Growth of AlN Single Crystals

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Aluminum nitride isa synthetic compound, it does not occur in nature. Itwas first synthesized [1] in 1862 by reacting molten aluminum with nitrogen gas. The first macroscopic crystals were probably produced accidentally in a large Serpek furnace [2] around 1910. Ottokar Serpek made AIN by reacting bauxite, coke, and nitrogen gas at 18000 C to 1900 0 C. He was interested in producing NI-I3 for fertilizer [3]. The first intentional crystals were produced by Fichter and Osterheld [4] in Base,, in 1915 in a water cooled electrically heated tungsten tube furnace in which AIN was sublimed in N2

gas.

The world research interest in AIN declined after 1920 when the Haber process for NH 3 production from N2+3H2 turned out to be better than the Serpek process. In 1956 Kohn, Potter, and Cotter [5] accidentally grew some small crystals of AIN in a graphite furnace. This generated renewed interest in AIN as a potential semiconductor. Ott's [6] x-ray measurements established the covalent, adamantine structure of AIN, and produced a reasonable basis for semiconductor speculations [7]. HISTORY Since 1956 a large number of different laboratories have worked on the problems of growing AlN crystals and studying their properties. Table I lists a number of these studies [4, 5, 8-25], the type of crucibles employed, and the approximate volume of the largest crystals produced. All of the authors, except Dugger, have grown crystals by the sublimation - recondensation method. Dugger employed molten Ca 3N2 as a flux. Table I shows that only recently have crystals of 50mm3 or larger been grown.

35 Mat. Res. Soc. Symp. Proc. Vol. 512 01998 Materials Research Society

GROWTH FROM THE MELT Growth of AIN crystals from the melt was first attempted in 1968 by Class [27] who found a melting temperature of about 2800'C. More recent work by Vinogradov et al [28] give 2750'C. The nitrogen pressure needed to control excessive aluminum vaporization is about 100 bars. The dissociation pressure is about 9 bars. No serious efforts to grow AIN crystals from the melt have been reported. CRUCIBLES FOR SUBLIMATION GROWTH Almost all of the AIN crystals have been grown employing carbon crucibles, references 4, 23, and 24 have used tungsten, and a few have used nitrides such as TiN or Ta 2N. Many of the crystals grown in graphite crucibles contain carbon impurities and are often colored blue, green, or black. This effect was first discovered by Fichter and Oesterheld [4] in 1915 who observed graphite precipitates inside their crystals. This problem was re-emphasized by Slack and McNelly [29], but many people still use carbon crucibles. The carbon is often present in solid solution in AIN as A12OC. This produces a broad, characteristic optical absorption band [24] peaking at 1.86 ev. The carbon in the furnaces can be transported as CN, C2N2, CO, AICN and other gaseous molecules. It is quite obvious that carbon or graphite crucibles or furnaces should be avoided if high purity AIN crystals are desired. For this reason Slack and McNelly [24] chose tungsten, tungs

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