Growth of nearly stoichiometric MgAl2O4 spinel single crystals by a gradient furnace technique

Abstract

Nearly stoichiometric MgAl2O4 spinel single crystals can be grown by a gradient furnace technique in molybdenum crucibles with an inert-gas atmosphere of industrial-grade argon, high-purity argon, or technical-grade helium. There is a reaction between MgO in the melt and the molybdenum crucibles which results in bands of molybdenum metal inclusions in the boule, although molybdenum concentration away from the bands is less than 100 ppm. This reaction can be minimized by ensuring that the starting materials are well reacted by high-temperature heat treatment to form MgAl2O4 before melting. Similarly, a greenish-brown discoloration which arises from oxygen deficiency in the boules may be eliminated by reacting the starting materials completely to MgAl2O4. A loss of nearly 2 mole% MgO generally occurs during the growth of a boule. This loss results in the formation of a ``skin'' of nonstoichiometric spinel on the outside of the boule. This ``skin'' is the last material to solidify and is highly strained upon cooling. It cracks and these cracks propagate throughout the boule. The loss of MgO also causes the formation of a second phase, α-Al2O3, when the spinel is annealed at 1150 and/or 1550°C for periods of time that are less than 30 h. This second-phase precipitate causes variations in the Knoop100 hardness of spinel between 1360 and 1820 kg/mm2. Because of the difficulty in controlling composition, cracking, and the changes in hardness upon annealing at temperatures where silicon is commonly deposited on single-crystal insulating substrates, it is questionable whether spinel will find widespread applicability as an insulating substrate material.