Defects that generate energy levels deep within the bandgap play a pivotal role in influencing the performance of GaN/AlGaN devices. Despite their critical impact, the understanding of many of these deep-level defects remains limited [1]. This limitation primarily stems from the challenge of deciphering the intricate interplay among various types of defects and impurities present in typical GaN/AlGaN structures.

In this work, we study defects with deep levels in high-quality dilute Al_xGa_1-xN samples grown by metal-organic vapor phase epitaxy (MOVPE) on highly conductive native ammono-GaN substrates. The Al_xGa_1-xN samples have been characterized by using a combination of deep-level transient spectroscopy (DLTS), high-resolution Laplace DLTS (L-DLTS) and low-temperature photoluminescence (PL) spectroscopy.

Using L-DLTS, we find that the electron emission signal associated with the Fe_Ga acceptor level in Al_xGa_1-xN splits into individual components attributed to fluctuations in aluminum concentration within the second-nearest neighbor (2NN) shell around the Fe_Ga impurity atoms (Fig. 1a). Calculations of the probability of encountering a specific number of aluminum atoms in the 2NN shell of the Fe_Ga defect align closely with experimental concentrations determined from Laplace DLTS peak intensities (Fig. 2). This observation indicates that in MOVPE-grown dilute Al_xGa_1-xN layers, aluminum and iron atoms exhibit a random distribution in the material. Furthermore, our findings demonstrate that the energy level of the Fe_Ga acceptor with no Al atoms in the 2NN shell in the Al_xGa_1-xN samples shifts linearly with the aluminium content and the shifts are 28 and 55 meV relative to that in GaN for the samples with x = 0.025 and 0.05, respectively (Fig.1b). 

We also observe that the position, shape and relative intensities of the PL peaks associated with the Fe_Ga(0/-) defect change with aluminum concentration (Fig. 3). The relative intensities of the PL peaks associated with different numbers of Al atoms in the 2NN shell around Fe defects are also found to be consistent with a random distribution of Al and Fe atoms. The variation of the intensity of these PL peaks with excitation photon energy is also used to determine the position of the Fe_Ga acceptor level relative to the valance band.

 

[1] M. Meneghini et al. J. Appl. Phys. 130, 181101 (2021).