wearable and implantable applications. Among the WBG semiconductors, the ZnO

stands out due to its biodegradability. ZnO can be dissolved not only in water, but also in

NaOH solution, ammonia, and blood serum of horses [2]. Besides the ZnO biodegrad­

ability in the environmental impact, it is also useful for biomedical devices, which can

degrade in the body after serving its purpose [2]. In contrast, due to their chemical in­

ertness, GaN and SiC have been extensively studied for wearable and implantable de­

vices, where they can work properly for several decades in situ [2]. Although Al is known

for its toxicity, studies showed that in the AlGaN/GaN heterostructures, a low compo­

sition of Al does not result in adverse effects on the cell’s culture [22]. Further, SiC

electronic devices are able to work efficiently in a biofluid without any encapsulation

layers [21].

13.4 Which Techniques Have Been Used to Fabricate These Devices?

Even though WBG devices require specific properties already mentioned, a factor as

important as their application in bioelectronics is manufacturing. The mechanical capa­

city of these materials interferes directly with their effectiveness, such as stretchability

and flexibility [2]. Therefore, the biggest challenge over the years is to manufacture these

devices on flexible substrates [23] since each mechanism has its limitations. Thus, the

preparation of WBG semiconductors can be performed by different techniques directly on

the flexible substrate or pre-prepared on another material and transferred to the main

substrate. Several methods have been developed and improved, and the most common

techniques are described (Table 13.2) with their highlights and limitations.

13.4.1 Direct Growth of Nanostructures on Flexible Substrates

Various methods were developed aiming to promote the growth of nanostructures di­

rectly on flexible substrates. The main challenge here is the reaction temperature that the

nucleation process requires once the soft substrate does not tolerate such high tempera­

tures as those of the direct growth reactions. To overcome this obstacle, techniques were

developed and reported involving a combined process.

Examples of low-temperature techniques that are promising are electrochemical de­

position (ECD) and chemical solution growth along with atomic layer deposition, for in­

stance. These processes can be combined in three steps of seeding-annealing-growth (SAG).

Reddy et al. [24] related the combined SAG method in which seed layers of ZnO were

deposited on a nickel-coated flexible substrate using ECD. Then the material was annealed

in the air to obtain a pure crystalline ZnO phase, and in the third step, the ZnO nanorods

were developed using the chemical deposition process. Pradhan et al. [25] report a suc­

cessfully direct synthesis of two different structures, nanopillars and nanowalls, of ZnO on

a plastic substrate (polyester) by using ECD at low temperature without templates.

Another widely used method is the hydrothermal process (Figure 13.4), which consists of

a process where the substrate is coated and dried, and subsequently, it is soaked in a so­

lution with nanoparticles of interest, repeating the deposition process for the concentrated

film. The nucleation of these sites occurs, and at a determined ideal temperature, it favors

the growth of nanostructures, which depends directly on the main parameters as bath

temperature, growth time, and seed coating condition [26].

Wide Bandgap Semiconductors

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