Reliability of sputter deposited aluminum-doped zinc oxide under harsh environmental conditions

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Solar Energy 89 (2013) 54–61 www.elsevier.com/locate/solener

Reliability of sputter deposited aluminum-doped zinc oxide under harsh environmental conditions Mohammad M. Hamasha a,b, Tara P. Dhakal a,⇑, Parag Vasekar a, Khalid Alzoubi a, Susan Lu b, Daniel Vanhart a, Charles R. Westgate a b

a Center for Autonomous Solar Power, State University of New York at Binghamton, Binghamton, NY 13902, USA Department of Systems Science and Industrial Engineering, State University of New York at Binghamton, Binghamton, NY 13902, USA

Received 19 July 2012; received in revised form 7 November 2012; accepted 6 December 2012 Available online 19 January 2013 Communicated by: Associate Editor Takhir Razykov

Abstract We report the reliability of aluminum-doped zinc oxide (AZO) thin films exposed to harsh environmental conditions of damp heat. Identical sets of the AZO films grown on float glass substrates were subjected to four different controlled environments of temperature and relative humidity. The selected high and low levels of the temperature were 100 °C and 20 °C respectively and the selected high and low relative humidity levels were 100% and 20% respectively. The total exposure time for this controlled environment was 120 h. Electrical resistance was measured every 24 h for each sample. Material composition, crystallinity, optical transmittance and surface morphology of the films were investigated after 120 h exposure time. A mixed, full-factorial design of experiment was used in this study, with the response being the electrical resistance. The factors were exposure time, temperature and humidity. The ANOVA results showed that all the considered factors were significant. Although the electrical resistance of the film exposed to high temperature and high relative humidity increased by 20%, the microstructure in the bulk of the film, crystallinity and optical properties of the films exposed to all combinations of temperature and humidity were unchanged. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Reliability; Aluminum-doped zinc oxide; Harsh environmental conditions

1. Introduction Zinc oxide (ZnO) is an inorganic material that is insoluble in water. The material can be found as a component in materials, such as glass, ceramic, plastics, cement, lubricants, paints, adhesives, ointments, pigments, sealants, and foods. Furthermore, it is abundant in the Earth’s crust as a zincite ore. ZnO is found in different technologies, such as optoelectronic devices (Vispute et al., 1998), piezoelectricity (Corso et al., 1994) photocatalysis (Dutta and Basak, 2009), gas sensing (Li et al., 2005), solar cells (Scheer et al., 2004; Hua et al., 2008) and many others. ⇑ Corresponding author. Tel.: +1 352 213 6445.

E-mail address: [email protected] (T.P. Dhakal). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.12.006

Although ZnO is a dielectric material, it can be doped with metallic atoms to produce conductive zinc oxide (Chopra et al., 1983). The conductive zinc oxide is transparent and belongs to the family of transparent conductive oxide (TCO) Dhakal et al., 2010. The most popular ZnO-based TCO are aluminum-doped zinc oxide (AZO) and galliumdoped zinc oxide (GZO). Moreover, doping improves the mechanical and optical properties of the ZnO (Liu et al., 2010). AZO is an environment-friendly TCO (Suzuki et al., 1996) with excellent electrical and optical properties suitable for different electronic applications, especially when transparency is required, such as photovoltaics (Tanaka et al., 2005; Dincalp et al., 2010) and touch screen (Fernandez and Naranjo, 2010) applications. The dominantly used

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TCOs that may be replaced with AZO are indium tin oxide (ITO) Yang et al., 2008 and fluorine-doped tin oxide (FTO) Yang and Forrest, 2006. Specifically, AZO is more cost effective as compared to ITO and has better optical characteristics as compared to FTO (Yang and Forrest, 2006). AZO has already found application as a TCO layer in solar cells, such as CdTe (Compaan et al., 2004) and CIGS (Vasekar et al., 2009). An area where AZO needs improvement as compared to ITO is its stability in moist environments. It is reported that for applications that involve reducing atmospheres and high temperatures, doped ZnO films are more stable than ITO films (Minami et al., 1989). In contrast, ITO is more stable for use in oxidizing atmospheres (e.g., air and moisture) and high temperatures (Sato et al., 1993). AZO films can be grown by several techniques, such as sol–gel (Musat et al., 2004), chemical spray (Suarez et al., 2002), pulsed laser deposition (Mass et al., 2003), electron beam evaporation (Ma et al., 1999), DC and RF magnetron sputtering (Jeong et al., 2003) and atomic layer deposition (George et al., 1996). The conductivity, transparency, surface morphology and the growth rate depend on the deposition technique and the parameters used. Although electrical and optical properties of AZO pertaining to its use in photovoltaic and other applications have been vigorously studied, an investigation of its reliability in harsh environmental conditions has not been adequately discussed. Tohsophon et al. (2006) studied the effect of damp heat and subsequent vacuum annealing on AZO films. They observed an increase in the film electrical resistivity after the damp heat. They observed a slight increase in the electrical resistivity in the large grain and thicker films as opposed to a larger increase in the electrical resistivity in the thinner film. Ando and Miyazaki (2001) studied the behavior of sputtered low-emissivity coatings with single silver layer structures of ZnO/Ag/ZnO/glass and AZO/Ag/AZO/glass under high humidity. They found that with a moderate aluminum percentage, the AZO/Ag/ AZO coatings expressed a better humidity resistance. Tai et al. (2003) studied the effect of humidity on the thin films with nano-structures, such as AZO, AZO/TiO2 and TiO2/ AZO. They observed an increase in the electrical resistivity when exposed to a humid environment. Statistical analysis plays a vital role to reach a conclusion of an experimental result (Mayyas et al., 2009; Montgomery, 2000). Design of experiments (DOE), as one of the most popular statistical tools, is a powerful method to study influence of different parameters of complex systems, such as electronic package and solar panel. It helps to study the effect of operational loads, mechanical load and environmental condition effects in the solar photovoltaic system, which is important to improve the product. Furthermore, DOE has been used in all fields of science and engineering (Dasgupta et al., 1998; Liu et al., 2004; Huang et al., 2003; Holland et al., 2008). In this research paper, we report the analysis of our data using DOE.

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2. Experimental details Commercially available AZO thin films were used for this study. The AZO films were coated on a float glass substrate using DC magnetron sputtering system in a large scale production setting using a 126 inch diameter rotating target in a reactive gas (Ar/O) atmosphere of 5 mTorr during the growth. The power applied to the target was in the range of 40–60 kW. The substrate was not heated during the growth but the temperature on the substrate was around 70–80 °C due to the plasma heating. Postheating in air using tempering furnace was done at 630 °C for 90 s. The thickness of the film used was about 600 nm. The environmental stability experiments were conducted by applying constant temperature and humidity on the samples over time. The films were exposed to four different environmental conditions. The four test conditions were: high temperature high humidity (HH); high temperature low humidity (HL); low temperature high humidity (LH); and low temperature low humidity (LL). For each environmental condition, four sets of the AZO samples (replicates) were used. The selected high and low temperatures were 100 °C and 20 °C respectively and the selected high and low relative humidity values were 100% and 20% respectively. The total exposure time was 120 h. The electrical resistivity for each sample was measured every 24 h to follow the progress of the electrical resistance degradation in response to the harsh environmental conditions. In order to reduce the measurement errors, the electrical resistance was measured at five different spots (duplicates) of each sample and the average was considered as a final reading. The environmental stability experiment was designed and conducted according to the DOE principles. The considered factorial design was a mixed, full factorial with three design factors: time of exposure; temperature and relative humidity; and a response of electrical resistance, which were measured in ohm. The levels of each factor were selected as shown in Table 1. Four different samples (replicates) were tested at each test condition to reduce the effect of the randomized error (increase the degree of freedom of error). According to the DOE principle, the actual electrical resistance of the sample can be represented by the following equation: Y ijkl ¼ l þ si þ bj þ ck þ ðsbÞij þ ðscÞik þðbcÞjk þ ðsbcÞijk þ eijkl i ¼ 1; 2; 3; 4; 5; 6 j ¼ 1; 2

ð1Þ

k ¼ 1; 2 l ¼ 1; 2; 3; 4 where yijkl is the actual value of resistance, l is the overall mean, si, bj, ck, are the main effects of 3 factors, e.g., time, temperature, and relative humidity. (sb)ij, (sc)ik and (bc)jk represent corresponding two factor interactions, (sbc)ijk represents three factor interactions between all design fac-

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Table 1 Factors and levels. Factor

Level Level 1

Level 2

Level 3

Level 4

Level 5

Level 6

Time (h) Temperature (°C) Relative humidity (%)

0 20 20

24 100 100

48

72

96

120

tors and eijkl is a random error component with a normal distribution with zero mean and constant variance. For more information, a detailed description of the design of experiments method is outlined in Montgomery and Runger (2003) and Montgomery et al. (2006). The resistances of the AZO films were measured at the interval of 24 h exposure to the different environmental conditions using standard four-probe technique. Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) were used to study the change in surface morphology and material composition respectively after 120 h of exposure. Film crystallinity was assessed by standard theta–2theta technique. The transmittance of the exposed films was also measured and the band gap thereafter was derived.

Fig. 1. Electrical resistances in ohm as a function of exposure time to a different combinations of temperature and relative humidity.

Table 2 ANOVA table (where DF, degree of freedom, SS: sum of square, MS: mean sum of square, F: F-value and P: P-value). Source

DF

SS

MS

F

P

T (°C) RH (%) Time (h) T (°C)  RH (%) T (°C)  Time (h) RH (%)  Time (h) T (°C)  RH (%)  Time (h) Error Total S = 0.0231660

1 1 5 1 5 5 5 72 95 R-Sq = 77.84%

0.0381535 0.0044349 0.0211526 0.0310734 0.0176163 0.0115580 0.0117356 0.0386396 0.1743639 R-Sq(adj) = 70.76%

0.0381535 0.0044349 0.0042305 0.0310734 0.0035233 0.0023116 0.0023471 0.0005367

71.09 8.26 7.88 57.90 6.57 4.31 4.37

0.000 0.005 0.000 0.000 0.000 0.002 0.002

Fig. 2. Main Effects plot for resistance.

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3. Results and discussion In this section, the effect of various environmental conditions on the electrical resistance, film surface morphology, material composition, film crystallinity and transmittance are discussed in detail. The science behind each effect is presented and discussed as well. 3.1. Electrical resistance The as obtained AZO film had resistance of 2.35 X, which corresponded to the resistivity value of 6.35  104 X cm. Fig. 1 shows the response of the AZO film’s electrical resistance to temperature and humidity exposure over time. Each data point is an average of five measurements (duplicates). A 95% confidence interval error bars were added at each data point to show the deviation of the measurements. The resistance increased gradually over time under high temperature and high humidity (HH) conditions, although the change was not very significant in terms of the resistance values. However, it was constant over time under the condition of high temperature and low humidity (HL), low temperature and high humidity (LH), and low temperature low high humidity (LL). Therefore, it can be suggested that combination of high temperature and high humidity must be avoided in the manufacturing and application of the AZO thin film. The effect of temperature and humidity will be explained in depth in the next section. The significance of temperature, relative humidity and exposure time and the interaction between them on the electrical resistance was analyzed using ANOVA as shown

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in Table 2. The factors and factors’ interactions are listed in the first column of the table followed by the degree of freedom in the second column. Similarly, sum of square, mean sum of square, F-value, and P-value are given in 3rd, 4th, 5th and the 6th column respectively. The significance of the factor or the interaction between factors was concluded based on the P-value, where it is considered significant if the P-value is less than 0.05 (the first type error probability a is set at 0.05 for the hypothesis test). All P-values less than 0.05 indicate that all factors or two and three factor interactions are significant in terms of affecting the electrical resistance. The data are a good fit to the introduced DOE model with an R2(adjustable) of 70.76%. Although the change in resistance was small to have any effect in the application, all the factors were found significant to affect the electrical resistance. It can be assumed from this observation that an exposure longer than 120 h may bring about the larger change in the electrical resistance. After 120 h of exposure in high temperature and high humidity, the resistance of the sample increased by 8% to 2.5 O, corresponding to the resistivity value of 6.75  104 O cm. After concluding that all factors were significant, the type of relationship between each factor and the electrical resistance was required. Therefore, main effect plot was constructed as shown in Fig. 2. The “Main Effects” plot can draw a conclusion about the type of relationship between the factors and response, such as increasing, decreasing and the concavity (in the case of three or more considered levels). It can be concluded from the main effect plot of Fig. 2 that the response increases with each one of the three considered factors.

Fig. 3. SEM images at 5 KX magnification of the films exposed to different environmental conditions.

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3.2. Film morphology As mentioned before, SEM images were obtained to investigate the change in microstructure and surface morphology. Figs. 3 and 4 show the microstructure of thin films that were exposed to different environmental conditions at 5 KX and 200 KX magnifications respectively. The microstructure taken from clean area of all the films (as shown in the 200 KX magnifications image in Fig. 4) did not show any change with the temperature and humidity. However, circular dry spots were observed all over the film surface after high humidity exposure as shown in Fig. 3 in the 5 KX magnifications images. Although the

spots seen on the film exposed to LH condition were relatively clean and undamaged, the ones on the film exposed to HH appeared damaged and contaminated. This was due the condensation and drying of the water on the thin film during the exposure. In the films exposed to HH, the spots were fewer in number, but the corrosion on the surface was severe, while in the film exposed to LH, the spots were cleaner and greater in number. The circular spots without any microstructural damage seen on the film exposed to LH is due to the slow and non-invasive drying of the water droplets that were sitting there for a long time because the temperature was low (20 °C). The high temperature at HH condition prevented the forming of large drop-

Fig. 4. SEM images at 200 KX magnification of the films exposed to different environmental conditions.

Fig. 5. An EDX-spectra of the films exposed to LL, LH, HL, HH conditions. Blue graph corresponds to the EDX spectrum taken on the spot seen on the HH film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M.M. Hamasha et al. / Solar Energy 89 (2013) 54–61

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lets, but corroded the film surface due to the combined effect of dampness and heat. EDX was performed on all tested films. The spectrum for all the films was collected at low magnifications to cover a larger area and then evaluate the composition. Fig. 5 shows a comparison between the tested films under the condition of HH, HL, LH and LL for each elemental peak. An additional spectrum taken from the spot area of HH film is also shown. These data suggest that the AZO composition does not change with the various exposure conditions as evidenced by no observable change in the film’s microstructure as verified by SEM images in Fig. 4. However, more carbon (C) and oxygen (O) and an additional contaminants like silicon (Si), were observed inside the spot area of the film exposed to HH condition. The higher intensity of oxygen could be due to the additional oxygen associated with bonding with carbon or chemisorbed species (Dhakal et al., 2012). The silicon contaminant was from the measurement chamber. Surface morphology was also studied by atomic force microscopy (AFM). The sample exposed to low temperature and low humidity (LL) and the sample exposed to the high temperature and high humidity (HH) were imaged by AFM. For HH-film, two areas were used. The area which looked relatively clean and the area inside a spot as shown in Fig. 3 were used. Fig. 6 shows 3D images of LL, HH (clean area) and HH (inside the spot) films. As shown in Fig. 6a and b, the surface topography is similar between the LL film and the clean area of the HH film. However, the surface topography of Fig. 6c shows that the features seen on the clean area of the HH film (Fig. 6b) have melted away in the area inside the spot (Fig. 6c). Additional detail on the growth morphology was obtained from the roughness parameters derived from the AFM images. The root mean squared (rms) roughness, which is a mathematical description of the average deviaqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn ffi tion from a mean surface value (Rq ¼ 1n i¼1 z2i ; zi = the vertical distance from the mean line to the ith data point), and maximum peak to valley height (Rmax) were derived from the 1 lm  1 lm AFM images shown in Fig. 6. As shown in Table 3, the Rq and Rmax increase for HH-film compared to the LL film. The roughness gets even larger in the spot area compared to the clean area of the HH-film and the LL film. The slight increase in resistance of HHfilm could be due to this roughness increase.

Fig. 6. AFM images of the surface of the film exposed to (a) LL, (b) HH (clean area), and (c) HH (inside the spot).

3.3. Film crystallinity (XRD)

Table 3 Surface roughness of the AZO films exposed to LL and HH conditions.

The film crystallinity and orientation were assessed with a h2h X-ray diffraction (XRD) in grazing angle incidence geometry. The angle of incidence used was 0.6°, which was an optimized value to avoid reflection and the substrate peaks. The XRD spectra of the film showed all the characteristics of the ZnO hexagonal lattice with space group P63mc (186) with some traces of unidentified impurity peaks. The film texture was predominantly (0 0 2) oriented

Roughness type

LL (nm)

HH (clean area) (nm)

HH (inside the spot) (nm)

Rq Rmax

4.74 34.0

6.6 52.9

14.1 127

(Fig. 7), although all characteristic ZnO lattice peaks are observed. XRD scan was performed in LL and HH-films.

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M.M. Hamasha et al. / Solar Energy 89 (2013) 54–61 n

ðahtÞ ¼ Aðhm  Eg Þ

1100

(103) 25

30

35

40

45

(110)

(102)

(101)

(100)

500 200

(004)

(103) (110)

(102)

AZO-HH

50

55

2θ

60

(200) (112) (201)

800

Because T, R and d are known; the absorption coefficient can be calculated from Eq. (2). Thus using n = 2 for direct band gap transition in Eq. (3), band gap was calculated from the usual plot of (ahm)2 as a function of photo energy (hm) and extrapolation of the graph linearly to zero. Fig. 8 inset shows a plot of (ahm)2 vs. hm and a linear extrapolation showing the band gap. The band gap of the LL-film was 3.47 eV. This value was essentially the same for the films exposed to LH, HL and HH conditions. This shows that the crystallinity and microstructure of the bulk of the film are not affected by the exposure to the moisture.

65

70

(004)

1100

(101)

(100)

200

(002)

Intensity (a.u.)

500

(200) (112) (201)

(002)

800

75

4. Conclusion

o

Fig. 7. XRD scan of the films exposed to LL and HH conditions.

100

Float glass AZO-LL AZO-HH

60 3

AZO-LL

2

-2

(αhν) (10 eV cm )

2

10 2

Normalized Trans. (T) (%)

80

40

20

1

Eg= 3.47 eV

0 3.0

3.3

350

450

550

3.6

hν (eV)

0 250

ð3Þ

AZO-LL

650

750

850

Wavelength (nm)

AZO thin films deposited on glass by sputtering method were exposed to different environmental conditions of temperature and humidity. The results showed that the condition of high temperature and high humidly led to an increase in the electrical resistivity, although slightly, due to the residual spots created on the surface of the film. However, the microstructures of the bulk of the films were unchanged as observed by SEM/AFM and XRD measurements. The ANOVA showed that temperature, humidity, exposure times and the interaction between them are all significant in terms of affecting the electrical resistance. The optical property of the films also did not change. A nominal decrease in the transmittance of the HH-film in visible regime was observed, which is attributed to the absorption at the rough residual spots on the surface. However, the band gap of the films exposed to all the conditions was same.

Fig. 8. Transmittance of AZO films after the exposure to a different environmental condition.

Acknowledgment

No appreciable change in the crystallinity was observed except a nominal decrease of peak intensities for HH film. So, the increase in resistance observed in HH-film was due to the roughness and changes in the morphology at the surface only as seen in the SEM and AFM images.

This work was supported in part by Defense Advanced Research Projects Agency (DARPA) award number HR0011101002.

3.4. Optical properties of the films Fig. 8 shows the normalized transmittance of the AZO films exposed to HH, and LL conditions. For comparison, the transmittance of the substrate (Float glass) is added to the figure as well. There was no significant difference in transmittance between the HH and LL film except that the HH film had slightly lower transmission (2% less as compared to LL) in the visible, which is due to the surface roughness resulting from deposits on the HH film. To calculate the band gap, the following equations were used (Jiang et al., 2011), where A is a constant, d is the thickness of the film and hm is the photon energy. T ¼ ð1  RÞ expðadÞ

ð2Þ

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