**6. Characterization of sol-gel-derived HfO2 thin films on Si(001) wafers dependent on sol solution: "HCOOH sol" and "HNO3 sol" HfO2 thin films**

#### **6.1 Crystallization temperature dependent on sol solution of HfO2 thin films on Si(001) wafers**

In sol-gel-derived HfO2 thin films on Si(001) wafers, the crystallization temperature depends on the composition of the sol solution. Upon preparing the sol solution, (a) a formic acid (HCOOH) or (b) a nitric acid (HNO3) is used as the catalyst for Hf(OH)4 to form a soluble sol in H2O, resulting in a hafnia sol solution.

Fig. 12. (a) XRD patterns of "HCOOH sol" HfO2 films on Si fired at 560 and 570 oC and (b) XRD patterns of "HNO3 sol" HfO2 films on Si fired at 460 and 470 oC (Shimizu et al., 2010).

In the X-ray pattern for "HCOOH sol" HfO2 films fired at temperatures below 560 °C, only halo patterns representing the amorphous state were observed. At 560 °C [Fig. 12(a)], a small diffraction peak was observed at 2*θ* = 28° in the halo pattern. At 570 °C, the diffraction peak at 2*θ* = 28° became clearer and higher, indicating that partial crystallization from the amorphous state commenced at 560 °C. The observed diffraction peak was identified to correspond to monoclinic (1- 11) (JCPDS card) and full crystallization was attained at 700 °C.

In contrast, "HNO3 sol" HfO2 films fired at 460 °C showed no diffraction peaks (only halo patterns were observed). At 470 °C, two diffraction peaks were identified, corresponding to the monoclinic structures (111 ) and (111) according to the JCPDS card [Fig. 12(b)]. By using

until 350 °C in the first TPD measurement. In contrast, in process ③, the small protrusions between 100 and 200 °C appeared again. This result provides evidence that the small peaks (small protrusions) were caused by adsorption of H2O immediately after the samples were taken out of the furnace and that the amount of desorbed water (i.e., adsorbed water) saturated during the exposure time. Thus, the small protrusions in the TPD curves can be

attributed to physisorbed H2O and/or chemisorbed Zr-OH bonds at the surface area.

**6. Characterization of sol-gel-derived HfO2 thin films on Si(001) wafers dependent on sol solution: "HCOOH sol" and "HNO3 sol" HfO2 thin films** 

**wafers** 

Intensity (arb. unit)

in H2O, resulting in a hafnia sol solution.

20 25 30 35 40 45 50

2θ (deg)

correspond to monoclinic (1-

**6.1 Crystallization temperature dependent on sol solution of HfO2 thin films on Si(001)** 

In sol-gel-derived HfO2 thin films on Si(001) wafers, the crystallization temperature depends on the composition of the sol solution. Upon preparing the sol solution, (a) a formic acid (HCOOH) or (b) a nitric acid (HNO3) is used as the catalyst for Hf(OH)4 to form a soluble sol

(a) (b) Fig. 12. (a) XRD patterns of "HCOOH sol" HfO2 films on Si fired at 560 and 570 oC and (b) XRD patterns of "HNO3 sol" HfO2 films on Si fired at 460 and 470 oC (Shimizu et al., 2010).

Intensity (arb.unit)

**570 o C**

**:monoclinic(-111)**

**560 <sup>o</sup> C**

In the X-ray pattern for "HCOOH sol" HfO2 films fired at temperatures below 560 °C, only halo patterns representing the amorphous state were observed. At 560 °C [Fig. 12(a)], a small diffraction peak was observed at 2*θ* = 28° in the halo pattern. At 570 °C, the diffraction peak at 2*θ* = 28° became clearer and higher, indicating that partial crystallization from the amorphous state commenced at 560 °C. The observed diffraction peak was identified to

In contrast, "HNO3 sol" HfO2 films fired at 460 °C showed no diffraction peaks (only halo patterns were observed). At 470 °C, two diffraction peaks were identified, corresponding to the monoclinic structures (111 ) and (111) according to the JCPDS card [Fig. 12(b)]. By using

11) (JCPDS card) and full crystallization was attained at 700 °C.

20 25 30 35 40 45 50

**470 <sup>o</sup> C**

:monoclinic(111) :monoclinic(-111)

> **460 <sup>o</sup> C**

2θ(deg)

a HNO3 solution as the catalyst, the crystallization temperature was reduced to less than 470 °C compared with 560 °C for the "HCOOH sol" HfO2 films. The lattice interplanar distances calculated using the Bragg equation were 0.319 and 0.286 nm, in contrast to the reported values of 0.314 and 0.288 nm, respectively (Shimizu et al., 2004). These results probably differed from the crystallization temperature for the monoclinic structure (1 - 11) (Nishide et al., 2000) because different sol solutions were employed in each case. The "HCOOH sol" HfO2 films remained in the amorphous state up to a higher temperature (560 °C) than the "HNO3 sol" films (crystallized at 470 oC). Based on TPD measurements, HCOOH and HNO3 desorb at temperatures below 350 oC, indicating that an intrinsic amorphous HfO2 film without using a catalyst for either film stably exists above 350 oC (Shimizu et al., 2010).
