**1. Introduction**

The chemical effect of ultrasonic waves derives primarily from hot spots formed during acoustic cavitation in a chemical mixture. Due to locally achieved extreme conditions, an unusual chemical environment is attained in such experiments [1]. It is therefore not surprising that a growing interest in simple and cheap sonochemical syntheses of materials is observed, particularly in nanophases [2–4]. Given the fact that ultrasonic irradiation, or sonication, of reaction mixtures is easily controllable, sonochemical fabrication of high-quality materials becomes a particularly interesting subject. One of the goals of this article is to discuss the recent excitement about sonochemically modified semiconductor materials.

Quite recently, sonochemical techniques have been used in processing of semiconductor surfaces [5–14]. In this method, the main phenomenon is the acoustic cavitation, which enhances chemical reactions in a solution. The growth of cavitation bubbles occurs due to the diffusion of solute vapor in the volume of the bubbles. After the growth process, the bubbles collapse, breaking the chemical bonds on the material surface.

Over the last years different industries have therefore become increasingly drawn to sonochemistry because it provides a green and clean alternative to conventional technologies, particular in the areas of processing of silicon-based materials. The aim of this work is to provide a cohesive presentation of the related efforts. Two techniques related to ultrasonic cleaning of Si wafers and sonochemical modification of Si and SiGe surfaces in hydrocarbon solutions will be discussed. In both cases, the occurrence of cavitation and bubble implosion is necessary for ultrasonic cleaning and surface processing as it is known today. The use of higher ultrasonic frequencies to expand the range of ultrasonic cleaning and processing capabilities will be emphasized. Although exact mechanisms of an improved photoelectric behavior of Si-based micro- and nanostructures subjected to power ultrasound are not yet clarified in many cases, the likely scenarios behind the observed photovoltaic performances will be proposed to involve the surface chemistry of oxygen and hydrogen molecules as well hydrocarbon chains.

In this respect, various surface-passivation layers have been employed. In particular, SiO2 surface passivation coating layer has been proven to offer outstanding passivation [18–20]. In general, two oxide growth methods are employed termed as the dry and wet oxidation. Dry oxygen and water vapor are used in the former and latter cases, respectively. Dry oxidation typically forms thin oxide layers in practical structures due to perfect Si–SiO2 interface developed in this case. In turn, wet oxidation yields greater growth rates, which is necessary for depositing thicker SiO2

*Ultrasonic Processing of Si and SiGe for Photovoltaic Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.96939*

Furthermore, a hydrogenated amorphous silicon a-Si:H layer can saturate the dangling bonds by hydrogen termination [21–23]. The atomic hydrogen is also able to effectively passivate dangling bonds at the c-Si/SiO2 interface thus drastically suppressing the interface state density and surface recombination velocities [24]. An important aspect is that the treatment in HF can produce an inversion layer on p-type Si surfaces while an accumulation of majority carriers is observed on n-type Si after treatment in either NH4F or HF [25]. These both are due to positive charges induced by electronegative surface groups such as –H, –O–H and –F bonded to the

Most of the oxidizing solutions, e.g. H2SO4/H2O2, HCl, HNO3, RCA-2 (the mixture of HCl, H2O2 and H2O), lead to surface depletion of holes in p-Si and a weakly depleted majority carriers in n-Si that appear due to the positive fixed oxide charge. Surface processing in the RCA-1 solution, which contains the mixture of NH4OH, H2O2 and H2O, can be considered, in turn, noting a strong depletion in n-Si and a weak one on p-type Si surface thus implying the negative surface charge arisen from the dissociation of �Si–OH groups in the SiO2 film during the oxidation

) [25, 26]. Using an amorphous silicon layer should also prove useful in GexSi1–x/Si structures [27]. It is seen in **Figure 1** that the surface photovoltage (SPV) is enhanced as the structure is covered with a-Si (curves 3 and 4 compared with curves 1 and 2 at time *t* ¼ 0). Roughly a 10 times larger value of the SPV magnitude is observed in the

*Time-dependent SPV of Si wafer (1), structures of GexSi1–<sup>x</sup> islands on Si (2), 10 nm a-Si/GexSi1–x/Si (3), and 10 nm a-Si/GexSi1–x/Si annealed for 5 minutes at 400 °C in an O2 ambient atmosphere (4). The concentration of Ge atoms in the islands is about 80%. Reprinted with permission from Podolian A, Nadtochiy A, Korotchenkov O, Romanyuk B, Melnik V, Popov V. Journal of Applied Physics. 2018;124:095703. Copyright*

layers.

**Figure 1.**

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*2018, AIP publishing.*

Si surface atoms �Sis– [26].

in the solution (�Si–OH ⇆ �Si–O� + H+
