Local atomic structure in strained interfaces of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}$ heterostructures

We present a structural study of the interfaces between ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ and InP (and vice versa) by x-ray absorption fine structure spectroscopy (XAFS); the samples investigated are a set of nominally matched ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}$ short-period superlattices. We find that the coordination numbers around As and Ga deviate significantly from those expected in an abrupt superlattice structure even if interface bonds are taken into account; this demonstrates the presence of unwanted interface layers between InP and ${\mathrm{In}}_x{\mathrm{Ga}}_x\mathrm{As}$ (and vice versa). Based on the growth sequence employed and on indications from other techniques, we model the structure as composed of the two nominal layers plus ${\mathrm{InAs}}_x{\mathrm{P}}_{1-x}$ and ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ strained interface layers. XAFS is a chemically sensitive probe of the local structure in these strained layers. We find that each bond length measured (As-In, Ga-As, and Ga-P) has a different value, with small variations among the different samples. This implies the presence of structural distortions that accommodate strain at the local level. We find good agreement between the XAFS results and high-resolution x-ray diffraction data that probe the structure in an average way. The results are discussed also with reference to the problem of the band offsets at ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}$ heterojunctions and to theoretical simulations.