channels, but also produce higher effective mobility. If the lateral field is small, as shown by the inset of Fig. 3.9, DG-mode current almost equals to the summary of TG-mode current and BG-mode current. No mobility enhancement can be observed.
3.5 Barrier Lowering and electric field
We separate two parts to discuss the enhanced effective mobility in NDD devices with Ar treatment and the mechanism of additional drain current enhancement of dual gate measurement.
Firstly, there are some reasons to explain the enhanced effective mobility in NDD devices. The effective channel length is reduced due to the conductive dots regions in the channel. In previous researches, we estimate the intrinsic channel length
as the effective channel length. The effective channel length of 0.2wt% and 0.8 wt%
PS sphere density is reduced from 1000m to 684m and 500m, Fig 3.11.When we
use new effective channel length to modify the mobility extraction, the mobility of top gate measurement are 32.5 cm2/Vs and 39.6 cm2/Vs respectively, which is 8-10 times bigger than the STD top gate devices. It is not sufficient to explain the enhanced mobility in TG-NDD devices.
As we know, the electron transport in a-IGZO is governed by the percolation transport. The reduction electron mobility is formed by potential barriers which is caused by the random distribution of Ga3+ and Zn2+ ions in the network structure.
36
When high density conductive dots-like regions in device transport channel, the potential barrier is lower. As a result, the electron mobility of top gate measurement is larger than bottom gate measurement.
The barrier lowering effect is observed in many semiconductor devices. The Schottky barrier at the metal-organic interface shows a Schottky-barrier lowering effect when increasing the doping level of organic semiconductor. [35]. For short-channel MOSFETs, the built-in potential barrier between the heavily-doped source and the bulk suffers from the drain-induced-barrier-lowering effect [36-38].
For poly-Si TFTs, the grain boundary barrier is also lowered by the drain-to-source electric field [39].
Secondly, the barrier lowering effect is presumed to be strongly dependent on lateral electric field. the reduced vertical field in DG mode may enhance the barrier lowering effect and increase the mobility. As a result, When drain bias (VDS) is 0.1V , RI is around 1, representing that the dual gate operation simply turns on top channel and bottom channel simultaneously. When VDS = 20 V, however, RI increases to be 1.5, revealing that increasing lateral electric field the electric mobility and the drain current of dual gate mode are increasing. In contrast with NDD structure, STD device which without nano-dots structure does not show the drain current of dual gate mode is bigger than the sum of the other mode.
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Figure in Chapter 3
Fig. 3.1 Schematic device structures. (a) and (b) is Dual gate (DG) of a-IGZO TFTs without and with NDD.
Fig. 3.2 Schematic metal-insulator-metal (M-I-M) device structures of (a) bottom gate insulator, (b) bottom gate insulator.
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Fig. 3.3 The sheet resistance of a-IGZO thin films as a function of an Ar plasma treatment time.
Fig. 3.4 The transfer characteristics of different Ar plasma treatment time on TG-NDD 0.8 wt% TFTs.
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Fig. 3.5 The transfer characteristics of TG-NDD TFTs without and with different dot concentration doping.
Fig. 3.6 SEM images. (a) Cross sectional view of the a-IGZO TFTs. (b) 1.2 wt%
of polystyrene spheres with diameters of 200nm on the substrate.
VG (V)
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Fig. 3.7 The transfer characteristics of NDD a-IGZO TFT operated in top gate, bottom gate and dual gate modes.
Fig. 3.8 The transfer characteristics of STD a-IGZO TFT operated in top gate, bottom gate and dual gate modes.
-20 -15 -10 -5 0 5 10 15 20 25
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Fig. 3.9 The current amount bars under TG, BG and DG operation modes at the fixed bias VDS = 20 V and VDS -VTH= 10 V, and VDS = 0.1 V as shown in inset
chart.
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Fig. 3.10 The new method of mobility calculation process and schematic diagram
0.8 wt%
So, 1583/1000um (Diameter of PS = 0.2 um) Threrfore, L’= 1000-(1583*0.2) = 684um
So, 2500/1000um (Diameter of PS = 0.2 um) Therefore, L’= 1000-(2500*0.2) = 500um
Cross section view Top view about 45o
Taking SEM image
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Fig. 3.11 SEM images. (a) Cross sectional view of the a-IGZO TFTs. (b) 0.8 wt%
and (c) 0.2 wt% of polystyrene spheres with diameters of 200nm on the substrate.
The average of 0.8 wt% and 0.2 wt% PS density calculated in the 5um2 square are 6.8*106 mm-2 and 4.8*106 mm-2, respectively.
Al(100nm) (a)
PVP(420nm)
IGZO(50nm)
Highly conductive IGZO region
(b)
The 0.8 wt% PS density 6.8 ×106mm-2
(c)
The 0.2 wt% PS density 4.8 ×106mm-2
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Table 1. Typical parameters of TG-NDD TFTs with and without nano-dot doping.
TABLE II. Typical parameters of TG, BG, and DG a-IGZO TFTs with NDD structure.
On/Off ratio 1.6×106 1.2×104 2.5×106 Glass
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TABLE III. Typical parameters of TG, BG, and DG STD a-IGZO TFTs .
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Chapter 4
Conclusions and Future Work
4-1 Conclusions
We demonstrate that the NDD a-IGZO TFT exhibits a 1.5 times current increase and a corresponding high effective mobility as 272 cm2V-1s-1 by connecting the top-gate and bottom-gate together to form a dual-gate (DG) NDD a-IGZO TFT. The dual-gate enhancement effect is strongly dependent on drain bias, indicating that the enhancement effect is pronounced when lateral electric field plays an important role.
For NDD a-IGZO TFT, it has been proposed that the high effective mobility (100 cm2V-1s-1) is due to the suppressed potential barrier in intrinsic a-IGZO. In this work, the DG operation reduces the vertical electric field, enhances the lateral field induced barrier lowering effect and hence enlarges the effective mobility of NDD a-IGZO TFT.
4-2 Future Work
We propose the high mobility, low operating voltage and small sub-threshold swing of dual gate a-IGZO TFTs with nano dot-like doping . But, we still have some problem to solve. The Ar treatment a-IGZO film decline in ambient environment, maybe we could apply passivation layer on the NDD devices to maintain its reliability and stability. The distribution of PS spheres are random, we could replace
47
nano-imprint technique by present method.
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