Post by Professor Lake Shore on Jan 31, 2017 17:09:51 GMT -5
For measuring transport properties of graphene in my probe station, are there any general recommendations on how to prevent contamination of my samples when characterizing under vacuum conditions?
The high-vacuum option accelerates the pump-down time and significantly lowers the chamber pressure when compared with the compact turbo pumping system – helping to minimize contamination of sensitive samples. Whether using the standard or high vacuum options, a full-range vacuum gauge is needed to measure the chamber pressure from atmosphere to operating pressures. The full-range gauge uses a combination of Pirani (for high pressure) and Cold Cathode (for low pressure) gauges. Device measurements have shown that surface-sensitive materials like graphene and other 2D materials can be contaminated by phenomena originating from the Cold Cathode vacuum gauge. At low pressures, residual gasses are ionized by the vacuum gauge and a small portion of the ions can spread into the chamber – exposing graphene or similar samples to these chemical species and potentially changing the sample surface. To prevent this Cold Cathode ion gauge-induced contamination from occurring, it is necessary to unplug the gauge before the gauge switches from the high-pressure Pirani mode to the low-pressure ionization mode, specifically at the point when the chamber reaches 10-2 mbar pressure (as indicated by the pump controller or digital readout on the side of the pump). The turbo pump itself will continue to operate, but with gauge disabled, it will no longer be ionizing gasses in the chamber.