Optical signature of Mg-doped GaN: Transfer processes

Abstract

Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states which are involved in the donor acceptor pair band transitions situated at 3.26 eV -3.29 eV in GaN:Mg. The presented determination of the donor acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies which is put into a broader context based on Haynes's rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN bandedge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states we determine binding energies of 164±5 meV and 195±5 meV which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.

Properties of the main Mg-related acceptors in GaN from optical and structural studies

“…This correlation was made possible in this work via the use of homoepitaxy on bulk GaN substrates, thus avoiding the more complex spectral behavior in samples grown on sapphire. 52 The unusual unstable behavior in the optical spectra at low temperatures for high Mg doping may then be explained by the presence of another nonradiative recombination path via a deeper defect level which is metastable in this low temperature range (<300 K), and affects the PL of the Mg-related spectra via the quasi-Fermi level under optical excitation. Since the deeper radiative PL emission at 2.9 eV only occurs at high doping levels in MOCVD samples (and is not regularly observed in similarly Mg-doped MBE and HVPE samples), it is implausible that this emission is related to the substitutional Mg acceptor, as recently suggested in theoretical work.…”

Anisotropic polaron localization and spontaneous symmetry breaking: Comparison of cation-site acceptors in GaN and ZnO

“…Taking into account the Madelung correction of 0.14 eV, the transition energies for the Mg Ga defect are calculated to be about 0.21-0.23 eV, in reasonable agreement with the experimentally measured values. 20,[34][35][36] It should be noted that, given the small energy difference, the precise ordering of the three states could depend on the particular energy functional used in the calculation and other factors such as ionic potential and the size of the supercell. The finding of the existence of three acceptor transition levels, however, could provide an important guidance for experimental study of the Mg Ga defect.…”

“…Interestingly, recent experiments indicate a third acceptor state (or defect transition level) related to Mg Ga and possibly Li Zn . [20][21][22] Local structures associated with these levels remain to be identified.…”

Optical and structural characterisation of epitaxial nanoporous GaN grown by CVD

“…The lineshape of this band depends on the concentration of Mg, as a consequence of the complexity of the Mg acceptor configuration in the GaN lattice [36], and the difficulty to set up the conditions for the optimum Mg doping. Recently, different DAP transitions associated with three different Mg-related acceptor levels have been reported [37]. Thus, the shape of the DAP band depends on the relative intensity of these three DAP transitions and it can substantially differ from the shape of a standard DAP peak with its corresponding phonon replicas.…”