Phase and surface structures-optical transmittance correlation of sputtered Al-doped ZnO film on glass substrate

The electrical and optical properties of sputtered Al-doped ZnO films prepared on a glass substrate with different thicknesses (97, 127, 161, 211, and 276 nm) were systematically investigated. The 97 nm film showed only the main peak of the AZO (002) phase, whereas the rest films exhibited AZO (002) and (004) phases, and the peak intensities were obviously increased with increasing thickness. The films displayed a granular grain surface and columnar-like structure with different sizes and distributions depending on film thickness. Surface roughness was increased, whereas the electrical resistance was decreased with increasing film thickness. The smallest crystallite size of about 26 nm with the highest resistivity and lowest carrier concentration was observed on a 127 nm film, whereas the crystallite size of about 29 nm was observed on the 97, 161, 211, and 276 nm films. All AZO films exhibited good electrical properties and transparency with an averaged optical transmittance higher than 80% in the visible wavelength. The 162 nm film showed the highest transmittance of 86% in the wavelength range of 350–900 nm and a wide energy band gap of 3.52 eV because of the highest mobility and crystallite size with a columnar structure and random size distribution. The figure of merit (FOM) was strongly related to the optical band gap and tended to increase with increasing thickness. The results are attributed that the optical energy band gap was altered by film thickness by improving phase structure and surface morphology.


Introduction
Aluminum-doped zinc oxide (AZO) thin films, an n-type semiconductor with a hexagonal wurtzite crystal structure, have attracted noteworthy interest from the researcher because of their distinctive optical and electrical properties with a wide range of applications and good stability.It is a non-toxic, lower-cost, and sustainable alternative to using as transparent conductive oxide (TCO) or an electron-transport layer (ETL) in applications for optoelectronic devices [1], and a coating layer for UV sensors [2], gas sensors [3], and heaters [4].Various methods such as RF-sputtering [1][2][3][4], sol-gel dip coating [5,6], pulse laser deposition [7,8], and atomic layer deposition [9] have been used to prepare the AZO thin film on both a glass and flexible substrates.Sputtering parameters such as power [10][11][12][13][14][15][16], pressure [11,12], and temperature [14] with substrate temperature [11,17] and substrate roughness [18] were important keys that have been intensively investigated to improve the optical and electrical properties of the AZO films.It has been reported that the working pressure, RF power, and substrate temperature affected the crystallinity and the preferred orientation of the crystallites [11].The increase in RF power from 60 W to 180 W led to the evolution of the plane texture of the zinc oxide phase and increased grain size from 14 nm to 23 nm, and affected the electrical resistivity of AZO film [14].Moreover, the substrate surface roughness with the post-annealing mechanism modified the optical band gap of RFsputtered AZO thin film and improved the performance of a thin film solar cell [18].
In this work, the effect of thickness on the phase structure, surface morphology, electrical properties and optical properties of sputtered AZO films was intensively studied to explore the appropriate film properties and thickness in an application for a good transparent conductive.

Experiment
Al-doped Zinc Oxide (AZO) films with different sputtering times were deposited on a glass substrate by Leybold-Heraeus, Univex 300 RF-sputtering machine under an Ar atmosphere of about 3.0 × 10 −2 mbar with the power of 200 W.The ZnO: Al ceramic target (99.99%purity with ZnO: 98 wt%, Al 2 O 3 : 2 wt%) with a diameter of 3 inches and a distance between the target to the substrate of 3.8 cm.The glass substrates were cleaned ultrasonically using acetone to remove the dust particles and then to dry before deposition.
The cross-section thickness of the deposited films was measured by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) using FEI, QUANTA 450.The crystal structure and the crystallinity of these films were analyzed by x-ray diffraction (XRD) spectroscopy using a Bruker with Cu Kα radiation (λ = 1.54060Å, 2θ = 10-80°).Surface morphology and roughness were investigated by atomic force microscopy (AFM) using ASYLUM RESEARCH, AR MFP-3D AFM (Bio).The optical spectra of the films were measured by UV-vis spectrophotometer in the spectral range 250-900 nm using Shimadzu UV-1800.The electrical properties were examined by the four-point and hall measurement.

Thickness and surface morphology
The thickness of the prepared films calculated from SEM images is 97, 127, 161, 211, and 276 nm.The sputtering rate was an exponential fit with an equation of y = 50.65e 0.026x and an R-square value of 0.9996.Where y is the film thickness (nm), and x is the sputtering time (min).An example of the cross-section images of AZO film on a glass substrate is shown in figure 1(a).It reveals a smooth interfacial area between AZO film and glass substrate.The irregularity is significantly observed at the film surface.
SEM images in figures 1(b)-(f) indicated the granular grain surface with different sizes, shapes, and distributions depending on the thickness.The 97 nm film showed a spherical grain shape with the narrowest size distribution.Additionally, the 127, 161, and 211 nm films exhibit continuous quasi-spherical grain shapes with different sizes depending on thickness.In contrast, the 276 nm film possessed an elongated grain shape with the largest grain size and broadest distribution.These results inferred a strong correlation between surface morphology and thickness of sputtered AZO film.
Figure 2 shows 2D and 3D AFM results of the AZO films over a scan area of 2 × 2 μm 2 .All AZO films display a fine columnar-like structure with size and distribution dependent on film thickness.The regular and stocky column size and distributions are observed on the 97 nm film giving rise to the lowest surface roughness.The 127, 161, and 211 nm films exhibit smaller and irregular column sizes and distributions, with column height and surface roughness increasing with thickness.In addition, the 127 nm film showed the smallest columnar structure with the narrowest size distribution and a moderate RMS roughness of 2.15 nm over a scan area of 10 × 10 μm 2 .Moderate surface roughness of 2.22 nm with columnar size and broad normal distributions were observed on 161 nm film.Moreover, the 276 nm film showed the sharpest and largest columnar size and height with the broadest size distribution resulting in a relatively high roughness of 3.30 nm.
AFM results indicate that the surface roughness of the films increased with increasing thickness, as shown in figure 3. It inferred that the 97 nm film has the homogenous surface with the lowest roughness and the 127, 161, and 211 nm films possess an irregular surface with moderated roughness.The 276 nm film shows the heterogenous surface with the sharp column with the lowest surface packing.

Structure and crystallite size
The XRD results in figure 4 confirmed that all sputtered AZO films on a glass substrate showed a hexagonal wurtzite structure of the AZO phase with the dominant (002) and minor (004) planes except on a 97 nm film, and the peak intensities were gradually increased with increasing thickness.However, with increasing thickness, the AZO (004) peak tendency shited from 72.187 to 72.432 and the AZO (002) peak exhibited a higher angle shift from 34.294°to 34.417°.This shift indicated substituting Al atoms in the ZnO lattice sites, decreasing lattice parameters [19], and generating compressive stress and lattice distortions within the film.It demonstrates that an appropriate thickness gives rise to the most substitution of Al atoms in the Zn site developing an excellent crystalline structure of the AZO film.An increment of the (002) intensity indicates the fast crystal structure growth on preferred (002) planes along the c-axis.The good c-axis preferred orientation with columnar morphology of AZO film glass substrate originated from intrinsic stress and thermal mismatch between film and substrate [20].
The crystallite size (D) of the AZO (002) peak was calculated from FWHM using Scherrer's formula [21] D K cos where λ is the x-ray wavelength, β is the full width at half maximum (FWHM), crystallite size in radians, K is a constant (0.9) related to crystallite shape and θ is the Bragg angle.
The crystallite size is plotted as a function of thickness, shown in figure 5.The crystallite sizes are between 25.9 and 29.3 nm.The 127 nm film has the smallest crystallite size of about 25.9 nm, whereas the 97 and 161 nm films show the largest crystallite size of about 29.3 nm.Additionally, the 211 and 276 nm films exhibit a moderate size of 28.8 nm.

Electrical properties
Electrical resistance with surface morphology of all AZO films is shown in figure 6.The resistance is rapidly decreased with increasing film thickness.The maximum and minimum resistances were observed on 97 and 276 nm films.These results indicate that AZO film resistance is reduced with increasing thickness and surface roughness.The smallest crystallite size might be due to the initial stage of the AZO (004) plane growth, resulting in higher strain and distortions in the AZO (002) plane.The largest crystallite size was ascribed to the rapid growth of the AZO (002) plane, reducing the strain, imperfection, and distortion of the lattice.The decrease in the resistance may be due to a lower imperfection and a higher substitution of Al atoms in the thicker films.In addition, this result is consistent with resistivity, mobility, and carrier concentration measured from hall measurement, as shown in figure 7, and also correlated to the crystal structure.The bigger crystallite size gives rise to a lower resistivity because of a lower scattering rate at the grain boundary, whereas the smaller one provides a higher resistivity due to a higher scattering rate.The results confirm that the film resistivity is inversely varied with the crystallite size because the small grain size offers a higher grain boundary resulting in more scattering at the edges.
Resistivity, mobility, and carrier concentrations of AZO films as a function of thickness are shown in figure 7. The lowest and highest mobility observed on 97 and 161 nm films are varied with thickness.Hall mobility is a  crucial factor strongly related to imperfection, dislocation, and defects.In addition, the carrier concentration increases with increasing thickness because of the higher substituted Al atoms in a thicker film.These results showed that resistivity is inversely proportional to carrier concentration and mobility.It informed that the highest substituted atoms and crystallite size with moderate resistivity and carrier concentration caused the most increased mobility of a 161 nm film.The sudden increment in electrical resistivity of 127 nm film was attributed to a decrement in crystallized size from 29.3 nm to 25.9 nm when the thickness was increased from 97 nm to 127 nm.An abrupt change in crystallized size can be described by the structural evaluation of sputtered films was changed from island nucleation mode to growth and impingement mode [22].It confirmed that the resistivity is inversely related to crystallite size, whereas the resistance is significantly associated with surface morphology and thickness.
The structural and hall measurement parameters are summarized in table 1.The results indicate that all film thickness is about a perfect number of crystallites, and the intensity ratio increased with increasing thickness.The 127 nm film shows the smallest crystallite size, intensity ratio, and carrier concentration, resulting in the highest resistivity.The 161 nm film has the largest crystallite size, and mobility gives rise to low resistivity.The crystallized size tends to decrease with increasing film thickness because a thicker film gives higher intrinsic stress during the sputter deposition process.

Optical properties
The optical properties of the AZO film are shown in figure 8(a).The transmittance of the AZO film fluctuated in the visible wavelength range, with the transmittance and the wavelength peak being thickness independent.The  wavelengths at the transmission peak of 97, 127, 161, 211, and 276 nm films are 394, 486, 602, 752, and 488 nm, respectively.The maximum transmitted wavelength increases with increasing thickness, except for the 276 nm film, which is reduced to 488 nm.The average optical transmission in the 350-900 nm wavelength range, 161 and 211 nm films have the highest transmission of about 86%, film 127 and 276 nm film transmits 85%, and 97 nm film transmits light as low as 83%.
The energy band gap was calculated using Tauc's plot equation [23], where α is the absorption coefficient, hv is photon energy, E g is the energy bandgap, and B is a constant.The absorption coefficient was calculated from the transmittance using the Beer-Lambert law [2], where d is the film thickness, and T is the transmittance.
Figure 8(b) shows that the energy band gap is in the range of 3.41 to 3.57 eV, and it tends to increase with increasing film thickness.The increase of the energy band gap compared to that of ZnO is the Burstein-Moss effect regarding an increase in mobility and carrier concentration.The increase of band gap with increasing film thickness infers higher Al atoms substitute in the Zn site.However, the optical and electrical properties are strongly affected by the intensive properties of the film.The 127 nm film possesses the lowest energy gap, carrier concentration, and smallest crystallite size, whereas the 276 nm film exhibits the highest energy gap of 3.57 eV and lowest resistivity.It can be attributed that the highest transmission with a bandgap of 3.52 eV of the 161 nm film is intentionally due to the highest mobility, the largest crystallite size of about 29 nm, an irregular columnar structure, and moderate roughness.
In order to categorize the transparent electrode properties of sputtered AZO film, the figure of merit (FOM) was defined as [24]:  Where T is an optical transmission, and R s is sheet resistance.FOM tends to increase with increasing thickness dues to the decrease in the sheet resistance and strongly relates to the energy band gap.It also implies that the sheet resistance strongly depends on the crystallinity and imperfection of the film and then modifies the band gap of the AZO film.
Transmittance, FOM, and energy band gap of sputtered AZO films with different thicknesses are shown in figure 9.It clearly shows that the energy gap and FOM are increased with film thickness.The electrical and optical parameters and FOM values are summarized in table 2.
These results imply that film thickness can be divided into 3 ranges: thickness less than 100 nm, 100-200 nm, and greater than 200 nm.In the initial stage, the film is less than 100 nm thick, and the Al atoms replacement is not yet complete, consisting of the AZO phase in the (002) plane and the amorphous phase of the segregated Al atoms in the grain boundary.In the middle stage, a thickness is between 100 nm and 200 nm, the largest number of Al atoms replacements at about 160 nm, inducing the AZO phase of the (002) and (004) planes with the highest crystallinity.In the final stage, Al Atoms substitution preferably occurs along the (002) direction than that of the (004) direction, possibly due to a large amount of Al to replace in the ZnO site.These results infer that crystallite structure, including substitution and preferred orientation of the sputtered AZO film that is thicker than 200 nm, was modified by anisotropic growth along the specific and surface nucleation resulting in surface and roughness and an elongated grain shape on the film surface.Additionally, the columnar structure with different sizes and distributions of the 127, 161, and 211 nm films strongly results in electrical properties and trapping visible wavelength range.The results attributed that the thickness not only manipulated the preferred structure plane and crystallite size but also modified surface morphology, affecting the electrical property and optical transmittance.
Additionally, table 3 provides the electrical and optical properties of the AZO film on different substrates and processes, a comparative tabulation of the thickness, deposition process, substrate electrical resistivity, carrier concentration, and mobility in comparison to previous reports.

Conclusions
Sputtered AZO film with thicknesses deposited on a glass substrate showed only the AZO (002) plane for a thickness lower than 100 nm and exhibited AZO (002) and (004) planes for a thickness higher than 100 nm.All  AZO films displayed granular grain surfaces and columnar structures with different sizes, shapes, and distributions.The electrical and optical properties strongly depended on phase structure and surface morphology.The highest energy gap of 3.57 eV and lowest resistivity were observed in the 276 nm film.The lowest energy bandgap, carrier concentration, and smallest crystallite size were found in the 127 nm film.The AZO films.The optical transmittance of all AZO films was higher than 80%, and the 161 nm film exhibited the highest transmittance of 86% and 92% in the visible wavelength range and at 602 nm, respectively.The results indicated that The FOM of sputtered AZO film is closely related to the band gap and thickness dependent.In this study, all sputtered AZO films with a thickness between 100 and 300 nm on a glass substrate are good transparent conductive oxides and transparent electrodes in solar cells and optoelectronics devices.

Figure 3 .
Figure 3. Surface roughness of the sputtered AZO films on a glass substrate with different thicknesses.

Figure 4 .
Figure 4. XRD patterns of sputtered AZO films deposited on a glass substrate with different thicknesses.

Figure 5 .
Figure 5. Crystallite size and FWHM of sputtered AZO (002) phase of the films with different thicknesses.

Figure 6 .
Figure 6.Electrical resistance and surface morphology of AZO films on a glass substrate with different thicknesses.

Figure 7 .
Figure 7. Resistivity, mobility, and carrier concentrations of sputtered AZO films on a glass substrate as a function of thicknesses.

FOM T R s 10 =Figure 8 .
Figure 8.(a) Transmittance and absorbance with (b) inset of the energy band gap of sputtered AZO films as a function of thicknesses.

Figure 9 .
Figure 9. Transmittance, FOM, and energy band gap of sputtered AZO films on a glass substrate as a function of thicknesses.

Table 1 .
Crystallize size, intensity ratio, roughness, resistivity, carrier concentration, Hall mobility, and resistance of AZO films at different thicknesses.

Table 2 .
Crystallize size, wavelength peak, average transmittance, sheet resistance, a figure of merit, and energy bandgap of the sputtered AZO films at different thicknesses.