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) characteristics

The 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 find the slope of the leakage current equation.

kB 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: 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 find the slope of the leakage current equation.

k

[ln( )J 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 [51]:

(

_{FN ox}

)

are usually considered to be constant. and are given as the following:

AFN B_FN

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

mox q

A plot of ln

(

JFN ε versus ox²

) (

1 ε should be a straight line if the conduction ox

)

through the oxide is pure Fowler-Nordheim conduction [51].

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 [49]. The equation of leakage current density is [50]:

(

_ox

)

ox exp E

E

J =α −β (3.11) From the equations as shown above, leakage current behaviors of insulate films can be investigated further on the leakage current density electric field J E characteristics such as J vs. 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 HfO2 film. Fig. 3-3(a) plots ln (J/E) versus reciprocal of electric field variation for the baking-only treated HfO2 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 [52] occurs in the electric fields higher than 0.7 MV/cm. Also, it is consistent with the electrical behavior of

baking-only treated HfO2 film in Fig. 3-1 that leakage current density sharply increases, while gate bias voltage larger than 0.7 MV/cm. This could be attributed to the trap-assisted tunneling due to numerous traps inside the 150°C- baking treated HfO2 film [53]. For the 3000 psi-SCCO2 treated HfO2 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. 3-3(b). The leakage current density of the 3000 psi-SCCO2 treated HfO2 is linearly related to the square root of the applied electric field, demonstrating Schottky-Richardson emission transport mechanism [54]. The Schottky-type conduction can be verified by comparing the theoretical value of

(

3 0

)

¹²

SR q 4πε ε

β = with the calculated one obtained from the slope of the experimental curve ln J versus E^1/2 [55], 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. 3-3(b), and independent of traps. From the slope of ln J versus E^1/2, the calculated value of relative dielectric constant (ε) is 26.4, and which is close to the determined value of 29.4 in capacitance-voltage (C-V) measurement (referring to table 3-1). This also proves, for 3000spi-SCCO2 treated HfO2 film, the conduction mechanism is really Schottky emission, but not trap-dependent Poole-Frenkel emission [55]. Additionally, the evolution of conduction mechanisms from trap-assisted tunneling to Schottky emission can confirm these defects inside low-temperature-deposited HfO2 film is minimized effectively by implementing the proposed SCCO2 technology. The leakage current densities of HfO2 films after different treatments are shown as a function of

be acquired after 3000 psi-SCCO2 and H2O vapor treatment, especially treated with SCCO2 fluids. This could be attributed to the influence of traps in the interface between parasitical SiOx and Si wafer. Generally, in positive gate bias, the sources of electron are (1) the interface states, (2) defects in depletion region, (3) back electrode of substrate, [56] and the later two source are negligible due to the p-type signal-crystal Si wafer is used in this work. For baking-only treated HfO2 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. 3-4. After 3000 psi-SCCO2

treatment, the interface states were deactivated, hence the leakage current is reduced.

The reduction of interface states would be proved in capacitance-voltage measurement.

3.1.3 The capacitance-voltage (C-V) characteristics

The capacitance-voltage (C-V) characteristics are also generally used to judge the quality of dielectric films. Figure. 3-5 shows capacitance-voltage characteristics of HfO2 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 HfO2 and Si wafer. In Fig. 3-5, the baking-treated HfO2

film presents the worst C-V curve and lower capacitance. 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 3-1, could be referred to the influence of defects in HfO2 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 HfO2

psi-SCCO2 treatment. This exhibits that the SCCO2 treatment possesses excellent ability to passivate the defects, including Hf dangling bonds and interface states.

Besides, from Fig. 3-5, the shift of C-V curve under forward and reverse swing is also appears in baking-treated and H2O vapor-treated HfO2 films. It is resulted from the trapped carrier in defects of HfO2 films, and that is not expected for gate insulator of transistors. Under negative gate bias, the electric inject from Al gate into HfO2

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 = 50%, and the shift of the flat-band voltages under forward and reverse swing is shown in table 3-1. It is evidently observed that the baking-treated HfO2 film hold numerous defects because of the extensive shift of flat-band voltage, and the defects almost disappear after 3000 psi-SCCO2 treatment.

These results conform to the tendency in current-voltage characteristics and again verify that the SCCO2 technology could effectively deactivate defects in HfO2

films.

Another interesting detection, in Fig. 3-5, is the change of flat-band voltage of different-treated HfO2 films under forward swing, also shown in table 3-1. For baking- treated HfO2 film, the flat-band voltage (= -3.2 volt.) is away from ideal gate bias voltage (about 0~0.3 volt.), and that of 3000 psi-SCCO2 treated HfO2 is zeroed nearly. The main reason could be referred to (1) the positively charged Hf dangling bonds are passivated, (2) the fixed positive charges are removed by SCCO2 fluids.

The mechanism of extracting of fixed charge is shown in Fig. 3-6, including positive and negative fixed charge [57]. The polarized-H2O molecule is taken as a dipole which would attract the fixed charge in HfO2 films. Afterward, the H2O molecule and fixed charge are carried away by SCCO2 fluids mixed with propyl alcohol. For H2O

vapor-treated HfO2 film, the un-zeroed flat-band voltage could be attributed to (1) partial positively charged Hf dangling bonds remain, (2) the poorer capability for H2O vapor to remove fixed charge. Hence, it is necessary for H2O molecule to be driven into HfO2 films and carried away by SCCO2 fluids.

As a matter of fact, upon reducing the oxide thickness, it is difficult to calculate the density of interface states by using the high-low frequency method because of the substantially increased gate leakage current. Therefore, the interface capacitance (Cit) was employed instead of interface states to investigate the interfacial property.

Figure 3-7 (a) and (b) illustrates the equivalent capacitance models of MOS structure without and with Cit, respectively [56]. For higher measuring frequency, fewer interface states could respond to the ac switching signal, so suiting to the model in Fig. 3-7 (a) and presenting lower measured capacitance. For lower measuring frequency, more interface states could respond to the ac switching signal, so suiting to the model in Fig. 3-7 (b) and presenting higher measured capacitance. Therefore, the separation of Cmax under different measuring frequency appears if the interface states existing. Figure 3-8 shows the capacitance-voltage characteristics of HfO2 films after different treatment, measuring at 1M Hz and 100k Hz with forward gate bias swing. A conspicuous separation occurs in baking-treated HfO2 films, and the higher density of interface states is supposed. However, with 3000 psi-SCCO2 treatment, the value of Cmax has only very slight rise under different measuring frequency, proofing that the effectively reducing interface states during SCCO2 process. The sharp slope in transient region of C-V curve thereby is also reasonable.

3.1.4 Breakdown voltage measurement and gate bias stress

Figure 3-9 (a) and (b) show the breakdown characteristic curves of HfO2 films

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. 3-10 (a) and (b), whether at negative or positive gate bias, the baking-treated HfO2 film presents the worst performance in breakdown voltage because of the high density of defects, and the improvements of breakdown voltage are gradually achieved via H2O vapor and 3000 psi-SCCO2 treatment. This result exhibits clearly that the density of defects in HfO2 films are effectively reduced, and the breakdown voltage of 7nm HfO2 film thereby could be substantially ameliorated from 1 V to 24 V at negative gate bias, from 30 V to 55 V at positive gate bias. It also indicates that the SCCO2 fluids technology is greatly useful to enhance the low-temperature deposited HfO2 films by passivating defects, and allows the treated HfO2 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 stress, the carriers of leakage current and high electric field would impact the weak bonding, leading to more defects, higher leakage current and the degradation of transistor [58]. Therefore, the reliability of dielectric under gate bias stress would judge whether agrees with the application of gate dielectric. Figure 3-10 shows the variation of leakage current of different-treated HfO2 films as a function of stress time at a high electric field = 5 MV/cm, where I0 is the initial leakage density. As well as the tendency of the measurement of breakdown voltage, the baking-treated HfO2 film

behaves the most rises in the degree of leakage current as the stress time increasing, because of the great amount of defects and weak bonding. However, after treating with 3000 psi-SCCO2 process, the sputter-deposited HfO2 film performs a fine reliability under high electric field stress, hence it is extremely suitable for the application of gate dielectric.

3.2 Thin Film Analysis of Material and Discussion

3.2.1 Fourier Trans-form Infrared Spectroscopy (FTIR) Analysis

Fig. 3-11 shows the FTIR spectra of HfO2 films after various post-treatments, including Baking-only, H2O vapor and 3000 psi-SCCO2 treatment. The functional group referred to Hf-O-Hf bonding is at 509 cm^-1 and 690 cm^-1, and the absorption peak at around 1070 cm^-1 attributes to the Si-O-Si bond. The Si-O-Si bond originates form the formation of interface layer (SiOx) between HfO2 film and silicon wafer during fabricating HfO2 films in Ar/O2 ambient. The peak intensity of Si-O-Si bond for different treatments is almost the same, meaning that these post-treatments would not make different influence on the thickness and quality of the interfacial SiOx film.

For the H2O-vapor-treated HfO2 film, however, the peak intensity of Hf-O-Hf bands (509 cm^-1 and 690 cm^-1) raises apparently in comparison with the baking-only-treated HfO2 film. This is believed well that the H2O vapor would permeate into HfO2 film and makes reaction with Hf dangling bonds (i.e. traps) forming Hf-O-Hf bands. These traps in the low-temperature deposited HfO2 film could be thereby passivated by H2O vapor molecules. Furthermore, with 3000 psi- SCCO2 treatment, obvious increase in the intensity of Hf-O-Hf bonding is observed in the FTIR. It indicates that the best transport efficiency of H2O molecules into HfO2 film is achieved by the SCCO2 fluids, potentially modifying the dielectric properties of HfO2 film, and the transporting

mechanism for SCCO2 fluids taking H2O molecule into HfO2 film is shown in Fig.

3-12.

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

The TDS measurement, as shown in the Fig. 3-13, was carried out upon heating these treated HfO2 films from 50 to 800 °C at a heating rate of 10 °C/min in vacuum (10⁻⁵ Pa.). In Fig. 3-13 (a), m/e (mass-to-charge ratio) = 32 peak that is attributed to O2 was monitored to evaluate the content of oxygen outgassing form HfO2 films. It is clearly found the highest oxygen content is detected in the 3000 psi-SCCO2 treated HfO2 film, certainly consistent with the FTIR observation. From Fig. 3-13 (b), m/e (mass-to- charge ratio) = 44 peak that is attributed to CO2, the residual carbon dioxide in HfO2 is equal after various post-treatments. This is result from SCCO2 fluid not only employed to transport the CO2 molecule into HfO2 film but the CO2 molecule is not remain in addition [21, 22].

3.2.3 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

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

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