Mechanical bending effect on the photo leakage currents characteristic of amorphous silicon thin film transistors

(1)

Short Communication

Mechanical bending effect on the photo leakage currents characteristic

of amorphous silicon thin ﬁlm transistors

M.C. Wang

a

, S.W. Tsao

b

, T.C. Chang

c,*

, Y.P. Lin

d

, Po-Tsun Liu

e

, J.R. Chen

f

a_{Physics Division, Institute of Nuclear Energy Research, Taiwan} b

Institute of Electro-Optical Engineering, National Sun Yat-set University, Taiwan

c

Department of Physics and Institute of Electro-Optical Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-set University, Taiwan

d

Department of Physics, National Sun Yat-set University, Taiwan

e

Department of Photonics and Display Institute, National Chiao Tung University, Taiwan

f

Department of Materials Science and Engineering, National Tsing Hua University, Taiwan

a r t i c l e

i n f o

Article history:

Received 12 November 2009

Received in revised form 31 March 2010 Accepted 19 April 2010

Available online 14 May 2010

The review of this paper was arranged by Dr. Y. Kuk

Keywords:

Photo leakage current a-Si:H TFTs Mechanical strain

a b s t r a c t

The photo leakage current (IPLC) characteristic of a-Si:H TFTs under different bending strains has been

studied. The larger IPLCof a-Si:H TFTs under the outward bending strain is due to larger conductivity

of a-Si:H, stemmed from the shift up of Fermi level (EF). Experimental results show the IPLCof a-Si:H TFTs

under the outward bending strain is larger than that of ﬂattened and inward bending a-Si:H TFTs in the density of states (DOS) limited region, stemmed from the lower recombination centers present in out-ward bending a-Si:H material. Furthermore, the extracted smaller activity energy (Ea) of a-Si:H TFTs

under the outward bending strain also conﬁrmed the shift of EF.

1. Introduction

a-Si:H TFTs have been widely used as switching device of AM-LCD. Recently, a portable communication system is spotlighted such as electronic paper, smart labels[1]. For these applications, the traditional glass substrate of large-area electronics must be re-placed with flexible and light-weighted substrates[2]. Except for the substrates, the electrical characteristic is also a critical issue for a-Si:H TFTs. The main objectives for flat panel display applica-tion are to enhance the field-effect mobility and to reduce the off-state leakage current under back light illumination[3]. Further-more, for flexible display application, display panels are required to sustain a certain degree of bending. Bending effect would induce strain in the electronic circuits and may affect TFT device charac-teristics. According to previous studies [4,5], the change in the resistance of a-Si:H was found as a function of strains. However, the lack of long-range order should attenuate any piezoresistive ef-fect. As a result, the transport mechanisms specific to a-Si:H would not apply to single crystal silicon[6,7]. The electrical characteristic of mechanical bending effect has been reported by Gleskova and Won et al.[8,9]. Won et al. reported that both the threshold voltage

(Vth) and the mobility increase under the tensile stress, while Vth

increases and mobility decreases under the compressive stress [8]. The IPLCcharacteristic of a-Si:H TFTs under different bending

strains, however, has not been reported yet. In this paper, the IPLC

characteristic of a-Si:H TFTs under different bending strains was investigated.

2. Experimental

Inverted–staggered a-Si:H TFTs with back-channel-etched structure were fabricated on a 50

l

m thick polyether-sulphone (PES) plastic substrate at the maximum process temperature of 150 °C, as shown inFig. 1. The device fabrication process was as follows. After a 200 nm-thick Cr gate electrode was deposited and patterned on the plastic substrate, a 300 nm-thick silicon–ni-tride, a 200 nm-thick a-Si:H active layer, and a 50 nm-thick n+

-a-Si:H layer were subsequently deposited by plasma enhanced chemical vapor deposition (PECVD) method. Finally, a 300 nm-thick Al source/drain electrode was deposited and patterned. The n+-a-Si:H layer in the TFT channel region was etched with the source/drain pattern electrodes as the mask. The channel length of TFT devices was ﬁxed at 5

l

m and the channel width was ﬁxed at 50

l

doi:10.1016/j.sse.2010.04.006

* Corresponding author. Tel.: +886 7 5252000x3708; fax: +886 7 5253709. E-mail address:[email protected](T.C. Chang).

Solid-State Electronics 54 (2010) 1485–1487

Contents lists available atScienceDirect

Solid-State Electronics

(2)

threshold voltage (Vth) with 5.52 V (extracted from the linear ID–VG

plot, where the VDS= 0.1 V), and the ION/IOFF ratio of 106 at

VDS= 10 V. The leakage current through the gate insulator is less

than 10 13_{A. The four devices were measured and reversible}

changes in the TFT transfer characteristics were observed in this study.

The a-Si:H TFTs were mechanically strained at the radius of 10 mm. Inward (outward) cylindrical bending produces the com-pressive (tensile) strain, by deﬁnition negative radius Rn (positive radius Rp). The strain in the top surface was estimated about ±0.09%[8]. The IPLCmeasurement was carried by light illumination

from the back side of substrate to compare the difference in the off-state IPLCbetween the different bending strains. The intensity

of cold cathode ﬂuorescent lamp (CCFL) back light source was ﬁxed at 3300 cd/m2_.

3. Results and discussion

Fig. 2shows the comparison of a-Si:H TFT transfer characteris-tics under different bending strains in the back light illumination. The off-state IPLCof a-Si:H TFT with the inward bending strain is

similar to the ﬂattened a-Si:H TFT in the DOS limited region. How-ever, the off-state IPLC of a-Si:H TFTs with the outward bending

strain has shown larger IPLCcharacteristic than that with the

ﬂat-tened a-Si:H TFTs in the DOS limited region. According to our pre-vious study [10], the channel sheet conductance has shown the variation under different mechanical bending strains. The variation under mechanical bending strains is originated from the evolution of defect state density in a-Si:H channel material. Because a-Si:H TFTs under the back light illumination were in the non-equilibrium

state (pn > n2

i), the trap states played the role of recombination

centers. The similar IPLCbetween the ﬂattened and inward bending

a-Si:H TFTs was resulted from the low fabrication process temper-ature (150 °C), and the larger DOS has been created during the low process temperature. As a result, the electric ﬁeld is not large en-ough to separate the photo induced electron–hole pairs in the neg-ative gate voltage region and VDS= 10 V. However, the IPLC of

outward bending a-Si:H TFTs operated in the same condition is 50% larger than the IPLC of the ﬂattened and inward bending

a-Si:H TFTs. Because the decrease of recombination centers in a-Si:H channel material, the larger IPLCof outward bending a-Si:H

TFTs has been observed in the DOS region. Furthermore, the Vth

of different mechanical strains on a-Si:H TFTs has also observed inFig. 2. The larger Vthof the inward bending a-Si:H TFT was also

indicated that the larger DOS in the a-Si:H channel material. Oppo-sitely, the smaller Vthof the outward bending a-Si:H TFT was due to

the smaller DOS. Thus, a reasonable explanation is the shift of the EF. The increase of Vthmay originate from the increase of defect

density by inward bending strain and lead to a EF shift toward

the valence band edge[11]. Because the EFof a-Si:H is determined

from the charge neutrality condition. Some trap states within the band gap are positively charged and other states are negatively charged by the same amount. a-Si:H TFTs under the inward bend-ing strain have shown the larger Vthand decrease on current ID

than those of ﬂattened and outward bending a-Si:H TFTs, as shown inFig. 2. In order to make sure the increase of DOS in a-Si:H chan-nel material, the Eaof a-Si:H TFTs under different bending strains

was extracted from varied temperature measurement in the tem-perature range from room temtem-perature to 125 °C[12]. As shown in the inset ofFig. 3, the Eaof ﬂattened and outward bending

a-Si:H TFTs are larger than inward bending a-a-Si:H TFTs. At VG= 0 V,

the EFof ﬂattened and inward bending a-Si:H TFT are situated

be-low about 0.72 eV and 0.75 eV from the conduction band edge EC,

respectively. However, the EFof outward bending a-Si:H TFT is

sit-uated below about 0.5 eV from the conduction band edge EC. The

small Ea of a-Si:H TFT under outward bending also resulted in

the larger conductivity and increased the IPLC.

The distribution of DOS g(Ea) was estimated from the inset of

Fig. 3with the slope of Ea–VGcurve[13]. The DOS are nearly about

3.08 1016_cm 3_eV 1_{and 3.51 10}16_cm 3_eV 1_{for both the}

ﬂat-tened and the inward bending a-Si:H TFTs near the deep state re-gion. Furthermore, the DOS is nearly about 2.81 1016cm 3eV 1 for the outward bending a-Si:H TFT near the deep state region. Thus, the smaller Eaof outward bending a-Si:H TFT also indicated

the less DOS in the a-Si:H channel material. As a result, the larger IPLCof outward bending a-Si:H TFTs has been observed inFig. 2.

Fig. 2. The comparison of IPLCcharacteristics between a-Si:H TFTs under different

mechanical bending strains (W/L = 50lm/5lm).

Fig. 3. The extracted distribution of DOS for the a-Si:H TFT under different strains and the inset showed the Eain different strains.

Fig. 1. The illustration of the e–h pairs generation region and the inverted– staggered bottom gate TFT structure.

(3)

Furthermore, the inward bending a-Si:H TFTs also demonstrated the higher DOS distribution from the ECto EF,as shown inFig. 3.

4. Conclusion

The larger IPLCof a-Si:H TFTs under the outward bending strain

is due to larger conductivity of a-Si:H, stemmed from the shift up of EF. The outward bending strain in a-Si:H channel material,

how-ever, appears to shift the EFtowards the conduction band edge due

to the decrease of DOS in a-Si:H material. Furthermore, the inward bending a-Si:H TFTs have shown the similar IPLCcharacteristic to

the ﬂattened a-Si:H TFTs due to the large DOS created during the low temperature fabrication process. Although the decrease of DOS has resulted in the better device characteristic, it also resulted in the large off-state IPLC.

Acknowledgements

This work was supported by National Science Council, the Republic of China under Contract Nos. NSC-98-3114-M-110-001

and NSC-97-2112-M-10-009-MY3. Also, authors thank the ERSO/ ITRI and AUO, Taiwan, for their support.

References

[1] Baeuerle R, Baumbach J, Lueder E, Siegordner J. SID’99 Dig 1999; 14–7. [2] Gleskova H, Konenkamp R, Wagner S, Zhang Q, Shen DS. IEEE Electr Dev Lett

1996;17:264–6.

[3] Yoon JK, Jang YH, Kim BK, Choi HS, Ahn BC, Lee C. J Non-Cryst Solids 1993;164– 166:747–50.

[4] Minomura S, Drickamer HG. J Phys Chem Solids 1962;23:451–6. [5] Spear WE, Heintze M. Philos Mag B 1986;54:343–58.

[6] Menard E, Nuzzo RG, Rogers JA. Appl Phys Lett 2005;86(9):093507. [7] Zhu ZT, Menard E, Hurley K, Nuzzo RG, Rogers JA. Appl Phys Lett

2005;86(13):133507.

[8] Won SH, Chung JK, Lee CB, Nam HC, Hur JH, Jang J. J Electrochem Soc 2004;151(3):G167–70.

[9] Gleskova H, Hsu PI, Xi Z, Sturm JC, Suo Z, Wagner S. J Non-Cryst Solids 2004;338–340:732–5.

[10] Wang MC, Chang TC, Liu PT, Tsao SW, Chen JR. Electrochem Solid-State Lett 2007;10:J49–51.

[11] Spear WE. J Non-Cryst Solids 1983;59–60:1–13.

[12] Schropp REI, Snijder J, Verwey JF. J Appl Phys 1986;60:643–9.

[13] Globus T, Slade HC, Shur MS, Hack M. Mat Res Soc Proc 1994;336: 823.