Mg is the most efficient p-type dopant for the GaN semiconductor. However, to achieve significant hole concentrations in GaN, the Mg doping levels should exceed 1019 cm-3, which induces microstructural planar defects [1]. Various pyramidal structures have been reported in GaN films grown by metal-organic chemical vapour deposition (MOCVD) wherein they depended on the growth polarity. They have been identified as Mg-rich pyramidal inversion domains, resulting from phase segregation effects [2]. Although the origin is not completely understood, it seems that the nucleation occurs at the sample surface, inducing changes in the stacking sequence from hexagonal to cubic structures or formation of Mg3N2 precipitates [3]. However, no direct experimental evidence has been provided so far to support the above mechanism and there is insufficient understanding of the underlying structural process. Therefore, the aim of this research was to study the three-dimensional Mg-rich hexagonal pyramids formed with Ga polarity in MOCVD grown GaN:Mg films by hard X-ray nanoprobe.

Figure 134 shows the XRF data collected at ID22NI, both optical micrograph and scanning electron microscopic (SEM) images. The presence of elemental traces of Cr and Fe is revealed. A blue-red-yellow (BRY) plot displays the Ga-, Cr- and Fe-K intensity distributions. As expected, the Ga arrangement presents equally spaced and periodic planes sequentially stacked from the hexagonal base. However, the impurities Cr and Fe exhibit a close correlation on their spatial locations without the 3D pyramidal shape, suggesting a possible Cr-Fe related secondary phase formation. However, no evidence for such a phase has been observed. A rough estimation of the elemental trace content yields [Fe] ~ [Cr] ~ 0.03% in the lowest concentrated areas, whilst [Fe] ~ 0.41% and [Cr] ~ 0.24% for the highest concentrated areas. Our observations emphasise the underlying diffusion mechanism, indicating local impurity agglomeration predominantly on the hexagonal base, supporting the occurrence of such pyramids by the kinetics of a number of additional impurities (not only light elements like O, C and H) that accompanied the Mg incorporation [4]. This is consistent with earlier reports that suggest dopant or impurity segregation is responsible for the defect formation in GaN:Mg [5].


Fig. 134 : a) Optical micrograph of the Mg-rich hexagonal pyramids in GaN. b) XRF images: BRY plot displaying the Ga-, Cr- and Fe-K intensity distributions with their corresponding concentrations in the colour scales. c) SEM image of the pyramidal defect.

Figure 135 shows XANES probed inside and outside of such pyramids. For GaN, typically two polytypes exist: zincblende (cubic, Td) and wurtzite (hexagonal, C6v). Thus, in the case of the cubic GaN, an isotropic material, the XANES spectra should not depend on the angle of incidence , and the allowed transitions 1a1 t*2 are expected to be invariant with . Contrary to this, in the case of the hexagonal GaN, the XANES data are expected to depend on , more specifically the intensities of the resonances and not their linewidths and energy positions. The allowed transitions 1a1 a*1 will be strongest when the electron field vector is parallel to the c axis, while 1a1 e*1 will be strongest when the electron field vector is parallel to the (x,y) plane. While the e*1 final state results from mixing of px and py orbitals and can be considered as a plane orbital, the a*1 state can be considered as a vector orbital along the z axis (pz orbitals). Here, the data do not show a clear superposition of the hexagonal spectrum plus a contribution associated with GaN having cubic symmetry. Since the energy positions and linewidths of the resonances are found to be independent of , it can be concluded that the pyramidal defects are quite pure hexagonal. From the comparison of the X-ray linear dichroism (XLD), our findings also show that these defects exhibit excellent crystallographic alignment. There is no remarkable damping effect revealing a strong influence of the Cr and Fe impurities in any preferential crystallographic direction.

Fig. 135 :a) Calculated and measured XANES data around the Ga K-edge for perpendicular/parallel incidence on the pyramid centre and outside. b) Calculated and measured XLD recorded at the Ga K-edge with the beam focused on the pyramid centre and outside of it.

In summary, we have investigated three-dimensional Mg-rich hexagonal pyramids in GaN by hard X-ray nanoprobe. Elemental maps of Ga, Cr and Fe were acquired and impurity concentrations estimated on the sub-micrometre scale. Structural analysis based on XANES and XLD revealed Ga atoms in tetrahedral coordination in the wurtzite structure. XLD collections around Ga atoms have shown no local atomic distortion inside the hexagonal defects, providing direct evidence for the highly short range structural order.



[1] W. V. Lundin et al., Semiconductors 43, 963 (2009).
[2] M. Leroux et al., Phys. stat. sol. (a) 192, 394 (2002).
[3] M. Hansen et al., Appl. Phys. Lett. 80, 2469 (2002).
[4] Hongbo Yu et al., J. Cryst. Growth 289, 419 (2006).
[5] Qiang Sun et al., Phys. Rev. B 73, 155337 (2006).


Principal publication and authors

G. Martinez-Criado, R. Tucoulou, P. Cloetens, J.A. Sans and J. Susini, Appl. Phys. Lett., 95, 151909 (2009).