Growth and Characterization of Epitaxial GaN Thin Films on 4H-SiC (11.0) Substrates
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Growth and Characterization of Epitaxial GaN Thin Films on 4H-SiC (11.0) Substrates Brian P. Wagner1, E.A. Preble3, Z.J. Reitmeier1, R.F. Davis1, D.N. Zakharov2, and Z. Liliental-Weber2, North Carolina State University, Raleigh, NC 27695, 2Lawrence Berkeley National Laboratory, MS 62-203, Berkeley, CA 94720, 3Kyma Technologies, Inc., Raleigh, NC 1
ABSTRACT GaN thin films were grown via metalorganic vapor phase epitaxy on a-plane 4HSiC substrates on which had been deposited an AlN buffer layer. Atomic force microscopy images revealed that the microstructure of the AlN buffer layer and the subsequently deposited GaN had a highly oriented growth structure where parallel growth features propagated in the [1-1.0] direction. Scanning electron microscopy showed that the interfaces between the substrate, buffer layer, and epi-layer were continuous. Cracking was observed in GaN films having a thickness greater than 800 nm. Plan-view transmission electron microscopy analysis revealed stacking faults and threading dislocations with densities of ~1.6x106cm-1 and ~ 3.3x1010cm-1, respectively. X-ray diffraction confirmed that the GaN was deposited epitaxially in the same orientation as the substrate. The average on- and off-axis x-ray full-width half-maxima of the (11.0) and the (10.0) reflections were 948 arcsec and 5448 arcsec, respectively. INTRODUCTION Group III-nitrides are inherently piezo- and pyroelectric materials due to their wurtzite structure. The polar axis in a wurtzitic crystal is aligned along the [0001] direction. The polarization fields of this orientation result in reduced quantum recombination efficiency in quantum well structures, and a red-shift in optical emission wavelengths. Electrostatic fields and piezo- and pyro-electric characteristics are absent along all directions perpendicular to [0001]. The absence of an electric field yields a distinct advantage for high-efficiency GaN-based LED’s and laser diodes compared to conventionally [0001] oriented structures similarly grown on sapphire and SiC.1-3 Recently, growth of GaN in non-polar directions has been attempted on tetragonal (γ) LiAlO2 4, and on (1-1.0) r-plane sapphire.5 The principal objective of the research reported herein has been the growth and characterization of non-polar GaN(11.0) on aplane SiC (11.0) via metalorganic vapor phase epitaxy (MOVPE). EXPERIMENTAL DETAILS The epitaxial AlN buffer layers and the subsequently deposited epitaxial GaN films were grown in-situ in a cold-wall, vertical, pancake-style metalorganic vapor phase epitaxy (MOVPE) system on research grade, on-axis 4H-SiC(11.0) substrates which had been previously etched in a 10% HF solution to remove the native oxide. The AlN buffer layers and the GaN films were grown at 20 Torr and at platter temperatures of 1100°C
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and 1015°C, respectively. The precursor species (mass flow rates) of trimethylaluminum (TMA; 5.4 µmol/min), tri-ethylgallium (TEG; 101 µmol/min) and ammonia (NH3; 0.14 mol/min) were mixed with a purified H2 diluent (3 slm) in a twoinch intern
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