Structural Characterization of Reactive Ion Etched Semiconductor Nanostructures Using X-ray Reciprocal Space Mapping
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Institut fur Halbleiterphysik, Johannes Kepler Universitat, A-4040 Linz, Austria
ABSTRACT We have studied GaAs/AlAs periodic quantum dot arrays using high resolution x-ray diffraction (reciprocal space mapping) around the (004) and (113) reciprocal lattice points. From the distribution of the diffracted intensities we deduced the average strain status of the dots. From the numerical simulations it is evident that random elastic strain fields are present, which extend through almost the entire volume of the quantum dot. The simulations of the x-ray measurements revealed that the crystalline part of the dots is considerably smaller as scanning electron micrographs would indicate.
INTRODUCTION Quantum wires and quantum dots have been predicted to exhibit unique optical and electrical properties. 1 Quantum wire lasers were expected to show a significantly lower threshold current than two-dimensional quantum well laser, the luminescence efficiency of quantum dots should be even higher. The performance of real, etched nanostructures often does not come up to the expectations. 2 The most probable reason for this fact is that the etching process induces deep electronic traps, which act as nonradiative recombination centers, up to a distance of several nanometers away from the surface. During the etching, structural defects are introduced into a layer at the sidewalls of the wires or dots, and the perfection of the crystal lattice in the wires and dots is diminished due to random strains propagating from these defects. Such random strain fields may be decisive for the electrical and optical properties of laterally patterned quantum confined systems. Moreover, the crystal lattice in the dot is inhomogeneously strained, mainly due to the internal stresses in the multilayer structure, in which the dot array is fabricated. These stresses are relieved during patterning, which leads to elastic strain relaxation 2 deforming significantly the crystal lattice in the dots. Another source of non-random inhomogeneous strains is the lattice expansion due to foreign interstitial atoms or vacancies incorporated into the dots during the etching process itself. Both the non-random and the random strains can be studied by x-ray diffraction. Several groups demonstrated the applicability of x-ray diffraction for the structural characterization of periodic quantum wires and quantum dots in the past years. 4 - 10 Emphasis was laid on nonrandom strains which may cause shifts or deformations of the maxima of the diffracted intensity distribution in reciprocal space. Random strains in wire and dot structures give rise to incoherent (diffuse) x-ray scattering that can be studied by means of the reciprocal space mapping method as well. I I It is well-established that the distribution of the diffusely scattered intensity in reciprocal space yields information on the correlation properties of the random deformation field. 12 From the symmetry of this distribution, the nature of the structural defects can be deduced, the quantitative comparison of
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