**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 (Peters et al., 2009).

### **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

Characterization of Sol-Gel-Derived and Crystallized

in the amount of H2O in the film.

(Shimizu & Nishide 2011).

et al., 2004).

**8.4 TPD analyses of sol-gel-derived ZrO2-Y2O3 thin films** 

and 400 oC are attributed to equipment noise.

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 343

quantities are plotted as absolute values. The leakage current of the Al/ZrO2-Y2O3/Si capacitors was approximately five orders of magnitude lower than that of the ZrO2 thin films for forward bias at an electric field of 2 MV/cm and three orders of magnitude lower for reverse bias at -2 MV/cm, respectively (Shimizu & Nishide, 2011). This improvement of the leakage current is noteworthy. For the sample fired at 700 oC, a similar reduction was observed for the Al/ZrO2-Y2O3/Si capacitor. This is because the lower surface roughness and crack-free state of the ZrO2-Y2O3 film surface may reduce the leakage current in comparison with the ZrO2 thin films as described in subsection 5.4. For the ZrO2-Y2O3 thin films fired between 350 and 700 oC, the leakage current of the latter was two orders of magnitude smaller than that of the former [Fig. 29(b)]. This is probably due to the film quality caused by crystallization such as packing density and/or a considerable difference

The leakage current (forward bias) for the sample fired at 700 oC was approximately 5×10-7 A/cm2 in an electric field of 1 M/cm (Shimizu & Nishide, 2011), which is one or two orders of magnitude lower than previously reported results (Chim et al., 2003). The latter results may be for densely compacted ZrO2 thin films, because they were fabricated by sputtering in an argon-plus-oxygen gas ambient and annealed at 400 oC in a nitrogen ambient for 5 min. For reverse bias, the leakage current at 700 oC was superior to that of the other measured films. Therefore, there is some possibility for sol-gel-derived ZrO2- Y2O3 thin films to be used as an alternative high-*k* material for gate insulators in miniaturized CMOS devices. However, the film quality must be improved further

TPD was used to investigate the desorption of H2O (*m*/*z* = 18) that evolved from sol-gelderived ZrO2-Y2O3 thin films on Si(001) wafers, which were fired at 350 and 700 °C for 30 min (Figure 30). The vertical axis indicates the current value of QMS. The film thicknesses were 11.1 and 6.9 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-Y2O3 thin films on Si(001) wafers. For the ZrO2-Y2O3 thin film fired at 350 oC, the peaks seen at 370

Two TPD curves are close to those of ZrO2 thin films (Shimizu & Nishide, 2011), except that the sample fired at 350 oC does not show any similar protrusions between 100 and 200 oC like those seen for the ZrO2 thin film (Figure 22) (Shimizu et al., 2009). The peak was separated into several components using a Gaussian-type waveform (Figure 23), and the waveform indicated by the dashed line is shown as a function of temperature (Figure 30). The desorption temperature of the main peak of the ZrO2-Y2O3 thin film was approximately between 100 and 200 oC. This implies that the TPD peak may be due to physisorbed H2O (mere adsorption of H2O). In contrast, at 700 oC, the TPD curves for H2O desorption are similar in shape to that of the ZrO2 thin film. The peak from 100 to 200 oC is due to the adsorption of physisorbed H2O and the main peak at approximately 250 oC is caused by Zr-OH (chemisorbed) (Nishide et al., 2005, Takahashi & Nishide, 2004). The relative permittivity of ZrO2 formed by atomic layer deposition has been reported to be 23 (Niinisto

Fig. 28. AFM images of the surface microstructures of ZrO2-Y2O3 thin films fired at (a) 350 and (b) 700 oC (Shimizu & Nishide, 2011).

Fig. 29. *I-V* characteristics (i.e., current density vs electric field relationship) for sol-gelderived ZrO2-Y2O3 thin films fired at (a) 350 and (b) 700 oC in air, respectively, in comparison with those reported for sol-gel-derived ZrO2 thin films (Shimizu & Nishide, 2011).

(a) (b)

(a) (b)

(a) (b)

**10-10**

**-6 -4 -2 0 2 4 6**

**Electric Field (MV/cm)**

**10-8**

**10-6**

**ZrO2 -Y2 O3**

**ZrO2**

**Current density (A/cm2**

**)**

**10-4**

**10-2**

**10<sup>0</sup>**

Fig. 29. *I-V* characteristics (i.e., current density vs electric field relationship) for sol-gelderived ZrO2-Y2O3 thin films fired at (a) 350 and (b) 700 oC in air, respectively, in

comparison with those reported for sol-gel-derived ZrO2 thin films

**-6 -4 -2 0 2 4 6**

**Electric Field (MV/cm)**

(Shimizu & Nishide, 2011).

**ZrO2 -Y2 O3**

**ZrO2**

**10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10<sup>0</sup>**

**Current density (A/cm2**

**)**

Fig. 28. AFM images of the surface microstructures of ZrO2-Y2O3 thin films fired at

100 nm 100 nm

(a) 350 and (b) 700 oC (Shimizu & Nishide, 2011).

quantities are plotted as absolute values. The leakage current of the Al/ZrO2-Y2O3/Si capacitors was approximately five orders of magnitude lower than that of the ZrO2 thin films for forward bias at an electric field of 2 MV/cm and three orders of magnitude lower for reverse bias at -2 MV/cm, respectively (Shimizu & Nishide, 2011). This improvement of the leakage current is noteworthy. For the sample fired at 700 oC, a similar reduction was observed for the Al/ZrO2-Y2O3/Si capacitor. This is because the lower surface roughness and crack-free state of the ZrO2-Y2O3 film surface may reduce the leakage current in comparison with the ZrO2 thin films as described in subsection 5.4. For the ZrO2-Y2O3 thin films fired between 350 and 700 oC, the leakage current of the latter was two orders of magnitude smaller than that of the former [Fig. 29(b)]. This is probably due to the film quality caused by crystallization such as packing density and/or a considerable difference in the amount of H2O in the film.

The leakage current (forward bias) for the sample fired at 700 oC was approximately 5×10-7 A/cm2 in an electric field of 1 M/cm (Shimizu & Nishide, 2011), which is one or two orders of magnitude lower than previously reported results (Chim et al., 2003). The latter results may be for densely compacted ZrO2 thin films, because they were fabricated by sputtering in an argon-plus-oxygen gas ambient and annealed at 400 oC in a nitrogen ambient for 5 min. For reverse bias, the leakage current at 700 oC was superior to that of the other measured films. Therefore, there is some possibility for sol-gel-derived ZrO2- Y2O3 thin films to be used as an alternative high-*k* material for gate insulators in miniaturized CMOS devices. However, the film quality must be improved further (Shimizu & Nishide 2011).
