Ammonothermal Crystallization of AlN Crystals
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Ammonothermal Crystallization of AlN Crystals Alexander I. Motchanyy, Alexey A. Reu, Vladimir S. Kovalenko, Vladimir G. Balakirev Russian Research Institute for the Synthesis of Materials (VNIISIMS), 1 Institutskaya St., Alexandrov 601650, Vladimir District, Russia ABSTRACT III-Nitrides (GaN, AlN and other compounds) have attracted vast interest due to their unique properties and potential applications in optoelectronic devices operating in the blue and UV spectral regions and for the construction of electronic devices capable of operating under high power and high temperature conditions. Nanocrystalline AlN powder were obtained by AMMONO method, in which nitridization of Al metal occurs in highly chemical active supercritical ammonia using both NH4Cl as mineralizer. The experiments were performed in the temperature range of 350-550 oC and pressure of 80-120 MPa in stainless steel autoclaves for up to 5 days. Nanocrystalline AlN were spontaneously nucleated on the lower walls of the autoclaves. The obtained AlN powder was characterized by X-ray diffraction. Nanocrystalline AlN powders with average crystallite size 20-30 nm were produced in the temperature range of 450-550 oC. INTRODUCTION In the last ten years, the family of III-nitride has attracted considerable interest due to their importance for optoelectronic devices and also high power electronic device applications [1-10]. One of such materials is aluminum nitride. AlN is wide band gap semiconductor (6.2 eV) and possesses attractive properties including low thermal expansion coefficient, high thermal conductivity and chemical stability [11]. The main problem concerning growth of the aluminum nitride is that bulk growth has not been developed, i.e. bulk wafers of 3-5 nitrides for homoepitaxial growth still do not exist. Epitaxial growth is therefore still limited to heteroepitaxy on foreign substrates, mainly sapphire and SiC. Heteroepitaxial AlN layers, commonly grown on these substrates, suffer from structural defects. Therefore, AlN crystales are necessary for homopitaxial growth. Ammonothermal growth of bulk AlN is a promising method to obtain high-quality single crystals. In the paper, we report on the synthesis of nanocrystalline AlN in supercritical ammonia at low temperature by ammonothermal method (AMMONO method), which utilizes ammonia as fluid rather than water as in the hydrothermal process. EXPERIMENTAL DETAILS Nanocrystalline AlN powder were obtained by ammonothermal (AMMONO) method, in which nitridization of Al metal occurs in highly chemical active supercritical ammonia using NH4Cl as mineralizer. The experiments were performed at 350-550 oC and 80-120 MPa in stainless steel autoclaves with an internal volume of 14 ml.
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An amount of Al (99.9% purity) metal and NH4Cl mineralizer was loaded into a stainlesssteel autoclave. Then the autoclave was filled with liquid ammonia. The space factor of the vessel was varied in the interval of 52 to 70 % of the “free” volume of autoclave. The autoclave was located in an ordinary tubular fur
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