COMMUNICATION www.rsc.org/materials | Journal of Materials Chemistry

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ZnO based field-effect transistors (FETs): solution-processable at lowtemperatures on flexible substrates

Friederike Fleischhaker,* Veronika Wloka and Ingolf Hennig

Received 17th May 2010, Accepted 23rd June 2010

DOI: 10.1039/c0jm01477j

A well-performing ZnO field-effect transistor (FET) on poly-

ethylenenaphthalate (PEN) foil with solution-processed ZnO

semiconductor and dielectric material is presented for the first time.

In addition, we developed a route that allows preparation of the ZnO

semiconductor layer simply from commercially available ZnO dis-

solved in aqueous ammonia in a single processing step. The material

performance in FETs exceeds that of state of the art solution-

processable ZnO materials at comparably low processing temper-

atures (#150 �C). Polysilsesquioxanes are identified as physically

and chemically suitable dielectric materials that also fulfill the

criteria solution processability at low temperatures. Water and

ethyllactate are used as ‘‘green’’ solvents.

Electronic devices on mechanically flexible substrates have attracted

increasing attention in the last years. The idea is to turn many ‘‘daily

life’’ products in a cost-effective process into ‘‘smart’’ functional

objects. Among others, especially integrated circuit technologies

based on field effect transistors (FETs) such as bendable displays,

radio-frequency identification tags or smart cards are of high interest.

The application of solution-processable materials enables low-cost,

large-area printing technologies for manufacturing. Low-temperature

processing of the materials is crucial to allow the use of flexible,

inexpensive polymer substrates such as polyethylenenaphthalate

(PEN) or polyethyleneterephthalate (PET) foils.1 Consequently, there

is a great potential in the development of FETs that are based on low-

temperature and solution-processable semiconductors and dielectric

materials. ZnO is a very promising inorganic n-type semiconductor

material for FETs. It can also be combined with p-type semi-

conductor FETs for low-power consuming inverters. In addition

ZnO shows several advantages compared to organic n-type semi-

conductors: it is for example inexpensive, oxidation stable and

transparent in the visible region.2,3 Several strategies how to manu-

facture ZnO and other inorganic films from solution and the use in

FETs have been published. Basically three strategies are described:

(1) deposition of a precursor solution onto a substrate followed by the

conversion into inorganic material;4–6 (2) direct deposition of stabi-

lized inorganic nanoparticles onto a substrate followed by tempera-

ture treatment;7–9 and (3) a combination of the first two methods.8,9

The published solution-processable ZnO FETs primarily use silicon

wafers or glass as test substrates and silica as dielectric material.

Many of the described methods still require processing temperatures

between 200 and 600 �C. In the case of ZnO nanoparticles, these

processing temperatures are necessary to enable sintering and coa-

lescence of the particles and to remove the stabilizer. Regarding ZnO

BASF SE, 67056 Ludwigshafen, Germany. E-mail: friederike.fleischhaker@basf.com; Fax: +49 62160 43205; Tel: +49 621 60 21759

6622 | J. Mater. Chem., 2010, 20, 6622–6625

precursors, most zinc salts and complexes only convert into ZnO at

elevated temperatures. Just for very few systems — mainly based on

the precursor method — temperature treatments #150 �C are

sufficient.5,6

As far as we know the preparation of solution-processed ZnO

based transistors on low-temperature flexible polymer substrates such

as PET or PEN foil has not been described yet. The challenge here is

not only the necessity of a low-temperature (T # 150 �C), solution-

processable ZnO material, but also the identification of a chemically

and physically suitable dielectric. In addition, the dielectric should be

solution-processable at plastic compatible temperatures — analo-

gously to the ZnO semiconductor material.

Here we present a well-performing ZnO FET on PEN foil with

solution-based ZnO semiconductor and suitable dielectric material.

Also we developed an extremely facile and inexpensive method for the

preparation of ZnO films from solution at temperatures # 150 �C.

Keszler et al. use the ammine–hydroxo complex [Zn(NH3)x](OH)2

(predominantly x ¼ 4) in aqueous solution as precursor material to

prepare ZnO films for FETs. In contrast to many other ZnO precur-

sors such as zinc acetate, zinc nitrate or organometallic zinc complexes

(e.g. zinc alkylamines), [Zn(NH3)x](OH)2 leads to high-performance

ZnO FETs on silicon wafer at temperatures as low as 150 �C.6

Compared to the publication we developed a simplified and improved

preparation method of the [Zn(NH3)x](OH)2 complex: according to

literature, a Zn(OH)2 agglomerate is precipitated from ultrapure

Zn(NO3)2 and NaOH. In order to remove the counter ions Na+ and

NO3�, four centrifugation and decantation steps are necessary.

Afterwards aqueous ammonia is added. In our approach, inexpensive

commercially available ZnO (pharma quality) is simply dissolved in

aqueous ammonia (7 M). So, we use fewer reactants, less expensive

reactants and the number of reaction steps is reduced. Technical

upscale — especially after eliminating the centrifugation steps — is

easilypossible.Also impurificationof thematerial byadsorptionofNa

ions at the chemically reactive ZnO (0001) surface is avoided.

[Zn(NH3)x](OH)2 prepared by the simplified approach leads to ZnO

FETs with an electron mobility m of 1.2 cm2 V�1 s�1, an on/off ratio of

107 and a threshold voltage of 10 V. Silicon wafers with silica dielectric

layer (200 nm) are used as test substrate, [Zn(NH3)x](OH)2 (0.17 M) is

spin-coated from aqueous solution at 3000 rpm (30 s) anda calcination

temperature of 150 �C is applied. FETs are prepared in a bottom gate,

top contact fashion with aluminium source and drain contacts. Cor-

responding output and transfer curves are shown in Fig. 1. Compared

to FETs obtained from [Zn(NH3)x](OH)2 described in literature,6

electron mobility and on/off ratio are improved by one order of

magnitude. The developed ZnO preparation method does not only

impress by its facileness, but also the material performance in FETs

exceeds that of all state of the art solution-processable ZnO materials

at comparably low processing temperatures.

This journal is ª The Royal Society of Chemistry 2010

Fig. 1 Output (a) and transfer (b) curves of a ZnO FET prepared from

aqueous [Zn(NH3)x](OH)2 solution (1.1 wt% Zn, simplified synthesis) on

silicon wafer with silica dielectric. Calcination temperature: 150 �C.

Bottom gate, top contact architecture, Al source/drain contacts, and

channel width/length: 20 (width: 1.5 mm). m: 1.2 cm2 V�1 s�1, on/off ratio:

107, and threshold voltage VT: 10 V.

Scheme 1 Schematic illustration of the transfer from a bottom gate/top

contact ZnO FET on a silicon wafer with silica dielectric to a PEN foil

substrate with suitable solution-processable dielectric that has to be

identified.

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According to the published [Zn(NH3)x](OH)2 synthesis6 as well as

by our simplified approach, the highest concentration achieved was

0.17 M with respect to Zn (1.1 wt% Zn). Longer stirring, application

of ultrasound, variation of ammonia concentration or a hydro-

thermal treatment of ZnO with NH4OH solution at elevated

temperatures in an autoclave system did not lead to more concen-

trated solutions. Consequently the thickness of the final ZnO films

achieved after one spin-coating step on a silicon wafer is limited to

about 15 nm. Higher precursor concentrations are necessary to

generate ZnO films with increased thickness and to keep up the film-

thickness under different surface wetting conditions or when different

substrate topographies are used. This is for example the case when

using PEN foil as substrate with structured gate lines on the surface

or when using bottom-contact source and drain architectures. Here,

a 0.17 M precursor solution does not ensure complete covering of the

substrate with ZnO. Several time-consuming coating and tempera-

ture steps are necessary to achieve a desired thickness. A one-step

coating and temperature treatment procedure is clearly favored. For

this reason a further modified [Zn(NH3)x](OH)2 synthesis was

developed. The improved route also starts from 0.17 M

[Zn(NH3)x](OH)2 solution based on commercially available ZnO and

aqueous ammonia. The clear complex solution is then evaporated at

This journal is ª The Royal Society of Chemistry 2010

room temperature under vacuum and the resulting white precipitate

is isolated and dried. We suggest the formation of highly reactive

form of Zn(OH)2 (note that zinc hydroxide has one amorphous and

six different crystalline phases)10 showing improved solubility in

aqueous ammonia solution. In contrast to other precipitation

methods that start e.g. from zinc nitrate and sodium hydroxide and

that create Zn(OH)2 in a rapid step, here Zn(OH)2 is generated slowly

by evaporation of water and ammonia. Stirring the powder into

aqueous ammonia solution (7 M) results in a [Zn(NH3)x](OH)2

solution with up to 0.5 M Zn, corresponding to 3.3 wt% Zn.

In order to successfully accomplish the ZnO FET transfer from

wafer to polymer foil, not only the substrate needs be changed, but

the major challenge is the identification of a chemically and physically

suitable dielectric material that additionally can be processed from

solution (Scheme 1). The dielectric material suitable for our ZnO

FET system on foil was chosen according to the following require-

ments: it is aimed to be solution-processable at temperatures lower

than 150 �C but simultaneously has to be resistant to the ZnO pro-

cessing temperature #150 �C. Furthermore the dielectric layer

should exhibit low specific leakage currents (I/A < 10�8 A cm�2 at

0.5 MV cm�1), a low frequency dependence of the dielectric permit-

tivity (3(20 Hz)/3(100 kHz) z 1) and it should lead to good electron

mobilities with ZnO (m > 10�2 cm2 V�1 s�1). Also homogeneous films

and resistance to [Zn(NH3)x](OH)2 solution are required. Before the

actual fabrication of ZnO FETs with a chosen dielectric material on

PEN foil, we tested the dielectric material separately in a capacitor

and as additional dielectric layer to silica in a ZnO FET on wafer.

From the capacitor pretest (performed with an Agilent LCR-meter

4284A) we gained information about the insulation and polarization

behavior of the dielectric material. The dielectric permittivity 3, the

frequency dependence of the dielectric permittivity and the specific

leakage current of the dielectric material were determined. From the

FET pretest (performed with an Agilent SMU 5273A) we obtained

information about the electron mobility at the ZnO/dielectric

interface. The respective data for various tested dielectric materials

are listed in Table 1. The tested organic polymer dielectric

materials polysulfone (Ultrason�) as well as the polystyrene

derivatives (poly(4-tert-butylstyrene), poly(4-vinylbiphenyl), poly(2,6-

dichlorstyrene), poly(4-vinylphenol), poly(vinylcyclohexane), and

poly(vinylphenol) with poly(melamin-co-formaldehyde)) were

chosen because of their high glass temperatures exceeding 150 �C

or at least 130 and 135 �C for poly(4-tert-butylstyrene) and

poly(vinylcyclohexane), respectively. In the latter two cases, the ZnO

processing temperature was lowered to 130 and 135 �C, respectively.

Although the tested organic polymers showed mainly good results in

the capacitor test (Table 1), the electron mobilities m in the FET

pretest were rather poor (m: 10�6–10�3 cm2 V�1 s�1). We also

J. Mater. Chem., 2010, 20, 6622–6625 | 6623

Table 1 Dielectric films from various materials pretested in a capacitor system and as an additional dielectric layer in a ZnO FET on silicon wafer. Filmthickness, calcination temperature of the FET, electron mobility determined from the FET system, dielectric permittivity, specific leakage current andfrequency dependence of the dielectric permittivity are listed.

Dielectric materialCalcinationtemp./�C

Thickness/nm

Mobility/cm2 V�1 s�1

Dielectricpermittivity 3

�lg (I/A) at0.5 MV cm�1

3(20 Hz)/3(100 kHz)

Ultrason S 3010 150 200 4 � 10�3 3.36 8.91 1.03Ultrason S 6010 150 240 4 � 10�3 3.45 8.91 1.03Poly(4-tert-butyl-styrene) 130 210 3 � 10�4 2.67 8.51 1.05Poly(4-vinylbiphenyl) 150 210 2 � 10�5 b b b

Poly(2,6-dichlorstyrene) 150 290 2 � 10�6 b b b

Poly(4-vinylphenol) 150 240 No transistor b b b

Poly(vinylcyclohexane) 135 140 2 � 10�5 b b b

Poly(4-vinylphenol)/poly(melamine-co-formaldehyde)

150 340 3 � 10�3 4.1 7.89 1.07

SOG FG65a 150 420 3 � 10�2 Short Short ShortSOG GR 653LPPa 150 870 2 � 10�2 2.23 9.42 4.74SOG GR 150a 150 690 3 � 10�2 4.79 9.16 1.05SOG B512a 150 500 9 � 10�2 6.76 6.52 2.33

a SOG: ‘‘spin-on glass’’ (solution-processable polysilsesquioxanes); FG65, Filmtronics, no specifications; GR 653LPP, Techneglas,polymethylsilsesquioxane in n-butanol/methanol mixture; GR 150, Techneglas, poly(methyl-co-phenylsilsesquioxane), 50 mol% phenyl groups, 50mol% methyl groups; and B512, Honeywell, no specifications. b Not measured because of low FET mobility.

Fig. 2 Output (a) and transfer (b) curves of a ZnO FET on PEN foil.

ZnO semiconductor prepared from aqueous [Zn(NH3)x](OH)2 solution

(3.3 wt% Zn, 1 coating step). Solution-processable poly(methyl-co-phe-

nylsilsesquioxane) is used as dielectric material. Calcination

temperature: max. 150 �C. Bottom gate, top contact architecture, Al

source/drain contacts, and channel width/length: 25 (width: 2 mm). m:

7 � 10�2 cm2 V�1 s�1, on/off ratio: >104, and threshold voltage VT: 1 V.

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investigated different polymer layer thicknesses, but did not observe

an influence on the electron mobilities. Since thermally evaporated

silica as dielectric material combined with our ZnO semiconductor

exhibited very good electronic properties in an FET on wafer

(see Fig. 1), different solution-processable polysilsesquioxanes11,12

were investigated as dielectric material (see Table 1). Besides, Si is

known to favor the formation of Si–O–Zn bonds and interactions.

Si–O–Zn interactions can for example be found in natural minerals

such as hemimorphite.13 With the empirical formula (RSiO1.5)n (R is

an organic rest), silsesquioxanes show a structural similarity to silica,

but contrary to silica they can be dissolved in common organic

solvents. On the market, polysilsesquioxanes are also available as

so-called ‘‘spin-on glasses’’ (SOGs). The electron mobilities in the

FET pretest on wafer significantly improved and were determined to

be in the range 10�2–10�1 cm2 V�1 s�1. Possible reasons for this

observation can be the following: the used polysilsesquioxanes are

very unlikely to lead to electron traps in the FET at the dielectric–

semiconductor interface. The inorganic/organic hybrid materials may

also induce a better crystallization of ZnO compared to the organic

polymers, especially at the dielectric–semiconductor interface where

the electron transport takes place in an FET. Among the pretested

polysilsesquioxanes, poly(methyl-co-phenylsilsesquioxane) (SOG GR

150, Techneglas) processed in ethyllactate as environmentally friendly

solvent led to the best capacitor test results and was used as dielectric

material in a ZnO FET on PEN foil. A 690 nm thick spin-coated film

showed specific leakage currents lower than 10�9 A cm�2 at 0.5 MV

cm�1 and a very low frequency dependence of the dielectric permit-

tivity (3(20 Hz)/3(100 kHz)¼ 1.03) (Table 1). For a FET on PEN foil

with GR 150 poly(methyl-co-phenylsilsesquioxane) solution-

processable dielectric and ZnO semiconductor layer from aqueous

[Zn(NH3)x](OH)2 precursor solution, the following results were

achieved: electron mobility m: 7� 10�2 cm2 V�1 s�1, on/off ratio: >104

and a threshold voltage VT: 1 V. The corresponding output and

transfer curves are shown in Fig. 2. The transfer curve also shows

very little hysteresis and we observe good saturation behavior in the

output curve. In order to achieve a sufficient ZnO film thickness,

6624 | J. Mater. Chem., 2010, 20, 6622–6625 This journal is ª The Royal Society of Chemistry 2010

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either three spin-coating and heating steps of a 0.17 M

[Zn(NH3)x](OH)2 precursor solution are needed or one coating and

heating step of a 0.5 M [Zn(NH3)x](OH)2 precursor solution

prepared according to our modified and improved synthesis

described above. Regarding the ZnO calcination step, we noted that

a heating ramp of about half an hour prevents the poly(methyl-co-

phenylsilsesquioxane) dielectric layer from potential cracking that can

appear when the material is directly exposed to 150 �C. A more

detailed investigation on the silsesquioxane (RSiO1.5) residues R and

investigations on siloxane/silsesquioxane composites are likely to

further improve the electronic properties of corresponding ZnO

FETs.

In summary, a well-performing ZnO field-effect transistor (FET)

on a flexible PEN substrate with solution-processable ZnO n-semi-

conductor and solution-processable dielectric at temperatures

#150 �C has been presented for the first time. [Zn(NH3)x](OH)2 in

water, solely based on commercially available ZnO and aqueous

ammonia, is used as ZnO precursor. Solution-processable poly-

silsesquioxanes are identified as physically and chemically suitable

dielectric. The best results were achieved for poly(methyl-co-phenyl-

silsesquioxane). Water and ethyllactate are used as ‘‘green’’ solvents.

A clearly simplified and improved method for the preparation of

[Zn(NH3)x](OH)2 compared to a literature process has been devel-

oped in addition. Also a method for the preparation of more

concentrated [Zn(NH3)x](OH)2 solutions reducing the number of

ZnO film processing steps was generated. The material performance

in FETs exceeds that of state of the art solution-processable ZnO

materials at comparably low processing temperatures.

This journal is ª The Royal Society of Chemistry 2010

Acknowledgements

We thank Dr A. Karpov, Dr I. Domke, Dr M. Kastler and

Dr R. Parashkov for valuable discussions. We also thank Th. Kaiser,

T. Bauer, V. Budde, P. Hubach, D. Sch€afer and Th. Kolb for

technical support.

Notes and references

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9 B. Sun and H. Sirringhaus, Nano Lett., 2005, 5, 2408.10 Gmelins Handbuch der anorganischen Chemie, Zn Erg€anzungsband,

System Nr. 32, 8th edn, Verlag Chemie, Weinheim, 1956, p. 819.11 Z. Bao, V. Kuck, J. A. Rogers and M. A. Paczkowski, Adv. Funct.

Mater., 2002, 12, 526.12 Y. Wu, P. Liu and B. S. Ong, Appl. Phys. Lett., 2006, 12, 013505.13 A. F. Holleman and E. Wiberg, in Lehrbuch der anorganischen

Chemie, ed. N. Wiberg, Walter de Gruyter, Berlin, New York,101th edn, 1995, p. 931.

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