Atomic distribution in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ single quantum wells studied by extended x-ray absorption fine structure

The anisotropy of the local structure around In atoms in high quality ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ $(x=0.145,0.20,0.275)$ single quantum wells (SQWs) was investigated by fluorescence extended x-ray absorption fine structure measurements using linearly polarized x ray whose electric field was set to be both horizontal and vertical to the SQWs. The interatomic distance was isotropic for the nearest In-N, whereas for the second nearest In-Ga and In-In, the interatomic distance for out-of-plane atoms was longer than that for in-plane atoms. From the analyses of the coordination number for In-In and In-Ga, the In atom distribution in the ${\mathrm{In}}_{0.275}{\mathrm{Ga}}_{0.725}\mathrm{N}$ SQW was found to be statistically random in both directions. On the other hand, In atoms in the ${\mathrm{In}}_{0.20}{\mathrm{Ga}}_{0.80}\mathrm{N}$ SQW, corresponding to green light-emitting diodes (LEDs), were randomly distributed in the horizontal direction but aggregated in the vertical direction. Correlation was suggested between the vertical aggregation and the higher efficiency of InGaN-based blue to green LEDs.

Figure 1

Schematic view of the sample setup for fluorescence XAFS measurements in the horizontal and vertical configurations. $\mathbf{E}$ is the electric field vector of the incident x ray.

Figure 2

EXAFS $k\chi \left(k\right)$ spectra of $\mathrm{In}K$ edge for the ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ SQWs in the horizontal (solid line) and vertical (dashed line) configurations.

Figure 3

Fourier transforms of the EXAFS spectra shown in Fig. 2. The solid and dashed lines are for the horizontal and vertical configurations, respectively.

Figure 4

Theoretical fitting to the $\mathrm{In}K$-edge spectrum of the ${\mathrm{In}}_{0.20}{\mathrm{Ga}}_{0.80}\mathrm{N}$ SQW in the horizontal configuration. Fittings for the first and second peaks were performed independently.

Figure 5

(a) In-plane and (b) out-of-plane local InN molar fraction $y$ as a function of average InN molar fraction $x$. The broken line represents $y=x$.