Effects of fabrication conditions and post-annealing on the characteristics of thin film
transistors with Zr-Al-Zn-O channel layers
Hyun-Sik Jun, Sang-Hyuk Lee, Ju-Hee Park, Won Kim, and Jin-Seok Park
Department of Electronic Systems Engineering, Hanyang University at ERICA Campus
1271, Sa 3-dong, Sangnok-gu, Ansan, Gyeonggi-do 426-791, Republic of Korea
E-mail:
Thin film transistors (TFTs) are important electronic devices that are used in a wide range
of applications in the field of electronics such as flat panel displays (FPDs), laser printers,
and image sensors of scanners. So far, hydrogenated amorphous silicon (a-Si:H) has been
used in most conventional TFTs as a channel. However, the device performance of the a-Si:H
TFT is sometimes not suitable for transparent and flexible displays due to its relatively low
field-effect mobility (< 1 cm
2
/V·s), light sensitivity, nonflexibility and opaqueness. Recently,
amorphous oxide semiconductors (AOSs) as the channel layer of TFTs have attracted much
attention as alternatives for a-Si:H due to their superior electrical properties that include a
wide band gap, high field-effect mobility, and high uniformity over large areas. Particularly,
indium (In) doped AOSs such as indium-gallium-zin-oxide (IGZO) and indium-zin-oxide
(IZO) were used as the active layer of TFTs with excellent field-effect mobility (> 10
cm
2
/V·s), turn-on voltage (~ 0 V), on/off current ratio (> 10
7
), and subthreshold swing (<
0.20 V/dec). However, the IGZO-based TFTs still suffer from instability due to a weak
binding force between gallium (Ga) and oxygen (O). In principle, to improve the bias-stress
stability of oxide TFTs, defect states such as oxygen vacancies must be reduced in the active
layers. Recently, a number of reports have been published about materials with low standard
electrode potential (SEP), such as yttrium (Y), hafnium (Hf), and zirconium (Zr). These
elements have strong bonds with oxygen and are believed to efficiently suppress oxygen-
vacancy formation in the oxide thin films. Particularly, Zr with low SEP (~ 1.45 eV) plays a
role in decreasing the carrier concentration of thin films through the suppression of oxygen
vacancy generation. The Zr doped oxide semiconductor, however, suffers from low channel
mobility due to the reduction in the carrier concentration. In is widely used in the ZnO-based
AOS as a dopant in order to compensate for the reduction in the carrier concentration. The In
doping in the ZnO-based AOS systems induces an increase in electron carriers through the
replacement of In
3+
ions with Zn
2+
ions at substitution sites. However, In is rare on earth,
which results in high prices; thus, there are many studies seeking to replace In with other
elements such as aluminum (Al), tin (Sn), and silicon (Si). Particularly, Al is abundant on
earth and harmless to humans, thereby resulting in a very low manufacturing cost.
In this study, we have fabricated a TFT using the Zr-Al-Zn-O (ZAZO) channel layers.
Glass (Corning 1737) and Si (with the resistivity ≤ 2×10
-3
Ωcm) were prepared for the
substrates of ZAZO thin films and TFTs, respectively. The ZAZO thin films were deposited
on Glass and SiO
2
-grown (with 300 nm of thickness) Si substrates at room-temperature. AZO
thin films (with 50 nm of thickness) for source/drain electrodes were deposited via RF
sputtering, after which lift-off was conducted. To analyze the roles of Zr in the ZAZO films,
the RF power applied to the ZrO
2
target was varied within the ranges of 0-100 W and the
flow of O
2
additive gases was varied during the deposition of ZAZO thin films. The post-
annealing in the furnace was also conducted. The structural, optical, and electrical
characteristics as well as the chemical bonds of ZAZO films were evaluated using various
methods, such as X-ray diffraction (XRD), UV/visible spectrophotometer, I-V characteristics
analyzer, and X-ray photoelectron spectroscopy (XPS) respectively and analyzed in terms of
the TFT fabrication conditions and post-annealing conditions. It was suggested from this
study that the ZAZO-based TFTs possessed desirable characteristics for transparent displays.
PS1 31
-187-
