First-principles calculations of stability, optical properties, and non-radiative hole capture rates of carbon defects in wurtzite AlGaN alloys

AlGaN alloy has a wide range of applications in ultraviolet photodetectors. Carbon defects, though detrimental as carrier capture centers, offer a tunable pathway for defect-mediated optoelectronic engineering in AlGaN alloys. By combining first-principles calculations with configurational sampling, we systematically resolve the thermodynamic stability of Al_xGa_1-xN (x = 0.33, 0.50, 0.61) alloy, and determine that the carbon atom will occupy the nitrogen point position. Taking into account c-axis polarity of wurtzite structure and carbon-on-nitrogen substitutional defect (C_\rm N) nearest neighbor atomic arrangement, the eight possible configurations are constructed, and the polarity has little effect on the total energy (\Delta E is 0.02 eV-0.07 eV). After ignoring the c-axis polarity, the formation energy of 8 possible C_\rm N defect configurations increases linearly with the number of Al neighbors (n = 0, 1, 2, 3, 4, meaning the number of Ga neighbors is 4, 3, 2, 1, 0), and the influence of different configurations on the transition level is less than 0.2 eV. Because the formation energy difference of five configurations is less than 1.22 eV, all these defect configurations may appear in the actual growth process of AlGaN alloy. In addition, the calculation results of the optical transition process show that the photoabsorption (PA) and photoluminescence (PL) energies are also linear with the Al contents. For the non-radiative recombination process of defect transition levels, since the 5 possible configurations produce a transition level fluctuation within 0.18 eV, each 0.1-eV energy fluctuation corresponds to an order of magnitude in the hole capture cross-section fluctuation. Strategic Al content modulation enables precise tuning of defect transition levels, offering a direct route to suppress non-radiative recombination in ultraviolet (UV) photodetectors.