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

A K Singh,M J Kappers,M Boćkowski,K Lorenz,K P O’Donnell,P R Edwards

doi:10.1038/srep41982

A K Singh, M J Kappers + Show 4 more

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https://doi.org/10.1038/srep41982

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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.

Highlights

The 4 f electrons of rare earth (RE) ions are strongly localized, the weak perturbation caused by the local crystal field of a given host crystal determines the spectral fine structure of the sharp intra-4f transitions by splitting the ground state 7FJ and excited state 5DJ multiplets[14,15,16,17,18,19]; in particular, the energy level shifts and splitting of Eu3+ in GaN are very sensitive probes of the local crystal field symmetry[19]
Upon cooling a sample of GaN(Mg):Eu, the PL spectrum shows a striking transformation: as the temperature decreases below 40 K, the intensity of Eu0 decreases rapidly, contrary to normal behaviour, while an Eu1-like spectrum rises to replace it; this photochromic transformation was attributed to the structural instability of the Mg acceptor in GaN, an actual displacement of the associated Mg atom leading to a switch from a low-symmetry Eu0 spectrum to a more symmetric one, Eu1(Mg)[18,19]
To study the photochromic dynamics systematically, we cool a sample in the dark to a set temperature between 12.5 and 45 K, allow it to stabilize for 30 min and illuminate the sample

Summary

Introduction

The 4 f electrons of rare earth (RE) ions are strongly localized, the weak perturbation caused by the local crystal field of a given host crystal determines the spectral fine structure of the sharp intra-4f transitions by splitting the ground state 7FJ and excited state 5DJ multiplets[14,15,16,17,18,19]; in particular, the energy level shifts and splitting of Eu3+ in GaN are very sensitive probes of the local crystal field symmetry[19]. Upon cooling a sample of GaN(Mg):Eu, the PL spectrum shows a striking transformation: as the temperature decreases below 40 K, the intensity of Eu0 decreases rapidly, contrary to normal behaviour, while an Eu1-like spectrum rises to replace it (see Fig. 1); this photochromic transformation was attributed to the structural instability of the Mg acceptor in GaN, an actual displacement of the associated Mg atom leading to a switch from a low-symmetry Eu0 spectrum to a more symmetric one, Eu1(Mg)[18,19]. We investigate the dynamics of the transformation of Eu0 to Eu1(Mg), at a number of fixed temperatures below 50 K, as a function of excitation density. These studies allow a ‘dynamic’ interpretation of the temperature-dependent switching described in previous work[18,19]

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