Time-Resolved Cathodoluminescence Spectroscopy of Oxygen Related Defects in AlN Layers

For the fabrication of AlGaN UV LEDs, a high quality AlN buffer layer on sapphire substrate is a prerequisite for increasing the quantum efficiency. However, in MOVPE it is challenging to achieve a low threading dislocation density (TDD), high transparency of the AlN buffer layer and high light outcoupling efficiencies at the same time. Additional processing challenges arise due to excessive wafer bow caused by differences in thermal expansion coefficients between LED stack and substrate. AlN is one of the few compound semiconductors where annealing at high temperatures (HTA) leads to a substantial improvement of the crystal quality without dissociation of the compound. At the same time, the optical transparency is increased and bowing problems are reduced, since much thinner AlN HTA layers can be used in comparison to thick MOVPE-grown buffers. Typical HTA AlN thicknesses are below 500 nm. However, during the annealing process, a large number of point defects is introduced. Among those, oxygen is one of the most prominent ones due to the higher chemical affinity of aluminum for oxygen than for nitrogen.<br/>We analyzed the influence of oxygen defects in AlN layers with cathodoluminescence (CL) spectroscopy. For all experiments, we used an AlN sample fabricated by PVD on <i>c</i>-oriented sapphire substrates. The 425 nm thick AlN layer was treated with HTA at 1680°C for 2 hours in N₂ atmosphere at normal pressure. SIMS measurements revealed a high oxygen concentration of 2×10²⁰cm^-3 for the studied AlN sample.<br/>At room temperature, CL spectroscopy shows a broad (FWHM ~ 400 meV) oxygen related luminescence band centered at about 3.54 eV (350 nm). To study the charge carrier dynamics of the oxygen-related defects in AlN, time-resolved CL measurements have been performed. The defect related emission is characterized by a complex multiexponential decay with a fast component of less than 100 ns and a slow component of longer than 10 µs. We attribute the short decay time to the radiative transition from the conduction band-edge states to (V_Al-O_N)^- or (V_Al-O_N)^2-. The long recombination times might be related to carriers deeply trapped in spatially separated regions of the sample by the impurity-derived electric fields. Further we will discuss the influence of the oxygen concentration in AlN on the recombination times.