The enormous demand for superior optoelectronic devices has systematically stimulated the application of new optical characterisation techniques. Here, we propose a photoluminescence approach based on X-ray core-level excitation using a hard X-ray microbeam. So far, a typical X-ray microscope provides significant information in terms of elemental specificity, chemical mapping, coordination chemistry, and site symmetry. If a photoluminescence spectrum is also recorded at each point, patterns of electron-hole recombinations may be produced simultaneously that show complementary properties in a spatially-resolved manner. Usually, the effect of doping, strain, and disorder are studied with high sensitivity by radiative transitions. The experimental layout of the optical-sensitive scheme at ID22 is shown in Figure 129a. To study the recombination channels in highly-demanded wide-band-gap materials, UV-visible wavelength detection was prioritised. Thus, aspheric collection optics and fibre optic accessories have been incorporated to bring the UV-visible light into an optical spectrometer coupled to a linear Si CCD. Based on freestanding GaN and epitaxially-grown GaN:Mn on a-Al2O3, the applications of the scanning X-ray excited luminescence technique will be described.

Figure 129b displays a red-green-blue (RGB) map of the intensities associated to the transitions located at 3.0, 2.6, and 1.9 eV from the GaN:Mn/Al2O3. The X-ray excited luminescence signal was recorded with an excitation energy of 13 keV over a 100x65 mm2 scanned area. The detailed spectral analysis suggested the presence of different peaks, where both Al2O3 and GaN:Mn-related transitions are overlapped. The emission at 1.9 eV has been assigned to the trace impurity Fe3+ in Al2O3, while a luminescence band around 3.0 eV accompanied with another transition at 2.5 eV was tentatively attributed to Mn-related recombinations in GaN. The typical donor-acceptor pair recombinations, attributed to oxygen related complexes and vacancy defects in nitrides, are superimposed at 2.8 and 3.2 eV, respectively. Although the common yellow band associated with intrinsic defects (VGa) is a competitive GaN recombination channel, the luminescence does not exhibit any transition at 2.2 eV. Therefore, within the experimental accuracy, a visible nonuniformity characterises the Mn centres in good correlation with a former X-ray fluorescence map [1].

Fig. 129: a) Schematic layout of the scanning X-ray excited optical luminescence arrangement. b) Spatial variation of the different transition intensities (defined in the plots) recorded on GaN:Mn/Al2O3. The corresponding colour scales are included in the graphs (light represents low counts, dark high counts).


Expanding the microprobe versatility, XANES data in both photon collection modes (X-ray excited luminescence and X-ray fluorescence) are also recorded. To avoid the overlapping of both GaN and substrate-related emissions, in Figure 130, the Ga K edge XANES have been plotted for a freestanding GaN layer. The inset displays the luminescence excited with X-ray energies above and below the Ga K-edge. The spectra consist of two broad bands with insensitive shapes to the excitation energy. The blue emission band (BB) dominates at 2.8 eV, whereas the green band (GB) at 2.5 eV commonly attributed to defects is observed as a shoulder. The bound exciton (BX) lines appear at higher energies with very low intensity. As expected for a thick and totally absorbing sample (~100 mm thick) under the used experimental geometry, the luminescence shows a negative edge jump with inverted oscillations, suggesting that the high-energy electrons excited from outer core states contribute more to the optical luminescence than those from inner core states [2].

Fig. 130: Normalised XANES spectra around the Ga K edge taken in both X-ray excited optical luminescence and X-ray fluorescence modes from a freestanding GaN layer. The inset shows the luminescence spectra excited with X-ray energies below and above the Ga K edge.


In summary, despite the energy relaxation and transfer processes involved, relevant optical transitions have been examined spatially and spectrally resolved in GaN:Mn/a-Al2O3. The luminescence scanning probe illustrated the nonuniform distribution of the Fe3+ ions and defect-related centres in sapphire, also showing a good agreement between the Mn related luminescence map and formerly reported Mn pattern by X-ray fluorescence [1]. Furthermore, the luminescence from a freestanding GaN film was monitored around the Ga K edge, demonstrating that no additional radiative channels are created above the Ga K shell.



[1] G. Martinez-Criado, A. Somogyi, S. Ramos, J. Campo, R. Tucoulou, M. Salome, J. Susini, M. Hermann, M. Eickhoff, M. Stutzmann, Appl. Phys. Lett., 86, 131927 (2005).
[2] C. Gauthier, H. Emerich, and J. Goulon, Jpn. J. Appl. Phys., Part 1, 32, 226 (1992).

Principal publication and authors

G. Martinez-Criado (a), B. Alen (b), A. Homs (a), A. Somogyi (a), C. Miskys (c), J. Susini (a), J. Pereira-Lachataignerais (d), J. Martinez-Pastor (e), Appl. Phys. Lett., 89, 221913 (2006).
(a) ESRF
(b) Microelectronics Institute, Madrid (Spain)
(c) Walter Schottky Institute, Garching (Germany)
(d) Chemical and Environmental Research Institute, Barcelona (Spain)
(e) Materials Science Institute, Valencia (Spain)