Al x Ga 1  x N bulk crystal growth: Crystallographic properties and p–T phase diagram Andrey Belousov a,n , S. Katrych a , K. Hametner b , D. G¨ unther b , J. Karpinski a,n , B. Batlogg a a Laboratory for Solid State Physics, Schafmattstr. 16, ETH Zurich 8093, Zurich, Switzerland b Laboratory for Inorganic Chemistry, Wolfgang-Pauli-Strasse 10, ETH Zurich, Zurich 8093, Switzerland article info Available online 9 April 2010 Keywords: A1. Phase diagrams A2. Growth from solutions A2. Single crystal growth A2. High-pressure crystal growth B1. AlGAN B2. Semiconducting III-V materials abstract The recent results on the growth of the Al x Ga 1x N bulk single crystals (0.22 rx r0.91) from solution in liquid Ga under high nitrogen pressure are discussed. We focus on the influence of temperature and the choice of the Al source on the crystal growth. The experiments involving different sources of aluminum such as Al metal, pre-reacted polycrystalline Al y Ga 1y N and AlN powder are compared. The best results were achieved using pre-reacted polycrystalline Al y Ga 1y N or/and AlN. Single-crystal structure refinement data of these Al x Ga 1 x N crystals are presented. We also update the p–T phase diagram of (Al,Ga)N compound at high N 2 pressure for various Al content, which is the basis for (Al,Ga)N synthesis. & 2010 Elsevier B.V. All rights reserved. 1. Introduction In the last years, the interest in III-nitride materials have become of technical significance due to their optical and electrical properties. In particular, the ternary Al x Ga 1 x N alloy is a key material in ultra-violet light emitting diodes, power-switching HEMTS and lasers since the band gap energy can be changed from 3.4 to 6.2 eV, depending on the atomic composition. The application of III-nitride materials is still expanding, since significant progress in synthesis has been achieved in recent years. Many methods have been developed for depositing Al x Ga 1 x N thin films such as MBE [1], HVPE [2] or well- established MOVPE [3,4] and recent research is mostly concen- trated on the optimization of the deposition techniques and on the improvement of the structural properties. Although the growth of bulk AlN by sublimation or GaN using ammonothermal method or low/high nitrogen pressure solution methods has been studied by many groups [5–7], relatively little is known about the growth of bulk (Al,Ga)N [8]. Recently, we reported on the growth of bulk single Al x Ga 1 x N crystals with 0.5 ox o0.9 from solution under high nitrogen pressure (up to 10 kbar) and at high temperature (up to 1800 1C) [9]. We have used Ga metal as the solvent and we have introduced the proper amount of Al in the form of previously synthesized polycrystalline Al y Ga 1 y N. A high-pressure nitrogen gas autoclave was used which allowed precise control of p–T growth conditions. Several experiments in a wide range of temperatures and pressures were carried out in order to determine optimal p–T conditions for the growth of Al x Ga 1 x N. We found that by varying the p–T conditions, Al x Ga 1 x N crystals with a desired Al composition can be grown. In this paper we report the detailed aspects of the crystal growth, in particular the influence of the amount of aluminum introduced into the melt (up to 1.2at%) and of p–T conditions on the composition of the grown crystals. We also focus on the crystallographic characterization of the crystals that provide information about their structural quality. On the basis of the growth experiments and the calculation of the thermodynamic functions we have derived the equilibrium lines for the (Al,Ga)N for the whole range of Al content and have updated the experimentally estimated p–T equilibrium phase diagram of (Al,Ga)N compound at high N 2 pressure for various compositions reported previously [9]. 2. Experimental details Crystals were grown in a high-pressure nitrogen gas autoclave, in which a maximum working pressure of 12 kbar and a temperature of about 1800 1C can be reached. The high-pressure system consists of a compressor, a pressure intensifier and a 40 mm internal diameter high-pressure chamber with an internal, three zone furnace. The nitrogen gas was first pumped up to 3 kbar to the experimental chamber by using a commercial membrane gas compressor (Nova Swiss), and then compressed to the desired value with a pressure intensifier. The crystal growth has been carried out from the solution in Ga melt under high nitrogen pressure (up to 10 kbar) and at high temperature (up to Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.04.017 n Corresponding authors. E-mail addresses: belousov@phys.ethz.ch (A. Belousov), karpinski@phys.ethz.ch (J. Karpinski). Journal of Crystal Growth 312 (2010) 2585–2592