Journal of Electronic Materials, Vol. 22, No, 12, 1993 Special Issue Paper Structure and Carrier Lifetime in LT-GaAs ZUZANNA LILIENTAL-WEBER, H.J. CHENG,* S. GUPTA,* J. WHITAKER,* K. NICHOLS,* and F.W. SMITH* Center for Advanced Materials, Materials Science Division, Lawrence Berkeley Laboratory, 62/203, University of California, Berkeley CA 94720 *Center for Ultrafast Optical Science, The University of Michigan, Ann Arbor MI 48109 *MIT Lincoln Laboratory, Lexington MA 02173-0073 The relationship between the structural quality of low-temperature GaAs layers and the photoexcited carrier lifetime has been studied. Transmission electron microscopy, x-ray rocking curves, time-resolved reflectance methods, and photo- conductive-switch-response measurements were used for this study. For a variety of samples grown at temperatures in the vicinity of200~ subpicosecond carrier lifetimes were observed both in as-grown layers, as well as in the same layers after post-annealing and formation of As precipitates. These results suggest that the carrier lifetime, which was found to be shorter in the as-grown layers than in the annealed ones, might be related to the density ofAsca antisite defects present in the layers. The annealed layers which contained structural defects before annealing appeared to exhibit the longest carrier lifetime due to gettering of As on these defects (and formation of relatively large As precipitates) and depletion of extra As (ASGa) defects from the layer. It was found as well that the responsivity of detectors fabricated on these layers depended strongly on the structural quality of the layers, with the greatest response obtained not for the layers with the fewest defects, but for the layers with 107-108/cm 2 of pyramidal defects. Key words: Carrier lifetime, low-temperature-grown GaAs, MBE, TEM INTRODUCTION The key to understanding the properties of GaAs layers grown by molecular beam epitaxy (MBE) at low temperatures (180-300~ (LT-GaAs) 1~ seems to be in the incorporation of high concentrations (up to 1.5%) of extra As, which leads to an increase in lattice parameter (up to 0.15%). 5,~ The crystal perfection of these layers is strongly related to parameters such as growth temperature, As/Ga flux ratio, and growth rate2 Layers grown at 200~ or above are monocrys- talline, with no line or extended defects. However, breaking of crystallinity can be observed, especially for layers grown below 200~ 5,~At a specific critical layer thickness (related to the concentration of incor- porated As), crystallinity breaks down, due to forma- tion of polycrystalline grains or characteristic "pyra- (Received April 12, 1993) midal" defects with polycrystalline cores (mixture of GaAs and As grains)surrounded by dislocations, stacking faults, and microtwins. Annealing of LT layers, either due to the growth at higher temperature of a cap layer on the top of the LT layer, or ex-situ annealing, leads to the formation of hexagonal arsenic precipitates2 ,7-10 These precipi- tates are semi-coherent with the matrix, and due to their formation, strain is relieved in the layer, and the lattice parameter decreases to the substrate value. Such annealed layers exhibit semi-insulating be- havior and thus may be employed as the active mate- rial for photodetector applications in which short lifetimes on the order of half a picosecond can be obtained. 4 After several years of study, however, it is still not clear if point defects (As antisite defects) or structural defects (precipitates, and line and plane defects) are responsible for the short reported life- time. Some researchers tend to interpret short life- 1465