Fully unstrained GaN on sacrificial AlN layers by nano‐heteroepitaxy

Abstract

Usually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer, since compressive strain causes 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 fully 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 heterostructure1 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 substrates such as sapphire or silicon carbide (SiC) exhibit an extraordinary chemical stability and inhibit the realization of freestanding functional layers by selective underetching. One possibility to evade these difficulties is the insertion of a sacrificial layer between the functional layer and the substrate. In this paper, we report on the nanoheteroepitaxy (NHE) of unstrained high quality epitaxial GaN layers and AlGaN/GaN heterostructures by metal-organic vapor deposition (MOCVD) on templates consisting of thick sputtered nanocrystalline AlN on sapphire. AlN can be removed selectively to GaN by wetchemical etching techniques [3] and, thus, is suitable as sacrificial layer. The resulting structures combine the advantages of a novel sacrificial layer technique with the possibilities of an AlGaN/GaN heterostructure with an inherent 2D elecron gas (2DEG). AlN has smaller lattice parameters compared to GaN, resulting in highly compressive strained GaN on AlN in conventional planar epitaxy. 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 irreproducible mechanical behaviour [6]. NHE is a new approach to gain unstrained epitaxial layers on heterosubstrates despite a large lattice mismatch [7]. It takes advantage of a three dimensional stress release mechanism which is only available on a nanostructured template. In conventional planar epitaxy, the substrate is considered as non-compliant and rigid due

Micro‐ and nano‐electromechanical resonators based on SiC and group III‐nitrides for sensor applications

“…Fig. 9a), which was achieved by etching in a solution of 25% NaOH at 50 °C with etch rates ∼150 nm/min 108.…”

Group III nitride and SiC based MEMS and NEMS: materials properties, technology and applications

“…First studies were performed by halide vapour phase epitaxy (HVPE) of GaN on sputtered AlN on sapphire substrates [164,165]. By the MOCVD overgrowth of sputtered nanocrystalline AlN films on sapphire, fully relaxed GaN layers and AlGaN/GaN heterostructures were realized [166]. The two-dimensional surface morphology of the overgrown heterostructure is shown in figure 3.…”

AlGaN/GaN Based Heterostructures for MEMS and NEMS Applications

“…More details about the growth can be found elsewhere for sapphire [29,31], SiC [31], silicon [19] and silicon with thin SiC interlayers [32]. On the sapphire substrates, nanocrystalline AlN layers with a thickness of 500 nm were grown as sacrificial layer prior to the MOCVD of AlGaN/GaN [29] A summary of the used technologies as well as the advantages and disadvantages for the application of the most common substrates for wide band gap semiconductor based MEMS is given in table 1. Optical lithography was used to define bridges with beam lengths l of 10 to 1000 µm and widths of 2 to 10 µm, respectively.…”