Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the Mg acceptor

Abstract

Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a 'spectator ion' to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on 'switchdown' from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm² to (an estimated) several hours at 50 K, 1 mW/cm². Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.

Figure 1. Mean PL Signal of ⁵D₀ → ⁷F₀ transition of Eu³⁺ for Eu0 and Eu1(Mg) configurations of Eu-Mg defect as a function of temperature under ~1 mW/cm² excitation showing hysteretic photochromic switching between Eu0 and Eu1(Mg) configurations.

Figure 2. Showing a kinetic series of 300 PL spectra (⁵D₀ → ⁷F₀ transitions of Eu³⁺) under ~10 mW/cm² excitation at 18.5 K (see text).

The photochromic switching of Eu0 (587 nm) to Eu1(Mg) (588.9 nm) takes place in about 1 s. The inset shows that normalized PL signals of the Eu0 and Eu1(Mg) configurations are complementary in time.

Figure 3. Showing the time evolution of mean Eu0 PL signal under 10 mW/cm², 355 nm excitation at 18.5, 30 and 40 K.

The solid lines show fits to Eqn. 1 with the characteristic times τ as indicated.

Figure 4. The Arrhenius plot (ln(τ) vs. inverse temperature) of Eu0/Eu1(Mg) switching at 3 different excitation densities.

Figure 5. Showing the likely disposition of the Mg-Eu defect in Ga(Mg)N:Eu; on the right, the nanoscale phase changes that occur during the Eu0 to Eu1(Mg) transformation, with bond lengths taken from ref. .