Structural and optical properties of a radio frequency magnetron-sputtered ZnO thin film with different growth angles
© Ko et al; licensee Springer. 2012
Received: 19 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
This study introduces optical properties of a columnar structured zinc oxide [ZnO] antireflection coating for solar cells. We obtained ZnO films of columnar structure on glass substrates using a specially designed radio frequency magnetron sputtering system with different growth angles. Field-emission scanning electron microscopy was utilized to check the growth angles of the ZnO films which were controlled at 0°, 15°, and 30°. The film thickness was fixed at 100 nm to get a constant experiment condition. Grain sizes of the ZnO films were measured by X-ray diffraction. A UV-visible spectrometer was used to measure the transmittance and reflectance of the ZnO film columnar structures as a function of the growth angles.
To achieve a high efficient solar cell, one of the most important processes is antireflection coating [ARC] which also has a function of passivation . ARCs generally reduce the reflection of sunlight and increase the intensity of radiation on the inside of solar cells. With the antireflection layer, Choi et al.  demonstrated that solar cell efficiency can be increased by around 10%.
In general, the refractive index of a thin film is variable according to the kind of material and thickness of the films. It is addressed that a medium refractive index material between air (n = 1) and Si (n ≈ 3.4) is optimal for the ARC . However, with glass-based solar cells, such as dye-sensitized and thin film solar cells, it is hard to get a good antireflection effect due to a low refractive index of the glass substrate (n ≈ 1.7). Therefore, with the glass base, a structural modification of the ARC is a better approach than the refraction effect scheme.
ZnO thin films are used in various applications due to their high optical transmittance in the visible light region . ZnO, one of the most important binary II-VI semiconductor compounds, has a hexagonal structure and a natural n-type electrical conductivity . Moreover, ZnO thin films doped with Al, Ga, or In have low electrical resistivity and high optical transmittance due to their high carrier concentrations above 1020 cm-3 and wide optical bandgap energy above 3.3 eV. Also, it has merits on having a low material cost, on being nontoxic, and on having a better stability under hydrogen plasma compared with ITO .
In this paper, we introduce the optical properties of columnar structured ZnO films formed with several different growth angles. The films were deposited with radio frequency [RF] magnetron sputtering. During the sputtering, the angle between the sample and the target (ZnO) is changed to get several different growth angles. Field-emission scanning electron microscopy [FE-SEM] was applied to check the growth angles of ZnO films, controlled at 0°, 15°, and 30°, and to measure the thickness of the film. The film thickness was fixed at 100 nm to get the same mechanical condition of the columnar structured thin films. The grain sizes of the ZnO films were obtained by X-ray diffraction [XRD]. A UV-visible [UV-vis] spectrometer was used to measure the transmittance and reflectance of the columnar structured ZnO films, as a function of the growth angles.
To get good quality samples, the glass substrates were cleaned in trichloroethylene, acetone, methanol, and distilled water for 10 min, respectively. The sputtering chamber was vacuumed to a base pressure of 1 × 10-5 Torr. A pre-sputtering treatment was performed to clean the target surface for 10 min using argon plasma. A distance between the target center and the sample substrate was kept at 9.5 cm, and we manually tilted the sample substrate with angle measurement. The thickness and cross-sectional images of the films were measured by FE-SEM (Hitachi, S-4800, Hitachi High-Tech, Minato-ku, Japan); the grain sizes of the films were measured using XRD (Max 2500H, Rigaku Corporation, Tokyo, Japan), and the optical properties were observed in a UV-vis spectrometer (S-3100, Scinco, Gangnam-gu, Seoul, South Korea).
Results and discussion
We investigated the optical properties of antireflection coating on columnar structured ZnO films. The ZnO films were deposited on glass substrates inside a specially designed RF magnetron sputtering system. We studied the growth angle effect of the films for optical properties. The thickness of the ZnO thin films was checked by FE-SEM and was fixed at 100 nm. Three growth angles (0°, 15°, and 30°) of the columnar ZnO films were carefully selected. The intensities of the main peaks were different according to the growth angles. The highest intensity was obtained at the 0° angled columnar structured ZnO film. The 15° angled columnar structured film had the largest grain size of 59.01 nm, and the 0° angled columnar structured film had the lowest grain size of 25.95 nm. These results showed that intensity and grain sizes varied according to the growth angles. Transmittance of the ZnO thin films was changed according to the wavelength of incidence rays and the growth angle. The lowest average reflectance at 550 nm was measured with the 15° angled columnar thin film with a value of 8.67%. The best optical properties of the columnar structured ZnO films were obtained from the 15° angled growth columnar thin film.
This work was supported by the 2011 Hanbat National University academic research funding.
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