(Invited) New Avenues for Exploration and Aplications of GaN
Authors: Tiginyanu Ion 
Published: February 2020

Gallium nitride, a wide-band-gap semiconductor compound (E
g = 3.4 eV at 300 K), has in the last two decades registered a fascinating increase in the crystalline quality of epitaxial layers determining its leading role in the development of the modern solid-state lighting industry. The demonstration and successful commercialization of GaN-based blue light emitting diodes resulted in the physics Nobel prize to I. Akasaki, H. Amano and S. Nakamura in 2014. Besides, in the year of 2014 Bhattacharya et al reported on the first room temperature electrically injected GaN polariton laser [1]. Exhibiting an impressive number of unique properties such as high breakdown voltage, high switching frequencies, enhanced power efficiency, high electrical conductivity, excellent thermal stability and radiation hardness, over the last decade GaN has been remarkably successful in the area of high-power/high-frequency electronic applications and is now considered the second most important semiconductor material after Silicon. In this paper, we report on new fields of research and applications of gallium nitride. First, we describe the Surface Charge Lithography (SCL) developed by us which enables one to fabricate ultrathin GaN suspended membranes for multifunctional applications. Second, we report on GaN biocompatibility and on possibility to mark living cells with hollow GaN nanoparticles exhibiting both piezoelectric and magnetic properties. Third, we report on three-dimensional nanoarchitectures of GaN for nano/microfluidic, microrobotic and biomedical applications.
The SCL is a maskless approach based on direct writing of negative charges on the surface of semiconductors by a focused ion beam. The negative charges were found to shield the material against photoelectrochemical etching. Using direct writing of negative charges with subsequent photoelectrochemical etching of specimens under UV irradiation, ultrathin GaN single-crystalline membranes suspended on specially designed GaN micro/nano-structures have been fabricated. Further, these suspended membranes were used to fabricate networks of memristor devices exhibiting basic learning mechanisms such as habituation and dishabituation to a certain electrical stimulus [2]. Interestingly, the duration of the learning process proves to depend upon the number of memristors connected in parallel.
Nanoperforation by design of GaN ultrathin membranes allowed one to fabricate ultrathin flexible photonic crystals with embedded waveguides, beam splitters and cavities [3]. It is shown that flexible photonic crystals based on nanometer-thick membranes exhibit both surface and bulk modes, the degree of localization of the electromagnetic field of surface modes in nanoperforated membranes being orders of magnitude stronger as compared to non-perforated membranes.
It is demonstrated experimentally that hollow GaN:Fe nanoparticles suspended in the growth medium are taken up by endothelial cells and, with the time, they penetrate in the cells and are stored within vesicles while the cells show no sign of cellular damage [4]. We show that the living cells marked by GaN:Fe nanoparticles can be guided in a controlled fashion using applied magnetic fields.
We report on fabrication of three-dimensional architectures based on GaN micro-tubular structures with nanoscopic thin walls which exhibit dual hydrophilic-hydrophobic behavior [5]. The micro-tubular structures are shown to self-organize when interacting with water, forming self-healing waterproof rafts with impressive cargo capabilities (cargo up to 500 times heavier than the floating raft). Along with this, we demonstrate self-propelled liquid marbles with exceptional mechanical robustness which may find applications as bioreactors for scalable in vitro cell growth. The physical properties of the new material based on three-dimensional GaN architectures will be presented in the context of its prospects for biomimetic applications in nano/microfluidics, biomedicine, microrobotics etc.
We will discuss the feasibility to use the nano/microtubular structures on GaN for the fabrication of light-driven nano/microengines with performances higher than those inherent to microengines based on arrays of TiO2 nanotubes [6].
[1] P. Bhattacharya et al, Room temperature electrically injected polariton laser, Phys. Rev. Lett. 112, 236802 (2014).
[2] M. Dragoman, I. Tiginyanu et al, Learning mechanisms in memristor networks based on GaN nanomembranes, J. Appl. Phys. 124, 152110 (2018).
[3] M. Dragoman, D. Dragoman, I. Tiginyanu, Atomically thin semiconducting layers and nanomembranes, Semicond. Sci. Technol. 32, 033001 (2017).
[4] T. Braniste, I. Tiginyanu et al, Targeting endothelial cells with multifunctional GaN/Fe nanoparticles, Nanoscale Res. Lett. 12, 486 (2017).
[5] I. Tiginyanu et al, Self-organized and self-propelled aero-GaN with dual hydrophilic-hydrophobic properties, Nano Energy 56, 759 (2019).
[6] M. Enachi et al, Light-induced motion of microengines based on microarrays of TiO2 nanotubes, Small 12, 5497 (2016).