A high spatial resolution, near infrared absorption method has been used to determine [EL2] concentrations in semi-insulating undoped Czochralski liquid encapsulated (LEC) GaAs. Fine structure superimposed on the well-known ‘‘W’’ distribution across 〈110〉 diameters of {001} wafers has been detected. Fine structure has also been directly observed in {110} sections of ingots grown along 〈001〉 using a CCTV vidicon system to observe transmitted infrared radiation. The structure consists of narrow bands of increased absorption which correspond with high densities of dislocations in the same specimens.
The influence of dislocations on the electrical properties of 1" partially dislocation-free LEC GaAs has been studied as a function of the fraction of melt solidified ( g ) . Microscopic electrical characterisation was performed using spreading resistance and anodisation imaging techniques, and Hall measurements at 294-400 K were employed for macroscopic characterisation.The zero-dislocation-density seed end regions are microscopically uniform with consistently high mobility, resistivity and thermal activation energy. However, dislocated regions are microscopically non-uniform with lower mobilities and activation energies. At g>0.4, carrier concentrations increase due to the presence of a shallow donor, possibly at €,-0.42 eV, at the dislocations, and this causes large microscopic fluctuations in resistivity. Radial dependences of the electrical parameters vary with dislocation distribution and sub-structure along the crystals, with the mobility having an 'M' shaped radial variation at g = 0.5 where dislocations lie in slip bands and a 'W' shape at g-0.97 where the resistivity is low and dislocations form cellular networks.Good correlation of electrical inhomogeneity with slip bands, cell structure and lineage on a microscopic scale is obtained.
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