Ar plasma time=
2.3.4. The Plausible Mechanism of Increased Mobility
The drastically enhanced the effective field-effect mobility in NDD structures may be because of two reasons. Firstly, the effective channel length is reduced due to the conductive dot regions inside the channel. The effective channel length for TG-NDD devices can be estimated by calculating the dots concentration[shown in Figure 2.12]. From these two SEM images in Figure 2.12, we estimated the average PS spheres in different dots concentration. The average dots of 0.2 wt% and 0.8 wt%
are 1583 and 2500 per 1000μm, respectively. Then, the effective channel length is equal to the channel length definition by shadow mask minus the high conductive region.
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Figure 2.12 The SEM images of different dots concentration.
For TG-NDD devices with high-density dots (with 0.8 wt% PS spheres), the effective channel length reduces from 1000 m to 500 m. For TG-NDD devices with low-density dots (with 0.2 wt% PS spheres), the effective channel length reduces from 1000 m to 684 m. If the effective intrinsic channel length is used to estimate the mobility inside the intrinsic a-IGZO region, the intrinsic mobility is 39.6 cm2 V-1 s-1 for TG-NDD devices with high-density dots and is 32.5 cm2 V-1 s-1 for TG-NDD devices with low-density dots.(shown in Table 2.2) The mobility for TG-STD devices is only around 3.8 cm2 V-1 s-1, which is 8-10 times smaller than the mobility in the intrinsic channel region for TG-NDD devices. The reduction of the effective channel length is not sufficient to explain the enhanced mobility in TG-NDD devices.
Table 2.2 The intrinsic mobility of a-IGZO with 0.2 and 0.8 wt% nano dot doping.
*L
int=intrinsic channel length, μ
int=intrinsic mobility
TG-NDD (0.8 wt%) Ltotal(μm) Lint(μm) μ(cm2V-1s-1) μint(cm2V-1s-1)
1000 500 79.2 39.6
TG-NDD (0.2 wt%) Ltotal(μm) Lint (μm) μ(cm2V-1s-1) μint(cm2V-1s-1)
1000 684 47.5 32.5
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Therefore, we proposed the second reason to explain the enhanced mobility in the intrinsic channel region for TG-NDD devices is the field-induced barrier lowering effect. It is known that the electron transport in a-IGZO is governed by the percolation transport.[16, 17] The random distribution of Ga3+ and Zn2+ ions in the network structure forms potential barriers around the conduction band and then reduces electron mobility.[18] The potential barrier can be significantly reduced when carrier concentration is increased.[16-18] When high-density conductive dot-like regions are introduced into the intrinsic a-IGZO film, the potential barrier in the intrinsic a-IGZO is lowered by the neighboring high conductive regions. Increasing the dot concentration leads to a more pronounced barrier lowering effect. As a result, when dot density increases from 4.8106 mm-2 to 6.8106 mm-2, the mobility in the intrinsic channel increases from 32.5 cm2 V-1 s-1 to 39.6 cm2 V-1 s-1.
The barrier lowering effect is well observed in many semiconductor devices. For example, the Schottky barrier at the metal-organic interface exhibits a Schottky-barrier lowering effect when increasing the doping level of the organic semiconductor.[19] 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.[20-22] For poly-Si TFTs, the grain boundary barrier is also lowered by the drain-to-source electric field.[23] Drain-induced barrier lowering effect is also observed in short channel ZnO TFT.[24] The high density dot-like doping in channel region of field-effect transistors was not reported in previous studies. However, in our work, it is believed that the effective potential barrier in the intrinsic a-IGZO surrounded by heavily-doped dots is lowered when the dot density is increased and when the doping level is increased. Since the electron mobility in a-IGZO is exponentially dependent on the minus of the potential barrier height[shown in Eq. 1], the reduction of potential barrier leads to a significant
56
improvement of the electron mobility. [16, 17]
( )n oe eeff( )/n k TB , eff
is effective potential barrier height (Eq. 1) Also, The transfer characteristics of TG-NDD devices when we repeating the measurement for seven times are shown in Figure 2.13. The stability is acceptable.
The performance of the devices, however, gradually degrades after several days when stored in ambient. Because the dot-like regions with a large amount of oxygen deficiencies are exposed to the oxygen-rich ambient, the oxygen deficiencies are gradually decreased and the conductivity is reduced. Passivation is required to solve this issue and the related process is currently developed.
Figure 2.13 The transfer characteristics of TG-NDD devices when we repeating the measurement for seven times.
Finally, the NDD process is utilized on the back interface of conventional bottom-gate (BG) a-IGZO TFTs. The bottom-gate (BG) a-IGZO TFTs with and without NDD are denoted as BG-NDD and BG-STD, respectively. The schematic diagrams of BG-STD and BG-NDD a-IGZO TFTs are shown in Figures 2.14(a) and 2.14(b), respectively.
-20 -15 -10 -5 0 5 10 15 20 25
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For bottom-gate devices, process flow is similar to that shown in Figure 2.3(a) except for two differences. The first difference is that the glass substrate is replaced by a heavily-doped silicon substrate capped with a 100 nm silicon nitride. The second is that the top aluminum electrode above the PVP layer is replaced by a thermal-evaporated SiOx with a thickness of 40 nm. The SiOx is served as a mask and the PVP without the SiOx coverage is etched by oxygen plasma. The channel width and length of the bottom gate devices are 1000 μm and 100 μm, respectively. The channel width is defined by the edge of the a-IGZO pattern. For bottom-gate devices (BG-STD and BG-NDD), the channel length is defined by the edge of the source/drain electrodes. Four typical parameters, including threshold voltage (Vth), on/off ratio, field-effect mobility (μ), and subthreshold swing (S.S.), are extracted and plotted as a function of Ar plasma treatment time, as shown in Figures 2.14(c). For BG-STD devices, when the Ar plasma time increases from 0 sec to 180 sec, the field-effect mobility slightly increases from 10.76 to 15.6 cm2 V-1 s-1 and the threshold voltage decreases from 3.7 to -0.42 V. The subthreshold swing and the on/off current ratio are almost unchanged. The decrease of the threshold voltage and the increase of the field-effect mobility after Ar plasma treatment was also reported by Park et al. and was explained by the improvement of the contact resistance between the source/drain electrodes and a-IGZO semiconductor.[11]
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Figure 2.14 The schematic device structures of (a) standard (STD) and (b) nano-dot doping (NDD) bottom gate (BG) a-IGZO TFTs. (c) Four typical parameters (threshold voltage, on/off ratio, mobility, and subthreshold swing) of BG-STD and BG-NDD devices are extracted and plotted as a function of Ar plasma treatment time. Each data point was extracted from the transfer characteristics measured at VD = 20 V. The device channel width is 1000 μm and the channel length is 100 μm.
For BG-NDD devices, when the Ar plasma time increases from 0 sec to 180 s, the field-effect mobility significantly increases from 10.8 to 32.7 cm2 V-1 s-1. The threshold voltage decreases from 3.1 to -5.9 V. Also, the parameters is summarized
(a) (b)
On/Off ratioMobility (cm2 V-1 s-1 )
S.S. ( V decade-1 )
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in Table 2.3. The NDD structure influences the electric field distribution in the front channel because the a-IGZO film is only 30 nm thick. As a result, the field-effect mobility is enlarged by NDD treatment due to the reduced effective channel length together with the barrier-lowering-effect. The shift of the threshold voltage is not clearly understood. In our previous report, the removal or the injection of electrons into body region causes a positively-shifted or a negatively-shifted threshold voltage, respectively.[25] When electron concentration in body region is increased, a more negative gate bias is required to deplete the channel. In this work, the dot doping creates localized high electron concentration regions. The three-dimensional potential distribution in channel region is still not clearly investigated. However, the negatively shift of threshold voltage is consistent with the phenomenon reported in our previous work when electrons are injected into back channel by capping calcium/aluminum layer onto the back interface of a bottom-gate a-IGZO TFT.[25]
Table 2.3 Typical parameters of BG-NDD TFTs with and without nano-dot doping.
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2.4. Conclusion
To summarize, this study proposes a top-gate self-aligned a-IGZO TFT with nano-meter-scale dotted channel doping. With a simple, low-cost, and lithography-free process, the effective mobility level of TG a-IGZO TFT becomes 19 times higher than that of the control sample and the maximum effective mobility reaches 79 cm2 V-1 s-1. If the effective intrinsic channel length is used to estimate the mobility inside the intrinsic a-IGZO region, the maximum intrinsic mobility of TG a-IGZO TFT reaches 39.6 cm2 V-1 s-1 and increases 10 times than controls (STD). The nano dot doping (NDD) structure reduces the effective channel length and lowers the potential barrier in the intrinsic a-IGZO by the neighboring high conductive regions.
Increasing the dot concentration leads to a more pronounced barrier lowering effect.
According to the percolation conduction model, the decrease of the potential barrier leads to a significant increase of the field-effect mobility in a-IGZO semiconductor.
The high mobility and the self-aligned structure of the proposed a-IGZO TFTs with NDD are promising for the development of low cost circuit-like RFID tags, smart cards, and transparent circuits on windows.
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Chapter 3. Increasing Organic Vertical Carrier Mobility