Layer- and strain-dependent optoelectronic properties of hexagonal AlN

Motivated by the recent synthesis of layered hexagonal aluminum nitride ($h$-AlN), we investigate its layer- and strain-dependent electronic and optical properties by using first-principles methods. Monolayer $h$-AlN is a wide-gap semiconductor, which makes it interesting especially for usage in optoelectronic applications. The optical spectra of 1-, 2-, 3-, and 4-layered $h$-AlN indicate that the prominent absorption takes place outside the visible-light regime. Within the ultraviolet range, absorption intensities increase with the number of layers, approaching the bulk case. On the other hand, the applied tensile strain gradually redshifts the optical spectra. The many-body effects lead to a blueshift of the optical spectra, while exciton binding is also observed for 2D $h$-AlN. The possibility of tuning the optoelectronic properties via thickness and/or strain opens doors to novel technological applications of this promising material.

Figure 1

Optical absorption spectrum of (a) bulk hexagonal-AlN and (b) bulk wurtzite-AlN compared to experimentally obtained imaginary dielectric function [19, 58].

Figure 2

(a) Imaginary dielectric function and (b) layer-dependent absorbance spectra of single-layer, bilayer, three-layer, and four-layer (1L, 2L, 3L, and 4L) $h$-AlN. The comparison with bulk $h$-AlN crystal is shown as an inset.

Figure 3

Electronic band structures of 1L, 2L, 3L, and 4L $h$-AlN. Zero level is set to the valence band maximum. Red arrows indicate important band-to-band (direct) transitions.

Figure 4

(a) Frequency-dependent optical conductivity, (b) absorption coefficient, and (c) reflectivity of 1L, 2L, 3L, and 4L $h$-AlN as a function of $\hslash \omega$.

Figure 5

The variation of electron (red line) and hole effective mass (blue line) with respect to the number of layers. The bulk values are shown by dashed lines.

Figure 6

(a) Imaginary dielectric function, (b) absorption coefficient, and (c) frequency-dependent reflectivity of single-layer $h$-AlN under strain. Black solid curve stands for the unstrained case and dashed lines for strain values of 1%, 3%, 5%, and 7%, respectively.

Figure 7

(a) Strain-dependent band structures indicating the variation of the band gap values (E) as a function of strain $\left(\epsilon \right)$ and (b) partial densities of states of monolayer $h$-AlN.

Figure 8

Many-body calculations of the optical response of monolayer and bilayer $h$-AlN. (a) Comparison of RPA spectra based on PBE, HSE, and $G_0{W}_0$ calculations for monolayer $h$-AlN and (b) optical spectra of mono- and bilayer $h$-AlN with electron-hole interactions taken into account.