Chapter 1 Introduction
1.4 Motivation
In recent records, Metal oxide dielectrics, such as HfO2, Ta2O5, and Al2O3 have attracted much attention for memory cell capacitors and gate dielectric applications in the ultra large scale integration (ULSI) technology [7, 8]. Among several metal oxide film formation methods [7, 8, 17],in general, low-temperature technology is welcome due to a low thermal and low price process. Also, it is suitable for thin film transistor liquid crystal displays (TFT-LCDs) technology on the base of glass substrates or plastics. However, the low-temperature-deposited dielectric films perform inferior properties and larger current leakage due to numerous traps inside the metal oxide film [18, 19]. Hence, the low-temperature-deposited metal oxide film to reduce electrical traps by implementing a post-treatment process is needed. The supercritical
fluids technology has been applied to remove photoresist and impurity in integrated circuit (IC) fabrications [20]. It is also operative method to extract moisture from structures of nanoscale, such as porous dielectric-material and carbon nano-tube [21, 22]. By the liquid-like property, it is allowed for supercritical fluids to own fine transport capability [23]. 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.
In addition, OTFTs will extensively research 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)[24, 25]. The performance of low-temperature process for OTFTs, however, due to the poor gate dielectrics with great quantity of defects is unsuitable for application to display technology, [26]. 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 low-temperature traps passivation technology 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
Experiment Procedures and Principle
2.1 Fabrication of Metal-Insulator-Silicon and Experiment Process
At first experiment, a metal-oxide HfO2 film layer was deposited on p-type (100) silicon wafers by reactive DC magnetron sputtering at room temperature under Ar/O2ambient. The thickness of as-deposited HfO2 films was 7nm, which was measured by an ellipsometer system. Subsequently, the wafers with 7nm-thick HfO2 film were split into three groups, and processed with different post-treatments to study the properties of low-temperature-deposited HfO2 film. The first group labeled as Baking-only treatment, was designed as the control sample, and was only baked on a hot plate 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 100cm3. The third sample marked as 3000psi-SCCO2
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 HfO2 film for passivating defects.
The supercritical fluid system is shown in Fig. 2-1.
After these different treatments, the thickness of HfO2 films is almost intact, checked with the ellipsometer measurement. 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 HfO2 films, respectively. Electrical measurements were conducted on metal insulator
semiconductor (MIS) capacitors by thermally evaporating Al electrodes on the front surface of the HfO2 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 HfO2 film. The experiment processes of thin HfO2 film with various treatments are exhibited in Fig. 2-2.
2.2 Fabrication of OTFTs and Experiment Process
At second experiment, a metal-oxide HfO2 film layer was deposited on p-type (100) silicon wafers by E-beam thermally evaporating at room temperature. The thickness of as-deposited HfO2 films was 150nm, which was measured by an ellipsometer system. Subsequently, the wafers with 150nm-thick HfO2 film were split into three groups, and processed with different post-treatments to study the properties of low-temperature-deposited HfO2 film. The first group labeled as Baking-only treatment, was designed as the control sample, and was only baked on a hot plate 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 100cm3. The third sample marked as 3000psi-SCCO2
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 HfO2 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 wafer was put into the oven with HMDS steam for 20mins at 150 degree. 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 fabrication the same process to comparison with insulation of SiO2 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, current stress and DC bias 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 Properties of Organic Thin Film Transistors
2.3.1 Characteristics of the organic materialsOrganic conjugated materials can be generally sorted small molecules and polymers in OTFTs. The one single bond exhibit contiguous sequences of double bonds separated. The π orbital in the conjugated system is linked with the neighboring π orbital, and spreads in the whole molecule. The π electrons are delocalized across
the molecule, which makes conductive or semiconducting characteristics of the conjugated system. Unlike the case in inorganic semiconductors, the carrier transport is band-like transport which is determined by the Bloch wave states. Carrier transport is via hopping between localized states in disordered organic semiconductors.
Mobility is the most important parameter when we mention about OTFTs. The localized wave functions in organic semiconductors lead to small inter-molecular interactions, typically the weak π-π overlap or van der Waals. That is the main cause of the low mobility. But it can be improved by modifying the materials and device architectures. Pentacene is one of the most widely-used in OTFTs materials. Its mobility has reached the fundamental limit (> 3 cm2/Vs) [27.28] which is obtained with a single crystal at room temperature. Organic semiconductors are soluble in vaporizable or organic solvents at low temperatures due to such small inter-molecular interactions. This implies that there may be a trade-off between the mobility and processability in organic semiconductors of OTFTs. The mobility of organic semiconductors ranged from 0.001to 3cm2/Vs is comparable to that of amorphous silicon transistors which is widely developed and used in flexible displays.
First introduces the polymer material. Conjugated polymers are more suitable than small molecules for solution processability due to their high viscosity. However, the mobility of conjugated polymer is generally smaller than that of small molecule semiconductors for the more random orientation of molecular units or relatively short conjugated length in polymer backbones. The mobility as high as 0.1cm2/Vs has been achieved with regioregular head-to-tail poly(3-hexylthiophene) [29]. More recently, it has been shown that the mobility can be raised to 0.2cm2/Vs when the polymer film is applied by dip-coating to a thickness of only 2-4nm [30]. Polymer-based TFTs offer the advantage of spin-coating or inkjet printing, but in turn decreases the mobility, the
purity defects in material may give rise to charge-trapping sites.
In addition, introduces the small molecules material. Since the spin-coated polymer is disordered in structure, the structure of thermal evaporated small molecule is well ordered. Small molecules can be classified into linear fused ring compounds, 2-D fused ring compounds, oligomers, and 3-D molecules.
Most organic materials tend to transport holes better than electrons. This is because the OTFTs is p-type semiconductors are more stable in air and larger mobility.
Most n-type organic semiconductors are sensitive to moisture and air due to the organic anions. Especially, the carbanions can easily react with water and oxygen under operating conditions, which causes the low mobility of n-type organic semiconductors.
Thereupon, we choose pentacene as material of the semiconductor region. Because pentacene material is have the largest mobility and more stability.
Pentacene is an aromatic compound with five condensed benzene rings, the chemical formula is with molecular weight 278.3. The volume of the unit cell is about 705Å [31]. The permittivity is 4 [32], and the electron affinity is about 2.49eV. Silinish et al. determined the adiabatic energy gap ( ) by using the threshold function of intrinsic photoconductivity of pentacene [33]. The second transition is from the excited state to the ionic state, which is called the optical energy gap ( ).The energy band of pentacene are exhibited in Fig. 2.4[34]
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EG
Opt
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2.3.2 Operation of OTFTs
Organic thin-film transistors are basically similar to traditional thin-film transistors in structure. Two common device structures are used in organic TFTs.
Although there is not large difference between inorganic TFTs and organic TFTs, the
operation modes of these two kinds oftransistors are not the same at all.
Since pentacene is a p-type semiconductor. First, a negative bias is applied to the gate, the voltage drops over the insulator and semiconductor regions, which gives rise to band bending in the semiconductor. The additional positive charges provided by the source and drain electrodes accumulate charges 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 direct the current from source to drain. The conduction is determined by mobility μ which represents how the electrical field drives the accumulated charges. Therefore, the increased gate voltage δVG accounts for the increased charges COXδVG and the total charges increased over the channel are WLCOXδVG, where L and W correspond to the channel length and width. The increased drain current δID then represented as
G
In general, we can divide the operation of OTFTs into two regions, linear and saturation regions. The drain current in the linear region is determined from the following equation Since the drain voltage is quite small, sometimes equation (2.2) can be simplified as
D layer. The current equation is modified as
2
D ( )
I 2 COX VG VTH L
W −
= μ (2.4)
The energy band diagrams for n-type and p-type OTFTs are shown in Fig. 2.5 [35].
2.3.3 Transportation Mechanisms
Latterly, two principal types of theoretical model are used to describe the transport in organic semiconductors : “The band-transport model” and “The hopping models”. Multiple trapping and release (MTR) model assumes that most of the carriers injected in the semiconductor are trapped in states localized in the forbidden gap. MTR model is widely used in amorphous silicon TFTs [36.37] and explains reasonably well the observed characteristics in vapor-deposited polycrystalline pentacene films. The model assumes that the intrinsic charge transport mechanism is the one involving extended states, and a distribution of traps exists in the forbidden gap above the valence-band edge. At low gate biases, most of the holes injected in the semiconductor are trapped into these localized states. The deepest traps are first filled and carriers can be thermally released. As the negative gate bias increases in p-type materials, the Fermi level approaches the valence-band edge and more traps are filled.
At an appropriately high gate voltage, all trap states are filled and subsequently injected carriers move with the microscopic mobility in the delocalized (valence) band [38.39]. Several trap levels have been reported for polycrystalline vapor-deposited pentacene films at depths ranging from 0.06eV to 0.68eV [40], which can account for the MTR model. Traps are sometimes caused from the impurities and structure defects in the crystalline pentacene film which include dislocations, point defects and most importantly, the grain boundaries [41]. The concept of grain boundaries has been used to explain the gate-voltage dependence of mobility in polycrystalline oligothiophene
films [42.43]. The energy barrier created in the grain boundaries is a function of trapped charge states, carrier concentration within the grains and temperature.
At high temperatures, the charge transport is dominated by the thermionic emission over the potential barrier at grain boundaries. At low temperatures, the carrier transport is dominated by tunneling. However, mobility in molecular crystal is still moderate at very low temperatures. The corresponding mean free path does not exceed the inter-molecular distance, which is not physically acceptable for a diffusion-limited transport. Polaron models have been proposed to rationalize the discrepancy [44]. In spite of the recent efforts, the best explanation and exact phenomenon describing the carrier transport in molecular crystals are still under investigation.
The concept of variable range hopping (VRH) [45]is usually used in organic transistors, where the carriers transport by hopping: thermally activated carriers tunneling between localized states including percolation, rather than by the activation of carriers to a transport level.
The model describes the conductivity in the polymer as equivalent to transport through a resistor network. The percolation criterion through the network is then related to the temperature, the position of Fermi level, and the width of the exponential tail of the density of states.
A carrier may either hop over a long distance with low activation energy or hop over a small distance with high activation energy. As the accumulated charges fill the lower lying states, any additional charges will occupy states with relatively higher energy. Therefore, these additional charges just need less energy to hop away to neighboring sites, and the mobility will rise as the gate voltage increases.
2.3.4 Parameters extraction
In this section, we introduce four parameters are used to evaluate the performance of OTFTs. They are field effect mobility, threshold voltage, sub-threshold slope and on/off current ratio.
Mobility
Field effect carrier mobility is usually considered the most critical part of these four parameters. The behavior of an OTFT is revealed with the observation of carrier mobility. Mobility of an OTFT is affected by many factors, such as the trap density of active layer, ambient temperature, and carrier concentration. In OTFTs, the mobility mainly depends on the ordering of molecules and trap density.
Generally, mobility can be extracted from the trans-conductance in gm the linear region :
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. –VD > –(VG - VTH) :
Devices with high on/off current ratio represent ratio of the 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 the low current when the device is turned off.
Threshold voltage
Threshold voltage is related to the power consumptions and the operation voltage of OTFTs. Many researches on OTFTs are suffered from the large threshold voltage.
Threshold voltage is influenced by the ratio of the mobile and trapped carriers at the interface between the organic semiconductor layer and insulator. There are also researches on lowering the threshold voltage by adjusting the insulator layer [46]. In our experiments, we extract the threshold voltage from equation (2.6), the intersection point of the square-root of drain current versus gate voltage when the device is in saturation mode operation.
Sub-threshold Slope
Sub-threshold voltage is defined to evaluate the sensitivity of drain current to gate voltage in OTFTs. The following equation is used to define sub-threshold voltage. Obviously, a well-performed TFT will have a smaller value of sub-threshold voltage. This means that the relatively large swing of drain current can be achieved with a relatively small gate voltage.
Chapter 3 Analysis and Result
3.1 Thin Film Analysis of Electrical Characteristics and Discussion
3.1.1 The current density-electric field (J-E) characteristicsThe leakage current densities of HfO2 films after different treatments are shown as a function of applied negative gate bias voltage in Fig. 3-1. Among various post-treatments, the baking-treated HfO2 film exhibits the most serious leakage current, inferentially due to its poor dielectric characteristics with numerous traps inside the HfO2 film and the interface between parasitical SiOx and Si wafer. The improvement of electrical characteristics is observed by using H2O vapor process, however, a high leakage current density still appears at larger applied voltages. It could be inferred reasonably dependent on the defect passivation efficiency. The most indicating that H2O vapor can passivate the traps (or defects) and alter dielectric properties of the low-temperature-deposited HfO2 film. After H2O vapor treatment, effective improvement of electrical characteristic is obtained by the 3000 psi-SCCO2
treatment, exhibiting the lowest leakage current density among all samples. Low leakage current density (~2×10-7 A/cm2) is kept constantly, even biased at an electric field of 3 MV/cm. The electrical performance agrees with FTIR analysis, in which 3000 psi-SCCO2 treatment modified HfO2 dielectrics even effectively.
3.1.2 Conduction Mechanism
There may be different conduction mechanisms in the insulator thin film, including Schottky-Richardson emission [47], Frenkel-Poole emission [47,48], Fowler-Nordheim tunneling [47,48], and trap assisted tunneling [49,50] illustrated in
Fig 3-2. The Schottky-Richardson emission generated by the thermionic effect is caused by the electron transport across the potential energy barrier via field-assisted lowering at a metal-insulator interface. The leakage current governed by the Schottky-Richardson emission is as following:
⎟⎠
φSR is the contact potential barrier,E is the applied electric field, ε0 is the permittivity in vacuum, ε is the high frequency relative dielectric constant, Tis the absolute temperature, and is the Boltzmann constant. We can
φSR is the contact potential barrier,E is the applied electric field, ε0 is the permittivity in vacuum, ε is the high frequency relative dielectric constant, Tis the absolute temperature, and is the Boltzmann constant. We can