A simple means of ensuring the purity of your cells’ source material is to include a mini regeneration cycle at the start of each day. Under some circumstances you may operate your MBE system 24/7 and never actually cool down your sources. Indeed in the ideal world of batch processing and in situ growth monitoring with a dream machine (see MBE: Dream machine) an MBE system could be run non-stop for 3 months. However even under this utopia of operation you do not need all the sources to be hot all of the time, doing so is simply too wasteful.
When you cool down a cell, any impurities in the system can condense on the shutter, cell divider, water-cooling jacket and worse still alloy with the source material. Simply reheating your cell to operating temperature and growing will result in the cell outgassing onto your sample at the start of the growth. To avoid this you simply need to “over heat” the cell by ~25°C for half an hour at the start of each day. The impurities will outgas more readily at the higher temperature, and upon cooling to operating temperature the source will be ultra-pure once more. This extra step also has the added bonus of reducing the settling time between the cell’s thermocouple registering the operating temperature and the cell’s source material equilibrating at the desired flux.
One’s first thought is: “this is going to take up my valuable MBE time”. Quench that thought immediately! Consider instead the ability to set a “wake up” time for the MBE system at say 0700. The system will then execute a recipe to ramp all the cells to outgas temperature, outgas all the cells for 30 minutes and then ramp them down to operating temperature. At around 0800 the system can then start a flux check on each of the cells and tune the beam equivalent pressure (BEP) to the desired values. At around 0845 the auto flux tuning is complete and the machine sits waiting patiently for you to grow your first sample. My software does this, what you need to ask yourself is: why doesn’t yours?