Time-resolved luminescence studies on AlN and high AlN mole fraction AlGaN alloys

Impacts of point defects and impurities on the recombination dynamics for the near-band-edge (NBE) emission in AlN and high AlN mole fraction Al_xGa_1-xN epilayers are revealed by means of deep ultraviolet (DUV) time-resolved luminescence and positron annihilation measurements. Extremely radiative nature of AlN is identified, as the radiative lifetime (τ_R) for a free excitonic polariton emission is as short as 10 ps at 7 K and 180 ps at 300 K, which are the shortest ever reported for spontaneous emission of bulk semiconductors. However, τ_R increases with the increase in impurity and Al-vacancy (V_Al) concentrations up to 530 ps at 7 K, irrespective of the threading dislocation (TD) density. Continuous decrease in τ_R with temperature rise up to 200 K for heavily-doped samples may reflect the carrier release from band-tail states. Similar to these AlN, low-temperature τ_R for low-temperature-grown high-x Al_xGa_1-xN are longer than that for low-x Al_xGa_1-xN, AlN, or GaN due to the contribution of bound and localized tail-states. However, τ_R shows little change with temperature rise, and is still a few ns at 300 K. The results essentially indicate an excellent radiative performance, although the luminescence efficiency of AlN and Al_xGa_1-xN DUV light-emitting-diodes (LEDs) reported so far is limited by short nonradiative lifetimes (τ_NR). To increase τ_NR, high temperature growth with appropriate defect management is preferable (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)