The optical and electrical effects of high concentrations of defects in irradiated crystalline gallium arsenide

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Abstract
The damage produced by fast neutron irradiation of gallium arsenide has been studied by a number of techniques. The electrical resistivity, which increases with dose at low doses to semi-insulating values, shows a remarkable, specimen-independent decrease for doses greater than 1017 n cm-2 from values of ca. 109 Ω cm to 3 Ω cm for the highest dose of 1.5 × 1020 n cm-2. In this high dose region the temperature dependence of the resistivity at low temperatures is given by exp [b/T 1/4] and it is suggested that in this highly disordered state conduction occurs by tunnel-assisted hopping between defect levels in the band gap. The presence of such levels is indicated by the strong optical absorption tail which is produced from 0.1 eV to the crystalline edge at 1.5 eV. Although at the highest doses the samples contain a high degree of disorder, X-ray diffraction shows that they are basically crystalline. Lattice parameter determinations show that it increases with dose, linearly at first, then tending to saturation for doses above 2 × 1018 n cm-2. The dose dependence of the lattice parameter is the same as that of the integrated optical absorption and both effects may be expected to relate to the total defect concentrations. The existence of small-angle neutron scattering at the highest dose shows that the defects are not uniformly distributed. It is shown that this state is the high dose manifestation of the production of defects by each primary knock-on atom in fairly localized regions. The overlap of the wings of these regions for doses greater than 1017 n cm-2 provides a conducting path and a fairly constant b value with dose, while the resistivity decreases rapidly. The total defect concentration, which is heavily weighted to the central regions of the individually damaged volumes, does not show overlap until a higher dose, as observed for the optical absorption and lattice parameter. Estimates of the total defect concentration have been obtained by measuring the total diffuse neutron scattering as well as from the small-angle scattering results. It is shown that each primary knock-on produces ca. 103 atomic displacements. This means that the amount of damage in gallium arsenide is close to that predicted by radiation damage theory. Measurements of photoconductivity show that the sharp photoconductivity edge is eliminated at quite low doses. At high doses the photoconductivity becomes very small and is much less than in unirradiated samples of similar resistivity. It appears that the ionized state lifetime of photo-induced carriers is very small in the high dose state. The effects anneal more or less completely after heating to 700°C. The hopping conductivity effect anneals between room temperature and 400°C and the remaining electrical effects between 500 and 600°C. The optical effects anneal more or less continuously throughout the temperature range up to 700°C. It is shown that the doses at which the tunnel-assisted hopping sets in are consistent with similar effects recently observed in ion implantation. We suggest that a similar mechanism applies in implanted layers for ion doses greater than 1012–1013 ions cm-2. Density measurements show an expansion identical with the lattice parameter expansion and indicate an expansion of 2.93 Å3 per defect. This is relatively small and is consistent with the tentative conclusion from the neutron-scattering data that the main defect is the close interstitial-vacancy pair in which the strain is minimized by being of opposite sign around each component.

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