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Review

Robust and Electrically Conductive ZnO Thin Films andNanostructures: Their Applications in Thermally and ChemicallyHarsh Environments
Ruolin Yan, Tsunaki Takahashi,* Hao Zeng, Takuro Hosomi, Masaki Kanai, Guozhu Zhang,
Kazuki Nagashima, and Takeshi Yanagida*

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ABSTRACT: “Harsh electronics”, which are designed to operate under harsh environments,
have garnered signiﬁcant attention to collect various physical and chemical information insurroundings toward the Internet of Things era. Among various electronic materials andstructures, ZnO thin ﬁlms, which consist of an abundant resource, have been intensively
investigated because of their unique electrical and optical properties. However, ZnO thinﬁlms have been regarded as chemically nonresistive to harsh environments (e.g., hightemperatures, high humidity, and acidic and basic conditions). Herein, we present recent
progress and advances in electrically conductive ZnO thin ﬁlms and nanostructures forapplications in harsh electronics. First, various fabrication methods and progresses forachieving high-quality ZnO nanomaterials are introduced. Subsequently, previously reported
approaches for enhancing the reliability and stability of ZnO nanostructures in harshelectronics are compared. Strategies for fabricating robust ZnO materials and ZnO-based
electronics are discussed on the basis of several proposed mechanisms. Finally, we describe
the current limitation, perspective, and outlook for future developments of ZnOnanostructures for use in harsh electronics.
KEYWORDS: conductive ZnO nanoﬁlms, thermal stability, chemical stability, harsh electronics, transparent conductive oxides
materials at high temperatures.14,15 Furthermore, in open-top
applications such as chemical sensors, chemical reactions withchemically active species in atmospheric air (e.g., O2, H2O, andCO2) must be suppressed at high operating temperatures.16−18
Chemical stability is another essential challenge for materials
used in harsh electronics. Biosensors and photoelectrochemical
devices (e.g., water splitting and CO2 reduction) require stableelectronic materials under a wide pH range from acidic to basic
conditions.19−21
Among the various electronic materials, ZnO thin ﬁlms andnanostructures have been intensively investigated for many
decades because of their unique electrical and opticalproperties, including wide bandgap (∼3.7 eV), tunable
semiconducting electrical conduction via impurity doping,high exciton binding energy (60 meV), and high visible-lighttransmission.22,23 As impurity ions for tuning electrical

1. INTRODUCTION“Harsh electronics”, which are electronic devices designed tooperate in harsh environments, have garnered signiﬁcantattention owing to increasing demands for chemical imaging,energy harvesting, and conventional electrical applications in
extreme conditions.1−6 In these harsh electronics, such as bioand chemical sensors, stable electronic materials in harshenvironments (e.g., high temperature, high humidity, chemically active medium) are necessitated. The intrinsic stability ofelectronic materials under harsh conditions is diﬃcult to securein harsh electronics because conventional encapsulation
packaging cannot be used in bio- and chemical sensors,which must be exposed to the surroundings.7−10 The thermal
stability of the electrical properties is one of the mostimportant requirements for materials used in harsh electronics
because chemical sensor, chemical industry, and aerospace
applications are typically operated at temperatures exceeding
200 °C.11−13 Wide bandgap semiconductors are suitable forelectron devices operated at high temperatures because theintrinsic carrier density increases signiﬁcantly with temperaturein conventional narrow bandgap semiconductors (Si, III−V).
Morphological changes such as crystallization and phasetransition can cause the electrical degradation of electronic© 2021 American Chemical Society

Received: May 12, 2021
Accepted: July 2, 2021
Published: July 16, 2021

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conductivity of ZnO, Al has been intensively studied becauseof its high electrical conductivity, visible-light transmittance,
and abundance of resources of Al-doped ZnO (AZO). Thus,
AZO is a promising alternative to indium tin oxide (ITO),
which is mass-produced transparent oxide electric material. In
addition to these attractive properties, the abundance ofresources of pure and doped ZnO is advantageous for rareelement-free electronics.24,25 However, ZnO thin ﬁlms andnanostructures have been widely known as thermally andchemically unstable because of the instability of mobile carriers
and their amphoteric nature, which limit their utilization inharsh electronics.26−28 In recent years, a lot of eﬀort has been
devoted to achieving stable ZnO thin ﬁlms and nanostructures
toward applications in harsh environments, including chemicalsensors, biosensors, and photoelectrochemical devices.26,29−33
Some promising strategies for stable and conductive ZnO havebeen proposed, accelerating developments of ZnO-based harshelectronics.
Herein, we present a comprehensive review of strategies forenhancing the thermal and chemical stability of electricallyconductive ZnO thin ﬁlms and nanostructures for applicationsin harsh environments. Among a wide variety of harshenvironments such as high or low temperatures, acidic orbasic solutions, high humidity, radiation, and vibration, we
focus on the thermal and chemical stability because of their
particular importance related to promising applications of ZnO(chemical sensors, biosensors, and photoelectrochemical
applications). First, various fabrication methods for ZnO thinﬁlms and nanostructures are reviewed in terms of their quality(e.g., feature size and crystallinity) and parameters forcontrolling their electrical conductivity. Subsequently, strategies for achieving robust and conductive ZnO thin ﬁlms are
introduced and compared with the proposed mechanisms forenhancing thermal and chemical stability. Strategies forenhancing the stability of conductive ZnO thin ﬁlms andnanostructures can be categorized into defect compensation,
impurity doping, and capping layers. Moreover, studies
regarding applications using robust ZnO under harsh environments were performed. Finally, we present our conclusions andoutlook regarding the current limitations as well as further
research directions for robust and conductive ZnO thin ﬁlms
and nanostructures toward harsh electronics.

observed at 34°, which corresponds to the (002) orientation of
ZnO.50 Two typical sputtering methods are used depending on theproperties of the target. The DC voltage is applied between thecathode and anode for conductive materials, whereas a high-frequency
RF generator is applied to nonconductive materials.50 In both DC andRF sputtering, a magnetic ﬁeld is typically used to conﬁne charged
plasma to the target surface. For the oxidation of the metallic Zntarget, a gas mixture comprising Ar and O2 is introduced into a
sputtering chamber, where the ratio of Ar and O2 signiﬁcantly aﬀects
the properties of the deposited ZnO thin ﬁlms.51
2.1.1. DC-Reactive Sputtering. In DC sputtering, the characteristics of the deposited ﬁlms are associated closely with the substratematerial, Ar/O2 partial pressure, substrate temperature, and sputteringtargets. Among these parameters, the eﬀects of oxygen partial pressure
and substrate temperature have been intensively investigated. ZnO
deposited on Al foil, Si, and corning glass exhibits polycrystalline(0001) crystallographic orientation.52−54 It was shown that thesurface roughness increased and the grain size decreased as the O2
partial pressure decreased in cases involving a Zn metallic target and a
ZnO ceramic target.52 A systematic study by Ellmer et al. revealed
that both the carrier mobility and concentration increase with thedecrease of O2 partial pressure.54 Moreover, it was revealed that boththe carrier mobility and concentration increased with O2 partialpressure and that the lowest electrical resistivity was achieved using a
low O2 partial pressure of approximately 7 × 10−4 mbar.52
Furthermore, it was discovered that the crystallinity increased when
the substrate temperature was increased up to 200 °C; subsequently,
the crystallinity decreased when the substrate temperature exceeded
200 °C.55 To enhance the electrical conductivity and opticaltransparency, an Al dopant is typically introduced into ZnO. Huang
et al. demonstrated that the crystallinity increased as the substratetemperature increased up to 250 °C in DC sputtering and the increase
became indistinctive in the range of 250 to 300 °C.56 Al-doped ZnO(AZO) deposited at a temperature higher than 250 °C demonstrated
improved crystallinity, which can be attributed to the suﬃcient
diﬀusion of atoms and growth of grains. A similar tendency has beenreported in an earlier study.55 Notably, AZO deposited at a hightemperature demonstrated enhanced thermal stability, which is
attributed to the suppression of oxygen absorption and dopant
activation around grain boundaries due to the higher crystallinity. The
AZO thin ﬁlms deposited at 300 and 350 °C exhibited much higher
electrical stability than those prepared at lower substrate temperatures
after 30 min of annealing at 400 °C in ambient air. Moreover,Nomoto et al. reported that AZO deposited via DC sputtering with athin buﬀer layer showed eﬀective enhancement in the c-planeorientation and carrier transport properties.47 Adatoms with excess
free energy on the deposition surface can be incorporated into thebuﬀer layer during deposition. Thus, the buﬀer layer suppresses the
movement of these adatoms to grain boundaries, resulting in apreferential formation of (0001) orientation and reducing the residualcompressive stress in the ﬁlm.
2.1.2. RF Sputtering. In RF sputtering, the charging up of
insulating targets during deposition can be suppressed.57,58 Because itis easier to achieve stable deposition conditions using insulating ZnOtargets than using metallic Zn targets, RF sputtering has been typicallyutilized to deposit ZnO thin ﬁlms.59 Dominant deposition parametersin RF sputtering include substrate temperature, Ar/O2 gas pressure,RF power, and the distance between the target and substrate.
Prepelita et al. demonstrated that ZnO thin ﬁlms exhibited apolycrystalline structure with (0001) surface orientation on Kapton
ﬁlms, glass, and Si substrates. Furthermore, ZnO deposited on Kapton
ﬁlms demonstrated a more prominent orientation with smaller
crystallites and a higher surface roughness compared with the othermaterials investigated.60 According to Besleaga et al., ZnO thin ﬁlms
deposited at 400 °C showed higher crystallinity than another sample
deposited at 200 °C.59 Regarding the deposition atmosphere, Lee etal. reported the optimal condition of a 50% O2 concentration in an
Ar/O2 mixture, which aﬀorded ZnO ﬁlms with a good c-axis-preferredorientation, surface roughness (root-mean-square, RMS) of 1.1 nm,
and resistivity of 106 Ω·cm.61 In a pure Ar atmosphere, ZnO thin ﬁlms

2. FABRICATION METHODS
Various fabrication methods have been investigated to obtain highquality ZnO thin ﬁlms and nanostructures on substrates. Thesemethods can be classiﬁed into the following three types:34−36 (a)physical vapor deposition, including thermal evaporation, molecularbeam epitaxy, sputtering, and laser ablation;37−40 (b) chemical vapor
deposition (CVD), including metal−organic CVD, mist CVD, hot
ﬁlament CVD, and atomic layer deposition;34,41,42 (c) chemicalsolution synthesis including sol−gel synthesis, hydrothermal growth,and spray pyrolysis.43−46 Generally, the parameters of these synthesis
methods are optimized to obtain ZnO thin ﬁlms and nanostructureswith the required physical and chemical properties, high stability, andcompatibility using various substrates. However, parameter optimization generally involves a trade-oﬀ between the required properties.
For example, the introduction of dopant elements for high electricalconductivity causes material instability at high temperatures.15 To
achieve the desired properties (e.g., high electrical conductivity) withlong-term stability, signiﬁcant eﬀort has been expended.
2.1. Sputtering. Sputtering is a widely used method for
fabricating ZnO thin ﬁlms as it enables controllable compositionand large-scale deposition.47−49 Single-phase hexagonal ZnO is
obtained with optimized parameters. An X-ray diﬀraction peak is
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pubs.acs.org/acsaelmwith higher crystallinity were obtained at higher Ar pressures in therange from 0.25 to 0.45 Pa.59 Additionally, a relatively low O2 partialpressure was utilized to deposit AZO thin ﬁlms with low electricalresistivity because of the suppression of Zn vacancy (VZn)
formation.62 Unlike the resistance of AZO thin ﬁlms deposited atroom temperature and 200 °C, that of AZO thin ﬁlms deposited at
400 °C increased signiﬁcantly.63 A decrease in the carrier mobility wasobserved in AZO deposited at 400 °C, which resulted in degradedelectrical properties. Lee et al. investigated the eﬀect of RF power on
AZO thin ﬁlms deposited on glass and polycarbonate (PC)substrates.64 As the RF power increased, the resistivity of AZO thinﬁlms on the PC substrates increased, whereas that on glass substrates
decreased.
2.2. CVD. 2.2.1. Metal−Organic Chemical Vapor Deposition(MOCVD). In MOCVD, reactants of metal−organic precursors are
introduced to a deposition chamber by a carrier gas.35 The selectionof appropriate precursors is essential because of the close relationshipbetween the prereaction of the precursors and the properties of thedeposited ZnO thin ﬁlms. Dimethyl zinc (CH3)2Zn (DMZn) or
diethyl zinc (C2H5)2Zn (DEZn) are typically used as Zn precursors,
although other precursors have been investigated.51 By using DMZn
and oxygen, the grown ZnO exhibit a highly (0001)-textured wurtzite
structure. The broad deep-level emission bands around 2.48 or 2.36
eV were observed.65
Boukadhaba et al. revealed the morphology of ZnO grown via
MOCVD at a high temperature of 900 °C with helium as the carriergas. DEZn and nitrous oxide N2O were selected as zinc and oxygen
sources, respectively.66 XRD patterns of the ZnO thin ﬁlms grown on
c-plane sapphire substrates exhibited the (0001) orientation with thewurtzite phase. Kirchner et al. investigated the eﬀects of reactor
pressure and deposition temperature on ZnO growth rate; they
discovered that the ZnO growth rate was proportional to the reactor
pressure and reached a maximum rate at 400 mbar.41 In contrast, thedeposition temperature imposed a diﬀerent eﬀect on the growth rate.
In depositions where tert-butyl alcohol was used as a precursor, thehighest growth rate of 2.2 μm/h was achieved at a wide temperature
range from 380 to 510 °C. In depositions where isopropyl alcohol wasused as a precursor, the temperature range for the high growth rate
reduced to approximately 380 °C. Heinze et al. systematicallyinvestigated the eﬀects of temperature, total carrier gas pressure, andoxygen/zinc ratio on the morphology of deposited ZnO;67 they
analyzed the eﬀect of growth temperature in the range of 800−1000°C and demonstrated that lower temperatures degraded the surface
morphology. In addition, a smooth and compact surface morphology
was observed in ZnO thin ﬁlms deposited at 1000 °C and 200 mbar.
For the precursor oxygen/zinc atomic ratio of 400, the dominantsurface morphology was hexagonal pyramids with a rough surface
between the pyramids. It is noteworthy that the surface morphology
can be improved signiﬁcantly by increasing the oxygen/zinc atomic
ratio to 25 000. Zhang et al. reported that ZnO thin ﬁlms with a 15
nm ZnO buﬀer layer grown via low-pressure MOCVD showed
optimized crystallinity and smooth surface morphology comparedwith samples without a buﬀer layer or with a thicker buﬀer layer.68
Illiberi et al. obtained AZO using trimethylaluminum (Al(CH3)3,
TMA) as a dopant gas. Their results indicated that the grain size
decreased when the thin ﬁlms were deposited at higher temperatures
because of the occurrence of a secondary gas phase and/or surface
desorption reactions.69 The resistivity of the deposited AZO thin ﬁlms
decreased as the thickness increased, and a low resistivity of 2 × 10−3
Ω·cm was achieved in 260 nm-thick ﬁlms.
2.2.2. Mist CVD. Mist CVD is a simple, environmentally friendly,
and cost-eﬀective method for fabricating ZnO thin ﬁlms andnanostructures. A water or alcohol solution of zinc species is usedas the source, and aerosol or mist particles formed via ultrasonicatomization are introduced to the reaction area by a carrier gas.70,71
The dominant parameters in mist CVD include the Zn source type,
carrier gas ﬂow rate, growth temperature, and growth pressure.72
Instead of DEZn and DMZn, which are typically used in MOCVD,
zinc acetate (ZA) is generally used as an alternative zinc source inmist CVD, owing to its advantages of low cost and low reactivity (lessdangerous).72,73 Pure ZnO thin ﬁlms with high transparency havebeen prepared via mist CVD using an aqueous solution of ZA. SEM
observations showed that the ZnO obtained using a Zn source of
0.0050 mol/L concentration in ZA solutions showed smaller grains
than that using a Zn source of 0.0025 mol/L concentration.73 Mist
CVD allowed a relatively low-temperature growth of ZnO thin ﬁlms
compared with the MOCVD method, and the prepared ZnO thinﬁlms showed a mixture of (100), (002), and (101) planes.72 It was
revealed that ZnO thin ﬁlms grown at 300 °C exhibited no
preferential orientation, whereas ZnO grown at 450 °C exhibited ahighly c-axis-oriented polycrystalline structure. However, the preferential growth along the [0002] direction became insigniﬁcant in ZnOgrown at 500 °C.
Lu et al. deposited n-type AZO and p-type N-doped ZnO thinﬁlms.72 The n-type AZO ﬁlms showed a low resistivity of 1.08 × 10−3
Ω·cm at the optimal Al content of 4 at. %. When the Al concentration
was varied, the maximum carrier concentration value of 9.4 × 1020
cm−3 was achieved at 4 at. %. Meanwhile, the carrier mobility
decreased gradually from 7.2 to 1.7 cm2/V·s as the Al concentration
increased from 2 to 6 at. %. Moreover, p-type conductivity wasreported in ZnO/N thin ﬁlms with a resistivity of 930 Ω·cm, and theresistivity reduced to 72.8 Ω·cm via thermal annealing.
2.2.3. Atomic Layer Deposition (ALD). ALD is an ideal techniquefor depositing an extremely thin composite ﬁlm at a low temperature.35,74,75 During ALD, the precursors are sequentially introduced
into the reactor, and between the precursor pulses, the reactor ispurged by an inert gas.74 Therefore, the precursors can only react withthe surface of the substrate, and the thin ﬁlms can be deposited in alayer-by-layer manner. The thickness of the ﬁlms can be controlled byvarying the number of reaction cycles to yield a precise growth ofultrathin layers and excellent stoichiometry. In ALD, the surface
chemistry and nucleation mechanisms are critical aspects.
Baji et al. investigated the nucleation and growth modes on various
substrates at various temperatures.74 The morphology of the ZnO
layers deposited on GaN substrates was determined by the substratemorphology. For the ZnO layers deposited on Si substrates, islandlike growth was observed at all growth temperatures investigated.
Furthermore, the morphology of the ZnO layers deposited on
sapphire substrates can be tuned by varying the growth temperature.
At 150 °C, the morphology of the ZnO layers on sapphire substrates
was similar to that on Si substrates, whereas layer-by-layer growthoccurred at temperatures exceeding 220 °C in the c-axis growthorientation. Banerjee et al. investigated the eﬀects of ALD parameterson the properties of AZO thin ﬁlms76 and obtained optimized
parameters that yielded conductive (4.5 × 10−3 Ω·cm) andtransparent AZO thin ﬁlms for 3 at. % Al doping. Illiberi et al. grew
In-doped ZnO by varying In content via ALD.77 AZO ﬁlms with low
In content (In/(In + Zn] < 15%) showed a polycrystalline structure,
which was characterized by the presence of diﬀerent crystallographic
orientations, including (100), (002), (101), and (110).
2.3. Pulsed Laser Deposition (PLD). PLD is suitable for material
investigations of high-quality thin ﬁlms because of its wide variety of
independent deposition parameters, such as target species, oxygen
partial pressure, substrate temperature, laser energy, ﬂux intensity, andpulse frequency.78−80 Similar to sputtering, oxygen partial pressure
signiﬁcantly aﬀects the electrical properties of deposited thin ﬁlms.81
In n-type oxide semiconductors such as ZnO, low oxygen partialpressure, which generates oxygen vacancies as electron donors, isfavorable for achieving low resistivity. Oxygen partial pressure also
inﬂuences the direct band gap of ZnO. The direct band gap energy ofgrown ZnO thin ﬁlms decreases as oxygen partial pressure increases.82
The correlation between oxygen partial pressure and band gap is
attributed to the change in carrier density in addition to the Moss−
Burstein shift. The substrate temperature is a crucial parameter fordepositing high-quality conductive ZnO thin ﬁlms. Using a ZnOtarget, polycrystalline ZnO thin ﬁlms with low surface roughness canbe grown at relatively low substrate temperatures of 200−400 °C.83 In
contrast, depositions using a Zn target require high substratetemperatures of 400−750 °C to fully oxidize metal Zn.84 Althoughhigh substrate temperatures can eﬀectively improve crystallinity, the2927

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Review

deposition rate decreases as the substrate temperature increases. Zenget al. investigated the eﬀects of PLD deposition parameters on theproperties of ZnO thin ﬁlms from room temperature to 750 °C on Si
substrates, demonstrating a highly c-axis-oriented hexagonal wurtzite
structure except for the thin ﬁlms deposited at room temperature and
100 °C.79 Liu et al. reported similar results using Si (111)substrates.78 The grain size increased with the deposition temperatureowing to enhanced atomic diﬀusion on the surface at hightemperatures.
Dopant species can be varied using a composite target for PLD, andAl is one of the most attractive dopants in ZnO owing to its highconductivity and visible-light transmittance as well as its abundance as
an elemental source. Various conditions of AZO thin ﬁlm deposition
via PLD are listed in Table 1, where Al conc., ρmin, Tsub, and PO2

Table 1. Various Experimental Conditions of ZnO Thin
Films Deposited via PLD
Al conc.

ρmin (Ω·cm)

Tsub
(°C)

PO2 (Pa)

thickness(Å)

ref1−2 at. %3 wt %
2 wt %
1.8 wt %0.8 wt %2 wt %
2 wt %

9.7 × 10−4
1.9 × 10−4
6.6 × 10−3
8.5 × 10−5
3.7 × 10−4
1.0 × 10−3
5.2 × 10−3

300
300
600
230
200
150
350

1.0 × 10−1
4.0 × 10−2
0
3 × 10−4
6.7 × 10−1
11
1.3

∼1000
∼3000
N/A
∼2800
∼3000
N/A
∼1100

86
14
15
85
88
40
89

Figure 1. Eﬀects of PLD parameters on resistivity of AZO thin ﬁlms,
ρAZO. (a) Oxygen pressure and growth temperature, (b) Al atomic
concentration, CAl, in AZO target, (c) ﬁlm thickness, dt, and (d)deposition rate, vd. As standard parameters, oxygen pressure, growthtemperature, CAl, dt, and vd were set to 0.1 Pa, 300 °C, 1 at. %, 100
nm, and 100 nm/h, respectively. Reproduced from ref 87. Copyright2020 American Chemical Society.

denote the Al dopant concentration, minimum reported resistivity,
substrate temperature, and oxygen partial pressure, respectively. Aguraet al. reported AZO thin ﬁlms with a low resistivity of 10−5 Ω·cm anda visible-light transmittance of 88% by optimizing the parameters.85
When the substrate temperature was changed from 250 to 500 °C, it
was demonstrated that the carrier concentration decreased slightly as
the temperature increased. In contrast, the Hall mobility ﬁrst
increased from 8.49 to 22.6 cm2/(V s) when the temperature
increased from 250 to 400 °C and then decreased to 9.88 cm2/V·swhen the temperature further increased to 500 °C.14 A lowest
resistivity of 2.03 × 10−4 Ω·cm was obtained at 350 °C. Meanwhile,
Kodu et al. discussed the mechanisms by which oxygen partialpressure aﬀected Al dopant activation.86 They discovered that thecarrier concentration increased and the mobility decreased as theoxygen partial pressure decreased. This is because the O2-deﬁcientenvironment induced oxygen vacancies and Al interstitials, both ofwhich served as electron donors. The mobility degradation was due tothe increase in carrier scattering caused by ionized impurities. Yan etal. systematically investigated the eﬀects of the deposition parameterson the resistivity of AZO thin ﬁlms, as shown in Figure 1. A minimum
resistivity of ∼10−4 Ω·cm was achieved for the deposition in PO2 (0.1
Pa) at Tsub = 300 °C, which is consistent with other reports.87
2.4. Solution-Based Method. 2.4.1. Sol−Gel. As a typically usedfabrication method for nanomaterials, sol−gel is promising because of
its low growth temperature (∼100 °C), enabling ZnO thin ﬁlm andnanostructure fabrication on a ﬂexible substrate. The typical sol−gel
growth of ZnO includes the following procedures: (a) preparation ofa precursor solution, (b) deposition of the precursor solution on asubstrate, and (c) heat treatment to form xerogel ZnO thin ﬁlms.90
Additionally, the sol−gel method comprises two typical routes: metal
alkoxides in organic solvents and metal salts in aqueous solutions.
Precursors of ZnO growth include nitrate, chloride, perchlorate,ethoxide, propoxide, acetylacetonate, and acetate dehydrate. Amongthese precursors, inorganic acetate dehydrates are frequently usedbecause of their facile and low-cost synthesis.
In the 1990s, ZnO thin ﬁlms were successfully synthesized throughthe sol−gel process.91−93 It was discovered that the as-synthesizedZnO ﬁlms exhibited a nonoriented polycrystalline structure with asmooth and dense surface. The ZnO ﬁlms synthesized on Pyrex glass
substrates show a polycrystalline structure with a c-axis orientation.
When annealed in air at 525 °C, the resistivity decreased from 110 Ω·

cm (as-grown ZnO) to 28.2 Ω·cm.94 These ZnO ﬁlms are transparent
in a visible-light range above the wavelength of 400 nm and show
sharp ultraviolet absorption edges at approximately 380 nm. Sagar et
al. investigated the eﬀect of the pH of solutions on the quality of ZnO
synthesized using zinc acetate in methanol, demonstrating that thehigher pH of the solution resulted in a lower surface roughness andless c-axis oriented growth.95 Compared with physical or chemical
deposition methods, such wet-chemical methods are signiﬁcantly
hindered by low crystallinity and unsatisfactory densiﬁcation. To
enhance the properties of the as-synthesized thin ﬁlms, posttreatments such as thermal treatments or ultraviolet irradiation are
generally performed. Thermal annealing at 400 °C and higher
temperature enhances the c-axis orientation of the ﬁlms.92,93 Raouﬁ
and Raouﬁ reported that the grain size of ZnO thin ﬁlms increased
from 19 nm at 300 °C annealing to 28 nm at 500 °C annealing with ahigh light transmission exceeding 80%.43 Meanwhile, Han et al.
reported that nanostructured ZnO thin ﬁlms that underwent
ultraviolet treatments exhibited improved electrical properties owing
to higher densiﬁcation and the passivation of surface oxygen
vacancies.96 Liu et al. have grown a porous ZnO nanostructured
ﬁlm by the so-gel method using polyethylene glycol as the template as
shown in Figure 2. The high transmittance of 80% was achieved forthese porous ZnO nanostructured ﬁlms.97 Duan et al. reported thatsol−gel synthesized AZO thin ﬁlms indicated a resistivity of 2.52 ×

Figure 2. SEM images of (a) gel ﬁlm and (b) ZnO ﬁlm calcined at
500 °C for 1 h. Reproduced with permission from ref 97. Copyright2008 Elsevier.
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10−1 Ω cm and a light transmission of 70% after postvacuum
annealing.98 Tabassum et al. achieved electrically stable AZO thinﬁlms under 85% damp humidity for more than 7 d using a thin metal
cap layer in the sol−gel method.99 Furthermore, they achieved
reduced resistivities using higher Al dopant concentrations, i.e., theminimum resistivity of 1.65 × 10−3 Ω·cm with 2 wt % Al dopant
concentration.
2.4.2. Hydrothermal Synthesis. Hydrothermal synthesis has been
intensively investigated for the fabrication of nanostructures at low
temperatures for applications such as light-emitting devices, solar
cells, and sensors.22,75,100,101 In the hydrothermal growth of ZnOnanostructures, several parameters, including the pH of growthsolutions, temperature, and ionic species, should be optimized toobtain the desired morphology and properties. Understanding theeﬀects of these parameters is crucial for fabricating ZnOnanostructures to be used in harsh electronics.16,44,45,102
The mechanisms of hydrothermal growth for ZnO nanostructures
have been comprehensively investigated.103,104 Podlogar et al.
synthesized (0001)-oriented 90 nm thick ZnO by heating a zincacetate solution on a glass substrate for 30 min at 300 °C. The visiblelight transmittance of these hydrothermally grown ZnO nanostructured ﬁlms varied from 82% to 72% at the wavelength of 450
nm. The electrical resistivity was measured as approximately 10−1 Ω·
cm. Note that the morphology of the ZnO nanostructures can be
designed on the basis of the growth conditions as shown in Figure3.105 Sun et al. reported the hydrothermal synthesis of crystalline ZnO

thermally harsh electronics is low electrical resistivity with highthermal stability.106 Although oxygen vacancies (VO) inwurtzite ZnO crystal induce electrical conductivity throughthe generation of mobile electrons, these VO-induced carriers
are annihilated at high temperatures in air via V O
compensation.107,108 Therefore, extrinsic ion impurity doping
in ZnO as electron donors, such as Al, Ga, and In doping, isnecessary to achieve thermally stable conductive ZnO.109−111
In addition to the increase in carrier density by doping, theincrease of carrier mobility by suppressing electron scattering
centers is eﬀective for achieving low resistivity.62 Generally, theelectrical conductivity thermal degradation of doped ZnO is
attributable to the following three factors (also shown in Figure4): (1) diﬀusion of donor ions from ZnO crystal to the outside

Review

Figure 4. Possible causes of electrical conductivity thermal
degradation of doped ZnO. (a) Diﬀusion of dopant ions from ZnO
crystal to the outside region, (b) formation of acceptor levels that trap
carriers, (c) aggregation of donor ions that form an insulator in ZnO.

region, (2) formation of acceptor levels that trap carriers, and(3) aggregation of donor ions that form an insulator in
ZnO.15,112,113 The improvement of crystallinity is eﬀective forenhancing thermal stability because all three above-mentioned
factors are promoted by defects in ZnO.86,114−116 However,crystal defects, including interstitial ions, vacancies, and lattice
distortion, are inevitably incorporated in impurity-doped ZnO
thin ﬁlms because of the charge neutrality principle. Zhan et al.
systematically investigated the electrical properties of AZO
thin ﬁlms deposited using magnetron sputtering.15 Theysuggested a growth model of AZO thin ﬁlms that depended
on the oxygen partial pressure, on the basis of the charge
neutrality principle during the sputtering process. In a nooxygen condition, most Al dopants form interstitial Al0 andhence generate free carriers. Insulative Al2O3 formation caused
by interstitial Al diﬀusion degrades the thermal stability ofelectrical conductivity at high temperatures. In an oxygen-lesscondition such as 0.1 Pa of oxygen partial pressure, Al candirectly substitute Zn as Al3+. In this case, oxygen vacancies
and other lattice defects cannot satisfy charge neutrality,
thereby resulting in low thermal stability and low mobility. In
an oxygen-rich condition where VO is suﬃciently compensated,the formation of zinc vacancies (VZn) and Al2O3 is enhanced. It
is noteworthy that VZn forms an acceptor level in the bandgap
of ZnO, thereby reducing the concentration of free electrons
via trapping.80,113 The scenario above indicates that robust andconductive ZnO cannot be obtained on the basis of deposition
parameter optimization. Therefore, post-treatment of as-grown
ZnO thin ﬁlms is generally required to enhance both thermal
stability and electrical conductivity.89,117,118
3.1.1. Defect Compensation. To enhance thermal stability,
the compensation of defects in ZnO, which are typically VZn
and VO,119,120 has been investigated. Thermal annealing in a
Zn vapor atmosphere (Zn vapor annealing) has been proposed
to compensate VZn in doped ZnO thin ﬁlms, as shown in
Figure 5.15,121,122 Zn vaporizes and diﬀuses into the ZnO
crystal at a relatively low temperature because of the highvapor pressure of pure Zn and the relatively smaller atomic size

Figure 3. FE-SEM images of ZnO nanostructures grown via
hydrothermal synthesis at 90 °C for 18 h. (a) Cross-sectional and(b) top view of ZnO nanowire array grown on glass substrate frompure Zn-nitrate aqueous solution. (c) Cross-sectional and (d) top
view of ZnO nanostructured ﬁlm grown from Zn-nitrate solution withthe addition of Na-citrate. Reproduced with permission from ref 105.
Copyright 2012 John Wiley and Sons.
thin ﬁlms with Al doping.103 Through SEM images, it was shown thatthe initial process of hydrothermal growth involved the formation of
ZnO nanorods. These nanorods grew in the axial and radial directions
and then coalesced into ZnO thin ﬁlm structures. A sharp (0002)
peak was observed in the XRD patterns of the grown ZnO, indicating
the c-axis oriented crystalline structure of the ZnO thin ﬁlm. The
optical transparency of the thin ﬁlm was superior to that of aconventionally sputtered thin ﬁlm. The addition of 2% Al dopantsigniﬁcantly reduced the thin ﬁlm resistivity to 2.5 × 10−2 Ω·cm.

3. ROBUSTNESS AND STABILITY OF PROPERTIESUNDER HARSH ENVIRONMENTS
3.1. Thermal Stability. The essential requirement for
conductive ZnO thin ﬁlms and nanostructures to be used in2929

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In addition to AZO, VZn compensation treatment enhanced
the thermal stability of Ga-doped ZnO during a 500 °C, 10
min thermal stress treatment in air.120
Recently, Yan et al. reported a sequential defectcompensation eﬀect by investigating sequential annealing in
air and Zn vapor.87 Figure 6a shows several annealing
procedures investigated in their study, including thermal
annealing in air and/or Zn vapor atmosphere. Only theannealing sequence of ﬁrst in air followed by in Zn vapor (Air/Zn vapor) eﬀectively compensated both VO and VZn in AZO
thin ﬁlms, yielding stable and low resistivity AZO thin ﬁlms ofless than 10−3 Ω·cm even after thermal tolerance testing at 500°C in ambient air (Figure 6b). Photoluminescence (PL)spectra (Figure 6c,d) show low excitonic emission at
approximately 380 nm, supporting successful defect compensation via Air/Zn vapor annealing. The origin of the sequential
eﬀect was discussed on the basis of sequential energy beneﬁts
during VO and VZn compensations, which minimized thedeviation of the valence of Zn ions from the stable 2+ charge.
In addition, undoped ZnO and room-temperature-depositedZnO were annealed in air and Zn vapor sequentially, but nothermal stability enhancement was observed. This phenomenon indicates that the improvement by sequential annealing is
attributable to doping-related defects in a polycrystalline
structure.
A relatively high oxygen partial pressure during growth was
proposed to suppress VO in the doped ZnO thin ﬁlms. When
CVD was performed at an oxygen partial pressure of 7 Torr,
Ga-doped ZnO thin ﬁlms exhibited a low resistivity of ∼6 ×
10−4 Ω·cm with a high light transmission after annealing at 500°C.123 This high thermal stability was attributed to thestoichiometric structure because of the low VO of the highoxygen partial pressure (7 Torr) samples compared with

Figure 5. Schematic illustration of Zn vapor annealing treatment.

of Zn (13 Å). In typical Zn vapor annealing, pure metal Zn isheated with ZnO thin ﬁlm samples in a vacuum or low-oxygen
partial pressure environment at a temperature of approximately
600 °C. After Zn vapor treatment, a decrease in the resistivityof AZO thin ﬁlms without observable morphological changes
has been reported, owing to the release of electrons trapped by
VZn.87 Hu et al. reported the low resistivity of AZO thin ﬁlms
via Zn vapor annealing (3.2 × 10−4 Ω·cm), which iscomparable to the best-reported value.121 More importantly,
Zn vapor annealing successfully enhanced the thermal stabilityof the electrical properties of doped ZnO, e.g., stablecharacteristics for 4 weeks at 85 °C in 85% relative humidity.15

Figure 6. (a) Flowchart of annealing procedures including sequential annealing of sequences I and II. (b) Thermal tolerance tests of resistivity ofannealed AZO thin ﬁlms at various test temperatures for 1 h in air. All resistivity measurements were performed at room temperature. (c) PL
spectrum of AZO thin ﬁlms for sequence I annealed, sequence II annealed, and as-grown samples. (d) PL spectrum of AZO nanoﬁlms for sequence
I annealed, sequence III annealed, and as-grown samples after a thermal tolerance test. Reproduced from ref 87. Copyright 2020 American
Chemical Society.
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Figure 7. (a) Optical bandgap of ZnO/Zr thin ﬁlms at various annealing temperatures. (b) Resistivity, carrier concentration, and mobility of ZnO/
Zr ﬁlms as a function of annealing temperature. Reproduced with permission from ref 136. Copyright 2016 MDPI.

NiOx26,140,141 The thickness of the capping layer is crucial
when optimizing the electrical conductivity and thermal
stability. Kim et al. reported that ZTO-coated AZO thinﬁlms indicated a low resistivity of 1.0 × 10−3 Ω·cm.142 Theirresults indicate that a 100 nm ZTO capping layer is more
eﬃcient for suppressing the thermal degradation of AZO thinﬁlms at 400 °C in ambient air compared with a 300 nm ZTO
capping layer. Furthermore, ATO has been applied as a
capping layer owing to its stable properties.140 Previous studies
suggest that an ATO capping layer prevents oxygen diﬀusionfrom ambient into ZnO thin ﬁlms, thereby suppressing thecompensation of electron-donor V O at high temperatures.140,143 When the ATO capping layer thickness (130nm) was optimized, thermally stable electrical properties up to
450 °C in the air for 15 min were achieved. A stable and highly
transparent AZO was achieved using an extremely thin Al
capping layer (2.2 nm).141 The resistivity of the Al/AZO
bilayer thin ﬁlms was stable in air at 400 °C for at least 1 h
without changes in light transmittance. The desorption spectraof these samples indicate that the thin Al capping layer
suppressed Zn desorption, which occurred at approximately
320 °C in the AZO thin ﬁlms without a capping layer. Felizco
et al. investigated ZnO thin ﬁlms sandwiched between Hfdoped or Ti-doped ZnO, demonstrating that the Hf-doped
ZnO capping layer enhanced the electrical conductivity
thermal stability during a thermal cycle test at 178 °C.129
Hüpkes et al. reported a speciﬁc annealing procedure to growhighly stable AZO ﬁlms.144 In this study, a Si capping layer wasﬁrst deposited on as-deposited AZO thin ﬁlms; subsequently,
the bilayer ﬁlms were annealed at 650 °C for 24 h, followed by
NF3 plasma etching to remove the Si capping layer. The AZO
thin ﬁlms grown via this method exhibited improved electricalproperties with a low resistivity of 2.0 × 10−4 Ω·cm and highcarrier mobility of 70 cm2/(V s) even after 1000 h of dampheat treatment (Figure 8). In addition to a single capping layer,
a multiple capping layer structure is proposed herein. Zhou etal. fabricated transparent multiple-layer electrodes of AZO/Al/
Ag/Al/AZO stacked structures, which exhibited stable
resistivity at 400 °C for 10 min in ambient air.145
3.1.4. UV Ozone. An ultraviolet ozone (UV ozone)
treatment is an eﬀective method to remove surface
contaminants from surfaces.146 Recently, several studies
reported the eﬀect of UV ozone treatments on the stabilityof ZnO nanostructures.147−149 Mahmud et al. investigated a
ZnO electron transport layer with an UV ozone treatment,

conventional conditions (∼0.1 Torr). Several studies reported
the higher stability of hydrogenated ZnO thin ﬁlms.124−127
Zhu et al. demonstrated the eﬀect of H2 postannealing on thestability of ZnO thin ﬁlms.128 The increase in resistivity wasevaluated to be 1.7 times within 1 year for H2-annealed ZnO
thin ﬁlms. The introduced hydrogen passivated VO near thegrain boundaries and formed a VO−H complex (H0), resulting
in the suppression of oxygen adsorption on VO. It wassuggested that the formation of H0 contributed to the highstability.
3.1.2. Impurity Doping. The doping of group IV elements(Ti, Hf, Zr, etc.) has been investigated to increase theconductivity and improve the thermal stability of ZnO thinﬁlms owing to their higher electronegativity and weaker ioniccharacteristics than conventional dopant elements (Al, Ga, andIn).129−132 These group IV elements have been suggested tosuppress VO formation because of their relatively high metal−
oxygen binding energy.133,134 As shown in Figure 7, a previous
study reported that Zr-doped ZnO thin ﬁlms exhibited stable
optical and electrical properties in the temperature range of
350−550 °C.136 In this sample, the resistivity increased
signiﬁcantly under thermal stresses above 550 °C, owing to
the signiﬁcant degradation in both carrier concentration andmobility (Figure 7b). The mobility degradation is attributable
to the increase in carrier scattering at the grain boundaries.
Additionally, the formation of VZn caused by Zn evaporation athigh temperatures might have contributed to the decrease in
the carrier density. Sato et al. investigated the eﬀects ofelement IV doping (Ti, Zr, Hf, Sn, etc.) on the electricalproperties of ZnO and reported improvements in thermal
stability.135 Güntner et al. reported Ti-doped ZnO nanostructured porous ﬁlms for a isoprene-selective chemical sensor
at high relative humidity and temperature of 325 °C. The
sensor resistance decreased from 37.3 to 27.5 mΩ (area size:
15 mm × 13 mm) in 20 ppb isoprene. The similar sensor
response value to the initial one was obtained after a thermalload at 325 °C in 90% relative humidity for 100 min.137
3.1.3. Capping Layers. The evaporation of Zn atoms from
ZnO thin ﬁlms and nanostructures is a primary cause ofelectrical conductivity degradation at high temperatures.138,139
Therefore, the deposition of a capping layer on ZnO thin ﬁlms,
which suppresses Zn evaporation, has been proposed as asimple method to enhance thermal stability. Several strategies
have been reported on the basis of capping layers, including
Sb-doped SnO2 (ATO), Sn-doped ZnO (ZTO), TiOx, and2931

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demonstrating the enhancement of the electron transport
stability of ZnO.148 The UV ozone-treated ZnO perovskite
solar cells showed superior device stability: approximately 88%
of initial power conversion eﬃciency (PEC) after the 1 month
systematic degradation experiment. The UV ozone treatments
passivate the interstitial defects (oxygen vacancies) on the ZnO
surface, suppressing the water-induced degradation. Chan et al.
reported an enhancement of stability of organic photovoltaic
devices using an UV ozone-treated ZnO buﬀer layer. The dropof PEC of the UV-ZnO device was 5%, whereas that of astandard device was 35%. The enhancement was attributed tothe stable high work function of the UV-ZnO layer.149
3.2. Chemical Stability. Chemical stability is anotherchallenging issue in conductive ZnO used in chemically harshelectronics such as chemical sensors, biosensors, and watersplitting devices because of the amphoteric nature of
ZnO.1,32,150−156 It is widely accepted that ZnO thin ﬁlms
and nanostructures grown on substrates are easily dissolved inchemically active media, such as acidic or basic water
solutions.46,157,158 In aqueous solutions, zinc ion species such
as Zn2+(aq), ZnO(aq), Zn(OH)2(aq), and Zn(OH)−(aq) may
appear, as shown in the following reactions. Under acidic
conditions, the dominant reactions of solid ZnO dissolution
are shown in eqs 1 and 2. In neutral conditions at a pH ofapproximately 7, the major reactions are those shown in eqs
3−9. Under basic conditions, the reactions shown in eqs 8−11
are more critical.21(1)

ZnO(s) + H+(aq) ↔ Zn(OH)+(aq)(2)

Zn(OH)+(surf) + OH−(aq) ↔ Zn(OH)2(s)(3)

Zn(OH)2(s) ↔ Zn(OH)+(aq) + OH−(aq)(4)

Zn(OH)2(s) ↔ Zn 2 +(aq) + 2OH−(aq)(5)

Zn(OH)2(s) ↔ Zn(OH)2(aq)(6)

Zn(OH)2(s) ↔ ZnO(s) + H 2O(l)(7)

ZnO(s) + OH−(aq) + H 2O(l) ↔ Zn(OH)3−(aq)(8)

ZnO(s) + 2OH−(aq) + H 2O(l) ↔ Zn(OH)4 2 −(aq)(9)

Zn 2 +(s) + OH−(aq) ↔ Zn(OH)+(aq)(10)

Zn(OH)+(aq) + OH−(aq) ↔ Zn(OH)2(s)(11)

The surface crystal orientations of ZnO thin ﬁlms
signiﬁcantly aﬀect the chemical dissolution of the depositedZnO thin ﬁlms. Many experimental studies have reported that
deposited ZnO thin ﬁlms exhibit a c-plane ZnO (0001)
orientation on substrates such as Si, sapphire, and glass.
Valtiner et al. investigated the chemical stability of Zn- and Opolar surfaces of ZnO (0001)-oriented thin ﬁlms.159 The
dangling bonds on the Zn- and O-polar surfaces are charged as
δ+ and δ−, respectively. In aqueous solutions, the ion species
charged oppositely to the polar surface can attack the surface
dangling bonds. Therefore, H3O+ (acidic) etching and OH−(basic) etching occur on the O- and Zn-polar surfaces,
respectively (Figure 9).27

Figure 8. (a) Resistivity and (b) carrier mobility of diﬀerent Si−ZnO:
Al ﬁlms as a function of climatic chamber damp heat treatment time
applied to as-deposited ZnO/Al (solid red triangles, solid line), to
NF3 etched ZnO/Al (open green triangles, dotted line), after
deposition and NF3 etch removal of the Si capping layer (black
circles, dashed line), and after annealing under Si capping layer and
NF3 decapping (open blue squares, solid line). Reproduced withpermission from ref 144. Copyright 2014 Elsevier.

ZnO(s) + 2H+(aq) ↔ Zn 2 + + H 2O

Review

Figure 9. Surface polarity-dependent dangling bond model forchemical etching of wurtzite ZnO (0001) surfaces.

3.2.1. Defect Compensation. The sequential annealing
method, which is proposed for thermally stable AZO thin ﬁlms
(Figure 6), can eﬀectively enhance the chemical stability of the
AZO thin ﬁlms. Yan et al. reported that sequentially annealed
AZO thin ﬁlms (ﬁrst annealed in air and then in a Zn vapor
atmosphere) demonstrated superior pH tolerance in a wide pH
range from 3 to 11 (Figure 10).29 The etching rate of thesequentially annealed AZO thin ﬁlms at pH 3 was 0.2 nm/min,whereas that of as-grown AZO thin ﬁlms was 3.2 nm/min. It isnoteworthy that the chemical stability of sequentially annealed
AZO thin ﬁlms at pH 3 is comparable to that of the singlecrystalline Zn-polar ZnO (0001) surface, which is known to bemuch more stable than O-polar ZnO (0001) in an acidic2932

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Reviewa resistance to the proton-promoted dissolution, which occurs
on the other crystal planes ((1120) or (1010)) at the edges.160
3.2.2. Impurity Doping. Olvera et al. investigated thechemical stability of ZnO thin ﬁlms with varied dopant ions inhydrochloric acid (pH ∼ 3).46 Cr-doped ZnO showed the best
chemical stability, i.e., etching rate of 2.5 nm/min in ahydrochloric acid solution. The enhancement in the chemical
stability was attributed to the formation of chromium oxidearound the grain boundaries. However, this chromium oxide
served as a scattering center for electron transport, thereby
increasing the resistivity. Therefore, high chemical stability andhigh electrical conductivity coexist, which is a challenging
phenomenon. Choi et al. reported the improved chemical
stability of AZO thin ﬁlms in water and acidic aqueous
solutions (pH ∼ 4) by optimizing the Al dopant concentration.156 After 30 d in acidic solutions, the amount of Zn ionsreleased from the ZnO ﬁlms was 3.2 mg/L, whereas that of the
AZO ﬁlms with 10 at. % Al doping concentration was 0.3 mg/
L. Minami et al. investigated the chemical stability of AZO thinﬁlms with Co or Cr codoping.161 Both Co and Cr codoping
suppressed the chemical etching of AZO compared with noncodoped AZO in HCl or KOH solutions. For Cr codoped
AZO thin ﬁlms, the optimum Cr doping concentration was 3
wt %, which yielded the lowest etching rate of 2 nm/s in 0.2
mol/L HCl. However, the optimized Cr-codoped AZO
showed a higher resistivity than the as-grown AZO. In
contrast, Co codoping yielded an etching rate of 5 nm/s inHCl, which was lower than that of the as-grown AZO, with alow resistivity of 5 × 10−4 Ω·cm for 0.5 wt % CoO content.
The etching rates for undoped and doped ZnO thin ﬁlms are
summarized in Table 2.

Figure 10. pH dependence of (a) etching rate and (b) resistanceincrease of as-grown, sequence I annealed (ﬁrst Zn vapor, then air
annealing) and sequence II annealed (ﬁrst air, then Zn vapor
annealing) AZO thin ﬁlms during the chemical stability tests. Etchingrate was calculated using etched thickness at 25 min immersion ineach pH buﬀer solution. Resistance increase was calculated as (R −R0)/R0, where R and R0 are the resistance at 25 min immersion ineach pH buﬀer solution and the initial resistance before the stability
tests, respectively. Reproduced from ref 29. Copyright 2021 American
Chemical Society.

environment (Figure 9). This signiﬁcant improvement inchemical stability is attributed to the Zn-polar rich ZnO(0001) surface and the less-defective crystal structure ofsequentially annealed AZO thin ﬁlms. Valtiner et al.
demonstrated that chemical etching on the Zn-polar ZnO(0001) surface begins from the surface VZn, forming etchingpits during etching (Figure 11).159 The sequential annealing

Table 2. Etching Rates of Undoped and Doped ZnO Films
in Previous StudiespHetching rate (nm/min)

growth method

treatment

3

2.546
6.546
6.046
5.246
4.546
3.946
9.167211201620.046210.09221magnetron sputtering
magnetron sputtering
magnetron sputtering
magnetron sputtering
magnetron sputtering
magnetron sputteringhydrothermal growthmagnetron sputteringhydrothermal growthhydrothermal growth

Cr doping
Al dopingF doping
Ga doping
In doping
Zr dopingundopedZn addition
undopedundoped9
11

Figure 11. (a) Photomicrograph of the surface after etching of theZn-polar ZnO (0001) surface in a solution at pH 3. Hexagonal
etching pits were observed. (b) Ex situ AFM topography scan on thesurface between the etching pits. Triangular features were visible on
the surface. Triangular features on “A” terraces rotated 60° withrespect to triangles on “a” terraces. The line proﬁle (measured asindicated) shows step heights corresponding to the expected Zn−O
double-layer height. Reproduced from ref 159. Copyright 2008
American Chemical Society.

3.2.3. Capping Layers. The deposition of a chemicallystable capping layer on conductive ZnO thin ﬁlms is a typicalstrategy for preventing surface chemical etching. Zhong et al.
demonstrated the enhanced chemical stability of GaN-coatedZnO nanowires during photoelectrochemical processes.33 The
formation of Ga−O and Ga−N bonds on the surface of ZnO
nanowires enhances chemical stability in photoelectrochemical
water oxidation conditions. The stability of the uricaseimmobilized ZnO nanorods matrix biosensor was investigatedby Zhang et al. The stable electrocatalytic activity to oxidationof uric acid up to 85 °C was reported. This biosensor
maintained 80.5% of its electrocatalytic activity with anexcellent response to uric acid after storage at 4 °C for 20
d.163 TiO2 coating is also utilized to enhance the chemical
stability of other ZnO-based nanostructures. The TiO2 coating
signiﬁcantly reduces the dissolution of ZnO nanorods undereﬀectively suppresses the chemical etching of the AZO thinﬁlms because of the sequential defect compensation eﬀect.87
Therefore, the sequential annealing method is a practicalstrategy for fabricating conductive ZnO thin ﬁlms for harshelectronics because it enhances both the thermal and chemical
stability of highly conductive AZO thin ﬁlms. The polar surface
eﬀect was also investigated for ZnO nanoparticles, indicating
the positively charged surface at a pH range of 5.5−3.8 for the(0001) terraces. The positively charged surface on (0001) has
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Figure 12. UV−vis transmission spectra of (a) AZO thin ﬁlm deposited via 1000-cycle ALD (1000c AZO); (b) TiO2 capping layer deposited via
100-cycle ALD on 1000c AZO (1000c AZO + 100c TiO2); (c) TiO2 capping layer deposited via 300-cycle ALD on 1000c AZO (1000c AZO +300c TiO2) as a function of immersion time in 1 M NaOH (pH 13.5). UV−vis transmission spectra of (d) 1000c AZO, (e) 1000c AZO + 100c
TiO2, and (f) 1000c AZO + 300c TiO2 as a function of immersion time in 1 M H2SO4 (pH 0.5). (g) Cross-sectional SEM image of ALD-deposited
1000c AZO + 300c TiO2 ﬁlm on n+-Si substrate. Reproduced from ref 165. Copyright 2018 American Chemical Society.

operations at 350 °C was observed for ZnO nanoparticle
sensors containing 2.5 mol % Y.
Huang et al. reported that surface ﬂuorine modiﬁcation
enhances the chemical stability of ZnO memory devices.32 The
ﬂuorination of ZnO signiﬁcantly suppressed chemicalcorrosion and the variation of resistance of high resistancestates in acid vapor environments. ZnGaON-coated ZnOnanorods grown on ZnO thin ﬁlms were proposed as a watersplitting cell.33 The ZnO-ZnGaON photoanodes showedstable photocurrent and morphology in the amperometricstudy after 5.5 h at pH 13. In contrast, ZnO photoanodes were
fully etched within 5.5 h. Surface acoustic wave (SAW) devices
based on ZnO thin ﬁlm have been investigated toward ﬂexible
sensor and microﬂuidic applications.166,167 Xiong et al.
demonstrated SAW device operation based on a ZnO thinﬁlm in acid and alkali environments.167 Grain defects in ZnO
thin ﬁlms increase within 45 s in HCl and NaOH solution,
resulting in the failure of device operations.

UV radiation.164 In TiO2-coated and calcined ZnO nanorods,
dissolution was suppressed from 2.86% (bare ZnO nanorods)
to 2.44%. Reed et al. reported the eﬀect of an ALD-deposited
ultrathin TiO2 capping layer on the chemical stability of AZO
thin ﬁlms.165 Figure 12 shows the UV−vis transmission
characteristics of AZO thin ﬁlms with varying thicknesses of
the TiO2 capping layer during chemical stability tests in 1 M
NaOH (pH 13.5) and H2SO4 (pH 0.5) solutions; as shown,
AZO thin ﬁlms with a 10 nm TiO2 (1000c AZO + 300c TiO2)
capping layer indicated stable superior transmission properties
in solutions with pH values of 0.5 and 13.5 (Figure 12f,c,respectively).

4. APPLICATIONS OF ZNO IN HARSH ELECTRONICS
Recent advances in the thermal and chemical stability of ZnO
thin ﬁlms and nanostructures enable electrical applications of
ZnO in harsh environments. Long-term stability of a chemicalsensor performance of SnO2 nanochannel devices was achieved
using AZO transparent electrodes with the Air/Zn vapor
sequential annealing method.87 The sequentially annealed
AZO electrodes enabled stable sensor response during sensor
operations at 200 °C in ambient air for more than 100 h. In
contrast, the sensor response of the devices with conventional
Ti/Pt bilayer electrodes or as-deposited AZO electrodes
rapidly degraded within 100 h. Formica et al. reported aninverted polymer solar cell (PSC) using AZO transparent
electrodes with TiO2 capping layer, demonstrating stableproperties during 1000 h of operation in a damp environment
at 85 °C.26 The power conversion eﬃciency of the PSC using
the AZO/TiO2 bilayer electrodes was 6.3%, which iscomparable to that of the PSC using ITO electrodes. ZnOnanorods with a Pd shell layer were proposed as H2 gas
sensors. The Pd shell layer successfully prevented surface
contamination of ZnO nanorods, resulting in the stable sensor
response during 5 months.30 Yttrium-doped ZnO nanoparticle
ﬁlms fabricated using ﬂame spray pyrolysis were investigated asa sensor channel to detect acetic acid in humid environments.31 The enhancement of thermal stability during sensing

5. CONCLUSIONS AND OUTLOOK
Herein, we reviewed recent advances in robust and electricallyconductive ZnO thin ﬁlms and nanostructures for applicationsin harsh environments, such as high temperatures and/or
chemically active media. In terms of thermal stability, theformation of an insulating oxide via dopant diﬀusion and anincrease in VZn at high temperatures contributed primarily tothe degradation of the electrical conductivity of conductive
ZnO. The deposition of a stable capping layer on ZnO thinﬁlms is the most typical strategy for enhancing the thermal andchemical stability. In this strategy, the deposition of an
extremely thin transparent capping layer on doped ZnO thinﬁlms yielded chemically stable and transparent conductive
ZnO thin ﬁlms. However, the capping layers degraded the
intrinsic properties of ZnO, including the surface chemical
activity for chemical sensors and visible-light transmittance forphotoelectrochemical applications. Postdeposition treatments
to compensate for defects in ZnO thin ﬁlms are fundamental
approaches for suppressing key processes aﬀecting thermal andchemical degradation. Thermal annealing in air and Zn vapor
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atmospheres, which compensates both VO and VZn in doped
ZnO thin ﬁlms, is a promising strategy for enhancing both
thermal and chemical stability with a suﬃciently low electricalresistivity of doped ZnO thin ﬁlms. A further understanding of
the mechanisms of defect formation and compensation isrequired to design conductive ZnO thin ﬁlms under extremely
harsh conditions, such as at temperatures exceeding 500 °C
and/or strong acid solutions (pH < 3). We believe that robust
and conductive ZnO thin ﬁlms and nanostructures, which havebeen achieved using the reviewed strategies, are promising
candidates as transparent electronic materials for forthcoming
harsh electronics.

T.T. was supported by JST PREST Grant Number
JPMJPR19M6, Japan. T.T., G.Z., K.N., and T.Y. were
supported by JST CREST (Grant Number JPMJCR19I2,
Japan). T.Y. and K.N. were supported by the CAS-JSPS Joint
Research Projects (Grant Number JPJSBP120187207) andMirai R&D of JST. R.Y. was supported by the program of the
China Scholarships Council (No. 201706090261). This work
was performed under the Cooperative Research Program of
the “Network Joint Research Center for Materials and Devices”
and the MEXT Project of “Integrated Research Consortium on
Chemical Sciences”.

■■

AUTHOR INFORMATION

Review

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Corresponding Authors

Tsunaki Takahashi − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Bunkyo, 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,
Bunkyo, Tokyo 113-8656, Japan; Institute for Materials
Chemistry and Engineering, Kyushu University, Kasuga,
Fukuoka 816-8580, Japan; orcid.org/0000-0003-48375701; Email: yanagida@g.ecc.u-tokyo.ac.jp

Authors

Ruolin Yan − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Bunkyo,
Tokyo 113-8656, Japan; Institute for Materials Chemistry
and Engineering, Kyushu University, Kasuga, Fukuoka 8168580, Japan
Hao Zeng − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Bunkyo,
Tokyo 113-8656, Japan
Takuro Hosomi − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Bunkyo, 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, Bunkyo,
Tokyo 113-8656, Japan; Institute for Materials Chemistry
and Engineering, Kyushu University, Kasuga, Fukuoka 8168580, Japan
Guozhu Zhang − Department of Applied Chemistry, Graduate
School of Engineering, The University of Tokyo, Bunkyo,
Tokyo 113-8656, Japan
Kazuki Nagashima − Department of Applied Chemistry,
Graduate School of Engineering, The University of Tokyo,
Bunkyo, 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.1c00428
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by KAKENHI (Grant Numbers:
JP20H02208, JP18H01831, JP18H05243, and JP18KK0112).
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