Magnetron sputter epitaxy (MSE) is the standard process for the deposition of a wide range of industrially relevant coatings, operating with a low-energy ion assistance to minimize ion damage of the material, thus enabling fabrication of high-quality semiconductor materials for the optoelectronics. However, reports on the growth of GaN nanostructures by MSE are much less in comparison with molecular-beam epitaxy and metal-organic chemical vapor deposition. Here, we will present the study on the growth of single-crystal GaN nanorods by MSE using liquid Ga target. The talk will contain three parts: 1) the handling of liquid Ga target during sputtering; 2) self-assembled growth of GaN nanorods on cost-effective substrates; and 3) selective-area growth of GaN nanorods assisted by patterned substrates. Characterizations on structural and optical properties of the MSE-grown nanorods as well as the growth mechanism from nucleation stage to well-developed nanorods will be discussed.
The scarab beetle Cetonia aurata is known to reflect light with brilliant colors and a high degree of circular polarization. Both color and polarization effects originate from the beetles exoskeleton and have been attributed to a Bragg reflection of the incident light due to a twisted laminar structure. Our strategy for mimicking the optical properties of the Cetonia aurata was therefore to design and fabricate transparent, chiral films. A series of films with tailored transparent structures of helicoidal InxAl1-xN nanorods were grown on sapphire substrates using UHV magnetron sputtering. The value of x is tailored to gradually decrease from one side to the other in each nanorod normal to its growth direction. This introduces an in-plane anisotropy with different refractive indices in the direction of the gradient and perpendicular to it. By rotating the sample during film growth the in-plane optical axis will be rotated from bottom to top and thereby creating a chiral film. Based on Muellermatrix ellipsometry, optical modeling has been done suggesting that both the exoskeleton of Cetonia aurata and our artificial material can be modeled by an anisotropic film made up of a stack of thin layers, each one with its in-plane optical axis slightly rotated with respect to the previous layer. Simulations based on the optical modeling were used to investigate how pitch and thickness of the film together with the optical properties of the constitutive materials affects the width and spectral position of the Bragg reflection band.
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