Download 1. Introduction

A study of Zinc Oxynitride Thin Film Transistor: mobility and stability
Kyung-Chul Ok, Hyun-Jun Jeong, Hyun-Mo Lee, and Jin-Seong Park*
Division of Materials Science and Engineering, Hanyang University, 222 wangsmini-ro,
Seoul, 133-719, Republic of Korea
Abstract
Amorphous ZnON thin film transistors were investigated
in terms of semiconducting behavior, fabrication process
and light induced bias stability. The electrical
characteristics such as superior mobility and light
induced bias stability were systemically analyzed by
specific physical and chemical properties.
1.
Introduction
Amorphous oxide semiconductor thin film transistors
(AOS TFTs) have been attracted many attention in active
matrix liquid crystal display (AMLCD) and organic
light-emitting diode (AMOLED) display because of their
good electrical properties and high on/off ratio compared
to conventional a-Si TFTs1. In spite of various
advantages, AOS TFTs was faced with limitation for the
applications of ultra high definition (UHD) display and
fast response in the large area.
Fig. 1. (a) cross-sectional device configuration, (b)
transfer curve and (c) saturation mobility of ZnO and
ZnON TFTs
Since the report of a-ZnON TFTs in 20092, amorphous
zinc-oxynitride semiconductor has been suggested as a
new potential candidate material to overcome above
issues due to its high mobility (≥ 50cm2/Vs) and good
stability3-4. Unfortunately, there are several reports of aZnON in particular due to very short period of
investigation. In this paper, we investigate the a-ZnON
TFTs focused on the semiconducting behavior and
fabrication process such as oxygen partial pressure and
effective annealing time. The amorphous phase and low
effective mass of ZnON lead to high filed effect mobility
in large area uniformity.
Figure 1 (b) and (c) show transfer characteristics and
saturation mobility of ZnO and ZnON TFTs,
respectively. After post annealing process at 250oC in
vacuum (~10-1 Torr), electrically metallic properties of
as-deposited a-ZnON TFTs was changed into
semiconducting properties, while ZnO TFTs was
changed from insulating to semiconducting properties.
Especially, the two order of saturation mobility of aZnON TFTs (~56.3cm2/Vs) was higher than ZnO TFTs.
The main difference between the ZnO and ZnON could
be suggested that the amorphous phase and low effective
mass of ZnON lead to high filed effect mobility in large
area uniformity. As a result of XPS analysis, N-rich
ZnxNy states which can be act as a electron donor were
effectively suppressed by post annealing process.
Therefore, the post annealing process is important to
fabricate semiconducting a-ZnON film and is available
to control the carrier concentration in the channel layer.
2.
2.2
2.1
Highly stable and high mobility
amorphous ZnON TFTs
Comparative studies of ZnO and
amorphous ZnON TFTs
First of all, the fabrication process of a-ZnON TFTs
was investigated by comparing with conventional ZnO
TFTs. Figure 1. (a) shows the cross-sectional device
configuration as a inverted staggered structure. The
different Ar/O2 and Ar/O2/N2 gas mixture were used to
reactive gas source to fabricated ZnO and ZnON film
during the DC reactive sputtering.
Effect of Oxygen Partial Pressure:
High mobility
The effect of oxygen partial pressure of a-ZnON TFTs
was investigate in order to optimize fabrication process
during the DC reactive sputtering. The device structure
and annealing process were carried out identical
conditions in 2.1. In this part, 30nm-thick ZnON films
were fabricated with different oxygen partial pressures
(O2/Mixture gas (Ar+N2+O2) = 0.013: A, 0.026: B,
0.038: C and 0.051: D).
Figure 2. (a) shows the representative transfer
characteristics of ZnON TFTs as a function of oxygen
partial pressure and (b) shows the average TFT
parameters at different oxygen partial pressure.
1
Fig. 2 (a) Representative transfer curves of ZnON TFTs
as a function of oxygen partial pressure. (b) The average
saturation mobility, threshold voltage and subthreshold
voltage of seven ZnON TFTs at different oxygen partial
pressure.
Fig. 3 (a) Representative transfer curves of a-ZnON TFT
(5hrs) under NBIS (VG = -20 V, 1500 lux @ white) as a
function of stress time. (b) Time-dependent threshold
voltage shift (∆Vth) of a-ZnON (1~5 hrs) under NBS and
NBIS conditions.
Interestingly, as oxygen partial pressure increased from
A to B, TFT mobility increased from 47.03 to 71.69
cm2/Vs. However, oxygen partial pressure increased
from B to D, mobility was degraded to 17.51cm2/Vs.
Generally, in oxide TFT, as oxygen partial pressure is
increased, mobility is decreased. However, ZnON TFTs
show different conventional tendency, mobility show
parabolic trend as a function of oxygen partial pressure.
This phenomena explained by XPS O 1s analysis (not
shown). The binding energy at 529.8eV (peak 1) and
531.1eV (peak 2) represent metal-oxide (Zn-O) and
oxygen related defects (ex. oxygen vacancy),
respectively. As oxygen partial pressure increased from
A to B, ratio of peak 1 was decreased from 74.42% to
73.2% in spite of increase of peak 2 from 16.64% to
17.64%. However, oxygen partial pressure changed from
B to D, peak 1 was increased up to 79.12% and peak 2
was decreased to 14.23%. The optimum oxygen partial
ratio of ZnON TFTs leads to suppress the oxygen related
defects as well as high TFT performance.
After post annealing process as enough time (5hrs),
enhancement of bonding properties between Zn metal
and two different anions (O, N) can result in the stable
O-Zn-N hybridization.
3.
The DC reactive sputtered a-ZnON TFTs were
investigated in focused on the semiconducting behavior
and fabrication process. In order to turn into the
semiconducting properties for the as-deposited film, post
annealing process is available to suppress the carrier
concentration. The optimized oxygen partial pressure
during the DC reactive sputtering could affect to the high
saturation mobility (~50cm2/Vs). After suitable
annealing time (5hrs), considerable improvement of
light-induced bias stability of a-ZnON TFTs was
investigated with stable O-Zn-N bonding properties.
4.
2.3 Effect of Annealing time: Reliability
The effect of annealing process of a-ZnON TFTs was
investigated as a function of annealing time (1, 3 and 5
hours). The a-ZnON TFTs were fabricated by optimized
Ar/O2/N2 mixture. Figure 3. (a) shows representative
transfer curve of a-ZnON (5 hrs) and (b) shows evolution
of threshold voltage under negative bias stress (NBS)
and negative bias illumination stress (NBIS). All the
TFTs were shown as similar device characteristics of Vth
(~ -3 V) and μsat (~50cm2/Vs) but the light-induced
device instability was significantly improved by
increasing the annealing time from 1 hr (-10.88 V) to
5hrs (-2.28 V). These phenomenon could be the stable
nitrogen related states near the valence band maximum
(VBM) affect to the improved light induced instability
due to the screening the oxygen related defects.
Conclusions
Acknowledgements
This work was
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