Investigation of the Morphology of AlN Films Grown on Sapphire by MOCVD Using Transmission Electron Microscopy
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& Nuclear Engineering, University of Maryland, College Park, MD Research Center of Excellence, Howard University, Washington, D.C. U.S. Army Research Laboratory, Adelphi, MD
ABSTRACT To determine the effect of growth conditions on AIN film morphology, we investigated several AIN films grown on sapphire by MOCVD with various V/Ill ratios. Transmission electron microscopy was used to characterize the film's crystalline quality and defect morphology. TEM results show that the resulting film morphology depends on the V/Ill ratio. Films grown with NH 3 flow rates below 170 sccm have high crystalline quality. In contrast, we observe columnar growth and secondary interfaces in films grown with NH 3 flow rates at or above 170 sccm. The secondary interfaces are likely to be inversion domain boundaries (IDBs) and may be associated with strain relaxation. We discuss the V/Ill ratio's effect on crystalline quality, surface roughness, and IDB and columnar structure formation. INTRODUCTION Because of its large energy gap, AIN is a good candidate for the fabrication of high temperature semiconductor devices and as the dielectric material in high power, high frequency devices operating at high temperatures. However, the quality of AIN films still needs to improve before this material can be used in practical applications. Due to the large lattice mismatch, AIN films grown on sapphire tend to grow three dimensionally, leading to a high defect density and in
many instances, a columnar structure. In order to reduce the lattice mismatch from 35% to an effective mismatch of 13%, AIN films grown on sapphire have a 300 in plane rotation with respect to the substrate. Three dimensional growth negatively affects the optical and electrical properties of AIN, therefore inhibiting its potential for high power, high frequency, and high temperature applications. The goal of this experiment was to determine which growth parameters result in the highest quality films. In a previous study, we have obtained the optimum growth temperature range (11 10°C - 1 150'C) for high quality AIN films. In this study, we obtain the optimum V/Ill ratio for this temperature range. EXPERIMENT Several AIN films were grown on (0001) sapphire in a low pressure MOCVD reactor. All of the films were grown for 15 minutes and at a pressure of 10 Torr. Samples were grown at four different temperatures ranging from 1110' C - 11600 C. The growth precursors, trimethylaluminum (TMA), and ammonia, the hydrogen flow rate and the temperature were held constant during the growth of each individual sample. Various NH 3 flow rates, ranging from 130 - 190 sccm, were used for different samples. Since the TMA flow rate was 15 sccm for each sample, an increase in the ammonia flow results in an increased V/Ill ratio.
339 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
Cross-sectional TEM samples were prepared using a tripod polishing and ion milling, and were examined in a JEOL 4000FX TEM operated at 300 KV. The film's surface was examined with atomic force mic
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