**3. Sonochemistry: basic principles**

The use of ultrasound for accelerating chemical reactions in liquid–solid heterogeneous systems is very attractive since ultrasound is capable of increasing the reactivity by more than a factor of 10<sup>5</sup> due to the fact that the reagent particles clash at such a high speed that they melt at the point of collision and generate microscopic flames in cold liquids [1, 38]. In ultrasonically irradiated slurries, turbulent flow and shock waves are produced by acoustic cavitation [39] resulting in many tiny gas bubbles. The bubbles expand and contract in accordance with the pressure oscillations of the ultrasonic wave. When the bubble radius is of a certain size and the acoustic amplitude is above a given threshold value, the bubbles collapse violently and the temperature inside a bubble increases dramatically due to the quasiadiabatic compression [40]. At the final stage of the collapse, the vapor, which often is water vapor, dissociates inside the collapsing bubble due to the high bubble temperature. This generates H and OH radicals as well as other kinds of oxidants, which are assumed to produce a variety of chemical reactions [3, 41–45]. The reactions involve the formation of primary radicals from the ultrasound-initiated dissociation of water within a collapsing cavity as

$$(\text{H}\_2\text{O}())) \to \text{ H} + \text{OH},\tag{4}$$

where the brackets stand for the sonolysis of water. The intermediate hydroxyl and hydrogen radicals can form H2O2 and O2 products.

In aqueous media, these reactions occur in different regions surrounding the collapsed bubble. One of these regions is e.g. the interfacial liquid region between the cavitation bubbles and the bulk solution. The temperature in this region is lower than the one in the interior of the bubbles. The reaction is therefore a liquid phase reaction but the temperature is believed to be high enough to rupture chemical bonds. Apart from these oxidants, considerable concentrations of local hydroxyl radical have been reported [43, 46]. Another reactant region is the bulk solution. Here, the reaction between reactant molecules and OH or H takes place at ambient temperatures.

Since a quantitative analysis of the chemistry involved into the sonochemical reactions is yet difficult to perform [47], it is not certain whether or not the chemical effects indeed originate from acoustic cavitation. The implosive collapse of

### *Ultrasonic Processing of Si and SiGe for Photovoltaic Applications DOI: http://dx.doi.org/10.5772/intechopen.96939*

capped GexSi1–x/Si structure (curve 3 at time *t* ¼ 0) compared to that of bare GexSi1–x-on-Si islands (curve 2). This enlargement is even 5 to 10 times greater after

Presuming the use of effective hydrogenation, a-Si:H/c-Si heterojunction solar cells (HET) represent one of the most promising solar cell structures that enable high efficiencies due to high open-circuit values coming from the excellent passivation properties of a-Si:H combined with the beneficial effect of the *a*-Si:H/c-Si heterojunction on the built-in voltage and reduced charge carrier loss at the interface [28]. HET cells also have reduced costs compared with systems installed today

Moreover, in contrast to dielectric passivation materials such as SiO2 and amorphous Si nitride (a-SiNx:H) [29], a-Si:H is simultaneously suitable for good passivation and electrical conduction. However, the surface passivation quality worsens both at low and high processing temperatures because of the porous medium grown in the amorphous silicon phase at excess amounts of hydrogen [30] and growing crystalline Si film instead of forming a-Si:H [31, 32], respectively. As a consequence, discrepancy exists in the literature as to the passivation ability of a-Si:H [33–37].

The use of ultrasound for accelerating chemical reactions in liquid–solid hetero-

where the brackets stand for the sonolysis of water. The intermediate hydroxyl

In aqueous media, these reactions occur in different regions surrounding the collapsed bubble. One of these regions is e.g. the interfacial liquid region between the cavitation bubbles and the bulk solution. The temperature in this region is lower than the one in the interior of the bubbles. The reaction is therefore a liquid phase reaction but the temperature is believed to be high enough to rupture chemical bonds. Apart from these oxidants, considerable concentrations of local hydroxyl radical have been reported [43, 46]. Another reactant region is the bulk solution. Here, the reaction between reactant molecules and OH or H takes place at ambient temperatures. Since a quantitative analysis of the chemistry involved into the sonochemical

reactions is yet difficult to perform [47], it is not certain whether or not the

chemical effects indeed originate from acoustic cavitation. The implosive collapse of

H2OÞÞÞ ! H þ OH, (4)

geneous systems is very attractive since ultrasound is capable of increasing the reactivity by more than a factor of 10<sup>5</sup> due to the fact that the reagent particles clash at such a high speed that they melt at the point of collision and generate microscopic flames in cold liquids [1, 38]. In ultrasonically irradiated slurries, turbulent flow and shock waves are produced by acoustic cavitation [39] resulting in many tiny gas bubbles. The bubbles expand and contract in accordance with the pressure oscillations of the ultrasonic wave. When the bubble radius is of a certain size and the acoustic amplitude is above a given threshold value, the bubbles collapse violently and the temperature inside a bubble increases dramatically due to the quasiadiabatic compression [40]. At the final stage of the collapse, the vapor, which often is water vapor, dissociates inside the collapsing bubble due to the high bubble temperature. This generates H and OH radicals as well as other kinds of oxidants, which are assumed to produce a variety of chemical reactions [3, 41–45]. The reactions involve the formation of primary radicals from the ultrasound-initiated dissociation

subsequent sample annealing in O2 (curve 4 at time *t* ¼ 0).

based on conventional silicon technologies.

*Solar Cells - Theory, Materials and Recent Advances*

**3. Sonochemistry: basic principles**

of water within a collapsing cavity as

**140**

and hydrogen radicals can form H2O2 and O2 products.

bubbles during cavitation produces local transient temperatures of about 5000 K and pressures of about 500 atm, with heating and cooling rates exceeding 10<sup>10</sup> K/s [1, 38]. These conditions create high-velocity collisions between suspended particles and the estimated speed of colliding particles approaches almost the speed of sound in the liquid. Interestingly, sonochemical reactions in cavitational fields occur more slowly at elevated than at lower temperatures. Even so it is surprising, this counterintuitive property makes sense, because at higher temperatures more solvents vaporize into the bubble and, hence, cushion the collapse.

In cold liquids, ultrasound is able to drive reactions that normally occur only under extreme conditions. Examples [45] include intercalation, activation of liquid– solid reactions, and the synthesis of amorphous and nanophase materials. The sonochemical syntheses of nanophase metals, alloys, metal carbides, supported heterogeneous catalysts, and nano-colloids derives from the sonochemical decomposition of volatile organo-metallic precursors during cavitation, which produces clusters of a few nm in diameter.

Various types of sonochemical cells using ultrasound baths and ultrasound horn systems have been reported [48–50]. Most frequently, an ultrasonic transducer and the ultrasound horn are placed directly in the solution. One example is shown in **Figure 2**. An oscillating rf voltage from an oscillator (1 in **Figure 2**) with the amplitude of *U*<sup>0</sup> is amplified by an amplifier (2) and applied to a Langevin transducer, which consists of a piezoelectric transducer (3), back (4) and front (5) masses. The vibrating transducer is loaded to a glass flask (6) filled with a reactant solution (7) thus delivering an acoustic power at a resonance frequency of the transducer-solution system. The lowest-mode resonance frequency is defined by the solution thickness *h* such that *h* ¼ *λ=*2 with *λ* the sound wavelength in the solution at the resonant frequency.

The samples can freely levitate near the pressure antinode zone at operating frequencies in the dozens kHz range. They tend to reside on the flask bottom when increasing the frequency up to several hundreds of kHz.

#### **Figure 2.**

*Schematic of the sonochemical cell: 1 – Oscillator, 2 – Amplifier, 3 – Piezoelectric transducer, 4 – Back mass, 5 – Front mass, 6 – Boiling flask, 7 – Reactant solution, 8 – Sample.*
