Study of annealing effect on the physical properties of AlGaN epitaxial layers

Abstract

The low Al content (x ≤ 8%) AlGaN epitaxial layer was grown on GaN/sapphire pseudo-substrate using atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE). The growth and thermal annealing of the samples were in situ monitored using laser reflectometry. Three samples of AlGaN/GaN were grown at a different composition, x, of aluminum. The effects of annealing temperature and gas ambient environment on the surface morphology, structural and optical properties of AlxGa1−xN layers were investigated. This study was undertaken using a scanning electron microscopy (SEM), an optical microscope (OM), and cathodoluminescence (CL). The thermal annealing process was performed via the MOVPE vertical reactor in mixed flows of H2 and N2 at the temperature range of 900-1200°C. Under N2 ambient, the AlGaN layers were annealed up to the temperature (T) of 1200 °C for 30 min. The in situ laser reflectometry showed a constant reflectivity signal for a long time synonymous of a thermal stability. However, under these same conditions, the GaN epitaxial layers began to decompose at 1100 °C and lose a large part of their thickness at 1200 °C. The SEM micrographs displayed very small pits on the surface of the samples at 1050 °C and 1200 °C probably associated with etch pits revelation. On the other hand, the GaN/AlGaN/GaN/sapphire hetero-structure was annealed at 1200 °C under a nitrogen atmosphere. We observed that the GaN cap layer was completely decomposed at 1200 °C, while the AlGaN layer underneath was stable at this temperature. In H2 atmosphere, a critical decomposition temperature of AlGaN was found to be approximately 980 °C. At 1200 °C, the decomposition rate reached a value of about 10.3μm/h. However, the annealing under H2 gas ambient at T<1200 °C led to the formation of cracks on AlxGa1−xN surface. Above 1200 °C, the OM images showed the extremely poor quality of AlGaN surface due to the total decomposition. Moreover, the addition of N2 into the mixture flow N2 + H2 partially blocks the decomposition. Room temperature CL intensity increased for the sample annealed at 1200 °C in an N2 gas atmosphere indicating the improvement of the optical properties of AlGaN layer. On the other hand, at the same temperature, annealing under pure hydrogen atmosphere led to a complete disappearance of CL in the GaN and AlGaN epitaxial layers.