It is a well known fact in the MBE community that the group V element should be over supplied during III-V MBE. If pressed a grower will probably recommend a 1.6:1 As:Ga ratio when discussing optimal GaAs growth conditions. When asked why (?) the answer is simple: because these are the optimum conditions. But why ?
Well upon heating GaAs in a vacuum a number of notable phenomena occur. Arsenic is the more volatile of the two species and this fact underpins the observed phenomena. Firstly, the sublimation of As bulk: any amorphous As that condensed on the surface will readily evaporate at around 300 °C resulting in a c(4×4) reconstruction. Secondly, sublimation of As-As dimers: further heating in the absence of an external As flux results in a (2×4) reconstruction appearing in favour of c(4×4) at ~400°C; this represents the loss of As dimers back bonded onto As lattice sites. Thirdly, breaking of the As-Ga bond: continued heating under no external As flux results in a very rough surface as As-Ga bonds break and the upper GaAs surface thermally decomposes into sublimed As and Ga droplets.
To prevent thermal damage above 400°C it is therefore necessary to supply an As over pressure. The actual magnitude is almost irrelevant so long as it is large enough to compensate As loss from the surface. This minimum As overpressure depends on the substrate temperature. Supplying a flux close to the minimum is advantageous since it allows the surface to readily anneal and flatten; and also (from an economical point of view) results in minimal wastage of As.
Supplying a Ga flux to the mix results in GaAs growth. Now the required As flux has two parts, firstly it needs to compensate for As loss, secondly it needs to compensate for the additional Ga on the surface and ensure stoichiometric crystal growth. The total required As flux can therefore be thought of as:
Astotal = AsAs + AsGa
AsGa is temperature independent. The AsGa flux required to compensate the Ga flux is by definition 1:1. A slightly higher flux is recommended to prevent unwanted Ga-Ga interactions on the surface, though fluxes greater than 1.2:1 start to impair adatom mobility (increasing roughness) and increase the chance of point defects (such as anti sites). Conversely AsAs is temperature dependent, but Ga independent. At the optimum MBE growth temperature for GaAs formation (~600 °C) the AsAs flux required to maintain the surface is ~0.3 ML/s; which is a sizable fraction of the typical MBE growth rates of 0.5 – 1.0ML/s. Combining these two constituents means the actual optimum As:Ga flux ratio is (…as an MBE grower I would say…) about 1.6:1.