Little known MBE facts: Temperature determination and RHEED

Faebian Bastiman

MBE surface characterisation benefits greatly from reflection high energy electron diffraction (RHEED) in situ monitoring. RHEED can give information regarding roughness, surface order, growth rate and even polycrystalline grain size. It also proves a highly repeatable means of temperature determination with reasonable accuracy. The secret is differentiating the three different types of temperature dependence.

Type 1 is flux independent, substrate independent. Being independent of flux and material is very useful, as it means you are also system independent. Unfortunately only a limited number of these points exist. The most obvious one on a III-V MBE system is As cap removal. This is the evaporation of As bulk from any substrate surface. Since this is a property of the As and not the substrate it tells you when any substrate is at ~300 ± 10°C. So you can quickly compare the temperature of S.I. and n+ GaAs and also InAs, InP and GaSb. The RHEED transition is amorphous to single crystalline.

Type 2 is flux independent, substrate dependent. A well known example of a type 2 RHEED transition is oxide removal. All substrates have a specific temperature at which the native oxide thermally decomposes. GaAs is 590 ± 10°C. InAs is 500 ± 10°C. A less well known type 2 transition can be utilised if the substrate has both c(4×4) and (2×4) reconstructions. GaAs and InAs are good examples. Under no external As flux a RHEED transition occurs where As-As bonds supporting the 1.75ML of As of the c(4×4) thermally destabilise and only 0.5ML of As on the (2×4) remains. For GaAs this happens at 400 ± 10°C. For InAs it seems to occur at a similar temperature. For AlAs the c(4×4) to (2×4) appears to be somewhat higher. Regardless of the absolute temperature, type 2 transiton always occur at the same temperature for a given material system, so it can be used as a quick temperature calibration point.

Type 3 is both flux and substrate dependent. A number of static reconstructions exist on III-V substrates. Each happens at a specific temperature under a specific As flux. The reconstructions are c(4×4) to (2×4) to (4×2) and each represents the loss of As from the surface. Hence the larger the As flux the higher the temperature at which the reconstruction transition occurs. Accurate and repeatable temperature determination relies on accurate and repeatable flux determination. However if you calibrate your III:V ratio using the steps in Little known MBE facts: Growth rate and flux the c(4×4) to (2×4) transition can be used to estimate 500 ± 20°C for a 0.5 ML/s As flux.

Which means for GaAs(100) we can determine with reasonable accuracy 300, 400, 500 and 600°C