Nanocrystalline silicon thin film transistor (nc si h tf-ts)-a device

International Journal of Electronics and Communication Engineering & Technology (IJECET),
INTERNATIONAL JOURNAL OF ELECTRONICS AND
ISSN 0976 – 6464(Print), ISSN 0976 – 6472(Online), Special Issue (November, 2013), © IAEME

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ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Special Issue (November, 2013), pp. 115-127
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Nanocrystalline Silicon Thin Film Transistor (nc-Si:H TFTs)-A Device
for Display Panels
Prachi Sharma1, Navneet Gupta2
Department of Electrical & Electronics Engineering, Birla Institute of Technology and Science
Pilani, Rajasthan, India
1prachi.sharma@pilani.bits-pilani.ac.in, 2ngupta@pilani.bits-pilani.ac.in

ABSTRACT: Nanocrystalline silicon (nc-Si:H) has recently emerged as a possible alternative to
both hydrogenated amorphous silicon and polycrystalline silicon for high speed large area
displays. In this paper, a global overview is presented on the nanocrystalline silicon TFT
technology, which includes TFT structure, operation, channel materials, characteristics and
device performance. The various physical effects involved in the conduction mechanism of ncSi:H are also discussed.

KEYWORDS: Nanocrystalline silicon (nc-Si:H), Thin-film transistors (TFTs), Threshold-voltage
shift (∆VT), Density of states (DOS), Hydrogenated amorphous silicon (a-Si:H), Polycrystalline
silicon (poly-Si)

I.

INTRODUCTION

From the mid-1980s the silicon-based thin film transistors (TFTs) are used as switching
elements and for fabrication of their peripheral driving circuits in large area electronics [1-3].
Large- area electronics involves various applications in which low speed, micrometer size
transistors are sufficient in spite of sophisticated transistors. The aim of these applications is to
spread electronic components over large area substrate at low fabrication cost. Active matrix
liquid crystal displays (AMLCDs) [1], organic light-emitting diode displays (OLEDs) [2], radiofrequency identification (RFID) tags [3], medical X-ray imager [4] are the important
applications of large area electronics. The most beneficial aspect of TFT technology is a
separate transistor for each pixel on the display. As each transistor is small, the amount of
charge needed to control it is also small. This allows for very fast re-drawing of the display.
For fabricating thin film transistors, different materials are used as an active layer. The
commonly used materials are amorphous hydrogenated silicon (a-Si:H) and polycrystalline
silicon (poly-Si). The main advantage of a-Si:H is the possibility for deposition over large area
at relatively low temperature (below 450˚C) [5]. Plasma Enhanced Chemical Vapour Deposition
(PECVD) is commonly used for the deposition of amorphous silicon over large area at low
temperatures. The material obtained by PECVD is a-Si:H. The a-Si:H is an amorphous silicon
International Conference on Communication Systems (ICCS-2013)
B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India

October 18-20, 2013
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alloy with incorporated hydrogen atoms. However the disadvantages of a-Si:H are low electron
mobility and instability. The low temperature PECVD process used for a-Si:H deposition as well
as the amorphous nature of non-crystalline substrate (like glass), lead to the formation of
amorphous material having missing atoms. Deep defect states in the forbidden energy gap of
the a-Si:H are associated with these missing atoms, i.e. dangling bonds. Deep defect states are
also associated with the deviation in bond length and angle which results in states below the
conduction band, known as band tail states. The density of states (DOS) in a-Si:H is in the range
1015-1018 cm-3eV-1. The electrons are frequently trapped into and released from these band tail
states leading to the low mobility. Thus a-Si:H has electron mobility as low as 0.1-1 cm2V-1s-1
[6] and hole mobility is so low that the p-type devices are not used in any application. Although
this low electron mobility is sufficient for pixel switch applications and LCDs [7].
In addition to low electron mobility, a-Si:H also suffers from bias stress induced degradation
and light induced degradation. As a consequence, the electrical characteristics of TFTs (like VT,
field effect mobility and sub threshold slope) change and this damages the overall device
performance [9, 10]. As TFTs in LCD and X-ray imager pixel devices are not subject to
prolonged gate voltages thus for these devices instability is not a major issue.
When a-Si:H goes under annealing process, it can be transformed into poly-Si. Poly-Si is having
low hydrogen atoms in large grained silicon films and thus poly-Si is not called as
“hydrogenated”. The main advantages of poly-Si are high electron mobility and better stability.
Since poly-Si is a network of randomly oriented crystalline grains interconnected by thin grain
boundaries, the defects in poly-Si are concentrated in the grain boundaries whereas in a-Si:H
materials they are uniformly distributed in the bulk. In case of poly-Si, large grains are
preferred, as the larger grain size results in lesser grain boundaries across the channel. This
results in lower density of defect states [11] in poly-Si, leading to lesser trapping of carriers at
grain boundaries and hence higher mobility (~100 cm2V-1s-1) [7] than a-Si:H materials. Poly-Si
TFTs provide sufficient electron and hole mobility in n-type and p-type devices for CMOS
(complementary metal oxide semiconductor) operation. Due to high mobility, poly-Si can also
be applicable to peripheral circuits like multiplexers etc. The main problem with poly-Si is that
the crystallization process requires a much higher temperature (usually higher than 300˚C)
than the a-Si:H deposition temperature. In addition to this problem, poly-Si also suffers from
poor spatial uniformity as random positioning of grain boundaries causes irregularity across
the channel. This results in mobility and threshold voltage non-uniformity over large area
substrate [7, 12].
Nanocrystalline silicon (nc-Si:H) has been proposed as a promising alternative material to aSi:H and poly-Si. The advantages of nc-Si:H over a-Si:H are high carrier mobility and better
stability. It can have higher field effect mobility than a-Si:H due to the presence of higher silicon
crystallites [13] as well as increased doping efficiency. For top gated devices, field effect
mobility (µFET) is in range of 40 cm2/Vs [14] to 150 cm2/Vs [15] and for bottom gate, it is in
the range of 0.5-3 cm2/Vs [16,17].Due to the presence of lower hydrogen concentration, ncSi:H have improved stability under bias and illumination stress [13] than a-Si:H. The
advantages of nc-Si:H over poly-Si are low processing temperatures (as low as 150˚C), low
manufacturing cost and better uniformity [18]. Hot wire chemical vapour deposition (HWCVD)
allow direct obtainment of nc-Si:H at very low substrate temperature over large area and at
high deposition rates (about 1nm/s) [18].The resulting nc-Si:H film consists of small silicon
crystallites than poly-Si, with an average grain size of a few nanometers, embedded into a-Si.
However, the main problem with nc-Si TFT is that it is affected from high drain leakage
currents, i.e. off-current.

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Attribute
Mobility
Circuit type
Stability
Drive capacity (ION)

nc-Si:H
Poly-Si
Low
Much higher than a-Si:H High
NMOS
NMOS/PMOS
NMOS/PMOS
Issue
More stable than a-Si:H
Stable
Large W/L
Small W/L
Small W/L
to reduce VG
at small VG
at small VG
VT uniformity
High
High
Improving
Mobility uniformity High
Potentially high
Improving
Manufacturability
Mature
RF PECVD
New
Cost
Low
Low
High
Table 1: Comparison between different materials used for the TFT fabrication

II.

a-Si:H

DEVICE STRUCTURE AND OPERATION

The TFTs consist of three electrodes (i.e. source, drain and gate), gate insulator, and thin
semiconductor layer. These elements can be arranged in many ways. Based on the level of the
gate electrode, the TFTs are divided into two types, top gate TFT and bottom gate TFT. In top
gated TFT, the gate electrode is located above the semiconductor layer whereas in bottom
gated TFT, the gate electrode is located below the semiconductor layer. These two types are
further subdivided into coplanar and staggered devices, giving a total of four types of basic
TFTs structures. The term coplanar/staggered describes the location of source and drain
electrodes with respect to the gate electrode. In coplanar case, the source and drain electrodes
are located at the same side as the gate electrode while in staggered case, the source and drain
electrodes are located at the opposite side to the gate electrode separated with the
semiconductor layer.

Fig. 1: Illustration of different TFT structures [19]

III.

ELECTRICAL INSTABILITY OF TFTS

The a-Si:H and nc-Si TFTs are generally suffers from bias stress degradation due to two major
instability mechanisms i.e. defect creation in the a-Si:H active layer and charge trapping in the
gate dielectric. The creation of defect states leads to an increasing of the density of states in the
band-gap of the channel material and also causes the degradation of electrical characteristics
like increasing of the threshold voltage and decreasing of the sub-threshold slope. When defect

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state creation take place, it is necessary to apply thermal or bias annealing in order to recover
the initial device performance [20, 21]. When charge trapping occurs in the gate insulator or at
the channel/insulator interface, then the threshold voltage increases under positive stress and
decreases under negative stress. Annealing under special thermal and/or bias conditions is
necessary to release the trapped charge from the gate insulator whereas when charge trapped
at the channel/insulator interface, it can be released at room temperature without need of
annealing. The interface trapped charge has the similar effect on the characteristics as the
charge trapped in the bulk of the insulator. The only difference is that the charge trapped in the
interface requires less energy in order the charge to be released.
Various authors [22, 23] presented the model for carrier-induced defect creation and proposed
that hydrogen atom that is available in the channel material diffuses to the weak Si-Si bond and
participate in the process where upon the breakage of the weak bond, two dangling bonds are
created which one of them is passivated by the hydrogen. Hydrogen diffusion has been found
to follow stretched-exponential time dependence and to be dispersive. Authors examined that
the resulting dispersive motion of the hydrogen leads to the stretched-exponential time
dependence of carrier-induced creation of defects and consequently, the shift in the threshold
voltage. Thus authors assumed that defect state creation is the dominant mechanism of the
shift in threshold voltage.
∆

=

1−

−

(1)

Where C ≈ VGS - VT0, VGS is the applied gate-source voltage, also called gate bias stress, VTO is the
threshold voltage at time t = 0, and τ and β are a time constant and ﬁtting exponent,
respectively. The parameter β has been found to be around 0.5 at room temperature [22, 23].

IV.

DRAIN LEAKAGE CURRENT OR OFF-CURRENT

A. Top-Gated TFT
Various authors examined the leakage current in the top-gated nc-Si:H TFT at various
temperatures. Authors proposed that under high
and high
(=10V), the behaviour is
reminiscent of the Poole-Frenkel emission in the drain depletion region and Eacontinues to
decrease as
increase upto 10V [24].
=
Where,

exp

√

(2)

is the generation current at zero electric field.
= |

−

−

is the peak electric field [25].

|

(3)

is the PF coefficient [25]
=

/


(4)

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And under high
(> 0V) and low
(=5V), the leakage current can be attributed to the
thermal emission of trapped carriers present at the mid-gap grain boundary states and Ea
becomes independent of
.The value of Ea i.e. 0.72±0.002eV is retrieved from the average and
standard deviation between
= 0V and 10V
=
Where

−

(5)

is independent of temperature and constant.

Lee et. al. [25] also proposed that under low gate bias, the leakage current is mainly due to the
ohmic conduction through the bulk nc-Si:H channel layer and IL has a weaker gate and drain
bias dependence. The increase in resistive leakage current with
can be referring to the
increase in peak holes concentration in the bulk. This is the main reason behind the decrement
of activation energy from 0.77 to 0.72eV for
=5V.
B. Bottom-Gated TFT
Authors examined the leakage current in n-channel nc-Si TFTs and proposed that when a high
drain voltage (Vds> 0) is applied, the large electric field generated at the reverse n+-p drain
junction. This electric field would assist the electrons in the valence band to travel by band to
band tunnelling. The use of a-Si and nc-Si double layer beneath the n+ a-Si drain contact is
required in order to reduce the leakage current since the a-Si:H has the larger band gap as
compare to nc-Si [26]. According to the tunnelling theory, at large drain bias voltages, the
tunnelling leakage current is given as [24]
=

exp −

(6)

Where, A is constant in AcmV-1.
F is the maximum electric field at the drain junction [27]
=

(

)

+

(7)

Where
is the flat band voltage, = Vd-Vg. and is the permittivity and thickness of SiNX,
respectively and w is the depletion region of the p-n junction [24]
E =

(

) /

/

(8)

Where h is the Planck’s constant divided by 2 , mr is the tunnelling effective electron mass and
Egis the energy gap of the silicon.
At low electric fields (E<2.2 MV/cm) [28], the leakage current is dominated by PF conduction.

V.

PERFORMANCE OF NC-SI:H TFT

Various stochastic effects which affect the performance of nc-Si:H TFT and thus changes the
characteristic of the device are:

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A. Temperature
The defect state creation and thus the shift in the threshold voltage are a function of time and
temperature. When device operating temperature increases, the rate of hydrogen diffusion
increases which results in an increase in ∆VT. The hydrogen dependence is reflected in the
parameter τ as
=

(9)

Where τ0 has been found to be around 10-10 sec and EA= 0.95eV is the activation energy [22].
Various authors [29] compared the threshold voltage shift of the nc-Si TFTs with that of the aSi:H counterpart, under similar operation conditions. For example, they electrically stressed
the TFTs under constant drain currents of 2, 10, and 15 µA at two temperatures of 22 and 75◦C
for 50 hours. And found two fundamental differences in the behavior of nc-Si TFTs compared
to that of the a-Si:H TFTs. First, ∆VT in nc-Si TFT saturates at prolonged stress times, but that of
a-Si:H TFT does not. Second, ∆VT in nc-Si TFT is weakly temperature dependent, in contrast to
that of a-Si:H device. For example, after 50 hours stressing at 15 µA, ∆VT in nc-Si TFT is 3 V and
4 V at 22 and 75◦ C, respectively, whereas that for the a-Si:H TFT is 7.6 V and 21 V, respectively.
The observed behavior of ∆VT indicates absence of defect state creation in the nc-Si TFTs. Its
weak temperature dependence is consistent with the mechanism proposed by Powell et al.
[20], implying that the instability mechanism is charge trapping in the nitride. The kinetics of
∆VT does also follow the stretched exponential time dependence predicted for charge trapping
[30, 31].
To further support the conclusion that defect creation is absent in nc-Si TFTs, authors [29]
investigated the other attribute of the charge trapping, i.e. its reversibility. It is known that
charge trapping is reversible, but defect creation is indeﬁnitely stable and irreversible at room
temperature. Authors performed the relaxation test, in which nc-Si TFT was electrically
stressed for some time to induce some shift in its threshold voltage. Subsequently, the TFT was
relaxed, i.e. bias voltages were removed and device was turned off. From time to time, authors
performed a quick test to retrieve its I-V characteristics to see whether the induced ∆VT was
disappeared and initial I-V curves were obtained. Authors found that after 5 days relaxation at
room temperature, the initial I-V curves can be obtained. This observation is an another
evidence indicating that charge trapping in the nitride causes ∆VT in nc-Si TFTs. If defect
creation were the source of instability, it may take around a year at room temperature to
anneal the created defects and retrieve the initial I-V curves.
B. Grain Size
Mao [32] proposed a model for the shift of valence and conduction band edges and proposed
that valence or conduction band edges are affected by grain size which in turn affects the
barrier height at nc-Si/SiO2 interface. This is because it equals to the difference between the
barrier height at the bulk-Si/SiO2 interface for electrons and the shift of conduction band edge.
.

∆ ( )=

∆

=

.

(eV)


(10)

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.

∆

( )=

∆

.

=

(eV)

(11)

According to above equations [32], the edge of the conduction band or valance band will be
shifted by the grain size. Thus barrier height will decrease.
The gate leakage current of nc-Si TFTs strongly depends on the silicon-grain size. This results
from the large change in the band gap and dielectric constant due to size effects. The screening
dielectric constant of nc-Si can be theoretically calculated with the formula [33] below,
( )= 1+

.

(12)

.

.
×

In above equation, the unit of d is meter.
The model of quantum confinement, within the effective mass approximation predicts a shift of
the valence band edge with size i.e. the valence band edge shifts to larger binding energy in
smaller cluster. This shift corresponds to a narrowing of the valence band and correlates well
with the optical measurements of band gap broadening. The experimental data of the band gap
of nc-Si obey with the formula [34] below,
∆

=

( )−

(∞ ) =

.
×

+

.
( ×

)

(

)

(13)

WhereEg(d) is the quantum-dot band gap as a function of radius and Eg(∞) is the bulk band
gap.
Various authors [35] examined the impact of the grain size of nc-Si on the surface potential of
doped nc-Si TFTs and proposed that the change in both dielectric constant and band gap of ncSi caused by quantum size effects can largely affect on the channel surface potential in nc-Si
TFTs. The surface potential affected by the crystal size also has an effect on the tunneling
current under a given gate voltage [32].
Mao [35] calculated the surface potential with the formula below,
(

.
.

.

)
⎡
⎢
⎢
⎢
⎢
⎢
⎢
⎣

=

+

+


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Page 121

(

⎛
⎜
ln ⎜
⎜
⎜

.

.

−
.

⎛
⎜
⎜

.

(∞)

) /

) (

(∞)

exp
.

.

.

⎤

⎞
⎟ ⎞⎥
⎟

⎟⎥

⎠ ⎟⎥

⎝

.

⎝

−1 +

⎛
⎜
⎜

.

(∞)

.

.

⎝

.

⎞
⎟
⎟
⎠

(14)

⎟⎥
⎟⎥
⎥
⎠⎦

It was proposed that the surface potential of nc-Si TFTs is largely affected when the diameter of
nc-Si is smaller than 10 nm because the corresponding dielectric constant of nc-Si largely
changes with its diameter. At the same time, the surface potential of nc-Si TFTs is little affected
by the change in the band gap when the diameter of nc-Si is smaller than 6 nm. This implies
that the effects of grain-boundary traps on the surface potential of nc-Si TFTs become more
and more important due to the rapid increase of the area of grain-boundary. Thus the change
in the surface potential due to the band gap of nc-Si can be neglected, compared to the change
caused by grain-boundary traps.

Fig. 2: Comparison of surface potential for an nc-Si TFT [35]
Mao [36] proposed that the threshold voltage in nc-Si TFTs is also strongly depends on the size
of silicon grain when the size of silicon grain is less than 20 nm. It was analyzed and proposed
that the size of silicon grain impacts on the threshold voltage weakly dependent on the active
dopant density and is mainly depends on the change in bandgap and dielectric constant due to
the quantum size effects. The threshold voltage or turn-on voltage is defined as the voltage at
which strong inversion occurs [37].Strong inversion begins at
≈2

≈2

(15)

Thus the threshold voltage is given as [36]
=
+2

+

×


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+

2

+

−1 +
(16)

By using the assumption that the q
threshold voltage can be given as [36]

=

+2

+

×

and

−

+

−

are much larger than of kT, the

2

+

(17)

C. Contact Resistance
The nc-Si:H TFTs made by plasma-enhanced chemical vapor deposition have higher carrier
mobilities than a-Si:H TFTs. However, the contact resistance limits the effective carrier
mobilities as it reduces the voltage drop across the channel of nc-Si TFT and distorts the
injected current [38]. Contact resistance from source to drain has been attributed to a number
of mechanisms such as current crowding [39], constant parasitic resistance,defects at the
contact interface [40] and transport through barrier [41]. Contact resistance has a vital impact
on the TFTs transconductance and on the electrical characteristics of the device.
Some of the authors [42] determined the total resistance of nc-Si TFT by the following
equations
=2 + 2 +
= × /( × )
(18)
RTOT is the total resistance of nc-Si TFT, RC is the contact resistance between source/drain and
metal, RS is the resistance of source/drain, RCH is the channel resistance, ρ is the resistivity of
nc-Si TFT channel, W, L are the width and length of nc-Si channel and t is the thickness of nc-Si
active layer. RS and RCis low. Authors analyzed that if t is increased and grain size is increased.
Then, ρ is decreased and causes the reduction of RCH resulting in reduction of RTOT [42]. The
reduced RCH can increase the field effect mobility and off-state current. Authors [38] proposed
that the omission of contact resistance effects can result in an incorrect extraction of the
maximum µFET by a factor of 2.
Some other authors [43] measured the total resistance RTOT = VDS/IDS, composed of contact
resistance and channel resistance, across the TFTs output terminals. And proposed that the
contact resistance R0, composed of ohmic and non-ohmic components, is inversely
proportional to channel width W. However the channel resistance, RCH = rCH × (L/W) where L is
the channel length and rCH is the channel resistance per L/W, in the linear regime follow a
purely ohmicbehaviour.
Authors [43] estimated the ohmic contact resistance from the intercept with the Y-axis when
RTOT ×W (Y -axis) is plotted versus L (X-axis) and the slope denotes rCH. The results show that
the source/drain current at VDS = 10 V becomes limited by the contact resistance when the L is
less than 10 μm for n-channel and less than 25 μm for p-channel. Authors also proposed that
the contact resistance is strongly depends on the gate voltage VGS.

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Other authors [38] estimated the non-ohmic contact resistance using the current dependent
polynomial, from which the total voltage drop across the TFT can be calculated
=

=

+
∑

+

=
(19)

+

+

Here,
represent the measured voltage drop, Ais the constant coefficients of a polynomial, IDS
the drain-source current,
the channel resistance and
the
ohmic and non-ohmic components, respectively.
Contact resistance can also be due to transport over a barrier with electronic defect states
[41].As a gate voltage is applied the defects are filled and the barrier height changes. Authors
proposed that for low trap state density the mobility saturates. The combined effect of charge
accumulation in the channel and drop in the contact resistance can increase the trapping state
density and thus the mobility.
=

VI.

.

(20)

CONCLUSION

The recent progress in nc-Si:H TFTs has been reviewed in this paper. The different type of
materials such as a-Si:H, poly-Si and nc-Si:H which are used as an active layer for the
fabrication of thin film transistors and also the various structures and operation of TFTs were
discussed in this review paper. In case of bottom-gated nc-Si:H TFT, the device performance is
determined by the quality of bottom layers of nc-Si, where the conduction channel is formed
whereas in case of top-gated nc-Si:H TFT, the conduction channel will be formed in the highly
crystalline part of the nc-Si ﬁlm. Hence, it is expected that top-gate nc-Si TFTs render better
performance than bottom-gate devices. The main limitations of the nc-Si:H TFTs are electrical
instability and the drain leakage current. Various physical parameters like temperature, grain
size and contact resistance affects the device performance and thus affects the device
characteristics. It was also discussed that defect state creation is the dominant mechanism of
the shift in threshold voltage.

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BIOGRAPHY
Prachi Sharma is currently pursuing PhD from Department of Electrical &
Electronics Engineering, Birla Institute of Technology and SciencePilani,
Rajasthan, India. She received M-Tech in VLSI Design in 2011 from Banasthali
University, Rajasthan and B-Tech in Electronics and Telecommunication in
2009 from UPTU, Lucknow. Her research interests are Semiconductor Device
Modelling, VLSI Design, Microelectronics.

October 18-20, 2013
Page 126


Dr. Navneet Gupta obtained M.Sc. (Physics-Electronics) in 1995 from H.N.B
Garhwal Central University (HNBGU), Srinagar, India with first rank in the
University. He received M.Tech in Materials Technology in 1998 from Indian
Institute of Technology (IIT-BHU) (formerly IT-BHU). He did his Ph.D. in the
field of Semiconductor Devices in 2005 from HNBGU. Presently, he is Assistant
Professor and Convenor-Departmental Research Committee in Electrical and
Electronics Engineering Department, Birla Institute of Technology and
Science, Pilani, (BITS-Pilani) Rajasthan, India. He has guided 1 Ph.D. student and is currently
guiding 4 Ph.D. candidates. He is on doctoral advisory committee for 6 Ph.D. students. He
completed 2 sponsored research projects from UGC and DST. His research interests include
Semiconductor Device Modelling, RF-MEMS, Material Selection and Antenna Design. He is life
member of several international and national professional bodies. He has over 50 research
publications (of which 21 are in reputed peer reviewed international and national journals
with good impact factors, and 29 in conference proceedings.). He has published six books in the
areas of engineering physics and electronics engineering. He received the Bharat Jyoti Award
in 2011 by IIFS, New Delhi, India, DST Young Scientist Award (Fast track Scheme) in Physical
Sciences in 2007 and Gold Medal in M.Sc. His biography is included in Marquis Who's Who in
World and Marquis Who's Who in Science and Engineering. He is expert reviewer of over 5
International Journals. He reviewed three books of Oxford University Press, Pearson Education
and Tata McGraw Hill publishers.


October 18-20, 2013
Page 127

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