Effects of NIR annealing on the characteristics of al-doped ZnO thin films prepared by RF sputtering
Nanoscale Research Letters
Effects of NIR annealing on the characteristics of al-doped ZnO thin films prepared by RF sputtering
Min-Chul Jun 0
Jung-Hyuk Koh 0
0 Department of Electronic Materials Engineering, Kwangwoon University , Seoul 139-701 , South Korea
Aluminum-doped zinc oxide (AZO) thin films have been deposited on glass substrates by employing radio frequency (RF) sputtering method for transparent conducting oxide applications. For the RF sputtering process, a ZnO:Al2O3 (2 wt.%) target was employed. In this paper, the effects of near infrared ray (NIR) annealing technique on the structural, optical, and electrical properties of the AZO thin films have been researched. Experimental results showed that NIR annealing affected the microstructure, electrical resistance, and optical transmittance of the AZO thin films. X-ray diffraction analysis revealed that all films have a hexagonal wurtzite crystal structure with the preferentially c-axis oriented normal to the substrate surface. Optical transmittance spectra of the AZO thin films exhibited transmittance higher than about 80% within the visible wavelength region, and the optical direct bandgap (Eg) of the AZO films was increased with increasing the NIR energy efficiency.
Al-doped ZnO; Transparent conducting oxide; Thin films; NIR; RF sputtering
Background
Transparent conducting oxide (TCO) has been widely
applied for various optoelectronic devices, such as
flatpanel and liquid crystal displays [
1
], organic
lightemitting diodes [
2
], and thin-film solar cells [
3
]. Most of
the TCO materials are based on tin oxide (SnO2) and
indium tin oxide (In2O3) with group III elements, such
as boron, gallium, indium, or aluminum-doped zinc oxide
(AZO) [
2
]. In particular, Al-doped ZnO-based TCO have
been extensively researched because of its low cost and
useful properties, such as non-toxicity, excellent electrical
and optical properties, and high thermal and chemical
stability [
4,5
].
To fabricate the AZO thin films, several deposition
techniques, such as chemical vapor deposition, electron
beam evaporation, thermal plasma, pulsed laser
deposition, metal organic chemical vapor deposition, sol–gel
method, and DC, radio frequency (RF) sputtering have
been developed and studied [
6-14
]. RF sputtering
technique is one of the most widely used because of its
reproducibility, efficiency, and reliability. There have
been lots of reports concerning substrate temperature,
oxygen pressure, target to substrate distance, and
postdeposition annealing on the AZO thin-film quality using
RF sputtering methods [
15-18
]. Because one of the main
factors affecting the deposited film is optical energy, we
have employed advanced optical processing by near
infrared ray (NIR) annealing method to get better quality
AZO thin films. NIR curing method has some
advantages. Heat transfer coefficients are high; the process
time is short; and the cost of energy is low. Since air is
primarily a mixture of oxygen and nitrogen, neither of
which absorbs NIR radiation, energy is transferred from
the heating source to the sample without heating the
surrounding air [19].
In this paper, the NIR annealing effects on the
microstructure and the electrical and optical properties of the
AZO thin films are reported.
Experimental details
In this experiment, the AZO thin films were fabricated
by RF sputtering on glass substrates from a ZnO:Al2O3
(98:2) target with 99.999% purity and a 2 in. diameter.
The glass substrates (Corning 1737) were ultrasonically
cleaned using acetone, methanol, and deionized water,
and then dried by blowing nitrogen over them before
being introduced into the sputtering chamber. The
chamber was initially evacuated to a base pressure under
1 × 10−6 Torr, and the deposition was carried out at a
working pressure of 2 × 10−3 Torr. The argon gas was
used as the plasma source, and the gas flow rate was
controlled at 40 sccm, using the mass flow controller.
The RF power was fixed at 100 W. Prior to the film
deposition, pre-sputtering was performed for 10 min to
remove any contamination on the target surface. The
films were annealed by RTA at 450°C for 10 min and
then annealed by a NIR at 20%, 40%, 60%, and 80%
energy efficiency for 10 min to investigate the NIR effect.
The thicknesses of the deposited films were investigated
by using α-step, and the thickness of the final film was
approximately 150 nm. The crystalline structures of the
specimens were analyzed by X-ray diffraction (XRD) patterns.
XRD 2θ scans were carried out by employing an X-ray
diffractometer Rigaku with a Cu-Kα source (λ = 0.154056 nm).
The surface microstructure was observed by a scanning
electron microscope (SEM). The electrical properties
were measured by a four-point probe method, and optical
transmittance measurements were carried out using a UV–
VIS spectrometer. Photoluminescence (PL) spectra were
recorded using a PL spectrometer excited with a 325 nm
He-Cd laser at room temperature. (...truncated)