Anions relative location in the group-V sublattice of $\mathrm{Ga}\mathrm{As}\mathrm{Sb}\mathrm{N}∕\mathrm{Ga}\mathrm{As}$ epilayers: XAFS measurements and simulations

We investigated the local structure around N and Sb atoms in $\mathrm{Ga}\mathrm{As}\mathrm{Sb}\mathrm{N}∕\mathrm{Ga}\mathrm{As}$ epilayers as a function of growth conditions and annealing time via soft and hard x-ray absorption spectroscopies in order to find out if short range ordering (SRO) in the group-V sublattice is present. SRO is one of the potential origins of the huge blueshift of the band gap observed upon annealing in these materials. By combining a Sb $K$- and $L$- and N $K$-edge x-ray absorption fine structure spectroscopy analysis, we demonstrate that neither strong Sb clustering nor preferential Sb-N association is possible, and that Sb atoms see a random number of N next nearest neighbors except for growth temperatures smaller than $400°\mathrm{C}$, for which Sb-N neighbors in the type-V sublattice are in excess with respect to statistical disorder. On the other hand, the evolution of SRO around N anions (breaking of nitrogen pairs and randomization) can play a role in the annealing-induced band gap blueshift. Varying growth conditions and concentration modifies the band gap but, surprisingly, it does not affect the position of the conduction band minimum when Sb is incorporated.

Figure 1

Band gap of as-grown samples A, B, C, and D determined by the PL peak position (band gap of R is far away at $1.38\mathrm{eV}$) as a function of the bulk lattice mismatch determined by XRD. Inset: band gap energy shift as a function of annealing time for sample A and for the reference R.

Figure 2

Sb $K$-edge background subtracted XAFS spectra: (a) spectra taken at RT for the different growth conditions; (b) spectra of sample A taken at $80\mathrm{K}$ for different annealing times (together with a spectrum of R300). Spectra are not $k$-weighted.

Figure 3

N $K$-edge XANES of GaAsSbN epilayers treated with different annealing times; the topmost spectrum represents the XANES of N2 used for the energy calibration. All spectra are normalized via average postedge, the spectrum of N2 is then multiplied by a factor 0.2 for better integration in the figure.

Figure 4

FMS simulation of the N $K$-edge XANES of GaAsN performed using a cluster in which all atomic positions have been relaxed via a VFF potential: the simulation reproduces very well the main features of the experimental spectrum.

Figure 5

(Color online) Fits on the XAFS Fourier transform for samples C (bottom) and R (top). Continuous black line: experimental data, red (dark gray) full circles: fit with unique average Sb-As distance in second shell, blue open circles: simulation with strategy II for $\frac{1}{2}$ N atoms in the second shell, violet (dark gray) diamonds: strategy II with one N in the second shell, black squares: fit with variable N occupancy in second shell, olive (light gray) triangles: fit with strategy I. The two last fits for sample C are artificially shifted towards the bottom of the figure for the sake of clarity.

Figure 6

Sb $L3$ XANES for samples A (as grown and annealed), B, and FMS simulation for different SRO configurations.

Figure 7

Fits on the FT of the Sb $K$-edge EXAFS signal for selected annealings of sample A; FT of sample R300 (black dots) is also reported superposed to that of sample A300. Inset: variation of the signal amplitude (with respect to the as-grown sample) as a function of annealing time.

Figure 8

(Color online) (a) N $K$-edge XANES FMS simulations in the presence of NN1-NN2 nitrogen pairs and other SRO configurations, and experimental spectrum for sample A. (b) Zoom on the pre-edge region of the experimental spectra for the different samples, spectra B and D have been vertically shifted by a constant offset. (c) Zoom on the pre-edge region of the simulated spectra for different SRO configurations.

Figure 9

(a) N absorption edge position measured for sample A as a function of annealing time; (b0) calculated edge positions for different N pairs and other SRO configurations; (c) N $K$-edge position as a function of the PL emission for all the samples; d) very near-edge XANES simulation for the different SRO configurations.