Before attempting the first self-seeded GaAs nanowire growth on epiready Si(111)-n, the MBE reactor was thoroughly calibrated. The Group III cells were outgassed (see Post-bake tasks: Cell outgas) and the beam equivalent pressure (BEP) data for each cell was obtained (see Post bake tasks: Group III flux: Arrhenius plots). The BEP data for the As valved cracker source was similarly obtained (see Post bake tasks: As cracker flux). Growth was then performed on an undoped GaAs(100) sample in order to put some meaning to the arbitrary mBar or nA values. The mBar values were converted to ML/s and atoms/nm2/s using the method outlined in Little known MBE facts: Flux determination and the articles linked therein.
GaAs self seeded NW growth is very sensitive to substrate temperature and the As:Ga flux ratio. The actual growth rate is not so important, somewhere around 0.12 ± 0.03 ML/s is a good starting value. This equates to 0.75 ± 0.19 atoms/nm2/s. The sample temperature was monitored by a pyrometer at normal incidence to the substrate. The generated undoped GaAs(100) static reconstruction map (see Little known MBE facts: Making a static reconstruction map) was used to verify the pyrometer reading to with ±10°C and the emissivity of the pyrometer was adjusted to read this absolute value. The emissivity equalled 0.65 in this case.
The flux ratio was established at a substrate temperature of 580°C. At this temperature only 0.05 ML/s (0.31 atoms/nm2/s) of As is required to maintain a (2×4) reconstruction. With the Ga cell set to 0.11 ML/s (0.69 atoms/nm2/s) the As:Ga ratio was tuned such that the RHEED pattern immediately turned (3×6) upon opening the Ga shutter AND THEN turned to (4×2) after around 15 seconds on Ga deposition. This is arguably around 1:1 for this substrate temperature, perhaps slightly Ga rich in fact, but a good starting point for self-seeded NW growth nonetheless.
With all that done, we are ready to attempt our first self-seeded GaAs NW growth on Si(111) –n epiready substrates. Importantly here for a reference the pyrometer emissivity is not changed (even though the emissivity of Si is indeed different to GaAs). The BEP values of the established 1:1 fluxes were checked and recorded for later reference and the Si(111)-n (CrysTec Si(111) n type (Phosphorus doped), 275um thick SS polished) sample was loaded. You can find out how the first NW sample proceeded in the following article: Nanowires: First NW sample.