Fully Unstrained GaN on Thick AlN Layers for MEMS Application
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Fully Unstrained GaN on Thick AlN Layers for MEMS Application Katja Tonisch, Florentina Niebelschuetz, Volker Cimalla, Henry Romanus, and Oliver Ambacher Institute of Micro- and Nanotechnologies, Technical University Ilmenau, Gustav-Kirchhoff-Str. 7, Ilmenau, D-98693, Germany
ABSTRACT Usually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer. Compressive strain would lead to instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure. INTRODUCTION Group III-nitrides and their ternary alloys are favourite materials for many devices due to their exceptional electrical, optoelectronic and chemical properties [1]. So far only few applications in microelectromechanical systems (MEMS) have been reported [2], since the commonly used sapphire heterosubstrate exhibits an extraordinary chemical stability and hinders the realization of freestanding functional layers by selective undercutting. The insertion of a sacrificial layer between the functional layer and the substrate is one possibility to evade these difficulties. Growing GaN epitaxially on AlN usually results in compressive strained GaN, since AlN has the smaller lattice constants. However, for the fabrication of MEMS not only the structural quality but also the strain of the active layer, in our case the GaN film, is an important parameter. While a tensile strained active layer results in both, a higher resonant frequency and a higher quality factor [4, 5], compressive strain leads to buckling effects and an irreproducible mechanical behaviour [6]. NHE is an approach to gain unstrained epitaxial layers on heterosubstrates [7], which takes advantage of a three dimensional stress release mechanism which is only available on nanostructured templates. In conventional planar epitaxy, the substrate is considered as non-compliant and rigid due to its large thickness. Consequently, the whole strain caused by the lattice mismatch is accumulated in the epilayer. In contrast, considering the overgrowth of nanoscale islands, both the islands and the epitaxial layer can deform vertically and laterally in the initial state of growth, leading to reduced strain accumulation in the epilayer.
As shown by Zubia et al. [7], the strain decays exponentially with distance to the interface in
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