Nonstoichiometric GaAs thin layers can be produced in molecular beam epitaxy if they are grown at temperatures below 400 degrees C [low-temperature (LT)-GaAs]. Due to the incorporation of excess As in the form of native point defects, namely As antisite defects (As-Ga). these layers exhibit ultrashort time response and, after annealing at 600 degrees C, excellent semi-insulating behavior. The ultrashort time response, however, is governed by the concentration of ionized antisites ([As-Ga(+)]), which are just a few percent of the total concentration of antisites ([As-Ga]) Additionally, thermal annealing leads to As precipitate formation and out-diffusion of point defects into adjacent layers. Recent studies have shown that p-type doping with Be increases the thermal stability of point defects and shortens the time response due to an increase in ionized antisites, while maintaining the high electrical resistivity in as-grown material. We report on the studies of p doping of LT-GaAs with Be and, alternatively, with C in order to enhance the thermal stability in semi-insulating thin layers with ultrashort carrier trapping times. The epilayers were characterized and their electronic properties investigated by time-resolved reflectivity transients and Hall measurements. The properties of as-grown and annealed thin layers will be discussed based on the results of defect concentrations. These results will be compared to those obtained in undoped LT-GaAs.