Properties of Crucible Materials for Bulk Growth of A1N
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Y10.74.1
Properties of Crucible Materials for Bulk Growth of AlN Glen A. Slack, Jon Whitlock, Ken Morgan, and Leo J. Schowalter Crystal IS Inc., Latham, NY 12110, USA Abstract: A variety of different crucible materials have been suggested and/or employed for the sublimation-recondensation growth of AlN single crystals above 2000 C. Representative materials all have melting points well above 2300 C, a reasonable degree of chemical compatibility with AlN, relatively low vapor pressures, and relatively small thermal expansion coefficients. We analyze the current state of knowledge on crucible materials such as C, W, Re, W-Re alloys, BN, HfN, HfC, NbC, TaC, Ta2C, TaN, ZrC and ZrN with respect to published bulk AlN growth conditions. Crucible materials pyrolytic graphite, pyrolytic BN, and W have integrated thermal contraction values (upon cooling from growth temperatures) that are less than that of AlN; the other materials have larger values. The lowest vapor pressure materials in a nitrogen atmosphere are W, TaC, and Re; thus they are expected to yield higher purity crystals than the other candidates. The materials C, BN, Hf, and ZrN are expected to contribute to higher impurity levels in the AlN crystals. Introduction: Based upon demonstrated material properties aluminum nitride substrates are expected to and have begun to show results in emerging electronics applications. Devices that are expected to benefit from aluminum nitride substrates include: high power RF (wireless base stations), UV LED’s and lasers (high-density data storage, homeland security – bio-detection), UV light sources (instrumentation packages, water purification), and mature applications including solidstate lighting alternatives that would reduce global energy consumption. Although promising work has been done to produce demonstration devices on alternative substrate materials there are inherent quality and economic issues that are expected to drive commercial device production toward a bulk source AlN substrate. Several bulk growth techniques have been demonstrated for AlN including flux, sublimationrecondensation, skull melt, and high nitrogen pressure solution growth. To date the most developed method is the sublimation-recondensation method first advanced by Slack and McNelly [10]. This method relies upon solid AlN to sublime from the source, migrate to the seed end as vapor species, and condenses as solid AlN at the seed end of the crucible. In this system the crucible acts as a container for the source, vapor, and seed AlN. The interaction between the growth environment and the crucible is important in determining the quality of the grown boule. Discussion: Crucible materials of construction are a very important consideration for successful crystal growth, particularly at the high temperatures and relatively high aluminum partial pressures needed for reasonable growth rates of AlN. A review of the current literature [1-12] shows that, in general, temperatures above 2000 C and pressures near or less than 1 atm. are preferred.
Y10.74.2
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