UV Illumination Technique for Leakage Current Reduction in a-Si:H Thin-Film Transistors

ultraviolet (UV) laser. An 85% reduction in the leakage current

in a-Si:H TFTs is experimentally observed. The general SPICE

model (such as the RPI model) lacks the proper term to capture the

photo-induced phenomena; therefore, the physical mechanisms

that are associated with the illumination of a-Si:H TFTs under

UV, including the energy state and the density of traps, are

ana-lyzed using device simulation. The I–V characteristics of the

inverted staggered a-Si:H TFTs under different magnitudes of UV

exposure are calibrated with experimentally measured data. The

preliminary results show the change of trap states in amorphous

silicon film and a shift of the Fermi level with UV illumination.

UV illumination may induce traps in the active layer of the device

and thereby reduce the

-state leakage current.

Index Terms—Amorphous silicon thin-film transistors (a-Si:H

TFTs), band-to-band tunneling, device simulation and

character-ization, leakage current, trap-assisted tunneling, ultraviolet (UV)

illumination.

I. I

H

YDROGENATED amorphous silicon thin-film

transis-tors (a-Si:H TFTs) have recently been used widely as

switching devices in large-area electronics such as active matrix

liquid crystal displays (LCDs) [1] and memory devices [2].

When the TFT turns on, both the liquid crystal capacitance

and the associated capacitance are charged; they have to

main-tain sufficient voltage for the rotation of the liquid crystal.

Unfortunately, an a-Si:H TFT has high photoconductivity [3],

which may result in a high leakage current under visible light

illumination, particularly those projectors and displays with

Manuscript received May 28, 2008. Current version published October 30, 2008. This work was supported in part by the National Science Council under Contracts NSC-96-2221-E-009-210 and NSC-96-2752-E-009-003-PAE and in part by the InnoLux Display Corporation, Science-Based Industrial Park, Chu-Nan 350, Miao-Li County, Taiwan, R.O.C., under a 2006–2008 Grant. The review of this brief was arranged by Editor H. S. Tae.

Y. Li is with the Department of Communication Engineering, the Mod-eling and Simulation Center, and the Parallel and Scientific Computing Laboratory, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: [email protected]).

C.-H. Hwang is with the Department of Communication Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C.

C.-L. Chen and J.-C. Lou are with the Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C.

S. Yan is with the Technology Development Division, InnoLux Display Corporation, Chu-Nan 350, Taiwan, R.O.C.

Color versions of one or more of the figures in this brief are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2008.2005133

posed to suppress the

-state leakage current by increasing

the acceptorlike density of states in a-Si:H (fluorine) material

to shift the Fermi level toward the valence band edge [9]–[11].

Observations of the increase in acceptorlike states motivated

this exploration of the mechanism by which the

-state

leakage current of a-Si:H TFTs is reduced. The influence of

prolonged illumination with intense light (600–900 nm) on the

metastable changes in a-Si:H film has been reported elsewhere

[12], [13]. However, little attention has been paid to study

the ultraviolet (UV) illumination-induced metastable increase

in hydrogenated materials. The effect of UV exposure on the

passivation quality of SiN

/a-Si:H stacked layers in solar cell

fabrication has been examined [14]. The increase of trap density

reduces the carrier lifetime caused by UV illumination, which

may reduce the

-state leakage current in a-Si:H TFTs.

This brief presents a leakage current reduction approach, in

which inverted staggered a-Si:H TFTs are exposed to a UV

laser. The UV illumination may produce traps in the active layer

of the device and thus reduce the

-state leakage current.

A general SPICE model, such as the well-known RPI model

[15], [16], for simulating the a-Si:H TFT circuit lacks a term for

photo-induced phenomena. Therefore, the physical mechanism

that governs the characteristics of the device should be studied

qualitatively and quantitatively. To further study the metastable

changes in a-Si:H film, caused by UV illumination, a set of

ther-modynamic transport equations coupled with trap state models

is simultaneously solved using our own simulation platform

[17]–[19]. The calculated current–voltage (I–V ) characteristics

of the inverted staggered a-Si:H TFTs illuminated with different

intensities of UV are calibrated with experimentally measured

results. The shift in the threshold voltage, the reduction of the

leakage current, and the shift in the Fermi level are observed

and discussed.

This brief is organized as follows. Section II introduces

the experimental measurement and the physical models used

for numerical simulation. In Section III, the measurement and

simulation results are calibrated for the best accuracy of the

analysis. The observed results and the associated phenomena

are then studied. Finally, conclusions are drawn.

II. E

S

Fig. 1(a) shows the inverted staggered a-Si:H TFTs exposed

to a UV laser (355 nm) from the topside of the a-Si:H layer.

Fig. 1. (a) Schematic illustration of the inverted staggered a-Si:H TFTs. (b) SEM image.

Fig. 1(b) shows a SEM picture of the fabricated sample. The

device is with an 18-μm channel width, a 5-μm channel length,

and a 100-nm channel thickness. The thickness of the nitride

is 330 nm. The top of the inverted staggered a-Si:H TFTs is

exposed to a UV laser. In the UV laser illumination experiment,

the I–V curves of the devices are measured after fabrication

(denoted as 0 shot). Then, the a-Si:TFTs are exposed to a UV

laser with the particular shot, and their I–V characteristics are

measured again. This experiment includes 1, 10, 22, 50, and

101 shots. The I–V characteristics of the thin-film transistor

devices were measured using an HP4156C with the source

grounded and the body floating. A set of thermodynamic

trans-port equations consisting of the Poisson equation,

electron-hole current continuity equations, and the lattice temperature

equation are solved numerically to calculate the device

char-acteristics [17]–[19]. The variations of the charge distributions

induced by the density and distribution of trap states in the

a-Si:H layer are included in the device simulation to estimate

the variation of the device characteristics accurately. The

den-sity of states in the a-Si:H film are grouped into two types—tail

and deep states. Most of the deep states of the inverted

stag-gered TFT with silicon nitride gate insulator [20] are located

in the lower part of the amorphous silicon gap. The localized

Fig. 2. Measured drain current (ID) versus the gate voltage (VG) for the

source–drain voltage of 12 V with different magnitudes of UV light illumi-nation. The inset shows the prethreshold characteristics of the TFTs under UV light.

acceptor- and donorlike states in the a-Si:H mobility gap can be

modeled by exponentially distributed deep and tail states [21]

N

= g

exp



E

− E

E



+ g

exp



E

− E

E



(1)

N

= g

exp



E

− E

E



+ g

exp



E

− E

E



(2)

where E

is the conduction band edge; g

and g

are the

densities of states at the conduction band edge for the tail

and deep acceptorlike states, respectively. E

and E

are the

associated gradients of the exponential distributions of the tail

and deep acceptorlike states. E

is the valence band edge; g

and g

are the densities of states at the valence band edge

for the tail and deep donorlike states. E

and E

are the

associated gradients of the exponential distribution of the tail

and deep donorlike states, respectively, and E is the energy in

the a-Si:H mobility band gap. The recombination models that

incorporate trap-assisted [22] and band-to-band [23] tunneling

effects are included to capture the characteristics of a parasitic

Schottky contact under negative gate bias.

III. R

D

Fig. 2 shows the measured I–V characteristics of the inverted

staggered a-Si:H TFTs that are exposed with different shots

from the UV laser. An 85% reduction of the leakage current

after multiple shots from the UV laser is observed. Table I

summarized the dependences of the characteristics of the device

with various numbers of shots from the UV laser, such as the

threshold voltage, the

-state current, the leakage current, the

mobility, and the on/off current ratio. The threshold voltage is

determined from the current criterion that drain current (I

)

is larger than 10

(W/L) A, where W and L are the width

and length of the studied device, respectively. The

-state

current is defined as the drain current at which the gate voltage

is 20 V, and the leakage current is defined as the drain current

at which the gate voltage is

−20 V. The results demonstrate

that the threshold voltage increased as the UV exposure time is

increased. Increasing the threshold voltage reduces the leakage

Fig. 3. Activation energy as a function of gate voltage (VG). The activation

energy is increased as the number of UV shot increases, which indicates the shift of Fermi level toward valence band edge after UV illumination.

current at the cost of the decreased mobility and

-state

current. The UV exposure approach improves the on/off current

ratio by a factor of five. The increase of the on/off current

ratio reveals the effectiveness of this leakage current reduction

approach. Fig. 3 shows the activation energy as a function of

gate voltage (V

), where the activation energy E

decreases

when the V

increases. The activation energy is strongly related

to the position of the Fermi level, which is estimated using the

following:

I

∼

= exp



−

qE

kT



.

(3)

As shown in Fig. 3, the activation energy increases with the

number of UV shots, indicating a shift of the Fermi level toward

the valence band edge upon UV illumination. Based on the

charge neutrality, the Fermi level can be lowered in two ways

to produce a positive threshold voltage shift [24]—by reducing

the density of donorlike states or increasing the density of

acceptorlike states. The inset of Fig. 2 shows the prethreshold

characteristics of the TFTs under UV light. Since the

prethresh-old slope is sensitive to the acceptorlike states and illumination

causes a metastable increase in the bulk density of states in the

semiconductor, the change of the electrical characteristics are

mainly caused by an increase of acceptorlike states. The charge

state and energy position of the defect states cause the Fermi

Fig. 4. Calibrated ID–VGcharacteristics for all shots, where the source–drain

voltage is equal to 12 V. Without loss of generality, only 0, 22, and 101 shots are shown. The inset is a zoom-in plot.

level to move away from the conduction band upon illumination

and therefore increase the threshold voltage of the device. We

noted that the UV exposure affects the switching characteristic

of the device due to the increase of the defect density of states.

To explore further the influence of UV illumination on

the metastable changes in a-Si:H films, the measured I

–V

characteristics were used to calibrate the simulator, as shown in

Fig. 4. The calibration result agrees closely with the measured

data, in which the acceptorlike states from 0 to 101 shots are

calibrated. Fig. 5 shows the simulated extracted density of

states in the a-Si:H layer for 0, 22, and 101 shots. A shift in

the Fermi level is observed, and the simulated position of the

Fermi level is similar to the Fermi level shown in Fig. 3. Fig. 6

shows the calibrated tail (g

) and deep (g

) acceptorlike states

in the a-Si:H layer after UV illumination. Both g

and g

increase with the number of shots and then saturate after 22

shots due to the limited trap states in silicon. The tail states are

the silicon conduction band states, which are broadened and

localized by the disorder, to form a “tail” of localized states just

below the conduction band mobility edge. These states are

so-called weak silicon bonds [25]. The deep states originate from

defects in the a-Si:H network. They are thought to consist of

Si dangling bonds, which have a wide range of energies [25].

UV laser illumination weakens the bonds in the a-Si:H network

and then breaks them. Therefore, both g

and g

increase. The

Fig. 5. Modeled density of states used in a-Si:H with 0, 22, and 101 shots, respectively.

Fig. 6. Simulated tail (gtc) and deep (gdc) acceptorlike states with different

magnitudes of UV light illumination.

illumination causes a metastable increase in the bulk density

of states in the semiconductor, which may account for the

aforementioned findings.

IV. C

This brief presented a leakage current reduction approach in

which the tops of inverted staggered a-Si:H TFTs are exposed to

a UV laser. While exposure to visible light produced a leakage

current in the a-Si:H film, UV illumination produced traps in

the active layer of the device, reducing the

-state leakage

current. The reduction of the device leakage current by UV

illumination was then studied by solving the thermodynamic

transport equations with trap state and recombination models.

The simulations suggest that the density of acceptorlike states

in a-Si:H increase with UV exposure. The shift in Fermi level

away from the conduction band to the valence band upon UV

illumination is responsible for the shift of threshold voltage and

the reduction of the

-state leakage. Although the

-state

current, mobility, and switching characteristic of the a-Si:H

TFTs are affected, the device on/off current ratio is significantly

improved. The proposed UV illumination approach is useful

for reducing leakage current and can be incorporated into the

industrial manufacturing process.

R

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University (NCTU), Hsinchu, Taiwan, R.O.C., in 1996, 1998, and 2001, respectively.

In 2001, he was with the National Nano Device Laboratories (NDL), Hsinchu, as an Associate Re-searcher, and with the Microelectronics and Infor-mation Systems Research Center (MISRC), NCTU, as an Assistant Professor, where he was engaged in the field of computational science and engineering, particularly in modeling, simulation, and optimization of nanoelectronics and very large scale integration (VLSI) circuits. In the fall of 2002, he was a Visiting Assistant Professor at the Department of Electrical and Computer Engineering, University of Massachusetts, Amherst. From 2003 to 2004, he was the Research Consultant of the System on a Chip (SOC) Technology Center, Industrial Technology Research Institute, Hsinchu. From 2003 to 2005, he was the Director of the Departments of Nanodevice and Computational Nanoelectronics, NDL, and an Associate Professor with the MISRC, NCTU, from the fall of 2004. He is currently an Associate Professor with the Department of Communica-tion Engineering, NCTU, where he is the Deputy Director of the Modeling and Simulation Center and conducts the Parallel and Scientific Computing Laboratory. His current research areas include computational electronics and physics, physics of semiconductor nanostructures, device modeling, parameter extraction, VLSI circuit simulation, development of technology computer-aided design and electronic CAD tools and SOC applications, bioinformatics and computational biology, advanced numerical methods, parallel and scientific computing, optimization techniques, and computational intelligence. He has authored or coauthored over 120 research papers appearing in international book chapters, journals, and conferences. He has served as a Reviewer, Guest Associate Editor, Guest Editor, Associate Editor, and Editor for many interna-tional journals. He was an Editor for proceedings of internainterna-tional conferences.

Dr. Li is a member of Phi Tau Phi, Sigma Xi, the American Physi-cal Society, the American ChemiPhysi-cal Society, the Association for Comput-ing Machinery, the Institute of Electronics, Information and Communication Engineers, Japan, and the Society for Industrial and Applied Mathematics. He is included in Who’s Who in the World. He has organized and served on several international conferences and has served as a Reviewer for the IEEE TRANSACTIONS ON NANOTECHNOLOGY, the IEEE TRANSACTIONS ON MICROWAVETHEORY AND TECHNIQUES, the IEEE TRANSACTIONS ONCOMPUTER-AIDEDDESIGN OFINTEGRATEDCIRCUITS ANDSYSTEMS, the IEEE ELECTRONDEVICELETTERS, and the IEEE TRANSACTIONS ON

ELECTRONDEVICES. He was the recipient of the 2002 Research Fellowship Award presented by the Pan Wen-Yuan Foundation, Taiwan, and the 2006 Out-standing Young Electrical Engineer Award from Chinese Institute of Electrical Engineering, Taiwan.

Chung Cheng University, Chiayi, Taiwan, R.O.C., and the M.S. degree from the Institute of Elec-tronics Engineering, National Chiao Tung Univer-sity (NCTU), Hsinchu, Taiwan, in 2003 and 2008, respectively.

He is currently with the Institute of Electronics, NCTU. His research interests focus on modeling and simulation of amorphous thin-film transistors.

Shuoting Yan received the B.S. degrees from the Department of Physics and the Institute of Elec-tronics Engineering, National Chiao Tung Univer-sity (NCTU), Hsinchu, Taiwan, R.O.C., in 2000 and 2001, and the Ph.D. degree from NCTU in 2004.

Currently, he is dedicated to high-resolution TFTLCD panel design and pixel circuit simula-tion with the Technology Development Division, InnoLux Display Corporation, Chu-Nan, Taiwan. His research interests focus on a-Si TFT devices, microcrystalline-Si TFT devices, poly-Si TFT de-vices, and high-resolution TFTLCD panel and pixel design.

Jen-Chung Lou received the B.S. and the M.S. degrees in physics from the National Tsing Hua University, Hsinchu, Taiwan, R.O.C., in 1975 and 1977, respectively, and the Ph.D. degree in electrical engineering and computer sciences from the Univer-sity of California, Berkeley, in 1991.

He was a Faculty Member with the Department of Electrical Engineering, University of Tsing Hua, from 1979 to 1990. He is currently with the Insti-tute of Electronics, National Chiao Tung University, Hsinchu. His research interests include MOCVD of GaAs, LPE of HgCdTe, Schottky devices of III-V, and Si semiconductors. His current research topic is the selectively epitaxial growth of silicon at low temperatures.