Electronic structure and chemical bonding anisotropy investigation of wurtzite AlN

The electronic structure and the anisotropy of the AlN $\pi$ and $\sigma$ chemical bonding of wurtzite AlN has been investigated by bulk-sensitive total fluorescence yield absorption and soft x-ray emission spectroscopies. The measured $\text{N}K$, $\text{Al}{L}_1$, and $\text{Al}{L}_{2,3}$ x-ray emission and $\text{N}1s$ x-ray absorption spectra are compared with calculated spectra using first-principles density-functional theory including dipole transition matrix elements. The main $\text{N}2p\text{-Al}3p\text{-Al}$ $3d$ and $\text{N}2p\text{-Al}3s$ hyridization regions are identified at $-1.0$ to $-1.8\text{eV}$ and $-5.0$ to $-5.5\text{eV}$ below the top of the valence band, respectively. In addition, $\text{N}2s\text{-Al}3p$ and $\text{N}2s\text{-Al}3s$ hybridization regions are found at the bottom of the valence band around $-13.5$ and $-15\text{eV}$, respectively. A strongly modified spectral shape of $\text{Al}3s$ states in the $\text{Al}{L}_{2,3}$ emission from AlN in comparison to Al metal is found, which is also reflected in the $\text{N}2p\text{-Al}3p$ hybridization observed in the $\text{Al}{L}_1$ emission. The differences between the electronic structure and chemical bonding of AlN and Al metal are discussed in relation to the position of the hybridization regions and the valence-band edge influencing the magnitude of the large band gap.

Figure 1

(Color online) X-ray diffractograms (XRDs) from the AlN (0001) thin-film sample in comparison to bulk Al(100).

Figure 2

(Color online) Top right: $\text{N}1s$ SXA-TFY spectra and left: resonant and nonresonant $\text{N}K$ SXE spectra of $w\text{-AlN}$. The SXA-TFY spectra were normalized by the step edge below and far above the $\text{N}1s$ absorption thresholds. All spectra were plotted on a photon energy scale (top) and a relative energy scale (bottom) with respect to the top of the valence band. The SXE spectra were resonantly excited at the first $\text{N}1s$ absorption peak maximum at 400.9 eV and nonresonant far above the $\text{N}1s$ absorption threshold at 430.0 eV. The excitation energy (400.9 eV) of the resonant SXE spectrum is indicated by the vertical tick in the SXA spectrum and by the small elastic recombination peak at 400.9 eV. To account for the experimental bandgap of 6.2 eV, the calculated SXA spectra were rigidly shifted by $+2.24\text{eV}$. The calculated $\text{N}2p$ charge occupation of $w\text{-AlN}$ is $2.286e$.

Figure 3

(Color online) Experimental and calculated $\text{Al}{L}_1$ and $\text{Al}{L}_{2,3}$ SXE spectra of $w\text{-AlN}$ in comparison to Al(100) spectra. The experimental spectra were excited nonresonantly at 140 and 110 eV, respectively. The vertical dotted line indicates the valence-band edge. A common energy scale with respect to the top of the valence band is indicated in the middle. For comparison of the peak intensities and energy positions, the integrated areas of the experimental and calculated spectra of the two systems were normalized to the calculated $\text{Al}3p$ and $3d+3s$ charge occupations of $w\text{-AlN}$: ($\text{Al}3s$: $0.524e$; $3p$: $0.701e$; and $3d$: $0.282e$) and pure Al metal ($3s$: $0.592e$; $3p$: $0.526e$; and $3d$: $0.090e$). The area for the $L_2$ component was scaled down by the experimental branching ratio and added to the $L_3$ component.