MBE Setup and Material Growth

The growth was carried out in an Omicron MBE (Lab 10) with a ∼10^–10 mbar base pressure. Cleaved 1 × 1 cm² GaAs(111)B substrates were outgassed at up to 500 °C for at least 30 min. The native oxide was removed from the surface of GaAs(111)B by heating it to 350 °C under a flux of atomic hydrogen. Hydrogen molecules were dissociated by a tungsten filament with Joule heating at 70 W and were introduced into the chamber via a leak valve. The procedure lasted 30 min or more at base pressure of ∼3 × 10^–7 mbar, resulting in sharp streaks in RHEED. A Kundsen cell and an electron beam source (EFM-3 from Omicron) were used for Se and Mo evaporation, respectively. The flux rates were calibrated using a quartz crystal microbalance, and the flux ratio of Se/Mo was optimized to be ∼40 for growth. A RHEED camera (Staib Co.) was used to monitor the growth in situ. The growth temperature was optimized in the 470–530 °C temperature range. Post-annealing at up to 550 °C was performed in Se atmosphere. Higher temperature leads to GaAs decomposition and increased surface roughness.