The data files are named for consistency with the associated paper "Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the {Mg} acceptor" by A. Singh et al., Scientific Reports (2017). Further analysis and discussion of these results can also be found in this paper.

A GaN layer, 2 µm thick, grown on a 2-inch sapphire wafer by metalorganic vapour phase epitaxy, was doped in-situ to a concentration of 1.1–1.2 × 10^19 Mg cm-3, as verified by secondary ion mass spectrometry. The wafer was sectioned into 1 cm2 samples and implanted with various fluences of Eu (8 × 10^12 cm-2 @ 70 keV, 1.7 × 10^13 cm-2 @ 150 keV and 6 × 10^13 cm-2 @ 380 keV) along the surface normal, in order to produce an approximately uniform Eu concentration of 1 × 10^19 at/cm3 from 20 to 75 nm below the sample surface. The sample was annealed at high temperature (1673 K) and high pressure (1 GPa of N2), during which the sample surface was covered with bulk GaN crystallites to prevent out-diffusion of nitrogen.

PL measurements were carried out in a closed-cycle helium cryostat (base temperature 12.5 K) with sample excitation by a 355 nm CW laser (with maximum output of 20 mW in a 1.5 mm spot). The incident light intensity was controlled by inserting calibrated neutral density filters in the beam path. Sample luminescence was dispersed by a 2/3-m spectrometer and recorded using a cooled 1024 × 127 pixel CCD camera.

Figure2.txt shows the time evolution of the photoluminescence (PL) spectrum of the sample at 18.5 K. The excitation source power density was 10 mW/cm2. The spectra are background-corrected and were recorded at 0.1 s intervals using a silicon charge-coupled device (CCD) array.

Figure2Inset.txt shows a subset of the above PL data, showing the time response of the intensities of the peaks at 587 nm ("Eu0") and 588.9 nm ("Eu1(Mg)").

Figure3.txt shows the decay of the Eu0 peak at three different temperatures: 18.5 K, 30 K and 40 K. In each case the laser excitation density was kept at 10 mW/cm2.

Figure4.txt gives the hyperbolic decay time constants obtained by fitting to the decay of the Eu0 peak intensity as a function of both temperature and excitation density.