Contents lists available at ScienceDirect Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp Freestanding patterned polycrystalline GaN substrate by a straightforward and affordable technique N. Zainal a, ⁎ , M.E.A. Samsudin a , Muhamad Ikram Md Taib a , M.A. Ahmad a , A. Ariff a , N. Alwadai b , I.S. Roqan b a Institute of Nano Optoelectronics Research and Technology (INOR), Universiti Sains Malaysia, 11800 Penang, Malaysia b Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia ABSTRACT A new process for producing a freestanding patterned polycrystalline GaN substrate by applying a straightforward and affordable technique is presented here. Such substrate was fabricated by depositing ~ 50 μm thick bulk GaN layer on porous Si/Si substrate by e-beam evaporator with successive ammonia annealing to improve the material quality of the GaN layer. The GaN layer was then separated from the porous Si/Si substrate by immersing in an acidic solution. The surface of the freestanding patterned polycrystalline GaN substrate that was in contact with the porous Si/Si substrate consisted of cubic-like structures, as inherited from the porous Si/Si substrate. Although the cubic-like structures were almost uniformly distributed on the surface, they were formed in various heights due to irregular degree of symmetry of the porous Si/Si substrate. X-ray diffraction results suggested that β-Ga 2 O 3 inclusions are inside the freestanding patterned polycrystalline GaN substrate but not on its surface. This was supported by micro-photoluminecsence (PL) measurement, whereby only the GaN PL signals were observed. Furthermore, Raman spectroscopy revealed a small amount of compressive stress (0.23 GPa), suggesting that the substrate was almost relaxed. 1. Introduction Due to its low electron affinity, and strong chemical and mechanical stability, polycrystalline GaN material has been regarded as a prefer- able candidate for electron field emitters [1] and photodetectors [2]. This material is also reported to enhance diluted magnetic semi- conductor memory [3] due to its ferromagnetic properties at room temperature. The main advantage of polycrystalline GaN over its single GaN counterpart is that it can be easily produced in variety of size through simple and cost-effective means. If a large thickness could be attained, it can serve as the native substrate for electronic devices of various size. Nonetheless, polycrystalline GaN substrate is less attrac- tive for optoelectronic devices (e.g. light emitting diodes (LEDs) and laser diodes) as such substrate is always characterized by grain boundaries and contains high density of threading dislocations. Such dislocations could propagate into the active region of the device structure and act as non-recombination centers, which trap most of the carriers and hence prevent radiative recombination to happen. This process leads to a significant degradation of the device performance [4]. In relation to this issue, there is a need to develop the poly- crystalline GaN substrate for optoelectronic devices. Fabricating patterned polycrystalline GaN substrate could address the aforementioned issues, allowing the polycrystalline GaN substrate to be exploited in a wider range of applications. In general, patterned structures on a surface of substrate/template promote lateral epitaxial growth that hinders the propagation of the threading dislocations into the next layers/structures of the devices [5]. Patterned structures also increase the light extraction efficiency of an LED, with nearly 30% improvement [6,7] by enabling photon scattering effect. However, patterned polycrystalline GaN substrate has not been demonstrated so far and is presently not available in the market. The lack of progress in this domain could be due to the absence of the established technology for fabricating the patterned freestanding GaN substrate, especially through straightforward and affordable means. This work will focus on the effort aimed at producing the free- standing patterned polycrystalline GaN substrate using a new straightforward and affordable technique that begins with the pre- paration of porous structures on the Si substrate, followed by e-beam evaporator deposition of a ~ 50 μm thick bulk GaN layer on the porous Si/Si substrate. The subsequent steps comprise of annealing the bulk GaN layer in ammonia ambient, followed by separation of the layer from the substrate through acidic etching. The structural and optical characterizations will be investigated using scanning electron micro- scopy (SEM), atomic force microscopy (AFM), x-ray diffraction (XRD), micro-photoluminescence (micro-PL) and Raman spectroscopy mea- surements. The preparation procedures of the porous Si/Si substrate were optimized in our previous study and further details can be found in [8]. https://doi.org/10.1016/j.mssp.2018.07.029 Received 18 May 2018; Received in revised form 13 July 2018; Accepted 19 July 2018 ⁎ Corresponding author. E-mail address: norzaini@usm.my (N. Zainal). Materials Science in Semiconductor Processing 88 (2018) 40–44 1369-8001/ © 2018 Elsevier Ltd. All rights reserved. T