Surface Morphology and Composition Characterization at the Initial Stages of AlN Crystal Growth
- PDF / 2,490,079 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 43 Downloads / 245 Views
Surface Morphology and Composition Characterization at the Initial Stages of AlN Crystal Growth B.Liu, Y. Shi, L. Liu, J.H. Edgar, and D.N. Braski1 Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506, U.S.A. 1 Oak Ridge National Laboratory, Oak Ridge, TN 37831-6139, U.S.A. ABSTRACT The morphology and composition of AlN crystals on 6H-SiC (0001) at the initial stage of crystal growth by sublimation re-condensation technique were investigated by SEM and SAM. Discontinuous AlN coverage occurred after 15 minutes growth. The AlN nuclei size, and growth rate increased as temperature increased or pressure decreased. The SiC substrate decomposed leaving hexagonal hillocks; simultaneously, the AlN nucleated on these SiC hillocks apparently o o rotated by 15 to 30 . The chemical composition of the substrate and different AlN crystal facets were characterized by SAM. The bare substrate area was stoichiometric SiC with insignificant conversion to silicon nitride, while Si and C preferentially incorporated in the AlN at the initial stages of growth on specific crystal planes. 1. INTRODUCTION Aluminum nitride (AlN) is a good candidate substrate for gallium nitride (GaN) epitaxial films due to its good thermal stability, high resistivity, relatively small lattice constant mismatch, and similar coefficient of thermal expansion with GaN (see Table 1 [1,2]). AlN is also an excellent material for many other applications such as passive barrier layers, high-frequency acoustic wave devices, high temperature windows, and dielectric optical enhancement layers in magneto-optic multi-layer structures [3-5]. Bulk AlN crystal are most commonly produced by vapor transport (sublimation) from an AlN source. This technique was most successfully developed by Slack and McNelly [6,7] in the mid-1970’s. Large AlN single crystals, up to 1cm long and 0.4cm in diameter, were grown by self-seeding from the sharp tip of sealed tungsten crucibles heated in an rf induction furnace. More recently, self-seeded crystal growth of AlN has been used by Schowalter and Slack [8] to produce AlN crystal up to 1.5cm in diameter and subsequently used for AlGaN epitaxy. Table 1 Crystalline properties of GaN, AlN and 6H-SiC [1,2] Crystalline Properties Lattice parameter (a, Å ) D(1200k-298k)/a298k(%) Band gaps(eV) Melting points(oC) Thermal conductivity (W/cm K)
GaN 3.186 0.46 3.45 ~2000 1.4
G3.13.1
AlN 3.1114 0.49 6.28 2750 3.2
6H-SiC 3.081 0.43 2.9 2830 4.9
Seeding bulk AlN growth on silicon carbide substrates instead of self-seeding offers several potential advantages including the availability of large diameter seed crystals (up to 100nm) for rapid process scale-up, and better control of the crystal polarity, through the choice of the SiC seed crystal polarity (C or Si polarity). Balkas et al [9] first produced thick (PP $O1ILOPVRQ 10×10mm2 6H-SiC substrates in a resistively heated graphite furnace. Their results presented three problems with 6H-SiC as seed crystals: the decomposition of the SiC substrate at high crystal g
Data Loading...
