Synthesis of AlN needles by nitridation of Al–Si melt

Abstract

Needle-like single crystals of aluminum nitride (AlN) were synthesized by the direct nitridation of an AlSi binary melt. The growth behavior and shape of the needles were changed by varying the Al:Si ratio. With lower Al content, needles with high aspect ratio (>20) preferentially grew on the AlSi melt and crucible wall. With increasing Al content, needles with lower aspect ratios having various shapes such as ribbon-like and bended shapes tended to grow. The longer directions of the needles could mainly correspond to the c-axis of a wurtzite-type AlN.©2016 The Ceramic Society of Japan. All rights reserved.Key-words : Aluminum nitride, Whisker, Growth from melt, Single crystal [Received May 26, 2016; Accepted August 5, 2016] Aluminum nitride (AlN) has high refractoriness, high hardness, high thermal conductivity, good corrosion resistance, and good insulation property. 1),2) AlN ceramics have been widely used as substrates and structural components for semiconductor devices and manufacturing. AlN is a typical sintering-resistant material; therefore, high-temperature sintering with sintering additives is necessary to obtain dense AlN samples. Rare-earth and alkaliearth oxides such as Y 2 O 3 and CaO have been found to be effective for developing dense AlN ceramics with high thermal conductivity. 3)9)The growth of AlN single crystals and thin films is another important area of research. AlN single crystals have been used in substrates and buffer layers in the growth process of GaN and AlGaN. 10)13) AlN also has attracted attention for use in widebandgap devices such as a deep ultraviolet light-emitting diode (UV-LEDs).14) In addition, AlN single crystals with needle or whisker shapes are used as seed crystals in the growth of largesized AlN single crystals, 15) has been studied. The direct nitridation of Al is a simple process for obtaining large-sized AlN crystals at relatively low temperatures. In this process, the crystal tends to have a needle-like shape from 1800 to 1900°C and a platelet-like shape at higher temperatures. 20) However, the low melting point (660°C) and high heat of nitridation (¦H 298K = ¹318 kJ mol ¹1, as calculated using the FACT-Web program) of Al often make the evaporation and nitridation rate of Al uncontrollable; this limits the ability to control the morphology of AlN crystals by the growth temperature alone. One of the solutions to this problem is to use a binary melt containing Al 25) instead of a pure Al melt. Si has a much higher melting point (1414°C) and lower heat of nitridation (¦H 298K = ¹246 kJ mol ¹1 , as calculated using the FACT-Web program) than Al. Therefore, the morphology of AlN can be controlled by changing the Al:Si ratio in the AlSi melt. In this study, we synthesized needle-like AlN crystals by the direct nitridation of an AlSi binary melt with various Al:Si ratios.Commercially available Al powder (Rare Metallic Co., Ltd., 99.9%) and Si powder (Rare Metallic Co., Ltd., 99.9%) were used for raw powders. These powders were weighted with Al:Si molar ratios of 1:3, ...

Growth of Spontaneous Nucleation AlN Crystals by Al-Base Alloy Evaporation in Nitrogen Atmosphere

“…Twinned AlN platelets have been observed within the temperature range of 1900-2100 • C, and AlN c-plates have also been synthesized at temperatures above 2100 • C by using BN crucibles [12]. However, the growth process is hindered by uncontrollable morphology due to the difference between the evaporation and nitridation rates of Al [13]. Since N 2 struggled to react directly with aluminum, it often requires extremely high temperatures to ensure the sufficient nitriding of Al vapor.…”

Thermal conductivity and mechanical strength of low-temperature-sintered aluminum nitride ceramics containing aluminum nitride whiskers

“…The granules were carefully mixed with 5 % (by volume) of AlN whiskers synthesized by direct nitridation of the Al melt by using alumina mortar. 7) The synthesized AlN whiskers were between 20 and 30¯m in diameter and between 500 and 2000¯m in length.…”