enhancement of long-term stability of pentacene thin-film transistors encapsulated with transparent...

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Applied Surface Science 252 (2005) 1332–1338

Enhancement of long-term stability of pentacene thin-film

transistors encapsulated with transparent SnO2

Woo Jin Kim a, Won Hoe Koo a, Sung Jin Jo a, Chang Su Kim a,Hong Koo Baik a,*, Jiyoul Lee b, Seongil Im b

aDepartment of Metallurgical Engineering, Yonsei University, Seoul 120-749, Republic of Koreab Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea

Received 18 January 2005; received in revised form 14 February 2005; accepted 16 February 2005

Available online 11 April 2005

Abstract

The long-term stability of pentacene thin-film transistors (TFTs) encapsulated with a transparent SnO2 thin-film prepared by

ion beam-assisted deposition (IBAD) was investigated. After encapsulation process, our organic thin-film transistors (OTFTs)

showed somewhat degraded field-effect mobility of 0.5 cm2/(V s) that was initially 0.62 cm2/(V s), when a buffer layer of

thermally evaporated 100 nm SnO2 film had been deposited prior to IBAD process. However, the mobility was surprisingly

sustained up to 1 month and then gradually degraded down to 0.35 cm2/(V s) which was still three times higher than that of the

OTFT without any encapsulation layer after 100 days in air ambient. The encapsulated OTFTs also exhibited superior on/off

current ratio of over 105 to that of the unprotected devices (�104) which was reduced from �106 before aging. Therefore, the

enhanced long-term stability of our encapsulated OTFTs should be attributed to well protection of permeation of H2O and O2

into the devices by the IBAD SnO2 thin-film which could be used as an effective inorganic gas barrier for transparent organic

electronic devices.

# 2005 Elsevier B.V. All rights reserved.

Keywords: OTFT; Ion beam-assisted deposition; Encapsulation; Lifetime; Pentacene

1. Introduction

Organic thin-film transistors (OTFTs) have drawn

much attention due to their great compatibility with

plastic substrates available for a flexible display, smart

* Corresponding author. Tel.: +82 2 2123 2838;

fax: +82 2 312 5375.

E-mail address: [email protected] (H.K. Baik).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.02.100

cards and rf tags application over the last decades

[1–4]. Organic semiconductors also make it possible

to fabricate electronic devices at room temperature

through simple methods without using complicated

equipments [5]. Among organic semiconductors

considered as active materials in organic thin-film

transistors, pentacene was widely used for its mobility

as high as that of a-Si or in some cases even exceeding

that value [6,7]. However, when the OTFTs are

.

W.J. Kim et al. / Applied Surface Science 252 (2005) 1332–1338 1333

Fig. 1. (a) Schematic drawing of IBAD process and (b) photo-

graphic plan view (length, L = 100 mm and width, W = 1000 mm)

and cross-section of the device structure.

operated in air ambient, they tend to easily degrade in

the similar way that the lifetime of organic light-

emitting diodes (OLEDs) is limited [8]. In order to

enhance the stability of devices, some encapsulation

layers capable of protecting H2O or O2 in the air from

penetrating through the organic layers have been

employed. SiOx, AlOx, AlOxNy and SiOxNy are

common amorphous oxides used as inorganic gas

barrier [9–11]. But, during the deposition process of

the above-mentioned oxides, some serious damages

might be induced on the organic layers, which would

be attributed to energetic ions, X-rays and electron

beam [12,13]. Encapsulation with an organic material,

such as a vacuum deposited parylene was reported to

have an excellent conformal coating and successfully

protect the organic active layer from solvents in

patterning process [14]. For the sake of more reliable

operation of the devices, it is quite desirable for an

organic layer to be used along with an inorganic oxide,

because an organic material itself could not provide

sufficient capability of blocking permeation of H2O or

O2. Among the transparent amorphous oxides, which

can perform function of gas barrier deposited by

simple thermal evaporation, SiOx and SnO2 would be

taken into account. It was previously reported by our

group that SnO2 was more desirable one due to its high

polarizability resulting in effectively suppressing

permeation of water vapor by strong chemical

interaction, particularly when the film could be made

to have much denser microstructures [15]. Therefore,

the SnO2 thin-film prepared by ion beam-assisted

deposition (IBAD) is a promising candidate for

transparent thin-film encapsulation adaptable for

top-emitting organic light-emitting diodes (TE-

OLEDs) since it has relatively high transmittance

(�85% over 450 nm) as well as quite low water vapor

transmission rate (WVTR) of below 10�3 g/(m2 day).

In order to prevent possible damages induced from

energetic ions during IBAD process, it is also desir-

able that a proper buffer layer should be employed

[16].

In this paper, we report on the investigation of the

long-term stability of our OTFTs in terms of electrical

properties associated with materials characterization

by X-ray photoemission spectroscopy (XPS) when the

TFT was encapsulated with the IBAD SnO2 thin-film

having a buffer layer of same kind of material on top of

the device.

2. Experiment

Our OTFTs with the pentacene thin-film as an

active layer were fabricated on SiO2/p+-Si substrate.

The thickness of thermally grown SiO2 was 100 nm.

Prior to the deposition of pentacene, the surface of

SiO2 (to be used as a gate dielectric) was ultrasonically

cleaned with trichloroethylene, acetone, isopropylal-

cohol and deionized water sequentially. Our thermal

evaporation chamber was evacuated to a base pressure

of 1 � 10�7 Torr. We then deposited the pentacene

film through a shadow mask by heating the effusion

cell containing pure pentacene powder (99.8%,

Aldrich Chem. Co.) at a rate of 0.5 A/s at room

temperature (RT). The resulting film thickness of the

pentacene film was adjusted to 50 nm as measured

with a quartz crystal monitor. The source and drain

contacts were patterned by a second shadow mask,

through which Au was thermally evaporated to the

thickness of about 100 nm. The channel length,

defined as the spacing between the source and drain

pads, was 100 mm and the channel width was

1000 mm. SnO2 (to be used as a buffer layer) was

W.J. Kim et al. / Applied Surface Science 252 (2005) 1332–13381334

then deposited on top of the device by thermal

evaporation through a third shadow mask to the film

thickness of about 100 nm. Finally, another 80 nm

SnO2 thin-film (to be used as an encapsulation layer)

was consecutively deposited without breaking vacuum

by IBAD process shown in Fig. 1(a). The end-hall ion

gun was used to increase SnO2 ad-atom mobility. Ar+

ion energy and current density were 150 eV and

45 mA/cm2, and the deposition rate was 1 nm/s. In all

cases of ion beam irradiation conditions, the incident

angle of the ion beam was 458. Fig. 1(b) shows

photographic plan view and schematic cross sectional

view of our pentacene OTFT with encapsulation.

Fig. 2. (a) The output characteristics of the reference OTFTwithout

any encapsulation and (b) degradation of the drain current in the

saturation regime when VG is �40 V after 100 days in air.

3. Results

The I–V measurements of our OTFTs were

performed with an HP 4145B parameter analyzer in

the dark at RT. Fig. 2(a) displays the output

characteristics of our OTFT without encapsulation

and one can see the typical drain current–voltage (ID–

VD) curves of the FET devices. The drain current in the

saturation regime at a gate bias of �40 V was around

�120 mA and the leakage current measured between

the gate and source electrode (IG) was below 2 nA. It is

then clear that the measured drain current could be

used for evaluation of the electrical performance of

our OTFTs. The curves in Fig. 2(b) are representing

the drain current of the OTFT without encapsulation

(open circle) and with encapsulation (close circle)

when the gate bias of VG is�40 V. According to Fig. 2

(b), the saturation current of the unprotected OTFT is

decreased to �35 mA from 120 mA after 100 days in

air ambient, whereas the encapsulated device showing

a relatively low initial current of�100 mAmaintained

considerable current level of �60 mA. The initial

degradation of the OTFTwith encapsulation might be

attributed to some possible damages induced on the

pentacene active layer during the encapsulation

process in which small portion of the energetic ions

could penetrate through the thermally evaporated

buffer layer [16].

The plots offfiffiffiffiffiffiffiffiffi

�IDp

versus VG and log10 (� ID)

versus VG with and without encapsulation are

presented in Fig. 3(b and c), respectively. The field-

effect mobility of holes in the pentacene layer was

obtained from the slope in VG versusffiffiffiffiffiffiffiffiffi

�IDp

plot in the

saturation regime where the source–drain voltage of

VD = �40 V was applied. The initial field-effect

mobility of the encapsulated and unprotected OTFT

was calculated to be 0.5 and 0.62 cm2/(V s), respec-

tively. As expected from serious degradation of

saturation current of the OTFT without encapsulation

in Fig. 2(b), the mobility of the device decreased

sharply from 0.62 to 0.10 cm2/(V s) after 100 days.

On/off current ratio and sub-threshold slope also

showed a similar degradation trend according to the

log10 (� ID) versus VG plots. The on/off current ratio

decreased down to �104 from initial value of �106

with a very inferior sub-threshold slope of 7.2 V/dec.

In the mean while, the encapsulated device maintained

an on/off current ratio of �105 even after 100 days

with similar sub-threshold slope to the initial value. It

is also interesting to note in theffiffiffiffiffiffiffiffiffi

�IDp

versus VG plot

that the threshold voltage (VT) changes toward positive


gate bias showing early turn-on upon degree of

degradation. Although the initial VT values of both

devices were almost same as approx. �2.5 V, the

unprotected device displayed about 7 V as shown in

Fig. 3(a), while the same-period-aged but encapsulated

one showed only �0.1 V of VT after 100 days in air.

Since the pentacene channel/SiO2 interfaces should

be in the same condition for all OTFTs, we suspected

if water molecules exist in the channel layer. Horowitz

demonstrates that the surface of gate dielectrics in

contact with organic channel layer has so many

defects that it would strongly aggravate the charge

transport between the source and drain electrode [17].

Fig. 3. The plots offfiffiffiffiffiffiffiffiffi

�IDp

vs. VG and log10 (� ID) vs. VG. for the

OTFTs without encapsulation (a) and with encapsulation (b).

Hence, we should take into account the organic

molecular layer beyond the first monolayer in the

channel region to find out what would be responsible

for the total charge transport. The water molecules in

air are known to be adsorbed on pentacene surface or

even penetrate into the channel layer during aging

period [18]. Then, they could be one of the main

leakage sources or active deep level traps when the

OTFT is particularly under a high electric field of

depletion state [19].

The property-endurance limit or aging effect of our

OTFTs in terms of field-effect mobility and on/off

current ratio upon time are shown in Fig. 4(a and b),

respectively. The encapsulated TFT shows the best

performance even after a long time exposure to air.

The initial degradation of mobility from 0.62 to

0.5 cm2/(V s) is the expected behavior in line with

Fig. 4. The property-endurance limit or aging effect with time of our

OTFTs in terms of field-effectmobility (a) and on/off current ratio (b).


reduction of the drain current of the encapsulated

OTFT in Fig. 2(b). However, the SnO2 film was

deposited by using IBAD on top of the device without

any buffer layer, the device was found to seriously

degrade due to direct penetration of energetic ions into

the organic active layer [16]. In order to avoid

deterioration of the device during IBAD process, it is

strongly required to employ a proper buffer layer of

which is thermally evaporated SnO2 in this study.

Unfortunately, thermally evaporated SnO2 film had

quite open structure resulting in a loose microstructure

and this would also be known from the fact that, when

being used alone, it could not effectively suppress H2O

permeation into the device as indicated in Fig. 4(a).

Therefore, the buffer layer itself could not play an

important role in blocking permeation of water vapor

and it is the IBAD SnO2 layer that performs this

function.

4. Discussion

It was once reported that the main reason for

degradation of the OTFTs with time was adsorption

and penetration of H2O into the active layer of a

pentacene film [18]. Fig. 5 represents O 1s peaks that

we obtained from the both surfaces of aged and un-

aged pentacene films as they were scanned by X-ray

photoemission spectroscopy. All the peak energies

were calibrated with C 1s peak. Since OH molecular

groups in pentacene are thought to be involved during

Fig. 5. XPS spectra for O 1s peaks obtained from the pentacene

thin-films before and after exposed to air for 100 days.

pentacene film deposition, the O 1s peaks from the OH

groups in aged and un-aged pentacene should be the

same each other in intensity. It is then clear that the

100-day-aged pentacene film has much higher density

of H2O than the un-aged film. Total amounts of H2O

and OH group inside of the pentacene film could be

estimated from corresponding O 1s peaks and

calculated to be �1.7 and �5% for the un-aged and

aged one, respectively. It is thus, considered that the

water molecules are crucial clues for the observed

property-degradation.

With application of encapsulation to the OTFTs,

the long-term stability in terms of the electrical

properties was tremendously enhanced. This should be

attributed to well protection of the devices from water

vapor or oxygen in air due to our IBAD SnO2 film.

Fig. 6 displays XPS O 1s spectrum of the IBAD SnO2

film comprised of two main peaks which were

originated from the OH groups at around 531.6 eV

and the O 1s peak corresponding to the Sn4+ at around

530.3 V while the inset in Fig. 6 represents the peak

from Sn 3d3/2 (495.3 eV) to 3d5/2 (486.9 eV). It was

clearly shown that the SnO2 film had O 1s peak from

OH group inside of the film, which is a result of higher

polarization and refractive index of the SnO2 film [20].

The H2O has a large dipole moment and lone-pair

electrons, thus is a good donor. Molecular adsorption

occurs by acid/base interaction between water vapor

and the oxides [21]. Therefore, the continuous

transmission of water vapor is restrained by the

interaction between the formed OH groups and water

vapor. However, although the thermally evaporated

SnO2 film has the high refractive index of about 1.9

and considerable amount of OH groups inside of film,

it is obvious that this layer could not prevent

permeation of H2O effectively as shown in

Fig. 4(a). Here, we should consider the influence of

the film density on the water vapor permeation,

because if the pore size and density are too large and

high, the permeating water vapor would be expected to

penetrate the films without interaction through the

central pore space away from the surface OH groups

within pores [15]. Consequently, when encapsulated

with the IBAD SnO2, the device displays much

enhanced performance in terms of long-term stability

due to its quite low WVTR, which should come from

strong chemical interaction between penetrating polar

water vapor and hydroxyl elements inside of the film


Fig. 6. XPS spectra for O 1s peaks and Sn 3d2/3 and 3d2/5 spectra (inset) from the SnO2 thin-film prepared by IBAD.

in conjunction with a dense microstructure by IBAD

process.

However, even the encapsulated devices finally

began to degrade its field-effect mobility with time

after 1 month. This might be attributed to our

pentacene films with a very rough surface of rms

9.3 nm as measured by atomic force microscope

(AFM) in the channel region shown in Fig. 7(a). Since

the IBAD SnO2 thin-film with a buffer layer was

deposited on the rough pentacene surface, the

encapsulation layer was also found to be rough

following underlying layer. Fig. 7(b) shows the top

surface of the encapsulation layer prepared by IBAD

Fig. 7. AFM images of 50 nm pentacene thin-film (a) and 80 nm IBAD

and according to a section analysis of AFM profile, the

vertical distance between marks was found to reach as

high as 50 nm where the thickness of the IBAD SnO2

layer was 80 nm. Therefore, in addition to a dense

microstructure, the barrier film should have a flat

surface because vapor permeation should be mainly

dominated by diffusion via the shortest path in the

film. For this reason, a thin-film encapsulation

consisted of multi-layer was more desirable to

enhance the barrier properties and some polymer

smoothing layers could be used to make a surface flat

on which the inorganic barrier film will be deposited.

Hence, a further study would be required to make our

SnO2 encapsulation layer on top of 100 nm SnO2 buffer layer (b).


encapsulation meet the requirements needed for stable

operation of a real device in air ambient.

5. Conclusion

We have fabricated pentacene TFTs with the

transparent SnO2 encapsulation layer prepared by

IBAD to enhance long-term stability of the devices.

Although the encapsulated OTFTs showed somewhat

degraded field-effect mobility of 0.5 cm2/(V s), the

devices exhibited much enhanced electrical perfor-

mances sustaining the mobility up to 1 month. On/off

current ratio also prevails in the case of the

encapsulated devices showing around 105 that is just

one order of magnitude lower than that of its initial

value of�106, while the unprotected OTFT shows on/

off current ratio of �104 after 100 days. The IBAD

SnO2 was found to effectively prevent H2O or O2 from

permeating into the devices by means of densification

of microstructures through IBAD process and che-

mical interaction between water vapor and SnO2

which resulted in quite low WVTR value accounting

for enhancement of the long-term stability of the

OTFTs. It is thus, concluded that a transparent SnO2

thin-film prepared by IBAD is a promising material

that could be used as an inorganic gas barrier for

transparent organic electronic devices with an appro-

priate buffer layer including some smoothing layers.

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enhancement of long-term stability of pentacene thin-film transistors encapsulated with transparent...

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