**6.2 Thicknesses and refractive indexes dependent on sol solution of HfO2 thin films**

The thickness of the sol-gel-derived HfO2 films decreased with increasing firing temperature (Figure 13). It is seen that the smallest thickness was 6 nm for the "HCOOH sol" HfO2 film fired at 700 °C, which is about 1 nm thinner than the thinnest "HNO3 sol" HfO2 film. The difference is due to the properties of the catalyst used and this result shows that the "HCOOH sol" HfO2 film may be suitable for use as the gate insulator of highly integrated CMOS devices. However, its electrical performance should be superior to that of conventional HfO2 films.

Fig. 13. Thicknesses and refractive indexes of sol-gel-derived HfO2 films based on both (a) "HCOOH sol" and (b) "HNO3 sol" fired at 350, 450, 550, and 700 °C for 30 min in air (Shimizu et al., 2010).

The refractive indexes began to increase at approximately 550 oC for the "HCOOH sol" film and at 450 oC for the "HNO3 sol" film. These temperatures are in good agreement with those at which crystallization occurs, as obtained by XRD analysis [Figures 12(a) and 12(b)]. The maximum refractive indexes obtained were 1.70 for the "HCOOH sol" film and 1.95 for the "HNO3 sol" film, although the reported value for the HfO2 crystal (monoclinic) is 2.19. The

Characterization of Sol-Gel-Derived and Crystallized

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 329

Fig. 14. AFM images showing the surface microstructures of "HCOOH sol" HfO2 thin films

fired at (a) 350, (b) 450, (c) 550 and (d) 700 oC (Shimizu et al., 2010).

**100nm 100nm**

**100nm 100nm**

packing densities of the HfO2 films were calculated using the Lorentz-Lorentz equation (Nishide et al., 2001),

$$p = \frac{n\_{\text{f}}^2 - 1}{n\_{\text{f}}^2 + 2} \times \frac{n\_{\text{m}}^2 + 2}{n\_{\text{m}}^2 - 1},\tag{2}$$

where *p* is the packing density, *n*f is the refractive index of the film, and *n*m is the refractive index of the crystal. The calculated packing densities indicated that nanopores remained in the amorphous state. However, upon the crystallization of the film, the packing densities became greater than those in the amorphous state in both the "HCOOH sol" and "HNO3 sol" HfO2 films.

#### **6.3 Surface morphologies of HfO2 layers for both "HCOOH sol" and "HNO3 sol" HfO2 films**

Images of surface microstructures were obtained with an atomic force microscope (AFM) for both "HCOOH sol" and "HNO3 sol" HfO2 films fired at 350, 450, 550, and 700 oC (Figures 14 and 15). The progress of the microstructure development depended on the firing temperature. The surface of the "HCOOH sol" HfO2 thin films fired at 350, 450, and 550 oC showed homogeneous glass-like structures. The root mean square (RMS) surface roughness was determined to be 0.13, 0.14, and 0.15 nm at firing temperatures of 350, 450 and 550 oC, whereas it was 0.34 nm at 700 oC, which indicated the presence of grain boundaries caused by crystallization. These values are in good agreement with the XRD and refractive index results. For the "HNO3 sol" HfO2 films, the RMS values were 0.14 and 0.15 nm at firing temperatures of 350 and 450 oC, respectively. At firing temperatures of 550 and 700 oC, the RMS values were 0.17 and 0.34 nm, resulting in grain boundaries due to crystallization. In this case, the surface roughness was also due to crystallization.

#### **6.4 TPD spectral analyses of sol-gel-derived HfO2 thin films based on "HCOOH sol" and "HNO3 sol"**

The desorption of H2O (*m*/*z* = 18) was analyzed by TPD for both "HCOOH sol" and "HNO3 sol" HfO2 films on Si(001) wafers fired at 350, 450, 550, and 700 °C for 30 min [Figures 16(a) and 16(b)]. The vertical axis indicates the QMS current and the horizontal axis shows the heating temperature of the samples in the TPD chamber. The film thicknesses ranged approximately between 6 to 10 nm. The overall intensities of the desorption of H2O in the TPD curves are related to both the "HCOOH sol" and "HNO3 sol" HfO2 films. The intensity of both TPD curves decreased with increasing firing temperature. The "HCOOH sol" HfO2 films fired at 350, 450, and 550 °C, which were amorphous, showed small peaks at approximately 500 °C in the TPD curves. These peaks are presumably associated with crystallization during heating in the TPD chamber, because no corresponding peak was observed in the film fired at 700 °C (crystallization temperature: 560 °C).

TPD curves for the HfO2 thin films fired at 350 °C using the "HCOOH sol" and "HNO3 sol" on Si are separated into five Gaussian waveforms shown by dashed lines [Figure 17(a) and 17(b)]. Component (a) is thought to be due to H2O physically adsorbed (simple adsorption of H2O) on the surface of the HfO2. Component (e) can be ascribed to the desorption of H2O through nanopores in the crystallized HfO2 film. Component (b) is probably due to the

packing densities of the HfO2 films were calculated using the Lorentz-Lorentz equation

2 2 f m 2 2 f m

where *p* is the packing density, *n*f is the refractive index of the film, and *n*m is the refractive index of the crystal. The calculated packing densities indicated that nanopores remained in the amorphous state. However, upon the crystallization of the film, the packing densities became greater than those in the amorphous state in both the "HCOOH sol" and "HNO3

*n n*

= ×

*n n*

**6.3 Surface morphologies of HfO2 layers for both "HCOOH sol" and "HNO3 sol" HfO2**

**6.4 TPD spectral analyses of sol-gel-derived HfO2 thin films based on "HCOOH sol"** 

The desorption of H2O (*m*/*z* = 18) was analyzed by TPD for both "HCOOH sol" and "HNO3 sol" HfO2 films on Si(001) wafers fired at 350, 450, 550, and 700 °C for 30 min [Figures 16(a) and 16(b)]. The vertical axis indicates the QMS current and the horizontal axis shows the heating temperature of the samples in the TPD chamber. The film thicknesses ranged approximately between 6 to 10 nm. The overall intensities of the desorption of H2O in the TPD curves are related to both the "HCOOH sol" and "HNO3 sol" HfO2 films. The intensity of both TPD curves decreased with increasing firing temperature. The "HCOOH sol" HfO2 films fired at 350, 450, and 550 °C, which were amorphous, showed small peaks at approximately 500 °C in the TPD curves. These peaks are presumably associated with crystallization during heating in the TPD chamber, because no corresponding peak was

TPD curves for the HfO2 thin films fired at 350 °C using the "HCOOH sol" and "HNO3 sol" on Si are separated into five Gaussian waveforms shown by dashed lines [Figure 17(a) and 17(b)]. Component (a) is thought to be due to H2O physically adsorbed (simple adsorption of H2O) on the surface of the HfO2. Component (e) can be ascribed to the desorption of H2O through nanopores in the crystallized HfO2 film. Component (b) is probably due to the

Images of surface microstructures were obtained with an atomic force microscope (AFM) for both "HCOOH sol" and "HNO3 sol" HfO2 films fired at 350, 450, 550, and 700 oC (Figures 14 and 15). The progress of the microstructure development depended on the firing temperature. The surface of the "HCOOH sol" HfO2 thin films fired at 350, 450, and 550 oC showed homogeneous glass-like structures. The root mean square (RMS) surface roughness was determined to be 0.13, 0.14, and 0.15 nm at firing temperatures of 350, 450 and 550 oC, whereas it was 0.34 nm at 700 oC, which indicated the presence of grain boundaries caused by crystallization. These values are in good agreement with the XRD and refractive index results. For the "HNO3 sol" HfO2 films, the RMS values were 0.14 and 0.15 nm at firing temperatures of 350 and 450 oC, respectively. At firing temperatures of 550 and 700 oC, the RMS values were 0.17 and 0.34 nm, resulting in grain boundaries due to crystallization. In

*p*

this case, the surface roughness was also due to crystallization.

observed in the film fired at 700 °C (crystallization temperature: 560 °C).

1 2 2 1

, (2)

− +

+ −

(Nishide et al., 2001),

sol" HfO2 films.

**and "HNO3 sol"** 

**films** 

Fig. 14. AFM images showing the surface microstructures of "HCOOH sol" HfO2 thin films fired at (a) 350, (b) 450, (c) 550 and (d) 700 oC (Shimizu et al., 2010).

Characterization of Sol-Gel-Derived and Crystallized

(Shimizu et al., 2010).

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Intensity (

×10-11A)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

(a)

(b)

(c)

Intensity (

×10-11A)

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 331

 (a) (b) Fig. 16. TPD curves of H2O (*m*/*z* = 18) released from sol-gel-derived HfO2 thin films fired at 350, 450, 550, and 700 °C for 30 min: (a) HfO2 films using "HCOOH sol" and (b) HfO2 films using "HNO3 sol" on Si. The vertical axis indicates the QMS current and the horizontal axis shows the heating temperature of the samples in the TPD chamber

Intensity (

4.0 5.0

6.0 7.0

8.0

0.0 1.0 2.0 3.0

50

×10-11A)

(a) (b)

Intensity (

(a)

(b)

(c)

×10-11A)

8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

100 200 300 400 500 600 700 Heating Temperature (<sup>o</sup>

100 200 300 400 500 600 700

HeatingTemperature (<sup>o</sup>

(d) (e)

C)

*m/z 18*

C)

350 o C 450 o C

> 550 o C 700 o C

(a) "HCOOH sol" and (b) "HNO3 sol" on Si, separated into five Gaussian waveforms shown

desorption of H2O and/or chemically adsorbed Hf-OH bonds on the surface. On the other hand, the desorption of O in the TPD curves has main peaks at ~260 oC and subpeaks at ~350 oC, corresponding to the peak temperatures of components (c) and (d). For components (c) and (d), the desorption of the chemically adsorbed Hf-OH bonds and/or a small amount

Fig. 17. TPD curves for sol-gel-derived HfO2 thin films fired at 350 °C using

C)

*m/z 18*

C)

350 o C 450 o C 550 o C 700 o C

by the dashed lines (Shimizu et al., 2010).

100 200 300 400 500 600 700

(e)

(d)

100 200 300 400 500 600 700

Heating Temperature (<sup>o</sup>

Heating Temperature (<sup>o</sup>

Fig. 15. AFM images showing the surface microstructures of "HNO3 sol" HfO2 thin films fired at (a) 350, (b) 450, (c) 550 and (d) 700 oC (Shimizu et al., 2010).

Fig. 15. AFM images showing the surface microstructures of "HNO3 sol" HfO2 thin films

fired at (a) 350, (b) 450, (c) 550 and (d) 700 oC (Shimizu et al., 2010).

**100nm 100nm**

**100nm 100nm**

Fig. 16. TPD curves of H2O (*m*/*z* = 18) released from sol-gel-derived HfO2 thin films fired at 350, 450, 550, and 700 °C for 30 min: (a) HfO2 films using "HCOOH sol" and (b) HfO2 films using "HNO3 sol" on Si. The vertical axis indicates the QMS current and the horizontal axis shows the heating temperature of the samples in the TPD chamber (Shimizu et al., 2010).

Fig. 17. TPD curves for sol-gel-derived HfO2 thin films fired at 350 °C using (a) "HCOOH sol" and (b) "HNO3 sol" on Si, separated into five Gaussian waveforms shown by the dashed lines (Shimizu et al., 2010).

desorption of H2O and/or chemically adsorbed Hf-OH bonds on the surface. On the other hand, the desorption of O in the TPD curves has main peaks at ~260 oC and subpeaks at ~350 oC, corresponding to the peak temperatures of components (c) and (d). For components (c) and (d), the desorption of the chemically adsorbed Hf-OH bonds and/or a small amount

Characterization of Sol-Gel-Derived and Crystallized

**Si(001) wafers** 

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100

350 <sup>o</sup> C

550 <sup>o</sup> C

700 <sup>o</sup> C

> 450 <sup>o</sup> C

Current Density (A/cm2

)

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 333

**6.5 Electrical characterization of both "HCOOH sol" and "HNO3 sol" HfO2 thin films on** 

To measure the electrical characteristics (i.e., *I-V* and *C-V* characteristics) of both "HCOOH sol" and "HNO3 sol" HfO2 thin films on Si(001) wafers, a 0.4-mm-diameter aluminum (Al) electrode was deposited on the surface of the films. Al/HfO2/SiO2/n-Si capacitors were fabricated on the Si wafers using a shadow mask in a vacuum. Using these capacitors, the *I-V* characteristics, i.e., the current vs electric field relationships, were investigated for the "HCOOH sol" and "HNO3 sol" HfO2 thin films fired at 350, 450, 550, and 700 oC in air.

(a) (b) Fig. 19. *I-V* characteristics of HfO2 thin films fired at 350, 450, 550, and 700 oC in air using

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100

Current density (A/cm2

)


550 <sup>o</sup> C

700 <sup>o</sup> C

550 <sup>o</sup> C

450 <sup>o</sup> C

350 <sup>o</sup> C 700 <sup>o</sup> C

> 450 450 ℃ o C

350 <sup>o</sup> C

Electric Field (MV/cm)

The absolute values of the reverse bias are plotted in Figure 19. For the "HCOOH sol" HfO2 thin films, a small bias dependence on firing temperature was detected for the forward bias condition. In contrast, for the reverse bias condition, the smallest leakage current was observed at a firing temperature of 550 oC (amorphous film) for which the leakage current was ~10-7 A/cm2 in an electric field of -2 MV/cm. These data indicate that the amorphous film is more promising than the crystallized film as a gate insulator. The leakage current was comparable to previously reported results (Suzuki & Kato, 2007, 2009), but was smaller than that of a HfO2 film deposited using atomic layer deposition (Chiou et al., 2007). At 700 oC, crystallization roughens the layer structure of the film as described in section 4.2 and provides a short leakage path through grain boundaries (Chiou et al., 2007, Zhu et al., 2002). The *I-V* characteristics for both forward and reverse biases in the Al/HfO2/SiO2/n-Si structure are unsymmetrical against bias voltages. This is probably because the potential barrier in the band diagram between the Al electrode and the HfO2 film and that between the HfO2 film and the SiO2 film may differ between forward and reverse bias conditions. For the reverse bias condition, the energy slope of each band diagram of the Al/HfO2/SiO2/n-Si structure goes upwards and the difference between the work function of Al and the electron affinity of HfO2 presumably creates a potential barrier against the flow of carriers. Therefore, the flow of electrons or holes may be suppressed by the barrier, resulting in

both (a) "HCOOH sol" and (b) "HNO3 sol (Shimizu et al., 2010)".

700 <sup>o</sup> C 550 <sup>o</sup> C

350 <sup>o</sup> C


450 <sup>o</sup> C

of O can occur from the nanopores of the HfO2 film via the reaction OH + OH→H2O + O. In addition, the H2O desorption of chemically adsorbed Hf-OH may occur by the abovementioned reaction ( ≡ Hf-OH +HO-Hf ≡ → ≡ Hf-O-Hf ≡ +H2O).

HCOOH and/or HNO3 adsorbs or bonds on the surface of the sol. In the "HCOOH sol", HCOOH ions form a bridge structure coordinated with Hf ions (Nishide et al., 2004). In the TPD measurements performed after the firing process, HCOOH and HNO3 were found to desorb at less than 350 oC. Thus, "HCOOH sol" and "HNO3 sol" HfO2 films in the amorphous state without an acid exist stably on Si wafers above 350 oC. This result may affect the *I-V* characteristics, as will be discussed later.

On the basis of the foregoing results, a speculative schematic diagram of physically adsorbed H2O and the chemically adsorbed Hf-OH attached to a sol-gel-derived HfO2 film is shown in Fig. 18. The gaps in the figure correspond to nanopores.

Fig. 18. Speculative schematic diagram of physically adsorbed H2O and chemically adsorbed Hf-OH attached within the sol-gel-derived HfO2 film (Shimizu et al., 2010).

of O can occur from the nanopores of the HfO2 film via the reaction OH + OH→H2O + O. In addition, the H2O desorption of chemically adsorbed Hf-OH may occur by the

HCOOH and/or HNO3 adsorbs or bonds on the surface of the sol. In the "HCOOH sol", HCOOH ions form a bridge structure coordinated with Hf ions (Nishide et al., 2004). In the TPD measurements performed after the firing process, HCOOH and HNO3 were found to desorb at less than 350 oC. Thus, "HCOOH sol" and "HNO3 sol" HfO2 films in the amorphous state without an acid exist stably on Si wafers above 350 oC. This result may

On the basis of the foregoing results, a speculative schematic diagram of physically adsorbed H2O and the chemically adsorbed Hf-OH attached to a sol-gel-derived HfO2 film

Fig. 18. Speculative schematic diagram of physically adsorbed H2O and chemically adsorbed

Hf-OH attached within the sol-gel-derived HfO2 film (Shimizu et al., 2010).

abovementioned reaction ( ≡ Hf-OH +HO-Hf ≡ → ≡ Hf-O-Hf ≡ +H2O).

affect the *I-V* characteristics, as will be discussed later.

is shown in Fig. 18. The gaps in the figure correspond to nanopores.

#### **6.5 Electrical characterization of both "HCOOH sol" and "HNO3 sol" HfO2 thin films on Si(001) wafers**

To measure the electrical characteristics (i.e., *I-V* and *C-V* characteristics) of both "HCOOH sol" and "HNO3 sol" HfO2 thin films on Si(001) wafers, a 0.4-mm-diameter aluminum (Al) electrode was deposited on the surface of the films. Al/HfO2/SiO2/n-Si capacitors were fabricated on the Si wafers using a shadow mask in a vacuum. Using these capacitors, the *I-V* characteristics, i.e., the current vs electric field relationships, were investigated for the "HCOOH sol" and "HNO3 sol" HfO2 thin films fired at 350, 450, 550, and 700 oC in air.

Fig. 19. *I-V* characteristics of HfO2 thin films fired at 350, 450, 550, and 700 oC in air using both (a) "HCOOH sol" and (b) "HNO3 sol (Shimizu et al., 2010)".

The absolute values of the reverse bias are plotted in Figure 19. For the "HCOOH sol" HfO2 thin films, a small bias dependence on firing temperature was detected for the forward bias condition. In contrast, for the reverse bias condition, the smallest leakage current was observed at a firing temperature of 550 oC (amorphous film) for which the leakage current was ~10-7 A/cm2 in an electric field of -2 MV/cm. These data indicate that the amorphous film is more promising than the crystallized film as a gate insulator. The leakage current was comparable to previously reported results (Suzuki & Kato, 2007, 2009), but was smaller than that of a HfO2 film deposited using atomic layer deposition (Chiou et al., 2007). At 700 oC, crystallization roughens the layer structure of the film as described in section 4.2 and provides a short leakage path through grain boundaries (Chiou et al., 2007, Zhu et al., 2002). The *I-V* characteristics for both forward and reverse biases in the Al/HfO2/SiO2/n-Si structure are unsymmetrical against bias voltages. This is probably because the potential barrier in the band diagram between the Al electrode and the HfO2 film and that between the HfO2 film and the SiO2 film may differ between forward and reverse bias conditions. For the reverse bias condition, the energy slope of each band diagram of the Al/HfO2/SiO2/n-Si structure goes upwards and the difference between the work function of Al and the electron affinity of HfO2 presumably creates a potential barrier against the flow of carriers. Therefore, the flow of electrons or holes may be suppressed by the barrier, resulting in

Characterization of Sol-Gel-Derived and Crystallized

(Ragnarsson et al., 2009).

gate insulator material.

Intensity (arb.unit)

**Si(001) wafers** 

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 335

requires a relatively higher permittivity and a smaller film thickness to achieve a reasonable EOT for highly integrated CMOS devices. For EOT scaling, the necessity of suppressing the liberation of H2O from the HfO2 film at Si oxidation temperatures has been emphasized

The reported *C-V* curves in Fig. 20 show a small reduction with increasing frequency. The relative permittivity decreases with increasing growth temperature of the high-*k* film (ZrO2) and frequency (Kukli et al., 2001, 2002). In general, the relative permittivity is essentially governed by the polarization of the material, and decreases with increasing frequency. In the present sol-gel-derived HfO2 films, H2O, OH groups in the nanopores, and other impurities probably caused electronic and ionic polarizations, thereby giving rise to the possibility of the frequency dependence of the capacitance. One possible way of refining the electrical performance of sol-gel-derived HfO2 films is to use a firing environment of oxygen, inert gas, or forming gas. Thus, the amount of H2O, defects, and impurities in solgel-derived HfO2 films must be reduced to make the films applicable as a semiconductor

**7. Characterization of sol-gel-derived and crystalline ZrO2 thin films on** 

XRD patterns were obtained for sol-gel-derived ZrO2 films on Si(001) wafers fired at 450, 550, and 700 °C for 30 min (Figure 21). For the ZrO2 film fired at 450 °C, a halo-like pattern

monoclinic

Si

Si

Si

450 <sup>o</sup> C

550 o C

700 <sup>o</sup> C

monoclinic tetragonal

20 25 30 35 40 2θ (deg)

Fig. 21. XRD patterns obtained for ZrO2 films on Si fired at 450, 550, and 700 °C for 30 min. The XRD pattern for the Si substrate is also shown for reference (Shimizu et al., 2009).

**7.1 Crystallinity of sol-gel-derived ZrO2 thin films on Si(001) wafers** 

current lower than that in the forward bias condition in which a potential barrier may not exist.

The unsymmetrical *I-V* characteristics are true in the "HNO3 sol" case. The smallest leakage current in the "HNO3 sol" HfO2 thin films was seen for the amorphous films fired at 450 oC, which might be attributable to the smooth surface structure of the film. At 450 oC, the H2O in the HfO2 thin film desorbed less compared with that in the amorphous film fired at 350 oC. Therefore, there is some possibility for sol-gel-derived HfO2 thin films to be used as alternative high-*k* materials for gate insulators in CMOS devices; however, the amount of H2O should be reduced to a minimum (Ragnarsson et al., 2009).

Fig. 20. *C-V* curves for Al/HfO2/SiO2/n-Si capacitors with HfO2 films using (a) "HCOOH sol" at a firing temperature of 550 oC and (b) "HNO3 sol" at 450 oC (Shimizu et al., 2010).

The *C-V* curves for Al/HfO2/SiO2/n-Si capacitors were examined in relation to the "HCOOH sol" HfO2 film fired at 550 oC and to the "HNO3 sol" HfO2 film fired at 450 oC, respectively. The *C-V* curves are plotted in Figure 20 from – 2 to 2 V, representing the practical range for device operation. The *C-V* curves show a well-defined transition from depletion and inversion to accumulation as the applied voltage was varied from – 2 to 2 V, similar to the *C-V* curves for normal Al/SiO2/Si capacitors (Nicollian & Brews, 1981). The *C-V* characteristics do not show any dependence on firing temperature, but the capacitance decreases with increasing frequency. On the basis of the well-defined capacitance in the plotting of a *C-V* curve at a frequency of 100 kHz, the relative permittivity *ε*HfO2 of the "HCOOH sol" HfO2 film was calculated to be 11, with an effective oxide thickness (EOT) of 2.1 nm (HfO2 film thickness: 7.4 nm). The SiO2 film thickness was 2 nm, so the relative permittivity *ε*HfO2 was calibrated using that of SiO2. The relative permittivity was much higher than that of silicon dioxide (SiO2, 3.9), but is comparable to previously reported results (10~11) (Suzuki & Kato, 2009). The difference in the relative permittivity *ε*HfO2 between the sol-gel HfO2 film and bulk HfO2 may be due to the presence of the SiO2 film and nanopores in the HfO2 film.

For the "HNO3 sol" HfO2 film, the relative permittivity was calculated to be 11 and the EOT was 3.9 nm (HfO2 film thickness: 10.9 nm). The "HCOOH sol" HfO2 film is promising, but it

current lower than that in the forward bias condition in which a potential barrier may not

The unsymmetrical *I-V* characteristics are true in the "HNO3 sol" case. The smallest leakage current in the "HNO3 sol" HfO2 thin films was seen for the amorphous films fired at 450 oC, which might be attributable to the smooth surface structure of the film. At 450 oC, the H2O in the HfO2 thin film desorbed less compared with that in the amorphous film fired at 350 oC. Therefore, there is some possibility for sol-gel-derived HfO2 thin films to be used as alternative high-*k* materials for gate insulators in CMOS devices; however, the amount of

(a) (b)


0


1 MHz

10 kHz

100 kHz

Voltage (V)

200

400

Capacitance (pF)

600

800

1000

The *C-V* curves for Al/HfO2/SiO2/n-Si capacitors were examined in relation to the "HCOOH sol" HfO2 film fired at 550 oC and to the "HNO3 sol" HfO2 film fired at 450 oC, respectively. The *C-V* curves are plotted in Figure 20 from – 2 to 2 V, representing the practical range for device operation. The *C-V* curves show a well-defined transition from depletion and inversion to accumulation as the applied voltage was varied from – 2 to 2 V, similar to the *C-V* curves for normal Al/SiO2/Si capacitors (Nicollian & Brews, 1981). The *C-V* characteristics do not show any dependence on firing temperature, but the capacitance decreases with increasing frequency. On the basis of the well-defined capacitance in the plotting of a *C-V* curve at a frequency of 100 kHz, the relative permittivity *ε*HfO2 of the "HCOOH sol" HfO2 film was calculated to be 11, with an effective oxide thickness (EOT) of 2.1 nm (HfO2 film thickness: 7.4 nm). The SiO2 film thickness was 2 nm, so the relative permittivity *ε*HfO2 was calibrated using that of SiO2. The relative permittivity was much higher than that of silicon dioxide (SiO2, 3.9), but is comparable to previously reported results (10~11) (Suzuki & Kato, 2009). The difference in the relative permittivity *ε*HfO2 between the sol-gel HfO2 film and bulk HfO2 may be due to

For the "HNO3 sol" HfO2 film, the relative permittivity was calculated to be 11 and the EOT was 3.9 nm (HfO2 film thickness: 10.9 nm). The "HCOOH sol" HfO2 film is promising, but it

Fig. 20. *C-V* curves for Al/HfO2/SiO2/n-Si capacitors with HfO2 films using (a) "HCOOH sol" at a firing temperature of 550 oC and (b) "HNO3 sol" at 450 oC

the presence of the SiO2 film and nanopores in the HfO2 film.

H2O should be reduced to a minimum (Ragnarsson et al., 2009).

1 MHz

10 kHz

100 kHz


Voltage (V)

exist.

(Shimizu et al., 2010).


0

200

400

Capacitance (pF)

600

800

1000

requires a relatively higher permittivity and a smaller film thickness to achieve a reasonable EOT for highly integrated CMOS devices. For EOT scaling, the necessity of suppressing the liberation of H2O from the HfO2 film at Si oxidation temperatures has been emphasized (Ragnarsson et al., 2009).

The reported *C-V* curves in Fig. 20 show a small reduction with increasing frequency. The relative permittivity decreases with increasing growth temperature of the high-*k* film (ZrO2) and frequency (Kukli et al., 2001, 2002). In general, the relative permittivity is essentially governed by the polarization of the material, and decreases with increasing frequency. In the present sol-gel-derived HfO2 films, H2O, OH groups in the nanopores, and other impurities probably caused electronic and ionic polarizations, thereby giving rise to the possibility of the frequency dependence of the capacitance. One possible way of refining the electrical performance of sol-gel-derived HfO2 films is to use a firing environment of oxygen, inert gas, or forming gas. Thus, the amount of H2O, defects, and impurities in solgel-derived HfO2 films must be reduced to make the films applicable as a semiconductor gate insulator material.
