pubs.acs.org/acsaelm

ArticleEnhancement of pH Tolerance in Conductive Al-Doped ZnO
Nanoﬁlms via Sequential Annealing
Ruolin Yan, Tsunaki Takahashi,* Hao Zeng, Takuro Hosomi, Masaki Kanai, Guozhu Zhang,
Kazuki Nagashima, and Takeshi Yanagida*

Downloaded via UNIV OF TOKYO on February 28, 2021 at 06:43:52 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Cite This: ACS Appl. Electron. Mater. 2021, 3, 955−962

ACCESS

Metrics & More

Read Online

Article Recommendations

sı Supporting Information*

ABSTRACT: Chemically stable and electrically conductive metaloxide nanoﬁlms are promising as robust electrodes for chemical/
biosensors and for photoelectrochemical applications, which
require harsh conditions (e.g., acidic or basic environments).
Among the various conductive metal oxides, impurity-doped ZnO
nanoﬁlms deposited on substrates are chemically nonresistive toacidic and basic environments because of the inevitable etchingeﬀects. Herein, we demonstrate a strategy to enhance the pH
tolerance of Al-doped ZnO (AZO) nanoﬁlms using a sequential
annealing technique for ﬁlm preparation. This technique involves
ﬁrst annealing in air followed by annealing in Zn vapor atmosphere. Although the as-grown AZO nanoﬁlms rapidly dissolved inacidic and basic solutions, the sequentially annealed AZO nanoﬁlms exhibited excellent pH tolerance toward the chemical etchingrate and electrical resistance in buﬀer solutions, with pH ranging from 3 to 11. This enhancement eﬀect of pH tolerance wasconsiderably weakened when sequential annealing was performed in reverse (Zn vapor/air). The origin of the enhanced pH
tolerance of the sequentially annealed AZO nanoﬁlms is discussed in terms of the compensation of the anion/cation vacancies andthe surface polarity of the ZnO(0001) surface.
KEYWORDS: Al-doped ZnO nanoﬁlms, chemical stability, sequential annealing, transparent conductive oxides, defects compensation■

INTRODUCTION
Electronic devices interacting with chemical species, such as
chemical/biosensors and photoelectrochemical devices, haveattracted signiﬁcant attention because of the increasing social
demands for collecting chemical and biological data andaddressing energy and environmental issues.1−3 Theseemerging electronic devices inherently require electronicmaterials whose electrical characteristics are highly stableeven under chemically harsh conditions. For example, aﬂuorescent sensor to detect toxic Al3+ ions is typically operated
under acidic conditions (pH ≈ 4.5).4 Biological applications,such as neural recording and use of antimicrobial devices, areperformed in acidic aqueous solutions (pH 4−7).5−8 In
contrast, basic environments are required for photoelectrochemical devices, including water splitting and CO2 reduction(pH 6.8−13).9−11 Thus, chemical stability under acidic orbasic conditions is strongly desired for these sensing electronicmaterials, in addition to other requirements of physicalproperties, stability in atmospheric air, and/or stability athigh temperatures.12
Among various electric materials, electrically conductive
impurity-doped metal oxides are promising for emerging
electronics because of their inherent atmospheric stability andtemperature resistance.13−17 Al-doped ZnO (AZO) ﬁlms havebeen intensively studied as conductive metal oxide electrodes© 2021 American Chemical Society

because of their high electrical conductivity and visible-lighttransmittance as well as abundance of resources.18−21
However, it has been reported that deposited AZO nanoﬁlms
are easily dissolved in acidic or basic solutions,8,22 limiting theirutilization under harsh conditions. To overcome the chemical
instability of AZO, the formation of a chemically stablecapping layer (TiO2) on AZO nanoﬁlms has been proposed.22
Although the capping layer can enhance the chemical stabilityof the fabricated layered structures in acidic or basic solutions,the intrinsic properties of AZO, including optical transmissivity
and surface chemical activities, are partially sacriﬁced.
Recently, we reported that sequential thermal annealing in
air and in Zn vapor atmosphere signiﬁcantly enhanced thethermal stability of AZO nanoﬁlms without a capping layer.23
In the sequential annealing method, AZO nanoﬁlms depositedon Al2O3 or SiO2/Si substrates were ﬁrst annealed in air andsubsequently annealed in Zn vapor atmosphere. The long-term
stability and low-resistivity characteristics of the sequentially
Received: November 30, 2020
Accepted: January 25, 2021
Published: February 3, 2021

955

https://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962

ACS Applied Electronic Materials

pubs.acs.org/acsaelm

ArticleFigure 1. (a) Schematics of the procedure of the chemical stability tests of AZO nanoﬁlm devices. Etched thickness and resistance were measured
in ambient air at room temperature. (b) Etched thickness and (c) resistance of the as-grown AZO nanoﬁlm devices during chemical stability tests at
a pH of 3. The inset in (b) shows the etched thickness at pH 3 for the as-grown AZO nanoﬁlms deposited at room temperature as a function oftime.

Figure 2. (a) Flowchart of the sequential annealing steps including sequence I and II annealing. (b) UV−vis transmission spectra of as-grown,
sequence-I-annealed, and sequence-II-annealed AZO nanoﬁlms.

the Methods section. Figure 1a illustrates the procedure for thechemical stability tests. The fabricated AZO nanoﬁlm devices
were immersed in buﬀer solutions with a controlled pH of 3−
11 for a controlled duration. The etched thickness and fourprobe electrical resistance of the AZO nanoﬁlms were
measured in ambient air at room temperature after each
immersion. Detailed fabrication processes and characterizationof the AZO nanoﬁlm devices are discussed in the Methods
section. Note that both the etched thickness and resistancemust be constant for chemically stable samples during stability
tests, whereas those values will increase as the test duration
increases for chemically unstable samples because of theetching of AZO nanoﬁlms in acidic/basic solutions. Figure 1b,cshows the etched thickness and resistance values of the asgrown AZO nanoﬁlm devices during the chemical stability
tests at pH 3. The as-grown AZO nanoﬁlm, whose initial
thickness was approximately 140 nm, was rapidly etched andfully dissolved within 35 min, indicating the chemical
instability of the as-grown AZO nanoﬁlms in acidic solutions.
This rapid chemical etching phenomenon of AZO nanoﬁlms inacidic aqueous solutions is consistent with previous reports.8,22,24,25 Although the present as-grown AZO nanoﬁlms
show a suﬃciently low resistance (resistivity: ∼1.9 × 10−3 Ω·cm) before the stability tests, the resistance signiﬁcantly
increased as the test duration increased because chemical
etching reduced the cross-sectional area of the electricalcurrent ﬂow (Figure 1c). The rapid resistance increase near 2orders of magnitude can be attributed to the nonuniform
chemical etching of AZO surfaces during the stability tests at

annealed AZO nanoﬁlms were demonstrated through thermal
tolerance tests at 400 °C in ambient air. The improvement ofthermal stability was attributed to the sequential compensationof dual-anion/cation defects in as-grown AZO nanoﬁlms
through sequential annealing. This method could enhance thechemical stability of AZO nanoﬁlms. However, the eﬀect of thesequential annealing on the chemical stability of AZO
nanoﬁlms is unclear because the surface properties ofsequentially annealed AZO nanoﬁlms, especially the chemical
etching process that occurs on the surface, have not been
investigated in the previous study. In this study, weinvestigated the eﬀects of the sequential annealing methodon the chemical stability of AZO nanoﬁlms by evaluating themorphological and electrical variations of AZO nanoﬁlms inacidic and basic solutions. Both chemical etching and electricaldegradation were signiﬁcantly suppressed for the devices basedon sequentially annealed AZO nanoﬁlms over a wide pH rangeof 3−11, indicating the high chemical stability of thesequentially annealed AZO nanoﬁlms. The origin of thechemical stability of AZO nanoﬁlms is discussed in terms ofdefect compensation eﬀects during sequential annealing andthe surface polarity of the ZnO(0001) surface.

■

RESULTS AND DISCUSSIONFirst, we examined the chemical stability of the as-grown AZO
nanoﬁlms. AZO nanoﬁlms were deposited on Al2O3(110)substrates through pulsed laser deposition (PLD) withdeposition parameters optimized to obtain low electricalresistivity.23 The details of ﬁlm deposition are provided in956

https://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962

ACS Applied Electronic Materials

pubs.acs.org/acsaelm

ArticleFigure 3. Etched thickness of the as-grown, sequence-I-annealed (Zn vapor/air), and sequence-II-annealed (air/Zn vapor) AZO nanoﬁlms duringchemical stability tests over pHs 3−11: (a) pH 3.00, (b) pH 4.01, (c) pH 5.00, (d) pH 7.01, (e) pH 9.18, (f) pH 10.00, and (g) pH 11.00.

vapor annealing steps are provided in the Methods section. In
this study, the eﬀects of sequence I annealing (annealing in Zn
vapor atmosphere followed by that in air) and sequence IIannealing (annealing in air followed by that in Zn vapor
atmosphere) on the chemical stability of AZO nanoﬁlms were
investigated (Figure 2a). Figure 2b shows the ultraviolet−
visible (UV−vis) transmission spectra of the as-grown,
sequence-I-annealed, and sequence-II-annealed AZO nanoﬁlms. Excellent visible-light transparency, which agrees withprevious reports,29,30 was observed in all ﬁlms, including
sequentially annealed AZO nanoﬁlms. Therefore, the present
sequentially annealed AZO nanoﬁlms are compatible withapplications based on optical transmissivity of AZO, such as
photoelectrochemical devices.22,31,32
Figure 3 shows the etched thickness of the as-grown,
sequence-I-annealed, and sequence-II-annealed AZO nanoﬁlms during the chemical stability tests over a wide pH rangeof 3−11. First, the as-grown AZO nanoﬁlms started to dissolve
in both acidic and basic solutions within 1 h. Furthermore, theas-grown AZO nanoﬁlms were etched gradually even in aneutral solution (Figure 3d, pH 7). This chemically unstableproperty of the as-grown AZO nanoﬁlms is consistent with thepreviously reported chemical etching eﬀects of doped or
undoped ZnO nanostructures, which is attributed to theamphoteric character of ZnO.26,33 Second, in the sequence-Iannealed (Zn vapor/air) AZO nanoﬁlms, the etched thickness
decreased slightly compared with that of the as-grown samples.
However, the rapid dissolution of the AZO nanoﬁlms in acidic
solution (pH 3) still existed even for the sequence-I-annealed
samples (Figure 3a). Third, in the sequence-II-annealed (air/Zn vapor) AZO nanoﬁlms, chemical etching was signiﬁcantly

pH 3. The nonuniform chemical etching causes a signiﬁcantresistance increase because partially survived current paths
within AZO nanoﬁlms determine measured resistance. Notethat changes in the contact resistance between Pt electrodesand AZO nanoﬁlms do not aﬀect the measured resistancebecause the resistance was measured by the four-probe
technique. The deposition parameters of the as-grown AZO
nanoﬁlms were optimized to obtain low resistivity, resulting ininevitable crystal defects within AZO.23 Thus, the as-grown
AZO nanoﬁlms, which are deposited to be electricallyconductive, are not stable in acidic aqueous solutions.
To overcome the above limitation of the chemical stabilityof the as-grown AZO nanoﬁlms, we focus on reducing the
crystal defects within the as-grown AZO nanoﬁlms during ﬁlm
deposition. This is because such crystal defects inherently
accelerate chemical etching events.26 The inset in Figure 1b
shows the etched thickness of the as-grown AZO nanoﬁlms
deposited at room temperature, which has a much lower
crystallinity than optimized samples deposited at 300 °C(Figure S1). The AZO nanoﬁlms deposited at room
temperature rapidly dissolved within 40 s at pH 3, verifying
the strong correlation between crystallinity and chemical
stability. As reported previously,23 we performed sequential
annealing to compensate for crystal defects in the as-grown
AZO nanoﬁlms. Figure 2a illustrates the sequential annealing
processes employed for the AZO nanoﬁlms before thechemical stability tests. The sequential annealing method
consists of a set of two thermal annealing steps: annealing in air
and in Zn vapor atmosphere. These air and Zn vapor annealing
steps were employed to compensate for oxygen and Zn defects
in AZO nanoﬁlms, respectively.23,27,28 Details of air and Zn
957

https://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962

ACS Applied Electronic Materials

pubs.acs.org/acsaelm

ArticleFigure 4. Resistance of the as-grown, sequence-I-annealed (Zn vapor/air), and sequence-II-annealed (air/Zn vapor) AZO nanoﬁlms duringchemical stability tests over pHs 3−11: (a) pH 3.00, (b) pH 4.01, (c) pH 5.00, (d) pH 7.01, (e) pH 9.18, (f) pH 10.00, and (g) pH 11.00.

suppressed over the entire pH range, indicating superiorchemical stability under both acidic and basic conditions. Notethat the surface topology of the sequence-II-annealed AZO
nanoﬁlms also did not change during the chemical stability
tests at pH 3 (Figure S2). Interestingly, the chemical stabilityof the sequence-II-annealed samples is consistent with ourpreviously reported thermal stability tests of a series ofsequentially annealed AZO nanoﬁlms. Similar to Figure 3, only
the sequence-II-annealed AZO nanoﬁlms showed thermal
stability and suﬃciently low resistivity during thermal tolerance
tests up to 500 °C in ambient air.23 Figure 4 shows theelectrical resistance of the as-grown, sequence-I-annealed, andsequence-II-annealed AZO nanoﬁlm devices during thechemical stability tests. Note that the high initial resistanceof the sequence-I-annealed AZO nanoﬁlms originates from theremaining Zn defects in the AZO nanoﬁlms, which trap mobile
charges.23 Each sample and data point in Figure 4 corresponds
to that of the chemical etching evaluation shown in Figure 3. A
clear correlation between chemical etching and resistanceincrease was observed in all samples and tested pH values
because chemical etching causes reduction of the crosssectional area of the current ﬂow. Note that the enhancementof chemical stability induced by sequence II annealing can beobserved in the AZO nanoﬁlms deposited on SiO2/Si substrate(Figure S3), indicating the general applicability of this strategy.
Thus, these series of data on the chemical stability of AZO
nanoﬁlms highlight that sequence II annealing (air/Zn vapor)only enhances the chemical stability of AZO nanoﬁlms.
Figure 5 summarizes the pH dependence on the chemical
etching rates and the increase in the electrical resistances of the
as-grown, sequence-I-annealed, and sequence-II-annealed AZO
nanoﬁlms during the chemical stability tests. An enhancementof the pH tolerance toward chemical etching and electricalresistance of the conductive AZO nanoﬁlms can be achieved
only through sequence II annealing (air/Zn vapor). The
etching rate at pH 3 of the sequence-II-annealed AZO

Figure 5. pH dependence of (a) etching rate and (b) resistanceincrease of the as-grown, sequence-I-annealed (Zn vapor/air), andsequence-II-annealed (air/Zn vapor) AZO nanoﬁlms during chemical
stability tests. The etching rate was calculated using etched thickness
at 25 min immersion in each pH buﬀer solution. Resistance increase
was calculated as (R − R0)/R0, where R and R0 are the resistances at
25 min immersion in each pH buﬀer solution and the initial resistance
before the stability tests, respectively.

nanoﬁlms was calculated to be 0.2 nm/min. We compared
the etching rates of the sequence-II-annealed AZO nanoﬁlms
to previously reported etching rates25,34−36 as shown in Table
1. For example, Olvera et al. investigated the chemical stabilityof doped ZnO microcrystalline ﬁlms deposited using the spraypyrolysis technique in hydrochloric acid solution (pH ≈ 3),
reporting that Cr-doped ZnO exhibits the lowest etching rateof 2.5 nm/min and Al-doped ZnO (AZO) shows the highest
chemical etching rate of 6.5 nm/min.25 Heinonen et al.
reported an etching rate of 9.167 nm/min in solution at pH 3
for hydrothermally grown polycrystalline ZnO ﬁlms.34 The
etching rates of 15 nm/min in NH4Cl (pH ≈ 4) solution36 has
been reported for PLD-deposited ZnO. Thus, these comparisons of etching rates of previous and present studies highlight958

https://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962

ACS Applied Electronic Materials

pubs.acs.org/acsaelm

ArticleTable 1. Etching Rates of Sequence-II-Annealed AZO
Nanoﬁlms (Shown as “This Work”) and Doped or Undoped
ZnO Films in Previous StudiespHetching rate (nm/min)

growth method

treatment

3

0.20
2.5
6.5
9.167
120
0.25
3.056
15
0.084
0.046
0.053
0.092

PLD
spray pyrolysis
spray pyrolysis
hydrothermal growthmagnetron sputtering
PLD
hydrothermal growth
PLD
PLD
hydrothermal growth
PLDhydrothermal growth

this work
Cr doping25
Al doping25
n/a34
Zn addition35
this work
n/a34
n/a36
this work
n/a34
this work
n/a34

Figure 6. Etched thickness of O-terminated and Zn-terminatedsurfaces of single-crystalline ZnO(0001) substrate during chemical
stability tests at pH 3. Etched thicknesses of the as-grown, sequence-Iannealed (Zn vapor/air), and sequence-II-annealed (air/Zn vapor)
AZO nanoﬁlms are also shown.

that sequence-II-annealed AZO nanoﬁlms prepared in thisstudy exhibit superior pH tolerance compared to previous
AZO nanoﬁlms to the best of our knowledge.
Finally, we discuss the origin of the enhanced pH toleranceof AZO nanoﬁlms prepared via the sequential annealing
method, which should correlate with the chemical etchingprocess of ZnO crystals because of the low Al content of AZO
nanoﬁlms (1% in sintered tablets used for PLD). In general,the deposited ZnO ﬁlms form wurtzite (0001)-oriented crystal
structures on substrates, and the presence of this ZnO(0001)
structure has been conﬁrmed in our AZO nanoﬁlms (FigureS1). The chemical etching process of single-crystalline ZnO
can be explained by a model based on a dangling bond onpolar Zn-terminated and O-terminated surfaces of the wurtzite
structure.26,37 The dangling bonds on Zn-terminated and Oterminated surfaces are charged as δ+ and δ−, respectively.
These charged dangling bonds are easily attacked by etchantions such as H3O+ and OH−, resulting in the chemical etchingof ZnO(0001) surfaces. In an ideal single-crystalline ZnO
wurtzite crystal structure, only ions having the charge opposite
to that of polar surfaces can attack these dangling bonds, i.e.,
H3O+ (acidic) etching on the O-terminated surface and OH−(basic) etching on the Zn-terminated surface. To investigate
the eﬀects of polar surface etching on the stability of thesequentially annealed AZO nanoﬁlms, the chemical stability ofsingle-crystalline ZnO(0001) substrates was evaluated. Figure
6 shows the etched thickness characteristics of a singlecrystalline ZnO(0001) substrate, where surface polarity ispredeﬁned by the supplier, during the chemical stability tests at
pH 3. The O-terminated surface is etched much faster than the
Zn-terminated surface, supporting the chemical etching model
based on the charged dangling bonds. Note that the chemical
etching of the as-grown and sequentially annealed AZO
nanoﬁlms was slower than that of the O-terminated surface,whereas it was faster than that of the Zn-terminated surface(Figure 6), indicating the Zn-terminated-rich surface of these
AZO nanoﬁlms. This speculation is consistent with previous
studies that reported the faster formation of Zn-terminatedsurfaces than that of O-terminated surfaces during conventional ZnO ﬁlm deposition.26,38−40 However, the signiﬁcant
increase in the chemical stability of the AZO nanoﬁlms
prepared by sequential annealing in this study cannot be
explained by only the model based on dangling bonds on polar
surfaces. This is because the diﬀerence between sequence I(Zn vapor/air) and II (air/Zn vapor) annealings is only thesequence of the postdeposition annealing atmosphere. We
have proposed a model of sequential defect compensationeﬀects during the sequential annealing in a previous study.23
Dual defects of cation (Zn2+) and anion (O2−) in as-grown
AZO nanoﬁlms can be compensated successfully only in theannealing sequence of ﬁrst in air and subsequently in Zn vapor
atmosphere because of a sequential energy beneﬁt during
annealing.23 The high electrical conductivity of the as-grown ntype AZO nanoﬁlms (resistivity: ∼1.9 × 10−3 Ω·cm) suggests
excess cation (Zn ion) and insuﬃcient anion (oxygen ion) in
the as-grown AZO nanoﬁlms because oxygen defects generate
mobile electrons. Therefore, cation defect compensation via
Zn vapor annealing must be limited because the initial state,
namely, as-grown AZO nanoﬁlms, is already cation excess. In
contrast, air annealing of the as-grown AZO nanoﬁlms caneﬀectively compensate anion defects because cation and anion
contents are balanced via air annealing. Zn vapor annealing cansuﬃciently compensate Zn defects in these ion-balanced AZO
nanoﬁlms after air annealing, resulting in defectless AZO
nanoﬁlms via sequence II annealing. Therefore, the as-grown
and sequence-I-annealed AZO nanoﬁlms contain many crystal
defects, and these crystal defects are compensated in thesequence-II-annealed AZO nanoﬁlms. More importantly, these
defects degrade chemical stability because the surface defects
on ZnO ﬁlms act as starting points for the chemical etchingprocess, even for Zn-terminated surfaces in acidic solutions.41−43 Defect-triggered chemical etching occurs in theas-grown and sequence-I-annealed AZO nanoﬁlms, resulting infaster etching compared to that of the single-crystalline Znterminated surface and sequence-II-annealed AZO nanoﬁlms(Figure 6). Density functional theory (DFT) calculations
reported that Fermi-level shift near conduction band edge
decreases the formation energy of Zn defects in wurtzite ZnO
crystal.44 Therefore, n-type (Al) doping generally increases theamount of Zn defects in as-grown AZO ﬁlms, supporting the
proposed model for the enhancement of chemical stability
caused by defects compensation via sequential annealing. Notethat annealing in only Zn vapor atmosphere could not suppress
the chemical etching of the AZO nanoﬁlms (Figure S4), thus
supporting our model (which requires two-step sequential
annealing). Therefore, the signiﬁcant improvement of thechemical stability of the sequence-II-annealed AZO nanoﬁlms
is attributed to the suppression of defect-triggered chemical
etching on the Zn-terminated wurtzite (0001) surface throughsequential compensation of dual-anion/cation defects. Because

4

9
11

959

https://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962

ACS Applied Electronic Materials

pubs.acs.org/acsaelm

sequence II annealing also signiﬁcantly enhances the thermal
stability of highly conductive AZO nanoﬁlms up to 500 °C,23
the sequential annealing method facilitates the utilization ofabundant AZO nanoﬁlms in emerging applications under awide variety of harsh conditions.

Sample Fabrication for Chemical Stability Tests. The
deposited AZO nanoﬁlms were ﬁrst patterned by photolithography
and chemical etching using a 0.5% HCl solution. Pt (100 nm) was
then deposited and patterned by radio frequency (RF) sputtering anda liftoﬀ process with photolithography as four-terminal electrodes on
the AZO nanoﬁlm channel. The channel length and width of the AZO
nanoﬁlms were 500 and 200 μm, respectively. For the chemical
stability tests of the single-crystalline ZnO surface, 100 nm of SiO2
was deposited and patterned on a single-crystalline ZnO(0001)
substrate (K&R Creation Co., Ltd.) by RF sputtering and a liftoﬀ
process with photolithography to protect a part of the ZnO surface
from chemical etching during stability tests. The window on the SiO2deposited ZnO substrate, which was chemically etched during stability
tests, was designed to be the same size as the AZO channels (500 ×
200 μm2).
Characterizations. UV−vis transmission spectroscopy was
performed using a spectrometer (Jasco V-770). The electricalresistance of the AZO nanoﬁlms was measured by the four-probe
technique using a semiconductor parameter analyzer (Keithley
4200SCS) with a probe station in ambient air at room temperature.
The etched thickness was calculated from the measured step values
using a surface proﬁler (Alpha-Step IQ, KLA). In the etched thickness
measurements and resistance measurements, the averaged values ofthe etched thickness and resistance were evaluated using at least threesamples. Error bars are also calculated using standard deviation for allof the etched thickness and resistance data. The averaged values of theresistance and etched thickness with error bars of standard deviation
were calculated using at least three samples. In the chemical stability
tests, the samples were immersed in pH buﬀer solutions (HANNA
Instruments, pH: 3.00, 4.01, 5.00, 7.01, 9.18, 10.00, and 11.00) for acontrolled period at room temperature.

■

CONCLUSIONS
We demonstrate chemically stable and highly conductive Aldoped ZnO (AZO) nanoﬁlms in both acidic and basic aqueous
environments; the ﬁlms are prepared via a sequential annealing
method. When AZO nanoﬁlms are thermally annealed ﬁrst in
air and subsequently in Zn vapor atmosphere (sequence II),
the chemical etching of AZO nanoﬁlms in pH buﬀer solutions
was signiﬁcantly suppressed over a wide pH range of 3−11.
The highly conductive characteristics of the AZO nanoﬁlms
are also maintained in the sequence-II-annealed AZO nanoﬁlms during the chemical stability tests. Conversely, in the asgrown and sequence-I-annealed (ﬁrst annealed in Zn vapor
atmosphere and subsequently in air) samples, the AZO
nanoﬁlms start to dissolve in both acidic and basic solutions
within 1 h, resulting in a signiﬁcant increase in resistance.
Chemical etching analysis of single-crystalline ZnO(0001)
substrates reveals that the AZO nanoﬁlms have a Znterminated-rich surface with a wurtzite crystal structure,
where chemical etching is enhanced by surface defects serving
as attacking sites of etchant ions. Because sequential
compensation eﬀects successfully compensate for dual defectsof cation (Zn2+) and anion (O2−) in the AZO nanoﬁlms, thesequence-II-annealed samples contain much fewer defects than
the as-grown and sequence-I-annealed samples. Therefore,
defect-triggered chemical etching on the Zn-terminated surface
is considerably suppressed in the sequence-II-annealed AZO
nanoﬁlms, resulting in their superior chemical stability. The
sequential annealing method also enhances the thermal
stability of conductive AZO nanoﬁlms and does not require
the addition of a capping layer, which possibly sacriﬁces otherproperties such as surface reaction and transparency. Thus, theproposed sequential annealing method can facilitate emerging
applications using abundant AZO in harsh environments suchas biosensors operated in acidic or basic environments.

■■

ArticleASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsaelm.0c01052.
X-ray diﬀraction (XRD) data of AZO nanoﬁlms;
scanning electron microscopy (SEM) images ofsequence-II-annealed AZO nanoﬁlms; chemical stability
tests of AZO nanoﬁlms deposited on SiO2/Si substrate;and chemical stability tests of AZO nanoﬁlms after Zn
vapor annealing (PDF)■

METHODSAUTHOR INFORMATION

Corresponding Authors

Tsunaki Takahashi − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan; JST, PRESTO, Kawaguchi,
Saitama 332-0012, Japan; orcid.org/0000-0002-28408038; Email: takahashi-t@g.ecc.u-tokyo.ac.jp
Takeshi Yanagida − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan; Institute for Materials Chemistry
and Engineering, Kyushu University, Kasuga, Fukuoka 8168580, Japan; orcid.org/0000-0003-4837-5701;
Email: yanagida@g.ecc.u-tokyo.ac.jp

AZO Nanoﬁlm Fabrication. AZO nanoﬁlms were deposited on
Al2O3(110) or 100 nm SiO2/Si substrates using the PLD method.23,45
A sintered tablet (1 atom % of Al2O3 in ZnO, 99.99% purity, Kojundo
Chemical Laboratory) was utilized as a target for ablation by an ArF
excimer laser (λ = 193 nm, Coherent, COMPex Pro). The chamber
for deposition was initially evacuated below 5 × 10−6 Pa, following
which oxygen gas (99.9999% purity) was introduced. The oxygenpressure, growth temperature, and deposition rate were set to 10−1 Pa,
300 °C, and 100 nm/h, respectively. A part of the deposited nanoﬁlms
was then sequentially annealed under two types of conditions: (i)ambient air, 500 °C, and 1 h or (ii) Zn vapor, 600 °C, and 1 h. For Zn
vapor annealing, samples were placed in a small crucible with a Zn
tablet (99.99% purity, Kojundo Chemical Laboratory) and maintained at 0.1 Pa at 600 °C for 1 h.28 The distance between the samples
and the Zn tablet was 3 cm, and the AZO-deposited side of thesample substrates faced downward to prevent direct Zn deposition on
the AZO surface. The crucible was then sealed in a quartz tube andevacuated to 0.1 Pa, followed by a temperature increase to 600 °C at aheating rate of 20 °C/min. In sequence I annealing, the samples wereannealed ﬁrst in Zn vapor atmosphere and then in air (Zn vapor/air).
In sequence II annealing, the samples were annealed ﬁrst in air andthen in Zn vapor atmosphere (air/Zn vapor). The AZO nanoﬁlms
were deposited on a glass substrate for UV−vis spectroscopy.

Authors

Ruolin Yan − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Tokyo 1138656, Japan; Institute for Materials Chemistry andEngineering, Kyushu University, Kasuga, Fukuoka 816-8580,
Japan
Hao Zeng − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Tokyo 1138656, Japan960

https://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962

ACS Applied Electronic Materials

pubs.acs.org/acsaelm

Takuro Hosomi − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan; JST, PRESTO, Kawaguchi,
Saitama 332-0012, Japan; orcid.org/0000-0002-56496696
Masaki Kanai − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Tokyo 1138656, Japan; Institute for Materials Chemistry andEngineering, Kyushu University, Kasuga, Fukuoka 816-8580,
Japan
Guozhu Zhang − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Tokyo 1138656, Japan
Kazuki Nagashima − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan; JST, PRESTO, Kawaguchi,
Saitama 332-0012, Japan; orcid.org/0000-0003-0180816X
Complete contact information is available at:https://pubs.acs.org/10.1021/acsaelm.0c01052

Environmentally Friendly Glass Panels. J. Alloys Compd. 2017, 719,
271−280.
(9) Liu, M.; Nam, C. Y.; Black, C. T.; Kamcev, J.; Zhang, L.
Enhancing Water Splitting Activity and Chemical Stability of Zinc
Oxide Nanowire Photoanodes with Ultrathin Titania Shells. J. Phys.
Chem. C 2013, 117, 13396−13402.
(10) Liu, C. F.; Lu, Y. J.; Hu, C. C. Effects of Anions and PH on the
Stability of ZnO Nanorods for Photoelectrochemical Water Splitting.
ACS Omega 2018, 3, 3429−3439.
(11) Gerischer, H. On the Stability of Semiconductor Electrodes
against Photodecomposition. J. Electroanal. Chem. Interfacial Electrochem. 1977, 82, 133−143.
(12) Zhou, Z.; Lan, C.; Wei, R.; Ho, J. C. Transparent Metal-Oxide
Nanowires and Their Applications in Harsh Electronics. J. Mater.
Chem. C 2019, 7, 202−217.
(13) Kolhe, P. S.; Shinde, A. B.; Kulkarni, S. G.; Maiti, N.; Koinkar,
P. M.; Sonawane, K. M. Gas Sensing Performance of Al Doped ZnOThin Film for H2S Detection. J. Alloys Compd. 2018, 748, 6−11.
(14) Benwadih, M.; Coppard, R.; Bonrad, K.; Klyszcz, A.; Vuillaume,
D. High Mobility Flexible Amorphous IGZO Thin-Film Transistors
with a Low Thermal Budget Ultra-Violet Pulsed Light Process. ACS
Appl. Mater. Interfaces 2016, 8, 34513−34519.
(15) Tomeda, A.; Ishibe, T.; Taniguchi, T.; Okuhata, R.; Watanabe,K.; Nakamura, Y. Enhanced Thermoelectric Performance of GaDoped ZnO Film by Controlling Crystal Quality for Transparent
Thermoelectric Films. Thin Solid Films 2018, 666, 185−190.
(16) Li, L.; Hosomi, D.; Miyachi, Y.; Hamada, T.; Miyoshi, M.;
Egawa, T. High-Performance Ultraviolet Photodetectors Based on
Lattice-Matched InAlN/AlGaN Heterostructure Field-Effect Transistors Gated by Transparent ITO Films. Appl. Phys. Lett. 2017, 111,
No. 102106.
(17) Huang, T.-H.; Yang, P.-K.; Lien, D.-H.; Kang, C.-F.; Tsai, M.L.; Chueh, Y.-L.; He, J.-H. Resistive Memory for Harsh Electronics:
Immunity to Surface Effect and High Corrosion Resistance via
Surface Modification. Sci. Rep. 2015, 4, No. 4402.
(18) Rotella, H.; Mazel, Y.; Brochen, S.; Valla, A.; Pautrat, A.;
Licitra, C.; Rochat, N.; Sabbione, C.; Rodriguez, G.; Nolot, E. Role of
Vacancy Defects in Al Doped ZnO Thin Films for Optoelectronic
Devices. J. Phys. D: Appl. Phys. 2017, 50, No. 485106.
(19) Nomoto, J.; Makino, H.; Nakajima, T.; Tsuchiya, T.;
Yamamoto, T. Improvement of the Properties of Direct-Current
Magnetron-Sputtered Al-Doped ZnO Polycrystalline Films Containing Retained Ar Atoms Using 10-Nm-Thick Buffer Layers. ACS
Omega 2019, 4, 14526−14536.
(20) Nebatti, A.; Pflitsch, C.; Atakan, B. Unusual Application of
Aluminium-Doped ZnO Thin Film Developed by Metalorganic
Chemical Vapour Deposition for Surface Temperature Sensor. Thin
Solid Films 2017, 636, 532−536.
(21) Ghosh, J.; Ghosh, R.; Giri, P. K. Tuning the Visible
Photoluminescence in Al Doped ZnO Thin Film and Its Applicationin Label-Free Glucose Detection. Sens. Actuators, B 2018, 254, 681−
689.
(22) Reed, P. J.; Mehrabi, H.; Schichtl, Z. G.; Coridan, R. H.
Enhanced Electrochemical Stability of TiO 2 -Protected, Al-DopedZnO Transparent Conducting Oxide Synthesized by Atomic Layer
Deposition. ACS Appl. Mater. Interfaces 2018, 10, 43691−43698.
(23) Yan, R.; Takahashi, T.; Kanai, M.; Hosomi, T.; Zhang, G.;
Nagashima, K.; Yanagida, T. Unusual Sequential Annealing Effect inAchieving High Thermal Stability of Conductive Al-Doped ZnO
Nanofilms. ACS Appl. Electron. Mater. 2020, 2, 2064−2070.
(24) Kim, I. H.; Ku, D. Y.; Ko, J. H.; Kim, D.; Lee, K. S.; Jeong, J.-h.;Lee, T. S.; Cheong, B.; Baik, Y.-J.; Kim, W. M. Improvement of the
Thermal and Chemical Stability of Al Doped ZnO Films. J.
Electroceram. 2006, 17, 241−245.
(25) Olvera, M. L.; Maldonado, A.; Asomoza, R. Chemical Stabilityof Doped ZnO Thin Films. J. Mater. Sci.: Mater. Electron. 2005, 11, 1−
5.

Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by KAKENHI (grant nos.:
JP20H02208, JP18H01831, JP18H05243, and JP18KK0112).
T.T. was supported by JST PREST grant no. JPMJPR19M6,
Japan. T.T., G.Z., K.N., and T.Y. were supported by JST
CREST, grant no. JPMJCR19I2, Japan. T.Y. and K.N. were
supported by CAS-JSPS Joint Research Projects (grant no.
JPJSBP120187207) and Mirai R&D of JST. R.Y. was
supported by the program of China Scholarships Council(no. 201706090261). This work was performed under the
Cooperative Research Program of the “Network Joint ResearchCenter for Materials and Devices” and the MEXT Project of“Integrated Research Consortium on Chemical Sciences”.

■

REFERENCES(1) Park, J. W.; Kang, B. H.; Kim, H. J. A Review of LowTemperature Solution-Processed Metal Oxide Thin-Film Transistors
for Flexible Electronics. Adv. Funct. Mater. 2020, 30, No. 1904632.
(2) Sberveglieri, G. Recent Developments in Semiconducting ThinFilm Gas Sensors. Sens. Actuators, B 1995, 23, 103−109.
(3) Kegel, J.; Povey, I. M.; Pemble, M. E. Zinc Oxide for Solar Water
Splitting: A Brief Review of the Material’s Challenges and Associated
Opportunities. Nano Energy 2018, 54, 409−428.
(4) Jiang, X. H.; Wang, B. D.; Yang, Z. Y.; Liu, Y. C.; Li, T. R.; Liu,
Z. C. 8-Hydroxyquinoline-5-Carbaldehyde Schiff-Base as a Highly
Selective and Sensitive Al3+ Sensor in Weak Acid Aqueous Medium.
Inorg. Chem. Commun. 2011, 14, 1224−1227.
(5) Gupta, S.; Lacour, S. P. Performance of Indium Gallium Zinc
Oxide Thin-Film Transistors in Saline Solution. J. Electron. Mater.
2016, 45, 3192−3194.
(6) Benfenati, V.; Toffanin, S.; Bonetti, S.; Turatti, G.; Pistone, A.;
Chiappalone, M.; Sagnella, A.; Stefani, A.; Generali, G.; Ruani, G.;
Saguatti, D.; Zamboni, R.; Muccini, M. A Transparent Organic
Transistor Structure for Bidirectional Stimulation and Recording ofPrimary Neurons. Nat. Mater. 2013, 12, 672−680.
(7) Raghupathi, K. R.; Koodali, R. T.; Manna, A. C. Size-Dependent
Bacterial Growth Inhibition and Mechanism of Antibacterial Activityof Zinc Oxide Nanoparticles. Langmuir 2011, 27, 4020−4028.
(8) Choi, H.-J.; Pammi, S. V. N.; Park, B.-J.; Eom, J.-H.; An, H.;
Kim, H. Y.; Kim, M.; Seol, D.; Kim, Y.; Yoon, S.-G. Resistance against
Water and Acid Water (pH = 4.0) via Al-Doped ZnO Thin Films for
961

Articlehttps://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962

ACS Applied Electronic Materials

pubs.acs.org/acsaelm(26) Hüpkes, J.; Owen, J. I.; Pust, S. E.; Bunte, E. Chemical Etchingof Zinc Oxide for Thin-Film Silicon Solar Cells. ChemPhysChem
2012, 13, 66−73.
(27) Zhan, Z.; Zhang, J.; Zheng, Q.; Pan, D.; Huang, J.; Huang, F.;
Lin, Z. Strategy for Preparing Al-Doped ZnO Thin Film with High
Mobility and High Stability. Cryst. Growth Des. 2011, 11, 21−25.
(28) Hu, Q. C.; Ding, K.; Zhang, J. Y.; Yan, F. P.; Pan, D. M.;
Huang, F.; Chiou, J. W. On the Variations of Optical Property andElectronic Structure in Heavily Al-Doped ZnO Films during DoubleStep Growth Process. Appl. Phys. Lett. 2014, 104, No. 021913.
(29) Kim, H.; Gilmore, C. M.; Horwitz, J. S.; Piqué, A.; Murata, H.;
Kushto, G. P.; Schlaf, R.; Kafafi, Z. H.; Chrisey, D. B. Transparent
Conducting Aluminum-Doped Zinc Oxide Thin Films for Organic
Light-Emitting Devices. Appl. Phys. Lett. 2000, 76, 259−261.
(30) Meyer, J.; Görrn, P.; Hamwi, S.; Johannes, H.-H.; Riedl, T.;
Kowalsky, W. Indium-Free Transparent Organic Light Emitting
Diodes with Al Doped ZnO Electrodes Grown by Atomic Layer andPulsed Laser Deposition. Appl. Phys. Lett. 2008, 93, No. 073308.
(31) Cheng, C.; Ren, W.; Zhang, H. 3D TiO2/SnO2 HierarchicallyBranched Nanowires on Transparent FTO Substrate as Photoanodefor Efficient Water Splitting. Nano Energy 2014, 5, 132−138.
(32) Li, X.; Liu, S.; Fan, K.; Liu, Z.; Song, B.; Yu, J. MOF-BasedTransparent Passivation Layer Modified ZnO Nanorod Arrays forEnhanced Photo-Electrochemical Water Splitting. Adv. Energy Mater.
2018, 8, No. 1800101.
(33) Kluth, O.; Rech, B.; Houben, L.; Wieder, S.; Schöpe, G.;
Beneking, C.; Wagner, H.; Löffl, A.; Schock, H. W. Texture Etched
ZnO:Al Coated Glass Substrates for Silicon Based Thin Film Solar
Cells. Thin Solid Films 1999, 351, 247−253.
(34) Heinonen, S.; Nikkanen, J. P.; Huttunen-Saarivirta, E.;
Levänen, E. Investigation of Long-Term Chemical Stability ofStructured ZnO Films in Aqueous Solutions of Varying Conditions.
Thin Solid Films 2017, 638, 410−419.
(35) Kim, Y. H.; Lee, K. S.; Lee, T. S.; Cheong, B. ki.; Seong, T. Y.;
Kim, W. M. Electrical, Structural and Etching Characteristics of
ZnO:Al Films Prepared by Rf Magnetron. Curr. Appl. Phys. 2010, 10,
S278−S281.
(36) Sun, J.; Bian, J.; Liang, H.; Zhao, J.; Hu, L.; Zhao, Z.; Liu, W.;
Du, G. Realization of Controllable Etching for ZnO Film by NH 4 Cl
Aqueous Solution and Its Influence on Optical and Electrical
Properties. Appl. Surf. Sci. 2007, 253, 5161−5165.
(37) Gatos, H. C. Dangling Bonds in III-V Compounds. J. Appl.
Phys. 1961, 32, 1232−1234.
(38) Kato, H.; Sano, M.; Miyamoto, K.; Yao, T. High-Quality ZnO
Epilayers Grown on Zn-Face ZnO Substrates by Plasma-Assisted
Molecular Beam Epitaxy. J. Cryst. Growth 2004, 265, 375−381.
(39) Zhang, W.; Bunte, E.; Luysberg, M.; Spiekermann, P.; Ruske,F.; Hüpkes, J. Pretreatment of Glass Substrates by Ar/O2 Ion Beams
for the as-Sputtered Rough Al Doped Zinc Oxide Thin Films. Surf.
Coat. Technol. 2011, 205, S223−S228.
(40) Lee, S. H.; Minegishi, T.; Park, J. S.; Park, S. H.; Ha, J.-S.; Lee,
H.-J.; Lee, H.-J.; Ahn, S.; Kim, J.; Jeon, H.; Yao, T. Ordered Arrays of
ZnO Nanorods Grown on Periodically Polarity-Inverted Surfaces.
Nano Lett. 2008, 8, 2419−2422.
(41) Valtiner, M.; Borodin, S.; Grundmeier, G. Stabilization andAcidic Dissolution Mechanism of Single-Crystalline ZnO(0001)
Surfaces in Electrolytes Studied by in-Situ AFM Imaging and ExSitu LEED. Langmuir 2008, 24, 5350−5358.
(42) Jo, W.; Kim, S.-J.; Kim, D.-Y. Analysis of the Etching Behavior
of ZnO Ceramics. Acta Mater. 2005, 53, 4185−4188.
(43) Owen, J. I.; Hüpkes, J.; Zhu, H.; Bunte, E.; Pust, S. E. Novel
Etch Process to Tune Crater Size on Magnetron Sputtered ZnO:Al.
Phys. Status Solidi A 2011, 208, 109−113.
(44) Oba, F.; Kumagai, Y. Design and Exploration of Semiconductors from First Principles: A Review of Recent Advances. Appl.
Phys. Express 2018, 11, No. 060101.
(45) Kodu, M.; Arroval, T.; Avarmaa, T.; Jaaniso, R.; Kink, I.;
Leinberg, S.; Savi, K.; Timusk, M. Effect of Oxygen on Active Al

Concentration in ZnO:Al Thin Films Made by PLD. Appl. Surf. Sci.
2014, 320, 756−763.

962

Articlehttps://dx.doi.org/10.1021/acsaelm.0c01052
ACS Appl. Electron. Mater. 2021, 3, 955−962