Paper Field Effect Transistor

Invited Paper

Paper Field Effect Transistor E. Fortunato*a, Nuno Correiaa, Pedro Barquinhaa, Cláudia Costaa,b, Luís Pereiraa, Gonçalo Gonçalvesa, Rodrigo Martinsa a Materials Science Department, CENIMAT/I3N and CEMOP/UNINOVA, Faculty of Sciences and Technology of New University of Lisbon, Campus de Caparica, 2829-516 Caparica, Portugal b YDreams - Informática S.A., YLabs, Campus de Caparica, 2829-516 Caparica, Portugal ABSTRACT In this paper we report the use of a sheet of cellulose fiber-based paper as the dielectric layer used in oxide based semiconductor thin film field-effect transistors (FETs). In this new approach we are using the cellulose fiber-based paper in an “interstrate” structure since the device is build on both sides of the cellulose sheet. Such hybrid FETs present excellent operating characteristics such as high channel saturation mobility (>30 cm2/Vs), drain-source current on/off modulation ratio of approximately 104, near-zero threshold voltage, enhancement n-type operation and sub-threshold gate voltage swing of 0.8 V/decade. The cellulose fiber-based paper FETs characteristics have been measured in air ambient conditions and present good stability. The obtained results outpace those of amorphous Si TFTs and rival with the same oxide based TFTs produced on either glass or crystalline silicon substrates. The compatibility of these devices with large-scale/large-area deposition techniques and low cost substrates as well as their very low operating bias delineates this as a promising approach to attain high-performance disposable electronics like paper displays, smart labels, smart packaging, RFID and point-of-care systems for self analysis in bio-applications, among others.

Keywords: Oxide field-effect transistor (FET), cellulose fibers, thin films, rf magnetron sputtering; oxide semiconductors.

1. INTRODUCTION Today there is a strong interest in the use of biopolymers for electronic applications, mainly driven by low-cost applications. Cellulose is the earth’s major biopolymer and is of tremendous global economic importance. This natural polymer represents about one-third of plant tissues and can be restocked by photosynthesis. The biosynthesis of this polymer is about 1 000 tones by year in the world. Cellulose is the structural component of the primary cell wall of green plants. About 33 percent of all plant matter is cellulose (the cellulose content of cotton is 90 percent and that of wood is 50 percent). For industrial use, cellulose is mainly obtained from wood pulp and cotton. It is mainly used to produce cardboard and paper; to a smaller extent it is converted into a wide variety of derivative products such as cellophane and rayon. Cellulose is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand (14) linked D-glucose units. Some bacteria can convert cellulose into ethanol which can then be used as a biofuel as an alternative fuel source[1] The possibility to integrate electronic and optoelectronic functions within the production methods of the paper industry is therefore of current interest, in order to enhance and to add new functionalities to conventional cellulose fiber based-paper. To fulfill these demands materials and methods should be developed for cheap and mass production. An example is printed electronics using ink-jet technology [2-4] or even classical printing methods like reel-to-reel processing [5]. Some reports have been presented recently using cellulose based paper as either a substrate for physical support of the processing devices, like organic thin-film transistors [6], logic circuits [7] and electrochromic displays [8] or as an active media in thin film flexible Li batteries [9], originating what we call Paper Electronics as opposed to Electronic Paper (e-paper) [10]. Recently it was demonstrated by the firsts time the

*

[email protected]; phone +351 21 2948562; fax +351 21 2948558. Zinc Oxide Materials and Devices IV, edited by Ferechteh Hosseini Teherani, Cole W. Litton, David J. Rogers, Proc. of SPIE Vol. 7217, 72170K · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.816547

Proc. of SPIE Vol. 7217 72170K-1

possibility to produce FETs paper.

[11]

as well as Random Access Memories (RAMs)

[12]

using a conventional sheet of

In this paper we report the use of a conventional sheet of cellulose fiber based paper simultaneously as the substrate and the dielectric layer in oxide FETsfield effect transistors, fully processed at room temperature. A detailed physical and chemical analysis of two kinds of papers has been done in order to correlate their properties with the device performance. In this new device approach we are using the cellulose fiber-based paper as an “interstate” structure since the device is build on both sides of the cellulose sheet: the gate electrode, based on a transparent conductive oxide (TCO), is deposited on one side and on the other the active semiconductor, to be used as the channel layer, and the highly conductive drain and source regions [13-15].

2. EXPERIMENTAL DETAILS The proposed FETs were produced using two different cellulose fiber-based papers (with different finishing surfaces), acting as the gate dielectric, without any kind of post surface treatment. On one side of the paper sheet a 40 nm thick GIZO (Ga2O3-In2O3-ZnO; 1:2:1 mol) layer (the active oxide semiconductor) was deposited by r.f. magnetron sputtering, at room temperature, in a Pfeiffer Vacuum Classic 500 system. The Aluminum source and drain regions were deposited afterwards by e-beam evaporation (180 nm) over the patterned semiconductor region. On the other side of the paper sheet an IZO (In2O3-ZnO; 5:2 mol) film (160 nm) was deposited by r.f. sputtering at room temperature, to serve as the gate electrode. The paper sheet works simultaneously as the substrate and the device’s dielectric layer. Figure 1 shows a schematic representation of the proposed device configuration illustrating the different layers and corresponding thicknesses. The surface characterization of the cellulose paper was done by scanning electron microscopy (SEM) using a SU-70 FE from Hitachi. X-ray diffraction experiments were performed with the Bruker-AXS D8Disvover in -2 Cu KD1 line collimated with a Gobël mirror, a 2 bounce Ge(220) asymmetry monochromator and a divergent slit of 0.6 mm. A 2 range from 10º to 90º with a step size of 0.02 º, a 0.2 mm detector slit and an acquisition time of 1s was used. Contact angle measurements were performed using a CAM 100 system, the liquid used was desionized water. The contact angle value was obtained by the software using the picture taken immediately when the drop (manually deposited, 5 Pl) touches the surface of the sample. Standardless chemical analysis (semi-quantive using the IQ+ software package) was performed using a PANalytical XRF-WDS 4 kW AXIOS spectrometer under He flow. Thermogravimetry measurements were performed by using a Rheometric Scientific TG 1000 thermobalance operating under a flowing argon atmosphere in order to prevent any thermoxidative degradation. A heating rate of 10 °C.min-1 was used to analyze all the samples. The electrical characteristics of the oxide films were determined from Hall effect measurements using the Van der Pauw geometry (Biorad HL5500) at a constant magnetic field of 0.5 T. The FETs were electrically characterized in air, at room temperature and in the dark using a Cascade Microtech M150 microprobe station connected to a semiconductor parameter analyzer (Agilent 4155C) controlled by the Metrics ICS software. The films thicknesses were measured with a Sloan Tech Dektak surface profilometer.

Fig. 1. Schema of the FET structure using the cellulose sheet as the gate dielectric.


3. RESULTS AND DISCUSSION 3.1 Paper characterization Prior to the deposition of semiconductor films the substrates of paper were characterized. Figure 2 shows the SEM micrographies of the two papers. From the images we can see that paper A has a better definition of the fibers while in the case of paper B the fibers are not so individualized, maybe due to a different manufacturing process, including finishing surface. Besides that it is also possible to infer from the surface analysis that paper B shows a more compact and homogeneous structure. Concerning the surface roughness we have measured it by profilometry and the two papers present similar average values in the range of 3-4 Pm.

Paper A

/

Paper B

;

Sub 150KV 60mm x300 SE(M)

Paper A

SU7O 150KV 5.9mm x300 SE(M)

Paper B

Fig. 2. SEM images showing the type of microstructure exhibited by cellulose fibers based paper with two different magnifications.

In order to analyze the crystallinity of the two cellulose based papers X-ray diffractometry was carried out. The results are shown in figure 2. From the data it is possible to qualitatively evaluate that both samples present the same diffraction features as those observed for semi-crystalline cellulose type I, which is evidenced by the lattice plane (101) at 2Tdegrees equal to 15.6 and the lattice plane (200) at 2Tdegrees equal to 22.5 [16]. These results are also in accordance with data available from commercial cellulose [17, 18].


Intensity (u.a)

(200)

(101)

paper B paper A

10

15

20 25

30

35

40

45 50

55

60

2T (degrees) Fig. 3. XRD diffractograms for the two cellulose based papers. The inset shows the three-dimensional structure of [19] cellulose, with four glucose units visible. (Black=carbon; red=oxygen; white=hydrogen.) .

We can also determine the degree of crystallinity, crystallinity index (Ic) using the empirical method [20]:

Ic

I

( 002 )

I ( am )

I ( 002)

u 100

(1)

where I(002) is the maximum intensity of (002) lattice diffraction and I(am) corresponds to the minimum between (002) and (101) close to 18° representing amorphous the material in cellulosic fibers. The results show that the crystallinity was similar for the two cellulose based papers, 75% for paper A and 79% for paper B. Considering that hemicelluloses has a random amorphous structure we can conclude that the samples have a small quantity of hemicelluloses. Figure 3 shows the water contact angle measurement for a drop of water on the surface of the two cellulose papers. From the observation we can conclude that paper A presents a more hydrophobic nature (the contact angle is higher than 90°), while for paper B a more hydrophilic surface is obtained. This observation was supported by elemental analysis carried out by X-Ray fluorescence, where it was found that paper A has about 2% of fluor, while no traces have been detected for paper B, which confirms the hydrophobic nature of paper A.

Paper A

a Paper B

Fig. 3. Shape of water drop on surface of the two papers: contact angle of paper A is 123.69º and for paper B is 83.48º.


The thermogravimetry of the two papers is shown in fig. 4 a) where it is represented the residual mass (wt%) as a function of temperature, which permits the study of the decomposition of the two commercial papers. For both cases two main weight loss regions are observed: the first one in the 25-150 °C region and the other one in the 260-400 °C region. The first decomposition region is associated with moisture and desorption of low molecular weight compounds remaining from the isolation manufacture process. Both papers show moisture losses in amounts of 6 wt% in the 25-150 °C range. For the second weight loss region, it is possible to observe that the main decomposition process occurs in the 300-400 °C range, where cellulose decomposition takes place [21]. Figure 4 b) presents the moisture uptake for the first decomposition region where a faster initial mass reduction is observed for paper A which is related to the hydrophobic tendency presented by this paper [22].

paper B

100

Residual mass (%)

90 paper A 80

degradation (~260 ºC)

70 60 50 40 30 20

0

100

200

300

400

Temperature (ºC)

Residual mass (%)

105

100

95

90 25

50

75

100

125

150

Temperature (ºC)

Fig. 4. a) Thermogravimetry analysis of the two commercial cellulose based papers; b) First weight decomposition region for the two commercial cellulose based papers.


3.2 Films characterization In order to evaluate the electrical properties presented by cellulose based substrates as well as to determine the effectiveness of their coverage an IZO thin film (gate electrode) was deposited on top of the two different substrates. Electrical resistivity and Hall mobility measurements were carried out at room temperature. The obtained values are presented in table 1 and compared to those obtained for IZO deposited on regular soda-lime glass. The IZO films deposited on cellulose paper substrates show some degradation of their electrical performance when compared with films deposited on glass substrates. The resistivity increases about 2.5 times with a significant decrease associated to the Hall mobility, from 35 cm2/Vs to around 8 cm2/Vs in average, due to the chemical nature of the surface of the cellulose and to the high roughness exhibited by these surfaces. The roughness is responsible for a decrease on the electrical mobility. Still, the IZO properties obtained on paper are still adequate for its usage as a TCO for low cost applications. These results are in agreement with previous work [11].

Table. 1. Comparison of the electrical properties of amorphous IZO thin films deposited on different substrates.

Substrate

Resistivity (:cm)

Hall mobility (cm2/Vs)

Soda-lime glass Cellulose fiber Type A Cellulose fiber Type B

4.0u10-4 1.0u10-3 1.3u10-3

35.0 6.7 10.0

Carrier concentration (cm-3) 4.4u1020 9.3u1020 4.8u1020

In order to observe the cellulose fibers surface coverage by the IZO film a high magnification SEM was performed (30k). Figure 5 a) shows a fracture that occurred due to the overheating induced by the microscope electron beam. The film presents an amorphous structure and covers all the free surface of the cellulose fibers. An Energy Dispersive X-ray Spectroscopy (EDXS) spectrum acquired on a fiber’s surface reveals that the In, Zn and O elements are indeed predominant in its composition (fig. 5 b)).

Counts (impulse/eV)

250

b)

200

O

150 100 50 0

In

C ZnAl In 0

In

2

In Zn 4

6

8

10

12

14

Energy (keV)

Fig. 5. a) IZO crack due to the different thermal expansion coefficient of IZO and the cellulose fiber, revealing the high adhesion coefficient of the oxide material to the cellulose fiber. b) EDXS analysis of the paper surface, revealing the IZO elements.

Both IZO and GIZO films deposited on glass or silicon at room temperature are amorphous, remaining in most cases without appreciable crystallization until 500 °C. Figure 6 a) shows a SEM image of the surface of an IZO film on glass, being notorious that the films present a very smooth surface. Only for ternary oxides (i.e., IZO) with a high In content was verified evidence of crystallization at 500 °C (fig. 6 b)) [23]. For GIZO, regardless of its composition, the


films remain always amorphous, with the transmission electron microscopy (TEM) analysis revealing only diffuse and broad electron diffraction rings, typical of amorphous films with some short range order.

Counts (a.u.)

IZO 1:2 IZO 2:1 GIZO 1:4:2 GIZO 1:2:1

0

Films annealed @ 500ºC

20

30

40

50

60

70

80

Annealing @ 500°C

2*Theta (º)

Fig. 6. Structural and morphological characterization on IZO and GIZO films on glass and silicon: a) SEM of an IZO film; b) XRD of IZO and GIZO films annealed at 500 °C; c) Electron diffraction pattern obtained by TEM of a GIZO film.

Concerning the electrical performance, the IZO films produced at room temperature show good electrical properties, namely low resistivity and high Hall mobility (table 1), but as the films are annealed at higher temperatures their electrical properties are degraded as a consequence of the crystallization process, since the grain boundaries inhibit carrier transport in oxide semiconductors [23, 24] The addition of gallium (GIZO) turns the films deposited under the same conditions as IZO more resistive, as a result of oxygen vacancy annihilation, since gallium forms stronger bonds with oxygen than zinc or indium. Since oxygen vacancies are the main source of free carrier generation in oxide semiconductors, the carrier concentration of GIZO is lower than IZO, turning these films more suitable for active layer application on transistors, like demonstrated in this work.

3.3 Device characterization Figure 7 shows the transfer characteristics of two typical GIZO FETs, with W/L = 10.6, in the saturation region (VD = 15 V) using the two types of cellulose based fibers as the dielectric layer. Saturation mobility (μsat) and threshold voltage (VT)were calculated from the derivative and the x-axis interception of the ID(VG) plot, respectively. The subthreshold gate swing value (S) was obtained at the maximum slope of dVG/d(logID). Table 2 presents a comparison of the electrical parameters of the two series of paper-based GIZO FETs and of a typical GIZO FET fabricated on crystalline silicon, using thermal SiO2 as the dielectric layer (100 nm thick).


-3

10

VD= 15 V -4

10

W/L = 10.6

ID (A)

-5

10

-6

10

Cellulose type A

-7

10

Cellulose type B

-8

10

-20

-10

0

10

20

VG (V)

~

Fig. 7. ID–VG transfer characteristics obtained at VD = 15 V (saturation region) for GIZO FETs using two types of cellulose fibers as dielectric layers, fully produced at room temperature.

Table. 2. Comparison of the electrical properties of GIZO based FETs with different dielectric materials. Substrate (dielectric) Si/SiO2 Cellulose Type A Cellulose Type B

μsat (cm2/Vs) 30 30 34

VT (V) + 1.8 - 0.6 + 1.9

ION/IOFF 8.0×108 5.9×103 2.9×104

S (V/dec) 0.4 0.8 0.8

The obtained electronic performance of the devices do not differ significantly when the cellulose fibers are used as dielectric. The main difference is in the IOFF value, which is around four orders of magnitude higher than that of GIZO FETs using SiO2 as the dielectric layer. This is mainly related to the fact that cellulose fibers typically have an open structure (macroporosity), as can be seen in fig. 8.


Cellulosefiber surface

Crosssection

Fig. 8. SEM image of a cross section of a cellulose fiber, revealing the internal morphology.

Figure 9 shows some photographs taken on the paper based FETs.

Fig. 9. Optical micrographs of some paper transistors.

4. CONCLUSIONS We have produced high performance hybrid flexible field effect transistors using cellulose fiber (without any treatment) as the dielectric layer and a semiconductor oxide (GIZO), deposited by r.f. magnetron sputtering at room temperature, as the channel layer. The transistors processed under this way have an enhancement n-type operation mode and exhibit an almost zero threshold voltage, a channel saturation mobility exceeding 30 cm2/Vs, a drainsource current ION/IOFF modulation ratio above 104 and a sub-threshold gate voltage swing of about 0.8 V/ decade. Even two months after processing the device performances were, was unchanged revealing that they are environmentally stable (stored in air ambient conditions).


The obtained results outpace those of amorphous Si TFTs and rival with the actual state of art concerning oxide based thin film transistors produced on either glass or crystalline silicon substrates, even the ones processed or annealed at temperatures as high as 200-300 °C. The compatibility of these devices with large-scale/large-area deposition techniques and low cost substrates as well as their very low operating bias delineates this as a promising approach to attain high-performance disposable electronics like paper displays, smart labels, smart packaging, RFID and point-of-care systems for self analysis in bio-applications, among others.

ACKNOWLEDGEMENTS This work was funded by by the Portuguese Science Foundation (FCT-MCTES) through projects PTDC/CTM/73943/2006, PTDC/EEA-ELC/64975/2006. The authors would also like to thank Portuguese Science Foundation (FCT-MCTES) for the fellowships SFRH/BD/17970/2004, SFRH/BD/27313/2006 and SFRH/BDE/33291/2008 given to tree of the authors (Pedro Barquinha and Gonçalo Gonçalves, Cláudia Costa). Thanks are also due to Lucelinda Cunha for the FRX analysis, Eduardo Alves for the XRD measurements and Manuela Silva for the TGA analysis.

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