Impact of the substrate and of the nucleation layer on the properties of AlGaN/GaN HEMTs on SiC P. Gamarra n , C. Lacam, M. Tordjman, M.-A. di Forte-Poisson Thales Research and Technology, 3–5lab, GaN Microelectronics Group, Route de Nozay, 91460 Marcoussis, France article info Article history: Received 11 May 2012 Received in revised form 26 September 2012 Accepted 1 October 2012 Available online 11 October 2012 Keywords: A1. Nucleation A1. Surface structure A3. Metalorganic vapour phase epitaxy B1. Nitrides B3. High electron mobility transistors abstract This work reports on the influence of the substrate properties and of the nucleation layer on the growth mode of AlGaN/GaN HEMT (high electron mobility transistor) structures, grown by low pressure metal organic vapour phase epitaxy (LP-MOVPE) on 3 in. 4H-SiC and 6H-SiC semi-insulating substrates. In-situ normal incidence reflectance at 633 nm and 950 nm was used to monitor the evolution of the surface morphology during the growth. The analysis reveals that the growth of the GaN buffer layer on the AlN wetting layer is a critical key step of the process. Reflectance measurements are discussed in term of roughness-related light scattering and absorption. The influence of substrate surface properties has been studied extensively. Physical and electrical properties of the HEMT structures, as evaluated by a complete set of ex-situ characterisation techniques, were found to be strongly influenced by the quality of this interface. A significant improvement of the surface morphology during the growth was obtained by the insertion of an AlGaN/GaN multiquantum well (MQW) between the AlN wetting layer and the GaN layer. Smooth interfaces and an excellent coalescence of AlGaN on AlN and of GaN on the top of the MQW are reported. In addition the technique provides a powerful tool to manage the strain of the structure and to reduce the resulting curvature of the wafer. & 2012 Elsevier B.V. All rights reserved. 1. Introduction High electron mobility transistors (HEMTs) based on the AlGaN/GaN structure have demonstrated a great potential for microwave power amplification. In this field, SiC is the substrate of choice, thanks to its high thermal conductivity and the relatively low lattice mismatch with GaN and AlN. However, Lahreche et al. [1] reported that the growth of GaN directly on SiC under typical growth conditions results in a rough, islanded GaN layer due to poor surface wetting. Therefore high tempera- ture (HT) AlGaN or AlN nucleation layers are commonly used for the growth of GaN based devices on SiC. The importance of the nucleation control on SiC has been investigated by several groups [1–6]. Normal angle in-situ reflectance is a characterisation techni- que which allows real-time monitoring of the growth process in MOVPE reactors. Through the analysis of the reflectance traces, information on growth rates, surface morphology and growth mechanisms can be extracted. Despite the widespread use of the technique, there are relatively few analyses of the growth of GaN on SiC [1,4,5], while extensive analyses and simulations based on suitable models and algorithms have been performed to study GaN growth on sapphire [7–9]. In that case, the need of a fast GaN coalescence on the nucleation layer to obtain highly resistive GaN layer has been demonstrated [8]. In this work, we report on the analysis by in-situ reflectance of the growth of AlGaN/GaN HEMT structures on SiC, using a HT AlN nucleation layer. The growth of GaN on AlN and the effect of the substrate polishing have been evaluated as well as their correlation with the resulting physical properties of the heterostructures. The insertion of an AlGaN/GaN MQW between the AlN wetting layer and the GaN buffer is proposed as an efficient method to avoid the formation of islands and undulations. In addition it provides a source of compressive strain which allows managing the stress induced in the structure and the final wafer bow. 2. Experiment 2.1. Experimental procedure The epitaxy of AlGaN/GaN HEMT structures on semi-insulating 4H-SiC and 6H-SiC 3 in. substrates was carried out by LP-MOVPE using an AIXTRON Close Coupled Showerhead s (CCS) 7  3 in. 2 reactor. Trimethylgallium (TMGa), trimethylaluminium (TMAl), Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.10.001 n Corresponding author. Tel.: þ33 130777148; fax: þ33 130776786. E-mail address: piero.gamarra@3-5lab.fr (P. Gamarra). Journal of Crystal Growth 370 (2013) 282–287