Operating of Organic Thin-Film Transistors

The operation of the pentacene-based OTFT is described below [16]. Pentacene is a p-type semiconductor. At first, a negative bias is applied to the gate, the voltage drops over the insulator region and semiconductor one, which gives rise to bend the band in the semiconductor. The additional positive charges provided from the source and drain electrodes will accumulate in this region. The insulator serves as a capacitance which stores charges and can be represented as COX. It is assumed that a little voltage drop across the semiconductor is negligible. In this situation, the applied drain-bias can force the current from the source to drain. The conduction is determined by mobility μ, which represents how the electrical field drives the accumulated charges. Therefore, the increased in gate voltage δVG accounts for the increased charges COXδVG and the total charges induced over the channel are (W/L) COXδVG, where W and L correspond to the channel width and length. The increased drain current δID is then represented as

( / )

D OX D G

I W L C V V

δ ≈ μ δ (1-1) In general, we can divide the operation of OTFTs into two regions, linear and saturation regions. At low drain voltages, the current increases linearly with drain voltage, following Ohm’s law. The drain current in the linear region is determined from the following equation

( / ) ( / 2)

D OX G TH D D

I = W L C μ V −V −V V (1-2) Since the drain voltage is quite small, sometimes equation (1-2) can be simplified as

( / ) ( )

D OX G TH D

I = W L C μ V −V V (1-3) For –VD > - (VG - VTH), I tends to saturate due to the pinch-off of the _D accumulation layer. The current equation is modified as

( / 2 ) ( )2

D OX G

I = W L C μ V −V_TH (1-4)

1.3 Background of Metal Oxide Dielectric (High dielectric constant material, High-k)

The continuous decrease of the thickness of gate oxides used in modern electronics has to confront the limit of material itself.In order to make a breakthrough on what is aforementioned, it is necessary to increase the physical thickness as well as the dielectric constant of the gate dielectrics. The Majority of metal oxide dielectric films, such as Al2O3, Ta2O5 and HfO2, hold higher dielectric constant than oxide, and studied to replace the gate dielectric layer for future generation CMOS devices because of the lower leakage current and thicker physics thickness than thermal oxide (SiO2) under identical equivalent oxide thicknesses (EOT) [17, 18]. For thin film transistors, in pervious records, the use of low-temperature-deposited metal oxide films as gate dielectrics not only reduces the threshold voltage to near zero for pentacene and zinc oxide (ZnO2) TFTs but also the thickness of the gate insulator to nanometer scale [19, 20]. Although reduced leakage current density has been observed in some devices with high-k gate dielectrics, there are still several problems to be solved, including thermal stability, interface quality between high-k gate dielectrics and Si-substrate, mobility degradation, reliability, charge trapping, and fabrication integrity with IC technology, etc. For extending the application, the novel method to improve the quality of metal oxide films is necessary.

1.4 Supercritical Fluid Technology

The attractiveness of supercritical fluids for commercial applications is their unique combination of liquid-like and gas-like properties. Supercritical fluids are compounds above their critical temperatures and pressure, as shown in Fig 1-1 [21, 22]. The Table 1-1 shows critical temperature and pressure for some common fluids.

Supercritical fluids [23-25], particularly supercritical CO2 (SCCO2), have unique

characteristics and high potential in material processing [26-28]. The supercriticality is a strange and intriguing state in which solids can dissolve in gases, and liquids can alternate between reflectivity and transparency. CO2-based supercritical fluids are particularly attractive because CO2 is nontoxic, inflammable, not corrosive, not explosive, recyclable and inexpensive. Besides, its critical conditions are easily achievable with existing process equipment (31 °C, 1072 psi =72.8 atm).

Figure 1-2 shows the density-pressure-temperature surface for pure CO2. It can be discovered that relatively small changes in temperature or pressure near the critical point, resulting in large changes in density. Table 1-2 shows the comparison of several physical properties of typical liquid, vapor, and supercritical fluid state for CO2. It could be seen that SCCO2 fluids possesses liquid-like density, so that SCCO2 fluids are analogous with light hydrocarbon to dissolve most solutes and own exceptional transport capability [29, 30]. On the other hand, SCCO2 fluids hold gas-like characteristic due to their viscosity and surface tension are extremely low, it allows SCCO2 fluids to keep fine diffusion capability and enter the nano-scale pores or spaces without damage. These properties are the reasons for SCCO2 fluids to employ in many commercial applications, including the extraction of caffeine from coffee, fats from foods, and essential oils from plants for using in perfumes. Furthermore, considerable attention is now paid to using SCCO2 for semiconductor processing , such as wafer cleaning, photoresist-stripping, photoresist drying, and to manufacturing porous low-dielectric-constant thin films, by means of its high mass transfer rates and infiltration capabilities [24, 25].

1.5 Motivation

In recent records, metal oxide dielectrics, such as Al2O3, Ta2O5 and HfO2, have attracted much attention for thin film transistor liquid crystal displays (TFT-LCDs) technology on the base of glass substrates or plastics. Among several metal oxide film

formation methods [17, 18, 31],in general, low-temperature technology is welcome due to a low thermal budget process. However, the low-temperature-deposited dielectric films perform inferior properties and larger current leakage due to numerous traps inside the metal oxide film [32]. It is thereby required for the low-temperature-deposited metal oxide film to reduce electrical traps by implementing a post-treatment process. High-temperature (>600 °C) annealing is typically used to diminish the traps in metal-oxide films [33-34].Nevertheless, there are several considerable issues present for high-temperature annealing process. For example, crystallizing phenomenon would occur possibly during the process duration, and leads to unexpected leakage current through grain boundaries [34-36].

Additionally, the high-temperature process is not applicable to the substrates with low glass transition temperature (Tg), such as glasses and plastics [37]. By the liquid-like property, it is allowed for supercritical fluids to own fine transport capability [38].

Supercritical fluids, in addition, hold gas-like and high-pressure properties to efficiently diffuse into thin films with no damage. Here, these advantages would be adequately employed to passivate the defects in low- temperature-deposited metal oxide dielectric film at 150 °C.

Besides, OTFTs will be used in the flexible displays. The excellent transfer characteristics are thereby demanded, such as high mobility and lower threshold voltage. Especially, in recent years, the fabrication of OTFTs tends to being implemented at low temperature processes for cost down and comparable with plastic substrates (120~250 °C)[39, 40]. The performance of low-temperature-fabricated OTFTs, however, is unsuitable for application to display technology, due to the poor gate dielectrics with plenty of defects [41]. For improving electrical characteristics of OTFTs, it is necessary to passivate the defect-states in the gate dielectrics. Therefore, it is critical to develop a traps passivation technology at low temperature for extending the application of OTFTs. In this work, therefore, the supercritical fluids treatment is also proposed to effectively decrease the defects in the gate dielectrics of OTFTs at low temperature.

Chapter 2

Application of Supercritical Fluid Technology on Metal-Oxide Dielectric Thin Film

2.1 Process Flow of Metal-Insulator-Silicon (MIS) Fabrication and Experiment Process

In this experiment, a metal-oxide Al2O3 film layer was deposited on p-type (100) silicon wafers by E-gun evaporation deposition at room temperature. The thickness of as-deposited Al2O3 films was 16nm, which was measured by an ellipsometer system.

Subsequently, the wafers with 16nm-thick Al2O3 film were split into three groups, and processed with different post-treatments to study the properties of low-temperature-deposited Al2O3 film. The first group labeled as SCCO2-only treatment, was designed as the control sample, and was only treated with 3000psi- SCCO2 that without co-solvent at 150 °C for 2 hrs. The second group labeled as H2O vapor treatment, was immersed into a pure H2O vapor ambience at 150 °C for 2 hrs in a pressure-proof stainless steel chamber with a volume of 100 cm³. The third group marked as SCCO2 with co-solvent treatment, was placed in the supercritical fluid system at 150°C for 2 hrs, where was injected with 3000psi of SCCO2 fluids mixed with 5 vol.% of propyl alcohol and 5 vol.% of pure H2O. The propyl alcohol plays a role of surfactant between nopolar-SCCO2 fluids and polar-H2O molecules, so that the H2O molecule uniformly distributes in SCCO2 fluids and be delivered into the Al2O3

film for passivating defects. The supercritical fluid system is shown in Fig. 2-1.

After these different treatments, fourier transformation infrared spectroscopy (FTIR) and thermal desorption spectroscopy (TDS) were also used to investigate the evolution of chemical functional bonding and the content of oxygen in Al2O3 films, respectively. Electrical measurements were conducted on metal insulator

semiconductor (MIS) capacitors by thermally evaporating Al electrodes on the front surface of the Al2O3 films and the backside of the silicon wafer. The current density-electric field (J-E) characteristics, capacitance-voltage (C-V) characteristics, breakdown voltage and gate bias stress were measured with HP4156C semiconductor parameter analyzer for investigating the transformation of Al2O3 film. The experiment processes of thin HfO2 film with various treatments are exhibited in Fig. 2-2.

2.2 Process Flow of Organic Thin Film Transistors Fabrication and Experiment Process

At second experiment, a metal-oxide Al2O3 film layer was deposited on p-type (100) silicon wafers by E-gun evaporating system at room temperature. The thickness of as-deposited Al2O3 films was 240nm, which was measured by an ellipsometer system. Subsequently, the wafers with 240nm-thick Al2O3 film were split into three groups, and processed with different post-treatments to study the properties of low-temperature-deposited Al2O3 film. The first group labeled as SCCO2-only treatment, was designed as the control sample, and was only treated with 3000psi- SCCO2 that without co-solvent at 150 °C for 2 hrs. The second group labeled as H2O vapor treatment, was immersed into a pure H2O vapor ambience at 150 °C for 2 hrs in a pressure-proof stainless steel chamber with a volume of 100 cm³. The third sample marked as SCCO2 with co-solvent treatment, was placed in the supercritical fluid system at 150°C for 2 hrs, where was injected with 3000psi of SCCO2 fluids mixed with 5 vol.% of propyl alcohol and 5 vol.% of pure H2O. The propyl alcohol plays a role of surfactant between nopolar-SCCO2 fluids and polar-H2O molecules, so that the H2O molecule uniformly distributes in SCCO2 fluids and be delivered into the Al2O3

film for passivating defects.

After these different treatments, the gate contact by thermally evaporating Al electrodes on the backside surface of the silicon wafer. Then, the samples were put

into the oven with HMDS steam for 20mins at 150 °C. Pentacene was used as an active layer. This was deposited using ULVAC thermal evaporator. The deposition is started at a pressure lower than 3×10^-6 torr. The deposition rate is controlled at 0.1Å/s.

The temperature we use in depositing pentacene films is 70℃. We use shadow mask to define the active region of each device. The resulting thickness of the pentacene thin film was 70 nm, which was measured by a quartz-crystal thin film thickness monitor. After pentacene deposition, we use shadow mask to define top contact of each device. The top electrodes are Au. We deposited the Au (100nm) via the thermal evaporator as the source and drain electrode pad. The deposition pressure was at 3×10^-6 torr with the deposition rate of 0.5Å/sec. In addition, we fabricated the same process to comparison with insulation of SiNx by Plasma Enhanced Chemical Vapor Deposition (PECVD).

After fabrication OTFTs, The current density-electric field (J-E) characteristics, capacitance-voltage (C-V) characteristics, Current-Voltage (I-V) characteristics, gate-bias stress and current stress were measured with HP4156C semiconductor parameter analyzer for investigating the OTFTs. The experiment processes of OTFTs with various treatments are exhibited in Fig. 2-3.

2.3 Organic Thin Film Transistors Parameters extraction

In this section, the methods of extraction the mobility, the threshold voltage, the on/off current ratio and the sub-threshold swing is characterized, respectively.

Mobility

Generally, mobility can be extracted from the transconductancein gm the linear region:

D

D O X m

G V CONSTANT

WC g I

V L μV^D

=

⎡∂ ⎤

=⎢⎣∂ ⎥⎦ = (2-1)

Mobility can also be extracted from the slope of the curve of the square-root of

drain current versus gate voltage in the saturation region, i.e. For –VD > - (VG - VTH) : 2

(

D OX G

I W C V

Lμ

= −VTH

)

Threshold voltage is related to the operation voltage and the power consumptions of an OTFT. We extract the threshold voltage from equation (2-2), the intersection point of the square-root of drain current versus gate voltage when the device is in the saturation mode operation.

On/Off current ratio

Devices with high on/off current ratio represent large turn-on current and small off current. It determines the gray-level switching of the displays. High on/off current ratio means there are enough turn-on current to drive the pixel and sufficiently low off current to keep in low power consumption.

Sub-threshold Slope

Sub-threshold swing is also important characteristics for device application. Its is a measure of how rapidly the device switches from the off state to the on state in the region of exponential current increase. Moreover, the sub-threshold swing also represents the interface quality and the defect density [42].

, when VG < VT for p-type. (2-3)

If we want to have good performance TFTs, we need to lower sub-threshold swing of transistors.

Chapter 3

Results and Discussions

3.1 Analysis of Material and Discussion

3.1.1 Fourier Trans-form Infrared Spectroscopy (FTIR) Analysis

Figure 3-1 shows the FTIR spectra of Al2O3 films after various post-treatments, including SCCO2-only, H2O vapor and SCCO2 with co-solvent treatment. The functional group referred to Al-O-Al bonding is at 476 cm^-1 and 600 cm^-1, and the absorption peak at around 1080 cm^-1 attributes to the Al-O-H bond [43, 44]. The peak intensity of Si-O-Si bond for different treatments is not obtained, meaning that these post-treatments would not make influence on the thickness and quality of the interfacial SiOx film. For the H2O-vapor-treated Al2O3 film, however, the peak intensity of Al-O-Al bands (476 cm^-1 and 600 cm^-1) raises apparently in comparison with the SCCO2-only-treated Al2O3 film. This is believed well that the H2O vapor would permeate into Al2O3 film and makes reaction with Al dangling bonds (i.e. traps) forming Al-O-Al bands. These traps in the low-temperature deposited Al2O3 film could be thereby passivated by H2O vapor molecules. By the way, the band at 1080 cm^-1 is due to Al-O-H bending vibration o bridge hydroxyl anions. The broad intensity peak centered at approximately 3400 cm^-1 is typical of O-H vibration from adsorbed water molecules [45]. Furthermore, with SCCO2 with co-solvent treatment, obvious increase in the intensity of Al-O-Al bonding is observed in the FTIR. It indicates that the H2O molecules into Al2O3 film is achieved by the SCCO2 fluids, potentially modifying the dielectric properties of Al2O3 film, and the transporting mechanism for SCCO2 fluids taking H2O molecule into Al2O3 film is shown in Fig.

3-2.

3.1.2 Thermal Desorption System – Atmospheric Pressure Ionization Mass Spectrometer (TDS-APIMS) Analysis

The TDS measurement, as shown in the Fig. 3-3, was carried out upon heating these treated Al2O3 films from 50 to 800 °C at a heating rate of 10 °C/min in vacuum (10⁻⁵ Pa.). In Fig. 3-3 (a), m/e (mass-to-charge ratio) = 32 peak that is attributed to O2

was monitored to evaluate the content of oxygen outgassing form Al2O3 films. It is clearly found the highest oxygen content is detected in the SCCO2 with co-solvent treated Al2O3 film, certainly consistent with the FTIR observation. From Fig. 3-3 (b), m/e (mass-to- charge ratio) = 18 peak that is attributed to H2O, the residual moisture in Al2O3 film is much more after H2O vapor treatment. This is result from SCCO2

fluid not only employed to transport the H2O molecule into Al2O3 film but a suitable method to remove H2O molecule in addition [46, 47].

3.1.3 X-ray Photoelectron Spectroscopy (XPS) Analysis

XPS involves measuring the photoelectron spectra obtained when a sample surface is irradiated with x-rays. The kinetic energy (peak position) of the photoelectrons can be written as

K B s

E =h -E - -qν φ φ

where hν is the x-ray energy, EB is the binding energy (the difference between the Fermi level and the energy level being measured), φ_s is the work function of the electron spectrometer, q is the electronic charge, and φ is the surface potential.

We have also performed XPS measurements using an Mg Kα X-ray source (1253.6 eV photons) to determine the bonding environments of the Al and O atoms.

Figure 3-4 shows the XPS spectra for Al 2p level that was calibrated from C 1s peak

at 284.5 eV. Each spectrum was represented the result at different post-treatments, including SCCO2-only, H2O vapor and SCCO2 with co-solvent treatment. As shown in Figure 3-4 , the Al 2p peak, which has binding energy of 75.8 eV, related to Al-O bonding in Al2O3. However, the binding energy of Al 2ppeak shown in Figure 3-4 varied from 75.8 eV for SCCO2-only sample to 74.7 eV [48] for SCCO2 with co-solvent treatment sample. The origins of binding energy shift (ΔBE) are suggested as a number of factors such as charge transfer effect, presence of electric field, environmental charge density, and hybridization. Among these, charge transfer is regarded as a dominant mechanism causing a binding energy shift. According to the charge transfer mechanism, removing an electron from the valence orbital generates the increment in core electron’s potential and finally leads a chemical binding energy shift [49]. Therefore, it is considered that the Al 2p peak shift originated from the enhanced charge transfer with different post-treatments, i.e., the larger portion of Al atoms was fully oxidized with SCCO2 with co-solvent treatment. Figure 3-5 shows the O 1s core level peaks also demonstrated binding energy shift with changing of different post-treatments. For the H2O-vapor-treated Al2O3 film, however, the peak intensity of Al-O bands raises apparently in comparison with the SCCO2-only treated Al2O3 film. This is believed well that the H2O vapor would permeate into Al2O3 film and makes reaction with Al dangling bonds (i.e. traps) forming Al-O bands. These traps in the low-temperature deposited Al2O3 film could be thereby passivated by H2O vapor molecules. Furthermore, with SCCO2 with co-solvent treatment, obvious increase in the intensity of Al-O bonding is observed in the XPS. It indicates that the best transport efficiency of H2O molecules into Al2O3 film is achieved by the SCCO2

fluids, potentially modifying the dielectric properties of Al2O3 film, and the transporting mechanism for SCCO2 fluids taking H2O molecule into Al2O3 film is shown in Fig. 3-2. Summary of binding energies for Al2O3 films are shown in Table 3-1.

3.1.4 Auger Electron Spectroscopy (AES) Analysis

In order to analyze the composition of the Al2O3 film after various post-treatments, including SCCO2-only, H2O vapor and SCCO2 with co-solvent treatment, we performed the Auger electron spectroscopy analysis. The first group labeled as SCCO2-only treatment, was designed as the control sample, and was only treated with 3000psi-SCCO2 that without co-solvent at 150 °C for 2 hrs. The second group labeled as H2O vapor treatment, was immersed into a pure H2O vapor ambience at 150 °C for 2 hrs in a pressure-proof stainless steel chamber with a volume of 100 cm³. The third group marked as SCCO2 with co-solvent treatment, was placed in the supercritical fluid system at 150°C for 2 hrs, where was injected with 3000psi of SCCO2 fluids mixed with 5 vol.% of propyl alcohol and 5 vol.% of pure H2O. As shown in Figure 3-6, the SCCO2-only treated and H2O vapor treated films have oxygen composition lower than that of the Al2O3 film after SCCO2 with co-solvent treatment. The propyl alcohol plays a role of surfactant between nopolar-SCCO2

fluids and polar-H2O molecules, so that the H2O molecule uniformly distributes in SCCO2 fluids and be delivered into the Al2O3 film for passivating defects.

3.2 Analysis of Metal-Insulator-Silicon (MIS) Electrical Characteristics and Discussion

3.2.1 The current density-electric field (J-E) characteristics

The leakage current densities of Al2O3 films after different treatments are shown

The leakage current densities of Al2O3 films after different treatments are shown