TEM Study of Bulk AlN Growth by Physical Vapor Transport
- PDF / 398,939 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 104 Downloads / 185 Views
Dept. of Materials & Nuclear Engineering, University of Maryland, College Park, MD, Materials Science Research Center of Excellence, Howard University, Washington, D.C., 3 U.S. Army Research Laboratory, Adelphi, MD 2
ABSTRACT We are attempting to grow bulk AlN that would be suitable as a substrate for nitride film growth. Bulk AlN films were grown by physical vapor transport on 3.5º offaxis and on-axis 6H SiC seed crystals and characterized by TEM, x-ray-diffraction, Auger electron microscopy, and SEM. TEM images show that the bulk AlN does not have the columnar structure typically seen in AlN films grown by MOCVD. Although further optimization is required before the bulk AlN is suitable as a substrate, we find that the structural characteristics achieved thus far indicate that quality bulk AlN substrates may be obtained in the future. INTRODUCTION Despite the rapid progress made in nitride based semiconductor film growth, there are still no ideal substrates for high quality epitaxial growth. AlN and GaN are typically grown on SiC, which is expensive, or on Sapphire, which is cheaper but has a large lattice mismatch with the nitride based films. Minimizing the lattice mismatch and enhancing the growth conditions improves the nitride film morphology by lowering the density of defects such as misfit dislocations. Other defects, such as inversion domain boundaries, result from the film’s non-isomorphism with the substrate rather than the lattice mismatch [1]. The ideal substrate needs to be as lattice matched and isomorphic to the film as possible. Nitride buffer layers are often grown on sapphire or SiC in an attempt to provide a lattice matched and isomorphic surface for film growth. While buffer layers have helped improve film quality, films grown on buffer layers are usually columnar and contain a higher density of dislocations than desired. AlN or GaN substrates are necessary to achieve further film quality improvement. Due to the high melting temperatures and high dissociation pressures of III-N compounds, bulk nitride crystal growth is difficult. Although the exact nitrogen dissociation pressure is not known, values cited by Landolt and Börnstein [2] indicate that the nitrogen dissociation pressure of AlN is orders of magnitude smaller than that of GaN or InN [3]. Bulk AlN, therefore, should be easier to grow than bulk GaN or InN. In this study we examine the structural quality of bulk AlN grown by physical vapor transport and compare it to AlN buffer layers grown by MOCVD. EXPERIMENT Bulk AlN was grown by physical vapor transport by the decomposition of AlN powder in the presence of ambient nitrogen. The growth temperature range was 2150º-
F99W5.5 Downloaded from https://www.cambridge.org/core. IP address: 83.171.252.83, on 14 Sep 2020 at 12:44:27, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/S1092578300004543
2200º C with nitrogen pressures of 400~410 Torr. The separation between the seed and AlN powder was approximately 4 mm under
Data Loading...
