**4.2 Applications on NCT: functional devices of Si-NWs**

It is of great interest to find applications for Si-NWs, which could be as standalone innovative structures such as in photovoltaic (PV) cells (**Figure 7**) or integrating with conventional structures such as Atomic Force Microscopy (AFM) (**Figure 8**) [26], and MOSFET (**Figure 9**), for the purpose of developing and miniaturizing.

**PV Cells** made of Si-NWs have several potential benefits over conventional bulk Si one- or thin-film devices related primarily to cost reduction. It is possible to form the p–n core–shell junctions in high-density arrays, which have the advantages of decoupling the absorption of light from charge transport by allowing lateral diffusion of minority carriers to the p–n junction which is at most 50–500 nm away rather than many microns away as in Si conventional bulk photovoltaic cells. Based on this, the potential cost benefits come from lowering the purity standard and the amount of semiconductor material needed to obtain sophisticated efficiencies, increasing the defect tolerance, and lattice-matched substrates [6]. The concept of

**Figure 7.**

*Schematic illustration of the main types of heterojunction nanowires. (a) Axial heterojunction. (b) Radial heterojunction (core-shell). (c) a core–shell PV cell [3].*

#### **Figure 8.**

*Schematic demonstration of detailed steps involved in Si-NWs integration with AFM tip [26].*

**Figure 9.**

*(a) Shows a schematic representation of an (npn) MOSFET, the conventional one in parallel with the innovative one of NWs; (b) shows the migration of charges based on the applied voltages; and (c) presents the formation of the inversion layer, the channel across the diameter of the NW.*

nanowire-based solar cells has attracted significant attention because of their potential benefits in carrier transport, charge separation, and light absorption. The Lieber [1] and Atwater [28] and other groups [29–31] have developed core–shell growth and contact strategy for their silicon p–i–n nanowire solar cells, with sophisticated efficiencies. Moreover, the ability to make single-crystalline nanowires on low-cost substrates, such as Al foil, and to relax strain in subsequent epitaxial layers removes two more major cost hurdles associated with high-efficiency planar solar cells. A schematic representation is shown in **Figure 7** where SiO2 has been used as a separation layer between the planar defective growth, which occurs during NWs growth, and the substrate to enhance the performance of the PV core–shell junctions [3].

**AFM** was invented in 1968, which has opened new perspectives for various micro- and nanoscale surface imaging in science and industry. Nanotechnology has benefited from the invention of the AFM, and in turn AFM is developing based on the progress of Si-NWs growth techniques. Based on the NCT technique that is based on catalyzing the growth of Si-NWs with Al, it has been proposed to improve the resolution of AFM tips in a production scale [26]. The concept of "Production Scale Fabrication Method for High Resolution AFM Tips" is demonstrated in **Figure 8**, along with the various steps of the Si-NW growth on the tip of the available Si(100) or Si(111) AFM tips. The grown Si-NW on the squared-base Si(100) tip is 45° tilted, while Si-NW grow perpendicularly on the AFM tip of the triangular base of Si(111) [32–35] where further reduction of the average wire diameter to the nanometer scale can be done via hydrogen annealing or oxidation [8, 36–41]. As the diameter decreased, the tensile strength tended to increase from 4.4 to 11.3 GPa. Under bending, the Si-NWs demonstrated considerable plasticity [42].

The proposed structure in **Figure 8** has not yet been experimentally demonstrated [26]; the fabrication of the structure could be achieved by the described processes where it illustrates the potential mass scalability of this technique. A strategy has been *Semiconductor Epitaxial Crystal Growth: Silicon Nanowires DOI: http://dx.doi.org/10.5772/intechopen.100935*

presented to equip microcantilever beams with single Si-NW scanning tips that were directly grown by Au-catalyzed VLS synthesis. It was evident from AFM measurements evidently that the assembled Si-NW scanning tips are suitable for topography reconstruction as well as for overall comparison with conventional pyramidal scanning tips besides their high aspect-ratio nature and a superior durability [39, 43–45].

**MOSFET** can be designed in the form of NWs, as shown in **Figure 9** [8]. The channel can be altered using NWs as shown when a positive voltage is applied to the gate. The holes in the p-type Si are repelled from the surface, and minority carrier conduction electrons are attracted to the surface. If the gate voltage exceeds the threshold value, then an inversion layer is created near the surface. In this layer, the material behaves as an n-type and provides a conducting channel between the source and the drain. The width of the conduction channel is dominated by the diameter of the NW.

Because of the enhanced surface-to-volume ratio of NWs, their transport behavior may be modified by changing their surface conditions, and this property may be utilized for sensor applications to provide improved sensitivity compared to conventional sensors based on bulk material. Si-NWs sensors will potentially be smaller, more sensitive, demand less power, and react faster than their macroscopic counterparts [42, 43, 46].

### **5. Future remarks of nanowires research**

In this chapter, we attempt to summarize progresses made in this field during the last several years, ranging from nanowire growth with precise control at the atomic level [41]. Probing novel properties in 1D systems using a stand-alone innovative novel device was presented, in addition to integration and assembly methods of large numbers of NWs for practical applications.

We conclude this chapter with some outlooks for future research. Will nanowires research lead to new science or discovery of new phenomena? Will it lead to new applications? [47–50]. The answer is clearly yes based on the research activity done on the topic of nanowires. Studies are among the most potential in the topic of nanoscience, as shown in **Figure 10**. The cumulative published studies starting from 2010 up to 2020 on the topic of nanowires have been increased, thus markedly reflecting the technological importance of this topic.

The ever-growing demand for smaller electronic devices is prompting the scientific community to produce circuits whose components satisfy the size and weight requirements. The well-controlled NW growth process, with distinct chemical composition, structure, size, and morphology, implies that semiconductor nanowires can be integrated within the process of the development of nanodevices. Control of the synthesis and the surface properties of Si-NWs may open new opportunities in the field of silicon nanoelectronics and use them as nanocomponents to build nanocircuits and nanobiosensors. Moreover, Si-NWs possess the combined attributes of cost effectiveness and mature manufacturing infrastructures [51–55].

The conventional thin-film technologies grown at MBE have technical limitations, mainly the interfacial lattice mismatch issues that often result in highly defective optical materials. In this regard, Si-NWs growth provides a natural mechanism for relaxing the lattice strain at the interface and enables dislocation-free semiconductor growth on lattice-mismatched substrates, where radial strain relaxation allows for uncharted combinations of semiconductor materials (III–V on Si). In this regard, efforts must be made to break new grounds in this promising research field to stimulate more creative ideas about nanowire research and applications. Many

#### **Figure 10.**

*Cumulative nanowires publications in 11 years 2010–2020 (there are no data available for 2021, where the x-axis represents the number of articles in kilo. In 2020, there were more than 100,000 articles on the topic on nanowires, according to the number of nanotechnology-related articles indexed in web of science (WoS) (ISI web of knowledge). https://statnano.com/report/s29/3.*

promising applications are now at the early demonstration stage but are moving ahead rapidly because of their promise for new functionality, not previously available, to the fields of electronics, optoelectronics, biotechnology, magnetics, and energy conversion and generation, among others [56, 57].

Integration of nanoelectronic units, such as Si-NWs, and biosystems is a multidisciplinary field that has the potential for multilateral impact on various scientific fields including biotechnology. The combination of these multidisciplinary research backgrounds promises to yield revolutionary advances in our everyday life through, for example, the creation of new and powerful tools that enable direct, sensitive, and rapid analysis of biological and chemical species [49, 50].

To simulate future research on NWs, one would look at the progress achieved by the industries of semiconductors which have produced devices and systems that are part of our daily lives, including transistors, sensors, lasers, light-emitting diodes, solar panels, computers, and cell phones [51–54, 58]. Then, imagine changing the morphology of semiconductors from the bulk to the nanowire form; one might wonder how much fundamental difference there is. Where, sometimes the intersection of top-down and bottom-up approaches toward building nanostructures for practical functionality is also possible.

#### **Acknowledgements**

I would like to thank Materials Science Research Institute, KACST, for the kind professional support and fruitful discussion.

#### **Conflict of interest**

The author declares no conflict of interest.

*Semiconductor Epitaxial Crystal Growth: Silicon Nanowires DOI: http://dx.doi.org/10.5772/intechopen.100935*
