Versatile 3D GaN processing for integrated high-aspect-ratio vertical nanodevices

Since the birth of high-power 2D gallium nitride (GaN) light-emitting diodes (LEDs), immense optimization and investigation have been carried out in the last few years, not only to enhance the material quality and device performance but also to further extend its application realm. InGaN/GaN LEDs have been broadly employed in general illumination and backlight units because of their higher luminous efficacy and longer lifetime compared to conventional light sources. By forming the materials in 3D nanowire (NW) structures, light-switchable sensors can be developed for photoelectrochemical detection of NADH in life sciences [1]. Meanwhile, because of their superior characteristics in switching speed, operation temperature, and power loss, GaN field-effect transistors (FETs) also gain remarkable research interests for power switching applications, in which vertical 3D GaN NW FETs have recently been introduced as a novel device concept [2]. Therefore, by sharing a common material platform, a monolithic integration of multiple 3D GaN-based nanodevices (i.e., LEDs, sensors, and FETs) on a single chip has been considered as an ultimate goal to yield compact smart-optoelectronic sensing systems.

Although GaN nanostructures with a vertical architecture have been successfully created, their processing reliability remains a major challenge. Here, we report on the advanced development of versatile 3D GaN processing that has been applied in various nanodevices (i.e., nanoLED arrays, nanoFETs, and biochemical nanosensors). To fabricate the well-ordered high-aspect-ratio nanostructures, a top-down approach consisting of nanophotolithography and hybrid etching was employed. In this case, both GaN NWs and fins, with smooth a-plane sidewalls, fine morphology, and different material stacks, could be realized down to 50 nm feature size from the subsequent processes of SF₆/H₂-based inductively coupled plasma dry reactive ion etching (ICP-DRIE) and KOH-based wet chemical etching (Fig. 1). As top and bottom surfaces of the structures are distantly separated by about 3.5 – 5 µm, device planarization plays a critical role for the feasibility of top metal contact deposition. Thus, polymer filling with different insulating materials has been carefully investigated (e.g., positive photoresist, spin-on-glass (SOG), and benzocyclobutene (BCB)), in which their results were characterized in scanning electron microscope and confocal laser scanning microscope (Fig. 2). In this case, the height of the spin-coated and cured polymer should be adjusted in regard to the targeted process steps. For the photoresist, the revealed etch rate was ~123nm/min in O₂-plasma based ICP etching at a pressure of 10 Pa (Fig. 3). Moreover, in FET processing, the Cr gate contact was applied on the NW and fin sidewalls as the third electrode besides top (drain) and bottom (source) electrodes after atomic layer deposition (ALD) of thin SiO₂ or Al₂O₃ gate dielectric (Fig. 4(a)). Whereas, for the nanosensors, the filling polymer during planarization has to be removed to realize hanging top contact, so that the freely exposed NW or fin sidewalls can be functionalized with organic molecules as sensitive sensing layers (Fig. 4(b)). Besides their detailed process flow, the performances of the developed 3D devices will be discussed.

1. M. Riedel, S. Hölzel, P. Hille, J. Schörmann, M. Eickhoff, F. Lisdat, InGaN/GaN nanowires as a new platform for photoelectrochemical sensors – detection of NADH. Biosensors and Bioelectronics 94 (2017) 298-304, doi: 10.1016/j.bios.2017.03.022.
2. F. Yu, S. Yao, F. Römer, B. Witzigmann, T. Schimpke, M. Strassburg, A. Bakin, H.W. Schumacher, E. Peiner, H.S. Wasisto, A. Waag, GaN nanowire arrays with nonpolar sidewalls for vertically integrated field-effect transistors, Nanotechnology 28(9) (2017) 095206, doi: 10.1088/1361-6528/aa57b6.