On the character of defects in GaAs

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Abstract
Positron lifetime measurements on GaAs are presented and discussed and former measurements are reviewed. The limitations and appropriate criteria for adequate spectrum analyses are considered in detail. It is shown that exceptionally good statistical accuracy is necessary for a reliable and consequential decomposition, and that source corrections are very important. Results for liquid phase electro-epitaxially grown GaAs conclusively show that the bulk lifetime for GaAs is 220+or-1 ps. All other GaAs samples (grown by either the liquid encapsulated Czochralski method or by the horizontal Bridgman method) show variable bulk lifetimes up to 232 ps. Shallow traps are concluded to be the cause for this. The binding energy of positrons in such traps is estimated to be about 23 meV, and the trapping rate into deep traps from such shallow traps is found to be approximately five times smaller than from the bulk state. The shallow trap is likely to be substitutional boron or nitrogen. The shallow traps anneal mainly around 700 K. Deep traps are found in all samples, yielding lifetimes tau DI approximately=260 ps and 290 psDII<or=315 ps. These lifetimes occur in n-type, p-type and semi-insulating materials. The results support the authors' earlier contention that the 290-315 ps components are due to divacancies and the 260 ps lifetime is due to monovacancies, in complex form with some impurity. The divacancy anneals around 620 K while the mono-vacancy-impurity complexes generally anneal around 800 K.