**2. Sample preparation**

Continued device scaling for future technology nodes requires reduction in equivalent oxide thickness (EOT) of gate dielectrics to maintain electrostatic control of the charges induced in the channel. The use of amorphous SiO2 as a gate dielectric offers several key advantages in complementary metal-oxide semiconductor (CMOS) processing, including thermal and chemical stability as well as superior electrical isolation properties (high band gap of nearly 9 eV, and a Si–SiO2 potential barrier for electrons of about 3 eV). The continuous miniaturization of Si electronics has imposed severe constraints on the performance of the SiO2 gate oxide, with its thickness now approaching the quantum tunneling limit [1,2]. To continue the downward scaling, dielectrics with a higher dielectric constant (high-*k*) are being suggested as a solution to achieve the same transistor performance while maintaining a relatively thick physical thickness. Following this roadway, many materials systems (viz. lead–free non-ferroelectric) are currently under consideration as alternatives to conventional silicon oxide films as the gate dielectric material for sub-0.1 μm CMOS technology. Such an approach allows one to employ the best available materials for each phase, whose properties are known a priority due to the scarcity of high-*k* materials, to suit the desired application. Recent reports of giant dielectric constant have directed considerable attention to several new material systems, such as perovskite–related materials ACu3Ti4O12 (A = Ca, Bi2/3, Y2/3, La2/3) [3,4], La2/3Li*x*Ti1-*x*Al*x*O3 [5], Nd2O3 doped (1-*x*)Bi0.5Na0.5TiO3-*x*Bi0.5K0.5TiO3 [6], Fe-containing complex perovskites A(Fe1/2B1/2)O3 (A = Ba, Sr, Ca; B = Nb, Ta, Sb) [7,8], non-perovskite material Li0.05Ti0.02Ni0.93O [9], percolative BaTiO3-Ni composites [10], electron-doped manganites Ca1-*x*La*x*MnO3 and holedoped insulators La2Cu1-*x*Li*x*O4 and La2-*x*Sr*x*NiO4 [11–13]. The sensitivity of these complex oxides to strain, stoichiometry, phase heterogeneities, oxidation state, disorder, etc. can lead to drastic modifications in their magnetic and electric properties at the nanoscale. Besides that, as the key guidelines for replacing alternative dielectrics with high-*k* materials are required to (i) remain thermodynamically and chemically stable between the metal-oxide and Si substrate; (ii) kinetic stability against Si and the metal gate, in particular during high temperature processing and annealing; (iii) insulating properties: band offsets with Si over 1 eV to assure low leakage currents; (iv) a passivated, low-defect-density interface with Si to ensure large carrier mobility in the Si channel and good breakdown properties; and (v) interface quality between the high-*k* dielectrics and Si substrate: a low defect density in the high-*k* dielectric itself to prevent flat band and threshold voltage shifts and instabilities. Many dielectrics appear favorable in some of these areas, but very few materials are promising with respect to all of these guidelines. The ranking of HfO2-based system as a desired high-*k* gate dielectric material to replace amorphous SiO2 drops considerably, as HfO2 suffers crystallization at a relatively low process temperature (< 500°C), resulting high leakage current along the grain boundaries [14]. Therefore, the exploitation of new type of amorphous phase pure high-*k* gate dielectrics

176 Ferroelectric Materials – Synthesis and Characterization

candidates as a replacement of SiO2 still faces several daunting challenges.

Besides the aforementioned consideration, the superior electrical characteristics of the Si– SiO2 interface in ideal gate dielectric stack compatible with planarization technology has not achieved with any other alternative semiconductor–dielectric combination. Despite several key advantages of SiO2, the continual scaling of CMOS technologies has pushed the Si–SiO2 system in formidable challenge. One promising alternative approach to overcome the scaling limit has been proposed to substitute by silica-based single-valence nanoparticles (NPs) as gate The preparation of RE2O3:SiO2 nano-glass composite system (RE ~ La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu) consists of three consecutive processes: (a) preparation of wet gel in which rare earth ions were doped by sol–gel way, (b) drying of the gel and (c) densification of the dry gel to a dense glass in which RE2O3 NPs embedded by calcining at selective temperatures [15]. The process was based on the hydrolysis of precursors, such as tetraethylorthosilicate {Si(OC2H5)4} (TEOS) and subsequent condensation of hydrolyzed TEOS in a medium contain‐ ing a hydroalcoholic solution of rare earth salt [16] (Figure 1(a)) having different mol% concentrations following essentially the method developed by Sakka and Kamiya [17]. Water was required for the hydrolysis. The molar ratio of water and TEOS was kept at 20 while that of TEOS and catalyst HCl at 100. Dry ethanol was used for diluting the alkoxide. The following composition of the Si(OC2H5)4 solutions used in the study (Table 1):


**Table 1.** Compositions of the Si(OC2H5)4 (TEOS), H2O, and C2H5OH solutions used in the sol–gel process.

There are two distinct chemical reactions involved in the sol–gel process, describing Eqn. (1) for hydrolysis of the alcohol groups, Eqns. (2) and (3) for polycondensation of hydroxyl groups.

**a.** Hydrolysis:

$$\text{=Si-OR} + H\_2\text{O} \rightarrow \equiv \text{Si-OH} + \text{ROH} \tag{1}$$

**b.** Condensation (water/alcohol condensation):

**i.** Water condensation:

$$\text{H} \equiv \text{Si-OH} + \equiv \text{Si-OH} \rightarrow \equiv \text{Si-O-Si} \equiv + H\_2O \tag{2}$$

3

Alcohol condensation: Colossal dielectric and MD response of RE2O3 nanoparticles in SiO2 glass matrix

3 groups.

$$\equiv \text{Si-OH} + \equiv \text{Si-OR} \rightarrow \equiv \text{Si-O-Si} \equiv + \text{ROH} \tag{3}$$

The clear solutions without any precipitaion are prepared with the mixing of half amount of ethanol in alkoxide and the solution consisting of the specified amount of water with another half of the ethanol containing HCl and dopant. The mixure solutions continued stirring for 2– 3 hours at room temperature. The clear solution was kept in pyrex beaker at the atmospheric condition for 7/8 days to form stiff monolithic transparent gel. Further, the gels were allowed to dry for 4–5 weeks at room temperature. The dried (liquid removed by thermal evaporation) monolith is termed as xerogel. The oven–dried gel (temperature range 100–200°C) still contains large concentration of chemisorbed hydroxyls. Heat treatment in the temperature range 500– 800°C desorbs the hydroxyls, forming a stabilized gel. At 1000°C, it transformed to a dense glass. Heat treatments of samples were performed according to preselected calcination temperature schedule [16] (Figure 1(b)). 4 (a) Hydrolysis: ������ ��2� � ������ � ��� �1� 5 (b) Condensation (water/alcohol condensation): 6 (i) Water condensation: ������ � ������ � ��������� � �2� �2� 7 (ii) Alcohol condensation: ������ � ������ � ��������� � ��� ��� 8 The clear solutions without any precipitaion are prepared with the mixing of half amount of 9 ethanol in alkoxide and the solution consisting of the specified amount of water with another half of 10 the ethanol containing HCl and dopant. The mixure solutions continued stirring for 2–3 hours at room 11 temperature. The clear solution was kept in pyrex beaker at the atmospheric condition for 7/8 days to 12 form stiff monolithic transparent gel. Further, the gels were allowed to dry for 4–5 weeks at room 13 temperature. The dried (liquid removed by thermal evaporation) monolith is termed as xerogel. The 14 oven–dried gel (temperature range 100–200°C) still contains large concentration of chemisorbed 15 hydroxyls. Heat treatment in the temperature range 500–800°C desorbs the hydroxyls, forming a 16 stabilized gel. At 1000°C, it transformed to a dense glass. Heat treatments of samples were performed

17 according to preselected calcination temperature schedule [16] (Figure 1(b)).

19 calcination process. **Figure 1.** (Color online) (a) Sol–gel process. (b) Gel–glass embedded with rare earth nanoparticle calcination process.

18 Figure 1. (Color online) (a) Sol–gel process. (b) Gel–glass embedded with rare earth nanoparticle

It is relevant to mention here the important findings of Raman spectroscopic studies including measurements of pore size, density and specific surface area on the densification of undoped SiO2 gel as a function of heat treatment up to 900o C [18]. With increasing temperature from 700 to 800o C, the average pore size increases abruptly from 1.0 nm to 2.3 nm, whereas, the specific surface area decreases from 550 m2 /g to 160 m2 /g and the pore volume/gm decreases from 0.19 cc/g to 0.12 cc/g. The surface energy for a siloxane surface is higher than for a hydroxyl surface. The Si–OH groups condense to Si–O–Si bonds with increasing temperature, thus increasing the surface energy as well as enhancing pore collapse. In these rare earth elements doped gel-glass specimens, a large number of small pores collapse at 700o C and an agglomeration of individual RE3+ are set free after small pores collapse to form NPs. The remaining pores join to form larger pores. At still higher temperature (i.e., 800o C), collapse of larger pores also takes place with similar observation in undoped sample, which is indicated by rapid fall of pore volume/g from 0.12 cc/g to 0.026 cc/g in going from 800o C to 900o C [18]. Thus it is possible at the highest temperature in the present case (i.e., 1200o C), the rare earth oxide NPs grow to a maximum size because of complete annihilations of pores, leading to a disappearance of Si – OH groups to condensation of Si–O–Si bonds. To this end, we have systematically synthesized nano-glass composites systems with different doping concentrations of rare earth elements to prepare the sols, since the gelation process (rate of the hydrolysis and condensation reactions) strongly affect with varying the kind and/or the amount of the starting solvent and the final outcome of the preparation.
