Epitaxial growth of AlN on c-plane sapphire by High Temperature Hydride Vapor Phase Epitaxy: Influence of the gas phase N/Al ratio and low temperature protective layer R. Boichot a, ⁎, N. Coudurier a,b , F. Mercier a , S. Lay a , A. Crisci a , S. Coindeau c , A. Claudel b , E. Blanquet a , M. Pons a a SIMaP CNRS, Grenoble INP, UJF, 1130 Rue de la Piscine, 38402 Saint Martin d'Hères, France b ACERDE, 354 Voie Magellan-Alpespace, 73800 Ste Hélène du Lac, France c CMTC, Grenoble INP-CNRS, 38402 Saint Martin d'Hères, France abstract article info Available online 22 August 2013 Keywords: AlN High Temperature-HVPE III–V heteroepitaxy c-Plane sapphire AlN is epitaxially grown on c-plane sapphire by High Temperature Hydride Vapor Phase Epitaxy (HT-HVPE) at constant growth rate and thickness, while varying the N/Al ratio in the gas phase at 1500 °C. The influence of an additional low temperature (1200 °C) protective layer on AlN crystal quality is also assessed. The experiments and thermodynamic calculations show that the sapphire substrate is unstable at high temperature under hydro- gen and ammonia while it is stable at low temperature or under a few hundred nanometers of AlN protective layer even at high temperature. In terms of AlN crystal quality, the optimal process developed here consists in depositing a 170 nm low temperature protective AlN layer with N/Al = 3 followed by a high temperature thick AlN layer grown with N/Al = 1.5. In this case, the interface between AlN and sapphire remains continuous (no etching) and the stress in the grown layer at room temperature is minimized by a balance of the growing ten- sile stress with the cooling compressive stress. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Aluminum nitride is a promising substrate for AlGaN-based UV LED and piezoelectric applications (Micro Electro Mechanical Systems, MEMS and Surface Acoustic Waves, SAW devices). The UV LED industry requires high quality single crystals (i.e. deep UV transparency, low den- sity of defect) [1]. The requirements are less restrictive for piezoelectric applications, for which highly oriented c-axis layers are needed [2,3]. Among the different available processes for AlN growth (Physical Vapor Transport, PVT [4–7]; Metal–Organic Chemical Vapor Deposition, MO-CVD [8] and High Temperature Hydride Vapor Phase Epitaxy, HT- HVPE), High Temperature HVPE (N 1200 °C) becomes the most prospec- tive technique to produce the required quality for both piezoelectric and semiconductor industry [9–21]. A better understanding of the phenom- ena leading to high quality AlN layers grown on sapphire is the key point to allow HT-HVPE becoming a new industrial reference in thick AlN layers processing. Indeed, it is currently one of the cheapest way to produce industrial grade AlN single crystals. Currently, the main concern is the lack of cheap compatible seed sub- strates that could present a thermodynamic stability at the temperature and gas mixture used for AlN growth. In particular, the deep UV transpar- ent sapphire undergoes severe etching under hydrogen atmosphere at high temperature [21,22]. The main solution to overcome this issue is to protect the sapphire surface with a thin (b 200 nm) epitaxial AlN layer grown at low temperature prior to the thick high temperature de- posit (N 1 μm). This buffer intermediate layer is called “protective layer”. Growth of AlN templates on sapphire by HT-HVPE was historically assessed since 2001, in majority on c-plane surfaces [9–16]. Some growths were also attempted on a-plane, tilted c-plane or semi-polar orientations [17]. The poor thermodynamic stability of sapphire in HT- HVPE growth conditions was noticed and several authors worked on processes that are able to avoid sapphire decomposition by means of templates, protective or nucleation layers [18–20]. On the contrary, the thermodynamic instability of sapphire substrates in HVPE condi- tions was used to promote mechanical fragility of the sapphire/AlN in- terface to produce freestanding substrates [21]. Due to the high dislocation density obtained by direct growth on flat substrates, the current trend is to transpose the ELO (Epitaxial Layer Overgrowth) technique, mastered with GaN, to AlN layers [23–26]. The aim of this study is to assess the feasibility of a one step growth of AlN on c-plane sapphire by only varying the N/Al ratio in the gas phase and compare the crystalline quality obtained with layers grown on a thin low temperature protective layer (in situ grown template). Results are analyzed with the help of thermodynamic considerations. 2. Experiments Experiments are conducted in a quartz, cold wall, and vertical CVD reactor. The experimental apparatus is depicted in [27]. Pres- sure is maintained at 1333 Pa during experiments. The reactor is Surface & Coatings Technology 237 (2013) 118–125 ⁎ Corresponding author at: SIMAP/Phelma Bâtiment Recherche, 1130 Rue de la Piscine, 38402 Saint Martin d'Hères, France. Tel.: +33 476826537; fax: +33 476826677. E-mail address: raphael.boichot@simap.grenoble-inp.fr (R. Boichot). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.08.016 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat