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

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

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).

Characterization of Sol-Gel-Derived and Crystallized

+H2O).

Intensity (

(a)

(b)

×10-10A)

0.6

0.5

0.4

0.3

0.2

0.1

0

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 337

they were classified into three groups on the basis of the TPD results for SiO2 formed by chemical vapor deposition (Hirashita et al., 1993): α, small peaks (small protrusions) between 100 and 200 °C; β, major peaks between 200 and 350 °C; and γ, small sharp peaks at approximately 410 °C for the samples fired at 350 and 450 °C. The measured TPD curve of H2O had the main peak at a temperature of 260 oC with an unsymmetrical shape, providing

In a detailed analysis, the TPD curve for the sample fired at 350 °C was separated into five peak components using a Gaussian-type waveform (Figure 23). Component (a) is presumably due to physisorbed H2O (mere adsorption of H2O) on the surface of the ZrO2 thin films. This was confirmed experimentally as discussed in the next subsection. Component (e) can be attributed to the desorption of H2O through nanopores of the crystallized ZrO2 thin film. Component (b) can be ascribed to the desorption of H2O and/or chemisorbed Zr-OH bonds at the surface area. For components (c) and (d), H2O desorption may have occurred because of the following reaction ( ≡ Zr-OH +HO-Zr ≡ → ≡ Zr-O-Zr ≡

0 80 160 240 320 400 480 560 640

(c)

(d)

o C)

(e)

Heating Temperature (

Fig. 23. TPD curve for the sol-gel-derived ZrO2 thin film fired at 350 °C separated into five peak components using a Gaussian-type waveform as a function of the temperature measured with a thermocouple inside the TPD chamber (Shimizu et al., 2009).

The refractive indexes and film thicknesses were determined for sol-gel-derived ZrO2 films fired at temperatures from 350 to 700 °C (Figure 24). The refractive indexes converged at 2.0, which is in good agreement with deposited ZrO2 thin films (Moulder, 1995) and monoclinic

**7.3 Refractive indexes and film thicknesses of sol-gel-derived ZrO2 thin films** 

m/z 18 a b c d e

evidence that several desorbed components were present during heating.

was observed at approximately 2*θ* = 30°, indicating that the film was still amorphous (Liu et al. 2002, Shimizu et al., 2009). The diffraction peak of 33° is ascribed to the Si (001) wafer. At 550 °C, a new peak appeared at 2*θ* = 30.3°, which was determined to be tetragonal (011) (JCPDS card, Liu et al., 2002, Shimizu et al., 2010), and the lattice interplanar distance was calculated to be 0.295 nm. In addition, at 700°C, three peaks at 2*θ* = 28, 30.3, and 31.3° were observed. The two peaks at 2*θ* = 28 and 31.3° were determined to be monoclinic (1 - 11) and monoclinic (111), respectively, because the calculated lattice interplanar distances were 0.319 and 0.286 nm, which correspond to the reported values of 0.316 and 0.284 nm. The ZrO2 thin films fired at 700 °C consisted of a mixed crystal of tetragonal and monoclinic structures. Rapid temperature annealing (RTA) above 700 oC results in a mixture of monoclinic and tetragonal phases (Liu et al., 2002).

## **7.2 Spectral analyses of sol-gel-derived ZrO2 thin films by TPD**

Figure 22 shows the TPD curves of H2O (*m*/*z* = 18) that evolved from the sol-gel-derived ZrO2 thin films on Si, which were fired at 350, 450, 550, and 700 °C for 30 min. The vertical axis indicates the current value of QMS. The film thicknesses were determined to be 10.2, 9.9, 7.6, and 8.1 nm, respectively. The intensity of the TPD curves decreased as the firing temperature increased, indicating that the amount of H2O was reduced in the ZrO2 films on Si(001)wafers. Since the TPD curves were unsymmetrical against the heating temperature,

Fig. 22. TPD curves of H2O (*m/z* = 18) that evolved from sol-gel-derived ZrO2 thin films on Si fired at 350, 450, 550, and 700 °C for 30 min. The film thicknesses were 10.2, 9.9, 7.6, and 8.1 nm, respectively (Shimizu et al., 2009).

was observed at approximately 2*θ* = 30°, indicating that the film was still amorphous (Liu et al. 2002, Shimizu et al., 2009). The diffraction peak of 33° is ascribed to the Si (001) wafer. At 550 °C, a new peak appeared at 2*θ* = 30.3°, which was determined to be tetragonal (011) (JCPDS card, Liu et al., 2002, Shimizu et al., 2010), and the lattice interplanar distance was calculated to be 0.295 nm. In addition, at 700°C, three peaks at 2*θ* = 28, 30.3, and 31.3° were

monoclinic (111), respectively, because the calculated lattice interplanar distances were 0.319 and 0.286 nm, which correspond to the reported values of 0.316 and 0.284 nm. The ZrO2 thin films fired at 700 °C consisted of a mixed crystal of tetragonal and monoclinic structures. Rapid temperature annealing (RTA) above 700 oC results in a mixture of monoclinic and

Figure 22 shows the TPD curves of H2O (*m*/*z* = 18) that evolved from the sol-gel-derived ZrO2 thin films on Si, which were fired at 350, 450, 550, and 700 °C for 30 min. The vertical axis indicates the current value of QMS. The film thicknesses were determined to be 10.2, 9.9, 7.6, and 8.1 nm, respectively. The intensity of the TPD curves decreased as the firing temperature increased, indicating that the amount of H2O was reduced in the ZrO2 films on Si(001)wafers. Since the TPD curves were unsymmetrical against the heating temperature,

β γ

350 ℃ (tZrO2=10.2nm)

450 ℃ (tZrO2=9.9nm)

> 550 ℃ (tZrO2=7.6nm) 700 ℃ (tZrO2=8.1nm)

> > o C)

100 200 300 400 500 600 700

Heating Temperature (

Fig. 22. TPD curves of H2O (*m/z* = 18) that evolved from sol-gel-derived ZrO2 thin films on Si fired at 350, 450, 550, and 700 °C for 30 min. The film thicknesses were 10.2, 9.9, 7.6, and


11) and

observed. The two peaks at 2*θ* = 28 and 31.3° were determined to be monoclinic (1

**7.2 Spectral analyses of sol-gel-derived ZrO2 thin films by TPD** 

tetragonal phases (Liu et al., 2002).

0

8.1 nm, respectively (Shimizu et al., 2009).

0.2

0.4

Intensity

(×10-10A)

0.6

0.8

1.0

H2O

α

they were classified into three groups on the basis of the TPD results for SiO2 formed by chemical vapor deposition (Hirashita et al., 1993): α, small peaks (small protrusions) between 100 and 200 °C; β, major peaks between 200 and 350 °C; and γ, small sharp peaks at approximately 410 °C for the samples fired at 350 and 450 °C. The measured TPD curve of H2O had the main peak at a temperature of 260 oC with an unsymmetrical shape, providing evidence that several desorbed components were present during heating.

In a detailed analysis, the TPD curve for the sample fired at 350 °C was separated into five peak components using a Gaussian-type waveform (Figure 23). Component (a) is presumably due to physisorbed H2O (mere adsorption of H2O) on the surface of the ZrO2 thin films. This was confirmed experimentally as discussed in the next subsection. Component (e) can be attributed to the desorption of H2O through nanopores of the crystallized ZrO2 thin film. Component (b) can be ascribed to the desorption of H2O and/or chemisorbed Zr-OH bonds at the surface area. For components (c) and (d), H2O desorption may have occurred because of the following reaction ( ≡ Zr-OH +HO-Zr ≡ → ≡ Zr-O-Zr ≡ +H2O).

Fig. 23. TPD curve for the sol-gel-derived ZrO2 thin film fired at 350 °C separated into five peak components using a Gaussian-type waveform as a function of the temperature measured with a thermocouple inside the TPD chamber (Shimizu et al., 2009).

### **7.3 Refractive indexes and film thicknesses of sol-gel-derived ZrO2 thin films**

The refractive indexes and film thicknesses were determined for sol-gel-derived ZrO2 films fired at temperatures from 350 to 700 °C (Figure 24). The refractive indexes converged at 2.0, which is in good agreement with deposited ZrO2 thin films (Moulder, 1995) and monoclinic

Characterization of Sol-Gel-Derived and Crystallized

plotted as absolute values (Shimizu et al., 2009).

gate insulators in densely packed CMOS devices.

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 339

Fig. 25. *I-V* characteristics ( i.e., current density vs electric field relationship) for sol-gelderived ZrO2 thin films fired at 350, 450, 550, and 700 oC in air. The reverse biases are

current at 550 oC was suppressed more than in the other films measured. Thus, there is some possibility for sol-gel-derived ZrO2 thin films to be used as an alternative high-*k* material of

To determine the relative permittivity *ε*ZrO2 of the sol-gel-derived ZrO2 films, the *C-V* curves of the Al/ZrO2/n-Si capacitors were obtained for the ZrO2 thin film fired at 550 oC for 30 min. The *C-V* curves are plotted in Figure 26 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* characteristics of normal Al/SiO2/Si capacitors (Nicollian & Brews, 1981). The *C-V* characteristics did not show any dependence on firing temperature, but the capacitance decreased with higher frequency. On the basis of the well-defined capacitances in the accumulation region of the *C-V* curves at a frequency of 100 kHz, the relative permittivity *ε*ZrO2 of the sol-gel-derived ZrO2 film was calculated to be 12 and the EOT was 2.4 nm (ZrO2 film thickness: 7.4 nm). The relative permittivity was higher than that of silicon dioxide (SiO2; 3.9) and the EOT was comparable to previously reported results (~2.5 nm) (Chim et al., 2003). The relative permittivity of ZrO2 formed by atomic layer deposition has been reported to be 23 (Niinisto et al., 2004). The *C-V* curves decline slightly with increasing frequency. The relative permittivity decreases with the growth temperature of ZrO2 thin films and increasing frequency (Kukli et al., 2001). Relative permittivity is essentially governed by the polarization of the material, so it decreases as the frequency increases. In the sol-gel-derived ZrO2 film, H2O, OH groups in nanopores and other impurities probably induced electronic and ionic polarizations, so there is the possibility of the frequency dependence of capacitance. To refine the electrical performance of sol-gel-derived ZrO2 films, an alternative firing environment such as

Fig. 24. Refractive indexes and film thicknesses of sol-gel derived ZrO2 films at firing temperatures from 350 to 700 °C (Shimizu et al., 2009).

ZrO2 crystals (Niinisto et al., 2004). The packing densities of the ZrO2 films were calculated using the Lorentz-Lorentz equation (1) (Nishide et al., 2001). The refractive indexes were 1.62 at 350 °C, 1.70 at 450 °C, 2.01 at 550 °C, and 2.00 at 700 °C. The basic refractive index of the ZrO2 crystal (monoclinic) for calculating the packing density was 2.22 (Yamada et al., 1988). Using this value, the packing densities were estimated to be 0.62 at 350 °C, 0.68 at 450 °C, 0.89 at 550 °C and 0.88 at 700 °C. The packing density of the films increased with increasing firing temperature. This is because more H2O desorbed at higher firing temperatures and the small gaps of the nanopores were squeezed or evaporated.

#### **7.4 Electrical characteristics of sol-gel-derived ZrO2 films on Si(001) wafers**

The *I-V* characteristics (current density vs electric field) were examined for sol-gel-derived ZrO2 thin films on Si(001) wafers fired at 350, 450, 550, and 700 oC in air (Figure 25). For the sample fired at 550 oC, the leakage current was smaller than that of the amorphous ZrO2 thin films fired at 350 and 450 oC. Leakage current deterioration was partially due to the considerable amount of H2O in the film, but at 700 oC, crystallization was completed, and small surface cracks and surface relief observed with the AFM were responsible for the deterioration. The leakage current (forward bias) for the sample fired at 550 oC was approximately 4 ×10-3 A/cm2 in an electric field of 1 M/cm, which is one or two orders of magnitude higher than that previously obtained (Chim et al., 2003). This difference is due to the densely compacted ZrO2 thin film (Chim et al., 2003). For reverse bias, the leakage

Oxide thickness Refractive index

350 400 450 500 550 600 650 700

ZrO2 crystals (Niinisto et al., 2004). The packing densities of the ZrO2 films were calculated using the Lorentz-Lorentz equation (1) (Nishide et al., 2001). The refractive indexes were 1.62 at 350 °C, 1.70 at 450 °C, 2.01 at 550 °C, and 2.00 at 700 °C. The basic refractive index of the ZrO2 crystal (monoclinic) for calculating the packing density was 2.22 (Yamada et al., 1988). Using this value, the packing densities were estimated to be 0.62 at 350 °C, 0.68 at 450 °C, 0.89 at 550 °C and 0.88 at 700 °C. The packing density of the films increased with increasing firing temperature. This is because more H2O desorbed at higher firing

The *I-V* characteristics (current density vs electric field) were examined for sol-gel-derived ZrO2 thin films on Si(001) wafers fired at 350, 450, 550, and 700 oC in air (Figure 25). For the sample fired at 550 oC, the leakage current was smaller than that of the amorphous ZrO2 thin films fired at 350 and 450 oC. Leakage current deterioration was partially due to the considerable amount of H2O in the film, but at 700 oC, crystallization was completed, and small surface cracks and surface relief observed with the AFM were responsible for the deterioration. The leakage current (forward bias) for the sample fired at 550 oC was approximately 4 ×10-3 A/cm2 in an electric field of 1 M/cm, which is one or two orders of magnitude higher than that previously obtained (Chim et al., 2003). This difference is due to the densely compacted ZrO2 thin film (Chim et al., 2003). For reverse bias, the leakage

Firing Temperature (

Fig. 24. Refractive indexes and film thicknesses of sol-gel derived ZrO2 films at firing

temperatures and the small gaps of the nanopores were squeezed or evaporated.

**7.4 Electrical characteristics of sol-gel-derived ZrO2 films on Si(001) wafers** 

1.5

o C) 1.6

1.7

1.8

Refractive Index

1.9

2

2.1

2.2

6

temperatures from 350 to 700 °C (Shimizu et al., 2009).

7

ZrO

2 Thickness (nm)

8

9

10

11

Fig. 25. *I-V* characteristics ( i.e., current density vs electric field relationship) for sol-gelderived ZrO2 thin films fired at 350, 450, 550, and 700 oC in air. The reverse biases are plotted as absolute values (Shimizu et al., 2009).

current at 550 oC was suppressed more than in the other films measured. Thus, there is some possibility for sol-gel-derived ZrO2 thin films to be used as an alternative high-*k* material of gate insulators in densely packed CMOS devices.

To determine the relative permittivity *ε*ZrO2 of the sol-gel-derived ZrO2 films, the *C-V* curves of the Al/ZrO2/n-Si capacitors were obtained for the ZrO2 thin film fired at 550 oC for 30 min. The *C-V* curves are plotted in Figure 26 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* characteristics of normal Al/SiO2/Si capacitors (Nicollian & Brews, 1981). The *C-V* characteristics did not show any dependence on firing temperature, but the capacitance decreased with higher frequency. On the basis of the well-defined capacitances in the accumulation region of the *C-V* curves at a frequency of 100 kHz, the relative permittivity *ε*ZrO2 of the sol-gel-derived ZrO2 film was calculated to be 12 and the EOT was 2.4 nm (ZrO2 film thickness: 7.4 nm). The relative permittivity was higher than that of silicon dioxide (SiO2; 3.9) and the EOT was comparable to previously reported results (~2.5 nm) (Chim et al., 2003). The relative permittivity of ZrO2 formed by atomic layer deposition has been reported to be 23 (Niinisto et al., 2004). The *C-V* curves decline slightly with increasing frequency. The relative permittivity decreases with the growth temperature of ZrO2 thin films and increasing frequency (Kukli et al., 2001). Relative permittivity is essentially governed by the polarization of the material, so it decreases as the frequency increases. In the sol-gel-derived ZrO2 film, H2O, OH groups in nanopores and other impurities probably induced electronic and ionic polarizations, so there is the possibility of the frequency dependence of capacitance. To refine the electrical performance of sol-gel-derived ZrO2 films, an alternative firing environment such as

Characterization of Sol-Gel-Derived and Crystallized

**6**

**7**

**8**

**9**

**Thickness (nm)**

(Peters et al., 2009).

**10**

**11**

**12**

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 341

Thickness Refractive index

**300 400 500 600 700 800**

**Firing Temperature (<sup>o</sup>**

Fig. 27. Film thicknesses and refractive indices of sol-gel-derived ZrO2-Y2O3 films fired at

**8.2 AFM-observed surface morphologies of sol-gel-derived ZrO2-Y2O3 thin films** 

The surface microstructures of ZrO2-Y2O3 thin films fired at 350 and 700 oC were observed with the AFM [Figs. 28(a) and 2(b)]. The morphology depended on the firing temperature. The surface of the ZrO2-Y2O3 thin film fired at 350 oC showed a homogeneous structure [Fig. 28(a)]. The RMS surface roughness was 0.15 nm at 350 oC. The RMS value at 700 oC was 0.24 nm and the surface structure was slightly wavy, but it did not show grain boundaries and/or cracks caused by crystallization [Fig. 28(b)]. Similar results have been reported for crack-free nano- and microcrystalline ZrO2-Y2O3 thin films deposited on sapphire substrates

**8.3 Electrical characteristics of sol-gel-derived ZrO2-Y2O3 thin films on Si(001) wafers**  The *I-V* characteristics (current density vs electric field) were investigated for sol-gelderived ZrO2-Y2O3 thin films fired at 350 and 700 oC in air, in comparison with those obtained for sol-gel-derived ZrO2 thin films [Figure 29(a) and 29(b)]. The reverse bias

350, 450, 550 and 700 °C for 30 min (Shimizu & Nishide, 2011).

**0.9**

**C)**

**1.0**

**1.1**

**1.2**

**Refractive index**

**1.3**

**1.4**

**1.5**

Fig. 26. *C-V* curves for Al/ZrO2 /n-Si capacitors, showing a well-defined transition from depletion and inversion to accumulation as a function of the applied voltage. The firing temperature of the ZrO2 film was 550 °C for 30 min (Shimizu et al., 2009).

oxygen, inert gas, or forming gas must be used. Thus, there is some possibility for applying sol-gel-derived ZrO2 thin films as a semiconductor gate insulator material. To fabricate improved ZrO2 films, further experiments should be conducted to find an effective way of reducing impurities. Sol-gel-derived Y doped ZrO2(ZrO2-Y2O3 ) thin films on Si(001) wafers are also promising.
