Chapter 3 Result and Discussion
3.4 Summary
In this chapter, we confirm that after the supercritical fluid passivation achieve better device performances, including the on-current, the off-current, the threshold voltage and the subthreshold swing, and also better filed effect mobility. In addition, back channel is also repaired by supercritical fluid passivation. These improvements are attributed to the oxygen atoms passivation of the dangling bonds and it is observed by material analysis (i.e. FTIR, TDS and Contact angle). These mean that density of defect states in the channel region (amorphous silicon) are decreased after supercritical fluid passivation interfuse H2O.
Vg
-10 0 10 20
Log(NId)
1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-9 1e-8 1e-7
Before After
Fig. 3-1 Transfer characteristics of a-Si TFTs before and after annealing for 2 hours at 150
℃
(was measured at Vd=0.1V and T=30℃
)Vg
-10 0 10 20
NId
0.0 2.0e-9 4.0e-9 6.0e-9 8.0e-9 1.0e-8 1.2e-8 1.4e-8 1.6e-8
Before After
Fig. 3-2 Transfer characteristics of a-Si TFTs before and after annealing
Device name W/L=20/10 Mobility (cm2/VS)
S.S.
(V/dec.)
Vth
(V)
Sample 1 Before 0.4018 2.0616 2.6603 After 0.3774 1.7849 1.5966 Sample 2 Before 0.4020 2.0539 2.2678 After 0.3938 1.8941 0.8000 Sample 3 Before 0.4032 2.0773 2.7078 After 0.3738 1.7901 1.1619
Table 3-1 Parameters of a-Si TFTs before and after annealing for 2 hours at 150
℃
(was measured at Vd=0.1V and T=30℃
)Vg
-10 0 10 20
Log(NId)
1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-9 1e-8 1e-7 1e-6 1e-5
Before After
Fig. 3-3 Transfer characteristics of a-Si TFTs before and after annealing for 2 hours at 150
℃
(was measured at Vd=10V and T=30℃
)Vd
Fig. 3-4 Output characteristics of a-Si TFTs before and after annealing for 2 hours at 150
℃
(was measured at T=30℃
)Fig. 3-5 Transfer characteristics of a-Si TFTs before annealing for 2
Vg
Fig. 3-6 Transfer characteristics of a-Si TFTs after annealing for 2 hours at 150
℃
(was measured at Vd=0.1V and T=30℃
~60℃
)Fig. 3-7 Transfer characteristics of a-Si TFTs before annealing for 2 hours at 150
℃
(was measured at Vd=10V and T=30℃
~60℃
)Vg
-10 0 10 20
Log(NId)
1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-9 1e-8 1e-7 1e-6 1e-5
30度 40度 50度 60度
Fig. 3-8 Transfer characteristics of a-Si TFTs after annealing for 2 hours at 150
℃
(was measured at Vd=10V and T=30℃
~60℃
)1/T
0.00295 0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330 0.00335
Log (NId)
-28 -26 -24 -22 -20 -18 -16
Fig. 3-9 Temperature activation of the drain-source current of the only annealing 2 hours at 150
℃
sample (Fig. 3-6) for different gate voltagesVg
-6 -4 -2 0 2 4 6
Ea
0.0 0.2 0.4 0.6 0.8
Before After
Fig. 3-10 Activation energy vs. gate voltage for the samples before and after annealing in supercritical fluids treatment system (2 hours, 150
℃
)Et
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Density of states
1e+16 1e+17 1e+18 1e+19
Before After
Fig. 3-11 Density of states (DOS) vs. Et for the samples before and after annealing in supercritical fluids treatment system (2 hours, 150
℃
)Vg
-10 0 10 20
Log(NId)
1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-9 1e-8 1e-7
Before(Vd=0.1V) After(Vd=5V)
Fig. 3-12 Transfer characteristics of a-Si TFTs before and after H2O passivation for 2 hours at 150
℃
(was measured at linear region, T=30℃
)Vg
-10 0 10 20
Log(NId)
0.0 2.0e-9 4.0e-9 6.0e-9 8.0e-9 1.0e-8 1.2e-8 1.4e-8 1.6e-8
Before(Vd=0.1V) After(Vd=5V)
Fig. 3-13 Transfer characteristics of a-Si TFTs before and after H2O
Device name W/L=20/10 Mobility (cm2/VS)
S.S.
(V/dec.)
Vth
(V)
Sample 1 Before 0.4234 1.6098 2.3000 After 0.0350 3.3959 3.7452 Sample 2 Before 0.4068 1.7174 3.3095 After 0.0419 3.1355 3.5212 Sample 3 Before 0.4108 1.6480 2.7408
After 1.05x10-5 X X
Table 3-2 Parameters of a-Si TFTs before and after H2O passivation for 2 hours at 150
℃
(was measured at linear region and T=30℃
)Vg
-10 0 10 20
Log(NId)
1e-15 1e-14 1e-13 1e-12 1e-11 1e-10 1e-9 1e-8 1e-7 1e-6 1e-5
Before(Vd=10V) After(Vd=20V)
Fig. 3-14 Transfer characteristics of a-Si TFTs before and after H2O passivation for 2 hours at 150
℃
(was measured at sat. region, T=30℃
)Vd
Fig. 3-15 Output characteristics of a-Si TFTs before and after H2O passivation for 2 hours at 150
℃
(was measured at T=30℃
)Fig. 3-16 Transfer characteristics of a-Si TFTs before and after SCCO2 passivation for 2 hours at 150
℃
. (was measured at Vd=0.1V and T=30℃
)Vg
-10 0 10 20
Log(NId)
0.0 2.0e-9 4.0e-9 6.0e-9 8.0e-9 1.0e-8 1.2e-8 1.4e-8 1.6e-8 1.8e-8
Before After
Fig. 3-17 Transfer characteristics of a-Si TFTs before and after SCCO2 passivation for 2 hours at 150
℃
. (was measured at Vd=0.1V and T=30℃
)Device name W/L=20/10 Mobility (cm2/VS)
S.S.
(V/dec.)
Vth
(V)
Sample 1 Before 0.4265 2.0447 2.8018 After 0.4375 1.3272 0.4224 Sample 2 Before 0.4249 1.9111 1.6530 After 0.4358 1.6278 0.2700 Sample 3 Before 0.4230 1.8460 1.1170
After 0.4390 1.4542 0.2673
Table 3-3 Parameters of a-Si TFTs before and after SCCO2 passivation for 2 hours at 150
℃
(was measured at Vd=0.1V and T=30℃
)Vg
Fig. 3-18 Transfer characteristics of a-Si TFTs before and after SCCO2 passivation for 2 hours at 150
℃
. (was measured at Vd=10V and T=30℃
)Fig. 3-19 Output characteristics of a-Si TFTs before and after SCCO2 passivation for 2 hours at 150
℃
(was measured at T=30℃
)Vg
Fig. 3-20 Transfer characteristics of a-Si TFTs before SCCO2 passivation for 2 hours at 150
℃
(measured at Vd=0.1V, T=30℃
~60℃
)Fig. 3-21 Transfer characteristics of a-Si TFTs after SCCO2 passivation for 2 hours at 150
℃
(measured at Vd=0.1V, T=30℃
~60℃
)Vg
Fig. 3-22 Transfer characteristics of a-Si TFTs before SCCO2 passivation for 2 hours at 150
℃
(measured at Vd=10V, T=30℃
~60℃
)Fig. 3-23 Transfer characteristics of a-Si TFTs after SCCO2 passivation for 2 hours at 150
℃
(measured at Vd=10V, T=30℃
~60℃
)Vg
-6 -4 -2 0 2 4 6
Ea
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Before After
Fig. 3-24 Activation energy vs. gate voltage for the samples before and after SCCO2 fluid interfuse H2O passivation (2 hours, 150
℃
)Et
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Density of states
1e+16 1e+17 1e+18 1e+19
Before After
Fig. 3-25 Density of states (DOS) vs. Et for the samples before and after SCCO2 fluid interfuse H2O passivation (2 hours, 150
℃
)Wavenumber
500 1000 1500 2000 2500
Intensity
-0.02 0.00 0.02 0.04
Anneal H2O
SCCO2 / Propyl alcohol+H2O
Fig. 3-26 Infrared absorption spectra after passivation 2 hours
Wavenumber
500 1000 1500 2000 2500
Intensity
-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04
Anneal H2O
SCCO2 / Propyl alcohol+H2O
Fig. 3-27 Infrared absorption spectra after passivation 2 hours and
Temperature
80 100 120 140 160 180 200
Intensity
2e-8 3e-8 4e-8 5e-8
6e-8 Annealing
H2O
SCCO2/H2O+Propyl alcohol
Fig. 3-28 Thermal desorption spectroscopy after passivation 2 hours
Temperature
80 100 120 140 160 180 200
Intensity
2e-8 3e-8 4e-8 5e-8
6e-8 Annealing
H2O
SCCO2/H2O+Propyl alcohol
Fig. 3-29 Thermal desorption spectroscopy after passivation 2 hours and then hot plate baking 1 hour at 200
℃
Substrates α
Fig. 3-30 Contact angle measurement
Poly - Si (without any
treatment)
Annealing H2O
SCCO2 / H2O +Propyl
alcohol Before (hot plate)
Baking 48.1 38 5 17.4
After (hot plate)
Baking 40.9 40.7 7.5 20.9
Table 3-4 Contact angle after passivation
Fig. 3-31 Density of states (DOS) in the amorphous silicon
Fig. 3-32 Schematic illustration of the basic operation of the a-Si TFT
-20 -15 -10 -5 0 5 10 15
emission Sub-threshold Above Threshold
Fig. 3-33 Drain-source current vs. gate-source voltage of the a-Si TFT
Fig. 3-34 Band diagram for a cross section in the channel area of the
E
FSChapter 4 Conclusion
Amorphous silicon thin film transistors (a-Si TFTs) have been widely applied to fabrication of liquid crystal flat-panel display. Plasma enhancement chemical vapor deposition (PECVD) has been used for fabrication of a-Si TFTs at low temperatures.
Inevitable amorphous silicon films among deposition can have electrically active defect states due to dangling bonds and lattice disorder. The defects reduce electrical current due to trapping carriers. One of most important problems on fabrication of a-Si TFTs is the reduction of densities of those defect states.
In this thesis, we report fabrication of a-Si TFTs using defect reduction of supercritical fluid passivation interfuse H2O. This method of the defect reduction treatments is essential for the higher on-current, the lower off-current and threshold voltage as well as the subthreshold swing, and also better filed effect mobility. In addition, back channel is also repaired by supercritical fluid passivation. These improvements are attributed to the oxygen atoms passivation of the dangling bonds and it is observed by material analysis (i.e. FTIR, TDS and Contact angle). These mean that density of defect states in the channel region (amorphous silicon) are decreased after supercritical fluid passivation interfuse H2O. Incidentally, it is first confirmed that the a-Si TFTs after the supercritical fluid passivation could achieve better device performances. In order to compare with the result of supercritical fluid passivation, we have also reported that the only annealing and the use of H2O passivation at temperature of 150℃.
Additionally, supercritical fluids technology will have advantage for integrating the fabrication of TFT-LCD’s, because of the low cost and the process easily.
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簡 歷
姓 名：王 建 文 ( Chen-Wen Wang )
性 別：男
出生年月日：民國
69年
8月
16日
住 址：台北縣樹林市名園街54號3樓
學 歷：
國立台北科技大學電機學系學士
(89.9-91.6)國立交通大學光電工程學系顯示科技研究所碩士
(93.9-95.6)碩士論文題目
:超臨界流體技術應用於非晶矽薄膜電晶體之研究
Application of Supercritical Fluids Technology for Amorphous Thin Film Transistors