3-1 Metal-Insulator-Metal Leakage Test
Follow previous sections, we can get two polymer dielectric between Al and Au electrodes, and the cross-section were 1×1 mm2. The thickness of the polymer films measured by using a surface profiler (Alpha-step) was about 420nm. So we can investigate the electrical insulating properties and capacitance of PVP and PVP-PMMA.
We applied one electrode voltage sweep from 0 to ±50v and the other electrode was ground. Fig. 3-1 showed the MIM leakage current density were 1.71×10-8 A/cm2 and 2.85×10-8 A/cm2 in an electric field of 1MV/cm. Showing that PVP and PVP-PMMA both have enough insulating properties for OTFT operation. The measured capacitance of the MIM structure with PVP and PVP-PMMA was approximately 8.8nF/cm2 and 5.6nF/cm2.
3-2 Characteristics of Pentacene-based TFTs with PVP and PVP-PMMA Dielectrics
Electrical transfer characteristics of pentacene-based organic thin film transistors (OTFTs) with PVP and PVP-PMMA dielectrics measured in the ambient air and in the dark are shown in the Fig 3-2. We operated the device in the linear region |VD|
|VG VT| and got field effect mobility (μFE) and threshold voltage by extracting from transfer of characteristics devices. The μFE and VT of PVP OTFTs were about 0.5~0.6cm2/Vs and -15 V. And the μFE and VT of PVP-PMMA-OTFTs were close to 0.32 cm2/Vs and -14 V. Ion/Ioff ratio of devices of PVP-OTFT was 4×104 worse than
PVP-PMMA was 3×105. In our research, if we measured transfer characteristics (ID-VG) more times, we could know threshold voltage of PVP OTFT easily shift to positive, but PVP-PMMA OTFT was much more stable only shift less. Apparently, PVP-PMMA OTFT has fine characteristics for display application than PVP OTFT. In the next sections, we would explain why the PVP OTFT has higher mobility but poor stability than PVP-PMMA OTFT.
3-3 Morphology of PVP, PVP-PMMA and Pentacene Thin Films
Before and after depositing pentacene thin films on organic gate dielectrics, using atomic force microscope (AFM) to observe the morphology of PVP, PVP-PMMA and pentacene films deposited on PVP and PVP-PMMA. Surface roughness is one of factors that can effect semiconductor crystallization. Pentacene grow on smooth surface usually have better morphology and mobility than grow on rough surface. Fig. 3-3(a) and (b) are atomic microscopy images of PVP and PVP-PMMA gate dielectric before depositing pentacene thin films. Both PVP and PVP-PMMA have smooth surface roughness about 0.5 nm. Fig. 3-3 (c) and (d) are atomic microscopy images of pentacene thin films on PVP and PVP-PMMA gate dielectrics. By telling (c) and (d) images, pentacene films have similar grain size no matter it was deposited on PVP or PVP-PMMA surface. However, according to the study proposed by Sangyun Lee et al, OH-terminated interface may increase the mobility by supplying the hopping sites, the carriers then can move easily through the channel. Fig. 3-3 (e) show the AFM images of pentacene films morphology during
PVP-PMMA rather than PVP. It is reported the large grain size of pentacene-based TFTs on the much hydrophobic surface have higher mobility is observed during initial pentacene film growth state [23]. In our experiments, OH groups’ effects on OTFTs need further experiment to study why the PVP-PMMA OTFT also have OH group but not present high mobility like PVP OTFT.
3-4 Wet-ability of PVP and PVP-PMMA Dielectrics
PVP and PVP-PMMA films were fabricated by using spin-coating process on the glass substrates. Water contact angles of PVP and PVP-PMMA dielectric surfaces were 67.3o and 63.9o as shown in Fig. 3-4 (a) and (b). Previous researches mentioned that water contact angle strongly dependents the chemical composition of dielectric surface. Fig. 3-4 (a) and (b) shows that PVP dielectric surface has indistinct difference with PVP-PMMA because OH groups also consist in PVP and PVP-PMMA molecule structure. But they also provide a good surface energy for pentacene deposition.
Surface energy of PVP and PVP-PMMA calculated were 50.3 mJ/m2 and 48.3 mJ/m2.
3-5 Moisture Effect on OTFTs Operation
Of the polymer gate dielectrics reported in the literature, PVP is perhaps the dielectric with the highest mobility for pentacene-based TFTs. We try to clarify why PVP-OTFT has higher mobility than PVP-PMMA OTFT. In our work, we do encapsulation in the anhydrous oxygen environment for PVP OTFT and PVP-PMMA OTFT. Fig. 3-5(a) and (b) show the transfer characteristics of OTFTs with or w/o encapsulation. By the Table 2, we can get information of moisture effect to OTFTs.
PVP OTFT with encapsulation has the lowest mobility and the lowest threshold voltage
in the Table 2. We can infer the moisture contributes large mobility to PVP OTFT, because OH groups closed to the channel easily interact with water vapor to become electron traps to induce extra hole. In contrast, the mobility of PVP-PMMA OTFT has less change. It means the methyl methacryl groups not only prevent moisture invade but also perform good behavior for OTFTs such as stable electrical properties, high on/off ratio, low leakage current.
3-6 Moisture Effect on Hysteresis Phenomenon Investigation
Though PVP OTFT have high mobility than PVP-PMMA OTFT in ambient air, but its instability has been widely discussed in previous researches [13-17], hysteresis and bias stress effect were observed in the transfer characteristic of pentacene-based TFTs with PVP dielectric. In this section, we firstly discuss the hysteresis of PVP-OTFT and PVP-PMMA-OTFT in ambient air, and then we encapsulate two devices to observe the hysteresis phenomenon in anhydrous oxygen environment.
3-6.1 Hysteresis Phenomenon Investigation on OTFTs in Ambient Air
Devices with PVP and PVP-PMMA dielectrics were prepared to measure by semiconductor parameter analyzers HP4156A (Agilent). Every time we fabricated the device completely and vacuumed them in boxing quickly to prevent moisture intrusion.
First, we measured them in the dark box in the ambient air with a relative humidity of 65% at room temperature 25°C. We given symmetric sweeps from VG=+30V to
electrical transfer characteristics of devices. We observed hysteresis behavior appeared for the PVP-OTFT, but no hysteresis was observed for the PVP-PMMA OTFT. It was found that back scan current (on to off) was large than forward scan current (off to on) (or called clockwise hysteresis), it’s different with hysteresis due to hole trapping in the channel/dielectric interface [14]. But it is considered to relate with residual dipole-induced effect caused by slow polarization in the bulk organic dielectric with hydroxyl groups. This behavior is often found in the PVP or terminated-OH (hydroxyl groups) dielectric, which can be slowly reoriented by an applied electric field [5].
When PVP OTFT swept from off to on, extra hole would be stay in the channel/dielectric interface. Then we sweep gate voltage from on to off mode, the extra hole can’t be released rapidly by slow polarization due to dipole reorientation. That is why back scan current is large than forward scan current, so the hysteresis is formed.
Respect to PVP-PMMA-OTFT, no hysteresis is observed in the on and subthreshold swing region. Hydroxyl groups in PVP-PMMA might difficultly be charged or be interfered the polarity of vinyl phenol by MMA groups. It seems that PVP-PMMA OTFT has no memory effect on device operation.
3-6.2 Hysteresis Phenomenon Investigation on OTFTs with Encapsulation
In previous section, we find PVP OTFT has hysteresis but PVP-PMMA OTFT has hysteresis-free in dark and ambient environment. Although PVP and PVP-PMMA dielectrics both have OH groups, the hysteresis phenomenon is not found for the PVP-PMMA OTFT. In previous researches [5], water vapor might diffuse to PVP bulk dielectric results in slow polarization when OTFT operates. We try to investigate water vapor how to influence two devices both have OH groups. Put devices in the anhydrous
glove box after achieve high vacuum pressure in the chamber, and then encapsulate the devices in the anhydrous oxygen glove box. Also give symmetric sweeps from VG=+30V to VG=-30V (off to on) and back scan from VG=-30V to VG=+30V (on to off) and VD=-5V, VS=0V. Fig. 3-6 (c) and (d) are electrical transfer characteristics of devices. Both OTFTs with PVP and PVP-PMMA are hysteresis-free after encapsulating the devices in the anhydrous oxygen environment. It is clearly saying that hysteresis behavior due to water vapor for PVP-OTFT. The amount of OH groups would increase water vapor absorption in the bulk dielectric. As PVP-OTFT in the anhydrous oxygen environment, no water vapor absorption results in slow polarization.
On the contrary, PVP-PMMA-OTFT shows hysteresis-free even in ambient air, so we guess the MMA group is much hydrophobic for moisture that made water vapor is difficult to invade dielectric bulk.
3-7 Bias Stress Effect on OTFTs in Ambient/Anhydrous Oxygen Environment
As above description, pentacene thin film deposited on organic dielectrics with PVP and PVP-PMMA groups had different interface states between gate dielectric and pentacene thin film owing to grain size of pentacene initial growth state. In this section, we discuss the OH group influence on the device reliability, and the defect generation mechanism. However, one of main degradation mechanisms is OH group exists and interacts with water vapor.
3-7.1 Bias Stress Effect on OTFTs in Ambient Air
In this section, the bias stress effect on devices with PVP and PVP-PMMA groups in the dark and ambient air environment was discussed. Because Au/pentacene contact barrier is prefer to hole transportation, pentacene-based TFTs in this study is p-type.
The positive gate bias was applied to stress the device at off-state region and the negative gate bias was applied to stress the device at on-state region.
For positive gate-bias stress, VG was kept at +15 V and VD = VS were 0 V for 2000 sec. The linear-region transfer characteristics of devices before stress and after 2000-sec stress are depicted in Fig. 3-7 (a). Then, threshold voltage shift (△VT) curves of devices with PVP and PVP-PMMA gate dielectrics were plotted as a function of bias stress time as shown in Fig. 3-7 (b). We found that positive gate-bias stress influences on devices with PVP and PVP-PMMA gate dielectrics were similar but different with pentacene-based TFT on SiO2 dielectric [8]. Obviously, the gate-bias stress causes a negative shift of the transfer characteristics of devices with PVP and PVP-PMMA gate dielectric. For the negative gate-bias stress, VG was kept at -15V minus initial threshold voltage (-15V-VT) while VD = VS were 0 V for 2000 sec. The linear-region transfer characteristics of devices before stress and after 2000-sec stress are depicted in Fig. 3-7 (c). Then, threshold voltage shift (△VT) curves of devices with PVP and PVP-PMMA gate dielectrics were plotted as a function of bias stress time as shown in Fig. 3-7 (d). The negative bias stress caused a △VT of both transfer characteristics of device with PVP and PVP-PMMA gate dielectrics. The △Vth of device with PVP gate dielectric is much larger than with PVP-PMMA gate dielectric, even make device situate normally on-state. Compare to the positive gate bias stress, the negative gate bias stress caused large △Vth of device with PVP-PMMA dielectric.
The field-effect mobility μFE as a function of stress time is shown in Fig. 3-7 (e).
3-7.2 Bias Stress Effect on OTFTs with Encapsulation
After discuss pentacene-based TFTs in ambient air, we encapsulate the devices with
PVP and PVP-PMMA dielectrics and measure the same gate-bias stress condition like previous section. For positive gate-bias stress, the transfer characteristics of both devices are depicted in Fig. 3-8 (a). Both transfer characteristics of devices keep almost unchanged after applied VG=+15V stress 2000-seconds. Apparently, polymer dielectrics with OH groups in anhydrous ambient hardly form electron traps with water vapor.
For negative gate-bias stress, the transfer characteristics of both devices are depicted in Fig. 3-8 (b). PVP OTFT still was suffered for negative gate bias stress, but transfer characteristics of PVP-PMMA OTFT almost keep unchanged. It’s a incredible result for stressing device hold on high carrier concentration in the channel. For OTFTs with or without encapsulation, comparison to threshold voltage shift is shown in Fig. 3-8 (c) and (d).
3-8 Superior Stability of PVP-PMMA OTFTs with Encapsulation
In the last part, negative gate bias stress effect on PVP-PMMA OTFTs with encapsulation has been decreased drastically. Even give voltage -30volts (VG-VT=-15volts) bias stress ten thousand seconds to gate electrode on PVP-PMMA devices with encapsulation maintain initial electrical properties. Gate bias stress on the transfer characteristics of device are depicted in Fig. 3-9 (a) shows high stability for PVP-PMMA OTFTs. It might be caused in no water environment which OH group is hard to form negative-chareged with water vapor. Further, we use LED backlight to
characteristics with 1.2mW white light illumination as shown is shown in Fig. 3-9 (b).
PVP-PMMA device is much more stable with light. Even give gate bias stress on device companied with 1.2mW light. The threshold voltage slightly shifted. Its high stability saying that OH group is too difficult to form negative-charge with water vapor to be electron traps.The similar result is reported for high stability use Cytop [24] as gate dielectric owing to its ultralow gate leakage current.
Figures of Chapter 3
Fig. 3-1 The leakage current density of MIM with PVP and PVP-PMMA dielectrics.
Electric Field ( MV/cm )
-40 -30 -20 -10 0 10 20
Table. 3-1 OTFT Parameters
Dielectric PVP PVP-PMMA
Ci(nF/cm2) 8.8 5.6
Mobility (cm2/V*s) 0.5~0.6 0.31
Threshold Voltage (V) About -15 About - 14 V
Ion/Ioff Near 5×104 >5×105
PVP
Roughness Rms:5.2A
PVP‐PMMA
Roughness Rms:5.6A
(a) (b)
PVP PVP‐PMMA
(c) (d)
Fig. 3-3 AFM image and surface roughness of (a) PVP (b) PVP-PMMA AFM image of 50-nm pentacene on (c) PVP (d) PVP-PMMA
AFM image of 5-nm pentacene on(e) PVP and PVP-PMMA
Fig. 3-4 Water contact angle and surface energy of (a) PVP (b) PVP-PMMA
PVP PVP‐PMMA
(e)
PVP
Water contact angle 67.32°
Surface Energy 50.3 mJ/m2
PVP‐PMMA
Water contact angle 63.96 ° Surface Energy 48.3mJ/m2
(a) (b)
Fig. 3-5 The transfer characteristics of (a) PVP OTFT (b) PVP-PMMA OTFT with encapsulation or not
-40 -30 -20 -10 0 10 20
Fig. 3-6 The transfer characteristics with forward and back scan of devices with (a) PVP (b) PVP-PMMA in ambient air
(c) PVP (d) PVP-PMMA with encapsulation
w/o encapsulation with encapsulation
Gate Voltage ( V )
w/o encapsulation with encapsulation
Gate Voltage ( V )
-40 -30 -20 -10 0 10 20
0 500 1000 1500 2000
-10
0 500 1000 1500 2000
0
Fig. 3-7 (a) The transfer characteristics of devices before and after 2000 seconds positive gate bias stress (b) Threshold voltage shift (△Vth) of PVP and PVP-PMMA devices during positive gate bias stress in air (c) The transfer characteristics of devices before and after 2000 seconds negative gate bias stress
(d) Threshold voltage shift (△Vth) of PVP and PVP-PMMA devices during negative gate bias stress in air
(e) The field-effect mobility plotted as a function of stress time for positive bias and negative bias on the PVP and PVP-PMMA devices
0 500 1000 1500 2000
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Mobility ( cm2 /Vs )
S tress Tim e ( sec )
P V P (V
G= + 15V ) (V
G-V
T= -15V ) P V P -P M M A (VG= + 15V ) (VG-VT= -15V )
(e)
Fig. 3-8 The transfer characteristics of devices before and after 2000 seconds (a) positive gate bias stress (b) negative gate bias stress on PVP and PVP-PMMA
devices with encapsulation.
And comparison to threshold voltage shift (△Vth) of PVP and PVP-PMMA devices during positive and negative gate bias stress (c) in air (d) with
encapsulation.
0 500 1000 1500 2000
-12
Threshold Voltage Shift ( V )
Stress Time ( sec )
PVP (VG-VT=-15V) (VG=+15V) PVP-PMMA (VG-VT=-15V) (VG=+15V)
0 500 1000 1500 2000
-0.5
Threshold Voltage Shift ( V )
Stress Time ( sec )
PVP (VG-VT=-15V) (VG=+15V) PVP-PMMA (VG-VT=-15V) (VG=+15V)
w/o Encapsulation with Encapsulation
Fig. 3-9 The transfer characteristics of devices (a) before and after 1000, and 10000 seconds under negative gate bias stress (b) light illumination combined
-40 -30 -20 -10 0 10 20 PVP-PMMA OTFT with Encapsulation VG-VT=-15V
10-11