Chapter 2 Property of P3HT and Spin-Coating technique
4.4 Summary
The effect of contact resistance between source/drain electrodes and P3HT has been experimentally investigated for different channel length. Due to contact resistance effect, the field-effect mobility and the saturation current decreases with decreasing channel length.
Work function of metals higher than that of P3HT (e.g. Ni, Au and Pt) would form ohmic contact between S/D electrodes and P3HT OTFTs, a kind of p-channel device. On the contrary, metal work function lower than that of P3HT, such as Ti and Al, would form Schottky contact.
The electrical characteristic of P3HT OTFT is not FET-like with a Schottky-type S/D contacts, because a high potential barrier at the interface between S/D electrodes and P3HT, causing carriers can not inject from S/D electrodes to P3HT.
Because interfacial layer formed at Ni surface leads to a large contact resistance, comparing to noble metal (Au and Pt), the crowding effect was occurred when the OTFTs were fabricated by Ni as contact electrodes. Therefore, using Pt or Au as S/D contact materials would result in better I-V characteristics.
Next, we applied a simple model to estimate the contact resistance between source/drain electrodes and P3HT and found that the contact resistance is typically greater than the contact resistance of inorganic transistors. Regardless of adhesion/contact thickness ratio, Pt and Au would form a good ohmic contact between S/D contact electrodes with a value of contact resistance of approximate 0.3MΩ-cm. Besides, the extracted field-effect mobility of OTFTs with Pt as contact electrodes is 2.22×10-3 cm2/Vs, which is well agreement with those values described in Chap.2 and Chap.3.
10 20 30 40 50
Figure 4-1: The variation of threshold voltage and mobility in the linear regime as a function of the channel length (a) S/D metal is Ti/Au (b) S/D metal is Ti/Pt
0 -10 -20 -30 -40 -50 0.0
1.0x10-7 2.0x10-7 3.0x10-7 4.0x10-7
5.0x10-7 VG=-25V
VG=-25V VG=-25V VG=-20V VG=-20V VG=-20V
L=15µm L=25µm Ti/Au=200/1000AO
S ouc e C u rrent I
S(Amp)
Drain Bias(Volt)
L=50µm
Figure 4-2: Output characteristics IS vs VD for OTFTs with different channel lengths of 50,25,15µm.W/L=20
0 -10 -20 -30 -40 -50
Figure 4-3: Output characteristics IS vs VD of OTFT with (a)Ti (b)Ti/Ni (c)Ti/Au (d)Ti/Pt Source and Drain contact (continue)
0 -10 -20 -30 -40 -50
Figure 4-3: Output characteristics IS vs VD of OTFT with (a)Ti (b)Ti/Ni (c)Ti/Au (d)Ti/Pt Source and Drain contact
0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0
Figure 4-4: Current-voltage characteristics of OTFT with (a) Ti/Au (b)Ti/Pt electrodes at small drain voltage
-40 -30 -20 -10 0 10 20 30 40 10-12
10-11 10-10 10-9 10-8 10-7
10-6 VDS=-5V
Ti/Au Ti/Pt Ti/Ni Ti
Source Current I
S(A )
Gate Voltage V
G(volt)
Figure 4-5: Transfer characteristics IS vs VG of OTFT in the linear regime with different S/D contact metal (W/L=1000/50µm)
-10 0 10 20 30 40 50
Channel length L(µm)
Ti/Au=1000/200A0
Channel length L(µm)
Ti/Au=1000/200A0
(b)
Figure 4-6: Width-normalized ON resistance as a function of channel length at different gate voltage and at (a) VDS=-1V (b)VDS=-5V .The solid lines represent the linear least square fit of the data. (Ti/Au=1000/200Å)
-10 0 10 20 30 40 50
Channel length L(µm)
Ti/Au=200/1000A0
Channel length L(µm)
Ti/Au=200/1000A0
(b)
Figure 4-7: Width-normalized ON resistance as a function of channel length at different gate voltage and at (a) VDS=-1V (b) VDS=-5V .The solid lines represent the linear least square fit of the data. (Ti/Au=200/1000Å)
-10 0 10 20 30 40 50
Channel length L(µm)
Ti/Pt=1000/200A0
Channel length L(µm)
Ti/Pt=1000/200A0
(b)
Figure 4-8: Width-normalized ON resistance as a function of channel length at different gate voltage and at (a) VDS=-1V (b) VDS=-5V .The solid lines represent the linear least square fit of the data. (Ti/Pt=1000/200Å)
-20 -10 0 10 20 30 40 50
Channel length L(µm)
Ti/Pt=200/1000A0
Channel length L(µm)
Ti/Pt=200/1000A0
(b)
Figure 4-9: Width-normalized ON resistance as a function of channel length at different gate voltage and at (a) VDS=-1V (b) VDS=-5V .The solid lines represent the linear least square fit of the data. (Ti/Pt=200/1000Å)
(a)
(b)
Figure4-10: I-V characteristics of contacts between metal and semiconductor in integrated circuits (a) Ideal ohmic contact (b) nonlinear ohmic contact
-25 -20 -15 -10 -5 4
6 8 10 12 14 16
Gate Voltage(Volt)
Ti/Au=200/1000Ao
[∆( R
on.W)/ ∆ L]
-1(10
-6S)
Y=-(0.5137x10
-6)X+2.5494x10
-6Figure 4-11: OTFT channel sheet conductance as a function of gate voltage (S/D contact metal is Ti/Au)
-25 -20 -15 -10 -5 4
6 8 10 12 14 16
Gate Voltage (Volt)
Ti/Pt=200/1000Ao
Y=-(0.5574x10
-6)X+1.4476x10
-6[∆( R
on.W) / ∆ L]
-1(10
-6S)
Figure 4-12: OTFT channel sheet conductance as a function of gate voltage (S/D contact metal is Ti/Pt)
VDS=-5V VDS=-1V Source /Drain Metal
Contact Resistance (×106Ω．cm)
Contact Resistance (×106Ω．cm)
Ti/Pt=200/1000Å 0.3 1.0~2.5
Ti/Au=200/1000Å 0.25 0.4~0.5
Ti/Pt=1000/200 Å 0.3~0.4 4.5~6.0
Ti/Au=1000/200 Å 0.3~0.35 1.0
Table4-1: The contact resistance between source/drain electrodes and P3HT with different composition of adhesion/contact materials.
Chapter 5
Conclusion and Future work
5.1 Conclusions
The feasibility study of Spin-Coating technique, physical and the electrical characteristics of P3HT OTFT are investigated. Here, we conclude our study into three parts: (1) OTFTs Fabricated by Different Solvents and Weight Percentages of P3HT (2) Reliability Characteristics of P3HT OTFTs (3) Contact Resistance of P3HT OTFTs
5.1.1 OTFTs Fabricated by Different Solvents and Weight Percentages of P3HT
It was found that chloroform is a good solvent to dissolve P3HT, the anomalous gate leakage current was suppressed by chloroform solution, and the high ON-OFF ratio of about 4 order magnitudes and the field-effect mobility of 10-3 cm2/Vs were attributed to chloroform solution. Next, we investigate the performance of P3HT OTFTs which was fabricated different weight percentage of P3HT in chloroform.
The surface root-mean-square roughness of organic thin film deposited by 0.3% of P3HT is 8.24Å. That is much smoother than RMS roughness of organic thin film deposited by others, 0.1%, 0.8% and 2.0%. As weight percentage of P3HT in chloroform is above 0.3%, the bulk current effect would affect IS-VG curves and IS-VD curves. Therefore, in order to acquire an OTFTs with good mobility, high ON-OFF current, appropriate threshold voltage, the optimal weight percentage of P3HT would be 0.3%.
5.1.2 Reliability Characteristics of P3HT OTFTs
Given the sensitivity of P3HT to oxygen absorbed during the fabrication process, it is
expected that vacuum and N2 treatments can be used to recover some of the lost performance through vacuum-induced expulsion of absorbed oxygen.
The threshold voltage shift of OTFTs with P3HT as active material has been studied from our experiments. There are two elements for resulting in the threshold voltage shift. One is the diffusion of oxygen atoms into P3HT polymer; the other is the electric field of the polarization effect in the P3HT polymer.
Under O2 treatment, since the diffusion of oxygen into P3HT polymer causes an increase to the conductivity of P3HT polymer and bulk leakage current, the device was hardly turned off.
Therefore, the threshold voltage shift is dependent on O2 treatment time. Nevertheless, the variation of field-effect mobility increases in the first 3hours O2 treatment, and then it decreases after 3 hours O2 treatment. The phenomenon is owing to bulk leakage current. Due to the bulk leakage current, the extraction of field-effect mobility was overestimated in the first 3 hours.
However, after 3 hours O2 treatment oxygen continues diffusing into P3HT polymer, and it would degrade P3HT polymer characteristics and cause serious carriers scattering. Therefore, the field-effect mobility decreased after 3 hours O2 treatment.
Under stress measurement, the threshold voltage shift is dependent on the polarity of gate bias. It was found that positive bias stress causes a positive threshold voltage shift and negative bias stress causes a negative threshold voltage shift. Nevertheless, the variation of field-effect mobility was independent on the polarity of gate bias.
Additionally, it was proved that humidity will not affect the performance of P3HT OTFTs.
5.1.3 Contact Resistance of P3HT OTFTs
The effect of contact resistance between source/drain electrodes and P3HT has been experimentally investigated for different channel length. Due to contact resistance effect, the
field-effect mobility and the saturation current decreases with decreasing channel length.
Work function of metals higher than that of P3HT would form ohmic contact between S/D electrodes and P3HT for p-type OTFTs, such as Ni, Au and Pt. On the contrary, work function of metals lower than that of P3HT, such as Ti, would form Schottky contact. The electrical characteristic of P3HT OTFT is not FET-like with a Schottky-type S/D contacts, because a high barrier at the interface between S/D electrodes and P3HT, causing the charges can not inject from S/D electrodes to P3HT.
Because interfacial layer formed at Ni surface leads to large contact resistance, comparing to noble metal (Au and Pt), the crowing effect was occurred as the OTFTs were fabricated by Ni as contact electrodes. Therefore, using Pt or Au as S/D contact materials would form a good ohmic contact.
Next, we applied a simple model to estimate the contact resistance between source/drain electrodes and P3HT and found that the contact resistance is typically greater than the contact resistance of inorganic transistors. Regardless of adhesion/contact thickness ratio, Pt and Au would form a good ohmic contact between S/D contact electrodes and P3HT and the contact resistance is approximate 0.3MΩ-cm. By this method, the field-effect mobility with Pt as contact electrodes is 2.22×10-3 cm2/Vs and is very well in agreement with those values from chap2 and chap3.
5.2 Future work
An in-situ pacivation layer for protecting the P3HT film
From our experimental results, P3HT OTFTs are sensitive to ambient conditions. Protection from the environment by encapsulation is critical to the stability of P3HT OTFTs. Therefore, using a suitable material as pacivation to protect P3HT film from environmental effect is another
important topic.
A new method to deposit P3HT thin film
There are three methods to deposit P3HT thin films: (1) spin-coating (2) dip-coating (3) drop-casting. In our experiment, we made use of spin-coating method to deposit P3HT thin films and attain an optimized deposition parameter for producing P3HT thin films. However, among the three methods to deposit P3HT thin films, the best method is drop-casting. Therefore, in the future we will make use of drop-casting to deposit P3HT thin films, and study the deposition parameters of drop-casting.
Thermal stability of P3HT OTFTs
In addition to studies of device lifetime and the stability of P3HT in different ambient, thermal stability is another topics .This is an important topic for various reasons. First, poly (3-hexylthiophenes) devices will be likely exposed to elevated temperatures during the fabrication process, due to the annealing requirements of other layers. Second, thermal cycling studies provide crucial insights into device lifetime and stability. [30]
New gate insulator materials for P3HT OTFTs
From the performance point of view, the most important parameters are charge carrier mobility, ON-OFF current ratio and the operational voltage range. However, the operating voltages of P3HT OTFT required to produce such performance were impractically high, around 50~60V. Although decreasing the thickness of SiO2 could reduce the operating voltages of P3HT OTFT, the gate leakage current would increase with decreasing the thickness of SiO2 and affect the performance of P3HT OTFT. Therefore, the use of high dielectric gate insulator materials is possible for reducing operating voltages of P3HT and gate leakage current. [31], [32], [33]
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簡歷
姓名: 林榮祥 性別: 男
出生地: 台灣省台南縣
出生日期: 民國 68 年 01 月 11 日
住址: 台南縣歸仁鄉辜厝村中正路 193 巷 102 弄 30 號 學歷: 國立高雄師範大學物理系
(86 年 6 月至 90 年 6 月)
國立交通大學電子工程研究所碩士班 (91 年 6 月至 93 年 6 月)
Publications
[1] Shuo-Cheng WANG, Jen-Chung Lou, Bo-Lin Liou, Ron-Xion Lin, and Ching-Fa Yeh “ Process Improvement and Reliability Characteristics of Spin-on Poly-3-hexylthiophene Thin-Film Transistor＂Electron Devices and Materials Symposia, 2003