Invited original talkPolarization dependent X-ray absorption studies of the chemical bonds anisotropy in wurtzite GaN grown at different conditions
Introduction
The interest in III–V nitride semiconductors has steadily increased during the last decade particularly due to their application for blue light emitting devices. Although the crystal structure of wurtzite group-III nitrides is well known, the nature of bonds in these crystals has not been widely studied. The wurtzite structure is characterized by the lattice constants a and c and by the parameter u=b/c, with b as the bond length along the c-axis as shown in Fig. 1. For the ideal wurtzite structure, c/a=1.633 and u=0.375. In real bulk crystals and epilayers of group-III nitrides, deviations from these values have been reported ([1] and references therein). Strain and defects, which can distort the lattice, might be the reason for the wide scatter of the reported values of lattice constants for GaN. The commonly accepted values of the c/a and u parameters are 1.626 and 0.377, respectively [1]. Two kinds of Ga–N bonds have been distinguished in the GaN lattice in diffraction and electron density investigations [2], [3]: a longer single bond along the c-axis (b) and three shorter bonds slightly inclined with respect to the c-plane (d) as shown in Fig. 1. For simplicity we call the latter bonds ‘in the c-plane’. The unpolarized extended X-ray absorption fine structure (EXAFS) analysis of N K-edges showed also the existence of two different N–Ga distances, but they were not correlated with the wurtzite structure but with the inhomogeneous strain distribution due to the point and extended defects in the investigated GaN layers [4]. The length and the number of nearest-neighbor atoms should influence a particular chemical bond, i.e. the distribution of the electron density in the given direction. To examine the bond anisotropy, polarization dependent X-ray absorption spectroscopy [5], [6] (XANES and EXAFS) was used in the presented studies.
Proper adjusting of the energy of the analyzed X-ray radiation to the X-ray absorption edge allows us to monitor selectively the distribution of the density of states of the conduction band around Ga and N atoms. Additionally, selection rules for the electron transitions separate the states of different symmetry. For example, the K-edge absorption spectra enable us to monitor the p-symmetry conduction band states, which in the case of GaN dominate over all states with other symmetry, both from the anion and the cation site, as has been shown recently [7]. Therefore, a XANES spectrum provides the energy distribution of the electron density in the conduction band corresponding to a particular bond of the studied atom with its nearest neighbor. Moreover, the EXAFS analysis gives information on the local atomic structure, e.g. interatomic distances R, Debye–Waller factors σ and the coordination number N [8]. When these well established techniques are performed with linearly polarized synchrotron radiation and are applied to both oriented bulk samples and epitaxial layers, a direct insight into the shape and anisotropy of the chemical bonds can be obtained.
Our studies on the polarization-dependent X-ray absorption mainly concerned single crystaline wurtzite GaN in the form of a bulk sample and layers grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on different substrates such as SiC, sapphire, and bulk GaN. This enabled us to examine both the influence of the crystal growth technique and the substrate type on the bond lengths and the bond structure of the formed GaN.
Section snippets
Experimental
The X-ray absorption measurements for the N-edge were performed at the beamline 6.3.2 of the Lawrence Berkeley National Laboratory (LBNL) Advanced Light Source. The total photocurrent measurement technique was applied for recording the spectra. The intensity of the incoming radiation was monitored by the photocurrent (I0) generated at the focusing mirror or in a gold mesh. A Hettrick–Underwood type varied line space grating monochromator [9] allowed the measurement of spectra in the 50–1000 eV
XANES
The results of the XANES investigations of the Ga K-edges and N K-edges are presented in Fig. 2, Fig. 3, respectively. The structure of these edges is related to the distribution of the Ga and N p-states in the conduction band. The Ga K-edges shown in Fig. 2a were measured under normal incidence and can thus be related to the three bonds formed in the c-plane (see Fig. 1). The intensity of the spectra was normalized to the same maximum value and for the sake of clarity the spectra were shifted
Conclusion
Standard EXAFS analysis of the polarization dependent spectra allowed us to distinguish between the long and the short Ga–N bonds in the wurtzite GaN crystals in form of the bulk sample and the layers epitaxially grown on different substrates. To our knowledge, this is the first direct evidence of the anisotropy of the chemical bonds derived from EXAFS in this class of materials. Therefore, the bond in the c-direction is weaker and easy to break. This may be a reason that most of the observed
Acknowledgments
The authors would like to thank T. Suski and I. Grzegory for supplying the samples and J. Libera for help in the performed measurements. This work was partially supported by the State Committee for Scientific Research (Republic of Poland) Grant No. 2 P03B 101 14. This work was also supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under contract No. DE-ACO3-76SF00098.
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