300 AZO/Ag-300 AZO/N2-300 AZO/Ar-300

XRR thickness (nm) 307+13 307+13 307+13 307+13

Table 1: Baseline Readings Film Roughness Interface (nm) roughness (nm) 3.35+0.1 3.29+0.6 3.35+0.1 3.29+0.6 3.35+0.1 3.29+0.6 3.35+0.1 3.29+0.6

Film roughness (Ra) (nm) 4.50 3.83 3.47 3.51

Rq roughness (nm) 3.21 3.19 3.20 3.10

The first stage of preparation was to get a section of the TCO material cut out and cleaned using standard cleaning methods to eliminate any possible contamination using distilled water and compressed air. Immediately after the sample was cleaned, it was placed on a holding stab before it was introduced into the scanning probe microscopic device. The AZO thin films used in this research were laboratory or home-made.

3. METHODS In this work, a single atomic force microscope (AFM), a Bruker Dimension ICON offering operation modes (with an amplifier current range of 1 pA to 1 μA), was used. The current through the probe was not measured, thus, only topography imaging was performed. All measurements were made under ambient conditions of 20 °C temperature and 50 % to 60% humidity. The addition of super-sharp silicon probes of 2 nm nominal radius can help get higher-resolution topography images. The aluminium doped zinc oxide (AZO) conductive thin films analysed in this study were: AZO:Al/Car, AZO:Al/N2 AZO:Al/Ag, and AZO:Al/Ar. These AZO were all grown by the same technique, which is chemical vapour deposition. Table 1 above shows that each of the thin films at baseline demonstrated a high quality of zinc oxide deposited on a glass substrate and nanostructure substrate (AZO_Al/Ar). However, after 16 months of storage in the laboratory, these TCO films started to degrade; the defects were studied by means of structural and morphological analysis. Each film had a thickness of 300 um to start with, so the study was to investigate why the ZnO layer deposited thickness changes with time as a result of oxidation [27]. Using a scanning probe microscope (SPM) in non-contact mode, the morphological degradations were investigated in a way that preserved the structural properties of the AZO. In all measurements, a conductive Pt-coated tip with a resonant frequency of 70 kHz spring constant 2N/m was used. This technique was used to acquire topography of the sample surface.

4. RESULTS Figures 1 to 4 show the SPM surface images of different AZO thin films deposited on flat glass substrates and a nanostructure substrate in the case of AZO-Al/Ar. There are various degrees of variation in the homogeneity of the thin films surface morphology. The structure of some of the AZO samples are seen to be well distributed in some areas while others are not, which can be observed in their surface roughness. The morphology of some of the samples exhibit small granular structures, with height and width changes to some extent. Their surface morphologies are characterised by different degrees of granular structure. As the scanning tip approaches area of the thin films sample, the surface structure’s lateral size and height disappears, and the surface becomes rougher with large tip circumference. Figures 1 to 4 show SPM images of the different AZO thin films used in this work.

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Figure 1: a–b 2-D Top View and c-d 3-D view SPM Image of the AZO:Al/Car thin Film grown on Flat Glass Substrates

Figure 2: a–d 2-D Top View and 3-D view SPM image of the AZO:Al/ N2 Thin Film Grown on Flat Glass Substrates

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Figure 3: a–d 2-D top view and 3-D view SPM image of the AZO:Al/Ag thin film grown on Flat Glass Substrates

Figure 4: a–d 2-D top view and 3-D view SPM image of the AZO:Al/Ar Thin Film Grown on Flat Glass Substrates b) the Back side.

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An SPM image of the films is shown in Figures 1 to 4. The uniformity of each of AZO:Al/Car, AZO:Al/N2, AZO:Al/Ag, and AZO:Al/Ar thin films surfaces differ in terms of structures and voids. The 3-D image in Figure 1(b) shows that the AZO:Al/Car film has the lowest roughness while the highest roughness is AZO:Al/Ar. The largest feature size is about 10 nm in height for AZO-Al/Car and 50 nm in height for AZO-Al/N2 Analysing the structural degradation of a film gives a direct clue to its conductivity because surface structure with uniform coverage exhibits better conductivity than nonuniform coverage spin/sputter-coated films [28]. Judging from its structural degradation, it is fair to deduce that the AZOAl/Car film will perform better than the other due to its smooth surface 5.1 Morphology and Roughness The surface morphological features of AZO samples with various materials of equal percentage are presented in Figures 1 to 4. The doped zinc oxides exhibited different densely pack and well covered structure on the substrate with a porous appearance (cracks) at certain places in the samples. It is important to note that this crack concentration varied from one thin film to another due to different material composition. The doping of AZO with different materials was experimented with to ascertain which dopant would be better as a thin film layer. However, we saw that not only efficiency needed to be investigated, but also stability. The SPM micrographs revealed drastic changes in the morphology of the samples from one position to another and from one sample to the other. There are two possible explanations for the morphological change, the first being linked to defect, and the second being due to differences in the doping material. As such, we can analyse the defect susceptibility of each material and compare their stability. Arguably, all the doped material exhibited uniform and compact surface grains (shape) at the initial stage. This compact feature gradually eroded as degradation time increased. It is obvious that AZO thin film doped with Car exhibited densely spherical slightly big size grains with uniform distribution for small area while the uniformity became more distorted as the area of consideration increased, as seen in Figure 1(b) and (c).

5.2. Surface Topography Scanning probe microscope (SPM) images are vital in characterising the morphology and roughness of thin film surfaces through root means square (RMS). Also possible is grain size evaluation. Figure 3(a) and (b) show the SPM images of AZO thin film doped with N2 at 5 µm and 10 µm. The images confirmed changes in the surface morphological features as the scanned area increased. The roughness as indicated by the increase in RMS as the scan size increased was due to nonuniform degradation. The RMS values for AZO_Al/Car were 10.21 nm, 11.69 nm and 14.21 nm for size 1 µm, 5 µm and 10 µm respectively. The 5 µm image demonstrated weak granular morphology of size range of 60 nm to 120 nm (Figure 1), whereas for a scan area of 1 um the grain size changed to a range of 30 nm 79 nm (Figure 1). Figure 2 (a) and (b) show the SPM images of AZO thin film doped with N2. From these images it is clear that the thin film exhibits surface morphological features of a large base, far more than what was observed in AZO_Al/Car (Figure 1). The grain RMS value for size 1 µm, 5 µm, and 10 µm are 1.95 nm, 13.39 nm and 11. 88 nm respectively. From its 5 µm scan image the granular morphology has a size range of 69 nm to 110 nm (Figure 2(a)). In the case of AZO thin film doped with N2 shown in Figure 3 (a) and (b), the small granular surface morphological features exhibit rough surface with an amorphous nature.

The trough and crest of the grain are small compared to AZO_Al/Car and AZO_Al/N2. Its 5 µm scanned image gives a grain size in the range of 25 nm to 40 nm, while the RMS values range from 4.47 nm, 10.81 nm and 14.04 nm for 1 µm and 5 µm and 10 µm scanned areas. Finally the SPM images of Al doped AZO in Ar show surface morphological features that are densely compacted with uniform structure except place where deep crash is observed. Its RMSs are 4.09 nm, 8.17 Impact Factor (JCC): 9.6246

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nm and 24.10 nm which are relatively high value when compared to AZO_Al/N2 and AZO_Al/Ag due to crashes observed in Figure 4 (b). The grain granular morphology has a size range of 25 nm to 40 nm (Figure 4 (a-b)). Table 2 shows the RMS and Ra (roughness) of the AZO doped thin films. The RMS was calculated from the SPM images presented in Figures 1 to 4.

Table 2: The RMS and Ra (roughness) of the AZO Doped Thin Films AZO_Al/Car AZO_Al/N2 AZO_Al/Ag AZO_Al/Ar size Rsm Ra Rsm Ra Rsm Ra Rsm Ra 1 µm 1.843 4.472 2.091 4.091 5.674 10.212 1.886 1.947 5 µm 10.809 11.155 8.167 61.998 11.691 3.522 13.397 2.098 10 µm 72.012 14.212 5.405 11.878 10.084 14.039 26.401 24.101

The illustrations presented in Figures 1 to 6 provide an avenue for correlation between defect concentration and surface morphology through grain sizes and roughness. The fact that the doping materials were different provided a platform for defect concentration analysis. Another recognition is that each different doping material/substance had significant long term effects on grain size and surface roughness of the conductive zinc oxide film. The doped conductive zinc oxide films showed variation in the homogeneous distribution of the crystal grain which are polycrystalline in nature except in the case of AZO_Al/Ar which shows some amorphous characteristics. The 3D image of the TCO film shows the extent of each columnar grain that exists alone its axis, a fact that corroborates Schuler et al. [29] findings. As we move from AZO_Al/Car to AZO_Al/Ar there appears to be a decrease in the grain columnar structures which is a sign of reduction in the preferred or expected axis growth [1, 30].

5.3. Stability and Adaptability Figure 5 shows the SPM line patterns of all the AZO_Al used in this study. Each peak corresponds to the trough and the lowest part of the grain valley. The grain texture of each thin film under perfect conditions is expected to have a certain uniformity of spacing from one another. The huge structural deviation in the pattern is due to degradation and it is evident that these films have degraded differently from one other.

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Figure 5: Line Pattern Analysis of: AZO_Al/Car, AZO_Al/Ar, AZO_Al/N2 and AZO_Al/Ag Thin Films

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The AZO film analysed via morphology shown in Figure 3(d) clearly indicates that the roughness of the film has degraded from what can be deduced from Table 1. The film’s roughness is affected by degradation, and the effect of degradation varies from one film to the other. The AZO_Al/Car was the most degraded followed by AZO-Al/N2, while AZO-Al/Ar showed less degradation as can be seen from Figure 3. Doping of AZO is definitely a way to increase film stability hence resulting in reduced degradation. These results show that even though all the films were susceptible to degradation effects after a long time, in AZO_Al/Car the consecutive grain happens to collapse into a bigger grain without a crack. While AZO_Al/Ag and AZO_Al/Ar showed signs of cracks, the cracks in AZO_Al/Ar were less severe compared to AZO-Al/Ag as shown in the SPM images in Figure 3(c) and (d) [31].

From these results, one may conclude that even though AZO_Al/Ag and AZO_AL/Ar show low degradation signs, the presence of Ag and Ar are less advantageous to film adhesion. This assumption was previously predicted by Ho [32] when he analysed the composition and morphology of AZO films. In order to further understand the roughness of the samples after degradation, their centralised values and resultant probability models over their hypothetical centre was analysed through medium and mean. Figure 6 presents the median and mean value of each film; for better comparison, the same scale was used in each case. Figure 6(a) shows the medium and mean of 5 µm of AZO_Al/Car, AZO_Al/N2, AZO_Al/Ar and AZO_Al/Ag transparent conductive films. The 10 µm scanned area was only of AZO_Al/N2, AZO_Al/Ar and AZO_Al/Ag. Distance (mm) Roughness (nm)

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Figure 6: AZO_Al/X Thin Film as a Function of Degradation (X: Car/Ar/N2/Ag)

As seen in Figure 6(a), AZO_Al/N2 shows slightly higher roughness value when compared to AZO_Al/Car. This can be explained from the fact that AZO_Al/Car has a larger cluster of grain and less grain structure which tends to reduce its roughness. The results show that AZO_Al/Car has the highest non-uniform roughness judging from the high discrepancy between its roughness mean and medium. For AZO_Al/Car the mean is above the medium while for the others, the mean and the medium coincide at the middle point. To further analyse the finding, the scanned area was increased to 10 µm, as shown in fig 6(b). Note that only AZO_Al/N2, AZO_Al/Ar and AZO_Al/Ag are presented in Figure 6 (b). Here AZO_Al/N2 shows the highest value while the least is AZO_Al/Ar; this result is similar to what was found in Figure 4(a). While the medium and mean value of AZO_Al/Ar and AZO_Al/Ag appear to be at the same point, that of AZO_Al/N2 is different from the mean showing high deviation. It appears to be the same with the high roughness value which, when highly degraded, have different medium and mean points. This observation can be linked to the degradation

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experienced by each film.

In summary, the least degraded film is AZO_Al/Ar followed by AZO_Al/Ag though adhesive strength degrades more than that of AZO_Al/Car and AZO_Al/N2. Figure 4(a) and (b) show the SPM roughness data analysis, which is a function of degradation. The roughness value is in agreement with root mean square roughness values presented in Table 1.

The lowest roughness AZO_Al/Ag corresponds to the least root mean square observed in Table 1. These results can be compared to what was reported previously in DC magnetron sputtering grown doped zinc oxide [24, 30]. The lower the roughness of AZO films after degradation, the more useful and advantageous for improving the stability of microelectronic devices.

6. CONCLUSIONS In this study, scanning probe microscopic measurements of aluminium doped zinc oxide (AZO) thin films were carried out with different doping materials on flat glass and nanostructure substrates. The mean and medium analysis provides degradation analysis on different TCO films compared to just the conventional root mean square roughness values. The grain and microstrain observed vary with AZO/Car having the highest and AZO/Ar the lowest value. Cracks and microstrain dislocation were more common in AZO/N2, while AZO/Ar had less roughness linked to low degradation. This make AZO/N2, and AZO/Ar better candidates for electronic devices. The various thin films have film thickness and interface roughness of 3.35 nm and 3.29 nm each, however, after degradation, the roughness of the thin films increased to 11.69 nm, 3.52 nm, 2.09 nm and 8.17 nm for AZO/Car, AZO/N2, AZO/Ag and AZO/Ar respectively. The results revealed that roughness along with the average grain size of all the samples (films) rises with degradation and the use of various doping materials offered extra stability to the samples. Some material showed better surface roughness and reduced degradation.

Funding This research received no external funding.

ACKNOWLEDGMENTS The authors would like to express their gratitude to the following organizations: UFH, UNISA and UKZN. Declaration of conflicting interests The authors declare no potential conflicts of interest in respect of this research, authorship and/or publication of this article.

Data availability The data used to support the findings of this study are included within the article. However, in support of open access research, all underlying data can be accessed upon request via email to the corresponding author.

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