Nitrogen-hydrogen complex in $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{N}}_{1-x}$ revealed by x-ray absorption spectroscopy

GaAsN alloys belong to a class of semiconductors with fascinating physical properties. Indeed, a small amount of nitrogen incorporation in GaAs leads to a counterintuitive and large band-gap reduction, and to an unexpected sudden increase in the effective mass of electrons. Even more surprisingly, both electronic and structural changes can be reversed fully and in a tunable manner by hydrogen incorporation. In this paper, we combine x-ray absorption spectroscopy at the nitrogen edge with ab initio simulations to investigate the atomic geometry of $\mathrm{N}-\mathrm{H}$ complexes in hydrogenated GaAsN. In this way, we provide experimental evidence that dihydrogen-nitrogen complexes with $C_{2\mathrm{v}}$ symmetry are the most abundant species in hydrogenated GaAsN. This finding contradicts previous predictions of “in-line” $\mathrm{N}-\mathrm{H}_2{}^{*}$ complexes as the predominant species, and accounts for recent infrared absorption experiments.

Figure 1

XRD data near the (004) reflection for $\mathrm{Ga}{\mathrm{As}}_{0.986}{\mathrm{N}}_{0.014}$ before (dashed line) and after (continuous line) exposure to a hydrogen dose equal to $3.0\times {10}^{18}\text{ions}∕{\mathrm{cm}}^2$. The inset reports the low-temperature $\left(10\mathrm{K}\right)$ photoluminescence (PL) spectra for the same samples shown in the main part of the figure. In the hydrogenated sample, the low-energy peak is due to the passivated GaAsN epilayer, the high-energy shoulder is likely related to the GaAs substrate. PL multiplication factors are given.

Figure 2

(a) Sketch of the $\mathrm{N}\mathrm{H}_2{}^{*}$ (a) and $C_{2\mathrm{v}}$ symmetric and asymmetric complexes. The $\mathrm{N}\mathrm{H}_2{}^{*}$ (b) complex has an $\mathrm{N}{\mathrm{H}}_{\mathrm{BC}}\mathrm{Ga}{\mathrm{H}}_{\mathrm{AB}}$ configuration; similar in-line configurations characterize the $\mathrm{N}{\mathrm{H}}_{\mathrm{BC}}$ and $\mathrm{N}{\mathrm{H}}_{\mathrm{AB}}$ monohydrogen complexes.

Figure 3

(a) Experimental and simulated XANES for the as-grown sample before exposure to hydrogen; (b) experimental N $K$-edge XANES for the hydrogenated sample of Fig. 1 (top) and corresponding FMS simulations based on the atomic coordinates calculated via DFT in presence of the most stable monohydrogen- and dihydrogen-nitrogen complexes.