Grain Expansion and Subsequent Seeded Growth of AlN Single Crystals

Abstract

ABSTRACTOngoing efforts of growing large AlN single crystals at NCSU using an induction-heated, high-temperature reactor are based on (1) engineered expansion of single crystalline grains with increasing boule length, as well as (2) the development of a growth process that enables seeded growth on AlN surfaces previously exposed to air. The growth process is based on physical vapor transport (PVT), where AlN powder is sublimed in a high purity nitrogen atmosphere. The growth temperature was typically in the range of 2250 to 2300 °C. In this study, tungsten crucibles were used in combination with graphite insulation and were found to be durable for AlN growth. Boule growth was interrupted several times in order to refill the AlN powder source and the growth surface was subjected to surface preparation to facilitate epitaxial re-growth. Grain expansion was studied as a function of process parameters. Crystalline quality of large single crystalline grains was correlated with their surface morphology.