Response of the wurzite GaN surface to swift heavy ion irradiation

The passage of a SHI through a solid material can result in permanent nanoscale damage called ion track. The most common description of the SHI track formation, the thermal spike model, suggests that the kinetic energy of the SHI projectile that is deposited as dense electronic excitation along the SHI trajectory, can lead to nanoscale melting of the material. Irradiation of a flat solid surface by SHI under grazing incidence angle can result in the formation of surface SHI tracks. These ion tracks can be observed directly using atomic force microscopy (AFM). However, to extract statistical information (average ion track length, length distribution etc.), structural investigations of this type are very time consuming. In the present work, we report the results of our investigations regarding SHI irradiation of wurzite GaN surface, and show that grazing incidence small angle X-ray scattering (GISAXS) can be utilized for acquiring an excellent statistics during short measuring times.
Wurzite GaN thin film samples were grown by low-pressure metalorganic vapor phase epitaxy on c-plane sapphire substrates. SHI irradiations were performed at the IRRSUD beamline at GANIL using 92 MeV Xe, and at the RBI using 23 MeV I. Surface modifications were investigated using tapping mode AFM. Complementary GISAXS analysis was carried out at the synchrotron facilities of Elettra-Sincrotrone Trieste, on the SAXS beamline, using synchrotron radiation with wavelength λ=0.154 nm (photon energy of 8 keV). To investigate possible stoichiometric changes of the GaN, in situ TOF-ERDA measurements were performed at the RBI using the same 23 MeV I beam.
In contrast to previous works where nanohillocks were found within the surface ion track, the morphology of 92 MeV Xe ion tracks consists of both nanohillocks and nanoholes. A sample irradiated at high fluence with 92 MeV Xe, but still under conditions when ion tracks are not much overlapped, produces a strong GISAXS signal (Figure 1.). For lower energy irradiation using 23 MeV I, ion tracks consist only of nanoholes. TOF-ERDA measurements performed using 23 MeV I at the same grazing incidence angle of 1° show a significant loss of nitrogen already at the fluence of 2×10¹¹/cm².

 
Figure:  GaN surface irradiated at IRRSUD with 92 MeV Xe²³⁺, grazing angle 1°, fluence= 10¹⁰/cm² (a) AFM image and GISAXS spectra taken at different azimuthal angles with respect to orientation of the surface ion tracks: 0° (b), 2° (c), 10° (d).

While the hillocks are generally interpreted as a signature of molten material, the occurrence of holes indicates a loss of material. Very recently, it was shown that in case of another wide band gap material, silicon carbide (6H-SiC), grooves with a depth of ~ 0.3 nm instead of chains of nanohillocks appear when irradiated by SHI under grazing incidence angle. In a broader context, the observation of nitrogen loss reported here and the loss of silicon from the SiC surface upon SHI irradiation reported earlier opens up the question of the composition of SHI tracks.

The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) -CALIPSO under grant agreement nº 312284.

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