Challenges in AlN crystal growth and prospects of the AlN-based technology

B. Moodyb, S. Craftb, R. Schlesserb, R. Dalmaub, J. Xieb, T. Ricea, J. Tweedya, R. Collazoa, Z. Sitara*
*corresponding author: sitar@ncsu.edu
a Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, USA
b HexaTech, Inc., Morrisville, North Carolina, USA

Abstract
Any crystal growth from the vapor phase relies on one of the two strategies to achieve large-area wafers: growth of thick layers on non-native seeds and subsequent seed removal, or gradual crystal-size expansion through an iterative re-growth process. While the former technique produces large diameter wafers faster, the latter approach yields crystals of much higher quality. This presentation contrasts the two approaches and focuses on challenges inherent to high-temperature growth of AlN.

AlN boule expansion was pursued in an inductively-heated reactor, capable of temperatures in excess of 2400°C. Sintered AlN powder was used as a source material and growth was performed in a nitrogen atmosphere. Detailed finite element analysis was used to design radial and axial gradients to follow the desired boule expansion; realistic expectations and limitations of this technique will be discussed.
Stable facets observed in AlN crystals grown close to the thermal equilibrium are c, m and r. Single crystalline AlN boules were grown on c+, c-, a and m oriented high-quality AlN seeds. The growth on the N-polar seeds was controlled by one growth center leading to a mirror-like growth facet, while the growth on the Al-polar seeds resulted in numerous competing growth centers. A surface energy model in conjunction with the BCF theory, developed as a framework for AlN crystal growth on different facets, supported experimental observations. AlN boule growth on non-polar seeds was controlled by step-flow growth emanating from one to a handful of growth center and was always terminated with m-facets, regardless of seed orientation. AFM measurements were performed on the as-grown c- and m-surfaces and showed step-flow growth mechanism.

Despite the excellent FWHM of x-ray rocking curves and low dislocation density, which were typically below 20 arcsec and 103 cm-2, respectively, formation of low-angle grain boundaries during crystal expansion and control of point defects (intrinsic and extrinsic) seem to be the remaining issues.

AlN and AlGaN films with varying compositions and doping levels were deposited on single crystal AlN substrates. A hydroxide passivation layer that could be removed in situ prior to MOCVD growth, was crucial for achieving homoepitaxial growth. TEM studies confirmed epitaxial relationship and absence of interfacial oxide or defects; AlxGa1-xN films with x ≥ 0.7 grew peseudomorphically on AlN substrates. When doped with Si, these films showed several orders of magnitude higher conductivity as compared to those grown on sapphire substrates. UV LED structures and Schottky diodes were fabricated on these materials and turn-on voltages below 5 V and breakdown fields greater than 10 MV/cm were achieved, respectively.

Paper 8b.2.pdf