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

)

(2-2) Threshold voltage

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 as a function of applied negative gate bias voltage in Fig. 3-7. Among various post-treatments, the SCCO2-treated Al2O3 film exhibits the most serious leakage current, inferentially due to its poor dielectric characteristics with numerous traps inside the Al2O3 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 Al2O3 film. After H2O vapor treatment, effective improvement of electrical characteristic is obtained by the SCCO2 with co-solvent treatment, exhibiting the lowest leakage current density among all samples.

Low leakage current density (~3.9×10^-9 A/cm²) is kept constantly, even biased at an electric field of 1.7 MV/cm. The electrical performance agrees with XPS analysis, in which SCCO2 with co-solvent treatment modified Al2O3 dielectrics even effectively.

3.2.2 Conduction Mechanism

There may be different conduction mechanisms in the insulator thin film, including Schottky-Richardson emission [50], Frenkel-Poole emission [50,51], Fowler-Nordheim tunneling [50,51], and trap assisted tunneling [52,53] illustrated in Fig 3-8. The Schottky-Richardson emission generated by the thermal ionic 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:

(

¹

)

* 2 2

SR SR

J =A T exp β E −φ_SR k T_B

where βSR =

(

q³ 4πε ε0

)

¹² , q is the electronic charge, is the effective Richardson constant,

A*

φSR is the contact potential barrier, is the applied electric field,

E

ε0 is the permittivity in vacuum, ε is the high frequency relative dielectric constant, is the absolute temperature, and is the Boltzmann constant. We can find the slope of the leakage current equation.

T k_B

( )

12 * 2

lnJ_SR =β_SRE k T_B +⎡⎣ln A T −φ_SR k_BT⎤⎦

SR B

Solpe=β k T

The Frenkel-Poole emission is due to field-enhanced thermal excitation of trapped electrons in the insulator into the conduction band. The leakage current equation is:

(

¹²

)

FP 0 FP

J =J exp β E −φ_FP k T_B

where J₀ =σ₀E is the low-field current density, σ₀ is the low-field conductivity, ^βFP=

(

^{q3 πε ε0}

)

^12, q is the electronic charge, φ_FP is the contact potential barrier, is the applied electric field, E ε₀ is the permittivity in vacuum, ε is the high frequency relative dielectric constant, is the absolute temperature, and

is the Boltzmann constant. We can find the slope of the leakage current equation.

T kB

1

( )

2 0

lnJ_FP =β_FPE k T_B +⎡⎣ln J −φ_FP k T_B ⎤⎦

FP B

Solpe=β k T

The Fowler-Nordheim tunneling is the flow of electrons through a triangular potential barrier. Tunneling is a quantum mechanical process similar to throwing a ball against a wall often results that the ball goes through the wall without damaging the wall or the ball. It also loses no energy during the tunnel event. The probability of this event happening, however, is extremely low, but an electron incident on a barrier typically several nm thick has a high probability of transmission. The Fowler-Nordheim tunneling current I_FN is given by the expression [54]:

( )

2

FN G FN FN

I =A A ε_oxexp −B ε_ox

where the A_G is the gate area, ε_ox is the oxide electric field, and and are usually considered to be constant. and are given as the following:

AFN B _FN AFN B_FN

( ) ( )

3 6

AFN =q m m_ox 8πhΦ =_B 1.54 10× ⁻ m m_ox Φ _B

(

³

)

^{12 7}

^{( )}

¹²

BFN =8π 2m_oxΦ_B 3eh=6.83 10× ^⎡⎣ m m_ox Φ_B^3⎤⎦

where is the effective electron mass in the oxide, m is the free electron mass, is the electronic charge, and is the barrier height at the silicon-oxide interface given in units of eV in the expression for

mox q

ΦB

B .FN Φ_B is actually an effective barrier height that take into account barrier height lowering and quantization of electrons at the semiconductor surface. Rearranging I_FN formula gives by:

(

²

) (

²

) ( )

ln I_FN A_{G ox}ε =ln J_FN ε_ox =ln A_FN −B_FN ε_ox

A plot of ^ln

(

^{JFN εox2}

)

^versus

(

^1εox

)

should be a straight line if the conduction through the oxide is pure Fowler-Nordheim conduction [54].

In the trap assisted tunneling model, it is assumed that electrons first tunnel through the SiOX interfacial layer (direct-tunneling). Then, electrons tunnel through traps located below the conduction band of the high-k thin film and leak to substrate finally [52]. The equation of leakage current density is [53]:

( )

J=α E exp_ox −β E_ox

From the equations as shown above, leakage current behaviors of insulate films can be investigated further on the leakage current density J electric field characteristics such as vs.

E J E¹² plots.

The plot of the nature log of leakage current density versus the square root of the applied electric field was observed. It is found that the leakage current density is linearly related to square root of the applied electric field. The linear variations of the current correspond either to Schottky-Richardson emission or to Frenkel-Poole conduction mechanism. For trap states with coulomb potentials, the expression is virtually identical to that of the Schottky-Richardson emission. The barrier height, however, is the depth of the trap potential well, and the quantity β_FP is larger than in the case of Schottky-Richardson emission by a factor of 2.

Leakage conduction mechanism is also investigated to support the comments on the electrical improvement of Al2O3 film. Fig. 3-9(a) plots ln (J/E) versus reciprocal of electric field variation for the SCCO2-only treated Al2O3 film, and a schematic

energy band diagram accounting for leakage transport mechanism shown in the inset.

A good linear fitting explains Fowler-Nordheim (F-N) tunneling [55] occurs in the electric fields higher than 0.5 MV/cm. Also, it is consistent with the electrical behavior of SCCO2-only treated Al2O3 film in Fig. 3-9 that leakage current density sharply increases, while gate bias voltage larger than 0.5 MV/cm. This could be attributed to the trap-assisted tunneling due to numerous traps inside the SCCO2-only treated Al2O3 film [56]. For the SCCO2 with co-solvent treated Al2O3 film, a plot of leakage current density versus the square root of the applied field (E^1/2) gives a good representation of the leakage behavior at high electric fields, as shown in Fig. 3-9(c).

The leakage current density of the SCCO2 with co-solvent treated Al2O3 is linearly related to the square root of the applied electric field, demonstrating Schottky-Richardson emission transport mechanism [57]. The Schottky-type conduction can be verified by comparing the theoretical value of βSR =

(

q³ 4πε ε0

)

¹²

with the calculated one obtained from the slope of the experimental curve ln J versus E^1/2 [58], where q is the electronic charge, ε₀ the dielectric constant of free space, ε is the high frequency relative dielectric constant. The Schottky emission generated by the thermionic effect is caused by electron transport across the potential energy barrier via field-assisted lowering at a metal-insulator interface, shown in the insert of Fig.

3-9(c), and independent of traps. Additionally, the evolution of conduction mechanisms from trap-assisted tunneling to Schottky emission can confirm these defects inside low-temperature-deposited Al2O3 film is minimized effectively by implementing the proposed SCCO2 technology. The leakage current densities of Al2O3 films after different treatments are shown as a function of applied positive gate bias voltage in Fig. 3-10, and the lower leakage current still could be acquired after SCCO2 with co-solvent and H2O vapor treatment, especially treated with SCCO2

fluids. Generally, in positive gate bias, the sources of electron are (1) the interface states, (2) defects in depletion region, (3) back electrode of substrate, [59] and the later two source are negligible due to the p-type signal-crystal Si wafer is used in this work. For SCCO2-only treated Al2O3 film, the great quantity of interface states still

exist which generate electron-hole pair and lead to higher leakage current, as described in the inset of Fig. 3-10. After SCCO2 with co-solvent treatment, the interface states were deactivated, hence the leakage current is reduced. The reduction of interface states would be proved in capacitance-voltage measurement.

3.2.3 The capacitance-voltage (C-V) characteristics

The capacitance-voltage (C-V) characteristics are also generally used to judge the quality of dielectric films. Figure 3-11 shows capacitance-voltage characteristics of Al2O3 films after different treatment, measuring at 1M Hz with gate bias swing from negative voltage to positive voltage (forward) and from positive voltage to negative voltage (reverse). The slope of C-V curve in transient region, i.e. from Cmax to Cmin, is relative to the interface states, for example, the sharp slope indicates fewer defects exist in the interface between Al2O3 and Si wafer. In Fig. 3-11, the SCCO2-treated Al2O3 film presents the worst C-V curve . This expresses the larger number of interface states exist and lead to the smooth C-V curve. Additionally, the lower dielectric constant, as shown in Table 3-2, could be referred to the influence of defects in Al2O3 film. With H2O vapor treatment, the sharper C-V curve and higher capacitance are obtained, and it could be attributed to the reduction of defects in Al2O3 film and the interface.

Besides, from Fig. 3-11, the shift of C-V curve under forward and reverse swing is also appears in SCCO2-treated and H2O vapor-treated Al2O3 films. It is resulted from the trapped carrier in defects of Al2O3 films, and that is not expected for gate insulator of transistors. Under negative gate bias, the electric inject from Al gate into Al2O3 films and trapped by defects, leading to the larger gate bias is required for inducing electron-inversion layer. For describing clear, we define the flat-band voltage is the gate bias as C/Cmax = 0.5, and the shift of the flat-band voltages under forward and reverse swing is shown in Table 3-2. It is evidently observed that the SCCO2-treated Al2O3 film hold numerous defects because of the extensive shift of

flat-band voltage, and the defects almost disappear after SCCO2 with co-solvent treatment.

These results conform to the tendency in current-voltage characteristics and again verify that the SCCO2 technology could effectively deactivate defects in Al2O3

films.

3.2.4 Breakdown voltage measurement and gate bias stress

Figure 3-12 show the breakdown characteristic curves of Al2O3 films after various treatments at negative gate bias region. The breakdown voltage is mainly relative to the qualities of dielectric films and the density of defects in the dielectric films. A large number of traps lead to the trap-assisted tunneling early occurs and a high leakage current appears at small electric field, such that the lower breakdown voltages of dielectric films comes up. In Fig. 3-13, at negative gate bias, the SCCO2-treated Al2O3 film presents the worst performance in breakdown electric field because of the high density of defects, and the improvements of breakdown electric field are gradually achieved via H2O vapor and SCCO2 with co-solvent treatment.

This result exhibits clearly that the density of defects in Al2O3 films are effectively reduced, and the breakdown electric field of 16 nm Al2O3 film thereby could be substantially ameliorated from 2.3 MV/cm to 5.1 MV/cm at negative gate bias. It also indicates that the SCCO2 fluids technology is greatly useful to enhance the low-temperature deposited Al2O3 films by passivating defects, and allows the treated Al2O3 film holding good reliability as the gate dielectric.

Another important property of dielectric films is the reliability under gate bias stress. Due to the gate dielectric is stressed at a high field when the transistors are operating, so that it is demanded for gate dielectric to have excellent resistance to the impairment under long time stress at operating electric field. During high electric field

Another important property of dielectric films is the reliability under gate bias stress. Due to the gate dielectric is stressed at a high field when the transistors are operating, so that it is demanded for gate dielectric to have excellent resistance to the impairment under long time stress at operating electric field. During high electric field

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