Heterojunction oxide thin-film transistors with ... · Heterojunction oxide thin-film transistors...

SC I ENCE ADVANCES | R E S EARCH ART I C L E

MATER IALS SC I ENCE

1Department of Physics and Centre for Plastic Electronics, Blackett Laboratory,Imperial College London, London SW7 2AZ, U.K. 2Department of Physics, AristotleUniversity of Thessaloniki, GR-54124 Thessaloniki, Greece. 3Division of PhysicalSciences and Engineering, King Abdullah University of Science and Technology,Thuwal 23955-6900, Saudi Arabia.*Corresponding author. Email: [email protected] (T.D.A.); [email protected] (H.F.); [email protected] (P.A.P.)†These authors contributed equally to this work.

Faber et al., Sci. Adv. 2017;3 : e1602640 31 March 2017

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Heterojunction oxide thin-film transistors withunprecedented electron mobility grown from solutionHendrik Faber,1*† Satyajit Das,1† Yen-Hung Lin,1 Nikos Pliatsikas,2 Kui Zhao,3 Thomas Kehagias,2

George Dimitrakopulos,2 Aram Amassian,3 Panos A. Patsalas,2* Thomas D. Anthopoulos1,3*

Thin-film transistorsmadeof solution-processedmetal oxide semiconductors hold great promise for application in theemerging sector of large-area electronics. However, further advancement of the technology is hindered by limitationsassociated with the extrinsic electron transport properties of the often defect-prone oxides. We overcome this limita-tion by replacing the single-layer semiconductor channel with a low-dimensional, solution-grown In2O3/ZnO hetero-junction. We find that In2O3/ZnO transistors exhibit band-like electron transport, with mobility values significantlyhigher than single-layer In2O3 and ZnOdevices by a factor of 2 to 100. Thismarked improvement is shown to originatefrom the presence of free electrons confined on the plane of the atomically sharp heterointerface induced by the largeconduction band offset between In2O3 and ZnO. Our finding underscores engineering of solution-grownmetal oxideheterointerfaces as an alternative strategy to thin-film transistor development and has the potential for widespreadtechnological applications.

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INTRODUCTIONTransparent metal oxides have emerged as a promising family of com-pound semiconductors for a range of applications in the field of large-area optoelectronics because of a variety of assets, including tunableenergy band structure, high charge carrier mobility, optical transparen-cy, mechanical flexibility and durability, and outstanding chemical sta-bility (1–5). A further prominent asset is their ease of processing fromsolution phase at low temperatures (6, 7) and under ambient conditionsthat would alter our perception and modus operandi in semiconductorindustry by gradually abolishing established vacuum-based processtechnologies for widespread technological applications, especially inthe field of thin-film transistor (TFT) electronics. Currently,metal oxideTFTs with electronmobilities exceeding 10 cm2/V·s have been achieved(8–13), and numerous new approaches to further enhance the deviceperformance have been proposed and demonstrated (6, 14, 15). It isnowwell established that themeasured carrier mobility may be affectedby several factors, including (i) the intrinsic mobility of the semi-conductor used, (ii) the microstructural quality of the semiconductinglayer, and (iii) the device architecture itself.

A notable strategy that has been successfully used to enhance theperformance of metal oxide TFTs in the past few years is based onthe use of multilayer channels (16–21). Although the exact mecha-nism(s) underpinning performance enhancement is still under debate,it has been proposed that under certain circumstances, electron transferand confinement at the heterointerface can occur because of conductionband (CB) offset (17, 21–23) in a process similar to that observed inconventional high electron mobility transistors (HEMTs) made ofAlGaAs/GaAs heterointerface channels (24–29). In these devices,the formation of a two-dimensional electron gas (2DEG) at the criticalheterointerface enables the realization of transistors with electron mo-bilities close to the theoretical limit set by phonon scattering in the ab-

sence of impurity scattering in the semiconductor (27–29). In terms ofthe underling transport physics, metal oxide–based TFTs that resembletraditional HEMTs have been produced by various growth techniques,including radio-frequency (RF) sputtering (16, 30), molecular beam ep-itaxy (MBE) (31), metalorganic chemical vapor deposition (MOCVD)(32), and atomic layer deposition (21).

One of the most extensively studied metal oxide heterointerfacesystems that is known to lead to 2DEG formation is that of ZnO/ZnMgO (33–37). The latter relies on electron confinement at the het-erointerface due to a large CB offset and polar surfaces (34, 35). For op-timal interface quality, epitaxial material growth is commonly carriedout viaMBEor pulsed laser deposition on single-crystal substrates, suchas sapphire or ScAlMgO4 (33, 37). However, it was shown that 2DEGcould also form in ZnO/ZnMgO heterointerfaces grown on glass sub-strates via less elaborate techniques, such as RF sputtering (36). De-spite their polycrystalline nature, the heterointerfaces exhibitedenhanced sheet electron concentration andmobilitywith a characteristictemperature-independent behavior (36).

We recently reported evidence of quantized energy states in low-dimensional, solution-grown In2O3 (38) andZnO layers (39) anddem-onstrated metal oxide superlattice TFTs with electron mobilities inthe range of 25 to 45 cm2/V·s (22). However, these archetypal devicesrely on complex manufacturing and involve elaborate multilayeredarchitectures (21, 22). Thus, a major challenge in further advancingthe performance of metal oxide TFTs lies in the development ofhigh-quality oxide heterostructures via scalable manufacturing pro-cesses (7).

Here, we report the growth of low-dimensional metal oxide hetero-structures consisting exclusively of In2O3 and ZnO layers grown fromsolution phase and their implementation in high electron mobilityTFTs. We show that ultrathin (≤13-nm) In2O3/ZnO heterojunctionscan be grown reliably over large areas at <250°C via sequential ultra-sonic spray pyrolysis and spin coating. Remarkably, charge transportin In2O3/ZnO heterojunction transistors is found to be consistently su-perior to single-layer devices withmaximum electronmobility values inexcess of 45 cm2/V·s. The high electron mobility is accompanied by amarked change in the charge transport mechanism, both of which areattributed to the synergistic effects of the atomically sharp In2O3/ZnO

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heterointerface and the presence of a 2D-confined layer of free electronsat its vicinity. The In2O3/ZnO heterointerface studied here provides anexcellentmodel that could be further exploited for the rational design ofadvanced transistor channels (17).

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RESULTSWe fabricated bottom-gate, top-contact transistors based on solution-grown ZnO, In2O3, and In2O3/ZnO heterostructures using aluminum(Al) source-drain (S-D) electrodes. Epitaxial-like 4- to 7-nm-thickIn2O3 layers were grown by ultrasonic spray pyrolysis (Fig. 1A) in airat 250°C (8), whereas 4- to 6-nm-thick polycrystalline layers of ZnOwere processed via spin casting and thermal annealing at 210°C (Fig.1B) (39). In2O3/ZnO heterostructures were fabricated via sequentialdeposition of In2O3 by ultrasonic spray pyrolysis and spin casting ofZnO at 250° and 210°C, respectively. TFT fabrication was completedwith the deposition of the topAl S-D electrodes via vacuum sublimation(Fig. 1C).

Figure 1D shows the transfer characteristics (drain current ID versusgate voltage VG) of single-layer In2O3 and ZnO TFTs, as well as of anIn2O3/ZnO heterojunction device. First, we consider the characteristicsof In2O3 and ZnO transistors to benchmark the level of performance ofthe single-layer semiconductor devices. All In2O3 transistors exhibit atypical n-channel behavior with electron field-effect mobility (mFE) of~16 cm2/V·s and channel current on/off ratios of 106, in accordancewith previous work (8). On the other hand, ZnO transistors processedvia spin coating exhibit high turn-on voltages VON (+40 V) and a rela-tively low mFE of <1 cm

2/V·s. Although the In2O3/ZnO heterojunctiontransistor exhibits a channel current on/off ratio andVON similar to theIn2O3 device, it can sustain significantly higher currents, most likely dueto the increased electron channelmobility. Analysis of the transfer char-acteristics in Fig. 1D reveals that In2O3/ZnO transistors exhibit nearly100× higher mFE than ZnO devices and at least a factor of 2 higher thanIn2O3 transistors with average and peak mFE of ~30 and >45 cm2/V·s,respectively. The impact of mFE on the current driving capabilities of theTFTs is better illustrated in Fig. 1E, where the output characteristics ofIn2O3 and In2O3/ZnO transistors are shown. The threshold voltage VT

of the heterojunction transistors is also found to be consistently morenegative (−7.3 V) when compared to In2O3 TFTs (~1.3 V) (Fig. 1F),suggesting the existence of a higher electron concentration in the chan-nel, the origin of which will be discussed below. Evidently, the In2O3/ZnO heterojunction TFT can sustain significantly larger currents thanthe In2O3 device, which is a direct consequence of the greatly enhancedelectron mobility.

To elucidate the origin of the stark difference in electron transportmeasured between In2O3, ZnO, and In2O3/ZnO transistors, we ana-lyzed the gate-voltage dependence of mFE according to the power lawequation (30)

mFE ¼ KðVG � VT;PÞg ð1Þ

where VG, VT, and VP are the gate, percolation, and threshold voltages,respectively, whereas the prefactor K and the exponent g relate to thenature of the transportmechanism. Specifically, the prefactorK includesthe percolation term from the Thomas-Fermi approximation (40) forboth trap-limited conduction (TLC) and percolation conduction (PC).For the latter case, K also includes the contribution of the spatial char-acteristics of the potential barriers. On the other hand, the power


exponent g depends either on the temperature T for TLC or the spatialcharacteristics of the potential barriers for PC. Consequently, its valueis indicative of the different charge transport processes that dominatechannel conduction. Lee et al. (30) have recently shown that a g valueclose to 0.7 indicates TLC, whereas a value of ~0.1 indicates a PC-domi-nated process. As shown in Fig. 1G and fig. S1, the mFE evolution for theIn2O3 device can be fitted over a wide range ofVG values using g = 0.66,indicating TLC as the dominant conduction process. On the otherhand, for In2O3/ZnO heterojunction TFTs, g varies from 0.7 to 0.11for lower and higher gate fields, respectively, suggesting a transitionfrom a TLC-dominated to a PC-dominated process.

Further experimental evidence on the existence of starkly differentelectron transport mechanisms between the two types of devices isprovided by the temperature dependence of mFE evaluated in the linearbias regime mLIN. Figure 1H displays the Arrhenius plots of mLIN be-tween 78 and 300 K for In2O3 and In2O3/ZnO transistors. Evidently,the In2O3 device exhibits a temperature-activated behavior, which is atypical characteristic of TLC, where mLIN remains constant (3 to 4 cm2/V·s)in the temperature range of 78 K ≤ T ≤ 160 K, followed by an ex-ponential increase for T >160 K to a maximum value of ~12 cm2/V·sat room temperature. The activation energy EA determined from the160 K≤ T≤ 300 K region is ~40 meV and is associated with the pres-ence of electron traps in the In2O3 layer. A similar behavior is observedfor single-layer ZnO TFTs and is consistent with TLC because the gexponent of mLIN does depend on the temperature (30). In contrast,In2O3/ZnO heterojunction devices exhibit a temperature-independentmLIN across the entire temperature range investigated (78 to 300 K).These results indicate that electron transport in In2O3/ZnO TFTs ismost likely governed by PC, which is exclusively associated with thespatial distribution of the potential barriers (30). Therefore, the elec-trical measurements point to the fact that the electron conduction pro-cess in single-layer In2O3 and ZnO TFTs is fundamentally differentfrom that in In2O3/ZnO heterojunction devices.

To determine the origin of the different electron transport mecha-nisms, we investigated the surface topography of each oxide layer bymeans of intermittent contact mode atomic force microscopy (AFM).Figure 2 (A to D) shows AFM images for Si++/SiO2, Si

++/SiO2/In2O3,Si++/SiO2/ZnO, and Si++/SiO2/In2O3/ZnO. The differences between thevarious surface topographies are better illustrated in the height histo-grams of Fig. 2E. As expected, SiO2 exhibits a featureless and atomicallysmooth topographywith a rootmean square (RMS) roughness (sRMS) of~0.18 nm. Remarkably, spray-grown In2O3 layers (Fig. 2B) also exhibitvery flat surfaceswithsRMS = 0.42 nm.Wenote that the latter value is lessthan half of the unit cell size of cubic In2O3 (41), and as a result, thesample’s surface appears almost featureless over several micrometers.The latter is comparable to the surface characteristics of semiconductorquantum-well structures grown via sophisticated vacuum-based techni-ques, such as MBE (42), MOCVD (43), and metalorganic vapor phaseepitaxy (44), despite being processed by ultrasonic spray pyrolysis in am-bient atmosphere. Contrary to In2O3, the surface topography of single-layer ZnO (Fig. 2C) reveals the existence of crystallites with a relativelyhigh surface roughness ofsRMS~1.57nm.WhenZnO is spin-coatedontoIn2O3 to form the In2O3/ZnO heterojunction (Fig. 2D), a similar surfacetopography is observed but with the ZnO crystallites appearing slightlyenlarged.We attribute this to the different contact angles of aqueous solu-tions on SiO2 and In2O3 surfaces and the different conditionsunderwhichZnO crystallization and growth occur (45, 46). These larger ZnO crystal-lites are responsible for the increased surface roughness (sRMS = 2.32 nm)of the In2O3/ZnO heterojunction (Fig. 2E).

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The microstructure of the In2O3/ZnO heterojunctions was furtherexamined by high-resolution transmission electron microscopy(HRTEM). Figure 3A shows the cross-sectionalHRTEM image of a rep-resentative In2O3/ZnO heterojunction grown on SiO2. The thickness ofthis particular heterojunction is approximately 10 nm and is character-ized by the atomically sharp In2O3/ZnO heterointerface—a truly re-markable finding if one considers the simplicity of the solutiondeposition techniques used. For this particular sample, the ZnO layerwas approximately 6 nm thick and featured randomly oriented nano-


crystals on the surface of In2O3.On the contrary, the In2O3 layer had theform of highly crystalline elongated platelets that are 4 to 5 nm thick.Geometric phase analysis (GPA) performed with a spatial resolution of0.4 nm confirms that the microstructure of In2O3 is that of elongatedplatelets with an average length of 35 ± 5 nm along the lateral direction(see section S2). Although no persistent growth texture for the In2O3

crystals could be identified, there is a trend amid them to grow withthe [001] direction of their Ia�3 body-centered cubic (bcc) crystal latticeroughly parallel (fig. S2), or occasionally perpendicular, to the SiO2

In O /ZnO

In O

μFE = (VG – VP)

μFE = (VG � V T)

μ FE (c

m/V

. s)

VG (V)

μFE = (VG � VT)

In O /ZnO

μ LIN

mc(

)V.

s/

/T ( /K)

In O

In2O3

In2O3/ZnO

I DA(

)

VG (V)

VT = 1.3 V

VT = –7.3 V

ZnO precursor solution

Pipet

ZnO

In2O3 In2O3 /ZnO

I D)A(

VG (V)0 20 40 60 80 100

0

2

4

6

8

10

12

V

V

ID

)A

m(

VD (V)

ΔVG = V

0 20 40 60 80 100

VD (V)

D E

G H

S D

Si++/SiO2

Sample holder

Semiconductor

Shadow mask

Si++/SiO2

Hot plate (210 °C)

Annealing in air

Al crucible

ZnO

Si++/SiO2Si++/SiO2

Hot plate (250 °C)

In2O3

Ultrasonic nozzle

In2O3 precursor

A B C

ΔT4–7 nm 4–6 nm

F

Fig. 1. Material growth and transistor fabrication and characterization. (A) Schematic of the ultrasonic spray pyrolysis process used to grow the In2O3 layers. (B) Spincasting of ZnO onto spray-deposited In2O3 followed by thermal annealing at 210°C in air. (C) Transistor fabrication is completed with the deposition of the Al S-Delectrodes via vacuum sublimation through shadow masks. (D) Transfer characteristics measured in the saturation regime (VD = 100 V) for single-layer ZnO and In2O3

and In2O3/ZnO heterojunction transistors. The channel widthW and length L were 1 mm and 100 mm, respectively, for all devices. (E) Output characteristics of In2O3 andIn2O3/ZnO transistors. (F) Plots of ID

1/2 versus VG for In2O3 and In2O3/ZnO heterojunction transistors. (G) Gate-voltage dependence of the electron mobilities in single-layer In2O3 and In2O3/ZnO heterojunction transistors. The solid lines represent fits according to their respective power law equations. Note that a double-logarithmicplot of the data can additionally be found in fig. S1. (H) Arrhenius plot of the electron mobility (linear) of single-layer In2O3 and In2O3/ZnO heterojunction transistors as afunction of inverse temperature measured between 78 and 300 K and extracted according to Eq. 3 at VG − VT = 20 V.

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surface. Fast Fourier transform (FFT) analysis (Fig. 3B) of the high-lighted regions in Fig. 3A provided further evidence that the ZnO layeris nanocrystalline with the three prominent spacings corresponding tothe [202], [110], and [101] planes of the hexagonal wurtzite structure,whereas for the In2O3 layer, the lattice spacing of 0.36 nm correspondsto the [220] crystal plane of the bcc structure of In2O3. Although nostraightforward structural relation between the In2O3 and ZnO layerscould be identified, the heterointerface is continuous with no evidenceof materials intermixing (Fig. 3A and fig. S3). This interesting micro-structural feature may be due to the chemistry and low viscosity ofthe ammonium hydroxide–based precursor formulation used to grow


the ZnO layers. Thus, the HRTEM data verify that sequential solutiondeposition of In2O3 and ZnO leads to the formation of a seamlessIn2O3/ZnO heterojunction with low interface roughness and withoutevidence of alloying (see section S2).

We have combined x-ray photoelectron spectroscopy (XPS) withmild in situ Ar+ etching (Fig. 4A) to study the electronic states in bareIn2O3 and In2O3/ZnO heterojunction as a function of etching depth.The same method was also used to acquire the elemental profile andenergy band structure of the In2O3/ZnO heterointerface. The integralarea of the In-3d, Zn-2p, O-1s, C-1s (adventitious carbon), and Si-2pphotoelectron lines was used after background subtraction and taking

ZnO ( nm)

SiO ( nm)

In O ( nm)

).u.a( ycneuqerF

Height (nm)

In O /ZnO ( nm)

E

SiO2z = 1.5 nm

100 nm

In2O3z = 3 nm

100 nm

ZnOz = 9 nm

100 nm

D In2O3/ZnOz = 15 nm

100 nm

A B C

Fig. 2. Surface topography of single and heterojunction metal oxide transistor channels. AFM images of the surfaces of (A) SiO2, (B) single-layer In2O3, (C) single-layer ZnO, and (D) In2O3/ZnO heterojunction. The indium oxide layer exhibits very flat areas by itself with only hints of crystalline features visible, whereas both the neatZnO and bilayer surface show pronounced nanocrystals. (E) Height distributions extracted from the AFM images in (A) to (D). a.u., arbitrary units.

0.36 nm[220]

0.12 nm[202]

0.16 nm[110]

0.24 nm[101]

SiO2

AB

5 nm

ZnO

0.36 nm

In2O3

Protective layer

In2O3

ZnO

Fig. 3. HRTEM analysis of solution-processed In2O3/ZnO heterostructures. (A) Representative HRTEM image of the In2O3/ZnO heterostructure deposited on SiO2.(B) FFT patterns obtained from the selected areas of the In2O3 and ZnO layers highlighted in (A).

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into account the relevant sensitivity factors. ZnO exhibit a slight oxygendeficiency, whereas In2O3 is nearly stoichiometric. For the chemicalanalysis of the layers, special attention was given to the O-1s core-levelspectra (fig. S4), which are very sensitive to the chemical state of Oand to the neighboring atoms (8, 47–49). Ultrathin layers of ZnOand In2O3 were found to contain Zn–O and In–O bonds in their re-spective lattice sites, with minor contributions from Zn–OH and In–OH detected on their surfaces.

Figure 4B shows the depth-resolved elemental composition of thelow-dimensional In2O3/ZnO heterostructure acquired through succes-sive XPS spectra at varying etching time (depth). The small amount ofIn detected at the surface of the heterojunction (etching time, 0 s) isattributed to the small layer thickness of ZnO (4 to 6 nm), which iscomparable to the escape depth of the In-3d photoelectrons. The sameapplies for the In2O3 layer because it is transparent to the Si-2p photo-electrons, resulting in the apparent presence of Si at an etching time of~150 s. The XPS data are in agreement with HRTEM analysis (Fig. 3)and verify the structural and chemical composition of the In2O3/ZnOheterojunction.

Figure 4C shows the In-3d core-level spectra recorded at differentetching times for bare In2O3 (dashed lines) and In2O3/ZnO hetero-


junction (solid lines) (more details on the In-3d and Zn-2p core-levelspectra are presented in figs. S5 to S8 and in the relevant discussionfound in the online Supplementary Materials). No spectral variation inthe core-level spectra of bare In2O3 is observed (dashed lines), withincreasing etching time indicating a size-independent electronic struc-ture. On the contrary, there is a strong dependence of the In-3d core-level spectra with etching depth for the SiO2/In2O3/ZnO heterojunction(solid lines). The latter is attributed to the Fermi level pinning dueto electron transfer from ZnO to In2O3 (22). To quantify this effect,we studied the valence band offset (VBO) between ZnO and In2O3

using the experimentally determined relative spectral positions ofthe Zn-2p and In-3d core levels, taking into account the value of theupper edge of the VB spectra of bulk In2O3 and ZnO (fig. S9) andfollowing the method proposed by Kraut et al. (50). The VBO betweenthe In2O3 and ZnO can be determined using the equation

VBO ¼ ðEbulkIn�3d � Ebulk

In�VBÞ�

ðEbulkZn�2p � Ebulk

Zn�VBÞ � ðEexpIn�3d � Eexp

Zn�2pÞ ð2Þ

Ion gun

Hemispherical sector analyzer

Electron detector

Focusing monochromator Aperture

Electrostatic lensing

Photoelectrons

Magnetic lensing

ZnOX-ray source

In2O3

Sample holder

Si++/SiO2

)Ve( BOV

ZnO thickness (nm)

Approaching In2O3 interface

A

In O /ZnO

sss

).u.a( ytisnetni nortceleotohP

Binding energy (eV)

s s s s s s s s

In O

C D E

Etching depth t = 0 s t = 600 s

2 nm

ZnO In2O3 SiO2

B

In

Si

O

Etching time (s)

)% ta( noitartnecno

C

Zn

C

1 eV

0.36 eV

2.4 eV

-

3.4 eV

VB

-

Ener

gy w

.r.t. E

F(e

V)

Ener

gy w

.r.t.

vacu

um (–

eV)

VB

CBCB

ZnOIn2O3

- 0.93 eVEF

–1

–2

–3

–4

0

1

5

6

7

8

4

3

Fig. 4. XPS analysis of single-layer In2O3 and In2O3/ZnO heterojunctions. (A) Schematic drawing of the XPS depth profiling measurement setup. (B) Elemental depthprofile of the bilayer film stack on the Si/SiO2 substrate. Inset: HRTEM image of the heterointerface studied by XPS where the vertical dashed lines indicate the In2O3/ZnOand SiO2/In2O3 interfaces. at %, atomic %. (C) The evolution of the spectral position of the In-3d core level with respect to the etching time for SiO2/In2O3/ZnO (solid lines)and for SiO2/In2O3 (dashed lines); note that the times indicated for the SiO2/In2O3/ZnO system correspond exclusively to ZnO etching. (D) The VBO between ZnO andIn2O3 versus the thickness of the ZnO overlayer determined from Eq. 2. (E) Energy band diagram of the In2O3/ZnO heterointerface reconstructed on the basis of XPS,optical absorption, and Kelvin probe measurements. Energies are shown with respect to (w.r.t.) Fermi energy, EF, and vacuum level.

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where In-3d and Zn-2p are the spectral positions of the correspondingcore-level spectra, and VB indicates the upper edge of the VB spectra ofbulk In2O3 and ZnO; “bulk” and “exp” superscripts indicate the valuesobtained frombulk (>20nm)andultrathin samples (<10nm), respective-ly. Figure 4Dshows the evolutionof theVBObetween In2O3 andZnO forvarying ZnO thickness, which was determined by monitoring the Zn-2pand In-3d core levels after each etching step. VBO reaches a constant val-ue of ~1 eV for a ZnO thickness of >5 nm. The corresponding CB offsetcanbedeterminedby considering the bandgap energyEg of the individualIn2O3 (3.76 eV) andZnO(3.33 eV) layers (see fig. S10). Figure 4E shows aschematic of the resulting energy band diagram of the In2O3/ZnO het-erojunction. The electron transfer from the CB of ZnO to that of In2O3

is attributed to the CB offset, resulting in the formation of a spatiallyconfined sheet of free electrons at the vicinity of the interface on theIn2O3 side. To verify the existence of the confined charge, we evaluatedthe electron concentration across the heterojunction using capacitance-voltage (C-V) analysis of metal-insulator-semiconductor capacitorstructures. The C-V technique is often used to verify the presenceand location of the confined changes across the heterostructures be-cause it enables quantitative analysis of the apparent free carrier concen-tration with high spatial resolution (33, 34). In heterointerfaces where a2D-confined sheet of electrons is present, a clear spike in the carrierprofile should exist, indicating the exact position of the critical interface(33, 34). The experimental results shown in fig. S11 (A and B) reveal thepresence of an increased concentration of free electrons located right atthe expected position of the In2O3/ZnO heterointerface. The spike inthe electron profile reaches a maximum value of ~3.1 × 1019 cm−3

andhas awidth of 1 to 2nm.The latter value is expected to be determinedprimarily by the interface roughness and is in agreement with the surfaceheight histograms in Fig. 2E. TheCBdiscontinuity evaluated via XPS andoptical absorption spectroscopy measurements, combined with the highstructural quality of the In2O3/ZnO heterointerface and the presence of a2D-confined sheet of electrons at the heterointerface, supports the PCmechanism observed in In2O3/ZnO transistors.

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DISCUSSIONFor the first time, low-dimensional and atomically sharp In2O3/ZnOheterojunctions have been grown from solution phase at low tempera-tures and incorporated in TFTs. Although heterojunction oxide TFTshave been previously realized using various solution-processed amor-phous ternary and quaternary metal oxide compounds, such as InZnO,AlInZnO, or InGaO (19, 20, 51–53), it has been proven challenging toincrease the electron mobility beyond values obtained from the indi-vidual constituent materials (23). The In2O3/ZnO heterojunction tran-sistors developed in this work not only surpass the performance ofsingle-layer In2O3 and ZnO TFTs but also compare favorably tostate-of-the-art vacuum-processed devices (4, 9, 54). The exceptionallevel of performance is attributed to three main factors: (i) the abilityto grow highly crystalline In2O3 layers via ambient ultrasonic spraypyrolysis (Fig. 3), (ii) the formation of a seamless and dislocation-freeIn2O3/ZnO heterointerface upon sequential deposition of the top ZnOlayer via spin casting (Fig. 3A), and (iii) the existence of a sheet of freeelectrons confined in the xy plane of the atomically sharp In2O3/ZnOheterointerface due to the large CB discontinuity (Fig. 4E). Additionalfactors that could potentially contribute to the enhanced performanceinclude the passivation of In2O3 surface defects by ZnO and/or the sup-pression of the injection barrier for electrons due to Schottky barrierlowering because of the existence of surface electron accumulation.


The combined effects of these unique attributes results to themarked change in the electron transport mechanism manifested firstas an increase in electron mobility (Fig. 1D) and, second, in the tran-sition from a trap-limited to a percolation-dominated conductionprocess (Fig. 1G). Further evidence supporting this fundamentalchange is provided by the temperature dependence of mLIN, wherethe temperature-activated electron transport in single-layer In2O3

devices (Fig. 1H) transforms to a temperature-independent behaviorfor In2O3/ZnO heterojunction TFTs, which is a characteristic indicativeof trap-free band-like electron transport not previously observed inmetal oxide TFTs.

The presence of a smooth and highly crystalline In2O3 layer is ofcritical importance because it facilitates the formation of the atomical-ly sharp In2O3/ZnO heterointerface upon sequential processing of thetop ZnO layer without alloying (Fig. 3A). The latter feature combinedwith the significant CB energy offset between In2O3 and ZnO (Fig. 4E)results in the formation of a space charge layer that is confined in theout-of-plane z direction but highly delocalized along the 2D heteroin-terface. This quasi-2D free-electron system, which was experimentallydetermined for the first time by depth-resolved XPS (Fig. 4, C to E)and C-V carrier profiling measurements (fig. S11), facilitates a sub-stantial increase in the concentration of free electrons within the crys-talline In2O3 layer while avoiding adverse effects associated withCoulomb scattering that are often seen in conventional doped semi-conductor transistors (17, 28). To this end, one may also argue thatthe higher electron concentration would shift the Fermi energy of theheterojunction closer the CB of In2O3 (Fig. 4E), facilitating access tothe extended states which would otherwise have remained in-accessible. On the other hand, the ultrathin nature of the In2O3 layer(Fig. 3A) may lead to quantization of the energy states (subbands) (38)perpendicular to the transport plane (z direction) without affectingtheir in-plane delocalization. The low-dimensional and exceptionalstructural quality of these solution-grown In2O3 layers could oneday lead to the observation of transport processes similar to those re-ported for traditional 2DEG heterojunction devices (18, 28). However,more work is required to reveal the true potential of these systems anddevices based on them.

Although this work focuses on the In2O3/ZnO heterointerface, ourfindings provide guidelines for the design of metal oxide TFTs withperformance characteristics potentially superior to any existing thin-film semiconductor technology, either via further heterointerface en-gineering or through the use of different materials and/or advanceddoping schemes (27–29). For example, incorporation of suitable seedlayers predeposited from solution onto the substrate could assist incontrolling and hence improving the crystal quality of the subsequent-ly deposited In2O3 layer with direct impact on the quality of the re-sulting heterointerface. Similarly, n-type doping of ZnO using newlydeveloped routes (55) could provide an additional parameter that couldbe explored to tune the confined electrons, and hence the TFT char-acteristics, in a similar manner to modulation-doped AlGaAs/GaAsheterojunction field-effect transistors (27–29, 56). More broadly, the useof high-mobility In2O3/ZnO heterointerfaces could be extended totransparent conductive electrodes, where the incorporation of mul-tiple “charged” high-mobility heterointerfaces could be exploited toincrease the layer conductivity of solution-processed transparentsuperlattices, hence enabling their use as conductive electrodes inlarge-area optoelectronics.

Last, our results identify the use of solution-grown In2O3/ZnO het-erointerfaces as a powerful method to enhance the electron transport

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in metal oxide TFTs beyond single material–based devices. It also sug-gests that solution-processed, low-dimensional metal oxide semicon-ductors could well be entering the realm of quantum electronics andthat the resulting devices may one day compete directly with maturesemiconductor technologies but with the added advantage of beingproduced at a small fraction of the cost.

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MATERIALS AND METHODSPrecursor solutions and deposition techniquesMetal oxide semiconductors were deposited in ambient air either viaultrasonic spray pyrolysis (In2O3) or spin coating (ZnO). For spraypyrolysis of In2O3, indium nitrate [(In(NO3)3] (99.99%, IndiumCorporation of America) was dissolved in deionized (DI) water ata concentration of 30 mg/ml. The resulting solution was stirredat room temperature for 30 min and subsequently sprayed onto theheated sample surface (250°C) using a liquid flow rate of 2 ml/minthrough a stencil mask to achieve rudimentary patterning. The totaltime for the cyclic spray deposition step was 10 min. For spin coatingdeposition of ZnO films, the precursor solution was formed by dis-solving zinc oxide hydrate (ZnO·H2O, 97%, Sigma-Aldrich) in am-monium hydroxide [50% (v/v) aqueous solution, Alfa Aesar] at aconcentration of 10 mg/ml and stirred at room temperature for 2to 3 hours. The spin coating parameters used for the spin coatingof ZnO was 4000 rpm for 30 s. As-deposited films were subjectedto thermal annealing at 210° to 250°C for 30 min. All chemicals wereused as received.

Transistor fabrication and characterizationTransistors were fabricated in bottom-gate, top-contact architectureon wafers of highly doped Si (Si++) equipped with a 400-nm-thickthermally grown layer of SiO2 acting as the gate electrode and the gatedielectric, respectively. The Si++/SiO2 wafers were cleaned by subsequentultrasonication in DI water, acetone, and isopropanol for 10 min each,followed by ultraviolet/ozone treatment for 15 min. Deposition ofthe metal oxide layers was performed using the methods describedabove. Transistor fabrication was completed with the deposition oftop Al S-D electrodes under high vacuum (~10−6 mbar). Thechannel width (W) and length (L) of the resulting devices were1000 and 100 mm, respectively. The electrical characterization ofthe transistors was carried out at room temperature in a nitrogenglove box using an Agilent B2902A parameter analyzer or at differ-ent temperatures in a cryogenic probe station (ST-500, Janis Re-search) under high vacuum (10−5 mbar) using a Keithley 4200semiconductor parameter analyzer. The field-effect mobility wasextracted from the transfer characteristics in the linear and saturationregimes using the gradual channel approximation model

mLIN ¼ LW CiVD

∂ID;lin∂VG

ð3Þ

msat ¼L

W Ci

∂2ID;sat∂ V2

Gð4Þ

where L and W are the channel length and width, respectively, Ci isthe capacitance of the gate dielectric, VD is the S-D voltage, and VG isthe gate voltage.


Material characterization techniquesTopography and roughness of semiconductor samples were measuredvia intermittent contact mode AFM (Agilent 5500 AFM system). ATitan 80-300 Super Twin microscope (FEI Company) operating at300 kV and a JEOL 2011 electron microscope operating at 200 kVwere used to perform HRTEM imaging and analysis. Two TEM sam-ple preparation techniques were used as follows: (i) Ion beam milling(Ga ion beam, 30 kV, and 9 nA) to cut a specimen from the bulk sam-ple, which was then attached to a Cu grid in a lift-out method and fur-ther thinned down to ca. 50 nm thick and cleaned (2 kV and 28 pA) toremove areas that were damaged during the thinning process. Beforethe milling process, the sample surface was protected against ion bom-bardment via electron beam–assisted carbon and platinum deposition.(ii) The standard sandwich technique, followed by tripod polishing toreach a 30-nm-edge thickness. Electron transparency to less than 15 nmwas achieved by precision low-voltage (5 to 1 keV) Ar+ ion milling in aGatan precision ion polishing system.

Core-level and VB XPS spectra were acquired in a KRATOS AXISUltra DLD system equipped with a monochromic AlKa x-ray source,a hemispherical sector electron analyzer, and a multichannel electrondetector. Depth profile analysis was performed by mild, destructivein situ sputter etching using a 500-eV defocused Ar+ beam to achievethe required depth resolution. The XPS measurements were acquiredusing 20-eV pass energy, resulting in a full width at half maximum ofthe Ag-3d peak of less than 500meV. Spectral shifts due to charging ofthe surface were evaluated and subtracted on the basis of the spectralpositions of the C-1s peak of adventitious carbon and Ar-2p peak aftersputter etching.

SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/3/e1602640/DC1section S1. Power law fits of electron mobilitysection S2. Geometric phase analysissection S3. Core-level spectrasection S4. Tauc analysis of optical absorption spectrasection S5. Carrier depth profiling via C-V analysisfig. S1. Log10-log10 plots of the field-effect electron mobility (mFE) as a function of gate voltage(VG) for single-layer In2O3 and In2O3/ZnO heterojunction transistors.fig. S2. Composite diffractogram obtained by FFT of an HRTEM image of an In2O3/ZnO sample,along the growth direction of the heterojunction.fig. S3. Electron microscopy analysis.fig. S4. O-1s core-level spectra.fig. S5. Depth profile of the In-3d core-level spectra.fig. S6. Depth profile of the Zn-2p core-level spectra.fig. S7. Variation of the spectral position of the In-3d peak versus the [Zn] composition, whichis proportional to the ZnO thickness.fig. S8. Variation of the spectral position of the Zn-2p peak versus the [Zn] composition, whichis proportional to the ZnO thickness.fig. S9. The XPS valence band spectra of thick and pure In2O3 (blue) and ZnO (black); the reddashed lines are used to determine the upper edge of the VB.fig. S10. Optical absorption spectra.fig. S11. Capacitance-voltage (C-V) measurements.References (57, 58)

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AcknowledgmentsFunding: H.F., Y.-H.L., and T.D.A. were supported by the European Research Council AMPROproject no. 280221. Author contributions: H.F., Y.-H.L., and T.D.A. conceived the ideaof a solution-processed bilayer metal oxide channel for the fabrication of electron-transporting


TFTs. H.F., S.D., Y.-H.L., and T.D.A. fabricated the metal oxide samples, designed and conductedthe charge transport experiments, analyzed the experimental data, and wrote the manuscript.N.P. and P.A.P. designed and conducted the XPS measurements and wrote part of themanuscript. T.K., G.D., K.Z., and A.A. conducted the electron microscopy measurementsand relevant data analysis and provided feedback on the manuscript. Competing interests:The authors declare that they have no competing interests. Data and materials availability:All data needed to evaluate the conclusions in the paper are present in the paper and/orthe Supplementary Materials. Additional data related to this paper may be requested fromthe authors.

Submitted 26 October 2016Accepted 10 February 2017Published 31 March 201710.1126/sciadv.1602640

Citation: H. Faber, S. Das, Y.-H. Lin, N. Pliatsikas, K. Zhao, T. Kehagias, G. Dimitrakopulos,A. Amassian, P. A. Patsalas, T. D. Anthopoulos, Heterojunction oxide thin-film transistorswith unprecedented electron mobility grown from solution. Sci. Adv. 3, e1602640 (2017).

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solutionHeterojunction oxide thin-film transistors with unprecedented electron mobility grown from

Amassian, Panos A. Patsalas and Thomas D. AnthopoulosHendrik Faber, Satyajit Das, Yen-Hung Lin, Nikos Pliatsikas, Kui Zhao, Thomas Kehagias, George Dimitrakopulos, Aram

DOI: 10.1126/sciadv.1602640 (3), e1602640.3Sci Adv

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