Recently, organic thin-film transistors (OTFTs) have received great attention due to their low-cost and large-area array application. In numerous organic materials, pentacene is promising candidate due to its high mobility. Pentacene is made up of five benzene rings as shown in Fig. 1.1.
In previous studies, there are many superior groups to promote the electrical characteristic of pentacene-based thin-film transistors such as field-effect mobility, subthreshold slope, Ion/Ioff ratio, and low operation voltage.
OTFT arrays to drive liquid crystal (LC) [1] [2] or organic light emitting diode (OLED) [3] which showed full-color moving pictures had been demonstrated. In these reports, OTFTs were encapsulated by passivation layer to avoid exposing to oxygen or moisture in air, and to avoid damage from the subsequent LC or OLED process.
However, even when devices are encapsulated or operated in an inert environment, OTFTs are known to suffer from bias stress effect (BSE) that causes significant threshold voltage shift.
The bias-stress effect in OTFTs had been studied by using different organic active materials or different gate insulators on different device structures [4]. It was found that, for p-type OTFTs under DC stress, positive gate bias stress caused a positively-shifted Vth and negative gate bias stress caused a negatively-shifted Vth. The BSE was reversible by removing gate bias or by applying opposite polarity gate bias. Light irradiation also enhanced the reversal process.
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Charge trapping, ion migration, charged-state creation and the formation of bound hole pairs (bipolaron) are several proposed mechanisms to explain the BSE [5].
Charge trapping and ion migration were found to be dominant mechanisms in OTFTs with an organic dielectric [6]. When using thermally-grown SiO2 as the gate dielectric to study OTFTs reliability, charged-state creation is usually believed to be responsible for ΔVth. John E. Northrup and Michael L. Chabinyc used density functional calculation to simulate defect states generation in pentacene film and found that it was due to the formation of oxygen- and hydrogen-related defects such as C-H2, OH, and C-HOH in organic semiconductors [7]. Gu et al. also studied the response time of the defect states in pentacene. Long-lifetime deep electron traps were proposed to explain the hysteresis effect in pentacene-based OTFTs.
1.2 Operation of OTFTs
A thin film transistor is composed of three basic elements: (i) a thin semiconductor film; (ii) an insulating layer; and (iii) three electrodes (source, drain and gate). Fig.1.2 show two kinds of standard OTFT device structure Fig. 1.2(a) is the top-contact device and Fig. 1.2(b) is the bottom-contact device, respectively. The general operation concepts are originated from MOSFET theory. But there is a slight difference, traditional MOSFET are usually operated in inversion mode while the OTFTs are generally operating in accumulation mode.
Since the pentacene is a p-type semiconductor, negative bias is applied on the gate to turn on our OTFTs. The voltage-drop across dielectric causes the energy band bending in the organic semiconductor and additional positive charge carriers will accumulate at the interfaces. The dielectric serves as a capacitance and can store charges. Then we apply a drain bias to drive the accumulated charges from source to drain and from the drain current. The conduction is determined by the field effect
mobility (μFE) which represents charges’ driving ability by the electrical field.
In general, we can divide the operation of OTFTs into two regions: linear region and saturation region. If we add gate bias at turn on state, beginning with small drain voltage, OTFTs are operated in linear region, as given drain become larger the drain current will gradually saturate and into saturation region.
Understand how OTFTs normally operate, parameters such as the threshold voltage, field effect mobility can be extracted according to the measured electric characteristic. In addition, how environment effect devices can also be told by analyzing abnormal changing of these parameters.
1.3 Defect Generation Mechanism
Until now, the device reliability issue has been a greatest barrier to realize the organic electronic application. Even when devices are encapsulated, the threshold voltage (Vth) tends to shift under continuous bias and the field effect mobility degrades after prolonged storage in normal environment. The device threshold voltage shift (ΔVth) is generally attributed to hole/electron trapping in the interface between pentacene and dielectric. Although the field effect mobility degradation mechanism is not clearly understood, the permeation of H2O and O2 in pentacene film is the usually proposed mechanism. These two phenomena seriously strict the organic TFTs application ranges. Therefore, in following section, mechanisms caused device ΔVth
and field effect mobility degradation are explained in detail
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1.3.1 Threshold Voltage Shift Mechanism
The ΔVth of OTFTs is believed due to the carrier trapping by the defect states.
However, there are only a few explanations on the micro process of the defect creation, which can be observed in bias stress experiment. Bias tress experiment can be divided into two kinds: negative bias stress and positive bias stress.
First, micro process of defect creation under negative bias stress is introduced.
The formation of bipolaron proposed by R. A. Street et al. (Phys. Rev. B, vol.68, 085316, 2003) is one of the plausible mechanisms. The deep states slowly trap holes to form bipolarons. The formation of bipolarons would cause the ΔVth due to the reduction of mobile holes. The reaction can be expressed as:
hh BP
h
h++ + →( )2+
The other possible mechanism was proposed by John John E. Northrup et al. (Phy.
Rev. B, vol.68, 041202, 2003) They studied the formation of hydrogen- and oxygen-related defects (C-H2, OH, and C-HOH) in pentacene film based on the density functional calculation. The defect creation reactions were given as follows:
+
When the pentacene film is in a hole-rich environment, both these two reactions tend react to the right-hand side and produce positive-charged states that cause the ΔVth. Either bipolaron formation or hydrogen-, oxygen-related defect creation, these studies need more experimental results to support their theories. Both mechanisms assume that the reaction rate is proportional to the carrier concentration.
However, compare with negative bias stress effect, there are fewer studies focused on positive bias stress effect. Applying a prolonged positive bias to the device usually
causes electrons trapping in the channel and a threshold voltage shift forward positive bias. After removing the negative bias, the recovery of trapped electrons can be observed and the device threshold voltage comes back to the original value. Until now, the micro process of electron trap generation under the positive bias stress is not discussed in detail.
The reversible positive ΔVth not only caused by positive bias stress but also can be induced by H2O and O2 in ambient air. When there are lots of OH groups on the SiO2
surface, SiOH is generated. H2O and O2 are easily absorbed by SiOH to cause electron traps at the pentacene/dielectric interface. This generation process can be shown as chemical reaction:
2 3
We can see H2O and O2 contained in air promote the reaction to the right-hand side and produce negative-charged states that cause the ΔVth. Therefore, if we perform the measure in vacuum or eliminating OH groups on the dielectric surface, the prolonged positive bias influence on device Vth may be drastically reduced.
1.3.2 Resources of trapped carrier
Besides how defects form in OTFTs and cause ΔVth, another question is where the charges that be trapped in these defects come from? Two resources had been suggested: carrier injection from electrode and photogenerated carriers. Carrier injection proposes that when gate is given bias stress, extra carrier can inject from electrode into organic semiconductor and accumulate. But with the pentacen-based OTFTs be used in this experiment, there is a high energy barrier at the junction
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between Au electrode and LUMO of pentacene to block the electron injecting from electrodes. Therefore, the possibility that election can inject from electrode is rare.
The other is resource is photogenerated carriers, When OTFTs is under illumination, the proper light intensity can induced excitons in the pentacene film. These photo-induced exctions then disassociate by gate bias into electrons and holes, which can be trapped by defects.
1.3.3 Field-Effect Mobility Degradation
Mobility degradation is also caused by defects. These defects can originally exist (i) in the semiconductor or grain boundary, defects can also generate by (ii) giving bias stress, (iii) absorbing H2O and O2 in the air. Mobility degradation is usually permanent damage while ΔVth is often reversible, this means defects cause mobility degradation are in deeper state, trapped carriers are difficult to get out from these states.
Optimize the semiconductor deposition temperature, deposit rate, film thickness…etc, may diminish defects originally exist in active layer. On the other hand, passivation layer is usually used to protect organic semiconductor from H2O and O2 after OTFTs are fabricated. In short, prevent defects from forming is the best method to postpone mobility degradation.
1.4 Surface Treatment
The growth process of pentacene thin film can be described by diffusion limited aggregation. When initially growing pentacene thin film, molecules were vapored to the gate dielectric surface. Before meeting critical nuclei, molecules drifted on the surface. It is as well known that pentacene consists of thin film phase and bulk phase.
Therefore, surface states of the gate dielectric greatly affect the pentacene growth.
When most components of pentacene film are thin film phase, pentacene-based TFTs have the best electrical performance.
There are many surface treatments proposed to improve the surface states of gate dielectric. Fundamental functions of using surface treatments are: (i) lowering leakage current, (ii) reducing surface trap states to enhance the field-effect mobility (iii) improving the device stability in ambient air and (ix) obtaining better device sense ability.
1.5 Motivation
Defects can induce the threshold voltage shift and mobility degradation. This leads Short lift time of OTFTs, and prohibit OTFTs from further application. We want to find out defects forming mechanism, and study how these defects react in different environment (ex. in dark/light, in air/vacuum)
PVP (with OH groups) and PMMA (without OH groups) were used as surface treating materials, bias stress measurement was taken in air/vacuum helping to know how H2O and O2 effect OTFTs operation. In order to observe how light effects OTFTs, measurement was also taken in dark, under light, illuminate with different wavelength light.
Combine above bias stress measurement results and material analysis. We hope this experiment can help us better understand how carriers be trapped and make some contribution to realize OTFTs application.
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Figure of Chapter 1
Fig. 1. 1
Pentacene molecular structure
Fig. 1. 2
OTFT device configurations. (a) cross section view of top contactdevice. (b) cross section view of bottom contact device.
Chapter 2
EXPERIMENTAL SETUP
2.1 Device Fabrication
In this study, conventional top-contact pentacene-based TFTs with dual dielectric layers were used. 100-nm-thick thermal oxide was grown on heavily doped Si wafers to serve as the first layer of gate dielectric. The back of heavily doped Si wafer is served as the gate electrode. Poly (methyl methacrylate) (PMMA) and PVP (poly-4-vinyl phenol) were separately used as second dielectric layers to provide different surface states.
Device Fabrication Process Flow:
Step1. Clean the oxide surface
Before fabricating device on wafer, the native oxide on the back of wafer must be etched by using BOE (NH4F : HF = 10 : 1) solution. Then, the oxide surface of wafer was cleaned by 5 mins DI water, 5 mins acetone and 5 mins DI water, sequentially. Using hot plate bakes the wafer to remove the moisture on the oxide surface.
Step2. Organic dielectric layer fabrication
PMMA fabrication condition: PMMA was obtained from MicroChem. Corp.
with molecular weight of 95000 and was dissolved in anisole at 10 wt%. Fig. 2.1 is the molecular structure of PMMA. The spin speed was accelerated from 0 to 1000 rpm during the first 10 sec and further increased the spin speed from 1000 rpm to
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6000 rpm in following 10 sec. After keeping 6000 rpm of the spin speed for 40 sec, the spin speed was decreased from 6000 rpm to 0 rpm in following 10 sec. Then, using hot plate baked the sample for 30 mins at 70°C.
PVP fabrication condition: PVP was obtained from Aldrich with molecular weight about 20000. PVP have OH function groups on its molecule structure as shown in Fig. 2.2, poly (melamine-co-formaldehyde) MMF was used as cross-linker and dissolved in PGMEA (propylene glycol 1-monomethyl ether 2-acetate, C6H12O3 ).
Fig. 2.3 is schematic picture of MMF cross-link with PVP. Most of PVP spin conditions were similar to above PMMA fabrication process except the initial acceleration from 0 to 1000 rpm was finished in 5 seconds. Then, PVP was cross-linked by thermal curing through a hot plate in air. At temperatures of 100 °C for 10 min and 200 °C for 50 min to remove PGMEA in the PVP film [1-3]. The spin conditions of PMMA and PVP was organized in Table 2.1
Step3. Pentacene film deposition through shadow mask
Pentacene obtained from Aldrich (purity: 99.9%) without purification was evaporated through a shadow mask onto thermal oxide to form the active layer. The deposition rate was set at 0.5 Å/s. The substrate temperature and the pressure were kept at room temperature and at around 3 × 10-6 Torr during deposition process.
Step4. Depositing Au to form source and drain contact
After depositing a 100-nm-thick pentacene, 100-nm-thick gold was deposited through the shadow mask to form source/drain contacts. The thickness of Au layer was 100 nm. The device channel length varied from 100 μm to 600 μm while channel width was fixed as 1000 μm.
The structure scheme of pentacene-based TFTs with PMMA or PVP dielectric layers are shown in Fig. 2.4.
2.2 Material Analysis Instrument 2.2.1 Contact angle system
Contact angle system is used to estimate wetting ability of a localized region on a solid surface. The angle between the baseline of the drop and drop boundary is measured. Comparing the result can tell the material surface is relatively hydrophile or hydrophobic, then can further analysis surface chemical composition. Since PVP have OH groups on its molecular structure while PMMA do not have, PVP surface should be more hydrophile and have smaller contact angle compare with PMMA surface. Fig. 2.5 is the picture of contact angle system used in this experiment.
2.2.2 Fourier Transform Infrared Spectroscopy (FTIR)
FTIR is powerful tool for identifying types of chemical bonds (function groups) in organic molecular. The wavelength of light absorbed is characteristic of the chemical bond, by analyzing the infrared absorption spectrum, the chemical bonds in a molecule can be determined. In this experiment, OH groups’ absorption spectrum can be find in library of known compounds, using FTIR is the more accurate method to check if PVP surface have OH groups. Fig. 2.6 shows some common function groups’
absorption spectrum.
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2.2.3 X-Ray Diffraction (XRD)
X-ray diffraction (XRD) is a non-destructive technique that reveals detailed information about the chemical composition and crystal structure of materials. The crystal lattice is a regular three-dimensional distribution of atoms in space. Atoms are arranged and form a series of parallel planes separated from one another by a distance d, which varies according to the nature of the material. When monochromatic X-ray project onto a crystalline material at particular angle (Θ), diffraction will occurs because the ray traveled distances reflected from successive planes differs by a complete number n of wavelength. Plotting the angular positions and intensities of the diffracted peaks produces a pattern, which is characteristic of the sample. In this experiment, XRD is used to check two things. First is to find if pentacene is successfully deposited on PMMA and PVP surface, this can be told by diffracted peaks exists at certain angles. Second is to know the crystallization of pentacene grow on PMMA and PVP, this can be told by pentacene peaks intensity. Fig. 2.7 is the picture of Shimadzu XRD-6000.
2.2.4 Atomic force microscope (AFM)
Since PVP treated surface may exist OH groups, its surface morphology can also be different compare with PMMA surface. In addition, pentacene deposits on PVP (hydrophile) and PMMA (hydrophobic) influenced by these two kinds of surfaces may have different grain size or morphology, too. Atomic force microscope (AFM) is used to measure surface morphology on a scale from angstroms to 30 microns. It scans samples through a probe or tip, with radius about 20 nm. The tip is held several nanometers above the surface and using feedback mechanism that measured interactions between tip and surface on the scale of nanoNewtons. Variations in tip height are recorded when the tip is scanned repeatedly across the sample, then
producing morphology image of the surface. In this experiment, the used equipment is Digital Instruments D3100 as shown in Fig. 2.8 and the used active mode is tapping mode.
2.3 Device Electrical Characteristic Measurement
Two measurement systems were used in this study. In ambient air condition, using semiconductor parameter analyzers HP4156 or Keithley 5270 measured devices in metal box at room temperature. Another measurement system includes semiconductor parameter analyzer Keithley 4200 and vacuum chamber. The chamber pump can lower the pressure in chamber from 760 Torr to 0.5 Torr. Therefore, measurements can be performed in vacuum at room temperature to compare H2O and O2 influences on device stability. Fig. 2.9 shows the measurement system includes semiconductor parameter analyzer Keithley 4200 and vacuum chamber.
2.4 Illumination Setup
There are four different light sources to illuminate the device in this experiment.
The white light source comes from light-emitting diode (LED) backlight with a broad wavelength range. Blue, green and red light sources are light-emitting diodes with 467 nm, 536 and 631 nm wavelengths. These spectrums of four light sources are shown in Fig. 2.10. The light source was set up above the device to irradiate the sample from the top. The light power was controlled by the power supply (PPT3615). The light intensity was adjusted by changing the applied voltage.
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2.5 Device Electrical Parameters Extraction
Field effect mobility, threshold voltage, subthreshold slope and Ion/Ioff ratio are usual used to compare different devices’ performance. In the following section, extraction methods would present how to extract parameters from electrical transfer characteristic of pentacene.
2.5.1 Field Effect Mobility
The field effect mobility (μFE) was determined by the orientation of pentacene molecules near gate dielectric. Therefore, gate dielectric surface states strongly affect the device μFE. The device μFE variation can be used to compare the difference between PMMA and PVP dielectric layers. In our experiment, μFE were extracted by using the linear region equation. Because the electrical transfer characteristic of pentacene-based thin film transistor is similar to those conventional single crystalline MOSFETs, the linear region equation can be applied to pentacene-based thin film transistor and can be expressed as
1 2
where Ceff and Vth are effective capacitance per unit area and the threshold voltage. W and L are device channel width and channel length. When operating device at low drain bias, the linear region equation can be modified to
( )
DS eff FE GS th DS
I C W V V V
μ
L= −
After differentiating Eq.(*) with respect to (VG - Vth), the device transconductance can be written as M eff EFW DS
G C V
μ
L=
The field effect mobility can be extracted from the transconductance and this equation can be expressed as FE M
DS eff
L G
μ =WV C
In this study, we used max GM value to calculate and define the field effect mobility.
2.5.2 Threshold voltage
Threshold voltage (Vth) determines the device operation voltage and smaller Vth
can help to lower power consumption. Because Vth strongly dependents on dielectric surface states, environmental and fabrication process variations easily cause a shift on the Vth. Based on this phenomenon, the device Vth shift is usually used as an importance parameter when pentacene-based TFTs applied to Photo detector or Chem-Bio detector. In this study, we used the linear region equation to extract the device Vth.
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Figure of Chapter 2
Fig. 2. 1
PMMA molecular structure
Fig. 2. 2
PVP molecular structure
Fig. 2. 3
Schematic picture of MMF cross-link with PVP
Fig. 2. 4
Schematic structure of PVP-OTFTs and PMMA-OTFTs
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Fig. 2. 5
image of contact angle system
Fig. 2. 6
Absorbance spectrum of different function groups
Fig. 2. 7
Picture of Shimadzu XRD-6000
Fig. 2. 8
Picture of Digital Instruments D3100
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Fig. 2. 9
Picture of vacuum chamber
wavelength (nm)
400 500 600 700 800
0
Fig. 2. 10
Intensity of LED (Red, Green ,Blue and White) wavelength spectrum
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Table of Chapter 2
PMMA Time (second) spin speed (rpm)
0 ~ 10 0
10 ~ 20 0 ~ 1000 20 ~ 30 1000 ~ 6000
30 ~ 70 6000
70 ~ 80 6000 ~ 0
PVP Time (second) spin speed (rpm)
0 ~ 10 0
10 ~ 15 0 ~ 1000 15 ~ 25 1000 ~ 6000
25 ~ 65 6000
65 ~ 75 6000 ~ 0
Table 2. 1
PMMA and PVP spin coating parameters
Chapter 3
Analysis and Results
3.1 Material Analysis of PVP and PMMA
3.1.1 Wettability of PVP and PMMA Dielectrics
PMMA and PVP were fabricated by using spin-coating process on silicon wafer.
Contact angles of PMMA and PVP dielectric surfaces were 61.7o and 51.8o as shown in Fig. 3.1. Previous researches mentioned that the moisture contact angle strongly
Contact angles of PMMA and PVP dielectric surfaces were 61.7o and 51.8o as shown in Fig. 3.1. Previous researches mentioned that the moisture contact angle strongly