Solid-State Electronics 126 (2016) 170–174
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Solid-State Electronics
journal homepage: www.elsevier.com/locate/sse
Ambient effect on thermal stability of amorphous InGaZnO thin film
transistors
Jianeng Xu, Qi Wu, Ling Xu, Haiting Xie, Guochao Liu, Lei Zhang, Chengyuan Dong ⇑
National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e
i n f o
Article history:
Received 17 March 2016
Received in revised form 7 August 2016
Accepted 9 August 2016
Available online 9 August 2016
The review of this paper was arranged by Dr.
Y. Kuk
Keywords:
Amorphous InGaZnO (a-IGZO)
Thin film transistor (TFT)
Thermal stability
Ambient gas
a b s t r a c t
The thermal stability of amorphous InGaZnO thin film transistors (a-IGZO TFTs) with various ambient
gases was investigated. The a-IGZO TFTs in air were more thermally stable than the devices in the ambient argon. Oxygen, rather than nitrogen and moisture, was responsible for this improvement.
Furthermore, the thermal stability of the a-IGZO TFTs improved with the increasing oxygen content in
the surrounding atmosphere. The related physical mechanism was examined, indicating that the higher
ambient oxygen content induced more combinations of the oxygen vacancies and adsorbed oxygen ions
in the a-IGZO, which resulted in the larger defect formation energy. This larger defect formation energy
led to the smaller variation in the threshold voltage for the corresponding TFT devices.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) have
been considered one of the most promising candidates for use in
next-generation displays including active-matrix liquid crystal displays (AMLCDs) and active-matrix organic light-emitting diodes
(AMOLEDs); this is largely due to their high field-effect mobility,
which is greater than that found in traditional hydrogenated amorphous silicon (a-Si:H) TFTs, and good large-area uniformity, which
is superior to conventional polycrystalline silicon (p-Si) TFTs [1,2].
However, from a practical point of view, some critical issues, such
as the instability of a-IGZO TFTs, still need to be resolved [3,4]. The
thermal instability of a-IGZO TFTs caused by the prolonged temperature stressing is especially important for the display products
that utilize these novel semiconductor devices [5–7]. Several
research groups ascribed the negative threshold voltage shifts of
the a-IGZO TFTs at high temperatures to the generation of oxygen
vacancies [8–10]. The gas molecules in the surrounding atmosphere also play an important role [11,12]. Chang et al. observed
the suppression of the thermal instability of the a-IGZO TFTs in
the ambient air with respect to the a-IGZO TFTs in a vacuum
[11]. Our group demonstrated that the passivation layers, which
prevented the interaction between the active layers and the
⇑ Corresponding author at: 800 DongChuan Rd., Shanghai 200240, China.
E-mail address: [email protected] (C. Dong).
http://dx.doi.org/10.1016/j.sse.2016.08.001
0038-1101/Ó 2016 Elsevier Ltd. All rights reserved.
ambient air, could markedly improve the thermal stability of the
a-IGZO TFTs. We built a simple model to describe the related physical mechanism; for the sake of simplicity, only oxygen was considered in the discussion of the ambient effects [12]. However, the
other ambient gases, such as moisture, nitrogen, and inert gases
could also influence the instability of a-IGZO TFTs. Our recent studies confirmed this assumption and proved that moisture was the
main origin of the storage instability for the a-IGZO TFTs at room
temperature (RT) [13]. These facts inevitably make us wonder
how the ambient gases exactly influence the thermal instability
of a-IGZO TFTs. We believe the answer to this question might lead
us to discover how to improve the thermal stability of these novel
semiconductor devices.
In this research, we measured the transfer characteristics of the
a-IGZO TFTs in the temperature range of 323–473 K with various
ambient gases including air, Ar, O2, N2, and H2O. Interestingly, O2,
one of the main components of air, was the only gas that enhanced
the thermal stability of the a-IGZO TFTs.
2. Experiment
The inverted-staggered a-IGZO TFTs, as shown in Fig. 1(a), were
fabricated on the substrates of n-type silicon wafers (gate electrodes) with 300-nm-thick layers of thermal SiO2 (gate insulators).
After the substrates were thoroughly cleaned, the 50-nm-thick
a-IGZO films were deposited as the active layers and the
J. Xu et al. / Solid-State Electronics 126 (2016) 170–174
171
respectively. A heating stage was used to modulate the measurement temperature (298–423 K) of the samples.
Based on the measured transfer curves, the threshold voltage
(Vth) was obtained from the gate voltage value (Vgs) where
Ids/(W/L) = 100 nA. Here, Ids is the drain current of the TFT
devices. In order to quantitatively characterize the thermal
stability of the a-IGZO TFTs, we defined the essential term DVth
as DVth = Vth(T) Vth(298 K), where T represents the ambient
temperature. Apparently, smaller values of the DVth imply the
improved thermal stability of the a-IGZO TFTs.
3. Results and discussion
Fig. 1. (a) Schematic cross section of the inverted-staggered a-IGZO TFTs used in
this study. (b) Schematic diagram of the unsealed probe station and electrical
meters for the current-voltage (I-V) measurements.
200-nm-thick Indium Tin Oxide (ITO) films were prepared as the
source/drain (S/D) electrodes by the magnetron sputtering
technique. Patterned by shadow masks, the channel length (L)
and width (W) of the devices were fixed to 250 lm and
1000 lm, respectively. Finally, the samples were annealed in air
at 683 K for 1 h to improve their electrical properties.
We measured the transfer characteristics (VDS = 10 V) of the
devices in an unsealed probe station chamber using a Keithley
2636A parameter analyzer (Fig. 1(b)). It is worth noting that the
chamber maintained the atmospheric pressure and room temperature and little gas circulation, which created the conditions strikingly similar to those of the natural atmosphere. We directly fed
argon, nitrogen, and oxygen into the unsealed chamber, but
injected the moisture by feeding the water molecules with the
argon flow. The moisture content was quantitatively controlled
by the water level in the container, i.e., the humidity in the
chamber increased proportionally with the rise of the water level.
We defined three water levels, low, middle, and high, and
characterized them as ‘‘Low H2O”, ‘‘Middle H2O”, and ‘‘High H2O”,
Fig. 2 shows the transfer characteristics of the a-IGZO TFTs at
different temperatures in the ambient air. As shown in Fig. 2(a),
the Vth of the transfer curves shifted negatively and the drain
current increased gradually with the increasing temperature. This
degradation was mainly associated with the generation of the oxygen vacancies [8–10] and/or the ambient effects [11,12]. The
intrinsic excitation (i.e. the thermally generated electrons and
holes) cannot work efficiently unless T > 473 K [12], so we ignored
this mechanism. In addition, we observed the recovery process of
the transfer characteristics when we shut down the heater, which
resulted in the gradual fall of the temperature. Fig. 2(b) shows that
both Vth and Ids recovered to the initial values within 90 min. The
slow recovery process agreed well with the assumption that the
ambient effects (instead of the intrinsic excitation) dominated
the thermal instability of the a-IGZO TFTs observed in this study.
Fig. 3 shows the DVth of the a-IGZO TFTs as a function of the
temperature in the ambient argon, air, and nitrogen, respectively.
For argon and nitrogen, their flow rates were fixed at 15 L per minute (LPM) to ensure that the original air in the unsealed chamber
could be completely removed. It is generally believed that argon,
a kind of inert gas, hardly interacts with semiconductors due to
its inertness. Hence, the thermally induced DVth of the a-IGZO TFTs
in the ambient argon was mainly associated with the inside generation of oxygen vacancies, i.e. the thermally excited oxygen atoms
leaving their original sites, moving into the interstitial sites, and
causing the vacancies with the remained free electrons at the corresponding sites [8–10]. Therefore, the Vth reductions observed at
high temperatures for the a-IGZO TFTs in the ambient argon (as
shown in Fig. 3) could be attributed to the additional generation
of these free electrons along with the oxygen vacancies. For simplicity, we regarded the performance of the devices in argon as a
reference to elucidate the ambient effects on the thermal stability
Fig. 2. Transfer characteristics of the a-IGZO TFTs (a) at different temperatures and (b) in the recovery process in the ambient air.
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J. Xu et al. / Solid-State Electronics 126 (2016) 170–174
Fig. 3. Temperature dependence of DVth for the a-IGZO TFTs in the ambient air,
argon, and nitrogen.
Fig. 4. Temperature dependence of DVth for the a-IGZO TFTs in the ambient argon
(no H2O) and moisture. Here, the ‘‘Low H2O”, ‘‘Middle H2O”, and ‘‘High H2O” are the
various water levels, as shown in Fig. 1(b).
of the a-IGZO TFTs. Then, as shown in Fig. 3, the ambient air apparently improved the thermal stability of the devices. Although
Chang et al. [11] reported a similar result, it is still unknown which
gas (i.e. nitrogen, moisture, or oxygen) was responsible for this
improvement. In this study, we measured the temperature dependence of DVth on the ambient nitrogen, moisture, and oxygen. As
shown in Fig. 3, the temperature dependence of DVth on the ambient nitrogen exhibited a similar tendency with that found with the
ambient argon, indicating that nitrogen hardly interacted with the
back channels of the devices, even at high temperatures. This was
consistent with the results for the a-IGZO TFTs under both the bias
stresses and ambient nitrogen [14].
Next, we measured the thermal stability of the a-IGZO TFTs in
the ambient moisture. Fig. 4 shows the DVth of the a-IGZO TFTs
as a function of the temperature under the ambient ‘‘Low H2O”,
‘‘Middle H2O”, and ‘‘High H2O” conditions, respectively. For comparison, the figure also shows the case in the ambient argon (no
H2O). Fig. 4 indicates that the Vth reduction under the ‘‘Low H2O”
conditions exhibited a similar tendency to that found with the
argon gas (no H2O). Even when the moisture content increased
(‘‘Middle H2O”/‘‘High H2O”), the DVth still remained almost the
same. It seemed that the a-IGZO TFTs at high temperatures were
not sensitive to the moisture, a result that was quite different from
our pervious study that showed the a-IGZO TFTs stored at RT were
fairly sensitive to the ambient moisture [13].
Based on the aforementioned investigations, we examined the
physical mechanism concerning the effect of the ambient moisture
on the thermal stability of the a-IGZO TFTs by comparing the cases
at RT and a high temperature. Fig. 5(a) shows the mechanism of the
moisture adsorption for the a-IGZO TFTs at RT. The surrounding
moisture was easily adsorbed onto the back channel surface, which
donated electrons to the a-IGZO in a form of H2O = H2O+ + e, and
thus led to the gradual negative shift of the Vth for the devices
stored at RT until the system reached equilibrium [13,15]. However, when the a-IGZO TFTs were heated, as shown in Fig. 5(b),
apart from the inside generation of the oxygen vacancies, the high
temperature suppressed the moisture adsorption onto the a-IGZO
films; as a result, the moisture donated few electrons to the active
layers of the a-IGZO TFTs. Fig. 5(c) shows that even with the
increase in the moisture content, the moisture adsorption effect
was still negligible. So, the negative Vth shifts of a-IGZO TFTs at a
high temperature in the ambient moisture were mainly caused
by the inside generation of the oxygen vacancies rather than the
extra electrons donated by the ambient moisture. Therefore,
the ambient moisture did not influence the thermal stability of
the a-IGZO TFTs.
Neither nitrogen nor moisture was proven to significantly influence the thermal stability of the a-IGZO TFTs. We therefore assume
that oxygen should exhibit different performance, which was
confirmed by the following experimental results. Fig. 6 shows the
temperature dependence of DVth for the a-IGZO TFTs in the ambient oxygen with different flow rates; we include the results for the
ambient argon for comparison. One may observe much smaller Vth
reductions with the increase in the surrounding oxygen content.
For example, the DVth at 423 K improved to only 7 V when the oxygen flow rate increased to 15 LPM. It seemed that the a-IGZO TFTs
at high temperatures were fairly sensitive to oxygen, contrary to
what our pervious results indicated (i.e. the a-IGZO TFTs stored
at RT were insensitive to the ambient oxygen [13]).
Since the thermal instability of the a-IGZO TFTs was closely connected to the oxygen content in the surrounding atmosphere,
Fig. 5. Schematic diagram of the a-IGZO films in the ambient moisture (a) without heating, (b) with heating (little H2O), and (c) with heating (more H2O).
J. Xu et al. / Solid-State Electronics 126 (2016) 170–174
Fig. 6. Temperature dependence of the DVth for the a-IGZO TFTs in the ambient
argon and oxygen with different flow rates.
Fig. 7. Experimental plots and the proximate straight lines for ln(|DVth|) e/kBT in
the temperature range of 323–423 K for the a-IGZO TFTs in the ambient argon with
a flow rate of 15 LPM or the oxygen with the flow rate of 5, 10, and 15 LPM,
respectively. Inset: the dependence of the defect formation energy W on the
different gases in the surrounding atmosphere.
investigating the exact physical essence of the oxygen vacancy formation during the thermal stability tests became necessary. Here,
we employed a theoretical analysis based on the model proposed
by Takechi et al. [9], where the temperature dependence of DVth
can be expressed as the following equation:
lnðjDV th jÞ ¼ W
Cg
;
ln
3kB T
2 e t IGZO C 1
ð1Þ
173
where W stands for the defect formation energy, kB is the Boltzmann constant, Cg is the capacitance of the gate insulator per unit
area, e is the electronic charge, tIGZO is the thickness of the a-IGZO
layer, and C1 is the constant related to the entropy for the formation
of one vacancy and two free electrons. By using the experimental
data (shown in Fig. 6) and the least square method to fit these
points, we extracted W and C1 from the slope and the intercept of
the straight lines expressed by Eq. (1), as shown in Fig. 7. Here,
the defect formation energy W rather than C1 was calculated since
the later was beyond the scope of this article. As shown in the inset
of Fig. 7, the experimental plots led to the proximate straight lines
with W of 0.696, 0.750, 0.786, and 0.831 eV when the argon flow
rate was fixed at 15 LPM or the oxygen flow rate was fixed at 5,
10 and 15 LPM, respectively. The defect formation energy clearly
increased with the rise in the ambient oxygen content. It is well
known that the electron concentration in a-IGZO is closely related
to the concentration of the oxygen vacancies determined by the
defect formation energy W [9,16]. Generally speaking, the higher
W values lead to the smaller variations in the carrier concentration.
Therefore, a conclusion might be that the higher ambient oxygen
content could result in the larger defect formation energy in the
a-IGZO films and thus lead to a smaller variation in the carrier concentration and DVth value.
Why did the higher ambient oxygen content cause the larger
defect formation energy in the a-IGZO? We discuss the related
physical mechanism based on some earlier reports in the literature
[11,13,17–19]. Since the a-IGZO TFTs were fabricated and stored in
the ambient air, the oxygen molecules were adsorbed onto the
back channel surface, as reported by several researchers [17–19].
The adsorbed oxygen on the back channel surface existed as O
or O
2 , indicating that the adsorbed oxygen could accept electrons
from the back channel and cause a positive shift of Vth [17,18].
Our pervious results proved that this oxygen adsorption would
achieve saturation at RT, as shown in Fig. 8(a); it was characterized
by the phenomenon in which the oxygen molecules could no
longer be adsorbed onto the back surface anymore with the further
increase in the ambient oxygen content [13]. When the a-IGZO
TFTs were heated, a large amount of oxygen vacancies were generated by thermal excitation, which led to the large negative Vth
shifts as the temperature increased. Apart from the inside generation of the oxygen vacancies, the recombination of the oxygen
vacancies and oxygen ions was accelerated at high temperatures
in the ambient oxygen [11]; this process could be divided into
two steps, as shown in Fig. 8(b). First, the saturation of the oxygen
adsorption was broken at high temperatures and more oxygen
molecules were adsorbed onto the back channel surface, which
additionally accepted electrons and created the oxygen ions
(O or O
2 ). It is likely that the oxygen ions existed as O at high
temperatures, and the related reaction can be described as
O2 + 2e = 2O [19]. Second, the thermally excited oxygen vacancies
recombined with the oxygen ions in the a-IGZO films, which can be
described as Metal-V+o + O = Metal-O, where Metal-V+o and
Fig. 8. Schematic diagram of the a-IGZO films in the ambient oxygen (a) without heating, (b) with heating (little O2), and (c) with heating (more O2).
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J. Xu et al. / Solid-State Electronics 126 (2016) 170–174
Metal-O stand for the oxygen vacancy and the recovered oxygen
atom, respectively. In addition, the higher oxygen content in the
ambience accelerated this recombination process, indicating that
more oxygen atoms might turn into oxygen ions and then recombine with the oxygen vacancies as the ambient oxygen flow rate
increased, as shown in Fig. 8(c). Therefore, the generation of oxygen vacancies in the a-IGZO films at high temperatures needed
more energy (a higher W value) if the devices were exposed to
more oxygen molecules. In other words, the higher ambient oxygen content made the thermal generation of the oxygen vacancies
in the active layers more difficult, resulting in more thermally
stable a-IGZO TFTs.
4. Conclusion
We investigated the influence of the ambient gases including
argon, air, nitrogen, moisture, and oxygen on the thermal stability
of a-IGZO TFTs. It was found that the ambient nitrogen and moisture had little effect on the thermal stability of the a-IGZO TFTs,
whereas oxygen caused more thermally stable devices. Furthermore, a higher oxygen content in the surrounding atmosphere
led to a better thermal stability for the devices. We also discussed
the related physical mechanism to elucidate how the ambient
gases affected the thermal stability of the a-IGZO TFTs.
Acknowledgment
This work was supported by National Natural Science Foundation of China (Grant nos. 61136004 and 61474075).
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