Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full length article Growth and thermal annealing for acceptor activation of p-type (Al)GaN epitaxial structures: Technological challenges and risks Sebastian Zlotnik ⁎ , Jakub Sitek 1 , Krzysztof Rosiński, Paweł P. Michałowski, Jarosław Gaca, Marek Wójcik, Mariusz Rudziński Łukasiewicz Research Network - Institute of Electronic Materials Technology, Wólczyńska 133, 01-919 Warsaw, Poland ARTICLE INFO Keywords: III-nitride AlGaN Doping Epitaxy Annealing ABSTRACT III-nitride materials, such as ternary alloys of gallium nitride (GaN) and aluminum nitride (AlN), are the pro- minent semiconductor systems in research and industry due to their importance for optoelectronic applications using ultraviolet (UV) spectral range. Although significant efforts have been made over the last two decades, the main drawback of epitaxial structures hindering their full potential in devices is still associated with obtaining reasonably good p-doping control. Here, an effect of acceptor activation by post-growth treatments, that is conventional and rapid thermal annealing, was studied, revealing that while selecting inappropriate conditions p-type AlGaN structures with microstructural degradation, surface precipitation, Mg out-diffusion and poor electrical properties are achieved. The observed planar segregation in a form of pyramidal domains (Mg-rich features), associated with Mg overdose and its limited solubility in AlGaN (~5 × 10 19 cm -3 ) results in a de- crease of the hole concentration. However, rapid thermal annealing in oxidizing and then reducing atmospheres leads to controlled oxygen co-doping of a p-type layer, and at the same time acceptor activation is enhanced and the carrier concentration is increased, > 10 18 cm -3 . Therefore, rapid thermal annealing of Mg-doped AlGaN structures, in particular using oxygen atmosphere, is advantageous to obtain relatively high carrier concentra- tion and p-type conduction. 1. Introduction III-nitride compounds, namely gallium nitride (GaN) and its alloys with In and Al, are considered to be crucial semiconductors for optoe- lectronic devices because of their very large spectral range of bandgaps [1–4], thus they are uniquely suited for solid state lighting to light emission in the green, blue and ultraviolet (UV) regions of the solar spectrum. Despite of using for lighting applications, GaN is also con- sidered to be a contender for power electronic applications as an al- ternative to the established Si and emerging SiC power devices [5]. Significantly, the nitrides are commercially attractive because of their physical and optical properties, thus the devices exhibit long lifetimes and high-efficiency optical emission, and the materials are en- vironmentally friendly due to the lack of harmful arsenic, mercury and lead [6]. Nowadays, GaN-based blue, green and white light emitters, such as light emitting diodes (LEDs), have become efficient alternatives for conventional light sources mainly due to low size, weight and op- erating power as lighting components with precisely adjustable light emission wavelength. Despite visible light emission, by adding alu- minum nitride (AlN) to the GaN alloy system, the emission wavelength of AlGaN-based LEDs can be tuned into wide UV range (400–210 nm), strongly developing the UV-LEDs market. However, the technological use of Al-rich AlGaN alloys to realize high efficiency LEDs and lasers operating in the deep UV region (DUV; UVC range, < 300 nm) still remains very limited. In spite of a number of challenges, AlGaN-based UV-LEDs are already applied or expected to be highly applicable in many crucial areas, such as sterilization, water and air purification, medicine and biochemistry. At the moment DUV LEDs are manu- factured only by few companies, Crystal IS, Dowa Electronics Materials, Nichia, LG Innotek or SETi, but still some key issues for their further development need to be overcome in order to be efficient for the pro- posed applications. Typical LED epitaxial structures are based on a p-n junction, thus they rely upon the ability to achieve both n- and p-type doping. However, it is well known that for GaN-based alloys a p-type doping (normally via magnesium doping) is the major obstacle for the ultimate https://doi.org/10.1016/j.apsusc.2019.05.306 Received 8 November 2018; Received in revised form 21 May 2019; Accepted 26 May 2019 ⁎ Corresponding author at: Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland. E-mail addresses: sebastian.zlotnik@itme.edu.pl (S. Zlotnik), pawel.michalowski@itme.edu.pl (P.P. Michałowski), jaroslaw.gaca@itme.edu.pl (J. Gaca), marek.wojcik@itme.edu.pl (M. Wójcik), mariusz.rudzinski@itme.edu.pl (M. Rudziński). 1 Present address: Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland. Applied Surface Science 488 (2019) 688–695 Available online 28 May 2019 0169-4332/ © 2019 Published by Elsevier B.V. T