Chapter 2 Fabrication and Characterization
2.4 Measurement Setup
In this study, the devices were characterized by current–voltage ( I–V ) and capacitance–voltage ( C–V ) measurements using 4155B semiconductor parameter analyzer and Agilent E4980A precision LCR meter, respectively.
In the section of metal–insulator–semiconductor capacitors, we did capacitance–
voltage measurement from the typical top electrode -to-ground bias which was swept forward from V = 2 V to V = -4 and then reverse from V = -4 V to V = 2 V in 100,300 and500KHZ frequency. In current–voltage measurement, the typical top electrode -to-ground bias was swept from V = 0 V to V = 2 V and V = 0 V to V = -2 V.
Next, we did capacitance–voltagemeasurement from the typical top electrode
-to-16
ground bias which was swept from V = 2 V to V = -2 V and current–voltage measurement that the typical top electrode -to-ground bias was swept from V = 0 V to V = 2 V and V = 0 V to V = -2 V in the section of metal–insulator–metal capacitors.
Finally, we did IDS-VGS measurement that the typical drain-to-source bias was swept from VGS = -0.5 V to VGS = 2 V, VDS= 0 to VDS= 1 V and IDS-VDS measurement that the typical drain-to-source bias was swept from VDS = 0 V to VDS = 2 V, VGS= 0V to VGS= 1 V.
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Table2.1 The experimental flow path
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Table2.2 The experimental conception in this study
What’s the quality about SiO
2and
LaAlO
3MIM Capacitors Fabrication
Conclusion a-IGZO TFTs
Fabrication
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Figure 2.1 Schematic cross-sectional view of a Ni/SiO2/P-type Si MOS capacitors
Figure 2.2 Schematic cross-sectional view of a Ni/LaAlO3/P-type Si MOS capacitors
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Figure 2.3 Schematic cross-sectional view of a Al/SiO2/TaN MIM capacitors
Figure 2.4 Schematic cross-sectional view of a Al/LaAlO3/TaN MIM capacitors
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Figure 2.5 Schematic cross-sectional view of a-IGZO TFT with an inverted staggered bottom-gate structure
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Figure 2.6 Extrapolation method in the saturation region (ESR) implemented on the measured I D0:5 –Vg characteristics of the test bulk device measured at Vd = Vg. This method consists of finding the gate-voltage axis intercept (i.e., I D 0:5 = 0)
of the linear extrapolation of the ID0:5 –Vg curve at its maximum slope point.
[ref 2.6]
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Chapter 3
Experimental Results and Discussion
3.1 Metal–Insulator–Semiconductor Capacitors
3.1.1 Analyzing C-V curves of SiO2 MOS and LaAlO3 MOS Capacitors
Figure 3.1 shows capacitance density-voltage (C-V) characteristics. A voltage shift of 1.2V for the SiO2 MOS capacitors during forward and reverse swept in 100KHZ was observed. Figure 3.3 shows capacitance density-voltage (C-V) characteristics. A voltage shift of 0.4V for the LaALO3 MOS capacitors during forward and reverse swept in 100KHZ was observed. Clearly, the voltage shift of SiO2 MOS was larger than LaAlO3 MOS. We had known that the quality of SiO2 insulator layer is poor because the SiO2/Si interface state was so rough that a lot of carriers were trapped at interface and oxide layer had many defects to trap charges. On the other hand, in Figure 3.1 and Figure 3.2, compared with the C-V curves from 100KHZ to 300KHZ, it indicates that the flat-band voltage has been shifted from -0.2 V to -0.8 V. It can be seen that the metal/interface may be also poor to change the flat-band voltage. since the SiO2
insulator layer was deposited by dual E-GUN evaporation system and just followed by a low temperature as 400◦C O2 annealing for 10 min, it causes poor quality for SiO2
layer but not for LaAlO3 layer. Besides, the defects of MOS capacitors was shown as Figure 3.5.
3.1.2 Summary
The SiO2 MOS capacitor has a large flat band voltage shift in forward and reverse swept measurements and the flat-band voltage is changed from -0.2 V to -0.8 V in frequency-varied measurement. The poor quality for SiO2 layer is related to the low
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temperature PDA process and E-GUN evaporation system.
3.2 Metal–Insulator–Metal Capacitors
3.2.1 C-V characteristics of SiO2 MIM and LaAlO3/SiO2MIM Capacitors
The capacitance density-voltage (C-V) characteristics of the SiO2 MIM capacitors are shown in Figure 3.6. Figure 3.7 shows the capacitance density-voltage (C-V) characteristics of the LaAlO3/SiO2 MIM capacitors. The average capacitance density of SiO2 MIM capacitors and LaAlO3/SiO2 MIM capacitors are about 0.31µf/cm2,and 0.43µf/cm2, respectively The capacitance density of LaAlO3/SiO2 MIM capacitors is higher than SiO2 MIM capacitors. We can know that the higher capacitance density can decrease Vt and S.S. from eq. (6) and eq. (7).
3.2.2 I-V characteristics of SiO2 MIM and LaAlO3/SiO2MIM Capacitors
Figure 3.8 and Figure 3.9 show the current-voltage (I-V) characteristics of SiO2 MIM and LaAlO3/SiO2 MIM capacitors. Here, the leakage current of LaAlO3/SiO2 MIM capacitors is smaller than that of the SiO2 MIM capacitors, indicating that the SiO2 layer has a lot of defects deposited by dual E-GUN evaporation system and low temperature annealing treatment. The small leakage current can reduce the power consumption which plays an important role in green power devices.
3.3 Characteristics of a-IGZO TFTs
Our a-IGZO TFTs device has a channel length of 50 um and width of 500 um, respectively. The output ID–VD characteristics of the high-κ LaAlO3/SiO2 a-IGZO TFT
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is shown in Figure 3.10. Well-behaved transistor characteristics were observed even under a low operation voltage of 1 V, which is important for low-power application.
Figure 3.11 shows the transfer ID–VG characteristics of the high-κ LaAlO3/SiO2 a-IGZO TFT. A low Vt of 0.5V was determined from the linear I D1/2 versus VG plot as shown in Figure 3.12. Low S.S. of 95 mV/dec is reached. Such small S.S. is essential to turn on the transistor fast at low voltage. The acceptable µFE mobility of 3.08cm2/V.s is obtained simultaneously. The gm and mobility are shown in Figure 3.13 This better performance is attributed to the high gate capacitance density, the larger conduction band offset for SiO2 / IGZO than that of LaAlO3/IGZO and good a-IGZO material.
Figure 3.14 shows the band diagram of the TFT device.
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Figure 3.1 C-V characteristics of Ni/SiO2/p-type Si MOS capacitors in 100KHZ
-4 -3 -2 -1 0 1 2
Figure 3.2 C-V characteristics of Ni/SiO2/p-type Si MOS capacitors in 300KHZ
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Figure 3.3 C-V characteristics of Ni/LaAlO3/p-type Si MOS capacitors in 100KHZ
Figure 3.4 C-V characteristics of Ni/LaAlO3/p-type Si MOS capacitors in
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300KHZ
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Figure 3.5 The defects of MOS capacitors
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Figure 3.6 C-V characteristics of Al/SiO2/TaN MIM capacitors
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Figure 3.7 C-V characteristics of Al/SiO2-LaALO3/TaN MIM capacitors
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Figure 3.8 I-V characteristics of Al/SiO2/TaN MIM capacitors
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Figure 3.9 I -V characteristics of Al/SiO2-LaALO3/TaN MIM capacitors
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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0
0.2 0.4 0.6 0.8 1.0 1.2
Drai n Cu rr ent (
A )
Drain Voltage (V)
Vg=0V Vg=0.2V Vg=0.4V Vg=0.6V Vg=0.8V Vg=1.0V
Figure 3.10 ID-VD characteristics of a-IGZO TFTs
Figure 3.11 ID-VG characteristics of a-IGZO TFTs
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Figure 3.12 The threshold voltage of a-IGZO TFTs Determinated by the extrapolation method in the saturation region
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Figure 3.13 Determinate the gm and mobility of a-IGZO TFTs
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Figure 3.14 The band diagram of a-IGZO TFTs from channel to gate
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Chapter 4 Conclusion
In conclusion, incorporating LaAlO3 /SiO2 as gate dielectrics, the IGZO TFTs show a small S.S. of 95 mV/dec, a low Vt of 0.5 V, and an acceptable μFE of 3.08 cm2/V∙
sec at operation voltage as low as 1.7 V. These good performances were related to the high gate capacitance density by introducing the high-κ LaAlO3 dielectric. The present results show that these low operation voltage IGZO TFTs with LaAlO3 /SiO2 as gate dielectrics have high potential for future high speed and low power applications.
Since the IGZO TFT performances still need improvements, we will do more work to enhance the device characteristics in the future.
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