Formation of anion vacancies by Langmuir evaporation from InP and GaAs (110) surfaces at low temperatures

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A low-temperature Langmuir-evaporation regime is observed by scanning tunneling microscopy on InP and GaAs (110) surfaces. It is characterized by the formation of positively charged anion monovacancies even at room temperature. This vacancy formation has been studied as a function of time on InP between 293 and 480 K and on GaAs at room temperature. On InP the maximum vacancy concentration is obtained at 435 K. At this temperature 1.2% of the P surface sites are vacant. At higher temperatures the concentration decreases. The observations are explained by a competition between, on the one hand, P-vacancy--adatom pair production followed by P₂ molecule formation and desorption and, on the other hand, phosphorus outdiffusion from the bulk.