Growth of Bulk Polycrystalline Gallium Nitride at Subatmospheric Pressures

摘要

Gallium nitride is receiving intense interest throughout the world because of its efficient emission of blue light. An area of concern, both for applications and for scientific study, is the inability to grow large, bulk crystals of GaN. Current methods for the growth of GaN can be broadly divided into two categories: 1) bulk growth at high pressures using N_2 as a source gas, and 2) growth of thin films on (partially) lattice-matched substrates by MBE or MOCVD. Typical conditions for bulk growth are 1200C and 10,000 bar [1-4]. Only millimeter size crystals are now available. We have recently shown that the thermodynamic requirement of high pressures can be circumvented by the use of atomic nitrogen, N, rather molecular nitrogen, N2 [5]. A beam of N, N~+, and N_2~+ from a microwave ECR source is directed at a pool of liquid gallium held between 700 and 1000C. After an initial induction period, the specularly reflecting gallium surface becomes rough as solid GaN forms. A "skull" of polycrystalline GaN covers the remaining unreacted Ga. A typical "skull" is 0.1 mm thick, 70 mm~2 in surface area, and weighs 40 mg.